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Different CO2 absorbents-modified SBA-15 sorbent for highly selective CO2 capture
Accepted Manuscript Research paper Different CO2 Absorbents-modified SBA-15 Sorbent for Highly Selective CO2 Capture Xiuwu Liu, Xinru Zhai, Dongyang Liu, Yan Sun PII: DOI: Reference:
S0009-2614(17)30267-1 http://dx.doi.org/10.1016/j.cplett.2017.03.048 CPLETT 34648
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
10 January 2017 13 March 2017 16 March 2017
Please cite this article as: X. Liu, X. Zhai, D. Liu, Y. Sun, Different CO2 Absorbents-modified SBA-15 Sorbent for Highly Selective CO2 Capture, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.03.048
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Different CO2 Absorbents-modified SBA-15 Sorbent for Highly Selective CO2 Capture Xiuwu Liu a,*, Xinru Zhai a, Dongyang Liu a, Yan Sun b (aSchool of Chemical and Technology, Hebei University of Technology, Tianjin, 300130, b
Adsorption Laboratory of Chemical Engineering, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China)
Abstract Different CO2 absorbents-modified SBA-15 materials are used as CO2 sorbent to improve the selectivity of CH4/CO2 separation. The SBA-15 sorbents modified by physical CO2 absorbents are very limited to increasing CO2 adsorption and present poor selectivity. However, the SBA-15 sorbents modified by chemical CO2 absorbents increase CO2 adsorption capacity obviously. The separation coefficients of CO2/CH4 increase in this case. The adsorption and regeneration properties of the SBA-15 sorbents modified by TEA, MDEA and DIPA have been compared. The SBA-15 modified by triethanolamine (TEA) presents better CO2/CH4 separation performance than the materials modified by other CO2 absorbents.
Keywords: CO2 capture; CO2/CH4 separation; adsorption stability; reversibility stability; high selectivity
1.
Introduction Excessive greenhouse gas emission has become one of the most serious global
environmental problems, which arises more and more attention by public authorities worldwide. The global temperature is predicted to rise 1 - 5℃ to 2050 if there is no proper action to reduce greenhouse gas emission [1-4]. Among the greenhouse gases, CH4 and CO2 contribute most to human being generated greenhouse. The recovery and utilization of CH4 and CO2 is becoming an urgent task to the world 1
[5]. In vary sources of gas emission, such as landfill gas, biogas, and savage gas, whose major components are CO2 and CH4 gases, contribute a large proportion of greenhouse gas emissions. For example, landfill generates at least 600 million m3/year gas in China, and is still increasing from 7% to 9% every year [6-7]. If the emission from those typical sources could be recovered and utilized, the emission of greenhouse gases could be massively reduced, and a large number of energy can be recovered in the meantime. In order to mitigate global warming progress, many countries are paying more attention to control and utilize the emissions. Methane content of landfill emission gas must be higher than 80% to increase heat value in order to meet methane pipeline transfer requirement and reaction to produce chemical feedstock. CO2 in landfill emission gas must be removed to increase methane concentration [8-11]. The sizing of landfills in lots of cities in China is small and decentralized distributes, operation flexible separation techniques are required in urgent matter. PSA technology has been paid much attention in the industry due to its low capital investment and low energy consumption requirement. High selectivity of adsorbent is a vital segment of the technology [12-15]. The adsorbent must perform selectively to separate carbon dioxide and methane mixture. The adsorption method attracts many researchers’ interest in order to enrichment methane. The conventional adsorbents, such as activated carbon, carbon molecular sieve, silica gel, and zeolite molecular
sieves, are futile
to
recover
methane
from
landfill
gas
[16-19]. Ordered mesoporous materials emerge in recent years and their applications of methane enrichment have also been studied. The surface modification of mesoporous materials, such as MCM-48, MCM-41, SBA-15, can significantly improve adsorption capacity and selectivity of CO2 [20-29]. Most adsorbents can be regenerated easily at ambient temperature when they are applied for a PSA operation. Whereas, the adsorbents with good selectivity and facile reversibility are difficult for CO2/CH4 separation at ambient temperature. In
previous
studies, several
commercial
CO2
absorbents
(DIPA, MDEA, NMP, DGDE, TEP and PC) previously were described effective 2
to absorb H2S gas [30, 31], and are selected to test modified SBA-15. The SBA-15 modified by TEA has been validated to CO2/CH4 separation [32]. In this study, we still focus on identifying the validity of adsorbents for CO 2/CH4 separation through SBA-15 materials modified by several commercial CO2 absorbents which are effective to absorb H2S gas at ambient temperature. Different commercial CO2 absorbents modify SBA-15, the separation and the adsorption-desorption properties of CO2/CH4 are investigated to identify as valid adsorbents for separation.
2.
Experiment
2.1. CO2 absorbents-modified the SBA-15 The detailed procedure of triethanolamine (TEA) modified mesoporous silica material SBA-15 synthesized is previously described [32]. A definite weight of SBA-15 is immersed in acetone solution of CO2 absorbents, and acetone solvent is slowly evaporated at about 60℃. The coating ratio of a sample is evaluated from weight and specific pore volume of SBA-15, the coated weight and density of CO2 absorbents. Rc (the coating ratio), is ratio of the fractional filling of pore space and the entire pore space, it can be expressed with formula (1) as below: Rc
Mass CO2 absorbents Mass SBA 15 Vpore SBA 15 Density CO2 absorbents
(1)
2.2. Breakthrough and Regeneration Tests The detail of experimental device, two gas mixtures for the separation experiments (one mixture of He (79.04%), CO2 (9.74%), and CH4 (11.22%) to determine the separation coefficient between CO2 and CH4, another mixture consisting of CH4 (63.8%) and CO2 (36.2%) to study regeneration performance of saturated adsorbent.) and adsorbent regeneration tests are previously described [32]. Breakthrough curves of the gas mixture are collected at 298 K and pressure 0.5 MPa respectively. Flow rate of the stream is kept at 100 cm3/min during the experiment. The adsorption and regeneration tests are first fed with the CO2 and CH4 (without helium) mixture at 0.5 MPa, flow rate kept at 100 cm3/min. When the column is saturated with CO2, its pressure releases down to the atmospheric, and then the 3
column is purged with a pure methane stream, flow rate of the purging stream is kept at 50 cm3/min.
3. Result and discussion 3.1. The structural changes of the SBA-15 modified by TEA In previous study [33], the synthesized SBA-15 material is examined with XRD, TEM, and the nitrogen adsorption at 77K, and the ordered channel structure is confirmed. The BET specific surface area, pore volume and pore size distribution are 780 m2/g, 1.31 cm3/g and 7.7 nm respectively. In order to test the impact of TEA coating to the structural properties, 77 K nitrogen adsorption isotherms are collected with different TEA coating ratios (0, 0.14, 0.29, 0.41, 0.55, 0.68, 0.82, 1.00 and 1.16) to calculate the corresponding surface area and pore volume. The result is shown in Fig.1. The pore volume and surface area decrease with the coating ratio increasing. The pore volume (0.05 cm3 g-1) reaches nearly zero when Rc =1.0, which means that the coated TEA goes inside pore space of SBA-15 and the pore space was fully filled with TEA. Another test is carried out to prove the impact of coating TEA to the selectivity of adsorbent to CO2. The breakthrough curve is generated to summarize CO2 and CH4 passing SBA-15 bed together with carrier gas (helium) in condition 0.5 MPa, flow rate 100 cm3/min, which is shown in Fig.2. The adsorption capacity of CO2 appears increase obviously, but there is no help for the increasing of CH4. The separation coefficient increases more than seven times. In conclusion, the separation coefficient between CH4 and CO2 enlarges remarkably with TEA coating. TEA-coating is capable to enhance the separation between CO2 and CH4, but the enhancement is not remarkable when coating ratio, Rc, is less than 0.4. It reaches maximum at Rc=1.0 (the breakthrough curve at Rc =1.0 was shown as in Fig.2.). Consequently, Rc=1.0 was selected as the coating ratio (Rc) of other CO2 absorbents modified SBA-15.
3.2. CO2 /CH4 separation property of different CO 2 absorbent modified SBA-15 In order to further enhance CO2 adsorption selectivity of the SBA-15 modified, several commercial CO2 absorbents (DIPA, MDEA, NMP, DGDE, TEP 4
and PC) that are effective to absorbed H2S gas [30, 31], are selected to test modified SBA-15. The test result is shown in Table 1. The adsorption capacity of SBA-15 modified to methane gas is lower than that of SBA-15 unmodified. Different SBA-15 materials modified by different CO2 absorbent have nearly the same methane adsorption capacity. This might be explained that the solubility of methane is similar in different CO2 absorbent. It is very limited to increasing CO2 adsorption capacity of SBA-15 modified by physical CO2 absorbents, such as NMP, DGDE, TEP, and PC. The CO2/CH4 separation coefficients of those samples are very low and perform poor selectivity at the same time. On the contrary, CO2 adsorption capacity of SBA-15 modified by chemical CO2 absorbent performs very well. Chemical CO2 absorbents-modified the SBA-15 with numerous CO2-anity site such as amine groups is loaded into the pores of SBA-15 to increase the affinity between adsorbent and CO2. It improves the selectivity of CH4/CO2 separation and increases CO2 adsorption capacity obviously[26]. CO2 adsorption is the result of an acid-base reaction, therefore, the separation coefficient between CH4 and CO2 enhances remarkably and performs high selectivity. For example, the CO2/CH4 separation coefficient of SBA-15 modified by DIPA reaches 46.75. Its CH4 and CO2 breakthrough curve is shown in Fig.3.
3.3. The separation stability of the SBA-15 materials modified by TEA, DIPA and MDEA Following the findings from tests, the separation coefficient of CH4 and CO2 performs considerably enhancement in SBA-15 materials modified by TEA, DIPA and MDEA. Their CO2 and CH4 adsorption stability is further studied. Saturated adsorption and purged tests are repeated in the condition of single column filled with SBA-15 modified by chemical CO2 absorbent. In condition pressure 0.5 MPa and flow rate 100 cm3/min, the breakthrough curve of the column filled with the SBA-15 material is first saturated with a stream of CH4, CO2, and He (carrier gas). The bed pressure is discharged to the atmospheric. Following the steps above, the adsorbent bed is purged with helium gas at flow rate of 50 cm3/min at ambient temperature and 5
pressure. The test results are summarized in Fig.4. The adsorbent would be considered as regenerated completely and can transfer to the next adsorption process if CO 2 gas cannot be detected by MS at the outlet of the bed in purged process. As shown in Fig.4, the CO2/CH4 separation coefficients of the SBA-15 modified by various CO2 absorbents are a bit low at the beginning of several operation cycles. It present a quite stable trend after 5 cycles of consecutive saturation and regeneration. Among those curves, the SBA-15 modified by DIPA presents most obvious characteristic. By comparing the stability test experimental breakthrough curves, it can be found that methane adsorption capacity remains basically constant during whole stability test, but CO2 adsorption capacity decreases in the first batch four cycles. It present stability from the fifth cycle stability experiments. A conclusion can be drawn that the change of CO2/CH4 separation performance is caused by the changes of
adsorption
capacity
of regenerated methane
and
of
CO2. Meanwhile, the
carbon
dioxide
remains
adsorption capacity stable
after
5
times cycles. As shown in Fig.4, the separation coefficient of CO2/CH4 presents less variation in condition SBA-15 modified by TEA, MDEA than modified by DIPA. However, the CO2 adsorption capacity of SBA-15 modified by DIPA decreases quickly in the prior several adsorption and regeneration tests. The separation coefficient of CO2/CH4 decreases remarkably. DIPA is a secondary amine and its amine groups have stronger force to CO2 than tertiary amine. In addition, DIPA's melting point is 42℃ , which is higher than that of TEA and MDEA, DIPA is solid at the ambient condition, which causes CO2 adsorbed irreversibly. Result in the decrease of CO 2 absorption capacity in stable experiment. This is the result of CO2 adsorption in acid-base reaction, in this condition, secondary amines react with CO2 via a zwitterion mechanism, it forms carbamate and tertiary amines react with CO2 in the presence of water to form bicarbonate. For the zwitterion mechanism, an additional free base is required to deprotonate the zwitterion to form a carbamate, which can be supplied by a neighboring amine groups[34]. The reaction of secondary amines and CO2 in anhydrous condition is shown as following: 6
Part of generating carbamate doesn’t decompose in the desorption process at ambient condition, which causes a sharp fall of CO2 adsorption capacity before the fourth regeneration and remains stable after the fifth cycle regeneration. Tertiary amines (TEA and MDEA) react with CO2 in the presence of water to form bicarbonate, the reaction is shown as following:
If water is not available, TEA and MDEA are tertiary amine as tertiary amines lack a transferable proton and therefore cannot form carbamates, it results that can’t react with CO2 in anhydrous condition. Their amine groups have relatively weak force to CO2. Carbon dioxide can be nearly completely recycled and the separation coefficient of CO2/CH4 varies limitedly. The changes of methane and CO2 adsorption capacity presented by the SBA-15 modified by DIPA are shown in Fig.5. After the stable experiment, CH4/CO2
separation
coefficients
of
the
SBA-15
modified
by
TEA, MDEA, and DIPA are 16.73, 12.7, and 15.5 respectively.
3.4. The adsorption and regeneration properties of the SBA-15 which are modified by TEA, DIPA, MDEA Fast regeneration of the adsorbent is a key factor to decide its further application. A sorbent with high selectivity but difficulty in regeneration cannot be applied as a PSA adsorbent product. In order to test the adsorption and regeneration of the sorbents modified by TEA, DIPA, MDEA, a bed filled with the modified sorbent is first saturated with a stream CO 2-CH4 mixture, under pressure and flow rate at 0.5 MPa and 100 cm3 /min. The pressure is discharged to the atmospheric
afterwards, and
then the
column
is
purged
with
methane
gas at flow rate of 50 cm3 /min. The breakthrough curves of CO 2 are shown in Fig.6, and
purged
curves
are
shown
in
Fig.7.
C
represents
concentrations at adsorption bed outlet. C0 is feeding CO2 raw concentrations. 7
CO2
As shown in Fig.6, MDEA-modified SBA-15 has maximum breakthrough time, and TEA-modified SBA-15 has shortest one. However, CO2 breakthrough times among three CO 2 absorbents-modified SBA-15 have little different. As shown in Fig.7，among all SBA-15 materials modified by CO 2 absorbents, the CO2 concentration in the purging stream remains stable at the initial 20 seconds, but decreases very fast. The initial value of C/C0 is higher than 1.0 due to fast replacement of the adsorbed CO 2 by CH4 . After a comparison among three CO 2 absorbents-modified
SBA-15
regeneration
curves, it
can
be
found
that
TEA-modified SBA-15 is the easiest adsorbent to be regenerated. As CO 2 concentration at bed outlet is highest at the initial 20 seconds, and the complete regeneration time is the shortest. Regeneration of DIPA-modified SBA-15 is the most difficult among those absorbents. The concentration of C/C 0 is still 0.1 and it is regenerated incompletely after purge time has reached 1190s. Therefore, it can be concluded from the above tests that the TEA-modified SBA-15 has the best regeneration performance at ambient pressure and temperature.
4. Conclusion The SBA-15 materials modified by physical CO 2 absorbents perform quite limited CO2 adsorption capacity, it presents poor selectivity also. On the contrary, the SBA-15 materials modified by chemical CO 2 absorbents increase capacity greatly and present better separation coefficient as well as better selectivity. The SBA-15 adsorbent modified by TEA has the best regeneration performance through the adsorption and regeneration experiments, which can be used as a highly selective adsorbent for CO2/CH4 separation.
Acknowledgement This test is financial supported by Hebei Province Natural Science Foundation [B2008000023], Hebei Province Education Foundation [2006332], and Tianjin 8
Municipal Science and Technology Commission [10JCZDJC23900].
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12
Figure Captions
800
1.6
●
2
Pore volume
1.4
700
○Surface area
1
600 -1
Surface area/m .g
500
1.0
2
3
Pore volume/cm . g
-1
1.2
0.8
400
0.6
300
0.4
200
0.2
100
0.0 0.0
0.2
0.4
0.6
0.8
1.0
0 1.2
Rc
Fig.1. Effects of TEA-modifying on the structural property of samples.
n/mmol.g
-1
1.5
● CH4 adsorption capacity ○ CO2 adsorption capacity ▲ separation coefficient
21 18 15
1.2
3
12
0.9
9
0.6 0.3 0.0
6
2
3
1
0.0
0.2
0.4
0.6
Separation coefficient
1.8
0.8
1.0
1.2
0
Rc
Fig.2. Effects of TEA-coating ratio on CH4 and CO2 adsorption capacity and separation coefficient of CH4 /CO2.
13
1.0
He
Composition
0.8
0.6
0.4
0.2
0.0
CH4 CO2 0
500
1000
1500
2000
2500
3000
Time/SEC
Fig.3. Breakthrough curves of a gas mixture passing the DIPA-modified SBA-15 bed (at 0.5 MPa and with flow rate 100 cm3/min).
48
DIPA TEA MDEA
Separation coefficient
42
36
30
24
18
12 0
2
4
6
8
10
12
14
16
Cycles
Fig.4. Variations of CH4/CO2 separation upon CO2 absorbent-modified SBA-15 adsorbent with continues saturation and regeneration cycles.
14
4.8 0.14
CH4 3.2 0.10
n/mmol.g-1
n/mmol.g-1
4.0 0.12
2.4
0.08
CO2 1.6
0.06 0
2
4
6
8
10
12
14
16
Cycles
Fig.5. The adsorption capacity of CH4 and CO2 on the DIPA-modified SBA-15 adsorbent change with the consecutive saturation and regeneration cycles.
1.0
0.8
C/CO
0.6
0.4
TEA MDEA DIPA
0.2
0.0
0
100
200
300
400
500
600
Time/SEC
Fig.6. The breakthrough curves of CO 2 over the adsorbent bed of SBA-15 sorbent modified by different CO2 absorbents.
15
TEA
2.0
C/C0
1.5
1.0
0.5
DIPA MDEA 0.0
0
100
200
300
400
500
600
Time/SEC
Fig.7. Regeneration curves of the saturated SBA-15 sorbent modified by different CO2 absorbents at ambient temperature and pressure.
16
Table 1 The effects upon adsorption capacity and the separation coefficient of different CO2 absorbents modified SBA-15. nCO2
α
Absorbent nCH4
nCO2
α
Unmodified 0.160
0.388
2.80
PC
0.116
0.284
2.04
NMP
0.060
0.152
2.92
DIPA
0.117
4.776
46.75
DGDE
0.102
0.200
2.25
MDEA
0.108
1.316
14.13
TEP
0.118
0.207
2.02
TEA
0.096
1.719
20.71
Absorbent
nCH4
nCH4, nCO2－the adsorption capacity of CH4 and CO2 ; α－CO2/CH4 separation coefficient.
17
Highlights ► Different CO2 absorbent modified SBA-15 affects to CO2 /CH4 separation. ► The separation stability of TEA/DIPA/MDEA modified SBA-15 is studied. ►The adsorption and regeneration of TEA/DIPA/MDEA modified SBA-15 are studied. ► TEA-modified SBA-15 has better CO2/CH4 separation performance than others. Table Captions
18
S0009-2614(17)30267-1 http://dx.doi.org/10.1016/j.cplett.2017.03.048 CPLETT 34648
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
10 January 2017 13 March 2017 16 March 2017
Please cite this article as: X. Liu, X. Zhai, D. Liu, Y. Sun, Different CO2 Absorbents-modified SBA-15 Sorbent for Highly Selective CO2 Capture, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett.2017.03.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Different CO2 Absorbents-modified SBA-15 Sorbent for Highly Selective CO2 Capture Xiuwu Liu a,*, Xinru Zhai a, Dongyang Liu a, Yan Sun b (aSchool of Chemical and Technology, Hebei University of Technology, Tianjin, 300130, b
Adsorption Laboratory of Chemical Engineering, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China)
Abstract Different CO2 absorbents-modified SBA-15 materials are used as CO2 sorbent to improve the selectivity of CH4/CO2 separation. The SBA-15 sorbents modified by physical CO2 absorbents are very limited to increasing CO2 adsorption and present poor selectivity. However, the SBA-15 sorbents modified by chemical CO2 absorbents increase CO2 adsorption capacity obviously. The separation coefficients of CO2/CH4 increase in this case. The adsorption and regeneration properties of the SBA-15 sorbents modified by TEA, MDEA and DIPA have been compared. The SBA-15 modified by triethanolamine (TEA) presents better CO2/CH4 separation performance than the materials modified by other CO2 absorbents.
Keywords: CO2 capture; CO2/CH4 separation; adsorption stability; reversibility stability; high selectivity
1.
Introduction Excessive greenhouse gas emission has become one of the most serious global
environmental problems, which arises more and more attention by public authorities worldwide. The global temperature is predicted to rise 1 - 5℃ to 2050 if there is no proper action to reduce greenhouse gas emission [1-4]. Among the greenhouse gases, CH4 and CO2 contribute most to human being generated greenhouse. The recovery and utilization of CH4 and CO2 is becoming an urgent task to the world 1
[5]. In vary sources of gas emission, such as landfill gas, biogas, and savage gas, whose major components are CO2 and CH4 gases, contribute a large proportion of greenhouse gas emissions. For example, landfill generates at least 600 million m3/year gas in China, and is still increasing from 7% to 9% every year [6-7]. If the emission from those typical sources could be recovered and utilized, the emission of greenhouse gases could be massively reduced, and a large number of energy can be recovered in the meantime. In order to mitigate global warming progress, many countries are paying more attention to control and utilize the emissions. Methane content of landfill emission gas must be higher than 80% to increase heat value in order to meet methane pipeline transfer requirement and reaction to produce chemical feedstock. CO2 in landfill emission gas must be removed to increase methane concentration [8-11]. The sizing of landfills in lots of cities in China is small and decentralized distributes, operation flexible separation techniques are required in urgent matter. PSA technology has been paid much attention in the industry due to its low capital investment and low energy consumption requirement. High selectivity of adsorbent is a vital segment of the technology [12-15]. The adsorbent must perform selectively to separate carbon dioxide and methane mixture. The adsorption method attracts many researchers’ interest in order to enrichment methane. The conventional adsorbents, such as activated carbon, carbon molecular sieve, silica gel, and zeolite molecular
sieves, are futile
to
recover
methane
from
landfill
gas
[16-19]. Ordered mesoporous materials emerge in recent years and their applications of methane enrichment have also been studied. The surface modification of mesoporous materials, such as MCM-48, MCM-41, SBA-15, can significantly improve adsorption capacity and selectivity of CO2 [20-29]. Most adsorbents can be regenerated easily at ambient temperature when they are applied for a PSA operation. Whereas, the adsorbents with good selectivity and facile reversibility are difficult for CO2/CH4 separation at ambient temperature. In
previous
studies, several
commercial
CO2
absorbents
(DIPA, MDEA, NMP, DGDE, TEP and PC) previously were described effective 2
to absorb H2S gas [30, 31], and are selected to test modified SBA-15. The SBA-15 modified by TEA has been validated to CO2/CH4 separation [32]. In this study, we still focus on identifying the validity of adsorbents for CO 2/CH4 separation through SBA-15 materials modified by several commercial CO2 absorbents which are effective to absorb H2S gas at ambient temperature. Different commercial CO2 absorbents modify SBA-15, the separation and the adsorption-desorption properties of CO2/CH4 are investigated to identify as valid adsorbents for separation.
2.
Experiment
2.1. CO2 absorbents-modified the SBA-15 The detailed procedure of triethanolamine (TEA) modified mesoporous silica material SBA-15 synthesized is previously described [32]. A definite weight of SBA-15 is immersed in acetone solution of CO2 absorbents, and acetone solvent is slowly evaporated at about 60℃. The coating ratio of a sample is evaluated from weight and specific pore volume of SBA-15, the coated weight and density of CO2 absorbents. Rc (the coating ratio), is ratio of the fractional filling of pore space and the entire pore space, it can be expressed with formula (1) as below: Rc
Mass CO2 absorbents Mass SBA 15 Vpore SBA 15 Density CO2 absorbents
(1)
2.2. Breakthrough and Regeneration Tests The detail of experimental device, two gas mixtures for the separation experiments (one mixture of He (79.04%), CO2 (9.74%), and CH4 (11.22%) to determine the separation coefficient between CO2 and CH4, another mixture consisting of CH4 (63.8%) and CO2 (36.2%) to study regeneration performance of saturated adsorbent.) and adsorbent regeneration tests are previously described [32]. Breakthrough curves of the gas mixture are collected at 298 K and pressure 0.5 MPa respectively. Flow rate of the stream is kept at 100 cm3/min during the experiment. The adsorption and regeneration tests are first fed with the CO2 and CH4 (without helium) mixture at 0.5 MPa, flow rate kept at 100 cm3/min. When the column is saturated with CO2, its pressure releases down to the atmospheric, and then the 3
column is purged with a pure methane stream, flow rate of the purging stream is kept at 50 cm3/min.
3. Result and discussion 3.1. The structural changes of the SBA-15 modified by TEA In previous study [33], the synthesized SBA-15 material is examined with XRD, TEM, and the nitrogen adsorption at 77K, and the ordered channel structure is confirmed. The BET specific surface area, pore volume and pore size distribution are 780 m2/g, 1.31 cm3/g and 7.7 nm respectively. In order to test the impact of TEA coating to the structural properties, 77 K nitrogen adsorption isotherms are collected with different TEA coating ratios (0, 0.14, 0.29, 0.41, 0.55, 0.68, 0.82, 1.00 and 1.16) to calculate the corresponding surface area and pore volume. The result is shown in Fig.1. The pore volume and surface area decrease with the coating ratio increasing. The pore volume (0.05 cm3 g-1) reaches nearly zero when Rc =1.0, which means that the coated TEA goes inside pore space of SBA-15 and the pore space was fully filled with TEA. Another test is carried out to prove the impact of coating TEA to the selectivity of adsorbent to CO2. The breakthrough curve is generated to summarize CO2 and CH4 passing SBA-15 bed together with carrier gas (helium) in condition 0.5 MPa, flow rate 100 cm3/min, which is shown in Fig.2. The adsorption capacity of CO2 appears increase obviously, but there is no help for the increasing of CH4. The separation coefficient increases more than seven times. In conclusion, the separation coefficient between CH4 and CO2 enlarges remarkably with TEA coating. TEA-coating is capable to enhance the separation between CO2 and CH4, but the enhancement is not remarkable when coating ratio, Rc, is less than 0.4. It reaches maximum at Rc=1.0 (the breakthrough curve at Rc =1.0 was shown as in Fig.2.). Consequently, Rc=1.0 was selected as the coating ratio (Rc) of other CO2 absorbents modified SBA-15.
3.2. CO2 /CH4 separation property of different CO 2 absorbent modified SBA-15 In order to further enhance CO2 adsorption selectivity of the SBA-15 modified, several commercial CO2 absorbents (DIPA, MDEA, NMP, DGDE, TEP 4
and PC) that are effective to absorbed H2S gas [30, 31], are selected to test modified SBA-15. The test result is shown in Table 1. The adsorption capacity of SBA-15 modified to methane gas is lower than that of SBA-15 unmodified. Different SBA-15 materials modified by different CO2 absorbent have nearly the same methane adsorption capacity. This might be explained that the solubility of methane is similar in different CO2 absorbent. It is very limited to increasing CO2 adsorption capacity of SBA-15 modified by physical CO2 absorbents, such as NMP, DGDE, TEP, and PC. The CO2/CH4 separation coefficients of those samples are very low and perform poor selectivity at the same time. On the contrary, CO2 adsorption capacity of SBA-15 modified by chemical CO2 absorbent performs very well. Chemical CO2 absorbents-modified the SBA-15 with numerous CO2-anity site such as amine groups is loaded into the pores of SBA-15 to increase the affinity between adsorbent and CO2. It improves the selectivity of CH4/CO2 separation and increases CO2 adsorption capacity obviously[26]. CO2 adsorption is the result of an acid-base reaction, therefore, the separation coefficient between CH4 and CO2 enhances remarkably and performs high selectivity. For example, the CO2/CH4 separation coefficient of SBA-15 modified by DIPA reaches 46.75. Its CH4 and CO2 breakthrough curve is shown in Fig.3.
3.3. The separation stability of the SBA-15 materials modified by TEA, DIPA and MDEA Following the findings from tests, the separation coefficient of CH4 and CO2 performs considerably enhancement in SBA-15 materials modified by TEA, DIPA and MDEA. Their CO2 and CH4 adsorption stability is further studied. Saturated adsorption and purged tests are repeated in the condition of single column filled with SBA-15 modified by chemical CO2 absorbent. In condition pressure 0.5 MPa and flow rate 100 cm3/min, the breakthrough curve of the column filled with the SBA-15 material is first saturated with a stream of CH4, CO2, and He (carrier gas). The bed pressure is discharged to the atmospheric. Following the steps above, the adsorbent bed is purged with helium gas at flow rate of 50 cm3/min at ambient temperature and 5
pressure. The test results are summarized in Fig.4. The adsorbent would be considered as regenerated completely and can transfer to the next adsorption process if CO 2 gas cannot be detected by MS at the outlet of the bed in purged process. As shown in Fig.4, the CO2/CH4 separation coefficients of the SBA-15 modified by various CO2 absorbents are a bit low at the beginning of several operation cycles. It present a quite stable trend after 5 cycles of consecutive saturation and regeneration. Among those curves, the SBA-15 modified by DIPA presents most obvious characteristic. By comparing the stability test experimental breakthrough curves, it can be found that methane adsorption capacity remains basically constant during whole stability test, but CO2 adsorption capacity decreases in the first batch four cycles. It present stability from the fifth cycle stability experiments. A conclusion can be drawn that the change of CO2/CH4 separation performance is caused by the changes of
adsorption
capacity
of regenerated methane
and
of
CO2. Meanwhile, the
carbon
dioxide
remains
adsorption capacity stable
after
5
times cycles. As shown in Fig.4, the separation coefficient of CO2/CH4 presents less variation in condition SBA-15 modified by TEA, MDEA than modified by DIPA. However, the CO2 adsorption capacity of SBA-15 modified by DIPA decreases quickly in the prior several adsorption and regeneration tests. The separation coefficient of CO2/CH4 decreases remarkably. DIPA is a secondary amine and its amine groups have stronger force to CO2 than tertiary amine. In addition, DIPA's melting point is 42℃ , which is higher than that of TEA and MDEA, DIPA is solid at the ambient condition, which causes CO2 adsorbed irreversibly. Result in the decrease of CO 2 absorption capacity in stable experiment. This is the result of CO2 adsorption in acid-base reaction, in this condition, secondary amines react with CO2 via a zwitterion mechanism, it forms carbamate and tertiary amines react with CO2 in the presence of water to form bicarbonate. For the zwitterion mechanism, an additional free base is required to deprotonate the zwitterion to form a carbamate, which can be supplied by a neighboring amine groups[34]. The reaction of secondary amines and CO2 in anhydrous condition is shown as following: 6
Part of generating carbamate doesn’t decompose in the desorption process at ambient condition, which causes a sharp fall of CO2 adsorption capacity before the fourth regeneration and remains stable after the fifth cycle regeneration. Tertiary amines (TEA and MDEA) react with CO2 in the presence of water to form bicarbonate, the reaction is shown as following:
If water is not available, TEA and MDEA are tertiary amine as tertiary amines lack a transferable proton and therefore cannot form carbamates, it results that can’t react with CO2 in anhydrous condition. Their amine groups have relatively weak force to CO2. Carbon dioxide can be nearly completely recycled and the separation coefficient of CO2/CH4 varies limitedly. The changes of methane and CO2 adsorption capacity presented by the SBA-15 modified by DIPA are shown in Fig.5. After the stable experiment, CH4/CO2
separation
coefficients
of
the
SBA-15
modified
by
TEA, MDEA, and DIPA are 16.73, 12.7, and 15.5 respectively.
3.4. The adsorption and regeneration properties of the SBA-15 which are modified by TEA, DIPA, MDEA Fast regeneration of the adsorbent is a key factor to decide its further application. A sorbent with high selectivity but difficulty in regeneration cannot be applied as a PSA adsorbent product. In order to test the adsorption and regeneration of the sorbents modified by TEA, DIPA, MDEA, a bed filled with the modified sorbent is first saturated with a stream CO 2-CH4 mixture, under pressure and flow rate at 0.5 MPa and 100 cm3 /min. The pressure is discharged to the atmospheric
afterwards, and
then the
column
is
purged
with
methane
gas at flow rate of 50 cm3 /min. The breakthrough curves of CO 2 are shown in Fig.6, and
purged
curves
are
shown
in
Fig.7.
C
represents
concentrations at adsorption bed outlet. C0 is feeding CO2 raw concentrations. 7
CO2
As shown in Fig.6, MDEA-modified SBA-15 has maximum breakthrough time, and TEA-modified SBA-15 has shortest one. However, CO2 breakthrough times among three CO 2 absorbents-modified SBA-15 have little different. As shown in Fig.7，among all SBA-15 materials modified by CO 2 absorbents, the CO2 concentration in the purging stream remains stable at the initial 20 seconds, but decreases very fast. The initial value of C/C0 is higher than 1.0 due to fast replacement of the adsorbed CO 2 by CH4 . After a comparison among three CO 2 absorbents-modified
SBA-15
regeneration
curves, it
can
be
found
that
TEA-modified SBA-15 is the easiest adsorbent to be regenerated. As CO 2 concentration at bed outlet is highest at the initial 20 seconds, and the complete regeneration time is the shortest. Regeneration of DIPA-modified SBA-15 is the most difficult among those absorbents. The concentration of C/C 0 is still 0.1 and it is regenerated incompletely after purge time has reached 1190s. Therefore, it can be concluded from the above tests that the TEA-modified SBA-15 has the best regeneration performance at ambient pressure and temperature.
4. Conclusion The SBA-15 materials modified by physical CO 2 absorbents perform quite limited CO2 adsorption capacity, it presents poor selectivity also. On the contrary, the SBA-15 materials modified by chemical CO 2 absorbents increase capacity greatly and present better separation coefficient as well as better selectivity. The SBA-15 adsorbent modified by TEA has the best regeneration performance through the adsorption and regeneration experiments, which can be used as a highly selective adsorbent for CO2/CH4 separation.
Acknowledgement This test is financial supported by Hebei Province Natural Science Foundation [B2008000023], Hebei Province Education Foundation [2006332], and Tianjin 8
Municipal Science and Technology Commission [10JCZDJC23900].
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12
Figure Captions
800
1.6
●
2
Pore volume
1.4
700
○Surface area
1
600 -1
Surface area/m .g
500
1.0
2
3
Pore volume/cm . g
-1
1.2
0.8
400
0.6
300
0.4
200
0.2
100
0.0 0.0
0.2
0.4
0.6
0.8
1.0
0 1.2
Rc
Fig.1. Effects of TEA-modifying on the structural property of samples.
n/mmol.g
-1
1.5
● CH4 adsorption capacity ○ CO2 adsorption capacity ▲ separation coefficient
21 18 15
1.2
3
12
0.9
9
0.6 0.3 0.0
6
2
3
1
0.0
0.2
0.4
0.6
Separation coefficient
1.8
0.8
1.0
1.2
0
Rc
Fig.2. Effects of TEA-coating ratio on CH4 and CO2 adsorption capacity and separation coefficient of CH4 /CO2.
13
1.0
He
Composition
0.8
0.6
0.4
0.2
0.0
CH4 CO2 0
500
1000
1500
2000
2500
3000
Time/SEC
Fig.3. Breakthrough curves of a gas mixture passing the DIPA-modified SBA-15 bed (at 0.5 MPa and with flow rate 100 cm3/min).
48
DIPA TEA MDEA
Separation coefficient
42
36
30
24
18
12 0
2
4
6
8
10
12
14
16
Cycles
Fig.4. Variations of CH4/CO2 separation upon CO2 absorbent-modified SBA-15 adsorbent with continues saturation and regeneration cycles.
14
4.8 0.14
CH4 3.2 0.10
n/mmol.g-1
n/mmol.g-1
4.0 0.12
2.4
0.08
CO2 1.6
0.06 0
2
4
6
8
10
12
14
16
Cycles
Fig.5. The adsorption capacity of CH4 and CO2 on the DIPA-modified SBA-15 adsorbent change with the consecutive saturation and regeneration cycles.
1.0
0.8
C/CO
0.6
0.4
TEA MDEA DIPA
0.2
0.0
0
100
200
300
400
500
600
Time/SEC
Fig.6. The breakthrough curves of CO 2 over the adsorbent bed of SBA-15 sorbent modified by different CO2 absorbents.
15
TEA
2.0
C/C0
1.5
1.0
0.5
DIPA MDEA 0.0
0
100
200
300
400
500
600
Time/SEC
Fig.7. Regeneration curves of the saturated SBA-15 sorbent modified by different CO2 absorbents at ambient temperature and pressure.
16
Table 1 The effects upon adsorption capacity and the separation coefficient of different CO2 absorbents modified SBA-15. nCO2
α
Absorbent nCH4
nCO2
α
Unmodified 0.160
0.388
2.80
PC
0.116
0.284
2.04
NMP
0.060
0.152
2.92
DIPA
0.117
4.776
46.75
DGDE
0.102
0.200
2.25
MDEA
0.108
1.316
14.13
TEP
0.118
0.207
2.02
TEA
0.096
1.719
20.71
Absorbent
nCH4
nCH4, nCO2－the adsorption capacity of CH4 and CO2 ; α－CO2/CH4 separation coefficient.
17
Highlights ► Different CO2 absorbent modified SBA-15 affects to CO2 /CH4 separation. ► The separation stability of TEA/DIPA/MDEA modified SBA-15 is studied. ►The adsorption and regeneration of TEA/DIPA/MDEA modified SBA-15 are studied. ► TEA-modified SBA-15 has better CO2/CH4 separation performance than others. Table Captions
18