Study a new atmospheric freeze drying system incorporating a vortex tube and multi mode heat input 2

Chapter-2 Literature Review
CHAPTER 2
LITERATURE REVIEW

The first part of review explains atmospheric freeze drying as a modifications
technique of the vacuum freeze drying process and covers relevant information on
modeling and experimental investigations. This chapter also contains some basic
information on vibrating bed dryers along with a discussion of the vortex tube and
multimode heat transfer, which are applied in this research to develop a new integrated
atmospheric freeze drying system.

2.1 Freeze Drying Under Vacuum or Atmospheric Pressure

Vacuum freeze-drying is a well known process for highly heat-sensitive materials.
This method is used as a benchmark of product quality as it often gives the best quality
dried products. For example, Marques et al. (2006) investigated the physical properties
of vacuum freeze drying of tropical fruits and showed that this process gives high
quality products. They also proved that vacuum freeze-dried (VFD) foods have high
porosity and low apparent density. VFD also conserves color, flavor, and taste and
provides rapid rehydration.

The main disadvantages of the freeze drying technique are its high fixed and operating
costs as demonstrated by Matteo et al. (2003). The latter are due to the series of
energy-intensive operations involved in the process: freezing of the fresh product,
heating of the frozen foods at low temperature to induce sublimation, condensation of
water vapor and mechanical energy needed to maintain high vacuum. Moreover,
vacuum operations are mainly carried out batchwise, which represent an additional
cost together with requirements of the apparatus operated under vacuum.

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Chapter-2 Literature Review

Liapis et al. (2007) showed that the limiting step of the traditional freeze-drying
process under vacuum is the transfer of heat to the product due to the decrease in
thermal conductivity with decreasing the pressure of the freeze-drying chamber.

Efforts have been made to improve the vacuum freeze drying method but none has
given economically satisfactory results as far as industrial applications are concerned.
In particular, as disclosed in U.S. Patent specification No. 3,319,344, an attempt has
been made to fluidize the product using vibration to be freeze-dried under vacuum in
order to improve the heat and mass exchanges, but the major drawbacks resulting from
operating under vacuum are not overcome in this design.

2.2 Atmospheric Freeze Drying – Fixed Bed Dryer

The early workers in atmospheric freeze drying, namely the works of Meryman (1959),
Lewin and Matelas (1962) and Woodward (1963), reported varying degrees of success
when fixed beds of desiccants were employed to freeze dry foods and other biological
materials in the absence of vacuum. The potential for atmospheric freeze-drying was
demonstrated by Meryman (1959). He showed that the drying rate of a material
undergoing freeze drying is a function of ice temperature and the vapor pressure
gradient between the site of water vapor formation and the drying media, rather than
the total pressure in the drying chamber. He invented process using either a fixed bed
water vapor adsorbent adjacent to the frozen product or a condenser in a stream of cold
air.

However, drying periods were observed to be very long. In order to reduce drying time,
other researchers focused attention on reducing product dimensions and on utilizing
fluidized beds through modeling and experimental investigations.

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Chapter-2 Literature Review

2.2.1 Experimental studies of atmospheric freeze drying-fluidized bed dryer


A number of researchers have performed experimental studies on atmospheric freeze-
drying using a fluidized bed to investigate the drying performance on different size and
shape of products. Quality parameters of the dried products have also been investigated.

Malecki et al. (1969) carried out atmospheric fluidized-bed freeze drying of apple juice
and egg white. Their work supports the conclusion of Dunoyer and Larpusse (1961)
and Woodward (1963) that the drying rate in atmosphere can reach that under vacuum
if the particle size is sufficiently small. However, Malecki et al. (1969) found that for
apple particles the bed had to be at -34
o
C to prevent sticking and, at this temperature,
only 1 percent ice was sublimated per hour, which is extremely low and hence not
attractive from practical standpoint.

Boeh-Ocansey,O. (1985) conducted experiments to investigate the drying kinetics at
different product thicknesses, chamber temperature (-5
o
C, -10
o
C and -15
o
C) and using
different adsorbents on carrot slices (1.8 x 33, 3.4 x 27, 4.8 x 26 and 5.8 x 22 mm).
They carried out their experiments in a drying chamber consisting of a vertical
cylindrical column 10 cm in diameter and 100 cm high at atmospheric pressure. They
obtained higher freeze drying rates with particles of activated alumina (0.4 mm
average diameter) than with activated carbon. Their results showed that a higher drying

temperature (-5
o
C) is preferable to increase the drying rate. They compared their
results with conventional vacuum drying and found that product thickness is more
sensitive in drying kinetics to a great extent in AFD then in VFD.

An apparatus and a technique for spray freeze aqueous solution at very low
temperatures (-90
o
C) and for subsequent dehydration of the resulting frozen particles

11
Chapter-2 Literature Review
in a stream of cold, desiccated air was developed by Mumenthaler and Leuenberger
(1991).They investigated the influence of various operating variables on the drying
kinetics as well as the quality of the products. They found a dry, stable and intact cake
of same shape and size as the original frozen mass, with sufficient strength to prevent
cracking and powdering or collapse. They also observed uniform color and rapid
solubility upon reconstitution in water; increased inner surface area and good crystal
structure of the active substance. They also reported an enhanced heat and mass
transfer between the circulating drying medium and the frozen sample.

Alves-Filho et al. (1998) used a fluidized bed as the first-stage freeze-dryer at
atmospheric pressure in a two-stage heat-pump system without adsorbent. Adjusting
the heat pump dryer components to keep the air temperature below the drying
product’s freezing point controlled drying condition in the first stage fluid bed dryer.
Their control strategy was based on the specific enthalpy curves developed by Alves-
Filho et al. (1996). The product residence time in the first-stage dryer was selected to
reduce the moisture content to the critical values. Afterwards the semi-dry product was
transferred to the second stage fluid bed to be dried at higher temperatures. The

advantages of their two-stage system are that low-temperature drying reduces the
moisture content while maintaining product quality while higher-temperature drying
increases the overall heat-pump dryer capacity. These authors reported excellent
quality of dried shrimp, apple pieces, carrot slices, etc., at a relatively high cost,
however.

Donsi et al. (2000) here demonstrated the feasibility of atmospheric freeze-drying for
shrimp and showed that the drying time is indeed an order-of-magnitude longer than
that for the vacuum process. To shorten the drying time, part of the water was removed

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Chapter-2 Literature Review
by osmotic dehydration. Freeze-drying was carried out in a fluidized bed using a
mineral adsorbent (Zeolite particles 88 μm mean diameter) or an organic adsorbent
(wheat bran). The AFD dried shrimp dehydration properties that are similar to those
obtained in conventional freeze-drying. The economics of the process were not
reported.

Bussmann et al. (2003) invented an apparatus for drying a product using a regenerative
adsorbent which can be carried out in an energy-saving manner. According to their
invention, the product is dried by bringing it into contact with adsorbent, water being
taken up from product by the adsorbent. Subsequently, the adsorbent is regenerated
with superheated steam.

A detailed investigation was carried out by Matteo et al. (2003) of atmospheric freeze-
drying in a fluidized bed mixed with different compatible adsorbent particles. They
fluidized 1 cm long potato cylinders of various diameters with various particulate
adsorbents. They measured higher heat and mass transfer coefficients compared with
vacuum freeze drying due to convective heat and mass transfer, which is absent in
VFD. They showed that higher freezing (-10

o
C) and fluidized bed temperatures (-6
o
C),
compatible adsorbent (bran & bentonite), reduced sample size (6mm), smaller
adsorbent particles (400 μm), higher product/adsorbent weight ratio (1/8), moderate
regeneration temperature (60
o
C) are conducive to enhancing the dehydration rate.
However, fluidization velocity had no significant effect on the dehydration rate. Their
work also revealed that the size of the product is the key parameter in atmospheric
freeze-drying. Finally, they showed that there is a significant reduction of the energy
cost relative to vacuum freezes drying due to the absence of a vacuum chamber and
ancillary equipment.

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Chapter-2 Literature Review
Stawczyk et al. (2005) investigated the kinetics of atmospheric freeze-drying and
quality of apple dried cubes; all properties such as dehydration rate, shrinkage, color,
antioxidant content etc. were reportedly very good. They conducted experiments at
three different temperature increasing strategies; namely, constant inlet air temperature
(CT); inlet air at different temperature (DT) option and inlet air of ascending
temperature (AT) at a fixed airflow. They found ascending inlet temperature condition
maintained a stable drying rate during the whole drying process to obtain an
economical AFD process and generally better quality dried product. Their results
showed that AFD dried products at lower temperature (-10
o
C), had characteristics of
rehydration kinetics and hygroscopic properties similar to products obtained by
vacuum freeze drying. They also found that AFD products are better than hot air-dried

products in terms of their anti-oxidative activity.

Strommen et al. (2005) carried out an experimental study of atmospheric freeze drying
of cod using a fluidized bed dryer coupled with a heat pump. They found lower bulk
densities, higher rehydration, and light color, which is similar to vacuum freeze drying,
when dried at low temperature (-5
o
C) over a longer residence time of about 10 hours.
They also noted that the typical specific moisture extraction rate (SMER) for
atmospheric freeze drying with heat pumps is in the range 4.6 to 1.5 kg of water per
kWh.

Claussen et al. (2005) analyzed the physical and quality parameters (color, water
content, rehydration properties and sorption isotherms) of traditional Norwegian
stockfish and compared them with atmospheric freeze dried (AFD) cod fillets.
Sorption isotherms of several stockfish and AFD cod samples were measured with a
CIsorp water analyzer to determine the optimal storage conditions. They found 4 and 5

14
Chapter-2 Literature Review
times higher rehydration index for atmospheric freeze dried cod compared with
naturally dried stockfish. In addition they observed the atmospheric freeze dried fish
had brighter color than that of the naturally dried cod.

The influence of AFD on the physiochemical properties, quality, and functional
properties (color, water content, bulk density, rehydration properties, sorption isotherm,
specific enzyme activity, solubility, protein denaturizing) of potato was investigated by
Claussen et al. (2007). Their results showed that atmospheric freeze drying is a gentle
drying process than spray or vacuum freeze drying. The solubility measurement gave
better results for AFD potato protein samples at pH between 3.5 and 5, while the

lowest value was obtained for spray dried samples over the whole pH range. Moreover,
both enthalpy measurements and sorption isotherms indicate reduced protein
denaturizing of AFD samples, while specific enzyme activity was at same level for all
dried samples.

Drying kinetics, sorption properties, shrinkage, and freezing point depression were
determined by Claussen et al. (2007) in atmospheric freeze drying (AFD) of pieces of
apple, turnip cabbage, and cod. They observed that drying at -5
o
C resulted in a greater
shrinkage than drying at –10
o
C. Claussen et al. (2007) also carried out measurement of
the physical properties of atmospheric freeze-dried cod and turnip cabbage. True
density, apparent density and pore size distribution were measured using helium
pycnometry, geopycnometry and light microscopy. They concluded that thawing
during drying and product shrinkage affects the drying rate and the diffusion of water
leading to poor product quality of the end product.



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Chapter-2 Literature Review
2.2.2 Modeling of atmospheric freeze drying


Heldman et al. (1974) proposed a simple mathematical model and validated it with
their results experimentally. They concluded that the rate of drying, as expected, is
higher for smaller particles and by increasing the surface mass transfer coefficient.
Since the drying kinetics for AFD are determined by internal resistance to heat and

mass transfer. Their conclusion about effect of external mass transfer coefficient is
surprising.

Boeh-Ocansey et al. (1983, 1985) reported measurements of kinetics of ice sublimation
in vacuum and in a fluidized bed drying system under atmospheric condition. They
showed that for ice sublimation the recommended partial vapor pressure, temperature
of drying chamber and relative humidity of air are: 4.58 mm Hg, approximately 0
o
C
and below 20 ppm, respectively. Since there is freezing point depression with soluble
components in drying material is 0
o
C really the optimum temperature.

A further kinetics study of ice sublimation in a fluidized-bed dryer operating under
atmospheric conditions was reported by Boeh-Ocansey and Wachet (1986).
Mathematical expressions were formulated to account for mass variation and
dimensions of ice samples during sublimation. They investigated the influence of
chamber temperature and air flow rate on sublimation and showed that sublimation of
ice was obtained at drying chamber temperatures greater than 0
o
C (7.6
o
C maximum).
They also noted that there exists a correlation ice sublimation temperature and the
temperature of the fluidized bed. Ice sublimation temperature can be predicted
accurately for a given fluidized-bed temperature. Finally they showed that the kinetics
of ice sublimation was regulated by the expression: M/M
0
= (1 – t/t

T
)
2
where M and t

16
Chapter-2 Literature Review
represent mass of product and time, respectively. Subscript o and T stand for initial
and final time, respectively.

Wolf and Gilbert (1990b) proposed a model for atmospheric freeze-drying in a
fluidized bed of a particulate adsorbent (starch) incorporated in different mass ratios.
Their model was based on uniformly retreating ice front (URIF). They found spherical
shape, minimum thickness (2mm), higher drying temperature (-5
o
C) and minimum
regeneration temperature of adsorbent (50
o
C) are most advantageous for drying. They
also concluded that a mass fraction between the mass of water sublimated and the mass
of adsorbent used (m
w
/ m
a
), of the order of 0.10, or even 0.05 at higher temperatures,
seems satisfactory. They validated their model with experiments with potato
parallelepipeds of different thicknesses (2, 3 & 5 mm).

Joseph et al. (1996) presented two sets of nonlinear coupled heat and mass transfer
models to describe the absorption and desorption process. Model I describes the

temperature and moisture distribution in a porous medium with a moving evaporation
front. They reported that in the two phase system with moving boundary condition, the
rate of movement of the evaporation front decreased with deepening of the evaporation
front in the porous body. They showed that the higher the value of nondimensional
vaporization parameter γ, the slower is the movement of the evaporation front. The
temperature decreased and the moisture content increased as the nondimensional
vaporization parameter γ increased. In model II they described a set of simultaneous
heat and mass transfer equations describing moisture adsorption during the steeping of
barely kernels.


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Chapter-2 Literature Review
A study of freeze drying, by immersion in an adsorbent medium both at atmospheric
pressure and under vacuum was carried out by Lombrana and Villaran (1996). They
used spherical moistened particles of commercial cereal food paste as a drying product.
They employed zeolite as adsorbent particles (0.63g/cm
3
density, diameter 0.7 mm)
with adsorbent to product mass ratio of 10:1 in a fluidized bed dryer. They evaluated
the effect of pressure and temperature on the drying kinetics through a model by
considering a uniformly retreating ice front in spherical geometry. Their model
calculated the pressure and temperature at the sublimations front in terms of the
product moisture. Conditions of -5
o
C and pressures of 310 and 410 mm Hg, but
without adsorbent, were also investigated to analyze the possibility elimination of
adsorbent when vacuum was employed. They found that values of total time and
shrinkage varied from 400 to 390 min and from 0.567 to 0.573, respectively, they
concluded that adsorbent usage is recommended. Also they proposed some operational

strategies which can reduce the process duration without damage to product quality.
First step starts at atmospheric pressure and a temperature of -10
o
C followed by a
second step at where low pressure with temperature in the range 0
o
C to 15
o
C and a
third step at atmospheric pressure with temperature below 15
o
C.

Lombrana and Villaran (1997) developed a mathematical model for AFD in a fluid bed
dryer. They evaluated the effect of pressure and temperature on the drying kinetics
through a model by considering a uniformly retreating ice front in spherical geometry.
Their model calculated the pressure and temperature in the sublimations front in terms
of the product moisture. Good agreement was found between predicted and
experimental results.


18
Chapter-2 Literature Review
Li et al., (2007) presented a film sublimation model that couples a fluid dynamic
model for the ice-vapor interface with a URIF model for vapor transport through the
dry porous zone. The interface model considered a vapor film resulting from
sublimation from a virtual ice wall under local thermodynamics equilibrium. They
used the Fluent CFD package to describe the diffusivity vapor transport from the
interface. Their simulation captured the drying phenomena of a plate of ice, a chitosan
membrane, and a 10-mm-wide cube of apple at temperatures of -4

o
C, -8
o
C, -12
o
C,
and -16
o
C. Comparison with experimental results showed good agreement mainly for
medium and low moisture contents and for highest drying temperatures.

A simplified mathematical model was developed based on uniformly retreating ice
front (URIF) considerations by Claussen et al, (2007). The model used theoretical
drying curves for atmospheric freeze dried foods in a tunnel drier. They proposed that
the model can be used to simulate industrial atmospheric freeze drying of different
foodstuffs in a tunnel dryer. They also found good accordance with their experimental
results.

2.2.3 Technical feasibility and economic viability

Technical feasibility and economic viability of atmospheric freeze-drying system have
been examined by a number of investigators through experimental analysis.
Winter (1968), Gunn et al. (1969) and Simatos et al. (1974) investigated the
applications of various adsorbents in freeze-drying. They demonstrated that activated
carbon, silica gel, activated alumina and molecular sieves are suitable for the drying
application. Their work agreed with the conclusion of Meryman (1962) that the
method could be improved by circulating the air stream through a bed of molecular

19
Chapter-2 Literature Review

sieve or over a refrigeration coil to remove water vapor from air. Sandall et al. (1967)
reported that the thermal conductivity of the porous dry layer increases as total
pressure increases while the diffusivity of water vapor across the same layer decrease.

Gibert (1979) experimented with atmospheric freeze drying using a fluidized bed. He
used pieces of carrot in the form 3 cm diameter discs which were 3.5 mm thick.
Activated aluminas (mean diameter between 250 to 500 microns) were used as the
adsorbent. Water vapor pressure inside the drying chamber, flowrate of the carrier gas,
freezing and freeze drying temperature were about 0.1 mm of mercury, approximately
1.3 V.sub.mf to 1.7 V.sub.mf, -30
o
C and –5
o
C, respectively. His method revealed that
the rate of freeze drying was higher at the beginning of the test.

A detailed investigation of heat and mass transfer co-efficient under vacuum and in
atmospheric fluidized-bed dryer was carried by Osei Boeh-Ocansey (1988). They used
ice samples of cylindrical (2.8 x 4.6, 3.0 x 4.8 and 5.4 x 1.9 cm) and block (2.3 x 5.0 x
12 cm) shapes as a product mixed with activated alumina granules as adsorbent (0.4
mm average diameter) in their experiments. The fluidized bed AFD dryer consisted of
a 10 cm diameter column, a refrigeration unit and an air blower, while the vacuum
dryer was made of a cylindrical stainless steel chamber, 28 cm in diameter and 26 cm
high. They found the ratio between the external heat and mass transfer coefficients
(h/K) for the fluidized bed dryer was constant at 398.8 kcal torr kg
-1

o
C
-1

but varied for
the vacuum dryer in the range 9.5 to 16.2 kcal torr kg
-1

o
C
-1
. They calculated the
individual heat and mass transfer co-efficient. The mass transfer co-efficient, k, for the
vacuum unit was 1.0 kg h
-1
m
-2
torr
-1
; for the atmospheric fluidized bed dryer, k varied
slightly with temperature, being 0.7, 0.8, and 1.0 kg h
-1
m
-2
torr
-1
, for a fluidized bed
dryer at 0, -10 and 20
o
C, respectively. At these temperatures, the heat transfer co-

20
Chapter-2 Literature Review
efficient for the fluidized bed dryer were 287, 321, and 402 kcal h

-1
m
-2

o
C
-1
,
respectively. For the vacuum dryer h was 9.5-16.2 kcal h
-1
m
-2

o
C
-1
.They showed that
values of the mass transfer co-efficient were comparable in the two drying systems but
heat transfer co-efficient in the fluidized-bed dryer was about 20-40 times greater than
that in the vacuum drying system.

The feasibility of a freeze drying process based on a fluidized bed dryer with
adsorbent at atmospheric pressure was demonstrated by Wolff and Gibert (1990a).
Experiments were performed in a fluidized bed freeze-drying column fitted with a
double jacket for the cooling liquid. In the course of drying, a 1x1 cm square base with
different thicknesses of potato sample (0.3 kg) was immersed into a 5-kg bed of corn
starch fluidized by air at with a superficial velocity of to 0.04 m/s. They also calculated
the cooling and heating energy requirements for the vacuum system to be about 3550
kJ/kg and 3780 kJ/kg, respectively, and for atmospheric freeze drying system to be
2250 kJ/kg and 3440 kJ/kg, respectively. Their results revealed that the energy cost is

lower by 38% and 34% for the cooling and heating requirements for atmospheric
freeze drying, respectively.

A review of the literature on atmospheric freeze-drying has recently been carried out
by Claussen et al. (2007). This review includes technological aspects, product
possibilities, and physical properties of products, drying kinetics, modeling and
simulation of an AFD system. However, there is still scope to enhance the drying rate
in an atmospheric freeze drying system using a vibrating bed and multimode heat input
to enhance heat transfer rate without causing melting of the ice. In addition, osmotic
dehydration can generally be used to reduce the energy consumption as well as to

21
Chapter-2 Literature Review
improve the drying rate and quality of the dried products on AFD system. As will be
seen later, this hypothesis did not turn out to be true.

2.3 Use of Vortex Tube to Obtain Subzero Temperature Air

Conventional atmospheric freeze dryers utilize a system of a mechanical heat pump to
lower temperature and a large condenser to reduce humidity of the air. At least two
mechanical agents are required for this operation, which is not economical from the
energetic point of view. It also takes time to setup, de-humidify and cool the drying
chamber. A vortex chiller tube can be used as a viable alternative to achieve required
characteristics of the carrier gas supplied to the drying chamber at atmospheric
pressure. The vortex tube is a simple device with no moving parts that is capable of
separating a high pressure flow into two lower pressure flows of different temperatures.
The vortex tube was first discovered by Ranque (1933). Later it was improved in
efficiency by Hilsch (1947). Ahlborn et al. (1994) developed a two-component model
to determine the limits of the increase and decrease in temperature within the standard
vortex tube. Their experimental data with air as the working fluid were within the

calculated limits.

Frohlingsdorf and Unger (1999) studied the phenomenon of velocity and energy
separation inside the vortex tube using CFX commercial code. They developed a 2D
axisymmetric model which allowed successful prediction of the experimental results.
They found that the application of the k-є model leads to substantial differences
between measured and calculated tangential velocity profiles; they replaced the k-є
model with a correlation from Keey (1972) for the calculation of the turbulence
viscosity. Their results show that the strength of energy separation can be fitted to both

22
Chapter-2 Literature Review
measured total temperature differences (cold/hot gas to the inlet total temperature) by
increasing the turbulent Prandtl number value, leading to damping of turbulent
diffusion in favor of the transfer of mechanical work. Mechanical work is transferred
from the cold to the hot gas by viscous friction, intensifying the dissipation processes
there; this results in a temperature rise of the hot gas portion.

Behera et al (2005) conducted a 3D computational fluid dynamic (CFD) and
experimental study of optimization of the Ranque-Hilsch vortex tube. Different types
of nozzle profiles and number of nozzles were evaluated by CFD analysis. They also
analyzed the swirl velocity, axial velocity, and radial velocity components as well as
the flow patterns including secondary circulation. They proposed optimum cold end
diameter (d
c
=7mm) and the length to diameter ratios (L/d =10 to 35). Furthermore,
optimum parameters for obtaining the maximum hot gas temperature (391 K for
L/D=30) and minimum cold gas temperature (267 K for L/D=35) were obtained
through CFD analysis and validated through experiments.

Aljuwayhel et al. (2005) also studied the energy separation mechanism and flow in a
counter-flow vortex using the CFD code FLUENT with the standard k-є and the RNG
k-є models of turbulence. A two-dimensional axi-symmetric model was developed that
exhibits the general behavior expected of a vortex tube. They reported that the RNG k-
є model provided better predictions. The vortex-tube flow can be divided into three
regions that correspond to: flow through the hot exit (hot flow exit), flow through the
cold exit (cold flow region), and flow within the device (re-circulation region). This is
contrary to the results of skye et al. (2006), who claimed that, the standard k-є model
with swirl performs better than the RNG k-є model, although both used the same

23
Chapter-2 Literature Review
commercial CFD code FLUENT. They validated their model results with experimental
data obtained from a laboratory scale vortex tube operated with room temperature
compressed air. They demonstrated that the energy separation exhibited by the vortex
tube can be explained primarily by the work transfer caused by a torque produced by
viscous shear acting on a rotating control surface that separates the cold flow region in
the opposite direction and therefore tends to reduce the temperature separation effect.

Eiamsa-ard and Promovonge (2006) carried out a 2D axi-symmetrical numerical
calculation using the algebraic Reynolds stress model (ASM) and the standard k-є
model for the simulation of a strongly swirling flow in a vortex tube. They used the
TEFESS code, based on a staggered finite volume approach with the standard k-є
model and first-order-numerical schemes. They noted that the use of ASM results in
more accurate prediction than does the k-є model. They proposed that the predicted
results for strongly swirling turbulent compressible flow in a vortex tube suggests that
the use of the ASM model which leads to better agreement between the numerical
results and experimental data.

The application of a mathematical model for simulation of thermal separation in a

Ranque-Hilsch vortex tube was carried out by Eiamsa-ard and Promovonge et al.
(2007). A staggered finite volume approach with the standard k-є turbulence model
and an algebraic stress model (ASM) was used to provide an understanding of the
physical behavior of the flow, pressure, and temperature in a vortex tube. They used
2D; second order upwind (SOU) and the QUICK numerical schemes. They compared
the first-order upwind and hybrid schemes. Their computations showed good
agreement between the results of the algebraic stress model (ASM) and experimental

24
Chapter-2 Literature Review
results. They concluded that the diffusive transport of mean kinetic energy has a
substantial influence on the maximum temperature separation occurring near the
region.

The fundamental mechanism of energy separation has been well documented by some
investigators (Aljuwayhel et al. 2005). However, due to lack of reliable measurements
of the internal temperature and velocity distributions, there is still need for more effort
to capture the real phenomena in a vortex tube. A 3D simulation is clearly a good
option to capture well the complex flow phenomena in the vortex tube. Literature
results revealed that only a few investigators have been worked on 3D simulation of
vortex tube. Therefore effort is needed to apply a 3-D model for more reliable
prediction of the complex vortex-tube flow.

2.4 Vibrating Bed Dryer

The principle of vibrating beds can be used as a strategy to improve fluidization
quality of irregular and cohesive materials which are hard-to-fluidized and to avoid
problems channeling, defluidization and slug flow. To improve drying rates by de-
agglomeration with, consequent increase of specific area for gas-solid contact, and to
accelerate the migration rate of moisture from the interior to the surface of the particles

in vibrofluidized bed was demonstrated by Pakowski et al. (1984).

Dong et al. (1991) carried out an experimental studied on the drying characteristics in
vibrated fluidized beds of corn plu-mule, silica gel and citric acid. They found that
application of vibration enhance the drying rate during the falling rate period. In the
optimum range of vibration parameters, the critical moisture content decreased

25
Chapter-2 Literature Review
significantly from 50% to 28% for corn plu-mule. They also demonstrated that effect
of amplitude is not clearly defined and no trend is discernible for corn plumule drying
in the VFD.

The behavior of a sawdust dryer in a vibrating fluidized bed dryer was analyzed by
Rogelio et al. (2000). Using mechanical vibration of the drying chamber, they found
significant reductions in the required air velocities for drying sawdust in a fluidized
bed. They proposed that this reduction can be up to 50% in relation to the minimum
fluidization velocity and up to a 70% with respect to the operating velocity used in
conventional fluid beds. They also demonstrated that sawdust with moisture content
higher than 2 kg/kg moisture can be vibro-fluidized with high degree of bed
homogeneity.

Soponronnarit et al. (2001) tested a commercial scale vibro-fluidized bed dryer for
paddy. The operating conditions were as follows: paddy feed rate, 4.82 t/h; paddy bed
height, 11.5 cm; airflow rate, 1.7m
3
/s; bed velocity, 1.4m/s; fraction of air recycled,
0.85; vibration factor, approximately 1 (frequency, 7.3 Hz and amplitude, 5mm) and
average inlet air temperature of 140
o

C. They concluded that vibro-fluidized bed could
reduce the moisture content of paddy from 28% to 23% d.b. with head rice yield and
rice whiteness of 37% and 41.2, respectively , while the paddy dried with ambient air
drying provided head rice yield and rice whiteness of 32% and 42.5 , respectively.
They also illustrated that the total electrical power of blower motor and vibration
motor that used in vibro-fluidized bed dryer was approximately 55% of electrical
power motor used in fluidized bed dryer without vibration.


26
Chapter-2 Literature Review
Roger et al. (2005) investigated a wide range of dimensionless vibrating number (Γ) to
quantify the vibration energy of a vibro-fluidized bed. It is defined as a function of the
amplitude (A) and of the frequency of vibration (f). They illustrated that the pressure
drop, standard deviations of pressure drop and minimum fluidization velocity were
strongly affected by both the amplitude and the frequency of vibrations. They
concluded that the dimensionless vibration number (Γ) must be used carefully when
applied as the only parameter to characterize the vibration effects in a vibro-fluidized
bed and the application of Γ on other studies involving vibro-fluidized beds deserves
further evaluation.

2.5 Multi-mode Heat Supply

Infrared and conduction heat can be used as heat sources in atmospheric freeze drying.
In recent years intermittent multimode drying has been reported as an effective means
of energy savings at enhanced drying rates and often yielding better quality dried
products.

Dostie (1992) found that intermittent IR drying is a viable option for drying processes
to effect large reductions in drying time. Nastaj (1994) found that application of

combined conductive-radiative heating in vacuum drying is advantageous and gives
considerable process intensification, i.e. shortening duration of the process. Ratti and
Mujumdar (1995) summarized the advantages of IR drying: high efficiency to convert
electrical energy into heat for electrical IR; radiation penetrates directly into the
product without heating the surroundings; uniform heating of the product; easy to
program and manipulate the heating cycle for different products and to adapt to
changing conditions; leveling of the moisture profiles in the products and low product

27
Chapter-2 Literature Review

28
deterioration; IR sources are inexpensive compared to dielectric and microwave
sources; they have a long service life and low maintenance.

Islam and Mujumder (2003) have given a comprehensive overview of their work on
multi-mode drying with a one-dimensional liquid diffusion model. Heat of wetting,
temperature and moisture dependent effective diffusivity and thermal conductivity as
well as changes in product density were accounted for in the model. They analyzed the
relative advantages of combining various modes of heat transfer e.g., convection,
conduction, radiation and volumetric heating in a microwave field. They found that
some combinations of heat inputs did not exhibit the best drying performance during
the whole drying period which reflects the need of appropriate switching between
different modes of heat input to get the optimum and energy efficient drying condition.
An experimental study was conducted using a batch heat pump dryer designed to
permit simultaneous application of conduction and radiation using potato as a model
heat sensitive drying object by Lan et al. (2005). They also developed a two-
dimensional model to predict the drying rate and temperatures within the slab during
drying and showed good agreement with their measurements.

Therefore, it can be concluded from the review of literature that atmospheric freeze
drying method with combination of vortex tube, vibrating bed dryer and multimode or
intermittent heat input would be a good alternative method to traditional vacuum
drying.
Osmotic dehydration can also be implemented preceding AFD to reduce the
initial moisture content prior to drying. As will be noted later, this pre-treatment in fact
leads to longer drying times and lower quality dried product.