A fairly rigorous mechanistic model of a continuous fluidized-bed dryer has been developed. It depicts the dynamic interactions between gaseous and solid phases in detail. The performance of the dryer has been simulated numerically based on the model. The effects of the operating parameters on the performance characteristics of the dryer have been investigated. These parameters include the superficial gas velocity, the inlet temperature of the drying gas, the mean residence time of solids and the dryer-wall temperature. The results of simulation based on the present model are compared with those based on an existing model. This comparison shows that the former is a substantial improvement over the latter. INTROD%JCTION The fluid&d-bed dryer possesses many significant features over the conventional packed-bed or movingbed dryer [see, for example, Vlnecbk et al. (1966), Nonhebei and Moss (197 l)]. These include the following: (i) drying gas is locally mixed intensively during its passage through the bed; consequently the rate of mass and heat transfer between the gas and solids are high, (ii) the extremely rapid heat transfer enables a relatively high inlet gas temperature to be used; (iii) the time of drying is relatively short. Because of its numerous advantages, fluidized-bed drying has been increasingly applied in diverse industries in either the batch or continuous mode (VaniZccCk et al., 1966, Viswanathan et al., 1982). In fact, several papers have been published on the subject of continuous fluid&d-bed drying since the late 1950s. A comprehensive account of these and other related publications is available (Viswanathan et al., 1982). Conventional design procedures for a continuous fluidized-bed dryer have been developed mainly under the assumptions that the bed temperature is uniform, the outlet streams are in thermal or concentration equilibrium, and that fluid mechanistic behaviour of the drying gas is homogeneous; in other words, the drying gas is not partitioned into different phases of the fluidized bed, such as the emulsion and bubble phases [see, for example, Nonhebel and Moss (1971), Palann and Parti (1973)]. Although these assumptions are valid in some circumstances, they may not hold under certain actual situations. The aim of this work is to develop a fairly rigorous and comprehensive mechanistic model without imposing such assumptions. The model can predict the temperature and ~To whom correspondence should be addressed. moisture content of the outlet gas and also the average moisture content and temperature-of the solids at the exit. It will be amply demonstrated that the proposed model represents a significant improvement over an existing mechanistic model for the continuous fluidized-bed dryer proposed by Palancz (1983). MATHEMATICAL MODELLING A schematic diagram of the model is shown in Fig. 1. The present model is based on the two-phase theory of fluidization (see, for example, Davidson and Harrison, 1963). The underlying assumptions of this theory are SOLIDS INPUT