Vapor–liquid equilibrium for tetrahydrofuran + methanol + tetrafluoroborate-based ionic liquids at 101.3 kPa
Graphical abstract
Potential entrainers for the separation of THF and methanol in extractive distillation.
Introduction
In the separation process of chemical industries, there are some close-boiling components forming azeotropic mixtures which are difficult to be separated by conventional distillation techniques. Special distillation, e.g. extractive distillation or salt distillation is a valid way to solve this problem among all kinds of separation methods [1]. The effectiveness of solvent or salt is the most important factor in designing the special distillation process. Compared with traditional toxic and volatile organic solvents, ionic liquids (ILs) have many interesting properties such as their extremely low volatilities, good thermal stabilities, and in particular good performance in changing the relative volatility of the mixtures [2], [3]. It was Arlt and co-workers who first studied the use of ILs as solvents for the separation of azeotropic mixtures [4]. Since that, many thermodynamic data for IL-containing systems have been reported [5], [6], [7], [8], [9]. However, the isobaric VLE data for such systems are extremely scarce.
Tetrahydrofuran (THF) is a good solvent for a number of polymers and is therefore often used when measuring polymer solution properties [10]. THF is often mixed with methanol and other substances in the reaction process [11]. Some publications available show the VLE data of THF (1) + methanol (2) binary system [12], [13]. THF forms an azeotrope with methanol, so it is difficult to separate it from the mixtures for recycling. If the mixtures cannot be effectively separated, it will result in a waste of resources and environmental pollution. Water or glycol [14] is often used as traditional entrainer for separating the mixture of THF and methanol. However, it often makes the process complicated, long and high energy consumption. Because of all the excellent properties mentioned above, it is promising to separate azeotropic mixtures using ILs as entrainers. However, as far as we know, there is no published article reporting the VLE data of THF + methanol + ILs.
[EMIM][BF4] and [BMIM][BF4] are preferred in this work because they have been widely applied for the separation of close boiling mixtures [15], [16] and their synthetic methods have been mature. In this work, the VLE data for the system of THF (1) + methanol (2) + [EMIM][BF4] (3) and THF (1) + methanol (2) + [BMIM][BF4] (3) are measured at 101.3 kPa. The performance of the ILs in enhancement of the relative volatility of THF (1) + methanol (2) azeotrope system is related with their effects on the activity coefficients and their molecular structures. In the end, all the experimental data are correlated by the nonrandom two-liquid (NRTL) model.
Section snippets
Materials
The chemicals of THF and methanol, which are both chromatographic grade, are respectively purchased from Tianjin Jinke Fine Chemical Research Institute and Tianjin Siyou Fine Chemical Reagents Factory, China. The mass fraction of the above chemical reagents are both 99.9% and no impurities are detected by gas chromatography (GC). The ILs ([EMIM][BF4] and [BMIM][BF4]) are supplied by Shanghai Chengjie Chemical Reagents Factory and their mass fraction purities are higher than 99.0%. The water
Experimental data
In order to check the reliability of the experimental apparatus, the VLE data of THF and methanol are obtained at 101.3 kPa. The THF (1) + methanol (2) system shows that the azeotropic point is at x1 = 0.513 and the equilibrium temperature is 332.5 K at 101.3 kPa. The VLE data from this work are listed in Table 2 and compared with literature in Fig. 2. It can be seen that our experimental data are in good agreement with those values in Tong et al. [18]. Hence the apparatus is reliable for measuring
Conclusions
The VLE data of THF (1) + methanol (2) + [EMIM][BF4] (3) and THF (1) + methanol (2) + [BMIM][BF4] (3) are obtained at 101.3 kPa. The result shows that the addition of ILs to THF (1) + methanol (2) system enhance the relative volatility of THF to methanol. Especially, when the mole fraction of [EMIM][BF4] is up to 0.20, the azeotrope disappears. The azeotropic temperature is increasing when the concentrations of ILs in the liquid phase are raised. Comparing two ILs, [EMIM][BF4] has a stronger effect on the
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