Redlich-Kwong variant

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. 

The new ideas exposed in these studies (and in a few others like them) inspired great activity in the modification of the RK equation parameters a and b were treated as functions of temperature various constraints on the equation were relaxed new ones were added and many extensions to mixture behavior were proposed. This period (1967-1977) may be well remembered as the Redlich-Kwong Decade. In the midst of all this, Soave (13) published a RK variant which appeared to be a great improvement over other versions. Not surprisingly, the idea behind the new variation was a twist on an older one—the one exploited by Wilson in 1964. 

Given its simplicity, the van der Waals equation is remarkable for its ability to reproduce the qualitative features of red-fluid behavior. However, it is never used for engineering calculations more flexible equations are required for quantitative work. These may be generated from Eq. (1.3-24) by incorporating other assignments for parameters 0, 5, e, and i). It is customary to sett) = b and to express parameters 5 and e as specified multiples of b and 5 , respectively. By this procedure, one generates two-parameter variants of the van der Waals equation. Modem examples of cubic equations obtained in this way are the Redlich-Kwong equation of state, ... 

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