Feed and process effects on the in situ reduction of sulfur in FCC gasoline

Abstract

In this study we investigated the effects of various types of fluid catalytic cracking (FCC) feedstocks (VGO, FCC gasoline and FCC gasoline cuts) on sulfur compound distribution in the gasoline produced from FCC process. A bench scale short contact time microactivity test unit (SCT-MAT) and an FCC pilot plant unit were found to be satisfactory for the gasoline sulfur studies. Based on these studies reaction mechanisms are proposed for the formation of sulfur compounds during FCC process. Hydrogen transfer reactions play an important role on gasoline sulfur and thus the cracking temperature affects gasoline sulfur removal. As expected the FCC feed sulfur strongly affects the gasoline sulfur. FCC feeds with higher sulfur give gasoline with higher sulfur concentration. However, the percent feed sulfur ending up in gasoline increases with decreasing sulfur in feed content. The cracking of various FCC gasoline cuts, enriched in specific sulfur compounds, was used to indicate reaction networks through which these compounds are desulfurized in the FCC environment. The most interesting result of this study is the observation that re-cracking the gasoline product in the FCC unit significantly reduces total sulfur. This reduction is mainly due to the cracking and/or cyclization of long chain alkyl-thiophene compounds.