Experimental study of droplet-side mass transfer under slug flow in microreactors

Droplet-side mass transfer was experimentally investigated in slug flow capillary microreactors using the model system of pure ethyl acetate extraction into water. Mass transfer coefficients were found to decrease with the increasing residence time in microreactors of different diameters and lengths, consistent with the trend of the penetration theory. Moreover, mass transfer coefficients were improved at higher mixture velocities due to enhanced internal circulation and were mostly independent of two-phase flow ratio. However, deviations existed especially for cases of short droplets under extended residence times, where mass transfer coefficients could be lower than (or more precisely 0.5–1.2 times) the penetration theory predictions. In these cases, the penetration of the droplet bulk was accelerated by appreciable interaction of diffusion processes between the droplet cap and film regions, and the mass transfer enhancement by circulation had a more important role causing the outlet solute concentration to increase with the decreasing residence time in a given microreactor. Consequently, a mass transfer model was proposed based on the penetration theory incorporating separate contributions from the cap and film regions. In the model, Reynolds numbers were further introduced to account for the internal circulation enhancement and Fourier numbers were used to describe the cap-film interaction effect. In addition, this model was validated with methyl isobutyl ketone or butyl acetate as the slug phase, covering moderate variations in viscosity and interfacial tension.