Modeling and simulation of steam-enhanced extraction: Parameter effect of injected steam-air mixture on NAPL remediation at contaminated sites.

Steam enhanced extraction (SEE) shows great potential for the remediation of sites contaminated by non-aqueous phase liquids (NAPLs). The condensation of the injected steam, which is induced by subsurface temperature gradients, is recognized as crucial in hindering the remediation of NAPLs. To quantitatively examine the complex process for an optimal design, this study formulates a thermo-hydro-concentration-moisture coupled two-phase flow model. This model takes into consideration the volatilization and dissolution of NAPLs during phase transition and mass transfer, as well as water evaporation and steam condensation. The injection of the steam-air mixture under various thermophysical conditions is subsequently analyzed. The findings indicate that, under a constant relative humidity, increasing injection temperature decreases the contaminant removal efficiency. Under saturated steam conditions, the NAPL removal rate decreases from 89.0 % at 303 K to 74.3 % at 353 K. A decrease in the relative humidity of the steam-air mixture improves the NAPL remediation efficiency, and this trend becomes more distinct as the relative humidity further decreases. In addition, a threshold of relative humidity of steam-air mixture is proposed, and a reasonable injection temperature is matched according the threshold. This study provides a vital reference for optimizing the thermal parameters of steam-air mixed in SEE remediation.