- Pneumatic or flash dryers, one of the most common types of industrial drying units, are characterized by continuous convective heat and mass transfer processes. To improve design procedures for these dryers, reliable models are needed. A two- dimensional, two-fluid model was developed for modeling the steady-state flow of particulate materials through a vertical pneumatic dryer. Two-dimensional flow field interpretations provided information about the properties of the continuous and dispersed phases at every point of the flow field. Axial and radial profiles were obtained for the flow variables. The gas phase was treated as a mixture of gas and liquid vapor. The wet particles were composed of a porous solid structure that contained a saturated liquid. The drying model considered a two-stage drying process. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. In the second stage, the particles are assumed to have a wet core and a dry outer crust. The evaporation of the liquid from a particle is assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks as the particle dries. Following the models successful validation the influence of tube diameter on the properties of the continuous and dispersed phase in the axial and radial direction was investigated.