59
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
ISSN: 0974- 6846
A simple pH-based method for estimation of CO2 absorbed in alkanolamines
Asha Masohan1, Mamshad Ahmed1, Sushil Kumar Nirmal1, Ajay Kumar2 and Madhukar Onkarnath Garg1
1
Indian Institute of Petroleum, Mohkampur, Dehradun, Uttarakhand, 248005, India.
Chemical Metrology Section, National Physical Laboratory, New Delhi, 110012, India.
2
[email protected], [email protected]
Abstract: CO2 is recognized as a major contributor of
Typically primary and secondary amines form acidic
global warming. We report a facile, sensitive and anion species like bicarbonate, carbonate and carbamate
accurate method for estimating CO2 content in the with CO2 (equations 1 to 5).
alkanolamine solvents. It can be applied to a host of R2NH + H2O ←→ R2NH2+ + OH… (1)
alkanolamine solvents as well as solvent mixtures for CO2 + 2 H2O ←→ HCO3- + H3O+
… (2)
screening for prospective absorbent for CO2 capture. In HCO3- + H2O ←→ CO3-- + H3O+
… (3)
this method, pH of the CO2-loaded absorbent sample is CO + R NH ←→ R NH+COO- (zwitterions) … (4)
2
2
2
measured and referred to a “reference pH graph” of R NH+COO- + R NH ←→
2
2
corresponding amine with sulphuric acid that is R NCOO- + R NH +
… (5)
2
2
2
constructed under specific conditions. The authenticity of (carbamate) (protonated amine)
the method was validated from known amounts of CO2
Tertiary amines cannot form carbamate species,
added in the form of CO2-saturated water to three because they do not have hydrogen attached to the
different types of alkanolamines i.e. the primary nitrogen atom. Typically tertiary amines react according
(monoethanolamine, MEA), secondary (diethanolamine, to equation 6.
DEA) and tertiary (triethanolamine, TEA) alkanolamines.
CO2 + R3N + H2O ←→ HCO3- + R3NH+
… (6)
The method was compared with known methods and
One of the main criteria for selection of suitable
found to be better in terms of accuracy.
solvent requires estimation of CO2 in both the gas and
Keywords: CO2 estimation, pH, pKa, Alkanolamines, Gas
liquid phases for ascertaining the mechanism and
absorption.
material balance for rate and capacity estimation of the
Introduction
solvent. Accurate amounts of CO2 in liquid solvents are
Carbon dioxide (CO2) has been proven to be
also required to remove discrepancies in parameter
responsible for 80% of green house gases, contributing to
estimation in simulation and modeling by mathematical
the increase of earth’s temperature. Half of the CO2
model for plant design and feasibility studies.
emission is produced by industries and power plants
There are many methods available for estimation of
using fossil fuel (Desideri et al., 1999). Consequently,
carbon dioxide in absorbing liquids specifically the
modern research geared towards developing new CO2
alkanolamines. The UOP method 826-81 (UOP Method,
capture technologies worldwide.
1981) uses phosphoric acid to liberate the dissolved CO2
Traditionally, absorption/stripping via circulated
in amine solutions. The method involves quite a few steps
aqueous alkanolamines have been the most favored
like use of mercury for determining the volume of
technology for removing CO2 from process and waste gas
calibrated tube, introducing sample of CO2-containing
streams. The existing commercial processes based on
amine, adding phosphoric acid to liberate CO2 from
alkanolamines are highly energy- and cost- intensive. As
sample and use of mercury leveling bulb to force the
much as 80% of the total energy consumption in an
liberated gas into the measuring burette repeatedly till no
alkanolamine absorption process occurs during solvent
further CO2 remains dissolved in the amine and washing
regeneration (White. et al., 2003). Therefore, search is on
of carbonimeter. Regarding precision of the method no
for new improved solvents with higher rates and
estimated standard deviation is reported due to lack of
capacities for absorption of CO2, high degradation
sufficient data to permit this calculation with at least 4
resistance, low corrosiveness and low energy use for
degrees of freedom. Also the elapsed time for one
regeneration. Such a solvent will decrease both capital
analysis is 1.0 hour.
and operating costs of the process thereby reducing the
Currently the most popular analytical method for
cost of electricity at thermal power plant when CO2
determining the acid gas concentration in aqueous amine
removal is integrated in the plant. Many new solvents are
solutions employs a wet chemistry titration. In the
being researched for potential commercial use to remove
method for CO2, the amine sample is mixed with an
CO2 from flue gas streams.
excess of standard base and heated to boiling. Since the
Three classes of alkanolamines, primary (MEA),
amine-acid gas complex is thermally unstable, the acid
secondary (DEA) and tertiary (TEA and methyl
gas is converted into an ionic species and is precipitated
diethanolamine, MDEA) or their mixtures, are generally
by an appropriate metal salt like BaCl2. The filtrate is
used as absorbing liquids. The different categories of
titrated with a standard acid to determine the
amines differ in the type of mechanisms with which they
concentration of uncarbonated amine in the sample (here
react with CO2 as well as the reaction products and the
onwards termed as “BaCl2 method”). Indicators like
heats of reaction.
bromocresol green, cresol red and phenolphthalein etc.
Research article
Indian Society for Education and Environment (iSee)
“pH-based CO2 estimation”
http://www.indjst.org
Asha Masohan et al.
Indian J.Sci.Technol.
60
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
ISSN: 0974- 6846
pH
pH
pH
are commonly used to indicate the end points (Coldrey & verified daily before use. Blank runs should also be made
Harris, 1976; Ellis et al., 1963; Hikita et al., 1977; Jensen to properly execute the titration. The solution analysis by
et al., 1954; Gas Conditioning Fact Book, 1962; Jou et al., titration is often complicated by the presence of impurities
1982; Weiland &Trass,1969). The CO2 content of the and degradation products (Arnold & Pearce, 1960). Most
solution is calculated as the difference between the total chemical methods of ethanolamine analysis depend on
amine and uncarbonated amine. However, the amine functionality and are nonspecific and inaccurate
calculations require assumption of 1:1 stoicheometry (Brydia & Pearce, 1967).
A method based on GC has been reported by
between amine and CO2 (Weiland et al., 1969) which is
not true. Another titrimetric method for determination of Robbins and Bullin (Robbins & Bullin, 1984) which is fast,
CO2 in ethanolamine is the
accurate and analyses the amine
solution for acid gases like H2S and
UOP method 829-82 [here
Fig. 1. pH graph of 2.6M AMP vs H 2 SO 4
CO2, hydrocarbons, water and
onwards termed as “MeONa
13
amine content.
However, the
method”]
(UOP
Method,
12
example cited is with MDEA only
1982). The method involves
11
10
and the structural characteristics of
dissolution of CO2-absorbed
9
other alkanolamines are bound to
sample
in
anhydrous
8
affect the retention time of the
methanol.
The
7
constituent components and lot of
amine/methanol solution is
6
work needs to be done on their
titrated
with
standard
5
separation using the prospective
methanolic sodium hydroxide
4
solvents used in CO2 separation for
solution
using
3
thymolphthalein
as
the
this method to become a standard
2
indicator and calculating the
method. Moreover the effects of
1
CO2 content.
amine salts, that result when the
0
alkanolamine reacts with CO2, on
The
wet
chemistry
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
methods
involving
retention time is not known.
moles of H2SO4 mole -1 amine
precipitations and titrations
Thus, the analytical methods
Fig. 2. pH graph of MEA, DEA, TEA and MDEA
have serious disadvantages.
reported above can provide only
vs H 2 SO 4
The total time required for an
the directional values for total
13
experienced technician to
amount of CO2 in the lean and rich
MEA- H2SO4
12
analyze
one
carbonated
amines but are not suited for
DEAH2SO4
11
TEA- H2SO4
amine solution could be as
studies involving small changes in
10
MDEA- H2SO4
long as 1-2 hours. During
CO2 content as is required in
9
part of this time, the CO2studies
like kinetics of absorption in
8
the
stirred
contactor/wetted-wall
amine solution is open to the
7
column. The number of data points
atmosphere so both flashing
6
in these studies is large and the
of CO2 and degradation of the
5
CO2 build up that starts from just a
amine can occur. Also
4
stoicheometry for different
few mg is very gradual. Present
3
types
of
alkanolamines
studies were, therefore, aimed to
2
(primary,
secondary
and
0.0 0.1 0.2 0.3
0.4 0.5 0.6 0.7 0.8 develop a sensitive and fast
-1
tertiary) for CO2 is usually
method for estimation of CO2 for
moles of acid mole of amine
assumed as one which is not
analyzing sequential build up of
Fig. 3. pH graph of 2.6M AMP vs acetic acid
true and leads to erratic
CO2 in the alkanolamine solvents
13
results for CO2 content. This
ranging from ppm to percent level
12
for absorption-based studies. The
has serious bearing on results
11
method was applied to CO2-loaded
for the weight of CO2
10
estimated
because
the
alkanolamines which were obtained
species formed upon reaction
after absorption of CO2 in the
9
of alkanolamine with CO2 may
alkanolamines of different chemical
8
structures (MEA, DEA, TEA etc.).
vary from monoionic HCO37
Experimental
and carbamate to di-ionic
6
CO3-- ions (equations. 2, 3 and
1. Reagents and equipments: All
5
5) in different proportions.
the chemicals used in the present
4
Freshly prepared solutions of
study were 97-99% pure. H2SO4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 (98%), DEA (98%), TEA (97%) and
standardized acid and base
moles of acetic acid mole -1 amine
must also be available and
NaOH (97%) were purchased from
Research article
“pH-based CO2 estimation”
Asha Masohan et al.
Indian Society for Education and Environment (iSee)
http://www.indjst.org
Indian J.Sci.Technol.
61
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
ISSN: 0974- 6846
pH
SD Fine – Chem. Ltd., India. HCl, MEA (98%), oxalic acid addition of acid (0.5 ml) the pH was noted. Addition of
ampules (volumetric solution N/10) and BaCl2 (99%) were acid increased the temperature of mixture and was
purchased from Qualigens, India while MDEA (99+%) allowed to cool to 25oC before taking the pH reading.
was purchased from Aldrich Organics, New Jersy, USA. Initial pH of amine was nearly 12. Readings were taken
Methanol (>99%) from Merck Limited, India; Acetic acid till the pH fell to nearly 3 (pH range of acid). Weight of
(99.8%) from Rankem Fine
acid added was calculated from
Chemicals Ltd., India and
its normality and the volume
Fig. 4. pH graph of MEA, DEA and TEA vs
oxalic acids
Na2CO3 from BDH, E. Merck,
added. The weight thus obtained
13
was converted to moles of acid
India and 2-amino-2-methyl-112
by dividing with 98 and
propanol (AMP) from Merck
MEA- Oxalic acid
11
DEA- Oxalic acid
subsequently to moles acid moleSchuchardt OHG, Germany
TEA- Oxalic acid
10
1
were purchased. All solutions
amine by dividing the moles of
9
were prepared in double distilled
acid by moles of amine. A graph
8
water. Thymolphthalein indicator
was plotted for moles acid mole-1
7
solution taken was from Loba
amine vs. corresponding pH for
6
Chemie Pvt. Ltd., India.
the whole pH range. This was the
5
pH meter used was of
“reference pH graph” (Fig. 1).
4
Thermo Orion, Model 420,
3
Similarly
reference
pH
2
Beverly, MA 01915, USA. All
graphs were obtained for MEA,
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
titrations were carried out at
DEA, TEA and MDEA with
-1
moles of oxalic acid mole of amine
25oC.
H2SO4 (Fig. 2). Graphs were
also plotted for 2.6M AMP (Fig.
2. Preparation of solutions
3) with acetic acid and MEA,
100 ml of desired strengths
DEA and TEA with oxalic acid
(1M or 2.6M) of MEA, DEA, TEA,
(Fig. 4) to study the effect of
MDEA, and AMP absorbents were
structure of acid on pH curve and
prepared.
Methanolic NaOH
stoicheometry.
(0.05M)
was
prepared
by
dissolving 2.1 g NaOH in 100 ml
4. Preparation of CO2-loaded
water and making up the solution
alkanolamine samples
to 1L by adding methanol as given
4.1. Absorption of CO2 in
in reference (UOP Method, 1982).
alkanolamine: 50 ml of 1M
BaCl2 used was 0.68 M while
solution of MEA was taken in a
100 ml three-neck round bottom
H2SO4, acetic acid and oxalic
flask. CO2 absorption in amine
acids used for pH titration were
0.1N.
was started by passing CO2 gas
250 ml each of 0.05N NaOH,
at a flow rate of 5.5 sec for 10 ml
0.06M BaCl2 and 0.02N HCl were
of CO2 gas. Absorption was
prepared for use for estimation of
continued for 1h. This resulted in
CO2 in CO2-saturated water.
the CO2-loaded amine sample.
3. Titration for
reference graphs
plotting
of
10 Drops of thymolphthalein
indicator were added to 20 ml of
methanol in a 100 ml beaker.
This solution was titrated against
methanolic NaOH (termed as
MeONa) till light blue color
appeared. 3 ml of (2.6M) solution
of AMP was added to this
mixture. Moles of amine taken
were calculated by dividing
weight of amine in 3 ml solution
by its molecular weight. pH of this
solution was noted. The resulting
methanolic solution of amine was
then titrated against H2SO4 with
constant stirring by magnetic stirrer. After every small from the graph.
Research article
“pH-based CO2 estimation”
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4.2. pH Measurement of CO2loaded amine samples: Alkaline
solution of 20 ml methanol was
prepared in the same way as
under 3 by titrating with MeONa
till blue to thymolphthalein
indicator. 1 ml of the CO2-loaded
MEA sample prepared under 4.1
was immediately added to this
solution.
The blue solution
turned colorless because of
acidic CO2 in amine. pH of this
solution was noted and referred
to the corresponding pH value on
the reference pH graph plotted
under 3. Corresponding moles
acid mole-1 amine was noted
Asha Masohan et al.
Indian J.Sci.Technol.
62
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
4.3. Calculations for weight of CO2 in loaded amine: The
weight of CO2 is calculated from the moles acid mole-1
amine obtained from reference pH graph in 4.2 as given
below (equations 7 to 10):
Moles of CO2
Moles CO2 mole-1 amine =
… (7)
Moles of amine
Weight of CO2/44
=
… (8)
Moles of amine
Weight of CO2 = Moles CO2 mole-1 amine × Moles of
amine × 44
… (9)
=Moles CO2 mole-1 amine ×
Weight of amine *
Mol. Wt. of amine
× 44 ..(10)
(*Weight of amine is the weight in the volume taken for
pH estimation)
5. Validation of results
5.1. Reparation of saturated solution of CO2 in water:
100 ml of distilled water was taken and degasified
over ultrasonic bath for 1h. CO2 was then passed through
this water (maintained at 25oC) at the same flow rate as
under 4.1 for 1h. Estimation of CO2 was done by taking 5
ml of this CO2-saturated water in a mixture of 10 ml
NaOH and 10 ml BaCl2 and titrating excess NaOH with
HCl. Amount of CO2 was calculated by the difference.
ISSN: 0974- 6846
noted that addition of 5 ml water (as CO2-saturated
solution) results in dilution effect on concentration of
amine from 3 ml to 8 ml i.e. by a factor of 2.66. So the
strength of amine reduces to 1M after addition of CO2saturated water. Therefore, the “reference pH graph” to
be used for reference of pH for this sample was that
constructed with 1M MEA solution. From the pH value of
the sample moles of acid mole-1 amine was noted from
pH graph and amount of CO2 calculated by using
equations 7-10. Similar estimations were also made by
adding 5 ml of the above mentioned CO2-saturated water
in 2.6M solutions of DEA and TEA and referring to the
“reference pH graphs” of 1M solutions of corresponding
amines. The results obtained (Table 1) were compared
with the amount of CO2 actually present in 5 ml of CO2saturated water (estimated earlier as under 5.1).
The estimations for mixtures containing same
volumes of CO2-saturated water in the three amines were
also done by two of the reported, the MeONa and the
BaCl2, methods (Table 1).
6. Estimation of CO2 in loaded amines
Based on the accuracy of the results, the present pHbased method was then used in estimating CO2 content
in the CO2-loaded amine samples prepared as described
under 4.1.
Completely loaded samples of alkanolamines: 3 ml of the
5.2. Adding known volume of CO2-saturated water to CO -absorbed amine sample was added to 20 ml of
2
amine and estimating by the method developed: 3 ml of alkaline
methanol as under 3. pH of the sample is noted
alcoholic amine solution of 2.6M MEA was prepared in
and moles acid mole-1 amine obtained from the MEAsimilar way as mentioned under 2. A 5 ml sample of CO2H2SO4. Weight of CO2 absorbed in amine is then
saturated water prepared as above was added to this
calculated from the value of moles acid mole-1 amine.
amine solution. pH of this mixture was noted. It is to be
Results were compared with those obtained by the
MeONa and BaCl2 methods. Similar
Table 1. Estimation of saturated amount of CO2 in water as such and after
estimations were made for CO2
addition in different amines
absorbed in DEA and TEA. The
Wt. of CO2 in 5 mL of CO2-saturated water (mg)
results are shown in Table 2.
Strength CO2 estimated
Partially
loaded
samples
of
of
alkanolamines containing variable
In water In amine
Amin
amine
amounts of CO2: That the method is
from pH
Methanolic
e
solution, Alkaline BaCl2
suitable for any content of CO2 in
NaOH Method reference graph
BaCl2
Method
(M)
alkanolamines was tested by
(UOP method, of amine with
(Literature)
Method
conducting kinetic experiments in
H2SO4
1982)
stirred cell. CO2 was passed at the
MEA
2.67M
5.701
5.5
6.1813
5.984
interphase of the alkanolamine with
DEA
2.67M
8.11297
2.816
stiiring of the liquid phase and
(× 1.9=5.35)
collecting CO2-absorbed samples at
TEA
2.67M
11.44
8.11297
5.104
an interval of few seconds (30
seconds) to minutes till the end of
MEA
2.67M
5.632
6.41
4.699
5.55
experiment. The results are shown in
DEA
2.67M
8.223
3.168
Fig. 5. Similar estimations were
(× 1.9=6.02)
made for CO2 in DEA and TEA also.
TEA
2.67M
7.32
9.3985
5.346
In case of secondary amine like DEA
MEA
2.67M
5.700
6.64
5.9
it was observed that the results
neede to be multiplied by a factor of
DEA
2.67M
6.18
2.99
1.9 for exact amounts (perhaps
(× 1.9=5.68)
because
of
stoicheometric
TEA
2.67M
7.78
5.34
requirements).
Research article
“pH-based CO2 estimation”
Asha Masohan et al.
Indian Society for Education and Environment (iSee)
http://www.indjst.org
Indian J.Sci.Technol.
63
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
ISSN: 0974- 6846
Therefore, its “reference pH graph” for the three amines
Results and discussions
Because of the large number of new
solvents could be used for estimation of acidic component like CO2
that are being
in amine.
screened
today
for
C. Validation: Based on the
Table 2. Estimation of CO2 in the loaded amine samples
translating into a costabove
reasoning
the
(mg)
BaCl2
MeONa
Reference pH graph
effective technology and the
validation
was
done
for
the
Amines
method
method
method
inherent
disadvantages
results obtained for adding
MEA
66.17
76.75
46.28
associated
known volumes of saturated
with
the wet chemistry
solution of CO2 containing
MEA
71.1
98.7
46.8
methods as is realized by the
known
amount of CO2.
MEA
72.5
94
47.7
scientific community, a new
Repeat values of 0.025
33.16
method developed in this
moles CO2 L-1 were obtained
DEA
64.22
52.87
× 1.9 = 62.89
laboratory is being reported.
for CO2-saturated water as
31.1
determined
by alkaline BaCl2
A. Structures and pKas of the DEA
62.4
65.4
× 1.9 = 61.88
materials used: Structures of
precipitation. Same volumes
(5mL) of this CO2-saturated
the alkanolamines used in
34
DEA
62.8
71.3
water, with known CO2
present study are shown in
× 1.9 = 64.6
Table 3. These structures
content, were added to each
TEA
34.18
20.36
12.67
include all the three types i.e.
of the MEA, DEA and TEA
the primary, secondary and
samples simultaneously so
Table 3 – Structures of alkanolamine absorbents used for
tertiary alkanol monoamines.
that there was no time gap
present studies
The pKa values for the
for escape of CO2 from
amine-acid systems of the
water. Estimation of this CO2
present studies are provided
in each amine was done by
Monoethanolamine
Diethanolamine (DEA)
in Table 4. The reported
all the three (MeONa, BaCl2
(MEA)
values are for aqueous
and
the
present
pH)
solutions
while
the
methods. As seen in Table 1
experimentally
determined
values of CO2 estimated in
values are for the methanolic
amine by the pH graph
solutions of alkanolamines.
method are of very high
2-amino-2- methyl-1The latter are being reported
accuracy as it matches with
propanol (AMP)
Triethanolamine, TEA
for the first time.
As
the alkaline BaCl2 results in
expected pKa of MEA, DEA
water (error of only 4.4, 0.88
and -5.5% for MEA, DEA
and triethanolamine (TEA)
and TEA respectively for
goes on decreasing as the
such low amount as 5.6 mg
substitution on nitrogen atom
N-methyl
2-(2-aminoethylamino)
CO2). For secondary amine
increases. However, these
diethanolaomine (MDEA)
ethanol (AEAE)
values are lower than
like DEA it was observed that
corresponding values of aqueous solutions.
the values obtained from pH graph needed to be
B. Basic principle behind the method developed: multiplied by a factor of 1.9 for correct results. In case of
Alkanolamine being the basic molecule reacts with acidic BaCl2 method the error range was 17.5; 9.4 and 37.7%
components like H2SO4 or CO2 stoicheometrically and in case of MeONa method the error was -16.5, 45.8
(equations 1-6). When an acid is added to a known and 66.7%. These values of errors are rather high for
amount of amine the resulting pH is the measure of moles dependable use.
It was also finalized that the reference pH graph of
acid mole-1 amine reacted and indirectly the moles of
amine left un-reacted. The stoicheometry is obtained after alkanolamine with H2SO4 gave most accurate results as
complete neutralization of amine with the acid by plotting compared to the pH graphs with organic acids.
pH against moles acid mole-1 amine added.
D. Application of method to amines loaded with CO2 after
The stoicheometry of alkanol monoamines for absorption: Tabulated results of the amount of CO2
sulphuric acid is 0.5 suggesting that two moles of amine estimated in loaded alkanolamines after CO2 absorption
are reacting with one mole of H2SO4 (Fig.1, 2). In case of (Table 2; Fig. 6) show a lot of variation in the repeat
neutralization with weak monobasic organic acid, like experiments for primary and secondary alkanolamines
acetic acid, a stoicheometry of nearly 0.31 was obtained (MEA and DEA) by MeONa and BaCl2 methods. Results
from graph while for dibasic organic acid like oxalic acid by the present pH method show good repeatability and
the stoicheometry is nearly double as compared to that of high accuracy. Also the trend observed in case of the
monobasic organic acid i.e. >0.6. It was finalized that present method shows that capacity for absorption for
H2SO4 being strong acid dissociated completely. secondary amine DEA is higher than primary amine
Research article
“pH-based CO2 estimation”
Asha Masohan et al.
Indian Society for Education and Environment (iSee)
http://www.indjst.org
Indian J.Sci.Technol.
64
Indian Journal of Science and Technology
Vol.2 No 4 (Apr. 2009)
ISSN: 0974- 6846
which is the observed trend for CO2 absorption. TEA
Table 4 – Experimental and literature values of pKas of the
shows lot of variation in CO2 content by the three
alkanolamines and acids
methods. These results can throw an insight into the
Amines
experimental results of absorption/desorption with
Range of
pKa in
pKa in
different solvents when applied to the simulation
moles of acid
Amine – acid
methanolic
aqueous
used mole-1
system
solution
solution
modeling of the data by eNRTL where the predictions
(Exp.)
(Perrin
amine
(max.)
et
based on liquid analysis are still lacking because of
al.,1965)
dependable experimental data.
MEA - H2SO4
9.4
9.51
0.50
It is envisaged that the developed method can also
MEA - Oxalic acid
9.29
0.60
be extended to mixtures of alkanolamines. Use of such
mixtures is the latest state of the art in CO2 capture.
DEA - Oxalic acid
8.78
0.60
Estimation of absorbed CO2 in the mixture will again
DEA - H2SO4
8.72
8.95
0.5
require construction of the “reference pH-graph” by
DEA - HCl
8.66
0.5
-1
plotting pH vs. moles of acid mole amine mixture as
TEA - Oxalic acid
7.71
0.60
under 3 followed by validation and then estimating the
TEA - H2SO4
7.5
7.78
CO2 content (4.2 and 4.3) in the mixture.
Acids
Conclusion
A novel, versatile and facile method has been
Literature value of pKa
developed for estimation of CO2 loaded into the
pKa1
pKa2
alkanolamine with very good accuracy for use in data
Acetic Acid
4.74
analysis. A pH graph of the methanolic solution of
(CH3COOH)
specific concentration of the amine (that is to be tested
Carbonic Acid
6.37
10.25
for CO2 absorption) with sulphuric acid is plotted as
(H2CO3)
Oxalic Acid
reference pH graph. The CO2 content is then obtained
1.23
4.19
[(COOH)2]
by noting pH of the CO2-loaded amine and referring to
-1
Sulfuric Acid
the pH graph for moles of CO2 mole amine and
none
1.92
(H2SO4)
subsequently the weight of CO2 absorbed can be
calculated using the molecular weight of amine. This 7. Hikita H, Asai S, Ishikawa H and Honda M (1977) The
kinetics of reactions of carbon dioxide with
method has the potential of application to other
monoethanolamine,
diethanolamine
and
alkanolamines and to mixtures of alkanolamines as well.
triethanolamine
by
a
rapid
mixing
method.
Chem.
The method is applicable to full range of CO2 loading
Eng. J. 13, 7-12.
(from very low to high) in the solvent.
8.
Jensen MB, Jorgensen E and Faurholt C (1954)
Acknowledgement
Reactions between carbon dioxide and amino
The authors thank Dr. S. Chandra, Jt. Advisor,
alcohols. Acta Chem. Scand. 8, 1137-1140.
Ministry of Chemicals and Fertilizers, New Delhi, India,
9.
Jou
FY, Mather AE and Otto FD (1982) Solubility of
for his constant encouragement and the Ministry of
solution
and
carbon
dioxide
in
aqueous
Chemicals and Fertilizer, Govt. of India, for financial
Methyldiethanolamine solution. Ind. Eng. Chem.
assistance to carry out this study.
Process Des. Dev. 21, 539-544.
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10.
Perrin RG (1965, Supplement 1972) Dissociation
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Research article
“pH-based CO2 estimation”
Asha Masohan et al.
Indian Society for Education and Environment (iSee)
http://www.indjst.org
Indian J.Sci.Technol.