Ind. Eng. Chem. Res. 2010, 49, 11067–11072
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Kinetics of Carbon Dioxide Removal by Aqueous Alkaline Amino Acid Salts Prakash D. Vaidya, Prashanti Konduru, and Mukanth Vaidyanathan Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India
Eugeny Y. Kenig* Chair of Fluid Process Engineering, Faculty of Mechanical Engineering, UniVersity of Paderborn, D-33098, Paderborn, Germany
Aqueous solutions containing alkaline salts of carboxylic or sulfonic amino acids represent candidate solvents with good potential for carbon dioxide (CO2) capture. In the present work, the CO2 reactions with potassium salts of glycine (aminoacetic acid) and taurine (2-aminoethanesulfonic acid) in aqueous solutions are investigated using a stirred-cell reactor. The reaction pathways are comprehensively described using the zwitterion and the termolecular mechanism. The investigated reactions belong to the fast pseudo-first-order reaction regime systems. The second-order rate constant for the CO2 reaction with potassium glycinate is determined, and its value at 303 K is evaluated to be 6.29 m3/(mol s). The liquid-side mass-transfer coefficient is estimated, and its value (0.006 cm/s) is consistent with those typical for stirred-cell reactors. Finally, it is determined that potassium glycinate promotes the activity of tertiary amines (e.g., N,N-diethylethanolamine). 1. Introduction Carbon dioxide (CO2) is a major greenhouse gas responsible for global warming, and, hence, much effort is being put on the development of technologies for its capture from process gas streams. Currently, several different CO2 separation technologies are available; here, absorption performed with chemical solvents represents the most feasible option.1 In such operations, alkanolamine-based absorbents and their blends are applied extensively. Along with amines, carbonate-bicarbonate buffers also are used in bulk CO2 removal. Aqueous solutions containing salts of amino acids represent further candidate solvents with good potential for CO2 capture.2 They are either used individually or added as promoters to carbonate solutions. Because of their ionic nature, these salt solutions exhibit low volatility, high surface tension, and increased resistance to oxidative degradation.3,4 Their reactivity and CO2 absorption capacity are comparable to those of aqueous amines of the related classes.5,6 Examples of amino acids of commercial interest are given by glycine (Giammarco-Vetrocoke), alanine, and diethyl or dimethyl glycine (Alkacid, BASF). In the present work, the CO2 reactions with potassium salts of glycine (or aminoacetic acid) and taurine (or 2-aminoethanesulfonic acid) in aqueous solutions are investigated. The reaction with potassium glycinate (PG) was earlier studied by Jensen et al.7 at 291 K, by Caplow8 at 283 K, by Penny and Ritter6 over a temperature range of 278-303 K, and, more recently, by Kumar et al.3 at 295 K and by Portugal et al.4 at 293-303 K. However, the agreement between the reaction rate constants estimated in these works is poor. By now, there exists just scarce information on the reaction with potassium taurate (PT) available in the literature; a recent study by Kumar et al.3 at 285 and 305 K is the only reported investigation on reaction kinetics. Thus, a comprehensive study on the reaction mechanism and kinetics appears desirable. N,N-Diethylethanolamine (DEEA), which can be prepared from renewable resources, represents a promising solvent for CO2 capture.9 Because of its tertiary amine characteristics, * To whom correspondence should be addressed. Tel.: +49 5251 60 2408. Fax: +49 5251 60 3522. E-mail:
[email protected].
DEEA has low reactivity, with respect to CO2. Therefore, finding absorption activators to promote the reaction with DEEA is essential. PG was selected as a possible activator for this study, and the absorption of CO2 into aqueous mixtures of DEEA and PG was investigated. 2. Theory In an aqueous salt solution, CO2 may simultaneously react with the amino acid salt, OH-, and H2O. The amino acids dissolved in water exist as zwitterions that have the completely protonated amino group.3,10 The ionic equilibrium of the amino acids (viz, glycine and taurine) is as follows: HOOC-CH2-NH3+ T H+ + -OOC-CH2-NH3+ T 2H+ + -OOC-CH2-NH2
(1)
HO3S-C2H4-NH3+ T H+ + -O3S-C2H4-NH3+ T 2H+ + -O3S-C2H4-NH2
(2)
The addition of an alkali (e.g., potassium hydroxide) to an aqueous amino acid solution results in the deprotonation of the zwitterion into the deprotonated amino acid salt solution (KOOC-CH2-NH2 or KO3S-C2H4-NH2). Consequently, this results in high reactivity with CO2. A detailed knowledge of the CO2 reaction mechanism for such solvents is essential for a fundamental understanding of the reaction systems. Since the functional groups in amino acids are similar to those in amines of the related classes, it is expected that the reaction mechanisms are similar.3,4,10 In a recent paper, Vaidya and Kenig11 presented a comprehensive overview on the kinetics of the CO2 reaction with amines. Generally, the reaction kinetics can be described by either the two-step zwitterion mechanism (originally proposed by Caplow12 and later reintroduced by Danckwerts13) or by the single-step termolecular mechanism (originally proposed by Crooks and Donellan14 and recently revisited by da Silva and Svendsen15). In the following sections, these mechanisms will be extended to aqueous amino acid salt solutions.
10.1021/ie100224f 2010 American Chemical Society Published on Web 06/21/2010
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The CO2 reactions with OH- and H2O can be represented as follows: -
1+
kOH-
-
CO2 + OH 798 HCO3 kH
2O
(3)
+
(4)
The rates of the reactions described by eqs 3 and 4 are given by: r ) kOH-(CO2)(OH-)
(5)
-
r ) kH2O(CO2)(OH )
k1, k-1
CO2 + RNH2 798 RNH+ 2 COO
(7)
This zwitterion undergoes deprotonation by a base (or bases) B, thereby resulting in carbamate formation: kB
-
RNH2 COO + B 98 RNHCOO + BH
+
(8)
Applying the steady-state principle to the intermediate zwitterion in eq 7, the rate of reaction of CO2 in aqueous amino acid salt solutions can be generally expressed as r)
k1(CO2)(RNH2) k-1 1+ kB(B)
(9)
where the kinetic constant kB(B) represents deprotonation of the zwitterion by either any base (such as H2O, OH-, or the amino acid salt), or by a combination of bases, and (RNH2) denotes the salt concentration. The overall rate of all CO2 reactions in an aqueous salt solution is given by the sum of eqs 5, 6, and 9: roverall )
k1(CO2)(RNH2) + {[kOH-(OH-) + k-1 1+ kB(B) kH2O(H2O)](CO2)}
(10)
The reaction represented by eq 4 is very slow, compared to the other reactions. When the contribution of the reaction given by eq 3 to the overall rate is negligible (as was suggested by Kumar et al.3 for aqueous PG solutions), the overall rate is given by eq 9: roverall )
k1(CO2)(RNH2) k-1 1+ kB(B)
(13)
where kobs denotes the observed reaction rate constant, which can be measured and is expressed by kobs ) 1+
(
k1(RNH2) k-1 kRNH2(RNH2) + kOH-(OH-) + kH2O(H2O)
)
(14)
The reaction rate given by eq 11 exhibits a fractional order between 1 and 2, with respect to the salt concentration. When deprotonation is almost instantaneous, compared to the reverse reaction in eq 7 (k-1 , kB(B)) and zwitterion formation is ratedetermining, eq 11 takes the form roverall ) k1(CO2)(RNH2)
(15)
thereby suggesting that the reaction is first order, with respect to both CO2 and salt, and hence, the overall reaction is second order. When zwitterion deprotonation is rate-determining (k-1 . kB(B)), eq 11 takes the form roverall )
k1kB(B) (CO2)(RNH2) k-1
(16)
Similar to eq 11, the latter expression suggests a fractional reaction order between 1 and 2, with respect to the salt concentration. In the limiting case, when the contribution of the salt to zwitterion deprotonation is much more significant than that of other bases, such as H2O and OH-, the overall reaction is second order, with respect to the salt. 2.2. Termolecular Mechanism. This mechanism, which was earlier used by Kumar et al.3 to describe the CO2 reaction kinetics with aqueous solutions of PG and PT, assumes that the amino acid salt 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 complex dissociates to form reactant molecules (CO2 and salt), while its small fraction reacts with a second molecule of the amino acid salt or a water molecule to give ionic products (carbamate). This can be represented as CO2 + RNH2 + B T [Encounter complex] f RNHCOO- + BH+
(17)
The forward reaction rate for this mechanism, for the case where H2O, OH-, and RNH2 are the dominating bases, is given by r ) {kRNH2(RNH2) + kOH-(OH-) + kH2O(H2O)}(RNH2)(CO2) (18) ) kobs(CO2)
(11)
When RNH2, H2O, and OH- are the dominant bases that contribute to zwitterion deprotonation, the overall rate is represented as
)
kRNH2(RNH2) + kOH-(OH-) + kH2O(H2O) (12)
(6)
2.1. Zwitterion Mechanism. A few complete reviews on the zwitterion mechanism are available.16,17 Assuming this mechanism, the reaction between CO2 and the carboxylic acid salt (viz, PG or PT) proceeds through the formation of a zwitterion as an intermediate (here, the salt is denoted as RNH2, where R equals +K-OOC-CH2 for PG and +K-O3S-C2H4 for PT):
-
k1(CO2)(RNH2) k-1
) kobs(CO2) -
CO2 + H2O 798 HCO3 + H
+
(
roverall )
(19)
where kobs is given by kobs ) {kRNH2(RNH2) + kOH-(OH-) + kH2O(H2O)}(RNH2) (20)
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010
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18
Figure 1. Experimental setup, according to Vaidya and Mahajani.18
Equation 18 suggests that H2O, OH-, and RNH2 can influence the reaction in parallel. Its form is similar to that of the limiting case of the zwitterion mechanism represented by eq 16, and it also can explain fractional and higher-order kinetics.15 When the solvent (water) is the dominant base and the contribution of RNH2 and OH- to carbamate formation is negligible, the reaction is first order, with respect to the amino acid salt, and the rate is given by r ) kH2O(H2O)(CO2)(RNH2) ) kˆ(CO2)(RNH2)
(21)
where kˆ ) kH2O(H2O). When RNH2 is the most dominant base, the reaction is second order, with respect to the salt, and the rate is given by r ) kRNH2(CO2)(RNH2)2
(22)
It is clear from eq 18 that the number of fitting parameters in the termolecular mechanism is fewer than that in the zwitterion mechanism. Kumar et al.3 found that the kinetic behavior of these reaction systems is complex; the reaction order, with respect to PG and PT, changes from one at low concentrations to 1.5 at salt concentrations approaching 3 kmol/m3. It is probable that such reactions can be represented equally well by the zwitterion and the termolecular mechanisms, whereas the kinetic data can be fitted to either of those. 3. Experimental Section 3.1. Materials. Glycine, taurine, and potassium hydroxide used in experiments were purchased from S.D. Fine Chemicals Ltd., Mumbai. Potassium salts of glycine and taurine were prepared by neutralizing an aqueous solution of the amino carboxylic or sulfonic acid with an equimolar quantity of potassium hydroxide. The neutralization reaction was performed, along with continuous cooling. N,N-Diethylethanolamine was purchased from Sisco Research Laboratories Pvt. Ltd., Mumbai. Carbon dioxide, nitrous oxide, and nitrogen cylinders, with a given purity of 99.95%, were purchased from Inox Air Products Ltd., Mumbai. 3.2. Experimental Setup. A glass stirred-cell reactor (Figure 1) with a planar horizontal gas/liquid interface was used for
the absorption studies (see Vaidya and Mahajani ). 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 of 97 mm, height of 187 mm) was operated batchwise. The total volume of the reactor was 1.45 dm3, and the interfacial surface area was 7.5 × 10-3 m2. The reactor was equipped with a flange made of stainless steel (Sharad Autoclave Engineers, Mumbai). A pressure transducer (Trans Instruments, U.K., 0-1 bar), mounted on this flange and coupled with a data acquisition system, enabled measurement of the total pressure inside the reactor, with the uncertainty in this measurement being (1 mbar. 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 120 rpm, to ensure that the gas/ liquid interface was undisturbed. 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 solute gas passed through a coil, also kept in the water bath, before being charged inside the reactor. 3.3. Experimental Procedure. In each experiment, the reactor was charged with 0.4 dm3 of the absorbent. The gas inside the reactor was then purged with nitrogen to ensure an inert atmosphere. Thereafter, nitrogen was released through the gas outlet port. All the lines were closed and the reactor content attained the desired temperature. CO2 from the gas cylinder was then 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. 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 least-squares 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. The reproducibility of experiments was checked and the error in all experimental measurements was determined to be 5, because of the low salt concentrations used in this work, the values presented in Table 2 and the independence of CO2 absorption rates on
Table 2. Equilibrium and Kinetic Characteristics of the CO2-Aqueous PG System at 303 K (PG)0 (kmol/m3)
HCO2(DCO2k1)1/2 (× 107 kmol1/2/ (m1/2 s kPa))
HCO2 (× 104 kmol/ (m3 kPa))
DCO2 (× 109 m2/s)
k1 (m3/(mol s))
M
Ei
0.1 0.2 0.3 0.4 0.5
12.03 11.01 9.16 10.74 8.67
3.11 2.94 2.71 2.68 2.57
2.17 2.16 2.16 2.15 2.15
6.90 6.49 5.29 7.47 5.29
18 27 30 41 38
28 64 89 141 157
Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010
Figure 2. Plot of kobs vs (PG)0 at 303 K and comparison with the kobs values reported by Portugal et al.4 Table 3. Kinetic Studies on the CO2-Aqueous PG Reaction System temperature (K)
(PG)0 (kmol/m3)
k1 (m3/(mol s))
reference
303 295 293-303 278-303 291 283
0.10-0.50 0.20-3.0 0.10-3.0
