Chinese Journal of Chemical Engineering, 19(5) 733—737 (2011)
A Research Note on the Adsorption of CO2 and N2
ZHANG Zhongzheng (张中正)1, RUAN Hongzheng (阮红证)2, ZHOU Yaping (周亚平)2, SU
Wei (苏伟)1, SUN Yan (孙艳)1 and ZHOU Li (周理)1,*
1
Chemical Engineering Research Center, School of Chemical Engineering & Technology, Tianjin University, Tianjin
300072, China
2
Department of Chemistry, School of Science Tianjin University, Tianjin 300072, China
Abstract Experiments were made for the adsorption of CO2 and N2 on typical adsorbents to investigate the effects
of porous structure and surface affinity of adsorbents as well as those of adsorption temperature and pressure that
might cause the variation of adsorption mechanism. It is shown that polar surface tends to enlarge the adsorption
difference between CO2 and N2, and the difference is more sensitive to temperature than the adsorbents with
non-polar surface. The adsorbents with non-polar surface are not much sensitive to the effect of water vapor, though
the water vapor interferes the separation remarkably. The separation coefficient linearly increases with the micropore volume per unit surface area of activated carbons, but no rule is shown on mesoporous silicon materials. The
function of adsorption mechanism on the separation is not as much as expected.
Keywords adsorption, CO2, N2, comparison
1
INTRODUCTION
Both CO2 and N2 are important industrial gases
because they are mixture components in many production or processing industries. They are also the
major components of flue gas emitted from coal-fired
power plants, so the separation between them or the
capture of CO2 attracts global research interest. A
huge quantity of CO2 emits into atmosphere every day,
so the industrial technology for CO2 capture must be
efficient and cheap in cost. The separation based on
adsorption cannot compete with the technology based
on amine-absorption presently [1]. However, studies
on alternative techniques are necessary since each
method has its best cases to apply and the advance in
science may change the suitability sequence of technologies for a specified separation. The separation
based on adsorption difference of components has
been an important technology in industry especially
for mixtures composed of light gases since 1970’s.
Many adsorbents have been tested for the capture of
CO2, from activated carbons [2, 3] and zeolites [4, 5] to
novel materials [6-8]. However, there is still a long
way to go to find an efficient adsorbent suitable for
industrial application. Therefore, the results of a set of
fundamental experiments are presented in the present
report and hopefully it may help the development of
advanced adsorbents in the future. Attention of the
experiments is especially paid to investigate the effect
of pore structure and surface affinity of adsorbents as
well as the adsorption mechanism on the adsorption of
CO2 and N2. The selectivity difference of adsorbents
for the two gases is reflected in the thermodynamically defined separation coefficient evaluated based on
the breakthrough curves collected with a gas mixture
containing the components of interest [9].
2
2.1
EXPERIMENTAL
Apparatus
Breakthrough curves of component gases passing
through an adsorption bed are the basis to evaluate an
adsorbent for a specified separation. The apparatus for
collecting breakthrough curves is schematically shown
in Fig. 1. The adsorbent was packed in a column of
length 250 mm and of inner diameter 10 mm. A section of length 230 mm was filled with adsorbent. Two
SY-9312 type mass flow controllers with precision
±1% were used to control the flow rates in the two
incoming passages of the column: one passage for gas
Figure 1 Schematic diagram of the setup to collect breakthrough curves
PR: pressure regulator; MFC: mass flow controller; BP: back
pressure regulator; P: pressure transducer; T: thermocouple;
QMS: quadrapole mass spectrograph
Received 2010-06-02, accepted 2011-08-13.
* To whom correspondence should be addressed. E-mail: [email protected]
734
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
mixture and another for carrier gas (helium). A regulator was used to maintain the back-pressure of adsorption bed and a SY-9411 type pressure transducer
with accuracy ±0.1% was used to detect the pressure.
The zero point of the transducer was automatically
adjusted to compensate for the fluctuation of room
temperature. Pressure at both the entrance and exit of
the adsorption bed was detected with the same transducer in terms of two T-way valves, and the pressure
drop over the adsorption bed was thus determined. A
QMS Series Gas Analyzer purchased from the Stanford Research In. was used to analyze the composition
of effluent streams. All parts of the setup were connected by stainless steel capillary tubes of inner diameter 2 mm and wall thickness 0.5 mm. Signals of
pressure, temperature, flow rates and composition
were transferred to a computer in terms of PC-lab
cards PCLD-880 and PCL-812PG purchased from the
Advantech Co., Ltd. The computer recorded the variation of signals with time and issued commands of experiments according to a prescribed program.
Characterization of adsorbents was based on the
adsorption/desorption isotherms of N2 at 77 K collected
on Micromeritics ASAP 2020. The BET (Brunauer,
Emmett, Teller) theory [10] was applied to evaluate the
specific surface area. The BJH (Barrett, Joyner, Halenda)
equation [11] was used to determine the pore size distribution (PSD) of mesoporous materials, and the
NLDFT (Non-local density function theory) method
[12] was used to determine the PSD of other materials.
2.2
Material
To observe the effect of pore size on adsorption
mechanism, a set of activated carbons (AC) was prepared from carbonized corncobs through a stepwise
activation with steam. A definite quantity of water that
just fully fills the pore space was dropped in the sample at each step, and the water-loaded carbon was then
put in a furnace at 750 °C for 1 h while a nitrogen
stream kept flowing at a rate of 100 cm3·min−1. The
AC samples 1# to 7# were thus obtained. Silica gels
(SG) of Type A, B, and C were purchased from Ocean
Chemicals, Qingdao. The silicon mesoporous material
MCM-41 and SBA-15 were prepared in the lab following the procedure reported in literature [13-16].
Major parameters of adsorbents calculated with the
afore-mentioned methods based on the adsorption/
desorption isotherms of nitrogen at 77 K are listed in
Table 1. It is shown that AC-1, AC-2 and AC-3 are
microporous because the volume of mesopores is negligibly small. Similarly, the volume of micropores in
the sample SG-B, SG-C, MCM-41 and SBA-15 is
negligibly small, so they are mesoporous.
A gas mixture composed of He (52.41%), CO2
(25.78%) and N2 (22.81%) was used in breakthrough
experiments. The purity of all gases is higher than
99.99 %. The flow rate was kept at 100 cm3·min−1 when
the gas mixture passed through the adsorption bed.
Table 1
Basic porous structure parameters
of the tested adsorbents
Sample
ABET/m2·g−1
Vp/cm3·g−1
Percentage of
micropores/%
Percentage of
mesopores/%
AC-1
236
0.100
100
0
AC-2
717
0.318
100
0
AC-3
1050
0.419
96.81
3.19
AC-4
1414
0.657
68.76
31.24
AC-5
2236
1.051
52.09
47.91
AC-6
2803
1.840
24.87
75.13
AC-7
3400
1.827
31.94
68.06
SG-A
649
0.404
77.49
22.51
SG-B
483
0.817
—
—
SG-C
463
1.077
—
—
MCM-41
934
1.155
—
—
SBA-15
546
0.768
—
—
3 EVALUATION OF ADSORPTION DIFFERENCE
The adsorption difference of mixture components
is the basis of adsorptive separation. The difference is
reflected in the adsorbed amounts on adsorbent under
a specified condition and is usually shown on isotherms. However, the adsorbed amount of a pure gas
is different from that of the gas as a component from a
mixture because of the adsorption competition among
components. Therefore, breakthrough curves were
collected with the given gas mixture on a selected adsorbent in addressing separation problems. The adsorbed amount of each component can be calculated
with assumption [9]:
∫ 0 ( ui yk ,i − ue yk ,e ) Acdt − ε ALyk ,i p / RT
t
nk =
(1)
m
where nk is the adsorbed amount of component k,
mmol·g−1; t is the breakthrough time of component k,
s; ui and ue are the flow speed of the gas stream at the
entrance and exit of adsorption bed, cm·s−1; yk,i and yk,e
are the molar fraction of component k in the entrance
and exit streams, respectively; c is the total concentration of components, mol·cm−3; A and L are the sectional area of adsorption bed, cm2, and the bed length,
cm, respectively; ε and m are the void space and mass
of adsorbent, g, respectively; p and T are the pressure
and temperature of adsorption, respectively; and R is
the gas constant. The value of the integral corresponds
to the area underneath the breakthrough curve of
component k. The thermodynamically defined separation coefficient [17] can then be calculated:
⎛ ni ⎞ ⎛ y j ⎞
⎟⎜ ⎟
⎟
⎝ n j ⎠ ⎝ yi ⎠
α ij = ⎜
⎜
(2)
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
735
where y is the molar fraction of components, i and j, in
the gas phase at equilibrium.
4
RESULTS AND DISCUSSION
4.1 Effect of porous structure on adsorption selectivity
Different correlations were tried on the separation coefficient (α) with porous structure parameters
of activated carbons, and a linear correlation (correlation coefficient = 0.93) was found for α vs. Vmp/ABET,
the micropore volume per unit specific surface area of
adsorbents, as shown in Fig. 2. However, no rule was
shown for α with the structure parameters of mesoporous silicon materials.
Figure 3
perature
4.2
4.3
Effect of surface polarity
Because the molecular polarity of CO2 is larger
than that of N2, the adsorbent with polar surface may
enlarge the adsorption difference between the two
gases. As is expected, the separation coefficient on
silica gel Type-A is much above that of activated carbons as shown in Fig. 2. The α values on the tested
silicon mesoporous adsorbents at 0.4 MPa and 283 K
are: 9.82 (SG-A), 5.32 (SG-B), 2.99 (SG-C), 3.05
(MCM-41) and 2.33 (SBA-15). However, as
afore-mentioned, there was no rule between the separation coefficient and the pore parameters of silicon
mesoporous materials. The effect of surface polarity is
also shown in the variation of α with temperature.
While α does not change much with the increase of
temperature as observed on activated carbons, remarkable change in the α value is observed on silica
gels as shown in Fig. 3.
Figure 2 Correlation of separation coefficients (at 0.4 MPa,
283 K) with pore parameters of AC adsorbents
Variation of separation coefficient with tem-
Effect of pressure
The critical temperature of carbon dioxide is 304 K
while that of nitrogen is 126 K. Therefore, the two
gases may show different adsorption mechanisms at
283 K. While N2 always follows the mechanism of
supercritical gases, it is possible for CO2 to follow the
mechanism of sub-critical gases. It was expected that
higher adsorption pressure might enlarge the adsorption difference, but this expectation was not observed
as shown in Fig. 4. Both activated carbons AC-4 and
AC-7 possess a large portion of mesopores as indicated in Table 1. However, capillary condensation did
not occur with CO2 at relatively high pressures, so the
separation coefficient was not enlarged. The separation coefficient on the two silica gels generally showed
a decreasing trend as pressure increased.
Figure 4 Effect of adsorption pressure on the separation
coefficient at 283 K
1—AC-4; 2—AC-7; 3—SG-A; 4—SG-B
736
4.4
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
Effect of water vapor
The water vapor contained in the mixture of CO2
and N2, as is the case of flue gas, may affect the separation between them. Zeolite 13X may be a good
adsorbent for CO2 capture from the point of view of
adsorption selectivity, but its adsorption capacity for
CO2 is totally lost in a humid stream, and it is difficult
to recover the adsorption capacity after being saturated by water. Therefore, the experiments were carried out on 9 g of activated carbon with a specific surface area of 1120 m2·g−1. The water vapor carried by
flue gas will condensate on the surface of adsorbent. It
was estimated that 0.3 g of water will mono-molecularly
cover the surface of carbon sample, and the experiment was carried out consecutively with 0.1, 0.2 and
0.3 g of water loaded on the carbon corresponding to
33%, 67% and 100% percentage wetting of carbon
surface. The water vapor affects the adsorption of CO2
more remarkably than that of N2 and the separation
coefficient decreases with the increase of wetting percentage of carbon surface as shown in Fig. 5. However,
the adsorption capacity for CO2 and the adsorption
difference is still reasonable for separation even if the
carbon adsorbent is totally wetted. In addition, it is
much easier to get water off from the carbon surface
than from a polar surface.
not observed experimentally.
NOMENCLATURE
A
c
L
m
n
p
R
T
t
u
V
y
α
ε
surface area of adsorbents (m2·g−1) or the sectional area of an adsorption bed, cm2
total concentration of components, mol·cm−3
bed length, cm
mass of adsorbent in bed, g
adsorbed amount of component, mmol·g−1
adsorption pressure, MPa
gas constant
adsorption temperature, K
breakthrough time of a component, s
flow speed of a gas stream passing through adsorption bed, cm·s−1
pore volume, cm3·g−1
molar fraction of components in gas phase
separation coefficient
void space of adsorption bed
Subscripts
BET
e
i
i
j, k
m
p
according to the BET theory
exit
inlet
component index
component index
micro
pore
REFERENCES
1
2
3
4
5
Figure 5 Effect of water vapor in gas mixture on the adsorption and separation
6
5
CONCLUSIONS
Polar surface may yield larger separation coefficient between CO2 and N2, but no regular effect of
pore parameters was observed while a linear correlation was found for activated carbons, on which water
vapor present in the gas mixture did not show a serious effect on the separation. Although a temperature
between the critical temperatures of two gases may
cause a differential adsorption mechanism that may
enlarge the adsorption difference, a positive effect was
7
8
9
Ian Murray & Company Ltd., “Alberta CO2 capture cost survey and supply
curve”, Ian Murray & Company Ltd., December 31, 2008 [2011-06-02],
http://www.canadiancleanpowercoalition.com/pdf/GS25%20-%20I
MC%20Report%20_%20CO2%20Capture%20in%20Alberta.pdf.
Guo, B., Chang, L., Xie, K., “Adsorption of carbon dioxide on activated carbon”, Nat. Gas Chem., 15 (3), 223-229 (2006).
Grande, C.A., Rodrigues, A.E., “Electric swing adsorption for CO2
removal from flue gases”, Int. J. Greenhouse Gas Control., 2 (2),
194-202 (2008).
Zhao, Z., Cui, X., Ma, J., Li, R., “Adsorption of carbon dioxide on
alkali-modified zeolite 13X adsorbents”, Int. J. Greenhouse Gas
Control., 1 (3), 355-359 (2007).
Zhang, J., Webley, P.A., Xiao, P., “Effect of process parameters on
power requirements of vacuum swing adsorption technology for CO2
capture from flue gas”, Energy Conversion and Management, 49 (2),
346-356 (2008).
Xu, X.C., Andresen, J.M., Song, C.S., Miller, B.G., Scaroni, A.W.,
“Preparation and characterization of novel CO2 “molecular basket”
adsorbents based on polymer-modified mesoporous molecular sieve
MCM-41”, Micro. Meso. Mater., 62, 29-45 (2003).
Liu, X.W., Zhou, L., Fu, X., Sun, Y., Su, W., Zhou, Y.P., “Adsorption
and regeneration study of the mesoporous adsorbent SBA-15
adapted to the capture/separation of CO2 and CH4”, Chem. Eng. Sci.,
62 (4), 1101-1110 (2007).
Glover, T.G., Dunne, K.I., Davis, R.J., Le Van, M.D., “Carbon-silica
composite adsorbent: Characterization and adsorption of light gases”,
Micro. Meso. Mater., 111 (1-3), 1-11 (2008).
Yang, R.T., Gas Separation by Adsorption Process, Butter Worths,
Boston (1987).
Chin. J. Chem. Eng., Vol. 19, No. 5, October 2011
10
11
12
13
Brunauer, S., Emmett, P.H., Teller, E.J., “Adsorption of Gases in
Multimolecular Layers”, Am. Chem. Soc., 60, 309-319 (1938).
Barrett, E.P., Joyner, L.G., Halenda, P.P., “The determination of pore
volume and area distributions in porous substances. I. Computations
from nitrogen isotherms”, J. Am. Chem. Soc., 73, 373-380 (1951).
Ravikovitch, P.I., Vishnyakov, A., Russo, R., Neimark, A.V., “Unified approach to pore size characterization of microporous carbonaceous materials from N2, Ar, and CO2 adsorption isotherms”, Langmuir, 16, 2311-2320 (2000).
Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T.,
Schmitt, K.D., Chu, C.T.W., Olson, D.H., Sheppard, E.W., “A new
family of mesoporous molecular sieves prepared with liquid crystal
templates”, J. Am. Chem. Soc., 114, 10834-10843 (1992).
14
15
16
17
737
Kresge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C., Beck, J.S.,
“Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism”, Nature, 359, 710-712 (1992).
Zhao, D.Y., Feng, J.L., Huo, Q., Melosh, N., Fredrickson, G.H.,
Chemlka, B.F., Stucky, G.D., “Triblock copolymer syntheses of
mesoporous silica with periodic 50 to 300 angstrom pores”, Science,
279, 548-552 (1998).
Zhao, D.Y., Huo, Q.H, Feng, J.L., Chmelka, B.F., Stucky, G.D.,
“Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered hydrothermally stable mesoporous silica structures”, J. Am. Chem. Soc., 120, 6024-6036 (1998).
Ruthven, D.M., Principles of Adsorption and Adsorption Processes,
John Wiley & Sons, New York (1984).