Image analysis assessment of the effect on mixing on aqueous dissolution of CO2 and air bubble swarms in a bubble column

Many processes for capture and use of carbon dioxide, CO2, involve aqueous solutions containing salts, the dissolution of CO2 being rate-determining. For bubble column reactors, mixing the solution may increase the dissolution rate, reducing the necessary height. For this paper, extending an experimental cylindrical bubble tower set-up by the inclusion of mixers enables to observe the performance of multiphase bubble reactors. These experiments were done to gain deeper understanding on the dissolution and later chemical conversion of CO2 bubble swarms in agitated vessels. Bubble swarms of 100 bubbles were tracked using a high-speed camera. Comparison of the dissolution rate of bubble size distributions at different heights in a 2 m column revealed significant rates of CO2 mass transfer in contrast with hardly any change in the bubble size distribution for air. Results are consistent with the dissolution of single rising bubbles, and previous one-way coupling simulation shows a fair agreement with the experimental results. CO2 bubbles showed a smaller average size while increasing their bubble sphericity as function of height in the column. Initially, bigger CO2 bubbles present a wobbly behavior and trajectories, becoming more spherical while dissolving. The effect of the stirred environment increases as bubbles dissolve, affecting trajectories and velocities, as smaller spherical bubbles don't show a linear rising trajectory. Comparing results without and with mixing showed that increased mixing rates bring down the height needed for dissolution, especially in the middle section of the column, where bubbles are no longer wobbly yet are not yet very small. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.