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Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line water-based
solid Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
s physically resembling
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
, in which small
non-polar In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain one or more pola ...
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s (typically
gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
es) or
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates * Polar climate, the c ...
molecules with large hydrophobic moieties are trapped inside "cages" of
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
ed, frozen
water molecules Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "univer ...
. In other words, clathrate hydrates are
clathrate compound A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
s in which the host molecule is
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
and the guest molecule is typically a gas or liquid. Without the support of the trapped molecules, the
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
structure of hydrate clathrates would collapse into conventional ice crystal structure or liquid water. Most low molecular weight gases, including , , , , , , , , and , as well as some higher
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s and
freon Freon ( ) is a registered trademark of the Chemours Company and generic descriptor for a number of halocarbon products. They are stable, nonflammable, low toxicity gases or liquids which have generally been used as refrigerants and as aerosol prop ...
s, will form
hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
s at suitable temperatures and pressures. Clathrate hydrates are not officially chemical compounds, as the enclathrated guest molecules are never bonded to the lattice. The formation and decomposition of clathrate hydrates are first order phase transitions, not chemical reactions. Their detailed formation and decomposition mechanisms on a molecular level are still not well understood. Clathrate hydrates were first documented in 1810 by
Sir Humphry Davy Sir Humphry Davy, 1st Baronet, (17 December 177829 May 1829) was a British chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, several elements for the ...
who found that water was a primary component of what was earlier thought to be solidified chlorine. Clathrates have been found to occur naturally in large quantities. Around 6.4 trillion (6.4×1012) tonnes of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
is trapped in deposits of
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
on the deep
ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
. Such deposits can be found on the
Norwegian continental shelf The Norwegian continental shelf ( no, Den norske kontinentalsokkelen) (abbreviated as NCS) is the continental shelf over which Norway exercises sovereign rights as defined by the United Nations Convention on the Law of the Sea. The area of the s ...
in the northern headwall flank of the
Storegga Slide The three Storegga Slides ( no, Storeggaraset) are amongst the largest known submarine landslides. They occurred at the edge of Norway's continental shelf in the Norwegian Sea, approximately 6225–6170 BCE. The collapse involved an estimated ...
. Clathrates can also exist as
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
, as at the
Mallik gas hydrate site The Mallik Methane Hydrate Site (or ''Mallik Gas Hydrate Production Research Well'', ''Mallik test well'') is located in the Beaufort Sea, Canada. Site highlights * First dedicated scientific and technical research site investigating permafrost g ...
in the Mackenzie Delta of northwestern
Canadian Arctic Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories and N ...
. These natural gas hydrates are seen as a potentially vast energy resource and several countries have dedicated national programs to develop this energy resource. Clathrate hydrate has also been of great interest as technology enabler for many applications like seawater desalination, gas storage, carbon dioxide capture & storage, cooling medium for data centre and district cooling etc. Hydrocarbon clathrates cause problems for the petroleum industry, because they can form inside gas pipelines, often resulting in obstructions. Deep sea deposition of carbon dioxide clathrate has been proposed as a method to remove this
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
from the atmosphere and control
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. Clathrates are suspected to occur in large quantities on some outer
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s,
moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
and
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically ...
s, binding gas at fairly high temperatures.


Structure

Gas hydrates usually form two
crystallographic Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The word ...
cubic structures: structure (Type) I (named ''sI'') and structure (Type) II (named ''sII'') of space groups Pm\overlinen and Fd\overlinem respectively. A third hexagonal structure of space group P6/mmm may also be observed (Type H). The unit cell of Type I consists of 46 water molecules, forming two types of cages – small and large. The unit cell contains two small cages and six large ones. The small cage has the shape of a pentagonal
dodecahedron In geometry, a dodecahedron (Greek , from ''dōdeka'' "twelve" + ''hédra'' "base", "seat" or "face") or duodecahedron is any polyhedron with twelve flat faces. The most familiar dodecahedron is the regular dodecahedron with regular pentagon ...
(512) (which is not a regular dodecahedron) and the large one that of a
tetradecahedron 240px, A tetradecahedron with ''D2d'' symmetry, existing in the Weaire–Phelan structure A tetradecahedron is a polyhedron with 14 faces. There are numerous topologically distinct forms of a tetradecahedron, with many constructible entirely wi ...
, specifically a
hexagonal truncated trapezohedron In geometry, the truncated hexagonal trapezohedron is the fourth in an infinite series of truncated trapezohedra. It has 12 pentagon and 2 hexagon faces. It can be constructed by taking a hexagonal trapezohedron and truncating the polar axis v ...
(51262). Together, they form a version of the
Weaire–Phelan structure In geometry, the Weaire–Phelan structure is a three-dimensional structure representing an idealised foam of equal-sized bubbles, with two different shapes. In 1993, Denis Weaire and Robert Phelan found that this structure was a better solution ...
. Typical guests forming Type I hydrates are CO2 in carbon dioxide clathrate and CH4 in
methane clathrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
. The unit cell of Type II consists of 136 water molecules, again forming two types of cages – small and large. In this case there are sixteen small cages and eight large ones in the unit cell. The small cage again has the shape of a pentagonal dodecahedron (512), but the large one is a
hexadecahedron A hexadecahedron (or hexakaidecahedron) is a polyhedron with 16 faces. No hexadecahedron is regular; hence, the name is ambiguous. There are numerous topologically distinct forms of a hexadecahedron, for example the pentadecagonal pyramid, tetr ...
(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 ones of different types, and one "huge". In this case, the unit cell consists of three small cages of type 512, two 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 Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name but ...
,
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
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.'', trying to interpret the
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
deficiency in
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...
s, stated most of the conditions for hydrate formation in the protoplanetary nebulae, surrounding the 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
radiometric Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...
continuum Continuum may refer to: * Continuum (measurement), theories or models that explain gradual transitions from one condition to another without abrupt changes Mathematics * Continuum (set theory), the real line or the corresponding cardinal number ...
of circumstellar discs around \tau -Tauri and Herbig Ae/Be stars suggest massive dust disks consisting of millimeter-sized grains, which disappear after several million years (e.g.,). A lot of work on detecting water ices in the Universe was done on the
Infrared Space Observatory The Infrared Space Observatory (ISO) was a space telescope for infrared light designed and operated by the European Space Agency (ESA), in cooperation with ISAS (now part of JAXA) and NASA. The ISO was designed to study infrared light at wavelengt ...
(ISO). For instance, broad emission bands of water ice at 43 and 60 μm were found in the disk of the isolated
Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young () star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines ...
HD 100546 in
Musca Musca () is a small constellation in the deep southern sky. It was one of 12 constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman, and it first appeared on a celestial globe in ...
. 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. There is also another broad ice feature between 87 and 90 μm, which is very similar to the one in
NGC 6302 NGC 6302 (also known as the Bug Nebula, Butterfly Nebula, or Caldwell 69) is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae. The spectrum of NG ...
(the Bug or Butterfly nebula in
Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gre ...
). Crystalline ices were also detected in the proto-planetary disks of ε-Eridani and the isolated Fe star HD 142527 in
Lupus Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Comm ...
. 90% of the ice in the latter was found crystalline at temperature around 50 K. HST demonstrated that relatively old
circumstellar disks A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the re ...
, as the one around the 5-million-year-old B9.5Ve
Herbig Ae/Be star A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young () star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines ...
HD 141569A, are dusty. Li & Lunine found water ice there. Knowing the ices usually exist at the outer parts of the proto-planetary nebulae, Hersant ''et al.'' proposed an interpretation of the volatile enrichment, observed in the four
giant planets The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. The ...
of the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, with respect to the Solar abundances. They assumed the
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ' ...
had been trapped in the form of hydrates and incorporated in the
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...
s flying in the protoplanets’ feeding zones. Kieffer ''et al.'' (2006) hypothesized that the geyser activity in the south polar region of
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
's moon
Enceladus Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
originates from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space through the " Tiger Stripe" fractures found in that area. However, subsequent analysis of plume material makes it more likely that the geysers on Enceladus derive from a salty subsurface ocean. Carbon dioxide clathrate is believed to play a major role in different processes on Mars.
Hydrogen clathrate A hydrogen clathrate is a clathrate containing hydrogen in a water lattice. It is not possible to store commercial quantities of hydrogen by this method hydrogen in a hydrogen economy. A recent review that accounts the state-of-the-art and future ...
is likely to form in condensation nebulae for gas giants. Kamata ''et al.''(2019) of the University of Hokkaido have proposed that a thin layer of clathrate hydrates thermally insulates the subsurface ocean of liquid water of
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...
, the existence of which is suggested by data from the ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...
'' probe.


Hydrates on Earth


Natural gas hydrates

Naturally on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
gas hydrates can be found on the
seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
, in ocean sediments, in deep lake sediments (e.g.
Lake Baikal Lake Baikal (, russian: Oзеро Байкал, Ozero Baykal ); mn, Байгал нуур, Baigal nuur) is a rift lake in Russia. It is situated in southern Siberia, between the federal subjects of Irkutsk Oblast to the northwest and the Repu ...
), as well as in the
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
regions. The amount of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
potentially trapped in natural
methane hydrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large am ...
deposits may be significant (1015 to 1017 cubic metres), which makes them of major interest as a potential energy resource. Catastrophic release of methane from the decomposition of such deposits may lead to a global climate change, referred to as the "
clathrate gun hypothesis The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The idea is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulate ...
", because CH4 is a more potent greenhouse gas than CO2 (see
Atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year globa ...
). The fast decomposition of such deposits is considered a
geohazard A geologic hazard or geohazard is an adverse geology, geologic condition capable of causing widespread damage or loss of property and life. These hazards are geological and environmental conditions and involve long-term or short-term geological ...
, due to its potential to trigger
landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated grade (slope), slope failures, mudflows, and debris flows. Landslides occur in a variety of ...
s,
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s and
tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...
s. However, natural gas hydrates do not contain only methane but also other
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
gases, as well as H2S and CO2. Air hydrates are frequently observed in polar ice samples.
Pingo Pingos are intrapermafrost ice-cored hills, high and in diameter. They are typically conical in shape and grow and persist only in permafrost environments, such as the Arctic and subarctic. A pingo is a periglacial landform, which is defin ...
s are common structures in permafrost regions. Similar structures are found in deep water related to methane leakages. Significantly, gas hydrates can even be formed in the absence of a liquid phase. Under that situation, water is dissolved in gas or in liquid hydrocarbon phase. In 2017, both Japan and China announced that attempts at large-scale
resource extraction Extractivism is the process of extracting natural resources from the Earth to sell on the world market. It exists in an economy that depends primarily on the extraction or removal of natural resources that are considered valuable for exportation w ...
of methane hydrates from under the seafloor were successful. However, commercial-scale production remains years away. The 2020 Research Fronts report identified gas hydrate accumulation and mining technology as one of the top 10 research fronts in the geosciences.


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 plug the line and cause flow assurance failure and damage valves and instrumentation. The results can range from flow reduction to equipment damage.


Hydrate formation, prevention and mitigation philosophy

Hydrates have a strong tendency to
agglomerate Agglomerate (from the Latin ''agglomerare'' meaning "to form into a ball") is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records flui ...
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 under 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 order of priority: # Avoid operational conditions that might cause formation of hydrates by depressing the hydrate formation temperature using
glycol dehydration Glycol dehydration is a liquid desiccant system for the removal of water from natural gas and natural gas liquids (NGL). It is the most common and economical means of water removal from these streams. Glycols typically seen in industry include tr ...
; # 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 ( inhibitors) 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 Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
, monoethylene glycol (MEG), and
diethylene glycol Diethylene glycol (DEG) is an organic compound with the formula (HOCH2CH2)2O. It is a colorless, practically odorless, and hygroscopic liquid with a sweetish taste. It is a four carbon dimer of ethylene glycol. It is miscible in water, alcohol, ...
(DEG), commonly referred to as
glycol A diol is a chemical compound containing two hydroxyl groups ( groups). An Aliphatic compound, aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common ...
. All may be recovered and recirculated, but the economics of methanol recovery is not 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 Triethylene glycol, TEG, or triglycol is a colorless odorless viscous liquid with molecular formula HOCH2CH2OCH2CH2OCH2CH2OH. It is used as a plasticizer for vinyl polymers. It is also used in air sanitizer products, such as "Oust" or "Clean and ...
(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 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, but 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
zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. : With amino acids, for example, in solution a chemical equilibrium wil ...
ic — usually ammonium and COOH — surfactants being both attracted to hydrates and hydrocarbons.


Empty clathrate hydrates

Empty clathrate hydrates are thermodynamically unstable (guest molecules are of paramount importance to stabilize these structures) with respect to ice, and as such their study using experimental techniques is greatly limited to very specific formation conditions; however, their mechanical stability renders theoretical and computer simulation methods the ideal choice to address their thermodynamic properties. Starting from very cold samples (110–145 K), Falenty et al. degassed Ne-sII clathrates for several hours using vacuum pumping to obtain a so-called ice XVI, while employing neutron diffraction to observe that (i) the empty sII hydrate structure decomposes at T ≥ 145 K and, furthermore, (ii) the empty hydrate shows a negative thermal expansion at T < 55 K, and it is mechanically more stable and has a larger lattice constant at low temperatures than the Ne-filled analogue. The existence of such a porous ice had been theoretically predicted before. From a theoretical perspective, empty hydrates can be probed using Molecular Dynamics or Monte Carlo techniques. Conde et al. used empty hydrates and a fully atomic description of the solid lattice to estimate the phase diagram of H2O at negative pressures and T ≤ 300 K, and obtain the differences in chemical potentials between ice Ih and the empty hydrates, central to the van der Waals−Platteeuw theory. Jacobson et al. performed simulations using a monoatomic (coarse-grained) model developed for H2O that is capable of capturing the tetrahedral symmetry of hydrates. Their calculations revealed that, under 1 atm pressure, sI and sII empty hydrates are metastable regarding the ice phases up to their melting temperatures, T=245 ± 2 K and T=252 ± 2 K, respectively. Matsui et al. employed molecular dynamics to perform a thorough and systematic study of several ice polymorphs, namely space fullerene ices, zeolitic ices, and aeroices, and interpreted their relative stability in terms of geometrical considerations. The thermodynamics of metastable empty sI clathrate hydrates have been probed over broad temperature and pressure ranges, 100 ≤ T (K) ≤ 220 and 1 ≤ p (bar) ≤ 5000, by Cruz et al. using large-scale simulations and compared with experimental data at 1 bar. The whole p−V−T surface obtained was fitted by the universal form of the Parsafar and Mason equation of state with an accuracy of 99.7–99.9%. Framework deformation caused by applied temperature followed a parabolic law, and there is a critical temperature above which the isobaric thermal expansion becomes negative, ranging from 194.7 K at 1 bar to 166.2 K at 5000 bar. Response to the applied (p, T) field was analyzed in terms of angle and distance descriptors of a classical tetrahedral structure and observed to occur essentially by means of angular alteration for (p, T) > (2000 bar, 200 K). The length of the hydrogen bonds responsible for framework integrity was insensitive to the thermodynamic conditions and its average value is r(̅O H) = 0.25 nm.


CO2 Hydrate

Clathrate hydrate, which encaged CO2 as guest molecule is termed as CO2 hydrate. The term CO2 hydrates are more commonly used these days with its relevance in anthropogenic CO2 capture and sequestration. A nonstoichiometric compound, carbon dioxide hydrate, is composed of hydrogen-bonded water molecules arranged in ice-like frameworks that are occupied by molecules with appropriate sizes and regions. In structure I, the CO2 hydrate crystallizes as one of two cubic hydrates composed of 46 H2O molecules (or D2O) and eight CO2 molecules occupying both large cavities (tetrakaidecahedral) and small cavities (pentagonal dodecahedral). Researchers believed that oceans and permafrost have immense potential to capture anthropogenic CO2 in the form CO2 hydrates. The utilization of additives to shift the CO2 hydrate equilibrium curve in phase diagram towards higher temperature and lower pressures is still under scrutiny to make extensive large-scale storage of CO2 viable in shallower subsea depths.


See also

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Clathrate A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning ‘with bars, latticed’. Most clathrate compounds are polymeric and completely envelop t ...
* Star formation and evolution *
Clathrate gun hypothesis The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The idea is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulate ...


References


Further reading

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External links


Gas hydrates
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Leibniz Institute of Marine Sciences
Kiel (IFM-GEOMAR)
The SUGAR Project (Submarine Gas Hydrate Reservoirs)
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Leibniz Institute of Marine Sciences
Kiel (IFM-GEOMAR)

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(by manufacturer of hydrate autoclaves) {{Authority control Ice Gases Industrial gases Natural gas