Effects of bubble coalescence and breakup on CO₂absorption performance in nanoabsorbents

The complexity and dynamics of bubble coalescence and breakup have fascinated scientists for decades. In this study, we performed experiments and simulations involving successively rising bubbles in a rectangular bubble column for carbon dioxide absorption in nanoabsorbents (methanol with various concentrations of alumina nanoparticles). The variations of the bubble behavior were captured by a high-speed camera in experiments and simultaneously visualized via simulations. Firstly, we distinguished the orifice region and the bulk liquid region. The bubble diameter was determined by only the gas flow rate and orifice diameter in the orifice region; however, it changed significantly in the bulk liquid region, experiencing approximately four stages. Wake entrainment and eddy capture dominated the bubble coalescence in the bulk liquid region; however, the bubble breakup was mainly caused by eddy collision and the coalescence of two bubbles with a small surface tension force. Both coalescence and breakup were more likely to occur in liquids with a higher concentration of nanoparticles. Additionally, we considered the mass transfer coefficient in the liquid phase and found that it can be enhanced by the coalescence and breakup, through the generation of polydisperse bubble swarms. A correlated parameter was deduced that can be substituted in the Hughmark equation to predict the mass transfer coefficient of the CO2-methanol-nanoparticle system.