Reduce Salt Corrosion Rates With Stronger Base Amines
Reduce Salt Corrosion Rates With Stronger Base Amines
Reduce Salt Corrosion Rates With Stronger Base Amines
Special Report
Refining Developments
J. LACK and B. HARRELL, Baker Hughes,
Sugar Land, Texas
Reduce salt corrosion rates with stronger base amines
When processing discounted crude oils, one of the primary
challenges refiners face is under-salt corrosion in crude unit
distillation overhead systems. The combination of higher over-
head chloride levels and the presence of amine contaminants
in discounted crudes can greatly increase the risk of salt forma-
tion. For those overhead systems where amine salt corrosion
cannot be mitigated by conventional methods, the application
of nontraditional neutralizing amines can be beneficial. In par-
ticular, under-salt corrosion rates produced by stronger base
amines are significantly less than those of weaker bases found
as contaminants, such as ammonia or monoethanolamine.
Corrosion rates can also be reduced when the weaker base con-
taminant salts are mixed with stronger base amine salts. Signifi-
cant reductions in corrosion rates have been confirmed in both
laboratory and field operating environments.
Controlling corrosion in the overhead condensing section
of crude atmospheric distillation units is a constant challenge
for refiners. The typical treatment strategy involves a delicate
balancing act of neutralizing acids in condensed waters with
ammonia or amines while avoiding the formation of corrosive
salts via a vapor phase reaction with hydrogen chloride (HCl).
Recent trends of declining crude quality, which result in less
effective chloride removal and the frequent presence of amine
contamination, have increased the risk of forming corrosive
salts. Neutralizing amine selection that reduces the risk of salts
caused by the neutralizer does not affect the risk of salt forma-
tion by contaminant amines or ammonia.
Contaminant reduction techniques, such as caustic addi-
tion for chlorides and desalter acidification for amines, can be
successful at reducing risk of salt formation.
1,2
However, these
approaches have limitations if significant reduction is required.
Water wash applications have had mixed results as a means for
reducing salt formation and are not always viable in some over-
head configurations. With these options exhausted, a refiner is
left with operational changes or enduring frequent bundle fail-
ures, both of which can be costly. There is a need for additional
options, including chemical treatment at the overhead, which
can reduce corrosion from salts.
Chemical suppliers have focused on filming inhibitors to re-
duce corrosion rates from acidic water and amine neutralizers
to control pH. Various strategies have been used to optimize
neutralizer performance and reduce the risk of salt formation.
These have included the use of volatile amines, weaker base
amines, amine blends and thermodynamic model guidance.
37
While these techniques have been successful at reducing salt
formation by the neutralizer, they do not address the corrosion
from salts formed by ammonia or contaminant amines. To re-
duce corrosion from salts formed by contaminants, first con-
sider the chemical drivers that make salts corrosive.
Theory. Ammonia or amines can react with HCl to form a
solid or liquid salt:
RNH
2
+ HCl o RNH
3
Cl (1)
These salts are highly hydroscopic and easily absorb water
from the steam used for stripping the distillation column. In
the presence of water, the salts ionize, forming ammonium or
aminium ions with chloride ions:
RNH
3
Cl o RNH
3
+
+ Cl
(2)
According to the Brnsted theory, which describes acid-
base interactions, the reaction of an acid with a base produces a
conjugate base and a conjugate acid.
8
The strength of the con-
jugate acid is inversely related to the strength of the base from
which it is derived:
AH + B
o
A
+ BH
+
(3)
(Acid) (Base) (Conjugate base) (Conjugate acid)
The resulting ammonium or aminium ion is the conjugate
acid of ammonia or the amine. This conjugate acid is weak
by definition in that there is an equilibrium established, which
depends on its acid constant:
RNH
3
+
o H
+
+ RNH
2
(4)
where:
(5) K
a
=
H
+
RNH
2
RNH
3
+