(153b) High Pressure Multiphase Equilibria Modeling with GCA-EoS

The Group-Contribution with Association equation of state (GCA-EoS) has been successfully applied to represent the global phase behavior of carbon dioxide mixtures with normal and branched alkanes and alcohols under a wide range of conditions. Using a single set of parameters, the model is able to predict vapor-liquid, liquid-liquid and solid-liquid equilibria of binary mixtures not included in the parameterization procedure. In addition, liquid-vapor and liquid-liquid critical locus were included in the optimization. Special emphasis was set on describing the transformation between types of fluid phase behavior in each homologous series of organic compounds to reduce the jeopardy of predicting incorrect liquid split. This family of binary systems has been investigated in previous works; however, this is the first time that GCA-EoS was shown to be able to represent the transformation between types of phase behavior as the length of the alkyl chain increases. The wide range of conditions and the parameterization strategy are keys to develop a robust thermodynamic tool for predicting multiphase behavior.

The GCA-EoS is the first equation of state of the SAFT family that uses a group contribution approach of the Wertheim association thoery. The free volume and attractive contributions are based on Carnahan-Starling and Non Random Two Liquids (NRTL) models respectively. The density-dependent mixing rules of this model are laborious to fit. However, they give enough flexibility to reproduce the global phase diagrams without affecting the accuracy of bubble and dew curves calculations. It is worth to highlight that the same set of parameters accomplishes accurate results, both in subcritical and critical regions, for the evaluated homologous series. Prediction of multiphase behavior with a single set of parameters is a key point when developing a model for new processes and products design.