A recently developed model, called the Reynolds mass flux (RMF) model is adopted for the simulation of multiphase flow involving chemical reaction in gas-particle CFB downers. The newly deduced Reynolds mass flux equation is utilized for closing the turbulent mass transfer equation, thereby realizing the simulation of anisotropic turbulent mass transfer. The simulations are validated with experimental data in literatures. The results indicate that the turbulent mass diffusion in downers would show significant anisotropic character. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.