Mixing toplogy of microchannel plug flow with an Ionic Liquid, experimental and computation investigations.*

Small channel contactors alongside alternative solvents such as Ionic Liquids (IL) offer a novel double process intensification for mass transfer applications. The superior mass transfer in part due to intensified mixing in a plug flow regime (Angeli & Gavriilidis, 2008). However, the comparatively high viscosities of IL are observed to alter flow hydrodynamics with potential mixing limitations (Balestra et al., 2018). Here we investigate experimentally with Bright field Particle Image Velocimetry (PIV) and three-dimensional solver that uses a high-fidelity hybrid Front-Tracking/Level-Set for treating the interface the mixing topology of an IL in small channels with increased mixture velocity.

3 vortices were seen, a dominant central vortex and secondary vortices at the front and rear of the plug. The structure, vorticity and location of which all altered with increase in mixture velocity from 0.01 – 0.05 m/s (Capillary number = 0.005 – 0.15). The central primary vortex increased in length and vorticity tending to total dominance of the plug area. The front vortex exhibited consistently lower vorticity than the rear with predicted total loss of the vortex at Ca = 0.24. The rear vortex maintained higher vorticity but reduced in size with predicted total loss at Ca = 0.19. The loss of the rear vortex aligned with the predictions of (Balestra et al., 2018) however we predicted additional earlier loss of the front vortex. This reduction in secondary vortices in part explains the reductions in mass transfer seen by (Pheasey & Angeli, 2023; Vereycken et al., 2022) where, in spite of an increase in interfacial area changes in radial mixing impeded increase in mass transfer.