A penetration model description for liquid–liquid mass transfer under slug flow in microreactors

• Overall mass transfer coefficient in slug flow declines at a longer residence time. • A modified penetration model was developed to describe mass transfer coefficient. • The model includes acceleration factors to represent internal circulation effects. • Longer slugs and shorter droplets contributed more to mass transfer acceleration. • Acceleration factor is analyzed by circulation vortex geometry in droplets or slugs. Liquid-liquid mass transfer characteristics were investigated under slug flow through capillary microreactors via experiments with several physical extraction systems. In most experiments, acetanilide was extracted from water to octanol in a polytetrafluoroethylene microreactor. The influence of capillary inner diameter, phasic flow ratio, and diffusion/partition coefficient (by using heptanol and decanol as the alternative solvent) on the extraction performance was studied in order to develop a mass transfer model applicable under a wide range of operating conditions. The measured physical mass transfer coefficient can be described by the penetration theory, with the inclusion of two acceleration factors based on the mass transfer enhancement by internal circulation inside the aqueous droplets and organic slugs. In the slug, the factor increased with the increasing slug length. However, an increasing droplet length resulted in a decrease of the acceleration factor for mass transfer, which is explained by a more localized inner circulation intensity that enhanced mass transfer in the droplet. The values of the two acceleration factors can be approximated from the increased fictitious interfacial areas within droplets and slugs, depending on the effective inner circulation patterns therein. The developed mass transfer model predicts the experimental results well under all conditions tested, and was further validated in a glass microreactor with inverted slug and droplet phases.