A Systematic Study of Short and Long Range Interactions in Associating Fluids Using Molecular Theory
Chapman, Walter G
Doctor of Philosophy
Parameters needed for the Statistical Associating Fluid Theory (SAFT) equation of state are usually fit to pure component saturated liquid density and vapor pressure. In this thesis, other sources of information such as quantum mechanics, infinite dilution properties, Fourier transform infrared (FT-IR) spectroscopy and molecular dynamic (MD) simulation are used to obtain a unique set of parameters for complex fluids such as water and alcohols. Consequently, the equation of state can be more predictive and the parameters are not anymore system dependent. Moreover, the four vertices of the molecular thermodynamic tetrahedron (phase equilibrium experiments, spectroscopy, MD simulation and molecular theory) are used to study the distribution of hydrogen bonds in water and alcohol containing mixtures. The new sets of physical parameters and the knowledge gained in studying hydrogen bonding are then applied to model water content of sour natural gas mixtures as well as the phase behavior of alcohol + n-alkane and alcohol + water binary systems. Accurate determination of the water content in hydrocarbons is critical for the petroleum industry due to corrosion and hydrate formation problems. Experimental data available in the literature on the water content of n-alkanes (C5 and higher) is widely scattered. The perturbed chain form of the SAFT equation of state (PC-SAFT) was used to accurately correlate water mole fraction in n-alkanes, C1 to C16, which are in equilibrium with liquid water or ice. In addition, a list of experimental data is recommended to the reader based on its agreement with the fundamental equation of state used in this dissertation. The proposed molecular model was then applied to predict water content of pure carbon dioxide (CO2), hydrogen sulfide (H2S), nitrous oxide (N2O), nitrogen (N2) and argon (Ar) systems. The theory application was also extended to model water content of acid gas containing mixtures in equilibrium with an aqueous or a hydrate phase. To model accurately the liquid-liquid equilibrium (LLE) at subcritical conditions, cross association between CO2, H2S and water was included. The hydrate phase was modeled using a modified van der Waals and Platteeuw (vdWP) theory. The agreement between the model predictions and experimental data measured in our lab was found to be good across a wide range of temperatures and pressures. Modeling the phase behavior of liquid water can be quite challenging due to the formation of complex hydrogen bonding network structures at low temperatures. However, alcohols share some similarities with water in terms of structure and physical interactions. As a result, studying alcohol + n-alkane binary systems can provide us with a better understanding of water-alkane interactions. Besides, the application of alcohols in the petroleum and the biodiesel industry is of great importance. As a result, Polar PC-SAFT was used to model short chain 1- alcohol + n-alkane mixtures. The ability of the equation of state to predict accurate activity coefficients at infinite dilution was demonstrated as a function of temperature. Investigations show that the association term in SAFT plays an important role in capturing the right composition dependence of the activity coefficients in comparison to excess Gibbs free energy models (UNIQUAC in this case). Results also show that considering long range polar interactions can significantly improve the fractions of free monomers predicted by PC-SAFT in comparison to spectroscopic data and molecular dynamic (MD) simulations. Additionally, evidence of hydrogen bonding cooperativity in 1-alcohol + n-alkane systems is discussed using spectroscopy, simulation and theory. In general, results demonstrate the theory’s predictive power, limitations of Wertheim’s first order thermodynamic perturbation theory (TPT1) as well as the importance of considering long range polar interactions for better hydrogen bonding thermodynamics. Furthermore, the thermodynamics of hydrogen bonding in 1-alcohol + water binary mixtures is studied using MD simulation and Polar PC-SAFT. The distribution of hydrogen bonds in pure saturated liquid water is computed using TIP4P/2005 and iAMOEBA simulation water models. Results are compared to spectroscopic data available in the literature and to predictions using Polar PC-SAFT. The distribution of hydrogen bonds in pure alcohols is also computed using the OPLS-AA force field. Results are compared to Monte Carlo (MC) simulations available in the literature and to predictions using Polar PC-SAFT. The analysis show that hydrogen bonding in pure alcohols is best predicted using a two-site model within the SAFT framework. On the other hand, simulations show that increasing the concentration of water in the mixture increases the average number of hydrogen bonds formed by an alcohol molecule. As a result, a transition in association scheme occurs at high water concentrations where hydrogen bonding is now better captured using a three site alcohol model within the SAFT framework. The knowledge gained in understanding hydrogen bonding is applied to model the vapor-liquid equilibrium (VLE) and LLE of 1-alcohol + water mixture using Polar PC-SAFT. Predictions are in good agreement with experimental data, thus exhibiting the equation of state predictive power.
SAFT; water content; gas hydrates; hydrogen bonding;