Gas Hydrates Prof. Attilio Citterio Dipartimento CMIC “Giulio Natta” http://iscamap.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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Gas Hydrates Structure

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

• 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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Structure of Gas Hydrates

S-I S-H S-II

46 H2O 136 H2O 34 H2O

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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 Description 512 51262 512 51264 512 435663 51268

Number/ unit cell 2 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, °C THF·2Xe·17H2O

THF·0.5Pr4NF·17H2O

THF·5H2O

Xe·6H2O

CH4·xH2O CH4·6H2O

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 Sample

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Methane Hydrate

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 recovered black dot = hydrate inferred from seismic imaging dotted 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 25 20 10 0

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

Tem

pera

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

DEP

TH (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

DEP

TH (k

m)

b)

1.40

1.20

1.00

0.80

0.60

0.40

0.20

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Progression of Hydrate Formation/Stabilization

Disseminated Nodular Layered Massive Disseminated Nodular Layered

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 7.0 0

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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Recovery of Methane with Sinking of CO2

Park et al., PNAS, 2006

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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 O

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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