GAS DEHYDRATION SYSTEMS
Natural gas normally contains a large amount of dissolved water and, at the various processing stages of separation and compression after cooling, it is typically water saturated. In effect, this means that any subsequent cooling will result in water condensing in the piping and downstream equipment, which can create corrosion and hydrate problems that are potentially very expensive issues to resolve.
In most cases, it is not practical to cool the gas temperature below pipeline operating temperatures or injection temperatures. Therefore, the formation of free water must be prevented by removing some of the water from the gas to ensure that the water dew point or hydrate formation temperature is not reached during operation.
The most popular gas treatment technologies utilise aqueous solutions and dehydration therefore typically follows gas treating and/or gas compression. Techniques for dehydrating natural gas include:
- Absorption using liquid glycol desiccants (MEG / TEG)
- Adsorption using solid desiccants (Mol Sieve / Silica Gel / Activated Alumina)
- Inhibition by injection of hydrate point depressants (Glycol / Methanol Injection)
- Dehydration by expansion refrigeration (Low-Temperature Separation)
Capital and operating cost economics usually favour glycol dehydration over other processes where this process will meet the dehydration specifications required. TEG is the most widely used glycol because of lower vapour losses combined with a greater dewpoint suppression. Solid desiccant processes such as molecular sieves can produce outlet water content as low as 1ppm. As a result, they are widely used for feed streams to cryogenic processing systems.
All dehydration processes have advantages for specific applications but also have disadvantages and potential issues for the inexperienced.