13C NMR Studies on the Dissolution Mechanisms of Carbon Dioxide

Dec 22, 2009 - Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, ...
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Ind. Eng. Chem. Res. 2010, 49, 1222–1228

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C NMR Studies on the Dissolution Mechanisms of Carbon Dioxide in Amine-Containing Aqueous Solvents at High Pressures toward an Integrated Coal Gasification Combined Cycle-Carbon Capture and Storage Process Kin-ya Tomizaki,*,§,† Mitsuhiro Kanakubo,‡ Hiroshi Nanjo,‡ Shinkichi Shimizu,† Masami Onoda,† and Yuichi Fujioka† Chemical Research Group, Research Institute of InnoVatiVe Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai 983-8551, Japan

Carbon dioxide (CO2) capture and storage (CCS) technology has emerged and become a promising tool for the control of greenhouse gas emissions. CCS is applicable to the integrated coal gasification combined cycle (IGCC) equipped with a water-gas shift reaction (IGCC-CCS). In our previous studies, we obtained novel chemical absorbents suitable for IGCC-CCS and examined vapor-liquid equilibria of the absorbents. However, the mechanisms of dissolution of pressurized CO2 into the solvents (e.g., determination of the fractions of CO2 absorbed chemically and physically under high CO2 pressure conditions) were not clear, even though this information is very important for estimation of the energy requirements for the CCS process. We examined the usefulness of 13C NMR spectroscopy to determine CO2 solubility and the components of inorganic carbon species (bicarbonate/carbonate and molecular CO2) in six different amine-containing aqueous solvents, at temperatures ranging from 40 to 70 °C and CO2 pressures ranging from 0.5 to 4 MPa. We found that (i) the amounts of CO2 physically absorbed into the solvents increased with increasing CO2 pressure and comprised 15-30% of the total CO2 in all the solvents at 40 °C and 4 MPa, and (ii) the solubility determined by 13C NMR spectroscopic and vapor-liquid equilibrium measurements were in good agreement over the CO2 pressure range examined. The results indicate that we could not only obtain CO2 solubility data but also identify the inorganic carbon species in the solvents by quantitative 13C NMR spectroscopy. 1. Introduction In recent years, carbon dioxide (CO2) capture and storage (CCS) technology has emerged and become a promising tool to control greenhouse gas emissions. The CCS process can be applied not only in coal-fired power plants and steel factories but also in the integrated coal gasification combined cycle (IGCC) process equipped with a water-gas shift reaction (IGCC-CCS). The water-gas shift reaction generates a pressurized gas stream (∼5 MPa) containing a volume fraction of CO2 (φ ≈ 40%) that affords a CO2 partial pressure pCO2 ≈ 2 MPa (eq 1).1,2 CO + H2O f H2 + CO2

(1)

A purified hydrogen product could be used for fuel cells but the other product, CO2, needs to be stored or preferably converted into an alternative, useful product. In this context, although other CCS technologies, including physical absorption, adsorption, membranes, and cryogenics are available,3,4 we are focusing on chemical absorbents that absorb and release pressurized CO2 for the following reasons: (i) CO2 absorption into a liquid agent is most commonly used for the bulk removal of CO2; (ii) although at high partial pressures of CO2 physical absorption in polar organic solvents (e.g., polyethylene glycol derivatives) has been preferred,5,6 reduction of * To whom correspondence should be addressed. Tel.: +81-77-5437469. Fax: +81-77-543-7483. E-mail: [email protected]. † Research Institute of Innovative Technology for the Earth (RITE). ‡ National Institute of Advanced Industrial Science and Technology (AIST). § Current address: Innovative Materials and Processing Research Center and Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan.

a pressure level (sometimes with elevating temperature) is necessary to release CO2 from the CO2-rich solution and regenerate the active absorbents. This causes a loss in the pressure level of released CO2 and an increase in CO2 compression energy and cost for subsequent transportation; (iii) operation of the physical absorption process at low temperature conditions (below ambient temperature) is an issue. Development of novel chemical absorbents that work adequately above ambient temperature and release highly pressurized CO2 from CO2-rich solutions can save the energy requirements for a series of CO2 removal and subsequent compression of separated CO2. CO2 absorption processes depend strongly upon basicity of amines, which is commonly characterized using acidic dissociation constants (pKa).7-13 Thus, we screened novel chemical absorbents suitable for IGCC-CCS conditions on the basis of pKa values and obtained several candidate absorbents.14 Subsequently, we examined the heats of reaction (∆Hr) and vapor-liquid equilibria of the candidate absorbents and summarized the data in Table 1.15 It was concluded that 1,2dimethylimidazole (12DMIm) and 4-(2-hydroxyethyl)morpholine (HEMO) would be suitable candidates to absorb and release CO2 with pCO2 ) 1.6 MPa (or possibly higher) without loss in CO2 pressure level.15 A common chemical absorbent, Nmethyldiethanolamine (MDEA)16-25 exhibited greater pKa and ∆Hr values and smaller CO2 capacity than those for both 12DMIm and HEMO.15 These results suggest that the novel chemical absorbents are favorable to remove and compress CO2 with smaller energy requirements than that for MDEA. However, the mechanisms of dissolution of pressurized CO2 into such solutions (e.g., the fractions of CO2 absorbed chemically and physically under high CO2 pressure conditions) are not clear, even though this information is very important for estimation

10.1021/ie900870w  2010 American Chemical Society Published on Web 12/22/2009

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Table 1. Summary of the Data of pKa, ∆Hr, and CO2 Capacity for 10 Different Amine-Containing Aqueous Solutions Reported in the Literature15 aminea

pKab

-∆Hr [kJ/mol-CO2]c

R [mol/mol-amine]d

CO2 capacity [mol/mol-amine]e

MDEA 12DMIm 2MeIm TEA EMO 4MeIm 1MeIm Im HEMO Bis-Tris

8.51 8.00 7.87 7.72 7.71 7.54 7.03 6.99 6.93 6.47

58.9 ( 3.3 56.7 ( 1.8 59.1 ( 1.9 56.5 ( 2.2 54.3 ( 3.2 59.2 ( 0.8 46.8 ( 2.0 55.0 ( 1.4 46.6 ( 1.6 45.3 ( 0.9

0-0.73 0-0.35 0-0.42 0-0.40 0-0.29 0-0.31 0-0.13 0-0.19 0-0.12 0-0.16

0.571 0.700 0.705 f f f 0.496 0.571 0.623 f

a Abbreviations: MDEA ) N-methyldiethanolamine; 12DMIm ) 1,2dimethylimidazole; 2MeIm ) 2-methylimidazole; TEA ) triethanolamine; EMO ) 4-ethylmorpholine; 4MeIm ) 4-methylimidazole; 1MeIm ) 1-methylimidazole; Im ) imidazole; HEMO ) 4-(2-hydroxyethyl)morpholine; Bis-Tris ) 2,2-bis(hydroxymethyl)-2,2′,2′′-nitrilotri ethanol. b pKa values were measured at 25 °C and atmospheric pressure ([amine] ) 10 mM in water). c Heats of reaction in the range of the solvent loading (-∆Hr) were measured at 40 °C and atmospheric pressure (mean ( SD, n ) 3-4, [amine] ) 3 M or [Bis-Tris] ) 1.5 mM in water). d CO2 loading during CO2 absorption into aqueous solutions. e CO2 capacity (at pCO2 ) 1.6 MPa) ) [CO2 rich-loading (40 °C)] - [lean-loading (120 °C)]. f Not applied.

of energy requirements and for optimization of conditions for CCS processes. 13 C NMR spectroscopy may be one of the most suitable analytical methods for detecting inorganic carbon species in solutions directly and specifically,26-28 whereas total organic carbon (TOC) analysis acidifies an aqueous sample to convert bicarbonate/carbonate into CO2 and is not applicable. In this paper, we confirm the usefulness of the 13C NMR spectroscopic method and present the results of CO2 solubility and identification of the components of inorganic carbon species (bicarbonate/ carbonate and molecular CO2, which are absorbed chemically and physically, respectively) in six different amine-containing aqueous solvents, obtained previously,14,15 at temperatures ranging from 40 to 70 °C and CO2 pressures ranging from 0.5 to 4 MPa (Figure 1).

Figure 1. Chemical structures used in this study. Abbreviations: EG ) ethylene glycol; MDEA ) N-methyl diethanolamine; 12DMIm ) 1,2dimethylimidazole; 2MeIm ) 2-methylimidazole; 1MeIm ) 1-methylimidazole; Im ) imidazole; HEMO ) 4-(2-hydroxyethyl)morpholine.

2. Experimental Section 2.1. Materials. All chemicals and solvents were purchased from Aldrich or Wako Pure Chemical Industries (Japan) and used as received. Water used in all experiments was purified with a water distillation apparatus (RFD240NA, Advantec) and an ion-exchange apparatus (RFU424CA, Advantec) in series. 2.2. Measurements of 13C NMR Spectra of Aqueous Solutions under Various Temperature and CO2 Pressure Conditions. 13C NMR spectra were acquired on an NMR spectrometer (Varian Inova 500, 125 MHz). A high-pressure cell composed of a PEEK (poly(etherether ketone)) tube (o.d., 10 mm, i.d., 6 mm) was used and connected with a digital pressure indicator (Druck, DPI145) through SS316 tubes, as previously detailed in the literature29-31 (Figure 2). Absorbents were dissolved in 50% D2O/H2O to 0.5 M for ethylene glycol (EG) and 3 M for amines [MDEA, 12DMIm, 2-methylimidazole (2MeIm), 1-methylimidazole (1MeIm), imidazole (Im), and HEMO]. A small amount (∼1.2 mL) of each sample solution was transferred into the PEEK cell and placed in the NMR spectrometer. The sample volume sufficiently covered the observation area of NMR. 1H and 13C NMR spectra of the sample solution were measured at atmospheric pressure without spinning the PEEK cell. CO2 gas (Showa Tansan Co., Ltd., 99.990 vol. % up) was injected into the PEEK cell from a

Figure 2. 13C NMR spectroscopic apparatus used in this study.

cylinder, and the cell was shaken carefully outside the magnet at room temperature until no appreciable pressure decrease was observed. Then, the PEEK cell was reinstalled in the NMR spectrometer and kept at a constant temperature, 40 °C. After the vapor-liquid equilibrium was regarded as attained by checking that the pressure change was smaller than (0.01 MPa during 3-4 h, 1H and 13C NMR measurements were carried out at a given CO2 pressure. The very small amount of the sample solution can shorten the total equilibrium time, ∼10 h or less. This procedure was repeated to record NMR spectra under various temperature and CO2 pressure conditions. The temperature and pressure were controlled within (0.1 °C and (0.018 MPa during acquisition of each spectrum. Pressure was expressed in absolute scale. To obtain quantitative 13C NMR spectra, the gated decoupling technique was used to eliminate the nuclear Overhauser effect. Before each series of highpressure experiments, NMR parameters were properly adjusted; the pulse width was fixed at 5 µs, corresponding to 30°, and the delay time for repetition was longer than 30 s. The number of acquisition for each spectrum was 120 or more. The

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Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010 Table 2. Integral Ratios in 13C-NMR Spectra of 3 M MDEA in 50% D2O/H2O at 40 °C under Various CO2 Pressure Conditions integral ratios in 13C NMR spectrab P [MPa]a

peak at 160 ppmc

peak at 125 ppmd

0.10 0.53 1.28 2.04 3.14 4.03

e 0.949 1.033 0.992 1.006 0.964

e n.d.f 0.053 0.101 0.132 0.181

a Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 41 ppm. c Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. f Not detected (below lower limit of detection). b

Figure 3. Measurements of CO2 absorption in 0.5 M EG in 50% D2O/H2O at 40 °C upon CO2 pressurization. 13C NMR spectrum of the EG solution (A) without feeding CO2 gas (p ) atmospheric pressure) and (B) at p ) 4.12 MPa, and (C) integral ratios (125 ppm/63 ppm) as a function of CO2 pressure.

uncertainty of the integral ratios obtained from 13C NMR spectra was estimated within (5% at 40 °C, but became worse (7-10% for smaller peaks as well as at higher temperatures. NMR data were reproducible within the experimental errors mentioned above. 3. Results and Discussion 3.1. 13C NMR Analysis of CO2 Solubility in 0.5 M Ethylene Glycol Aqueous Solution. During the absorption/ release cycle of pressurized CO2 with amine-containing aqueous solutions, CO2 dissolves into the solutions both chemically and physically so the amount of physically absorbed molecular CO2 must be taken into account to investigate CO2 absorption mechanism at high pressures. Thus, we first attempted to obtain the chemical shift of molecular CO2 in 13C NMR spectra under various CO2 pressure conditions. A 0.5 M EG in 50% D2O/ H2O solution was employed as a CO2-capturing solvent for the following reasons: (i) EG is a symmetric molecule, affording just one 13C NMR signal originating from the two symmetric ethylene carbons; (ii) EG is a neutral molecule, without acidic and basic moieties within the molecule, causing CO2 to be dissolved into EG predominantly as molecular CO2; and (iii) a defined concentration of EG solvent is an internal standard for determination of CO2 concentration in the solvent directly, without taking sample solutions containing pressurized CO2 for analysis. It is known that deuteration of water from H2O to D2O sometimes has an effect on the kinetic process such as proton exchange, but the thermodynamic equilibrium state discussed in this work is assumed to be less affected. Figure 3A shows a 13C NMR spectrum of 0.5 M EG in 50% D2O/H2O at 40 °C at atmospheric pressure before feeding CO2 gas. A singlet peak can be observed at 63 ppm, corresponding to the symmetric ethylene carbon of EG. After feeding CO2 gas into the PEEK tube and pressurizing it with CO2 to 4.12 MPa, another singlet peak was observed at 125 ppm, which

originated from molecular CO2 physically absorbed into the EG solvent (Figure 3B). The chemical shift of molecular CO2 remained almost unchanged at different pressures between 1 and 4 MPa. Figure 3C shows the change in integral ratio (125 ppm/63 ppm) as a function of CO2 pressure. It was found that the integral ratio was proportional to CO2 pressure at least up to 3 MPa. The experimental errors hinder the limit of this linear dependence at high pressure, but the data point at 4 MPa seems to drop slightly as expected from the straight line at lower pressure. The concentration of CO2 in the EG solvent at 40 °C and 4.12 MPa was estimated with the integral ratio (0.978) and the value of the internal standard ([carbon] ) 1.0 M) to be 43 g CO2/L. On the basis of the preliminary 13C NMR spectroscopic measurements, we have reached the conclusion that 13C NMR spectroscopy is very useful to reveal CO2 absorption mechanisms under pressurized conditions, where both chemical and physical absorption mechanisms work simultaneously. 3.2. 13C NMR Analyses of CO2 Absorption in 3 M Amine-Containing Aqueous Solvents at Various Pressures. With such a promising method in hand, we examined CO2 absorption in six different amine-containing aqueous solutions (MDEA, 12DMIm, 2MeIm, 1MeIm, Im, and HEMO) at 40 °C and at CO2 pressures up to 4 MPa. In 13C NMR spectra for six amine solutions, only two peaks newly appeared at approximately 125 and 160 ppm due to CO2 absorption. The former peak is assigned to be molecular CO2 physically absorbed in view of the results in EG solution, whereas the latter peak is attributable to bicarbonate and/or carbonate. There was no appreciable peak observed for carbamate species in every solution, which could give a distinct signal at a higher chemical shift. Unfortunately, it is not possible to distinguish such bicarbonate/carbonate species because of the fast proton exchange rather than NMR time scale. The present observation was in a good agreement in the previous NMR study on aqueous alkanolamines at atmospheric pressure, though some highly basic amines showed one more peak at ∼165 ppm resulting from carbamate species.28 The bicarbonate/carbonate equilibrium could be also influenced by CO2 pressure; however, no significant change in the chemical shift was observed even at high pressure. In the following discussion, the peak at ∼160 ppm was treated as total chemically absorbed CO2 without difference between bicarbonate and carbonate. To obtain the concentrations of CO2 in the solvents, the integral ratios of 13C NMR signals originating from CO2 against that corresponding to a carbon atom in the amine were calculated and are summarized in Tables 2-7 and Figure 4. All 13C NMR spectral data are shown in the Supporting Information (Figures S1-S40).

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Figure 4. Comparison of CO2 absorption into 3 M amine-containing aqueous solutions at 40 °C under various CO2 pressure conditions detected by 13C NMR spectroscopy, (A) MDEA, (B) 12DMIm, (C) 2MeIm, (D) 1MeIm, (E) Im, and (F) HEMO. Gray and white bars show the amounts of chemically- and physically absorbed CO2 as bicarbonate/carbonate and molecular CO2, respectively. Closed circles show fractions of physically absorbed CO2 against total CO2 in the solutions.

3.2.1. CO2 Absorption in 3 M MDEA. MDEA is a wellknown chemical absorbent that is considered to work effectively for CO2 removal at high CO2 partial pressures.16-25 In the 13C NMR spectrum at 40 °C before feeding CO2 gas, overlapping signals originating from MDEA were observed at 60 and 40 ppm. After elevating the CO2 pressure, two singlet peaks resulted at 160 and 125 ppm and were assigned to be bicarbonate/ carbonate and molecular CO2, respectively (Table 2 and Figure 4A). CO2 loadings (CO2 solubility) for chemically absorbed CO2 were almost unity and constant over the entire CO2 pressure range used in this study. In contrast, CO2 loadings for physically absorbed CO2 increased with increasing CO2 pressure. Fractions of physically absorbed CO2 against the total amount of CO2 (bicarbonate/carbonate + molecular CO2) in the solution also increased with increasing CO2 pressure, reaching 0.158 at 4.03 MPa. 3.2.2. CO2 Absorption in 3 M 12DMIm. 12DMIm is one of the most highly basic compounds among the imidazole derivatives used in this study and has the potential to absorb/

release pressurized CO2 effectively.15 In 13C NMR spectra at 40 °C under pressurized CO2 conditions, signals at 145, 122, 119, 33, and 10 ppm originated from 12DMIm. Additional signals at 160 and 125 ppm, assigned to be bicarbonate/ carbonate and molecular CO2, respectively, were observed (Table 3 and Figure 4B). CO2 loadings for chemically absorbed CO2 increased upon increasing the CO2 pressure up to 3.12 MPa and reached unity at higher CO2 pressures. CO2 loadings for physically absorbed CO2 increased upon increasing the CO2 pressure over the entire CO2 pressure range used in this study. Fractions of physically absorbed CO2 against the total amount of CO2 in the solution also increased with increasing CO2 pressure, reaching 0.207 at 4.00 MPa. 3.2.3. CO2 Absorption in 3 M 2MeIm. 2MeIm is a strong base, similar to 12DMIm, but the heat of reaction is greater than that for 12DMIm.15 In 13C NMR spectra at 40 °C under pressurized CO2 conditions, signals at 145, 119, and 12 ppm originated from 2MeIm, and additional signals at 160 and 125 ppm, assigned to be bicarbonate/carbonate and molecular CO2,

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Table 3. Integral Ratios in 13C-NMR Spectra of 3 M 12DMIm in 50% D2O/H2O at 40 °C under Various CO2 Pressure Conditions

Table 6. Integral Ratios in 13C-NMR Spectra of 3 M Im in 50% D2O/H2O at 40°C under Various CO2 Pressure Conditions

integral ratios in 13C NMR spectrab

integral ratios in 13C NMR spectrab

P [MPa]a

peak at 160 ppmc

peak at 125 ppmd

P [MPa]a

peak at 162 ppmc

peak at 126 ppme

0.10 1.11 2.15 3.12 4.00

e 0.837 0.914 1.020 0.957

e 0.125 0.146 0.199 0.249

0.10 1.05 2.03 3.11 3.98

e 0.456 0.586 0.678 0.692

e 0.122 0.206 0.273 0.319

a Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 10 ppm. c Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. b

Table 4. Integral Ratios in 13C-NMR Spectra of 3 M 2MeIm in 50% D2O/H2O at 40 °C under Various CO2 Pressure Conditions

a Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 137 ppm. c Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. b

Table 7. Integral Ratios in 13C-NMR Spectra of 3 M HEMO in 50% D2O/H2O at 40°C under Various CO2 Pressure Conditions integral ratios in 13C NMR spectrab

integral ratios in 13C NMR spectrab P [MPa]a

peak at 161 ppmc

peak at 125 ppmd

0.10 0.53 1.07 2.13 3.27 4.18

e 0.743 0.852 0.905 0.932 1.004

e 0.068 0.111 0.181 0.193 0.258

Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 12 ppm. Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. a

b c

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Table 5. Integral Ratios in C-NMR Spectra of 3 M 1MeIm in 50% D2O/H2O at 40 °C under Various CO2 Pressure Conditions integral ratios in 13C NMR spectrab a

P [MPa]

peak at 162 ppmc

peak at 124 ppmd

0.10 0.89 1.98 3.13 4.02

e 0.491 0.558 0.640 0.695

e 0.084 0.130 0.210 0.293

a Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 141 ppm. c Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. b

respectively, were observed (Table 4 and Figure 4C). CO2 loadings for chemically absorbed CO2 increased upon increasing the CO2 pressure and reached unity at 4.18 MPa. CO2 loadings for physically absorbed CO2 also increased upon increasing the CO2 pressure over the entire CO2 pressure range used in this study. Fractions of physically absorbed CO2 against the total amount of CO2 in the solution also increased with increasing CO2 pressure, reaching 0.205 at 4.18 MPa. 3.2.4. CO2 Absorption in 3 M 1MeIm. 1MeIm is categorized within the imidazole derivatives with the smallest heat of reaction, enabling to reduce energy requirements for the CCS process.14,15 In 13C NMR spectra at 40 °C under pressurized CO2 conditions, signals at 141, 123, 121, and 30 ppm originated from 1MeIm, and additional signals at 162 and 124 ppm, assigned to be bicarbonate/carbonate and molecular CO2, respectively, were observed (Table 5 and Figure 4D). CO2 loadings for both chemically and physically absorbed CO2 increased over the entire CO2 pressure range used in this study. Fractions of physically absorbed CO2 against the total amount of CO2 in the solution also increased with increasing CO2 pressure, reaching 0.297 at 4.02 MPa.

P [MPa]a

peak at 161 ppmc

peak at 126 ppmd

0.10 0.94 2.16 3.16 4.03

e 0.550 0.736 0.885 0.892

e 0.113 0.191 0.265 0.258

Total pressure (p) [MPa] ) gauge pressure + atmospheric pressure. Peak ratios were calculated on the basis of the peak at 60 ppm. c Signals derived from bicarbonate/carbonate (chemically absorbed CO2). d Signals derived from molecular CO2 (physically absorbed CO2). e Measurements before feeding CO2 gas. a

b

3.2.5. CO2 Absorption in 3 M Im. Im is a weak base, similar to 1MeIm, but exhibiting somewhat greater heat of reaction.14,15 In 13C NMR spectra at 40 °C under pressurized CO2 conditions, signals at 137 and 121 ppm originated from Im, and additional signals at 162 and 126 ppm, assigned to be bicarbonate/carbonate and molecular CO2, respectively, were observed (Table 6 and Figure 4E). CO2 loadings for both chemically and physically absorbed CO2 increased over the entire CO2 pressure range used in this study. Fractions of physically absorbed CO2 against the total amount of CO2 in the solution also increased with increasing CO2 pressure, reaching 0.315 at 3.98 MPa. 3.2.6. CO2 absorption in 3 M HEMO. HEMO is the weakest base and exhibits the smallest heat of reaction among the chemical absorbents used in this study.14,15 In 13C NMR spectra at 40 °C under pressurized CO2 conditions, signals at 67, 60, 58, and 54 ppm originated from HEMO, and additional signals at 161 and 126 ppm, assigned to be bicarbonate/ carbonate and molecular CO2, respectively, were observed (Table 7 and Figure 4F). CO2 loadings for both chemically and physically absorbed CO2 increased over the entire CO2 pressure range used in this study. Fractions of physically absorbed CO2 against the total amount of CO2 in the solution also increased with increasing CO2 pressure, reaching 0.225 at 4.03 MPa. 3.2.7. Summary of CO2 Absorption in Amine-Containing Aqueous Solvents Detected by 13C NMR Spectroscopy. The CO2 solubility data of the solvents containing strong bases such as MDEA, 12DMIm or 2MeIm indicate almost unity at CO2 pressures of 1 MPa or higher. With increased CO2 pressure, the fractions of physically absorbed CO2 against the total amount of CO2 in all the solvents increased in every case. The differences in the amounts of physically absorbed CO2 among the solvents were less significant (15-30%) than expected over the entire pressure range used in this study. This implies that the use of absorbents with smaller heats of reaction with CO2 is important for recovery of larger amounts of highly pressurized CO2 from the CO2-rich solutions with smaller heat

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Figure 5. CO2 absorption into 3 M HEMO aqueous solution at 4 MPa depending upon temperature ranging from 40 to 70 °C, detected by 13C NMR spectroscopy. Gray and white bars show the amounts of chemically and physically absorbed CO2 as bicarbonate and molecular CO2, respectively. Closed circles show fractions of physically absorbed CO2 against total CO2 in the solutions.

requirements. Thus, we attempted to determine CO2 solubility at 4 MPa and temperatures ranging from 40 to 70 °C by 13C NMR spectroscopy. 3.3. Temperature Dependence of CO2 Absorption in 3 M HEMO. HEMO has been nominated as one of the suitable candidates for absorbing/recovering pressurized CO2 with a small heat requirement.15 It is very interesting to reveal this behavior of pressurized CO2 release under heated conditions, which should enable the development of an innovative IGCC-CCS process with minimum energy consumption. Throughout this study, the quantitative 13C NMR spectroscopic technique had the ability to analyze the solubility of pressurized

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CO2 in amine-containing aqueous solutions at 40 °C and was also helpful to assess the behavior of pressurized CO2 under heated conditions. Figure 5 shows the CO2 absorption in a 3 M HEMO aqueous solution at 4 MPa and temperatures ranging from 40 to 70 °C, detected by 13C NMR spectroscopy. Although the experimental errors involved in NMR measurements at higher temperatures could lead to the unreasonable result in Figure 5 that the physical solubility of CO2 slightly increases from 40 to 50 °C, we consider that the total CO2 loading decreased gradually with increasing temperature. Interestingly, however, the fraction of physically absorbed CO2, compared with the total amount of CO2 in the solution, increased from 0.240 to 0.289, with a maximum (0.332) at 60 °C over the entire temperature range in Figure 5. These results suggest that HEMO, with a small heat of reaction, releases chemically absorbed CO2 at elevated temperatures even at higher CO2 pressures and is able to absorb and release pressurized CO2 while maintaining the pressure level, leading to the mitigation of the compression energy required for transportation. 3.4. Correlations between 13C NMR Spectroscopic and Vapor-Liquid Equilibrium Measurements for CO2 Solubility in Amine-Containing Aqueous Solvents. We now have two powerful methods to determine the solubility of pressurized CO2, 13C NMR spectroscopy and vapor-liquid equilibrium measurements. To verify the applicability of the 13 C NMR experimental setup and the procedures used in this study, correlations determined by both methods for CO2 solubility in six different amine-containing aqueous solutions at 40 °C and CO2 partial pressures up to 4 MPa are shown in Figure 6. In all cases, solubility data determined by the both

Figure 6. Correlations for CO2 solubility in amine-containing aqueous solutions (A, MDEA; B, 12DMIm; C, 2MeIm; D, 1MeIm; E, Im; and F, HEMO), determined by 13C NMR spectroscopy presented herein (open circles) and vapor-liquid equilibrium measurements in our previous report (closed circles).15 CO2 partial pressures were calculated on the assumption that used gases were ideal and vapor pressures of amines were negligible.

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methods are in good agreement over the pressure range examined, indicating that 13C NMR spectroscopy is useful not only for obtaining CO2 solubility data but also identifying the components of inorganic carbon species in the solvents. 4. Conclusions We examined the usefulness of 13C NMR spectroscopy for determining CO2 absorption in six different amine-containing aqueous solutions and for identifying the inorganic carbon species in the solutions at temperatures ranging from 40 to 70 °C and CO2 pressures ranging from 0.5 to 4 MPa. We found that (i) the amounts of physically absorbed CO2 in all the solutions increased with increasing CO2 pressure and were 15-30% of the total CO2 at 40 °C and 4 MPa, and (ii) the solubility determined by 13C NMR spectroscopy and vapor-liquid equilibrium measurements were in good agreement over the wide pressure range studied. This method allows us to obtain not only CO2 solubility data but also to identify the inorganic carbon species in the solutions by a single 13C NMR spectroscopic measurement. To minimize energy requirements and costs in the IGCC-CCS process, it is necessary to optimize the combinations of heat of reaction of the absorbents and the temperature and pressure at the stripper in the CCS process, from the viewpoint of thermodynamics. The rate of absorption of pressurized CO2 into amine-containing aqueous solutions is also an important factor in the design of an innovative IGCC-CCS process, and such measurements of the kinetics are now under consideration. Acknowledgment This study was financially supported by Grant-in-Aid from the Ministry of Economy, Trade and Industry (METI) of Japan. Supporting Information Available: 13C NMR spectral data. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Go¨ttlicher, G.; Pruschek, R. Comparison of CO2 removal systems for fossil-fuelled power plant processes. Energy ConVers. Manage. (Suppl.) 1997, 38, S173. (2) Ekstro¨m, C.; Cavani, A.; Ericson, S.-O.; Hinderson, A.; Westermark, M. Technology and cost options for capture and disposal of carbon dioxide from gas turbines. A system study for Swedish conditions. ASME 1998, 98–GT-443. (3) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. Separation and capture of CO2 from large stationary sources and sequestration in geological formationssCoalbeds and deep saline aquifers. J. Air Waste Manage. Assoc. 2003, 53, 645. (4) Nair, S. CO2 removal systems for ammonia plantssA survey. Nitrogen Methanol 2003, 264, 19. (5) Gainar, I.; Anitescu, G. The solubility of CO2, N2, and H2 in a mixture of dimethylether polyethylene glycols at high pressures. Fluid Phase Equilib. 1995, 109, 281. (6) Shah, V. A. CO2 removal from ammonia synthesis gas with Selexol solvent process. Energy Progress 1988, 8, 67. (7) Blauwhoff, M. M.; Versteeg, G. F.; van Swaaij, W. P. M. A study on the reaction between CO2 and alkanolamines in aqueous solutions. Chem. Eng. Sci. 1984, 39, 207. (8) Barth, D.; Tondre, C.; Delpuech, J.-J. Kinetics and mechanisms of the reactions of carbon dioxide with alkanolamines: A discussion concerning the cases of MDEA and DEA. Chem. Eng. Sci. 1984, 39, 1753. (9) Linek, V.; Sinkule, J.; Havelka, P. Empirical design method of industrial carbon dioxide-mixed solvent absorbers with axial dispersion in gas. Ind. Eng. Chem. Res. 1994, 33, 2731. (10) Rinker, E. B.; Ashour, S. S.; Sandall, O. C. Kinetics and modelling of carbon dioxide absorption into aqueous solutions of N-methyldiethanolamine. Chem. Eng. Sci. 1995, 50, 755.

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ReceiVed for reView May 26, 2009 ReVised manuscript receiVed October 30, 2009 Accepted December 7, 2009 IE900870W