Reactions of Organic Sulfur and Nitrogen Compounds in the FCC Pretreater and the FCC Unit

The effect of hydrotreating VGO (FCC pretreatment) at different severities was examined with respect to FCC performance. Furthermore, the reaction mechanisms for conversion of the organic sulfur and nitrogen compounds found in the pretreated FCC feedstocks during FCC processing were studied. The studies were carried out under realistic conditions in a large pilot plant hydrotreating unit and in an FCC pilot plant operating with a circulating catalyst. It was found that the sulfur compounds in the FCC gasoline (i.e., thiophenes and benzothiophenes) were formed from recombination reactions between H2S and olefins generated in the FCC unit. Similarly, the nitrogen compounds found in the FCC gasoline (i.e., anilines and indoles) were formed from recombination reactions between ammonia and olefins. These results show that the sulfur and nitrogen compounds in the FCC gasoline are mostly the same with only minor differences in the relative concentration of the individual compound. This phenomenon is not dependent on the feed charged to the FCC unit but is a result of equilibrium reactions between olefins, H2S and ammonia, respectively, and the temperature in the FCC reactor.