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

A robust flash calculation algorithm can increase thepossibilityof correctly performing multiphase equilibrium calculations. Thiswork develops a new hybrid multiphase flash calculation algorithmadopting both equilibrium ratios and phase fractions as iterationvariables. Different from the standalone implementation of the Newton-Raphsonmethod documented in the literature, the hybrid algorithm properlyhybridizes Newton-Raphson and trust-region methods to achievea higher computational efficiency and better robustness. We show thefast convergence behavior of the hybrid algorithm in some scenarioswhere the conventional Newton-Raphson method encounters convergencedifficulties. Specifically, the hybrid algorithm is capable of reducingthe number of iterations consumed in the flash calculations from severalhundreds to only tens. We also demonstrate the good performance ofthe hybrid algorithm by constructing phase diagrams for several reservoirfluid mixtures.