Experimental Analysis of Fluidized Bed Freeze Drying

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Drying 2004 Proceedings of the 14th International Drying Symposium (IDS 2004)

So Paulo, Brazil, 22-25 August 2004, vol. A, pp. 526-532

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EXPERIMENTAL ANALYSIS OF FLUIDIZED BED FREEZE DRYING

P. Tomova, W. Behns, H. Haida, M. Ihlow and L. Mrl

Otto-von-Guericke-University of Magdeburg, Institute of Process Equipment andEnvironmental Technology, Post office box 4120, D 39016 Magdeburg,

[email protected]

Keywords: freeze drying, fluidized bed, adsorption isotherm, modelling

ABSTRACT

The present work deals with freeze drying in a fluidized bed operating at atmosphericpressure. This fluidized bed freeze drying process with integrated adsorption dryer

combines the advantages of the fluidized bed (high heat and mass transfer rates) with

the possibility of adsorption drying at low temperatures. The new process was experi-

mentally studied. Experiments have been carried out with variation of the gas inlet tem-

perature, the gas flow velocity, the bed mass and the particle diameter using a model

substance -Al2O3 and one biological product. The experimental investigations were

carried out in a fluidized bed plant with 100 mm nominal diameter which was built inan industrial freezer container. The adsorption isotherms of the model material were

measured using the desiccator method at 1C and 5 C. The Brunauer-Emmett-Teller

(BET) and Guggenheim-Anderson-de Boer (GAB) models were applied to describe the

adsorption isotherm. This supplies information for a later mathematical modelling of the

fluidized bed freeze drying process.

INTRODUCTION

Freeze drying is the best method for drying temperature-sensitive products with regards to their com-

position. This process is widespread in food and pharmaceutical industry, when the natural properties of

the products do not allow other drying technologies. The most conspicuous properties of freeze dried

products are the voluminous state and high specific surface allowing a fast dissolution in solvents. An-

other important difference to conventional drying processes is the avoidance of denaturation which can

occur in case of conventional drying by thermal stress or by concentration increase in the liquid phase.

(Hayashi, 1989; Oetjen, 1999) The great benefits of the standard vacuum freeze drying process, like high

product quality, are faced by high investments in equipment and high operation costs, caused by long

drying times. To compare, spray drying is four times and vacuum drying is two times cheaper than the


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standard vacuum freeze drying process (Wolff et al., 1990 Part 1). Therefore the new atmospheric freeze

drying alternative has been experimentally and theoretically investigated in order to determine energetic

and economic advantages.

MATERIALS AND METHODS

Two products have been used in the study of the new atmospheric fluidized bed freeze drying: the

spherical model substance -Al2O3with a mean diameter of

dP=1.93 mm and one biologi-

cal product. The experiments

were prepared adjusting the

desired initial load of the

model substance and freezing

it at 18 C. The biological

product was peeled, reduced to

small pieces of cubic shape

and frozen also at 18 C. Fig-

ure 1 shows a flowsheet of the

experimental equipment usedin the study of this process.

The fluidized bed freeze dryer

is built in an industrial freezer

container facilitating drying

experiments at low tempera-

tures. The fluidized bed cham-

ber had a nominal diameter of 0.1 m. The drying medium i. e. the fluidizing medium was air. It is sucked

from the cold environment by means of a pressure fan and is introduced into the fluidizing chamber

through the flow distributor bottom. It leaves the apparatus at the top and flows through a pipework and a

dust filter into the adsorber. The dehumidification is performed there. Finally the dry fluidizing medium

with the desired temperature flows back into the fluidizing chamber. In case of a direct flow operation the

plant can be used for the adsorbent regeneration. The used adsorbent materials is molecular sieve with a

mean diameter of dP=2 mm. The adsorbent particles maintain the driving force for drying the water va-

pour pressure difference between the air and the product. The air velocity, air inlet and outlet temperature

and humidity were measured. The experiments were carried out as a batch process with periodic sam-

pling. Therefore the process had to be interrupted regularly in order to determine the load values by

means of drying in an oven and weighing. In this manner, the particle load can be recorded in the course

of time.

RESULTS

The feasibility of atmospheric freeze drying was demonstrated by drying 0.450 kg model material -

Al2O3with an initial moisture content of 0.56 g/g. The bed mass in this experiment was fluidized at fluidflow velocity of 5.3 m/s. The air temperature below the flow distributor was controlled to be at 6 C.

The experimentally determined drying curve is shown in Figure 2. The course of the air humidity with

time is shown in Figure 3. The drying period in this case was approximately 4 hours. During the first 1

hour, a distinct decrease in the product moisture content and an increase in the air relative humidity at the

outlet have been observed. As in normal drying processes, the following 3 hours are characterized by de-

creasing the drying velocity which is an attribute of the second drying period. This second drying period

ends when the hygroscopic load is reached. The inlet humidity of fluidizing air was almost 0 % during the

Figure 1 - Atmospheric freeze-drying experimental set up


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whole experiment which confirms the adsorber efficiency. The air temperature at the inlet and outlet of

the fluidized bed apparatus is shown in Figure 4. The air inlet temperature has been adjusted at 1 C by

the controller and maintained constant during the course of the process. The air outlet temperature drops

during the first drying period because of the sublimation heat consumption and, then, this temperature

increases to a steady state value because of the reduction in the drying velocity. The obtained temperature

curves show that the period of time necessary for reaching the steady outlet temperature at the apparatus

exit is equal to the period of time necessary for reaching the stationary relative humidity and the station-

ary product moisture content. The temperature and humidity curves correspond to those under normaldrying conditions. The difference between the inlet and the outlet temperature is ascribed to the heat

transfer through the apparatus wall.

Figure 2 Experimentally determined

drying curveFigure 3 Experimentally determined gas

inlet and outlet humidityFigure 4 Experimentally determined

gas inlet and outlet temperature

In order to better understand the mass and heat transfer processes in atmospheric fluidized bed freeze

drying, experiments have been carried out with variation of the gas inlet temperature, the gas flow veloc-

ity, the bed mass and the particle diameter using a model substance -Al2O3. Table 1 shows the main pa-

rameters of all experiments carried out.

Table 1: Main parameters of atmospheric fluidized bed freeze drying experiments

Experiment code Tair [C] wair [m/s] mP [kg]

air inlet temperature variation

V1 1 3 0.450

V2 4 3 0.450

V3 6 3 0.450

fluid flow velocity variation

V4 6 3 0.450

V5 6 4 0.450

V6 6 5 0.450

product mass variation

V7 6 3 0.225

V8 6 3 0.450V9 6 3 0.600

product diameter variation

V11 1 4 0.500

V12 1 4 0.500

The influence of the gas inlet temperature on the drying kinetics is shown in Figures 5 and 6. The

curves represent freeze drying at temperatures of 1 C, 4 C and 6 C in the case of a constant gas


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flow velocity of 3 m/s for the three experiments, corresponding to a gas mass flow rate of 108 kg/h. The

initial bed mass was 0.450 kg with a load of 0.6 kg/kg for all experiments. As shown in Figure 5, the in-

crease in the gas inlet temperature causes a decrease in drying time, which is expressed by a faster de-

crease in the particle moisture content values and a faster reaching of a stationary gas outlet temperature

(Figure 6). The reason is that at equal gas mass flow rates the higher gas inlet temperature makes that the

higher average gas temperature is achieved causing a greater driving force for the drying process.

The influence of the gas flow velocity on the drying period, the temperature and the humidity curves isdisplayed in Figures 7 and 8. The experiments were performed with a gas inlet temperature of 6 C. The

bed mass was 0.450 kg with an initial moisture content of 0.6 kg/kg. The increase in the gas flow veloc-

ity, while keeping other process parameters constant, leads to a decrease of the drying period and a faster

reaching of stationary gas outlet humidity and gas outlet temperature values. This can be explained by the

higher enthalpy flow rate introduced by the higher gas mass flow rate.

Figure 5 Comparison of the measured drying

curves for the experiments V1, V2and V3Figure 6 Comparison of the measured gas outlet

temperature for the experiments V1, V2, V3


curves for the experiments V4, V5and V6

Figure 8 Comparison of the measured gas outlet

temperatures for the experiments V4, V5, V6


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The influence of the bed mass on the drying process is illustrated in Figures 9 and 10. The experimen-

tally obtained drying curves of three experiments with a gas flow velocity of 3 m/s and a gas temperature

of 1 C are compared in Figure 9. The bed mass was varied from 0.225 kg to 0.4 kg and 0.6 kg. The ini-

tial product moisture content was 0.6 kg/kg. In all cases the same enthalpy flow rate is introduced into the

system by the fluidizing gas. Because of the same initial product moisture content, the fluidized bed con-

tains different frozen water masses. Therefore a longer drying period is needed in the case of higher bed

masses because a higher ice mass has to be sublimated. The load is related to the dry mass so that the par-

ticle load decreases faster in case of smaller bed masses.

Experiments with a diced biological product

have been performed in order to verify the ap-

plicability of the new process for temperature

sensitive products. The particle diameter was

varied from 5 to 8 mm for the experiment V 11and from 8 to 12 mm for the experiment V 12.

The air inlet temperature was 1 C and the gas

flow velocity was 4 m/s for both experiments.

As shown in Figure 11, the drying period is

shorter in the case of the smaller fraction (V11),

which can be explained by the greater transfer

area and the higher drying rate.

Previous knowledge and the analogy be-

tween conventional drying processes and at-

mospheric fluidized bed freeze drying serves as

the basis for the mathematical description of

this new process. The water vapor adsorption

isotherms of the product serve as the basis for theoretical considerations (Wolff et al., 1990 Part 2). The

needed adsorption isotherm of the model material was measured using the desiccator method at 1 C and

5C. The adsorption isotherm was described by the Brunauer-Emmett-Teller (BET) model:

( )BET

eq m

BET

CX X

(1 ) 1 C

=

+ (1)

Figure 11 Comparison of the measured drying curves for

the experiments V11and V12


curves for the experiments V4, V5and V6

Figure 10 Comparison of the measured gas outlet

temperatures for the experiments V4, V5and V6


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and the Guggenheim-Anderson-de Boer (GAB) model. This is a modified BET-equation with an addi-

tional factor k and is valid in the relative humidity range from 0 % up to 90 % (Sablani et al., 2001; San

Martin et al., 2001):

( )GAB

eq m

CAB

CX X

(1 k ) 1 k C k

=

+ .

(2)

The GAB-model is often used in the literature in order to describe adsorption equilibria of biological

matter and showed better conformity with the experimental results than the BET-model Figure 13.

CONCLUSIONS

Based on experimental investigations, the technical feasibility of the new atmospheric fluidized bed

freeze drying has been confirmed in this paper. The experimental results with the model substance -Al2O3have shown that the drying rate can be increased by increasing the gas flow velocity and gas tem-

perature and by decreasing the bed mass as in conventional drying processes using heated air. The ex-

periments with a diced biological material have demonstrated the applicability of this process for the dry-

ing of temperature sensitive products. The adsorption isotherms of the -Al2O3 material have been ex-

perimentally measured and mathematically described. These will be used to develop a mathematical

model for describing the freeze drying process.

ACKNOWLEDGMENT

The research has been supported by the InnoRegio-Netzwerk REPHYNA and by the scholarship pro-gram of the Deutsche Bundesstiftung Umwelt (German Federal Environmental Foundation).

NOTATION

C sorption isotherm parameter -

k sorption isotherm parameter -

m mass kg

Figure 12 Adsorption isotherm of the model

material -Al2O3at 1 C and 5 C

Figure 13 Experimental and predicted by BET

and GAB models isotherms of -Al2O3at 1 C


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T temperature C

t time s w velocity m / s

X particle moisture content, dry basis g / g

Greek Symbols relative air humidity, %

Subscripts

BET BET- equationbed fluidized bedeq equilibrium

GAB GAB- equationgas gas

m monolayer

P product

LITERATURE

Hayashi H. (1989), Drying technology of toods their history and future, Drying Technology 7, 2 pp.315-369

Oetjen G. W. (1999), Freeze drying, Weinheim, Wiley-VCH, pp. 200-230

Sablani, S.S., Myhara R. M., Mahgoub, Al-Attabi, Z. H. and Al-Mugheiry, M. M. (2001), Water sorptionisotherms of freeze dried fish sardines, Drying Technology, 19 (3&4), pp. 673-680

San Martin, M. B., Mate, J. I., Fernandez, T. and Virseda, P. (2001), Modelling adsorption equilibriummoisture characteristics of rough rice, Drying Technology, 19 (3&4), pp. 681-690

Wolff, E. and Gibert, H. (1990), Atmospheric Freeze Drying Part 1: Design, experimental investigationand energy saving advantages, Drying Technology, 8 (2), pp. 385-404

Wolff, E. and Gibert, H. (1990), Atmospheric Freeze Drying Part: 2 Modelling drying kinetics usingadsorption isotherms, Drying Technology, 8 (2), pp. 405-428

Experimental Analysis of Fluidized Bed Freeze Drying

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