The Effect of Detaching Bubbles on Aluminum-Cryolite Interfaces: An Experimental and Numerical Investigation
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G the operation of commercial aluminum reduction cells, gaseous bubbles will form underneath the anode, setting the molten constituents of the cell in motion. In addition to bubble induced circulation in the side channels and interpolar regions of the cell, detaching bubbles from the anode are known to induce perturbations on the aluminum–cryolite interface. Deformations and instabilities, especially vertical oscillations, are important when considering the overall energy efficiency in the cell.[1] Parasitic side reactions such as the reduction of sodium also are known to play a role in the loss of current efficiency. Haarberg et al.[2] studied the effect of gas-induced deformations for this particular process, concluding that the current efficiency decreased when the interface deformation was accounted for. Whatever the cause, it is likely that reduced interfacial stability increases the loss of mechanisms and lowers the overall current efficiency.[3] As pointed out by Fortin et al.,[4] interfacial instabilities tend to increase as the interpolar distance decreases. An increasing demand for aluminum products as well as increasing costs for energy, forces the aluminum industry to push the Hall–He´roult process to its limits by reducing the interpolar distance. In this limit the effects of bubble-induced flow become important, and a KRISTIAN ETIENNE EINARSRUD, Ph.D. Student, is with the Department of Energy and Process Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Contact e-mail: [email protected]. Manuscript Submitted June 2, 2009. Article published online March 2, 2010. 560—VOLUME 41B, JUNE 2010
greater understanding of the deformations of the interface is essential in order to improve cell performance, motivating the present work. Two driving forces act on the electrolyte in reduction cells, and practical models often are based on the judicious decoupling of some parts of the physics, treating the gas-driven flow and magnetohydrodynamics (MHD) as separate phenomena. The importance of this uncoupling can be checked and rectified to some extent a posterior.[5] Numerical and experimental studies on a stationary two-dimensional, two-fluid model were carried out by Solheim et al.,[6] who concluded that gas-induced circulation is of the same order of magnitude as magnetically induced convection. The work of Solheim et al.[6] was generalized further by Haarberg et al.,[2] allowing for deformation of the bath–metal interface, who observed stationary deformations with amplitudes up to 40 mm. The effect of MHD, however, was not included in these studies. Increasing computational resources have given researchers the possibility to investigate numerically the joint effects of gas- and MHD-driven flows. An early three-dimensional stationary study was carried out by Bilek et al.[7] As well as studying the joint effects of gasand MHD-driven flows, Bilek et al.[7] compared each driving force individually, concluding that the bubbledriving force dominates mixing and convective transport. A similar
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