Kinetics of CO2 Absorption in Aqueous Sodium Glycinate Solutions

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Ind. Eng. Chem. Res. 2007, 46, 1578-1583

Kinetics of CO2 Absorption in Aqueous Sodium Glycinate Solutions Seungmoon Lee, Ho-Jun Song, Sanjeev Maken, and Jin-Won Park* Department of Chemical Engineering, Yonsei UniVersity, 134 Shinchon-dong, Seodaemun-ku, Seoul, 120-749, Republic of Korea

The physical solubility and diffusivity of N2O and CO2 in aqueous sodium glycinate (SG) solutions of various concentrations (1.0-3.5 kmol/m3) and temperatures (303.15-323.15 K) were reported. The kinetics of the reaction between CO2 and SG has been studied using absorption data measured by a wetted-wall column apparatus at various temperatures and concentrations. The second-order rate constant of CO2 with SG was determined to be k2(m3 kmol-1 s-1) ) 1.95 × 1013 exp(-7670/T), where T is the absolute temperature (given in Kelvin). The activation energy for the reaction of CO2 with SG was calculated to be 63.8 kJ/mol. 1. Introduction

k2

CO2 + RNH2 y\ z RNH+ 2 COO k -1

Carbon dioxide (CO2) is a very important greenhouse gas that is emitted during the burning of fossil fuels.1 The most likely options for CO2 separation and capture include chemical absorption, physical and chemical adsorption, gas-separation membranes, mineralization/biomineralization, and vegetation. Aqueous alkanolamines have been widely used chemical absorbents, for the removal of acid gases (CO2, H2S).2-6 Amino acid or salts of amino acids are also reported to be used for the selective removal of acid gases in some industrial processes.7 The amino acid salts have certain advantages over alkanolamines, such as higher surface tension, an almost nonvolatile nature, resistance to degradation in oxygen-rich flue gas, and better absorption.8,9 Knowledge of the reaction kinetics between CO2 and the amine solution is important for the designing of a gas-liquid contactor. A literature survey reveals that the kinetics of CO2-alkanolamine was studied by many researchers,10-14 whereas data about CO2-salt of amino acid is scarce.19-21 This observation prompted us to study the CO2 absorption in sodium glycinate (SG) solutions. In our previous work, we have reported the physicochemical properties and solubility of CO2 in SG solutions.9,22,23 In this paper, we have studied the kinetics of CO2 absorption using the previously published23 and currently measured diffusivity, physical solubility, and absorption rate of CO2 in aqueous SG solutions (1.0-3.5 kmol/m3) at 303.15, 313.15, and 323.15 K. 2. Theory The reaction mechanism of the CO2 absorption in amine has been reported in literature.23-27 It was also expected that similar to primary and secondary amines, CO2 reacts with a solution of alkaline salt of amino acid through a zwitterionic mechanism, because of the similar functional groups.21 The overall reaction of CO2 in an aqueous amine solution consists of the following steps: (i) carbamate formation, (ii) bicarbonate formation, and (iii) carbonic acid formation. 2.1. Carbamate Formation. The carbamate formation reaction of primary and secondary amines, which are sterically unhindered, with CO2 occurs through the zwitterionic mechanism28,29 and has the following relations: * To whom correspondence should be addressed. Tel: +82-2-3641807. Fax: +82-2-312-6401. E-mail address: [email protected].

kb

+ RNH+ 2 COO + B 798 RNHCOO + BH

(1) (2)

The carbamate formation is very unstable, because of the alkyl group that is attached to the amine, and this result in a fast hydrolysis reaction. The concentration of the carbamate is on the order of 10-4 lower than the amine concentration.30 During deprotonation, bases such as the water, amine, or hydroxyl ion can contribute to the deprotonation of the zwitterions24 in aqueous solutions in the following way: kH

2O

+ RNH+ 2 COO + H2O 798 RNHCOO + RNH3 kAM

+ RNH+ 2 COO + RNH2 798 RNHCOO + H3O kOH-

RNH+ 2 COO + OH 798 RNHCOO + H2O

(3) (4) (5)

2.2. Bicarbonate Formation. k*OH-

CO2 + OH- 798 HCO3

(6)

This reaction of bicarbonate formation is fast, which can enhance mass transfer, even with low hydroxyl ion concentration, and it has a significant contribution to the overall reaction rate. Therefore, the reaction rate of CO2 with OH- in SG solution31 becomes

rCO2-OH- ) k*OH-[CO2][OH-]

(7)

2895 T

(8)

log10(k*OH-) ) 13.635 -

where k*OH- is given in units of m3 kmol-1 s-1 and T is the absolute temperature (given in Kelvin). 2.3. Carbonic Acid Formation. k* H O

+ CO2 + H2O 798 HCO3 +H 2

(9)

The reaction is very slow (k*H2O ) 0.026 s-1 at 298.15 K) and can be neglected.31 The reaction rate of quasi-steady state between CO2 and SG for the zwitterionic mechanism can be described as

10.1021/ie061270e CCC: $37.00 © 2007 American Chemical Society Published on Web 01/27/2007

Ind. Eng. Chem. Res., Vol. 46, No. 5, 2007 1579

rCO2-SG ) kapp[CO2]

(10)

The apparent reaction rate constant (kapp) for the reaction of CO2 with SG in aqueous solution could be calculated from the overall rate constant (kov) by correction for the contribution of the bicarbonate formation via reaction 6: -

kapp ) kov - k*OH [OH ] -

(11)

The OH- contribution is low in the reaction of CO2 with amine. Hence, the reaction of CO2 with H2O is usually neglected in the overall reaction rate. As a result, the overall reaction rate constant kov has the following expression:

kov ) kapp + k*OH-[OH-] ) [SG] + k* OH [OH ] k-1 1+ kH2O[H2O] + k*OH-[OH-] + kSG[SG] (12) 3. Experimental Section SG (Sigma-Aldrich, mass purity of >99%) was used in this study, and its aqueous solutions were prepared from doubly distilled water. All solutions were prepared on a mass basis, with a balance precision of (1 × 10-4 g. The uncertainty in the concentration was 0.01 mol %. Nitrous oxide and carbon dioxide of high purity (>99.8%) were used during the experiment. 3.1. Physical Solubility. Physical solubility of gas in liquids (A) can be calculated from Henry’s law, which is defined as

HA (kPa m3/kmol) )

PA C*A

(13)

where HA is Henry’s law constant, PA the partial pressure in the gas phase (expressed in units of kPa), and C*A the equilibrium concentration of gas (A) absorbed by liquid (expressed in units of kmol/m3). A higher Henry’s constant value corresponds to a lower solubility. The physical solubility of N2O was measured in the manner described by Al-Ghawas et al.32 Physical solubility was measured in a glass flask with a volume of 500 mL at atmospheric pressure. Approximately 30 mL of solution, which was weighed with a balance precision of (1 × 10-4 g, was injected into the flask. The apparatus was kept in a water thermostat, and the temperature of the bath was controlled to within (0.05 K. The partial pressures of N2O in the experiments were obtained from the measured total pressure, corrected for water vapor pressure, using the following equation:32

(-5243.04 ) T

PHvap2O (kPa) ) 1.35337 × 106 exp

(14)

where the absolute temperature T is given in Kelvin. The experimental uncertainty in the solubility measurements was estimated to be less than (2%. The physical solubility of CO2 in various SG solutions was estimated using a N2O analogy33 from the relation

HCO2 HN2O

)

0 HCO 2

HN0 2O

(15)

where HN2O is the physical solubility of N2O in aqueous SG 0 solutions, and HCO and HN0 2O are the physical solubilities of 2 CO2 and N2O in water, calculated from eq 13. These latter values compared well with those calculated from eqs 16 and 17, as proposed by Versteeg and van Swaaij.5 0 HCO (kPa m3/kmol) ) 2.8249 × 106 exp 2

(-2044 T ) (16) (-2284 T ) (17)

HN0 2O (kPa m3/kmol) ) 8.5470 × 106 exp

where the absolute temperature T again is given in Kelvin. 3.2. Diffusivity and Kinetics. A schematic diagram of experimental setup was shown in Figure 1. The diffusion coefficients were measured using the wetted-wall column absorber, which had an outer diameter of 2.54 cm and a length of 10.05 cm and was composed of SS316 stainless steel in an air bath. The temperature of the air thermostat was kept constant, within (0.1 K, and the temperature of the air bath was recorded using a digital thermometer (Hanyang model AT3) with an accuracy of 0.1 K. The SG solution was made to flow using gear pumps (Cole-Parmer, model 7553-70), which were calibrated with experimental liquids of different concentrations and at different temperatures. The liquid sample was distributed uniformly as a thin film on the outside of the cylinder. To prevent the ripple on the liquid surface, 0.04% (v/v) of a surface active agent (Tween 80) was added. The effect of the surface active agent Tween 80 was negligible (according to the report by Yoon et al.34). The absorption rate was measured via a gasuptake method, using a soap-film meter. The input flow rate of input gas was controlled with an accuracy of (0.03% by a massflow controller (Tylon model FC-280S). The composition of exhaust gases was determined using a gas chromatograph (Hewlett-Packard, model 5890 Series II) equipped with a thermal conductivity detector (TCD) and column that was packed with Porapak Q. The analysis conditions of the gas chromatograph were a detector temperature of 140 °C, a column temperature of 30 °C, and carrier gas total flow rate of 75 mL/ min. Molecular diffusivity of CO2 in various SG solutions was estimated using N2O analogy33 from the relation

DCO2 DN2O

)

0 DCO 2

(18)

DN0 2O

where DCO2 and DN2O are the diffusion coefficients of CO2 and 0 N2O in aqueous SG solutions, respectively, and DCO and DN0 2O 2 are the diffusion coefficients of CO2 and N2O in water and compared well (not shown here) with those calculated from eqs 19 and 20, as proposed by Versteeg and van Swaaij.5

(-2119 T ) -2371 exp( T )

0 (m2/s) ) 2.35 × 10-6 exp DCO 2

(19)

DN0 2O (m2/s) ) 5.07 × 10-6

(20)

The absolute temperature T again is given in Kelvin. The same experimental setup was used to measure the absorption rate of CO2 in SG solution. The experimental procedure is the same as that described by Yoon et al.34

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Figure 1. Schematic diagram of the diffusivity apparatus. Legend is as follows: 1, N2O gas; 2, CO2 gas; 3, mass-flow controller; 4, saturation flask; 5, sodium glycinate (SG) solution; 6, gas input; 7, wetted-wall column; 8, absorbent input; 9, absorbent output; 10, gas output; 11, wet gas meter; 12, gas chromatograph-thermal conductivity detector (GC-TCD); 13, pump; and 14, reservoir. Table 1. Physical Solubility and Diffusivity of N2O and CO2 in Aqueous Sodium Glycinate (SG) Solutions temperature (K) 303.15 303.15 303.15 303.15 303.15

concentration of sodium glycinate, [SG] (kmol/m3) 1.065 1.622 2.226 2.816 3.466

HN2O (kPa m3/kmol) 4788.19 4912.55 4998.19 5010.15 5043.56

HCO2 (kPa m3/kmol) 3539.42 3631.35 3679.13 3703.49 3728.18

DN2O (× 109 m2/s) 1.516 1.457 1.424 1.357 1.273

DCO2 (× 109 m2/s) 1.613 1.550 1.515 1.444 1.355

313.15 313.15 313.15 313.15 313.15

1.061 1.617 2.218 2.806 3.455

5833.57 5921.95 6107.02 6355.27 6797.37

4202.74 4266.41 4399.74 4578.59 4897.10

1.969 1.801 1.711 1.578 1.478

2.040 1.867 1.773 1.636 1.531

323.15 323.15 323.15 323.15 323.15

1.056 1.610 2.209 2.794 3.440

7695.46 7769.40 7885.55 7999.04 8119.00

5386.72 5464.04 5543.61 5625.54 5709.92

2.309 2.188 2.092 1.846 1.817

2.333 2.212 2.115 1.866 1.837

Table 2. Kinetics Parameters Obtained for CO2 in Aqueous Sodium Glycinate (SG) Solutions PA (kPa)

tc (s)

R (kmol/kmol)

303.15 303.15 303.15 303.15 303.15

concentration of sodium glycinate, [SG] (kmol/m3) 1.065 1.622 2.226 2.816 3.466

97.0997 97.0997 97.0997 97.0997 97.0997

2.34 2.32 2.29 2.61 2.67

0.266 0.240 0.142 0.185 0.114

concentration of water, [H2O] (kmol/m3) 51.667 49.58 48.000 45.54 43.594

313.15 313.15 313.15 313.15 313.15

1.061 1.671 2.218 2.806 3.455

93.9789 93.9789 93.9789 93.9789 93.9789

2.22 2.27 1.84 2.42 2.63

0.237 0.233 0.185 0.155 0.126

323.15 323.15 323.15 323.15 323.15

1.056 1.610 2.209 2.794 3.440

89.0039 89.0039 89.0039 89.0039 89.0039

2.19 2.25 1.91 2.40 2.38

0.300 0.243 0.191 0.176 0.142

temperature (K)

kL (× 105 m/s)

NA (× 105 kmol m-2 s-1)

kov (s-1)

kapp (s-1)

Ha

E

3.30 2.88 2.85 2.67 2.87

1.73 2.03 2.32 2.54 2.87

238.3 353.0 499.1 623.6 765.0

233.7 352.5 489.0 616.2 752.0

21.3 26.3 28.9 35.3 37.6

21.3 26.3 28.9 35.3 37.6

51.500 49.39 47.822 45.39 43.478

3.70 3.15 3.69 2.84 3.12

2.09 2.38 3.21 3.21 3.50

608.3 902.1 1260.9 1610.2 2088.5

597.9 891.5 1245.7 1592.6 2066.2

25.3 34.1 39.5 55.1 58.4

25.3 34.1 39.5 55.1 58.4

51.26 49.19 47.64 45.19 43.28

3.81 3.54 3.83 3.12 3.47

2.64 3.18 3.47 3.73 3.97

1178.6 1647.9 2325.4 2946.9 3622.2

1164.4 1628.9 2299.6 2918.3 3585.3

41.9 58.7 60.8 70.2 73.3

41.9 58.7 60.8 70.2 73.3

According to Higbie penetration theory, the absorption rate of a sparingly soluble gas for a short contact time with a degassed liquid is related to the diffusion coefficient by the following relation:

DA (m2/s) )

( )

NAHA πtc 2PA

where tc is the contact time (given in seconds) and can be computed from the wetted-wall column hydrodynamics, using the relation

tc (s) )

2

(21) where

( )( )

2h πds 3 L

2/3

3η Fg

1/3

(22)

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NA )

NTA πdsh

(23)

and

ds ) d +

3ηL πFd

(24)

Under fast pseudo-first-order reaction conditions, the kinetics reaction was given by

2 < Ha , E∞

(25)

where the Hatta number Ha is expressed as

Ha )

xkovDCO

2

(26)

kL

The specific rate of mass transfer of CO2 under fast pseudofirst-order reaction conditions is given as35,36

NA ) -DCO2 ) [CO2]i

(

)

d[CO2] dx

Figure 2. Effect of aqueous sodium glycinate (SG) concentration on the absorption rate.

x)0

xDCO kov 2

tanh(DCO2kov/kL)

(27)

For the Hatta number Ha > 2, the specific rate of mass transfer of CO2 becomes

NA ) [CO2]ixDCO2kov )

pCO2

D k HCO2 x CO2 ov

(28)

4. Results and Discussion The measured physical solubility and diffusivity of N2O and CO2 in pure water at various temperatures (303.15-323.15 K) compared well (not shown here) with the previous literature values for the physical solubility and diffusivity of N2O and CO2 in pure water.32,37-40 Variations in the diffusivity and physical solubility of N2O and CO2 in aqueous SG solutions of various concentrations (1.056-3.466 kmol/m3) and temperatures (303.15-323.15 K) were recorded in Table 1. It was determined that the diffusivity and solubility decrease as the SG concentration increases. However, an increase in temperature increases the diffusivity and decreases the solubility of CO2 in SG solutions. This is due to the fact that the solubility of gases is inversely proportional to the temperature and, according to the Arrhenius equation (D ) Do exp[-Q/(RT)]), thediffusivity decreases with decreasing temperature and also with decreasing viscosity, because of the decrease in SG concentration. The total absorption rate (NTA) of N2O and CO2 in aqueous SG solutions of various concentrations (1.056-3.466 kmol/m3) at different temperatures (303.15-323.15 K) was measured using the wetted-wall column absorber. NA can be calculated from the total absorption rate (eq 23). The density and viscosity data used in these calculations were taken from the literature.22 The specific absorption rate of CO2 in aqueous SG solutions was recorded in Table 2 and is shown graphically in Figure 2. The absorption rate was determined to increase as the temperature and concentration of SG each increased. The overall pseudo-first-order rate constant kov and apparent reaction rate constant kapp in aqueous SG solutions were also

Figure 3. Plot of the apparent reaction rate constants with sodium glycinate (SG).

tabulated in Table 2. The kov parameter of CO2 in aqueous SG solutions is calculated using eq 28. The kapp parameter (eq 12) is calculated from the overall rate constant kov by correction for the contribution of the bicarbonate formation. The experimental result of kapp, as a function of the SG concentration for the different temperatures is shown in Figure 3. It was observed that, although kapp increases linearly as the SG concentration increases, it also increases linearly with increasing temperature. Similar trends were reported in the literature for the other absorbents.34,41-43 The reaction rate between CO2 and SG for the zwitterionic mechanism can be calculated using the overall reaction rate constant kov (eq 12). The reaction rate parameter is determined by a nonlinear regression method. The second-order reaction constants for aqueous solutions are recorded in Table 2. The parameter kOH-[OH-] is neglected, considering the low OHcontribution in the reaction of CO2 with amine.14,34,38,41,44 Using

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Figure 4. Arrhenius plot for CO2 in the aqueous sodium glycinate (SG) absorption reaction.

an Arrhenius plot, the second-order reaction constant, as a function of temperature, was determined as follows:

k2 (m3 kmol-1 s-1) ) 1.95 × 1013 exp

(-7670 T )

(29)

where T again is given in Kelvin. The calculated Arrhenius plot of the second-order reaction constant for SG, as a function of temperature, was determined, and the results are shown in Figure 4. 5. Conclusion We have studied the kinetics reaction of CO2 with aqueous sodium glycinate (SG) solutions of various concentrations at different temperatures. The physical solubility and diffusivity of N2O and CO2 in pure water was determined to be in agreement with the literature data. The second-order reaction constant between CO2 and SG for the zwitterionic mechanism was determined for each parameter. The values of the secondorder reaction constant at 303.15, 313.15, and 323.15 K were 218, 576, and 1034 m3 kmol-1 s-1, respectively. The activation energy for the reaction of CO2 with SG was calculated to be 63.8 kJ/mol . These results would be applied in the basic absorption process such as the design of equipment for acid gas removal in our future work. Acknowledgment The authors are grateful to the Korea Electric Power Research Institute, Daejeon for funding this research work. Nomenclature A ) gaseous species d ) outside diameter of the wetted wall column (m) DCO2 ) diffusion coefficients of CO2 in aqueous sodium glycinate solutions (m2/s) DN2O ) diffusion coefficients of N2O in aqueous sodium glycinate solutions (m2/s) 0 ) diffusion coefficients of CO2 in water (m2/s) DCO 2 DN0 2O ) diffusion coefficients of N2O in water (m2/s)

E∞ ) value of the enhancement factor in an instantaneous reaction region g ) gravitational constant h ) height of the wetted-wall column (m) Ha ) Hatta number HCO2 ) physical solubility of CO2 in aqueous sodium glycinate solutions (kPa m3/kmol) HN2O ) physical solubility of N2O in aqueous sodium glycinate solutions (kPa m3/kmol) 0 HCO2 ) physical solubility of CO2 in water (kPa m3/kmol) HN0 2O ) physical solubility of N2O in water (kPa m3/kmol) k-1 ) reverse first-order reaction rate constant (s-1) k2 ) second-order reaction rate constant (m3 kmol-1 s-1) 3 k* H2O ) reaction rate constant for CO2 hydration in eq 9 (m -1 -1 kmol s ) k*OH- ) reaction rate constant in eq 6 (m3 kmol-1 s-1) kapp ) apparent rate constant (s-1) kov ) overall pseudo-first-order rate constant (s-1) kAM ) reaction rate constant in eq 3 kH2O ) reaction rate constant in eq 4 kOH- ) reaction rate constant in eq 5 L ) liquid flow rate (m3/s) NA ) specific absorption rate (kmol m-2 s-1) NTA ) total absorption rate (kmol/s) Q ) activation energy for diffusion R ) gas constant tc ) contact time (s) T ) absolute temperature (K) Literature Cited (1) United Nations Framework Convention on Climate Change, Counting Emissions and Removals. Available via the Internet at http://unfccc.int/ resource/docs/publications/counting.pdf. (2) Al-Juaied, M.; Rochelle, G. T. Absorption of CO2 in Aqueous Blends of Diglycolamine and Morpholine. Chem. Eng. Sci. 2006, 61, 3830. (3) Mandal, B. P.; Bandyopadhyay, S. S. Absorption of Carbon Dioxide into Aqueous Blends of 2-Amino-2-Methyl-1-Propanol and Monoethanolamine. Chem. Eng. Sci. 2006, 61, 5440. (4) Rinker, E. B., Ashour, S. S., Sandall, O. C., 1995. Kinetics and Modeling of Carbon Dioxide Absorption into Aqueous Solutions of N-Methydiethanolamine. Chem. Eng. Sci. 1995, 50, 755. (5) Versteeg, G. F., van Swaaij, W. P. M. Solubility and Diffusivity of Acid Gases (CO2, N2O) in Aqueous Alkanoamine Solutions. J. Chem. Eng. Data 1988, 33, 29. (6) Wang, R.; Li, D. F.; Liang, D. T. Modeling of CO2 Capture by Three Typical Amine Solutions in Hollow Fiber Membrane Contactors. Chem. Eng. Proc. 2004, 43, 849. (7) Kohl, A. L.; Nielsen, R. B. Gas Purification; 5th Edition; Gulf Publishing: Houston, TX, 1997. (8) Hook, R. J. An Investigation of Some Sterically Hindered Amines as Potential Carbon Dioxide Scrubbing Compounds. Ind. Eng. Chem. Res. 1997, 36, 1779. (9) Song, H. J.; Lee, S.; Maken, S.; Park, J. J.; Park, J. W. Solubilities of Carbon Dioxide in Aqueous Solutions of Sodium Glycinate. Fluid Phase Equilib. 2006, 246, 1. (10) Cents, A. H. G.; de Bruijn, F. T.; Brilman, D. W. F.; Versteeg, G. F. Validation of the Danckwerts-Plot Technique by Simultaneous Chemical Absorption of CO2 and Physical Desorption of O2. Chem. Eng. Sci. 2005, 60, 5809. (11) Derks, P. W. J.; Kleingeld, T.; van Aken, C.; Hogendoorn, J. A.; Versteeg, G. F. Kinetics of Absorption of Carbon Dioxide in Aqueous Piperazine Solutions. Chem. Eng. Sci. 2006, 61, 6837. (12) Jamal A.; Meisen, A.; Lim, C. J. Kinetics of Carbon Dioxide Absorption and Desorption in Aqueous Alkanolamine Solutions using a Novel Hemispherical ContactorsII: Experimental Results and Parameter Estimation. Chem. Eng. Sci. 2006, 61, 6590. (13) Vaidya, P. D.; Mahajani, V. V. Kinetics of the Reaction of CO2 with Aqueous Formulated Solution Containing Monoethanolamine, N-Methyl-2-Pyrrolidone, and Diethylene Glycol. Ind. Eng. Chem. Res. 2005, 44, 1868.

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ReceiVed for reView October 2, 2006 ReVised manuscript receiVed November 30, 2006 Accepted December 13, 2006 IE061270E