Page 45 - Fluid Catalytic Cracking Handbook

Process Description 27

ammonium salts, that can cause equipment fouling. This mixture Is
partially condensed and flashed in the HPS.
The vapor from the HPS flows to the primary absorber and the
liquid is pumped to the stripper. The HPS is essentially a separation
stage with an external cooler located between the primary stripper and
absorber. In some units, they are a single tower,

Primary Absorber

The HPS overhead vapor contains appreciable amounts of C 3's and
heavier components. The primary absorber recovers these components.
The HPS vapor enters below the bottom tray and proceeds up the
tower contacting absorption oil. Heavy components are absorbed in
the oil.
Two sources of absorption oil are normally utilized in this tower.
The first is the hydrocarbon liquid from the main fractionator overhead
receiver. This stream, often called "wild," or unstabilized, naphtha,
enters the absorber a few trays below the top tray. The second absorbent
is cooled debutanized gasoline, which generally enters on the top tray.
It has a lower vapor pressure and can be considered a trim absorbent.
The expression "lean oil" generally refers to the debutanized gasoline
plus the unstabilized naphtha from the overhead receiver.
The absorption process is exothermic. To improve C 3+ recovery,
liquid from one or more of the middle trays is pumped through an
intercooler and returned to the tray below. In some FCC units, the lean
oil feed is chilled.
To enhance C 3+ recovery, some units have installed presaturator
drums that function as an additional absorption stage. In this operation,
the cooled debutanized gasoline is mixed (presaturated) with the
absorber overhead gas. The mixture is cooled and flashed in the
presaturator drum. The liquid from this drum is then pumped to the
top of the primary absorber.

Sponge OH Absorber

The vapor from the primary absorber or the presaturator contains a
small quantity of gasoline. The sponge oil absorber recovers this
gasoline. "Sponge oil" is stripped or unstripped light cycle oil. It is
used for final absorption of the dry gas stream. Instead of LCO, a