Determination of H2S in Crude Oil via a Rapid, Reliable and
Sensitive Method
Amir Vahid
=
a
Email: avahid753@gmail.com; vahida@ripi.ir
Abstract
Determination of hydrogen sulfide (H2S) in crude oil is very important due to the environmental impacts, industrial problems, and legal international limitat= ion of transportation. In the present work, H2S of crude oil is determined by liquid-liquid extraction followed by potentiometric titration= . Moreover, three factors including dilution ratio of crude oil with toluene, extraction time of H2S into the caustic phase and API of crude oil were inv= estigated via factorial design. The ANOVA results have revealed that the dilution rat= io, crude type, and extraction time have the highest effect of the recovery of = H2S from crude oil. In addition, the linear dynamic range of the method was fro= m 1 up to 2000 ppm which can be manipulated for lower or higher concentration by further optimization of the above-mentioned parameters. Finally, this metho= d is rapid, reliable, operator-independent, which these characteristics make it a useful technique for the field test of crude oil and overcome extreme uncertainty of H2S measurement.
Keywords: Crude Oil;
Determination; Experimental Design; Field test; Hydrogen Sulfide.
1. Introduction
H2S is one of the most hazardous compounds which is colorless
gas and can dissolved in aqueous and organic sol=
vents.
In addition, it is a poisonous and corrosive compound. In addition, it caus=
es
environmental damages (1-3). H2S can be present in petroleum=
and
petroleum products including asphalt, residual fuel oil, mid-distillates and
their blend, natural gas and LPG (4-6). H2S is evolved and produced in
production of fossil fuels in oil and gas industry. There is very tight int=
ernational
regulations in the transportation of crude oil and hydrocarbons (7). Due to this regulations, determination of =
H2S
at low level and also high level is very critical from industrial and
environmental point of view (8, 9). There are many methods for the determinati=
on of
H2S in crude oil and its derivatives. The method of measurements=
is given
in Table 1. As can be seen, there is no a valid and standard method for the
determination of H2S in crude oil specially in the presence of <=
span
class=3DSpellE>mercaptan, chlorine and some basic additives and scav=
engers
(10-17). Furthermore, there is demand for a method =
which
comprises requirements of a good method. These requirements including easy
operation in field or laboratory, low health and safety problem, operator
independent, low cost, repeatable, reproducible and providing very low
detection limit as well as wide linear dynamic range. Among the methods of =
H2S
determination, each of them has its own limitation (18-20). The health and
safety hazards related to H2S are summarized in Table 2 <=
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":"1089-8611
(Electronic)\\r1089-8603 (Linking)","ISSN":"10898611&qu=
ot;,"PMID":"24932545","abstract":"Hydrog=
en
sulfide (H2S) is an important signaling molecule with physiological endpoin=
ts
similar to those of nitric oxide (NO). Growing interest in its physiological
roles and pharmacological potential has led to large sets of contradictory
data. The principle cause of these discrepancies can be the common neglect =
of
some of the basic H2S chemistry. This study investigates how the experiment=
al
outcome when working with H2S depends on its source and dose and the
methodology employed. We show that commercially available NaHS should be
avoided and that traces of metal ions should be removed because these can
reduce intramolecular disulfides and change protein structure. Furthermore,
high H2S concentrations may lead to a complete inhibition of cell respirati=
on,
mitochondrial membrane potential depolarization and superoxide generation,
which should be considered when discussing the biological effects observed =
upon
treatment with high concentrations of H2S. In addition, we provide chemical
evidence that H2S can directly react with superoxide. H2S is also capable of
reducing cytochrome c3+with the concomitant formation of superoxide. H2S do=
es
not directly react with nitrite but with NO electrodes that detect H2S. In
addition, H2S interferes with the Griess reaction and should therefore be
removed from the solution by Cd2+or Zn2+precipitation prior to nitrite
quantification. 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTI=
O)
is reduced by H2S, and its use should be avoided in combination with H2S. A=
ll
these constraints must be taken into account when working with H2S to ensure
valid data. © 2014 Elsevier Inc. All rights
reserved.","author":[{"dropping-particle":"&q=
uot;,"family":"Wedmann","given":"Rudolf&=
quot;,"non-dropping-particle":"","parse-names"=
;:false,"suffix":""},{"dropping-particle":&qu=
ot;","family":"Bertlein","given":"S=
arah","non-dropping-particle":"","parse-names=
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;:"","family":"Macinkovic","given":=
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iven":"Jan
Lj","non-dropping-particle":"","parse-names&q=
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E.","non-dropping-particle":"","parse-names&q=
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t;Martin","non-dropping-particle":"","parse-n=
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uot;Nitric
Oxide - Biology and
Chemistry","id":"ITEM-1","issued":{"=
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"title":"Working
with \"H2S\": Facts and apparent
artifacts","type":"article-journal","volume&q=
uot;:"41"},"uris":["http://www.mendeley.com/docume=
nts/?uuid=3D9d845041-381a-4528-a933-661dec29d393"]}],"mendeley&qu=
ot;:{"formattedCitation":"(Wedmann
et al. 2014)","plainTextFormattedCitation":"(Wedmann et=
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2014)","previouslyFormattedCitation":"(Wedmann et al.
2014)"},"properties":{"noteIndex":0},"schema&=
quot;:"https://github.com/citation-style-language/schema/raw/master/cs=
l-citation.json"}(21). Furthermore, among the properties of the c=
rude
oil reported for the sale and refinery designing, H2S may be the
most unstable parameter, even in comparison with Reid vapor pressure and
specific gravity. So far, to the best of our knowledge, there is not an
appropriate analytical method for determination of H2S in crude =
oil
for use in both oil field and laboratory test. In the present work, a rapid
method was developed and investigated using factorial experimental design f=
or
the determination of H2S of crude oil which can be used in
laboratory or field test and led to the repeatable and accurate results.
Table1.
The health and safety hazards related to H2S are summarized =
in
Table 2 (Wedmann et=
al.
2014).
Table 2.
Furthermore, among the properties of the crude oil reported for the sal=
e and
refinery designing, H2S may be the most unstable parameter, even=
in
comparison with Reid vapor pressure and specific gravity. So far, to the be=
st
of our knowledge, there is not an appropriate analytical method for
determination of H2S in crude oil for use in both oil field and
laboratory test. In the present work, a rapid method was developed and
investigated using factorial experimental design for the determination of H=
2S
of crude oil which can be used in laboratory or field test and led to the
repeatable and accurate results.
2. Materials and Methods
All chemicals were purchased from Merck and used as received. In a typi=
cal
determination, 50 mL of crude oil and predetermined amount of toluene as Di=
luting
ratio were poured into a decanting funnel and shaken for a about 5 minutes.
Then 25 mL of 5 w/w% aqueous caustic solution was used as extracting agent =
and was
shaken for a predetermined time. The mixture was aged for 10 minutes to
separate oil and aqueous solution. Afterwards, 10 mL of aqueous solution was
decanted on a paper filter, and the filtrate was collected in a titration
beaker. After addition of 2 mL on concentrated solution of ammonia, the fil=
trate
was titrated with 0.05 N of silver nitrate solution. The first equivalent p=
oint
at about -600 mV is related to the H2S. Titration was carried out
using a Metrohm titrando
880 according to the UOP 209.
For the optimization of the condition of determination, a factorial des=
ign
was applied for the investigation of three main effects, including time req=
uired
for extraction, diluting ratio ration and crude type according to its API. =
12
experimental runs were designed and carried out. Calculation and modeling of
results were done using Design Expert 7.
Recovery is defined as the ratio of the concentration of the H2S
obtained in the designed test to H2S obtained at 2 hours and 60 C
with diluting ratio of 5.
3. Results and Discussion
The condition of statistically designed runs and their corresponding re=
sults
are given in Table 3.
Table 3.<=
b>
Analysis of variance of obtained results is
calculated and give in T=
able
4. The obtained Equation for model is given as Eq. 1 in terms of coded fact=
ors.
Table 4.
Recovery =3D +91.75 - (1.25 * A[1]) + (0=
.50 *
A[2]) +(4.75 * B) + (1.75 * C) + (=
0.42 *
BC) (Eqn. 1)
<= o:p>
As seen in Figure 1, and according to the Equation 1 and ANOVA (Table 4=
),
it can be said that Time has very little effect on the recovery of H2<=
/sub>S.
It is a very good property for an analytical method to carry out in short t=
ime.
Figure 1.
Figure 2.
Fig. 2 displays the effect of Diluting ratio which is the most effective
factor among all. Moreover, it is generally known that crude oil has very b=
road
range of properties in term of density and viscosity. H2S is tra=
pped
in the complex matrix of crude oil. Thus, the presence of toluene is essent=
ial
for the dilution of the crude oil and breaks its complex matrix and semi
ordered array of large molecules (wax, resin, and asph=
altene)
to easily evolve the trapped H2S.
As the crude oil becomes heavier as well as viscose, the
evolve of H2S becomes more difficult and led to the reduc=
tion
of recovery. So higher Diluting ratio is necessary in c=
ase of
heavy crude oil. This means that Diluting ratio is the most important
factor, as could be deduced from its F value in Table 4.
Figure 3.
Fig. 3 shows the effect of crude type on the recovery of H2S=
. As
the API of crude oil increases, the crude oil become lighter and recovery is
higher due to the better contact of oil phase and caustic phase an so need =
to
low Diluting ratio and vice versa.
Figure 4.
Fig. 4 illustrates the interaction of the crude type and diluting ratio=
in
a 3D curve. In case of heavier crude oil, i.e. at lower API, the effect of =
diluting
ratio is very large while at higher API, this effect is lower.
It can overall said that higher diluting ratio and lower extraction time
are in favor of recovery despite the fact that higher time, 10 minutes, is
steel low for a typical analytical method. For the determination of other
merits of this analytical method, additional test was done. In addition, on=
e of
the most important properties of a robust analytical me=
thods
is Linear Dynamic Range. Four crude oils containing H2S concentr=
ation
ranged from 0.2 to 1902 ppmw were examined. Mor=
eover,
obtained results showed that any concentration of H2S can be
determined by manipulation of amount of aliquot of crude oil used for test.=
For
the Crude oil contains 1902 ppmw, only 2 grams =
of
crude oil is adequate while for sample containing 0.2 =
ppmw
of H2S, 100 grams of crude oil is needed. RSD for the 148 ppmw H2S, in four determinations, was obta=
ined 98%.
Another advantage of this methods is its
compatibility for field determination because H2S is very volati=
le
and unstable. It is generally known that long delay between sampling and
determination led to the loss of H2S and cusses lower result.
Furthermore, for field test, operator can pour the fresh crude sample i=
nto
the decanter containing toluene and caustic solution. After that, vigorous
stirring of this mixture led to the absorption of H2S into the
caustic phase and prevent loss of the analyte b=
efore
titration or transport to laboratory.
It is noteworthy to say that for further improvement of quantification
limit and RSD of method, once can increase the absolute amount of crude oil=
and
extraction time. Furthermore, decreasing the amount of caustic solution, wh=
ich
results in higher concentration factor, and using centrifuge instead of
decanter for separation of caustic phase prior to titration can improve
quantification limit and RSD.
It is well known for those who work in petroleum laboratories that H
Conclusions
In this work, a method comprising the merits of very good analytical me=
thod
is reported. The method is rapid because the time needed for extraction is
lower than 10 minutes and a traditional potentiometric titration. Quantific=
ation
limit of the method is low enough. Linear Dynamic Range of the method is ve=
ry
broad and this range can be
tuned by changing the amount of crude oil. In addition, RSD of the method is=
up
to 98% which is very good for a volatile and unstable compound, i.e. H=
2S
in a complex matrix of crude oil. The equipment needed for this method is v=
ery
simple, accessible and cheap. Finally, the procedure of determination,
including extraction and titration, is very traditional and don't need high
level of expertise and skill. Furthermore, the method is capable of perform=
ing
for field test which is very important from industrial point of view. Also,
main factors of the method can be tuned easily according to the properties =
of
the crude oil to manipulate lower and upper limit of quantification.
Acknowledgment
This work was financially supported by Research Institute of Petroleum
Industry under the grant number of 1647601.
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Figures
Figure 1.
Figure 2.
Figure
3.
Figure 4.
Figure captions
Figure 1. Effect of factor A: Time on recovery.
Figure 2. Effect of factor B: Diluting ratio on recovery.
Figure 3. Effect of factor C:Crude type on recovery.<= o:p>
Figure 4. 2D representation of
interaction of Crude type and Diluting ratio.
Tables
Table 1.
|
Method |
Disadvantage |
Scope of work |
Dynamic range |
|
ASTM D5623 |
Narrow dynamic range, High cost, Limited sam=
ple
boiling range |
GC of Liquid distillates with FBP < 230 <=
/span>°C |
0.1 - 100 ppm |
|
UOP 212 |
Limited sample boiling range, High cost |
Potentiometric titration, ethane to
such gasoline |
1 to several thousand p=
pmw
of H2S |
|
ASTM D5705 |
Only applicable to vapor phase, Poor
repeatability |
H2S in the vapor
phase (equilibrium) of a residual fuel oil for field test using gas detection tubes and Can t=
est |
5 ppm v/v to 4000 ppm v/v <= o:p> |
|
UOP 163 |
Not applicable to crude oil, Complex data
interpretation |
H2S in Liquid hydrocarbons by
potentiometric titration |
1 up to =
100 ppmw. |
|
ASTM D7621 IP 570 ISO 8217 |
Not applicable to crude oil, mercaptan
interference |
H2S in Fuel Oils by Rapid Liquid Phase
Extraction |
1 up to =
50 ppmw |
|
UOP 41 ASTM D4952 Doctor Test |
Only for qualitative analysis, Using of
poisonous metal |
qualitative test of H2S in gasoline, jet fue=
l, kerosine and similar petroleum products and solvent=
s |
|
|
ASTM 6021 |
skilled operator and complex
calculations=
, High
cost |
H2S in Residual Fuels by Multiple Headspace
Extraction and Sulfur Specific Detection |
0.01 to 100 ppmw |
|
IP 399 |
Complex procedure and pure materia=
ls
needed. Oxidation and absorption may occur. |
H2S in Residual Fuels by sprctrophotometric determination. |
0.50 to 32 ppmw |
Table 2.
|
Concentration mg/kg |
Health & Hazard Effect |
|
<
0.02 |
Odor Detection L=
imit |
|
10 |
8 Hours Exposure=
Limit |
|
15 |
15 Min. STEL |
|
100 |
Common Ship Head=
space
Spec. |
|
300 |
Considered Immediately Hazardous |
|
713 |
LC50 Concentrati=
on |
|
1000 |
Common Tank, Ship
Headspace Concentration |
Table 3.
|
=
Std.
run order |
=
A: =
Time
(min) |
=
B: =
Diluting
ratio |
=
C: =
Crude
type (API) |
=
Recovery
% |
|
=
1 |
=
2 |
=
1 |
=
22 |
=
84 |
|
=
2 |
=
6 |
=
1 |
=
22 |
=
86 |
|
=
3 |
=
10 |
=
1 |
=
22 |
=
87 |
|
=
4 |
=
2 |
=
2 |
=
22 |
=
93 |
|
=
5 |
=
6 |
=
2 |
=
22 |
=
95 |
|
=
6 |
=
10 |
=
2 |
=
22 |
=
95 |
|
=
7 |
=
2 |
=
1 |
=
36 |
=
87 |
|
=
8 |
=
6 |
=
1 |
=
36 |
=
89 |
|
=
9 |
=
10 |
=
1 |
=
36 |
=
89 |
|
=
10 |
=
2 |
=
2 |
=
36 |
=
98 |
|
=
11 |
=
6 |
=
2 |
=
36 |
=
99 |
|
=
12 |
=
10 |
=
2 |
=
36 |
=
99 |
Table 4.
|
=
Source |
=
Sum of
Squares |
df |
=
Mean
Square |
F
Value |
=
P-value
Prob > F |
=
Remark |
|
=
Model |
=
319.1 |
=
5 |
=
63.8 |
=
328.2 |
=
<
0.0001 |
=
significant |
|
=
A-Time |
=
9.5 |
=
2 |
=
4.8 |
=
24.4 |
=
0.0013 |
|
|
=
B-Diluting
ratio |
=
270.8 |
=
1 |
=
270.8 |
=
1392.4 |
=
<
0.0001 |
|
|
=
C-Crude
type |
=
36.8 |
=
1 |
=
36.8 |
=
189.0 |
=
<
0.0001 |
|
|
=
BC |
=
2.1 |
=
1 |
=
2.1 |
=
10.7 |
=
0.0170 |
|
|
=
Residual |
=
1.2 |
=
6 |
=
0.2 |
|||
|
=
Corr.
Total |
=
320.3 |
=
11 |
Table Legends
Table1. Standard methods of H2S determination and their
properties (Nadkarni 2=
000).
Table 2. Exposure limit and its related hazards.
Table 4. ANOVA of obtained results of H2S analys=
is.