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Avoiding FCCU Emergency shutdowns

There have been circumstances where catalyst losses have been very severe and a shutdown was imminent. However, it is very unusual for the cause of an immediate/emergency shutdown to be cyclone damage due to normal “wear-and-tear.” The normal “wear-and-tear” damage can usually be observed, monitored, and anticipated well in advance by operations and technical personnel, thus avoiding an emergency outage; a good monitoring program will allow operations and technical personnel to work with planning and scheduling to plan an outage as the damage and losses become unsustainable.

FCCU dipleg erosion CatCrackerThe rapid and imminent causes are usually the result of a significant event or a catastrophic failure.  The usual causes are refractory failure/loss following an upset or a thermal cycle which plugs or restricts one or more cyclone diplegs leading to massive catalyst carry-over.  A second example is the use of “smear-coating” or “butter-coating” the refractory in the cyclones.  “Smear-coats” or “butter-coats” do not adhere to the base refractory and spall off almost immediately upon start-up. Since the secondary cyclones are relatively small in diameter to most units (usually 8-10 inches), a major spall of this type can easily plug (and has) the dipleg.   An example unique for the reactor side, would be coke spalling from reactor cyclone gas outlet tubes following a thermal cycle and again plugging or restricting the cyclone dipleg.  These situations can be observed in several manners:

The failure mentioned previously may not be easily observed upon the restart / dry-circulation period when catalyst circulation rates are generally very low and the catalyst loadings to the cyclones are extremely low (generally less than 10% of operating catalyst circulation rates).  The situation will manifest itself once feed has been introduced into the unit and catalyst circulation rates, and thus catalyst loadings to the cyclones, are increased.

A more unusual, but still possible (possible because it has occurred) circumstance is the loss of a cyclone.  A loss in this case means the cyclone failed or dropped from its supports.  This can happen following an extreme thermal excursion (usually in the regenerator) or following some seismic activity for either or both the reactor and/or the regenerator sides (usually discovered upon a re-start as the seismic activity most likely took the unit off-line). Again, the same five previously mentioned items will be key indicators of a problem or issue. An FCCU/RFCCU process engineer should know the following critical pieces of information for their unit:

  1. At what rate would catalyst be lost from the unit if a regenerator primary cyclone were to plug?
  2. At normal catalyst circulation rates.
  3. At minimum feed rate catalyst circulation rates (minimum feed rate is usually the point required for main fractionator operational and product yield stability).
  4. At what rate would catalyst be lost from the unit if a regenerator secondary cyclone were to plug?
  5. At normal catalyst circulation rates.
  6. At minimum feed rate catalyst circulation rates.
  7. At what rate would catalyst be lost from the unit if a reactor primary/rough-cut cyclone were to plug?
  8. At normal catalyst circulation rates.
  9. At minimum feed rate catalyst circulation rates (minimum feed rate is usually the point required for main fractionator operational and product yield stability).
  10. At what rate would catalyst be lost from the unit if a reactor secondary cyclone were to plug?
  11. At normal catalyst circulation rates?
  12. At minimum feed rate catalyst circulation rates.

The ability of process personnel to quickly analyze and respond to any of these is the difference between a short five-to-eight day disruption or two-to-three week outage.

-K.P.

Find more technical articles at the CatCracking.com Blog and in the FCCU Forum.

Stay current by attending RefComm® CatCracking conferences around the world.

In RefComm® Past Presentations, find these articles.

Troubleshooting Catalyst Losses in the FCC Unit

Twenty Questions Identify Probable Cause of High FCC Catalyst Loss

Monitoring Mitigating and Troubleshooting FCC Catalyst Losses

 

 

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Posted by: Paul R Orlowski

Paul Orlowski is General Manager for the Refining Community which includes Coking.com, CatCracking.com, SulfurUnit.com and Resid Hydrocracking. They consult at refineries around the world. They've hosted 36 technical conferences around the globe, trained 1,000's and completed very beneficial consulting and troubleshooting projects. Paul co-founded Coking.com Inc in 1998 with Gary Pitman. Besides being an educator and software applications engineer, he worked 18 years at ARCO and BP refineries near Seattle, WA USA. Previously he worked for Science Applications International Corporation and Dealer Information Systems. In 2019 look for #RefComm Galveston Coking | CatCracking | Resid Hydrocracking and RefComm® Rotterdam Coking | CatCracking. In 2020 Galveston; Gdansk, Poland and Rio de Janeiro, Brazil.

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