Adsorption Storage

A viable alternative to compression

for natural gas powered vehicles ?

David Quinn

Royal Military College of Canada

Presented to

ALL-CRAFT Columbia, MoJuly,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 / epoxycylinders required.

Store ~220 - 240 v/v based on internal volume.No consideration of wall thickness or envelopebox.

Internal volume is ~ 70% of envelope, so storageis 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)

A B

Adsorption

Solid placed in B

then,

PAVA > PAB(VA + VB)

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 298Kfor experimental studies.

2. Adsorption potential of surface

Different materials give different 298Kmethane 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 areaporous carbon.

They have lower adsorption potentials.

0

20

40

60

80

100 Up

take (

mg/g)

0 100 200 300 400 500 600 Pressure (psia)

BPL Dow Resin MCM-41 Zeolite

Methane 298K Isothermson various porous materials

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.

[Fm-3m, a=25.6690(3)Å]

1,4-Benzenedicarboxylate(BDC)

Yaghi, University of Michigan

Porous Carbons

Highly disordered carbon, unlike diamond orgraphite

Described as like a pile of potato chips

“Chips”, small crystallites with graphite likestructure

Space between chips are the “pores”

Pore Definitions (IUPAC)

Micropore 2 - 20 ÅMesopore 20 - 50 ÅMacropore > 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.

0

50

100

150

200

250

Uptak

e (mg

/g)

0 100 200 300 400 500 600 700 800 900 1000 1100 Pressure (psia)

AX-21 BPL PVDC

Methane 298K Isotherms on a mass basis

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

Carbon

Void

Micropore

Macropore

Carbon Filled Vessel

Vessel Volume Utilization

AX-21 Carbon

Carbon12%

Micro14%

Void42%

Macro32%

Vessel Volume Utilization

PVDC Carbon

Carbon46%

Micro44%

Void8%Macro

2%

AX-21 Carbon PVDC Carbon

Vessel Volume Utilization

Carbon12%

Void42%

Macro32%

Micro14%

Carbon46%

Micro44%

Void8%

Macro2%

0

50

100

150

200

250

Uptak

e (mg

s/mL)

0 100 200 300 400 500 600 700 800 900 1000 1100 Pressure (psia)

AX-21 BPL PVDC

Methane 298K Isothermson a volume basis

0

50

100

150

200

250

Uptak

e (mg

/g)

0 100 200 300 400 500 600 700 800 900 1000 1100 Pressure (psia)

AX-21 BPL PVDC

Methane 298K Isotherms on a mass basis

0

50

100

150

200

250

Uptak

e (mg

s/mL)

0 100 200 300 400 500 600 700 800 900 1000 1100 Pressure (psia)

AX-21 BPL PVDC

Methane 298K Isothermson a volume basis

Mass

Volume

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 psistore 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 VehicleAGLARG / DOEDodge Dakota

Four 80L Aluminum ANG Tanks

installed on bed of Dodge Dakota

0

50

100

150

200

250

0 5 10 15 20

Pressure (MPa)

V/V

De

live

red

CNGAGLARG ANG ANG Adsorbent delivers 3 times

the volume of CN gas at 5 MPa

At ~ 10MPa ANG Adsorbentreaches capacity

CNG at 20MPa would appear todeliver ~30% more gas than ANGat that pressure

CH4 DeliveryAGLARG ANG Vessel vs. CNG Vessel

Porous Carbon Models

Based on a slit shaped pore.

Keith Gubbins, Density Functional TheoryAlan Myers, Grand Canonical Monte Carlo

Two different approaches, both concludeHighest adsorbed methane densityis found in pores of slit width 11.2 (7.4) Å0.17 g CH4 / mL of pore at 3.4 Mpa0.23 g CH4 / mL of pore at infinite P

0.000.020.040.060.080.100.120.140.160.18

0100

200300

400 0204060

80100

120140

Methane density (g/m

L)

Pressure (PSI)

Pore Size [A]

Database derived from Gubbins DFT

11.2 Å

7.4 Å

Everett and Powl (1976)Distance between carbon layers“Effective Pore Width”

Quirke (2002) uses the term,“Chemical Pore Width” 7.4 Åas distinct from the“Physical Pore Width” of 11.2 Å

So “Ideal” carbon would have only pores of7.4 Å effective pore width,

Pore fraction = 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 andpore 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

AX-21 Carbon

Carbon12%

Micro14%

Void42%

Macro32%

Micropore Volume

Various methods in use for determination ofmicropore volume.

Most common, Dubinin-Radushkevich (1947) plotusing the low pressure 77K nitrogen isotherm.Has also been applied to 273K CO2 isotherms.

Very different conditions to relatively highpressure methane at 298K.

These methods only give overall microporevolume but give no clue or indication of the rangeof micropore widths.

Pore Size Distribution

Again, there are several methods used toobtain PSDs, some more widely accepted thanothers.

Mostly determined using 77K nitrogen or 273Kcarbon dioxide low pressure isotherms. Bothsub-critical conditions.

Wide variation in the result depending onmethod.

Unlike nitrogen or carbon dioxide, methane isnon-linear (tetrahedral) and at 298K is super-critical.

298K Methane Pore Size Distribution

Method for determination of porous carbon PSDhas been developed by Sosin and Quinn.

Database derived from Gubbins DFT model for298K methane isotherm at pressures to 3.4 MPa.

Simple to use spreadsheet method forQuattro or Excel, (Solver “add in” needed).

Clearly shows the different PSDs of differentcarbons.Valuble in showing how changes in carbonpreparation affect change in PSD.Useful in determining how close to “ideal” thecarbon 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 toxicmolecules.

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 foracetylene storage.

Enhanced storage of semiconductor gases,BF3 , AsH3 , GeH4 using carbon monoliths.

Xenon adsorption cooling for space Infra-redtelescope 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