Separation Effects of Renewable Solvent Ethyl Lactate on the Vapor

Article pubs.acs.org/jced

Separation Effects of Renewable Solvent Ethyl Lactate on the Vapor− Liquid Equilibria of the Methanol + Dimethyl Carbonate Azeotropic System Hiroyuki Matsuda,* Koji Inaba, Keiji Nishihara, Hirofumi Sumida, Kiyofumi Kurihara, Katsumi Tochigi, and Kenji Ochi Department of Materials and Applied Chemistry, Nihon University, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan ABSTRACT: Ethyl lactate, which is an expected renewable solvent, was tested as an entrainer candidate for the separation of the binary methanol + dimethyl carbonate (DMC) azeotropic system by extractive distillation. Isobaric vapor−liquid equilibria (VLE) for two binary constituent systems, that is, methanol + DMC and DMC + ethyl lactate of the methanol + DMC + ethyl lactate ternary system, were determined by an ebulliometric method at pressures of (40.00 to 101.3) kPa. The experimental VLE data were fitted by the nonrandom two-liquid (NRTL) model. Predictions of the binary systems were also performed by the NIST-modified universal functional activity coefficient (UNIFAC) group contribution model. The separation effects of ethyl lactate were examined by two methods: residue curve map and relative volality α12 using the binary NRTL parameters. Both sets of calculated results indicated that ethyl lactate can be used as entrainer. Compared to other entrainers reported in the previous works, ethyl lactate would be a more potential entrainer for breaking the methanol + DMC azeotropic system by extractive distillation, considering not only the minimum liquid mole fraction of the entrainer, but also the corresponding boiling point of the ternary system, viscosity, molecular weight, and price. Finally, the measurements of VLE for the methanol + DMC + ethyl lactate ternary system were also done, and these behaviors were compared with the predictions using binary NRTL parameters and the NIST-modified UNIFAC model. at the industrial scale.8 The viscosity of ethyl lactate is lower than that of ILs. Therefore, ethyl lactate can be used without any modification of equipment or procedure in the separation and purification processes.3 In addition, the cost of ethyl lactate is quite lower than that of ILs. So, if effective separation could be achieved using ethyl lactate, it would have a high advantage as a green solvent for the separation and purification processes. Previously, we investigated ethyl lactate as an entrainer for the separation of binary azeotropic systems by extractive distillation. Vapor−liquid equilibrium (VLE) data are of importance in the design and development of extractive distillation. Especially, knowledge of enhancement in the relative volality, α12 of a binary azeotropic mixture by adding entrainer is particularly significant. In our previous work, the binary azeotropic system methyl acetate + methanol was investigated, and the VLE of the methyl acetate + methanol + ethyl lactate ternary system and its binary constituent systems were measured at pressures of (40.00 to 101.3) kPa. The obtained residual curve map and calculated α12 showed that ethyl lactate could be a suitable entrainer for the separation of this azeotropic system.9

1. INTRODUCTION Alternative solvents for green chemistry have been paid a great deal of attention as promising candidates to replace toxic organic solvents and volatile organic compounds (VOCs). One type of alternative solvents, renewable solvents, can be easily produced from biomass feedstocks. One such renewable solvent, ethyl lactate, has excellent properties, such as low toxicity, relatively high boiling point, high solvency power, good biodegradability, and recyclability.1−3 Because of these features, ethyl lactate has been applied as a green solvent in several areas. One example in the applications of ethyl lactate is as a replacement solvent for toxic organic solvents and VOCs such as hazardous air pollutants (2-butanone, 4-methyl-2-pentanone, and toluene) for magnetic tape coating,4 and N-methyl pyrrolidone (NMP), toluene, acetone, and xylene.3,5 Another application is as a solvent for pharmaceutical manufacturing processes as a pharmaceutical and food additive with an approval by the US Food and Drug Administration.1,2 Thus, it has been employed as an environmentally benign reaction solvent for the synthesis of pharmaceutical compounds.6,7 This work aims for a use of ethyl lactate as a green solvent for separation and purification processes, for which VOCs have been used as entrainer for azeotropic or extractive distillation, and solvent for liquid−liquid extraction, as an example. Ionic liquids (ILs) have also become popular for these processes in recent years because of their unique properties; however, the high viscosities of some ILs sometimes limit their potential use © 2017 American Chemical Society

Special Issue: Memorial Issue in Honor of Ken Marsh Received: February 17, 2017 Accepted: June 29, 2017 Published: July 14, 2017 2944

DOI: 10.1021/acs.jced.7b00185 J. Chem. Eng. Data 2017, 62, 2944−2952

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in this Work component

CASRN

source

grade

molecular sieve

purity (mass fraction)

water content (ppm)

methanol DMC ethyl lactate

67-56-1 616-38-6 97-64-3

Wako Pure Chemical Industries

special grade

3A 4A 13×

>0.999 >0.997 >0.995

7 81 11

type ebulliometer (a detail description is presented in Apparatus and Procedure section). Densities, ρ/kg m−3 at T = 298.15 K and P = 0.1 MPa were measured using a oscillating U-tube density meter (DMA 4500, Anton Paar, Graz, Austria) with a reproducibility of 10−2 kg m−3. The experimental Tb and ρ at 298.15 K of the chemicals used are presented in Table 2 with the literature values.12,25,26

The present work continues our study of the use of ethyl lactate as entrainer candidate of extractive distillation. In this work, we selected a methanol + dimethyl carbonate (DMC) binary azeotropic system, which forms a minimum boiling point azeotrope. In the DMC production process involving transesterification of cyclic carbonates with methanol, azeotropic point of the methanol + DMC system10−22 causes a separation problem. To our knowledge, several salts or organic solvents such as tetramethylammonium bicarbonate,14,15 2-ethoxyethanol,13 and 4-methyl-2-pentanone13 have been employed as entrainers for the separation of methanol + DMC azeotrope. Also, different kinds of ILs have also been studied as the entrainers, such as 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]),16 tetrafluoroborate-based ILs such as 1butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4])16 or 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM][BF4]),17 phosphate-based ILs such as 1-methyl-3methylimidazolium dimethylphosphate ([MMIM][DMP]),18 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]),18 and 1-butyl-3-methylimidazolium dibutylphosphate ([BMIM][DBP]),19 trifluoromethanesulfonate-based ILs such as 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf])20 and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]),20 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]),21 and halogenbased ILs such as 1-ethyl-3-methylimidazolium bromide ([EMIM][Br])22 and 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]).22 Here, isobaric VLE data of two binary constituent systems, that is, the methanol + DMC and DMC + ethyl lactate systems and the methanol + DMC + ethyl lactate ternary system were measured at (40.00 to 101.3) kPa by an ebulliometric method. The experimental results of VLE were represented by the nonrandom two-liquid (NRTL) model.23 In addition, we used the NIST-modified universal functional activity coefficient (UNIFAC) group contribution model24 for the predictions of these binary systems, and the prediction accuracy was evaluated. The separation effects of ethyl lactate as entrainer for the separation of methanol + DMC were evaluated by calculating the residue curve map and α12 using the NRTL model coupled with binary parameters obtained from binary VLE data. The minimum liquid composition of ethyl lactate for breaking the azeotropic point of the system was compared with those of entrainer candidates studied in the reported works.13,16−22 Finally, we determined the VLE data of the methanol + DMC + ethyl lactate ternary system. These behaviors were predicted using the NRTL and NIST-modified UNIFAC models.

Table 2. Normal Boiling points (Tb) and Densities at 298.15 K (ρ) of the Pure Components Used in this Worka ρ (298.15 K)/kg m−3

Tb/K component

experimental

literature

experimental

literature


337.60b 363.00 426.10b

337.696c 363.39d 427.7c 424.98e

786.56 1063.29 1028.08

786.37c 1063.28c 1027.2c 1028.0e

a Standard uncertainties are u(ρ) = 0.01 kg/m3, u(Tb) = 1 K, u(P) = 1 kPa. bReference 9. cReference 25. dReference 12. eReference 26.

Table 3 lists the Antoine constants of the chemicals used in this work. Those of methanol and ethyl lactate were the same Table 3. Antoine Equation Constantsa for the Pure Components Used in this Work Antoine constants

a

component

A

B

C


7.1323b 6.1081 6.7499b

1553.46b 1209.50 1776.94b

−34.588b −68.210 −51.719b

log(Psi /kPa) = A − B/[(T/K) + C]. bReference 9.

values as those used in our previous work.9 Regarding DMC, they were estimated from the experimental vapor pressures. The experimental vapor pressures of DMC are shown in Table 4. Antoine constants A, B, and C were determined by the Table 4. Experimental Vapor Pressures of DMCa

a

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals used in this work are given in Table 1. Purities of the chemicals were verified by a gas chromatograph (GC-4000, GL Sciences Co., Ltd., Tokyo, Japan) with a thermal conductivity detector. Water contents were determined by a Karl Fischer moisture meter (CA-200, Mitsubishi Chemical Co., Ltd., Tokyo, Japan). Normal boiling points, Tb/K were determined by a modified Swietoslawski-

P/kPa

T/K

40.00 53.33 66.66 79.99 63.32 101.3

336.61 344.29 350.50 355.83 360.49 363.00

Standard uncertainties are u(P) = 0.03 kPa, u(T) = 1 K.

Marquardt method27 using the following objective function Fobj: NDP

Fobj =

∑ k=1

2945

⎛ Pexptl − Pcalcd ⎞2 ⎜⎜ ⎟⎟ Pexptl ⎝ ⎠k

(1) DOI: 10.1021/acs.jced.7b00185 J. Chem. Eng. Data 2017, 62, 2944−2952


Article

Table 5. Experimental Boiling Points, Liquid Phase Mole Fraction (x1), and Temperature (T), for the Methanol (1) + DMC (2) Systema 40.00 kPa

a

53.33 kPa

66.66 kPa

79.99 kPa

93.32 kPa

101.3 kPa

x1

T/K

x1

T/K

x1

T/K

x1

T/K

x1

T/K

x1

T/K

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

336.61 326.03 320.55 318.06 316.24 315.46 314.89 314.57 313.93 314.53 315.49b

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

344.29 333.42 327.92 325.01 323.28 322.14 321.56 321.17 320.64 320.99 321.98b

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

350.50 339.62 333.74 330.72 328.87 327.57 326.97 326.41 326.06 326.24 327.23b

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

355.83 344.81 338.68 335.49 333.59 332.32 331.54 330.92 330.63 330.67 331.66b

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

360.49 349.38 343.07 339.71 337.79 336.43 335.53 334.93 334.62 334.53 335.51b

0.0000 0.1001 0.2003 0.3001 0.4003 0.5001 0.5999 0.7002 0.7999 0.9000 1.0000

363.00 351.94 345.64 342.01 339.99 338.71 337.69 337.14 336.76 336.63 337.60b

Standard uncertainties are u(P) = 0.03 kPa, u(x) = 0.0001, u(T) = 1 K. bReference 9.

Table 6. Experimental Boiling Points, Liquid Phase Mole Fraction (x1), and Temperature (T) for the DMC (1) + Ethyl Lactate (2) Systema 40.00 kPa

a

53.33 kPa

66.66 kPa

79.99 kPa

93.32 kPa

101.3 kPa

x1

T/K

x1

T/K

x1

T/K

x1

T/K

x1

T/K

x1

T/K

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

396.91b 380.50 370.28 362.56 355.82 351.28 347.34 344.79 341.72 338.60 336.61

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

405.49b 390.45 379.32 371.42 364.81 359.93 355.61 352.95 349.85 346.46 344.29

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

412.44b 397.83 386.48 379.67 370.56 367.40 360.69 359.50 356.33 352.77 350.50

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

418.34b 403.88 392.69 385.68 377.99 372.70 368.00 365.26 361.82 358.18 355.83

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

423.47b 409.13 397.76 391.18 383.07 377.89 373.03 370.15 366.71 362.88 360.49

0.0000 0.1001 0.2000 0.2998 0.4000 0.4996 0.6000 0.7000 0.7999 0.8999 1.0000

426.10b 411.74 400.54 394.05 386.00 380.80 375.60 372.78 369.26 365.46 363.00

Standard uncertainties are u(P) = 0.03 kPa, u(x) = 0.0001, u(T) = 1 K. bReference 9.

where NDP is the number of experimental vapor pressures, and Pexptl/kPa and Pcalcd/kPa are the experimental and calculated vapor pressures, respectively. 2.2. Apparatus and Procedure. We used a modified Swietoslawski-type ebulliometer for the measurements of boiling points. Detailed descriptions of the experimental apparatus and its procedure can be found in our previous works.9,12,28 Boiling points of the mixture were determined with a calibrated platinum resistance thermometer with an accuracy of ±0.01 K. The pressure in the ebulliometer was established with a pressure controller (DPI515, Druck Co., Kirchentellinsfurt, Germany). The liquid composition of the mixture was determined by weighing using a balance (AX504, MettlerToledo Inc., Columbus, OH, USA) with a sensitivity of 0.1 mg. The uncertainty in the liquid composition was estimated to be 0.0001 mole fraction. The equilibrium state in the boiling point of the mixture was assumed when a fluctuation range of boiling point showed ±0.02 K/min. In the measurements at 101.3 kPa, the ebulliometer was open to atmosphere, and the pressure in the ebulliometer was determined by a Fortin barometer with an accuracy of ±0.013 kPa (±0.1 mmHg). The experimental boiling points were corrected to 101.3 kPa by using eq 2 as with our previous work:29

T /K = Texptl /K + ×

1 NC

2.303 ∑i = 1 Bi xi /(Texptl /K + Ci)2

101.3 − Pexptl /kPa Pexptl /kPa

(2)

where NC is the number of components, Texptl/K is the experimental boiling point at the actual atmospheric pressure, Pexptl/kPa, Bi, and Ci are the Antoine constants of component i, as listed in Table 3, xi is the mole fraction in the liquid phase.

3. RESULTS AND DISCUSSION 3.1. Experimental VLE for Binary Systems. Isobaric VLE data for two binary systems, methanol (1) + DMC (2) and DMC (1) + ethyl lactate (2), were measured at (40.00 to 101.3) kPa. We previously reported those for the methanol (1) + ethyl lactate (2) system.9 The experimental VLE data are summerized in Tables 5 and 6, and are illustrated in Figures 1 and 2 together with the literature values.11,12,14,17,19,20 Literature VLE data of methanol + the DMC system at 101.3 kPa pass two constituency tests, the point-to-point test of Van Ness et al.,30 in the version of Fredenslund et al.,31,32 and the area test by Herington et al.33 From Figure 1, the experimental VLE data of methanol + DMC system agreed well with the literature values. Literature values for DMC + ethyl lactate system are not available. 2946



Article

For the methanol + DMC and DMC + ethyl lactate systems, binary parameters g12−g22/J mol−1, g21−g11/J mol−1, and α12 in the NRTL model were determined using the Marquardt method27 for each data set of the experimental VLE, considering the correlation accuracies of the binary systems. Parameter optimization was done by minimizing Fobj as follows: NDP

Fobj =

∑ (Texptl − Tcalcd)k2

(4)

k=1

where NDP is the number of experimental boiling points, and Texptl/K and Tcalcd/K are the experimental and calculated boiling points, respectively. Table 7 presents the estimated Table 7. Parameters and Deviations between the Experimental and Calculated Boiling Points (|ΔT|) for the Methanol (1) + DMC (2) and DMC (1) + Ethyl Lactate (2) Systems Using the NRTL Modela Figure 1. Temperature−composition relationships for the methanol (1) + DMC (2) system. This work: ●, 40.00 kPa; ▲, 53.33 kPa; ■, 66.66 kPa; ▼, 79.99 kPa; ◆ 93.32 kPa; ★, 101.3 kPa; , NRTL; ---, NIST-modified UNIFAC; ⊗, azeotropic points. Literature: ○, ◇, Fukano et al.;12 ☆, Rodriguez et al.;11 ×, Yang et al.;14 gray circle, Li et al.;17 gray triangle, Chen et al.;19 gray box, Li et al.20

P/kPa

a

g12 − g22/ J mol−1

g21 − g11/ J mol−1

α12

40.00 53.33 66.66 79.99 93.32 101.3 overall

3329.79 4136.91 4469.77 4570.84 3902.19 4876.34

Methanol (1) + DMC (2) 1069.84 0.41 −12.1965 0.23 −384.273 0.20 −500.260 0.20 166.793 0.30 −799.155 0.20

40.00 53.33 66.66 79.99 93.32 101.3 overall

3054.46 2697.97 4964.76 3656.53 3611.46 3874.29

DMC (1) + Ethyl Lactate (2) −2045.83 0.20 −1813.47 0.25 −2917.90 0.31 −2639.23 0.22 −2658.65 0.20 −2253.46 0.39

|ΔT|av/K |ΔT|max/K 0.13 0.07 0.06 0.06 0.08 0.07 0.08

0.24 0.15 0.14 0.18 0.20 0.18 0.24

0.27 0.22 0.65 0.35 0.41 0.73 0.44

0.40 0.42 1.39 0.69 0.98 1.56 1.56

NDP

|ΔT |av = ∑k = 1 |Texptl − Tcalcd|k /NDP , where NDP is the number of data points.

parameters in the NRTL model and the average and maximum deviations between the experimental and calculated boiling points, |ΔT|av/K and |ΔT|max/K, respectively. From Table 7, the NRTL model gave good correlation accuracies for both binary systems. The VLE diagrams calculated by the the NRTL model are shown in Figures 1 and 2. In the methanol + DMC system, calculated vapor phase compositions were compared with those of the literature data.11,14,17,19,20 Average and maximum deviations between the literature and calculated boiling points, |ΔT|av/K, |ΔT|max/K, and those in vapor phase compositions, |Δy1|av and |Δy1|max, are summarized in Table 8. When these deviations are compared to the ones between their literature and regressed values by activity coefficient models,11,14,17,19,20 our calculation accuracies are nearly the same to their regression ones. Azeotropic points in the methanol + DMC system were calculated from the determined binary NRTL parameters. The determined azeotropic mole fraction y1,az and temperature Taz/K are listed in Table 9 and Figure 1. The VLE prediction for two binary systems was also tested by the NIST-modified UNIFAC model. Group interaction parameters anm, bnm, and cnm of this model were determined by the group of Kang et al. by using critically evaluated phase equilibrium data.24 van der Waals volumes Rk, surface areas Qk

Figure 2. Temperature−composition relationships for the DMC (1) + ethyl lactate (2) system. This work: ●, 40.00 kPa; ▲, 53.33 kPa; ■, 66.66 kPa; ▼, 79.99 kPa; ◆, 93.32 kPa; ★, 101.3 kPa; , NRTL; ---, NIST-modified UNIFAC.

3.2. Data Correlation and Prediction of Binary Systems. The NRTL model was employed to correlate the experimental VLE data of binary constituent systems. For the methanol + ethyl lactate system, the calculated results in our previous work4 were used. The following criterion was used for the calculation of VLE assuming ideal gas behavior: Pyi = γiPiSxi

(3)

where xi and yi are the mole fractions of component i in the liquid and vapor phase, respectively, γi is the liquid phase activity coefficient, P/kPa is the total pressure, and PSi /kPa is the saturated vapor pressure of pure component i. PSi in eq 3 was calculated using the Antoine equation with the Antoine constants listed in Table 3. 2947



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Table 8. Deviations between the Literature and Calculated Boiling Points (|ΔT|) and Vapor-Phase Mole Fractions (|Δy1|) for the Methanol (1) + DMC (2) System at 101.3 kPa Using the NRTL Modela,b |ΔT|av/K

|ΔT|max/K |Δy1|av ́ Rodriguez et al. (2002)11 0.57 0.005 Li et al. (2012)17 0.44 0.007 Li et al. (2014)20 0.62 0.004

0.29 0.24 0.17 a

|ΔT |av =

NDP ∑k = 1 |Texptl

b

− Tcalcd|k /NDP |Δy1|av =

|Δy1|max

|ΔT|av/K

0.012

0.33

0.025

0.39

|ΔT|max/K

|Δy1|av

Yang et al. (2012)14 0.78 0.006 Chen et al. (2013)19 0.89 0.010

|Δy1|max 0.018 0.023

0.012 NDP ∑k = 1 |y1,exptl

− y1,calcd |k /NDP, where NDP is the number of data points.

where ξ is the dimensionless measure of time. xi was determined by the Runge−Kutta fourth-order method in combination with the NRTL model with the parameters of binary constituent systems. The NRTL parameters of methanol + ethyl lactate system were obtained from our previous work.9 Figure 3 shows the calculated residue curve map at 101.3 kPa

Table 9. Azeotropic Temperature (Taz) and Composition (y1,az) in Mole Fraction for Methanol (1) + DMC (2) System Calculated with the NRTL Modela P/kPa

y1,az

Taz/K

40.00 53.33 66.66 79.99 93.32 101.3

0.827 0.835 0.842 0.848 0.855 0.859

314.16 320.77 326.10 330.60 334.50 336.65

a

Standard uncertainties are u(P) = 0.03 kPa, u(y1,az) = 0.0001, u(Taz) = 1 K.

of the functional groups, and the group interaction parameters anm, bnm, and cnm were cited from the Dortmund Data Bank (DDB) 2016.34 |ΔT|av/K and |ΔT|max/K are presented in Table 10. Also, Figures 1 and 2 show the predicted results. This Table 10. Deviations between the Experimental and Calculated Boiling Points (|ΔT|) for the Methanol (1) + DMC (2) and DMC (1) + Ethyl Lactate (2) Systems Using the NIST-Modified UNIFAC Modela methanol (1) + DMC (2)

a

DMC (1) + ethyl lactate (2)

P/kPa

|ΔT|av/K

|ΔT|max/K

|ΔT|av/K

|ΔT|max/K

40.00 53.33 66.66 79.99 93.32 101.3 Overall

0.21 0.18 0.21 0.22 0.21 0.20 0.21

0.50 0.38 0.45 0.35 0.31 0.37 0.39

4.35 4.51 4.34 4.51 4.42 4.34 4.41

10.26 10.20 10.07 9.90 9.96 9.86 10.04

Figure 3. Residue curve map and isovolatility line for the methanol (1) + DMC (2) + ethyl lactate (3) system at 101.3 kPa. , residue curve using NRTL model; ---, isovolatility line; ⊗, unstable node [azeotropic point of methanol (1) + DMC (2)]; ●, stable node; ○, saddle node.

for the methanol + DMC + ethyl lactate system. In Figure 3, the azeotropic point of the methanol + DMC system is an unstable node, that of ethyl lactate is a stable node, and those of methanol and DMC are saddle points. Also, the isovolatility lines intersect the methanol (1)−ethyl lactate (3) edge. Therefore, from the literature of Laroche et al.,35 methanol can be distilled from the top in an extractive distillation column, and DMC can be distilled in the entrainer-recovery column. Next, α12 is defined as follows:36

NDP

|ΔT |av = ∑k = 1 |Texptl − Tcalcd|k /NDP, where NDP is the number of data points.

model provided good prediction results of methanol + the DMC system, while somewhat large errors between the experimental and predicted boiling points could be observed compared to |ΔT|av/K in Table 7. However, in the DMC + ethyl lactate system, this model gave large prediction errors, giving lower boiling points in comparison with the experimental data especially in the liquid mole fraction range x1 < 0.4. 3.3. Separation Effect as Entrainer. The separation effect of ethyl lactate as entrainer was examined by analyzing the residue curve map and α12. First, the residue curve can be calculated by solving the following differential equation: dxi = xi − yi , dξ

i = 1, ...n − 1

α12 =

y /x1 K1 = 1 K2 y2 /x 2

(6)

where xi and yi are the liquid and vapor phase mole fractions of the ternary system, respectively. α12 was calculated on the basis of the prediction of ternary VLE for three liquid compositions of ethyl lactate, x3 = 0.000, 0.050, and 0.120, as is the case with the residue curve map. Figure 4 shows the calculated α12 versus ethyl lactate-free liquid mole fraction, x1sf. Without entrainer (x3 = 0.00), the binary azeotropic system cannot be separated

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But the effect of it is inferior to that of [EMIM][EtSO4], [BMIM][BF4], [MMIM][DMP], [EMIM][DEP], [BMIM][DBP], and [EMIM][TCB]. On the other hand, when the boiling points at x3,min are compared, the value of ethyl lactate is the lowest. This boiling point Tb = 340 K is almost the same value as that of [EMIM][TCB], which has the second lowest value of x3,min. A low boiling point of the ternary mixture needs less energy consumption to achieve equilibrium. In addition, according to Blahut and Dohnal,21 a good entrainer should have some criteria, for example, “melting temperature below ambient (to avoid possibility of crystallization), low viscosity (to ensure a sufficiently high mass transfer rate), low heat capacity (to minimize energy demands for heating/cooling), low molecular mass (to minimize transportation costs), also non-corrosive, nontoxic, and cheap”. Table 11 shows that the viscosity and molecular weight of ethyl lactate are quite lower than those of ILs. Also, as described in the Introduction section, it is less expensive than the ILs. Therefore, ethyl lactate would have more potential as a green entrainer for the separation of methanol + DMC system by extractive distillation. 3.4. Experimental VLE and Prediction for Ternary System. Isobaric VLE for the methanol + DMC + ethyl lactate ternary system were determined at 101.3 kPa with fixed liquid mole fraction of ethyl lactate x3 = 0.050 and 0.120. x3 = 0.120 is where the azeotropic point of the methanol + DMC system disappears, as mentioned in the previous section. The experimental results of VLE are reported in Table 12. Figure 5 shows the experimental boiling points versus ethyl lactate-free liquid mole fraction, x1sf. VLE predictions of the system were carried out by using the NRTL parameters of binary constituent VLE data. Predicted vapor mole fractions y1,calcd, y2,calcd, and the ethyl lactate-free basis vapor mole fraction, y1sf,calcd are also presented in Table 12. |ΔT|av./K and |ΔT|max/K are given in Table 13. Predicted boiling points of the system were in reasonable agreement. We also tested the VLE predictions using the NIST-modified UNIFAC model. The predicted results are presented in Table 13. The prediction accuracy obtained with this model was somewhat better compared to those obtained by the NRTL model. Figure 5 shows the results predicted by both models.

Figure 4. Predicted relative volalities α12 for the methanol (1) + DMC (2) + ethyl lactate (3) system at 101.3 kPa via the NRTL model for different liquid mole fractions of ethyl lactate x3. ---, x3 = 0.000; ·, x3 = 0.050; , x3 = 0.120.

by normal distillation, because the dotted line in Figure 4 intersects with α12 = 1 at the azeotropic point. When one added ethyl lactate, the apparent azeotropic point broke at x3 = 0.120, and α12 was >1 over the entire mole fraction range of x1sf. Therefore, ethyl lactate can be a possible entrainer for the extractive distillation of the methanol + DMC system. The minimum liquid composition of ethyl lactate for breaking the azeotropic point of the methanol + DMC system was compared with that of entrainer candidates studied in the reported works.13,16−22 Table 11 summarizes the minimum mole fraction of entrainer (x3,min) in which the azeotropic point of the system disappears, and corresponding pressure and boiling point of the mixture, viscosity at 298 K, and molecular weight of ethyl lactate, and entrainers studied in the previous works, that is, two organic solvents,13 and 11 ILs.16−22 From the value of x3,min, ethyl lactate has better separation effect in comparison with 2-ethoxyethanol, 4-methyl-2-pentanone, [OMIM][BF4], [EMIM][OTf], [BMIM][OTf], [EMIM][Br].

Table 11. Minimum Mole Fraction of Entrainer (x3,min) for Breaking the Azeotropic Point of the Methanol + DMC System and Corresponding Boiling Point of the Mixture (Tb), Viscosity (η), and Molecular Weight (M) entrainer organic solvents

ILs

ethyl lactate 2-ethoxyethanol 4-methyl-2-pentanone [EMIM][EtSO4] [BMIM][BF4] [OMIM][BF4] [MMIM][DMP] [EMIM][DEP] [BMIM][DBP] [EMIM][OTf] [BMIM][OTf] [EMIM][TCB] [EMIM][Br] [BMIM][Cl]

ref

x3,min

P/kPa

Tb/Ka

η/mPa sb

ref

M/g mol−1

this work 13 13 16 16 17 18 18 19 20 20 21 22 22

0.120 0.423c 0.170c 0.100c 0.100c 0.200d 0.074c 0.090c 0.096c 0.202d 0.133d 0.080c 0.1461c 0.1168c

101.3 93.32 93.32 n.a.e n.a.e 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3

340 349f 337f n.a. n.a. 341d 353d 354d 355d 348d 344d 341d 350f 353f

2.44 2.05 0.546 98 103 341 291 410 n.a. 43 90 18 n.a. 29

25 25 25 37 38 39 40 41 n.a. 42 43 44 n.a. 45

118.13 90.12 100.16 236.29 226.02 282.13 222.18 264.26 348.42 260.23 288.29 226.05 191.07 174.67

a

At 101.3 kPa, x3,min, and x1 corresponding to the azeotrope break point. bAt 298.15 K. cReported value in the given reference. dCalculated value by Blahut and Dohnal.21 eIsothermal condition (T = 333.15 K). fCalculated in this work from the given reference. 2949



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Table 12. Experimental Boiling Points, Liquid Phase Mole Fractions (xi), Ethyl Lactate-Free Liquid Mole Fractions (x1sf), Temperatures (T), Vapor Phase Mole Fractions (yi,calcd), and Ethyl Lactate-Free Vapor Mole Fractions (y1sf) Calculated with NRTL Model for the Methanol (1) + DMC (2) + Ethyl Lactate (3) Ternary Systema x1

x2

0.0000 0.0949 0.1901 0.2849 0.3800 0.4751 0.5701 0.6650 0.7603 0.8550 0.9500

0.9500 0.8551 0.7599 0.6651 0.5700 0.4749 0.3799 0.2850 0.1897 0.0950 0.0000

0.0000 0.0881 0.1760 0.2638 0.3522 0.4401 0.5280 0.6161 0.7042 0.7921 0.8800

0.8800 0.7919 0.7040 0.6162 0.5278 0.4399 0.3520 0.2639 0.1758 0.0879 0.0000

x1sf

T/K

y1,calcd

x3 = 0.050; 101.3 kPa 0.0000 364.91 0.0000 0.0999 354.90 0.2904 0.2001 348.73 0.4563 0.2999 344.63 0.5616 0.4000 342.40 0.6376 0.5001 340.17 0.6988 0.6001 339.62 0.7535 0.7000 338.64 0.8072 0.8003 338.22 0.8641 0.9000 338.44 0.9270 1.0000 339.52 0.9988 x3 = 0.120; 101.3 kPa 0.0000 366.97 0.0000 0.1001 358.56 0.2435 0.2000 352.58 0.4085 0.2998 348.34 0.5257 0.4002 344.83 0.6158 0.5001 342.90 0.6896 0.6000 342.02 0.7552 0.7001 340.99 0.8171 0.8002 340.20 0.8780 0.9001 340.25 0.9385 1.0000 342.02 0.9966

y2,calcd

y1sf,calcd

0.9954 0.7071 0.5421 0.4373 0.3615 0.3004 0.2458 0.1922 0.1352 0.0721 0.0000

0.0000 0.2911 0.4570 0.5622 0.6381 0.6993 0.7541 0.8077 0.8647 0.9279 1.0000

0.9875 0.7487 0.5861 0.4703 0.3810 0.3076 0.2423 0.1805 0.1195 0.0586 0.0000

0.0000 0.2454 0.4107 0.5278 0.6178 0.6915 0.7570 0.8191 0.8802 0.9412 1.0000

Table 13. Deviations between the Experimental and Calculated Boiling Points (|ΔT|) for the Methanol (1) + DMC (2) + Ethyl Lactate (3) Systems Using the NRTL and NIST-Modified UNIFAC Modelsa NRTL

NIST-modified UNIFAC

x3

|ΔT|av/K

|ΔT|max/K

|ΔT|av/K

|ΔT|max/K

0.05 0.12

0.59 1.06

1.17 2.19

0.39 0.69

1.03 1.85

NDP

a

|ΔT |av = ∑k = 1 |Texptl − Tcalcd|k /NDP , where NDP is the number of data points.

experimental VLE of their binary systems were correlated well by the NRTL model. The NIST-modified UNIFAC model was also applied for the prediction of these systems. Good prediction accuracies could be obtained for the methanol + DMC system. However, in the DMC + ethyl lactate system, large prediction errors of the boiling points were observed. The separation effect of ethyl lactate as entrainer was evaluated using a residue curve map and α12 calculated using the NRTL model with the parameters of the binary constituent systems. Results of two calculations indicated that ethyl lactate could be a possible entrainer for the separation of the azeotropic system. From a comparison of x3,min of ethyl lactate with that of other entrainers studied in the reported works, the separation effect of ethyl lactate was inferior to that of some ILs. However, ethyl lactate could be a more suitable and green entrainer, considering the corresponding boiling point of the ternary system, viscosity, and molecular weight, and price. Finally, VLE data for the ternary methanol + DMC + ethyl lactate system were also measured at x3 = 0.050 and 0.120. Predicted results of the VLE of the ternary system using the NRTL model with the parameters of binary constituent systems and NIST-modified UNIFAC model represented well the experimental ternary data.

a Standard uncertainties are u(P) = 0.03 kPa, u(x) = 0.0001, u(T) = 1 K.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-3259-0814. Fax: +81-3-3293-7572. E-mail: [email protected]. ORCID

Hiroyuki Matsuda: 0000-0003-3580-8483 Notes

The authors declare no competing financial interest.

■

REFERENCES

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Figure 5. Temperature−composition relationships for the methanol (1) + DMC (2) + ethyl lactate (3) system at x3 = 0.000, 0.050, and 0.120 at 101.3 kPa. This work: ■, x3 = 0.000; ▲, x3 = 0.050; ●, x3 = 0.120; , NRTL; ---, NIST-modified UNIFAC.

4. CONCLUSIONS We determined isobaric VLE of two binary constituent systems of the methanol + DMC + ethyl lactate ternary system, that is, methanol + DMC and DMC + ethyl lactate at pressures of (40.00 to 101.3) kPa using an ebulliometric method. The 2950



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