Kinetic Model Based on a Molecular Description for Catalytic Cracking of Vacuum Gas Oil

Authors: Isabelle Pitault ^a, David Nevicato ^a, Michel Forissier ^a, and Jean-René Bernard ^a
a Génie catalytique des Réacteurs de Raffinage, CNRS/ELF, Centre de Recherches Elf Solaize, BP 22, F69360 Saint Symphorien d’Ozon, France

Source: This paper was published in 2005 in Chemical Engineering Science, Volume 49, Issue 24, Part A.

Abstract: A kinetic model based on a molecular approach is proposed to represent the cracking reactions of industrial feedstocks on a commercial equilibrium catalyst.Special attention is given to the definition of lumps to represent correctly the effect of the feedstock composition on the gasoline composition through the most important reactions occurring between lumps. The latter are defined not only by their boiling range but also by their chemical composition.The kinetic model is built from experiments with a small fixed bed reactor (the micro-activity test). The analysis of cracked products is done by chromatography simulated distillation and a detailed gas chromatographic analysis of gases and gasoline. The study is done at 803 K with various reaction conditions (catalyst hold-up, initial catalyst coke content) to observe primary and successive products. Three typical commercial feedstocks (aromatic, naphthenic or paraffin) are used to distinguish the effects of feedstock components.The selected reactions are grouped as: (i) cracking reactions by β scission of paraffin, alkyl-aromatic or naphthenic chains, (ii) condensation reactions of olefins possibly with aromatic hydrocarbons, (iii) cyclization of olefins, (iv) hydrogen transfer reactions.The attention is focused on the deactivation function of the catalyst. This function is experimentally determined by testing previously coked catalysts. It takes into account chemical deactivation by coke fouling and diffusional limitation due to pore plugging by coke. The results show that this function depends on the feedstock composition. The reactor is modeled as isothermal, plug flow and unsteady because of the deactivation by coke. The kinetic expressions suppose generally an order one versus each reactant. The kinetic constants are determined by non-linear adjustment with experimental data. The obtained set of kinetic constants describes satisfactorily the experimental results. The kinetic model shows the importance of the condensation and hydrogen transfer reactions for coke formation and gasoline quality. It can be used for modeling a commercial FCC riser.