Page 236 - Fluid Catalytic Cracking Handbook

204 Fluid Catalytic Cracking Handbook

* ZSM-5 additive. Seasonal or regular use of ZSM-5 catalyst will
center-crack the low-octane paraffin fraction of the FCC gasoline.
The results are increases in propylene, butylene, and octane—all
at the expense of FCC gasoline yield.
* Catalyst cooler(s). Installing a catalyst cooler(s) is a way to con-
trol and vary regenerator heat removal and thus allow processing of
a poor quality feedstock to achieve increased product selectivity,
* Feed segregation. Split feed injection involves charging a portion
of the same feed to a different point in the riser. This is another
tool for increasing light olefins and boosting gasoline octane.

An example of increasing FCCU capacity is oxygen enrichment:
* Oxygen enrichment. In a cat cracker, which is either air blower
or regenerator velocity limited, enrichment of the regenerator air
can increase capacity or conversion provided there is good air/
catalyst distribution and that the extra oxygen does not just burn
CO to CO 2.
In recent years, numerous mechanical improvements have been
implemented to increase the run length and minimize maintenance
work during turnarounds. Examples include:
* Expansion joints. Improvement in bellows metallurgy to Alloy
800H or Alloy 625 has reduced the failures caused by stress
corrosion cracking induced by polythionic acid. Additionally,
placing fiber packing in the bellow-to-sleeve annulus, instead of
purging with steam, has reduced bellows cracking. Reliability has
also increased with the use of dual ply bellows.
* Slide or plug valves. Cast-vibrating of the refractory lining and
stern/guide modifications have minimized stress cracking and
erosion.
« Air distributors. Improvements in the metallurgy, refractory lining
of the outside branches, and better air nozzle design, combined
with reducing L/D of the branch piping, have reduced thermal
stresses, particularly during start-ups and upset conditions.
* Cyclones. Changes in refractory anchor and material, the hanging
system, longer L/D, and using more welds in the anchors have
improved cyclone performance.