Numerical solution of many non-isothermal flow problems requires robust isenthalpic flash. However, isenthalpic flash is challenging when the total enthalpy is sensitive to temperature, which is referred to as narrow-boiling behavior.

This paper presents a robust algorithm for three-phase isenthalpic flash for water-containing hydrocarbon mixtures. The algorithm is an extension of the direct substitution (DS) presented in the literature and our previous research for two phases. A detailed analysis is given for the narrow-boiling behavior and its effects on the DS algorithm. A new method is also presented for K-value estimates for three phases for water-containing hydrocarbon mixtures. The thermodynamic model used is the Peng-Robinson equation of state with the van der Waals mixing rules.

The narrow-boiling behavior occurs when the Gibbs free energy varies in temperature-composition space in such a way that a small temperature change yields significant changes of equilibrium phase compositions relative to the overall composition. The system of equations used in the DS algorithm becomes nearly degenerate for narrow-boiling fluids. The three-phase DS algorithm in this paper adaptively switches between Newton's iteration step and the bisection step depending on the Jacobian condition number. The bisection algorithm solves for temperature based solely on the enthalpy constraint only when narrow-boiling behavior is identified. The DS algorithm is tested with a number of different isenthalpic flash calculations. Results show the robustness of the DS algorithm for narrow-boiling fluids.

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