Separation of methanol + dimethyl carbonate azeotropic mixture using ionic liquids as entrainers
Introduction
Dimethyl carbonate (DMC) has been a high-profile agent in recent years because of its excellent properties and extensive applications [1], [2], [3], [4], [5], [6], [7]. For example, DMC has favorable solubility and nontoxicity which make it an environmentally friendly solvent. Due to its high oxygen content, DMC could be used as gasoline additives to improve octane number. As a basic organic raw material, methanol has a very wide range of applications [8], such as medicine, solvent, and organic synthesis. In the synthesis process of DMC, methanol is one of the raw materials. Due to the use of excessive methanol, DMC and methanol could form azeotrope which is difficult to be separated.
For separating azeotropic systems, extractive distillation [9], [10], [11] is an efficient method and the selection of entrainer is the key to it. Inorganic salts [12], [13], [14] and organic solvents [15], [16] are traditionally used as entrainers but some bad influence could be caused. For example, the use of inorganic salts could lead to pipeline corrosion and organic solvents may cause environmental pollution and water pollution. Ionic liquids (ILs) as new entrainers have become more and more popular in recent years because of their unique properties, such as negligible vapor pressure, good chemical and thermal stability and tunable structures [17], [18]. Now, plenty of ILs have been employed as entrainers to separate azeotropes and their performances reported in literature are uplifting. Zhang et al. [19] investigated the ternary systems of methyl acetate and methanol containing 1-butyl-3-methylimidazolium chloride ([C4MIM][Cl]), or 1-(2-chloroethyl)-3-methylimidazolium chloride ([ClC2MIM][Cl]), or 1-butyl-3-methylimidazolium bromide ([C4MIM][Br]). Li et al. [20–21] measured the VLE data based on methanol and DMC containing 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM][BF4]), or 1-ethyl-3-methylimidazolium trifluor ([EMIM][OTf]), or 1-buthyl-3-methylimidazolium trifluor ([BMIM][OTf]). Cai and Chen et al. [22–23] compared the separation effect of different phosphoric-based ionic liquids on the mixture of methanol and DMC. Ales Blahut et al. [24] studied the effect of 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]) on the azeotropic system of methanol + DMC. The VLE data for the system DMC + methanol with different concentration of 1-buthyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-ethyl-3-methylimidazolium ethyl sulfate (EMISE) were measured by Hyeon-Deok Kim et al. [25]. [BMIM][Cl] has been tried to separate the azeotropic mixture of methanol and DMC, but only a speculative conclusion that [BMIM][Cl] has remarkable azeotrope breaking capacity was made and no details was given [26].
In this work, two ILs, namely [EMIM][Br] and [BMIM][Cl], were used as entrainers to separate methanol and DMC mixture at 101.3 kPa. The separation effects of these two ionic liquids were compared and the reason for their different performance was analyzed.
Section snippets
Chemicals
In this experiment, the solvents were methanol, DMC, [EMIM][Br], and [BMIM][Cl]. Methanol and DMC were purchased from Sinopharm group and examined by gas chromatography. The results proved that their mass fraction was higher than 99.5%. [EMIM][Br] and [BMIM][Cl] were obtained from our own laboratory. Their purities were greater than 99% (mass fraction) which were authenticated by liquid chromatography (Wayee IC6000). The water content checked by Karl Fischer was lower than 0.005 (mass
Experimental data
The VLE data of the binary system of methanol + DMC were measured to check the reliability of our apparatus. Detailed data are shown in Table 2, where T is equilibrium temperature, x1 is the mole fraction of methanol in the liquid phase and y1 is the mole fraction of methanol in the vapor phase. It is obvious in Fig. 1 that the experimental data agree well with the data calculated by the NRTL model and the data in literature [20], [23]. Therefore, the apparatus in this work is reliable.
The
Conclusions
In this work, the isobaric VLE data for the ternary systems of methanol (1) + DMC (2) + [EMIM][Br] (3) and methanol (1) + DMC (2) + [BMIM][Cl] (3) were measured at 101.3 kPa. The experimental data were well correlated using the NRTL model. Both ILs could enhance the relative volatility of DMC to methanol and break the azeotrope. But the separation effect of [BMIM][Cl] was stronger than that of [EMIM][Br]. When mole fraction of [BMIM][Cl] calculated by NRTL model was about 0.1168, the azeotropic
Acknowledgement
This work is financially supported by National Science Foundation of China (Project No. 21076126), National Science Foundation of China (Project No. 21576166), and Liaoning Province Science Foundation of China (Project No. 2014020140).
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