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

• H2S, olefins recombination reactions to mercaptans and thiophenes in FCC lab scale reactor.
• Cracking of dymethyl-dysulfide (DMDS) doped sulfur free diesel in an FCC lab scale reactor.
• Cracking DMDS doped diesel with zinc oxide based gasoline sulfur reduction additives.

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.

Olefin-H2S recombination to form mercaptans and thiophenes was simulated in the lab by cracking ultra-low sulfur diesel spiked with dymethyl-dysulfide (DMDS) as a proposed model reaction for FCC sulfur chemistry in a micro-reactor system producing gasoline range thiophenes. When the experiment was repeated in the presence of ZnO containing catalyst thiophene formation was inhibited by zinc, which acted as an H2S trap.Figure optionsDownload as PowerPoint slide