Page 46 - Fluid Catalytic Cracking Handbook

28 Fluid Catalytic Cracking Handbook

few FCC units use cooled heavy naphtha from the main column as
sponge oil.
The lean sponge oil enters the absorber on the top tray. The gas
from the presaturator or from the primary absorber enters below the
bottom tray. The rich sponge oil from the bottom is then returned to
the main fractionator. The lean gas leaves the top of the absorber
to an amine unit for H 2S removal prior to entering the refinery fuel
gas system.

Stripper or De-ethanizer

The HPS liquid consists mostly of C 3's and heavier hydrocarbons;
however, it also contains small fractions of C 2's, H 2S, and entrained
water. The stripper removes these light ends. The liquid enters the
stripper on the top tray. The heat for stripping is provided by an
external reboiler, using steam or debutanizer bottoms as the heat
medium. The vapor from the reboiler rises through the tower and strips
the lighter fractions from the descending liquid. The rich overhead
vapor flows to the HPS via the condenser and is fed to the primary
absorber. The stripped naphtha leaves the tower bottoms and goes to
the debutanizer. Usually, at least one draw is installed in the tower to
remove the entrained water.

Debutanizer

The stripper bottoms contain C 3's, C 4's, and gasoline; the debuta-
nizer separates the C 3's and C 4's from the gasoline. In some units,
the hot stripper bottoms can be further preheated before entering the
debutanizer. In a number of units, the stripper bottoms is sent directly
to the debutanizer. The feed enters about midway in the tower. Debuta-
nizer feed is always partially vaporized because the debutanizer
operates at a lower pressure than the stripper. A control valve that
regulates stripper bottoms level is the means of this pressure drop. As
a result of this drop, part of the feed is vaporized across the valve.
The debutanizer separates the feed into two products. The overhead
product contains a mixture of C 3's and C 4's. The bottoms product is
the stabilized gasoline. Heat for separating these products comes from
an external reboiler. The heating source is usually the main fractionator
heavy cycle oil or slurry. Steam can also be used.