In this paper, a multiphase model applied to a downer reactor is proposed, where the gas-solid modeled with some special characteristics, such as: unsteady and 3D flow for two fluids on an Eulerian-Eulerian approach; dissipation of turbulent kinetic energy only on the gas phase modeled as real fluid; and the assumption that the solid phase behaves like a hypothetical fluid without shear stress from molecular causes. Studies based on first (HIPWIND) and second order (HIGHER UPWIND) interpolation schemes were carried out. The difference in the results using the two schemes was found to be negligible. The Computational Fluid Dynamic Technique (CFD) was adopted as a tool to predict downer reactor dynamic, comparing the proposed model with experimental data available in the literature. Good agreement between experimental data and model predictions was verified. Therefore CFD techniques proved to be a powerful tool to predict the fluid dynamic of inlet changing of downer reactors, and it can be used with success in chemical process optimization. (c) 2005 Elsevier B.V. All rights reserved.