Enhancing the steam-reforming process with acoustics

ENHANCING THE STEAM-REFORMING PROCESS WITH ACOUSTICS: AN
INVESTIGATION FOR FUEL CELL VEHICLE APPLICATIONS

By

PAUL ANDERS ERICKSON

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2002

LD
j780

20

UNIVERSITY OF FLORIDA
3 1262 08554 4335

Copyright 2002

by

Paul Anders Erickson

This dissertation is dedicated to my wife, Dena.

ACKNOWLEDGMENTS

Anyone with a semblance of true wisdom knows that one in and of oneself is a

very small thing. Such is the case with this study, as many people have contributed

directly or indirectly to this dissertation. I acknowledge the following individuals and

institutions who have extended themselves to give support to the present study.

I acknowledge the support, encouragement, and patience put forth by my advising

professor, Dr. Vernon P. Roan. Dr. Roan is truly a scholar and I am honored to have had

him as my mentor for the duration of my time at the University of Florida. Similarly, I

would like to thank my doctoral committee (Dr. Jean M. Andino, Dr. D.Yogi Goswami,

Dr. Ulrich H. Kurzweg and Dr. William E. Lear, Jr) for their support in this research

project and in my personal education.

I acknowledge the State of Florida, through the University of Florida and the

Mechanical Engineering Department for funding my education and allowing me to serve

as an instructor and teaching assistant. The research has been funded in part by Ford

Motor Company with their generous grant to the University of Florida Fuel Cell Research

and Training Laboratory for studies dealing with reformation of hydrocarbon fuels.

I acknowledge the support and guidance I have received from my parents Lloyd

and Virginia Erickson and my siblings. Words cannot express my thanks to my wife

Dena, to whom I have dedicated this work, and my children Timothy, Eliani, and Kari.

They provide me with inspiration to fulfill my potential. My family has sacrificed much

that I might pursue my education. Truly my family is the center of my joy and happiness.

I also acknowledge support from my in-laws Fred and Jacque Wright as well as Les

Meinzer who provided computer hardware for the publication of the project. I

acknowledge my colleagues at the Fuel Cell Research and Training Laboratory, with

special mention of Timothy Simmons, Daniel Betts, and Michael Heckwolf who gave

much needed assistance with the project and encouragement at critical times.

No acknowledgment would be complete without thanking God for enlightenment,

wisdom, and vision to see me through this project. My sincere hope is that He will find

that I have used my knowledge, time and resources to help His children here on earth, and

that I will be found worthy of His approval through the Messiah. Despite any small

amount of earthly knowledge I have obtained, I consider myself a "fool before God" and I

understand that eternal knowledge and wisdom is His to give to those who humbly and

diligently seek His will.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ..................

LIST OF TABLES ........................

LIST OF FIGURES .......... .............

LIST OF SYMBOLS AND NOMENCLATURE

ABSTRACT .............................

. iv

. ix

. xi

xiv

.. ... ...... ................ xv

CHAPTER

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

Background .... ................ ... ....... ......
Fuel Cells .....................................
Vehicle Applications ....... .......................
Fueling Options ..... ... ................... ...
Reform ing .. ............ .................. ... ....
Problem Definition ...... ...............................
Research Objectives ............... ....................

2 LITERATURE REVIEW AND THEORETICAL APPROACH .

Steam Reforming .. ..................................
Limiting Steps in the Reformation Process ..................
Acoustics in Past Processes ..............................
Steam-Reforming in Combination with Acoustics ............
Increased Particle Path Length due to Acoustics ..........
Increased Heat Transfer Rate due to Acoustics ...........
Increased Overall Reaction Rate due to Acoustics .........
Extension of Catalyst Life by Using Acoustics ...........
Summary ........................................
Contribution .............................. ..........

.12
.13
.18
.21
.22
.25
.27
.29
.30
.31

3 EXPERIMENTAL APPROACH AND FACILITY

Exnprimental Annrnach

Experiments
Pumpin
Vapori2
Reactor
Ca
Ac
Conden
Instrum
Te
Ac
Analysis of
Gas Ch
Conver
Space N
Remov
Procedures

il Facility ...................... ........
g Subassem bly .........................
zer Subassembly ........................
Subassem bly ..........................
talyst bed .................. ...........
oustic components ......................
sing Unit Subassembly .......................
entation and Control Subassembly ..........
mperature control .......................
oustic control ..........................
Data ..................................
romatography ..........................
sion ..................................
velocity ...............................
al of Data Points ........................
. . . . . . . . . . .

Modeling the Acoustically Enhanced Reformation Process
Factorial Experiment Design ....................
Further Data and Verification of the Model .........

4 PROPERTIES OF THE ACOUSTIC FIELD ...........

. 32

. .32
. 33
S.35
.. 36
. 39
..39
S.41
.. 43
S. 44
. 45
. 50
S.51
. 52
S.55
. 56
. 57
S.57
S. 57
S. 58
S. 60

...................62

The Use of Resonance within the Catalyst Bed ...........
Transfer Functions .................................
M odal A analysis ...................................

5 EMPIRICAL MODEL DEVELOPMENT AND ANALYSIS

.62
.65
.70

Experimental Results and Model
Uncertainty Analysis .........
Midpoint Curvature .........
Flow Rate .............
Sound Pressure ..........
Catalyst Bed Length ......
The Effect of Frequency .......
Summary ..................

Correlation Factors
................
................
................
................
................
................
. . . . .

6 THE EFFECTS OF ACOUSTICS ON THE REFORMATION PROCESS

The Effect on Temperature Profile ...............................
The Effect on Conversion ......................................
The Effect on Controllability ........................ ... ........

....... 84

The Effect on Heat Band Power Draw ................ .................. 93
The Effect on Power Output ......................................... 95
The Effect on Catalyst Degradation ................................... 98
Potential Benefits for a Fuel Cell Vehicle ................................ 100

7 CONCLUSIONS AND RECOMMENDATIONS .......................... 103

Conclusions ...................................................... 103
Recommendations ................................................. 104

APPENDIX

A COMPUTER PROGRAMS .......................................... 107

B CHECKLISTS AND PROCEDURES ................................. 123

C STATISTICAL DESIGN OF EXPERIMENTS ........................... 127

D EXPERIMENT RUN ORDERS ....................................... 135

E TRANSFER FUNCTIONS ........................................ 137

F TEMPERATURE PROFILES ....................................... 147

REFERENCES ...................................................... 153

BIOGRAPHICAL SKETCH ............................................ 158

LIST OF TABLES

Table page

1.1 Types of fuel cells and their operating parameters ........................... 3

1.2 Energy densities of fuels (LHV). ........ ............. .................. 7

3.1 Control setpoint matrix. .......................................... 48

3.2 Average and standard deviation for all catalyst bed temperatures utilizing a
250C setpoint.................................................... 49

3.3 Gas chromatograph calibration levels. ..................... ............... 54

3.4 Volume percent species of the dry product outlet gas. ...................... 55

3.5 Factorial input matrix and run order. ................................. 59

4.1 Waveforms and boundary conditions for a 33 C reactor containing only air .... 72

5.1 The input matrix and methanol conversion percentage data obtained from the
factorial experiments .............. ......... ................. ..... .. 74

5.2 Total average conversion, pooled standard deviation, standard error, and total
degrees of freedom from the factorial experiments ......................... 75

5.3 The effects and interactions found from the factorial experimentation. ......... 75

5.4 Signal-to-noise t-ratios and statistical significance. ..................... .. 77

5.5 Static and kinematic Reynolds numbers and methanol conversion levels for
three frequencies. ................................................. 82

6.1 Extent of conversion for catalyst bed length with and without acoustics. ....... 89

6.2 Average and standard deviation of the centerline temperatures with a setpoint
temperature of 250C at various locations both with and without a 165dB
acoustic field. ..................................................93

6.3 Overall average catalyst bed power draw and pooled standard deviation for
conditions with and without a 165 dB acoustic field. ....................... 94

6.4 A summary of significant performance parameters with and without acoustic
enhancement for two space velocities. ................................. 97

6.5 Auxiliary requirements for generating the acoustic fields in the current facility. 102

LIST OF FIGURES

Figure page

1.2 Simplified fuel cell schematic. ...................................... 2

1.2 Hondas' hydrogen-fueled experimental fuel cell vehicle ................... 5

1.3 Methanol-fueled PAFC transit bus at the University of Florida .............. 9

2.1 Simplified representation of a packed bed methanol-steam reformer. ......... 14

2.2 Simplified representation of the reactants moving to the open catalyst site and
products moving out to the bulk stream....................... ........... 16

2.3 Flow fields at different times for a plane wave superimposed onto a steady
flow field. ........................................................ 19

2.4 A 15 kW diffusion propane flame. .................................... 21

2.5 Total slip displacement and slip displacement due to acoustics for increasing
velocity ratio. ................................................ 24

2.6 Nusselt number ratio versus velocity ratio for a single particle in an empty duct. 26

3.1 Reforming facility at the University of Florida Fuel Cell Research and
Training Laboratory. ....... ....................................... 33

3.2 Simplified schematic of the methanol steam reforming processor ............ 34

3.3 Artistic representation of the pumping subassembly ....................... 36

3.4 Artistic representation of the vaporizer and liquid trap .............. ...... 37

3.5 Artistic representation of the superheater .............................. 38

3.6 Artistic representation of the catalyst bed housing. ............ ...... 4. 40

3.7 Artistic representation of the acoustic driver, driver housing. mounting plate
and fittings ....................................................... 42

xi

3.8 Artistic representation of the condenser subassembly ....................... 44

3.9 Input and output signals for temperature and acoustical control .............. 45

3.10 Control circuit for heating elements................................... 46

3.11 Power distribution box.............................. ........ ......... 47

3.12 Heating element control logic ...................................... 47

3.13 Time plot of catalyst bed temperatures at zone exit utilizing two control
schemes with a 250C setpoint. ..................................... .49

3.14 Typical time trace of pressure in the catalyst bed while generating hydrogen
with an acoustic input of 650 Hz ................................... 51

3.15 Flow paths in the gas chromatograph. ................... ............. 53

3.16 Silica gel-packed column temperature and approximate specie elution times
for the species expected with the gas chromatograph. ...................... 53

4.1 Standing wave pattern of displacement and velocity in a waveguide and an
analogous pattern of displacement and velocity in a driven string. ............ 63

4.2 Transmission and reflection of an acoustic plane wave at a pipe junction. ...... 64

4.3 Transfer function magnitude for the catalyst bed housing containing only
33C air. ........................................................ .66

4.4 Transfer function magnitude for a catalyst bed housing containing catalyst
and air at 33C ................................................... 67

4.5 Transfer function magnitude for a reactor containing reduced catalyst and CO,
at operating temperature (250 C) .......................... ........... 67

4.6 Reactor transfer function magnitude while generating hydrogen at 2500C....... 68

4.7 Frequency response of the acoustic driver as measured in a plane wave tube
by the manufacturer. .............................................. 69

4.8 Normalized velocity magnitude for six waves found in a 33 C reactor
without catalyst. ................................................. 70

5.1 Predicted and actual conversions for various flow rates with and without
acoustics....................................................... 79

xii

5.2 Predicted and actual conversion for maximum pressure amplitude with a
0.568 m catalyst bed length and 9.5 g/min premix flow rate. ....... ........ 80

5.3 Predicted and actual conversions for bed length at a flow rate of 9.5 g/min
premix and a 165 dB acoustic field. .................................. 81

6.1 Temperature profile for the catalyst bed at premix flow rate 4.9 g/min without
an acoustic field .... ............... ............... ..... ..... .... 86

6.2 Temperature profile for the catalyst bed at premix flow rate of 4.9 g/min with
an acoustic field at 165 dB............................................ 86

6.3 Temperature profile for the catalyst bed at a premix flow rate of 9.5 g/min
without an acoustic field .......................................... 87

6.4 Temperature profile for the catalyst bed at premix flow rate of 9.5 g/min with
an acoustic field at 165 dB ........................................... 87

6.5 Conversion versus bed length with and without acoustic enhancement. ........ 90

6.6 Difference in conversion as a function of bed length for 4.8 and 9.5 g/min
premix flow rates .................................................. 91

6.7 Conversion versus space velocity with and without a 165 dB acoustic field. .... 91

6.8 Average power draw for zone band heaters at two space velocities with and
without a 165 dB acoustic field. ..................................... .94

6.9 Average maximum pressure as a function of driver power at 650 Hz with a
0.568 m catalyst bed length while generating hydrogen ..................... 95

6.10 Average maximum pressure as a function of driver power at 650 Hz with a
0.095 m catalyst bed length while generating hydrogen ..................... 97

6.11 Catalyst degradation with and with out acoustics over a 50 hour time period
with an average premix flow rate of 9.3 g/min. .......................... ..98

xlll

LIST OF SYMBOLS AND NOMENCLATURE

Symbol Definition Symbol Definition Symbol Definition

Pressure

po Non-oscillating
density

6 Boundary layer
thickness (99%)

Rek Kinetic reynolds
number

t time

Ym Acoustic
displacement
amplitude

Nu, Nusselt number
with pulsations

Nu, Nusselt number
without pulsations
(steady)

k Thermal
conductivity

S Cross sectional
area

n Mode

X Input variable

n, Number of
factorials

E Effects

Non-oscillating
Velocity

Acoustic velocity

f Frequency (Hz)

ro Pipe radius or
characteristic
length

x position

L Length or
characteristic
length

Sh Sherwood number

H Enthalpy

A Area

Sonic velocity

um Acoustic velocity
amplitude

j ^T

Pm Acoustic pressure
amplitude

v Kinematic
viscosity

rh Mass flow rate

D Diffusivity

T Temperature

SPL

T, Transmitted
acoustic power

x Wavelength

Y Output variable

o, Pooled standard
deviation

I Interaction

Sound pressure
level

Root mean square
pressure

Reference pressure

Reference

Conversion

SE Standard Error

xiv

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

ENHANCING THE STEAM-REFORMING PROCESS WITH ACOUSTICS: AN
INVESTIGATION FOR FUEL CELL VEHICLE APPLICATIONS

By

Paul Anders Erickson

August 2002

Chairperson: Dr. Vernon P. Roan
Major Department: Mechanical Engineering

An investigation was made into the possibility of enhancing steam reformation

processes through superposition of an acoustic field on the catalyst bed of a methanol

steam reformer. The enhancement scheme presented is of particular interest for

application in fuel cell vehicles or similar applications where size and mass of a reactor is

to be minimized. As part of this study, background is given outlining the difficulties and

liabilities of steam-reformation for transportation applications. Proven acoustic

enhancement of various processes is reviewed and a theory-based model of acoustic

enhancement of the steam-reforming process is developed. The experimental facility

includes a steam-reforming reactor that has been modified to accept an acoustic field.

Relc\eant parameters of the acoustic field are quantified and discussed. The effect of the

acoustic field is investigated experimentally with relation to the reactor output parameters

of temperature profile, controllability, fuel conversion, species concentration, power

demand, and catalyst degradation. Additionally, an empirical model is developed that

predicts the fuel conversion in the reactor for a given set of fuel flow rate, catalyst bed

length, and acoustic pressure parameters.

Although the test facility used was not optimized for utilizing acoustic N\ aves,

significant acoustic enhancement of the steam-reformation process is demonstrated by the

study and such enhancement is statistically proven to have a positive effect on the

reformation process. Potential benefits resulting from acoustic enhancement of steam

reforming as shown by this study are an increased reactor capacity for a given size and

mass, smoothing of the temperature profile, better temperature control of the catalyst bed,

and increased catalyst life. It is expected that for different fuels and/or reforming

methods, similar results would be obtained for comparable process constraints.

Conclusions and recommendations for further study and application are drawn from the

presented results.

CHAPTER 1
INTRODUCTION

This chapter gives the background for the present study and is an introduction to

fuel cells, including a brief historical sketch of the technology. The different types of fuel

cells are outlined and the main difficulties in applying fuel cells for transportation are

mentioned. Fueling options for fuel cell vehicles are discussed and the difficulties in

reforming liquid fuels are presented. This background allows a formulation of the

problem definition and a statement of the study's objective.

Background

Because of high energy conversion efficiencies and low emissions, fuel cell power

plants (or "engines") are currently receiving a high degree of interest as potential

replacements for the internal combustion engines in transportation applications.

Fuel Cells

The fuel cell was first demonstrated in 1839 by Sir William Grove. However, the

majority of practical work on fuel cells did not happen until Francis Bacon demonstrated

a 5 kW working system in 1959. NASA became interested in the technology, and fuel

cells were further developed under General Electric and United Technologies Corporation

for the Gemini and Apollo space flights. The Arab oil embargo in 1973 stimulated the

United States Department of Energy (DOE) to look for ways to conserve

petroleum-derived fuels and utilize fuel cells in terrestrial applications. A major

breakthrough in fuel cell technology came in the 1980s when Los Alamos National

1

2
Laboratory and Texas A&M University found a way to decrease the platinum loading for
certain types of fuel cells (Norbeck, 1996). Since that time vehicle manufacturers have

been looking to the fuel cell power plant as a way to comply with ever-increasing

emissions standards, yet keep the high level of performance expected with the mature

internal combustion engine.

The fuel cell is an electrochemical cell that uses an oxidation reaction to generate

electricity directly from a fuel and an oxidizer. Figure 1.1 shows how an acid or polymer

electrolyte fuel cell operates. Other fuel cell types operate in a comparable manner yet

these other types of fuel cells can use conductive oxide, alkali, or molten carbonate as

electrolytes. For the acid or polymer type fuel cells as shown in Figure 1.1, hydrogen

fuel is brought to the anode of the fuel cell. At the anode, typically a platinum-type

catalyst splits the hydrogen into protons and electrons. The protons are free to migrate

H2

e-
Anode 2H

Electrolyte H+ ,o
e-
t+02+4e 2HO "

A
V

02 H20
Figure 1.1: Simplified fuel cell schematic.

across the electrolyte and combine with oxygen at the cathode to form water. To

complete this reaction, the electrons that are left at the anode must be brought to the

reaction site at the cathode. A conductive path is made available through a load and the

electrons are passed through the circuit. The end products are water, heat, and direct

current electrical power. Because a single fuel cell only generates about 0.75 volts,

practical applications arrange multiple fuel cells in series to form a fuel cell stack that

yields usable voltages.

Several different types of fuel cells exist. The different types of fuel cells are

normally distinguished by their electrolyte. Table 1.1 (adapted from Norbeck, 1996) lists

several types of fuel cells along with their operating temperature, typical application, and

chemical tolerance to carbon monoxide and carbon dioxide.

Table 1.1: Types of fuel cells and their operating parameters.

Fuel cell Typical stack
operating Typical applications CO tolerance o
type temperature tolerant?
temperature C
Solid oxide Steam cogenerating power
(SOFC) 1000 plants Good Yes

Molten
olten Steam cogenerating power
carbonate
car e 600 plants Good Yes
(MCFC)

Phosphoric Smaller stationary power
Phosphoric
acid supplies
AF 150-205 Heavy transport (buses, Fair Yes
trucks, ships, trains)

Solid Stationary and mobile power
r supplies
(PEM) 25-120 Heavy transport, light Poor Yes
vehicles

Space Vehicles, submarines,
Alkali 65-220 specialty vehicles carrying Poor No
hydrogen and oxidi/er

Vehicle Applications

Transportation applications place demands on the power source that currently rule

out certain types of fuel cells. Some of the more significant demands on the fuel cell

system for these applications are the ability to use air as the oxidizer, and quick start-up

and shutdown periods. Because of the development of the Apollo and Space Shuttle

Programs, alkali-type fuel cells have been thoroughly studied, produced, and successfully

operated for over 40 years. These fuel cells have provided astronauts with electrical

power, heat and drinking water. Excellent as they may be for such space vehicles

carrying hydrogen and oxidizer, these types of fuel cells are of little use in terrestrial

applications because the Alkali-type fuel cells are not tolerant of the low levels of carbon

dioxide in the air or in reformed liquid fuels. Solid Oxide and Molten Carbonate type

cells are excellent in terms of carbon dioxide and carbon monoxide tolerance; however,

their high temperatures and associated long start-up times currently prohibit their use in

automotive applications. The lower-temperature fuel cells that are carbon dioxide

tolerant such as the Polymer Electrolyte Membrane (PEM) and Phosphoric Acid Fuel

Cells (PAFC) are more suitable for transportation applications. PEM (then referred to as

solid polymer electrolyte or SPE) fuel cells were first developed in the 1960s for NASA

and were used in the Gemini Program (Prater, 1990). These fuel cells were not further

developed for NASA as alkali-type cells were used in later space missions. PEM fuel

cells can have high power densities because of the thin polymer electrolyte and thus have

an advantage over the bulkier PAFCs for terrestrial applications. On the other hand,

PAFCs which operate at a higher temperature have a better tolerance for carbon

monoxide in the fuel stream. Both of these cell types (indeed all fuel cells, except for

5

direct methanol) best utilize pure hydrogen as the fuel. As it has little infrastructure, this

use of hydrogen is a significant drawback to widespread implementation of hydrogen-

fueled fuel cell vehicles. As shown in Figure 1.2 (Photograph by Tom LaRocque and

.. .. .. .

f .. l

Figure 1.2: Hondas' hydrogen-fueled experimental fuel cell vehicle.

NREL/DOE), this limited availability has not prevented development and demonstration

of hydrogen prototype fuel cell vehicles by most auto manufacturers. Auto manufacturers

see fuel cell powerplants as perhaps the only feasible technology that can match the

requirements of both consumers and increasingly stringent emission regulations while

reducing dependence on petroleum and reducing the generation of CO,.

Fueling Options

In a fuel cell chicle that uses hydrogen as a primary fuel, there are no chicle

emissions as the only end products are water, heat, and electrical power. As hydrogen is

also the most abundant element in the known universe, it may seem that hydrogen would

be the ideal primary fuel for a fuel cell vehicle. Unfortunately, virtually all of the

hydrogen on earth is bound up in compounds such as water that are at a low energy state.

Because there are no direct hydrogen sources available, with the exception of small

quantities of hydrogen found with some natural gas deposits, hydrogen fuel must be

derived from other sources. Although there are some projects studying and

demonstrating renewable hydrogen production (Erickson and Goswami, 2001), the

majority of hydrogen production currently utilizes fossil fuels, primarily natural gas, for

the feedstock. The production of hydrogen from fossil fuels creates emissions where the

hydrogen or the energy to produce the hydrogen is generated. For a hydrogen fuel cell

vehicle there are no vehicle emissions but the production of the hydrogen fuel creates

emissions upstream for the vehicle. While there are places such as the Los Angeles

Basin, where transferring pollution outside of the area would be beneficial, the overall

efficiency is changed only slightly by production of hydrogen from a hydrocarbon fuel

on-board the vehicle (Louis, 2001). Adamson and Pearson (2000) have shown that the

"well-to-wheel" efficiency and emissions for fossil-derived hydrogen-fueled fuel cell

vehicles over the entire use-phase life-cycle are only slightly better than that of fossil-

derived methanol-fueled fuel cell vehicles over the same life cycle.

With these similarities in the overall energy conversion understood, one must

consider which fuel would be a wise choice for use in fuel cell vehicles. Table 1.2

(Adapted from Ogden and Nitsch, 1993; and Turns, 1996) shows the energy densities,

based on the lower heating value available from various fuels. These energy densities

demonstrate a practical area where hydrogen fuel is clearly not the best choice. While

Table 1.2: Energy densities of fuels (LHV).
Fuel Volumetric energy Mass energy density
density (MJ/liter) (MJ/kg)

Diesel 33.3 44.5

Gasoline 32.3 43.0

Liquified propane (690 kPa) 23.6 46.3

Methane gas 5.51 50.0
(16.5 MPa)

Liquified methane 2. .
(-161 C)21.5 39.1
(-161C)
Methanol 15.9 19.9

Ethanol 21.6 27.2

Hydrogen gas (16.5 MPa) 1.76 120.2

Liquified hydrogen (-253 C) 8.4 120.2

Metal hydride 4.9 1.80

hydrogen gas has the highest specific energy (mass energy density) of all practical fuels,

it has the lowest volumetric energy density. The volumetric energy density of hydrogen is

still paltry compared to liquid hydrocarbon fuels even in cryogenic liquid form near

absolute zero temperature. This issue of poor volumetric energy density suggests

prohibitively large fuel tanks for hydrogen fuel cell vehicles to achieve current vehicle

ranges (-560 km, -350 miles). In terms of volumetric energy density, a more feasible

alternative is to use a liquid hydrocarbon fuel such as liquified methane, liquified

propane, methanol, ethanol, gasoline, or diesel.

As previously mentioned, an even more significant problem hindering the

introduction of direct hydrogen fuel cell vehicles is the lack of hydrogen fueling

infrastructure. The current United States fuel infrastructure has evolved for use of liquid

fuels, notably gasoline and diesel. Because of the infrastructure and energy density

issues, near-term true consumer (as opposed to limited fleet operation) fuel cell vehicles

will likely utilize a liquid hydrocarbon fuel. Although smaller environmental benefits are

projected for using these liquid fuels, rather than using hydrogen from a renewable

resource, the use of fossil-derived liquid hydrocarbon fuels can serve as a precursor to

enable the move toward a renewable energy economy.

Reforming

Liquid hydrocarbon fuels need to be decomposed or "reformed" into a hydrogen-

rich stream before they can be used by a Polymer Electrolyte Membrane (PEM)' or a

Phosphoric Acid Fuel Cell (PAFC). This can be accomplished by a steam reforming,

partial oxidation, or an autothermal reaction. For comparisons of the reforming methods,

see Ahmed and Krumpelt (2001), Brown (2001), and Docter and Lamm (1999). The on-

board reforming process further complicates the fuel cell vehicle and increases the cost,

weight, and under-hood volume requirements of the fuel cell engine. The reforming

process also penalizes the overall on-board energy conversion efficiency because of an

associated increase in entropy. Furthermore, the liquid-fueled vehicle may require

clean-up units to purify the reformate stream before hydrogen-rich mixture can be utilized

in the fuel cell. The reforming process also may further complicate the overall vehicle

system by requiring long start-up (Erickson et al., 2000) and shutdown (IHeck\x olf et al.,

2001) periods. Although liquid-fueled fuel cell vehicles are demonstrated, as shown in

Figure 1.3 (Photograph by Daniel Betts and the University of Florida Fuel Cell Research

There are some exceptions such as the direct methanol PEM fuel cell but it is
still in the laboratory research phase.

Figure 1.3: Methanol-fueled PAFC transit bus at the University of Florida.

and Training Laboratory), the efficient reforming of the liquid fuel utilizing practical on-

board reformers is considered one of the major technological hurdles delaying widespread

introduction of fuel cell vehicles (Erickson and Roan, 1999; Romney, 2001). Steam

reforming, where fuel and steam are externally heated and catalytically reacted is

currently the most widespread method of reforming a hydrocarbon fuel into a hN drogcn-

rich gas. Although steam reforming is widely used, typical steam reformers are not

designed for transportation application and thus they have large volume with

corresponding high mass, slow transient response (especially at start-up), and are often

characterized by ha\ ing hot and cold spots throughout (deWild and Verhaak, 2000).

Problem Definition

As introduced above, the problem of reforming liquid hydrocarbon fuels to

hydrogen is currently one of the major technological hurdles delaying the introduction of

liquid-fueled fuel cell vehicles. While it has been demonstrated that the endothermic

steam-reforming process can be accomplished in a catalytic converter (Idem and Bakhshi,

1994: Ledjeff-Hey et al., 1998: Peppley et al., 1997), the process has liabilities of weight,

size, and complexity, in addition to long start-up and transient response times (Ohl et al.,

1996). Because of these liabilities any enhancement scheme for the steam-reforming

process which could lead to reducing size, weight, or response time of the reformer

should be investigated.

Research Objectives

A steam-reformation process that utilizes acoustic fields in critical fluid paths is

proposed. This study develops a theoretical approach for the use of acoustic fields in

conjunction with steam-reforming of methanol and demonstrates that the resulting

acoustic enhancement of reforming is one approach that can reduce the above liabilities

of the steam-reforming process.

The objectives of the present study are to develop theory-based models for

acoustic enhancement of the reformation process, to experimentally demonstrate a steam-

reformation process under the influence of an acoustic field, and to quantify the effects of

acoustic fields on the steam-reforming process. This includes an analysis of acoustic

fields and investigation of the effect of acoustic fields with relation to the reactor output

parameters of temperature profile and controllability, power demand, fuel conversion,

and catalyst degradation. The objectives also include a demonstration of the overall

hydrogen power output relative to acoustic power input. In connection with the objective

to quantify the effect of the acoustic field on the reformation process, an empirical model

is developed that predicts the conversion for a given set of space velocity, and acoustic

11

pressure parameters. Fulfillment of the objectives provides understanding of how

acoustic fields can be used to enhance the reformation of hydrocarbon fuels, resulting in

smaller, more effective reformers, especially relevant for transportation applications.

CHAPTER 2
LITERATURE REVIEW AND THEORETICAL APPROACH

This chapter provides a theoretical basis and background for the strategy of using

acoustic waves as a means of enhancing performance in conjunction with the

hydrocarbon fuel steam-reforming process. As part of the theoretical approach, the

literature dealing with steam-reforming and acoustic enhancement of related processes is

reviewed and the potential contribution of the present study is identified.

Steam Reforming

Steam reforming is the conversion of a hydrocarbon and steam into hydrogen-rich

gas. The steam reforming process was developed in the 1920s to produce a mixture of

fuel gas containing carbon monoxide and hydrogen known as "syngas" or synthetic gas

(Cox and Williamson, 1977). Steam reforming can occur with a variety of hydrocarbon

fuels including methanol and is used extensively for the commercial production of

hydrogen from natural gas. Equation 2.1 shows the ideal steam-reforming reaction for

CH3OH+ H20-+ 3H2 + CO2 (2.1)

production of hydrogen and carbon dioxide from methanol and steam. The change in

enthalpy of the reaction is +49.0 kJ/mol. This net gain in enthalpy indicates that

methanol-steam reformation is endothermic. Practical product species typically include,

in addition to hydrogen and carbon dioxide, carbon monoxide, unreacted methanol and

steam. Significant methane concentrations and solid carbon formation have also been

noted under certain conditions. The reforming process typically occurs at a temperature of

12

200-3000C and in the presence of a catalyst (usually copper and zinc on an alumina

substrate) that aids the reaction kinetics and shifts selectivity to carbon dioxide. Excess

water (1.3:1 -1.5:1 steam:carbon ratio) is used to inhibit formation of solid carbon (Cheng

et al., 1999; Amphlett et al., 1993). Models have been developed for methanol steam

reforming processors (Asprey et al., 1999) and reaction mechanisms have been proposed

(Amphlett et al., 1988). It has been proposed that a simplified model of the methanol

steam reforming process can be seen as two series reactions which are the endothermic

decomposition of methanol, followed by the exothermic water-gas shift reaction. These

steps are shown in Equations 2.2 and 2.3 respectively. The decomposition of methanol is

thought to be the rate-limiting step as its rate is approximately one order of magnitude

less than the water-gas shift reaction (Pour et al., 1975).

CH3OH o CO+ 2H2 (2.2)

CO+ H20 CO, + H, (2.3)

With its high hydrogen yield from a liquid fuel, the methanol-steam reforming

reaction is being studied as a possible method to provide hydrogen for fuel cell vehicles.

Methanol-steam reforming has been shown to successfully produce hydrogen for \vehicle

applications (Kaufman, 1992), yet the process is hindered in transportation applications

by characteristics of slow transient response (Dusterwald, 1997), temperature gradients

(Wild and Verhaak, 2000), and large volume and mass (Ohl et al.,1996).

Limiting Steps in the Reformation Process

For fuel cell vehicle applications, size, weight, and transient response are vital

factors. In order to design an optimal reformer for fuel cell vehicles one would desire the

smallest possible, lightweight reformer that would have enough catal st surface area to

react the fuel into a hydrogen-rich stream at the maximum design flow at the end of the

useful lifetime. Steam-reforming of hydrocarbons can be limited by heat transfer rates,

diffusion or mass transport, chemical kinetics and/or by degradation of the catalyst with

time. These limitations are evidenced by restrictions in reformer capacity, slow transient

response and/or limited lifetime.

Because of its endothermic nature, the reformation of liquid hydrocarbon fuels

requires heat transfer to the reactants and the catalyst. In a steam reformer heat comes

from an external source and is transferred through the catalyst bed housing wall and into

the catalyst bed as shown in Figure 2.1. Because of this endothermic nature and irregular

Methanol-Steam

Heat Heat

Hydrogen
Rich Product
Gas

Figure 2.1: Simplified representation of a packed
bed methanol-steam reformer.

15

flow patterns associated with packed bed catalysts, the typical steam reformer is plagued

with temperature gradients. Typically high temperatures are found nearest to the heating

surface, and low temperatures are found toward the midpoint of the catalyst bed.

Furthermore, if the heat transfer rate to the midpoint of the catalyst bed is not high

enough, unreacted species may pass through the reactor at this location resulting in a

limited reformer capacity. Additionally the catalyst may be damaged by condensing

species where heat transfer is insufficient. Heat transfer can take place as convection,

conduction, or radiation. Radiation is typically negligible, as the local temperature

difference is relatively small. Heat conducts through the catalyst bed housing wall and

possibly into the first layer of catalyst particles coming into contact with the wall.

Because of point-to-point contact between the catalyst pellets, conduction from catalyst

pellet to catalyst pellet is minimal, and the bulk of the heat transfer to the center of the

packed bed is via convection through the fluid inside the reactor. As the explained

temperature gradient manifests, heat transfer via convection is typically the limiting

factor. Heat transfer limitations can be compensated for by an increased size of the

reformer to increase the heat transfer area (Ohl et al., 1996). Such design results in

undesirable large processors and corresponding high mass. The large size and thermal

mass then penalize the process for transportation applications.

Diffusion can also limit the reformation process. The processes of transport

within the catalyst bed and conversion to a hydrogen-rich gas is represented in Figure 2.2.

As this figure shows, the hydrocarbon and steam mixture must first be transported to the

surface of the catal st. Next the mixture must diffuse through the pores to an open

catalyst site to react. The reaction rate on the catalyst is then governed by chemical

Reactants Products

Step 1 Step 7

Step 1- Reactants diffuse through bulk
stream to catalyst surface
Step 2- Reactants diffuse through
catalyst pores to open reaction site
Step 3- Reactants absorb onto catalyst Step 6
Step 4- Reaction time governed by Step 24
chemical kinetics on the catalyst
Step 5-Products adsorb from catalyst
Step 6-Products diffuse through catalyst
pores to surface
Step 7- Products diffuse from surface to
bulk stream

Sleps 3-5

Figure 2.2: Simplified representation of the reactants moving to the open catalyst site
and products moving out to the bulk stream.

kinetics. Products of the reaction must then diffuse back out of the catalyst pores and into

the bulk flow stream. Increasing the amount of catalyst sites available can compensate for

limitations in diffusion. Normally this means increasing the amount of catalyst to provide

sufficient reaction area and open reaction sites closer to the surface of the catalyst,

allowing for a decreased diffusion distance. This typically requires an increase in the

reactor volume or conversely, a decrease of mass flow rate of the reactants. This is a

decrease in what is known as "space velocity" as shown in Equation 2.4. This decrease of

space velocity once again results in larger reforming processors or a more limited

processing capacity for a set reformer volume.

Volumetric Flow Rate (.
Space Velocity = (2.4)
Volume of Reactor

Slow chemical kinetics can usually be alleviated by increasing the temperature.

Because temperature gradients exist within the catalyst bed, temperature cannot be

increased at the midpoint of the catalyst bed without possibly damaging the catalyst near

the wall surfaces by overheating. Thus, restriction by chemical kinetics is normally a

manifestation of convective heat transfer limitations in the center portion of the catalyst

bed.

In methanol-steam reformation, performance decreases as the catalyst ages. The

lifetime requirements of transportation applications require the reforming processor to be

over-designed to order to compensate for performance degradation. Once again this

overdesign results in larger processors and increases under-hood volume requirements in

transportation applications. Catalyst lifetime can be affected by poisoning, fouling, or

sintering of the catalyst. Deactivation due to poisoning occurs with chemisorption of

impurities on reaction sites (Farrauto and Bartholomew, 1997). In reduced copper-zinc

catalysts, oxygen is a poison as the metals are changed to an oxidized state and thus are

unavailable for the steam reforming process. Fouling is the physical blockage of reaction

sites and can take place when fluid phase molecules contact the catalyst or as solid

material (e.g., carbon) collects on the surface (Sterchi, 2001). In pelletized catalysts

%where the interior area is dominant, fouling at the catalyst surface or pores can

significantly change the available catalyst area. Sintering or a change in catalyst structure

normally occurs with thermal stress of the substrate material. The change in structure

reduces the effective area of the catalyst. In order to avoid sintering. o\ erheating. rapid

thermal cycling, and thermal gradients should be minimized.

It is proposed that some of these limiting steps in the methanol-steam reformation

process may be minimized by imposing an acoustic field in the critical flow paths. The

proposed benefits due to this acoustic field are expected as a result of increased heat

transfer and mass transfer rates.

Acoustics in Past Processes

In several past studies (see overview by White, 1991) an acoustic wave

superimposed on a flow field has been shown to change the characteristics of the flow

field. Periodic plane waves in a wave guide can be represented by their pressure (p) and

velocity (u) components as shown in Equations 2.5 and 2.6 (Kinsler et al., 1982). In these

equations x is position, t is time, j is the imaginary constant, po is the non-oscillating

( 2nf 2nf
p=Ael c +Be c(2.5)
p- Ae+ 2ft2xf2

A j(2nt- 2fx) B (it 2+2-f x)
u -- e c + -- e (2.6)
Poc poc

density and A and B are found from the initial and boundary conditions. A general

representation of an acoustic wave at a frequency (f) in combination with a steady flow

component within a tube is shown in Figure 2.3. In this figure the acoustic pressure

(shown as the shading variation) drives the acoustic velocity. When the acoustic velocity

component (u') is larger than a left-to-right steady flow component (U), the motion of the

fluid periodically changes direction depending on space and time. The use of resonance

can allow for high acoustic pressures and velocities to build with the resulting standing

Steady Flow (U) with a Superimposed Acoustic Wave
Instananeous Velocity U+U U-U

Motion direction for
< .U t=0

Acoustic Velocity
Amplitude (U) Trace

$ Motion direction for
t= 1/2f U

Instananeous Velocity U-U U+U

Acoustic Pressure
Maximum ; Minimum

Figure 2.3: Flow fields at different times for a plane wave superimposed onto a steady
flow field.

waves, yielding a periodic change of motion although now dependant on the spatial

position associated with the specific mode of the standing wave.

In the development of fluid mechanics it has been noted that acoustic waves can

influence several parameters of the flow field. These parameters include the boundary

layer and flow characterization. Stokes' 2nd problem solved for the velocity profile near a

flat plate with an oscillating stream (see discussion in White, 1991). The solution of this

problem allows calculation of the laminar boundary layer thickness in this oscillating

flowfield. The boundary layer is represented by Equation 2.7, where 6 is the boundary

S= 6.5 (2.7)
V21Lt

layer thickness, v is the kinematic viscosity of the fluid, and f is the frequency of the

wave oscillation. The acoustic field may also change the flow regime as discussed by

White (1991). The kinetic Reynolds number (Rek), which is the Reynolds number for

oscillating flowfields is shown in Equation 2.8 for pipe flow. In this equation, ro is the

radius of the pipe or a characteristic length. Like the standard Reynolds number the flow

may become turbulent when this value approaches 2000. Turbulent flow regimes are

2nfx r'(
Rek (2.8)
V
characterized by chaotic motion of the fluid resulting in increased mixing, and for pipe

flow a flattened velocity profile as opposed to a parabolic profile for a fully developed

laminar regime.

The acoustically thinned boundary layer in laminar flows as well as a jump to the

turbulent flow regime, due to the employment of an acoustic field has direct positive

implications on heat transfer (Fraenkel et al., 1998). In combustion studies acoustic wave

amplitude has been correlated to heat transfer (Kwon and Lee, 1985), temperature

(Crocco, 1969), emissions (Keller and Hongo, 1990), particle dispersion (Barrere and

Williams, 1969), mixing of species (Dubey et al., 1996), and flame structure (McManus

et al., 1990). An example of the effect of acoustics on flame structure is shown in Figure

2.4 (Photographs by author). These photographs of propane diffusion flames show

marked changes in the flow field due to acoustic oscillation. Enhanced mixing is

demonstrated by the compacted flame zone for the case with the oscillating flow field. A

change of flow field direction is manifested by the flame zone being moved downward

below the burner arms. Some of these changes seen in past processes can be expected

with implementation of an acoustic field in the steam-reforming process.

(A)

(B)

Figure 2.4: A 15 kW diffusion propane flame without
oscillating flow (A) and with oscillating flow (B).

Steam-Reforming in Combination with Acoustics

Superimposing an oscillating flow in the axial direction of the reformer (also

referred to as organ pipe oscillations) with the steady flow component is proposed

(Erickson and Roan, 2000). High acoustic pressures and corresponding local particle

____

velocity perturbations are achieved by establishing resonance in the reactor. If the

acoustic velocity component is high enough, the steady flow component carries the

overall flow forward and the oscillating flow portion incrementally steps the fluid

forward and backward across portions of the catalyst bed. This allows for a "mixing" of

the boundary layer along the reactor walls and past the catalyst particles. Under the

proper conditions, increased heat and mass transfer rates also occur as a result of the

thinned boundary layers and acoustic-induced mixing in the reactor. With increased heat

and mass transfer, temperature gradients are also reduced throughout the acoustically

enhanced reactor. The projected effects of acoustics on the limiting steps of heat transfer,

mass transfer and chemical kinetics of the overall process are discussed as potential

theory-based models are developed below.

Increased Particle Path Length due to Acoustics

A simplified model of the acoustics shows a significant increase in the particle

path length, resulting in a decreased effective space velocity, without changing the overall

bulk residence time. In order to illustrate this effect, consider an empty open-ended pipe

(without obstruction from the catalyst). Acoustic displacement amplitude is given in

Equation 2.9 (Halliday and Resnick, 1986), where Ym is the maximum acoustic

P
Y (2.9)
2 2nfpc

displacement amplitude (m), Pm is the maximum acoustic pressure (Pa), f is frequency

(Hz), p is the non-oscillating density of the gas (kg/m3), and c is the sonic velocity in the

gas (m/sec). The equation for maximum particle velocity due to acoustic oscillation (U,)

(Kinsler et al., 1982) is shown in Equation 2.10. This particle velocity is subsequently

Um = (2.10)
pc
referred to as acoustic velocity. In order to derive the total path length seen by a particle

traveling through the reactor (effective particle path length), one must consider the length

of the reactor (original particle path length with no acoustic oscillation) and distance

traveled by a particle due to acoustic oscillation.

A simulation of the particle path can be made by investigating the velocity

components during the residence time. The slip velocity, or velocity relative to a

stationary catalyst pellet or wall is the sum of the oscillating velocity component Um

(multiplied by a spatial factor (Sf) resulting from the mode shape of the standing wave)

and average velocity (Vs). The spatial factor is assumed to follow a cycle sine wave

pattern within the reactor. The absolute value of the total velocity multiplied by the

residence time yields the slip displacement (Xsii,) past the catalyst or wall. If the time is

decreased to a small time step (At), then summed over the residence time (t,) the slip

displacement can be represented by Equation 2.11. The slip displacement can be

nondimensionalized by dividing by the reactor length. Plotting this dimensionless slip

displacement versus the velocity ratio (Um/Vs) Equation 2.11 yields the total theoretical

slip displacement in Figure 2.5

Xsp = I(SU,m sin2ntft + U,) At (2.11)

This figure shows that an slope asymptote of 0.407 is dee eloped at large velocity

ratios. This limiting slope can be developed through investigation of the displacement

due to the acoustic field within the reactor during the residence time. As before,

assuming that a standing wave pattern equivalent to a single cycle sine pattern for

velocity is developed in the reactor, a spatial average displacement amplitude of 0.64 of

9
S8
S 7
rrI 6
) 5
E Total Theoretical
004
S4 Slip Displacement
3
SQ 2 Slip Displacement
S 1 due to Acoustics
0
0 5 10 15 20
Velocity Ratio (Um/Vs)

Figure 2.5: Total slip displacement and slip displacement due to acoustics for increasing
velocity ratio.

the maximum amplitude will occur. The equation showing the displacement of the

oscillating portion while residing in the reactor is shown as Equation 2.12 and further

simplified in Equation 2.13, where Xs,,p is the slip displacement (m), L is the length of the

P L
Xsi = (4) x x 0.64 x f
21tfp oc V

U L
X,,p 0.407 x
V,

(2.12)

(2.13)

reactor (m), and Vs is the steady flow component of the reactor (m/s). One can see that

the slip displacement is independent of frequency in this model. This results from the

fact that while an increased frequency does cause more oscillations during the residence

time the amplitude of those oscillations is decreased proportionally for a given acoustic

pressure.

As Figure 2.5 indicates, the model predicts large increases in particle path length

for decreasing average velocity or increasing particle acoustic velocity by higher acoustic

pressure levels. For a reasonable example, a sound pressure level of 165 dB (re 20pPa) in

air and a steady flow component of 1 m/s yields a particle path length of 7 times the

particle path length with non-oscillating flow. If employed in a reforming process the

effective space velocity would be 7 times smaller than the actual space velocity or the

space velocity without acoustics. This effect is diminished with the longer flow path and

wave attenuation due to obstruction from catalyst pellets. Also because of the fact that

the catalyst is not continuous throughout the catalyst bed these results must be scaled by

the catalyst packing factor and a factor accounting for the effective area of the catalyst

bed seen by the acoustic wave (ie. Exterior of the catalyst pellet). However this model

yielding the effective space velocity gives an upper boundary of the expected benefits due

to an acoustic field in conjunction with methanol-steam reformation in an other\\ ise

undisturbed and unobstructed (without catalyst pellets) flow field.

Increased Heat Transfer Rate due to Acoustics

A more realistic expectation may be that of modeling an increase in capacity due

to increased heat transfer in the reformer. As presented above, heat transfer rate is seen as

a fundamental resistive step in the fuel reformation process. Recent papers have

addressed this issue (Ohl et al., 1996; Daister\nald et al., 1997; Nagano et al., 2001) and

certain methods of increasing heat transfer rate have been reported, including the use of

fins to increase the heat transfer area (Shiizaki et al., 1999). However, increasing heat

transfer rates with acoustic fields for other applications has been well documented over

the years (Fraenkel et al., 1998: Erickson et al., 1997: Zinn, 1992: Yavuzkurt et al., 1991)

Space and time averaged Nusselt numbers have been shown by Ha and Yavuzkurt

(1993), for a single spherical particle in a pulsating flow with a steady flow component, to

be represented by Equations 2.14 and 2.15. In these equations, V, is velocity ratio

Nu 0570
for V, > 1 =0.73839V, +0.16161 (2.15)
Nu

Nu
for V < 1 = 1+0.009608 V, -0.109608 V2 (2.14)
Nu,

(previously defined as acoustic velocity U, divided by average velocity Vj), Nu, is the

Nusselt number due to pulsating flow, and Nus is the steady flow Nusselt number. Nusselt

number is a direct indicator of convective heat transfer rate as shown in Equation 2.16,

where h is the convective heat transfer rate coefficient, k is the thermal conductivity of

hL
Nu =hL (2.16)
k

the gas, and L is a characteristic length. Figure 2.6 is a plotted representation of

Equations 2.14 and 2.15. While interactions between particles in a fixed bed reactor are

m 7
z

0 5
4_
3 .
E
Z 2

Z 0
0 10 20 30 40 50
Acoustic Velocity IAverage Velocity

Figure 2.6: Nusselt number ratio versus velocity ratio for a single particle in an
empty duct.

clearly much more complex than the single particle example, Figure 2.6 shows the

potential for increasing convective heat transfer rates with acoustic fields. For the

previous example of a SPL of 165 dB and a 1 m/s steady or average velocity component,

the convective heat transfer rate is increased by a factor of 4 by using an acoustic wave.

Fraenkel et al. (1998) have found similar results for velocity ratios above I when flow

oscillations are used in grain drying.

The increase in heat transfer rate, by definition, allows a faster response to

transients including load following and start-up. Mass flow capacity can also been seen

to increase by referring to a simplified energy balance within the catalyst bed. From an

energy balance in the reformer (taking into account convective heat transfer from the

walls to the gas in the catalyst bed), allowable reactant mass flow rate is directly

proportional to the heat transfer rate from the hot wall to the gas as shown in Equation

2.17 where rh is mass flow rate, h is the convective heat transfer rate coefficient, A is the

hA(T )
m = l oca' (2.17)
AH

heat transfer area, T is temperature, and AH is the change in enthalpy between reactants

and products. This relation applies for heat transfer limited cases that are not limited by

diffusion or chemical kinetic rates.

Increased Overall Reaction Rate due to Acoustics

With a higher heat transfer rate, the rate limiting steps of diffusion and chemical

kinetics become more important to the reformation process. Although mass transfer rate

has not been a perceived limitation in previous work (Asprey et al.. 1999: Pcppley et al.,

1997), smaller reactors with high flow rates will deviate from the plug flow reactor

assumption. As described and quantified by Ha and Yazavkurt (1993), acoustic mixing

increases the mass flow rate at the surface of a particle; thus in a reformer, acoustics

would reduce the diffusion time of a mixture to reach the outside of a catalyst. Equations

2.14 and 2.15 apply to the Sherwood number ratio as well as to the Nusselt number ratio

as presented. The Sherwood number (Sh), as shown in Equation 2.18 is analogous to the

hmL
Sh = (2.18)
DAB

Nusselt number in relation to convective mass transfer rate. In Equation 2.18 hm is the

convective mass transfer rate coefficient, DAB is the diffusivity of gas A into the gas B,

and L is a characteristic length. This increase in convective mass transfer should

increase the diffusion of both the reactants to the catalyst particle surface and the products

to the bulk stream. Internal diffusion or mass transfer into the core catalyst area is not

expected to be significantly affected by the acoustic waves due to extremely small

dimensions and velocities, thus overall diffusion rate enhancement by acoustics may be

limited. If the internal diffusion is not the limiting factor, but rather the process is limited

by external diffusion, acoustics may have a significant effect.

While increased mixing from acoustics may decrease a rate limiting diffusion step

to a catalyst surface, there is probability that the chemical kinetics will be increased as

well. Considering Arrhenius type rate equations, reactions will proceed more quickly for

higher temperatures. With the reduction of local temperature gradients due to acoustically

induced mixing and increased heat transfer, the reactor controlled by local temperature

feedback (as discussed by Sterchi, 2001 and Amphlett et al., 1999) would have higher

temperatures near the entrance of the reactor and thus higher chemical kinetic rates would

result. Because of the rate limiting step of methanol decomposition occurs toward the

entrance of the catalyst bed, minimization of the temperature gradient at this point is vital.

Additionally, Tonkovich et al. (1999) have found that for the water gas shift reaction.

intrinsic reaction times can be on the order of milliseconds rather than the observed

reaction times of 6-9 seconds. They exploit the fast intrinsic reaction times by using

micro-channel reactors where fast heat and mass transfer rates govern. In a similar

approach, acoustic vibrations should increase heat and mass transfer rates at the exposed

catalyst surface and thus may decrease the required contact time on the catalyst surface.

Although it is obvious that not all of the area in a catalyst would be affected by the

acoustic wave (due to internal pores etc.), bounds of the reaction rate can be established.

The lower bound would be the case where the reaction rate is limited only by internal

diffusion rate and thus the overall reaction rate would not change when acoustics are

applied. The upper bound would be where the rate is wholly limited by diffusion rates at

the surface and the total effective area (derived from the effective length) would be used

in the reaction rate models. It is expected that with catalyst pellets, internal area and

diffusion to those sites should dominate, thus deviating from the upper boundary

predicted by use of the effective area. With screen or monolith type catalysts, where

acoustic waves would have more of an effect on the active reaction sites, the actual rate

may follow predicted rate using the effective area (derived from effective length in Eq.

2.11) more closely.

IExension of Catal st .ife by Using Acoustics

In some aspects, catalyst life is also expected to be positively affected by using an

acoustic field in the reformation process. Acoustics may hinder catalyst degradation due

to poisoning, fouling and sintering. While all types of poisoning may not be affected by

the acoustic wave, poisoning by oxygen with reduced catalysts could be mitigated by the

oscillating flow under the proper conditions. With an oscillating flow, hydrogen

produced in the reformer would be recirculated to the upstream catalyst particles,

effectively re-reducing any oxidized catalyst. This recirculating characteristic of the flow

field would happen at velocity ratios (V,) above 1. Fouling is also a degradation

mechanism that would theoretically be minimized by employing an acoustic field. Much

like the oscillations of a sonic cleaning application, clogging at the surface reaction sites

and catalyst pores could be reduced by the oscillating action of the passing fluid.

Reduction of certain types of sintering may also occur as spatial temperature gradients

throughout the reactor are alleviated and temperature fluctuations are minimized by

increased control.

There also may be a mechanism for catalyst degradation due to acoustic fields.

Theoretically, oscillating pressure fields may break apart the catalyst structure or drive

particles into the pores of the catalyst increasing the fouling mechanism or creating the

effects of catalyst sintering.

Summary

In theory, the possible benefits which arise from employing acoustic waves in the

reforming process are an increase in capacity for a given catalyst mass, a decrease in

transient response time, an increase in the overall efficiency of the reformation process,

and potential extension of catalyst life. The benefits of using acoustics in related

processes including combustion reactors have been shown in the literature as increased

convective heat transfer rates, increased mass transfer rates, and increased specie mixing.

While the acoustic field may not significantly affect diffusion and chemical kinetics for

all catalysts, especially those with high amounts of internal area not affected by the

acoustic wave, the theoretical investigation points toward potential for enhancing

reformation processes with acoustic waves.

Contribution

The present study contributes to the technical field in that it is both a theoretical

development as presented above and an empirical quantification of the benefits that arise

from using an acoustic field in conjunction with the steam-reforming of a hydrocarbon

fuel. As shown by the literature review, a properly applied acoustic field can benefit other

processes. The acoustic field in the reformation process results with similar enhancement

and is expected to allow smaller size and lower weight of the reforming reactor, which in

turn would yield quicker start-up times and faster dynamic response. The lifetime of the

catalyst is expected to be extended by utilizing acoustic waves. The present investigation

quantifies these benefits and may contribute towards a practical method of reforming a

liquid fuel with high efficiency for fuel cell vehicles.

CHAPTER 3
EXPERIMENTAL APPROACH AND FACILITY

This chapter discusses the experimental approach and facility. A description of

the methanol steam reforming facility is presented. Various subassemblies of the facility,

as well as instrument and control interfaces are described. This includes discussion of the

acoustic field generator and schemes for controlling the temperature and acoustic field.

The instrumentation is reviewed along with the methods used for collecting and analyzing

data. Modeling of the reformation process is discussed, which discussion includes the set

up of a factorial experimental design.

Experimental Approach

The experimental approach was designed to quantify the effect of the acoustic

field on the reformation process. This included adaptation of a reformer facility to accept

acoustic waves, development of procedures for operation of the facility, establishment of

temperature and acoustic control in the catalyst bed, the acoustical analysis of that

facility, and the design of an experiment from which, a statistically valid empirical model

could be developed. Further data were taken to show curvature of the model, verify the

model results, demonstrate the effect of acoustics on catalyst degradation, power demand.

as well as on the reactor temperature profile and controllability. Analysis of the data

enabled construction of a hydrogen power output per acoustic power input ratio which

quantifies the energy benefit/cost on the reformation process from the acoustic field.

Experimental Facility

A reforming facility was previously constructed at the University of Florida Fuel

Cell Research and Training Laboratory. This reforming facility, shown in Figure 3.1

(photograph by J. Patrick Sterchi) has been used in the past to study the effects of

Figure 3.1: Reforming facility at the University of Florida Fuel Cell Research and
Training Laboratory.

impurities in methanol on the steam-reforming process (Sterchi, 2001). The reforming

facility hardware consists of pumps. vapori/ers, superheaters, two electrically heated

reforming reactors, a condensing unit. a po\cer relay box, and thermal instrumentation in

connection with computer control for the power relay box and pumps. As the facility \was

originally designed to simultaneously operate two reforming reactors, the present study

utilized only one side of the total facility. Figure 3.2 is a simplified schematic of the

Water:Methanol
Premix Reservior

1-4
0

Liquid Pump
_Q -J

4- 0o
I +
C2O,
(-1

I I
o

'-

F. ^Ga
Samp
Poi

Electric
Heaters

Acoustic
Driver

1 ^-)

u

_ t.,

To
Exhaust

s
ling
rt

I

Condensing
Unit

Figure 3.2: Simplified schematic of the methanol steam reforming processor.

system used. In this system methanol:water premix is pumped through an electrically

heated vaporizer. The vaporized gas is routed through an electrically heated liquid trap or

secondary vaporizer. This liquid trap insures complete vaporization of the premix

entering the reformer at high flow rates. The gaseous premix then may be routed to the

exhaust or directed to the superheater by manipulation of the valves shown. If routed to

the electrically heated superheater the premix follows the path shown to the catalyst bed.

An oscillating pressure is directed through the catalyst bed from a nitrogen-purged and

I

35

water-cooled acoustic driver attached at the lower end of the catalyst bed. After traveling

through the electrically heated catalyst bed the reacted mixture can be routed through the

condensing unit or to the exhaust by manipulation of the valves shown. After passing

through the condensing unit the dry gas may be sampled through a gas sampling port as it

travels to the exhaust. A carbon dioxide purge unit attaches to the superheater assembly

to keep the superheater and catalyst bed free from oxygen during non-operating periods.

The overall system was divided into separate subassemblies. The methanol:water

premix reservoir, pump, and flow metering devices were grouped into the pumping

.%uba..sembhly. The vaporizer, liquid trap (or secondary vaporizer) and the superheater

comprise the vaporizer subassembly. The reactor subassembly consists of the catalyst bed

and acoustic components. These acoustic components are the acoustic driver, a nitrogen

purge, a water cooling system, a pressure transducer, and adapters. The condensing unit

and gas sample port make up the condenser subassembly. Computer control and logic for

both temperature and acoustic field regulation is explained in the control uha~vremhly.

Each subassembly is more fully described as a separate section below.

Pumping Subassembly

The pumping subassembly consists of a reservoir placed on a scale, a peristaltic

pump and associated tubing as shown in Figure 3.3. The scale and a timing device was

used to calculate the actual mass flow rate of premix fed into the reactor. The scale used

was an Acculab VI-4kg that had a resolution of 1 gram. The peristaltic pump was a

lasterflex L/S Model 7519-20 cartridge pump. Rough control of premix mass flow rate

%\as enabled by changing the volumetric flow setting on this peristaltic pump. The

volumetric flow setting could be set at increments of 0.1 ml/min. The actual mass flow

rate was found by measuring the mass change of the reservoir per unit time.

Premix
Reservior
Peristaltic
Pump
To
Vaporizer
Scale

Figure 3.3: Artistic representation of the pumping
subassembly.

Vaporizer Subassembly

The vaporizer subassembly consisted of a vaporizer and a liquid trap or secondary

vaporizer as shown in Figure 3.4, and the superheater shown in Figure 3.5. The vaporizer

was a 1.27 cm (0.5 in.) O. D. stainless steel tube cut to 85 cm (33.5 in.) in length. The

length of the tube was wrapped in a 244 cm (96 in.) 650 Watt heat tape. The liquid trap

or secondary vaporizer was also made of 1.27 cm (0.5 in.) O. D. stainless steel tubing

that was cut to 64 cm (25.2 in). This tube was wrapped in a 183 cm (72 in.) length 435

Watt heat tape. The temperatures at the pipe surfaces and the exits were monitored by

stainless steel sheathed k-type thermocouples. The superheater assembly as shown in

Figure 3.5 is a 2.68 cm O.D. (nominal 3/4" schedule 40) stainless steel pipe cut to a 38

cm (15 in.) length. Receptacles for fittings (7/8 in.-14) were machined into both ends of

the pipe. The pipe was wrapped in a 366 cm (144 in.) 430 Watt ceramic beaded heater

To Superheater
or Exhaust
A

1.27cm O.D.
Stainless Steel
Tubing
85 cm length

650W Heat Tape

Vaporize

r

1.27 cm O.D.
Stainless Steel
Tubing
64 cm length

435W Heat Tape

Liquid Trap or
Secondary
Vaporizer

Insulation

Insulation

A

From Pump

0.635 cm O.D.
Stainless Steel
Tubing

*Not to Scale

Figure 3.4: Artistic representation of the vapori/er and liquid trap.

and outer surface temperature was measured near the bottom of the assembly by a

sheathed k-type thermocouple. The exit of the pipe was made by allowing a 15.25 cm

Check Valve

CO, i -:

From Vaporizers

v

2.68 cm O.D. Stainless
Steel Pipe 38 cm length

430W Ceramic beaded
heater

Insulation

V
To Catalyst Bed

Recessed Outlet
0.635 O.D. Stainless
Steel Tubing
15.25cm length

Figure 3.5: Artistic representation of the superheater

(6 in.) long 0.635 cm (0.25 in.) O.D. stainless steel tube to be placed at the bottom of the

assembly recessing through the lower fitting into the superheater cavity. This recessed

outlet insured that no liquid phase fluid accumulating at the bottom of the superheater

would be allowed into the catalyst bed. The exit temperature was monitored at the exit of

*Not to Scale

Jib

the recessed outlet pipe by a stainless steel sheathed k-type thermocouple inside the

fitting directly above the catalyst bed.

Reactor Subassembly

The reactor subassembly consisted of the catalyst bed, and acoustic components.

These two sections are discussed individually below.

Catalyst bed

As shown in Figure 3.6, The catalyst bed was housed in a 2.68 cm O.D. (nominal

3/4 in. schedule 40) stainless steel pipe cut to a 63.5 cm (25 in.) length. Receptacles for

fittings were made at both ends of the pipe. The bed was filled with a pelletized low-

temperature water-gas shift catalyst manufactured by ICI Katalco (ICI catalyst 51-3). The

catalyst was designed for a temperature range of 200-3000C. This catalyst is a

proprietary combination of copper oxide, zinc oxide, and aluminum oxide and had to be

reduced before use. The reduction process for this catalyst takes place over 40 hours at

1800C with a 2.3% hydrogen, 97.7% nitrogen gas mixture, at a gas hourly space velocity

of 300 hr'. The 40 hour period is followed by an hour at 210C with a 6.5% volume

fraction of hydrogen. The reduction finishes with a final hour at 2100C with a 12 %

volume fraction of hydrogen. This reduction process is explained by Sterchi (2001). The

pelletized catalyst was dimensionally 0.52 cm in length and 0.54 cm in diameter and had

a bulk density of 1.2-1.3 kg/liter. In order to avoid blockage of the exit, the catalyst bed

was supported by a stainless steel screen fixed at the top of the adapter for the acoustic

generator. The catalyst bed was divided into 6 zones. At the end of each zone and before

the first zone a 0.93 cm diameter (1/8 in. NPT) port was machined into one or both sides

1.0 cm O.D. Stainless
Steel Pipe
12.7 cm length

Kistler 206 Piezoelectric
Pressure Transducer

Ak, ;

From Superheater

Y

Adapter

Water Cooling Jacket

Thermocouples
placed 9.525 cm
apart on center

250W Heating bands
(6)

Insulation

2.68 cm O.D. Stainless
Steel Pipe
63.5 cm length

(

*Not to Scale

To Condenser or Exhaust
To Condenser or Exhaust

Adapter for Acoustic Field
Generator

Figure 3.6: Artistic representation of the catalyst bed housing.

of the stainless steel pipe. Fittings could be placed into the machined holes to extract

temperature or pressure information at each respective location in the bed.

The fittings were sealed by a high temperature silicone sealant. Above the first

zone, two ports are used for monitoring dynamic pressure and monitoring the exit

jl

:;ii
i I

l.j

iti

1 f
i
I

temperature of the superheater. Each zone was individually heated by a 250 Watt band

heater. Control for these heaters was determined by the temperature at the respective

zone exit as explained in the control subassembly below. Temperature was monitored

using k-type stainless steel sheathed thermocouples. Each fitting was machined to be

flush with the interior surface of the pipe at the zone exits. The control thermocouples

were aligned as to be near the interior wall location in the fitting. These temperatures

represented the interior wall temperature at the specified axial location. At the bottom of

the pipe an adapter was threaded into the machined receptacle to allow an acoustic driver

to modulate the catalyst bed pressure.

Acoustic components

Transmission of acoustic pressure and associated velocity fluctuations are a

product of geometric conditions, and also the fluid properties. Additionally, transmission

is typically inhibited by obstruction in the acoustic path. In a packed catalyst bed, the

catalyst itself partially obstructs the propagation of acoustic waves. Notwithstanding this

difficulty, standing acoustic waves can build up within the bed as a result of wave

reflection at the ends of the reactor.

An acoustic field was generated by pulsating an acoustic driver attached to an

adapter at the bottom of the reactor as shown in Figure 3.7. The acoustic driver used was

a Klipsch Pro 1133 Horn Driver typically used in amphitheater applications. The method

of using loudspeakers to modulate pressure in a chemical reaction has been pro\ cn by

several researchers including McManus et al. (1990), Poinsot et al. (1989), Heckl (1988),

.ang et al. (1987), Zikikiout et al. (1986), and many others. The horn driver was attached

to mounting devices and directed into the catalyst bed as show n. As the horn driver was

*Not to Scale

,' Electrical
~) Connections

Nitrogen
Purge

Water Cooling \
Jacket
Adapter (7/8 in.-14
To Exhaust x 3/4 in. NPT Pressure
x 1/8 in. NPT) AcotEnclosure
Acoustic Driver
Mounting Plate
Figure 3.7: Artistic representation of the acoustic driver, driver housing, mounting plate
and fittings.

not designed to be used in relatively high temperature chemical applications, a water

cooling jacket was designed around a fitting to eliminate conduction from the catalyst bed

walls. This water-cooled section was also a heat sink for the reactor as it pulled heat

away from the bottom zone. A nitrogen purge was directed to both the front of the

diaphragm and the back of the driver housing. The nitrogen at the front of the diaphragm

was used to inhibit hot reacted species from flowing into the driver. The nitrogen purge

at the back of the housing was a safety precaution as there was possibility of electrical

ignition in the voice coils in the presence of hydrogen. The entire housing and fittings

were tilted upward at a 100 angle to allow any condensed species forming on the water

cooled section to flow down toward the exit of the reactor rather than condensing and

43

pooling in the driver or fittings. The driver housing and fittings were sealed with the same

silicone sealant as the port fittings in the high temperature zone.

The acoustic wave was monitored by a Kistler piezoelectric A206 pressure

transducer. This piezoelectric transducer used a Kistler piezotron coupler and was factory

calibrated at 15.3 mV/kPa. As previously shown in Figure 3.6, the pressure transducer

was mounted at the port above the first zone in the catalyst bed for most of the

experiments performed. This position was selected so that the acoustic wave would have

to travel through the entire length of the catalyst bed. The mounting took place through a

1.0 cm O.D. (1/8 in. schedule 40) stainless steel pipe cut to a 12.7 cm (5 in.) length and

an adapter to connect the pressure transducer. A water cooling jacket was constructed for

the mounting section but after monitoring temperatures at the pressure transducer it was

determined that water cooling was not necessary. Preliminary experimentation found that

water cooling at the transducer condensed liquid species directly above the catalyst bed.

Experiments performed for catalyst degradation required the elimination of this

condensation and possible intrusion thus the pressure transducer was removed for

experiments dealing with degradation. When the pressure transducer was absent in the

degradation experiments, acoustic driver feedback came from the voltage input to the

acoustic driver.

Condensing Unit Subassembly

A condensing unit was also used that cooled the exit gases from approximately

2500 C to 0 C. As shown in Figure 3.8, This unit consisted of a coiled 0.635 cm (0.25

in.) O. D. copper tubing through which, the exit gases flowed. The exterior of this copper

tubing was flooded with water from an ice bath. Reacted species would condense in the

TC
Refrigeration
Unit Dry Gas
Sampling Port

From
Catalyst To Exhaust
Bed
TC

0.635 cm O.D.
Copper Tubing

Water IceBath Condensate V
; Pump Trap

*Not to scale

Figure 3.8: Artistic representation of the condenser subassembly.

tubing and follow the flow path to a liquid trap where the liquid species would remain

and the dry product gases would be directed to the exhaust. The above condensing

components were housed in a refrigeration unit that decreased the heat losses of the ice

bath. Temperatures were monitored at the dry gas exit and at the interior of the

refrigeration unit with sheathed k-type thermocouples. A dry gas sampling port was

available as the dry gas products were directed to the exhaust.

Instrumentation and Control Subassembly

Control of the temperatures and acoustic pressure took place in the form of an

active closed loop and open loop schemes, respectively. The input and output signals for

these two control schemes are found in Figure 3.9. The instrumentation for these systems

were the output signals coming from the reactor. A description of the control schemes is

given in the following subsections.

Input Signal
Output Signal ..."".....

Thermocouple Signals (14)

V
Power
ADC/DAC .. Distribution
Box
.. ........... .

Computer 1 Computer 2

A A
V.
LabVIEW User LabVIEW Us
Interface Interface

Heating
Elements (9)

I-* |1|* II* II ** l -l -l^lIlll I ***

V

; ADC

er Waveform
Generator

ci
J^
0
0
O^

SP Acoustic
SDriver

0

Pressure
Transducer

................,,,' .Amplifier

Figure 3.9: Input and output signals for temperature and acoustical control.

Temperature control

The heating elements discussed previously were computer controlled by feedback

from the thermocouple outputs. The 14 thermocouple signals were processed using a

lotech DBK-52 Thermocouple module that was linked to a data acquisition and control

program that was built by the author in LabVIEW. This temperature control program can

be found in Appendix A. Control of the 9 heating elements took place through an lotech

DBK-24 Digital Output module linked to the same program. Depending on the

indi\ idual location and temperature history sensed by the respective thermocouples, the

output module switched individual relays that in turn switched 120 V to the heating

elements through a solid state rela\ as shown in the control circuit display ed in Figure

46

3.10. The relays were contained in a power distribution box exhibited in Figure 3.11. As

shown by the control logic diagram found in Figure 3.12, control occurred as the

thermocouple signals were compared to a setpoint temperature. The slope of the

temperature signals was also monitored and compared to predetermined set points as was

the duration of the on position for each heating element. The ability to override the

control was given to the user but this option was not used for any of the experiments

presented. The setpoint matrix used for the automatic control of the temperatures is

found in Table 3.1. The temperature control scheme with the matrix in Table 3.1 was

found to regulate temperature more precisely than other control schemes previously

utilized with this steam reforming reactor. While previous studies did not require precise

Figure 3.10: Control circuit for heating elements.

Computer Power
Controlled Meter
Relay

Solid
+ Relay State 120 VAC \
5V DC Input Relay

Heating
Element

Figure 3.11: Power distribution box.

Current
Temperature

Y
Temperature
History

Switch
History
A

T

T
> T
dT/dt
> dT/dt < dT/dts, ,mt

Duration < Durationpm,,

Figure 3.12: 1 leading element control logic.

T

F

Logical
AND

Heater
On

Table 3.1: Control setpoint matrix.
Setpoint DT/dt Maximum Duration
C (C / sec) (% time)

Vaporizer Surface 420 0.3 100

Vaporizer Exit 290 20 100

Trap Surface 480 0.3 100

Trap Exit 250 20 100

Superheater Surface 305 0.2 100

Superheater Exit 250 0.1 100

Zone 1 Exit 250 0 100

Zone 2 Exit 250 0 100

Zone 3 Exit 250 0 100

Zone 4 Exit 250 0 100

Zone 5 Exit 250 0 100

Zone 6 Exit 250 0 100

control of the instantaneous temperature, it was expedient in the present study to

eliminate possible variations to clearly identify factors affecting the reforming process.

Figure 3.13 shows 15 minute time scans of the Zone 1 exit temperatures with a control

scheme previously developed by Sterchi (2001) and the control scheme used in the

present study. Both temperature plots were acquired under similar operating conditions

with a control setpoint of 2500C with no acoustic wave, and a premix flow rate of 4.78

g/min. The major differences between these two control schemes are the control matrix,

the location of the feedback thermocouples, and the use of the liquid trap (or secondary

vaporizer) for the present study. Previous operation utilized feedback thermocouples at

the radial centerline of the catalyst bed while the control scheme used in the present study

obtains feedback from thermocouples at the interior wall positions. This position near the

49

200

275
x x
270

265

0 2 4 6 8 10 12 14
Minutes
x Cortrol Scheme by Stech : Corro Scheme for Present Study

Figure 3.13: Time plot of catalyst bed temperatures at zone 1 exit utilizing two
control schemes with a 250C setpoint.

wall allows a faster response time to the heat input and thus the overshoot in temperature

is reduced. The fluctuations in temperature over the period shown in Figure 3.13 for the

present study are representative of temperature measurements over an entire experiment

period and throughout the catalyst bed. These temperatures and fluctuations are also

characteristic for all the experiments performed in this study. The average and standard

deviation for all zone exit temperatures under the conditions described above are shown

in Table 3.2.

Table 3.2: Average and standard deviation for all catalyst bed temperatures utilizing a
2502C setpoint._

Average Temperature Standard Deviation

Zone 1 Exit 251.4 1.6

Zone 2 Exit 251.2 1.5

Zone 3 Exit 250.9 0.9

Zone 4 Exit 251.2 1.3
Zone 5 Exit 254.1 1.0

Zone 6 Exit 251.8 3.4
__ Zone 6 Exit 251.8 3.4

Acoustic control

An acoustic field was generated with an open loop control scheme. As previously

shown by the input and output signals in Figure 3.8, a monotone sine wave or swept sine

wave reference signal was induced with a Wavetek Sweep function generator. This

reference wave then was directed to the channel input of a QSC Mx-700 amplifier. The

amplifier was in turn connected to the Klipsch Pro 1133 Horn Driver through a pressure

housing. The amplified AC voltage seen by the driver was monitored by connecting a

handheld Fluke multimeter in parallel with the driver. The acoustic output was probed by

observing the conditioned output voltage from the Kistler piezotron coupler that was

connected to the piezoelectric pressure transducer as described previously. Both the input

(from the function generator) and the output signal (from the piezotron signal

conditioner) were simultaneously monitored by an analog oscilloscope and a digital data

acquisition unit programmed in LabVIEW by the author. These digital data acquisition

programs can be found in Appendix A. These programs included a spectral analysis

program for measuring the acoustic properties of the reactor via transfer functions and

anoscilloscope program for measuring and recording the sound pressure level data. The

frequency of oscillation was held constant throughout the experiment. The resonant

frequencies and acoustic field properties within the catalyst bed are presented and

discussed in Chapter 4. As shown in Figure 3.14, which is a typical time trace of the

acoustic pressure, there were both fluctuations in the acoustic pressure amplitude levels

and 60 Hz noise present in the transducer output signal. Because the noise fluctuations

increased the observed maximum amplitude, the average noise level amplitude found

after an experiment run was removed from the average maximum found during the

0000
1/60 Hz 1/60 Hz 160 Hz

-2000 1
.i.

m 0 j -* .. -- --- --
I I

.4000 4U -. ---. -4 -( -- 4-- -- U-- -

-6000
0 001 002 003 0.04 0,05 0 000 C7 006
Seconds

Figure 3.14: Typical time trace of pressure in the catalyst bed while generating hydrogen
with an acoustic input of 650 Hz.

experiment run. This post processing yielded a more accurate representation of maximum

acoustic pressure and the corresponding sound pressure level found inside the reactor.

The sound pressure level (SPL) in dB was found by applying Equation 3.1 where P,,s is

the root mean square of the pressure wave amplitude and Pref is a standard reference of

20~Pa used throughout the results.

SPL= 20log :ns
\ ref

(3.1)

Analysis of Data

There were multiple steps and sub-processes required for analysis of the collected

data. The sub-processes required for analysis were use of gas chromatography,

calculations of conversion, and calculations of space velocity.

Gas Chromatography

An SRI model 8610-C Gas Chromatograph (GC) was used to analyze dry

products of the reactor. The GC was computer controlled using Peak Simple software as

provided by the manufacturer. The GC was equipped with a 91.44 cm (36 in.) molecular

sieve-packed column and a 182.88 cm (72 in.) silica gel-packed column. Tedlar bags (1.5

liter) were used to collect dry gas samples. After the tedlar sampling bag was connected

to the sampling valve on the gas chromatograph, a 40-50 ml gas sample was drawn

through the sampling valve with a syringe. The automated gas sampling valve was kept

heated to 100C and was used to inject a 1 ml sample into the columns upon command

from the computer user interface. A Bourdon type pressure gauge was used in connection

with the gas sampling valve to ensure that the sample in the valve was near ambient

pressure before the sample was injected. The flow paths for the GC sampling valve,

columns and detectors are shown in Figure 3.15. As the gas sampling valve would rotate

from inject back to load position (at 9.0 min), the flow in the columns would reverse.

This allowed separation of H, 02, N, CH4, and CO specie passing through the mol sieve

and separation of CO2 and higher hydrocarbons trapped in either the molecular sieve-

packed column or silica gel-packed column after the flow reversal (SRI, 1997).

Separation occurred and the silica gel-packed column was heated (molecular sie\ e-

packed column was kept at 100C for all experiments) as shown in Figure 3.16. The

detectors used were a thermal conductivity detector (TCD) and a flame ionization

detector (FID) in conjunction with a methanizer. The voltage traces from these detectors

and a calibration file were used by the software to calculate the volume percent of species

in the gas sample. The TCD was used to detect H2, 0, and N,. as well as high levels

TCD Methanizer- FID

Exhaust

Carrier Gas
.4

A

Sample Gas
In

Gas
Sampling
Valve in
Load
Position

Gas
Sampling
Valve in
Inject
Position

Figure 3.15: Flow paths in the gas chromatograph.

240
220
0 200
180
S160
E 140
0.
E 120
100
E Flow Reversal
J 80

0

SH2 0 N2 CH4 CO CO2 Higher Hydrocarbons
20 ....
0 5 10 15 20 25 30

Time (min)

Figure 3.16: Silica gel-packed column temperature and approximate specie elution times
for the species expected with the gas chromatograph.

Sample Gas
Out

. A -

(above 1%) of CH4, CO, and CO,. The FID was utilized to detect low levels of

carbonaceous compounds including CH4, CO, CO, and non-methane hydrocarbons

(NMHC). All of the above specie (except hydrogen) were detected using a ultra high

purity grade helium carrier gas and a high TCD current setting. Detection of H, levels

with the GC required use of an ultra high purity grade nitrogen carrier gas and a low TCD

current setting.

Calibration of the GC took place as standardized gas samples were tested in the

GC. The calibration levels for each gas is shown in Table 3.3. With the exception of the

higher levels of hydrogen, these calibration mixtures were acquired from calibrated

sample gas cylinders.

Table 3.3: Gas chromatograph calibration levels.
Specie H2 02 N2 CH4 CO CO2 NMHC

1%, 4%, 10%,
S5%, 20 10%,
Calibration -65%,-70%, 5%, 20% PPM 100 PPM, Ethylene
levels -75%,-80%, 21% 5% 20 PPM
79% 4% 25%
100%

Rough calibration of the higher hydrogen volume percentages was made by filling

a tedlar bag with metered hydrogen and nitrogen. As there was some fluctuation in the

metered values and these points were not independently verified, this calibration was

considered approximate. This rough calibration did however verify that the balance of

products not acquired using the helium carrier was hydrogen. This is shown by an

example of the volume percentages found for the dry product gases during preliminary

data at various operating conditions in Table 3.4 below. Table 3.4 also demonstrates the

variability due to the gas chromatograph sampling as Experiments 1 and 2 N\ere gathered

under the same operating conditions. Because of this demonstrated variability in o\ crall

species concentration, the overall average concentration of hydrogen was used to

calculate hydrogen output power in the analysis. Thus the hydrogen output power was a

function only of the dry product gas flow rate rather than being affected by the variable

concentrations during a particular experiment.

Table 3.4: Volume percent species of the dry product outlet gas.
Specie H2 02 N2 CH4 CO CO2 NMHC Total

Experiment 1 68.76 ND 6.52 ND 0.54 22.83 ND 98.65

Experiment 2 72.65 ND 4.04 ND 0.55 23.92 ND 101.16

Experiment 3 71.19 ND 5.21 ND 0.50 23.11 ND 100.01
ND=Not Detected

Conversion

Conversion of the methanol is defined as shown in Equation 3.2 where X is the

conversion of the methanol fuel, and methanol is determined on a mass basis. The

calculated conversion is then independent of the sample time period. The time period and

SCH3OHnp,,, CH3OH,,p (3.2)
CHlfOHi,,,,

corresponding amounts of condensate did, however, affect the variability associated with

the conversion. Shorter time periods yielded less condensate and thus these small

amounts of condensate were more prone to contamination from water droplets that

formed on the liquid trap when exposed to the atmosphere prior to analysis or \\aler that

would leak into the liquid trap upon opening. It was determined from comparing small

amounts of condensate from shorter experiment times to those of longer experiments

under the same conditions that a minimum of 40 grams of condensate should be collected

to insure negligible biasing of conversion data by sample collection time. The conversion

equation shown above in Equation 3.2 required that the methanol mass fraction in the

condensate be known. The amount of methanol in a condensate sample was determined

by utilizing the condensate density. The condensate density was found by way of a Anton

Paar density meter that had a 0.0001 g/cm3 resolution. The density (p) was input to an

emperical formula developed by Sterchi (2001) that yields methanol mass fraction

(mfCHIOH) of the condensate. This empirical relation is found in Equation 3.3 and was

verified for this study. From the methanol mass fraction of the condensate, the mass of

unused methanol (output) is found, and the conversion can then be determined.

mfcHnOH = 3859.9p2 823 .lp + 4370.0 (3.3)

Space Velocity

Space velocity for this investigation was calculated from the mass flow rate of

methanol:water premix. Flow rate data was gathered as masses of both premix consumed

and condensate accumulated could be known over a unit of time. Space velocity is found

by measuring the volume of the reactor (measured catalyst bed length was used for

determination of the reactor volume), and the volumetric flow rate of the reactants. Space

velocity, as previously presented in Equation 2.4 can be defined in several ways. This

includes gaseous space velocity or liquid space velocity. Space velocity can also be

determined with regards to a single specie or all reactants traveling through a processor.

Additionally, considering an acoustic field within a processor, an effective space \ clocity

can be defined basing an effective reformer volume on a typical particle path length. In

order to avoid confusion, the space velocity with regards to liquid methanol is used.

57

Liquid Hourly Space Velocity of Methanol (LHSV-M), as defined in Equation 3.4 will be

used throughout the presentation of the results.

mhr liquid methanol input
mL reactor volume (3

Removal of Data Points

As multiple sets of data were used in the present study with variability in both

input and output parameters, output data were analyzed to identify outlying points using a

first application of Chavenets criterion (Holman, 1994).

Procedures

Specific procedures and checklists for start-up, data collection, and shutdown

were made in order to collect all pertinent data as well as to avoid hazardous situations

and damage to equipment. These procedures were made by both consulting past studies

(Sterchi, 2001) foresight in the operating process, and experience. The checklists used,

which outline the general procedures for start-up, data collection, and shutdown can be

found in Appendix B.

Modeling the Acoustically Enhanced Reformation Process

As discussed in the theoretical approach, there are several analytical models that

may be applied to the various steps of the reformation process with acoustics. These are

increase of effective space velocity (Equation 2.11), heat transfer (using Equations 2.12

and 2.13 for Nusselt number), mass transfer (using Equation 2.12 and 2.13 for Sherwood

number) and chemical kinetics (using Equation 2.11 for the effective area). While each

of these models may represent a portion of the overall enhancement due to the acoustic

waves, an actual reactor does not fit the ideal models developed and other reactor

58

parameters must be considered. This is especially true in the packed bed reactor utilized

for this investigation because the models developed do not account for obstruction in the

acoustic path or the volume that the catalyst pellets occupy. While a comprehensive

model for the actual conditions may not be currently possible, empirical modifications of

the ideal model for the experimental facility can be developed. One should note that the

facility is not an optimal design for employment of acoustics or reforming and that the

resulting model is specific to the facility used. Although this is a significant limitation,

one gains understanding of how acoustics could be applied in different reactors and just

what the demonstrated benefits are in a particular application.

Factorial Experiment Design

A fraction of the data collected was used as a standard 2" factorial experiment

design. This type of experiment design is common for statistically identifying effects of

certain factors on a process and yields a model of the process with respect to the variables

investigated (Lawson and Erjavec, 2000). A brief overview of factorial experiment

design and analysis is given in Appendix C.

This factorial experiment design is used to develop an empirical model.

Although the model developed yields linear relationships for the variables investigated,

residuals or curvature of the output with relation to the inputs was also investigated with

further data. The output (dependent variable) for the factorial experiment design was

conversion of methanol. After consulting previous experimentation with the facility,

analysis of preliminary data, and alignment with the objectives of the study, the input (or

independent) variables were chosen to be fuel (premix) flow rate, acoustic pressure, and

catalyst bed length. Other possible variables were held constant for the experiments

59

including control setpoint temperature (2500C), frequency (650 Hz), and water-methanol

stoichiometry (1.5:1 on a molar basis). High and low levels were determined for each of

the varying parameters. The levels for fuel flow rate were a high of 9.5 g'mnin. and a low

of 4.8 g/min. The levels for acoustic pressure were a high of 5000 Pa, and a low of 0 Pa.

The levels for catalyst bed length were a high of 0.576 m, and a low of 0.279 m. This

allowed for a 23 factorial experiment design yielding 8 combinations of the input

variables at high and low settings. The input matrix and run orders can be found for the

factorial experiments in Table 3.5.

Table 3.5: Factorial input matrix and run order.
Flow Acoustic Catalyst Bed Run Order
Flow Run Order
Pressure Length
+ + + 3,6,10

-+ + 1,4,11

+ -+ 2,7,9

+ 5,8,12

+ + 14,18,19

+ 13,17,22

+ -- 15,23,24

16,20,21

Although other parameters were held "constant", variability in these settings of

high and low levels was apparent. These fluctuations yielded variability in the measured

output of conversion. Two replications of the study were performed to allow statistical

analysis and identification of outlying points. As shown by the run order, the ariables

of fuel (premix) flow rate and acoustic pressure were randomized while catalyst bed

length was taken sequentially at a high then low level. The sequential order of catalyst

bed length was required because of the time-intensive reduction process between

changing bed lengths. Inadvertently the model includes variability from both the

reduction process and catalyst arrangement as a bed. This came as a result of rupturing

the seals of the facility halfway through the full (high level) catalyst bed experiments.

The consequences of this disturbance are that the variation in fuel conversion may be

higher in the empirical model than would be observed normally.

Further Data and Verification of the Model

Further data were required in order to verify the empirical model constructed and

to demonstrate different effects of the acoustic waves on the reformation process.

Curvature for the input variables of the model was found by investigating intermediate

sound pressure levels, flow rates and various bed lengths including those bed lengths

lower than the low level in the factorial experiment. This bed length data enabled one to

not only find the curvature and limits of the proposed model but also to estimate how the

local conversion of methanol progressed though the full catalyst bed.

Other data not directly accounted for in the empirical model were also collected.

As frequency was suspected to change the boundary layer thickness for laminar flow, and

possibly induce the flow into the turbulent regime, conversion at two frequencies was

investigated. Because the temperature within the processor was expected to significantly

affect the conversion of fuel, temperature data were collected for acoustic field and flow

conditions. While holding the temperature control setpoint at 2500C, actual temperature

data at certain locations throughout the catalyst bed were collected. These temperature

data were collected by utilizing thermocouples extending into the bed at specified lengths

through the unused sampling ports shown in Figure 3.6. Other parameters of interest,

61

including conversion change with time (indicating catalyst degradation), hydrogen power

output, and acoustic driver input electrical power were also collected. Specific

configurations of all controllable parameters for these further data is explained in the

discussion of those data in Chapter 6. The run orders for the additional data collected is

found in Appendix D.

CHAPTER 4
PROPERTIES OF THE ACOUSTIC FIELD

This chapter is a presentation of the results found by investigating the acoustic

field within the catalyst bed and the analysis of those results. As the acoustic velocity

must be higher than the average velocity in order to enhance the reforming process,

standing acoustic waves built at resonance with their associated pressures and velocities

can be utilized. The analysis of the acoustic field provides the basis for understanding

what resonant acoustic modes can be employed in enhancing the reformation process.

Transfer functions were gathered for frequency analysis. These data were compared to

theory and enabled an acoustic modal analysis to be performed.

The Use of Resonance within the Catalyst Bed

As explained in the theoretical approach, enhancement of the reforming process is

expected from increased mixing within the catalyst bed when the acoustic velocity is

sufficiently high. High amplitude sound waves are made possible by building up standing

or resonant waves within the reactor. Resonant frequencies and sound pressure levels in a

wave guide such as the catalyst bed is a function of acoustic boundary conditions, the

speed of sound in the wave guide and the damping characteristics of any obstruction

within the waveguide. A simple example of a standing wave pattern of an acoustic field

within a wave guide and the analogous resonant motion of a string is shown in Figure 4.1.

The boundary conditions shown in these simple examples are the same as those expected

within the catalyst bed studied. The acoustically closed condition was expected at the

62

Displacement Amplitude trace in a wave
guide with closed-open boundary conditions
2nd mode resonant oscillation

A Open or Driven
Condition
V

Displacement and
Velocity Anti-node

Displacement and
Velocity Node

String displacement analogy with
fixed-free boundary conditions
2nd mode resonant oscillation
AA
Fixed End

Displacement and Displacement and
Velocity Anti-node Velocity Node

Figure 4.1: Standing wave pattern of displacement and velocity in a waveguide and
an analogous pattern of displacement and velocity in a driven string.

entrance to the catalyst bed. This entrance is where a 0.635 cm (0.25 in.) O.D. tube opens

to a 2.68 cm 0. D. (0.75 in. schedule 40) pipe previously shown in Figure 3.5. Acoustic

waves propagate upward from the catalyst bed to this junction. By investigating the

theoretical acoustic power transmitted from a large pipe opening to a small pipe opening

as shown in Figure 4.2, the expectation of a closed acoustic condition at the catalyst bed

entrance can be justified. The transmitted acoustic po\ er is shown in Equation 4.1,

%\here Tp is the transmitted acoustic power, S, is the large pipe cross-sectional area and S,

in the small pipe cross-sectional area (Kinsler et al., 1982). For the pipe junction

described above, the acoustic power transmitted through the entrance is calculated to be

Closed End

CA

^A

Free or Driven
Condition

64

A
Incident Acoustic
Wave

A
Transmitted
S < Pipe Cross- > S, Acoustic Wave
sectional Areas

Reflected Acoustic
Wave

Figure 4.2: Transmission and reflection of an acoustic plane wave at a pipe
junction.

4S,S2
T (s, + s)2 (4.1)

less than 18% of the incident power. Thus acoustically the entrance location acts as a

closed end. The other end of the waveguide is where the acoustic driver is attached.

Although physically this is not a straight through path, acoustically it appears to be so. It

is expected that because of the displacement associated with the voice coil of the driver

that this location would act as an acoustically open condition with associated large

acoustic displacements and velocities. There are several resonant modes and frequencies

associated with these boundary conditions. The predicted resonant frequencies for these

closed-open conditions are shown in Equation 4.2 where n is the mode, c is sonic velocity

and L is the length of the tube. The pressure and velocity equations for any location and

time for the closed-open acoustic boundary conditions in an idealized a\ ve guide are

(2n- I)c
f= 4L n= 1, 2, 3,... (4.2)

given in Equations 4.3 and 4.4 where x is the position from the closed end (entrance) and

t is time. With the theory established one can measure the actual resonant properties of

the catalyst bed via transfer functions and perform a modal analysis to verify the predicted

boundary conditions.

(2n 7) nx
P = P co (2n- 1)n sin(2ft) (4.3)
= P2L )

u = U, sin (2n- )n) cos(2nf.t) (4.4)

Transfer Functions

In order to observe what resonant frequencies were available in the catalyst bed

several transfer functions were measured in the reactor. These transfer functions were

measured from the signal generator or driver input signal to the pressure transducer

output signal. Excitation of a range of frequencies was accomplished by a swept sine

method. Frequencies were input from 10 Hz up to 15 kHz. The spectrums of the input

and output were measured and compared. This resulted in a transfer function magnitude

plot. The transfer function phase plot and coherence plot were also measured. The

coherence was checked to verify that the output signal was due to the input signal and not

due to noise. The number of samples acquired for the transfer function was 8192. These

samples were taken at a sampling rate of 35 ksamples/sec resulting in a frequency

resolution of 4.27 Hz. Thirty single transfer functions were averaged to ensure the

excitation and capture of all desired frequencies. In this manner, transfer functions were

measured for a variety of temperatures and reactor conditions. These conditions were

66

ambient (-33 C) and operating temperature (2500C Control Setpoint), empty and full of

catalyst, as well both stagnant (no premix flow) and while generating hydrogen (4.9 g/min

premix flow)(see Figures 4.3- 4.6). The transfer functions for these conditions presented

in Figures 4.3 4.6 are a composite of two transfer functions for each figure. One transfer

functions is for the frequencies from 40 to 2100 Hz and the second transfer function is for

2100 Hz to 15 kHz. There are no significant frequency peaks beyond 7000 Hz in all

cases. The most basic of the configurations tested was an air filled ambient temperature

reactor with no catalyst. The transfer function for this condition is found in Figure 4.3.

0.2-
Transfer
Function
Magnitude

0.0-

0.0- I 1 I 1 I I 1 I ,
40 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
Hz

Figure 4.3: Transfer function magnitude for the catalyst bed housing containing only
33C air.

This transfer function shows the frequencies at which resonance is theoretically possible.

Transfer functions were also found for a 33 C reactor containing catalyst and air as

shown in Figure 4.4. The amplitude peaks are lower than without the catalyst because of

the acoustic attenuation of the obstructing catalyst particles, yet the frequency peaks

corresponding to the highest amplitude modes were still well established. After reducing

Transfer
Function
Magnitude

U.I

0.0-1
40

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Figure 4.4: Transfer function magnitude for a catalyst bed housing containing catalyst
and air at 33C

the catalyst and holding carbon dioxide in the reactor at 250C, the transfer function, as

shown in Figure 4.5 revealed a small shift in frequencies due to a slight change of the

sonic velocity of the gas within the catalyst bed. The sonic velocity in 33 C air is 351 m/s

0.1 -

Transfer
Function
Magnitude

0.0-
40 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
Hz

Figure 4.5: Transfer function magnitude for a reactor containing reduced catalyst and
CO, at operating temperature (250 oC).

~-Ll

while the sonic velocity in 2500C carbon dioxide is 357 m/s. With this slight difference

in sonic velocity only a small change in frequency positioning is expected. These

frequency peaks, under the conditions described were also found positioned at similar

relative locations as the air filled reactor with catalyst, indicating no shift in primary

oscillation modes. With a flow of reactants and products in the catalyst bed (premix flow

of 4.8 g/min) under operating conditions (250C control setpoint) Transfer functions

were acquired as displayed in Figure 4.6. As seen in this figure, the transfer function

shifted slightly in magnitude and frequency positioning when water/methanol premix was

being reacted because of both the slight differences in overall reactor temperature,

pressure, and gas composition (and thus sonic velocity) due to the reaction. The highest

amplitude peak previously near 550 Hz was shifted to 650 Hz and the peak previously at

1700 Hz shifted to 1900 Hz, while two low amplitude peaks became evident near 1400

Hz and 2300 Hz. An example of the time domain pressure signal within the catalyst bed

0.1- - -

Transfer
Function
Magnitude

0.0- ,l l li l
40 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000
Hz

Figure 4.6: Reactor transfer function magnitude while generating h1 drogen at 250'C.

69

while being driven at 650 Hz and while reaction is taking place was presented previously

in Figure 3.14. It was determined that while generating hydrogen rich products, an

acoustic sound pressure level of 165 dB (re20 iPa) could be maintained in the catalyst

bed if driven at 650 Hz and approximately 9 watts of electrical power to the driver.

One can compare these experimental results of resonant frequencies found by the

transfer functions with theoretical resonant frequencies for the assumed boundary

conditions. Allowable resonant frequencies (f) for an closed-open waveguide have been

given in Equation 4.1. Noticeably, the frequencies as predicted by Equation 4.1 are not

all present in the measured transfer functions. This deviation from the analytical solution

is due to insufficient driving power at certain frequencies and/or to the actual boundary

conditions present in the reactor. The midrange driver has a frequency response as shown

in Figure 4.7 (Klipsch, 2001). Because the lower frequencies (below 200 Hz) are not

well excited by the midrange horn driver, it was not expected that the lower frequencies

would be excited in the reactor.

o -------------------
0
-2

-4

S-6

I -8

S10
-12

-14
100 1000 10000
Hz

Figure 4.7: Frequency response of the acoustic driver as measured in a plane
wave tube by the manufacturer.

The actual resonant frequencies from the transfer functions can be translated to

their associated wavelengths (1) through Equation 4.2 With the wavelengths and at least

one boundary condition known, modal shapes can be plotted and the location of actual

boundary conditions can be found.

c
A .- (4.2)
fn

Modal Analysis

Wavelengths in the 330C air-filled catalyst bed housing without catalyst were

calculated from the measured transfer function previously shown in Figure 4.3. With the

acoustically closed condition assumed as the catalyst bed housing entrance, the velocity

and displacement waveforms for six frequencies (amplitude has been normalized) were

plotted. These plotted waveforms are shown in Figure 4.8. This figure plots normalized

velocity magnitude versus position from the entrance, and has the temperature and

pressure ports along with suspected physical boundary conditions marked along the x-

axis.

MODAL SHAPES IN REFORMER
1.0- wave 328 Hz
0.8-I wave 2 550Hz
0.6 wave3- 756Hz
0.4- wave 4 982 Hz
02- \ wave5-1200Hz
Voi 0.0-e wave 6 1414 Hz
Amnpude -0.2
-04-1
-0.6- -OPEN 57500 000 00i
/ OPEN. a05000 00 E
**-to-, 1, 1 1, ,,1 1 OPEN 87500 000 00 VfS
0 50 100 150 200 250 300 350 0 0 450 500 60 o s650o 750 80 80 eSo0 OPEN 87500 000
P1 P2 P3 P4 PS P6 and o refrm Adapter drive
20.30 115-125 210-220 305-315 395-405 490-500 epaion oa throat
575 800-815 875

Figure 4.8: Normalized velocity magnitude for six waves found in a 33C reactor without
catalyst.

From Figure 4.8, it is evident that an acoustically open condition exists near

800-810 mm inside the reactor. This open condition directly corresponds to the driver

mounting plate where the driver is attached to the pipes leading to the catalyst bed. This

location is termed the adapter throat. This acoustically open condition at this point

indicates that the mass of fluid inside the acoustic driver oscillates in unison with the

voice coils and diaphragm. Considering the large diameter (7.62 cm) of the diaphragm

and the relatively short distance from the diaphragm to the adapter throat (8.9 cm), this

unison in oscillation is not surprising. In addition to this boundary condition at the

adapter throat, there are other contributing boundary conditions for some of the frequency

peaks found in the transfer functions. Most notably at 550 Hz, 1700 Hz, and 2100 Hz for

the empty air filled reactor there is contribution from an open condition at the pressure

transducer (PT) tube and housing (see Figure 3.6). As expected, the frequency peaks with

contributing boundary conditions are higher than those without. This amplification of

certain frequencies with contributing boundary conditions explains why the real reactor

deviates from the idealized case presented by Equation 4.2. As can be seen from Figure

4.6, the contributing conditions also widen the frequency peaks which allows for better

control of the sound pressure level. As noted in the explanation of the experimental

facility there are fluctuations in overall temperature and gas composition with time that

can change the resonant characteristics of the catalyst bed. The \wide bands of resonance

found for frequencies with contributing boundary conditions allow for minimization of

the changes in sound pressure when acoustically dri\ ing at these frequencies.

The modal analysis was verified by moving the pressure transducer down the

reactor length through the six access ports in the air-filled reactor without catal) st. The

transfer functions, measured through the ports are shown in Appendix E for the lower

frequencies. As expected from the standing wave pressure amplitude trace, which can be

thought of as 90 leading the velocity trace in Figure 4.8, the transducer picked up higher

magnitudes pressures at the corresponding pressure anti-nodes. These shifts in the

transfer function magnitude and also in the corresponding phase plots were noted and

found to follow the expected modes as predicted by the waveforms in Figure 4.8. A

summary of the frequencies, wavelengths and boundary conditions for the air-filled 33 C

reactor without catalyst is shown in Table 4.1 below.

Table 4.1: Waveforms and boundary conditions for a 33 C reactor containing only air.
Frequency (Hz) Wavelength Boundary Contributing
(m) Condition Conditions
145 2.41 Open at reformer end
328* 1.06 Open at adapter throat
550* 0.63 Open at adapter throat Open at PT Tube
756* 0.46 Open at adapter throat
982* 0.36 Open at adapter throat
1196* 0.29 Open at adapter throat
1414* 0.25 Open at adapter throat
1550 0.23 Open at adapter throat
1717 0.20 Open at adapter throat (1650 Hz) Open at PT Tube
2050 0.17 Open at adapter throat
2285 0.15 Open at adapter throat
2555 0.14 Open at reformer end
2896 0.12 Open at adapter throat Open at PT Tube
3123 0.11 Open at adapter throat
3358 0.10 Open at adapter throat
3708 0.09 Open at driver throat
3900 0.09 Open at driver throat Open at PT Tube
5383 0.06 Open at adapter throat Open at PT Tube
6669 0.05 Closed-closed transverse wave Open at PT Tube
velocity amplitude mode shape is plotted in Figure 4.8

CHAPTER 5
EMPIRICAL MODEL DEVELOPMENT AND ANALYSIS

This chapter presents and discusses the results of the 23 factorial experiment

design used to develop empirical correlation factors to model the reformation process

under the influence of a resonant acoustic field. The reactor was investigated for the

input variables of premix flow rate, sound pressure amplitude, and catalyst bed length at

high and low levels as described in the experimental approach. This investigation allowed

for correlation factors of an empirical model to be found. Model correlation factors are

presented, along with an uncertainty analysis of those correlation factors. Midpoint

curvature for each variable and the effect of frequency on the conversion level of the

steam-reforming process were also examined.

Experimental Results and Model Correlation Factors

By using a standard factorial design of experiments one can develop an empirical

model of a certain design space with relatively few experimental trials. An overview of

the general setup of such a process, formulation of the correlation factors, and statistical

analysis of hypothetical results are presented in Appendix C. Actual values for the high

(+) and low (-) levels investigated and the randomized run order were previously

presented in Chapter 3. The results of the experiments for the configurations specified

yielded the output data shown in Table 5.1. In these experiments the output or

independent variable is conversion of methanol in percent. The temperature control

setpoint was held constant as \Ncre all other controllable parameters not specified. As

shown in Table 5.1 the standard deviations are relatively low indicating good

repeatability for each configuration. The data also show that with only a few exceptions

the conversion decreases over the trials run. Because the trial run is correlated to the

relative sample time, these data suggest that catalyst degradation was causing the bulk of

variability.

Table 5.1: The input matrix and methanol conversion percentage data obtained from the
factorial experiments.

Flow Sound Catalyst Conversion
Bed Trial Trial Trial Standard
Run Rate Pressure % Average
(X) (X2) Length 1 2 3 Deviation
(X1) (X2) (Y)
(X3)
1 + + + 96.33 96.28 94.94 95.85 0.79

2 + + 99.74 99.61 99.44 99.60 0.15

3 + + 93.06 92.22 91.84 92.37 0.62

4 + 99.13 98.81 98.90 98.95 0.17

5 + + 70.58 70.19 70.34 70.37 0.20

6 + 92.57 91.46 91.33 91.79 0.68

7 + 61.74 60.82 59.77 60.78 0.99

8 87.61 86.45 86.37 86.81 0.69

Equation 5.1 is used to find the standard error (SE) from the

deviation (op) and the total number of factorials (nf) for these data.

pooled standard

The total average

conversion ( Y ), pooled standard deviation, standard error, and total degrees of freedom

for the data are given in Table 5.2. These data suggest that mean values within the

conditions studied are expected to deviate less than or equal to the standard error (0.251

%) and any individual point should not deviate by more than twice the pooled standard

deviation (1.22 %) from its mean value using a 95% confidence interval. The correlation

sE- (5.1)
fl",

Table 5.2: Total average conversion, pooled standard deviation, standard error, and total
de rees of freedom from the factorial experiments.
Total Average Pooled Standard Deviation Standard Error Total Degrees of
Conversion % Freedom

87.06 0.61 0.251 16

factors or effects (E) and interactions (I) were calculated for each variable (X) and set of

variables by taking the average of the average conversion outputs (Y) when the variable

or set of variables were high and subtracting the same average when the variable or set of

variables were low. This statement can be simplified to yield Equation 5.2 for the effects.

8
Sun Xi run (5.2)
E Run=l
4

Interactions are found in a similar manner where the combination of the level X,,ru

multiplied by Xj,rn is used instead ofX,,, in Equation 5.2 find the Interaction (I,). This

calculation, using the data presented in Table 5.1 resulted in the correlation factors for the

model as the effects and interactions displayed in Table 5.3

Table 5.3: The effects and interactions found from the factorial experimentation.
Effect of Flow Rate (E,) -14.44

Effect of Sound Pressure (E2) 4.67

Effect of Catalyst Bed Length (E3) 19.26

Interaction of Flow Rate and Sound Pressure (I12) 1.86

Interaction of Flow rate and Catalyst Bed Length (I,3) 9.28

Interaction of Sound Pressure and Catalyst Bed Length (I23) -2.61

Interaction of Flow Rate, Sound Pressure and Catalyst Bed Length (I,23) -0.45

These correlation factors show the magnitude and direction each variable has on

the conversion of methanol in the reformer. These model correlation factors yield the

equation for methanol conversion (C) as shown in Equation 5.3. However, not all effects

El E2 E3 I2 I3 123, 1.3
C = Y + X2 X, + X3 + X+ X, X + XX X3 + XXX X+' X3 (5.3)
2 2 2 2 2 2 2

or interactions presented above are necessarily statistically significant or need to be

included in the working model equation. In order to find the statistical significance of the

correlation factors, one must perform an uncertainty analysis on the effects and

interactions.

Uncertainty Analysis

In order to formulate the proper model correlation equation one must test each

effect and interaction for statistical significance. This uncertainty analysis can also be

used to test the significance level of the individual correlation factors. This is particularly

important for statistically proving that the acoustic wave has a significant effect on the

conversion in this reactor. The signal to noise t-ratio (t*) was found using Equation 5.4.,

E or I
t* = (5.4)

As with a standard t-test, statistical significance for each effect and interaction was found

at the desired confidence level (both 95% and 99.9% are presented) for the total degrees

of freedom, by comparing the absolute value of the signal-to-noise t-ratio to the Student t-

value. The Student-t value yields the maximum noise ratio at the specified degrees of

freedom and confidence interval. The effects and interactions with absolute values of the

signal to noise t-ratios larger than the Student t-value are considered to be significant at

the confidence interval stated. The results of this analysis are shown in Table 5.4.

Table 5.4: Signal-to-noise t-ratios and statistical significance.

t* Significant at Significant at 99.9%
95% Confidence? Confidence?

Student t-value at 95% Confidence 2.120

Student t-value at 99.9% Confidence 4.015

Effect of Flow Rate (E,) -57.60 Yes Yes

Effect of Sound Pressure (E2) 18.64 Yes Yes

Effect of Catalyst Bed Length (E3) 76.80 Yes Yes

Interaction of Flow Rate and Sound
7.42 Yes Yes
Pressure (I,12)

Interaction of Flow Rate and Catalyst 37.02 Yes Yes
37.02 Yes Yes
Bed Length (1,3)

Interaction of Sound Pressure and -11
-10.41 Yes Yes
Catalyst Bed Length (123)

Interaction of Flow Rate, Sound 78
-1.78 No No
Pressure and Catalyst Bed Length (I,23)

These data also show that all of the variables investigated including sound

pressure has a statistically significant effect on the conversion for this reactor at a

confidence level of 99.9%. The fact that the interaction of flow rate. sound pressure and

catalyst bed length can be neglected allows for a slight simplification of the con\ crsion

equation. The statistically valid conversion model for the inputs of flow rate, sound

pressure amplitude and catalyst bed length is found in Equation 5.5. This equation is

used by finding where the actual flow rate, sound pressure and bed length variables fit

into the high (+1) and low (-1) settings for each parameter. These values can be found by

C(%) = 87.06 7.22X, + 2.34X, + 9.63X3 (
+ 0.93XX,2 + 4.64X,X, 1.31XX,

utilizing Equations 5.6 5.8 for specific premix flow rates (g'min), sound pressure

amplitudes (Pa), and catalyst bed lengths (m), respectively. These values are then utilized

in Equation 5.5 as X,, X2, and X3, where C is the conversion of methanol in percent.

SFlow(g / min)- 7.13(g / min)
2.35(g / min)

Pressure (Pa)- 2680(Pa)
X2 = 2680(Pa) (5

Bed Length (m) 0.424(m)
X3 0.145(m)

While sound pressure (X2) does not influence the conversion as much as the flow rates

(Xi) or catalyst bed lengths (X3) studied, the equation does show how much an effect

sound pressure can have in relation to the other variables. In order to demonstrate the

some of the limitations of the model, midpoint curvature data were obtained and the

effect of frequency was observed.

Midpoint Curvature

The model presented in Equation 5.5 was further examined for curvature near the

center points of the variables investigated.

Flow Rate

Flow rate curvature in the model was investigated at two sound pressure levels

keeping catalyst bed length constant as shown in Figure 5.1. For these investigations,

catalyst bed length was set at 0.568 m and the high level of sound pressure was

approximately 5000 Pa or 165 dB. Four intermediate points were taken between the high

flow setting (+1) of 9.5 g/min and the low flow setting (-1) of 4.8 g'min. Comparing the

conversion at various flow rates with the conversion predicted by the model, for cases

101
100
99 --Model Prediction No
98 *Acoustics
9 97 a Actual No Acoustics
96 "
S95 Model Prediction
E 94 With Acoustics
0 93
93 e Actual with
2 Acoustics
91
90
4 6 8 10
Premix Flow Rate (glmin)

Figure 5.1: Predicted and actual conversions for various flow rates with and without
acoustics.

with and without the 165dB acoustic field show some visual curvature near the

midpoints. Although one can see this slight trend upward in conversion at the midpoints,

no statistically significant curvature is found. This is because differences between the

predicted and actual conversion data (also known as the residuals) are in the acceptable

noise range ( 1.22 %) for the individual data points collected.

Sound Pressure

Curvature for the sound pressure was investigated at a high level of premix flow

rate (9.5 g/min) and a high level of catalyst bed length (0.576 m) at a frequency of 650

Hz. The actual conversion at various sound pressure amplitudes, together with the

conversion predicted by the model, are shown in Figure 5.2. Although some of the actual

Sales are slightly lower than expected, these values do not show significant curvature as

the residuals are less than 1% for the cases investigated.

100
99
98
S97
c 96 _
.9 Model prediction
Z 95
4 w Actual
> 94
o 93
92
91
90
0 2000 4000 6000 8000
Maximum Pressure Amplitude (Pa)

Figure 5.2: Predicted and actual conversion for maximum pressure amplitude with a
0.568 m catalyst bed length and 9.5 g/min premix flow rate.

Catalyst Bed Length

Curvature in the catalyst bed length variable was investigated at the high level of

premix flow rate (9.48 g/min) with a 165 dB (re20Pa) acoustic field as shown in Figure

5.3. This figure shows the predicted conversion of the model and actual conversion data

points gathered at different catalyst bed lengths. Note that the conversion predicted by the

model has been extrapolated beyond the low level of catalyst bed length (0.278 m) in the

figure. Data shown has significant curvature as it deviates from the model predictions

away from the high (0.568 m) and low test levels (0.278 m). The linear model prediction

is in error for catalyst bed length and it appears that a logarithmic fit, shown as the

regression line on the figure is more suitable and realistic, especially considering the

actual entrance conditions. Further data on conversion with respect to catalyst bed length

is presented in Chapter 6.

100
90
S80
70 .
S60 model prediction
S50 au actual
40 n
40 -Log. (actual)
S30 0 C% = 38.286Ln(length) + 119.61Log(actual)
0 20 R2 = 0.9612
10
0
0 0.2 0.4 0.6
Bed Length (m)

Figure 5.3: Predicted and actual conversions for bed length at a flow rate of 9.5 g/min
premix and a 165 dB acoustic field.

The Effect of Frequency

As previously described in the theoretical approach, for an ideal setting, frequency

has been linked to boundary layer thickness for the laminar regime (Equation 2.7) and the

kinetic Reynolds number (Equation 2.8). Because of this, the acoustically enhanced

methanol-steam reforming process was also investigated to find any correlation bet\\ een

frequency and conversion of fuel. Frequency effects were measured at a high leI el of

premix flow rate (9.5 g/min) and a high level catalyst bed length (0.568 m). With the

acoustic driver power held constant at 9.3 Watts, the average observed sound pressure

level dropped to 1434 Pa when the reference frequency was increased to 1980 Hz. High

acoustic pressures (5000 Pa Max. Amplitudes) were not possible at this frequency with

the same driver power due to the acoustic resonant properties as previously presented.

Conversion. as predicted by the model created from 650 Hz data for this sound pressure

amplitude %was 93.2%. This value matches the experimental conversion found at 1980 Hz

exactly. However, care should be taken in the interpretation of this data point because

this value cannot necessarily be considered out of the noise band of the data without

acoustics. Notwithstanding, this data suggests that frequency was not a significant

variable for the acoustic fields investigated.

Reynolds numbers were estimated for different frequencies using the properties of

ambient pressure steam at 2500C (v = 4.4018 x 10-5 m2/s) and the diameter of a catalyst

pellet (0.54 cm) as the characteristic length. The average velocity was estimated at 0.89

m/s from the flow rate, density steam at 250 C, and volume of the reactor. The Reynolds

numbers are shown in Table 5.5. Static Reynolds number (Res) for the non-oscillating

flow indicates a laminar flow regime with no acoustics. Kinetic Reynolds numbers

(Rek)were calculated for an oscillating field at the two frequencies investigated using the

properties above in Equation 2.8. The kinetic Reynolds number at 650 Hz is 2705

suggesting that the flow is in the transitional or turbulent regime at this frequency. An

increase of frequency to 1980 Hz increases the kinetic Reynolds number to 8241 which is

also in the turbulent regime. A comparison of the turbulent regime to turbulent regime

Table 5.5: Static and kinematic Reynolds numbers and methanol conversion levels for
three frequencies.
Hz Reynolds Number Conversion %

0 110 (Re,) 92.4

650 2705 (Rek) 93.2 (predicted from model at 1434 Pa)

1980 8241 (Rek) 93.2 actual (1434 Pa)

with the two frequencies yields no difference in conversion as shown by the data.

Furthermore, increased frequency yields lower pressure for the same driver input power

83

thus the overall power benefit was adversely influenced by increasing acoustic frequency

in this system.

Summary

Factorial design of experiments has been used to find the effects of the parameters

of premix flow rate, sound pressure amplitude, and catalyst bed length. Sound pressure

amplitude in the study was statistically significant at a 99.9% confidence level. From the

data collected a model was formulated that predicts methanol conversion for the above

parameters. The model had negligible curvature for fuel flow rate and maximum pressure

amplitude and significant error for catalyst bed lengths. Conversion for catalyst bed

length is better modeled by a logarithmic relationship. Acoustic field frequency did not

produce a noticeable change in conversion.

	Title Page
	Dedication
	Acknowledgement
	Table of Contents
	List of Tables
	List of Figures
	List of symbols and nomenclatu...
	Abstract
	Introduction
	Literature review and theoretical...
	Experimental approach and...
	Properties of the acoustic...
	Empirical model development and...
	The effects of acoustics on the...
	Conclusions and recommendation...
	Appendix A: Computer programs
	Appendix B: Checklists and...
	Appendix C: Statistical design...
	Appendix D: Experiment run...
	Appendix E: Transfer functions
	Appendix F: Temperature profil...
	References
	Biographical sketch

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