Experimental study of the contribution of liquid film for liquid-liquid Taylor flow mass transfer in a microchannel

Liquid-liquid two-phase Taylor flow mass transfer in microchannels, which has many advantages for heat and mass transfer operations, should be studied to develop practical applications in various chemical processes. To understand the effects of various parameters on mass transfer and develop an estimation method of mass transfer performance, mass transfer in microchannels was examined by an evaluation of phenol extraction from dodecane to water. Under stable Taylor flow conditions in a 1 mm inner diameter glass channel, the organic phase (dodecane) formed liquid droplets and the aqueous phase (water) formed liquid slugs with a liquid film enclosing the organic phase droplets. The effect of the droplet length on mass transfer was investigated with a microchannel device that consists of a circular channel with a 1 mm inner diameter and union tees with inner diameters of 2.0 mm and 2.4 mm. Although the droplets lengths were varied in the range of 7.4 mm to 30.5 mm, approximately the same volumetric mass transfer coefficients (Ka) were obtained. This result suggested that the side interface of the droplets that contact the thin liquid film becomes a mass transfer interface as well as an end interface. This occurs because the side interface area increases with increasing droplet length and droplet volume while the specific interfacial area remains approximately constant. The volumetric mass transfer coefficients were obtained for various cross-sectional shapes and hydraulic equivalent channel diameters at various flow velocities. The mass transfer coefficients were derived by dividing the volumetric mass transfer coefficients by the specific interfacial area. It was found that the Sherwood number (Sh), which is a dimensionless expression of the mass transfer coefficient, has excellent correlation with the Reynolds number (Re); thus, the estimation of the mass transfer performance for a given channel and given operational conditions can be determined using the correlation between Sh and Re.