Nonequilibrium mass transfer between the vapor, aqueous, and solid phases in unsaturated soils during vapor extraction

Vapor extraction is a commonly used method for removing nonaqueous phase liquid volatile organic compounds (VOC) from the vadose zone. Experience indicates that in the absence of liquid VOC, the efficiency of vapor extraction systems decreases dramatically with time as effluent concentrations approach zero asymptotically. When such systems are restarted after a temporary shutdown, effluent concentrations are often found to recover for a short period before dropping back to preshutdown levels. This behavior is generally attributed to kinetic processes which limit the transfer of contaminant into the moving air. A numerical model is developed to simulate the rate-limited extraction of volatile compounds governed by first-order kinetic mass transfer processes. A sensitivity analysis is performed to identify model responses to various kinetic and equilibrium partitioning processes. The model is calibrated using experimental data collected from a pilot-scale experiment involving vapor extraction of trichloroethylene from fine sand. An analysis of the relationships between airflow rates and the kinetic mass transfer coefficients under various pumping schemes shows that for a given condition, increasing the flow rate has little effect beyond a certain point. It is also shown that pulsed pumping is generally less efficient than continuous pumping at a low rate.