Henrys Law and the Mass-Transfer Coefficient

Such processes as mechanical surface aeration, diffused aeration, spray fountains, spray or tray towers, open-channel cascades, and countercurrent packed towers are encompassed by the term air stripping. These procedures produce a condition in which a large surface area of the water to be treated is exposed to air, which promotes transfer of the contaminant from the liquid phase to the gaseous phase. This occurs because under normal conditions the concentration of the contaminant in ambient air is much lower than the concentration in contaminated water. 

The ratio of the contaminant at equilibrium in the liquid phase, C, to the contaminant in the gaseous phase, C, is a relationship known as Henry s law  

The general form of the equation for the rate of mass transfer across the gas/liquid interface in a gas stripper is (4) 

Several models exist for the characterization of gas transfer across the gas/liquid interface (5). According to the two-film model (6), laminar films exist at the gas/liquid interface. The resistance to the rate of mass transfer is given by Rj, and is estimated by summing the resistances offered by the liquid- and gas-phase boundary layers, and R, respectively (4)  

The rate constants for the local liquid and gas phase transfers, and k, respectively, are related to the overall transfer rate constant by 

---