According to Rebecca Kuo, technical service engineer for BASF Corp., almost all fluid cat cracker (FCC) units have experienced a catalyst problem, with the main causes of elevated catalyst losses being cycloning problems and catalyst attrition.
Specifically, a recent industry survey found that cyclone problems account for 12 percent of unplanned shutdowns of FCC units.
High catalyst losses can lead to a unit shutdown due to erosion and/or pluggage in the slurry circuit, erosion of the flue gas equipment, violation of environmental regulations, catalyst circulation instability or inability to fluidize, and excessive catalyst additions to maintain inventory -- all of which "can be pretty expensive," Kuo said, discussing effective troubleshooting at the AFPM Cat Cracker Seminar held recently in Houston.
Causes of attrition, Kuo explained, include excessive velocities that occur when feed injectors malfunction.
"Excessive velocities above 300 feet per second are the usual cause of high attrition in the unit, or if there's damage or erosion to torch oil nozzles on the air grid," Kuo said.
Higher catalyst loading to cyclones can also cause attrition, Kuo said.
"The primary cyclones are more highly loaded, so there you're going to see more of the particle-to-particle type of attrition," Kuo continued, "whereas the secondary cyclones are less loaded and can handle the particle interaction attrition better. But you are going to see more erosion on refractory walls of secondary cyclones."
The most common places for attrition on an FCC unit, Kuo said, are jets, nozzles and in bends. "We like to say, as a rule of thumb, one-third of attrition occurs on the aeration and about two-thirds of attrition occurs in the cyclones," she said.
"Some units are going to be more severe on the catalyst than others," Kuo said, reminding listeners attrition is just one aspect of catalyst design and formulation, and that low catalyst attrition may be needed in some cases, due to local emission limits. "It is really important that the fresh catalyst properties are suitable for your specific unit," she said.
Baseline monitoring is essential
"Always, always, No. 1 is to establish good baseline monitoring," Kuo emphasized. "This is because, if your unit is having a problem, you are not going to be able to answer what has changed if you don't know what your unit looks like when it is running well. It is not really useful to start collecting samples when you already have your health problem. This really lets you be able to pick up the problem early on, and you can take corrective measures before the problem gets worse and you have to get to a unit shutdown."
For example, if units are showing losses to a slurry and elevated slurry ash is indicated, "then you can enter that you have good maintenance done on the standby slurry pump," Kuo said.
Kuo recommended a catalyst mass balance be performed on a monthly basis.
"We also highly recommend collecting ECAT [equilibrium catalyst] and fines samples regularly and sending these to multiple catalyst vendors to make sure that whenever you are getting those particle size-distribution results back, you are able to cross-track new trends," Kuo said.
BASF Account Manager Tiffany Clark agreed, saying "recognizing you have a problem" is essential to effective troubleshooting.
Clark recommended consistent reviews of operating conditions to determine changes, as well as verifying catalyst balance calculations and pressure survey variances.
The nature of any losses should be investigated, Clark said, explaining a gradual loss increase can indicate attrition or a cyclone hole. Step changes point to a mechanical failure or operating problem, she said, and intermittent losses indicate operation is running close to the cyclone-flooding limit.
Identifying a catalyst loss issue can be a long process, Clark said.
"It could take several weeks to identify a problem. If it is severe and you are losing pressure in your vessels or your scrubber, that's obvious," she said. "But if it is just a light problem, that can somehow be difficult to identify."
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