Page 47 - Fluid Catalytic Cracking Handbook
P. 47
Process Description 29
The overhead product is totally liquefied in the overhead condensers.
A portion of the overhead liquid is pumped and returned to the tower
as reflux. The remainder is sent to a treating unit to remove H 2S and
other sulfur compounds. The mixed C 3's and C 4's stream can then be
fed to an ether or an alkylation unit. It can be fed to a depropanizer
tower where the C 3's are separated from C 4's. The C 3's are processed
for petrochemical feedstock and the C 4's are alkylated.
The debutanized gasoline is cooled, first by supplying heat to the
stripper reboiler or preheating the debutanizer feed. This is followed
by a set of air or water coolers. A portion of the debutanizer bottoms
is pumped back to the presaturator or to the primary absorber as lean
oil. The balance is treated for sulfur and blended into the refinery
gasoline pool.
Gasoline Splitter
A number of refiners split the debutanized gasoline into "light" and
"heavy" gasoline. This optimizes the refinery gasoline pool when
blending is constrained by sulfur and aromatics. In a few gasoline
splitters, a third "heart cut" is withdrawn. This intermediate cut is low
in octane and it is processed in another unit for further upgrading.
Water Wash System
The cat cracker feedstock contains low concentrations of organic
sulfur and nitrogen compounds. Cracking of organic nitrogen com-
pounds liberates hydrogen cyanide (HCN), ammonia (NH 3), and other
nitrogen compounds. Cracking of organic sulfur compounds produces
hydrogen sulfide (H 2S) and other sulfur compounds.
A wet environment exists in the FCC gas plant. Water comes from
the condensation of process steam in the main fractionator overhead
condensers. In the presence of H 2S, NH 3, and HCN, this environment
is conducive to corrosion attacks. The corrosion attack can be any or
all of the following types [2]:
* General corrosion from ammonium bisulfide
* Hydrogen blistering and/or embrittlement
* Pitting corrosion under fouling deposits
Ammonium bisulfide is produced by the reaction of ammonia and
hydrogen sulfide [2]:
