Two-phase flow, heat transfer, and reaction in fluid catalytic-cracking riser-type reactors are studied using a 3-D mathematical model This study was carried out based on the model of Theologos and Markatos, which incorporates a detailed ten-lump reaction kinetics scheme and accounts for gradual feedstock vaporization inside the reactor Predictions obtained using the new model are compared against industrial reactor operating darn. A design study was also carried out to illustrate that the model developed is capable of predicting feed-injector geometry effects on overall reactor performance It shows that by increasing the number of feed-injection operating nozzles at the bottom of the reactor, selectivity of primary products is improved.