Fluidized-bed drying

An overall material balance across a fluidized bed drier may be written 

Equation 4.16 indicates that the feed rate of wet solids must be below that which would result in saturation of the exhaust air stream. Failure to satisfy the moisture balance leads to wet quenching of the bed (see Chapter 5). 

Both the enthalpy change in the solids and associated moisture and the enthalpy of the water vapour in the fluidizing air are relatively small. Neglecting these terms simplifies the energy balance to 

Equafion 4.19 indicafes fhaf fhe enthalpy change of fhe fluidizing gas is approximafely equal fo fhe sum of the latent heat of the water which is evaporated from fhe wef solids feed and fhe heat losses from fhe bed. 

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Similar to processing mined rock salt, solar salt may be cmshed, screened, and kiln dried or fluidized-bed dried. Coarse solar salt is a premium product because of high purity and relatively large crystal size. It is in particular demand for use to regenerate the resin in cation-exchange water softeners... 

Drying, e.g. spray, drum, fluidized bed drying Powder mixing... 

In the last years, several publications have tackled these problems Kahn-Wyler [ 1.791 lists four reasons which prove, that fluidized-bed drying (solving problem 2 above) is not suitable ... 

VANficfiK, J., MARKVART, M. and Drbohlav, R. Fluidized Bed Drying (Leonard Hill, London, 1966). 

In addition to the vertical mixing of solids, a degree of horizontal mixing occurs and this is especially important in long, shallow plug flow beds of the type used in fluidized bed drying or freezing (see Chapter 3). 

Despite these limitations, fluidized bed drying is used extensively for a very wide range of food applications including fruits, diced vegetables, grain and sugar these are listed in greater detail in Table 4.1. 

Table 4.1 Some reported applications of fluidized bed drying of foods.

Reay and Baker (1985) suggest that there are two types of particle which are encountered in fluidized bed drying fype A parficles which dry easily and where the water vapour in the gas leaving the bed is in equilibrium with the moisture in the particle and type B particles which have a high internal resistance to the mass transfer of water and therefore dry very slowly. Wheaf is an example of fhe latter (Giner and Galvelo, 1987). 

Han, W., Mai, B. and Gu, T., Residence time distribution and drying characteristics of a continuous vibro-fluidized bed. Drying Tech., 9 (1991) 159-181. 

Jumah, R., Modelling and simulation of continuous and intermittent radio frequency-assisted fluidized bed drying of grains. Food Bioprod. Proc., 83 (2005) 203-210. 

Senadeera, W., Bhandari, B., Young, G. and Wijesinghe, B., Physical properties and fluidization behaviour of fresh green bean particulates during fluidized bed drying. Food Bioprod. Proc., 78 (2000) 43-47. 

Bakhshi, N.N. and Nihilani, A., Fluidized bed drying of sodium sulphate solutions fluidisation and its applications. Proceedings of an International Symposium on Fluidisation, Toulouse, 1973, 534-544. 

Haida, H., Kroyer, G.T., Kuenne, H.J., Washuettl, J. and Winker, N., Anwendung der Wirbelschichttechnologie zur Herstellung eines Apfeltrockenproduktes [Use of fluidized bed drying for manufacture of a granulated dried apple product], Deutsche Lebensmittel Rundschau, 90 (1994) 9-15. 

Boeh-Ocansey, O., Low temperature fluidized-bed drying of mushroom, carrot, beef and shrimp samples, Acta Alimentaria, 15 (2) (1986) 79-92. 

Therefore, product temperature should be monitored closely to control the fluidized bed drying process. During fluid-bed drying, the product passes through three distinct temperature phases (Fig. 21). At the beginning of the drying process, the material heats up from the ambient temperature to approximately the wet-bulb temperature of the air in the dryer. This temperature is maintained until the granule moisture content is reduced to the critical level. At this point, the material holds no free surface water, and the temperature starts to rise further. 

Figure 9.5. Residence time distribution in particle dryers, (a) Four types of dryers (McCormick, 1979). (b) Residence time distribution cf air in a detergent spray tower example shows that 27% (difference between the ordinates) has a residence time between 24 and 32 sec [Place et al., Trans. Inst. Chem. Eng. 37, 268 (1959)]. (c) Fluidized bed drying of two materials (Vanacek et al., Fluidized Bed Drying, 1966).

Much ingenuity has been applied to the design of fluidized bed drying. Many different arrangements of equipment are illustrated and described in the comprehensive book of Krdll (1978) for instance. Figure 9.13(a) depicts the basic kind of unit and the other items are a few of the many variants. Tables 9.14 and 9.15 are selected performance data. 

A wide range of operating conditions is used commercially. Performance data are in Table 12.19. Gas velocities cover a range of 3-20 times the minimum fluidizing velocity or 0.1-2.5 m/sec. Bed expansion ratios are up to 3 or so. As in fluidized bed drying, bed depths are low, usually between 12 and 24 in. Evaporation rates are in the range 0.005-1.0 kg/(sec)(mz). 

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