Page 49 - Fluid Catalytic Cracking Handbook

Process Description 31

Ammonia sulfide is not corrosive, but it can precipitate. Under-
deposit corrosion and pitting can occur.
Typically, sour water from the FCC contains a mixture of ammo–
nium sulfide and ammonium bisulfide with an ammonia-to-hydrogen
sulfide ratio between 0.5 and 1.0
Most refiners employ continuous water wash as the principal method
of controlling corrosion and hydrogen blistering. The best source of
water is either steam condensate or well-stripped water from a sour
water stripper. A number of refiners use ammonium polysulfate to
neutralize hydrogen cyanide and to control hydrogen stress cracking.
In the gas plant, corrosive agents (H 2S, HCN, and NH 3) are most
concentrated at high-pressure points. Water is usually injected into the
first and second-stage compressor discharges. The water contacts the
hot gas and scrubs these agents. There are two common injection
methods: forward cascading and reverse cascading.
In forward cascading (Figure 1-14A), the water is normally injected
into the discharge of the first-stage compressor and condenses in the
interstage cooler. From the interstage drum, the water is pumped to
the second-stage discharge, condenses in the cooler, and collects in
the EPS, From the high pressure separator, the water is then pressured
to the sour water stripper.
In reverse cascading (Figure 1-14B), fresh water is injected into the
second-stage discharge. The water containing corrosive agents is
pressured to the first-stage discharge and then back to the main
fractionator overhead. From the overhead receiver, the water is then
pumped to the sour water stripper. Reverse cascading requires one less
pump, but a portion of cyanide captured in the second stage is released
in the interstage, forming a cyanide recycle. Consequently, forward
cascading is more effective in minimizing cyanide attack.

TREATING FACILITIES

The gas plant products, namely fuel gas, C 3's, C 4's, and gasoline,
contain sulfur compounds that require treatment. Impurities in the gas
plant products are acidic in nature. Examples include hydrogen sulfide
(H 2S), carbon dioxide (CO 2), mercaptan (R-SH), phenol (ArOH), and
naphthenic acids (R-COOH). Carbonyl and elemental sulfur may also
be present in the above streams. These compounds are acidic.
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