bottoms must be designed for this service. A typical design is three pumps of 50% capacity that allow a pump to be taken out of service without impacting the unit’s capacity. Locating these pumps close to the bottom of the tower minimises the chance of plugging the inlets to the pumps. A basket filter in front of the pumps is necessary to prevent large amounts of coke from blocking the line. Heat exchangers are used to remove the heat from the bottoms circuit and provide the cold reflux that removes the superheat coming in from the reactor. These can exchange heat with feed, products or produce high-pressure steam. The slurry is on the tube side of the exchangers, and the velocity through the tubes should be at least 6 ft/sec. Lower velocities have been used in the past, but coking has been an issue. The minimum tube diameter should be at least 1 to 1.5 inches to prevent plugging. Velocities of 10 ft/sec have been used, but higher velocities could cause erosion. Coking of the tubes may start with a loss of heat transfer as a soft coke forms in the tubes. A wash with hot LCO may remove this type of deposit before it hardens to the tubes. Anti-foulants have been used in this service to pro- long the time between cleanings. Refiners can monitor the fouling by calculating the heat transfer coefficients for the exchangers. This is useful for monitoring the additive effec- tiveness and usage rates. Continuous use of these additives generally works better than intermittent doping. Some coking in FCC units is inevitable due to the pres- ence of thermal cracking. Processing resids increases the probability that this phenomenon will occur. To minimise the amount and effects of unwanted coke formation, refin - ers have found some of the design and operation practices mentioned here to be very effective. Operation during transit conditions is very important since the low temperatures can precipitate coke formation. Large temperature changes can cause the coke formed in the unit to spall off and cause operating problems. Using lighter feeds for start-ups helps to avoid these problems. Steady- state operation with a minimum of temperature excursions is important for long, trouble-free runs. Literature cited 1 McPherson, L. J., Causes of FCC reactor coke deposits identified, Oil and Gas Journal , October 9, 1984, 139-143. 2 Brevoord, E., Wilcox, J. R., Coke formation in FCCU reactor vapour lines, ACS 206th National Meeting, Chicago Illinois, Aug 22-27, 1993. 3 Dean, C. F., Golden, S. W., FCC vapour line coking, PTQ Revamps 2003, 3-7. 4 Hunt, D. A., Minyard, B., Koebel, J., Understanding and minimizing FCC slurry exchanger fouling, Grace Catalagram 101, Spring 2007. 5 NPRA Transcripts, 2007, 1972-1990. 6 Wilson, J. W., Fluid Catalytic Cracking Technology and Operation , Pennwell Books, 1997. 7 Minton, P., FCC reactor and transfer line coking, The Catalyst Report, Engelhard. Warren Letzsch is President, Warren Letzsch Consulting PC. He has 53 years’ experience in refining catalysts, petroleum refining and engineer - ing design, and technology development for a major technology licensor. Email: wletzsch@verizon.net
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