SEQUESTRATION OF CO 2 BY CHEMICALLY REACTIVE AQUEOUS K 2 CO 3 IN HIGH EFFICIENCY ADSORBENTS USING MICROFIBROUS MEDIA ENTRAPPED SUPPORT PARTICULATES Except where reference is made to the work of others, the work described in this thesis is my own or was done in collaboration with my advisory committee. This thesis does not include proprietary or classified information. ________________________________ Noppadon Sathitsuksanoh Certificate of Approval: _____________________ _____________________ Robert P. Chambers Bruce J. Tatarchuk, Chair Professor Professor Chemical Engineering Chemical Engineering _____________________ _____________________ Gopal A. Krishnagopalan Joe F. Pittman Professor Interim Dean Chemical Engineering Graduate School SEQUESTRATION OF CO 2 BY CHEMICALLY REACTIVE AQUEOUS K 2 CO 3 IN HIGH EFFICIENCY ADSORBENTS USING MICROFIBROUS MEDIA ENTRAPPED SUPPORT PARTICULATES Noppadon Sathitsuksanoh A Thesis Submitted to the Graduate Faculty of Auburn University in Partial Fulfillment of the Requirements for the Degree of Master of Science Auburn, Alabama May 10, 2007 iii SEQUESTRATION OF CO 2 BY CHEMICALLY REACTIVE AQUEOUS K 2 CO 3 IN HIGH EFFICIENCY ADSORBENTS USING MICROFIBROUS MEDIA ENTRAPPED SUPPORT PARTICULATES Noppadon Sathitsuksanoh Permission is granted to Auburn University to make copies of this thesis at its discretion, upon the request of individuals or institutions and at their expense. The author reserves all publication rights. _____________________ Signature of Author _____________________ Date of Graduation iv VITA Noppadon Sathitsuksanoh, son of Thawat Sathitsuksanoh and Lawan Sae Koo, was born on January 28, 1980 in Bangkok, Thailand. He attended Chinorot High School and graduated in 1995. He enrolled in the Chemical Engineering program at Thammasat University in Bangkok, Thailand where he graduated with a Bachelors of Science in Chemical Engineering in May 1999 and then enrolled in Graduate School at Southern Illinois University shortly thereafter to pursue a Masters of Science in Mechanical Engineering. He graduated from Southern Illinois University with his Masters in May 2004. In August 2003, he enrolled in Graduate School at Auburn University. v THESIS ABSTRACT SEQUESTRATION OF CO2 BY CHEMICALLY REACTIVE AQUEOUS K2CO3 IN HIGH EFFICIENCY ADSORBENTS USING MICROFIBROUS MEDIA ENTRAPPED SUPPORT PARTICULATES Noppadon Sathitsuksanoh Master of Science, May 10, 2007 (B. S. ChE, Thammasat University, 1999) 123 Typed Pages Directed by Bruce Tatarchuk This work is mainly focused on developing a new adsorptive material and regenerable system for CO2 removal to supply CO2-free gas stream for low temperature and low CO2 concentration applications, such as Alkaline Fuel Cells, Metal-Air batteries, and portable air-purifying respirators. A novel microfibrous media has been introduced for CO2 filtration from wet gas streams at room temperature. The microfibrous media was prepared by uniformly dispersing activated carbon particulates in the nickel fiber matrix via wet layer paper-making/sintering processes. The use of microfibrous media in a composite bed maximizes the breakthrough capacity per unit volume and promotes high accessibility. The microfibrous media synergically combines the high contacting efficiency of the microfibrous matrix and the small internal mass transfer resistance of the small particulates. The capacity of the microfibrous media can be reversibly vi recovered. The incorporation of microfibrous media to the Sodalime was observed. The result shows 120% improvement in the breakthrough capacity compared with the packed bed of the Sodalime with the same volume. This approach can be applied to miniaturize the reactor size, reduce thermal mass, and enhance the process intensification. vii ACKNOWLEDGEMENTS The author would like to thank the Sathitsuksanoh family; my mother, Lawan who has always been there for me; my sister, Varisa who encouraged me not to give in to my obstacles; my father, Thawat who always believes in me. I would like to also give the deepest thanks to my advisor, Dr. Bruce Tatarchuk who provided support and guidance throughout the learning processes. He allowed me to have freedom to learn more than course works could ever teach. In addition, I would like to thank the following: ? Drs. Don Cahela and Yong Lu for their interesting discussions and advices throughout my course of study ? Hongyun Yang for his helps and suggestions in completing this research ? Drs. Robert Chamber and Gopal Krishnagopalan for taking the time to be on my thesis committee. viii Style Manual Used: Chemical Engineering Progress Style Guide, available at www.cepmagazine.org/editorial/refstyle.htm Computer Software Used: Microsoft Word, Microsoft Excel, Adobe Photoshop, Microsoft Visio, Win-Transfer ix TABLE OF CONTENTS LIST OF TABLES........................................................................................................... xii LIST OF FIGURES .........................................................................................................xiii I. INTRODUCTION .............................................................................................. 1 II. LITERATURE REVIEWS .................................................................................. 3 Sources of Carbon Dioxide Current Technologies for Carbon Dioxide Removal Chemical absorption Physical absorption Adsorption Membrane processes Banefield Process Selected Commercially Available Sorbents/Solvents Heat of Reactions from Selected Sorbents/Solvents Microfibrous Media (MM) III. EXPERIMENTAL............................................................................................... 9 Support Particulates and Their Physical Properties SiO 2 , Al 2 O 3 , Activated Carbon Particulates (ACP) Sorbent Preparation by Impregnation Adsorption System Breakthrough tests Microfibers and Their Physical Properties Polymer fibers Nickel fibers Glass fibers Adsorbent Characterization X-ray diffraction (XRD) Scanning electron microscopy (SEM) Differential Scanning Calorimetry (DSC) x IV. PRELIMINARY RESULTS................................................................................ 45 Commercial Adsorbents Molecular sieves ? 3A, 4A, 5A, 13X Hydroxide of alkali metals ? Sofnolime, Carbolime, Baralime Summary V. INFLUENCE OF POROUS HOST MATRIX .................................................... 54 Breakthrough Test of SiO 2 -based sorbents Chemical Reaction between K 2 CO 3 and SiO 2 Breakthrough Test of Al 2 O 3 -based sorbents Chemical Reaction between K 2 CO 3 and Al 2 O 3 Breakthrough Test of ACP-based sorbents Thermal Stability of Sorbents at Elevated Temperatures Summary VI. BREAKTHROUGH PERFORMANCE OF K 2 CO 3 /ACP SORBENTS AND THEIR REGENERABILITY .............................................................................. 67 Adsorption Capacity and Utilization of ACP-based sorbents Effects of Water on Adsorption Free water Crystallization water Additional water from gas stream Determination of Regeneration Condition ?G and DSC of KHCO 3 DSC of K 2 CO 3 Impregnated ACP Phase Transformation Fresh sorbents Spent sorbents Regenerated sorbents VII. MICROFIBROUS MATERIALS AND THEIR APPLICATIONS.................... 82 Microfibrous Media Adsorption Interparticle diffusion Intraparticle diffusion Composite Bed Design Multicyclic Breakthrough Performance xi SEM Micrographs of MM after 8 Regeneration Cycles Sodalime? Incorporation of Sodalime? in a Composite Bed Summary VIII. STUDIES ON THE FORMATION OF THE SOLIS SOLUTION COMPOUND IN K 2 CO 3 IMPREGNATED AL 2 O 3 SORBENTS .............................................. 94 Reaction between K 2 CO 3 and Al 2 O 3 at Different Temperatures IX. CONCLUSIONS..................................................................................................101 REFERENCES ................................................................................................................103 xii LIST OF TABLES Table 1 : Current CO 2 removal technologies............................................................ 7 Table 2 : Comparison of Fuel Cell Technologies..................................................... 9 Table 3 : Selected currently available CO 2 -sorbents Liquid solvents .......................................................................................... 13 Solid sorbents ............................................................................................ 14 Table 4 : Chemical composition of Baralime and Sodalime .................................... 19 Table 5 : Selected molecules and their critical diameters ........................................ 21 Table 6 : Properties of Selected Molecular sieves.................................................... 22 Table 7 : Adiabatic heat of selected CO 2 sorbents ................................................... 24 Table 8 : Physical properties of porous host matrix ................................................. 34 Table 9 : Physical properties of microfibers............................................................. 36 Table 10 : Composition of 8? Preform ....................................................................... 37 Table 11 : Characteristics of Selected Adsorbents ..................................................... 45 Table 12 : Breakthrough Data of Selected Adsorbents .............................................. 47 Table 13 : Solubility of selected alkali metals............................................................ 50 Table 14 : Solubility and liquid density of K 2 CO 3 and KHCO 3 at 25 o C.................... 74 Table 15 : Adsorption characteristics of composite sorbents ..................................... 87 Table 16 : Characteristics of Sofnolime in a composite bed design .......................... 92 xiii LIST OF FIGURES Figure 1 : Atmospheric CO 2 concentrations in parts per million by volume, ppm, at Mauna Loa, Hawaii ................................................................................... 4 Figure 2 : CO 2 concentration levels in selected applications..................................... 5 Figure 3 : Basic configuration of different types of fuel cells ................................... 11 Figure 4 : Cross-sectional view of the basic zinc-air batteries .................................. 12 Figure 5 : Adsorption enhanced catalytic processes .................................................. 17 Figure 6 : Concentration Profiles in a Composite Bed .............................................. 30 Figure 7 : Equal Area Design Rule............................................................................ 31 Figure 8 : SEM micrographs of Microfibrous Media ................................................ 39 Figure 9 : Experimental apparatus ............................................................................. 40 Figure 10 : Non-dispersive Infrared CO 2 Analyzer ..................................................... 41 Figure 11 : XRD patterns of K 2 CO 3 *1.5H 2 O, K 2 CO 3 , and KHCO 3 ............................ 42 Figure 12 : Composite Bed Design.............................................................................. 43 Figure 13 : Breakthrough curves of selected adsorbents ............................................. 46 Figure 14 : Breakthrough curves of Molecular Sieves 5A in wet and dry gas ............ 48 Figure 15 : Breakthrough curves of Sofnolime in wet and dry gas ............................. 52 Figure 16 : Breakthrough curves of K 2 CO 3 /ACP at different K 2 CO 3 loadings??? 55 xiv Figure 17 : CO 2 adsorption capacity of SiO 2 composite sorbents as a function of K 2 CO 3 loadings ......................................................................................... 56 Figure 18 : XRD patterns of SiO 2 and K 2 CO 3 impregnated SiO 2 ................................ 57 Figure 19 : Breakthrough curves of K 2 CO 3 /Al 2 O 3 at different K 2 CO 3 loadings ......... 59 Figure 20 : CO 2 adsorption capacity of Al 2 O 3 composite sorbents as a function of K 2 CO 3 loadings ......................................................................................... 60 Figure 21 : XRD patterns of Al 2 O 3 and K 2 CO 3 impregnated Al 2 O 3 with different loadings ..................................................................................................... 61 Figure 22 : Breakthrough curves of K 2 CO 3 /ACP at different K 2 CO 3 loadings........... 63 Figure 23 : DSC spectra of spent K 2 CO 3 /ACP and K 2 CO 3 /Al 2 O 3 ............................... 64 Figure 24 : CO 2 adsorption capacity and utilization of K 2 CO 3 /ACP sorbents at different K 2 CO 3 loadings. ........................................................................................ 67 Figure 25 : Breakthrough curves of K 2 CO 3 in dry and wet gas streams...................... 69 Figure 26 : CO 2 adsorption capacity at various moisture contents.............................. 71 Figure 27 : Schematic phase diagram of K 2 CO 3 in the pores of activated carbon. ..... 73 Figure 28 : Breakthrough curves of K 2 CO 3 /ACP in dry and wet gas streams............. 75 Figure 29 : DSC spectrum and calculated ?G as a function of temperature................ 77 Figure 30 : DSC spectra of ACP and K 2 CO 3 impregnated ACP. ................................ 78 Figure 31 : Breakthrough curves of K 2 CO 3 /ACP in wet and dry regeneration conditions. ................................................................................................. 79 Figure 32 : XRD patterns of K 2 CO 3 /ACP sorbents under wet and dry regeneration conditions. ................................................................................................. 80 Figure 33 : SEM micrograph of microfibrous media................................................... 83 xv Figure 34 : A schematic of a composite bed design. ................................................... 85 Figure 35 : Comparative BT curves of a composite bed and a packed bed................. 86 Figure 36 : BT curves of composite beds from cycle 1 to cycle 8............................... 88 Figure 37 : SEM micrographs of microfibrous media................................................. 90 Figure 38 : BT curves of composite bed using Sodalime. ........................................... 91 Figure 39 : XRD patterns of Al 2 O 3 -composite sorbents under adsorption and regeneration. .............................................................................................. 92 Figure 40 : DSC spectra of spent K 2 CO 3 /Al 2 O 3 at different K 2 CO 3 loadings............. 93 Figure 41 : XRD patterns of K 2 CO 3 /Al 2 O 3 calcined at different temperatures. .......... 94 1 I. INTRODUCTION Carbon dioxide sequestration has been of interest as the increment of CO2 emissions contributes to the global warming and climate change [1]. Carbon dioxide is a product from many sources, such as human activities, vehicle emissions, and combustion in power plants. To develop a material for carbon dioxide control, a basic understanding of the individual system and the chemistry behind the system is needed. There are a number of techniques that have been used to remove CO2, such as Rectisol process, Selexol process, adsorption by solid sorbents, and absorption by liquid solvents. Such processes might be suitable for one application, but not another. Monoethanolaine (MEA) and hot potassium carbonate (K2CO3), for example, are often used in recovery of CO2 in the ammonia industries, oil refineries, and petrochemical plants [2], both of which have high CO2 removal capacity and efficiency. The same method can not be applied for scrubbing CO2 in fuel cells for portable electronic devices or in space cabin or submarines due to the large unit size and zero gravity limitation. An alternative, such as a use of solid sorbents and membrane processes has been of interest to overcome these disadvantages. The membrane processes are effective, but costly due to the high pressure required inside the membrane reactors to allow the gas or liquid to 2 permeate through the membrane. In the case of solid sorbents, CaO is one of the most common carbon dioxide removal sorbents in power stations and coal gasification. However, the reaction between CaO and CO2 is a slow reaction at room temperature. As a result, CaO is usually operated at 500-600oC. Molecular sieves often known as drying agents due to their moisture adsorption capacity by means of physical adsorption are often used to adsorb carbon dioxide in medical applications, such as during low-flow sevoflurane anaesthesia [3]. However, due to the high water adsorption, many beds of molecular sieves or silica are often incorporated in order to remove moisture prior to adsorption of carbon dioxide. In this study, the combination of the liquid phase absorption and gas phase adsorption has been used to remove CO2 from the wet gas stream at room temperature. The goal of this research is to synthesize a new carbon dioxide adsorbent that work at room temperature with regeneration capability and thermal stability. Many sorbents are considered in this study, such as Molecular sieves (3A, 4A, 5A, and 13X), CaO, Sodalime, potassium superoxide, and potassium carbonate. Energy dispersive x-ray spectroscopy, scanning electron microscopy, differential scanning calorimetry and x-ray diffraction are employed to examine chemical and structural changes of sorbents before and after breakthrough tests. 3 II. LITERATURE REVIEWS A development of the greenhouse gas control technology has attracted much interest due to the concern of the possibility of global warming induced by the atmospheric accumulation of greenhouse gases. The current average carbon dioxide level has increased during the past century as shown in Figure 1 [4]. Carbon dioxide comes from many sources including the respiration processes of living; man-made sources of carbon dioxide come mainly from the burning of various fossil fuels for power generation and transport use, all of which induced an increase in greenhouse gases that is the cause of global warming. 4 Figure 1: Atmospheric CO2 concentrations in parts per million by volume, ppm, at Mauna Loa, Hawaii Some combustion applications release a large quantity of carbon dioxide, which needs to be removed prior to storage or being released into atmosphere such as coal gasification, fossil fuel power plants, and syngas production [5,6]. As a result, in recent years many carbon dioxide removal processes have been developed to meet the requirement of many applications as shown in Figure 2 [7]. 5 0.0001 0.001 0.01 0.1 1 CO2 (vol.%) Te mp er atu re (o C) Facepiece air-purifying respirators 100 Selexol Rectisol 200 450 500 0 150 -100 Alkaline Fuel Cells 400 Water Gas Shift (HTS) (LTS) 250 300 350 Zn-Air Batteries CO + H2O = CO2 + H2 Syngas & Ammonia IGCC Selexol (amines) Rectisol (cold methanol) Figure 2: CO2 concentration levels in selected applications Applications of CO2 Removal Processes Fuel Cells and Batteries The reformation of logistic fuels (i.e. JP8) involves many steps in order to obtain high purity hydrogen to power on-board Proton Exchange Membrane Fuel Cells (PEMFCs). Large amounts of carbon dioxide are obtained along the reformation and clean-up processes, which has to be reduced prior to entering the PEMFCs due to the possibility of side reactions, such as reverse water-gas shift, COS formation, or equipment corrosion [8,9]. Many technologies have been developed for CO2 removal processes, such as 6 physical absorption, chemical absorption, adsorption, and membrane processes as shown in Table 1. 7 Table 1: Current CO2 removal technologies Technologies Examples Advantages Disadvantages Chemical absorption ? MEA (monoethanolamine) ? DEA (diethanolamine) ? MDEA (methyldiethanolamine) ? Low temperature operation (inlet gas can be around 40oC) ? Corrosion by amines ? Degradation of amines ? CO2 removal dependent on type of amines ? Low heat efficiency Physical absorption ? RECTISOL (cold methanol) ? SELEXOL (dimethyl ether of polyethylene) ? Sulfinol (mixture of ? aqueous amine and sulfolane) ? Fluor process (propylene carbonate) ? Low temperature operation (-70o to 20oC) ? High pressure operation ? High capital cost ? High energy consumption to maintain the solvents Adsorption ? Alumina ? Zeolite ? Activated carbon ? Ease of use and storage ? Usually use for trace contamination ? May require landfilling Membrane processes ? Inorganic membrane (zeolite) ? Polymeric membranes (microporous) - Polypropylene - Polyphenyleneoxide - Polydimethylsiloxane ? Ease of use ? Compact equipment size ? Expensive ? Low regeneration rate ? Often required pre- treatment 8 Fuel Cells have been around for over 150 years, but it was in 1960s when the potential of fuel cells was recognized after NASA demonstrated in Space Shuttle Program. The development of fuel cells was slow due to the technical barriers and high investment cost. To date, there are many types of fuel cells suitable for different applications. Table 2 shows a comparison of different types of fuel cell technologies and their applications. The major concern of fuel cells is mainly on economic competition with existing technologies. Among these fuel cells, Alkaline Fuel Cells offer highest efficiency and has been used since the mid-1960s by NASA in the Apollo and Space Shuttle Program due to their high efficiency, which allows the electricity to be generated nearly 70%. Figure 3 shows simple diagrams of different types of fuel cells and their operations. It is shown that for AFCs, high purity O2 and H2 are required to obtain high efficiency and extend the life of electrolyte (i.e. KOH) from poisoning. As a result, AFCs are very costly; mainly due to the operational costs from frequent replacement of CO2 scrubbers to maintain high purity H2 and O2. Carbon dioxide can be used to generate water once separated from air in Bosch and Sabatier processes [10], which is one of the main reasons this type of fuel cells is used mainly under Life Support Program (LSS) in military and space programs. 9 Table 2: Comparison of Fuel Cell Technologies [11] Fuel Cell Type Common Electrolyte Operating T.(oC) System Output Efficiency Applications Advantages Disadvantages Polymer Electrolyte Membrane (PEM) Solid organic polymer polyperflu- orosulfonic acid generation 50-100 <1kW- 250kW 50-60% electric - Back-up power - Portable power - Transportation - Solid electrolyte reduces corrosion & electrolyte management problem - Quick start-up - Low temperature - Expensive catalysts - Sensitive to impurities - Low temperature waste heat Alkaline (AFC) Aqueous solution of potassium hydroxide soaked in a matrix 90-100 10kW- 100kW 60-70% electric - Military - Space - Cathode reaction is faster in alkaline electrolyte so high performance - Expensive to remove CO2 from fuel and air is required Phosphoric (PAFC) Liquid phosphoric acid soaked in a matrix 150-200 59kW- 1MW (250k W module typical) 36-42% electric (80-85% overall with CHP) - Distributed generation - High efficiency - High tolerance to impurities in hydrogen - Suitable for CHP - Pt catalyst - Low current and power - Large size/weight Molten Carbonate (MCFC) Liquid solution of lithium, sodium, and/or potassium carbonates, soaked in a matrix 600-700 <1kW- 1MW (250k W module typical) 60% electric (85% overall with CHP) - Electric utility - Large distributed generation - High efficiency - Fuel flexibility - Variety of catalysts can be used - Suitable for CHP - High temperature speeds corrosion and breakdown of cell components - Complex electrolyte management - Slow start-up Solid Oxide (SOFC) Solid zirconium oxide to which a small amount of yttitra is added 650-1000 5kW- 3MW 60% electric (85% overall with CHP) - Auxiliary power - Electric utility - High efficiency - Fuel flexibility - Variety of catalysts can be used - Suitable for CHP - Solid electrolyte reduces electrolyte management problem - Suitable for CHP - High temperature enhances corrosion and breakdown of cell components - Slow start-up * CHP = Combined Heat and Power 10 Figure 3: Basic configuration of different types of fuel cells 11 Zn-Air Batteries Similar to Alkaline Fuel Cells, the use of liquid solvent to remove CO2 is not viable in practice for Zn-air batteries. Zinc-air batteries, an example of metal-air batteries are energized only when atmospheric oxygen is absorbed into the electrolyte through a gas- permeable, liquid-tight membrane. They use the oxygen from air at the cathode as shown in the basic chemical reaction below [12]. At Air cathode: 0.5O2 + H2O+ 2e- barb2right 2OH- At Zn anode: Zn barb2right Zn2+ + 2e- Zn2+ + 2OH- barb2right Zn(OH)2 Zn(OH)2 barb2right ZnO + H2O Overall : Zn + 0.5O2 barb2right ZnO E0 = 1:65V The basic configuration of the zinc-air batteries is shown in Figure 4. Figure 4: Cross-sectional view of the basic zinc-air batteries 12 Zn-Air batteries are often used in many portable applications, such as hearing aids, pagers, and mobile electronic devices. The essential components of a Zn-air cell are the negative terminal, anode, separator, cathode catalyst, cathode, positive terminal, and electrolyte. The electrolyte is typical potassium hydroxide utilizing OH- as a charge carrier. Atmospheric oxygen is reduced at the cathode. The size of the air vent determines the energy density of the cell. However, zinc-air batteries are very sensitive to temperature, moisture and carbon dioxide from air. The carbon dioxide from air usually leaks through the membrane forming carbonate compound, which reduces the OH-. The formation of this carbonate results in low conductivity and premature capacity reduction. Among current CO2 removal technologies, liquid phase absorption is not possible for this application due to the fact that it is not amendable for ransom orientation. As a result, many other sorption based technologies have been developed, such as molecular sieves, dolomite, LiOH, and potassium super oxide as shown in Table 3. Molecular sieves/zeolites are typically used to remove CO2 via physical adsorption in the pore structure of the zeolites during anesthesia. The use of chemical compounds, such as Sodalime, Baralime, Carbolime, lithium hydroxide, potassium superoxide, and dolomite (CaO) causes a concern of the reaction between the sorbent and anesthetic. 13 Table 3: Selected currently available CO2-sorbents Liquid Solvents Candidates Amine (absorption) K2CO3 K2CO3+CO2+H2O=2KHCO3 (reaction) Saturation Capacity g CO2/g sorbent 0.0882 (PEI: polyethylenimine) 0.318 Regeneration Temp. (oC) 150 180-250 Use -IGCC power plants - Ammonia and syngas production Disadvantages - Excess moisture increases ?P and reduces capacity - Liquid phase absorption - Large space required - Not readily amendable to random orientation 14 Solid Sorbents Candidates LiOH 2LiOH+CO2=Li2CO3 +H2O (reaction) Dolomite CaO+CO2=CaCO3 (reaction) Potassium superoxide 2KO2 + CO2 = K2CO3 + 1.5O2 (reaction) Molecular Sieves (adsorption) Saturation Capacity g CO2/g sorbent 0.920 0.786 0.31 plus 1.5 mol O2 generated 0.066 (MS 5A) Regeneration Temp. (oC) 1310 600-900 >2000 250-350 Use - AR Systems -Coal gasification -IGCC plants -AR systems (Drager?) - Natural gas treat. - AFCs Disadvantages -High regen. T. -Short duration -One time use - Kinetically slow at low temp. - High regen. T - High regen. T - One time use - Protective bed is needed to remove H2O - Easy to degrade 15 Gasification Today?s most commercial CO2 removal units for power industries utilize a chemical absorption of aqueous solvents due to their high CO2 removal capacity and their ability to regenerate. However, the aqueous solvent units usually suffer from large unit size and their complexity from recycling spent solvents and maintaining the operating temperatures. Rectisol process, for example, has to maintain the temperature around - 50oC. In the fossil fuel power plant, integrated gasification combined cycle (IGCC) power plants are considered very efficient and relatively clean. The natural gas field is found to contain at least 50% by volume [13]. With the combination of Rectisol or Selexol process, 50-60% energy conversion can be reached with ?zero? CO2 emissions [14-16]. However, these technologies are costly and high energy consumption. As a result, several other processes have been introduced into power plants, such as solid sorbents and membrane processes. Membrane Processes While membrane processes are usually light and compact, membranes usually operates at high partial pressure of CO2 to ensure the permeability through the membrane, which causes high energy consumption. Moreover, the gas has to be relatively clean from small particulates to prevent the blockage from particles. The gas selectivity of the current 16 membrane technology is still inadequate. As a result, a large area of membranes is required to remove a small amount of CO2. Reforming CaO has long been used for CO2 removal associated with power stations and coal gasification [17]. Recently CaO has been used in steam reforming and water-gas shift to break the thermodynamic barrier by simultaneously removing CO2 and thus pushes the reaction further towards H2 production. This increases the production and purity of hydrogen, increases the CO conversion, and effectively removes carbon species from the gas phase product as shown in methane reforming in Figure 5. Steam Reforming: CH4 + H2O bleftright 3H2 + CO ?H = 206 kJ/mol Water-gas shift: CO + H2O bleftright H2 + CO2 ?H = -41 kJ/mol Overall: CH4 + 2H2O bleftright 4H2 + CO2 ?H = 165 kJ/mol A separated CO2 product can be recovered from the adsorbent by regenerating the bed under reduced gas phase pressures, i.e. via pressure swing adsorption (PSA) concepts or temperature swing adsorption (TSA), which normally takes place around 800oC. The advantages of combining the water-gas shift reaction with CO2 adsorption is the high conversion since the equilibrium limited reaction is increased through the removal of a product of reaction. 17 Figure 5: Adsorption enhanced catalytic processes Other Low Carbon dioxide concentration Applications The objective of this study is mainly focused on the low concentration carbon dioxide applications. The use of aqueous solvents is not plausible for such applications as mentioned. This is not only because of the fact that aqueous solvents are not amendable to random orientation, but also due to high operating cost, large unit size, low heat efficiency, solvent degradation, and equipment corrosion. Thus, the development of a new material for a cost-effective filtration with high CO2 adsorption capacity is needed. The use of solid sorbents attracts much interest to overcome these disadvantages due to its low cost, low energy consumption, and wide range of operating temperatures and pressures. Moreover, the use of solid sorbents for adsorption combines with chemical reactions results in promoting reactions, which is normally limited by equilibrium such as steam reforming and water-gas shift. In case of waster gas shift reaction in IGCC processes, CO2 removal process results in shifting the reaction toward lower CO concentration, generating more CH4 + H2O H2 +CO +CO2 catalyst catalyst catalyst Typical Methane Reforming Sorption Enhanced Reforming CH4 + H2O H2 CO2 CO2 CO2 CO 2 CO2 CO2 sorbent sorbent catalyst CO2 18 hydrogen, which can be used for fuel cells. However, many solid sorbents usually require high regeneration temperatures, such as LiOH and dolomite compared to the liquid solvent counterparts. Air-purifying Respirator Many methods are employed for CO2 removal from a close-circuit breathing apparatus in military and medical applications, such as the use of monoethaoamine, LiOH, and the recent KO2. However, the use of potassium superoxide is one of the most interesting methods for breathing atmosphere. The reaction between KO2 and CO2 at 25oC is as follows: 2KO2 + H2O barb2right 2KOH + 1.5O2 ?H = 5.502 kJ 2KOH + CO2 barb2right K2CO3 + H2O ?H = -193.154 kJ 2KO2 + CO2 barb2right K2CO3 + 1.5O2 ?H = -187.652 kJ Based on the chemical reaction, it is shown that one mole of carbon dioxide reacts with two moles of potassium superoxide forming the emission of 1.5 mole of oxygen. However, the reaction is highly exothermic and the potassium superoxide is highly unstable. As a result, solid sorbents based on hydroxides of alkali metals is often used to remove CO2 from the closed-circuit atmosphere, such as Baralime? and Sodalime?. The chemical composition of Baralime and Sodalime is shown in Table 4. 19 Table 4: Chemical composition of Baralime and Sodalime Sorbents Baralime Sodalime Chemical Composition ? 80% Ca(OH)2 ? 20% Ba(OH)2 ? 94% Ca(OH)2 ? 5% NaOH ? 1% KOH This type of sorbents is a mixture of Ca(OH)2, and sodium-, potassium, and barium hydroxides. For reaction with carbon dioxide, water is rather necessary and the mixture of this type of sorbents usually contains 12-20% H2O. Baralime demonstrated to have high carbon dioxide adsorption capacity; however, barium hydroxide is rather corrosive and the presence of potassium hydroxide causes degradation of anesthetic agent in medical applications. Current packed-bed technologies for CO2 recovery from flue gases utilize zeolites/molecular sieves for the pressure-swing adsorption in a dry gas stream. The flue gases usually contain 8-17% moisture, which affect the adsorption capacity of these solid sorbents. As a result, the moisture traps have to be used to remove moisture prior to removing carbon dioxide resulting in large adsorbers. Molecular sieves are another type of sorbents that have been used for carbon dioxide control from the cabin air for space station by NASA [18]. This system utilizes a combination of silica and molecular sieve 13X to remove water vapor and molecular sieve 5A to remove carbon dioxide. 20 Molecular Sieves Molecular sieves are crystalline metal aluminosilicates with interconnecting tetrahedral network of silica and alumina. The uniform cavities of the molecular sieves selectively adsorb molecules of a specific size. Table 5 shows some selected molecules and their critical diameters. 21 Table 5: Selected molecules and their critical diameters [19] Molecule Critical diameter (?) Helium 2.0 Hydrogen 2.4 Acetylene 2.4 Oxygen 2.8 Carbon monoxide 2.8 Carbon dioxide 2.8 Nitrogen 3.0 Water 3.2 Ammonia 3.6 Hydrogen sulfide 3.6 Argon 3.8 Methane 4.0 Ethylene 4.2 Ethylene oxide 4.2 Ethane 4.4 Methanol 4.4 Methyl mercaptan 4.5 Propane 4.9 n-Butane to n-docosane 4.9 Propylene 5.0 Ethyl mercaptan 5.1 1-Butene 5.1 trans-2-Butene 5.1 1,3-Butadiene 5.2 Chlorodi fluoromethane (Freon 22?) 5.3 Thiophene 5.3 Isobutane to isodocosane 5.6 Cyclohexane 6.1 Benzene 6.7 Toluene 6.7 p-Xylene 6.7 Carbon tetrachloride 6.9 Chloroform 6.9 Neopentane 6.9 m-Xylene 7.1 o-Xylene 7.4 Triethylamine 8.4 22 A 4 to 8-mesh sieve is normally used in gas phase applications, while the 8 to 12-mesh type is common in liquid phase applications. The concept of the molecular sieves is to adsorb species smaller than the pore size of the molecular sieves. 3A molecular sieves have the effective pore size ~3A. As a result, 3A adsorbs species with diameter less than 3A, e.g. ethane. Table 6: Properties of Selected Molecular sieves Type Composition Pore size (A) Adsorbed species 3A 0.6 K2O: 0.40 Na2O : 1 Al2O3 : 2.0 ? 0.1SiO2 : x H2O 3 NH3 and H2O from a N2/H2 flow 4A 1 Na2O: 1 Al2O3: 2.0 ? 0.1 SiO2 : x H2O 4 H2O, SO2, CO2, H2S, C2H4, C2H6, and C3H6 5A 0.80 CaO : 0.20 Na2O : 1 Al2O3: 2.0 ? 0.1 SiO2: x H2O 5 H2S, CO2 and mercaptans removal from natural gas 13X 0.80 CaO : 0.20 Na2O : 1 Al2O3: 2.0 ? 0.1 SiO2: x H2O N/A H2O, CO2, H2S, mercaptan removal from hydrocarbon/natural gas Composite Sorbents The current approach is to combine the advantages of liquid solvent, membrane process and adsorption for regenerable CO2 removal sorbents for low concentration applications such as in Alkaline Fuel Cells (AFCs), Metal-Air batteries, and portable life 23 support systems by adapting the Banefield process. The Banefield process uses hot K2CO3(aq) to react with CO2 in the presence of moisture as follows: H2O + CO2 bleftright H2CO3 H2CO3 bleftright H+ + HCO3- Ka = 4.3x10-7 HCO3- bleftright H+ + CO32- Ka = 5.6x10-11 2H+ + CO32- + K2CO3 bleftright 2KHCO3 ___________________________ K2CO3 + CO2 + H2O bleftright 2KHCO3 (1) The reaction is initiated by diffusion of carbon dioxide from the gas phase to the water layer on the surface. This water layer can be formed by water adsorption or excess water provided during sorbent preparation. Carbonic acid, a product from carbon dioxide dissolved in water, is a weak acid and dissociated in two steps and then neutralized by K2CO3 forming KHCO3. Many commercial processes for CO2 removal employ the Banefield process. However, the use of liquid solvents is difficult in practice especially to downsize the unit. Thus, the development of a new material for a cost-effective filtration with high CO2 adsorption capacity and ability to regenerate is needed. The current approach is to utilize the liquid potassium carbonate in packed bed operations for CO2 removal under the moist conditions. The porous microfibrous media [20-22] is used to entrap K2CO3(aq) as an ?apparent solid? by incipient wetness impregnation. The use of small particulates significantly enhances the intraparticle diffusion and allows high contacting efficiency. 24 The nano-dispersed nature of K2CO3 combined with the use of small support particulates promotes high K2CO3 utilization, high contacting efficiency, and high accessibility of K2CO3 while minimizing the pressure drop and lowering the heat of reaction generated as shown in Table 7. Table 7: Adiabatic heat of selected CO2 sorbents Sorbents Chemical Reaction ?H (kJ/mol) @25oC ?T (K) LiOH 2LiOH + CO2 bleftright Li2CO3 + H2O -88.53 120 CaO CaO + CO2 bleftright CaCO3 -178.33 259 Ba(OH)2 Ba(OH)2 + CO2 bleftright BaCO3 + H2O -162.31 218 K2CO3 + CO2 + H2O bleftright 2KHCO3 -100.20 120 K2CO3 K2CO3 + CO2 + H2O + ACP bleftright 2KHCO3 + ACP -100.20 15 A high heat generation of sorbents is a drawback since it will require a cooling system meaning high energy consumption and capital cost. Based on the data in Table 7, LiOH system has a fairly low heat of reaction compared to CaO and Ba(OH)2; however, the system is not regenerable. An alternative, K2CO3 system with the same adiabatic temperature rise, the K2CO3 system has a slightly higher heat of reaction than LiOH. However, the K2CO3 system can be regenerated. By utilizing incipient wetness impregnation, K2CO3 can be entrapped in the porous support matrix, e.g. activated carbon particulates (ACP). The high dispersion nature of the support materials will allow the reduction in adiabatic temperature rise. 25 Methods for Sorbent Preparation It is known that the preparation procedures affect sorbent characteristics, such as compositions, crystal structure, and homogeneity of the sorbents. By tailoring the preparation procedure, the desire characteristics of the sorbents can be controlled. There are two main methods for sorbent preparation: precipitation and incipient wetness impregnation. Precipitation This method involves the use of two or more chemically reactive chemical compounds in the aqueous solutions or suspension to form precipitation. The general rule of thumb is that the chemical reagents have to have high solubility in water. These precipitates can present in the form of single metallic compounds, alloys, metal hydroxide, or metal carbonate depending on the reactions between the metal salts. They can be converted into metal oxides by calcination in a presence of oxygen (air) or in a reducing atmosphere (i.e. He) to reduce the metal oxide to metal. This method is suitable for sorbent preparation in bulk; however, it is difficult to control the composition and the amount of the sorbent formed due to the possibility of side reactions. Impregnation Impregnation is another way of forming a composite sorbents by utilizing the porous support materials. The sorbents are prepared by dissolving a metallic compounds, which then come into contact with the support materials. The impregnated materials are then calcined or dried at selected temperature and atmosphere to control the formation of the 26 compound. The choice of the support materials and calcination temperature is one of the most important factors for a desire composite sorbent formation. Zhang et al [23] studied the metal oxide-support interaction of Li2O and ?-Al2O3. The result suggested that Li2O diffuses into the bulk ?-Al2O3 forming ?-LiAlO2 at 733-793K. Incipient wetness impregnation method allows the control over the amount of the active materials added and the sorbents usually are generated in the porous structure of the support materials. Adsorption Adsorption is a process that materials at the interface between two phases accumulate. These phases can be any combination of liquid, solid, and gas. The adsorbing phase is called adsorbent and the materials being adsorbed are called adsorbate. Adsorption is the result of interactive forces of an attraction at the surface between porous solids and adsorbate molecules removed from the fluid surrounding the solids. This interactive force at the surface consists of permanent dipole, induced dipole, quadrupole electrostatic effects known as van der Waal?s forces. Physical adsorption is a phenomenon due to van der Waal?s forces, or the forces of attraction between non-polar molecules. Electrostatic forces, or forces due to charged species, also play an important role in physical adsorption especially when the adsorbent is a charged species such as a zeolite. Physical adsorption is always an exothermic process and the heat of adsorption provides a direct measure of the strength of the bond between adsorbate and adsorbent. 27 Physical adsorption is separate from chemical adsorption or chemisorption, where a chemical bond is formed and electrons shared between the adsorbent and adsorbate. Physical adsorption is considered a weak force and normally characterized by forces between molecules less than R*T, where R is the universal gas constant and T is the absolute temperature. Chemisorption involves stronger forces with energies normally larger than R*T. At room temperature R*T is about 2.5 kJ/mol. Adsorption on the solid adsorbents has great potential for environmental applications as the process can effectively remove pollutants from both liquid or gas environments, which allows many possibilities for a wide range of applications, such as CO, CO2, and H2S. Due to the high degree of purification, adsorption on solid sorbents is usually used at the end of the treatment sequence as a packed bed. Adsorption should not be confused with absorption, as they are two very different phenomena. Adsorption is the accumulation of concentration on the surface of a solid; absorption is the accumulation of concentration within the bulk of a solid or liquid. The most common means of gas-phase adsorption is by passing contaminated gas through an immobilized bed of particulates, also known as a packed bed or fixed bed adsorber. In a packed bed, adsorption occurs in the mass transfer zone (MTZ). This is the zone where adsorbate molecules from the feed are transferred to the solid adsorbent. As the adsorbent particles in this zone become saturated, the zone moves slowly through the bed in the direction of gas flow. Breakthrough occurs when the zone reaches the exit of the bed. Areas upstream and downstream of the MTZ are not involved in the adsorption process. Upstream of the MTZ, adsorbent particles are saturated and in equilibrium with the gas carrying the contaminant through the bed. Downstream of the 28 MTZ, no challenge is present in the gas passing through the bed and thus no adsorption is occurring. Microfibrous Media (MM) Microfibrous media (MM) have great potential to enhance mass and heat transfer of the adsorption and catalytic processes compared to the use of the packed beds of particulates [24,25]. For conventional adsorption process using porous adsorbents, the rate of adsorption depends on two major factors: interparticle diffusion and intraparticle diffusion. Interparticle resistance is the mass transport resistance between the flowing gas and the adsorbent particle, also called the film resistance. This resistance is often considered as a constant and modeled as a stagnant film around the particle. Intraparticle resistance is the mass transport resistance inside the adsorbent particle and is a function of the pore type and pore structure [26]. As a result, most adsorption processes are intraparticle-limited mass transport. A decrease in particle size increases the proportion of interior surface area to exterior surface area and results in an increase in the overall adsorption rate. Adsorption will take place faster and allow more adsorbents to be utilized. However, small particle size leads to high pressure drops in packed bed adsorbers as seen in the Ergun equation: ( ) p o p o D v D v L P ? ? ?? ? ? ?+ ??? ? ??? ? ?=? 175.11150 3 22 3 where ?P is total pressure drop, L is bed depth or length, ? is viscosity, vo is the linear gas velocity through the bed, ? is the void fraction of the bed (commonly 0.6), Dp is particle diameter, and ? is gas density. 29 In industrial adsorption processes, the use of small adsorbents results in high adsorption efficiency, but it is not practical due to high pressure drop or high energy consumption. The current effort is to utilize small particulates to reduce the intraparticle- limited mass transport and still maintain the high adsorption efficiency by means of microfibrous media. Composite Bed A large packed bed usually has a high capacity due to high volume loading of sorbent. However a packed bed of large particulates makes poor use of the sorbent as evidenced by a slow, sigmoidal breakthrough curve as shown in Figure 6. A decrease in the size of the particle increases the breakthrough time of the packed bed [27], but at the same time the pressure drop through the packed bed also increases. It is evident that the particle size affects the pressure drop through the bed and the rate of diffusion into the particles. As a result, most commercial packed bed adsorbers sacrifice high adsorption capacity to maintain low pressure drop. The concept of composite beds is then introduced. A composite bed refers to the combination of a standard packed bed with a thin layer of MM at the outlet. MM acts as a polisher removing small amounts of challenge as they break through the packed bed. The polishing layer in a composite bed has high contacting efficiency from the small entrapped particles in the MM. The breakthrough curve of the MM layer alone is very sharp indicative of high sorbent utilization, but breakthrough times are low as expected due to a low volume loading of the sorbent. The synergistic effect of the composite bed is much greater than the simple addition of the breakthrough times of each layer. Low outlet concentrations exiting the packed bed are 30 introduced into the high contacting efficiency polishing layer. The first bed adsorbs a large percentage of the inlet challenge, so the MM layer only needs to polish or adsorb the remainder. Figure 6: Concentration Profiles in a Composite Bed This composite bed can be described by a bed depth service time equation (BDST) [28], which is usually used for packed bed of particles with a strong physical adsorption. The BDST equation is often used to determine the required time in the bed for a given challenge concentration to be adsorbed. ??? ? ??? ? ??? ? ??? ? ??= 1ln b o o o oo o C C kN vL vC Nt Where t is time required for adsorption, vo is linear gas velocity through the bed (also known as face velocity), L is adsorbent bed depth, k is the overall adsorption rate Co nc en tra tio n ( C/ C 0 ) Trace CO2 Contaminant 1 0 Dimensionless Bed Distance (z) CO2-free gas Packed Bed + Polishing Layer 31 constant, No is the mass per unit volume of adsorbent, Co is influent concentration, and Cb is concentration at breakthrough. A thicker bed for higher adsorption effectiveness is balanced with the fact that pressure drop increases linearly with bed thickness as evident in the Ergun equation. Cahela and Tatarchuk [29] adapted this bed depth service time equation to describe the composite bed of microfibrous media incorporation. If the amount of challenge removed until breakthrough by the MM layer alone (concentration*time area) is converted to an equal area underneath the packed bed breakthrough curve, a good estimate of the composite bed breakthrough time is obtained as shown in Figure 7. This is known as the Equal Areas Design Rule. Figure 7: Equal Area Design Rule Equal Areas 5-Log Breakthrough of packed bed (PB) Time Elapsed (minutes) Co nc en tra tio n 5-log breakthrough of Microfibrous Media Composite bed: 5-Log Breakthrough of PB + MM Co Co 32 Modeling Adsorption Breakthrough Adsorption systems for gas or liquid purification usually employ a packed bed adsorber. To design adsorbers effectively, the dynamics and characteristics of the systems are required. The three main parameters are the adsorption isotherm, the external mass transfer coefficient, and the intraparticle mass transfer coefficient. Many theoretical models have been developed for modeling adsorption breakthrough curves, such as the effective diffusion model, the combined diffusion model under the assumptions of: (1) negligible pressure drop, (2) constant hydraulic loading, (3) negligible dispersion, (4) isothermal conditions, and (5) constant surface diffusion coefficient. Yoon and Nelson [30] have developed a model describing adsorption breakthrough curves on activated charcoal based on assumption that the rate of adsorption decreases as a function of adsorbate adsorption. This equation is simple and required no data on characteristics of the adsorbates and adsorbents. As a result, this model will be applied throughout this study. The Yoon and Nelson equation can be expressed as follows: bi b CC C kt ?+= ln 1 '? where k? is the rate constant (1/min), ? is the time required for 3% adsorbate breakthrough(min), t is the (sampling) breakthrough time (min), Cb is the breakthrough (effluent) concentration of adsorbate (ppm), Ci is the initial inlet concentration of adsorbate (ppm). 33 These values are determined from the experimental data by plotting ln[Cb/(Ci-Cb)] vs. time (t) according to Yoon and Nelson equation. If the accurate experimental data are obtained, this plot will result in a straight line with a slope of k? and the intercept of ? k??. Critical Bed Depth Critical bed depth (CBD), or theoretical minimum bed thickness required for adsorption of a given challenge. Critical bed depth is obtained from the bed depth service time equation by setting t = 0 and solving for L: ??? ? ??? ? ?= 1ln b o o o c c kN vCBD If the thickness of a packed bed is less than critical bed depth, there will be an immediate breakthrough. Critical bed depth is useful in comparing different adsorber designs as it gives a measure of overall effectiveness. Sorbent Utilization Overall efficiency of an adsorbent bed can be evaluated by different parameters. In this study, sorbent utilization is used by the following definition: theo BT ttnUtilizatio = where tBT is the time required for the 3% adsorbate breakthrough, ttheo is the time required for the 100% adsorbate known as saturation time. 34 II. EXPERIMENTAL Many types of sorbents were tested in this study, such as Molecular sieves (3A, 4A, 5A, ad 13X), Baralime?, Carbolime?, Sodalime?, and K2CO3 impregnated onto various support materials. Adsorbent Preparation Supported K2CO3 sorbents were prepared using K2CO3*1.5H2O as a precursor, which is loaded onto the support by pseudo-incipient wetness impregnation. Three types of support materials were selected based on their liquid holding capacity (pore volume): Al2O3, SiO2, and activated carbon. The characteristics of these support materials are shown in Table 8. Table 8: Physical properties of porous host matrix Host Matrix Density (g/cc.) Pore Volume (cc./g) Manufacturer ACP 0.48 0.60 PICA USA SiO2 0.512 0.60-0.70 Selecto Scientific, Inc. Al2O3 0.387 1.14 Alfa Aesar 35 The sorbents were then prepared by filling the pores of the porous supports at various loadings by varying the solution concentration to examine the effect of K2CO3 loading on CO2 adsorption performance. The impregnation solution was prepared by adding potassium carbonate sesquihydrate and dionized water to obtain a desire concentration. After impregnation, the sorbents were then dried at 100oC for 30 minutes. The volume of the sorbents was maintained at 10 cc; otherwise stated. The influence of the nature of host matrix on the adsorption capacity was studied. Microfibers Three types of microfibers can be used: Polymer fibers, nickel fibers, and glass fibers. Physical properties of microfibers are shown in Table 9. 36 Table 9: Physical properties of microfibers Factors\Fiber type Metal Ceramic Polymer Volume Loading 3% 4% 4% Fiber Density 8 g/cc 2.4 g/cc 1 g/cc Fiber Diameter 2-12 ?m <1-12 ?m 10-20 ?m Operating Temperature 150-600 oC 600-900 oC <150 oC Cost $60/lb $7/lb $1/lb Different microfibers are used mainly based on the type of operating temperature ranges. Polymer fibers, for example, are light and cheap. As a result, they are often used in air-purifying respirators to scrub trace contaminants, such as H2S and CO. Due to their sensitivity to temperature (<150oC), the use of polymer fibers often limits to disposable filters or regeneration has to be done through PSA to maintain the robust structure of the polymer microfibrous media. For Metal fibers, the operating temperature is between 150- 600oC, which allows this type of fibers to be used in combination with adsorbents or catalysts of high exothermic reactions [31,32]. Even though the use of nickel fibers allows higher operational temperature ranges, it should be noted that nickel fibers can get oxidized in the presence of H2O and/or O2. As a result, the microfibrous media formed from nickel 37 fibers tend to gradually corrode over adsorption/regeneration in the presence of H2O and O2. The glass fibers offer the operational temperatures (900oC), which allows this type of fibers to be used in a wide range of applications [33]. The nickel microfibers were selected in this study due to operating temperature of the adsorption tests and the range of regeneration temperatures as well as the high thermal conductivity of the nickel fibers that allows the reduction in thermal effects. Preparation of Microfibrous Entrapped Sorbents A sintered metal microfibrous carrier was used to entrap 150-250 ?m diameter support particulates by wet layer paper-making/sintering procedure. The composition of 8? preform media is shown in Table 10. Table 10: Composition of 8? Preform Component Weight (g) 8 ?m nickel fibers 1 12 ?m nickel fibers 3 Cellulose 1 150-250 ?m particulates 10 38 1g of 8 ?m nickel fibers, 3 g of 12 ?m nickel fibers, and 1 g of cellulose were added into water and stirred vigorously to produce a uniform suspension. The produced suspension and 10 g of 150-250 mm ACP were mixed in an 8? perform former under aeration. 8? perform was then formed by vacuum filtration followed by drying in a heat drum. The preform is then sintered in hydrogen atmosphere at 900oC for two hours to get rid off the cellulose in the preform and nickel fibers to come into contact and form a sinter-locked matrix. By utilizing the wet layer papermaking process, these particulates can be entrapped uniformly in the fibrous matrix as shown in the SEM micrograph in Figure 8. 39 Figure 8: SEM micrographs of Microfibrous Media A: Microfibrous entrapped ACP B: Microfibrous Matrix Activated carbon particulates (150-250 ?m) 12 ?m nickel fiber 8 ?m nickel fiber 90 ?m A 10 nm B 8 ?m nickel fiber 12 ?m nickel fiber 40 Figure 8B shows 12 ?m and 8 ?m nickel fibers. Figure 8A shows a microfibrous media consisting of 12 ?m and 8 ?m nickel fibers forming a 3-D sinter-locked matrix structure to entrap activated carbon particles of 150-250 ?m. Breakthrough Tests The performance of adsorptive materials used for carbon dioxide removal was studied at 20-30oC in a packed-bed adsorber (1.5 cm ID) using simulated gas containing 1.5 vol.% CO2 (C0). The gas flux was then saturated with water vapor up to 2 vol.% (84% RH) prior to entering the reactor inlet at room temperature as shown in Figure 9. Figure 9: Experimental apparatus He CO 2 - 150 o C Air Mass flow controller Thermal controller Moisture trap ~ Furnace Mass flow controller Vent CO2 Analyzer 010 H2O saturator 41 The outlet CO2 concentration was determined by CO2 analyzer (PP Systems, USA) shown in Figure 10. Figure 10: Non-dispersive Infrared CO2 Analyzer The CO2 analyzer was calibrated against air and can detect CO2 concentration as low as 1 ppm by utilizing Non-dispersive infra-red gas analysis to detect CO2. The downstream concentration (C) was recorded simultaneously at the sampling rate of 1 data point per second. The test results are expressed in term of variations of C/C0 over time (breakthrough curves). The breakthrough concentration is defined at 50 ppm; otherwise stated. 42 Sorbent Characterization The structural phase composition of the sorbents was then characterized by X-ray diffraction (XRD) using CuK? radiation from 20o to 60o with the scanning rate of 4o/min. Figure 11 shows XRD patterns of K2CO3*1.5H2O, K2CO3, and KHCO3, all of which are used as references for phase identification. 20 25 30 35 40 45 50 2? (degree) Int en sit y/a .u. KHCO3 K2CO3 K2CO3*1.5H2O Figure 11: XRD patterns of K2CO3*1.5H2O, K2CO3, and KHCO3 Differential Scanning Calorimetry (DSC) was then used to determine the thermal stability of the sorbents at temperatures between 35 and 500oC at the heating rate of 10oC/min to determine the regeneration temperature. Scanning Electron Microscopy (SEM) was then 43 employed to examine the robustness and structural integrity of the microfibrous media after cyclic adsorption/desorption. Composite Bed The composite bed consists of a packed bed of sorbent particulates followed by a polishing sorbent layer as shown in Figure 12. The influence of microfibrous media incorporating adsorption of solid sorbents on the breakthrough characteristics was studied. Figure 12: Composite Bed Design A series of supported-K2CO3 sorbents were prepared in order to examine and compare the performance of a packed bed of K2CO3/ACP sorbents, composite, and the packed bed of K2CO3/ACP with the same volume as the composite bed. A packed bed of Sodalime was then applied in a composite bed utilizing microfibrous media entrapped K2CO3/ACP (Packed Bed) Inlet Effluent (MM) 44 sorbents as a polisher and performed breakthrough tests against the packed bed of Sodalime of the same volume for comparison. 45 IV. PRELIMINARY BREAKTHROUGH TESTS OF SELECTED SORBENTS Selected solid sorbents, such as molecular sieves 3A, 4A, 5A, 13X, Sofnolime, Baralime, and Carbolime undergo breakthrough tests to measure their carbon dioxide adsorption capacity. The basic properties of these sorbents are shown in Table 11. Table 11: Characteristics of Selected Adsorbents Adsorbent Particle Size Density Manufacturer 3A 1.6 mm 0.6550 g/cc. Alfa Aesar 4A 1.6 mm 0.7045 g/cc Alfa Aesar 5A 1.6 mm 0.6685 g/cc. Alfa Aesar 13X 1.6 mm 0.5963 g/cc. Alfa Aesar Sofnolime 1.0-2.5 mm 0.8715 g/cc. Molecular Product (UK) Baralime 2.3-4.7 mm 0.9207 g/cc. Allied Healthcare Products Carbolime 2.5-5.0 mm 0.7805 g/cc. Allied Healthcare Products The breakthrough tests were conducted by loading five grams of each sorbent in a packed bed adsorber (1.5 cm. ID) using 1.5% CO2 challenge gas as a test gas. The breakthrough results are shown in Figure 13. 46 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 Time (min.) C/ C 0 3A 4A 5A 13X Sofnolime Baralime Carbolime Test Condition: - 6 cm/s face velocity - 1.5% CO2/He - 5g sorbents Figure 13: Breakthrough curves of selected adsorbents The breakthrough curves obtained in Figure 13 show a various shapes of breakthrough curves indicating differences in adsorption mechanisms of sorbents. Among molecular sieves, 3A and 13X offer sharp breakthrough curves indicating high sorbent utilization. In the case of Sodalime, Sofnolime offers a sharp breakthrough curve compared to those of Carbolime and Baralime indicative a higher sorbent utilization. The carbon dioxide adsorption capacity was then calculated and tabulated as shown in Table 12. 47 Table 12: Breakthrough Data of Selected Adsorbents Weight Volume BT time Saturation Capacity Utilization g cc. min g CO2/g sorbent g CO2/cc. % 3A 5.008 7.519 0.00 0.0000 0.0000 N/A 4A 5.009 7.097 3.04 0.0538 0.0379 24 5A 5.009 7.479 5.75 0.0660 0.0441 36 13X 5.009 8.385 2.75 0.0245 0.0146 54 Sofnolime 5.006 5.737 2.92 0.0296 0.0258 47 Baralime 5.011 5.431 2.50 0.0830 0.0764 15 Carbolime 5.003 6.406 1.08 0.0650 0.0507 11 Molecular Sieves (MS) It is shown that molecular sieves exhibit CO2 adsorption capacity in the sequence: 5A>4A>13X>3A. The use of MS utilizes the idea of physical adsorption of gas molecules with critical diameter less than the effective pore size of the MS. The number usually indicates the average pore size of the MS. In the case of carbon dioxide, CO2 molecules have an average critical diameter of 2.8A. As a result, the MS with the effective pore size larger than 2.8A have potential to adsorb such species. The use of molecular sieves for carbon dioxide removal in commercial applications is normally incorporated with other drying agents such as silica and 13X- MS. This is due to the competitive adsorption between CO2-MS and H2O-MS. NASA used two canister molecular sieves containing 5A for carbon dioxide removal and 13X for water removal during SKYLAB. The desorbed carbon dioxide was vacuumed to space. Figure 14 shows breakthrough curves of molecular sieves 5A in dry gas and wet gas of 1.5%CO2. 48 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 20 40 60 80 100 Time (min.) C/ C 0 with 84%RH dry gas stream Test Condition: - 6 cm/s face velocity - 1.5% CO2/He - 5g of 5A-MS Figure 14: Breakthrough curves of Molecular Sieves 5A in wet and dry gas It is shown that additional water caused a decrease in CO2 adsorption capacity as shown in the breakthrough curves. However, the breakthrough curve of additional water in the gas stream shows an increase in outlet concentration over the initial concentration (C/C0>1) indicating carbon dioxide enrichment. In the presence of water, a competitive adsorption on the MS-5A between H2O and CO2 take place. However, molecular sieves adsorb water preferentially. As a result, once the packed bed of MS-5A is fully saturated; MS-5A is then released carbon dioxide in order to accommodate water as shown in an overshoot of the carbon dioxide. 49 Sodalime Sodalime is usually used for carbon dioxide removal processes from a close- circuit breathing systems by utilizing chemical reaction on the solid sorbents based on hydroxides of alkali metals. The major component of these sorbents is calcium hydroxide activated with sodium hydroxide or potassium hydroxide as shown in the following reactions: 2NaOH + CO2 barb2right Na2CO3 + H2O KOH + CO2 barb2right K2CO3 + H2O Ca(OH)2 + CO2 barb2right CaCO3 + H2O There are many types of Sodalime depending on the hydroxide of alkali metal components, most of which are often used in military applications, such as in submarines and diving apparatus as well as in medical and chemical applications to purify anesthetic and industrial gases. The breakthrough data in Figure 13 show that carbon dioxide adsorption capacity in a sequence: Baralime>Carbolime> Sofnolime. Baralime is another type of sorbents with barium hydroxide as an additive to increase the adsorption rate. Baralime consists of Ba(OH)2*8H2O and Ca(OH)2 and reacts with the carbon dioxide by the following reactions: Ba(OH)2*8H2O + CO2 barb2right BaCO3 + 9H2O H2O + CO2 bleftright H2CO3 H2CO3 bleftright H+ + HCO3- Ka = 4.3x10-7 HCO3- bleftright H+ + CO32- Ka = 5.6x10-11 50 2H+ +CO32- + Ca(OH)2 barb2right CaCO3 + 2H2O 2H+ +CO32- + KOH barb2right K2CO3 + 2H2O K2CO3 + Ca(OH)2 barb2right CaCO3 + 2KOH Due to the crystallization water of barium hydroxide, the reaction between barium hydroxide can take place rapidly; however, calcium hydroxide has to form a water environment of dissolve in water surface for chemisorption with carbon dioxide to take place. Carbonic acid, a product of carbon dioxide dissolved in water is chemically bonded on the surface of the sorbents as a surface layer. As a result, the diffusion through the surface layer controls the rate of adsorption. The adsorption rate depends on the solubility of the main hydroxide of alkali metals (Ca(OH)2). Selected solubilities of some hydroxide of alkali metals are shown in Table 13. Table 13: Solubility of selected alkali metals Solubility in water (20oC) Hydroxide of alkali metal g/100 g H2O Ba(OH)2*8H2O 3.89 KOH 112 Ca(OH)2 0.165 NaOH 109 51 For the sorbents consisting of sodium hydroxide, the chemical reactions are similar to those presented in the case of Baralime. The difference is on the activation reaction at the beginning of the reaction: H2O + CO2 bleftright H2CO3 H2CO3 bleftright H+ + HCO3- HCO3- bleftright H+ + CO32- 2H+ +CO32- + 2NaOH barb2right Na2CO3 + 2H2O Na2CO3 + Ca(OH)2 barb2right CaCO3 + 2NaOH A better solubility of sodium hydroxide than calcium hydroxide is important for the chemisorption of carbon dioxide. Form the above reactions, water and sodium hydroxide have to be dissociated in order to yield high reaction efficiency by controlling the proper concentration of sodium hydroxide and water. As a result, some manufacturers add moisture (12-19%) in the sorbents to improve dissociation of sodium hydroxide and enhance the adsorption capacity. Figure 15 shows breakthrough curves of Sofnolime under wet and dry conditions. It is shown that in the presence of water, the dissociation of sodium hydroxide is enhanced and results in higher carbon dioxide adsorption capacity. 52 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 50 100 150 200 250 300 Time (min.) C/ C 0 Test Condition: - 6 cm/s face velocity - 1.5% CO2/He - 8g of Sofnolime dry gas stream with 84%RH Figure 15: Breakthrough curves of Sofnolime in wet and dry gas 53 Summary: Based on the data obtained from preliminary results, it is shown that molecular sieves 5A offers highest carbon dioxide adsorption capacity; however, for the use of molecular sieves, a guard bed is needed to remove moisture prior to removing carbon dioxide. In the case of Sodalime, water is rather necessary to create the water later on the surface of the sorbents and allows carbon dioxide adsorption reaction. As a result, many manufacturers add water to form crystallization water in the hydroxides of alkali metal to enhance the carbon dioxide adsorption capacity of the sorbents. 54 III. INFLUENCE OF POROUS HOST MATRIX The development of the nano-dispersed chemically active compound into pores of the porous matrix reduces the internal mass transfer diffusion, improves utilization of the active compound, and lowers thermal effects (low regeneration temperature and ?H of reactions). As a result, the composite sorbents were developed based on different types of porous support materials. The support materials were obtained from different manufacturers as shown in Chapter III. The support materials were then pulverized to obtain the size of 150-250 ?m and 1 mm. The prepared composite sorbents consist of different amount of K2CO3 by varying the concentration of the impregnation solution. The composite sorbents were then drained the excess water prior to drying at 100oC for 30 minutes. The carbon dioxide adsorption capacity tests were conducted at 20-30oC in a packed bed adsorber of 60 cm length and 1.5 cm in diameter. The amount of the composite sorbent was maintained at 10 cc; otherwise stated. The adsorber was purged by He gas with the flow rate of 600 cc/min to remove the remaining of carbon dioxide in the adsorber. The breakthrough tests were conducted under wet gas (84 %RH) containing 1.5% CO2. From the breakthrough curves, the CO2 saturation capacity of the composite sorbents was then calculated as a ratio of the amount of CO2 adsorbed by the sorbents and the amount of the sorbents loaded. 55 RESULTS AND DISCUSSION K2CO3-on-SiO2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 1 2 3 4 5 6 time(min.) C/ C0 10 wt.% 20 wt.% 30 wt.% 50 wt.% Challenge: 1.5% CO2 in He Flow rate = 586 cc/min. bed thickness = 5.34 cm. Cross sectional area = 1.767 cm2 5 wt.% SiO2 Figure 16: Breakthrough curves of K2CO3/ACP at different K2CO3 loadings From the breakthrough curves shown in Figure 16, it is shown that the silica composite sorbents show sharp breakthrough curves indicative of high utilization of the composite sorbents, but low carbon dioxide adsorption capacity was observed on all samples. An increase in K2CO3 loading causes a decrease in carbon dioxide adsorption capacity as seen in the shift in breakthrough curves to the left side. It should be noted that there was an increase in volume after drying the prepared sorbents at 100oC for 30 minutes. The carbon dioxide adsorption capacity as a function of K2CO3 loading was then calculated from the breakthrough curves as shown in Figure 17. 56 0 0.002 0.004 0.006 0.008 0.00 0.10 0.20 0.30 0.40 0.50 0.60 K2CO3 Loadings (wt.%) CO 2 ca p. (g C O2 /to tal g so rb en ts) Figure 17: CO2 adsorption capacity of SiO2 composite sorbents as a function of K2CO3 loadings Silica particles often used as a drying agent offer carbon dioxide adsorption capacity even though it is considerably low (residence time = 0.5 min) as shown in Figure 17. This might be due to the competitive adsorption between SiO2-H2O and SiO2-CO2. An increase in 5 wt.% K2CO3 loading shows a slight increase in carbon dioxide adsorption capacity, which does not exceed 0.0069 g CO2/ total g of the composite sorbent. An increase in K2CO3 loaded onto silica from 5 wt.% to 10 wt.% caused a decrease in carbon dioxide adsorption capacity. A further increase in K2CO3 loading shows a further decrease in carbon dioxide adsorption capacity. The drop in capacity might be related to the chemical transformation during sorbent preparation between 57 potassium carbonate and silica forming an inactive phase on the surface of the composite sorbents. As a result, the silica composite sorbents were then analyzed by XRD to examine the chemical phase transformation. XRD patterns in Figure 18 show the results of SiO2 particles (for comparison) and K2CO3 impregnated SiO2 with different K2CO3 loadings between 20 and 60 degrees. Silica particulates exhibit an amorphous structure with a broad peak indicating a presence of silica between 20 and 25o. An addition of K2CO3 onto silica particulates causes the surface reaction between SiO2 and K2CO3 forming inactive K2SiO3 on the surface of SiO2 as shown in the following reaction [34]: K2CO3 + SiO2 bleftright K2SiO3 + CO2 20 25 30 35 40 45 50 55 60 2? (degree) int en sit y/a .u. SiO2 50 wt.% 5 wt.% 20 wt.% 40 wt.% Figure 18: XRD patterns of SiO2 and K2CO3 impregnated SiO2 58 The formation of potassium silicate is shown in a decrease in silica peak (23o); however no other phases were observed. Based on the breakthrough data and the XRD results, it is shown that silica alone can adsorb a low amount of carbon dioxide; however an increase in K2CO3 loading higher than 5 wt.% causes the carbon dioxide adsorption capacity to decrease lower than that of silica. This indicates the chemical phase transformation on the surface of silica forming an inactive material. With 5 wt.% K2CO3 loading only a small amount of potassium silicate is formed in the pores of the composite sorbents and the rest of the composite sorbents can still be utilized as shown in an slight increase in carbon dioxide adsorption capacity compared to that of silica. A low activity of the silica based sorbents probably derives from the large pore size of the silica, which lowers the dispersion of the impregnated salts. As a result, a further increase in K2CO3 loading causes the reduction in active composite sorbents (less pore volume, most of which were occupied by potassium silicate) due to the formation of inactive potassium silicate on the surface of silica, which results in a decrease in carbon dioxide adsorption capacity. 59 K2CO3-on-Al2O3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 50 100 150 200 time (min.) C/ C0 Al2O3 16.24 wt.% 30.43 wt.% 42.75 wt.% 63.50 wt.% 53.68 wt.% Figure 19: Breakthrough curves of K2CO3/Al2O3 at different K2CO3 loadings Figure 19 shows the breakthrough curves of the Al2O3-composite sorbents at various K2CO3 loadings. The alumina composite sorbents show high carbon dioxide adsorption capacity compared to those of silica composite sorbents. The composite sorbents were observed to increase in volume after drying in the oven at 100oC for 30 minutes. The carbon dioxide adsorption capacity was then calculated as a function of K2CO3 loading as shown in Figure 20. 60 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0% 10% 20% 30% 40% 50% 60% 70% K2CO3 loading (wt.%) CO 2 ca p. (g C O2 /to tal g so rb en t) Figure 20: CO2 adsorption capacity of Al2O3 composite sorbents as a function of K2CO3 loadings Based on Figure 20, it is shown that an increase in K2CO3 loading shows an increase in carbon dioxide adsorption capacity. The composite sorbents can reach 0.17 g CO2/total g of sorbent. These alumina based composite sorbents might be the best candidate for carbon dioxide adsorption application due to their high adsorption capacity. However, an increase in volume after drying might be due to chemical reaction between potassium carbonate and alumina causing a chemical phase transformation. As a result, the composite sorbents were then analyzed by XRD. 61 20 25 30 35 40 45 50 2? (degree) Int en sit y/ a.u . Al2O3 50 wt.% K2CO3/Al2O3 10 wt.% K2CO3/Al2O3 20 wt.% K2CO3/Al2O3 30 wt.% K2CO3/Al2O3 K2CO3*1.5H2O Al2O3 KAl(CO3)2*1.5H2O Figure 21: XRD patterns of Al2O3 and K2CO3 impregnated Al2O3 with different loadings The XRD patterns in Figure 21 shows an amorphous nature of alumina particles as shown in the broad peaks between 35-40o and 45-50o respectively. Figure 21 shows that at 10 wt.% K2CO3, the XRD pattern of the composite sorbent is similar to that of the pure Al2O3 particles. This is due to the high dispersion of the K2CO3 in the porous matrix of alumina and the low K2CO3 loading resulting in no K2CO3 reflections detected. An increase in K2CO3 loading (>10 wt.%) leads to the mixture of the phase between Al2O3 (37o) and K2CO3*1.5H2O (32.4o). The K2CO3 can incorporate into the surface lattice of Al2O3 and diffuse into the bulk of Al2O3 with the formation of potassium alumino carbonate, but the reaction is limited by the amount of K2CO3 added due to the pore volume. Shaeronov et al [35] 62 investigated the same system and concluded that the K2CO3 and Al2O3 react at low temperature forming potassium alumino carbonate as shown in the following reaction: xK2CO3 + yAl2O3 bleftright xK2O*yAl2O3*(x-y)CO2 + yCO2 At 10 wt.% K2CO3 loading, the formation of potassium alunino carbonate is confined in the pores of alumina and due to the low K2CO3 loading only a small amount of potassium alumino carbonate is formed. A further increase in K2CO3 loading promotes the formation of potassium alumino carbonate in the pores of alumina resulting in a decrease in pore volume. The reduction in pore volume causes a condensation of K2CO3(aq) out of the pores of alumina forming K2CO3*1.5H2O(s). A further increase in K2CO3 loading causes more K2CO3*1.5H2O to precipitate as shown in a decrease in weaker Al2O3 reflections and stronger K2CO3*1.5H2O reflections. These precipitates react with CO2 as shown in the following reaction: K2CO3*1.5H2O + CO2 barb2right HCO3- + H+ + K2CO3 + 0.5H20 The amount of CO2 adsorbed depends on the formation of potassium sesquihydrate. A result of additional K2CO3 loading causes potassium sesquihydrate to precipitate and yields higher CO2 capacity. The process is not reversible. This transformation will be further discussed in Chapter VIII. 63 K2CO3-on-Activated Carbon Particles 0 0.2 0.4 0.6 0.8 1 0 2 4 6 8 10 12 Time (min.) C/ C0 ACP 10 wt.% 20 wt.% 30 wt.% 40 wt.%50 wt.% Figure 22: Breakthrough curves of K2CO3/ACP at different K2CO3 loadings The ACP composite sorbents underwent breakthrough tests as shown in Figure 22. It was found that an increase in K2CO3 loading yields an optimal K2CO3 loading around 30 wt.%--- this composite sorbent can reach 0.01 g CO2/total g of the sorbent and yield 30% utilization at room temperature. Unlike K2CO3/SiO2 and K2CO3/Al2O3, an increase in volume after drying ACP composite sorbents at 100oC for 30 minutes of the K2CO3/ACP sorbents was not observed. The ACP composite sorbents yield higher carbon dioxide adsorption capacity than that of silica composite sorbents, but lower than that of alumina composite sorbents. The ACP composite sorbents show sharp breakthrough curves indicative of high utilization of the sorbents. This type of composite sorbents will be further discussed in Chapter VI. 64 Thermal stability One of the most important attributes for adsorbents is the regenerability. To determine the regenerability and their mechanisms at elevated temperatures, spent K2CO3/ACP and K2CO3/Al2O3 composite sorbents were then tested by DSC at the heating rate of 10oC/min from 35 to 400oC as shown in Figure 23. 50 100 150 200 250 300 Temperature (oC) He at Flo w/ a. u. Exo spent K2CO3/Al2O3 spent K2CO3/ACP 1 32 Figure 23: DSC spectra of spent K2CO3/ACP and K2CO3/Al2O3 1: removal of physically adsorbed water 2: side reaction 3: decomposition of KHCO3 in Ar It was found that the K2CO3/ACP sorbents exhibit three exothermic peaks. The first broad peak between 50 and 130oC is associated with the removal of physically adsorbed water. The second peak between 130-160oC is associated with the dehydration of 65 crystallization water in the composite sorbent. The third peak between 160 and 220oC corresponds to the decomposition of KHCO3. For K2CO3/Al2O3 sorbents, the DSC study exhibits similar phenomenon as the K2CO3/ACP sorbents. The DSC spectrum shows three exothermic peaks at 50-130oC, 130-160oC, and 170-220oC corresponding to the removal of physically adsorbed water, dehydration of crystallization water, and decomposition of KHCO3 respectively. However, the K2CO3/Al2O3 composite sorbent exhibits a strong exothermic reaction between 130 and 160oC, which results from the reaction between KHCO3 and Al2O3 as follows: 2xKHCO3 + yAl2O3 bleftright xK2O*yAl2O3*(x-y)CO2 + (x+y)CO2 The product of this reaction is inactive and it will be further discussed in Chapter VIII. 66 Summary: The nature of the host matrix has a strong influence on the CO2 adsorption capacity and sorbent regenerability. The use of three host matrix supports shows an increase in CO2 adsorption capacity in a sequence: SiO27.5 ACP K2CO3(aq) pore K2CO3(s) 1.5