Influence of Temperature on the LiquidâLiquid Phase Equilibria of

Article pubs.acs.org/jced

Influence of Temperature on the Liquid−Liquid Phase Equilibria of Ternary (Water + Alcohol + Entrainer) Systems Riccardo M. Swanepoel and Cara E. Schwarz* Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa ABSTRACT: The effect of temperature on the liquid−liquid equilibrium (LLE) phase behavior of ternary (water + alcohol + entrainer) systems comprised of the alcohols ethanol, isopropanol, and n-propanol and the entrainers diisopropyl ether (DIPE), cyclohexane, and isooctane (excluding (water + n-propanol + DIPE)) was investigated for application to the decanter in heterogeneous azeotropic distillation. LLE data were measured at ambient pressure for the (water + isopropanol + cyclohexane), (water + isopropanol + isooctane), and (water + n-propanol + isooctane) systems at 308.2 and 318.2 K and for the (water + n-propanol + cyclohexane) and (water + ethanol + isooctane) systems at 318.2 K. These data, in conjunction with literature LLE data, show that temperature has an effect on all systems investigated. As temperature increases, the aqueous phase becomes depleted of and the organic phase becomes enriched in alcohol. It appears that component polarities play an important role in explaining the phase behavior. The systems were correlated with the NRTL and UNIQUAC ACMs in Aspen Plus V8.2, but reliable correlations were only obtained for the (water + ethanol + DIPE/cyclohexane/isooctane) and (water + ethanol/isopropanol/n-propanol + cyclohexane) systems. These correlations were used to simulate the decanter water recoveries over a range of temperatures. equilibrium (LLE) data for thermodynamic modeling,6 and these data have to be available over a range of temperatures. Several entrainers have been employed in heterogeneous azeotropic distillation. Examples include benzene, which had been used extensively as an entrainer for alcohol dehydration,3 but due to its carcinogenic nature,7 it has largely been replaced by cyclohexane.8 Despite its widespread use, however, cyclohexane does not perform as well as benzene, and consequently other entrainers have been sought. Hexane, heptane, isooctane, di-n-propyl ether (DNPE), and diisopropyl ether (DIPE) have also been proposed as alternative entrainers to benzene and cyclohexane.9 For an entrainer to be feasible for heterogeneous azeotropic distillation, it is imperative that it forms a ternary heterogeneous azeotrope, which is indicated by vapor−liquid− liquid equilibrium (VLLE) data with a temperature minimum or maximum. Based on the available VLLE data in literature, DIPE, cyclohexane (c-C6H12) and isooctane (i-C8H18) were selected in this study as candidate entrainers for C2 and C3 alcohol dehydration. The most thorough comparison will be possible between these entrainers, even though a heterogeneous azeotrope is absent for the (water + n-propanol + DIPE) system.9 DIPE can therefore not be used as an entrainer for the dehydration of n-propanol; the (water + n-propanol + DIPE) system was subsequently excluded.

1. INTRODUCTION The C2 alcohol ethanol (EtOH) and C3 alcohols isopropanol (IPA) and n-propanol (NPA) have found several applications, both in industry and domestic use. All three alcohols can be present in the wastewater stream of the Fischer−Tropsch process, and economic incentives exist in order to retrieve them.1 Additionally, bioethanol production is also currently receiving intense attention as it has been mooted as a renewable alternative to conventional automobile fuels.2 In both of these process routes, the alcohols are produced in aqueous form and separation is required in order to yield pure alcohols. While distillation is the oldest and best-understood separation technique,3 conventional distillation cannot be used as these alcohols form homogeneous azeotropes with water.4 Consequently, advanced distillation techniques are required, one of which is heterogeneous azeotropic distillation. With this technique, an entrainer is added to the (alcohol + water) feed, which then forms a minimum-boiling heterogeneous azeotrope that is approached in the overheads of an azeotropic column. When the vapor from the azeotropic column is condensed, two immiscible liquid phases form, namely, a water-rich aqueous phase and an entrainer-rich organic phase. The separation of these phases is the key in overcoming the azeotrope and is performed by decantation in a decanter. Control and simulation of the decanter is, however, difficult5 as it is often sensitive to small changes in operating conditions such as temperature, which strongly influences the liquid− liquid equilibria (LLE) in the decanter. It was therefore the overarching aim of this work to improve the understanding of the effect of temperature on the LLE of (water + alcohol + entrainer) systems. This necessitates accurate liquid−liquid © 2017 American Chemical Society

Special Issue: Memorial Issue in Honor of Ken Marsh Received: January 31, 2017 Accepted: June 8, 2017 Published: June 22, 2017 2740

DOI: 10.1021/acs.jced.7b00110 J. Chem. Eng. Data 2017, 62, 2740−2754

Journal of Chemical & Engineering Data

Article

Figure 1. Availability of liquid−liquid equilibrium data of (water + alcohol + entrainer) systems at ambient pressure: literature (●); systems measured in this work (○).

determine phase behavior, while the sixth was a water blank used for temperature measurement. A 2 kW Julabo MA heating immersion circulator with PID control and a temperature stability of ±0.01 K (8) was used for temperature control. The pump (8.2) circulated water at 16 L/ min. A Testo 720 temperature instrument, calibrated by a SANAS (South African National Accreditation System) laboratory, connected to a two-wire Pt100 probe with uncertainty of ±0.2 K was used to ensure that the immersion heater maintained the water bath at the correct temperature. No measurable cold or hot spots were identified within the water bath. 2.2. Experimental Procedure. Components were charged to the separating funnel using a buret, and the quantity of each component added to the funnel was determined using an electronic scale with error 0.01 g. The components were added such that the estimated volumes of both the aqueous and organic phases were approximately equal. The inventory of each funnel was approximately 100 mL. After the three components were added to each separating funnel, the funnel was shaken vigorously and placed into the water bath preheated to the temperature of interest for that particular run. Once the charged separating funnels were submerged and allowed to reach thermal equilibrium for at least 1 h, they were shaken vigorously, one after the other, each for 15 s. The flasks were replaced in the water bath immediately after the 15 s agitation to limit the decrease in temperature. After each flask, including the water blank, was shaken at least 10 times, the flasks were again allowed to reach thermal equilibrium for at least 2 h before the entire procedure was repeated. After the second agitation session, the stoppers were removed from the separating funnels and the custom-made PVC plugs inserted. The aqueous sampling needles were then inserted into the mixture, purged with air, and sealed using the gastight valves, after which the mixture was allowed to separate into two phases at constant temperature for at least 15 h before sampling. The aqueous sampling needles were inserted before equilibration to prevent disturbance of the equilibrium interface during the sampling procedure, and the needles were purged with air before sealing to ensure that no phase separation occurs within them. Samples were taken with the separating funnels firmly secured in the water bath to prevent any changes in temperature during sampling. Before sampling the aqueous

Figure 1 shows the temperatures at which LLE data are available, and the sources of the data are summarized in Table 1. The table also shows the ternary vapor−liquid equilibrium (VLE) and VLLE data available for the systems of interest. The LLE data are, to the best of the authors’ knowledge, the only LLE data available for the systems of interest, but other sets of VLE and VLLE data may be found. For accurate modeling of the decanter, LLE data are required at at least three different temperatures, at or above ambient temperature (298.2 K). Based on the available LLE data, and to complete the investigation into the temperature dependence, the objectives of this work are (i) to present newly measured experimental data for the LLE of the (water + isopropanol + cyclohexane), (water + isopropanol + isooctane), and (water + n-propanol + isooctane) systems at 308.2 and 318.2 K, and the (water + npropanol + cyclohexane) and (water + ethanol + isooctane) systems at 318.2 K; (ii) to determine the temperature dependence of the LLE of the (water + alcohol + entrainer) systems; and (iii) to correlate the LLE phase behavior using the NRTL and UNIQUAC activity coefficient models and subsequently simulate the decanter.

2. MATERIALS AND METHODS 2.1. Experimental Setup. The experimental setup is shown in Figure 2. The setup consisted of six 125 mL glass separating funnels (1) with polytetrafluoroethylene (PTFE) stopcocks (1.3). The funnels were submerged into the water bath (10) so that the water level was above the separating funnel body (1.2) at all times, which minimized condensation on the inner glass surfaces above the organic layer (6.1). The stems (1.4) were sealed with silicone plugs (5) before submerging them in the water bath to prevent water from wetting the inner surface of the stem, which would have influenced the mass balance when phases were decanted. Custom poly(vinyl chloride) (PVC) plugs (2) were manufactured to house two stainless steel sheaths (2.1) that supported the sampling needles (3, 4). A 115 mm stainless steel sampling needle (3) was used to sample the aqueous phase, and this needle was sealed off with a gastight valve (3.1). Sampling needles (70 mm; 4) were used for sampling of the organic phase. The six separating funnels were submerged simultaneously in the water bath (10), which had a water inventory of approximately 30 L. Five of the separating funnels contained (water + alcohol + entrainer) mixtures which were sampled to 2741



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Table 1. Summary of the Ternary Liquid−Liquid Equilibrium (LLE), Vapor−Liquid Equilibrium (VLE), and Vapor−Liquid− Liquid Equilibrium (VLLE) Data Available in Literature As Used in This Worka system

data type

(water + ethanol + DIPE)

LLE

(water + isopropanol + DIPE)

VLE VLLE LLE

(water + ethanol + cyclohexane)

VLE VLLE LLE

(water + isopropanol + cyclohexane)

VLE VLLE LLE

(water + n-propanol + cyclohexane)

VLE VLLE LLE

(water + ethanol + isooctane)

VLLE LLE

(water + isopropanol + isooctane)

VLE VLLE LLE

(water + n-propanol + isooctane)

VLE VLLE LLE VLE VLLE

T (K)

no. of tie lines

ref

298.2 298.2 298.2 308.2 318.2

7 7 9 9 9 18 21 7 8 14 9 12 18 9 6 22 18 6 13 22 10 19 18 21 27 11 8 7 8 6 14 7 10 10 10 20 12 7 5 9 9 23 7 27 7 15 6 15 17 10 14 6 7 11 22 8 8 17 17

Letcher et al.39 Hwang et al.40 Arce et al.41 Arce et al.41 Arce et al.41 Pienaar et al.9 Pienaar et al.9 Letcher et al.39 Hwang et al.40 Arce et al.42 Arce et al.42 Arce et al.42 Verhoeye43 Pienaar et al.9 Plačkov and Štern14 Hong-Fang et al.21 Moriyoshi et al.44 Letcher et al.12 De Doz et al.45 Hong-Fang et al.21 Stephenson46 Hong-Fang et al.21 Moriyoshi et al.44 Hong-Fang et al.21 Gomis et al.8 Gomis et al.8 Verhoeye47 Plačkov and Štern14 Washburn et al.13 Letcher et al.12 Nikurashina and Sinegubova48 Battler et al.31 Choi et al.32 Choi et al.32 Choi et al.32 Verhoeye47 Verhoeye47 Plačkov and Štern14 Letcher et al.12 Washburn et al.13 Washburn et al.13 Lee and Shen33 Nowakowska et al.49 Wagner and Sandler22 Nowakowska et al.49 Peschke and Sandler50 Huber et al.11 Wagner and Sandler22 Font et al.51 Font et al.51 Arda and Sayar52 Rastegar and Jessen53 Otero et al.54 Chen and Zhang55 Font et al.56 Font et al.56 Wang et al.57 Pienaar et al.9 Pienaar et al.9

298.2 298.2 298.2 308.2 318.2

298.2 298.2 298.2 298.2 303.2 308.2 313.2 318.2 323.2 328.2

298.2 298.2 298.2 298.2 298.2 303.2 303 313 323

298.2 298.2 298.2 308.2 273.2 278.2 293.2 298.2 298.2 313.2

293.2 293.2 298.2 298.2

298.2

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Table 1. continued a

Unless stated otherwise, P = 0.1 MPa.

the temperature was increased at a rate of 10 K/min to a final temperature of 393 K. The water content of all samples was determined by means of Karl Fischer titrations which were performed on a Metrohm 701 KF Titrino unit using Hydranal Composite-5 and Hydranal−Methanol Dry (both from Sigma-Aldrich). The unit was calibrated using distilled water before each set of samples was analyzed, and all samples were analyzed at least four times. 2.3. Materials. Table 2 shows the chemicals used in this work, as well as the chemical purities, their suppliers, and Figure 2. Experimental equipment showing (a) the construction of the modified separating funnel and (b) the setup of the water bath: 1, 125 mL glass separating funnel; 1.1, separating funnel neck; 1.2, separating funnel body; 1.3, polytetrafluoroethylene (PTFE) stopcock; 1.4, separating funnel stem; 2, poly(vinyl chloride) (PVC) plug; 2.1, stainless steel sheath; 3, 115 mm stainless steel aqueous sampling needle; 3.1, stainless steel pushbutton gastight valve; 4, 70 mm stainless steel organic sampling needle; 5, silicone plug; 6.1, organic phase; 6.2 interface; 6.3, aqueous phase; 7, six separating funnels; 8, thermostat with PID temperature control; 8.1, heating element; 8.2 water pump; 9, polystyrene cover; 10, 30 L water bath.

Table 2. Purities (Mole Fractions) of the Materials Used in This Work, As Determined by Karl Fischer Titrations and Gas Chromatography material

puritya (mole fraction)

supplier

product no.

ethanol isopropanolb n-propanol cyclohexane isooctane acetonec 2-pentanol

0.9992 0.9983 0.9980 0.9912 0.9912 0.9963 0.9985

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Fluka

34852 34863 34871 C100307 59045 34850 76942

a

Determined from Karl Fischer titrations, assuming the only components are water and the hydrocarbon, as no impurity peaks were identified on the chromatograms. bAverage of the two bottles. c Average of the three bottles.

phase, the aqueous sampling needle was purged with a 200 μL aqueous phase sample using a 1 mL gastight syringe. An aqueous phase sample of approximately 2 mL was then drawn into a 5 mL gastight syringe containing a known amount (approximately 2 mL) of acetone solvent. A second 5 mL gastight syringe, also with a known amount of approximately 2 mL of acetone, was fitted with a 70 mm sampling needle and inserted into the organic sampling sheath. An organic sample of approximately 2 mL was then withdrawn. Care was taken during the sampling procedure to sample the aqueous and organic phases as quickly as possible after each other in order to minimize the departure from equilibrium. It was not necessary to preheat the syringes before sampling as the acetone solvent in the syringes always ensured the presence of a homogeneous phase. Samples were refrigerated as soon as possible. Once sampled, the aqueous and organic phases were decanted into separate glass flasks and weighed so that an overall mass balance could be performed. All samples were analyzed by means of gas chromatography (GC), which furnished the mass ratios of the organic components, and Karl Fischer titrations, which quantified the samples’ water content. All samples were analyzed on the same day that they were taken. Duplicate GC samples were prepared for each sample taken, where 2-pentanol was used as an internal standard for quantitative results and acetone used as a solvent. Each prepared GC sample was manually injected once on a HP 5890 GC fitted with a flame ionization detector (FID) and coupled to a computer equipped with Delta 5.5 software. The injection volume was 0.8 μL, and components were separated on a Zebron ZB-5 wax capillary column with dimensions 60 m × 0.32 mm × 0.25 μm, with a helium carrier flow of 34.7 kPa and split ratio of 1:70. The injector and detector were operated at 493 and 523 K, respectively. For separation, the column temperature was initially kept at 303 K for 5 min, after which

product numbers. All chemicals were used as supplied, and hydrocarbons were injected on the GC-FID described earlier. Impurity peaks were not detected on the resulting chromatograms, and Karl Fischer titrations were performed at least four times on all feedstocks in order to determine the shown purities. The results confirmed that water was the principal impurity in the hydrocarbons. The water content of the acetone solvent was accounted for in the sample analysis. Water with a resistivity of 18.2 MΩ·cm was used from a Millipore Milli-Q water purification system. For gas chromatography, high purity helium and technical grade air were supplied by Afrox. 2.4. Uncertainty. Even though the immersion heater had a quoted temperature stability of ±0.01 K, the temperature was estimated to be stable to at least 0.1 K. The deviations of the Pt100 probe with which the temperature was measured in the water bath, was 0.2 K according to the most recent calibration performed 3 months prior to the start of the experimental work. The maximum uncertainty in the temperature reported in this work is therefore u(T) = 0.2 K. All experiments were conducted at atmospheric pressure, and no attempt at pressure control was made as LLE are sparingly affected by pressure.10 Barometric readings at Stellenbosch University showed that the pressure varied between 101.4 and 102.9 kPa during the experimental period, with an average and standard deviation of 101.9 and 0.4 kPa, respectively. The pressure is therefore quoted as P = 0.1 MPa with an uncertainty, based on the 99% confidence interval, of u(P) = 3 kPa. 2743



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Figure 3. Comparison of the experimentally measured tie lines (red ▼) of the (water + n-propanol + cyclohexane) system at P = 0.1 MPa and T = 298.2 K with tie lines reported by Washburn et al.13 (●), Letcher et al.12 (■), and Plačkov and Štern14 (▲). All data are in mole fractions.

These values are less than the estimated analytical error of 0.013 mole fraction. These deviations are therefore likely due to errors in the analytical procedure in either this work or the literature data. The measured verification data also show a regular trend according to the Othmer−Tobias15 plot given in Figure 4 (see the following section), and the trend observed in the plot is similar to that exhibited by the literature data.

To determine the uncertainty in the compositions, four representative (water + alcohol + entrainer) samples of known compositions were prepared and analyzed according to the same method used for analysis of the experimental samples. The samples were prepared such that they lie on the two ends of tie lines published in the literature.11,12 The average absolute error of the 12 compositions was 0.005 mole fraction with a standard deviation of 0.003 mole fraction, and a maximum error of 0.012 mole fraction. Analytical errors are therefore taken to be no more than u(x) = 0.013 mole fraction, based on the 99% confidence interval.

3. VERIFICATION 3.1. Equipment and Method Verification. In order to verify the experimental procedure and analytical method, the (water + n-propanol + cyclohexane) system at 298.2 K was measured and compared to published data. This system and temperature were chosen as at least three sets of published data12−14 are available, with these three sets being in excellent agreement with one another. The data are provided in Table 4 and are compared to the published data in Figure 3. The markers close to the centers of the tie lines measured in this work show the overall composition charged to the separating funnels. The target overall compositions of the five measured tie lines were chosen such that they lie on, or very close to, the published tie lines, which allows for quantitative comparison between the measured and published compositions of the phases. Figure 3 shows excellent agreement between the measured and published data as the measured data points all lie on the published binodal curves. The slopes of the tie lines are also nearly identical to those reported in the literature. Material balances close sufficiently as all loading points are coincident with the tie lines (93−103% of the component masses originally charged were recovered according to the analyses), except for the lowest tie line for which the determination of the very small amounts of n-propanol was difficult. Quantitatively, the largest average absolute deviation was 0.010 mole fraction for the entrainer composition in the organic phase, and the overall average absolute deviation was 0.007 mole fraction.

Figure 4. Othmer−Tobias plot for the measured compositions w (in mass fraction) of the {water (1) + n-propanol (2) + cyclohexane (3)} system at P = 0.1 MPa and T = 298.2 K (red ▼), compared with the compositions reported by Washburn et al.13 (●), Letcher et al.12 (■), and Plačkov and Štern14 (▲).

Based on the visual agreement with the published data, the small deviations of the 10 measured data points from published data and the regular trend of the Othmer−Tobias plot, it is therefore believed that the experimental and analytical method used in this work is accurate. 3.2. Thermodynamic Consistency and Data Validity. It appears that there exists no thermodynamic consistency test for experimentally determined LLE data as the individual activity coefficients cannot be determined from experimental data and the composition range is not continuous.10,16 Consequently, the Othmer−Tobias correlation,15 given in eq 1, was used to determine whether the data follow a regular trend. 2744



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⎡ 1 − w3org ⎤ ⎡ 1 − w1aq ⎤ ⎥ k = log⎢ log ⎥ + cOT ⎢ OT org ⎣ w1aq ⎦ ⎣ w3 ⎦

Table 4. Experimentala (Liquid + Liquid) Equilibrium Mole Fractions (x) for {Water (1) + n-Propanol (2) + Cyclohexane (3)} at Temperatures T = 298.2 and 318.2 K and Pressure P = 0.1 MPa

(1)

w1aq

Here, refers to the weight fraction of water (component 1) in the aqueous phase, w3org refers to the weight fraction of entrainer (component 3) in the organic phase, and kOT and cOT are correlating constants. Note that the Othmer−Tobias plot does not necessarily have to be linear, as is frequently claimed in literature, but it should exhibit a regular trend.16 The Othmer−Tobias plots of all data sets measured in this work were found to have a regular trend and no distinct outliers were present. Additionally, the validity of the data was also checked by inspecting the differences between duplicate sample analyses, by performing mass and component balances, and by comparing the measured data with published data at other temperatures. The composition differences between duplicate sample analyses were, in all cases, less than the experimental error, and when plotted, these differences are negligible on the ternary diagrams, even for the largest difference observed. The loading points were also coincident on the tie lines, within experimental error, which showed that the experimentally determined compositions and phase masses were consistent with the masses originally charged. Comparison with published data also showed that the measured LLE data fit the gaps between the LLE data at lower temperatures and VLLE data reported in literature. It is therefore believed that the measured data are correct and suitable to allow for an investigation into the temperature dependence and modeling of ternary (water + alcohol + entrainer) systems.

aqueous phaseb

x2aq

0.949 0.880 0.820 0.704 0.581

0.051 0.119 0.175 0.271 0.350

0.952 0.885 0.836 0.748 0.649

0.048 0.114 0.159 0.235 0.302

x1org

T = 308.2 K 0.000 0.007 0.001 0.049 0.005 0.093 0.025 0.162 0.069 0.252 T = 318.2 K 0.000 0.009 0.001 0.059 0.005 0.116 0.017 0.209 0.049 0.324

x2org

x3org

0.041 0.206 0.288 0.329 0.376

0.952 0.745 0.619 0.509 0.372

0.050 0.233 0.309 0.390 0.387

0.941 0.709 0.576 0.401 0.288

0.080 0.078 0.061 0.047 0.006

0.985 0.964 0.950 0.945 0.939

0.015 0.036 0.050 0.055 0.061

x1org

T = 298.2 K 0.001 0.275 0.000 0.189 0.000 0.065 0.000 0.012 0.000 0.009 T = 318.2 K 0.000 0.010 0.000 0.019 0.000 0.033 0.000 0.116 0.000 0.253

x2org

x3org

0.435 0.363 0.224 0.080 0.002

0.289 0.449 0.710 0.908 0.989

0.020 0.091 0.191 0.300 0.405

0.971 0.890 0.777 0.584 0.341

Table 5. Experimentala (Liquid + Liquid) Equilibrium Mole Fractions (x) for {Water (1) + Ethanol (2) + Isooctane (3)} at Temperature T = 318.2 K and Pressure P = 0.1 MPa aqueous phaseb x1aq

x2aq

0.838 0.637 0.486 0.338 0.222

0.162 0.360 0.500 0.616 0.643

organic phasec x3aq

x1org

x2org

x3org

T = 318.2 K 0.000 0.011 0.003 0.016 0.014 0.020 0.046 0.027 0.135 0.062

0.028 0.048 0.086 0.149 0.295

0.961 0.936 0.894 0.824 0.643

a

u(x) = 0.013 mole fraction, u(T) = 0.2 K, and u(P) = 3 kPa. bLiquid phase rich in water. cLiquid phase rich in isooctane.

Table 6. Experimentala (Liquid + Liquid) Equilibrium Mole Fractions (x) for {Water (1) + Isopropanol (2) + Isooctane (3)} at Temperatures T = 308.2 and 318.2 K and Pressure P = 0.1 MPa aqueous phaseb

organic phasec x3aq

0.920 0.922 0.939 0.953 0.994

x3aq

u(x) = 0.013 mole fraction, u(T) = 0.2 K, and u(P) = 3 kPa. bLiquid phase rich in water. cLiquid phase rich in cyclohexane.

Table 3. Experimentala (Liquid + Liquid) Equilibrium Mole Fractions (x) for {Water (1) + Isopropanol (2) + Cyclohexane (3)} at Temperatures T = 308.2 and 318.2 K and Pressure P = 0.1 MPa x1aq

x2aq

a

4. RESULTS AND DISCUSSION The LLE data measured in this work are reported in Table 3−Table 7, and a representative ternary diagram for the (water + isopropanol + cyclohexane) system is presented in Figure 5. For brevity, only the ternary diagram of this system is presented, but the temperature-dependent trends of the other seven (water + alcohol + entrainer) systems of interest are similar.

aqueous phaseb

x1aq

organic phasec

x1aq

x2aq

0.976 0.911 0.830 0.745 0.615

0.024 0.089 0.168 0.248 0.354

0.977 0.901 0.829 0.747 0.621

0.023 0.099 0.169 0.245 0.347

organic phasec x3aq

x1org

T = 308.2 K 0.000 0.009 0.000 0.028 0.002 0.046 0.008 0.066 0.031 0.105 T = 318.2 K 0.000 0.017 0.000 0.034 0.002 0.053 0.008 0.072 0.032 0.116

x2org

x3org

0.012 0.114 0.183 0.225 0.296

0.979 0.858 0.772 0.709 0.599

0.016 0.140 0.202 0.245 0.317

0.967 0.826 0.746 0.682 0.568

a u(x) = 0.013 mole fraction, u(T) = 0.2 K, and u(P) = 3 kPa. bLiquid phase rich in water. cLiquid phase rich in isooctane.

4.1. Effect of Temperature on the Composition of Coexisting Phases. The ternary diagrams show that the alcohol concentration of the aqueous phase decreases and that of the organic phase increases with an increase in temperature

a

u(x) = 0.013 mole fraction, u(T) = 0.2 K, and u(P) = 3 kPa. bLiquid phase rich in water. cLiquid phase rich in cyclohexane. 2745



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Table 7. Experimentala (Liquid + Liquid) Equilibrium Mole Fractions (x) for {Water (1) + n-Propanol (2) + Isooctane (3)} at Temperatures T = 308.2 and 318.2 K and Pressure P = 0.1 MPa aqueous phaseb x1aq

x2aq

0.974 0.932 0.893 0.842 0.781

0.026 0.068 0.106 0.155 0.211

0.975 0.943 0.906 0.856 0.786

0.025 0.057 0.093 0.141 0.206

organic phasec x3aq

x1org

T = 308.2 K 0.000 0.015 0.000 0.029 0.001 0.053 0.003 0.066 0.008 0.074 T = 318.2 K 0.000 0.010 0.000 0.027 0.001 0.056 0.003 0.075 0.009 0.085

x2org

x3org

0.022 0.117 0.191 0.225 0.244

0.964 0.854 0.755 0.709 0.682

0.023 0.123 0.221 0.251 0.280

0.967 0.850 0.723 0.674 0.635

Figure 6. Distribution of isopropanol in the aqueous and organic phases of the {water (1) + isopropanol (2) + cyclohexane (3)} system at P = 0.1 MPa as a function of temperature: T = 298.2 K, Plačkov and Štern14 (blue ▲); T = 303.2 K, Battler et al.31 (turquoise ▼); T = 308.2 K, this work (green ⧫); T = 318.2 K, this work (gold ■); VLLE liquid phase data, Verhoeye47 (red ●); azeotrope at 337.4 K, Verhoeye47 (red ○).

a

u(x) = 0.013 mole fraction, u(T) = 0.2 K, and u(P) = 3 kPa. bLiquid phase rich in water. cLiquid phase rich in isooctane.

decrease is more pronounced at higher alcohol concentrations. This was observed for all eight systems investigated. In order to estimate the magnitudes of these alcohol concentration changes, the azeotrope of each system is taken as a reference point as the decanter will operate in this region. The alcohol distribution plots are then used to estimate the composition change of an infinitely small amount of aqueous phase in equilibrium with an infinite amount of organic phase at the heterogeneous azeotropic composition of the organic phase, as the temperature increases from 298.2 K to the boiling point. This change, designated by Δx2aq, is indicated by the horizontal interpolated line shown on Figure 6. The same is also done for the organic phase, designated by Δx2org, which is indicated by a vertical line on the figure. These magnitudes are then expressed relative to the alcohol concentration of the vapor phase of the azeotrope (y2AZTP), which constitutes the overall composition

for all systems investigated. This is evident from the fact that the tie lines slope more toward the left (i.e., the slope becomes more positive) as temperature increases. Inspection of the ternary diagrams is, however, difficult, and in order to clarify the trends observed above, alcohol distribution plots are prepared, as shown in Figure 6 for the (water + isopropanol + cyclohexane) system. These plots show the alcohol content of the organic phase against that of the aqueous phase at different temperatures. The distribution plots show more clearly than the ternary diagrams that the organic phase alcohol concentration increases and aqueous phase alcohol concentration decreases with increasing temperature. The isotherms are also spread further apart at higher alcohol concentrations, which shows that the

Figure 5. Liquid−liquid phase behavior of the {water (1) + isopropanol (2) + cyclohexane (3)} system at P = 0.1 MPa as a function of temperature: T = 298.2 K, Plačkov and Štern14 (blue ▲); T = 303.2 K, Battler et al.31 (turquoise ▼); T = 308.2 K, this work (green ⧫); T = 318.2 K, this work (gold ■); VLLE liquid phase data, Verhoeye47 (red ●); azeotrope at 337.4 K, Verhoeye47 (red ○). Tie lines measured in this work are presented in bold. All data are in mole fractions. 2746



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Figure 7. Estimated magnitudes of aqueous (Δx2aq, black) and organic (Δx2org, gray) phase alcohol composition changes, relative to the alcohol content of the vapor phase of the azeotrope (y2AZTP).

Further, the greater apparent temperature dependence for the propanol systems as opposed to the ethanol systems can be explained similarly. Figure 8 shows that the polarities of the propanols decrease much quicker with temperature than that of ethanol. Consequently, their solubility in the nonpolar entrainer increases more quickly with increasing temperature than for ethanol and they move more preferentially to the organic phase. The polarities of isopropanol and n-propanol are similar and decrease at similar rates with increasing temperature, which explains why their temperature-dependent behavior is similar. It appears from Figure 7, however, that the temperature dependence of isopropanol is slightly larger than that of npropanol. This is possibly due to the fact that the polarity of isopropanol decreases slightly more with temperature than that of n-propanol, but it is speculated that this difference is due to the very low solubility of n-propanol in the aqueous phase. The low solubility is brought on by the larger alkyl group, which decreases water’s ability to solvate n-propanol. This leads to a smaller range over which the aqueous compositions can change. 4.2. Effect of Temperature on the Mutual Solubility of the Aqueous and Organic Phases. In terms of the sizes of the heterogeneous regions, it appears that the ethanolcontaining systems exhibit a more significant dependence on temperature. Both the cyclohexane and isooctane systems show a distinct decrease in the size of the heterogeneous region, with increasing temperature, which has also been noted previously.21,22 This trend is shown for the (water + ethanol + isooctane) system in Figure 9. For the (water + ethanol + DIPE) system, however, the VLLE phase envelope is similar to that of the LLE phase envelope at 298.2 K, which shows a smaller temperature dependence for this system compared to the cyclohexane and isooctane systems. Comparison of the heterogeneous regions of the (water + isopropanol + DIPE/ cyclohexane/isooctane) and (water + n-propanol + cyclohexane/isooctane) systems as functions of temperature are inconclusive as the differences between phase envelopes are too small to be significant when the experimental errors are taken into account. Changes in polarity can also explain the decrease in the size of the heterogeneous region with temperature for the (water + ethanol + cyclohexane/isooctane) systems. With increasing temperature, the polarity of the water molecules decreases, whereas that of the entrainer remains relatively constant. Thus, the mutual solubility of the water and entrainer increases, and therefore also the mutual solubility of the aqueous and organic

of the aqueous and organic phases at the azeotrope. The results are shown in Figure 7. From the figure, and for a particular entrainer, it is seen that the isopropanol-containing systems are more temperaturedependent than the ethanol-containing systems. A similar, but not as conclusive, observation is also made for the n-propanol systems when compared to the ethanol-containing systems. These observed trends can be explained through the component polarities, which are related to the intermolecular polar forces and solvation effects, both of which are important phenomena in the (water + alcohol + entrainer) systems investigated here.17 The polarities of the components involved, in terms of their dielectric constants,18,19 are shown in Figure 8

Figure 8. Relative dielectric constants, εr (relative to the permittivity of a vacuum, εo), as functions of temperature on a semilog scale. The equations to plot εr for all components except DIPE are taken from Wohlfarth;18 data for DIPE are taken from Kiselev et al.19

as functions of temperature. According to the like-dissolves-like rule, a solute (in this case the alcohol) will solvate more easily in a solvent (water or entrainer) of similar polarity.20 The polarity of water decreases more with increasing temperature than that of the alcohols and much more than that of the entrainers (see Figure 8). Consequently, with an increase in temperature, water’s ability to solvate the alcohol decreases. At the same time, the polarity of the alcohol also decreases whereas that of the entrainer remains relatively constant. Thus, the alcohol becomes more soluble in the organic phase than in the aqueous phase. The effect occurs to a greater extent at the higher alcohol concentrations as more alcohol transfers from the aqueous to the organic phase, which explains why the changes in composition appear more pronounced at higher alcohol concentrations. 2747



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Figure 9. Liquid−liquid phase behavior of the {water (1) + ethanol (2) + isooctane (3)} system at P = 0.1 MPa as a function of temperature: T = 298.2 K, Peschke and Sandler50 (blue ▲); liquid phase VLLE data, Font et al.51 (red ●); azeotrope at 341.9 K, Font et al.51 (red ○). Dashed lines show the NRTL correlations for the 298.2 K LLE (blue --) and the liquid phase VLLE data (red --). All data in mole fractions.

eq 3 above. These references contain the original formulations of the NRTL and UNIQUAC models, respectively. To determine the aij and bij correlating parameters, the following data were used: binary VLE of (water + alcohol) and (alcohol + entrainer) systems; binary LLE of (water + entrainer) systems; and ternary VLE, LLE, and VLLE of (water + alcohol + entrainer) systems. The binary VLE and LLE data used are given in Table 8, and the ternary VLE, LLE, and VLLE were given previously in Table 1. These data, as well as the LLE data measured in this work, were correlated in the Aspen Plus V8.2 data regression system (DRS) using the maximum likelihood objective function and the Britt−Luecke algorithm to determine the optimal parameters. Aspen Plus was used for the correlations as the ultimate goal of the research was to perform process simulations, for which Aspen Plus is a convenient tool. For each system, a total of 12 parameters (13 including the NRTL ACM’s nonrandomness parameter α) were determined: two energy parameters for each binary pair. Using the default Aspen Plus binary parameters as starting values, new temperature-dependent parameters were determined using the binary data given in Table 8. These new parameters were then used as starting values to determine parameters using all of the binary and ternary data listed previously. As far as possible, α was set to the values suggested by Schefflan,26 but it was regarded as a temperature-independent correlating parameter if these suggested values were insufficient. When correlating with the α parameter, it was seen purely as a correlating parameter and allowed to be negative, as discussed by Walas.27 Parameters were correlated for each ternary system, and no attempt was made to keep binary parameters constant over different ternary systems. The goodness-of-fit of the correlated models was determined using ternary diagrams similar to the one shown in Figure 9, which shows the correlation of the (water + ethanol + isooctane) system with the NRTL model, the most successful correlation obtained in this work. Similar diagrams were prepared for each (water + alcohol + entrainer) system and

phases. It appears that the (water + ethanol + cyclohexane/ isooctane) systems’ phase envelopes exhibit a greater dependence on temperature than those of the (water + ethanol + DIPE) or other alcohol systems. This is speculated to be due to the fact that the (water + ethanol + cyclohexane/isooctane) systems exhibit lower mutual solubilities between aqueous and organic phases (i.e., larger phase envelopes). The higher mutual solubilities in the other systems likely minimize any further increases in mutual solubility due to temperature. The higher binary mutual solubility of (water + DIPE) and the surfactantlike properties of the propanols lead to higher mutual solubilities between the aqueous and organic phases in these systems.

5. THERMODYNAMIC MODELING 5.1. Data Correlation. The majority of the LLE data reported in the literature are correlated using the nonrandom two liquid (NRTL)23 and universal quasi-chemical (UNIQUAC)24 activity coefficient models (ACMs). In most cases the data are only correlated at a single temperature with the NRTL and UNIQUAC energy parameters (Δgij and Δuij, respectively) assumed to be independent of temperature. For accurate simulations of the temperature-dependent behavior of the decanter, however, a single set of parameters should correlate LLE phase behavior over a range of temperatures. Consequently, the energy parameters were assumed to vary linearly with temperature, as given in eqs 2 and 3:23,25 τij ≡

(gij − gjj)/R T

=

Δgij /R T

=

aijT + bij T

(2)

⎛ (uij − ujj)/R ⎞ ⎛ Δuij /R ⎞ ⎛ aijT + bij ⎞ ⎟⎟ = exp⎜ − τij ≡ exp⎜⎜ − ⎟ = exp⎜ ⎟ T T ⎠ T ⎝ ⎝ ⎠ ⎝ ⎠ (3)

The reader is referred to eq 36 in the seminal paper of Renon and Prausnitz23 for the context of eq 2 above and to eq 23a in the seminal paper of Abrams and Prausnitz24 for the context of 2748



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Here, Ntie lines and Ncmpnts refer to the number of tie lines and components in each regression set, respectively; (xjπ)exp and (xjπ)calc refer to the experimental and calculated mole fractions of component j in the π phase, respectively. The parameters of the qualitatively correct correlations are presented in Table 10 and Table 11. Both the NRTL and

Table 8. Additional Binary Literature Data Used in Regressionsa system

data type

no. of points

ref

(water + ethanol)

VLE

23 21 18 26 27 21 48 9 26 16 21 10 19 21 11 21 32 14 27 11 21 31 17 21 21 14 19 21 21 18 8 5 3

Arce et al.58 Kojima et al.59 Kurihara et al.60 Zemp and Francesconi61 Brunjes and Bogart62 Kojima et al.59 Lebo63 Verhoeye47 Wilson and Simons64 Dawe et al.65 Kojima et al.59 Murti and Van Winkle66 Ku and Tu67 Lladosa et al.68 Verhoeye43 Zhang et al.69 Yuan et al.70 Zhao et al.71 Yuan et al.70 Verhoeye47 Hiaki et al.72 Łenka et al.73 Hiaki et al.74 Ku and Tu75 Wen and Tu76 Bures et al.77 Hiaki et al.74 Huang and Tu78 Lin and Tu79 Hiaki et al.74 Hwang et al.40 Mac̨ zyński et al.80 Mac̨ zyński et al.80

(water + isopropanol)

a

VLE

(water + n-propanol)

VLE

(ethanol + DIPE) (isopropanol + DIPE)

VLE VLE

(ethanol + cyclohexane)

VLE

(isopropanol + cyclohexane)

VLE

(n-propanol + cyclohexane)

VLE

(ethanol + isooctane)

VLE

(isopropanol + isooctane)

VLE

(n-propanol + isooctane) (water + DIPE) (water + cyclohexane) (water + isooctane)

VLE LLE LLE LLE

Table 10. Regressed UNIQUAC Parameters for the (Water + Alcohol + Entrainer) Systems ij 12 13 23 12 13 23 12 13 23 12 13 23 12 13 23

ACM, and inspected visually. For a fit to be sufficient, both the change in the size of the heterogeneous region and slopes of the tie lines with temperature should be represented correctly. Quantitatively, the goodness-of-fit was judged using the average absolute deviation (AAD), as defined in eq 4. The AADs are summarized in Table 9, and in all cases the AADs agreed with the visual inspections (i.e., a poor visual fit agreed with a high AAD). Ncmpnts

Ntie lines

1 6Ntie lines

∑

[

i

org

∑

{|(xj aq)exp − (xj aq)calc |

j org

+ |(xj )exp − (xj )calc |}]

aji

bij (K)

bji (K)

UNIQUAC correlations provide qualitatively correct representations of the ethanol-containing systems as well as the (water + isopropanol + isooctane) system. The NRTL correlations for the (water + ethanol + cyclohexane/isooctane) and (water + isopropanol + isooctane) systems, however, provide better representations for the change in the heterogeneous region and tie lines with increasing temperature. The UNIQUAC correlation of the (water + ethanol + DIPE) system is slightly better than the NRTL correlation close to the azeotropic tie line. For the (water + isopropanol + DIPE) system, a qualitatively correct correlation was obtained with the UNIQUAC ACM only, whereas the (water + isopropanol + cyclohexane), (water + n-propanol + cyclohexane), and (water + n-propanol + isooctane) systems were only successfully correlated with the NRTL ACM. Difficulties in the correlation of the latter four systems have been reported previously. Considering the (water + isopropanol + DIPE) system, Pienaar28 notes that neither the NRTL nor the UNIQUAC ACM was able to predict the VLLE of this system accurately. Lladosa et al.29 attempt to predict the VLLE

Unless stated otherwise, P = 0.1 MPa.

AAD =

aij

Water (1) + Ethanol (2) + Diisopropyl Ether (3) 1.801 −1.620 −890.4 584.9 −1.509 1.684 364.7 −1198.0 −0.209 1.118 142.4 −739.5 Water (1) + Isopropanol (2) + Diisopropyl Ether (3) −3.679 1.287 909.1 −311.3 −1.642 0.329 411.1 −814.5 6.339 −18.534 −1875.7 5420.4 Water (1) + Ethanol (2) + Cyclohexane (3) −8.870 −1.618 3043.9 584.9 −1.845 4.510 −12.3 −2003.2 −1.657 −2.762 609.6 490.2 Water (1) + Ethanol (2) + Isooctane (3) −8.291 1.271 2762.8 −205.4 10.042 −16.833 −3545.7 4161.0 1.585 −6.609 −355.7 1550.1 Water (1) + Isopropanol (2) + Isooctane (3) −1.464 −0.538 142.7 336.5 −1.803 10.987 38.8 −4404.5 −0.323 −0.252 271.1 −404.0

(4)

Table 9. Average Absolute Deviations (AAD in Mole Fractions) for the Correlation of the Eight (Water + Alcohol + Entrainer) Systems Using the NRTL and UNIQUAC Activity Coefficient Models water

NRTL UNIQUAC

water

water

water

water

water

water

water

EtOH

IPA

EtOH

IPA

NPA

EtOH

IPA

NPA

DIPE

DIPE

c-C6H12

c-C6H12

c-C6H12

i-C8H18

i-C8H18

i-C8H18

0.030 0.039

0.151 0.019

0.014 0.016

0.051 0.136

0.068 0.116

0.020 0.025

0.016 0.034

0.021 0.101

2749



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account. The ACMs appear more suited to molecules in which either the nonpolar or polar behavior should dominate in a molecule. It is also seen in this work that the tie lines of the propanol-containing systems are more dependent on temperature, which also likely contributes to the difficulties experienced in correlating. In addition to the complex chemical nature of the systems involved, the correlation difficulties are suspected to be due to limitations in the numerical methods employed in the Aspen Plus V8.2 DRS. These difficulties have also been experienced previously by the present research group in the correlation of VLLE data.35 Without access to the Aspen source code, however, this suspicion cannot be proven. Further work is required in the modeling of these systems, and it is suggested that the modeling should be performed in a numerical environment such as MATLAB instead of the Aspen Plus DRS, to provide more control over the numerical methods used in the parameter estimation. If the exact same structures of the models are used in the numerical environment, the determined parameters can easily be transferred to Aspen Plus for use in the process simulation. 5.2. Decanter Simulations. Since several different flowsheets exist for heterogeneous azeotropic distillation, no specific flowsheet will be considered here and the reader is referred elsewhere for different distillation sequences.36 In all of these sequences, however, the decanter is fed with a heterogeneous liquid stream obtained by condensing the overheads from a column in which the heterogeneous azeotrope was approached. Because the decanter is nearly always preceded by a condenser, the two units are sometimes referred to as a condenser−decanter by some authors,37 but in this work we will refer only to a decanter. To investigate the influence of temperature on the decanter’s performance, it was simulated in Aspen Plus V8.2 using the simulator’s DECANT block. In all simulations, the decanter was fed with a stream with the experimental vapor composition of the ternary heterogeneous azeotrope (see Pienaar et al.9), at its calculated bubble point. The temperature of the decanter Tdecant is then varied from 298.2 K to the calculated bubble point in increments of 2.5 K, and the flow rates and compositions of the organic and aqueous product streams at each temperature were recorded. All simulations were performed at P = 101.3 kPa. The process efficiency of heterogeneous azeotropic distillation sequences is improved if the amount of water removed in the decanter is increased.37 Consequently, the decanter performance is defined here in terms of the water recovery (WR), or fraction of water in the decanter feed that reports to the aqueous stream:

Table 11. Regressed NRTL Parameters for the (Water + Alcohol + Entrainer) Systems ij 12 13 23 12 13 23 12 13 23 12 13 23 12 13 23 12 13 23 12 13 23

aij

aji

bij (K)

bji (K)

Water (1) + Ethanol (2) + Diisopropyl Ether (3) −1.214 2.896 1249.1 −1057.3 11.093 −1.844 −1798.6 1063.9 0.313 3.621 −55.6 −801.4 Water (1) + Ethanol (2) + Cyclohexane (3) −8.204 3.970 3751.8 −1833.2 1.818 −3.518 2329.5 2539.4 −3.642 2.172 1452.3 −262.7 Water (1) + Isopropanol (2) + Cyclohexane (3) 6.960 6.896 −1401.3 −2489.4 3.855 −5.778 1536.0 3331.8 −0.777 2.996 202.9 −809.0 Water (1) + n-Propanol (2) + Cyclohexane (3) 4.335 −1.026 −1591.9 826.7 7.525 3.874 393.2 −430.6 −5.995 −0.582 1767.7 3008.1 Water (1) + Ethanol (2) + Isooctane (3) 6.286 −4.800 −2175.9 1621.3 −19.962 4.432 9986.3 −386.2 −4.540 1.187 1887.5 132.4 Water (1) + Isopropanol (2) + Isooctane (3) 2.500 7.333 −265.3 −2489.4 11.932 −14.808 337.2 5993.9 −0.603 7.401 585.2 −2285.0 Water (1) + n-Propanol (2) + Isooctane (3) 15.465 3.862 −7645.4 530.0 3.865 −5.109 −1099.9 3737.3 24.869 −14.553 −7573.7 6948.5

αij = αji 0.30 0.20 0.20 0.30 0.20 0.44 0.30 0.20 −2.70 0.30 0.20 0.47 0.30 0.20 0.47 0.30 0.20 0.30 −0.071 −0.072 0.660

of the (water + isopropanol + DIPE) system using the NRTL and UNIQUAC ACMs regressed with LLE and VLE data. They also report liquid phase deviations of similar magnitudes to those obtained in this work for the UNIQUAC ACM, but they fail to report their parameters. Considering the (water + propanol + hydrocarbon) systems, Plačkov and Štern14,30 report failed NRTL and UNIQUAC correlations for the LLE of (water + isopropanol/n-propanol + cyclohexane) systems at 298.2 K, whereas Battler et al.31 report failed NRTL and UNIQUAC correlations for the LLE of (water + isopropanol + cyclohexane) over the temperature range 293.2 to 303.2 K. Choi et al.32 also report very large AADs (in the order of 0.2 mole fraction) for their correlation of the (water + isopropanol + cyclohexane) system with the UNIQUAC model. Lee and Shen33 further report a failed UNIQUAC correlation for the VLLE of the (water + npropanol + cyclohexane) system. Moreover, Pienaar28 fails to obtain a successful correlation for the VLLE of the (water + npropanol + isooctane) system with neither the NRTL nor UNIQUAC model. It is therefore evident that the LLE of (water + propanol + entrainer) systems are particularly difficult to correlate. Magnussen et al.34 refer to this phenomenon as “The Propanol Problem”, which they report to be due to a rapid change in tieline slopes over a small distance on the binodal curve. Additionally, Plačkov and Štern14,30 conclude that the polar hydroxyl and nonpolar alkyl group in the propanol molecules are of similar influence, other than ethanol where the hydroxyl group dominates the molecule’s behavior. This then leads to complex behavior for which the models cannot sufficiently

WR =

Laq x1aq Dy1

(5)

Here, Laq and x1aq are the molar flow rate and composition of the aqueous stream leaving the decanter, respectively; D and y1 are the molar flow rate and composition of the feed stream to the decanter, respectively. Component 1 refers to water. It appears that, according to the available experimental data as analyzed in Figure 6, with an increase in temperature, the alcohol concentration in the organic phase uniformly increases whereas the concentration in the aqueous phase decreases. The simulations were performed with both the NRTL and UNIQUAC models for all systems, using the parameters 2750



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Figure 10. Decanter simulation at P = 101.3 kPa using the NRTL model with regressed parameters for the dehydration of ethanol using isooctane as an entrainer: experimental tie line at 298.2 K, Peschke and Sandler50 (blue ▲); experimental azeotrope at 341.9 K, Font et al.51 (red ○); tie lines showing the compositions of the aqueous and organic phases leaving the decanter at different compositions (black ○). Not all simulated tie lines are shown. The thick arrow indicates the direction in which the simulated tie lines slope as the temperature increases from 298.2 K to the simulated azeotropic temperature of 338.4 K.

Figure 11. Decanter simulations showing water recovered in the decanter as a function of temperature for (a) ethanol dehydration using different entrainers and (b) C2 and C3 alcohol dehydration using cyclohexane as entrainer. Regressed parameters for the simulations are given in Table 10 and Table 11.

The (water + ethanol + DIPE) simulations with UNIQUAC and (water + ethanol + cyclohexane/isooctane) simulations with NRTL were, however, reasonable, and the water recoveries for these systems are plotted in Figure 11a, which allows for comparison of the three entrainers. When increasing the decanter temperature from 298.2 K to the simulated azeotropic temperature, it appears that the alkane entrainers have much higher water recoveries (cyclohexane, 97.6 to 94.7%; isooctane, 96.0 to 96.6%) than DIPE (60.1 to 57.5%). Water recovery for the isooctane system also appears to be practically independent of temperature as it decreases by only 0.6% over the temperature range investigated. The lower recovery for DIPE as opposed to the alkanes is believed to be due to the increased mutual solubility of the aqueous and organic phases in DIPE-containing systems in comparison with the alkane systems. In terms of practical implications for ethanol dehydration, it therefore appears that the decanter of distillation sequences using cyclohexane or DIPE as entrainers should be operated at lower temperatures, whereas the water recovery of isooctane is

from the reasonable regressions, and it was investigated whether this behavior is accurately represented. This was done on figures similar to the one shown in Figure 10, which shows the experimental azeotrope, a tie line at 298.2 K which intersects the azeotropic tie line close to feed composition, and the simulated tie lines which relate the compositions of the aqueous and organic phases leaving the decanter. It was found that the following simulations, however, erroneously predicted a uniform increase in aqueous phase alcohol concentration with increasing temperature: (water + ethanol + DIPE) with NRTL and (water + isopropanol + DIPE/isooctane) with UNIQUAC. Simulations of the (water + isopropanol + isooctane) systems with UNIQUAC grossly overpredicted the increase in mutual solubilities with temperature. Further still, (water + ethanol + cyclohexane) simulations with UNIQUAC and (water + isopropanol/n-propanol + isooctane) simulations with NRTL first predicted an increase in aqueous phase alcohol concentrations, followed by a decrease, with increase in temperature. These simulations are therefore regarded as unrealistic and not considered further. 2751



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decanter. When using cyclohexane as an entrainer, as the decanter temperature increases, water recoveries decreased for ethanol dehydration and increased for isopropanol and npropanol dehydration. The alcohol thus plays an important role in decanter temperature dependence using cyclohexane as an entrainer, and the decanter should be operated at temperatures close to the azeotropic temperature for propanol-rich streams, and at low temperatures for ethanol-rich streams.

practically temperature-independent. It also appears as if isooctane and cyclohexane are more effective entrainers than DIPE, based only on the water recoveries, but an in-depth energy and economic analysis is required to determine conclusively the most effective entrainer. The (water + ethanol/isopropanol/n-propanol + cyclohexane) simulations with the NRTL correlations were also reasonable, but the magnitudes of the composition changes with temperature in the two liquid phases were slightly overpredicted. Water recoveries for the dehydration of the C2 and C3 alcohols using cyclohexane as entrainer are shown in Figure 11b. As the temperature increases, water recoveries of the ethanol system (97.6 to 94.7%) are higher than those of the isopropanol (67.9 to 78.4%) and n-propanol (73.9 to 76.1%) systems. The higher water recoveries for the ethanol systems are due to the fact that the vapor of the ethanol system’s heterogeneous azeotrope contains less water (0.188 mole fraction) than the isopropanol (0.212 mole fraction) or npropanol (0.256 mole fraction) systems. The ethanol system’s azeotropic organic liquid also contains less water with a mole fraction of 0.02 versus that of 0.07 and 0.03 for isopropanol and n-propanol, respectively. Consequently, less water reports to the organic phase in the ethanol system. The fact that water recoveries of the ethanol-containing system appear to decrease while those of the propanolcontaining systems increase with increasing temperature has interesting industrial implications. Cyclohexane is widely used as an entrainer in ethanol dehydration38 but ethanol-containing streams from the Fischer−Tropsch process are likely to contain significant amounts of propanol.1 Consequently, it appears that the decanter should be operated at low temperatures for distillate streams rich in ethanol, but as close as possible to the azeotropic temperature for streams rich in propanol.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +27 21 808 4487. Fax: +27 21 808 2059. 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. 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. Notes

The authors declare no competing financial interest.

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

(1) Nel, R. J. J.; De Klerk, A. Fischer−Tropsch Aqueous Phase Refining by Catalytic Alcohol Dehydration. Ind. Eng. Chem. Res. 2007, 46, 3558−3565. (2) Devarapalli, M.; Atiyeh, H. K. A review of conversion processes for bioethanol production with a focus on syngas fermentation. Biofuel Res. J. 2015, 2, 268−280. (3) Seader, J. D.; Henley, E. J.; Roper, D. K. Separation Process Principles, 3rd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2011. (4) Gmehling, J.; Menke, J.; Krafczyk, J.; Fischer, K.; Fontaine, J.-C.; Kehiaian, H. V. Azeotropic Data for Binary Mixtures. In CRC Handbook of Chemistry and Physics, 86th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, USA, 2005; pp 6-180−6-200. (5) Udeye, V.; Mopoung, S.; Vorasingha, A.; Amornsakchai, P. Ethanol heterogeneous azeotropic distillation design and construction. Int. J. Phys. Sci. 2009, 4, 101−106. (6) Ciric, A. R.; Mumtaz, H. S.; Corbett, G.; Reagan, M.; Seider, W. D.; Fabiano, L. A.; Kolesar, D. M.; Widagdo, S. Azeotropic distillation with an internal decanter. Comput. Chem. Eng. 2000, 24, 2435−2446. (7) Gomis, V.; Pedraza, R.; Francés, O.; Font, A.; Asensi, J. C. Dehydration of Ethanol Using Azeotropic Distillation with Isooctane. Ind. Eng. Chem. Res. 2007, 46, 4572−4576. (8) Gomis, V.; Font, A.; Pedraza, R.; Saquete, M. D. Isobaric vapor− liquid and vapor−liquid−liquid equilibrium data for the system water + ethanol + cyclohexane. Fluid Phase Equilib. 2005, 235, 7−10. (9) Pienaar, C.; Schwarz, C. E.; Knoetze, J. H.; Burger, A. J. Vapor− Liquid−Liquid Equilibria Measurements for the Dehydration of Ethanol, Isopropanol, and n-Propanol via Azeotropic Distillation Using DIPE and Isooctane as Entrainers. J. Chem. Eng. Data 2013, 58, 537−550. (10) Raal, J. D.; Mühlbauer, A. L. Phase Equilibria: Measurement and Computation, 1st ed.; Taylor & Francis: Washington, DC, USA, 1998. (11) Huber, J. F. K.; Meijers, C. A. M.; Hulsman, J. A. R. J. New Method for Prediction of Partition Coefficients in Liquid-Liquid Systems and Its Experimental Verification for Steriods by Static Chromatographic Measurements. Anal. Chem. 1972, 44, 111−116.

6. CONCLUSIONS The LLE data generated in this work and the LLE and VLLE data gathered from literature were used to determine the effect of temperature on the LLE of ternary (water + alcohol + entrainer) systems comprised of the alcohols ethanol, isopropanol, and n-propanol and the entrainers DIPE, cyclohexane, and isooctane. Temperature was found to have an effect on each of the systems investigated as the alcohol concentration of the aqueous phase uniformly decreases, whereas that of the organic phase uniformly increases with increasing temperature. The effect is more pronounced for the propanol-containing systems, especially isopropanol, than for the ethanol-containing systems. Additionally, the heterogeneous regions of systems with low mutual solubilities of the aqueous and organic phases (i.e., the (water + ethanol + hydrocarbon) systems) decrease more rapidly with temperature than the other systems. All of these observations can be explained by considering the component polarities, which are related to the polar intermolecular forces and solvation effects. The complexity of the intermolecular forces between the polar molecules as well as the changes of these forces with temperature, however, complicate correlations with the NRTL and UNIQUAC ACMs, especially in Aspen Plus V8.2, which warrants further attention. Subsequent simulations performed on the decanter showed that the water recoveries of cyclohexane, and isooctane are very similar, and much higher than those of DIPE. It therefore appears that the hydrocarbon entrainers provide better performance than DIPE in terms of water recovered in the 2752



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