Wednesday, March 6, 2013

Process Wednesday: filter cloth choices

I didn't know that you can choose different kinds of filter cloth materials during centrifugation of a slurry. I'll be frank and say that I did not know that there were choices to make, but Francis X. McConville's "The Pilot Plant Real Book" makes it clear that some choices are better than others (emphases mine):
Cloth filter media are widely used because they are relatively inexpensive. But because of the great variety available, selected the optimum cloth for a given application requires some thought. The wrong cloth can hurt the entire operation. Yields and cycle times will suffer, costing a great deal in the long run.  
Woven fabrics are a very common choice for filtration media because they are strong and available with a wide variety of characteristics. Waves are usually of the plain one-over-one-under basket weave (very tight), the twill weave (medium porosity) or the stain weave (most open). 
[snip] Where monofilament-fiber weaves are applicable (usually for coarse crystalline products with large particle sizes) they are advantageous because they do not blind easily and product is easily removed. Multifilament yard weaves are capable of retaining much smaller particles, while spun-staple yarns as depth filters and have the highest fine retentions... 
[snip] Bench-scale tests should be carried out by using samples of the material in a number of repeat filtrations. If the resistance increases after several filtrations, it is a sign that the cloth is becoming clogged or blinded and may not be a good choice for operations such as multiple-load centrifuge batches.
I think I have always approached centrifugation as "my problem", i.e. if the filtration is slow, I need to change the crystallization parameters. Now, I've found a way to deflect criticism! (kidding.)

(Obviously, the take-home message of Mr. McConville's comments is the suggestion of a laboratory-scale test for filtration.) 

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