Differential equations for fluid phase equilibria: Isothermal–isobaric case

Differential equations for two-phase equilibria under isothermal–isobaric conditions are derived. These equations can be used in connection with arbitrary equations of state (Helmholtz energy models) for fluid mixtures to compute phase envelopes. In contrast to conventional computation methods, which solve the (nonlinear) algebraic equations describing phase equilibrium by means of iterative methods and which often suffer from convergence problems, the differential equations merely have to be integrated, but not solved. Convergence problems are thus avoided. The computation of phase envelopes from differential equations is rapid, reliable, and advantageous in connection with complicated equations of state. • Algorithm for calculation of 2-phase envelopes of ternary and higher fluid mixtures. • Using differential equations. • Avoids convergence problems. • Very rapid. • Can be used with any Helmholtz energy model.