Catalysis 2024 Issue

reactor is adjusted. The most common situation where this option is considered is FCC units that have oversized feed risers due to a reduction in recycle or feed rate. The latter can occur if a hydrocracker is added to the refinery. Terminator designs for unwanted reactions Reactor termination is very important since it is desirable to end the cracking reactions as quickly as possible before many secondary reactions can occur. Any dilute phase reac- tions will be largely thermal, and delta coke will increase. Rapid separation of the spent catalyst from the products is desirable, so the primary separator should be very efficient. A close-connected terminator system provides the fastest stoppage of unwanted reactions. The underflow from the reactor cyclones should also be minimised since hydrocar- bons going down the diplegs go into the stripper bed and increase the delta coke. Another approach to minimising secondary reactions is to use a vapour quench of the gas leaving the primary reactor cyclones. By injecting the quench after the primary separator, the catalyst is not cooled, and the coke yield does not increase. The operation of the main column needs to be adjusted to satisfy the heat balances of the column. LCO is used because it is relatively inert and does not overload the main column overhead system. Quench is more effective at high reactor temperatures and high dilute phase residence times. Better stripper design can lower the regenerator temper- ature. Baffles can be used to improve the steam-catalyst contacting. In some cases, more baffles for an existing

unit might be needed to get the improved performance. A reduction in steam usage might provide other opportuni- ties to increase profits. Switching from baffles to packing or other internals may be the best way for refiners to reduce the hydrogen in coke since the shell of the existing stripper can be used. Paybacks for these revamps can be a year or less. The hydrodynamics of the stripper may suggest ways to improve its performance. High regenerator temperatures can be controlled by add- ing a catalyst cooler. The older methods of using cooling coils and dilute phase catalyst coolers have been replaced with dense phase catalyst coolers that are reliable and have turn-down capabilities to optimise the unit operation. They increase the coke make, but the higher cat-to-oil ratio may give a more favourable product distribution. If a higher unit pressure is required to accommodate the increased gas rate, the vaporisation of the feed and the desorption rates of the products in the stripper will need to be reviewed. The solutions to low or high regenerator temperatures are unit dependent. All of the factors mentioned in this dis- course need to be examined to arrive at the best short- and long-term fixes to the FCC unit. 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. He is a Fellow of the AIChE. Email: wletzsch@verizon.net

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Catalysis 2024