A mechanistic model that to provide a quantitative understanding of the interplay of hydrodynamics, heat/mass transfer, and cracking reactions in the feed injection zone of a fluid catalytic cracking (FCC) riser reactor with a single nozzle spray. With the injection of an oil spray into a gas-solid flow, the collision between cold oil droplets and hot catalyst particles results in a strong momentum transfer that affects the spray hydrodynamics in terms of penetration and scattering. It also causes a significant heat transfer giving rise to rapid droplet vaporization and the attendant cooling of the catalyst. The presence of cracking reactions introduces volume expansion, changes in gas composition and volume fraction, and a cooling effect due to endothermicity. Accordingly, we in this study present an analysis of chemically-induced "entrance effects" in an FCC riser with a single nozzle spray. The cracking reaction network is described by a four-lump reaction model, while the ambient gas-solid transport is represented by a dense-phase riser flow. A Lagrangian modeling approach is adopted to track the spray trajectory as cracking reactions proceed. It is shown that cracking reactions play an important role in dictating the spray behavior, reaction and heat/mass transfer characteristics in the feed injection zone of an FCC riser.