Multiphase equilibrium calculation using reduced variables

A new method for multiphase equilibrium calculations with cubic equations of state (EoS) has been developed. The minimum of the Gibbs free energy and of the tangent plane distance function should be found in a space with a significantly reduced number of dimensions. The number of independent variables for phase equilibrium calculations does not depend on the number of components in the mixture and non-zero binary interaction coefficients (BICs) are taken into account by our approach. The robustness and efficiency of the proposed method is tested for several systems of interest in reservoir and production engineering. The reduction in computer time as compared to conventional methods is all the more important as the number of the components is greater. The most important savings in computer time are expected for mixtures with many components and relatively few BICs. The proposed method using reduced variables represents a new, efficient, and reliable tool for many practical situations when multiphase equilibria calculations are needed. To the best of our knowledge, it is the first implementation of the reduced variables to multiphase equilibrium calculation.