A distributed-site model for non-equilibrium dissolution of multi-component residually trapped NAPL

An approach for modeling the dissolution of residually-trapped multi-component non-aqueous phase liquids (NAPLs) from a physically heterogeneous porous medium is presented. The Distributed-Site (DS) model presented uses the beta probability density function to characterize the initial distribution of residual NAPL blob volumes. Mass transfer from the trapped idealized NAPL blobs is simulated using as a basis a correlation for mass transfer coefficients, a linear concentration driving force, and multicomponent partitioning. The main advantage of the approach outlined in this work is that by using reasonable assumptions the Distributed-Site model is able to model the mass transfer behavior of a complex dissolution phenomena using fewer fitted parameters than current multi-site models. Application of this model to column experimental data for the dissolution of benzene, toluene, and m-xylene from a tridecane NAPL source is presented. The incorporation of several NAPL blob sizes (and corresponding mass transfer rate coefficients) was able to account for the behavior of the effluent concentrations as they passed through the exponential decay and into the asymptotic region, an important feature lacking in the two-site modeling approach. When fitted to data for benzene, the DS model was found to predict experimental data satisfactorily for the dissolution of toluene and m-xylene from tridecane without adjustment of the parameters. This indicates that the dissolution process can be generalized for various compounds even when mass transfer limitations are present and that dissolution data obtained for one compound can be useful for predicting the long term dissolution history.