(558d) Design and Operation Characteristics of Liquid-Liquid Extractors with Disc Supported Taylor-Couette-Flow

Authors: 
Aksamija, E., Graz University of Technology
Siebenhofer, M., Graz University of Technology

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 of Taylor-Couette extractors 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 of Rotating Disc Contactors (RDC) led to the suggestion that the characteristics of the Taylor-Couette flow pattern can seemingly be obtained in an improved 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 the state of the art RDC design with stator discs. On the one hand compartments are separated more efficiently by enlarged rotor discs, on the other hand higher energy dissipation rate induced by enlarged rotor discs leads to narrow drop size distribution and a uniform axial velocity of the dispersed phase. The geometrical stabilization of the toroidal flow pattern minimizes vortex inversion, which is observed in state of the art RDC design for low compartment height. Vortex inversion leads to inhomogeneous hold-up distribution in the column.

Mass transfer and flooding limit experiments were performed in a pilot scale RDC of 100 mm diameter and compared with the improved design, to quantify optimization in terms of separation efficiency and throughput. The height of one transfer unit (HTU) was experimentally determined to be 40% less for the improved design, while the hydraulic load capacity was increased.

Beside lower investment cost, increased mass transfer efficiency and increased hydraulic load capacity, several operation advantages in cleaning, maintenance and assembly arise from the improved design of RDC extraction columns without stator rings.

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