Taylor-Couette vortices are formed in the gap between concentrically rotating cylinders. They have been subject of investigations for decades and some attempts have been made for implementation in liquid-liquid extraction. Problems with vortex instability at high axial flow rate, high angular velocity difference, large diameter difference of the cylinders and the presence of a second phase made this hydrodynamic phenomenon difficult to apply in liquid-liquid extraction.
Experimentally validated CFD-Simulations of dual phase and single phase flow patterns in the Rotating Disc Contactor (RDC) led to the suggestion that the characteristics of Taylor-Couette flow pattern can seemingly be obtained in a simplified RDC design without stator rings and appropriate rotor disc diameter. As long as the compartment height is large enough to accommodate two toroidal eddies, vortices are dynamically stabilized by the rotor discs, allowing smaller shaft diameters and larger active space in the apparatus. CFD-Simulations show less axial mixing for both phases compared to a state of the art RDC design with stator discs. One the one hand compartments are separated more efficiently by the enlarged rotor discs, on the other hand higher energy dissipation rates caused by the enlarged rotor discs lead to narrower drop size distributions and more uniform sedimentation velocity of the dispersed phase. The geometrical stabilization of the toroidal flow pattern via well placed rotor discs also supports a well-defined path of the dispersed phase, minimizing the tendency for vortex inversion, which can be observed in state of the art RDC design. Experiments in a pilot scale RDC100 confirmed that the increased stability of dual-phase flow pattern results in a broader operating range in terms of flooding limits. Mass transfer experiments quantified the optimization in terms of separation efficiency, the height of one transfer unit (HTU) was found to be significantly lower for the simplified internals geometry.
Beside lower investment cost, increased flooding limit and increased mass transfer efficiency, several practical operation advantages in cleaning, maintenance and assembly may arise from statorless design of RDC extraction columns.