Page 95 - Handbook of Natural Gas Transmission and Processing Principles and Practices

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The alpha function used in the SRK equation is often named Soave's alpha function. The
accuracy of this model was tested by comparing vapor pressures of a number of hydrocarbons
calculated with the SRK equation to experimental data. Contrary to the Redlich–Kwong equation,
the SRK equation was able to fit well the experimental trend. After Soave's proposal, many
modifications were presented in the literature for improving the prediction of one or another
property. One of the most popular ones is certainly the modification proposed by Peng and
Robinson (1976) (their equation is named PR76 in this chapter). They considered the same alpha
function as Soave, but they recalculated coefficients of the m function. In addition, they also
modified the volume dependency of the attractive term:

(2.30)

The accuracy of the PR76 equation is comparable to the one of the SRK equations. Both these
models are quite popular in the hydrocarbon industry and offer generally a good representation of
the fluid phase behavior of nonassociated molecules (paraffins, naphthenes, aromatics, permanent
gases, and so on). Robinson and Peng (1978) proposed to slightly modify the expression of the m
function to improve the representation of heavy molecules i such that . This
model is named PR78 in this chapter.

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