Dynamic behaviour of the CO₂ bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO₂) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO₂ bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO₂ concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO₂ bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eötvös number and the behaviour of the CO₂ bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO₂ bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO₂ concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO₂ transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.