Mechanisms of CO2 Capture into Monoethanolamine Solution with

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Mechanisms of CO2 Capture into Monoethanolamine Solution with Different CO2 Loading during the Absorption/Desorption Processes Bihong Lv, Bingsong Guo, Zuoming Zhou, and Guohua Jing* Department of Environmental Science & Engineering, College of Chemical Engineering, Huaqiao University, Xiamen, Fujian 361021, China ABSTRACT: Though the mechanism of MEA-CO2 system has been widely studied, there is few literature on the detailed mechanism of CO2 capture into MEA solution with different CO2 loading during absorption/ desorption processes. To get a clear picture of the process mechanism, 13C nuclear magnetic resonance (NMR) was used to analyze the reaction intermediates under different CO2 loadings and detailed mechanism on CO2 absorption and desorption in MEA was evaluated in this work. The results demonstrated that the CO2 absorption in MEA started with the formation of carbamate according to the zwitterion mechanism, followed by the hydration of CO2 to form HCO3−/CO32−, and accompanied by the hydrolysis of carbamate. It is interesting to find that the existence of carbamate will be influenced by CO2 loading and that it is rather unstable at high CO2 loading. At low CO2 loading, carbamate is formed fast by the reaction between CO2 and MEA. At high CO2 loading, it is formed by the reaction of CO3−/CO32− with MEA, and the formed carbamate can be easily hydrolyzed by H+. Moreover, CO2 desorption from the CO2-saturated MEA solution was proved to be a reverse process of absorption. Initially, some HCO3− were heated to release CO2 and other HCO3− were reacted with carbamic acid (MEAH+) to form carbamate, and the carbamate was then decomposed to MEA and CO2. calculations, Hwang et al.13 found that both the CO2 capture by MEA and the solvent regeneration followed a zwitterionmediated two-step mechanism. Also, from the zwitterionic intermediate, the relative probability between deprotonation (carbamate formation) and CO2 removal (MEA regeneration) largely depends on the interaction between the zwitterion and adjacent H2O molecules. To date, there is no direct evidence to prove the existence of the zwitterion. Da Silva and Svendsen14 reported that no stable zwitterion species was formed and a single-step reaction mechanism would be more compatible for the reaction of the primary alkanolamine and CO2. In this mechanism, the bond formation and proton transfer to the base take place simultaneously, giving a single-step, third order reaction. In this mechanism, water molecule would act as a proton acceptor instead of MEA.

1. INTRODUCTION It has been widely accepted that carbon dioxide (CO2) emissions that are produced as a result of human activity is responsible for rapid global climate change.1,2 Chemical absorption is one of the most promising technologies to capture CO2.3,4 Due to its high reactivity with CO2, monoethanolamine (MEA) has been used in industrial processes to capture CO2 for many years. In the past few decades, lots of experimental and theoretical works have been carried out to investigate the reaction mechanism of CO2 absorption into MEA solution.5−8 Nevertheless, there is still a controversy regarding to the details of the reaction mechanism.9 There were three reaction mechanisms proposed for the reaction between CO2 and MEA. The zwitterion mechanism suggested that primary and secondary alkanolamines were first reacted with CO2 to form zwitterions10,11 and then the intermediate was instantaneously neutralized by the base (such as amine, OH−, or H2O) to form carbamate.12 CO2 + RNH 2 ⇌ RNH 2+COO−

(1)

RNH 2+COO− + RNH 2 ⇌ RNHCOO− + RNH3+

(2)

CO2 + RNH 2 ···B ⇌ RNHCOO− ···BH+

(3)

However, MEA is known to have overall second-order kinetics in an aqueous solution. Afterward, Shim et al.15 also proved that MEA was a more suitable base than water and doubted the single-step reaction mechanism.

9

Recently, Xie et al. used molecular orbital reaction pathway calculations to compute the reaction free energy landscapes for the reaction steps, and the results proved that the most favorable reaction channel was to form carbamate via a zwitterion intermediate. Using static quantum chemical © XXXX American Chemical Society

Received: May 12, 2015 Revised: July 22, 2015 Accepted: August 3, 2015

A

DOI: 10.1021/acs.est.5b02356 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology The third mechanism is the carbamic acid mechanism.16 Based on this mechanism, MEA first reacts with CO2 to form carbamic acid that is then catalyzed by another MEA to form carbamate. CO2 + RNH 2 ⇌ RNHCOOH

(4)

RNHCOOH + RNH 2 ⇌ RNHCOO− + RNH3+

(5)

2.2. CO2 Absorption and Desorption. The absorption/ desorption experiments were carried out in the three-neck flask, and the schematic diagram of the experimental setup was shown in Figure 1. During the absorption, CO2 was

Among these three mechanisms, the zwitterion mechanism is the most commonly accepted mechanism. Meanwhile, it is also widely used to explain the reaction of CO2 absorption into other solvents, for example, mixed amine and functionalized ionic liquids.17−20 Iliuta et al. analyzed the kinetics of DEA dispersion in [hmim][Tf2N] for efficient CO2 capture based on zwitterion mechanism.21 Though a large number of works have been carried out to clarify the reaction between CO2 and MEA, its mechanism is still controversial. In fact, CO2 capture into MEA solution is a complex process that is influenced by the chemical absorption and other interactions. As the most important intermediates, the changes of carbamate and carbamic acid (MEAH+) are crucial for elucidating the mechanism of CO2 capture into MEA solution. Bottinger et al.22 suggested that amine, carbamate, bicarbonate, and carbon dioxide were all obtained in the absorption of MEA solution by using both 1H and 13C NMR analysis. McCann et al.23 found that there were three parallel and reversible reactions of free amine with CO2, carbonic acid, and bicarbonate ions in the absorption of CO2 by MEA. Though carbamate was unstable in their work, they assumed the possible hydrolysis reaction path of carbamate without verifying them. Han et al.24 used molecular dynamics simulations to unveil the reaction pathways for CO2 capture in 30 wt % MEA aqueous solution. Their calculations found that the reaction between MEA and CO2 was affected by the partial pressure or CO2 loadings in the solution. Nevertheless, there are few experimental explorations about the influence of CO2 loading on the reaction mechanism. Besides, most reported works are focused on CO2 absorption/desorption performance and the absorption mechanism,25−27 but there are few researches on the desorption mechanism. A fundamental understanding of the desorption mechanism has crucial significance for designing more effective MEA process for CO2 capture, and further research is needed in the mechanism of CO2 desorption. To get a clearer picture of the reaction mechanisms, 13C nuclear magnetic resonance (NMR) was used to analyze the reaction intermediates under different CO2 loadings. The effects of intermediates, such as HCO3−/CO32− and H+, on the hydrolysis of carbamate were then investigated. Also, the reaction mechanism of CO2 capture into MEA solution with different CO2 loadings was proposed during the absorption and desorption processes.

Figure 1. Schematic diagram of the experimental setup 1, CO2 cylinder 2, mass flowmeter 3, magnetic stirring apparatus 4, the three-neck flask 5, acidimeter 6, sampling port.

continuously supplied to the absorber with a flow rate of 30 mL/min and the initial mass fraction of MEA solution was 30%. The CO2-saturated solutions were regenerated in the flask with oil bath at 110 °C. The absorption/desorption experiments were finished when pH value of the system was tended to be stable. 2.3. 13C NMR Analysis. 13C NMR spectroscopy is one of the most suitable analytical methods for detecting inorganic carbon species in solutions.28 Thus, 0.5 mL of each MEA solvent with different CO2 absorption loading were characterized by 13C NMR (Bruker AVIII500 MHz), using an internal standard of 0.1 mL D2O for the deuterium lock. The chemistry of MEA-CO2 system using the 13C NMR technique is expressed in Figure 2.29,30

Figure 2. Molecular structure and type of carbon nuclei in MEA-CO2 system.

3. RESULTS 3.1. CO2 Absorption into MEA Solution. Normally, reaction temperature during CO2 capture has been set as constant and the change of temperature caused by the chemical reaction can be neglected. In present work, CO2 absorption into MEA solution was carried in ambient atmosphere. The result is given in Figure 3. CO2 is an acidic gas. With its continuous supply to the absorber, CO2 loading in the solution was increased with the decreasing pH as the absorption time increased. It was interesting to find that CO2 capture into MEA solution could be divided into two stages. In the first stage, pH decreased from 12.54 to 9.0 and temperature gradually increased from 291 to 305 K. In the second stage (pH less than 9.0), the pH continued to decrease (from 9.0 to 7.83) and the temperature began to decrease until reaching to the room temperature. It was noted that the reaction rate of the first stage was faster than that of the second stage. Meanwhile, the CO2 loading in the first stage was up to 0.40 mol CO2/ mol MEA, contributing to nearly 80% of the total loading (0.53 mol CO2/ mol MEA).

2. MATERIALS AND METHODS 2.1. Chemicals. Analytical reagents of monoethanolamine (MEA), NaHCO3, Na2CO3 and HCl were all purchased from the Sinopharm Group Chemical Reagent Co. Ltd., China. CO2 (>99.999%) was supplied by Fujian Nanan Chenggong Gas Co., Ltd., China. D2O was provided by J&K Scientific Ltd. Ultrapure water (≥18.2 MΩ·cm) was used in all the experiments and no further purification was performed on the materials used. B


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Environmental Science & Technology

was 12.54 and decreased as the CO2 loading increased. There were new signals (3′) appeared at 164.5 ppm when the pH value was 11.00, which was assigned to the carbamate of MEACO2−.6,23 As the CO2 loading continuously increased, pH value varied from 11.00 to 9.00 and the peak intensity of the carbamate was also gradually enhanced. In the first stage, the formation of carbamate was the main reaction between CO2 and MEA. There was no signal that proved the existence of the zwitterion in Figure 4. When pH value was less than 8.5, both of the pH value and the peak intensity of the carbamate were decreased as CO2 loading increased. The results indicated that the carbamate began to decompose in this period, and the decomposition rate was faster than that of the formation rate. Meanwhile, a new signal at 160.2 ppm was assigned to the bicarbonate/carbonate (HCO3−/CO32−) species,23 and the peak intensity was enhanced as CO2 loading increased. Unfortunately, it was impossible to separate such HCO3−/ CO32− species because of the fast proton exchange rather than NMR time scale. The hydration reaction of CO2 was the main reaction in the second stage of the absorption. Finally, the signals of carbamate and HCO3−/CO32− species all existed in the CO2 saturated solution. In order to verify whether this finding is consistency in other alkanolamine, CO2 capture into diethanolamine (DEA) and methyldiethanolamine (MDEA) were also investigated. The 13 C NMR spectra of DEA and MDEA at different CO2 loadings are shown in Figure 5. Similar with the results of MEA (Figure 5(a)), the peak intensity of the carbamate (164.5 ppm) were first increased and then decreased as the CO2 loading increased. There was also a new signal at 160.2 ppm assigned toHCO3−/ CO32− appeared when pH decreased from 9.5 to 9.0. Its peak intensity was enhanced as CO2 loading increased. The results indicated that CO2 capture into DEA solution also began with the formation of carbamate, followed by the hydration reaction of CO2 to form HCO3−/CO32−, and accompanied by the hydrolysis reaction of carbamate. Nevertheless, MDEA is a tertiary amine and can not form carbamate in the absorption. As given in Figure 5(b), the peak intensity of HCO3−/CO32− (160.2 ppm) was increased as the absorption reaction occurred. This result indicated that CO2 absorption into MDEA solution only contained the hydration reaction of CO2. This agreed well with the existing reported works.31

Figure 3. Performance of CO2 absorption into MEA solution.

3.2. 13C NMR Spectra of CO2 Absorption into Amine Solution. As shown in Section 3.1, pH decreased as CO2 was continuously supplied to the absorber. Thus, the pH value was used to estimate the CO2 loading of the system. The spectra for different CO2 absorption loadings into MEA solution are shown in Figure 4. The initial pH value of the MEA solution

CO2 + R1R 2R3N + H 2O ⇌ R1R 2R3NH+ + HCO3−

(6)

3.3. Carbamate Hydrolysis. As reported in most works, the zwitterion of RNH + COO − was easy to undergo deprotonation by a base B (base catalysis) to form carbamate, where B could be an amine, OH−, or H2O.12 RNH 2+COO− + B ↔ RNHCOO− + BH+

(7) −

Figure 4. loadings.

That indicates that the presence of amine, OH , or H2O would promote the formation of carbamate.32,33 However, the concentrations of MEA and OH− were quite low in the solution under a high CO2 loading condition. Besides, we also found that the peak intensity of carbamate did not change even after 12 h while pH value was 9.00. The results showed that H2O could not affect the formation of carbamate in weakly alkalinity. It seemed that HCO3−/CO32− and H+ were the main species in the solution, affecting the hydrolysis of carbamate. Based on the results of Section 3.2, the hydrolysis of carbamate began when pH value of the solution was less than 9.0 under a high CO2 loading (0.40 mol/mol MEA). At this

13

C NMR spectra of MEA solution with different CO2 C


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Figure 6. 13C NMR spectra at different conditions. (1) MEA + CO2, pH 9.00; (2) MEA + CO2+ HCl, pH 7.83; (3) MEA+CO2+ NaHCO3, pH 8.92; (4) MEA + CO2+Na2CO3, pH = 9.23.

be hydrolyzed into HCO3− and reacted with MEA to form carbamate. The change of carbamate in these three conditions presented that H+ was the most important factor for promoting the hydrolysis of carbamate, and HCO3−/CO32− could react with MEA to form carbamate. 3.4. 13C NMR Spectra of CO2 Desorption from CO2Saturated MEA Solution. Since CO2 absorption into MEA solution was complex and did not simply obey the zwitterion mechanism, further research was needed to investigate the mechanism of CO2 desorption. The CO2-saturated MEA solution was recovered at 110 °C under atmospheric pressure. Similarly, pH value was used to evaluate the regeneration extent of the solvent. The 13C NMR spectra of the CO2-saturated MEA solution after regeneration with different CO2 loadings were compared in Figure 7. The carbamate and HCO3−/CO32− were both presented in the CO2-saturated solution. As the regeneration reaction continued, the pH increased and the CO2 was gradually desorbed from the solution. Similarly, the regeneration could also be divided into two stages. At the initial stage of the regeneration, the pH increased from 7.93 to 9.72. The peak intensity of HCO3−/CO32− was decreased and that of the carbamate was increased significantly. The signals of HCO3−/CO32− disappeared in the 5th spectrum when pH value was 9.72, and the peak intensity of carbamate reached the maximum value. In the second stage, pH increased from 9.72 to 10.83, and the peak intensity of carbamate was decreased and the peak intensity of MEA was increased as the regeneration continued. The regeneration experiment of the CO2-saturated MEA solution was stopped when there was no gas desorbed from the system. The final pH value was 10.83, which was lower than that of the fresh MEA solution (12.54). The signal of carbamate still existed in the eighth spectrum but the peak intensity was weak. The results indicated that MEA solution could not be completely regenerated under the current experimental condition. Most reported works also found that the absorption capacity of the amine-based solvent was decreased with the regeneration cycle increased.19,20,25

Figure 5. 13C NMR spectra of (a) DEA and (b) MDEA with different CO2 loadings.

moment, there was carbamate produced in the solution, but there was no peak of HCO3−/CO32−, and the absorption experiments stopped. The saturated solutions of Na2CO3 and NaHCO3 (1 mL), and HCl were separately added to these MEA solvents to obtain the desired conditions. The results are given in Figure 6. The pH value of the solution was adjusted from 9.00 to 7.83 with the addition of HCl. Based on the results of the first and second spectrum, it could be obviously observed that the peak intensity of carbamate was decreased and that of the HCO3−/CO32− was increased significantly in second spectrum. It seemed that the carbamate was hydrolyzed by H+ to form HCO3−/CO32−. The pH decreased from 9.00 to 8.92 when NaHCO3 was added to the system. Compared with first spectrum, the peak intensity of carbamate in the 3th spectrum was enhanced but the peak intensity of MEA was decreased in the spectrum, and a signal of HCO3− was appeared at 160.2 ppm. The extra formation of the carbamate and the decrease of MEA concentration indicated that MEA could react with HCO3− to form carbamate. In the fourth spectrum, the pH value increased from 9.00 to 9.23 when Na2CO3 solution was added to the system. Similar with the results of NaHCO3, the peak intensity of carbamate was increased and that of the MEA was decreased. CO32− could D


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stages. In the first stage (low CO2 loading), CO2 reacted with MEA to form the carbamate of MEA-CO2−. The reaction rate was fast and the reaction was exothermic.34,35 Based on the results of the 13C NMR analysis, CO2 absorption into MEA was found to form carbamate first and then to form MEAH+. Since MEA is known to have overall second-order kinetics in an aqueous solution, the single-step mechanism is not suitable due to its third order reaction. In this stage, though there was no signal that proved the existence of the zwitterions, the zwitterion mechanism seemed to be more suitable to describe the chemical reaction between MEA and CO2. As the absorption was carried on, the CO2 loading in the solution was increased while the concentration of MEA was decreased. The chemical reaction became weak. In the second stage, CO2 hydration was the main reaction. In this period (high CO2 loading), carbamate was rather unstable and would be decomposed easily. The results proved that CO2 absorption into MEA solution under high CO2 loading condition was different with that under low CO2 loading. Therefore, a single reaction mechanism was not enough to clarify the real reaction during the absorption process. The reaction of CO2 capture into DEA solution was similar to that of MEA solution and was also affected by the CO2 loading. However, CO2 absorption into MDEA solution was different with that of MEA, and there was only CO2 hydration carried out in MDEA solution. 4.2. Carbamate Hydrolysis. The above results showed that the reaction process of CO2 capture into MEA solution was rather complicate, and the formation of carbamate varied as the reaction was carried out. It was also reported in other works that the carbamate was highly unstable and was easy to decompose.36 However, there is few direct evidence to prove the hydrolysis mechanism of carbamate. Base on the results in Section 3.2 and Section 3.3, carbamate was quickly formed during the chemical reaction between MEA and CO2 at low CO2 loading, and the formed carbamate started to hydrolyze at high CO2 loading due to an enhanced CO2 hydration. The species of H+ and HCO3−/CO32− were the key factors to influence the stability of carbamate. HCO3−/CO32− could react with MEA to form carbamate, whereas H+ could promote the hydrolysis of carbamate to form MEAH+ and HCO3−.The results proved the reaction pathway of carbamate hydrolysis in the MEA solution at high CO2 loading, and it seemed that H+ was the most significant factor. 4.3. Desorption of CO2-Saturated MEA Solution. Similar to the absorption process, desorption of the CO2saturated MEA solution could also be divided into two stages.

Figure 7. 13C NMR spectra of MEA solution after different regeneration extents.

4. DISCUSSION 4.1. CO2 Absorption into MEA Solutions. On the basis of the preliminary 13C NMR spectroscopic measurements, the results presented that CO2 absorption into MEA solution contained chemical and physical absorption simultaneously in the whole process. The reaction could be divided into two

Table 1. Literature Data on the Reaction Between CO2 and MEA Aqueous Solution investigators ́ Garcia-Abui ń et al.6 Han et al.,24 McCann et al.,23 da Silva et al.,14 Arstad et al.,16 this work

concentration

CO2 capture

analytic technique

results

0.7 mol/L

absorption

13

1

30 wt %

absorption

2.28 mol/L 4.19 mol/L 3.01 mol/L 

absorption

molecular dynamics simulations 13 C, 1H NMR and UV−vis spectroscopy

at low CO2 partial pressure, produce [MEAHt][MEACOO−], at high CO2 partial pressure, water directly reacts with CO2 there are three parallel, reversible reactions of the free amine with CO2, carbonic acid, and the bicarbonate ion

absorption

ab initio calculations

water molecule would act as a proton acceptor instead of MEA



absorption

30 wt %

absorption and desorption

DFT and high-level ab initio calculations 13 C NMR and experimental observations

MEA first reacts with CO2 to form carbamic acid and then catalyzes by another MEA to form carbamate at low CO2 loading, form carbamate according by the zwitterion mechanism, at high CO2 loading, form HCO3−/CO32−

C and H NMR

MEA produce a mixture of carbamate and bicarbonate as reaction products

E


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Environmental Science & Technology At the initial stage of the regeneration (with high CO2 loading), some of the HCO3−/CO32− desorbed CO2 under thermolysis and some of them reacted with MEAH+ to form carbamate. While HCO3−/CO32− was completely desorbed, the desorption reaction went into the second stage (with low CO2 loading), in which carbamate began to react with H+ to form MEA and desorbed CO2 during regeneration. The pH value of the solution increased as CO2 was continuously released from the CO2-saturated MEA solution. Lastly, the signals of carbamate were still existed in the regeneration solution, indicating that MEA solution could not be completely regenerated under the current conditions. 4.4. Mechanism of CO2 Capture into MEA Solution. As mentioned above, a large number of works had been carried out to find out the reaction mechanism of CO2 absorption into MEA solution. Table 1 shows some investigations on the reaction between CO2 and MEA. The differences among each work included the analytic technique, MEA concentration and CO2 loading of the solution. In most of the existing works, CO2 reacted with MEA to form carbamate according the zwitterion ́ mechanism. Garcia-Abui ń et al.6 observed that MEA produced a mixture of carbamate and bicarbonate as the main reaction products during absorption. McCann et al.23 also found that there were carbamate, carbonic acid, and the bicarbonate ion in MEA-CO2 system. Though these products existed in all our investigations, their content and change trend were different during different CO2 loading. The reaction between MEA and CO2 changed as the CO2 loading increased. The absorption reaction started with the reversible reactions between MEA and CO2 to form carbamate at low CO2 loading, followed by the CO2 hydration to form HCO3−/CO32− under high CO2 loading, and accompanied by the hydrolysis of carbamate. The mechanism of CO2 capture into MEA solution with different CO2 loading is shown in Figure 8.

Besides the hydration reaction of CO2, the intermediate products of carbamate and MEAH+ were found to be affected by other species in the solution. McCann et al.23 also reported the hydrolysis reaction of carbamate, while Han et al.23 calculated the reaction route. However, they did not prove the reaction mechanism via experiments. Based on our results by 13C NMR and experimental observations, carbamate would be easily influenced by the species in the solution at high CO2 loading. In this period, some of MEA reacted with CO2 to form carbamate, and some of MEA also could react with HCO3− to form carbamate. HCO3− + RNH 2 → RNHCOO− + H 2O 2−

Meanwhile, CO3 could be hydrolyzed into similarly reacted with MEA to form carbamate.

(10)

HCO3−

CO32 − + H+ + RNH 2 → RNHCOO− + H 2O

and (11)

At high CO2 loading, pH value of the solution decreased significantly. In this condition, carbamate would react with H+ to form HCO3−. RNHCOO− + H+ + H 2O → HCO3− + RNH 2H+

(12)

Thus, if the initial pH value of the solution and the concentration of MEA were all quite low in the system, the reversible reaction of MEA with CO2 was weak. The carbamate would be decomposed fast by H+ to carbonic acid, or to HCO3−. Thus, the mechanism may be misleading because of the unstable formation of carbamate. This might be the main reason why the carbamic acid mechanism suggested that MEA first reacted with CO2 to form carbamic acid and then was catalyzed by another MEA to form carbamate. Meanwhile, CO2 desorption from the CO2-saturated MEA solution was found to be a reverse process of the absorption. Initially, some HCO3− was heated to release CO2 and other HCO3− reacted with MEAH+ to form carbamate. heating

CO32 ‐ + 2H+ ⎯⎯⎯⎯⎯⎯→ CO2 (g) + H 2O

(13)

heating

HCO3− + H+ ⎯⎯⎯⎯⎯⎯→ CO2 (g) + H 2O

(14)

2HCO3− + RNH3+ heating

⎯⎯⎯⎯⎯⎯→ RNHCOO− + 2H 2O + CO2 (g)

(15)

Whereafter, the carbamate was decomposed to MEA and CO2 under thermolysis and the solvent was regenerated.

Figure 8. Mechanism of CO2 capture into MEA solution.

heating

RNHCOO− + H+ ⎯⎯⎯⎯⎯⎯→ RNH 2 + CO2 (g)

At low CO2 loading, CO2 absorption into MEA was an exothermic reaction and the reaction rate was fast. In this stage, the absorption capacity of the solvent was found to be 0.40 mol CO 2 /mol MEA, shown a similarity to the zwitterion mechanism. Subsequently, the pH value decreased and the CO2 loading increased as the experiment went on. In this period (with high CO2 loading), the chemical reaction became weak and the hydration reaction of CO2 was enhanced. The reaction could be expressed as follows: CO2 + H 2O ↔ H+ + HCO3−

(8)

CO2 + H 2O ↔ 2H+ + CO32 −

(9)

(16)

In summary, the mechanism of CO2 capture into MEA solution was different from the existing reported works due to the influence of CO2 loading. CO2 absorption into MEA solution started with the chemical reaction between MEA and CO2, and followed by the hydration of CO2 in the solution. Moreover, desorption of the CO2-saturated MEA solution was proved to be the reverse process of the absorption. First, some of HCO3− reacted with MEAH+ to form carbamate and some of them desorbed CO2. After that, the carbamate was decomposed to MEA by themolysis. The results indicated that the mechanisms of CO2 capture into MEA solution varied with CO2 loading. F


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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-592-6166216; fax: +86-592-6162300; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was sponsored by the National Natural Science Foundation of China (21277053), the Program for New Century Excellent Talents in the University of China (NCET11-0851), the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-YX104) and the Scientific Research Funds of Huaqiao University (15BS106).

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Mechanisms of CO2 Capture into Monoethanolamine Solution with

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