Industrial fluidized beds usually show a wide particle size distribution which significantly affects the performance of gas-solid two-phase reactors. However, in traditional CFD models the particle size distribution is often represented by the Sauter mean diameter no matter whether the distribution is wide or narrow, which cannot reflect the effects of the particle size distribution on its operation. In this study, a simplified population balance model (PBM) is combined with two-fluid model (TFM) and the Energy Minimization Multi-Scale (EMMS) drag model for simulating heterogeneous gas-solid flow. The EMMS drag model is used to characterize the effects of meso-scale clustering structures on the inter-phase drag force. The current model is compared with the model with the assumption of single particle diameter, and is used to study the segregation and mixing characteristics in a riser. It was shown that (i) for the specific cases we studied, particle polydispersity has a negligible effect on the time-averaged axial solid concentration profiles but has a notable effect on the particle velocity distributions, (ii) the experimental hydrodynamics as well as segregation and mixing patterns in a riser with a continuous particle size distribution can be predicted reasonably well by present combined TFM-EMMS-PBM method. (C) 2013 Elsevier B.V. All rights reserved.