The clathrate hydrates (clathrates or alternatively gas, gas hydrates, clathrates, hydrates, etc..) Are a class of supramolecular chemistry of solids in which gas molecules occupy "cages" made up of water molecules connected by hydrogen bonds. Once emptied, known as "cages" candies become unstable and collapse into ordinary ice crystals, but can be stabilized with the inclusion of molecules of appropriate size inside. Most of the low molecular weight gases (eg O2, N2, CO2, CH4, H2S, argon, krypton, and xenon) as well as some in higher weight hydrocarbons such as freon and clathrate hydrates formed under certain conditions of pressure and temperature. The clathrate hydrates are not chemical compounds. The formation and decomposition of clathrate hydrates are first order phase transitions and chemical reactions.
It is thought that clathrate hydrates are present in large quantities on some outer planets, moons and objects trans-uranium in the form of associated gas at relatively high temperatures. Clathrates were discovered in large quantities on Earth in large deposits of methane clathrates in the deep ocean (eg on the northern flank of the Storegga submarine landslide, which is part of the Norwegian continental shelf) and permafrost (eg, gas fields Mallik hydrates in the Mackenzie Delta in the Canadian North). The hydrocarbon clathrates are a problem in the oil industry, since their formation in gas pipelines frequently leads to their occlusion. The deposition of clathrates of carbon dioxide in the deep ocean has been proposed as a method to remove this greenhouse gas from the atmosphere.
The gas hydrates usually form two cubic crystal structures - structure (type) and the structure (type) of space groups and II respectively. Less often observed is a third hexagonal structure of space group P6 / mmm (Type H).
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 are two to six large. The small cage in the shape of a pentagonal dodecahedron (512) and the large tetrakaidecaedro (51262). The molecules typically form hydrates of type I are CO2 and CH4.
The unit cell of Type II consists of 136 water molecules, which are also two types of cages - large and small. In this case the small cages in the unit are sixteen against eight largest. The small cage was still in the form of pentagonal dodecahedron (512) while the big one this time is a hexakaidecaedro (51264). Type II hydrates are formed by gases such as O2 and N2.
The unit cell of Type H consists of 34 water molecules, forming three types of cages - two small and different type and a huge. In this case, the unit cell consists of three small cages of the type 512, twelve small type 435663 and a huge type 51268. The formation of Type H requires the cooperation of the two gases guests (large and small) to be stable. It is the large cavity that allows the structure H hydrates to be part of large molecules (butane, hydrocarbons), given the presence of other auxiliary and support small to fill the remaining cavity. It is believed that the structure H hydrates are present in the Gulf of Mexico, where production of thermogenic heavy hydrocarbons is common.
Hydrates In The Universe
Iro et al trying to interpret the loss of nitrogen of comets, formulated most of the conditions for hydrate formation in the protoplanetary nebula, around the pre-main sequence and main. The key was to provide enough microscopic particles of ice exposed to a gaseous environment. Radiometric observations of the continuum of circumstellar disks around T Tauri stars and Herbig Ae / Be disks suggest the presence of massive dust consisting of millimeter-sized grains, which disappear after several million years.Many work for detection of water ice has been made sull'Infrared Universe Space Observatory (ISO). For example, the emission spectra of large water ice at 43 and 60 μm are detected in the disks of the isolated star Herbig Ae / Be HD 100546 Moscow. The one at 43 μm is much weaker than that at 60 mM, which means that the water ice is placed in the outer disk temperatures below 50 K. There is also another characteristic of ice between 87 and 90 mM, which is very similar to the one in NGC 6302 (the nebula in Scorpius worm or butterfly). The ice crystals are also found in protoplanetary disks of ε-Eridani and isolated Fe in the star HD 142527 in the constellation of the Wolf.
Hydrates on Earth
Hydrates Of Natural Gas
Of course the Earth's gas hydrates can be detected on the seabed, sediments deposited on the ocean floor, or in deep lakes (eg Lake Baikal), and in permafrost regions. The potential amount of methane trapped in methane hydrate deposits may be significant, making them more attractive as potential energy sources in the future. The catastrophic release of methane from the decomposition of these deposits can lead to global climate change, as methane is a greenhouse gas than CO2 is also effective. For its part, the rapid decomposition of these deposits is considered a geohazard, because of its ability to cause landslides, earthquakes and tsunamis. However, natural gas hydrates contain not only methane but also other hydrocarbons as well as H2S and CO2. The clathrates of air are commonly observed in polar ice cores. Pings are common structures in permafrost regions. Such structures are detectable in deep water in relation to escaping methane.
Hydrates In Gas Pipelines
The thermodynamic conditions that favor the formation of clathrates are often found in gas pipelines. This is highly pernicious because the clathrate crystals can agglomerate interrupting the flow of gas, damaging the valves and instrumentation. The results can range from flow reduction to physical damage to the plant.
Preventing The Formation Of Clathrates And Technical Mitigation
The clathrates have a strong tendency to agglomerate and to adhere to the pipe wall blocking the pipeline. Once formed, can be decomposed by increasing the temperature and / or decreasing the pressure. Even under these conditions, the dissociation of the clathrates is a slow process.
Therefore, to prevent the formation of clathrates appears to be the best solution. A philosophy of preventing the formation of clathrates may be based on three criteria of safety, in order of priority:
1.Avoid operating Conditions That May Cause the formation of clathrates;
2.Conditions change Temporarily operating in order to Prevent the formation;
3. Prevent training by adding Substances that (a) shift the equilibrium of clathrate to lower temperatures and higher Pressures and (b) Increase the time of formation of clathrates (inhibitors).
The technique used depends on the operating conditions such as pressure, temperature, fluid type (gas, liquid, presence of water, etc..)
Working within the parameters of a system in which the clathrates may form, however, there are ways to avoid their formation. Altering the composition of the gas with the addition of substances can lower the temperature of clathrate formation and / or delay the formation. There are generally two options:
-motion sickness inhibitors / anti-caking
The most common thermodynamic inhibitors are:
-Ethylene glycol (MEG)
-Diethylene glycol (DEG).
All are available, but the economics of the recovery of methanol is not favorable in many cases. The MEG DEG is preferred for applications where the temperature is expected to be -10 ° C or less, for the high viscosity at low temperatures. Triethylene glycol (TEG) has a vapor pressure is too low to be used as an inhibitor injected into a pipeline.
The use of kinetic inhibitors and anti-caking agents in the practical field is a new and evolving technology. Requires extensive testing and optimizing the real system. While the kinetic inhibitors work by slowing the kinetics of nucleation, the anti-caking not interrupt the nucleation, rather than stopping the agglomeration of gas hydrate crystals. These two types of inhibitors are also known as inhibitors of clathrates at low doses because they require much lower concentrations than conventional thermodynamic inhibitors. The kinetic inhibitors (which do not require a mixture of water and hydrocarbons to be effective) are usually polymers or copolymers as anti-caking agents (which require the mixture) are polymers or zwitterionic surfactants (usually ammonium COOH) that are attracted to both hydrates that hydrocarbons.
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