Reliable phase stability analysis for excess Gibbs energy models

Because models used to represent the Gibbs energy of mixing are typically h ighly nonlinear, the reliable prediction of phase stability from such model s is a challenging computational problem. The phase stability problem can b e formulated either as a minimization problem or as an equivalent nonlinear equation solving problem. However, conventional solution methods are initi alization dependent, and may fail by converging to trivial or nonphysical s olutions or to a point that is a local but not global minimum. Since the co rrect prediction of phase stability is critical in the design and analysis of separation processes, there has been considerable recent interest in dev eloping more reliable techniques for stability analysis. Recently we have d emonstrated a technique that can solve the phase stability problem with com plete reliability. The technique, which is based on interval analysis, is i nitialization independent, and if properly implemented provides a mathemati cal guarantee that the correct solution to the phase stability problem has been found. In this paper, we demonstrate the use of this technique in conn ection with excess Gibbs energy models. The NRTL and UNIQUAC models are use d in examples, and larger problems than previously considered are solved. W e also consider two means of enhancing the efficiency of the method, both b ased on sharpening the range of interval function evaluations. Results indi cate that by using the enhanced method, computation times can be substantia lly reduced, especially for the larger problems. (C) 2000 Elsevier Science Ltd. All rights reserved.