A new reduction method for phase equilibrium calculations

Abstract

Phase equilibrium calculations require the most important computational effort in many process simulators and in reservoir compositional simulations. In this work, a new reduction method for phase equilibrium calculations is proposed. A new set of independent variables (and the related set of error equations) is introduced, based on the observation that, under certain conditions, the equilibrium ratios can be related only to some component properties (elements of the reduction matrix) and to equation of state parameters. The new formulation leads to simpler expressions of the elements of the Jacobian matrix. Some important features are presented, and an interesting and useful link with classical flash calculation methods is revealed. The reliability and efficiency of the proposed method are tested on several synthetic and reservoir hydrocarbon mixtures. The proposed method proves to be robust and it performs in all cases better than previous reduction methods. Finally, it is discussed how the new set of independent variables can be used for a variety of phase equilibrium calculations.