LIQUID-LIQUID EXTRACTION BASED ON A NEW FLOW PATTERN: TWO-FLUID TAYLOR-COUETTE FLOW

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 ii of countercurrent axial flow, efficient continuous processing is also possible. It is proposed that this two-fluid Taylor-Couette 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. With this goal, the present study of two-fluid Taylor-Couette flow with countercurrent axial flow includes: • A review of aqueous-aqueous and reversed micelle extraction techniques, the commercially available centrifugal extractors, and one fluid TaylorCouette flow and its variations. • A theoretical analysis to predict the onset of the two-fluid Taylor-Couette instability in the presence of countercurrent axial flow. • Theoretical predictions for interphase mass transfer using a combination of penetration theory and computational fluid dynamics. • The demonstration of two-fluid Taylor-Couette flow with countercurrent axial flow in the laboratory, including: (1) fluid mechanics studies to determine the onset of two-fluid Taylor-Couette flow, and (2) mass transfer studies to determine intraphase and interphase mass transfer characteristics. The agreement between the theoretical analyses and the experimental results is good for both the fluid mechanics and the mass transfer. Furthermore, the iii extraction performance of two-fluid Taylor-Couette flow with countercurrent axial flow is very promising 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 or cylinder modification to promote vortex formation. Besides two-fluid Taylor-Couette flow, other instabilities can occur that degrade the extraction performance and should be avoided in the design and operation of an extractor. With low viscosity fluids at low rotation rates, twofluid Taylor-Couette flow is not observed experimentally, but rather the barber pole pattern, which is believed to be a lingering gravitational effect. At high countercurrent axial flowrates, the linear stability analysis predicts a KelvinHelmholtz instability related to the countercurrent flow profile. When axial flow is not present, two computational fluid dynamics packages calculate that vortices paired across the interface can corotate, rather than counterrotate, with each other.