A predictive power sequence equation for vapor pressures of pure organic fluids and partial pressures in multicomponent systems in equilibrium

Abstract

The simplified Clausius-Clapeyron equation with the assumption of a constant molar enthalpy of vaporization is not adequate for general applications, despite the desirable need of using only two experimental values for the two integration constants. Therefore many correlations and models of equations of state with more than two necessary empirical constants are used. Here the critical state equation is used as a starting point, with a power sequence as a mathematical model for the estimation of the critical constants for small molecules and for homologous series. With the n-alkanes as reference class for all organic compounds, a reference vapor pressure equation has been defined as function of the relative molecular mass M, which represents principally the correct temperature dependence over the entire liquid state of n-alkanes. With an additional, temperature dependent structural increment U, with the same dimensionless unit as M, a generally applicable vapor pressure equation results for all organic fluids which needs only two specific experimental values. In this investigation principally liquids with freezing point, Tf < 298 K are discussed. The two experimental vapor pressures used are for T1 = 298.15 K and T2 = Tb (boiling point). The predicted values are reliable for the most part of the liquid state. The equation has been validated for alkanes, alkyl-benzenes, 2-ketones, esters, primary alcohols and carboxylic acids by comparing of predicted values with published data of high quality. The increment values for specific functional groups and branching groups can be used for pressure predictions only when the structure or the boiling point is known. An important application of the increments is the prediction of partial pressures in binary mixtures with specific fractions δ of the U-values. The determination of the corresponding δ-values from a set of experimental partial vapor pressures is very easy in comparison with other models. Prediction of partial pressures in multicomponent systems at equilibrium is possible with a good first approximation, without additional empirical constants.Most remarkably, for the application of the developed equation only a small number of easily available empirical parameters are needed as structure increments U and fractions δ of U. The qualitative relation of these parameters to the polarities and other properties of the investigated compounds allow a better understanding of the main reasons for the relative magnitude of vapor pressures and their interactions in mixtures.