1
THE INFLUENCE OF NAPHTHENIC ACID AND
SULFUR COMPOUND STRUCTURE ON GLOBAL
CRUDE CORROSIVITY UNDER VACUUM
DISTILLATION CONDITIONS
Heather D. Dettman, N. Li and D. Wickramasinghe (NRCan)
J. Luo (U. of Alberta)
Presented to:
COQA/CCQTA Joint Meeting
New Orleans, LA
February 10 – 11, 2010
Mechanisms of Refinery Corrosion
 Refinery corrosion occurs at temperatures
between 220°C and 400°C
 Naphthenic (organic) acids (RCOOH) reach their
boiling points and condense on metal surfaces,
removing iron [Fe] and eventually causing pits
 Sulfur-containing compounds decompose to form
hydrogen sulfide (H2S), where iron removal causes
general corrosion but can form protective films
 Acids and hydrogen sulfide work together:
Fe + 2RCOOH → Fe(RCOO)2 (oil soluble) + H2
Fe + H2S → FeS (oil insoluble) + H2
Fe(RCOO)2 + H2S → FeS + 2RCOOH
2
Corrosivity, TAN, and Sulfur
3
 Corrosivity does not always correlate with total
acid number (TAN) (Derungs, 1956; Messer
2004)
 Are organic acid molecular species in some oils “less
active” that those in other oils?
 Does high sulfide content result in iron sulfide film that
protects the plant metallurgy?
 Another reason?
 This project was conducted to improve the
understanding of the contributions of specific
structural features of organic acids and sulfur
compounds to corrosivity at refinery temperatures
Corrosion Test Unit
4
Simulates corrosion in
vacuum distillation unit
Coupon locations
in vapor phase
where vapor
condenses on
metal surface
Coupon location
in liquid
Features:
1. Volume: 250 mL
2. Charge: Any crude or
refinery feed blend
3. No. of coupons: 4
4. Operates under
vacuum throughout the
temperature range
When Does TAN Correlate with Corrosivity?
5
10
Corrosion Rate (mm/y)
9
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
TAN (mg KOH/g)
Liquid - 300 C
Liquid - 350 C
Vapor -300 C
Vapor -350 C
Liquid - 330 C
Vapor -330 C
TAN correlates with corrosivity when different concentrations
of the same acids are tested. (Corrosion rates of carbon steel
coupons for commercial naphthenic acids [CMNA] in white oil.)
6
When Does TAN NOT Correlate with Corrosivity?
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
CMNA ATHB
SA1
3.22
3.76
1.36
3.85
1.31
2.51
2.33
0.78
4.15
0.10
Corrosion Rate (mm/y)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
SA2
As Produced
AB16 AB17 INT22 INT30
Oil
Liquid Phase
Commercial Products
Vapor Phase
When comparing corrosion rates for different crude oils.......?
Corrosivity of Organic Acid Compounds
Liquid Phase
Vapor Phase
Corrosion rates of carbon steel coupons for organic acid
compounds in white oil (TAN= 5.0mg KOH/g) at atmospheric
equivalent temperature (AET) of 300°C (250°C actual)
7
8
What about Sulfur?
 Sulfur compounds R-S-R
H2S + by-products
Thermal Cracking
Sulfidic Corrosion
FeS
 Model sulfur compounds were chosen to represent the different C-S
bonds found in petroleum. For example:
H
R
C
H
H
S
C
H
Octyl sulfide
R
S
Dibenzothiophene
 Thermal decomposition studies of nine sulfur compounds dissolved in
white oil (1 wt% sulfur) were performed
Thermolysis of Sulfur Compounds
70
H2S Yield (wt% Feed Sulfur)
60
s
S
50
s
40
30
S
S
20
S
10
S
0
200
220
240
260
S
280
300
320
340
360
380
400°C
Dicyclohexyl disulfide
℃
Dibenzothiophene
Sec-butyl disulfide
Benzyl sulfide
1,3-Dithiane
Diphenyl sulfide
Benzyl phenyl sulfide
Dodecyl sulfide
Octyl sulfide
Temperature (
)
At temperatures as low as 200°C (392°F), within 2
hours -CH2-S- bonds (sulfides) crack and form H2S
9
Corrosion
Corrosion Rate (mm/y)
(mm/y)
H2S Effects on Corrosivity of Organic
Acids under Refinery Conditions
9
8
7
6
5
4
3
2
1
0
10
Fe(RCOO)2 + H2S → FeS + 2RCOOH
Low H2S generation
Enhanced corrosion in vapor phase
with little film formation
Fe + H2S → FeS + H2
High H2S generation
Inhibited corrosion in liquid
phase due to significant
film formation
Fe + 2RCOOH → Fe(RCOO)2 + H2
No H2S present
CMNA
octyl sulfide
in whiteSul
oil +f phenyl
sulfide
Octinywhite
l Suloilf +i de+CM
NA CMNA
Di phenyl
i de+CM
NA
Liquid
i n Li q
CMNA
inNA
white oil
CM
Vapor
Phase
i n Cond
Influence of presence (1wt% S) or absence of sulfur compounds on the
corrosion rates of commercial naphthenic acids (CMNA) in white oil (Total
acid number = 5.0mg KOH/g) for carbon steel coupons at atmospheric
equivalent temperature of 300°C (250°C actual)
11
Why Does TAN Not Correlate with Corrosivity?
Model compound studies show that:
 Small organic acid molecules (boiling point <
300°C) are significantly more corrosive than larger
molecules (boiling point > 300°C)
 In liquid phase, chain and 1-ring acids are the most
corrosive; in vapor phase, chain and 1-ring cycloalkane
(naphthenic) acids are most corrosive
 Vapor phase corrosion due to organic acids is
greatest at temperatures above the boiling point of
the acid
 Sulfur compounds can decompose to form
hydrogen sulfide at temperatures as low as 200°C;
acid corrosion can be inhibited or enhanced
depending on how much hydrogen sulfide is
present
12
Crude Oil Analyses
Crude Oil
ATHB
SA1
SA2
C
83.07
86.77
83.67
H
10.51
10.64
10.31
AB16
AB17
INT22
INT30
81.89
84.75
86.34
86.10
12.66
11.70
11.77
12.29
Elemental (wt%)
N
S
0.52
4.77
0.60
0.94
0.83
3.76
0.44
0.25
0.34
0.32
3.85
2.51
0.78
0.10
O
1.14
1.05
1.43
Density
(g/mL)
1.0100
0.9934
1.0090
1.16
0.79
0.76
1.19
0.9304
0.9319
0.9379
0.9282
TAN
(mg KOH/g)
3.39
0.60
3.22
As produced:
ATHB Athabasca bitumen
SA
South America
Commercial products:
AB
Alberta heavy oil or bitumen
INT
Non-Canadian crude of non-disclosed
geographical location
* Crudes were topped at 204°C for corrosion testing
1.04 *
1.27 *
2.11 *
4.15
13
HTSD of Crudes
8 00
6 00
A
700
Temperature (C)
Temperature (C)
7 00
800
5 00
4 00
3 00
2 00
1 00
0
-1 00 0
600
B
500
400
300
200
100
0
20
40
60
80
10 0
-100 0
20
Weight % Off
A THB
SA 1
40
60
80
Weight % Off
S A2
AB 16
AB 17
INT22
High temperature simulated distillation (HTSD) of:
A – Crudes, as produced
B – Crudes, commercial products
INT30
10 0
Analyses of Extracted Organic Acids
Organic
Acids
CMNA
ATHB-OA
SA1-OA
SA2-OA
Content in
Crude (wt%)
2.68
0.99
2.22
C
74.35
78.06
81.18
78.14
H
11.97
10.29
9.97
10.48
AB16-OA
AB17-OA
INT22-OA
INT30-OA
1.11
1.43
2.37
4.83
78.80
77.96
81.48
81.09
10.15
10.06
10.98
11.66
CMNA
Elemental (wt%)
N
S
0.00
0.00
0.37
3.75
0.44
0.99
0.82
3.54
0.57
0.58
0.36
0.34
3.72
4.75
0.90
0.27
14
O
13.68
7.54
7.42
7.02
6.76
6.64
6.29
6.64
Commercial naphthenic acids
Organic Acids Extracted from Crude Oils (Mediaas et.al., 2003)
As produced:
Commercial products:
ATHB
Athabasca bitumen
AB
Alberta heavy oil or bitumen
SA
South America
INT
Non-Canadian crude of nondisclosed geographical
location
Crude Oil TAN versus Organic Acid Yield
6.00
TAN (mg KOH/g)
5.00
4.00
3.00
2.00
1.00
0.00
0.00
1.00
2.00
3.00
4.00
Yield (wt% Crude)
5.00
6.00
15
16
Distillation of Organic Acids from Crudes
800
800
A
600
500
400
300
200
100
600
500
400
300
bp<300°C
“most
corrosive”
200
100
0
-100
B
700
Temperature (C)
Temperature (C)
700
0
0
20
40
60
80
100
0
Weight % Off
CMNA
ATHB-OA
SA1-OA
20
40
60
80
100
Weight % Off
SA2-OA
AB16-OA
AB17-OA
INT22-OA
Organic acids from:
A – CMNA and crudes, as produced
B – Crudes, commercial products
INT30-OA
Corrosivity Results – CMNA & Crude Oils
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
3.22
3.76
1.36
3.85
1.31
2.51
2.33
0.78
17
4.15
0.10
Corrosion Rate (mm/y)
1.8
Corrosivity does not
correlate with TAN
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CMNA ATHB
SA1
SA2
As Produced
AB16 AB17 INT22 INT30
Oil
Liquid Phase
Commercial Products
Vapor Phase
Corrosion rates of carbon steel coupons for CMNA in white oil
and crude oils at AET of 300°C (250°C actual)
CorrosivityHigher
Results
– CMNA
corrosivity
of CMNA is& Crude Oils
TAN
S (wt%)
Corrosion Rate (mm/y)
1.8
of lower
3.00 explained
3.39 0.60by its
3.22higher content
1.36 1.31
2.33
0.00 boiling
4.77 acid
0.94 components
3.76
3.85
2.51
0.78
(i.e. 50wt%
18
4.15
0.10
have bp<300°C)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CMNA ATHB
SA1
SA2
As Produced
AB16 AB17 INT22 INT30
Oil
Liquid Phase
Commercial Products
Vapor Phase
Corrosion rates of carbon steel coupons for CMNA in white oil
and crude oils at AET of 300°C (250°C actual)
Corrosion Rate (mm/y)
Boiling point distributions of organic acids in
Corrosivitythese
Results
CMNA
& Crude
crudes do–not
explain why
SA2 and Oils
INT30
corrosivity
than4.15
the
TAN
3.00 have
3.39 higher
0.60 liquid
3.22 phase1.36
1.31
2.33
S (wt%)
0.00 other
4.77 crudes;
0.94
3.76
3.85 2.51
0.78 &/or
0.101-ring
higher contents
of chain
naphthenic acids in lowest boiling species are
1.8
implicated
1.6
19
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CMNA ATHB
SA1
SA2
As Produced
AB16 AB17 INT22 INT30
Oil
Liquid Phase
Commercial Products
Vapor Phase
Corrosion rates of carbon steel coupons for CMNA in white oil
and crude oils at AET of 300°C (250°C actual)
Boiling point
Corrosivity Results – CMNA & Crude
Oils
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
CMNA ATHB
SA1
3.22
3.76
1.36
3.85
Corrosion Rate (mm/y)
1.8
1.6
1.4
1.2
1.0
1.31
2.51
20
distributions of
2.33 4.15
organic
acids in
0.78 0.10
these crudes do not
explain why AB16
has high vapor
phase corrosivity
0.8
0.6
0.4
0.2
0.0
SA2
As Produced
AB16 AB17 INT22 INT30
Oil
Liquid Phase
Commercial Products
Vapor Phase
Corrosion rates of carbon steel coupons for CMNA in white oil
and crude oils at AET of 300°C (250°C actual)
Identification of Sulfidic Corrosion
Influences in Crude Oils
 Replace the oil matrix with a sulfur-free
medium i.e. white oil
 Organic acids isolated from each of the Global
crudes were dissolved in white oil at TAN values
similar or slightly lower than those of the original
crude oils tested
 If the oil matrix has no influence on corrosion, the
corrosion rates of the white oil mixtures should be
the same as that of the original oil
 If the oil matrix influences corrosion, the corrosion
rates of the white oil mixtures could be either
greater or lesser than those of the original oil
21
Corrosion Rate (mm/y)
Corrosivity Results – Extracted Acids
22
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CMNA ATHB- SA1OA
OA
SA2OA
AB16- AB17- INT22- INT30OA
OA
OA
OA
Extracted Organic Acids in White Oil
Liquid Phase
Vapor Phase
Corrosion rates of carbon steel coupons for CMNA and extracted
organic acids in white oil at AET of 300°C (250°C actual)
Differences in Corrosivity Results
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
3.22
3.76
CMNA ATHB
SA1
SA2
1.36
3.85
1.31
2.51
2.33
0.78
4.15
0.10
Corrosion Rate (mm/y)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
As Produced
AB16 AB17 INT22 INT30
Commercial Products
-0.8
-1.0
-1.2
-1.4
Difference (Organic Acids in White Oil - Oil)
Liquid Phase
Vapor Phase
23
Differences in Corrosivity Results
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
3.22
3.76
CMNA ATHB
SA1
SA2
1.36
3.85
1.31
2.51
2.33
0.78
4.15
0.10
Corrosion Rate (mm/y)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
AB16 AB17 INT22 INT30
As Produced
Commercial Products
Negative-0.8
difference for AB16 vapor phase
corrosion-1.0
rate shows that sulfidic corrosion is
-1.2
the predominant
corrosion mechanism
-1.4
Difference (Organic Acids in White Oil - Oil)
Liquid Phase
Vapor Phase
24
Differences in Corrosivity Results
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
3.22
3.76
CMNA ATHB
SA1
SA2
1.36
3.85
1.31
2.51
2.33
0.78
4.15
0.10
Corrosion Rate (mm/y)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
AB16 AB17 INT22 INT30
As Produced
Commercial Products
-0.8
Positive differences
for corrosion rates
-1.0
suggest sulfidic
film formation provides
protection-1.2
for original crudes; protection does
not correlate
-1.4 with sulfur content.
Difference (Organic Acids in White Oil - Oil)
Liquid Phase
Vapor Phase
25
Differences in Corrosivity Results
TAN
S (wt%)
3.00
0.00
3.39
4.77
0.60
0.94
3.22
3.76
CMNA ATHB
SA1
SA2
1.36
3.85
1.31
2.51
2.33
0.78
4.15
0.10
Corrosion Rate (mm/y)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
As Produced
AB16 AB17 INT22 INT30
Commercial Products
-0.8
Positive differences
for vapor phase
corrosion-1.0
rates for SA1, SA2 and INT30 also
-1.2
indicate higher
contents of chain &/or 1-ring
naphthenic
-1.4acids in lowest boiling species of
these crudes compared
to those(Organic
of Alberta
Difference
Acids in White Oil - Oil)
and INT22 crudes
Liquid Phase
Vapor Phase
26
CONCLUSIONS
 TAN values of crude oils are not reliable
indicators of crude oil corrosivity
 Crude corrosivity appears to be determined by:
 Low boiling acids (i.e. bp<350°C) where content of
chain and/or 1-ring naphthenic acids will be
important
 Content of thermally-labile sulfur species
 Hydrogen sulfide-generating ability of the crude
will be influenced by:
 Content of CH2-S bonds in sulfur species
 Thermal history of crude oil (field and plant)
27
IMPLICATIONS OF RESULTS
 If TAN does not correlate with crude
corrosivity, why is it used for setting crude
prices?
 How does production method influence
corrosivity (i.e. SAGD versus mined?)
 How does the content of low-boiling chain and 1ring naphthenic acids compared to the total organic
acid content, and the contents of thermally-labile
sulfur species work together to influence corrosivity
 When can blending a high TAN crude with a
low sulfur, low TAN crude (or diluent?) result in
enhanced corrosion?
28
Funding Acknowledgements
 Alberta Science and Research Authority
(COURSE/Alberta Energy Research Institute
[AERI])
 Canadian Association of Petroleum
Producers (CAPP)
 Natural Resources Canada through partial
funding by the Canadian Program for Energy
Research and Development, and the
Technology and Innovation Program
29