Rachford-Rice iterations are widely used in equation-of-state (EOS) flash calculations to determine phase equilibrium compositions from constant equilibrium ratios (K-values). The method, however, can be slow to converge and in some cases may not converge at all.

This paper presents a simpler and more robust method to replace Rachford-Rice and similar methods in flash calculations. The new objective function has two fewer asymptotes compared to Rachford-Rice and is based on one of the equilibrium phase mole fractions, typically the lightest component.

Results with the new objective function demonstrate that rapid convergence using Newton-Raphson iterations (NR) is assured because the correct solution is determined within a small range (window). In that window, no poles exist no matter the overall composition and the objective function is often linear. Improved linearity of the objective function contributes to increased convergence speed even near critical points. The method can be easily incorporated into any compositional simulator without significant modifications.

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