Filtration / Drying

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. Also, where physical disruption of the filter cake is required this can in turn damage delicate solids.　

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.

Considerable work has been carried out on applying process tomography to filtration:

• Tomography can be used to monitor and hence track the filtration process, providing a basis for optimizing process design and providing a on-line tool where precise control is required
• 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.
   
• 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 p2+ and m3cinstruments can be used for filtration monitoring, depending on the levels of dryness to be observed and whether the mother liquor is aqueous or organic. 

The above video 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.

Key benefits include:

• Characterise regional variations in drying of filter cakes.
   
• Determine structures in filter cake (eg cracks and rat holes).
   
• Determine bulk level liquor within full filter cake for use as liquid level detection. 

Batch to batch comparaison (drying process time) using Electrical Tomography

Register to access our "Online monitoring of a pressure filter vessel" case study available from the Download section on the right.

Publications:

Davidson, Ruffino, Stephenson, Mann, Grieve and York (2004) Three-dimensional electrical impedance tomography applied to a metal-walled filtration test platform, Measurement Science and Technology, Vol. 15, 2263-2274

For more information about this paper, please contact ITS.  

In the Press:

• European Pharmaceutical Review - Issue 1 2003 - Improving pressure filter performance
• Professional Engineering - Volume 16 Number 3 - FIlters check in for a scan
• Connectingindustry.com  - April 2003 - The chance to unplug your filter with tomography
• Filtration + Separation - Nov 2004 - ERT: tracking the process of pressure filtration

If you would like to receive a copy of an article, please contact ITS.