Hybrid Multiphase Flash Calculation Algorithm Based on Simultaneous Solution of Equilibrium Ratios and Phase Fractions

Abstract

A robust flash calculation algorithm can increase the possibility of correctly performing multiphase equilibrium calculations. This work develops a new hybrid multiphase flash calculation algorithm adopting both equilibrium ratios and phase fractions as iteration variables. Different from the standalone implementation of the Newton–Raphson method documented in the literature, the hybrid algorithm properly hybridizes Newton–Raphson and trust-region methods to achieve a higher computational efficiency and better robustness. We show the fast convergence behavior of the hybrid algorithm in some scenarios where the conventional Newton–Raphson method encounters convergence difficulties. Specifically, the hybrid algorithm is capable of reducing the number of iterations consumed in the flash calculations from several hundreds to only tens. We also demonstrate the good performance of the hybrid algorithm by constructing phase diagrams for several reservoir fluid mixtures.