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Article 2
Mechanism and kinetics study of CO absorption into blends of MDEA and [COHmim][Gly] aqueous solution 2
Cheng Sun, Shujing Wen, Jingkai Zhao, Chongjian Zhao, Wei Li, Sujing Li, and Dongxiao Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01942 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017
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Mechanism and kinetics study of CO2 absorption into blends
2
of MDEA and [C2OHmim][Gly] aqueous solution
3
Cheng Suna, Shujing Wena, Jingkai Zhaoa, Chongjian Zhaoa, Wei Lia, Sujing Li*a,
4
Dongxiao Zhangb
5
a
6
of Industrial Ecology and Environment, College of Chemical and Biological
7
Engineering, Zhejiang University, Yuquan Campus, Hangzhou 310027, China
8
b
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute
Henan Tianguan Group Co., Ltd. Nanyang 473001, China
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Highlights
11
1. The addition of [C2OHmim][Gly] promoted CO2 absorption rate and capacity of MDEA aqueous solution in a double stirred-cell absorber.
12 13
2. The blends of MDEA/[C2OHmim][Gly] had a better antioxidant ability and regeneration efficiency comparing to individual MDEA and MEA.
14 15
3. Reaction mechanism was investigated in details by 13CNMR and described as a shuttle mechanism.
16 17
4. Reaction mechanism and kinetics study indicated that the addition of [C2OHmim][Gly] enhanced CO2 absorption process into MDEA aqueous solution.
18 19
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Abstract
21
In this work, the blends of 1-hydroxyethyl-3-methylimidazolium glycine
22
([C2OHmim][Gly]) synthesized by our laboratory and MDEA in aqueous solution
23
were prepared for CO2 capture, and the maximum absorption performance of the
24
blends was obtained at the mole ratio of 8:2 of MDEA to [C2OHmim][Gly] with a
25
total concentration of 1.0mol L-1. CO2 loading of the blended absorbent was less
26
adversely influenced by temperature and O2 concentration than that of MDEA
27
aqueous, and it had a good performance in regeneration ability. The reaction
28
mechanism of the CO2 absorption into MDEA/[C2OHmim][Gly] was investigated by
29
13
30
carbamate promoted the reaction between CO2 and MDEA, which could be described
31
as a shuttle mechanism. The kinetics of CO2 absorption was investigated in a double
32
stirred-cell absorber at different temperatures and some important kinetic parameters
33
were obtained, such as the reaction rate constant (k2) and the overall rate constant (kov).
34
Experimental results indicated that the addition of [C2OHmim][Gly] enhanced CO2
35
absorption of MDEA under low CO2 partial pressure, which could improve the
36
application of MDEA in industry.
CNMR. CO2 firstly reacted with [C2OHmim][Gly] to form carbamate, then
Keywords: CO2 absorption, ionic liquid, MDEA, mechanism, kinetics
37 38
1. Introduction
39
The rise of carbon dioxide concentration in the atmosphere is one of the most
40
pressing environmental problems of our time1, leading to frequent extreme climate,
41
ocean acidification2, crop reduction3 and many other environmental issues. To develop
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efficient and economic methods for the capture of carbon dioxide from coal-fired
43
power plants is an important task for control and reduction of carbon dioxide
44
emissions.
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Chemical absorption into monoethanolamine (MEA) aqueous solution is most
46
widely used for carbon dioxide capture of flue gas in industry because of its fast
47
absorption rate and high degree of purification at low CO2 partial pressure4-6.
48
However, the disadvantages are obvious as well, such as high regeneration energy
49
consumption7, corrosion of equipment8 and degradation at the presence of oxygen9,
50
limiting the application and development of MEA. Another widely used absorbent is
51
N-methyldiethanolamine (MDEA). As a tertiary amine, MDEA has significant
52
advantages of less degradation, corrosion and regeneration cost comparing to primary
53
and secondary amines. As early as 1970s, the German BASF SE has used MDEA for
54
CO2 absorption. In 1980, Donaldson proposed alkali catalytic reaction mechanism
55
between MDEA and CO2, and Versteeg also obtained the reaction kinetics model of
56
CO2 absorption into MDEA aqueous solution at different temperatures by a double
57
stirred-cell absorber10. Benamor studied the models of solubility and concentration of
58
carbamate for MDEA aqueous solution at 303-323K under the pressure of
59
0.09-100kPa11. However, the disadvantage of MDEA absorbent is the slow absorption
60
rate under atmospheric pressure, thus amines with rapid absorption rate are usually
61
mixed with MDEA for making up the shortage. Hagewiesche established the mass
62
transfer model for CO2 absorption into the blend of MDEA and MEA, and obtained its
63
mass transfer coefficient equation at 313K12. Zhang blended MDEA with DEA at
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mass ratios of 50:3-50:10 and studied the CO2 absorption rate respectively by a plate
65
column absorber at 313-343K, and established the kinetics model of CO2 absorption
66
into MDEA/DEA mixture13. Although the hybrid amines could partially compensate
67
for the defects of individual absorbent, the flaws shared to them are still difficult to
68
overcome, such as oxidation and volatility.
69
Ionic liquids (ILs) have attracted much attention in recent years, because of their
70
prominent characteristics of low vapor pressure, high thermal and chemical stability
71
and adjustable properties 14, 15. CO2 capture by conventional ionic liquids is a physical
72
process with a quite low reaction rate and low CO2 capacity under atmospheric
73
pressure, thus some functional groups are introduced to ILs to improve capture
74
properties, such as amino acid functionalized ionic liquids. Gurkan synthesized
75
[P66614][Pro] and [P66614][Met] with CO2 absorption capacities of about 0.9 mol
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CO2/mol IL16. Zhang’s group synthesized a series of amino acid functionalized ionic
77
liquids, such as [AP4443][Gly], [AP4443][Ala] and [AP4443][Val] etc., and the capacity
78
was 1.20, 1.15 and 1.10 mol CO2/mol IL, respectively17. However, it is still hard to
79
realize the industrial application of ionic liquids for CO2 capture directly at present
80
because its high viscosity18 and production cost limit the large-scale development.
81
Considering the advantages of blends of absorbent, more researchers began to focus
82
on mixing alkanolamines with ILs to develop economical and efficient CO2
83
absorbents. In 2008, Camper mixed ILs with MEA and DEA for CO2 absorption, and
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pointed out the feasibility and optimistic application prospect of CO2 absorption into
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blends of ILs and alkanolamines19. Yang utilized blends of MEA, [bmim][BF4] and
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H2O at a mass ratio of 3:4:3, and discovered that regeneration energy consumption of
87
the blends was 37.2% less than that of MEA aqueous solution, and the loss of MEA
88
was also decreased by 67.3% in the absorption process20. Gao established the mixed
89
system of MDEA/PZ/ILs/H2O at mass ratios of 30:3:10:57, and found that
90
[bmim][BF4] could decrease both the average enthalpy and the sensible heat
91
obviously21. Zhang synthesized four kinds of tetramethylammonium amino acid
92
functionalized ionic liquids and clarified that the addition of ionic liquids in MDEA
93
aqueous solution could enhance the CO2 absorption process22. Fu’s group blended
94
MDEA with [N1111][Gly]23, [Bmim][Gly]24 and [Bmim][Lys]25 respectively for CO2
95
absorption and found that the addition of small amount of these ionic liquids could
96
increase absorption rate obviously. They established viscosity models and concluded
97
that Weiland equation can correctly capture the effects of CO2 loading, mass fraction
98
of blended systems. Although many works focus on the hybrid systems of
99
alkanolamines and ionic liquids for CO2 absorption, there is not much research on the
100
absorption mechanism and kinetics study of MDEA in the blended system. Besides,
101
many other solvents have been developed for CO2 capture, such as enzyme-mediated
102
solvent. Zhang26 investigated the kinetics of CO2 absorption into a 20 wt% potassium
103
carbonate solution promoted by the enzyme carbonic anhydrase in a stirred tank
104
reactor, and found that absorption rate into a lean potassium carbonate with 3 g·L-1
105
carbonic anhydrase was 50% lower than that into 5M MEA in a packed-bed column.
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Our group successfully synthesized a hydrophilic amino acid functionalized ionic
107
liquid ([C2OHmim][Gly]) based on imidazolium in our previous work, which had a
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good performance in CO2 absorption 27. Taking into account the considerable defects
109
and advantages of MDEA, this ionic liquid is proposed to blend with MDEA to
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enhance the absorption process of MDEA under low CO2 partial pressure. Herein, we
111
optimized the synthesis process of [C2OHmim][Gly], explored the optimum
112
proportion of blending MDEA/[C2OHmim][Gly] aqueous solution to achieve the
113
optimal performance on CO2 absorption, and investigated effects of different factors
114
on CO2 absorption of the system. 13CNMR was employed to investigate the reaction
115
mechanism between CO2 and the blended absorbent. In addition, reaction kinetics on
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CO2 absorption into the blends in aqueous solution was studied at 303.15-333.15K in
117
a double stirred-cell absorber.
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2. Materials and Methods
119
2.1 Chemicals
120
N-methylimidazole (99% purity), methyldiethanolamine (99% purity) and
121
glycine (99% purity) were purchased from Aladdin Chemical Co. Ltd.. 2-Chlorine
122
ethanol (99.5% purity) was supplied by Xiya Chemical Co. Ltd.. CO2 (99.999%
123
purity) and N2 (99.999% purity) were provided by Zhejiang Jin-gong Gas Co.. The
124
anion-exchange resin (Dowex Monosphere 550A) was purchased from Dow Chemical
125
Company. The ionic liquid [C2OHmim][Gly] was synthesized in our laboratory, and
126
the synthesis and optimization method are discussed in details below. Aqueous
127
solutions were prepared with ultrapure water. No further purification was conducted
128
for all of the materials.
129
2.2 Optimization of [C2OHmim][Gly] synthesis
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The synthesis process of [C2OHmim][Gly] was reported in our previous work27.
131
The synthesis was divided into three steps, including the reaction between
132
1-Methylimidazole
133
anion-exchange of [C2OHmim][Cl] to form [C2OHmim][OH], and the neutralization
134
reaction between [C2OHmim][OH] and glycine to form [C2OHmim][Gly].
135
Experimental conditions were optimized for high yield and purity in this work.
136
Reaction between 1-Methylimidazole and 2-Chlorine-1-ethanol achieved highest yield
137
of 96.21% and purity of 96.27% at mole ratio of 1:1.3 at 353.15K. For ion-exchange
138
process, static ion-exchange method was more suitable than dynamic method because
139
of less time and water consumption. And the neutralization reaction was carried out
140
under stirring at room temperature for 8 hours.
141
2.3 Experimental procedures
and
2-Chlorine-1-ethanol
to
form
[C2OHmim][Cl],
the
142
CO2 absorption experiments were carried out in two kinds of absorbers. For
143
saturated absorption loading measurements, a bubbling glass absorber with volume of
144
100 cm3 was used for its faster mass transfer and higher reaction rate. 50 mL of 1 mol
145
L-1 MDEA and [C2OHmim][Gly] blended at different mole ratios was fed into the
146
reactor at constant temperature of 323.15K in an oil bath. Pure nitrogen was supplied
147
continuously to the bottle before absorption in order to eliminate the effects of air. 1
148
L·min-1 of gas stream with 15% (v/v) CO2 and 85% (v/v) N2 was bubbled into the
149
bottom of reactor. The concentration of CO2 was measured by gas chromatography
150
(GC-7890, Agilent, USA) at gradually increasing intervals, and the absorption process
151
ended when the inlet and outlet concentration of CO2 was equal or close. For
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absorption rate and reaction kinetics measurements, a double stirred-cell absorber was
153
used as introduced in our previous work27. More details could be found in Lv’s work28,
154
29
.
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The desorption experiments of the saturated absorbents were conducted in a 500
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mL three-neck flask with a condenser in an oil bath at 383.15K. The flow rate of
157
escape CO2 dried by concentrated sulfuric acid was measured by a soap-membrane
158
flowmeter, and the regenerated solution was used for CO2 absorption again.
159
2.4 Theoretical analysis
160
2.4.1 Reaction mechanism. In the past two decades, a lot of studies have been
161
done on the kinetics of CO2 absorption into MDEA aqueous solution, and the mass
162
transfer reaction mechanism has been established30, 31. The reaction of CO2 with
163
MDEA aqueous solution can be described by alkali catalytic mechanism32.
164
The reaction of CO2 with MDEA4:
165
CO2 + R1 R2 R3 N + H 2 O = R1 R2 R3 NH + + HCO3
166
RCO2 = k R1R2 R3 N C R1R2 R3 N CCO2
167
The reaction of CO2-[C2OHmim][Gly]-H2O system was analyzed in detail in our
168
previous work, and the reaction and rate equations were listed as following based on
169
the zwitterions mechanism29:
−
(1) (2)
+
170
CO2 + 2 IL − R1 NH 2 ↔ IL − R1 NHCOO − + IL − R1 NH 3
171
RCO2 = k IL− R1NH 2 C IL − R1NH 2 CCO2
172
CO2 absorption process into blends of MDEA and amine additives was described
173
as a shuttle mechanism33: Along the way diffusing from interface to the bulk liquid,
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CO2 firstly reacts with the reactive amine and produces carbamate, then it dissociates
175
to carbonate and the freed H+ reacts with MDEA, finally the released additive could
176
react with CO2 again. 2.4.2 Mass transfer. The mass transfer rate of CO2 absorption with chemical
177 178
reaction can be expressed by Eq.5 and Eq.6.
179
N = k G ( PCO2 − PCO2i )
(5)
180
N = k L ' (CCO2i − CCO2 )
(6)
181
According to our previous work, CO2 concentration in the bulk of liquid is
182
approximately zero, and the value of H CO2
183
Ek L
1 could be ignored comparing to that of kG
, then the total mass transfer rate of absorption could be simplified as Eq.729:
PCO2 PCO2 PCO 2 = Ek L = H CO2 H CO2 H CO2 Ek L
184
N=
k overall × DCO2
185
Here kL is the liquid-phase mass transfer coefficient, E is the enhancement factor,
186
and DCO2 and H CO2 represents the diffusion coefficient and solubility respectively.
187
2.5 Physicochemical parameters
(7)
188
2.5.1 Viscosity and density. Viscosity measurements in this work were
189
performed using a rotary viscometer (HAAKE VI550), and density measurements
190
were carried out by using a densimeter (Anton Paar, DMA-5000M).
2.5.2 Diffusion coefficient. The diffusion coefficient of CO2 in organic amines
191 192
could be obtained by N2O analogy method as shown below34-36:
DCO2 ,amine = DN 2O ,amine ×
193
DCO2 ,water DN 2O ,water
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− 2119 ) T − 2371 = 5.07 × 10 −6 × exp( ) T
194
DCO2 ,water = 2.35 × 10 −6 × exp(
195
DN 2O ,water
196
According to revised Stoke-Einstein equation:
197
γ γ DN2O ,amine × µ amine = DN2O , water × µ water
198
Where µ MDEA and µ water represent viscosities of MDEA and water under the
199
same conditions, respectively. For MDEA aqueous solution, the value of γ is usually
200
0.8. This equation was also used for blends of MDEA and ionic liquid aqueous
201
solution.
(10)
(11)
2.5.3 Solubility. The solubility of CO2 in organic amines was obtained by N2O
202 203
(9)
analogy36.
H CO2 ,water
(12)
204
H CO2 ,amine = H N 2O ,amine ×
205
H N 2O ,amine = (5.52 + 0.7C ) ×10 6 × exp(
206
H CO2 ,water = 2.8249 ×10 6 exp(
(14)
207
H N 2O , water
(15)
208
The H CO2 ,mix of the mixed system of MDEA/[C2OHmim][Gly] could refer to
209
H N 2O , water − 2166 ) T
− 2044 ) T − 2284 = 8.5470 ×10 6 exp( ) T
(13)
the solubility of [N1111][Gly]/AMP37: H CO2 ,mix = 509.77C IL − R1NH 2 + H CO2 ,amine
210
211
3. Results and Discussion
212
3.1 Optimum proportion of MDEA/[C2OHmim][Gly]
(16)
213
Fig.1 compares CO2 absorption rates of different absorbents under the same
214
operation conditions. The absorption rates of MEA, AMP and [C2OHmim][Gly] were
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all much higher than that of MDEA aqueous solution, thus all these absorbents could
216
blend with MDEA as additives for better CO2 absorption performance. CO2
217
absorption performances of the blended aqueous solutions of MDEA and
218
[C2OHmim][Gly] at different mole ratios were investigated at 323.15 K in a double
219
stirred-cell absorber. The total concentration of solution was 1.0 mol L-1, and the mole
220
ratio of MDEA to [C2OHmim][Gly] was 1:0, 9:1, 8:2 and 7:3 respectively. As shown
221
in Fig.2, compared to single MDEA aqueous solution, CO2 absorption rates increased
222
obviously in the presence of [C2OHmim][Gly] at beginning, then they all decreased
223
rapidly with the consumption of absorbents, and finally trended to be stable after
224
about 200 minutes, The reaction rate of blends was still higher than that of MDEA at
225
this stage. In addition, the initial absorption rate increased with the increase of
226
[C2OHmim][Gly] concentration because of the chemical absorption of primary amine
227
with high reaction rate. Table 1 shows CO2 absorption loading of blends at different
228
mole ratios under the same operation time in the double stirred-cell absorber. The
229
blend with a mole ratio of 8:2 of MDEA to [C2OHmim][Gly] achieved the highest
230
CO2 absorption loading. Fu24 investigated the influence of blends proportion of
231
MDEA and [Bmim][Gly] on absorption rate. They found that absorption rate
232
corresponding to w[Bmim][Gly] =0.05 was higher than that corresponding to w[Bmim][Gly] =
233
0.10, and they proposed that such phenomenon may be caused by change of viscosity.
234
In MDEA/[N1111][Gly] system23, they concluded that addition of small amount of
235
[N1111][Gly] could increase the absorption rate obviously, but with the increase of
236
concentration of [N1111][Gly], the absorption rate may decrease because increasing
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viscosity could decrease the diffusion of CO2 in the absorbent. Similar conclusions
238
were drawn in Zhang38 and Gao’s39 work. So the increase of concentration of
239
[C2OHmim][Gly] may lead to higher viscosity, limiting gas diffusion and blocking
240
absorption. This is why molar ratio of 8:2 of MDEA to [C2OHmim][Gly] had a higher
241
absorption than that of 7:3 in definite time . Thus this blend was chosen for further
242
experiment.
243
3.2 Effects of different factors on CO2 absorption
244
3.2.1 Effect of temperature. The effects of temperature on CO2 absorption were
245
studied under atmospheric pressure in a temperature range of 303.15-333.15 K in the
246
double stirred-cell absorber and the bubbling glass absorber. The results in Fig 3
247
agreed with the Arrhenius’s Law that higher temperature leads to a higher absorption
248
rate. When temperature changed from 303.15K to 333.15K, the initial absorption rate
249
increased from 0.17 to 0.46 ×10-3 mol·min-1, and the rate varied from 0.066 to 0.145
250
×10-3 mol·min-1 in 350 minutes. Saturated CO2 loading of the absorbent was 0.572,
251
0.542, 0.523, and 0.501 mol CO2/mol absorbent at 303.15-333.15K, which decreased
252
a bit with the increase of temperature, and this result was consistent with some other
253
investigations22. In general, CO2 loading of the absorbent was hardly affected by
254
temperature.
255
3.2.2 Effect of O2 concentration. As the flue gas generated by coal-fired power
256
plants contains 3-4% (v/v) oxygen generally, effect of O2 concentration was
257
investigated in a range of 0-10%. Fig.4 shows effect of O2 on CO2 absorption capacity
258
for blended absorbent and MDEA aqueous solution. With the increase of O2
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concentration, the CO2 loadings of the two aqueous solutions decreased to various
260
extents, proving the significant influence of O2 on CO2 absorption of these absorbents.
261
When
262
MDEA/[C2OHmim][Gly] was decreased by 11.6% compared with that without O2,
263
while that of MDEA was decreased by 17.2%. This indicated that the addition of
264
[C2OHmim][Gly] weakened the effect of O2 concentration on absorption capacity.
265
Besides, CO2 loading of MEA/[C2OHmim][Gly] system was decreased by 12.6% at
266
8% concentration of O2 in our previous work28, while that was decreased by 9.1% for
267
MDEA/[C2OHmim][Gly] system at the same O2 concentration, suggesting that CO2
268
capacity of the blends was less adversely influenced by O2 in the stream.
269
3.3 Regeneration of absorbent
the
concentration
of
O2
was
up
to
10%,
CO2
loading
of
270
The saturated solution after absorption was regenerated at 383.15K in a magnetic
271
stirrer with a condenser. Fig.5 compares regeneration efficiency of 1.0 mol·L-1 MDEA
272
and the blended aqueous solution of MDEA/[C2OHmim][Gly] at a mole ratio of 8:2.
273
For the absorbents tested, with the increase of regeneration cycles, lower regeneration
274
efficiency was achieved. After the third cycle, the regeneration efficiency of the two
275
absorbents was 88.3% and 93.1% respectively, indicating that blended solution
276
possessed a higher regeneration capacity than MDEA. For MEA/[C2OHmim][Gly]
277
system28, regeneration efficiency was 91.7% after the third cycle, suggesting that the
278
blended system has a good regeneration ability.
279
Several detailed parameters after the first regeneration cycle are listed in Table 2.
280
The CO2 escape temperature of MDEA and MDEA/[C2OHmim][Gly] was 345.15 and
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343.15 K, respectively, and the desorption temperatures were maintained at about
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375.15 K. As ionic liquids are less volatile and easy to be regenerated40, the addition
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of [C2OHmim][Gly] enhanced the regeneration efficiency even after several cycles.
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3.4 Reaction mechanism
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In order to explore the reaction mechanism of CO2 absorption into the blended
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system, 100ml of the blended aqueous solution was prepared for absorption reaction,
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and pH and CO2 loading were measured as the absorption proceeded. Fig.6 described
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the changes of pH and CO2 loadings throughout the absorption process at 323.15K.
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With the increase of CO2 loading, pH of the solution decreased gradually.
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was conducted to investigate the changes of chemical structure of the process. As
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shown in Fig.7, when pH>10.00, the peaks assigned to MDEA (61.20ppm, 60.75ppm
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and 44.31ppm) were not shifted nearly, while peak 4 and peak 5 which respectively
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belonged to carboxylic and methylene group of glycine anion were shifted visibly.
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Besides, a new peak was found at 166.70 ppm which assigned to carboxyl carbon of
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carbamate41. It is indicated that [C2OHmim][Gly] firstly reacted with CO2 in the
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system to form carbamate when pH>10.00, and CO2 loading was about 0.08 mol
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CO2/mol absorbent at 323.15K at pH 10.00. When pH
