Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers

Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers

Accepted Manuscript Title: Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers Author: Zhigang Zhang Angran H...

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Accepted Manuscript Title: Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers Author: Zhigang Zhang Angran Hu Tao Zhang Qinqin Zhang Mingyao Sun Dezhang Sun Wenxiu Li PII: DOI: Reference:

S0378-3812(15)00223-X http://dx.doi.org/doi:10.1016/j.fluid.2015.04.018 FLUID 10541

To appear in:

Fluid Phase Equilibria

Received date: Revised date: Accepted date:

20-1-2015 13-4-2015 23-4-2015

Please cite this article as: Zhigang Zhang, Angran Hu, Tao Zhang, Qinqin Zhang, Mingyao Sun, Dezhang Sun, Wenxiu Li, Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers, Fluid Phase Equilibria http://dx.doi.org/10.1016/j.fluid.2015.04.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Separation of methyl acetate + methanol azeotropic mixture using ionic liquids as entrainers Zhigang Zhang, Angran Hu, Tao Zhang, Qinqin Zhang, Mingyao Sun, Dezhang Sun, Wenxiu Li* Liaoning Provincial Key Laboratory of Chemical Separation Technology, Shenyang University of Chemical Technology, Shenyang 110142, China.

Graphical abstract Highlights 

Isobaric VLE data for three ternary systems containing ILs were measured at 101.3 kPa.



The addition of ILs produced a strong salting-out effect on methyl acetate and enhanced the relative volatility of methyl acetate to methanol.



The salting-out effect of three ILs follows the order: [C4MIM][Cl] > [ClC2MIM][Cl] > [C4 MIM][Br]



Ternary VLE data were well correlated by NRTL model.

Abstract: Three ionic liquids (ILs), namely 1-butyl-3-methylimidazolium chloride ([C4MIM][Cl]), 1-(2-chloroethyl)-3-methylimidazolium chloride

([ClC2MIM][Cl])

and 1-butyl-3-methylimidazolium

bromide ([C4MIM][Br]), were investigated as entrainers for the separation of azeotropic mixture of methyl acetate and methanol. Isobaric vapor-liquid equilibrium (VLE) data for the ternary systems containing ILs were measured in a recirculating still at 101.3 kPa. It was found that all the three ILs produced a salting-out effect on methyl acetate, thus enhancing the relative volatility of methyl acetate to methanol. The salting-out effect of the ILs employed in this study follows the order: [C4MIM][Cl] > [ClC2 MIM][Cl] > [C4MIM][Br]. Moreover, [C4MIM][Cl] could eliminate the azeotropic point at a mole fraction of 0.134. The experimental data were well correlated with the nonrandom two-liquid (NRTL) model. Keywords: vapor-liquid equilibrium, ionic liquid, methyl acetate, methanol, NRTL model. 1. Introduction The separation of methyl acetate and methanol mixture is a challenging task due to the fact that they form an azeotrope at atmospheric pressure. This azeotropic mixture is widely involved in the industrial

1

manufacturing process of poly (viny1 alcohol) [1]. Extractive distillation is the most common technique for the separation of azeotropic or close-boiling mixtures. It is evident that the selection of a suitable entrainer is crucial in extractive distillation [2]. Traditional entrainers such as organic solvents [3] or inorganic salts [4-7] have been employed in separating the mixture of methyl acetate and methanol, but they often make the process complicated, long and huge emerge consumption.

* Corresponding Author: Wenxiu Li, Email address: [email protected], Fax: 86-024-89383736

Efforts to make existing separation methods more efficient and eco-friendly may get a boost from the use of a relatively new class of compounds known as ionic liquids (ILs) [8]. ILs have negligible vapor pressure, low flammability, chemical stability at high temperatures, high selectivity, and excellent solvency for a variety of materials, which are favorable and attractive properties to serve as entrainers for extractive distillation. Moreover, the use of ILs as entrainers in extractive distillation integrates the advantage of a liquid solvent (easy operation) and a solid salt (high separation ability) [9, 10]. In terms of methyl acetate and methanol system, Orchillés et al. [11] measured isobaric VLE data using 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][Triflate] at 100 kPa. Cai et al. reported VLE for the ternary systems containing 1-ethyl-3-methylimidazolium acetate [EMIM][OAc] [12] or 1-octyl-3-methylimidazolium hexafluorophosphate [OMIM][PF6] at 101.3 kPa [13]. Matsuda et al. [14] measured the VLE data using 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtSO4]. Furthermore, Dhanalakshmi et al. [15] compared the separation effects of several ILs with different cations and anions on this azeotropic system using COSMO-RS model. Dohnal [16] reported the isothermal VLE of the ternary systems

with

1-butyl-1-methylpyrrolidinium

dicyanamide

[BMPYR][DCA]

and

1-ethyl-3-methylimidazolium thiocyanate [EMIM][SCN] at 327.31 K. The VLE data are essential for the design of extractive distillation process, but they are not enough up to now, making still it necessary to 2

further study the VLE for systems containing ILs. In this work, three ILs, namely [C4MIM][Cl], [ClC2MIM][Cl] and [C4MIM][Br], were employed as entrainers for the separation of methyl acetate + methanol azeotropic system. Firstly, the isobaric VLE data for three ternary systems, namely methyl acetate (1) + methanol (2) + [C4MIM][Cl] (3), methyl acetate (1) + methanol (2) + [ClC2 MIM][Cl] (3), and methyl acetate (1) + methanol (2) + [C4MIM][Br] (3) were measured at 101.3 kPa. The separation effects of the three ILs were tested by comparing their effects on the relative volatility of methyl acetate to methanol and the reason for differences of their separation ability was briefly analyzed. Additionally, the IL that has the best performance among the three ILs in separating the methyl acetate and methanol mixture is compared with ILs that have been reported in literature. Finally, the experimental data were correlated with nonrandom two-liquid (NRTL) model proposed by Renon and Prausnitz [17]. 2. Experimental section 2.1. Materials Methyl acetate and methanol were obtained from Sinopharm group. Their purities were higher than 99.5% (mass faction) examined by gas chromatography. The ILs [C4MIM][Cl], [ClC2MIM][Cl] and [C4MIM][Br] were synthesized in our own laboratory with mass fraction purities higher than 99% checked by liquid chromatography, also no impurities were detected by an ion chromatograph. The water content of ILs was less than 0.005 (mass fraction) determined with Karl Fischer titration. The ILs were dried further by vacuum vaporation at about 333 K for approximately 48 h prior to use. The final water content of ILs was determined with Karl Fischer titration, indicating a water mass fraction lower than 0.003. The specifications of the chemicals used in this study are provided in Table 1. Table 1 The specifications of chemical samples. Chemical name

Source

Purity

Purification method

Analysis method

Methyl acetate

Sinopharm group

0.995

None

GCa

3

Methanol

Sinopharm group

0.995

None

GCa

[C4MIM][Cl]

Synthesized own

0.990

Vacuum desiccation

LCb, KFc

[ClC2MIM][Cl]

Synthesized own

0.990

Vacuum desiccation

LCb, KFc

[C4MIM][Br]

Synthesized own

0.990

Vacuum desiccation

LCb, KFc

[C4MIM][Cl] = 1-butyl-3-methylimidazolium chloride. [ClC2MIM][Cl] = 1-(2-chloroethyl)-3-methylimidazolium chloride. [C4MIM][Br] = 1-butyl-3-methylimidazolium bromide. a

GC = gas chromatography.

b

LC = liquid chromatography.

c

KF = Karl Fischer titration.

2.2. Apparatus and procedure An all-glass dynamic recirculating still (NGW, Wertheim, Germany) was employed to conduct VLE experiments, a detail description of this still is available in previous literature [18]. The pressure of this apparatus was kept constant at 101.3 kPa by controlling a gas buffer connected with the still, and measured by a manometer with a standard uncertainty of 0.1 kPa. The temperature was determined using an accurate and calibrated thermometer with a standard uncertainty of 0.01 K. All the solutions were gravimetrically prepared with a digital balance (CAV264C OHAUS America), and the standard uncertainty is 0.0001 g. For the binary system of methyl acetate + methanol, some methyl acetate was added to the pure methanol until a very diluted solution was obtained. For each ternary system, the mixture of methanol and scheduled mass fraction of IL was prepared, at which some other mixtures of methyl acetate and the same mass fraction of IL were added, trying to keep a constant mass fraction of IL in each series. Only when the temperature was constant for more than 30 min, was the VLE assumed. 2.3. Sample analysis The condensed vapor and liquid phase samples were inserted into a headspace sampler (HS-sampler) (G1888 Network headspace sampler, Agilent Technologies) and then detected by a gas chromatograph (Model 7890A, Agilent Technologies). Since ILs have negligible vapor pressure, only the peaks of methyl acetate and methanol can be observed. The concentration of IL was calculated with the aid of the digital

4

balance after vaporizing the volatile components from a known mass of sample until constant mass. Each sample was analyzed at least three times, and the standard uncertainty of compositions in vapor and liquid phase is 0.001 (mole fraction). 3. Results and discussion 3.1. Experimental data To check the reliability of our apparatus, the VLE data of methyl acetate (1) + methanol (2) system were measured at 101.3 kPa. The VLE data for IL-free system are listed in Table 2. It can be observed in Table 2 that the measured data are consistent with the calculated ones using NRTL model, and the maximum absolute deviation between the experimental and calculated mole fractions of methyl acetate in the vapor phase is no more than 0.003. Fig. 1 illustrates that VLE data for methyl acetate + methanol measured in this work are in good agreement with those reported by M. C. Iliuta et al. [5] and M. Topphoff et al [6]. Hence the apparatus is adequate to measure the VLE data of the systems containing ILs. Table 2 VLE data for the binary system of methyl acetate (1) + methanol (2) at 101.3 kPa.a

a

γ2

Δy1

T/K

x1

y1

γ1

γ2

Δy1

T/K

0.999

0.000

337.69

0.424

0.546

1.425

1.189

0.002

327.58

2.630

0.998

0.001

337.10

0.495

0.581

1.317

1.273

0.001

327.17

0.091

2.481

1.000

0.000

336.03

0.576

0.620

1.220

1.392

0.000

326.89

0.061

0.166

2.369

1.002

0.000

334.62

0.647

0.656

1.155

1.519

0.000

326.77

0.081

0.208

2.287

1.005

0.000

333.81

0.737

0.707

1.089

1.733

-0.001

326.85

0.121

0.279

2.155

1.012

0.001

332.41

0.828

0.772

1.043

2.024

-0.003

327.27

0.162

0.336

2.014

1.024

0.001

331.28

0.910

0.856

1.020

2.359

-0.002

328.14

0.212

0.394

1.878

1.041

0.001

330.16

0.950

0.911

1.013

2.553

-0.002

328.87

0.263

0.440

1.742

1.067

0.001

329.28

0.980

0.960

1.009

2.772

-0.002

329.60

0.303

0.472

1.655

1.089

0.002

328.72

1.000

1.000

1.008

0.000

330.20

0.354

0.506

1.550

1.125

0.002

328.16

x1

y1

0.000

0.000

0.010

0.033

0.030

γ1

Δy1 = y1exptl - y1calcd.. Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.1 kPa, u(x1) = u(y1) = 0.001.

5

Fig. 1 Comparison of VLE data for the binary system of methyl acetate (1) + methanol (2). ■, this work; ▽, from Ref [5]; ○, from Ref [6]; solid line, correlated using the NRTL model.

The VLE data for ternary systems of methyl acetate (1) and methanol (2) containing [C4MIM][Cl] (3), [ClC2MIM][Cl] (3), or [C4MIM][Br] (3) were measured at 101.3 kPa to examine the separation effect of ILs on methyl acetate and methanol mixture. The mass fractions of ILs were fixed at w3 = 0.1, 0.2, and 0.3 in each series, respectively. The measured VLE data are listed in Tables 3-5, where T is the equilibrium temperature, x'1 is the mole fraction of methyl acetate in the liquid phase expressed on an IL-free basis, and y1 refers to the mole fraction of methyl acetate in the vapor phase, γ1 and γ2 are activity coefficients of methyl acetate and methanol, respectively. α12 is the relative volatility of methyl acetate to methanol, which is calculated by the following the equations. Due to the low total pressure in this work, the assumption of ideal vapor-phase is made, and VLE equation is simply written as follows: yi P = xi γi Pio

(1)

y1 / x1  1 P1o 12   • y2 / x2  2 P2o

(2)

where yi and xi present the mole fraction of component i in the vapor and liquid phase, respectively; γi refers to the activity coefficient of component i; P is the total pressure of the system, and Pio is the saturated vapor pressure of pure component i at equilibrium temperature which is calculated by Antoine equation. The Antoine equation is expressed as follows: 6

o

log Pi 

AB T C

(3)

where Pio is the saturated vapor pressure in kPa, T is equilibrium temperature in K, A, B, and C the Antoine parameters. The Antoine parameters [19], as well as the normal boiling point Tb of pure component measured in this work and that reported in literature [20] are presented in Table 3. All of the binary and ternary experimental VLE data comply with thermodynamic consistency by the Herington test (|D - J| < 10) [21].

Table 3 Antoine parameters and the normal boiling points of pure components. Component

Antoine parameters

Tb (101.3 kPa)/K

A

B

C

exptl

Lit.

Methyl acetate

6.0078 a

1076.05 a

-61.91 a

330.20

330.018 b

Methanol

7.0949 a

1521.23 a

-39.18 a

337.69

337.696 b

u(T) = 0.05 K. a From ref [19]. b From ref [20]. Table 4 VLE data for ternary system of methyl acetate (1) + methanol (2) + [C4MIM][Cl] (3) at 101.3 kPa.a T/K

x'1

y1

γ1

γ2

α12

T/K

x'1

y1

γ1

γ2

α12

328.10

0.426

0.569

1.498

1.141

1.783

w3 = 0.1 338.26

0.000

0.000

0.997

336.80

0.022

0.078

2.871

0.996

3.705

327.88

0.499

0.599

1.359

1.231

1.503

336.08

0.035

0.116

2.809

0.995

3.643

327.82

0.560

0.640

1.298

1.263

1.399

335.33

0.049

0.155

2.719

0.996

3.540

327.90

0.633

0.679

1.218

1.346

1.231

334.30

0.071

0.208

2.611

0.997

3.421

328.15

0.709

0.724

1.150

1.452

1.076

332.86

0.109

0.281

2.431

1.001

3.201

328.47

0.766

0.759

1.106

1.553

0.965

331.71

0.147

0.339

2.266

1.008

2.984

329.16

0.846

0.823

1.062

1.687

0.849

330.17

0.216

0.419

2.006

1.029

2.614

330.00

0.911

0.884

1.029

1.856

0.744

329.17

0.285

0.479

1.805

1.057

2.305

331.77

1.000

1.000

0.999

7

328.50

0.356

0.529

1.637

1.093

2.031 w3 = 0.2

339.21

0.000

0.000

0.984

329.04

0.576

0.687

1.355

1.130

1.619

336.74

0.038

0.134

2.957

0.979

3.883

329.24

0.632

0.718

1.284

1.169

1.481

335.14

0.070

0.217

2.787

0.976

3.707

329.51

0.685

0.748

1.227

1.207

1.369

333.80

0.103

0.287

2.621

0.975

3.519

329.83

0.733

0.777

1.181

1.247

1.273

331.91

0.165

0.385

2.337

0.979

3.162

330.28

0.785

0.812

1.136

1.292

1.178

330.79

0.221

0.450

2.134

0.986

2.889

330.75

0.832

0.846

1.102

1.327

1.109

329.77

0.297

0.519

1.900

1.002

2.548

331.28

0.875

0.879

1.071

1.383

1.030

329.24

0.367

0.568

1.723

1.025

2.267

332.04

0.927

0.925

1.039

1.426

0.965

329.00

0.429

0.599

1.575

1.068

1.992

333.37

1.000

1.000

1.001

328.92

0.505

0.648

1.457

1.090

1.806

330.54

0.524

0.696

1.500

0.963

2.082

w3 = 0.3

a

340.71

0.000

0.000

0.957

336.76

0.062

0.215

3.018

0.941

4.123

330.78

0.585

0.731

1.407

0.973

1.931

335.93

0.080

0.258

2.899

0.939

3.987

331.12

0.644

0.762

1.324

0.995

1.773

334.82

0.107

0.315

2.748

0.936

3.820

331.62

0.710

0.799

1.245

1.017

1.625

333.77

0.140

0.372

2.596

0.932

3.649

332.17

0.769

0.834

1.182

1.040

1.505

332.21

0.206

0.462

2.317

0.928

3.304

332.66

0.814

0.862

1.140

1.055

1.425

331.24

0.274

0.531

2.087

0.925

3.002

333.23

0.862

0.894

1.099

1.074

1.345

330.75

0.331

0.578

1.920

0.928

2.761

333.89

0.910

0.928

1.062

1.088

1.278

330.48

0.395

0.620

1.756

0.939

2.501

335.34

1.000

1.000

0.999

330.42

0.461

0.661

1.615

0.949

2.276

w3 is the mass fraction of IL in the liquid phase, and x'1 is the mole fraction of methyl acetate in the liquid phase expressed

on an IL-free basis. Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.1 kPa, u(x'1) = u(y1) = 0.001. Table 5 VLE data for ternary system of methyl acetate (1) + methanol (2) + [ClC2MIM][Cl] (3) at 101.3 kPa.a T/K

x'1

y1

γ1

γ2

α12

T/K

x'1

y1

γ1

γ2

α12

328.56

0.383

0.536

1.552

1.127

1.864

w3 = 0.1 338.34

0.000

0.000

0.997

336.88

0.023

0.078

2.823

0.997

3.636

328.39

0.413

0.556

1.502

1.144

1.780

335.96

0.039

0.126

2.726

0.998

3.528

328.05

0.485

0.601

1.403

1.190

1.602

333.79

0.087

0.238

2.493

1.001

3.262

328.05

0.594

0.660

1.262

1.291

1.328

332.50

0.125

0.299

2.286

1.013

2.979

328.23

0.685

0.710

1.168

1.410

1.124

8

331.67

0.155

0.345

2.191

1.015

2.865

328.55

0.756

0.746

1.106

1.579

0.949

330.89

0.190

0.390

2.085

1.019

2.729

329.29

0.844

0.813

1.050

1.755

0.806

330.22

0.226

0.424

1.949

1.036

2.520

329.97

0.898

0.864

1.030

1.907

0.725

329.33

0.292

0.480

1.767

1.064

2.239

330.72

0.939

0.911

1.016

2.030

0.669

328.88

0.338

0.511

1.653

1.092

2.045

332.15

1.000

1.000

0.996

329.49

0.409

0.576

1.581

1.082

1.968

w3 = 0.2 339.30

0.000

0.000

0.989

337.44

0.029

0.100

2.898

0.988

3.754

329.37

0.457

0.606

1.498

1.103

1.830

336.51

0.046

0.152

2.845

0.985

3.719

329.34

0.506

0.631

1.416

1.141

1.672

334.71

0.086

0.245

2.621

0.985

3.463

329.39

0.556

0.660

1.348

1.174

1.547

333.53

0.119

0.308

2.476

0.985

3.297

329.57

0.620

0.694

1.270

1.227

1.393

332.81

0.144

0.344

2.354

0.990

3.133

330.12

0.725

0.753

1.163

1.347

1.157

331.93

0.180

0.394

2.214

0.995

2.950

330.56

0.779

0.791

1.125

1.395

1.078

330.82

0.244

0.458

1.987

1.013

2.618

331.15

0.836

0.832

1.083

1.482

0.974

330.08

0.309

0.516

1.822

1.024

2.385

332.75

0.955

0.942

1.017

1.765

0.760

329.77

0.351

0.540

1.700

1.055

2.165

333.76

1.000

1.000

1.005

331.49

0.302

0.530

1.915

0.976

2.607

w3 = 0.3

a

340.85

0.000

0.000

0.966

339.47

0.017

0.067

3.130

0.969

4.081

331.13

0.362

0.574

1.763

0.988

2.377

338.28

0.037

0.132

3.010

0.966

3.966

330.97

0.391

0.593

1.698

0.996

2.272

337.49

0.052

0.174

2.922

0.965

3.876

330.98

0.426

0.616

1.629

1.003

2.165

336.53

0.072

0.226

2.822

0.961

3.777

330.98

0.478

0.646

1.531

1.022

1.998

335.03

0.111

0.307

2.625

0.958

3.559

331.15

0.549

0.683

1.411

1.063

1.768

333.87

0.150

0.371

2.448

0.958

3.346

331.79

0.665

0.751

1.268

1.110

1.514

333.16

0.180

0.412

2.321

0.959

3.182

332.88

0.787

0.823

1.148

1.190

1.270

332.64

0.208

0.443

2.212

0.963

3.031

334.59

0.890

0.901

1.070

1.243

1.122

332.27

0.232

0.471

2.144

0.960

2.953

336.86

1.000

1.000

1.002

w3 is the mass fraction of IL in the liquid phase, and x'1 is the mole fraction of methyl acetate in the liquid phase expressed

on an IL-free basis. Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.1 kPa, u(x'1) = u(y1) = 0.001. Table 6 VLE data for ternary system of methyl acetate (1) + methanol (2) + [C4MIM][Br] (3) at 101.3 kPa.a T/K

x'1

y1

γ1

γ2

α12

T/K

x'1

y1

γ1

γ2

α12

328.21

0.407

0.540

1.470

1.168

1.709

w3 = 0.1 338.09

0.000

0.000

0.999

9

333.91

0.080

0.216

2.434

1.008

3.162

327.82

0.499

0.589

1.329

1.258

1.438

333.40

0.093

0.243

2.390

1.008

3.113

327.68

0.601

0.643

1.213

1.383

1.195

332.85

0.109

0.274

2.344

1.007

3.067

327.81

0.695

0.692

1.126

1.556

0.985

332.38

0.124

0.298

2.284

1.009

2.991

328.10

0.765

0.738

1.081

1.699

0.865

331.45

0.159

0.342

2.125

1.023

2.761

328.63

0.835

0.790

1.042

1.904

0.741

330.64

0.195

0.388

2.013

1.030

2.614

329.36

0.897

0.855

1.024

2.047

0.674

330.53

0.201

0.394

1.996

1.031

2.588

329.96

0.934

0.895

1.010

2.245

0.604

329.37

0.275

0.461

1.776

1.064

2.250

331.50

1.000

1.000

1.001

329.01

0.306

0.481

1.689

1.086

2.101

328.70

0.472

0.581

1.387

1.207

1.555

w3 = 0.2 338.67

0.000

0.000

336.38

0.038

0.126

2.787

0.993

3.617

328.62

0.528

0.613

1.314

1.257

1.416

334.89

0.070

0.201

2.578

0.996

3.366

328.71

0.636

0.676

1.206

1.364

1.197

334.33

0.083

0.227

2.476

1.002

3.227

328.90

0.690

0.707

1.157

1.440

1.087

333.49

0.107

0.276

2.422

0.997

3.187

329.17

0.743

0.738

1.113

1.542

0.974

332.44

0.142

0.325

2.232

1.011

2.916

329.65

0.807

0.790

1.081

1.620

0.898

331.33

0.189

0.390

2.084

1.016

2.730

330.17

0.856

0.831

1.055

1.718

0.823

330.09

0.267

0.461

1.835

1.048

2.348

331.30

0.933

0.908

1.022

1.901

0.715

329.32

0.343

0.513

1.637

1.094

2.016

332.78

1.000

1.000

1.001

328.91

0.412

0.561

1.515

1.126

1.818

329.80

0.530

0.636

1.356

1.178

1.547

0.996

w3 = 0.3

a

339.49

0.000

0.000

336.00

0.062

0.189

2.676

0.986

3.506

329.89

0.585

0.665

1.288

1.226

1.411

334.48

0.101

0.268

2.468

0.990

3.251

330.06

0.637

0.694

1.231

1.278

1.293

333.36

0.137

0.330

2.331

0.991

3.090

330.44

0.709

0.736

1.164

1.360

1.145

331.61

0.220

0.427

2.019

1.011

2.652

331.00

0.782

0.785

1.110

1.448

1.023

330.97

0.266

0.469

1.881

1.026

2.447

331.24

0.806

0.802

1.092

1.496

0.971

330.56

0.306

0.499

1.768

1.045

2.262

331.95

0.867

0.853

1.058

1.579

0.888

330.13

0.365

0.539

1.630

1.075

2.033

332.94

0.931

0.917

1.029

1.666

0.813

329.91

0.419

0.574

1.531

1.100

1.870

334.36

1.000

1.000

1.001

329.80

0.475

0.605

1.433

1.141

1.688

0.989

w3 is the mole fraction of IL in the liquid phase, and x'1 is the mole fraction of methyl acetate in the liquid phase expressed

on an IL-free basis. Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.1 kPa, u(x'1) = u(y1) = 0.001.

10

3.2. Regression analysis In this work, the nonrandom two-liquid (NRTL) model is used to correlate the ternary VLE data since it is often suitable for the correlation of systems containing ILs [22-24]. There are nine adjustable parameters for three pairs in the model. Firstly, the IL-free binary parameters were obtained from the binary VLE data of methyl acetate and methanol system. The remaining model parameters were obtained from the VLE data of ternary systems containing ILs. The NRTL model parameters were regressed by Levenberg–Marquardt method using the following objective function: exptl

ARD(%) 

1 i i  exptl n n i

calcd

 100

(4)

where γi is the activity coefficient of the solvent i, the indices exptl and calcd denote the experimental and calculated values, respectively; n is the number of experimental data points. All the model parameters and the average relative deviations (ARDs) between the experimental data and the calculated values by NRTL model are listed in Table 7. A good agreement between the experimental data and correlated results is shown in Figs. 2-4. Table 7 Binary energy parameters for NRTL model. Component i

Component j

αij

g ij

g ji

J mol

J mol

ARD(%)

Methyl acetate

Methanol

0.302

1507.7

1626.3

1.57

Methyl acetate

[C4MIM][Cl]

0.884

-96.9

22519.6

1.79

Methanol

[C4MIM][Cl]

0.277

230.2

-6715.7

Methyl acetate

[ClC2MIM][Cl]

0.324

130.6

23308.3

Methanol

[ClC2MIM][Cl]

0.397

600.8

-3970.4

11

1.85

Methyl acetate

[C4MIM][Br]

0.834

72.8

20046.7

Methanol

[C4MIM][Br]

0.307

106.63

-3970.4

Δgij = gij - gii, τij = (gij - gii)/RT.

12

1.76

Fig. 2 Composition diagram for the

Fig. 3 Composition diagram for the

Fig. 4 Composition diagram for the

VLE of methyl acetate (1) + methanol

VLE of methyl acetate (1) + methanol

VLE of methyl acetate (1) + methanol

(2) + [C4MIM][Cl] (3) system at 101.3

(2) + [ClC2MIM][Cl] (3) system at

(2) + [C4MIM][Br] (3) system at 101.3

kPa: dotted line, IL-free system; ■, w3

101.3 kPa: dotted line, IL-free system;

kPa: dotted line, IL-free system; ■, w3

= 0.1; ▲, w3 = 0.2; ▼, w3 = 0.3; solid

■, w3 = 0.1; ▲, w3 = 0.2; ▼, w3 = 0.3;

= 0.1; ▲, w3 = 0.2; ▼, w3 = 0.3; solid

line, correlated using NRTL model.

solid line, correlated using NRTL

line, correlated using NRTL model.

model.

It can be observed from Figs. 2-4 that the addition of an IL produces a salting-out effect on methyl acetate which makes the content of methyl acetate in the vapor phase enlarge, and this effect is more notable with the increase of mass fraction of IL. The increasing content of ILs also shifts upward the azeotropic point. Moreover, the azeotropic point is eliminated by [C4MIM][Cl] with its mass fraction up to 0.3. According to the NRTL model, to totally break the azeotropic point of methyl acetate and methanol mixture at 101.3 kPa, the mole minimum fractions of [C4MIM][Cl], [ClC2MIM][Cl] and [C4MIM][Br] are x3 = 0.134, x3 = 0.204, and x3 = 0.269, respectively. Therefore, the conclusion that [C4MIM][Cl] has more remarkable azeotrope breaking capacity for methyl acetate + methanol system than [ClC2MIM][Cl] and [C4MIM][Br] can be made.

13

Fig. 5 Relative volatility of methyl

Fig. 6 Relative volatility of methyl

Fig. 7 Relative volatility of methyl

acetate (1) to methanol (2) with

acetate (1) to methanol (2) with

acetate (1) to methanol (2) with

different

different

different

mass

fractions

of

mass

fractions

of

mass

fractions

of

[C4MIM][Cl] at 101.3 kPa: dotted line,

[ClC2MIM][Cl] at 101.3 kPa: dotted

[C4MIM][Br] at 101.3 kPa: dotted line,

IL-free system; ■, w3 = 0.1; ▲, w3 =

line, IL-free system; ■, w3 = 0.1; ▲, w3

IL-free system; ■, w3 = 0.1; ▲, w3 =

0.2; ▼, w3 = 0.3; solid line, correlated

= 0.2; ▼, w3 = 0.3; solid line,

0.2; ▼, w3 = 0.3; solid line, correlated

using NRTL model.

correlated using NRTL model.

using NRTL model.

As relative volatility is an important indicator of the separation efficiency of an entrainer, the influences of ILs on the relative volatility of methyl acetate to methanol are investigated and presented in Fig. 5-7. It can be seen that a small quantity of ILs significantly improves the relative volatility of methyl acetate to methanol. The more ILs, the greater the relative volatility. Furthermore, among the three ILs investigated, [C4MIM][Cl] gives the highest α12 while the enhancement of α12 produced by [ClC2MIM][Cl] is greater than that produced by [C4MIM][Br]. Hence the salting-out effect produced by three ILs follows the order: [C4MIM][Cl] > [ClC2MIM][Cl] > [C4MIM][Br]. It is worth noting that the volatility of methyl acetate to methanol far surpasses 1 on the whole investigated concentration range when the mass fraction of [C4MIM][Cl] is 0.3, which verifies the good separation efficiency of [C4MIM][Cl] to the azeotrope.

14

Fig. 8 Activity coefficient of methyl acetate γ1 in relation with the mole fraction of methyl acetate on an IL-free basis for the mixtures containing different ILs at 101.3 kPa: dotted line, IL-free system; ■, w3 = 0.1; ▲, w3 = 0.2; ▼, w3 = 0.3; solid line, correlated using NRTL model.

The relative volatility α12 is related to the ratio of activity coefficients as shown in Eq. (2) [25]. The ratio of saturated vapor pressures, P1o/P2o, is insensitive to temperature holding constant at 1.25 to 1.37 in the present work, and makes almost no difference to the relative volatility, thus α12 mainly depends on the ratio of γ1/γ2. The activity coefficients of methyl acetate (γ1) and methanol (γ2) in the ternary systems with different amounts of ILs, against the mole fraction of methyl acetate expressed on an IL-free basis, are presented in Figs. 8-9 to illustrate the effects of ILs on the solution non-ideality. As can be seen in Fig. 8, the activity coefficient of methyl acetate γ1 increases with the content of ILs. However, as shown in Fig. 9, the activity coefficient of methanol γ2 decreases with the increase of ILs. This phenomena shows strong interactions between ILs and methanol, forcing methyl acetate to the vapor phase, thus increasing relative volatility of methyl acetate to methanol. When the mass fractions of ILs are at the same level, the value of γ1 in the ternary systems containing different ILs follows the order: [C4MIM][Cl] > [ClC2MIM][Cl] > [C4MIM][Br]. But the opposite applies for the value of γ2. This versed orders of the activity coefficients for 15

methyl acetate and methanol contribute to understanding the differences of three ILs in enhancing the relative volatility α12. The anion of [Cl] has a stronger role in interaction with methanol than the anion of [Br], thus the salting-out effect produced by [C4MIM][Cl] is stronger than [C4MIM][Br]. When comparing [C4MIM][Cl] with [ClC2MIM][Cl], the introducing of the strong polar chlorine in the cation is unfavorable, probably because it cancels the effect of the IL in some extent, thus the salting-out effect produced by [ClC2MIM][Cl] is weaker than [C4MIM][Cl].

Fig. 9 Activity coefficient of methanol γ2 in relation with the mole fraction of methyl acetate on an IL-free basis for the mixtures containing different ILs at 101.3 kPa: dotted line, IL-free system; ■, w3 = 0.1; ▲, w3 = 0.2; ▼, w3 = 0.3; solid line, correlated using NRTL model.

The separation effect of [C4MIM][Cl] on the methyl acetate and methanol system is compared with those of ILs reported in literature. The minimum mole fraction of [C4MIM][Cl] to break the azeotrope at 327.31 K is 0.124 calculated using NRTL model with parameters listed in Table 7, since Dohnal [16] has compared the azeotrope breaking capacity of previous reported ILs at the same condition. The minimum mole fractions of ILs (3) to break the methyl acetate (1) + methanol (2) azeotrope at 327.31 K of [C4MIM][Cl] and those of the reported ILs are summarized in Table 8. It can be seen that the azeotrope breaking capacity 16

of [C4MIM][Cl] is superior to those of [EMIM][Triflate] [11] and [OMIM][PF6] [13] but not so strong as those of [EMIM][OAc] [12], [BMPYR][DCA] and [EMIM][SCN] [16]. On the other hand, the synthesis is easier and the price is lower than [EMIM][OAc], [BMPYR][DCA] and [EMIM][SCN], from this standpoint, [C4MIM][Cl] is a potential entrainer for the separation of methyl acetate and methanol system. Table 8 The minimum mole fractions of ILs (3) to break the methyl acetate(1) + methanol(2) azeotrope at 327.31 K.

a

Ionic liquids

x3,min

Reference

[C4MIM][Cl]

0.123

This work

[EMIM][Triflate]

0.125 a

From [16]

[EMIM][OAc]

0.096

From [16]

[OMIM][PF6]

0.343

From [16]

[BMPYR][DCA]

0.043

From [16]

[EMIM][SCN]

0.087

From [16]

Electrolyte NRTL (eNRTL).

4. Conclusions In this work, the isobaric VLE data for methyl acetate (1) and methanol (2) with [ClC2MIM][Cl] (3), [C4MIM][Cl] (3) or [C4MIM][Br] (3) were measured in a recirculating still at 101.3 kPa. The results show that the addition of [C4MIM][Cl], [ClC2MIM][Cl] or [C4MIM][Br] produces a notable salting-out effect on methyl acetate and significantly improves the relative volatility of methyl acetate to methanol. The more ILs, the stronger salting-out effect is produced. Moreover, the azeotropic point can be eliminated by [C4MIM][Cl] at a mole fraction of 0.134 at 101.3 kPa. The salting-out effect of the three investigated ILs follows the order: [C4MIM][Cl] > [ClC2MIM][Cl] > [C4MIM][Br]. Additionally, the experimental data were correlated with NRTL model, and the correlated results are in good agreement with the experimental data. 17

5. Acknowledgments This work is financially supported by the National Science Foundation of China (Project No. 21076126), Program for Liaoning Excellent Talents in University (LR2012013) and Liaoning Province science foundation of China (Project No. 2014020140). List of symbols Δgij

binary energy parameter of NRTL model

w3

mass fraction of IL in ternary mixture

xi

mole fraction of solvent i in the liquid phase

yi

mole fraction of solvent i in the vapor phase

yiexptl

mole fraction of solvent i in the vapor phase measured by experimental data

yicalcd

mole fraction of solvent i in the vapor phase calculated with the NRTL model

T

equilibrium temperature

Tb

normal boiling point of pure component

P

total pressure in the equilibrium system

Pio

saturated vapor pressure of component i at equilibrium temperature

Greek letters α12

relative volatility of component 1 to component 2

αij

non-randomness parameter of NRTL model

γi

activity coefficient of component i

γiexptl

the activity coefficient of component i measured by experimental data

γicalcd

the activity coefficient of component i calculated with the NRTL model

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21