The Riedel vapor pressure correlation and multi-property optimization

The Riedel vapor pressure correlation is a useful model that has been employed for decades. The shape of the temperature dependent correlation is a strong function of the value of the final coefficient (denoted as E), particularly at the low temperature end. Vapor pressure data can be accurately correlated with different values of E, but properties derived from vapor pressure are more sensitive to the value of E. Such properties include enthalpy of vaporization and liquid heat capacity. Because these three properties are linked through rigorous thermodynamic relationships (including the Clapeyron equation and the derivative of enthalpy of vaporization), a multi-property optimization is possible to obtain the value of E that will give the best thermodynamic picture for the compound. To determine the effects of the final coefficient on subsequent properties, E was varied from 1 to 6 in integer steps and for E = 0.5, and vapor pressure data were fit for several well-known compounds. These correlations were then tested to see how well they predict enthalpy of vaporization and liquid heat capacity data. The results indicate that the traditional practice of setting E = 6 rarely produces the best thermodynamic consistency, and that E = 2 is superior for many compounds. The improved vapor pressure correlations were then used to predict triple point pressures with favorable results that compare well with the literature.