To deeply understand the effect of coupled configuration between two-stage separators on the gas flow behavior in fluid catalytic cracking disengager space, the gas flow field and residence time distribution in a bench-scale disengager is numerically studied on the platform of the commercial computational fluid dynamics software package. Fluent 6.1. Two conventionally used coupling configurations in refineries and a newly-designed configuration are investigated. The Reynolds stress model is applied to simulate the gas flow field and the results agree well with the experimental values measured by a smart five-hole probe. Coupled with the Reynolds stress model, a scalar transport equation is used to obtain the gas residence time distribution. It is shown that the coupling configuration between the primary and secondary cyclones has an apparent effect on the gas flow behavior. The newly-designed configuration is better to quickly discharge the gas into the secondary cyclone inlet with a maximum residence time of 0.38 s, compared to 7.72 s and 279s for the two conventional configurations. The simulation results indicated that the appropriate modification of the coupling configuration can help to improve the gas flow. The knowledge of the gas flow field also help to understand the coke formation process in the disengager space. (C) 2010 Elsevier B.V. All rights reserved.