A clathrate (lat.: clatratus = lattice) is a inclusion compound of a solid phase which forms a lattice-like crystall structure with cavities. In these cavities a second (or more) liquid or gaseous guest substance gets included. The compound is bonded only via intermolecular forces and not via chemical bonds. This differs clathrates from interstitial solution of metals and nonmetals and from complex compounds.
Clathrates are non-stoichiometric, they do not have a fix composition because not every cavity of the lattice has to be or could be occupied. The structure of the lattice depends on the size of the included guest molecule, the bigger it is the more voluminous the lattice polyhedron has to be. Typical lattice-forming host molecules besides water are urea, thiourea, hydroquinone and other organic molecules.
Inclusion compounds of gases in a lattice of water molecules are called gas hydrates. The structure of the lattice depends strongly on the included guest molecule, the physical conditions (pressure and temperature) and the chemical conditions (chemical composition of the alloy).
The single cells of the lattice form a polyhedron which is assambled from regular polygons. The nomenclature of these polyhedron according to Jeffrey is , where ni is the number of angles of the polygonic face i and mi is the number of faces with ni angles. For example a cell with 12 similar pentagons as faces is denoted as 512.
At gas hydrates three different structures can be found, they are denoted as S-I, S-II and S-H. These structures differ in the number and form of included polyhedrons. In the following grafic the included polyhedrons of one unit cell for every structure are shown. Structure S-I contains for example 2 polyhedrons of the type 512 (=dodecahedron) and 6 polyhedrons of the type 51262 (= hexagonal truncated trapezohedron) from: http://www.ifm-geomar.de
Methane hydrate is a clathrate compound of water as lattice and methane as guest molecule. The density of methane hydrate is about 0.9 g/cm3 with a maximum mole ratio of 5.75 : 1 water to methane. One liter of hydrate contains therefore up to 168 liters of methane at standard temperature and pressure. The hydrate forms structures of the type S-I, an unit cell contains 46 water and 8 methane molecules.
Methane hydrate causes big problems especially at the transport of natural gas. The conditions of temperature and pressure inside pipelines mainly in cold areas and submarine allow the formation of hydrates. These hydrate agglomerate after their formation to bigger clusters and could plug valves, pumps and other narrow parts or even the whole pipeline.
Conditions for gas hydrate formation
Avoidance of hydrates:
Avoidance of hydrate formation is preferable to removal of excisting hydrate from an economical view and safety concerns. To achieve this, the hydrate formation process can be influenced on several points:
• Dehydration of the gas before transport
• Controlling of pressure and temperature during the transport
• Use of thermodynamic hydrate inhibitors, THI
• Use of kinetic hydrate inhibitors, KHI
• Use of anti agglomerate, AA
Dehydration: The less water the transported gas contains the lower is the risk of hydrate formation. Natural gas is dehydrated with triethylene glycol (TEG) or with molecular sieves. The choosen desiccant depends of the wanted dew point. Disadvantageous at the dehydration is the extensive needed technic directly or near by the drill hole. Pressure and temperature control: If the conditions can be held outside of the limits for hydrate formation, wet gas can be transported, too. But it is to be considered, that local pressure and particularly temperature fluctuations can occure. The required technic for temperature control of the whole pipeline or for larger sections is costly, reduced pressure leads directly to lower feed rates. Thermodynamic hydrate inhibitors, THI: With thermodynamic inhibitors the chemical potential between the water moleculs is changed. The pressure and temperature range in which hydrate formation is possible gets shift to higher pressure and to lower temperature. Used as inhibitors are salts (esp. alkali halogenides), alcohols (methanol) and glycols.
The advanages of THI are their proven application, their recycling possibilities, their availabilities and that they works with any type of hydrocarbon.
Disadvantages are the required high volume percentages of THI. For example the ratio methanol to water is between 0.5 : 1 and 1:1. This leads to an extensive logistic support for the transport of THI to the drill hole, the feed rate decreases due to the added THI volume and the recycling of THI is costly. Also lead the chemical properties of the THI to an higher corrosion rate and to higher safety requirements. Too small amounts of THI can support the hydrate formation instead of preventing it. Kinetic hydrate inhibitors, KHI: With kinetic inhibitors the formation of the cell structure is strongly decreased in its speed. The inhibitors interfere in the chemical equillibrium of the formation reaction and decrease the reaction rate. So they are the opposite of catalysts which interfere to reactions by increasing their reaction rates. Anti agglomerate, AA: Anti agglomerate do not prevent the formation of hydrate particles but they inhibit the agglomeration of these particels to bigger clusters. The hydrate particles stay dispersed in the hydrocarbon phase and can not plug the pipeline. KHI and AA are combined denoted as Low Dosage Hydrate Inhibitors, LDHI. Advantages of these inhibitors are their low dosage rates in a range of only a few mass or volume percentages what causes lower costs for chemicals, injection technique and logistics. Also higher feed rates can be obtained. Unfavourable is, that the LDHI can only be used under limited conditions and that they mostly have higher enviromental risks and therefore require higher safety standards.
Removal of gas hydrates:
The removal of existing hydrate plug is very extensive in time and money. The hydrates are very stable after their formation and break down only slowly. The easiest way for decomposing hydrates is a pressure reduction and if possible an increased gas or pipeline wall temperature. This process requires due to the hydrate stability a high expentiture of time.
With chemical additives the hydrates can be dissolved but at completely plugged parts the contact between chemical and hydrate is possible only at the surface layer of the plug and the mixture is insufficient.
It must allways be kept in mind that the abrupt decomposition of hydrates release enormous amounts of included gas. The appearing pressure waves can cause great damage at the pipeline and at technical installations up to pipeline raptures.
Artificial gas hydrates:
The production of artificial gas hydrates can be done in special autoclaves like the PSL Gas Hydrate Autoclave System. The formation of gas hydrate only occur under specific pressure and temperature conditions (formation envelope). At standard conditions (room temperature and normal atmospheric pressure) the methane hydrate is unstable and it dissociates into water and gas. The released methane forms an inflammable gas-air mixture over the surface, this can be ignited (see picture on the left, click to enlarge)
Methane normally burns with a weak blue or nearly invisible flame, therefore some lithium and potassium salts were added to get a visible, red flame color.