Filter aid pretreatment can improve filtration properties and efficient removal of fine solids. Whether the filter aids are used in Plate-and-frame filter presses, horizontal and vertical pressure leaf filters, candle or tubular filters, Nutsche filters, or rotary vacuum drum filters, these practical tips can help this part of the process run smoothly.
We typically see diatomite, perlite and cellulose filter aids today. They meet the requirements of a filter aid in that they:
- Consist of rigid, complex shaped, discrete particles;
- Form a permeable, stable, incompressible filter cake;
- Remove fine solids at high flow rates; and
- Remain chemically inert and insoluble in the process liquid.
You’ll want to test different approaches to determine the best aid for your process and which of the methods — precoat or body feed — offers the greatest benefits. Once you’ve done so, though, it’s important to keep these troubleshooting tips in mind.
Practical Pointers for Using Filter Aids
Whether the process is precoating or body feeding, the filter aid slurry tank and pump are critical to the operation.
In precoating, the mix tank should be a round, vertical tank with a height twice its diameter. Set the usable volume of the precoat tank at ≈1.25–1.5 times the volume of the filter plus the connecting lines. Use a mixer or agitator with large slow-speed impellers to avoid filter aid degradation and the creation of fines — otherwise you’ll dramatically change the filter aid process filtration.
The precoating pumps almost always are centrifugal pumps because they produce no pulsations to disturb precoat formation and their internal parts usually have hardened surfaces and open impellers to reduce wear. For body feeding, you’ll use positive displacement pumps.
Yet even when the feed tank and pump are correct, several typical issues with filtration/filter-aid systems can arise.
Bleed-through is common where the filter aid is bypassing the filter media. It may stem from mechanical, operational or process causes. Check a couple of mechanical points:
- Is the filter medium secured to the filter correctly?
- Does the filter medium have a tear or pinholes?
- Is the type of filter aid correct for the filter medium mesh size and the particle size distribution of the process solids?
- Is the pump working correctly (flow, pressure, etc.)?
- Is the proper amount of filter aid being added?
Another issue may be reduced filtration cycles — i.e., the time to reach the maximum pressure drop becomes shorter and shorter. This may occur:
- if the cake isn’t being discharged completely, then each new batch has residual solids in the filter, resulting in lower capacities. Increasing precoat height or lengthening cake drying time may help improve cake discharge.
- if the precoat doesn’t completely cover the filter medium, then the process solids may begin to blind the medium.
- if you’re using body feed, inadequate mixing with the process solids may result in filter medium blinding. This also can happen if the velocity in the filter vessel is too low, which will allow the filter aid to settle out before reaching the filter elements. A bypass at the top of the filter vessel can help keep the solids suspended within the vessel.
On filters with vertical elements, precoat pump flowrate or pressure may cause loss of the precoat from the filter medium, Improper valve sequencing creating a sudden change in the pressure or flowrate may also be to blame. Finally, a mechanical issue with the filter may prompt a pulsation or pressure change that impacts the cake structure.
Apply Filter Aids Wisely
Employing filter aids to help filtration is tricky; most process operations try to eliminate or minimize their use. However, sometimes they are unavoidable.
To succeed with filter aids, a process engineer should take three essential steps:
- Conduct lab testing to examine the filtration operation (vacuum or pressure), cake thickness, filter aid quantities, filter medium and other parameters that are crucial to the process design;
- Ensure correct mechanical design to provide optimum precoat or body feed handling and distribution; and
- Arrange for operator training on the filtration technology as well as on filter aid operation.
This blog is an edited version of an article I co-authored with Garrett Bergquist, BHS-Sonthofen Inc. for Chemical Processing.