The determination of mass transfer rates from individual small bubbles

Abstract

The time-dependent mass transfer rates from individual air bubbles into water of differing quality was measured with high accuracy by a gas chromatograph. The bubbles were released at the bottom of a plexiglass column which was filled with either distilled, tap, or sewage water. They were collected at different heights in a diver and analyzed in the gas chromatograph. The necessary repeatability in the production of equally sized bubbles was achieved by a specially developed device in which a flexible membrane with a small hole is used. The bubble frequency was low enough to guarantee that the bubbles did not interact with each other. The newly developed measuring technique has the following advantages: (1) accurate measurements can be made even for gases which have a low solubility in the liquid and for small bubbles; (2) the time-dependent mass transfer rates and the associated transfer coefficients can be determined; (3) the mass transfer processes of the bubbles into an opaque liquid can be studied. To demonstrate the accuracy and the wide applicability of this novel measuring principle for studying the mass transfer of gases from individual bubbles into the surrounding liquid, several measurements are presented for different bubble sizes and liquids. In the case of sewage water only but 20% of the oxygen was transferred, while with distilled water a transfer rate of 40% was reached, with bubbles of 1.3 mm diameter at a column height of 2 m. This result indicates the importance of detailed mass transfer studies for the optimization of sewage water aeration systems. Furthermore, the resulting time-dependent mass transfer coefficients were calculated from the measured degassing rates. For the aeration of liquids with low water level (e.g. between 1 and 2 m), approximately 80% higher mass transfer coefficients were obtained, compared to the stationary value which is often used to design aeration systems.