GLOPEQ - A NEW COMPUTATIONAL TOOL FOR THE PHASE AND CHEMICAL-EQUILIBRIUM PROBLEM

Calculation of phase and chemical equilibrium represents a crucial pha se in the modeling of many separation processes. For conditions of con stant temperature and pressure, a necessary and sufficient condition f or the true equilibrium solution is that (i) the total Gibbs free ener gy of the system be at its global minimum, or (ii) the minimum of the tangent plane distance function be nonnegative for all phase models us ed to represent the system, In this work, the goal is to obtain equili brium solutions corresponding to a global minimum of the Gibbs free en ergy as efficiently as possible, for cases where the liquid phase or p hases can be modeled by the NRTL, UNIQUAC, UNIFAC, Wilson, modified Wi lson and ASOG equations. Vapor phases whose behavior can be described as ideal can also be handled. In achieving this goal, there are two di stinct problems of relevance: (i) the minimization of the Gibbs free e nergy, denoted (G), and (ii) the minimization of the tangent plane dis tance function, or the tangent plane stability criterion, denoted (S). For all these activity coefficient models, GLOPEQ (GLobal OPtimizatio n for the Phase and chemical EQuilibrium problem) can guarantee global solutions for problems (G) and (S), but a combined algorithm employs them in tandem, using (G) to generate candidate equilibrium solutions which can then be verified for thermodynamic stability by solving (S). Two key features of the combined algorithm are that (i) as much infor mation as is possible is obtained from local searches, and (ii) it is preferable to verify a globally stable equilibrium solution using the tangent plane criterion, as this problem contains fewer variables than the minimization of the Gibbs free energy. Results for several exampl es are presented, and all but one of them are for the case of phase eq uilibrium, due to the paucity of examples for reacting systems that em ploy excess Gibbs free energy models. Copyright (C) 1996 Elsevier Scie nce Ltd