RELIABLE PREDICTION OF PHASE-STABILITY USING AN INTERVAL NEWTON METHOD

A key step in phase equilibrium calculations is determining if, in fac t, multiple phases are present. Reliably solving the phase stability a nd, ultimately the phase equilibrium problem, is a significant challen ge for high pressure vapor/liquid, liquid/liquid and vapor/liquid/liqu id equilibrium. We present the first general-purpose computational met hod, applicable to any arbitrary equation of state or activity coeffic ient model, that can mathematically guarantee a correct solution to th e phase stability problem. In this paper, we demonstrate the use of th is new method, which uses techniques from interval mathematics, for th e van der Waals equation of state to determine liquid/liquid and liqui d/vapor phase stability for a variety of representative systems. Speci fically, we describe and test interval methods for phase stability com putations for binary mixtures that exhibit Type I and Type II behavior , as well as for a relatively simple ternary mixture. This shows that interval techniques can find with absolute certainty all stationary po ints, and thus solve the phase stability problem with complete reliabi lity.