September Process Puzzler: Fix a Fast Fouling Filter

Readers suggest ways to improve operation and safety. Also included is November's Process Puzzler Question.

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Dirk Willard, senior process engineer
Ambitech Engineering, Hammond, Ind.

November’s Puzzler

A batch process that consists of a series of feed tanks for a fluidized rotary dryer relies on pH probes to control product quality (Figure 1). There are two steps: pH adjustment, then chemical addition. The pH is adjusted from 1.0 to about 6.5. If the pH is too low, the acid bound to the solid will corrode the dryer and the customer’s equipment. Also, low or high pH will cause a problem with another ingredient added to the customer’s product. Unfortunately, we can’t depend on the probes. They suffer from slow response, especially with negative drops, for example, from 1.0 to 1.1. Also, because of poor residence time in the process, some chemicals added during the pH adjustment have caused problems such as silicon gelling of the probes (in acid solution). The quality assurance department is convinced that we should be able to meet our customer’s needs if we can achieve an accuracy of 0.25 pH, i.e., both probes agreeing that closely. The pH probes last about a week with some product runs and only a few hours with others. Can you suggest improvements in this process or how it’s controlled?

Send us your comments, suggestions or solutions for this question by October 10, 2008. We’ll include as many of them as possible in the November 2008 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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