Newly developed zeolite catalysts for the fluid catalytic cracking (FCC) process are typically characterized by high activity and rapid deactivation. Modifications of conventional FCC reactors are necessary to avoid severe back mixing which decreases selectivities and yields and to take full advantage of new catalysts. Recent studies suggest that more uniform flow, better reaction control, shorter residence times with narrower residence time distributions (RTD), and higher catalyst/oil ratios can be achieved in downflow systems (downers) compared to upflow systems (risers). In this work, a novel computer simulation of a FCC downer reactor is introduced. Comparison of downer and riser models for the FCC process shows that downers are slightly more beneficial than risers when commercial silica-alumina catalysts are used. In the case of the FCC process using zeolite catalysts, the benefits of downers become more significant.