PAPER
www.rsc.org/ees | Energy & Environmental Science
Comparative study of solvent properties for carbon dioxide absorption
Ortrud Aschenbrenner and Peter Styring*
Received 10th February 2010, Accepted 9th June 2010
DOI: 10.1039/c002915g
Several inexpensive and non-toxic solvents with low vapour pressures were investigated for their
suitability as alternative solvents for the absorption of carbon dioxide from flue gas. The solvents
include poly(ethylene glycol)s, poly(ethylene glycol) ethers, poly(ethylenimine) and glycerol-based
substances. Solvent properties such as thermal stability, solubility of carbon dioxide and selectivity over
nitrogen were investigated in a systematic study using a thermogravimetric analyser. Absorption results
are reported for pure carbon dioxide and nitrogen as well as a mixture of both gases. Desorption and
long-term sorption behaviour are also discussed. Glycerol and poly(ethylene glycol)s show a high
solubility of carbon dioxide. Due to the high viscosity of the solvent, carbon dioxide absorption in
poly(ethylenimine) is very slow in spite of the presence of favourable amine groups. PEG 300 was found
to be the best solvent in this study and shows a high carbon dioxide solubility as well as good selectivity
over nitrogen. The advantages of high stability, low solvent loss and low desorption energy of PEG 300
may outweigh its lower absorption capacity compared to the state-of-the-art solvent
monoethanolamine, making it a potentially advantageous solvent for industrial carbon dioxide
absorption processes.
Introduction
Due to international efforts for the reduction of greenhouse gas
emissions, the capture of carbon dioxide (CO2) from flue gas has
gained increasing interest in recent years.1 Carbon capture
technologies are still far from being the industrial standard, and
processes vary from pre-combustion to post-combustion technologies. Current approaches are targeted at carbon capture and
storage (CCS) where the gas is transported, post-capture, and
stored in underground reservoirs or aquifers. However, this is
a net loss of carbon from the economy and so the need has been
identified to not only capture carbon dioxide but also to re-cycle
it by using it in reactions to synthesise value-added chemicals.
Various alternative methods have been developed for the fixation
of carbon dioxide in commercially useful products. Recent and
ongoing research includes the chemical conversion of carbon
dioxide to a wide range of potential products, photochemical
reduction or artificial photosynthesis and biological transformation using bacteria or algae.2 Although carbon dioxide
separation processes have been widely used in industry, for
Department of Chemical & Process Engineering, The University of
Sheffield, Sir Robert Hadfield Building, Sheffield, UK S1 3JD. E-mail:
[email protected]
example in the purification of natural gas, capturing carbon
dioxide from flue gas presents an additional challenge.1 Flue gas
usually discharges at atmospheric pressure and at a partial
pressure of approximately 0.15 bar in nitrogen. Membrane
processes, which have been used for the effective separation of
carbon dioxide from natural gas, are therefore not viable due to
the high pressures required.3 One of the most widely used
methods for carbon dioxide capture to date is the absorption in
a solvent at near-ambient temperature and pressure and subsequent solvent regeneration at elevated temperature and/or
reduced pressure.4
The selection of a suitable solvent is crucial for the economic
viability of the process.5 The main selection criteria are high
solubility of carbon dioxide and, equally important, high
absorption selectivity of carbon dioxide over nitrogen (N2).
Furthermore, easy desorption is highly desirable, as it reduces
the necessary regeneration temperature and pressure difference.
In order to prevent the loss of solvent, a low vapour pressure and
high thermal stability as well as long-term stability are beneficial.
Additionally, the cost and environmental toxicity of the solvents
have to be taken into account, especially when evaporative loss
and chemical degradation are taken into account.
Currently, the most widely used solvents for carbon dioxide
separation are amine solutions.5,6 Amines selectively absorb
Broader context
The absorption capacity and selectivity for carbon dioxide over nitrogen have been investigated for a number of low cost liquid
polymers. Thermogravimetric analysis was used in order to minimise the quantity of adsorbent required and provide a rapid
screening procedure. While the best of these (PEG 300) shows around 30% efficiency compared to the industry standard MEA, the
desorption and regeneration costs are significantly lower and less energy intensive. Furthermore, the liquids investigated have very
low vapour pressures and so solvent loss by evaporation is significantly reduced. The liquids are also shown by TGA to be thermally
stable for prolonged periods over the expected range of operation. Therefore, the overall environmental impact may be reduced.
1106 | Energy Environ. Sci., 2010, 3, 1106–1113
This journal is ª The Royal Society of Chemistry 2010
carbon dioxide over nitrogen with a high absorption capacity, as
they react with CO2 to form carbamates. The most commonly
used amine-based absorbent is monoethanolamine (MEA).7
However, the amine-based absorption process is still at the
research stage,8,9 albeit at the pilot plant scale, as the use of amine
solutions for CO2 absorption has some disadvantages. The main
problems are the high energy requirement for solvent regeneration, their high vapour pressure and subsequent mass loss
through evaporation, degradation of the solvent and associated
plant corrosion.4,7,10
If physical absorption is used rather than chemical absorption,
solvent regeneration is easier and less energy intensive. Solvents
currently used for physical absorption of carbon dioxide are, for
example, methanol, sulfolane and poly(ethylene glycol) ethers.11
The best-known example is Selexol, a commercial mixture of
poly(ethylene glycol) dimethyl ethers (poly ¼ 1 to 11) with
optimised properties.12,13 The main disadvantage of these physical solvents is the energy needed for gas adsorption and solvent
regeneration via pressure or temperature swing and potential
solvent loss due to volatility.14 When carbon dioxide capture and
activation pathways are considered,2 physical adsorption
solvents have an additional benefit by potentially directly
combining the absorption process with a subsequent or simultaneous catalytic conversion of carbon dioxide to useful
products.
Ionic liquids have been suggested as alternative physical
solvents for carbon dioxide absorption due to their extremely low
vapour pressures. Various ionic liquids were found to absorb
CO2 with high selectivity over N2.15,16 Polymers of ionic liquids
have also been reported to have high CO2 absorption capacity
and selectivity over N2, with fast and completely reversible
absorption.17,18 However, ionic liquids are at present considered
to be too expensive for large-scale industrial applications.
In this study, more commonly available solvents with low
vapour pressures for the absorption of CO2 from flue gas are
proposed: poly(ethylene glycol), for example, has been reported
to exhibit high CO2 solubility selectivity over N2.19 Liquid
poly(ethylene glycol) enhances CO2 adsorption over aminebased solids.20 Poly(ethylene glycol)s have low vapour pressures
and are stable and non-toxic. Glycerol is another stable and nontoxic liquid with low vapour pressure that is available in vast
quantities as a by-product of bio-diesel production, that can be
used as solvent for CO2. Kovvali and Sirkar reported21 a low
selectivity of CO2 over N2 for glycerol, whereas a much higher
selectivity was found for glycerol carbonate, which is also stable,
non-volatile and non-toxic. It is, however, difficult to compare
the suitability of all these substances for CO2 absorption, because
the reported data for solubility and selectivity were obtained with
very different experimental methods. A systematic study of the
relevant properties of these substances is required in order to
evaluate their performance as solvents for CO2 capture from flue
gas.
The aim of this study was to investigate and compare the
suitability of several commercially available alternative solvents
for the capture of carbon dioxide from flue gas. The study
includes glycerol-based substances (glycerol and glycerol
carbonate) as well as polymeric liquids, namely poly(ethylene
glycol)s, poly(ethylene glycol) ethers and poly(ethylenimine).
These solvents have low vapour pressures, low toxicity and low
This journal is ª The Royal Society of Chemistry 2010
or moderate cost. Solubilities of carbon dioxide and nitrogen,
measured using a thermogravimetric method, are reported along
with further properties such as thermal stability and vapour
pressure. Liquids were selected rather than solid adsorbents due
to their ease of transportation around processes through
pumping through pipes. The performance of the solvents was
evaluated and compared to the state-of-the-art solvent monoethanolamine, which shows a CO2 solubility of 43.8 g l1. To our
knowledge, this is the first reported systematic study of carbon
dioxide and nitrogen solubility for these solvents.
Materials and methods
Materials
Glycerol (98%) was supplied by Prolabo. 4-Hydroxymethyl-1,3dioxolan-2-one (glycerol carbonate), tetra(ethylene glycol)
dimethyl ether (tetraglyme, 99%), poly(ethylene glycol) 150
dimethyl ether (PEGDME 150), branched poly(ethylenimine),
poly(ethylene glycol) 200 (PEG 200) and poly(ethylene glycol)
300 (PEG 300) were obtained from Sigma Aldrich. Poly(ethylene
glycol) 600 (PEG 600) was obtained from Acros. All solvents
were used as received. The structures of the solvents are shown in
Fig. 1. Dry carbon dioxide (99.8%) and nitrogen were supplied
by BOC.
Method
A thermogravimetric analyser (Perkin Elmer Pyris 1 TGA) was
used for the experiments. All experiments were performed at
atmospheric pressure with carbon dioxide, nitrogen or a 50% v/v
mixture of both gases, the ratio being maintained by flow
controllers. The gases were obtained dry and were also passed
through in-line silica gel drying tubes. The flow rate was 50 cm3
min1 for each gas. A small amount of solvent (20–40 mg) was
placed in a ceramic sample pan and suspended in the furnace of
the thermogravimetric analyser.
For the thermal stability experiments, the sample was heated
up to 700 C at a heating rate of 10 K min1 with a flow of
nitrogen or carbon dioxide. For some experiments, a heating rate
of 20 K min1 was used. There was no significant effect of the
heating rate on the results.
For the absorption experiments, the sample was heated to
100 C and held at this temperature for at least 30 min prior to
the experiment in order to remove any absorbed gas and water.
The sample was then cooled at 200 K min1 to the absorption
temperature 25 C and held at this temperature until the weight
remained constant. This took up to 5 hours. Each experiment
was performed at least three times and the average value
determined.
Results and discussion
Stability
Thermal stability of the solvents for carbon dioxide absorption is
important with respect to solvent regeneration at elevated
temperatures. All the substances were therefore examined using
thermogravimetric analysis.
Energy Environ. Sci., 2010, 3, 1106–1113 | 1107
Fig. 1 Structures of the selected solvents for absorption studies: (a) glycerol, (b) glycerol carbonate, (c) poly(ethylene glycol), (d) poly(ethylene glycol)
dimethyl ether and tetraglyme (n ¼ 4), and (e) poly(ethylenimine).
For all the substances used in this study, the temperature scan
graphs show only one step where the weight is rapidly reduced to
approximately zero, as shown in Fig. 2 for PEG 300. Generally
no residue was found in the sample pan. This behaviour was
independent of the carrier gas used and can be attributed to total
evaporation or decomposition of the sample. The onset
temperature of the step was determined for the various
substances and is listed in Table 1 along with the vapour pressure
at 95 C. The type of carrier gas used (carbon dioxide or
nitrogen) had no influence on the onset temperature. As the onset
temperature marks the beginning of a weight loss of 100%, this
temperature can be regarded as the approximate boiling point of
the substances. In case of the polymeric substances that do not
exist as a vapour, it represents the approximate decomposition
temperature. As seen from Table 1, the obtained boiling or
decomposition temperatures of the substances are consistent
with the vapour pressure data, since in most cases the substances
with higher vapour pressure at 95 C show the lower boiling or
decomposition temperature.
As can be seen from Table 1, PEG 600 and poly(ethylenimine)
have the highest boiling/decomposition temperatures and lowest
Table 1 Thermogravimetrically obtained onset temperatures for total
evaporation/decomposition and vapour pressure at 95 C
Substance
Glycerol
Glycerol
carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
Molar mass
in g mol1
Onset
temperature/ C
Vapour
pressure/Pa
92.09
118.09
237
237
15.8a
7.6b
222.28
150
200
300
600
ca. 10 000
177
205
202
272
404
370
60.5b
78.8b
9.9b
1.5b
0.2b
0.2b
a
Data from: Cammenga, et al.,22 Ross and Heideger;23
Aschenbrenner, et al.24
b
Data from:
vapour pressures of the substances used. Both have a very high
viscosity and are paste-like at room temperature, whereas the
other solvents are liquids. The high viscosity of PEG 600 and
poly(ethylenimine) is due to their high molar masses compared to
the other substances in this study.
Solubility of carbon dioxide
Fig. 2 Thermogravimetric data for PEG 300 under nitrogen at
a temperature scan rate of 10 K min1.
1108 | Energy Environ. Sci., 2010, 3, 1106–1113
The solubility of carbon dioxide at 25 C in the various
substances was measured using the thermogravimetric method
described in the Experimental section. The results are shown in
Table 2 together with the solvent densities. The solubility data
are shown in mg g1 as obtained directly from the experiments.
The listed densities were then used to calculate the solubility in g
l1. Data for the common solvents methanol and water are also
included in the table for reference.
For most of the solvents, equilibrium adsorption was
reached in less than four hours. However, in the case of poly(ethylenimine) the absorption of carbon dioxide was so slow that
equilibrium was not reached even after six hours. This may be
explained by the high viscosity of poly(ethylenimine) compared
This journal is ª The Royal Society of Chemistry 2010
Table 2 Solubility of carbon dioxide at 25 C. Solvent density data as
provided by supplier
Substance
Solvent
density in g l1
Solubility
in mg g1
Solubility
in g l1
Glycerol
Glycerol carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
Methanol25,26
Water27,28
1250
1400
1011
1089
1124
1124
1124
1030
788
997
13.8
7.9
4.8
6.4
13.4
13.5
7.7
>3.0
7.7
1.5
17.2
11.0
4.9
6.9
15.1
15.1
8.7
>3.1
6.1
1.5
to most of the other substances in this study, making diffusion of
carbon dioxide into the solvent more difficult. The data for
poly(ethylenimine) in Table 2 represent the average concentration after six hours when the weight was still increasing, albeit
much more slowly than at the start of the absorption process.
From the shape of the uptake curve, the equilibrium solubility
value can be estimated to lie between 4.5 and 9.0 mg g1 and
therefore in the same range as the other substances in this study.
Poly(ethylenimine) contains –NH2 and –NH groups that can be
expected to increase the absorption of carbon dioxide due to
strong polar interactions and reaction between the functional
groups and the carbon dioxide molecules. However, due to the
branched network structure of poly(ethylenimine) shown in
Fig. 1e, many of these groups may be located in the interior and
may therefore not be accessible to carbon dioxide molecules. The
presence of these functional groups therefore does not present an
advantage for carbon dioxide absorption in polymeric liquids.
However, a lower degree of polymerisation and a higher number
of amine groups may lead to increased carbon dioxide absorption, especially if porous structures can be engineered.
Rolker et al. obtained high carbon dioxide absorption29 for
a hyperbranched poly(ethylenimine) with a molar mass of just
615 g mol1 as opposed to a molar mass of ca. 10 000 g mol1 for
the poly(ethylenimine) used in this study. The hyperbranched
poly(ethylenimine) investigated by Rolker et al. showed far
higher absorption of carbon dioxide than poly(ether)s and
poly(ester)s of similar molar mass.29 A recent study by
Ismael et al. indicates that the presence of water is essential for
the reaction of amine groups with carbon dioxide.9 The total
absence of water in our experiments may therefore be another
reason for the poor performance of poly(ethylenimine) for
carbon dioxide absorption. Carbamates are notoriously insoluble in non-protic solvents, often leading to the formation of
a precipitate film at the liquid surface. In these studies, no
precipitate was observed on carbon dioxide adsorption with
poly(ethylenimine).
According to Table 2, all of the solvents in this study exhibit
far better carbon dioxide solubility than water, and the majority
of the solvents perform significantly better than methanol. The
best carbon dioxide solubility is obtained for glycerol, PEG 200
and PEG 300. These are the solvents with the highest density of
free –OH groups. Although carbon dioxide is a non-polar gas,
the polarity of the individual C–O bonds in the molecule allows
for interaction with polar groups. Thus, carbon dioxide can act
This journal is ª The Royal Society of Chemistry 2010
as Lewis acid or Lewis base and participate in hydrogen
bonding.30 Previously, the high solubility of carbon dioxide in
liquid poly(ethylene glycol) membranes found in several studies
was attributed to acid–base reactions of the acidic carbon dioxide
with the electron-rich ether oxygen in the PEG molecules.31
However, this cannot be the only explanation, as solvents such as
poly(ethylene glycol) dimethyl ethers show lower solubility of
carbon dioxide in spite of their ether groups, as seen in Table 2. It
is therefore likely that the terminal –OH groups have a higher
affinity for carbon dioxide molecules and increase absorption
compared to substances which contain only ether groups. The
high polarity of the bonds in the carbon dioxide molecule allows
a strong electron interaction with the highly polar –OH groups in
the solvent molecules as shown in Fig. 3.
It is interesting to note the dependence of the carbon dioxide
solubility on the average molar mass of the different poly(ethylene glycol)s. The solubility of CO2 in PEG 200 and PEG
300 is almost identical. Obviously, these two solvents have
a similar number of –OH and –O– groups, as well as a similar
average chain length of 8 and 12 carbon atoms, respectively, and
therefore similar affinity to carbon dioxide. PEG 600 has much
larger molecules with an average chain length of 26 carbon
atoms. This drastically reduces the density of available –OH
groups with high affinity to carbon dioxide, compared to
the number of less favourable ether –O– groups. Furthermore,
the high number of carbon atoms per molecule increases the
viscosity of PEG 600 compared to PEG 200 and PEG 300,
making diffusion of carbon dioxide to the functional groups
more difficult. Both effects result in lower carbon dioxide
absorption for PEG 600. PEGs with higher molar mass will most
likely show similarly low carbon dioxide solubility.
Some literature data were found for the solubility of carbon
dioxide in similar solvents at 25 C. Kovvali and Sirkar found
a value of 2.8 g l1 for the solubility of carbon dioxide in glycerol
carbonate,21 which is a low value compared to the 11.0 g l1
found in this study. However, the value given by Kovvali and
Sirkar was calculated from membrane permeability rather than
direct absorption and can therefore not be compared directly.
The same authors present a carbon dioxide solubility of 6.5 g l1
in PEG dimethyl ether, which is close to the values of 4.9 g l1 and
6.9 g l1 found in this study for tetraglyme and PEG dimethyl
ether 150, respectively. Unfortunately, Kovvali and Sirkar do
not specify the molar mass of the PEG dimethyl ether. A solubility value of 4.4 g l1 carbon dioxide in PEG given by Saha and
Chakma does not specify the molar mass of PEG,31 making it
also difficult to compare with the values obtained in this study for
PEG 200, 300 and 600.
The solubility of carbon dioxide in various PEG dimethyl
ethers was investigated by Henni et al.13 as well as Sciamanna and
Fig. 3 Polar interaction of a terminal hydroxyl group with a free carbon
dioxide molecule.
Energy Environ. Sci., 2010, 3, 1106–1113 | 1109
Table 3 Solubility of carbon dioxide in mg g1 at 25 C. The substances
are listed in order of increasing molar mass
Substance
Henni13
Sciamanna32
Present study
Di(ethylene glycol)
dimethyl ether
PEGDME 150
Tri(ethylene glycol)
dimethyl ether
Tetraglyme
PEGDME 250
6.8
8.9
—
—
5.6
—
7.3
6.4
—
6.6
5.5
6.6
—
4.8
—
Lynn.32 Table 3 shows the solubility data at 25 C together with
the corresponding values found in this study. The substances are
listed in order of increasing molar mass. PEGDME 150 is
a mixture of poly(ethylene glycol) ethers which has a molar mass
higher than di(ethylene glycol) dimethyl ether but lower than
tri(ethylene glycol) dimethyl ether.
The literature values lie in the range of 5 to 9 mg g1 for the
substances in Table 3. This is in agreement with the results found
in this study. The deviation between the values in this study and
the literature values is of the same magnitude as the deviation
between the values of the two literature sources. The literature
values show a general trend of a slight decrease in carbon dioxide
solubility with increasing molar mass of the PEG dimethyl ether.
This trend is confirmed in the results from this study.
It is important to compare the carbon dioxide solubility of the
solvents in this study with state-of-the-art solvents for carbon
dioxide absorption. At the present time, the most widely used
solvent for carbon dioxide absorption is an aqueous solution of
monoethanolamine (MEA).4 In a state-of-the-art carbon dioxide
absorption process described in the literature using MEA as the
solvent the concentration of MEA in solution is 0.3 g/g.5,33 This
gives a typical concentration of carbon dioxide in the MEA
solution was calculated to 43.8 g l1 using a solution density of
1013 g l1 (ref. 34) and showing a typical carbon dioxide uptake
of 0.2 mol mol1.33,35 The calculated adsorption value of 43.8 g l1
is approximately three-times higher than the 15 to 17 g l1
obtained in this study for the best absorbents PEG 200, PEG 300
and glycerol. It also has to be taken into account that the typical
uptake for MEA solutions is still relatively inefficient and below
the maximum uptake concentration which can theoretically be
five times as high, corresponding to 1 mol carbon dioxide per mol
MEA as obtained in a stoichiometric reaction. This would result
in a maximum concentration of 219 g l1, which is more than 10
times the maximum concentration reached in glycerol. In reality,
such high uptake values are not achieved and in addition the
regeneration energy is high as the carbamates need to be broken
down. However, it is possible that the lower energy requirement
for solvent regeneration and the reduced loss of solvent in the
case of glycerol, PEG 200 or PEG 300 can compensate the
disadvantage of lower carbon dioxide solubility.
Selectivity over nitrogen
In order to gain knowledge about the selectivity of carbon
dioxide absorption over nitrogen absorption, the solubility of
nitrogen was measured for comparison. The results for pure, dry
nitrogen are shown in Table 4.
1110 | Energy Environ. Sci., 2010, 3, 1106–1113
Table 4 Solubility of nitrogen at 25 C
Substance
Solubility in mg g1
Solubility in g l1
Glycerol
Glycerol carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
7.5
4.4
1.5
2.3
8.9
4.7
4.7
6.3
9.3
6.2
1.5
2.5
10.1
5.3
5.3
6.5
Table 5 Comparative molar solubilities of carbon dioxide and nitrogen
at 25 C for the pure gases in the test adsorbents under identical conditions. Data for methanol and water are included for reference
Substance
Solubility of
CO2/mmol l1
Solubility of
N2/mmol l1
Glycerol
Glycerol carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
Methanol25,26
Water27,28
391
251
111
158
343
344
197
>71
138
34
333
220
52
91
359
189
190
233
7
1
Glycerol and PEG 200, which are among the substances with
the highest molar solubility for carbon dioxide, also have the
highest solubility for nitrogen. Low nitrogen solubility is found
for tetraglyme and PEGDME 150, but these also have low
solubility for carbon dioxide. From the solubility data in mg per
solvent unit, however, it is not possible to assess the different
affinities of the carbon dioxide and nitrogen molecules for the
solvents. Table 5 shows the solubilities of carbon dioxide and
nitrogen in mmol per litre of solvent. Again a general trend can
be observed that solvents with high carbon dioxide solubility also
exhibit comparatively high nitrogen solubility.
However, the data for pure gases give no indication of selectivity as adsorption in a mixed gas system may be competitive. In
order to examine the possibility of competitive adsorption,
studies were carried out on a 1 : 1 by volume mixture of the two
gases over the different adsorbents. The results are presented in
Table 6 as the mass of gas adsorbed per mol of adsorbent for
pure carbon dioxide, pure nitrogen and the 1 : 1 mixture for the
different adsorbents under the same conditions. The value for
poly(ethylenimine) is an estimated value based on a relative
molecular mass of 10 000. The final entry is the mass adsorption
per monomer unit of the polymer in order to relate the performance back to the low molecular mass materials. An experimental error of 10% based on uncertainties in the data has also
been included. It is recognised that the partial pressure of CO2 in
this mixed gas system is high (0.5 bar) in comparison with
a typical flue gas, however this is a model system. Other partial
pressure compositions are currently under investigation, both on
a bench-scale and in a scaled-up system.
Tables 5 and 6 show that those adsorbents possessing terminal
hydroxyl groups have the highest affinity for carbon dioxide with
This journal is ª The Royal Society of Chemistry 2010
Table 6 Solubility by mass of carbon dioxide, nitrogen and a 1 : 1
volumetric mixture of the gases per mole of adsorbent at 25 C. All data
have an accuracy of 10%
Substance
CO2/g mol1
N2/g mol1
CO2/N2/g mol1
Glycerol
Glycerol carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
in g/momomer
1.27
0.93
1.07
0.96
2.68
4.04
4.63
63.44
1.92
0.69
0.52
0.32
0.35
1.79
1.42
2.84
30.27
4.02
1.13
0.72
0.81
0.61
2.25
4.70
6.56
19.13
1.21
glycerol having the highest volumetric capacity than PEG 300
and PEG 200. However, the highest molar capacity is shown by
PEG 600 then PEG 300. The adsorption of nitrogen is highest for
PEG 200 and glycerol volumetrically and the methyl ethers in
terms of moles of adsorbent. Table 6 shows the mass of gas
adsorbed per mol of adsorbent for the mixed gas system as TGA
does not permit the analysis of gas composition so that molar
values cannot be determined. The absorbed mass is therefore the
sum of both gases absorbed. The highest gas capacity is observed
for PEG 600 and PEG 300. Due to the high molecular weight of
poly(ethylenimine) the mass of adsorbed gas per monomer unit is
given as this relates more closely to the other solvents, however
these values are very low and surprisingly suggest a greater
selectivity for nitrogen.
A theoretical value for the solubility of the gas mixture,
calculated from the solubility of the pure gases, is also included in
Table 7. This theoretical solubility is based on the assumption
that both gases obey Henry’s law, where the solubility is
proportional to the partial pressure of the gas, and that the
absorption occurs independently for each gas. Henry’s law has
generally been used to describe the solubility of carbon dioxide in
a wide range of liquids.13,32,36
Most of the substances show good agreement of the experimental value with the theoretical value calculated from Henry’s
law. However, in the cases of glycerol, PEG 300 and PEG 600 the
experimental value is higher than the value found from theory.
This can be due to competitive absorption, one of the gases
absorbing faster than the other and so inhibiting the absorption
of the second gas. If the experimental solubility value is higher
than the predicted one, as is the case here, the gas with higher
solubility (carbon dioxide) will then be the compound with faster
absorption. This behaviour can be explained by a stronger
interaction of carbon dioxide with the solvent molecules due to
the strong polarity of the molecular bonds in carbon dioxide and
the functional groups of the solvent. Unfortunately, an experimental confirmation of this kinetic behaviour was not possible as
the thermogravimetric method used was not accurate enough for
kinetic studies.
Another possible reason for the high experimental values
compared to the theoretical calculation is a deviation of the
behaviour of the gas mixture from Henry’s law. This is particularly obvious for PEG 300 where the solubility of the gas mixture
is higher than the solubility of pure carbon dioxide. This can only
be explained by the assumption that both gases absorb to
This journal is ª The Royal Society of Chemistry 2010
Table 7 Absorption of carbon dioxide, nitrogen and a 1 : 1 mixture of
both gases at 25 C. The theoretical values were calculated from the
solubility of the pure gas with the assumption of independent absorption
and the validity of Henry’s law for both gases
Solubility of CO2/N2
mixture in g l1
Substance
Glycerol
Glycerol
carbonate
Tetraglyme
PEGDME 150
PEG 200
PEG 300
PEG 600
Poly(ethylenimine)
Solubility of Solubility of
CO2 in g l1 N2 in g l1 Experimental Theoretical
17.2
11.0
9.3
6.2
15.3
8.5
13.3
8.6
4.9
6.9
15.1
15.1
8.7
>3.1
1.5
2.5
10.1
5.3
5.3
6.5
3.7
4.4
12.6
17.6
8.5
>2.0
3.2
4.7
12.6
10.2
7.0
>4.8
a higher extent than the 50% of the value for the pure gas
expected from Henry’s law. This also means that each of the two
gases interact with different parts of the solvent molecules, so
that the sum of the dissolved gas molecules is more than the
number of molecules corresponding to saturated solution for
each individual gas. This is in agreement with the different
mechanisms for molecular interaction as discussed in the
previous section, with strong electron interaction between the
carbon dioxide and the –OH and –O– groups of the solvent, and
far weaker interaction between the nitrogen molecules and the
entire surface of the solvent molecules.
For poly(ethylenimine), the absorption of the gas mixture was
very slow and equilibrium was not reached during the experimental time. This is similar to the result with pure carbon dioxide
and due to the very high viscosity of poly(ethylenimine), as
mentioned earlier. However, one would expect to see a fast
absorption of the nitrogen present in the gas mixture. This was
not the case, indicating that nitrogen absorption is inhibited by
the presence of carbon dioxide, which may react selectively but
very slowly with the amine groups in the polymer. It is possible
that carbon dioxide is selectively adsorbed at the surface of the
polymer and that this inhibits mass transfer into the interior.
Desorption and absorption cycles
For industrial applications, it is necessary to know the desorption and long-term absorption behaviour of the solvents.
Therefore, experiments were performed with two subsequent
absorption cycles using pure carbon dioxide. After equilibrium
absorption was reached, the sample was heated to a desorption
temperature of 100 C for 30 min and then cooled for a second
absorption cycle. The gas was not changed during the procedure.
This means desorption was conducted in the same carbon
dioxide atmosphere, the driving force for desorption being only
the higher temperature. The experiments were only performed
with tetraglyme and PEGDME 150 because of their relatively
short absorption time (1 h compared to up to 5 h for the other
solvents in this study). Fig. 4 shows the two absorption cycles of
carbon dioxide in PEGDME 150.
As seen in Fig. 4, a sharp decrease in weight is observed during
the desorption period. The weight loss is far higher than the
Energy Environ. Sci., 2010, 3, 1106–1113 | 1111
Fig. 4 Thermogravimetric data for PEGDME 150 under carbon dioxide
in two subsequent absorption cycles. The temperatures are the set values.
Table 8 Absorption of carbon dioxide at 25 C in two subsequent
absorption cycles
Solubility of CO2 in mg g1
Substance
1st cycle
2nd cycle
Tetraglyme
PEGDME 150
4.8
6.4
3.7
5.0
absorbed amount of carbon dioxide. This effect arises from
evaporation of the solvent. It is supposed that most of the
absorbed carbon dioxide is desorbed at 100 C. However, this is
not evidently clear due to the underlying weight loss caused by
evaporation. It is possible that some carbon dioxide remains in
the solvent, especially as there was still a constant flow of carbon
dioxide during desorption in these experiments.
Table 8 shows the average absorption of carbon dioxide for
PEGDME 150 and tetraglyme in the first and second absorption
cycle. The absorbed amount was always lower in the second
cycle. This may indicate that desorption at 100 C was not
complete. It might also indicate degradation of the solvent over
time. It is possible that PEG 300 as the best solvent in this study
regarding solubility and selectivity shows similar problems in
long-run absorption and desorption experiments. A thorough
further investigation of the desorption and long-term absorption
behaviour will be necessary in order to draw conclusions about
the solvent’s potential for industrial absorption of carbon
dioxide from flue gas. The TGA method is not suitable for this
investigation, as the experiments should be performed under real
desorption conditions without presence of carbon dioxide and
possibly at reduced pressure. It is obvious that such conditions
will make the desorption of carbon dioxide much easier and will
probably lead to relatively fast and complete desorption for all
the solvents in this study, with exception of poly(ethylenimine)
due to its high viscosity as well as chemical reactions between its
amine groups and carbon dioxide.
Conclusions
In this study, a range of alternative solvents for the absorption of
carbon dioxide from flue gas was investigated. All the solvents
have high thermal stabilities and low vapour pressures, reducing
the loss of solvent in the process to a minimum.
1112 | Energy Environ. Sci., 2010, 3, 1106–1113
In spite of its potential to react with carbon dioxide, poly
(ethylenimine) as the only solvent with amine groups in this study
showed very slow absorption due to its high viscosity. It performed particularly badly in a mixed gas system when compared
to adsorption of pure gases.
The best substance in this study was shown to be PEG 300 with
a high carbon dioxide solubility and good selectivity over
nitrogen. The affinity for pure carbon dioxide was higher than
for pure nitrogen, and the mixed gas system showed a positive
deviation from Henry’s Law with a higher mass of gas being
adsorbed than for either of the pure gas streams. The enhanced
performance has been attributed to a combination of internal
ether oxygen groups together with terminal hydroxyl groups. It is
proposed that the latter show enhanced interactions with the
carbon dioxide molecules leading to stronger adsorptive interactions. The long-term sorption behaviour of PEG 300 still needs
to be investigated in order to be able to compare the overall
performance and costs with state-of-the-art solvents such as
monoethanolamine solutions. Although monoethanolamine has
a higher carbon dioxide absorption capacity than PEG 300, it is
possible that the advantages of PEG 300 such as the high
stability, reduced solvent loss and lower desorption energy will
outweigh this disadvantage in comparison with monoethanolamine, making PEG 300 a suitable and favourable alternative
solvent for industrial processes.
Acknowledgements
We gratefully acknowledge support from the EPSRC for OA
under the ‘C-Cycle’ project (EP/E010318/1).
References
1 IPCC Special Report: Carbon Dioxide Capture and Storage, ed. B.
Metz, O. Davidson, H. de Coninck, M. Loos and L. Meyer,
Cambridge
University
Press,
2005,
www.ipcc.ch/
publications_and_data_reports.html#1, accessed 02.02.2010.
2 M. Mikkelsen, M. Jorgensen and F. C. Krebs, Energy Environ. Sci.,
2010, 3, 43–81.
3 A. Meisen and X. Shuai, Energy Convers. Manage., 1997, 38, S37.
4 C. M. White, B. R. Strazisar, E. J. Granite, J. S. Hoffman and
H. W. Pennline, J. Air Waste Manage. Assoc., 2003, 53, 645–715.
5 R. Notz, N. Asprion, I. Clausen and H. Hasse, Chem. Eng. Res. Des.,
2007, 85(A4), 510–515.
6 S. Ma’mun, H. F. Svendsen, K. A. Hoff and O. Juliussen, Energy
Convers. Manage., 2007, 48, 251–258.
7 J. C. Abanades, E. S. Rubin and E. J. Anthony, Ind. Eng. Chem. Res.,
2004, 43, 3462.
8 M. Mikkelsen, M. Jorgensen and F. C. Krebs, Int. J. Greenhouse Gas
Control, 2010, 4, 452–458.
9 M. Ismael, R. Sahnoun, A. Suzuki, M. Koyama, H. Tsuboi,
N. Hatakeyama, A. Endou, H. Takaba, M. Kubo, S. Shimizu,
C. A. Del Carpio and A. Miyamoto, Int. J. Greenhouse Gas
Control, 2009, 3, 612–616.
10 M. Wilson, P. Tontiwachwuthikul, A. Chakma, R. Idem, A. Veawab,
A. Aroonwilas, D. Gelowitz, J. Barrie and C. Mariz, Energy, 2004, 29,
1259.
11 Y. J. Heintz, L. Sehabiague, B. I. Morsi, K. L. Jones and
H. W. Pennline, Energy Fuels, 2008, 22, 3824–3837.
12 Selexol Patents, US Pat., 3 594 985, 1971; Selexol Patents, UK Pat.,
1 277 139, 1985.
13 A. Henni, P. Tontiwachwuthikul and A. Chakma, Can. J. Chem.
Eng., 2005, 83, 358.
14 M. Kanniche and C. Bouallou, Appl. Therm. Eng., 2007, 27, 2693–
2702.
This journal is ª The Royal Society of Chemistry 2010
15 J. L. Anthony, J. L. Anderson, E. J. Maginn and J. F. Brennecke, J.
Phys. Chem. B, 2005, 109, 6366.
16 J. Jacquemin, P. Husson, V. Majer and M. F. Costa Gomes, Fluid
Phase Equilib., 2006, 240, 87.
17 J. Tang, H. Tang, W. Sun, M. Radosz and Y. Shen, Polymer, 2005, 46,
12460.
18 P. Styring, O. Aschenbrenner and S. Supasitmongkol, Prepr. Pap. Am. Chem. Soc., Div. Fuel Chem., 2009, 54(1), 002.
19 H. Lin and B. D. Freeman, J. Mol. Struct., 2005, 739, 57.
20 S. Satyapal, T. Filburn, J. Trela and J. Strange, Energy Fuels, 2001,
15, 250.
21 A. S. Kovvali and K. K. Sirkar, Ind. Eng. Chem. Res., 2002, 41, 2287.
22 H. K. Cammenga, F. W. Schulze and W. Theuerl, J. Chem. Eng. Data,
1977, 22, 131.
23 G. R. Ross and W. J. Heideger, J. Chem. Eng. Data, 1962, 7, 505.
24 O. Aschenbrenner, S. Supasitmongkol, M. Taylor and P. Styring,
Green Chem., 2009, 11, 1217–1221.
25 E. Wilhelm and R. Battino, Chem. Rev., 1973, 73, 1–9.
This journal is ª The Royal Society of Chemistry 2010
26 P. L€
uhring and A. Schumpe, J. Chem. Eng. Data, 1989, 34, 250.
27 F. Daniels and R. A. Alberty, Physical Chemistry, Wiley, New York,
1980.
28 S.-W. Park, J.-W. Lee, B.-S. Choi and J.-W. Lee, J. Ind. Eng. Chem.,
2005, 11, 202.
29 J. Rolker, M. Seiler, L. Mokrushina and W. Arlt, Ind. Eng. Chem.
Res., 2007, 46, 6572–6583.
30 P. Raveendran, Y. Ikushima and S. L. Wallen, Acc. Chem. Res., 2005,
38, 478–485.
31 S. Saha and A. Chakma, J. Membr. Sci., 1995, 98, 157–171.
32 S. F. Sciamanna and S. Lynn, Ind. Eng. Chem. Res., 1988, 27, 492–
499.
33 A. B. Rao and E. S. Rubin, Environ. Sci. Technol., 2002, 36, 4467–
4475.
34 R. H. Weiland, J. C. Dingman, D. B. Cronin and G. J. Browning,
J. Chem. Eng. Data, 1998, 43, 378–382.
35 A. Chakma, Energy Convers. Manage., 1997, 38, S51–S56.
36 Y. Miyano and I. Fujihara, Fluid Phase Equilib., 2004, 221, 57–62.
Energy Environ. Sci., 2010, 3, 1106–1113 | 1113