Energy & Fuels 2005, 19, 1153-1159
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Adsorption of CO2 on Zeolites at Moderate Temperatures Ranjani V. Siriwardane,* Ming-Shing Shen, and Edward P. Fisher U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880
James Losch Parsons Infrastructure & Technology Group, Inc., Morgantown, West Virginia 26507-0880 Received June 28, 2004. Revised Manuscript Received March 7, 2005
Pressure swing adsorption (PSA) and temperature swing adsorption (TSA) are potential techniques for removing carbon dioxide (CO2) from high-pressure fuel gas streams. Zeolites are suitable candidate sorbents for use in these processes; however, the systems would be even more energy efficient if the sorbents were operational at moderate or high temperatures, especially for the removal of CO2 from high-pressure gas streams, such as those from integrated gasification combined-cycle (IGCC) systems. Competitive gas adsorption tests with gas mixtures representing both coal combustion and coal gasification gas streams were conducted in an atmospheric flow reactor with five zeolites at 120 °C. Promising results of preferential adsorption of CO2 were observed with two of these zeolites. However, the CO2 adsorption capacity was significantly lower at 120 °C than at ambient temperature. Volumetric gas adsorption tests of CO2 and nitrogen (N2) on these two zeolites were conducted at 120 °C, up to a pressure of 300 psi (2 × 106 Pa). Both showed high CO2 adsorption capacity at high pressure. High-pressure flow reactor studies also indicated the preferential adsorption of CO2 from gas mixtures at 120 °C. CO2 adsorption rates were measured utilizing thermogravimetric analysis, and the rates were similar for the two zeolites.
Introduction Fossil fuels supply more than 98% of the world’s energy needs. However, the combustion of fossil fuels is one of the major sources of the greenhouse gas carbon dioxide (CO2). Technologies are needed that will allow us to utilize fossil fuels while reducing greenhouse gas emissions. Existing commercial CO2 capture technology is very expensive and energy-intensive. Improved technologies for CO2 capture are necessary to achieve low energy penalties. Pressure swing adsorption (PSA) and temperature swing adsorption (TSA) are possible techniques that may remove CO2 from gas streams. The PSA process is based on preferential adsorption of the desired gas (e.g., CO2) on a porous adsorbent at high pressure, and recovery of the gas at low pressure; TSA is based on preferential adsorption of the desired gas at low temperature and desorption at high temperature. Thus, the porous sorbent can be reused for subsequent adsorption. PSA technology has gained interest, because of the low energy requirements and low capital investment cost.1,2 Applications of the PSA process to separate and capture CO2 are also being reported.3-6 The low recovery rate of CO2 is one of the problems reported with the PSA process. For the PSA * Author to whom correspondence should be addressed. Telephone: 304-285-4513. Fax: 304-285-4403. E-mail: ranjani.siriwardane@ netl.doe.gov. (1) Skarstrom, C. W. U.S. Patent No. 2,944,627, 1960. (2) Guerrin de Montgareuil, P.; Domine, D. U.S. Patent No. 3,155,468, 1964.
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process to be successful, regenerable sorbents that have high selectivity, high adsorption capacity, and high adsorption and desorption rates for CO2 capture must be developed. Zeolites have shown promising results for separating CO2 from gas mixtures and can potentially be used in the PSA process.3-10 According to an International Energy Agency (IEA) coal research report,11PSA/TSA systems would be even more energy efficient if the sorbents were operational at moderate (125 °C) or high temperatures (325 °C). In our research, zeolites 4A, 5A, 13X, APG-II, and WE-G 592 were studied for CO2 adsorption at 120 °C. Volumetric gas adsorption tests of CO2 and nitrogen (N2) on the five sorbents were conducted at 120 °C and up (3) Cheu, K.; Jong-Nam, K.; Yun-Jong, Y.; Soon-Haeng, C. Fundamentals of Adsorption: Proceedings of the Fifth International Conference on Fundamentals of Adsorption; Le Van, M. D., Ed.; Kluwer Academic Publishers: Boston, 1996; pp 203-210. (4) Dong, F.; Lou, H.; Goto, M.; Hirose, M. Sep. Purif. Technol. 1990, 15, 31-40. (5) Sircar, S.; Golden, T. C. Ind. Eng. Chem. Res. 1995, 34, 28812888. (6) Daeho, K.; Siriwardane, R. V.; Biegler, L. T. Ind. Eng. Chem. Res. 2003, 42, 339-348. (7) Siriwardane, R. V.; Shen, M.; Fisher, E. P.; Poston, J. P. Energy Fuels 2001, 15, 279-284. (8) Siriwardane, R. V.; Shen, M.; Fisher, E. P. Energy Fuels 2003, 17, 571-576. (9) Inui, T.; Okugawa, Y.; Yasuda, M. Ind. Eng. Chem. Res. 1988, 27, 1103-1109. (10) Akten, E. D.; Siriwardane, R.; Sholl, D. S. Energy Fuels 2003, 17, 977-983. (11) Smith, I. M. CO2 ReductionsProspects for Coal, IEA Coal Research Report, 1999, pp 29-32.
This article not subject to U.S. Copyright. Published 2005 by the American Chemical Society Published on Web 04/20/2005
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to a pressure of 300 psi (∼2 × 106 Pa), to determine the equilibrium adsorption capacity of these materials. Tests were conducted on the competitive adsorption of CO2 from gas mixtures, utilizing both atmospheric and high-pressure microreactors. Adsorption rates were determined using thermogravimetric analysis (TGA). The heat of CO2 adsorption and other physical and chemical properties of the zeolites were also measured. Experimental Section Zeolites 13X (Z10-02) and 4A (Z4-01) were obtained from Zeochem, Inc., whereas zeolites WE-G 592 and APG-II were obtained from UOP and zeolite 5A was obtained from Aldrich Chemical Co. A volumetric adsorption apparatus7,8 was used to obtain adsorption isotherms at 120 °C in pure CO2 (99.5%, Jackson Welding Supply Co.), N2 (99.5%, Jackson Welding Supply Co.), and oxygen (O2, 99.6%, Jackson Welding Supply Co.) on the five sorbents, up to an equilibrium pressure of ∼300 psi (∼2 × 106 Pa). Approximately 10 mL of the sorbent materials was placed in the sample chamber, and then the chamber was evacuated to ∼5 × 10-5 Torr. The amount of CO2 adsorbed was calculated using the pressure measurements before and after exposure of the sample chamber to CO2. Baseline data with CO2 were obtained using 10 mL of glass beads (2 mm in diameter). Surface areas and information about micropores were determined using a Micromeritics model ASAP 2010 micropore-volume analyzer. The samples were evacuated initially at 90 °C for 1 h and then at 350 °C for 24 h. N2 adsorption was measured at the liquid N2 temperature (-196 °C). Micropore analyses of the data were conducted using the Horvath-Kawazoe (HK) method; cylindrical pores were assumed in the calculations.8 Density functional theory (DFT) was used to calculate the total pore volume and surface area. Competitive gas adsorption studies and temperatureprogrammed desorption (TPD) studies were conducted in a laboratory-scale fixed-bed reactor (Micromeritics model Autochem 2910 atmospheric flow reactor) at 14.7 psi (∼1.01 × 105 Pa), using gas mixtures with the following compositions: (a) 15 vol % CO2, 82 vol % N2, and 3 vol % O2 (certified gas mixture by Butler Gas Products Co.), in the presence of water vapor at ambient temperature, to represent a coal-combustion gas stream; and (b) 12 vol % CO2, 35.9 vol % CO, 27.1 vol % H2, and 25 vol % helium (a certified gas mixture, supplied by the Messer Gas Technology and Service Group) saturated with water to represent a coal-gasification gas stream. The premixed gas mixtures were passed through a water bubbler, to introduce moisture into the gas stream before introduction of the gas mixture to the reactor. The outlet gas stream from the reactor was analyzed using a mass spectrometer (Pfeiffer Vacuum Thermostar). The samples (with a volume of 1.7 × 10-3 L) were pretreated at 120 °C for 1 h in the flow reactor, under helium, prior to exposure to the gas mixtures. A switching valve was used to direct the gas stream to the mass spectrometer through either the sample or sample bypass loop. During the pretreatment of the zeolites with helium, the gas mix that contained CO2 was introduced to the mass spectrometer through the sample bypass loop. It was possible to establish the initial composition of the gas mix during this time. After the helium pretreatment, the gas mix was directed to the zeolite sample, using the switching valve. There was an abrupt decrease in the CO2 concentration when the gas mix was directed to the sample, from either 12 or 15 vol % to
