gas hydrates energy
TRANSCRIPT
Gas Hydrates.
Prof. Attilio Citterio
Dipartimento CMIC “Giulio Natta”
http://iscamapweb.chem.polimi.it/citterio/education/course-topics/
School of Industrial and Information Engineering
Course 096125 (095857)
Introduction to Green and Sustainable Chemistry
Attilio Citterio
Overview.
• Hydrates – What they are?
• Clathrates and Supramolecular Chemistry
• Structure and Stoichiometry of gas hydrates
• Example: methane hydrate
• Other gas hydrates
• Occurrence and distribution
• Uses
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Gas Hydrates: Examples of Clathrates.
Gas hydrates: They are solids formed from hydrocarbon gas and liquid
water. They resemble wet snow and can exist at temperatures above
the freezing point of water.
They belong to a form of solid
complexes known as clathrates
inclusion compounds, existing
at low T and high P.
The supramolecular assembly is
made by two parts:
1) Host molecules of water
arranged in rigid cages
2) Mobile guest molecules (gas)
of appropriate dimensions
gas molecule
water
molecules
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Supramolecular Chemistry.
Supramolecular Connections :
• wide aggregates of molecules
• Weak non covalent
interactions
• Interactions by association
Auto organization
Auto assembling
Molecular Supramolecular
Connection by
Covalent Bonds
Interaction via
intermolecular
bonds
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Structure of Gas Hydrates.
Ice like crystalline substances made up of two or more
components
Host molecule - forms an expanded framework with void spaces
Guest component(s) - fill the void spaces
Van der Waals forces hold the lattice together
• Natural gas hydrates
• Host - water
• Guest - one or more gases
• pure methane hydrates REQUIRE -
4-6oC, 50 Atm (500 m), AND correct
concentration of gas
• Guest gases in marine seds. - methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide Methane I Hydrate
ideal formula is X(gas):5.75 water
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Rules for Solid Water.
According to some authors
the crystal structure of ice
follows these two rules:
1. tetrahedral O atoms
surrounded by H atoms
2. H atoms insert between
two adjacent O atoms.
These geometrical constrains
allow to produce several
different network.
Normal Ice
structure
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Nomenclature.
Edges – hydrogen bonds
Vertices – oxygen atoms
Nomenclature Xn
X: Number of edges of side surfaces
n: Number of sides with X edges
in the cage
512 2 pentagonal dodecahedra (12 sides)
51262 6 tetrakaidecahedra (14 sides)
- methane and ethane can be accommodated - nothing bigger
- ideal formula is X(gas):5.75 water
- only one third of spaces need to be filled
- rarely are voids - 100% filled
- non-stoichiometric structures
512 68
vertices
edges
side surface
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Structure of Gas Hydrates (2).
• Three non stoichiometric
cage structures
• S-H host two molecules
Double hydrates
cubic
hexagonal
2/cell
512 51262 (T)
51264 (H)512
512 51268 (E)435663 (D)
sI
sII
sH
6/cell
16/cell 8/cell
3/cell 2/cell 1/cell
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II Hydrate Structure.
II Hydrate
17-angstrom cell, 136 water molecules,
512 pentagonal dodecahedra (12 sided)
51268 hexakaidecahedra (16 sided)
accommodates molecules up to 4.8 and 6 Å
16 small, 8 large void spaces
X(gas):17 water molecules
ideal formula will not occur because when all 8
large spaces are filled the small ones can’t be filled
void spaces are diamond shaped - can
accommodate propane and isobutane.
II Hydrate
51268 435663
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Property of Gas Hydrates.
*http://www.netl.doe.gov/scng/hydrate/about-hydrates/chemistry.htm
I I II II H* H* H*
Cavity size small med small large small small huge
Cavity shape round oblate round round round round oblate
Cavity
Description5
125
126
25
125
126
45
124
35
66
35
126
8
Number/ unit
cell2 6 16 8 3 12 1
Average radius
(Angstrom)3.91 4.33 3.902 4.683 3.91 4.06 5.71
Rel. size of
CH4 (%)88.6 75.7 88.9 67.5 88.6
Coordination
No. 20 24 20 28 20 20 36
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Influence of Host Molecules.
3
4
5
Size (Å) Hydrate
former
Cavities
occupied
No Hydrates
Structure II
Structure II
Structure I
512 + 51262
512 + 51264
7 ⅔ H2O
5¾ H2O
CO2
Xe H2S
CH4
O2
N2
Kr
Ar
N° water
5
7
Structure II
Structure I
51264
51262
No SI or SII
Hydrates
17 H2O
7⅔ H2O
C-C3H6
C2H6
(CH2)2O
C3H8
Iso-C4H10
n-C4H10
Size (Å)
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Stoichiometry.
Ratio of occupation
dependent on Pressure
and Temperature
120
100
80
60
40
20
0
-20
0
T, °CTHF·2Xe·17H2O
THF·0.5Pr4NF·17H2O
THF·5H2O
Xe·6H2O
CH4·xH2OCH4·6H2O
THF·7H2O
THF·17H2O
i1hl
i3l
i5l
i6l
■
■
■
■
■
■■
THF·2CH4·17H2O
4000 8000 12000 15000
P, bar
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Example: Methane Hydrate.
«firing ice »
Is a potential Energy Source:
• A 1 m3 block of hydrate at normal
temps and pressures will release
~ 163 m3 of methane (if filled to 90%)
• Methane hydrate energy content of ~
6860 Mj·m-3
• Can occur at water T up to 30°C (↑P)
Cryogenic SEM images
taken at very low T to
keep the hydrate stable.
Source: L. Stern / USGS
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Methane Hydrate Deposits.
Deposits in permafrost regions (1.4 × 1013 to 3.4 × 1016 m3)
Submarine sediments in oceans (3.1 × 1015 to 7.6 × 1018 m3)
kerogen is the only pool of carbon greater than hydrates
natural gas in hydrates 2 times greater than total fossil fuel reserves
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Distribution of Gas Hydrate.
white dot = gas samples recoveredblack dot = hydrate inferred from seismic imagingdotted lines = hydrate-containing permafrost
http://walrus.wr.usgs.gov/globalhydrate/images/browse.jpg
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Methane Hydrate Formation.
Formation:
• Sufficient gas molecules
• Water saturated by methane
• Appropriately low
temperatures
• Appropriately high pressures
if we know where these conditions
are met, it is possible to predict
where gas hydrates will form.
Not fully understood and is
dependent on methane source.
Temperature (°C)
See depth (m)
1500
2000
1000
500
-5 0 5 15 2520100
1000 m
100 m
400 m
Gas hydrate =
Stable
Gas hydrate =
Unstable
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Phase Diagram.
Scheme: isobars
in the phase
diagram.
In green the
methane hydrate
region.
H2O CH4·?H2O CH4
M-H
LM-H
H-V
H-I
LW-I
H-LW
V-LW
Te
mp
era
ture
V Vapor
Lw Liquid water
H Hydrate
M Solid methane
LM Liquid methane
I Ice
V
LM-H
M-LM
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Hydrates can form as -
finely disseminated
crystals, nodules,
layers and mass
accumulations
Progression of smaller
to larger accumulations
Gas Hydrate Formation.
• Thermogenic gas produced at high temperatures
migrates from place of origin to the ‘HYDRATE STABILITY ZONE’
• Hydrates - most likely form at the ‘gas-liquid’ interface
• BUT - could also form from gases dissolved in the liquid phase
• Gas must migrate to the ‘ZONE’ and be presented in ‘SUPERSATURATED’
quantities
Source: Stanford Univ.TEMPERATURE (°C)
PERMAFROST
TEMPERATURE (°C)
-30 -15 0 15 30
IICE-WATER
PHASE-BOUNDARY
DE
PT
H(k
m)
SEDIMENT
a)
GEOTHERMAL
GRADIENT IN
PERMAFROST
DEPTH OF PERMAFROST
1.40
1.20
1.00
0.80
0.60
0.40
0.20
GEOTHERMAL
GRADIENT
OCEAN
WATER
HYDROTHERMAL
GRADIENT
CONCENTRAZION
OF METHANE
INSUFFICIENT PER
FOR HYDRATE
FORMATION
ICE-WATER
PHASE-BOUNDARY
OCEAN FLOOR
SEDIMENT
GEOTHERMAL
GRADIENT
-30 -15 0 15 30
DE
PT
H(k
m)
b)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
Attilio Citterio
Progression of Hydrate Formation/Stabilization.
Disseminated Nodular Layered Massive DisseminatedNodularLayered
Source: Brooks (1986)
hydrate lattices stabilized OR de-stabilized by co-occurring organic and
inorganic pore water constituents
hydrate is effectively ‘freshened’ as these are ‘excluded’ from the lattice
in the presence of propane hydrates can be stable at higher T and lower P
clays can stabilize hydrates (and other solids)
rapid sedimentation rates and influenced by sediment texture
authigenic carbonate rubble formation with shallow fracturing of sediments
Hydrate
Matrix
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Inhibition of Formation.
• NaCl - “anti-freeze” - lowers the temp at which a hydrate can form
3.5% solution reduces stabilization temp by 2-3 °C
• Inclusion of heavier gases increases temp (hence off-sets the salinity
problem)
• Gas hydrates can alter the concentrations of pore waters by excluding
salts or melting (’freshens’ the pore water)
• >50% Nitrogen can increase pressure required by 30%
• butane (1 - 5.8%) is included in a hydrate when pressures are less
than 103 Atm
does not form hydrates at its own vapor pressure
does not inhibit the formation of methane hydrates
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Lab. Experiment on Methane Hydrate.
• Feeding of water and methane • Hexagonal Ice and Gas
CH4
200
bar
methane
cylinder
Compressor
(0….3000 bar)
Tank
(0…250 bar)
Sensor
Drain valve Shaft
Release valve
Isolation
Thermostatic bath
(+150°C to -40°C)
Pressure cell
(Al 7075)
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Uses of Gas Hydrates.
As energy source
• Relevant reserves of
methane
• Country mainly
interested: Siberia and
Canada
• Possible use as fuel
storage
Methane hydrate energy content of
~ 6860 MJ·m-3
Methane gas – 42.8 MJ·m-3
Liquefied natural gas 16000 MJ·m-3
Gas Hydrates; 10000
Fossil fuels; 5000
Earth (includes soil,
biota, peat and detritus);
2790
Oceans (includes dissolved organic and biota); 983
Atmosphere; 3,6
Distribution of organic carbon in Earth reservoirs
(excluded dispersed carbon in rocks and sediments,
which equals nearly 1000 times this amount.
Numbers in gigatons (1015 tons) of carbon
1 cubic meter of gas hydrate
(90% site occupied) = 163 m3 of gas
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PROBLEMS with Methane Hydrates.
hydrate dissociation upon recovery; engineering challenge
expense of long pipelines across continental slope, subject to blockage with solid hydrate
methane release into atmosphere problem for climate change (20x more potent than CO2)
fragile ecosystems surround sediment surface hydrates & seeps
dissociating methane hydrate at sediment/water interface
Environmental issues:
Increase in temperature on Earth will
induce significant release of methane
from methane hydrates.
Archer et al., 2007
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Environmental Concern on Methane Escape.
lots of CH4 escaping from melting gas hydrates
powerful positive feedback on global warming
CH4 is a powerful greenhouse gas
most likely oxidizes to CO2 before it enters the atmosphere… but still!
A detailed investigation of methane hydrate dissociation during global warming was reported (Archer et al., 2007)
Westbrook et al., 2009
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00
Potential temperature (°C)
50
100
150
200
250
Dep
th (
m)
250 m
0 10 20 30 40 50
Methane (nM)
Upper limit of weak
part of acoustic image
of plume intersected
by the ctd cast.
Upper limit of strong
part of acoustic image
of plume intersected
by the ctd cast.
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Large, expensive pilot programs
focus on drilling in frozen
permafrost areas
Ex: Mallik, Canada
http://energy.usgs.gov/other/gashydrates/mallik.html
Extractive Activities.
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Uses of Hydrogen Clathrates
as Energy Storage and Carriers.
Hydrogen Storage Potential
• Binary inclusions of gas hydrate and THF
• Cage occupation
(2 H2)2·(4 H2)x·THF(1-x)17H2O
• Capacity: 4 % by weight
THF = Source: Nature 434, 743-746, 2005
Attilio Citterio
References.
1. E. Dendy Sloan, Jr., Nature, 426, 353-363, 2003
2. H. Lee, J.-w. Lee, D. Y.Kim, J. Park, Nature 434, 743-746, 2005
3. Dr. Jörg Bialas, IFM-GEOMAR, Submarine Gashydratlagerstätten
als Deponie für die CO2-Sequestrierung, 2007
4. Gabitto, J. F. and Tsouris C. “Physical Properties of Gas Hydrates: A
Review.” J. of Thermodynamics, Hindawi Pub., Vol. 2010, ID271291.
5. ] J. Caroll. Natural gas hydrates. A Guide for Engineers. Gulf
Professional Publishing, ISBN 0-7506-7569-1, 2003
6. E. D. Sloan and C. A. Koh. Clathrate hydrates of natural gases. Third
edition, CRC Press, 6000 Broken Sound Parkway NW, Suite 300, Boca
Raton, FL 33487-2742, U.S.A., 2008
7. www.ifm-geomar.de
8. http://www.imc.tuwien.ac.at/download/supramolekular_01.pdf
9. http://www.sfv.de/lokal/mails/wvf/methanhy.htm
10. http://www.cup.uni-muenchen.de/ac/kluefers/homepage/L_ac1.html