Liquid-liquid equilibria and mechanism exploration for the extraction of sulfides from FCC naphtha via organic solvent as extractant

Abstract

Separation of sulfides from fluid catalytic cracking (FCC) naphtha is of great significance for environmental protection. In this work, the organic solvents N-methyl-2-pyrrolidone (NMP), sulfolane (SUL), and their mixtures were selected to extract sulfides (3-methylthiophene) from model FCC naphtha (n-heptane). Correspondingly, the liquid-liquid phase equilibrium (LLE) data of the systems n-heptane +3-methylthiophene + SUL/NMP/(NMP + SUL) were experimentally obtained at 298.15 K and 101.3 kPa. The distribution coefficient and separation factor were calculated to assess the extraction performance of the solvents. The results showed that mixed solvent (NMP + SUL) could be used as a potential solvent to extract 3-methylthiophene due to its suitable selectivity and solubility. Meanwhile, the density functional theory (DFT) method was applied to explore the extraction mechanism of 3-methylthiophene from n-heptane by NMP or SUL. Electrostatic potential (ESP) analysis, interaction energies, and reduced density gradient (RDG) analysis showed that the main interactions between NMP/SUL and 3-methylthiophene were van der Waals and weak hydrogen bond forces. In addition, the LLE data were correlated by the NRTL and UNIQUAC thermodynamic models with the RMSD values were all <0.30%. Also, the binary interaction parameters were regressed which could be applied for the optimization and design of the separation process.