Liquid feed vaporization and vapor cracking reaction in the feed zone plays an important role in the overall reaction performance of fluid catalytic cracking (FCC), an important petroleum refining process for converting heavy hydrocarbon feed stocks to high-value products such as gasoline and light olefins. The process starts with injecting atomized oil feed into the bottom of a gas-solid riser reactor, followed by feed vaporization and vapor cracking. Modeling investigation on parametric effects of operation conditions (e.g. catalyst temperature, catalyst-to-oil ratio (CTO), atomized droplet size, and feed injection velocity) becomes essential to the optimization of riser reaction performance. In this study, we have developed a theoretical framework for quantifying the liquid feed transport, vaporization and cracking in the feed injection zone. Specifically, the multi-sprays are assumed to be mutual penetrating and symmetrically structured. The spray trajectories and vaporization distributions are tracked via Lagrangian modeling approach; whereas the gas and solids, coupled with vapor cracking reactions, are accelerated towards the downstream of riser flow. The results of feed-zone modeling provide the needed hydrodynamics and reaction conditions for the modeling of downstream gas-solids transport and cracking reactions in the remaining part of the riser reactors. To illustrate the modeling capability, we have investigated the three-phase transport in a feed zone with feed injected from four square nozzles. The catalytic reaction is based on a four-lump cracking model. Our spray transport model shows a complicated overlapping structure of mutual-penetrating sprays, with each individual spray spreading and vaporizing in its associated injection plane. This spray structure not only defines the spray coverage and cross-sectional penetration but also defines the feed zone length (as the farthest spray traveling distance along the riser). Other important modeling results include the radial profiles of transport variables at the end of the feed-zone. Among the key variables investigated are the catalysts concentration, phase velocities and reaction species concentrations. (C) 2013 Elsevier Ltd. All rights reserved.