Gas-liquid-liquid slug flow and mass transfer in hydrophilic and hydrophobic microreactors

Gas-liquid-liquid three-phase slug flow was generated in both hydrophilic and hydrophobic microreactors with double T-junctions. The bubble-droplet relative movement and the local mass transfer within the continuous slug and the dispersed droplet were investigated. It was found that bubbles moved faster than droplets under low capillary number (Ca), while droplets moved faster upon the increase of Ca due to the increased inertia. For the first time, we observed that the increased viscosity of droplets fastened the droplet movement. The mass transfer in the continuous slug was dominated by convection, leading to nearly constant global mass transfer coefficient (k_La); while that in the dispersed droplet was dominated by diffusion, resulting in k_L decreasing along the channel. Such features are analogical to the corresponding gas-liquid or liquid-liquid two-phase slug flow, but the formation of bubble-droplet clusters caused by relative movement lowered the absolute mass transfer coefficient. These results provide insights for the precise manipulation of gas-liquid-liquid slug flow in microreactors towards process optimization.