The key parameters for selecting suitable ILs are theavailability of VLE data and selectivity.
Research studies on the extractivedistillation of ethanol and water have involved a pool of various ILs,particularly imidazolium-based ones 8 28. However, neither theircomparison of performances in vapour-liquid laboratory experiments, processsimulations, or pilot scale experiments produces a conclusive choice of IL as thebest entrainer. Different papers are contradictory of one another in theselection of suitable IL, as they are judged using different solvent criteria. Usingthe available comparisons and results from literature, a fair choice of IL ismade based on the overall analysis, with the following justifications.The first step in the selection is to ascertain theavailability of VLE data of the ILs for ternary system involving ethanol andwater. The respective VLE data of ILs that is available in literature iscompiled by Figueroa et al.
29.Among all the possible choices of ILs for ethanol dehydration, the most frequentlydiscussed cations for are Emim+ and Bmim+, whereas theanions are BF4-, Cl-, OAc-. The second step for IL screening is to compare theselectivities at infinite dilution between the ILs, which is reported byseveral papers 8 27 17.ILs with higher selectivity will lead to a greater difference in volatilitybetween the components in the feed, therefore increasing the separation.
With agreater separation, the operation would require a lower entrainer mass andnumber of column stages, which reduces the operating and capital costs 14.As inferred by Ge et al. 17, the selectivitydepends on both the cation and anion of the IL. The larger the cation size, thelower the difference in relative volatility. By comparing the relativevolatility between ethanol and water using Emim+ and Bmim+,Ge et al.
found that the enhancement effect of Emim+ was larger 17. This was furthersupported by a more recent study by Pereiro et al. 11, which investigatedthree cations, namely (in ascending order of chain length) Emim+,Bmim+ and Hmim+, with Cl- as cation. Thesame result was found and attached in Figure3,where the relative volatility decreased with size of cation at mole fractions ofIL above 0.03.
Furthermore, when the molar fraction of IL increased, there wasa greater enhancement in relative volatility. Figure 3: Relative volatility of chlorinated ILs with different cations at different IL molar fractions 11. Figure 4: Relative volatility of ILs 11. As for anions, they have a greater impact on the relativevolatility than cations 1728.
The relative volatility between ethanol and water was plotted against thedifferent ILs studied by Pereiro et al. 11in Figure4.EmimCl had the highest selectivity among the ILs listed in the figure, followedby EmimOAc. This was also supported by other VLE study, by Ge et al. 17. Both papers reportedresults of the relative volatility enhancement effect by the ILs to be in thisdescending order: Cl-, OAc-, BF4, pairedwith cations Emim+ or Bmim+.
However, the viscositiesof chlorinated ILs are high, and would have the mass transfer resistance,leading to the poor mass and heat transfer performance in separation units.This would also incur a high operating cost as more energy would be spent to overcomethese resistances in agitation and other operations 8. In addition tothat, chlorinated ILs are also corrosive 28.Therefore, they are not chosen. The IL with the second highest selectivity is EmimOAc,and is claimed to be a promising entrainer due to its lower viscosity comparedto EmimCl 17.However, several other papers have disputed its use because of its thermalinstability 8 12.
It is has a lowflash point of 164°C and is therefore infeasible to berecycled under high temperature conditions. Meindersma et al. 27 explained that thestrong attachment of the acetate IL to water would cause the separation to bedifficult during solvent recovery. The flash drum would need to operate at highvacuum conditions (0.1 mPa in Meindersma et al.
‘s study) in order for therecovery to be feasible 27.This would incur a high energy demand, which leads to costly operation.Arlt et al. was the pioneer for the research of ILs in ethanoldehydration.
Their patent for BASF was published in 2004, concluding that it wasfeasible to break the azeotrope using EmimBF4 as the entrainer. Sincethen, EmimBF4 has become a popular choice to be used in multiplestudies for the process design of the ethanol dehydration and comparison withother solvents 8 13.This is because EmimBF4 has favourable characteristics overother ILs such as a high thermal stability (up to 450°C), high selectivity and is commercially available 24.
Hence, EmimBF4is selected as the IL to be studied in this dissertation.1.1.1.
Process designand optimisationThis section of the literature review focuses on twoaspects, the feasibility of EmimBF4 in ethanol dehydration andthe process economics. The IL is evaluated to determine the achieved purityunder set operating parameters, and its feasibility in meeting the baselinepurity in industry. As for the economics, the differences in the capital costsand the utilities cost are analysed, with comparison to conventional solvents.EmimBF4 was used in the research by Seileret al. to carry out process simulation and optimisation of the extractivedistillation of ethanol and water, with comparison to 1,2-ethanediol 13. The extractivecolumn was operated at Patm whereas the solvent recovery unitconsisted of a flash vessel operating at 10 kPa, and was connected to astripping column operating at Patm.
The results showed that theoverall heat consumption for IL was about 25% lower than 1,2-ethandiol at thesame purity of ethanol (99.8 mol%). The capital cost was not taken into accountin the economic analysis. Nevertheless, this 13-year-old study has presentedthe feasible design of the IL in the extractive distillation process and itsenergy advantages. It provided a good base for future work such as exploringmore aspects of economic analysis and more optimisation parameters.
A more recent study was done by Meindersma et al. in 2016 27, which compared theenergy consumption by different types of configuration, where the solventrecovery unit consisted of either flash drum only or flash drum and strippingcolumn. The configuration with only flash drum required about 4% lower in energythan flash drum+stripping column. As the comparison was done under the same setof operating conditions, this configuration with flash drum appears to be wellvalidated to be the superior choice. The process configuration was modified andin accordance with other literature, such as Zhu et al.
8. Zhu et al. focused on the process design and optimisationof the ethanol dehydration using different solvents, including EmimBF4.The general process design consists of two units, the extractive distillationcolumn and the flash drum for solvent recovery.
The IL is added to theextractive distillation column along with the ethanol and water feed. The topproduct obtained would be the anhydrous ethanol whereas the bottom product wouldcontain the mixture of IL and water. The bottom product is subsequently fedinto the solvent recovery unit. As ILs have negligible vapour pressure and donot vaporise, a conventional distillation column cannot be used to recover IL.
Instead,it is recovered using a flash vessel instead under vacuum conditions (P