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The initial phase of the project involved an investigation into the modeling of binary carboxylic acid vapour-liquid equilibrium (VLE) data. This stemmed from the Masters research that led into the current study, in which the conventional gamma-phi formulation of VLE was found to inadequately describe the complicated acid chemistry. In an effort to correctly describe the dimerization occurring in both the liquid and vapour phases, the chemical theory of vapour-phase imperfections was applied. The chemical theory technique allowed the experimental liquid-phase activity coefficients to be accurately calculated by taking the vapour phase dimerization into account. Once these activity coefficients had been determined, standard Gibbs excess energy models were fitted to permit analysis of the VLE data's thermodynamic consistency. In addition, the typical bubble-point iteration scheme used for VLE data regression was adapted to include the chemical theory expressions necessary for satisfactory modeling of the carboxylic acids. The primary focus of this study was to determine the ability of currently available computer simulation techniques and technology to correctly predict the phase equilibria of polar molecules. Thus, Monte Carlo simulations in the NVT- and NPT- Gibbs ensembles were used to predict pure component and binary phase equilibrium data (respectively), for a variety of polar compounds. The average standard deviations for these simulation results lay between 1 and 2 % for the saturated liquid densities, and varied between 5 and 10 % for the saturated vapour pressures and densities. Pure component data were simulated for alcohols, carboxylic acids, hydrogen sulfide (ELS), sulfur dioxide (SO2) and nitrogen dioxide (NO2). For H2S, S02 and NO2, a potential model parameterized as part of this project was used to describe the molecular interactions. All the other compounds were simulated using the TraPPE-UA force field. The simulation results for the alcohols and acids showed a consistent saturated vapour pressure over-prediction of 5 - 20 % depending on the species and the system temperature. The liquid density predictions were, in general, good and on average differed from experiment by 1 - 2 %. The critical temperatures and densities were estimated from the pure component data by fitting to the scaling law and the law of rectilinear diameters. They were found to lie within 1 and 2 % of the experimental values for the carboxylic acids and alcohols, respectively. Clausius-Clapeyron plots of the saturated vapour pressures allowed the critical pressure and normal boiling points to be determined. The critical pressures were, as expected, over-predicted for both compound classes and the normal boiling points were under-estimated somewhat for the acids, but deviated from experiment by less than 0.5 % for the alcohols. A Lennard-Jones 12-6 plus Coulombic potential energy surface was parameterized for H2S, SO2 and NO2. For FbS, the proposed force field offers improved saturated vapour pressure and vapour density predictions when compared to the existing NERD force field, and comparable accuracy with the recent models of Kamath and co workers. SO2 and NO2 had not previously been parameterized for a Lennard-Jones 12-6 based force field. For SO2, there was excellent agreement with experimental data. In the case of NO2, the saturated liquid density predictions were very good, but the vapour pressures and densities were over-predicted. Binary VLE simulations were carried out for systems consisting purely of carboxylic acids, and also for H2S and SO2 with a selection of alkanes and alcohols. The liquid and vapour composition predictions were good for the acid systems, but the anticipated pressure and temperature deviations were observed in the isothermal and isobaric simulations, respectively. The H2S + alkane systems were generally good, as were the SO2 + alkane systems. For both H2S and SO2, the systems involving an alcohol displayed a characteristic pressure over-estimation. The azeotropes were, in most cases, predicted fairly well; the exception was the SO2 + methane binary. A sensitivity analysis of the Lennard-Jones unlike interaction parameters was also conducted. It was demonstrated that even minor changes to these parameters can have a significant effect on the final simulation results. The considerable affect that these parameters have on the simulation outputs was emphasized by studying the influence of different combining rules on the H2S + methane and H2S + ethane binary systems. Analysis of the radial distribution functions indicated that hydrogen bonding and dimerization were occurring in the alcohol and carboxylic acid systems, respectively. The H2S, SO2 and NO2 distribution functions showed little sign of any association, except for a small plateau in that of SO2. A radial distribution function from one of the carboxylic acid binary simulations was also analysed, and supported the assumption made in the chemical theory modeling work of using a geometric mean (instead of twice the geometric mean, which is favoured by some researchers) to determine the heterodimerization constant, KAB- / Thesis (Ph.D.)-University of KwaZulu-Natal, 2006.

High pressure vapour-liquid equilibrium experiments were undertaken with a static high-pressure apparatus designed by Prof. J. D. Raal and commissioned by Prof. D. Ramjugernath. Isothermal VLE binary data data was measured at moderate temperatures and pressures ranging from atmospheric to 7.2 bar. The equipment had a combined operating pressure and temperature limit of approximately 150 bar and 215° C respectively. The apparatus was initially designed for the measurement of gas-liquid binary systems- where one of the components was supercritical at the operating conditions. Test data were measured for the pentane + ethanol system at 100.41°C. The 2-methyl-2-butene + TAME, hydrocarbon + olefin system, was observed at 70°C, 94.6°C and 104.5°C. The apparatus was modified for the measurement of binary systems containing sub-critical components at the operating conditions specified. An injection port was installed on the apparatus assembly such that the second component of the binary system could be introduced into the equilibrium cell. The binary VLE data was regressed using various thermodynamic models. The direct method or phi-phi approach was considered. The equations of state models used in the regression were the Peng-Robinson-Stryjek-Vera (PRSV) and Soave-Redlich-Kwong (SRK). The 1-fluid van der Waals, Wong and Sandler mixing rules were selected to estimate binary interactions. The excess Gibbs energy equations coupled with the Wong and Sandler mixing rules were the NRTL and WILSON equations. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009.

Thermodynamics plays an important role for separation processes in chemical industries. Phase equilibrium is of special interest in chemical engineering as separation processes such as distillation and extraction involve phase contacting. The main focus of this research was the measurement of new phase equilibrium data for acetonitrile and water with heavy hydrocarbons that included: heptanoic acid, 1-nonanol, dodecane and 1-dodecene. Hence, binary vapour-liquid equilibrium (VLE), liquid-liquid equilibrium (LLE) and vapour-liquid-liquid equilibrium (VLLE) data were investigated. The VLE and VLLE data were measured with the modified apparatus of Raal (Raal and Miihlbauer, 1998). The modification, undertaken by Ndlovu (2005), enabled measurement for VLLE systems. Isothermal binary VLE data for the (nonanol + 1-dodecene) system at 403.15 K was measured and VLLE data for the systems (acetonitrile + 1-dodecene) at 343.15 K, and (nonanol + water) at 353.15 K were investigated. The LLE data were measured with the modified apparatus of Raal and Brouckaert (1992). The modification, introduced by Ndlovu (2005), improved thermal insulation and the sampling procedures. Binary LLE data for the systems (acetonitrile + 1-dodecene) at 1 atm and (water + 1-nonanol) at 1 atm were measured. Furthermore, ternary data at 323.15 K and 1 atm were also measured for the systems containing water + acetonitrile with the each of the following components: heptanoic acid, 1-nonanol, dodecane and 1-dodecene. The experimental VLE data were regressed using two different methods: the combined method and the direct method. For the combined method, the second virial coefficients were calculated from the methods of Pitzer and Curl (1957) and Tsonopoulos (1974). The activity coefficients were calculated using three local-composition based activity coefficients models: the model of Wilson (1964), the NRTL model of Renon and Prausnitz (1968) and the modified UNIQUAC model of Anderson and Prausnitz (1978). For the direct method, the equation of state of Stryjek and Vera (1986) and the alpha function of Twu et al. (1991) in the equation of state of Peng and Robinson (1976) were employed. In addition, the mixing rules of Wong and Sandler (1992) and Twu and Coon (1996) were utilised. Furthermore, the point test of Van Ness et al. (1973) and the direct test of Van Ness (1995) were employed to test the thermodynamic consistency of the experimental VLE data measured in this work. The experimental binary LLE data were regressed using the three-suffix Margules model, Van Laar (1910) model and the NRTL model of Renon and Prausnitz (1968) to obtain the temperature dependence of the model parameters. The experimental ternary LLE data were subjected to a two part correlation: the tie-line correlation and the binodal curve correlation. The tie-lines were correlated with the NRTL model of Renon and Prausnitz (1968) and the modified UNIQUAC model of Anderson and Prausnitz (1978). The binodal curves were correlated with the Hlavaty (1972) equation, B-density function equation of Letcher et al. (1989) and the log y equation of Letcher et al. (1986). / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2006.

Phase equilibrium data are vital in process design for chemical industries. Development of / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006.

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