Sizing-up effect on the flow pattern and mass transfer of gas–liquid-liquid three-phase flow in microchannels

•Gas agitation signally intensifies the liquid–liquid mass transfer in microchannels.•Emulsification become more significant in liquid plug in wider microchannels.•Inverse effect of sizing-up appears at low and high flow velocities for mass transfer.•Mass transfer coefficient model is developed considering microchannel diameter. One of the important strategies for the scale-up of microreactors is sizing-up, which is conducted by increasing the hydrodynamic diameter of microreactors. However, the interphase mass transfer deteriorates seriously in the sizing-up. This work aimed to probe the possibility of adding an inert gas phase to offset the adverse effect of microreactor sizing-up on the mass transfer between two immiscible liquid phases. Using a high-speed camera, four flow patterns were observed in three capillaries with their diameters ranging from 0.8 to 3.0 mm. Empirical equations were given to describe the flow-pattern transitions. The influencing mechanism of the capillary diameter on the liquid–liquid mass transfer was analyzed by taking the effect of adding the inert gas phase into account. Finally, the evaluation of the energy consumption suggested that adding an inert gas phase to agitate the flow could utilize the input energy more efficiently to intensify the liquid–liquid mass transfer in the microchannel with a larger hydrodynamic diameter. Therefore, the method of inert gas agitation is a meritorious assistive technology in the sizing-up of microreactors.