Measurement and Modeling of Vapor–Liquid Equilibria for Systems

Article pubs.acs.org/jced

Measurement and Modeling of Vapor−Liquid Equilibria for Systems Containing Alcohols, Water, and Imidazolium-Based Phosphate Ionic Liquids Junfeng Wang* and Zhibao Li* Key laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Vapor−liquid equilibiria (VLE) were measured for systems containing 1-methyl-3-ethylimidazolium diethylphosphate ([EMIM][DEP]) ionic liquid (IL) in the temperature range from 310 K to 390 K. The systems include 1-propanol + [EMIM][DEP], 2-propanol + [EMIM][DEP], water +1propanol + [EMIM][DEP], and water + 2-propanol + [EMIM][DEP]. [EMIM][DEP] was divided into one 1,3-dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and three methylene (CH2) electrically neutral groups. And then the experimental VLE data for binary systems containing [EMIM][DEP] were regressed using the modified UNIFAC model with the maximum average relative deviation (ARD) of 2.7 %. The newly obtained interaction parameters between groups allowed the reliable prediction of other binary and ternary systems containing [MMIM][DMP] and [EMIM][DEP] without parametrization. It was found that both of the ILs can give rise to the salting-out effect, and lead to a breaking of the azeotropic behavior of alcohols + water mixtures. [MMIM][DMP] shows a higher separation ability for the azeotropic mixture studied than [EMIM][DEP]. experimental composition range.9 Lladosa et al. have used the Wilson, NRTL, and UNIQUAC models to predict VLE for binary and ternary mixtures containing ethanol, 2-propanol, 2butanone, and butyl propionate at 101.3 kPa.10 The results showed that the Wilson equation gave the best correlation results for all systems. The modified UNIFAC model which has been applied successfully for prediction of the phase equilibria and excess properties for systems with ionic liquids, allowed the reliable prediction of binary and multicomponent systems containing ionic liquids and could be applied successfully for the synthesis and design of the different process.11 Among these models mentioned above, the group contribution-based UNIFAC (Dortmund) is widely used. The advantages of the modified UNIFAC model include the reliable prediction for a large number of systems requiring only fewer group interaction parameters.12−14 Also, the model has already been embedded in the Aspen Plus software with a process modeling tool. Therefore, the modified UNIFAC model is chosen for this study. Following our previsou work,7 VLE data for 1-propanol + [EMIM][DEP], 2-propanol + [EMIM][DEP], water + 1propanol + [EMIM][DEP], and water + 2-propanol + [EMIM][DEP] systems were measured using the quasi-static ebulliometer apparatus. The experimental data for three binary systems containing [EMIM][DEP] were regressed by the

1. INTRODUCTION Azeotropic mixtures, for example, water + ethanol, water +1propanol, and water +2-propanol, are nonideal solutions that can be separated under the condition of higher energy consumption. Among the separation techniques of these mixtures, the extractive distillation is widely employed. However, entrainer selection limits its further application since solid salts corrode the pipeline and organic substances demand high energy. Ionic liquids have shown great potential as a potential alternative to conventional volatile organic solvents in separation processes.1−4 A major reason is that its negligible vapor pressure avoids the loss of volatile solvent and decreases the risk of worker exposure.5,6 Imidazolium-based phosphate ionic liquids are probable for separation processes, and VLE data for several systems containing the kind of IL had been measured in our previous work.7,8 The results showed that the kind of ionic liquid can enhance the relative volatility of light components in water + ethanol, ethanol + methanol, water + 1-propanol, and water + 2-propanol azeotropic mixtures. Besides the experimental data, reliable prediction methods are also a prerequisite to describe satisfactorily the experimental data and find the suitable ILs for separating the mixtures. To estimate the activity coefficients of solvents in systems, a number of thermodynamic models have been used to make calculations for systems containing ILs. The nonrandom two-liquid (NRTL) equation has been used by Geng et al. to correlate the VLE for the water + ethanol + 1butyl-3-methylimidazolium chloride system, indicating that the equation was adequate for the ternary system in the © XXXX American Chemical Society

Received: December 28, 2012 Accepted: May 10, 2013

A

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modified UNIFAC model embedded in the AspenPlus. And then the other seven systems containing [MMIM][DMP] and [EMIM][DEP] were predicted using the resulting interaction parameters.

Φi =

xiri ∑j xjrj z

θi =

2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals of 1-propanol and 2propanol were supplied by Beijing Red Star Reagents Company and were used without further purification in the experiments. All were analytical research grade with a purity of 99.7 %. [EMIM][DEP] was prepared in the laboratory according to literature procedures,8 and its purity is 98.0 %. Distilled water with specific conductivity (< 0.1 μS·cm−1) was used. 2.2. Apparatus and Procedure. VLE data were measured by the quasi-static ebulliometric method; its working principle as well as the operation procedure was described in previous work.8 The apparatus was composed of a working ebulliometer and a reference one, and the two ebulliometers shared the same equilibrium pressure. The equilibrium pressure of the reference system was calculated by the equilibrium temperature of pure water according to the temperature−pressure relation.15 The sample of mixture (approximately 85 cm3) was placed in the working ebulliometer, and the same volume of deionized water was charged into the reference ebulliometer. The composition of sample was determined gravimetrically by a digital balance (model JA5003). The uncertainty of the mole fraction was estimated within ± 0.002. The system was adjusted to an appropriate degree of pressure. And then both the sample and the pure water were heated and stirred vigorously with magnetic stirrers to enhance the liquid-phase circulation. After the vapor−liquid equilibrium was attained (the temperatures of both ebulliometers were kept constant for 10 min), the two equilibrium temperatures which were measured using twochannel four-wire 25 Ω calibrated platinum resistance thermometers (type CST6601) with an uncertainty of ± 0.02 K, were recorded. The uncertainty of vapor pressure arising from the uncertainty of temperature measurement was estimated within ± 0.04 kPa. The next measurement was performed at elevated temperatures.

xi 2 qi z

∑j xj 2 qj

(4)

(i) where ri = ∑v(i) k Rk, qi = ∑vk Qk, and the coordination number z is set to 10. The parameter Φ′i /xi in eq 2 can be calculated using the following equation:

Φ′i ri 3/4 = xi ∑j xjrj 3/4

(5)

and the residual activity coefficient of component i arising from intermolecular forces can be calculated by the following equation: m

ln γi R =

∑ vk(i)(ln Γk − ln Γ(ki))

(6)

k=1

⎛ ln Γk = Q k ⎜⎜1 − ln ∑ θmτmk − ⎝ m

⎛ θτ ⎞⎞ m km ⎟⎟⎟⎟ ng ⎝ ∑n θnτnm ⎠⎠

∑ ⎜⎜ m

(7)

where θ m = (X m Q m )/(∑ n X n Q n ) and X m (∑j∑nv(j) n xj). To accurately describe the phase equilibrium behavior of multicomponent systems, the temperature-dependent parameters have been introduced into the modified UNIFAC (Dortmund) model.16

=(∑ j v m(i) x j )/

⎛ a + b T + c T2 ⎞ nm nm ⎟ τmn = exp⎜ − nm T ⎝ ⎠

(8)

The relative volume R and surface area Q parameters were taken from the literature,11,18 and are listed in Table 1. There Table 1. Group Parameters of Relative Volume R and Relative Surface Area Q group a

water CHa CH2a CH3a OH(p)a OH(s)a [MMIM][DMP]b

3. THERMODYNAMIC BACKGROUND 3.1. The Modified UNIFAC Model. The modified UNIFAC (Dortmund) model embedded in AspenPlus has introduced the temperature-dependent group interaction parameters and different van der Waals quantities.16 In this model, the excess Gibbs free energy is made up of a combinatorial term and a residual term, and accordingly the activity coefficient of a solvent i, γi, can be expressed:16 ln γi = ln γiC + ln γi R

(3)

R

Q

1.7334 0.6325 0.6325 0.6325 1.2302 1.0630 7.1620

2.4561 0.3554 0.7081 1.0608 0.8927 0.8663 5.8440

a

Group parameters taken from ref 11. bGroup parameters taken from ref 18.

(1)

are six fitted interaction parameters in this model for each group−group interaction. The group interaction parameters, amn, bmn, cmn, anm, bnm, and cnm for OH−CH2, OH-H2O, and CH2−H2O were taken from the literature,19 and are listed in Table 2. The group interaction parameters for OH-[MMIM][DMP], CH2−[MMIM][DMP], and H2O-[MMIM][DMP] can be obtained by correlating the VLE data for the three binary systems of water + [EMIM][DEP], 1-propanol + [EMIM][DEP], and 2-propanol + [EMIM][DEP] in this work (in the following section 4.2). 3.2. Group Segmentation of Ionic Liquid. The modified UNIFAC model is a group-contribution model.17 To calculate

17

In comparison to the original UNIFAC method, the modified UNIFAC (Dortmund) model has a slightly different combinatorial part: ⎛ Φ′ ⎞ Φ Φ⎞ z ⎛ ⎛Φ ⎞ ln γiC = ln⎜ i ⎟ + 1 − i − qi⎜⎜ln⎜ i ⎟ + 1 − i ⎟⎟ xi 2 ⎝ ⎝ θi ⎠ θi ⎠ ⎝ xi ⎠ (2)

The molecular volume and surface fractions can be calculated by the following equations: B

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Table 2. Group Interaction Parameters for Modified UNIFAC (Dortmund) main groups

group interaction parameters

1

a12 (K)

b12

c12 (K−1)

a21 (K)

b21

c21 (K−1)

2777.0 1606.0 1391.3 −17.253 −2727.3 1833.7 −801.90 1460.0 −1997.5 −421.58 1623.8 1792.9

−4.6740 −4.7460 −3.6156 0.8389 7.4509 −5.4576 3.8240 −8.6730 4.3100 1.7203 −11.469 −12.081

1.551·10−3 9.181·10−4 1.144·10−3 9.021·10−4 0 2.705·10−5 −7.514·10−3 1.641·10−2 0 8.386·10−5 1.843·10−2 1.630·10−2

2

CH2

OH(p)

CH2

water

CH2

[MMIM][DMP]

OH(p)

water

OH(p)

[MMIM][DMP]

water

[MMIM][DMP]

⎡ (B − V1)(P − P1s) ⎤ ϕc = exp⎢ 1 ⎥ ⎣ ⎦ RT

the activity coefficient of solvents in systems containing ILs using the model, ILs should be divided into electrically neutral groups. In our previous work,18 a novel group segmentation method which can represent the infinite dilution activity coefficients for six molecular solutes in different ionic liquids very well, has been put forward. On the basis of this method, [EMIM][DEP] was divided into one [MMIM][DMP] group and three CH2 groups in this study. For solvents, 1-propanol was divided into one CH3 group, two CH2 groups, and one OH (p) group, 2-propanol was divided into two CH3 groups, one CH group, and one OH (s) group, and water as a whole group was not divided. 3.3. Vapor−Liquid Equilibrium. Vapor pressure Psi of water, 1-propanol, and 2-propanol can be calculated with the Antoine equation:15 ln(Pis/kPa) = A −

B (T /K + C)

By substituting eq 11 into eq 10, the expression P1sx1γ1 = y1Pϕc

(12)

is obtained. The parameter in eq 11, B1, can be calculated as follows: BP B1 = c (13) RT B = 0.08664035/Pc − 0.4274802Tc/PcT

(14)

2/7

(9)

The Antoine constants for the three solvents taken from the literature20 are listed in Table 3. The vapor phase composition can be calculated using the following equation: ⎡ Vi (P − Pis) ⎤ s s ϕ = P ϕ x γ exp yP ⎢ ⎥ i i i i i i ⎣ ⎦ RT

(11)

V1 =

RTcZc1 + (1 − Tr) Pc

(15)

T=

T Tc

(16)

The critical parameters, Tc, Pc, and Zc, for 1-propanol and 2propanol used in the following calculation are listed in Table 4. Table 4. Critical Parameters Tc, Pc, and Zc21

(10)

Table 3. Antoine Coefficients A, B, and C20

component

Tc/K

Pc/kPa

Zc

1-propanol 2-propanol

536.75 535.55

5168 4153

0.253 0.248

Antoine coefficients component

A

B

C

1-propanol 2-propanol water

16.0353 16.4089 16.5700

3415.56 3439.60 3984.92

−70.7330 −63.4170 −39.7240

The correction of fugacity coefficient were calculated, the range of φc for 1-propanol and 2-propanol was from 1.00001 to 1.003 and 1.00001 to 1.004, respectively. And the average error was 0.15 % and 0.2 %, respectively. That is, for this binary system interest, the correction of fugacity coefficient was equal to one at low pressure or the relatively small pressure difference (P − Psi ). Thus, eq 10 can be simplified as follows:

For a binary system containing IL, because the vapor pressure of IL is safely assumed to be zero, y1 = 1. The Poynting factor, exp[(V1(P−Ps1)/RT)], takes into account the change of the fugacity upon expansion or compression of the pure liquid chosen as the standard state as a function of its saturation vapor pressure on the system pressure. The ratio of fugacity coefficient ϕ1/ϕs1 can also be expressed as exp[(B1(P−Ps1)/ (RT))]. Thus, the correction of fugacity coefficient, ϕc, can be written as

γ1 = P /(P1sx1)

(17)

The experimental activity coefficient of solvent in eq 17 is based on the Raoult’s law. For a ternary system containing IL, the vapor pressure and mole fraction of solvent in the vapor phase at low pressure can be calculated using the following equations: p = p1s x1γ1 + p2s x 2γ2 C

(18)

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yi =

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pis xiγi p1s x1γ1

+

p2s x 2γ2

using eq 17, are less than 1. The vapor pressure of 1-propanol in the binary 1-propanol + [EMIM][DEP] system at IL mass fraction of 10 %, 30 %, 50 %, and 70 % is shown in Figure 1. It can be seen that the ln(P/kPa) against 1/(T/K + C) relation is linear at constant liquid composition.

(19)

4. RESULTS AND DISCUSSION 4.1. Experimental Data. The vapor pressure data for two binary systems of 1-propanol + [EMIM][DEP] and 2-propanol + [EMIM][DEP] as well as two ternary systems of water + 1propanol + [EMIM][DEP] and water + 2-propanol + [EMIM][DEP] were measured, and are listed in Tables 5 to 8, respectively. Note that the correction of fugacity coefficient for solvents (as mentioned above) is equal to 1 at low pressure, so that eq 17 is sufficient to describe the relationship between pressure and activity coefficient. The experimental activity coefficients, γexp 1 , of the 1-propanol and 2-propanol which can be calculated Table 5. Vapor Pressure with Standard Uncertainties of Temperature, Pressure, and Compositions for Binary System 1-Propanol (1) + [EMIM][DEP] (2)a T/K

Pexp/kPa

329.801 336.595 342.574 349.562 355.785 361.073 366.307 370.052

16.816 23.562 31.304 42.864 55.947 69.841 85.865 99.252

331.478 340.285 343.511 350.227 354.846 360.082 365.210 370.654

16.026 24.634 28.654 38.741 47.263 58.776 72.350 89.339

340.106 346.336 350.972 356.759 363.702 368.689 372.833 375.463

16.672 22.295 27.741 35.872 47.766 58.393 68.541 75.849

343.789 352.026 357.817 363.113 369.033 376.387 382.889 388.347

10.617 15.613 20.405 26.440 34.896 47.380 60.918 74.662

Pcal/kPa x1 = 0.9753 16.775 23.499 31.175 42.717 55.816 69.430 85.510 98.811 x1 = 0.9112 15.961 24.540 28.527 38.593 47.113 58.610 72.036 88.973 x1 = 0.8147 17.961 24.092 29.728 38.284 51.186 62.539 73.498 81.246 x1 = 0.6533 10.836 16.264 21.372 27.208 35.300 48.125 62.551 77.312 ARD(P) %b = 2.7 %

γexp 1

γcal 1

0.9961 0.9964 0.9980 0.9974 0.9963 0.9999 0.9981 0.9984

0.9936 0.9938 0.9939 0.9939 0.9940 0.9940 0.9940 0.9940

0.9335 0.9353 0.9365 0.9372 0.9373 0.9378 0.9399 0.9402

0.9297 0.9317 0.9324 0.9336 0.9344 0.9351 0.9358 0.9364

0.7139 0.7168 0.7266 0.7342 0.7367 0.7409 0.7432 0.7459

0.7692 0.7746 0.7786 0.7836 0.7894 0.7936 0.7969 0.7990

0.4779 0.4872 0.4984 0.5206 0.5449 0.5617 0.5723 0.5812

0.4877 0.5075 0.5220 0.5358 0.5513 0.5706 0.5876 0.6018

Figure 1. The experimental and correlative vapor pressure data of binary system 1-propanol (1) + [EMIM][DEP] (2) at different mass fraction of [EMIM][DEP]: Symbols are experimental data at different mass fraction of [EMIM][DEP]: ■, 0.10; ▲, 0.30; ●, 0.50; △, 0.70; ---, pure 1-propanol; , calculated by modified UNIFAC model.

Figure 2 shows the experimental vapor pressure of water + 2propanol + [EMIM][DEP] system at IL mass fraction of 30 %

Figure 2. The experimental vapor pressure data of ternary system water (1) + 2-propanol (2) + [EMIM][DEP] (3) at mass fraction of [EMIM][DEP] = 0.3. Symbols are experimental data at different mass fraction of water: ■, 0.10; ▲, 0.30; ●, 0.50; □, 0.70; △, 0.70.

as a function of temperature. It can be observed that the vapor pressure increases with the increasing temperature at constant composition, and varies slightly with the increase of concentration of light component at constant temperature. 4.2. Model Parameterization. The unknown model parameters were estimated by minimization of the following objective function:

a

Standard uncertainties of temperature, pressure, and compositions are ± 0.01 K, ± 0.04 kPa, and ± 0.002, respectively. bARD(P) = (∑i n= 1|PNRTL − Pexp|/Pexp)/n). D

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⎡ exp 2 ⎛ P exp − P cal ⎞2 ⎢⎛ T − Tical ⎞ i ⎟⎟ + ⎜⎜ i ⎟⎟ OBF = Min ∑ ⎢⎜⎜ i σ σ ⎠ ⎝ ⎠ T P i ⎢⎝ ⎣

Table 6. Vapor Pressure with Standard Uncertainties of Temperature, Pressure, and Compositions for Binary System 2-Propanol (1) + [EMIM][DEP] (2)a

N

⎛ x exp − x cal ⎞2 i,j i,j ⎟ + + ∑ ⎜⎜ ⎟ σ x ⎠ j ⎝ C

⎛ y exp − y cal ⎞2 ⎤ i,j ⎟ ⎥ ∑ ⎜⎜ i ,j ⎟⎥ σy j ⎝ ⎠ ⎥⎦

T/K

Pexp/kPa

314.606 319.962 326.929 334.302 341.255 347.802 352.275 355.902

14.708 19.581 27.916 39.832 54.712 72.654 87.726 101.719

317.115 325.307 329.958 334.912 338.557 342.086 347.760 354.761

15.115 23.131 29.112 36.889 43.857 51.410 65.477 87.587

320.544 325.525 330.213 336.997 343.875 348.280 354.695 359.000

12.963 16.777 21.223 29.302 40.156 48.887 64.437 76.916

330.502 335.188 340.506 345.853 351.601 356.576 362.928 369.831

10.947 13.908 17.995 23.540 30.979 39.110 51.418 68.541

C

(20)

On the basis of the group interaction parameters of OH−CH2, OH-H2O, and CH2−H2O listed in Table 2, the interaction parameters aH2O‑[MMIM][DMP], bH2O‑[MMIM][DMP], cH2O‑[MMIM][DMP], a[MMIM][DMP]−H2O, b[MMIM][DMP]−H2O, and c[MMIM][DMP]−H2O can be obtained by regressing the activity coefficients of water in the water + [EMIM][DEP] system taken from the literature8 and are listed in Table 2. The calculated values agree very closely with the experimental vapor pressure data with the ARD of 1.3 %. Also, the group interaction parameters of OH− [MMIM][DMP] and CH2−[MMIM][DMP] can be obtained by regressing the activity coefficients of 1-propanol and 2propanol in the 1-propanol + [EMIM][DEP] and 2-propanol + [EMIM][DEP] systems. The obtained group interaction parameters are listed in Table 2, and the calculated values cal (activity coefficients γcal 1 and vapor pressure data P ) for the binary systems 1-propanol + [EMIM][DEP] and 2-propanol + [EMIM][DEP] are listed in Table 5 and Table 6, respectively. The results show that the obtained group interaction parameters satisfactorily represent the VLE data for the two binary systems with the ARD of 2.7 % and 0.6 %, respectively. Figure 1 shows the calculated vapor pressure for the binary system 1-propanol + [EMIM][DEP]. It can be further proved that the correlated vapor pressure is quite agreeable. 4.3. Evaluation of Parameters. To evaluate whether the group interaction parameters listed in Table 2 perform equally well in the other systems (not used in model parametrization process), the experimental data for the water + [MMIM][DMP], 1-propanol + [MMIM][DMP] and 2-propanol + [MMIM][DMP] systems taken from our previous work7 were predicted with the ARD of 1.3 %, 2.7 %, and 5.8 %, respectively. The results show that the model parameters obtained in this study are adequate to describe these systems. Also, the experimental vapor pressure for other four ternary systems of water + 1-propanol + [MMIM][DMP], water + 2-propanol + [MMIM][DMP], water + 1-propanol + [EMIM][DEP], and water + 2-propanol + [EMIM][DEP] at varying liquid composition and temperature were predicted without introducing any new group interaction parameters with the ARD of 3.7 %, 4.4 %, 2.0 %, and 4.0 %, respectively. The experimental data for ternary systems containing [MMIM][DMP] were taken from our previous work.7 The calculated activity coefficients cal cal (γcal 1 and γ2 ) and vapor pressure (P ) data for the two ternary systems containing [EMIM][DEP] are listed in Table 7 and Table 8, respectively. To show the overall prediction performance, Figure 3 compares the experimental and predicted vapor pressure data. The results indicate that it is enough to predict vapor pressure data of multicomponent systems using model parameters obtained from the binary system. 4.4. Predictions for Phase Behavior of Ternary IL System. As proved above, the modified UNIFAC model can represent well the phase equilibrium behavior of multicomponent systems studied in this work. To show the salt effect of [MMIM][DMP] and [EMIM][DEP] on the

Pcal/kPa x1 = 0.9753 14.650 19.501 27.803 39.670 54.516 72.500 87.437 101.352 x1 = 0.9122 14.731 22.569 28.418 36.009 42.626 49.977 64.007 85.704 x1 = 0.8147 12.979 16.837 21.322 29.588 40.591 49.311 64.787 77.302 x1 = 0.6533 10.783 13.760 17.989 23.349 30.608 38.395 50.765 67.934 ARD(P) %b = 0.6 %

γexp 1

γcal 1

0.9977 0.9980 0.9982 0.9984 0.9980 0.9966 0.9979 0.9982

0.9938 0.9940 0.9942 0.9943 0.9944 0.9945 0.9946 0.9946

0.9575 0.9588 0.9596 0.9608 0.9658 0.9664 0.9621 0.9624

0.9332 0.9355 0.9367 0.9379 0.9387 0.9395 0.9405 0.9417

0.7674 0.7702 0.7737 0.7759 0.7812 0.7867 0.7949 0.7989

0.7683 0.7730 0.7773 0.7835 0.7897 0.7936 0.7992 0.8029

0.4908 0.4994 0.5068 0.5241 0.5410 0.5578 0.5717 0.5884

0.4834 0.4941 0.5066 0.5198 0.5345 0.5475 0.5645 0.5832

a

Standard uncertainties of temperature, pressure, and compositions are ± 0.01 K, ± 0.04 kPa, and ± 0.002, respectively. bARD(P) = (∑i n= 1|PNRTL − Pexp|/Pexp)/n).

azeotropic mixtures of water + 1-propanol and water + 2propanol, VLE for such mixtures with ILs mass fractions of 30 % and 50 % at 101.3 kPa were predicted in the whole concentration range. The x′y (Tx′y) diagrams of the ternary systems containing [MMIM][DMP] and [EMIM][DEP] are plotted in Figures 4 to 7. The liquid phase composition (x′) is given on an IL-free basis. Also VLE data of the water +1propanol and water +2-propanol mixtures at atmosphere are also plotted in these figures. It can be observed that x2′ = 1 corresponds to neat 2-propanol even when IL is present, further indicating that IL does not appear in the vapor phase due to its nonvolatility. The influence of [MMIM][DMP] and [EMIM][DEP] on the azeotropic mixture of water + 1-propanol at IL mass fraction of 50 % and atmosphere is illustrated in Figure 4. It can be seen that [EMIM][DEP] can increase (but does not eliminate) the azeotropic point of water + 1-propanol mixture, E

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Table 7. The Experimental and Predictive Vapor Pressure with Standard Uncertainties of Temperature, Pressure, and Compositions for Ternary System Water (1) + 1-Propanol (2) + [EMIM][DEP] (3)a T/K 324.952 332.201 336.773 342.182 346.480 351.959 358.246 362.861 321.592 326.265 331.583 338.128 343.591 347.753 352.424 358.503 320.578 324.329 329.768 336.225 342.616 346.870 351.977 356.237 318.276 325.077 331.871 337.499 344.131 347.247 352.583 358.088 321.386 326.045 333.909 339.894 346.399 350.324 355.138 360.313

Pexp/kPa

Pcal/kPa

γexp 1

x1 (water) = 0.2506, x2 (1-propanol) = 0.6761 15.200 15.798 1.8607 21.620 22.577 1.8183 26.838 28.018 1.7948 34.462 35.855 1.7699 41.807 43.334 1.7524 52.889 54.733 1.7328 68.400 70.801 1.7137 82.071 84.942 1.7019 x1 (water) = 0.5565, x2 (1-propanol) = 0.3893 14.968 15.305 1.4004 19.337 19.405 1.3870 24.986 25.183 1.3740 33.879 34.250 1.3613 43.241 43.800 1.3532 51.680 52.492 1.3484 62.810 63.913 1.3444 79.907 81.792 1.3411 x1 (water) = 0.7363, x2 (1-propanol) = 0.2207 14.827 14.721 1.1698 17.878 17.877 1.1640 23.310 23.467 1.1570 31.506 31.959 1.1508 41.914 42.762 1.1470 50.280 51.513 1.1455 62.103 63.918 1.1449 73.562 76.048 1.1452 x1 (water) = 0.8545, x2 (1-propanol) = 0.1098 12.375 12.485 1.0473 17.804 17.848 1.0415 24.765 25.034 1.0377 32.648 32.688 1.0359 43.724 44.098 1.0353 49.900 50.488 1.0355 61.963 63.173 1.0366 77.110 78.846 1.0385 x1 (water) = 0.9383, x2 (1-propanol) = 0.0313 12.993 12.713 0.9843 16.502 16.132 0.9829 24.052 23.663 0.9817 32.077 31.199 0.9817 42.405 41.552 0.9825 49.751 49.065 0.9834 60.429 59.769 0.9848 74.154 73.328 0.9868 ARD(P) %b = 2.0 %

Table 8. The Experimental and Predictive Vapor Pressure with Standard Uncertainties of Temperature, Pressure, and Compositions for Ternary System Water (1) + 2-Propanol (2) + [EMIM][DEP] (3)a

γcal 1

T/K

1.0466 1.0532 1.0566 1.0598 1.0618 1.0636 1.0646 1.0646

313.566 320.399 325.833 332.406 337.299 344.535 350.538 354.403

1.4691 1.4848 1.4994 1.5127 1.5196 1.5222 1.5224 1.5187

313.587 320.275 325.441 331.016 336.857 341.029 346.542 350.961

2.3003 2.3300 2.3646 2.3924 2.4055 2.4065 2.3999 2.3881

315.815 321.218 327.883 332.542 337.537 343.200 348.525 353.414

3.7651 3.9015 3.9957 4.0420 4.0611 4.0575 4.0338 3.9878

314.502 323.385 329.777 336.009 340.454 345.245 350.410 355.258

6.6325 6.8407 7.0813 7.1752 7.1962 7.1713 7.1057 6.9961

319.773 326.337 333.685 338.959 344.056 349.194 355.381 362.675

a

Pexp/kPa

Pcal/kPa

γexp 1

x1(water) = 0.2506, x2 (2-propanol) = 0.6761 13.637 13.199 1.8278 19.278 18.869 1.7795 24.976 24.783 1.7450 34.236 34.021 1.7082 42.783 42.694 1.6840 58.595 58.964 1.6528 75.139 76.219 1.6310 87.478 89.457 1.6187 x1 (water) = 0.5565, x2 (2-propanol) = 0.3893 13.556 13.550 1.3877 19.053 19.280 1.3647 24.594 25.200 1.3573 32.051 33.041 1.3433 41.841 43.402 1.3310 50.290 52.383 1.3238 63.766 66.612 1.3160 76.272 80.236 1.3112 x1 (water) = 0.7363, x2 (2-propanol) = 0.2207 14.698 14.732 1.1590 19.326 19.613 1.1494 26.628 27.496 1.1393 33.103 34.457 1.1342 41.474 43.505 1.1301 53.093 56.073 1.1270 66.357 70.489 1.1255 80.829 86.275 1.1252 x1 (water) = 0.8545, x2 (2-propanol) = 0.1098 12.953 12.557 1.0442 20.339 20.202 1.0356 27.604 27.882 1.0313 36.660 37.535 1.0290 44.507 45.982 1.0282 54.710 56.765 1.0281 67.655 70.600 1.0288 82.220 85.941 1.0302 x1 (water) = 0.9383, x2 (2-propanol) = 0.0313 13.095 13.153 0.9835 18.237 18.436 0.9814 25.907 26.351 0.9803 33.176 33.626 0.9803 41.349 42.156 0.9808 50.904 52.474 0.9818 65.024 67.534 0.9836 86.655 89.590 0.9864 ARD(P) %b = 4.0 %

γcal 1 1.0373 1.0454 1.0511 1.0569 1.0604 1.0645 1.0666 1.0675 1.4269 1.4514 1.4774 1.4898 1.4992 1.5035 1.5061 1.5056 2.1678 2.2139 2.2623 2.2847 2.3005 2.3083 2.3063 2.2967 3.4277 3.6048 3.7001 3.7512 3.7687 3.7706 3.7540 3.7221 5.9170 6.1703 6.3523 6.4223 6.4431 6.4209 6.3419 6.1846

a

Standard uncertainties of temperature, pressure, and compositions are ±0.01 K, ± 0.04 kPa, and ± 0.002, respectively. bARD(P) = (∑i n= 1|PNRTL − Pexp|/Pexp)/n)

Standard uncertainties of temperature, pressure, and compositions are ± 0.01 K, ± 0.04 kPa, and ± 0.002, respectively. bARD(P) = (∑i n= 1|PNRTL − Pexp|/Pexp)/n).

while [MMIM][DMP] can break the azeotropic phenomena under the same conditions. The case can be ascribed to the fact that the polarity and selectivities of ILs decrease with the increase of alkyl chain length on the cation since the selectivities are in the order [MMIM][DMP] > [EMIM][DEP].

Figure 5 indicates the influence of [EMIM][DEP] concentration on the VLE of the water + 2-propanol system. It can be seen that the addition of [EMIM][DEP] to the mixture leads to a breaking of the azeotropic behavior of the system at its mass fraction of 30 %, and the greater is the [EMIM][DEP] concentration, the higher is the mole fraction of 2-propanol in the vapor phase. Figure 6 indicates the F

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Figure 3. Predicted versus experimental vapor pressure for three binary and four ternary systems.

Figure 5. Isobaric VLE diagram for water (1) + 2-propanol (2) + [EMIM][DEP] (3) ternary systems at 101.3 kPa:: ---, IL-free mixture of water and 2-propanol; ×, water + 2-propanol mixture at IL mass fraction = 0.30; Δ, water + 2-propanol mixture at IL mass fraction = 0.50.

Figure 4. Isobaric VLE diagram for water (1) + 1-propanol (2) + ILs (3) ternary systems at 101.3 kPa: ---, IL-free mixture of water and 1propanol; ×, water + 1-propanol mixture at [EMIM][DEP] mass fraction of 0.50; Δ, water + 1-propanol mixture at [MMIM][DMP] mass fraction of 0.50.

Figure 6. T, x, y diagram for the ternary system of water (1) + 2propanol (2) + [MMIM][DMP] (3) at 101.3 kPa and different mass fraction of IL: □, x1 = x1′ (x3 = 0); ■, y1 (x3 = 0); △, x1′ (mass fraction of IL = 0.30); ▲, y1 (mass fraction of IL = 0.30); ○, x1′ (mass fraction of IL = 0.50); ●, y1 (mass fraction of IL = 0.50).

influence of [MMIM][DMP] concentration on the equilibrium temperature of the water + 2-propanol mixture. As can be seen from the figure, the equilibrium temperature increases with the increase of [MMIM][DMP] concentration. In addition, the comparison of separation ability on the azeotropic mixture of water + 2-propanol between [MMIM][DMP] and [EMIM][DEP] is also made. Figure 7 indicates the influence of both ILs on the VLE of the water + 2-propanol mixture at IL mass fraction of 30 %. Compared to [EMIM][DEP], [MMIM][DMP] exhibits a higher selectivity. The results may be ascribed to a decrease of IL-water interaction with an increase of alkyl chain length on the cation of IL.

of water + [EMIM][DEP], 1-propanol + [EMIM][DEP], and 2-propanol + [EMIM][DEP] systems were correlated by the modified UNIFAC model. And the newly interaction parameters obtained could also represent vapor pressure of water + [MMIM][DMP], 1-propanol + [MMIM][DMP], 2propanol + [MMIM][DMP], water + 1-propanol + [MMIM][DMP], water + 2-propanol + [MMIM][DMP], water + 1propanol + [EMIM][DEP], and water + 2-propanol + [EMIM][DEP] systems (not used in the regression stage) very well. With an increase of alkyl chain length on the cation of IL, the interaction between IL and water decreases, implying that [MMIM][DMP] is a more promising solvent candidate used for separating the mixtures of 1-propanol + water and 2propanol + water. The modified UNIFAC model parameters obtained will provide thermodynamic basis for the separation of the two azeotropic mixtures.

5. CONCLUSIONS VLE data for systems containing [EMIM][DEP] at different ILcontent were successfully measured using a quasi-static method. The experimental results revealed that [EMIM][DEP] can reduce the vapor pressure of 1-propanol and 2-propanol due to the affinity between [EMIM][DEP] and solvent. The VLE data G

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REFERENCES

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Figure 7. Isobaric VLE diagram for water (1) + 2-propanol (2) + ILs (3) ternary systems at 101.3 kPa: ---, IL-free mixture of water and 2propanol; ×, water + 2-propanol mixture at mass fraction of [EMIM][DEP] = 0.30; Δ, water + 2-propanol mixture at mass fraction of [MMIM][DMP] = 0.30.

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γexp = activity coefficient of component i determined by i experimental vapor pressure data γcal i = activity coefficient of component i calculated with the modified UNIFAC model γCi = combinatorial activity coefficient γRi = residual activity coefficient φ̂ i = fugacity coefficient of component i in the vapor mixture ϕsi = fugacity coefficient of pure component i in its saturated state Γk = group activity coefficient of group k in the mixture Γ(i) k = group activity coefficient of group k in the pure substance v(i) k = number of structural groups of type k in molecule i σT, σP, σx, and σy = estimated standard deviations for T, P, x, and y, respectively

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-62551557. E-mail: [email protected]. cn; [email protected]. Funding

The authors are grateful for financial support from the National Natural Science Foundation of China (21077213 and 21206165), and the National Basic Research Program of China (973 Program, 2009CB219904). Notes

The authors declare no competing financial interest.

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NOTATIONS P = vapor pressure (kPa) Pexp = experimental vapor pressure (kPa) Pcal = calculated vapor pressure (kPa) Psi = vapor pressure of pure component i at system temperature (kPa) T = temperature (K) xi = mole fraction of component i in liquid phase xi′ = mole fraction of component i in liquid phase on a salt free basis yi = mole fraction of component i in vapor phase A, B, and C = Antoine coefficients anm, bnm, cnm = modified UNIFAC (Dortmund) group interaction parameters between main groups n and m Φi = molecular volume fraction for component i θi = molecular surface fraction for component i z = coordination number ri = relative van der Waals volume of component i qi = relative van der Waals surface of component i Qk = relative van der Waals surface of subgroup k Rk = relative van der Waals volume of subgroup k Tc, Pc, Zc = critical parameters N = number of data points C = number of components B1= the second virial coefficient

Greek Letters

γi = activity coefficient of component i H

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I

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