Establishing Consistent Thermodynamic Data on Vaporization Equilibria for Organic Compounds

Vapor pressure data are abundant and accurate near the normal boiling point while they are scarce and unreliable in the low pressure range. Data on thermal properties (enthalpy of vaporization and difference in the heat capacities of an ideal gas and the liquid) are available, however at temperatures far below the normal boiling point. All three properties are related by exact thermodynamic relationships and can be correlated simultaneously. This procedure can be used in the evaluation of thermodynamic data on vaporization equilibria as a rigorous consistency test and for producing recommended data sets. When a suitable correlation equation is selected a single set of parameters permits generation of consistent data for several properties between the triple and normal boiling points. The procedure is especially useful for calculating vapor pressures and/or enthalpies of vaporization far below the normal boiling point. In combination with the group contribution methods for estimation of thermal properties, the principle of simultaneous correlation can serve as a base in formulation of a new approach to the prediction of vapor pressures and/or enthalpies of vaporization at low reduced temperatures. Such a procedure would not require use of any traditional parameters (critical constants, acentric factor, etc.) which are not available for most high boiling compounds.