Liquid-Liquid Equilibrium For Extractive Separation Of Benzene And Cyclohexane Using Ionic Liquids

Benzene and Cyclohexane are two important commodity chemicals, their difference in boiling point is only as low as 0.6℃, which makes their separation via traditional distillation invalid. Therefore, an effective alternative separation technique is highly required in industry. This paper is mainly focused on the following two aspects, (1) applicability of imidazolium based dialkylphosphate ionic liquids (ILs) for the extractive separation of benzene-cyclohexane mixtures, and (2) measurement of liquid liquid equilibria (LLE) data for the ternary systems of IL (1)-Benzene (2)-cyclohexane (3), on this basis, the extractive selectivity of different ILs with respect to bezene can be assessed. The ILs studied here include 1-methyl-3-methylimidazolium dimethylphosphate ([Mmim] [DMP]), 1-ethyl-3-ethylimidazolium diethylphosphate ([Emim] [DEP]) and N-methylimidazole hydrogen diethylphoaphate {[Hmim] [DBP] (H=hydrogen, DBP=dibutylphosphate)}. From the experiment data we found that the extraction selectivity of different ILs with respect to benzene followed the order of [Mmim] [DMP] > [Emim] [DEP]) >[Hmim][DBP], and their selectivities are all higher than unity, suggesting their potential use as an effective extractant for the separation of benzene from its cyclohexane mixtures. Within the temperature range of 298.2K to 313.2K studied, the temperature dependence of extraction selectivity for different ILs is negligiable. Besides, as the increase of benzene content in the feed solution, the selectivity of ILs for benzene becomes lower and the coexisting immisible phases region becomes narrower. The experimental LLE data for the ternary system is correlated using NRTL equation with satisfaction, which suggested that ILs in the solution is presented as neutral molecular entity rather than ionic species, due to the nonpolarity and extremely low dielectric permeativity of the benzene-cyclohexane mixtures. This explained why NRTL equation still holds for the 'ionic' liquid.The applicability of some imidazole series solvents for the extractive separation of benzene from its cyclohexane mixture is studied. It is found that imidazole is immiscible to both benzene and cyclohexane, and N-butylimidazole is mutual soluble with benzene and cyclohexane, thus they are not suitable to be used as extracting solvent. However, N-methylimidazole and N-ethylimidazole shows good extraction selectivity for benzene. With the size increase of alkyl substitutes to the imidazole ring (H→CH₃→C₂H₅), the solubility of benzene in the solvent increases, resulting in the following extraction selectivity order for benzene, i.e N-methylimidazole> N-ethylimidazole. In addition, LLE data for alkylimidazole-benzene-cyclohexane ternary systems at 298.2K is measured, and the data are correlated with NRTL equation. The results indicate that N-methylimidazole and N-ethylimidazole can be used as a potential extractant for the extraction separation of benzene from its cyclohexane mixture with their extraction selectivity located in the ranges of (10.7-413.6) and (1.5-5.4), respectively.