Process Puzzler: Sidestep a Scale-up Scare

Readers grapple with a reactor production change.

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Avoid a temperature so low that a phase change occurs as this will be a barrier to heat transfer.

An alternative coolant could be 30% propylene glycol (PPG) in water. This could avoid the cost of replacing the heat exchangers because of corrosion. PPG is a safer choice than ethylene glycol and less of an environmental risk.

Because it is too expensive to move the reactor volume through a pump, it may be possible to capture the solvent as it evaporates, chill it, and inject it into the reactor. Inerting the reactor may also help. By careful experimentation it may be found that additional solvent is best to control the temperature.

Now, we turn to product quality. Because it may not be desirable to have a product with the solvent, the temperature could be used to drive off the solvent at the end of the batch. This could work best under vacuum. It may be necessary to add an additional step of drying, perhaps vacuum drying with a drum.

All of these changes require a radical change in the process steps but might be managed with a modest investment of time, capital and research.
Dirk Willard, consultant
Highland, Ind.

We react two organic chemicals in a stirred tank. The reaction is exothermic and highly sensitive to temperature. We control the temperature by adjusting the feed flows, particularly reactant A, which makes up 75% of the flow. Reactant B is ratioed off reactant A.  (See Figure 1: Our new control engineer thinks we need to program some lag in the control valve for A. His first idea was to install an electric valve positioned on control valve A. The product quality has declined. Is he right about the lag? What other improvements should we consider?

Send us your comments, suggestions or solutions for this question by April 13, 2009. We’ll include as many of them as possible in the May 2009 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, 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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