A robust algorithm for isenthalpic flash of narrow-boiling fluids

• Narrow-boiling behavior is analyzed using the Peng–Robinson EOS.
• Equations in direct substitution become degenerate for narrow-boiling behavior.
• Bisection can robustly solve for the temperature when the equations are degenerate.
• A robust algorithm is developed for isenthalpic flash of narrow-boiling fluids.

Numerical solution of many non-isothermal reservoir 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. The direct substitution (DS) algorithms proposed in the literature have convergence difficulties for narrow-boiling fluids.This research presents a detailed analysis of the narrow-boiling behavior and its effects on the robustness of the DS algorithms. A new DS algorithm is developed that addresses the direct reason for the convergence issues associated with narrow-boiling behavior. The main focus of the research is on robust isenthalpic flash for two hydrocarbon phases, although mathematical derivations are given for a general multicomponent multiphase system. The thermodynamic model used is the Peng–Robinson equation of state.The new DS algorithm is tested for various isenthalpic flash problems, which include the cases for which the prior DS algorithms exhibit non-convergence. Results show that the narrow-boiling behavior causes the system of equations solved in the DS algorithms to be degenerate. The degenerate equations can be robustly handled by the bisection algorithm developed in this research. Case studies demonstrate that the new DS algorithm exhibits significantly improved robustness for isenthalpic flash of narrow-boiling fluids.