Page 38 - Fluid Catalytic Cracking Handbook

Process Description 21

peratures. At low temperatures, regeneration is always partial, carbon
on regenerated catalyst is high, and increasing combustion air results
in afterburn. At intermediate temperatures, carbon on regenerated
catalyst is reduced. The three normal "operating regions" are indicated
in Table 1-2.
There are some advantages and disadvantages associated with full
and partial combustion:
Advantages of full combustion
• Energy efficient
• Heat-balances at low coke yield
• Minimum hardware (no CO boiler)
• Better yields from clean feed
Disadvantages of full combustion
• Narrow range of coke yields unless some heat removal system
is incorporated
• Greater afterburn, particularly with an uneven air or spent catalyst
distribution system
• Low cat/oil ratio
The choice of partial versus full combustion is dictated by FCC feed
quality. With "clean feed," full combustion is the choice. With low
quality feed or resid, partial combustion, possibly with heat removal,
is the choice.

Catalyst Handling Facilities

Even with proper operation of the reactor and regenerator cyclones,
catalyst particles smaller than 20 microns still escape from both of
these vessels. The catalyst fines from the reactor collect in the frac-
tionator bottoms slurry product storage tank. The recoverable catalyst
fines exiting the regenerator are removed by the electrostatic pre-
cipitator or lost to the environment. Catalyst losses are related to:
« The design of the cyclones
« Hydrocarbon vapor and flue gas velocities
• The catalyst's physical properties
• High jet velocity
• Catalyst attrition due to the collision of catalyst particles with the
vessel internals and other catalyst particles