Gas absorption and reaction in a wet pneumatic foam

Abstract

Results for the rate of absorption of carbon dioxide gas into a pneumatic (i.e. continuously rising) foam formed from a sodium carbonate–bicarbonate buffer solution stabilised by a polyglycol frother are presented. Previous studies upon the use of gas–liquid foam for gas absorption operations have demonstrated limited potential for the technique, mainly because the liquid films rapidly become saturated with the absorbing gas due to the relatively low liquid fraction within the foam. However, by adding washwater to the free surface of the foam we created a wetter and more stable foam that (1) exhibits a high liquid fraction and therefore avoids film saturation, and (2) creates greater liquid advection past the gas-liquid surfaces, thereby enhancing the mass transfer coefficient. The interfacial area achieved within the foam was in the range 2100–3200m2m−3 (i.e. much higher than in conventional gas-liquid contactors and the liquid-side mass transfer coefficient was the same magnitude as that achieved in packed beds (i.e. 4–8×10–5ms−1). Indeed, by modelling the foam as a packed-bed of solid spheres (with no adjustable constants), the mass-transfer coefficient in a bed of closely packed solid spheres was found to be in satisfactory agreement with the mass-transfer measured in the wet gas-liquid foam. The hydrodynamic state and the slip velocity between gas and liquid phases is required for the design of wet foam absorption columns, and this can be estimated using a theory of pneumatic foam (Stevenson, 2007).