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Amine mixtures and the effect of additives on the CO2 capture rate
Available online at www.sciencedirect.com
Energy Procedia 00 (2010) 000–000 Energy Procedia 4 (2011) 195–200 www.elsevier.com/locate/XXX
www.elsevier.com/locate/procedia
GHGT-10
Amine mixtures and the effect of additives on the CO2 capture rate R.Rowland1, Q.Yang2, P.Jackson1 and M.Attalla1* 1
2
CSIRO Energy Technology, P.O. Box 330, Newcastle, NSW 2300, Australia
CSIRO Material Science & Engineering Private Bag 33, Clayton South, Vic. 3169, Australia Elsevier use only: Received date here; revised date here; accepted date here
Abstract The mass transfer of CO2 into aqueous ammonia with a series of promoters was studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. Of these classes the alkanolamine showed the greatest enhancement of CO2 mass transfer, followed by the amino acid. The inorganic base did not show any enhancement. Two factors were identified that lead to the enhancement of CO2 mass transfer. The first factor was the alkanolamine and amino acid used have faster reactions with CO2 than ammonia. The second was the presence of ammonia in a high concentration provides additional sites for proton accepting; allowing more of the added alkanolamine or amino acid to react with CO2. This theory is supported by C13 NMR data. c 2010 ⃝ 2011Elsevier Published Elsevier Ltd. © Ltd.byAll rights reserved
Keywords: ammonia; mass transfer; promoters; CO2. 1. Introduction The capture, reversible release and storage of carbon dioxide (CO2) from combustion flue gases (post combustion capture, PCC) is recognised by government and industry as a viable near-term option for greenhouse gas abatement [1,2]. It is relevant to electricity generation from fossil fuels (coal, oil and gas) which accounts for approximately 25% of global CO2 emissions [3]. This figure has been forecast to increase drastically in the next 25 years [4]. PCC has two distinct advantages over other power station CO2 mitigation options such as oxy-firing and integrated gasification combined cycle (IGCC) with pre-combustion capture [5]. The first advantage is that being an end-of-pipe technology means it can be retrofitted to existing power stations with minimal modification, or easily integrated into new ones. The second advantage is the ability to dynamically control the energy demand of the PCC plant, allowing additional electricity output to the grid in times of peak load or optimal electricity pricing. PCC technology is also suitable for CO2 capture from other point sources such as steel and cement manufacturing. The most mature PCC technology is reactive chemical absorption/desorption of CO2 into/from an aqueous alkanolamine absorbent [5]. It is a temperature swing process where CO2 is absorbed at doi:10.1016/j.egypro.2011.01.041
196
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low temperature (~313 K) and released at high temperature (~393 K), with regenerated absorbent returned to the absorption process. Gas-liquid contacting takes place via packed columns with counter-current gas and liquid flows. The application of PCC to combustion flue gases from electricity generation or other point sources poses a number of technical challenges. The two main issues are the energy requirements of the process and capital cost. The main energy requirements are for heating the absorbent to release CO2, and the electricity to pump the absorbent around the system. The largest contribution to the capital cost are the materials for construction of the absorption columns. The size of the absorption columns are defined by the rate of CO2 absorption. The faster the absorption rate, the smaller the gas-liquid contact area required, and thus smaller absorption columns are required. In an attempt to address these issues, aqueous ammonia (NH3) solutions are now being proposed as an alternative to aqueous alkanolamine absorbents for PCC. Aqueous ammonia has been shown to achieve higher CO2 loadings (on a molar and mass basis) than sterically free primary alkanolamines such as monoethanolamine (MEA) [6]. This is due to the CO2-NH3-H2O system favouring bicarbonate over carbamate formation, particularly as CO2 loading increases [7]. Aqueous ammonia has also been shown to require less heat input for desorption than MEA [8]. This is due to the smaller reaction enthalpy for CO2 absorption and higher CO2 partial pressure at elevated temperature compared to MEA. Ammonia is also resistant to oxidative degradation, which is a major benefit when treating oxygen containing gas streams such as those from coal fired power stations. The other main attractive feature is that in the presence of sulfur and nitrogen oxides in the gas stream, the ammonium salts that form have commercial value as fertilisers. A major drawback in the use of ammonia is its vapour pressure. Due to its small molecular weight ammonia vapour pressure is high compared to alkanolamines [9,10]. To address this it has been proposed that the absorption process take place at lower temperatures to reduce losses via volatilization (slip). While reducing the temperature of the absorption process lowers the ammonia slip, it also slows the kinetics of the absorption reaction. For the ammonia system to be comparable to amine systems it needs to absorb CO2 from a flue gas at similar rates. Currently the rates of absorption for low temperature ammonia (e.g. 283 K) are much slower than amines such as MEA at 313 K [11]. One approach to increasing this rate is to add other compounds to ‘promote’ the absorption of CO2. In this work the mass transfer of CO2 into aqueous ammonia with a series of promoters is studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. All three of these classes have been previously used to absorb, or promote the absorption of CO2 [12-14]. C13 NMR was used to identify the reaction product species. This information was then used to identify the reaction mechanisms of the CO2 absorption. 2. Method 2.1. Mass Transfer The reactive chemical absorption of CO2 into a thin film can be described as a combination of diffusion and chemical reaction processes. The CO2 diffuses from the gas phase, across the gasliquid interface, into the liquid phase where it undergoes chemical reaction. It is assumed that the film is uniform and the continuous replenishment of the film means there are no long range diffusion processes taking place in the liquid phase between the interface and the bulk liquid [15]. The mass transfer processes taking place can be described as a combination of those taking place on the gas side of the interface and on the liquid side. The concentration of CO2 in the gas phase falls
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from its bulk gas partial pressure, to its partial pressure at the gas-liquid interface, according to the gas side mass transfer coefficient kg. The dissolved CO2 concentration at the interface then falls by diffusion and chemical reaction to the bulk dissolved CO2 concentration, according to the liquid side mass transfer coefficient kl. The liquid side mass transfer coefficient is a function of mass transfer of CO2 without reaction, k°l, and enhancement by chemical reactions occurring in the liquid film that act to consume CO2. The overall mass transfer co-efficient KG is related to the inverse sum of the liquid and gas side mass transfer co-efficient, Eq. 1.
1 1 1 = + KG kg kl
Eq. 1
A wetted-wall column was used to study the absorption rate of CO2 into aqueous solutions of ammonia and either piperazine, glycine or boric acid. All compounds were purchased from Sigma Aldrich. The purities of the compounds were; ammonia (28% v/v), piperazine (99%), glycine (99%), and boric acid (99%). The solutions studied were; 3 mol/L ammonia with 0.5 mol/L piperazine, 3 mol/L ammonia with 0.5 mol/L boric acid, 3.35 mol/L ammonia with 0.5 mol/L glycine, and 0.5 mol/L piperazine. All solutions were studied at a temperature of 283K. The carbon dioxide loadings studied are outlined in Table 1. Table 1 List of solution and CO2 loadings studied. Solution 0 mol 0.6 mol 1.2 mol CO2 CO2 CO2 3 mol/L ammonia with x x x 0.5 mol/L piperazine 3 mol/L ammonia with x 0.5 mol/L boric acid 3.35 mol/L ammonia x x x with 0.5 mol/L glycine 0.5 mol/L piperazine
1.8 mol CO2
2.4 mol CO2
2.8 mol CO2
x
x
x
x x
x
x
The increased ammonia concentration used with the glycine study (pKa = 2.35 at 298 K) was to offset the free ammonia lost to the formation of ammonium glycinate in solution. Due to higher pKa values the ammonia concentration in the piperazine and boric acid studies did not need to be increased. The pKa values at 298 K for piperazine and boric acid are 9.73 and 9.24 respectively. The design and operation of the wetted wall column has been previously presented [11]. Flux rates (NCO2) were measured for each solution at bulk carbon dioxide partial pressures of 0, 4, 8, 12, 16 and 20 kPa. A plot of NCO2 versus the applied CO2 partial pressure PCO2 (log mean of the inlet and outlet CO2 partial pressure) yields a linear relationship. With the slope equal to KG and an xintercept equal to the equilibrium partial pressure (P*CO2), see Eq 2. * NCO2 = K G (PCO2 -PCO2 )
Eq. 2
Due to ammonia having a high vapour pressure ammonium bicarbonate formed in the condenser down-stream from the wetted-wall column. After measurement at each CO2 partial pressure the condenser was dried at <283 K and rinsed with distilled water. The amount of ammonia in the collected solution was then measured using an ammonium ion selective electrode (Orion). This data was then used to correct the measured CO2 concentration exiting the wetted-wall column by up to 5%. This correction was applied to all CO2 partial pressures for the following test solutions; 3 mol/L ammonia with 0.5 mol/L piperazine (loadings of 0, 26.4 and 52.8g of CO2); 3 mol/L
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ammonia with 0.5 mol/L boric acid (loadings of 0g of CO2); 3.35 mol/L ammonia with 0.5 mol/L glycine (loadings of 0 and 26.4g of CO2). At all other operating conditions no ammonium bicarbonate formation was detected. 2.2. NMR Aqueous solutions of 3 mol/L ammonia with 0.5 mol/L piperazine, and 0.5 mol/L piperazine were prepared in 15 mL volumetric flasks with deionised water. The solutions were decanted into reaction flasks and warmed with a thermostatted bath to 298 K. After the solutions reached the required temperature, CO2 was introduced to the reaction flask above the stirring bean at 5 mL/min. Samples of the reaction were taken in NMR tubes at 0 and 10 minutes. An external standard of 1,4dioxane was added to each tube and samples were then analysed via C13 NMR spectroscopy at 298 K. 3. Results & Discussion The overall mass transfer co-efficients (KG) for the promoted solutions are shown in Figure 1.
-1
-2
-1
KG (mmols m kPa )
2.5 2.0 1.5 1.0 0.5 0.0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
CO2 Loading (mol CO2/mol NH3 + Promoter) 3M NH3
3M NH3 + 0.5M Pz
3.35M NH3 + 0.5M Glycine
3M NH3 + 0.5M Boric Acid
0.5M Pz
Figure 1 Plot of overall mass transfer co-efficient (KG) versus CO2 loading for 3 mol/L ammonia [11] and the promoted ammonia solutions at 283 K. The overall mass transfer of CO2 into 3 mol/L ammonia with 0.5 mol/L boric acid shows no increase in absorption over 3 mol/L ammonia. Without nitrogen within the molecule boric acid is incapable of forming a carbamate. In this system boric acid is only capable of accepting protons. The lack of increase in CO2 absorption rates with boric acid present indicates the rate limiting step of absorption by ammonia is not proton accepting, it is the ammonium carbamate formation. The overall mass transfer of CO2 into 3.35 mol/L ammonia with 0.5 mol/L glycine shows an increase in absorption of CO2 over 3 mol/L ammonia. This increase in absorption rate is seen at loadings from 0-0.45. Glycine, an amino acid, is capable of both forming a carbamate and proton accepting. The increased CO2 absorption rate can be explained by glycine having a faster reaction with CO2 than ammonia, see Table 2. The faster reaction of glycine with CO2 is why we see faster CO2 absorption at loadings of less than 0.45. Having another base in high concentration (ammonia) will allow more glycine to react with CO2 than glycine on its own, as the other base provides alternative sites for proton accepting.
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Table 2 Second order rate constants for ammonia, glycine and piperazine. Ammonia Glycine Piperazine
Second Order Rate Constant at 283K (m3/mol.s) 0.9 [11] 3.0 [12] 26 [16]
The overall mass transfer of CO2 into 3 mol/L ammonia with 0.5 mol/L piperazine shows an increase in absorption of CO2 over 3 mol/L ammonia. This increase in absorption rate is seen at loadings from 0-0.8. Piperazine, a secondary diamine is capable of both forming a carbamate and proton accepting. Being a diamine piperazine can form a dicarbamate, diprotonate or a combination of the two. The increased CO2 absorption rate can be explained by piperazine having a faster reaction with CO2 than ammonia, see Table 2. The faster reaction of piperazine with CO2 is why we see faster CO2 absorption at loadings of less than 0.8. Having another base in high concentration (ammonia) will allow more piperazine to react with CO2 than piperazine on its own, as the other base provides alternate sites for proton accepting.
Figure 2 C13 NMR for and 3 mol/L ammonia with 0.5 mol/L piperazine (left) and 0.5 mol/L piperazine (right) before and after exposure to CO2. The C13 NMR data for 0.5 mol/L piperazine and 3 mol/L ammonia with 0.5 mol/L piperazine are shown in Figure 2. At T = 0 min in both sets of data we see a sharp peak at a shift of 45 ppm. This peak corresponds to the four NCH2 carbons in the piperazine ring. At T = 10 min we see this peak shift to the right, we also see a small peak appear due to the NCH2 carbons in piperazine carbamate, a reaction product. The ratio of the unreacted piperazine peak heights (T = 10 min divided by T = 0 min) for the 3 mol/L ammonia with 0.5 mol/L piperazine is 0.98. For the 0.5 mol/L piperazine with no ammonia present it is 0.79. The difference in these ratios confirms that there is more unreacted piperazine present in the blend with ammonia after 10 minutes than there is with piperazine on its own. This supports the theory that having a second base in high concentration allows more piperazine to react with CO2 as the second base (ammonia) provides alternate sites for proton accepting. 4. Conclusion The mass transfer of CO2 into aqueous ammonia with a series of promoters was studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. Of these classes the alkanolamine showed the greatest enhancement of CO2 mass transfer, followed by the amino acid. The inorganic base did not show any enhancement.
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There are two factors that lead to the enhancement of CO2 mass transfer. The first factor is the alkanolamine and amino acid used have faster reactions with CO2 than ammonia. The second factor is the presence of ammonia in a high concentration. This provides additional sites for proton accepting; allowing more of the added alkanolamine or amino acid free to react with CO2. The C13 NMR data presented supports this theory. 5. Acknowledgements This project is part of the CSIRO Coal Technology Portfolio and received funding from the Australian Government as part of the Asia-Pacific Partnership on Clean Development and Climate. The views expressed herein are not necessarily the views of the Commonwealth, and the Commonwealth does not accept responsibility for any information or advice contained herein. 6. References [1] IEA, Capturing CO2, D. Adams and J. Davison, Editors. 2007, IEA Greenhouse Gas R&D Program: Cheltenham. p. 23. [2] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change, B. Metz, et al., Editors. 2005, Cambridge University Press: Cambridge. [3] WRI. Climate Analysis Indicators Tool. 2007 [cited 2007; Available from: http://www.wri.org/. [4] IEA, World Energy Outlook 2007. 2008: Paris, France. [5] Davison, J., Performance and costs of power plants with capture and storage of CO2. Energy, 2007. 32(7): p. 1163-1176. [6] Yeh, A.C. and H. Bai, Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse gas emissions. The Science of the Total Environment, 1999. 228(2-3): p. 121-133. [7] Mani, F., Peruzzini, M. & Stoppioni, P., CO2 absorption by aqueous NH3 solutions: speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study. Green Chemistry, 2006. 8: p. 6. [8] Yeh, J.T., et al., Semi-batch absorption and regeneration studies for CO2 capture by aqueous ammonia. Fuel Processing Technology, 2005. 86(14-15): p. 1533-1546. [9] Edwards, T.J., et al., Vapor-Liquid Equilibria in Multicomponent Aqueous Solutions of Volatile Weak Electrolytes. AIChE Journal, 1978. 24(6): p. 966-976. [10] Goppert, U.M., G., pour-liquid equilibria in aqueous solutions of ammonia and carbon dioxide at temperatures between 333 and 393 K and pressures up to 7 MPa. Fluid Phase Equilibria, 1998. 41: p. 33. [11] Puxty, G., R. Rowland, and M. Attalla, Comparison of the rate of CO2 absorption into aqueous ammonia and monoethanolamine. Chemical Engineering Science, 2010. 65(2): p. 8. [12] Penny, D.E. and T.J. Ritter, KINETIC-STUDY OF THE REACTION BETWEEN CARBONDIOXIDE AND PRIMARY AMINES. Journal of the Chemical Society-Faraday Transactions I, 1983. 79: p. 2103-2109. [13] Bishnoi, S., Carbon Dioxide Absorption and Solution Equilibrium in Piperazine Activated Methyldiethanolamine. 2000, University of Texas, Austin: Austin, Tx. [14] Astarita, G., PROMOTION OF CO2 MASS-TRANSFER IN CARBONATE SOLUTIONS. 1981. p. 581-588. [15] Danckwerts, P.V., Gas-Liquid Reactions. 1970, New York: McGraw-Hill. [16] Bishnoi, S. and G.T. Rochelle, Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chemical Engineering Science, 2000. 55(22): p. 55315543.
Energy Procedia 00 (2010) 000–000 Energy Procedia 4 (2011) 195–200 www.elsevier.com/locate/XXX
www.elsevier.com/locate/procedia
GHGT-10
Amine mixtures and the effect of additives on the CO2 capture rate R.Rowland1, Q.Yang2, P.Jackson1 and M.Attalla1* 1
2
CSIRO Energy Technology, P.O. Box 330, Newcastle, NSW 2300, Australia
CSIRO Material Science & Engineering Private Bag 33, Clayton South, Vic. 3169, Australia Elsevier use only: Received date here; revised date here; accepted date here
Abstract The mass transfer of CO2 into aqueous ammonia with a series of promoters was studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. Of these classes the alkanolamine showed the greatest enhancement of CO2 mass transfer, followed by the amino acid. The inorganic base did not show any enhancement. Two factors were identified that lead to the enhancement of CO2 mass transfer. The first factor was the alkanolamine and amino acid used have faster reactions with CO2 than ammonia. The second was the presence of ammonia in a high concentration provides additional sites for proton accepting; allowing more of the added alkanolamine or amino acid to react with CO2. This theory is supported by C13 NMR data. c 2010 ⃝ 2011Elsevier Published Elsevier Ltd. © Ltd.byAll rights reserved
Keywords: ammonia; mass transfer; promoters; CO2. 1. Introduction The capture, reversible release and storage of carbon dioxide (CO2) from combustion flue gases (post combustion capture, PCC) is recognised by government and industry as a viable near-term option for greenhouse gas abatement [1,2]. It is relevant to electricity generation from fossil fuels (coal, oil and gas) which accounts for approximately 25% of global CO2 emissions [3]. This figure has been forecast to increase drastically in the next 25 years [4]. PCC has two distinct advantages over other power station CO2 mitigation options such as oxy-firing and integrated gasification combined cycle (IGCC) with pre-combustion capture [5]. The first advantage is that being an end-of-pipe technology means it can be retrofitted to existing power stations with minimal modification, or easily integrated into new ones. The second advantage is the ability to dynamically control the energy demand of the PCC plant, allowing additional electricity output to the grid in times of peak load or optimal electricity pricing. PCC technology is also suitable for CO2 capture from other point sources such as steel and cement manufacturing. The most mature PCC technology is reactive chemical absorption/desorption of CO2 into/from an aqueous alkanolamine absorbent [5]. It is a temperature swing process where CO2 is absorbed at doi:10.1016/j.egypro.2011.01.041
196
Author name et / Energy Procedia 00 (2010) 000–000 R. Rowland al. / Energy Procedia 4 (2011) 195–200
low temperature (~313 K) and released at high temperature (~393 K), with regenerated absorbent returned to the absorption process. Gas-liquid contacting takes place via packed columns with counter-current gas and liquid flows. The application of PCC to combustion flue gases from electricity generation or other point sources poses a number of technical challenges. The two main issues are the energy requirements of the process and capital cost. The main energy requirements are for heating the absorbent to release CO2, and the electricity to pump the absorbent around the system. The largest contribution to the capital cost are the materials for construction of the absorption columns. The size of the absorption columns are defined by the rate of CO2 absorption. The faster the absorption rate, the smaller the gas-liquid contact area required, and thus smaller absorption columns are required. In an attempt to address these issues, aqueous ammonia (NH3) solutions are now being proposed as an alternative to aqueous alkanolamine absorbents for PCC. Aqueous ammonia has been shown to achieve higher CO2 loadings (on a molar and mass basis) than sterically free primary alkanolamines such as monoethanolamine (MEA) [6]. This is due to the CO2-NH3-H2O system favouring bicarbonate over carbamate formation, particularly as CO2 loading increases [7]. Aqueous ammonia has also been shown to require less heat input for desorption than MEA [8]. This is due to the smaller reaction enthalpy for CO2 absorption and higher CO2 partial pressure at elevated temperature compared to MEA. Ammonia is also resistant to oxidative degradation, which is a major benefit when treating oxygen containing gas streams such as those from coal fired power stations. The other main attractive feature is that in the presence of sulfur and nitrogen oxides in the gas stream, the ammonium salts that form have commercial value as fertilisers. A major drawback in the use of ammonia is its vapour pressure. Due to its small molecular weight ammonia vapour pressure is high compared to alkanolamines [9,10]. To address this it has been proposed that the absorption process take place at lower temperatures to reduce losses via volatilization (slip). While reducing the temperature of the absorption process lowers the ammonia slip, it also slows the kinetics of the absorption reaction. For the ammonia system to be comparable to amine systems it needs to absorb CO2 from a flue gas at similar rates. Currently the rates of absorption for low temperature ammonia (e.g. 283 K) are much slower than amines such as MEA at 313 K [11]. One approach to increasing this rate is to add other compounds to ‘promote’ the absorption of CO2. In this work the mass transfer of CO2 into aqueous ammonia with a series of promoters is studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. All three of these classes have been previously used to absorb, or promote the absorption of CO2 [12-14]. C13 NMR was used to identify the reaction product species. This information was then used to identify the reaction mechanisms of the CO2 absorption. 2. Method 2.1. Mass Transfer The reactive chemical absorption of CO2 into a thin film can be described as a combination of diffusion and chemical reaction processes. The CO2 diffuses from the gas phase, across the gasliquid interface, into the liquid phase where it undergoes chemical reaction. It is assumed that the film is uniform and the continuous replenishment of the film means there are no long range diffusion processes taking place in the liquid phase between the interface and the bulk liquid [15]. The mass transfer processes taking place can be described as a combination of those taking place on the gas side of the interface and on the liquid side. The concentration of CO2 in the gas phase falls
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from its bulk gas partial pressure, to its partial pressure at the gas-liquid interface, according to the gas side mass transfer coefficient kg. The dissolved CO2 concentration at the interface then falls by diffusion and chemical reaction to the bulk dissolved CO2 concentration, according to the liquid side mass transfer coefficient kl. The liquid side mass transfer coefficient is a function of mass transfer of CO2 without reaction, k°l, and enhancement by chemical reactions occurring in the liquid film that act to consume CO2. The overall mass transfer co-efficient KG is related to the inverse sum of the liquid and gas side mass transfer co-efficient, Eq. 1.
1 1 1 = + KG kg kl
Eq. 1
A wetted-wall column was used to study the absorption rate of CO2 into aqueous solutions of ammonia and either piperazine, glycine or boric acid. All compounds were purchased from Sigma Aldrich. The purities of the compounds were; ammonia (28% v/v), piperazine (99%), glycine (99%), and boric acid (99%). The solutions studied were; 3 mol/L ammonia with 0.5 mol/L piperazine, 3 mol/L ammonia with 0.5 mol/L boric acid, 3.35 mol/L ammonia with 0.5 mol/L glycine, and 0.5 mol/L piperazine. All solutions were studied at a temperature of 283K. The carbon dioxide loadings studied are outlined in Table 1. Table 1 List of solution and CO2 loadings studied. Solution 0 mol 0.6 mol 1.2 mol CO2 CO2 CO2 3 mol/L ammonia with x x x 0.5 mol/L piperazine 3 mol/L ammonia with x 0.5 mol/L boric acid 3.35 mol/L ammonia x x x with 0.5 mol/L glycine 0.5 mol/L piperazine
1.8 mol CO2
2.4 mol CO2
2.8 mol CO2
x
x
x
x x
x
x
The increased ammonia concentration used with the glycine study (pKa = 2.35 at 298 K) was to offset the free ammonia lost to the formation of ammonium glycinate in solution. Due to higher pKa values the ammonia concentration in the piperazine and boric acid studies did not need to be increased. The pKa values at 298 K for piperazine and boric acid are 9.73 and 9.24 respectively. The design and operation of the wetted wall column has been previously presented [11]. Flux rates (NCO2) were measured for each solution at bulk carbon dioxide partial pressures of 0, 4, 8, 12, 16 and 20 kPa. A plot of NCO2 versus the applied CO2 partial pressure PCO2 (log mean of the inlet and outlet CO2 partial pressure) yields a linear relationship. With the slope equal to KG and an xintercept equal to the equilibrium partial pressure (P*CO2), see Eq 2. * NCO2 = K G (PCO2 -PCO2 )
Eq. 2
Due to ammonia having a high vapour pressure ammonium bicarbonate formed in the condenser down-stream from the wetted-wall column. After measurement at each CO2 partial pressure the condenser was dried at <283 K and rinsed with distilled water. The amount of ammonia in the collected solution was then measured using an ammonium ion selective electrode (Orion). This data was then used to correct the measured CO2 concentration exiting the wetted-wall column by up to 5%. This correction was applied to all CO2 partial pressures for the following test solutions; 3 mol/L ammonia with 0.5 mol/L piperazine (loadings of 0, 26.4 and 52.8g of CO2); 3 mol/L
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198
ammonia with 0.5 mol/L boric acid (loadings of 0g of CO2); 3.35 mol/L ammonia with 0.5 mol/L glycine (loadings of 0 and 26.4g of CO2). At all other operating conditions no ammonium bicarbonate formation was detected. 2.2. NMR Aqueous solutions of 3 mol/L ammonia with 0.5 mol/L piperazine, and 0.5 mol/L piperazine were prepared in 15 mL volumetric flasks with deionised water. The solutions were decanted into reaction flasks and warmed with a thermostatted bath to 298 K. After the solutions reached the required temperature, CO2 was introduced to the reaction flask above the stirring bean at 5 mL/min. Samples of the reaction were taken in NMR tubes at 0 and 10 minutes. An external standard of 1,4dioxane was added to each tube and samples were then analysed via C13 NMR spectroscopy at 298 K. 3. Results & Discussion The overall mass transfer co-efficients (KG) for the promoted solutions are shown in Figure 1.
-1
-2
-1
KG (mmols m kPa )
2.5 2.0 1.5 1.0 0.5 0.0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
CO2 Loading (mol CO2/mol NH3 + Promoter) 3M NH3
3M NH3 + 0.5M Pz
3.35M NH3 + 0.5M Glycine
3M NH3 + 0.5M Boric Acid
0.5M Pz
Figure 1 Plot of overall mass transfer co-efficient (KG) versus CO2 loading for 3 mol/L ammonia [11] and the promoted ammonia solutions at 283 K. The overall mass transfer of CO2 into 3 mol/L ammonia with 0.5 mol/L boric acid shows no increase in absorption over 3 mol/L ammonia. Without nitrogen within the molecule boric acid is incapable of forming a carbamate. In this system boric acid is only capable of accepting protons. The lack of increase in CO2 absorption rates with boric acid present indicates the rate limiting step of absorption by ammonia is not proton accepting, it is the ammonium carbamate formation. The overall mass transfer of CO2 into 3.35 mol/L ammonia with 0.5 mol/L glycine shows an increase in absorption of CO2 over 3 mol/L ammonia. This increase in absorption rate is seen at loadings from 0-0.45. Glycine, an amino acid, is capable of both forming a carbamate and proton accepting. The increased CO2 absorption rate can be explained by glycine having a faster reaction with CO2 than ammonia, see Table 2. The faster reaction of glycine with CO2 is why we see faster CO2 absorption at loadings of less than 0.45. Having another base in high concentration (ammonia) will allow more glycine to react with CO2 than glycine on its own, as the other base provides alternative sites for proton accepting.
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Table 2 Second order rate constants for ammonia, glycine and piperazine. Ammonia Glycine Piperazine
Second Order Rate Constant at 283K (m3/mol.s) 0.9 [11] 3.0 [12] 26 [16]
The overall mass transfer of CO2 into 3 mol/L ammonia with 0.5 mol/L piperazine shows an increase in absorption of CO2 over 3 mol/L ammonia. This increase in absorption rate is seen at loadings from 0-0.8. Piperazine, a secondary diamine is capable of both forming a carbamate and proton accepting. Being a diamine piperazine can form a dicarbamate, diprotonate or a combination of the two. The increased CO2 absorption rate can be explained by piperazine having a faster reaction with CO2 than ammonia, see Table 2. The faster reaction of piperazine with CO2 is why we see faster CO2 absorption at loadings of less than 0.8. Having another base in high concentration (ammonia) will allow more piperazine to react with CO2 than piperazine on its own, as the other base provides alternate sites for proton accepting.
Figure 2 C13 NMR for and 3 mol/L ammonia with 0.5 mol/L piperazine (left) and 0.5 mol/L piperazine (right) before and after exposure to CO2. The C13 NMR data for 0.5 mol/L piperazine and 3 mol/L ammonia with 0.5 mol/L piperazine are shown in Figure 2. At T = 0 min in both sets of data we see a sharp peak at a shift of 45 ppm. This peak corresponds to the four NCH2 carbons in the piperazine ring. At T = 10 min we see this peak shift to the right, we also see a small peak appear due to the NCH2 carbons in piperazine carbamate, a reaction product. The ratio of the unreacted piperazine peak heights (T = 10 min divided by T = 0 min) for the 3 mol/L ammonia with 0.5 mol/L piperazine is 0.98. For the 0.5 mol/L piperazine with no ammonia present it is 0.79. The difference in these ratios confirms that there is more unreacted piperazine present in the blend with ammonia after 10 minutes than there is with piperazine on its own. This supports the theory that having a second base in high concentration allows more piperazine to react with CO2 as the second base (ammonia) provides alternate sites for proton accepting. 4. Conclusion The mass transfer of CO2 into aqueous ammonia with a series of promoters was studied. Three promoters were chosen to represent three different chemical classes. These classes were alkanolamine, amino acid and inorganic base. Of these classes the alkanolamine showed the greatest enhancement of CO2 mass transfer, followed by the amino acid. The inorganic base did not show any enhancement.
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There are two factors that lead to the enhancement of CO2 mass transfer. The first factor is the alkanolamine and amino acid used have faster reactions with CO2 than ammonia. The second factor is the presence of ammonia in a high concentration. This provides additional sites for proton accepting; allowing more of the added alkanolamine or amino acid free to react with CO2. The C13 NMR data presented supports this theory. 5. Acknowledgements This project is part of the CSIRO Coal Technology Portfolio and received funding from the Australian Government as part of the Asia-Pacific Partnership on Clean Development and Climate. The views expressed herein are not necessarily the views of the Commonwealth, and the Commonwealth does not accept responsibility for any information or advice contained herein. 6. References [1] IEA, Capturing CO2, D. Adams and J. Davison, Editors. 2007, IEA Greenhouse Gas R&D Program: Cheltenham. p. 23. [2] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change, B. Metz, et al., Editors. 2005, Cambridge University Press: Cambridge. [3] WRI. Climate Analysis Indicators Tool. 2007 [cited 2007; Available from: http://www.wri.org/. [4] IEA, World Energy Outlook 2007. 2008: Paris, France. [5] Davison, J., Performance and costs of power plants with capture and storage of CO2. Energy, 2007. 32(7): p. 1163-1176. [6] Yeh, A.C. and H. Bai, Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse gas emissions. The Science of the Total Environment, 1999. 228(2-3): p. 121-133. [7] Mani, F., Peruzzini, M. & Stoppioni, P., CO2 absorption by aqueous NH3 solutions: speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study. Green Chemistry, 2006. 8: p. 6. [8] Yeh, J.T., et al., Semi-batch absorption and regeneration studies for CO2 capture by aqueous ammonia. Fuel Processing Technology, 2005. 86(14-15): p. 1533-1546. [9] Edwards, T.J., et al., Vapor-Liquid Equilibria in Multicomponent Aqueous Solutions of Volatile Weak Electrolytes. AIChE Journal, 1978. 24(6): p. 966-976. [10] Goppert, U.M., G., pour-liquid equilibria in aqueous solutions of ammonia and carbon dioxide at temperatures between 333 and 393 K and pressures up to 7 MPa. Fluid Phase Equilibria, 1998. 41: p. 33. [11] Puxty, G., R. Rowland, and M. Attalla, Comparison of the rate of CO2 absorption into aqueous ammonia and monoethanolamine. Chemical Engineering Science, 2010. 65(2): p. 8. [12] Penny, D.E. and T.J. Ritter, KINETIC-STUDY OF THE REACTION BETWEEN CARBONDIOXIDE AND PRIMARY AMINES. Journal of the Chemical Society-Faraday Transactions I, 1983. 79: p. 2103-2109. [13] Bishnoi, S., Carbon Dioxide Absorption and Solution Equilibrium in Piperazine Activated Methyldiethanolamine. 2000, University of Texas, Austin: Austin, Tx. [14] Astarita, G., PROMOTION OF CO2 MASS-TRANSFER IN CARBONATE SOLUTIONS. 1981. p. 581-588. [15] Danckwerts, P.V., Gas-Liquid Reactions. 1970, New York: McGraw-Hill. [16] Bishnoi, S. and G.T. Rochelle, Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chemical Engineering Science, 2000. 55(22): p. 55315543.