ENHANCED INTERVAL-ANALYSIS FOR PHASE-STABILITY - CUBIC EQUATION OF STATE MODELS

The reliable prediction of phase stability is a challenging computatio nal problem in chemical process simulation, optimization, and design. The phase stability problem can be formulated either as a minimization problem or as an equivalent nonlinear equation-solving problem. Conve ntional solution methods are initialization dependent and may fail by converging to trivial or nonphysical solutions or to a point that is a local but not a global minimum. Thus, there has been considerable rec ent interest in developing more reliable techniques for stability anal ysis. Recently, we have demonstrated, using cubic equation of state mo dels, a technique that can solve the phase stability problem with comp lete reliability. The technique, which is based on interval analysis, is initialization independent and, if properly implemented, provides a mathematical guarantee that the correct solution to the phase stabili ty problem has been found. However, there is much room for improvement in the computational efficiency of the technique. In this paper we co nsider two means of enhancing the efficiency of the method, both based on sharpening the range of interval function evaluations. Results ind icate that, by using the enhanced method, computation times can be red uced by nearly an order of magnitude in some cases.