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Avoiding FCC regenerator upsets

Examination of all unit operating conditions indicates that lowering the regenerator bed level will eliminate catalyst upsets and increase unit charge rates

David A Hunt and Adam Kasle, Grace Davison
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Article Summary
An FCC unit operating at normal rates experienced a sudden loss of catalyst from the regenerator. Two days later, this Kellogg Model III unit experienced a similar incident, releasing approximately 10 tons of catalyst from the regenerator. After an extensive investigation by the refinery’s technical department and Grace Davison, it was recommended that the regenerator bed level be reduced by 25in water gauge or approximately 4ft of physical bed level. After reducing the regenerator bed level, there were no further catalyst upsets.

Troubleshooting investigation
The refiner requested that Grace Davison provide on-site assistance and work with the refinery technical department to investigate the reason for the unit upset and make recommendations to eliminate further incidents. The investigation included a pressure survey around the regenerator, a review of the regenerator vessel and cyclone drawing, and an examination of all unit operating conditions.

During the time of the upset, the regenerator conditions shown in Table 1 were estimated. Grace Davison’s calculations showed that the secondary cyclone diplegs were flooded. Submerging the diplegs in the regenerator bed provides sufficient sealing to prevent gas from flowing up the diplegs. However, dipleg flooding is a condition where the estimated catalyst dipleg level is at the top of the dipleg. Any further load to the system, resulting in a higher dipleg level, would lead to significant catalyst losses.

Contributing to the flooding were high primary cyclone inlet velocities and large dipleg burial depth. Regenerator cyclones handle large quantities of catalyst. Calculations also showed that the primary cyclone dipleg flux rates were high. Typical design values for primary cyclones are listed in a previous publication.1 With these conditions, it was a surprise that the unit was not experiencing more problems than the two earlier incidents and at best was operating near upset conditions. It was surmised that brief level fluctuations could have resulted in the catalyst losses under these conditions.

It was concluded that the regenerator bed level was excessive. At a lower bed level, the secondary cyclone diplegs would no longer be at flood and would be more forgiving of any operating shifts. The calculations are described in previously published documents.2

Problem resolution
A recommendation was made to refinery operations to reduce the normal bed height from a nominal 150in of water gauge to 125in, effectively lowering the normal regenerator bed height by approximately 4ft. Calculations showed that at these conditions the regenerator would still have adequate regenerator catalyst residence time to ensure low coke on regenerated catalyst. The lower bed level would reduce the regenerated catalyst slide valve pressure drop by a corresponding 25in of water. However, the resulting slide valve differential pressure was still above the minimum value set by operations. A review of the regenerator drawings also showed that at the lower bed level adequate bed height would still exist above the regenerated catalyst withdraw hopper.

With the lowering of the bed level, the refinery has been able to operate at increased charge rates without any further regenerator catalyst upsets. Carbon-on-regenerated catalyst remained at low levels and no increased levels of afterburn were observed.

It should be noted that in this instance the unit configuration and operation were such that the regenerator bed level could be lowered without significantly affecting other aspects of the unit operation. However, each case is different and should be evaluated, taking into consideration the following:
• Catalyst regeneration residence time
• Regenerated catalyst slide valve pressure drop
• Regenerator cyclone dipleg submergence
• Regenerator level relative to the withdraw hopper
• Effect of lower operating pressure at air grid on nozzle outlet velocities.
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