QUESTIONS & ANSWE
pressure. Note the very sharp break in vapour load, which occurs as the condensing temperature causes the water vapour pressure to approach the condensers operating pres- sure. This is a result of condenser underperformance, possi- bly due to the conditions listed previously, and can also cause ejector break. It will not, however, result in the vapour outlet temperature exceeding the condensate outlet temperature. The X-shell first-stage inter-condenser long air baffle design principle is to minimise the gas outlet temperature, thereby reducing second-stage gas load for a given non- condensable gas load. First-stage inter-condensers hot vapour bypass of the long air baffle is a major cause of ejec - tor system break. Baffle sealing A key feature of the long baffle design is the baffle edge seal - ing. Seal strips are a critical component of TEMA X-type con- densers with removable bundles. In removable bundles, the baffle cannot be welded to the shell. To seal the long baffle and prevent vapour bypassing, sealing strips are attached at the baffle to shell junctions. These strips are made from thin corrosion-resistant material. These can be easily damaged during bundle installation. Seals should be replaced, not reused, every time the bundle is removed. The installer should never allow the bundle to twist or rotate during installation. If the bundle is not sealed properly, some fraction of the vapour entering the exchanger will bypass the condensing area and flow directly to the outlet nozzle. This adds additional load to the next stage, increasing suction pressure and back pressure on the first stage. In practice, when cooling water temperature is lower than design and as long as cooling water flow rates are at design the extra load due to bypass of the long air baffle may still be within the capabilities of the second-stage ejector and not lead to broken operation. As cooling water inlet temperature increases seasonally and reaches its design, second-stage gas load often exceeds the second-stage capacity, resulting in excessive back pressure on the first stage. In this case, higher pressure in the first-stage condenser is not due to exchanger heat transfer underperformance, but a faulty long baffle as indicated by vapour temperatures higher than the liquid condensate. Once the second-stage pressure exceeds the maximum discharge pressure of the first-stage ejector, the ejector breaks and, once broken, further increases in first-stage ejector discharge pressure continue to increase vacuum col- umn operating pressure. It is not unusual for the first-stage ejector operating in broken mode to generate higher noise levels than the already high noise levels generated by large first stage ejectors. Depending on the severity of the situation, it is sometimes possible to compensate for the higher bypass with first- stage inter-condenser motive steam nozzle changes. Either nozzle location in the steam chest or larger nozzle sizes have been used to increase the ejector MDP and eliminate system break. For more severe cases, rather than replace the entire system installing larger second-stage ejectors and condens- ers to handle high gas load has been a successful solution.
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120 With respect to the processing of residual feedstock in FCC, perhaps the most important change in modern FCC catalyst design is the quantification and subsequent optimisation of catalyst accessibility. Data from about 20 commercial experiences show that when contaminants like iron, vanadium, calcium and sodium increase, cata- lyst accessibility decreases rapidly. When catalysts with high accessibility (as measured by the AAI – Akzo Acces- sibility Index) are used, very marked improvements in activity and selectivity are achieved. 110 115 105 100 95 90 Vapour temperature Scott Golden is President of Process Consulting Services. He has more than 40 years of experience including refinery process design, refinery technical service, and distillation equipment design. Tony Barletta is Vice President of Process Consulting Services, Inc. He has over 34 years of experience with refinery revamps and process design for heavy oil units. To enhance the production of light olefins, especially propylene, in the FCCU stable narrow pore zeolites, eg ZSM-5, are required. This has to be combined with a host FCC catalyst featuring high propensity to produce olefinic precursors, which are subsequently cracked to light olefins. Steve White is a chemical engineer with Process Consulting Services. He has more than 46 years of process design experience for revamps and grass roots units, including crude/vacuum, FCC, hydrotreating, alkylation, isomerisation, reforming and other processes. Darrell Campbell is a process engineer specialising in refinery revamps. He received a degree in Aerospace Engineering from Texas A&M University in 1994 and earned his PE in Chemical Engineering in 2014. Increased ability of the FCC catalysts system to make lower sulphur-containing products is necessary for an overall more profitable refining operation. Reduced NO x and SO x emissions from the FCC stack are also required. The introduction of new FCC catalyst additives as Figure 6 Condenser vapour rate upgrading process for the oil refining industry, producing vast quantities of transportation fuels. It is also expected that it will gain importance as supplier of propylene worldwide and occasionally ethylene. The FCC process and the products it produces will have to meet strict emission standards. High accessibility and accessibility retention are required the make the processing of even more contam- inated residual feedstock possible. Akzo Nobel has intro- duced the Opal, Sapphire and Coral catalysts line featuring enhanced accessibility.
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PTQ AUTUMN 2002 18 13
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Revamps 2022
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