Microbubbles, oscillating flow, and mass transfer coefficients in air-water bubble columns

Mass transfer coefficient k_La determines the flux of dissolved oxygen DO in bubble columns. The k_La values can be determined experimentally or predicted using a correlation. Common correlations link k_La with superficial air velocity, air hold-up volume, bubble contact time, bubble rising velocity, or bubble Sauter diameter. We compared experimentally found and predicted k_La values to evidence that none of the above correlations is applicable for microbubbles flow. The only way to accurately predict the flow is by using mass transport models needed to calculate k_L. The transition from k_L to k_La requires a redefinition of the specific interfacial area a for microbubbles. Microbubbles distribute DO quicker than macrobubbles. Oscillating flow can raise the DO level to saturation. The generation of oscillating flow does not require extra energy turning the subsurface aeration more energy-efficient than the surface aeration.