Clathrate hydrate

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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, 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.

Clathrates are believed to occur in large quantities on some outer planets, moons and trans-Neptunian objects, binding gas at fairly high temperatures. Clathrates have also been discovered in large quantity on Earth, i.e. in giant natural methane clathrate deposits on the deep ocean floor (e.g. in the northern headwall flank of the Storegga Slide, which is a part of the Norwegian continental shelf) and in permafrost regions (e.g. the Mallik gas hydrate field in the Mackenzie Delta of northwestern Canadian Arctic). Hydrocarbon clathrates are a problem for the petroleum industry, since their formation inside gas pipelines frequently leads to plug formation in the latter. Deep sea deposition of carbon dioxide clathrate to remove this greenhouse gas from the atmosphere has also been proposed.

Contents 1 Structure 2 Hydrates in the Universe 3 Hydrates on Earth 3.1 Natural gas hydrates 3.2 Gas hydrates in pipelines 3.2.1 Hydrate formation prevention and mitigation philosophy 3.2.2 Hydrate inhibitors 4 See also 5 References

[edit] Structure

Gas hydrates usually form two crystallographic cubic structures – structure (Type) I and structure (Type) II^[1] of space groups and respectively. Rather seldom, a third hexagonal structure of space group P6 / mmm maybe observed (Type H).^[2]

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 pentagonal dodecahedron (5¹²) and the large one that of tetrakaidecahedron (5¹²6²). Typical guests forming Type I hydrates are CO₂ and CH₄.

The unit cell of Type II consists of 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 pentagonal dodecahedron (5¹²) but the large one this time is hexakaidecahedron (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. There thermogenically-produced supplies of heavy hydrocarbons are common.

[edit] Hydrates in the Universe

Iro et al. ^[3], 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 meter scale. The key was to provide enough microscopic ice particles exposed to a gaseous environment. Observations of the radiometric continuum of sircumstellar discs around τ-Tauri and Herbig Ae/Be stars suggest massive dust disks consisting of millimeter-sized grains, which disappear after several millions of years (e.g. ^[4], ^[5]). 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 then the one at 60 μm, which means the water ice, is located in the outer parts of the disk at temperatures below 50 K ^[6]. There is also another broad ice feature between 87 and 90 μm, which is very similar to the one in NGC 6302 ^[7] (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^[8][9] 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^[10] Herbig Ae/Be star HD 141569A, are dusty^[11]. Li & Lunine^[12] found water ice there. Knowing the ices usually exist at the outer parts of the proto-planetary nebulae, Hersant et al.^[13] 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) 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]

Carbon dioxide clathrate is believed to play a major role in different processes on Mars.

[edit] Hydrates on Earth

[edit] Natural gas hydrates

Main article: Methane hydrate

Naturally on Earth gas hydrates can be found on the seafloor, in ocean 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 for the future. Catastrophic release of methane from the decomposition of such deposits may lead to a global climate change, since CH₄ is more efficient greenhouse gas even than CO₂(see Methane). On its turn, 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 perma frost regions. Simllar structures are found in deep water related to methane leakages. Methane Leakage from Pingo-like Features on the Arctic Shelf, Beaufort Sea, NWT, Canada Authors: Ussler, W.; Paull, C. K.; Lorenson, T.; Dallimore, S.; Medioli, B.; Blasco, S.; McLaughlin, F.; Nixon, F. M. [[1]]

[edit] 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.

[edit] 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:

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.)

[edit] 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.

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 then 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.

[edit] See also

• Clathrate
• Star Formation and evolution

[edit] References