The exploitation of flow instabilities that can occur in rotating flows is investigated as a new approach to liquid-liquid extraction. Two immiscible liquids are radially stratified by centrifugal force in the annulus between corotating coaxial cylinders. When the inner cylinder is then rotated above a critical speed, Taylor vortices form in one or both of the fluids. Although the flow pattern yields a relatively small amount of interfacial surface area, the surface is highly active for interphase mass transfer due to the local vortex motion. With the addition of countercurrent axial flow, efficient continous processing is also possible. It is proposed that this flow yields a viable extraction process, particularly for fluid pairs that are easily emulsifiable and therefore have limited processing options with the current commercially available equipment.

The present study demonstrates that two-fluid Taylor-Couette flow with countercurrent axial flow is achievable in practice and explores, experimentally and computationally, the mass transfer characteristics of the flow. Experimentally, we observe that when the vortices first appear, axial dispersion decreases and the interphase mass transfer starts to increase. Upon further increase in differential rotation rate, the extraction performance continues to improve, with the mass transfer coefficient proportional to the strength of Taylor vortices. This suggests that very high extraction efficiencies can be obtained with even larger relative rotation rates. Furthermore, mass transfer boundary layer theory, in combination with computational fluid dynamics, provides a reliable method to predict the extraction performance.