Page 39 - Fluid Catalytic Cracking Handbook

22 Fluid Catalytic Cracking Handbook

The activity of catalyst degrades with time. The loss of activity is
primarily due to impurities in the FCC feed, such as nickel, vanadium,
and sodium, and to thermal and hydrothermal deactivation mechanisms.
To maintain the desired activity, fresh catalyst is continually added to
the unit, Fresh catalyst is stored in a fresh catalyst hopper and, in most
units, is added automatically to the regenerator via a catalyst loader.
The circulating catalyst in the FCC unit is called equilibrium
catalyst, or simply E-cat. Periodically, quantities of equilibrium catalyst
are withdrawn and stored in the E-cat hopper for future disposal. A
refinery that processes residue feedstocks can use good-quality E-cat
from a refinery that processes light sweet feed. Residue feedstocks
contain large quantities of impurities, such as metals and requires high
rates of fresh catalyst. The use of a good-quality E-cat in conjunc-
tion with fresh catalyst can be cost-effective in maintaining low
catalyst costs.

MAIN FRACTIONATOR

The purpose of the main fractionator, or main column (Figure 1-13),
is to desuperheat and recover liquid products from the reactor vapors.
The hot product vapors from the reactor flow into the main fractionator
near the base. Fractionation is accomplished by condensing and
revaporizing hydrocarbon components as the vapor flows upward
through trays in the tower.
The operation of the main column is similar to a crude tower, but
with two differences. First, the reactor effluent vapors must be cooled
before any fractionation begins. Second, large quantities of gases will
travel overhead with the unstabilized gasoline for further separation.
The bottom section of the main column provides a heat transfer
zone. Shed decks, disk/doughnut trays, and grid packing are among
some of the contacting devices used to promote vapor/liquid contact.
The overhead reactor vapor is desuperheated and cooled by a pump-
around stream. The cooled pumparound also serves as a scrubbing
medium to wash down catalyst fines entrained in the vapors. Pool
quench can be used to maintain the fractionator bottoms temperature
below coking temperature, usually at about 700°F (370°C).
The recovered heat from the main column bottoms is commonly
used to preheat the fresh feed, generate steam, serve as a heating medium
for the gas plant reboilers, or some combination of these services.