Phase-Equilibrium Calculations

Abstract

The quantitative prediction of phase behaviour is the central problem in chemicalengineering thermodynamics and a very important consideration in the design of chemical process plant. The almost infinite number of possible mixtures and wide ranges of temperature and pressure encountered in process engineering is such that no single thermodynamic model is ever likely to be applicable in all cases. Conse quently, knowledge and judgement are required to select the most appropriate methods by which to estimate the conditions under which two or more phases will be in equilibrium.In this chapter, some basic algorithms for the determination of vapour-liquid equilibrium conditions in mixtures are discussed together with their application to certain kinds of flash calculation. A short discussion of liquid-liquid equilibrium is also included. For every topic considered, a computer routine will be given and used as the basis of an example. As elsewhere in this book, the computational methods used are not the most efficient possible (instead, they are designed to be easy to understand and modify) and this may be a consideration if the routines are to be called many times as a part of a larger calculation. Elementary Phase BehaviourBefore embarking on the quantitative aspects of phase equilibrium, it would be as well to have a qualitative understanding of the most common types of phase behaviour. The subject of phase behaviour in mixtures is a complicated one and we shall consider here only the most elementary kinds of phase diagram for vapourliquid and vapour-liquid-liquid systems.The simplest kind of vapour-liquid equilibrium (VLE) in a binary system is illustrated in the phase diagrams of Figure 8.1. Here, we show either the pressures at which vapour and liquid coexist under conditions of constant temperature (the P-x-y Thermophysical Properties of Fluids Downloaded from www.worldscientific.com by UNIVERSITY OF BIRMINGHAM LIBRARY -INFORMATION SERVICES on 03/22/15. For personal use only.