Generalized Vapor Pressure Prediction Consistent with Cubic Equations of State

Laura L. Petrasky and Michael J. Misovich. Engineering, Hope College, 27 Graves Pl, Holland, MI 49422-9000

Equations of state may be used to calculate pure component vapor-liquid equilibrium properties such as vapor pressure, heat of vaporization, liquid density, and vapor density. The standard approach requires coupling the EOS with a phase equilibrium criterion such as free energy, chemical potential, or fugacity. The resulting equations are nonlinear and must be solved by numerical methods.

An alternative approach is applicable to cubic EOS such as the commonly used Soave-Redlich-Kwong and Peng-Robinson equations. Equilibrium properties may be explicitly expressed as power series in reduced temperature or related functions. These results are more convenient than numerical calculations, but are subject to truncation error in many practical situations.

Results from such a power series method were used to generate generalized dimensionless vapor pressure equations which were extensions of the commonly used Antoine equation but valid over wider temperature ranges. The substance-specific adjustable constants of the vapor pressure equations were expressed as functions of the acentric factor of the substance and its critical temperature and pressure. The deviations between these results and the exact equilibrium property predictions from the EOS were quantitatively characterized.