Effects of mass transfer resistance and coking on yield during fluid catalytic cracking of heavy hydrocarbon fractions

Abstract

A transient model for an industrial fluid catalytic cracking (FCC) riser is here presented. The FCC riser models of previous researchers were mostly based on the assumption of negligible mass transfer resistance. This assumption reduced the accuracy of the models. In this work, the effects of mass transfer resistance and coking on the yield of FCC reactions were modeled and simulated. A five-lump reaction scheme was used to model the cracking reactions. Catalyst deactivation was modeled based on the exponential decay function. The mass transfer coefficient and the catalyst effectiveness factor were estimated from empirical correlations obtained from literature. The model parameters that were used in this investigation were sourced from the field as well as from literature. The reaction model was solved using COMSOL Multiphysics 4.0. The major difficulty encountered in this investigation was the unavailability of experimental data from which the mass transfer resistance, catalyst effectiveness factor, kinetic constants of the cracking reactions and effective diffusivities of species could be determined. This challenge was overcome by using empirical correlations from literature to estimate the terms. Model data which could not be obtained from the plant were sourced from literature and duly referenced. The proposed model vividly showed that mass transfer resistance significantly affects FCC reactions. Previous models over-predicted the residence time of the riser while under-predicting the over-cracking point of gasoline (the key product). A residence time of 2s and a gasoline yield of 45% were predicted by this model. Key words: Fluid catalytic cracking (FCC), Transient model, mass transfer resistance, catalyst deactivation, Riser models.