Adsorption Storage
A viable alternative to compression
for natural gas powered vehicles ?
David Quinn
Royal Military College of Canada
Presented to
ALL-CRAFT Columbia, Mo
July,2005
Natural Gas as a Vehicular Fuel
Excellent fuel,
Clean burning, no deposits
No additives,
High Octane Number, 130 RON
Worldwide, more than a million vehicles
operate using CNG as their fuel source.
Used as a vehicular fuel for nearly a century!
Scottish bus in World War I
running on gas stored in balloon on roof.
CNG, Compressed natural gas
Storage at pressures >200 atmos (3000psi)
Expensive 4 stage compression needed using
~15% energy of the gas.
Heavy walled steel or carbon-fiber / epoxy
cylinders required.
Store ~220 - 240 v/v based on internal volume.
No consideration of wall thickness or envelope
box.
Internal volume is ~ 70% of envelope, so storage
is really about 160 v/v.
ANG as an alternative to CNG
What is ANG ? Adsorbed Natural Gas
What is adsorption ?
Gas Law
A
B
Total moles gas ∝ PAVA (Valve closed, B evacuated)
Valve opened,
Total moles gas ∝ PAB(VA + VB)
PAVA = PAB(VA + VB)
Adsorption
Solid placed in B
then,
PAVA
>
PAB(VA + VB)
A
B
Molecules removed from gas phase,
“Adsorbed” onto surface of solid.
Amount adsorbed ∝ PAVA - PAB(VA + VB)
Extent of adsorption
dependent on,
1.
Temperature
2.
Adsorption potential of surface
3.
Amount of available surface
1. Temperature
Lower temperature, greater adsorption,
Higher temperature, lower adsorption.
Simplify to realistic temperature for vehicular use,
constant temperature, (isothermal), of 298K
for experimental studies.
2.
Adsorption potential of surface
Different materials give different 298K
methane isotherms.
Porous organic compounds,
e.g. Amberlite (Rohm & Haas), Dow resins
Zeolites, (Davidson molecular sieves)
Silica based compounds,
Xerogels, aerogels, MCM41 etc.
All adsorb less methane than similar area
porous carbon.
They have lower adsorption potentials.
Methane 298K Isotherms
on various porous materials
100
BPL
Dow Resin
MCM-41
Zeolite
Uptake (mg/g)
80
60
40
20
0
0
100
200
300
400
Pressure (psia)
500
600
However,
Some high methane uptake claims made for
cavity based crystalline salts.
Ni++ Cu++ salts
by Seki, Osaka Gas
Zn++ salts by Yaghi, University of Michigan
Never independently verified.
1,4-Benzenedicarboxylate
(BDC)
[Fm-3m, a=25.6690(3)Å]
Yaghi, University of Michigan
Porous Carbons
Highly disordered carbon, unlike diamond or
graphite
Described as like a pile of potato chips
“Chips”, small crystallites with graphite like
structure
Space between chips are the “pores”
Pore Definitions (IUPAC)
Micropore
2 - 20 Å
Mesopore
20 - 50 Å
Macropore
> 50 Å
Adsorption
Pore wall of carbon atoms provides attractive
force for the adsorbate molecules.
Influence of both “walls” in narrow pores
so adsorption potential is greater.
Rule of thumb,
Narrow pored adsorbents, good for gas
adsorption, small molecules.
Larger pored materials, better for liquids
and larger sized molecules.
Methane 298K Isotherms
on a mass basis
250
AX-21
BPL
PVDC
Uptake (mg/g)
200
150
100
50
0
0
100 200 300 400 500 600 700 800 900 1000 1100
Pressure (psia)
Gas Storage
Adsorption uptakes usually expressed as
mass uptake,
e.g. Grams adsorbate / gram adsorbent
Porous carbons differ greatly in density.
Storage vessels have finite volume.
For storage, uptake must be considered
from a volume perspective.
Container Volume
Micropore
Macropore
Carbon
Void
Carbon Filled Vessel
Vessel Volume Utilization
Micro
14%
Macro
32%
Carbon
12%
Void
42%
AX-21 Carbon
Vessel Volume Utilization
Micro
44%
Carbon
46%
Void
8%
Macro
2%
PVDC Carbon
Vessel Volume Utilization
Micro
14%
Carbon
12%
Micro
44%
Macro
32%
Carbon
46%
Void
42%
Void
8%
Macro
2%
AX-21 Carbon
PVDC Carbon
Methane 298K Isotherms
on a volume basis
250
AX-21
BPL
PVDC
Uptake (mgs/mL)
200
150
100
50
0
0
100 200 300 400 500 600 700 800 900 1000 1100
Pressure (psia)
Methane 298K Isotherms
on a mass basis
Mass
250
AX-21
BPL
PVDC
Uptake (mg/g)
200
150
100
50
0
0
100 200 300 400 500 600 700 800 900
Pressure (psia)
Methane 298K Isotherms
1000 1100 on a volume basis
250
AX-21
BPL
PVDC
Uptake (mgs/mL)
200
150
100
50
0
Volume
0
100 200 300 400 500 600 700 800 900 1000 1100
Pressure (psia)
Volumetric Storage
Maximise micropore volume in vessel
Minimise void space in vessel
Density of molecules in macropore nearly
the same as the gas phase,
so carbon adsorbent should have as few
macropores as possible.
Some mesopore structure needed to aid
kinetics of adsorption / desorption.
Natural Gas Storage
Natural Gas Vehicles
CNG Tanks, Heavy wall cylindrical steel
Gas compressed to 3000 psi (21 MPa)
store / deliver ~220 V/V
ANG Tanks, Extruded aluminum
Carbon monolith filled tank at 500 psi
store 185 V/V, deliver ~150 V/V
ANG at 1/6 the pressure store 85%,
deliver 70% that of CNG
CNG 3000psi Storage Tank
AGLARG ANG Extruded Aluminum Tank
ANG Demonstration Vehicle
AGLARG / DOE
Dodge Dakota
Four 80L Aluminum ANG Tanks
installed on bed of Dodge Dakota
CH4 Delivery
AGLARG ANG Vessel vs. CNG Vessel
250
CNG
AGLARG ANG
ANG Adsorbent delivers 3 times
the volume of CN gas at 5 MPa
V/V Delivered
200
150
At ~ 10MPa ANG Adsorbent
reaches capacity
100
CNG at 20MPa would appear to
deliver ~30% more gas than ANG
at that pressure
50
0
0
5
10
15
Pressure (MPa)
20
Porous Carbon Models
Based on a slit shaped pore.
Keith Gubbins, Density Functional Theory
Alan Myers, Grand Canonical Monte Carlo
Two different approaches, both conclude
Highest adsorbed methane density
is found in pores of slit width 11.2 (7.4) Å
0.17 g CH4 / mL of pore at 3.4 Mpa
0.23 g CH4 / mL of pore at infinite P
Database derived from Gubbins DFT
0.18
0.16
0.14
0.12
0.10
0.08
M
e
th
a
n
e
d
e
n
sity (g/mL)
0.06
0.04
0.02
0.00
)
PSI
(
e
300r
ssu
e
r
P
200
400
100
0
140
120
100
80
Por
60
eS
40
ize
20
[A]
0
Everett and Powl (1976)
Distance between carbon layers
“Effective Pore Width”
Quirke (2002) uses the term,
“Chemical Pore Width” 7.4 Å
as distinct from the
7.4 Å
“Physical Pore Width” of 11.2 Å
So “Ideal” carbon would have only pores of
7.4 Å effective pore width,
Pore fraction
11.2 Å
= 0.66
Carbon fraction = 0.34
Density of this porous carbon = 0.75 g/mL
Maximum methane capacity at 298K
152 g/L , ~ 230 V/V
Porous Carbons are far from “ideal”
Great range of densities, pore volumes and
pore size distribution.
How do we characterise a carbon ?
“Particle Density”
Usually determined by mercury at 1 Bar
“Pack Density”
Density carbon can be packed in storage tank
From these, void volume can be found.
“ 420 Bar mercury Density”
Macropore filled at this pressure.
Vessel Volume Utilization
Micro
14%
Macro
32%
Carbon
12%
Void
42%
AX-21 Carbon
Micropore Volume
Various methods in use for determination of
micropore volume.
Most common, Dubinin-Radushkevich (1947) plot
using the low pressure 77K nitrogen isotherm.
Has also been applied to 273K CO2 isotherms.
Very different conditions to relatively high
pressure methane at 298K.
These methods only give overall micropore
volume but give no clue or indication of the range
of micropore widths.
Pore Size Distribution
Again, there are several methods used to
obtain PSDs, some more widely accepted than
others.
Mostly determined using 77K nitrogen or 273K
carbon dioxide low pressure isotherms. Both
sub-critical conditions.
Wide variation in the result depending on
method.
Unlike nitrogen or carbon dioxide, methane is
non-linear (tetrahedral) and at 298K is supercritical.
298K Methane Pore Size Distribution
Method for determination of porous carbon PSD
has been developed by Sosin and Quinn.
Database derived from Gubbins DFT model for
298K methane isotherm at pressures to 3.4 MPa.
Simple to use spreadsheet method for
Quattro or Excel, (Solver “add in” needed).
Clearly shows the different PSDs of different
carbons.
Valuble in showing how changes in carbon
preparation affect change in PSD.
Useful in determining how close to “ideal” the
carbon sample is.
Strategies for Enhancing NG Storage / Delivery
1. Tank is vital to success
2. Guard bed
3. Monolith
4. Micropore volume
5. Adsorbent preparation
1. Tank
Should possess good box (envelope)
characteristics.
Must be suitable for packing monoliths.
Internal web structure, not only for strength,
but for good heat exchange.
Multiple tanks, switchable and programmed to
operate as isothermally as possible.
2. Guard Bed
Impurities in natural gas can build up in the
micropores and over many fill / empty cycles
can result in a decrease in storage capacity.
Water is particularly difficult to desorb.
3. Monolith
Carbon adsorbent should be capable of being
produced as monoliths to minimise void space.
If a binder is used, it should not block
micropores.
Binder should also occupy minimal volume.
4. Micropore Volume
Methane isotherm should be used to determine
micropore volume.
It should be in excess of 0.7 mL / mL of monolith,
since it is unlikely to be all “optimal pore”.
5.
Carbon Preparation
Directed towards methods that create new
micropore, not to conventional “activation”
methods which merely enlarge existing pore.
Military Interest in Porous Carbon
Protection from CW agents,
Sarin, VX, mustard, HCN, phosgene etc.
Carbon in respirator canisters and clothing,
attempts made to “tailor” carbon for various toxic
molecules.
Difficulties with water saturation in respirators.
Other gaseous adsorbate applications.
Cigarette filters for toxic gas removal.
Adsorbent heat pumps and air conditioners,
ammonia, HFCs such as R134a.
Replacing acetone asbestos with carbon for
acetylene storage.
Enhanced storage of semiconductor gases,
BF3 , AsH3 , GeH4 using carbon monoliths.
Xenon adsorption cooling for space Infra-red
telescope detector with carbon monolith.
Mars rock and soil recovery vehicle.
Other Uses of Porous carbon
Help !!! I can’t get home !!
X-ray P51
Rosalind Franklin
John Randall, University College, London
Rosalind Franklin, University College, London
Maurice Wilkins, University College, London
Max Perutz, MRC, Cambridge
Francis Crick, Cambridge University
James Watson, Cambridge University
Aaron Klug, UCL and MRC
Independent Books on Rosalind Franklin by :
Anne Sayre, Brenda Maddox, Lynne Elkin