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Heat and Mass Transfer within the Diffusion Driven Desalination Process with Heated Air

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PAGE 1

HEAT AND MASS TRANSFER WITHIN THE DIFFUSION DRIVEN DESALINATION PROCESS WITH HEATED AIR By JESSICA KNIGHT A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006

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Copyright 2006 by Jessica Knight

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To my parents, for their infinite supp ort and love throughout my college career.

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iv ACKNOWLEDGMENTS I would like to thank everyone who has cont ributed to the successful completion of this work. First, I would like to thank Dr James Klausner for his constant guidance throughout the past two years a nd for providing me the opportuni ty to do research on the diffusion driven desalination pr oject. His patience, academic supervision, and technical assistance are greatly appreciated. I would also like to extend thanks to Dr. Renwei Mei. His academic lectures and technical discussi ons were always a welcome challenge. I would also like to thank Dr. David Hahn for serving on my supervisory committee. I would also like to thank all of my peers who helped me through my graduate studies career. A special thanks goes to Yi Li for his assistance and advice. This research was supported by the U.S. Department of Energy under Award No. DE-FG26-02NT41537. Without thei r financial assistance this research would not be possible. Lastly, I would like to thank my parent s for their unceasing love and support throughout my academic career. Their consta nt encouragement and understanding gave me the motivation necessary to achieve my goals.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii NOMENCLATURE..........................................................................................................xi ABSTRACT.....................................................................................................................xi v CHAPTER 1 INTRODUCTION........................................................................................................1 Methods of Desalination...............................................................................................1 Membrane Processes.............................................................................................2 Thermal Processes.................................................................................................3 Advancements in Desalination.....................................................................................4 Desalination and the Environment..............................................................................10 Comparison of DDD with RO and MSF....................................................................11 Scope of Work............................................................................................................12 2 EXPERIMENTAL FACILITY..................................................................................14 System Overview........................................................................................................14 Description of Indi vidual Components.......................................................................17 3 HEAT AND MASS TRANSFER WITH IN THE DIFFUSION TOWER.................26 Theoretical Heat and Mass Transfer Model...............................................................26 Results and Discussion...............................................................................................33 Case 1: Heated Air/Ambient Water.....................................................................34 Case 2: Heated Air/Heated Water.......................................................................39 4 PARAMETRIC ANALYSIS......................................................................................47 Parametric Analysis of Heated Air/Heated Water DDD Process...............................47 Heated Air/Heated Water Results and Discussion.....................................................49 Comparison of Different DDD Processes...................................................................55

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vi Heated Water/Ambient Air at 60 C...................................................................55 Heated Air/Heated Water using Q-PAC..............................................................59 Industrial Plant Application........................................................................................61 Operation Conditions...........................................................................................62 Economic Analysis..............................................................................................65 5 CONCLUSION...........................................................................................................73 APPENDIX A ONDAS CORRELATION........................................................................................77 B EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED AIR/AMBIENT WATER...........................................................................................78 C EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED AIR/HEATED WATER.............................................................................................80 LIST OF REFERENCES...................................................................................................82 BIOGRAPHICAL SKETCH.............................................................................................84

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vii LIST OF TABLES Table page 1-1 Comparison of energy consump tion for desalination processes................................5 1-2 Cost estimation per cubic meter of water for various processes................................6 4-1 DDD optimal operating conditi ons for industrial site..............................................66 4-2 Summary of DDD Plant Costs.................................................................................69

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viii LIST OF FIGURES Figure page 1-1 Reverse Osmosis (RO) system ..................................................................................2 1-2 Multistage Flash (MSF) plant....................................................................................4 1-3 Flow chart of the DDD process..................................................................................8 2-1 Diffusion driven desalin ation (DDD) facility..........................................................15 2-2 Air heating section...................................................................................................16 2-3 Diffusion driven desa lination (DDD) system...........................................................18 2-4 Diffusion tower .......................................................................................................19 2-5 An Allspray water distributor...................................................................................20 2-6 Shaped heater...........................................................................................................20 2-7 The HD Q-PAC packed bed.....................................................................................21 2-8 Water flow meter calibration curves........................................................................22 2-9 Main control.............................................................................................................24 2-10 Diffusion tower data.................................................................................................24 2-11 Histogram view of the DDD data acquisition..........................................................25 3-1 Differential control volume for heated air conditions..............................................27 3-2 Calibration curve of the h eat loss flux for the heated air/heated water case............34 3-3 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 0.77 kg/m2-s.....................................................................36 3-4 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 1.15 kg/m2-s.....................................................................37

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ix 3-5 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 1.55 kg/m2-s.....................................................................38 3-6 Repeatability of different experi ments for different exit parameters.......................39 3-7 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 0.79 kg/m2-s.....................................................................41 3-8 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 1.15 kg/m2-s.....................................................................42 3-9 Comparison of predicted and measured exit temperatures and humidity for similar air mass flux G = 1.55 kg/m2-s.....................................................................43 3-10 Repeatability of experiments for different exit parameters......................................45 4-1 Tower height at the maximum exit abso lute humidity as a function of the inlet air mass flux G.........................................................................................................50 4-2 Exit air temperature at maximum absolu te humidity as a function of the air mass flux........................................................................................................................... 51 4-3 Maximum exit absolute humid ity with varying air mass flux.................................52 4-4 Fresh water production flux w ith varying air mass flux..........................................53 4-5 Diffusion tower energy consumption rate...............................................................54 4-6 Fresh water production effi ciency with varying air mass flux realized for the lower air and liquid mass fluxes...............................................................................56 4-7 Fresh water production efficiency an d energy consumption rate for varying liquid mass flux for 60 C inlet conditions..............................................................58 4-8 Fresh water production flux and energy consumption rate for varying liquid mass flux for 60 C inlet conditions.........................................................................58 4-9 Fresh water production flux and energy c onsumption rate for varying liquid mass flux for HD Q-PAC a nd Q-PAC packed bed..................................................60 4-10 Fresh water production flux and energy consumption rate for varying liquid mass flux for HD Q-PAC a nd Q-PAC packed bed..................................................61 4-11 Sample schematic of the DDD system coupled with an industrial site....................63 4-12 Fresh water production flux for vary ing diffusion tower liquid mass flux..............64 4-13 Energy consumption rate of the DDD sy stem for varying diffusion tower liquid mass flux..................................................................................................................64

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x 4-14 Fresh water production efficiency for varying diffusion tower liquid mass flux.....65 4-15 Net fresh water profit as a function of energy retail price for varying water retail price.................................................................................................................71 4-16 The bottled water market in the United States.........................................................72 4-17 Net fresh water product as a function of energy retail price for varying distilled bottled water prices..................................................................................................72

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xi NOMENCLATURE A Cross sectional area (m2) a Overall specific volume of the packed bed (m2/m3) aw Wetted specific area of the packed bed (m2/m3) CP Specific heat (kJ/kg-K) D Molecular diffusion coefficient (m2/s) f Plant availability d Diameter of the tower (m) dp Diameter of the packed bed (m) G Air mass flux (kg/m2-s) g Gravitational acceleration (m/s2) hfg Latent heat of vaporization (kJ/kg) h ` Enthalpy (kJ/kg) i Interest rate K Thermal conductivity (W/m-K) k Mass transfer coefficient (m/s) L Water mass flux (kg/m2-s) Mv Molecular weight of vapor (kg/kmol) m Mass flow rate (kg/s) n Plant life P Pressure (kPa) Pw Electrical power cons umption for pumps (kW) Q Retail cost ($)

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xii qHL Heat loss flux (kW/m2) R Universal gas constant (kJ/kg-K) T Temperature (C or K) U Heat transfer coefficient (W/m2-K) VG Volumetric flow rate (m3/s) Specific cost of operating labor ($/m3) Dynamic viscosity (Pa-s) Density (kg/m3) c Critical surface tension of the packed bed (N/m) L Liquid surface tension (N/m) Absolute humidity Relative humidity Profit ($) Subscripts a Air elec Electricity evap Portion of liquid evaporated fixed Fixed cost fw Fresh water G Air/vapor mixture GA Gas side parameter based on the specific area of the packed bed i Liquid/vapor interface in Inlet parameter

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xiii L Liquid LA Liquid side parameter based on th e specific area of the packed bed Labor Labor Cost LW Liquid side parameter based on th e specific wet area of the packed bed mix Air/vapor mixture out Outlet parameter sat Saturated state unit,p Unit amount in terms of production z Fluid flow direction

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xiv Abstract of Thesis Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science HEAT AND MASS TRANSFER WITHIN THE DIFFUSION DRIVEN DESALINATION PROCESS WITH HEATED AIR By Jessica Knight December 2006 Chair: James F. Klausner Major Department: Mechanic al and Aerospace Engineering The purpose of this research is to examin e the performance of the diffusion driven desalination process (DDD) with heated ai r inlet conditions. A laboratory scale DDD facility has been constructed and fully in strumented. Experiments were conducted and data were collected for two different cas es: heated air/ambient water and heated air/heated water. The experiments were conducted over a range of liquid and air mass fluxes. A theoretical heat and mass transfer model was compared against the experimental data collected. The experime ntal values agree quite well with the theoretical model, provided the fraction of area wetted is corr ectly specified. The heated air/heated water case is demonstrated to be a more efficient process than the heated air/ambient water case. A parametric study re veals that for every liquid mass flux there is an air mass flux value where the diffusion to wer energy consumption is minimal and an air mass flux where the fresh water production flux is maximized. A study was also performed to compare the DDD process with di fferent inlet operating conditions as well

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xv as different packing. It is shown that the heat ed air/heated water case is more capable of greater fresh water production with the sa me energy consumption than the ambient air/heated water process at high liquid mass flux. It is al so shown that there can be significant advantage when using the heated air/heated water process with a less dense less specific surface area packed bed. A case study with the DDD process was coupled to an industrial site that produces 850,000 acfm heated waste air at 82C. For inlet air conditions at 82C, the fresh water produc tion and system energy consumption were both optimal at a diffusion to wer liquid mass flux of 0.15 kg/m2-s, a system air mass flux of 1.5 kg/m2-s, and a fresh water to air mass flow ratio of 2 in the direct contact condenser. It was determined that with the available energy and operations at the optimal conditions the plant can produce 201,800 gal/d ay with a total electrical energy consumption of 0.0012 kW-hr/kg and a fresh wa ter production efficiency of 0.224. The total footprint area required is 526.2 m2. An economic study revealed that the most financial gain can be achieved when the product distilled water is sold as bottled water. Without considering the cost of bottling, if a gallon of distilled water costs $1.00, then a profit of $0.99 per gallon is expected. T hus the DDD process is economically viable when driven by waste he at carried by air.

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1 CHAPTER 1 INTRODUCTION Water is an essential part of sustaining life on earth, and its use is widespread for industrial, irrigation, mining, a nd domestic use. While water is abundant on earth, 97% of the water is saline, while on ly 3% is freshwater. Of the 3% freshwater, 70% of that is frozen in glaciers, ice, or permafrost; 30% is groundwater, while only a mere 0.25% is available above the ground in the form of la kes and rivers. In 2000 Clarke and King [1] estimated that of the 6 billion people on Earth, 0.5 billion people live d in countries that were chronically short on water. It is projected by the year 2050 that the world population will grow to 8.9 billion people and that 4 billion people may live in countries that are chronically short on water. A growing population is accompanied by an increasing need for agricultural and industrial output. Agriculture currently accounts for nearly 70% of freshwater withdrawals, a nd the demand for food will only increase with increasing population [1]. Growth in industry is also expected. The industrial use of water is expected to grow steeply over the ne xt 25 years as more countries industrialize [1]. Given the fact that the population on earth continues to in crease and industrial growth shows no signs of slowing down, it is inevitable that conventional sources of freshwater are not sustainabl e. The only water resource th at is inexhaustible is the oceans. Thus a solution for sustainabili ty may lie in desalination technologies. Methods of Desalination Desalination is the process by which salt is removed from water to produce fresh water. There are several different types of desalination, however the two main types are

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2 membrane and thermalor phase change de salination. Membrane desalination involves the use of a membrane to remove the salt usi ng either electrical force or mechanical force in the separation process. There are two ma in types of membrane processes: reverse osmosis (RO) and electrodialysis. Unlike membrane processes, thermal desalination removes the salt by causing the solution to undergo a change in phase. Multistage flash (MSF), multiple-effect distillation (MED), and vapor compression (VC) are the most common thermal desalination processes. Membrane Processes The most common membrane process is reverse osmosis (Fig. 1-1). Reverse osmosis utilizes a semi-permeable membrane to separate the unwanted ions from the solution. The force drivi ng the solution through the membrane is provided by a feedwater pump whose pressure depends on the concentration of the solid, desired recovery, and overall performance of the memb rane. The RO membranes are arranged in pressure vessels often containing 1-7 spir al-wound membrane elements. These vessels can be placed in series or parallel dependi ng on the desired concentration of the final desalted water product. Figure 1-1. Reverse Osmosi s (RO) system [2].

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3 Another type of membrane process is el ectrodialysis, (ED). Unlike RO, ED uses an electrical force to drive the ions through the membrane. It is an electrochemical process where electrodes are placed in a soluti on of the dissolved solid and a dc power is supplied. The ions will migrate towards the el ectrode of opposite charge of the given ion. The movement of ions is controlled by ionselective membranes that form watertight compartments. These membranes are electrically conductive and impermeable to water under pressure. Thermal Processes Multistage flash (MSF) is the most widely used thermal process. The MSF process is initiated by heating seawater using steam flowing over the water tubes. The water tubes are enclosed in a vessel called the brin e heater. The water then flows to another chamber where the pressure is lowered to the point where the water will either boil or flash into steam. In general there is not a sufficient amount of vapor formed thus several evaporative stages are needed to produce more steam. The steam produced is usually condensed on the heat exchanger tubes r unning through each stage producing freshwater. Fig 1-2 depicts a schematic diagram of a sample MSF desalination plant. Multi-effect distillation (M ED) and vapor compression (VC) evaporation are other types of thermal distillation processes that are used. MED is quite similar to MSF except in MED, except feedwater is sprayed on the outer surface of the steam tubes to enhance boiling or flashing. VC is generally used fo r small scale freshwater production and is very similar to MED. However unlike MED, VC typically utilizes mechanical energy, via a mechanical compressor, rather than di rect energy to supply the thermal energy to heat the incoming feedwater. The mechanical compressor creates a vacuum in the vessel

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4 and compresses the vapor removed from the vessel. As the vapor exits the vessel it condenses on the inside of a tube bundle and re leases heat. The feed water is then sprayed on the tube bundle where boili ng and evaporation occurs. Figure 1-2. Multistage Fl ash (MSF) plant [2]. Advancements in Desalination Desalination is a rapidly growing technol ogy. As noted by Mielke and quoted in Dore [3], there are approximately 11,000 desa lination plants in 120 countries around the world with a combined capacity of 13.25 Mm3/d. He also noted the three factors which have the greatest impact on the overall cost of desalination per unit of fresh water produced: salinity of inlet feedwater, energy costs, and plant size. While RO and MSF are still the more widely used methods of desalination there ar e drawbacks to the processes. The most important is the fact that all of the processes are very energy intensive and are generally only useful fo r large scale production. With large energy requirements, cost is certainly an issue. John Keys, commissioner of the Bureau of

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5 Reclamation, has said that cost reduction is the single most important factor to increase the implementation of desalination [4]. Tabl e 1-1 presents a comparison for the costs of the various desalination proce sses [5]. As the table shows energy demands of the various processes can be extensive. Thus a need em erges for a more energy efficient desalination process. Table 1-1. Comparison of energy consum ption for desalination processes. Process Type Distillation Membrane Desalination Process MSF MED MVC RO ED Thermal Consumption, kWh/kg 0.0700.084 0.0420.061 None None Electrical Consumption, kWh/kg 0.00350.005 0.00150.0025 0.00150.002 0.0050.009 Of recent interest is a fairly new proce ss entitled humidification dehumidification (HDH). Bourouni et al. [6] de scribes the process as simple and flexible with low installation and operating costs and encompasses a possibility of utilizing a low temperature energy source. In this process ai r is drawn through a packed bed tower, the humidifier, where air and prehea ted seawater will meet and heat and mass transfer will occur creating a saturated air/vapor mixture. The air/vapor mixture then moves to the dehumidifier where the vapor is condensed out. The extractio n of the vapor from the air can be done in several ways including mechanical, refrigeration, adsorption, and absorption methods. The most common met hod is through film condensation used by Bourouni et al. [7]. He reported on an aero-evapo-conde nsation process using waste geothermal heat and commented on the economic competitiveness of the process. Table 1-2 summarizes his results. From the tabl e it can be seen that the HDH principle is competitive when coupled with the energy from waste heat.

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6 Bourouni et al. [6] commented on the num erous advantages of HDH over other common forms of desalination. First, HDH is flexible in capacity. In general, the only components required are an evaporator and a condenser which can be designed to be compact. Also, due to the low temperature and pressure operating range, components are primarily plastics which are light, inexpe nsive, and easy to clean. Perhaps most important however is the fact that it can run off of low grade heat which implies that the only energy required for the system is the pumping power. Table 1-2. Cost estimation per cubic me ter of water for various processes. Plant Unit Water Cost/m3 MSF with back-pressure steam turbine 1.57 MSF with gas turbine an d waste-heat boiler 1.44 MSF/TVC with gas turbine and waste heat boiler 1.31 RO single-stage with energy recovery 1.39 Aero-evapo-condensation pro cess (geothermal energy) 1.15 Aero-evapo-condensation process (fuel) 4.80 Employing the principles of HDH, a simila r process, multiple effect humidification, was developed. This process utilizes the sa me principles of HDH with the addition of multiple evaporation and condensation cycles Al-Hallaj et al. [8] reported on a MEH unit utilizing solar collection panels to provi de the heating source for the water. They tested both a pilot and a bench unit over a ra nge of operating conditi ons. They concluded that an increase in water flow rate will maximize the production of the unit to an optimum point, but will also decrease the ope rating water temperature which leads to a decrease in efficiency of the evaporator and condenser. They also determined that the amount of freshwater produced was directly rela ted to the season. They were also able to increase the production at night by utilizing the hot water reje cted from the humidifier.

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7 Vlachogiannis et al. [9] re ported on mechanically intensified evaporation (MIE) utilizing the HDH principle in combination wi th vapor compression and a heat pump. In the MIE process air enters an evaporation chamber through a porous media and is dispersed in small bubbles through the liquid. The exiting stream is then compressed by a blower and directed to a shel l and tube heat exchanger. They reported adequate results, but improvements in the condenser surface area were needed to yield a cost effective process. While HDH shows promise it has its downfa lls in comparison with the more well known methods of desalination. First, th e overall production rate is smaller in comparison with RO and MSF and thus ca nnot compete for large scale production. Second, in general natural draft is relied upon for the air. This results in lower heat and mass transfer coefficients as well as an incr ease in the area of the humidifier. Finally, a shell and tube type heat exchanger is typically used as the de-humidifier. This method of heat exchange directly depends on the am ount of surface area. For large freshwater production, more condenser surface area is required which translates to an increase in the amount of land needed. Thus a more efficient means of desali nation should be used to overcome the limitations of the HDH method. Klausner et al. [10] described a diffusion driven desalination (DDD) process that may provide an economically feasible solution to the shortcomings of HDH. Li et al. [11] describes the DDD process, wh ich is designed to r un off of waste heat and is depicted in Figure 1-3.

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8 Main Feed Water Heater (a) Main Feed Pump Seawater Reservoir Fresh Water Pump Water Cooler (d) Cooler Pump Diffusion Tower (b) Direct Contact Condenser (c) Exhaust Fresh Water Production Fresh Water Storage Tank Low Pressure Steam Seawater Air/Vapor Fresh Water Forced Draft Blower Power Plant Figure 1-3. Flow chart of the DDD process [11]. The process consists of three main flow lines: the inlet seawater, the air/vapor mixture, and the freshwater. The seawater is drawn from near the surface of a geothermally stratified seawater source and then pre-heated in a water cooler (d). The importance of drawing the water from the stratif ied source is that the water that is on the surface is much warmer than that at deeper depths due to the solar absorption at shallower depths and thermal stra tification. It then flows to the main heater (a) where it is heated using waste heat. Low pressure c ondensing steam from a thermoelectric power plant is one possible source of waste heat. Once heated, the seawater is then sprayed through the diffusion tower (b) that contains low-pressure drop, high surface area packing material. Simultaneously, a forced draft bl ower impels the air/vapor mixture through the diffusion tower where a portion of the seawater will evaporate in the air/vapor mixture. The seawater that is not evaporated will exit the diffusion tower as brine discharge. The

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9 saturated air/vapor mixture will enter the direct contact condens er (c) which is the innovative idea behind DDD. As compared to HDH, a direct contact condenser approach is taken to overcome the amount of surface ar ea that would otherwise be required for a shell and tube type heat exchanger. The di rect contact condenser allows direct contact between the air/vapor mixture and water ther eby improving the heat and mass transfer of the process. A portion of the freshwater product is used to condense the saturated air/vapor as the cooling agent. The freshw ater produced will exit the condenser and be pumped to the water cooler where it will be cooled by the feed seawater. A portion of the cooled freshwater is delivered back to th e condenser, and the remaining freshwater is then dumped into the freshwat er reservoir as product. The key differences in the DDD process and the HDH process are: 1. A direct contact condenser, in lieu of a shell and tube h eat exchanger, is used to decrease the amount of surface area requi red to condense the air/vapor mixture and increase the condensation effectiven ess. The decrease in size of the condenser contributes to less required material as well as less required land space. 2. The DDD process is driven by waste heat from power plants, however, it can be designed to be geography specific depe nding upon the location. For example, other forms of waste heat may be used su ch as solar heating, geothermal spas, or wind turbines may be used. 3. Forced draft is used in combination with a packed bed tower as the humidifier. The forced draft provides a constant s ource of dry air. The packed bed is designed to be made of plas tic due to the low operating temperature and pressure. Plastics are easier to maintain as well as cheaper to replace. The packed bed also provides an increase in surface area for more heat and mass transfer between the air and water and thus greater production. 4. The HDH process is compatible for only small scale production whereas the DDD process can be used for larger flow rates and thus at a larger scale for increased production. 5. The components used for the DDD process are commonly available from a variety of different manuf acturers or retailers.

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10 Given the advantages of DDD over the HDH pr ocess it is worthwhile to research the overall production rates and economic feasibility of the process. Desalination and the Environment Water desalination ha s increased substant ially throughout the 20th century. With this increase has come an improvement in the quality of life. This improvement, however, comes at a price. The price is paid through the damage done to the environment. Water desalination contributes in a multiple of different ways to the degradation of the environm ent. These include incr eased occupation of land by desalination plants, the contamination of groundwater, damage done to marine biology, noise pollution, and the energy and discharg e of combustion of products all have a negative impact on the environment. In a world where the population is stea dily increasing and total land remains constant, land use and occupation is always an issue. For example, Sadhwani et al. [12] noted that a typical RO plant requires a land area of about 10,000 m2 (2.47 acres) to produce between 5,000-10,000 m3/day (1.32 million gal/day to 2.6 million gal/day). In addition to the land footprint occupied by the pl ant, the infrastructure of the plant is a concern. The feed seawater pipes, the elec trical transmission lines, and the product water pipes are all important parts of the infrastructure that require space and land use. The groundwater could also face contaminati on from a desalination plant. A plant that is built over or near an aquifer will have pipes to transport the inlet seawater as well as those for product discharge. These pipe s could leak, allowing saltwater to seep through to an aquifer. Care must be ta ken to ensure no leak s are present. [12]. Morton et al. [13] commented that the marine life can su ffer in several ways from the discharge of desalination plants. The brine discharge from desalination plants is

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11 typically of increased temp erature and of higher salinity and depending upon treatments used during the process, rich in other chemi cals as well. The warmer water and increased salinity can also reduce the amount of dissolved oxygen in the water and therefore restricts marine life in the vi cinity of the plant. The pr esence of other chemicals could intensify this effect. The temperature increas e could result in death of marine life or a reduction in the rate of metabolic proce sses which retards maturity, life stage development, and size. Morton et al. [13] c oncluded that thermal desalination processes are more prone to have a greater thermal impact on the environment whereas membrane processes typically have a greater salt concentration of the discharge. The noise from a desalination plant is a concern in the vicinity surrounding the plant as well as to the workers of the plant. This acoustic contamination is primarily an issue for RO plants where high pressure pum ps and energy recovery systems can produce noise in excess of 90 dB(A) [12]. Of perhaps greatest importance is the atmospheric emissions from the input energy. Recall Table 1-1 and the substantial amount of input energy required for various desalination processes. Thes e processes generally rely on the combustion of fossil fuels to supply the necessary energy. The principl e emissions from the combustion of fossil fuels are sulfur dioxide, nitrogen oxi des, carbon dioxide, carbon monoxide, and suspended particulate matter [13]. From Tabl e 1-1 it can be seen that the MSF is more energy intensive when compared with RO a nd thus more likely to contribute to air contamination. Comparison of DDD with RO and MSF It is obvious that economics as well as environmental concerns play an important role in the viability of desa lination processes. One of the biggest advantages of DDD

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12 over RO and MSF is that DDD is driven by waste heat, therefore the only energy required is that to pump the air and water th rough the system. Since low pressure drop packing is used the energy required for pumping is not prohibitively expensive. This has very important implications for the environment. DDD takes advantage of energy that would have otherwise been discarded. Since it does not require as much energy as RO or MSF it is expected that the atmospheric emissi ons will be significantly less. Breschi [14] reported on the performance of a desalination unit called LTF (low-temperature flash) that ran off of waste heat. The unit consists of a vacuum flash chamber (the evaporator), a shell-and-tube exchanger (the distillate c ondenser), and a vacuum system. The process is driven by the small temperature difference in the warm seawater exiting a steam turbine. The environmental impact of that facility was considered negligible due to the low change in salinity of the brine and the lack of chemical additive to reduce scaling since the operating temperature is low. Another advantage of DDD is that th e main components are inexpensively manufactured. The operating te mperatures and pressures are low; therefore most parts can be constructed from simple plastics. Un like RO, there are no membranes that must be changed and maintained. While RO and MSF require specialized high pressure and large pumps respectively. The DDD process has the potential to be economically competitive while having less impact on the environment. The scope of th is research is to explore the feasibility of the DDD process under specific operating conditions. Scope of Work Li et al. [11] reported on the economic f easibility of the DDD process with heated input feedwater and ambient dry air. The pur pose of this research is to explore the

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13 concept of a heated dry air i nput. It is known that the amount of vapor carried by the air increases with increasing temperature, but further studies need to be conducted to determine the effect of a heated air input on the DDD process. Thus the goals of this study are as follows: 1. Design and install an air heated section to the DDD facility that is fully instrumented to measure the appropri ate heat and mass transfer rates. 2. Take experimental measurements over a range of flow conditions for two separate cases: the case where both inle t air and inlet feedwater are heated and the case with a heated air i nput with an ambient water input. 3. Develop and implement a mathematical model to correctly predict the heat and mass transfer behavior of the two cases. 4. Perform a parametric study to determ ine the optimal operating conditions. 5. Perform an economic study to determ ine the practicability of the DDD process for the two different cases. Chapter 2 describes the DDD process constructed and maintained for the current research. The instrumentation, the individual components, and softwa re used to collect the data are all described in detail. A theoretical model for a heated air input for both an ambient water input as well as a heated water input is presented in detail in Chapter 3. A control volume approach is taken to reduce the conservation equations to determine the governing equations of the two processes. The results for the two different cases will then be compared to the model and then discussed. Finally, in Chapter 4, the results from a study to determine the optimal conditions for both cases are presented. The economics of the process are also explored in an example of the DDD process coupled with an industrial plant.

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14 CHAPTER 2 EXPERIMENTAL FACILITY A laboratory scale diffusion driven desalin ation (DDD) facility has been designed and constructed to collect the heat and mass transfer data fo r the experimental analysis. The data collected will be used to verify the theoretical model for the diffusion tower for two different cases: heated air/heated water a nd heated air/ambient water. Data for both cases will be taken over a range of differe nt flow conditions. Th e performance of both cases will then be explored using the data collected. System Overview Figure 2-1 presents a schematic diagra m of the current DDD facility. The municipal water line, serving as the inlet so urce water, flows through a series of valves which determines the flow meter to measure the inlet mass flow rate. The water then flows through a series of heaters, the prehea ter and the main heater. The preheater can raise the temperature to a maximum of 50 C and the main feedwater heater is a PID temperature controlled heater. Once the water is heated, its temperature is measured and then it flows into the top of the diffusion to wer. The water is sprayed with a spray nozzle and flows through the tower packing via gravit y. The water not evaporated will exit the tower from a drain at the bottom where the temperature is measured using a type E ungrounded thermocouple. The dry air is drawn through a 3.68 kW ( 5.0 horsepower) centrifugal blower whose speed is regulated using a three phase autotran sformer. The air exiting the blower flows through a 10.2 cm nominal vertical duct where a thermal mass flow rate meter measures

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15 the air flow rate. The major modification to the system described by Li [15] is the addition of an air heating section. The air heat ing section is shown pictorially in Fig. 2-2. The U-shaped air heater sect ion is required to ensure enough pipe length for fully developed flow for the air flow measurement. The air flow meter is placed before the air heater since it was calibrated using ambient air. The air flows down the duct where a 4 kW tubular heater is installed. A thin sheet of aluminum lin es the inside of the duct to guarantee that the duct does not exceed its maximum operating temperature. The amount of power supplied to the air heat er is controlled by a single-phase autotransformer. The Figure 2-1. Diffusion driven de salination (DDD) facility.

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16 air relative humidity and temperature are then measured using a resistance type humidity gauge located downstream of the flow meter and heater in th e horizontal section. The air then enters the diffusion tower and is forced through the packed bed. The air exits the diffusion tower where the exit relative humidity and temperature are measured in a similar manner as at the entrance. Figure 2-2. Air heating section. The condenser is comprised of two differe nt stages: a countercurrent stage and a co-current stage. The condenser is designed in a twin tower structure where both towers are identically constructed. Both towers are made of 25.72 cm inner diameter acrylic tubing connected via two schedule 80 PVC elbows. Though this study only focuses on the performance of the diffusion tower, the co ndenser is included for completeness of the system.

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17 Fresh cold water is drawn from a diffe rent municipal line and split into two different lines, one line for each section of the condenser. The water flow rate is measured with two different tu rbine flow meters. The water temperature is measured and then the water is sprayed from the top of both of the towers using additional spray nozzles. The air leaving the diffusion tower is at an elevated temperature and humidity. The air enters the co-current condenser stage where the air flows co-currently with water and is cooled and de-humidified. Upon exiti ng the co-current stage, the air will flow through a 90 PVC elbow where it travels thr ough a 25.4 cm nominal diameter duct. The air temperature and humidity is measured with another resistance type humidity gauge as was used in the diffusion tower. The air th en enters the countercurrent stage where the air flows upward in the opposing direction of the falling water and continues to further cool and dehumidify. The ex it air temperature and humidity are measured with another gauge and the air will then exit via a duct at the top of the c ountercurrent condenser stage. The water used to cool the air in the condenser and the condensate product from the air will flow down in both towers to a drain where the exit water temperature is measured. Fig. 2-3 depicts a pict orial view of the DDD facility. Description of Individual Components The diffusion tower, shown in Fig. 2-4, is composed of three primary sections: the top, middle, and bottom. The bottom consists of the air entrance and drain, the middle houses the packing, and the top contains the water spray and exit air duct. The middle portion of the tower is compos ed of 27 cm outer diameter 0.64 cm thickness R-Cast cast acrylic tube. The packed bed of the tower can occupy up to 1 m of height in the acrylic tube. The bottom and the top portions ar e schedule 40 25.4 cm nominal PVC pipe.

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18 The bottom and middle portions of the towe r are connected via a schedule 80 PVC 25.4 cm nominal bolted flange. Figure 2-3. Diffusion driven de salination (DDD) system. The condenser stage of the DDD system is comprised of two different stages each with a top segment connected to a common bo ttom segment. The top segments include the actual tower, the water spray, and the ai r duct, while the bottom segment contains the drain and packed bed sections. The towers of both stages are also composed of 27 cm outer diameter 0.64 cm thickness R-Cast cas t acrylic tube. The two segments are connected via schedule 80 PVC 25.4 cm nomi nal sized bolted flanges. The bottom portion contains two schedule 80 90 25.4 cm PVC elbows connected using a 25.4 cm nominal size schedule 40 PVC pipe. Up to 50 cm of packing material can be accommodated in each vertical portion of the bottom segment.

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19 Figure 2-4. Diffusion tower [15]. The three water distributors used in the diffusion tower and direct contact condenser are manufactured by Allspray. They are brass full cone with a 65 spray angle and are designed for uniform solid cone spray. The two spray nozzles used for the direct contact condenser have a capacity of 7.57 lpm (2.00 gpm). Fig. 2-5 illustrates a typical Allspray nozzle. The water preheater is a DHC-E tankless electric water heater manufactured by Stiebel Eltron. It is a 240 V heater where the input of heat is electr onically controlled. Its operating range is from 30 C to 52 C.

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20 Figure 2-5. An Allspray water distributor. The main water heater consists of two 3 kW electric coil he aters wrapped around a copper pipe where the water flows. The heat er is PID controlled with a 240 V output. The feedback is controlled with a type J th ermocouple located at th e exit of the heater. The air heater is a 4 kW 1.21 cm diameter round cross section tubul ar heater. It has a 240 V rating and has a watt density of 194 W/cm2. The sheath is Incoloy, which has a maximum temperature of 815 C. It has a sheath length of 254 cm and a heated length of 236 cm. The heater has been shaped to fit in side the 9.5 cm inner diameter pipe. Figure 2-6 shows the heater shape. The power to th e heater is controlled with a single-phase autotransformer. Figure 2-6. Shaped heater. The packed bed is high density, low pressu re drop HD Q-PAC type from Lantec. It is made of polypropylene and is available in 30 cm x 30 cm x 30 cm square pieces. The packing material was cut to fit the round acryl ic tubes of the diffusion tower and direct

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21 contact condenser towers using a specially de signed hot wire setup. It has a specific diameter of 18 mm and a specific area of 267 m2/m3. Figure 2-7 shows a portion of the packed bed cut for use in the system. Figure 2-7. The HD Q-PAC packed bed. The three turbine water flow meters used to measure the water flow rate in the DDD system are manufactured by Proteus Industr ies Inc. Two flow meters have a flow range of 5.7-45.4 lpm (1.5-12.0 gpm) while one flow meter has a range of 0.4-3.8 lpm (0.1-1.0 gpm). They require a 24 VDC input and have a 0-5 V or 0-20 mA output. All have an accuracy of .5% full scale and were calibrated using the catch and weigh method. Figure 2-8 shows the calibration curv es obtained for the three different flow meters. The air flow meter is a thermal insertion mass flow meter (Sierra Series 620S FastFlo) from Sierra Instruments Inc. It ha s a microprocessor-based transmitter for 0-10

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22 VDC output and a 200 s response time. It ha s a 15.24 cm 304 stainl ess steel probe to measure velocity as well as temperature. The flow meter range is 0-1125 SCFM of air with an accuracy of % full scale. The flow meter was delivered as factory calibrated. 201 Series Calibration CurveVoltage (V) 012345 Flow Rate (gpm) 0.0 0.2 0.4 0.6 0.8 1.0 Serial No: 00101543 Calibration Curve Q = 0.1915V + 0.0195 250 Series Calibration CurvesVoltage (V) 012345 Flow Rate (gpm) 0 2 4 6 8 10 12 14 Serial No: 00091465 Calibration Curve Q = 2.7128V + 0.3042 Serial No: 00091467 Calibration Curve Q = 2.6805V + 0.203 Figure 2-8. Water flow meter calibration curves. There are four Vaisala Corp. HMD70Y resi stance type humidity gauges to measure the relative humidity as well as temperat ure. Both temperature and humidity have transmitters for 0-10 V output. All four gauge s were factory calibrated. The operating range for temperature is -20 to +80 C, while the relative humidity has an operating range of 0-100%. The uncertainty of th e gauges is C for temperature and % relative humidity. The thermocouples used in the facility are type E and were manufactured by Omega. They are factory calibrated with an uncertainty of .2 C. The data acquisition system consists of a 16-bit PCI Analog-Digital converter and a 32 channel multiplexer card manufactured by Measurement Computing. The board is calibrated for type E thermocouples and has a 0-10 V input range.

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23 The data acquisition system uses the progr am SoftWIRE to collect the necessary heat and mass transfer data. The SoftWIRE editor uses constructed flow diagrams to represent the flow of data a nd control with icons and wires. The program developed for the DDD system includes five different interf aces: the main control, the diffusion tower view, the direct contact c ondenser view, the diffusion tower histogram view and the direct contact condenser histogram view. Th e SoftWIRE program sends all of the data directly to an Excel spreadsheet where the data is collected. The main control tab of the DDD program allows direct control over the program. A view of the main control is depicted in Fi gure 2-9. There is an on/off switch to turn the program on as well as a frequenc y box to input the desired da ta sampling rate. It also depicts specific values important to the overa ll process. Values specific to the diffusion tower are depicted in the diffusion tower tab while those specific to the direct contact condenser are shown in the diffusion tower tab. Figure 2-10 shows the diffusion tower tab. As the picture illustrates, values at ce rtain locations of the diffusion tower are visible and are easily available. Since steady state is an important assump tion in the DDD analysis, it is important to obtain measurements at steady state c onditions. Two of the interfaces in the DDD programs include a series of histograms whic h indicate the degree to which steady-state is achieved. Figure 2-11 shows a view of the diffusion tower histogram view. The x-axis is the time coordinate while the y-axis is the given measured value. All measurements recorded were taken at steady state conditions at a fr equency of 1 Hz.

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24 Figure 2-9. Main control. Figure 2-10. Diffusion tower data.

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25 Figure 2-11. Histogram view of the DDD data acquisition.

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26 CHAPTER 3 HEAT AND MASS TRANSFER WITH IN THE DIFFUSION TOWER An in depth theoretical m odel for both the diffusion tower [16] and direct contact condenser [17] based on heated water/ambient air inputs has already been explored. The purpose of this analysis is to experimenta lly explore the performance of the diffusion tower for two different cases: heated air/heated water i nput and a heated air/ambient water input. The heat and ma ss transfer model, proposed by Kl ausner et al. [16] has been investigated for both cases. The theoretical model is compared with the experimental data collected and agreement is satisfactory. Theoretical Heat and Mass Transfer Model The theoretical model is a one-dimensional two fluid film model for a packed bed. The conservation equations for mass and ener gy are applied to a differential control volume to obtain the governing equations for the process. In order to determine the governing equations certain assumptions must be made. The assumptions made are: 1. The process operates at steady-state. 2. Air and water vapor ar e both perfect gases. 3. The changes in kinetic and pot ential energy are negligible. 4. The pumping power required for water is solely that required to overcome gravity. Fig. 3-1 shows the differential control volume analyzed for the two cases. As it can be seen the problem is one-dimensional with th e only variation lying in the z-direction.

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27 Figure 3-1. Differential control vol ume for heated air conditions. The z-direction is taken as positive in the axial direction. The c onservation of mass on the control volume for the air/vapor mixture yields, ,,()()VzVevapdd mm dzdz (3.1) where mrepresents the mass flow rate, the subs cripts V and evap denote vapor and the vapor evaporated from the liquid respectivel y. Similarly, conservation of mass on the liquid side yields, ,,()()LzVevapdd mm dzdz (3.2) where the subscript L denotes liquid. The humidity ratio, and relative humidity, are defined for an air/vapor mixture as follows, 0.622() ()Vsata asatamPT mPPT (3.3) q HL Packed Bed dmVevap ma+mV mL Liquid Air/Vapor z z + dz

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28 where P is the total pressure of the system, and Psat(Ta) is the saturation pressure of the vapor evaluated at the air temperature Ta. The small change in system pressure is not accounted for in evaluating the properties. Th e definition of the mass transfer coefficient is applied to the differential control volume to obtain the following, ,,()(()())Vevap GwVsatLVad mkaTTA dz, (3.4) where kG is the gas mass transfer coefficient, aw is the wetted specific area, and A is the cross sectional area of the diffusion tower. It should be noted that the total specific area of the packing, a, is the total surface area of packing per unit volume of space occupied. The rate of change of evaporation can be further reduced by considering the perfect gas law. By applying the perfect gas law [18] to Equation 3.4, the rate of evaporation becomes, ,()() ()Vsatisata Vevap Gw iaMPTPT d mkaA dzRTT (3.5) where MV is the molecular weight of vapor, R is the universal gas constant, and Ti is the liquid/vapor interfacial temperat ure. By combining Equations 3.2-3.5 the gradient of the humidity ratio is, () 0.622GWVsati iakaMPT dP dzGRTT (3.6) where am G A is the air mass flux. Equation 3.6 is a first order ordinary differential equation with dependent variable When solved, the humidity ratio along the axial z direction is obtained. Equa tion 3.6 requires a value of the liquid/vapor interfacial

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29 temperature, Ti. The interfacial temperature is found by recognizing that the energy convected from the liquid is the same as that convected to the gas, ()()LLiGiaUTTUTT (3.7) where UL and UG are the heat transfer coefficients of liquid and gas respectively. The interfacial temperature is obtaine d by solving Equation 3.7 and is, (/) 1(/)LGLa i GLTUUT T UU (3.8) The conservation of energy on the liquid si de of the differential volume yields the following, ,() ()()Vevap L LfgLaddm mhhUaTTA dzdz (3.9) where h is the enthalpy, U is the overall heat transfer coefficient, and hfg is the latent heat of evaporation. Equation 3.9 can be furt her manipulated by utilizing the following: LpLLdhCdT and ()L L LL LLdh ddm mhhm dzdzdz and ()()()fgaVaLahThThT The gradient of the liquid temperature, TL, then reduces to the following, () ()fgL La L PLPLhh UaTT dT Gd dzLdzCCL (3.10) where L m L A is the liquid mass flux, CP is the specific heat, and a is the overall specific area of the packing material. Equation 3.10 is also a first order ordinary differential that when solved will yield the temperature dist ribution of the water throughout the diffusion tower.

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30 Likewise, conservation of energy of the ai r/vapor mixture is obtained from the differential control volume and yields, ,()()aV aVVevapfgLaHLdd mhmhmhUaTTAqd dzdz (3.11) As in the liquid energy equation, Equation 3.11 can be simplified by utilizing the fact that the air mass flow rate is held constant throughout the entire process such that: ....()V aV aVaV avVdhdh ddm mhmhmmh dzdzdzdz aa PadhdT C dzdz and Va PVdhdT C dzdz Equation 3.11 then becomes, "()()()v a av PaPVLaLaHLdT dm mCmChTUaTTAqd dzdz (3.12) Equation 3.12 can be simp lified by noting that the CPmix, specific heat of the mixture, is evaluated as, aV PmixPaPV aVaVmm CCC mmmm (3.13) Recalling the evaluation of the latent heat of evaporizati on from the liquid conservation equation, and combining Equations 3.12 and Equation 3.13 the gradient of air temperature through the diffus ion tower is evaluated as, "()() 4 1 1(1)(1)aLaLa HL P mixPmixPmixdThTUaTT q d dzdzCCGGdC (3.14) where d is the diameter of the diffusion tower and H Lq is the heat flux loss from the air. Equation 3.14 is also a first order ordinary differential equation with dependent variable Ta. Equations 3.6, 3.10, and 3.14 are a set of coupled ordinary differ ential equations that when solved simultaneously give solutions fo r the distributions of humidity ratio, air

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31 temperature, and water temperature throughout the diffusion tower. However, since a one-dimensional model is utilized, closure must be achieved. This requires that both the heat transfer coefficient and mass transfer coefficient be known. Directly measuring the heat transfer coefficients is not possible b ecause of the fact that the interfacial film temperature cannot be measured. Therefore to overcome this difficulty the heat and mass transfer analogy [19] has been utilized. The h eat transfer coefficient for the liquid side is evaluated using, 1/21/2PrLL LLNuSh Sc (3.15) 1/2 L LLLPL LK UkC D (3.16) Similarly, the heat transfer coeffici ent for the gas side is calculated as, 1/31/3PrGG GGNuSh Sc (3.17) 2/3 1/3G GGGPG GK UkC D (3.18) where D is the molecular diffusion coefficien t and K is the thermal conductivity. Thus the overall heat transfer co efficient is evaluated as, 11 LLGUUU (3.19) The mass transfer coefficient is evaluate d using a widely known and well tested correlation. Ondas correl ation [20] allows for evaluation of the mass transfer coefficients in packed beds. Ondas co rrelation, found in Appendix A, is used to calculate the mass transfer coefficients, kG and kL. In the correlation the coefficient, C, can take on two possible values C=5.23 for 15pd mm and C=2.00 for 15pd mm.

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32 The difference in C values accounts for the fact that kGa for the smaller packing (15pd mm) tends to increase monotonously with increas ing specific area, a. Li et al. [17] provided an explanation for th e phenomena of the decreased mass transfer coefficient and is believed to be the cause of liquid hold-up in the packed bed which is responsible for liquid bridging and reduced area for mass transfer. The current packed bed has a diameter of 18 mm which is close to the cut off for both sizes. Thus for the packed bed used in the current investigati on, either constant value woul d be appropriate. Similar to the analysis described by Klausn er et al. [16], the heated ai r/ambient water uses C=5.23. The coefficient for the heated air/heated water case, however, uses C=2.0. This change in constant can perhaps be attributed to the fact that at higher air temp eratures the air that enters the packed bed is drye r and thus more water is eva porated. However, due to the increase in evaporation there is an increased hold up of the liquid in the packing due to an accelerating gas stream. The increase in hold up could possibly cause more liquid bridging in the packing thereby decr easing the local mass transfer. The wetted area for the current experiments differs from that computed via Ondas correlation. It was found that the specific wett ed area remains nearly constant for varying air to water mass flow ratios. This was de termined by first using Ondas correlation to calculate the wetted specific area. There was slight variation in the comparison of the theoretical and experimental da ta. An analysis was then performed to determine what the specific wetted area should be to obtain adequate results. Interestingl y, over the range of operating conditions considered in this wo rk, the specific wetted area is found to be simply a constant, 0.5waa (3.20)

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33 The value of the heat loss, H Lq is experimentally determined and is diffusion tower specific. There was negligible heat loss fo r the heated air/ambient water case thus the heat loss flux term is taken to be zero. Th e heat loss for the heated air/heated water case is experimentally calibrated for various air mass fluxes. Figure 3-2 shows the calibration curve for the heated air/heated water h eat loss flux for varying air mass flux. To solve the three coupled equations for the humidity ratio, air temperature, and water temperature distributions in the diffusi on tower, the following solution procedure is followed: 1. Specify the water mass flux, air mass flux, inlet water temperature, inlet air temperature, and inlet humidity ratio. 2. Guess a value of the exit water temperature. 3. Compute the exit humidity ratio, exit wate r temperature, and exit air temperature utilizing Equations 3.6, 3.10, and 3.14 until z reaches the height of the packed bed. 4. Compare the values of the calculated inle t water temperature and specified inlet water temperature. If they match, the analysis is complete. If they differ, repeat the procedure beginning from step 2. Results and Discussion Experiments were conducted to obtain da ta for the two different cases: heated air/ambient water and heated air/heated wate r. For both cases, the air mass flux was held constant at about 0.77 kg/m2-s, 1.16 kg/m2-s, and 1.55 kg/m2-s while the liquid mass flux was varied between 0.6 kg/m2-s to 1.3 kg/m2-s. The height of the packed bed was held constant at 0.38 m. All experiments c onducted were performed in a parameter space beneath the flooding curve for the packed bed. For the model anal ysis, the inlet water temperature, inlet air temperature, and inlet absolute humidity were all used to compute

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34 the exit conditions. Comparisons between the predicted and measured exit conditions from the model are described next. Air Mass Flux (kg/m2-s) 0.60.81.01.21.41.6 Heat Loss Flux (kW/m2) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Figure 3-2. Calibration curve of the heat loss flux for the he ated air/heated water case. Case 1: Heated Air/Ambient Water Six different data sets were recorded fo r the heated air/ambient water case. Two different data sets per air mass flux were taken to ensure the repeatability of the experiments. For all experiments, the inlet air, water, and humidity were held constant at about 60.9 C, 25.2 C, and 0.0060 respectively. Figures 3-3 to 3-5 show the comparison between the predicted exit values and the m easured exit values. The comparison between the two is quite good. The exit absolute humidity and exit water temperature are predicted with fairly good accuracy while the exit air temperature is slightly underpredicted for all data sets.

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35 From the data collected it can be seen that the heated air/ambient water case does not yield good production. Th e exit air temperature is ap proximately 25-26 C for each value of the liquid mass flux as well as air mass flux, which indicates that the air is cooled to the temperature of the water. This is supported by the fact that the temperature difference of the exit water and exit air is nearly a constant 1-2 C. Thus it is clear that reliance on heated air is inefficient because upon entering the tower the air is immediately cooled close to the water temperature. All of the energy is being used to heat up the water and as a result the mass transfer is poor As the absolute humidity shows, there is no optimum value as the exit hum idity is essentially constant Further, the change in humidity from the inlet to th e outlet essentially remains a c onstant at about 0.0125. This indicates that regardless of the diffusion tower liquid mass fl ux only a small fraction of water will be evaporated. Figure 3-6(a-c) shows the repeat ability of the six different experiments for the different flow conditions. As shown in the figures, th e repeatability of the experiments is very good. However, the figures also elucidate the fact that the heated air/ambient water case is very inefficient. There is little vari ation between the values recorded for the six different experiments despite the different air and water mass fluxes used in the experimental measurements. For example, consider the exit air temperature shown in Fig. 3-6(a). For the three values of air ma ss flux the exit air temper ature remains almost a constant despite the varying liquid mass flux. The exit water temperature and the exit humidity also demonstrate similar behavior. The process will exhibit the same behavior despite the operating conditions chosen.

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36 (a) ou t outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 0.77 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.10 Tw,in (C) = 25.32 = 0.007515 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 0.78 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 61.79 Tw,in (C) = 25.19 = 0.004064 (b) Figure 3-3. Comparison of pred icted and measured exit temperatures and humidity for similar air mass flux G = 0.77 kg/m2-s: a) Set 1 b) Set 2.

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37 (a) ou t outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 1.15 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.92 Tw,in (C) = 25.12 = 0.006148 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 1.17 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.02 Tw,in (C) = 25.15 = 0.006841 (b) Figure 3-4. Comparison of pred icted and measured exit temperatures and humidity for similar air mass flux G = 1.15 kg/m2-s: a) Set 1 b) Set 2.

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38 (a) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 1.55 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 61.11 Tw,in (C) = 25.14 = 0.007112 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 15 20 25 30 35 Absolute Humidity 0.00 0.01 0.02 0.03 0.04 0.05 G = 1.54 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 61.79 Tw,in (C) = 25.19 = 0.004064 (b) Figure 3-5. Comparison of pred icted and measured exit temperatures and humidity for similar air mass flux G = 1.55 kg/m2-s: a) Set 1 b) Set 2.

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39 Details of the experimental data can be found in Appendix B. Case 2: Heated Air/Heated Water As in the previous case, six different data sets were taken, two for each different air mass flux. For all six experiments the inle t air temperature, water temperature, and humidity were held constant at about 60.9 C, 60.6 C, a nd 0.0077 respectively. Figures 3-7 to 3-9 show the comparison between the predicted and measured exit temperatures and humidity. For all sets of data the ex it water temperature and exit humidity are predicted with considerable accuracy. The air temperature is slightly overpredicted in all cases. (a) Water Mass Flux L (kg/m2-s) 0.50.60.70.80.91.01.11.21.3 Exit Air Temperature (C) 20 22 24 26 28 30 GSet 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (a) Figure 3-6. Repeatability of di fferent experiments for different exit parameters: a) Exit air temperature, b) Exit water temperature, c) Exit absolute humidity.

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40 (b) Water Mass Flux (kg/m2-s) 0.50.60.70.80.91.01.11.21.3 Exit Water Temperature (C) 10 15 20 25 30 35 40 GSet 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (b) (c) Water Mass Flux L (kg/m2-s) 0.50.60.70.80.91.01.11.21.3 Exit Absolute Humidity 0.00 0.01 0.02 0.03 0.04 GSet 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (c) Figure 3-6. Continued.

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41 (a) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 0.79 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.99 Tw,in (C) = 60.48 = 0.006854 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 0.80 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 61.41 Tw,in (C) = 60.75 = 0.006207 (b) Figure 3-7. Comparison of pr edicted and measured exit temperatures and humidity for similar air mass flux G = 0.79 kg/m2-s: a) Set 1 b) Set 2.

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42 (a) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 1.15 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 61.29 Tw,in (C) = 60.87 = 0.007919 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 1.16 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.90 Tw,in (C) = 60.79 = 0.008120 (b) Figure 3-8. Comparison of pr edicted and measured exit temperatures and humidity for similar air mass flux G = 1.15 kg/m2-s: a) Set 1 b) Set 2.

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43 (a) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 1.55 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.52 Tw,in (C) = 60.61 = 0.008882 (a) (b) out outin Water Mass Flux L (kg/m2-s) 0.60.81.01.21.4 Temperature (C) 0 10 20 30 40 50 Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 G = 1.55 kg/m2-s PredictedMeasured Ta,out Tw,out Ta,out Tw,out Inlet Conditions Ta,in (C) = 60.42 Tw,in (C) = 60.27 = 0.008409 (b) Figure 3-9. Comparison of pr edicted and measured exit temperatures and humidity for similar air mass flux G = 1.55 kg/m2-s: a) Set 1 b) Set 2.

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44 For all air mass fluxes studied, the exit ai r temperature and exit water temperature increase with increasing water mass flux. As the liquid mass flux increases the exit humidity also increases due to th e increase in the amount of liquid available to evaporate. Figures 3-10(a-c) demonstrat es the repeatability of th e six experiments for the heated air/heated water case. As shown in the figures the repeatab ility for all of the experiments is quite reasonable. These figur es show that at the lower air mass flux the maximum exit humidity is obtained. It is also interesting that both the exit air temperature and exit water temperature decr ease with increasing air mass flux. This suggests that the residence tim e plays a key role in the heat and mass transfer. A decrease in residence time implies that there is less time for heat and mass transfer to occur thereby explaining the decreased temper atures as well as humidity with increasing air mass flux. As demonstrated, the theo retical model develope d is obviously a good design tool that can be utilized to achieve the desired production rate. Details of the experimental data collected for the heated air heated water case can be viewed in Appendix C.

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45 (a) Water Mass Flux L (kg/m2-s) 0.40.60.81.01.21.4 Exit Air Temperature (C) 30 35 40 45 50 GSet 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (a) (b) Water Mass Flux (kg/m2-s) 0.40.60.81.01.21.4 Exit Water Temperature (C) 25 30 35 40 45 GSet 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (b) Figure 3-10.Repeatability of e xperiments for different exit parameters: a) Exit air temperature, b) Exit water temperature, c) Exit absolute humidity.

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46 (c) Water Mass Flux L (kg/m2-s) 0.40.60.81.01.21.4 Exit Absolute Humidity 0.03 0.04 0.05 0.06 0.07 G Set 1Set 2 0.79 kg/m2-s 1.55 kg/m2-s 1.79 kg/m2-s (c) Figure 3-10. Continued.

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47 CHAPTER 4 PARAMETRIC ANALYSIS The previous chapters have focused on the experimental facility, data collection, the heat and mass transfer model, and comparis ons of the predicted a nd measured data. It has been shown that the DDD process has good performance for the heated air/heated water case, while the heated air/ambient water case is inefficient. In order to fully explore the feasibility of the heated air/he ated water DDD process a parametric analysis is considered. This chapter details the parametric analysis including the methodology, analysis of results, and comparisons between several different DDD processes. At the end of the chapter an economic analysis of the DDD facility for coupling with an industrial plant that produces large quantities of waste heat in air is explored. Parametric Analysis of Heated Air/Heated Water DDD Process The heat and mass transfer model describe d in Chapter 3 is used to model the heated air/heated water DDD process. The assumptions made in this analysis are: 1. There is no heat lost to the surrounding environment. 2. The only energy consumed is the pumpi ng power required to pump the air and water through the system. 3. The air/vapor mixture is tr eated as a perfect gas. 4. Changes in potential and kinetic energy are consider ed to be negligible. 5. The process operates at steady-state conditions. Equations 3.6, 3.10, and 3.14 are used to evaluate the absolute humidity, water temperature, and air temperat ure respectively through the diffusion tower. The heat and

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48 mass transfer analogy is used to evaluate th e heat transfer coefficients and Ondas correlation is used to evaluate the mass transf er coefficients assuming a constant wetted area. In order to fully explore the bounds of th e heated air/heated water DDD process the energy consumption must be evaluated and certa in parameters are needed to effectively assess the process. The fresh wate r production rate is computed from, ()fwoutinmGA (4.1) where the subscripts fw, in, and out refer to the fresh water and diffusion tower inlet and outlet respectively. It should be noted that it is assumed that the carrier air circulates in a closed loop and the inlet humid ity to the diffusion tower is the outlet humid ity from the condenser. The major energy consumption of the pro cess is assumed to be the pumping power required to pump the air and water through th e system. The pumping power required for the diffusion tower gas is G GGGGG GGm GA EVPPP (4.2) In order to evaluate the pressure drop on the gas side a corre lation provided by the packing material company, Lantec, is used. The pressure drop, GP across the HD QPAC packed bed is computed from, 24 274 2[0.0354654.48()1.17610()]G GllGP GLLG z (4.3)

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49 where GP is the pressure drop across the packed bed (kPa) and z is the height of the tower. The validity of the correlation wa s explored by Li [15], and it has excellent agreement with the experimental data. The pumping power required for the di ffusion tower liquid is calculated as, L LL Lm EPLAgH (4.4) Thus the total pumping power is calculated as, totalLGEEE (4.5) The energy consumption rate per unit of fresh water production is defined as, total fw fwE E m (4.6) The fresh water production rate and ener gy consumption rate are two important parameters that characterize the performance of the DDD process. The fresh water production rate characterizes the quantity of fresh water produced at a given set of operating conditions while the energy consum ption rate denotes how much energy is consumed per unit of fresh water produced. The two values are important in finding the optimal operating conditions. An ideal DDD process would have a high fresh water production rate and a low energy consumption rate. Heated Air/Heated Water Results and Discussion Using the analysis described above and the theory discussed in Chapter 3, a parametric analysis is performed to determin e the effects of certain operating variables on the performance of the heated air/heated wate r process. In perf orming the analysis, the air inlet temperature, water inlet temperatur e, specific packing area, diameter of the packing material, and inlet humidity ratio were all held constant at 60 C, 60 C, 267

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50 m2/m3, 0.018 m, and 5.25% respectively. Nine different values of the inlet feedwater mass flux, L, were considered 0.15, 0.25, 0.5, 0.75, 1.20, 1.55, 2.0, 2.5, 3.0 kg/m2-s. The inlet air mass flux, G, was varied continuously from 0.04 to 23.2 kg/m2-s for each inlet feedwater flux. For each air mass flux, the maximum absolute humidity was determined, and the tower height, air exit te mperature, and water exit temp erature were recorded. All calculations were performed in a parame ter space below the flooding curve of the packing material. Figure 4-1 depicts the tower height as a function of th e inlet air mass flux. The tower height reported is the computed tower height required to achieve the maximum exit absolute humidity. For all inlet liquid ma ss fluxes, the tower height decreases with increasing inlet air mass flux. This is impor tant because as the air mass flux increases, Air Mass Flux G (kg/m2-s) 0510152025 Tower Height (m) 0 1 2 3 4 5 6 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 Figure 4-1. Tower height at th e maximum exit absolute humidity as a function of the inlet air mass flux G.

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51 the tower height decreases which translates to less required materials and thus reduced cost. However, as the air mass flux increas es the power required to pump the air also increases. It should be noted that the towe r height for high liquid mass flux and low air mass flux was restricted to values less than 5 m. This is to ensure that the tower heights considered are realistic. Figure 4-2 shows the exit air temperature wi th varying air mass flux. The exit air temperature decreases with increasing ai r mass flux until it reaches a minimum value then increases. It is also worthy to note that the highest exit air temp eratures are realized when the air mass flux is low. Air Mass Flux G (kg/m2-s) 0510152025 Exit Air Temperature (C) 25 30 35 40 45 50 55 60 65 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 Figure 4-2. Exit air temperature at maximum absolute humidity as a function of the air mass flux. Figure 4-3 depicts the maximum exit absolu te humidity as a function of air mass flux for varying liquid mass flux values. For all liquid mass flux values, the exit humidity decreases with increasing air mass flux. The maximum exit humidity is

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52 Air Mass Flux G (kg/m2-s) 0510152025 Exit Absolute Humidity 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 Figure 4-3. Maximum exit absolute hu midity with varying air mass flux. achieved with the higher liquid mass fluxes and can be attributed to the fact that the heat capacity of the water is large with larger mass fluxes, and th e water temperature will not decrease as much with a given amount of evaporation. Thus as the liquid mass flux decreases the exit absolute hum idity decreases as well. The maximum exit humidity is realized for low values of the air mass flux. It is important to note that the maximum exit air temperature and maximum humidity for al l liquid mass fluxes are achieved for values of the air mass flux less than about 2.00 kg/m2-s. While the exit air temperature and exit absolute humidity are maximum at low air ma ss flux, this does not imply that the fresh water production will also be high. The fresh water production is an important parameter in evaluating the economy of the process. Figure 4-4 shows the fresh wate r production flux with va rying air mass flux. For each value of the liquid mass flux the fr esh water production flux increases until it

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53 reaches an optimum condition. This is importa nt because it indicates that for every liquid mass flux there is a value of the air mass flux that can produce the maximum amount of fresh water. It is also important to notice that as the liquid mass flux increases, the fresh water production flux increases Thus a higher production ca n be achieved at higher liquid mass flux. Interestingly, for al l liquid mass flux the maximum fresh water production does not occur below 2.00 kg/m2-s where the maximum exit humidity is realized. Air Mass Flux G (kg/m2-s) 0510152025 Fresh Water Production Flux (kg/m2-s) 0.00 0.05 0.10 0.15 0.20 0.25 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 Figure 4-4. Fresh wate r production flux with varying air mass flux. The energy consumption rate is also an important parameter in evaluating the economy of the DDD process. Figure 4-5(a-b) shows the energy consumption rate for the diffusion tower with vary ing air mass flux for the differe nt values of the liquid mass flux. Figure 4-5(a) shows th e full range of air mass flux while Figure 4-5(b) shows a smaller range. For all values of liquid ma ss flux, the energy consumption rate increases

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54 Air Mass Flux G (kg/m2-s) 0510152025 Energy Consumption Rate (kW-hr/kgfw) 0 1 2 3 4 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 (a) Air Mass Flux G (kg/m2-s) 0.00.51.01.52.02.53.0 Energy Consumption Rate (kW-hr/kg) 0.000 0.002 0.004 0.006 0.008 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 (b) Figure 4-5. Diffusion tower energy consumption rate (a) wi th varying air mass flux (b) for low air mass flux. with increasing air mass flux. The higher the air mass flux, the more pumping power required to drive the process. As the graphs reveal there is a minimum energy

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55 consumption rate for each liquid mass flux. Beyond that optimum value, the energy consumption steadily increases. From Figure 4-5(a) it can be seen that operating at higher air mass flux is impractical due to the hi gh energy consumption rate. Li et al. [11] reported that the ideal operat ing condition in the condenser is for air mass flux below 1.5 kg/m2-s to ensure low energy consumption. Figure 4-5(b) reiterates this condition for the diffusion tower. Below an air mass flux of 1.5 kg/m2-s, the energy consumption is low. Figure 4-6 shows the fresh water producti on efficiency versus the air mass flux for varying liquid mass flux. The fresh wa ter production efficiency increases with increasing air mass flux until it reaches a ma ximum condition, and then it steadily decreases. As the liquid mass flux incr eases the maximum fr esh water production efficiency decreases. The maximum efficien cy occurs for low liquid mass flux while the maximum fresh water production occurs for hi gh liquid mass flux. An optimal operating condition is one that has high fresh water production and low energy consumption. Comparison of Different DDD Processes Next the heated air/heated water DDD pro cess will be compared against other DDD configurations. The heated air/heated wate r DDD process will first be compared against the process described by Klausn er et al. [16], heated water/ ambient air for a 60 C water inlet. The process will then be compared against the heated air/heated water DDD process with Q-PAC, a packed bed with a sma ller specific area and lower pressure drop. Heated Water/Ambient Air at 60 C In this analysis the heated air/heated water DDD process is compared against the heated water/ambient air DDD process described by Klausner et al. [16]. In performing the analysis of the heated air/heated water the analysis described in Chapter 3 is used

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56 Air Mass Flux G (kg/m2-s) 05101520 Fresh Water Production Efficiency (mfw/mL) 0.0 0.1 0.2 0.3 0.4 Liquid Mass Flux L (kg/m2-s) 0.15 0.25 0.50 0.75 1.20 1.55 2.00 2.50 3.00 Figure 4-6. Fresh water produc tion efficiency with varying air mass flux realized for the lower air and liquid mass fluxes. with the air inlet temperature, water inlet temp erature, specific packing area, diameter of the packing material, and inle t absolute humidity held constant at 60 C, 60 C, 267 m2/m3, 0.018 m, and 0.0065 respectively. The values obtained for the heated water/ambient air case are calcu lated using the model proposed by Klausner et al. [16] where the inlet water temperature, inlet air temperature, inlet humidity ratio, specific area, and diameter of the packing ar e held at 60 C, 26 C, 0.023, 267 m2/m3, and 0.018 m respectively. To obtain the predictions fo r comparison calculations were run for nine different liquid mass fluxes: 0.15, 0.25, 0. 5, 0.75, 1.20, 1.55, 2.0, 2.5, 3.0 kg/m2-s. For each liquid mass flux, the gas mass flux was varied. For each liquid mass flux, the minimum energy consumption ra te was recorded over the range of air mass flux. The values of the fresh water pr oduction flux, and fresh water pr oduction efficiency reported

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57 are those corresponding to the point of minimum energy cons umption. Figures 4-7 and 4-8 show the fresh water production flux, energy consumption rate, and fresh water production efficiency for the two different configurations. Figure 4-7 shows the fresh water produc tion efficiency and energy consumption rate with varying liquid mass flux for both of the processe s. The energy consumption rate for both shows little difference for liq uid mass fluxes greater than about 1.3 kg/m2-s. However at low mass flux the heated water/ ambient air case has a considerably less energy consumption rate. The fresh water produ ction efficiency of the heated air/heated water process is greater for all values of liquid mass flux, although at large liquid mass flux the difference is not signi ficant. It should be noted that when comparing the two configurations there is more thermal energy input for the heated air/heated water case than the heated water/ambient air case, and the energy consumption ra te only reflects the electrical energy consumed. Figure 4-8 shows the fresh water producti on flux and energy consumption rate for varying liquid mass flux for both processes. It is observed that the heated air/heated water process has a greater fresh water produc tion flux for all values of the liquid mass fluxes considered. However at low liquid ma ss flux, the energy consumption rate is higher for the heated air/heated water configur ation. Thus the decision to use one process over the other depends upon the source of waste heat and operating conditions. This comparison reveals that for higher liqui d flow rates the heated air/heated water case is equally comparable in energy cons umption rate but has a higher fresh water production efficiency and fresh wa ter production flux. On a sma ll scale, this equates to a

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58 Liquid Mass Flux L (kg/m2-s) 0.00.51.01.52.02.53.03.5 Fresh Water Production Efficiency (mfw/mL) 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Energy Consumption Rate (kW-hr/kg) 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 Hot Air/Hot Water Ambient Air/Hot Water Hot Air/Hot Water Ambient Air/Hot Water Inlet Conditions: Ta,in = 60 CTw,in = 60 C in 5.25% Hot Air/Hot Water Ambient Air/Hot Water Ta,in = 26 CTw,in = 60 C in % Hot Air/Hot Water Figure 4-7. Fresh water produc tion efficiency and energy co nsumption rate for varying liquid mass flux for 60 C inlet conditions. Liquid Mass Flux L (kg/m2-s) 0.00.51.01.52.02.53.03.5 Fresh Water Production Flux (kg/m2-s) 0.00 0.02 0.04 0.06 0.08 0.10 Energy Consumption Rate (kW-hr/kg) 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 Ambient Air/Hot Water Hot Air/Hot Water Ambient Air/Hot Water Hot Air/Hot Water Inlet Conditions: Ta,in = 60 CTw,in = 60 C in 5.25% Hot Air/Hot Water Ambient Air/Hot Water Ta,in = 26 CTw,in = 60 C in % Figure 4-8. Fresh water pr oduction flux and energy consump tion rate for varying liquid mass flux for 60 C inlet conditions.

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59 smaller tower size by utilizing the heated air/heated water DDD process. Less tower height is required for the heated air/heated water process in order to generate the same amount of fresh water produced in the heated wa ter/ambient process. This translates to lower production cost and less space required. Heated Air/Heated Water using Q-PAC For the next analysis two di fferent types of packed bed are used with the heated air/heated water process. The theoretical model proposed in Chapter 3 is used to calculate the parameters, however a different wetted specific area is used. A modified Ondas correlation, which is found in A ppendix A, is used for the mass transfer coefficients and the wetted sp ecific area for both configurat ions. HD Q-PAC, the packing material described in Chapter 2 and used in the experimental facility, is compared with Q-PAC. The Q-PAC material is also produced by Lantec a nd is manufactured to have lower pressure drop, reduced incidence of f ouling, and flooding at higher air mass flow rates giving it a wider range of operation. Lantec supplied the gas side pressure drop, GP (kPa), across the Q-PAC packed bed as 24 2 2[0.00781.3788()0.30715()]G GllGP GLLG z (4.7) Figures 4-9 and 4-10 show the fresh water production, fresh wa ter efficiency, and energy consumption of the heated air/heated water process for the two configurations. Again the computations show n in the graph correspond to the points of minimum energy consumption rate for each liquid mass flux. Figure 4-9 shows the fr esh water production efficiency and energy consumption rate wi th varying liquid mass flux. The Q-PAC packed bed configuration is obviously much more energy efficient than the HD Q-PAC configuration. As the liquid mass flux increa ses the variation in the energy consumption

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60 rate of the two configurations increases. There is little variation in the fresh water production efficiency, however the Q-PAC appear s to have a slightly better fresh water production efficiency for all values of the liquid mass flux explored. Liquid Mass Flux L (kg/m2-s) 0.00.51.01.52.02.53.03.5 Fresh Water Production Efficiency (mfw/mL) 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 Energy Consumption Rate (kW-hr/kg) 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 Q-PAC Material HD Q-PAC Material HD Q-PAC Material Q-PAC Material Inlet Conditions: Ta,in = 60 CTw,in = 60 C in 5.25% Figure 4-9. Fresh water produc tion flux and energy consumption rate for varying liquid mass flux for HD Q-PAC a nd Q-PAC packed bed. Figure 4-10 shows the varia tion in the fresh water production flux and energy consumption rate for varying liquid mass flux. There is very little variation in the fresh water production flux between the HD Q-PAC and Q-PAC packed bed configurations. However, at high liquid mass flux, the Q-PAC appears to have a slightly higher fresh water production flux. Despite the minimal difference in fresh wa ter production between the two beds, the energy consumption rate remains the key difference. Figure 4-10 demonstrates that the Q-PAC packed can be utilized to produce the same quantity of fresh water product but at a lo wer energy cost. The downside of the Q-PAC is that much more footprint area is needed to achieve th e same quantity of produc tion. Despite this

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61 Liquid Mass Flux L (kg/m2-s) 0.00.51.01.52.02.53.03.5 Fresh Water Production Flux (kg/m2-s) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Energy Consumption Rate (kW-hr/kg) 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 Q-PAC Material HD Q-PAC Material HD Q-PAC Material Q-PAC Material Inlet Conditions: Ta,in = 60 CTw,in = 60 C in 5.25% Figure 4-10. Fresh water produc tion flux and energy consumpti on rate for varying liquid mass flux for HD Q-PAC a nd Q-PAC packed bed. fact, however, the Q-PAC could prove to be especially important for large scale DDD facilities. If space is not an issue then the Q-PAC configur ation would be advantageous for the lesser energy consumption rate. Furt her research should be conducted on the QPAC to determine the exact wetting and performance. Industrial Plant Application In order to fully understand the feasibili ty of the DDD process with a heated air/heated water input, an economic study must be performed. Consider an industrial plant site that can supply 850, 000 acfm of hot air at 82 C (180 F). Figure 4-11 shows a flow diagram of the DDD process coupled w ith an industrial plant. As the drawing depicts, the waste energy in the form of heat ed air from the plant site enters the diffusion tower at 82 C. The source brackish/run off/ sea water enters the diffusion tower at 30 C. The source water is heated from th e water exiting the diffusion tower in a

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62 regenerative heater. The source water will flow down through a packed bed where it will meet the heated air and a portion of the wate r will evaporate into the air stream. The saturated air exiting the diffusion tower will enter a countercurrent flow oriented direct contact condenser. The fres h water exiting the condenser wi ll enter a fresh water storage tank where a portion is the pr oduct and a portion is pumped th rough a heat exchanger to be cooled and then cycled back to the condens er. For all calculations it is assumed that the sink temperature is 15 C. Operation Conditions Using the analysis described at the beginning of the chapter, Figure 4-12 shows the fresh water production flux for varying diffusion towe r liquid mass fluxes. For the analysis the condenser air mass flux is 1.5 kg/m2-s and the fresh water mass flux is 3.0 kg/m2-s. These values were determined by Li et al. [ 11] to be the optimal operating conditions of the direct contact condenser using the HD Q-PAC packing mate rial. It is also assumed that the surface areas of the condenser and diffusion tower are the same. The condenser was analyzed using the model pr oposed by Li et al. [17]. As seen from Figure 4-12, there is an optimal fresh water production flux for an inlet water temperature of 30 C at a liquid mass flux of about 0.20 kg/m2-s. Figure 4-13 depicts th e total energy consumption for the DDD process with varying diffusion to wer liquid mass flux. From the graph the total energy consumption for an inlet water te mperature is optimal in the range of liquid mass fluxes from 0.1-0.2 kg/m2-s. It is fortunate that the optimal total energy consumption and maximum fresh water producti on occur in the same range. The optimal diffusion tower liquid mass flux is therefore ta ken to be 0.15 kg/m2-s. It is worthy to note that the energy consumption of the DDD process demonstrated is nearly five times

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63 less than that of reverse osmosis desalination. This is a very distinct advantage to the DDD process. Figure 4-11. Sample schematic of the DDD sy stem coupled with an industrial site. Figure 4-14 depicts the fresh water producti on efficiency as a function of diffusion tower liquid mass flux. As the graph shows th e fresh water efficien cy increases steeply with decreasing diffusion tower liquid mass fl ux. The fresh water production efficiency in the range of the optimal total energy cons umption and fresh wate r production is quite high and is about 0.224. The optimal operating conditions of the proposed DDD process coupled with an industrial plant are summarized in Table 4-1. If operating at these conditions there is a potential to produce about 201,800 Gal/day of di stilled water with sm all total electrical

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64 Diffusion Tower Liquid Mass Flux (kg/m2-s) 0.00.20.40.60.81.01.2 Fresh Water Production Flux (kg/m2-s) 0.010 0.015 0.020 0.025 0.030 0.035 0.040 Inlet Conditions: Tai= 82 C Water Inlet Temperature: Twi = 40 C Twi = 35 C Twi = 30 C = 0.0147 Gdiff = Gcond = 1.5 kg/m2-s Lfw = 3.0 kg/m2-s Figure 4-12. Fresh water pr oduction flux for varying diffu sion tower liquid mass flux. Diffusion Tower Liquid Mass Flux (kg/m2-s) 0.00.20.40.60.81.01.2 System Energy Consum ption Rate (kW-hr/kg) 0.000 0.001 0.002 0.003 0.004 Inlet Conditions: Tai= 82 C Water Inlet Temperature: Twi = 40 C Twi = 35 C Twi = 30 C = 0.0147 Gdiff = Gcond = 1.5 kg/m2-s Lfw = 3.0 kg/m2-s Figure 4-13. Energy consumption rate of the DDD system for va rying diffusion tower liquid mass flux.

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65 Diffusion Tower Liquid Mass Flux (kg/m2-s) 0.00.20.40.60.81.01.2 Fresh Water Production Efficiency (mfw/mL) 0.0 0.2 0.4 0.6 0.8 1.0 Inlet Conditions: Tai= 82 C Water Inlet Temperature: Twi = 40 C Twi = 35 C Twi = 30 C = 0.0147 Gdiff = Gcond = 1.5 kg/m2-s Lfw = 3.0 kg/m2-s Figure 4-14. Fresh water produc tion efficiency for varyi ng diffusion tower liquid mass flux. energy consumption of 0.0012 kW-hr/kg and a fresh water production efficiency of 0.224. A total foot print area of 562.3 m2 is required. Economic Analysis Next the cost of the DDD process facility when coupled with an industrial plant is evaluated. The unit cost of a desalinati on plant is highly depe ndent upon the site characteristics. Factors such as, plan t capacity, pumping units, chemicals, and pretreatment all depend on the gi ven location [21]. According to Ettouney et al. [21] the costs of a desalination facil ity can be broken down into th ree segments: direct capital costs, indirect capital costs, and annual opera ting costs. The direct capital costs include the costs of purchasing the necessary equipment, land, and construction of the plant.

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66 Table 4-1. DDD optimal operating conditions for industrial site. DDD System Operating Conditions General Operating Conditions: Diffusion Tower: Condenser: G = 1.5 kg/m2-s G = 1.5 kg/m2-s L = 0.15 kg/m2-s L = 3.0 kg/m2-s Tai = 82C Tai = 46.05C Twi = 29.89 C Twi = 19.82 C i = 0.01469 i = 0.0290 Tao = 46.05 C Tao = 21 C Two = 36.95 C Two = 29.2 C o = 0.0290 o = 0.0066 Height = 0.265 m Height = 0.822 m mfw/mL = 0.224 Econsumption= 0.001218 kW-hr/kg mfw = 201,817 Gal/day Flow Conditions using Avai lable Energy (850,000 acfm) Diffusion Tower: Condenser: ma = 395 kg/s ma = 395 kg/s mw = 39 kg/s mw = 789 kg/s Adiff = 263.16 m2 Acond = 263.16 m2 1. Land The cost of land is very region specific. It depends on the demand for land in a given area. 2. Well ConstructionRecent studies estimat e the cost of construction per meter depth to be $650. The average well capacity is approximated as 500 m3/d. 3. Process EquipmentThis category consis ts of some of th e most expensive equipment all of which will depend upon the plant capacity and process. Instrumentation and controls, pumps, el ectric wiring, pre-and post-treatment equipment, pipes, valves, and process cleaning systems are all included. The equipment for a RO plant are generally less than that of the distillation processes of MSF. A RO plant equipment can cost as little as $1,000 whereas a MSF plant with a 27,000 m3/d capacity can cost $40 million.

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67 4. Auxillary EquipmentGenerators, transformers, pumps, pipes, valves, wells, storage tanks, and transmi ssion piping are all consider ed auxillary equipment. 5. Building constructionThe building costs are site dependent and can vary from $100 to $1,000/m2. The indirect costs of a plan t are expressed as percentage s of the total direct capital cost. The indirect costs include freight and insurance, constr uction overhead, owners costs, and contingency costs. The freight a nd insurance is about 5% of the direct capital costs, construction overhead is about 15% of the direct materi al and labor costs, owners costs is 10% of direct materi al and labor costs, and the c ontingency costs are about 10% of the total direct costs. The annual opera ting costs are the cost s incurred during plant operation and include electricity, labor, maintenance, insurance, chemicals, and amortization or fixed charges. The costs for the DDD process operating at the conditions described in Table 4-1 are estimated. The estimates based on the following assumptions: 1. The interesting rate, i, is 5%. 2. The plant life is approximated as n=30 years. 3. The plant availability is estimated as f=0.9. 4. The cost of land is neglected as it assu med that the DDD system will be coupled with an aluminum smelting plan t and is readily available. 5. The cost of the chemicals used in preor post-treatment of the water is neglected. The cost is neglected because the chemical used in treatment is highly dependent upon the use of the fresh water produced. 6. The specific cost of the operating labor is estimated to be = $0.01/m3 for the thermal processes and $0.05/m3 for RO. Thus for the DDD process the specific cost of the operating labor is approximated as = $0.025/m3. 7. According to the American Water Works Association [22] th e direct costs are estimated to be $3-$6/gal per day insta lled capacity and are determined to be typical seawater desalting plant construc tion costs. This value does not include any of the off-site construc tion costs including engineering, legal, or financial

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68 fees. It also does not include the cost to transport the water to the plant, or incidental or contingency co sts. However the cost to develop the site is included and is estimated to be 5% of the material and construction costs. The amortization factor is calculated as, (1) (1)1n nii a i (4.8) where i is the interest rate a nd n is the plant life. The annua l fixed costs are calculated as, ()()fixed A aDC (4.9) where DC is the direct capital cost. Las tly, the annual labor co st is evaluated as, ()()()(365)laborAfm (4.10) where is the specific cost of operating labor and m is the plant capacity (m3/day). Therefore the total annual co st can be calculated as, totalfixedlaborAAA (4.11) It should be noted that the cost of the waste heat, for this case in the form of heated air, is not included since it is wast e that would have otherwis e been discarded into the environment. Therefore the unit product cost of the system is calculated as follows, ,365total unitpA A fm (4.12) Table 4-2 summarizes the estimate of the dir ect costs for a full scale diffusion driven desalination facility operating in conjunction with an industria l plant. The analysis shows that the production cost, negl ecting the electricity charges, is between 0.19-0.36 $/m3. In terms of $/103 gal the production cost is 0.72-1.34 $/103 gal. The fresh water profit is the most important consideration in the analysis of the DDD facility. One of the most expensive operating costs is the pumping power required

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69 Table 4-2. Summary of DDD Plant Costs. Unit Calculated Result Direct Costs (103 $) 636-1,271 ACfixed (103 $) 41-83 Alabor (103 $) 6.3 Alabor+ACfixed (103 $) 47.7-89 ACunit,p($/m3) 0.19-0.36 to operate the pumps and the blowers. It is not included in the summary of costs listed in Table 4-2. Since the cost of electricity is highly fluctuating it is factored into the determination of the profit. The fres h water profit can be calculated as, ,ffunitpfelecQACPwQ (4.13) where f ($/103gal) is the net fresh water profit, ACunit,p ($/103gal) is the pr oduction cost calculated above, Qf ($/103gal) is the retail price of fresh water, and Qelec ($/kW-hr) is the retail price of electricity. Here ACunit,p is taken to be an intermediate value of $0.99/103gal. The profit to be gain ed is highly dependent on th e retail cost of water and the cost of water strongly depends on how th e water is transported to the customer. Two different scenarios will be evaluated. First, the profit will be evaluated when the water is transferred to the customer via municipal pipe lines. For this case Figure 4-15 depicts the calculated net fresh water profit as a function of the retail price of electricity for varying costs of water. On average, the retail pr ice of water in the United States is about $3/103gal. For this price, the largest profit to be earned is for the lowest energy retail price of $0.04/kW-hr. As the retail price of energy increases, the profit linearly decreases. A similar trend is observed for the other water retail prices. If the retail price

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70 of water is $3/103gal and the cost of elect ricity is $0.08/kW-hr, then the profit is about $1.80/103gal. For the production summarized in Ta ble 4-1, this amounts to a profit of only $363/day. Given this profit outlook it adva ntageous to consider other prospects to market the product distilled water. It is of importance to recall that the product of the DDD process is high purity distilled water, which is of s uperior quality to that circulat ing through the municipal lines. Another possible solution ther efore would be to consider selling the distilled water as bottled water. Figure 4-16 shows the growth of the bottled water industry. As the graph shows, the sale of bottled water has been st eadily increasing since the late 1970s. Clarke and King [1] estimate that the bottled water market was worth an estimated $20 billion a year for 2002 alone. The reasons for th e increase in bottled water consumption vary from distaste of tap water, distrust of the sa fety of tap water, and the overall realization that water is a healthier choi ce of beverage than soda and other carbonated beverages. The price per gallon of bottled distilled wa ter can vary between $0.90-$1.50 per gallon. Without taking into account the cost of bo ttling, Figure 4-17 shows the net fresh water profit that can be achieved with the DDD fres h water product. The graph shows that the operating and production costs have little effect on the fres h water profit. Nearly all revenue generated from sales is considered profit If the distilled bottled water is sold at $1.00/gal and the DDD process can produce 201, 800 gal/day then the profit generated is about $200,000/day or $73 million/yr. As it can be seen there is the possibility of enormous profit because the pr ofit is basically th e retail price of th e distilled bottled water. Given the continuous gr owth of the bottled water indu stry that shows no signs of slowing down, an enormous profit can be rendered from the DDD process.

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71 Energy Retail Price ($/kW-hr) 0.040.060.080.100. 120.140.160.180.20 Net Fresh Water Profit ($/103 gal) 0 1 2 3 4 5 6 7 Water Retail Price ($/103 gal) 2 3 4 5 6 7 Figure 4-15. Net fresh water profit as a functi on of energy retail price for varying water retail price. Overall the DDD process utilizing a heated air/heated water inlet conditions has proven to be an economically reasonable pro cess when coupled with an industrial plant that can provide waste heat. While trans porting the distilled fr esh water product to customers predicted minimal profit, the bottl ed water industry provi des the opportunity to sell the distilled water for enormous profit.

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72 Year 197519801985199019952000 Millions of Gallons 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Figure 4-16. The bottled water mark et in the United States [23]. Energy Retail Price ($/kW-hr) 0.040.060.080.100.120.140.160.180.20 Net Fresh Water Profit ($/gal) 0.6 0.8 1.0 1.2 1.4 Distilled Bottled Water Retail Price ($/gal) 0.80 0.90 1.00 1.10 1.20 1.30 Figure 4-17. Net fresh water product as a f unction of energy retail price for varying distilled bottled water prices.

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73 CHAPTER 5 CONCLUSION The diffusion driven desalination process pr ovides an economic means for utilizing and converting saltwater to high purity distilled water A laboratory scale DDD facility has been designed and constructed which is fully instrumented and includes a data acquisition system. The laborator y facility has been modified to include an air heating section which heats the inlet air flowing to the diffusion tower. Experiments were conducted on the diffusion tower for two diffe rent cases: the heated air/ambient water and heated air/heated water. For each cas e, a set of experiments were conducted by holding the air mass flux constant and vary ing the liquid mass flux over a range of values. The heat and mass transfer for the DDD process were analyzed. A one dimensional liquid film model was used to obtain three c oupled ordinary differential equations with dependent variables of humidity, water temp erature and air temper ature. Closure was achieved by using the heat and mass transfer analogy and Ondas correlation was used to evaluate the mass transfer coefficients. Through experimental analysis, the wetted specific area of the packed bed was found to be a constant in the range of operating conditions studied. It was also found that fo r the heated air/heated water case there is moderate heat loss from the system that mu st be accounted for in the energy balance. The experimental values obtained were compar ed to those obtained from the theoretical heat and mass transfer model. Agreemen t between the two was quite good for both cases. It was also found that the repeatability of the experi ments is satisfactory. The

PAGE 89

74 heated air/ambient water case proved to be ine fficient with very little change in exit conditions over the range of liquid and air mass fluxes. Th e heated air/heated water process proved to give better performance. The theoretical model is a necessary tool needed to design an efficient DDD facility. A parametric study to determine the effect of certain operating variables on the maximum freshwater production was complete d for the heated air/heated water DDD process. For each liquid mass flux there is an optimal air mass flux where the maximum fresh water production flux is produced. There is also a location where the energy consumption for pumping through the diffusion tower is minimized. The heated air/heated water process was compared to other different DDD processes, specifically the heated water/ambien t air case and the heated air/heated water process utilizing the Q-PAC packed bed, a less dense and lower pr essure drop packing material. The comparison showed that the heated air/heated water DDD process has its advantages and disadvantages at different operating conditions. In comparison for the heated water/ambient air it was shown that th e heated air/heated water process requires more electrical energy than the heated wate r/ambient air process at low liquid mass flux however the fresh water producti on efficiency for the heated air/heated water is greater for all values of liquid mass flux. The fres h water production for the heated air/heated water process is greater for all liquid mass flux. In the comparison with the heated air/heated water using the Q-PAC packing, it was shown that similar fresh water production can be achieved at a lower energy cost by using Q-PA C. Further studies are required to determine the wettability of Q-PAC and its wetted fraction as part of the DDD process.

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75 An economic study was performed to determ ine the cost effectiveness of the DDD process using a heated air/heated water i nput when coupled with an industrial plant producing waste heat in an air stream. If 850,000 acfm of 82 C heated waste heat in air is available from a plant the optimal ope rating conditions are a diffusion tower liquid mass flux of 0.15 kg/m2-s, an air mass flux of 1.5 kg/m2-s, and a fresh water mass flux of 3.0 kg/m2-s in the condenser. At these conditio ns with the given amount of waste heat available, the plant can produce 201,800 gal/d ay of high purity di stilled water while consuming a miniscule 0.0012 kW-hr/kg and ha ving a fresh water conversion efficiency of 0.224. Through an economic study it was dete rmined that a reasonable profit can be realized by selling the product as distilled bot tled water. Without taking into account the cost of bottling, assuming that a gallon of distilled water costs $1.00 per gallon, a profit of $0.99 per gallon can be achieved. Extensive research has already been performed on the DDD process and while the future of the process looks optimistic, fu rther research needs to be done. The DDD process needs to be further examined on a la rger scale. A pilot facility should be constructed and the DDD performance should be analyzed over long term operation in an industrial setting. As of yet, the DDD process has not been analyzed using a saline feed water source in the diffusion tower. Using such a source, water quality testing of the fresh water produced in the DDD process shoul d be conducted. This is important in determining what type of post treatment of the product wate r is required. The DDD process has been thoroughly studied using a laboratory scale facility. The diffusion tower performance has been stud ied for heated air/heated water, heated air/ambient water, and heated water/ambient air inputs [16]. On a small scale, further

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76 experiments should be conducted to determin e the performance of DDD system utilizing a different type of packed bed. On a larger scale, the DDD process now needs to be examined with a pilo t scale facility coupl ed with a low grade waste heat source to determine its performance under an i ndustrial load and continuous operation.

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77 APPENDIX A ONDAS CORRELATION 1/3 0.4 2/30.50.0051ReL LLwLp Lg kScad 2 0.71/3ReGGAGpGkCScadaD, (C=5.23 if dp > 15 mm; C=2 if dp 15 mm) 3/4 1/20.051/51exp2.2Rec wLALL LaaFrWe ReLA LL a 2 L LLa Fr g 2 L LLL We a ReLW wLL a ReGA GG a L L LLSc D G G GGSc D This equation has been modified from Ondas original correlation

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78 APPENDIX B EXPERIMENTAL DATA OF THE DI FFUSION TOWER FOR HEATED AIR/AMBIENT WATER mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.031 0.040 25.26 59.83 0.0075 26.93 26.07 0.0206 0.040 0.040 25.36 60.26 0.0075 26.98 25.95 0.0205 0.047 0.040 25.30 59.88 0.0075 26.87 25.80 0.0203 0.056 0.040 25.36 60.27 0.0075 26.94 25.75 0.0203 0.063 0.040 25.30 60.27 0.0075 26.80 25.70 0.0203 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.028 0.040 25.28 61.82 0.0041 25.70 26.04 0.0201 0.035 0.040 25.30 61.91 0.0040 25.70 25.58 0.0198 0.044 0.041 25.09 61.66 0.0041 25.42 25.11 0.0194 0.059 0.041 25.10 61.76 0.0041 25.38 24.99 0.0193 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.029 0.060 25.30 60.57 0.0060 26.73 26.04 0.0204 0.039 0.060 24.77 60.82 0.0061 26.45 25.66 0.0200 0.051 0.060 25.06 60.95 0.0062 26.40 25.65 0.0200 0.062 0.059 25.33 61.33 0.0062 26.40 25.70 0.0201 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.032 0.060 24.90 60.38 0.0068 26.29 26.09 0.0205 0.039 0.061 25.25 59.78 0.0067 26.38 25.86 0.0203 0.043 0.061 25.20 59.83 0.0068 26.43 25.86 0.0203 0.049 0.060 25.20 59.95 0.0069 26.48 25.83 0.0203 0.055 0.060 25.17 60.03 0.0069 26.35 25.81 0.0203 0.064 0.061 25.16 60.16 0.0069 26.28 25.79 0.0203

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79 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.029 0.080 24.66 61.09 0.0068 27.49 26.34 0.0207 0.037 0.081 25.09 61.03 0.0072 27.46 26.32 0.0208 0.048 0.081 25.39 60.96 0.0071 27.37 26.25 0.0208 0.055 0.080 25.17 61.09 0.0072 27.27 26.16 0.0207 0.065 0.081 25.38 61.39 0.0073 27.33 26.21 0.0208 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.035 0.080 25.28 60.38 0.0080 27.51 26.67 0.0213 0.042 0.080 25.27 60.30 0.0081 27.40 26.68 0.0213 0.048 0.080 25.34 60.43 0.0080 27.37 26.61 0.0213 0.056 0.079 25.24 60.76 0.0080 27.31 26.55 0.0212 0.065 0.081 25.27 60.46 0.0080 27.13 26.42 0.0211

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80 APPENDIX C EXPERIMENTAL DATA OF THE DI FFUSION TOWER FOR HEATED AIR/HEATED WATER mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.034 0.040 60.82 61.03 0.0066 37.21 42.28 0.0550 0.039 0.039 60.99 61.15 0.0066 37.81 43.57 0.0591 0.047 0.039 59.99 61.71 0.0066 38.84 44.74 0.0632 0.054 0.040 60.58 60.94 0.0069 39.96 44.76 0.0631 0.061 0.041 61.02 60.61 0.0071 40.41 45.54 0.0661 0.068 0.041 59.48 60.52 0.0074 40.91 45.03 0.0642 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.036 0.041 61.00 61.35 0.0059 35.99 42.30 0.0566 0.043 0.040 60.79 61.57 0.0059 36.27 44.03 0.0624 0.050 0.040 60.75 61.69 0.0061 37.30 44.71 0.0649 0.056 0.041 60.89 61.53 0.0064 39.12 44.10 0.0627 0.064 0.042 60.31 60.94 0.0066 39.63 44.89 0.0656 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.032 0.059 61.01 61.10 0.0078 33.65 37.74 0.0420 0.041 0.060 61.66 61.08 0.0078 34.46 39.70 0.0474 0.047 0.061 61.12 60.93 0.0080 35.02 40.20 0.0489 0.055 0.060 60.05 61.23 0.0079 36.14 40.98 0.0511 0.063 0.059 60.52 62.10 0.0081 39.87 42.18 0.0550

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81 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.033 0.061 60.79 60.52 0.0080 32.46 37.51 0.0416 0.038 0.060 60.76 60.69 0.0080 33.51 38.44 0.0448 0.046 0.060 60.97 61.03 0.0081 34.65 39.70 0.0482 0.056 0.061 60.86 61.10 0.0082 35.95 40.85 0.0498 0.062 0.060 60.57 61.16 0.0083 37.06 41.33 0.0510 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.030 0.080 60.94 60.11 0.0088 30.75 34.51 0.0353 0.039 0.080 61.01 60.29 0.0090 31.88 35.91 0.0387 0.048 0.080 60.45 60.48 0.0088 33.04 37.40 0.0426 0.056 0.081 60.40 60.78 0.0089 34.88 38.02 0.0442 0.066 0.080 60.26 60.94 0.0090 36.02 39.01 0.0468 mL ma TL,in Ta,in in TL,out Ta,out out (kg/s) (kg/s) ( C) ( C) ( C) ( C) 0.032 0.080 59.69 59.94 0.0080 31.03 35.02 0.0350 0.036 0.080 61.29 59.73 0.0083 30.82 35.97 0.0378 0.046 0.080 60.83 61.09 0.0084 32.97 37.12 0.0410 0.055 0.081 60.57 60.96 0.0086 34.43 37.90 0.0429 0.064 0.081 58.98 60.40 0.0088 34.99 38.97 0.0458

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82 LIST OF REFERENCES 1. Clarke, R., and King, J., 2004, The Water Atlas The New Press, New York. 2. Semiat, R., 2000, Desalination: Presen t and Future, Water International, 25(1), pp. 54-65. 3. Dore, M., 2005, Forecasting the Economic Costs of Desalination Technology, Desalination., 172, pp. 207-214. 4. Ehrenman, G., April 2003, Mapping the Road to Water, Mechanical Engineering, 125(4), pp. 23. 5. Economic and Social Commission for West ern Asis, 2001, Energy Options for Water Desalination in Selected ESCWA Countries, Publication E/ESCWA/ENR/2001/17, Unite d Nations, New York. 6. Bourouni, K., Chaibi, M.T., Tadrist, L., 2001, Water Desalination by Humidification and Dehumidification of Ai r: State of the Art, Desalination, 137, pp. 167-176. 7. Bourouni, K., Martin, R., Tadrist, L., Chaibi, M.T., 1999, H eat Transfer and Evaporation in Geothermal Units, Applied Science, 64, pp. 129-147. 8. Al-Hallaj, S., Farid, M.M., Tamimi, A.R., 1998, Solar Desalination with a Humidification-Dehumidificat ion Cycle: Performance of the Unit, Desalination, 120, pp. 273-280. 9. Vlachogiannis, M., Bontozoglou, V., Georgalas, C., Litinas, G., 1999, Desalination by Mechanical Compre ssion of Humid Air, Desalination, 122, pp. 35-42. 10. Klausner, J.F., Li Y., Darwish, M., and Mei, R., 2004, Innovative Diffusion Driven Desalination Process, J. of Energy Resources Technology, 126(3), pp. 219225. 11. Li, Y., Klausner, J.F., Mei, R., 2006, Per formance Characteristics of the Diffusion Driven Desalination Process, Desalination, 196, pp. 188-209. 12. Sadhawani, J.J., Veza, J.M., Santana, C., 2005, Case Studies on Environmental Impact of Seawater Desalination, Desalination, 185, pp. 1-8.

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83 13. Morton, A.J., Callister, I.K., Wade, N.M., 1996, Environmental Impacts of Seawater Distillation and Reverse Osmosis Processes, Desalination, 108, pp. 1-10. 14. Breschi, D., 1999, Seawater Distillation from Low-Temperature Streams: A Case History, Desalination, 122, pp. 247-254. 15. Li, Y., 2006, Heat and Mass Transfer for the Diffusion Driven Desalination Process, Ph.D. Thesis, University of Florida, Gainesville. 16. Klausner, J. F., Li, Y., Mei, R., 2006, E vaporative Heat and Mass Transfer for the Diffusion Driven Desalination Process, J. of Heat and Mass Transfer, 42, pp. 528536. 17. Li, Y., Klausner, J., Mei, R., Knight, J., 2006, Direct Condensation in Packed Beds, International J. of Heat and Mass Transfer, 49, pp. 4751-4761. 18. Kays, W.M., Crawford, M.E., Weigand, B., 2005, Convective Heat and Mass Transfer McGraw Hill, New York. 19. Eckert, E.R.G., Goldstein, R.J., 1976, Measurements in Heat Transfer Hemisphere Publishing Corp, Washington, Chap. 9. 20. Onda, K., Takechi, H., Okumoto, Y., 1968, Mass Transfer Coefficients Between Gas and Liquid Phases In Packed Columns, J. Chem. Eng. Jpn., 1, pp. 56-62. 21. Ettouney, H. M., El-Dessouky, H. T., Raib ish, R. S., Gowin, P. J., December 2002, Evaluating the Economics of Desa lination, Chemical Eng. Prog., 98(12), pp. 3239. 22. Water Desalting Committee of the Amer ican Water Works Association., 2004, Water Desalting Planning Guide For Water Utilities John Wiley & Sons, Inc, New Jersey. 23. International Bottled Water Association, 2000, U.S. Bottled Water Market Volume, Growth, Consumption -1976-1999 , 2000 Marketing Report Findings, Retrieved on November 17, 2006 from http://www.bottledwater.org/public/statistics_main.htm.

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84 BIOGRAPHICAL SKETCH Jessica Knight is the dau ghter of Donald and Renee Knight. She was born and raised in Jacksonville, Florida. She began attending the University of Florida in August 2000 where she later earned her bachelors degree in mechanical engineering in December 2004. She began working on the di ffusion driven desalination project in her senior year of her undergraduate studies. She continued research on the diffusion driven desalination process as a gra duate student in January 2005 in pursuit of a Master of Science degree.


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Material Information

Title: Heat and Mass Transfer within the Diffusion Driven Desalination Process with Heated Air
Physical Description: Mixed Material
Copyright Date: 2008

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Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
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HEAT AND MASS TRANSFER WITHIN THE DIFFUSION DRIVEN
DESALINATION PROCESS WITH HEATED AIR














By

JESSICA KNIGHT


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006
































Copyright 2006

by

Jessica Knight









































To my parents, for their infinite support and love throughout my college career.
















ACKNOWLEDGMENTS

I would like to thank everyone who has contributed to the successful completion of

this work. First, I would like to thank Dr. James Klausner for his constant guidance

throughout the past two years and for providing me the opportunity to do research on the

diffusion driven desalination project. His patience, academic supervision, and technical

assistance are greatly appreciated. I would also like to extend thanks to Dr. Renwei Mei.

His academic lectures and technical discussions were always a welcome challenge. I

would also like to thank Dr. David Hahn for serving on my supervisory committee.

I would also like to thank all of my peers who helped me through my graduate

studies career. A special thanks goes to Yi Li for his assistance and advice.

This research was supported by the U. S. Department of Energy under Award No.

DE-FG26-02NT41537. Without their financial assistance this research would not be

possible.

Lastly, I would like to thank my parents for their unceasing love and support

throughout my academic career. Their constant encouragement and understanding gave

me the motivation necessary to achieve my goals.





















TABLE OF CONTENTS


page

ACKNOWLEDGMENT S .............. .................... iv


LI ST OF T ABLE S ................. .............. vii...___.....


LIST OF FIGURES ............_...... .__ ..............viii...


NOMENCLATURE .............. .................... xi


AB STRAC T ................ .............. xiv


CHAPTER


1 INTRODUCTION ................. ...............1.......... ......


M ethod s of Desalination............... .............
M embrane Processes .............. ...............2.....
Thermal Processes ................. ...............3.................
Advancements in Desalination .............. ...............4.....
Desalination and the Environment ................. ...............10................

Comparison of DDD with RO and MSF ................. ...............11........... ..
Scope of W ork ................. ...............12........... ....

2 EXPERIMENTAL FACILITY .............. ...............14....


System Overview............... ..... ...........1
Description of Individual Components ...._._ ....__ ................. ...............17

3 HEAT AND MASS TRANSFER WITHIN THE DIFFUSION TOWER ........._......26


Theoretical Heat and Mass Transfer Model .............. ...............26....
Results and Discussion ............... ....... ...............33
Case 1: Heated Air/Ambient Water. ....__ ......_____ .......___ ...........3
Case 2: Heated Air/Heated Water .............. ...............39....


4 PARAMETRIC ANALYSIS ............ .....___ ...............47...


Parametric Analysis of Heated Air/Heated Water DDD Process ............... ..............47
Heated Air/Heated Water Results and Discussion .............. ..... ............... 4

Comparison of Different DDD Processes ....._. ................. ........._.. .......5











Heated Water/Ambient Air at 60 OC .............. ...............55....
Heated Air/Heated Water using Q-PAC ................. .............. ......... .....59
Industrial Plant Application............... ..............6
Operation Conditions............... ...............6
Economic Analysis ............ ..... ._ ...............65....

5 CONCLU SION................ ..............7


APPENDIX


A ONDA' S CORRELATION ............ ..... .__ ...............77...


B EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED
AIR/AMBIENT WATER ............ ..... .__ ...............78....


C EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED
AIR/HEATED WATER ............ ..... ._ ...............80....


LIST OF REFERENCES ............ ..... .__ ...............82...

BIOGRAPHICAL SKETCH .............. ...............84....

















LIST OF TABLES

Table pg

1-1 Comparison of energy consumption for desalination processes. ............. ................5

1-2 Cost estimation per cubic meter of water for various processes. ............. ..............6

4-1 DDD optimal operating conditions for industrial site. .............. ....................6

4-2 Summary of DDD Plant Costs. ........._.. _....._ ...............69.



















LIST OF FIGURES


Figure pg

1-1 Reverse Osmosis (RO) system ............. ...............2.....

1-2 Multistage Flash (MSF) plant. ............. ...............4.....

1-3 Flow chart of the DDD process ................. ...............8............ ..

2-1 Diffusion driven desalination (DDD) facility. ............. ...............15.....

2-2 Air heating section. ............. ...............16.....

2-3 Diffusion driven desalination (DDD) system ................. .............................18

2-4 Diffusion tower ............ ...............19.....

2-5 An Allspray water distributor ................. ...............20........... ...

2-6 Shaped heater. ............. ...............20.....

2-7 The HD Q-PAC packed bed ................. ...............21...............

2-8 Water flow meter calibration curves. ............. ...............22.....

2-9 M ain control. ............. ...............24.....

2-10 Diffusion tower data. .........._.... ...............24.._.__. ....

2-11 Histogram view of the DDD data acquisition. ............__......_ ................25

3-1 Differential control volume for heated air conditions .................... ...............2

3-2 Calibration curve of the heat loss flux for the heated air/heated water case...........34

3-3 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 0.77 kg/m2-S ................. ...............36..............

3-4 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.15 kg/m2-S ................. ...............37...........










3-5 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.55 kg/m2-S ................. ...............38......_.. .

3-6 Repeatability of different experiments for different exit parameters............._.._......39

3-7 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 0.79 kg/m2-S. ....._.._.. .... ..._. _. ...._.._..........4

3-8 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.15 kg/m2-S. ....._.._.. .... ..._. _. ........_.......4

3-9 Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.55 kg/m2-S. ....._.._.. .... ..._. _. ...._.._.........4

3-10 Repeatability of experiments for different exit parameters. ........._..... ........._......45

4-1 Tower height at the maximum exit absolute humidity as a function of the inlet
air m ass flux G. ............. ...............50.....

4-2 Exit air temperature at maximum absolute humidity as a function of the air mass
flux. ............. ...............51.....

4-3 Maximum exit absolute humidity with varying air mass flux. ............. .................52

4-4 Fresh water production flux with varying air mass flux. ............. ....................53

4-5 Diffusion tower energy consumption rate. .............. ...............54....

4-6 Fresh water production efficiency with varying air mass flux realized for the
lower air and liquid mass fluxes ................. ...............56...............

4-7 Fresh water production efficiency and energy consumption rate for varying
liquid mass flux for 60 oC inlet conditions. ............. ...............58.....

4-8 Fresh water production flux and energy consumption rate for varying liquid
mass flux for 60 oC inlet conditions ................. ...............58........... ..

4-9 Fresh water production flux and energy consumption rate for varying liquid
mass flux for HD Q-PAC and Q-PAC packed bed. ............. .....................6

4-10 Fresh water production flux and energy consumption rate for varying liquid
mass flux for HD Q-PAC and Q-PAC packed bed. ............. .....................6

4-11 Sample schematic of the DDD system coupled with an industrial site. .................. .63

4-12 Fresh water production flux for varying diffusion tower liquid mass flux. .............64

4-13 Energy consumption rate of the DDD system for varying diffusion tower liquid
m ass flux. ............. ...............64.....











4-14 Fresh water production efficiency for varying diffusion tower liquid mass flux.....65

4-15 Net fresh water profit as a function of energy retail price for varying water
retail price............... ...............71.

4-16 The bottled water market in the United States. ............. ...............72.....

4-17 Net fresh water product as a function of energy retail price for varying distilled
bottled water prices. ............. ...............72.....









NOMENCLATURE


A Cross sectional area (m2)

a Overall specific volume of the packed bed (m2/m3)

aw Wetted specific area of the packed bed (m2/m3)

CP Specific heat (kJ/kg-K)

D Molecular diffusion coefficient (m2/S)

f Plant availability

d Diameter of the tower (m)

d, Diameter of the packed bed (m)

G Air mass flux (kg/m2-S)

g Gravitational acceleration (m/s2)

hfg Latent heat of vaporization (kJ/kg)

h 'Enthalpy (kJ/kg)

i Interest rate

K Thermal conductivity (W/m-K)

k Mass transfer coefficient (m/s)

L Water mass flux (kg/m2-S)

My Molecular weight of vapor (kg/kmol)

m Mass flow rate (kg/s)

n Plant life

P Pressure (kPa)

Pw Electrical power consumption for pumps (kW)

Q Retail cost ($)









qHL") Heat loss flux (kW/m2)

R Universal gas constant (kJ/kg-K)

T Temperature (OC or K)

U Heat transfer coefficient (W/m2-K)

VG Volumetric flow rate (m3/S)

Y Specific cost of operating labor ($/m3)

CL Dynamic viscosity (Pa-s)

p Density (kg/m3)

ca Critical surface tension of the packed bed (N/m)

GL Liquid surface tension (N/m)

co Absolute humidity

(D Relative humidity

H Profit ($)

Sub scripts

a Air

elec Electricity

evap Portion of liquid evaporated

Eixed Fixed cost

fw Fresh water

G Air/vapor mixture

GA Gas side parameter based on the specific area of the packed bed

i Liquid/vapor interface

in Inlet parameter










L Liquid

LA Liquid side parameter based on the specific area of the packed bed

Labor Labor Cost

LW Liquid side parameter based on the specific wet area of the packed bed

mix Air/vapor mixture

out Outlet parameter

sat Saturated state

unit,p Unit amount in terms of production

z Fluid flow direction















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science


HEAT AND MASS TRANSFER WITHIN THE DIFFUSION DRIVEN
DESALINATION PROCESS WITH HEATED AIR

By

Jessica Knight

December 2006

Chair: James F. Klausner
Major Department: Mechanical and Aerospace Engineering

The purpose of this research is to examine the performance of the diffusion driven

desalination process (DDD) with heated air inlet conditions. A laboratory scale DDD

facility has been constructed and fully instrumented. Experiments were conducted and

data were collected for two different cases: heated air/ambient water and heated

air/heated water. The experiments were conducted over a range of liquid and air mass

fluxes. A theoretical heat and mass transfer model was compared against the

experimental data collected. The experimental values agree quite well with the

theoretical model, provided the fraction of area wetted is correctly specified. The heated

air/heated water case is demonstrated to be a more efficient process than the heated

air/ambient water case. A parametric study reveals that for every liquid mass flux there is

an air mass flux value where the diffusion tower energy consumption is minimal and an

air mass flux where the fresh water production flux is maximized. A study was also

performed to compare the DDD process with different inlet operating conditions as well









as different packing. It is shown that the heated air/heated water case is more capable of

greater fresh water production with the same energy consumption than the ambient

air/heated water process at high liquid mass flux. It is also shown that there can be

significant advantage when using the heated air/heated water process with a less dense

less specific surface area packed bed. A case study with the DDD process was coupled to

an industrial site that produces 850,000 acfm heated waste air at 820C. For inlet air

conditions at 820C, the fresh water production and system energy consumption were

both optimal at a diffusion tower liquid mass flux of 0. 15 kg/m2-S, a system air mass flux

of 1.5 kg/m2-S, and a fresh water to air mass flow ratio of 2 in the direct contact

condenser. It was determined that with the available energy and operations at the optimal

conditions the plant can produce 201,800 gal/day with a total electrical energy

consumption of 0.0012 kW-hr/kg and a fresh water production efficiency of 0.224. The

total footprint area required is 526.2 m2. An economic study revealed that the most

Financial gain can be achieved when the product distilled water is sold as bottled water.

Without considering the cost of bottling, if a gallon of distilled water costs $1.00, then a

profit of $0.99 per gallon is expected. Thus the DDD process is economically viable

when driven by waste heat carried by air.















CHAPTER 1
INTTRODUCTION

Water is an essential part of sustaining life on earth, and its use is widespread for

industrial, irrigation, mining, and domestic use. While water is abundant on earth, 97%

of the water is saline, while only 3% is freshwater. Of the 3% freshwater, 70% of that is

frozen in glaciers, ice, or permafrost; 30% is groundwater, while only a mere 0.25% is

available above the ground in the form of lakes and rivers. In 2000 Clarke and King [1]

estimated that of the 6 billion people on Earth, 0.5 billion people lived in countries that

were chronically short on water. It is proj ected by the year 2050 that the world

population will grow to 8.9 billion people and that 4 billion people may live in countries

that are chronically short on water. A growing population is accompanied by an

increasing need for agricultural and industrial output. Agriculture currently accounts for

nearly 70% of freshwater withdrawals, and the demand for food will only increase with

increasing population [1]. Growth in industry is also expected. The industrial use of

water is expected to grow steeply over the next 25 years as more countries industrialize

[1]. Given the fact that the population on earth continues to increase and industrial

growth shows no signs of slowing down, it is inevitable that conventional sources of

freshwater are not sustainable. The only water resource that is inexhaustible is the

oceans. Thus a solution for sustainability may lie in desalination technologies.

Methods of Desalination

Desalination is the process by which salt is removed from water to produce fresh

water. There are several different types of desalination, however the two main types are










membrane and thermal- or phase change desalination. Membrane desalination involves

the use of a membrane to remove the salt using either electrical force or mechanical force

in the separation process. There are two main types of membrane processes: reverse

osmosis (RO) and electrodialysis. Unlike membrane processes, thermal desalination

removes the salt by causing the solution to undergo a change in phase. Multistage flash

(MSF), multiple-effect distillation (MED), and vapor compression (VC) are the most

common thermal desalination processes.

Membrane Processes

The most common membrane process is reverse osmosis (Fig. 1-1). Reverse

osmosis utilizes a semi-permeable membrane to separate the unwanted ions from the

solution. The force driving the solution through the membrane is provided by a

feedwater pump whose pressure depends on the concentration of the solid, desired

recovery, and overall performance of the membrane. The RO membranes are arranged

in pressure vessels often containing 1-7 spiral-wound membrane elements. These vessels

can be placed in series or parallel depending on the desired concentration of the final

desalted water product.

Post Treatment
Low pressure purnp Pre-treatment
Mlembranes Prdt




Energy


Figure 1-1. Reverse Osmosis (RO) system [2].











Another type of membrane process is electrodialysis, (ED). Unlike RO, ED uses

an electrical force to drive the ions through the membrane. It is an electrochemical

process where electrodes are placed in a solution of the dissolved solid and a dc power is

supplied. The ions will migrate towards the electrode of opposite charge of the given ion.

The movement of ions is controlled by ion-selective membranes that form watertight

compartments. These membranes are electrically conductive and impermeable to water

under pressure.

Thermal Processes

Multistage flash (MSF) is the most widely used thermal process. The MSF process

is initiated by heating seawater using steam flowing over the water tubes. The water

tubes are enclosed in a vessel called the brine heater. The water then flows to another

chamber where the pressure is lowered to the point where the water will either boil or

flash into steam. In general there is not a sufficient amount of vapor formed thus several

evaporative stages are needed to produce more steam. The steam produced is usually

condensed on the heat exchanger tubes running through each stage producing freshwater.

Fig 1-2 depicts a schematic diagram of a sample MSF desalination plant.

Multi-effect distillation (MED) and vapor compression (VC) evaporation are other

types of thermal distillation processes that are used. MED is quite similar to MSF except

in MED, except feedwater is sprayed on the outer surface of the steam tubes to enhance

boiling or flashing. VC is generally used for small scale freshwater production and is

very similar to MED. However unlike MED, VC typically utilizes mechanical energy,

via a mechanical compressor, rather than direct energy to supply the thermal energy to

heat the incoming feedwater. The mechanical compressor creates a vacuum in the vessel










and compresses the vapor removed from the vessel. As the vapor exits the vessel it

condenses on the inside of a tube bundle and releases heat. The feedwater is then sprayed

on the tube bundle where boiling and evaporation occurs.






Steam Preatreatment
heater ~~~~Condensate collection traysofeawtrfd




Primary



Flash chambers Productou


Heat recovery stages Concentrate Heat rejection stages
out



Figure 1-2. Multistage Flash (MSF) plant [2].

Advancements in Desalination

Desalination is a rapidly growing technology. As noted by Mielke and quoted in

Dore [3], there are approximately 11,000 desalination plants in 120 countries around the

world with a combined capacity of 13.25 Mm3/d. He also noted the three factors which

have the greatest impact on the overall cost of desalination per unit of fresh water

produced: salinity of inlet feedwater, energy costs, and plant size. While RO and MSF

are still the more widely used methods of desalination there are drawbacks to the

processes. The most important is the fact that all of the processes are very energy

intensive and are generally only useful for large scale production. With large energy

requirements, cost is certainly an issue. John Keys, commissioner of the Bureau of










Reclamation, has said that "cost reduction is the single most important factor to increase

the implementation of desalination [4]." Table 1-1 presents a comparison for the costs of

the various desalination processes [5]. As the table shows energy demands of the various

processes can be extensive. Thus a need emerges for a more energy efficient desalination

process.

Table 1-1. Comparison of energy consumption for desalination processes.

Process Type Distillation Membrane
Desalination Process MSF MED MVC RO ED
Thermal Consumption, 0.070- 0.042- -None None
kWh/kg 0.084 0.061
Electrical 0.0035- 0.0015- 0.0015- 0.005-
Consumption, kWh/kg 0.005 0.0025 0.002 0.009

Of recent interest is a fairly new process entitled humidification dehumidification

(HDH). Bourouni et al. [6] describes the process as simple and flexible with low

installation and operating costs and encompasses a possibility of utilizing a low

temperature energy source. In this process air is drawn through a packed bed tower, the

humidifier, where air and preheated seawater will meet and heat and mass transfer will

occur creating a saturated air/vapor mixture. The air/vapor mixture then moves to the de-

humidifier where the vapor is condensed out. The extraction of the vapor from the air

can be done in several ways including mechanical, refrigeration, adsorption, and

absorption methods. The most common method is through film condensation used by

Bourouni et al. [7]. He reported on an aero-evapo-condensati on process using waste

geothermal heat and commented on the economic competitiveness of the process. Table

1-2 summarizes his results. From the table it can be seen that the HDH principle is

competitive when coupled with the energy from waste heat.









Bourouni et al. [6] commented on the numerous advantages of HDH over other

common forms of desalination. First, HDH is flexible in capacity. In general, the only

components required are an evaporator and a condenser which can be designed to be

compact. Also, due to the low temperature and pressure operating range, components are

primarily plastics which are light, inexpensive, and easy to clean. Perhaps most

important however is the fact that it can run off of low grade heat which implies that the

only energy required for the system is the pumping power.

Table 1-2. Cost estimation per cubic meter of water for various processes.

Plant Unit Water Cost/m3
MSF with back-pressure steam turbine 1.57
MSF with gas turbine and waste-heat boiler 1.44
MSF/TVC with gas turbine and waste heat boiler 1.31
RO single-stage with energy recovery 1.39
Aero-evapo-condensation process (geothermal energy) 1.15
Aero-evapo-condensation process (fuel) 4.80

Employing the principles of HDH, a similar process, multiple effect humidification,

was developed. This process utilizes the same principles of HDH with the addition of

multiple evaporation and condensation cycles. Al-Hallaj et al. [8] reported on a MEH

unit utilizing solar collection panels to provide the heating source for the water. They

tested both a pilot and a bench unit over a range of operating conditions. They concluded

that an increase in water flow rate will maximize the production of the unit to an

optimum point, but will also decrease the operating water temperature which leads to a

decrease in efficiency of the evaporator and condenser. They also determined that the

amount of freshwater produced was directly related to the season. They were also able to

increase the production at night by utilizing the hot water rej ected from the humidifier.










Vlachogiannis et al. [9] reported on mechanically intensified evaporation (MIE)

utilizing the HDH principle in combination with vapor compression and a heat pump. In

the MIE process air enters an evaporation chamber through a porous media and is

dispersed in small bubbles through the liquid. The exiting stream is then compressed by

a blower and directed to a shell and tube heat exchanger. They reported adequate results,

but improvements in the condenser surface area were needed to yield a cost effective

process.

While HDH shows promise it has its downfalls in comparison with the more well

known methods of desalination. First, the overall production rate is smaller in

comparison with RO and MSF and thus cannot compete for large scale production.

Second, in general natural draft is relied upon for the air. This results in lower heat and

mass transfer coefficients as well as an increase in the area of the humidifier. Finally, a

shell and tube type heat exchanger is typically used as the de-humidifier. This method of

heat exchange directly depends on the amount of surface area. For large freshwater

production, more condenser surface area is required which translates to an increase in the

amount of land needed.

Thus a more efficient means of desalination should be used to overcome the

limitations of the HDH method. Klausner et al. [10] described a diffusion driven

desalination (DDD) process that may provide an economically feasible solution to the

shortcomings of HDH.

Li et al. [1 l] describes the DDD process, which is designed to run off of waste heat

and is depicted in Figure 1-3.

































-*-* Low Pressure Steamn -- 4ir Iapor
SSemawaer ----------- Fresh Water


Figure 1-3. Flow chart of the DDD process [1 l].

The process consists of three main flow lines: the inlet seawater, the air/vapor

mixture, and the freshwater. The seawater is drawn from near the surface of a

geothermally stratified seawater source and then pre-heated in a water cooler (d). The

importance of drawing the water from the stratified source is that the water that is on the

surface is much warmer than that at deeper depths due to the solar absorption at

shallower depths and thermal stratification. It then flows to the main heater (a) where it is

heated using waste heat. Low pressure condensing steam from a thermoelectric power

plant is one possible source of waste heat. Once heated, the seawater is then sprayed

through the diffusion tower (b) that contains low-pressure drop, high surface area packing

material. Simultaneously, a forced draft blower impels the air/vapor mixture through the

diffusion tower where a portion of the seawater will evaporate in the air/vapor mixture.

The seawater that is not evaporated will exit the diffusion tower as brine discharge. The










saturated air/vapor mixture will enter the direct contact condenser (c) which is the

innovative idea behind DDD. As compared to HDH, a direct contact condenser approach

is taken to overcome the amount of surface area that would otherwise be required for a

shell and tube type heat exchanger. The direct contact condenser allows direct contact

between the air/vapor mixture and water thereby improving the heat and mass transfer of

the process. A portion of the freshwater product is used to condense the saturated

air/vapor as the cooling agent. The freshwater produced will exit the condenser and be

pumped to the water cooler where it will be cooled by the feed seawater. A portion of the

cooled freshwater is delivered back to the condenser, and the remaining freshwater is

then dumped into the freshwater reservoir as product.

The key differences in the DDD process and the HDH process are:

1. A direct contact condenser, in lieu of a shell and tube heat exchanger, is used to
decrease the amount of surface area required to condense the air/vapor mixture
and increase the condensation effectiveness. The decrease in size of the
condenser contributes to less required material as well as less required land space.

2. The DDD process is driven by waste heat from power plants, however, it can be
designed to be geography specific depending upon the location. For example,
other forms of waste heat may be used such as solar heating, geothermal spas, or
wind turbines may be used.

3. Forced draft is used in combination with a packed bed tower as the humidifier.
The forced draft provides a constant source of dry air. The packed bed is
designed to be made of plastic due to the low operating temperature and pressure.
Plastics are easier to maintain as well as cheaper to replace. The packed bed also
provides an increase in surface area for more heat and mass transfer between the
air and water and thus greater production.

4. The HDH process is compatible for only small scale production whereas the DDD
process can be used for larger flow rates and thus at a larger scale for increased
production.

5. The components used for the DDD process are commonly available from a
variety of different manufacturers or retailers.









Given the advantages of DDD over the HDH process it is worthwhile to research

the overall production rates and economic feasibility of the process.

Desalination and the Environment

Water desalination has increased substantially throughout the 20th century. With

this increase has come an improvement in the quality of life. This improvement,

however, comes at a price. The price is paid through the damage done to the

environment. Water desalination contributes in a multiple of different ways to the

degradation of the environment. These include increased occupation of land by

desalination plants, the contamination of groundwater, damage done to marine biology,

noise pollution, and the energy and discharge of combustion of products all have a

negative impact on the environment.

In a world where the population is steadily increasing and total land remains

constant, land use and occupation is always an issue. For example, Sadhwani et al. [12]

noted that a typical RO plant requires a land area of about 10,000 m2 (2.47 acres) to

produce between 5,000-10,000 m3/day (1.32 million gal/day to 2.6 million gal/day). In

addition to the land footprint occupied by the plant, the infrastructure of the plant is a

concern. The feed seawater pipes, the electrical transmission lines, and the product water

pipes are all important parts of the infrastructure that require space and land use.

The groundwater could also face contamination from a desalination plant. A plant

that is built over or near an aquifer will have pipes to transport the inlet seawater as well

as those for product discharge. These pipes could leak, allowing saltwater to seep

through to an aquifer. Care must be taken to ensure no leaks are present. [12].

Morton et al. [13] commented that the marine life can suffer in several ways from

the discharge of desalination plants. The brine discharge from desalination plants is










typically of increased temperature and of higher salinity and depending upon treatments

used during the process, rich in other chemicals as well. The warmer water and increased

salinity can also reduce the amount of dissolved oxygen in the water and therefore

restricts marine life in the vicinity of the plant. The presence of other chemicals could

intensify this effect. The temperature increase could result in death of marine life or a

reduction in the rate of metabolic processes which retards maturity, life stage

development, and size. Morton et al. [13] concluded that thermal desalination processes

are more prone to have a greater thermal impact on the environment whereas membrane

processes typically have a greater salt concentration of the discharge.

The noise from a desalination plant is a concern in the vicinity surrounding the

plant as well as to the workers of the plant. This acoustic contamination is primarily an

issue for RO plants where high pressure pumps and energy recovery systems can produce

noise in excess of 90 dB(A) [12].

Of perhaps greatest importance is the atmospheric emissions from the input energy.

Recall Table 1-1 and the substantial amount of input energy required for various

desalination processes. These processes generally rely on the combustion of fossil fuels

to supply the necessary energy. The principle emissions from the combustion of fossil

fuels are sulfur dioxide, nitrogen oxides, carbon dioxide, carbon monoxide, and

suspended particulate matter [13]. From Table 1-1 it can be seen that the MSF is more

energy intensive when compared with RO and thus more likely to contribute to air

contamination.

Comparison of DDD with RO and MSF

It is obvious that economics as well as environmental concerns play an important

role in the viability of desalination processes. One of the biggest advantages of DDD









over RO and MSF is that DDD is driven by waste heat, therefore the only energy

required is that to pump the air and water through the system. Since low pressure drop

packing is used the energy required for pumping is not prohibitively expensive. This has

very important implications for the environment. DDD takes advantage of energy that

would have otherwise been discarded. Since it does not require as much energy as RO or

MSF it is expected that the atmospheric emissions will be significantly less. Breschi [14]

reported on the performance of a desalination unit called LTF (low-temperature flash)

that ran off of waste heat. The unit consists of a vacuum flash chamber (the evaporator),

a shell-and-tube exchanger (the distillate condenser), and a vacuum system. The process

is driven by the small temperature difference in the warm seawater exiting a steam

turbine. The environmental impact of that facility was considered negligible due to the

low change in salinity of the brine and the lack of chemical additive to reduce scaling

since the operating temperature is low.

Another advantage of DDD is that the main components are inexpensively

manufactured. The operating temperatures and pressures are low; therefore most parts

can be constructed from simple plastics. Unlike RO, there are no membranes that must be

changed and maintained. While RO and MSF require specialized high pressure and large

pumps respectively.

The DDD process has the potential to be economically competitive while having

less impact on the environment. The scope of this research is to explore the feasibility of

the DDD process under specific operating conditions.

Scope of Work

Li et al. [1 l] reported on the economic feasibility of the DDD process with heated

input feedwater and ambient dry air. The purpose of this research is to explore the










concept of a heated dry air input. It is known that the amount of vapor carried by the air

increases with increasing temperature, but further studies need to be conducted to

determine the effect of a heated air input on the DDD process. Thus the goals of this

study are as follows:

1. Design and install an air heated section to the DDD facility that is fully
instrumented to measure the appropriate heat and mass transfer rates.

2. Take experimental measurements over a range of flow conditions for two
separate cases: the case where both inlet air and inlet feedwater are heated
and the case with a heated air input with an ambient water input.

3. Develop and implement a mathematical model to correctly predict the heat
and mass transfer behavior of the two cases.

4. Perform a parametric study to determine the optimal operating conditions.

5. Perform an economic study to determine the practicability of the DDD
process for the two different cases.

Chapter 2 describes the DDD process constructed and maintained for the current

research. The instrumentation, the individual components, and software used to collect

the data are all described in detail.

A theoretical model for a heated air input for both an ambient water input as well as

a heated water input is presented in detail in Chapter 3. A control volume approach is

taken to reduce the conservation equations to determine the governing equations of the

two processes. The results for the two different cases will then be compared to the model

and then discussed.

Finally, in Chapter 4, the results from a study to determine the optimal conditions

for both cases are presented. The economics of the process are also explored in an

example of the DDD process coupled with an industrial plant.















CHAPTER 2
EXPERIMENTAL FACILITY

A laboratory scale diffusion driven desalination (DDD) facility has been designed

and constructed to collect the heat and mass transfer data for the experimental analysis.

The data collected will be used to verify the theoretical model for the diffusion tower for

two different cases: heated air/heated water and heated air/ambient water. Data for both

cases will be taken over a range of different flow conditions. The performance of both

cases will then be explored using the data collected.

System Overview

Figure 2-1 presents a schematic diagram of the current DDD facility. The

municipal water line, serving as the inlet source water, flows through a series of valves

which determines the flow meter to measure the inlet mass flow rate. The water then

flows through a series of heaters, the preheater and the main heater. The preheater can

raise the temperature to a maximum of 50 oC and the main feedwater heater is a PID

temperature controlled heater. Once the water is heated, its temperature is measured and

then it flows into the top of the diffusion tower. The water is sprayed with a spray nozzle

and flows through the tower packing via gravity. The water not evaporated will exit the

tower from a drain at the bottom where the temperature is measured using a type E

ungrounded thermocouple

The dry air is drawn through a 3.68 kW (5.0 horsepower) centrifugal blower whose

speed is regulated using a three phase autotransformer. The air exiting the blower flows

through a 10.2 cm nominal vertical duct where a thermal mass flow rate meter measures











the air flow rate. The maj or modification to the system described by Li [15] is the

addition of an air heating section. The air heating section is shown pictorially in Fig. 2-2.


The U-shaped air heater section is required to ensure enough pipe length for fully

developed flow for the air flow measurement. The air flow meter is placed before the air


heater since it was calibrated using ambient air. The air flows down the duct where a 4

kW tubular heater is installed. A thin sheet of aluminum lines the inside of the duct to


guarantee that the duct does not exceed its maximum operating temperature. The amount

of power supplied to the air heater is controlled by a single-phase autotransformer. The

v-1 Muricipal Water Line 2
MunlcrgalWater Line 1 .J


E'. Direct....... Contact._ .. .

Transformer V-2 DI[rain V-5

--Air DP Differential pressure Lv Water level -A Air side
------Drin G Air flow rate P Pressure -L Water side
WaterH Relative humidity T Temperature -W Wall
L Liquid flow rate V Valve


Faln 1


Figure 2-1. Diffusion driven desalination (DDD) facility.











air relative humidity and temperature are then measured using a resistance type humidity


gauge located downstream of the flow meter and heater in the horizontal section. The air

then enters the diffusion tower and is forced through the packed bed. The air exits the


diffusion tower where the exit relative humidity and temperature are measured in a

similar manner as at the entrance.


Water Flow ~cTo
Water
,rJ, MetrCX~h Hater odne




Transformer 2

Air Flow Meter
Air Heater

Diffusion
Centrifugal BlowerToe






Valve
Transformer 1
Drain



Figure 2-2. Air heating section.

The condenser is comprised of two different stages: a countercurrent stage and a


co-current stage. The condenser is designed in a twin tower structure where both towers


are identically constructed. Both towers are made of 25.72 cm inner diameter acrylic


tubing connected via two schedule 80 PVC elbows. Though this study only focuses on


the performance of the diffusion tower, the condenser is included for completeness of the

sy stem.










Fresh cold water is drawn from a different municipal line and split into two

different lines, one line for each section of the condenser. The water flow rate is

measured with two different turbine flow meters. The water temperature is measured and

then the water is sprayed from the top of both of the towers using additional spray

nozzles.

The air leaving the diffusion tower is at an elevated temperature and humidity.

The air enters the co-current condenser stage where the air flows co-currently with water

and is cooled and de-humidified. Upon exiting the co-current stage, the air will flow

through a 900 PVC elbow where it travels through a 25.4 cm nominal diameter duct. The

air temperature and humidity is measured with another resistance type humidity gauge as

was used in the diffusion tower. The air then enters the countercurrent stage where the

air flows upward in the opposing direction of the falling water and continues to further

cool and dehumidify. The exit air temperature and humidity are measured with another

gauge and the air will then exit via a duct at the top of the countercurrent condenser stage.

The water used to cool the air in the condenser and the condensate product from

the air will flow down in both towers to a drain where the exit water temperature is

measured. Fig. 2-3 depicts a pictorial view of the DDD facility.

Description of Individual Components

The diffusion tower, shown in Fig. 2-4, is composed of three primary sections: the

top, middle, and bottom. The bottom consists of the air entrance and drain, the middle

houses the packing, and the top contains the water spray and exit air duct. The middle

portion of the tower is composed of 27 cm outer diameter 0.64 cm thickness R-Cast cast

acrylic tube. The packed bed of the tower can occupy up to 1 m of height in the acrylic

tube. The bottom and the top portions are schedule 40 25.4 cm nominal PVC pipe.










The bottom and middle portions of the tower are connected via a schedule 80 PVC 25.4

cm nominal bolted flange.






Co~untercurrentn
Codenser



C o-curraent
a Condenser



D~iffusio~n Tower



Air Heating
S~ectiorn







Figure 2-3. Diffusion driven desalination (DDD) system.

The condenser stage of the DDD system is comprised of two different stages each

with a top segment connected to a common bottom segment. The top segments include

the actual tower, the water spray, and the air duct, while the bottom segment contains the

drain and packed bed sections. The towers of both stages are also composed of 27 cm

outer diameter 0.64 cm thickness R-Cast cast acrylic tube. The two segments are

connected via schedule 80 PVC 25.4 cm nominal sized bolted flanges. The bottom

portion contains two schedule 80 900 25.4 cm PVC elbows connected using a 25.4 cm

nominal size schedule 40 PVC pipe. Up to 50 cm of packing material can be

accommodated in each vertical portion of the bottom segment.






































Figure 2-4. Diffusion tower [15].

The three water distributors used in the diffusion tower and direct contact

condenser are manufactured by Allspray. They are brass full cone with a 650 spray angle

and are designed for uniform solid cone spray. The two spray nozzles used for the direct

contact condenser have a capacity of 7.57 1pm (2.00 gpm). Fig. 2-5 illustrates a typical

Allspray nozzle.

The water preheater is a DHC-E tankless electric water heater manufactured by

Stiebel Eltron. It is a 240 V heater where the input of heat is electronically controlled.

Its operating range is from 300 C to 520 C.





Figure 2-5. An Allspray water distributor.

The main water heater consists of two 3 kW electric coil heaters wrapped around a

copper pipe where the water flows. The heater is PID controlled with a 240 V output.

The feedback is controlled with a type J thermocouple located at the exit of the heater.

The air heater is a 4 kW 1.21 cm diameter round cross section tubular heater. It has

a 240 V rating and has a watt density of 194 W/cm2. The sheath is Incoloy, which has a

maximum temperature of 8150C. It has a sheath length of 254 cm and a heated length of

236 cm. The heater has been shaped to fit inside the 9.5 cm inner diameter pipe. Figure

2-6 shows the heater shape. The power to the heater is controlled with a single-phase

autotransformer.










Figure 2-6. Shaped heater.

The packed bed is high density, low pressure drop HD Q-PAC type from Lantec. It

is made of polypropylene and is available in 30 cm x 30 cm x 30 cm square pieces. The

packing material was cut to fit the round acrylic tubes of the diffusion tower and direct










contact condenser towers using a specially designed hot wire setup. It has a specific

diameter of 18 mm and a specific area of 267 m2/m3. Figure 2-7 shows a portion of the

packed bed cut for use in the system.

























Figure 2-7. The HD Q-PAC packed bed.

The three turbine water flow meters used to measure the water flow rate in the

DDD system are manufactured by Proteus Industries Inc. Two flow meters have a flow

range of 5.7-45.4 1pm (1.5-12.0 gpm) while one flow meter has a range of 0.4-3.8 1pm

(0.1-1.0 gpm). They require a 24 VDC input and have a 0-5 V or 0-20 mA output. All

have an accuracy of +1.5% full scale and were calibrated using the catch and weigh

method. Figure 2-8 shows the calibration curves obtained for the three different flow

meters.

The air flow meter is a thermal insertion mass flow meter (Sierra Series 620S Fast-

Flo) from Sierra Instruments Inc. It has a microprocessor-based transmitter for 0-10











VDC output and a 200 s response time. It has a 15.24 cm 304 stainless steel probe to


measure velocity as well as temperature. The flow meter range is 0-1125 SCFM of air


with an accuracy of 1% full scale. The flow meter was delivered as factory calibrated.




201 Series Calibration Curve 250 Serles Callbration Curves

1 0 14
121 -


06- 8 8


g O4-4-

02-J



Voltage (V)Votg()
*SenalNo 00101543 SenalNo 00091465 m SenalNo 00091467
Calibration Curve Calibration Curve --- Calibration Curve
Q = 0 1915V + 0 0195 Q = 2 7128V + 0 3042 Q = 2 6805V1 + 0 203


Figure 2-8. Water flow meter calibration curves.

There are four Vaisala Corp. HMD70Y resistance type humidity gauges to measure


the relative humidity as well as temperature. Both temperature and humidity have


transmitters for 0-10 V output. All four gauges were factory calibrated. The operating


range for temperature is -200 to +800 C, while the relative humidity has an operating


range of 0-100%. The uncertainty of the gauges is 10 C for temperature and 2%


relative humidity.


The thermocouples used in the facility are type E and were manufactured by


Omega. They are factory calibrated with an uncertainty of 10.20 C.


The data acquisition system consists of a 16-bit PCI Analog-Digital converter and a


32 channel multiplexer card manufactured by Measurement Computing. The board is


calibrated for type E thermocouples and has a 0-10 V input range.









The data acquisition system uses the program SoftWIRE to collect the necessary

heat and mass transfer data. The SoftWIRE editor uses constructed flow diagrams to

represent the flow of data and control with icons and wires. The program developed for

the DDD system includes five different interfaces: the main control, the diffusion tower

view, the direct contact condenser view, the diffusion tower histogram view and the

direct contact condenser histogram view. The SoftWIRE program sends all of the data

directly to an Excel spreadsheet where the data is collected.

The main control tab of the DDD program allows direct control over the program.

A view of the main control is depicted in Figure 2-9. There is an on/off switch to turn the

program on as well as a frequency box to input the desired data sampling rate. It also

depicts specific values important to the overall process. Values specific to the diffusion

tower are depicted in the diffusion tower tab while those specific to the direct contact

condenser are shown in the diffusion tower tab. Figure 2-10 shows the diffusion tower

tab. As the picture illustrates, values at certain locations of the diffusion tower are visible

and are easily available.

Since steady state is an important assumption in the DDD analysis, it is important

to obtain measurements at steady state conditions. Two of the interfaces in the DDD

programs include a series of histograms which indicate the degree to which steady-state

is achieved. Figure 2-11 shows a view of the diffusion tower histogram view. The x-axis

is the time coordinate while the y-axis is the given measured value. All measurements

recorded were taken at steady state conditions at a frequency of 1 Hz.










rM _~,,

m m a 11 L
mmp ~






Figure 2-9. Main control.


I


Figure 2-10. Diffusion tower data.


MMM~ ee~

MM ~~w~~
.~~~~


~edF~































Air Flow Rate (kg/s)









08 60s 120s 180s 240s


Water Out Temp (C)









08 60s 120s 180s 240s


Air Out Temp (C)









08 60s 120s 180s 240s



RH out (%)


WaterIn Temp (C)









os 60s 120s 180s 240s


Air In Temp (C)


ClnnrlWlm~lllr~m3s~l~1~;11 II~


Water Flow Rate (kg/s)





08s 60 12s 10 4s


40 0
2 0


2 1O0 0



















3 O00


us180s 240s


RH in (%)


1 1 0


50
2 1


-


Ann 12nR 1Rns 24nR


Ann 12nR 1Rns 24nR


200
1 0


Os 60s


Figure 2-11i. Histogram view of the DDD data acquisition.















CHAPTER 3
HEAT AND MASS TRANSFER WITHIN THE DIFFUSION TOWER

An in depth theoretical model for both the diffusion tower [16] and direct contact

condenser [17] based on heated water/ambient air inputs has already been explored. The

purpose of this analysis is to experimentally explore the performance of the diffusion

tower for two different cases: heated air/heated water input and a heated air/ambient

water input. The heat and mass transfer model, proposed by Klausner et al. [16] has been

investigated for both cases. The theoretical model is compared with the experimental

data collected and agreement is satisfactory.

Theoretical Heat and Mass Transfer Model

The theoretical model is a one-dimensional two fluid film model for a packed bed.

The conservation equations for mass and energy are applied to a differential control

volume to obtain the governing equations for the process. In order to determine the

governing equations certain assumptions must be made. The assumptions made are:

1. The process operates at steady-state.

2. Air and water vapor are both perfect gases.

3. The changes in kinetic and potential energy are negligible.

4. The pumping power required for water is solely that required to overcome
gravity.


Fig. 3-1 shows the differential control volume analyzed for the two cases. As it can

be seen the problem is one-dimensional with the only variation lying in the z-direction.










mL
Liquid Air/Vapor
z + dz

Packed | vp~
Bed dmea






ma+my


Figure 3-1. Differential control volume for heated air conditions.

The z-direction is taken as positive in the axial direction. The conservation of mass on

the control volume for the air/vapor mixture yields,

d d
(nar,=)= (n?',evap, (3.1)
dz dz


where na represents the mass flow rate, the subscripts V and evap denote vapor and the

vapor evaporated from the liquid respectively. Similarly, conservation of mass on the

liquid side yields,

d d
(nL,z)= HT,evap) (3.2)
dz dz

where the subscript L denotes liquid.

The humidity ratio, co, and relative humidity, @, are defined for an air/vapor

mixture as follows,

nz, 0.62204s,<(T)
m = ,(3.3)









where P is the total pressure of the system, and Psat(Ta) is the saturation pressure of the

vapor evaluated at the air temperature Ta. The small change in system pressure is not

accounted for in evaluating the properties. The definition of the mass transfer coefficient

is applied to the differential control volume to obtain the following,


(mV,,,,) = kGa pf,~sat (7L) T',rrT ())A, (3.4)


where kG; is the gas mass transfer coefficient, aw is the wetted specific area, and A is the

cross sectional area of the diffusion tower. It should be noted that the total specific area

of the packing, a, is the total surface area of packing per unit volume of space occupied.

The rate of change of evaporation can be further reduced by considering the perfect gas

law. By applying the perfect gas law [18] to Equation 3.4, the rate of evaporation

becomes,


(mVevap) = kG w A, T (3.5)


where My is the molecular weight of vapor, R is the universal gas constant, and Ti is the

liquid/vapor interfacial temperature. By combining Equations 3.2-3.5 the gradient of the

humidity ratio is,

de kG~ 2T M ,, E,(7) P
dz G R 7 0.622 +0 (i ; 3



where G = 0I is the air mass flux. Equation 3.6 is a first order ordinary differential


equation with dependent variable co. When solved, the humidity ratio along the axial z

direction is obtained. Equation 3.6 requires a value of the liquid/vapor interfacial









temperature, Ti. The interfacial temperature is found by recognizing that the energy

convected from the liquid is the same as that convected to the gas,

1,7L 71L G U(71 Ta) (3.7)

where UL and UG; are the heat transfer coefficients of liquid and gas respectively. The

interfacial temperature is obtained by solving Equation 3.7 and is,


T7 = TL G L(3.8)


The conservation of energy on the liquid side of the differential volume yields the

following,


d(mL hL) dnwp)h +Ua(TL -T)A, (3.9)
dz dz f

where h is the enthalpy, U is the overall heat transfer coefficient, and hfg is the latent heat

of evaporation. Equation 3.9 can be further manipulated by utilizing the following:


d d nL dhL
dhL= CpdT~and (nLhL)-=hL- 9 2L ,and h g(, )=h(,(()- hL (7,). The
PLdz dz dz

gradient of the liquid temperature, TL, then reduces to the following,

dT TL G d co (h~ hL) L~T -,
+ (3.10)
dz L dz C C~L


where L = -I4 is the liquid mass flux, CP is the specific heat, and a is the overall specific


area of the packing material. Equation 3.10 is also a first order ordinary differential that

when solved will yield the temperature distribution of the water throughout the diffusion

tower.









Likewise, conservation of energy of the air/vapor mixture is obtained from the

differential control volume and yields,

d d
dz no ha +n ,- h, I + (nT'wra)hfg = -Ua(TL Ta)A + qHLad (3.11)

As in the liquid energy equation, Equation 3.11 can be simplified by utilizing the fact that

the air mass flow rate is held constant throughout the entire process such that:


d dha dh, d nv dh dT dh,- dT
(nz ha +nz, h,,) =n m +nz +h, ,-= CPa and = CPT
dz dz dz dz dz dz dz dz

Equation 3.11 then becomes,


dT d n,
a(nza C~a PT-)C,, = hL(a ~TL Ta)A + qua~d .
dz dz


(3.12)


Equation 3.12 can be simplified by noting that the CPmix, Specific heat of the mixture, is

evaluated as,


Cnnx 82aPa PTC (3.1


Recalling the evaluation of the latent heat of evaporization from the liquid conservation

equation, and combining Equations 3.12 and Equation 3.13 the gradient of air

temperature through the diffusion tower is evaluated as,

dT, 1 doi hL(, U~TL () 4quL
+ (3.1
dz 1+@i dz Cnx Pnx(+) d1m),n


3)


where d is the diameter of the diffusion tower and qm is the heat flux loss from the air.

Equation 3.14 is also a first order ordinary differential equation with dependent variable

Ta. Equations 3.6, 3.10, and 3.14 are a set of coupled ordinary differential equations that

when solved simultaneously give solutions for the distributions of humidity ratio, air


4)









temperature, and water temperature throughout the diffusion tower. However, since a

one-dimensional model is utilized, closure must be achieved. This requires that both the

heat transfer coefficient and mass transfer coefficient be known. Directly measuring the

heat transfer coefficients is not possible because of the fact that the interfacial film

temperature cannot be measured. Therefore to overcome this difficulty the heat and mass

transfer analogy [19] has been utilized. The heat transfer coefficient for the liquid side is

evaluated using,

Nu, Sh
L (3.15)
Prl/2 Scl/2
L L


U, =k, pCPL L (3.16)


Similarly, the heat transfer coefficient for the gas side is calculated as,

NuG ShG
G (3.17)
PrG/3 ScG/3



UG k GPG1/ K (3.18)

where D is the molecular diffusion coefficient and K is the thermal conductivity. Thus

the overall heat transfer coefficient is evaluated as,

U, = (U,-+UG1) (3.19)

The mass transfer coefficient is evaluated using a widely known and well tested

correlation. Onda' s correlation [20] allows for evaluation of the mass transfer

coefficients in packed beds. Onda' s correlation, found in Appendix A, is used to

calculate the mass transfer coefficients, kG and kL. In the correlation the coefficient, C,

can take on two possible values C=5.23 for dp > 15 mm and C=2.00 for dp <;15 mm.










The difference in C values accounts for the fact that kGa for the smaller packing (dp < 15

mm) tends to increase monotonously with increasing specific area, a. Li et al. [17]

provided an explanation for the phenomena of the decreased mass transfer coefficient and

is believed to be the cause of liquid hold-up in the packed bed which is responsible for

liquid bridging and reduced area for mass transfer. The current packed bed has a

diameter of 18 mm which is close to the cut off for both sizes. Thus for the packed bed

used in the current investigation, either constant value would be appropriate. Similar to

the analysis described by Klausner et al. [16], the heated air/ambient water uses C=5.23.

The coefficient for the heated air/heated water case, however, uses C=2.0. This change in

constant can perhaps be attributed to the fact that at higher air temperatures the air that

enters the packed bed is dryer and thus more water is evaporated. However, due to the

increase in evaporation there is an increased hold up of the liquid in the packing due to an

accelerating gas stream. The increase in hold up could possibly cause more liquid

bridging in the packing thereby decreasing the local mass transfer.

The wetted area for the current experiments differs from that computed via Onda' s

correlation. It was found that the specific wetted area remains nearly constant for varying

air to water mass flow ratios. This was determined by first using Onda' s correlation to

calculate the wetted specific area. There was slight variation in the comparison of the

theoretical and experimental data. An analysis was then performed to determine what the

specific wetted area should be to obtain adequate results. Interestingly, over the range of

operating conditions considered in this work, the specific wetted area is found to be

simply a constant,


aw = 0.5a .


(3.20)










The value of the heat loss, qu, is experimentally determined and is diffusion tower

specific. There was negligible heat loss for the heated air/ambient water case thus the

heat loss flux term is taken to be zero. The heat loss for the heated air/heated water case

is experimentally calibrated for various air mass fluxes. Figure 3-2 shows the calibration

curve for the heated air/heated water heat loss flux for varying air mass flux.

To solve the three coupled equations for the humidity ratio, air temperature, and

water temperature distributions in the diffusion tower, the following solution procedure is

followed :

1. Specify the water mass flux, air mass flux, inlet water temperature, inlet air
temperature, and inlet humidity ratio.

2. Guess a value of the exit water temperature.

3. Compute the exit humidity ratio, exit water temperature, and exit air temperature
utilizing Equations 3.6, 3.10, and 3.14 until z reaches the height of the packed
bed.

4. Compare the values of the calculated inlet water temperature and specified inlet
water temperature. If they match, the analysis is complete. If they differ, repeat
the procedure beginning from step 2.

Results and Discussion

Experiments were conducted to obtain data for the two different cases: heated

air/ambient water and heated air/heated water. For both cases, the air mass flux was held

constant at about 0.77 kg/m2-S, 1.16 kg/m2-S, and 1.55 kg/m2-S while the liquid mass flux

was varied between 0.6 kg/m2-S to 1.3 kg/m2-S. The height of the packed bed was held

constant at 0.38 m. All experiments conducted were performed in a parameter space

beneath the flooding curve for the packed bed. For the model analysis, the inlet water

temperature, inlet air temperature, and inlet absolute humidity were all used to compute










the exit conditions. Comparisons between the predicted and measured exit conditions

from the model are described next.


0.8 1 .0 1.2 1 .4

Air Mass Flux (kg/m2-S)


Figure 3-2. Calibration curve of the heat loss flux for the heated air/heated water case.

Case 1: Heated Air/Ambient Water

Six different data sets were recorded for the heated air/ambient water case. Two

different data sets per air mass flux were taken to ensure the repeatability of the

experiments. For all experiments, the inlet air, water, and humidity were held constant at

about 60.9 OC, 25.2 OC, and 0.0060 respectively. Figures 3-3 to 3-5 show the comparison

between the predicted exit values and the measured exit values. The comparison between

the two is quite good. The exit absolute humidity and exit water temperature are

predicted with fairly good accuracy while the exit air temperature is slightly

underpredicted for all data sets.









From the data collected it can be seen that the heated air/ambient water case does

not yield good production. The exit air temperature is approximately 25-26 OC for each

value of the liquid mass flux as well as air mass flux, which indicates that the air is

cooled to the temperature of the water. This is supported by the fact that the temperature

difference of the exit water and exit air is nearly a constant 1-2 oC. Thus it is clear that

reliance on heated air is inefficient because upon entering the tower the air is immediately

cooled close to the water temperature. All of the energy is being used to heat up the

water and as a result the mass transfer is poor. As the absolute humidity shows, there is

no optimum value as the exit humidity is essentially constant. Further, the change in

humidity from the inlet to the outlet essentially remains a constant at about 0.0125. This

indicates that regardless of the diffusion tower liquid mass flux only a small fraction of

water will be evaporated.

Figure 3-6(a-c) shows the repeatability of the six different experiments for the different

flow conditions. As shown in the figures, the repeatability of the experiments is very

good. However, the figures also elucidate the fact that the heated air/ambient water case

is very inefficient. There is little variation between the values recorded for the six

different experiments despite the different air and water mass fluxes used in the

experimental measurements. For example, consider the exit air temperature shown in

Fig. 3-6(a). For the three values of air mass flux the exit air temperature remains almost

a constant despite the varying liquid mass flux. The exit water temperature and the exit

humidity also demonstrate similar behavior. The process will exhibit the same behavior

despite the operating conditions chosen.
















G = 0.77 kg/m2-s
Predicted


(a)
Measured


Inlet Conditions

Ta,In (C) = 60 10
Tw,In (C) = 25 32
man = 0007515


Ta,out Ta,out
Tw,out a Tw,out
Wout A mout


0 05



0 04



0 03








0 01



0 00


H- m
**


m
e s


06 08 10 12

Water Mass Flux L (kg/m -s)


G = 0.78 kg/m2-s
Predicted

Ta,out
Tw,out

At


Measured

O Ta,out
o Tw,out
a mout


Inlet Conditions


Ta,In (C)
Tw,In (C)
man


61 79
= 25 19
=0 004064


35 -





30 -
-









20 -





15 -


002










000


8--._. p
-------~ = -~--1- = =_a

a


Water Mass Flux L (kg/m -s)





Figure 3-3. Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 0.77 kg/m2-S: a) Set 1 b) Set 2.
















G = 1.15 kg/m2-s
Predicted


(a)
Measured


Inlet Conditions


Ta,out Ta,out
Tw,out a Tw,out
mout A mout


Ta,In (C)= 60 92
Tw,In (C)= 2512
man = 0006148


0 03



0 02


- *- -


* *----

A A


06 08 10 12 14

Water Mass Flux L (kg/m -s)


G = 1.17 kg/m2-s
Predicted


(b)
Measured


Inlet Conditions

Ta,In (C) = 60 02
Tw,In (C) = 25 15
man = 0 006841


-Ta,out O Ta,out
-Tw,out o Tw,out
mo A mot












- - -


35





30





i 25-





20





15


)
003 U
S
I
s
002


000


Water Mass Flux L (kg/m -s)





Figure 3-4. Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.15 kg/m2-S: a) Set 1 b) Set 2.
















G = 1.55 kg/m2-s
Predicted


(a)
Measured


Inlet Conditions


Ta,out Ta,out
Tw,out Tw,out
Wout A mout


Ta,In (C) = 61 11
Tw,In (C) = 25 14
man = 0 007112


0 05



0 04



0 03








O 01



0 00


r~~~~~~~~~ I -1-_____


06 08 10 12

Water Mass Flux L (kg/m -s)


G = 1.54 kg/m -s
Predicted

Ta,out
Tw,out
At


(b)
Measured


Inlet Conditions


Ta,In (C)
Tw,In (C)
man


61 79
= 25 19
=0 004064


Ta,out
Tw,out
mout


35 -




30 -




i 25




20 -




15 -


0 03



002


O O O O 0

oo o o o


Water Mass Flux L (kg/m -s)





Figure 3-5. Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.55 kg/m2-S: a) Set 1 b) Set 2.











Details of the experimental data can be found in Appendix B.

Case 2: Heated Air/Heated Water

As in the previous case, six different data sets were taken, two for each different air

mass flux. For all six experiments the inlet air temperature, water temperature, and

humidity were held constant at about 60.9 OC, 60.6 OC, and 0.0077 respectively. Figures

3-7 to 3-9 show the comparison between the predicted and measured exit temperatures

and humidity. For all sets of data the exit water temperature and exit humidity are


predicted with considerable accuracy. The air temperature is slightly overpredicted in all

cases.


(a)


G
0.79 kg/m2-S
1.55 kg/m2-S


30


28




26


0.5 0.6 0.7 0.8 0.9 1.0
Water Mass Flux L (kg/m2-S)


1.1 1.2 1.3


Figure 3-6. Repeatability of different experiments for different exit parameters: a) Exit air
temperature, b) Exit water temperature, c) Exit absolute humidity.















































luI
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.:

Water Mass Flux (kg/m2-S)




(c)
G Set 1 Set 2

0.79 kg/m2-s O

1.55 kg/m2-S O
1.79 kg/m2-S
0.04


G
0.79 kg/m2-S
1.55 kg/m2-S

1.79 kg/m2-S


40



35

-
30



E 25
1--

-
20


ar a g] O m
O~ O O


0.02- o O O 5~ O o,, eO d


0.01


0.00
0.5 0.6 0.7 0.8 0.9 1.0

Water Mass Flux L (kg/m2-S)


1.1 1.2 1.3


Figure 3-6. Continued.

















G = 0.79 kg/m2-s
Predicted


(a)
Measured


Inlet Conditions

Ta,In (C)= 60 99
Tw,In (C) = 60 48
man = 0 006854


Ta,out Ta,out
Tw,out Tw,out
mout A mout


0 10


0 08


----C
C




A


0 06



0 04



0 02



0 00


06 08 10 12


Water Mass Flux L (kg/m -s)


G = 0.80 kg/m2-s
Predicted


Measured


Inlet Conditions


Ta,out O Ta,out Ta,In (C) = 61 41
Tw,out o Tw,out Tw,In (C) = 60 75
mout a mout man = 0 006207






--- a


50 -



40 -



30 -



S2-

E 0 -



0-


010



0 08


O 04


Water Mass Flux L (kg/m -s)





Figure 3-7. Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 0.79 kg/m2-S: a) Set 1 b) Set 2.


















G =1.15 kghn'-s
Predicted


(a)
Measured


Inlet Conditions


Ta~out Ta~out

Tw,out Tw,out
Wout A mout


Tayin(C)= 61 29
Tw,In (C) = 60 87
man = 0007919


50




40




30




S20




10




0


010




008




006




0 04



002




000


-----~ go m





rA


06 08 10 12

Water Mass Flux L (kg/m -s)


G =1.16 kghn'-s
Predicted

-- Ta,out
Tw,out

At


Measured


O Ta,out
o Tw,out
a mout


Inlet Conditions


Tayin(C)
Tw,In (C)
man


6090
= 60 79

= 0 008120


50 -




40 -




30 -




S2-

20 -




0-


010




008


006




O 04


aa
a


Water Mass Flux L (kg/m -s)






Figure 3-8. Comparison of predicted and measured exit temperatures and humidity for

similar air mass flux G 1.15 kg/m2-S: a) Set 1 b) Set 2.
















G = 1.55 kg/m2-s
Predicted


(a)
Measured


Inlet Conditions


Ta,out Ta,out
Tw,out Tw,out
Wout A mout


Ta,In (C) = 60 52
Tw,In (C) = 60 61
man = 0 008882


50



40



30



S20



10



0


010



0 08



0 06




0 04



002



000


J
I-------
_c -
J-- ---
L ----


06 08 10 12

Water Mass Flux L (kg/m -s)


G = 1.55 kg/m -s
Predicted


Measured


Inlet Conditions


-Ta,out O Ta,out Ta,In (C) = 60 42
-Tw,out o Tw,out Tw,In (C) = 60 27
Wout a mout man = 0 008409








o _O


50 -



40 -



30 -



S2-

20 -



0-


010


U ub








O 04


Water Mass Flux L (kg/m -s)





Figure 3-9. Comparison of predicted and measured exit temperatures and humidity for
similar air mass flux G 1.55 kg/m2-S: a) Set 1 b) Set 2.









For all air mass fluxes studied, the exit air temperature and exit water temperature

increase with increasing water mass flux. As the liquid mass flux increases the exit

humidity also increases due to the increase in the amount of liquid available to evaporate.

Figures 3 -10(a-c) demonstrates the repeatability of the six experiments for the

heated air/heated water case. As shown in the figures the repeatability for all of the

experiments is quite reasonable. These figures show that at the lower air mass flux the

maximum exit humidity is obtained. It is also interesting that both the exit air

temperature and exit water temperature decrease with increasing air mass flux. This

suggests that the residence time plays a key role in the heat and mass transfer. A

decrease in residence time implies that there is less time for heat and mass transfer to

occur thereby explaining the decreased temperatures as well as humidity with increasing

air mass flux. As demonstrated, the theoretical model developed is obviously a good

design tool that can be utilized to achieve the desired production rate.

Details of the experimental data collected for the heated air heated water case can

be viewed in Appendix C.
















G

0.79 kg/m2-s
1.55 kg/m2-s

1.79 kg/m2-S


Set 1 Set 2
O
5 O

r


50




45




S40


1--

w 35


0.6 0.8 1 .0 1 .2

Water Mass Flux L (kg/m2-S)


G
0.79 kg/m2-S
1.55 kg/m2-S

1.79 kg/m2-S


45




o~40







x 30


O*
HO
O*0


0.6 0.8 1.0

Water Mass Flux (kg/m2-S)


Figure 3-10.Repeatability of experiments for different exit parameters: a) Exit air
temperature, b) Exit water temperature, c) Exit absolute humidity.








46



(c)
(c)
G Set 1 Set 2
0.79 kg/m2-s O
1.55 kg/m2-S O

1.79 kg/m2-S
0.07



CO O
oO O

m
0.05 -



0.04-



0.03-


0.4 0.6 0.8 1 .0 1 .2 1 .4

Water Mass Flux L (kg/m2-S)


Figure 3-10. Continued.















CHAPTER 4
PARAMETRIC ANALYSIS

The previous chapters have focused on the experimental facility, data collection,

the heat and mass transfer model, and comparisons of the predicted and measured data. It

has been shown that the DDD process has good performance for the heated air/heated

water case, while the heated air/ambient water case is inefficient. In order to fully

explore the feasibility of the heated air/heated water DDD process a parametric analysis

is considered. This chapter details the parametric analysis including the methodology,

analysis of results, and comparisons between several different DDD processes. At the

end of the chapter an economic analysis of the DDD facility for coupling with an

industrial plant that produces large quantities of waste heat in air is explored.

Parametric Analysis of Heated Air/Heated Water DDD Process

The heat and mass transfer model described in Chapter 3 is used to model the

heated air/heated water DDD process. The assumptions made in this analysis are:

1. There is no heat lost to the surrounding environment.

2. The only energy consumed is the pumping power required to pump the air
and water through the system.

3. The air/vapor mixture is treated as a perfect gas.

4. Changes in potential and kinetic energy are considered to be negligible.

5. The process operates at steady-state conditions.

Equations 3.6, 3.10, and 3.14 are used to evaluate the absolute humidity, water

temperature, and air temperature respectively through the diffusion tower. The heat and









mass transfer analogy is used to evaluate the heat transfer coefficients and Onda's

correlation is used to evaluate the mass transfer coefficients assuming a constant wetted

area.

In order to fully explore the bounds of the heated air/heated water DDD process the

energy consumption must be evaluated and certain parameters are needed to effectively

assess the process. The fresh water production rate is computed from,

mrw = GA(q,,,u 0,),,), (4.1)

where the subscripts fw, in, and out refer to the fresh water and diffusion tower inlet and

outlet respectively. It should be noted that it is assumed that the carrier air circulates in a

closed loop and the inlet humidity to the diffusion tower is the outlet humidity from the

condenser.

The maj or energy consumption of the process is assumed to be the pumping power

required to pump the air and water through the system. The pumping power required for

the diffusion tower gas is


EG GM _( G = ~PG GA APG (4.2)


In order to evaluate the pressure drop on the gas side a correlation provided by the

packing material company, Lantec, is used. The pressure drop, APG, across the HD Q-

PAC packed bed is computed from,


G ,=,[0.0354+654.48( )) +1.176x10'( ) ],, (4.3)
: pG pl pl Gc










where APG is the pressure drop across the packed bed (kPa) and z is the height of the

tower. The validity of the correlation was explored by Li [15], and it has excellent

agreement with the experimental data.

The pumping power required for the diffusion tower liquid is calculated as,


E, = "m P = LA gH (4.4)


Thus the total pumping power is calculated as,

E,,,a1 = E, + EG (4.5)

The energy consumption rate per unit of fresh water production is defined as,


Ef, =totax (4.6)
m,,

The fresh water production rate and energy consumption rate are two important

parameters that characterize the performance of the DDD process. The fresh water

production rate characterizes the quantity of fresh water produced at a given set of

operating conditions while the energy consumption rate denotes how much energy is

consumed per unit of fresh water produced. The two values are important in finding the

optimal operating conditions. An ideal DDD process would have a high fresh water

production rate and a low energy consumption rate.

Heated Air/Heated Water Results and Discussion

Using the analysis described above and the theory discussed in Chapter 3, a

parametric analysis is performed to determine the effects of certain operating variables on

the performance of the heated air/heated water process. In performing the analysis, the

air inlet temperature, water inlet temperature, specific packing area, diameter of the

packing material, and inlet humidity ratio were all held constant at 600 C, 600 C, 267







50


m2/m3, 0.018 m, and 5.25% respectively. Nine different values of the inlet feedwater

mass flux, L, were considered 0.15, 0.25, 0.5, 0.75, 1.20, 1.55, 2.0, 2.5, 3.0 kg/m2-S. The

inlet air mass flux, G, was varied continuously from 0.04 to 23.2 kg/m2-S for each inlet

feedwater flux. For each air mass flux, the maximum absolute humidity was determined,

and the tower height, air exit temperature, and water exit temperature were recorded. All

calculations were performed in a parameter space below the flooding curve of the

packing material.

Figure 4-1 depicts the tower height as a function of the inlet air mass flux. The

tower height reported is the computed tower height required to achieve the maximum exit

absolute humidity. For all inlet liquid mass fluxes, the tower height decreases with

increasing inlet air mass flux. This is important because as the air mass flux increases,






\ Liquid Mass Flux
L (kg/m2-S)
4-
E 'i 0.15
E- 0.25
0.50
Ia 0.75
1.20
o \ 1 1.55
1-- ** **** *** 2.00
\ 2.50


0.0


0 5 10 15 20 25

Air Mass Flux G (kg/m2-S)



Figure 4-1. Tower height at the maximum exit absolute humidity as a function of the inlet
air mass flux G.











the tower height decreases which translates to less required materials and thus reduced

cost. However, as the air mass flux increases the power required to pump the air also

increases. It should be noted that the tower height for high liquid mass flux and low air

mass flux was restricted to values less than 5 m. This is to ensure that the tower heights

considered are realistic.


Figure 4-2 shows the exit air temperature with varying air mass flux. The exit air

temperature decreases with increasing air mass flux until it reaches a minimum value

then increases. It is also worthy to note that the highest exit air temperatures are realized

when the air mass flux is low.

65

60-


55 \1 LiqudMass Flux

50 0.15
-0.25
'145) / 0.50
45-/ / .. 0.75
1-- \ / -- 1.20
40 \ q .- - 1.55

m~ 35 2.50
.. 3.00
30-

25
0 5 10 15 20 25

Air Mass Flux G (kg/m2-S)


Figure 4-2. Exit air temperature at maximum absolute humidity as a function of the air
mass flux.

Figure 4-3 depicts the maximum exit absolute humidity as a function of air mass

flux for varying liquid mass flux values. For all liquid mass flux values, the exit

humidity decreases with increasing air mass flux. The maximum exit humidity is










0.10

0.16-

0.14 Liquid Mass Flux
s li~ IL (kg/m2-S)
S0.12 \01
0.1 \
0.50
-0.75
o 0.08 -\\

0.06 ****** 2.00
2.50
0.04 -._ 30



0.00
0 5 10 15 20 25

Air Mass Flux G (kg/m2-S)



Figure 4-3. Maximum exit absolute humidity with varying air mass flux.

achieved with the higher liquid mass fluxes and can be attributed to the fact that the heat


capacity of the water is large with larger mass fluxes, and the water temperature will not

decrease as much with a given amount of evaporation. Thus as the liquid mass flux

decreases the exit absolute humidity decreases as well. The maximum exit humidity is

realized for low values of the air mass flux. It is important to note that the maximum exit

air temperature and maximum humidity for all liquid mass fluxes are achieved for values

of the air mass flux less than about 2.00 kg/m2-S. While the exit air temperature and exit

absolute humidity are maximum at low air mass flux, this does not imply that the fresh

water production will also be high.

The fresh water production is an important parameter in evaluating the economy of


the process. Figure 4-4 shows the fresh water production flux with varying air mass flux.

For each value of the liquid mass flux the fresh water production flux increases until it







53


reaches an optimum condition. This is important because it indicates that for every liquid

mass flux there is a value of the air mass flux that can produce the maximum amount of

fresh water. It is also important to notice that as the liquid mass flux increases, the fresh

water production flux increases. Thus a higher production can be achieved at higher


liquid mass flux. Interestingly, for all liquid mass flux the maximum fresh water

production does not occur below 2.00 kg/m2-S where the maximum exit humidity is

realized.


0.25



S0.20 .
.* Liquid Mass Flux
I / ///~ '\L (kg/m2-S)
~ / \ 0.15
0.15 -/ \ -- 0.25

/\ -- 0.75

0.10 ------ 1.20
-1.55
P I ,' .' I..........* 2.00
2.50
.c 0.05 -11 -'- 3.00



0.00
0 5 10 15 20 25

Air Mass Flux G (kg/m2-S)



Figure 4-4. Fresh water production flux with varying air mass flux.

The energy consumption rate is also an important parameter in evaluating the

economy of the DDD process. Figure 4-5(a-b) shows the energy consumption rate for

the diffusion tower with varying air mass flux for the different values of the liquid mass

flux. Figure 4-5(a) shows the full range of air mass flux while Figure 4-5(b) shows a

smaller range. For all values of liquid mass flux, the energy consumption rate increases












4





3







co
1


0


0 5 10 15 20 25

Air Mass Flux G (kg/m2-S)


0.008




_
0.006


-


co

S0.002


(b)


Liquid Mass Flux
L (kg/m2-S)
0.15
-0.25
0.50
-0.75
------1.20
-1.55
********* 2.00
2.50
-3.00


/


0000


0.0 0.5 1 .0 1 .5 2.0 2.5 3.0

Air Mass Flux G (kg/m2-S)


Figure 4-5. Diffusion tower energy consumption rate (a) with varying air mass flux (b)
for low air mass flux.


with increasing air mass flux. The higher the air mass flux, the more pumping power


required to drive the process. As the graphs reveal there is a minimum energy









consumption rate for each liquid mass flux. Beyond that optimum value, the energy

consumption steadily increases. From Figure 4-5(a) it can be seen that operating at

higher air mass flux is impractical due to the high energy consumption rate. Li et al. [l l]

reported that the ideal operating condition in the condenser is for air mass flux below 1.5

kg/m2-S to ensure low energy consumption. Figure 4-5(b) reiterates this condition for

the diffusion tower. Below an air mass flux of 1.5 kg/m2-S, the energy consumption is

low.

Figure 4-6 shows the fresh water production efficiency versus the air mass flux

for varying liquid mass flux. The fresh water production efficiency increases with

increasing air mass flux until it reaches a maximum condition, and then it steadily

decreases. As the liquid mass flux increases the maximum fresh water production

efficiency decreases. The maximum efficiency occurs for low liquid mass flux while the

maximum fresh water production occurs for high liquid mass flux. An optimal operating

condition is one that has high fresh water production and low energy consumption.

Comparison of Different DDD Processes

Next the heated air/heated water DDD process will be compared against other DDD

configurations. The heated air/heated water DDD process will first be compared against

the process described by Klausner et al. [16], heated water/ambient air for a 60 oC water

inlet. The process will then be compared against the heated air/heated water DDD

process with Q-PAC, a packed bed with a smaller specific area and lower pressure drop.

Heated Water/Ambient Air at 60 oC

In this analysis the heated air/heated water DDD process is compared against the

heated water/ambient air DDD process described by Klausner et al. [16]. In performing

the analysis of the heated air/heated water the analysis described in Chapter 3 is used










0.4




0.3 _Liquid Mass Flux
L (kg/m2-S)
I 5I 0.15
I / ~~.- ---....-.. -- 0.25
.9 0.2 -0.50
I // -- 0.75
-0- 1.20
\- 1.55
a ... /. i\\ ........... 2.00
0.1 .. ****. \2.50
-3.00



0.0
0 5 10 15 20

Air Mass Flux G (kg/m2-S)



Figure 4-6. Fresh water production efficiency with varying air mass flux realized for the
lower air and liquid mass fluxes.



with the air inlet temperature, water inlet temperature, specific packing area, diameter of


the packing material, and inlet absolute humidity held constant at 600 C, 600 C, 267

m2/m3, 0.018 m, and 0.0065 respectively. The values obtained for the heated

water/ambient air case are calculated using the model proposed by Klausner et al. [16]

where the inlet water temperature, inlet air temperature, inlet humidity ratio, specific

area, and diameter of the packing are held at 600 C, 260 C, 0.023, 267 m2/m3, and 0.018

m respectively. To obtain the predictions for comparison calculations were run for nine

different liquid mass fluxes: 0.15, 0.25, 0.5, 0.75, 1.20, 1.55, 2.0, 2.5, 3.0 kg/m2-S. For

each liquid mass flux, the gas mass flux was varied. For each liquid mass flux, the

minimum energy consumption rate was recorded over the range of air mass flux. The

values of the fresh water production flux, and fresh water production efficiency reported









are those corresponding to the point of minimum energy consumption. Figures 4-7 and

4-8 show the fresh water production flux, energy consumption rate, and fresh water

production efficiency for the two different configurations.

Figure 4-7 shows the fresh water production efficiency and energy consumption

rate with varying liquid mass flux for both of the processes. The energy consumption

rate for both shows little difference for liquid mass fluxes greater than about 1.3 kg/m2-S.

However at low mass flux the heated water/ambient air case has a considerably less

energy consumption rate. The fresh water production efficiency of the heated air/heated

water process is greater for all values of liquid mass flux, although at large liquid mass

flux the difference is not significant. It should be noted that when comparing the two

configurations there is more thermal energy input for the heated air/heated water case

than the heated water/ambient air case, and the energy consumption rate only reflects the

electrical energy consumed.

Figure 4-8 shows the fresh water production flux and energy consumption rate for

varying liquid mass flux for both processes. It is observed that the heated air/heated

water process has a greater fresh water production flux for all values of the liquid mass

fluxes considered. However at low liquid mass flux, the energy consumption rate is

higher for the heated air/heated water configuration. Thus the decision to use one process

over the other depends upon the source of waste heat and operating conditions.

This comparison reveals that for higher liquid flow rates the heated air/heated water

case is equally comparable in energy consumption rate but has a higher fresh water

production efficiency and fresh water production flux. On a small scale, this equates to a














Inlet Conditions
Hot Alr/Hot Water
Ambient Alr/Hot Water


S60 C T
S26 C T


,C n= 5 25%
,C $n= 80.17%

Ambient Alr/Hot Water
Hot Alr/Hot Water


Ambient Alr/Hot Water
Hot Alr/Hot Water


009


0 008
-


0 007


~006-


0-
e







0 02 -


-00008


- O0007


- 0006


- 0005

- O0004
E
- O0003








-00000


//




,, /
, /


00 05 10 15 20 25 30 35

Liquid Mass Flux L (kg/m2-S)





Figure 4-7. Fresh water production efficiency and energy consumption rate for varying

liquid mass flux for 60 oC inlet conditions.


Inlet Conditions
Hot Alr/Hot Water
Ambient Alr/Hot Water


T ,= 60 C T ,= 60 C $tn= 5 25%
T ,= 26 C T ,= 60 C $tn= 80.17%


Ambient Alr/Hot Water
Hot Alr/Hot Water


Ambient Alr/Hot Water
Hot Alr/Hot Water


01-




S008-

-








006

a 0-

0 2


-0 0007


-0 0006


-0 0005 a


-0 0004


-0 0003


-0 0002


-0 0001


',


7,


,, *^


00 05 10 15 20 25 30 35

Liquid Mass Flux L (kg/m2-S)





Figure 4-8. Fresh water production flux and energy consumption rate for varying liquid
mass flux for 60 oC inlet conditions.









smaller tower size by utilizing the heated air/heated water DDD process. Less tower

height is required for the heated air/heated water process in order to generate the same

amount of fresh water produced in the heated water/ambient process. This translates to

lower production cost and less space required.

Heated Air/Heated Water using Q-PAC

For the next analysis two different types of packed bed are used with the heated

air/heated water process. The theoretical model proposed in Chapter 3 is used to

calculate the parameters, however a different wetted specific area is used. A modified

Onda' s correlation, which is found in Appendix A, is used for the mass transfer

coefficients and the wetted specific area for both configurations. HD Q-PAC, the packing

material described in Chapter 2 and used in the experimental facility, is compared with

Q-PAC. The Q-PAC material is also produced by Lantec and is manufactured to have

lower pressure drop, reduced incidence of fouling, and flooding at higher air mass flow

rates giving it a wider range of operation. Lantec supplied the gas side pressure drop,

APG (kPa), across the Q-PAC packed bed as


G =C[0.0078+1.3788( .)+0.30715( )2 2 ], (4.7)
z pG 10l 1l PG

Figures 4-9 and 4-10 show the fresh water production, fresh water efficiency, and

energy consumption of the heated air/heated water process for the two configurations.

Again the computations shown in the graph correspond to the points of minimum energy

consumption rate for each liquid mass flux. Figure 4-9 shows the fresh water production

efficiency and energy consumption rate with varying liquid mass flux. The Q-PAC

packed bed configuration is obviously much more energy efficient than the HD Q-PAC

configuration. As the liquid mass flux increases the variation in the energy consumption







60


rate of the two configurations increases. There is little variation in the fresh water


production efficiency, however the Q-PAC appears to have a slightly better fresh water

production efficiency for all values of the liquid mass flux explored.


Inlet Conditions: Tain = 60 C Tw~,,n = 60 C n,= 5.25%
S HD Q-PAC Material ---HD Q-PAC Material
Q-PAC Material Q-PAC Material
0.065 0.0007

S0.060-
5 1 .- 0.0006 ~

0.0005

0.04 0.0004
0.040 -r

0.0003
0.035 -4

S0.030 000


0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Liquid Mass Flux L (kg/m2-S)



Figure 4-9. Fresh water production flux and energy consumption rate for varying liquid
mass flux for HD Q-PAC and Q-PAC packed bed.

Figure 4-10 shows the variation in the fresh water production flux and energy

consumption rate for varying liquid mass flux. There is very little variation in the fresh

water production flux between the HD Q-PAC and Q-PAC packed bed configurations.

However, at high liquid mass flux, the Q-PAC appears to have a slightly higher fresh

water production flux. Despite the minimal difference in fresh water production between

the two beds, the energy consumption rate remains the key difference. Figure 4-10

demonstrates that the Q-PAC packed can be utilized to produce the same quantity of

fresh water product but at a lower energy cost. The downside of the Q-PAC is that much

more footprint area is needed to achieve the same quantity of production. Despite this















0.12 0.0007


0.10 -000

E 0.0005

S0.06
L: I C 0.0004
~i0.04 -

.c 0.0003

S0.02 -


0.00 0.0002
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Liquid Mass Flux L (kg/m2-S)



Figure 4-10. Fresh water production flux and energy consumption rate for varying liquid
mass flux for HD Q-PAC and Q-PAC packed bed.

fact, however, the Q-PAC could prove to be especially important for large scale DDD

facilities. If space is not an issue then the Q-PAC configuration would be advantageous

for the lesser energy consumption rate. Further research should be conducted on the Q-

PAC to determine the exact wetting and performance.

Industrial Plant Application

In order to fully understand the feasibility of the DDD process with a heated

air/heated water input, an economic study must be performed. Consider an industrial


plant site that can supply 850,000 acfm of hot air at 820 C (1800 F). Figure 4-11 shows a

flow diagram of the DDD process coupled with an industrial plant. As the drawing


depicts, the waste energy in the form of heated air from the plant site enters the diffusion

tower at 820 C. The source brackish/run off/sea water enters the diffusion tower at 300


C. The source water is heated from the water exiting the diffusion tower in a










regenerative heater. The source water will flow down through a packed bed where it will

meet the heated air and a portion of the water will evaporate into the air stream. The

saturated air exiting the diffusion tower will enter a countercurrent flow oriented direct

contact condenser. The fresh water exiting the condenser will enter a fresh water storage

tank where a portion is the product and a portion is pumped through a heat exchanger to

be cooled and then cycled back to the condenser. For all calculations it is assumed that

the sink temperature is 15 oC.

Operation Conditions

Using the analysis described at the beginning of the chapter, Figure 4-12 shows the fresh

water production flux for varying diffusion tower liquid mass fluxes. For the analysis the

condenser air mass flux is 1.5 kg/m2-S and the fresh water mass flux is 3.0 kg/m2-S.

These values were determined by Li et al. [l l] to be the optimal operating conditions of

the direct contact condenser using the HD Q-PAC packing material. It is also assumed

that the surface areas of the condenser and diffusion tower are the same. The condenser

was analyzed using the model proposed by Li et al. [17]. As seen from Figure 4-12, there

is an optimal fresh water production flux for an inlet water temperature of 30 oC at a

liquid mass flux of about 0.20 kg/m2-S. Figure 4-13 depicts the total energy consumption

for the DDD process with varying diffusion tower liquid mass flux. From the graph the

total energy consumption for an inlet water temperature is optimal in the range of liquid

mass fluxes from 0.1-0.2 kg/m2-s. It is fortunate that the optimal total energy

consumption and maximum fresh water production occur in the same range. The optimal

diffusion tower liquid mass flux is therefore taken to be 0.15 kg/m2-s. It is worthy to

note that the energy consumption of the DDD process demonstrated is nearly five times








63



less than that of reverse osmosis desalination. This is a very distinct advantage to the


DDD process.

Tc= 28 oC Air-Cooled Heat
me =2283 kg/s Exchanger


Tw= 30 oC Tc 5o
m, =39.5 kgis E s
STw= 20 oCm 7 k/
Diffusion m=79k/
Tower
i f L Industrial Tc 46009 o
Plant a=006

Packed Bec ~m I ~m Packed Bec

Ta,= 82 oC
,Forced Draft = 00147 aj 0.2C Direct
Blower m = 395 kg/s Contact
Condenser

Water Cooler Two= 3C oC w=2
::::::::::::::::::::::::::::::::::::::::. Two =37 oC I Fresh W ater Storage

Tw = 17 oC T = 15 oC ...........
Fresh Wt
Main Feed ProductionFrsWaePun
Pump

|| m = .8 g/s Subsc its
I m, 8.8kg/s a-Air
Seawater, Brackish, or Process Off Air/Vapor w-Water
Run-Off Reservoir Gas i-inlet
Seawater Fresh Water a-Outlet
** w denotes absolute
humidity




Figure 4-11. Sample schematic of the DDD system coupled with an industrial site.


Figure 4-14 depicts the fresh water production efficiency as a function of diffusion


tower liquid mass flux. As the graph shows the fresh water efficiency increases steeply


with decreasing diffusion tower liquid mass flux. The fresh water production efficiency


in the range of the optimal total energy consumption and fresh water production is quite


high and is about 0.224.


The optimal operating conditions of the proposed DDD process coupled with an


industrial plant are summarized in Table 4-1. If operating at these conditions there is a


potential to produce about 201,800 Gal/day of distilled water with small total electrical









64



Water Inlet Temperature


T, = 40 C
T, = 35 C
----- T= 30 C


Inlet Conditions
T,,= 82 C
0 = 0 0147
Gdif = Geond = 1 5 kg/m2-S
L, =30 kg/m2-S


00 02 04 06 08 10 12

Diffusion Tower Liquid Mass Flux (kg/m2-S)




Figure 4-12. Fresh water production flux for varying diffusion tower liquid mass flux.


Inlet Conditions
T,,= 82 C
0 = 0 0147
Gdif = Geond = 1 5 kg/m2-S
L, =30 kg/m2-S


Water Inlet Tem peratu re


T, = 40 C
T, = 35 C
----- T= 30 C


0 004










0 001
-




0 000 -
-n


00 02 04 06 08 10 12

Diffusion Tower Liquid Mass Flux (kg/m2-S)




Figure 4-13. Energy consumption rate of the DDD system for varying diffusion tower
liquid mass flux.











Inlet Conditions Water Inlet Temperature
T,,= 82 C
Co = 00147 T, = 40 C
Gdiff = Geond = 1 5 kg/m2-S T,,= 35 C
L,=30kg/m2-S- T,,=30C
10-
















00 02 04 06 08 10 12
Diffusion Tower Liquid Mass Flux (kg/m2-S)



Figure 4-14. Fresh water production efficiency for varying diffusion tower liquid mass
flux.


energy consumption of 0.0012 kW-hr/kg and a fresh water production efficiency of

0.224. A total footprint area of 562.3 m2 is required.

Economic Analysis

Next the cost of the DDD process facility when coupled with an industrial plant is

evaluated. The unit cost of a desalination plant is highly dependent upon the site

characteristics. Factors such as, plant capacity, pumping units, chemicals, and


pretreatment all depend on the given location [21]. According to Ettouney et al. [21] the

costs of a desalination facility can be broken down into three segments: direct capital

costs, indirect capital costs, and annual operating costs. The direct capital costs include

the costs of purchasing the necessary equipment, land, and construction of the plant.









Table 4-1. DDD optimal operating conditions for industrial site.
DDD System Operating Conditions


General Operating Conditions:


Condenser:


Diffusion Tower:

G = 1.5 kg/m2-S
L = 0.15 kg/m2-S
Tai = 82oC
Twi = 29.89 oC
wi= 0.01469
Tao = 46.05 oC
Two = 36.95 oC
o,= 0.0290
Height = 0.265 m


G = 1.5 kg/m2-S
L = 3.0 kg/m2-S
Tai = 46.05oC
Twi = 19.82 oC
wi= 0.0290
Tao = 21 oC
Two = 29.2 oC
o,= 0.0066
Height = 0.822 m


mfw/mL = 0.224
Econsumption= 0.001218 kWl~-hr/kg
mfw = 201,817 Gallday

Flow Conditions usina Available Eneray (850,000 acfm)


Diffusion Tower:

m, = 395 kg/s
mw = 39 kg/s
Adiff = 263.16 m2


Condenser:


m, = 395 kg/s
mw = 789 kg/s
Acond = 263.16 m2


1. Land The cost of land is very region specific. It depends on the demand for
land in a given area.

2. Well Construction- Recent studies estimate the cost of construction per meter
depth to be $650. The average well capacity is approximated as 500 m3/d.

3. Process Equipment- This category consists of some of the most expensive
equipment all of which will depend upon the plant capacity and process.
Instrumentation and controls, pumps, electric wiring, pre-and post-treatment
equipment, pipes, valves, and process cleaning systems are all included. The
equipment for a RO plant are generally less than that of the distillation processes
of MSF. A RO plant equipment can cost as little as $1,000 whereas a MSF plant
with a 27,000 m3/d capacity can cost $40 million.









4. Auxiliary Equipment- Generators, transformers, pumps, pipes, valves, wells,
storage tanks, and transmission piping are all considered auxiliary equipment.

5. Building construction- The building costs are site dependent and can vary from
$100 to $1,000/m2

The indirect costs of a plant are expressed as percentages of the total direct capital

cost. The indirect costs include freight and insurance, construction overhead, owner's

costs, and contingency costs. The freight and insurance is about 5% of the direct capital

costs, construction overhead is about 15% of the direct material and labor costs, owner' s

costs is 10% of direct material and labor costs, and the contingency costs are about 10%

of the total direct costs. The annual operating costs are the costs incurred during plant

operation and include electricity, labor, maintenance, insurance, chemicals, and

amortization or fixed charges.

The costs for the DDD process operating at the conditions described in Table 4-1

are estimated. The estimates based on the following assumptions:

1. The interesting rate, i, is 5%.

2. The plant life is approximated as n=30 years.

3. The plant availability is estimated as f=0.9.

4. The cost of land is neglected as it assumed that the DDD system will be coupled
with an aluminum smelting plant and is readily available.

5. The cost of the chemicals used in pre- or post-treatment of the water is neglected.
The cost is neglected because the chemical used in treatment is highly dependent
upon the use of the fresh water produced.
6. The specific cost of the operating labor is estimated to be y = $0.01l/m3 for the
thermal processes and $0.05/m3 for RO. Thus for the DDD process the specific
cost of the operating labor is approximated as y = $0.025/m3

7. According to the American Water Works Association [22] the direct costs are
estimated to be $3-$6/gal per day installed capacity and are determined to be
typical seawater desalting plant construction costs. This value does not include
any of the off-site construction costs including engineering, legal, or financial









fees. It also does not include the cost to transport the water to the plant, or
incidental or contingency costs. However the cost to develop the site is included
and is estimated to be 5% of the material and construction costs.

The amortization factor is calculated as,

i( + i)"
a = (4.8)


where i is the interest rate and n is the plant life. The annual fixed costs are calculated as,

A~fixed = (a)(DC), (4.9)

where DC is the direct capital cost. Lastly, the annual labor cost is evaluated as,

Alabor = (7)( f)(m)(365), (4.10)

where y is the specific cost of operating labor and m is the plant capacity (m3/day).

Therefore the total annual cost can be calculated as,

ALtotal = Afixed + Alabor (4.11)

It should be noted that the cost of the waste heat, for this case in the form of heated air, is

not included since it is waste that would have otherwise been discarded into the

environment. Therefore the unit product cost of the system is calculated as follows,


A,, Aos (4.12)
""tpf-m-365

Table 4-2 summarizes the estimate of the direct costs for a full scale diffusion driven

desalination facility operating in conjunction with an industrial plant. The analysis shows

that the production cost, neglecting the electricity charges, is between 0.19-0.36 $/m3 I

terms of $/103 gal the production cost is 0.72-1.34 $/103 gl

The fresh water profit is the most important consideration in the analysis of the

DDD facility. One of the most expensive operating costs is the pumping power required









Table 4-2. Summary of DDD Plant Costs.

Unit Calculated Result

Direct Costs (103 $) 636-1,271

ACixed (103 $) 41-83

Alabor (103 $) 6.3

Alabor+ACfixed (103 $) 47.7-89

ACunit~p($/m3) 0.19-0.36


to operate the pumps and the blowers. It is not included in the summary of costs listed in

Table 4-2. Since the cost of electricity is highly fluctuating it is factored into the

determination of the profit. The fresh water profit can be calculated as,

nf = Q, AC,,,,~, PwQeeec (4.13)

where 0, ($/103gal) is the net fresh water profit, ACunit,p ($/103gal) is the production cost

calculated above, Q, ($/103gal) is the retail price of fresh water, and Qelec ($/kW-hr) is the

retail price of electricity. Here ACunit,p is taken to be an intermediate value of

$0.99/103gal. The profit to be gained is highly dependent on the retail cost of water and

the cost of water strongly depends on how the water is transported to the customer. Two

different scenarios will be evaluated. First, the profit will be evaluated when the water is

transferred to the customer via municipal pipelines. For this case, Figure 4-15 depicts the

calculated net fresh water profit as a function of the retail price of electricity for varying

costs of water. On average, the retail price of water in the United States is about

$3/103gal. For this price, the largest profit to be earned is for the lowest energy retail

price of $0.04/kW-hr. As the retail price of energy increases, the profit linearly

decreases. A similar trend is observed for the other water retail prices. If the retail price









of water is $3/103gal and the cost of electricity is $0.08/kW-hr, then the profit is about

$1.80/103gal. For the production summarized in Table 4-1, this amounts to a profit of

only $363/day. Given this profit outlook it advantageous to consider other prospects to

market the product distilled water.

It is of importance to recall that the product of the DDD process is high purity

distilled water, which is of superior quality to that circulating through the municipal lines.

Another possible solution therefore would be to consider selling the distilled water as

bottled water. Figure 4-16 shows the growth of the bottled water industry. As the graph

shows, the sale of bottled water has been steadily increasing since the late 1970s. Clarke

and King [1] estimate that the bottled water market was worth an estimated $20 billion a

year for 2002 alone. The reasons for the increase in bottled water consumption vary

from distaste of tap water, distrust of the safety of tap water, and the overall realization

that water is a healthier choice of beverage than soda and other carbonated beverages.

The price per gallon of bottled distilled water can vary between $0.90-$1.50 per gallon.

Without taking into account the cost of bottling, Figure 4-17 shows the net fresh water

profit that can be achieved with the DDD fresh water product. The graph shows that the

operating and production costs have little effect on the fresh water profit. Nearly all

revenue generated from sales is considered profit. If the distilled bottled water is sold at

$1.00/gal and the DDD process can produce 201,800 gal/day then the profit generated is

about $200,000/day or $73 million/yr. As it can be seen there is the possibility of

enormous profit because the profit is basically the retail price of the distilled bottled

water. Given the continuous growth of the bottled water industry that shows no signs of

slowing down, an enormous profit can be rendered from the DDD process.

























___
__
__
___
_
___
___











~

___
___
____
____
___
___
____
____
_____

____
__
__
__
____
__
__
__
___
__
__


0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20



Energy Retail Price ($/kW-hr)




Figure 4-15. Net fresh water profit as a function of energy retail price for varying water

retail price.




Overall the DDD process utilizing a heated air/heated water inlet conditions has




proven to be an economically reasonable process when coupled with an industrial plant




that can provide waste heat. While transporting the distilled fresh water product to




customers predicted minimal profit, the bottled water industry provides the opportunity to




sell the distilled water for enormous profit.


I I I I I I I _


6-





5-





4 -





3-





2-





1-


Water Retail Price

($1103 gl



-~2




5

--- 6
-7




































Year



Figure 4-16. The bottled water market in the United States [23].


5000 -

4500-

4000-

3500-

3000-

2500-

2000-

1500-

1000-


500-

1975


1980


1985


1990


1995


2000


Distilled Bottled
Water Retail Price
($1gal)
0.80
-0.90
-----1.00
1.10
-1.20
-1.30


0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Energy Retail Price ($/kW-hr)


Figure 4-17. Net fresh water product as a function of energy retail price for varying
distilled bottled water prices.


1 rr














CHAPTER 5
CONCLUSION

The diffusion driven desalination process provides an economic means for utilizing

and converting saltwater to high purity distilled water A laboratory scale DDD facility

has been designed and constructed which is fully instrumented and includes a data

acquisition system. The laboratory facility has been modified to include an air heating

section which heats the inlet air flowing to the diffusion tower. Experiments were

conducted on the diffusion tower for two different cases: the heated air/ambient water

and heated air/heated water. For each case, a set of experiments were conducted by

holding the air mass flux constant and varying the liquid mass flux over a range of

values.

The heat and mass transfer for the DDD process were analyzed. A one dimensional

liquid film model was used to obtain three coupled ordinary differential equations with

dependent variables of humidity, water temperature and air temperature. Closure was

achieved by using the heat and mass transfer analogy and Onda' s correlation was used to

evaluate the mass transfer coefficients. Through experimental analysis, the wetted

specific area of the packed bed was found to be a constant in the range of operating

conditions studied. It was also found that for the heated air/heated water case there is

moderate heat loss from the system that must be accounted for in the energy balance.

The experimental values obtained were compared to those obtained from the theoretical

heat and mass transfer model. Agreement between the two was quite good for both

cases. It was also found that the repeatability of the experiments is satisfactory. The









heated air/ambient water case proved to be inefficient with very little change in exit

conditions over the range of liquid and air mass fluxes. The heated air/heated water

process proved to give better performance. The theoretical model is a necessary tool

needed to design an efficient DDD facility.

A parametric study to determine the effect of certain operating variables on the

maximum freshwater production was completed for the heated air/heated water DDD

process. For each liquid mass flux there is an optimal air mass flux where the maximum

fresh water production flux is produced. There is also a location where the energy

consumption for pumping through the diffusion tower is minimized.

The heated air/heated water process was compared to other different DDD

processes, specifically the heated water/ambient air case and the heated air/heated water

process utilizing the Q-PAC packed bed, a less dense and lower pressure drop packing

material. The comparison showed that the heated air/heated water DDD process has its

advantages and disadvantages at different operating conditions. In comparison for the

heated water/ambient air it was shown that the heated air/heated water process requires

more electrical energy than the heated water/ambient air process at low liquid mass flux

however the fresh water production efficiency for the heated air/heated water is greater

for all values of liquid mass flux. The fresh water production for the heated air/heated

water process is greater for all liquid mass flux. In the comparison with the heated

air/heated water using the Q-PAC packing, it was shown that similar fresh water

production can be achieved at a lower energy cost by using Q-PAC. Further studies are

required to determine the wettability of Q-PAC and its wetted fraction as part of the DDD

process.









An economic study was performed to determine the cost effectiveness of the DDD

process using a heated air/heated water input when coupled with an industrial plant

producing waste heat in an air stream. If 850,000 acfm of 82 OC heated waste heat in air

is available from a plant the optimal operating conditions are a diffusion tower liquid

mass flux of 0. 15 kg/m2-S, an air mass flux of 1.5 kg/m2-S, and a fresh water mass flux of

3.0 kg/m2-S in the condenser. At these conditions with the given amount of waste heat

available, the plant can produce 201,800 gal/day of high purity distilled water while

consuming a miniscule 0.0012 kW-hr/kg and having a fresh water conversion efficiency

of 0.224. Through an economic study it was determined that a reasonable profit can be

realized by selling the product as distilled bottled water. Without taking into account the

cost of bottling, assuming that a gallon of distilled water costs $1.00 per gallon, a profit

of $0.99 per gallon can be achieved.

Extensive research has already been performed on the DDD process and while the

future of the process looks optimistic, further research needs to be done. The DDD

process needs to be further examined on a larger scale. A pilot facility should be

constructed and the DDD performance should be analyzed over long term operation in an

industrial setting. As of yet, the DDD process has not been analyzed using a saline feed

water source in the diffusion tower. Using such a source, water quality testing of the

fresh water produced in the DDD process should be conducted. This is important in

determining what type of post treatment of the product water is required.

The DDD process has been thoroughly studied using a laboratory scale facility.

The diffusion tower performance has been studied for heated air/heated water, heated

air/ambient water, and heated water/ambient air inputs [16]. On a small scale, further










experiments should be conducted to determine the performance of DDD system utilizing

a different type of packed bed. On a larger scale, the DDD process now needs to be

examined with a pilot scale facility coupled with a low grade waste heat source to

determine its performance under an industrial load and continuous operation.















APPENDIX A
ONDA' CORRELATION





kG = C Reg ScG/3 (ad,)-2 aDG, (C=5.23 if d, > 15 mm; C=2 if d,<~ 15 mm)

a =a 1-x -. Re u2 Froo'Weus5


Re =L FrL We, =L
a p, p, g p ~a

Re = Re, c ScG
aw~p, apG LD, pGDG
SThis equation has been modified from Onda's original correlation
















APPENDIX B
EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED
AIR/AMBIENT WATER


TL,in
(" C)
25.26
25.36
25.30
25.36
25.30



TL,in
(" C)
25.28
25.30
25.09
25.10



TL,in
(" C)
25.30
24.77
25.06
25.33



TL,in
(" C)
24.90
25.25
25.20
25.20
25.17
25.16


Tai,i moin TL,out
(" C) (" C)
59.83 0.0075 26.93
60.26 0.0075 26.98
59.88 0.0075 26.87
60.27 0.0075 26.94
60.27 0.0075 26.80



Ta,in moin TL,out
(" C) (" C)
61.82 0.0041 25.70
61.91 0.0040 25.70
61.66 0.0041 25.42
61.76 0.0041 25.38



Tai,i moin TL,out
(" C) (" C)
60.57 0.0060 26.73
60.82 0.0061 26.45
60.95 0.0062 26.40
61.33 0.0062 26.40



Ta,in moin TL,out
(" C) (" C)
60.38 0.0068 26.29
59.78 0.0067 26.38
59.83 0.0068 26.43
59.95 0.0069 26.48
60.03 0.0069 26.35
60.16 0.0069 26.28


Ta~out m~out
(" C)
26.07 0.0206
25.95 0.0205
25.80 0.0203
25.75 0.0203
25.70 0.0203



Ta,out m~out
(" C)
26.04 0.0201
25.58 0.0198
25.11 0.0194
24.99 0.0193



Ta~out m~out
(" C)
26.04 0.0204
25.66 0.0200
25.65 0.0200
25.70 0.0201



Ta~out m~out
(" C)
26.09 0.0205
25.86 0.0203
25.86 0.0203
25.83 0.0203
25.81 0.0203
25.79 0.0203


mL
(kg/s)
0.031
0.040
0.047
0.056
0.063



mL
(kg/s)
0.028
0.035
0.044
0.059



mL
(kg/s)
0.029
0.039
0.051
0.062



mL
(kg/s)
0.032
0.039
0.043
0.049
0.055
0.064


ma
(kg/s)
0.040
0.040
0.040
0.040
0.040



ma
(kg/s)
0.040
0.040
0.041
0.041



ma
(kg/s)
0.060
0.060
0.060
0.059



ma
(kg/s)
0.060
0.061
0.061
0.060
0.060
0.061













TL,in
(" C)
24.66
25.09
25.39
25.17
25.38



TL,in
(" C)
25.28
25.27
25.34
25.24
25.27


Ta,in moin TL,out
(" C) (" C)
61.09 0.0068 27.49
61.03 0.0072 27.46
60.96 0.0071 27.37
61.09 0.0072 27.27
61.39 0.0073 27.33



Ta,in moin TL,out
(" C) (" C)
60.38 0.0080 27.51
60.30 0.0081 27.40
60.43 0.0080 27.37
60.76 0.0080 27.31
60.46 0.0080 27.13


Ta~out m~out
(" C)
26.34 0.0207
26.32 0.0208
26.25 0.0208
26.16 0.0207
26.21 0.0208



Ta~out m~out
(" C)
26.67 0.0213
26.68 0.0213
26.61 0.0213
26.55 0.0212
26.42 0.0211


mL
(kg/s)
0.029
0.037
0.048
0.055
0.065



mL
(kg/s)
0.035
0.042
0.048
0.056
0.065


ma
(kg/s)
0.080
0.081
0.081
0.080
0.081



ma
(kg/s)
0.080
0.080
0.080
0.079
0.081
















APPENDIX C
EXPERIMENTAL DATA OF THE DIFFUSION TOWER FOR HEATED
AIR/HEATED WATER


TL,in
(" C)
60.82
60.99
59.99
60.58
61.02
59.48



TL,in
(" C)
61.00
60.79
60.75
60.89
60.31



TL,in
(" C)
61.01
61.66
61.12
60.05
60.52


Tai,i moin TL,out
(" C) (" C)
61.03 0.0066 37.21
61.15 0.0066 37.81
61.71 0.0066 38.84
60.94 0.0069 39.96
60.61 0.0071 40.41
60.52 0.0074 40.91



Tai,i moin TL,out
(" C) (" C)
61.35 0.0059 35.99
61.57 0.0059 36.27
61.69 0.0061 37.30
61.53 0.0064 39.12
60.94 0.0066 39.63



Ta,in moin TL,out
(" C) (" C)
61.10 0.0078 33.65
61.08 0.0078 34.46
60.93 0.0080 35.02
61.23 0.0079 36.14
62.10 0.0081 39.87


Ta~out m~out
(" C)
42.28 0.0550
43.57 0.0591
44.74 0.0632
44.76 0.0631
45.54 0.0661
45.03 0.0642



Ta~out m~out
(" C)
42.30 0.0566
44.03 0.0624
44.71 0.0649
44.10 0.0627
44.89 0.0656



Ta~out m~out
(" C)
37.74 0.0420
39.70 0.0474
40.20 0.0489
40.98 0.0511
42.18 0.0550


mL
(kg/s)
0.034
0.039
0.047
0.054
0.061
0.068



mL
(kg/s)
0.036
0.043
0.050
0.056
0.064



mL
(kg/s)
0.032
0.041
0.047
0.055
0.063


ma
(kg/s)
0.040
0.039
0.039
0.040
0.041
0.041



ma
(kg/s)
0.041
0.040
0.040
0.041
0.042



ma
(kg/s)
0.059
0.060
0.061
0.060
0.059










TL,in
(" C)
60.79
60.76
60.97
60.86
60.57


TL,in
(" C)
60.94
61.01
60.45
60.40
60.26



TL,in
(" C)
59.69
61.29
60.83
60.57
58.98


Ta,in moin TL,out
(" C) (" C)
60.52 0.0080 32.46
60.69 0.0080 33.51
61.03 0.0081 34.65
61.10 0.0082 35.95
61.16 0.0083 37.06


Ta,in moin TL,out
(" C) (" C)
60.11 0.0088 30.75
60.29 0.0090 31.88
60.48 0.0088 33.04
60.78 0.0089 34.88
60.94 0.0090 36.02



Ta,in moin TL,out
(" C) (" C)
59.94 0.0080 31.03
59.73 0.0083 30.82
61.09 0.0084 32.97
60.96 0.0086 34.43
60.40 0.0088 34.99


Ta~out m~out
(" C)
37.51 0.0416
38.44 0.0448
39.70 0.0482
40.85 0.0498
41.33 0.0510


Ta~out m~out
(" C)
34.51 0.0353
35.91 0.0387
37.40 0.0426
38.02 0.0442
39.01 0.0468



Ta~out m~out
(" C)
35.02 0.0350
35.97 0.0378
37.12 0.0410
37.90 0.0429
38.97 0.0458


mL
(kg/s)
0.033
0.038
0.046
0.056
0.062


mL
(kg/s)
0.030
0.039
0.048
0.056
0.066



mL
(kg/s)
0.032
0.036
0.046
0.055
0.064


ma
(kg/s)
0.061
0.060
0.060
0.061
0.060


ma
(kg/s)
0.080
0.080
0.080
0.081
0.080



ma
(kg/s)
0.080
0.080
0.080
0.081
0.081

















LIST OF REFERENCES


1. Clarke, R., and King, J., 2004, The Water Atla~s, The New Press, New York.

2. Semiat, R., 2000, "Desalination: Present and Future," Water International, 25(1),
pp. 54-65.

3. Dore, M., 2005, "Forecasting the Economic Costs of Desalination Technology,"
Desalination., 172, pp. 207-214.

4. Ehrenman, G., April 2003, "Mapping the Road to Water," Mechanical Engineering,
125(4), pp. 23.

5. Economic and Social Commission for Western Asis, 2001, "Energy Options for
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BIOGRAPHICAL SKETCH

Jessica Knight is the daughter of Donald and Renee Knight. She was born and

raised in Jacksonville, Florida. She began attending the University of Florida in August

2000 where she later earned her bachelor' s degree in mechanical engineering in

December 2004. She began working on the diffusion driven desalination project in her

senior year of her undergraduate studies. She continued research on the diffusion driven

desalination process as a graduate student in January 2005 in pursuit of a Master of

Science degree.