EQUILIBRIUM AND MASS TRANSFER BEHAVIOUR OF CO...
Transcript of EQUILIBRIUM AND MASS TRANSFER BEHAVIOUR OF CO...
EQUILIBRIUM AND MASS TRANSFER BEHAVIOUR OF CO2 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
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 CO2 Adsorption on Zeolites, Carbon Molecular Sieve, and Activated Carbons, in an oral examination held on September 6, 2012. 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: Dr. Fanhua Zeng, Petroleum Systems Engineering
Supervisor: Dr. Adisorn Aroonwilas, Industrial Systems Engineering
Committee Member: Dr. David deMontigny, Industrial Systems Engineering
Committee Member: Dr. Amornvadee Veawab, Environmental Systems Engineering
Chair of Defense: Dr. Shaun Fallat, Department of Mathematics & Statistics *Not present at defense
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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 (CO2) 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 CO2 removal in this PSA process. The objective of this study was to
characterize the CO2 adsorption equilibrium and kinetics of commercial adsorbents that
have potential for use in the PSA process and also to provide a better understanding of
CO2 adsorption behaviour under wide range of operating conditions.
A comprehensive set of data and analysis for CO2 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 293 – 333 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 CO2 adsorption was also estimated for individual adsorbents according to
the Clausius-Clapeyron equation. The CO2 adsorption kinetic, presented in terms of mass
transfer coefficients (k), were experimentally measured at a temperature range of 293 –
333 K and pressure up to 11 atm. The mass transfer was analyzed from the plots of CO2
uptake rate using the well-recognized linear driving force (LDF) model. The mass
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transfer coefficients were correlated by non-linear regression to reveal the effects of
adsorption temperature and pressure. Activation energies of CO2 adsorption on the
individual adsorbents were also calculated and correlated according to the Arrhenius
equation.
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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.
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TABLE OF CONTENTS
Page
ABSTRACT
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ACKNOWLEDGEMENTS
iii
TABLE OF CONTENTS
iv
LIST OF TABLES
vii
LIST OF FIGURES
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NOMENCLATURE
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1. INTRODUCTION 1
1.1 Need for CO2 removal from natural gas 4
1.2 Technology for CO2 separation 6
1.3 Gas adsorption technology 8
1.4 Research motivation 9
1.5 Objective and scope 11
2. CO2 ADSORPTION CHARACTERISTICS AND LITERATURE 12
REVIEWS
2.1 Adsorption equilibrium 12
2.1.1 Langmuir isotherm 13
2.1.2 Volmer Isotherm 14
2.1.3 Hill de-Boer & Fowler-Guggenheim Isotherm 14
2.1.4 Freundlich isotherm 15
2.1.5 Sips isotherm 15
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2.1.6 Toth isotherm 16
2.1.7 Prausnitz isotherm 16
2.1.8 Unilan isotherm 16
2.2 Adsorption kinetics 18
2.3 Isosteric heat of adsorption 22
2.4 Activation energy 23
2.5 Literature review on adsorbents for CO2 removal 24
2.5.1 CO2 adsorption by zeolite 13X 24
2.5.2 CO2 adsorption by zeolite 5A 25
2.5.3 CO2 adsorption by zeolite 4A 26
2.5.4 CO2 adsorption by carbon molecular sieve 26
2.5.5 CO2 adsorption by activated carbon 26
3. CO2 ADSORPTION EXPERIMENTS AND PROCEDURES 33
3.1 Materials 33
3.2 Experimental apparatus 34
3.3 Experimental procedures 39
3.3.1 Preparation of adsorption cell 39
3.3.2 Determination of void volume 39
3.3.3 Determination of CO2 adsorption performance 41
3.4 Experimental condition 42
3.5 Validation of experimental method 44
4. RESULTS AND DISCUSSION 46
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4.1 CO2 adsorption equilibrium 46
4.1.1 Isotherm curves 46
4.1.2 Isotherm correlations 56
4.1.3 Isosteric heat of adsorption 64
4.2 CO2 adsorption kinetics 69
4.2.1 Mass transfer coefficient for CO2 adsorption 76
4.2.2 Mass transfer coefficient and activation energy 82
5. CONCLUSION AND FUTURE WORK 88
5.1 Conclusions 88
5.2 Recommendations for future work 90
REFERENCES 91
APPENDIX A: Experimental results of pure CO2 adsorption equilibrium 101
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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 CO2 adsorption on zeolite 13X 28
Table 2.2 Previous works for the CO2 adsorption on zeolite 5A 29
Table 2.3 Previous works for the CO2 adsorption on zeolite 4A 30
Table 2.4 Previous works for the CO2 adsorption on carbon molecular sieve 31
Table 2.5 Previous works for the CO2 adsorption on activated carbon 32
Table 3.1 Physical properties of different adsorbents 36
Table 3.2 CO2 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 CO2 adsorption on the adsorbents 66
Table 4.8 Mass transfer coefficients of CO2 adsorption on the tested adsorbents 81
Table 4.9 Activation energy (Ea) and frequency factor (A) for the tested 85
adsorbents
Table A.1 CO2 adsorption equilibrium data on zeolite 13X at different pressure 101
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and temperature
Table A.2 CO2 adsorption equilibrium data on zeolite 5A at different pressure 102
and temperature
Table A.3 CO2 adsorption equilibrium data on zeolite 4A at different pressure 103
and temperature
Table A.4 CO2 adsorption equilibrium data on MSC-3R at different pressure 104
and temperature
Table A.5 CO2 adsorption equilibrium data on GCA-830 at different pressure 105
and temperature
Table A.6 CO2 adsorption equilibrium data on GCA-1240 at different pressure 106
and temperature
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LIST OF FIGURES
Page
Figure 1.1 Primary sources of energy in the world in 2003 2
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 CO2 adsorption on 38
adsorbents
Figure 3.3 Comparison of CO2 adsorption isotherm data with published data 45
for (a) zeolite 13X and (b) GCA -1240
Figure 4.1 Isotherm curves of CO2 adsorption on zeolite 13X at different 47
temperatures
Figure 4.2 Isotherm curves of CO2 adsorption on zeolite 5A at different 48
temperatures
Figure 4.3 Isotherm curves of CO2 adsorption on zeolite 4A at different 49
temperatures
Figure 4.4 Isotherm curves of CO2 adsorption on MSC-3R at different 50
temperatures
Figure 4.5 Isotherm curves of CO2 adsorption on GCA-830 at different 51
temperatures
Figure 4.6 Isotherm curves of CO2 adsorption on GCA-1240 at different 52
x
temperatures
Figure 4.7 Comparison of isotherm curves among the adsorbents tested (a) 55
293 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 CO2 uptake for zeolite 13X at (a) 3.4 atm; (b) 5.4 atm; (c)
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6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
Figure 4.11 Plots of CO2 uptake for zeolite 5A at (a) 3.4 atm; (b) 5.4 atm; (c) 71
6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
Figure 4.12 Plots of CO2 uptake for zeolite 4A at (a) 3.4 atm; (b) 5.4 atm; (c)
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6.8 atm; (d) 8.8 atm.
Figure 4.13 Plots of CO2 uptake for MSC-3R at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8
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atm; (d) 8.8 atm; (e) 10.9 atm.
Figure 4.14 Plots of CO2 uptake for GCA-830 at (a) 3.4 atm; (b) 5.4 atm; (c)
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6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
Figure 4.15 Plots of CO2 uptake for GCA-1240 at (a) 3.4 atm; (b) 5.4 atm; (c) 75
6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
Figure 4.16 Linear plot of ln (1- ∗) versus time of CO2 adsorption at 293 K and
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different pressures
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Figure 4.17 Linear plot of ln (1- ∗) versus time of CO2 adsorption at 313 K and 79
different pressures
Figure 4.18 Linear plot of ln (1- ∗) versus time of CO2 adsorption at 333 K and 80
different pressures
Figure 4.19 Linear plot of lnk versus ( ) for CO2 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
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NOMENCLATURE
A frequency factor or collision factor, sec-1
Å angstrom
Ap 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
Cힵi amount adsorbed by the component i, mol/kg
Cμ adsorbed amount of pure component i at the hypothetical
pressure, mol/kg
CힵT total amount adsorbed, mol/kg
CO2 carbon dioxide
Dc intra-crystalline concentration dependant diffusion
coefficient, m2/sec
DP macro-pore diffusion coefficient, m2/sec
DEA diethanol amine
DGA diglycol amine
DIPA di-isopropanol amine
Ea activation energy of adsorption, kJ/mol
∆H isosteric heat of adsorption, kJ/ mol
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He helium gas
k overall mass transfer coefficient, sec-1
kf external film mass transfer coefficient, m/sec
KH Henry’s constant, mol/kg.atm
k temperature dependant isotherm constant
MDEA methyl diethanol amine
MEA monoethanol amine
n total number of moles per unit mass of the adsorbent
n number of mole of pure component j per unit mass of the
adsorbent, mol/Kg
N loading of gas molecule, mol/Kg
P equilibrium pressure, atm
PS pressure sensor
PSA pressure swing adsorption
P hypothetical pressure of the pure component i, atm
P adsorption pressure of the pure component j, atm
q amount of adsorbed gas, mol/kg
q* equilibrium amount of adsorbed gas, mol/kg
푞∗ maximum capacity at corresponding temperature, mol/kg
R gas constant, m3 atm K−1 mol−1
RTD resistance temperature detector
RP macro particle radius, m
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rc micro-crystal radius, m
s quantity of heterogeneity of the system
t temperature dependant isotherm constant
T adsorption temperature, K
TS temperature sensor
TSA thermal swing adsorption
VSA vacuum swing adsorption
w interaction energy between adsorbed molecules, kJ/mol
xi molar fraction of component i in the adsorbed phase
xj molar fraction of component j in the adsorbed phase
yi molar fraction of component i in the gas phase
yj molar fraction of component j in the gas phase
z reduced spreading pressure, mol/m3
Greek Letters
γi activity coefficient of the component i
휺p porosity of adsorbent particle
θ fractional coverage
π spreading pressure, kJ/m3
ɸ surface potential of the adsorbed phase per unit mass of the
adsorbent, kJ
ɸ surface potential of the pure component j, kJ
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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 (CO2),
one of the greenhouse gases contributing to global warming and climate change. The
increasing world energy demand could lead to a substantial increase in CO2 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 CO2 emissions compared to other
fossil fuels (Mokhatab et al., 2006). Natural gas is commonly produced from
underground reservoirs. It is composed of methane (CH4) 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 CO2. The typical natural gas
compositions are listed in Table 1.1. Note that raw natural gas can contain significant
amounts of CO2 that must be removed prior to the delivery to customers.
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Figure 1.1 Primary sources of energy in the world in 2010
(Redrawn from key world energy statistics, IEA, 2011)
Coal/peat, 20.2%
Natural Gas, 24.5%Oil, 36.3%
Biofuels, 4.7%
Nuclear, 11.0%
Hydro, 2.1% Other, 1.2%
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Table 1.1: Typical natural gas composition (in mol%) worldwide
Gas Canada Western Southwest Bach Ho Miskar Rio Arriba CliffsideComponents (Alberta) Colorado Kansas Fielda Field County, Field,
Vietnam Tunisia New Mexico Texas
Carbon dioxide 1.7 42.7 0.0 0.1 13.6 0.8 0.0
Hydrogen sulfide 3.3 0.0 0.0 0.0 0.1 0.0 0.0
Pentanes & heavier 3.0 0.3 0.6 2.6 0.6 0.0 0.5
Helium 0.0 0.0 0.5 0.0 0.0 0.0 1.8
0.7 25.6
Methane 77.1 30.0 72.9 70.9 63.9 96.9 65.8
Nitrogen 3.2 26.1 14.7 0.2 16.9
1.3 3.8
Propane 3.1 0.3 3.7 7.5 1.0 0.2 1.7
Ethane 6.6 0.6 6.3 13.4 3.3
a Tabular mol% data is on a wet basis (1.3 mol% water)Source: Kidnay and Parrish, 2006
0.1 0.8Butanes 2.0 0.2 1.4 4.0 0.5
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1.1 Need for CO2 removal from natural gas
CO2 is an incombustible gas, providing no thermal energy during the combustion
process. The presence of CO2 in natural gas product, therefore, results in the reduced
heating value per unit volume of natural gas. The presence of CO2 can also present
several challenges regarding pipeline transportation of natural gas. The large amount of
CO2 in the gas product simply occupies partial volume of the pipeline, reducing the
transport efficiency per unit volume of natural gas. In addition, CO2 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 CO2, 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 CO2 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 CO2, and its CO2 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 CO2 should be
maintained below 2 vol% (or mole %).
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Table 1.2: Typical pipeline gas specifications (Younger, 2004)
Specification Trans Alberta West coast West coast CanadianItem Canada & Southern for US use for BC use Western
Minimum heating value (MJ/m3) 35.4 36.3 37.3 37.3 35.4
Hydrocarbon Dew point (K at 54.4 atm)
‒
Hydrogen Sulphide (Grains per 2.8 m3)
Mercaptans (Grains per 2.8 m3)
Total Sulphur (Grains per 2.8 m3) 20 1 20 20 10
Carbon dioxide (mol %)
Oxygen content (mol %) ‒ 0.40 0.20 1 ‒
Delivery Temperature (K) 320 320 ‒ ‒ 320
Delivery Pressure (atm) 61.2 61.2 varies varies 34
Water Content (kg/MMm3) 64 64 64 64
264 264 Free of Liquids Free of Liquids ‒
‒ 0.20 5 ‒ ‒
1 0.25 0.25 1 1
2 2 1 ‒ ‒
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1.2 Technology for CO2 separation
Technology selection for CO2 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 CO2 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 CO2 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 CO2 (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 CO2 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 CO2 from natural gas. In this
study, focus has been limited to the adsorption process for separating CO2 from high-
pressure natural gas.
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Figure 1.2 Carbon dioxide gas removal technologies
(Redrawn from Kohl and Nielsen, 1997)
CO2 Removal Technology
Solvent Absorption
Chemical
Amine
MEA
DGA
DEA
DIPA
MDEA
Alkali Salt
Benfield
Catacarb
Vetro-coke
Flexorb HP
Physical
Selexol
Rectisol
Ifpexol
PurisolHybrid
Solid Adsorption
PSA
TSA
VSACryogenic
Membrane
Cellulose acetatePoly-imidePoly-
sulfoneDirect
Conversion Stretford
Molecular Gate
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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
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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 carbon-
based adsorbents for gas separation are carbon molecular sieves and activated carbons.
1.4 Research motivation
The design and operation of the CO2 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 CO2 from the natural gas
succeeds through the use of appropriate adsorbent.
At present, there are a number of adsorbents capable of removing CO2 from natural
gas. As mentioned earlier, zeolites, carbon molecular sieves, and activated carbons have great
potential to remove CO2 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 CO2 separation from
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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 CO2 gas on different adsorbents were reported. Most of these projects focused on
adsorption equilibrium (or adsorption isotherms) at different temperatures and pressures. The
CO2 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 CO2 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 CO2 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 CO2 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 CO2 feed pressures. Rutherford et al.
(2003) and Bae and Lee (2005) reported the CO2 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 CO2 adsorption on zeolite 5A below atmospheric
pressure. Perez and Armenta (2010) and Yucel and Ruthven (1980) presented CO2 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 CO2
adsorption process, the behaviour of CO2-adsorption kinetics and equilibrium must be
characterized systematically for the selected adsorbents.
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1.5 Objective and scope
The objective of this study is to characterize the CO2 adsorption equilibrium and CO2
adsorption kinetics of the six adsorption materials selected to determine the most suitable
adsorbent for CO2 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 CO2 gas adsorption using different adsorbents, and the adsorbents’ potential
in applicability for CO2 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 CO2 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 CO2 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 CO2 adsorption, are provided in Chapter 2. Details of
the experimental set up, materials used, experimental procedures, and test conditions for the
CO2 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 CO2 adsorption. Conclusions and
recommendations for future work are given in Chapter 5.
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2. CO2 ADSORPTION CHARACTERISTICS AND LITERATURE REVIEW
Carbon dioxide adsorption is a mass-transfer process taking place when a gas stream
containing CO2 and particles of a porous material (or adsorbent) are brought into direct
contact in order to allow the CO2 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 CO2 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 CO2 is released under controlled pressure and temperature, and the adsorbent
capacity is restored for more adsorption. The performance of CO2 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
13
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 CO2 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.
2.1.1 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, qm 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
14
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" (qi) can be calculated using this model equation for a
multi-component system.
2.1.2 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.
2.1.3 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)
15
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.
2.1.4 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* = 푘 푃 / (2.9)
where 푘 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).
2.1.5 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.
16
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.
2.1.7 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.
2.1.8 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)
17
where the parameter 푏 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 multi-
component gas adsorption. The ideal framework can be written as (Do, 1998):
= ∑ (2.14a)
= = -∫ d푃 (2.14b)
Pyj = 푃 xj (2.14c)
∑ 푥 =1 (2.14d)
where n is the total number of moles per unit mass of the adsorbent, xj is the molar fraction of
component j in the adsorbed phase, 푛 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, 푃 is the adsorption pressure of the pure component j, P is the equilibrium
pressure, and yj 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 CO2 on zeolite-based adsorbents. Real
Adsorption Solution Theory (RAST), which includes an activity coefficient for non-ideality
18
of systems, can provide better prediction. The equations of the RAST model can be written
as (Costa et al., 1981; Talu and Zwiebel, 1986):
Pyi = xi γi (푥) 푃 (Z) (2.15a)
∑ 푥 = 1 (2.15b)
z = = ∫ ( ) dP0 (2.15c)
= ∑ + ∑ 푥 ( )X (2.15d)
Cµi = xi CµT (2.15e)
where P is the equilibrium pressure, yi is the molar fraction of component i in the gas phase,
xi is the molar fraction of component i in the adsorbed phase, γi is the activity coefficient of
the component i, z is the reduced spreading pressure, 푃 is the hypothetical pressure of the
pure component i, 퐴 is the adsorption potential, π is the spreading pressure, 퐶 is the
adsorbed amount of pure component i at the hypothetical pressure, CᆌT is the total amount
adsorbed, and Cᆌi 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
19
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.
20
Figure 2.1 Internal view of adsorbent particles
(Modified from Do, 1998).
21
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, kf is the external film mass transfer coefficient, KH is the Henry’s
constant, RP is the macroparticle radius, DP is the macropore diffusion coefficient, Dc is the
intra-crystalline concentration dependant diffusion coefficient, rc is the microcrystal radius,
and 휺p 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)
22
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, 푘is
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 푙푛 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
23
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 of푙푛푃
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 (Ea) can be expressed using the following Arrhenius equation (Zhang et al.,
2010):
푘 = 퐴푒 / (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 = - + 푙푛퐴 (2.21)
The activation energy (Ea) can be analyzed directly from the slope of a plot between 푙푛푘 and
reciprocal of temperature (1/T).
24
2.5 Literature review on adsorbents for CO2 removal
This section provides a review of studies that were carried out to evaluate the
performance of potential CO2 removal adsorbents in terms of equilibrium, kinetics, and other
associated adsorption characteristics.
2.5.1 CO2 adsorption by zeolite 13X
Most research studies on CO2 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 CO2 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 CO2 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 CO2 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 CO2 from flue gas containing humidity and other impurities. They also provided
the adsorption isotherm at 1.2 atm and breakthrough analysis of CO2 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 CO2 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
25
pressures up to 4.9 atm and reported the adsorption isotherm, as well as breakthrough curves,
of CO2 adsorption.
For studies at high pressure ranges, Cavenati et al. (2004) measured the CO2 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.
2.5.2 CO2 adsorption by zeolite 5A
The adsorption of CO2 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 CO2 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 CO2 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 CO2 from H2 gas at 27.5 atm.
26
2.5.3 CO2 adsorption by zeolite 4A
Eagan and Anderson (1975) investigated the CO2 adsorption isotherm for zeolite 4A at
pressures up to 1.02 atm. Yucel and Ruthven (1980) measured the CO2 adsorption kinetics at
different temperatures at 0.4 atm. Siriwardane et al. (2001) reported the use of zeolite 4A for
the separation of CO2 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.
2.5.4 CO2 adsorption by carbon molecular sieve
Kikkinides and Yang (1993) reported the equilibrium capacity of a carbon molecular
sieve for CO2 adsorption from flue gas. Mochida et al. (1995) investigated the CO2
adsorption isotherm of a carbon molecular sieve for the removal of CO2 from methane at 1
atm. Also, for CO2 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 CO2 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 CO2 adsorption performance at 39.5 atm. Castello et al. (2004)
reported the CO2 adsorption isotherm at 29.6 atm. Bae and Lee (2005) investigated the
adsorption equilibrium and kinetics at 14.9 atm.
2.5.5 CO2 adsorption by activated carbon
Most research studies on CO2 adsorption by activated carbon have been carried out at
high pressure ranges. Amoros et al. (1996) studied the CO2 adsorption characteristics of
27
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 CO2 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 CO2 adsorption equilibrium.
Sudibandriyo et al. (2003) investigated CO2 adsorption on activated carbon at a very high
pressure of 133.3 atm. Drage et al. (2009) reported the equilibrium capacity and kinetics of
CO2 adsorption from synthetic gas at 39.5 atm.
For low pressure applications, Chue et al. (1995) investigated the CO2 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 CO2 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 2.5.
28
Table 2.1: Previous works on CO2 adsorption on zeolite 13X
Author Technique 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 & 308 0 - 1 X - - - -
Calleja et al., 1994 Volumetric 293 0 - 1 X - - - -
Chue et al., 1995 Volumetric 288 0 - 1 X - - - -
Lee et al., 2002 Volumetric 273-353 0 - 1 X - X - -
Cavenati et al., 2004 Gravimetric 298, 308 & 323 0 - 49 X - X - -
Harlick & Tezel, 2004 Micrometrics 295 0 - 2 X - - X -
Siriwardane et al., 2005 Volumetric 393 0 - 20 X - - X -
Merel et al., 2008 - 298 & 323 0 - 5 X - - - -
Li et al., 2008 Micrometrics 293, 313 & 363 0 - 1 X - - - -
Zhang et al., 2009 Micrometrics 293, 313 & 363 0 - 1 X - - - -
Wang & Levan, 2009 Volumetric 228 - 448 0 - 1 X - - - -
Zhang et al., 2010 Gravimetric 298 - 328 0 - 30 X X - - X
X = Data Available
Experimental conditions
29
Table 2.2: Previous works on CO2 adsorption on zeolite 5A
Author Technique 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 193 - 372 0 - 0.5 X - - - X
Chen et al., 1990 Volumetric 298 0 - 54 X - X - -
Kim et al., 1995 - 288 - 313 0 - 27 X - - X -
Triebe & Tezel, 1995 Volumetric 243 - 383 - - - - X -
Pakseresht et al., 2002 Volumetric 303 - 573 0 - 10 X - - X -
Tlili et al., 2009 Gravimetric 298 - 473 0 - 1 X - - - -
Saha et al., 2010 - 298 0 - 1 X - - - -
Liu et al., 2011 Gravimetric 303 - 423 0 - 1 X - X - -
Experimental conditions
X = Data Available
30
Table 2.3: Previous work on CO2 adsorption on zeolite 4A
Author Technique 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 - 0 - 1 X - - - -
Yucel & Ruthven, 1980 Gravimetric 273 - 371 0 - 0.5 X X - - X
Siriwardane et al., 2001 Volumetric 298 0 - 20 X - - - -
Ahn et al., 2004 Gravimetric 273 - 313 0 - 0.9 X X - - X
Experimental conditions
X = Data Available
31
Table 2.4: Previous works on CO2 adsorption on carbon molecular sieves
Author Technique 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 303 0 - 1 - - - - -
Amoros et al., 1998 Gravimetric 273 0 - 39 X - - - -
Reid & Thomas, 1999 Gravimetric 303 - 343 0 - 1 X X X - -
Jayaraman et al., 2002 Gravimetric 303 - 343 1 - 5 X X - X X
Rutherford et al., 2003 Volumetric 323 & 343 0 - 3 X X - X X
Castello et al., 2004 Gravimetric 273 0 - 30 X - - - -
Tan & Ani, 2004 Micrometrics 298 - X - - - -
Rodil et al., 2005 Volumetric 273 - X X - - -
Castello et al., 2005 Gravimetric 298, 313 & 328 0 - 1 X X - X -
Bae & Lee, 2005 Volumetric 293, 303 & 313 0 - 15 X X - - -
Experimental conditions
X = Data Available
32
Table 2.5: Previous works on CO2 adsorption on activated carbon
Author Technique 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 288 - 343 0 - 1 X - - - -
Amoros et al., 1996 Gravimetric 298 0 - 39 X - - - -
Dreisbach et al., 1999 Gravimetric 298 0 - 59 X - - - -
Vaart et al., 2000 Gravimetric 292 - 349 0 - 8 X - - - -
Siriwardane et al., 2001 Volumetric 298 0 - 20 X - X - -
Sudibandriyo et al., 2003 Gravimetric 318 0 - 134 X - - - -
Millward & Yaghi, 2005 Volumetric - 0 - 44 - - - - -
Guo et al., 2006 Vacuum 303 - 333 0 - 0.5 X - X - -
Drage et al., 2009 Volumetric 303 0 - 39 X - - - -
Experimental conditions
X = Data Available
33
3. CO2 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 CO2 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 CO2 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 Na2O: 1 Al2O3: 2.8 ± 0.2 SiO2: xH2O. 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/nNa12-2n [(AlO2)12(SiO2)12]
·xH2O. 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 Na12 [(AlO2)12(SiO2)12] · xH2O. The carbon molecular sieve (MSC
3R) used in this study has uniform super-micropores with a diameter of less than 10 Å.
34
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 12-40 mesh for GCA-1240. 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 CO2 gases used in this study were ultra pure with purity grades of
99.999%. 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 CO2 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 122.5 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: 68072-58) 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: 68072-60) connected to the adsorption cell could measure up to
35
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: 09082411) 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-79200-00) 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: 137097-10). 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.
36
Table 3.1: Physical properties of different adsorbents
Adorbent Physical properties ReferencesZeolite 13X Nominal pore diameter : 8 Å Sigma Aldrich
Particle Diameter : 1.6 x 10-3 mBulk density : 640.7 kg/m3
Crush Strength : 3.2 kgSurface area : 7.2 x 105 m2/kgpore volume : 3.2 x 10-4 m3/kg
Zeolite 4A Pore diameter : 4 Å Sigma AldrichBulk density : 720.8 kg/m3
Regeneration temp : 473-588 KHeat of Adsorption : 4.18 MJ/kg H2O
Zeolite 5A Pore diameter : 5 Å Sigma AldrichBulk density : 704.8 kg/m3
Regeneration temp : 473-588 KHeat of Adsorption : 4.18 MJ/kg H2O
GCA-830 Surface area : 1.15 x 106 m2/kg Norit Americas Inc.Bulk density : 464.5 kg/m3
GCA-1240 Surface area : 1.15 x 106 m2/kg Norit Americas Inc.Bulk density : 496.6 kg/m3
MSC-3R Pellet diameter : 1.8 X 10-3 m Japan Enviro-Chemicals, Ltd.
37
Figure 3.1: Photographs of adsorbents used in this study (original in colour)
(b) Zeolite 5A (a) Zeolite 13x
(c) Zeolite 4A (d) MSC-3R
(e) GCA-830 (f) GCA-1240
38
Data AcquisitionSystem
Thermostatic Water Bath
PS(Pressure Sensor)
CO2 He
Storage Cell
Pressure Regulator
Vacuum Pump
TS(Temperature
Sensor)
PS
TS
Adsorption Cell
Manualvalve
Computer
Data Acquisition
Vacuum Pump
Gas Storage
ThermostaticWater Bath
Adsorption Cell
Pressure Regulator
Figure 3.2: Experimental apparatus for the measurement of CO2 adsorption on
adsorbents
(a) Schematic diagram of gas adsorption apparatus
(b) Photograph of the experimental setup (original in colour)
39
3.3 Experimental procedures
The pressure difference technique was used to determine the CO2 adsorption
isotherms and uptake rate. The amount of gas adsorbed in the adsorption cell could be
evaluated from the change in pressure of the storage vessel at a fixed temperature. As
CO2 from the storage vessel was adsorbed by the adsorbent in the adsorption cell, the
pressure of the storage vessel decreased rapidly and then became stable as soon as the
adsorption cell reached pressure equilibrium. The difference between the initial pressure
and final pressure was used for isotherm analysis while the reduction in pressure with
time was used for uptake rate analysis. Details of the experimental procedures starting
from adsorbent preparation are given below.
3.3.1 Preparation of adsorption cell
Prior to the adsorption experiment, the selected adsorbent was heated in a
convection oven so as to remove any moisture and unwanted gases. After being heated
overnight, the adsorbent was cooled down at room temperature in a desiccator. The
adsorbent was then weighed and loaded into the adsorption cell. The loaded adsorption
cell was kept under vacuum to minimize the amount of gas retained within the void
volume of the bed.
3.3.2 Determination of void volume
After the adsorption cell was properly prepared, its void volume was determined
and further used for estimating the amount of gaseous component that did not
participated in the adsorption process but occupied this empty space during each
adsorption experiment. The measurement of void volume was done through the
40
displacement technique using helium gas supplied from the storage vessel. Using the data
acquisition (DAQ) system, pressure and temperature of the storage were measured and
recorded before and after the displacement. The difference in pressure and temperature
was translated into the void volume (푉 ) using the following ideal gas equations:
Vvoid = ∗ ∗ ∗
(3.1)
푛 = 푛 − 푛 = −
(3.2)
where Pf and Tf are the pressure and temperature of the adsorption vessel, respectively; Zf
is the compressibility factor at Pf and Tf; R is the universal gas constant; 푛 is the
number of moles of helium gas released from the storage vessel at the initial and final
conditions; 푉 is the volume of the storage vessel; and P1, T1, and Z1 are the
pressure, temperature, and compressibility factor of the storage vessel at the initial
conditions while P2, T2, and Z2 are those at the final conditions. The compressibility
factor can be calculated from the following equations (Salehi et al., 2007):
Z = 1+(퐵 + 0.01퐵 ) ( ) (3.3a)
B0 = 0.083 − .
. (3.3b)
B1 = 0.139 – .
. (3.3c)
Pr = (3.3d)
Tr = (3.3e)
where Tc and Pc are the critical temperature and critical pressure, respectively.
41
3.3.3 Determination of CO2 adsorption performance
To measure the adsorption performance of the adsorbent, the storage vessel was
first charged with a CO2 gas, and its pressure and temperature were recorded as the initial
conditions. The CO2 gas from the storage was then introduced into the adsorption cell
with a specific pressure controlled by the pressure regulator. At this point, adsorption
activity started to take place in the adsorption cell, as was observed from the reduction in
pressure of the storage vessel. This adsorption activity continued until the storage
pressure reached a new equilibrium value, which was set as the final pressure and
temperature. Depending upon the test conditions, it could take from minutes to hours to
reach the new pressure equilibrium. Note that the pressure reduction of the storage vessel
was measured and recorded continuously as a function of time. This real-time
information was used for calculating the CO2 adsorption uptake rate. The following are
the equations for calculating the amount of gas adsorbed on solid adsorbent (푛 ):
nadsorbent = 푛 − 푛 (3.4)
nvoid = (3.5)
where 푛 is the number of moles of CO2 released from the storage vessel, 푛 is the
number of moles of CO2 that occupies the void space in an adsorption vessel, Peq and Teq
are the pressure and temperature of an adsorption vessel under equilibrium or final
conditions, and Zeq is the compressibility factor of the adsorbed gas under equilibrium
conditions.
42
3.4 Experimental conditions
Experimental conditions for the measurement of adsorption isotherm and kinetics
are given in Table 3.2.
43
Table 3.2: CO2 adsorption capacity test conditions for different adsorbents
AdsorbentsTemperature (K) Pressure (atm) Temperature (K) Pressure (atm)
Zeolite 13X 293, 303, 313, 0 - 35.4 293, 313 & 333 3.4, 5.4, 6.8,323 & 333 8.8 & 11
Zeolite 5A 293, 303, 313 0 - 35.5 293, 313 & 333 3.4, 5.4, 6.8,& 333 8.8 & 11
Zeolite 4A 293, 303, 323 0 - 35.4 293, 313 & 333 3.4, 5.4, 6.8,& 333 & 8.8
MSC-3R 293, 303, 313, 0 - 35.4 293, 313 & 333 3.4, 5.4, 6.8,323 & 333 8.8 & 11
GCA-830 293, 303, 313, 0 - 35.6 293, 313 & 333 3.4, 5.4, 6.8,323 & 333 8.8 & 11
GCA-1240 293, 303, 313, 0 - 35.4 293, 313 & 333 3.4, 5.4, 6.8,323 & 333 8.8 & 11
Adsorption Isotherm Adsorption kinetics
44
3.5 Validation of experimental method
The experimental method was validated by comparing the CO2 adsorption
isotherms obtained in this study with those reported in the open literature. Based on the
availability of the published data, the comparison was made at 293 K and pressure up to
34 atm for two adsorbents (i.e. zeolite 13X and activated carbon GCA-1240). It is clear
from Figure 3.3 that the CO2 adsorption isotherms in this study remained within the range
of the published data, thereby validating the experimental method and analysis carried
out in this study. Figure 3.3 also indicates significant deviations in the isotherm data
among the published works as the difference in adsorbent physical property (such as pore
volume, pore diameter and available surface area) results in the variation of CO2
adsorption capacity on the adsorbent surface.
45
Figure 3.3: Comparison of CO2 adsorption isotherm data with published data for (a)
zeolite 13X and (b) GCA -1240
0.0
2.0
4.0
6.0
8.0
0 10 20 30 40
Amou
nt of
ads
orbe
d CO
2(m
mol
/g)
Pressure (atm)
This StudyCavenati et al., 2004Ko et al., 2005Siriwardane et al., 2005Zhang et al., 2010
0
2
4
6
8
10
0 10 20 30 40
Am
ount
of ad
sorb
ed C
O2
(mm
ol/g
)
Pressure (atm)
This StudyBerlier & Frere, 1997Dreisbach et al., 1999Himeno et al., 2005Goetz et al., 2006
(a)
(b)
46
4. RESULTS AND DISCUSSION
This chapter presents the experimental results of CO2 adsorption on the selected
adsorbent materials: zeolite 13X, zeolite 5A, zeolite 4A, carbon molecular sieve (MSC-
3R) and two activated carbons (GCA-830 and GCA-1240). The CO2 adsorption
experiments were conducted under test conditions provided earlier in Chapter 3. The
results reported here can be categorized into two main aspects: i) CO2 adsorption
equilibrium indicating the maximum capacity of each adsorbent material and ii) CO2
adsorption kinetics indicating the rate of adsorption activity. The effects of process
parameters (temperature and pressure) on the adsorption behaviour are also discussed in
this chapter. The equilibrium data were analyzed further for heat of adsorption while the
kinetic data were analyzed for mass transfer coefficient and activation energy.
4.1 CO2 adsorption equilibrium
4.1.1 Isotherm curves
In this study, the CO2 adsorption equilibrium for each adsorbent was measured
under a series of isothermal conditions (i.e., 293 K, 303 K, 313 K, 323 K and 333 K). For
a given temperature, the adsorption equilibrium was obtained as a series of data pairs that
shows the maximum amount CO2 adsorbed (or CO2 loading in the unit of mmol CO2/ gm
of adsorbent) at the corresponding pressure of CO2 gas. The measured equilibrium data
for all adsorbents are given in Appendix A. The relationships between the adsorbed
amount of CO2 and equilibrium pressure, commonly known as isotherm curves, are given
in Figures 4.1 through 4.6 (for zeolite 13X, zeolite 5A, zeolite 4A, MSC-3R, GCA-830
and GCA-1240, respectively).
47
Figure 4.1: Isotherm curves of CO2 adsorption on zeolite 13X at different temperatures.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K313 K323 K333 K
48
Figure 4.2: Isotherm curves of CO2 adsorption on zeolite 5A at different temperatures.
0.0
1.0
2.0
3.0
4.0
5.0
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K313 K333 K
49
Figure 4.3: Isotherm curves of CO2 adsorption on zeolite 4A at different temperatures.
0.0
1.0
2.0
3.0
4.0
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K323 K333 K
50
Figure 4.4: Isotherm curves of CO2 adsorption on MSC-3R at different temperatures.
0.0
1.5
3.0
4.5
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K313 K323 K333 K
51
Figure 4.5: Isotherm curves of CO2 adsorption on GCA-830 at different temperatures.
0.0
2.1
4.2
6.3
8.4
10.5
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K313 K323 K333 K
52
Figure 4.6: Isotherm curves of CO2 adsorption on GCA-1240 at different temperatures.
0.0
2.1
4.2
6.3
8.4
10.5
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
293 K303 K313 K323 K333 K
53
Note that the isotherm curves reported here were obtained from the experiments
using pure CO2 gas. These pure component isotherms can be used together with the real
adsorption solution theory (RAST) model for the design of a multi-component adsorption
system. From Figures 4.1 through 4.6, all isotherm curves exhibit common behaviour
regardless of temperature and adsorbent type (i.e., the amount of CO2 adsorbed on the
adsorbent increases very rapidly with the increase in equilibrium pressure over the low
pressure range, and it tends to stabilize as the pressure continues to increase). This
isotherm behaviour follows the type-I isotherm category according to IUPAC adsorption
isotherm classification (Keller et al., 2005), which indicates a monolayer adsorption
mechanism, commonly applied to microporous adsorbents (Thomas and Crittenden,
1998). The rapid increase in the isotherm curve demonstrates the proper range of
equilibrium pressure that promotes the majority of adsorption activity. This is important
information that can be used to identify both adsorption pressure and regeneration
pressure in the pressure swing adsorption process. The stabilized amount of CO2
adsorbed in the curve shows that all adsorption sites are occupied by CO2 gas, which
represents the capacity limit of the adsorbent at the particular temperature.
Figures 4.1 through 4.6 also reveal the typical behaviour showing the effect of
temperature on the CO2 adsorption capacity. That is, an increase in adsorption
temperature leads to a reduction in the amount of adsorbed CO2. Rising temperature
simply provides more internal energy to CO2 molecules in the gas phase. It should be
noted that the increasing energy allows gaseous molecules to diffuse at a greater rate, but,
at the same time, it reduces the chance for the CO2 to be restrained or trapped by fixed-
energy adsorption sites on the adsorbent surface.
54
Figure 4.7 shows a comparison of the CO2 adsorption isotherms among the
adsorbents tested in this study. The comparison was made at the lower-bound and upper-
bound temperatures: 293 K and 333 K, respectively. At 293 K, the activated carbons
(GCA-830 and GCA-1240) offered an adsorption capacity of 10.0 mmol/g, the highest
capacity among all the adsorbents tested. Zeolite 13X, zeolite 5A, and MSC-3R offered
CO2 adsorption capacities of 7.0, 4.7 and 4.2 mmol/g, respectively. Zeolite 4A provides
the lowest capacity of 3.8 mmol/g. The ranking of CO2 adsorption capacity, then, can be
written as GCA-1240 ˃ GCA-830 ˃ zeolite 13X ˃ zeolite 5A ˃ MSC-3R ˃ zeolite 4A.
This ranking remains unchanged as the temperature increases to 333 K. At the higher
temperature, the CO2 adsorption capacity for each adsorbent was reduced by 20 to 30 per
cent. The capacities at 333 K are 6.8 mmol/g for GCA-1240, 6.5 mmol/g for GCA-830,
5.2 mmol/g for zeolite 13X, 3.5 mmol/g for zeolite 5A, 3.1 mmol/g MSC-3R and 3.0
mmol/g for zeolite 4A.
In addition to the comparison of CO2 adsorption capacity, Figure 4.7 also reveals
the strength of interaction between CO2 molecules and adsorption sites for individual
adsorbents, which can be observed from the slope of the isotherm curves (during the
rapid increase). It appears that the zeolite-based adsorbents (zeolite 13X, zeolite 5A, and
zeolite 4A), exhibiting greater slope of the isotherm curves, have stronger adsorption sites
compare to the carbon molecular sieve (MSC-3R) and activated carbons (GCA-830 and
GCA-1240). The specific values of such interactions are reported in a later subsection
addressing the isosteric heat of adsorption for individual adsorbents.
55
Figure 4.7: Comparison of isotherm curves among the adsorbents tested (a) 293 K; (b)
333 K.
0.0
2.1
4.2
6.3
8.4
10.5
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
Zeolite 13x Zeolite 5AZeolite 4A MSC-3RGCA-830 GCA-1240
0.0
1.4
2.8
4.2
5.6
7.0
0 10 20 30 40
Am
ount
of a
dsor
bed
CO
2(m
mol
/g)
Pressure (atm)
Zeolite 13x Zeolite 5AZeolite 4A MSC-3RGCA-830 GCA-1240
(a)
(b)
56
4.1.2 Isotherm correlations
To allow the use of the obtained adsorption isotherms for different purposes, the
equilibrium data were correlated into different isotherm models. These models are the
Langmuir, Toth, Sips, and Prausnitz equations presented previously in Chapter 2. Some
other isotherm models (Such as Volmer, Hill de Boer, Fowler-Guggenheim and Unilan)
which were discussed in Chapter 2 were not correlated with experimental isotherm data
in this thesis work because of the complexity of calculation method. Nonlinear regression
analysis was performed to determine the model parameters for individual adsorbents. The
obtained regression results for individual temperatures are given in Tables 4.1 through
4.5. The average percent deviation quantity, ∆q, or the adsorbed amounts were calculated
using the following formula (Morse et al., 2010):
∆q% = ∑ ⃒ ⃒ (4.1)
where h is the number of experimental data and Nexp and Ncal are the experimental and
calculated number of moles that are adsorbed by the adsorbent pellet. From Tables 4.1 to
4.5, it can be seen that the Sips model equation provides the best fit with the isotherm
curves for zeolite 13X, zeolite 5A, zeolite 4A, and carbon molecular sieve (MSC-3R),
which indicates that an adsorbed molecule can occupy more than one adsorption site
during the adsorption process (Morse et al., 2010). On the other hand, the Prausnitz
model equation provides the best fit for activated carbon (GCA-830 and GCA-1240),
which indicates that the adsorbent surface is not ideal but, rather, it is energetically
heterogeneous. It should be noted that both the Sips and Prausnitz models correlated the
obtained isotherm data well for temperatures ranging from 293 to 333 K. This enabled us
57
to develop a generic isotherm equation for each adsorbent that takes into account the
effect of adsorption temperature. The generic equations for corresponding adsorbent
materials are listed in Table 4.6.
58
Table 4.1: Regression parameters for different model equations at 293 K
Adsorbent a B t ∆q%Zeolite 13X Langmuir ‒ 3.40 ‒ 0.97 6.66
Toth ‒ 4.79 0.83 0.97 6.81Sips ‒ 3.47 1.15 0.97 6.98Prausnitz 28.29 6.44 0.02 0.98 5.23
Zeolite 5A Langmuir ‒ 4.38 ‒ 0.99 3.91Toth ‒ 6.00 0.84 0.99 3.73Sips ‒ 4.54 1.13 0.99 3.83Prausnitz 23.81 4.39 0.01 0.99 3.07
Zeolite 4A Langmuir ‒ 1.76 ‒ 0.98 5.29Toth ‒ 2.92 0.76 0.99 4.44Sips ‒ 1.77 1.25 0.98 4.72Prausnitz 9.18 3.19 0.05 0.99 3.42
MSC-3R Langmuir ‒ 0.83 ‒ 0.96 7.19Toth ‒ 0.65 1.21 0.98 4.82Sips ‒ 0.83 0.85 0.99 3.96Prausnitz 3.11 4.67 -0.02 0.98 4.69
GCA-830 Langmuir ‒ 0.44 ‒ 0.96 7.40Toth ‒ 0.31 1.28 0.99 3.93Sips ‒ 0.42 0.85 0.99 3.48Prausnitz 3.70 10.81 0.00 1.00 2.13
GCA-1240 Langmuir ‒ 0.43 ‒ 0.96 11.20Toth ‒ 0.30 1.32 0.97 5.91Sips ‒ 0.42 0.83 0.98 4.91Prausnitz 3.59 12.01 -0.02 0.98 5.94
Model R2Regression Parameters
59
Table 4.2: Regression parameters for different model equations at 303 K
Adsorbent a B t R2 ∆q%Zeolite 13X Langmuir ‒ 1.90 ‒ 0.95 11.79
Toth ‒ 1.18 1.58 0.97 9.13Sips ‒ 1.86 0.66 0.97 7.68Prausnitz 9.52 7.07 -0.04 0.96 10.77
Zeolite 5A Langmuir ‒ 1.52 ‒ 0.93 8.11Toth ‒ 0.82 1.68 0.97 5.63Sips ‒ 1.32 0.65 0.98 4.56Prausnitz 4.85 5.32 -0.06 0.96 7.00
Zeolite 4A Langmuir ‒ 0.85 ‒ 0.97 5.62Toth ‒ 0.61 1.23 0.99 4.25Sips ‒ 0.77 0.84 0.99 3.75Prausnitz 2.53 3.96 -0.03 0.98 4.81
MSC-3R Langmuir ‒ 0.75 ‒ 0.95 9.18Toth ‒ 0.40 1.63 1.00 2.49Sips ‒ 0.65 0.71 1.00 1.93Prausnitz 1.80 5.30 -0.09 1.00 2.74
GCA-830 Langmuir ‒ 0.36 ‒ 0.96 9.18Toth ‒ 0.20 1.55 1.00 2.20Sips ‒ 0.32 0.75 0.99 3.34Prausnitz 2.04 11.92 -0.07 1.00 1.05
GCA-1240 Langmuir ‒ 0.36 ‒ 0.97 11.08Toth ‒ 0.21 1.57 1.00 2.81Sips ‒ 0.33 0.75 0.99 4.48Prausnitz 2.17 12.71 -0.07 1.00 1.76
ModelRegression Parameters
60
Table 4.3: Regression parameters for different model equations at 313 K
Adsorbent a B t R2 ∆q%Zeolite 13X Langmuir ‒ 1.65 ‒ 0.96 8.58
Toth ‒ 1.09 1.40 0.97 6.14Sips ‒ 1.56 0.73 0.98 5.61Prausnitz 7.58 6.30 -0.04 0.97 7.18
Zeolite 5A Langmuir ‒ 1.33 0.90 16.35Toth ‒ 0.65 2.22 0.96 9.91Sips ‒ 1.21 0.52 0.98 6.88Prausnitz 3.64 5.71 -0.10 0.94 12.59
Zeolite 4A Langmuir ‒ ‒ ‒ ‒ ‒Toth ‒ ‒ ‒ ‒ ‒Sips ‒ ‒ ‒ ‒ ‒Prausnitz ‒ ‒ ‒ ‒ ‒
MSC-3R Langmuir ‒ 0.68 0.95 7.70Toth ‒ 0.36 1.64 1.00 1.27Sips ‒ 0.59 0.70 1.00 2.06Prausnitz 1.51 4.92 -0.09 1.00 0.94
GCA-830 Langmuir ‒ 0.31 ‒ 0.95 11.41Toth ‒ 0.16 1.66 1.00 2.74Sips ‒ 0.27 0.72 0.99 4.55Prausnitz 1.58 11.73 -0.07 1.00 1.57
GCA-1240 Langmuir ‒ 0.33 ‒ 0.96 11.43Toth ‒ 0.19 1.58 0.99 3.88Sips ‒ 0.30 0.75 0.99 5.48Prausnitz 1.84 10.94 -0.05 1.00 1.40
ModelRegression Parameters
61
Table 4.4: Regression parameters for different model equations at 323 K
Adsorbent a B t R2 ∆q%Zeolite 13X Langmuir ‒ 1.44 ‒ 0.95 11.92
Toth ‒ 0.91 1.53 0.97 8.14Sips ‒ 1.40 0.68 0.98 6.73Prausnitz 6.14 6.27 -0.05 0.96 9.74
Zeolite 5A Langmuir ‒ ‒ ‒ ‒ ‒Toth ‒ ‒ ‒ ‒ ‒Sips ‒ ‒ ‒ ‒ ‒Prausnitz ‒ ‒ ‒ ‒ ‒
Zeolite 4A Langmuir ‒ 0.53 ‒ 0.96 22.67Toth ‒ 0.29 2.11 1.00 4.66Sips ‒ 0.53 0.65 1.00 6.66Prausnitz 1.14 6.43 -0.18 0.99 10.11
MSC-3R Langmuir ‒ 0.59 ‒ 0.94 11.97Toth ‒ 0.28 1.84 1.00 2.38Sips ‒ 0.49 0.66 1.00 2.05Prausnitz 1.12 5.63 -0.13 1.00 2.94
GCA-830 Langmuir ‒ 0.30 ‒ 0.94 9.91Toth ‒ 0.15 1.72 0.99 3.66Sips ‒ 0.26 0.71 0.99 5.21Prausnitz 1.37 10.75 -0.07 1.00 0.56
GCA-1240 Langmuir ‒ 0.29 ‒ 0.96 13.12Toth ‒ 0.16 1.66 1.00 3.83Sips ‒ 0.27 0.74 0.99 6.07Prausnitz 1.37 10.67 -0.08 1.00 1.46
ModelRegression Parameters
62
Table 4.5: Regression parameters for different model equations at 333 K
Adsorbent a B t R2 ∆q%Zeolite 13X Langmuir ‒ 1.35 ‒ 0.96 12.08
Toth ‒ 0.84 1.53 0.98 7.15Sips ‒ 1.29 0.71 0.99 6.28Prausnitz 5.41 6.36 -0.06 0.97 8.69
Zeolite 5A Langmuir ‒ 1.66 ‒ 0.95 6.33Toth ‒ 0.92 1.61 0.98 3.41Sips ‒ 1.42 0.67 0.99 2.61Prausnitz 4.44 4.24 -0.05 0.97 4.53
Zeolite 4A Langmuir ‒ 0.34 ‒ 0.95 22.74Toth ‒ 0.19 2.73 1.00 2.47Sips ‒ 0.38 0.62 1.00 9.79Prausnitz 0.67 9.41 -0.30 0.99 7.93
MSC-3R Langmuir ‒ 0.56 ‒ 0.93 11.21Toth ‒ 0.26 1.90 1.00 2.42Sips ‒ 0.47 0.64 1.00 1.90Prausnitz 0.94 5.51 -0.15 1.00 2.64
GCA-830 Langmuir ‒ 0.27 ‒ 0.93 11.14Toth ‒ 0.13 1.84 0.99 3.74Sips ‒ 0.23 0.69 0.98 4.92Prausnitz 1.05 12.49 -0.12 1.00 0.76
GCA-1240 Langmuir ‒ 0.27 ‒ 0.95 13.16Toth ‒ 0.65 0.76 0.86 27.12Sips ‒ 0.24 0.72 0.99 5.89Prausnitz 1.11 12.00 -0.11 1.00 1.15
ModelRegression Parameters
63
Table 4.6: Isotherm correlation equations for adsorbents tested
Adsorbent Model Equation
Temperature Range (K)
Generic Isotherm Equation
Zeolite 13x
Sips
293 - 333
q = qT∗ P1.44 (71.43e−0.012 T )1.44
1+ P1.44(71.43e−0.012 T )1.44
Zeolite 5A
Sips
293- 333
q = qT∗ P1.64(6.3783e−0.005T )1.64
1+ P1.64(6.378e−0.005 T )1.64
Zeolite 4A
Sips
293 -333
q = qT∗ P1.43 (911.85e−0.023T )1.43
1+ P1.43(911.85e−0.023 T )1.43
MSC-3R
Sips
293 - 333
q = qT∗ P1.48 (8.473e−0.009 T )1.48
1+ P1.48(8.473e−0.009 T )1.48
GCA-830
Prausnitz
293 - 333
1q
= 11351.1e−0.021 T P
+ 1(58.13e−0.005 T )P−0.08
GCA-1240
Prausnitz
293 - 333
1q
= 12336.8e−0.023T P
+ 1(175.85e−0.009 T )P−0.08
64
4.1.3 Isosteric heat of adsorption
According to the Clausius-Clapeyron equation (2.19), it is possible to analyze
isosteric heat of adsorption from a series of isotherm curves. The Clausius-Clapeyron
equation can be rewritten as:
lnP = -∆ + constant (4.2)
Here, T is the adsorption temperature, P is the pressure, R is the gas constant, and ∆H is
the isosteric heat of adsorption. Analyzing Equation (4.2) helped identify the regeneration
energy as well as the interaction between adsorbent materials and gaseous molecules,
which provided important information on the level of heterogeneity of the solid surface.
As per the ideal Langmuir model (Langmuir, 1918), isosteric heat of adsorption should
be constant with the adsorbate or surface loading, suggesting that the solid surface is
energetically homogeneous. However, in the real system, the adsorbent’s surface is not
energetically homogeneous and the isosteric heat varies with the amount of adsorbate
retained on the surface (surface loading). Consequently, it is important to extract the
isosteric heat (∆H) from the slope of lnP ‒ (1/T) plot at a given adsorbent loading. Figure
4.8 show a series of these plots for individual adsorbents. The isosteric heats of
adsorption obtained from Figure 4.8 are listed in Table 4.7. The corresponding heat of
adsorption data are also presented in Figure 4.9 as a function of surface loading (or the
amount of CO2 adsorbed).
65
Figure 4.8: Plots of lnP versus 1/T for (a) zeolite 13x, (b) zeolite 5A, (c) zeolite 4A, (d) molecular sieve carbon MSC-3R, (e) activated carbon GCA-830, and (f) activated carbon GCA-1240
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
ln P
1/T x 103 (K-1)
0.5 mmol/g 1 mmol/g1.5 mmol/g 2 mmol/g2.5 mmol/g 3 mmol/g
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
lnP
1/T x 103 (K-1)
0.5 mmol/g 1 mmol/g1.5 mmol/g 2 mmol/g2.5 mmol/g 3 mmol/g
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
lnP
1/T x 103 (K-1)
0.5 mmol/g 0.75 mmol/g1 mmol/g 1.25 mmol/g1.5 mmol/g 1.75 mmol/g
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
ln P
1/T x 103 (K-1)
0.5 mmol/g 1 mmol/g1.5 mmol/g 2 mmol/g2.5 mmol/g
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
lnP
1/T x 103 (K-1)
0.5 mmol/g 1 mmol/g1.5 mmol/g 2 mmol/g2.5 mmol/g 3 mmol/g
-6.0
-4.5
-3.0
-1.5
0.0
1.5
3.0
2.8 3.0 3.2 3.4 3.6
ln P
1/T x 103 (K-1)
0.5 mmol/g 1 mmol/g1.5 mmol/g 2 mmol/g2.5 mmol/g 3 mmol/g
(a) (b) (a) (b)
(c) (d)
(e) (f)
66
Table 4.7 Isosteric heat of CO2 adsorption on the adsorbents
Adsorbent Adsorbate loading ∆H Average ∆H(mmol/g) (kJ/mol) (kJ/mol)
Zeolite 13X 0.50 40.101.00 39.701.50 39.302.00 39.002.50 38.703.00 38.60
Zeolite 5A 0.50 24.201.00 23.801.50 23.502.00 23.202.50 23.003.00 23.00
Zeolite 4A 0.50 46.400.75 45.001.00 43.601.25 42.201.50 40.601.75 38.90
MSC-3R 0.50 23.101.00 23.101.50 23.302.00 24.102.50 26.40
GCA-830 0.50 23.801.00 23.801.50 23.902.00 24.002.50 24.203.00 24.50
GCA-1240 0.50 23.401.00 22.401.50 22.102.00 22.002.50 22.203.00 24.30
22.73
39.23
23.45
42.78
24.00
24.03
67
It appears from Figure 4.9 that isosteric heat of CO2 adsorption on zeolite 13X,
zeolite 5A, and zeolite 4A decreases with the increasing adsorbate loading, indicating that
the interaction between adsorbent material and CO2 gas is rather strong at the initial stage
of adsorption and becomes weaker when the adsorption process continues. This
decreasing trend demonstrates that the regeneration of adsorbent material will be energy
intensive. Figure 4.9 also shows that the isosteric heat for activated carbons and the
carbon molecular sieve increases with the adsorbate loading. The reason for this
increasing trend is the Lennard-Jones potential energy of interaction between the
adsorbate molecules and adsorbent surface atoms, which depends on the position of the
adsorbate molecule and the size and shape of the pore of the adsorbent (Nguyen & Do,
1999). The increase in heat of adsorption with loading suggests that the adsorption is
reversible and the regeneration is not energy intensive. In addition, Figure 4.9 also
reveals the degree of difficulty regarding the adsorbent regeneration. The greater the heat
of adsorption, the more difficult and energy intensive the regeneration process. It is clear
that zeolite-based adsorbents would require more energy to regenerate compared to the
carbon based materials. Zeolite 4A clearly requires the highest regeneration energy
among all adsorbents tested.
68
Figure 4.9 Correlation plot of isosteric heat of adsorption versus adsorbate loading
20
25
30
35
40
45
0.0 2.0 4.0 6.0
Isos
teri
c he
at o
f ads
orpt
ion
(kJ/
mol
)
Amount of adsorbed CO2(mmol/g)
Zeolite 13XZeolite 5AZeolite 4AMSC-3RGCA-830GCA-1240
69
4.2 CO2 adsorption kinetics
It is very important to measure the kinetics of an adsorbent so as to evaluate its
suitability for the adsorption application required. An adsorbent having high capacity
with slow kinetics or an adsorbent having fast kinetics with low capacity might not be the
ideal choice for industrial applications. Slow kinetics result in long residence time in the
adsorption column and low capacity requires more adsorbent to attain the desired product
(Do, 1998). The CO2 adsorption kinetics for the six adsorbents were measured through a
series of uptake rate experiments at temperatures from 293 to 333 K and pressures up to
11 atm.
Figures 4.10 through 4.15 show plots of the CO2 uptake rates of the six adsorbents
at different pressures. From the figures, it can be seen that, in all cases, CO2 was adsorbed
quickly on the adsorbent surface at the initial stage of adsorption, then the rate became
slower with time, and, finally, it reached adsorption equilibrium. It was also observed that
the CO2 adsorption processes at the higher pressures reached equilibrium to a greater
degree and, yet, in a shorter period compared to those at the lower pressures. This
suggests that an increase in pressure promotes CO2 adsorption rate.
70
Figure 4.10: Plots of CO2 uptake for zeolite 13X at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2
(mm
ol / g
)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(a) (b)
(c) (d)
(e)
71
Figure 4.11: Plots of CO2 uptake for zeolite 5A at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
0.0
1.3
2.5
3.8
5.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.3
2.5
3.8
5.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.3
2.5
3.8
5.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.3
2.5
3.8
5.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.3
2.5
3.8
5.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(a) (b)
(c) (d)
(e)
72
Figure 4.12: Plots of CO2 uptake for zeolite 4A at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm.
0.0
1.0
2.0
3.0
4.0
0 150 300 450 600 750 900
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
0 150 300 450 600 750 900
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
0 150 300 450 600 750 900
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.0
2.0
3.0
4.0
0 150 300 450 600 750 900
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(a) (b)
(d) (c)
73
Figure 4.13: Plots of CO2 uptake for MSC-3R at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
0.0
0.9
1.8
2.7
3.6
4.5
0 170 340 510 680 850
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
0.9
1.8
2.7
3.6
4.5
0 170 340 510 680 850
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
0.9
1.8
2.7
3.6
4.5
0 170 340 510 680 850
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
0.9
1.8
2.7
3.6
4.5
0 170 340 510 680 850
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
0.9
1.8
2.7
3.6
4.5
0 170 340 510 680 850
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(a) (b)
(c) (d)
(e)
74
Figure 4.14: Plots of CO2 uptake for GCA-830 at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
0.0
1.7
3.4
5.1
6.8
8.5
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.7
3.4
5.1
6.8
8.5
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.7
3.4
5.1
6.8
8.5
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.7
3.4
5.1
6.8
8.5
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.7
3.4
5.1
6.8
8.5
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(b) (a)
(d) (c)
(e)
75
Figure 4.15: Plots of CO2 uptake for GCA-1240 at (a) 3.4 atm; (b) 5.4 atm; (c) 6.8 atm; (d) 8.8 atm; (e) 10.9 atm.
0.0
1.5
3.0
4.5
6.0
7.5
9.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.5
3.0
4.5
6.0
7.5
9.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.5
3.0
4.5
6.0
7.5
9.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.5
3.0
4.5
6.0
7.5
9.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
0.0
1.5
3.0
4.5
6.0
7.5
9.0
0 150 300 450 600 750
Am
ount
of a
dsor
bed
CO
2(m
mol
/ g)
Time (second)
293 K313 K333 K
(a) (b)
(c) (d)
(e)
76
4.2.1 Mass transfer coefficient for CO2 adsorption
It is essential to analyze the CO2 adsorption rate in terms of the mass transfer
coefficient for individual adsorbents for the purposes of comparison. In this study, the
mass transfer coefficient was analyzed from the plots of CO2 uptake rate presented
previously. The linear driving force (LDF) model representing the first order kinetics of
the adsorption process was used for the analysis. The LDF model was described in
Chapter 2. The integral form of the model was given earlier as Equation (2.18):
ln ∗∗
= - kt (2.18)
where k is the overall mass transfer coefficient presented as a function of adsorption
pressure and temperature. Here, the plots of CO2 uptake rate were translated into plots of
ln (1-q/q*) versus adsorption time (t) for the 70-80% of CO2 adsorption capacity of the
six commercial adsorbents, of which the slope represents the coefficient k at the
corresponding pressure and temperature. Figures 4.16 to 4.18 show plots of ln(1-q/q*)
versus time for all adsorbents tested at 293, 313, and 333 K, respectively. These plots
show linear behaviour, which confirms that the obtained experimental data follow the
LDF model and first order adsorption kinetics, validating the assumption of the data
analysis through LDF model. Mass transfer coefficients obtained from the LDF analysis
are reported in Table 4.8. From the table, it can be seen that, at a given pressure, mass
transfer coefficient increases with adsorption temperature. An increase in temperature
results in higher diffusion of CO2 molecules, providing greater mass transfer activities. It
can be seen from Figures 4.16 through 4.18 that zeolite 4A provided different mass
transfer behaviour, especially at the initial stage of adsorption, compared to the other
77
adsorbents tested. It seems that the mass transfer process took place relatively faster once
the adsorption started, and it was retarded as time progressed. This behaviour is probably
due to the micropore properties of zeolite 4A. The micropore diameter of zeolite 4A is
approximately 3.9 Å, and the molecular diameter of CO2 gas is 3.996 Å. Because of the
equivalent dimensions between adsorption pore and adsorbate molecule, the initial
adsorption occurred at a faster rate on the outer surface of zeolite 4A, and then the
diffusion through the pores took place later, at a slower rate. Table 4.8 show that the mass
transfer coefficient increased with an increased in pressure for the six commercial
adsorbents.
78
Figure 4.16: Linear plot of ln (1- ∗) versus time for CO2 adsorption at 293 K and
different pressures
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
(a) Zeolite 13X (b) Zeolite 5A
(c) Zeolite 4A (d) MSC-3R
(e) GCA-830 (f) GCA-1240
79
Figure 4.17: Linear plot of ln (1- ∗) versus time for CO2 adsorption at 313 K and
different pressures
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm
5.4 atm
6.8 atm
8.8 atm
10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
(a) Zeolite 13X (b) Zeolite 5A
(c) Zeolite 4A (d) MSC-3R
(e) GCA-830 (f) GCA-1240
80
Figure 4.18: Linear plot of ln (1- ∗) versus time for CO2 adsorption at 333 K and
different pressures
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm
5.4 atm
6.8 atm
8.8 atm
10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
-0.8
-0.6
-0.4
-0.2
0.0
0 150 300 450
ln (1
-q/q
*)
Time (second)
3.4 atm5.4 atm6.8 atm8.8 atm10.9 atm
(a) Zeolite 13X (b) Zeolite 5A
(c) Zeolite 4A (d) MSC-3R
(e) GCA-830 (f) GCA-1240
81
Table 4.8: Mass transfer coefficients of CO2 adsorption on the tested adsorbents
Pressure Temperature(atm) (K) Zeolite 13X Zeolite 5A Zeolite 4A MSC-3R GCA-830 GCA-12403.41 293 1.79 1.73 0.77 1.79 1.39 1.46
313 2.00 2.01 0.99 2.13 1.61 1.66333 2.23 2.23 1.56 2.31 1.82 1.90
5.44 293 2.84 2.14 0.92 2.22 2.86 2.95313 3.00 2.39 1.19 2.46 3.09 3.16333 3.20 2.65 1.74 2.72 3.29 3.34
6.81 293 3.60 2.74 1.31 3.04 4.46 4.68313 3.72 2.94 1.71 3.24 4.61 4.80333 3.93 3.20 1.95 3.48 4.72 4.93
8.84 293 5.34 3.52 1.65 3.97 5.53 6.00313 5.57 3.82 1.90 4.18 5.63 6.13333 5.65 3.98 2.10 4.38 5.72 6.20
10.88 293 6.29 5.16 ‒ 5.13 6.86 7.15313 6.41 5.35 ‒ 5.27 6.93 7.22333 6.50 5.65 ‒ 5.44 7.07 7.31
Mass transfer coefficients, k x 103 (1/sec)
82
4.2.2 Mass transfer coefficient and activation energy
From the previously shown Table 4.8, it can be inferred that an increase in
adsorption temperature results in an increase in mass transfer coefficient k regardless of
adsorbent material. This temperature effect can be demonstrated through the Arrhenius
equation shown earlier in Chapter 2:
lnk = - + lnA (2.21)
Here, the activation energy (Ea) can be extracted from the slope of the plot between lnk
and the reciprocal of temperature ( ). Figure 4.19 shows the linear plots of lnk versus ( )
based on the experimental data obtained in this study for zeolite 13x, zeolite 5A, zeolite
4A, MSC 3R, GCA-830, and GCA-1240. Table 4.9 summarizes the activation energy and
frequency factor A that were analyzed from Figure 4.19. It is clear that the activation
energy decreases with pressure, which suggests strong adsorbate-adsorbent interaction
potential at high pressure, and considering the heterogeneity of the adsorbent surface, the
CO2 adsorption at low pressure occurs mostly on adsorption sites with strong energy
barriers while adsorption at high pressure takes place on the sites with weak energy
barriers. Table 4.9 also shows that the frequency factor A increases with increasing
pressure, thereby promoting increased collisions between adsorbate molecules and
adsorbent materials, which results in a reduction in activation energy for the adsorption
process. Figure 4.20 shows the significance of the pressure effect on the activated energy
and the frequency factor. The effect of pressure was then correlated and included in the
Arrhenius-based mass transfer equation shown below from Equation (4.3) to Equation
(4.8):
83
Zeolite 13X: k = (0.01P2 + 0.48P + 0.67) exp (‒ . . . ) (4.3)
Zeolite 5A: k = (0.05P2 ‒ 0.28P + 2.94) exp (‒ . . . ) (4.4)
Zeolite 4A: k = (‒ 0.01P2 + 0.12P + 1.73) exp (‒ . – . . ) (4.5)
MSC-3R: k = (0.03P2 + 0.05P + 2.13) exp (− . – . . ) (4.6)
GCA-830: k = (‒ 0.02P2 + 0.96P - 0.96) exp (− . – . . ) (4.7)
GCA-1240: k = (‒ 0.03P2 + 1.10P - 1.36) exp (‒ . – . . ) (4.8)
Figure 4.21 shows a comparison of activation energy amongst the adsorbents tested
in this study. Regardless of pressure, the activation energy of zeolite 4A was much higher
than the activation energy of the other adsorbents tested. At pressures below 3.4 atm,
zeolite 5A and MSC-3R offered lower activation energies than zeolite 13X and the
activated carbons. However, at the higher pressure range, zeolite 13X and the activated
carbons required less activation energy than zeolite 5A and the carbon molecular sieve.
This comparison suggests that, for high pressure applications, zeolite 13X and the
activated carbons are the more suitable adsorbents for fast removal of CO2 from
industrial gas streams. However, for a specific application of CO2 removal from high-
pressure natural gas, zeolite 13X appears to be the best adsorbent because it tends to
adsorb less hydrocarbons compared to the activated carbons.
84
Figure 4.19: Linear plot of lnk versus ( ) for CO2 adsorption activation energy on (a)
zeolite 13X; (b) zeolite 5A; (c) zeolite 4A; (d) MSC-3R; (e) GCA-830; (f)
GCA-1240
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm10.9 atm
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm10.9 atm
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm10.9 atm
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm10.9 atm
-8.0
-7.0
-6.0
-5.0
-4.0
2.8 3.0 3.2 3.4 3.6
ln k
1/T x 103 (K-1)
3.4 atm 5.4 atm6.8 atm 8.8 atm10.9 atm
(a) (b)
(c) (d)
(e) (f)
85
Table 4.9: Activation energy (Ea) and frequency factor (A) for the tested adsorbents
Adsorbent Pressure Ea A x 103
(atm) (kJ/mol) (1/sec)Zeolite 13X 0.50 10.26 ‒
3.40 4.46 2.485.44 2.41 3.386.80 1.77 4.088.84 1.16 5.84
10.88 0.67 6.61Zeolite 5A 0.50 8.08 ‒
3.40 5.17 2.545.44 4.34 2.946.80 3.14 3.448.84 2.51 4.25
10.88 1.83 5.88Zeolite 4A 0.50 29.94 ‒
3.40 14.25 2.115.44 12.82 2.316.80 8.12 2.428.84 4.91 2.58
MSC-3R 0.50 7.80 ‒3.40 5.21 2.665.44 4.12 3.006.80 2.74 3.718.84 1.99 4.60
10.88 1.19 5.59GCA-830 0.50 12.04 ‒
3.40 5.47 2.085.44 2.85 3.536.80 1.15 4.868.84 0.69 5.82
10.88 0.61 7.16GCA-1240 0.50 11.17 ‒
3.40 5.34 2.155.44 2.52 3.556.80 1.05 5.058.84 0.67 6.31
10.88 0.45 7.38
86
Figure 4.20: Effect of pressure on activation energy and frequency factor for (a) zeolite
13X; (b) zeolite 5A; (c) zeolite 4A; (d) MSC-3R; (e) GCA-830; (f) GCA-
1240
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x10
3
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol )
Pressure (atm)
Activation energyFrequency factor
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x 1
03
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol )
Pressure (atm)
Activation energyFrequency factor
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x 1
03
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol)
Pressure (atm)
Activation energyFrequency factor
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x 10
3
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol )
Pressure (atm)
Activation energyFrequency factor
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x 1
03
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol )
Pressure (atm)
Activation energyFrequency factor
0
3
6
9
12
0
5
10
15
20
25
30
35
0 3 6 9 12 15
Freq
uenc
y fa
ctor
x 1
03
(1/se
c)
Act
ivat
ion
ener
gy( k
J/ m
ol)
Pressure (atm)
Activation energyFrequency factor
(b)
(c)
(a)
(d)
(e) (f)
87
Figure 4.21: Comparison of activation energy of six different adsorbents
0
3
6
9
12
15
0 3 6 9 12 15
Act
ivat
ion
ener
gy( k
J/ m
ol )
Pressure (atm)
Zeolite 13XZeolite 5AZeolite 4AMSC-3RGCA-830GCA-1240
88
5. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK
5.1 Conclusions
This thesis has significantly extended the knowledge of carbon dioxide (CO2)
separation from natural gas using a pressure swing adsorption process. A large set of
experimental data on CO2 adsorption equilibrium and kinetics was produced for six
commercial adsorbents including zeolite 13X, zeolite 5A, zeolite 4A, a carbon molecular
sieve (MSC-3R), and two activated carbons (GCA-830 & GCA-1240). The CO2
adsorption experiments were conducted using the volumetric method at different
temperatures and pressures. From this study, the following conclusions can be drawn:
The CO2 adsorption isotherm obtained in this study followed general gas
adsorption behaviour, demonstrating that the CO2 adsorption capacity increases
with increasing pressure and decreases with increasing temperature. The
adsorption isotherm follows a type-I isotherm classification according to IUPAC,
representing a monolayer adsorption mechanism. Among the six commercial
adsorbents tested, activated carbon GCA-1240 offers the highest adsorption
capacity, and zeolite 4A provides the lowest capacity for temperatures ranging
from 293 to 333 K and pressures up to 35 atm.
The equilibrium data of CO2 adsorption were correlated to fit with different model
equations (i.e., the Langmuir, Toth, Sips, and Prausnitz equations). It was found
that the Sips model showed the best fit with the equilibrium data for zeolite 13X,
zeolte 5A, zeolite 4A, and the carbon molecular sieve (MSC-3R) while the
Prausnitz model provided excellent fit with the data for the activated carbons
89
(GCA-830 and GCA-1240). The correlations for individual adsorbents were
reported as a function of temperature and pressure.
Isosteric heat of adsorption of CO2 adsorption on the six commercial adsorbents
was calculated using the Clausius-Clapeyron equation. It was found that the
isosteric heat of adsorption varied with CO2 loading, indicating the heterogeneity
of the adsorbent surface. The heat of adsorption of zeolite 13X, zeolite 5A, and
zeolite 4A decreased with increasing CO2 loading while the heat of adsorption of
the activated carbons and carbon molecular sieve increased with increasing CO2
loading.
A comprehensive set of mass transfer coefficients for CO2 adsorption on the six
adsorbents was produced for a temperature range of 293 to 333 K and pressure up
to 11 atm. Among the tested adsorbents, zeolite 4A showed the lowest mass
transfer coefficient while activated carbon GCA-1240 offered the highest
coefficient. Regardless of adsorbents, the effect of temperature on the mass
transfer coefficient followed the Arrhenius equation and the pressure exhibited a
non-linear effect on the mass transfer coefficient.
Activation energy of CO2 adsorption on the six adsorbents was analyzed using the
Arrhenius equation. This information revealed the amount of energy needed for
regenerating of the CO2 saturated adsorbent. It was found that the energy
decreased with pressure regardless of adsorbent. Among the six adsorbents, the
carbon molecular sieve had the lowest activation energy below pressure 3.4 atm
whereas zeolite 4A required the highest activation energy.
90
5.2 Recommendations for future work
To improve selection of the best adsorbent for the removal of CO2 from natural gas,
the following future research activities are recommended:
The kinetic and equilibrium selectivity for CO2 adsorption compared to other
gases (such as methane and ethane) should be investigated in the future for the six
adsorbents as selectivity is an important criterion to choose an adsorbent for the
separation of a gas from the mixture of gases.
The performance of combined adsorption-desorption cycles should be examined
to identify the proper cycling operation of the adsorbents which will provide the
necessary information regarding how long the adsorbent will show proper
separation efficiency for the separation of desired gas component.
Cost analysis (capital and operating expenditures) should be performed for each
adsorbent to be used in the natural gas purification applications so that it would be
possible to select an adsorbent from technological as well as economical point of
view.
91
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101
APPENDIX A
Experimental results of pure CO2 adsorption equilibrium
Table A.1 CO2 adsorption equilibrium data on zeolite 13X at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.06 0.54 0.29 1.13 0.35 1.22 0.37 0.97 0.36 0.920.06 1.05 0.34 1.98 0.44 2.13 0.47 1.86 0.48 1.810.07 1.61 0.44 2.76 0.66 2.98 0.66 2.71 0.74 2.740.09 2.17 0.57 3.51 0.90 3.75 0.87 3.35 1.23 3.510.12 2.63 0.80 4.19 1.59 4.28 1.60 3.96 1.90 3.910.13 3.13 1.34 4.71 2.77 4.66 3.33 4.50 3.32 4.380.19 3.65 2.53 5.17 4.28 4.91 5.70 4.81 6.30 4.760.30 4.11 5.24 5.55 5.74 5.07 10.06 5.05 12.86 5.050.45 4.50 9.35 5.80 11.00 5.32 15.28 5.18 20.64 5.120.63 4.78 14.75 5.94 18.60 5.43 23.88 5.22 29.40 5.150.83 5.04 22.72 6.00 23.94 5.49 31.10 5.25 34.19 5.151.27 5.31 30.67 6.01 31.23 5.49 35.00 5.262.21 5.61 34.48 6.06 34.12 5.523.84 5.896.61 6.14
10.55 6.3518.16 6.5325.02 6.6531.95 6.7833.79 6.8935.41 7.02
T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K
102
Table A.2 CO2 adsorption equilibrium data on zeolite 5A at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.11 0.68 0.47 1.13 0.47 0.74 0.48 1.170.14 1.64 0.71 2.26 0.76 2.14 0.93 2.280.23 2.55 1.44 3.24 1.73 3.23 1.77 2.850.58 3.48 2.32 3.66 3.45 3.61 3.72 3.161.25 3.85 3.96 3.85 6.12 3.78 6.55 3.333.96 4.15 6.63 3.98 16.74 3.94 11.22 3.448.23 4.33 10.22 4.09 25.06 3.99 16.57 3.49
15.39 4.51 15.69 4.18 32.65 4.02 22.60 3.5022.67 4.56 24.27 4.21 35.49 4.03 28.35 3.5228.82 4.60 32.80 4.28 34.63 3.5434.59 4.68 34.41 4.30
T = 293 K T = 303 K T = 313 K T = 333 K
103
Table A.3 CO2 adsorption equilibrium data on zeolite 4A at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.21 1.00 0.62 0.91 0.25 0.21 0.25 0.140.30 1.47 1.62 2.16 0.35 0.30 0.35 0.190.53 1.99 2.87 2.62 0.65 0.55 1.25 0.691.06 2.48 4.38 2.84 0.95 0.80 2.25 1.243.59 3.01 6.83 3.03 1.25 1.05 3.09 1.645.82 3.24 10.46 3.18 1.75 1.45 5.89 2.53
12.77 3.47 23.65 3.41 2.25 1.81 12.69 2.7524.10 3.59 32.02 3.49 2.75 2.12 24.05 2.9031.43 3.71 35.43 3.55 3.05 2.25 30.49 2.9534.51 3.82 5.82 2.75 34.73 3.01
12.78 2.9724.15 3.1130.57 3.1534.69 3.21
T = 293 K T = 303 K T = 323 K T = 333 K
104
Table A.4 CO2 adsorption equilibrium data on MSC-3R at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.34 0.63 0.60 0.74 0.84 0.95 0.69 0.54 1.07 0.750.54 1.10 1.11 1.50 1.38 1.47 1.48 1.29 2.31 1.580.75 1.57 1.82 2.04 2.30 2.03 2.74 1.94 4.89 2.230.95 1.93 2.78 2.50 4.56 2.65 4.37 2.40 8.12 2.601.25 2.23 4.60 2.96 7.97 3.05 6.63 2.74 12.59 2.892.11 2.79 6.58 3.22 10.61 3.21 10.46 3.02 21.24 3.044.92 3.43 9.64 3.44 15.42 3.34 14.24 3.15 30.50 3.087.98 3.73 11.88 3.54 18.86 3.39 19.07 3.21 34.78 3.1112.60 3.93 16.66 3.63 23.67 3.40 24.31 3.2218.62 4.06 22.85 3.63 27.01 3.40 28.14 3.2327.27 4.11 30.48 3.64 31.64 3.40 32.39 3.2333.72 4.18 34.53 3.66 34.48 3.41 35.03 3.2435.36 4.22
T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K
105
Table A.5 CO2 adsorption equilibrium data on GCA-830 at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.40 1.14 1.01 1.66 0.93 1.17 1.20 1.46 1.57 1.480.74 2.16 1.79 2.83 1.81 2.35 3.20 3.03 3.42 2.701.05 3.03 3.55 4.38 3.30 3.56 4.72 3.86 6.65 4.051.83 4.17 5.68 5.56 4.61 4.33 7.56 4.86 8.91 4.682.91 5.35 8.59 6.47 7.31 5.40 11.34 5.70 12.35 5.364.42 6.45 10.37 6.82 10.67 6.21 15.72 6.31 15.79 5.836.76 7.49 15.27 7.49 14.94 6.85 24.31 6.83 27.36 6.4410.69 8.47 23.75 8.00 19.86 7.29 32.07 7.03 34.35 6.5615.89 9.08 32.85 8.21 27.27 7.62 34.51 7.0923.98 9.50 35.56 8.27 33.68 7.7732.05 9.77 35.37 7.8135.02 9.88
T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K
106
Table A.6 CO2 adsorption equilibrium data on GCA-1240 at different pressures and temperatures
Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount Adsorption Amount pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed pressure adsorbed
(atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g) (atm) (mmol/g)
0.40 0.87 0.60 1.03 0.70 1.08 0.77 0.91 1.08 1.060.65 1.72 1.02 1.86 1.19 1.87 1.47 1.73 1.88 1.820.85 2.55 1.76 3.02 2.02 2.80 2.65 2.69 4.80 3.501.16 3.28 2.65 3.90 3.45 3.92 4.75 3.85 7.44 4.421.95 4.52 3.81 4.80 6.35 5.33 8.77 5.13 10.65 5.173.45 6.05 4.98 5.52 10.12 6.38 16.32 6.26 13.23 5.634.77 6.90 6.62 6.23 13.78 7.00 24.42 6.71 20.46 6.357.88 8.09 9.64 7.10 20.12 7.58 33.31 6.92 31.30 6.6711.95 8.96 13.83 7.78 25.74 7.84 35.37 6.7716.32 9.47 19.62 8.28 32.67 7.9923.20 9.87 25.30 8.54 34.44 8.0230.33 10.07 31.49 8.6434.07 10.25 34.76 8.69
T = 293 K T = 303 K T = 313 K T = 323 K T = 333 K