Chemical Processing - August 2007 - (Page 40) >> PROCESS PUZZLER go somewhere and they will go down. If you are putting solids in the process your choices are simple: remove them upstream, remove them in the process or remove them downstream. Have you considered a wiped-film evaporator on the heavy oil feed? This would eliminate the solids and send a vapor stream to the tower. Once the solids enter the tower the best you can do is keep them moving. Electropolish the trays; install fixed, directional valve trays with large openings; and make sure the tray flow patterns don’t have dead spots. This moves the solids to the tower bottoms and exchangers. Some towers have successfully used trash collectors on a bottom tray sump or other locations to keep solids out of the reboilers and bottoms pumps, with manholes or handholes to ease cleaning. Once the solids get past the tower bottoms they’ll end up in the reboilers and bottoms pumps. At that point, you need to have bundles that are easy to clean. From your description, it sounds like you have the tower side (process service) in the tubes. I don’t see any simple solutions to improving exchanger performance here. Polishing the tubes to prevent sticking may help but the solids still go somewhere. The soundest approach to preventing reboiler problems is probably reboiler replacement. Helical baffle and spiral-plate exchangers as well as conventional nonfixed-head exchangers may all be good choices. Reboiler pumps also lack clear solutions. Solid crusher pumps are available if your pressure and temperature are in the correct ranges. Filters installed between the tower and pump may help but be careful of pump NPSH. Once filters are installed you need to clean them as well. Heavy oil with moderate or high relative volatility. Heavy oil with a moderate vapor fraction of the heavy oil in the feed implies that the heavy oil can go overhead easily. Moving the feed location up would increase stripping effectiveness of the heavy oil. If the problems arise from the temperature-driven kinetics of reactions of the heavy oil, moving the feed point up and the heavy oil into a cooler section of the tower may slow reactions down, helping the tower stay clean. Check the vapor/liquid equilibrium as well as other process and equipment constraints. Heavy oil with low (or no) vapor fraction in the feed. If the heavy oil has a low (or no vapor) fraction in the feed and is difficult to vaporize (low relative volatility) a high concentration of the oil will accumulate in the reboiler regardless of the tower feed point. Reboiler temperatures rise. This increases the rate of many fouling mechanisms and stresses machinery seals. Even so, you may be able to improve the situation. — and impurity reactive at tower conditions. Cooling the tower and keeping the contaminants moving may be the best approaches. Running at minimum pressure may 40 • August 2007 drop tower temperatures enough to slow fouling. Keeping the fouling material moving has already been discussed. — and reactive (or reaction initiated) at reboiler conditions. More scope for improvement is possible if elevated surface temperatures in the reboiler are required to initiate fouling. Approaches may include setting the controls to minimize reboiler temperatures (steam chest operation instead of flooded tubes on the reboiler utility side); minimizing tower pressure; running the tower as a steam distillation (adding water to the system) if possible; recycling light material to the bottoms (from the product, for example); or increasing reboiler area. — and inherently reactive with itself or other process components. This situation doesn’t leave much scope for action. Minimizing temperature may help but will not make the problem go away. The best methods here keep the contaminants completely out of the process, change the system chemistry or make equipment changes to deal with the inevitable shutdowns. Keeping the contaminants out returns to the idea of the wiped-film evaporator or other dedicated equipment. Additives can change the system chemistry. Along with equipment changes, new arrangements for easier start-up may be required. CP Andrew Sloley, principal engineer VECO USA, Inc., Bellingham, Wash. >> OCTOBER’S PUZZLER A spray condenser tank uses a float as a level instrument. Metal chlorides settle in the tank and are eventually pumped to waste treatment. Unfortunately, solids build up, especially during start-up, causing failure of the float. Last year alone, eight floats were replaced. Two months ago, two floats were replaced during a single outage, causing more than 30 hours of unexpected downtime. The current instrument is loop-powered. There are no additional points left in the PLC for self-powered instruments. Can you propose alternatives for measuring level? Send us your comments, suggestions or solutions for this question by September 7, 2007. We’ll include as many of them as possible in the October 2007 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, 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. www.chemicalprocessing.com http://CP.com http://www.chemicalprocessing.com
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