Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P1.26 - Drying Behavior of High Load Multiphase Droplets at High Temperature Conditions
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 Material Information
Title: P1.26 - Drying Behavior of High Load Multiphase Droplets at High Temperature Conditions Industrial Applications
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Mondragón, R.
Juliá, J.E.
Hernández, L.
Jarque, J.C.
Chiva, S.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: spray drying
multiphase droplet
droplet drying
acoustic levitator
porcelain tiles
 Notes
Abstract: The drying behavior of suspension droplets is important in many industrial applications for the material processing, ceramic, chemical or food industry involving spray dryers. This fact is particularly significant for high load and temperature conditions close to those found in the mentioned industrial applications (Masters, 1991). In this work, the drying behavior of acoustically levitated multiphase droplets has been experimentally investigated. The experiments have been performed using ceramic suspensions like those used in the manufacture of porcelain tiles. The initial composition has a mean particle size and relative density of 3.25 μm (standard size) and 2.7 respectively. An acoustic tube levitator modified to allow experiments at high temperature conditions has been used (Mondragon et al., 2009). The flow rate, temperature and relative humidity of this air stream can be controlled by an air conditioning system. A CMOS camera and a back-light illumination system are used to measure the droplet cross-sectional area and vertical position of the droplet during the drying process (Yarin et al., 1999, 2002; Kastner et al., 2001; Brenn, 2005). The effect of the flocculation state (flocculated-deflocculated), solid mass load (0.65 < YS < 0.70), particle size distribution of the porcelain composition (1.95 μm < dP50 < 3.25 μm), ambient air temperature (70ºC < T < 100ºC) and initial droplet volume (0.4 μl < V0 < 0.7 μl) on the mean porosity of the grain and its mechanical strength has been studied. The experimental results obtained indicate that the most important parameters to be considered for the porosity are the particle size of the composition and the initial solid mass load. The most important parameters to be considered for the mechanical strength are the initial droplet volume, the particle size of the composition and the initial solid mass load. High initial solid mass load and standard particle size distribution are the optimal conditions to obtain low porosity and high mechanical strength.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00442
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P126-Mondragon-ICMF2010.pdf

Full Text

Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Drying Behavior of High Load Multiphase Droplets at High Temperature Conditions


Rosa Mondragdn*, J. Enrique Juliat, Leonor Hernandezt, Juan Carlos Jarque* and Sergio Chivat

Institute de Tecnologia Cerimica, Asociaci6n de Investigaci6n de las Industrias Cerimicas. Universitat Jaume I.
Campus de Riu Sec. 12071- Castell6n de la Plana. Spain

Departamento de Ingenieria Mecinica y Construcci6n, Universitat Jaume I.
Campus de Riu Sec, 12071 Castell6n de la Plana. Spain

rosa.mondragon@itc.uji.es, bolivar@emc.uji.es, lhemand@emc.uji.es, juancarlos.jarque@itc.uji.es, schiva@emc.uji.es


Keywords: spray drying, multiphase droplet, droplet drying, acoustic levitator, porcelain tiles




Abstract

The drying behavior of suspension droplets is important in many industrial applications for the material processing, ceramic,
chemical or food industry involving spray dryers. This fact is particularly significant for high load and temperature conditions
close to those found in the mentioned industrial applications (Masters, 1991). In this work, the drying behavior of acoustically
levitated multiphase droplets has been experimentally investigated. The experiments have been performed using ceramic
suspensions like those used in the manufacture of porcelain tiles. The initial composition has a mean particle size and relative
density of 3.25 utm (standard size) and 2.7 respectively. An acoustic tube levitator modified to allow experiments at high
temperature conditions has been used (Mondragon et al., 2009). The flow rate, temperature and relative humidity of this air stream
can be controlled by an air conditioning system. A CMOS camera and a back-light illumination system are used to measure the
droplet cross-sectional area and vertical position of the droplet during the drying process (Yarin et al., 1999, 2002; Kastner et al.,
2001; Brenn, 2005). The effect of the flocculation state (flocculated-deflocculated), solid mass load (0.65 < Ys< 0.70), particle size
distribution of the porcelain composition (1.95 utm < dp50 < 3.25 utm), ambient air temperature (700C < T < 1000C) and initial
droplet volume (0.4 tl < Vo < 0.7 p l) on the mean porosity of the grain and its mechanical strength has been studied. The
experimental results obtained indicate that the most important parameters to be considered for the porosity are the particle size of
the composition and the initial solid mass load. The most important parameters to be considered for the mechanical strength are the
initial droplet volume, the particle size of the composition and the initial solid mass load. High initial solid mass load and standard
particle size distribution are the optimal conditions to obtain low porosity and high mechanical strength.


Introduction

Spray drying is the process by which a fluid feed material is
transformed into a dry powder by spraying the feed into a hot
drying medium. This is a well-known method of drying and
nowadays covers large number of applications for products
ranging from food to ceramic and chemical industry. It is an
efficient means of drying due to the large surface area
available for heat and mass transfer as a result of atomizing
the liquid into very small droplets. Properly done, spray
drying is an economical and continuous operation which
produces a powder of uniform and repeatable characteristics.
The spray drying is a complex industrial process that includes
physical processes such as spray atomization, heat and mass
transport from the droplets to the surrounding gas, drop-wall
interactions, etc. Drying behavior of droplets of liquid-solid
suspensions in a gas is of significant importance. This fact is
particularly significant for high load and temperature
conditions close to those found in industrial applications.
Droplet drying models can be used to relate the final powder
properties (such as the grain diameter distribution, mean
porosity, morphology, etc) with the spray dryer design and


process parameters.
The drying process of a liquid-solid suspension droplet is
characterized by two drying periods. In the first period, the
evaporation is produced in the droplet surface and the droplet
diameter, d, decreases following the d2-law. In the second
drying period a crust is formed in the droplet surface and the
evaporation is produced through the pores of the crust. In this
stage the droplet (or grain) diameter is constant and a droplet
collapse, hollow or explosion can occur, modifying the final
grain morphology.
It is not possible to obtain detailed information about the
droplet drying process in industrial or laboratory-scaled
dryers. For the study of the drying behavior of the suspension
droplets single droplets experiments are needed. In this
regard levitator tubes present some advantages with respect
to conventional methods since there is no contact between
the droplet and the solid. In the last decades acoustic levitator
tubes have been extensively used to study the drying
behavior of pure liquid, multi-component liquid and
liquid-solid suspension droplets. In this regard, Yarin et al.
(1999) predicted the droplet shape and evaporation rate of
acoustic levitated liquid droplets. Kastner et al. (2001)





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


developed the experimental procedure to measure
evaporation rates in both drying periods. Yarin et al. (2002)
modeled the drying behavior, obtaining the duration of the
first drying period and an experimental correlation for the
final grain porosity. Finally, Brenn (2005) modeled the solid
concentration fields inside the droplet; predicting the
appearance of hollow grains. Most of the available data on
the drying behavior of suspension droplets in acoustic
levitators were obtained for moderate temperatures (T<800C).
However, higher temperature conditions can be found in
important industrial applications. Recently, Mondragon et al.
(2009) have developed a levitator tube modified to allow
experiments at high temperature condition. This system
allows temperatures up to 150 C.
In case of using spray dryers in the ceramic industry, it is very
important that the drying process produces certain final
powder characteristics which favor later stages of the
manufacturing process, increase the production and also
improve the quality of the resulting ceramic tiles.
Porcelain tile is a product characterized by low water
absorption (usually less than 0.1%) and excellent mechanical
properties. To enhance tile aesthetic qualities, much of the
porcelain tile production is polished to provide a high-gloss
surface finish, in which certain closed pores in the tile body
become visible. This apparent porosity of the polished tile,
which had been closed porosity before polishing, sometimes
lowers the product's stain resistance (Amoros et al. 2007).
In this work experiments have been performed using ceramic
suspensions like those used in the manufacture of porcelain
tiles. The effect of the flocculation state
(flocculated-deflocculated), solid mass load (0.65 < Ys <
0.70), particle size distribution of the porcelain composition
(1.95 gum < d5so < 3.25 pim), ambient air temperature (700C <
T < 1000C) and initial droplet volume (0.4 gl < Vo < 0.7 gl)
on the mean porosity of the grain (s) and its mechanical
strength (oR) has been studied. The optimal conditions
established for this material are low porosity and high
mechanical strength. The former is the most important and
the lowest porosity is desirable, while the latter has to be high
enough to resist handling and processing but not so high to
deform easily during pressing process.


Nomenclature


diameter
force
radius
temperature
volume
mass fraction


Greek letters
E porosity
gt viscosity
p density
CR mechanical strength

Subscripts
D droplet
G grain
max maximum


particle
solid
initial
10% (mass) of particles below dplo
50% (mass) of particles below dp50
90% (mass) of particles below dp90


Experimental Facility and Measurement Techniques

The experimental facility has also been detailed previously in
Mondragon et al. (2009). The experimental setup is
composed of three systems (see Fig. 1 a): an acoustic
levitator consisting of an ultrasonic 58 kHz horn and a
concave reflector, an optical system consisting of a white
light source with a diffuser and a CMOS camera with a
macro lens and a gas conditioning system (not shown in the
figure) controlling the temperature, flow rate and relative
humidity of the air inside the levitator tube.
The levitator tube (tec5 AG Sensorik und Systemtechnik)
produces a standing wave and pressure nodes where the
droplet can be located. The original levitator tube has been
modified in order to allow experiments at high temperature
conditions (see Fig. 1 b). In this way, two separated metallic
chambers can be found in the levitator tube. The section
identified as "cold chamber" contains the ultrasonic
transducer of the levitator. The temperature of this chamber
can not be higher than 60C in order to prevent damages in
the piezoelectric transducer and it is controlled by forced
convection using cold air. The section identified as "hot
chamber" contains the levitator reflector and the multiphase
droplet. The temperature of this chamber is controlled by an
electric heater at the wall and by an air stream that enters the
levitator tube through an array of holes located in the
reflector. The flow rate of the air stream is set to 0.5 1/min and
it is used to ensure constant drying conditions around the
droplet and deplete the acoustic vortex system around the
droplet from liquid vapor. The droplet is inserted into the
acoustic field with a syringe. The needle of the syringe can be
introduced into the acoustic field using a hole centered in the
horn (dotted line in Fig. 1 b). Using this levitator tube
configuration it is possible to work with temperatures up to
1500C. However, the droplet insertion method limits the
maximum temperature working condition to 1200C (for
higher temperatures the droplet is dried before it is expelled
from the syringe tip).
The air conditioning system (CEM System W-202A,
Bronkhorst High-Tech B.V) is composed of an air-drying
cartridge, a two-mass flow controllers and a
mixer/evaporation unit. This system allows temperatures up
to 2000C and a humidity up to a dew point of T=800C. In
order to prevent the air stream cooling from the conditioning
system to the levitator tube the tube that connect both devices
as well as the reflector need to be heated using electric
heaters.
A CMOS camera (UI-1220M, IDS-Imaging Development
Systems GMBH) (752x480 pixels, 87 frames per second)
and a back-light illumination system are used to measure the
droplet cross-sectional area and vertical position of the
droplet during the drying process. The spatial resolution of
the images can be varied with the macro lens. A resolution
range between 250 and 500 pixels/mm has been used in the
experiments depending on the initial droplet volume value.


Paper No






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


F
R 07max


COOLING AIR

LIGHT SOURCE


HOT CHAMBER --- ELECTRIC HEATER

|INSULATOR


<- AIR SUPPLY
b)

Figure 1: a) General sketch of the experimental set-up,
b) modified acoustic levitator tube.

The processing of the images has been implemented in
Matlab. Thus, the equivalent diameter and position of the
droplet during the drying process are measured. With this
information is possible to obtain the mean porosity of the
grain. It can be obtained using the following equation:


-1 "s
=\----
V
G&


where Vs and VG are the volumes occupied by the solid phase
and the dried grain, respectively and,


V, = Vo (2)
Ps

where pD and Ps are the densities of the liquid-solid
suspension and solid phase respectively.
In order to obtain the mechanical strength of the granules,
diametric compression of single granules was performed.
Typical load-displacement curves were obtained for each
experimental condition. With this information the
mechanical strength of the grains can be calculated by means
of the following equation:


where Fm, is the maximum force resisted by the grain before
breaking and R is its radius.


Materials

All the experiments were carried out with water-based
ceramic suspensions. The composition is the same used in the
manufacture of porcelain tiles (clays, sand and feldspar in the
proper proportion). A standard suspension with mean particle
size, initial solid mass load and flocculation state similar to
those found in industrial processes is established with values
3.25 gim, 0.65 and deflocculated suspension respectively.
These variables have been modified to analyze their effect on
the output variables (e and aR).
In order to achieve a mean particle size of the composition
less than the standard value, the milling time has been
increased. Fig. 2 shows the particle size distribution of the
standard composition (milling during 10 minutes) and the
reduced size composition (milling during 4 hours). The mean
particle size, dpso, is the diameter value corresponding to a
50 % of cumulative mass.


100- -

80

60

40-- -- -


20
4__ _Standard (10 min)
--- o --Reduced size (4 hours)


01 1 10
Diameter, dp [pm]


Figure 2: Particle size distribution of the standard and the
reduced size composition.

The flocculation state has been modified by addition of
different amounts of deflocculant additive. The deflocculant
used has been a Na5P3010 : Na20SiO2 (1:3) mixture. For
each composition and solid mass load the deflocculation
curve has been obtained as shown for dp50 = 3.25 gLm, Ys =
0.65 in Fig. 3 a). The amount of deflocculant needed to
achieve the deflocculated state for each case was that one
which provided the minimum viscosity and thixotropy
(difference in viscosity measured after one and six minutes),
while to achieve the flocculated state an amount far from the
minimum was chosen. For each condition the rheogram was
measured (Fig. 3 b). In these curves the different theological
states can be seen at low shear rates. At high shear rate,
suspensions with the same mass load have the same viscosity
independently of its state.


Paper No


TRANSDUCER


COLD CHAMBER






Paper No


3000


--- 1 mm
- 6 mln


E 2000



> 1000



0


0 01 02
Deflocculant [%]


- Deflocculated
- Flocculated


0.01
0.01


1 10 100 1000 10000
Shear rate [1/s]
b)


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

three variables have significant effects on the porosity. On
the contrary, the influence of ambient air temperature and
initial droplet volume on the porosity is not relevant.
The significant variables for porosity are analyzed in Fig. 4
b), which shows the variation of the porosity when main
effects change from lower to upper values.


0 1 2 3 4 5 6
Standardized effects

a)

Main Effects Plot for Porosity


Flocc. Deflocc. 0.65 0.70
Flocculation state Ys


1.95 3.25
dP50


Figure 3: a) Deflocculation curve and b) rheogram for the
standard suspension, do50 = 3.25 Lm, Ys = 0.65.


Results and Discussion


Mean particle size of the composition, initial solid mass load,
flocculation state, ambient air temperature and initial droplet
volume have been modified in order to study their effect on
the mean grain porosity and its mechanical strength. The
range of variation of the input variables is the following: 1.95
gm < dpso < 3.25 Lm, 0.65 < Ys < 0.70,
flocculated-deflocculated, 700C < T < 1000C and 0.4 gl < Vo
< 0.7 gl respectively.
An experimental matrix based in a factorial design 2k has
been chosen. As a result, a total number of 32 test cases were
carried out. All the results were analyzed using Anova
method by means of Statgraphics. The effects of inputs on
outputs are shown in the Pareto charts, where the most
sensitive input variables can be identified. Each of these
graphs shows an ordered bar chart of the absolute effects
scaled by P-values, essentially the number of standardized
effects beyond the mean response. The line at 2.31 P-value
represents a significant level for achieving 95% confidence
that a given effect did not just occur by chance.

Influence of variables on porosity

Fig. 4 a) shows the Pareto chart for porosity, where the
following codes have been used for the input variables: A for
flocculation state, B for initial solid mass load, Ys and C for
particle size of the composition, dp5o. It can be seen that these


Figure 4: a) Standardized effects and b) main effects for
porosity

The degree to which particles are able to rearrange during the
drying process influences the final granules porosity.
Deflocculation of the suspension involves an increase in the
mean grain porosity. The granule formation is determined by
the strength of the floc structure. When the slurry is
deflocculated, a crater may form from the inward collapse of
the surface of a forming granule when the particle-packing
density in a droplet continues to increase after the droplet size
become fixed by the formation of a rigid shell, leaving an
internal void and a hollow grain (Walker et al. 1999).
Moreover, the deflocculant chosen acts by a steric
mechanism in which sodium silicate forms a protector
colloid and sodium tripolyphosphafe adsorbs over the
particles leading to an increase in the effective particle
volume. In this case the particle-packing is worst than in the
flocculated one.
The increase in the initial solid mass load leads to a decrease
in the porosity. Duffie and Marshall (1953) demonstrated that
the variation of the grain density between two different
conditions depends on the relationship between mass load
ratio and grain diameter ratio, the granules being denser
when the former is bigger than the latter. It is obvious that in
droplets with equal initial volume, the higher the mass load,
the higher the number of particles inside the droplet and the
contacts among them. As a result, the arrangement of the
particles when the droplet dries is more uniform and the






Paper No


packing leads to a less porosity.
Finally, the most important effect on the porosity is the
particle size of the composition. It is observed from the
particle size distribution (Fig. 2) that the composition with
higher mean particle size (standard composition) has also a
wider size distribution, defined by the ratio between dp90 and
dplo. It means that there are more particles with different sizes
and thus, the small ones can fill the voids created between the
big ones when these latter approach and contact them during
the drying process. Therefore, an increase in the mean
particle size leads to a better particle-packing and a decrease
in final grain porosity.

Cross section of some grains has been observed by Scanning
Electron Microscopy (SEM). The biggest grains with Vo
= 0.7 gl (Fig. 5 a, b) present hollow morphology while the
smallest one with Vo= 0.4 gl (Fig. 5 c) present a bigger shell
thickness which makes the grain almost solid with a more
homogeneously distributed porosity. Fig. 5 a) and b) also
show the difference porosity obtained when the flocculation
state is changed, as mentioned before.


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

conditions, the expected value for porosity obtained by
means of the Anova method is 0.294 0.014.

Influence of variables on mechanical strength

Fig. 6 a) shows the Pareto chart for mechanical strength,
where the following codes have been used for the input
variables: A for flocculation state, B for initial solid mass
load, Ys, C for particle size of the composition, d5so and D for
ambient air temperature, T. It can be seen that the most
significant effect that influence the mechanical strength is the
particle size of the composition. Effects also important are
the initial solid mass load, the ambient air temperature and
the cross effect of flocculation state and initial solid mass
load.
The main effects for mechanical strength are shown in Fig. 6
b). Fig. 6 b) and c), where the variation of the mechanical
strength when main and cross effects change from lower to
upper values, can be respectively seen.


0 2 4 6 8
Standardized effects

a)

Main Effects Plot for Strength


0.65 0.70 1.95 3.25
Ys dP50


70
T


Figure 5: Cross section of grains under different conditions:
a) Vo = 0.7 pl, Deflocculated, b) Vo = 0.7 l, Flocculated and
c) Vo = 0.4 1l, Deflocculated.


With all this information, if a low porosity condition is
established as the optimum for the manufacturing process,
the input variables must be the following: standard particle
size distribution (wider distribution with higher mean particle
size), high initial solid mass load and flocculated suspension.
Moreover, although the initial droplet volume is not of
significance, it is desirable a small droplet volume in order to
achieve a more uniformly distributed porosity. Under these


b)

Interaction Plot for Strength

Ys=0.70


Ys=0.65
Ys=0.70 <


Ys=0.65
Flocculated Deflocculated
Flocculation state


c)

Figure 6: a) Standardized effects, b) main
cross effect for mechanical strength.


effects and c)






Paper No


The obtained results are related to the effect that the same
variables have on the porosity. As mentioned before, an
increase in the particle size of the composition and in the
initial solid mass load leads to a decrease in porosity. This
leads to denser grains with higher mechanical strength. Also,
an increase in the particle size involves more attractive Van
der Waals forces between particles which make grains less
breakable.
The increase in the ambient air temperature leads to a
decrease in the mechanical strength of the grains. This is
consequence of the increase in the drying rate that generates
an internal stress in the grains which break easier under an
external force.
Finally, from the cross effect (Fig. 6 c) it can be seen that
when the suspension is flocculated, the initial solid mass load
has no effect on the mechanical strength of the grains.
However, when the suspension is deflocculated two opposite
effects appear. These effects are also related with the porosity.
As mentioned before, a deflocculated suspension has a higher
porosity and, thus, a lower strength. That happens when the
initial solid mass load is low, but increasing the initial solid
mass load has the opposite effect and this is predominant
when the solid content is high.

With all this information, if a high mechanical strength
condition is searched as the optimum for the manufacturing
process, the input variables must be the following: standard
particle size distribution (wider distribution with higher mean
particle size), high initial solid mass load, low ambient air
temperature and deflocculated suspension. Under these
conditions the expected value for mechanical strength
obtained by means of the Anova method is 442 + 24 kPa.

The two first conditions are the same that those required for a
low porosity, but the deflocculation state is different.
However, in this case the highest strength is not desirable.
Actually, it has to be high enough to resist handling and
processing but small enough to allow an easy deformation
during pressing process. Therefore, is more important to
know the conditions that provide the lowest porosity and
check if the mechanical strength is appropriate for the
process. Considering that the maximum load that supports a
grain inside a conventional silo is 100 kPa, the optimum
value obtained is a very good one.


Conclusions

The drying behavior of liquid-solid suspension droplets has
been investigated in experimental conditions close to those
found in ceramic industry. In order to achieve high
temperature conditions a standard levitator tube modified
(drying temperature up to 120 OC) has been used. A ceramic
suspension with the same characteristics that those used in
the manufacture of industrial porcelain tiles has been used as
a reference. The flocculation state, the initial solid mass load,
the particle size distribution of the porcelain composition, the
ambient air temperature and the initial droplet volume have
been modified to study their effect on the mean porosity of
the grain and its mechanical strength:


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

- The significant effects that influence porosity are the
particle size of the composition, the initial solid mass load
and the flocculation state.
If low porosity grains are desirable, standard particle size
distribution (wider distribution with higher mean particle
size), high initial solid mass load and flocculated suspension
is needed. Moreover, small initial droplet volume gives a
more uniformly distributed porosity.

- For the mechanical strength, the significant effects are the
particle size of the composition, the initial solid mass load,
the ambient air temperature and the cross effect of
flocculation state and initial solid mass load.
If grains with high mechanical strength are desirable,
standard particle size distribution (wider distribution with
higher mean particle size), high initial solid mass load, low
ambient air temperature and deflocculated suspension is
required.


Acknowledgements

The authors gratefully acknowledge the financial support
from Fundaci6 Caixa Castell6-Bancaixa (project:
P11B2006-37).

R. Mondrag6n thanks the Spanish Ministry of Education for
a predoctoral fellowship (FPU program, Ref.
AP2008-01077).


References

Amoros, J.L., Orts, M.J., Garcia-Ten, J., Gozalbo, A. &
Sanchez, E. Journal of the European Ceramic Society, 27,
2295-2301 (2007)

Brenn, G. International Journal of Heat and Mass Transfer, 48,
395-402 (2005)

Duffie, J.A. & Marshall, W.R. Chemical Engineering
Progress, 49, 480-486 (1953)

Kastner, O., Brenn, G, Rensink, D. & Tropea, C. Chem. Eng.
Technol., 24, 335-339 (2001)

Masters, K. Spray Drying Handbook. Longman Scientific &
Technical. (1991)

Mondragon, R., Hernandez, L., Julia, J. E., Chiva, S., Jarque,
J. C. & Cantavella, V llth Triennial International Annual
Conference on Liquid Atomization and Spray Systems, Vail,
Colorado USA (2009)

Walker, W.J., Reed, J.S. & Verma, S.K. Journal of the
American Ceramic Society, 82 [7], 1711-1719 (1999)

Yarin, A.L., Brenn, G, Kastner, O., Rensink, D. & Tropea, C.
J. Fluid Mech., 399, 151-204 (1999)

Yarin, A.L., Brenn, G, Kastner, O. & Tropea, C. Physics of
Fluids, 14, 2289-2298 (2002)




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