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Kinetics, thermodynamics, and mechanism of a novel biphasic solvent for CO2 capture from flue gas Shihan Zhang, Yao Shen, Peijing Shao, Jianmeng Chen, and Lidong Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05936 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018
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Kinetics, thermodynamics, and mechanism of a novel
2
biphasic solvent for CO2 capture from flue gas
3
Shihan Zhanga,*, Yao Shena, Peijing Shaoa, Jianmeng Chena, Lidong Wangb,*
4
a
5
China
6
b
7
University, Baoding 071003, China
8
ABSTRACT: The main issue related to the deployment of amine-based absorption
9
process for CO2 capture from flue gas is its intensive energy penalty. Therefore, this
10
study screened a novel biphasic solvent, comprising a primary amine e.g.,
11
triethylenetetramine (TETA) and a tertiary amine e.g., N, N-dimethylcyclohexylamine
12
(DMCA), to reduce the energy consumption. The TETA-DMCA blend exhibited high
13
cyclic capacity of CO2 absorption, favorable phase separation behavior, and low
14
regeneration heat. Kinetic analysis showed that the gas- and liquid-side mass transfer
15
resistances were comparable in the lean solution of TETA-DMCA at 40oC, whereas
16
the liquid-side mass transfer resistance became dominant in the rich solution. The rate
17
of CO2 absorption into TETA-DMCA (4M, 1:3) solution was comparable to 5M
18
benchmark monoethanolamine (MEA) solution. Based on a preliminary estimation,
19
the regeneration heat with TETA-DMCA could be reduced by approximately 40%
20
compared with that of MEA. 13C NMR analysis revealed that the CO2 absorption into
21
TETA-DMCA was initiated by the reaction between CO2 and TETA via the zwitterion
22
mechanism, and DMCA served as a CO2 sinker to regenerate TETA, resulting in the
23
transfer of DMCA from the upper to lower phase. The proposed TETA-DMCA
24
solvent may be a suitable candidate for CO2 capture.
College of Environment, Zhejiang University of Technology, Hangzhou, 310014,
School of Environmental Science and Engineering, North China Electric Power
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1 INTRODUCTION
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Carbon dioxide (CO2) is a major contributor to the global warming, which incurs
27
severely environmental issues such as weather extremes and a rise in sea level. The
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Paris Agreement, ratified by 159 nations as of summer 2017, aims to mitigate the
29
global warming. Because CO2 accounts for over 77% of greenhouse gas emission, the
30
development of the advanced technologies for CO2 capture is critical.1-4
31 32
The monoethanolamine (MEA)-based absorption process is regarded as a state-of-art
33
technology for near-term CO2 capture,5-7 but costly due to the high energy
34
requirement. As estimated by Department of Energy of the United States, the
35
MEA-based process incurs an increase in the cost of electricity by approximately
36
80%.8 The cost of the steam usage for the solvent regeneration and required CO2
37
compression accounts for 60-70% of the total.8-10 Therefore, it is crucial to develop
38
novel solvents and tailored absorption processes to reduce the energy penalty of CO2
39
capture. 11-15
40 41
Currently, the biphasic solvents are proposed to capture CO2.16 Upon CO2 absorption
42
or temperature swing, the CO2-laden solvent undergoes liquid-liquid or liquid-solid
43
phase transition, and over 90% of the absorbed CO2 is concentrated in a CO2-rich
44
phase. Therefore, only the CO2-rich phase is sent to the stripper for the solvent
45
regeneration, and the CO2-lean phase is directly sent back to the absorber. As a result,
46
the sensible heat is expected to be reduced due to a smaller amount of solution for
47
regeneration.17 Moreove, because a more concentrated CO2-rich phase is used for
48
solvent regeneration, a higher CO2 partial pressure in the stripper can be achieved,
49
resulting in a lower stripping heat and less compression work. On the other hand, the 2
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absorption heat may also be reduced by optimizing composition of biphasic solvent.
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Therefore, the biphasic solvent-based process possesses a great potential to
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substantially lower the energy consumption of CO2 capture.17
53 54
Recent studies on biphasic solvents predominantly focused on liquid-liquid phase
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transition solvents, because their corresponding processes are straightforward to
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operate.18 Most of the reported biphasic solvents are amine blends, comprising a
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primary or secondary amine as a CO2 absorption accelerator and a tertiary amine as a
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CO2 sinker. For example, the N-methyl-1,3-propane-diamine (MAPA) and
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2-(diethylamino)-ethanol (DEEA) blend,19,
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blend,21 diethylenetriamine (DETA) and pentamethyldiethylenetriamine (PMDETA)
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blend,13
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(DMCA) blend,22 and DMXTM solvent have been proposed as biphasic solvents for
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CO2 absorption.23, 24 MAPA-DEEA, BDA-DEEA, and DETA-PMDETA solvents are
64
CO2-triggered biphasic solvents, whereas MCA-DMCA and DMXTM solvent are
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temperature swing-triggered biphasic solvents. Compared with the CO2-triggered
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biphasic
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solvent-based process requires a phase separator to be positioned downstream of the
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heat exchanger which may result in higher sensible heat.25 In addition, the temperature
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swing-triggered biphasic solvent is regenerated at approximately 90 oC which results
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in a relatively low CO2 partial pressure before the compression and thus increases the
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required compression work.25 However, the critical issue related to CO2-triggered
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biphasic solvents is their phase separation behavior. The liquid-liquid phase
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separation has been reported to disappear with an increase in CO2 loading.19-21 As a
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result, a tradeoff of the phase separation behavior and cyclic absorption capacity
N-methylcyclohexylamine
solvent-based
process,
20
1,4-butanediamine (BDA) and DEEA
(MCA) and
the
N,N-dimethylcyclohexylamine
temperature
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should be considered to develop a novel CO2-triggered biphasic solvent. Moreover,
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most of the previous researches on biphasic solvent only focused on absorption
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capacity and phase transitional behavior. Therefore, the energy penalty of the
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developed biphasic solvents requires further investigation.
79 80
The phase transition of CO2-triggered biphasic solvents is due to a sharp increase in
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ionic strength in the CO2-rich phase after the formation of carbamate or bicarbonate.26
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Therefore, to achieve phase separation, selection of a primary or secondary amine
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with multiple protonatable sites (e.g., diamine and triamine) is preferred to increase
84
the ionic strength upon CO2 absorption. Additionally, a tertiary amine with suitable
85
hydrophobicity is beneficial to ensure phase separation even at high CO2 loading.
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Therefore, the triethylenetetramine (TETA) with two primary amino groups and two
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secondary amino groups was used as the CO2 absorption accelerator in this study,
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which possesses a fast absorption rate and high CO2 absorption capacity as reported in
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the literature.27 Five tertiary amines with different hydrophobicity such as DEEA,
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DMCA,
91
bis(2-dimethylaminoethyl)ether (BDMAEE) were analyzed as CO2 sinkers to achieve
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excellent phase separation behavior.
PMDETA,
3-(Diethylamino)-1,2-propanediol
(DEAPD),
and
93 94
The aim of this study was to develop a novel biphasic solvent with high CO2
95
absorption capacity, fast absorption rate, desirable phase separation behavior, and low
96
energy penalty. The kinetics of CO2 absorption into the novel biphasic solvent was
97
analyzed using a double-stirred cell reactor. The vapor-liquid equilibrium (VLE) data
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were determined, and the energy penalty of the biphasic solvent was evaluated using
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an estimation method reported in the literature.28, 29 Furthermore, the mechanism of 4
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CO2 absorption into the developed biphasic solvent was investigated through
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speciation analysis. This study provides insight into development of an energy-saving
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biphasic solvent.
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2 EXPERIMENTAL METHODOLOGY
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2.1 Chemicals
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Monoethanolamine (MEA, purity ≥99%), triethylenetetramine (TETA, purity ≥68%),
107
2-(diethylamino)ethanol
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(DMCA, purity≥98%) and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA,
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purity ≥99%) were obtained from Aladdin Industrial Corporation, China.
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Bis(2-dimethylaminoethyl)ether (BDMAEE, purity ≥98%) was obtained from Nine
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Ding Chemistry, China. 3-(Diethylamino)-1,2-propanediol (DEAPD, purity ≥98%)
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was obtained from Dibo Chemistry, China. CO2 (purity ≥99.99 vol%) and N2 (purity
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≥99.99 vol%) were supplied by Jingong Gas Co., China. The hydrophobicity (LogP)
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and alkalinity (pKa) of the tested amines were provided in Table S1.
(DEEA,
purity≥99%),
N,N-dimethylcyclohexylamine
115 116
2.2 Experimental procedure
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The CO2 absorption capacity and phase separation behavior of the TETA-DEEA,
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TETA-DMCA, TETA-PMDETA, TETA-BDMAEE, and TETA-DEAPD solvents
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were tested in a 50 mL glass bubbler reactor. In a typical test, 13vol% of CO2
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balanced with N2, at a flow rate of 200 mL min-1, was bubbled into 30 ml of the tested
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solvents with a total amine concentration of 4 M at a desired blend ratio for 120 min
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at 40oC and atmospheric pressure. The effect of the ratio of primary to tertiary amines
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(0.5:3.5, 1.0:3.0, and 1.5:2.5) on absorption capacity and phase separation behavior
124
was also investigated. The solvent with the promising absorption capacity and phase 5
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separation behavior was selected to determine the VLE data at the different
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temperatures (40, 50, and 60oC) and inlet CO2 concentrations (0.09-13vol%).
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Equilibrium was achieved and indicated by an equal concentration of CO2 in the inlet
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and outlet. After reaching equilibrium, both of the two liquid phases, if formed, were
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sampled to determine the CO2 loading by the Chittick apparatus.30
130 131
The absorption rates of CO2 into the desired solvents were tested in a 500 mL
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double-stirred cell reactor (DSCR) with an internal cross-section of 28.3cm2 (Figure
133
S1). Two different concentrations of CO2 balanced by N2 (1.3 and 13vol%), at a flow
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rate of 1 L min-1, were introduced to 250 mL of the promising biphasic solvent at
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different CO2 loadings (0.25 and 0.75 mol mol-1) and temperatures (40, 50, and 60oC).
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The stirring rate of the gas phase was 250 rpm and that of the liquid phase was varied
137
from 250 to 400 rpm to ensure a smooth gas-liquid interface and homogeneous
138
mixing.
139 140
The CO2 absorption rate in DSCR was determined via the difference between the inlet
141
and outlet gas flow rate, measured by a soap-film flowmeter, and was calculated as
142
follows: 31 =−
143 144 145 146 147
( − ) (1) ,
where is the CO2 absorption rate, kmol m-2 s-1; and are the standard and
room temperatures, respectively, K; and are the gas flow rates in the outlet and inlet, respectively, m3 s-1; is the interfacial area between the gas and liquid
phase, m2; and , is the molar volume of the gas at the standard temperature and atmospheric pressure, m3 kmol-1.
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2.3 Data interpretation
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2.3.1 Mass transfer resistance determination
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According to the two-film theory of gas absorption with reaction, the overall
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liquid-side mass transfer resistance can be determined as follow:32
154
where is the overal l liquid-side mass transfer coefficient, m s-1; is the
155 156 157 158
1 1 = + (2)
Henry’s law constant of CO2, kmol m-3 kPa-1; is the temperature of the solution,
K; is the universal gas constant, kPa m3 kmol-1 K-1; is the individual gas-side
mass transfer coefficient, m s-1; is the enhancement factor; and is the
individual liquid-side mass transfer coefficient, m s-1.
159 160
161
The enhancement factor can be determined by:33, 34 ∗ ( − ) ! − !"# = − = (3) 1 1 , + ∗ where ! is the partial pressure of CO2 in the gas phase, kPa; !"# is the partial
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pressure of CO2 in equilibrium which can be obtained from the VLE data, kPa. The
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estimation of the physiochemical parameters in the gas and liquid phases was
164
provided in Supporting Information. The estimated physiochemical parameters used
165
in this study were summarized in Table S2.
166 167
For a pseudo-first-order reaction, the enhanced factor can also be calculated as: ≈ ' =
168
() ,*+ , (4)
where ' is the Hatta number; ) ,*+ , is the diffusion coefficient of CO2 in the 7
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amine solution, m2 s-1; - is the overall first-order rate constant, s-1. The chemistry
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of CO2 absorption into the biphasic solvent is provided in Supporting Information.
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Therefore, - can be calculated.
172 173
To meet the pseudo-first-order reaction assumption, this general criterion should be
174
satisfied: 35 3 < ' ≪ 1 (5) 1 = 1 +
' = 175 176
34*+ , )*+ , (6) 4 ) ,*+ ,
() ,*+ , (7)
where 1 is the infinite enhancement factor; 3 is the stoichiometric reaction ratio
of CO2 to amines; 4*+ , is the concentration of amines, kmol m-3; )*+ , is the
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diffusion coefficient of amines in the solution, m2 s-1; and The values of )*+ , in
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the fresh amine solution were determined according to the approach reported by
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Snijder et al.36 Additionally, the values in the CO2-laden amine solution were
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estimated based on a modified Stokes–Einstein relation.37 The values of ) ,*+ ,
181
were estimated based on the method developed by Sada et al.38 Under the tested
182
conditions in this work, the criterion (Eq. 5) of the pseudo-first-order assumption was
183
satisified.
184 185
2.3.2 Thermodynamic data evaluation
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Regeneration heat (7,8, ) was estimated using the method reported by Kim et al..28
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A schematic diagram of the biphasic solvent-based process is provided in Figure S2,
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and the selected temperatures at the top ( 9) and bottom (: ) of the stripper for the
189
estimation are presented in Table S3. 8
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7,8, comprises heat of reaction (7; ), sensible heat (
TETA-DMCA
>
TETA-BDMAEE
>
246 247
The phase separation behavior of the blends with high CO2 absorption capacity was
248
also investigated. Experimental results revealed that the phase transition behavior
249
depended upon the hydrophobicity (LogP in Table S1) of the tertiary amines. The
250
more hydrophobic the tertiary amine was, the greater the liquid-liquid phase
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separation appeared (Figure S3). For example, since DEAPD has a low 11
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hydrophobicity, the TETA-DEAPD blend did not undergo a phase transition
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throughout the entire absorption process (data not shown). Although the phase
254
separation occurred in TETA-DEEA blend during the CO2 absorption, the phase
255
separation disappeared and a homogenous solvent was formed at a high CO2 loading
256
of 0.95 mol mol-1 (Figure S3). Similar phase separation behavior was also obtained in
257
the reported biphasic solvent.41 Although the fresh TETA-DMCA separated into
258
two-liquid phases, its phase separation behavior was improved compared with the
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previous reported biphasic solvent.41 For instance, the CO2-laden phase accounted for
260
65vol% even once it had reached its maximum CO2 loading (Figure S3). Notably, lots
261
of the reported biphasic solvents at their lean loadings had two-liquid phases even
262
though their fresh solutions were one homogeneous phase.13, 21, 27, 42 Therefore, the
263
TETA-DMCA blend can be regarded as a promising biphasic solvent because two
264
distinct liquid phases formed at the maximum CO2 loading. Furthermore, over 98% of
265
the absorbed CO2 was concentrated into the lower phase.
266 267
The composition of the TETA-DMCA solvent was optimized in terms of its phase
268
separation behavior and absorption capacity. With an increase in TETA concentration,
269
the absorption capacity increased from 0.58 to 0.88 mol mol-1 (Figure S4). However,
270
with a high TETA: DMCA ratio (e.g., 1.5:2.5), a liquid-solid phase separation
271
occurred rather than a two-liquid phase separation. Therefore, the TETA-DMCA
272
solvent with a ratio of 1:3 was selected as the optimum composition for CO2
273
absorption and used in the further investigation.
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3.2 Kinetic analysis
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CO2 absorption into TETA-DMCA solvent with normalized lean and rich solution
277
loadings of 0.25 and 0.75 mol mol-1 was performed to simulate the solvent
278
composition at the top and bottom of an absorber, respectively. Figure 2 revealed that
279
the absorption rates into both solutions increased with the absorption temperatures
280
despite their CO2 loadings. The overall absorption rate of CO2 into lean and rich MEA
281
solution was of the same order of magnitude as the reported data in the previous
282
work,32, 43 indicating that the method used in this study for determining the CO2
283
absorption rate was relevant. The CO2 absorption rates into the TETA-DMCA
284
solvents were slightly slower than those into MEA solutions (Figure 2). For example,
285
at 50oC, the absorption rates into the lean and rich TETA-DMCA solutions were 16%
286
and 8% slower than those into MEA solutions, respectively. The slower absorption
287
rates into the TETA-DMCA blend were due to their higher CO2 loadings compared
288
with MEA solutions. As the absorption rates into the TETA-DMCA and MEA
289
solvents were comparable, the sizes of the absorbers with TETA-DMCA and MEA as
290
solvents for CO2 capture would be comparable. However, it should be noted that the
291
capacity of the TETA-DMCA solvent is 70% higher than MEA, which will reduce the
292
energy penalty of CO2 capture.
293 294
A Kinetic analysis was conducted to determine the enhancement factor when the
295
TETA-DMCA blend was used to identify the main mass transfer resistance during
296
CO2 absorption in a DSCR. The total mass transfer resistance (1/KL) was calculated
297
according to Eqs. 2 and 3, and the gas- ( ⁄ ) and liquid-side (1⁄ ) mass
298
transfer resistances were also determined.
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As depicted in Table 1, for the lean TETA-DMCA solution, the individual gas-side
301
and liquid-side mass transfer resistance were comparable at 40oC, indicating that the
302
absorption rate was controlled by both gas- and liquid-side mass transfer. These
303
results were in good agreement with the lean MEA solvent reported by Dang.44 For
304
the rich TETA-DMCA solution, the liquid-side mass transfer resistance was dominant
305
at 40 and 50 oC, revealing that the overall absorption rate was limited by the
306
liquid-side mass transfer (including reactions). Notably, the enhancement factors of
307
the lean and rich solution increased with the absorption temperature, resulting in a
308
decrease of the liquid-side mass transfer resistance. For example, when the absorption
309
temperature was 60oC, the gas-side mass transfer resistance was dominant for the
310
TETA-DMCA lean solution. The total mass transfer resistances of the lean solutions
311
were considerably smaller than those of the rich solutions, suggesting that the average
312
absorption rate in the absorber was limited by the rich solution. However, within the
313
tested absorption temperatures, the total mass transfer resistances of the rich solution
314
decreased as the absorption temperature was increased, indicating that a high
315
absorption temperature (e.g., 60oC) may be beneficial for CO2 absorption.
316 317
3.3 Thermodynamics and regeneration heat estimation
318
In this study, the VLE data of TETA-DMCA blends with a total amine concentration
319
of 4 M and ratio of 1:3 were measured at 40, 50, and 60oC, respectively. Moreover,
320
the VLE data of the 5M benchmark MEA were also determined at 40oC. As shown in
321
Figure 3, the measured VLE data of MEA solution were in good accordance with the
322
data reported in the literature,45 indicating that the set-up used in this study for VLE
323
data determination was relevant. For the TETA-DMCA solvent, the VLE curves as a
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function of CO2 loading were reverse S-shaped. The CO2 partial pressure in
325
equilibrium gradually increased with the CO2 loadings.
326 327
The regeneration heat of the benchmark MEA solution was estimated according to
328
Eqs. 8-11. The result was in good agreement with the reported data (Table S4),
329
suggesting that the method used in this work is relevant to estimate the regeneration
330
heat. For the estimation of regeneration heat of TETA-DMCA solvent, the main
331
parameters, such as reboiler temperatures, CO2 loadings of the rich and lean solutions,
332
CO2 partial pressures at the top of the stripper, ?*+ , and ?P are listed in Table S3.
333
To calculate the sensible heat, approximately 72wt% of the solution (lower phase)
334
was assumed for the regeneration because over 98% of the absorbed CO2 was
335
concentrated in the lower phase. The preliminary estimation results indicated that the
336
regeneration heat of the TETA-DMCA solvent was dependent upon the normalized
337
lean loading (the loading at the top of the absorber calculated based on the total amine
338
amount) at a stripping temperature of 120oC (Figure 4a). When the lean loading was
339
increased from 0.25 to 0.45 mol mol-1, the regeneration heat substantially decreased
340
from 3.92 to 2.07 GJ t-1 CO2. As depicted in Figure 4a, the decrease in regeneration
341
heat was attributed to a sharp drop in the latent heat due to the high CO2 partial
342
pressure at the top of the stripper (Table S3). Conversely, the sensible heat gradually
343
increased with an increase in the lean loading because of the decrease in the cyclic
344
capacity. The regeneration heat did not noticeably vary when the stripping
345
temperature was increased from 90 to 130oC (Figure 4b), indicating that a relatively
346
low stripping temperature (90-110oC) can be used for TETA-DMCA solvent
347
regeneration.
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As expected, lower sensible and latent heat were achieved using a biphasic solvent
350
compared with the MEA solution (Figure 4a). The lowest regeneration heat of the
351
TETA-DMCA solvent (2.07 GJ t-1 CO2) was achieved with rich and lean loadings of
352
0.75 and 0.45 mol mol-1, and this regeneration heat was approximately 40% lower
353
than that of MEA (3.7 GJ t-1 CO2)46. Furthermore, the regeneration heat determined in
354
this study was comparable to that of the reported biphasic solvents as shown in Figure
355
4a and Table S4. Therefore, the TETA-DMCA solvent may be a promising candidate
356
for CO2 absorption.
357 358
3.4 Mechanism
359
Three pairs of the upper and lower phases of the solvents with different CO2 loadings
360
(0, 0.20, and 0.55 mol mol-1) were collected for 13C NMR analysis to investigate the
361
mechanism of CO2 absorption into the TETA-DMCA blend. Becasue TETA
362
possesses two primary amine groups and two secondary amine groups, four types of
363
carbamate can be formed during CO2 absorption (Figure S5).
364 365
As shown in Figure 5, for the neat DMCA solution, characteristic peaks
366
corresponding to the carbons of DMCA were detected at the signals of 27.2, 28.0,
367
30.5, 42.8, and 65.1 ppm. Interestingly, the upper phases of the solutions with
368
different CO2 loadings exhibited the same characteristic peaks as neat DMCA,
369
indicating that the component of the upper phase was DMCA/DMCA+.
370
Distinguishing signals of amine and its protonated one using 13C NMR is difficult due
371
to the rapid proton exchange between the amines and water.21 In the lower phase,
372
characteristic peaks assigned to both DMCA and TETA were detected. Moreover, the
373
intensity of peaks corresponding to DMCA gradually increased with CO2 loadings, 16
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suggesting that DMCA was gradually transferred from the upper to lower phase
375
during CO2 absorption. Furthermore, at the CO2 loading of 0.2 mol mol-1, the primary
376
carbamate formed through the reaction between CO2 and TETA was detected at the
377
resonance of 161.12 ppm according to the ACD-LAB 6.0 analysis. With further
378
increase in the CO2 loading to 0.55 mol mol-1, the characteristic peaks of both
379
secondary carbamate and CO32-/HCO3- were detected at 157.94 and 161.30 ppm,
380
respectively, revealing that the reaction between CO2 and the amines followed the
381
order of primary amines, secondary amines, and tertiary amines, as expected.27, 41
382
Moreover, as the CO2 loading was increased, the acidity of the solvents increased,
383
resulting in a slight change in the chemical shift of the characteristic peaks of DMCA
384
and TETA as shown in Table S5.
385 13
386
Based on the
387
TETA-DMCA was proposed (Figure 6). Overall, the main reactions can be described
388
as follows:
389 390 391 392 393 394 395 396
C NMR analysis, the mechanism of CO2 absorption into
+ 4gh ↔ []l 4ggm
[]l 4ggm + ↔ []l + []4ggm
[]l 4ggm + )]4 → [)]4] l + []4ggm
[]l 4ggm + h g → []l + 4gom
(R1) (R2) (R3) (R4)
397
As shown in Figure 6, CO2 absorption was initiated by the reaction between CO2 and
398
the primary amine groups in TETA through the zwitterion mechanism. With the
399
consumption of primary amine groups, the secondary amine groups participated into
400
CO2 absorption, as indicated by the 13C NMR spectra. Furthermore, with the depletion
401
of TETA, DMCA acted as a “sinker” to regenerate TETA for sustaining continuous
402
CO2 absorption. Therefore, DMCA gradually transferred from the upper to lower 17
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phase, resulting in a volume expansion of the lower phase.
404 405
In summary, the biphasic solvent, TETA-DMCA (4M, 1:3), exhibited an absorption
406
rate comparable with that of MEA, a high cyclic absorption capacity, and a
407
considerably low heat regeneration. Kinetic analysis revealed that the absorption rate
408
into lean solution was limited by both the gas- and liquid-side mass transfer, whereas
409
the liquid -side mass transfer rate determined the absorption rate into the rich solution.
410
Furthermore, the CO2 absorption into TETA-DMCA occurred with TETA as an
411
accelerator and DMCA as a sinker. The DMCA in the upper phase gradually
412
transferred to the lower phase to regenerate TETA, resulting in a high absorption rate
413
even at a high CO2 loading. The regeneration heat of the TETA-DMCA blend was
414
approximately 40% lower than that of 5M benchmark MEA solution. Therefore, this
415
novel TETA-DMCA solvent is a promising candidate for CO2 capture from flue gas.
416 417
AUTHOR INFORMATION
418
Corresponding Authors
419
*(S.H.Z.) Tel: +86 571 8832 0853; E-mail address:
[email protected] 420
*(L.D.W.) Tel: +86 312 752 5511; E-mail address:
[email protected] 421
Note
422
The authors declare no competing financial interest.
423
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ACKNOWLEDGMENTS
425
We appreciate the financial support from National Natural Science Foundation of
426
China (No. 21606204) and Zhejiang University of Technology Initial Research
427
Foundation (No. 2017129000729).
428 429
SUPPORTING INFORMATION
430
Additional details on physiochemical parameters evaluation and chemistry of CO2
431
absorption into biphasic solvent. Additional tables giving hydrophobicity and the
432
alkalify of the tested amines, physiochemical parameters, thermodynamic data,
433
comparison of regeneration heat of TETA-DMCA with other reported biphasic
434
solvents, and carbon chemical shift of the carbons in TETA and DMCA.
435
Additional figures depicting schematic diagram of the double-stirred cell reactor
436
and typical phase change absorption process, influence of TETA: DMCA ratios on
437
the performance, and the potential carbamates formed in TETA-DMCA solvent.
438 439
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440
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579
Table 1. Kinetic analysis of TETA-DMCA under different temperatures in a DSCR.
T o
( C)
40 50 60
CO2 loading (mol mol-1)
0.25
pqrs
(kPa) 1.30
tuv
N (kmol m-2
E
-1
(s )
s-1)
1.34E-06
82
763
Liquid-sid
Total
Gas-side
resistance
(w⁄xy)
resistance
288
46.5
53.5
13.8 62.7 23.6 74.0 47.0
86.2 37.3 76.4 26.0 53.0
0.75
13.0
2.05E-06
18
84
841
0.25
1.30
1.63E-06
124
1407
274
0.75
13.0
2.36E-06
26
139
633
0.25
1.30
1.8E-06
167
2110
294
0.75
13.0
2.53E-06
60
554
402
580 581
27
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(%)
e resistance (%)
Environmental Science & Technology
582
Figure captions:
583
Figure 1. CO2 absorption capacity of the tested five blends and 5M benchmark MEA.
584
(total amine concentration of the blends: 4M, amine ratio: 1:3, 40oC, 13vol% CO2,
585
and 200 mL min-1)
586
Figure 2. Comparison of the absorption rates between TETA-DMCA and MEA under
587
the simulated conditions corresponding to the top and bottom of an absorber (1 atm,
588
300 rpm, and 1 L min-1). The blue solid and dash lines represent the absorption rates
589
into the rich and lean MEA solutions at 50oC reported in the literature, respectively.32
590
Figure 3. Vapor-liquid equilibrium (VLE) of the TETA-DMCA blend and MEA
591
solution. MEA 40 oC represents VLE data of MEA solution measured at 40oC.45
592
Figure 4. Regeneration heat estimation of TETA-DMCA from the rigorous
593
simulation (a) under different lean loadings (Treboiler=393 K, ∆T=10oC, normalized
594
rich loading=0.75 mol mol-1); (b) under various temperature at the reboiler (∆T=10oC,
595
normalized rich loading=0.75 mol mol-1). The lean loading represents the loading of
596
the lean solution at the top of the absorber; the data of MEA obtained from Kim et al.;
597
28
598
reported biphasic solvent such as DEEA-MAPA and DMX-A solvent.46, 47
599
Figure 5. Quantitative
600
loading levels. (a) the upper phase and neat DMCA; (b) the lower phase and neat
601
TETA.
602
Figure 6. Schematic diagram of the mechanism of CO2 absorption into TETA-DMCA
603
solution.
the red dashed bar showed in Fig. 4a representing the regeneration heat of the
13
C NMR spectra of TETA-DMCA solvent with various CO2
604 605
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CO2 loading (mol/mol)
1.0 0.8 0.6 0.4 0.2 0.0
TETA/ TETA- TETA- TETAMEA TETADEAPD DEEA DMCA BDMAEE PMDETA 606 607
Figure 1. CO2 absorption capacity of the tested five blends and 5M benchmark MEA.
608
(total amine concentration of the blends: 4M, amine ratio: 1:3, 40oC, 13vol% CO2,
609
and 200 mL min-1)
610
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Absorption rate ×108 (kmol·m-2·s-1)
Environmental Science & Technology
350 300
TETA-DMCA 0.25 TETA-DMCA 0.75 MEA 0.2 MEA 0.45
Lean solvent TETA-DMCA (0.25 mol CO2 mol-1 amine)
250
Page 30 of 35
Cleaned gas 1.30% CO2
200 150 100 50
Rich solvent TETA-DMCA (0.75 mol CO2 mol-1 amine)
0 40
50
60 o
Temperature ( C)
Flue gas 13.0% CO2
611 612 613
Figure 2. Comparison of the absorption rates between TETA-DMCA and MEA under
614
the simulated conditions corresponding to the top and bottom of an absorber (1 atm,
615
300 rpm, and 1 L min-1). The blue solid and dash lines represent the absorption rates
616
into the rich and lean MEA solutions at 50oC reported in the literature, respectively.32
617
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CO2 partial pressure (kPa)
618
10
1
0.1 40oC 50oC 60oC MEA 40oC MEA 40oC lit.
0.01
0.001 0.2
0.3
0.4
0.5
0.6
40oC 50oC 60oC MEA 40oC MEA 40oC lit.
0.7
0.8
0.9
CO2 loading (mol/mol)
619 620
Figure 3. Vapor-liquid equilibrium (VLE) of the TETA-DMCA blend and MEA
621
solution. MEA 40 oC represents VLE data of MEA solution measured at 40oC.45
622
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Regeneration heat (GJ/t CO2)
5.0 4.5
(a)
Qlatent Qsens Qrxn
4.0 3.5
Reported regeneration heat
3.0 2.5 2.0 1.5 1.0 0.5 0.0
MEA lit. 0.25
0.3
0.35
0.4
0.42
0.45
Lean loading (mol/mol)
623
Regeneration heat (GJ/t CO2)
4.0 3.5
(b)
3.0 2.5 2.0 1.5 Lean loading=0.35mol mol-1 Lean loading=0.40mol mol-1 Lean loading=0.45mol mol-1
1.0 0.5 0.0 360
370
380
390
400
410
Temperature (K)
624 625
Figure 4. Regeneration heat estimation of TETA-DMCA from the rigorous
626
simulation (a) under different lean loadings (7,:=,7 =393 K, ∆T=10oC, normalized
627
rich loading=0.75 mol mol-1); (b) under various temperature at the reboiler (∆T=10oC,
628
normalized rich loading=0.75 mol mol-1). The lean loading represents the loading of
629
the lean solution at the top of the absorber; the data of MEA obtained from Kim et al.;
630
28
631
reported biphasic solvent such as DEEA-MAPA and DMX-A solvent.46, 47
the red dashed bar showed in Fig. 4a representing the regeneration heat of the
32
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2'
3* 2* 3^ 2" 4^
1* 6' 1" 6^ 1'
5' 4' 5^2^ 1
9 α=0.55
10
11 13 12
7" 7* 147^7' α=0.55 8" 8'
2
10
5^
4^ 3^
2^
9 10
11 13 12
α=0.20 7^7*
12
9 3" 3' 1^
11
1 1* 6^
23
2* 3*
α=0.20
3
13 11
13 12
1^ 9
10
1
2 3
10
11 13 12
10
11 1213
9 α=0.00
α=0.00
10
9 2 3
9 DMCA
12 11 13
1
TETA
170 165 70 65 60 55 50 45 40 35 30 25
170 165 70 65 60 55 50 45 40 35 30 25
f1 (ppm)
f1 (ppm)
(a)
(b)
632 633 634
Figure 5. Quantitative
635
loading levels. (a) the upper phase and neat DMCA; (b) the lower phase and neat
636
TETA.
13
C NMR spectra of TETA-DMCA solvent with various CO2
637
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638 639 640
Figure 6. Schematic diagram of the mechanism of CO2 absorption into TETA-DMCA
641
solution.
642
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643 644
TOC art: CO2
Clean Gas
CO2
645 646 647
Flue Gas
Heat Exchanger
Qrenge=2.07 GJ/t CO2
DMCA TETA Carbamate H2O HCO3-
Absorber
Stripper
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