Efficient Phase Equilibrium Calculation for Compositional Simulation: The Direct Reduced Flash
Résumé
A possibility for reducing the dimensionality of the phase equilibrium problem is given by the reduction method. The number of independent variables for reduced phase equilibrium calculations is independent of the number of components in the mixture. For two-phase flash calculations, the number of independent variables is m + 2, where m is the number of non-zero (or non-negligible) eigenvalues of the matrix with (1 - kij) elements (kij denotes the binary interaction coefficient between components i and j). In this work, a new extrapolation procedure, the direct reduced flash (DRF), is formulated for the selected set of independent variables, taking into account both changes in pressure and composition. This non-iterative extrapolation algorithm is particularly efficient when results of previous flash calculations, performed at some close reference conditions (pressure and composition), are available. This is the case for most of the phase equilibrium calculations performed during a compositional simulation run. The efficiency and reliability of the proposed formulation are proved by several test problems, with emphasis on the near-criticality and the vicinity of phase boundaries.