By Curtis Marsh, Aughinish Alumina, Aughinish Island, Ireland

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Aughinish Alumina, a subsidiary of Glencore, is an alumina refinery situated on Aughinish Island between Askeaton and Foynes, on the west coast of Ireland. Aughinish Alumina uses the Bayer process to produce over 1.4 million tonnes per annum of alumina (Al₂O₃), a substance that is used in the production of aluminum and also in industrial ceramics. In the Bayer process, bauxite ore is ground up and dissolved in sodium hydroxide at high temperature and pressure, forming a liquor of sodium aluminate. The liquor is placed in a precipitator and seeded with alumina hydrate particles. Crystallization takes place, and larger particles are formed that are subsequently separated from the liquid. The final product is formed by calcining the alumina hydrate at very high temperatures to remove the water molecules, producing the end product of alumina.

Contours of density for a gas/oil-propane-coal (liquid-gas-particle) mixture in a separator

A joint University-Industry research project has recently been announced, where Aughinish Alumina will invest ¤5.2m over the next five years in research at the University of Limerick. The grant will support 32 researchers in a number of programs, including one CFD program. The CFD program has been in progress for the last two years and currently includes five researchers.

During the dissolving phase of the Bayer process, the sodium hydroxide and bauxite are heated to 250°C at a pressure of 5,000 kPa. In order to recover the thermal energy from the liquid, it is passed through a series of flash tanks where the excess energy in the superheated liquid is “flashed" off as steam. The quality of the steam drawn off the top of the flash tank depends on how effectively the liquid droplets are separated from it. Steam quality is an important factor in the rate of scale build-up on downstream piping and equipment, and scaling affects the performance of the heat exchangers used to recover the energy from the steam.

The flow of the droplet-laden vapor into the flash tank was investigated using the Eulerian multiphase model in FLUENT, in a transient simulation with relatively small time steps. The model contained approximately 700,000 tetrahedral cells and took several days to converge to a stable solution. The results gave new insights into the interaction between the inlet jet and liquid surface, and demonstrated the effects of operating at different liquid levels in the tank. The results also highlighted the importance of the internal geometry of the inlet tee, and how the direction of the vapor jet affects the velocity and turbulence in the upper sections of the flash tank. The vertical rise velocities in the upper section of the vessel showed regions of high velocity, a pattern that leads to poor separation of the liquid droplets. Alternative designs were modeled, and the improvements in the velocity profile in the upper section of the vessel indicated that improved separation and cleaner vapor can be achieved.

The approach is now being applied to different flash tanks with different internal vessel geometries. This will allow the optimum operating levels for liquid separation to be identified and will also enable new designs to be optimized prior to installation. The discrete phase model (DPM) in FLUENT is also being used in conjunction with the Eulerian model, allowing for the erosion rates from entrained sand particles to be predicted. This will allow operating conditions with potentially high erosion rates to be identified before the actual equipment experiences any premature wear.

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