Effects of droplet length and inner circulation on mass transfer in liquid-liquid slug flow microreactors: experimental study and penetration theory-based modelling

The mass transfer characteristics and internal circulation patterns during liquid-liquid slug flow through capillary microreactors were investigated experimentally, using both physical and reactive extraction of acetic acid into aqueous phases as a model system. Four inlet mixing junctions of varying sizes were employed to generate droplets of different lengths under the same phasic flow rate and constant aqueous to organic volumetric flow ratio at 1:1. The results of physical extraction show that the overall mass transfer coefficient decreased with increasing the residence time, in qualitative agreement with the penetration theory. Furthermore, the overall mass transfer coefficient increased for shorter droplets or higher mixture velocities at a constant residence time. These observed dependencies can be attributed to two principal factors: (i) different internal circulation patterns being stronger and more uniform in slugs, but weaker and more length-dependent in droplets, as quantitatively characterized through color visualization in reactive extraction experiments; and (ii) more limited mass transfer from the liquid film region for longer droplets, caused by less sufficient supply of solute from the surrounding slugs. Consequently, a mass transfer model based on the penetration theory was proposed to well describe the physical extraction data. In the model, stagnant slugs and droplets that incorporate distinct transfer mechanisms for the droplet cap and film regions are assumed, with the Reynolds number further used to characterize the mass transfer enhancement by internal circulation. • Average mass transfer coefficient drops at lower velocities or longer residence times. • Mass transfer rate is low for long droplets due to insufficient film replenishment. • Internal circulation is strong in slugs, but weak and length-dependent in droplets. • A slug flow mass transfer model based on the penetration theory was proposed. • The model accounts for mass transfer in cap/film regions and circulation effects.