Co-injection of air for the prevention of the downward migration of DNAPLs during steam-enhanced extraction operations

The steam enhanced extraction process is an in-situ thermal remediation process specifically designed to treat source zones of volatile and semi-volatile NAPL contamination. However, there is significant concern that as a steam condensation front propagates through a contaminated medium, the vaporized NAPL will condense with the steam and accumulate ahead of the front, increasing its mobility to an extent that gravitational forces will cause denser than water non-aqueous phase liquids (DNAPLs) to migrate to a location below the steam zone.; In this work, the influence of air co-injection on the steam enhanced extraction process is identified. A semi-analytical solution for the temperature and saturation profiles associated with an advancing condensation front is developed. Increasing the injection ratio of air to steam is shown to increase the width of temperature fronts. As a result, any contaminant condensation that might occur will be distributed over a larger volume, reducing the maximum DNAPL saturations and thus the mobility, providing added protection against downward migration of DNAPLs. For a constrained injection rate, increasing the ratio of air to steam will reduce the condensation front velocity. Therefore, co-injecting air will increase the time required to heat a volume of contaminated soil.; An experiment was conducted to evaluate the optimum injection ratio prediction. Injection at the predicted optimum injection ratio for a volumetric average NAPL saturation, reduced accumulation of the contaminant ahead of the condensation front by over 90%, as compared to steam injection alone. Injection at the optimum injection ratio also resulted in earlier recovery than steam injection alone. Co-injection of steam and air is also shown to result in much higher recovery rates than air injection alone.; Numerical simulations of a proposed remediation project are included to illustrate how air-co injection will influence field operations. Simulation results confirm that the co-injection of air can prevent the accumulation of NAPL ahead of the temperature front for geometries of practical interest. Air co-injection is also shown to reduce the potential for spreading NAPL outside the initial source zone. Simulation results suggest that air co-injection has the potential to spread contaminant to groundwater. (Abstract shortened by UMI.)...