Ammonium Bi-sulphide Corrosion in Hydrocrackers
Transcript of Ammonium Bi-sulphide Corrosion in Hydrocrackers
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7/25/2019 Ammonium Bi-sulphide Corrosion in Hydrocrackers
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REFINING
.
t i f I t j j J G : t 4 e : ; : 4 : 4
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mmon um bi sulphide ;
corrosion in hydrocrackers
Factors in the
corrosion
of effluent
condensers
in
hydrocrackers are
the
subject of this
artiele, which
ineludes
a case
study of the problems experienced
at
Riyadh refinery, Saudi Arabia,
and the corrective actions
that were
taken
Walid A Al-Naim
Saudi Arameo
W
orldwide hydrocracker opera
tional experience has drawn
attention to
unique corros ion
problem
that
is . developing in reactor's
effluent condensers. This corros ion prob
. . . .m is defined
as ammonium
bi-sulphide
. H
4
(H5))
corrosion. Ammonium bi-sul
phide corrosion is
considered very risky
and may lead to a catastrophe because
hydrocrackers operate at high pressure,
high t e m p e r a ~ e , and in a hydrogen
atmosphere. Numerous incidents of efflu
ent condenser failures and consequent
fires have been experienced in a number
of refineries around the world due to this
form of corrosion.
Ammonium bi-sulphide's inherent
corrosivity
is
a function of the organic
nitrogen (pyridines, quinolines, and
acridines)
and sulphur
(thiophene,
benzo and di-benzo-thiophene) content
in
the
hydrocracker fresh feed that is
finally corrcrted intc Nll
3
al1d H
2
S
in
the hydrotreating reactors. As organic
sulphur and nitro gen levels increase in
the
fresh feed to the hydrocracker, more
ammonium bi-sulphide is
produced,
ading to corrosion attack in the efflu
~ o n d e n s e r . ..
The ammonium bi-sulphide corrosion
is further w o r s ~ n e d
when
hydrocracker
feed
is
heavier than the conventional vac
uum gasoil (VGO) feed. An example of
heavier feed stock
is
VGO with demet
allised oil (DMO >
1050F)
feed which
contains high levels of organic nitrogen,
and asphaltene which
is
a precursor to
carbon lay
down
on
the
catalyst.
NH
4
(HS) formation
Ainmonium bi-sulphide corros ion is con
sidered a main source of potential hazard
to hydrocrackers in all parts of the world.
T ~ e recipitation of
the ammonium
bi
sulphide on t e effluent con enser tu es
causes under-deposit corrosin and tube
pitting. Thereafter, condenser failure is
bouna
to occur, with a potential disaster.
AS a result, remedial measures
and
correc
ive
actions at design stages
and
unit oper-
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IN THIS AREA, DEPOSITION OF SOUD
:
NH4HS
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OCCUR
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Temperature, Deg.
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Figure
1
Impact
of p
factor on ammonium bi sulphide formation
ation re being take!1
and
implementecl in
an attempt to mitigate this phenomenon
as
much
as possible
and
thereby ensuring
the plant safety
and
integrity. Hydroc
racker operational experience has led to
the identification of several parameters
that are considered as driving forces for
ammonium
bi-sulphide corrosion attack,
as
listed in Table 1.
The table shows
the
optimum parame
ters to operate the hydrocracker reactor
effluent condenser free from
ammonium
bi-sulphide attack. These values were
brought into practice after extensive sur-
Optimum parameters
Operational
Optimum
parameters
value
1. Piehi Kp Factor 0.15
2.
NH.(HS) wt % in wash water 4.8
3.
Wash
water rate,
vol%
leed 5.2
4. Oxygen (ppm) in wash water 0.05
5_Condenser tube velocity,
Ips
15
Table 1
veys and e x p e r i ~ n d u c t e d by NACE
and analysed
~
Effect of Kp factor
The levels of nitrogen
and
sulphur con
tent in feedstocks mainly dictate
the
extent of ammonium bi-sulphide forma
tion in the reactor effluent condenser. In
hydrc-processing
plants
that process
heavy feed stocks such as VGO
and
DMO,
NH
3
and
HzS
are produced in fairly high
concentrations, leading to a
highKp
fac-
tor (mol% NH
3
x mol% H
2
S).
The ammonium bi-sulphide fouling
rate at the effluent condenser is classified
as severe, modera te,
and
low based on
the
Kp
factor levels. The severity of corrosion
is indicated as follows:
Severe
(5) Kp
>
0.5
M o d e r a ~ e
M) Kp