Best Practices for Filtration Testing for Solid-Liquid Separation

filtration testing
Photo by Pacific Northwest National Laboratory – PNNL on Foter.com / CC BY-NC-SA

Testing in school has a negative connotation. Students dread tests. Parents bemoan “teaching to the test.” Teachers chafe against the curriculum parameters defined by testing expectations. Yet, the word “testing” should resonate much more positively with process engineers. After all, testing is the key for selecting the most suitable filtration tech for any individual solid-liquid separation task.

Although there is only limited theoretical background available, and even specialized engineering education at universities leaves many theoretical questions open, it is beneficial to have a minimum understanding of the theory of filtration itself. By identifying the role of each influencing part, the process engineer gains a potential tool to work with when it comes to understanding testing findings and developing a path forward in determining the best filtration procedure.

Just from experience, and for the benefit of engineers, some overview observations are necessary:

  • Don’t stop testing just because the first results suit your target
  • Don’t accept samples without verifying the parameters in the description
  • Never change more than one parameter at a time
  • One result is no result => verification is a must
  • Take a break and check the conformity of the results before you call it a day

Filtration Testing Requires Decision Making

In testing it’s essential to train yourself to stop and repeat. Don’t succumb to perceived certainty. After all, many parameters of the liquid and the solids have an influence on the filtration process.

  • Form and size of particles
  • Particle size distribution (PSD)
  • Agglomerate building behavior
  • Deformability
  • Compressibility
  • Liqiud viscosity
  • Solid content
  • Zeta-potential

While all of the above may not be known for all filtration applications, the final target is to find a theoretical approach together with a practical method of testing.

Sampling in Filtration Testing

Filtration tests need to be done with a “representative sample” defined as a sample “as close as possible” to the real production product.  Yet the specific characteristics of a slurry from the point of filtration are not obvious to everyone. That’s where testing comes in: the list of parameters is quite extensive and in many cases only a few are available.

Still, the more you can get the better. Although for the first tests, the ph-value, temperature, particle shape, size distribution, etc. are not necessary right from the beginning, these parameters are normally quickly measured and complete the picture of the suspension. It is obvious that solid content and viscosity do have an impact on the filterability.

“Suspending” Judgment in Filtration Testing

The characteristics of suspensions are not only caused by the liquid phase but also by the particles, the other half of a slurry. The solids can be of crystalline nature or amorphous, which means their structure is not really defined. They can also be organic (i.e. cell debris), fibrous, in-organic, compressible or incompressible, generate agglomerates or not, may have a zeta potential or not…. there are many possibilities.

An easy way to verify the type of solids is a sample check. If possible, the original suspension should be checked under the microscope. Then, the behavior of the solids can also be seen:

  • Do they tend to build agglomerates or stay on their own?
  • How is the distribution of the solids?
  • Is the structure of the solids needle-shape, potato shape, snow crystal or even fibrous?

The best practice in filtration testing is to consider all of these angles thoroughly before deciding on a filtration procedure.

I am a big fan of Sherlock Holmes who always warns “don’t jump to conclusions.”  This is one of the biggest risks we face during tests in the daily work of process engineering.  Let me know if you need help!

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