Air sparging: Air‐water mass transfer coefficients

Abstract

Abstract. Experiments investigating the mass transfer of several dissolved volatile organic compounds (VOCs) across the air-water interface were conducted using a single-airchannel air-sparging system. Three different porous media were used in the study. Air velocities ranged from 0.2 cm s-• to 2.5 cm s-•. The tortuosity factor for each porous medium and the air-water mass transfer coefficients were estimated by fitting experimental data to a one-dimensional diffusion model. The estimated mass transfer coefficients K G ranged from 1.79 x 10 -3 cm min -• to 3.85 x 10 -2 cm min -•. The estimated lumped gas phase mass transfer coefficients KGa were found to be directly related to the air diffusivity of the VOC, air velocity, and particle size, and inversely related to the Henry's law constant of the VOCs. Of the four parameters investigated, the parameter that controlled or had a dominant effect on the lumped gas phase mass transfer coefficient was the air diffusivity of the VOC. Two empirical models were developed by correlating the Damkohler and the modified air phase Sherwood numbers with the air phase Peclet number, Henry's law constant, and the reduced mean particle size of porous media. The correlation developed in this study may be used to obtain better predictions of mass transfer fluxes for field conditions. IntroductionIn situ air sparging has been used for more than 10 years with varying success for the remediation of aquifers contaminated with dissolved volatile organic compounds (VOCs) and nonaqueous phase liquids (NAPLs). In a typical air-sparging system, clean air is injected below the water table to Even though the VOC concentrations at the air-water interface may more accurately represent the physics of the system, measurement of the VOC concentrations at the air-water interface is not possible. Therefore the standing issue in the estimation of the mass transfer coefficients between gas and liquid phases during air sparging is to define in a practical way, the driving force (i.e., concentration gradient) for the volatilization of VOCs. Since an air-sparged porous medium is not a completely mixed system, the task of defining the concentration gradient is more challenging. The objective of this work was to investigate the air-water mass transfer process for dissolved VOCs using an experimental setup where the interfacial area between the air and liquid phases was held constant. Mass transfer coefficients deter- Materials and MethodsThe approach taken was to use an experimental setup which would simulate air flow in a single discrete air channel in saturated porous media. Table 2. Partition coefficients of the VOCs for the three porous media were assumed to be linear for the concentrations tested. The VOC partition coefficients were determined individually using the head space technique as described by Ong and Lion [1991]. Linear partition coefficients were found to range between 0.018 mL g-• and 0.082 mL g-• and

Mass transfer of VOCs in laboratory-scale air sparging tank

“…The sparging screen was a porous stone tube with a diameter of 5 cm and a length of 10 cm. The air sparging experiments were conducted under stagnant water conditions which was reasonable for the laboratory-scale study [7][8][9].…”

“…Air sparging is expected to increase the rate of volatilization by increasing contact between the contaminants and sparged air. To quantify mass removal, diffusion of contaminants in aqueous phase along with mass-transfer across the air-water interface has been invoked [7][8][9][10][11][12]. Air-water mass transfer is usually described by a first-order kinetic process as a function of the mass transfer coefficient and air-water interfacial surface area.…”

Numerical modeling of kinetic interphase mass transfer during air sparging using a dual‐media approach

“…Recent laboratory studies have demonstrated that dissolved volatile organic compound (VOC) removal during air sparging is limited by the mass transfer of the VOC into the flowing gas phase [Braida and Ong, 1998; Hein et al, 1998; Semer and Reddy, 1998; Adams and Reddy, 1999]. Mass transfer limitations may occur at several scales because of the heterogeneous nature of gas distributions during air sparging.…”

Effect of system variables and particle size on physical characteristics of air sparging plumes

“…Mixing induced by this circulation should reduce restrictions on mass transfer relative to conditions in a stagnant state, where diffusion in the interfacial and bulk pore water can control the rate of removal (Braida and Ong 1998;. The circulation patterns observed with pulsing suggest that mixing probably is an important mechanism contributing to the enhanced mass transfer observed immediately after the air pressure is reapplied during a pulsing operation (Heron et al 2002).…”

“…However, the rate of increase in mass removal diminishes as the air flow rate or the duration of sparging increase due to a shift from local equilibrium conditions in the air channel to control of mass transfer by diffusive resistance at the airwater interface or in the bulk pore water (Braida and Ong 1998;). …”