Intra-particle Mass Transfer and Contact Time Effects in Fluidized Catalytic Cracking

Certain yield changes in FCC due to changes in catalyst contact time, reactor configuration, and/or catalyst properties are consistent with the hypothesis that mass transfer in the pores external to the zeolite is additionally limiting the catalysis. We attempted to validate this hypothesis by evaluating catalyst effectiveness factors and their parametric sensitivity to catalyst contact time. The rate constants required to obtain a given conversion vary as the reciprocal of catalyst contact-time, so that the potential importance of mass transfer increases for processes with shorter contact times. We found that vapor-phase gas-oil cracking is diffusion-limited at the riser entrance for contact times and conversions typical of many operations, a result consistent with commercial observations.