Performance and Properties of a Solid Amine Sorbent
for Carbon Dioxide Removal in Space Life Support
Applications
Sunita Satyapal, Tom Filburn,*
,‡
John Trela,
†
and Jeremy Strange
‡
United Technologies Research Center, 411 Silver Lane, East Hartford, Connecticut, and
Hamilton Sundstrand Division, One Hamilton Road, Windsor Locks, Connecticut
Received October 25, 2000. Revised Manuscript Received January 15, 2001
NASA is currently using a solid amine sorbent known as HSC
+
for regeneratively removing
CO
2
in space shuttle applications. This sorbent may also be of value for CO
2
removal in various
industrial processes such as greenhouse gas control, industrial syntheses, and natural gas
purification. To design novel sorbents and to design a CO
2
scrubber based on HSC
+
, physical
and thermochemical property data are required. In this paper, we present a detailed experimental
investigation of property data and long-term performance results using HSC
+
as a CO
2
sorbent.
Differential scanning calorimetry was used to determine the heat capacity of the material. Cyclic
and equilibrium capacities of the material for CO
2
pickup were determined and long-term test
data show excellent performance. In addition, we have determined the heat of adsorption
associated with CO
2
pickup by HSC
+
and the effect of moisture, using isothermal flow calorimetry.
We have also performed thermal gravimetric analyses on the materials to gain insight into the
stability of the material and determine the temperatures at which CO
2
and constituents of HSC
+
leave the surface of the material.
Introduction
The primary method used to remove CO
2
in space life
support systems until the early 1990s was based on
lithium hydroxide (LiOH).
1-7
This was used, for ex-
ample, in the environmental control and life support
system of space suits and the space shuttle to absorb
CO
2
from the air. Although the storage capacity of LiOH
is high (
∼30 wt %), the material cannot be practically
regenerated. Therefore, the long-term occupation of a
space station would require a CO
2
sorbent that can be
easily regenerated to reduce launch weight and storage
volume. Hamilton Sunstrand Space Systems Interna-
tional (HSSSI) has developed a regenerable sorbent
consisting of solid amine beads, known as HSC
+
. This
proprietary material consists of a liquid amine, poly-
ethyleneimine (PEI), bonded to a high-surface-area,
solid polymethyl methacrylate polymeric support. The
material also consists of a second liquid phase coating,
poly(ethylene glycol) (PEG), to enhance CO
2
adsorption
and desorption rates.
NASA has used HSC
+
in the Regenerable CO
2
Re-
moval System (RCRS) of the space shuttle, to remove
CO
2
from the crew compartment. In this application two
equally sized beds of sorbent are used to maintain CO
2
within tolerable limits (generally below 7.6 mm Hg).
These beds operate in a semi-batch, Pressure Swing
Adsorption (PSA) mode. One bed uses a fan to pass
cabin air through the bed to remove CO
2
and water
vapor, while the second bed is exposed to space vacuum
to regenerate the sorbent. This arrangement continues
for a preset half-cycle time period of roughly 13 min at
which point the bed functions alternate.
This system has demonstrated its CO
2
removal ca-
pability both in ground testing and while in orbit. Its
capacity has been shown to exceed the space shuttle
seven-person crew specification, which requires an
average CO
2
removal rate of 7 kg per day. As a pressure
swing adsorption system it typically adsorbs CO
2
at a
partial pressure of 4.5 mmHg. Despite being exposed
to space vacuum, restrictions in the outlet exhaust duct
limit the final desorb pressure to only
∼1 mmHg.
Therefore, the PSA system alternates between the 4.5
mmHg of the cabin pCO
2
and 1 mmHg during the
desorption mode.
To design sorbents with higher capacities and favor-
able kinetics, a detailed study of the currently used
sorbent, HSC
+
, is essential. In this paper, we report a
* Author to whom correspondence should be addressed.
†
United Technologies Research Center.
‡
Hamilton Sunstrand Division.
(1) Johnson, S. R.; Garrard, G. Regenerative Trace Contaminant
Control: New Test Method for Effects on Solid Amine. ICES paper
921349, July 1992.
(2) Johnson, S. R., et al. Multifunction Air Revitalization Systems:
Combined CO
2
-Trace Contaminant Removal Using Solid Amines. 30th
Space Congress Proceedings, Cocoa Beach, Fl, April 1993.
(3) Kazemi, A. R.; Mitchell, S. M. Advanced Testing and Modeling
of a Modified Solid Amine Regenerative CO
2
& H
2
O Removal System.
ICES paper 932293, p 1624.
(4) Kissenger, L. D. Experimental & Analytical Techniques for
Multiple Gas-Adsorbent Equilibrium (on HSC). M.S. Thesis, Rice
University, May 1977.
(5) Yieh, D. T. Trace Contaminant Studies of HSC Adsorbent. M.S.
Thesis, December, 1978.
(6) Snowdon, D. Trace Contaminant Testing of HSC
+
. Hamilton
Standard Memo, Anal. 95-100, July 20, 1995.
(7) Saiyapal, S.; Filburn, T.; Michels, H.; Graf, J. A Unique Solid
Amine Sorbent Useful for Capturing Low Concentrations of Carbon
Dioxide. Proceedings of the 4th International Greenhouse Gas Control
Conference; submitted August 1998.
250
Energy & Fuels 2001, 15, 250-255
10.1021/ef0002391 CCC: $20.00
© 2001 American Chemical Society
Published on Web 02/20/2001
thorough experimental investigation of thermodynamic
data for HSC
+
such as heat capacities, heats of adsorp-
tion, adsorption capacity, and thermal gravimetric
analyses. Due to the exothermic nature of the CO
2
absorption reaction and the variability of CO
2
capacity
with temperature, physical property data is essential
to optimize system parameters (power, weight, and
volume). Our overall goal is to maximize the cyclic CO
2
capacity of the system while adhering to system power,
weight, and volume specifications.
By reporting on fundamental property data, this
sorbent may also be found applicable for other CO
2
removal applications.
Experimental Setup and Results
Performance. Figure 1 shows a schematic of the RCRS, the
system presently used for regenerative CO
2
removal on-board
the space shuttle. This system has been successfully used on
15 separate shuttle missions operating for over 188 days. This
system occupies less than 0.32 m
3
on board the orbiter and
weighs less than 150 kg. While operating, it draws a meager
65 W of power. This combination of low weight, power, and
volume makes it very attractive for this aerospace application.
Heat Capacity Measurements. Differential Scanning
Calorimetry (DSC; TA Instruments; model 2920) was used to
determine the heat capacities of HSC
+
and the substrate
material over a range of temperatures. The heat capacities are
measured by subjecting a sample of material to a linear
temperature rise. Table 1 shows the heat capacity of HSC
+
between 25 and 50 °C. Several sample sizes between roughly
2 and 8 mg were tested due to the small sample size and
potential inhomogeneities in the material. Each data point is
the average of 6 trials. The procedure involved cooling the DSC
sample holder enclosure to below room temperature (
∼15 °C)
and purging the sample with nitrogen. A ramp rate of 5 °C
per minute was used until a final temperature of roughly 60
°C was obtained. A nitrogen purge of 50 mL/min was used
throughout the runs. By measuring the rate of heat flow and
temperature rise at each temperature, the specific heat of the
material may be calculated. Determining the heat capacity of
a sapphire standard validated the procedure.
As shown in Table 1, the presence of the amine and coating
on the substrate appears to have an effect on the heat capacity.
The substrate consists of porous polymeric beads and the heat
capacity is not expected to change significantly within the
given temperature range. The heat capacity of HSC
+
appears
to be substantially different from the heat capacity of the
substrate due to the presence of a thick amine and poly-
(ethylene glycol) coating. A heat transfer material with low
specific heat would be beneficial in transferring heat away
from the material. The RCRS on the space shuttle employs
such a heat transfer material. The material is a reticulated
aluminum foam that has a 95% open area and therefore adds
negligible weight to the system. We have shown that the cyclic
performance of a dual bed system is improved by 20% when
using the heat transfer material. The foam allows heat
released from the exothermicity of the adsorption process to
be transferred to the desorbing bed, thereby aiding in desorp-
tion.
Temperature Profiles and CO
2
Concentration Break-
through Data. The adsorption capacity for CO
2
and temper-
ature increase associated with adsorption was determined
using a flow apparatus. The apparatus consisted of a Pyrex
tube packed with HSC
+
. A thermocouple was placed at the
center of the bed and the concentration of CO
2
was measured
at the inlet and exit of the tube using an infrared detector
(Horiba, model PIR-2000). Figure 2 shows a typical break-
through curve using an initial mass of 11.4 g of HSC
+
, packed
in a 2.5 cm diameter tube. The flow rate of CO
2
(2% in N
2
)
was varied between
∼0.5 and 2 slpm. The maximum temper-
ature determined in the center of the bed (with no aluminum
Figure 1. RCRS simplified flight schematic.
Table 1. Heat Capacity (J/g-
°
C)
HSC
+
substrate
25 °C
1.73 ( 12
2.78 ( 10.14
30 °C
1.74 ( 0.12
2.76 ( 0.14
35 °C
1.75 ( 0.13
2.75 ( 0.14
40 °C
1.78 ( 0.15
2.75 ( 0.15
45 °C
1.80 ( 0.15
2.76 ( 0.15
50 °C
1.81 ( 0.15
2.76 ( 0.15
HSC
+
in CO
2
Removal in Space Shuttle Applications
Energy & Fuels, Vol. 15, No. 2, 2001
251
foam packing material) was 52.9 °C, and the maximum
capacity was found to be roughly 4% by mass. An improved
version of the amine has shown a
∼8% capacity for CO
2
absorption but has not yet been flight qualified for use in space
applications
7
. Molecular sieve type 5a could be expected to
have a 3.5 wt % capacity in a PSA system operating with a
CO
2
partial pressure between 7.6 and 1 mmHg. However, this
same mole sieve system would need to dry the air stream
before exposing the zeolite to the gas.
Thermal Gravimetric Analysis (TGA). The solid amine
sorbent, HSC
+
, is an improved version of an amine previously
developed at HSSSI which was known as HSC. That material
consisted of a solid polymeric support, coated with an amine.
The HSC
+
version uses a poly(ethylene glycol) coating along
with the amine coating to improve the cyclic capacity of the
sorbent material. To determine the thermal stability of HSC
+
,
thermal gravimetric methods (TA Instruments, model 2950)
were used. TGA (thermal gravimetric analysis) data allow us
to determine the range of temperatures at which CO
2
, the
coating material, and the amine leave the surface of HSC
+
under controlled conditions. Figures 3-8 show TGA data of
HSC
+
and its constituents. All experiments were taken under
consistent conditions and typical sample sizes varied between
20 and 50 mg. The runs were initiated at room temperature
(approximately 25˚C), and the temperature was increased at
a ramp rate of 1 0˚C per minute. Nitrogen was used as a purge
gas at a flow rate of roughly 70 mL/min, throughout all
experiments.
TGA data for the three individual constituents of HSC
+
(substrate, poly(ethylene glycol) coating, and amine) are shown
in Figures 3-5. Figure 3 shows TGA data for the polymeric
substrate and it is clear that significant loss of material does
not occur until
∼300 °C. The slight loss in mass between room
temperature and 300 °C may be due to the evaporation of
adsorbed moisture. The steep slope seen between
∼300 and
∼330 °C, and again between ∼390 and ∼420 °C, is indicative
of two different processes taking place and complete decom-
position is seen by
∼500 °C. As seen in Figure 4, the poly-
(ethylene glycol) coating material by itself, begins to lose
significant mass at temperatures as low as 150 °C. The most
noteworthy point is that although the coating does not have a
high boiling point by itself, it enhances the stability of the
overall material by enhanced intermolecular interaction with
the amine. In other words, the vapor pressure of the coating
by itself is not indicative of the stabilizing effect it produces
when combined with the other materials. Figure 5 shows TGA
data for the liquid amine without any additional constituents.
As seen in Figure 5, the amine has lost only
∼10% of its
original mass even at a temperature of
∼250 °C. The slope of
the mass-loss curve changes from a somewhat gradual slope
to a steep slope at roughly 300 °C. There is another abrupt
change in slope at
∼350 °C, indicative of yet another change
in the mechanism of thermal loss. The amine is completely
disintegrated by roughly 600 °C. It is evident that the amine
has an extremely low vapor pressure, unlike commonly used
amines such as MEA. This property makes it suitable for
coating a substrate and for longevity under temperatures
slightly above ambient conditions. Figures 6 through 8 show
additional TGA data that characterize the sorbent’s thermal
stability. Figure 6, for example, shows the effect of combining
the amine and the poly(ethylene glycol) coating. If one
compares the mass loss for pure coating at 200 °C (see Figure
4) with the mass loss for coating and amine at the same
temperature (see Figure 6), it is clear that there is a difference
of 60% loss (for pure coating) versus
∼15% loss (for amine +
coating). It is also possible that the 15% loss includes trace
Figure 2. Typical breakthrough curve showing CO
2
concen-
tration and temperature profiles.
Figure 3. TGA data for polymeric substrate.
Figure 4. TGA data for coating.
Figure 5. TGA data for amine.
252
Energy & Fuels, Vol. 15, No. 2, 2001
Satyapal et al.
amounts of moisture and CO
2
. Therefore, the TGA data show
that the presence of the amine increases the thermal stability
of the coating material. However, as anticipated, combining
low-boiling and high-boiling components results in a mixture
that usually boils at an intermediate temperature (unless an
azeotrope is formed). Finally, all components are combined to
form the material known as HSC
+
, and the thermal stability
of this combined material is shown in Figure 7. The initial
peak in the mass loss derivative curve (at
∼60 °C) is due to
removal of trace amounts of CO
2
. The second peak at
∼200
°C is due to removal of both amine and coating. There is a
change in the mechanism beginning at roughly 230 °C, and
complete decomposition occurs at
∼450 °C. The HSC
+
sample
was purged using CO
2
, just prior to obtaining TGA data, and
the results are shown in Figure 8. The maximum rate of
desorption of CO
2
is seen to be between 60 and 70 °C. It is
clear that loss of additional components (e.g., amine or coating)
do not begin to take place until over 100 °C. The material
therefore appears to be thermally stable under the operating
conditions we employ for space life support systems. All TGA
data are summarized in Table 2. This solid amine sorbent has
been used on space shuttle missions over the last several years
and no degradation of material has been shown.
Heats of Reaction. To configure coupled adsorption and
desorption beds, the heat of adsorption during the reaction of
HSC
+
and CO
2
must be known. We have measured the heat
of adsorption using isothermal flow microcalorimetery. This
highly sensitive technique is valuable for thermochemical
measurements in which equilibrium is attained in a relatively
short time. The technique we have used is a conductometric
method as opposed to an adiabatic method. Rather than
maintaining adiabatic conditions (i.e., eliminating heat flow
to or from the sample cell), we maintain isothermal conditions
and measure heat flow to or from the cell. Integration of the
heat flow over the time period of the adsorption process
provides the heat of adsorption. The instrument (CSC, model
4400) is a differential (dual cell) unit and can measure heat
flows as low as 0.1 µW (25 nanocalories/s). Operating temper-
atures range from 0 to 100 °C with an adsorbent bed volume
of approximately 3 cm
3
.
Figure 9 shows the heat generated during the adsorption
of CO
2
(2% in air) on HSC
+
. In this example, an approximately
0.6 g sample of HSC
+
was exposed to a 2% mixture of CO
2
in
N
2
, at a flow rate of 30 mL/min. The heat of adsorption was
calculated to be -94 ((8) kJ/mol CO
2
which is consistent with
results anticipated for amine + CO
2
reactions. This value
represents an average for 5 sample trials. The mass % of CO
2
adsorbed was 3.7 ((0.4) %. For instance primary amines have
heat of adsorption values around 84 kJ/mol, secondary amines
have values of about 72 kJ/mol, and tertiary amines form a
much weaker bond of about 48 kJ/mol.
9
One of the major
sources of error is believed to be incomplete degassing to
(8) Goodridge, F. Kinetic Studies in Gas-Liquid Systems. Trans.
Faraday Soc. 1955, 1703.
(9) Kohl, A.; Nielsen, R.; Gas Purification, 5th ed.; Gulf Publishing
Co.: Houston, TX, 1997.
Figure 6. TGA data for mixture of coating and amine.
Figure 7. TGA data for HSC
+
.
Figure 8. TGA data for HSC
+
pretreated with CO
2
.
Figure 9. Heat of adsorption for HSC
+
.
Table 2. Mass Loss Data as a Function of Temperature
temperature °(C)
wt %
amine
coating
amine +
coating
substrate
HSC*
HSC +
CO
2
90
229.8
157.9
188.6
307.7
177.2
165.2
80
264.1
173.3
205.9
315.5
208.3
200.4
70
285.0
182.8
219.2
320.2
283.6
269.4
60
299.5
189.9
237.4
326.6
330.3
323.6
50
315.0
195.9
269.3
354.1
364.3
359.6
40
329.4
201.0
295.6
398.1
391.2
388.1
30
349.4
205.4
328.6
408.0
410.9
409.5
20
475.4
209.4
364.6
412.8
423.6
422.7
10
545.5
213.4
515.8
455.2
437.6
437.0
HSC
+
in CO
2
Removal in Space Shuttle Applications
Energy & Fuels, Vol. 15, No. 2, 2001
253
remove CO
2
and moisture. The sample cell is not directly
connected to a vacuum pump and therefore adsorption of trace
CO
2
or moisture is anticipated during sample transfer. The
flow rates were varied from 5 to 30 mL/min to verify that
the instrument response time was sufficient for the extent of
heat flow.
We also measured the heat released as a stream of humid
air passed through the sample cell containing HSC
+
. A water
adsorption accessory is available for the instrument, which
allows a controlled humidity stream to pass through the
sample cell. Water vapor was continuously injected into a
stream of nitrogen such that the relative humidity was
stabilized at 80%, and the heat released as water adsorbed
onto HSC
+
was determined. We also injected water vapor into
a stream of 2% CO
2
in N
2
, and measured the overall heat
released due to both reaction and adsorption. All results are
tabulated in Table 3. The average values represent data from
either 4 or 5 sample runs. Due to the long times required for
stabilizing the calorimeter temperatures, each run was up to
50 h long.
Discussion and Conclusions
Measuring heats of adsorption provides important
information on the interaction between CO
2
and the
HSC
+
sorbent. We also measured the heat of adsorption
for water vapor on HSC
+
. Our results show a ∆H of -
47.2 ((1.0) kJ/mol H
2
O and we found that nearly 17%
of the initial mass of HSC
+
is absorbed water. The
material is therefore an efficient dehumidification sor-
bent. The heat of condensation of water vapor is -44.0
kJ/mol (at 25 °C) and the ∆H we observe is slightly
larger. As anticipated, a physisorptive process typically
results in a ∆H similar to the heat of condensation of
the adsorbed gas. Due to the potential of additional
chemisorption and the fact that the coating on HSC
+
is
hydrophilic, we expect the heat of adsorption to be larger
than the heat of condensation. The difference, however,
is minor and the predominant mechanism for moisture
adsorption appears to be similar to capillary condensa-
tion. The heat released during adsorption of CO
2
is
significantly larger than the heat of condensation of
water vapor, indicative of a strong interaction between
the CO
2
and the amine surface. These results indicate
that even though the temperature required for desorb-
ing CO
2
is low, the mechanism of adsorption is not
simple physisorption.
To understand the effect of water vapor on the
adsorption process, we also measured the heat released
when a mixture of 2% CO
2
in N
2
, at a relative humidity
of 80%, is adsorbed onto HSC
+
. The important difference
to note is the mass % adsorbed in the presence of water
versus in the absence of water. In the case of pure CO
2
(no water), there is only a 3.67 (( 0.41) % mass gain; in
the case of pure water, there is a 16.8 (( 3.6)% mass
gain; and in the case of CO
2
+ water, the mass gain is
27.3 (( 2.2)%. It is therefore clear that higher CO
2
capabilities are achieved by coadsorption of water.
The mechanism for CO
2
removal using amines is
known to be dependent on the presence of water.
8-9
Without moisture present, the main reaction believed
to account for CO
2
removal is carbamate formation:
This shows that for every one mole of amine, only
1
/
2
a
mole of CO
2
is removed. However, when moisture is
present, further reaction of the carbamate ion to form
bicarbonate occurs:
Bicarbonate may also form directly from the amine +
CO
2
+ water reaction:
Therefore, in the presence of water, one mole of amine
is effective in removing one mole of CO
2
. This mecha-
nism has been discussed in the literature for several
years.
10
In our results, it appears as if roughly 3 times
more CO
2
is removed in the presence of water as
compared to the absence of water. We did not differenti-
ate between the mass adsorbed in terms of the molar
fraction of water and CO
2
. Therefore, there may also
be an enhanced adsorption of water in the experiments
tabulated in the last section of Table 3. Another pos-
sibility is that reaction on the surface, and/or moisture
adsorption, allows more of the active sites of the
material to be available for CO
2
removal. The fraction
of CO
2
removed on a molar basis as compared to the
number of moles of active amine available on the surface
has not been determined. We have theorized the pres-
ence of the poly(ethylene glycol) coating has two poten-
tial effects. The first may be to simply attract more
water to the surface of the support due to the hygro-
scopic nature of the chemical. The second may be an
enhancement due to the preponderance of OH
-
ions
from the polyethlene glycol molecules.
To design CO
2
scrubbers for various applications,
detailed performance studies of the sorbent are es-
sential. We have performed a set of detailed experiments
to determine the physical and thermochemical proper-
ties of the solid amine sorbent HSC
+
. The material has
been used on the space shuttle for several years, and
improved materials are currently under development.
The material may also be of value for other CO
2
removal
(10) Otsubo, K., et al. International Symposium on Space Technology
and Science, Proceedings, Vol. 2; 1992; pp 1431-1438, and references
therein.
Table 3. Heat of Adsorption Data
HSC
+
2% Adsorption in N
2
∆H
initial
mass
(g)
wt %
adsorbed
J/g(mi)
KJ/g(Am)
kJ/mol
CO
2
average
0.531
3.67
78.1
2.15
94.6
std. dev.
0.030
0.41
6.4
0.19
8.2
HSC
+
Water Adsorption in N
2
∆H
initial
mass
wt %
adsorbed
J/g(mi)
KJ/g(Am)
kJ/mol
H
2
O
average
0.546
16.8
504
2.62
47.2
std. dev.
0.057
3.6
157
0.06
1.0
HSC
+
Water Adsorption with 2% CO
2
in N
2
∆H
initial
mass
wt %
adsorbed
J/g(mi)
KJ/g(∆m)
average
0.535
27.3
688
2.52
std. dev.
0.027
2.2
57
0.05
CO
2
+ 2R
2
NH ) R
2
NH
2
+
+ R
2
NCOO
-
R
2
NCOO- + 2H
2
O + CO
2
) R
2
NH
2
+
+ 2HCO
3
-
CO
2
+ R
2
NH + H
2
0 ) R
2
NH
2
+
+ HCO
3
-
254
Energy & Fuels, Vol. 15, No. 2, 2001
Satyapal et al.
applications and therefore property data such as heat
capacities and heats of adsorption are necessary.
The key advantage of this sorbent as opposed to
membrane separations is that pressurization of the CO
2
-
rich stream is not required. The sorbent is capable of
removing low concentrations of CO
2
(
∼1 Torr) under
ambient temperatures and pressures, and is therefore
less cost intensive than membrane separations. Liquid
amines have been the most predominant method for
CO
2
removal but require substantial equipment for
circulating liquids and must deal with the corrosive
nature of the carbamates formed in the process. The use
of solid amines provides ease of handling, applicability
to microgravity environments, and regenerability in
either pressure swing or temperature swing modes. As
opposed to metal oxides for CO
2
removal, we have shown
that the solid amine may desorb CO
2
at temperatures
as low as
∼40 °C. We have measured the CO
2
absorption
capacity for HSC
+
to be
∼4% at ambient pressures, and
have shown that the sorbent may be regenerated using
vacuum desorption at
∼1 mmHg. Thermal gravimetric
analysis was used to show that the amine is strongly
bonded to the substrate and the material does not begin
to lose amine/coating components until over 100 °C. The
material has been tested for hundreds of cycles with no
loss in performance. These cycles have consisted of a
period of CO
2
adsorption followed by an equal time
period of vacuum desorption. Vacuum levels generally
range from 5 mmHg to 1 mmHg at the end of the
desorption period.
Acknowledgment. We thank Harvey Michels, Phil
Birbara, and Joe Genovese for detailed discussions on
this work. We also thank Larry Pryor for the setup of
theTGA instrument.
EF0002391
HSC
+
in CO
2
Removal in Space Shuttle Applications
Energy & Fuels, Vol. 15, No. 2, 2001
255
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