A bi-dispersed bubble model incorporating lateral forces and bubble-induced turbulence is employed to simulate hydrodynamics in an up flow jet-loop reactor (JLR) operated in gas-liquid co-injection mode. In the model the bubble swarm effect on the drag force was differently described for the large and small bubble phases, where the drag force for the large bubble decreases with the gas holdup in accordance with the wake acceleration effect, while for the small bubble the drag force increases with the gas holdup in consideration of the hindrance effect of the bubble swarm. The simulated radial profiles of gas holdup and axial liquid velocity agree with the cold experiments, which shows that the different description of drag forces for the large and small bubbles is reasonable for correctly simulating the effect of superficial gas velocity on liquid circulation velocity. Effects of draft tube position (H-c) and cross-sectional area ratio of riser to downcomer (A(r)/A(d)) are evaluated by model simulation. The results show that with the increase of H-C and A(r)/A(d), both overall gas holdup and liquid circulation velocity reach a maximum at He = 0.06 m and A(r)/A(d) = 0.44, respectively. Recommendatory geometric parameters for the JLR design is: 0 < H-C <0.09 m, 0.4 < Ar/Ad < 0.7. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.