A New Pure-Component Equation of State Designed for Accurate Reproduction of Phase-Equilibrium, Caloric and Volumetric Properties with Emphasis on Supercritical-Property Prediction

摘要

Numerous modifications to the Van der Waals model have been presented over the years with the aim of representing with increasing accuracy the thermodynamics of complex systems. As a matter of facts, the most of those do not incorporate a substantial difference in the functional combination of attractive and repulsive forces, with respect to the original formulation introduced by Van der Waals. Although the analytical expressions of the repulsive and attractive terms proposed in literature do not correctly quantify the actual repulsive and attractive contribution to pressure, their sum results in a quantitative representation of fluid properties being sufficiently accurate to make their combination the 'cornerstone of the generalized Van der Waals theory'. From the Van der Waals proposal, even the most successful two-parameter cubic equations of state (EoS) still express their attractive and repulsive term by introducing a parameter a, which is a measure of the attractive forces ('energy parameter') between molecules, and the parameter b, which is a measure of the size ('intrinsic volume' or 'co-volume') of the molecules. The purecomponent a energy parameter is directly proportional to the temperature-dependent alpha function α(T) which is a measure of how the a parameter deviates from its critical value. A wide variety of α-functions has been proposed over the past years. Those contain parameters which are adjusted according to different criteria: either over a whole set of components or for each specie ('generalized' versus 'component-specific' α-function); either considering the whole temperature domain or specifically in the subcritical or supercritical regions ('overall' versus 'domain-specific' α-function). Furthermore, among the mostly applied formulations, two different functional forms are identifiable: polynomial and exponential forms.