Axial gas and solids mixing behavior in a down flow circulating fluidized bed reactor (downer reactor) was systematically studied by means of a numerical approach. A 2D Eulerian-Eulerian model based on the kinetic theory of granular flow with a k-epsilon turbulence model was adopted to simulate the flow. The virtual tracer method based on CFD simulation was used to study the mixing of gas and solids phases. The effects of the operating conditions (i.e., inlet gas velocity and solids circulation rate) and the particle properties (i.e., density and diameter) on the mixing behavior were investigated. The predicted hydrodynamics and mixing behavior were validated with previously reported experimental data from the literature. The simulation results show that flow behavior of both the gas and solids phases is approximately the plug flow. The axial Peclet numbers increase with increased superficial gas velocity and decreased solids circulation rate. Furthermore, larger and/or heavier particles potentially increase the axial Peclet number. In addition, correlations for predicting the axial gas and solids Peclet numbers based on the operating conditions and the physical properties of the gas and solids phases were proposed. (C) 2012 Elsevier Ltd. All rights reserved.