Clathrate hydrate
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/* Structure */ +[[Weaire-Phelan structure]]
'''Clathrate hydrates''' (or ''gas clathrates, gas hydrates, clathrates, hydrates'' etc) were first documented in 1810 by [[Sir Humphrey Davy]]<ref>{{cite web |url=http://ethomas.web.wesleyan.edu/ees123/clathrate.htm |title=Clathrates: little known components of the global carbon cycle |accessdaymonth=13 December |accessyear=2007 |year=2004 |month=11 |author=Ellen Thomas |publisher=Wesleyan University }}</ref>; they are [[crystal|crystalline]] water based [[solid]]s physically resembling [[ice]], in which small [[Chemical_polarity|non polar]] [[molecule]]s (typically [[gas]]es) are trapped inside "cages" of [[hydrogen bond]]ed [[water (molecule)|water molecules]]. Without the support of the trapped molecules, the [[Crystal_structure|lattice]] structure of hydrate clathrates would collapse into conventional ice crystal structure or liquid water. Most low molecular weight gases (including {{oxygen}}<sub>2</sub>, {{hydrogen}}<sub>2</sub>, {{nitrogen}}<sub>2</sub>, [[carbon dioxide|CO]]<sub>2</sub>, [[methane|CH]]<sub>4</sub>, [[hydrogen sulfide|H]]<sub>2</sub>[[hydrogen sulfide|S]], {{argon}}, {{krypton}}, and {{xenon}}), as well as some higher [[hydrocarbon]]s and [[freon]]s will form [[hydrate]]s at suitable temperatures and pressures. Clathrate hydrates are not chemical compounds as the sequestered molecules are never bonded to the lattice. The formation and decomposition of clathrate hydrates are [[Phase_transition#Classification_of_phase_transitions|first order phase transitions]], not chemical reactions.
Clathrates have been found to occur naturally in large quantities. Around 6.4 trillion (i.e. 6.4x10<sup>12</sup>) tonnes of [[methane]] is trapped in deposits of [[methane clathrate]] on the deep [[ocean floor]].<ref>{{Cite paper | first = B. | last = Buffett | first2 = D. | last2 = Archer | title = Global inventory of methane clathrate: sensitivity to changes in the deep ocean. | year = 2004 | pages = 185-199 | id = Earth Planet. Sci. Lett. 227 (2004)}}</ref> Such deposits can be found on the [[Norwegian continental shelf]] in the northern headwall flank of the [[Storegga Slide]]. Clathrates can also exist as [[permafrost]], as at the [[Mallik gas hydrate field]] in the [[Mackenzie River|Mackenzie Delta]] of northwestern [[Canadian Arctic]]. These natural gas hydrates are seen as a potentially vast energy resource, but an economic extraction method has so far proven elusive. [[Hydrocarbon]] clathrates cause problems for the petroleum industry, because they can form inside [[Pipeline transport|gas pipelines]] often resulting in plug formation. Deep sea deposition of [[carbon dioxide clathrate]] has been proposed as a method to remove this [[greenhouse gas]] from the atmosphere and control [[climate change]].
Clathrates are suspected to occur in large quantities on some outer [[planet]]s, [[natural satellite|moons]] and [[trans-Neptunian object]]s, binding gas at fairly high temperatures.
== Structure ==
[[Image:HydrateStructures.jpg|thumb|360px|Cages building the different gas hydrate structures.]]
Gas hydrates usually form two [[Crystallography|crystallographic]] cubic structures – structure (Type) I and structure (Type) II<ref>von Stackelberg, M. & Müller, H. M. (1954) ''Zeitschrift für Elektrochemie'' '''58''', 1, 16, 83</ref> of space groups <math>Pm\overline{3}n</math> and <math>Fd\overline{3}m</math> respectively. Seldom, a third hexagonal structure of space group <math>P6/mmm</math> may be observed (Type H).<ref>Sloan E. D., Jr. (1998) Clathrate hydrates of natural gases. Second edition, Marcel Dekker Inc.:New York.</ref>
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]] (5<sup>12</sup>) and the large one that of a [[tetradecahedron]], specifically a [[hexagonal truncated trapezohedron]] (5<sup>12</sup>6<sup>2</sup>), together forming a [[Weaire-Phelan structure]]. Typical guests forming Type I hydrates are [[carbon dioxide|CO<sub>2</sub>]] in [[carbon dioxide clathrate]] and [[methane|CH<sub>4</sub>]] in [[methane clathrate]].
The unit cell of Type II consists of 136{{Fact|date=May 2007}}<!-- 136? --> 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 (5<sup>12</sup>) but the large one is a [[hexadecahedron]] (5<sup>12</sup>6<sup>4</sup>). Type II hydrates are formed by gases like O<sub>2</sub> and N<sub>2</sub>.
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 5<sup>12</sup>, twelve small ones of type 4<sup>3</sup>5<sup>6</sup>6<sup>3</sup> and one huge of type 5<sup>12</sup>6<sup>8</sup>. 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]], [[hydrocarbon]]s), 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.
== Hydrates in the Universe ==
Iro ''et al.'' <ref>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</ref>, trying to interpret the [[nitrogen]] deficiency in [[comet]]s, stated most of the conditions for hydrate formation in the [[Protoplanetary disk|protoplanetary nebulae]], surrounding the [[Main sequence|pre-main and main sequence]] stars were fulfilled, despite the rapid grain growth to meter scale. The key was to provide enough microscopic ice particles exposed to a gaseous environment. Observations of the [[Radiometry|radiometric]] [[Continuum (theory)|continuum]] of [[Protoplanetary disk|circumstellar discs]] around <math>\tau</math>[[T Tauri star|-Tauri]] and [[Herbig Ae/Be stars]] suggest massive dust disks consisting of millimeter-sized grains, which disappear after several million years (e.g. <ref>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</ref>, <ref>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
</ref>). A lot of work on detecting water ices in the Universe was done on the [[Infrared Space Observatory]] (ISO). For instance, broad [[Spectral bands|emission bands]] of water ice at 43 and 60 μm were found in the disk of the isolated [[Herbig Ae/Be stars|Herbig Ae/Be star]] HD 100546 in [[Musca]]. The one at 43 μm is much weaker than the one at 60 μm, which means the water ice, is located in the outer parts of the disk at temperatures below 50 K <ref>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</ref>. There is also another broad ice feature between 87 and 90 μm, which is very similar to the one in [[NGC 6302]] <ref>Barlow, M.J., In the proceedings of ‘ISO’s view on stellar evolution’, Noordwijkerhout, July, 1-4, 1997</ref> (the Bug or Butterfly nebula in [[Scorpius]]). Crystalline ices were also detected in the proto-planetary disks of [[Epsilon Eridani|ε-Eridani]] and the isolated Fe star HD 142527<ref>Li, A., Lunine, J. I. & Bendo, G. J. (2003) Modeling the infrared emission from the ε-Eridani disk. ''Astrophys. J.'' '''598''', pp. L51-L54</ref><ref>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</ref> in [[Lupus (constellation)|Lupus]]. 90 % of the ice in the latter was found crystalline at temperature around 50 K. [[Hubble Space Telescope|HST]] demonstrated that relatively old [[Protoplanetary disk|circumstellar disks]], as the one around the 5 million year old B9.5Ve<ref>Jaschek, C. & Jaschek, M. (1992) ''Astron. Astrophys.'', '''95''', p. 535</ref> [[Herbig Ae/Be stars|Herbig Ae/Be star]] HD 141569A, are dusty<ref>Clampin, M. ''et al.'' (2003) Hubble Space Telescope ACS Coronagraphic Imaging of the Circumstellar Disk around HD 141569A. ''Astron. J.'' '''126''', pp. 385-392</ref>. Li & Lunine<ref>Li, A. & Lunine, J. I. (2003) Modeling the infrared emission from the HD 141569A disk. ''Astrophys. J.'' '''594''', pp. 987-1010</ref> found water ice there. Knowing the ices usually exist at the outer parts of the [[Protoplanetary disk|proto-planetary nebulae]], Hersant ''et al.''<ref>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</ref> proposed an interpretation of the [[Volatility (physics)|volatile]] enrichment, observed in the four [[Gas giant|giant planets]] of the [[Solar System]], with respect to the Solar [[Abundance of the chemical elements|abundances]]. They assumed the [[Volatility (physics)|volatiles]] had been trapped in the form of hydrates and incorporated in the [[planetesimal]]s flying in the [[Protoplanet|protoplanets’]] feeding zones.
Kieffer ''et al.'' (2006) suggest that the geyser activity in the south polar region of [[Saturn (planet)|Saturn]]'s moon [[Enceladus (moon)|Enceladus]] originates from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the "[[Tiger Stripes (Enceladus)|Tiger Stripe]]" fractures found in that area.<ref name=Kieffer2006>{{cite journal| first=Susan W.| last= Kieffer| coauthors= Xinli Lu, Craig M. Bethke, John R. Spencer, Stephen Marshak, Alexandra Navrotsky| year=2006| doi=10.1126/science.1133519| title=A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume| journal=Science| volume=314| issue=5806| pages=1764–1766| pmid=17170301 }}</ref>
[[Carbon dioxide clathrate]] is believed to play a major role in different processes on Mars.
== Hydrates on Earth==
===Natural gas hydrates===
{{main|Methane clathrate}}
Naturally on [[Earth]] gas hydrates can be found on the [[seafloor]], in ocean sediments, in deep lake sediments (e.g. [[Lake Baikal]]), as well as in the [[permafrost]] regions. The amount of [[methane]] potentially trapped in natural [[Methane clathrate|methane hydrate]] deposits may be significant, which makes them of major interest as a potential energy resource for the future. Catastrophic release of methane from the decomposition of such deposits may lead to a global climate change, because [[methane|CH]]<sub>4</sub> is more efficient greenhouse gas even than [[carbon dioxide|CO]]<sub>2</sub> (see [[Methane#Methane_in_Earth.27s_atmosphere|Methane in Earth's atmosphere]]). On its turn, the fast decomposition of such deposits is considered a [[geohazard]], due to its potential to trigger [[landslide]]s, [[earthquake]]s and [[tsunami]]s. However, natural gas hydrates do not contain only methane but also other [[hydrocarbon]] gases, as well as [[Hydrogen sulfide|H<sub>2</sub>S]] and [[Carbon dioxide|CO<sub>2</sub>]]. [[Air hydrates]] are frequently observed in polar ice samples.
[[pingo|Pingos]] are common structures in permafrost regions<ref>{{cite web | last = Ussler, W.; Paull, C. K.; Lorenson, T.; Dallimore, S.; Medioli, B.; Blasco, S.; McLaughlin, F.; Nixon, F. M. | title = Methane Leakage from Pingo-like Features on the Arctic Shelf, Beaufort Sea, NWT, Canada | work = Physics Abstract Service | publisher = SAO/NASA ADS | date = 12/2005 | url = http://adsabs.harvard.edu/abs/2005AGUFM.C11A1069U | accessdate = 2008-03-09 }}</ref>. Similar structures are found in deep water related to methane leakages.
===Gas hydrates in pipelines===
Thermodynamic conditions favouring hydrate formation are often found in [[Pipeline transport|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 damage 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.
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:
# Avoid operational conditions that might cause formation of hydrates;
# Temporarily change [[operating conditions]] in order to avoid hydrate formation;
# 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 ([[Reaction inhibitor|inhibitor]]s)
The actual philosophy would depend on operational circumstances such as pressure, temperature, type of flow (gas, liquid, presences of water etc.)
====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]], [[ethylene glycol|monoethylene glycol]] (MEG) and [[diethylene 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. [[Triethylene glycol]] (TEG) has too low vapour pressure to be suited as an inhibitor injected into a gas stream. More methanol is lost in the gas phase when compared to MEG or DEG.
The use of [[kinetic inhibitor]]s 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.
==See also==
* [[Clathrate]]
* [[Star#Formation and evolution|Star Formation and evolution]]
==References==
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[[Category:Clathrates]]
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[[Category:Hydrates]]
[[Category:Water ice]]
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