The Combined Method for Hpvle Computation
J. David Raal, Andreas L. Mühlbauer · 2023
The direct (EOS) approach to high-pressure vapor liquid equilibrium (HPVLE) computation requires that a single equation be capable of describing both liquid-and vapor-phase behavior at all compositions, including the critical region. In the combined method, advantage is taken of the extensive availability of modeling equations for the excess Gibbs free energy, from which the liquid-phase fugacity may be computed. The main feature of the combined method is the use of separate auxiliary functions to describe the nonideality of liquid and vapor phases, namely, the activity and fugacity coefficients, respectively. A semiempirical equation is used to calculate the activity coefficient, which is only completely defined once the standard-state fugacity is specified. The Chao-Seader method, although reasonably simple by present standards, was a turning point in HPVLE calculation procedures. The carbon dioxide and propane components in the carbon dioxide/toluene and propane/1-propanol systems, respectively, were treated as condensable components and their activity coefficients were normalized using the symmetric convention.