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.

The 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:

• A rapid loss in catalyst level or inventory, usually in the regenerator as the reactor/stripper level is typically in level control and the regenerator level/inventory “takes the swing”
• Significant increase in the catalyst content in the main fractionator bottoms (MFB) slurry loop sample (if it is a reactor cyclone)
• Significant increase in the catalyst content in the flue gas scrubber (FGS) effluent loop sample (if it is a regenerator cyclone)
• Significant increase in the catalyst being dumped to the electrostatic precipitator (ESP) bins (if it is a regenerator cyclone)
• Significant increase in flue gas stack opacity if there is an electrostatic precipitator (ESP) in service (if it is a regenerator cyclone).

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.

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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