Page 41 - Fluid Catalytic Cracking Handbook

24 Fluid Catalytic Cracking Handbook

either gravity or centrifugal, to remove catalyst fines. A slipstream
of FCC feed was used as a carrier to return the collected fines from
the separator to the riser. Since then, improvements in the physical
properties of FCC catalyst and in the reactor cyclones have lowered
catalyst carry-over. Most units today operate without separators. The
decant oil is sent directly to the storage tank. Catalyst fines accumulate
in the tank, which is cleaned periodically. Some units continue to use
some form of slurry settler to minimize the ash content of decanted oil.
Above the bottoms product, the main column is often designed for
three possible sidecuts:

* Heavy cycle oil (HCO)—used as a pumparound stream, some-
times as recycle to the riser, but rarely as a product
* Light cycle oil (LCO)—used as a pumparound stream, sometimes
as absorption oil in the gas plant, and stripped as a product for
diesel blending; and
* Heavy naphtha—used as a pumparound stream, sometimes as
absorption oil in the gas plant, and possible blending in the
gasoline pool

In many units, the light cycle oil (LCO) is the only sidecut that
leaves the unit as a product. LCO is withdrawn from the main column
and routed to a side stripper for flash control. LCO is sometimes
treated for sulfur removal prior to being blended into the heating oil
pool. In some units, a slipstream of LCO, either stripped or unstripped,
is sent to the sponge oil absorber in the gas plant. In other units,
sponge oil is the cooled, unstripped LCO.
Heavy cycle oil, heavy naphtha, and other circulating side pump-
around reflux streams are used to remove heat from the fractionator.
They supply reboil heat to the gas plant and generate steam. The
amount of heat removed at any pumparound point is set to distribute
vapor and liquid loads evenly throughout the column and to provide
the necessary internal reflux.
Unstabilized gasoline and light gases pass up through the main
column and leave as vapor. The overhead vapor is cooled and partially
condensed in the fractionator overhead condensers. The stream flows
to an overhead receiver, typically operating at <15 psig (<1 bar).
Hydrocarbon vapor, hydrocarbon liquid, and water are separated in
the drum.