Redlich-Kwong successes with

The Redlich-Kwong equation with modified temperature dependence of the a parameter has been found to successfully represent the vapor-liquid equilibria of the methane-... 

Gillespie and Beattie [89] (see also [33]) were by for the most successful experimentally in establishing a firm basis for an analytical expression of the equilibrium constant in the range of industrial interest. The values in Tables 10 and 11 were calculated using their equation. A detailed description, with literature data and many tables, appears in [33]. A description of the equilibrium using the Redlich-Kwong equation of state is given in [90]. 

Several cubic equations of state such as Redlich-Kwong, Soave-Redlich-Kwong, and Peng-Robinson have been used to calculate vapor liquid equilibria of fatty acid esters in supercritical fluids. Comparisons are made with experimental data on n-butanol, n-octane, methyl oleate, and methyl linoleate in carbon dioxide and methyl oleate in ethane. Two cubic equations of state with a non quadratic mixing rule were successful in modeling the experimental data. 

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. 

All cubic equations of state proposed to date are special cases of a general five-parameter expression. The apparent flexibility of such an expression is partly illusory, however, because of the inherent limitations imposed by its density dependence. Historically, the greatest successes with cubic equations had been achieved with variants of the Redlich-Kwong equation of state. Recent work, e.g., that of Peng and Robinson, has demonstrated the suitability of other cubic forms, and has inspired renewed efforts to identify the best cubic expression. One must approach such efforts with realistic expectations, and with an understanding of what cubic equations can and cannot do. 

In some cases the phase behaviour of the appropriate solvent/solute systems can be collated with success using the simple equation of state techniques based on cubic equations of state such as the Redlich-Kwong or Peng-Robinson equations. 

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