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Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3 kPa
Journal Pre-proof Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3�kPa Wenxiu Li, Ting Guan, Ying Cao, Yu Zhang, Tao Zhang PII:
S0378-3812(19)30474-1
DOI:
https://doi.org/10.1016/j.fluid.2019.112412
Reference:
FLUID 112412
To appear in:
Fluid Phase Equilibria
Received Date: 12 August 2019 Revised Date:
11 November 2019
Accepted Date: 14 November 2019
Please cite this article as: W. Li, T. Guan, Y. Cao, Y. Zhang, T. Zhang, Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3�kPa, Fluid Phase Equilibria (2019), doi: https://doi.org/10.1016/j.fluid.2019.112412. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3 kPa Wenxiu Li, Ting Guan, Ying Cao, Yu Zhang, Tao Zhang* Liaoning Provincial Key Laboratory of Chemical Separation Technology, Shenyang University of Chemical Technology, Shenyang, 110142, China. *corresponding author. Tel.: +86 024 89381082. E-mail address: [email protected]
ABSTRACT In order to enhance the separation ability of ionic liquid (IL) in the toluene-methanol binary azeotrope, the vapor-liquid equilibrium (VLE) data for the ternary systems of toluene + methanol + IL (1-decyl-3-methylimidazolium acetate [DMIM][OAC], 1-tetradecyl-3-methylimidazolium acetate [C14MIM][OAC] or tri octyl methylammonium acetate [N8,8,8,1][OAC]) were measured at 101.3kPa. The VLE data were well correlated by the nonrandom two-liquid (NRTL) model. The azeotrope of the toluene-methanol mixture can be fully eliminated when IL content is increased to a specific value. The relative volatility (α12) of toluene to methanol is increased with the increase of [DMIM][OAC] or [C14MIM][OAC], while it is decreased with the increase of [N8,8,8,1][OAC]. The separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. The separation abilities of the three ILs were analyzed with the help of their σ-profiles.
Keywords: Vapor liquid phase equilibrium; ionic liquid; toluene; methanol; NRTL model
1. Introduction Methanol and toluene are common organic solvents [1,2], widely used in fine chemical, pharmaceutical and other fields [3-5]. The azeotrope of methanol and toluene is formed at atmospheric pressure [6,7]. The azeotropic temperature is 63.8 ℃, and the mole fraction of methanol in the azeotrope is 88.2% [8]. Thus, it is difficult to completely separate methanol and/or toluene from their mixture by common distillation. There are two main methods for separating methanol-toluene mixture. Pervaporation membrane separation method has been studied by many researchers [9-12]. High cost is the major constraint on the industrialization of the method. Another effective separation method for toluene-methanol mixture is extractive distillation [13,14]. Extractive distillation is widely used to separate azeotropic or
close-boiling mixtures. The key to extractive distillation is to select an effective entrainer. Three traditional organic solvents (cyclohexanol, furfural and dimethyl sulfoxide) have been used as entrainers to separate the methanol-toluene azeotrope by extractive distillation. The separation ability of the three extractants is: cyclohexanol > furfural > dimethyl sulfoxide [15]. The ternary vapor-liquid equilibrium (VLE) data of methanol + toluene + o-xylene were measured and the azeotropy of methanol-toluene mixture can be completely eliminated by o-xylene [16]. However, due to the toxic or corrosive nature of the traditional entrainers and their high solvent consumption, it is extremely important to find an entrainer with better performance. Ionic liquids (ILs) can be used to replace traditional entrainers because of their extremely low volatility, good chemical and thermal stability, and good solubility and selectivity [17-26]. The isothermal VLE data for the methanol-toluene azeotropic mixture containing IL have been used to assess the separation ability of three ILs ( Triphenylbenzylphosphonium Chloride, Tetrabutylammonium Tetraphenylborate, and N‑Butylpyridinium Bromide ) at 318.15 K [27-29]. The relative volatility (α12) of toluene to methanol was decreased with the increase of IL content in the methanol-toluene-IL mixture. The minimum mole fractions of Triphenylbenzylphosphonium Chloride, Tetrabutylammonium Tetraphenylborate and N‑Butylpyridinium Bromide needed to break the azeotrope were 0.1 mol•kg-1, 0.3 mol•kg-1, and 1.2 mol•kg-1, respectively. In this text, three ILs with acetate anion (1-decyl-3-methylimidazolium acetate [DMIM][OAC], 1-tetradecyl-3-methylimidazolium acetate [C14MIM][OAC] and trioctyl methylammonium acetate [N8,8,8,1][OAC]) were selected by a molecular design method using the COSMOthermX software. The range of screening was 420 ILs composed of 21 common anions and 20 common cations. The basis of screening included the selectivity of IL to the mixture to be separated, the solubility of IL in the mixture to be separated and the difficulty of obtaining ILs. The specific screening process was described in the supplementary section. Three ILs with acetate anion ([N8,8,8,1][OAC], [DMIM][OAC], and [C14MIM][OAC]) were selected for further study. The VLE data of the toluene + methanol + IL system were measured at 101.3 KPa. The VLE data were correlated by NRTL model [30]. The ability of ILs to separate the toluene-methanol mixture was analyzed with the aid of the σ-Profiles. COSMOtherm (version C2.1, release 01.11) software with BP_TZVPD_FINE_C30_1601 parameterization was used in this work which is based on the COSMO-RS model. The data predicted was obtained through density functional theory (DFT) calculations, utilizing the Becke-Perdew (BP) functional with the resolution of identity (RI) approximation and the Triple-ζ Zeta Valence Potential (TZVP) basis set. The ILs were treated as two individual ions in an equimolar mixture. The selectivity of ILs composed of these anions and cations at infinite dilution ( S )
were obtained by Ionic Liquids Screening of COSMOtherm. S separation efficiency of IL to some extent [31-33].
can represented the
2. Experimental 2.1. Chemicals Toluene (>99.5 mass %) and methanol (>99.7 mass %) were purchased from Sinopharm Group. The purities of the two reagents were confirmed by gas chromatography, and they were used without further purification. The ILs ([DMIM][OAC], [C14MIM][OAC] and [N8,8,8,1][OAC]) were provided by Yulu Group (>98.0 mass %). In order to remove volatile compounds, it is necessary for IL to be purified at 373 K and 2 kPa for 48 hours before the experiment. The water content in the ILs determined by Karl Fischer titration was below 500ppm. The specifications of the chemicals used are given in Table 1. Table 1
The specifications of chemical samples. Chemical name
source
Water Content (ppm).
Purity ( wt% )
Purification method
Analysis method
Toluenea
50
0.997
None
GCf KFg
50
0.995
None
GCf KFg
[DMIM][OAC]c
Sinopharm Group Sinopharm Group Yulu Group
500
0.980
KFg LCh
[C14MIM][OAC]d
Yulu Group
500
0.980
[N8,8,8,1][OAC]e
Yulu Group
500
0.980
Vacuum desiccation Vacuum desiccation Vacuum desiccation
Methanolb
a
KFg LCh KFg LCh
toluene: the experimental densities (at 298.15 K) is 0.8700 g/cm³, normal boiling point temperature is 383.75 K.[6] b methanol: the experimental densities (at 298.15 K) is 0.7918 g/cm³, normal boiling point temperature is 337.85 K. [6] c [DMIM] [OAC] = 1-decyl-3-methylimidazolium acetate d [C14MIM][OAC] = 1-tetradecyl-3-methylimidazolium acetate e [N8,8,8,1][OAC] = trioctyl methylammonium acetate f GC = gas chromatography. g KF = Karl Fischer titration. h
LC = liquid chromatography
2.2. Apparatus and Procedure
The equilibrium apparatus is an all-glass dynamic recirculating still (NGW, Wertheim, Germany) and has been described in a previous literature [34]. The still pressure was kept at 101.3 kPa by a pressure controller (MKS, USA) and measured by a manometer (TDGC2-0.5 CHNT China) with a standard uncertainty of 0.1 kPa. The equilibrium temperature was measured by a calibration thermometer with a standard uncertainty of 0.01 K. The VLE experimental sample are prepared by an electronic balance (PTX-FA300S) with a standard uncertainty of 0.0001 g. The toluene and methanol contents were measured using an Agilent gas chromatography (GC) (Model 7820A) connected to a headspace autosampler (G1888 Network Headspace Sampler, Agilent Technologies). The GC was equipped with an Agilent 19091 J-413 capillary column (0.32 mm in diameter, 30m in length, and 0.25 µm in thickness) and a flame ionization detector (FID). The temperature of the detector, column, and injector were 443 K, 339 K, and 423 K, respectively. N2 was the carrier gas in the experiment. The content of IL was determined by mass difference before and after volatile components were evaporated (373 K, 2 kPa, and 48h) due to the nonvolatility of IL. The standard uncertainty of IL mole fraction is 0.001. Each sample was analyzed at least three times.
2.3. Data treatment Because the experiment is conducted at low vapor phase pressure (101.3 kPa), the assumption of ideal vapor phase is made. The activity coefficient of component i ( ) can be computed by Eq. (1).
=
(1)
Where xi and yi are the mole fraction of component i in the liquid phase and the vapor phase, respectively. P is the total pressure (101.3 kPa), is the saturated vapor pressure of component i and calculated by the Antoine equation. =
−
+
(2)
In which Ai, Bi and Ci are the Antoine parameters of component i. t is equilibrium temperature in ℃. These parameters are listed in Table 2. Table 2 Antoine parameters for toluene and methanol.a Antoine parameters component A B C Tolueneb 6.955 Methanolb 8.072 a Antoine equation: ( /
1344.800 219.482 1574.990 238.870 ) = − /( /℃ + )
Temp range (K) 279.00-409.00 257.00-364.00
b
Parameters obtained from ref [35].
The relative volatility α12 is calculated by the following equation:
=
⁄ ⁄
=
(3)
1 indicates toluene and 2 indicates methanol. The average relative volatilities ( are calculated by Eq. (4) #% &
= "# %$& $
where
(
'
represents
(
)
(4)
the mole content of toluene without IL in liquid phase.
The binary interaction parameters (∆g12 and ∆g21) and the nonrandomness parameter (α12) for toluene-methanol system are regressed from the binary VLE data in Table 3 by the Levenberg-Marquardt method [36] through minimizing the objective function (ARD) according to Eq. (5), and the rest of the nonrandomness parameters (α13, α23) and binary interaction parameters (∆g13, ∆g31, ∆g23 and ∆g32) are regressed from the VLE data in Tables 4−6. )*(%) =
1 --. , 4
/#0
−
/#0
1231
. × 100 (5)
In which n represents the number of experiment points. the superscripts “exp” and “calc” represent experimental and calculated values, respectively.
3. Results and discussion 3.1. Experimental data To test the dependability of the equilibrium apparatus, the binary VLE data for toluene (1) + methanol (2) mixture were measured at 101.3 kPa and are shown in Table 3 and Fig. 1. The reported data [37] are also listed in Fig. 1. Where T presents the VLE temperature. It can be seen from Fig. 1 that the VLE data measured in this work are in good agreement with those reported in the literature. In addition, the experimental data are consistent with thermodynamic consistency. The area test shows that |D-J|= 8.90 < 10. The point test shows that the absolute deviation between ycalc and yexp is less than 0.01, and the absolute deviation between Pcalc and Pexp is less than 1.33 kPa. Therefore, the experimental method and the apparatus used in this study are reliable. The thermodynamically consistency of the literature data have also
been tested through area test method in Ref. [37]. The result of area test is: |D-J| = 8.32 < 10. Table 3
Isobaric VLE data for the binary system of toluene (1) + methanol (2) at 101.3kPaa
a
T/K
x1
y1
α12
γ₁
γ2
337.64 337.13 336.72 336.71 336.77 336.84 337.03 337.27 337.34 337.48 337.87 338.49 339.31 340.02 342.51 345.33 346.97 350.64 355.26 382.95
0.000 0.031 0.096 0.125 0.177 0.204 0.268 0.332 0.381 0.426 0.558 0.676 0.752 0.794 0.86 0.899 0.913 0.939 0.958 1.000
0.000 0.048 0.107 0.123 0.143 0.151 0.163 0.171 0.175 0.178 0.186 0.194 0.203 0.212 0.238 0.271 0.290 0.343 0.413 1.000
1.587 1.134 0.986 0.778 0.692 0.532 0.415 0.345 0.293 0.181 0.115 0.084 0.070 0.051 0.042 0.039 0.034 0.031
7.298 5.325 4.700 3.845 3.497 2.858 2.397 2.136 1.934 1.515 1.273 1.162 1.118 1.058 1.039 1.030 1.041 1.046
1.003 1.024 1.040 1.078 1.102 1.172 1.260 1.349 1.441 1.827 2.408 3.012 3.488 4.507 5.373 5.717 6.592 7.256
Standard uncertainty u(x1) =u(y1) = 0.001, u(T) = 0.01 K, u(P) =0.1 kPa.
Fig. 1. VLE data for the binary system of toluene (1) + methanol (2) at 101.3kPa: ●, experimental data in this work; △, from Ref [37]; solid line, correlated using NRTL model.
3.2 Ternary data The experimental VLE data for toluene (1) + methanol (2) + [DMIM][OAC] (3), [C14MIM][OAC] (3) or [N8,8,8,1][OAC] (3) are shown in Tables 4-6 and Figs. 2-4. The regressed parameters and the values of ARD are listed in Table 7. The correlated curves are drawn in Figs. 2-4. The values of ARD in the ternary systems for [C14MIM][OAC], [DMIM][OAC] and [N8,8,8,1][OAC] are 0.99%, 1.04% and 1.05%, respectively. The experimental data are quite well correlated by the NRTL model. Table 4
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [DMIM][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100
360.46 359.37 358.74 358.42 358.47
0.044 0.089 0.136 0.184 0.227
0.049 0.099 0.151 0.204 0.252
0.114 0.197 0.263 0.317 0.358
2.490 2.229 2.002 1.808 1.658
5.261 4.669 4.172 3.765 3.444
0.449 0.446 0.444 0.443 0.442
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
358.68 359.21 360.10 361.08 362.41 363.93 365.14 366.29 368.60 370.34 372.70 375.29 378.07 382.06 373.11 371.87 371.10 370.44 370.06 370.02 370.09 370.46 370.58 371.09 371.84 372.63 373.49 374.54 375.81 377.23 378.57 380.00 382.19 384.48 383.01 381.92 381.07 380.48 380.06 379.92 379.74 379.80 379.97 380.38
0.272 0.321 0.370 0.413 0.462 0.509 0.555 0.585 0.646 0.689 0.742 0.790 0.835 0.880 0.043 0.086 0.129 0.171 0.216 0.262 0.302 0.343 0.384 0.430 0.475 0.517 0.560 0.600 0.647 0.690 0.728 0.768 0.824 0.038 0.081 0.123 0.167 0.210 0.257 0.300 0.347 0.390 0.434 0.479
0.302 0.357 0.411 0.459 0.513 0.565 0.617 0.650 0.718 0.766 0.824 0.878 0.928 0.978 0.051 0.101 0.152 0.201 0.254 0.308 0.355 0.404 0.452 0.506 0.559 0.608 0.659 0.706 0.761 0.812 0.857 0.904 0.969 0.048 0.101 0.154 0.209 0.263 0.321 0.375 0.434 0.488 0.542 0.599
0.397 0.436 0.472 0.503 0.537 0.571 0.606 0.629 0.679 0.719 0.772 0.828 0.889 0.962 0.118 0.206 0.279 0.338 0.394 0.445 0.486 0.525 0.563 0.604 0.643 0.679 0.716 0.751 0.793 0.832 0.869 0.909 0.969 0.110 0.207 0.288 0.359 0.421 0.480 0.530 0.582 0.626 0.669 0.712
1.524 1.394 1.282 1.193 1.103 1.023 0.955 0.912 0.832 0.781 0.721 0.668 0.619 0.565 2.484 2.313 2.161 2.034 1.912 1.801 1.715 1.632 1.563 1.488 1.420 1.361 1.306 1.255 1.200 1.150 1.108 1.066 1.009 2.448 2.327 2.221 2.122 2.036 1.954 1.883 1.816 1.759 1.707 1.654
3.163 2.887 2.635 2.434 2.228 2.045 1.913 1.817 1.652 1.552 1.441 1.342 1.255 1.148 3.716 3.413 3.143 2.940 2.743 2.556 2.415 2.269 2.166 2.043 1.924 1.823 1.729 1.639 1.545 1.458 1.387 1.319 1.231 2.804 2.622 2.464 2.322 2.199 2.080 1.976 1.883 1.799 1.721 1.638
0.442 0.441 0.437 0.433 0.428 0.423 0.423 0.420 0.418 0.418 0.418 0.419 0.422 0.420 0.313 0.309 0.305 0.304 0.301 0.298 0.296 0.292 0.291 0.288 0.284 0.280 0.277 0.273 0.269 0.264 0.260 0.257 0.253 0.236 0.233 0.230 0.227 0.224 0.221 0.218 0.215 0.212 0.209 0.205
0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
380.95 381.56 382.04 383.00 383.63 384.57 384.82 385.68
0.522 0.569 0.603 0.659 0.692 0.732 0.742 0.777
0.653 0.711 0.754 0.824 0.865 0.915 0.928 0.971
0.752 0.793 0.824 0.874 0.903 0.939 0.948 0.979
1.607 1.561 1.530 1.480 1.453 1.419 1.410 1.382
1.560 1.486 1.436 1.355 1.310 1.254 1.240 1.194
0.201 0.197 0.194 0.189 0.186 0.182 0.181 0.178
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 5
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [C14MIM][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150
364.03 363.07 362.45 362.08 361.95 362.01 362.23 362.66 363.16 363.28 364.10 365.17 366.24 368.43 370.58 372.29 374.22 376.51 381.88 378.06 377.08 376.32 375.77 375.38 375.14 375.02 375.04 375.17 375.40
0.048 0.097 0.145 0.201 0.218 0.254 0.305 0.385 0.425 0.437 0.483 0.523 0.562 0.626 0.686 0.724 0.760 0.803 0.881 0.041 0.086 0.136 0.180 0.218 0.279 0.318 0.324 0.369 0.414
0.053 0.108 0.161 0.223 0.242 0.282 0.339 0.428 0.472 0.486 0.537 0.581 0.624 0.695 0.762 0.804 0.844 0.892 0.979 0.048 0.101 0.160 0.212 0.256 0.328 0.374 0.381 0.434 0.487
0.095 0.172 0.234 0.296 0.314 0.349 0.396 0.466 0.500 0.511 0.550 0.583 0.617 0.673 0.731 0.769 0.808 0.859 0.968 0.080 0.155 0.230 0.289 0.336 0.407 0.451 0.458 0.506 0.553
1.882 1.721 1.592 1.465 1.431 1.363 1.278 1.167 1.119 1.104 1.053 1.009 0.969 0.905 0.849 0.813 0.778 0.737 0.658 1.717 1.639 1.565 1.510 1.469 1.409 1.376 1.371 1.337 1.306
3.628 3.323 3.086 2.853 2.797 2.664 2.498 2.298 2.199 2.173 2.062 1.953 1.859 1.701 1.576 1.491 1.407 1.322 1.160 2.303 2.199 2.098 2.024 1.971 1.880 1.833 1.824 1.764 1.707
0.408 0.409 0.411 0.413 0.415 0.415 0.415 0.417 0.416 0.417 0.414 0.409 0.405 0.396 0.389 0.384 0.378 0.373 0.365 0.279 0.279 0.279 0.280 0.280 0.279 0.278 0.278 0.275 0.273
0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
375.74 376.23 376.39 377.04 377.93 379.07 380.14 381.14 383.53 390.46 389.50 388.69 388.02 387.47 387.04 386.70 386.45 386.29 386.20 386.17 386.27 386.40 386.56 386.83 387.06 387.39 387.58 387.90
0.477 0.530 0.541 0.576 0.620 0.665 0.709 0.745 0.824 0.042 0.079 0.130 0.177 0.215 0.262 0.311 0.346 0.380 0.426 0.518 0.569 0.595 0.626 0.670 0.741 0.750 0.753 0.786
0.561 0.624 0.637 0.678 0.729 0.782 0.834 0.876 0.969 0.053 0.099 0.162 0.221 0.269 0.328 0.389 0.433 0.475 0.532 0.648 0.711 0.744 0.782 0.838 0.886 0.938 0.959 0.982
0.618 0.673 0.683 0.718 0.761 0.807 0.851 0.888 0.971 0.080 0.144 0.224 0.294 0.347 0.410 0.472 0.516 0.556 0.610 0.715 0.770 0.798 0.829 0.875 0.913 0.953 0.969 0.987
1.268 1.239 1.229 1.208 1.186 1.163 1.141 1.120 1.078 1.564 1.531 1.494 1.465 1.445 1.424 1.406 1.396 1.387 1.377 1.362 1.358 1.356 1.354 1.352 1.354 1.350 1.344 1.341
1.639 1.582 1.547 1.497 1.444 1.390 1.338 1.286 1.187 1.576 1.550 1.507 1.474 1.454 1.426 1.398 1.381 1.364 1.339 1.288 1.261 1.245 1.224 1.197 1.158 1.141 1.125 1.096
0.270 0.266 0.262 0.258 0.253 0.248 0.243 0.238 0.228 0.206 0.207 0.207 0.207 0.207 0.206 0.205 0.204 0.202 0.200 0.195 0.191 0.189 0.186 0.182 0.176 0.174 0.172 0.168
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 6
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [N8,8,8,1][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100
358.64 358.41 358.34 358.42 358.65 359.00 359.49 360.10 360.84
0.044 0.094 0.143 0.193 0.242 0.283 0.332 0.364 0.422
0.049 0.104 0.159 0.214 0.269 0.314 0.369 0.404 0.469
0.051 0.103 0.151 0.196 0.239 0.274 0.316 0.343 0.394
1.053 0.993 0.941 0.895 0.855 0.825 0.791 0.771 0.737
2.529 2.411 2.309 2.220 2.141 2.076 2.007 1.948 1.883
0.511 0.517 0.523 0.528 0.533 0.536 0.539 0.537 0.542
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300
361.88 362.71 363.63 365.15 366.62 368.27 370.65 374.26 378.34 382.38 381.17 381.05 381.01 381.05 381.15 381.31 381.54 381.84 382.19 382.62 383.11 383.68 384.26 384.97 385.60 386.38 387.00 388.17 389.13 399.55 399.63 399.74 399.94 400.13 400.24 400.50 400.59 400.95 401.35 401.50 401.70 402.05 402.35 402.60
0.490 0.521 0.553 0.602 0.643 0.683 0.724 0.782 0.832 0.877 0.039 0.082 0.126 0.168 0.204 0.247 0.290 0.326 0.369 0.412 0.455 0.514 0.558 0.601 0.628 0.663 0.690 0.750 0.790 0.034 0.072 0.111 0.150 0.189 0.220 0.258 0.282 0.328 0.381 0.405 0.430 0.468 0.501 0.531
0.544 0.579 0.614 0.669 0.714 0.759 0.804 0.869 0.924 0.974 0.049 0.103 0.157 0.210 0.255 0.309 0.363 0.407 0.461 0.515 0.569 0.643 0.697 0.751 0.785 0.829 0.863 0.937 0.988 0.049 0.103 0.159 0.214 0.270 0.314 0.369 0.403 0.469 0.544 0.578 0.614 0.669 0.715 0.759
0.456 0.485 0.516 0.566 0.609 0.655 0.703 0.783 0.861 0.947 0.040 0.085 0.131 0.177 0.216 0.265 0.315 0.357 0.409 0.463 0.518 0.595 0.653 0.712 0.750 0.799 0.838 0.925 0.986 0.033 0.072 0.115 0.159 0.207 0.246 0.297 0.330 0.396 0.474 0.511 0.550 0.612 0.664 0.715
0.703 0.686 0.670 0.645 0.624 0.603 0.578 0.543 0.509 0.477 0.812 0.809 0.807 0.806 0.806 0.806 0.807 0.808 0.809 0.811 0.813 0.817 0.819 0.821 0.821 0.822 0.821 0.825 0.826 0.667 0.676 0.685 0.695 0.705 0.713 0.723 0.729 0.741 0.756 0.762 0.769 0.780 0.789 0.798
1.814 1.766 1.717 1.646 1.585 1.522 1.434 1.322 1.211 1.124 1.103 1.115 1.126 1.136 1.143 1.150 1.156 1.158 1.160 1.161 1.159 1.162 1.158 1.150 1.134 1.120 1.101 1.091 1.074 0.631 0.650 0.669 0.685 0.703 0.717 0.732 0.742 0.758 0.775 0.783 0.790 0.799 0.806 0.812
0.548 0.546 0.543 0.540 0.536 0.532 0.520 0.509 0.495 0.488 0.282 0.286 0.289 0.292 0.294 0.296 0.297 0.297 0.297 0.296 0.295 0.294 0.292 0.289 0.285 0.280 0.276 0.272 0.267 0.192 0.195 0.197 0.199 0.202 0.203 0.205 0.206 0.206 0.207 0.207 0.207 0.207 0.206 0.205
0.300 0.300 0.300 0.300
403.00 403.43 403.90 404.19
0.564 0.601 0.639 0.675
0.805 0.859 0.913 0.964
0.769 0.833 0.897 0.957
0.807 0.819 0.830 0.841
0.815 0.818 0.819 0.822
0.203 0.201 0.199 0.197
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 7
Parameters and correlation statistics for the NRTL model
component
component
∆gij
∆gji
ARD
(J/mol)
(J/mol)
(%)
αij i toluene
j methanol
0.4370
4290.3
3857.3
0.83 1.04
toluene
[DMIM][OAC]
0.4589
-12640.1
62355.0
methanol
[DMIM][OAC]
0.2834
-7504.0
29869.2
toluene
[C14MIM][OAC]
0.4383
-13726.0
62354.9
methanol
[C14MIM][OAC]
0.1967
-12818.3
45652.0
toluene
[N8,8,8,1][OAC]
0.4679
-12584.1
20311.1
methanol
[N8,8,8,1][OAC]
0.2028
-12513.1
46674.8
1.15
1.05
Fig. 2. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [DMIM] [OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.15; ▲, x3 = 0.20; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 3. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [C14MIM][OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.15; ▲, x3 = 0.20; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 4. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [N8,8,8,1][OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.20; ▲, x3 = 0.30; dotted line, IL-free system; solid line, correlated using NRTL model.
It can be seen from Figs. 2-4, the effects of the three ILs on the azeotropic point of the toluene-methanol mixture are different. For [DMIM][OAC] and [C14MIM][OAC], the azeotropic point is shifted upward by increasing the mole fraction of IL, and the azeotropy is completely eliminated when the mole fraction of IL is increased to a specific value [38-40]. After the azeotropic phenomenon is eliminated, the equilibrium content of toluene in the vapor phase (y1) is higher than that in the liquid phase (x'1). Therefore, toluene becomes a light component when the two ILs are used. For [N8,8,8,1][OAC], the azeotropic point is moved downward by increasing the mole fraction of IL, and the azeotropy is completely eliminated when the mole fraction of IL is increased to a specific value. After the azeotropic phenomenon is eliminated, the equilibrium content of toluene in the vapor phase (y1) is lower than that in the liquid phase (x'1). Therefore, toluene becomes a heavy component when [N8,8,8,1][OAC] is used. According to the NRTL model, The minimum mole fractions of [DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC] needed to eliminate the azeotrope of the toluene-methanol mixture are 0.1519, 0.1401 and 0.1299, respectively. The abilities of [DMIM][OAC] and [C14MIM][OAC] to eliminate azeotropy are very close. It is not sufficient to evaluate the separation abilities of the two ILs by only using their abilities to eliminate azeotropy [41]. Further research is needed.
Fig. 5. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.10: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 6. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.15: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 7. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.20: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 8. The relative volatility of toluene (1) to methanol (2) with different mole fractions of [N8,8,8,1][OAC] (3) at 101.3 kPa: ■, x3 =0.10; ●, x3 =0.20; ▲, x3=0.30; dotted line, IL-free system; solid line, correlated using the NRTL model.
In order to further compare the separation ability of [DMIM][OAC] and [C14MIM][OAC], the relative volatilities (α12) of toluene to methanol caused by [DMIM][OAC] and [C14MIM][OAC] are calculated and drawn in Figs. 5-7. The IL mole fractions are 0.10, 0.15 and 0.20, respectively. When the mole fraction of IL is 0.20, α12 value caused by [DMIM] [OAC] is significantly higher than that caused by [C14MIM][OAC]. When the mole fractions of ILs are 0.10 and 0.15, the intersection points of the x'1-α12 curves are generated because the curve corresponding to [DMIM][OAC] has a larger slope than that corresponding to [C14MIM][OAC]. The average relative volatility ( ) calculated by Eq. (4) is shown in table 8. The values corresponding to [DMIM][OAC] is higher than that corresponding to [C14MIM][OAC] at all the three ILs concentrations. From Table 8 and Fig. 7, it can be seen that the separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. The result is consistent with that predicted by COSMOtherm in the supplementary section. The larger slope of the x'1-α12 curves corresponding to [DMIM][OAC] than that corresponding to [C14MIM][OAC] is the reason why the minimum breaking azeotropic mole fraction of [DMIM][OAC] is greater than that of [C14MIM][OAC]. Table 8
The average relative volatility of toluene to methanol produced by different ILs.
Mole fraction of ILs 0.10 0.15 0.20
[DMIM][OAC]
[C14MIM][OAC]
1.388 1.664 1.839
1.165 1.339 1.412
The α12 caused by [N8,8,8,1][OAC] is calculated and shown in Fig. 8. The IL mole fractions are 0.10, 0.20 and 0.30, respectively. Because toluene is a heavy component, the α12 values are less than 1 when azeotrope is eliminated. The smaller the α12 value, the better the separation effect. It can be seen from Fig. 8, when the mole fraction of [N8,8,8,1][OAC] is 0.2 or 0.3, the azeotrope of the toluene-methanol mixture is completely eliminated. After the azeotrope is completely eliminated，the value calculated by Eq. (4) is shown in table 9. It can be observed that ( ) is decreased with the increase of [N8,8,8,1][OAC] concentration. Table 9
The average relative volatility of toluene to methanol produced by different ILs. Mole fraction of ILs [N8,8,8,1][OAC] 0.20 0.30
0.814 0.749
3.3 Analysis of the σ-profiles The ability of IL to separate the toluene-methanol azeotropic mixture can be explained by their σ-profiles [42-45]. The σ-profiles of toluene and methanol are displayed in Fig. 9, and the σ-profiles of [DMIM]+, [C14MIM]+, [N8,8,8,1]+ and [OAc]are displayed in Fig. 10. The cut-off value (dashed line) is -0.0082e / Å for the donor and 0.0082e / Å for the acceptor. The non-polar region is located from -0.0082 e / Å2 to 0.0082 e / Å2 [46,47].
Fig. 9. σ-Profiles for toluene and methanol.
Fig. 10. σ-Profiles for [DMIM]+, [C14MIM]+, [N8,8,8,1]+ and [OAc]- .
It can be observed from Fig. 9 and Fig. 10, the peaks of toluene are all in non-polar regions. The peaks of methanol are distributed in the donor, non-polar and acceptor regions. The peak value of toluene is obviously larger than that of methanol in the non-polar region. The peaks of anion in ILs ([OAc]-) are located in the non-polar region and the acceptor regions, while the peaks of cations in ILs ([DMIM]+, [C14MIM]+, and [N8,8,8,1]+) are distributed in the donor region and non-polar region. The peaks of the three cations are much larger than that of toluene and methanol in the non-polar region. The peak values of the three cations in the non-polar region are: [DMIM]+ < [C14MIM]+ < [N8,8,8,1]+. Because the peak of toluene is not appeared in the polar region, there is no intermolecular polar interaction between toluene and other molecules. The azeotrope of toluene-methanol mixture is due to the intermolecular non-polar interaction. Because the peak value of methanol is obviously smaller than that of toluene in the non-polar region, the intermolecular non-polar interaction between methanol and IL is also obviously smaller than that between toluene and IL. Therefore, the intermolecular interaction between methanol and IL is mainly characterized by polar interaction. The average relative volatility ( ) value of toluene to methanol is
increased with the increase of [DMIM][OAc] content, indicating that the intermolecular polar interaction between [DMIM][OAc] and methanol is larger than the intermolecular non-polar interaction between toluene and methanol and that between toluene and IL [23,25,41]. Since the anions of the three ILs are the same, their effect on the system to be separated is reflected in their cations. The peak of [C14MIM]+ is substantially coincident with that of [DMIM]+ except that [C14MIM]+ has larger peak value in the non-polar region. The nonpolar interaction between [C14MIM][OAc] and toluene is larger than that between [DMIM][OAc] and toluene because of the larger non-polar peak of [C14MIM]+, while the polar interaction between [C14MIM][OAc] and methanol is substantially the same as that between [DMIM][OAc] and methanol. Because the volatility of toluene is reduced by the lager nonpolar interaction and the volatility of methanol has hardly changed at the same time, the values of toluene to methanol caused by [C14MIM][OAc] are smaller than that caused by [DMIM][OAc]. The peak of [N8,8,8,1]+ is larger than that of [C14MIM]+ in the non-polar region, while the polar peak of [N8,8,8,1]+ is substantially the same as that of [C14MIM]+ and [DMIM]+. When [N8,8,8,1][OAc] is used as an entrainer, the non-polar interaction between IL and toluene continues to increase and the polar interaction between IL and methanol remains unchanged. When [N8,8,8,1][OAc] is added to the toluene-methanol solution, the toluene becomes a heavy component with greater interaction with the IL, indicating that the unaltered polar interaction between IL and methanol is exceeded by the increasing non-polar interaction between IL and toluene.
4. Conclusions Isobaric VLE data for the ternary systems of toluene + methanol + ILs ([DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC]) were measured at 101.3 kPa. The VLE data were well correlated with the NRTL model. Based on the NRTL model, the minimum mole fractions of [DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC] needed to eliminate the azeotrope of the toluene-methanol mixture were 0.1519, 0.1401 and 0.1299, respectively. When [N8,8,8,1][OAC] was used as an entrainer, toluene was a heavy component and when [DMIM][OAC] or [C14MIM][OAC] was used as entrainer, toluene was a light component. After the toluene-methanol azeotrope was completely eliminated, the separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. Acknowledgments This work is financially supported by the National Science Foundation of China (Project No. 21576166), the National Science Foundation of China (Project No. 21978173) List of symbols xi
Mole fraction of solvent i in the liquid phase
x'i
The liquid-phase mole fraction of component i excluding IL
yi
Mole fraction of solvent i in the vapor phase
T
Equilibrium temperature
∆gij
Binary energy parameter of NRTL model
P
Total pressure in the equilibrium system
Pi0 Greek letters α12
Relative volatility of component 1 to component 2
αij
Non-randomness parameter of NRTL model
γi
Activity coefficient of component i
γiexptl
The activity coefficient of component i measured by experimental data
γicalcd
The activity coefficient of component i calculated with the NRTL model
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Author Contributions Section Wenxiu Li : Conceptualization, Formal analysis, Validation, Investigation, Writing - Original Draft, Ting Guan: Development or design of methodology; creation of models, Validation, Data Curation, Ying Cao: Development or design of methodology; creation of models Yu Zhang: Software, Tao Zhang: Conceptualization, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition
S0378-3812(19)30474-1
DOI:
https://doi.org/10.1016/j.fluid.2019.112412
Reference:
FLUID 112412
To appear in:
Fluid Phase Equilibria
Received Date: 12 August 2019 Revised Date:
11 November 2019
Accepted Date: 14 November 2019
Please cite this article as: W. Li, T. Guan, Y. Cao, Y. Zhang, T. Zhang, Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3�kPa, Fluid Phase Equilibria (2019), doi: https://doi.org/10.1016/j.fluid.2019.112412. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Isobaric vapor-liquid equilibrium for toluene-methanol system including three ionic liquids with acetate anion at 101.3 kPa Wenxiu Li, Ting Guan, Ying Cao, Yu Zhang, Tao Zhang* Liaoning Provincial Key Laboratory of Chemical Separation Technology, Shenyang University of Chemical Technology, Shenyang, 110142, China. *corresponding author. Tel.: +86 024 89381082. E-mail address: [email protected]
ABSTRACT In order to enhance the separation ability of ionic liquid (IL) in the toluene-methanol binary azeotrope, the vapor-liquid equilibrium (VLE) data for the ternary systems of toluene + methanol + IL (1-decyl-3-methylimidazolium acetate [DMIM][OAC], 1-tetradecyl-3-methylimidazolium acetate [C14MIM][OAC] or tri octyl methylammonium acetate [N8,8,8,1][OAC]) were measured at 101.3kPa. The VLE data were well correlated by the nonrandom two-liquid (NRTL) model. The azeotrope of the toluene-methanol mixture can be fully eliminated when IL content is increased to a specific value. The relative volatility (α12) of toluene to methanol is increased with the increase of [DMIM][OAC] or [C14MIM][OAC], while it is decreased with the increase of [N8,8,8,1][OAC]. The separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. The separation abilities of the three ILs were analyzed with the help of their σ-profiles.
Keywords: Vapor liquid phase equilibrium; ionic liquid; toluene; methanol; NRTL model
1. Introduction Methanol and toluene are common organic solvents [1,2], widely used in fine chemical, pharmaceutical and other fields [3-5]. The azeotrope of methanol and toluene is formed at atmospheric pressure [6,7]. The azeotropic temperature is 63.8 ℃, and the mole fraction of methanol in the azeotrope is 88.2% [8]. Thus, it is difficult to completely separate methanol and/or toluene from their mixture by common distillation. There are two main methods for separating methanol-toluene mixture. Pervaporation membrane separation method has been studied by many researchers [9-12]. High cost is the major constraint on the industrialization of the method. Another effective separation method for toluene-methanol mixture is extractive distillation [13,14]. Extractive distillation is widely used to separate azeotropic or
close-boiling mixtures. The key to extractive distillation is to select an effective entrainer. Three traditional organic solvents (cyclohexanol, furfural and dimethyl sulfoxide) have been used as entrainers to separate the methanol-toluene azeotrope by extractive distillation. The separation ability of the three extractants is: cyclohexanol > furfural > dimethyl sulfoxide [15]. The ternary vapor-liquid equilibrium (VLE) data of methanol + toluene + o-xylene were measured and the azeotropy of methanol-toluene mixture can be completely eliminated by o-xylene [16]. However, due to the toxic or corrosive nature of the traditional entrainers and their high solvent consumption, it is extremely important to find an entrainer with better performance. Ionic liquids (ILs) can be used to replace traditional entrainers because of their extremely low volatility, good chemical and thermal stability, and good solubility and selectivity [17-26]. The isothermal VLE data for the methanol-toluene azeotropic mixture containing IL have been used to assess the separation ability of three ILs ( Triphenylbenzylphosphonium Chloride, Tetrabutylammonium Tetraphenylborate, and N‑Butylpyridinium Bromide ) at 318.15 K [27-29]. The relative volatility (α12) of toluene to methanol was decreased with the increase of IL content in the methanol-toluene-IL mixture. The minimum mole fractions of Triphenylbenzylphosphonium Chloride, Tetrabutylammonium Tetraphenylborate and N‑Butylpyridinium Bromide needed to break the azeotrope were 0.1 mol•kg-1, 0.3 mol•kg-1, and 1.2 mol•kg-1, respectively. In this text, three ILs with acetate anion (1-decyl-3-methylimidazolium acetate [DMIM][OAC], 1-tetradecyl-3-methylimidazolium acetate [C14MIM][OAC] and trioctyl methylammonium acetate [N8,8,8,1][OAC]) were selected by a molecular design method using the COSMOthermX software. The range of screening was 420 ILs composed of 21 common anions and 20 common cations. The basis of screening included the selectivity of IL to the mixture to be separated, the solubility of IL in the mixture to be separated and the difficulty of obtaining ILs. The specific screening process was described in the supplementary section. Three ILs with acetate anion ([N8,8,8,1][OAC], [DMIM][OAC], and [C14MIM][OAC]) were selected for further study. The VLE data of the toluene + methanol + IL system were measured at 101.3 KPa. The VLE data were correlated by NRTL model [30]. The ability of ILs to separate the toluene-methanol mixture was analyzed with the aid of the σ-Profiles. COSMOtherm (version C2.1, release 01.11) software with BP_TZVPD_FINE_C30_1601 parameterization was used in this work which is based on the COSMO-RS model. The data predicted was obtained through density functional theory (DFT) calculations, utilizing the Becke-Perdew (BP) functional with the resolution of identity (RI) approximation and the Triple-ζ Zeta Valence Potential (TZVP) basis set. The ILs were treated as two individual ions in an equimolar mixture. The selectivity of ILs composed of these anions and cations at infinite dilution ( S )
were obtained by Ionic Liquids Screening of COSMOtherm. S separation efficiency of IL to some extent [31-33].
can represented the
2. Experimental 2.1. Chemicals Toluene (>99.5 mass %) and methanol (>99.7 mass %) were purchased from Sinopharm Group. The purities of the two reagents were confirmed by gas chromatography, and they were used without further purification. The ILs ([DMIM][OAC], [C14MIM][OAC] and [N8,8,8,1][OAC]) were provided by Yulu Group (>98.0 mass %). In order to remove volatile compounds, it is necessary for IL to be purified at 373 K and 2 kPa for 48 hours before the experiment. The water content in the ILs determined by Karl Fischer titration was below 500ppm. The specifications of the chemicals used are given in Table 1. Table 1
The specifications of chemical samples. Chemical name
source
Water Content (ppm).
Purity ( wt% )
Purification method
Analysis method
Toluenea
50
0.997
None
GCf KFg
50
0.995
None
GCf KFg
[DMIM][OAC]c
Sinopharm Group Sinopharm Group Yulu Group
500
0.980
KFg LCh
[C14MIM][OAC]d
Yulu Group
500
0.980
[N8,8,8,1][OAC]e
Yulu Group
500
0.980
Vacuum desiccation Vacuum desiccation Vacuum desiccation
Methanolb
a
KFg LCh KFg LCh
toluene: the experimental densities (at 298.15 K) is 0.8700 g/cm³, normal boiling point temperature is 383.75 K.[6] b methanol: the experimental densities (at 298.15 K) is 0.7918 g/cm³, normal boiling point temperature is 337.85 K. [6] c [DMIM] [OAC] = 1-decyl-3-methylimidazolium acetate d [C14MIM][OAC] = 1-tetradecyl-3-methylimidazolium acetate e [N8,8,8,1][OAC] = trioctyl methylammonium acetate f GC = gas chromatography. g KF = Karl Fischer titration. h
LC = liquid chromatography
2.2. Apparatus and Procedure
The equilibrium apparatus is an all-glass dynamic recirculating still (NGW, Wertheim, Germany) and has been described in a previous literature [34]. The still pressure was kept at 101.3 kPa by a pressure controller (MKS, USA) and measured by a manometer (TDGC2-0.5 CHNT China) with a standard uncertainty of 0.1 kPa. The equilibrium temperature was measured by a calibration thermometer with a standard uncertainty of 0.01 K. The VLE experimental sample are prepared by an electronic balance (PTX-FA300S) with a standard uncertainty of 0.0001 g. The toluene and methanol contents were measured using an Agilent gas chromatography (GC) (Model 7820A) connected to a headspace autosampler (G1888 Network Headspace Sampler, Agilent Technologies). The GC was equipped with an Agilent 19091 J-413 capillary column (0.32 mm in diameter, 30m in length, and 0.25 µm in thickness) and a flame ionization detector (FID). The temperature of the detector, column, and injector were 443 K, 339 K, and 423 K, respectively. N2 was the carrier gas in the experiment. The content of IL was determined by mass difference before and after volatile components were evaporated (373 K, 2 kPa, and 48h) due to the nonvolatility of IL. The standard uncertainty of IL mole fraction is 0.001. Each sample was analyzed at least three times.
2.3. Data treatment Because the experiment is conducted at low vapor phase pressure (101.3 kPa), the assumption of ideal vapor phase is made. The activity coefficient of component i ( ) can be computed by Eq. (1).
=
(1)
Where xi and yi are the mole fraction of component i in the liquid phase and the vapor phase, respectively. P is the total pressure (101.3 kPa), is the saturated vapor pressure of component i and calculated by the Antoine equation. =
−
+
(2)
In which Ai, Bi and Ci are the Antoine parameters of component i. t is equilibrium temperature in ℃. These parameters are listed in Table 2. Table 2 Antoine parameters for toluene and methanol.a Antoine parameters component A B C Tolueneb 6.955 Methanolb 8.072 a Antoine equation: ( /
1344.800 219.482 1574.990 238.870 ) = − /( /℃ + )
Temp range (K) 279.00-409.00 257.00-364.00
b
Parameters obtained from ref [35].
The relative volatility α12 is calculated by the following equation:
=
⁄ ⁄
=
(3)
1 indicates toluene and 2 indicates methanol. The average relative volatilities ( are calculated by Eq. (4) #% &
= "# %$& $
where
(
'
represents
(
)
(4)
the mole content of toluene without IL in liquid phase.
The binary interaction parameters (∆g12 and ∆g21) and the nonrandomness parameter (α12) for toluene-methanol system are regressed from the binary VLE data in Table 3 by the Levenberg-Marquardt method [36] through minimizing the objective function (ARD) according to Eq. (5), and the rest of the nonrandomness parameters (α13, α23) and binary interaction parameters (∆g13, ∆g31, ∆g23 and ∆g32) are regressed from the VLE data in Tables 4−6. )*(%) =
1 --. , 4
/#0
−
/#0
1231
. × 100 (5)
In which n represents the number of experiment points. the superscripts “exp” and “calc” represent experimental and calculated values, respectively.
3. Results and discussion 3.1. Experimental data To test the dependability of the equilibrium apparatus, the binary VLE data for toluene (1) + methanol (2) mixture were measured at 101.3 kPa and are shown in Table 3 and Fig. 1. The reported data [37] are also listed in Fig. 1. Where T presents the VLE temperature. It can be seen from Fig. 1 that the VLE data measured in this work are in good agreement with those reported in the literature. In addition, the experimental data are consistent with thermodynamic consistency. The area test shows that |D-J|= 8.90 < 10. The point test shows that the absolute deviation between ycalc and yexp is less than 0.01, and the absolute deviation between Pcalc and Pexp is less than 1.33 kPa. Therefore, the experimental method and the apparatus used in this study are reliable. The thermodynamically consistency of the literature data have also
been tested through area test method in Ref. [37]. The result of area test is: |D-J| = 8.32 < 10. Table 3
Isobaric VLE data for the binary system of toluene (1) + methanol (2) at 101.3kPaa
a
T/K
x1
y1
α12
γ₁
γ2
337.64 337.13 336.72 336.71 336.77 336.84 337.03 337.27 337.34 337.48 337.87 338.49 339.31 340.02 342.51 345.33 346.97 350.64 355.26 382.95
0.000 0.031 0.096 0.125 0.177 0.204 0.268 0.332 0.381 0.426 0.558 0.676 0.752 0.794 0.86 0.899 0.913 0.939 0.958 1.000
0.000 0.048 0.107 0.123 0.143 0.151 0.163 0.171 0.175 0.178 0.186 0.194 0.203 0.212 0.238 0.271 0.290 0.343 0.413 1.000
1.587 1.134 0.986 0.778 0.692 0.532 0.415 0.345 0.293 0.181 0.115 0.084 0.070 0.051 0.042 0.039 0.034 0.031
7.298 5.325 4.700 3.845 3.497 2.858 2.397 2.136 1.934 1.515 1.273 1.162 1.118 1.058 1.039 1.030 1.041 1.046
1.003 1.024 1.040 1.078 1.102 1.172 1.260 1.349 1.441 1.827 2.408 3.012 3.488 4.507 5.373 5.717 6.592 7.256
Standard uncertainty u(x1) =u(y1) = 0.001, u(T) = 0.01 K, u(P) =0.1 kPa.
Fig. 1. VLE data for the binary system of toluene (1) + methanol (2) at 101.3kPa: ●, experimental data in this work; △, from Ref [37]; solid line, correlated using NRTL model.
3.2 Ternary data The experimental VLE data for toluene (1) + methanol (2) + [DMIM][OAC] (3), [C14MIM][OAC] (3) or [N8,8,8,1][OAC] (3) are shown in Tables 4-6 and Figs. 2-4. The regressed parameters and the values of ARD are listed in Table 7. The correlated curves are drawn in Figs. 2-4. The values of ARD in the ternary systems for [C14MIM][OAC], [DMIM][OAC] and [N8,8,8,1][OAC] are 0.99%, 1.04% and 1.05%, respectively. The experimental data are quite well correlated by the NRTL model. Table 4
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [DMIM][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100
360.46 359.37 358.74 358.42 358.47
0.044 0.089 0.136 0.184 0.227
0.049 0.099 0.151 0.204 0.252
0.114 0.197 0.263 0.317 0.358
2.490 2.229 2.002 1.808 1.658
5.261 4.669 4.172 3.765 3.444
0.449 0.446 0.444 0.443 0.442
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
358.68 359.21 360.10 361.08 362.41 363.93 365.14 366.29 368.60 370.34 372.70 375.29 378.07 382.06 373.11 371.87 371.10 370.44 370.06 370.02 370.09 370.46 370.58 371.09 371.84 372.63 373.49 374.54 375.81 377.23 378.57 380.00 382.19 384.48 383.01 381.92 381.07 380.48 380.06 379.92 379.74 379.80 379.97 380.38
0.272 0.321 0.370 0.413 0.462 0.509 0.555 0.585 0.646 0.689 0.742 0.790 0.835 0.880 0.043 0.086 0.129 0.171 0.216 0.262 0.302 0.343 0.384 0.430 0.475 0.517 0.560 0.600 0.647 0.690 0.728 0.768 0.824 0.038 0.081 0.123 0.167 0.210 0.257 0.300 0.347 0.390 0.434 0.479
0.302 0.357 0.411 0.459 0.513 0.565 0.617 0.650 0.718 0.766 0.824 0.878 0.928 0.978 0.051 0.101 0.152 0.201 0.254 0.308 0.355 0.404 0.452 0.506 0.559 0.608 0.659 0.706 0.761 0.812 0.857 0.904 0.969 0.048 0.101 0.154 0.209 0.263 0.321 0.375 0.434 0.488 0.542 0.599
0.397 0.436 0.472 0.503 0.537 0.571 0.606 0.629 0.679 0.719 0.772 0.828 0.889 0.962 0.118 0.206 0.279 0.338 0.394 0.445 0.486 0.525 0.563 0.604 0.643 0.679 0.716 0.751 0.793 0.832 0.869 0.909 0.969 0.110 0.207 0.288 0.359 0.421 0.480 0.530 0.582 0.626 0.669 0.712
1.524 1.394 1.282 1.193 1.103 1.023 0.955 0.912 0.832 0.781 0.721 0.668 0.619 0.565 2.484 2.313 2.161 2.034 1.912 1.801 1.715 1.632 1.563 1.488 1.420 1.361 1.306 1.255 1.200 1.150 1.108 1.066 1.009 2.448 2.327 2.221 2.122 2.036 1.954 1.883 1.816 1.759 1.707 1.654
3.163 2.887 2.635 2.434 2.228 2.045 1.913 1.817 1.652 1.552 1.441 1.342 1.255 1.148 3.716 3.413 3.143 2.940 2.743 2.556 2.415 2.269 2.166 2.043 1.924 1.823 1.729 1.639 1.545 1.458 1.387 1.319 1.231 2.804 2.622 2.464 2.322 2.199 2.080 1.976 1.883 1.799 1.721 1.638
0.442 0.441 0.437 0.433 0.428 0.423 0.423 0.420 0.418 0.418 0.418 0.419 0.422 0.420 0.313 0.309 0.305 0.304 0.301 0.298 0.296 0.292 0.291 0.288 0.284 0.280 0.277 0.273 0.269 0.264 0.260 0.257 0.253 0.236 0.233 0.230 0.227 0.224 0.221 0.218 0.215 0.212 0.209 0.205
0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
380.95 381.56 382.04 383.00 383.63 384.57 384.82 385.68
0.522 0.569 0.603 0.659 0.692 0.732 0.742 0.777
0.653 0.711 0.754 0.824 0.865 0.915 0.928 0.971
0.752 0.793 0.824 0.874 0.903 0.939 0.948 0.979
1.607 1.561 1.530 1.480 1.453 1.419 1.410 1.382
1.560 1.486 1.436 1.355 1.310 1.254 1.240 1.194
0.201 0.197 0.194 0.189 0.186 0.182 0.181 0.178
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 5
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [C14MIM][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150
364.03 363.07 362.45 362.08 361.95 362.01 362.23 362.66 363.16 363.28 364.10 365.17 366.24 368.43 370.58 372.29 374.22 376.51 381.88 378.06 377.08 376.32 375.77 375.38 375.14 375.02 375.04 375.17 375.40
0.048 0.097 0.145 0.201 0.218 0.254 0.305 0.385 0.425 0.437 0.483 0.523 0.562 0.626 0.686 0.724 0.760 0.803 0.881 0.041 0.086 0.136 0.180 0.218 0.279 0.318 0.324 0.369 0.414
0.053 0.108 0.161 0.223 0.242 0.282 0.339 0.428 0.472 0.486 0.537 0.581 0.624 0.695 0.762 0.804 0.844 0.892 0.979 0.048 0.101 0.160 0.212 0.256 0.328 0.374 0.381 0.434 0.487
0.095 0.172 0.234 0.296 0.314 0.349 0.396 0.466 0.500 0.511 0.550 0.583 0.617 0.673 0.731 0.769 0.808 0.859 0.968 0.080 0.155 0.230 0.289 0.336 0.407 0.451 0.458 0.506 0.553
1.882 1.721 1.592 1.465 1.431 1.363 1.278 1.167 1.119 1.104 1.053 1.009 0.969 0.905 0.849 0.813 0.778 0.737 0.658 1.717 1.639 1.565 1.510 1.469 1.409 1.376 1.371 1.337 1.306
3.628 3.323 3.086 2.853 2.797 2.664 2.498 2.298 2.199 2.173 2.062 1.953 1.859 1.701 1.576 1.491 1.407 1.322 1.160 2.303 2.199 2.098 2.024 1.971 1.880 1.833 1.824 1.764 1.707
0.408 0.409 0.411 0.413 0.415 0.415 0.415 0.417 0.416 0.417 0.414 0.409 0.405 0.396 0.389 0.384 0.378 0.373 0.365 0.279 0.279 0.279 0.280 0.280 0.279 0.278 0.278 0.275 0.273
0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.150 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200
375.74 376.23 376.39 377.04 377.93 379.07 380.14 381.14 383.53 390.46 389.50 388.69 388.02 387.47 387.04 386.70 386.45 386.29 386.20 386.17 386.27 386.40 386.56 386.83 387.06 387.39 387.58 387.90
0.477 0.530 0.541 0.576 0.620 0.665 0.709 0.745 0.824 0.042 0.079 0.130 0.177 0.215 0.262 0.311 0.346 0.380 0.426 0.518 0.569 0.595 0.626 0.670 0.741 0.750 0.753 0.786
0.561 0.624 0.637 0.678 0.729 0.782 0.834 0.876 0.969 0.053 0.099 0.162 0.221 0.269 0.328 0.389 0.433 0.475 0.532 0.648 0.711 0.744 0.782 0.838 0.886 0.938 0.959 0.982
0.618 0.673 0.683 0.718 0.761 0.807 0.851 0.888 0.971 0.080 0.144 0.224 0.294 0.347 0.410 0.472 0.516 0.556 0.610 0.715 0.770 0.798 0.829 0.875 0.913 0.953 0.969 0.987
1.268 1.239 1.229 1.208 1.186 1.163 1.141 1.120 1.078 1.564 1.531 1.494 1.465 1.445 1.424 1.406 1.396 1.387 1.377 1.362 1.358 1.356 1.354 1.352 1.354 1.350 1.344 1.341
1.639 1.582 1.547 1.497 1.444 1.390 1.338 1.286 1.187 1.576 1.550 1.507 1.474 1.454 1.426 1.398 1.381 1.364 1.339 1.288 1.261 1.245 1.224 1.197 1.158 1.141 1.125 1.096
0.270 0.266 0.262 0.258 0.253 0.248 0.243 0.238 0.228 0.206 0.207 0.207 0.207 0.207 0.206 0.205 0.204 0.202 0.200 0.195 0.191 0.189 0.186 0.182 0.176 0.174 0.172 0.168
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 6
The experimental VLE data of the ternary system toluene (1) + methanol (2) + [N8,8,8,1][OAC] (3) at 101.3kPaa x3
T/K
x1
x'1
y1
α
γ1
γ2
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100
358.64 358.41 358.34 358.42 358.65 359.00 359.49 360.10 360.84
0.044 0.094 0.143 0.193 0.242 0.283 0.332 0.364 0.422
0.049 0.104 0.159 0.214 0.269 0.314 0.369 0.404 0.469
0.051 0.103 0.151 0.196 0.239 0.274 0.316 0.343 0.394
1.053 0.993 0.941 0.895 0.855 0.825 0.791 0.771 0.737
2.529 2.411 2.309 2.220 2.141 2.076 2.007 1.948 1.883
0.511 0.517 0.523 0.528 0.533 0.536 0.539 0.537 0.542
0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.200 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300
361.88 362.71 363.63 365.15 366.62 368.27 370.65 374.26 378.34 382.38 381.17 381.05 381.01 381.05 381.15 381.31 381.54 381.84 382.19 382.62 383.11 383.68 384.26 384.97 385.60 386.38 387.00 388.17 389.13 399.55 399.63 399.74 399.94 400.13 400.24 400.50 400.59 400.95 401.35 401.50 401.70 402.05 402.35 402.60
0.490 0.521 0.553 0.602 0.643 0.683 0.724 0.782 0.832 0.877 0.039 0.082 0.126 0.168 0.204 0.247 0.290 0.326 0.369 0.412 0.455 0.514 0.558 0.601 0.628 0.663 0.690 0.750 0.790 0.034 0.072 0.111 0.150 0.189 0.220 0.258 0.282 0.328 0.381 0.405 0.430 0.468 0.501 0.531
0.544 0.579 0.614 0.669 0.714 0.759 0.804 0.869 0.924 0.974 0.049 0.103 0.157 0.210 0.255 0.309 0.363 0.407 0.461 0.515 0.569 0.643 0.697 0.751 0.785 0.829 0.863 0.937 0.988 0.049 0.103 0.159 0.214 0.270 0.314 0.369 0.403 0.469 0.544 0.578 0.614 0.669 0.715 0.759
0.456 0.485 0.516 0.566 0.609 0.655 0.703 0.783 0.861 0.947 0.040 0.085 0.131 0.177 0.216 0.265 0.315 0.357 0.409 0.463 0.518 0.595 0.653 0.712 0.750 0.799 0.838 0.925 0.986 0.033 0.072 0.115 0.159 0.207 0.246 0.297 0.330 0.396 0.474 0.511 0.550 0.612 0.664 0.715
0.703 0.686 0.670 0.645 0.624 0.603 0.578 0.543 0.509 0.477 0.812 0.809 0.807 0.806 0.806 0.806 0.807 0.808 0.809 0.811 0.813 0.817 0.819 0.821 0.821 0.822 0.821 0.825 0.826 0.667 0.676 0.685 0.695 0.705 0.713 0.723 0.729 0.741 0.756 0.762 0.769 0.780 0.789 0.798
1.814 1.766 1.717 1.646 1.585 1.522 1.434 1.322 1.211 1.124 1.103 1.115 1.126 1.136 1.143 1.150 1.156 1.158 1.160 1.161 1.159 1.162 1.158 1.150 1.134 1.120 1.101 1.091 1.074 0.631 0.650 0.669 0.685 0.703 0.717 0.732 0.742 0.758 0.775 0.783 0.790 0.799 0.806 0.812
0.548 0.546 0.543 0.540 0.536 0.532 0.520 0.509 0.495 0.488 0.282 0.286 0.289 0.292 0.294 0.296 0.297 0.297 0.297 0.296 0.295 0.294 0.292 0.289 0.285 0.280 0.276 0.272 0.267 0.192 0.195 0.197 0.199 0.202 0.203 0.205 0.206 0.206 0.207 0.207 0.207 0.207 0.206 0.205
0.300 0.300 0.300 0.300
403.00 403.43 403.90 404.19
0.564 0.601 0.639 0.675
0.805 0.859 0.913 0.964
0.769 0.833 0.897 0.957
0.807 0.819 0.830 0.841
0.815 0.818 0.819 0.822
0.203 0.201 0.199 0.197
a
Standard uncertainty u (x1) = u (y1) = 0.001, u (T) = 0.01 K, u (P) = 0.1 kPa, y3 = 0 due to the extremely low volatility of the ionic liquid. Table 7
Parameters and correlation statistics for the NRTL model
component
component
∆gij
∆gji
ARD
(J/mol)
(J/mol)
(%)
αij i toluene
j methanol
0.4370
4290.3
3857.3
0.83 1.04
toluene
[DMIM][OAC]
0.4589
-12640.1
62355.0
methanol
[DMIM][OAC]
0.2834
-7504.0
29869.2
toluene
[C14MIM][OAC]
0.4383
-13726.0
62354.9
methanol
[C14MIM][OAC]
0.1967
-12818.3
45652.0
toluene
[N8,8,8,1][OAC]
0.4679
-12584.1
20311.1
methanol
[N8,8,8,1][OAC]
0.2028
-12513.1
46674.8
1.15
1.05
Fig. 2. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [DMIM] [OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.15; ▲, x3 = 0.20; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 3. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [C14MIM][OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.15; ▲, x3 = 0.20; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 4. x'1-y1 diagram for the ternary system of toluene (1) + methanol (2) + [N8,8,8,1][OAC] (3) at 101.3 kPa: ■, x3 = 0.10; ●, x3 = 0.20; ▲, x3 = 0.30; dotted line, IL-free system; solid line, correlated using NRTL model.
It can be seen from Figs. 2-4, the effects of the three ILs on the azeotropic point of the toluene-methanol mixture are different. For [DMIM][OAC] and [C14MIM][OAC], the azeotropic point is shifted upward by increasing the mole fraction of IL, and the azeotropy is completely eliminated when the mole fraction of IL is increased to a specific value [38-40]. After the azeotropic phenomenon is eliminated, the equilibrium content of toluene in the vapor phase (y1) is higher than that in the liquid phase (x'1). Therefore, toluene becomes a light component when the two ILs are used. For [N8,8,8,1][OAC], the azeotropic point is moved downward by increasing the mole fraction of IL, and the azeotropy is completely eliminated when the mole fraction of IL is increased to a specific value. After the azeotropic phenomenon is eliminated, the equilibrium content of toluene in the vapor phase (y1) is lower than that in the liquid phase (x'1). Therefore, toluene becomes a heavy component when [N8,8,8,1][OAC] is used. According to the NRTL model, The minimum mole fractions of [DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC] needed to eliminate the azeotrope of the toluene-methanol mixture are 0.1519, 0.1401 and 0.1299, respectively. The abilities of [DMIM][OAC] and [C14MIM][OAC] to eliminate azeotropy are very close. It is not sufficient to evaluate the separation abilities of the two ILs by only using their abilities to eliminate azeotropy [41]. Further research is needed.
Fig. 5. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.10: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 6. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.15: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 7. The relative volatility of toluene (1) to methanol (2) with different ILs (3) at the mole fraction of 0.20: ●, [C14MIM][OAC]; ▲, [DMIM][OAC]; dotted line, IL-free system; solid line, correlated using NRTL model.
Fig. 8. The relative volatility of toluene (1) to methanol (2) with different mole fractions of [N8,8,8,1][OAC] (3) at 101.3 kPa: ■, x3 =0.10; ●, x3 =0.20; ▲, x3=0.30; dotted line, IL-free system; solid line, correlated using the NRTL model.
In order to further compare the separation ability of [DMIM][OAC] and [C14MIM][OAC], the relative volatilities (α12) of toluene to methanol caused by [DMIM][OAC] and [C14MIM][OAC] are calculated and drawn in Figs. 5-7. The IL mole fractions are 0.10, 0.15 and 0.20, respectively. When the mole fraction of IL is 0.20, α12 value caused by [DMIM] [OAC] is significantly higher than that caused by [C14MIM][OAC]. When the mole fractions of ILs are 0.10 and 0.15, the intersection points of the x'1-α12 curves are generated because the curve corresponding to [DMIM][OAC] has a larger slope than that corresponding to [C14MIM][OAC]. The average relative volatility ( ) calculated by Eq. (4) is shown in table 8. The values corresponding to [DMIM][OAC] is higher than that corresponding to [C14MIM][OAC] at all the three ILs concentrations. From Table 8 and Fig. 7, it can be seen that the separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. The result is consistent with that predicted by COSMOtherm in the supplementary section. The larger slope of the x'1-α12 curves corresponding to [DMIM][OAC] than that corresponding to [C14MIM][OAC] is the reason why the minimum breaking azeotropic mole fraction of [DMIM][OAC] is greater than that of [C14MIM][OAC]. Table 8
The average relative volatility of toluene to methanol produced by different ILs.
Mole fraction of ILs 0.10 0.15 0.20
[DMIM][OAC]
[C14MIM][OAC]
1.388 1.664 1.839
1.165 1.339 1.412
The α12 caused by [N8,8,8,1][OAC] is calculated and shown in Fig. 8. The IL mole fractions are 0.10, 0.20 and 0.30, respectively. Because toluene is a heavy component, the α12 values are less than 1 when azeotrope is eliminated. The smaller the α12 value, the better the separation effect. It can be seen from Fig. 8, when the mole fraction of [N8,8,8,1][OAC] is 0.2 or 0.3, the azeotrope of the toluene-methanol mixture is completely eliminated. After the azeotrope is completely eliminated，the value calculated by Eq. (4) is shown in table 9. It can be observed that ( ) is decreased with the increase of [N8,8,8,1][OAC] concentration. Table 9
The average relative volatility of toluene to methanol produced by different ILs. Mole fraction of ILs [N8,8,8,1][OAC] 0.20 0.30
0.814 0.749
3.3 Analysis of the σ-profiles The ability of IL to separate the toluene-methanol azeotropic mixture can be explained by their σ-profiles [42-45]. The σ-profiles of toluene and methanol are displayed in Fig. 9, and the σ-profiles of [DMIM]+, [C14MIM]+, [N8,8,8,1]+ and [OAc]are displayed in Fig. 10. The cut-off value (dashed line) is -0.0082e / Å for the donor and 0.0082e / Å for the acceptor. The non-polar region is located from -0.0082 e / Å2 to 0.0082 e / Å2 [46,47].
Fig. 9. σ-Profiles for toluene and methanol.
Fig. 10. σ-Profiles for [DMIM]+, [C14MIM]+, [N8,8,8,1]+ and [OAc]- .
It can be observed from Fig. 9 and Fig. 10, the peaks of toluene are all in non-polar regions. The peaks of methanol are distributed in the donor, non-polar and acceptor regions. The peak value of toluene is obviously larger than that of methanol in the non-polar region. The peaks of anion in ILs ([OAc]-) are located in the non-polar region and the acceptor regions, while the peaks of cations in ILs ([DMIM]+, [C14MIM]+, and [N8,8,8,1]+) are distributed in the donor region and non-polar region. The peaks of the three cations are much larger than that of toluene and methanol in the non-polar region. The peak values of the three cations in the non-polar region are: [DMIM]+ < [C14MIM]+ < [N8,8,8,1]+. Because the peak of toluene is not appeared in the polar region, there is no intermolecular polar interaction between toluene and other molecules. The azeotrope of toluene-methanol mixture is due to the intermolecular non-polar interaction. Because the peak value of methanol is obviously smaller than that of toluene in the non-polar region, the intermolecular non-polar interaction between methanol and IL is also obviously smaller than that between toluene and IL. Therefore, the intermolecular interaction between methanol and IL is mainly characterized by polar interaction. The average relative volatility ( ) value of toluene to methanol is
increased with the increase of [DMIM][OAc] content, indicating that the intermolecular polar interaction between [DMIM][OAc] and methanol is larger than the intermolecular non-polar interaction between toluene and methanol and that between toluene and IL [23,25,41]. Since the anions of the three ILs are the same, their effect on the system to be separated is reflected in their cations. The peak of [C14MIM]+ is substantially coincident with that of [DMIM]+ except that [C14MIM]+ has larger peak value in the non-polar region. The nonpolar interaction between [C14MIM][OAc] and toluene is larger than that between [DMIM][OAc] and toluene because of the larger non-polar peak of [C14MIM]+, while the polar interaction between [C14MIM][OAc] and methanol is substantially the same as that between [DMIM][OAc] and methanol. Because the volatility of toluene is reduced by the lager nonpolar interaction and the volatility of methanol has hardly changed at the same time, the values of toluene to methanol caused by [C14MIM][OAc] are smaller than that caused by [DMIM][OAc]. The peak of [N8,8,8,1]+ is larger than that of [C14MIM]+ in the non-polar region, while the polar peak of [N8,8,8,1]+ is substantially the same as that of [C14MIM]+ and [DMIM]+. When [N8,8,8,1][OAc] is used as an entrainer, the non-polar interaction between IL and toluene continues to increase and the polar interaction between IL and methanol remains unchanged. When [N8,8,8,1][OAc] is added to the toluene-methanol solution, the toluene becomes a heavy component with greater interaction with the IL, indicating that the unaltered polar interaction between IL and methanol is exceeded by the increasing non-polar interaction between IL and toluene.
4. Conclusions Isobaric VLE data for the ternary systems of toluene + methanol + ILs ([DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC]) were measured at 101.3 kPa. The VLE data were well correlated with the NRTL model. Based on the NRTL model, the minimum mole fractions of [DMIM][OAC], [C14MIM][OAC], and [N8,8,8,1][OAC] needed to eliminate the azeotrope of the toluene-methanol mixture were 0.1519, 0.1401 and 0.1299, respectively. When [N8,8,8,1][OAC] was used as an entrainer, toluene was a heavy component and when [DMIM][OAC] or [C14MIM][OAC] was used as entrainer, toluene was a light component. After the toluene-methanol azeotrope was completely eliminated, the separation ability of [DMIM][OAC] is higher than that of [C14MIM][OAC]. Acknowledgments This work is financially supported by the National Science Foundation of China (Project No. 21576166), the National Science Foundation of China (Project No. 21978173) List of symbols xi
Mole fraction of solvent i in the liquid phase
x'i
The liquid-phase mole fraction of component i excluding IL
yi
Mole fraction of solvent i in the vapor phase
T
Equilibrium temperature
∆gij
Binary energy parameter of NRTL model
P
Total pressure in the equilibrium system
Pi0 Greek letters α12
Relative volatility of component 1 to component 2
αij
Non-randomness parameter of NRTL model
γi
Activity coefficient of component i
γiexptl
The activity coefficient of component i measured by experimental data
γicalcd
The activity coefficient of component i calculated with the NRTL model
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Author Contributions Section Wenxiu Li : Conceptualization, Formal analysis, Validation, Investigation, Writing - Original Draft, Ting Guan: Development or design of methodology; creation of models, Validation, Data Curation, Ying Cao: Development or design of methodology; creation of models Yu Zhang: Software, Tao Zhang: Conceptualization, Formal analysis, Investigation, Resources, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition