Optimising the design of agitated tanks for mitigating scale formation

Davoody, M 2019, Optimising the design of agitated tanks for mitigating scale formation, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Optimising the design of agitated tanks for mitigating scale formation
Author(s) Davoody, M
Year 2019
Abstract Scale build-up in slurry vessels in mineral processing costs the industry millions of dollars in the form of increased capital expenditure, reduced capacity, and production loss during the de-scaling operations. Majority of the available literature on descaling is focusing on the applications of antiscalants in the removal of scale. Very few studies have been devoted to preventing scale formation through optimal design and operation of stirred vessels mostly due to the lack of information on scale formation in various regions of the tank.

A qualitative and quantitative investigation on the formation of scale in mixing tanks was carried out in this work. A purpose-built tank that could be disassembled into nine segments (including a base, four walls, and four baffles, or four blanking pieces) was fabricated to facilitate the scale thickness measurement in various sections of the tank wall including the critical regions such as the impeller zone. A chemical reaction system involving calcium hydroxide, sodium carbonate and magnesium chloride reacting at 80°C was employed to form the scales on the tank wall. Experiments were carried out using an A310 impeller at different impeller speeds and reaction times under baffled and unbaffled conditions. The scale grown on the tank wall was physically scanned using a Coordinate Measuring Machine (CMM) for measuring the scale thickness at several locations. The CMM readings were then used to plot 3-D graphs of scale thickness distribution on the tank walls.

It was noticed that the overall mass of scale decreases with an increase in the impeller speed. At the same time, the average scale thickness near the liquid surface at the higher impeller speed increases, implying that the scale build-up near the liquid surface becomes denser as the impeller speed increases. Running the experiments for a longer time leads to the accumulation of a larger amount of scale on the wall-segments.

Under both baffled and unbaffled conditions, the bottom region of the reactor was almost free of scale while the top region had a  quite noticeable amount of scale. The results also suggested that, for a similar impeller speed, removal of baffles can lead to a significant reduction in scale thickness on the walls of the mixing tank. Overall, the present work established that the approach used in this work could be successfully applied to quantify the scale thickness distribution in mixing tanks. Besides, the results indicated that the scale formation and growth in stirred reactors could be influenced by modifying the liquid flow patterns within the vessel appropriately.

The scale thickness values measured indicated that scale growth occurs mainly in regions above the impeller zone, closer to the liquid surface. It was also noticed that the scale grown on the walls were uneven and asymmetric. To obtain further insight, CFD modelling was carried out to determine the transient and average liquid flow fields in the vessel. The pattern of scale deposition could then be interpreted in terms of the prevailing liquid flow pattern. The scale formation was found to occur at the highest rate on the wall sections where the near-wall liquid velocities are the least. The scale build-up was more prominent in the top left-hand corner of each wall-segment on the downstream side of the baffle. This region corresponds to a zone of weak liquid flow, according to the CFD model, where weak vortices form, thereby providing a better environment for the scale growth. CFD simulations also indicated that the significant reduction in scale growth in the unbaffled tank could be attributed to the increased liquid flow velocity near the wall due to the removal of baffles.

The scale growth and measurement approaches adopted in this work were also extended to investigate the scaling problem in neutralisation tanks employed in a mineral processing site in Australia. Test samples of the overflow liquor and the limestone supplied by the industry were used in the laboratory tests to grow the scale. The tests indicated that an impeller generating swirl flow leads to thinner scale compared to A310 impeller. The swirl-flow generating impeller was employed in a full-scale neutralisation reactor for mixing. After a few months of operation, the amount of scale formed in the reactor was found to be significantly lower compared to that obtained using a conventional impeller. This finding validates the scale growth and measurement approach proposed in this work as an efficient and reliable approach to study scale growth occurring in reactors used in process industries, including the mineral processing industry.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Chemical Engineering not elsewhere classified
Chemical Engineering Design
Keyword(s) Scale formation
Coordinate Measuring Machine
Fluid mechanics
Unbaffled tanks
Scale quantification
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Created: Mon, 02 Mar 2020, 09:41:42 EST by Adam Rivett
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