The computational fluid dynamics(CFD)-population balance equations(PBE) coupled model is employed to investigate the hydrodynamics in a gas-slurry internal loop reactor with external slurry circulation.The predicted radial profiles of local gas holdup and bubble diameter are in good agreement with the corresponding experimental data.The spatio-temporal velocity profile of the gas phase reveals that the upward movement of gas is slowed down and the residence time of gas is prolonged by the downward momentum of the slurry.Introduction of the external slurry can greatly improve the uniformity of gas holdup distribution in the reactor,especially in the downcomer-tube action region.Moreover,the interaction between the downward slurry and upward gas can lead to small bubble size and high interfacial area as well as good mass and heat transfer.The above results suggest the function of external slurry circulation for the internal loop reactor and would be helpful for optimizing the design and scale up of reactors. The computational fluid dynamics (CFD) -population balance equations (PBE) coupled model is employed to investigate the hydrodynamics in a gas-slurry internal loop reactor with external slurry circulation. The predicted radial profiles of local gas holdup and bubble diameter are in good agreement with the corresponding experimental data. spatio-temporal velocity profile of the gas phase reveals that the upward movement of gas is slowed down and the residence time of gas is prolonged by the downward momentum of the slurry. Introduction of the external slurry can marked improve the uniformity of gas holdup distribution in the reactor, especially in the downcomer-tube action region. Moreover, the interaction between the downward slurry and upward gas can lead to small bubble size and high interfacial area as well as good mass and heat transfer. above results suggest the function of external slurry circulation for the internal loop reactor and would be helpful for optimizing the design and scale up of reactors.