A. Introduction

Practical methods for the calculation of the density of gas and liquid mixtures as a function of temperature and pressure are presented in this section. The methods are illustrated with worked examples and the results compared with experimental data. Methods based on the corresponding states principle are quite accurate and can be used up to high pressure, of the order of several hundred bar. The virial approach is accurate at low pressures but its performance worsens with increasing pressure and is not recommended above about 50 bar. For saturated liquid mixtures the correlations proposed by Hankinson and Thomson (1979) and Spencer and Danner (1973) are recommended. For compressed liquid mixtures the method of Thomson et al. (1982) is quite accurate. Only methods which are easy to implement have been selected. More elaborate thermodynamic models such as equations of state, which in principle allow the calculation of all the thermodynamic properties of single substances or mixtures, are beyond the scope of this article because their implementation would require the development of computer programs with lengthy testing periods. Wherever possible guidelines on the accuracy of the recommended methods are provided.

B. Gas mixtures

(a) Corresponding states principle

In the application of the corresponding states principle to mixtures it is necessary first to determine the pseudocritical parameters of the mixture of interest. In Section 508 it is shown how to estimate these scaling parameters through a process of averaging of the constants of the pure constituent components. Once the pseudocritical parameters of a given mixture have been determined, the procedure outlined in Section 499B can be used. This procedure treats the mixture as a fictitious substance, or pseudo-component, when its compression factor Z_m(T_r,p_r) at reduced conditions of temperature and pressure is given by the following three-parameter correlation (Pitzer et al., 1955):