Evaluation of mass transfer correlations for packed column air stripping of volatile organic contaminants from water supplies

Abstract

The application of packed column air stripping systems to the removal of volatile organic contaminants (VOCs) from drinking water sources was investigated. The crucial element for the design of such systems exists in obtaining accurate predictions of mass transfer rates. The first phase of this study evaluated three semi-empirical correlations available for predicting packed column mass transfer rates. From this initial screening, the Onda model was selected for further investigation. A test data base was established from water treatment pilot study results reported in the literature. Ten separate studies were selected for evaluation, encompassing approximately 450 data points. Eleven different VOCs were encountered in these investigations, and the basic packing types tested included rings, saddles, Tri-Packs, and Tellerettes. Comparison of measured mass transfer rates with the corresponding rates predicted by the Onda correlation yielded a relative standard deviation of 17%. A ± 30% accuracy value was therefore assigned to the model based on 90% confidence limits. This assessment agrees with the observed accuracy of the correlation for the chemical engineering-based system results utilized in the model's original derivation. From the overall evaluation, no severe deficiencies and/or limitations with the Onda correlation were noted. In particular, gas-phase resistance predictions appeared reasonably accurate. However, further investigative studies involving observed column performance with larger packing materials (≥2 inches in nominal size) is encouraged to verify the accuracy of the correlation for such situations.

Within the context of the evaluation procedure, several related areas were investigated. First, Henry's constant temperature relations reported in the literature were established within approximately 20% for common VOCs at low concentration levels. Secondly, the transfer unit performance model for calculating observed mass transfer rates was found sensitive to experimental measurement errors below a stripping factor of 1.5. Therefore, measured results obtained under such operating conditions must be viewed with appropriate caution. Finally, economic design boundaries for column operational parameters were established based upon optimization study results reported in the literature.