In almost every gas processing plant, there will be natural gas dehydration unit. Sometimes, it is called dehydration unit (DHU) or dehydrator. When, I was working in Tripatra and involving in Senoro Gas Development Project, the facilities consisted of dehydration unit as one of them.
Natural gas dehydration unit is an important facilities in onshore and offshore gas processing plant. Its function are:
- To mitigate risk of water condensation, which leads to flow capacity issues (pipeline clogging and blocking)
- To prevent hydrate formation or to minimize corrosion
- To ensure smooth operation in downstream facilities. For example, gas pipeline is usually required 4-7 lb/MMSCF water content (87.2-152.6 ppm). For cryogenic unit (to produce LNG), water content in gas shall be less than 1 ppm. For CNG plant, before entering compressor unit, water content shall be reduced to maximum 3 lb/MMSCF to meet product specification.
Natural gas dehydration unit is a facility used to remove water in gas stream. There are at least three commercial methods of natural gas dehydration, which are absorption, adsorption, and condensation.
Absorption
Absorption is the most widely used industrial natural gas dehydration. Absorption is usually performed by using TEG (triethylene glycol). The process consist of absorption (of water in gas stream) and regeneration (of TEG). Absorption occurs at low temperature, while regeneration occurs at high temperature.
Absorption dehydration unit consists of glycol contactor, in form of tray column or packed column. In the contactor, wet natural gas and TEG flow counter-currently. TEG enriched in water flows out the bottom part of contactor, while dry natural gas flows out to the upper part of contactor. At the top part of contactor, filter is sometimes installed. The function is to capture remaining TEG in dry gas stream.
TEG enriched in water flows into flash drum, where the flash gases are released. Then, the TEG enters to cold side of glycol/glycol heat exchanger. Then, TEG enters reboiler, where it is heated by heat transfer fluid to regenerate the TEG. Regenerated TEG is pumped back to hot side of glycol/glycol exchanger and gas/glycol heat exchanger to the top of contactor. The cycle begins again.
Several solids desiccants posses physical characteristics to adsorb water from natural gas. Those are mole sieve, silica gel, and alumina.
Natural gas adsorption is usually consists of two towers, where adsorption occurs in one tower, and regeneration occurs in the other one. Hot gas is used to drive off the absorbed water from the desiccant. Then, the towers is cooled with unheated gas stream. The towers are switched before the on-stream tower become water saturated.
The third conventional dehydration method rely on condensation which turn water molecules into the liquid phase by means of cooling, and then removes them from gas stream. By using this method, natural gas liquids (NGL) and condensed higher hydrocarbons can also be recovered. Therefor, this method is usually applied for simultaneous dehydration and recovery of NGL.
Natural gas can be cooled using the Joule-Thompson effect (JT effect). The JT effect characterized how gas temperature changes with pressure alternation. During gas expansion, kinetic energy decrease, thus resulting in temperature decrease of natural gas. When wet natural gas is cooled, methane hydration formation can also occur. This phenomenon will cause plug flow. To prevent this, methanol or monoethylene glycol (MEG) hydrate inhibitor is usually injected.
Table below shows applicability of each dehydration method, its advantages, and its disadvantages.
When I read several literatures related to natural gas dehydration, the unit used to express gas water content can be varried. It can be in lb/MMSCF, ppm, mg/L, and so on. To make you easy, I share conversion table to convert those natural gas water content. I hope you find it useful.
Reference:
- Natural Gas Dehydration
- Dehydration, GPSA Engineering Data Book
- Technical Note
- Glycol Dehydration
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