The separation of the close-boiling point mixture: toluene - methylcyclohexane can be carried out by extractive distillation using the ionic liquid 1-hexyl-3-methylimidazolium tetracyanoborate ([hmim][TCB]), reported as promising solvent for the separation of this mixture. However, the polar nature of ionic liquids causes the formation of two liquid phases which can be overcome with high solvent-to-feed ratios (S/F) resulting in high liquid phase viscosities that cause mass transfer limitations. Experiments in an extractive distillation pilot plant were performed with the objective of firstly exploring different operating conditions and secondly to compare the mass transfer efficiencies produced by [hmim][TCB] and the reference organic solvent N-methyl-2-pyrrolidone. Pure viscosity, density and surface tension data as well as ternary viscosity and density data of tolune - methylcyclohexane - [hmim][TCB] were measured to compute the mass transfer efficiency. From pilot plant experiments it was found that all the studied operating conditions did not form two-liquid phases. However, the high solvent-to-feed ratios increased the liquid phase viscosities and consequently the use of this ionic liquid produces Height Equivalent to a Theoretical Plates (HETPs) twice as high as the reference solvent NMP causing slightly lower top purities and a longer required distillation column for a required separation.

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Ionic liquids (ILs) can replace conventional solvents in separation processes, such as extractive distillation (ED), because of their ability to selectively separate azeotropic/close boiling mixtures. Four case studies were selected: ethanol/water (1-ethyl-3-methylimidazolium dicyanamide, [emim][N(CN)2], and ethylene glycol, EG), 1-hexene/n-hexane (no suitable IL found), methylcyclohexane/toluene (1-hexyl-3-methylimidazolium tetracyanoborate, [hmim][B(CN)4], and N-methyl-2-pyrrolidone, NMP), and ethylbenzene/styrene (4-methyl-N-butylpyridinium tetrafluoroborate, [4-mebupy][BF4], and sulfolane). Pilot plant experiments proved that the developed models for ED could well describe the experimental results.

Conceptual processes were designed for the ED of three case studies. The ethanol/water process with [emim][N(CN)2] reduced the energy requirements with 16 % compared to the process with EG, provided that proper heat integration is implemented. The methylcyclohexane (MCH)/toluene process with [hmim][B(CN)4] required about 50 % less energy with heat integration than the conventional process with NMP with heat integration.

The IL [4-mebupy][BF4] reduced the energy requirement most compared to the conventional distillation for the ethylbenzene/styrene process (43.2 %), which is 5 % lower than with extractive distillation with sulfolane. However, the capital expenditures were about 23 % higher than for the sulfolane process. It can be concluded from the total annual costs that all studied ED processes outperform the current distillation process to obtain high purity styrene, but that ILs do not perform better than sulfolane.

The general conclusion of these four examples is that only in some special cases ILs can be more advantageously applied than conventional solvents in extractive distillation. The key performance points for ED are a high selectivity and high capacity, next to the solvent recovery and heat integration.@en