The gas flow behaviors and gas residence time distribution in industrial FCC disengager space are numerically studied with the combined consideration of the product gas, stripping gas and anti-coking steam. Although purge steam is used in the top dome in order to alleviate the coke formation in the refinery plants, its effectiveness is still obscure. In this study, the Reynolds stress model is applied to calculate the gas flow field and the scalar transport equations are used to obtain the gas residence time distribution. The species transport equations are used to get the local mass fractions of steam and oil vapors and further to verify the effectiveness of the anti-coking steam. In view of the wide use of "open" configuration, the optimizing position of the primary cyclone outlet is proposed for an industrial case in Qianguo refinery. Based on the results, suggestions are made for improved operation of the industrial units. (C) 2010 Elsevier B.V. All rights reserved.