New strategy for phase equilibrium and critical point calculations by thermodynamic energy analysis. Part I. Stability analysis and flash

Abstract

We have reformulated Michelsen's method of phase stability analysis, which employs the thermodynamic energy minimization in terms of the tangent plane criterion. In our method, we use the component molar densities as the primary variables in place of the mole numbers or mole fractions. Furthermore, we use a modified stability test procedure called the ‘semi-local’ stability test and a higher order initialization procedure in the two-phase region. We believe that these modifications improve the numerical stability and the speed of convergence of the iterative technique, particularly in the near-critical and the liquid-liquid regions. We demonstrate the successful use of this method with well-defined example fluid systems and near-critical and retrograde condensate petroleum reservoir fluids.