Gas hydrate
From Wikipedia, the free encyclopedia
A gas hydrate is a hydrate consisting of a water lattice in which light hydrocarbon molecules are embedded resembling dirty ice. Hydrates normally form when a gas stream is cooled below its hydrate formation temperature in the presence of free water, such as when the gas is colder than its water dew point temperature.
The two major conditions that promotes hydrate formation are thus:
- High gas pressure and low gas temperature
- The gas at or below its water dew point with "free water" present
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[edit] Natural hydrates
Methane, escaping from deep seabed formations, can form Methane clathrate. Such deposits may form a future hydrocarbon energy source.
[edit] Hydrates in pipelines
Conditions of pressure and temperature leading to hydrate formation are often found in pipelines. Secondary conditions such as high gas velocity, agitation and the formation of a nucleation site may also help form hydrates.
Hydrate formation is undesirable because the crystals might cause plugging of flow lines, valves and instrumentation. This can reduce line capacity and cause physical damage to pipelines and equipment.
[edit] Hydrate formation prevention and mitigation philosophy
The formation of hydrates should be avoided because hydrates do not dissociate at the same conditions at which they are created. Significantly higher temperature and/or lower pressure are required. Even at these conditions the dissocioation of hydrates is a slow process. Furthermore hydrates have a strong tendency to agglomerate and to adhere to the pipe wall and thereby plug the pipeline.
A hydrate prevention philosophy could typically be based on three levels of security listed in prioritised order:
- Avoid operational conditions that might cause formation of hydrates
- Temporarily change operating conditions in order to avoid hydrate formation
- Prevent formation of hydrates by addition of chemicals that lower the hydrate formation temperature or 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 the formation of hydrates. Altering the gas composition by adding chemicals can lower the hydrate formation temperature and/or delay the formation of hydrates. Two options generally exists:
- Thermodynamic inhibitors
- Kinetic inhibitors/anti-agglomerators
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.
Methanol is a colourless volatile liquid fully soluble in water. Synonyms are methyl alcohol, wood alcohol, wood spirits, or curbinol. Methanol is used primarily in anti-freeze compounds, paints, cements, inks, varnishes, shellacs, wood strippers, windshield wiper solvents, gasoline antifreeze and as a solvent in dyes. Methanol is highly toxic and readily absorbed from any routes of exposure. Symptoms include malaise, headache, dizziness, confusion, abdominal cramps with excruciating pain and tenderness, stupor, weakness, and acidosis. When methanol is swallowed, it is metabolized to formaldehyde. This formaldehyde is more toxic than the methanol itself. Blindness and death may occur following ingestion.
Ethylene glycol is a colourless, odourless, involatile, hygroscopic liquid. It is characterised by two hydroxyl groups, which contribute to its high water solubility, hygroscopic and reactivity with many organic compounds. Major applications for ethylene glycol are as an intermediate for the manufacture of polyester resins, fibres and surface coatings, as well as antifreeze in the automotive industry. Mono-, di- and triethylene glycols (MEG, DEG and TEG) are the first three members of a homologous series of dihydroxyalcohols. They are colourless, essentially odourless stable liquids with low viscosities and high boiling points that are poisonous when ingested. Ingestion may result in depres-sion followed by respiratory and cardiac failure, kidney damage and brain damage. (Mono) ethylene glycol is by far the largest volume of the glycol products and is used in a variety of applications.
Methanol, monoethylene glycol and di-ethylene glycol could all three be used as a hydrate inhibitor. Methanol would currently be the cheapest solution and the solutions preferred by most for applications where the inhibitor is not expected to be reused.
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-agglomerators in actual field operations is a new and evolving technology. The use 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, rather they stop the agglomeration(sticking together) of gas hydrates. These 2 kinds of inhibitors are also known as Low-Dosage-Hydrate-Inhibitors because they require much less 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.
