Effect of Ionic Liquids on the Isobaric Vapor–Liquid Equilibrium

Apr 9, 2013 - The influence of the variation of the cation's alkyl chain length of imidazolium tetrafluoroborate-based ILs on VLE of the azeotropic sy...
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Effect of Ionic Liquids on the Isobaric Vapor−Liquid Equilibrium Behavior of Methanol−Methyl Ethyl Ketone Qunsheng Li,† Xueting Sun,† Ling Cao,† Baohua Wang,*,‡ Zhaowen Chen,† and Yuxin Zhang† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 35, Beijing, 100029, China College of Chinese Pharmacology, Beijing University of Chinese Medicine and Pharmacology, Beijing, 100029, China

‡

ABSTRACT: Isobaric vapor−liquid equilibrium (VLE) data for methanol−methyl ethyl ketone (MEK) systems containing ionic liquids (ILs) have been measured with a modified Othmer still at atmospheric pressure (101.32 kPa), and they were correlated by the NRTL equation. Three ILs, composed of an anion tetrafluoroborate ([BF4]−) and a cation from 1-ethyl-3-methylimidazolium ([EMIM]+), over 1-butyl-3-methylimidazolium ([BMIM]+), to 1-octyl-3-methylimidazolium ([OMIM]+), were investigated, and they all gave rise to a change of the relative volatility of methanol to MEK. The results indicated that, among the three ILs studied, [BMIM]+[BF4]− and [OMIM]+[BF4]− eliminated the azeotropic point at mole fraction 30 % and 10 %, respectively, whereas IL [EMIM]+[BF4]− pulled down the azeotropic point. The influence of the variation of the cation’s alkyl chain length of imidazolium tetrafluoroborate-based ILs on VLE of the azeotropic system methanol−MEK was discussed.

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appears to be feasible. Shen et al.15 discussed the effect of mono-, di- and triethanolammonium tetrafluoroborate protonic ionic liquids on VLE of ethanol + water system, and screened out appropriate entrainers for extractive distillation. Orchillés et al.16 and Cai et al.17,18 reported the isobaric VLE for methanol + methyl acetate containing different ILs, and the results showed the ILs 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]+[triflate]−), 1-ethyl-3-methylimidazolium acetate ([EMIM]+[Ac]−), and 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIM]+[PF6]−) all can break the azeotropic point of methanol + methyl acetate. In addition, Zhou et al.30 has confirmed the separation capability of ILs in the extraction of benzene from cyclohexane, which have extremely close boiling points (ΔT = 0.6 K at 101.32 kPa). Privott et al.2 have reported the vapor−liquid equilibrium (VLE) data of methanol−MEK in the presence of diethyl ketone, and the results indicated diethyl ketone could eliminate the azeotropic point and gave the VLE data of methanol−MEK on solvent-free basis in the presence of 70 % mole fraction of diethyl ketone. However, to the best of our knowledge, there seems to be no VLE data on the ternary system of methanol− MEK containing ionic liquids, thus making further studies on phase behaviors of IL-containing systems of methanol−MEK is still necessary. In this work, the isobaric VLE data for the ternary system of methanol + MEK + ILs were measured at atmospheric pressure (101.32 kPa), and the influence of systematic cation variation of imidazolium tetrafluoroboratebased ILs on the phase behavior of methanol−MEK, including

INTRODUCTION Methanol and methyl ethyl ketone (MEK) are byproducts when pyroligneous acid is ordinarily obtained by the destructive distillation of wood. They constitute important commercial commodities from the wood distilling industry. However, methanol and MEK form a positive azeotrope at atmospheric pressure so that complete recovery of the individual compounds is not obtained by traditional distillation.1,2 Up to now, extractive distillation is the most widely utilized technology in chemical engineering to separate azeotropes and other close-boiling mixtures.3−5 Recently, ionic liquids (ILs) have attracted increasing attention for their potential application in extractive distillation because of their good thermal stabilities and their extremely low vapor pressure.6−11 Generally, ILs refer to a class of chemicals composed entirely of ions with a negligibly low vapor pressure. In contrast to conventional salts, they have very low melting points (mainly below 373 K) and a liquidus range of 300 K, and they are outstanding solvents for organic, inorganic, and polymeric materials. Typically, an organic greatly asymmetric substituted cation (pyridinium, imidazolium, pyrrolidinium, tetraalkylphosphonium, quaternary ammonium, etc.) and an inorganic anion (trifluoromethanesulfonate, tetrafluoroborate, hexafluorophosphate, acetate, halide, nitrate, etc.) form the ILs, indicating the ILs’ chemical, such as polar character, and physical properties can be changed at will according to their components.12−14 Thus, the studies carried out for a battery of ILs on the VLE of the mixtures with an azeotropic point or close boiling point are significant for the further investigation. Recently, Jork et al.13 reported the effects of systematic cation and anion variation of imidazolium-based ILs on the phase behavior of the two azeotropic systems, including tetrahydrofuran (THF) + water and ethanol + water, and indicated that selecting a suitable extraction agent by tailoring the IL for extractive distillation © 2013 American Chemical Society

Received: October 8, 2012 Accepted: April 1, 2013 Published: April 9, 2013 1133

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Table 1. Specifications of Chemical Samples chemical name methanol MEK [EMIM]+[BF4]−a [BMIM]+[BF4]−b [OMIM]+[BF4]−c

source Tianjin Shield Fine Chemicals Co. Ltd. (China) Beijing Chemical Reagents Company (China) Shanghai Cheng Jie Chemical Co. LTD (China) Shanghai Cheng Jie Chemical Co. LTD (China) Shanghai Cheng Jie Chemical Co. LTD (China)

mass fraction purity

purification method

final water mass fraction

analysis method

0.999

none

0.00047

GCd, KFf

0.996 0.990

none rotary evaporation under vacuum

0.00058 0.00055

GCd, KFf LCe, KFf

0.990

rotary evaporation under vacuum

0.00050

LCe, KFf

0.990

rotary evaporation under vacuum

0.00048

LCe, KFf

[EMIM]+[BF4]− = 1-ethyl-3-methylimidazolium tetrafluoroborate. b[BMIM]+[BF4]− = 1-butyl-3-methylimidazolium tetrafluoroborate. [OMIM]+[BF4]− = 1-octyl-3-methylimidazolium tetrafluoroborate. dGC = gas-chromatography. eLC = liquid-chromatography. fKF = Karl Fischer titration. a c

Table 2. Experimental (Vapor + Liquid) Equilibrium Data for Temperature T, Liquid-Phase Mol Fraction x, and Gas-Phase Mol Fraction y, for the System Methanol (1) + MEK (2) at 101.32 kPaa

Figure 1. Absolute deviations Δy1= y(exptl) − y(calcd) between the calculated and measured mole fractions of methanol in the vapor phase for the binary system of methanol (1) + MEK (2) at 101.32 kPa: ■, this work with error bars representing the extended uncertainty; ○, ref 2; Δ, ref 19.

whether they could break the azeotropic point and their separation efficiency in extractive distillation were investigated.

T/K

x1

y1

348.2 345.9 344.5 340.9 340.5 339.3 338.7 338.3 338.1 337.8 337.7 337.5 337.4 337.5 337.6 337.8 337.9 338.2

0.091 0.145 0.191 0.344 0.390 0.493 0.548 0.594 0.624 0.654 0.701 0.754 0.802 0.902 0.902 0.950 0.976 1.000

0.240 0.321 0.373 0.520 0.555 0.628 0.656 0.690 0.702 0.725 0.746 0.777 0.811 0.888 0.889 0.939 0.968 1.000

a

Standard uncertainties u are u(T) = 0.1 K, u(P) = 0.10 kPa, and the combined standard uncertainties uc are uc(x1) = uc(y1) = 0.002

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EXPERIMENTAL SECTION Chemicals. The ILs used in this work were [EMIM]+[BF4]−, [BMIM]+[BF4]−, and [OMIM]+[BF4]− provided by Shanghai Cheng Jie Chemical Co. LTD (China). The mass fraction purity of all purchased and treated ILs (mass fraction > 99 %), were checked by liquid chromatography and characterized by NMR spectroscopy. The ILs, before used in the experiments, were subjected to a vacuum by rotary evaporation at (383 to 411.15) K over 48 h, in order to remove the volatile components from ILs. The amount of water in all used ionic liquids is less than 550 ppm, determined by Karl Fisher titration. Methanol (mass fraction > 99.9 %, analytical grade) was purchased from Tianjin Shield Fine Chemicals Co. Ltd. (China). MEK (mass fraction > 99.6 %, analytical grade) was obtained from Beijing Chemical Reagents Company (China). Purities of methanol and MEK were checked by GC (SP 7800, China), and no impurities were detected, and were used without further purification. The specifications of all chemicals used are summarized in Table 1. Apparatus and Procedure. All vapor−liquid equilibrium measurements were carried out using gas chromatography (GC). The GC (SP 7800, China) was equipped with a packed

Porapak-Q column, which is 3 m long and 3 mm in external diameter, and a thermal conductivity detector (TCD). The operating conditions of GC were as follows: the injector temperature at 455.15 K, the oven temperature at 435.15 K, and the detector temperature at 475.15 K. The carrier gas was hydrogen with a flow rate of 40 cm3·min−1. A calibration correction factor line used to quantify the amounts of methanol and MEK in the samples as obtained from a series of standard solutions which were gravimetrically prepared by an electronic balance (Sartorius) with a standard uncertainty of 0.1 mg. Using this analytical method, the combined standard uncertainty of the mole fraction of the components methanol and MEK in the liquid and vapor phases was 0.002. The IL mole fraction content in the liquid phase was determined with gravimetric method by measuring the mass difference of prepared samples with and without IL; furthermore, the combined standard uncertainty of the ILs is 0.001 in mole fraction. The VLE data were measured via a modified Othmer still at 101.32 kPa. The detailed description of the apparatus is available in our previous publications.6,7 A calibrated mercury thermometer was used to measure the vapor−liquid equilibrium 1134

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Table 3. Experimental (Vapor + Liquid) Equilibrium Data for Temperature T, Liquid-Phase Mol Fraction of IL x3, Liquid-Phase Mol Fraction of Methanol x1 (Based on IL-Free), Gas-Phase Mole Fraction y, Activity Coefficient γ, and Relative Volatility α12, for Ternary Systems Methanol (1) + MEK (2) + [EMIM]+[BF4]− (3)/[BMIM]+[BF4]− (3)/[OMIM]+[BF4]− (3) at 101.32 kPaa 100x3

a

T/K

10.619 10.087 11.401 11.003 11.877 11.303 12.001 11.829 11.767 10.633 20.911 20.709 21.007 19.098 20.106 20.010 20.337 20.676 20.091 19.594 31.682 29.155 30.106 30.655 29.854 30.632 29.276 28.599 30.887 31.089

349.7 346.7 345.1 343.5 342.1 340.7 339.9 339.5 339.5 339.4 350.1 348.6 347.0 345.4 344.1 343.0 341.8 341.2 341.1 341.3 352.3 350.5 348.5 347.5 346.6 346.2 345.5 344.8 344.6 344.7

9.504 11.257 10.644 10.065 9.854 9.905 10.184 9.653 9.972 10.258 20.654 20.132 20.283 20.441 19.981 19.954

348.0 345.4 343.6 342.4 341.9 341.2 340.2 339.9 340.0 340.2 350.7 348.5 346.9 345.5 345.0 344.5

x1

y1

[EMIM]+[BF4]− 0.089 0.193 0.181 0.350 0.300 0.469 0.388 0.539 0.496 0.606 0.589 0.670 0.703 0.741 0.805 0.811 0.905 0.897 0.954 0.947 0.080 0.148 0.162 0.283 0.292 0.427 0.379 0.508 0.494 0.583 0.588 0.654 0.700 0.723 0.803 0.804 0.903 0.894 0.953 0.944 0.085 0.139 0.152 0.241 0.275 0.381 0.374 0.454 0.483 0.537 0.502 0.540 0.591 0.606 0.694 0.694 0.818 0.781 0.922 0.857 [BMIM]+[BF4]− 0.132 0.295 0.279 0.449 0.389 0.540 0.491 0.611 0.539 0.642 0.592 0.680 0.791 0.804 0.814 0.821 0.900 0.898 0.953 0.948 0.170 0.294 0.275 0.419 0.383 0.510 0.504 0.608 0.542 0.626 0.603 0.674

γ1

γ2

α12

1.540 1.522 1.330 1.247 1.170 1.140 1.099 1.066 1.049 1.038 1.464 1.451 1.294 1.232 1.153 1.134 1.104 1.100 1.087 1.072 1.383 1.379 1.315 1.205 1.127 1.120 1.075 1.066 1.059 1.030

1.091 1.075 1.099 1.148 1.260 1.352 1.522 1.714 1.914 2.000 1.274 1.233 1.235 1.248 1.368 1.449 1.667 1.837 2.020 2.142 1.398 1.358 1.401 1.492 1.558 1.648 1.722 1.821 2.274 3.453

2.452 2.431 2.067 1.842 1.567 1.415 1.208 1.038 0.915 0.866 2.001 2.036 1.802 1.687 1.433 1.324 1.116 1.006 0.904 0.841 1.735 1.770 1.624 1.391 1.242 1.165 1.067 0.998 0.794 0.508

1.673 1.349 1.237 1.153 1.124 1.112 1.027 1.023 1.012 1.007 1.331 1.264 1.176 1.126 1.091 1.075

1.048 1.094 1.138 1.196 1.236 1.273 1.580 1.640 1.737 1.853 1.144 1.150 1.206 1.260 1.317 1.347

2.755 2.107 1.844 1.629 1.533 1.469 1.088 1.043 0.975 0.910 2.029 1.901 1.676 1.527 1.414 1.359

100x3

T/K

19.304 19.785 19.710 19.060 30.913 30.237 30.656 30.347 30.152 30.490 30.298 29.469 30.101 29.602

343.8 343.3 342.9 342.7 355.6 354.0 352.2 350.7 349.7 348.8 348.2 347.6 347.3 347.0

10.475 10.311 11.128 9.870 10.353 10.222 9.765 11.049 10.383 9.875 9.760 20.901 20.173 19.874 19.321 20.713 20.679 19.873 20.665 19.776 19.838 30.494 31.955 29.763 30.487 29.886 30.263 30.223 28.728 29.427 29.652

350.0 347.1 347.2 345.0 343.5 342.3 340.8 340.6 340.4 340.3 340.3 355.4 352.8 350.8 348.8 347.5 346.6 345.8 344.8 344.3 344.1 360.9 358.5 356.6 355.5 354.4 353.4 352.6 350.3 348.7 347.5

x1

y1

γ1

[BMIM]+[BF4]− 0.707 0.748 1.037 0.804 0.819 1.024 0.903 0.903 1.018 0.956 0.954 1.016 0.068 0.135 1.474 0.142 0.233 1.280 0.254 0.367 1.209 0.347 0.446 1.132 0.434 0.523 1.096 0.577 0.630 1.029 0.688 0.719 1.006 0.787 0.808 0.999 0.891 0.897 0.997 0.949 0.950 0.995 [OMIM]+[BF4]− 0.131 0.287 1.533 0.201 0.380 1.472 0.225 0.397 1.378 0.336 0.505 1.259 0.427 0.572 1.195 0.521 0.653 1.169 0.650 0.729 1.102 0.753 0.795 1.059 0.855 0.872 1.024 0.906 0.912 1.010 0.953 0.953 1.003 0.090 0.176 1.277 0.160 0.299 1.325 0.230 0.383 1.269 0.379 0.517 1.109 0.500 0.608 1.054 0.601 0.701 1.048 0.697 0.772 1.014 0.803 0.844 1.006 0.900 0.914 0.982 0.952 0.959 0.982 0.141 0.235 1.029 0.309 0.431 0.951 0.401 0.517 0.911 0.444 0.550 0.918 0.504 0.612 0.929 0.548 0.649 0.944 0.593 0.700 0.970 0.682 0.768 0.980 0.799 0.847 0.989 0.896 0.921 1.007

γ2

α12

1.432 1.575 1.725 1.796 1.228 1.232 1.242 1.302 1.332 1.430 1.499 1.511 1.616 1.689

1.230 1.102 0.999 0.956 2.133 1.836 1.708 1.516 1.429 1.247 1.159 1.139 1.062 1.013

1.000 1.041 1.049 1.068 1.130 1.143 1.280 1.396 1.489 1.573 1.656 1.053 1.044 1.063 1.096 1.172 1.156 1.177 1.300 1.409 1.414 0.993 1.010 1.016 1.066 1.059 1.090 1.060 1.109 1.227 1.284

2.665 2.433 2.261 2.012 1.794 1.727 1.445 1.272 1.152 1.075 1.015 2.152 2.230 2.084 1.752 1.549 1.557 1.475 1.320 1.186 1.181 1.878 1.692 1.599 1.531 1.552 1.526 1.606 1.540 1.395 1.351

Standard uncertainties u are u(T) = 0.1 K, u(P) = 0.10 kPa, and the combined standard uncertainties uc are uc(x1) = uc(y1) = 0.002, uc(x3) = 0.001

with the literature data2,19 to test the performance of the equilibrium apparatus, as shown in Figure 1, and the VLE data are listed in Table 1. It can be seen from the Figure 1 that the maximum absolute deviation (Δy1) between the calculated and measured mole fraction of methanol in the vapor phase (y1) was 0.013 in mole fraction, and the average absolute deviation was 0.005. The measured VLE data were correlated by using the NRTL model, and they were in good agreement with those reported.2,19

temperature which has a standard uncertainty of 0.1 K. In the laboratory, the equilibrium pressure of the whole system was kept constant using an on−off pressure controller with a standard uncertainty of 0.10 kPa.

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RESULTS AND DISCUSSION

The binary vapor−liquid equilibria for the systems of methanol−MEK were measured at 101.32 kPa and compared 1135

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Table 4. Estimated Values of Binary Interaction Parameters Δgij and Δgji, Nonrandomness Factors αij in the NRTL Model, and the Corresponding Average Relative Deviation ARD i component

j component

αij

Δgij/J·mol−1

Δgji/J·mol−1

ARD/%

methanol (1) methanol (1) MEK (2) methanol (1) MEK (2) methanol (1) MEK (2)

MEK (2) [EMIM]+[BF4]− (3) [EMIM]+[BF4]− (3) [BMIM]+[BF4]− (3) [BMIM]+[BF4]− (3) [OMIM]+[BF4]− (3) [OMIM]+[BF4]− (3)

0.470 0.445 0.200 0.080 0.350 0.200 0.450

1868.82 5800.28 1186.38 9324.40 8256.89 −3849.81 375.21

429.80 −72.27 11957.09 −7952.55 −3417.44 512.27 −2445.46

1.53 3.00 2.26 2.84

Figure 3. Vapor−liquid equilibrium (VLE) curves of methanol (1) + MEK (2) + [BMIM]+[BF4]− (3) system at 101.32 kPa: ■, x3 = 0; ○, x3 = 0.10; Δ, x3 = 0.20; ◊, x3 = 0.30; solid lines, correlated using the NRTL model.

Figure 2. Vapor−liquid equilibrium (VLE) curves of methanol (1) + MEK (2) + [EMIM]+[BF4]− (3) system at 101.32 kPa: ■, x3 = 0; ○, x3 = 0.10; Δ, x3 = 0.20; ◊, x3 = 0.30; solid lines, correlated using the NRTL model.

Thus the experimental apparatus was reliable to investigate the effect of the ILs on the VLE of the methanol (1) + MEK (2) system at atmospheric pressure (101.32 kPa). Measurements were made for the ternary system of methanol (1) + MEK (2) + ILs (3) by keeping the ILs mole fraction constant in each set of experiments. Results of isobaric VLE data at different ILs mole fraction contents (x3 = 10 %, 20 %, and 30 %) are listed in Table 2 in which γ1 and γ2 were calculated by the eq 1 y φiP γi = i s s xi′φi Pi (1) α12 =

y1 /x1 y2 /x 2

(2)

where yi is the mole fraction of component i in the vapor phase, xi ̀ represents the mole fraction of component i in the liquid phase (including IL), φi is the fugacity coefficient of component i in the vapor mixture, and φis is the fugacity coefficient of pure component i in its saturated state. P the total pressure of the equilibrium system, 101.32 kPa; Pis is the vapor pressure of pure component i at the system temperature, which could be calculated by the Antoine equation using the Antoine constants from the literature.20 The fugacity coefficients φi and φis were calculated by R−K equation of state,21 and they were close to unity by means of this method, so they could be ignored for these systems. Hence, data reduction was performed neglecting vapor-phase corrections.

Figure 4. Vapor−liquid equilibrium (VLE) curves of methanol (1) + MEK (2) + [OMIM]+[BF4]− (3) system at 101.32 kPa: ■, x3 = 0; ○, x3 = 0.10; Δ, x3 = 0.20; ◊, x3 = 0.30; solid lines, correlated using the NRTL model.

The relative volatility of methanol to MEK was obtained in terms of the eq 2, where x1 and x2 are mole fractions of methanol and MEK, respectively, on an IL-free basis. 1136

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Figure 5. Relative volatility (α12) of methanol (1) to MEK (2) at different mole fraction of [EMIM]+[BF4]− at 101.32 kPa: ■, x3 = 0 (IL-free); ○ , x 3 = 0.10 ([EMIM] + [BF 4 ] − ); Δ, x 3 = 0.20 ([EMIM]+[BF4]−); ◊, x3 = 0.30 ([EMIM]+[BF4]−); solid lines, correlated using the NRTL model.

Figure 7. Relative volatility (α12) of methanol (1) to MEK (2) at different mole fraction of [OMIM] + [BF 4 ] − at 101.32 kPa: [OMIM]+[BF4]− at 101.32 kPa: ■, x3 = 0 (IL-free); ○, x3 = 0.10 ([OMIM]+[BF4]−); Δ, x3 = 0.20 ([OMIM]+[BF4]−); ◊, x3 = 0.30 ([OMIM]+[BF4]−); solid lines, correlated using the NRTL model.

Figure 6. Relative volatility (α12) of methanol (1) to MEK (2) at different mole fraction of [BMIM] + [BF 4 ] − at 101.32 kPa: [BMIM]+[BF4]− at 101.32 kPa: ■, x3 = 0 (IL-free); ○, x3 = 0.10 ([BMIM]+[BF4]−); Δ, x3 = 0.20 ([BMIM]+[BF4]−); ◊, x3 = 0.30 ([BMIM]+[BF4]−); solid lines, correlated using the NRTL model.

Figure 8. Isobaric vapor−liquid equilibrium (VLE) curves of VLE data for the ternary systems methanol (1) + MEK (2) + [EMIM]+[BF4]− (3)/[BMIM]+[BF4]− (3)/[OMIM]+[BF4]− (3) at fixed mole fraction of 30 %: ■, x3 = 0; ●, x3 = 0.30 ([EMIM]+[BF4]−); ▲, x3 = 0.30 ([BMIM]+[BF4]−);▼, x3 = 0.30 ([OMIM]+[BF4]−); solid lines, correlated using the NRTL model.

Isobaric vapor−liquid equilibrium (VLE) data for methanol− MEK systems containing ionic liquids (ILs) [EMIM]+[BF4]−, [BMIM]+[BF4]−and [OMIM]+[BF4]− were listed in Table 2, where x3 is mole fraction of ILs, which is described in the Experimental Section. There are two kinds of models which were used to correlate and predict the phase equilibria of systems containing ionic liquids: first, the molecular models: Wilson, NRTL, e-NRTL, and UNIQUAC, and second, the predictive thermodynamic models: the COSMO-RS model, the quantitative structure−property relationship (QSPR) method, the regular solution model, and the UNIFAC model.22−24,32 Plenty of papers7−11,15−18,26,31 have used the NRTL model to fit various VLE systems containing ILs, and it gives good agreement with the experimental results. Therefore, in this paper, we also used the NRTL model to correlate the binary and ternary VLE data. The following

objective function (average relative deviation, ARD) was used to combine with it. The Levenberg−Marquardt method was adopted during data fitting with NRTL model.25 ARD(%) =

γexptl i

1 n

∑ n

γi exptl − γi calcd γi exptl

× 100 (3)

γcalcd i

where and are the activity coefficient of component i from experiment and calculation, respectively; n is the number of data points. All of the parameters and the average relative deviations for the binary and the three ternary systems, are summarized in Tables 3 and 4. The x, y diagram of the three ternary systems are shown in Figures 2 to 4. At the same time, Figure 8 compares the VLE of the three ternary systems methanol and MEK containing 1137

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Figure 9. T, x1, y1 diagram of ternary systems methanol (1) to MEK (2) at different mole fraction of [EMIM]+[BF4]−: ■, x1 (x3 = 0); □, y1 (x3 = 0); ●, x1 (x3 = 0.10); ○, y1 (x3 = 0.10); ▲, x1 (x3 = 0.20); Δ, y1 (x3 = 0.20); ⧫, x1 (x3 = 0.30); ◊, y1 (x3 = 0.30); solid lines, correlated using the NRTL model.

Figure 10. T, x1, y1 diagram of ternary systems methanol (1) to MEK (2) at different mole fraction of [BMIM]+[BF4]−: ■, x1 (x3 = 0); □, y1 (x3 = 0); ●, x1 (x3 = 0.10); ○, y1 (x3 = 0.10); ▲, x1 (x3 = 0.20); Δ, y1 (x3 = 0.20); ⧫, x1 (x3 = 0.30); ◊, y1 (x3 = 0.30); solid lines, correlated using the NRTL model.

ILs ([EMIM]+[BF4]−, [BMIM]+[BF4]− and [OMIM]+[BF4]−) at fixed mole fraction of 30 %. These figures show that [BMIM]+[BF4]− and [OMIM]+[BF4]− enable to break the azeotropic point at mole fraction 30 % and 10 %, respectively. Alternatively, it is

noteworthy that the trend of curves in Figure 2 is different from the other two figures (Figures 3 and 4). The [EMIM]+[BF4]− pulled down the azeotropic point [from x1 ≈ (0.8 to 0.7)], and the effect is noticeable with the increase of IL̀s concentration. 1138

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Figures 5 to 7 show that the relative volatility (α12) tends to horizontal lines in the range of methanol from 0 to 1, especially for the system containing [OMIM]+[BF4]−, in which the α12 is about 1.5. In Figure 5, it can be observed that the [EMIM]+[BF4]− reduces the relative volatility (α12) within the whole range of methanol. The numerical difference of α12 between the binary system and the ternary system containing [EMIM]+[BF4]− follows the rule: 30 % > 20 % > 10 %. As we all know, with the increase of the alkyl chain length in the imidazolium ring from ethyl to octyl, the polarity of imidazolium tetrafluoroborate-based ILs decreases, which has been reported in the previous paper.9,11 Methanol and MEK have different polar characters, so the attractive interaction between ILs and solvent molecules is different, according to the principle that the similar substance is more likely to be dissolved by each other. It is noteworthy that, the IL [EMIM]+[BF4]− has a stronger interaction with methanol than MEK, exhibiting a salting-in effect for methanol. Whereas Figures 6 and 7 show that [BMIM]+[BF4]− and [OMIM]+[BF4]− produce a crossover effect over methanol and MEK, and enhance α12 obviously above original ones just in the methanol-rich region, the pivotal point is x1 ≈ 0.8 (to [BMIM]+[BF4]−) and x1 ≈ 0.7 (to [OMIM]+[BF4]−), respectively. The use of ILs in special separation of azetropic or closeboiling mixtures has obvious advantages over classical entrainers or inorganic salts. Generally, it is withdrawn with the heavier components from the bottom of a column, and it can be totally removed from the solvents by flash distillation owing to the fact that it presents a negligible vapor pressure. In addition, a pure IL liquid stream can be easily added to the reflux stream, and it keeps a higher concentration of electrolyte existing along the column because of its great solubility.16,27−29 Moreover, the stream from the overhead of the column usually contains a null IL without further separation. As Privott et al.2 have reported when the binary system were added 0.4 mol fraction of diethyl ketone, the relative volatility (α12) of methanol to MEK was 1.67 (x1 = 0.836, on diethyl ketone free basis). However, when the binary system contains 0.4 mol fraction of [OMIM]+[BF4]−, α12 = 1.41 (x1 = 0.836, on IL-free basis), from the calculation of our NRTL model. The organic extrainer diethyl ketone has a slight stronger separation ability than [OMIM]+[BF4]−. Given the advantages of ILs, [OMIM]+[BF4]− could have significant processing advantages over the organic extrainer diethyl ketone. As seen from Figures 2 to 8, the exchange of the cation results in different influences of the ILs on the separation performance of the methanol−MEK system, and the order is [OMIM] + [BF 4 ] − > [BMIM] + [BF 4 ] − > [EMIM] + [BF 4 ] − ([EMIM]+[BF4]− is not able to break the azeotropic point), demonstrating the possibility to tailor the entrainer properties of an IL by modification of its ion structures. Among the ILs investigated, [OMIM]+[BF4]− exhibits the most distinct entrainer for the methanol−MEK separation. A similar phenomenon that [OMIM]+[BF4]− exhibits the highest separation ability has been observed in the system of ethyl acetate-ethanol.9 The T, x, y diagram of the ternary systems containing [EMIM]+[BF4]−, [BMIM]+[BF4]−, and [OMIM]+[BF4]− are shown in Figures 9 to 11, respectively. It is remarkable that the equilibrium temperatures raise when the ILs are added into the system, and [OMIM]+[BF4]− causes the highest elevation of boiling point than the other two ILs. Therefore, in the distillation process, the heat energy needed in the extractive distillation column will increase. The reflux ratio can be reduced, owing to the elevated relative volatility of methanol to

Figure 11. T, x1, y1 diagram of ternary systems methanol (1) to MEK (2) at different mole fraction of [OMIM]+[BF4]−: ■, x1 (x3 = 0); □, y1 (x3 = 0); ●, x1 (x3 = 0.10); ○, y1 (x3 = 0.10); ▲, x1 (x3 = 0.20); Δ, y1 (x3 = 0.20); ⧫, x1 (x3 = 0.30); ◊, y1 (x3 = 0.30); solid lines, correlated using the NRTL model.

MEK. This indicates that the energy demand for the extractive distillation process can be saved if it is reasonably designed. The literature6−11,27−29,31 also listed the elevation of boiling temperature of the system containing ILs. The equilibrium temperature 1139

dx.doi.org/10.1021/je301062g | J. Chem. Eng. Data 2013, 58, 1133−1140

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is important to the distillation design such as ASPEN PLUS, PRO II, etc.

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CONCLUSIONS Isobaric VLE data for methanol−MEK systems containing ILs [EMIM]+[BF4]−, [BMIM]+[BF4]−, and [OMIM]+[BF4]− were obtained at atmospheric pressure (101.32 kPa). [EMIM]+[BF4]− shows a notable salting-in effect for methanol, whereas [BMIM]+[BF4]−and [OMIM]+[BF4]− show a salting-out effect and break the azeotrope of methanol−MEK when the mole fraction reaches 30 % and 10 %, respectively. [OMIM]+[BF4]− investigated in this work has the most significant ability to separate the mix of methanol−MEK. The VLE data were fitted using the NRTL model, and the ARD of the three ternary systems, including methanol (1) + MEK (2) + ILs (3) ([EMIM]+[BF4]−, [BMIM]+[BF4]−, and [OMIM]+[BF4]−) are 3.00 %, 2.26 %, and 2.84 %, respectively.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-1064433695. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/je301062g | J. Chem. Eng. Data 2013, 58, 1133−1140