A key operating parameter is the reactor pressure. Lower pressures favour desorption of the oil molecules, as does the hydrocarbon partial pressure. More dispersion steam can help start the stripping process in the riser reactor. As feeds get heavier, the impact on the stripper becomes more acute. Refiners are usually interested in processing more feed to improve profitability. Over the years, capacity increases of 50% or more have been made. This generally is accomplished by raising the reactor pressure to shrink the gases going to the gas plant. When resid is part of the new feed, the stripper may become the bottleneck. A catalyst cooler may be added to control the regenerator temperature, but more oxygen will be required for the additional coke burn. Higher catalyst cir- culation rates would call for a stripper design review. If the cat cracker makes 1 wt% coke more than necessary, the lost revenue would be about $800/day for every 1,000 barrels or $40,000/day for a 50,000 B/D FCC unit. This assumes a barrel of products yield at $100 and fuel costs of $20 per barrel. In a year, this represents more than $14 million. The value of designing a new unit for lower pressure should be considered when resid is going to be run. If the cat cracker makes 1 wt% coke more than necessary, the lost revenue would be about $800/day for every 1,000 barrels or $40,000/ day for a 50,000 B/D FCC unit Lower pressure makes more money by reducing the coke produced and improving product distribution. Higher octanes and olefins for alkylation are an additional bonus due to reduced hydrogen transfer reactions. Better strip- ping and feed vaporisation are part of the equation. The extra cost of the equipment due to larger vessel sizes is compensated for by the better yields. Between 14 and 20 million dollars a year translates to 140-200 million dollars in a decade of operation. The evaluation of a stripper’s performance is done in several ways. Lower hydrogen in coke is a typical indicator of better operation. The flue gas analysis is critical to the hydrogen analysis in coke because its value is determined by oxygen disappearance. When an Orsat analysis was made of the flue gas, the CO and CO₂ solutions could be weak and not pick all of these molecules in the solutions. The unaccounted-for oxygen is assumed to be in the water formed in the regenerator. If the CO and CO₂ are low, the calculated coke make is low, and the hydrogen-in-coke would be high. Good hydrogen- in-coke numbers range from 5.5 to 7.0 wt%. Later chroma- tographs were found to give more consistent results than Orsats, and flue gas sampling devices improved. Refiners control the excess oxygen in the flue gas to minimise after - burn and control SOx and NOx. Energy efficiency is also improved.
The lowest hydrogen content in the coke is 4 wt%, though this number is unlikely. Numbers below 5.5 wt% are rare. The numbers above 7 wt% suggest that the oper- ation be examined, and possibly a revamp can be made to consistently lower the number. Products are being burned to hold the regenerator temperature up, and a revamp can easily be justified if the hydrogen value is 8 wt% or higher. If the catalyst flux rate exceeds 1,000 lb/(ft² minute), increasing the steam rate can be tried, but it may not be effective. More variations of hydrogen content in coke seem to occur in FCC units adding oxygen to its air. Accurate air rates are crucial since this figure, along with the oxygen content, is used to get the total oxygen going to the unit. Flux rates through the stripper can also be too low. When units are oversized or running at 60-70% of capacity, the steam rate might be reduced to match the cat circulation rate. Residence time in the stripper would be longer, which increases the thermal cracking. The quality of fluidisation would be poorer because of inferior mixing. The steam bubbles are the engine that stirs the catalyst bed. Low hydrogen on coke may reflect excessive thermal cracking rather than better stripping. A higher steam rate as meas- ured by (lb steam)/(1,000 lb of catalyst circulated) might be warranted. Stripper review Another method of examining a stripper is to take samples from around it and test them for strippable hydrocarbons. If a problem is detected, the operating parameters can be checked for possible causes. Periodic stripper checks can be made by lowering the steam and watching for an increased regenerator temper- ature. The steam rate is increased by about 10% from the point where the temperature started to get higher. Any changes affecting coke make or delta coke warrant a strip- per review. Literature cited 1 Rall R, Kuralte R, Application of Koch-Glitsch KFBE Packing In FCCU Catalyst strippers, Grace European Technology Conference, Seville, Spain 2004. 2 Wilcox J, Optimizing Spent Catalyst Stripping to Enhance FCC unit Performance, Albemarle , Issue 71, Courrier. 3 Baptista C, Cerqueira H S, Fusco J M, Chamberlain O R, What Happens in the FCC Stripper , NPRA, AM-04-53. 4 Schnaith M W, Improved Catalyst stripping From Cold Flow Modeling , Grace European Technology Conference, Dublin, Ireland 2002. 5 Letzsch W S, Revitalize Stripping Operations with Structured Packings, H ydrocarbon Processing, Sept. 2003. 6 Private Communication, Lee Turpin. 7 Miller R, NPRA Transcript 2000, Heavy Oil Processing. Warren S Letzsch has 56 years of experience in petroleum refining, including petroleum catalysts, refining and engineering, and design. He has authored more than 100 technical papers and publications and holds eight patents in the field of FCC. He holds BS and MS degrees in chemical engineering from the Illinois Institute of Technology. Letzsch is a Fellow of the American Institute of Chemical Engineers. Email: wletzsch@verizon.net
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