Triethylene glycol (TEG) removes water vapor from natural gas by absorbing it into a liquid glycol stream. The water-rich glycol is then heated to boil off the water, and the regenerated glycol goes back into the process. TEG is the most widely used method for meeting pipeline water specifications, which typically call for about 7 lb of water per million standard cubic feet (roughly 112 mg/Sm³) or less.
Why does natural gas need to be dehydrated?
Natural gas leaves the wellhead saturated with water. As the gas cools and its pressure drops along a pipeline, that water can condense. Combined with CO₂ or H₂S, liquid water causes corrosion. Water can also form solid gas hydrates that block valves, meters, and pipelines. Dehydration lowers the water dew point so the gas stays dry at the lowest temperatures and pressures it will meet downstream.
Why is TEG used in Natural Gas Dehydration?
Triethylene glycol is hygroscopic, meaning it attracts and holds water molecules. It also has a high boiling point, about 285 °C at atmospheric pressure. Water can be boiled off during regeneration while the glycol stays liquid, so the same TEG can be reused many times. Compared with ethylene glycol, TEG has a higher boiling point and lower vapor losses, so less glycol is lost to the atmosphere.
How does TEG dehydration work, step by step?
- Inlet separation: Wet gas passes through a separator that removes free liquids and solids before it reaches the dehydrator.
- Absorption in the contactor: Gas enters at the bottom of the contactor and rises through trays or packing. Lean TEG, typically 98.5% to 99.5% pure, flows down from the top. Because gas and glycol move in opposite directions (counter-current flow), the driest glycol meets the driest gas, and the gas leaves with its water content reduced to the target specification.
- Rich glycol leaves the contactor: The rich glycol leaves the bottom carrying absorbed water and some dissolved hydrocarbons. It passes through a flash tank at lower pressure, which releases most of the dissolved hydrocarbons, and these can be used as fuel gas. Filters then remove solids and degradation products.
- Regeneration: A lean-rich heat exchanger warms the rich glycol and cools the lean glycol returning to the contactor. The rich glycol is then heated in a reboiler and still column to about 190 to 204 °C (375 to 400 °F). Water boils off at the top while the TEG stays liquid. The temperature is kept below about 204 °C to limit thermal breakdown of the glycol.
- Lean glycol returns to the contactor: The regenerated glycol is cooled, pumped back to the top of the contactor, and the cycle repeats. In short: wet gas enters the contactor and leaves as dry gas, while rich TEG goes to regeneration and returns as lean TEG.
Read more: What are Industrial Glycols?
What is the difference between lean TEG and rich TEG?
| Rich TEG | Lean TEG | Property |
|---|---|---|
| High | Low | Water content |
| After absorption | After regeneration | Process stage |
| Carries absorbed water to regeneration | Absorbs water from natural gas | Main function |
| Regeneration system | Contactor | Destination |
Lean TEG is regenerated glycol ready for reuse, while rich TEG is glycol that has already absorbed moisture from the gas.
What controls how dry the gas gets?
Four factors largely determine outlet dryness:
- TEG purity. Higher purity gives a lower outlet dew point, which is why regeneration quality matters.
- Circulation rate. A commonly cited rule of thumb is 3 to 6 gallons of triethylene glycol per pound of water removed, though the final figure depends on the design. Too little circulation leaves more moisture in the outlet gas.
- Contactor temperature and pressure. Lower contactor temperatures leave less water in the gas, and higher pressure improves absorption.
- Number of contact stages. More trays or packing give better gas-glycol contact and more complete water transfer.
When deeper drying is needed, stripping gas or vacuum regeneration can raise TEG purity above 99.5%.
What are the common problems in TEG units?
Most operating problems trace back to contamination or heat. Hydrocarbon liquids, solids, and some chemical additives can cause foaming, which reduces contactor efficiency and carries glycol out with the gas. Oxygen, excessive reboiler temperatures, and dissolved salts degrade the glycol over time. Routine filtration, temperature control, and purity checks on the lean glycol prevent most of these issues.
Frequently asked questions
1- What TEG concentration is typical?
Lean TEG is usually 98.5% to 99.5% by weight. Deeper dehydration generally requires purity above 99%.
2- Is TEG consumed during dehydration?
No. TEG circulates continuously between the contactor and the regeneration system. Small quantities are lost through vaporization, liquid carryover, leaks, and maintenance, so the glycol inventory is topped up over time.
3- What water content do pipelines require?
Many pipeline specifications call for about 7 lb per million standard cubic feet (112 mg/Sm³) or less. Each pipeline operator sets its own exact limit.
4- What happens if natural gas is not properly dehydrated?
Excess moisture can cause corrosion, ice formation, and methane hydrate accumulation in pipelines and processing equipment. It can also cause problems in downstream gas treatment and processing.
