Abstract: A Computational Particle Fluid Dynamics (CPFD) model, which has the ability of revealing the coupled flow-reaction behaviors on the scales ranging from an individual particle to the whole reactor, was developed for studying the gas-solids circulating fluidized bed (CFB) riser reactor handling Geldart A particles. The model was validated against available experimental data with respects to the hydrodynamics (e.g. distributions of solids holdup and solids velocity) and the reaction behaviors (e.g. distribution of ozone concentration for ozone decomposition), and acceptable agreements were achieved between the modeling results and the experimental data. The CPFD model was further extended for analyzing the gas-solids flow hydrodynamics and the cracking reactions in an industrial Fluid Catalytic Cracking (FCC) riser reactor. Modeling results indicate that the volume of gas in the CFB riser will vary due to the presence of interphase chemical reactions (e.g. catalytic cracking reactions in FCC), which can significantly influence the gas-solids flow hydrodynamics. It was demonstrated that the CPFD model can be applied to quantify the relationship between the flow hydrodynamics and the chemical reactions in multiphase flow-reaction systems.