(83b) Mass Transfer during Taylor Flow in Microchannels with and without Chemical Reaction

The characteristics of mass transfer from gas to liquid during Taylor flow in capillaries smaller than 1mm in diameter with and without chemical reaction were investigated using Computational Fluid Dynamics modelling. The effects on mass transfer of tube size, bubble velocity and bubble geometry, such as bubble diameter (or thickness of film between the bubble and the tube wall) and bubble/slug lengths, were studied. Separate gas and liquid domains were solved and the interfacial concentration profile was updated so that the concentration gradients caused by the gas mass transfer were better addressed than in the single (liquid) domain module [1]. The second order fast reaction of CO2 absorption in NaOH aqueous solution was modelled. It was found that mass transfer improves with decreasing channel size and increasing bubble velocity, as a result of decreasing diffusion path length and enhancement of liquid convection respectively. Mass transfer is also improved when liquid film at the wall thickens, because this increases the volume of the film region, i.e. increases its capacity to take-up CO2. In both cases with and without chemical reaction, increase of the liquid slug length reduces mass transfer because the liquid circulation frequency is decreased in long slugs. On the other hand, bubble length has an opposite effect on mass transfer in both cases. When chemical reaction takes place, longer bubbles contribute to larger contact areas along the film region thus improving mass transfer. In the absence of reaction, however, the increased amount of gas transferred species is not consumed and saturates the film region quickly, thus preventing any further mass transfer. The simulation results also reveal the existence of concentration gradients within the gas bubbles. This results in interfacial concentration lower than the average gas phase concentration which also affects the rate of mass transfer from gas to liquid.

Keywords: Taylor flow, mass transfer, microchannel, chemical reaction, CO2 absorption

[1] Vandu, C.O., Ellenberger, J., and Krishna, R., 2005. Hydrodynamics and mass transfer in an upflow monolith loop reactor. Chemical Engineering and Processing 44, 363-374.