Prediction of coexistence vapor-liquid densities for substances and mixtures



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Texas Tech University


Knowledge of the critical state is important in any study of phase behavior; therefore, the applications of critical state prediction methods can be found in many areas of the petroleum and chemical industries. However, the vapor-liquid and volumetric computations for reservoir fluid systems in the retrograde and near-critical regions still remain a challenge. As a precursor in establishing a predictive equation of state for compositional reservoir processes, a previously established four-parameter cubic equation of state reported by Lawal-Lake-Silberberg (LLS) is used to predict orthobaric densities, second virial coefficient and critical volumes of pure substances (hydrocarbon, non-hydrocarbon, polar and non-polar fluids). The prediction results are generally within 0.5% of the experimental measurements. A framework of the attractive temperature function is established for two parameter (Peng-Robinson and Soave-Redlich-Kwong) and four parameter LLS equations of state. The temperature function is demonstrated to be internally consistent with the critical behavior of fluids at sub- and super-critical conditions and the function does not suffer the difficulty encountered with Soave-type of temperature function which hitherto has been major source of research in equations of state development. An analysis of the thermodynamic constraint criteria of the critical state of pure substances and binary mixtures is used to establish a theoretical expression for the van der Waals critical point. The theoretical expression for the van der Waals criticality is validated by the prediction results of binary critical volumes of asymmetric substances and mixtures. This project offers an insight to the phase behavior of ternary and multicomponent mixtures and the challenge for the future work is to apply this robust method to the near-critical flash routine in ternary and multicomponent systems.