Highly efficient separation of phenolic compounds from low-temperature coal tar by composite extractants with low viscosity

Highly efficient separation of phenolic compounds from low-temperature coal tar (LTCT) is significant in the industry. In this work, composite extractants composed of imidazolium-based ionic liquids (ILs) and solvents were used to separate phenolic compounds from model oil and LTCT. First, the COSMO-RS model was used to screen composite extractants that combined the infinite dilution thermodynamic indexes and liquid–liquid equilibria (LLE) calculations. Meanwhile, the separation mechanism was analyzed by σ-profile and σ-potential. Then, the reliability of the COSMO-RS model was validated by LLE experiments and FT-IR. The viscosity of composite extractants with high separation performance was further determined. Among the composite extractants, 1-ethyl-3-methyl-imidazolium hydrogen sulfate ([emim][HSO4])/ethylene glycol (EG) was selected as the most suitable extractant in the separation process. The m-cresol extraction efficiency and cumene entrainment were 98.1% and 8.3%, respectively, at a [emim][HSO4]:EG molar ratio of 1:2, a temperature of 25 ℃, and a composite extractant: model oil mass ratio of 1:1. Moreover, the viscosity of [emim][HSO4]:EG (1:2) was only 51.8 cP at 25 ℃ and decreased by 95.6% compared with pure [emim][HSO4], which enhanced the mass transfer in the separation process. The [emim][HSO4]:EG (1:2) could be regenerated and reused without significant reduction in separation performance. Finally, [emim][HSO4]:EG (1:2) was applied to separate phenolic compounds from LTCT with a phenol extraction efficiency of more than 99.9% and a neutral oil entrainment of 8.3%, providing a promising prospect for the separation of phenolic compounds.