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.

Thinking Critically in Process Troubleshooting

process troubleshooting

In past blogs, I’ve talked about reactive and proactive process troubleshooting. Reactive troubleshooting requires quick action to look at mechanical issues, upstream and downstream equipment, and operational procedures.  In proactive troubleshooting, we ask probing questions and walk around the plant to uncover potential problems and offer solutions.  

Dirk Willard, Contributing Editor of Chemical Processing, in his recent article “Read and Think Critically” offered more to think about on this subject. 

Dirk used an example of trying to write a debottling report and discovering several missteps:

  • The product manual was poorly written
  • The plant engineer had not identified what was missing from the manual and what he didn’t understand
  • No logic was applied to decide who would know best how to run the equipment — “the engineer who built it or the process expert who operates it”
  • The engineer could have paid more attention to the section that was more detailed on the topic at hand.

Ultimately, what was missing? Critical thinking! He called for “questioning the value of information, its relevance and validity, the agenda of the source and, most importantly, the logic on which the data are based.”  

process troubleshooting
Photo by Bernal Saborio G. (berkuspic) on Foter.com / CC BY-SA

He offered real world examples of critical review, “identifying what’s being done, how it’s done, why it’s done, and who’s doing it” with a paper mill selecting a close-coupled water pump without considering all angles of the decision.

Take a Critical Approach to Process Troubleshooting

I bet you can easily add your own examples. For instance, there always seems to be a question about instrument and compressed air for the process. The process design for valve actuation has one pressure, but the operators know that in normal operation there is high demand. So the instrument air pressure drops and the valve actuates more slowly. Slow actuation results in lower production. Thus, we need to keep in mind the actual conditions versus the design conditions.  

As for compressed air, it used to be “free” at the plant. Not any more. Every plant now considers this a cost of operation. Once again, operators know that during high demand, the air pressure drops. But the process designers may not have considered this. Next time, design in a compressed air tank so that your process can meet the requirements.  

To do your best work, keep thinking, keep reading, and keep asking questions. If you have an area of expertise, let me know; I am always learning and maybe I can use your skills. I’m always interested in learning more about process troubleshooting.

Troubleshooting When the Filtration System is Not Working

 

filtration system
Image source

These are five dreaded words that no engineer wants to hear on a Saturday night or Sunday morning: “The filtration system is not working.” Of course, we never seem to get this call at 10 in the morning on a work day!

No matter the time of day, let’s not panic, take a deep breath and begin the analysis.  

There are normally three main areas that must be examined when you learn the filtration system is not working:

  1. The filter itself for mechanical reasons
  2. The equipment around the filter is not working
  3. The filter operational procedures are not correct.  

To fully understand the problem, it’s necessary to separate the symptoms from the causes. So, let’s examine each of these groups in more detail.

Troubleshooting Filter Problems

The first thing that should be checked is the filter itself. There could be a failure of the equipment mechanics such an internal components, seals, etc. Many of these issues will be described normally in the preventative maintenance section of the filter’s O & M manual. 

Second, keeping in mind, the filtration system is part of the entire process it’s important to examine the upstream and downstream equipment. For example, you might check:

  • Are the reactors performing correctly in terms of agitation, temperature control, etc. in order to produce the specified crystals?  
  • Are the precoat and body feed systems in tune for mixing, feeding, flow rates, solids loading, etc.?  
  • Are the valves and instruments operating correctly and reading the correct variables (calibrations), etc.?  

Next, take a look at the pumps that feed the slurry and washing liquids as well as the compressors the feed the gas streams for drying and cake discharge.  The pumps must produce the required pressure, flow rates, etc.  The compressors must also produce a certain gas flow at a specific pressure for a certain amount of time.  Are their interlocks in the control system or a control communication problem that are not being recognized that are causing the filter problem.  Finally, if flocculants and chemicals are being used, have these changed?  

Process Engineering Problems? 

The last place to look is the process or operational procedures. These could be responsible for the filtration problems.  For example, the particle size distribution may have changed, the amount of solids in the slurry may have changed, the cake compressibility may have changed, etc.  In terms of the operation, has the filtration pressure changed, timers changes, speed changed, etc.?  Finally, determine whether or not a process parameter has changed.  

Trouble shooting is not easy, but solving the problem brings a great sense of satisfaction. 

Let me know some of your troubleshooting horror stories! I’d love to share some in a future blog. Together, we can make it easier to handle the situation next time we hear those five dreaded words.  

filtration system
Image source