Hydrocarbon Processing - June 2021 - 55

Catalysts
S. RIVA, BASF, Italy; and S. CHALLIS,
Chalcat Consulting, UK
Understanding HCN in FCC:
Formation, effects and mitigating options
In these challenging times, fluidized catalytic cracking units
(FCCUs) aim to improve margins by processing poorer- or
different-quality feeds, while maintaining good yield performance.
These feeds may produce more coke and have elevated
nitrogen levels, which can then increase the potential for higher
cyanides and associated equipment damage in wet hydrogen
sulfide (H2
S) service.
A general overview of cyanide formation and associated corrosion
problems in the catalytic fractionator and gas plant is provided
to help explain the nature of the problem and what mitigating
actions can be taken. Many FCCUs have no history of cyaniderelated
problems, as potential cyanide effects are adequately controlled
by water washing and by using chemical additives.
In addition, unit operating parameters can be adjusted to
help control cyanide production. Application of this approach
is discussed for a unit that observed higher cyanides in the catalytic
fractionator overhead water following increased processing
of heavier feeds enabled by a catalyst change. An in-depth investigation
showed that the amount of cyanides was increasing
with the heaviness of the feed. This unit subsequently found no
cyanide-related problems during a unit turnaround inspection.
Wet H2S equipment damage mechanism. In wet H2
S in water inhibits the recombination
S service,
H° is formed in the steel corrosion reaction, as indicated in
FIG. 1. The presence of H2
of the H°, and potentially enables these atoms to penetrate into
and diffuse through the equipment steel.
The H° can combine inside the steel to form molecular H2
,
which causes these larger structures to be trapped and unable to
diffuse back out. The H2
the form of blistering, sulfide cracking, H2
stress-oriented H2
circumstances, the iron sulfide (FeS) scale formed on the steel
surface in a wet H2
produced, thereby generally mitigating the issue.
HCN and potential for wet H2
cyanide (HCN) and ammonia (NH3
(aq) ←→NH4
can then cause damage to the steel in
-induced cracking and
-induced cracking. However, under normal
S environment provides a barrier to the H°
S damage. Hydrogen
) from the FCC reactor
leave in the catalytic fractionator overhead product, and
free cyanide ions (CNHCN
(aq) + NH3
→ Fe(CN)6
4- + S2)
are formed in water by the reaction
+ (aq) + CN-
(aq). The CNtends
to remove the FeS scale on steel surfaces by forming a
ferrocyanide complex, which is soluble in water by the reaction
FeS + 6CN-
. Removal of the scale then
Metal
H
External (air)
Fe + H2S
2H° (atomic)
H2
H
Blister
FeS +2H° (atomic)
H2 (molecular)
FIG. 1. Steel corrosion reaction and damage caused by H2
.
Hydrocarbon Processing | JUNE 2021 55
allows H° to diffuse into the steel, leading to wet H2S damage,
as previously described.
Experience shows that hydrocyanic acid in the overhead water
does not cause significant metal loss, as NH3
absorbed with
the HCN acts as a buffer, typically keeping the water pH above 8.
Locations prone to cyanide corrosion. A typical catalytic
fractionator overhead system with water wash injection used as a
corrosion mitigation measure is shown in FIG. 2. Locations most
prone to cyanide corrosion problems are circled.
Industry experience shows that if wet H2
S damage occurs,
it is most frequently observed in the vicinity of the compressor
coolers and absorber tower due to high HCN in the gas streams
vs. hydrocarbon liquids and in the deethanizer tower. Reported
problems have been related to changes in feed quality (e.g., high
nitrogen, low sulfur), which required adjustment to corrosion
control measures, inadequate water wash rates, poor distribution
of water upstream of coolers, and poor separation of hydrocarbons
and water.
HCN limits and measurement of free cyanides. A typical
guideline is to maintain free cyanides (HCN and CN)
in sour
water below 20 ppm (mg/l) for corrosion control. However, it
is relatively difficult to accurately measure free cyanides due to a
number of potential interferences. Measuring total cyanides includes
both free cyanide plus stable cyanide compounds, which
Process side
H
H2
H
Crack
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