Bubble dynamics underneath a downward facing anode in Hall-Heroult cell.
1. Introduction
The basic characteristics of the Hall-Héroult cell have not changed much in the last century. However, the technology breakthrough enables to push their cells to work at higher amperage i.e. nearly at maximum current density. At such current density, there is strong interplay between Magneto hydrodynamics MHD forces (stability), convective and conduction mechanism (Ledge thickness) and bubble induced velocity of metal. It has been already argued that the complexity of the governing phenomena necessitates a simplified approach in the analysis of the process. It is for this reason, a consensus is made to tackle the above problem in a systematic manner.This process employs a cell comprising a vessel or pot containing a molten electrolyte bath comprising sodium cryolite (Na3AlF6) as the principal constituent. The interior of the vessel is lined with carbon. A pool of molten aluminium lies on the bottom of the vessel and forms the cathode for the cell, and consumable carbon anodes located above the electrolyte bath extend downwardly through the top of the electrolyte bath. Alumina is introduced into the molten electrolyte bath wherein the alumina dissolves and a number of reactions occur, eventually producing molten aluminium which accumulates at the bottom of the vessel and carbon dioxide, and some carbon monoxide from a side reaction, which are given off from the top of the cell.
The behaviour of bubble in liquids is of great interest and plays a significant character in various fields such as waste water treatment, oil processing, fermentation, electrolytic cells, mixtures processing, steaming, plastic foam processing, and environment remediation. It is believed that the rheological properties (viscosity, surface tension, density etc.) depend on the shape, volume and terminal velocity of bubbles to be of fundamental importance in the design of such multi fluid systems. In particular, the improvement of bubble induced flow rise by bubble distortion and its path motion is of applied interest in aluminium reduction cell of Hall-Héroult Process. However, in spite of its important industrial relation, many important hydrodynamics and heat transfer phenomena linked with bubble flow, such as bubble formation, bubble cohesion, bubble break-up and bubble wake impact the overall convective mechanism and are still not fully understood.
Bubble flow in aluminium cells is a complex phenomenon due to various operators such as MHD forces, current density, surface tension, voltage fluctuations, buoyant field, shape of anode; temperature gradient which leads to convective forces, Anode Cathode Distance ACD and the electrolysis process is the most important factor. The bubbles escape in the path with horizontal movement near by the edge of the anode. Current density leaded to raise the Bubble inhabitant's density under the anode surface. Occasionally, when the gas-hold up rate goes higher the bubble coverage rises which leads to minimise the local current density flow which increases with the available surface area of the anode.Higher current density has various effects such an unexpected progress of thin bubble layer nearby the anode 1, MHD instabilities due to complex local perturbations 2, joule heating and changes in the wettability of the electrode 3.However, the accurate mechanism leading to the formation of this thin layer may be different from case to case.
2. Project definition
2.1 Key objective/Issue(s) to be investigated
The main objective of this project is to study and analysis the electrolytic ally generated bubble and their morphology in Hall-Heroult cell process. From cost point of view reducing bubbles underneath a downward facing anode will help the current to flow through the liquid. However, in this report the morphology of the bubble in the process is going to be the interest point. High speed camera will be one of the ways to analyse random behaviour of the bubble. Using experiment and ANSYS model will help to get better results for mathematical analysis.That will clear the idea of formation bubble under the anode in Hall-Heroult process. Furthermore, the insulation and how the process covered make is the formation of the bubble in Hall-heroult cell invisible issue, which make understanding bubble behaviour too complex. In sum the objectives are:
• Experimental using Hall-heroult process.
• Analysis by using High Speed Camera and ANSYS Model.
• Founding a relation between the experimental and expected data.
2.2 Project Benefits
Knowing bubble morphology behaviour is a very important step which lead to analyse and improve the process, hence increasing the current efficiency by minimise bubble shapes.By expanding mathematical area in bubble morphology field is going to be an inconceivable and useful idea as it is going to be supported with ANSYS model optimisation.The output benefit will be massive from financial aspect by controlling the effect of bubbles. Therefore, to improve the electrolysis process of aluminium melting takes the excessive amount of money it involve so much of cost so even 10% improvement in the process will save millions of dollars.
2.3 Project Deliverable
The project attempts to deliver the following outcomes:
• Increasing the current efficiency in Hall-Heroult process by reducing the bubble shape.
• Bubble characteristics: shape, size and density in the process.
• Mathematical analysis based on experiments determines the morphology of the bubble.
• Bubble modal by using ANSYS and COMSOL.
• Binding capacity of Carbon.
3. Literature review/Research
3.1 Rational and Significance of the study
• The electrolysed process of Hall-Heroult cell for product aluminium.
• Bubble will create in the carbon anode with the high temperature which affects the current to flow.
• The chemical reaction between oxygen and carbon in the anode where lead to change the shape of the anode and then bubble scape inside this gaps.
• How the magnetic field created from the current flow through the cathode which leads to mix the alumina with the cryolite effects by LORENTZ force.
3.2 Brief research strategies
• Understanding project projective.
• Investigating key issues and benefits of project.
• Having a solid background about the project using Deakin Library website, Google Scholar, Books and documents provided by the supervisor.
• Organising and scheduling the project using Gantt chart e.g.(diary, logbook, outline and end date)
• Reading though some research papers and getting more information about latest progress on galvanic corrosion of magnesium alloys and steel.
• Gaining good information about recent studies.
• Improving gained information by advanced research.
• Having a weekly meeting with the supervisor with the update.
• Writing down brief literature review.
• Developing and expanding literature review.
• Identifying suitable materials for the experimental side of the project e.g.(carbon, copper sulphate)
• Knowing experimental procedure and being aware of safety issue might happen during the experiment.
• Having a session and practicing on equipment used in the experiment.
• Analysing whole geometry of the model.
• Using ANSYS modelling for more data analysis and clear understanding.
• Using COMSOL softer ware for getting better results in the experiment.
• Analysing the result using mathematical model.
• Matching predicted data with final result.
• Doing the final analysis.
• Writing final report.