The density function was developed for predicting the distribution of moisture content of compound dryers of flash and fluidization with a circulating system of solids The moisture content distribution of drying FeSO 4·7H 2O was calculated. The results show that there are three necessary conditions for the optimum design of fluidized dryers. The first is the heat supply for drying process. The bottom area of fluidized beds is deter mined in terms of the required heat supply. The second is the average residence time of particulate material in fluidized beds. The volume of fluidized dryers is calculated from the mean residence time. And the last is the uniformity of product moisture content. The number of rooms of fluidized beds is predicted in order to satisfy the required uniformity.msofthereq