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Environmental and Carbon Dioxide Issues
IL-DMEE non-water system for CO2 capture: absorption performance and mechanism investigations Yuning Meng, Xindian Wang, Feng Zhang, Zhibing Zhang, and Youting Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01348 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018
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IL-DMEE non-water system for CO2 capture: absorption performance and mechanism investigations Yuning Meng, Xindian Wang, Feng Zhang*, Zhibing Zhang, Youting Wu
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China KEYWORDS: CO2 absorption; Ionic liquid;
Non-water absorbents;
Mechanism analysis ABSTRACT
Dual amino functionalized ionic liquid tetramethylammonium lysinate ([N1111][Lys]) was synthesized and mixed with 2-[2-(Dimethylamino)ethoxy]ethanol (DMEE) to obtain a series of non-water absorbents for CO2 capture. The density and viscosity of the IL hybrids were measured. Using dual-vessel absorption system, CO2 absorption capacity and rate were investigated over a wide range of IL mass fractions (5-100%) to evaluate the influence of DMEE content and 20% was chosen for further studies including influences of temperature and regeneration performances. Mass transfer coefficients with constant absorption area were calculated from apparent absorption rate constant. FT-IR and 13C NMR spectra were applied to indicate the absorption process and mechanism. This new non-water mixed system was proved to have high absorption capacities and prominent regeneration efficiency under mild conditions (80°C). 1
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1. INTRODUCTION
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Carbon dioxide is regarded as one of the main greenhouse gases. With the development of modern
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industry, the excessive emission of CO2 has caused serious climate issues, therefore, CO2 capture and
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utilization is a hot research area. Various techniques have been developed for the removal of CO2, including
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chemical absorption, physical adsorption, membrane separation, biological fixation, novel nanomaterials1,
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ion-exchange materials2 and so on. Among them, chemical absorption of CO2 with absorbents, especially
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the organic amines and alkanolamines, that contain basic groups, are most commonly applied. Accordingly,
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in the past decades, detailed studies have been successfully performed on CO2 capture with aqueous
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solutions of monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), or
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methyldiethanolamine (MDEA)3,4.
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However, these conventional absorbents have obvious drawbacks. Primary and secondary amines, such as
12
MEA and DEA, can fast absorb CO2 in chemical ways, while the energy cost of regeneration is quite high
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due to strong chemical bonds formed between CO2 and the alkanolamines. Tertiary amine, such as MDEA,
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has lower regeneration cost, but it only act as enhancer of bicarbonate formation, leading to low absorption
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rate5.
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In recent years, CO2 capture with ionic liquids (ILs) have attracted much attention because of their
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superiorities including high thermal and chemical stability, low vapor pressure, and low causticity5,6.
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Additionally, due to their structural designability, functional ionic liquids, mostly based on imidazolium,
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quaternary ammonium and quaternary phosphonium7–11, have been successfully developed for CO2 capture
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by introducing amino groups to the anions and cations.
2
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Proved by many researches12,13, the application of amino acids provides a convenient way to synthesize
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amino based ILs through simple neutral reactions. Moreover, before applied in ILs, structural nature of
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amino acid salts have already showed superiorities in CO2 absorption including low toxicity, low prices and
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high absorption capacities14,15. Therefore, various amino acid based ILs (AAILs) with cations based on
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imidazolium16–18, quaternary ammonium19 and quaternary phosphonium20 were developed and showed
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relatively high absorption capacity and low regeneration temperature, compared to conventional absorbents.
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However, because of the zwitterion mechanism, in most cases, the theoretical absorption capacities of neat
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AAILs with one amino group were limited to 0.5 / .
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Efforts have been made to improve absorption capacity by introducing extra amino group on either cations
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or anions. However, with more complex structures, the viscosities of absorbent can be particularly high (up
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to 700 mPa s, room temperature21), making theoretical absorption capacity difficult to reach. For example,
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Zhimin et al. reported a novel AAIL22 with amino groups on both cation and anion, [aemmim][Tau], the
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absorption capacity of which turned out to be lower than 0.7 / under room temperature, due
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to high viscosity. Similar results were reported by Yanqiang et al. in 200921, who found that highly viscous
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dual-amino ILs [P4443][AA] reached ideal absorption capacity only when supported on porous SiO2 to
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increase surface area. The applications of ILs in current absorption equipment are also limited by high
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viscosity.
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According to the zwitterion mechanism, adding proper solvents to ILs can not only reduce viscosity but
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also improve absorption capacity by taking part in the deprotonation reaction. Aqueous solutions of AAILs
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are the most well-developed mixed absorbent systems. By adding water, the absorption capacities of
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multi-amino ILs including [N66614][Lys]23, [P66614][Lys]23,24, [APmim][Gly]25, [TETAH][Lys]26 and
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[C2NH2MIm][Lys]27 were increased to over 1.2 / . However, high heat capacity of water 3
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inevitably leads to high regeneration energy costs, reflected in either high desorption temperatures (up to
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130°C25–27) or potentially long desorption time23. To overcome the demerits of aqueous systems,
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alkanolamines28,29 as well as non-functionalized low-viscosity ILs30 were tried as non-aqueous solvents. The
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regeneration time and temperature were efficiently reduced to lower than 2 hours and 80°C28–30. In 2016,
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[N1111][Gly] non-water system was investigated by our group and proved to have high absorption capacity
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(up to 1.5 / ) and low desorption energy cost (lower than 80°C, 4h)31.
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In this work, a dual-amino IL, [N1111][Lys] was synthesized as poly-amino compounds and its physical
8
properties were measured. DMEE was selected as solvent to build the non-water CO2 absorption system
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with this IL. Absorption performances including absorption rate, mass transfer coefficient, absorption
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capacity and regeneration efficiency were investigated using dual-vessel absorption system. In addition,
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absorption process and mechanism of [N1111][Lys] were studied with the help of
12
spectra.
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2. MATERIAL AND METHODS
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2.1. Materials and absorbents preparation.
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C NMR and FT-IR
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DMEE (purity>98%), Lysine (purity>98%), and Tetramethylammonium hydroxide pentahydrate
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([N1111][OH], purity>97%) were supplied by Aladdin Co., Ltd. CO2 (purity>99.9%) was provided by
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Nanjing Gas supply Inc. The investigated amino acid based IL [N1111][Lys] was synthesized through
18
one-step method, as described by Jiang32. The synthesized IL was vacuum dried at 40°C for over one week
19
before experiments. To investigate the effect of absorbent proportion on adsorption, a series of weight
20
rations of IL were applied in the IL-DMEE mixed system, including 5%, 10%, 20%, 40%, 60% and 100%.
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2.2. Characterization of physical properties.
2
Densities is required in the calculation of absorption capacity. In this work, the densities of absorbents
3
were measured by METTLER TOLEDO DM40 density meter with a precision of ±0.0001 g/cm3.
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Viscosities were determined with viscometer HAAKE Rheostress 600, with an uncertainty of ±0.1% in
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relation of the full scale. In the investigation of absorption mechanism
6
recorded using Bruker 500 MHz spectrometer and Thermo Scientific Nicolet iS10 respectively.
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2.3. CO2 absorption performance study.
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C NMR and FT-IR spectra were
8 9
Figure 1. Dual-vessel absorption system.
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Dual-vessel absorption system was used to investigate the absorption performance, as illustrated in Figure
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1. The experimental procedure was the same as our previous study31. The mass of blended absorbents used
12
in every single experiment kept 1g. During absorption procedures, the temperature of vessels was kept
13
constant with a thermostatic water bath. Absorption temperature applied was 30°C and it was changed only
14
in temperature experiments. The deviations of temperature and pressure are 0.1 K and 0.1 kPa, respectively.
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The uncertainty of solution concentration is below 0.1%.
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The real-time moles of absorbed CO2 in an absorption experiment can be calculated with Equation (1): 5
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=
1 ( − ( + − +
(1)
1
where , and denote the volumes of storage vessel, absorption vessel and absorbent,
2
respectively. , and stand for the initial pressure in the storage vessel before absorption, the vapor
3
pressure of absorbent and the real-time pressure in the vessels during absorption, respectively. is perfect
4
gas constant and is temperature, all applied in ideal gas state equation to establish relation between
5
pressure and amount of substance.
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Two types of absorption experiments were performed to investigate the absorption performance of the
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hybrids of IL ad DMEE. The first one performed a single absorption process and the real time gas phase
8
pressure was constantly recorded. The calculated moles of CO2 absorbed was illustrated as functions of
9
time, which is called absorption curve. Absorption curves can be used to investigate absorption rate and
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capacity. The other one is saturated absorption experiments in which absorption processes were performed
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continuously for several times under different original pressures. The relation between absorption capacity
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and equilibrium pressure ( can be obtained.
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Regeneration performance was investigated using the same method as the previous study of our group31.
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The regeneration temperatures applied were 60°C, 70°C and 80°C.
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3. RESULTS AND DISSCUSION
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3.1. Physical properties of [N1111][Lys].
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The densities of absorbent solutions with different mass fraction of IL (wt.%) under temperatures ranging
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from 30°C to 60°C are listed in Table 1. Obviously, the higher mass fraction of IL and the lower
19
temperatures, the higher densities absorbents have.
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Table 1. Densities (g ∙ cm ) of IL-DMEE mixture under different temperatures. 6
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IL wt. (%)
T=30°C
T=40°C
T=50°C
T=60°C
5
0.9594
0.9515
0.9435
0.9354
10
0.9668
0.9591
0.9512
0.9432
20
0.9789
0.9713
0.9636
0.9559
40
1.0044
0.9971
0.9898
0.9824
60
1.0353
1.0284
1.0215
1.0144
100
1.086
1.0798
1.0734
1.0670
1
Viscosity significantly affects absorption rate by determining the diffusion of CO2 in the liquid absorbents.
2
Figure 2 illustrates variation of the absorbent viscosities with IL concentration and temperature. It can be
3
observed that when the IL mass fraction is lower than 60%, the exist of DMEE efficiently decreases the total
4
viscosity, which corresponds to the results of absorption rate investigation in Section 3.2.2. 110
IL mass fraction 5% 10% 20% 40% 60% 100%
100 90 80
Viscosity (cP)
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70 60 50 40 30 20 10 0 30
5 6
7
35
40
45
Temperature (
50
55
60
)
Figure 2. Viscosities of IL-DMEE mixed absorbents.
Results of FT-IR and C NMR are illustrated and discussed in Section 3.2.5.
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3.2. CO2 absorption performance investigations.
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3.2.1.
Effect of IL mass fraction on equilibrium time and CO2 absorption capacity.
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The absorption curves of absorbents with different mass fraction of IL at P0=101kPa and T=303K are
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illustrated in Figure 3. CO2 load per mole of IL ( / ) as a function of time was calculated to show the
5
absorption capacity. It can be found that existence of DMEE greatly reduced the time required to reach
6
absorption equilibrium, while increased absorption capacity per mole of IL. In the case with an IL mass
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fraction of 5%, the value of / reached as high as 1.22, indicating that DMEE significantly
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intensified the absorption capacity of [N1111][Lys]. The time required to reach 95% of the equilibrium CO2
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mass loading (t95) was calculated as a measurement of absorption rate. The calculated values are listed in
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Table 2. 1.2 1.0
nCO2/nIL (mol/mol)
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0.8 0.6 0.4 IL mass fraction 5% 10% 20% 40% 60% 100%
0.2 0.0 0
11 12
13 14
20
40
60
80
100
120
Time (min)
Figure 3. Absorption curves of IL-DMEE mixed absorbents with different IL mass fractions. Zwitterion mechanism33 has been well established to describe interactions between CO2 molecules and absorbents with amino groups, as shown in reaction (2)– (3).
−RNH# + CO# ↔ −RNH#' COO 8
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(2)
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−RNH#' COO + B ↔ RNHCOO + BH '
(3)
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Amino group first react with CO2 and form the zwitterion, followed by deprotonation reaction with the
2
existence of bases (denoted as )). Adding of alkaline substance (DMEE in this case) can reduce the amount
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of amino group consumed through reaction (3) by acting as proton accepter, thus increasing absorption
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capacity of IL. Moreover, the existence of DMEE intensifies the diffusion process of CO2 in the liquid
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absorbents thus increases the absorption rate by dramatically dropping the viscosity of IL. It should be noted
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that high mass fraction of DMEE leads to lower CO2 loading per unit mass of absorbent, due to the low
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content of IL ( /*+,-+./ , listed in Table 2).
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Table 2. CO2 absorption capacities and t95 of IL-DMEE mixed absorbents with different IL mass fractions. IL wt. (%)
/
/*+,-+./
t95 (min)
5
1.22
0.28
7.77
10
0.90
0.40
9.13
20
0.76
0.69
20.92
40
0.69
1.27
26.50
60
0.61
1.69
82.30
100
0.39
1.84
110.46
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Figure 4 illustrated the tendency of /*+,-+./ , t95 and viscosity as a function of IL mass fraction.
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Viscosity greatly influences absorption rate, reflected in the very similar tendencies of t95 and viscosity
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curves. The value of t95 kept low when IL mass fraction is lower than 40%, showing an optimized IL content
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range with high absorption rate. As for /*+,-+./ curve, the absorption capacity rises straight with
13
IL content ranging from 5% and 60%. With a further increase of IL concentration, the /*+,-+./
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value raises slightly. Taking both the two parameters into consideration, mass fraction value of 20% was
2
selected and applied in the following absorption experiments (also verified in Section 3.2.2.). 120 nCO2 /mabsorbent t95 Viscosity
1.5
90
t95 (min)
1.2 0.9
90
60 60 30
Viscosity (cP)
1.8 nCO2/mabsorbent (mol/kg)
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 0
0.3
0 0
10
20
30
40
50
60
70
80
90
100
110
IL mass fraction (%)
3 4
Figure 4. Absorption capacity per unit mass of absorbent and t95 as a function of IL mass fraction.
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3.2.2.
6 7
Calculation of apparent absorption rate constant and mass transfer constant.
Based on absorption curves illustrated in Figure 3, further calculations can be performed to quantify the absorption rate. According to the Damping-Film theory34, the mass transfer rate can be described as
0 = 1(2 ∗ − 2
(4)
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where 0 is the mass transfer rate and k is mass transfer coefficient. 2 is the concentration of CO2 in the
9
liquid phase, and 2 ∗ denotes the equilibrium value of 2 . The difference of 2 ∗ and 2 represents the
10
driving force of mass transfer. Using ideal gas state equation, both 2 ∗ and 2 can be calculated from the
11
partial pressure of CO2 in the gas phase, which equals to the gas phase pressure in this case.
− − 0 = 1 5 6 − 7
6
(5)
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where and 6 denote the volume of liquid phase and gas phase, respectively. , and
13
represent the gas phase pressure at the beginning of adsorption, during adsorption and at equilibrium,
14
respectively. is ideal gas constant and denotes temperature. 10
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Equation (5) can be transformed to
0 = 2
16 ( −
(6)
Moreover, the mass transfer rate at the area element can also be described as
0 =
8 1 8 6 = 89 : 89 :
(7)
3
where : is the gas-liquid interface area.
4
Combining Equation (6) and (7) and integrating lead to the following equation:
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The slope of
