Modelling Supercritical Phase Equilibria: Problems and Pitfalls

Ulrich K Deiters

doi:10.3311/ppch.12757

Abstract

This article addresses some recurring difficulties and problems of computing phase equilibria involving supercritical fluid phases. These difficulties prevent a full automatization of thermodynamic calculations and require human interference. Examples are the wrong interpretation of experimental data, phase inversion phenomena, or overlooking phases. While none of these insights are knew, publications dealing with them are scattered and sometimes hard to obtain. This article gives a short overview over some of the most common difficulties and pitfalls.

Highlights

While there exist powerful computer programs for the calculation of the thermodynamic properties of fluids, even for phase equilibria of mixtures, these programs usually cannot regarded as “fool-proof ” in the sense that they will always: arrive at a physically reasonable set of model parameters when fitting a model to experimental data, and find the correct, stable phases when calculating phase equilibria.All this means that human guidance for such thermodynamic software is—and will be for many years— indispensible
For the modelling of fluid phase equilibria it is advisable to look up the global phase diagrams of the model used— they are available for several popular equations of state
Independent of the availability of a global phase diagram it is good technique to compute the critical curves of a mixture, because this will hint the phase diagram class and indicate the existence of perhaps unexpected demixing phenomena

Summary

Introduction

While there exist powerful computer programs for the calculation of the thermodynamic properties of fluids, even for phase equilibria of mixtures, these programs usually cannot regarded as “fool-proof ” in the sense that they will always: arrive at a physically reasonable set of model parameters when fitting a model to experimental data, and find the correct, stable phases when calculating phase equilibria.All this means that human guidance for such thermodynamic software is—and will be for many years— indispensible. The diagram contains the vapor pressure curves of the pure fluids (labeled "(lg)i", with i = 1 (CO2) or 2 (caffeine)), binary critical curves ("l = g"), and three-phase curves.

Results

Conclusion