Extraction of methylcyclohexane−toluene mixture using imidazolium ionic liquids

Separation of aromatics from non-aromatic hydrocarbons was exemplified assuming the liquid-phase extraction process for methylcyclohexane−toluene model mixture separation using the newly tested 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMim][NTf₂]) ionic liquid (IL). For this process a separation unit was proposed consisting of separation part represented by a counter-current extraction column and regeneration part including a vacuum evaporator and a vacuum distillation column. Separation of feed composed of 10 mole% of toluene in methylcyclohexane was considered in the designed separation unit. Designed separation efficiency of the unit was based on the following specifications: minimum methylcyclohexane and toluene content in the product streams of 99.5 mole% and the purity of regenerated extraction solvent recycled to the extraction column above 99 mole%.

Mathematical model of a counter-current extractor was compiled and its operation was simulated in the Matlab environment. In extractor simulations, proprietary phase equilibrium data were employed. Ternary liquid–liquid equilibrium (LLE) of the methylcyclohexane−toluene−[EMim][NTf₂] system was estimated experimentally, from which the model parameters of the original NRTL equation were evauated and used for the ternary phase equilibrium description.

Aspen Plus was used to design a separation unit for the title hydrocarbons mixture separation, including the extraction solvent regeneration. Simulation of the proposed separation unit operation was focused not only on its separation efficiency but also on the evaluation of the unit energetic requirements.

Results of the unit design calculations (parameters of individual equipment, heat duties) obtained for the tested ionic liquid [EMim][NTf₂] were confronted with the simulation results obtained for two ILs recommended in the literature for the non-aromatic–aromatic hydrocarbon mixture separation, namely 1‑butyl‑3-methylimidazolium tetracyanoborate [BMim][TCB] and 1-hexyl-3-methylimidazolium tetracyanoborate [HMim][TCB]. Based on the energetic analysis, the heat integration of the suggested separation unit was carried out for ionic liquid [HMim][TCB], which appeared to be the most efficient extraction solvent among those tested in this study, The heat integration resulted in about 71% reduction of the heat demand of the proposed separation unit.