CO2 Mass Transfer Induced through an Airlift Loop by a Microbubble Cloud Generated by Fluidic Oscillation

Carbon dioxide is one of the most common gases produced from biological processes. Removal of carbon dioxide from these processes can influence the direction of biological reactions as well as the pH of the medium, which affects bacterial metabolism. Kinetics of carbon dioxide transfer mechanisms are investigated by sparging with conventional fine bubbles and microbubbles. The estimate of the concentrations of CO2(aq), H2CO3, HCO3-, and CO32- from pH measurement in an airlift loop sparged mixer is derived. The canonical estimate of overall mass transfer coefficient of CO2 has been estimated as 0.092 min(-1) for a microbubble size of 550 m compared with 0.0712 min(-1) for a fine bubble (mean bubble size of 1.3 mm) sparging. It is observed that the efficiency of CO2 removal has increased up to 29% by microbubble sparging compared with fine bubble sparging. Laminar bubbly flow modeling of the airlift loop configuration correctly predicts the trend of the change in overall mass transfer in both gas stripping with nitrogen and gas scrubbing for CO2 exchange, while demonstrating the expected separated flow structure. The models indicate that the macroscale flow structure is transient and pseudoperiodic. This latter feature should be tested by flow visualization, as preferential frequencies in the flow can be exploited for enhanced mixing.