Ionic liquids for separation of aromatics and aliphatics : extraction and using CO₂ as a gas antisolvent
In the production of aromatic compounds, it is necessary to separate aromatic products from aliphatic compounds, which is achieved through liquid-liquid extraction. Due to several drawbacks from the current industrial processes for this separation, ionic liquids (ILs) have drawn interest as potential solvents for this separation. Additionally, it has been shown that CO₂ can be used as a gas antisolvent to induce liquid-liquid phase splits in mixtures of aromatic hydrocarbons and ILs, which could potentially separate aromatics from ILs. Additional understanding and improvements are needed to see if these are reasonable separation processes. This dissertation aims to use experimental results to further understand the underlying phenomena driving phase behavior in these systems. Liquid-liquid equilibrium (LLE) experiments of n-heptane + toluene + IL were carried out with ILs selected based on various criteria: structure, size, modeling results from a colleague, cation differentiation, and wanting to test mixtures of ILs compared with individual ILs containing the same functional groups (hybrid ILs). All of the ILs tested exhibited selectivity towards toluene over heptane, with many having excellent selectivity and high capacity for dissolving toluene. Phosphonium and sulfonium ILs performed similarly or better as extraction solvents than the equivalent imidazolium, and some potential advantages of using mixture and hybrid ILs are presented. Vapor-liquid equilibrium (VLE) experiments were used to measure CO₂ solubility in xylene isomers, ILs and mixtures. Ternary systems exhibited vapor+liquid to vapor+liquid+liquid phase transitions, with significantly different amounts and pressure of CO₂ required to induce a phase split depending on the xylene isomer. Despite these differences, ILs showed no selectivity towards one xylene isomer over another in ambient pressure LLE experiments. Density and heat of mixing measurements for ILs with aromatics and aliphatics were used to calculate excess properties and either confirm or deny several of the hypotheses about what was causing different phase behavior in LLE and VLE experiments. The combined results of these various experiments offer insight into phase equilibria of ILs with nonpolar compounds and how volume expansion of the IL-rich phase plays a key role.