Glycerol-Based Deep Eutectic Solvents as Extractants for the

Article pubs.acs.org/jced

Glycerol-Based Deep Eutectic Solvents as Extractants for the Separation of MEK and Ethanol via Liquid−Liquid Extraction Nerea R. Rodriguez,† Jordi Ferre Guell,† and Maaike C. Kroon*,†,‡ †

Eindhoven University of Technology, Department of Chemical Engineering and Chemistry, Separation Technology Group, STO1.22, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates S Supporting Information *

ABSTRACT: Four different glycerol-based deep eutectic solvents (DESs) were tested as extracting agents for the separation of the azeotropic mixture {methyl ethyl ketone + ethanol} via liquid−liquid extraction. The selected DESs for this work were: glycerol/choline chloride with molar ratios (4:1) and (2:1), and glycerol/tetramethylammonium chloride with molar ratios (4:1) and (2:1). The selected DESs have been characterized by measuring the density and the viscosity at different temperatures. The liquid−liquid equilibria data of the ternary systems {methyl ethyl ketone + ethanol + DES} were experimentally determined at T/K = 298.15 and atmospheric pressure. The extraction efficiencies of the selected DESs were analyzed by determining the solute distribution coefficients and the selectivity values, which were calculated from the experimental results. The extraction performance of the DESs was compared to the extraction data of pure glycerol. The integrity of the DESs after recovery was also studied. Finally, the experimental LLE data were regressed using the nonrandom two liquid (NRTL) model. It was found that the addition of a hydrogen bond acceptor to glycerol, and therefore, the formation of a deep eutectic solvent, improves the extraction efficiency of this mixture via liquid−liquid extraction.

1. INTRODUCTION The separation of azeotropic mixtures is of great importance for the chemical industry due to the huge economic impact of these separations on the chemical process. Azeotropic distillation,1 extractive distillation,2 and liquid−liquid extraction,3 which are three of the most important industrial separation techniques for azeotrope breaking, involve the use of an extracting agent. Commonly used extracting agents are organic solvents. However, they are generally volatile, flammable, nonbiodegradable, and/or hazardous for both health and environment. Recently, the applicability of two different families of solvents as extracting agents has been studied.4−7 Ionic liquids (ILs) were first discovered, and it was found that their physicochemical properties make them suitable for the separation of azeotropic mixtures via extractive distillation8−11 and liquid− liquid extraction.12−16 Their negligible vapor pressure and flammability, together with other characteristics such as easy recyclability were the main reasons for their theoretical success.17 However, in practice, their high price (associated with the complicated synthesis and the purification requirements) make their industrial large scale applicability complicated. Deep eutectic solvents (DESs), discovered at the beginning of the 21st century, share most of their physicochemical properties with ILs;18 however, they can be synthesized at a much lower price, without purification requirements, just by © XXXX American Chemical Society

mixing the initial components upon heating. DESs are formed by mixing one or more hydrogen bond donors (HBDs) with one or more hydrogen bond acceptors (HBAs). The final solvent shows a lower melting point than the individual components.19−22 Another important similarity between DESs and ILs is the tunability. The physicochemical properties of an IL can be modified by changing the nature of the anion/cation. In the case of the DESs, the physicochemical properties can be modified by changing the nature of the HBD/HBA and also the molar ratio between them.23−25 This gives an extra degree of freedom for designing a solvent with desired properties. In recent years, DESs have already been considered for the separation of several azeotropic mixtures via extractive distillation and liquid−liquid extraction. The obtained results show that DESs are a feasible alternative to both organic solvents and ILs.6,26−31 In this work, the applicability of different DESs for the ketone−alcohol azeotropic separation has been studied for the first time. Several ketone−alcohol mixtures show an azeotrope, and as representation of this family of mixtures the {methyl ethyl ketone (MEK) + ethanol} system has been selected for further investigation.17 MEK and ethanol can be found in the same production trains in processes (e.g., Fischer−Tropsch), Received: August 25, 2015 Accepted: January 14, 2016

A

DOI: 10.1021/acs.jced.5b00717 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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which produce MEK and ethanol as valuable byproducts.32 At the moment this separation is industrially performed via azeotropic distillation with several organic solvents, including phenol and dimethyl sulfoxide.33 In this work, we aim to investigate “greener” and cheaper technologies for the separation of this mixture. A first screening study in our group indicated that most DESs are fully soluble in ethanol, while they are generally insoluble or even precipitate in the presence of ketones.26 Therefore, we chose to separate the {MEK + ethanol} system by selective ethanol extraction using a DES as extracting agent. Glycerolbased DESs were considered, because {MEK + ethanol} separation was previously studied using pure glycerol as extracting agent via liquid−liquid extraction.34 Thus, the extraction efficiency of the studied DESs could be compared to that of pure glycerol. The selected DESs for this work were: glycerol/choline chloride with molar ratios (4:1) and (2:1) and glycerol/tetramethylammonium chloride with molar ratios (4:1) and (2:1). The selected solvents have been characterized by measuring their density and viscosity at different temperatures. The LLE data of the ternary systems {MEK + ethanol + DES} were experimentally determined at T/K = 298.15 and atmospheric pressure. The solute distribution coefficient and the selectivity values were calculated from the experimental data and compared to pure glycerol data. The experimental LLE data were regressed using the nonrandom two liquid (NRTL) thermodynamic model. After the extraction the DES was recovered and the integrity of the solvent after the recovery was studied.

2.2. DESs Preparation. The DESs were prepared by placing the HBD and the HBA in a flask and heating it under stirring until a clear liquid was formed. The mixtures were prepared using a Mettler AX205 balance with a readability of 0.1 mg. The temperature was controlled using a thermostatic bath (IKA RCT basic) and a temperature controller (IKA ETSD6) with a temperature stability of ±0.1 K. The DESs used in this work were prepared at T/K = 333.15 and atmospheric pressure. The molecular structures of the DESs used in this work are presented in Table 2. 2.3. DESs Characterization. The density and viscosity of the selected DESs were measured at different temperatures using an Anton Paar SVM 3000/G2 Stabinger viscometer. The temperature uncertainty is 0.02 K and the relative uncertainty of the dynamic viscosity is 0.35%, while the absolute uncertainty in the density is 0.0005 g·cm−3. The viscometer used in this work was calibrated using an Anton Paar viscosity set of standard oils. The water content of the studied DESs was measured using Karl Fischer titration method, type Metrohm 795, and it was found to be less than 0.3% in all samples. 2.4. LLE Determination. The LLE data of the ternary systems were measured using the equilibrium cell method. Ternary mixtures with different compositions were placed in a shaker with temperature controller (IKA KS 4000 i-control) kept at T/K = 298.15 (temperature stability of T/K= ± 0.1 K) and mixed for 2 h. Thereafter, the samples were placed in a thermostatic bath (IKA RCT basic) with a temperature controller (IKA ETS-D6) at T/K = 298.15 (temperature stability is T/K = ± 0.1 K). The vials were kept in the thermostatic bath overnight to ensure that full phase separation was reached. Next, samples of the bottom and top phase were taken and prepared for analysis. Methanol was used as an internal standard for the sample analysis. The ethanol and MEK concentration in the samples was measured using a Varian 430 GC with flame ionization detector (T/K = 495.15) and a Varian CP-SIL 5CB column (30 m × 0.25 mm × 1.2 μm). The oven temperature was set to T/K = 352 for 2 min, then increased to T/K = 373.15 at a flow rate of 100 K/min. All the samples were measured three times and the relative standard deviation was found to be lower than 1% for the top phase and lower than 2.5% for the bottom phase. The DES concentration was determined from a mass balance calculation.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. All the chemicals used in this work, including purity and source, are listed in Table 1. The choline chloride was kept in a vacuum desiccator before it was used. All the chemicals were used without further purification. Table 1. Chemicals Used in This Work chemical ethanol butan-2-one (MEK) choline chloride tetramethylammonium chloride glycerol methanol

purity (wt %) ≥ ≥ ≥ ≥ ≥ ≥

99.5 99 98 98 99 99

source TechniSolv Sigma-Aldrich Sigma-Aldrich Merck Merck Merck

Table 2. Molecular Structure of the Used DESs: Abbreviation, HBD, HBA, and HBD/HBA Molar Ratio

B



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Table 3. Experimental Density and Viscosity Values at Different Temperatures and Atmospheric Pressure (p = 1.01 bar) ρ (g·cm−3)

μ (mPa·s)

T (K)

GC(4:1)

GC(2:1)

GT(4:1)

GT(2:1)

GC(4:1)

GC(2:1)

GT(4:1)

GT(2:1)

293.15 298.15 303.15 308.15 313.15 318.15 323.15

1.217 1.214 1.211 1.208 1.205 1.202 1.200

1.194 1.191 1.188 1.185 1.183 1.180 1.177

1.180 1.177 1.174 1.171 1.168 1.166 1.163

1.158 1.155 1.152 1.149 1.147 1.144 1.141

578.2 400.3 283.4 205.1 151.7 113.0 87.96

514.6 365.7 265.7 197.0 148.9 114.7 89.81

557.5 384.7 272.0 197.0 145.8 110.2 84.80

581.8 406.9 290.5 212.4 158.5 120.7 93.57

Standard uncertainties u are ur(ρ) = 0.002, ur(μ) = 0.0046, u(T) = 0.02 K, and u(p) = 0.02 bar.

3. RESULTS AND DISCUSSION 3.1. DESs Characterization. The density and the viscosity of the four studied DESs were measured at atmospheric pressure within a temperature range of T/K = 293.15 to T/K = 323.15 with steps of 5 K. All samples were measured twice, and the relative standard deviations for both the density and the viscosity were found to be ur(ρ) = 0.002 and ur(μ) = 0.003, respectively. The experimental values for the density and the viscosity at different temperatures are shown in Table 3. The water content of the DESs was measured prior to the density and viscosity measurements, the obtained values are as follows: H2O (wt %)GC(4:1) = 0.26, H2O (wt %)GC(2:1) = 0.14, H2O (wt %)TC(4:1) = 0.25, and H2O (wt %)TC(2:1) = 0.15. It can be observed that the DESs with higher glycerol content show higher water content too. This is because of the hygroscopicity of the glycerol. The density follows a linear trend with the temperature. This relationship can be expressed using eq 1. ⎛ g ⎞ ρ⎜ 3 ⎟ = a + bT (K) ⎝ cm ⎠

From Figure 1, it can be observed that the density of the four studied DESs decreases linearly with the temperature. It can also be noticed that for the four studied DESs the density is lower than the density of pure glycerol which has a value of ρ/ (g/cm3) = 1.26362 g/cm3 at T/K = 293.15.35 Therefore, the presence of both choline chloride and tetramethylammonium chloride decrease the density compared to that of pure glycerol. It was also found that GC(4:1) and TC(4:1) have higher densities than GC(2:1) and TC(2:1), respectively. Thus, it can be concluded that an increase of the HBA decreases the density for the DESs investigated in this work. This behavior was previously reported for other DESs formed by glycerol and a quaternary ammonium salts.36 The fact that choline chloridebased DESs show higher densities than tetramethylammonium chloride-based DESs at the same molar ratio, suggests that molecular interactions between choline chloride and glycerol are stronger than between tetramethylammonium chloride and glycerol. Most probably, this is due to the presence of the hydroxyl group in the choline chloride, leading to more H-bond interactions between the HBA and the HBD. The viscosity as a function of temperature can be expressed using the Vogel−Tammann−Fulcher equation (VTF), which is presented in eq 2:

(1)

where ρ is the density in g/cm3, T is the temperature in K, and a and b are adjustable parameters. The experimental and the regressed values obtained using eq 1 are depicted in Figure 1. The adjustable parameters and the relative standard deviation obtained by using eq 1 are shown in the Supporting Information in Table S1.

⎛ B ⎞ η(mPa. s) = A exp⎜ ⎟ ⎝ T − T0 ⎠

(2)

where η is the dynamic viscosity in mPa·s, T is the temperature in K, and A, B, and T0 are the adjustable parameters of the VTF equation. The experimental and correlated values are represented in Figure 2, and the calculated fitting parameters and relative standard deviation are presented in the Supporting Information in Table S2. From Figure 2, it can be observed that the VTF equation satisfactorily correlates the viscosity values as a function of the temperature for the four studied DESs. From Figure 2, it can be noticed that the viscosity values of the four studied DESs decrease when the temperature increases, as expected. The viscosity of the DESs was found to be lower than that of pure glycerol (μ/mPa·s = 1410).35 This behavior was also found in other publications36,37 and it was attributed to the loss of 3D intermolecular H-bond interactions in glycerol that are broken up upon addition of quaternary ammonium salts, resulting in a less ordered system. For the studied DESs the viscosity values follow the next trend: GT(2:1) > GC(4:1) > GT(4:1) > GC(2:1). On the one hand, we observe that for the choline chloride-based DESs, the viscosity increases with an increasing amount of glycerol in the DES, which suggests that the higher is the fraction of glycerol, the stronger is the molecular interactions, for example,

Figure 1. Experimental density values as a function of temperature for the studied DESs. Dashed lines correspond to the linear fitting. C



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fraction, which would show that the experimental tie-lines are closer to the plait point. Also, if Figure 3 was presented in molar basis, we will observe a change of the slope of the tielines, indicating distribution coefficients higher than unity. This is actually not realistic because from an industrial process point of view, the bigger concern is mass solvent-to-feed ratio and not the molar solvent-to-feed ratio. Therefore, to show more realistic values for the industrial application of this process, all the results and calculations concerning the experimental LLE data will be shown in mass fraction. The solute distribution coefficient (β) and also the selectivity (S) were calculated from the experimental data using the following equations: w2, E β= w2, R (3) w2,E

S=

Figure 2. Experimental viscosity values as a function of temperature for the studied DESs. Dashed lines correspond to the fitting using the VTF equation.

β2 β1

=

w2,R w1,E w1,R

(4)

where w1 and w2 refer to the mass fractions of MEK and ethanol, respectively; the subscripts E and R refers to the extract (DES-rich phase) and the raffinate (MEK-rich phase), respectively. The calculated solute distribution coefficient and the selectivity values are also presented in Table 4. The solute distribution coefficient and the selectivity values as a function of the ethanol concentration in the MEK-rich phase are presented in Figure 4 and Figure 5, respectively. From Figure 4, the solute distribution coefficient behavior of the four studied DESs can be analyzed and compared to that of pure glycerol. First, as usual, the solute distribution coefficient decreases with the ethanol concentration in the MEK-rich phase. Moreover, it can be observed that the solute distribution coefficient trend is GT(2:1) > GC(4:1) > GT(4:1) > GC(2:1) which is found to be the same trend found for the viscosity of the studied DESs. We found that the distribution coefficient decreases with an increasing amount of choline chloride in the DESs, while the opposite behavior was found for the tetramethylammonium chloride. From Figure 5, the effect of the four studied DESs on the selectivity values can be analyzed. The selectivity values decrease with the ethanol concentration in the MEK-rich phase. In this case, the trend is GC(4:1) > GC(2:1) > GT(4:1) > GT(2:1), which is the same trend found for the density. It is possible to observe that the choline chloride-based DESs show higher selectivity at any ratio compared to the tetramethylammonium choride DESs. It is also observed that the selectivity increases with the amount of glycerol (HBD) in both DESs. When comparing the experimental results to pure glycerol, it can be observed that the choline chloride-based DESs show higher selectivity values than the tetramethylammonium chloride-based DES. From the studied DESs, the option for the extraction of the {MEK + ethanol} mixture is GC(4:1) because have the highest selectivity and high solute distribution coefficient. When compared to that of pure glycerol, it is observed that for low ethanol concentration the solute distribution coefficient of the studied DESs is higher; however, for concentrations above wEtOH = 0.2 in the MEK-rich phase, the solute distribution coefficient for pure glycerol is higher than those of the DESs. Moreover, it was found that higher selectivity

hydrogen bonds between HBD and HBA and also between the glycerol molecules themselves. On the other hand, for the tetramethylammonium chloride-based DESs, the viscosity increases with an increasing amount of tetramethylammonium chloride. The reason for this observation could be that the presence of short alkyl chain lengths, implying low flexibility of the molecule, restricts the presence of chemical interactions, which increases the viscosity.38 Also in literature36 it was previously found that a minimum viscosity was reached at different HBA:HBD ratios for different glycerol-based DESs. The density of the DES GC(2:1) was previously reported in other works.39,40 The previously reported data are in good agreement with the data reported in this work, with standard deviations smaller than 0.1% in all cases. 3.2. Experimental LLE Data. The LLE data of the ternary systems {MEK + ethanol + GC(4:1)}, {MEK + ethanol + GC(2:1)}, {MEK + ethanol + GT(4:1)} and {MEK + ethanol + GT(2:1)} were experimentally determined at T/K = 298.15 and atmospheric pressure. The obtained experimental data can be found in Table 4 and the experimental data are depicted in Figure 3. In Figure 3, the LLE behavior of the four studied ternary systems is shown by means of a triangular diagram. The behavior was classified according to Sørensen classification.41 The four studied DESs belong to the Type 1 classification, which implies two completely miscible pairs ({MEK + ethanol} and {ethanol + DES}) and one partially miscible pair ({MEK + DES}). The next thing to be noticed from the four ternary diagrams is that all the tie-lines over the diagram show negative slope. This indicates that the solute distribution coefficient will be lower than unity over the whole range of compositions, so that high solvent-to-feed ratios will be needed for this extraction. Moreover, it can also be observed that the experimental tie-lines seem to be far away from the plait point, because in the experimental procedure, due to the small size of the phases, it was not possible to measure more tie-lines ensuring the proper sampling of the phases. However, it should also be considered that Figure 3 is presented in mass fraction (because of the big difference in the molar mass of the components, this is the more realistic way) instead of molar D



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Table 4. Experimental LLE Data (Presented in Mass Fractions w), Values of the Solute Distribution Coefficient (β) and Selectivity (S) for the Systems {MEK (1) + ethanol (2) + GC(4:1) (3)}, {MEK (1) + ethanol (2) + GC(2:1) (3)}, {MEK (1) + ethanol (2) + GT(4:1) (3)} and {MEK (1) + ethanol (2) + GT(2:1) (3)} at atmospheric pressure (p/bar = 1.01) and T/K = 298.15 MEK-rich phase w1

w2

DES-rich phase w3

w1

0.944 0.889 0.864 0.830 0.804 0.767 0.734 0.685

0.033 0.074 0.094 0.117 0.138 0.158 0.180 0.195

0.022 0.038 0.041 0.053 0.058 0.075 0.086 0.120

0.031 0.040 0.045 0.047 0.053 0.059 0.068 0.076

0.976 0.930 0.902 0.852 0.820 0.786 0.755 0.701

0.017 0.055 0.076 0.117 0.141 0.160 0.184 0.208

0.007 0.014 0.021 0.031 0.039 0.054 0.062 0.082

0.034 0.038 0.041 0.049 0.052 0.057 0.062 0.072

0.959 0.934 0.880 0.858 0.823 0.788 0.738 0.683

0.036 0.056 0.100 0.119 0.139 0.160 0.181 0.199

0.005 0.010 0.020 0.022 0.038 0.052 0.082 0.118

0.038 0.045 0.059 0.062 0.064 0.073 0.087 0.100

0.956 0.934 0.911 0.882 0.856 0.831 0.794 0.757

0.033 0.051 0.071 0.091 0.116 0.134 0.155 0.176

0.011 0.015 0.018 0.027 0.028 0.035 0.050 0.067

0.044 0.046 0.052 0.056 0.062 0.069 0.075 0.084

w2 GC(4:1) 0.024 0.049 0.058 0.067 0.079 0.090 0.100 0.113 GC(2:1) 0.013 0.034 0.044 0.063 0.077 0.088 0.099 0.121 GT(4:1) 0.025 0.036 0.057 0.065 0.080 0.093 0.103 0.118 GT(2:1) 0.024 0.037 0.048 0.062 0.071 0.080 0.091 0.105

w3

β

S

0.945 0.911 0.897 0.885 0.869 0.850 0.832 0.811

0.727 0.811 0.662 0.617 0.573 0.572 0.570 0.556

22.147 21.953 14.717 11.847 10.113 8.,684 7.405 5.997

0.953 0.928 0.915 0.889 0.871 0.855 0.839 0.806

0.765 0.618 0.579 0.538 0.546 0.550 0.538 0.582

21.952 15.129 12.737 9.363 8.612 7.584 6.552 5.658

0.937 0.920 0.885 0.872 0.856 0.834 0.809 0.782

0.694 0.643 0.701 0.570 0.546 0.576 0.581 0.569

17.526 13.343 13.563 8.502 7.559 7.401 6.274 4.827

0.933 0.917 0.900 0.882 0.867 0.851 0.834 0.811

0.727 0.725 0.676 0.681 0.612 0.597 0,587 0.597

15.802 14.731 11.844 10.731 8.451 7.190 6.215 5.376

Standard uncertainties u are ur(w) = 0.025, u(T) = 0.1 K and u(p) = 0.02 bar

recovery was proved via 1H NMR and the results can be found as Supporting Information. 3.3. LLE Data Correlation. The experimental LLE data were regressed using the NRTL model42 by minimizing the objective function shown in eq 5, in which the DESs were treated as a single component.

values were found for all DESs than for pure glycerol. Thus, it can be concluded that the usage of glycerol-based DESs instead of glycerol improves the {ethanol + MEK} separation. Although in terms of solute distribution coefficient the improvement is not completely clear (since is only present for low ethanol concentrations), the lower viscosity of the DESs compared to pure glycerol is also a factor to consider in terms of process economics. Moreover, the higher selectivity values found for the DESs imply a reduction in the equipment sizes, which consequently will have a positive economic impact. Finally, the recovery and recyclability of the extracting agent is another important factor to be considered. In this work, the DESs were recovered after the extraction via evaporation of the volatile components (ethanol and MEK) using a vacuum line. The recovery of the DESs via flash distillation (mainly due to their low volatility) is one of their main advantages compared to volatile organic solvents when used as extracting agents for liquid−liquid extraction. The integrity of the solvents after

⎛ ⎛ ⎞cal ⎛ ⎞exp ⎞2 1 1 − ⎜β ⎟ ⎟ m n ⎜⎜β ⎟ ⎝ ⎠ ⎝ ij ij ⎠ ⎜ ⎟ OF = ∑ ∑ ⎜ exp ⎟ ⎛ ⎞ 1 i=1 j=1 ⎜ ⎜ ⎟ ⎟⎟ ⎜ ⎝ βij ⎠ ⎝ ⎠

(5)

where m is the number of tie-lines, n is the number of components in the mixture, and (1/β)exp and (1/β)cal are the inverse of the experimental and calculated solute distribution ratio, respectively. E



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Figure 3. (●, solid line) experimental tie-lines, presented in mass fraction, for the system: (a) {MEK + ethanol + GC(4:1)} at T/K = 298.15 and atmospheric pressure; (b) {MEK + ethanol + GC(2:1)} at T/K = 298.15 and atmospheric pressure; (c) {MEK + ethanol + GT(4:1)} at T/K = 298.15 and atmospheric pressure; (d) {MEK + ethanol + GT(2:1)} at T/K = 298.15 and atmospheric pressure; (●, dashed line) calculated tie-lines using the NRTL model.

Figure 4. Calculated solute distribution coefficient values for the systems (■) {MEK + ethanol + GC(4:1)}; (□) {MEK + ethanol + GC(2:1)}; (▲) {MEK + ethanol + GT(4:1)}; (△) {MEK + ethanol + GT(2:1)}; (●) {MEK + ethanol + glycerol)}34 at T/K = 298.15 and atmospheric pressure, as a function of the mass fraction of solute in the ketone-rich phase.

Figure 5. Calculated selectivity values for the systems (■) {MEK + ethanol + GC(4:1)}; (□) {MEK + ethanol + GC(2:1)}; (▲) {MEK + ethanol + GT(4:1)}; (△) {MEK + ethanol + GT(2:1)}; (●) {MEK + ethanol + glycerol)}34 at T/K = 298.15 and atmospheric pressure, as a function of the mass fraction of solute in the ketone-rich phase.

The root-mean-square deviation of the composition, σx, which compares the experimental and calculated mole fractions

of the components for each tie-line is a way to evaluate the goodness of the fitting and is calculated as follows: F



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⎛ ∑m ∑n − 1 (x I ,exp − x I ,calc)2 + (x II,exp − x II,calc) ⎞ ij ij ij ij i j ⎟ σx = 100 ⎜ ⎟ ⎜ 2 mn ⎠ ⎝

low ethanol concentrations in the MEK-rich phase, the improvements in the extraction efficiency are clear. For higher ethanol concentrations, the economic effect of the reduction of the viscosity of pure glycerol by the formation of a DES over the reduction of the solute distribution coefficient should be further studied. The recyclability of the solvents has been studied by recovering the DESs in a vacuum line after the extraction process. The integrity of the solvent has been proven by analyzing the DESs before the extraction and after the recovery via 1H NMR. The NRTL thermodynamic model has been satisfactorily applied to correlate the LLE data of ternary systems containing DESs.

(6)

The obtained parameters and deviations for all the studied systems are shown in Table 5. The fitting results were added to Figure 3. Table 5. NRTL Parameters Obtained from the Fitting of the Experimental Data ij 1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3 1−2 1−3 2−3

T/K

Δgij (kJmol−1)

Δgji (kJmol−1)

{MEK (1) + ethanol (2) + GC(4:1) (3)} 298.15 7.128 −4.998 10.830 160.189 1.934 1780.520 {MEK (1) + ethanol (2) + GC(2:1) (3)} 298.15 8.858 −6.688 13.147 186.225 0.955 1780.520 {MEK (1) + ethanol (2)+GT(4:1) (3)} 298.15 −4.964 −2.114 13.568 1.721 −4.042 2059.000 {MEK (1) + ethanol (2) + TG(2:1) (3)} 298.15 −2.669 2.709 11.718 1.058 0.947 2059.000

αij

σx

0.1

0.142

■

ASSOCIATED CONTENT

S Supporting Information *

0.1

0.222

0.09

0.412

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00717. Density and viscosity fitting parameters, 1HNMR of the studied solvents, before and after extraction (PDF)

■

AUTHOR INFORMATION

Corresponding Author 0.09

*E-mail: [email protected]; [email protected]. Tel.: +31 40 247 5289.

0.112

Funding

The authors of this work want to thank the Dutch Organization for Scientific Research (NWO) for the financial support of this project (Grant No.: ECHO.11.TD.006).

From both Figure 3 and Table 5, it can be observed that the NRTL model is well adjusted to the experimental data. Therefore, it can be concluded that this model can accurately correlate the LLE data of ternary systems containing DESs. The application of this model is very interesting, because of its simplicity and low consumption of computer resources.

Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS The effect of the usage of glycerol-based DESs instead of pure glycerol for the separation of the azeotropic mixture ethanol + methyl ethyl ketone via liquid−liquid extraction has been studied in this work. The selected glycerol-based DESs were glycerol/choline chloride with molar ratios of (4:1) and (2:1) and glycerol/tetramethylammonium chloride with molar ratios of (4:1) and (2:1). The density and viscosity as a function of temperature for the selected DESs have been measured. For the density it was found that the choline chloride-based DESs show higher densities compared to the tetramethylammonium chloride-based DESs. It was also found that the density of the DESs was lower than the density of pure glycerol. Moreover the density decreases with the amount of ammonium salt added. In terms of viscosity it has been found that the addition of ammonium salts also reduces the viscosity of the pure glycerol, probably due to the destruction of the 3D intramolecular H-bonding networks of pure glycerol. The LLE data of the systems {MEK + ethanol + GC(4:1)}, {MEK + ethanol + GC(4:1)}, {MEK + ethanol + GT(4:1)}, {MEK + ethanol + GT(2:1)} was experimentally determined. It has been found the solute distribution coefficient obtained for the studied DESs is higher than that of pure glycerol for ethanol concentrations in the MEK-rich phase lower than wEtOH = 0.2, while the selectivity values are higher than those of pure glycerol over the whole range of concentrations. Therefore, for G



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