Design strategy of a microchannel device for liquid-liquid extraction based on the relationship between mass transfer rate and two-phase flow pattern

In this study, the liquid-liquid two-phase flow pattern and mass transfer rate in microchannels were investigated by an extraction experiment. Slug flow was observed at low flow velocities, which changed to annular and slug-annular flows at high flow velocities. The volumetric mass transfer coefficient (Ka) of the slug flow increased with increasing flow velocity; however, the flow pattern transition from slug to annular flow led to a discontinuous decline in Ka and reduced it to below that of slug flow. Since the difference in the specific interface areas (a) between the slug and annular flows was not large enough to address the difference in Ka for the slug and annular flows, it was suggested that the decrease in the mass transfer coefficient (K) was mainly responsible for the decrease in Ka with flow pattern transition. The highest Ka was obtained at the highest flow velocity within the slug flow region. Therefore, it was demonstrated that increasing the flow velocity as much as possible within the range where the stable slug flow is formed without causing the flow pattern transition from the slug to annular flows is an effective design strategy for minimizing the microchannel volume.