Steven Chiao-Chien Wei*, Graeme Puxty and Paul Feron

Transcription

1 Available online at Energy Procedia 37 (2013 ) GHGT-11 Amino acid salts for CO 2 capture at flue gas temperatures. Steven Chiao-Chien Wei*, Graeme Puxty and Paul Feron CSIRO Energy Technology, 10 Murry Dwyer Circuit, Steel River Estate, Mayfield West, NSW 2300, Australia Abstract An amino acid salt, potassium taurate has been chosen as a high temperature absorbent in this study due to its low volatility and high absorption rate. The densities and viscosities of 2M-6M taurate solution have been determined over the temperature range from 293K to 353K. The CO 2 solubility of taurate solutions has been measured using a stirred-cell reactor. It has been found that the CO 2 solubility of taurate solutions is comparable to that of alkanolamines at high temperature. The absorption rate of CO 2 into CO 2 free and CO 2 loaded taurate solutions were obtained using a wetted-wall column. The K G of 4M taurate at 353K is similar in magnitude to the K G of 5M MEA at 313K. It has also been found that the K G of taurate decreased with increased CO 2 loading, but the values for K G of taurate solutions are still comparable to CO 2 loaded 5M MEA solution The Authors. Published by by Elsevier Elsevier Ltd. Ltd. Selection and/or peer-review under responsibility of GHGT Selection and/or peer-review under responsibility of GHGT Keywords:Absorbent; Taurine; Wetted-Wall Column; VLE; CO 2 solubiity;overall mass transfer coefficient. 1. Introduction A general consensus from the work of many climate scientists has emerged indicating that global warming and climate change are the result of the anthropogenic emissions of greenhouse gases. It is believed that carbon dioxide is the most important anthropogenic greenhouse gas. The major source of anthropogenic carbon dioxide is the combustion of fossil fuels (coal, natural gas and oil) which currently supply over 85% of world energy use. Of this energy, approximately 40% is produced in power plants. [1, 2] The technology of post combustion capture (PCC) is well recognized by government and industry as a way to effectively absorb 80-90% of CO 2 emissions from fossil fuel-fired power plants. [3] The captured CO 2 can be stored in depleted oil and gas fields, deep saline aquifers and unmineable coal seams to reduce the CO 2 emissions to the atmosphere from power plants. Most commercially available PCC processes use liquid absorbents such as aqueous ammonia or alkanolamine solutions which typical absorb CO 2 at temperatures between 283K and 313K. [4] The temperatures of flue gases in fossil fuel fired power plants range from 393K to 433K and are obviously * Corresponding author. Tel.: ; fax: address: steven.wei@csiro.com The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi: /j.egypro

2 486 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) higher than the temperature of absorption in PCC processes. Therefore, additional cooling systems and equipment are required to cool flue gases down to lower temperatures for the PCC processes. This cooling load adds to the cost, water consumption and energy consumption of PCC processes. Peeters et al. have evaluated the capital cost and energy penalty for the additional cooling system and pumps in a natural gas-fired power station. The contributions to capital cost and energy penalty are 3% and 10%, respectively. [5] Fisher et al. have also found the cost of the additional cooling system and pumps in a coal-fired power station is about 4% of total equipment cost. The energy penalty to maintain the cooling system is about 10% of total energy consumption. [6] Clearly, absorbents which can absorb CO 2 above 313K and as close to flue gas temperature as possible, constitute a technological breakthrough with a high potential for a more economic and energy efficient capture process. In addition, the mass transfer between flue gases and absorbents could be faster at high temperature due to the lower viscosity and faster reaction kinetics under high temperature conditions. Most commercial absorbents are amine-based solvents. The solvents are not suitable to absorb CO 2 at high temperatures (>333K) due to absorbent volatility and thermal degradation. [7] Recently, researchers have found that functionalized ionic liquids (ILs) can maintain their CO 2 solubility at high temperature. Wang et al. [8] have found the mixture of IL and superbases can absorb CO 2 at temperature up to 353K. A new type of polyamine based IL can absorb CO 2 at temperatures between 383 and 403K. [9] However, to reach the maximum CO 2 capacity in the ionic liquids, it requires at least 30 minutes of contact time between gas and liquid. The absorption rate of the ionic liquids is much slower than amine solutions. In addition, the cost of ILs is currently much greater than amine solutions due to the complex synthesis procedure. These issues mean that in the near term ILs might not be practically useful as an absorbent. Amino acid salts have been widely used as absorbents in the field of CO 2 capture due to their low volatility and resistance to oxygen degradation in the absorption process. [10-15] Van Holst et al. have investigated the apparent rate constants for several amino acid salts at 298K to find suitable absorbents for CO 2 capture. They found the amino acid solutions such as glycinate, prolinate, sarcosinate and taurate exhibit relatively high reaction rate constants as compared to MEA solution. [12] Kumar et al. have measured the solubility of CO 2 in taurate solution at 298K and 313K for a range of CO 2 partial pressure from 0.1 to 6kPa. The kinetics of CO 2 absorption in taurate solution has also been investigated over a range of temperatures between 285K and 305K by using a stirred cell reactor. [11,15] However, their studies have only investigated the properties of amino acid salts at temperatures ranging from 283K to 333K. So far, there are very few studies using amino acid salts to absorb CO 2 at high temperatures. It has been noticed that higher energy consumption can be required for CO 2 capture at high temperature. Although CO 2 absorption at high temperatures could result in the increase of reboiler heat duty, the capital cost of flue gases cooling systems and the saving from in terms of space for the system also need to be considered. This study focused on the performance of CO 2 capture at temperatures between 323K and 373K by using potassium taurate solutions (2M ~ 6M). Taurate solutions were chosen due to their relatively low cost, low volatility and thermal stability at high temperature. The physical properties of taurate solutions such as density and viscosity have also been measured in the study. 2. Methodology 2.1 Density and Viscosity measurements Taurine ( 90%) and potassium hydroxide ( 90%) were purchased from Sigma-Aldrich and were used directly without further purification. Densities of unloaded and loaded taurate solutions were measured using a density meter (DMA 38, Anton Paar) at temperatures between 298K and 313K with an error of ±0.001 g cm -3. A traditional method measuring the mass and the volume of the solution was used to determine the densities of the taurate solutions at temperature above 313K due to the limit of the density

3 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) meter. The traditional method was carried out by filling the taurate solutions in a volumetric flask (Pyrex) and weighting the mass of the solution using a balance (GR 300, A&D Weighing). The volume of the volumetric flask had been calibrated at temperatures from 313K to 353K using deionized water. The expanded volume at different temperatures was calculated by using the mass of water in the flask divided by the density of water obtained from Perry s handbook [16] Dynamic viscosities of the taurate solutions were measured using a viscometer (AMVn, Anton Paar) at temperatures between 298K and 353K with a specified repeatability <0.1%. The integrated temperature of the measurements is specified with a resolution of ±0.01K and the accuracy is ± 0.05K. 2.2 Wetted-Wall Column The wetted-wall column consists of a stainless steel column with effective length 8.21cm and diameter 1.27cm. Liquid stored in a reservoir was pumped up the inside of the column, through boreholes at the top, and then flowed down the outer surface of the column. This created a thin liquid film flowing down the column surface. Once the liquid flow reached the base of the column, it was returned to the reservoir to form a closed loop. The column was located within a glass jacket cover with an internal diameter of 2.2cm and an external diameter of 4.8cm. Water was circulated between the glass jacket and a water bath to maintain a constant temperature for the entire system. The gas stream was a mixture of CO 2 and N 2. The proportion of each gas was controlled by Bronkhorst mass flow controllers to mimic typical CO 2 concentrations found in different sections of an absorption column. The mixture was first passed though a stainless steel coil immersed in a water bath and then introduced to a saturator, also immersed in the water bath, which contained a fritted bubbler under 23cm of water. The gas flow then entered at the base of the internal chamber of the glass jacket and moved upwards, contacting the liquid film on the column surface in a counter-current fashion, and was exhausted from the top of the glass jacket. The gas and liquid flow rates were maintained at 3L/min and ml/min, respectively. The amount of CO 2 absorbed from the gas phase into the liquid phase was determined by measuring the CO 2 content of the gas entering and exiting the column. This information combined with surface area of contact between the liquid film and the gas allowed to determination of the CO 2 absorption flux, (mmol s -1 m -2 ). The overall mass transfer coefficient K G (mmol s -1 m -2 kpa -1 ) was then determined via the equation below, where (kpa) is the logarithmic mean of the CO 2 inlet and outlet partial pressure and (kpa) is the equilibrium CO 2 partial pressure. A plot of versus yields a straight line with a slope equal to K G. A more detail description of the experimental setup and procedure has been reported by Puxty et al. [17] The K G of 2M, 4M and 6M taurate solutions were determined. For each solution the CO 2 loading (mole CO 2 / mole taurate) was varied over the values 0, 0.1, 0.2 and 0.3. The equilibrium CO 2 partial pressure of the CO 2 loaded solutions was determined using the stirred cell reactor as described below. 2.3 Stirred cell reactor A stirred cell reactor was used to investigate the solubility of CO 2 in taurate solution. The reactor (Parr model 5104) is a closed system containing both gas and liquid phase. The temperature of the reactor was maintained by water circulated between the glass jacket surrounding the reactor content and a water bath. Once loaded with liquid the reactor was purged with nitrogen at atmospheric pressure before starting the (1)

4 488 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) experiment. CO 2 was injected from a CO 2 reservoir suspended from a balance into the reactor. Following injection the pressure in the reactor increased and then decreased as CO 2 was absorbed into the liquid phase. The variation of pressure was monitored until the pressure reached an equilibrium value. The liquid phase was stirred by a Teflon coated stainless steel stirrer. The amount of CO 2 in the liquid phase was determined by the equation below, where the is the total moles of CO 2 injected into the reactor, is the moles of CO 2 in the gas phase and is the moles of CO 2 in the liquid phase. was determined by measuring the mass loss of the CO 2 reservoir. was determined by the compressibility factor (Z) below [20], the CO 2 partial pressure (determined from the difference between the initial and final pressure), and the gas volume as per the equation below, Z=PV m /RT (3) where the P is equilibrium pressure, R is the gas constant and V m is the molar volume in the gas phase. V m can be obtained by subtracting the liquid volume from the total reactor volume. It should be noted that the total reactor volume includes the reactor, inserts, valves and tubing. To determine the total reactor volume, a calibration was carried out where five known amounts of CO 2 were injected into the empty reactor. Using the measured pressures and temperatures, and known compressibility factor taken from Perry s Handbook, the volume was determined by using equation 3. A more detail description of the stirred cell setup and experimental process has been reported by Puxty et al. [18] 3. Results and discussion 3.1 Density and viscosity Figure 1 shows the densities of 30% MEA solution and taurate solutions with concentrations from 2M to 6M at temperatures between 353K and 323K. It can be seen that the densities of the taurate solutions is in the range between 1.1 and 1.4 g cm -3 while the densities of 30% MEA is in the range from 0.98 to 1.01 g cm -3. The densities of the taurate solutions increased with increasing taurate concentrations. The densities of the taurate solutions decreased as temperature increased due to the volume expansion of the solution. (2)

5 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) M Taurate 4M Taurate 2M Taurate 30% MEA Density (g cm -3 ) Temperature (K) Figure 1. Densities of taurate solutions and 30% MEA at temperatures between 298K and 353K. Figure 2. Viscosities of taurate solutions and 30% MEA at temperatures between 291K and 353K. The viscosities of taurate solutions have also been determined in this study. An absorbent with lower viscosity can result in less interfacial mass-transfer resistance between gas and liquid phases. It means the diffusion coefficient of CO 2 in the liquid phase can be increased, thereby increasing the CO 2 absorption rate. Figure 2 shows plots of viscosities ( ) of 30% MEA and taurate solutions on a log scale against 1/T. The effect of temperature on viscosities of MEA and taurate solutions can be described by the Arrhenius relationship (Eq. 4), = Aexp(B/RT) (4) where is the viscosity (mpa s), A (mpa s) and B (kj/mole) are constants for the given liquid and R is the gas constant ( J/mol K). The viscosities of 2M taurate solutions were lower than 30% MEA and the viscosities of 4M taurate solutions were similar to 30% MEA at temperatures between 293 and 353K. This indicates that the diffusion coefficient of CO 2 in the taurate solutions will be larger or similar to 30% MEA solution. Thus the liquid side mass-transfer resistance in taurate solutions at higher temperatures could be smaller than in 30% MEA solution at 313K. [19] It has also been found that the viscosities of taurate solutions remain constant after loading with CO 2 whereas the viscosity of CO 2 loaded 30% MEA solution was increased. It is believed that the increase of viscosity in the CO 2 loaded 30% MEA solution resulted from the increasing ionic concentration and the increasing strength of electrostatic interaction. In amino acid salt solutions, the interaction between CO 2 and amino acid was dominated by hydrogen bonds. [20] Due to the complex crystal structures in amino acid salts, the hydrogen bonds were short and thus lead a stable viscosity in the solutions. The characteristic of stable viscosity can be considered as an advantage of taurate solutions for CO 2 capture as the diffusion coefficient of CO 2 in the solutions is maintained constantly in the entire process. 3.2 CO 2 solubility CO 2 solubility in taurate solutions was investigated by using stirred cell reactor. Figure 3 shows the CO 2 solubility in taurate solutions at 333K. At a given CO 2 /taurate ratio, the CO 2 partial pressure increased with deceasing concentration. This is because less free taurate molecules were available to react with CO 2

6 490 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) at lower concentration of taurate solution and thus resulted in a higher CO 2 partial pressure in the gas phase i.e. lower CO 2 solubility. Figure 4 shows the CO 2 solubility in 4M and 6M taurate solutions at temperatures between 333K and 373K. The CO 2 solubility in the taurate solutions decreased with increasing temperature. A similar trend can be also found in another amino acid salt, glycinate solution. Portugal et al. have reported that the CO 2 solubility in 1M potassium glycinate significantly reduced when the absorption temperature increased from 293K to 323K. [21] However, the CO 2 solubility increased with increasing taurate concentration, indicating a high concentration of taurate could be used for high temperature absorption Pco 2, kpa 100 2M Taurate, 333K 4M Taurate, 333K 6M Taurate, 333K Pco 2, kpa 100 4M Taurate, 333K 4M Taurate, 353K 4M Taurate, 373K 6M Taurate, 333K 6M Taurate, 353K 6M Taurate, 373K CO 2 / Taurate, mol/mol Figure 3. The solubility of CO 2 in taurate solutions at 333K CO 2 / Taurate, mol/mol Figure 4. The solubility of CO 2 in 2M and 4M taurate solutions at 333K, 353K and 373K. The correlation between CO 2 solubility and absorbent concentration in taurate solution is different from in MEA solution. Lee et al. have found that the CO 2 solubility reduced with increasing MEA concentration. [22] As shown in Figure 5, the CO 2 solubility in terms of CO 2 /amine in MEA solutions decreased with increasing of MEA concentration at 373K. The decreases of the CO 2 solubility in MEA solution at high temperature can be resulted from the desorption of MEA solution. The results in this study show the superior CO 2 solubility in taurate solution at high temperature as comparing to MEA and glycinate solutions.

7 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) Pco 2, kpa M Taurate, 373K 4M Taurate, 373K 6M Taurate, 373K 1M MEA, 373K 5M MEA, 373K K G, mmol s -1 m -2 kpa M, 353K 6M, 343K 6M, 333K 6M, 323K 4M, 353K 4M, 343K 4M, 333K 4M, 323K 2M, 353K 2M, 343K 2M, 333K 2M, 323K 7m MEA, 333K 7m MEA, 313K CO 2 / amine, mol/mol Figure 5. The solubility of CO 2 in taurate and in MEA solutions at 373K Pco 2, kpa Figure 6. Overall mass transfer coefficient (KG) in taurate solutions at temperatures range from 323K to 353K and in 5M MEA solution at 313K and 333K. 3.3 Overall mass transfer coefficient (K G ) Figure 6 shows the overall mass transfer coefficient (K G ) of taurate solution (2-6M) at temperatures from 323K to 353K. K G is plotted against the equilibrium CO 2 partial pressure as determined from the CO 2 solubility data. As a comparison, the K G in 7m MEA (~5M MEA) solution at 313 and 333K determined by Dugas et al. is also shown in Figure 6. [23] Effect of taurate concentration on overall mass transfer coefficient K G Figure 6 shows the overall mass transfer coefficient (K G ) in taurate solutions has been affected by the concentration of taurate. The value of K G increases with increasing taurate concentration as more unreacted taurate ions are available to react with CO 2. Also, as would be expected, it was found that the K G values decreased with increasing CO 2 loading. As it was determined that the viscosity of taurate solutions was unaffected by CO 2 content, the decrease in K G must be primarily due to the reduced availability of unreacted taurate. The decrease of K G was more significant in 6M taurate solution than in the 2M and 4M solutions at the same temperature. In high concentration taurate solution, more protonated taurate can be produced and the unstable protonated form can be precipitated in the solution. Although the precipitation was not visually observed during the experiments, the K G in the high concentration taurate solution can be affected by the unstable protonated form Effect of absorption temperature on overall mass transfer coefficient K G The K G values in 2M and 4M taurate solutions were measured at absorption temperatures from 323K to 353K. As protonated taurate can be easily precipitated in high concentration taurate solution at low temperature, the K G values in 6M taurate solution was thus measured from 333K to 353K. It can be seen that the K G values in the taurate solutions increased with increasing absorption temperature. It has been found that the K G values in the CO 2 free 2-6M taurate solution at temperatures above 323K are similar in magnitude to the K G of 5M MEA at 313K which is about 2.8 mmol s -1 m -2 kpa -1. Although the K G values decreased with increased CO 2 loading, the K G values of taurate solutions at temperatures above 323K are

8 492 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) still comparable to CO 2 loaded 5M MEA solution at 313K. The taurate solutions show superior CO 2 absorption kinetics at higher temperatures. 4. Conclusion The physical thermodynamic and kinetic properties of taurate solutions have been investigated for their application in the field of PCC. It has been observed that the density of taurate solutions is higher than 30% MEA solution. The viscosities of 4M taurate solutions are similar to 30% MEA solution over the temperature range 293K and 353K. It has been found that the CO 2 solubility of taurate solutions can be increased by increasing the concentration of taurate. The CO 2 solubility of taurate solutions is comparable to that of alkanolamines at high temperature. It has been found that K G increased with an increase of taurate concentration and absorption temperature. The K G values in CO 2 free taurate solutions at temperatures above 323 K are similar in magnitude to the K G of 5M MEA at 313K. The K G values in CO 2 loaded taurate solutions at temperatures from 323K to 353K are higher than the K G values in CO 2 loaded 5M MEA at 313 and 323K. In summary, the study has shown the superior CO 2 solubility and the kinetics of CO 2 absorption in taurate solutions at high temperatures up to 353K. More studies will be focused on its application in a pilot plant scale. 5. References [1] Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO 2 capture technology--the U.S. Department of Energy's Carbon Sequestration Program. Int J Greenh Gas Con 2008;2:9-20. [2] Energy Information Administration Intnational Energy Outlook 2011;DOE/EIA-0484; [3] MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C. S.; Williams, C. K.; Shah, N.; Fennell, P. An overview of CO 2 capture technologies. Energ Environ Sci 2010; 3: [4] Bandyopadhyay, A. Amine versus ammonia absorption of CO 2 as a measure of reducing GHG emission: a critical analysis. Clean Technol Envir Policy 2011;13: [5] Peeters, A. N. M.; Faaij, A. P. C.; Turkenburg, W. C. Techno-economic analysis of natural gas combined cycles with postcombustion CO 2 absorption, including a detailed evaluation of the development potential. Int J Greenh Gas Con 2007;1: [6] Kevin S.Fisher; Carrie Beitler; Curtis Rueter; Katherine Searcy; Gary Rochelle; Majeed Jassim Integrating MEA Regeneration with CO 2 Compression and Peaking to Reduce CO 2 Capture Costs; US Department of Energy: National Energy Technology Labatory, Pittsburg, Pennsylvania, [7] Lepaumier, H.; Martin, S.; Picq, D.; Delfort, B.; Carrette, P. L. New Amines for CO 2 Capture. III. Effect of Alkyl Chain Length between Amine Functions on Polyamines Degradation. Ind Eng Chem Res 2010;49: [8] Wang, C.; Luo, H.; Luo, X.; Li, H.; Dai, S. Equimolar CO 2 capture by imidazolium-based ionic liquids and superbase systems. Green Chem 2010;12: [9] Ren, S.; Hou, Y.; Wu, W.; Tian, S.; Liu, W. CO 2 capture from flue gas at high temperatures by new ionic liquids with high capacity. RSC Adv 2012;2: [10] Kumar, P. S.; Hogendoorn, J. A.; Feron, P. H. M.; Versteeg, G. F. Equilibrium solubility of CO 2 in aqueous potassium taurate solutions: Part 1. Crystallization in carbon dioxide loaded aqueous salt solutions of amino acids. Ind Eng Chem Res 2003;42: [11] Kumar, P. S.; Hogendoorn, J. A.; Timmer, S. J.; Feron, P. H. M.; Versteeg, G. F. Equilibrium solubility of CO 2 in aqueous potassium taurate solutions: Part 2. Experimental VLE data and model. Ind Eng Chem Res 2003;42: [12] van Holst, J.; Versteeg, G. F.; Brilman, D. W. F.; Hogendoorn, J. A. Kinetic study of CO 2 with various amino acid salts in aqueous solution. Chem Eng Sci 2009,64:59-68.

9 Steven Chiao-Chien Wei et al. / Energy Procedia 37 ( 2013 ) [13] Vaidya, P. D.; Konduru, P.; Vaidyanathan, M.; Kenig, E. Y. Kinetics of Carbon Dioxide Removal by Aqueous Alkaline Amino Acid Salts. Ind Eng Chem Res 2010;49: [14] Simons, K.; Brilman, W.; Mengers, H.; Nijmeijer, K.; Wessling, M. Kinetics of CO 2 Absorption in Aqueous Sarcosine Salt Solutions: Influence of Concentration, Temperature, and CO 2 Loading. Ind Eng Chem Res 2010;49: [15] Kumar, P. S.; Hogendoorn, J. A.; Versteeg, G. F.; Feron, P. H. M. Kinetics of the reaction of CO 2 with aqueous potassium salt of taurine and glycine. Aiche J 2003;49: [16] Perry's Chemical Engineers' Handbook; 7th ed. McGraw-Hill; [17] Puxty, G.; Rowland, R.; Attalla, M. Comparison of the rate of CO 2 absorption into aqueous ammonia and monoethanolamine. Chem Eng Sci 2010;65: [18] Puxty, G.; Allport, A.; Attalla, M. Vapour liquid equilibria data for a range of new carbon dioxide absorbents. Energy Procedia 2009;1: [19] Wu, H.; Shah, J. K.; Tenney, C. M.; Rosch, T. W.; Maginn, E. J. Structure and dynamics of neat and CO 2-reacted ionic liquid tetrabutylphosphonium 2-cyanopyrrolide. Ind Eng Chem Res 2011;50: [20] Jeffrey, G. A.; Maluszynska, H. A survey of hydrogen bond geometries in the crystal structures of amino acids. Int J Biol Macromol 1982;4: [21] Portugal, A. F.; Sousa, J. M.; Magalhaes, F. D.; Mendes, A. Solubility of carbon dioxide in aqueous solutions of amino acid salts. Chem Eng Sci 2009;64: [22] Lee, J. I.; Otto, F. D.; Mather, A. E. Equilibrium Between Carbon Dioxide and Aqueous Monoethanolamine Solutions. J Appl Chem Biotechn 1976;26: [23] Dugas, R. E.; Rochelle, G. T. CO 2 absorption rate into concentrated aqueous monoethanolamine and piperazine. Journal of Chemical and Engineering Data 2011;56: