VaporâLiquid Equilibria Measurements for the Five Linear C6 Esters

Article pubs.acs.org/jced

Vapor−Liquid Equilibria Measurements for the Five Linear C6 Esters with n‑Octane Jamie T. Cripwell, Cara E. Schwarz, and Andries J. Burger* Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa ABSTRACT: Vapor−liquid equilibrium (VLE) data were measured for the five binary systems comprising each of the unbranched constitutional isomers of a C6 ester with n-octane at 60.0 kPa. All five systems exhibited strong positive deviations from ideality and minimum boiling azeotropes due to the fundamental differences in the intermolecular forces of each component in the pair, as well as the closeness of the pure component boiling points at the system pressure. Thermodynamic consistency of all data was shown using the Wisniak L/W and McDermott−Ellis consistency tests, and the data were correlated and predicted with the NRTL and UNIFAC activity coefficient models, respectively. The data illustrated the role of the location of the polar ester functional group on the phase behavior of respective ester structural isomers with a common second component.

secondary alcohols6−39 as well as with normal alkanes.40−51 These studies have adopted a systematic approach in assessing the effect of varying the alkanolic length of the ester while keeping the acidic chain length constant (or vice versa) on the phase behavior of these esters with a common second component. The choice of an n-alkane as the second component allowed these authors to attribute differences in the observed phase behavior to the dilution or concentration of polar effects due to the presence of the nonpolar alkane. This rationale will be employed in this work too. The systems considered in this work comprise the five constitutional isomers of a C6 ester paired with an n-alkane exhibiting a similar vapor pressure, specifically n-octane. Here, the choice of an n-alkane as the second component allows any differences in the binary phase behavior between isomers to be

1. INTRODUCTION Many of today’s thermodynamic models are finely tuned to account for different intermolecular forces, such as polar forces and hydrogen bonding, and the role of molecular structures on these forces. Thus, accurate phase equilibrium data are vital, not only in the practical sense for application to existing industrial separation processes, but also in the theoretical sense for scrutinizing and refining the performance of the thermodynamic models used to design and control these industrial processes. For such theoretical applications, there needs to be a systematic approach to generating VLE data, so that subtle differences in the application of these models may be rigorously tested. Constitutional, or structural, isomerism is one such structural subtlety that affects the role and magnitude of different intermolecular forces. This is particularly true of polar forces, where the location of the polar functional group in the molecule greatly influences the degree to which these forces manifest themselves in the macroscopic properties of the component. Such differences are notably evident in the vapor pressures of polar constitutional isomers with the more sterically hindered isomers exhibiting higher vapor pressures than isomers where the functional group is located terminally.1−4 Thus, of interest to this work is the extension of this analysis to mixtures and the role of polar functional group location on the VLE of these different isomers with a common second component. A previous publication5 highlighted the lack of systematic studies into the phase behavior of such systems under identical conditions of constant temperature or pressure. To our knowledge, no previous studies have systematically considered the VLE of all the different isomers of a single linear ester with a common second component. Extensive studies have been done into the phase behavior and mixing properties of binary mixtures of linear esters with various primary and © XXXX American Chemical Society

Table 1. Chemicals Used (Supplied by Sigma-Aldrich) and Manufacturer Purities component

purity (mass fraction)

n-heptane 2-butanone 2-ethyl-1-hexanol n-octane methyl valerate ethyl butanoate propyl propanoate butyl ethanoate pentyl formate 2-heptanone

0.99 ≥0.997 ≥0.996 ≥0.990 0.99 0.99 0.99 ≥0.995 0.95 0.98

Received: December 14, 2015 Accepted: May 6, 2016

A

DOI: 10.1021/acs.jced.5b01058 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

work5 was used for the generation of the phase equilibrium data presented here. Equilibrium temperatures and unit pressure were measured using a Pt-100 probe connected to a digital Hart Scientific thermometer and a Wika UT-10 pressure transmitter, respectively. The absolute uncertainty in temperature measurements was 0.1 K between 313 and 413 K according to calibration performed by a SANAS (South African National Accreditation System) approved laboratory. Pressure was recorded with an uncertainty of 0.1% of full scale output, that is, 1.6 mbar absolute uncertainty on 1.6 bar abs full scale output. The experiments were initiated by loading the still with approximately 110 cm3 of one of the pure components with additions of the second component made between experimental runs. The system pressure was maintained by manually throttling the amount of vacuum drawn from the still, a procedure that introduced lower pressure fluctuations (±0.2 kPa) than those evident under the automated on−off control of the Pilodist M101 control system (±0.5 kPa). Steady and continuous liquid and condensed vapor flows were apparent after 20 min of operation at which point the sampling wells of the still were flushed to remove remaining liquid films from previous runs and prevent contamination of experimental samples. Flushing was performed again after a further 20 min before the sampling vials were replaced in anticipation of the final experimental sample for the run. Thereafter, the still was left to run for a final 20 min to ensure a stable equilibrium before one sample of each equilibrium phase was drawn for the run. It is in this final step that the experimental procedure altered from our previous work.5 Previously, the still was flushed again immediately prior to sampling, but using this procedure, a small but systematic deviation was apparent in our vapor phase compositions when compared to reference data. Our rationale in changing the procedure is that flushing the still immediately prior to sampling disturbs the equilibrium sufficiently to manifest itself in these small compositional deviations. Thus, in this work, the still is left to reattain equilibrium after the disturbance caused by flushing. Experimental samples were used to prepare duplicate samples for analysis by gas chromatography. For this purpose, a Varian CP-3380 gas chromatograph (GC) with flame-ionization detector was used with a ZB Wax capillary column (dimensions 30 m × 0.32 mm × 1 μm) installed and operated at a temperature of 523 K. Helium was used as the carrier gas with 2-ethyl-1-hexanol as solvent. Quantitative results were obtained using 2-heptanone as an internal standard with an established

Table 2. Comparison between Measured Normal Boiling Pointsa and Accepted Values in the DIPPR 801 Database52 for the Five Linear Structural Isomers of C6 Ester and n-Octane

a

component

measured

literature

n-octane methyl valerate ethyl butanoate propyl propanoate butyl ethanoate pentyl formate

398.71 400.48 393.78 395.51 399.09 405.15

398.83 400.54 394.15 395.55 399.24 405.45

u(T) = 0.12 K and u(p) = 0.36 kPa.

Figure 1. Comparison between experimental and literature vapor pressures for the five linear structural isomers of a C6 ester: Methyl valerate (○, solid line47), ethyl butanoate (△, dotted line53), propyl propanoate (◇, double dot-dash line53), butyl ethanoate (□, dashed line54), and pentyl formate (+, dot−dash line55). Note that reference data are represented graphically as a correlation through the actual data points for ease of reference.

attributed solely to the shifting of the ester functional group from one isomer to the next.

2. MATERIALS AND METHODS 2.1. Apparatus and Procedure. The same Pilodist VLE 100 D all-glass dynamic recirculating still as used in our previous

Table 3. Experimentally Measured Vapor Pressuresa for the Five Linear C6 Ester Structural Isomers P/kPa

T/K

methyl valerate 101.3 100.4 95.08 88.17 77.71 69.02 59.35 52.83 45.81 40.47 35.06 29.41 a

400.48 400.07 398.03 395.42 391.14 387.18 382.55 378.84 374.53 370.86 366.70 361.82

P/kPa

T/K

ethyl butanoate 101.3 100.4 90.84 84.14 77.20 65.75 58.98 54.40 50.00 40.41

393.78 393.63 390.23 387.65 384.82 379.65 376.28 373.83 371.33 365.24

P/kPa

T/K

propyl propanoate 101.3 93.37 86.38 77.90 70.32 60.83 51.21 47.90 40.92 33.58 29.95

395.51 392.74 390.13 386.81 383.56 379.07 373.82 371.88 367.30 361.75 358.63

P/kPa

T/K

butyl ethanoate 101.3 99.03 88.69 80.09 72.71 60.15 56.43 45.02 41.42 32.95 29.92

399.09 398.35 394.58 391.20 388.06 382.08 380.10 373.32 370.90 364.47 361.85

P/kPa

T/K

pentyl formate 101.3 93.45 86.56 80.16 73.50 68.46 60.50 55.79 49.42 44.06 39.15 33.67

405.12 402.19 399.55 396.89 394.01 391.65 387.67 385.08 381.35 377.87 374.41 370.04

u(T) = 0.12 K and u(p) = 0.36 kPa. B



Article

Figure 2. Comparison of experimental data with literature data56 for the 2-butanone(1)/n-heptane system at 94.0 kPa. (a) T−x−y plot with experimental (○, vapor and △, liquid) and reference (●, vapor and ▲, liquid) data. (b) x−y plot with experimental (△) and reference (●) data.

Table 4. Experimental and Reference56 VLE Data for Reference System 2-Butanone(1)/n-Heptane(2) at 94.0 kPa (u(P) = 0.36 kPa)

calibration curve based on peak area from the DELTA 5.0 software of the GC. 2.2. Materials. All chemicals used in this work were supplied by Sigma-Aldrich with manufacturer purities as indicated in Table 1. Analysis by gas chromatography showed that the indicated purities were met or exceeded in all cases and as such all components except pentyl formate were used without any further purification. The pentyl formate was further purified by simple distillation. Residual concentrations of impurities in the distilled pentyl formate were of the same order of magnitude as those in the other components after GC analysis and thus deemed insignificant. Comparisons between experimentally determined normal boiling points and corresponding literature values52 are given in Table 2. A similar comparison for the vapor pressures of the five esters with literature predictions47,53−55 is presented in Figure 1 with the experimental values tabulated in Table 3. The deviations between the experimental and literature values are consistently less than 5% with the majority of the data exhibiting deviations of less than 3%. This good agreement between the experimental values and those reported in the literature indicate acceptable component purities for further work. 2.3. Uncertainty. Temperature deviations for the Pt-100 probe did not exceed 0.1 K over the temperature range of interest in this study, according to the most recent certificate of calibration. Furthermore, slight fluctuations of no more than 0.02 K in the equilibrium temperature were observed during sampling. Thus, the maximum absolute uncertainty in the temperature reported here, u(T), is 0.12 K. The pressure transmitter has a maximum deviation of 1.6 mbar at full scale output. This, combined with the aforementioned typical pressure deviations of 0.2 kPa due to manual throttling of the still, result in a maximum absolute uncertainty in the pressure, u(P), of 0.36 kPa. Uncertainty in compositional measurements was determined by running samples of known composition through the GC after all experimental samples for a given binary pair were analyzed. A comparison between known compositions and those predicted by the established calibrations served to illustrate any drift in the components’ response factors and by association any increase in compositional uncertainty. Deviations were typically in the range of 0.006 mole fraction but

reference56

experimental T/K u(T) = 0.12 K 368.85 364.71 361.65 359.92 357.54 357.12 354.80 354.26 354.52 351.60 350.51 350.21 349.74 349.14 348.69 348.77 348.32 348.04 347.86 347.75 347.60 347.61 347.76 347.92 348.04 348.44 348.95 349.24 349.89 350.25

x1

y1

u(x) = u(y) = 0.010 mol fraction 0.000 0.000 0.033 0.156 0.069 0.260 0.105 0.325 0.139 0.397 0.152 0.406 0.201 0.485 0.220 0.489 0.226 0.490 0.302 0.572 0.353 0.596 0.378 0.603 0.417 0.623 0.466 0.641 0.515 0.657 0.526 0.661 0.575 0.678 0.604 0.688 0.642 0.705 0.685 0.719 0.725 0.737 0.764 0.755 0.798 0.777 0.829 0.802 0.860 0.824 0.901 0.864 0.937 0.904 0.961 0.937 0.985 0.975 1.000 1.000

T/K

x1

y1

369.02 367.15 366.05 365.45 364.15 363.45 363.25 361.95 361.35 360.35 358.05 357.35 355.85 354.55 353.35 352.65 349.75 348.90 348.41 348.26 348.13 348.27 348.65 349.32 349.67 350.00 350.42

0.000 0.010 0.019 0.024 0.036 0.040 0.044 0.058 0.070 0.088 0.122 0.133 0.162 0.198 0.236 0.256 0.466 0.582 0.639 0.716 0.743 0.814 0.890 0.944 0.968 0.985 1.000

0.000 0.057 0.097 0.123 0.173 0.192 0.203 0.240 0.270 0.303 0.364 0.390 0.439 0.477 0.500 0.517 0.633 0.677 0.703 0.737 0.751 0.793 0.850 0.913 0.944 0.971 1.000

never exceeded 0.010 mole fraction. Thus, the maximum uncertainty in vapor and liquid mole fractions reported here, u(y) and u(x), is 0.010 mole fraction. C



Article

Table 5. Experimental VLE Dataa for Temperature T, Liquid Phase Mole Fraction x and Vapor Phase Mole Fraction y for the Systems Comprising One of the Five Linear C6 Ester Isomers (1) with n-Octane (2) at 60.0 kPa γ1

γ2

0.000 0.000 0.051 0.095 0.142 0.131 0.173 0.180 0.224 0.269 0.271 0.330 0.353 0.374 0.409

1.787 1.692 1.487 1.623 1.494 1.487 1.395 1.349 1.339 1.265 1.245 1.215 1.188

1.000 1.000 0.998 0.997 1.007 0.997 1.010 1.009 1.030 1.038 1.042 1.071 1.076 1.094 1.116

0.000 0.029 0.085 0.126 0.155 0.226 0.247 0.253 0.312 0.325 0.375 0.377 0.405 0.412 0.421 0.445 0.447

0.000 0.013 0.045 0.069 0.087 0.145 0.166 0.165 0.227 0.235 0.288 0.284 0.330 0.326 0.346 0.380 0.385

2.036 1.732 1.712 1.707 1.530 1.472 1.525 1.390 1.410 1.352 1.378 1.281 1.323 1.275 1.235 1.224

1.000 0.999 0.999 0.993 0.995 1.000 1.006 0.998 1.017 1.013 1.027 1.019 1.043 1.032 1.047 1.066 1.070

380.76 380.87 380.15 379.92 378.95 378.69 378.07 377.10 376.87 376.85 376.61 376.58 376.18 376.03

0.000 0.000 0.062 0.067 0.141 0.164 0.213 0.289 0.309 0.314 0.335 0.344 0.380 0.411

0.000 0.000 0.032 0.037 0.080 0.098 0.142 0.214 0.236 0.238 0.266 0.272 0.312 0.355

1.832 1.743 1.734 1.662 1.513 1.413 1.376 1.386 1.334 1.340 1.307 1.251

1.000 1.000 0.994 1.000 0.993 0.993 1.002 1.016 1.024 1.020 1.035 1.030 1.043 1.061

380.78 380.06 379.76 379.32 378.77 378.62 378.08 377.74 377.46

0.000 0.068 0.081 0.120 0.165 0.172 0.218 0.257 0.284

0.000 0.037 0.049 0.073 0.108 0.111 0.152 0.193 0.213

1.974 1.779 1.771 1.683 1.722 1.624 1.518 1.534

1.000 0.995 1.003 0.999 1.001 1.000 1.006 1.016 1.013

T/K

y1

x1

380.82 380.94 379.94 379.32 379.05 378.94 378.58 378.56 378.15 377.96 377.95 377.69 377.71 377.64 377.53

0.000 0.000 0.082 0.142 0.185 0.186 0.223 0.231 0.267 0.307 0.308 0.350 0.370 0.381 0.406

380.85 380.47 379.63 379.14 378.58 377.69 377.40 377.33 376.60 376.43 375.82 375.79 375.66 375.49 375.47 375.26 375.29

T/K

y1

x1

γ1

methyl valerate (1) + n-octane (2) 0.413 0.413 1.190 0.432 0.450 1.144 0.453 0.474 1.135 0.460 0.494 1.108 0.488 0.524 1.106 0.509 0.562 1.069 0.519 0.572 1.075 0.552 0.612 1.062 0.558 0.621 1.056 0.574 0.644 1.044 0.585 0.658 1.036 0.600 0.671 1.043 0.620 0.701 1.025 0.637 0.729 1.007 0.653 0.739 1.013 ethyl butanoate (1) + n-octane (2) 375.10 0.466 0.409 1.208 374.87 0.488 0.439 1.187 374.97 0.492 0.449 1.165 374.87 0.497 0.455 1.165 374.83 0.513 0.479 1.144 374.75 0.534 0.504 1.135 374.65 0.555 0.533 1.119 374.66 0.591 0.583 1.088 374.65 0.627 0.636 1.059 374.76 0.650 0.666 1.045 374.70 0.651 0.663 1.054 374.82 0.672 0.696 1.033 374.83 0.695 0.725 1.025 374.81 0.705 0.737 1.023 374.89 0.712 0.746 1.017 374.85 0.729 0.767 1.015 377.64 377.65 377.74 377.69 377.73 377.91 377.84 378.05 378.07 378.21 378.34 378.33 378.51 378.70 378.87

γ2

T/K

y1

x1

γ1

γ2

1.108 1.143 1.148 1.180 1.188 1.233 1.235 1.261 1.275 1.300 1.315 1.316 1.372 1.435 1.418

379.02 379.53 379.67 379.71 380.05 380.24 380.69 381.08 381.44 381.43 381.90 381.98 382.12 382.78 383.29

0.672 0.704 0.717 0.730 0.752 0.775 0.804 0.833 0.864 0.867 0.899 0.916 0.925 0.968 1.000

0.759 0.789 0.804 0.818 0.840 0.857 0.880 0.899 0.923 0.928 0.947 0.956 0.962 0.984 1.000

1.011 1.000 0.997 0.996 0.989 0.992 0.988 0.990 0.988 0.987 0.988 0.995 0.995 0.997 1.000

1.442 1.472 1.501 1.541 1.592 1.615 1.652 1.654 1.753 1.818 1.868 1.864 1.890 1.890

1.082 1.100 1.109 1.113 1.128 1.136 1.156 1.192 1.245 1.269 1.254 1.300 1.337 1.354 1.368 1.403

374.92 375.13 375.21 375.32 375.53 375.47 375.64 375.75 375.97 376.18 376.26 376.57 376.75 376.87 377.23 377.21

0.740 0.773 0.785 0.802 0.814 0.828 0.851 0.855 0.881 0.894 0.911 0.940 0.964 0.971 1.000 1.000

0.779 0.814 0.827 0.845 0.854 0.869 0.891 0.893 0.916 0.926 0.939 0.961 0.977 0.982 1.000 1.000

1.012 1.005 1.002 0.999 0.996 0.997 0.995 0.994 0.992 0.989 0.991 0.990 0.993 0.992 1.000 1.000

1.416 1.461 1.486 1.518 1.508 1.562 1.612 1.593 1.641 1.667 1.699 1.754 1.778 1.806

375.55 375.63 375.69 375.75 375.86 375.97 376.16 376.26 376.57 376.94 377.46 377.87 378.36 378.75

0.603 0.609 0.625 0.653 0.677 0.701 0.722 0.748 0.789 0.829 0.881 0.913 0.965 1.000

0.624 0.629 0.649 0.686 0.715 0.746 0.767 0.796 0.834 0.873 0.909 0.945 0.978 1.000

1.057 1.056 1.048 1.035 1.025 1.015 1.011 1.005 1.001 0.993 0.997 0.982 0.988 1.000

1.248 1.243 1.258 1.296 1.327 1.371 1.380 1.427 1.460 1.531 1.465 1.732 1.724

377.77 377.83 378.27 378.46 378.30 378.82 379.26 379.38 379.75

0.675 0.680 0.716 0.737 0.738 0.770 0.807 0.813 0.842

0.757 0.761 0.798 0.820 0.823 0.852 0.880 0.883 0.905

1.015 1.014 1.006 1.001 1.004 0.994 0.996 0.996 0.994

1.476 1.478 1.526 1.579 1.604 1.669 1.694 1.684 1.733

propyl propanoate (1) + n-octane (2) 376.02 0.415 0.364 1.230 1.069 375.90 0.440 0.395 1.206 1.080 375.76 0.473 0.438 1.172 1.101 375.71 0.474 0.443 1.166 1.108 375.53 0.500 0.478 1.147 1.130 375.58 0.514 0.496 1.132 1.139 375.51 0.523 0.508 1.128 1.146 375.53 0.528 0.518 1.114 1.159 375.47 0.534 0.527 1.113 1.164 375.48 0.548 0.561 1.071 1.218 375.44 0.552 0.557 1.090 1.196 375.45 0.571 0.581 1.078 1.213 375.59 0.590 0.608 1.061 1.233 375.55 0.593 0.603 1.076 1.211 butyl ethanoate (1) + n-octane (2) 376.69 0.431 0.417 1.219 1.112 376.60 0.443 0.436 1.201 1.129 376.55 0.460 0.464 1.173 1.154 376.57 0.462 0.464 1.177 1.150 376.72 0.473 0.476 1.171 1.145 376.70 0.477 0.498 1.130 1.186 376.65 0.501 0.530 1.118 1.209 376.77 0.522 0.556 1.104 1.224 376.61 0.531 0.575 1.092 1.261

D



Article

Table 5. continued T/K

a

y1

x1

377.19 377.12 377.10 376.83 376.92 376.69 376.76 376.64

0.318 0.323 0.334 0.363 0.364 0.394 0.388 0.421

0.260 0.267 0.284 0.318 0.322 0.365 0.355 0.402

380.84 380.82 380.39 379.98 379.97 379.21 379.10 378.89 378.82 378.65 378.50 378.32 378.39 378.35 378.37 378.42 378.47

0.000 0.000 0.045 0.086 0.088 0.147 0.162 0.192 0.211 0.228 0.266 0.278 0.298 0.310 0.318 0.342 0.345

0.000 0.000 0.027 0.060 0.049 0.111 0.123 0.157 0.181 0.196 0.249 0.256 0.290 0.311 0.323 0.360 0.368

γ1

γ2

1.419 1.410 1.370 1.339 1.324 1.273 1.286 1.234

1.034 1.038 1.046 1.060 1.061 1.088 1.079 1.107

2.112 1.828 2.289 1.723 1.722 1.611 1.533 1.543 1.423 1.450 1.372 1.333 1.317 1.269 1.251

1.000 1.000 0.999 1.002 0.989 1.012 1.011 1.021 1.029 1.030 1.053 1.053 1.069 1.084 1.090 1.110 1.117

T/K

y1

γ1

x1

butyl ethanoate (1) + n-octane (2) 376.82 0.545 0.587 1.091 376.94 0.565 0.614 1.077 377.03 0.572 0.629 1.061 377.02 0.580 0.640 1.055 377.09 0.602 0.669 1.047 377.18 0.605 0.674 1.042 377.60 0.648 0.723 1.027 377.56 0.654 0.729 1.029 pentyl formate (1) + n-octane (2) 378.51 0.369 0.402 1.221 378.54 0.388 0.432 1.192 378.65 0.388 0.432 1.189 378.69 0.417 0.483 1.142 378.71 0.431 0.502 1.132 378.98 0.448 0.532 1.105 379.05 0.460 0.545 1.104 379.18 0.470 0.561 1.089 379.37 0.493 0.597 1.067 379.83 0.531 0.645 1.049 379.91 0.534 0.653 1.039 380.16 0.565 0.689 1.034 380.41 0.575 0.699 1.028 380.71 0.594 0.720 1.021 380.48 0.594 0.722 1.027 380.90 0.604 0.733 1.014 381.17 0.618 0.753 1.002

γ2

T/K

y1

x1

γ1

γ2

1.250 1.273 1.300 1.320 1.353 1.360 1.408 1.417

379.99 380.16 380.58 381.17 381.31 382.01 381.99

0.865 0.873 0.904 0.947 0.949 1.000 1.000

0.921 0.927 0.947 0.973 0.973 1.000 1.000

0.996 0.994 0.995 0.996 0.993 1.000 1.000

1.779 1.798 1.817 1.940 1.911

1.137 1.161 1.156 1.208 1.226 1.251 1.257 1.277 1.322 1.368 1.389 1.433 1.440 1.466 1.483 1.490 1.540

381.30 381.49 381.77 382.62 382.53 383.00 383.44 383.79 384.22 384.90 384.69 385.78 386.28 386.51 387.46 387.40

0.635 0.660 0.668 0.712 0.716 0.731 0.761 0.792 0.805 0.848 0.848 0.897 0.927 0.947 1.000 1.000

0.768 0.790 0.798 0.834 0.840 0.856 0.874 0.892 0.903 0.926 0.928 0.955 0.969 0.978 1.000 1.000

1.005 1.010 1.003 0.996 0.996 0.984 0.989 0.999 0.990 0.995 0.999 0.994 0.997 1.001 1.000 1.000

1.563 1.597 1.606 1.654 1.705 1.761 1.773 1.773 1.823 1.841 1.909 1.970 2.036 2.072

u(T) = 0.12K, u(p) = 0.36 kPa, and u(x1) = u(y1) = 0.010.

3. VERIFICATION 3.1. Equipment Verification. Given the change to the experimental procedure since our previous work, it was necessary to reverify our method and equipment. For this purpose, we chose the 2-butanone/n-heptane system at 94.0 kPa56 as our verification system with a comparison between the experimental and literature data provided in Figure 2 and Table 4. It can be seen that the literature data is reproduced well, particularly evidenced in the x−y plot with only small temperature deviations at the azeotropic point. This degree of agreement serves to justify our alteration in the experimental procedure and verify our experimental methodology. Both the experimental and reference data sets were found to be thermodynamically consistent according to both the Wisniak L/W57 and McDermott−Ellis58 consistency tests, as presented below. 3.2. Thermodynamic Consistency. Adherence of the data to the thermodynamic limitations imposed by the Gibbs−Duhem equation was tested by applying both the Wisniak L/W57 and McDermott−Ellis58 consistency tests. According to the Wisniak L/W test, a value for D of less than 5 for each system is indicative of thermodynamic consistency of the data. All five ester/n-octane data sets were well within this margin, exhibiting values of 0.736 (methyl valerate), 0.861 (ethyl butanoate), 0.619 (propyl propanoate), 0.602 (butyl ethanoate), and 0.676 (pentyl formate), respectively. The point-to-point nature of the McDermott−Ellis consistency test serves to eliminate thermodynamically inconsistent data points in a pairwise comparative fashion. A given pair of data points is assigned a maximum deviation, Dmax, based on the experimental uncertainties that should not be exceeded by the D value of the pair. The D values of all data presented here

were lower than their respective maximum values and thus deemed thermodynamically consistent by this test.

4. EXPERIMENTAL RESULTS The VLE data for each of the five C6 ester structural isomers with n-octane at 60.0 kPa are tabulated in Table 5, along with their corresponding activity coefficients as calculated by Aspen Plus in accordance with the experimental T−P−x−y data. The T−x−y and x−y plots for these systems are presented in Figure 3a−e with calculated activity coefficients and dimensionless excess Gibbs energies shown in Figure 4a−e. The liquid phase activity coefficients are calculated by considering the γ−φ representation of phase equilibrium, as typified by eq 1 xiPisatγi = yP i

φi φisat

⎛ v sat(P − Pisat) ⎞ exp⎜ − i ⎟ RT ⎠ ⎝

(1)

sat where Psat i and vi are the saturated vapor pressure and liquid density of component i respectively, and φi and φsat are i the fugacties of component i in solution and at saturation, respectively. The fugacity ratio in eq 1 is calculated by means of eq 2

φi φi

sat

⎡ B (P − P sat) + P(1 − y)2 (2B − B − B ) ⎤ i 12 11 22 ⎥ = exp⎢ ii RT ⎣ ⎦ (2)

where Bii and Bij are the second virial coefficients. Calculation of the experimental activity coefficients was achieved by using the appropriate DIPPR correlations52 for the relevant pure E



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Figure 3. T−x−y (○, vapor and △, liquid) and x−y (◆) representations of experimental data for the five C6 ester (1)/n-octane (2) systems measured at 60.0 kPa with corresponding NRTL predictions (gray, long-dashed line) and UNIFAC predictions (red, short-dashed line). Stick model representations of the respective esters to show the location of the ester functional group. (a) Methyl valerate (1)/n-octane (2). (b) Ethyl butanoate (1)/n-octane (2). (c) Propyl propanoate (1)/n-octane (2). (d) Butyl ethanoate (1)/n-octane (2). (e) Pentyl formate (1)/n-octane (2). F



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Figure 4. Activity coefficient (ln γi) and gE plots of experimental data (○, ln γester; △, ln γn‑octane; ■, system gE), with corresponding NRTL correlations (gray, long-dashed line) and UNIFAC predictions (red, short-dashed line). (a) Methyl valerate (1)/n-octane (2). (b) Ethyl butanoate (1)/n-octane (2). (c) Propyl propanoate (1)/n-octane (2). (d) Butyl ethanoate (1)/n-octane (2). (e) Pentyl formate (1)/n-octane (2). G



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Table 6. Experimentally Determined Azeotropic Pointsa for Each of the Five C6 Ester/n-Alkane Systems Measured at 60 kPa methyl valerate (1)/n-octane (2) ethyl butanoate (1)/n-octane (2) propyl propanoate (1)/n-octane (2) butyl ethanoate (1)/n-octane (2) pentyl formate (1)/n-octane (2) a

given by eq 3, so the Aij and Bij parameters are regressed by minimizing the objective function (OF) shown in eq 4 τij = Aij +

T/K

x1

377.53 374.60 375.44 376.55 378.32

0.409 0.605 0.546 0.459 0.308

Bij (3)

T

n

OF =

n

∑ (γ1exp − γ1calc)i2 + ∑ (γ2exp − γ2calc)i2 1

(4)

1

The regressed parameters are reported in Table 7 along with the absolute average deviations (AAD) in y and T, as well as the root-mean-square errors (σ) in γ and gE/RT for both the NRTL correlation and the UNIFAC prediction. Comparison with the experimental data shown in Figure 3a−e indicates the quality of the NRTL correlation’s fit; the model provides excellent correlation of the phase behavior and, in particular, the azeotropic points. Notably, the quality of the model fit is unaffected by the shifting functional group of the ester, producing equally good correlations for all five isomers. The AAD values in y and T indicate that the correlation fit is consistent across the composition range with both of these values well within the experimental uncertainty. Modeling of the activity coefficients in Figure 4a−e is understandably good, given the nature of the regression objective function (eq 4). More notably, the quality of the correlation for the derived gE/RT and the consistently small values of σ in Table 7 underline the quality of the correlation. Considering the UNIFAC predictions, the witnessed positive deviations from ideal phase behavior are qualitatively accounted for in all systems, but an inability to accurately predict the azeotropic temperature means that the narrow phase envelopes are excessively shifted in all but the butyl ethanoate case (Figure 3d). This point is emphasized by the excellent fit of the prediction to the T-independent x−y data representation. The predictions of the methyl-, ethyl-, and propyl-esters are of particular interest given that these molecules are comprised of the exact same groups in the UNIFAC (Dortmund) framework; it would appear that the least polar propyl-propanoate system is best predicted in terms of the azeotropic temperature, the overall AAD values in Table 7 as well as the system gE/RT in Figure 4. Thereafter, the quality of the prediction appears to systematically decrease, albeit slightly, as the CH2COO group shifts terminally (to ethyl butanoate and finally methyl valerate). This may suggest that the group contribution approach of considering these

u(T) = 0.12K, u(p) = 0.36 kPa, and u(x1) = u(y1) = 0.010.

sat component properties (Psat i , vi , and Bii) and employing the 59 Tsonopoulos correlation for calculation of Bij. All five systems exhibit strong positive deviations from ideality and exhibit minimum boiling azeotropes in the middle of the mixture composition space. The azeotropic temperatures and compositions, inferred from the experimental data, are tabulated in Table 6. Such behavior is to be expected given how close the respective pure component boiling points are, and the differences in the intermolecular forces of the two components; that is, the strongly polar ester with the nonpolar alkane. The role of the location of the ester functional group in the carbon backbone on the mixture behavior is also readily apparent from the data. Of the five isomers, pentyl formate with its ester functional group on the terminal carbon atom in the chain, has both the highest boiling point and the highest activity coefficient at infinite dilution. Conversely, ethyl butanoate and propyl propanoate present the lowest boiling points and the smallest deviations from ideal behavior (viz. smallest gE/RT). These trends serve to re-enforce the idea that polar effects are strongest when these functional groups are terminally located. As these groups shift more centrally, their roles in the macroscopic fluid behavior are diminished by steric hindrances.

5. THERMODYNAMIC MODELING Experimental data were predicted using both the Dortmund modified UNIFAC model60 and correlated with the NRTL activity coefficient model with interaction parameters determined by regression using Aspen Plus. In the regression procedure, the nonrandomness parameter (α) is set equal to a constant value of 0.3 in all cases, adhering to the standard for systems of hydrocarbons with polar, associating components.61 Within the simulation package, the τij parameter is defined by the form

Table 7. Average and Root Mean Square Deviations for NRTL Correlation and UNIFAC Prediction of Experimental Data (Included Are the NRTL Correlation Parameters Regressed as Per Equation 2)a system

A12

B12 (K)

A21

MV/Oct EB/Oct PP/Oct BE/Oct PF/Oct

76.301 −7.078 9.511 25.090 −2.084

−28875.9 2814.1 −3535.8 −9410.8 748.7

−73.214 14.965 −3.368 −22.420 20.740

σ(F ) =

∑1n(F exp − F calc)2 (n)

σ (γ1)

σ (γ2)

σ (gE/RT)

0.002 0.001 0.001 0.001 0.002

0.16 0.19 0.09 0.15 0.04

0.03 0.03 0.02 0.02 0.05

0.02 0.01 0.03 0.02 0.02

0.012 0.013 0.011 0.012 0.009

0.011 0.022 0.000(1) 0.015 0.009

0.63 0.45 0.43 0.22 0.47

0.08 0.07 0.09 0.04 0.08

0.17 0.11 0.06 0.10 0.02

0.123 0.140 0.144 0.174 0.246

AAD (y1)

NRTL 27959.1 −5549.3 1449.0 8669.5 −7518.5 UNIFAC

MV/Oct EB/Oct PP/Oct BE/Oct PF/Oct a

AAD (T)

B21 (K)

where n is the number of data points excluding those for the pure component. H



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(3) Wolfová, J.; Linek, J.; Wichterle, I. Vapor-Liquid Equilibria in the Heptane − 2-Pentanol and Heptane − 2-Methyl-1-Butanol Systems at 75, 85 and 95°C. Fluid Phase Equilib. 1991, 64, 281−289. (4) Wolfová, J.; Linek, J.; Wichterle, I. Vapor-Liquid Equilibria in the Heptane − 3-Pentanol and Heptane − 2-Methyl-2-Butanol Systems at Constant Temperature. Fluid Phase Equilib. 1990, 54, 69−79. (5) Cripwell, J. T.; Schwarz, C. E.; Burger, A. J. Vapor-Liquid Equilibria Measurements for the Nine n -Alkane/Ketone Pairs Comprising 2-, 3-, and 4-Heptanone with n -Octane, n -Nonane, and n -Decane. J. Chem. Eng. Data 2015, 60, 602−611. (6) Susial, P.; Sosa-Rosario, A.; Rios-Santana, R. Vapour-Liquid Equilibrium with a New Ebulliometer: Ester + Alcohol System at 0.5 MPa. Chin. J. Chem. Eng. 2010, 18, 1000−1007. (7) Susial, P.; Rios-Santana, R.; Sosa-Rosario, A. VLE Data of Methyl Acetate + Methanol at 1.0, 3.0 and 7.0 bar with a New Ebulliometer. J. Chem. Eng. Jpn. 2010, 43 (8), 650−656. (8) Blanco, A. M.; Ortega, J. Isobaric Vapor-Liquid Equilibria of Methanol + Methyl Ethanoate, + Methyl Propanoate, and + Methyl Butanoate at 141.3 kPa. J. Chem. Eng. Data 1996, 41, 566−570. (9) Blanco, A. M.; Ortega, J. Densities and Vapor-Liquid Equilibrium Values for Binary Mixtures Composed of Methanol + an Ethyl Ester at 141.3 kPa with Application of an Extended Correlation Equation for Isobaric VLE Data. J. Chem. Eng. Data 1998, 43, 638−645. (10) Susial, P.; Sosa-Rosario, A.; Rios-Santana, R. Vapor-Liquid Equilibria for Ethyl Acetate + Methanol at (0.1, 0.5, and 0.7) MPa. Measurements with a New Ebulliometer. J. Chem. Eng. Data 2010, 55, 5701−5706. (11) Susial, P.; Rodríguez-Henríquez, J. J.; Sosa-Rosario, A.; RiosSantana, R. Vapor-Liquid Equilibrium of Ethyl Acetate + Cnh2n+1OH (n= 1,2,3) Binary Systems at 0.3 MPa. Chin. J. Chem. Eng. 2012, 20, 723−730. (12) Espiau, F.; Ortega, J.; Penco, E.; Wisniak, J. Advances in the Correlation of Thermodynamic Properties of Binary Systems Applied to Methanol Mixtures with Butyl Esters. Ind. Eng. Chem. Res. 2010, 49 (19), 9548−9558. (13) Susial, P.; Rios-Santana, R.; Sosa-Rosario, A. Vapor-Liquid Equilibrium Measurements for the Binary System Methyl Acetate +Ethanol at 0.3 and 0.7 MPa. Braz. J. Chem. Eng. 2011, 28, 325−332. (14) Ortega, J.; Susial, P.; de Alfonso, C. Isobaric Vapor-Liquid Equilibrium of Methyl Butanoate with Ethanol and 1-Propanol Binary Systems. J. Chem. Eng. Data 1990, 35, 216−219. (15) Susial, P.; Ortega, J.; de Alfonso, C.; Alonso, C. Vapor-Liquid Equilibrium Measurements for Methyl Propanoate-Ethanol and Methyl Propanoate-Propan-1-ol at 101.32 kPa. J. Chem. Eng. Data 1989, 34, 247−250. (16) Ortega, J.; Susial, P.; de Alfonso, C. Isobaric Vapor-Liquid Equilibrium of Methyl Butanoate with Ethanol and 1-Propanol Binary Systems. J. Chem. Eng. Data 1990, 35, 216−219. (17) Soto, A.; Hernández, P.; Ortega, J. Experimental VLE at 101.32 kPa in Binary Systems Composed of Ethyl Methanoate and Alkan-1ols or Alkan-2-ols and Treatment of Data Using a Correlation with Temperature-Dependent Parameters. Fluid Phase Equilib. 1998, 146, 351−370. (18) Ortega, J.; Peña, J.; de Alfonso, C. Isobaric Vapor-Liquid Equilibria of Ethyl Acetate + Ethanol Mixtures at 760 ± 0.5 mmHg. J. Chem. Eng. Data 1986, 31, 339−342. (19) Susial, P.; Sosa-Rosario, A.; Rodríguez-Henríquez, J. J.; RiosSantana, R. Vapor Pressure and VLE Data Measurements on Ethyl Acetate/Ethanol Binary System at 0.1, 0.5, and 0.7mpa. J. Chem. Eng. Jpn. 2011, 44, 155−163. (20) Falcón, J.; Ortega, J.; González, E. Densities and Vapor-Liquid Equilibria in Binary Mixtures formed By Propyl Methanoate + Ethanol, + Propan-1-ol, and + Butan-1-ol at 160.0 kPa. J. Chem. Eng. Data 1996, 41, 859−864. (21) Ortega, J.; Gonzalez, C.; Peña, J.; Galván, S. Thermodynamic Study on Binary Mixtures of Propyl Ethanoate and an Alkan-1-ol (C2−C4). Isobaric Vapor−Liquid Equilibria and Excess Properties. Fluid Phase Equilib. 2000, 170, 87−111.

molecules as identical from a functional group composition standpoint is insufficient for longer chain molecules and we must explicitly account for that location of the functional group. Excellent correlation of existing phase equilibrium data is vital for industrial research purposes when using semiempirical models. The correlative nature of these models means that the effects caused by any structural differences in the molecules of given mixtures can be accounted for if data for these systems are available. However, this need for experimental data limits the applicability of such models because, as previously noted, such data is rarely available. Therefore, to be able to successfully account for these changes in intermolecular interactions in lieu of experimental data a predictive model is what is needed. Group contribution models such as UNIFAC may fill this purpose but only if functional group interactions have been fit to similar such data, making even these models “correlative” in a sense. Purely predictive models should have no need of a correlative bias. Data such as that presented here can be used to test the capabilities of such predictive models and provide a confidence in their predictions; this is a goal of future work.

6. CONCLUSIONS Isobaric VLE data for linear C6 ester isomers with n-octane presented minimum boiling azeotropes for all cases with distinct and separate phase envelopes for each isomer in evidence. These results once again serve to underpin the significant role of functional group location in the behavior of isomers of polar components mixed with a common second component. This type of behavior is well correlated by semiempirical models and fairly predicted by group contribution methods but only when such data are available for proper regression, a caveat not often met for systems of isomers with a common second component. Behavioral subtleties, however, may be accounted for by predictive models, but related verification would again require data such as presented here.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +27 21 8084494. Fax: +27 21 8082059. Notes

The authors declare no competing financial interest. Funding

This work is based on the research supported in part by the National Research Foundation of South Africa (Grant specific 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 accepts no liability whatsoever in this regard. The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at are those of the author and are not necessarily to be attributed to the NRF. Aspen Plus is a registered trademark of Aspen Technology Inc.

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REFERENCES

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