ARTICLE pubs.acs.org/IECR
Reaction Kinetics of CO2 in Aqueous Methyland Dimethylmonoethanolamine Solutions Ganeshkumar N. Patil,† Prakash D. Vaidya,† and Eugeny Y. Kenig*,‡ † ‡
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai-400 019, India Faculty of Mechanical Engineering, Chair of Fluid Process Engineering, University of Paderborn, D-33098, Paderborn, Germany
bS Supporting Information ABSTRACT: In the present work, kinetics of the carbon dioxide (CO2) reactions with N-methylmonoethanolamine (MMEA) and N,N-dimethylmonoethanolamine (DMMEA) is investigated by using a stirred-cell reactor. The reaction with the linear secondary amine, MMEA, is described by both the zwitterion and termolecular mechanisms, whereas the reaction with the tertiary amine, DMMEA, is described by the base-catalyzed hydration of CO2. Densities and viscosities of both amines in aqueous solutions are measured at 298, 303, and 308 K over a wide range of amine concentrations. The investigated reactions are of the first order with respect to both CO2 and amine. On the basis of the temperature dependence of the second-order rate constants, the activation energy is evaluated. It is found that the increase in temperature and amine concentration causes the expected increase in the values of the observed reaction rate constants.
1. INTRODUCTION Monoethanolamine (MEA) remains the most popular absorbent for CO2 capture from industrial gaseous streams. A closedloop absorptiondesorption cycle comprising CO2 absorption in a MEA scrubber, regeneration of the CO2-rich solution by desorption and recycle of the lean regenerated MEA solution to the scrubber represents a common process configuration. However, MEA, this highly reactive primary alkanolamine, has several limitations, e.g., low CO2 loading capacity, high solvent regeneration cost, formation of toxic products due to degradation in oxygen-rich atmosphere (for instance, flue gas), reduced scrubbing efficiency due to amine decomposition, and corrosion in the equipment and piping.1,2 Therefore, identification of alternative candidate amines is desirable. Diglycolamine (DGA), diethanolamine (DEA), diisopropanolamine (DIPA) and N-methyldiethanolamine (MDEA) are further industrially important amines, according to Kohl and Nielsen.2 DGA and DIPA have lower steam regeneration requirement than MEA. Although MDEA is far less reactive than MEA, its CO2 loading capacity is higher. At present, much effort is being put on the investigation of the sterically hindered primary amine, 2-amino-2-methyl-1propanol (AMP), which represents the hindered form of MEA. According to Sartori and Savage,3 the CO2 loading capacity and reaction rates of AMP are higher than those for MEA. AMP and DEA are more resistant to solvent degradation and corrosion than MEA; furthermore, according to Aroonwilas and Veawab,4 degradation and corrosion do not appear in the case of MDEA application. Secondary alkanolamines linked to an alkyl group constitute a further class of candidate amines with good potential for gas purification.5 They are more resistant to corrosion than MEA even at high amine concentration; besides, the regeneration energy requirement is lower than that of MEA.6,7 In a previous work,8 we highlighted the efficacy of one such amine— N-ethylmonoethanolamine (EMEA)—for enhanced CO2 r 2011 American Chemical Society
capture. In this work, we investigated kinetics of the CO2 reaction with another solvent belonging to this class of amines, viz. N-methylmonoethanolamine (MMEA). In this linear secondary alkanolamine, a methyl group replaces the hydrogen atom of the amino group in MEA. As evident from previous kinetic studies,913 the presence of the electron-donating methyl group on the nitrogen atom increases the basicity of the amine without significantly increasing the hindrance around the nitrogen atom; consequently, this results in increased reaction rates. It is worthy of note that Sotelo et al.,9 Littel et al.,10 and Mimura et al.11 used indirect techniques (e.g., bubble column and stirred cell) for studying reaction kinetics, whereas Bavbek and Alper12 and Ali et al.13 applied the direct stopped-flow technique. Therefore, there exist some deviations in the estimated kinetic parameters. Ali et al.13 compiled literature data on reaction kinetics; obviously, kinetic data at high amine concentrations are scarce. In this work, we studied kinetics at temperatures and amine concentrations relevant to industrial applications. We considered both the zwitterion and termolecular mechanism for the description of carbamate formation during reaction. We established the values of the reaction order, rate constant and activation energy. The outcome of this investigation will support the design and operation of gas treating plants using MMEA for CO2 capture. N,N-Dimethylmonoethanolamine (DMMEA) is another amine investigated in this work. In this tertiary amine, a methyl group replaces the hydrogen atom of the amino group in MMEA. Special Issue: Nigam Issue Received: March 29, 2011 Revised: May 23, 2011 Accepted: June 1, 2011 Published: June 01, 2011 1592
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There is no hydrogen atom attached to the nitrogen atom, as in case of MEA and MMEA, and thus, the carbamation reaction cannot take place. Instead, DMMEA promotes the CO2 hydrolysis reaction forming bicarbonates. The reaction heat released in bicarbonate formation is lower than that of carbamate formation, thus resulting in lower solvent regeneration costs. Moreover, DMMEA has a high CO2 loading capacity of 1 mol of CO2/mol of amine.14 The reaction with DMMEA may be accelerated by the addition of promoters; alternately, DMMEA may be useful in selective gas sweetening applications. In either case, a detailed knowledge on the reaction kinetics with CO2 is essential. At present, there is just scarce information on the values of the kinetic parameters available,1517 and the kinetic behavior is not well established; thus, a comprehensive study is essential. We described the reaction pathway with DMMEA using the basecatalyzed hydration mechanism.18
2. THEORY In an aqueous amine solution, CO2 may simultaneously react with the amine, OH, and H2O. In our recent papers, we presented comprehensive overviews on the kinetics of the CO2 reaction with amines.5,19 The reaction of CO2 with sterically hindered and unhindered primary and secondary amines can be described either by the two-step zwitterion mechanism (originally proposed by Caplow20 and later reintroduced by Danckwerts21) or by the single-step termolecular mechanism (originally proposed by Crooks and Donnellan22 and recently revisited by da Silva and Svendsen23), whereas the reaction with tertiary amines is governed by the base-catalyzed hydration of CO2.18 2.1. Zwitterion Mechanism. There are a few comprehensive reviews on the zwitterion mechanism available.24,25 Assuming this mechanism, the reaction between CO2 and the amine (here denoted as AmH) proceeds through the formation of a zwitterion as an intermediate: k2 , k1
CO2 þ AmH T AmHþ COO
ð1Þ
formation is rate-determining, eq 3 takes the form r overall ¼ k2 ðCO2 ÞðAmHÞ
thereby suggesting that the reaction is of the first order with respect to both CO2 and amine and, hence, overall of the second order. When zwitterion deprotonation is rate-determining (k1 . ^kB (B)), eq 3 becomes r overall ¼
AmHþ COO þ B sf AmCOO þ BHþ
ð2Þ
Applying the steady-state principle to the intermediate zwitterion in eq 1, the overall rate of reaction of CO 2 in aqueous amine solutions can be expressed in general as r overall ¼
k2 ðCO2 ÞðAmHÞ k1 1þ ^kB ðBÞ
ð3Þ
where the kinetic constant ^kB (B) represents deprotonation of the zwitterion by any base, such as H 2 O, OH, or AmH, or by a combination of bases. Equation 3 does not account for the CO2 reactions with OH and H2 O, whose contributions to the overall rate are assumed to be negligible. The reaction rate represented by eq 3 exhibits a fractional order between one and two with respect to the amine concentration. When deprotonation is almost instantaneous as compared to the reverse reaction in eq 1 (k1 , ^kB (B)) and zwitterion
k2^kB ðBÞ ðCO2 ÞðAmHÞ k1
ð5Þ
Similar to eq 3, the latter expression suggests a fractional reaction order between one and two with respect to the amine concentration. In the limiting case when the contribution of AmH to zwitterion deprotonation is much more significant than that of other bases, such as H2O and OH, the overall reaction is of the second order with respect to AmH. 2.2. Termolecular Mechanism. The termolecular mechanism assumes that the amine reacts simultaneously with one molecule of CO2 and one molecule of a base. The reaction proceeds in a single step via a loosely bound encounter complex as the intermediate (rather than a zwitterion). This can be represented as CO2 þ AmH 3 3 3 B T AmCOO 3 3 3 BHþ
ð6Þ
This complex breaks up to form reactant molecules (CO2 and amine), while its small fraction reacts with a second molecule of the amine or a water molecule to yield ionic products (carbamates). The forward reaction rate for this mechanism, for the case that H2O, OH, and AmH are the dominating bases, is given by r ¼ ½kH2 O ðH2 OÞ þ kOH ðOH Þ þ kAmH ðAmHÞðAmHÞðCO2 Þ
ð7Þ ¼ kobs ðCO2 Þ
ð8Þ
where kobs is expressed by kobs ¼ ½kH2 O ðH2 OÞ þ kOH ðOH Þ þ kAmH ðAmHÞðAmHÞ
This zwitterion undergoes deprotonation by a base (or bases) B, thereby resulting in carbamate formation: ^k B
ð4Þ
ð9Þ Equation 7 suggests that H2O, OH, AmH, and any other bases can influence the reaction in parallel. Its form is similar to that of the limiting case of the zwitterion mechanism represented by eq 5, and it can describe fractional and higher-order kinetics, too.23 In aqueous solutions, deprotonation proceeds mainly via water and the alkanolamine.26 When the solvent (water) is the dominant base, the reaction is of the first order with respect to the amine, and the rate is given by r ¼ kH2 O ðH2 OÞðCO2 ÞðAmHÞ ¼ ^kðCO2 ÞðAmHÞ
ð10Þ
where ^k = kH2O(H2O). When AmH is the most dominant base, the reaction is of the second order with respect to the amine and the rate is given by r ¼ kAmH ðCO2 ÞðAmHÞ2
ð11Þ
For the intermediate case when the contribution of water is comparable to that of AmH, eq 9 can be rewritten as follows: kobs ¼ ½kH2 O ðH2 OÞ þ kAmH ðAmHÞðAmHÞ 1593
ð12Þ
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Table 1. CO2 Absorption Rates into Aqueous MMEA Solutions at 298, 303, and 308 K 106RCO2
(MMEA)0 3
(kmol/m )
PCO2 (kPa)
(kmol/(m s))
298
0.5 1.0
5.9 7.2
3.85 6.33
1.5
6.5
6.78
2.0
7.1
2.5
8.4
308
8.46
0.5
5.7
3.83
13.4
8.27
0.5
19.2
20.0
0.5 1.0
25.7 7.1
25.3 6.33
1.0
13.2
13.6
1.0
19.8
22.0
1.0
28.1
29.7
1.5
7.5
1.5
13.2
16.6
1.5
18.1
21.2
1.5 2.0
27.2 7.1
33.3 9.57
2.0
11.5
14.6
2.0
20.3
24.1
2.0
27.1
35.8
2.5
7.1
11.6
2.5
9.7
17.1
2.5
19.4
33.1
2.5 0.5
27 4.2
45.1 3.83
1.0
4.6
5.43
1.5
4.5
6.56
2.0
6.0
9.54
2.5
4.5
8.46
AmHCOO þ H2 O f AmH2 þ þ HCO3
ð16Þ
3. EXPERIMENTAL SECTION
8.90
It is clear from eq 9 that the number of fitting parameters in the termolecular mechanism is fewer than that in the zwitterion mechanism. 2.3. Base-Catalyzed Hydration Mechanism. Donaldson and Nguyen18 suggested that tertiary alkanolamines cannot react directly with CO2. Such amines have a base-catalytic effect on the hydration of CO2. This can be represented as AmH þ H2 O þ CO2 sf AmH2 þ þ HCO3
ð13Þ
Besides, contrary to the suggestion of Donaldson and Nguyen,18 a direct reaction between CO2 and tertiary amines still may occur at extremely high pH, thereby resulting in monoalkylcarbonate formation.27 However, at pH values lower than 12, the rate of this reaction is negligible.28 When the contributions of the CO2 reactions with OH and H2O to the reaction rate can also be neglected, the following expression holds: r overall ¼ k0 ðAmHÞðCO2 Þ
ð15Þ
Equation 15 represents a reaction of the amine with CO2 to form an unstable complex. Equation 16 describes the homogeneous hydrolysis reaction in which water reacts with the zwitterion-type complex to yield a bicarbonate.
10.8
0.5
k0
AmH þ CO2 T AmHCOO
2
temp (K)
303
The base-catalysis reaction can also be explained by a zwitterion-type mechanism earlier proposed by Yu and Astarita:29
ð14Þ
The form of eq 14 is similar to that of the limiting case of the zwitterion mechanism represented by eq 4.
3.1. Materials. N-Methylmonoethanolamine (purity 98%) and N,N-dimethylmonoethanolamine (purity 98%) used in experiments were purchased from S. D. Fine Chemicals Ltd., Mumbai, India. Carbon dioxide, nitrous oxide, and nitrogen cylinders with a given purity of 99.95% were purchased from Inox Air Products Ltd., Mumbai, India. 3.2. Experimental Setup. A glass stirred-cell reactor (design pressure 202.6 kPa) with a plane, horizontal gasliquid interface was used for the absorption studies (see Vaidya and Mahajani30). The main advantage of the stirred cell is that the rates of absorption can be measured using a liquid with a single, known composition. This easy-to-use experimental device (inner diameter 97 mm, height 187 mm) was operated batchwise. The total volume of the reactor was 1.45 dm3 and the interfacial surface area was 7.5 103 m2. The reactor was equipped with a flange made of stainless steel (Sharad Autoclave Engineers, Mumbai, India). A pressure transducer (Trans Instruments), mounted on this flange and coupled with a data acquisition system, enabled measurement of the total pressure inside the reactor, the uncertainty in this measurement being (1 mbar. The lower and higher limits of the pressure reading in the transducer were 0 and 1 bar, respectively. The reactor was also equipped with inlet and outlet ports for the gas and liquid phases. The entire assembly was proven to have no leak. The setup was supplied by a variable speed magnetic drive (Premex Instruments, Switzerland). The gas and liquid were stirred by two impellers, mounted on the same shaft. The speed of stirring could be adjusted to the desired value with an accuracy of (1 rpm. The impeller speed during kinetic measurements was limited to 90 rpm, to ensure that the gasliquid interface was undisturbed. This results in a geometrically simple and hence exactly known interfacial area. The reactor was immersed in a water bath to guarantee isothermal conditions. The temperature was adjusted to the desired value with an accuracy of (0.1 °C. The setup was supplied by a coil, which was kept inside the water bath. After passing through this coil, the solute gas entered the reactor. 3.3. Experimental Procedure. A series of experiments were conducted over a wide range of temperatures and amine concentrations. In each experiment, the reactor was charged with 0.4 dm3 of the fresh amine solution. The gas inside the reactor was then purged with nitrogen to ensure an inert atmosphere. Thereafter, nitrogen was released through the gas outlet port and all the lines were closed. After the temperature had been adjusted to the desired value, CO2 from the gas cylinder was charged inside the reactor. This was considered to be the starting point for the reaction. The reactor content was stirred at the desired speed of agitation. The decrease in system pressure due to reaction was monitored by the pressure transducer, and the “PCO2 vs t” data were recorded during 30 s using the data acquisition system. 1594
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Table 3. Reaction Rate Constant for the CO2MMEAH2O System at 303 K temp (MMEA)0 rate constant k2 order with experimental (K) (kmol/m3) (m3/(kmol s)) (MMEA)
technique
ref
293 313
00.1 00.1
5602 12103
1 1
bubble column
Sotelo et al.9
293
03.0
5340
1
stirred-cell
Littel et al.10
318
03.0
21800
1
298
0.92.5
7940
1
stirred-cell
Mimura et al.11
283
00.04
2350
1
stopped-flow Ali et al.13
288
00.04
3190
1
298
00.04
5010
1
308 298
00.04 0.52.5
7100 6425
1 1.2
303
0.52.5
8580
1.2
308
0.52.5
12864
1.2
stirred-cell
this work
Figure 1. Plot of log{RCO2/[(CO2)(DCO2)1/2]} vs log (MMEA)0 at 303 K.
Table 2. Equilibrium and Kinetic Characteristics of the CO2MMEAH2O System at 303 K (MMEA)0 3
106RCO2
104HCO2
(kmol/
(kmol/
2
109DCO2
(m kPa)) (m2/s) (m3/(kmol s))
Ha
Ei
4675 5024
70 104
148 254
1.95
5720
140
346
1.86
6610
180
520
1.78
8580
234
668
(kmol/m )
(m s))
0.5 1.0
3.83 6.33
2.99 2.78
2.18 2.04
1.5
8.90
2.90
2.0
9.57
2.72 2.64
2.5
11.6
k2
3
These data were plotted for the time interval between t = 5 s and t = 25 s and fitted to a third-degree polynomial using the leastsquares regression. The absorption rates were calculated from the values of the slope dPCO2/dt. This measurement method based on the fall-in-pressure technique enabled a simple and straightforward estimation of the absorption rates. Furthermore, no analysis of the liquid phase was required and the pressure decrease was the only factor necessary for the evaluation of the kinetic parameters. We found that, in the range of agitation speeds studied, the mass transfer rate was independent of the gasside mass transfer coefficient, kG. Therefore, we concluded that the CO2 absorption process was liquid-phase-controlled. The reproducibility of experiments was checked and the error in all experimental measurements was found to be less than 3%.
4. RESULTS AND DISCUSSION 4.1. Estimation of Physical Properties. Knowledge of physical properties is essential for the estimation of kinetic parameters. We measured the density (F) and viscosity (μ) of aqueous MMEA and DMMEA solutions at 298, 303, and 308 K, and these values are presented in the Supporting
Figure 2. Plots of kobs vs (MMEA)0 at 298, 303, and 308 K.
Information. The diffusion coefficients of N2O and CO2 in water, viz. 2.03 109 and 2.15 109 m2/s at 303 K, were earlier reported by Versteeg and van Swaaij.31 From our viscosity measurements, we estimated the values of the N2O diffusivity in the aqueous amine solutions by using the modified Stokes Einstein correlation: ðDN2 O μ0:80 ÞAmine ¼ const ¼ ðDN2 O μ0:80 ÞWater
ð17Þ
We measured the solubility of N2O in aqueous MMEA and DMMEA solutions, too (Supporting Information). The values of DCO2 and HCO2 in solutions were found using the N2O analogy.31 The physical absorption of CO2 in water at 303 K was studied, and it was found that the HCO2 value (2.75 104 kmol/ (m3 kPa)) agrees well with the value published by Versteeg and van Swaaij.31 Littel et al.32 earlier outlined a procedure for the estimation of the liquid-side mass-transfer coefficient, kL, in a 1595
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is given by
Table 4. CO2 Absorption Rates into Aqueous DMMEA Solutions at 298, 303, and 308 K 10 RCO2
(MMEA)0 temp (K)
(kmol/m3)
PCO2 (kPa)
(kmol/(m2 s))
298
1.5 2.0
6.8 7.2
3.84 5.03
2.5
3.6
2.88
3.0
4.7
4.24
3.5
4.6
4.52
1.5
5.2
3.33
1.5
9.6
4.72
1.5
16.2
7.50
1.5 2.0
21.1 6.2
9.89 4.55
2.0
10.4
5.55
2.0
16.4
2.0
23.8
2.5
5.9
2.5
9.3
2.5
19.7
10.3
2.5 3.0
29.1 5.8
13.6 4.94
3.0
16.9
10.0
3.0
22.0
13.9
3.0
30.6
18.2
3.5
5.8
3.5
17.0
13.9
3.5
24.5
18.9
3.5 1.5
31.2 6.2
22.5 4.10
2.0
6.1
4.70
2.5
7.5
6.50
3.0
6.2
6.01
3.5
5.7
5.46
303
308
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DCO2 km, n ðCO2 Þm 1 ðAmHÞn mþ1 Ha ¼ kL
7
where km,n denotes the reaction rate constant. The enhancement factor for an instantaneous reaction is given by ðAmHÞ DAmH Ei ¼ 1 þ ð21Þ zðCO2 Þ DCO2 It is worthy of note that eq 21 is valid only if the film theory is used. Using eqs 18 and 20, the rate of absorption in aqueous amine solutions can be expressed as rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DCO2 km, n ðCO2 Þm þ 1 ðAmHÞn ð22Þ R CO2 ¼ mþ1
8.55
There is unanimous agreement among all researchers that the reaction order with respect to CO2 is 1.24,25 Therefore, eq 22 can be rewritten as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R CO2 ¼ ðCO2 Þ DCO2 k1, n ðAmHÞn ð23Þ
11.9 4.67 5.55
where (CO2) = HCO2PCO2. Equation 23 can be expressed in the following form: ( ) R CO2 1 n logðk1, n Þ þ logðAmHÞ pffiffiffiffiffiffiffiffiffiffi ¼ log 2 2 ðCO2 Þ DCO2
5.05
ð24Þ
stirred cell reactor. We used this technique and found that the value of kL (0.0037 cm/s) is in line with those typical for stirred cell reactors. 4.2. Study of Reaction Kinetics. When CO2 concentration in the bulk liquid is negligible and the resistance to mass transfer is entirely in the liquid phase, it can be shown, on the basis of the two-film theory of mass transfer,33 that the following relation holds: R CO2 ¼ kL ðCO2 ÞE
ð18Þ
where the enhancement factor E is used to describe the enhancing effect of chemical reactions on mass transport. To study the reaction kinetics, it is essential that the system belongs to the fast reaction regime, where E equals the Hatta number.33,34 The necessary conditions for the fast reaction regime are 10 < Ha , ðEi 1Þ
ð20Þ
ð19Þ
For an irreversible reaction of mth order with respect to CO2 and nth order with respect to the amine, the Hatta number
If the variation in RCO2 with (AmH) is studied, a plot of log{RCO2/[(CO2)(DCO2)1/2]} vs log (AmH) enables the estimation of the value of k1,n and n. The use of such plots to investigate CO2 reaction kinetics is not uncommon.35,36 4.3. CO2MMEAH2O System. We studied the CO2 reaction with MMEA over the ranges of temperatures, 298308 K, and amine concentrations, 0.52.5 M. The CO2 absorption rates into aqueous MMEA solutions at various temperatures are presented in Table 1. It is clear that the increase in CO2 partial pressure and initial MMEA concentration cause the expected increase in the CO2 absorption rates. In the fast reaction regime, the rate of absorption is independent of the liquid-side mass transfer coefficient and hence it should not depend on the agitation speed. We studied this effect experimentally and found practically no change in the absorption rate, while varying the stirring speed in the range 4080 rpm at 308 K. Hence, it can be concluded that the CO2MMEAH2O system belongs to the fast reaction regime systems. All further experiments were conducted at a speed of 60 rpm. A plot of log{RCO2/[(CO2)(DCO21/2)]} vs log (MMEA)0 at 303 K is shown in Figure 1. The slope equals 0.6, thereby suggesting that the reaction order with respect to MMEA (n) is 1.2. Similar values were observed at 298 and 308 K. Thus, the reaction order is close to 1, which is consistent with previous works.911,13 At 303 K and 2.5 M MMEA concentration, the second-order rate constant was found to be 8580 m3/(kmol s). Using the values of DCO2 and HCO2 (Table 2), the parameters Ha and Ei were estimated. The conditions given by eq 19 were satisfied, thereby confirming that this system belongs to the fast reaction regime. From the temperature dependence of k2, the activation energy was found to be 53 kJ/mol. A comparison of the rate constant 1596
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Table 5. Equilibrium and Kinetic Characteristics of the CO2DMMEAH2O System at 303 K (DMMEA)0 3
107RCO2
104HCO2
(kmol/
(kmol/
2
109DCO2
k2
3
(m kPa)) (m2/s) (m3/(kmol s)) Ha
Ei
(kmol/m )
(m s))
1.5
3.33
2.83
1.51
22.6
9 1019
2.0
4.55
2.50
1.34
32.0
12 1287
2.5
4.67
2.33
1.12
42.0
15 1828
3.0
4.94
2.12
1.00
54.3
18 2445
3.5
5.05
2.01
0.89
60.3
21 3007
Table 6. Reaction Rate Constant for the CO2DMMEA H2O System at 303 K temp (DMMEA)0
rate
order
constant k2
with
experimental
(K) (kmol/m3) (m3/(kmol s)) (DMMEA) technique 293
02.2
7.5
1.0
303
02.2
23.0
1.0
318
02.2
55.0
1.0
333
02.2
120.0
1.0
298
0.31.0
27.4
1.04
303
0.31.0
39.5
1.02
308
0.31.0
56
0.97
313
0.31.0
77.8
0.96
298 303
1.53.5 1.53.5
34.0 42.0
1.0 1.0
308
1.53.5
56.2
1.0
ref
stirred-cell
Versteeg and
stirred-cell
van Swaaij15 Littel et al.16
stopped-flow Henni et al.17
stirred-cell
this work
with other values earlier reported in the literature at different temperatures is given in Table 3. Our value of k2 at 298 K is lower than that reported by Mimura et al.,11 despite that the stirred-cell device is used in both investigations. Some reasons for the deviation in the rate constant, even for identical conditions, are uncertainties in values of the physical properties used, the existence of interfacial turbulence in the absorber, lack of knowledge on the exact gasliquid interfacial area and the assumption of a pseudo first-order reaction. We measured the values of kobs at 298, 303, and 308 K; these are presented in Figure 2. The expected trend of increasing kobs values with rising temperature and concentration was observed. If the two-step zwitterion mechanism is appropriate, eq 3 can be rewritten as r overall
k2 ðCO2 ÞðMMEAÞ ¼ k1 1þ ^ ^ ½kMMEA ðMMEAÞ þ kH2 O ðH2 OÞ þ ^kOH ðOH Þ ð25Þ
and the observed reaction rate constant, kobs, could be expressed as kobs
Figure 3. Arrhenius plot for the CO2DMMEAH2O system.
k2 ðMMEAÞ ¼ k1 1þ ^ ^ ½kMMEA ðMMEAÞ þ kH2 O ðH2 OÞ þ ^kOH ðOH Þ ð26Þ
Figure 4. Plots of kobs vs (DMMEA)0 at 298, 303, and 308 K.
Bavbek and Alper,12 who reported a fractional order between 1 and 2 (1.56) using a stopped-flow technique, considered both zwitterion formation and deprotonation steps to be important. In our study, it was found that the dependence of reaction rate on the amine concentration is of the first order. In this case, it is obvious that k1 , ^kB (B) and zwitterion formation is ratedetermining, whereas eq 26 takes the form kobs ¼ k2 ðMMEAÞ
ð27Þ
Interestingly, Ali et al.13 found that water stabilizes and predominantly deprotonates the zwitterion at low amine concentrations. If the termolecular model were appropriate, it would be obvious that H2O is the dominant base (cf. eq 10). Therefore, 1597
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stirred-cell reactor. The reaction pathway with MMEA was represented equally well by both the zwitterion and termolecular mechanism, whereas the reaction with DMMEA was governed by the base-catalyzed hydration of CO2. The CO2 solubility and diffusivity in aqueous MMEA and DMMEA solutions were estimated using the N2O analogy. The estimation procedure for the kinetic parameters was outlined. It was found that the investigated reactions are of the first order with respect to both CO2 and amine. At 303 K, the second-order rate constant for the reactions with MMEA and DMMEA were equal to 8580 and 42 m3/(kmol s), respectively; the corresponding activation energies were 53 and 38.3 kJ/mol. The observed reaction rate constants were measured at 298, 303, and 308 K. The value of kL (0.0037 cm/s) estimated in this work was found to be in line with those typical for stirred cell reactors. Figure 5. Comparison of the kobs values of DMMEA, MMEA, and AEEA at 303 K.
kobs can be expressed as kobs ¼ ^kðMMEAÞ
ð28Þ
Thus, the CO2MMEA reaction system can be represented equally well by both the zwitterion and the termolecular mechanism. 4.4. CO2DMMEA Reaction Mechanism. In this work, the CO2 reaction with DMMEA was studied over the range of temperatures 298308 K and amine concentrations 1.53.5 M. The CO2 absorption rates in aqueous DMMEA solutions are presented in Table 4. As expected, the absorption rates are much lower than those in aqueous MMEA solutions. The values of density, viscosity, DN2O, and HN2O for aqueous DMMEA solutions are given in the Supporting Information. We found that the CO2DMMEAH2O system belongs to the fast pseudo firstorder reaction systems. Therefore, the specific rate of absorption was expressed as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð29Þ R CO2 ¼ ðCO2 Þ DCO2 k2 ðDMMEAÞ0 Using the values of DCO2 and HCO2 (Table 5), k2 was estimated. At 303 K and 2.5 M DMMEA concentration, the value of k2 was found to be 42 m3/(kmol s), which is lower than that for another tertiary amine, N,N-diethyl monoethanolamine (DEMEA).37 From the values of Ha and Ei, it is clear that this system also belongs to the fast reaction regime. Rate constants reported in this work are in good agreement with those published by Henni et al.17 (Table 6). From the temperature dependence of k2 shown in Figure 3, the activation energy was found to be 38.3 kJ/mol. The plots of kobs vs (DMMEA)0 at 298, 303, and 308 K are shown in Figure 4. The value of kobs increases together with the amine concentration at all temperatures. Recently, we investigated CO2 reaction kinetics in aqueous N-(2-aminoethyl) ethanolamine (AEEA) solutions.36 A comparison of the kobs values of DMMEA, MMEA, and AEEA at 303 K is shown in Figure 5.
5. CONCLUSIONS MMEA, which has high reactivity, and DMMEA, which has high CO2 loading capacity, represent interesting alternative candidate solvents for CO2 capture. In this work, the kinetics of the CO2 reactions with these amines was investigated using a
’ ASSOCIATED CONTENT
bS
Supporting Information. Physical properties of aqueous MMEA and DMMEA solutions at 298, 303, and 308 K. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ49 5251 60 2408. Fax: þ49 5251 60 3522. E-mail:
[email protected].
’ ACKNOWLEDGMENT G.N.P. is grateful to University Grants Commission, New Delhi, for the financial assistance. ’ NOMENCLATURE AmH = alkanolamine (AmH) = alkanolamine concentration, kmol/m3 B = base assisting in zwitterion deprotonation (B) = concentration of base B in liquid, kmol/m3 (CO2) = concentration of CO2, kmol/m3 DAmH = diffusivity of amine in liquid phase, m2/s DCO2 = diffusivity of CO2 in liquid phase, m2/s DN2O = diffusivity of N2O in liquid phase, m2/s (DMMEA)0 = initial DMMEA concentration, kmol/m3 E = enhancement factor due to chemical reaction Ei = enhancement factor for an instantaneous reaction HCO2 = Henry’s law constant, kmol/(m3 kPa) Ha = Hatta number (H2O) = concentration of water, kmol/m3 k0 = rate constant in eq 13 ^k = rate constant in eq 10 k2 = forward reaction rate constant in eq 1, m3/(kmol s) k1 = backward reaction rate constant in eq 1, 1/s kAmH = deprotonation constant for amine ^kB = reaction rate constant in eq 3 kH2O = forward reaction rate constant for the CO2 reaction with H2 O kL = liquid-side mass transfer coefficient, m/s km,n = reaction rate constant for m,nth order reaction kobs = observed reaction rate constant, 1/s kOH = forward reaction rate constant for the CO2 reaction with OH 1598
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Industrial & Engineering Chemistry Research (MMEA) = MMEA concentration, kmol/m3 (MMEA)0 = initial MMEA concentration, kmol/m3 m = reaction order with respect to CO2 n = reaction order with respect to amine (OH) = hydroxyl ion concentration, kmol/m3 PCO2 = partial pressure of CO2 in bulk gas phase, kPa r = rate of reaction RCO2 = specific rate of absorption of CO2, kmol/(m2 s) roverall = overall reaction rate in aqueous solutions, kmol/(m3 s) t = time, s z = stoichiometric coefficient Greek Symbols
F = density, kg/m3 μ = viscosity, kg/(m s) Abbreviations
AEEA = N-(2-aminoethyl)ethanolamine AMP = 2-amino-2-methyl-1-propanol DEA = diethanolamine DEMEA = N,N-diethylmonoethanolamine DGA = diglycolamine DIPA = diisopropanolamine DMMEA = N,N-dimethylmonoethanolamine EMEA = N-ethylmonoethanolamine MDEA = N-methyldiethanolamine MEA = monoethanolamine MMEA = N-methylmonoethanolamine
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