UNIFAC modeling of multicomponent nonaqueous phase liquids containing polycyclic aromatic hydrocarbons

Multicomponent nonaqueous phase liquid (NAPL) contaminants that are mixtures of polycyclic aromatic hydrocarbons (PAHs) are examined using UNIFAC. The NAPL phase activity coefficients for constituent compounds of four different coal tar materials are estimated. The group contribution approach is adapted to describe representative molecular structures of the uncharacterized fractions of the tar materials, determined from average molecular weights and nonparametric regressions of the functional group parameters against molecular weight. The molecular structure characterization of the tars elucidates the similar chemistry of the materials from different sites, despite their different chemical compositions. Nearly ideal behavior is predicted for most tar constituents, with the majority of activity coefficients in the range of 0.9 to 1.1. These results provide a firm theoretical basis for an assumption of NAPL-phase ideality for many tar constituents. The robustness of this conclusion is indicated through comparable results across different tar materials and through a sensitivity analysis of the effect of the estimated characteristics of the uncharacterized fractions.