Glycol Dehydrator
A glycol dehydrator is a gas processing unit that removes water vapor from natural gas by contacting the wet gas with a lean (water-lean) solution of triethylene glycol (TEG) or diethylene glycol (DEG) in an absorption contactor, where the hygroscopic glycol absorbs water from the gas stream, and then regenerating the rich (water-rich) glycol by heating it in a reboiler to drive off the absorbed water as steam — producing a dry gas stream meeting pipeline or sales specifications for water content (typically less than 4 to 7 lb water per million standard cubic feet of gas) and a regenerated lean glycol solution that is recirculated to the contactor.
Key Takeaways
- Triethylene glycol (TEG) is the dominant glycol type used in natural gas dehydration because it has a higher boiling point (285°C versus 245°C for DEG) that allows regeneration to higher glycol purity (99.5 to 99.9% TEG concentration) without thermal degradation, producing a lean TEG with high water absorption capacity, and it has lower vapor pressure than DEG at typical contactor temperatures (20°C to 60°C), reducing glycol carryover losses in the dehydrated gas stream.
- The water dew point of the dehydrated gas — the temperature at which water would condense from the gas — is the primary performance specification for glycol dehydrators; natural gas pipeline specifications typically require dew points of -10°C to -20°C (corresponding to water contents of 4 to 7 lb water per MMSCF), while LNG feed gas and cryogenic processing requires much lower dew points (-40°C to -70°C) that exceed the capability of conventional glycol dehydration and require molecular sieve (desiccant) dehydration instead.
- The glycol circulation rate (gallons of lean glycol per pound of water absorbed) determines the dehydration efficiency — higher circulation rates push the equilibrium further toward dry gas, reducing the outlet dew point, but increase reboiler heat duty and operating cost; typical design circulation rates are 2 to 5 US gallons of TEG per pound of water removed, with optimization balancing the required dew point specification against energy cost.
- BTEX (benzene, toluene, ethylbenzene, xylene) co-absorption from the gas stream into the glycol is an important environmental and operational issue — these aromatic hydrocarbons are absorbed along with water by TEG in the contactor, then volatilized with water vapor from the reboiler vent, creating a BTEX air emission that may require vapor recovery or combustion treatment to meet environmental regulations, particularly for glycol dehydrators at well sites in sensitive air quality areas.
- Glycol losses in a dehydrator occur through carryover in the gas stream (liquid droplets entrained by the gas leaving the contactor), vaporization of glycol from the reboiler vent, and leaks — typical TEG losses range from 0.01 to 0.1 US gallons per MMSCF of gas processed, with higher losses indicating contactor internals damage, reboiler temperature problems, or excessive gas velocity through the contactor that generates liquid entrainment; glycol losses are monitored as a performance indicator for glycol dehydrator maintenance scheduling.
Fast Facts
The glycol dehydration process for natural gas was commercialized in the 1940s and rapidly became the dominant gas dehydration technology worldwide, displacing solid desiccant (silica gel, alumina) processes because glycol regeneration is simpler and more economical at the flow rates typical of gas processing facilities. Modern glycol dehydrators range in size from small skid-mounted units processing 0.1 MMSCFD at remote wellheads to large contactors at major gas processing plants handling 500 MMSCFD or more. The glycol reboiler temperature — typically 175°C to 205°C for TEG regeneration — determines the lean glycol concentration and is the most important operating variable; too-low temperature produces incompletely regenerated (lean) glycol with reduced absorption capacity, while too-high temperature causes thermal degradation of glycol to corrosive acidic compounds. The Stahl column (stripping gas injection into the reboiler) extends the effective regeneration temperature equivalent by injecting a small stream of dry gas to strip residual water vapor from the hot glycol, achieving lean TEG concentrations above 99.9% without exceeding the thermal degradation temperature.
What Is a Glycol Dehydrator?
Natural gas produced from reservoirs is typically saturated with water vapor at reservoir temperature and pressure. As the gas flows from the reservoir through wellbore tubing, surface flowlines, and processing equipment, temperature and pressure decrease — and the gas's water-holding capacity decreases accordingly. When the gas cools to its dew point, liquid water drops out and can cause: hydrate formation (ice-like gas hydrate crystals that plug pipelines and valves), corrosion acceleration in the presence of CO₂ or H₂S, pipeline slugging and efficiency reduction, and failure to meet the water content specification required for transmission in natural gas pipelines or for sale to LNG facilities.
A glycol dehydrator removes water vapor from gas before these problems occur by exploiting the strong affinity of glycol molecules for water. Glycol is a hygroscopic alcohol — it absorbs water from the gas by chemical hydrogen bonding between glycol hydroxyl groups (-OH) and water molecules. In the contactor vessel, wet gas flows upward through a column of glycol solution flowing downward, and water transfers continuously from the gas phase into the glycol phase until the gas reaches equilibrium with the lean glycol — at a water content determined by the temperature and glycol concentration, which is set by the regeneration system. The water-rich glycol is then heated in a reboiler to drive off the absorbed water as steam, and the regenerated lean glycol is returned to the contactor to repeat the cycle.
Glycol dehydrators are ubiquitous in natural gas production and processing — essentially every natural gas well producing above minor volumes has or shares a glycol dehydrator unit. Their simplicity, reliability, and low operating cost relative to alternatives (desiccant beds, refrigeration units) have made glycol dehydration the default gas dehydration technology for the vast majority of applications where the required dew point can be met by glycol.
Glycol Dehydrator Design and Operation
The absorption contactor is the key vessel in the glycol unit. In a tray contactor (the most common design), a series of bubble-cap or valve trays provides intimate gas-liquid contact as the glycol flows downward from tray to tray while gas flows upward through the downflowing glycol. The number of theoretical stages (equilibrium contacts between gas and glycol) determines the approach to dew point equilibrium — more trays provide closer approach to the equilibrium dew point for a given glycol concentration and circulation rate, but add cost and height. In a packed contactor, structured packing or random packing provides continuous gas-liquid contact area rather than discrete tray stages, offering lower pressure drop and flexibility for difficult foaming or liquid-containing gas streams.
The regeneration system (still column, reboiler, and reflux condenser) processes the rich glycol from the contactor bottom. The rich glycol enters the top of the still column and flows downward as water vapor rises from the reboiler heating glycol at the bottom. The still column functions as a fractional distillation column, stripping water from glycol efficiently, with the reflux condenser at the top condensing some of the overhead vapor to reflux back into the column and prevent glycol carryover in the water vapor vented to atmosphere (or combusted in a vapor recovery unit). The lean glycol from the reboiler is cooled in a lean-rich heat exchanger (heating the incoming rich glycol to recover energy), filtered to remove degradation products and particulates, and pumped back to the contactor.
BTEX vapor recovery is increasingly required at glycol dehydrators near communities or in jurisdictions with stringent volatile organic compound (VOC) emission regulations. BTEX is absorbed from the gas into the glycol in the contactor and released with the water vapor from the reboiler vent — a glycol unit processing gas with 50 ppm BTEX may emit 50 to 200 kg per year of BTEX from the reboiler vent, which is a regulated emission source. Vapor recovery units (VRU) that compress and route the reboiler off-gas back into the gas sales line, or enclosed combustors that burn the BTEX as fuel, are the standard emission control solutions for glycol dehydrator vent streams.
Glycol Dehydrators Across International Jurisdictions
Canada (AER / WCSB): WCSB gas wells and gas processing facilities operate TEG dehydrators under AER Directive 060 (Upstream Petroleum Industry Flaring, Incinerating, and Venting) requirements that regulate BTEX and other VOC emissions from glycol dehydrators. The AER's Directive 039 (Revised Program to Reduce Benzene Emissions from Glycol Dehydrators) specifically targets glycol dehydrator benzene emissions, requiring operators to quantify benzene emissions from glycol dehydrators and implement emission reduction measures when emissions exceed threshold levels. Alberta Environment and Parks regulates criteria air contaminant emissions from processing facilities including glycol dehydrators under the Air Quality Management System and applicable approvals.
United States (API / BSEE): EPA New Source Performance Standards (NSPS) Subpart OOOO and OOOOa (Quad O and Quad Oa) regulate VOC emissions from glycol dehydrators at natural gas production facilities on federal and tribal lands and in areas of the US under NAAQS non-attainment for ozone. Glycol dehydrators with actual VOC emissions above regulatory thresholds must reduce emissions through combustion, recovery, or enhanced condenser systems. The EPA's Natural Gas STAR program publishes best practice guidelines for glycol dehydrator optimization, including reboiler temperature management and Stahl column application to minimize BTEX emissions while maintaining dehydration performance. BSEE offshore glycol dehydrators on Gulf of Mexico platforms are regulated under the Bureau of Ocean Energy Management air quality requirements.
Norway (Sodir / NORSOK): NCS offshore gas processing facilities use glycol dehydration as part of the gas treatment train before export through the Gassled transmission system, with TEG units operating on major platforms (Sleipner, Troll, Ormen Lange) processing export gas to meet the Gassled entry specification for water content (typically 30 ppm or less by volume). Norwegian Environment Agency regulations require minimization of glycol losses and BTEX emissions from NCS dehydration units, with closed-loop glycol regeneration systems and vapor recovery as standard requirements for offshore platform glycol units. NORSOK P-100 (Process Design) and associated process equipment standards provide design requirements for NCS glycol dehydration units.
Middle East (Saudi Aramco): Saudi Aramco's gas processing plants — including the Hawiyah, Uthmaniyah, and Shedgum gas-oil separation plants (GOSPs) — operate glycol dehydration units as part of the natural gas liquids (NGL) recovery and gas treatment train for the Master Gas System. Aramco's gas processing standards specify TEG dehydration requirements for gas entering the pipeline grid, with lean TEG concentration and circulation rate controlled to meet the water dew point specification for the Saudi Aramco gas system. Large-scale glycol regeneration units at major NGL fractionation facilities process hundreds of MMSCFD of gas, requiring sophisticated process control and monitoring to maintain consistent dehydration performance and minimize glycol losses and emissions.
Synonyms and Related Terminology
A glycol dehydrator is also called a glycol unit, TEG unit, gas dehydration unit, or glycol contactor. The regeneration side of the unit may be referred to as the glycol still, reboiler, or regenerator. Related terms include triethylene glycol (TEG), water dew point, gas hydrate, natural gas processing, BTEX, absorption, lean glycol, and gas dehydration. The Stahl column (also called a stripping column or reboiler enhancement) is a configuration addition to the standard glycol regeneration system that injects a small volume of dry stripping gas (typically 2 to 5 SCF per gallon of TEG) into the reboiler to achieve very high lean glycol concentration (greater than 99.95% TEG) without exceeding thermal degradation temperatures.