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The current design of mass transfer systems for gas bubbles absorbing into a liquid is mainly restricted to the use of empirical relations which involve a high level of uncertainty. This is due to a lack of understanding of the interactions of gas bubbles and the liquid phase, and of how this affects the mass transfer. This work set out to enhance our understanding of the mass transfer of CO2 from concentrated sources such as flue gases into the aqueous phase, for use in applications such as micro-algal biomass cultivation systems.Bubble characteristics were observed using high speed imaging for single bubbles and optical fibre sensors for bubble swarms. These techniques were combined with gas chromatographic analysis of input and output gas samples to obtain a mass balance and measurements of the mass transfer. The mass transfer rate in bubble swarms was observed to be greater than that of single bubbles. For larger bubble sizes, this is partly due to the increased bubble rise velocity in bubble swarms. This was observed to increase, in part, due to the reduced drag a bubble experiences when it follows in the wake of a preceding bubble. Smaller bubbles within bubble swarms did not experience the same inhibition of mass transfer as was evident for single bubbles. This inhibition of the gas-liquid interface of single bubbles is due to the accumulation of surfactants which attach to the bubble surface, transforming the properties of the gas-liquid interface and reducing the mass transfer rate.The reduced mass transfer in single bubbles compared to bubble swarms was more apparent at lower input concentrations of CO2. This suggested a possible reduction in the internal circulation within the bubble, due to surfactant accumulation which reduces the gas-side resistance to mass transfer and is more apparent at a dilute gas concentration. Finally the experimental results from this work were compared with a simple finite difference model which analysed the mass balance of a rising bubble. The mass transfer coefficient of single bubbles with a mobile gas-liquid interface could be approximated by the penetration theory of Higbie (1935), while with sufficient surfactant accumulation to transform the bubble surface to an immobile gas-liquid interface the rigid particle theory by Frossling (1938) provided a good approximation. In bubble swarms, however, the theory for a mobile gas-liquid interface based on Higbie (1935) provided a reasonable approximation throughout the range of bubble sizes studied in this work. |
