A 3D model was developed for simulating the hydrodynamics of polypropylene powders in multi-zone circulating reactors (MZCRs). This model was used to design the proper geometry of the reactor. The model was based on computational fluid dynamics (CFD) in which an Eulerian-Eulerian approach, incorporating the kinetic theory of granular flow (KTGF), was used to describe gas and solid flow equations. The re-normalization group (RNG) k-E model was applied for implementing the turbulency. For finding the best design of the reactor from the hydrodynamic point of view, four geometrical configurations were proposed and tested. It was found that the best configuration has a narrower downer (compared to riser), and a return leg with a side flow which results in a good solid circulation and proper solid distribution in the reactor. This configuration ensures fast fluidization and pseudo packed bed regimes within the riser and downer, respectively. Effect of inlet gas velocity on overriding parameters, such as pressure drop and solid volume fraction, was studied for the selected configuration. Upper and lower limits for the inlet gas velocity were determined.