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Microbubbles increase the mixing efficiency in airlift bioreactors. Dispersal of gas phase throughout the ALR occurs with decreasing the bubble size. Phase slip velocity decreases with smaller bubble size as gas rise rate decreases. a b s t r a c tAirlift bioreactors can provide an attractive alternative to stirred tanks, particularly for bioprocesses with gaseous reactants or products. Frequently, however, they are susceptible to being limited by gas-liquid mass transfer and by poor mixing of the liquid phase, particularly when they are operating at high cell densities. In this work we use CFD modelling to show that microbubbles generated by fluidic oscillation can provide an effective, low energy means of achieving high interfacial area for mass transfer and improved liquid circulation for mixing.The results show that when the diameter of the microbubbles exceeded 200 mm, the "downcomer" region, which is equivalent to about 60% of overall volume of the reactor, is free from gas bubbles. The results also demonstrate that the use of microbubbles not only increases surface area to volume ratio, but also increases mixing efficiency through increasing the liquid velocity circulation around the draft tube. In addition, the depth of downward penetration of the microbubbles into the downcomer increases with decreasing bubbles size due to a greater downward drag force compared to the buoyancy force. The simulated results indicate that the volume of dead zone increases as the height of diffuser location is increased. We therefore hypothesise that poor gas bubble distribution due to the improper location of the diffuser may have a markedly deleterious effect on the performance of the bioreactor used in this work.

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 CO3 2– 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.

In this study, the effect of microfluidic microbubbles on overall gas-liquid mass transfer (CO 2 dissolution and O 2 removal) was investigated under five different flow rates. The effect of different liquid substrate on CO 2 mass transfer properties was also tested. The results showed that the K L a can be enhanced by either increasing the dosing flowrate or reducing the bubble size; however, increasing the flow rate to achieve a higher K L a would ultimately lower the CO 2 capture efficiency. In order to achieve both higher CO 2 mass transfer rate and capture efficiency, reducing bubble size (e.g. using microbubbles) has been proved more promising than increasing flow rate. Microbubble dosing with 5% CO 2 gas showed improved K L a by 30% -100% across different flow rates, compared to fine-bubble dosing. In the real algal culture medium, there appears to be two distinct stages in terms of K L a, divided by the pH of 8.4.

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