Studies on phase equilibrium calculations from equations of state

Abstract

The topics covered in this dissertation are the following: First, a robust and fast algorithm to calculate phase equilibria for multicomponent mixtures based upon an indirect technique of minimizing the Gibbs energy of the mixtures was developed. The new algorithm, termed the Modified Area Method, includes a stability analysis of the mixture consisting of determining either cusp or inflection points on the mixture's Gibbs energy curve. Once the stability analysis identifies an unstable region, the algorithm proceeds in a rational way to search for the equilibrium compositions that minimize the area between the Gibbs energy surface and its tangent plane. The modified area method speeds up the original area method by avoiding the excessive number of function evaluations and numerical integrations. Examples of applications covering up to three component mixtures are given. Second: a study of the prediction of solubility of water in liquid hydrocarbons is presented. Binary interaction parameters used in the Peng-Robinson equation of state for the hydrocarbon-water mixtures were obtained from literature data. Later, this information was employed in modeling an isochoric flash experiment used to study three-phase equilibria of multicomponent hydrocarbon-water mixtures. The model is used to both reduce the experimental data and to complete the experimental information by predicting key variables not measurable in the experiment. Third: the isochoric collinearity at cricondentherm for the DEX mixing rules for the van der Waals equation of state was tested. The results showed these mixing rules are consistent with the collinearity phenomena. Fourth: the Gamma-Phi formulation used in vapor-liquid equilibria calculations was studied to determine the limits at which it can be applied consistently. Two restrictions were found: First, for the light components in the mixture the pure-liquid fugacity coefficient in some pressure-temperature regions can not be evaluated unless hypothetical liquid states are assumed. Second, the models employed for the liquid and vapor mixtures have inconsistent mixing rules that void the application of Gamma-Phi to high pressures because these models fail to converge the bubble and dew curves to the critical point of the mixture.