Fluid catalytic cracking (FCC) is the primary conversion process in oil refining. The performance of an FCC riser strongly depends on the interactions between oil/catalyst flow and cracking kinetics, but most FCC riser models do not consider such interactions. Accordingly, this work develops a computationally simple model capturing the dominant features of flow-reaction coupling in the riser's dense phase and acceleration zones. Specifically, the particle-particle collision force and the particle-fluid interfacial force are considered. With a four-lump kinetic model, the riser model predicts conversion and selectivity from the axial profile of the catalyst-to-oil ratio resulting from particle-fluid interfacial momentum transfer. The cracking intensity in the riser bottom zone is much greater than that calculated from conventional riser models, which neglects oil-catalyst hydrodynamic coupling and catalyst dilution due to volume expansion. The present model compares well with published data and predicts conversion-selectivity patterns that are qualitatively different from those obtained from conventional models. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3122-3131, 2011