EQUILIBRIUM AND MASS TRANSFER BEHAVIOUR OF CO 2 ADSORPTION ON ZEOLITES, CARBON MOLECULAR SIEVE, AND ACTIVATED CARBONS. A Thesis

Transcription

1 EQUILIBRIUM AND MASS TRANSFER BEHAVIOUR OF CO 2 ADSORPTION ON ZEOLITES, CARBON MOLECULAR SIEVE, AND ACTIVATED CARBONS A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science In Process Systems Engineering University of Regina Md Ariful Islam Sarker Regina, Saskatchewan October, 2012 Copyright 2012: Sarker

2 UNIVERSITY OF REGINA FACULTY OF GRADUATE STUDIES AND RESEARCH SUPERVISORY AND EXAMINING COMMITTEE Md Ariful Islam Sarker, candidate for the degree of Master of Applied Science in Process Systems Engineering, has presented a thesis titled, Equilibrium and Mass Transfer Behaviour of CO 2 Adsorption on Zeolites, Carbon Molecular Sieve, and Activated Carbons, in an oral examination held on September 6, The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: Supervisor: Committee Member: Committee Member: Dr. Fanhua Zeng, Petroleum Systems Engineering Dr. Adisorn Aroonwilas, Industrial Systems Engineering Dr. David demontigny, Industrial Systems Engineering Dr. Amornvadee Veawab, Environmental Systems Engineering Chair of Defense: Dr. Shaun Fallat, Department of Mathematics & Statistics *Not present at defense

3 ABSTRACT Natural gas is an important source of energy that usually requires purification steps to remove contaminants prior to pipeline transmission and industrial usage. By pressure swing adsorption process (PSA), carbon dioxide (CO 2 ) can be separated from natural gas using solid materials commonly known as adsorbents. Adsorption capacity (or equilibrium) and adsorption kinetics of the adsorption materials have great impacts on the efficiency of CO 2 removal in this PSA process. The objective of this study was to characterize the CO 2 adsorption equilibrium and kinetics of commercial adsorbents that have potential for use in the PSA process and also to provide a better understanding of CO 2 adsorption behaviour under wide range of operating conditions. A comprehensive set of data and analysis for CO 2 adsorption equilibrium and kinetics is presented in this study for zeolite 13x, zeolite 5A, zeolite 4A, carbon molecular sieve (MSC-3R), activated carbon (GCA-830), and activated carbon (GCA- 1240). By using volumetric measurement technique, adsorption equilibrium and kinetic data were taken at a temperature range of K and pressure up to 35 atm. The obtained experimental data were correlated as a function of temperature and pressure to fit with different model equations (i.e., Langmuir, Toth, Sips, and Prausnitz). The isosteric heat of CO 2 adsorption was also estimated for individual adsorbents according to the Clausius-Clapeyron equation. The CO 2 adsorption kinetic, presented in terms of mass transfer coefficients (k), were experimentally measured at a temperature range of K and pressure up to 11 atm. The mass transfer was analyzed from the plots of CO 2 uptake rate using the well-recognized linear driving force (LDF) model. The mass i

4 transfer coefficients were correlated by non-linear regression to reveal the effects of adsorption temperature and pressure. Activation energies of CO 2 adsorption on the individual adsorbents were also calculated and correlated according to the Arrhenius equation. ii

5 ACKNOWLEDGEMENTS First of all, I would like to give my gratitude to my supervisor, Dr. Adisorn Aroonwilas for his great support, direction, and encouragement to pursue my research work and study. I am also thankful to Dr. Amornvadee Veawab for her significant direction regarding my research work throughout my M.A.Sc. program. I owe much to SaskEnergy Incorporated in Regina for their financial support. I am grateful to the Faculty of Graduate Studies and Research at the University of Regina for the financial support through graduate scholarships and research awards. I am also thankful to the Faculty of Engineering and Applied Science for providing me this opportunity. I am also thankful to all my friends and research team at the Energy Technology Laboratory at the University of Regina for their assistance. Finally I would like to give thanks to my beloved parents, sisters, and brothers for their love and support throughout my M.A.Sc. program. iii

6 TABLE OF CONTENTS Page ABSTRACT ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES NOMENCLATURE i iii iv vii ix xii 1. INTRODUCTION Need for CO 2 removal from natural gas Technology for CO 2 separation Gas adsorption technology Research motivation Objective and scope CO 2 ADSORPTION CHARACTERISTICS AND LITERATURE 12 REVIEWS 2.1 Adsorption equilibrium Langmuir isotherm Volmer Isotherm Hill de-boer & Fowler-Guggenheim Isotherm Freundlich isotherm Sips isotherm 15 iv

7 2.1.6 Toth isotherm Prausnitz isotherm Unilan isotherm Adsorption kinetics Isosteric heat of adsorption Activation energy Literature review on adsorbents for CO 2 removal CO 2 adsorption by zeolite 13X CO 2 adsorption by zeolite 5A CO 2 adsorption by zeolite 4A CO 2 adsorption by carbon molecular sieve CO 2 adsorption by activated carbon CO 2 ADSORPTION EXPERIMENTS AND PROCEDURES Materials Experimental apparatus Experimental procedures Preparation of adsorption cell Determination of void volume Determination of CO 2 adsorption performance Experimental condition Validation of experimental method RESULTS AND DISCUSSION 46 v

8 4.1 CO 2 adsorption equilibrium Isotherm curves Isotherm correlations Isosteric heat of adsorption CO 2 adsorption kinetics Mass transfer coefficient for CO 2 adsorption Mass transfer coefficient and activation energy CONCLUSION AND FUTURE WORK Conclusions Recommendations for future work 90 REFERENCES 91 APPENDIX A: Experimental results of pure CO 2 adsorption equilibrium 101 vi

9 LIST OF TABLES Page Table 1.1 Typical natural gas composition (in mol%) worldwide 3 Table 1.2 Typical pipeline gas specifications 5 Table 2.1 Previous works for the CO 2 adsorption on zeolite 13X 28 Table 2.2 Previous works for the CO 2 adsorption on zeolite 5A 29 Table 2.3 Previous works for the CO 2 adsorption on zeolite 4A 30 Table 2.4 Previous works for the CO 2 adsorption on carbon molecular sieve 31 Table 2.5 Previous works for the CO 2 adsorption on activated carbon 32 Table 3.1 Physical properties of different adsorbents 36 Table 3.2 CO 2 adsorption capacity test condition of different adsorbents 43 Table 4.1 Regression parameters for different model equations at 293 K 58 Table 4.2 Regression parameters for different model equations at 303 K 59 Table 4.3 Regression parameters for different model equations at 313 K 60 Table 4.4 Regression parameters for different model equations at 323 K 61 Table 4.5 Regression parameters for different model equations at 333 K 62 Table 4.6 Isotherm Correlation equations for adsorbents tested 63 Table 4.7 Isosteric heat of CO 2 adsorption on the adsorbents 66 Table 4.8 Mass transfer coefficients of CO 2 adsorption on the tested adsorbents 81 Table 4.9 Activation energy (E a ) and frequency factor (A) for the tested 85 adsorbents Table A.1 CO 2 adsorption equilibrium data on zeolite 13X at different pressure 101 vii

10 and temperature Table A.2 CO 2 adsorption equilibrium data on zeolite 5A at different pressure 102 and temperature Table A.3 CO 2 adsorption equilibrium data on zeolite 4A at different pressure 103 and temperature Table A.4 CO 2 adsorption equilibrium data on MSC-3R at different pressure 104 and temperature Table A.5 CO 2 adsorption equilibrium data on GCA-830 at different pressure 105 and temperature Table A.6 CO 2 adsorption equilibrium data on GCA-1240 at different pressure 106 and temperature viii

11 LIST OF FIGURES Page Figure 1.1 Primary sources of energy in the world in Figure 1.2 Carbon dioxide gas removal technologies 7 Figure 2.1 Internal view of adsorbent particles 20 Figure 3.1 Photographs of adsorbents used in this study 37 Figure 3.2 Experimental apparatus for the measurement of CO 2 adsorption on 38 adsorbents Figure 3.3 Comparison of CO 2 adsorption isotherm data with published data 45 for (a) zeolite 13X and (b) GCA Figure 4.1 Isotherm curves of CO 2 adsorption on zeolite 13X at different 47 temperatures Figure 4.2 Isotherm curves of CO 2 adsorption on zeolite 5A at different 48 temperatures Figure 4.3 Isotherm curves of CO 2 adsorption on zeolite 4A at different 49 temperatures Figure 4.4 Isotherm curves of CO 2 adsorption on MSC-3R at different 50 temperatures Figure 4.5 Isotherm curves of CO 2 adsorption on GCA-830 at different 51 temperatures Figure 4.6 Isotherm curves of CO 2 adsorption on GCA-1240 at different 52 ix

12 temperatures Figure 4.7 Comparison of isotherm curves among the adsorbents tested (a) K; (b) 333 K. Figure 4.8 Plots of lnp versus 1/T for (a) zeolite 13x, (b) zeolite 5A, (c) 65 zeolite 4A, (d) molecular sieve carbon MSC-3R, (e) activated carbon GCA- 830 and (f) activated carbon GCA-1240 Figure 4.9 Correlation plot of isosteric heat of adsorption versus adsorbate 68 loading Figure 4.10 Plots of CO 2 uptake for zeolite 13X at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm; (e) 10.9 atm. Figure 4.11 Plots of CO 2 uptake for zeolite 5A at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm; (e) 10.9 atm. Figure 4.12 Plots of CO 2 uptake for zeolite 4A at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm. Figure 4.13 Plots of CO 2 uptake for MSC-3R at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm; (e) 10.9 atm. Figure 4.14 Plots of CO 2 uptake for GCA-830 at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm; (e) 10.9 atm. Figure 4.15 Plots of CO 2 uptake for GCA-1240 at (a) 3.4 atm; (b) 5.4 atm; (c) atm; (d) 8.8 atm; (e) 10.9 atm. Figure 4.16 Linear plot of ln (1- ) versus time of CO 2 adsorption at 293 K and 78 different pressures x

13 Figure 4.17 Linear plot of ln (1- ) versus time of CO 2 adsorption at 313 K and 79 different pressures Figure 4.18 Linear plot of ln (1- ) versus time of CO 2 adsorption at 333 K and 80 different pressures Figure 4.19 Linear plot of lnk versus ( ) for CO 2 adsorption activation energy 84 on (a) zeolite 13X; (b) zeolite 5A; (c) zeolite 4A; (d) MSC-3R; (e) GCA- 830; (f) GCA-1240 Figure 4.20 Effect of pressure on activation energy and frequency factor for (a) 86 zeolite 13X; (b) zeolite 5A; (c) zeolite 4A; (d) MSC-3R; (e) GCA- 830; (f) GCA-1240 Figure 4.21 Comparison of activation energy of six different adsorbents 87 xi

14 NOMENCLATURE A frequency factor or collision factor, sec -1 Å A p angstrom adsorption potential, kj/mol B affinity of the adsorbed molecules to the solid surface, atm -1 B affinity constant at infinite temperature b average affinity of the adsorbed molecules, atm -1 C celsius Cμi C μ amount adsorbed by the component i, mol/kg adsorbed amount of pure component i at the hypothetical pressure, mol/kg CμT CO 2 D c total amount adsorbed, mol/kg carbon dioxide intra-crystalline concentration dependant diffusion coefficient, m 2 /sec D P DEA DGA DIPA E a H macro-pore diffusion coefficient, m 2 /sec diethanol amine diglycol amine di-isopropanol amine activation energy of adsorption, kj/mol isosteric heat of adsorption, kj/ mol xii

15 He helium gas k overall mass transfer coefficient, sec -1 k f K H k MDEA MEA n n external film mass transfer coefficient, m/sec Henry s constant, mol/kg.atm temperature dependant isotherm constant methyl diethanol amine monoethanol amine total number of moles per unit mass of the adsorbent number of mole of pure component j per unit mass of the adsorbent, mol/kg N P PS PSA P P q q * q R RTD R P loading of gas molecule, mol/kg equilibrium pressure, atm pressure sensor pressure swing adsorption hypothetical pressure of the pure component i, atm adsorption pressure of the pure component j, atm amount of adsorbed gas, mol/kg equilibrium amount of adsorbed gas, mol/kg maximum capacity at corresponding temperature, mol/kg gas constant, m 3 atm K 1 mol 1 resistance temperature detector macro particle radius, m xiii

16 r c s t T TS TSA VSA w x i x j y i y j micro-crystal radius, m quantity of heterogeneity of the system temperature dependant isotherm constant adsorption temperature, K temperature sensor thermal swing adsorption vacuum swing adsorption interaction energy between adsorbed molecules, kj/mol molar fraction of component i in the adsorbed phase molar fraction of component j in the adsorbed phase molar fraction of component i in the gas phase molar fraction of component j in the gas phase z reduced spreading pressure, mol/m 3 Greek Letters γ i ε p θ activity coefficient of the component i porosity of adsorbent particle fractional coverage π spreading pressure, kj/m 3 ɸ surface potential of the adsorbed phase per unit mass of the adsorbent, kj ɸ surface potential of the pure component j, kj xiv

17 1. INTRODUCTION The world s primary energy sources can be classified into three categories: i) fossil fuels such as oil, coal, and gas, ii) renewable sources including biomass, geothermal energy, solar energy, hydro energy, tidal energy, and wind energy, and iii) nuclear energy (Fridleifsson, 2003). Fossil fuels share the largest energy contribution, accounting for more than 80 per cent (Figure 1.1). Burning fossil fuels for electricity and other forms of energy has been the most accepted and widespread practice in every industry for decades, and in some cases centuries. Recently, the use of fossil fuels has been recognized as a major threat to the environment regarding the associated excessive greenhouse gas emissions. The combustion of fossil fuels leases a large amount of carbon dioxide (CO 2 ), one of the greenhouse gases contributing to global warming and climate change. The increasing world energy demand could lead to a substantial increase in CO 2 emissions from 26.6 gigatonnes per year in year 2003 to 40.4 gigatonnes per year by 2030 (Quadrelli and Peterson, 2007). One strategy to reduce greenhouse gas emissions is to use so called clean fuel such as natural gas since it generates relatively low CO 2 emissions compared to other fossil fuels (Mokhatab et al., 2006). Natural gas is commonly produced from underground reservoirs. It is composed of methane (CH 4 ) as the primary constituent and heavier hydrocarbons such as ethane, propane, and butane, as well as non-hydrocarbons such as nitrogen, hydrogen sulfide, helium, and CO 2. The typical natural gas compositions are listed in Table 1.1. Note that raw natural gas can contain significant amounts of CO 2 that must be removed prior to the delivery to customers. 1

18 Hydro, 2.1% Other, 1.2% Biofuels, 4.7% Nuclear, 11.0% Coal/peat, 20.2% Oil, 36.3% Natural Gas, 24.5% Figure 1.1 Primary sources of energy in the world in 2010 (Redrawn from key world energy statistics, IEA, 2011) 2

19 Table 1.1: Typical natural gas composition (in mol%) worldwide Gas Canada Western Southwest Bach Ho Miskar Rio Arriba Cliffside Components (Alberta) Colorado Kansas Field a Field County, Field, Vietnam Tunisia New Mexico Texas Helium Nitrogen Carbon dioxide Hydrogen sulfide Methane Ethane Propane Butanes Pentanes & heavier a Tabular mol% data is on a wet basis (1.3 mol% water) Source: Kidnay and Parrish,

20 1.1 Need for CO 2 removal from natural gas CO 2 is an incombustible gas, providing no thermal energy during the combustion process. The presence of CO 2 in natural gas product, therefore, results in the reduced heating value per unit volume of natural gas. The presence of CO 2 can also present several challenges regarding pipeline transportation of natural gas. The large amount of CO 2 in the gas product simply occupies partial volume of the pipeline, reducing the transport efficiency per unit volume of natural gas. In addition, CO 2 in the gas product can form carbonic acid in the presence of water and then corrode pipeline material (Mokhatab et al., 2006). To overcome the corrosion problem, special materials are required for process equipment and pipelines, which leads to an increase in capital investment. If corrosion does occur due to the presence of CO 2, the natural gas pipeline system (including pipe, valves, and associated equipment) must be maintained and replaced. The replacement downtime hinders natural gas production and delivery, causing the production costs to rise. In some cases, natural gas product is liquefied to form so called liquid-natural-gas (LNG) to increase transport and storage efficiency. The liquification process usually takes place at an extremely low temperature of 112 K (-161 C). At this temperature, if CO 2 remains in the natural gas product, it will form dry ice that freezes on heat exchanger surfaces and clogs pipelines and process equipment (Kidnay and Parrish, 2006). To avoid all these difficulties, raw natural gas must be processed prior to commercial use to remove CO 2, and its CO 2 content must be kept within the recommended pipeline specifications. Table 1.2 shows the typical pipeline gas specifications summarized by Younger (2004). Notice that the presence of CO 2 should be maintained below 2 vol% (or mole %). 4

21 Table 1.2: Typical pipeline gas specifications (Younger, 2004) Specification Trans Alberta West coast West coast Canadian Item Canada & Southern for US use for BC use Western Minimum heating value (MJ/m 3 ) Hydrocarbon Dew point (K at 54.4 atm) Free of Liquids Free of Liquids Water Content (kg/mmm 3 ) Hydrogen Sulphide (Grains per 2.8 m 3 ) Mercaptans (Grains per 2.8 m 3 ) Total Sulphur (Grains per 2.8 m 3 ) Carbon dioxide (mol %) Oxygen content (mol %) Delivery Temperature (K) Delivery Pressure (atm) varies varies 34 5

22 1.2 Technology for CO 2 separation Technology selection for CO 2 removal depends on the type and concentration of the impurities in the natural gas, temperature and pressure of the feed gas, required specifications of the gas product, volume of gas to be treated, cost (capital and operating), and environmental regulations. From the technical viewpoint, the removal of CO 2 can be achieved by a number of approaches, as shown in Figure 1.2. Such techniques include absorption into a liquid solvent, adsorption onto a solid adsorbent, low-temperature cryogenic distillation, permeation through a membrane, and chemical conversion (Kohl and Nielsen, 1997). In general, gas absorption using a physical solvent and adsorption technologies are suitable for gas streams with high CO 2 partial pressure (Kidnay and Parrish, 2006). Raw natural gas is usually available in the gas reservoirs with pressures up to 5000 psig or 340 atm, which provides considerably high partial pressure of CO 2 (Younger, 2004). The absorption technology is commonly used for processing high-volume gas streams while the adsorption onto solids is more suitable for smaller-scale applications. Removing CO 2 by adsorption helps maintain the high pressure level of the gas stream without using an additional compression step before delivering the methane product to the customers. Also, adsorption technology provides low regeneration cost and non-corrosive behavior, which makes this technology more applicable for the removal of CO 2 from natural gas. In this study, focus has been limited to the adsorption process for separating CO 2 from highpressure natural gas. 6

23 MEA DGA Amine DEA DIPA Chemical MDEA Benfield Solvent Absorption Physical Hybrid PSA Alkali Salt Selexol Rectisol Ifpexol Purisol Catacarb Vetrocoke Flexorb HP CO 2 Removal Technology Solid Adsorption Cryogenic TSA VSA Membrane Direct Conversion Molecular Gate Cellulose acetate Polyimide Polysulfone Stretford Figure 1.2 Carbon dioxide gas removal technologies (Redrawn from Kohl and Nielsen, 1997) 7

24 1.3 Gas adsorption technology Gas adsorption is a mass-transfer process that at least one selective component in a gas mixture is driven towards and retained on the surface of adsorbent particles by Van der Waals or electrostatic forces. The efficiency of a gas adsorption system depends on several factors including pore size of adsorbent material, partial pressure of adsorbed component, system temperature, and interaction between adsorbed component and adsorbent material. In addition, the efficiency of an adsorption system is also controlled by how well the adsorbent material can be regenerated under specific conditions. Based on the regeneration mechanism, adsorption technology can be classified into four categories: pressure swing adsorption (PSA), thermal swing adsorption (TSA), vacuum swing adsorption (VSA), and electrical swing adsorption (Thomas and Crittenden, 1998). In PSA and VSA processes, the regeneration is achieved by reducing the bed pressure after adsorption service. In TSA and electrical swing processes, the regeneration is done by heating the adsorption bed using either thermal energy or electric current. Among these, PSA is suitable for bulk separation and for the system having weak adsorptive force between adsorbed component and adsorbent material. The continuous operation of a PSA process is commonly achieved through a cyclic procedure consisting of four sequential steps: i) pressurisation by feed gas, ii) adsorption at high pressure, iii) counter-current depressurisation, and iv) purge with product fraction (Thomas and Crittenden, 1998). Selecting the most suitable adsorbent materials for a PSA process depends on a number of adsorbent attributes including selectivity, regeneration performance, inter-particle diffusivity, adsorption capacity, packing density, physical and chemical stability, and cost. The adsorbent materials commonly used in the gas separation industry are zeolite-based 8

25 adsorbents and carbon-based adsorbents. Recently, metal organic framework adsorbents are receiving increasing attention due to their excellent pore volume, as pore volume along with proper pore diameter increases the total available adsorption surface area which results in increase in gas adsorption capacity (Tagliabue et al., 2009). Zeolite-based adsorbents can be classified into different categories depending upon pore size. The common zeolite-based adsorbents for gas separation applications are zeolite 13X, zeolite 5A, and zeolite 4A. Carbon-based adsorbents exhibit low polarity and heat of adsorption. The typical carbonbased adsorbents for gas separation are carbon molecular sieves and activated carbons. 1.4 Research motivation The design and operation of the CO 2 adsorption process depends on characteristics of the adsorbents used. To select the most appropriate adsorbent for any application, it is extremely important to analyze equilibrium data, which are the key screening criteria revealing the maximum amount of gaseous component to be removed per unit mass of the adsorbent. Kinetic properties are even more important for adsorbent selection because they provide information about the rate of adsorption under controlled conditions. Both equilibrium and kinetic information help ensure that the removal of CO 2 from the natural gas succeeds through the use of appropriate adsorbent. At present, there are a number of adsorbents capable of removing CO 2 from natural gas. As mentioned earlier, zeolites, carbon molecular sieves, and activated carbons have great potential to remove CO 2 from natural gas by pressure swing adsorption. In this work, zeolite 13X, zeolite 5A, zeolite 4A, a carbon molecular sieve (MSC 3R), and two activated carbons (GCA-830 and GCA-1240) were selected to test their performance for CO 2 separation from 9

26 natural gas. These adsorbent samples are commercially available and cover a wide variety of adsorbent families. In recent years, a number of research projects associated with the adsorption properties of CO 2 gas on different adsorbents were reported. Most of these projects focused on adsorption equilibrium (or adsorption isotherms) at different temperatures and pressures. The CO 2 adsorption isotherms of zeolite 13X were measured by Cavenati et al.(2004), Zhang et al. (2010), Siriwardane et al. (2005), and Ko et al. (2003). The CO 2 adsorption capacity of zeolite 5A was reported by Pakseresht et al. (2002), Chen et al. (1990), and Kim et al. (1995). Siriwardane et al. (2001) showed the CO 2 adsorption capacity of zeolite 4A. Castello et al. (2004) and Bae and Lee (2005) reported the capacity of carbon molecular sieves. For the kinetics of CO 2 adsorption, there is only a limited number of projects reporting data at service pressure of natural gas. Zhang et al. (2010), Rutherford et al. (2003), and Bae and Lee (2005) investigated the kinetics of zeolite 13x at high CO 2 feed pressures. Rutherford et al. (2003) and Bae and Lee (2005) reported the CO 2 adsorption kinetics for carbon molecular sieves under moderate pressure. On the other hand, Rutherford and Do (2000) and Yucel and Ruthven (1980) reported the kinetics of CO 2 adsorption on zeolite 5A below atmospheric pressure. Perez and Armenta (2010) and Yucel and Ruthven (1980) presented CO 2 uptake curves for zeolite 4A below atmospheric pressure. Because both adsorption rate and adsorption capacity are vital for evaluating the performance and practicality of the CO 2 adsorption process, the behaviour of CO 2 -adsorption kinetics and equilibrium must be characterized systematically for the selected adsorbents. 10

27 1.5 Objective and scope The objective of this study is to characterize the CO 2 adsorption equilibrium and CO 2 adsorption kinetics of the six adsorption materials selected to determine the most suitable adsorbent for CO 2 separation from natural gas using the pressure swing adsorption process. An adsorption equilibrium and kinetic study was performed on six adsorbents (i.e., zeolite 13X, zeolite 5A, zeolite 4A, carbon molecular sieve (MSC 3R), and two activated carbons (GCA-830 and GCA-1240)). The results of this research are expected to provide better understanding of CO 2 gas adsorption using different adsorbents, and the adsorbents potential in applicability for CO 2 gas separation from natural gas. The following research tasks were carried out to attain the objectives of this study: Adsorption isotherms and the kinetics of CO 2 adsorption on the selected adsorbents were measured at different temperatures and pressures. The measured data were correlated with the conventional adsorption isotherm models. The mass transfer coefficient and adsorption activation energy of CO 2 adsorption were analyzed. Also, the isosteric heat of adsorption was determined. This thesis consists of five chapters. The basic theories of adsorption equilibrium and kinetics as well as literature review of CO 2 adsorption, are provided in Chapter 2. Details of the experimental set up, materials used, experimental procedures, and test conditions for the CO 2 adsorption experiments are described in Chapter 3. Chapter 4 presents the experimental results of the adsorption equilibrium and kinetics studies, as well as the data analysis. This chapter also describes the isosteric heat of CO 2 adsorption. Conclusions and recommendations for future work are given in Chapter 5. 11

28 2. CO 2 ADSORPTION CHARACTERISTICS AND LITERATURE REVIEW Carbon dioxide adsorption is a mass-transfer process taking place when a gas stream containing CO 2 and particles of a porous material (or adsorbent) are brought into direct contact in order to allow the CO 2 gas to travel towards and then reside on the surface of adsorbent particles. This process is exothermic, occurring by means of diffusion driven by a non-equilibrium condition or a difference in CO 2 partial pressure between the gas and solid phases. In most cases, the adsorbent material is packed in a series of adsorption columns where the gas stream is introduced at a regular time period until the adsorbent reaches its saturation point. The saturated column then undergoes the desorption operation during which the adsorbed CO 2 is released under controlled pressure and temperature, and the adsorbent capacity is restored for more adsorption. The performance of CO 2 adsorption depends on two primary characteristics: adsorption equilibrium and adsorption kinetics. The following sections provide some basic background of these two important features. 2.1 Adsorption equilibrium Adsorption equilibrium is a very important adsorption characteristic because it reveals the ability of solid adsorbent to accommodate the adsorbed gaseous molecules under specific conditions. The equilibrium data are usually presented for a given gas-solid pair as a function of temperature (T) and pressure (P). q * = f (P, T) (2.1) where q is the amount of gas adsorbed per gram of adsorbent. The adsorption equilibrium is commonly reported at a given temperature as the relationship between the partial pressure of 12

29 adsorbed molecules in the gas phase and the adsorbed amount q. This relationship is referred to as the adsorption isotherm, which can be classified into different types, according to the International Union of Pure and Applied Chemistry (IUPAC). For CO 2 adsorption, there are a number of mathematical models proposed to describe experimental adsorption isotherm data. Some models have theoretical foundations and others are empirical in nature. The key isotherm models are highlighted below Langmuir isotherm This is the simplest and the most recognized theoretical isotherm model. It illustrates monolayer adsorption on an ideal surface where the surface energy fluctuates periodically (Do, 1998). The periodic fluctuation indicates that the adsorption surface is homogeneous and the adsorption energy is constant and distributed evenly over all adsorption sites. It is assumed that there is no interaction between the adsorbed molecules (Yang, 2003). The Langmuir isotherm was derived from the dynamic-equilibrium concept (i.e., both adsorption and desorption activities take place at the same rate per unit of surface area). The Langmuir isotherm equation can be written as (Langmuir, 1918; Morse et al., 2010): q * = (2.2) where B = B exp ( ) (2.3) Here, q * is the amount of gas adsorbed, P is the equilibrium pressure, T is the adsorption temperature, q m is the maximum amount of gas adsorbed, B is the affinity constant at infinite temperature, Q is the heat of adsorption, and R is the gas constant. The parameter, B, is the Langmuir constant measuring the affinity of the adsorbed molecules to the solid surface. The higher the value of B, the greater the affinity. The adsorbed amount of gaseous 13

30 component on adsorbent surface for a binary system can be expressed by equation (2.4), which is known as the extended Langmuir model (Markham and Benton, 1931)., = (2.4) The adsorbed amount of the species "i" (q i ) can be calculated using this model equation for a multi-component system Volmer isotherm In 1925, Volmer proposed an isotherm model based on molecular mobility on the adsorbent surface. The Volmer isotherm equation can be written as (Ruthven, 1984): BP = exp (2.5) Here, θ is the fractional coverage that can be defined as a ratio of the adsorbed amount q to the maximum amount q *, P is the equilibrium pressure, and B is the affinity of the adsorbed molecules to the solid. Unlike the Langmuir model, the parameter B in the Volmer model decreases with the amount of molecules adsorbed on the adsorbent (Do, 1998). This suggests that a change in adsorption pressure has an impact on the interaction between adsorbed molecules Hill de-boer & Fowler-Guggenheim isotherm Based on an equation of state describing the adsorbent surface, two isotherm models taking into account the mobility and interaction between adsorbed molecules were proposed by Hill (1946), De Boer (1953), and Fowler-Guggenheim (1965). These isotherm models can be written as: Hill de-boer: BP = exp exp (-cθ) (2.6) 14

31 Fowler-Guggenheim: BP = exp (-cθ) (2.7) where c = (2.8) Here, z is a co-ordination number and w is the interaction energy between adsorbed molecules Freundlich isotherm This is the first known empirical equation that can fit the adsorption isotherm data deviating from the ideal situation due to the complexity of the adsorbent surface. The mathematical form of the Freundlich model is (Siriwardane et al., 2005; Freundlich, 1907): q * = k P / (2.9) where k and t are isotherm parameters depending on the adsorption temperature (t>1). The Freundlich isotherm can be applied to adsorption systems with heterogeneous adsorbent. This model does not follow Henry s law behaviour at low pressure and presents no finite limit at the higher pressure (Do, 1998) Sips isotherm This model is the combined form of the Langmuir and Freundlich isotherms. In the Freundlich model, the amount of gas adsorbed is increased indefinitely with pressure. A combined model is proposed by Sips (1948) to avoid this limitation. The mathematical form of this model is: q * = ( ) / ( ) / (2.10) where q *, B and t are isotherm parameters. The Sips model is transformed from the empirical Freundlich isotherm into the theoretical Langmuir isotherm predicting the ideal adsorbent surface only when the parameter t is equal to unity. 15

32 2.1.6 Toth isotherm This model is a semi-empirical isotherm with three parameters. It can be used to describe an adsorption system with sub-monolayer coverage, and it can also predict the adsorption behaviour of gases at both low and high pressure. The mathematical form of this model is (Toth, 1971; Cavenati et al., 2004): q * = ( ) / (2.11) In 1996, the Toth model was modified by Keller et al. (1996) to include a pressure function instead of a constant parameter Prausnitz isotherm In 1972, Radke & Prausnitz proposed an empirical equation having three parameters to calculate the adsorbed amount; it can be written as (Radke & Prausnitz, 1972): = + (2.12) where q represents the adsorbed amount, a represents Henry s constant, B indicates affinity constant and t indicates Freundlich constant. The Prausnitz isotherm equation reduces to Henry s equation at lower adsorption pressure and the Freundlich equation at higher adsorption pressure. Equation 2.12 can be applied successfully to a wide range of adsorption pressures and loadings Unilan isotherm This model takes into account the topography of the adsorbent surface, demonstrating the surface heterogeneity. Based on the topographic data, this model equation that can be written as (Do, 1998): q * = ln ( ) (2.13) 16

33 where the parameter b indicates the average affinity of the adsorbed molecules and s defines the quantity of heterogeneity of the system. It should be noted that the above isotherm models were developed from systems with a single gaseous component. In multi-component systems, these equations are used together with a set of thermodynamic equations (or a thermodynamic framework) so as to calculate total adsorbed amount as along with the adsorbed amounts of individual gases. A thermodynamic framework known as Ideal Adsorption Solution Theory (IAST) was proposed by Myers and Prausnitz (1965) in order to represent the ideal behaviour of multicomponent gas adsorption. The ideal framework can be written as (Do, 1998): = (2.14a) = = - Py j = P x j dp (2.14b) (2.14c) x =1 (2.14d) where n is the total number of moles per unit mass of the adsorbent, x j is the molar fraction of component j in the adsorbed phase, n is the number of mole of pure component j per unit mass of the adsorbent, ɸ is the surface potential of the adsorbed phase per unit mass of the adsorbent, ɸ is the surface potential of the pure component j, R is the gas constant, T is the temperature, P is the adsorption pressure of the pure component j, P is the equilibrium pressure, and y j is the molar fraction of component j in the gas phase. According to Costa et al. (1989), the ideal framework (IAST) cannot offer accurate prediction of adsorption of hydrocarbons and CO 2 on zeolite-based adsorbents. Real Adsorption Solution Theory (RAST), which includes an activity coefficient for non-ideality 17

34 of systems, can provide better prediction. The equations of the RAST model can be written as (Costa et al., 1981; Talu and Zwiebel, 1986): Py i = x i γ i ( x ) P (Z) (2.15a) x = 1 (2.15b) z = = ( ) dp 0 (2.15c) = ( + x ) X (2.15d) C µi = x i C µt (2.15e) where P is the equilibrium pressure, y i is the molar fraction of component i in the gas phase, x i is the molar fraction of component i in the adsorbed phase, γ i is the activity coefficient of the component i, z is the reduced spreading pressure, P is the hypothetical pressure of the pure component i, A is the adsorption potential, π is the spreading pressure, C is the adsorbed amount of pure component i at the hypothetical pressure, CμT is the total amount adsorbed, and Cμi is the amount adsorbed by component i. 2.2 Adsorption kinetics Adsorption kinetics or adsorption rate is another important mass transfer characteristic needed for the design and operation of a gas adsorption column. An adsorbent with large adsorption capacity (or equilibrium) might not be the best choice for industrial applications if its adsorption activity takes place at a very slow rate. In most cases, the adsorbent offering a fast adsorption rate is considered to be the most suitable material. Because typical adsorbents are porous particles, the overall adsorption rate is usually controlled by the diffusion of adsorbate molecules from the gas phase into the interior of 18

35 adsorption sites (Do, 1998). In general, the diffusion of these molecules involves three sequential steps: i) transport to the external surface of the solid particles, ii) diffusion through the macropores, and iii) diffusion into micropores. Figure 2.1 shows a simplified diagram of typical adsorbent particles and the general structure associated with molecular diffusion. From the figure, it is clear that the diffusion of adsorbate molecules is subject to three mass transfer resistances: external film resistance and macropore and micropore diffusive resistances. Under no slip conditions at a solid boundary, it can be assumed that an adsorbent particle is surrounded by a laminar film through which mass transfer takes place by diffusion (Ruthven, 1984). Typically, this external resistance is negligibly small compared to the other two resistances. Macropore resistance is usually a rate determining step for the adsorption process. Macropore diffusion depends on the pore size of the particular adsorbent and the nature of the fluid-wall interaction. 19

36 Figure 2.1 Internal view of adsorbent particles (Modified from Do, 1998). 20

37 The overall mass transfer resistance (1/k), which was first proposed by Glueckauf and Coates (1947) and later modified by Haynes and Sharma (1975), can be written in a mathematical form as (Farooq et al., 2002): = + + (2.16) where the first, second, and third terms on the right represent external film resistance, macropore resistance, and micropore resistance, respectively. Here, k is the overall mass transfer coefficient, k f is the external film mass transfer coefficient, K H is the Henry s constant, R P is the macroparticle radius, D P is the macropore diffusion coefficient, D c is the intra-crystalline concentration dependant diffusion coefficient, r c is the microcrystal radius, and ε p is the porosity of adsorbent particle. It should be noted that the determination of individual mass transfer resistances is rather difficult in real practice. Under typical circumstances, it is more practical to express the adsorption rate in terms of the overall coefficient k and the overall mass transfer driving forces across both gas and solid phases. The overall mass transfer coefficient k can be analyzed from the uptake profiles that demonstrate the amount of gas adsorbed on the absorbent particles as a function of time. Today, there are a number of models, such as the linear driving force model (LDF), pore diffusion model, and dual resistance model, proposed to represent the uptake-rate behaviour. In this study, the LDF model was chosen to analyze the mass transfer coefficient and adsorption kinetics. The LDF model was derived from the pore diffusion model. The following equation was proposed for the LDF model (Glueckauf and Coates, 1947; Zhang et al., 2010): = k (q* q) (2.17) 21

38 where is the rate of mass transfer or adsorption rate, q* is the equilibrium amount of gas adsorbed, q is the amount of gas adsorbed with respect to any particular time, and, again, k is the overall mass transfer coefficient. It should be noted that the coefficient k is a function of adsorption pressure and temperature. By integrating Equation (2.17), the LDF model can be written as: ln = kt (2.18) The mass transfer coefficient can be determined from the slope of a plot of ln 1 versus adsorption time (t). 2.3 Isosteric heat of adsorption The isosteric heat of adsorption is an important parameter that reveals the level of energy released during adsorption activity. The heat of adsorption could have a great impact on the adsorption rate because the released exothermic energy can cause the temperature of the adsorbent particles to rise, reducing the adsorption capacity (Do, 1998). The isosteric heat of adsorption ( H) can be calculated from the following equation Clausius-Clapeyron equation (Hill, 1949; Lee et al., 2002): = ( ) (2.19) Here, T is the adsorption temperature, P is the pressure, N is the loading of the gas molecule, and R is the molar gas constant. This equation was derived from the Clausius-Clapeyron equation under the assumption that adsorbed phase volume is negligible and the gas phase is ideal. With a series of isotherm curves obtained at different temperatures, it is possible to 22

39 extract the equilibrium pressure as a function of adsorption temperature at any given loading of gas molecule (N). The heat of adsorption can be determined from the slope of a plot of lnp versus reciprocal of temperature (1/T). 2.4 Activation energy The adsorption activation energy is the potential energy barrier that must be overcome to cause adsorption activity on the solid surface. In a physical adsorption process, activation energy decreases as pressure increases because, at a higher pressure, adsorbate-adsorbent interaction becomes stronger due to an increase in gas density. The activation energy of adsorption (E a ) can be expressed using the following Arrhenius equation (Zhang et al., 2010): k = Ae / (2.20) where k is the overall mass transfer coefficient, A is the frequency factor or collision factor, R is the molar gas constant, and T is the temperature. Equation (2.20) can also be rearranged as: lnk = - + lna (2.21) The activation energy (E a ) can be analyzed directly from the slope of a plot between lnk and reciprocal of temperature (1/T). 23

40 2.5 Literature review on adsorbents for CO 2 removal This section provides a review of studies that were carried out to evaluate the performance of potential CO 2 removal adsorbents in terms of equilibrium, kinetics, and other associated adsorption characteristics CO 2 adsorption by zeolite 13X Most research studies on CO 2 adsorption by zeolite 13X have been aimed at determining the adsorption isotherm at low pressure ranges close to atmospheric pressure (up to 1.2 atm). Costa et al. (1991) measured the CO 2 adsorption isotherm at different temperatures and up to 1 atm. Calleja et al. (1994) also measured the isotherm at the low pressure range (up to 0.9 atm) and fitted the obtained data with different isotherm models. Chue et al. (1995) measured the CO 2 adsorption isotherm so as to evaluate the performance of zeolite 13X in a pressure swing adsorption process. The measurement was made up to 1.05 atm. Lee et al. (2002) investigated the CO 2 adsorption equilibrium on zeolite 13x at low pressure (1 atm). The isosteric heat of adsorption was also analyzed from the obtained equilibrium data. Li et al. (2008) and Zhang et al. (2009) reported the use of zeolite 13X for removing CO 2 from flue gas containing humidity and other impurities. They also provided the adsorption isotherm at 1.2 atm and breakthrough analysis of CO 2 removal. Wang and Levan (2009) measured the adsorption isotherm at different temperatures and up to 1 atm. The obtained data were fitted with the Toth model. There are only a few studies focusing on CO 2 adsorption by zeolite 13X at medium pressure ranges. Harlick and Tezel (2004) investigated the adsorption capacity and the heat of adsorption for a number of adsorbents including zeolite 13X at pressures up to 2.5 atm. Merel et al. (2008) compared the adsorption performance between zeolites 13X and 5A at 24

41 pressures up to 4.9 atm and reported the adsorption isotherm, as well as breakthrough curves, of CO 2 adsorption. For studies at high pressure ranges, Cavenati et al. (2004) measured the CO 2 adsorption isotherm and analyzed the isosteric heat of adsorption at pressures up to 49.4 atm. Zhang et al. (2010) investigated the adsorption equilibrium and kinetics at different temperatures and up to 29.6 atm CO 2 adsorption by zeolite 5A The adsorption of CO 2 by zeolite 5A has been studied since the 1970s. In 1980, Yucel and Ruthven investigated the adsorption isotherm and kinetics at pressure as low as 0.4 atm. The adsorption kinetics, activation energy, and heat of adsorption were reported by Triebe and Tezel (1995) for the removal of CO 2 from air. Pakseresht et al. (2002) investigated adsorption equilibrium at different temperatures at intermediate pressure (up to 9.9 atm). Tlili et al. (2009) reported the adsorption equilibrium and breakthrough curve at 1 atm. Saha et al. (2010) studied the adsorption equilibrium and kinetics of CO 2 adsorption from air and methane at 1.05 atm. Finally, Liu et al. (2011) investigated the equilibrium isotherm at different temperatures and the pressure up to 1 atm. For the high pressure range, Chen et al. (1990) reported the adsorption isotherm at pressures up to 54.5 atm. Kim et al. (1995) studied the isotherm, heat of adsorption, and breakthrough curves for removal of CO 2 from H 2 gas at 27.5 atm. 25

42 2.5.3 CO 2 adsorption by zeolite 4A Eagan and Anderson (1975) investigated the CO 2 adsorption isotherm for zeolite 4A at pressures up to 1.02 atm. Yucel and Ruthven (1980) measured the CO 2 adsorption kinetics at different temperatures at 0.4 atm. Siriwardane et al. (2001) reported the use of zeolite 4A for the separation of CO 2 from a high pressure flue gas stream at 20.4 atm. Ahn et al. (2004) investigated the equilibrium isotherm and kinetics at 0.8 atm CO 2 adsorption by carbon molecular sieve Kikkinides and Yang (1993) reported the equilibrium capacity of a carbon molecular sieve for CO 2 adsorption from flue gas. Mochida et al. (1995) investigated the CO 2 adsorption isotherm of a carbon molecular sieve for the removal of CO 2 from methane at 1 atm. Also, for CO 2 removal from methane, the adsorption equilibrium and kinetics were reported by Jayaraman et al. (2002) at 5.1 atm and by Rutherford et al. (2003) at 2.4 atm. The isotherm and kinetics of CO 2 removal from air were studied at different temperatures by Reid and Thomas (1999). For the high pressure range, Amoros et al. (1998) examined the effect of pore size of a carbon molecular sieve on the CO 2 adsorption performance at 39.5 atm. Castello et al. (2004) reported the CO 2 adsorption isotherm at 29.6 atm. Bae and Lee (2005) investigated the adsorption equilibrium and kinetics at 14.9 atm CO 2 adsorption by activated carbon Most research studies on CO 2 adsorption by activated carbon have been carried out at high pressure ranges. Amoros et al. (1996) studied the CO 2 adsorption characteristics of 26

43 activated carbon at pressures up to 39.5 atm. Dreisbach et al. (1999) measured and modeled the adsorption isotherm at 59.2 atm. The use of activated carbon for removing CO 2 from the flue gas was investigated by Siriwardane et al. (2001) at 20.4 atm and by Millward and Yaghi (2005) at 44.4 atm. They also reported the corresponding CO 2 adsorption equilibrium. Sudibandriyo et al. (2003) investigated CO 2 adsorption on activated carbon at a very high pressure of atm. Drage et al. (2009) reported the equilibrium capacity and kinetics of CO 2 adsorption from synthetic gas at 39.5 atm. For low pressure applications, Chue et al. (1995) investigated the CO 2 adsorption isotherm at 1.05 atm. Guo et al. (2006) measured and modeled the isotherm and isosteric heat of adsorption at 3.9 atm. Vaart et al. (2000) reported the CO 2 adsorption isotherm at a pressure of 7.9 atm. Details of the above literature reviews summarized according to the type of adsorbent materials are given in Tables 2.1 through

44 Table 2.1: Previous works on CO 2 adsorption on zeolite 13X Author Technique Experimental conditions Isotherm Mass Transfer Isosteric Heat of Activation Temperature Pressure coefficient Heat adsorption energy (K) (atm) (1/sec) (kj/mol) (kj/mol) (kj/mol) Costa et al., 1991 Volumetric 279, 293 & X Calleja et al., 1994 Volumetric X Chue et al., 1995 Volumetric X Lee et al., 2002 Volumetric X - X - - Cavenati et al., 2004 Gravimetric 298, 308 & X - X - - Harlick & Tezel, 2004 Micrometrics X - - X - Siriwardane et al., 2005 Volumetric X - - X - Merel et al., & X Li et al., 2008 Micrometrics 293, 313 & X Zhang et al., 2009 Micrometrics 293, 313 & X Wang & Levan, 2009 Volumetric X Zhang et al., 2010 Gravimetric X X - - X X = Data Available 28

45 Table 2.2: Previous works on CO 2 adsorption on zeolite 5A Author Technique Experimental conditions Isotherm Mass Transfer Isosteric Heat of Activation Temperature Pressure coefficient Heat adsorption energy (K) (atm) (1/sec) (kj/mol) (kj/mol) (kj/mol) Yucel & Ruthven, 1980 Gravimetric X X Chen et al., 1990 Volumetric X - X - - Kim et al., X - - X - Triebe & Tezel, 1995 Volumetric X - Pakseresht et al., 2002 Volumetric X - - X - Tlili et al., 2009 Gravimetric X Saha et al., X Liu et al., 2011 Gravimetric X - X - - X = Data Available 29

46 Table 2.3: Previous work on CO 2 adsorption on zeolite 4A Author Technique Experimental conditions Isotherm Mass Transfer Isosteric Heat of Activation Temperature Pressure coefficient Heat adsorption energy (K) (atm) (1/sec) (kj/mol) (kj/mol) (kj/mol) Eagan & Anderson, 1975 Volumetric X Yucel & Ruthven, 1980 Gravimetric X X - - X Siriwardane et al., 2001 Volumetric X Ahn et al., 2004 Gravimetric X = Data Available X X - - X 30

47 Table 2.4: Previous works on CO 2 adsorption on carbon molecular sieves Author Technique Experimental conditions Isotherm Mass Transfer Isosteric Heat of Activation Temperature Pressure coefficient Heat adsorption energy (K) (atm) (1/sec) (kj/mol) (kj/mol) (kj/mol) Mochida et al., 1995 Volumetric Amoros et al., 1998 Gravimetric X Reid & Thomas, 1999 Gravimetric X X X - - Jayaraman et al., 2002 Gravimetric X X - X X Rutherford et al., 2003 Volumetric 323 & X X - X X Castello et al., 2004 Gravimetric X Tan & Ani, 2004 Micrometrics X Rodil et al., 2005 Volumetric X X Castello et al., 2005 Gravimetric 298, 313 & X X - X - Bae & Lee, 2005 Volumetric 293, 303 & X X X = Data Available 31

48 Table 2.5: Previous works on CO 2 adsorption on activated carbon Author Technique Experimental conditions Isotherm Mass Transfer Isosteric Heat of Activation Temperature Pressure coefficient Heat adsorption energy (K) (atm) (1/sec) (kj/mol) (kj/mol) (kj/mol) Chue et al., 1995 Volumetric X Amoros et al., 1996 Gravimetric X Dreisbach et al., 1999 Gravimetric X Vaart et al., 2000 Gravimetric X Siriwardane et al., 2001 Volumetric X - X - - Sudibandriyo et al., 2003 Gravimetric X Millward & Yaghi, 2005 Volumetric Guo et al., 2006 Vacuum X - X - - Drage et al., 2009 Volumetric X X = Data Available 32

49 3. CO 2 ADSORPTION EXPERIMENTS AND PROCEDURES Adsorption experiments were carried out in this study to measure the adsorption equilibrium or isotherm and the kinetics or uptake rate of CO 2 on the selected commercial adsorbents under different temperatures and pressures. Details of materials, experimental equipment, and experimental procedures are provided in this chapter. 3.1 Materials In this study, six commercial adsorbents were used for CO 2 adsorption experiments. They are zeolite 13X, zeolite 5A, zeolite 4A, a carbon molecular sieve (MSC 3R), and granular activated carbons (GCA-830 and GCA-1240). The three zeolite adsorbents were purchased from Sigma-Aldrich Company Limited (Oakville ON, Canada). The carbon molecular sieve MSC 3R and granular activated carbons were donated by Japan EnviroChemicals Ltd. (Tokyo, Japan) and Norit Activate Carbon Company Ltd (Texas, USA), respectively. The zeolite 13X used in this study is a pellet type of which the molecular formula is 1 Na 2 O: 1 Al 2 O 3 : 2.8 ± 0.2 SiO 2 : xh 2 O. It is a sodium-modified molecular sieve with a pore diameter of 10 angstroms (Å). Zeolite 5A is a calcium form of molecular sieve with a pore diameter of 5 (Å), and its molecular formula is Ca /n Na 12-2n [(AlO 2 ) 12 (SiO 2 ) 12 ] xh 2 O. The zeolite 4A is a sodium-modified molecular sieve having a smaller pore size compared to zeolite 5A. It is a bead type with a particle size of 8-12 mesh, and its molecular formula is Na 12 [(AlO 2 ) 12 (SiO 2 ) 12 ] xh 2 O. The carbon molecular sieve (MSC 3R) used in this study has uniform super-micropores with a diameter of less than 10 Å. 33

50 For activated carbons, both GCA-830 and GCA-1240 were made from coconut shells by steam activation. They can be regenerated by thermal reactivation. The particle sizes are 8-30 mesh for GCA-830 and mesh for GCA The physical properties of these adsorbents were obtained from the manufacturers and are presented in Table 3.1. Their photographs are given in Figure 3.1. The helium and CO 2 gases used in this study were ultra pure with purity grades of %. They were purchased from Praxair Company Limited. 3.2 Experimental apparatus The adsorption isotherm and uptake rate (kinetics) for all adsorbents were measured using a volumetric gas adsorption apparatus designed and fabricated in the Energy Technology Laboratory, University of Regina. A schematic diagram and a photograph of this setup are given in Figures 3.2 (a) and 3.2 (b), respectively. The adsorption apparatus consisted of two stainless steel (SS 304) pressure vessels purchased from Swagelok. One vessel (Part No.: 304L-HDF4-150-T-PD) was used for gas storage, supplying gaseous components such as CO 2 and He to another vessel serving as an adsorption cell. The capacity of the storage vessel was 150 ml with an accuracy of ±5%, and the vessel could sustain pressure up to atm. The adsorption cell was a packed column filled with the adsorbent of interest. Both storage and adsorption vessels were equipped with highly accurate pressure transmitters and RTD temperature probes. The pressure transmitter (Cole Parmer, Part No: ) connected to the storage vessel was capable of measuring pressure up to 34 atm with an accuracy of ± 0.05 atm, and another transmitter (Cole Parmer, Part No: ) connected to the adsorption cell could measure up to 34

51 68 atm with an accuracy of ± 0.1 atm. The temperature probes for both the storage vessel and adsorption cell had an accuracy of ± 0.15 K or (±0.15 C). Both pressure transmitters and temperature probes were connected to a data acquisition system (OMEGA, Model No: OM-420, Serial No: ) and a computer, enabling accurate measurements of pressure and temperature over time. Such real-time measurements enabled monitoring of the rate and amount of gaseous component being transferred between the two vessels. A rotary vacuum pump capable of generating a 0.78 atm vacuum (Cole Parmer, Model No: L ) was connected to the adsorption cell to allow the regeneration of the used adsorbents and also to help evacuate any unwanted gas held by the adsorbent prior to the adsorption experiments. A stainless steel pressure regulator (Swagelok, Model No: KCYIJRH 425A96050) was installed between the storage vessel and adsorption cell so as to allow the control of feed pressure set for the adsorption cell. The entire system was assembled using Swagelok fittings and tested to be leak proof. It could be operated at up to 393 K and 40.8 atm. The temperature of the adsorption cell was controlled with a temperature-controlled water bath with a precision of ±0.01 K or ±0.01 C and set point accuracy of ±1 K or ±1 C (TECHNE, Model No: FTE10DP, serial No: ). An electric oven (VMR, Canada) was used for treating the adsorbents prior to the adsorption experiments. An auto-calibrated microbalance (Ohaus Corporation, Model: EP214C) was used for weighing the adsorbent samples. 35

52 Table 3.1: Physical properties of different adsorbents Adorbent Physical properties References Zeolite 13X Nominal pore diameter : 8 Å Sigma Aldrich Particle Diameter : 1.6 x 10-3 m Bulk density : kg/m 3 Crush Strength : 3.2 kg Surface area : 7.2 x 10 5 m 2 /kg pore volume : 3.2 x 10-4 m 3 /kg Zeolite 4A Pore diameter : 4 Å Sigma Aldrich Bulk density : kg/m 3 Regeneration temp : K Heat of Adsorption : 4.18 MJ/kg H 2 O Zeolite 5A Pore diameter : 5 Å Sigma Aldrich Bulk density : kg/m 3 Regeneration temp : Heat of Adsorption : K 4.18 MJ/kg H 2 O GCA-830 Surface area : 1.15 x 10 6 m 2 /kg Norit Americas Inc. Bulk density : kg/m 3 GCA-1240 Surface area : 1.15 x 10 6 m 2 /kg Norit Americas Inc. Bulk density : kg/m 3 MSC-3R Pellet diameter : 1.8 X 10-3 m Japan Enviro-Chemicals, Ltd. 36

53 (a) Zeolite 13x (b) Zeolite 5A (c) Zeolite 4A (d) MSC-3R (e) GCA-830 (f) GCA-1240 Figure 3.1: Photographs of adsorbents used in this study (original in colour) 37