Extractive distillation with ionic liquids as solvents : selection and conceptual process design

Extractive distillation technology is widely used in the chemical and petrochemical industries for separating azeotropic, close-boiling and low relative volatility mixtures. It uses an additional solvent in order to interact with the components of different chemical structure within the mixture. The activity coefficients are modified in such a way that the relative volatility is increased. Therefore, the choice of the solvent determines the effectiveness of this process. Several solvent selection methodologies had been developed in the literature. They are based on one-way interaction parameters, meaning interactions of the components to be separated with the solvent. It has been widely accepted to consider as a promising solvent the one which is able to increase the relative volatility the most. However, the total annual cost (TAC) and the energy demand influence the final selection. Ionic liquids (ILs) are promising replacements of existing volatile solvents in extractive distillation. However, at this moment insufficient knowledge exists on the optimal properties for ionic liquids to be employed and their implementation in actual process systems. The main goal of this research was to analyze the selection and performance of ionic liquids in extractive distillation processes for three separation cases which differ from each other in polarity and chemical structures: 1-hexene/n-hexane, methylcyclohexane/toluene and water/ethanol. Theoretical ionic liquid design and selection for each mixture is done using COSMOtherm software (version C2.1, release 01.11a) by predicting activity coefficients at infinite dilution. Experimental selectivities and relative volatilities of real solutions were measured in order to choose the most suitable ionic liquids. At last, different extractive distillation processes using ionic liquids were proposed and analyzed. 1-Hexene/n-hexane separation Olefins are important base chemicals used for the manufacture of poly(olefins), plasticizers, etc. and according to Sasol, the projected demand for 2012 of C6-C8 olefins is around 0.85x106 ton. Due to the small differences in boiling temperatures and to the low relative volatility of the system, the separation of olefins and paraffins is energy intensive, meaning that all the commercially available processes for the production of olefins use several fractional distillation columns. In this work, 1-hexene and n-hexane were chosen as representative olefin and paraffin components. Extractive distillation using N-methyl-2-pyrrolidone (NMP) has been used to separate this mixture. COSMOtherm activity coefficients at infinite dilution were used to select suitable ionic liquids for this case study. Non-cyclic, cyclic and aromatic-like cations were tested in this software in combination with 27 different anions. According to the activity coefficients predicted with COSMOtherm, the solubility and selectivity of ionic liquids in 1-hexene and n-hexane is very low. This was confirmed experimentally for the selected ionic liquids using vapor – liquid equilibrium data. None of the ILs studied in this work is able to significantly increase the relative volatility in comparison with the conventional solvent NMP. Only the ionic liquid 1-hexyl-3-methyl-imidazolium tetracyanoborate [HMIM][TCB] reached a slightly higher relative volatility (1.63) than the conventional solvent NMP (1.55). However, the increase is not large enough to consider this solvent as a suitable replacement. Besides this, the ionic liquids have solubility constraints which force the use of large solvent to feed ratios to avoid the formation of two liquid phases. Methylcyclohexane/toluene separation Aromatics are among the most important chemical raw materials for the manufacture of plastics, synthetic rubber and synthetic fiber. The total production in 2009 in Western Europe of benzene, toluene and p-xylene was about 7.2x106, 1.6x106 and 1.7x106 tons, respectively. Because of the low relative volatility, external agents are used to increase the economic feasibility of the distillation units, e.g. N-methyl-2-pyrrolidone, sulfolane. The activity coefficients at infinite dilution for the mixture methylcyclohexane (MCH) and toluene with ionic liquids predicted with COSMOtherm showed a clear compromise between selectivity (easiness of separation) and solubility. Aromatic-like cations in combination with bis((trifluoromethyl)sulfonyl)imide (BTI) and tetracyanoborate (TCB) anions were selected and experimentally investigated. The relative volatility of the mixture MCH + toluene increased when any of the selected solvents (including the conventional solvent NMP) was added. The results showed that the TCB anion performed better than BTI. Therefore, the ILs 1-hexyl-3-methyl imidazolium tetracyanoborate [HMIM][TCB] and 1-butyl-3-methyl imidazolium tetracyanoborate [BMIM][TCB] seem to be the most promising replacements of NMP in the extractive distillation of methylcyclohexane and toluene. Binary and ternary liquid-liquid experimental data for the systems methylcyclohexane + toluene + [HMIM][TCB] and [BMIM][TCB] were collected and correlated with the NRTL and UNIQUAC thermodynamic models. The binary correlations were less satisfactory than ternary correlations. The results showed that UNIQUAC represented the experimental data better than the NRTL model, with a root mean square error below 0.02. The parameters obtained from the regressions of liquid-liquid equilibrium data were used to predict the vapor-liquid equilibrium (VLE). These were compared with experimental data taken by a headspace technique which showed that UNIQUAC and its parameters are able to predict the VLE of the ternary systems with a maximum error of 0.2. The non-aromatic/aromatic selectivities and relative volatilities of the ionic liquids make them suitable solvents to be used in extractive distillation processes. After obtaining the parameters for the thermodynamic model, process simulations for the extractive distillation technology using the IL [HMIM][TCB] were performed and compared with the benchmark solvent NMP. The process variables (reflux, solvent flow and number of stages) are obtained such that the energy requirements of the process are minimized. Just in the extractive distillation column, the process using the ionic liquid requires 43% less energy than the conventional solvent. Several recovery technologies were analyzed (e.g. flash evaporation, stripping with hot nitrogen, supercritical CO2, and stripping with hot MCH). The most energy efficient process (using [HMIM][TCB]) saves up to 50% of the energy requirements compared to the conventional solvent. This optimized process requires an extractive distillation column of 22 equilibrium stages, using a molar reflux ratio of 0.2 and a solvent to feed mass ratio of 2.03. The recovery of [HMIM][TCB] is done in a stripping column using part of the distillate product of the extractive distillation column as the stripping agent. Ethanol/water separation Ethanol is an important base chemical which is produced from petrochemical streams or bioprocesses. It has been used as solvent, in cosmetic and food industry, among others. However, ethanol as a (partial) replacement of gasoline has influenced its worldwide demand. Just in USA, 42x106 m3 (33x106 tons) of ethanol were added to gasoline in 2009 accounting for about 8% of gasoline consumption by volume. Water is involved in the ethanol production chain. This mixture forms an azeotrope with an ethanol mass composition of 0.956 and its challenging energy-efficient separation has been widely reported. Extractive distillation using ethylene glycol (EG) is commonly used to separate this mixture. Selectivities and activity coefficients at infinite dilution were predicted using COSMOtherm. In this case, the activity coefficients showed high attractive forces among the most promising ionic liquids and water, meaning that these ionic liquids are highly hydrophilic. The experimental relative volatility can be increased up to 23% when the conventional solvent is replaced by 1-ethyl-3-methyl-imidazolium acetate [EMIM][OAc] or 1-ethyl-3-methyl-imidazolium dicyanamide [EMIM][DCA]. These ionic liquids seem to be promising solvents for the extractive distillation of water and ethanol. Ternary VLE data were collected for the systems ethanol + water + [EMIM][OAc] and [EMIM][DCA]. In this case, the NRTL model correlates the data better than UNIQUAC, with a value for the root mean square error below 0.02. The ionic liquids are able to increase the relative volatility of the mixture ethanol – water by strongly attracting the water and making it "less volatile" which makes the recovery of the solvent rather challenging and energy intensive. Only after heat integration, the use of ILs appeared to be more attractive, yielding 16% of energy savings compared to the heat integrated conventional process. The recovery conditions and the relatively low energy savings might limit the applicability of ILs for the separation of ethanol – water by extractive distillation. Overall Finally, it can be concluded that, although ionic liquids can be suitable extractive distillation solvents, special attention should be paid to the solvent recovery technology and its heat integration with the extractive distillation column. In this study the most successful case was the separation of toluene/methylcyclohexane, where tetracyanoborate-based IL [HMIM][TCB] yielded 50% energy savings compared to the conventional solvent.