Isobaric VaporâLiquidâLiquid Phase Equilibria Measurements of

Article pubs.acs.org/jced

Isobaric Vapor−Liquid−Liquid Phase Equilibria Measurements of Three Ternary Water + 2‑Butanone + Aliphatic Alcohol (Ethanol, 1‑Propanol, 2‑Propanol) Systems at 101.3 kPa Leanne Brits, A. Cédric Kouakou, Andries J. Burger, and Cara E. Schwarz* Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa ABSTRACT: Vapor−liquid−liquid equilibrium (VLLE) data were measured for the ternary systems water + 2-butanone + alcohol at 101.3 kPa by means of a Gillespie type still equipped with an ultrasonic homogenizer. VLLE were observed in the temperature ranges of 346.65−346.79 K, 346.74−352.70 K, and 346.52−346.75 K for the water + 2-butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2propanol systems, respectively. For all three systems, a region of liquid− liquid immiscibility was observed, and the vapor phase compositions were outside the liquid−liquid phase envelope. The absence of ternary heterogeneous azeotropes was thus established. The nonrandom two-liquid (NRTL), universal quasichemical (UNIQUAC), and universal functional (UNIFAC) activity coefficient models were used for comparison with experimental results. The parameters were fitted to the binary subsystems vapor−liquid and liquid−liquid equilibrium experimental data. While correct estimations of experimental vapor phase trends were obtained, all three activity coefficient models failed to provide a fair description of the size of the heterogeneous region. Overall, the thermodynamic modeling results were poor, emphasizing the need for experimental data and application of advanced thermodynamic models.

1. INTRODUCTION

of entrainer determines the separation sequence and the overall economics of the process.7 Due to the harmful and carcinogenic nature of benzene,8,9 cyclohexane has replaced benzene as the most popular entrainer in numerous processes that separate ethanol and water via heterogeneous azeotropic distillation. Other solvents, such as isooctane,10,11 diisopropyl ether (DIPE),10,12 and di-npropyl ether (DNPE)12 are also attractive entrainers, since they generally form heterogeneous azeotropes with alcohols and are favorable in fuels, therefore reducing the alcohol purity constraints in the entrainer recovery system.10 2-Butanone, commonly referred to as MEK (methyl ethyl ketone), may be a suitable entrainer for alcohols dehydration via azeotropic distillation. Indeed, 2-butanone can be used as a fuel tracer,13,14 and its partial miscibility with water could be favorable to the separation of water + alcohol mixtures.15,16 Moreover, it is affordable, readily available in sufficient quantity, and could easily be integrated into separation sequences.17 While the water + 2-butanone system has a region of liquid− liquid immiscibility, it has a homogeneous azeotrope;18,19 i.e., the azeotrope lies outside the limits of the liquid−liquid immiscibility region. However, the composition of the azeotrope is close to that of the limit of the liquid−liquid immiscibility region. It may thus be that, for the ternary system of water + 2-butanone + alcohol (ethanol, 1-propanol, or 2propanol) at certain overall compositions, the vapor

In today’s economic and environmental climate, the use of hydrocarbon-based fuels is being scaled back, and the necessity of renewable energy sources, like biofuels, has arisen. Bioalcohols derived from biological fermentation are important examples of biofuels. Bioethanol has a high energy value and is currently employed as a gasoline additive to enhance combustibility and the octane number.1 Ethanol and gasoline blending is beneficial when the water content is less than 0.5% (in volume).2 However, the water + ethanol system forms a minimum-boiling azeotrope at atmospheric pressure, so do propanol isomer + water systems,3 which also show great promise as fuel additives due to their higher energy densities and comparatively low affinity for water.4 Since azeotropic mixtures cannot be separated using standard distillation, several alternative separation techniques such as membrane, adsorption, chemical dehydration, and azeotropic distillation processes are employed to produce low molecular weight dehydrated alcohols.5 Heterogeneous azeotropic distillation is one of the most efficient methods used to separate close-boiling binary mixtures and binary azeotropes.6 A separating agent, or entrainer, is added to the azeotropic mixture to form additional minimumboiling ternary heterogeneous azeotropes. The heterogeneous azeotrope can be used to cross the binary azeotropic composition and, when doing so, it is imperative to know the composition of the heterogeneous azeotrope. Therefore, entrainer selection is a critical step in the design of heterogeneous azeotropic distillation processes, as the choice © XXXX American Chemical Society

Received: August 14, 2016 Accepted: May 5, 2017

A

DOI: 10.1021/acs.jced.6b00725 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

using a Wika UT-10 pressure transmitter with a quoted accuracy of 0.1% of its full scale output (1.6 bar abs), that is, 0.16 kPa. 2.2. Experimental Procedure. The experimental procedure was explained in detail by Pienaar et al.10 and can be summarized as follows. Approximately 110 cm3 of material was loaded into the still, after which the heater, pressure regulator, and cooling water were switched on to allow the system to equilibrate. The ultrasonic homogenizer was switched on once the mixture started boiling in order to properly mix the two liquid phases. Equilibrium was achieved after approximately 60 min, indicated by a steady registered vapor temperature as well as a visually observed constant liquid and vapor return. The vapor phase was collected at the top of the separation chamber in the gaseous state, and two overall liquid samples were collected from the still. The sampling of the vapor phase in gaseous state was performed through the vapor sample nozzle using a gas-tight syringe. Before sampling, the syringe was already filled with a known amount of a solvent (acetonitrile) completely miscible with all compounds used. The vapor sample thus condensed directly into the syringe ensuring quantitative sampling. The same procedure was applied while sampling the overall liquid sample through the liquid sampling nozzle. Additionally, a second liquid sample was collected via a liquid receiver vial. The latter liquid sample was kept in a heating bath for 2 h in order to quantify the aqueous and organic liquid phases after separation. A Varian CP-3380 gas chromatograph equipped with a flame ionization detector (FID) was used to determine the composition of each of the organic compounds in each of vapor/liquid samples. The samples were injected with a split ratio of 20:1 onto a ZB Wax column (30 m × 0.32 mm × 1 μm) with an oven temperature of 443 K and an injection temperature of 523 K. All samples were analyzed in triplicate, with 2-pentanol as an internal standard and acetonitrile as a solvent. Karl Fischer titration (701 Volumetric Titrino and 703 Titrino stand from Metrohm) was used to quantitatively determine the water content in each of vapor/liquid samples. Furthermore, a mass balance check ensured consistency between the overall liquid composition and the aqueous and organic liquid phases; i.e., on the ternary diagram, the overall liquid composition must lie on the tie-line connecting the compositions of organic and aqueous liquid samples. 2.3. Materials. All chemicals used, including their purities and suppliers, are listed in Table 1. Distilled water with conductivity lower than 2 μScm−1 was used. The presence of the impurities in the organic compounds was investigated through gas chromatography and Karl Fischer titration. Only water was identified, and its content was taken into account during analysis. Thus, all materials were used without further

composition lies within the liquid−liquid immiscibility region and a ternary heterogeneous azeotrope may also be present. Therefore, the determination and analysis of phase behavior of the water + 2-butanone + alcohol systems are required to determine if heterogeneous azeotropic distillation can be used and, if so, for the simulation and design of the separation processes. In particular, vapor−liquid−liquid equilibrium (VLLE) data are required. Such experimental data are of paramount importance, since activity coefficient models, such as the nonrandom two-liquid (NRTL),20 the universal quasichemical (UNIQUAC),21 and the universal functional (UNIFAC)22 models often fail to predict/correlate the VLLE data and the azeotropic points of these type of mixtures accurately.10,12,15 Modeling strategies of ternary VLLE mixtures imply the direct prediction of experimental data with UNIFAC. For NRTL and UNIQUAC direct data correlation or estimation, using a set of parameters obtained by regression of binary subsystem VLE or/and LLE experimental data15,16,23 is generally used. However, none of these approaches have shown to systematically provide reliable prediction/correlation. Accurate and reliable VLLE data are thus required for process simulation and to support further development and refinement of thermodynamic models. While VLE and LLE experimental data are relatively abundant in the literature for binary mixtures of water + alcohols,18,19,24−32 2-butanone + alcohols,16,33 and water + 2butanone,34,35 only a few data sets were found for the corresponding ternary systems.15,36−38 Publications reporting VLLE data for these systems are very scarce. Younis et al.38 and Lladosa et al.15 reported VLLE measurements at atmospheric pressure for the water + 2-butanone + ethanol and water + 2butanone + 1-butanol systems, respectively. However, no VLLE data were found in the open literature for the water + 2butanone + 1-propanol and water + 2-butanone + 2-propanol systems. Therefore, this paper aims to present experimental VLLE and azeotropic data for three water + entrainer + alcohol + low molecular weight alcohol systems at 101.3 kPa, namely, water + 2-butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2-propanol. In addition, the measured data were compared to NRTL, UNIQUAC, and UNIFAC activity coefficient model predictions calculated by using binary parameters regressed from VLE and LLE literature data of the binary subsystems.

2. EXPERIMENTAL METHOD 2.1. Experimental Setup. An all-glass dynamic recirculating still (Pilodist VLE 100 D) was used to determine the VLLE data experimentally. This Gillespie-type still with both vapor and liquid phase recirculating is equipped with an ultrasonic homogenizer connected to the boiling flask of the still, as described in previous works.10,39 This ensures thorough mixing of the recirculated liquid and vapor phases as well as the emulsification of the two liquid phases when a heterogeneous mixture occurs at equilibrium. The maximum operating temperature of the still is 523 K, with equilibrium temperatures measured by a Pt-100 probe connected to a digital Hart Scientific thermometer. This thermometer has an accuracy of 0.03 K at 273 K, 0.05 K at 323 K, and 0.3 K at 473 K, according to the calibration provided by a SANAS (South African National Accreditation System) approved laboratory. The pressure and heating power of the system is controlled by a Pilodist M101 control system. The pressure was measured

Table 1. List of Chemicals Used

a

B

component

CAS No.

source

puritya

product number

acetonitrile ethanol 1-propanol 2-propanol 1-butanol 2-pentanol 2-butanone

27522 64-17-5 71-23-8 67-63-0 71-36-3 6032-29-7 78-93-3

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich

≥99.8% ≥99.8% ≥99.5% ≥99.5% ≥99.9% ≥98.0% ≥99.7%

271004 676829 82090 278475 537993 P8017 34861

Molar purity as stated by the supplier. DOI: 10.1021/acs.jced.6b00725 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

Table 2. VLLE Experimental Data (Mole Fraction) for the Water (1) + 1-Butanol (4) + Ethanol (3a) System at 101.3 kPaa organic phase

a

aqueous phase

vapor phase

T/K

x1

x4

x3a

x1

x4

x3a

y1

y4

y3a

366.10 366.36 366.20 365.81 365.60 365.11 365.38

0.6274 0.6394 0.6366 0.6450 0.6499 0.6646 0.6851

0.3726 0.3606 0.3508 0.3321 0.3267 0.3013 0.2778

0.0000 0.0000 0.0127 0.0230 0.0234 0.0341 0.0371

0.9825 0.9780 0.9734 0.9638 0.9610 0.9446 0.9014

0.0175 0.0220 0.0175 0.0200 0.0210 0.0303 0.0567

0.0000 0.0000 0.0091 0.0162 0.0180 0.0251 0.0419

0.7470 0.7540 0.7547 0.7321 0.7283 0.7113 0.7162

0.2530 0.2460 0.2205 0.2132 0.2107 0.1983 0.2030

0.0000 0.0000 0.0248 0.0547 0.0610 0.0905 0.0808

u(T) = 0.15 K, u(p) = 0.36 kPa, ur(xi) = ur(yi) = 0.02.

purification. Technical grade nitrogen, supplied by Afrox, was used for overpressure control in the phase equilibrium still. 2.4. Uncertainty Estimation. The still never reached a temperature above 350 K during the experimental runs, which corresponds to a maximum deviation of 0.1 K measured by the Pt100 probe, according to the certificate of calibration. Small temperature fluctuations were observed during the experiments, and these were less than 0.05 K. The standard uncertainty in the reported temperature, u(T), was therefore 0.15 K. The pressure transmitter has a maximum deviation in accuracy of 0.16 kPa. As such, considering small pressure fluctuationsnever larger than 0.2 kPa during experiments the standard uncertainty in the pressure measurements, u(p), was estimated as 0.36 kPa. The uncertainty of compositional measurements by GC was determined by running three samples of known composition for each system. Deviations in the predictions for each system were determined, and a maximum relative deviation was quantified as 0.020 of the mole fraction value. A total of 38 samples with varying water content, to cover the entire composition range of the studied systems, were analyzed twice using the Karl Fisher titration method to determine the average associated error. The maximum relative deviation was found to be less than 0.005 of the mole fraction value. Thus, the largest standard relative uncertainty of the compositional measurements is 0.020 of the value, i.e., ur(xi) = ur(yi) = 0.02.

Figure 1. VLLE ternary phase diagram of the water (1) + 1-butanol (4) + ethanol (3a) system at 101.3 kPa, compared with literature data. This work: red ■, aqueous liquid; red ●, organic liquid; red ◆, vapor phase. Pienaar et al.:10 ▽, liquid phase envelope; ◇, vapor phase. Newsham and Vahdat:40 ○, liquid phase envelope; ×, vapor phase. Gomis et al.:41 △, liquid phase envelope; □, vapor phase.

liquid (aqueous or organic) data points. All L/W values range from 0.92 to 1.10, and D values were lower than their corresponding Dmax, respectively. The regularity of the liquid phases was also checked using the Othmer−Tobias correlation44 with their Pearson’s values ranging from r = 0.969 to r = 0.980. While these tests do not necessarily prove thermodynamic consistency, they will indicate when data are thermodynamically inconsistent, which is not the case here.

3. VERIFICATION The reliability of the experimental setup and method was tested by comparison with previously measured experimental VLLE data and by means of thermodynamic consistency tests. The ternary water + 1-butanol + ethanol system was chosen for comparison due to the fact that three sets of reliable VLLE data at atmospheric pressure were available in the literature for this mixture. These three sets of published data are in agreement, despite the fact that different types of setups were used for the measurements. Newsham and Vahdat40 used the flow method, while the data of Gomis et al.41 and Pienaar et al.10 were both measured by using a dynamic recirculating still fitted with an ultrasonic homogenizer. The newly generated experimental data for the system water + 1-butanol + ethanol are reported in Table 2 and are compared with the published data on a ternary phase diagram in Figure 1. Except for a slight scatter of some literature data, the vapor and both liquid phases (aqueous and organic) of the compared sets of data agree well with each other. Thermodynamic consistency of the newly measured and verification system VLLE data sets generated in this work were established according to the Wisniak L−W42 and McDermott− Ellis43 point-to-point consistency tests for each couple vapor−

4. RESULTS AND DISCUSSION 4.1. Experimental Data. The measured VLLE data for the three ternary systems (water + 2-butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2propanol) are reported in Table 3, Table 4, and Table 5, respectively. These tables contain the temperature, compositions (mole fraction) of the liquid phases, and vapor phase for each experimental point. The temperature and compositions of the binary azeotropes measured in this work are reported in Table 6, together with literature data and thermodynamic modeling results. The binary azeotropic compositions and temperatures were determined by loading the azeotropic composition indicated by literature and verifying the azeotrope by ensuring the composition of the liquid and vapor phases differ by less than the compositional uncertainty. Overall, good comparisons were obtained between the experimental measurements and the published data for the binary azeotropes of water + 2-butanone and water + ethanol. VLLE were observed in the temperature ranges of 346.65− 346.79 K, 346.74−352.7 K, and 346.52−346.75 K for the water C



Article

Table 3. VLLE Experimental Data (Mole Fraction) for the Water (1) + 2-Butanone (2) + Ethanol (3a) System at 101.3 kPaa organic phase

a

aqueous phase

vapor phase

T/K

x1

x2

x3a

x1

x2

x3a

y1

y2a

y3a

346.65 346.79 346.70 346.68 346.67 346.65 346.63 346.61 346.59 346.56 346.54 346.51 346.48 346.45 346.42 346.38 346.35 346.31 346.26 346.19 346.12 345.65

0.4136 0.4150 0.4163 0.4150 0.4208 0.4198 0.4264 0.4281 0.4365 0.4427 0.4461 0.4475 0.4479 0.4511 0.4651 0.4743 0.4821 0.4952 0.5166 0.5386 0.5566 0.5910

0.5864 0.5850 0.5837 0.5790 0.5692 0.5689 0.5612 0.5583 0.5481 0.5390 0.5348 0.5330 0.5315 0.5281 0.5108 0.4994 0.4883 0.4733 0.4478 0.4226 0.4033 0.3669

0.0000 0.0000 0.0000 0.0060 0.0100 0.0114 0.0124 0.0136 0.0155 0.0183 0.0191 0.0195 0.0206 0.0207 0.0241 0.0263 0.0296 0.0315 0.0357 0.0389 0.0401 0.0421

0.9520 0.9550 0.9557 0.9496 0.9474 0.9450 0.9428 0.9411 0.9389 0.9344 0.9304 0.9255 0.9234 0.9200 0.9143 0.9119 0.9098 0.9033 0.8992 0.8882 0.8831 0.8787

0.0480 0.0450 0.0443 0.0462 0.0472 0.0480 0.0480 0.0492 0.0510 0.0537 0.0562 0.0594 0.0608 0.0632 0.0666 0.0680 0.0695 0.0728 0.0761 0.0838 0.0872 0.0908

0.0000 0.0000 0.0000 0.0041 0.0053 0.0071 0.0092 0.0097 0.0101 0.0119 0.0134 0.0151 0.0158 0.0168 0.0191 0.0201 0.0206 0.0239 0.0247 0.0280 0.0297 0.0305

0.3527 0.3510 0.3487 0.3493 0.3490 0.3486 0.3477 0.3478 0.3469 0.3455 0.3457 0.3462 0.3465 0.3468 0.3421 0.3442 0.3433 0.3431 0.3423 0.3410 0.3385 0.3367

0.6473 0.6490 0.6513 0.6463 0.6449 0.6434 0.6395 0.6381 0.6375 0.6371 0.6358 0.6324 0.6300 0.6271 0.6165 0.6128 0.6000 0.5973 0.5892 0.5830 0.5723 0.5551

0.0000 0.0000 0.0000 0.0045 0.0060 0.0080 0.0128 0.0141 0.0156 0.0175 0.0185 0.0213 0.0235 0.0261 0.0414 0.0430 0.0567 0.0596 0.0685 0.0761 0.0892 0.1082


Table 4. VLLE Experimental Data (Mole Fraction) for the Water (1) + 2-Butanone (2) + 1-Propanol (3b) System at 101.3 kPaa organic phase

a

aqueous phase

vapor phase

T/K

x1

x2

x3b

x1

x2

x3b

y1

y2

y3b

346.75 346.77 346.74 346.84 347.39 347.69 348.63 348.99 349.21 349.76 350.19 350.43 350.61 350.88 351.70 352.65 352.65 352.70

0.4109 0.4150 0.4197 0.4171 0.4148 0.4236 0.4271 0.4323 0.4346 0.4486 0.4516 0.4603 0.4633 0.4708 0.4783 0.4939 0.5065 0.5149

0.5891 0.5850 0.5803 0.5778 0.5736 0.5647 0.5549 0.5458 0.5414 0.5229 0.5155 0.5032 0.4995 0.4884 0.4725 0.4547 0.4334 0.4256

0.0000 0.0000 0.0000 0.0050 0.0117 0.0117 0.0180 0.0219 0.0241 0.0285 0.0329 0.0365 0.0373 0.0409 0.0491 0.0513 0.0601 0.0596

0.9508 0.9525 0.9556 0.9540 0.9535 0.9512 0.9491 0.9469 0.9446 0.9430 0.9407 0.9382 0.9349 0.9317 0.9299 0.9228 0.9221 0.9177

0.0492 0.0475 0.0445 0.0454 0.0454 0.0464 0.0469 0.0473 0.0476 0.0486 0.0496 0.0496 0.0503 0.0516 0.0524 0.0544 0.0538 0.0575

0.0000 0.0000 0.0000 0.0005 0.0011 0.0023 0.0039 0.0058 0.0078 0.0084 0.0096 0.0122 0.0148 0.0166 0.0177 0.0228 0.0241 0.0248

0.3518 0.3510 0.3537 0.3505 0.3502 0.3520 0.3512 0.3511 0.3532 0.3531 0.3549 0.3559 0.3573 0.3586 0.3589 0.3596 0.3592 0.3619

0.6482 0.6490 0.6463 0.6438 0.6399 0.6369 0.6345 0.6307 0.6247 0.6218 0.6183 0.6115 0.6092 0.6035 0.6014 0.5985 0.5967 0.5908

0.0000 0.0000 0.0000 0.0057 0.0099 0.0111 0.0143 0.0182 0.0220 0.0251 0.0268 0.0325 0.0334 0.0379 0.0397 0.0419 0.0441 0.0473


between the equipment used as well as with the determination of water compositions. The equipment of Younis et al. was based on the Othmer principle, in which only the vapor phase was recirculated. Rectification effects as well as difficulty with recirculating the two liquids after vapor condensation have been identified as recurring drawbacks of this method.45,46 Indeed, the water content of vapor compositions by Younis et al. is greater than our measurements. Moreover, they employed GC analysis with a thermal conductivity detector (TCD) for all the compositions, while in our work, Karl Fischer titration was used to determine the water content, the latter being regarded as

+ 2-butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2-propanol systems, respectively. The experimental data are presented in Figures 2 to 4. These ternary phase diagrams depict the two liquid phases and vapor phase mole fractions, the binary azeotropes, and some of the measured isobaric tie-lines. Figure 2 includes VLLE experimental data of Younis et al.38 for comparison. Small deviations can be found between the two data sets for the liquid phase’s compositions; however, the measured vapor phase does not agree well with the published data. These discrepancies might be explained by the difference D



Article

Table 5. VLLE Experimental Data (Mole Fraction) for the Water (1) + 2-Butanone (2) + 2-Propanol (3c) System at 101.3 kPaa organic phase

a

aqueous phase

vapor phase

T/K

x1

x2

x3c

x1

x2

x3c

y1

y2

y3c

346.68 346.75 346.70 346.70 346.69 346.69 346.68 346.67 346.67 346.67 346.66 346.65 346.64 346.64 346.63 346.63 346.62 346.61 346.60 346.60 346.60 346.59 346.58 346.52

0.4172 0.4179 0.4189 0.4186 0.4186 0.4191 0.4211 0.4229 0.4247 0.4255 0.4267 0.4286 0.4275 0.4342 0.4384 0.4414 0.4469 0.4471 0.4583 0.4740 0.4955 0.5148 0.5466 0.5526

0.5828 0.5821 0.5811 0.5760 0.5720 0.5702 0.5643 0.5585 0.5523 0.5481 0.5454 0.5410 0.5349 0.5221 0.5103 0.5041 0.4928 0.4903 0.4720 0.4492 0.4233 0.3925 0.3576 0.3491

0.0000 0.0000 0.0000 0.0054 0.0094 0.0108 0.0146 0.0186 0.0230 0.0264 0.0280 0.0305 0.0375 0.0437 0.0513 0.0545 0.0602 0.0626 0.0697 0.0769 0.0812 0.0927 0.0959 0.0983

0.9534 0.9532 0.9546 0.9508 0.9498 0.9480 0.9465 0.9412 0.9390 0.9361 0.9331 0.9278 0.9246 0.9211 0.9173 0.9148 0.9133 0.9105 0.9060 0.8993 0.8944 0.8895 0.8798 0.8611

0.0466 0.0468 0.0454 0.0469 0.0468 0.0466 0.0468 0.0477 0.0491 0.0495 0.0502 0.0511 0.0514 0.0505 0.0507 0.0515 0.0519 0.0527 0.0551 0.0590 0.0612 0.0645 0.0704 0.0823

0.0000 0.0000 0.0000 0.0023 0.0033 0.0054 0.0067 0.0111 0.0119 0.0144 0.0167 0.0211 0.0240 0.0284 0.0319 0.0336 0.0348 0.0368 0.0388 0.0417 0.0443 0.0460 0.0497 0.0566

0.3524 0.3539 0.3532 0.3538 0.3543 0.3552 0.3559 0.3578 0.3574 0.3586 0.3608 0.3614 0.3652 0.3665 0.3671 0.3698 0.3735 0.3763 0.3836 0.3839 0.3875 0.3923 0.3975 0.4026

0.6476 0.6461 0.6468 0.6385 0.6369 0.6341 0.6291 0.6250 0.6230 0.6214 0.6158 0.6107 0.6041 0.5959 0.5873 0.5804 0.5680 0.5617 0.5331 0.5243 0.5107 0.4993 0.4875 0.4758

0.0000 0.0000 0.0000 0.0077 0.0088 0.0108 0.0150 0.0171 0.0195 0.0200 0.0234 0.0279 0.0307 0.0376 0.0456 0.0498 0.0586 0.0621 0.0833 0.0919 0.1018 0.1084 0.1150 0.1216


Table 6. Temperature and Compositions (Mole Fraction) of the Binary Azeotropes Measured in This Work at 101.3 kPa along with Literature Data and Calculated with NRTL, UNIQUAC, and UNIFAC Models Homogeneous Binary Azeotrope Water (1) + 2-Butanone (2) T/K

a

x2

x1

reference

0.670 Tanaka18 0.652 Cho et al.19 0.647 this worka 0.682 NRTL 0.649 UNIQUAC 0.693 UNIFAC Water (1) + Ethanol (3a)

346.65 346.52 346.65 345.15 348.32 346.79 Homogeneous

0.330 0.348 0.353 0.318 0.351 0.307 Binary Azeotrope

T/K

x1

x3a

reference

351.35 351.33 351.30 351.27 351.33 351.19

0.106 0.107 0.105 0.103 0.095 0.107

0.894 0.893 0.895 0.897 0.905 0.893

Gmehling3 Kurihara et al.24 this worka NRTL UNIQUAC UNIFAC

Figure 2. VLLE ternary phase diagram of the water (1) + 2-butanone (2) + ethanol (3a) system at 101.3 kPa. Experimental data: this work; blue △, aqueous liquid; blue ▽, organic liquid; blue ◇, vapor phase; blue □, measured azeotropes; Younis et al.; green ▲, aqueous liquid; green ▼, organic liquid; green ◆, vapor phase. Calculated values:  (gray), NRTL; ■ (gray), NRTL azeotropes; , UNIQUAC; ○, UNIQUAC azeotropes; - - - (red), UNIFAC; × (red), UNIFAC azeotropes.


the organic liquid phase composition has a much larger range than the aqueous phase. The vapor phase lies outside the liquid−liquid heterogeneous region for all of the systems. Hence no heterogeneous ternary azeotrope is present for any of these systems, as no vapor composition can lie on the tie-line between the two liquid phase compositions. As a result, 2-butanone cannot be considered as a suitable entrainer for the dehydration of C2and C3-alcohols using heterogeneous azeotropic distillation.

more accurate (maximum relative deviation less than 0.005 of the mole fraction value). For all three systems, 2-butanone and water exhibit a small liquid−liquid immiscibility region, while the alcohols are completely miscible with both water and 2-butanone. The immiscible region of the ternary mixtures occurs at water mole fractions greater than 0.5 and low, generally less than 0.1, alcohol mole fractions. The ternary phase diagrams shows that E



Article

(example for VLE data regression), with n being the number of data points, nc the number of components, and σ the standard deviation of the indicated data. τij = Aij +

Bij T

+ Cij ln(T )

(1)

⎡ ⎤ B′ij τ′ij = exp⎢A′ij + + C′ij ln(T )⎥ T ⎣ ⎦

(2)

⎡⎛ 2 ⎛ Pmeas − Pcalc ⎞2 Tmeas − Tcalc ⎞ ⎢ OF = ∑ ⎜ ⎟ ⎟ +⎜ ⎢ σT σP ⎠ ⎝ ⎠ j = 1 ⎣⎝ n

Figure 3. VLLE ternary phase diagram of the water (1) + 2-butanone (2) + 1-propanol (3b) system at 101.3 kPa. Experimental data: this work; blue △, aqueous liquid; blue ▽, organic liquid; blue ◇, vapor phase; blue □, measured azeotropes. Calculated values:  (gray), NRTL; ■ (gray), NRTL azeotropes; , UNIQUAC; ○, UNIQUAC azeotropes; - - - (red), UNIFAC; × (red), UNIFAC azeotropes.

nc − 1

+

∑ i=1

⎛ xi , j ,meas − xi , j ,calc ⎞2 ⎜ ⎟ + σx ⎝ ⎠

nc − 1

∑ i=1

⎛ yi , j ,meas − yi , j ,calc ⎞2 ⎤ ⎜⎜ ⎟⎟ ⎥ σ ⎝ ⎠ ⎥⎦ y (3)

The form of τij and τ′ij along with a variable NRTL non randomness parameter αij was used as it had been observed that it leads to a better description of the liquid−liquid phase envelope. The regressed BIP of the water + 2-butanone, 2butanone + alcohols, and water + alcohols subsystems are reported in Table 7. The prediction/correlation of the VLLE data by NRTL, UNIQUAC, and UNIFAC models were then determined and compared to measurements by plotting the data sets onto ternary phase diagrams as well as by statistical quantification. The ability of the models to predict/correlate the experimental data was assessed by the average absolute relative deviation (AARD) values, according to eq 4 for the liquid molar compositions or eq 5 for the vapor molar compositions, for n data points.

Figure 4. VLLE ternary phase diagram of the water (1) + 2-butanone (2) + 2-propanol (3c) system at 101.3 kPa. Experimental data: this work; blue △, aqueous liquid; blue ▽, organic liquid; blue ◇, vapor phase; blue □, measured azeotropes. Calculated values:  (gray), NRTL; ■ (gray), NRTL azeotropes; , UNIQUAC; ○, UNIQUAC azeotropes; - - - (red), UNIFAC; × (red), UNIFAC azeotropes.

AARDx(%) =

AARDy(%) =

4.2. Thermodynamic Modeling. Activity coefficient models such as NRTL, UNIQUAC, and UNIFAC are often used to estimate the equilibrium data of the water + entrainer + alcohol systems for design and process simulation purposes.6,11,23 A fully predictive approach was applied when using the UNIFAC model with surface and volume parameters as well as group interaction parameters of Hansen et al.22 On the other hand, the NRTL and UNIQUAC models are correlative and require parameters fitted to experimental data. For both correlative models (NRTL, UNIQUAC), VLLE data were calculated with a set of parameters based on binary data, both VLE and LLE data. The selected literature data sets (Table 7) agree well with each other, and VLE data were found to be thermodynamically consistent according to Wisniak L− W42 and McDermott−Ellis43 point-to-point tests. Aspen Plus V8.2 was used to regress the NRTL and UNIQUAC binary interaction parameters (BIP) from VLE data of 2-butanone + alcohols and water + alcohols systems as well as LLE data and VLE data water + 2-butanone systems. The τij and τ′ij parameters were determined in the form described by eq 1 for NRTL and eq 2 for UNIQUAC, respectively, by minimizing the maximum likelihood objective function given by eq 3

n

100 n

∑

100 n

∑

1 n 1

xi ,experimental − xi ,model xi ,experimental

(4)

yi ,experimental − yi ,model yi ,experimental

(5)

The values of the AARD obtained for the various models are tabulated in Table 8, Table 9, and Table 10 for the water + 2butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2-propanol systems, respectively, and the corresponding thermodynamic modeling results are plotted in Figures 2 to 4. For all the systems, NRTL and UNIQUAC model results give a much larger than measured liquid−liquid phase envelope and therefore incorrectly predict a homogeneous region in the VLLE region, particularly near the top of the phase envelope. The latter overestimation is less pronounced for UNIFAC, but this model predicts a considerably smaller heterogeneous region, with both aqueous and organic liquid experimental points far from the predicted phase envelope. In all cases the slopes of the tie-lines are represented with reasonable accuracy. However, the trends are still not well-reproduced when approaching the plait point, as can be seen in Figure 4 for the water + 2-butanone + 2-propanol system where this observation is most prominent. On the other hand, all of the models provide quite accurate representations of the vapor composition. UNIFAC shows a F



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Table 7. NRTL and UNIQUAC Binary Interaction Parameters Obtained for the Binary Subsystems Using VLE and LLE Literature Dataa NRTL Binary Interaction Parameters binary subsystems

(1) + (2)

(1) + (3a)

Aij Aji Bij (K) Bji (K) Cij (K) Cji (K) αij

3.32 100.90 −648.05 −3893.94 0.25 −15.04 0.43

10.51 6.38 −1209.29 310.67 −0.94 −1.18 0.59

binary subsystems

(1) + (2)

Aij′ Aji′ Bij′ (K) Bji′ (K) Cij′ (K) Cji′ (K) literature data used for regression

a

(1) + (3b)

(1) + (3c)

(2) + (3a)

(2) + (3b)

(2) + (3c)

2.75 −7.16 525.85 −2275.86 −0.67 2.42 1.08

0.45 −0.31 −327.89 416.37 0.11 −0.09 0.71

−0.29 −0.56 570.90 369.70 −0.10 −0.14 −0.05

35.45 68.99 −1.91 11.80 −9472.97 −1613.53 361.30 736.99 −1.14 −10.59 0.25 −2.29 0.48 0.48 UNIQUAC Binary Interaction Parameters

(1) + (3a)

(1) + (3b)

(1) + (3c)

(2) + (3b)

(2) + (3a)

23.66 4.46 −576.55 −1303.43 −3.81 −0.23 Ferino et al.34

80.00 −24.85 −1382.41 −718.99 −13.04 4.58 Kurihara et al.24

−12.05 24.81 8022.26 −4751.55 −1.80 −2.02 Iliuta et al.27

3.16 20.23 3143.98 −5505.36 −2.05 −0.88 Li et al.29

−19.17 54.73 −813.37 −695.41 3.60 −9.00 ́ Martinez et al.16

Stephenson35

Kamihama et al.25 Á lvarez et al.26

Kojima et al.28

Marzal et al.30 Arce et al.31 Verhoeye and de Schepper32

Wen and Tu33

(2) + (3c)

1.84 28.68 −13.86 −55.91 675.52 1520.10 −1.64 −91.90 −0.66 −5.66 2.35 9.58 ́ et al.16 Martinez

(1) denotes water, (2) denotes 2-butanone, (3a) denotes ethanol, (3b) denotes 1-propanol, and (3c) denotes 2-propanol.

Table 8. AARD between Calculated and Experimental VLLE Data with NRTL, UNIQUAC, and UNIFAC Models for the Water (1) + 2-Butanone (2) + Ethanol (3a) System AARD (%) for the Water (1) + 2-Butanone (2) + Ethanol (3a) System organic phase NRTL UNIQUAC UNIFAC

aqueous phase

vapor phase

x1

x2

x3a

x1

x2

x3a

y1

y2

y3a

8.2 6.8 27.8

7.8 6.8 24.3

30.4 22.4 4.3

1.8 1.4 2.0

16.5 13.8 42.4

35.6 25.9 33.9

5.5 3.8 1.5

2.0 4.2 1.7

60.2 33.5 36.5

Table 9. AARD between Calculated and Experimental VLLE Data with NRTL, UNIQUAC, and UNIFAC Models for the Water (1) + 2-Butanone (2) + 1-Propanol (3b) System AARD (%) for the Water (1) + 2-Butanone (2) + 1-Propanol (3b) System organic phase NRTL UNIQUAC UNIFAC

aqueous phase

vapor phase

x1

x2

x3b

x1

x2

x3b

y1

y2

y3b

7.1 25.5 47.6

5.9 2.1 24.3

9.8 8.6 13.9

0.5 1.3 2.9

5.4 7.6 54.6

40.9 48.5 33.2

3.9 6.4 2.5

3.9 3.7 1.3

23.0 35.5 14.1

Table 10. AARD between Calculated and Experimental VLLE Data with NRTL, UNIQUAC, and UNIFAC models for the Water (1) + 2-Butanone (2) + 2-Propanol (3c) System AARD (%) for the Water (1) + 2-Butanone (2) + 2-Propanol (3c) System organic phase NRTL UNIQUAC UNIFAC

aqueous phase

vapor phase

x1

x2

x3c

x1

x2

x3c

y1

y2

y3c

23.2 20.3 35.5

7.6 3.5 30.7

24.0 13.4 8.4

3.1 0.9 1.6

11.8 10.2 47.1

64.7 27.2 49.8

9.3 3.2 3.7

3.3 4.0 2.2

93.7 31.9 54.6

better ability to predict the vapor composition with generally lower AARD values (except for two values of the alcohol mole fraction; see Table 8 and Table 10), and at the same time provides a correct prediction of the experimental trends. Table

6 shows that all the models accurately describe (molar composition and temperature) the binary homogeneous azeotrope of water + ethanol measured in this work, while only UNIFAC agrees well with the experimental data of the G



Article

unique reference number (UID) 83966) and Sasol Technology (Pty) Ltd. The authors acknowledge that opinions, findings, and conclusions or recommendations expressed in any publication generated by the supported research are that of the authors and that the sponsors accept no liability whatsoever in this regard.

binary water + 2-butanone azeotrope within the experimental uncertainty ranges. Additionally, a ternary homogeneous azeotrope is obtained for the water + 2-butanone + ethanol system by NRTL, UNIQUAC and UNIFAC models (Figure 2), as well as for water + 2-butanone + 2-propanol system by UNIFAC and UNIQUAC (Figure 4). However, experimentally, the presence (or absence) of a ternary heterogeneous azeotrope had not been established experimentally. Since models such as NRTL, UNIQUAC, and UNIFAC may incorrectly predict the presence (or absence) of azeotropes, further experimental measurements, beyond the scope of the current work, are required. Further work may also have to resort to the application of more advanced thermodynamic models.

Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Aspen Plus is a registered trademark of Aspen Technology Inc.

5. CONCLUSIONS Isobaric VLLE data were measured for the ternary systems water + 2-butanone + ethanol, water + 2-butanone + 1propanol, and water + 2-butanone + 2-propanol at 101.3 kPa. None of the water + 2-butanone + aliphatic alcohol systems exhibit a ternary heterogeneous azeotrope, and 2-butanone is therefore not considered to be a suitable entrainer for the dehydration of C2 - and C 3-alcohols in heterogeneous azeotropic distillation. Modeling the VLLE data of these ternary mixtures by means of a predictive approach, using either a group contribution model or a set of parameters based on both VLE and LLE data, results in significant inaccuracies. NRTL, UNIQUAC, and UNIFAC models were unable to satisfactory describe both the liquid phase envelope and vapor composition at the same time when, in the case of the NRTL and UNIQUAC models, the parameters were fitted to binary data. This confirms that correlation parameters generated for these models from binary data are not readily transferable to predict the behavior of the relevant ternary systems. Therefore, future work should include incorporation of corresponding ternary data to assess the possibility of generating improved correlations for the NRTL and UNIQUAC models. Models that accurately take into account the associating and strong polar nature of the studied components may improve the modeling of the systems under investigation. Previous studies47,48 have shown that modeling of non-hydrogenbonding/hydrogen-bonding and hydrogen-bonding/hydrogenbonding binary systems with polar versions of sPC-SAFT (simplified perturbed chain−statistical associating fluid theory) and CPA (cubic plus association) have not significantly improved the simultaneous VLE and LLE predictions. Therefore, if these models (sPC-SAFT, CPA etc.) are not able to successfully model binary VLLE data, it is highly doubtful they will be able to model ternary VLLE data. Improved models are therefore required. For example, the use of a nonhard sphere model with incorporation of a polar term may yield improved results.

■

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +27 21 8084487. Fax: +27 21 8082059. E-mail: [email protected]. ORCID

Cara E. Schwarz: 0000-0001-5513-2105 Funding

This work is based on the research supported in part by the National Research Foundation of South Africa (Grant specific H



Article

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