The MIP (maximizing iso-paraffins) reactor, which was developed by SINOPEC to meet the challenge for clean fuels, consists of a series of reaction zones with different diameters, thus allowing coexistence of multiple flow regimes. This study is to probe the flow behavior in the MIP reactor through computational fluid dynamics (CFD) simulation, in which a new drag-coefficient relation based on the energy minimization multi-scale (EMMS) model was applied. A testing simulation of a lab-scale MIP reactor validates our approach first; further investigation of an industrial MIP reactor indicates that, the so-called "choking" phenomenon, which is characterized by an S-shaped profile of voidage and saturation carrying of particles, also exists in the MIP reactor. This choking phenomenon deserves our reemphasis, since most of the existent simulations take for granted that, a riser flow can be determined by specification of only gas and solids velocities, ignoring the effect of solids inventory. In practice, understanding of the "choking" may facilitate troubleshooting of MIP units, as the related flow and transition behaviors are critical to the reaction performance. In general, this simulation unfolds a fresh perspective of the EMMS-based multi-scale CFD approach. (c) 2007 Elsevier Ltd. All rights reserved.