Vapor–liquid equilibria for tetrahydrofuran + ethanol system containing three different ionic liquids at 101.3 kPa
Graphical abstract
Introduction
Ionic liquids(ILs) are substances consisting entirely of ions. They are non-volatile, non-flammable, thermally and chemically stable. Except for their remarkable physicochemical excellent properties, a lot of possible combinations of cation and anion, have made ILs satisfied for different chemical processes. Because of their almost undetectable vapor pressure, ILs have been considered as green solvents compared with traditional organic solvents [1], [2], [3], [4]. Since Arlt and co-workers firstly used ILs as entrainers for separating close-boiling homo and heteroazeotropic mixtures [5], ILs as promising separating agents have raised a recent upsurge [6]. The non-volatility of ILs in combination with their remarkable separation efficiency and selectivity makes them as potential entrainers to separate azeotropic mixtures.
Tetrahydrofuran (THF) is an excellent organic solvent, and often mixed with ethanol in the waste of pharmaceutical industry (the production of betamethasonum as an example) [7], [8]. The system of THF (1) and ethanol (2) forms a minimum boiling point azeotrope at atmospheric pressure at x1 ≈ 0.90, T ≈ 338.6 K [9], which leads to the unfeasibility of separating the THF + ethanol mixture by common distillation. Wang Shouyu reported the salting-out effect of lithium chloride on binary THF + ethanol system, and concluded that azeotropic point could be eliminated when the mole fraction of lithium chloride saturated in the liquid mixture [10]. Liu Weiming et al. proved that ethylene-glycol was an effective entrainer for separating the THF + ethanol mixture, and reported that when the solvent ratio was 3, the relative volatility reached 2.26 [8].
In this work, as a continuation of the work recently published for the vapor–liquid equilibrium (VLE) of THF + methanol + tetrafluoroborate based ionic liquids [11], we measure the VLE data of three ternary systems: THF + ethanol + 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), THF + ethanol + 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM][BF4]), and THF + ethanol + 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) at 101.3 kPa with a modified Othmer still, and discuss the extractive capacities of these three ILs to separate the THF + ethanol mixture. In view of the formation of small amounts of HF, ILs containing [BF4]− are not stable in contact with water in long-term application, especially at high temperatures [12], [13]. Caution should be given in the use of ILs containing [BF4]−. However, mature synthetic methods and comprehensive of understanding of ILs containing [BF4]− lead to extensive research about them as effective entrainers for breaking azeotropies [14], [15], [16], [17], [18]. Besides, Arlt et al. said that the disposal of a certain amount of halogen compounds was acceptable for industrial application of ILs containing [BF4]− [14]. At last, the VLE experimental data are correlated using the NRTL model proposed by Renon and Prausnitzis [19].
Section snippets
Materials
The chemical reagents used are THF and ethanol. The specifications of THF and ethanol are listed in Table 1. These chemicals are directly used without further purification. The ILs used are [BMIM][BF4], [OMIM][BF4] and [BMIM][DCA]. All of the ILs are dried by a rotary evaporator (400 K, 48 h) under a vacuum condition in order to eliminate volatile components before experiment. The water contents of ILs are tested by Karl-Fischer titration. The halogen contents in the studied ionic liquids were
Experimental data
The binary VLE data of THF and ethanol listed in Table 2 are obtained at 101.3 kPa. The binary THF (1) + ethanol (2) system shows an azeotropic point at x1 = 0.933 and T = 338.9 K at 101.3 kPa, which is interpolated from the binary experimental data. The binary VLE data from this work agree well with the data from Yoshikawa et al. [20], as shown in Fig. 1.
The VLE data of three ternary systems: THF + ethanol + [BMIM][BF4], THF + ethanol + [OMIM][BF4], and THF + ethanol + [BMIM][DCA] are measured at 101.3 kPa by
Conclusions
The VLE data are measured at 101.3 kPa with a modified Othmer still for three ternary systems: THF+ ethanol + [BMIM][BF4], THF + ethanol + [OMIM][BF4], and THF + ethanol + [BMIM][DCA]. The three ionic liquids all enhance the relative volatility of THF to ethanol. The NRTL model is used for correlation. The standard deviations for equilibrium temperatures are 0.57 K, 0.47 K, and 0.50 K, while the values for vapor phase mole fractions are 0.0045, 0.0059 and 0.0047 for [BMIM][DCA], [BMIM][BF4] and [OMIM][BF4],
List of symbols
- F
objective function for fitting the binary interaction parameters
- Δgij
binary energy parameters of the NRTL model
- xi
mole fraction of solvent i in the liquid phase containing ILs
- xi’
mole fraction of solvent i in the liquid phase expressed on an IL-free basis
- yi
mole fraction of solvent i in the vapor phase
- T
equilibrium temperature
- N
number of experimental data points
- P
total pressure in the equilibrium system
- Pi5
vapor pressure of pure component i at equilibrium temperature
- Texptl
equilibrium temperature
References (35)
- et al.
J. Chromatogr. A
(2010) - et al.
J. Chem. Thermodyn.
(2012) - et al.
Fluid Phase Equilib.
(2013) - et al.
Fluid Phase Equilib.
(2007) - et al.
J. Chem. Thermodyn.
(2008) - et al.
Fluid Phase Equilib.
(2012) - et al.
Fluid Phase Equilib.
(2011) - et al.
J. Chem. Thermodyn.
(2010) - et al.
J. Chem. Thermodyn.
(2009) - et al.
Fluid Phase Equilib.
(2009)