A comprehensive method of ionic liquid screening and experimental verification for simultaneous separation of multiple sulfides from oil

Ionic liquid (IL) as the extractant has shown an excellent performance in oil desulfurization. However, it is still a great challenge to select an appropriate IL that can simultaneously remove multiple sulfides from oils due to enormous ILs' quantities. In this work, an ILs' screening method based on the fuzzy comprehensive evaluation function and COSMO-RS method was developed for oil extraction desulfurization. Four typical aromatic thiophene sulfides – thiophene (TS), benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were considered as the removal targets, and n-heptane was chosen as the model oil. By this method, a single IL – 1-butyl-3-methylpyridinium dicyanamide ([BMPy][DCA]) was selected from 702 ILs for removing the four sulfides from n-heptane. Liquid-liquid equilibrium (LLE) data of n-heptane + TS/BT/DBT/4,6-DMDBT + [BMPy][DCA] systems at 298.15 K were measured and then correlated by the NRTL equation. A data processing method for LLE experimental indices, distribution coefficient and selectivity, was proposed for their crossed curves and different measuring ranges to facilitate the comparison of ILs' extraction performance. The comprehensive experimental indices based on this processing method were compared with the comprehensive predicted indices obtained by screening. The results showed a consistent trend between the two, and the excellent desulfurization performance of [BMPy][DCA] was confirmed. The separation mechanism was then analyzed through the quantum chemistry method. The results showed that the interactions of four sulfides with either n-heptane or [BMPy][DCA] both were the van der Waals interaction and steric effect. The [BMPy][DCA] could attract the sulfides more potently than the n-heptane, demonstrating the extraction's mechanism and characteristics. The stronger itself interactions of BT, DBT and 4,6-DMDBT as solid sulfides limited their solubility in liquid–liquid two phases. In addition, the decomposition of interaction energy showed that the attraction between the sulfides and the [BMPy][DCA] was dominated by dispersion, assisted by electrostatics, and equipped with a small amount of induction. The essence of repulsion between them was the exchange-repulsion.