Gas Dehydration is carried out to remove water from the natural gas stream in order to achieve the required water dew point for pipeline specification or for downstream processing. This process is required to prevent hydrates formation at low temperatures or corrosion problems due to the presence of carbon dioxide or hydrogen sulfide (regularly found in natural gas).
Triethylene Glycol (TEG)-based Gas Dehydration is the most widely used process to remove water vapor from gas streams.
In some cases, gas is processed in a cryogenic LPG recovery process which requires a much lower Water Dew Point. The lower dew point is achieved by Molecular Sieve-based Gas Dehydration Unit.
TEG Dehydration Unit
The wet gas is brought into contact with lean glycol in an absorber column. An Absorber column is normally provided with structured packing which provides mass transfer surface.
Water vapor is absorbed in the lean glycol and consequently, the dew point of the gas is reduced. The wet rich glycol then flows from the absorber to a regeneration system in which the entrained gas and moisture is fractionated in a column and Reboiler. The heating allows boiling off the absorbed water vapor and the lean glycol is cooled and pumped back to the absorber.
In order to achieve a higher degree of water removal, fuel gas is used for stripping in the regenerator in order to achieve a higher concentration of glycol.
Normal guarantee achieved using TEG Unit is:
- Water outlet < 3-7 lb/ MMscf (70-150 ppm(vol))
Molecular Sieve Dehydration Unit
The molecular sieve design uses a zeolite which is a crystalline structure containing very small pores of precise uniform size (3-5 Å). The pores in the zeolite act as a "trap" to remove water vapor from the gas as it passes through the zeolite bed.
Basic design consists of two or more identical vessels which are filled with molecular sieve. In a simple two-vessel design, one vessel operates in dehydration mode while the other is in regeneration mode. Switching from dehydration to regeneration is done by use of automatic switching valves. As the vessel that is operating in dehydration mode becomes saturated with water vapor, it is automatically switched to regeneration mode while the regeneration vessel becomes active in dehydration mode. A slip-stream of dried gas is heated and introduced to the vessel which is in regeneration mode, and the heating of the molecular sieve drives off the water.
Normal guarantee achieved using Molecular Sieve Dehydration Unit is
- Water outlet < 0.1 ppm (vol)