High pressure phase equilibrium studies.
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This project involved the design, construction and commissioning of a new isothermal multipmpose high-pressure vapour-liquid equilibrium (HPVLE) apparatus of the static type. The equilibrium cell was approximately 200 cm3 in volume, and by use of a stepper motor and piston, was also variable in volume. The equipment had a combined pressure and temperature operating limit of 175 bar and 175 °C respectively. The equilibrium cell contents could be viewed through two pairs of illuminated sapphire windows. The equilibrium cell was mounted in an air-bath, which provided the isothermal environment. The air-bath was constructed of mild steel and was copper-lined with a Fibrefrax sandwich that provided more than adequate insulation to ensure that there were no temperature gradients induced by conduction and heat element radiation. The sampling method and procedures utilized in this project caused no disturbance to the equilibrium .condition. The liquid and vapour phases were sampled by a novel means of circulating representative equilibrium samples through the sample loop of a V ALCO six-port two-position sampling valve. The sample loop contained approximately 300 μl of sample, which was homogenized in jet-mixers. Analysis of the equilibrium sample was by gas chromatography. Experimental measurements of isothermal HPVLE were undertaken for the systems, carbon dioxide + toluene, carbon dioxide + methanol and propane + 1-propanol. In addition, to show the versatility of the apparatus, P-V-T measurements were undertaken for the nitrogen, propane and the propane + nitrogen binary system. Isothermal measurements were undertaken for carbon dioxide + toluene at 38 °C, 80 °C and 118.3 °C. There was excellent agreement between literature and the data measured. For the carbon dioxide + methanol system, measurements were undertaken at 40 °C, 90 °C and 100 °C. The 90 °C isotherm had not previously been measured. The propane+ 1-propanol system was measured at 105.1 °C and 120 °C. It was compared to the experimental data of Muhlbauer . There was a slight difference in the vapour compositions between literature and the data measured in this project. Experimental data of Muhlbauer  showed a higher mole fraction of the volatile component (propane). Modeling of all the systems for the various isotherms measured were undertaken using both the direct and combined methods. The direct method involved the use of the Soave and PengRobinson equations of state with various mixing rules e.g. van der Waals, Wong and Sandler, Huron-Vidal and modifications thereof. The combined method, based on a liquid phase model and an equation of state model for the corresponding activity and fugacity coefficients was used as discussed in Prausnitz et al. . A new combined method model was proposed that incorporated the Peng-Robinson-StryjekVera equation of state with the Wong-Sandler mixing rule together with the NRTL activity coefficient model. The model modeled all systems measured as well or at times better than current models in literature. P-V-T measurements undertaken for propane, nitrogen and the propane + nitrogen binary system illustrated the versatility of the experimental apparatus. From the measured P-V-T data, second virial coefficients were computed for propane and nitrogen. Second virial cross coefficients for the binary system, propane + nitrogen, were also calculated. Critical property computations were undertaken for the three binary systems measured using the method citied by Deiters and Schneider . The critical properties were computed by the Soave, Peng-Robinson and Peng-Robinson-Stryjek-Vera equations of state. Vapour-liquid equilibrium data measured were tested for thermodynamic consistency using the test suggested by Chueh et al.  and residual plots. The consistency tests indicated that the data measured were not inconsistent. This was the findings for the carbon dioxide + toluene, carbon dioxide + methanol and propane + 1-propanol systems for all the isotherms measured.