PRECISE REDUCTION OF VAPOR/LIQUID EQUILIBRIUM DATA BY MOLECULARLY-INSPIRED EXPRESSIONS FOR THE EXCESS GIBBS ENERGY (CHEMICAL THEORY)

A set of molecularly-inspired expressions for the excess Gibbs energy (G('E)) suitable for precise reduction of vapor/liquid equilibrium (VLE) data is described. The expressions comprise three distinct terms, representing enthalpic, entropic and chemical effects, respectively. Several options are available for each term; by selecting different combinations of terms (the "menu" approach), sets of expressions for G('E) are developed with adequate flexibility for precise data reduction. Candidates for the enthalpic term include modified regular-solution theory, the enthalpic portion of the Wilson equation, and the expressions for H('E) implied by either the van der Waals equation of state (EOS) or the Redlich-Kwong EOS. Either the Flory-Huggins equation or the Guggenheim equation represents entropic effects. Chemical theory provides expressions for contributions to G('E) from specific interactions like hydrogen-bonding.; VLE data from 56 binary systems are reduced using our set of expressions for G('E); 37 systems are represented to within the estimated experimental error contained in the data. Guidelines for choosing an appropriate combination of terms to reduce data for a particular type of system are presented. The physical significance of the resulting parameter values is also considered.; The quality of the correlations obtained by regressing ten sets of ternary VLE data is not as good as that obtained for the binary system. When the adjustable parameters are determined solely from data for the constituent binaries, the expressions for G('E) predict ternary mixture behavior with reasonable accuracy, compared to rival procedures.; Data for the heats-of-mixing of ten binary systems are correlated by implied expressions for H('E) with mixed results. However, simultaneous reduction of P-x and H('E) data yields surprisingly good fits of the data and gives parameter values that are physically realistic.; A numerical study on the validity of expressing G('E) as a linear combination of terms is included. The results indicate that such an approximation is poorest when strong physical and chemical interactions exist in the mixture.; Recommendations are presented for future work on this study and on related projects.