Mass Transfer Study of a Batch Aluminum Degassing Ladle with Multiple Designs of Rotating Impellers

Abstract

A mathematical model has been developed to describe the mass transfer in a batch aluminum degassing ladle utilizing the rotor-injector technique. The model uses a Eulerian algorithm for the multiphase system and a dispersed renormalization group k–ε model to account for the turbulence in the system. The model was employed to evaluate the relative performance of the degassing kinetics of four impeller designs, i.e., nozzles with and without notches, and with four and five blades. The former two designs are commercially available, while the other two were proposed by the authors of this study. The results were validated against experimental measurements of the oxygen concentration in a physical model, and the flow pattern obtained was compared with image velocimetry. The model could satisfactorily predict the liquid velocity, gas holdup, vortex size and degassing kinetics, for the first time. The local kinetic parameter $k_{\text{L}} a$ predicted using the rigid model was found to be an important characteristic in terms of the degassing kinetics and depends on the bubble size distribution and the stirring of the liquid in the vessel. The proposed impeller designs help to distribute the bubbles more uniformly throughout the ladle, increasing momentum transfer from the impeller to liquid and thus improving gas–liquid mass transfer.