Experimental Measurement and Modeling of Ternary Vapor–Liquid

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Experimental Measurement and Modeling of Ternary Vapor−Liquid Equilibrium for Water + 1‑Propanol + 1‑Butyl-3-methylimidazolium Chloride Lianzhong Zhang,* Yuanyue Guo, Dongshun Deng, and Yun Ge Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China ABSTRACT: Isobaric vapor−liquid equilibrium (VLE) data for the ternary system water + 1-propanol + 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) are reported. Complete T, x, y data were measured over a relatively wide range of ionic liquid (IL) mass fractions up to 0.8. The experimental pressure varied from (30 to 100) kPa, depending on the IL mass fraction. Six data sets were obtained in which the 1-propanol liquid-phase mole fraction on an IL-free basis (x2′ ) was fixed at a value in the range from 0.1 to 0.98. With the addition of [bmim][Cl], the azeotrope of water + 1-propanol was removed at an IL mass fraction of 0.5. The electrolyte nonrandom two-liquid (eNRTL) activity coefficient model was used for correlation and found to be adequate for the ternary system over the experimental composition range. The ternary VLE behavior was also modeled by correlation of two data sets in which the mole fractions of the volatile quasi-binary pair were set at the two diluted ends. In this way, the six data sets were reproduced satisfactorily, with rootmean-square deviations of 0.93 K for temperature and 0.0082 for the vapor-phase mole fractions.

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INTRODUCTION In the past decade, ionic liquids (ILs) have received considerable attention for their use in extractive distillation.1−3 Compared with inorganic salts or conventional organic solvents, ILs are effectively nonvolatile and have a large liquidus range. The properties of ILs can be tuned for specific applications by using different anions and cations. Usually, with the addition of an IL, azeotropic components change their nonideality to a different extent. It is desirable that these changes lead to removal of the azeotrope. Therefore, it is necessary to understand the influence of the IL on the VLE behavior. In the literature, predictive methods for the selection of ILs for extractive distillation have been reported.4,5 At the present stage, however, such information is mainly obtained from experimental measurement and correlation of VLE data using a suitable activity coefficient model.6−10 As a continuation of our research on VLE behavior of aqueous mixtures containing ILs, we present in this paper isobaric VLE data for the ternary system water + 1-propanol + 1-butyl-3-methylimidazolium chloride ([bmim]Cl). One aim of this work is to show the composition dependence of the activity coefficients of the volatile components, namely, water and 1propanol. The water + 1-propanol mixture forms a minimum-boilingpoint azeotrope at a 1-propanol mole fraction of 0.43. ILs may be used as an alternative solvent for extractive distillation. In the literature, Orchillés et al.11,12 reported the VLE behavior of the azeotropic mixture at p = 100 kPa in the presence of three ILs © XXXX American Chemical Society

having a trifluoromethanesulfonate anion and different cations [1-ethyl-3-methylimidazolium (emim), 1-butyl-3-ethylimidazolium (beim), and 1-butyl-1-methylpyrrolidinium (bmpyr)]. The azetrope could not be removed over the experimental range of IL mole fractions up to 0.30 for [emim][triflate]11 and 0.31 for [beim][triflate] and [bmpyr][triflate].12 For the same azeotropic mixture, Zhang et al.13 reported ternary VLE data containing, respectively, 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). With the addition of [emim][BF4], the azeotrope could be removed at an IL mass fraction of 0.700. In the case of addition of [bmim][BF4], however, a larger amount of IL would be required to achieve the same effect. 1-Butyl-3-methylimidazolium chloride ([bmim]Cl) is inexpensive and easy to prepare. In our previous work,14 we showed that [bmim]Cl can remove the water + 2propanol azeotrope with a minimum mass fraction of 0.223. Consequently, another aim of this work is to show the performance of [bmim]Cl for breaking the azeotrope of water + 1-propanol.

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EXPERIMENTAL SECTION Materials. Water was double-distilled. 1-Propanol and 1chlorobutane were purchased from Sinopharm Chemical Received: July 5, 2012 Accepted: December 12, 2012

A

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Table 1. Experimental VLE Data for Temperature (T), Pressure (p), Liquid-Phase Mole Fraction on an IL-Free Basis (x′), Liquid-Phase Mass Fraction (w), and Vapor-Phase Mole Fraction (y) and Calculated Activity Coefficients (γ) and Relative Volatilities (α) for the Ternary System Water (1) + 1-Propanol (2) + [bmim]Cl (3)a x2′

w3

y2

T/K

p/kPa

γ1

γ2

α2,1

29.23 30.18 30.05 41.13 100.52 100.48 100.43 100.50

0.427 0.639 0.749 0.919 0.999 1.044 1.061 1.064

1.655 2.599 3.080 3.917 4.106 4.552 5.043 5.303

4.142 4.175 4.133 4.301 4.408 4.646 5.040 5.274

29.49 30.90 30.00 39.72 100.09 100.04 99.76 99.60

0.349 0.570 0.760 0.913 1.046 1.101 1.158 1.195

1.225 1.813 2.322 2.634 2.765 2.820 2.859 2.852

3.784 3.289 3.051 2.896 2.826 2.724 2.614 2.519

30.07 29.95 30.19 39.95 100.04 99.94 99.94 99.97

0.281 0.484 0.696 0.915 1.119 1.242 1.357 1.462

0.740 1.073 1.409 1.573 1.695 1.706 1.705 1.644

2.881 2.323 2.052 1.743 1.624 1.463 1.331 1.187

29.77 28.25 29.47 40.62 100.22 100.03 100.00 99.67

0.237 0.370 0.599 0.863 1.115 1.367 1.629 1.830

0.572 0.761 0.992 1.174 1.179 1.240 1.275 1.244

2.661 2.184 1.705 1.399 1.146 0.974 0.833 0.719

30.35 29.75 28.85 39.47 99.84 99.76 99.76 99.76

0.180 0.330 0.504 0.786 1.075 1.407 1.714 2.144

0.429 0.608 0.714 0.895 0.939 1.047 1.078 1.099

2.653 1.978 1.485 1.188 0.957 0.806 0.677 0.547

30.24 29.99 30.00 40.64 100.06 99.99 99.96 99.98

0.139 0.244 0.472 0.733 1.195 1.585 2.019 2.663

0.348 0.462 0.609 0.730 0.846 0.899 0.961 0.984

2.804 2.072 1.370 1.057 0.780 0.622 0.518 0.400

x′2 = 0.1 0.1001 0.0998 0.0998 0.0997 0.0999 0.0998 0.0998 0.1000

0.799 0.700 0.600 0.500 0.400 0.300 0.200 0.100

0.3155 0.3163 0.3143 0.3227 0.3286 0.3399 0.3584 0.3694

366.11 352.67 346.77 347.77 367.12 364.71 362.94 362.00

0.2007 0.2005 0.2001 0.1998 0.1993 0.2002 0.1999 0.2000

0.799 0.700 0.600 0.500 0.400 0.300 0.200 0.100

0.4872 0.4521 0.4328 0.4196 0.4130 0.4054 0.3950 0.3864

368.92 354.62 345.38 346.71 365.71 363.82 362.18 361.16

0.3999 0.3989 0.3996 0.3993 0.3999 0.4004 0.4001 0.4000

0.800 0.700 0.600 0.500 0.400 0.300 0.200 0.100

0.6575 0.6065 0.5772 0.5367 0.5197 0.4941 0.4703 0.4417

375.44 358.80 348.84 349.24 367.01 364.46 362.44 361.17

0.5955 0.6001 0.6007 0.5994 0.5995 0.6010 0.5998 0.5996

0.801 0.699 0.600 0.500 0.400 0.300 0.200 0.100

0.7966 0.7662 0.7195 0.6767 0.6318 0.5946 0.5552 0.5185

379.34 363.22 353.17 353.14 371.71 367.40 364.00 362.06

0.8008 0.7984 0.7994 0.7997 0.7999 0.7999 0.8002 0.8004

0.800 0.700 0.600 0.500 0.400 0.300 0.200 0.100

0.9143 0.8868 0.8555 0.8258 0.7928 0.7631 0.7304 0.6868

386.15 368.40 358.91 357.58 376.76 371.21 368.08 365.07

0.9723 0.9799 0.9802 0.9804 0.9800 0.9802 0.9800 0.9803

0.800 0.700 0.600 0.500 0.400 0.300 0.200 0.100

0.9899 0.9902 0.9854 0.9814 0.9746 0.9685 0.9621 0.9522

390.94 374.84 363.23 363.20 380.44 376.68 373.28 371.21

x′2 = 0.2

x′2 = 0.4

x′2 = 0.6

x′2 = 0.8

x′2 = 0.98

a

u(T) = 0.08 K, u(p) = 0.05 kPa, ur(x′2) = 0.01, u(w3) = 0.003, ur(y2) = 0.01.

Reagent Co. Ltd. and used without further purification. Their mass-fraction purities were checked by GC and found to be

above 0.995. The water content of 1-propanol was 480 ppm as determined by Karl Fischer analysis. 1-Methylimidazole was B

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purchased from Yancheng Medical Chemical Factory and had a mass-fraction purity of 0.995. [bmim][Cl] was prepared and purified as described previously.14 The water content of the IL was 1200 ppm. The IL was also checked by electrospray ionization mass spectrometry (ESI-MS) and exhibited a single positive ion at m/z 139 ([bmim]). The mass-fraction purity of the IL was estimated to be 0.98. Apparatus and Procedure. The experimental apparatus and procedures for the VLE measurements as well as the uncertainties have been described in detail in our previous works.14−16 Vapor-phase compositions were determined by measuring the water content using Karl Fischer analysis (SF-3 Titrator, Zibo Zifen Instrument Ltd.). When the water mole fraction was higher than 0.1, the sample was first diluted with 1propanol quantitatively, and the water content of the mixture was analyzed. The vapor-phase composition was then calculated from the dilution ratio and the measured water content. The uncertainty in the vapor-phase composition was estimated to be 0.0001 in water mole fraction or a relative uncertainty of 1 %, whichever is greater. As discussed in our previous work,14 the uncertainty in the IL mass fraction (w3) was estimated to be ± 0.003, and the relative uncertainties in the molar composition of the volatile binary pair (x2′ and x1′ ) were less than 1 %.

N

F=

∑ n=1

− 1)2 (γ1,calcd /γ1,exptl n n N

N

+

∑

− 1)2 (γ2,calcd /γ2,exptl n n N

n=1

(1)

where N is the number of data points. With the obtained parameters, ternary VLE data were calculated and compared with the experimental values. The results are shown in Table 2, Table 2. Root-Mean-Square Deviations δT and δy in the Calculation of VLE Data for the Water (1) + 1-Propanol (2) + [bmim]Cl (3) System Based on Correlations Using the eNRTL Modela data sets used in the correlation

δT/Kb

δyc

all six data sets in Table 1 the x′2 = 0.1 and 0.98 data sets

0.92 0.93

0.0074 0.0082

a

For both data sets used in the correlation, the root-mean-square deviations were calculated using the six data sets in Table 1. bδT/K = [∑(Tcalcd/K − Texptl/K)2/N]1/2. cδy = [∑(y2,calcd − y2,exptl)2/N]1/2.

in which δT and δy are the root-mean-square deviations of the equilibrium temperature and vapor-phase mole fractions, respectively. The eNRTL model with temperature-independent parameters appeared to be adequate for the ternary system over the experimental temperature and composition ranges used in this work, with δT = 0.92 K and δy = 0.0074. In our previous work,21 we proposed a procedure for modeling ternary VLE behavior that involves correlating two ternary data sets in which the mole fractions of the volatile quasi-binary pair are distributed at the two diluted ends. This procedure was also used for the present ternary system. A correlation using the two data sets for x′2 = 0.1 and x′2 = 0.98 was used to determine the optimized binary energy parameters (Table 3), which were used to calculate the ternary VLE data.

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RESULTS AND DISCUSSION Table 1 lists the experimental VLE data for the ternary system water (1) + 1-propanol (2) + [bmim]Cl (3), including the liquid-phase mole fraction of 1-propanol on an IL-free basis (x2′ ), the mass fraction of [bmim]Cl (w3), the vapor-phase mole fraction of 1-propanol (y2), the equilibrium temperature (T), the equilibrium pressure (P), the activity coefficients of water (γ1) and 1-propanol (γ2), and the volatility of 1-propanol relative to water (α2,1). The experimental measurements were designed to be distributed regularly at eight values of w3 (from 0.8 down to 0.1 in an interval of 0.1) and six values of x′2 (0.1, 0.2, 0.4, 0.6, 0.8, and 0.98). In the calculation of the activity coefficients, the vapor phase was regarded as an ideal gas and the saturated vapor pressures of water and 1-propanol were calculated using parameters taken from the literature.17 In the measurements, the pressure was kept at 30 kPa for w3 = 0.8, 0.7, and 0.6; 40 kPa for w3 = 0.5; and 100 kPa for w3 = 0.4 to 0.1. As has been commonly recognized, the activity coefficients in liquid mixtures depend strongly on composition but often depend only weakly on temperature and very weakly on pressure. Unless the pressure is high, we can usually neglect the effect of pressure on activity coefficients.18 In the present measurements, the pressure varied from (30 to 100) kPa, and the temperature varied from (345 to 391) K. Therefore, the present measurements provide mainly the composition dependence of the activity coefficients. The ternary VLE data were correlated using the electrolyte nonrandom two-liquid (eNRTL) model,19 for which expressions for the liquid-phase activity coefficients of volatile components in a ternary system containing a salt have been presented by Vercher et al.20 In the correlation, the binary energy parameters and nonrandomness factor α12 for the water + 1-propanol system were taken from the literature.20 For simplicity of application, the values of the nonrandomness factors α13 and α23 were set to 0.3. The remaining four binary energy parameters corresponding to water + [bmim]Cl and 1propanol + [bmim]Cl were obtained by minimization of the following objective function:

Table 3. Binary Energy Parameters (Δgij and Δgji) and Nonrandomness Factors (αij) for the eNRTL Model Obtained from the Correlation of the Ternary VLE Data for the Water (1) + 1-Propanol (2) + [bmim]Cl (3) System Using the Data Sets for x2′ = 0.1 and x2′ = 0.98a component i

component j

Δgij/J·mol−1

Δgji/J·mol−1

αij

water 1-propanol

[bmim]Cl [bmim]Cl

−4497.5 −3515.8

−2536.7 402.2

0.3 0.3

a

The binary energy parameters and nonrandomness factor for water (1) + 1-propanol (2) were fixed at Δg12 = 7896.7 J·mol−1, Δg21 = 1648.8 J·mol−1, and α12 = 0.477; these values were taken from ref 20.

These were then compared with the entire set of experimental data (all six data sets in Table 1) by calculating the root-meansquare deviations. As shown in Table 2, the results were quite good, with δT = 0.93 K and δy = 0.0082. Such deviations have the same magnitude as those obtained by direct correlation of the six data sets. The two data sets at x2′ = 0.1 and x2′ = 0.98 seem to be adequate for modeling the VLE behavior over the experimental composition range used in this work. The calculated VLE results and the experimental values are shown in Figures 1 to 3. Figure 1 shows γ1 and γ2 as functions of x′2 at various fixed IL mass fractions. The experimental and calculated values are in good agreement. For best illustration, typical results at w3 = 0 (no IL), 0.1, 0.3, 0.5, 0.7, and 0.8 are presented. The results demonstrate that γ2 decreases with increasing x′2 at all of the given w3 values, whereas γ1 varies with C

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Figure 1. Experimental and calculated activity coefficients of (a) water (γ1) and (b) 1-propanol (γ2) as functions of the liquid-phase mole fraction of 1-propanol on an IL-free basis (x′2) for the saturated mixture water (1) + 1-propanol (2) + [bmim]Cl (3): ●, w3 = 0.1, p = 100 kPa; ■, w3 = 0.3, p = 100 kPa; ▲, w3 = 0.5, p ≈ 40 kPa; ▼, w3 = 0.7, p ≈ 30 kPa; ◆, w3 = 0.8, p ≈ 30 kPa. Lines were calculated using the eNRTL model with the parameter values shown in Table 3: solid lines, calculated values for w3 = 0.1, 0.3, 0.5, 0.7, and 0.8 at the relevant pressures; dashed line, calculated values for the water (1) + 1-propanol (2) system at p = 100 kPa.

Figure 3. Composition diagram for the vapor−liquid equilibrium of water (1) + 1-propanol (2) + [bmim]Cl (3): ○, w3 = 0.1, p = 100 kPa; □, w3 = 0.3, p = 100 kPa; ◆, w3 = 0.5, p ≈ 40 kPa; ●, w3 = 0.7, p ≈ 30 kPa; ■, w3 = 0.8, p ≈ 30 kPa. Lines were calculated using the eNRTL model with the parameter values shown in Table 3: solid lines, calculated values for w3 = 0.1, 0.3, 0.5, 0.7, and 0.8 at the relevant pressures; dashed line, calculated values for the water (1) + 1-propanol (2) system at p = 100 kPa.

water + 1-propanol azeotrope is shown in Figure 3. The IL has a salting-out effect in the 1-propanol-rich region, and a saltingin effect in the water-rich region. The addition of [bmim]Cl moves the azeotropic point from x2′ = 0.43 to higher values and breaks the azeotrope at an IL mass fraction of 0.5, which is equivalent to an IL mole fraction of 0.2. The amount of [bmim] Cl needed is much less than that of [emim][BF4] (mass fraction of 0.7),13 [bmim][BF4] (mass fraction of more than 0.7),13 [emim][triflate] (mole fraction of more than 0.3),11 and [beim][triflate] and [bmpyr][triflate] (mole fractions of more than 0.31).12 From the point of view of azeotrope removal, the performance of [bmim]Cl appears to be more favorable.

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Figure 2. Experimental and calculated volatilities of 1-propanol relative to water (α2,1) as functions of the liquid-phase mole fraction of 1propanol on an IL-free basis (x′2) for the saturated mixture water (1) + 1-propanol (2) + [bmim]Cl (3): ○, w3 = 0.1, p = 100 kPa; □, w3 = 0.3, p = 100 kPa; ◆, w3 = 0.5, p ≈ 40 kPa; ●, w3 = 0.7, p ≈ 30 kPa; ■, w3 = 0.8, p ≈ 30 kPa. Lines were calculated using the eNRTL model with the parameter values shown in Table 3: solid lines, calculated values for w3 = 0.1, 0.3, 0.5, 0.7, and 0.8 at the relevant pressures; dashed line, calculated values for the water (1) + 1-propanol (2) system at p = 100 kPa.

CONCLUSIONS Isobaric VLE data were measured for the ternary system water (1) + 1-propanol (2) + [bmim]Cl (3). T, x, y data were obtained at six values of the liquid-phase mole fraction of 1propanol on an IL-free basis (x2′ ) and eight IL mass fractions (w3). With the addition of [bmim]Cl, the water + 1-propanol azeotrope can be removed at an IL mass fraction of 0.5. The experimental data were correlated using the eNRTL model, which appeared to be adequate for the ternary system over the experimental composition range. By correlation of two data sets at x′2 = 0.1 and 0.98, all six data sets were reproduced satisfactorily, with δT = 0.93 K and δy = 0.0082. The experimental and calculated results were compared graphically and showed good agreement.

x2′ differently for various IL concentrations. When the IL mass fraction is high (i.e., w3 = 0.5, 0.7 and 0.8), γ1 decreases rapidly with increasing x′2. On the contrary, γ1 increases with increasing x2′ at w3 = 0.1 and 0.3. The influence of the IL mass fraction on the activity coefficients is similar to that presented for the system water (1) + ethanol (2) + [hmim]Cl (3).21 Figure 2 shows the effect of the IL on the volatility of 1-propanol relative sat to water, which is defined as α2,1 = (γ2/γ1)(psat 2 /p1 ), where the sat sat ratio p2 /p1 depends weakly on the equilibrium temperature, remaining almost unchanged from 1.06 to 1.14 over the experimental temperature range. Therefore, the effect of the IL on α2,1 is mainly embodied in the ratio γ2/γ1, so decreases in γ1 are beneficial for enhancement of the relative volatility but decreases in γ2 are undesirable. The effect of the IL on the

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 571 88320892. E-mail: [email protected]. Funding

The authors acknowledge the financial support by the National Natural Science Foundation of China (20776132). Notes

The authors declare no competing financial interest. D

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(20) Vercher, E.; Rojo, F. J.; Martínez-Andreu, A. Isobaric vapor− liquid equilibria for 1-propanol + water + calcium nitrate. J. Chem. Eng. Data 1999, 44, 1216−1221. (21) Zhang, L.; Ge, Y.; Ji, D.; Ji, J. Experimental measurement and modeling of vapor−liquid equilibrium for ternary systems containing ionic liquids: A case study for the system water + ethanol + 1-hexyl-3methylimidazolium chloride. J. Chem. Eng. Data 2009, 54, 2322−2329.

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