Based on the single stage dryer model (Qi, 1996), a model for a multi-stage diffusion-controlled pneumatic-conveying dryer train was developed. The model consisted of a set of algebraic equations describing the relationship between the moisture level in solids and major process parameters (i.e., diffusivity, partition coefficient, particle size, number of stages, solids loading, residence time, etc.). The effects of these parameters on drying were studied using the model. Equilibrium and other special or asymptotical conditions (e.g., infinitesimal injection of drying gas, uniform partition coefficient and Fourier numbers, etc.) were analyzed. Model calculations have been shown to compare very well with data from an actual plant HDPE drying process.