Analysis of Infinite Dilution Activity Coefficients of Solutes in Hydrocarbons from UNIFAC

Molecular structural effects on infinite dilution activity coefficients of solutes in n-alkanes and other hydrocarbons are studied within the UNIFAC model. Characteristic chain-length dependencies and other structural relationships imbedded in the model are discussed with emphasis to the consequences this has for model development.

The cases treated have subtle but major implications for the correlation of activity coefficients and derivatives since these imply that combinatorial terms may not be small and they can be essential to the success of correlations based on UNIFAC. We have examined a number of infinite dilution properties and find that current expressions do not adequately describe these and other cases.

The analysis is described and comparisons of the expressions with data for important systems are presented. New models are not presented, but improvements with either modified group definitions or revised relationships are discussed. The importance of such adjustments, for adding new terms to the existing equations, is stressed.