Drying on Inert Particles
4.2 Drying Mechanism and Process Considerations
rials such as swirling stream dryers, impinging stream dryers, or
pneu-CRC_73877_Ch004.indd 29
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drying of pigments, fine chemicals, pharmaceuticals, and certain materials
rates competitive to spray, drum, and film-rotary dryers (Strumillo et al.,
matic dryers (Figure 4.1). Independent of the hydrodynamic configuration, lations (e.g., Anonymous, 1986; Grbavcic et al., 1998). In addition, fluid bed
fluidized mixture with inert polyethylene beads (Alsina et al., 2005).
Drying on inert particles is typically performed in a variety of fluid beds (classical fluid bed, spouted bed, spout-fluid bed, jet spouted bed, vibrated fluid bed, cyclone dryer, etc.) as well as in other dryers for dispersed
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30 Advanced Drying Technologies
FIGURE 4.1
with inner conveyor screw, (d) spouted (jet spouted) bed, (e) vortex bed, (f) swirling streams,
(a) (b) (c)
(d) (e) (f )
(g) (h) (i)
(j) Feed Air Air + powder
Inert particles
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Basic configurations of dryers with inert carriers: (a) Fluid bed, (b) spout-fluid bed, (c) fluid bed (g) vibrofluidized bed, (h) rotary dryer, (i) pneumatic dryer, and (j) impinging stream dryer.
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the principle behind this technology lies in dispersing the liquid feed the sole hydrodynamic impact of the hot air stream or by the combined impact of an air stream and a mechanical device such as a screw con-veyor, a vibrator, or lifters (Flick et al., 1990; Kudra et al., 1989; Pallai et al., 2007; Erdesz and Ormos, 1986; Kudra and Mujumdar, 1989, 2007; Pan et al.,
ferrite (Kovalev et al., 1989).
Depending on the hydrodynamic conditions, the liquid coat on the par-ticle surface dries by convective heat transfer from hot air and by contact heat transfer due to sensible heat stored in the inert particles. When the coat is dry enough to be brittle, it cracks because of particle-to-particle and particle-to-wall collisions and peels off from the surfaces of inert particles.
Because of intense attrition, a dry product is discharged from the dryer
produced, especially when drying brittle materials of biological origin.
Figure 4.2 presents the idealized mechanism of drying on inert particles, which boils down to the following sequence of kinetic processes: heating of inert particles, coating with dispersed liquid, drying of the coat, and cracking and peeling-off the dry product. Because of continuous supply coats at the same time not only the material-free particles but also par-ticles with a dry but not peeled-off material and parpar-ticles with a partially dry layer. Thus, quasi-equilibrium is established between the individual rates of the component processes. Stable operation of the dryer requires the combined rate of drying/peeling-off to be greater than the rate of coat-ing. Otherwise, the wet coat would build up on the inert particles and the bed would eventually collapse eventually. The bed would also collapse with excessive saturation of exhaust air (Schneider and Bridgwater, 1989).
Another condition for stable operation of the dryer with inert particles stems from the material properties—no elastic shell should be formed on the solid carrier at any stage of drying as impact due to particle collisions particles made of two bimetallic canopies, which change their shape when subject to temperature changes during drying, could facilitate cracking of a dry shell (Dmitriev et al., 1989). Fibrous materials (e.g., pulp and paper sludge), which could bridge solid particles and therefore immobilize the bed, are also not good candidates for drying on inert particles. The bed can also collapse when drying sticky materials such as meat-rendering sludge with excessive fat content. In such a case, the melted fat acts as a binder,
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magnetic field if they are made of ferromagnetic material such as barium
with the exhaust air as a fine powder of rounded particles. When
chip-of the liquid feed and a definite material residence time, the liquid spray
might not be sufficient to crack the shell. Here, the almond-shaped inert over the surface of an inert solid carrier. This carrier is fluidized either by
2000; Limaverde et al., 2000). Particles can also be fluidized by an external
ping due to the impact of inert particles prevails attrition, small flakes are Small flakes can also be obtained when using inert particles with a corru-grooves on the particle surface (Kutsakova et al., 1985).
gated surface. The size of the flakes is then proportional to the size of the
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32 Advanced Drying Technologies
which immobilizes particles and traps dry meat powder inside the dryer.
This problem can be solved when altering the process by contact-sorption drying (see Chapter 12). In this particular case, the meat-rendering sludge with fat content up to 30% w/w was successfully dried in the jet spouted bed dryer when using either calcium carbonate or wheat bran in the mass ratio of 4.6 and 2.5%, respectively (Amazouz and Benali, 2000). An alter-native solution to the problem of hydrodynamic stability is the use of a hydrocyclone to remove excess fat before thermal drying (Kudra, 2000).
Studies on drying of a single 8 mm ceramic particle coated with a 0.6–
0.8 mm layer of the pasty pigment have shown that drying kinetics are typical for convective drying of capillary porous material (Leontieva et al., 2002). The clearly marked plateau of the material temperature cor-responds well with the wet bulb temperature of the air stream during the
FIGURE 4.2
Process schematic and idealized mechanism for drying of liquids on inert particles.
Outlet gas + dry powder
Dry powder
Inert particles
Wet feed (paste, slurry)
Hot gas
Wet coat
Inert particle Convection Conduction
Drying mechanism—idealized sequence of processes
Heating Coating Drying Peel-off
I II III IV
Heat transfer mode
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material temperature during drying lies in-between the wet bulb tem-perature and the exhaust air temtem-perature. This is because not all inert particles in the bed are always fully covered with the wet coat; therefore, heat conduction supplements convective drying and thus the condition for the wet bulb temperature does not hold. Also, the material tempera-ture at the moment of peel-off is higher than the wet bulb temperatempera-ture because removal of a dry coat starts at a certain moisture content, which extracted literature data (Leontieva et al., 2002) to combine the curve rep-resenting the kinetics of the peel-off process with the drying and tem-perature curves.
The same conclusion holds for other literature data as well. For exam-ple, Figure 4.4 presents the relationship between the equilibrium material (bed) temperature and inlet and outlet air temperatures for the drying of egg melange in a vortex bed dryer with forced pulsation of inert particles (Kutsakova and Utkin, 1989). Clearly, the material temperature is higher than the respective wet bulb temperature. At the same time, the material temperature is lower than the outlet air temperature, and this difference tends to be larger at lower inlet air temperatures. These data are consis-tent with measurements by Markowski (1992), who found the difference between the particle surface temperature and the wet bulb temperature to ture is lower by 15°C than the temperature of the outlet air.
Moisture content (% w.b.)
Drying curve
Drying, material temperature, and peel-off curves for drying R-salt on inert particles.
(Adapted from Leontieva, A. I. et al., Drying Technol., 20(4&5), 729–747, 2002.)
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first drying period (Figure 4.3). An analysis of literature data on drying in
is close to the final one. This can be seen in Figure 4.3, in which we have spouted and fluidized beds of inert particles indicates, however, that the
be at least 20°C. Also, Grbavcic et al. (1998) found that fluid bed
tempera-© 2009 by Taylor & Francis Group, LLC
34 Advanced Drying Technologies
Assuming that drying the relatively thin layer of the wet material on inert carriers is externally controlled, Kutsakova and Utkin (1987, 1989) proposed the following equation for the moisture evaporation rate:
dX water vapor pressures at the material surface and in the gas core, respec-on the surface of inert particles given by the mass of the wet material that adheres to the unit surface area of inert particles. TS represents the average gas temperature in the boundary layer at the surface of the inert particle.
Integration of Equation 4.1 gives the following relationship for drying time:
Material temperature versus exhaust and inlet air temperatures. (Extracted from Kutsakova, V. E. and Utkin, Yu. V., Trans. VUZOV. Food Technol., 5, 92–93 (in Russian), 1989.)
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tively, and φ is the parameter that quantifies the distribution of the wet coat
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water vapor pressure at the material surface with the average temperature of the inert particles and the material moisture content.
For drying in a vortex bed of inert particles, Kutsakova and Utkin (1987) developed the following experimental equation:
P T X A BX DX
where the parameters in Equation 4.3 for selected protein-based materials are given in Table 4.1.
The average temperature of the bed (TTT ) can be determined experimen-p tally or can be calculated from the following relationship (Kutsakova, 2004; Kutsakova and Utkin, 1987, 1989):
T T T W c a
where TTT is the equilibrium temperature of evaporation (°C), h is the gas-eq
2 K), G is the dryer throughput with respect to the dry product (kg/s), and n is the number of inert par-ticles in the dyer.
The mean temperature of inert particles with an accuracy of 3°C can also be determined from the following empirical formula (Kutsakova, 2004):
Tp⫽Tout⫺b (4.5)
The constant b varies from 10 to 20, depending on the drying material;
for protein hydrolyzate b = 15, whereas for casein b = 10.
TABLE 4.1
Parameters in Equation 4.3
Material A B C D E F TTT (°C)r
Protein hydrolyzate
11.53 1.027 −0.8 4.605 −1.8 1.0 120
Skim milk 21.53 0.053 −1.0 0.313 −2.3 0.032 (112 − Tp) 100 Whey 11.53 0.00378 −1.7 6.620 −1.7 0.032 (112 − Tp) 100 Source: Extracted from Kutsakova, V. E. and Utkin, Yu. V., J. Appl. Chem. USSR, 60(5),
1077–1081, 1987.
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To calculate the drying time, one should first identify the variation of the
to-particle heat-transfer coefficient (W/m
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36 Advanced Drying Technologies
The water vapor pressure in the bulk of a drying gas can be calcu-lated based on the exhaust gas humidity. The φ parameter should be and material characteristics, and operating parameters. For the drying of the materials listed in Table 4.1, the following semiempirical formula can be used (Kutsakova and Utkin, 1987; Kutsakova, 2004):
When drying the protein hydrolyzate from Xi= 4 kg/kg to Xf= 0.05 kg/
kg at Tin= 300°C, TTTp= 105°C, and TTTeq= 100°C, the parameter φ was found to be 0.7 kg/m2 at dry coat thickness from 0.2 to 0.3 mm. The calculated drying time varied from 60 to 90 s, whereas the experimentally deter-mined material residence time was in the order of 200–400 s. This indi-cates that several wet coats were dried in this case before the dry material was cracked and peeled off from the inert carrier (Kutsakova and Utkin, 1987). Nomograms that facilitate design calculations can be found in the papers by Kutsakova and Utkin (1987) and Kutsakova (2004) for various proteins and in the book chapter by Kudra (2007) for chicken eggs.
Despite extensive studies on drying on inert particles, there is practi-cally no data on the material residence time except those cited here for a swirling bed and a classical spouted bed of inert particles (Berni and Freitas, 2007). Also, the material residence time from 30 to 85 s has been
®
compared to the residence time of the drying air, such a substantial resi-ganisms when combined with the material temperature in the range of 70–90°C (Rysin et al., 1981).
From another point of view, the residence time in the order of several minutes in combination with the relatively low material temperatures explains successful drying on inert carriers of some biomaterials such as Zn-bacitracine, animal blood, vegetable extracts, egg products, pea pro-tein, starch, and meat processing sludge (Re and Freire, 1989; Markowski, 1992; Pan et al., 1994, 1995; Amazouz et al., 2000; Amazouz and Benali, 2000). However, the drying of heat-sensitive products of biotechnology such as enzymes or vitamins as well as living microorganisms (e.g., yeast
* Registered trademark of E. I. duPont de Nemours and Company, Inc. (Wilmington, Delaware).
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determined experimentally as it depends on dryer configuration, particle
specified for the drying of animal blood and egg products as a film of
dence time was claimed to be sufficient to suppress undesirable microor-60–200 µm on a fluidized bed of 3–5 mm Teflon * cubes (Table 4.2). As
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and bacteria) is yet a challenging task because of product degradation.
The positive or negative results depend on the following main factors that result from the drying mechanism (Kudra and Strumillo, 1998):
1. Mechanical effect of inert particles due to wall-to-bed and particle-to-particle impacts and attrition owing to circulation of the bed.
erties of inert particles, dryer geometry, as well as dryer con-spouted bed, etc.).
2. Thermal effect due to convective and contact heat transfers that depends mainly on the heat-transfer rate, bed hydrodynamics, 3. The effect of material accumulation, which depends on rheological properties of the material being dried and bed loading, and there-fore it can be important only for some materials.
loss risk (QLR) as
QLR⫽1⫺ HT M A (4.7)
where ∆HTTT, ∆M, and ∆Aare the heat-transfer, mechanical, and accumu-lation effects, respectively. Detailed information and an example of the QLR analysis for drying in a jet spouted bed can be found in the reference literature (Markowski, 1992, 1993a, 1993b; Kudra and Strumillo, 1998).
TABLE 4.2
Drying of Food Products on Inert Particles
Parameter Egg Melange Egg White Animal Blood
Inlet air temperature (°C) 110–12 120–130 110–135
Outlet air temperature (°C) 63–68 67–80 63–75
Unit feed rate (kg product-h/kg bed)
1.35 1.35 1.4
Air consumption (kg/kg product)
2.0 2.0 1.7
Layer thickness (mm) 0.06–0.20 0.06–0.15 0.10–0.20
Material temperature (°C) 69–77 77–90 73–85
Residence time (s) 50 30 85
Source: Extracted from Rysin et al., Method of drying of pasty products, Russian Patent 1024668, 1981.
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figuration (jet spouted bed, pneumatic transport, mechanically
properties of drying material, and dryer configuration.
The risk of product degradation can be quantified in terms of the quality These two phenomena depend on airflow rate, mass and
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38 Advanced Drying Technologies
From Equation 4.5, it follows that success in drying of living microor-ganisms can be achieved when reducing any of the components that con-tribute to quality losses. An interesting option to alleviate thermal effect appears to be heat pump drying (Alves-Filho et al., 1998). With reference to Figure 4.5, the bacterial suspension (Streptococcus thermophilius in a potas-sium phosphate buffer with and without trehalose protectant) was dried air temperature was kept below 35°C by the use of a classical heat pump circuit (see Chapter 15). The powdery product, which was dried with the addition of 100 mM of trehalose, showed 86% viability at an inlet air tem-perature of 10°C. Thus, it was concluded that this technique could be com-offers 100% viability but when drying at −20°C for 15 h, then at −10°C for