Ionic liquids as solvents in separation processes.
Due to the ever increasing need for sustainable development, the chemical and allied industries have been at the focus of much change. Decreasing tolerances on pollution via waste streams has resulted in a re-examination of many chemical processes. This has ushered in the era of 'green chemistry' which incorporates the synthesis of a process in both a sustainable and economically viable manner. In the petroleum and chemical industries, this has led to the search for alternatives to volatile organic compounds. Ionic liquids provide one such alternative. With a wide liquid phase and no measurable vapour pressure, ionic liquids have been found to be successful as a medium for reactions. Ionic liquids differ from high-temperature molten salts in that they have a significantly lower melting point. This work investigates the use of ionic liquids as solvents in separations. The work focuses on the separation of alpha-olefins from complex mixtures. The ionic liquids used in this study were: • l-methyl-3-octyl-imidazolium chloride • 4-methyl-N-butyl-pyridinium tetrafluoroborate • trihexyl-tetradecyl-phosphonium chloride Three experimental techniques used to evaluate ionic liquids were: • gas-liquid chromatography • liquid-liquid equilibria measurements • vapour-liquid equilibria measurements l-Methyl-3-octyl-imidazolium chloride ((MOIM)C1) was used as a stationary phase in gas-liquid chromatography. The solutes used were: • Alkanes: n-Pentane; n-Hexane; n-Heptane; n-Octane • Alkenes: 1-Hexene; 1-Heptene; l-Octene • Alkynes: l-Hexyne; l-Heptyne; 1-0ctyne • Cycloalkanes: Cyclopentane; Cyclohexane; Cycloheptane • Aromatics: Benzene; Toluene Activity coefficients at infinite dilution were measured at temperatures (298.15, 308.15 and 318.15) K. Values at 298.15 K ranged from 1.99 for benzene to 26.1 for n-octane. From the temperature dependence of the activity coefficients, the partial excess molar enthalpies at infinite dilution were calculated. These range from 2.0 kJ.mol'l for l-octyne to 7.3 kJ.mol·1 for n-pentane. (MOIM)C1 shows reasonable ability to separate 1-hexene from the longer n-alkanes and aromatics. 4-Methyl-N-butyl-pyridinium tetrafluoroborate (BuMePyBF) was used as a solvent in liquid-liquid equilibria measurements. The following systems were measured at 298.2 K: • LLE System 1: BuMePyBF4 + 1-Hexene + Toluene • LLE System 2: BuMePyBF4 + 1-Hexene + Ethanol • LLE System 3: BuMePyBF4 + 1-Hexene + 2-Butanone • LLE System 4: BuMePyBF4 + 1-0ctene + Ethanol LLE System 1 is a type 11 system and the other systems being type I. All systems exhibit a large two-phase region. LLE System 1 shows low distribution. LLE System 3 show almost equal distribution between phases resulting in a distribution ratio of close to 1. LLE Systems 2 and 4 show high distribution ratios at low concentrations of solute. LLE Systems 1 and 3 show low to moderate selectivity of the solvent towards the solute. LLE Systems 2 and 4 show high to moderate selectivity, but decrease exponentially with increasing solute concentration in the organic phase. For all systems investigated, the solvent shows no miscibility with feed solutions of low to medium solute concentration. The binodial curves were correlated to the Hlavaty equation, the beta function and the log gamma function. The correlations yielded acceptable results for LLE Systems 2, 3 and 4. The tie-lines were correlated to the NRTL model, with LLE systems 2 and 4 giving acceptable results and LLE systems 1 and 3 give excellent results. The following binary vapour-liquid equilibrium systems were measured: • Acetone + Methanol at 99,4 kPa • l-Hexene + 2-Butanone at 74.8 kPa The acetone + methanol system exhibits a minimum boiling azeotrope at 0.78 mole fraction acetone. The l-hexene + 2-butanone system exhibits a minimum boiling azeotrope at 0.83 mole fraction l-hexene. Trihexyl-tetradecyl-phosphonium chloride (CJ3C1PhCl was then added to the above systems in order to evaluate it as a solvent in extractive distillation. (CJ3C1PhCI shifts the azeotrope of the acetone + methanol system to a higher acetone concentration, but does not remove it altogether. (CJ3C1PhCI has a negative effect on the relative volatility of the l-hexene + 2-butanone, thus rendering it ineffective as an extractive distillation solvent for this system. Another aspect that was considered in this work was the production of an ionic liquid. Synthesis steps and experimental considerations were discussed. A major factor in the use of ionic liquids is the cost of the ionic liquid itself. The major problem associated with ionic liquids is the general lack of available information that is necessary for them to be implemented in a process. Ionic liquids show potential as solvents in liquid-liquid extraction for a number of systems. Their potential as solvents in extractive distillation is probably limited, due to their miscibility/immiscibility properties, to systems involving slightly polar to highly polar compounds.