TEG Dehydration: How It Works

What is TEG Dehydration?

Triethylene Glycol (TEG) is a heavy, clear, highly viscous liquid that is extremely hygroscopic (it naturally attracts and absorbs water). In the natural gas industry, TEG is the universal liquid solvent used to strip water vapor out of raw natural gas to meet strict pipeline dew point specifications.

The TEG Process Loop: A Mass Transfer Masterpiece

A standard TEG unit is a classic absorption and regeneration loop, relying on fundamental mass transfer principles. It consists of two primary sections:

1. The Contactor (Absorption Tower)

Wet natural gas from the inlet separator enters the bottom of a tall, high-pressure vertical vessel called the Contactor (or Absorber). It flows upward through a series of bubble-cap trays or structured packing.

Simultaneously, highly pure "Lean" TEG (>99.0% purity) is pumped into the top of the tower and cascades downward across the trays. As the rising gas and falling TEG intimately mix, the TEG aggressively absorbs the water molecules out of the gas phase.

The dried gas (now called "Sales Gas") exits the top of the tower, ready for the pipeline. The TEG, now saturated with water (called "Rich TEG"), exits the bottom of the tower.

2. The Regenerator (Stripping Still)

TEG is an expensive specialty chemical; it cannot be discarded. To reuse the glycol, the absorbed water must be boiled off.

The high-pressure Rich TEG is depressurized, passed through a rich/lean heat exchanger to recover heat, and enters a distillation column mounted directly on top of a fired Reboiler. The Reboiler operates at near-atmospheric pressure and is heated (via a gas-fired firetube or hot oil coil) to a highly specific temperature: 204°C (400°F).

Why exactly 204°C?

• Water boils at 100°C, meaning it vaporizes easily and vents out the top of the still column as steam.
• TEG boils at a much higher 288°C, meaning it remains safely as a liquid in the reboiler.
• However, if you heat TEG above 206°C (404°F), the molecule begins to thermally degrade, breaking down into highly corrosive organic acids. 204°C is the maximum safe limit.

The regenerated, hot Lean TEG is then cooled, filtered, and pumped back up to the high-pressure Contactor to repeat the endless cycle.

Key Design Rules of Thumb for Process Engineers

• Glycol Circulation Rate: Typically, operators circulate 2 to 4 gallons of TEG for every single pound of water they need to remove from the gas stream.
• Contactor Temperature: The lean TEG entering the top of the contactor must be carefully controlled to be 5°C to 10°C hotter than the incoming gas. If the TEG is colder than the gas, heavy hydrocarbons will condense into the glycol, causing severe foaming and destroying the process.
• Stripping Gas for Deep Dehydration: Standard reboiling at 204°C can only purify TEG to about 98.5-99.0%. If a very low pipeline dew point is required, engineers inject a small amount of dry "stripping gas" into the reboiler. This gas sweeps the remaining water vapor away, boosting the TEG purity to >99.9%.