Study of the thermodynamics of fluid mixtures in the critical region using cubic equations of state

Phase equilibrium at high pressures is of considerable technical importance, e.g., in oil and gas processing, cryogenic engineering and supercritical separations. In principle, the phase behavior of fluid mixtures can be modeled with equations of state; therefore, calculations of phase equilibrium with equations of state have become an essential tool in chemical engineering.; The subject of this thesis is the application of cubic equations of state with mixing rules that include an excess Gibbs energy model for the modeling of systems at critical and supercritical conditions. Systematic work has been done on the application of this model for liquid-liquid equilibria, and critical and supercritical phenomena of fluid mixtures. Our results show for the first time that cubic equations of state can predict closed loop liquid-liquid miscibility gaps characteristic of type VI phase behavior when they are used in combination with mixing rules that include an excess Gibbs free energy model. Furthermore, a prediction of the critical lines showed that the PR EOS in combination with the WS mixing rule is able to give reasonable predictions of the liquid-liquid critical lines of type VI mixtures. Indeed, one of the most significant conclusions of this work is that simple equations of state can be used to describe complex phase behavior provided that free energy-based mixing rules are used.; One of the driving forces for this research was the potential application of cubic EOS to processes with supercritical fluids. Our work has shown that the performance of cubic EOS for the representation of supercritical extraction is considerably improved when they are used in combination with mixing rules that include an excess free energy model. Therefore, the model has proved to be a reliable tool for the understanding of processes with supercritical fluids.; Another problem that is addressed in this work is the known failure of cubic equations of state for the modeling of the critical region of fluids. While the classical cubic equation of state model predicts a quadratic dependence of the density with temperature along the coexistence curve in the critical region, both experimental data and the results of the modern critical point and group renormalization theories have shown that this is erroneous for real fluids. In this work we have used the results of group renormalization theory to develop a crossover EOS for pure fluids that is able to accurately describe the volumetric properties of the fluid in the critical region. The crossover EOS reduces to the classical EOS far from the critical point. However, our method failed to describe the critical behavior of mixtures. Our results show that this deficiency is most likely ascribed to an intrinsic inconsistency of the method that results in its failure to predict a critical point that we have not been able to satisfactorily resolve. This is an area for future research.