Clathrate hydrates, or gas hydrates, clathrates, or hydrates, are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded, frozen water molecules. [1] [2] In other words, clathrate hydrates are clathrate compounds in which the host molecule is water and the guest molecule is typically a gas or liquid. Without the support of the trapped molecules, the lattice structure of hydrate clathrates would collapse into conventional ice crystal structure or liquid water. Most low molecular weight gases, including O 2, H 2, N 2, CO2, CH4, H2S, Ar , Kr , Xe , and Cl 2 as well as some higher hydrocarbons and freons, will form hydrates 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. [3] [4] [5] Clathrate hydrates were first documented in 1810 by Sir Humphry Davy who found that water was a primary component of what was earlier thought to be solidified chlorine. [6] [7]
Clathrates have been found to occur naturally in large quantities. Around 6.4 trillion (6.4×1012) tonnes of methane is trapped in deposits of methane clathrate on the deep ocean floor. [8] 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 site in the Mackenzie Delta of northwestern Canadian Arctic. These natural gas hydrates are seen as a potentially vast energy resource and several countries have dedicated national programs to develop this energy resource. [9] Clathrate hydrate has also been of great interest as technology enabler for many applications like seawater desalination, [10] gas storage, [11] carbon dioxide capture & storage, [12] cooling medium for data centre [13] 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 from the atmosphere and control climate change. Clathrates are suspected to occur in large quantities on some outer planets, moons and trans-Neptunian objects, binding gas at fairly high temperatures. [14]
Gas hydrates usually form two crystallographic cubic structures: structure (Type) I (named sI) and structure (Type) II (named sII) [15] of space groups and respectively. A third hexagonal structure of space group may also be observed (Type H). [16]
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 (512) (which is not a regular dodecahedron) and the large one that of a tetradecahedron, specifically a hexagonal truncated trapezohedron (51262). Together, they form a version of the Weaire–Phelan structure. Typical guests forming Type I hydrates are CO2 in carbon dioxide clathrate and CH4 in methane clathrate.
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 (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, 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.
Iro et al., [17] trying to interpret the nitrogen deficiency in comets, 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 metre-scale. The key was to provide enough microscopic ice particles exposed to a gaseous environment. Observations of the radiometric continuum of circumstellar discs around -Tauri and Herbig Ae/Be stars suggest massive dust disks consisting of millimetre-sized grains, which disappear after several million years (e.g., [18] [19] ). A lot of work on detecting water ices in the Universe was done on the Infrared Space Observatory (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 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. [20] There is also another broad ice feature between 87 and 90 μm, which is very similar to the one in NGC 6302 [21] (the Bug or Butterfly nebula in Scorpius). Crystalline ices were also detected in the proto-planetary disks of ε-Eridani and the isolated Fe star HD 142527 [22] [23] in Lupus. 90% of the ice in the latter was found crystalline at temperature around 50 K. HST demonstrated that relatively old circumstellar disks, as the one around the 5-million-year-old B9.5Ve [24] Herbig Ae/Be star HD 141569A, are dusty. [25] Li & Lunine [26] found water ice there. Knowing the ices usually exist at the outer parts of the proto-planetary nebulae, Hersant et al. [27] proposed an interpretation of the volatile enrichment, observed in the four giant planets of the Solar System, with respect to the Solar abundances. They assumed the volatiles had been trapped in the form of hydrates and incorporated in the planetesimals flying in the protoplanets' feeding zones.
Kieffer et al. (2006) hypothesized 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 through the "Tiger Stripe" fractures found in that area. [28] However, subsequent analysis of plume material makes it more likely that the geysers on Enceladus derive from a salty subsurface ocean. [29]
Carbon dioxide clathrate is believed to play a major role in different processes on Mars. Hydrogen clathrate is likely to form in condensation nebulae for gas giants.[ citation needed ]
Kamata et al. [30] (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, the existence of which is suggested by data from the New Horizons probe.
Naturally on Earth gas hydrates can be found on the seabed, in ocean sediments, [31] in deep lake sediments (e.g. Lake Baikal), as well as in the permafrost regions. The amount of methane potentially trapped in natural methane hydrate deposits may be significant (1015 to 1017 cubic metres), [32] 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", because CH 4 is a more potent greenhouse gas than CO 2 (see Atmospheric methane). The fast decomposition of such deposits is considered a geohazard, due to its potential to trigger landslides, earthquakes and tsunamis. However, natural gas hydrates do not contain only methane but also other hydrocarbon gases, as well as H2S and CO2. Air hydrates are frequently observed in polar ice samples.
Pingos are common structures in permafrost regions. [33] Similar structures are found in deep water related to methane vents. 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. [34]
In 2017, both Japan and China announced that attempts at large-scale resource extraction of methane hydrates from under the seafloor were successful. However, commercial-scale production remains years away. [35] [36]
The 2020 Research Fronts report identified gas hydrate accumulation and mining technology as one of the top 10 research fronts in the geosciences. [37]
Thermodynamic conditions favouring hydrate formation are often found in pipelines. This is highly undesirable, because the clathrate crystals might agglomerate and plug the line [38] and cause flow assurance failure and damage valves and instrumentation. The results can range from flow reduction to equipment damage.
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 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:
The actual philosophy would depend on operational circumstances such as pressure, temperature, type of flow (gas, liquid, presences of water etc.).
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:
The most common thermodynamic inhibitors are methanol, monoethylene glycol (MEG), and diethylene glycol (DEG), commonly referred to as glycol. 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 (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 zwitterionic – usually ammonium and COOH – surfactants being both attracted to hydrates and hydrocarbons.
Empty clathrate hydrates [39] 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. [40] 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. [41] 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, [42] 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 [43] 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 [44] 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 K ≤ T ≤ 220 K and 100 kPa ≤ p ≤ 500 MPa, by Cruz et al. [45] using large-scale simulations and compared with experimental data at 100 kPa. 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 100 kPa to 166.2 K at 500 MPa. 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) > (200 MPa, 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.
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). [46] 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. [47]
In organic chemistry, an alkane, or paraffin, is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. Alkanes have the general chemical formula CnH2n+2. The alkanes range in complexity from the simplest case of methane, where n = 1, to arbitrarily large and complex molecules, like pentacontane or 6-ethyl-2-methyl-5-(1-methylethyl) octane, an isomer of tetradecane.
In chemistry, hydronium (hydroxonium in traditional British English) is the cation [H3O]+, also written as H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: the Eigen cation, which is a tetrahydrate, H3O+(H2O)3, the Zundel cation, which is a symmetric dihydrate, H+(H2O)2, and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.
Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), 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 amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth (approx. 1100m below the sea level). Methane hydrate is formed when hydrogen-bonded water and methane gas come into contact at high pressures and low temperatures in oceans.
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 understood.
Ethane is a naturally occurring organic chemical compound with chemical formula C
2H
6. At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production.
A micelle or micella is an aggregate of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension. A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.
In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon.
The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and to be excluded by water. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar substances, which maximizes the entropy of water and minimizes the area of contact between water and nonpolar molecules. In terms of thermodynamics, the hydrophobic effect is the free energy change of water surrounding a solute. A positive free energy change of the surrounding solvent indicates hydrophobicity, whereas a negative free energy change implies hydrophilicity.
A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word clathrate is derived from the Latin clathratus, meaning 'with bars, latticed'. Most clathrate compounds are polymeric and completely envelop the guest molecule, but in modern usage clathrates also include host–guest complexes and inclusion compounds. According to IUPAC, clathrates are inclusion compounds "in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules." The term refers to many molecular hosts, including calixarenes and cyclodextrins and even some inorganic polymers such as zeolites.
The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:
The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The hypothesis is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released methane clathrate on upper continental slopes. This would have had an immediate impact on the global temperature, as methane is a much more powerful greenhouse gas than carbon dioxide. Despite its atmospheric lifetime of around 12 years, methane's global warming potential is 72 times greater than that of carbon dioxide over 20 years, and 25 times over 100 years. It is further proposed that these warming events caused the Bond Cycles and individual interstadial events, such as the Dansgaard–Oeschger interstadials.
Carbon dioxide hydrate or carbon dioxide clathrate is a snow-like crystalline substance composed of water ice and carbon dioxide. It normally is a Type I gas clathrate. There has also been some experimental evidence for the development of a metastable Type II phase at a temperature near the ice melting point. The clathrate can exist below 283K (10 °C) at a range of pressures of carbon dioxide. CO2 hydrates are widely studied around the world due to their promising prospects of carbon dioxide capture from flue gas and fuel gas streams relevant to post-combustion and pre-combustion capture. It is also quite likely to be important on Mars due to the presence of carbon dioxide and ice at low temperatures.
The atmosphere of Titan is the dense layer of gases surrounding Titan, the largest moon of Saturn. Titan is the only natural satellite in the Solar System with an atmosphere that is denser than the atmosphere of Earth and is one of two moons with an atmosphere significant enough to drive weather. Titan's lower atmosphere is primarily composed of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, propane, PAHs and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon and helium. The isotopic study of nitrogen isotopes ratio also suggests acetonitrile may be present in quantities exceeding hydrogen cyanide and cyanoacetylene. The surface pressure is about 50% higher than on Earth at 1.5 bars which is near the triple point of methane and allows there to be gaseous methane in the atmosphere and liquid methane on the surface. The orange color as seen from space is produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.
Methane is a chemical compound with the chemical formula CH4. It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it is hard because it is a gas at standard temperature and pressure.
Arctic methane release is the release of methane from Arctic ocean floors, lake bottoms, wetlands and soils in permafrost regions of the Arctic. While it is a long-term natural process, methane release is exacerbated by global warming. This results in a positive climate change feedback, as methane is a powerful greenhouse gas. The Arctic region is one of many natural sources of methane. Climate change could accelerate methane release in the Arctic, due to the release of methane from existing stores, and from methanogenesis in rotting biomass. When permafrost thaws as a consequence of warming, large amounts of organic material can become available for methanogenesis and may ultimately be released as methane.
A Bjerrum defect is a crystallographic defect which is specific to ice, and which is partly responsible for the electrical properties of ice. It was first proposed by Niels Bjerrum in 1952 in order to explain the electrical polarization of ice in an electric field. A hydrogen bond normally has one proton, but a hydrogen bond with a Bjerrum defect will have either two protons or no proton. D-defects are more energetically favorable than L-defects. The unfavorable defect strain is resolved when a water molecule pivots about an oxygen atom to produce hydrogen bonds with single protons. Dislocations of ice Ih along a slip plane create pairs of Bjerrum defects, one D defect and one L defect.
A hydrogen clathrate is a clathrate containing hydrogen in a water lattice. This substance is interesting due to its possible use to store hydrogen in a hydrogen economy. A recent review that accounts the state-of-the-art and future prospects and challenges of hydrogen storage as clathrate hydrates is reported by Veluswamy et al. (2014). Another unusual characteristic is that multiple hydrogen molecules can occur at each cage site in the ice, one of only a very few guest molecule that forms clathrates with this property. The maximum ratio of hydrogen to water is 6 H2 to 17 H2O. It can be formed at 250K in a diamond anvil at a pressure of 300MPa (3000 Bars). It takes about 30 minutes to form, so this method is impractical for rapid manufacture. The percent of weight of hydrogen is 3.77%. The cage compartments are hexakaidecahedral and hold from two to four molecules of hydrogen. At temperatures above 160K the molecules rotate around inside the cage. Below 120K the molecules stop racing around the cage, and below 50K are locked into a fixed position. This was determined with deuterium in a neutron scattering experiment.
Kinetic inhibitors are a new and evolving technology of a class of Low Dosage Hydrate Inhibitors (LDHI) that are polymers and copolymers. The most common of which is polyvinylcaprolactam (PVCap). These inhibitors are primarily utilized to retard the formation of clathrate hydrates. This problem becomes most prominent in flow lines when hydrocarbons and water flow through a line. The pressure and cold temperatures that could be exposed to the flow lines provides an environment in which clathrate hydrates can form and plug up the flow line. The inhibitors generally slow the formation of the hydrates enough so the fluid reaches storage without causing blockage.
Nitrogen clathrate or nitrogen hydrate is a clathrate consisting of ice with regular crystalline cavities that contain nitrogen molecules. Nitrogen clathrate is a variety of air hydrates. It occurs naturally in ice caps on Earth, and is believed to be important in the outer Solar System on moons such as Titan and Triton which have a cold nitrogen atmosphere.
Praveen Linga PhD, a chemical engineer, is a professor at the National University of Singapore's Department of Chemical and Biomolecular Engineering. He is an expert in clathrate hydrates or gas hydrates. He is also the co-founder of NewGen Gas Pte Ltd, a spin-off company that specialises in solidified natural gas (SNG) technology via clathrate hydrates for natural gas storage and transport. He has been interviewed and has provided expert opinion and commentary in the media.