Troubleshooting Filter Aids and Filtration Systems

 

filter aids
Cellulose filter: Imerys Filtration Minerals Inc.

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:

  1. 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;
  2. Ensure correct mechanical design to provide optimum precoat or body feed handling and distribution; and
  3. 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.

Summer Workplace Safety & Testing Assumptions

workplace safety
Image source

Summer is here! That means swimming, barbecues, and watermelon. I’ve got to admit, though, I’ll be looking at watermelons a lot differently this season. 

Recently, I came across a Black & Veatch video illustrating the importance of wearing your hardhat. They did it by demonstrating structural bolt falling from 20 and 30 feet onto a watermelon. 

While physics is not my primary background, I thought it would be interesting to share Rhett Allain’s discussion of the video’s science.

Allain notes he’s skeptical of the video’s claim that the one-pound piece will have an impact force of about 2,000 pounds when it collides after falling 20 feet. He notes “it’s really difficult to calculate the impact force for a couple of reasons”: impact force is typically not constant plus impact force depends on the stopping distance. 

He suggests instead that the falling bolt problem is a “perfect situation in which to use the work-energy principle.” He goes on to discuss the many considerations such as the one pound bolt falling its distance, making contact with the watermelon and still moving some distance, and the backward-pushing force on the bolt. He puts it all together in a work-energy equation:

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Then he considers impact force, and tries to determine why the bolt dropped from 30 feet instead of 20 feet smashes through the watermelon. He notes, “Honestly, I have no idea where they are getting their values for this video. (They probably need a good science consultant.)”

Clearly, in the video, the melon breaks. Its structural integrity is disrupted and it falls apart. It’s a gooey mess, and no one wants to think of the same thing happening to their head.

Allain points out also that a hard hat will increase impact force so that “if the bolt hits the hard hat and stops over a shorter distance, this would produce a higher average force.” Yet he also notes, “the hard hat does do one thing that’s very nice. Since the hat has a rigid surface, it distributes the impact force over a larger area, which reduces the impact pressure. Lower pressure means there is less chance that the bolt will penetrate your head.”

Ah, what a relief! Even if you don’t get the physics.

Key Takeaway

Ultimately, this video and Allain’s discussion had me thinking again about the importance of workplace safety. At the same time, Allain’s questioning the science demonstrated reminds me of my consistent warning against assumptions. We need to always be testing our thinking, whether it’s about filtration technology or busting watermelons. Be safe this summer!