AMINE PLANT CORROSION REDUCED BY REMOVAL OF BICINE
Technical Article
Presented at
Gas Processors Association Annual Convention
San Antonio, Texas, USA, March 12, 2003
By
Gary L. Lawson (Presenter), Arthur L. Cummings, and Shade Mecum
MPR Services, Inc.
Dickinson, Texas
ABSTRACT
Bicine, an amino acid, has been found in numerous amine gas treating systems including
tail gas treating units, certain refinery services, and some natural gas processing plants. Bicines
(bicine and other similar amino acids) are formed in amine gas treating systems as a result of
amine degradation due to the presence of oxygen and/or sulfur dioxide. The presence of Bicine
alone in the amine system is not necessarily corrosive. However, Bicine is a strong chelator
with iron and can initiate, in the presence of H2S, a fast acting corrosion mechanism. Through
both laboratory analytical studies and numerous full scale in plant Bicine removal projects, it
has been demonstrated that amine system corrosion can be reduced when the amine system is
cleaned by ion exchange through the removal of Bicine and the precursors of Bicine. Numerous
examples and case histories are presented to demonstrate the benefits of removing Bicine from
contaminated amine solutions.
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
Page 1 of 7
AMINE PLANT CORROSION REDUCED BY REMOVAL OF BICINE
BICINE
Bicine is an amino acid that results from amine degradation due to the presence of
oxygen and/or sulfur dioxide. The amine solvents experiencing this degradation are MDEA,
DEA, TEA, and mixed amine solvents containing any of these amines as components.
Chemical Formula
H-O-C-C
\ /
\
N
C =O
/
/
H-O-C-C H-O
N,N-bis(2-hydroxyethyl)glycine (Bicine)
Formation
Bicine has been shown to form in amine systems utilizing MDEA-based amines when
subjected to O2 contamination. Plant operating experience indicates a slow but continuous
formation of Bicine in the amine system when low levels of O2 and/or SO2 are present in the
amine system feed gas.
Numerous Bicine formation mechanisms have been proposed. The generally accepted O2
degradation path involves one of two mechanisms: 1) a disproportionation reaction of two moles
of DEA to form one mole each of TEA and MEA followed by oxidation of the TEA to form
Bicine or 2) a disproportionation reaction of MDEA to TEA and other mixed amines followed by
further oxidation of the TEA to Bicine. Regardless of the actual formation mechanism, the
Bicine formation reaction can be generally described as depicted below.
O2
S2O3= + MDEA + heat + time
SO2
Bicine
Other amino acids
Formate
Acetate
DEA
TEA
Accelerated Bicine formation has been noted after up-sets in the SRU and the
hydrogenation section of the TGTU increased the amount of O2, H2S, and SO2 entering the
TGTU absorber. The resulting elevated levels of thiosulfate (S2O3=) contribute to formation of
Bicine and other amino acids.[1,2] Removal of the thiosulfate, a heat stable salt anion precursor
for Bicine, soon after a TGTU upset could prevent much of the resulting Bicine formation in the
TGTU amine because the Bicine formation mechanism may take several weeks to produce a
substantial amount of Bicine and the subsequent corrosion.[1]
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
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CORROSION
MPY (Mills per Year)
The primary corrosion mechanism in an amine system is H2S attack on carbon steel with
the resultant formation of FeS. In an ideal amine system with a clean amine solution, the
protective layer of FeS formed on the carbon steel prevents further corrosion. However, when
Bicine is present the protective layer is continuously destroyed or not formed at all. Bicine is a
strong chelator and will chelate iron maintaining the iron in a soluble form and preventing or
weakening the stable formation of the protective FeS layer. The unprotected carbon steel is once
again attacked by the H2S resulting in an accelerated corrosion rate. [3, 4,5]
Bicine by itself in an amine solution is not corrosive to carbon steel. As shown in Figure
1, there is no increase in the instantaneous corrosion rate of a 30 wt % MDEA solution when
H2S is not present. The corrosion data presented in Figures 1 and 2 were generated using a
Mini-Amine-Plant designed to simulate, as closely as possible, an actual amine plant in
operation. The corrosion probe was placed in the 250 oF reboiler section. The corrosion
studies were conducted using a linear polarization probe to measure instantaneous corrosion
rates. The triple electrode probe design eliminates interference by solution conductivity on the
indicated corrosion rate.[3]
50
40
Add Bicine
30
20
30 wt% MDEA
with Zero Bicine
10
0
0
10
12
HOURS
Figure 1 Corrosion Study
MDEA, Bicine, with Zero H2S
Bicine prevents the protective FeS passivation layer from forming. The chelating
corrosion mechanism involves the removal of the protective FeS layer by dissolution of the Fe ++
by the chelant (Bicine). The mechanism proposed is as follows:
FeS + Bicine Bicine-Fe++ + S=
S= + H2O
HS- + OH2 HS + Fe FeS + H2 + S=
H2S must be present and it is the corroding agent as indicated in Figure 2 below. To
develop the data presented in Figure 2, a sample of 30 wt % MDEA containing 2 wt % Bicine,
taken from a refinery, was utilized. Initially the corrosion probe was conditioned in the sample
for several hours, then H2S was added (the refinery amine had only 0.003 mol/mol Lean
Loading). Then additional Bicine was added. The H2S loading from the point at which Bicine
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
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was added to the end of the test decreased to 0.008 m/m yet the corrosion rate increased
significantly over the same period. The contrast of Figures 1 and 2 indicates the role of Bicine in
the corrosion process as a chelator rather than as a direct corroder.[3]
70
MPY (Mills per Year)
60
50
40
Add H2S
to 0.015 m/m
30
20
10
Add Bicine
0
0
10
20
30
40
50
60
70
80
HOURS
Figure 2 Corrosion Study
MDEA, Bicine, and H2S
As long as Bicine and H2S are present, Fe will be continuously chelated from any available source.
Bicine Removal
Because Bicine is such a strong corrosion enhancer, concentrations of Bicine in an amine
system should be kept very, very low in the low hundreds of ppm. Thus distillation, which
focuses on moving the solvent away from impurities, is not an efficient means of removing low
levels of Bicine from amine treating units. However, ion exchange plucks the Bicine out of
passing solvent with little energy expended to move the solvent. Ion Exchange Technology has
proven successful in removing Bicine and Bicine precursors when properly implemented.
Several Case Histories are presented below.
Case History 1
A Texas gas plant utilizing a specialty MDEA solvent was experiencing significant
corrosion shortly after plant start-up in the carbon steel sections of the amine plant. With the
assistance of their amine supplier, the plant determined that the corrosion was being caused by
the presence of Bicine generated due to O2 in the sour gas feed. The incoming oxygen incursion
into the amine system could not be eliminated. A combination corrosion inhibitor/oxygen
scavenger was added to the amine to scavenge the O2 and to prevent further degradation of the
MDEA to Bicine. The implementation of the O2 scavenger program did decrease the Bicine
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
Page 4 of 7
formation rate. However, this action was not totally adequate to control the corrosion. MPR was
requested to provide their mobile ion exchange equipment to reduce the Bicine The Bicine level
was reduced from 3,929 ppmw to 471 ppmw in the operating amine system.[6] Figure 3 depicts
the Bicine reduction during the mobile cleaning period. The corrosion was reduced by the
mobile cleaning service but returned when the mobile cleaning equipment was removed.
3500
3000
Bicine, ppm
2500
2000
1500
1000
500
0
0
10
12
14
16
18
20
Exhaustion Cycles
Figure 3 Bicine Removal
Case History 2
A western U. S. natural gas producer was concerned about corrosion in the amine system
due to Bicine. The amine contained about 5,200 ppm amino acids, including 1,825 ppm Bicine.
During a turn-around, the amine system was taken offline, and the entire system circulated
through mobile ion exchange unit. The Bicine content was reduced to less than 25 ppm (0.0025
wt %).
Case History 3
A tail gas unit located in a gulf coast refinery was experiencing unexplained corrosion.
The refiner contacted the amine supplier who identified Bicine as a potential cause. MPR was
requested to do additional laboratory analytical work and a laboratory control study on Bicine.
The lab work and study performed by MPR confirmed that Bicine was present and that it was the
root cause of the corrosion. A demonstration of the custom designed removal process was
performed in the lab followed by a successful clean up of the operating system. Bicine was
reduced from 3 % to 0.7 %.
Case History 4
In 2002, Pinnacle Gas Treating Inc. (a subsidiary of Anadarko Petroleum) at the Bethel
Gas Plant at Tennessee Colony, Texas became concerned when their Bicine level reached the
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
Page 5 of 7
500 ppm level in their specialty formulated MDEA solvent. The plant began experiencing high
corrosion rates, excessive foaming, equipment fouling and unstable plant operations. Chlorides
were also elevated and were an additional concern. A mobile ion exchange project successfully
reduced the Bicine to less than 40 ppm. Figure 4 shows the Bicine reduction during the mobile
cleaning job.
ppm
1,200
System Samples
Bicine, ppm
Chloride, ppm
1,000
800
600
400
200
0
Sample Date/MPR #
12-Sep
18-Sep
19-Sep
23-Sep
25-Sep
27-Sep
3120
3152
3164
3168
3172
3181
Figure 4 Bicine and Chloride Removal
The following Table 1 depicts the actual results of other operating systems that have been
cleaned by ion exchange technology.
Table 1 Examples of Bicine Removal Projects
System
A
B
C
D (Case 3)
E
F
G
H
I
J (Case 1)
K (Case 2)
L (Case 4)
Amine Initial Bicine, Wt. % or ppm Removal End Point, Wt % or ppm
MDEA
4.9
1.8
MDEA
3.9
1,200 ppm
MDEA
2.1
3,200 ppm
MDEA
3.1
0.7
MDEA
2.0
0.6
DEA
1.2
0.6
MDEA
1.0
2,300 ppm
MDEA
2.1
1,400 ppm
MDEA
3,000 ppm
200 ppm
MDEA
4,000 ppm
500 ppm
MDEA
1,825 ppm
24 ppm
MDEA
500 ppm
40 ppm
The wide range of final end point concentrations shown in Table 1 reflects the
specification requested by the plant operator. Several of the data points illustrate that Bicine can
be reduced down to the very low part per million range if desired.
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
Page 6 of 7
BICINE ANALYSIS
Bicine is identified and quantified by Ion Chromatography. Other amino acids are also
separated in the ion chromatograph but are not usually identified. Total amino acid content can
be estimated from the ion chromatograph and from a combination of charge balance of
measurements of ionic compounds in the solution and knowledge of the contribution of amino
acids to amine titrations and total heat stable salts tests.
SUMMARY
Bicine is a very corrosive degradation product produced in certain DEA and MDEA
systems. Amine system operators should take steps to minimize O2 and SO2 incursion into the
amine system and should monitor Heat Stable Salts and Amino Acids (Bicine) concentrations in
the amine. By skilled application of ion exchange, Bicine can be removed and maintained at
very low levels that will reduce or prevent corrosion.
REFERENCES CITED
1.
Critchfield, J.E. and Jenkins, J.L., "Evidence of MDEA degradation in tail gas treating
plants", Petroleum Technology Quarterly, Spring 1999, pp 87-95.
2.
Kohl, Arthur and Nielsen, Richard, Gas Purification, Fifth Edition, 1997, p. 233.
3.
Cummings, A.L., Veatch, F.C., Keller, A.E., Corrosion and Corrosion Control Methods in
Amine Systems Containing H2S, Paper 97341, NACE Corrosion/97, March 1997.
4.
Mecum, S.M., Veatch, F.C., and Cummings, A.L., Why Caustic Addition is Bad for Amine
Systems, Hydrocarbon Processing, October 1997, pp. 115-19.
5.
Rooney, P.C., Bacon, T.R., DuPart, M.S., "Effect of Heat Stable Salts on Solution
Corrosivity of MDEA-based Alkanolamine Plants. Part III", Proceedings of the Laurance
Reid Gas Conditioning Conference, March 1997.
6.
Howard, M. and Sargent, A., Operating Experience at Duke Energy Field Services Wilcox
Plant with Oxygen Contamination and Amine Degradation, Proceedings of the Laurance
Reid Gas Conditioning Conference, February 2001.
Lawson, et al, Amine Plant Corrosion Reduced by Removal of Bicine
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