Recently, room temperature ionic liquids (ILs) have been proposed to be used as extractive solvents for the separation of close-boiling substances and azetropic mixtures [Arlt et al. DE 10114734, DE 10136614, WO 2002074718]. Obviously, ILs have an intrinsic advantage over conventional extractive (entrainers) due to their extremely low volatilities and their good thermal stabilities.
This paper presents an extensive study on the feasibility of ionic liquids (IL) for the extractive distillation of close-boiling (alkene/alkane) and azeotropic mixtures (chlorinated alkane/alkane) composed of high-volatile components. Alkenes and alkanes of the same chain length have rather close boiling temperatures, while some of chlorinated alkanes/alkanes mixtures show an azeotropic behaviour (maximum pressure azeotrope). Obviously, both types of mixtures are hardly separable by a simple distillation. Due to their higher polarizability, chlorinated and unsaturated hydrocarbons (such as alkenes, dienes or aromatics) experience a stronger solvation in ionic liquids compared to saturated hydrocarbons. From an engineering point of view, it is a highly relevant question whether this difference is large enough to allow an effective separation technology to be based on these differences. Since alkenes are low boilers (the ene-to-ane separation factor of the pure compounds is slightly higher than 1) and show always higher solubilities in ILs, the task of the entrainer in this case is more difficult than e.g. for the ethanol-water system. In the latter case, water is the high boiler in the mixture of the pure components and stays the high boiler after addition of the ionic liquid [Arlt et al. DE 10114734, DE 10136614, WO 2002074718, Y. Beste et al. DE 10336555]. Thus, a suitable entrainer IL should (i) convert the low-boiling alkene to a high boiler shifting the separation factor below 1, (ii) dissolve the alkene so effectively that the separation factor becomes as low as possible, and additionally (iii) in the case of azetropic mixtures retain the separation factor below 1 at concentrations below and above the azeotropic point.
Obviously, when two or more high-volatile substances dissolve in one solvent, their solubility is not independent of one another, especially at high loads. Experimental studies on the solubility of gas mixtures are however rather tedious and time-consuming. In the present study, a new express experimental procedure for a direct screening of low volatile extractive agents such as ILs is reported. Based on head space chromatography, this method is fast, is applicable to high-volatile mixtures directly, and requires only small amounts of ILs.
More than 20 ILs and their binary mixtures have been screened at ambient temperature and low pressures. Selected ILs have been also studied at elevated temperatures and pressures in an autoclave. The screening results show that every tested IL turns the low boiler alkene to be the high boiler with the ene-to-ane separation factor much below 1 in the whole composition range. In the case of the azeotropic chlorinated alkane/alkane mixtures, IL-entainers are shown to be able to break the azeotrope and thus assure the separation in the whole range of chloroalkane/alkane ratios. By adjusting the cation/anion combination and/or their chemical functionalization, the solubility and separation efficiency can be effectively changed. The gas-mixture solubility increases by elongation of the side chain length in the imidazolium-based cation, the IL separation ability being vice versa decreased. For the IL mixtures studied, individual ILs contribute additively to the quality of the mixed entrainer. The autoclave experiments at elevated temperatures and pressures indicate that the separation ability decreases with growing temperature and loading. Generally, our study proves impressively the great potential of ILs to act as entrainers in the extractive distillation of the close-boiling and azeotropic mixtures composed of high-volatile substances