Clathrate hydrate
Clathrate hydrates, or gas hydrates, clathrates, or hydrates, are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen-bonded, frozen water molecules. 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,,, carbon dioxide|, methane|, hydrogen sulfide|,,,, and 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.
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.
Clathrates have been found to occur naturally in large quantities. Around 6.4 trillion tonnes of methane is trapped in deposits of methane clathrate on the deep ocean floor. 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. 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 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.
History and etymology
Clathrate hydrates were discovered in 1810 by Humphry Davy. Clathrates were studied by P. Pfeiffer in 1927 and in 1930, E. Hertel defined "molecular compounds" as substances decomposed into individual components following the mass action law in solution or gas state. Clathrate hydrates were discovered to form blockages in gas pipelines in 1934 by Hammerschmidt that led to increase in research to avoid hydrate formation. In 1945, H. M. Powell analyzed the crystal structure of these compounds and named them clathrates. Gas production through methane hydrates has since been realized and has been tested for energy production in Japan and China.The word clathrate is derived from the Latin , meaning 'with bars, latticed'.
Structure
Gas hydrates usually form two crystallographic cubic structures: structure I and structure II of space groups and respectively. A third hexagonal structure of space group may also be observed.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 and the large one that of a tetradecahedron, specifically a hexagonal truncated trapezohedron. 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, but the large one is a hexadecahedron. 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 to be stable. It is the large cavity that allows structure H hydrates to fit in large molecules, 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.
The molar fraction of water of most clathrate hydrates is 85%. Clathrate hydrates are derived from organic hydrogen-bonded frameworks. These frameworks are prepared from molecules that "self-associate" by multiple hydrogen-bonding interactions. Small molecules or gases can be encaged as a guest in hydrates. The ideal guest/host ratio for clathrate hydrates range from 0.8 to 0.9. The guest interaction with the host is limited to van der Waals forces. Certain exceptions exist in semiclathrates where guests incorporate into the host structure via hydrogen bonding with the host structure. Hydrates form often with partial guest filling and collapse in the absence of guests occupying the water cages. Like ice, clathrate hydrates are stable at low temperatures and high pressure and possess similar properties like electrical resistivity. Clathrate hydrates are naturally occurring and can be found in the permafrost and oceanic sediments. Hydrates can also be synthesized through seed crystallization or using amorphous precursors for nucleation.
Clathrates have been explored for many applications including: gas storage, gas production, gas separation, desalination, thermoelectrics, photovoltaics, and batteries.
Hydrates on Earth
Natural gas hydrates
Naturally on Earth gas hydrates can be found on the seabed, in ocean sediments, in deep lake sediments, as well as in the permafrost regions. The amount of methane potentially trapped in natural methane hydrate deposits may be significant, 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 CH4 is a more potent greenhouse gas than CO2. 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. 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.
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.
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 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;
- Temporarily change operating conditions in order to avoid hydrate formation;
- Prevent formation of hydrates by addition of chemicals that shift the hydrate equilibrium conditions towards lower temperatures and higher pressures or increase hydrate formation time
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 and anti-agglomerants
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 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 are polymers or zwitterionic – usually ammonium and COOH – surfactants being both attracted to hydrates and hydrocarbons.