Natural gas contains many contaminants, one of them is water. When the gas is transmitted to the surface from processing and finally pipeline transmission, its pressure and temperature reduced naturally in the well string. This reduce the capacity of natural gas to hold water vapor and free water is condensed. The water vapor must be reduced to meet sales gas requirement, which is usually around 2-7 lb/MMscf.
For many years, glycol solutions have been used for natural gas drying. Early glycol dehydration units utilized diethylene glycol (DEG). Triethylene glycol (TEG) came into use around 1950 primarily because its higher boiling point thus gives better separation of water and greater dew point depression without causing thermal decomposition of the glycol. Tetraethylene glycol (T4EG) has been used in some specialized cases, but in majority, triethylene glycol is used.