Three-phase free-water flash calculations using a new Modified Rachford–Rice equation

A novel Modified Rachford–Rice equation is developed for three-phase equilibrium calculations in hydrocarbon–water systems, based on the free-water assumption, i.e., the water-rich liquid phase consists of pure water only. In the inner loop of the flash algorithm, the three-phase problem (consisting in a system of two non-linear equations) is replaced by a pseudo-two-phase problem (consisting in a non-linear equation). Unlike previous formulations, the new Modified Rachford–Rice function is guaranteed to monotonically decrease between two adjacent asymptotes. The negative flash concept is used, and a search window is proposed for the vapor fraction. The new free-water flash method proved to be robust, and excellent agreement between full three-phase flash and pseudo-two-phase free-water flash was obtained for various test problems. The proposed method is very useful in compositional reservoir simulation of certain oil recovery methods and in process simulation.