Chemical Processing - August 2007 - (Page 39) PROCESS PUZZLER < > THIS MONTH’S PUZZLER Our fractionating column is notorious for its problems. Oil is injected into heated feed to separate an impurity. This approach eliminates the impurity well, perhaps too well. A sludge collects in the bottom of the reboiler feed tank, clogs the sieve column and plugs the reboiler and pre-heater. There’s some concern that separation efficiency is affected by plugging of the trays but the main problem is frequent fouling of the heaters, which have fixed heads and are difficult to bore out. The reboiler pump fails often and we must call in a vacuum truck frequently and unexpectedly to clean out the reboiler feed tank. Start-up is often a problem because the steam plant is far away. The oil frequently burns, which adds to our fouling woes. What can you recommend to reduce sludge build-up in the trays, heaters and reboiler feed tank? Consider three options It seems like the problem is that we’ve introduced a nonvolatile component into a distillation system and that the component is collecting in the low spot of the system over time. Once it builds up enough, the pump sucks it in and then distributes it to the heat exchangers. I would propose several solutions to this problem: 1. Does the oil need to enter the fractionating system or can it be removed in the feed prior to the column? From the description, the impurity combines with the oil to create a sludge that’s denser than the feed or the bottoms product, so perhaps install an extraction system in the feed line downstream of the mixing point that could be as simple as a holding tank that allows the sludge to drop out of suspension and collect in the bottom of the tank. The tank could then be sucked out periodically as necessary to remove the impurity or, if possible, the oil could be filtered and recycled in the process. This would be capital intensive but solves the column problems, exchanger problems, pump problems, and start up problems. It also limits operator involvement if potential exposure to the process fluids is unacceptable. 2. Run the reboiler feed tank at a higher level and shorten the suction line of the reboiler pump. Automate the drain or make it standard operating procedure to drain the bottoms of the reboiler www.chemicalprocessing.com http://www.chemicalprocessing.com
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