The reliable prediction of phase stability is a challenging computatio
nal problem in chemical process simulation, optimization and design. T
he phase stability problem can be formulated either as a minimization
problem or as an equivalent nonlinear equation solving problem. Conven
tional 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 global minimum. Thus there has been considerable recent
interest in developing more reliable techniques for stability analysi
s. In this paper we demonstrate, using cubic equation of state models,
a technique that can solve the phase stability problem with complete
reliability. The technique, which is based on interval analysis, is in
itialization independent, and if properly implemented provides a mathe
matical guarantee that the correct solution to the phase stability pro
blem has been found.