Estimation of Mass Transfer Coefficients Using Air-liquid Interfacial Area in Porous Media

Accidental spillages or leakages of organic compound within the environment, released as non-aqueous phase liquid (NAPL), result a widespread contamination of subsurface soils and groundwater. In such multi-phase flow system in porous media, interphase mass transfer mainly governs the overall transport processes that occur in the interface between the two fluid phases. Air-liquid interfacial area plays a significant role in mass transfer during the remediation of contaminated soil in unsaturated zone. Usually interphase mass transfer coefficient is expressed as a lumped parameter together with the specific interfacial area. In this paper, interphase mass transfer coefficients are estimated separately using the air-liquid interfacial area based on soil characteristics curve. A series of NAPL volatilization experiments was carried out for different NAPL saturation (13.8%~71%) and pore gas velocities (0.1~2 cm/sec) for different porous media. Three uniform size glass beads (d50=0.25mm, 0.50mm, 0.75mm) and a medium grain silica sand (d50=1.4mm) were selected as the porous media and Toluene was chosen as single component NAPL. The results revealed that the mass transfer coefficient increases with pore gas velocities and grain sizes. Dependency of mass transfer on pore gas velocity was found to be linear. Mass transfer coefficient was gradually decreased with saturation for uniform size glass beads but sharply decreased in case of silica sand for NAPL saturation of 40-50%.