DEVELOPMENT OF A STATISTICAL MECHANICAL EQUATION OF STATE FOR PRACTICAL APPLICATION

Equation of state development is investigated with the general objective of analyzing the statistical mechanical basis for current engineering equations of state. Although current equations accurately reproduce the macroscopic properties of fluids, the related description of the microscopic properties is often vague or even erroneous in some of the fundamental details. As attempts are made to model complicated mixtures, accurate representation of the microscopic scale is becoming increasingly important.;The use of the virial series as the basis for an equation of state is investigated first. For hard body fluids a modification of the series results in an improved equation of state which is capable of representing the entire density range with a small number of coefficients. The significance of this improvement is that an equation of state can be produced directly from the virial coefficients without appealing to more approximate and less general theories. For molecules possessing an attractive part of the potential, however, the improved virial series does not provide a reliable basis for an equation of state below the critical temperature where condensation is possible.;In order to represent condensable fluids, equation of state development is analyzed from the perspective of statistical mechanical perturbation theory. From this perspective it is shown that current engineering representations of the hard sphere diameter and the perturbation integral are not justified. As an alternative to current engineering equations of state, it is shown how the principle of three-parameter corresponding states can be applied to perturbation theory. The improved generality of the three-parameter corresponding states approach is demonstrated by application to regular solutions containing flexible, nonspherical molecules. The convenience of this approach is the establishment of a correspondence between the macroscopic engineering parameters and the microscopic potential parameters, eliminating the need for regression of optimal potential parameters for each compound. Three-parameter corresponding states also fixes significant points on the phase diagram yielding a standard basis for comparing different theories. Perturbation theory is evaluated showing that it produces qualitatively correct results but is not nearly as accurate as the Soave equation of state.