Quell a Quality Control Confrontation

Readers recommend improvements for a pH control process

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We replaced the PH probes with conductivity probes. By correlating conductivity with pH, we were able to make our process considerably more stable and reliable. These probes have been operating for more than three years without replacement or any major issues.
 Darwin De Los Santos, plant engineer
 IMTT-Bayonne, Bayonne, N.J.

CHANGE THE PROCESS
It seems to me that a change of a pH 1.0 to about 6.5 is quite wild. Assuming this is SiO2-to-SiO4 neutralization, if the remaining SiO2 is highly acidic, it might explain probe fouling. It seems that the stuff is clumping up and, as has been suggested, the reaction may be slow. Have you considered homogenization, dewatering or filter press? If water addition is not an option (because of dryer load or economics), have you considered depolymerization/polymerization?
Michael Waugh, consultant
Lutz, Fla.

JANUARY’S PUZZLER
We use a carbon-steel spiral-tube heat exchanger as a condenser in our spray tower. Operating conditions normally are 100 psig at about 150°F. Treated chilled water at 45°F runs in the tube side. The shell side handles metal chlorides, e.g., FeCl2, VCl4 and TiCl4. These chlorides condense to form liquids and evaporate to form gases. Severe corrosion from chlorine and erosion was the reason why the spiral exchanger was chosen over a shell-and-tube one. The chlorine is usually dry but, occasionally, spikes of water upstream of the exchangers cause corrosion. One of our engineers is pushing the idea of cladding Type 304L stainless steel with a high-nickel alloy. Coupon tests suggest this will improve service life. One concern is how bending the tube will affect cladding. How can we assure our nervous operations manager that this cladding will work? Do you have any other ideas? Keep in mind replacing the heat exchangers with another style will require a major change in piping around the units.

Send us your comments, suggestions or solutions for this question by December 12, 2008. We’ll include as many of them as possible in the January 2009 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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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