Aromatic Hydrocarbon Sampling and Extraction From Flames ... › bitstream › 1807 › 29506 › 1...

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


mm for a sample volume of ∼300 mL . . . . . . . . . . . . . . . . . . . . 99


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



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





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


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.


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


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.


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


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]


Figure 2.5: Names and structures of selected PAHs monitored under EPA and EU regu-

lations


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


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.


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.


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

SOFTbank E-Book Center Tehran, Phone: 66403879,66493070 For Educational Use.

Figure 2.6: Formation of first ring from acetylene [27]


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


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.


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


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


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.


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


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


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]


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.


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


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


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.


Compound Largest Discrepancy

Benzene a factor of ∼450Naphthalene a factor of


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]


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.


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]


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


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.


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


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.


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;


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.


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]:


• 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


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

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