Mass transfer properties in a bubble column associated with micro-bubble dispersions

• Dispersion of micro-bubbles with diameters<50 μm verifies a superior gas-absorption performance.
• The self-compression and shrinking effects enhanced the gas absorption from the micro-bubbles.
• The values of kLa were well predicted by the complete absorption model at high dispersion heights.
• Absorption efficiency can be correlated well with the ratio of dispersion height to linear gas velocity.
• Specific interfacial areas were extremely greater than those for conventional dispersion systems.

The goal of this study was to investigate the gas holdup, bubble size distribution, and Sauter mean diameter for oxygen micro-bubble dispersions in water in an acrylic-acid resin column with an inner diameter of 0.15 m, and with a working liquid height varying from 0.500 to 1.850 m. The micro-bubbles, which varied in their Sauter mean diameters from 32 to 40 μm depending on the gas velocity, were employed to measure their superior mass transfer properties, which are enhanced by the effects of self-compression and shrinking. The apparent liquid-side mass transfer coefficient, kLa, in the micro-bubble column was also measured using the transient absorption from oxygen micro-bubbles into degassed water or into nitrogen-desorbed water. The obtained values of kLa for oxygen absorption into the degassed water were significantly greater than those for absorption into nitrogen-desorbed water. The kLa values for the degassed water were represented well by the complete absorption model, and generally increased with increasing gas flow rate. It was found that the oxygen absorption efficiency, which was defined by the ratio of the absorption rate to the supply rate of oxygen, decreased with increasing gas flow rate and increased with increasing liquid depth. The oxygen absorption efficiency could be described well by an empirical correlation in terms of the ratio of the liquid height to the superficial gas velocity, h/UG, or in terms of the ratio of the liquid height to the linear gas velocity, h/(UG/εG).