Redlich-Kwong observations

Hence, the third parameter, co, implicitly contains information about the vapor pressure, making vapor pressure prediction something like a circular loop. But Soave went beyond this simple observation. Wilson had previously recognized these issues, but his equation met with limited success, especially at low reduced temperatures. Soave was careful to analyze the temperature dependence of his equation of state in great detail at the outset. He achieved this by introducing an adjustable parameter into the attractive contribution of the Redlich-Kwong equation. 

FIGURE 1.7-1 Isotherms VLE for methane-propane. A, O are experimental data A, are observed crilieul points. Curves are computed via the Soave-Redlich-Kwong equation with kl2 — 0.029. 

It is instructive to compare Equation (5.31) to Equation (5.33). Equation (5.31) was developed using T and v as the independent properties, while Equation (5.33) used T and P. We see that in the former case we get a partial derivative in P. Therefore, this form is amenable to a pressure-explicit equation of state. Conversely, Equation (5.33) gives a partial derivative for v and is more amenable to a volume-explicit equation of state. This observation holds generally that is, T and v are convenient independent properties when we have a pressure-explicit equation of state, while T and P are convenient for a volume-explicit equation. This rule of thumb is reinforced in Example 5.4, where the choices of independent properties T,v) and (T,P) are compared for a calculation using the pressure-explicit Redlich—Kwong equation of state. 

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