CO2 Capture by Tertiary Amine Absorbents - American Chemical

May 20, 2013 - Gas scrubbing, vapor−liquid equilibrium (VLE), and reaction calorimetry ... Carbon dioxide (CO2) is a greenhouse gas that contributes...
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CO2 Capture by Tertiary Amine Absorbents: A Performance Comparison Study Firoz Alam Chowdhury,*,† Hidetaka Yamada,† Takayuki Higashii,† Kazuya Goto,† and Masami Onoda‡ †

Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan ‡ Nippon Steel & Sumitomo Metal Corporation, 2-6-1 Marunouchi, Chiyoda-ku, Tokyo 100-8071, Japan S Supporting Information *

ABSTRACT: In the present paper, we investigated CO2 capture with 24 tertiary amine absorbents, including three synthetic amines, with systematic modification of their chemical structures. Aqueous solutions of the amines (mass fraction 30%) were used to evaluate the performance for CO2 capture. Gas scrubbing, vapor−liquid equilibrium (VLE), and reaction calorimetry experiments were conducted in the laboratory to obtain the absorption rate, the amount of CO2 absorbed, cyclic CO2 capacity, and heat of reaction for each absorbent. The results for these absorbents were compared with the conventional tertiary absorbent N-methyldiethanolamine (MDEA). Seven of the investigated absorbents performed well with high absorption rates and cyclic capacities. Among these absorbents, some showed lower heats of reaction than MDEA. These results provide basic guidelines for discovery of potential tertiary amine-based absorbents that may lead to development of new absorbent systems in the CO2 capture area. 2R1R2NH + CO2 ↔ R1R2NCOO− + R1R2NH 2+

1. INTRODUCTION Carbon dioxide (CO2) is a greenhouse gas that contributes to global warming and climate change problems.1 Carbon capture and storage from large point exhaust sources, such as fossil fuel fired power plants, steel production, chemical and petrochemical manufacturing, cement production, and natural gas purification, is promising for mitigation of these problems.2−4 Many technologies are currently employed for separation and capture of CO2 from gas streams. These techniques are based on different physical and chemical processes, including absorption, adsorption, and membrane separation.5−9 Compared with other processes, CO2 capture and release by a cyclic chemical absorption/regeneration process using an aqueous solution of an amine-based absorbent is the most mature and applied technology for postcombustion CO2 removal. In aminebased absorption processes, it is estimated that more than half of the capture cost arises from absorbent regeneration.10,11 Consequently, for practical application it is essential to reduce absorbent regeneration energy consumption by improving existing absorbents and developing new ones. Among the alkanolamines there are three main categories, including primary [monoethanolamine (MEA), 2-amino-2methyl-1-propanol (AMP)], secondary [diethanolamine (DEA), di-isopropanolamine (DIPA)] and tertiary [N-methyldiethanolamine (MDEA), triethanolamine (TEA)], that have been used widely as chemical absorbents for the removal of acidic gases (CO2, H2S).12,13 Generally, during a chemical absorption process, CO2 is absorbed into the amine solution at low temperatures (approximately 40 °C) and desorbed from the solution after heating (to approximately 120 °C). The chemistry of this process is complex, but the main reactions taking place depend on the type of alkanolamine (eqs 1 and 2).14−16 © XXXX American Chemical Society

(1)

Unhindered (primary or secondary) amines form a fairly stable carbamate, and only half a mole of CO2 is absorbed per mole of amine (eq 1). On heating, the carbamate dissociates to produce CO2 and amine. Since the carbamate formed during absorption is quite stable, it takes a lot of heat energy to break the bonds and to regenerate the absorbent. Hindered (tertiary) amines form an unstable carbamate, and an alternate reaction leads to formation of bicarbonate ions. Consequently, tertiary amines have a higher theoretical capacity of one mole of CO2 per mole of amine (eq 2). R1R2R3N + CO2 + H 2O ↔ R1R2R3NH+ + HCO3−

(2)

Regeneration of tertiary amines requires less heat energy than for unhindered amines. However, low rates of CO2 absorption make tertiary amines difficult to use for CO2 gas removal. If the absorption rates for the tertiary amines could be increased efficiently while maintaining small heats of reaction, the absorbent regeneration energy could be greatly reduced. For example, Idem et al.17 reported that a huge heat-duty reduction can be achieved by using a mixed MEA/MDEA solution instead of a single MEA solution in an industrial environment of a CO2 capture plant. Oyenekan and Rochelle18 showed that tertiary amine (MDEA) solvents promoted by piperazine (PZ) offered 15−22% energy savings. Numerous studies on chemical absorption phenomena with tertiary alkanolamines such as MDEA, TEA, and their related absorbents have been reported.19−22 Recently, interest in the Received: March 14, 2013 Revised: May 6, 2013 Accepted: May 20, 2013

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investigated various tertiary amines to find one with a higher absorption rate, higher cyclic capacity, and lower heat of reaction than conventional MDEA.

use of mixed amine absorbents, especially blends of primary and tertiary amines (such as MEA and MDEA) or secondary and tertiary amines (such as DEA and MDEA), has increased. These mixed absorbents combine the higher equilibrium capacity of the tertiary amine with the higher reaction rate of primary or secondary amine, and have been suggested for industrial gas treatment processes.23−26 Most studies have been limited to MEA-, DEA-, MDEA-, TEA-, or MDEA-based single or combined absorbents. There is very little information available regarding rationally designed new synthetic amines or amino alcohols. Recently, new secondary and tertiary alkanolamines, including some butanol derivatives synthesized by Tontiwachwuthikul and co-workers,27,28 have been applied to CO2 capture. These absorbents provided much higher CO2 absorption and cyclic capacities than the conventional amine MEA. In our previous studies, we designed and synthesized several secondary and tertiary amino alcohols by modification of the chemical structures, and developed screening methods for finding promising absorbents29,30 On the basis of the screening methods, in the present study, we examined 3 synthetic and 21 commercial tertiary amines, including the reference compound MDEA, and compared their performance to MDEA.

3. EXPERIMENTAL SECTION Commercially available amines and solvents were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO), Wako Pure Chemical Industries (Osaka, Japan) or Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) and used as received. The details and chemical structures for the investigated amine absorbents are shown in Table 1 and Figure 2, respectively. Selection of high performing (high absorption rates, high cyclic capacities, and low heats of reaction) absorbents was performed by tuning the chemical structures [e.g., tertiary monoalkanolamines (1−10), tertiary di- or trialkanolamines (11−16) and cyclic tertiary alkanolamines (17−23)] and alkyl substituents (e.g., methyl, ethyl, isopropyl, tert-butyl) around the amino group. For all amines, pKa values (dissociation constants) were determined by potentiometric titration with aqueous NaOH and HCl solutions using titration systems (AT-510, Kyoto Electronics Mfg. Co. Ltd., Kyoto, Japan and DL58, Mettler Toledo, Schwerzenbach, Switzerland). An aqueous solution (amine mass fraction 30%) was used for the gas scrubbing test, vapor−liquid equilibrium (VEL), and heat of reaction measurement. Water was purified with a water distillation apparatus (RFD240NA, Advantec, Tokyo, Japan) and an ion-exchange apparatus (RFU424CA, Advantec) in series. 3-1. Syntheses. Three of the amine absorbents (10, 12, and 21) were produced in rapid and economic one-pot syntheses in our laboratory (Scheme 1). These amines were >95% pure and their structures were established by gas chromatography (GC), liquid chromatography−mass spectrometry (LC−MS), and nuclear magnetic resonance (NMR) spectroscopy. Preparation of 4-Ethylmethylamino-2-butanol (4EMA-2B, 10). This compound was prepared as detailed in the literature.37 The product purity (96%) was determined by GC and the structural determination was performed by NMR and LC−MS. NMR data were consistent with literature values. MS (ESI+): Calculated for C7H17NO, mol. wt. 131; found, m/z 132 (M+1). Preparation of N-Isopropyldiethanolamine (IPDEA, 12). NIsopropyldiethanolamine was synthesized via a literature procedure38 with some modification. Diethanolamine (210.27 g, 2.0 mol) and 84.0 mL of distilled H2O were placed in a threenecked flask equipped with a mechanical stirrer and a reflux condenser, and the mixture was stirred and cooled to 5 °C. Then, K2CO3 (165.84g, 1.2 mol) was dissolved into a minimum volume of water and carefully added in one portion to the flask. To this mixture, 2-bromopropane (245.98g, 2.0 mol) was added slowly through a dropping funnel with continuous stirring. The reaction mixture was then stirred for an hour at room temperature, and then the bath temperature was increased to 50 °C with stirring for a further 16 h. Completion of the reaction was monitored by LC−MS. After the reaction reached completion, water was removed under reduced pressure at 40 °C using a rotary evaporator. The resulting residue was diluted with MeOH and the potassium salt (KBr) that formed was filtered off. Evaporation of MeOH gave the crude product IPDEA. The crude product was distilled to give 264.98 g (90% yield) of pure IPDEA as a colorless oil. The product purity was determined by GC and the structural determination was performed by NMR and LC−MS. GC product purity, 99.4%. NMR: 1H NMR (CDCl3, 400 MHz) δ

2. TARGET FOR TERTIARY AMINE ABSORBENTS Our previous research has provided useful information on various amine absorbents for CO2 capture.29−36 The CO2 loading capacities, heats of reaction, and absorption rates of these absorbents were examined, and we found a structure− performance relationship. The relationship between the heat of reaction and CO2 absorption rate, as determined experimentally, for a primary amine MEA, a secondary amine DEA and a tertiary amine MDEA are shown in Figure 1. Figure 1 shows

Figure 1. The experimental relationship between the CO2 absorption rate and heat of reaction for aqueous solutions of conventional amines (mass fraction 30%; MEA, DEA, and MDEA) at 40 °C.

MEA reacts with CO2 faster than DEA, which reacts faster than MDEA. This indicates that the heats of reaction and absorption rates of the alkanolamines are dependent on the substituents attached to the nitrogen atom. In aqueous solutions, CO2 and unhindered primary and secondary amines form stable carbamate anions, while tertiary amines produce bicarbonate ions as explained in eqs 1 and 2. Figure 1 also shows that the heat of reaction is lower for MDEA than DEA and MEA. However, low rates of CO2 absorption make use of tertiary amines for flue gas scrubbing difficult. In the present study, we B

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Table 1. Investigated Amine Absorbents commercial amine absorbents

commercial amine absorbents

1. 2-(dimethylamino)ethanol (DMAE) 2. 3-dimethylamino-1-propanol (DMA-1P) 3. 2-diethylaminoethanol (DEAE) 4. 3-diethylamino-1-propanol (DEA-1P) 5. 1-dimethylamino-2-propanol (DMA-2P) 6. 1-diethylamino-2-propanol (DEA-2P) 7. 2-(diisopropylamino)ethanol (DIPAE) 8. 2-(dimethylamino)-2-methyl-1-propanol (DMA-2M-1P) 9. 3-dimethylamino-2,2-dimethyl-1-propanol (DMA-2,2-DM-1P) 11. N-ethyldiethanolamine (EDEA) synthesized amine absorbents

13. 14. 15. 16. 17. 18. 19. 20. 22. 23.

N-tert-butyldiethanolamine (tBDEA) 3-(dimethylamino)-1,2-propanediol (DMA-1,2-PD) 3-diethylamino-1,2-propanediol (DEA-1,2-PD) triethanolamine (TEA) 1-(2-hydroxyethyl)pyrrolidine [1-(2HE)PRLD] 3-pyrrolidino-1,2-propanediol (PRLD-1,2-PD) 1-(2-hydroxyethyl)piperidine [1-(2HE)PP] 3-piperidino-1,2-propanediol (3PP-1,2-PD) 3-hydroxy-1-methylpiperidine (3H-1MPP) 1-ethyl-3-hydroxypiperidine (1E-3HPP) conventional amine absorbents 24. N-methyldiethanolamine (MDEA)

10. 4-ethyl-methyl-amino-2-butanol (4EMA-2B) 12. N-isopropyldiethanolamine (IPDEA) 21. 1-methyl-2-piperidineethanol (1M-2PPE)

Figure 2. Abbreviations for the investigated amine absorbents and their chemical structures.

3.57 (4H, t, J = 5.5 Hz), 3.02−2.92 (3H, m, 1H and 2 × OH), 2.61 (4H, t, J = 5.5 Hz), 1.02 (6H, d, J = 6.7 Hz); 13C NMR (CDCl3, 100 MHz) δ 60.2 (CH2 × 2), 51.7 (CH2 × 2), 51.1 (CH), 18.2 (CH3 × 2). MS (ESI+): Calculated for C7H17NO2, mol. wt. 147; found, m/z 148 (M+1). Preparation of 1-Methyl-2-piperidineethanol (1M-2PPE, 21). 1M-2PPE was synthesized via the literature procedure39 with some modification. 2-Piperidineethanol (129.2 g, 1.0 mol) was placed in a 2 L three-necked flask equipped with a mechanical stirrer, pressure equalizing dropping funnel, and a reflux condenser. The flask was cooled in an ice bath and formaldehyde (mass fraction 37%, 162.32 mL, 2.0 mol) was added slowly, followed by formic acid (volume fraction 96%, 59.93 mL, 1.5 mol). After the addition was complete, the reaction mixture was stirred for 1 h at room temperature, and then the bath temperature was increased to 80 °C. Sodium formate (136.01 g, 2.0 mol) was then added in one portion, and the resulting solution was stirred at 80 °C for 24 h. After this

Scheme 1. Synthetic Routes for the Preparation of 4EMA2B, IPDEA, and 1M-2PPE

C

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time, the solution was cooled to 20 °C and then basified to pH 12 with cooling using 2 mol/L aqueous sodium hydroxide. The aqueous layer was washed with diethyl ether, and the combined organic layers were dried over anhydrous MgSO4, filtered, and the solvent evaporated on a rotary evaporator. The crude product 1M-2PPE was obtained as a light yellow oil, and this was distilled to give 133.2 g (93% yield) of pure 1M-2PPE as a colorless oil. The product purity was determined by GC, and the structural determination was performed by NMR and LC− MS. GC product purity, 98.0%. NMR: 1H NMR (CDCl3, 400 MHz) δ 4.82 (1H, OH, br.s), 3.94−3.88 (1H, m), 3.71−3.65 (1H, m), 2.90−2.85 (1H, m), 2.34 (3H, s), 2.23−2.07 (2H, m), 2.07−1.71 (2H, m), 1.63−1.40 (4H, m), 1.46−1.24 (2H, m); 13 C NMR (CDCl3, 100 MHz) δ 62.3 (CH), 60.4 (CH2), 56.6 (CH2), 43.2 (CH3), 33.0 (CH2), 29.2 (CH2), 25.1 (CH2), 24.1 (CH2). MS (ESI+): Calculated for C8H17NO, mol. wt. 143; found, m/z 144 (M+1). 3-2. Equipment and Experimental Procedure. Gas Scrubbing Test. The goal of the gas scrubbing test was to clarify the overall reactivity of each absorbent with CO2. Figure 3 shows a schematic diagram of the scrubbing system for the

constant during the experiment. The concentration of the CO 2 /N 2 gas mixture was controlled using mass flow controllers, and then supplied to the autoclave after passing through a water saturator. The outlet gas was monitored by an infrared CO2 analyzer (VA-3001, Horiba) after passing through a condenser. Equilibrium was obtained when the CO2 analyzer indicated a constant CO2 concentration (±0.01%). To analyze the equilibrium data, the total pressure in the gas phase was measured. The CO2 loading in the liquid phase was measured by a total organic carbon analyzer (TOC-VCSH, Shimadzu, Kyoto, Japan) after collecting a sample from the autoclave. Heats of Reaction. In these measurements, we applied similar assumptions to those of Goto et al.31 CO2 absorption involves multiple electrolyte reactions in a liquid phase, but only the overall reaction was considered. The heat of reaction of each aqueous amine solution (mass fraction 30%) during CO2 absorption was acquired using a differential reaction calorimeter (DRC Evolution, Setaram, Pennsauken, NJ). The key components of the DRC system were two glass flasks, with one serving as the reaction cell and the other as the reference cell. A CO2 preloaded (approximately 0.45 mol-CO2/molamine) amine-containing aqueous solution was placed into the two flasks (150 mL each). Then, the flasks were connected in parallel to a water bath, from which water was supplied and circulated at a constant temperature to control the reaction temperature. For each measurement, 99.9% CO2 gas was added to the reaction flask at a flow rate of 120 mL/min at 40 °C and atmospheric pressure. As absorption occurred, the total energy flux released or absorbed during the CO2−amine reaction was precisely measured and reported as the temperature gradient versus time. The amount of absorbed CO2 in a solution was determined using the TOC analyzer. The heat of reaction of the amine-containing solution [kJ/mol-CO2] was calculated using the total heat generated during absorption [kJ] and the amount of CO2 absorbed [mol].

4. RESULTS AND DISCUSSION 4-1. Gas Scrubbing Test. Figure 4 shows typical results obtained from the gas scrubbing test. The amount of absorbed

Figure 3. Schematic diagram of the scrubbing system for the gas scrubbing test.

gas scrubbing test. The equipment was designed to operate at atmospheric pressure and temperatures up to 90 °C. Six units were used to simultaneously carry out six tests under the same conditions. First, a 250 mL glass scrubbing bottle was filled with 50 mL of an aqueous solution of the absorbent (mass fraction 30%), and this was placed in a water bath at 40 °C. Gas (20% CO2:80% N2) was then supplied to the bottle at a flow rate of 700 mL/min. After 60 min of CO2 absorption, the bottle was moved to another water bath at 70 °C and CO2 was regenerated from the absorbent for 60 min. The flow rate and CO2 concentration of the feed gas were constant for the absorption and regeneration tests. The flow rate and CO2 concentration of the feed gas were controlled by mass flow controllers (SEC-E40, Horiba, Kyoto, Japan). During the test, the outlet gas from the reactor was analyzed with a carbon dioxide analyzer (VA-3001, Horiba). Vapor-Liquid Equilibrium (VLE). VLE data were measured for 10 aqueous amine solutions (mass fraction 30%) at 40, 70, and 120 °C according to the following procedure. 36 Approximately 700 mL each aqueous amine solution was fed into an autoclave, followed by a N2 purge. Each solution was agitated with a mechanical stirrer, and the temperature was held

Figure 4. Typical results from a gas scrubbing test.

CO2 in the aqueous amine solution (CO2 loading) was calculated from the measured CO2 concentration in the outlet gas flow. As shown in Figure 4, the CO2 loading increased with time at 40 °C and then decreased at 70 °C. The gradient of the curve at 50% of the 60 min CO2 loading was defined as the absorption rate. The absorption rate is not the chemical D

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Figure 5. Experimental CO2 absorption-regeneration profiles for 23 different amine absorbents (1−23) compared with conventional absorbent (24 MDEA).

increase in bulky groups either directly adjacent to the amine (e.g., DIPAE) or hydroxyl chain on the amine (e.g., DAM-2M1P and DMA-2,2-DM-1P) decreased the absorption rate. The effect of steric hindrance on the amine-CO2 reaction rates has been discussed by Sartori and Savage.40 They explained that the introduction of steric hindrance around the amino group reduces the initial rate of reaction even though some hindered amines absorb CO2 to a higher level. A similar effect was observed in this work, especially for DEA-2P, DAM-2M-1P, and synthetic 4EMA-2B. Each of these amines has two hindered groups adjacent to the N-atom, and one or two methyl groups on the hydroxyl chain of the alkanolamine. Another parameter that is important for the CO2 removal is the cyclic CO2 capacity. The results in Table 2 show that five of the tertiary monoalkanolamine absorbents (DEAE, DEA-1P, DMA2P, DEA-2P and DMA-2,2-DM-1P) had higher cyclic capacities than MDEA. One of the dialkanolamine absorbents, DEA-1,2-PD, showed excellent enhancement of the absorption rate and CO2 loading compared with MDEA (Figure 5C). Four of the dialkanolamine absorbents (IPDEA, tBDEA, DMA-1,2-PD, and DEA-1,2-PD) shows similar cyclic capacities to MDEA. This parameter was low for EDEA and TEA. Among the eight cyclic alkanolamines, two have pyrrolidine rings and the rest have piperidine rings. Figure 5D and Table 2 clearly show that the absorption rates and CO2 loadings of the four cyclic absorbents (1-(2HE)PRLD, 1-(2HE)PP, 3PP-1,2-

reaction rate but the apparent CO2 transfer rate from the gas to liquid phase. This reference index was used to compare the behavior of the aqueous amine solutions. The difference between the maximum CO2 loading at 40 °C and the minimum CO2 loading at 70 °C was defined as the cyclic capacity. The reproducibility of the experiments was checked, and the error in all of the experimental measurements was less than 3%. The experimental CO2 absorption-regeneration profiles obtained in this work for the 23 different amine-based absorbents are compared with the conventional absorbent MDEA in Figure 5. On the basis of the chemical structure, all the tested absorbents were subdivided into three categories. Tertiary monoalkanolamines are shown in Figure 5A (1−5) and Figure 5B (6−10), tertiary di- or trialkanolamines in Figure 5C (11−16), and cyclic tertiary alkanolamines in Figure 5D (17−23). From the absorption−regeneration curves it was possible to obtain the absorption rates, amount of CO2 absorbed, and cyclic capacities for the absorbents. Table 2 summarizes the absorption rates, absorption amounts, cyclic capacities and pKa values for all the screened amines. The relative performances of all tested amine absorbents in terms of absorption rate versus cyclic capacity are compared in Figure 6. Figure 5 panels A and B (monoalkanolamine absorbents) show that the CO2 absorption amount and absorption rates of DMAE, DEAE, DEA-1P, DMA-2P, and DEA-2P were higher than that of MDEA. Synthetic amine 4EMA-2B had a slightly lower absorption rate but higher CO2 loading than MDEA. An E

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Table 2. Summary of the Results for Each Screening Experiment amine 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

DMAE DMA-1P DEAE DEA-1P DMA-2P DEA-2P DIPAE DMA-2M-1P DMA-2,2-DM-1P 4EMA-2B EDEA IPDEA t BDEA DMA-1,2-PD DEA-1,2-PD TEA 1-(2HE)PRLD PRLD-1,2-PD 1-(2HE)PP 3PP-1,2-PD 1M-2PPE 3H-1MPP 1E-eHPP MDEA

absorption ratea g-CO2/L-soln/min

absorption amountb g-CO2/ L-soln

cyclic capacityc g-CO2/L-soln

pKa valuesd

1.70 1.47 2.49 2.60 2.24 1.66 1.01 1.18 1.08 1.37 0.70 1.36 1.91 1.28 3.40 0.75 2.41 1.25 2.22 3.33 3.17 1.08 0.96 1.56

73 71 94 89 92 78 57 88 57 69 39 58 52 70 73 22 94 47 83 57 77 55 56 55

21 19 32 25 31 32 24 12 27 20 12 24 26 27 26 9 28 13 36 19 25 21 21 24

9.49 9.54 10.01 10.29 9.67 10.18 10.03 10.34 9.54 9.82 8.86 9.12 9.06 9.14 9.89 7.85 9.86 9.64 9.76 9.49 9.89 8.94 9.21 8.65

CO2 absorption rates were calculated at 50% of the 60 min CO2 loading and 40 °C. b60 min CO2 loading at 40 °C. cDifference between CO2 loadings at 40 and 70 °C. dValues measured at 23−26 °C. a

this study (1, 2, 3, 19, 20, and 24). They performed microscale isothermal gravimetric analysis experiments (100 μL, 40 °C) to monitor the increase in mass of an aqueous amine solution (mass fraction 30%) when exposed to mixed gases (15% CO2:85% N2) at ambient pressure. The trend of initial CO2 absorption rate (per unit mass of amine) was given as 20 > 3 > 19 > 24 > 1 > 2. This trend almost agrees with ours of the absorption rate in Table 2 (20 > 3 > 19 > 1 > 24 > 2) despite differences in the evaluation methods. The pKa value is an important fundamental property which affects the kinetics and possibly the mechanism of the capture process.14 Many previous studies also reported on a Brønsted relationship between the rate constant of the reaction of amines with CO2 and the basicity of such amines.46−48 In tertiary amines, the rate shows a strong dependence on pKa because of the base-catalyzed mechanism. Puxty et al.45 studied a relation between CO2 absorption rates and calculated pKa values among 76 amines and found that the larger is the value of pKa, the higher is the absorption rate, as a general trend. This trend was confirmed in Figure 7, where the absorption rates were plotted against the pKa values measured in the present study. However, Figure 7 indicates that the absorption rate is also governed by other factors such as steric hindrance around the amino moiety. Chemical absorbents that have high absorption rates and sufficient cyclic CO2 capacities are favorable for industrial applications. Table 2 shows a wide range of values for the CO2 absorption rates and cyclic capacities of the amine absorbents. To easily identify those with high absorption rates and cyclic capacities, the absorption rates for all the screened amines were plotted against the cyclic capacities (Figure 6). Seven absorbents (black circles, Figure 6) performed better than the others in terms of absorption rate and cyclic capacity. The screening results show clear trends for the structural effects of

Figure 6. Twenty-one commercial and three synthetic amine absorbents preliminary screening results compared with conventional MDEA.

PD, and 1M-2PPE (synthetic)) were higher than those of MDEA. Heterocycles such as piperidine and piperazine (secondary amine) are known to have fast reaction rates for carbamate formation.41−44 Similarly, methyl and hydroxyethyl substituted tertiary pyrrolidine and piperidine absorbents may also be good base catalysts for CO2 hydration, and this might increase their absorption rates and capacities. 3-Hydroxy methyl- and ethyl- substituted piperidine absorbents (3H1MPP and 1E-3HPP) decreased both the absorption rate and capacity. One synthetic absorbent (1M-2PPE) and two commercial cyclic absorbents (1-(2HE)PRLD and 1(2HE)PP) showed higher cyclic capacities than MDEA. Puxty et al.45 performed a screening study of the CO2 absorption performance of 76 amines including six amines in F

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Figure 7. Absorption rates versus pKa values. Figure 9. Equilibrium CO2 solubilities at 40 °C and 20 kPa CO2 partial pressure versus pKa values.

hindered amine absorbents, with less sterically hindered substituents on to N-atom and hydroxyethyl pyrrolidine and piperidine rings enhancing the absorption rate and cyclic capacity. 4-2. VLE. The VLE solubility of CO2 obtained from 10 tertiary amines are given in Supporting Information, Table S18. The results of the VLE experiment were compared with gas scrubbing test results (Figure 8). Under equilibrium condition

1,2-PD, 1-(2HE)PRLD, 1-(2HE)PP, and 1M-2PPE) had better absorption characteristics than other the absorbents tested. These absorbents showed relatively high absorption rates and high cyclic capacities for CO2 compared with conventional MDEA. Therefore, the heats of reaction (ΔHr) were measured for these seven amines at 40 °C. Generally, ΔHr values depend on the CO2 loadings. Consequently, we expressed the ΔHr values as the differential (average) enthalpies in the range of the absorbent loading. Our results and reported data are summarized in Table 3. The ΔHr value for MDEA obtained in this work was close to the ΔHr values reported by Ma’mum et al.20 and Jou et al.49 To reduce the energy requirements of the capture process, absorbents with high CO2 absorption rates, high cyclic capacities, and low heats of reaction are needed. In Figure 10, the CO2 absorption rates for the seven amine absorbents are plotted against the ΔHr values. As mentioned for Figure 1, there was a trade-off between the heat of reaction and absorption rate for primary, secondary and tertiary amines. In this study, DEA-1,2-PD and 1M-2PPE (synthetic) were unique in that they had high absorption rates and maintained moderately low heats of reaction (Figure 10). DMA-2P, 1(2HE)PRLD, and 1-(2HE)PP had ΔHr values close to that of MDEA, but with higher absorption rates and cyclic capacities than MDEA. Equating a low heat of reaction directly with a low overall regeneration heat duty would give a wrong idea of the solvent regeneration performances.50 In fact, the heat required to regenerate the solvent in the desorber column of the CO2 capture process can be approximated as the sum of three terms: the heat necessary to desorb CO2 from the solution (heat of absorption), the sensible heat to raise the solvent from absorber to desorber temperature and the heat of evaporation required to produce the stripping steam in the reboiler. If two solvents have the same capacity, the one showing the larger heat of absorption can profit from a greater temperature swing between absorber and desorber. Solvents with low heat of absorption might benefit from regeneration below atmospheric pressure and at low temperatures.18

Figure 8. Screening cyclic capacities in Table 2 versus equilibrium cyclic capacities ΔS40−70 (= S40 °C − S70 °C) and ΔS40−120 (= S40 °C − S120 °C). CO2 solubility (S) was obtained form VLE data at 20 kPa CO2 partial pressure.

cyclic capacity increases compared to screening cyclic capacity, because CO2 loading does not reach equilibrium in the screening of this work. As the CO2 equilibrium loading normally decreases with temperature, the cyclic capacity increased by increasing regeneration temperature. However, the gas scrubbing test results were positively correlated with the equilibrium cyclic capacities between 40 and 70 °C as well as between 40 and 120 °C. This means that it is possible to evaluate the CO2 capture ability concerning plant process by a simple gas scrubbing test as a first step. We also confirmed a linear correlation between equilibrium CO2 solubility and pKa as shown in Figure 9. 4-3. Heats of Reaction. The gas scrubbing test results suggested seven absorbents (DEAE, DEA-1P, DMA-2P, DEAG

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Table 3. Heats of Reaction of CO2 in Amine Containing Aqueous Solutions amine 24. MDEA

3. DEAE 4. DEA-1P 5. DMA-2P 15. DEA-1,2-PD 17. 1-(2HE)PRLD 19. 1-(2HE)PP 21. 1M-2PPE a

amine concentration [wt %]

heats of reactiona [kJ/mol-CO2]

CO2 loading [mol-CO2/mol-amine]

30 50 24 50 30 30 30 30 30 30 30

58.5 53.4 55.4 53.2 62.4 60.4 57.6 56.3 58.0 58.2 56.4

0.46−0.58 0.50 0.50 0.50 0.46−0.61 0.46−0.51 0.47−0.59 0.47−0.63 0.45−0.58 0.47−0.63 0.48−0.67

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work

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Heats of reaction measured at 40 °C and around 0.5 mol CO2/mol amine.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental results for gas scrubbing test and heats of reaction measurement used for Figure 1, 1H, 13C NMR, GC, and MS spectra for the three compounds synthesized, and VLE data for Figure 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-774752360. Fax: +81-774752318. E-mail: firoz@ rite.or.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A part of this work was financially supported by the COURSE 50 project founded by the New Energy and Industrial Technology Development Organization, Japan.

Figure 10. Absorption rates and heats of reaction of preferred absorbents found in this study.

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5. CONCLUSIONS We investigated 3 synthetic and 21 commercial amine-based absorbents with systematic modifications of their chemical structures (e.g., aliphatic monoalkanolamines, aliphatic dialkanolamines, and cyclic alkanolamines) and alkyl substituents (e.g., methyl, ethyl, isopropyl, tert-butyl) around the amino group. Placement of functional groups within the N-atom/ alkanolamine chain affects CO2 absorption−regeneration performance. Gas scrubbing, VLE, and reaction calorimetry experiments were conducted to obtain the absorption rates, CO2 loadings, cyclic capacities, and heats of reaction for the absorbents. The results were compared with those for the conventional absorbent MDEA. Seven high performing absorbents were found, including one synthetic amine, and these had high absorption rates and cyclic capacities compared with those of MDEA and lower or comparable heats of reaction. Lower reaction enthalpies and higher absorption rates should greatly reduce the absorbent regeneration energy. The promising absorbents in this work are now being used to investigate concentrations and blending optimizations with rate promoters, under equilibrium CO2 solubility conditions using VLE apparatus in the industrially important temperatures range of 40−120 °C and pressures. The results of this work indicate that advancements can be made with new materials (synthetic amines), and a broad investigation of existing amines is required to find energy efficient absorbents for CO2 capture.

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