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Aromatic Hydrocarbon Sampling and Extraction FromFlames Using Temperature-Swing Adsorption/Desorption
Processes
by
Hei Ka Chan
A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science
Graduate Department of Mechanical and Industrial EngineeringUniversity of Toronto
Copyright c© 2011 by Hei Ka Chan
Abstract
Aromatic Hydrocarbon Sampling and Extraction From Flames Using
Temperature-Swing Adsorption/Desorption Processes
Hei Ka Chan
Master of Applied Science
Graduate Department of Mechanical and Industrial Engineering
University of Toronto
2011
The measurement of Polycyclic Aromatic Hydrocarbons (PAHs) in flames is essen-
tial for the understanding of soot formation. In comparison to conventional aromatics-
sampling techniques, a new technique was proposed that involves fewer manual operations
and no hazardous extraction solvents. Apparatus and experimental procedures of the
newly proposed adsorptive-sampling and desorptive-extraction technique for aromatic-
hydrocarbon measurements were established in this study. The capabilities and lim-
itations of this new technique were assessed in terms of limits of detection, sampling
locations and data repeatability.
The accuracy of this technique was also evaluated. Aromatic-hydrocarbon species
concentrations were measured in laminar co-flow diffusion flames of ethylene (C2H4) and
synthetic paraffinic kerosene (SPK). The results obtained from the ethylene flame were
compared to its numerical simulation, with the goal of achieving agreement within an
order of magnitude. The differences between simulated values and experimental mea-
surements, along with the limitations of the technique, were used as an indication of the
accuracy of the technique.
ii
Acknowledgements
I owe the deepest gratitude to my supervisor, Professor Murray Thomson, who has sup-
ported me throughout the course of this thesis with his patience, guidance and knowledge.
Without his encouragement, this thesis would have never been written or completed, and
one simply could not wish for a friendlier advisor. I would also like to thank Dr. Seth
Dworkin, who provided his generous help on running the numerical models and numer-
ous valuable insights to my endeavours in this study. Parham Zabeti is a key member
in my research group who I could never thank enough. His support on the setup of the
experimental apparatus and help when I had troubles in my experiments are heartily
appreciated.
This study has also benefited from the valuable advices offered by Mr. Dan Mathers
and Ms. Ying Lei from the Analytical lab for Environment Science Research and Training
at the University of Toronto. I also want to show my appreciation to Prof. Jim Wallace
and Prof. Greg Evans for being members on my thesis defense committee.
Thanks to every member in the Combustion Research Group of M.I.E. at the Uni-
versity of Toronto, specifically Meghdad Saffaripour, Coleman Young and Dr. Subram
M. Sarathy, for the quality time we spent together. This study would be more difficult
without their support.
Finally, I would like to show my gratefulness to my family members. I thank them
for taking off so many of my responsibilities at home so that I could fully concentrate on
my academic work.
iii
Contents
Abstract ii
Acknowledgements iii
Table of Contents iv
List of Tables ix
List of Figures xv
Acronyms xvi
1 Introduction 1
1.1 Research Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Dissertation Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Background 4
2.1 Fuels in Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 Ethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Shell Gas-to-Liquids Synthetic Jet Fuel (GTL-SJF) . . . . . . . . 5
2.2 Polycyclic Aromatic Hydrocarbons (PAHs) and Their Effects . . . . . . . 9
2.3 Chemical Pathways: From Fuel to PAHs to Soot . . . . . . . . . . . . . . 12
2.3.1 From Fuel to the First Aromatic Ring . . . . . . . . . . . . . . . 12
iv
2.3.1.1 Oxidation of Alkanes (Paraffins) . . . . . . . . . . . . . 12
2.3.1.2 Oxidation of Alkenes (Olefins) . . . . . . . . . . . . . . . 13
2.3.1.3 Formation of the First Aromatic Ring . . . . . . . . . . 15
2.3.2 From 1-Ring Aromatic Hydrocarbons to PAHs to Soot . . . . . . 16
2.3.2.1 Formation of PAHs . . . . . . . . . . . . . . . . . . . . . 16
2.3.2.2 Formation of Soot . . . . . . . . . . . . . . . . . . . . . 17
2.4 Co-flow Burner & Numerical Modeling . . . . . . . . . . . . . . . . . . . 21
2.4.1 Co-flow Diffusion Flame and Burner . . . . . . . . . . . . . . . . 21
2.4.2 Brief Description of the Numerical Model for the Ethylene Flame 23
2.5 Conventional & Proposed Techniques of PAH Measurement . . . . . . . . 23
2.5.1 Conventional Techniques of PAH Measurement in Flames . . . . . 23
2.5.2 Newly Proposed Technique for PAH Measurement in Flames . . . 31
2.5.2.1 Thermal Adsorption/Desorption Processes . . . . . . . . 31
2.5.2.2 Working Principle of Thermal Adsorption/Desorption . 33
3 Experimental Apparatus and Methodology 42
3.1 Co-flow Diffusion Burner and its Peripheral Equipment . . . . . . . . . . 42
3.1.1 Co-flow Diffusion Burner Setup . . . . . . . . . . . . . . . . . . . 42
3.1.2 Peripheral Equipment . . . . . . . . . . . . . . . . . . . . . . . . 45
3.1.2.1 Ethylene Flame . . . . . . . . . . . . . . . . . . . . . . . 45
3.1.2.2 Shell GTL-SJF Flame . . . . . . . . . . . . . . . . . . . 48
3.2 Gas-sampling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2.1 Sampling Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2.2 Other Components along the Sampling Path . . . . . . . . . . . . 54
3.3 Sampling Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.1 Preliminary Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3.2 Flame Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.3 Dynamic Headspace Adsorption Sampling . . . . . . . . . . . . . 62
v
3.4 Analytical Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4.1 Preliminary Compound Identification and Standard-Solution Prepa-
ration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4.2 FID-Signals/Compound-Mass Calibration . . . . . . . . . . . . . 68
3.4.3 Sample Extraction by Thermal Desorption . . . . . . . . . . . . . 74
3.4.3.1 Pneumatic Adjustments . . . . . . . . . . . . . . . . . . 75
3.4.3.2 Trap Cleaning . . . . . . . . . . . . . . . . . . . . . . . 76
3.4.3.3 Thermal Desorption Method . . . . . . . . . . . . . . . . 77
3.4.4 Gas Chromatograph Method . . . . . . . . . . . . . . . . . . . . . 79
3.4.5 Determination of Concentrations of Target Aromatics from Exper-
imental Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3.4.5.1 Compound Identification . . . . . . . . . . . . . . . . . . 82
3.4.5.2 Compound Quantification . . . . . . . . . . . . . . . . . 84
3.4.6 Summary of the Determination of Species Concentrations . . . . 84
3.5 Adsorbent Rejuvenation . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4 Technique Capabilities and Validation 91
4.1 Validity of Chromatographic Data . . . . . . . . . . . . . . . . . . . . . . 91
4.1.1 Concentration-dependent Drifting of Retention Times . . . . . . . 92
4.1.2 Signal-to-noise Ratio . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.1.3 Analyte Co-elution . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.2 Limits of Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.3 Effects of Cartridge Placement . . . . . . . . . . . . . . . . . . . . . . . . 106
4.4 Adsorbent Breakthrough Volumes (BTVs) . . . . . . . . . . . . . . . . . 111
4.4.1 BTV Determination by Dynamic Headspace Sampling . . . . . . 112
4.4.2 BTV Determination by Flame Sampling . . . . . . . . . . . . . . 115
4.5 Linearity of Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
4.6 Statistical Analysis of Experimental Data . . . . . . . . . . . . . . . . . . 124
vi
4.6.1 Repeatability of Adsorptive Sampling in a Dynamic Headspace . . 125
4.6.2 Repeatability of Flame-Aromatics Adsorption . . . . . . . . . . . 126
5 Results and Discussion 128
5.1 Results for the Ethylene Flame . . . . . . . . . . . . . . . . . . . . . . . 128
5.2 Results for the GTL-SJF Flame . . . . . . . . . . . . . . . . . . . . . . . 133
6 Closure 135
6.1 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Recommendations and Future Works . . . . . . . . . . . . . . . . . . . . 137
7 Bibliography 140
8 Appendices 149
8.1 Appendix I—Composition Analysis of SPK Fuels . . . . . . . . . . . . . 150
8.2 Appendix II—Governing Equations of Diffusion Flames . . . . . . . . . . 151
8.2.1 Governing Equations of Diffusion Flames . . . . . . . . . . . . . . 151
8.2.2 Flame-Height Prediction . . . . . . . . . . . . . . . . . . . . . . . 155
8.3 Appendix III—Calculations for the flow conditions of the fuel/oxidizer
streams of the ethylene flame . . . . . . . . . . . . . . . . . . . . . . . . 157
8.4 Appendix IV—Engineering drawings of custom-built parts . . . . . . . . 159
8.5 Appendix V—Thermal Adsorption/Desorption Processes . . . . . . . . . 168
8.6 Appendix VI—Gas Chromatography . . . . . . . . . . . . . . . . . . . . 177
8.7 Appendix VII—Example of a Typical TD Sequence . . . . . . . . . . . . 183
8.8 Appendix VIII—Sample Chromatograms . . . . . . . . . . . . . . . . . . 186
8.9 Appendix IX—Additional Graphs . . . . . . . . . . . . . . . . . . . . . . 189
vii
List of Tables
2.1 Physiochemical properties of ethylene [9] . . . . . . . . . . . . . . . . . . 5
2.2 Chemical composition of Shell’s GTL-SPK fuel . . . . . . . . . . . . . . . 7
2.3 Advantages and disadvantage of SPK blend [17] . . . . . . . . . . . . . . 8
2.4 LIF detection limits (LODs) for selected PAHs with a signal-to-noise ratio
of 2 [35] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.5 Definitions of some frequent terms used in the description of adsorptive
behaviours [51] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.6 Typical applications of some common adsorbents [51] . . . . . . . . . . . 35
2.7 Properties of Tenax TA [53] . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1 Flow rates and pressures of the fuel and oxidizer for the ethylene flame . 46
3.2 Summary of the flow rates of fuel, oxidizer, fuel diluent and sampling
stream in the ethylene- and SPK-flame-sampling experiments . . . . . . . 60
3.3 Chemical compositions of the aromatic mixture and the standard solution
used for calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.4 Summary of the parameters used in the TD & GC methods; refer to Table
8.15 for the functions of the TD method parameters . . . . . . . . . . . . 87
4.1 Summary of the 4 target aromatic hydrocarbon species studied . . . . . . 94
viii
4.2 Drifting of retention times when aromatic compounds existed in different
combinations and amounts were collected from the ethylene flame and sent
to the FID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.3 Limits of detection for various aromatic hydrocarbon compounds in num-
ber of nmoles (10−9 mol) . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.4 Limits of detection for various aromatic hydrocarbon compounds expressed
in concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.5 Repeatability of the data obtained from dynamic headspace sampling; n is
number of moles collected, X is mole fraction, and the subscripts represent
species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
4.6 Averaged mole fractions (in ppm) of benzene and naphthalene, and their
standard deviations measured at various locations of the ethylene flame . 127
6.1 Summary of the 4 target aromatic hydrocarbon species studied (repeated) 136
ix
List of Figures
2.1 Chemical structure of ethylene, C2H4 . . . . . . . . . . . . . . . . . . . . 5
2.2 Gas-to-Liquids process of natural gas; SPK is one of the final products. . 6
2.3 Reduction in the emission of particulate matters from 2 military gas tur-
bines burning various SPK blends . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Typical emissions of PAHs by sectors [25] . . . . . . . . . . . . . . . . . . 10
2.5 Names and structures of selected PAHs monitored under EPA and EU
regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.6 Formation of first ring from acetylene [27] . . . . . . . . . . . . . . . . . 15
2.7 HACA mechanism of PAH formation [27] . . . . . . . . . . . . . . . . . . 16
2.8 Condensation process that leads to PAH growth [27] . . . . . . . . . . . 17
2.9 Typical locations of soot formation in a diffusion flame [30] . . . . . . . . 18
2.10 A schematic of PAH and soot formation in non-premixed flames [27] . . . 20
2.11 Over- and under-ventilated flames in a co-flow burner; ri is the fuel-tube
radius, ro is the annulus radius and zf is the flame height [26] . . . . . . 22
2.12 Comparison of model predictions on PAH concentrations for a premixed
ethylene flame and the experimental data obtained by on-line microprobe-
sampling/GC/MS [44] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.13 Comparison of model predictions on PAH concentrations for a counter-flow
diffusion ethylene flame and the experimental data obtained by on-line
microprobe-sampling/GC/MS [45] . . . . . . . . . . . . . . . . . . . . . . 28
x
2.14 Comparison of model predictions on PAH concentrations for premixed
flames of ethylbenzene & ethyl alcohol and the experimental data obtained
by XAD-4 adsorption sampling, followed by methylene chloride extraction
into GC/MS [42] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.15 Typical structure of an adsorbent cartridge [49] . . . . . . . . . . . . . . 32
2.16 Behaviour of adsorption according to the Langmuir model [51] . . . . . . 41
3.1 Isometric and section views of the burner; Ox represents the oxidizer
streams and Fuel Mix. represents the fuel/diluent mixture . . . . . . . 44
3.2 Experimental setup for the ethylene flame in this study . . . . . . . . . . 47
3.3 The fuel/nitrogen mixing and delivery system . . . . . . . . . . . . . . . 49
3.4 Connections for the GTL-SPK fuel and oxidizer to the burner . . . . . . 51
3.5 Assembly drawing of the sampling probe . . . . . . . . . . . . . . . . . . 53
3.6 Components along the sampling path; “A” and “B” represent the two
locations for the adsorbent-cartridge placement in this study . . . . . . . 56
3.7 Three orthogonal directions in the centering of the sampling probe and
how the probe tip was aligned with the centering fixture . . . . . . . . . 58
3.8 How the tip of the sampling probe was considered as being aligned with
the central axis of the burner . . . . . . . . . . . . . . . . . . . . . . . . 58
3.9 Typical temporal profile of sample flow rate at z=35.00 mm in the ethylene
flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.10 Experimental setup for dynamic headspace adsorption sampling . . . . . 64
3.11 Schematic of the entire analytic-instrument setup . . . . . . . . . . . . . 65
3.12 Internal structure of the sample introduction device used for calibration . 70
3.13 Typical calibration curves used to quantify the unknown aromatic com-
pounds in experimental samples . . . . . . . . . . . . . . . . . . . . . . . 72
3.14 Imaginary calibration curve showing end characteristics . . . . . . . . . . 73
xi
3.15 Pneumatic adjustment panel as shown in Figure 8.11; Refer to the text
below for their functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.16 Input parameters for the TDU method used for the cold-trap cleaning
process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
3.17 Input parameters for the TDU method used for sample extraction . . . . 78
3.18 Temperature profile of the GC oven and the retention times for target
aromatic HCs: RT1—benzene, RT2—naphthalene, RT3—phenanthrene
and RT4—pyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3.19 Chromatograms showing drifting of retention times when different volumes
of standard solution were analyzed in the same day . . . . . . . . . . . . 83
3.20 Internal structure of the multi-cartridge conditioner and its pneumatic con-
nections; only 2 connections are shown here, but there are 10 connections
in total. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.1 Drifting in retention times of target aromatics when different volumes of
standard solution were sent to the FID . . . . . . . . . . . . . . . . . . . 93
4.2 Drifting in retention times of target aromatics when different amounts of
each compound collected from the ethylene flame were detected by the FID 95
4.3 Chromatogram of the species withdrawn from the ethylene flame at z=15.00
mm for a sample volume of ∼300 mL . . . . . . . . . . . . . . . . . . . . 98
4.4 Chromatogram of the species withdrawn from the ethylene flame at z=35.00
mm for a sample volume of ∼300 mL . . . . . . . . . . . . . . . . . . . . 99
4.5 Chromatogram of the species withdrawn from the ethylene flame at z=35.00
mm for a sample volume of ∼1700 mL . . . . . . . . . . . . . . . . . . . 100
4.6 Chromatogram of the species withdrawn from the GTL-SJF flame at
z=18.00 mm for a sample volume of ∼1200 mL . . . . . . . . . . . . . . 101
4.7 Chromatogram of the species withdrawn from the GTL-SJF flame at
z=28.00 mm for a sample volume of ∼1300 mL . . . . . . . . . . . . . . 102
xii
4.8 Comparison of dynamic headspace adsorption sampling for A1 and A2 by
adsorbent-cartridge pairs connected before and after the heated valve pump108
4.9 Comparison of adsorptive sampling in the ethylene flame for A1 by adsorbent-
cartridge pairs connected before and after the heated valve pump . . . . 109
4.10 Comparison of adsorptive sampling in the ethylene flame for A2 by adsorbent-
cartridge pairs connected before and after the heated valve pump . . . . 110
4.11 Breakthrough volumes of benzene sampled from pure benzene (Top) and
benzene from benzene saturated with naphthalene (Bottom) . . . . . . . 114
4.12 Breakthrough volumes of benzene sampled from the ethylene flame at
z=20.00 mm (Top) and z=35.00 mm (Bottom) . . . . . . . . . . . . . . 116
4.13 Breakthrough volumes of benzene sampled from the ethylene flame at
various locations in 3 separate experiments . . . . . . . . . . . . . . . . . 117
4.14 Linearity of samples collected from the headspace of a solution of ben-
zene saturated with naphthalene kept at 25◦C: benzene (top); naphthalene
(bottom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.15 Linearity of samples collected from the ethylene flame at z=20.00 mm:
benzene (top); naphthalene (bottom) . . . . . . . . . . . . . . . . . . . . 122
4.16 Linearity of samples collected from the ethylene flame at z=35.00 mm:
benzene (top); naphthalene (bottom) . . . . . . . . . . . . . . . . . . . . 123
5.1 Model predictions of species concentrations along the centreline of the
ethylene flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.2 Comparison of the experimental and simulated concentration profiles for
aromatic species along the centreline of the ethylene flame . . . . . . . . 131
5.3 Concentration profiles of the 4 target aromatic species along the centreline
of the GTL-SJF-fuel flame . . . . . . . . . . . . . . . . . . . . . . . . . . 134
8.1 Composition Analysis of the Shell GTL-SPK and 4 other SPK Fuels [70] 150
xiii
8.2 Axial (z) and radial (r) directions in a cylindrical coordinate system used
to describe a flame generated on a co-flow diffusion burner [28] . . . . . . 152
8.3 Assembly drawing of the I-beam stand of sampling probe . . . . . . . . . 160
8.4 Engineering drawing of the I-beam stand . . . . . . . . . . . . . . . . . . 161
8.5 Engineering drawing of the base mount for sampling probe clamping . . . 162
8.6 Engineering drawing of the top mounting plate for sampling probe clamping163
8.7 Multi-cartridge conditioner . . . . . . . . . . . . . . . . . . . . . . . . . . 164
8.8 Engineering drawing of the top housing of the multi-cartridge conditioner 165
8.9 Engineering drawing of the bottom housing of the multi-cartridge conditioner166
8.10 Engineering drawing of the centering fixture . . . . . . . . . . . . . . . . 167
8.11 Exterior structure of the TDU used in this study [49] . . . . . . . . . . . 169
8.12 Internal structure of the TDU (standby mode); heated components are
shown in heavy dotted lines . . . . . . . . . . . . . . . . . . . . . . . . . 170
8.13 Sample flow path of the TDU during the primary desorption step; refer to
Fig. 8.12 for the component list . . . . . . . . . . . . . . . . . . . . . . . 171
8.14 Sample flow path of the TDU during the secondary desorption step; refer
to Fig. 8.12 for the component list . . . . . . . . . . . . . . . . . . . . . 173
8.15 Summary of the parameters adjustable on the TDU and their functions . 175
8.16 Schematic drawing of a GC system (courtesy of Hewlett-Packard) . . . . 178
8.17 Internal structure of an FID and the H2/air flame [64] . . . . . . . . . . . 182
8.18 A sample TDU carousel arrangement (top view) . . . . . . . . . . . . . . 184
8.19 Sample TDU sequence used in the experiments . . . . . . . . . . . . . . 185
8.20 Chromatogram before Cold-Trap Rejuvenation . . . . . . . . . . . . . . . 186
8.21 Chromatogram after Cold-Trap Rejuvenation . . . . . . . . . . . . . . . 187
8.22 Drifting in retention times when aromatic species exist in different combi-
nations and detected mass . . . . . . . . . . . . . . . . . . . . . . . . . . 188
xiv
8.23 Linearity of samples collected from the ethylene flame at z=25.00 mm:
benzene (top); naphthalene (bottom) . . . . . . . . . . . . . . . . . . . . 189
8.24 Linearity of samples collected from the ethylene flame at z=35.00 mm:
benzene (top); naphthalene (bottom) . . . . . . . . . . . . . . . . . . . . 190
xv
Acronyms
A1—Aromatic Hydrocarbon with 1 fused aromatic ring (Benzene, C6H6)
A2—Aromatic Hydrocarbon with 2 fused aromatic rings (Naphthalene, C10H8)
A3—Aromatic Hydrocarbon with 3 fused aromatic rings (Phenanthrene, C14H10)
A4—Aromatic Hydrocarbon with 4 fused aromatic rings (Pyrene, C16H10)
BP—Boiling Point
BTV—Breakthrough Volume
CEM—Controlled Evaporator Mixer
CF—GC Column Flow
DF—Desorb Flow
EPA—Environmental Protection Agency
EU—European Union
FID—Flame Ionization Detector
GC—Gas Chromatography
GTL—Gas-to-liquids
GTL-SJF—Gas-to-liquids Synthetic Jet Fuel
HC—Hydrocarbon
HPLC—High Performance Liquid Chromatography
HRV—Heated Rotor Valve
HVP—Heated Valve Pump
ID-Inner Diameter
xvi
ISF—Inlet Split Flow
JF—Jet Fuel
LIF—Laser-Induced Fluorescence
LOD—Limit of Detection
NDIR—Non-Dispersive Infrared
OD—Outer Diameter
OSF—Outlet Split Flow
PAH—Polycyclic Aromatic Hydrocarbon
PE—PerkinElmer
ppb—Part-Per-Billion
ppm—Part-Per-Million
ppt—Part-Per-Thousand
RF—Response Factor
RT—Retention Time
SLPM—Standard-Litre Per Minute
S/N—Signal-to-noise
SPK—Synthetic Paraffinic Kerosene
SS—Stainless Steel
SSV—Safe Sampling Volume
SVOC—Semi-volatile Organic Compound
SJF—Synthetic Jet Fuel
TCD—Thermal-Conductivity Detector
TDU—Thermal Desorption Unit
TWA—Time-weighted Average
VOC—Volatile Organic Compound
UV—Ultraviolet
xvii
Chapter 1
Introduction
The Industrial Revolution has led to a huge technological advancement for human so-
cieties when many efficient ways for harvesting energy from fuel combustion were de-
veloped. The combustion of fuel has brought human prosperity; yet it has also caused
adverse environmental and health effects. Many polycyclic aromatic hydrocarbon (PAH)
compounds, which exist as by-products of incomplete combustion of organic fuels, are
classified as pollutants with potential carcinogenic and mutagenic effects on living organ-
isms [1]. Furthermore, PAHs generated in flames are generally considered precursors of
soot, which is another air pollutant that also plays a significant role in global warming
[2].
Because PAH compounds serve as a bridge between fuel fragments and soot particles,
measurements of PAH species in flames are essential to the development and validation
of the numerical model of soot formation. Nonetheless, the data generated by existing
techniques on PAH concentrations in flames are very lacking and are only available in
flames of simple fuels.
1
Chapter 1. Introduction 2
1.1 Research Motivation
Conventional techniques for analyzing PAHs [3], [4] involve the trapping of these species
by filters or adsorbents, followed by sample extraction by solvents and the introduction
of the resultant mixture into the analytical instruments. These techniques are an in-
dispensable part of combustion simulation because they provide critical information of
flame-PAHs for comparison with the model predictions. However, they are very time
consuming and laborious to perform. As the number of newly developed fuels increases
at a fast pace, the necessity of measuring PAHs in flames has also intensified and this
increasing demand for effective and efficient measurement of PAHs in flames leads to a
quest for new technique development.
The technique of thermal adsorption/desorption has been used successfully to sample
atmospheric PAHs [5], [6] & [7]. This technique offers relatively simple sampling and
extraction procedures, and a short analysis cycle-time. Moreover, the small size and
portability of the entire sampling setup make this technique suitable for in-situ sam-
pling in an industrial environment. With the need of developing an effective and efficient
method for flame-PAH measurement, this technique is a logical choice for further devel-
opment.
1.2 Dissertation Objectives
This dissertation attempts to elucidate how this thermal adsorption/desorption tech-
nique can be utilized in sampling PAHs in flames and evaluates the accuracy of the
measurements obtained by this technique. The primary objective of this dissertation
is the establishment of the experimental setup and procedures for measuring aromatic-
hydrocarbon-species in flames using the temperature-swing adsorption/desorption tech-
nique. This study proceeds by performing experiments in which pure aromatic com-
pounds were collected and analyzed by the technique proposed and subsequently using
Chapter 1. Introduction 3
the results to assess the technique performance in terms of limits of detection (LODs),
extraction efficiency and data repeatability. This study then continues by using the
technique to measure the concentrations of a target group of aromatic hydrocarbons—
benzene (C6H6), naphthalene (C10H8), phenanthrene (C14H10) and pyrene (C16H10)—in
co-flow diffusion flames of ethylene (C2H4) and synthetic paraffinic kerosene (SPK), and
concludes by comparing the experimental results with the numerical-model predictions.
This dissertation is divided into 5 chapters:
Chapter 1 Presents the motivation and objectives of this dissertation.
Chapter 2 Provides some background information of the fuels involved in this study,
our concerns on aromatic hydrocarbons, the chemical pathways of PAH and soot
formation, the numerical combustion model, and the theories behind the proposed
sampling technique of thermal adsorption/desorption.
Chapter 3 Describes in detail about the analytical instruments, and the experimental
apparatus & procedures.
Chapter 4 Examines the validity of the technique and its limitations.
Chapter 5 Presents the experimental results and their comparison with the model pre-
dictions, and concludes the study with recommendations for future studies.
Chapter 2
Background
2.1 Fuels in Experiments
Ethylene and synthetic paraffinic kerosene produced by the gas-to-liquids process were
selected as the fuels for in-flame species sampling by the new proposed technique.
2.1.1 Ethylene
Ethene, more commonly referred to as ethylene, is the first member in the alkene (olefin)
family with the chemical formula of C2H4. It is the simplest unsaturated hydrocarbon
containing one carbon-to-carbon double bond. As the most produced organic compound
in the world [8], it is refined from petroleum via a cracking process in which large hy-
drocarbons are converted into smaller ones with unsaturation introduced. The molecular
structure and physiochemical properties of ethylene are shown in Figure 2.1 and Table
2.1, respectively.
Most practical fuels are hydrocarbon-based; therefore, the modeling of hydrocarbon
(HC) combustion has a long history in scientific research and the underlying reaction
mechanisms for most simple aliphatic HCs are well established. It is an established
observation that the oxidation of HC fuels is highly hierarchical and the oxidation mech-
4
Chapter 2. Background 5
Figure 2.1: Chemical structure of ethylene, C2H4
Table 2.1: Physiochemical properties of ethylene [9]
Properties Values
Molar mass 28.05 g/mol
Density 1.178 g/L@15◦C
Std. enthalpy of combustion -1387.4 kJ/mol
EU classification Extremely flammable
Autoignition temperature 350◦C
anisms of large HC fuels always include those of smaller HC fuels. Consequently, the
oxidation mechanism of ethylene is necessarily a subset in the combustion mechanisms
of all alkyl HC fuels. As a matter of fact, evidence exists [10], [11] that all alkyl HC fuels
primarily form ethylene and propene during oxidation and thereby, demonstrating the
crucial importance of understanding the combustion mechanism of ethylene. Its roles in
the oxidation of higher-order HC fuels are discussed in Chapter 2.3.
2.1.2 Shell Gas-to-Liquids Synthetic Jet Fuel (GTL-SJF)
Kerosene is one of the refined products made from petroleum-based feedstocks such as
conventional crude, oil sands and oil shale, and is widely used as jet fuel (JF). The price of
fossil-fuel-derived JF experienced a 6-fold increase from 1988 to 2008 [12], and the rising
fuel cost was largely transferred to passengers [13]. Therefore, to curb our dependence
on foreign oil, synthetic jet fuels (SJFs) have been developed.
In this study, synthetic paraffinic kerosene (SPK) supplied by Shell and derived by the
gas-to-liquids (GTL) process was selected as a representative of these SJFs for combustion
Chapter 2. Background 6
investigation.
Figure 2.2: Gas-to-Liquids process of natural gas; SPK is one of the final products.
The major feedstock in the production of this SJF is natural gas, which is converted
by a gas-to-liquids (GTL) process into SPK. In the GTL process, which is shown in
Figure 2.2, methane is first separated from natural gas and sent to the reformer unit.
During the reforming process, methane reacts with oxygen at around 1400-1600◦C to
form a mixture of hydrogen and carbon monoxide, which is known as synthesis gas or
syngas. The syngas then undergoes low-temperature Fischer-Tropsch (F-T) process while
flowing through a bed of catalysts such as cobalt (Co) and iron (Fe), and develops into
long-chain HCs, mostly alkanes:
(2m+ 1) H2 +m COcatalysts: Co, Fe−−−−−−−−−→ CmH2m+2 +m H2O (2.1)
where m is any integer.
Chapter 2. Background 7
At last, these long-chain HCs undergo hydrocracking and develop into HCs with
different carbon numbers and shapes, which are subsequently separated by distillation
into naphtha, kerosene, n-paraffins, gasoil and base oils, ranked in order of increasing
molecular weight.
SPK fuel produced by the above GTL process was a fuel used in this study and its
blend with 50% conventional JF, called GTL-JF, can be used in aviation. The feasibility
of using SPK fuel to power aircrafts was demonstrated in a test flight: in September
2009, a 50% blend of conventional JF and SPK was the first SJF to become certified by
ASTM D7566 (the world’s first semi-synthetic aviation fuel specification), and the blend
was tested on a commercial passenger flight flying a 7-hour trip from London to Doha in
the following month [14]. This test has improved the confidence of governments in the
progressive replacement of petroleum-based JFs by alternative jet fuels. The chemical
composition of GTL kerosene produced by Shell is shown in Table 2.2, which is only an
excerpt of its full composition breakdown shown in Appendix I. From the table, it is
noteworthy that this fuel is virtually free of aromatics and sulfur, as their mass fractions
are measured to be 0.0 and 0.06%, respectively [14].
Table 2.2: Chemical composition of Shell’s GTL-SPK fuel
Property Shell GTL Test Method
HC composition, mass%
Aromatics 0.0 D2425
Cycloparaffins 4.0 D2425
iso- & n-paraffins 96.0 D2425
Nitrogen (mg/kg) 1 D4629
Sulfur (mg/kg) 0.6 D5453
As a result of the absence of aromatics and sulfur, the burning of this SPK in flights
has shown reduced soot emission, as demonstrated by the data in Figure 2.3 [15], [16]. In
addition to this promising characteristic in favour of cleaner emissions, SPK fuels have
Chapter 2. Background 8
other advantages, which are shown in Table 2.3.
Figure 2.3: Reduction in the emission of particulate matters from 2 military gas turbines
burning various SPK blends
Table 2.3: Advantages and disadvantage of SPK blend [17]
Advantages
• Allows owners of remote gas reserves a way to bring their gas to
market;
• Products are compatible with existing tankers, pipelines, and storage
facilities;
• Engines running on SPK blends pollute less;
• Greater global use of SPK-fuels could slow down oil consumption.
Disadvantages• If feedstocks are imported, foreign dependence would not be reduced;
• Conversion plants are expensive to build.
Coal and biomass can also serve as the feedstocks for the synthesis process of SJFs.
Chapter 2. Background 9
The synthesis process with coal is called coal-to-liquids (CTL) and the one with biomass
is called biomass-to-liquids (BTL). While BTL-SJF is under testing for its certification
as an aviation fuel, CTL-SJF has already been used in flights. The production processes
and emission characteristics of these 2 fuels are discussed in [18].
2.2 Polycyclic Aromatic Hydrocarbons (PAHs) and
Their Effects
Polycyclic Aromatic Hydrocarbons (PAH) are chemical compounds with fused aromatic-
ring structures that do not contain heteroatoms or carry substituents. They are con-
sidered to be one of the most ubiquitous organic pollutants and there are 1896 possible
structures for PAHs containing 2–8 rings [19]. Many of the compounds in the PAH fam-
ily are confirmed to be carcinogenic, teratogenic and mutagenic. Moreover, most PAH
compounds are highly lipophilic, making them even more hazardous to living organisms.
PAHs exist naturally in oil, coal and tar deposits, and natural formation of PAHs
ranges from forest fires, volcano eruptions and soil diagenesis [20]. Anthropogenic sources
of PAHs include incomplete combustion of fossil fuels, waste incineration and industrial
operations such as a coke oven. In most developed countries, up to 35% of PAH emissions
are estimated to be contributed by motor-vehicle emission [21]. Figure 2.4 shows a
breakdown of the sources of PAH emission from human activities.
As a result of their high toxicity and ubiquitous presence, most countries in North
America and Europe have regulations to limit PAH concentrations in air. For instance,
the United States Environmental Protection Agency (USEPA) has listed 16 PAH com-
pounds (EPA16) as priority pollutants and the European Union (EU) has designated 6.
The names and structures of the PAHs designated as pollutants by the above 2 organiza-
tions are shown in Figure 2.5. As an example to illustrate the regulated concentrations
of PAHs in air, the Expert Panel on Air Quality Standards (EPAQS) of United Kingdom
Chapter 2. Background 10
has recommended an annual average of 0.25 ng/m3, using benzo[a]pyrene as a marker
[22].
PAHs play an important role [23], [24] in the formation of soot from combustion: they
act as a bridge between fuel fragments and the formation of soot. Therefore, a prediction
of the formation of PAHs could lead to a prediction of the emission of soot from a burning
fuel. However, the mechanisms in flames by which PAHs transform into soot is not well
understood even after decades of studies. Thus, PAH formation in flames is a subject
under intense research.
Other (Energy)Other Transport
Other (Non Energy)9%
Other (Energy)41%
Road Transport11%
4%
Waste2%
11%
Agriculture8%
Fugitive Emissions2%
Energy Industries1%
Industry (Processes)17%
Industry (Energy)5%
Figure 2.4: Typical emissions of PAHs by sectors [25]
Chapter 2. Background 11
Figure 2.5: Names and structures of selected PAHs monitored under EPA and EU regu-
lations
Chapter 2. Background 12
2.3 Chemical Pathways: From Fuel to PAHs to Soot
As pointed out in an earlier section, the oxidation of HC fuels is highly hierarchical in the
way that oxidation mechanisms of smaller fuel molecules are necessarily sub-mechanisms
of larger fuels. Hence to understand the underlying combustion behaviours of large
complicated fuels such as SJF, which is a mixture of a wide range of HCs with different
carbon numbers and shapes, the understanding of the oxidation of smaller fuels such as
ethylene is warranted.
In general, most higher-order HC fuels break down to similar fuel fragments via
a limited number of pyrolytic pathways and these pyrolyzed molecules undergo similar
chemical reactions to form aromatic HCs with 1 ring. These mono-aromatics are believed
to share similar growth routes to form PAHs and soot [26]. Consequently, this section
will be further divided into two subsections: the first explains how higher-order paraffinic
fuels break down to form the major precursors of mono-aromatic HCs, followed by the
formation of the first ring; the second explains how these mono-aromatic HCs further
develop into PAH and soot.
2.3.1 From Fuel to the First Aromatic Ring
Table 2.2 reveals that the GTL-SJF is largely composed of n- and i-paraffins, with a
small amount of cycloparaffins included. Hence its oxidation follows mainly the chemical
pathways of alkane oxidation, which is discussed next. The oxidation of ethylene is of
course a subset of alkane oxidation and its discussion follows sequentially.
2.3.1.1 Oxidation of Alkanes (Paraffins)
Methane (CH4) presents oxidation patterns that are different from other higher-order
alkanes (n>2), and a discussion of methane combustion is not included here, but can be
found in [26]. Relying heavily on [26], the oxidation mechanism of high-order alkanes is
Chapter 2. Background 13
discussed. At a sufficiently high temperature, the chain initiation reaction is started by
the breakup of the fuel molecules along a CH bond:
RH (+M)→ R + H (+M) (2.2)
where R represents an alkyl radical (CnH2n+1) and M represents a third body. The H
radicals subsequently react with oxygen gas to generate a pool of radicals of O, H, and
OH, which takes over the chain initiation:
RH + OH→ R + H2O (2.3)
RH + X→ R + HX (2.4)
where X can be H, O, OH or CH3. The alkyl radicals then undergo β-scission1 to form
olefins and alkyl radicals of lower order:
R (+M)→ Olefin + R’(+M) (2.5)
Starting from this point, the oxidation of alkenes begins, and is discussed next.
2.3.1.2 Oxidation of Alkenes (Olefins)
The discussion on alkene oxidation will be limited to that of ethylene (n = 2) only, as it
is a product that is always formed in the combustion of paraffins and olefins. Unlike the
initiation of fuel oxidation in paraffins, oxygen atom addition is an alternative to H-atom
abstraction in the initiation reaction of ethylene oxidation. In this alternative reaction,
the C=C double bond is attacked primarily by an O atom to form an adduct, which
immediately decomposes:
C2H4 + O→ CH3 + HCO (2.6)
C2H4 + O→ CH2 + H2CO (2.7)
1This rule dictates that the bond broken in the R radical is one position away from the radical site.
Chapter 2. Background 14
H-atom abstraction proceeds concurrently by the fuel reaction with a hydroxyl radical:
C2H4 + OH→ C2H3 + H2O (2.8)
The vinyl radical that forms in this reaction then decays into acetylene via:
C2H3 + M→ C2H2 + H + M (2.9)
C2H3 + O2 → C2H2 + HO2 (2.10)
Finally, in a similar fashion as ethylene oxidizes, acetylene decays primarily by going
through an O-atom addition reaction to form an adduct, which subsequently decomposes
into methylene and carbon monoxide:
C2H2 + O→ CH2 + CO (2.11)
The oxidation of methylene is through its reaction with O2:
CH2 + O2 → H2CO + O (2.12)
The oxidation of ethylene as the initial fuel only differs in the initiation reactions, with
reactions (2.6) and (2.7) replaced by (2.13) and (2.14):
C2H4 + M→ C2H2 + H2 + M (2.13)
C2H4 + M→ C2H3 + H + M (2.14)
Although the oxidation of methyl radical and aldehydes is not included here, the final
products they form are C2 hydrocarbons and carbon monoxide, along with radicals such
as O, H and OH. The CO formed in all of the above reactions finally reacts with OH,
forming water and carbon dioxide, and releasing the major portion of the energy involved
in the entire fuel-oxidation reaction.
Chapter 2. Background 15
2.3.1.3 Formation of the First Aromatic Ring
The significance of ethylene oxidation is the production of acetylene as an intermediate
because under fuel rich conditions, which are always satisfied on the fuel side of a co-
flow flame, acetylene forms in large concentrations and its polymerization leads to the
formation of the 1-ring aromatics [27]. Because most aliphatic HC fuels share the same
chemical mechanism in fuel oxidation and soot formation [26], aromatic HCs with 1 ring
are expected to form in the same reactions for most aliphatic fuels. The most widely
adopted chemical reactions leading to the formation of mono-aromatic HCs include those
between C4Hx and acetylene, and another between 2 propargyl (C3H3) radicals. The
route of formation of the propargyl radicals and C4 species from acetylene and their
subsequent interactions leading to the first ring are shown in Figure 2.6. The formation
of the first ring through reactions of C6 recyclization and those between methylacetylene
and allene [27] are two more proposed, though less common, reactions. The formation
of mono-aromatic HCs is important because it controls the rate of soot inception, which
further controls the final amount of soot formed.120 Oxidation Mechanisms of Fuels
C
CC
CH
H
H
+C3H3
C2H3
C2H2+C2H2
+CH2C3H3
(C6H5)
(C6H6)
+H
(C4H4)
C
CC
CH
H
H H
+C2H4
+CH2
(n-C4H5)
C
CCH
H H
CHH
(1,3-C4H6)
(n-C4H3)
+C2H2
+C2H2
+H+H, OH
C
CCH
H H
CH
-H -H
-H
-H
-H
+H, OH
-H2, H2O
+H, OH
-H2, H2O•
•
•
Figure 3.9.1. Selected pathways of benzene formation in hydrocarbon combustion.
made up of a large number of randomly arranged grains. Each grain consists of 5 to10 nearly parallel planes arranged in a turbostractic fashion. Each layer is between 1to 2 nm in dimension and contains on the order of 50 carbon atoms. The inner layerspacing is about 0.35 nm, which is of the same order as that of graphite.
It is well accepted that the physical and chemical coalescence of PAHs is responsi-ble for the inception of soot. (Frenklach et al. 1984; Frenklach & Warnatz 1987). Thegrowth of soot particles to the size observed in combustion exhaust is caused by thecoagulation of smaller incipient soot particles, by PAH surface condensation, and bysurface reactions between soot and gaseous species like acetylene. Soot producedfrom flames may be oxidized by OH, O, and O2 before the combustion gas reachesthe exhaust. Addition of steam and carbon dioxide to the combusting gas can alsoenhance soot oxidation, possibly achieved by the increase in OH concentration as aresult of increased concentrations of H2O and through the direct reaction betweenC and CO2 to produce CO.
Because PAHs are the precursors to soot, a basic understanding of the mechanismof soot formation must start with that of PAH formation. It is known that acety-lene forms in large concentrations in fuel-rich combustion, and its polymerization isthought to be responsible for the formation of PAHs. In particular, the first aromaticring may be produced from nonaromatic species in the reaction sequence depictedin Figure 3.9.1. It is seen that in addition to the importance of acetylene during theformation of the first aromatic ring, that is, benzene and phenyl, the H atom alsoplays a critical role in that it activates/deactivates the radical species from which thefirst aromatic ring forms. This is also the case during the growth of the aromatic ringto PAHs, as will be seen later.
When burning long-chain aliphatic fuels, methylene radicals are produced fromthe reaction between C2H2 and the O atom, that is, reactions (C213a), (C213b),and (C214), thus their presence is directly related to acetylene. Only in fuel-rich
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Figure 2.6: Formation of first ring from acetylene [27]
Chapter 2. Background 16
2.3.2 From 1-Ring Aromatic Hydrocarbons to PAHs to Soot
2.3.2.1 Formation of PAHs
The 1-ring species further grow in the number of rings into PAHs through Hydrogen-
Abstraction-Carbon-Addition (HACA) reactions with acetylene. In these HACA reac-
tions, acetylene reacts with a ring species and abstracts a hydrogen atom. Each time,
a new bonding of C2H is formed until a partial ring is completed. This process repeats
and the species grow in size. Figure 2.7 illustrates a HACA pathway in which benzene
grows into pyrene.
PAH condensation is another way that small aromatic species grow, especially when
the concentrations of PAHs are sufficiently high. The condensation process is competitive
to the HACA process and their relative dominance is determined by the ratio of acetylene
to benzene. If acetylene exists in abundance, then the HACA process would dominate.
On the other hand, when both species exist in similar concentrations, both processes
have equal dominance. The PAH ring-ring condensation process is illustrated in Figure
2.8. 3.9. Chemistry of Pollutant Formation 121
C2H2
C2H2(-H)
CC
H(-H2)
H
CC
C2H2(-H)
H
H(-H2)
H
H
H (-H2)
C
CC
C
H
C2H2
H
C2H Large PAHs2(-H)
•
•
•
•
•
Figure 3.9.2. The H-abstraction–C2H2-addition (HACA) mechanism of polycyclic aromatic hydro-carbon formation.
methane or natural gas flames are methylene radicals produced from the direct re-action between CH3 and OH through (M19), which enhances the propargyl (C3H3)recombination path to benzene.
The further growth of the aromatics is thought to proceed through the H-abstraction----C2H2-addition (HACA) mechanisms, as shown in Figure 3.9.2. In thismechanism, the addition of acetylene to an aromatic radical, like phenyl, leads toeither the bonding of an ethynyl (----C2H) group with the aromatic ring, or the forma-tion of an additional condensed aromatic ring. Depending on the neighboring ringstructure, the newly formed ring can either be a radical, which can grow readily withacetylene, or it may be a molecular species. The latter will have to be “activated”through the H-abstraction reaction to produce a PAH radical species, before it canundergo the further growth reaction with acetylene.
If the concentrations of aromatic species are sufficiently large, PAH growth throughthe direct ring-ring condensation is also possible. For example, benzene and phenylcan react to form biphenyl. Through the H-abstraction reaction, a biphenyl radicalforms and can react with acetylene to form the three-ring phenanthrene, or it canreact with benzene to form a four ring aromatic species. Such a reaction sequence isshown in Figure 3.9.3.
The competition between the HACA mechanism and the aromatic condensationmechanism is largely determined by the ratio of acetylene to benzene. If the concen-tration of acetylene is substantially larger than that of benzene, the HACA mech-anism dominates. However, if the acetylene concentration is about equal to that ofbenzene, as in the very early stage of a premixed benzene flame, then the aromatic–aromatic condensation mechanism may prevail.
The reaction pathways leading to PAH formation and growth as depicted in thethree diagrams just discussed is highly reversible. When the temperature exceeds
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Figure 2.7: HACA mechanism of PAH formation [27]
Chapter 2. Background 17122 Oxidation Mechanisms of Fuels
(-H)
H
H(-H2)
H
C2H2(-H)
H
H(-H2)
H
(-H)(-H)
(-H)
•
•
•
•
Figure 3.9.3. An alternate polycyclic aromatic hydrocarbon growth mechanism.
around 1,800 K, some of these reactions may proceed in the reverse direction in favorof the reactants. Hence, the same reactions that are responsible for PAH formationand growth also cause PAHs to thermally decompose at high temperatures. In fact,the reduction of PAH concentrations in the post-burning region of a premixed flameis caused by the thermal decomposition of the PAHs following the reverse of thereaction pathways shown in these diagrams, and to a lesser extent, due to oxidation.
When PAHs grow to the size of pyrene (a four-ringed PAH) or larger, they maybe able to condense onto each other upon collision and form small clusters. Theseclusters can continue to react with acetylene following the same mechanism of PAHgrowth, or they may coagulate to form larger clusters. These chemical and physicalprocesses eventually lead to the formation of soot particles. When the PAH concen-trations are sufficiently high, surface condensation may become a major source ofsoot mass growth. Detailed kinetic models formulated on the basis of these physi-cal and chemical processes can predict reasonably well soot production in laminarpremixed and nonpremixed flames of simple hydrocarbons such as acetylene andethylene.
3.10. MECHANISM DEVELOPMENT AND REDUCTION
3.10.1. Postulated Semiglobal MechanismsRecognizing the complexity of a detailed reaction mechanism and the intricacy offuel oxidation kinetics, rational modeling and simulation of combustion phenomenaare invariably faced with the need for simpler but chemically realistic mechanisms.An early attempt toward achieving this goal is to extend the concept of the one-stepoverall reaction between reactants and products, with constant kinetic parameters, bypostulating some global and semiglobal reactions characterized by additional majorintermediates and empirically determined kinetic parameters. Since the approach isbasically empirical, it does not require knowledge of the detailed mechanism.
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Figure 2.8: Condensation process that leads to PAH growth [27]
2.3.2.2 Formation of Soot
To form soot, PAH species first nucleate into incipient particles through collision and
sticking. Although soot nucleation is one of the least understood phenomena [28], it is
believed that incipient particles are formed via dimerization of PAH compounds. Once
these small nucleates are formed, they grow by reacting with the vapour-phase species
while coagulating with other particles. Finally, these particles are oxidized through the
journey across the flame. Partially oxidized soot exists as smoke. Figure 2.9 demonstrates
the most likely locations in a diffusion flame where soot tends to form, grow and oxidize.
Smoke point is a measure of the tendency of a fuel to form soot, and it is defined
as the maximum flame height that a fuel can burn without soot “breakthrough”. The
higher this value is, the smaller the tendency a fuel generates soot. From [18], the Shell
GTL-SJF used in this study has a smoke point of more than 50 mm, compared to 21.5
mm of regular Jet A-1 fuel, which again confirms the findings of [15], [16] as shown in
Figure 2.3 that the Shell GTL-SJF is much less sooting than Jet A1—a conventional
oil-based JF.
Soot is the chemical species responsible for the luminosity of flames, and is formed on
Chapter 2. Background 18
the fuel side of the flame front and oxidized while being transported through the flame
sheet. It can play both a beneficial or detrimental role in combustion devices: on one
hand, soot provides luminosity in flames and increases the effectiveness of radiative heat
transfer of a combustion device; on the other hand, soot can severely reduce the lifetime
of a device (e.g., clogging the exhaust passage) and post tremendous health threats by
emitting into the atmosphere toxic species absorbed from the flames—it was found that
85% of the PAHs were associated with soot particles less than 5 µm in diameter, which
can penetrate through the upper respiratory airways to the lower ones and even to the
alveoli [29].
Figure 2.9: Typical locations of soot formation in a diffusion flame [30]
To summarize, in non-premixed combustion of HC fuels, soot is generally formed in
the following steps [30]:
1. oxidation and pyrolysis of the fuel
Chapter 2. Background 19
2. formation of the first ring (benzene and phenol)
3. formation of multi-ring species (i.e., PAHs)
4. soot-particle inception
5. surface growth and particle agglomeration & coalescence
6. particle oxidation
Figure 2.10 illustrates how small pyrolyzed fuel molecules transform into soot. PAHs are
participants in steps 3-5 and their measurements and predictions provide direct informa-
tion on soot formation.
Chapter 2. Background 20
Res
iden
ce ti
me
Figure 2.10: A schematic of PAH and soot formation in non-premixed flames [27]
One major difficulty in PAH measurements is the fact that PAHs can partition into
gaseous and particulate phases in combustion exhaust. The tendency to partition in
favour of one phase is determined by the vapour pressure of a specific compound—the
higher the molecular weight of a PAH compound (or the more the number of fused
Chapter 2. Background 21
aromatic rings the compound has), the lower its vapour pressure and the higher its
tendency to partition into the particulate phase. Vapour pressures of PAHs with 2–7
fused rings can span 11 orders of magnitude and it is believed that 2-ring PAHs tend
to stay in the gaseous phase, 3–4-ring PAHs have a similar tendency to partition into
both phases, and those with more than 4 rings tend to stay in the particulate phase
[31]. This physical nature makes their sampling and the ensuing analysis challenging,
as gaseous PAHs can be “blown off” from particulate PAHs trapped by filters, whereas
gaseous PAHs can adsorb on the surfaces of particulates and the sampling devices.
2.4 Co-flow Burner & Numerical Modeling
2.4.1 Co-flow Diffusion Flame and Burner
Non-premixed or diffusion combustions are characterized by a faster rate of fuel-oxidation
reactions than the rate of fuel/oxidizer (F/O) mixing. In other words, a diffusion flame
is mixing controlled and the rate at which fuel and oxidizer are brought together in
stoichiometric proportions determines how fast the fuel burns (and energy is released).
Diffusion flames have been under intense scientific research because they have far more
practical applications than their premixed counterparts, especially when the fuel is a
solid or liquid. The most important reason that diffusion flames are employed in so
many combustion devices is safety-related—no premature ignition is possible outside the
combustion chamber, as the fuel stream contains no premixed oxidizer and is therefore
beyond its flammability limit.
In many experiments conducted on diffusion flames, a co-flow diffusion burner is em-
ployed. Burners of this type are composed of two concentric tubes, one for the oxidizer
stream, while the other for the fuel stream. In these burners, a closed, elongated flame is
formed when the fuel stream flowing in the central tube is oxidized by the oxidizer flowing
in the outer annulus at a rate in excess of the stoichiometric amount or when the fuel
Chapter 2. Background 22
stream burns in a quiescent oxidizer environment. These flames are called over-ventilated.
In the opposite scenario, when the oxidizer is supplied in less than the stoichiometric pro-
portion, the resulting flame is under-ventilated and is fan-shaped. Figure 2.11 illustrates
the shapes of both flames. They have been studied extensively for different fuels because
they offer a simple, yet multi-dimensional, flow field for numerical simulations and ex-
perimental studies. Furthermore, these flames include a wide region in which soot forms,
grows and oxidizes, thereby, facilitating the sampling of soot-forming species with better
spatial resolution. Several simplified theoretical and empirical formulas used to predict
the height zf of co-flow diffusion flames and the set of governing differential equations
describing such flames are given in Appendix II.
Figure 2.11: Over- and under-ventilated flames in a co-flow burner; ri is the fuel-tube
radius, ro is the annulus radius and zf is the flame height [26]
Chapter 2. Background 23
2.4.2 Brief Description of the Numerical Model for the Ethy-
lene Flame
In the numerical code developed by [28], equations 8.2, 8.3, 8.5, 8.9 and 8.10, as provided
in Appendix II, were used to calculate different parameters of the ethylene/air co-flow
diffusion flame in which PAH measurements were taken in this study. In equation 8.9,
the combustion mechanism of ethylene in air was included in the term m′′′i , which is
the production rate of species i. These species were taken from a mechanism developed
by Appel et al. [32], and included pyrolyzed and oxidized C1 and C2 species, higher
linear HCs up to C6, and aromatics from benzene (A1) to pyrene (A4) and their oxidized
products.
In the energy conservation equation 8.11, 2 more terms were added by [28]: one for the
enthalpy carried by soot entering a finite volume due to thermopherosis and the other for
the energy generated in the finite volume due to soot formation. Moreover, the radiative
heat transfer caused by soot was taken into account. An Optically Thin Approximation
(OTA) model was used to simulate soot radiative heat transfer in the numerical model.
Finally, soot transport equations in the radial and axial directions were included in
the model to account for the convection, normal diffusion, thermophoresis, nucleation,
coagulation, surface growth, oxidation, surface condensation and fragmentation of soot
particles in each finite volume section.
2.5 Conventional & Proposed Techniques of PAH
Measurement
2.5.1 Conventional Techniques of PAH Measurement in Flames
Techniques for measuring PAH in flames are very different from those in atmospheric
PAH-measurements due to factors such as physical constraints and concentration levels.
Chapter 2. Background 24
Today, many techniques for PAH measurements in flames have been established and they
can be categorized as intrusive and non-intrusive. In general, intrusive techniques are
capable of collecting a wide range of stable species for repeated analyses, but suffer by
the disturbances they cause on the combustion environment and by having extra post-
processing steps to analyze the samples. Non-intrusive techniques not only impose no
disruption on the combustion environment, but also are capable of measuring highly
unstable species such as radicals. In addition, they yield results in a much shorter time
than their intrusive counterparts. Nonetheless, these techniques are only responsive to a
limited number of species.
Non-intrusive flame-PAH-measurement techniques are mostly spectroscopic in nature.
Among these techniques, Laser-Induced Fluorescence (LIF) is a popular one, but only
serves on a qualitative screening level. This technique takes advantage of PAH com-
pounds’ high absorbance for ultraviolet (UV) wavelengths and excites the samples by
a laser of a fixed wavelength. The species absorb the energy from the laser and enter
their excited states. They later de-excite by emitting light of higher energies than the
incident laser, which is measured as fluorescence and compared to the spectra of known
compounds. LIF has the advantage of a high signal-to-noise (S/N) ratio and a high differ-
entiation power in distinguish compounds with similar chemical structures (i.e., isomers),
as the laser can be tuned to emit light with a wavelength that excites a single compound
solely.
In an attempt to investigate the fluorescence spectra of several PAH vapours at ele-
vated temperatures, [33] prepared an experimental setup in which a nitrogen laser first
went through a bandpass filter and a fibre optic cable, and excited the PAH vapours gen-
erated in a heated, sealed ampoule at 300◦C. The fluorescence fed back through another
optical fiber was then sent to a spectrograph connected with an intensified charge-coupled
device (ICCD). A Dichroic mirror and a few coupling lens were used to deflect the laser
into the desired direction. Although their work did not yield any quantitative results
Chapter 2. Background 25
in PAH measurement, it illustrated what a typical setup involves when flame PAHs are
measured by LIF.
In another experiment performed on PAH measurement by LIF in an ethylene turbu-
lent flame, [34] had a similar measurement setup as [33]. What [34] succeeded to quantify
was soot volume fraction using another technique called Laser-Induced Incandescence
(LII) and the LIF technique just allowed the global structure of the turbulent flame to
be captured as images of OH- and PAH-signal-intensity plots by an ICCD camera.
Both of the studies above were based on qualitative monitoring of PAHs by LIF and
no evaluation of the technique in terms of accuracy or limitations was mentioned. Hence,
the capability of LIF in flame-PAH quantification will be referenced from a study that
was conducted on aerosol-PAH measurement by LIF. In [35], the PAHs sampled from
air was extracted into a high-performance liquid chromatograph (HPLC) and detected
by LIF. Table 2.4 summarizes the limits of detection for 3 PAHs that were involved in
[35]. A recent study that measured PAHs by LIF and GC/MS together for comparison
purpose shows that the discrepancy between PAH-concentration measurements obtained
by LIF and by GC/MS can be as good as ±20% [36].
Table 2.4: LIF detection limits (LODs) for selected PAHs with a signal-to-noise ratio of
2 [35]
Compound LOD (ng)
Naphthalene 0.43
Phenanthrene 0.49
Pyrene 0.48
Intrusive techniques generally involve insertion of a probe to withdraw samples from
the flames, followed by chromatographic analyses. In the sampling part of intrusive tech-
niques, microprobe sampling is the most popular technique [18], [37] & [38]. In this
method, a microprobe is inserted into the flame to withdraw species that later undergo
Chapter 2. Background 26
a drop in pressure via free expansion along the sampling path, thereby, quenching any
further reactions. The perturbation to the flow field around the flame that is caused by
the physical presence of the probe has been addressed by [37] to be “acceptably small”.
The samples can be sent to the analytical instruments directly in an “on-line” fashion,
or they can be collected by another medium in large amounts and then extracted into
the analytical instruments for analysis. These separation instruments are typically chro-
matographic in nature [39] and the analytical instrument is usually a mass spectrometer
[40] (MS) or a flame-ionization detector (FID). Collection media include whole-gas canis-
ters/sampling bags [41] or polymeric adsorbents [42]. Extraction method from adsorbents
is usually sonication by solvents [43].
In a study of PAH measurement from a premixed ethylene flame [44] and another
from an counter-flow diffusion ethylene flame [45], flame species were withdrawn from
the flames via a heated quartz microprobe into an on-line GC/MS. Comparisons between
the data and models are shown in Figures 2.12 & 2.13.
In these 2 measurement studies, the limits of detection and the repeatability of the
analytical technique were not reported. The discrepancy between model and data was
observed to be within an order of magnitude for the counter-flow ethylene flame, but
increased to 2–3 orders of magnitude for the premixed flame. These figures only serve
as an indication of how accurate the technique of microprobe sampling can be, as no
comments were given on the credibility of the numerical models.
Chapter 2. Background 27
Compound Largest Discrepancy
Benzene a factor of ∼450Naphthalene a factor of
Chapter 2. Background 28
Figure 2.13: Comparison of model predictions on PAH concentrations for a counter-
flow diffusion ethylene flame and the experimental data obtained by on-line microprobe-
sampling/GC/MS [45]
Chapter 2. Background 29
In [42], gaseous PAHs were sampled from flat, premixed flames of ethylbenzene and
ethyl alcohol by a polymeric adsorbent called XAD-4 and quartz wool. The samples
were then extracted by methylene chloride and sent to a GC/MS for analysis. The limit
of detection of the analytical instruments involved was reported to be on the ppb level.
The standard deviations of the concentration measurements were not listed for all PAHs
and were claimed to be at least an order of magnitude smaller than the mean values.
For example, the standard deviation for naphthalene was reported as 1×10−5–8×10−5
when the mean concentration was between 2×10−4 and 4×10−4, all expressed as mole
fractions. The discrepancy between the model predictions and the experimental data can
be observed in Figure 2.14. The model and data for benzene concentrations were shown
in separate plots in [42]; hence, their comparison is not shown here.
Although the data were claimed to have a discrepancy of less than an order of mag-
nitude with the model, it is noticeable that a few data were exceptions; for example, the
concentration of naphthalene in the ethyl-alcohol flame was measured to be much higher
than the model prediction at a location close to the burner surface. In addition, most of
the data did not follow the trends of the models. It was not mentioned whether these
differences in trends were caused by the uncertainties in the model or in the experimental
technique.
Chapter 2. Background 30
Figure 2.14: Comparison of model predictions on PAH concentrations for premixed flames
of ethylbenzene & ethyl alcohol and the experimental data obtained by XAD-4 adsorption
sampling, followed by methylene chloride extraction into GC/MS [42]
Chapter 2. Background 31
2.5.2 Newly Proposed Technique for PAH Measurement in Flames
2.5.2.1 Thermal Adsorption/Desorption Processes
In environmental analysis, the technique of thermal desorption (TD), as it is commonly
called, is actually an experimental technique that combines the adsorption and desorption
processes in a unique way to achieve cyclic sample collection and extraction [46], [47] &
[48]. The manner it operates is based on temperature-swing-adsorption and desorption.
The following paragraphs elucidate the technique in details.
The TD technique takes advantage of the fact that, at low temperatures, gaseous
chemicals have an overall higher tendency to partition in the adsorbed phase on the
surface of an adsorbent that has high selectivities for them. A continuous stream of gas
is sent through an adsorbent bed that is kept at a sufficiently low temperature. When the
stream of sampling gas is sent by a pump, the process is called active sampling; whereas
when the gas is drawn through the adsorbent bed due to concentration gradients, the
process is called diffusive sampling. Target species adsorb on the surface of the adsorbents
due to a low temperature and the attractive forces exerted by the adsorbents, thereby,
achieving sample collection.
In the reverse process of the TD technique, adsorbates (i.e., adsorbed samples) escape
from the adsorption sites on the adsorbent surface at a high temperature. A continuous
stream of inert gas is sent through the adsorbent bed and serves as the carrier to trans-
port the desorbed species downstream and away from the adsorbent bed. Consequently,
adsorbed samples are extracted by heat and sent to the analytical instruments by the
carrier gas. In another process, this high temperature is used to rejuvenate the adsorbent
bed after it becomes contaminated by repeated use.
The TD technique utilizes both the adsorption and desorption processes in a combined
way that the adsorbents first undergo rejuvenation (desorption) at a high temperature
to remove residual carryovers from previous use. Target species are then trapped by the
Chapter 2. Background 32
adsorbents (adsorption) and later extracted (desorption) to the analytical instruments
for separation, identification and quantification. The cycle repeats from the rejuvenation
step.
To facilitate the storage and handling of the adsorbents, a glass or stainless steel (SS)
sampling tube called an adsorbent cartridge is packed with a bed of adsorbents, which
is kept in place by SS gauzes positioned at both ends. An end cap with an internal
O-ring is put on each end of each adsorbent cartridge to prevent ingress of contaminants
and sample loss. All of these components described are manufactured in standard sizes
so that they can be joined as a sampling train by standard fittings or used in Thermal
Desorption Units (TDU) made by different suppliers. The internal structure of a typical
adsorbent cartridge is shown in Figure 2.15.
Figure 2.15: Typical structure of an adsorbent cartridge [49]
This technique has been used in atmospheric aerosol sampling [47], [48] & [50] be-
cause it offers better sensitivity and simpler preparation procedures without toxic sol-
vents, such as methylene chloride or carbon disulfide, CS2, for extraction that can later
interfere with the analytical instruments. These toxic solvents also expose laboratory
technicians to greater health risks. More importantly, this technique consists of a sample
pre-concentration step that, when an FID or MS is used as the quantification instrument,
renders better resolved peaks in the chromatograms that permit more accurate quantifi-
cation of species [49]. The repeatability of the TD technique was found to be greater
than 84% from the 3 studies just described in aerosol PAH measurements.
Chapter 2. Background 33
2.5.2.2 Working Principle of Thermal Adsorption/Desorption
The proposed sampling technique functions based on the principle of physical adsorption.
Before the introduction of the phenomenon of adsorption, the following terminologies
warrant explanation to avoid confusion in the context that follows:
Table 2.5: Definitions of some frequent terms used in the description of adsorptive be-
haviours [51]
Term Definition
Adsorption Enrichment of one or more components in an interfacial layer
Adsorbate Substance in the adsorbed state
Adsorptive Adsorbable substance in the fluid phase
Adsorbent Solid material on which adsorption occurs
Chemisorption Adsorption involving chemical bonding
Physisorption Adsorption without chemical bonding
Monolayer capacity either Chemisobed amount required to occupy all surface sites
or Physisorbed amount required to cover surface
Surface coverage Ratio of amount of adsorbed substance to monolayer capacity
Adsorption is a surface phenomenon in which fluid/solid interactions take place and
fluid molecules are attracted by the solid phase due to physical forces such as van de
Waals attraction to form an adsorbed phase; this type of adsorptive behaviour is called
physisorption. Another important attraction force that induces adsorption is polar-polar
attraction. For example, adsorbents made of aluminosilicas and silica-alumina have high
affinity to water because they are polar and hence hydrophilic, whereas carbonaceous,
polymeric and silicalite adsorbents are nonpolar and have high affinity to oils, which
render them superior capacity to adsorb hydrocarbon compounds [52]. On the other
hand, attraction forces due to chemical reactions in which electrons are either exchanged
Chapter 2. Background 34
or shared between adsorbents and adsorbates lead to chemiosorption, which is a stronger
form of adsorption than physisorption.
Adsorption has been used as an industrial process in areas such as catalysis, bulk
separation and purification, pollution control and respiratory protection. In fact, the
separation technique that was used in this study—gas chromatography—relies on the
principle of adsorption to work. Some common adsorbents in use and their major appli-
cations to organic compounds are summarized in Table 2.6.
Chapter 2. Background 35
Table 2.6: Typical applications of some common adsorbents [51]
Type Typical Applications
Polymers
• Water purification, including removal of phenol, chloropenols,
ketones, alcohols, aromatics, indene, polynuclear aromatics,
nitro- and chlor-aromatics, polychlorinated biphenyls (PCB) and
pesticides;
• Separation of fatty acids from water and toluene;
• Separation of aromatics from aliphatics;
• Removal of organics from hydrogen peroxide;
Silica gel• Drying of gases, refrigerants, and organic solvents;
• Desiccant in packings and double glazing;
Activated alumina• Drying of gases, organic solvents and transformer oils;
• Removal of HCl from hydrogen;
Carbon
• Hydrogen from syn-gas and hydrogenation processes;
• Ethylene from methane and hydrogen;
• Removal of SOx and NOx;
•Water purification, including removal of phenol, halogenated com-
pounds, pesticides, and chlorine;
Zeolites
• Oxygen from air and drying of gases;
• Separation of ammonia and hydrogen;
• Recovery of carbon dioxide;
• Removal of acetylene, propane and butane from air;
• Separation of xylenes and ethyl benzene;
• Separation of normal from branched paraffins;
• Recovery of olefins and aromatics from paraffins;
• Recovery of carbon monoxide from methane and hydrogen;
• Pollution control, including removal of Hg, NOx and SOx;
Chapter 2. Background 36
Adsorbents are categorized into different types according to their surface properties
such as specific surface area and polarity. To provide a large number of adsorption
sites in a small volume of mass (i.e., a large specific surface area), most adsorbents are
highly porous or composed of very fine particles. Adsorbents that are polar have high
selectivity for polar analytes, whereas those that are non-polar have high selectivity for
non-polar analytes. For instance, Tenax TA, the adsorbent used in the development
of the current flame-sampling technique, are polymeric adsorbents and therefore, non-
polar. This non-polar nature renders its suitability in flame sampling since Tenax TA
is hydrophobic and has a very low retention capability for water—a major combustion
product in flames. Adsorbent pore size becomes important when separation is achieved
by sieving effects—compounds smaller than the pores are allowed to pass, while those
bigger are not. According to the definition of the International Union of Pure and
Applied Chemistry (IUPAC), adsorbents with a pore size between 0.3 and 2 nm are
classified as microporous materials, those with a pore size between 2 and 50 nm are
classified as mesoporous materials and those with pores larger than 50 nm are classified
as macroporous materials. Table 2.7 provides detailed information of the adsorbent used
in this study. It is noteworthy that Tenax TA has a smaller specific surface area (35
vs. 725 m2/g) and a larger pore size (200 vs. 4 nm) than the adsorbent, XAD-4, used
by [42]. The larger pore size of Tenax TA allows bigger molecules to pass through with
being captured by sieving effects. Although it is desirable for Tenax TA to have a large
surface area so that more aromatic compounds can be collected, it is selected in favour
of its high thermal stability during desorption at elevated temperatures and its sufficient
retention capacity towards aromatics.
Chapter 2. Background 37
Table 2.7: Properties of Tenax TA [53]
Chemical structure: 2,6-diphenylene oxide
Temperature limit: 350◦C
Specific surface area: 35 m2/g
Pore volume: 2.4 cc/g
Average pore size: 200 nm
Density: 0.25 g/cc
Mesh size: 60/80
There are several variables that dominate the effectiveness and efficiency of an ad-
sorption application, and they include the choice of adsorbents and the conditions under
which adsorption takes place. In the selection of adsorbents, the most important factor
to consider is the selectivities of the adsorbent for the target adsorbates. The selectivities
of an adsorbent to different adsorbates vary according to the following factors [51]:
• Equilibrium effects—differences in thermodynamic equilibria for each adsorbent-
adsorbate interaction
• Kinetic effects—differences in the rate at which different adsorbates travel to the
internal structure of the adsorbent
• Desorption effects—differences in the rate at which different adsorbates desorb from
the adsorbent
• Molecular-sieving effects—the dimension of pores of the adsorbent excludes adsor-
bate molecules larger than a certain size
Selectivity of an adsorbent toward a specific compound can be compared by the
adsorbent/adsorbate pair’s breakthrough volumes (BTV). BTV is typically defined in
two ways [46]:
Chapter 2. Background 38
• the volume of gas passed through the adsorbent bed when the outlet concentration
is 95% of that at the inlet, divided by the adsorbent mass(concentration method);
• the volume of inert gas necessary to elute the analyte of certain mass injected into
the adsorbents, divided by the adsorbent mass (elution method).
Hence BTVs are indications of how strongly target analytes are retained by a particular
adsorbent.
Adsorbents should be used in adsorption processes with the incorporation of a safe
sampling volume (SSV), usually taken as 2/3 of the minimum BTV, so that no sample
loss would occur. On the contrary, their selectivity of any adsorbate should be not too
strong such that irreversible adsorption takes place for any target analytes.
Another factor to consider is the surface area of an adsorbent. This factor affects
the adsorption capacity of the adsorbent by varying the number of adsorption sites. It
is desirable for adsorbents to have a large specific surface area, which is defined as the
ratio of surface area to mass, because the higher this value, the larger the number of
adsorption sites an adsorbent possesses and hence the larger the amount of adsorbates it
can trap. To achieve this high value of specific surface area, most modern adsorbent are
highly porous in nature and have a specific surface area ranging from 300-1200 m2/g [51].
Besides this value, adsorbents with a large network of pores would allow better adsorbate
transport to interior adsorption sites so that the maximum adsorption capacity can be
achieved more efficiently.
Adsorbents should have a high adsorption kinetic rate and a low desorption rate for
the target adsorbates so that the kinetic effects are maximized while the desorption effects
are minimized. Efficient regeneration methods should exist to restore maximum adsorp-
tion capacity of the adsorbent particles for repeated usage. Some common adsorbent
regeneration methods for gaseous phase adsorption include desorption at a low pressure
or high temperature, both conducted with an inert carrier stream passing through the
Chapter 2. Background 39
adsorbents. A detailed description of adsorbents and their surface characteristics can be
found in [51] & [52].
Adsorption is an extremely complex process that depends not only on the choice of
adsorbent but also the conditions of the environment under which the process takes place.
These conditions include temperature, pressure, and the concentrations of adsorbates and
their interactions. Kinetic and equilibrium theories have been proposed to described how
these experimental variables are inter-related; however, these existing theories, especially
those describing a multi-adsorbate system, involve idealizations that are rarely achievable
during actual experiments and only the simplest theories will be presented in this study.
The most fundamental theory in adsorption equilibrium is the Langmuir theory pro-
posed in 1918. This theory states that at equilibrium, the rate at which an adsorbate
occupies an adsorption site on the adsorbent equals the rate at which it desorbs from
the adsorbent. In other words, it is a theory founded on kinetic basis that assumes a
continual bombardment of adsorbate molecules onto the adsorbent surface along with a
corresponding desorption of these molecules so that there is no overall accumulation of
adsorbates. In order to apply this theory, the following idealizations are assumed [52]:
1. adsorbent surface is homogeneous such that adsorption energy is constant over all
adsorption sites
2. adsorption is localized, i.e., adsorbates are adsorbed at definite and localized sites
3. each site can only accommodate a single adsorbate, i.e., monolayer adsorption
In the Langmuir theory, the rate of adsorption is assumed to be equal to the product
of a sticking coefficient and the rate of adsorbate bombardment derived from the the-
ory of gas kinetics. Moreover, when an incoming adsorbate strikes an adsorption site
already occupied by an adsorbed molecule or atom, the incoming one will evaporate very
quickly. As a result, the adsorption rate also depends on the fraction of vacant adsorption
sites. The rate of desorption equals to the rate of desorption at full adsorbent coverage
Chapter 2. Bac