Phase equilibrium calculations with specified vapor fraction

Phase equilibrium calculations have been extensively explored over the years, with numerous industrial applications, where pressure and temperature specifications are the most common. Different problems, however, may require different specifications for solving phase equilibrium. This article aims to develop a flash calculation with specified temperature or pressure and vapor fraction, termed $\psi$ $\beta$-flash, which can be useful in studies of storage tanks and distillation columns. An algorithm is developed with an external loop for pressure or temperature optimization and an inner loop for the isobaric–isothermal-flash calculation. The method is efficient in predicting pressure for different binary and ternary mixtures, including refrigerants, hydrocarbons, and carbon dioxide, even in complex scenarios such as regions with retrograde condensation. The computational demand is investigated, revealing that calculations within the isobaric–isothermal-flash primarily contribute to the total computational cost, rather than pressure optimization. Finally, two case studies highlight the method’s efficiency: one involving a spherical storage tank, where we compute pressures based on liquid height to classify the safe operational region, and another focusing on a distillation tray, predicting temperatures driven by changes in liquid height to provide insights into separation performance.