Comments on the Estimation of Mixture Properties

The predictive techniques for pure compounds can be applied to mixtures as well. The quality of the obtained results, however, depends on the need for, and the availability of, interaction coefficient values. 

For the volumetric behavior of vapor and gaseous mixtures of hydrocarbons up to moderate pressures, the cubic equations of state discussed in Chapter 10 give reasonable results even with k j = 0. At very high pressures, the Pitzer correlation (with the Lee-Kesler Tables) should be used. 

In the presence of gases such as CO2, CO, H2S, N2, etc. in the hydrocarbon mixture, however, interaction coefficients should be used for accurate results, especially at high pressures. They are also needed, but to a lesser degree, in the presence of CH4. 

For the volumetric behavior of polar - and, of course, nonpolar -mixtures at low pressures, the virial equation gives reasonable accuracy, provided that values are available. 

For polar systems at high pressures the cubic equations of state modified for polar compounds (Section 14.7.3), and for still higher ones the Pitzer-Lee-Kesler approach, give the most reliable results. The accuracy is poorer, however, Aan that of the previous cases. 

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We examine the prediction of the volumetric behavior of binary mixtures in the next two Examples 11.5 and 11.6 and the evaluation of interaction coefficients, in Example 11.7. We proceed, then, with some general comments on the estimation of mixture properties, followed by the discussion of fiigacities in mixtures. 

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