Vapor Pressures from Equations of State with Similarity Variables

The Antoine and Wagner equations predict vapor pressures as a function of temperature. These equations are accurate over specified temperature ranges and require substance-specific empirical constants. Equation of state methods such as Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) also predict vapor pressures, but complicated algorithms are required. Therefore, methods of finding generalized Antoine and Wagner constants for vapor pressure consistent with SRK and PR were studied. Two similarity variables were introduced. The common adjusted temperature variable, h^*, combined the effects of reduced temperature and acentric factor, and the common adjusted pressure variable, ψ^*, combined reduced pressure and reduced temperature. An iterative vapor pressure algorithm using the SRK or PR equation with a fugacity criterion was used to generate a data set of the similarity variables. Nonlinear least squares was used to determine the best fit Antoine and Wagner constants. Antoine equation results matched the vapor pressure predictions with relative deviations less than 0.05 percent for five subranges of common adjusted temperature from the critical point down to 0.19 (SRK) and 0.24 (PR). Wagner equation results had the same accuracy from the critical point down to 0.21 (SRK) and 0.26 (PR) for a single set of constants without additional subranges. It was concluded that the generalized Antoine and Wagner equations were successful in estimating vapor pressures from SRK and PR. The method worked over slightly wider temperature ranges for SRK than PR, and for the version of the Wagner equation using exponents 2.5 and 5 rather than 3 and 6.