Fundamental research on the influence of mercaptan and thioether structure on the solvent extraction of fluid catalytic cracking naphtha

The thioether and mercaptan components (over 50-100 mg/kg) of fluid catalytic cracking (FCC) naphtha must be removed to satisfy the rigorous sulfur content standard of the vehicle gasoline. Owing to the difference between the structures of thioether and mercaptan and that of thiophenic sulfur, the desulfurization process is usually divided. A serious loss of octane number (ON) occurs during typical desulfurization when the sulfur content is reduced to 10 mg/kg. With the oriented separation of FCC naphtha critical components process, the standard of sulfur content with losing 1 unit of ON can be satisfied. However, thioether and mercaptan require individual technologies for removal. Thioether, mercaptan, and thiophenic sulfur must be synchronously removed to improve the efficiency, simplify the technology, and reduce the energy consumption. In this study, quantum chemical calculation and ternary liquid-liquid equilibrium experiment were combined to investigate the effect of thioether and mercaptan structures on the sulfoxide solvent extraction performance. The results showed that the Van der Waals' force was the main interaction between the thioether/mercaptan and the solvent because the peaks of solvent and sulfide by RDG analysis were located at -0.01 to 0.01 a.u. In addition, the dipole moment, steric hindrance, and displacement affected the solvent extraction performance. The selectivity coefficient theta of C3 thioether and mercaptan was 5.80 and 6.48 by DMSO solvent, respectively, which were greater than those of C4 sulfides (4.48 and 5.56). The interaction energy Eint of DMSO-ethyl methyl sulfide and DMSO- propanethiol were -32.75 and -33.68 kJ/mol while DMSO-diethyl sulfide and DMSO- butanethiol were -24.72 and -29.20 kJ/mol. It also proves that the C3 thioether and mercaptan could better extract into the solvent. In this way, the extractive desulfurization rate of the real FCC naphtha by DMSO was 80%. Therefore, the use of sulfoxide composite solvent can simultaneously extract the thioether, mercaptan, and thiophenic sulfur. This analytical method could be used to predict the synchronous extraction performance for FCC naphtha sulfides by other typical solvents.

Automated summary

The removal of thioether and mercaptan components from FCC naphtha is crucial to meet the sulfur content standard of vehicle gasoline. Utilizing a sulfoxide composite solvent can efficiently extract these components and improve desulfurization efficiency.