Energy efficient trace removal by extractive distillation

Separation processes contribute for about 40–70 % to the total energy requirements of the chemical process industry. Especially when trace removal is required to manufacture high purity products, traditional separation technologies become extremely expensive and are not providing satisfying solutions. In this work, trace removal is defined as the separation of impurities or (by) products at a level of 100–1000 ppm. The main reason for trace removal separation is the need to meet the purity demanded by customers further processing and/or legislation. Distillation is currently used to conduct 90–95 % of all separations in the chemical process industry. The drawbacks of distillation are the poor energy efficiency and selectivity. These drawbacks are above all applicable to cases where impurities have to be removed with molecular structures closely resembling the main component (and hence, having similar boiling points) and are present at ppm level in a highly concentrated, organic stream. Two representative industrial examples are the separation of ethylbenzene traces (~100 ppm) from styrene and dichloroacetic acid (DCA) traces (~1000 ppm) from monochloroacetic acid (MCA). Both are examples of close-boiling mixtures (= low relative volatility). A well-established technology to separate close-boiling mixtures is extractive distillation. In extractive distillation, a solvent is added to a distillation column to alter the relative volatility of the system to be separated, and, thereby, to reduce the capital and operational expenditures (OPEX and CAPEX). Nowadays, extractive distillation is already applied to produce high purity products in benzene, toluene, xylene processes with purities up to 99.995 wt%. Therefore, the main objective of the research described in this thesis was to investigate whether extractive distillation could be a promising technology to obtain high purity products for the two representative industrial examples. For both cases, the trace removal and bulk separation were both performed by extractive distillation processes to be able to replace the current separation processes, and thus also achieving process intensification. Sulfolane, which is a commonly applied organic solvent, and a wide range of ionic liquids (ILs) were investigated for the separation of ethylbenzene from styrene to obtain ethylbenzene impurity levels lower than 10 ppm in styrene. Complexing agents (extractants) were studied to separate DCA from MCA to obtain high purity MCA (wDCA [4-mebupy][BF4] > [3-mebupy][B(CN)4]. The selectivity for the ethylbenzene/styrene system decreases in the same order. Binary and ternary phase equilibrium data were obtained for the systems ethylbenzene + styrene + selected ILs, and ethylbenzene + styrene + sulfolane. The NRTL model was able to correlate the phase equilibrium data adequately, and the parameters that were obtained by regression of the experimental data were used in the following chapters in the process simulations. A very important factor in extractive distillation processes is the recovery of the solvent. If that cannot be done adequately and cost effectively, the process cannot be competitive. Therefore, a screening of several IL recovery technologies was performed, for which the IL [4-mebupy][BF4] was taken as a model IL. It was found that this IL should be purified to at least 99.6 wt% in the regeneration section to maintain the distillate and bottom purity requirements for the primary separation of ethylbenzene and styrene in the extractive distillation column. From the TAC, the overall conclusion can be drawn that evaporation using very low pressures (P