ARTICLE pubs.acs.org/jced
Isobaric VaporLiquid Equilibrium for Methanol + Dimethyl Carbonate + 1-Octyl-3-methylimidazolium Tetrafluoroborate Qunsheng Li,*,† Wei Zhu,† Yongquan Fu,† Haichuan Wang,† Lun Li,† and Baohua Wang‡ †
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 vaporliquid equilibrium data for {methanol (1) + dimethyl carbonate (2) + 1-octyl-3-methylimidazolium (3)} where 3 is an ionic liquid ([OMIM]+[BF4]) were obtained at 101.32 kPa using a modified Othmer still. The results indicated that [OMIM]+[BF4] can transfer the azeotropic point and totally eliminate the azeotropic phenomena when its mole fraction is up to x3 = 0.20. This shows that [OMIM]+[BF4] could be used as a alternative entrainer in the extractive distillation for methanol (1) + dimethyl carbonate (2) system. The measured ternary vaporliquid equilibrium data were fitted with the NRTL.
’ INTRODUCTION Extractive distillation is widely used in industry for azetropic and close-boiling mixtures.13 However, the traditional entrainers (solid salts and organic substances) used in extractive distillation comprise many disadvantages, which have been the handicaps for further application of extractive distillation. The solid salts may corrode the column and pipeline, and the organic solvents will not only give rise to volatile organic compound emission but also demand high energy. Fortunately, ionic liquid (IL) may be used as a suitable entrainer for its outstanding properties, such as nonvolatility, less causticity, and good performance of separation efficiency.46 Furthermore, the ILcontaining system thermodynamic data can be used for understanding and developing the IL-containing system thermodynamic models and for designing a new separation process. Dimethyl carbonate (DMC)713 has been drawing much attention as a safe, noncorrosive, and environmentally friendly building block for the production of polycarbonate and other chemicals. Also, DMC is of interest as an additive to fuel oil owing to a high octane number, reducing particulate emission from diesel engines. In addition, it has been used as an electrolyte in lithium batteries due to its high dielectric constant. Therefore, development of DMC production technology and fuel properties has become important for the oil and chemical industries. Industrial synthesis of DMC is often carried out by the gasoline route and the product (methanol + DMC) is an azeotrope. For the purpose of getting high purity product, some methods such as membrane separation, crystal in low temperature, distillation in high pressure, azeotropic distillation, and extractive distillation have been reported.14 Among them, extractive distillation method compared with others is considered to be a clear, safe and economical way. Moreover, r 2012 American Chemical Society
entrainer screening is an important step in extractive distillation before the distillation sequence is designed.1518 However, to the best of our knowledge, extractive distillation with ionic liquid as entrainer for the separation of methanol + DMC has not been reported yet. In this work, isobaric vaporliquid equilibrium (VLE) data for methanol + dimethyl carbonate system containing [OMIM]+[BF4] was presented at atmospheric pressure (101.32 kPa), and the effect of [OMIM]+[BF4] on the VLE of methanol + DMC system was also discussed.
’ EXPERIMENTAL SECTION Chemicals. The solvents used were DMC and methanol (AR grade, Beijing Chemical Reagents Company (China), mass fraction > 99.6 %). The purity of solvents was checked by GC (SP6890, China), and the solvents were used directly without further purification. The IL, [OMIM]+[BF4], was provided by Shanghai Cheng Jie Chemical Co. LTD (China). Its purity (mass fraction > 98 %) was observed by liquid chromatography. The water mass fraction content in the IL was xw < 0.054%, which was determined by Karl Fisher titration. Furthermore, the IL was dried before the experiment and reused after the experiment by a rotary evaporator (48 h, 403 K) under a vacuum condition. Apparatus and Procedure. A modified Othmer still was used for measuring the VLE data at atmospheric pressure (101.32 kPa). A detailed description of the equilibrium still is available in our previous publications.19,20 The GC (SP6900, China), which was used to analyze the equilibrium composition of vapor and liquid phase, was equipped Received: June 2, 2011 Accepted: August 3, 2011 Published: April 30, 2012 1602
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Table 1. VaporLiquid Equilibrium Data for the Methanol (1) + DMC (2) System at 101.32 kPa T/K
x1
y1
363.3
0.000
0.000
351.6 345.9
0.097 0.197
344.1
Table 2. VaporLiquid Equilibrium Data for the Ternary System Methanol (1) + DMC (2) + [OMIM]+[BF4] (3) at 101.32 kPa 100x3
T/K
x10
y1
γ1
γ2
R12
0.362 0.523
10.085
357.6
0.081
0.275
1.816
1.055
4.339
10.108
352.4
0.164
0.447
1.747
1.057
4.130
0.249
0.569
10.160
347.7
0.256
0.550
1.630
1.139
3.552
342.9
0.290
0.613
10.081
344.9
0.378
0.643
1.433
1.194
2.964
340.8
0.379
0.669
339.1
0.489
0.723
10.263 10.186
342.8 342.2
0.495 0.539
0.700 0.723
1.288 1.250
1.331 1.376
2.382 2.234
338.3
0.553
0.744
10.104
341.4
0.578
0.748
1.246
1.409
2.173
337.9
0.602
0.761
10.110
340.6
0.697
0.795
1.132
1.647
1.687
337.5 337.3
0.664 0.701
0.777 0.789
10.104
340.3
0.801
0.848
1.059
1.881
1.380
10.205
340.2
0.901
0.910
1.016
2.227
1.118
336.9
0.755
0.809
18.446
362.4
0.056
0.227
2.062
1.054
4.970
336.9
0.802
0.832
19.292
357.7
0.156
0.392
1.504
1.083
3.501
336.7
0.858
0.859
336.9
0.901
0.889
19.444 19.435
353.7 350.3
0.226 0.353
0.480 0.584
1.457 1.281
1.156 1.239
3.157 2.575
337.0
0.952
0.934
19.156
348.3
0.458
0.667
1.212
1.270
2.369
337.9
1.000
1.000
19.363
347.6
0.526
0.711
1.158
1.290
2.226
19.273
346.6
0.561
0.732
1.158
1.340
2.140
19.119
345.3
0.680
0.796
1.089
1.470
1.831
19.231
344.7
0.787
0.862
1.043
1.522
1.692
19.321
344.4
0.898
0.923
0.990
1.805
1.354
30.440 30.340
368.6 364.4
0.050 0.147
0.193 0.360
1.817 1.331
1.028 1.036
4.531 3.271
30.307
359.4
0.228
0.461
1.302
1.134
2.902
30.455
359.2
0.327
0.549
1.086
1.094
2.510
30.276
357.6
0.440
0.634
0.984
1.125
2.204
30.219
355.9
0.512
0.692
0.980
1.151
2.140
30.244
353.9
0.577
0.727
0.982
1.258
1.955
30.435
353.7
0.666
0.788
0.929
1.247
1.866
30.587 30.409
352.4 351.1
0.763 0.887
0.860 0.929
0.927 0.903
1.214 1.350
1.908 1.668
measuring the mass difference of prepared samples containing and without IL. An onoff pressure controller with an uncertainty of 0.10 kPa was used to keep the equilibrium pressure constant. Figure 1. Absolute deviations Δy1 = y(exptl) y(calcd) between the calculated by NRTL model and measured mole fraction methanol in the vapor phase for the binary system of methanol (1) + DMC (2) at 101.32 kPa: 9, this work with error bars representing the extended uncertainty; O, ref 21; ], ref 22.
with a TCD detector and a Porapak-Q chromatographic column. The GC column was 3 m long and 0.3 mm in diameter. The carrier gas was hydrogen, and its flow rate was 45 cm3 3 min1. The injector and oven temperature was 393 K, and the detector temperature was 453 K. The equilibrium temperature of Othmer still was measured by a calibrated thermometer with an uncertainty of 0.1 K. The amounts of solvents in the samples were quantified using correction factor obtained from a set of standard solution which was gravimetrically prepared by an electronic balance (Satorius) with an uncertainty of 0.1 mg. With this analysis method, the maximum deviation found between the measured and real composition in the samples was 0.002 mole fraction. The IL mole fraction content in the liquid phase was determined with gravimetric method by
’ RESULTS AND DISCUSSION The binary VLE for the systems of methanol + DMC were measured at 101.32 kPa. The VLE data for binary system of methanol (1) + DMC (2) are listed in Table 1 and compared with the literature data as shown in Figure 1. Our experimental results were in good agreement with those Luo et al.21 and Li et al.22 have reported. The maximum absolute deviations (Δy 1) between the calculated by NRTL model and measured mole fractions of methanol in the vapor phase are less than 0.025, Hence, the experimental apparatus used was reliable. Vaporliquid equilibria for the methanol (1) + DMC (2) + [OMIM]+[BF4] (3) system, at 101.32 kPa as well, were measured by trying to keep the IL mole fraction in the liquid constant in each of the ten series. The experiment results for the methanol + DMC system containing IL [OMIM]+[BF4] at IL mole fraction from x3 = 10 % to 30 % are listed in Table 2, in which x10 is on an IL-free basis. 1603
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Figure 2. Isobaric VLE diagram for methanol (1) + DMC (2) + [OMIM]+[BF4] (3) system at 101.32 kPa: b, x3 = 0; O, x3 = 0.10; Δ, x3 = 0.20; 0, x3 = 0.30; solid lines, correlated using the NRTL model.
Table 3. Estimated Values of Binary Interaction Parameters Δgij and Δgji in the NRTL Model i component
j component
methanol (1) DMC (2)
Rij
1
Δgij/J 3 mol
1
Δgji/J 3 mol
0.4724
3132.3
938.8
methanol (1) [OMIM]+[BF4] (3) 0.0980 DMC (2) [OMIM]+[BF4] (3) 0.3440
18131.2 15395.4
14228.1 6479.7
In order to study the ternary system of methanol (1) + DMC (2) + [OMIM]+[BF4] (3) thermodynamic behavior, activity coefficients of component i (γi) were used to express the effect of ionic liquid on the solution non-ideality. Besides, the relative volatility of methanol to DMC (R12) was used to indicate the salt effect of IL on methanol (1) + DMC (2). The equations for calculation γi and R12 are as follows, which have been reported in a previous paper.19,20 γi ¼
yi P xi PiS
R12 ¼
ð1Þ
y1 =x1 y2 =x2
ð2Þ
The results of calculated activity coefficients (γi) and relative volatilities (R12) are also given in Table 2. The NRTL model combined with the Marquardt method was used to correlate the binary and ternary VLE data as suggested in our previous works.23,24 During data fitting with NRTL model, the following objective function (average relative deviation, ARD) was used, which also has been reported in previous paper.19,20 exptl 1 γi γcalcd i ð3Þ ARDð%Þ ¼ 3 100 exptl n n γi
∑
In the NRTL model, all the parameters were obtained from data correlation. In this case, the binary interaction parameters of NRTL model were first obtained from the VLE data of methanol
Figure 3. Relative volatility of methanol (1) + DMC (2) at 101.32 kPa: b, x3 = 0 (IL-free); O, x3 = 0.10 ([OMIM]+[BF4]); Δ, x3 = 0.20 ([OMIM]+[BF4]); 0, x3 = 0.30 ([OMIM]+[BF4]); solid lines, correlated using the NRTL model.
(1) + DMC (2); the ARD is 1.70 % for the methanol (1) + DMC (2) system and 3.27% for methanol (1) + DMC (2) + [OMIM]+[BF4] system. The measured VLE containing the azeotropic system methanol + DMC and the IL [OMIM]+[BF4] are presented in a pseudobinary in Figure 2, where the liquid-phase composition of the low-boiling component is the amount of this substance in the volatile part of the liquid phase. The IL component is demonstrated for each curve separately. The x, y diagram is shown in Figure 2, and the T, x, y diagram is shown in Figure 4. To further investigate the salt effect of IL on methanol + DMC, relative volatilities of methanol to DMC were plotted in Figure 3 where the system contains (10 to 30) % mole fraction of IL. As shown in Figures 2 to 4, When x10 was above 0.6, [OMIM]+[BF4] showed a salting-out effect for methanol, and the relative volatility of methanol to DMC was increased by [OMIM]+[BF4]. With an increase in the IL component, the relative volatility of methanol to DMC increases (except for x10 below 0.6). And the salting-out effect follows the order: 30% > 20 % > 10 %. Furthermore, [OMIM]+[BF4] can totally eliminate the azeotropic phenomena at x3 = 0.20. With the addition of [OMIM]+[BF4],the azeotropic point (x1 ≈ 0.86) for methanol (1) + DMC (2) binary system is shifted upward. Figures 2 to 4 also indicate a very strong crossover effect of IL between salting-in and salting-out on the VLE of methanol + DMC system. The salting-out effect is only appreciable at methanol mole fraction in the liquid-phase higher than 0.6, whereas a sharp salting-in effect appears at lower composition. This effect is very common with ILs, and it has been pointed out in many papers.2533 In the methanol-rich region, an increase of the IL content leads to a higher methanol content in the vapor phase and, therefore, to a large relative volatility of methanol. This behavior is due to the selective interaction difference between ionic liquid and solvent molecules. ILs can be treated as a weak electrolyte in a solution of highly dielectric solvent.34 Plus [OMIM]+[BF4] has a long carbon chain, and thus its polarity is greatly reduced. In addition the polarity of methanol is greater than the polarity of DMC. Consequently, according to 1604
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leads to a decreased activity of DMC and an increased relative volatility of methanol to DMC. In the DMC-rich region, although the interaction between IL and methanol is smaller than IL and DMC, there is an interaction between IL and methanol. Therefore, when the IL content, x3, increases from 10 % to 30 %, more and more methanol was “bonded” by [OMIM]+[BF4] as the number of methanol molecule is relatively little, which results in the relative volatility of methanol to DMC decrease (at x10 below 0.6).
’ CONCLUSIONS The isobaric VLE data for methanol (1) + DMC (2) system as well as for the methanol (1) + DMC (2) + [OMIM]+[BF4] (3) system were measured at 101.32 kPa. The obtained VLE data can be used in designing a new separation process for methanol (1) + DMC (2) binary system in the future. Moreover, the VLE data were correlated using the NRTL model. The ARD was 1.70 % for the binary system and was 3.27 % for the ternary system. It is seen that the conventional NRTL model was applicable for modeling the VLE of IL-containing system. The results indicated that [OMIM]+[BF4] could eliminate the azeotropic phenomenon of methanol (1) + DMC (2) system and showed a very strong crossover effect on the VLE of methanol (1) + DMC (2) system. Therefore, [OMIM]+[BF4] could be used as a promising entrainer for the extractive distillation processes of methanol + DMC system. Besides, compared with the traditional entrainer, [OMIM]+[BF4] was nonvolatile, nonflammable, chemical stable, and reusable. ’ AUTHOR INFORMATION Corresponding Author
*Tel: +86 10 64433695. E-mail:
[email protected].
’ REFERENCES
Figure 4. T, x, y diagram for the ternary system of methanol (1) + DMC (2) containing [OMIM]+[BF4] (3) at different contents of IL: b, x1 = x10 (x3 = 0); O, y1 (x3 = 0); 2, x10 (x3 = 0.10); Δ, y1 (x3 = 0.10); 9, x10 (x3 = 0.20); 0, y1 (x3 = 0.20); 1, x10 (x3 = 0.30); 3, y1 (x3 = 0.30); solid lines, correlated using the NRTL model.
the principle of “like dissolves like”, the attractive interaction between IL and DMC is greater than IL and methanol, which
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