Influence of air channel spacing, porous media, and airflow rate on NAPL volatilization during air sparging
Date
1999
Authors
Rogers, Shane William
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Ong, Say Kee
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Abstract
In order to study NAPL removal during air sparging, the removal of a benzene NAPL was quantified in a lab-scale reactor with two isolated vertical air channels at three discrete distances in three saturated porous media and five airflow rates. The air channels increased vertical aqueous contaminant migration when the air channels were spaced at 30 mm, and contained lateral migration for all cases. Benzene removal was highly correlated (r2=O.96) with a dimensionless number consisting of the mean particle diameter, the square root of the uniformity index, the effective aqueous phase diffusivity,the inverse of the air channel spacing squared, and time. Media porosity and airflow rate did not correlate well to mass removal.
The experimental results were compared to predictions of a simple dissolution-diffusion-volatilization (DDV) model to test common air sparging modeling assumptions and determine rate limiting mass transfer mechanisms. Volatilization and dissolution mass transfer rate constants were estimated for each reactor condition by fitting the model to the experimental data. Estimated dissolution rate constants (O.000396cm/min to O.00182cm/min) compared well to values calculated from empirical correlations of several other researchers (0.000285 cm/min to 0.00355 cm/min) and increased slightly with decreasing air channel spacing. Estimated volatilization rate constants (0.00133 cm/min to 0.00287 cm/min) were an order of magnitude lower than values calculated from available empirical correlations (0.00602 cm/min and 0.0390 cm/min).
The model accurately predicted aqueous phase contaminant migration for all experiments except above the NAPL source in the 30 mm air channel spacing experiments. It predicted a lower mass removal than experimentally determined when the air channels were closer than 22.5 mm from the NAPL source. A sensitivity analysis performed on the model suggested that dissolution was the rate limiting mass removal mechanism. Furthermore, the analysis suggested that as the air channels converged closer than 22.5 mm from the NAPL source, momentum transfer from the advecting air to the soil water may become an important process.
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