Filtration
Key Benefits Include:
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characterise regional variations in drying of filter cakes
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determine structures in filter cake (eg cracks and rat holes)
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determine bulk level liquor within full filter cake for use as liquid level detection
Introduction
Filtration is used to remove particles of some form from a fluid.
For example pulp from water, impurities from drinks, waste products from chemicals and many other applications. In every case the fluid is passed though a medium whose pore size retains the particles to be removed while allowing the liquid to pass through. Operation may be by gravity or under pressure and the filter medium may range from a bed of sand, through filter paper to filter cloths and sintered metal beds. The plants may be batch operation, where the filter medium is replaced after a fixed time of drop in performance. In continuous filtration the filter medium is moved through the filter and then cleaned before being returned to operation.
As filtration occurs within the filtered medium, and often involves solids, measuring what is actually happening on the filter medium is very difficult with any conventional instruments. This tends to lead to over filtration to ensure the fluid is clean, with consequential extra costs and time involved.
Additional problems arise due to inconsistencies in the filter cake. This can lead to rat-holes and cracking (where liquor that is supposed to wash the cake short circuits through the cake). In addition, different inconsistencies in the cake can lead to over-heating which changes the particle size distribution of the materials being dried and purified.
These effects can add considerable time to the filtration process. This in turn has knock on effects both upstream and downstream from the filter. For example materials held back in solution can change their particle size characteristics which can further complicate processing.
Solution
Considerable work has been carried out on applying process tomography to filtration:
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both dielectric and resistivity can provide useful information on the bound and free water in a filtrate
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the spatial discrimination offered by process tomography can help determine wet and dry regions during filtration. In addition this information can then be used to help direct or verify sampling strategies
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tomography can also be used to determine liquor levels within a filter vessel. This can be used to help manage the overall filtration process
Both the p2000 and m3000 can be used for filtration monitoring, depending on the levels of dryness to be observed and whether the mother liquor is aqueous or organic.
Right Click to zoom
This shows the conductivity distribution through the filter cake of an industrial pressure filter as it dries over time. It can be seen that the cake dries preferentially in the centre as the image becomes blue indicating low conductivity.
References
Davidson, Ruffino, Stephenson, Mann, Grieve and York (2004) Three-dimensional electrical impedance tomography applied to a metal-walled filtration test platform, Measurment Science and Technology, Vol. 15, 2263-2274
For more information about this paper, please contact ITS.
