Liquid-liquid separation of hex-1-ene from hexane and cyclohexene from cyclohexane with ionic liquids
Introduction
Ionic liquids (ILs) are attracting significant attention as novel alternative to conventional organic solvents for biphasic catalysis, solvent extraction, extraction from biomass, synthesis, as herbicides, as absorptive cooling and in electrochemical investigations [1], [2], [3], [4], [5]. Through last years of experimental and modelling studies it has been clearly established that ILs reveal unique type of dispersive and non-dispersive forces as well as hydrogen bonding prevailing among ILs constituents lead to a characteristic structural organization itself and in the solution. In the next two decades, the increase of use of ILs for the separation processes is expected because of their large values of selectivity in comparison with conventional organic solvents [6], [7]. Thermodynamic optimization of ILs as solvents in separation based on COSMO-RS predictions has been reported by many authors [6], [8], [9]. From these works it can be understood that ILs are powerful solvents for specific extraction problems.
The liquid-liquid separation is applied in many industrial processes for the separation of close boiling compounds. This procedure has advantages such as mild processes conditions, which includes lower temperature and pressure. The separation efficiency depends on the suitable solvent such as IL, which has to reveal a large selectivity of separation, solute distribution ratio and with little entrainer loss.
Currently, two processes are important in commercial production of the hydrocarbons, the separation of olefins and paraffins, such as hex-1-ene/hexane, or cyclohexene/cyclohexane. The olefins are important chemicals in petrochemical, polymers and petroleum industries. The olefins are also basic row substances for many intermediate and fine chemicals. Thus the separation of olefins from hydrocarbon feed stocks is important in the oil industry. The separation of olefins and paraffins is a specific distillation process, which is economically non-feasible. The disadvantage is the close boiling points of olefins and paraffins. The boiling point of hexane is 341.9 K, which is a little higher than that of hex-1-ene, 336.6 K. The removal of olefins with a commercial separation methods used for the separation of organic compounds might be much easier with the liquid–liquid extraction (LLE), or extractive distillation, or azeotropic distillation. The liquid-liquid extraction with ILs is probably during the last decade the most studied technique and ILs are new solvents tested for different separation processes [6], [7], [8], [9]. The liquid-liquid separation process is used for the separation of aromatic and aliphatic hydrocarbon mixture [10]. The commercial solvents which are used for this process are acetonitrile, sulfolane, N-methylpyrrolidyne, N-formylmorpholine, glycols or polypropylene carbons [11], [12]. More effective are ILs used as solvents in the aromatic/aliphatic separation as well as in desulfurization and denitrogenation of gasoline and diesel fuels [2], [13], [14], [15]. However, the greatest difficulty is to find an adequate extractive solvent, very selective for the extraction of for example hex-1-ene (without affecting the hexane content) and easy to be recovered after the extraction step.
Although the large number of ILs was already tested for hexane/hex-1-ene, or cyclohexene/cyclohexane separation, several challenges related to the solvent structure have to be resolved before its industrial application. The first studies of hex-1-ene/hexane separation using 38 ILs based on imidazolium-, or pyridinium-, or quinolinium-cation by using COSMO-RS model revealed lower selectivity than N-methyl-2-pyrrolidone [16]. The crucial influence on extraction process is the structure of the anion of the IL. Aromatic and protic cations, as imidazolium- and pyridinium-, are favorable selections to increase the separation capacity, while adding alkyl- or allyl-, or benzyl- substituents to the cation structure enhance the selectivity and decrease the solute distribution ratio [17], [18], [19], [20], [21], [22], [23]. It was found in the activity coefficients at infinite dilution measurements [17], [18], [19], [20], [21] and in the LLE measurements [22], [23] that ILs with small and polar CN-functionalized cations, or anions, as dicyanamide [DCA], reveal large values of selectivity. Anions, such as thiocyanate, [SCN]−, dicyanamide, [DCA]−, or tricyanomethanide, [TCM]−, {C(CN)3−}, or tetracyanoborate, [TCB]- significantly improves different separation processes [17], [18], [19], [20], [21], [22], [23], [24], [25]. Selectivity of the hex-1-ene/hexane, or cyclohexene/cyclohexane separation greater than two was found for 1-ethyl-3-methylimidazolium dicyanamide, [EMIM][DCA] [17], for 1-butyl-4-methylpyridinium dicyanamide, [B4MPy][DCA] [26] and for the 1-butyl-1-methylmorpholinium tricyanomethanide, [BMMOR][TCM] [27].
Generally, ILs possess high selectivity in extraction but the solute distribution ratio, or large viscosity of the IL may limits its application to particular separation problem.
In our previous work, we investigated the ternary liquid–liquid equilibrium phase diagrams (LLE) using the 1-ethyl-3-methylimidazolium-base ILs with three anions, [DCA]-, [TCM]- and bis{(trifluoromethyl)sulfonyl}imide, [NTf2]- [22]. From the experimental values, the extraction selectivity and the solute distribution ratio were determined and discussed for two separation processes hex-1-ene/hexane and cyclohexene/cyclohexane [22].
The purpose of this study is to investigate applicability of three new ILs in the separation of hex-1-ene/hexane and cyclohexene/cyclohexane compounds. The study consists the LLE measurements for binary and ternary mixtures of {IL (1) + hex-1-ene (2) + hexane (3)} or {IL (1) + cyclohexene (2) + cyclohexane (3)} at T = 298.15 K and p = 0.1 MPa. Taking into account the good results previously obtained with the [DCA]− and [TCM]− anions [19], [20], [21], [22], [23], [24], [25] and 1-butyl-4-cyanopyridinium cation [21], these combinations of cations and anions were selected. The ILs chosen for this work are: 1-butyl-3-methylimidazolium dicyanamide, [BMIM][DCA], 1-butyl-3-methylimidazolium tricyanomethanide, [BMIM][TCM] and 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [BCN4PY][NTf2]. Finally, the correlation of the LLE data were presented with NRTL model.
Section snippets
Materials
All solvents and ILs investigated in this work with structure, name, abbreviation of name, CAS number, sources, molar mass (M), purification method and purity analysis are presented in Table 1, Table 2, respectively. The 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [BCN4PY][NTf2] was synthesized in our laboratory. Synthesis, analysis and purity were described in our earlier work [28]. The purity of the [BCN4PY][NTf2] was determined by the chemical analysis, DSC, 1H NMR Spectra
Results and discussion
The experimental results for six ternary systems are presented in this work: {[BMIM][DCA] (1) + hex-1-ene (2) + hexane (3)}, {[BMIM][TCM] (1) + hex-1-ene (2) + hexane (3)}, {[BCN4PY][NTf2] (1) + hex-1-ene (2) + hexane (3)}, as well as {[BMIM][DCA] (1) + cyclohexene (2) + cyclohexane (3)}, {[BMIM][TCM] (1) + cyclohexene (2) + cyclohexane (3)}, {[BCN4PY][NTf2] (1) + cyclohexene (2) + cyclohexane (3)}, at T = 298.15 K and ambient pressure. The tie-line results are reported in Table 5, Table 6 and are plotted in a form of
Data correlation
The experimental tie-line data in ternary LLE systems were correlated using the non-random liquid equation model, (NRTL) [30]. The equations and algorithms used for the calculation of the compositions in both phases followed the method described by Walas [31]. The objective function F(P), was used to minimize the difference between the experimental and calculated compositions:where P is the set of
Conclusions
The ternary liquid–liquid equilibrium experiments of hex-1-ene/hexane and cyclohexene/cyclohexane are presented. Six ternary systems {IL + hex-1-ene + hexane}, or {IL + cyclohexene + cyclohexane} were analytically determined using GC for the composition analysis at temperature T = 298.15 K at ambient pressure. Liquid-liquid equilibrium measurements demonstrated that new ILs [BMIM][DCA], [BMIM][TCM] and [BCN4PY][NTf2] offer good performance as separation agents of hex-1-ene/hexane and
Acknowledgements
This work has been supported by the Ministry of Science and Higher Education in Poland in years 2015–2017 (Project “Iuventus Plus” No. IP2014051373).
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