Phase equilibrium measurements at low-to-moderate pressures for systems containing n-Hexane, 1-Hexene and n-Methyl-2-pyrrolidone.
The primary focus of this study is the measurement and modeling of binary and ternary VLE data. The measurements of binary and ternary systems were undertaken on a fully automated dynamic VLE apparatus. The glass dynamic VLE still was modified to handle pressures ranging from 0 to 500 kPa, however, the safest maximum pressure to which tests had been conducted was 350 kPa. Thus, this limit was not to be exceeded during the measurement of experimental data. The systems under investigation included the binary and ternary combinations of the following chemicals: n-hexane, 1-hexene and n-methyl-2-pyrrolidone (NMP) at isothermal conditions. A test system consisting of ethanol + cyclohexane was measured at 40 kPa, as well as the system of 1-hexene + NMP at 363.15 K and n-hexane + NMP at 363.15 K. Published literature data for these test systems were employed to verify the measured data for the test systems complied with thermodynamic consistency. All other data constitutes new data, currently unavailable in literature. The following isotherms were measured: 1) 1-hexene (1) + NMP (2) at 323.15, 343.15, 353.15 and 363.15 K 2) n-hexane (1) + NMP (2) at 353.15, 363.15, 378.15 and 383.15 K 3) 1-hexene (1) + n-hexane (2) at 343.15, 363.15 and 373.15 K, and 4) 1-hexene (1) +n-hexane (2) + NMP (2) at 363.15 K All system measurements were carried out on the glass low-to-medium pressure VLE still of Lilwanth (2011), with the exception of the test system ethanol + cyclohexane, which was carried out on the low pressure VLE glass still of Hirawan (2007). The two VLE stills, utilized to carry out measurements in this work, can operate isobarically and isothermally. The temperature on the stills of Hirawan (2007) and Lilwanth (2011) were controlled to within ±0.425 and ±0.089 K respectively and the accuracy of pressure control is to within ±0.320 and ±0.440 kPa respectively. In addition, for the calibration of the various systems: ethanol + cyclohexane, 1-hexene + NMP, n-hexane + NMP, 1-hexene + n-hexane and 1-hexene + n-hexane + NMP, the accuracies are: ±0.002, ±0.0034, ±0.0033, ±0.0066 and ±0.0083 of a mole fraction respectively. The binary interaction parameters obtained from modeling the three binary systems were used to predict the ternary system data. Thereafter, the experimentally measured data for the ternary system was then compared to the model prediction, which was completed on Dortmund Data Bank (DDB, 2011). The measured binary data was regressed utilizing the combined and the direct methods. For the direct method, the cubic equations of state (CEoS) used to describe the vapour phase included the Peng-Robinson (1976) and Soave-Redlich-Kwong (1972) equations combined with the mixing rule of Wong and Sandler (1992) in conjunction with the Gibbs excess energy models, namely the NRTL (1968) and UNIQUAC (1975) models, to describe the liquid phase non-idealities. For the combined method, the Gibbs excess energy activity coefficient models mentioned above were employed to represent the liquid phase imperfections and the vapour phase nonidealities were represented by cubic equations of state, as mentioned above, as well as the Hayden and O‟Connell (1975) virial equation of state for the calculation of the virial coefficients. To verify whether the measured data is thermodynamically consistent the point and direct tests were applied. Even though the direct test is a more stringent approach to testing thermodynamic consistency, for the systems 1-hexene + NMP and n-hexane + NMP, the point test was utilized as the primary means by which to quantify the data, as the associative effects of the NMP molecule effect the results obtained. For the system 1-hexene + n-hexane the direct test was used as the primary means to test the consistency of data, as no cross- or self-association is present. After extensive modeling was carried out, it was found that for the systems 1-hexene + NMP and n-hexane + NMP the model which enabled the best fit of the experimental data are the NRTL activity coefficient model in conjunction with the Hayden and O‟Connell virial equation of state (EoS). For the system 1-hexene + n-hexane the overall best fit model is the Peng-Robinson EoS in conjunction with the Wong-Sandler mixing rule and the NRTL activity coefficient model. A single set of binary interaction parameters for each of the three binary systems was obtained (via regression on Aspen Plus®) using the NRTL-HOC models. However, since Aspen Plus® cannot predict ternary system behaviour using the binary interaction parameters of the constituent systems, DDB was utilized. Further, DDB did not have available the HOC virial EoS (for enabling predictions), thus, it was decided to use the ideal gas model for representation of the vapour phase in conjunction with the NRTL activity coefficient model. The use of the ideal gas model does not compromise the integrity of the prediction in any way since the ternary system measurements were carried out in the dilute NMP region. Thus, since the main components in the ternary mixture at any one instant were 1-hexene and nhexane, and these components behave ideally, the ideal gas model is applicable. After the predicted behaviour for the ternary system was compared to the experimental data for the same system, the maximum percentage error encountered between the two data sets is 5%.