Article pubs.acs.org/EF
Absorption of CO2 into Aqueous Potassium Salt Solutions of L-Alanine and L-Proline Jin-ah Lim,†,‡ Dong Hyun Kim,‡ Yeoil Yoon,† Soon Kwan Jeong,† Ki Tae Park,† and Sung Chan Nam*,† †
Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea Department of Chemical Engineering, Kyungpook National University, 80 Daehangno, Buk-gu, Daegu 702-701, Republic of Korea
‡
ABSTRACT: Aqueous potassium salt solutions of L-alanine and L-proline were investigated as carbon dioxide (CO2) absorbents. The CO2 absorption capacities and absorption heats (−ΔHabs) of the aqueous amino acid salts were measured in a semi-batch absorption system and differential reaction calorimeter (DRC). The solution experiments tested concentrations of 2.5 M and were carried out at 298 and 313 K. The 13C and 1H nuclear magnetic resonance (NMR) spectra were used to identify the species distributions in the CO2-loaded absorbents. The absorption properties were compared to those of the commercial monoethanolamine (MEA) absorbent, revealing that the CO2 loading capacity was higher than that of MEA (0.68 mol of CO2/mol of solute for the potassium salt of L-alanine > 0.5 mol of CO2/mol of solute for MEA). The absorption heat was lower than that of MEA at 298 K (53.26 kJ/mol of CO2 for the potassium salt of L-alanine < 81.77 kJ/mol of CO2 for MEA).
1. INTRODUCTION Anthropogenic activities since the industrial revolution have resulted in a sharp increase in the atmospheric greenhouse gas concentration, which is causing global warming. This problem is most likely caused by the increasing atmospheric carbon dioxide concentration because of the burning of fossil fuels for, among others, power generation.1 CO2, generated by the combustion of fossil fuels, constitutes 80% of the total greenhouse gas emissions and is considered to be the most important of the six major greenhouse gases. The development of CO2 capture technologies is critical for reducing greenhouse gases and coping with the climate change problem at its root.2 Three basic processes can be applied for capturing CO2 from flue gases: oxy-fuel combustion, pre-combustion, and post-combustion.3 The wet absorption process, which is a post-combustion CO2 capture technology using liquid absorbent, has been studied extensively in many countries for its ability to process high concentrations of CO2 using retrofits of the existing process. The most important consideration in any CO2 absorption process is the selection of the absorbent, because this will determine the performance of the process. In general, alkanolamine absorbents are effectively used to remove acid gas. Monoethanolamine (MEA) is the leading alkanolamine absorbent. Although it features a fast absorption rate and a high alkalinity, it has the drawbacks of the loss of absorbent because of degradation and corrosion of the equipment as well as the high amounts of energy required for absorbent regeneration.4 To solve these problems, the absorption of CO2 using an amino acid salt solution as an alternative absorbent was studied. The advantages of the amino acid salt solutions include a high surface tension similar to water and low evaporation of absorbent because of outstanding solubility as a result of its COOH and NH2 groups and low requirement of regeneration energy attributed to the salt generated by the reaction with CO2.5,6 The amino group in the dissolved amino acid cannot react with CO2. Therefore, Kumar et al.7 and Holst et al.1,8 employed the CO2 absorbent by neutralization of the metal elements of the © 2012 American Chemical Society
Figure 1. Structures of the potassium salts of L-alanine and L-proline.
carboxyl group using metal hydroxides, such as potassium, sodium, and lithium. Potassium is reported to show the highest CO2 capture efficiency because of its outstanding saturation concentration from high solubility.2 Kumar et al.,9 Lee et al.,10 and Aronu et al.11 conducted studies of CO2 capture using amino acid salt solutions comprised of such materials as taurine and glycine, but there have been no systematic studies of the CO2 loading capacity and heat of absorption. The data for the CO2 Received: November 10, 2011 Revised: April 18, 2012 Published: April 23, 2012 3910
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Figure 2. Schematic diagram of the semi-batch system: (1) CO2/N2 bomb, (2) chilled H2O saturator, (3) mass flow controller, (4) thermocouple, (5) pressure gauge, (6) water bath, (7) reactor, (8) sampling valve, (9) condenser, (10) magnetic stirrer, (11) GC, and (12) back-pressure regulator.
Figure 3. Schematic diagram of the DRC: (1) CO2/N2 gas, (2) water bath, (3) inlet gas port, (4) optional probe, (5) motor, (6) impeller, (7) solvent, (8) reactor (thermostated jacket), (9) calibration probe, (10) thermocouple, (11) thermostat, (12) reference reactor, and (13) GC.
loading capacity, absorption heat, and species distribution are important for the design of the CO2 absorption process. In this study, the CO2 absorption capacity and absorption heat of the aqueous potassium salts of L-alanine and L-proline were investigated using a semi-batch absorption system and a differential reaction calorimeter (DRC). L-Alanine has a similar structure to glycine, the simplest amino acid, as well as a similar molecular weight. Merger et al.12 and Holst et al.8 reported that L-alanine has a high absorption capacity. L-Proline is a secondary vamino acid, which has a distinctive structure of a five-membered ring, including an amine group. The speciation in the CO2-loaded absorbents was investigated using nuclear magnetic resonance (NMR) spectroscopy. The results were compared to primary amine, monoethanolamine (MEA) and secondary amine, diethanolamine (DEA).
2. EXPERIMENTAL SECTION 2.1. Materials. The amines used in these experiments were MEA (≥99.0%) and DEA (98.0%) from Samchun Chemicals, Korea. The amino acids were L-alanine (≥99%) and L-proline (≥98%) from Sigma-Aldrich. Also, HCl (0.1 N solution, factor = 1.000 ± 0.005 at 293 K) and KOH (8 N solution) were obtained from Samchun Chemicals, Korea, and Acros. Deuterium oxide (D2O) used for NMR measurements was obtained from Sigma-Aldrich (99.99%). CO2 (99.99%) and N2 (99.999%) gases were used as received. Figure 1 shows the molecular structures and molecular structures reacted with CO2 of the absorbents used in this study. All absorbents were prepared by mixing deionized water to a concentration of 2.5 M. The amino acid salts were prepared by neutralization of the amino acid dissolved in deionized water. An equimolar amount of KOH was added to the amino acids. A 0.1 N HCl solution for titration was used 3911
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Figure 5. NMR spectra of MEA solution (CO2 absorption capacity of 0.53): (a) 13C NMR spectra and (b) 1H NMR spectra. 2.2. Experiment and Procedure. 2.2.1. Semi-batch Absorption System. A semi-batch absorption system was used to measure the CO2 absorption capacities of the absorbents. Figure 2 shows a schematic diagram of the experimental apparatus. The apparatus consisted of a gas mixer for mixing CO2 and N2 and supplying the mixture at a certain concentration, the reactor, and the water bath for maintaining the temperatures of the reactor. The 30% CO2 gas was prepared by mixing 99.99% CO2 and 99.999% N2 using a mass flow controller (MFC), and the gases were then supplied to the reactor at a rate of 1 L/min. The internal volume of the stainless-steel reactor was 2 L. The temperature of the reactor was isothermally controlled at 298 or 313 K. The temperature and pressure in the reactor were measured using a K-type thermocouple (±0.3%) and a pressure transmitter (PSCH0030K, Sensys, ±0.25%). The volume of the absorbent was 0.5 L, and the solution was stirred. The concentration of CO 2 discharged after passing through the absorbent was measured using a gas chromatograph (GC, 6890N model, Agilent). A packed column (Porapak-Q, 0.32 m × 6 ft, Supelco) was used as the GC column, along with a thermal conductivity detector (TCD). The gas was continuously fed through the inlet at the bottom of the reactor to form fine bubbles, and the discharge gas was measured by the GC until the concentration of discharged CO2 was the same as that of inlet CO2. The CO2-loaded absorbents were collected and studied by NMR measurements. 1H and 13C NMR measurements were performed to identify the species in the absorbent solutions. 1,4-Dioxane was used as an external reference standard of the 13C NMR measurement. The solvent peak of 1,4-dioxine appeared at 66.5.13 NMR analysis parameters at room temperature were as follows: 1 H NMR: number of scans, 16; acquisition time, 3.172 s; relaxation delay, 1; and dwell time, 48.4 μL. 13 C NMR: number of scans, 64; acquisition time, 1.091 s; relaxation delay, 60; and dwell time, 16.65 μL.
Figure 4. CO2 absorption capacity of MEA, DEA, and amino acid salt solution at (a) 298 K and (b) 313 K.
Table 1. CO2 Absorption Capacity of the Aqueous Amines and Potassium Salts of Amino Acid at 298 and 313 K CO2 loading (mol of CO2/mol of solute) absorbents
298 K
313 K
MEA DEA potassium salt of L-alanine potassium salt of L-proline
0.53 0.54 0.68 0.66
0.50 0.47 0.57 0.62
Table 2. Heat of Absorption (−ΔHabs) at 298 and 313 K −ΔHabs (kJ/mol of CO2) absorbents
298 K
313 K
MEA DEA potassium salt of L-alanine potassium salt of L-proline
81.77 67.06 53.26 90.20
90.75 65.49 66.13 86.17
to measure the accurate concentration of the amino acid salts before the experiment. 3912
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Figure 6. NMR spectra of DEA solution (CO2 absorption capacity of 0.54): (a) 13C NMR spectra and (b) 1H NMR spectra. 2.2.2. DRC. The absorption heat of the absorbent was measured using the DRC from SERARAM. Figure 3 shows a schematic diagram of the apparatus. It consisted of two mechanically agitated glass reactors (250 mL volume). The cooling water was circulated between the reactor and jacket to maintain the temperature. An agitator and a thermocouple (±0.01 °C) were used to stir the solution and measure the temperature, respectively. The temperature probes and Joule effect calibration probe were additionally installed in the reactor. A total of 100 mL of absorbent was supplied to the reactor, and a gas mixture containing N2 and CO2 (30 vol %) was supplied at 100 mL/min. To promote fast contact between the absorbent and gas, the gas was fed in bubble form and the absorbent was stirred at 200 rpm. The 7890N model GC from Agilent and Porapak-Q (0.32 m × 6 ft, Supelco) column and TCD were used to measure the discharge gas. The experimental temperature was controlled to 298 and 313 K, the same temperatures as those used for testing the semi-batch absorption system. The measurement of the reaction heat was conducted by following three steps:14 calibration before reaction, reaction, and calibration after reaction. The principle of the reaction heat determination is that the exchanged heat between the reaction mixture and reactor wall is proportional to the area under the ΔT versus time curve. The temperature difference between the two reactors was recorded as a function of the time. While thermal phenomenon occurs, the heat flow is
∫0
Q flow = UA
Figure 7. NMR spectra of aqueous potassium salt of L-alanine solution (CO2 absorption capacity of 0.68): (a) 13C NMR spectra and (b) 1H NMR spectra.
determine the heat-transfer value (UA). However, the UA value depends upon the mixture and its composition; therefore, UA is unknown along the reaction. Thus, an average value (UAaverage) is calculated from two calibrations
UA average =
(2)
where UA1 and UA2 are heat-transfer values before and after reaction, respectively. After measurement, the calculated UAaverage was used to calculate the reaction heat (QR) with the integration of the ΔT versus time curve recorded during the reaction step.
t
ΔT dt
1 (UA1 + UA 2) 2
(1)
∫0
where UA is the overall heat-transfer coefficient of the reactor in W/K. The electrical calibration realized by the Joule effect allows us to
Q R = UA average 3913
t
ΔT dt
(3)
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Figure 9. HMQC spectrum of the aqueous potassium salt of L-proline solution.
Figure 8. NMR spectra of aqueous potassium salt of L-proline solution (CO2 absorption capacity of 0.66): (a) 13C NMR spectra and (b) 1H NMR spectra.
3. RESULTS AND DISCUSSION 3.1. CO2 Absorption Capacity. A comparison of the amines and potassium salts of the amino acid is shown in panels a and b of Figure 4. The experiments were conducted at 298 and 313 K. In the graph, Ci and Co refer to the concentration of the initial 30% CO2 and that of CO2 discharged out of the reactor after contact with the absorption liquid, respectively. The breakthrough point was assumed to be where Co/Ci = 1 for saturation of CO2. The time of the breakthrough point was assumed to be the equilibrium time. The CO2 loading capacity (mol of CO2/mol of solute) of each absorbent is shown in Table 1. The total amount of CO2 absorbed until Co/Ci = 1 was calculated by the accumulate sum of absorbed CO2 at each time from GC results. The result indicated that the amino acid salt solutions had a higher CO2 absorption capacity than the amine solutions. At 50 min, the absorption capacity of the amino acid salt solution was higher than that of MEA and DEA because the solution saturated more slowly, particularly with a gradual slope after Co/Ci = 0.180 in the case of the potassium salt of L-alanine (potassium L-alaninate) and Co/Ci = 0.069 in the case of the potassium salt
Figure 10. COSY spectrum of the aqueous potassium salt of L-proline solution.
of L-proline (potassium L-prolinate) at the temperature of 298 K. At 313 K, also, absorption saturation slowed at Co/Ci = 0.187 for potassium L-alaninate and at Co/Ci = 0.052 for potassium L-prolinate. Figure 4 shows the absorption capacity of the amino acid salt solutions as a function of the temperature. As the temperature increased, the breakthrough point of the amino acid salt solution was reached faster. The vapor increased as the temperature increased, and the amount of CO2 that reacted with the absorbent in the solution decreased.15 Also, the reaction rate increased and the solubility of CO2 decreased simultaneously with the increase of the temperature.16 The CO2 absorption mechanism of the primary (RNH2) and secondary (R2NH) amine solutions can be expected as shown 3914
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Table 3. Chemical Shifts (δ) of the Species in 13C NMR Spectra at 298 and 333 K
below.17−19 It is reported that the dissolved CO2 quickly reacts with the amine and generates zwitterions (RNH2+COO− and R2NH+COO−), an intermediate medium.20 CO2 + RNH 2 ⇔ RNH 2+COO−
(5)
RNH 2+COO− + B ⇔ RNHCOO− + BH+
(6)
Therefore, in the case of MEA, which has the high stability of carbamate, the amount of CO2 that can be absorbed by 1 mol of amine is limited to 0.5 mol. Amino acid salt solutions have been shown to follow a mechanism similar to that used by the amine solution.21 However, the result of this study indicates that the potassium salts of L-alanine and L-proline quickly initiate the hydrolysis reaction because of the bicarbonate formation as a result of the steric hindrance effect.7,12 Therefore, additional reactions with CO2 occurred in the presence of the free amine; the reaction rate was slower than in the amine solution. In the case of secondary amino acid, L-proline shows that the absorption rate is slower and unstable because the absorption of CO2 is more impeded as a result of the characteristics of pentagonal rings. The absorption capacity of L-proline, the secondary amino acid, will be similar to or better than that of L-alanine, the primary amino acid. 3.2. Heat of Absorption. The heat of regeneration depends upon the heat of absorption as well as sensible heat
Here, B denotes the base, such as the amine, H2O, and OH−. The generated carbamate then produces the free amine and bicarbonate through hydrolysis. RNHCOO− + H 2O ⇔ RNH 2 + HCO3−
(7)
As the generated carbamate is stabilized, the progress of the reaction shown in eq 7 slows sufficiently enough to be ignored. Thus, the overall reaction can be expressed as below. CO2 + 2RNH 2 ⇔ RNHCOO− + RNH3+
(8) 3915
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Table 4. Chemical Shifts (δ) of the Species in 1H NMR Spectra at 298 and 333 K
show the 13C and 1H NMR spectra of the CO2-loaded MEA, DEA, potassium L-alaninate, and potassium L-prolinate solutions. The carbamate and bicarbonate/carbonate are related to CO2. The carbon peaks appeared at the low field (165− 160 ppm) in the 13C NMR spectra. The oxygen atom has a large electronegativity. Therefore, the electrons of the carbons are deshielded by the oxygen atoms. In the 1H NMR spectrum, the four peak groups of hydrogen in the free MEA and MEA carbamate appeared as four triplets. The results show that the free amine, carbamate, and bicarbonate/carbonate are in the MEA and DEA solutions. The carbon peaks of the carboxyl group in L-alanine and L-proline appeared at the lower field (184.51 and 182.49 ppm) in Figures 7 and 8. Three hydrogens of the methyl group in free L-alanine were present in the peak as a doublet at δ = 1.24− 1.33 ppm in the 1H NMR spectra. Most peaks of potassium 1 L-prolinate overlap in the H NMR spectra. It is found that the aqueous potassium L-alaninate and potassium L-prolinate solutions react with CO2 and form the carbamate and bicarbonate/carbonate. The formation of the
and latent heat. Therefore, it can be used as useful data for the recycling process. Table 2 shows the heats of absorption of the absorbent at different CO2 absorption capacities. The units of −ΔHabs are kJ/mol of CO2. To validate the reliability of the experimental results, absorption heats of MEA and DEA were compared to the results from Chowdhury et al.22 under the same conditions. As results, MEA and DEA showed 86.04 kJ/mol of CO2 and 70.42 kJ/mol of CO2, respectively, and these values are almost the same as the values from the literature within 2%. The primary amine, MEA, had lower absorption capacity and higher absorption heat in comparison to potassium L-alaninate. The L-alanine carbamate is unstable; therefore, it is hydrolyzed to bicarbonate/carbonate. As a result, the amino acid is expected to yield high regeneration efficiency because of the high absorption capacity and low heat of absorption. 3.3. Speciation. NMR spectroscopy was used to identify the species in the aqueous potassium salts of L-alanine and L-proline. NMR analysis used a sample of the semi-batch absorption system measured at a temperature of 298 K. Figures 5−8 3916
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Additionally, the potassium salts of L-alanine and L-proline included larger amounts of bicarbonate/carbonate than carbamate. Therefore, the potassium salts of L-alanine and L -proline are deemed to be the potential CO 2 absorbent to replace the existing amines.
bicarbonate/carbonate in the aqueous potassium L-alaninate solution was predominant because the stability of the carbamate was low. The carbamate was hydrolyzed to the bicarbonate/ carbonate because of the methyl group in L-alanine. In this study, cyclic compounds in proline cannot be verified using one-dimensional (1D) NMR spectroscopy. The proline is a cyclic amine, which has a nonplanar structure by ring strain and various conformers by structural change of atoms. Thus, there are interferences between atoms for different structures. Therefore, two-dimensional (2D) NMR was used to verify cyclic compounds in proline. The 2D NMR measurement, typically using correlation spectroscopy (COSY) and heteronuclear multiple-quantum coherence (HMQC) methods, is useful to investigate the interferences between atoms. The interactions between protons themselves and protons and carbons can be verified by the COSY and HMQC methods, respectively. The correlation of HMQC between carbon and proton before reaction with CO2 was investigated as shown in Figure 9. In this figure, we can see that 1H NMR peaks of δ = 1.75 and 2.12 correspond to a 13C NMR peak of δ = 30.85 and peaks of two protons bonded with C1 appear at δ = 2.76 and 3.05. From the peak at δ = 1.75 related to both C2 and C3, we also confirmed that there was partial overlap between two protons of C2 and one proton of C3. The COSY result, the correlation between protons, supports the HMQC result in reliability, as shown in Figure 10, which indicates peak locations of each proton. Tables 3 and 4 represent the chemical shifts of the species in the 1H and 13C NMR spectra, respectively. Potassium L-alaninate were present in the peak at 16.24, 50.44, 176.04, 18.52, 52.48, 182.70, 163.78, and 160.52 ppm in the 13C NMR spectra. The result of 13C NMR appeared similar to that by Majchrowicz et al.23 The relative ratios were calculated to integrate the peak areas in the 13C NMR spectra. In the aqueous MEA, DEA, potassium L-alaninate, and potassium L-prolinate solutions, the relative ratios of the carbamate:bicarbonate/carbonate at 298.15 K were 1:1.177, 1:1.132, 1:6.570, and 1:2.036, respectively. The results show that aqueous potassium L-alaninate and potassium L-prolinate had a large amount of bicarbonate/ carbonate.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +82-42-860-3645. Fax: +82-42-860-3134. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Energy and Resource R&D Program (2008-C-CD27-P-01-0-0000) under the Ministry of Knowledge Economy, Republic of Korea.
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REFERENCES
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4. CONCLUSION The amino acid salt solutions, aqueous potassium salts of L-alanine and L-proline, were selected as an alternative absorbent to supplement the existing CO2 absorbent. The CO2 absorption capacity and absorption heat of the absorbents were investigated using a semi-batch absorption system and DRC. These experiments were conducted at 298 and 313 K, and the results were compared to MEA and DEA. The CO2 loading capacities were found to be 0.50 and 0.68 mol of CO2/mol of solute for aqueous MEA and potassium salt of L-alanine at 298 K, respectively. The CO2 absorption heats of the potassium salts of L-alanine and L-proline were lower than those of MEA. The absorption heats were found to be 81.77, 67.06, 53.26, and 90.20 kJ/mol of CO2 for aqueous MEA, DEA, potassium salt of L-alanine, and potassium salt of L-proline, respectively. It was found that the potassium salt of L-alanine was excellent for CO2 capture. The species distribution in the CO2-loaded absorbents was determined using NMR measurements. The 13C and 1H NMR were used for qualitative and quantitative analyses. It was found that the potassium salts of L-alanine and L-proline react with CO2 and form carbamate and bicarbonate/carbonate. 3917
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