(480d) Progress in RDC Design

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 done for implementation in liquid-liquid extraction.

Problems with vortex instability at high axial flow rates, high angular velocity differences, high diameter differences of the cylinders and the presence of a second phase made this hydrodynamic phenomenon appear unsuitable for liquid-liquid extraction.

Experimentally validated CFD-Simulations of phase flow and 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 discs 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 axial velocity of the dispersed phase. The geometrical stabilization of the toroidal flow pattern with the rotor discs also induces a straight path of the dispersed phase, minimizing the tendency for vortex inversion, which can be observed in state of the art RDC design.

Mass transfer experiments were performed in a pilot scale RDC100 to quantify optimization in terms of separation efficiency. The height of one transfer unit (HTU) was found to be 40% lower for the simplified internals geometry.

Beside lower investment costs and increased mass transfer efficiency several operation advantages in cleaning, maintenance and assembly may arise from statorless design of RDC extraction columns.