Extraction desulfurization with mixed solvents of organic solvent + organic solvent or deep eutectic solvent as extractants: Liquid-liquid equilibrium experiments and molecular dynamics simulations

To meet the stringent requirements for sulfur content in gasoline, fuel oil desulfurization has become an indispensable process in petrochemical production. This study aimed to extract thiophene from heptane using a novel mixed solvent. In this work, the performance of single solvent N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and sulfolane (SULF) was investigated for the separation of thiophene and heptane at 298.15 K or 308.15 K, respectively. Ternary liquid-liquid equilibrium (LLE) data (heptane + thiophene + DMF, heptane + thiophene + DMSO and heptane + thiophene + SULF) shows that DMF has the highest distribution coefficient and DMSO has the highest selectivity. Subsequently, compared to a single solvent, the mixed solvent composed of the molar ratio of 1:7 tetrabutylammonium bromide-SULF (DES1) and DMSO has the highest selectivity and applicable distribution coefficient. The conclusion can be obtained from the quaternary LLE data (heptane + thiophene + DMF/DMSO, heptane + thiophene + DES1/DMSO and heptane + thiophene + DES1/DMF). And the NRTL and UNIQUAC models were used to correlate the LLE data. The mechanism of thiophene extraction from thiophene/heptane was elucidated by molecular dynamics (MD) simulations. The analysis of non-bonded interaction energies shows that the electrostatic force plays a major role in the separation of thiophene/heptane for the mixed solvent, and radial distribution functions demonstrates that the oxygen atom of DMSO, nitrogen atom atom of DMF and bromide ion of DES1 have strong interaction with hydrogen atom of thiophene. Therefore, this study provides theoretical guidance for the application of mixed solvents in the desulfurization of fuel oil process.