Theoretical study of multicomponent soil vapor extraction: propagation of evaporation–condensation fronts

Theoretical analysis of the multicomponent soil vapor extraction (SVE) process is presented. It is based on general principles of transport in porous medium and the Raoultʹs law describing the equilibrium between multicomponent nonaqueous phase liquid (NAPL) and aqueous/gaseous phases. The principal simplifying assumptions of the theory include: isothermal conditions, homogeneous porous medium, radial advective transport of volatile organic compounds, steady-state radial soil gas flow through the contaminated zone, stagnant NAPL and aqueous phases, constant solid–water distribution and gas–water partitioning coefficients. The main emphasis of the study is on the description of propagating composition fronts under local equilibrium conditions. Construction of multicomponent front solutions is carried out by applying the concept of coherent waves, developed by Helfferich and Klein for multicomponent chromatography. It is shown that under typical conditions component volatilities control the sequence of propagating fronts. However, this sequence may change for very small NAPL saturation values ( 10−3–10−4 or less) and/or relatively high retardation factors (>102). Tailing effect of nonequilibrium (kinetically controlled) gas–water partitioning behind the NAPL zone is also addressed in the presented study. This effect is shown to diminish with the increase of NAPL saturation.