Liquid–Liquid Equilibrium Data and Mechanism for Separating Thiophene from Heptane Using Mixed Solvents

Solvent extraction is a promising method for removing organic sulfides from fuel oils in refineries. However, selecting an appropriate extractant requires a fundamental understanding of liquid–liquid equilibria. In this study, we collected liquid–liquid equilibrium (LLE) data for ternary heptane + thiophene + solvent systems at 298.2 K and 101.3 kPa. The extraction capabilities of the selected solvents (ethylene glycol (EG), N-formylmorpholine, 1,3-propanediol, 1,4-butanediol, 1,2-propanediol, N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide, and benzyl alcohol) for separating heptane and thiophene were assessed based on the distribution constant (D) and separation factor (S). NMP exhibited the highest D value, whereas EG exhibited the lowest. In contrast, the EG had the highest S value. Therefore, to balance D and S simultaneously, mixtures of EG and NMP at different molar ratios were used as extractants. Molecular-level investigation into the separation mechanism was conducted via molecular dynamics simulations and density functional theory calculations. The enhanced S value of EG was attributed to a significant difference in its interaction energies with thiophene and heptane, whereas the enhanced D value of NMP resulted from a more negative solvation-free energy upon binding thiophene. Finally, a comparison of the thermodynamic models showed that the nonrandom two-liquid (NRTL) model and the universal quasichemical (UNIQUAC) model were equally suitable for correlating the experimental data for the ternary and quaternary heptane + thiophene + solvent systems; the root-mean-square deviation (RMSD) values for the NRTL model were all <1%, whereas the maximum RMSD value for the UNIQUAC model was 1.57%. The obtained liquid–liquid equilibrium data and mechanistic insights can aid in improving extractive desulfurization using conventional organic solvents toward the efficient, low-cost, and simple production of clean fuel oils with ultralow sulfur contents.