ARTICLE pubs.acs.org/IECR
CO2 Absorption Characteristics in Aqueous K2CO3/Piperazine Solution by NMR Spectroscopy Young Eun Kim,† Jeong Ho Choi,† Sung Chan Nam,† and Yeo Il Yoon*,† †
Greenhouse Gas Research Center, Korea Institute of Energy Research, 102, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Korea ABSTRACT: This research studied the absorption of CO2 into the aqueous solution of piperazine (PZ) and K2CO3/PZ in the vaporliquid equilibrium (VLE) apparatus at 313, 333, and 353 K. NMR spectroscopy was used to identify the species distribution in PZ-H2O-CO2 and K2CO3-PZ-H2O-CO2 systems. The study of species distribution is important for designing new absorbents and absorption processes for CO2 capture. The CO2 loaded solutions of PZ 7.5 wt % and K2CO3 15 wt %/PZ 7.5 wt % were prepared by VLE apparatus at 333 K. 1H NMR and 13C NMR spectra were obtained for these systems at 293 K. This study shows that it is possible to use this method to confirm the reaction mechanism of aqueous PZ and K2CO3/PZ solutions with CO2.
1. INTRODUCTION The capture of carbon dioxide (CO2) from flue gases produced by the combustion of fossil fuels in the air is referred to as ‘postcombustion capture’. CO2 recovery from flue gases accounts for 75% of the total cost of CCS (Carbon Capture and Storage). Various technologies exist for CO2 recovery, such as cryogenics, adsorption, absorption, and membranes. Chemical absorption is the most mature technology, and liquid absorbents are typically used in the absorption process. In this process, selecting the absorbent is the most critical factor because characteristics of the absorbent influence the effectiveness of the overall process. Therefore, most studies focus on the development of new absorbents or the enhancement of existing absorbents. The potassium carbonate (K2CO3) is effective for treating gas at high temperatures. The hotpot process (hot potassium carbonate), Benfield, uses aqueous K2CO3 solutions for removal of CO2 and H2S from ammonia synthesis gas, natural gas, crude hydrogen, and town gas. In this process, the acid gases are absorbed at temperatures near the boiling point of aqueous K2CO3 solution and then stripped by flashing and steam. The high temperature of the absorber can increase the solubility of potassium bicarbonate (KHCO3), thus allowing operation with highly concentrated solution.13 The K2CO3 solution has the advantage of having a low heat duty in the reboiler. However, the slow rate of absorption and low solubility of K2CO3 at a lower temperature, make the process unsuitable for coal fired power plants when used for postcombustion at atmospheric pressure.4,5 These disadvantages can be solved by mixing amines such as piperazine (PZ). PZ was added to aqueous 2-amino-2-methyl-1-propanol (AMP), methyldiethanolamine (MDEA), and K2CO3 solutions, which can react with CO2 as rapid absorption rates. Bishnoi and Rochelle,6 Xu et al.,7 and Kamps et al.8 studied the aqueous blends of the MDEA/PZ solution system. Bishnoi and Rochelle claim that the absorption rate of aqueous MDEA/PZ solution is faster than MDEA/MEA or MDEA/DEA. Cullinane and Rochelle9,10 used an ENRTL model to predict the equilibrium r 2011 American Chemical Society
partial pressure of CO2, species distribution, and enthalpy. K2CO3 increases carbonate (CO32-)/bicarbonate (HCO3) buffers and delays the protonation of PZ to high loadings. Therefore, the reactive PZ concentration was increased. These studies showed that PZ is an activator in the K2CO3 or amine solution. The vaporliquid equilibrium and species distribution of solutions have been studied in order to describe the chemical reaction between CO2 and amine solutions.1115 Several researchers explain the speciation of CO2 loaded PZ solution using the NMR spectroscope. However, studies on the NMR of aqueous K2CO3/PZ solution according to changing CO2 loading are hard to find. This study, therefore, investigates the species distribution of CO2 loaded PZ and K2CO3/PZ solutions.
2. REACTION MECHANISM The chemical reaction of aqueous K2CO3 solution and CO2 takes place through two parallel mechanisms. In the pH > 8 of interest for commercial operations, eqs 3 and 4 are predominant16 2H2 OðlÞ T H3 O+ + OH
ð1Þ
K 2 CO3 ðsÞ + H2 OðlÞ T 2K + + HCO 3 + OH
ð2Þ
The direct formation of HCO3 by reaction of OH with dissolved CO2 (at pH > 8) OH + CO2 ðaqÞ T HCO 3 ðfastÞ
ð3Þ
2 HCO 3 + OH T CO3 + H2 O ðinstantaneousÞ
ð4Þ
Received: December 13, 2010 Accepted: June 10, 2011 Revised: May 31, 2011 Published: June 10, 2011 9306
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Figure 1. Schematic diagram of vaporliquid equilibrium apparatus.
Hydration of dissolved CO2 to form H2CO3 followed by reaction of the H2CO3 with CO2 (at pH < 8) H2 OðlÞ + CO2 ðaqÞ T H2 CO3 ðslowÞ H2 CO3 + OH T HCO 3 + H2 O ðinstantaneousÞ
ð5Þ ð6Þ
The chemical reaction of aqueous PZ solution and CO2, eqs 14, and the following reactions are considered1720 PZðsÞ + H2 OðlÞ T PZH+ + OH
ð7Þ
PZH+ + H2 OðlÞ T PZðlÞ + H3 O+
ð8Þ
Formation of carbamate and bicarbamate by reaction of PZ with CO2 PZðlÞ + CO2 ðaqÞ + H2 OðlÞ T PZCOO + H3 O+
ð9Þ
PZH+ + CO2 ðaqÞ + H2 OðlÞ T H+ PZCOO + H3 O+
ð10Þ
H+ PZCOO + H2 OðlÞ T PZCOO + H3 O+
ð11Þ
PZCOO + CO2 ðaqÞ + H2 OðlÞ T PZðCOO Þ2 + H3 O+ ð12Þ
PZ is dissolved in water and forms PZH+. In this case, the water serves as an acid and provides a proton with PZ. The basicity of amine is quantified by pKa of its conjugate acid. Some of PZH+ reacts with water and form free amine (PZ(l)). If the PZH+ have a high pKa value, PZ is the strong base. Aqueous PZ solution is reported to be a promising absorbent because PZ has a high pKa, similar to that of MEA.21
3. EXPERIMENT 3.1. Equilibrium Solubility Measurement. Aqueous PZ and K2CO3/PZ solutions were prepared using concentrations of PZ 7.5 wt % and K2CO3 15 wt %/PZ 7.5 wt %. PZ is a white solid at room temperature with 150 g/L water solubility. Therefore, it was necessary to heat the K2CO3/PZ solution before using it in the experiment in order to obtain a solution with the preferred concentration. The absorbents were used with their purity as received: K2CO3 (anhydrous 99.5%), PZ (anhydrous 99.0%). All absorbents were obtained from Samchun Chemicals, Korea. The CO2 gas (purity 99.99%) and nitrogen gas (purity 99.999%) were obtained from Special Gas, Korea. The difference between
the gas cylinder and reactor volumes was corrected with nitrogen gas. After the absorption, the liquid samples containing CO2 were collected in order to obtain NMR and pH measurements. The equilibrium partial pressures of CO2 and absorption rate measurements were performed in the VLE apparatus (see Figure 1). The VLE apparatus consists of a gas reservoir, reactor, pressure-measuring instrument, and recorder. The gas reservoir and reactor were made of 316 stainless steels with an internal volume of 300.29 cm3 and 322.56 cm3, respectively. The temperatures of the CO2 gas and solution were measured using a K-type thermocouple. Furthermore, the pressure of the gas was measured by a pressure sensor a PTB model (range: 110 kgf/ cm2) from Synsys Ltd., Korea. The gas reservoir and reactor were heated by water bath method. The experiments were conducted over the temperature range of 313, 333, 353 K, and CO2 injection pressure of 20785 kPa. The conditions were set up in accordance with CO2 capture at the PFBC (Pressurized Fluidized Bed Combustion) power plant. PFBC is an advanced clean-coal power production system with low emissions such as sulfur dioxide (SO2) and nitrogen oxides (NOx). It also has good efficiency compared to conventional coal fired plants. The inlet CO2 (99.99%) was heated in the reservoir prior to entering the reactor with 313 K. The solution (100 mL) was kept in the reactor. Before starting the experiment, residual gas in the reactor was removed using a vacuum pump. In each experiment, the stirring speed in the reactor was 170 rpm, in order to maintain a smooth interfacial area. As the desired temperatures were reached in the gas reservoir and reactor, the valve was opened to inject CO2 into the reactor. The pressure of CO2 in the reactor was decreased over time. When the pressure and temperature values were constant, a state of equilibrium was obtained. Before injecting of CO2 into the reactor, vaporliquid equilibrium of the absorbent was maintained in the experiment condition. The pressure at this point was subtracted from pressure that was obtained after CO2 injection; in other words, the partial pressure of CO2 in the reactor was calculated as the difference between the after and before CO2 injection. In this experiment, CO2 was injected into the reactor again when equilibrium was reached. Only the absorbent absorbed the CO2 injected into the closed reactor. Therefore, it is possible to calculate the absorption rate by measuring the pressure changes in five-second intervals. 9307
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Table 1. Solubilities of CO2 in Aqueous PZ and K2CO3/PZ Solutions at 333 Ka PZ 7.5 wt % P *CO2
a
(kPa)
R (mol CO2/mol PZ)
K2CO3 15 wt %/PZ 7.5 wt % pH
P
* 2 CO
(kPa)
R (mol CO2/(mol K2CO3 + mol PZ))
pH
0
0
11.76
0
0
13.34
4.903
0.470
9.84
0.981
0.208
11.66
65.901
0.896
8.16
1.079
0.410
10.75
379.027
1.011
7.95
4.217
0.631
9.85
594.774
1.076
7.53
38.638
0.791
9.28
186.621
0.959
8.72
387.559
1.013
8.49
598.108
1.052
8.30
R = loading (mol CO2/mol solute), PCO2* = equilibrium partial pressure of CO2.
Figure 2. Comparison of experimental and literature data for CO2 equilibrium into aqueous PZ 5 wt % solution at T = 313 K: b, this work; 3, Bishnoi;17 9, Aroua;22 ), Derk;19 2, Cullinane.23
Figure 3. CO2 equilibrium into aqueous PZ 7.5 wt % solution at various temperatures: b, T = 313 K; O, T = 333 K; 1, T = 353 K.
3.2. NMR Measurement. This study determined the species distribution of aqueous PZ and K2CO3/PZ solutions using a BRUKER AVANCE 500 MHz NMR spectroscope. 1H and 13C
Figure 4. CO2 equilibrium into aqueous K2CO3 15 wt %/PZ 7.5 wt % solution at various temperatures: b, T = 313 K; O, T = 333 K; 1, T = 353 K.
Figure 5. Absorption rate for CO2 into aqueous PZ solution at various temperatures: b, T = 313 K; 3, T = 333 K; 9, T = 353 K.
NMR measurements were performed at 293 K for CO2 loaded aqueous PZ 7.5 wt % and K2CO3 15 wt %/PZ 7.5 wt % solutions. 9308
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Industrial & Engineering Chemistry Research CO2 loaded solutions were prepared using a VLE apparatus at 313 K. Table 1 presents the CO2 loading of solutions and pH values. An internal standard reference of D2O was added to the NMR tube for analysis. The protons bound in the amine group (NH) were changed into deuterium. Therefore, the peaks of theses protons disappear in the 1H NMR spectra. The quantitative spectra were obtained 13C NMR with a delay time (D1) of 2 min. The number of scans (NS) was 1024 so that accurate peaks were obtained in the 13C NMR spectra.
4. RESULTS AND DISCUSSION 4.1. Equilibrium Solubility. To confirm the experimental apparatus and procedure, the equilibrium partial pressure of CO2 (PCO2*) into aqueous PZ 5 wt % solution was measured at 313 K. The experimental data of this work were compared with data found in the literature (see Figure 2). There is positive agreement between the result of this work and the results of the literature.
Figure 6. Absorption rate for CO2 into aqueous K2CO3/PZ solution at various temperatures: b, T = 313 K; 3, T = 333 K; 9, T = 353 K.
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However, the results reported by Aroua and Salleh22 are different from the findings of this work. The equilibrium partial pressure of CO2 in aqueous PZ 7.5 wt % and K2CO3 15 wt %/PZ 7.5 wt % solutions were measured at 313, 333, and 353 K. Figures 3 and 4 show the effects of the temperature on the equilibrium partial pressure of CO2. These graphs represent the CO2 loading (mol CO2/mol solute), absorption at the specific partial pressure of CO2. The equilibrium partial pressure of CO2 was increased at a given concentration with increasing reactor temperatures. Figures 5 and 6 show the absorption rates for aqueous PZ and K2CO3/PZ solutions, respectively. These figures represent the pressure change over the time after injecting of the CO2. The absorption rates of aqueous PZ and K2CO3/PZ solutions were increased according to the increasing temperature of the reactor.
Figure 8. COSY spectrum of PZ at T = 293 K.
Figure 7. Molecular structure of species in PZ-H2O-CO2 and K2CO3-PZ-H2O-CO2 systems. 9309
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Figure 11. 1H NMR spectrum of K2CO3-PZ-H2O-CO2 system at T = 293 K, R = 0.410 mol CO2/(mol K2CO3 + mol PZ).
Figure 9. HMQC spectrum of PZ at T = 293 K.
Figure 10. 1H NMR spectrum of PZ-H2O-CO2 system at T = 293 K, R = 0.470 mol CO2/mol PZ.
H and 13C NMR spectroscopy were applied to investigate speciation in CO2 loaded aqueous PZ 7.5 wt % and K2CO3 15 wt %/PZ 7.5 wt % solutions. Figure 7 shows the molecular structure of species in PZ-H2O-CO2 and K2CO3-PZ-H2O-CO2 systems. Two-dimensional NMR (2D NMR) methods, correlation spectroscopy (COSY), and heteronuclear multiple quantum coherence (HMQC) were used to obtain an accurate analysis of species. Figures 8 and 9 present the COSY and HMQC spectra. The COSY spectrum indicates that protons are coupling with each other. Both axes of Figure 8 correspond to the 1H NMR spectrum. The HMQC is selective for direct CH coupling and allows for the determination of carbon to hydrogen connectivity. The COSY and HMQC show that the protons and carbons of PZ have only one peak in the 1H NMR (δ = 2.6 ppm) and 13C NMR (δ = 45 ppm) spectra, respectively. The other peak in the 1H NMR spectrum is D2O 4.2. Speciation.
1
Figure 12. 13C NMR spectrum of PZ-H2O-CO2 system at T = 293 K, R = 0.470 mol CO2/mol PZ: (a) low field and (b) high field.
(δ = 4.8 ppm). This result is based on the chemical equivalence of protons and carbons. Figures 10 and 11 show the typical 1H NMR spectra of PZH2O-CO2 and K2CO3-PZ-H2O-CO2. The deuterium oxide (D2O) was added to the PZ-H2O-CO2 and K2CO3-PZ-H2OCO2 systems in order to exchange protons bound in the amine group with the deuteriums. The deuteriums in the amine group produced a single peak in the water region. The PZ/PZH+ and PZ(COO)2 peaks appeared as a singlet, whereas the 9310
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PZCOO/H+PZCOO peak was represented by two triplets. Due to the fast proton exchange with water, it is impossible to distinguish between the two species, protonated and unprotonated forms of PZ (PZ/PZH+, PZCOO/H+PZCOO). The PZ(COO)2 peak appeared in a lower field than the PZ/PZH+ peak because the electrons of the protons in the PZ(COO)2
were deshielded by the carbon and oxygen atoms. The 1H NMR spectra of the PZ-H2O-CO2 differed from the K2CO3-PZ-H2OCO2. The PZ(COO)2 peaks were not found in the 1H NMR spectra of the PZ-H2O-CO2 system. Figures 12 and 13 show the typical 13C NMR spectra of PZ-H2O-CO2 and K2CO3-PZ-H2O-CO2 systems. The peaks of the CO2 in the PZCOO/H+PZCOO, PZ(COO)2 and HCO3/CO32- species were represented in a lower field. However, the carbons of PZ were shown in a relatively high field. Tables 2 and 3 show the chemical shifts of species in 1H and 13 C NMR at the same temperature. The shift of peak locations is caused by a change in pH. Most of the species in PZ-H2O-CO2 and K2CO3-PZ-H2O-CO2 systems shifted from high field to low field, except for PZ(COO)2 in the 1H NMR spectra. In contrast, most of the species in the PZ-H2O-CO2 and K2CO3PZ-H2O-CO2 systems shifted from low field to high field in the 13 C NMR spectra. The trends of shifting chemicals were consistent with the study of Hilliard’s24 study of the CO2 loaded PZ solution. The carbamate (PZCOO/H+PZCOO), bicarbamate (PZ(COO)2), and bicarbonate (HCO3 ) are related to CO2. Figure 14 shows the relative ratios of these species. The relative integral areas of peaks correspond to the relative concentrations of the species. The graph in Figure 14 represents the relative ratio (%) of PZ/PZH+, PZCOO/H+PZCOO, PZ(COO)2, and HCO3/CO32-. In the PZ-H2O-CO2 system, the relative ratio of carbamate increased according to increasing CO2 loading. However, in the K2CO3-PZ-H2O-CO2 system, relative ratios of carbamate and bicarbamate fluctuated according to changing CO2 loading. Interestingly, the PZ(COO)2 peaks were not found in the 1H NMR and 13C NMR spectra of aqueous PZ solution. Generally, the reaction rate between PZ and CO2 increases with the concentration of PZ solution, and then the reaction product, carbamate, reacts with CO2 while producing bicarbamate. Furthermore, the rate of bicarbamate production is slower than that of carbamate. The amount of produced bicarbamate would be small, because the PZ concentration, used in this study, was relatively low. Therefore, it could be considered that the amount of produced bicarbamate was out of the NMR resolution range, fewer than 5%. In the aqueous K2CO3/PZ solution, large amounts of bicarbonate and carbonate formed by dissociation of K2CO3. These served as a buffer, reducing the PZH+ and a large quantity of PZ available for CO2 absorption. Therefore, the amount of free PZ in the aqueous K2CO3/PZ solution is larger than that of PZ. Furthermore, the formation of the bicarbamate formation is promoted.
Figure 13. 13C NMR spectrum of K2CO3-PZ-H2O-CO2 system at T = 293 K, R = 0.631 mol CO2/(mol K2CO3 + mol PZ): (a) low field and (b) high field.
Table 2. Chemical Shift and Peak Areas of 13C NMR Spectra for PZ-CO2-H2O at 293 K δ (ppm) high field R
system
PZ-CO2-H2O
PZ carbamate
low field free PZ
PZ carbamate
bicarbonate/carbonate
numbers in Figure 12 0.470
5 44.11
4 43.21
3 43.29
1 162.37
2 162.81
0.896
43.53
41.52
42.67
160.59
162.32
1.010
43.58
41.64
42.70
160.68
162.36
1.076
43.53
41.53
42.67
160.59
162.33
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Table 3. Chemical Shift and Peak Areas of 13C NMR Spectra for K2CO3-PZ-CO2-H2O at 293 K δ (ppm) high field R
PZ bicarbamate
numbers in Figure 13 0.208
7 44.54
6 44.40
0.410
44.30
43.79
0.631
44.32
43.96
42.81
43.00
0.791
44.31
43.72
42.12
0.960
44.31
43.56
1.013
44.31
1.052
44.32
system
K2CO3-PZ-CO2-H2O
low field
PZ carbamate
free PZ
PZ bicarbamate
PZ carbamate
bicarbonate/carbonate
4 44.33
2 166.85
1 163.21
3 163.27
164.68
163.03
163.26
163.26
162.39
162.75
42.78
163.25
161.40
162.54
41.62
42.68
163.25
160.80
162.40
43.51
41.47
42.65
163.20
160.67
162.35
43.38
41.37
42.64
160.56
162.33
5 44.27
43.54
solutions, the equilibrium partial pressure of CO2 was decreased according to decreasing temperatures. In contrast, the absorption rates of CO2 in the aqueous PZ and K2CO3/PZ solutions were increased according to increasing temperatures. In particular, the CO2 absorption rate of the aqueous K2CO3/PZ solution was faster than that of the aqueous PZ solution. The results show the effectiveness of adding of K2CO3 to aqueous PZ solution. This study obtained the 1H and 13C NMR spectra and identified the species distributions of PZ-H2O-CO2 and K2CO3-PZH2O-CO2 systems. The aqueous PZ solution absorbed CO2 in the form of PZCOO/H+PZCOO and HCO3/CO32-. However, the aqueous K2CO3/PZ solution reacted with the CO2 and formed PZCOO/H+PZCOO, HCO3/CO32, and PZ(COO)2. In the aqueous PZ solution, PZ reacted with CO2 and formed the carbamate rather than bicarbamate because of protons in the water. Moreover, the carbamate was a minor component in the PZ-H2O-CO2 system. In the aqueous K2CO3/ PZ solution, large quantities of HCO3 and CO32- were formed by dissociation of K2CO3. The HCO3/CO32- increased the reactivity of PZ because these anions served as a buffer, reducing the protonation of PZ. Therefore, the relative ratio of the carbamate and bicarbamate could be reached 50% in the aqueous K2CO3/PZ solution.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +82-42-860-3758. E-mail:
[email protected].
’ ACKNOWLEDGMENT This research was supported by a grant (CJ3-301) from the Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government. Figure 14. Relative ratio of species in 1H and 13C NMR: (a) PZ-H2O-CO2 system; (b) K2CO3-PZ-H2O-CO2 system; 9, PZ/PZH+; 0, HCO3/ CO32-; (hatched pattern), PZCOO/H+PZCOO; (crosshatched), PZ(COO)2.
5. CONCLUSIONS VLE data were acquired for CO2 in the aqueous PZ and K2CO3/PZ solutions. In the aqueous PZ and K2CO3/PZ
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