We overhauled our isobutene/n-butane fractionating column during a recent turnaround. We cleaned its sieve trays, replaced the reboiler (a circulating thermosyphon) and slightly lowered the suction nozzle to avoid splashing the bottom tray. The bottom-most thermocouple failed during start-up total reflux; it went full-scale. We switched reboiler control to an available thermocouple higher in the column. We replaced the failed thermocouple after the column was dehydrated. Now, we're having trouble with the temperature profile below the feed tray. The reboiler differential temperature is sporadic. What's causing this and what can we do? Send us your comments, suggestions or solutions for this question by August 10, 2012. You can post your answer right to this blog. Please include your name, title, location and company affiliation in the response. We'll include as many of them as possible in the September 2012 issue and all on ChemicalProcessing.com. You can also 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. 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.