FACTOID # 22: South Dakota has the highest employment ratio in America, but the lowest median earnings of full-time male employees.

 Home Encyclopedia Statistics States A-Z Flags Maps FAQ About

 WHAT'S NEW

SEARCH ALL

Search encyclopedia, statistics and forums:

(* = Graphable)

Encyclopedia > Clathrate hydrate

Clathrate hydrates (or alternatively gas clathrates, gas hydrates, clathrates, hydrates etc) are a class of solids in which gas molecules occupy "cages" made up of hydrogen-bonded water molecules. These "cages" are unstable when empty, collapsing into conventional ice crystal structure, but they are stabilized by the inclusion of appropriately sized molecules within them. Most low molecular weight gases (including O2, H2, N2, CO2, CH4, H2S, Ar, Kr, and Xe), as well as some higher hydrocarbons and freons will form hydrate under certain pressure-temperature conditions. Clathrate hydrates are not chemical compounds. The formation and decomposition of clathrate hydrates are first order phase transitions, not chemical reactions. For other uses, see Solid (disambiguation). ... Gas can also refer to gasoline and natural gas and also hydrogen. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... An example of a quadruple hydrogen bond between a self-assembled dimer complex reported by Meijer and coworkers. ... H2O and HOH redirect here. ... For other uses, see Crystal (disambiguation). ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) pale blue (liquid) Standard atomic weight 15. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Methane is a chemical compound with the molecular formula CH4. ... Hydrogen sulfide (hydrogen sulphide in British English) is the chemical compound with the formula H2S. This colorless, toxic and flammable gas is responsible for the foul odor of rotten eggs and flatulence. ... Hydrogen sulfide (hydrogen sulphide in British English) is the chemical compound with the formula H2S. This colorless, toxic and flammable gas is responsible for the foul odor of rotten eggs and flatulence. ... General Name, symbol, number argon, Ar, 18 Chemical series noble gases Group, period, block 18, 3, p Appearance colorless Standard atomic weight 39. ... For other uses, see Krypton (disambiguation). ... General Name, Symbol, Number xenon, Xe, 54 Chemical series noble gases Group, Period, Block 18, 5, p Appearance colorless Standard atomic weight 131. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ... Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerant. ... Hydrate is a term which means different things in inorganic chemistry and organic chemistry. ... This diagram shows the nomenclature for the different phase transitions. ...

Cages building the different gas hydrate structures.

Gas hydrates usually form two crystallographic cubic structures – structure (Type) I and structure (Type) II[1] of space groups $Pmoverline{3}n$ and $Fdoverline{3}m$ respectively. Seldom, a third hexagonal structure of space group P6 / mmm maybe observed (Type H).[2] Image File history File links Metadata Size of this preview: 476 Ã— 599 pixelsFull resolution (1800 Ã— 2265 pixels, file size: 891 KB, MIME type: image/jpeg) Author: GGenov, Source: GGenov, I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document under the terms... Image File history File links Metadata Size of this preview: 476 Ã— 599 pixelsFull resolution (1800 Ã— 2265 pixels, file size: 891 KB, MIME type: image/jpeg) Author: GGenov, Source: GGenov, I, the creator of this work, hereby grant the permission to copy, distribute and/or modify this document under the terms... Crystallography (from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and graphein = write) is the experimental science of determining the arrangement of atoms in solids. ...

The unit cell of Type I consists of 46 water molecules, forming two types of cages – small and large. The small cages in the unit cell are two against six large ones. The small cage has the shape of a pentagonal dodecahedron (512) and the large one that of a tetradecahedron, specifically a hexagonal truncated trapezohedron (51262). Typical guests forming Type I hydrates are CO2 in carbon dioxide clathrate and CH4 in methane clathrate. A dodecahedron is any polyhedron with twelve faces, but usually a regular dodecahedron is meant: a Platonic solid composed of twelve regular pentagonal faces, with three meeting at each vertex. ... A tetradecahedron is a polyhedron with 14 faces. ... The hexagonal truncated trapezohedron is the fourth in an infinite series of truncated trapezohedron polyhedra. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Zygmunt Florenty WrÃ³blewski Carbon dioxide hydrate is a Type I gas clathrate (Sloan 1998). ... Methane is a chemical compound with the molecular formula CH4. ... Burning ice. Methane, released by heating, burns; water drips (USGS). ...

The unit cell of Type II consists of 136[citation needed] water molecules, forming also two types of cages – small and large. In this case the small cages in the unit cell are sixteen against eight large ones. The small cage has again the shape of a pentagonal dodecahedron (512) but the large one is a hexadecahedron (51264). Type II hydrates are formed by gases like O2 and N2.

The unit cell of Type H consists of 34 water molecules, forming three types of cages – two small of different type and one huge. In this case, the unit cell consists of three small cages of type 512, twelve small ones of type 435663 and one huge of type 51268. The formation of Type H requires the cooperation of two guest gases (large and small) to be stable. It is the large cavity that allows structure H hydrates to fit in large molecules (e.g. butane, hydrocarbons), given the presence of other smaller help gases to fill and support the remaining cavities. Structure H hydrates were suggested to exist in the Gulf of Mexico. Thermogenically-produced supplies of heavy hydrocarbons are common there. Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ...

## Hydrates in the Universe

Kieffer et al. (2006) suggest that the geyser activity in the south polar region of Saturn's moon Enceladus originates from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the "Tiger Stripe" fractures found in that area.[14] Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Atmosphere Surface pressure: trace, significant spatial variability[8][9] Composition: 91% Water vapour 4% Nitrogen 3. ... Cassini view of Enceladus south pole and the tiger stripes Tiger Stripes consist of four, sub-parallel, linear depressions in the south polar region of Saturns moon Enceladus. ...

Carbon dioxide clathrate is believed to play a major role in different processes on Mars. Zygmunt Florenty WrÃ³blewski Carbon dioxide hydrate is a Type I gas clathrate (Sloan 1998). ...

## Hydrates on Earth

### Natural gas hydrates

Main article: Methane hydrate

### Gas hydrates in pipelines

Thermodynamic conditions favouring hydrate formation are often found in pipelines. This is highly undesirable because the clathrate crystals might agglomerate and cause plugging of the flow-line, valves and instrumentation. The results can range from reduction of the flow to physical damages of the equipment.

#### Hydrate formation prevention and mitigation philosophy

Hydrates have a strong tendency to agglomerate and to adhere to the pipe wall and thereby plug the pipeline. Once formed, they can be decomposed by increasing the temperature and/or decreasing the pressure. Even at these conditions, the clathrate dissociation is a slow process. Agglomerate - Wikipedia /**/ @import /skins-1. ...

Therefore, preventing hydrate formation appears to be the key to the problem. A hydrate prevention philosophy could typically be based on three levels of security, listed in prioritised order:

1. Avoid operational conditions that might cause formation of hydrates;
2. Temporarily change operating conditions in order to avoid hydrate formation;
3. Prevent formation of hydrates by addition of chemicals that (a) shift the hydrate equilibrium conditions towards lower temperatures and higher pressures or (b) increase hydrate formation time (inhibitors)

The actual philosophy would depend on operational circumstances such as pressure, temperature, type of flow (gas, liquid, presences of water etc.) A reaction inhibitor is a substance that prevents or decreases the rate of a chemical reaction. ...

#### Hydrate inhibitors

When operating within a set of parameters where hydrates could be formed, there are still ways to avoid their formation. Altering the gas composition by adding chemicals can lower the hydrate formation temperature and/or delay their formation. Two options generally exist:

• Thermodynamic inhibitors
• Kinetic inhibitors/anti-agglomerants

The most common thermodynamic inhibitors are, methanol, monoethylene glycol (MEG) and di-ethylene glycol (DEG) commonly referred to as glycol. All may be recovered and recirculated, but the economics of methanol recovery will not be favourable in most cases. MEG is preferred over DEG for applications where the temperature is expected to be −10 °C or lower due to high viscosity at low temperatures. TEG has too low vapour pressure to be suited as an inhibitor injected into a gas stream. More methanol will be lost in the gas phase when compared to MEG or DEG. Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naptha or wood spirits, is a chemical compound with chemical formula CH3OH. It is the simplest alcohol, and is a light, volatile, colourless, flammable, poisonous liquid with a distinctive odor that is somewhat milder and sweeter than ethanol (ethyl alcohol). ... Ethylene glycol (monoethylene glycol (MEG), IUPAC name: ethane-1,2-diol) is an alcohol with two -OH groups (a diol), a chemical compound widely used as an automotive antifreeze. ... Diethylene glycol (DEG) is an organic compound described by the structural formula HO-CH2-CH2-O-CH2-CH2-OH. It is a clear, hygroscopic, odorless liquid. ... Ethylene glycol (IUPAC name:ethane-1,2-diol) is a chemical compound widely used as an automotive antifreeze (coolant). ...

The use of kinetic inhibitors and anti-agglomerants in actual field operations is a new and evolving technology. It requires extensive tests and optimisation to the actual system. While kinetic inhibitors work by slowing down the kinetics of the nucleation, anti-agglomerants do not stop the nucleation, they rather stop the agglomeration (sticking together) of gas hydrate crystals. These two kinds of inhibitors are also known as Low-Dosage-Hydrate-Inhibitors because they require much smaller concentrations than the conventional thermodynamic inhibitors. Kinetic inhibitors (which do not require water and hydrocarbon mixture to be effective) are usually polymers or copolymers and anti-agglomerants (requires water and hydrocarbon mixture) are polymers or zwitterionic (usually ammonium and COOH) surfactants being both attracted to hydrates and hydrocarbons.

## References

1. ^ von Stackelberg, M. & Müller, H. M. (1954) Zeitschrift für Elektrochemie 58, 1, 16, 83
2. ^ Sloan E. D., Jr. (1998) Clathrate hydrates of natural gases. Second edition, Marcel Dekker Inc.:New York.
3. ^ Iro, N., Gautier, D., Hersant, F., Bockelée-Morvan, D. & Lunine, J. I. (2003) An interpretation of the Nitrogen deficiency in comets. Icarus, 161, p. 513
4. ^ Beckwith, S. V. W., Henning, T., & Nakagawa, Y. (2000) Dust properties and assembly of large particles in protoplanetary disks. Protostars and Planets IV, p. 533
5. ^ Natta, A., Grinin, V. & Mannings, V. (2000) Properties and Evolution of Disks around Pre-Main-Sequence Stars of Intermediate Mass. Protostars and Planets IV, p. 559
6. ^ Malfait, K., Waelkens, C., Waters, L. B. F. M., Vandenbussche, B., Huygen, E. & de Graauw, M. S. (1998) The spectrum of the young star HD 100546 observed with the Infrared Space Observatory. Letter to the Editor Astron. Astrophys. 332, p. L25-L28
7. ^ Barlow, M.J., In the proceedings of ‘ISO’s view on stellar evolution’, Noordwijkerhout, July, 1-4, 1997
8. ^ Li, A., Lunine, J. I. & Bendo, G. J. (2003) Modeling the infrared emission from the ε-Eridani disk. Astrophys. J. 598, pp. L51-L54
9. ^ Malfait, K., Waelkens, C., Bouwman, J., de Koter, A. & Waters, L. B. F. M. (1999) The ISO spectrum of the young star HD 142527. Astron. Astrophys. 345, p. 181
10. ^ Jaschek, C. & Jaschek, M. (1992) Astron. Astrophys., 95, p. 535
11. ^ Clampin, M. et al. (2003) Hubble Space Telescope ACS Coronagraphic Imaging of the Circumstellar Disk around HD 141569A. Astron. J. 126, pp. 385-392
12. ^ Li, A. & Lunine, J. I. (2003) Modeling the infrared emission from the HD 141569A disk. Astrophys. J. 594, pp. 987-1010
13. ^ Hersant, F., Gautier, D., Lunine, J. I. (2004) Enrichment in volatiles in the giant planets of the Solar System. Planetary and Space Science 52 , p. 623
14. ^ Kieffer, Susan W.; Xinli Lu, Craig M. Bethke, John R. Spencer, Stephen Marshak, Alexandra Navrotsky (2006). "A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume". Science 314 (5806): 1764-1766. doi:10.1126/science.1133519.

Results from FactBites:

 MBARI - Ocean Chemistry of Greenhouse Gases (517 words) Gas hydrates are a solid ice-like phase formed at low temperature and high pressure by van der Waals forces between gas and water molecules, with the "host" water molecules forming a molecular cage which confines the "guest" gas molecules through their mutual electrostatic interaction. In the ocean, gas hydrates composed dominantly of methane are common constituents of the shallow marine geosphere (Kvenvolden, 1993), and they occur both in deep sedimentary structures (Dickens et al., 1997), and as outcrops on the ocean floor (MacDonald et al., 1994). Gas hydrate formation in the ocean does not involve shaking, and ice crystals are not part of the natural deep sea environment, and therefore laboratory experiments carried out to date have not been able to simulate natural processes.
 Methane clathrate information - Search.com (1365 words) Methane clathrate, also called methane hydrate or methane ice, is a form of water ice that contains a large amount of methane within its crystal structure (a clathrate hydrate). Methane hydrates are believed to form by migration of gas from depth along geological faults, followed by precipitation, or crystallization, on contact of the rising gas stream with cold sea water. Methane clathrates remain stable at temperatures up to 18 °C. The average methane clathrate hydrate composition is 1 mole of methane for every 5.75 moles of water, though this is dependent on how many methane molecules "fit" into the various cage structures of the water lattice.
More results at FactBites »

Share your thoughts, questions and commentary here