A mathematical model is developed to predict the two-phase flow and fluid catalytic cracking (FCC) process inside a downer reactor by coupling a turbulent gas-particle multiphase flow model

which describes the flow behavior

with some lumping kinetic models

which simulate the cracking reaction inside the reactor. The model equations are coded and solved by CFX4.3 package

a general computer program for unit simulation involving fluid

heat transfer or chemical reaction. The model can predict some important engineering aspects for a reactor such as solids holdup

interphase slip velocity

pressure drop

acceleration zone for both phases and yields distribution all over the reactor. The predictions reveal that flow behavior is significantly changed when cracking reaction take place inside downer reactors and scale-up of reactors can cause reduction of yield.