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?
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