Adsorption evaporative emission control system for vehicles

ADSORPTION EVAPORATIVE EMISSION
CONTROL SYSTEM FOR VEHICLES






HE JING MING








NATIONAL UNIVERITY OF SINGAPORE
2009



ADSORPTION EVAPORATIVE EMISSION CONTROL
SYSTEM FOR VEHICLES

HE JING MING (MS)
(B.Eng, M.Eng, Tianjin University, China)









A THESIS SUBMITTED
FOR THE DEGREE OF PHILOSOPHY
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERITY OF SINGAPORE
2009
Acknowledgements

-
I
-
Acknowledgements
I would like to extend my sincere and heartfelt thanks to my supervisors, Prof. Ng Kim
Choon and Prof. Christopher Yap from the Department of Mechanical Engineering, for
their invaluable advice, guidance and constant encouragement throughout my whole
candidature study. Being an elder student and lacking of research background, without
their patience, understanding and tremendous support, I definitely would not have been
able to complete this tough yet enjoyable journey.

I also extend my sincere appreciation to Mr. Sacadevan Radhavan (from the Air
Conditioning Laboratory) for having kindly assisted me during the experimental set-up
and tests. My thanks are also extended to Mr. Tan (from the Energy Conversion
Laboratory) and Mrs. Ang (from the Air Conditioning Laboratory) for their kind
support in this research project. I am grateful to members of Prof. Ng’s research team:
Dr.B.B. Saha, Dr.Yanagi Hideharu, Dr.Anutosh Chakraborty, Messrs.M Kumja, Kyaw
Thu and Loh Wai Soong for their insightful suggestions, which have been greatly
helpful for the advance of my research.
In addition, I would like to express my heartfelt gratitude to my friend, Dr. Li
Jun (Department of Mechanical Engineering), who is from my home town, for his
constant encouragement and help throughout my whole study journey.
Last but not least, I take this opportunity to extend my deepest gratitude to my
husband and my parents for their unfailingly love, unconditional sacrifice and moral
support, which are far more than I could express in words. It is the encouragement
from my beloved son that leads me to the end of this journey. I owe every bit of my
happiness, satisfaction and achievement to my family.
He Jing Ming
31 July 2009
Table of Contents

-
II
-
Table of Contents

Acknowledgements I
Summary
IV
List of Figures
V

List of Tables
IX
List of Symbols
XI
Chapter 1
Introduction 1
1.1 Background 1
1.2 Motivation 1
1.3 Objectives 5
1.4 Scope of the Thesis 5
Chapter 2 Literature Review 8
2.1 Adsorption Mechanism and Measurement 8
2.1.1 Principle of Adsorption 8
2.1.2 Adsorption Equilibrium 9
2.1.3 Adsorption Kinetics 11
2.1.4 Pore-related Surface Characteristics of Adsorbent 12
2.1.5 Adsorption Measurement Technique 14
2.2 Adsorption Characteristic of Gasoline Vapor 19
2.3 Gasoline Evaporative Emission Control System 22
2.3.1 Onboard Evaporative Emission Control 22
2.3.2 Evaporative Emission Control at Gas Station 24
Chapter 3 Surface Characteristics of Carbon-based Adsorbents 27
3.1 Introduction 27
3.2 Carbon-based Adsorbent 28
3.3 Experimental 29
3.3.1 Nitrogen Adsorption Measurement for Surface Characteristics 29
3.3.2 Measurement of Thermal Conductivity 32
3.4 Results and Discussion 37
3.4.1 Nitrogen Adsorption Isotherms 37
3.4.2 BET Surface Area 40

3.4.3 Pore Size Distribution 41
3.4.4 Thermal Conductivity of Type Maxsorb III Activated Carbon 46
3.5 Chapter Summary 47
Chapter 4 Adsorption Characteristics of Gasoline Vapor 49
4.1 Introduction 49
4.2 Theoretical Model 50
4.2.1 Adsorption Isotherm - Dubinin-Astakhov (D-A) Model 50
4.2.2 Adsorption Kinetics - Linear Driving Force Model 51
4.2.3 Isosteric Heat of Adsorption 53
4.3 Experimental Set Up 57
4.3.1 Gasoline Adsorption Measurement 57
4.3.2 Gasoline Vapor Pressure Test 62
4.3.3 Gas Chromatography Test on Gasoline Composition 63
4.4 Results and Discussion 64
4.4.1 Gasoline Vapor Pressure Correlation 64
4.4.2 Gasoline Composition 66
Table of Contents

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III
-
4.4.3 Adsorption Isotherms of Gasoline Vapor onto Carbon-based Adsorbents 67
4.4.4 Adsorption Kinetics Correlation 79
4.4.5 Isosteric Heat of Adsorption 83
4.5 Effect of Initial Bed Pressure on the Adsorption Rate 86
4.5.1 Experimental 86
4.5.2 Theoretical 87
4.5.3 Effect of Helium Gas on the Adsorption Measurement 89
4.5.4 Adsorption Uptake 90
4.5.5 Pressure Effect on the Adsorption Rate Constant 92

4.6 Chapter Summary 98
Chapter 5 Numerical Simulation on Gasoline Vapor Adsorption
System
100
5.1 Introduction 100
5.2 Mathematical Modeling 101
5.2.1 Energy Balance 104
5.2.2 Overall Heat Transfer Coefficient 106
5.3 Results and Discussion 110
5.3.1 Adsorption 110
5.3.2 Desorption 112
5.3.3 Effect of Cooling Water Temperature 114
5.4 Chapter Summary 117
Chapter 6 Experimental Study on Gasoline Vapor Adsorption System

.
118
6.1 Introduction 118
6.2 Experimental Apparatus 118
6.2.1 Configuration of the Apparatus 118
6.2.2 Adsorption Chamber 122
6.2.3 Finned-Tube Adsorber 124
6.2.4 Measurement 125
6.2.5 Experimental Procedures 128
6.3 Results and Discussion 128
6.3.1 Adsorption Uptake of Gasoline Vapor 128
6.3.2 Effect of Cooling Water Temperature 130
6.3.3 Effect of Cooling on the Adsorption Uptake 132
6.3.4 Desorption 133
6.3.5 Adsorption Isotherm 135

6.3.6 Adsorption Kinetics 138
6.4 Chapter Summary 142
Chapter 7 Conclusions 143
7.1 Summary of the Thesis 143
7.2 Recommendations for Future Work 145
References 146
Appendix A Derivation of Surface Characteristics
155
Appendix B Experimental Data of Type Maxsorb III AC/Gasoline
Pair (by TGA Apparatus)
163
Appendix C Experimental Data of Finned-Tube Adsorption
Apparatus
185
Appendix D List of Publications during Ph.D Study
193
Summary
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IV
-
Summary
In recent years, hydrocarbon emissions, caused by evaporation of the gasoline during
vehicle operation, vehicle refueling at gas station and gasoline unloading, have drawn
increasing research attention because of environmental concerns. Firstly, in the current
work, the adsorption characteristics of gasoline vapor for four types of activated carbon
adsorbents are investigated using thermal gravimetric apparatus (TGA) under
isothermal conditions. The experimental results are correlated into D-R isotherm
model, LDF kinetics model and heat of adsorption, which are greatly lacking in the

published literature. The type Maxsorb III activated carbon is found to have
significantly high absorbability to the gasoline vapor (up to 1.2 g/g) owing to its high
surface area and pore volume. In addition, the effect of initial bed pressure on the
adsorption rate is investigated near the atmospheric condition and correlated in an
exponential form based on the transition theory, which is useful for practical system
design. Secondly, with the gasoline adsorption characteristic correlations, a numerical
simulation on an adsorption apparatus using type Maxsorb III activated carbon as
adsorbent and a finned-tube heat exchanger as adsorber (supplied alternatively with
cooling and heating fluid to aid in the adsorption and desorption process), is established,
and such adsorption apparatus is fabricated and tested for a range of cooling and
heating temperatures. Both the simulation and experimental results show a good
agreement and high gasoline vapor uptake (up to 1.12 g/g) can be achieved.
Experimental results are also correlated into isotherm and kinetic expressions, and a
sample of results compared with those of TGA experiments to check their accuracy.
List of Figures
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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V
-
List of Figures
Figure 1.1 Schematic of onboard (vehicle) evaporative emission control 2
Figure 2.1 The IUPAC classification of isotherm 10
Figure 2.2 Schematic of volumetric/manometric apparatus 15
Figure 2.3 Schematic of gravimetric apparatus 17
Figure 2.4 Schematic of ORNL isopiestic apparatus 18
Figure 2.5 Schematic of evaporative emission control at gas station 24
Figure 3.1 Specimens of carbon-based adsorbents 28
Figure 3.2 Scanning electron micrograph (SEM) of type Maxsorb III AC 29
Figure 3.3 Scanning electron micrograph of type ACF-1500 ACF 29

Figure 3.4 Pictorial and schematic view of AUTOSORB-1 apparatus 31
Figure 3.5 Sample cells used for nitrogen adsorption by AUTOSORB-1 32
Figure 3.6 Pictorial view of guarded hot plate conductance apparatus 33
Figure 3.7 Schematic of test section of guarded hot plate apparatus 34
Figure 3.8 Typical layout of thermocouples on the sample surface 35
Figure 3.9 Nitrogen adsorption isotherms (at 77.4 K) for the four adsorbents 38
Figure 3.10 Nitrogen adsorption isotherms in low pressure region 40
Figure 3.11 Total surface area determined by multi-points BET plot 41
Figure 3.12 Pore size distribution for the four adsorbents by QSDFT analysis 43
Figure 3.13 Cumulative pore volume distribution for the four adsorbents 45
Figure 3.14 Experimental thermal conductivity of the Maxsorb III AC at different
sample temperatures 47
Figure 4.1 Pictorial view of TGA system 59
Figure 4.2 Schematic diagram of the TGA system 59
Figure 4.3 Pictures of sample installation in TGA experiment 60
Figure 4.4 Pictorial view of gasoline vapor pressure test 63
Figure 4.5 Gas chromatography (HP 6890 series) 64
Figure 4.6 Gasoline vapor pressure and temperature vs. time 65
Figure 4.7 Experimental gasoline vapor saturation pressure vs. temperature 65
Figure 4.8 Experimental transient adsorption uptake of gasoline vapors onto the four
carbon-based adsorbents at assorted adsorption temperatures 69
Figure 4.9 Instantaneous adsorption uptake of the four adsorption pairs at adsorption
temperature of 20°C 70
List of Figures
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-
VI
-
Figure 4.10 Transient adsorption uptake versus time for Maxsorb III/gasoline pair

with stepwise changes of pressure at adsorption temperature of 30°C 71
Figure 4.11 Plots of ln (W) versus [T ln (Ps/P)]
n
for Maxsorb III/gasoline pair 75
Figure 4.12 Plots of ln (W) versus [T ln (Ps/P)]
2
for ACF-1500/gasoline pair 75
Figure 4.13 Plots of ln (W) versus [T ln (Ps/P)]
2
for PAC-1/gasoline pair 76
Figure 4.14 Plots of ln (W) versus [T ln (Ps/P)]
2
for GAC-1/gasoline pair 76
Figure 4.15 Adsorption isotherm for Maxsorb III/gasoline pairs 78
Figure 4.16 Variations of ln [(W-w)/W)] versus time for Maxsorb II/gasoline pair at
assorted adsorption temperatures 79
Figure 4.17 Variations of ln [(W-w)/W )] versus time for ACF-1500/gasoline pair at
assorted adsorption temperatures 80
Figure 4.18 Variation of ln(k
s
a
v
) versus (1/T) 81
Figure 4.19 Compression between measured and predicted uptake of gasoline vapor
onto type Maxsorb III activated carbon 82
Figure 4.20 Compression between measured and predicted uptake of gasoline vapor
onto type ACF-1500 activated carbon fiber 83
Figure 4.21 Isosteric heat of adsorption versus surface coverage for the four
adsorption pairs 84
Figure 4.22 Isosteric heat of adsorption versus surface coverage at assorted

temperatures for Maxsorb III/gasoline pair 85
Figure 4.23 Ratio of activation energy to the heat of adsorption versus surface
coverage 85
Figure 4.24 Adsorbent sample mass and adsorption chamber pressure versus time
during charging of helium gas 89
Figure 4.25 Adsorption uptakes vs. time at various pressure differences under
adsorption temperature of 30°C (Maxsorb III/gasoline pair) 91
Figure 4.26 Adsorption uptakes vs. time at various pressure differences under
adsorption temperature of 35°C (Maxsorb III gasoline pair) 91
Figure 4.27 ln [(W-W) /W ] vs. time under adsorption temperature of 30°C 92
Figure 4.28 ln [(W-W) /W ] vs. time under adsorption temperature of 35°C 93
Figure 4.29 Deviation between LDF predicted uptake and experiemtnal uptake at
various pressures differences,ΔP and two adsorption temperatures,T 93
Figure 4.30 ln(k
s
a
v

) vs pressure difference under adsorption temperatures of 30°C
and 35°C 95
List of Figures
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-
VII
-
Figure 4.31 D
so
*
vs pressure difference under adsorption temperatures of 30°C and

35°C 96
Figure 4.32 Adsorption uptake of experimental, predicted by proposed equation and
predicted using Arrhenius form at pressure differences of 32 kPa under
adsorption temperature of 30°C 97
Figure 4.33 Effective mass transfer coefficient, k
s
a
v
versus pressure difference at
adsorption temperature of 30 °C 98
Figure 5.1 Schematic of the gasoline vapor adsorption system 101
Figure 5.2 Sectional view of the finned-tube assembly containing the adsorbent in
between the fins 102
Figure 5.3 Schematic of typical finned-tube section 102
Figure 5.4 Schematic of thermal resistance for finned-tube configuration 107
Figure 5.5 Simulation results for transient adsorption uptake and temperature at
cooling water temperature of 30°C 111
Figure 5.6 Simulation results for transient adsorption uptake and pressure at cooling
water temperature of 30°C 111
Figure 5.7 Simulation results of desorbed amount and temperature for desorption at
heating water temperature of 85°C 112
Figure 5.8 Simulation results of desorbed amount and bed pressure for desorption at
heating temperature of 85°C 113
Figure 5.9 Desorption profile at assorted heating temperature 114
Figure 5.10 Simulation results for transient adsorption uptake and temperature for
gasoline vapor adsorption using finned-tube adsorber at cooling water
temperature of 25°C 115
Figure 5.11 Simulation results for transient adsorption uptake and temperature for
gasoline vapor adsorption using finned-tube adsorber at cooling water
temperature of 20°C 115

Figure 5.12 Comparison of transient adsorption uptake and bed temperature at initial
bed temperature of 30°C - (1) with cooling (2) without cooling 116
Figure 6.1 Schematic of experimental apparatus for gasoline vapor adsorption 120
Figure 6.2 Pictorial view of gasoline vapor adsorption apparatus 122
Figure 6.3 Pictorial view of adsorption chamber 123
Figure 6.4 Schematic section view of adsorption chamber 123
Figure 6.5 Picture of finned-tube assembly 124
Figure 6.6 Calibration of load cell 126
List of Figures
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VIII
-
Figure 6.7 Photography of measuring Instrument 127
Figure 6.8 Adsorption uptake and adsorbent temperature versus time at cooling
water temperature of 30°C ( lines in black represent the predicted results )
129
Figure 6.9 Adsorption uptake and bed pressure versus time at cooling water
temperature of 30°C (lines in black represent the predicted results) 130
Figure 6.10 Adsorption uptake and adsorbent temperature versus time at cooling
water temperature of 20°C 131
Figure 6.11 Adsorption uptake and adsorbent temperature versus time at cooling
water temperature of 25°C 131
Figure 6.12 Adsorption uptake and adsorbent temperature versus time at cooling
water temperature of 35°C 132
Figure 6.13 Comparison of adsorption uptake and adsorbent temperature versus time
without and with cooling (30°C) 133
Figure 6.14 Transient desorbed amount and adsorbent temperature at heating water
temperature of 95°C 134

Figure 6.15 Transient desorbed amount and adsorbent temperature at heating water
temperature of 85°C 134
Figure 6.16 Ln (W) versus [T ln (Ps/P)]
2
136
Figure 6.17 Predicted adsorption isotherm of gasoline vapor onto Maxsorb III using
finned- tube adsorber by D-R equation 137
Figure 6.18 Variation of ln [(W-w)/W)] versus time for Maxsorb II/gasoline vapor
with finned-tube adsorber at assorted cooling water temperatures 139
Figure 6.19 Comparison of experimental uptake and uptake predicted by using LDF
model at cooling temperature of 30°C 139
Figure 6.20 Comparison of experimental uptake and uptake predicted by using LDF
model at cooling temperature of 20°C 140
Figure 6.21 Variations of ln (k
s
a
v
) vs. (1/T) 141
Figure A.1 Comparison of experimental isotherm with fitted isotherm using QSDFT
and NLDFT models (for type Maxsorb III activated carbon) 159
List of Tables

-
IX
-
List of Tables
Table 3.1 BET surface area of the four carbon-based adsorbents 41
Table 3.2 Surface characteristic properties of the four adsorbents 46
Table 3.3 Experimental results for determination of thermal conductivity of type
Maxsorb III activated carbon 48

Table 4.1 Summary of sample weight 61
Table 4.2 Composition of gasoline Octane 98 66
Table 4.3 Thermodynamic properties of gasoline vapor 66
Table 4.4 Equilibrium adsorption uptakes for Maxsorb III/gasoline pair at assorted
pressures and temperatures 72
Table 4.5
o
W
Experimental values of and E for the four adsorption pairs 77
Table 4.6 Comparisons of v
o
and micropore volume, v
mic
79
Table 4.7 Overall mass transfer coefficients, k
s
a
v
( s
-1
)

at assorted adsorption
temperatures ( °C) for the four adsorption pairs 80
Table 4.8
a
E
Experimental values of and
*
so

D
for four adsorption pairs 82
Table 4.9 Adsorption rate constant k
s
a
v
for various pressure differences ∆P under
adsorption temperatures of 30°C and 35°C 94
Table 4.10 Comparison of the k
s
a
v
of experimental k
s
a
v
(exp); prediction by
proposed form k
s
a
v
(pro) and

prediction using Arrhenius form k
s
a
v
(Arr)
97
Table 5.1 Physical and thermal properties constants used in simulation model 109

Table 5.2 Comparison for various cooling water temperature 116
Table 6.1 Comparison of gasoline vapor compositions (% by Volume) 135
Table 6.2
o
W
Comparison of experimental values of n, and E by TGA and finned-
tube adsorber 137
Table 6.3 k
s
a
v
under assorted cooling temperatures for finned-tube adsorber 138
Table 6.4
a
E
Comparison of experimental values of and
*
so
D
for finned-tube
adsorber and small sample by TGA 141
Table A.1 Nitrogen isotherm data for type Maxsorb III powdered activated carbon
158
Table A.2 Nitrogen isotherm data for type PAC-1 pellet activated carbon 160
List of Tables

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X
-
Table A.3 Nitrogen isotherm data for type GAC-1 charcoal activated carbon 161

Table A.4 Nitrogen isotherm data for type ACF-1500 activated carbon fiber 162
Table B.1 Experimental uptake data at 20°C (by TGA apparatus) 163
Table B.2 Experimental uptake data at 30°C (by TGA apparatus) 171
Table B.3 Experimental uptake data at 40°C (by TGA apparatus) 178
Table B.4 Experimental uptake data at 50°C (by TGA apparatus) 181
Table B.5 Experimental uptake data at 60°C (by TGA apparatus) 183
Table C.1 Experimental uptake data at cooling water temperature of 20°C (by
finned-tube adsorption apparatus) 185
Table C.2 Experimental uptake data at cooling water temperature of 25°C (by
finned-tube adsorption apparatus) 187
Table C.3 Experimental uptake data at cooling water temperature of 30°C (by
finned-tube adsorption apparatus) 189
Table C.4 Experimental uptake data at cooling water temperature of 35°C (by
finned-tube adsorption apparatus) 191
List of Symbols

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XI
-
List of Symbols
a
Constant-gasoline saturation pressure correlation
1

a
Constant-gasoline saturation pressure correlation
2

a
Constant-gasoline saturation pressure correlation

3

A
Adsorption potential
A
Specific BET surface area, m
BET

2
A
/g
Cross sectional area of sample, m
c

A
2

Cross sectional area of adsorbate molecule, cm
cs

3
A
/mol
External area of evaporator, m
evp

A
2

Total surface area of fins, m

f

A
2

Inner area of metal tubes, m
i

A
2

Total BET surface area, m
t

B
2

Fin pitch, m
B
Constant
C
Constant
C'
Constant
C
Constant
1

C
Constant

2

C
Constant
T

C'
Constant
T

C
Specific heat of adsorbed phase adsorbate, J/kg K
p,a

C
Specific heat of activated carbon, J/kg K
p,ac

C
Specific heat of evaporator, J/kg K
p,evp

C
Specific heat of gasoline liquid, J/kg K
p,gl

C
Specific heat of gasoline vapor, J/kg K
p,gv

C
Specific heat of finned-tube assembly, J/kg K
p,hex

C
Specific heat of water, J/kg K
p,w

d
Inner diameter of metal tube, m
w

D
Particle diameter, m
p

D
Diffusivity, s
s

D
-1

Diffusivity at zero activation energy , s
so

D
-1

so

Pre-exponential constant, s
*

dV(r )
-1

Incremental pore volume, cm
3
E
/nm g
Adsorption characteristic energy, kJ/mol
List of Symbols

-
XII
-
E
Activation energy, kJ/mol
a

Ē
Average activation energy of gasoline vapor, kJ/mol
a

EI
Power of heater, watt
F
Constant
o

G
Specific gravity of gasoline vapor, equal to ρ
g

g
/ρ
ΔG
air

Gibbs free energy change, J
h
Enthalpy of adsorbate in adsorbed phase, J/kg
a

h
Heat of vaporization, J/kg
fg

h
Enthalpy of adsorbate in gaseous phase , J/kg
g

h
Enthalpy of gasoline liquid , J/kg
gl

h
Heat transfer coefficient between water fluid and metal tube wall,
W/m K
i

h
Heat transfer coefficient between adsorbent and fin wall, W/m K
sf

H
Pore width, nm
ΔH
Enthalpy change, J
k
Thermal conductivity, W/m K
k
Effective thermal conductivity of adsorbent with stagnant fluid,
W/m K
eo

k
Effective thermal conductivity of adsorbent with radical fluid flow,
W/m K
er

k
Thermal conductivity of evaporator wall, W/m K
evp

k
Thermal conductivity of fin, W/m K
fin

k

Thermal conductivity of gasoline vapor, W/m K
g

k
Thermal conductivity of metal tube, W/m K
m

k
Thermal conductivity of activated carbon, W/m K
s

k
Thermal conductivity of water, W/m K
w

k
s
a
Overall mass transfer coefficient / rate constant, s
v

K*
-1

Equilibrium constant
K
BET constant
B

L

Total length of finned-tube, m
L
Total length of tube sections attached to the fin, m
b

L
Total tube length excluding fin base section, m
o

m
Mass of adsorbed adsorbate , kg
a

m
Mass of activated carbon contained in the finned-tube, kg
ac

m
Mass of gasoline liquid in the evaporator, kg
el

m
Mass of evaporator, kg
evp

m
Mass of adsorbate in gaseous phase, kg
g

m

Mass of finned-tube assembly, kg
hex

List of Symbols

-
XIII
-
m
Initial mass of sample, kg
i

m
Final mass of sample, kg
o

m
Mass of adsorbent, kg
s

m
Mass of sample, kg
sp

.
e
m

Mass flow rate of gasoline vapor, kg/s
.

w
m

Mass flow rate of water, kg/s
M
Molar volume of the adsorbate
v

N
Avogadro’s number, molecules/mol
Δm
Transient weight change, kg
M
Molar weight of gasoline vapor (average), g/mol
g

n
D-A constant
P
Pressure, kPa
P
Ambient pressure, kPa
a

P
Adsorption chamber pressure, kPa
c

P
Evaporator pressure, kPa

e

P
Initial bed pressure, kPa
adi

P
relative pressure, kPa
r

P
Saturation pressure, kPa
s

ΔP
Pressure change, kPa
q
Volume of adsorbed adsorbate, m
q
3

Volume of adsorbed adsorbate forming monolayer, m
m

Q
3

Heat of adsorption, J/kg
st

r
Radius of fin, m
f

r
Inner radius of metal tube, m
i

r
Mean radius of fin, m
m

r
Outer radius of metal tube, m
o

r
Average pore radius, m
p

R
Gas constant, J/mol K
R
Particle radius, m
p

R
Thermal resistance of adsorbent in finned-tube, K/W
o

R
Thermal resistance of adsorbent in finned-tube, K/W
f

R
Thermal resistance between water and tube wall, K/W
i

R
Thermal resistance through fin wall, K/W
wf

R
Thermal resistance through tube wall, K/W
wo

s
Specific entropy, J/kg
List of Symbols

-
XIV
-
S
Entropy, J
S
Entropy of adsorbed phase adsorbate, J
a

S

Entropy of gas phase adsorbate, J
g

S
Entropy of solid adsorbent, J
s

ΔS
Entropy change, J
t
Time, s
T
Temperature, °C
T
Ambient temperature, °C
a

T
h1
, T
Hot, cold side temperature of upper sample,
c1

o
T
C
h2
, T
Hot, cold side temperature of lower sample,
c2

o
T
C
Evaporator temperature,
e

o
T
C
Initial adsorption bed temperature,
adi

o
T
C
Hot side temperature of upper sample,
h1

o
T
C
Hot side temperature of lower sample,
h2

o
T
C
Adsorption/desorption temperature,
j

o
T
C
Temperature of sample,
sp

o
T
C
Inlet water temperature,
wi

o
T
C
Outlet water temperature,
wo

o
T
C
Water bath temperature,
wt

o
∆T
C
Temperature difference,
o

∆T
C
Temperature differential of upper sample,
1

o
∆T
C
Temperature differential of lower sample,
2

o
U
C
Overall heat transfer coefficient, W/m
j

2
u
K
Superficial velocity, m/s
o

u
Velocity of water fluid, m/s
w

V
Volume, m
ΔV

3

Volume change, m
V
3

Volume of adsorbate in adsorbed phase, m
a

V
3

Volume of adsorbate in gaseous phase, m
g

V
3

Volume of space in between fins, m
o

v
3

Specific volume of adsorbed phase adsorbate, m
a

3
v
/g

Specific volume of adsorbed nitrogen, m
ads

3
v
/g
Specific volume of gas phase adsorbate, m
g

3
v
/g
Micropore volume, cm
mic

3
v
/g
Equilibrium adsorbed volume, cm
3
v
/g
Molar volume of liquid nitrogen, cm
m

3
/mol
List of Symbols

-

XV
-
v
Maximum equilibrium adsorbed volume, cm
o

3
v
/g
Total pore volume, cm
t

3
w
/g
Instantaneous adsorption uptake, g/g
W
Equilibrium adsorption uptake, g/g
W
Maximum equilibrium adsorption uptake, g/g
o



Subscripts
a
Adsorbed phase
ac
Activated carbon
g

Gaseous phase
gv
Gasoline vapor
gl
Gasoline liquid
j
Adsorption mode or desorption mode
p
Particle
s
Solid phase
sp
sample
w
Water


Dimensionless Number
Nu
Nusselt number
Pr
Prandtl number
Re
Reynolds number


Greek

αβ
Correlation factor in heat transfer coefficient

β
Ratio of activation energy to heat of adsorption
δ
Constant for numerical simulation
δ
Thickness of evaporator wall, m
evp

δ
Thickness of fin, m
fin

δ
Thickness of sample, m
sp

ε
Porosity
η
Fin efficiency
f

θ
Surface coverage
µ
Chemical energy, kJ/kg
µ
Chemical potential of adsorbed adsorbate, kJ/kg
a

µ
Chemical potential of gas phase adsorbate, kJ/kg
g

List of Symbols

-
XVI
-
µ
Chemical potential of adsorbent, kJ/kg
s

µ
1,
µ
Variables for pore shape distribution
2

ν
Kinematic viscosity of gasoline vapor, m
g

2
ν
/s
Kinematic viscosity of water, m
w

2

ξ
/s
Correlation factor in fin efficiency
ρ
Density of air, equal to 1.21 kg/m
air

ρ
3

Density of gasoline vapor, kg/m
gv

ρ
3

Density of gasoline liquid, kg/m
gl

ρ
3

Density of water, kg/m
w

σ
3

1,
σ

Variables for pore shape distribution
2

Φ
Correlation factor in effective thermal conductivity
ψ
Correlation factor in fin efficiency

Chapter 1 Introduction

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1
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Chapter 1
Introduction

1.1 Background
In recent years, air pollutions caused by gasoline vapor emissions from vehicles and at
gas stations have attracted much attention because of environmental concerns [1-3].
Gasoline automobiles produce two types of emissions: the exhaust emissions from the
by-products of combustion such as CO, CO
2
1.2 Motivation
, NOx and sulfur, and the evaporative
emission from the evaporation of the fuel during refueling. The control of the
hydrocarbon emissions caused by evaporation of the gasoline during vehicle operation,
vehicle refueling at gas station and gasoline unloading is the main objective of the
current research. A successful method of controlling refueling emission will help to
reduce the health and safety risk of personnel who are exposed to such emissions. The
contents of unburned gasoline vapor are benzene, dimethyl benzene, ethyl benzene and

other hydrocarbons. It has been reported [3] that such gases can react under ultraviolet
radiation in atmospheric air, produce more toxic photochemical smog and threaten the
health of people.

Evaporative emission accounts for about 20% of the emitted hydrocarbons from
vehicles. A recent European study found that 40% of man-made volatile organic
compounds come from vehicles [4], which are released to the atmosphere as follows:
(1) Refueling: As gasoline vapors are always present in fuel tanks, these fuel
vapors are forced out when the tank is filled with liquid fuel.
Chapter 1 Introduction

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(2) Running Losses: Heat from hot engine can vaporize gasoline when the
vehicle is running.
(3) Diurnal Losses: This occurs during the day time when the fuel is heated by
an increase in surrounding temperature. The temperature rise leads to
gasoline vaporization.
(4) Hot soak: After a vehicle is turned off, the engine remains hot for a period
of time and the radiant heat will also cause gasoline vaporization for an
extended period.
Strategies of evaporative emission controls include onboard evaporative emission
control and emission control at gas station as well as restriction on gasoline volatility.
Since legislation passed in the United States in 1970 to prohibit venting of fuel vapor
into the atmosphere, the onboard (vehicle) evaporative emission control apparatus,
called the carbon canister, has been developed to eliminate this source from vehicles
(Figure 1.1).
Engine
Activated

Carbon
Canister
Fuel Tank
Fuel Injection
Purge air
Air Intake

Figure 1.1 Schematic of onboard (vehicle) evaporative emission control

Chapter 1 Introduction

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3
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The function of the apparatus is to trap and store fuel vapors that are emitted from
the fuel tank. When the engine is started, the adsorbed gasoline vapor is purged out of
the adsorbent (carbon) by the ambient air and drawn by engine vacuum into the
manifold for combustion in the engine. However, the drawbacks of the apparatus are:
(1) the moisture drawn into the canister with purge air could freeze or even block the
purging in cold weather [5]; (2) a decrease in adsorption capacity due to exothermic
adsorption process on hot days; (3) insufficient purging caused by a large pressure
resistance across the densely packed carbons, leading to the deterioration of the
adsorption capacity and (4)
The adsorption method is generally categorized into pressure swing adsorption
(PSA) and thermal swing adsorption (TSA) types. In PSA systems, high pressure
carrier gas is used to accomplish the adsorption of adsorbate vapor into adsorption beds,
whilst using reducing pressure or vacuuming for desorption. A PSA system is suited to
rapid cycling and separation process, but it has high mechanical energy consumption
and operates at very low desorption pressures. In addition, it occupies a large foot-print
and has high operation and maintenance costs. In TSA systems, the adsorption bed is

sub-emission due to the exposed configuration to the
atmosphere.
Gasoline recovery at the gas station is another strategy that captures gasoline
vapor when a vehicle is refilled. The emitted vapors are drawn in and disposed by
central treatment facilities. The commonly used techniques are adsorption, absorption,
condensation, direct combustion and membrane. Adsorption technology is a promising
method because of the low energy consumption, no moving parts, and little
maintenance required. With the adsorption method, the evaporated gasoline vapor is
captured in the adsorption bed and adsorbed by activated carbon, activated carbon fiber
or silica gel adsorbent. The adsorbed vapors could be thus desorbed by means of
reducing pressure (vacuuming), thermal heating and vacuum-assisted thermal heating.
Chapter 1 Introduction

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4
-
regenerated by heating, either by a stream of hot gas or hot water available from waste
heat. TSA systems are preferred for strongly adsorbed species of adsorbates and for
systems containing several adsorbates of different adsorption affinities because the
thermal adsorption is more effective than that due to pressure swing from the view of
thermodynamic potential.
For a practical adsorption system, the preferred adsorbent, such as activated
carbon (AC), is predominantly microporous. AC is a versatile adsorbent as it has an
extremely high surface area and micropore volume. Its bimodal or trimodal pore size
characteristics allow good access of adsorbate molecules to the interior surfaces [6]. It
is thus widely used in many applications including decolorizing sugar, water
purification, solvent recovery, gas purification, fuel gas desulphurization, gas
separation and air purification. AC can be produced from a variety of carbonaceous raw
materials such as coal, coconut shells, wood and lignite. The intrinsic properties of the
activated carbon are dependent on the raw material source. As gasoline vapor contains

hydrocarbons, the nonpolar activated carbon surface that tends to be hydrophobic and
organophilic, has the affinity for organic pollutants like benzene and is therefore
suitable for the adsorption of gasoline vapors [7].
For the proper design of an adsorption system for gasoline evaporative emission
control, the adsorption characteristics of gasoline vapor on activated carbon adsorbents
are required. However, a survey of literature indicates a great lack of information
available in this regard. As gasoline vapor has many chemical species (up to two
hundred) such as groups of n-paraffin, iso-paraffin, cylco-paraffin, olefins and
aromatics, the adsorption characteristics study becomes challenging.

Chapter 1 Introduction

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5
-
1.3 Objectives
The current study is an experimental investigation of the adsorption characteristics of
gasoline using the best commercially available carbon-based adsorbents including
pitch-based powder type Maxsorb III activated carbon, anthracite-based pellet type
PAC-1 activated carbon, wood-based granular type GAC-1 activated charcoal and
activated carbon fiber felt type ACF-1500. The study covers the broader aspects of
adsorption characteristics including the adsorption isotherm and kinetics of gasoline/
carbon-based adsorbents pairs, heat of adsorption, thermal conductivity and pore-
related surface characteristics of the activated carbon adsorbents. A mathematical
model of the adsorption system using novel finned-tube adsorption bed for the gasoline
evaporative emission control is developed and presented in the thesis. Based on results
of simulations, a bench-scale gasoline vapor adsorption apparatus using finned-tube
heat exchanger has been designed and fabricated. The finned-tube adsorber shows high
adsorption capacity and thus has promising potential in evaporative emission control
for use onboard the vehicle and at gas stations.

1.4 Scope of the Thesis
In this thesis, Chapter 1 introduces the background and objectives of this study.
Chapter 2 presents a thorough review of the available literature on aspects of the
adsorption characteristics of gasoline vapor and gasoline evaporative emission control
systems. There is a dearth of information on the adsorption characteristics of gasoline
compounds, and published work is associated with the pressure swing rather than the
temperature swing method. It is the latter approach that is used in the current work.
Chapter 3 presents the experimental investigation of the surface characteristics
including surface area, pore radius, pore volume and pore size distribution of four types
of carbon-based adsorbent, viz. (i) the pitch-based powder type Maxsorb III activated
Chapter 1 Introduction

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6
-
carbon, (ii) the anthracite-based pellet type PAC-1 activated carbon, (iii) the wood-
based granular type GAC-1 activated charcoal and (iv) type ACF-1500 activated
carbon fibre. The pore surface characteristics are used in identifying a suitable
adsorbent for gasoline adsorption, which in turn determines the adsorption capacity. In
addition, the thermal conductivity of type Maxsorb III activated carbon, which is
needed for computation of the overall heat transfer coefficient for adsorbent-adsorbate
heat exchanger, is investigated experimentally.
Chapter 4 describes the experimental studies on the gasoline adsorption
isotherms, kinetics and heat of adsorption with the four selected carbon-based
adsorbents. This fundamental information is needed for the design and modeling of the
gasoline adsorption evaporative emission control system. Studies are firstly conducted
for vacuum conditions where the Dubinin-Radushkevich (D-R) isotherm model is
found to be suitable for correlating the experimental data and to predict the adsorption
isotherm. The linear driving force (LDF) model is employed successfully to represent

the adsorption kinetics. In addition, by applying classical thermodynamic theory, the
isosteric heat of adsorption is derived as a function of adsorption temperature and
adsorbent surface coverage, which may result in a more accurate approximation than
that from the generally used Clausius-Clayperon method. In the last part of this chapter,
experimental results for near atmospheric conditions are presented in aspect of
adsorption rate, which is applicable and useful for the practical vehicle evaporative
emission control system.
Chapter 5 presents the thermodynamic modeling and mathematical simulation
of the thermal swing adsorption system for gasoline evaporative emission control. A
finned-tube adsorption bed supplied alternatively with cooling and heating fluid to aid
in the adsorption and desorption processes, is modeled. The simulation results are
Chapter 1 Introduction

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7
-
discussed under a range of operating conditions of cooling and heating water
tempratures.
Using the simulation results, an experimental test rig comprising eight finned
tubes with vapor spaces in-between is designed. Details of the rig are described in
Chapter 6. Vapor is introduced into the chamber and diffused into carbon adsorbents
that are packed between circular fins. Experiments have been conducted under
conditions of varying cooling and heating water temperatures, and the results are
verified against the simulations. In addition, experiments are also conducted under the
“no cooling” condition. The experimental results confirm that adsorption capacity with
cooling can be significantly enhanced by almost 30%.
The conclusion of this thesis is presented in Chapter 7 and recommendations for
future improvements have been made.