1542
Ind. Eng. Chem. Res. 2005, 44, 1542-1546
SEPARATIONS
Development of Supported Ethanolamines and Modified
Ethanolamines for CO2 Capture
T. Filburn,* J. J. Helble, and R. A. Weiss
Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269-3222
Liquid amines can be immobilized within the pores of polymeric supports to provide a regenerable
CO2 sorbent. This paper describes the capture of CO2 by a range of ethanolamines (primary,
secondary, and tertiary) immobilized within the pores of high-surface-area poly(methyl methacrylate) beads. These supported amines were used to remove low concentrations (7.6 mmHg)
of CO2 by a pressure swing absorption process, where low-pressure vacuum was used to desorb
the CO2 and regenerate the sorbent. The effect on CO2 capture of modification of primary amines
to secondary amines by reaction with acrylonitrile was also evaluated. The modified amines
provided nearly a factor of 2 increase in CO2 removal capacity compared to the original primary
amines. These results suggest that modified amines could potentially be used for CO2 capture
in space life support systems as well as for terrestrial flue gas CO2 removal applications.
Introduction
Long-duration, human-occupied space missions require the use of regenerable sorbents for CO2 capture.
Regenerable sorbents provide a reduction in overall
system weight and volume compared to single-use
sorbents and, therefore, decrease the storage volume
and launch weight of the space vehicle. Liquid amines
are particularly suited to this purpose, as their efficacy
in removing carbon dioxide and their regenerability
have been demonstrated in a host of industrial applications.1,2 Liquid-amine-based scrubbing systems, however, generally require large towers for contacting the
liquid amine absorbent with the process gas stream,
making them impractical for confined-space applications. This problem has been circumvented by immobilizing liquid amines within the pores of a solid
support, thus permitting their use without requiring a
separate phase separation step.3,4 CO2 removal and
sorbent regeneration are subsequently accomplished
though pressure swing absorption.5 These supported
amines therefore provide an attractive means for using
liquid amines as CO2 removal agents in the microgravity environment of space.
In designing a supported-amine-based system, the
selection of the optimum liquid amine to be immobilized
within a support remains a challenging problem. Liquid
alkanolamine sorbents have been used for the removal
of carbon dioxide from gas streams for many years, since
Bottoms6 introduced the use of triethanolamine (TEA)
for the regenerative removal of CO2 from natural gas
streams. Since that time, numerous refinements have
been made in the use of these absorbents,7-9 including
the utilization of alternate amine formulations to pro* To whom correspondence should be addressed. Current
addresss: Department of Mechanical Engineering, University
of Hartford, West Hartford, CT 06117. E-mail:
[email protected].
vide higher CO2 removal capacities and lower regeneration energy costs. The first of these changes switched
from the relatively low capacity and low reactivity of
TEA (a tertiary amine) to monoethanolamine (MEA), a
primary amine. A more recent innovation has been the
use of secondary amines (e.g., diethanolamine, DEA) or
hindered primary amines, which have lowered the
regeneration energy necessary to reuse the amine
solutions.10
It is well-known that primary, secondary, and tertiary
amines have different pKa values and consequently
differing affinities for acid gases such as carbon dioxide.11 These pKa values will also be affected by the
physical state of the amines. In aqueous solutions, the
basicities of the amine increase from the least basic
primary amine to tertiary and finally to secondary. In
the gas phase, the basicity increases from primary to
secondary to tertiary amine.11 Most prior research on
CO2 removal by amines has concentrated on measuring
capacities for aqueous amine solutions. The research
described herein examined the CO2 removal capacities
of different amines immobilized on a solid support, and
the specific goal of the research was to ascertain how
the type of amine (primary, secondary, or tertiary)
affected steady-state CO2 capacity in a pressure swing
absorption system.
Reaction of Amines with CO2
In general, the industrial use of amine sorbents has
centered on aqueous solutions of primary and secondary
amines, which react directly with CO2 to form carbamate ions, RNHCOO-. Reaction 1 shows the formation
of the carbamate ion for a primary amine. A similar
reaction occurs for secondary amines.
2RNH2 + CO2 w RNHCOO- + RNH3+
(1)
RNHCOO- + H2O w RNH2 + HCO3-
(2)
10.1021/ie0495527 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/29/2005
Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 1543
Figure 2. Structure of the TEPA molecule (normal).
Table 1. Integral Heats of Solution for Absorption of
CO21
Figure 1. Chemical structures of ethanolamines (primary, MEA;
secondary, DEA; and tertiary, TEA).
Water can hydrolyze the carbamate and regenerate one
amine molecule (reaction 2), but because of the stability
of the carbamate ion, this reaction does not occur
readily.
The ability to form carbamate ions allows for direct
reaction of the amine with CO2, which produces faster
CO2 capture kinetics for the primary and secondary
amines. Although carbamate formation reaction proceeds rapidly, the overall capacity for CO2 capture for
both primary and secondary amines is reduced by the
stoichiometry requirement of two amine molecules for
each CO2 molecule reacted. The chemical structures of
example alkanolamines for each of the three amine
types are shown in Figure 1.
Tertiary amines are generally not used for CO2
capture, because they do not react with CO2 to produce
carbamate ions. Tertiary amines, however, can remove
a stoichiometric amount of CO2 by reaction with water
to produce hydroxyl ions that can then react with CO2
to produce bicarbonate ions, as shown in reactions 3 and
4.
H2O + R3N w R3NH+ + OH-
(3)
CO2(aq) + OH- w HCO3-
(4)
The formation of the bicarbonate ion (reaction 4) is
relatively slow compared to the carbamate ion formation
reaction (reaction 1), however, so that the kinetics of
CO2 removal by tertiary amine are generally slower
than for primary and secondary amines.
Energy costs play a significant role in the feasibility
of any commercial CO2 removal system. For aminebased systems, the most significant energy demand is
for the amine regeneration step. Primary amines typically have higher heats of absorption than secondary
and tertiary amines. This higher heat of absorption
produces a commensurate energy penalty for primary
amines during the regeneration step. Secondary amines,
therefore, provide a useful compromise between the low
reaction rates of tertiary amines and high heat of
absorption for primary amines. As a result, since the
amine
type
integral heat of
solution (cal/g of CO2)
MEA
DEA
DGA
MDEA
TEA
DIPA
primary
secondary
primary
tertiary
tertiary
secondary
1485
1260
1476
1035
837
1296
1980s, secondary amines have seen increasing use in
industrial applications for acid gas removal.1 At present,
however, most aqueous solutions of secondary amines
limit the amine concentration to ∼20% because of the
use of carbon steel in absorption vessels; a relatively
low amine concentration is required to reduce the rate
of corrosion within the process system.1 Table 1 lists
average heats of solution for capturing CO2 from
representative amines.
In this paper, we describe the use of supported amine
sorbents to capture CO2 and the subsequent regeneration of the supported amine using pressure swing
absorption at low (∼1 mmHg) vacuum pressure. Specifically, we examined the most common commercial
ethanolamines representing the three amine types,
monoethanolamine (MEA, primary), diethanolamine
(DEA, secondary), and triethanolamine (TEA, tertiary);
see Figure 1. In contrast to these single amine molecules, multiamine molecules can contain more than one
type of amine functionality, which suggests the possibility of developing multifunctional amine sorbents
that optimize their CO2 capture behavior, i.e., provide
tradeoffs between kinetic and heat of absorption limitations. In this paper, the development of new highercapacity solid amine sorbents by modification of the
amine functional group of the immobilized sorbent is
also described. MEA was modified by reaction with
acrylonitrile to convert some of the primary amine
groups into secondary amines.
The justification for using reaction-modified amines
is based on work described by Giavarini et al. and
Rinaldi et al.,12-14 who modified tetraethylenepentamine
(TEPA, Figure 2) to increase its working capacity for
CO2 removal. The working capacity refers to the amount
of CO2 that can be absorbed and successfully removed
during regeneration of the sorbent. Giavarini et al. and
Rinaldi et al. modified TEPA by reacting it with various
ratios of phenol, formaldehyde, and combinations of the
two. These modified TEPA molecules showed increased
working capacity for gas-phase CO2 removal in aqueous
systems.
In the present study, the amine sorbents were used
in an adsorbed state, immobilized on a nonionic polymeric support. The main objective was to discern the
amine type most effective in removing carbon dioxide
from a gas stream containing low levels of CO2. Low
levels of CO2, generally at about 7.6 mmHg in a gas
stream maintained at 760 mmHg total pressure, were
considered. These CO2 partial pressures and total
pressures mimic those typically found in enclosedenvironment life-support systems such as submarines
and the space shuttle.
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Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005
Experimental Section
Commercially available ethanolamines MEA (Baker,
100% reagent grade), DEA (Aldrich, 99%), and TEA
(Baker, 98.7%) were used as amine sorbents for CO2
capture. The ethanolamines were used as received. MEA
was also modified by reaction with acrylonitrile (Aldrich,
>99%). The acrylonitrile/amine reaction formed a Michael
adduct much like the TEPA amines modified by Rinaldi
et al.14 via phenol and formaldehyde. The acrylonitrile
tends to react predominantly with primary amines
converting them to secondary amines.15
Two different ratios of acrylonitrile to MEA were
examined to provide a mixture of primary and secondary
amines: (1) a Michael adduct formed with 1 mol of
acrylonitrile (AN) to 4 mol of MEA and (2) a Michael
adduct formed when 2 mol of AN were reacted with 3
mol of MEA. To produce the adducts, 188 mL of liquid
MEA was added to a standard 500-mL round-bottom
glass flask with a mechanical stirrer. An ice/water bath
surrounding the flask was used to moderate the reaction
exotherm.. An addition funnel was used to slowly (∼3-5
min) add 50 mL of acrylonitrile (for the 1:4 molar ratio
synthesis) into the stirred amine. The rate of acrylonitrile addition was limited to prevent the solution temperature from exceeding 45 °C. The solution temperature rose slightly to approximately 30 °C and then
slowly returned to the ice/water bath temperature. Two
batches of amine/acrylonitrile reactants were produced.
In the first, 0.25 mol of acrylonitrile was added per 1
mol of amine (MA14). In the second, 0.67 mol of
acrylonitrile was added per 1 mol of MEA (MA23). After
the addition had been completed, the solution was
slowly heated to 50 °C, and stirring was then continued
for 1 h to ensure complete reaction of the acrylonitrile
and amine.
The amine sorbents were impregnated into a nonpolar
commercial poly(methyl methacrylate) (PMMA) support.
This high-specific-surface-area (BET, 470 m2/g, as measured by the manufacturer) support bead also provided
a large pore volume (1.2 mL/g) with a mean pore
diameter of 17 nm (based on N2 adsorption). These
beads had a range of diameter from 0.35 to 0.84 mm.
The solid polymer beads containing immobilized liquid
amines were prepared using a solvent evaporation
process. The PMMA beads were initially wetted by
dispersing them in methanol to facilitate impregnation
of the amine into the pores. An amine solution (equal
volumes of amine and methanol) was then added to the
beads, and the amine solution was rotated within a
rotary evaporator flask at room temperature for 5 min
to produce a homogeneous slurry. The volatile methanol
was then removed by heating the rotary evaporator
flask in a 90 °C water bath. Care was taken within the
first few minutes of solvent evaporation to prevent
“bumping” of the slurry, which would carry solid support
material into the condensation tube.
The impregnation procedure produced sorbents consisting of a PMMA polymeric support with 30-85% of
its theoretical pore volume filled with liquid amine. The
ethanolamine loadings achieved were 0.34 g of MEA,
0.47 g of DEA, and 0.53 g of TEA per gram of dry PMMA
bead. Because of the differences in molecular weight,
those mass loadings produced nearly equal molar loadings of ∼2 mol of amine per liter of support. The use of
the volume concentration is useful, because the pressure
swing absorption experiments were conducted with a
constant volume (0.1 L) of sorbent.
The CO2 absorption measurements were made in a
semicontinuous two-bed adsorption system. The fixedbed reactor contained an open-cell reticulated aluminum
foam that allowed the conduction of heat from the
absorption chamber into the desorption bed.16 A schematic diagram of the experimental system is shown in
Figure 3. As indicated, nitrogen and carbon dioxide
supplies were mixed to produce a fixed inlet CO2
concentration (generally 1 kPa). Not shown are the
humidifiers, which permitted variation of the inlet dew
point from -40 °C to a fully saturated ambient-temperature condition. Inlet and outlet CO2 levels, temperatures, and dew points were all monitored as shown.
The small bed size (110 cm3) and the efficient thermal
conduction paths provided by the aluminum foam made
both the cyclic and equilibrium capacity measurements
operate isothermally, limiting temperature variations
between the absorbing and desorbing bed to less than
2 °C, and also limited run-to-run absorbing or desorbing
temperature variations to much smaller values (<1 °C).
Once the amines had been fabricated, tests were conducted to examine the CO2 removal capacity for all
sorbents. No capacity for CO2 removal was expected
from the PMMA support; this material is nonionic and
has no affinity for CO2 capture. Sorption measurements
were conducted only with the supported amine samples.
The carbon dioxide removal capacity of the solid
amines was measured using a fixed absorption time of
25 min, followed by a low-pressure vacuum desorption
step for another 25 min. During absorption, the gas flow
was such that a gas residence time of 1.75 s and an inlet
dew-point temperature of 7 °C were maintained. The
absorption/desorption cycle was repeated until steadystate conditions were achieved. That typically required
four cycles. The steady-state values are what are
reported in this article.
Results and Discussion
Initial tests of CO2 absorption capacity were conducted with each chamber exposed to a continuous
stream containing CO2 at a partial pressure of 7.6
mmHg (in N2) with an inlet dew point maintained at 7
( 1 °C. This stream passed through one bed of the
reactor while the second bed was exposed to vacuum
for regeneration. After 25 min of operating in this mode,
the functions of the two beds, i.e., sorption and desorption, were alternated. Figure 4 represents the steadystate CO2 loading on the amine bed after four cycles of
absorption and desorption. This graph presents a comparison of the breakthrough capacities for the three
ethanolamines (MEA, DEA, TEA) loaded onto supports
and operated at 20 °C.
Figure 4 demonstrates the higher capacity of the
secondary ethanolamine relative to the primary and
tertiary amines for removing CO2 in a PSA system.1 For
these experiments, the pressure reached 1 mmHg
within the desorption bed at the end of the 25-min
desorption cycle. In Figure 4, the slight change in slope
between the primary and secondary amines in the 0-10min time period results from a small variation in amine
loading within the support. These small slope changes
between MEA and DEA are not important; it is only
the larger variations and overall capacity differences
that are significant. The large variation at times greater
than 10 min comes from the marked change in capacity
between the primary and secondary amines. The large
difference between the primary and secondary amines
Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005 1545
Figure 3. Bench-scale system used in all CO2 capture experiments.
Figure 4. Comparison of suppported, pure amine working capacities.
Figure 5. Modified MEA working capacity, normalized.
and the tertiary amine demonstrates the large difference in affinity for low concentrations of CO2 between
amines that form carbamates and amines that do not.
The tertiary amines do not form carbamates and, as
demonstrated in Figure 4, showed little affinity for CO2
at the levels considered in these tests.
A further demonstration of the efficacy of secondary
amines for CO2 capture was obtained from the capacity
results of the modified MEA samples, i.e., where some
of the primary amine was converted to secondary amine
by reaction with acrylonitrile. Absorption tests were
conducted at the same loadings and test conditions for
each as discussed previously. Comparisons of the results
for pure MEA and the modified MEAs also allowed us
to ascertain the influence of the hydroxyl group on CO2
capacity. The hydroxyl ion is known to be an important
chemical base in CO2 capture. In fact, aqueous amine
solutions will also generate hydroxyl ions, as an intermediate in their CO2 capture mechanism. Although the
ratio of amine/hydroxyl in MEA, DEA, and TEA inherently varied, so that the effect of the hydroxyl could not
be unambiguously resolved, MEA and all of the modified
MEAs had the same ratio of amine/hydroxyl. In the
latter case, only the ratio of secondary to primary amine
changed. Moreover, comparisons between the fully
reacted MEA, which contained predominantly secondary
amines, and DEA (solely secondary amines) isolated the
effect of the hydroxyl group concentration. This allowed
us to confirm that it is the amine type (secondary amine)
and not the quantity of hydroxyl groups attached to the
amine molecule that governs the capture of CO2 in this
cyclic PSA process. No change in CO2 capacity appears
to be related to the quantity of hydroxyl groups on the
ethanolamines, as the data in Figure 5 show similar
capacities between MEA reacted with acrylonitrile and
DEA. Note that these molecules have a 2:1 ratio in the
number of hydroxyl groups present in the tests.
Normalization of the capacity data by the amount of
amine available on the support surface permits isolation
of capacity effects associated with the amine structure
(see Figure 5). All of the data are for samples in which
the total amount of amine on the support surface was
held constant. The only difference is in the mass loading,
which is due to the addition of the acrylonitrile, which
1546
Ind. Eng. Chem. Res., Vol. 44, No. 5, 2005
increased the molecular weight of the amine molecule.
The modification of the primary MEA amine with
acrylonitrile increased the cyclic capacity for CO2 capture. The pure MEA curve in Figure 5 plateaus at an
amine capacity of about 0.14 mol of CO2 per mole of
amine available within the support. Modification of the
MEA with a 1:4 ratio of AN/amine increased the cyclic
CO2 removal capacity of the supported amine to 0.16
mol of CO2 per mole of amine. Increasing the AN/amine
ratio to 2:3 further increased the cyclic CO2 removal
capacity to nearly 0.19 mol of CO2 per mole of amine.
Conclusions
Although liquid amines have long been used for
removing acid gases from pressurized gas streams, they
have generally been used as bulk liquids in aqueous
absorption systems. In this report, a novel method for
contacting the gas phase and absorbent by using amines
supported within the pores of a polymeric support has
been demonstrated. The results indicate that secondary
amines are most beneficial for removing low concentrations of CO2 from a gas stream with a supported
absorbent. This information comes from a comparison
of MEA, DEA, and TEA immobilized within the solid
support at the same molar loading. Working capacity
test results showed a large increase in capacity and
utilization of the secondary amine. In addition, MEA
was modified by reaction with acrylonitrile to convert
this primary amine into a secondary amine. The acrylonitrile molecule formed a Michael adduct with the
primary amine of the MEA, converting it into a secondary amine. The reaction mechanism was tailored to
react predominantly with primary amines. This new
molecule (MEA-AN, now a secondary amine) provided
a large increase in cyclic CO2 removal capacity compared to the unaltered primary amine molecule.
These experiments demonstrate the importance of
amine type in removing low levels of CO2 from a PSA
process while using a polymeric support. These immobilized amines clearly have the potential to provide
CO2 removal for life-support applications The further
ability of the support to resist corrosion and foaming
while increasing the concentration of amine compared
to pumped aqueous systems might provide utility in
other applications. By eliminating the need for additives
and increasing the amine concentration within the
support, these solid amine beads might be useful in
terrestrial applications, such as natural gas sweetening
or flue gas CO2 removal.
Acknowledgment
The authors thank Hamilton Sundstrand Space Systems International for their support of this research
effort.
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Received for review May 24, 2004
Revised manuscript received October 29, 2004
Accepted November 20, 2004
IE0495527