Glycol dehydration is a method used in the purification of natural gas . It is intended to aid in removing surplus water from raw natural gas in order to turn it to be suitable for transport in natural gas pipelines and for the utility consumer. Glycol dehydration is a liquid desiccant system for the removal of water from natural gas and natural gas liquids (NGL). A desiccant is a hygroscopic (the ability of a substance to attract and hold water molecules from the surrounding environment) substance that induces or sustains a state of dryness (desiccation) in its local surrounding area in a fairly well-sealed container. Glycol, the principal agent in glycol dehydration, has a chemical similarity for water. This means that, when in contact with a stream of natural gas that contains water, glycol will serve to 'steal' the water out of the gas stream. Glycol dehydration is a very common and cost-effective method of water removal from these streams of natural gas. Glycols that are used for industry purposes are triethylene glycol (TEG), diethylene glycol (DEG), ethylene glycol (MEG), and tetraethylene glycol (TREG). The most commonly used glycol in industry is TEG due to its highest boiling points, generally making it easier to remove the water vapor and reuse.
When designing the overall system, the particular glycol is taken into consideration as each glycol has its own specific moisture content that can be absorbed in the given process. They have to take into consideration the
Glycol has a chemical affinity for water and so when passed through any moisture saturated gas, absorbs the water molecules in it. As it gets heavier with the water that has been absorbed it sinks to the bottom of the tank which is then let in to a boiler which is boiled at about 212F which is the boiling point of water and the water vaporizes leaving behind the Glycol as Glycol has a higher boiling point of 400 F. It is due to this difference in the boiling points of water and glycol and the affinity of glycol for water that makes it the ideal solution to act as a dehydrating agent in natural gas.
There are many processes used in the industrial removal of water from natural gas.
But the most widely used process is the Absorption by Hygroscopic Liquids. In this process the Glycol with about 1-5% water weight is passed from the top of the absorption tank or "Glycol contactor" and the moisture saturated natural gas is let in from the bottom of the tank and as these both substances come in contact with each other the glycol absorbs the moisture from the natural gas which escapes out now in its pure form (without water ) from the top of the tank into a different chamber and the glycol flows down after absorbing the water into a boiler for regeneration. TEG or triehtylene glycol is the preferred glycol due to its thermal stability.
The natural gas also has some other hydrocarbons such as methane in it. And glycols had affinity for these hydrocarbons. So the glycol has to be treated to remove these gases before being passed into the Boiler chamber for removal of water. One of the method is to add potassium acetate and potassium formate which not only reduce the affinity of glycol towards aromatic hydrocarbons but also increase the hygroscopic quality of the glycol.