Separations Technology

Should You Use A Filter Or A Centrifuge?

Follow a simple procedure to make the correct choice

By Tom Blackwood, Contributing Editor

I’ve never seen a filter or centrifuge that wasn’t doing what it was supposed to do.

Your company has developed a new product and now it’s time to go from the laboratory to small-scale production. Your chemists have created a slurry and have separated the product most likely using laboratory filters or maybe a laboratory centrifuge. These units are great for demonstrating the effectiveness of the new product. However, they may not produce the same product quality in full-scale operation. So, it’s sensible to take several steps before firming up the process flow diagram (PFD) and making equipment selections.

The chemists likely have provided you with all the most common fundamental data such as particle size, density and other physical properties of the slurry. (Don’t just view the researchers as a source of data; they also sometimes can offer valuable insights for your scale-up efforts — see: “Talk to a Chemist.”) You may want to consider refining the product through a variety of drying methods or even liquid/liquid extraction, especially if the desired material is a liquid. However, the most common choice for separation usually involves either filtration or centrifugation.

How Do You Choose?

Several experimental tests will lead you to the most appropriate type of unit for the separation. These tests, done in a step-by-step sequence, can narrow the selection to just a few pieces of equipment, which can reduce the amount of time needed to select a manufacturer of the appropriate device. Most of these experiments don’t require any specialized test equipment and can be run by almost anyone.

Step One. Look closely at the overall behavior of the slurry and ask yourself the following questions:

• How fast do the solids settle in calm liquid?
• Do the solids form a cohesive cake or does it behave like jelly?
• What gravitational force is needed to form a cohesive cake and what’s the solids content of the cake?
• How fast does the slurry filter and what is the minimum liquid content?
• What wash rate or cake quality could be achieved after the cake is formed?

One well-defined procedure was published by Edward Davies in 1965 (“Selection of Equipment for Solid/Liquid Separations,” Trans. I.Chem.E., Vol. 43-8, pp. T256–T259). Most of this procedure can be conducted in a tube centrifuge, Buchner funnel and a graduated cylinder. While running these tests, it’s important to look at the particles after filtration to determine if they shear or break easily. A simple microscope suffices to observe if this will be a concern in equipment selection.

Step Two. Determine the scale and nature of the processing, i.e., production rate, and whether batch or continuous. Also, check whether some suitable equipment exists in your current operations; if so, this may allow you to campaign the product or utilize an already installed device. However, don’t treat availability of such a unit as an overriding factor in equipment selection. Every device behaves in a very predictable manner. I’ve never seen a filter or centrifuge that wasn’t doing what it was supposed to do. However, I’ve seen a lot of devices that weren’t suitable for the product they were handling. While it’s tempting to try to use an existing device, doing so isn’t always a good decision.

Step Three. Finally, before developing a PFD, determine the quality and suitability of using any filtrate or wash liquid and how to treat it. In the case where the product is the liquid phase, downstream processing of the solids will be important. Retention of some of the samples from the first step is an option but a small-scale test on real-world equipment is better. Try to select a manufacturer that offers your top three choices as determined in the first step. If the tests don’t produce acceptable product, that vendor should be able to evaluate your other selections without any prejudices. (Conducting tests at a vendor facility also can prove valuable when using pneumatic conveying — see: “Run a Pneumatic Conveyor Test,”

Don’t neglect another important variable — the human element; operator acceptance of the chosen device is an important factor in its successful use. So, if possible, include relevant operators in the selection process and any on-site testing. Their acceptance can make or break your new product.


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TOM BLACKWOOD is a Chemical Processing Contributing Editor. You can email him at TBlackwood@putman.net

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