A new look at multiphase Rachford–Rice equations for negative flashes

The elegant method proposed by Michelsen [M.L. Michelsen, Calculation of multiphase equilibrium, Comput. Chem. Eng. 18 (1994) 545–550] for solving multiphase Rachford–Rice (RR) equations as a bound constrained minimization is extended to the negative flash region; this method is definitely much more robust than any method based on equation solving. The problem is reformulated as an unbounded linearly constrained minimization. The feasible domain is limited by (only few of the) hyperplanes defined by equilibrium constants. The proposed method is applied to several three-phase cases.