Mass Transfer from a Shrinking Single Microbubble Rising in Water

Microbubble aeration is utilized usefully for chemical and biological processes which consume large amount of dissolved gas in liquid because the microbubbles have long residence time in liquid, large specific gas-liquid interfacial area and fast mass transfer rate. To design the industrial process using microbubble aeration, however, the fundamental characteristic and behavior of microbubbles have to be investigated at first. In this study, therefore, both shrinking and rising behaviors of a single microbubble are simultaneously observed. A single microbubble was induced from a fine nozzle into the bottom of a tall transparent vessel filled with ion-exchanged water in which dissolved gas was reduced previously with vacuum degassing. The single rising microbubble was chased with a high-speed video camera along the vessels height. The mass transfer rate was measured from the shrinking behavior of the single microbubble which was captured and analyzed from the video image. The smaller the diameter of a microbubble, the more rapidly it decreased. Finally, the microbubble was vanished. With reducing concentrations of oxygen and nitrogen dissolved in water, the shrinking rate of a microbubble became faster. The mass transfer from shrinking microbubble of either air or pure oxygen can be evaluated by the sum of oxygen transfer and nitrogen transfer, in which each mass transfer coefficient was estimated by the equation of Ranz and Marshall. The estimated behavior of the single shrinking microbubble was agreed well with the observation. It is understood in this study that both oxygen transfer and nitrogen transfer from shrinking microbubble into water occur independently in relatively low dissolved gas concentration in water.