NMR Study of Carbon Dioxide Absorption in Aqueous Potassium

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NMR Study of Carbon Dioxide Absorption in Aqueous Potassium Carbonate and Homopiperazine Blend Youngeun Kim,† Jeongho Choi,† Sungchan Nam,† Soonkwan Jeong,† and Yeoil Yoon*,† †

Greenhouse Gas Research Center, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Korea ABSTRACT: This study focuses on identifying the CO2 absorption mechanism in an aqueous potassium carbonate (K2CO3)/ homopiperazine (homoPZ) solution, at various CO2 loading. 1H nuclear magnetic resonance (NMR) and 13C NMR measurements were conducted at 295 K within the absorbent concentration of homoPZ 7.5 wt % and K2CO3 15 wt %/homoPZ 7.5 wt %. The CO2-loaded absorbents were prepared by using vapor−liquid equilibrium (VLE) apparatus at 333 K. The results show that the amount of carbamate and bicarbamate in the CO2-loaded K2CO3/homoPZ solution was larger than in the homoPZ solution. The free homoPZ that is able to react with CO2 increases in the aqueous K2CO3/homoPZ solution because the K2CO3 serves as a buffer. It was found that the NMR method can be used to determine the CO2 absorption mechanism of the absorbents.

1. INTRODUCTION Absorption using the chemical solvents is a mature technology that can be used for carbon dioxide (CO2) capture. Potassium carbonate (K2CO3) and alkanolamines such as monoethanolamine (MEA), 2-amino-2-methyl-1propanol (AMP), diethanolamine (DEA), triethanaolamine (TEA), and methyldiethanolamine (MDEA) are the most commonly used absorbent. 1 Aqueous MEA solution is considered an attractive absorbent at a low partial pressure of CO2 because it has a fast CO 2 absorption rate and the raw material is inexpensive compared to secondary and tertiary amines. However, operating costs for the MEA process are high because of the high reboiler heat duty and operation problems such as solvent degradation, corrosion, and solvent loss. Aqueous K2CO3 solution has a relatively low reboiler heat duty and the raw material is inexpensive. However, the slow absorption rate of CO 2 and the low solubility of K2CO3 at a lower temperature make the process unsuitable for coal-fired power plants when used for postcombustion at atmospheric pressure.2,3 These disadvantages can be resolved by blending amines such as piperazine (PZ). Several researchers have studied the kinetics, heat of absorption, and solubility of CO2 in aqueous PZ and K2CO3/PZ solutions.4−10 The results of these studies show that the rapid reaction of PZ with CO 2 is attributed to its unique molecular structure, cyclic diamine. They also found that the PZ carbamate in an aqueous solution increased in the presence of bicarbonate and carbonate ions. Recently, several researchers have studied the absorption of CO2 in new aqueous amines.11−13 The kinetics of CO2 absorption in aqueous 2-(1-piperazinyl)-ethylamine (PZEA) and MDEA/PZEA solutions were measured using the wetted wall column at 303, 313, and 323 K.14,15 PZEA is a piperazine derivative that has three amino groups. The overall reaction rate constant (kov) and second-order rate constant (k2) of PZEA were slightly higher than that of the © 2011 American Chemical Society

PZ. The CO2 absorption and desorption capacities of various amines were investigated using a screening apparatus.16−18 These studies investigated the structure and activity relationships of the amines. The chemical reaction between aqueous K2CO 3 solution and CO2 takes place via two parallel mechanisms (eqs 4 and 5). 19

2H2O(S) ↔ H3O+ + OH−

(1)

+ CO2 (aq) + 2H2O(S) ↔ HCO− 3 + H3O

(2)

− K2CO3(s) + H2O(S) ↔ 2K+ + HCO− 3 + OH

(3)

OH− + CO2 (aq) ↔ HCO− 3

(4)

2− − HCO− 3 + OH ↔ CO3 + H2O

(5)

The following reactions are considered in the chemical reaction between homoPZ and CO2 in the aqueous solution. The mechanism is similar to that of the aqueous PZ solution.4,6,8,19,20

homoPZ(s) + H2O(S) ↔ homoPZH+ + OH−

(6)

homoPZH+ + H2O(S) ↔ homoPZ(S) + H3O+

(7)

homoPZ(S) + CO2 (aq) + H2O(S) ↔ homoPZCOO− + H3O+

(8)

homoPZH+ + CO2 (aq) + H2O(S) ↔ H+homoPZCOO− + H3O+

(9)

Received: October 18, 2011 Revised: December 27, 2011 Published: December 27, 2011 1449

dx.doi.org/10.1021/ef201617b | Energy Fuels 2012, 26, 1449−1458

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Figure 1. Schematic diagram of a vapor−liquid equilibrium apparatus.

H+homoPZCOO− + H2O(S) −

+

↔ homoPZCOO + H3O

2. EXPERIMENTAL SECTION 2.1. Materials. Aqueous homoPZ and K 2 CO 3 /homoPZ solutions were prepared using deionized water with a concentration of homoPZ 7.5 wt %, K 2CO3 15 wt %/homoPZ 7.5 wt %. The following materials were used with their purities as received: K 2CO 3 (Samchun Chemicals, anhydrous 99.5%) and homoPZ (Alfa Asear, 98.0%). The CO2 gas (purity 99.99%) and N2 gas (purity 99.999%) were obtained from Special Gas Co., Korea. These materials were used without further purification. 2.2. Experimental Setup and Procedure. Figure 1 shows the schematic diagram of the experimental apparatus. The apparatus consisted of a gas reservoir, reactor, pressure-measuring instrument, and recorder. The gas reservoir and reactor were made of STS 316 with an internal volume of 300.29 cm3 and 322.56 cm 3, respectively. Nitrogen gas was used to correct the difference between the gas reservoir and the reactor volumes. The gas reservoir and reactor were placed in a circulating bath and heated by water. A pressure sensor PTB model (range: −1−10 kg f/cm 2) for Sensys Ltd., Korea measured the pressure of the gas. Furthermore, K-type thermocouples measured the temperature of the gas and the solution. The experiments were conducted at the temperatures of 313, 333, and 353 K, and the gas injection pressure was 490−785 kPa. The inlet gas (CO2 99.99%) was heated at 313 K in the reservoir prior to entering the reactor. The 100 mL of solution was fed into the reactor. Before the experiment began, residual gas was removed from the reactor using a vacuum pump. The stirring speed of the reactor was 170 rpm, and a smooth interfacial area was maintained throughout the experiment. As the gas reservoir and reactor reached the desired temperature, the valve was opened and CO2 gas was injected into the reactor. The pressure of the CO2 gas (P CO2) was calculated as the difference between the injected pressure and residual pressure in the reactor. The pressure of CO 2 gas in the reactor was decreased over time. A state of equilibrium was obtained when the pressure and temperature values were constant, at which point, the equilibrium partial pressure of the CO 2 was calculated. In this experiment, CO2 was injected into the reactor again when equilibrium was reached. The CO2 injected into the closed reactor was absorbed only by the absorbent. Therefore, the absorption rate was calculated by measuring changes in pressure every five seconds. The CO 2-loaded absorbents were collected for NMR measurements. 1H NMR and 13C NMR measurements were performed

(10)

homoPZCOO− + CO2 (aq) + H2O(S) ↔ homoPZ(COO−)2 + H3O+

(11)

homoPZCOO− + H2O(S) ↔ homoPZH+ + HCO− 3

(12)

homoPZ(COO−)2 + H2O(S) ↔ H+homoPZCOO− + HCO− 3

(13)

Aqueous homoPZ and K2CO3/homoPZ solutions are likely to absorb CO 2 as carbamate (homoPZCOO− / H + homoPZCOO− ), bicarbamate (homoPZ(COO− ) 2 ), and bicarbonate (HCO3−)/carbonate (CO32‑). Homopipeazine can absorb the 1 mol of CO2 per 1 mol of amine because the amine is a diamine. Furthermore, homopiperazine has an advantage that the boiling point is higher compared to piperazine. Therefore, the use of homopiperazine can reduce the solvent loss in the stripper. NMR spectroscopy was used to identify the species distribution and determine the reaction mechanism between absorbent and CO 2. The researchers performed a qualitative and quantitative analysis using 1 H NMR and 13 C NMR.21−27 The studies show that NMR analysis can be used to determine the species distribution and CO 2 absorption mechanism. In this study, an aqueous solution of potassium carbonate (K 2CO 3) and homopiperazine (homoPZ) blend was investigated as an absorbent. There is no literature on the CO 2 absorption mechanism of the aqueous K 2 CO 3 /homoPZ solution at various CO 2 loading. NMR spectroscopy was used to obtain a better understanding of CO 2 absorption mechanism of the absorbent. 1450


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Figure 2. CO2 equilibrium into aqueous solutions at various temperatures: (a) homoPZ 7.5 wt %; (b) K2CO315 wt %/homoPZ 7.5 wt %; ●, T = 313 K; ▽, T = 333 K; ■, T = 353 K.

by BRUKER AVANCE 500 MHz. The CO 2-loaded absorbents were prepared in NMR sample tubes with added deuterium oxide (D2O). The volumes of the sample and D2O (sample/D2O) were 30 μL/500 μL (1H NMR) and 300 μL/200 μL (13C NMR), respectively. D2O was a solvent and internal standard reference in 1H NMR and the hydrogen atoms bound in the amine group (NH) were changed into deuterium. Therefore, the peaks of these hydrogen atoms disappeared in the 1H NMR spectra. As an external standard reference, 1,4-dioxane was used for 13C NMR measurement. In 1H NMR and 13C NMR, the peaks of the D 2O and 1,4-dioxane were represented at 4.8 and 67.0 ppm, respectively. The 1H NMR spectra were obtained with a delay time (D1) of 1 s and the number of scans (NS) of 32. The 13C NMR measurements were performed with a delay time of 120 s and number of scans of 64.

15 wt %/homoPZ 7.5 wt % solutions. The CO2 loading means the moles of CO 2 per moles of solute (mol homoPZ or mol (K2CO 3 + homoPZ)). The equilibrium pressure of the CO 2 (P*CO2) increased according to the increased temperature. Figure 3 shows the CO 2 absorption rates of the homoPZ 7.5 wt % and K 2CO 3 15 wt %/homoPZ 7.5 wt % solutions. The data are obtained from first addition of CO2 into CO 2 free solution. The curves in the figure were obtained by first order linear regression. The slope of the linear curve represents the absorption rate of the absorbent. The absorption rate in this study just means the apparent rate to compare the performance of absorbents. The apparent rate constants (kapp) (kPa/min) of the homoPZ are 12, 20, and 30 at 313, 333, and 353 K. Furthermore, the apparent rate constants of the K2CO 3/ homoPZ are 24, 32, and 50 at 313, 333, and 353 K. The CO 2 absorption rates of the homoPZ and K2CO3/homoPZ

3. RESULTS AND DISCUSSION 3.1. Vapor−Liquid Equilibrium. Figure 2 shows the VLE curves beween the CO 2 and homoPZ 7.5 wt %, K2CO 3 1451


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Figure 3. Absorption rate for CO2 into aqueous solutions at various temperatures: (a) homoPZ 7.5 wt %; (b) K2CO3 15 wt %/homoPZ 7.5 wt %; ●, T = 313 K; ▽, T = 333 K; ■, T = 353 K; −, linear regression.

Figure 4. Molecular structures of the species in homoPZ-H2O-CO2 and K2CO3-homoPZ-H2O-CO2 systems.

absorption rates than the homoPZ solutions at 313, 333, and 353 K. These results show that the CO2 absorption capacity is

solutions increased according to the increased temperature. Furthermore, the K2CO3/homoPZ solutions had faster 1452


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Figure 5. COSY spectrum of the aqueous homoPZ 7.5 wt % solution at T = 298 K.

Figure 6. HMQC spectrum of the aqueous homoPZ 7.5 wt % solution at T = 298 K. 1453


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Figure 7. HMBC spectrum of the aqueous homoPZ 7.5 wt % solution at T = 298 K.

decreased, whereas the absorption rate is increased according to increased temperature. These data can be used to determine the appropriate temperature having an outstanding performance. 3.2. Species Distribution. 3.2.1. HomoPZ-H2O-CO2 System. Figure 4 shows the molecular structures of the species in the CO2-loaded homoPZ and K2CO3/homoPZ solutions. The NMR spectra of the CO2-loaded absorbents were analyzed based on the species. Two-dimensional NMR (2D NMR) spectoscopy was used to find the accurate peak location. Figures 5−7 show the correlation spectroscopy (COSY), heteronuclear multiple quantum coherence (HMQC) and heteronuclear multiple bond coherence (HMBC) spectra of the fresh homoPZ solution. The COSY spectrum indicates that hydrogens are coupling with each other. The HMQC and HMBC spectra determine the carbon to hydrogen connectivity. HMQC is selective for direct C−H coupling and HMBC gives longer range couplings (2−4 bond coupling). The species distributions in the 1D NMR spetra were determined based on these results of the 2D NMR analysis. Figure 8 shows the 1H NMR spectrum of the CO2-loaded homoPZ solution. The D2O was added to the homoPZ-H2OCO2 and K2CO3-homoPZ-H2O-CO2 systems to exchange hydrogen atoms that were bound to the amine group with the deuteriums. The deuteriums in the amine group produced a single peak in the water region. The hydrogen group (Number 3 in Figure 8) peak in the homoPZ appeared as a singlet. However, the hydrogen group (Number 2) peak in homoPZ appeared as a triplet because of the two hydrogen atoms (Number 1) that were adjacent to them. The hydrogen groups (Numbers 1 and 5) appeared as a

Figure 8. 1H NMR spectrum of homoPZ-H2O-CO2 system at T = 295 K, CO2 loading = 1.098 mol CO2/mol homoPZ.

quintet at the high field. Due to the fast hydrogen exchange with water, it is impossible to distinguish between the two species, protonated and unprotonated forms of h o m o P Z ( h o m o P Z / h o m o P Z H + , h o m o P Z C O O− / H+homoPZCOO−). The homoPZ dicarbamate (homoPZ(COO−)2) was only found in the 1H NMR spectrum at CO2 loading = 0.524. It was hard to find the peak loacation of the homoPZ dicarbamate because the peaks were very small. The results show that the free homoPZ and homoPZ carbamate were in the homoPZ-H2O-CO2 systems. Table 1 presents the peak locations and areas of 1454


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Table 1. Chemical Shifts and Peak Areas of 1H NMR Spectra for the HomoPZ-H2O-CO2 System at T = 295 K δ, ppm (area) system

CO2 loading

homoPZ-H2OCO2

hydrogen group in Figure 8 0.524 0.972 1.098 1.184

free homoPZ

homoPZ carbamate

1

2

3

4

5

6

1.914 2.008 2.003 2.027

3.043 3.147 (1.70) 3.169 (1.60) 3.171 (1.79)

3.068 3.197 (1.64) 3.227 (1.59) 3.229 (1.79)

3.068 3.277 3.275 3.276

1.914 2.033 2.037 2.038

3.541 3.651 3.654 3.655

7 (1.00) (1.00) (1.00) (1.00)

3.469 3.510 3.511 3.512

8 (1.00) (1.01) (1.00) (1.00)

3.068 3.296 3.301 3.302

spectra shifted to the lower field according to the increased CO2 loading. The pH level decreased as the CO2 loading increased. The peak locations shifted due to changes in the pH level, and some peaks on the spectra overlap because of this chemical shift.24 Figure 9 shows the 13C NMR spectrum of the CO 2-loaded homoPZ solution. 13C NMR spectra give information for the carbons in the carbamate and HCO 3−/CO 32‑. The carbon peaks directly related to CO2 appeared at the low field (163.69−160.64 ppm). The oxygen atoms in the carboxyl group have a large electronegativity. The electrons of the carbons are deshielded by the oxygen atoms. Therefore, the carbon peaks appeared at the lower field. 3.2.2. K2CO3-homoPZ-H2O-CO2 System. Figure 10 shows the 1H NMR spectrum of the CO2-loaded K2CO3/homoPZ

Figure 10. 1H NMR spectrum of K2CO3-homoPZ-H2O-CO2 system at T = 295 K, CO2 loading = 0.848 mol CO2/(mol K2CO3 + mol homoPZ).

solution. The species distribution of the CO 2-loaded K 2 CO 3 /homoPZ solution was similar to that of the CO 2-loaded homoPZ solution. The free homoPZ and c a r b a m a t e w e r e f o u n d in t h e h o m o P Z - H 2 O -C O 2 and K 2 CO 3 -homoPZ-H 2 O-CO 2 systems. However, the bicarbamate was found in the K2CO 3/homoPZ solutions. The bicarbamate hydrogens had three peaks, and one of the peaks in the hydrogen group (Number 9 in Figure 10) appeared as a quintet. Furthermore, two hydrogen groups (Numbers 10 and 11) appeared as a triplet and singlet, respectively. Table 2 presents the peak locations and areas in the K2CO3-homoPZ-H2O-CO2 system in the 1H NMR spectra. The peak locations show that all the peaks in the 1H NMR spectra shifted to the lower field according to the increased CO2 loading. Furthermore, some

Figure 9. 13C NMR spectrum of homoPZ-H2O-CO2 system at T = 295 K, CO2 loading = 1.098 mol CO2/mol homoPZ: (a) low field; (b); medium field; (c) high field.

the homoPZ-H2O-CO2 system in the 1H NMR spectra. The peak locations show that all the peaks in the 1H NMR 1455


3.364 (0.17)

3.366 (0.15) 3.306 (0.16)

3.303 (0.18) 2.001

2.002 3.251

3.248 3.480 (1.00)

3.484 (1.00) 3.609 (1.00)

3.615 (1.00)

peaks overlapped so it was difficult to find their accurate locations. Figure 11 shows the relative ratio of the species in the homoPZ-H2O-CO2 and K2CO3-homoPZ-H2O-CO2 systems.

3.238 3.227 (0.26)

1.962

3.233 3.220 (0.29)

3.114 (0.27) 1.764 (0.07) 1.055

3.109 (0.29) 1.762 (0.07) 1.030

3.223 3.213 (0.29) 1.762 (0.11) 1.000

3.106 (0.29)

1.962

3.364 (0.25)

3.364 (0.24) 3.291 (0.24) 1.989 3.240 3.479 (1.01) 3.611 (1.00)

3.370 (0.37)

3.213 1.750 (0.13) 0.946

3.199 (0.36)

1.955

3.303 (0.25) 1.984 3.224 3.476 (1.00) 3.610 (1.00)

3.374 (0.42)

3.100 (0.37)

1.954

3.309 (0.39)

3.313 (0.43) 1.935

1.978 3.204

3.120−3.132 3.466 (0.81)

3.478 (1.00) 3.598 (1.00)

3.559 (0.81) 1.935 3.120−3.132

3.193

3.118

3.141 (0.37)

3.074 (0.40)

3.098 (0.36)

1.760 (0.22)

1.755 (0.18)

0.657

1.859 (0.20) 0.439

0.848

1.785 0.218

1.954

3.372 (0.38)

3.373 (0.13) 3.313 (0.13)

3.312 (0.38) 1.900

1.808 2.851

2.984−3.025 3.440 (0.73)

3.414 3.426

2.984−3.025 3.037 (0.54) 2.981

3.493 (0.73) 1.884

2.832 2.912 2.912

1.796

11 10 7 6

homoPZ carbamate free homoPZ

5 4 3 2 1 hydrogen group in Figure 10

CO2 loading system

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Figure 11. Relative peak areas of species in 1H NMR: (a) homoPZH2O-CO2, (b) K2CO3-homoPZ-H2O-CO2; ■, homoPZ/homoPZH+; □, homoPZCOO−/H+homoPZCOO−; hatched pattern, homoPZ(COO−)2.

The relative ratios of the species were calculated using the peak areas in the 1H NMR spectra. The graphs show that the relative ratios of the homoPZCOO−/H+homoPZCOO− and homoPZ(COO−)2 to homoPZ/homoPZH+ were high in the aqueous K2CO3/homoPZ solution. In the aqueous K2CO3/homoPZ solution, large amounts of bicarbonate and carbonate formed by dissociation of K2CO3. These served as a buffer, reducing the homoPZH+ and a large quantity of homoPZ available for CO2 absorption. Therefore, the amount of free homoPZ in the aqueous K2CO3/homoPZ solution was larger than that of the homoPZ. Furthermore, the formation of bicarbamate formation was promoted. Figure 12 shows the 13C NMR spectrum of the CO2-loaded K2CO3/homoPZ solution. The carbamate, bicarbamate, and bicarbonate/carbonate peaks related to CO2 are presented in the lower field. The results of the 13C NMR analysis agreed with the 1H NMR. Some carbon peaks (e.g., Numbers 2 and 3) overlapped so their locations could not be found.

K2CO3-homoPZH2O-CO2

δ, ppm (area)

Table 2. Chemical Shifts and Peak Areas of the 1H NMR Spectra for the K2CO3-homoPZ-H2O-CO2 System at T = 295 K

8

9

homoPZ bicarbamate

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NMR analysis shows that the CO2-loaded K2CO3/homoPZ solution contained free homoPZ, homoPZ carbamate, homoPZ bicarbamate, and bicarbonate/carbonate. HomoPZ bicarbamate was not found in the CO2-loaded homoPZ solution. The relative ratios of the carbamate and bicarbamate to homoPZ in the K2CO3-homoPZ-H2O-CO2 system were larger than in the homoPZ-H2O-CO2 system. The relative ratios of carbamate and bicarbamate reached 50% and 80% in the aqueous homoPZ and K2CO3/homoPZ solutions, respectively. These results show that the bicarbonate/carbonate ions in the aqueous K2CO3/ homoPZ solution reduced the homoPZH+ and a large quantity of homoPZ was available for CO2 absorption.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-860-3758. Fax: +82-42-860-3134. E-mail: [email protected].

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ACKNOWLEDGMENTS This research was supported by a grant (code 2011K000077) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Education Science and Technology of Korean government.

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REFERENCES

(1) Kohl, A. L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Company: Huston, 1997. (2) Oexmann, J.; Hensel, C.; Kather, A. Post-combustion CO2capture from coal-fired power plants: Preliminary evaluation of an integrated chemical absorption process with piperazine-promoted potassium carbonate. Int. J. Greenhouse Gas Control 2008, 2, 539. (3) Cullinane, J. T.; Rochelle, G. T. Kinetics of carbon dioxide absorption into aqueous potassium carbonate and piperazine. Ind. Eng. Chem. Res. 2006, 45, 2531. (4) Bishnoi, S. G.; Rochelle, G. T. Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility. Chem. Eng. Sci. 2000, 55, 5531. (5) Pérez-Salado Kamps, Á .; Xia, J.; Maurer, G. Solubility of CO2 in (H2O + Piperazine) and in (H2O + MDEA + Piperazine). AIChE J. 2003, 49, 2662. (6) Derks, P. W. J.; Dijkstra, H. B. S.; Hogendoorn, J. A.; Versteeg, G. F. Thermodynamics of Aqueous Potassium Carbonate, Piperazine, and Carbon Dioxide. AIChE J. 2005, 51, 2311. (7) 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. (8) Samanta, A.; Roy, S.; Bandyopadhyay, S. S. Physical Solubility and Diffusivity of N2O and CO2 in Aqueous Solutions of Piperazine and (N-Methyldiethanolamine + Piperazine). J. Chem. Eng. Data 2007, 52, 1381. (9) Cullinane, J. T.; Rochelle, G. T. Carbon Dioxide Absorption with Aqueous Potassium Carbonate Promoted by Piperazine. Chem. Eng. Sci. 2004, 59, 3619. (10) Cullinane, J. T.; Rochelle, G. T. Thermodynamics of Aqueous Potassium Carbonate, Piperazine, and Carbon Dioxide. Fluid Phase Equilib. 2005, 227, 197. (11) Puxty, G.; Rowland, R.; Allport, A.; Yang, Q.; Bown, M.; Burns, R.; Attalla, M. Carbon dioxide postcombustion capture: A novel screening study of the carbon dioxide absorption performance of 76 amines. Environ. Sci. Technol. 2009, 43, 6427. (12) García-Abuín, A.; Gómez-Díaz, D.; Navaza, J. M.; Vidal-Tato, I. Kinetics of carbon dioxide chemical absorption into cyclic amines solutions. AIChE J. 2011, 57, 2244.

Figure 12. 13C NMR spectrum of K2CO3-homoPZ-H2O-CO2 system at T = 295 K, CO2 loading = 0.848 mol CO2/(mol K2CO3 + mol homoPZ): (a) low field; (b); medium field; (c) high field.

4. CONCLUSION The stirred cell reactor was used to obatin the VLE data and sample for NMR measurements. The VLE experiments were perfomed at 313, 333, and 353 K. The CO2 absorption capacity decreased according to the increased temperature, while the CO2 absorption rate increased according to the increased temperature. The VLE samples at 333 K were collected for use in the NMR measurements. The CO2 absorption rates in the aqueous K2CO3 15 wt %/homoPZ 7.5 wt % solution were faster than in the aqueous homoPZ 7.5 wt % soution. The 1457


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