Formation of thiophenic species in FCC gasoline from H2S generating sulfur sources in FCC conditions

Sulfur contamination of a bio-gasoline produced from very low sulfur soybean oil in a circulating pilot riser led to the investigation of the H2S-olefin recombination pathway by which any sulfur source capable of producing H2S in the FCC riser could in principle produce all of the typical thiophenic species present in the FCC cracked naphtha. The H2S-olefin recombination mechanism was confirmed in the laboratory by cracking ultra-low sulfur diesel spiked with 2 wt% dymethyl-dysulfide (DMDS) in a Short Contact Time Resid Test reactor to produce 60–80 ppm gasoline, in which the typical cracked naphtha sulfur species were major components. DMDS cracked to produce methyl-mercaptan and H2S as primary products, which continued to react with hydrocarbons derived from the diesel oil cracking to produce the intermediates and final products in the reaction pathway. When the reaction was repeated with catalyst containing 10% of a commercial gasoline sulfur reduction additive, the sulfur in gasoline was reduced by 40% and the sulfur in the spent catalyst went from very low levels, when no additive was used, to 0.5 wt% with the additive. The importance of the recombination pathway and the catalyst additive activity in interfering with the pathway by acting as an H2S scavenger may explain the functional mechanism of ZnO based gasoline sulfur reduction additives.