Article pubs.acs.org/EF
Kinetics of Carbon Dioxide Absorption into Mixed Aqueous Solutions of MEA + [Bmim]BF4 Using a Double Stirred Cell Bihong Lu,†,‡ Xiangqian Wang,†,‡ Yinfeng Xia,†,‡ Nan Liu,† Sujing Li,† and Wei Li*,†,‡ †
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute of Industrial Ecology and Environment, Department of Chemical and Biological Engineering, Zhejiang University (Yuquan Campus), Hangzhou 310027, China ‡ Institute of Environmental Engineering, Zhejiang University (Zijingang Campus), Hangzhou 310058, China ABSTRACT: An aqueous blend of monoethanolamine (MEA) and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) for CO2 absorption from simulated flue gas was investigated using a double stirred cell at a CO2 partial pressure of 15 kPa. It was found that the values of the enhancement factor (E) and the second-order reaction rate constant (k2,mix) for CO2 absorption into mixed solution were higher than those into a single MEA aqueous solution with the same MEA concentration. k2,mix and k2, IL were found to be 3487.6 m3·kmol−1·s−1 and 1936.7 m3·kmol−1·s−1 at 303.15 K, respectively. The Arrhenius equation of CO2 absorption was also estimated. The results proved the assumption that [Bmim]BF4 had an active effect on the CO2 hydration. The diffusion and solubility of the absorbent in the solution were the limiting factors of the reaction. work found that CO2 can be absorbed into ionic liquids14−16 and an aqueous blend of ionic liquids.17 Previous studies have been done mainly on the kinetics of the reactions taking place in the ionic liquid18,19 and reactive capture of CO2 by amino functionalized ionic liquids.20,21 A kinetic study on the reactive capture of CO2 in an aqueous blend of amines and ionic liquids has been rarely taken into consideration. Hence, based on the zwitterions mechanism and our previous assumption,12 the kinetics of CO2 absorption into an aqueous blend of MEA and [Bmim]BF4 was investigated using a double stirred cell at a CO2 partial pressure of 15 kPa .
1. INTRODUCTION Carbon dioxide (CO2), as one of the greenhouse gases (GHG), is currently responsible for over 60% of the enhanced greenhouse effect. The emission limits for CO2 in the flue gas from thermal power stations have become increasingly stringent. A wide range of technologies have been developed for CO2 separation and capture from gas streams in recent years, and alkanolamines are the most conventional absorbents.1 Many methods have been proposed to improve the alkanolamines process, and various CO2 capture options are developed to substitute the traditional technologies.2,3 An understanding of the kinetic phenomena of CO2 reaction with amine is essential for effective design. The kinetics of the reactions between CO2 and aqueous solutions containing primary, secondary, tertiary, and hindered amines have been extensively investigated.4−6 The monoethanolamine (MEA)based process is one of the most extensively used techniques for CO2 separation,1 and the reaction of MEA with CO2 has been widely studied by several researchers.7,8 The zwitterion mechanism6,9 is generally accepted as the reaction mechanism between CO2 with primary and secondary amines. For the rational design of gas absorption units, physical properties such as solubility and diffusivity of acid gases in amine solutions are required to establish the model of the absorption rate, and these have been reported in previous studies.10,11 From our previous work, a mixed aqueous solution of monoethanolamine (MEA) and 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF 4 ) was proposed for CO 2 absorption.12 The absorption capacity of the mixed absorbent was significantly higher than pure MEA aqueous, approaching 0.638 mol of CO2 per mole of amine. The mixed absorbent has a higher antioxidant activity and can be easily regenerated, which lends great potential to making up the deficiencies of the aqueous MEA. However, there is a general agreement that CO2 dissolution in ILs is a pure physical phenomenon with a low adsorption rate,13 which does not match our previous work.12 Some other © 2013 American Chemical Society
2. THEORY 2.1. Reaction Mechanism. The zwitterion mechanism, which was originally proposed by Caplow7 and was further introduced to chemical engineering literature by Danckwerts,9 was generally accepted as the reaction mechanism among CO2 with primary and secondary alkanolamines. In the MEA + H2O system, CO2 may react with MEA to form an unstable zwitterion according to this mechanism (here, MEA is denoted as R1R2NH). k 2,MEA
CO2 + R1R 2NH XooooooY R1R 2NH+COO− k −2
(1)
The zwitterion is instantaneously neutralized by any base, Bi: k Bi
R1R 2NH+COO− + Bi → R1R 2NCOO− + BiH+
(2)
where B corresponds to any species (such as amine, OH−, or H2O) in the solution that acts as a base to abstract the proton from the zwitterion. In the MEA + [Bmim]BF4 + H2O system, the reaction of CO2 may be more complex. It is possible that this IL is able to take an extra proton, like a base in eq 2, and the reaction mechanism can be explained as follows: Received: May 23, 2013 Revised: September 6, 2013 Published: September 9, 2013 6002
dx.doi.org/10.1021/ef400976j | Energy Fuels 2013, 27, 6002−6009
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Article k BMEA
R1R 2NH+COO− + R1R 2NH ⎯⎯⎯⎯⎯→ R1R 2NCOO− + R1R 2NH 2+ (3) k BIL
R1R 2NH+COO− + IL ⎯⎯⎯→ R1R 2NCOO− + ILH+
(4)
IL + R1R 2NH 2+ ⇄ R1R 2NH + ILH+
(5)
This mechanism indicates that more R1R2NH exists in the solution, and it can react with more CO2 to produce R1R2NCOO−, which explains the higher absorption capacity in our previous results.12 The reaction of CO2 with MEA follows a second order kinetic. By applying the steady-state principle to the intermediate zwitterion, the reaction rate of CO2 in MEA can be written as rCO2 − MEA =
C MEACCO2 1 k2
+
k −2 k 2 ∑i k BiC Bi
=
k 2,MEACMEACCO2 1+
k −2 ∑i k BiC Bi
Here, kBiCBi is very large and (k−2/(∑ikBiCBi)) can be neglected. Equation 6 can be simplified as follows:
rCO2 − MEA = k 2,MEACMEACCO2
Figure 1. Mechanism of CO2 absorption into MEA + [Bmim]BF4 + H2O system.
(6) 22
2.2. Physicochemical Properties. 2.2.1. Diffusion Coefficient. Because the free molecular diffusivity of CO2 in amine solution cannot be measured directly, the N2O analogy has been frequently used to estimate the diffusivity of CO2 in amine solutions.10,11 The N2O analogy for diffusivity of CO2 in an amine solution can be expressed as follows:
(7)
Meanwhile, many reactions may also take place in the aqueous solution: H 2O ⇄ H+ + OH− ks
CO2 + H 2O ⇄ HCO−3 + H+ k OH−
CO2 + OH− XoooooY HCO−3
(8)
⎛ DCO ,water ⎞ 2 ⎟⎟ DCO2,amine = DN2O,amine⎜⎜ D ⎝ N2O,water ⎠
(9) (10)
The diffusion coefficient of CO2 and N2O in water can be obtained from the following equations:25
The reaction of eq 8 is almost instantaneous and is not the rate limiting step. The reaction rates of CO2 in the aqueous solution can be rewritten as rCO2 − H2O = (ks + k OH−COH−)CCO2
(11)
There is a general agreement that CO2 dissolution in ILs is a physical phenomenon with no chemical reaction.13 However, ILs can be dissolved in the water and hydrolyzed as follows (here, [Bmim]BF4 is denoted as IL): IL + H 2O ⇄ ILH+ + OH−
k 2,IL
(13)
(14)
(22)
2.2.2. Solubility. The solubility of CO2 in amine solution is also estimated by N2O analogy:11,26
(15)
where kov represents the overall reaction rate constant and can be presented as follows: (16)
⎛ HCO ,water ⎞ 2 ⎟⎟ HCO2,amine = HN2O,amine⎜⎜ ⎝ HN2O,water ⎠
(23)
⎛ − 2166 ⎞ ⎟ HN2O,amine = (5.52 + 0.7C) × 106 exp⎜ ⎝ T ⎠
(24)
The solubility of CO2 and N2O in water can be obtained as follows:25
The contribution of eq 9 and eq 10 in water can be neglected.22−24 Then, kov is simplified as
kov = k 2,MEACMEA + k 2,ILC IL
(20)
DCO2,amine∞μ−0.82
roverall = [k 2,MEACMEA + k 2,ILC IL + (ks + k OH−COH−)]CCO2
kov = k 2,MEACMEA + k 2,ILC IL + (ks + k OH−COH−)
⎛ − 2371 ⎞ ⎟ DN2O,water = 5.07 × 10−6 exp⎜ ⎝ T ⎠
⎛ − 2371 − 93.4CMEA ⎞ 2 ⎟ + 2.78 × 10−7C MEA ) × exp⎜ ⎝ ⎠ T (21) The diffusion of CO2 in ionic liquids is slow, and the diffusion coefficient may be 10−1000 times lower than that in organic solvents. As one of the main components in the mixed solution, the presence of [Bmim]BF4 indicated a higher viscosity than that of the pure MEA solution. The viscosity may affect the diffusion coefficient of CO2 in the mixed absorbent and can be calculated as follows:11
The mechanism of CO2 absorption into the MEA + [Bmim]BF4 + H2O system is shown in Figure 1. The overall rate of all the CO2 reactions into mixed aqueous solution is given by the sum of the reaction rates expressed by eq 7, eq 11 and eq 14:
= kovCCO2
(19)
DN2O,MEA = (5.07 × 10−6 + 8.65 × 10−7CMEA
(12)
The hydrolyzation reaction rate of CO2 in IL aqueous solution can be written as rCO2 − IL = k 2,ILC ILCCO2
⎛ − 2119 ⎞ ⎟ DCO2,water = 2.35 × 10−6 exp⎜ ⎝ T ⎠
The diffusion coefficient of N2O in aqueous MEA solution can be described as10
This reaction would enhance the reaction eq 9, and then the hydrolyzation of CO2 in the IL aqueous solution can be expressed as
IL + H 2O + CO2 XooooY ILH+ + HCO3−
(18)
⎛ − 2044 ⎞ ⎟ HCO2,water = 2.8249 × 106 exp⎜ ⎝ T ⎠
(17) 6003
(25)
dx.doi.org/10.1021/ef400976j | Energy Fuels 2013, 27, 6002−6009
Energy & Fuels ⎛ − 2284 ⎞ ⎟ HN2O,water = 8.5470 × 106 exp⎜ ⎝ T ⎠
Article
was adjusted to 1 L·min−1 by using a mass flow controller. A series of experiments were carried out under different concentrations of initial mixed absorbent (1−3 kmol·m−3) and different temperatures (303.15−333.15 K), respectively. The liquid was also input continuously from an accumulator tank to the stirred-cell to ensure a steady state. The CO2 partial pressure was the only evaluation of the kinetic parameters in this process. Since gas and absorbent were both supplied continuously, the CO2 partial pressure would remain unchanged under each condition during the experiment. The inlet and outlet concentrations of CO2 were measured by using gas chromatography (GC-7890, Agilent, USA). All data shown in this paper were the mean values of duplicate or triplicate experiments.
(26)
Few studies about CO2 capture into an aqueous blend of MEA and [Bmim]BF4 have been reported. However, the solubility of N2O into MDEA and [Bmim]BF4 can be used as a reference.17 The effect of [Bmim]BF4 concentration on the solubility of N2O into the aqueous blend of MEA and [Bmim]BF4 can be estimated by using analogy and association. HCO2,mix = − 1068.6C IL + HCO2,amine(303.15 K)
(27)
3. EXPERIMENTAL SECTION
4. RESULTS AND DISCUSSION 4.2. Estimation of kL. According to the mass transfer theory,29,30 the mass transfer rate of CO2 in a distilled water system can be expressed as follows:
3.1. Chemicals. The chemicals employed in this study were similar with those in our previous work.12 Monoethanolamine (99.0%) was purchased from Shanghai Ling Feng Chemical Reagent Co. Ltd., China. 1-Butyl-3-methylimidazolium tetrafluoroborate (99.0%) was provided by Shanghai Cheng Jie Chemical Reagent Co. Ltd., China. All of the other chemicals were analytical grade and commercially available without further purification. 3.2. Viscosity Measurement. The viscosities of MEA aqueous solution and the MEA and [Bmim]BF4 mixture were obtained by using an Ubbelohde viscometer.27 The measurements were taken by using a constant temperature water basin with temperature stability within 0.05 K. 3.3. Experimental Apparatus. A double stirred-cell absorber used in this work has also been used in our previous work.12 The absorber has a defined gas/liquid interface as the mass transfer area, which is convenient for kinetic investigation. The schematic diagram of the absorber is presented in Figure 2, and the liquid−gas interfacial area (A) was found to be 2.88 × 10−3 m2.
N = kL(CCO2, i − CCO2,L)
(28)
Here, CCO2,i is given by the formula: CCO2, i =
PCO2, i HCO2
(29)
where HCO2 is the solubility of CO2 in the solution. Since pure CO2 was used for this investigation, the partial pressure of CO2 (PCO2,i) was equal to the pressure in the bulk gas phase (PCO2). The distilled water was input continuously from the accumulator tank to the stirred-cell absorber, and the concentration of CO2 in the bulk solution (CCO2,L) approached zero. The mass transfer rate of CO2 (eq 28) was kept at a certain value and can be calculated according to N=
P(Q in − Q out) ΔN = AT ART
(30)
where P represents the CO2 pressure in the gas phase (1.01 × 105 Pa) and Q represents the gas flow rate. From eqs 28, 29, and 30, the expression of kL can be simplified: kL =
PCO2·ΔQ CO
2
CCO2, i·ART
=
ΔQ CO
2
HCO2,water·ART
(31)
The stirring speed of the gas phase was adjusted to 250 rpm. The relation between nL and kL was obtained, and the formula was calculated as follows: kL = 2.98 × 10−6 ·nL 0.5739
Figure 2. Schematic diagram of the double stirred-cell absorber. (1) Gas cylinder, (2) mass flowmeter, (3) gas mixer, (4) absorber, (5) the double stirred-cell absorber, (6) impellers, (7) water bath for temperature control, (8) gas chromatography.
(32)
In the following experiments, nL was controlled as 100 rpm, and kL was calculated to be 4.19 × 10−5 m·s−1. 4.2. Estimation of kG. The gas phase mass transfer coefficient (kG) of SO2 absorbed in NaOH solution was measured in our previous work and is expressed as follows:30
3.4. Liquid Phase Mass Transfer Coefficient (kL) Measurement. The liquid phase mass transfer coefficient (kL) in distilled water was determined by using the plotting method28 in the absorber at 303.15 K. A pure CO2 (100 mL·min−1) was absorbed by distilled water under the desired experimental conditions, and the absorptive capacity was measured by using a soap-membrane flowmeter. The gas and liquid were both input continuously into the stirred-cell absorber to ensure a steady state. The stirring speed of the gas phase (nG) was set at 250 rpm, and the stirring speed of liquid phase (nL) was changed within a range of 60−140 rpm, which was the only variable for kL. 3.5. Kinetic Data Measurement. CO2 gas continuously supplied from a commercially available cylinder was mixed with N2 and 15% (V/V) of CO2 to simulate the typical flue gas. The total gas flow rate
k G,SO2 = 5.69 × 10−6(kmol · m−2· s−1· kPa−1) (298.15 K) (33)
An analogy on the basis of the diffusion coefficient was used to estimate the kG,CO2 in this system.31−33 The value of kG,CO2 can be expressed as follows: k G,CO2 6004
⎛ DCO ,N ⎞2/3 2 2 ⎟⎟ = k G,SO2⎜⎜ D ⎝ SO2,N2 ⎠
(34)
dx.doi.org/10.1021/ef400976j | Energy Fuels 2013, 27, 6002−6009
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Table 1. Diffusivity and Solubility of CO2 in MEA + [Bmim]BF4 + H2O T, K
CMEA, kmol·m−3
C[Bmim]BF4, kmol·m−3
μmix/μMEA
DN2O,MEA, 109 m2 ·s−1
DCO2,·MEA, 109 m2 ·s−1
DCO2,·mix, 109 m2 ·s−1
HCO2,·mix, kPa·m3·kmol−1
303.15
1.0 0.9 0.8 0.7 0.6 0.5 0.7 0.7 0.7
0.0 0.1 0.2 0.3 0.4 0.5 0.3 0.3 0.3
1.00 1.08 1.12 1.17 1.18 1.20 1.09 1.06 1.06
1.83 1.85 1.86 1.88 1.90 1.92 2.43 3.09 3.88
1.95 1.97 1.98 2.00 2.02 2.04 2.52 3.12 3.83
1.95 1.85 1.81 1.76 1.77 1.76 2.35 2.99 3.65
3579.2 3460.9 3342.7 3224.5 3106.2 2988.0 3939.7 4774.6 5664.5
313.15 323.15 333.15
Table 2. Kinetic Data for CO2 in different proportions of MEA + [Bmim]BF4 + H2O at 303.15 K P, kPa
CMEA, kmol·m−3
C[Bmim]BF4, kmol·m−3
11.40 11.56 11.65 11.75 11.86 12.00
1.0 0.9 0.8 0.7 0.6 0.5
0.0 0.1 0.2 0.3 0.4 0.5
N × 106, kmol·m−2·s−1
kov, s−1
k2,mix, m3·kmol−1·s−1
± ± ± ± ± ±
3549.2 3160.6 2791.6 2441.3 2070.6 1720.8
3549.2 3511.8 3489.5 3487.6 3451.0 3441.6
8.38 8.05 7.82 7.56 7.31 6.98
0.18 0.10 0.12 0.18 0.21 0.20
The temperature effect on the gas diffusion coefficient can be expressed as follows:34 DG∞T
1.75
N = EkL
(35)
(36)
N = kL′ (CCO2, i − CCO2,L)
(37)
2
2
kL′ kL
1.3 0.7 0.8 1.2 1.3 1.2
HCO2
(41)
(42)
DCO2kov kL
(43)
and E∞ can be measured based on the penetration theory:37 E∞ = 1 +
z ·CamineDamine CCO2DCO2
(44)
4.4. Reaction Kinetics. According to eqs 17, 41, and 43, the mass transfer rate of CO2 in the MEA + [Bmim]BF4 + H2O system is further transformed to
The concentration of CO2 in the interface can be calculated N= =
(39)
Since the mixed solution was input continuously from the accumulator tank to the stirred-cell absorber, CO2 concentration in the bulk liquid (CCO2,L) could be negligible. Based on eqs 37, 38, and 39, the following expression can be obtained: pCO 2 N= H 1 CO2 + k Ek (40) L
± ± ± ± ± ±
PCO2
Ha = E =
as CCO2, i
62.8 57.6 53.8 49.5 45.7 41.5
where the Hatta number Ha is given by
(38)
1 ⎛ N⎞ = ·⎜pCO − ⎟ HCO2 ⎝ 2 kG ⎠
E 1.3 0.7 0.8 1.2 1.3 1.2
3 < Ha <
