- Email: [email protected]
Study of the behaviour of the azeotropic mixture ethanol–water with imidazolium-based ionic liquids
Fluid Phase Equilibria 259 (2007) 51–56
Study of the behaviour of the azeotropic mixture ethanol–water with imidazolium-based ionic liquids Noelia Calvar, Bego˜na Gonz´alez, Elena G´omez, A. Dom´ınguez ∗ Departamento de Ingenier´ıa Qu´ımica, Universidad de Vigo, Edif. Isaac Newton, Lagoas-Marcosende 36310, Vigo, Spain Received 27 December 2006; received in revised form 16 March 2007; accepted 21 March 2007 Available online 27 March 2007
Abstract In this work, experimental data of isobaric vapour–liquid equilibria for the ternary system ethanol + water + 1-hexyl-3-methylimidazolium chloride ([C6 mim][Cl]) and for the corresponding binary systems containing the ionic liquid (ethanol + [C6 mim][Cl], water + [C6 mim][Cl]) were carried out at 101.300 kPa. VLE experimental data of binary and ternary systems were correlated using the NRTL equation. In a previous work [N. Calvar, B. Gonz´alez, E. G´omez, A. Dom´ınguez, J. Chem. Eng. Data 51 (2006) 2178–2181], the VLE of the ternary system ethanol + water + [C4 mim][Cl] was determined and correlated, so we can study the influence of different ionic liquids in the behaviour of the azeotropic mixture ethanol–water. © 2007 Elsevier B.V. All rights reserved. Keywords: Ethanol; Water; Ionic liquid; Binary; NRTL
1. Introduction Ionic liquids (IL) are substances formed by ions and they show a negligible vapour pressure at normal temperature and pressure conditions. This and other properties, such as their relatively low viscosities, their thermal stability and their capability as solvents, make the ionic liquids a new alternative for different processes. They can be applied as replacement for conventional toxic, flammable and volatile organic solvents. The use of IL in separation processes is one of the multiples alternatives. The prediction of the ideal IL for each separation process is nowadays impossible, since there is not enough information about the influence of the structure of IL in its physical and solvent properties. We have to take into account that there are about 1018 possible ionic liquids by combination of ions [1]. Besides, experimental phase equilibrium data is required for developing thermodynamic models and for understanding their thermodynamic behaviour VLE data permit checking the potential of gE -models which are applied for the description of the real behaviour of systems with ionic liquids. In this work, we study the possibility of separating the azeotropic mixture ethanol–water (x1(az) ≈ 0.90) using an ionic liquid, the 1-hexyl-3-methylimidazolium chloride ∗
Corresponding author. Tel.: +34 986812422; fax: +34 986812380. E-mail address: [email protected] (A. Dom´ınguez).
0378-3812/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2007.03.021
([C6 mim][Cl]). For this purpose, experimental data were determined and correlated. Nowadays, there are no VLE data of these binary and ternary systems available in literature. 2. Experimental 2.1. Chemicals Ethanol was purchased from Merck. It was degassed ultrasonically and dried over molecular sieves type 0.4 nm. Its purity was more than 99.8%. Water was bidistilled and deionized. The ionic liquid used in this work was synthesised in our laboratory. It was dried by heating (343.15–353.15 K) and stirring under high vacuum (2 × 10−1 Pa) for 48 h. To assure its purity, a RMN and Positive FABMS were made [4]. The water concentration (<100 ppm) was checked by Karl Fischer titration. Density, viscosity and boiling point of ethanol and water were measured and compared with literature (Table 1). These two components were analysed by gas chromatography too. The presented results show that there is a good agreement between experimental and literature data. 2.2. Apparatus and procedure A glass Fischer Labodest apparatus model 602/D was used in equilibrium determinations. The equilibrium vessel is a dynamic
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N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
Table 1 Comparison of data with literature data for pure liquids at 298.15 K Component
ρ (g cm−3 ) Exptl.
Ethanol Water a b
η (×10−3 Pa s)
Lit.a
0.78545 0.78546 0.99710 0.99705
Table 2 Vapour–liquid equilibrium data for ethanol (1) + [C6 mim][Cl] (2) system at 101.300 kPa
TB (K)
Exptl.
Lit.a
Exptl.
Lit.b
1.082 0.890
1.082 0.890
351.43 373.15
351.443 373.150
From Ref. [5]. From Ref. [6].
recirculating still and it is equipped with a Cottrell pump. A termometer Yokogawa model 7563, with an accuracy of ±0.01 K was used to measure the equilibrium temperature. For the pressure measurement, a digital pressure controller Ruska model 7218 with an accuracy of 0.001 kPa was used. The densities and speeds of sound of pure liquids and mixtures were measured with a densimeter Anton Paar DSA-5000, with a precision of ±2 × 10−6 g cm−3 . To measure refractive indices, an automatic refractometer Abbemat-HP Dr. Kernchen with a precision of ±4 × 10−5 was used. The liquid phase composition of binary systems was formed by a solvent (ethanol or water) and the ionic liquid. It was determined by density. The vapour phase was also measured to assure that only ethanol or water were evaporated. The liquid phase composition of ternary system was measured by density and refractive index, since being a ternary mixture two physical properties are necessary to determine its composition. The vapour phase, formed by a binary mixture ethanol–water, was measured by density. The uncertainty of the technique for the determination of liquid and vapour composition is ±0.001 mol fraction. Physical properties of these binary and ternary systems were determined in a previous work [7]. 3. Results and discussion Experimental isobaric VLE data of the binary systems ethanol + [C6 mim][Cl] and water + [C6 mim][Cl] are listed in Tables 2 and 3, respectively. As it can be observed in these tables, the activity coefficients of ethanol and water in the binary systems studied are less than one (γ < 1), showing a negative deviation from Raoult’s law. This negative deviation usually indicates that intermolecular attraction forces between different molecules (EtOH–[C6 mim][Cl], H2 O–[C6 mim][Cl]) are stronger than between similar ones (EtOH–EtOH, H2 O–H2 O, [C6 mim][Cl]–[C6 mim][Cl]). These experimental data were correlated using the NRTL equation [3]. To simplify, the ionic liquid was treated like a nondissociating component and the assumption of an ideal behaviour of the vapour phase was assumed. This correlation was made by minimizing the following objective function: OF =
np nc j=1 i=1
exp
2
(γij − γijcalc )
(1)
where np is the number of experimental points and nc is the number of the components.
T (K)
x1
γ1
351.44 351.74 351.91 352.18 352.50 352.95 353.76 354.51 355.40 357.63 358.77 360.16 364.98 367.56 371.45 377.59 379.97 386.47 391.16 397.00 405.53 416.78
1.000 0.992 0.985 0.975 0.965 0.952 0.935 0.919 0.903 0.875 0.862 0.847 0.808 0.785 0.751 0.706 0.687 0.643 0.618 0.584 0.540 0.503
1.000 0.996 0.997 0.996 0.994 0.990 0.977 0.965 0.949 0.900 0.874 0.845 0.742 0.696 0.634 0.547 0.520 0.450 0.404 0.359 0.302 0.238
The correlation parameters obtained are presented in Table 4 together with the T and y, representing the root mean-square deviations of temperature and vapour composition, respectively. Fig. 1(a) shows the boiling temperature diagram of the experimental and calculated data and Fig. 1(b) shows the experimental and calculated activity coefficients for both binary systems. In Fig. 1(c), the excess molar Gibbs energies (GE /RT) calculated from NRTL equation for both binary systems plotted against Table 3 Vapour–liquid equilibrium data for water (1) + [C6 mim][Cl] (2) system at 101.300 kPa T (K)
x1
γ1
373.15 373.30 373.38 373.48 373.59 373.69 374.39 374.51 374.83 374.98 375.37 375.70 375.97 376.39 376.90 377.80 379.15 384.09 389.30 396.05 407.59 415.51
1.000 0.998 0.996 0.994 0.990 0.986 0.965 0.961 0.952 0.947 0.941 0.934 0.931 0.925 0.917 0.909 0.894 0.847 0.803 0.740 0.640 0.580
1.000 0.997 0.996 0.994 0.994 0.995 0.992 0.992 0.990 0.990 0.982 0.978 0.972 0.964 0.955 0.934 0.907 0.809 0.719 0.629 0.514 0.452
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
53
Table 4 Correlation parameters and root-mean-square deviations for the binary systems ethanol (1) + [C6 mim][Cl] (2) and water (1) + [C6 mim][Cl] (2) at 101.300 kPa Parameters (J mol−1 )
Model
T (K)
y1
Ethanol + [C6 mim][Cl] NRTL g12
−8858.70
g21
−7442.60
α
0.225
0.58
0.019
Water + [C6 mim][Cl] NRTL g12
−1086.37
g21
−6776.11
α
0.412
0.51
0.017
mole fraction are presented. This figure is represented in mole fraction, and it can be observed that x1 reaches a composition of 0.5 approximately, corresponding to a weight fraction for the binary system ethanol + [C6 mim][Cl] of 0.1870 and for the system water + [C6 mim][Cl] of 0.1094. Experimental VLE data of the ternary system are listed in Table 5. To correlate the experimental phase equilibrium data of the ternary system, the NRTL equation was used, treating the ionic liquids like nondissociating components and assuming ideal behaviour of the vapour phase. In this case, the objective function to minimize is defined by OF =
np
(T exp − T calc )
(2)
i=1
This objective function was chosen because of the wide temperature range studied, since using the usual objective function based on activity coefficients, the deviation in temperature is too high. Minimizing the temperature difference, better results are achieved. The correlation parameters and the root-mean-squared deviations calculated in the correlation of the VLE ternary experimental data are shown in Table 6. As it can be seen in Tables 4 and 6, the correlation with the NRTL equation is in good agreement with the experimental data, so the simplification of treating the ionic liquid like a nondissociating component is a good approximation. With these parameters obtained from the correlation of the VLE experimental data of the ternary system, we can predict the behaviour of the ternary system depending on the quantity of ionic liquid present in the mixture. In Figs. 2 and 3 the predicted VLE containing the azeotropic system EtOH-H2 O and the ionic liquid [C6 mim][Cl] are presented in a pseudobinary way. In these figures, the liquid phase composition of the lowboiling component is the amount of this substance in the volatile part of the liquid phase. The ionic liquid concentration is stated for each curve separately from x3 = 0 to 0.5. For comparison purposes it is also included in these figures the predicted VLE of the binary azeotropic mixture containing the ionic liquid 1-butyl-3methylimidazolium chloride ([C4 mim][Cl]), determined in our laboratory and previously published [8]. In Fig. 2 we present the xy diagram, and Fig. 3 shows the Txy diagram. VLE experimental data of the binary system EtOH–H2 O were obtained from literature [2]. As it can be observed in these figures, the addition of [C6 mim][Cl] to this binary azeotropic mixture leads to a noticeable decrease in the molar vapour fraction of ethanol. On the other hand, the addition of [C4 mim][Cl] leads to a increase in the
Table 5 Vapour–liquid equilibrium data for the ternary system ethanol (1) + water (2) + [C6 mim][Cl] (3) at 101.300 kPa T (K)
x1
x2
y1
γ1
γ2
372.45 367.09 362.41 359.76 357.89 358.72 359.77 360.64 361.45 362.69 363.48 364.74 366.38 362.34 360.31 358.68 357.65 356.81 356.01 354.83 351.47 351.86 352.40 353.11 353.89 355.49 356.13 358.66 362.51 366.79 372.99 382.32 387.31 378.48 376.09 368.92 364.52 364.77 365.27 366.29 362.48 364.47 366.63 368.63 370.51
0.026 0.091 0.255 0.423 0.531 0.403 0.286 0.212 0.155 0.120 0.100 0.081 0.066 0.116 0.161 0.227 0.269 0.328 0.402 0.512 0.873 0.903 0.918 0.855 0.855 0.837 0.788 0.710 0.583 0.460 0.389 0.317 0.157 0.093 0.036 0.115 0.274 0.170 0.111 0.067 0.147 0.088 0.066 0.053 0.040
0.884 0.815 0.637 0.460 0.354 0.506 0.646 0.737 0.805 0.847 0.871 0.894 0.911 0.862 0.817 0.749 0.704 0.644 0.568 0.456 0.125 0.080 0.047 0.089 0.069 0.054 0.089 0.130 0.217 0.314 0.341 0.358 0.521 0.706 0.830 0.748 0.581 0.735 0.819 0.876 0.794 0.866 0.901 0.927 0.949
0.264 0.399 0.513 0.575 0.627 0.572 0.530 0.498 0.454 0.394 0.398 0.357 0.311 0.418 0.467 0.507 0.540 0.563 0.585 0.609 0.865 0.889 0.898 0.817 0.820 0.824 0.789 0.766 0.732 0.692 0.690 0.677 0.482 0.348 0.255 0.434 0.540 0.434 0.400 0.352 0.445 0.383 0.308 0.229 0.136
17.43 6.135 2.580 1.717 1.467 1.875 2.537 3.319 4.411 5.444 6.351 7.442 8.649 5.713 4.423 3.329 2.919 2.477 2.085 1.711 1.144 1.090 1.050 1.096 1.062 1.021 1.057 1.066 1.124 1.220 1.161 1.044 1.797 4.041 11.22 4.509 2.228 3.559 5.354 8.579 4.452 6.907 8.505 9.978 12.24
1.160 1.530 2.336 3.582 5.016 3.396 2.552 2.162 1.918 1.739 1.640 1.525 1.406 1.731 1.976 2.297 2.542 2.876 3.367 4.391 18.34 28.36 47.32 24.17 30.32 36.25 21.38 13.19 6.830 4.013 2.953 2.026 1.180 1.174 1.086 1.558 2.363 1.851 1.631 1.467 1.870 1.589 1.409 1.271 1.159
molar fraction of ethanol, breaking the azeotropic behaviour of the system. This study shows how we can modify the behaviour of the azeotropic binary system ethanol–water adding different ionic liquids.
54
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56 Table 6 NRTL correlation parameters, g (J mol−1 ) and α, obtained from the correlation of VLE experimental data of the ternary system and root-mean-square deviations for the ternary system ethanol (1) + water (2) + [C6 mim][Cl] (3) at 101.300 kPa
Fig. 1. (a) Boiling temperature diagram and (b) activity coefficients diagram of experimental VLE data (, EtOH + [C6 mim][Cl]; , H2 O + [C6 mim][Cl]) and calculated correlation curve (—, NRTL) for both binary systems; (c) excess molar Gibbs energies calculated from NRTL for the binary systems (—) EtOH + [C6 mim][Cl] and (– –) H2 O + [C6 mim][Cl].
Parameters g12 3159.86 −12 965.86 g13 g21 −618.37
g23 g31 g32
RMS T
y
1.11
−5324.27 −14 121.80 −4851.67
α12 α13 α23
−0.351 −0.131 0.725
0.057
Fig. 2. xy diagram of the system (a) ethanol (1) + water (2) + [C6 mim][Cl] (3) and (b) ethanol (1) + water (2) + [C4 mim][Cl] (3) at IL concentrations of 0 mol% (—), 10 mol% (– –), 30 mol% (· · ·) and 50 mol% (– · – ·). (䊉) Experimental VLE data from [2].
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
55
4. Conclusion In this work, we have determined the VLE of the binary systems ethanol + [C6 mim][Cl] and water + [C6 mim][Cl], and of the ternary mixture ethanol + water + [C6 mim][Cl]. The experimental data of the binary and ternary systems have been correlated using the NRTL equation. From the parameters obtained in the correlation of the ternary system, we predict the behaviour of the azeotropic mixture ethanol–water with different amounts of ionic liquid. The obtained results have been compared with the previously studied IL [C4 mim][Cl]. The [C4 mim][Cl] is capable of breaking the binary azeotrope ethanol–water, opening a new possibility as entrainer for this system, while the [C6 mim][Cl] moves the azeotrope composition to a smaller fraction of ethanol. It is noteworthy that, because of the nonvolatility of the IL, this can be regenerated by stripping, evaporation or drying. List of symbols A fitting parameters [C4 mim][Cl] 1-butyl-3-methylimidazolium chloride [C6 mim][Cl] 1-hexyl-3-methylimidazolium chloride EtOH ethanol g NRTL parameters H2 O water Lit literature data np number of experimental points nc number of components RMS root mean-square deviation T temperature w weight fraction x molar fraction in the liquid phase y molar fraction in the vapour phase z values of the property Greek letters α NRTL parameter γ activity coefficient η dynamic viscosity ρ density Subscripts b boiling i, j components
Fig. 3. Txy diagram of the system: (a) ethanol (1) + water (2) + [C6 mim][Cl] (3) and (b) ethanol (1) + water (2) + [C4 mim][Cl] (3) at IL concentrations of 0 mol% (—), 10 mol% (– –), 30 mol% (· · ·) and 50 mol% (– · – ·). (䊉) Experimental VLE data from [2].
Superscripts calc calculated exp experimental m polynomial degree Acknowledgments The authors are grateful to the Ministerio de Ciencia y Tecnolog´ıa of Spain (project CTQ2004-00454) and Xunta de Galicia (PGIDIT05PXIC38303PN) for financial support.
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N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
References [1] J.D. Holbrey, K.R. Seddon, Clean Products and Processes, vol. 1, SpringerVerlag, New York, 1999, p. 223. [2] A. Arce, J. Mart´ınez-Ageitos, A. Soto, Fluid Phase Equilib. 122 (1996) 117–129. [3] H. Renon, J.M. Prausnitz, AIChE J. 14 (1968) 135–144. [4] E. G´omez, B. Gonz´alez, A. Dom´ınguez, E. Tojo, J. Tojo, J. Chem. Eng. Data 51 (2006) 696–701.
[5] B. Gonz´alez, A. Dom´ınguez, J. Tojo, J. Chem. Eng. Data 49 (2004) 1590–1596. [6] J.A. Riddick, W.B. Bunger, T.K. Sakano, Organic Solvents, Willey, New York, 1986. [7] E. G´omez, N. Calvar, I. Dom´ınguez, A. Dom´ınguez, Phys. Chem. Liquids 44 (2006) 409–417. [8] N. Calvar, B. Gonz´alez, E. G´omez, A. Dom´ınguez, J. Chem. Eng. Data 51 (2006) 2178–2181.
Study of the behaviour of the azeotropic mixture ethanol–water with imidazolium-based ionic liquids Noelia Calvar, Bego˜na Gonz´alez, Elena G´omez, A. Dom´ınguez ∗ Departamento de Ingenier´ıa Qu´ımica, Universidad de Vigo, Edif. Isaac Newton, Lagoas-Marcosende 36310, Vigo, Spain Received 27 December 2006; received in revised form 16 March 2007; accepted 21 March 2007 Available online 27 March 2007
Abstract In this work, experimental data of isobaric vapour–liquid equilibria for the ternary system ethanol + water + 1-hexyl-3-methylimidazolium chloride ([C6 mim][Cl]) and for the corresponding binary systems containing the ionic liquid (ethanol + [C6 mim][Cl], water + [C6 mim][Cl]) were carried out at 101.300 kPa. VLE experimental data of binary and ternary systems were correlated using the NRTL equation. In a previous work [N. Calvar, B. Gonz´alez, E. G´omez, A. Dom´ınguez, J. Chem. Eng. Data 51 (2006) 2178–2181], the VLE of the ternary system ethanol + water + [C4 mim][Cl] was determined and correlated, so we can study the influence of different ionic liquids in the behaviour of the azeotropic mixture ethanol–water. © 2007 Elsevier B.V. All rights reserved. Keywords: Ethanol; Water; Ionic liquid; Binary; NRTL
1. Introduction Ionic liquids (IL) are substances formed by ions and they show a negligible vapour pressure at normal temperature and pressure conditions. This and other properties, such as their relatively low viscosities, their thermal stability and their capability as solvents, make the ionic liquids a new alternative for different processes. They can be applied as replacement for conventional toxic, flammable and volatile organic solvents. The use of IL in separation processes is one of the multiples alternatives. The prediction of the ideal IL for each separation process is nowadays impossible, since there is not enough information about the influence of the structure of IL in its physical and solvent properties. We have to take into account that there are about 1018 possible ionic liquids by combination of ions [1]. Besides, experimental phase equilibrium data is required for developing thermodynamic models and for understanding their thermodynamic behaviour VLE data permit checking the potential of gE -models which are applied for the description of the real behaviour of systems with ionic liquids. In this work, we study the possibility of separating the azeotropic mixture ethanol–water (x1(az) ≈ 0.90) using an ionic liquid, the 1-hexyl-3-methylimidazolium chloride ∗
Corresponding author. Tel.: +34 986812422; fax: +34 986812380. E-mail address: [email protected] (A. Dom´ınguez).
0378-3812/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2007.03.021
([C6 mim][Cl]). For this purpose, experimental data were determined and correlated. Nowadays, there are no VLE data of these binary and ternary systems available in literature. 2. Experimental 2.1. Chemicals Ethanol was purchased from Merck. It was degassed ultrasonically and dried over molecular sieves type 0.4 nm. Its purity was more than 99.8%. Water was bidistilled and deionized. The ionic liquid used in this work was synthesised in our laboratory. It was dried by heating (343.15–353.15 K) and stirring under high vacuum (2 × 10−1 Pa) for 48 h. To assure its purity, a RMN and Positive FABMS were made [4]. The water concentration (<100 ppm) was checked by Karl Fischer titration. Density, viscosity and boiling point of ethanol and water were measured and compared with literature (Table 1). These two components were analysed by gas chromatography too. The presented results show that there is a good agreement between experimental and literature data. 2.2. Apparatus and procedure A glass Fischer Labodest apparatus model 602/D was used in equilibrium determinations. The equilibrium vessel is a dynamic
52
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
Table 1 Comparison of data with literature data for pure liquids at 298.15 K Component
ρ (g cm−3 ) Exptl.
Ethanol Water a b
η (×10−3 Pa s)
Lit.a
0.78545 0.78546 0.99710 0.99705
Table 2 Vapour–liquid equilibrium data for ethanol (1) + [C6 mim][Cl] (2) system at 101.300 kPa
TB (K)
Exptl.
Lit.a
Exptl.
Lit.b
1.082 0.890
1.082 0.890
351.43 373.15
351.443 373.150
From Ref. [5]. From Ref. [6].
recirculating still and it is equipped with a Cottrell pump. A termometer Yokogawa model 7563, with an accuracy of ±0.01 K was used to measure the equilibrium temperature. For the pressure measurement, a digital pressure controller Ruska model 7218 with an accuracy of 0.001 kPa was used. The densities and speeds of sound of pure liquids and mixtures were measured with a densimeter Anton Paar DSA-5000, with a precision of ±2 × 10−6 g cm−3 . To measure refractive indices, an automatic refractometer Abbemat-HP Dr. Kernchen with a precision of ±4 × 10−5 was used. The liquid phase composition of binary systems was formed by a solvent (ethanol or water) and the ionic liquid. It was determined by density. The vapour phase was also measured to assure that only ethanol or water were evaporated. The liquid phase composition of ternary system was measured by density and refractive index, since being a ternary mixture two physical properties are necessary to determine its composition. The vapour phase, formed by a binary mixture ethanol–water, was measured by density. The uncertainty of the technique for the determination of liquid and vapour composition is ±0.001 mol fraction. Physical properties of these binary and ternary systems were determined in a previous work [7]. 3. Results and discussion Experimental isobaric VLE data of the binary systems ethanol + [C6 mim][Cl] and water + [C6 mim][Cl] are listed in Tables 2 and 3, respectively. As it can be observed in these tables, the activity coefficients of ethanol and water in the binary systems studied are less than one (γ < 1), showing a negative deviation from Raoult’s law. This negative deviation usually indicates that intermolecular attraction forces between different molecules (EtOH–[C6 mim][Cl], H2 O–[C6 mim][Cl]) are stronger than between similar ones (EtOH–EtOH, H2 O–H2 O, [C6 mim][Cl]–[C6 mim][Cl]). These experimental data were correlated using the NRTL equation [3]. To simplify, the ionic liquid was treated like a nondissociating component and the assumption of an ideal behaviour of the vapour phase was assumed. This correlation was made by minimizing the following objective function: OF =
np nc j=1 i=1
exp
2
(γij − γijcalc )
(1)
where np is the number of experimental points and nc is the number of the components.
T (K)
x1
γ1
351.44 351.74 351.91 352.18 352.50 352.95 353.76 354.51 355.40 357.63 358.77 360.16 364.98 367.56 371.45 377.59 379.97 386.47 391.16 397.00 405.53 416.78
1.000 0.992 0.985 0.975 0.965 0.952 0.935 0.919 0.903 0.875 0.862 0.847 0.808 0.785 0.751 0.706 0.687 0.643 0.618 0.584 0.540 0.503
1.000 0.996 0.997 0.996 0.994 0.990 0.977 0.965 0.949 0.900 0.874 0.845 0.742 0.696 0.634 0.547 0.520 0.450 0.404 0.359 0.302 0.238
The correlation parameters obtained are presented in Table 4 together with the T and y, representing the root mean-square deviations of temperature and vapour composition, respectively. Fig. 1(a) shows the boiling temperature diagram of the experimental and calculated data and Fig. 1(b) shows the experimental and calculated activity coefficients for both binary systems. In Fig. 1(c), the excess molar Gibbs energies (GE /RT) calculated from NRTL equation for both binary systems plotted against Table 3 Vapour–liquid equilibrium data for water (1) + [C6 mim][Cl] (2) system at 101.300 kPa T (K)
x1
γ1
373.15 373.30 373.38 373.48 373.59 373.69 374.39 374.51 374.83 374.98 375.37 375.70 375.97 376.39 376.90 377.80 379.15 384.09 389.30 396.05 407.59 415.51
1.000 0.998 0.996 0.994 0.990 0.986 0.965 0.961 0.952 0.947 0.941 0.934 0.931 0.925 0.917 0.909 0.894 0.847 0.803 0.740 0.640 0.580
1.000 0.997 0.996 0.994 0.994 0.995 0.992 0.992 0.990 0.990 0.982 0.978 0.972 0.964 0.955 0.934 0.907 0.809 0.719 0.629 0.514 0.452
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
53
Table 4 Correlation parameters and root-mean-square deviations for the binary systems ethanol (1) + [C6 mim][Cl] (2) and water (1) + [C6 mim][Cl] (2) at 101.300 kPa Parameters (J mol−1 )
Model
T (K)
y1
Ethanol + [C6 mim][Cl] NRTL g12
−8858.70
g21
−7442.60
α
0.225
0.58
0.019
Water + [C6 mim][Cl] NRTL g12
−1086.37
g21
−6776.11
α
0.412
0.51
0.017
mole fraction are presented. This figure is represented in mole fraction, and it can be observed that x1 reaches a composition of 0.5 approximately, corresponding to a weight fraction for the binary system ethanol + [C6 mim][Cl] of 0.1870 and for the system water + [C6 mim][Cl] of 0.1094. Experimental VLE data of the ternary system are listed in Table 5. To correlate the experimental phase equilibrium data of the ternary system, the NRTL equation was used, treating the ionic liquids like nondissociating components and assuming ideal behaviour of the vapour phase. In this case, the objective function to minimize is defined by OF =
np
(T exp − T calc )
(2)
i=1
This objective function was chosen because of the wide temperature range studied, since using the usual objective function based on activity coefficients, the deviation in temperature is too high. Minimizing the temperature difference, better results are achieved. The correlation parameters and the root-mean-squared deviations calculated in the correlation of the VLE ternary experimental data are shown in Table 6. As it can be seen in Tables 4 and 6, the correlation with the NRTL equation is in good agreement with the experimental data, so the simplification of treating the ionic liquid like a nondissociating component is a good approximation. With these parameters obtained from the correlation of the VLE experimental data of the ternary system, we can predict the behaviour of the ternary system depending on the quantity of ionic liquid present in the mixture. In Figs. 2 and 3 the predicted VLE containing the azeotropic system EtOH-H2 O and the ionic liquid [C6 mim][Cl] are presented in a pseudobinary way. In these figures, the liquid phase composition of the lowboiling component is the amount of this substance in the volatile part of the liquid phase. The ionic liquid concentration is stated for each curve separately from x3 = 0 to 0.5. For comparison purposes it is also included in these figures the predicted VLE of the binary azeotropic mixture containing the ionic liquid 1-butyl-3methylimidazolium chloride ([C4 mim][Cl]), determined in our laboratory and previously published [8]. In Fig. 2 we present the xy diagram, and Fig. 3 shows the Txy diagram. VLE experimental data of the binary system EtOH–H2 O were obtained from literature [2]. As it can be observed in these figures, the addition of [C6 mim][Cl] to this binary azeotropic mixture leads to a noticeable decrease in the molar vapour fraction of ethanol. On the other hand, the addition of [C4 mim][Cl] leads to a increase in the
Table 5 Vapour–liquid equilibrium data for the ternary system ethanol (1) + water (2) + [C6 mim][Cl] (3) at 101.300 kPa T (K)
x1
x2
y1
γ1
γ2
372.45 367.09 362.41 359.76 357.89 358.72 359.77 360.64 361.45 362.69 363.48 364.74 366.38 362.34 360.31 358.68 357.65 356.81 356.01 354.83 351.47 351.86 352.40 353.11 353.89 355.49 356.13 358.66 362.51 366.79 372.99 382.32 387.31 378.48 376.09 368.92 364.52 364.77 365.27 366.29 362.48 364.47 366.63 368.63 370.51
0.026 0.091 0.255 0.423 0.531 0.403 0.286 0.212 0.155 0.120 0.100 0.081 0.066 0.116 0.161 0.227 0.269 0.328 0.402 0.512 0.873 0.903 0.918 0.855 0.855 0.837 0.788 0.710 0.583 0.460 0.389 0.317 0.157 0.093 0.036 0.115 0.274 0.170 0.111 0.067 0.147 0.088 0.066 0.053 0.040
0.884 0.815 0.637 0.460 0.354 0.506 0.646 0.737 0.805 0.847 0.871 0.894 0.911 0.862 0.817 0.749 0.704 0.644 0.568 0.456 0.125 0.080 0.047 0.089 0.069 0.054 0.089 0.130 0.217 0.314 0.341 0.358 0.521 0.706 0.830 0.748 0.581 0.735 0.819 0.876 0.794 0.866 0.901 0.927 0.949
0.264 0.399 0.513 0.575 0.627 0.572 0.530 0.498 0.454 0.394 0.398 0.357 0.311 0.418 0.467 0.507 0.540 0.563 0.585 0.609 0.865 0.889 0.898 0.817 0.820 0.824 0.789 0.766 0.732 0.692 0.690 0.677 0.482 0.348 0.255 0.434 0.540 0.434 0.400 0.352 0.445 0.383 0.308 0.229 0.136
17.43 6.135 2.580 1.717 1.467 1.875 2.537 3.319 4.411 5.444 6.351 7.442 8.649 5.713 4.423 3.329 2.919 2.477 2.085 1.711 1.144 1.090 1.050 1.096 1.062 1.021 1.057 1.066 1.124 1.220 1.161 1.044 1.797 4.041 11.22 4.509 2.228 3.559 5.354 8.579 4.452 6.907 8.505 9.978 12.24
1.160 1.530 2.336 3.582 5.016 3.396 2.552 2.162 1.918 1.739 1.640 1.525 1.406 1.731 1.976 2.297 2.542 2.876 3.367 4.391 18.34 28.36 47.32 24.17 30.32 36.25 21.38 13.19 6.830 4.013 2.953 2.026 1.180 1.174 1.086 1.558 2.363 1.851 1.631 1.467 1.870 1.589 1.409 1.271 1.159
molar fraction of ethanol, breaking the azeotropic behaviour of the system. This study shows how we can modify the behaviour of the azeotropic binary system ethanol–water adding different ionic liquids.
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N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56 Table 6 NRTL correlation parameters, g (J mol−1 ) and α, obtained from the correlation of VLE experimental data of the ternary system and root-mean-square deviations for the ternary system ethanol (1) + water (2) + [C6 mim][Cl] (3) at 101.300 kPa
Fig. 1. (a) Boiling temperature diagram and (b) activity coefficients diagram of experimental VLE data (, EtOH + [C6 mim][Cl]; , H2 O + [C6 mim][Cl]) and calculated correlation curve (—, NRTL) for both binary systems; (c) excess molar Gibbs energies calculated from NRTL for the binary systems (—) EtOH + [C6 mim][Cl] and (– –) H2 O + [C6 mim][Cl].
Parameters g12 3159.86 −12 965.86 g13 g21 −618.37
g23 g31 g32
RMS T
y
1.11
−5324.27 −14 121.80 −4851.67
α12 α13 α23
−0.351 −0.131 0.725
0.057
Fig. 2. xy diagram of the system (a) ethanol (1) + water (2) + [C6 mim][Cl] (3) and (b) ethanol (1) + water (2) + [C4 mim][Cl] (3) at IL concentrations of 0 mol% (—), 10 mol% (– –), 30 mol% (· · ·) and 50 mol% (– · – ·). (䊉) Experimental VLE data from [2].
N. Calvar et al. / Fluid Phase Equilibria 259 (2007) 51–56
55
4. Conclusion In this work, we have determined the VLE of the binary systems ethanol + [C6 mim][Cl] and water + [C6 mim][Cl], and of the ternary mixture ethanol + water + [C6 mim][Cl]. The experimental data of the binary and ternary systems have been correlated using the NRTL equation. From the parameters obtained in the correlation of the ternary system, we predict the behaviour of the azeotropic mixture ethanol–water with different amounts of ionic liquid. The obtained results have been compared with the previously studied IL [C4 mim][Cl]. The [C4 mim][Cl] is capable of breaking the binary azeotrope ethanol–water, opening a new possibility as entrainer for this system, while the [C6 mim][Cl] moves the azeotrope composition to a smaller fraction of ethanol. It is noteworthy that, because of the nonvolatility of the IL, this can be regenerated by stripping, evaporation or drying. List of symbols A fitting parameters [C4 mim][Cl] 1-butyl-3-methylimidazolium chloride [C6 mim][Cl] 1-hexyl-3-methylimidazolium chloride EtOH ethanol g NRTL parameters H2 O water Lit literature data np number of experimental points nc number of components RMS root mean-square deviation T temperature w weight fraction x molar fraction in the liquid phase y molar fraction in the vapour phase z values of the property Greek letters α NRTL parameter γ activity coefficient η dynamic viscosity ρ density Subscripts b boiling i, j components
Fig. 3. Txy diagram of the system: (a) ethanol (1) + water (2) + [C6 mim][Cl] (3) and (b) ethanol (1) + water (2) + [C4 mim][Cl] (3) at IL concentrations of 0 mol% (—), 10 mol% (– –), 30 mol% (· · ·) and 50 mol% (– · – ·). (䊉) Experimental VLE data from [2].
Superscripts calc calculated exp experimental m polynomial degree Acknowledgments The authors are grateful to the Ministerio de Ciencia y Tecnolog´ıa of Spain (project CTQ2004-00454) and Xunta de Galicia (PGIDIT05PXIC38303PN) for financial support.
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