Catalytic cracking of thiophene and benzothiophene: Mechanism and kinetics

The scope of the present study is to describe the cracking behavior of sulfur compounds during the fluid catalytic cracking (FCC) process. Specifically, the cracking mechanism of thiophene and benzothiophene molecules was investigated using model compounds. Experiments were performed in a fixed bed reactor unit, short contact time-microactivity testing (SCT-MAT) using a commercially available FCC catalyst deactivated with two different methods: steam deactivation with no metals using a laboratory fluid bed reactor and deactivation occurring in an industrial FCC unit (equilibrium). The latter represents a relatively slow deactivation in the presence of metals deposited from the feedstock and it was used in order to investigate the impact of Ni and V on the reaction mechanism. The model compounds were run in small concentrations either with hexadecane or with FCC gas oil. The experimental results indicated that thiophene is a very stable molecule, which undergoes mainly desulfurization reactions (leading to H2S and S deposition in coke), while alkylation and saturation reactions take place to a lower extent. It was found that the equilibrium catalyst (Ecat) promotes the deposition of S in coke while steamed deactivated catalyst favors the alkylation and saturation reactions. Benzothiophene is more reactive than thiophene and it participates in alkylation reactions resulting in heavier sulfur compounds with a boiling point outside the gasoline range. The above reactions are mainly promoted using the steamed catalyst. Based on the experimental results, reaction networks were proposed for both model compounds and the reaction rate parameters were estimated. Parity plots of experimental and predicted yields, as well as statistical analysis of the estimated kinetic parameters indicated that the suggested models simulated satisfactorily the cracking behavior of the specific sulfur compounds under the FCC conditions.