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Study of formation of bicarbonate ions in CO2-loaded aqueous single 1DMA2P and MDEA tertiary amines and blended MEA-1DMA2P and MEA-MDEA amines for low heat of regeneration Rui Zhang, Zhiwu Liang, Helei Liu, Wichitpan Rongwong, Xiao Luo, Raphael O. Idem, and Qi Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03097 • Publication Date (Web): 06 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016
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Industrial & Engineering Chemistry Research
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Study of formation of bicarbonate ions in CO2-loaded aqueous
2
single 1DMA2P and MDEA tertiary amines and blended
3
MEA-1DMA2P and MEA-MDEA amines for low heat of
4
regeneration
5
Rui Zhang1, Zhiwu Liang1 *, Helei Liu1, Wichitpan Rongwong1, Xiao Luo1,
6
Raphael Idem1,2, Qi Yang3 1
7
Joint International Center for CO2 Capture and Storage (iCCS), Provincial Key
8
Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing
9
Carbon-dioxide Emissions, College of Chemistry and Chemical Engineering, Hunan
10
University, Changsha, Hunan, 410082, P.R. China
11 12 13
2
Clean Energy Technologies Institute (CETI), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada 3
CSIRO Manufacturing Flagship, Clayton VIC 3168, Australia
14 15 16 17 18 19 20 21
*CORRESPONDING AUTHOR: Tel.: +86-13618481627; fax: +86-731-88573033;
22
E-mail address:
[email protected] (Z. Liang).
23 24
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ABSTRACT
25 26
The formation of bicarbonate ions in an amine solution during CO2 absorption
27
results in lowering the heat duty for amine solvent regeneration in the CO2 capture
28
process because bicarbonate breakdown needs the lowest energy input to release CO2.
29
In this study, bicarbonate formation was conducted for two mixed solvents consisting
30
of tertiary amines (1DMA2P (1M) or MDEA (1M)) blended with MEA in order to
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determine both formation rate and capacity of bicarbonate ions as compared to MEA
32
alone. The amines and concentrations used in the study were: MEA (5M),
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MEA-MDEA (5:1 molar ratio, 6M total) and MEA-1DMA2P (5:1 molar ratio, 6M
34
total) at various CO2 loadings. The formation of bicarbonate ions was evaluated using
35
13
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system, higher concentrations of bicarbonate ions were formed for MDEA than for
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1DMA2P for the same CO2 loading. The results for the blended amine systems
38
showed that bicarbonate ions were generated at a lower CO2 loadings than MEA
39
alone, with MEA-1DMA2P generating bicarbonate ions at a lower CO2 loading (0.34
40
mol CO2/mol amine) than MEA-MDEA (0.38 mol CO2/mol amine). Thus, as an
41
additive in MEA, 1DMA2P has a better potential than MDEA to generate bicarbonate
42
ions at a leaner CO2 loading with the attendant lowering of the regeneration energy.
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Keywords:
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MEA-MDEA; MEA-1DMA2P, carbon dioxide capture.
C NMR technique at 293.15K. The results show that, for the single tertiary amine
Bicarbonate
formation,
NMR,
1-dimethylamino-2-propanol,
45 46
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1. INTRODUCTION
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Climate change and global warming issues have caught the attention of
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researchers and governments worldwide because of the risks they pose to human life
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and the environment. Greenhouse gases (GHG), and in particular, carbon dioxide
51
(CO2) is blamed for this change. In recent years, various aqueous amine solutions and
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their blends have been used as chemical absorbents in post combustion CO2 capture
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technology to remove CO2 from a variety of sources such as flue gases, natural gas,
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synthesis gas, and various refinery streams using various absorption apparatus.1-4
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Precise knowledge of the reaction chemistry in amine based CO2 capture processes is
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needed in order to make rational improvements to the efficiency of post-combustion
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capture (PCC) using amines or other reactive solvents.
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Primary amines such as monoethanolamine (MEA) and secondary amines such as
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diethanolamine (DEA) have faster kinetics in their reaction with CO2 but have a lower
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CO2 absorption capacity as compared to tertiary amines. On the contrary, tertiary
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amines such as MDEA have higher CO2 absorption capacity and are more easily
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regenerated but have lower reaction kinetics. Blends of primary amine and tertiary
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amines are typically used in order to obtain a faster reaction rate, higher CO2 capacity,
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and lower heat of solvent regeneration.5-8 To this end, a mixture of MEA and MDEA
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has been tested to evaluate its CO2 capture performance in pilot plants and the results
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showed that this mixture requires less energy for solvent regeneration than MEA
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alone.9 1DMA2P (1-dimethylamino-2-propanol), a tertiary amine, has been
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investigated for its kinetics in its reaction with CO2 using a stopped-flow apparatus in
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the temperature range of 293-313K and was found to react with CO2 faster than
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MDEA.10 There has been no research reported in the literature on MEA-1DMA2P
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mixture. In addition, it is known that the relatively high heat of formation of
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carbamate increases the energy needed for amine regeneration. On the other hand,
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there is no carbamate formation in the reaction of tertiary amines with CO2, implying
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that the lower heat of formation of bicarbonate than carbamate in tertiary
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amine-CO2-H2O system will result in a lower heat requirement in the regeneration of
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tertiary amine systems. This further implies that, if the rich amine has more
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bicarbonate in the CO2-loaded amine as it goes to the regeneration column and the
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CO2-lean amine in the regeneration column can still generate bicarbonate ions at this
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CO2 depleted stage, it will further decrease the energy costs for amine regeneration.
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Barzagli et al.11 investigated the absorption efficiency as well as the amine
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regeneration efficiency of AMP-MDEA, AMP-DEA, DEA and MDEA solvent
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systems with the 13C NMR technique. The results showed that blends of amines have
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enhanced absorption efficiency compared to single amines.11 Among the single amine
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systems, MDEA performed the best in terms of regeneration efficiency while
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AMP-MDEA
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amine regeneration than AMP-DEA due to the fact that the DEA carbamate has a
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higher thermal stability than AMP carbamate.11 Barzagli et al.12 investigated the
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absorption efficiency of CO2 in a series of aqueous solutions of primary, secondary
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and tertiary alkanolamines. The results showed that carbamate reduces the
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CO2 absorption efficiency. In addition, they also found that the carbamate ion is not
displayed
better
performance
in
both
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CO2 absorption
and
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only generated in CO2 absorption process, but also, in the solvent regeneration
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process.12
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MEA and MDEA have been widely used in commercial processes, and studies
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have shown that the blend of MEA with tertiary amines leads to a reduction of energy
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requirement and an increase in CO2 removal efficiency,13, 14 as well as absorption
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ability and absorption rate15. The 5M solution of MEA in the amine mixture was
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chosen for this study because it has been used widely in industrial plants. When the
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tertiary amine reacts with CO2 it acts as a base that catalyzes the hydration of CO2 to
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produce bicarbonate. It would be a significant development for the industry if the
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tertiary amine can promote the 5M MEA to generate bicarbonate at the low CO2
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loading because then the tertiary amine would have a positive catalytic effect on the
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solvent regeneration. 1DMA2P, a novel solvent, could then be used to test for the
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production of bicarbonate in 5M MEA solution compared to conventional MDEA
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solvent. However, the bicarbonate ion as a factor in solvent regeneration has not been
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investigated in detail in single and blended amines such as MDEA, 1DMA2P,
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MEA-MDEA and MEA-1DMA2P. In this work, the 13C NMR technique was used to
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investigate
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1DMA2P-CO2-H2O, MDEA-CO2-H2O, MEA-CO2-H2O amine systems, as well as
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blended binary MEA-MDEA-CO2-H2O and MEA-1DMA2P-CO2-H2O amine systems
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at various CO2 loadings at 293.15K was investigated in order to determine their
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potential to generate bicarbonate ions at lower CO2 loading for lower energy costs of
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amine regeneration.
bicarbonate/carbonate
and
carbamate
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formation
in
single
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To get useful data from our laboratory by NMR spectroscopy for 1DMA2P,
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DEAB, which has a similar molecular structure to that of 1DMA2P, was used for
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validation of the NMR spectroscopy and the experimental procedure. The results were
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then compared with the literature (Figure 1).
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2. THEORY
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2.1. Chemistry of aqueous CO2-loaded 1DMA2P system
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The reaction mechanism of CO2 with amines can be explained by several
120
theories based on different amine molecular structures. Caplow (1968) suggested a
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two-step zwitterion mechanism,16 later reintroduced by Danckwerts (1979).17 The first
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step is zwitterions formation while the second step is the deprotonation of the
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zwitterions. Crooks and Donnellan35 proposed the termolecular mechanism which is
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based on an amine reaction with CO2 in only one step with proton transfer taking
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place simultaneously. Both mechanisms can only be used to explain the reaction
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between CO2 and primary or secondary amines. 1DMA2P is a tertiary amine. Similar
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to MDEA and DEAB, no hydrogen-atoms are directly connected to the
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nitrogen-atoms in the molecular structure of 1DMA2P; as such, it cannot generate
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carbamate ions.10, 18 Instead, 1DMA2P just like MDEA, acts as a base that catalyzes
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the hydration of CO2. Consequently, the major component in the system is either
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protonated 1DMA2P or protonated MDEA (in the case of MDEA) and their
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bicarbonate ions. Reactions considered to be occurring when CO2 is introduced into
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aqueous primary amine solutions and aqueous tertiary amine solutions are given in
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Equations (1) to (13).
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MEA-CO2-H2O system.19
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Dissociation of water:
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2H 2O ↔ H 3O + + OH −
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Dissociation of dissolved CO2 through carbonic acid:
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CO 2 + 2H 2 O ↔ H 2 CO 3 + H 2 O ↔ H 3O + + HCO 3
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Zwitterion formation for MEA:
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MEA + CO 2 + H 2 O ↔ MEAH + COO − (zwitterio n) + H 2O
142
Carbamate formation from MEA zwitterion:
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MEAH+ COO− (zwitterion) + H 2 O ↔ MEACOO− (carbamate) + H3O +
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Formation of protonated amine:
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MEA + H 3O + ↔ MEAH+ + H 2O
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Formation of bicarbonate:
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CO 2 + OH - ↔ HCO 3
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Carbamate reversion to bicarbonate:
149
HO(CH2 )2 NHCOO- + H 2O ↔ HO(CH2 )2 NH 2 + HCO3
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Dissociation of protonated amine:
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MEAH+ + H 2O ↔ MEA + H 3O+
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For tertiary amines, the following reactions are considered to occur in the
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R1R2R3N-CO2-H2O system.20
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Dissociation of water:
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2H 2O ↔ H 3O + + OH −
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Dissociation of dissolved CO2 through carbonic acid:
(1)
−
(2)
(3)
(4)
(5)
−
(6)
-
(7)
(8)
(9)
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CO 2 + 2H 2 O ↔ H 2 CO 3 + H 2 O ↔ H 3O + + HCO 3
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Dissociation of bicarbonate:
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CO 2 + OH - ↔ HCO 3
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Formation of protonated amine:
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R1R 2 R 3 N + H 3O + ↔ R1R 2 R 3 NH + + H 2O
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Dissociation of bicarbonate:
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HCO 3 + H 2 O ↔ CO 3
−
−
−
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(10)
(11)
2−
+ H 3O +
(12)
(13)
164
For blended amine systems such as MEA-MDEA or MEA-1DMA2P, 7 ions are
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considered to exist in each of the loaded aqueous amine solutions, namely, MEA,
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MDEA, MEAH+, MDEAH+, HCO3-, CO32-, MEACOO- (for the former amine system),
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and MEA, 1DMA2P, MEAH+, 1DMA2PH+, HCO3-, CO32-, MEACOO- (for the latter
168
amine system).
169
2.2 The role of bicarbonate in solvent regeneration to strip CO2
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As indicated, the ions in aqueous amine solutions are: free amine, protonated
171
amine, carbonates, bicarbonates and carbamates (only for primary and secondary
172
amine). The heat duty of solvent regeneration is controlled mainly from reactions (3),
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(4) and (8) because these reactions are strongly endothermic.16, 21 Also, among these
174
ions, the bicarbonate ion is the most important as a result of its contribution to
175
lowering the heat duty for solvent regeneration. This is attributed to the bicarbonate
176
ion being able to act as a proton acceptor to release the proton from the MEAH+.
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Consequently, the deprotonation process needs less energy than that with water.21 Yeh
178
et al.22 used the semibatch reactor to simulate the thermodynamics of aqueous
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ammonia reacting with CO2 at absorption/regeneration conditions. Their results
180
showed that, among the ammonium compounds, ammonium bicarbonate required the
181
least energy for regeneration.22 The breakdown of carbamate into CO2 by heating
182
needs a much larger quantity of heat and/or protons.21, 23 In a study, the concentration
183
of carbamate ions increased from 1.34 mol/L to 1.55 mol/L after heating the
184
CO2-loaded aqueous MEA solution, implying that carbamate was not decomposed to
185
release CO2 by heating under the conditions of their experiments.24 In addition, their
186
experimental results suggested that the generation of CO2 from the solution mainly
187
originated from bicarbonate/carbonate ions because the concentration of HCO3-/CO32-
188
was decreased significantly after heating the loaded aqueous MEA amine solution.24
189
This is because bicarbonates are relatively easy to decompose by heating. As well,
190
breaking C-O bonds requires less energy (in bicarbonate and carbonate breakdown)
191
than that for breaking C-N bonds (in carbamate breakdown).25
192 193
3. EXPERIMENTAL SECTION
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3.1 Chemicals and apparatus
195
Reagent-grade 1-dimethylamino-2-propanol
(1DMA2P,
with
mass
196
purity of ≥99%) was purchased from Alfa Aesar, A Johnson Matthey Company,
197
China, and was then prepared to the desired concentration using deionized water.
198
N-methyldiethanolamine (MDEA, with purity of ≥99%) were purchased from Tianjin
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Hengxing Chemical Preparation Co., Ltd., China.. DEAB was synthesized in the
200
solvent synthesis laboratory at the Joint International Center for CO2 Capture and
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Storage (iCCS) according to the method described by Tontiwachwuthikul et al.26 The
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purity of synthesized DEAB solvent was tested using NMR and found to be 97%. 1M
203
aqueous solutions of hydrochloric acid (HCl), NaCO3 (with mass purities ≥99.8%)
204
and NaHCO3 (with mass purities ≥99.6%) were prepared in iCCS laboratory, for use
205
in titration and NMR reference analysis. Deuterium oxide (D2O) and 1,4-dioxane
206
were purchased from InnoChem Science&Technology Co. Ltd (Beijing, China) and
207
used for sample preparation. As well, commercial-grade CO2 (with purity ≥99%) was
208
supplied by Changsha Jingxiang Gas Co., Ltd., China.
209
In addition, a nuclear magnetic resonance (NMR) spectrometer (INOVA-400,
210
Varian company, USA) was used to test the samples. Several 5mm diameter NMR
211
tubes, produced by Wilmad Lab Glass and bought from Synthware glass Co. Ltd
212
(Beijing, China), were used for the NMR tests in this work.
213
3.2 Sample preparation
214
Each CO2-loaded amine solution was prepared by bubbling pure CO2 gas into
215
each
of
the
aqueous
1DMA2P
(1M),
MDEA
216
(nMEA:n1DMA2P=5:1, 6M), MEA-MDEA (nMEA:nMDEA=5:1, 6M) and MEA (5M)
217
solutions at 298K. The CO2 loading and total amine concentration of each sample was
218
ascertained by titration with 1M HCl solution. The titration apparatus and the
219
operating procedure are as described in the literature.27 The titration equipment in our
220
laboratory was validated in our previous work.28 A 0.5mL sample of each CO2-loaded
221
and unloaded amine solution was placed into a 5mm NMR tube, and then 10 mass%
222
of D2O was added into the sample to obtain a signal lock. A GVLab fixed-speed
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(1M),
MEA-1DMA2P
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vortex was used to mix all contents in the NMR tubes in order to minimize resolution
224
error of the NMR spectra. About 0.015mL of pure 1,4-dioxane was also added into
225
the tubes to provide a sharp peak with a known chemical shift in the
226
spectrum as an internal reference standard. All the tested sample spectra analyzed
227
were calibrated based on the chemical shift of 67.19 ppm for 13C NMR.29
228
3.3 Determination of ion concentration in single/blended amines-CO2-H2O
229
system
13
C NMR
230
In the present work, a calibration method proposed by Shi et al.30 was used for
231
determination of ion concentration in single/blended amines-CO2-H2O system. In the
232
reference method, a
233
system for a VLE model as well as the liquid phase speciation in a
234
MEA-DEAB-CO2-H2O system at absorption and solvent regeneration conditions.30, 31
235
Equations (14)-(17) were used to calculate the ion concentrations,32 where the 168.03
236
ppm and 161.45 ppm are the chemical shift of pure carbonate and bicarbonate in 13C
237
NMR spectra (at 293.15K), respectively, and which is in agreement with the results
238
from Jakobsen et al.20 with an absolute deviation of 0.27% (at 293.15K).
239
[AmCOO− ] 2−
−
[CO3 ] + [HCO3 ] 240
[AmCOO− ] =
241
[CO 3 ] =
242
[HCO 3 ] =
2-
-
13
C NMR spectroscopy was used to study the DEAB-CO2-H2O
=
Scarbamate = R [Sbicarbonate + Scarbonate ]
R [CO2 ]0 1+ R
(δ - 161.45) [CO 2 ]0 (168.03 - 161.45)(1 + R) (168.03 - δ) [CO 2 ]0 (168.03 - 161.45)(1 + R)
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(14)
(15)
(16)
(17)
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243
[CO2]0 is the total amount of CO2 in the loaded amine aqueous solution. There
244
are three kinds of carbon compound in the loaded aqueous amine solution, namely,
245
carbamate, carbonate and bicarbonate. The [CO2]0 can be calculated based on the
246
Equation (18). Scarbamate and Scarbonate and Sbicarbonate represent the peak area in the
247
NMR spectra.
248
[CO 2 ]0 = Camine × loadingα
13
C
(18)
249
However, the calculation method of each ion concentration was performed using
250
the above equations in conjunction with the chemical shift and peak area obtained
251
from the 13C NMR spectra. Since the 13C signal of carbonate and bicarbonate was not
252
detected because no carbonate and bicarbonate ions were produced at low CO2
253
loading, Equations (19) and (20) were used to calculate the concentration of
254
carbamate, bicarbonate and carbonate in the blended system.
255
[AmH + ] + [H + ] = [HCO 3 ] + 2[CO 3 ] + [OH − ]
256
[MEACOO − ] + [HCO 3 ] + [CO 3 ] = [CO 2 ]0
257
3.4 Validation of NMR spectrometer using novel calibration method
2−
−
−
2−
(19) (20)
258
Because both DEAB and 1DMA2P contain a tertiary amino group and a
259
hydroxyl group, the only difference between these two amine molecular structures is
260
the size of the group which is connected to the tertiary amino group. In DEAB, the
261
tertiary amino group is connected to two ethyl groups but in 1DMA2P, the tertiary
262
amino group is connected to two methyl groups. Therefore, using DEAB as a
263
reference solvent to validate the NMR spectroscopy and experimental procedure was
264
done to increase the accuracy of the system. In this work, the NMR spectrometer was
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validated using aqueous solution of DEAB at concentration of 1.5M at 297.65K. The
266
calibration method was applied to test the accuracy of the apparatus for evaluating the
267
exact protonation ratio in the loaded amine. There were 10 samples tubes prepared
268
with protonation ratios of DEAB (nHCl:nDEAB = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9,
269
1.0), respectively. The percentage H+ added into DEAB was calculated by titration
270
with 1M HCl. The 13C NMR analysis was performed on these 10 samples at 297.65K.
271
The reference data from the literature (Shi et al.33) were used to plot the curves of
272
chemical shifts against protonation ratio of DEAB. The experimental results obtained
273
in this work were then compared with those of Shi et al.33 as shown in Figure 1. It is
274
clearly shown from Figure 1 that the percentage of protonated amine based on the
275
chemical shift obtained from this work (at 297.65K) matched well with the
276
experimental results of Shi et al.33 at 297.65K. Therefore, the results obtained by the
277
NMR spectrometer in this work can be said to be accurate and reliable.
278 279
4. RESULTS AND DISCUSSION
280
4.1 Quantitative estimation of carbonate/bicarbonate ions in single aqueous
281
tertiary amine systems
282
The bicarbonate and carbonate concentration profiles of each of 1M aqueous
283
solutions of 1DMA2P and MDEA at various CO2 loadings, investigated by 13C NMR
284
technique at 293.15K, are plotted in Figure 2. It can be clearly seen from Figure 2 that
285
the concentration of bicarbonate increased as the CO2 loading increased. With the
286
CO2 introduced into both aqueous tertiary amine solutions, the hydration reaction of
287
CO2 with water occurs first, and then form H2CO3 (l) which then decomposes to
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bicarbonate and H+ ions as shown in reaction (10). The tertiary amine acts as a base
289
that catalyzes the hydration of CO2 to generate the bicarbonate and H+ ions.34 As a
290
result, the acidity of the solution increases due to the increase of the concentration of
291
H+ ions. In addition, it is also observed that the concentration of carbonate increases
292
until it reaches a maximum and then it starts to decrease as the CO2 loading increases.
293
At the low CO2 loading in the aqueous tertiary amine solution, the solution is strongly
294
basic (pH>10) because of the large portion of free tertiary amine that exists in the
295
solution, which results in the protons of the solution being accepted by free tertiary
296
amine, as described in reaction (12). This means that little HCO3- is converted to
297
CO32- based on reaction (13). The result is that the concentration of carbonate
298
increases as the CO2 loading increases. As more CO2 is introduced into the aqueous
299
tertiary amine solution (increased CO2 loading beyond a certain point), the
300
concentration of H+ ions increases resulting in basicity decrease. Then, the carbonate
301
ions start to accept protons and convert to bicarbonate ions as described in the reverse
302
of reaction (13). As a result, the concentration of carbonate has a maximum value, and
303
the concentration of bicarbonate increases monotonically as the CO2 loading increases.
304
The concentration profiles of carbonate/bicarbonate for the two tertiary amines
305
(MDEA and 1DMAP) are similar to that of DEAB.30
306
It can be seen from Figure 2 that the concentration of bicarbonate ions in the
307
single MDEA-CO2-H2O system is higher than that in the single 1DMA2P-CO2-H2O
308
system at the same CO2 loading, but conversely, its concentration of carbonate ions is
309
lower than that in 1DMA2P-CO2-H2O system. Rayer et al. (2014) investigated the
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heats of absorption of CO2 in aqueous solution MDEA and 1DMA2P. The results
311
showed that the heat of absorption for the single MDEA system is lower than that for
312
1DMA2P when the CO2 loading is below 0.9 mole CO2/mole amine.18 They also
313
suggested that the formation of carbonate is the main contributor to the heat of
314
reaction in aqueous tertiary amines.18 Therefore, the higher heat of reaction means
315
that a larger quantity of carbonate ions exists in the aqueous amine solution. This is in
316
agreement with the results of this work for single tertiary amine systems, as shown in
317
Figure 2. In addition, the pKa of 1DMA2P10 is higher than that of MDEA35, which
318
leads to the pH in 1DMA2P-CO2-H2O system being higher than that of
319
MDEA-CO2-H2O system at the same concentration and CO2 loading. As a result,
320
more bicarbonate ions are generated in the MDEA-CO2-H2O system than in the
321
1DMA2P-CO2-H2O system at the same CO2 loading.
322
4.2 The effect of MEA on the production of bicarbonate/carbonate in blended
323
MEA-tertiary amine systems
324
The blended amine systems MEA-1DMA2P and MEA-MDEA at various CO2
325
loadings were tested for their carbonate and the bicarbonate ions concentrations at the
326
temperature of 293.15K. The results were compared with those for single tertiary
327
amine systems, as shown in Figures 3a and 3b for MEA-MDEA and MEA-1DMA2P,
328
respectively. In these blended primary-tertiary amines systems, there are three types
329
of carbon compounds: carbamate, carbonate and bicarbonate.31 Figures 3a and 3b
330
show that, in the blended amine MEA-MDEA and MEA-1DMA2P systems,
331
carbonate and bicarbonate were generated starting at the CO2 loading of 0.38 mole
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332
CO2/mole amine (for MEA-MDEA) and 0.34 mole CO2/mole amine (for
333
MEA-1DMA2P). In the single tertiary amine system, however, the carbonate and
334
bicarbonate were generated immediately after CO2 was introduced into the aqueous
335
amine solution. Also, the concentration of bicarbonate increased as the CO2 loading
336
increased. It is clear that the blended amine system generates bicarbonate and
337
carbonate at a higher CO2 loading than the single tertiary amine system. This is
338
because the carbamate formation reaction as shown in reactions (3) and (4) is the
339
main reaction in the blended amine system at the low CO2 loading, and this is
340
attributed to the ability for MEA to react directly with CO2.16, 17 MEA reacts more
341
easily with CO2 than tertiary amines (such as MDEA and 1DMA2P). As such, if
342
MEA is added into a tertiary amine system, it delays the ability of the tertiary amine
343
in producing the carbonate and bicarbonate. As a result, the tertiary amine does not
344
generate bicarbonate ions at a low CO2 loading in the solution of blended amines.
345
With less bicabonate or even no bicarbonate in the rich amine, then the energy
346
consumption is higher when the rich blended amine solution is being regenerated.
347
4.3 The effect of tertiary amine on the production of carbamate in MEA
348
The
concentration
of
carbamate
profile
in
the
MEA-CO2-H2O,
349
MEA-1DMA2P-CO2-H2O and MEA-MDEA-CO2-H2O systems is plotted in Figure 4.
350
In the blended amine system, there are MEA-based compounds such as MEA
351
carbamate, MEAH+ and free MEA. MEA reacts with CO2 directly when CO2 is
352
introduced into aqueous MEA solution and forms MEA carbamate as shown in
353
reactions (3) and (4). Meanwhile, the MEAH+ is also generated by reaction (5). After
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MEA is consumed completely by reacting with CO2, HCO3- is produced by reaction
355
(2) in the MEA-CO2-H2O system. However, for the dissociation reaction of MEAH+
356
as shown in reaction (8) in the blended amine system, tertiary amine replaces the H2O
357
as a base to release the MEAH+ so as to become free MEA. Because the basicity of
358
tertiary amine is stronger than H2O, MEAH+ more easily releases the proton to free
359
tertiary amine to form protonated tertiary amine. Thus, more MEAH+ is being
360
deprotonated by tertiary amine resulting in the release of more MEA. After that, the
361
free MEA reacts with CO2 as more CO2 is introduced into the solution leading to
362
more carbamate being generated. This is attributed to MEA, a primary amine, having
363
faster reaction kinetics relative to tertiary amine. The concentration of carbamate
364
increases till it reaches a maximum, and then it decreases by the hydrolyzation
365
process as shown in reaction (7), as the CO2 loading increases.
366
In the two blended amine system on the other hand, the concentration of
367
carbamate increases as the CO2 loading increases until the CO2 loading is over 0.38 or
368
0.34
369
MEA-1DMA2P-CO2-H2O systems respectively. This is because the same amount of
370
MEA (5M) in the original blended amines solutions and the MEA is available to react
371
first with CO2. However, after the original free MEA has been completely consumed,
372
(i.e. CO2 loading increased above 0.34 mole CO2/mole amine), the amount of
373
carbamate in MEA-1DMA2P-CO2-H2O system becomes less than that in
374
MEA-MDEA-CO2-H2O system as shown in Figure 4. This is because a larger amount
375
of free MEA is released from MEAH+ by MDEA in MEA-MDEA-CO2-H2O system
mole
CO2/mole
amine
for
MEA-MDEA-CO2-H2O
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376
than that in MEA-1DMA2P-CO2-H2O system. More of the 1DMA2P is used to
377
generate bicarbonate rather than to release MEA from MEAH+ relative to MDEA.
378
This is explained in Figure 6 which shows that more bicarbonate was generated in
379
MEA-1DMA2P-CO2-H2O system when the CO2 loading was over 0.34 mole
380
CO2/mole amine implying that less 1DMA2P is used to release MEA from MEAH+.
381
When the MEA is consumed completely by CO2 at CO2 loadings over 0.34 mole
382
CO2/mole amine, the tertiary amine then reacts with CO2 to produce bicarbonate ions.
383
More 1DMA2P reacts with CO2 than MDEA because the 1DMA2P reacts faster with
384
CO2 than MDEA.10 Considering the above factors, more free MEA is released by
385
MDEA from MEAH+ than by 1DMA2P due to more 1DMA2P being consumed by
386
CO2 rather than by MEAH+. Consequently, there is more carbamate produced in
387
MEA-MDEA-CO2-H2O system than that in MEA-1DMA2P-CO2-H2O system. It
388
should be noted that less carbamate in the loaded amine solution leads to less energy
389
consumption in solvent regeneration.36 From Figure 4, it can be concluded that the
390
absorbent mixture of MEA-1DMA2P consumes less energy than MEA-MDEA in
391
solvent regeneration.
392
4.4 The effect of tertiary amine on the production of bicarbonate from MEA
393
The amounts of bicarbonate ions and carbonate ions obtained from aqueous
394
single MEA (5M) solution were respectively added to the ones obtained from aqueous
395
single MDEA (1M) solution. These sums were compared with the ones obtained in
396
aqueous blended amine solution (MEA-MDEA, nMEA:nMDEA=5:1, 6M), as shown in
397
Figure 5. In this process, Matlab software was used to calculate the total amounts of
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bicarbonate and carbonate concentrations in single aqueous MEA (5M) solution and
399
single aqueous MDEA (1M) solution. It is clear that the total amounts of bicarbonate
400
or carbonate ions obtained by combining the individual amounts in the single aqueous
401
MEA and MDEA amine solutions is higher than those of the corresponding ions in
402
the blended amine solution. This is because when CO2 is added in the single MDEA
403
solution, bicarbonate and carbonate ions are produced right away due only to the
404
tertiary amine. However, in the single aqueous MEA solution, there is no bicarbonate
405
produced because CO2 reacts with MEA and forms carbamate preferentially.
406
Therefore, the results for the blended MEA-MDEA system shows that MEA does
407
suppress the production of bicarbonate of MDEA, as discussed previously.
408
It can also be seen from Figure 6 that the bicarbonate concentration in single
409
aqueous MEA solution increases significantly when the CO2 loading is higher than
410
0.45 mole CO2/mole amine. It is to be recalled that the concentration of
411
bicarbonate ions in a single aqueous MDEA solution increases nearly linearly as
412
shown in Figure 3. These two facts result in the total bicarbonate ion concentration
413
derived from the two single aqueous amine solutions increasing significantly when
414
the CO2 loading is higher than 0.45 mole CO2/mole amine as shown in Figure 5. After
415
this loading, the total concentration of bicarbonate ions in the two single aqueous
416
amine solutions comes mainly from the MEA reaction with CO2.
417
The situation for a blended amine solution is totally different; the MEA reacts
418
with CO2 and forms carbamate first in the blended amine system when CO2 is added
419
into the blended amine aqueous solution, and almost no carbonate or bicarbonate ions
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420
are formed until the free MEA is reduced. This is seen in the blue circles, representing
421
the bicarbonate ion concentration in blended amine solution, which is almost zero
422
before the CO2 loading reaches 0.4 mole CO2/mole amine. However, it is also
423
apparent that the inflection point of concentration of bicarbonate is advanced in
424
aqueous blended amine solutions compared to the sum of both aqueous single primary
425
and aqueous single tertiary amine solutions. This can be explained using reaction (6),
426
which shows that tertiary amines exists in the aqueous blended amine solution, and
427
leads to more OH- being generated in the blended amine system because tertiary
428
amines have stronger basicity than H2O which is helpful for reaction (9). As a result,
429
the concentration of OH- is increased in the aqueous blended amine solution. In
430
addition, the reaction rate of the formation of carbamate, as shown in reaction (2) and
431
(3) in blended amine aqueous solution, is faster than that in aqueous single MEA
432
solution because more free MEA is released by tertiary amine, which leads to the
433
MEA of blended amine aqueous solution being consumed faster than that in single
434
aqueous MEA solution. Consequently, the bicarbonate is produced earlier in the
435
blended amine aqueous solution than that in the sum of both of single amine aqueous
436
solutions. Thus, after comparing the amounts of bicarbonate/carbonate ions between
437
blended amine system and the sum of the individual single amine systems, it can be
438
concluded that tertiary amine can promote MEA to produce bicarbonate/carbonate at
439
a lower CO2 loading.
440
A comparison of the generation of bicarbonate between the single aqueous MEA
441
solution system (5M, MEA) and aqueous blended amines solution system
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442
(nMEA:nMDEA=5:1, 6M; nMEA:n1DMA2P=5:1, 6M) system was carried out and the results
443
are shown in Figure 6. It shows that the blended amine system also generates
444
bicarbonate at a lower CO2 loading than the single MEA system as discussed earlier.
445
It also shows that the bicarbonate appears in MEA-1DMA2P-CO2-H2O system at a
446
lower CO2 loading than that in MEA-MDEA-CO2-H2O system. This occurs because
447
more 1DMA2P reacts with CO2 when the MEA is completely consumed relative to
448
MDEA due to its faster reaction kinetics than MDEA as discussed earlier.10
449
However, rich amine of relatively low CO2 loading but with more bicarbonate
450
will not only lead to higher efficiency of CO2 removal but also to lower heat duty of
451
solvent regeneration. Based on these results, the heat duty of MEA-1DMA2P
452
regeneration is expected be lower than MEA-MDEA because it generates more
453
bicarbonate.
454 455
5. CONCLUSIONS In this work, the tertiary amines MDEA and 1DMA2P were investigated using
456
13
457
the
C NMR technique at 293.15K. In addition, the single amine MEA (5M) and
458
blends of MEA-MDEA (nMEA:nMDEA=5:1, 6M), MEA-1DMA2P (nMEA:n1DMA2P=5:1,
459
6M) were also tested. The results show that blended amines can generate bicarbonate
460
at a lower CO2 loading than MEA. MEA delays the tertiary amine producing
461
carbonate and bicarbonate. As well, the MEA-1DMA2P not only generates
462
bicarbonate at a lower CO2 loading than MEA-MDEA and MEA, but also, it
463
generates more bicarbonate than MEA-MDEA at the same CO2 loading. In addition,
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464
less carbamate is generated in MEA-1DMA2P aqueous solution than that in
465
MEA-MDEA aqueous solution, which leads to MEA-1DMA2P needing less energy
466
for solvent regeneration.
467 468 469 470 471 472
ACKNOWLEDGMENTS
473
The support for this work provided by China's State “Project 985” in Hunan
474
University—Novel Technology Research & Development for CO2 Capture is
475
gratefully acknowledged. In addition, we would also like to acknowledge the research
476
support of Natural Sciences and Engineering Research Council of Canada (NSERC),
477
and Canada Foundation for Innovation (CFI).
478 479 480 481 482 483 484 485 486 487 488
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(31) Shi, H.; Naami, A.; Idem, R. O.; Tontiwachwuthikul, P., 1D NMR analysis of a
594
quaternary MEA-DEAB-CO2-H2O amine system: Liquid phase speciation and
595
vapor-liquid equilibria at CO2 absorption and solvent regeneration conditions.
596
Industrial & Engineering Chemistry Research 2014, 53, (20), 8577-8591.
597 598
(32) Holmes, P. E.; Naaz, M.; Poling, B. E., Ion concentrations in the CO2-NH3-H2O system from
13
C NMR spectroscopy. Industrial & engineering chemistry
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research 1998, 37, (8), 3281-3287.
600
(33) Shi, H.; Liang, Z.; Sema, T.; Naami, A.; Usubharatana, P.; Idem, R.; Saiwan, C.;
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Tontiwachwuthikul, P., Part 5a: Solvent chemistry: NMR analysis and studies for
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amine–CO2–H2O systems with vapor–liquid equilibrium modeling for CO2
603
capture processes. Carbon Management 2012, 3, (2), 185-200.
604
(34) Donaldson, T. L.; Nguyen, Y. N., Carbon dioxide reaction kinetics and transport
605
in
aqueous
amine
membranes.
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Fundamentals 1980, 19, (3), 260-266.
Industrial
&
Engineering
Chemistry
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(35) Rayer, A. V.; Sumon, K. Z.; Jaffari, L.; Henni, A., Dissociation Constants (pKa)
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of Tertiary and Cyclic Amines: Structural and Temperature Dependences. Journal
609
of Chemical & Engineering Data 2014, 59, (11), 3805-3813.
610
(36) Song, H.-Y.; Jeon, S.-B.; Jang, S.-Y.; Lee, S.-S.; Kang, S.-K.; Oh, K.-J., Impact
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of speciation on CO2 capture performance using blended absorbent containing
612
ammonia, triethanolamine and 2-amino-2-methyl-1-propanol. Korean Journal of
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Chemical Engineering 2014, 1-9.
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FIGURE CAPTIONS
617
Figure 1. Validation of NMR apparatus for NMR test using 4-diethylamino-2-butanol
618
(DEAB).
619
Figure 2. The concentration of bicarbonate and carbonate plot in 1DMA2P-CO2-H2O
620
system and MDEA-CO2-H2O system at 293.15K.
621
Figure 3a. The effect of MEA on the production of bicarbonate/carbonate in single
622
MDEA system.
623
Figure 3b. The effect of MEA on the production of bicarbonate/carbonate in single
624
1DMA2P system.
625
Figure 4. The effect of tertiary amine on the production of carbamate in MEA.
626
Figure 5. A comparison of the total amounts of bicarbonate ions and carbonate ions
627
obtained from individual single MEA and MDEA amine systems with corresponding
628
ions from the blended MEA-MDEA amine system.
629
Figure 6. Effect of the tertiary amine on the production of bicarbonate in MEA.
630 631 632 633 634 635 636 637
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FIGURES
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640 641 642
Figure 1. Validation of NMR apparatus for NMR test using 4-diethylamino-2-butanol (DEAB).
643 644
0.8 Concentration (mole/L)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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CO32- (1DMA2P, 1M) HCO3- (1DMA2P, 1M) CO32- (MDEA, 1M)
0.6
HCO3- (MDEA, 1M)
0.4
0.2
0.0 0.0 645 646 647
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Loading (mole CO2/mole amine)
Figure 2. The concentration of bicarbonate and carbonate plot in 1DMA2P-CO2-H2O system and MDEA-CO2-H2O system at 293.15K.
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648 1.0 CO32- (MDEA, 1M) Concentration (mole/L)
0.8
0.6
HCO3- (MDEA, 1M) CO32- (5M MEA:1M MDEA, 6M) HCO3- (5M MEA:1M MDEA, 6M)
0.4
0.2
0.0 0.0
0.1
650 651 652
0.2
0.3
0.4
0.5
0.6
Loading (mole CO2/mole amine)
649
Figure 3a. The effect of MEA on the production of bicarbonate/carbonate in single MDEA system. 1.0 CO32- (1DMA2P, 1M) 0.8 Concentration (mole/L)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.6
HCO3- (1DMA2P, 1M) CO32- (5M MEA:1M 1DMA2P, 6M) HCO3- (5M MEA:1M 1DMA2P, 6M)
0.4
0.2
0.0 0.0 653 654 655
0.1
0.2
0.3
0.4
0.5
0.6
Loading (mole CO2/mole amine)
Figure 3b. The effect of MEA on the production of bicarbonate/carbonate in single 1DMA2P system.
656 657 658
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Concentration of carbamate (mole/L)
659
3.0
Blend (5M MEA:1M MDEA, 6M) Blend (5M MEA:1M1DMA2P, 6M) Single amine (MEA, 5M)
2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.1
661
0.2
0.3
0.4
0.5
0.6
Loading (mole CO2/mole amine)
660
Figure 4. The effect of tertiary amine on the production of carbamate in MEA.
662 2.5 CO2, Sum of single amine solutions 3 2 Concentrations (mole/L)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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HCO-3, Sum of single amine solutions CO2, Blended solution 3 HCO-3, Blended solution
1.5
1
0.5
0 0
663 664 665 666
0.1
0.2 0.3 0.4 Loading (mole CO2/mole amine)
0.5
0.6
Figure 5. A comparison of the total amounts of bicarbonate ions and carbonate ions obtained from individual single MEA and MDEA amine systems with corresponding ions from the blended MEA-MDEA amine system.
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1.0 Concentration of bicarbonate (mole/L)
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0.8
0.6
0.4
0.2
0.0 0.0 668 669
Blend (5M MEA:1M MDEA, 6M) Blend (5M MEA:1M 1DMA2P, 6M) Single amine (MEA, 5M)
0.1
0.2
0.3
0.4
0.5
0.6
Loading (mole CO2/mole amine)
Figure 6. Effect of the tertiary amine on the production of bicarbonate in MEA.
670
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