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Simulation of the gas-liquid CO2 absorption in alkaline aqueous solutions from partially contaminated spherical bubbles and ellipsoidal clean bubbles

par Wylock, Christophe ;Larcy, Aurélie ;Colinet, Pierre ;Cartage, Thierry ;Haut, Benoît
Référence 10th Conference on Gas-Liquid and Gas-Liquid-Solid Reactor Engineering (GLS-10) (26-29/06/2011: Braga (Portugal))
Publication Non publié, 2011-06-26

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Résumé :

Several gas-liquid absorption processes are realized in bubble columns. It is commonly admitted that the global bubble-liquid mass transfer rate is controlled by phenomena occurring in layers close to the gas-liquid interface (diffusion, convection and possibly reactions). Therefore, a good quantification of these phenomena is required to achieve an optimization of these processes. This work deals with the mathematical modeling and the numerical study of the CO2 transfer rate from a gas bubble to an aqueous solution of Na2CO3 and NaHCO3. The main resistance to the bubble-liquid CO2 transfer is assumed to be located in a thin liquid phase layer close to the interface. The convective transport of CO2 by the liquid flow is coupled with its diffusive transport and with chemical reactions. Therefore, it is necessary to take these three phenomena into account to quantify the CO2 transfer rate. When the bubble diameter is not larger than 1 mm, the bubble shape is nearly spherical. The bubbles get the shape of an ellipsoid with the increase of their equivalent diameter. Furthermore, surfactants can be adsorbed on the bubble surface. These phenomena can have a strong influence on the mass transfer rate as they can change the flow field around the bubbles. The influences of the interface contamination state and the ellipsoidal shape on the mass transfer rate are studied using a two-dimensional (2-D) axisymmetrical model. The equations of the model are solved numerically using the COMSOL Multiphysics software. On the one hand, the case a spherical bubble with a partially contaminated interface is investigated. It is assumed that a steady stagnant cap lies on a part of the bubble surface at the rear of this bubble. Simulations are performed for several stagnant cap ratio of the bubble surface. On the other hand, the case of an ellipsoidal bubble with a clean interface is investigated. Simulations are performed for several minor axis/major axis ratio of the ellipsoidal bubble. The influences of the contamination interface state and the ellipsoidal shape on the bubble-liquid transfer rate are presented. In a first time, correlations to estimate the transfer rate without reaction are presented and compared to the 2-D model results. In a second time, the mass transfer rate is compute with chemical reactions for several reaction rates. The interactions between reaction rate and the contamination state and the ellipsoidal shape are then presented and discussed.