Dehydrator: Removing Water Vapor From Natural Gas

What Is a Dehydrator?

Dehydrator (also called a gas dehydration unit or glycol dehydrator) is a vessel or process system that removes water vapor from produced natural gas or liquid hydrocarbons to meet pipeline quality specifications and prevent operational problems caused by excess moisture. Water in natural gas forms hydrates — ice-like crystalline solids — that plug pipelines and equipment; it also causes corrosion and freezing in compressors, valves, and instrumentation. The most widely used dehydration technology absorbs water vapor using triethylene glycol (TEG) as a liquid desiccant, though molecular sieve, silica gel, and refrigeration systems are also used depending on the required water dew point and gas composition.

Key Takeaways

Dehydration is required when produced gas contains enough water vapor to form hydrates or exceed pipeline specifications, typically 7 lb/MMscf for gathering systems and 4 lb/MMscf or less for interstate pipelines.
TEG (triethylene glycol) dehydrators are the most common type; lean TEG absorbs water in a contactor column, then the rich TEG is regenerated by heating in a reboiler to drive off the water, and the lean glycol is recirculated.
Molecular sieve dehydrators achieve the lowest water dew points (below −100°F) and are essential upstream of cryogenic gas plants and LNG liquefaction trains.
Glycol dehydrators emit BTEX compounds (benzene, toluene, ethylbenzene, xylene) from the still vent during glycol regeneration — a regulated air emission source in most jurisdictions.
Water dew point — the temperature at which water vapor begins to condense — is the key specification: pipelines require the dew point to be below the coldest expected operating temperature of the line.

Why Gas Dehydration Is Required

Natural gas produced from reservoirs is almost always saturated with water vapor at reservoir temperature and pressure. As the gas cools and its pressure drops during surface processing and transportation, the water can condense as liquid or, at low temperatures and moderate pressures, form natural gas hydrates. Hydrates are not ice — they form at temperatures well above 0°C when sufficient pressure is present, and they can form rapidly in choke valves, subsea flowlines, and pipeline low points. A hydrate plug can take days or weeks to safely dissociate and represents a serious safety and production hazard.

Even below the hydrate formation temperature, liquid water causes corrosion of carbon steel pipelines, particularly in the presence of CO2 (forming carbonic acid) or H2S (forming sulfuric acid). Free water also causes measurement errors in orifice meters and turbine meters if it enters the metering tube. Pipeline tariff agreements specify maximum water content — typically 7 lb water per million standard cubic feet (MMscf) for gathering lines and 4 lb/MMscf or less for transmission pipelines. Failure to meet the water specification can result in the gas being rejected at the custody transfer point or the producer being charged for line drying costs.

Fast Facts: Dehydrator

Most common type: TEG (triethylene glycol) contactor-regenerator
Typical pipeline spec: 4–7 lb water per MMscf
TEG reboiler temperature: 190–205°C (374–401°F)
TEG circulation rate: 1–3 gallons per pound of water removed
Molecular sieve dew point: to −100°F (−73°C) or lower
Refrigeration dew point: typically −20°F to −40°F (−29°C to −40°C)
BTEX emission source: still column vent during TEG regeneration
Key measurement: Water dew point (°F or °C) at line pressure

Field Tip:

Foaming in the TEG contactor is one of the most common dehydrator problems and is caused by liquid hydrocarbons, methanol, corrosion inhibitors, or particulates entering the absorber with the wet gas. Install a gas-liquid separator (inlet scrubber) upstream of the contactor and monitor glycol appearance regularly — dark or foamy glycol indicates contamination. Anti-foam injection can provide short-term relief, but resolving the source of contamination is the only lasting fix.

TEG Contactor Operation

A TEG dehydration system has two main vessels: the absorber (contactor) and the regenerator (reboiler/still column). In the absorber, wet gas enters at the bottom and flows upward through a column of structured packing or bubble-cap trays. Lean TEG (low water content, typically 99%+ by weight) flows downward from the top, countercurrent to the gas. Water vapor transfers from the gas phase into the glycol phase because TEG has a strong affinity for water. The dry gas exits the top of the contactor and passes through a gas-glycol separator to capture glycol mist before entering the sales line.

The rich TEG (now containing 4%–8% absorbed water) flows from the bottom of the contactor through a flash separator where dissolved hydrocarbons are removed, then through a heat exchanger where it is preheated by lean glycol returning from the regenerator. In the still column atop the reboiler, water and hydrocarbons are vaporized and exit through the still vent. The reboiler operates at 190°–204°C (374°–400°F) — a temperature high enough to drive off water but below the decomposition temperature of TEG (~207°C). The regenerated lean glycol is cooled, pumped back to the contactor by a glycol pump (often a Kimray or similar pump), and the cycle repeats continuously.

Alternative Dehydration Technologies

Molecular sieve dehydrators use synthetic zeolite beads with pores precisely sized to adsorb water molecules while excluding most hydrocarbon molecules. They achieve water dew points far below what glycol systems can reach — to −150°F (−101°C) or lower — making them essential for LNG plants, cryogenic NGL extraction, and any process where water would freeze in heat exchangers. Molecular sieves operate in a two-vessel cycle: one vessel adsorbs while the other regenerates using a hot regeneration gas stream. The cycle time is typically 8–16 hours per vessel. Molecular sieves are more complex and costly to operate than glycol systems and are susceptible to degradation from liquid slugs and sour gas components.

Refrigeration dehydration cools the gas stream to a low temperature, condensing water (and heavy hydrocarbons) as liquids that are separated in a low-temperature separator. Mechanical refrigeration (propane or Joule-Thomson expansion) achieves dew points of −20°F to −40°F, adequate for most gas processing plants but not for cryogenic service. Refrigeration dehydration is often used in combination with glycol injection (injecting TEG or MEG upstream of the choke) to prevent hydrates during the cooldown process. Silica gel dehydrators, used primarily for instrument gas or small-volume applications, are solid desiccant systems that adsorb water and are regenerated by heated gas — simpler than molecular sieve but with higher dew points.

Dehydrator is also referred to as:

glycol dehydrator — specifies the TEG-based absorption type; the most common shorthand in field operations
gas dehydration unit (GDU) — formal process engineering designation used in design documentation and permit applications
TEG unit — field abbreviation referencing the triethylene glycol desiccant specifically
desiccant dehydrator — broader term covering molecular sieve and silica gel solid-desiccant systems, as distinct from glycol liquid-desiccant systems

Frequently Asked Questions About Dehydrators

What is the difference between water dew point and hydrocarbon dew point?

Water dew point is the temperature at which water vapor begins to condense from the gas stream at a given pressure. Hydrocarbon dew point is the temperature at which the heaviest hydrocarbon components begin to condense. Both specifications are important for pipeline quality: water dew point is controlled by dehydration, while hydrocarbon dew point is controlled by NGL extraction in a gas plant. Pipeline tariffs may specify both limits because condensed hydrocarbons can damage compressors and alter the gas heating value, similar to the problems caused by liquid water.

Why is BTEX a concern with glycol dehydrators?

When rich TEG is regenerated in the reboiler, aromatic hydrocarbons — benzene, toluene, ethylbenzene, and xylene (BTEX) — that were absorbed along with the water are driven off with the still vent vapor. Benzene is a known human carcinogen and is regulated as a hazardous air pollutant under the U.S. EPA's New Source Performance Standards (NSPS) for oil and gas (40 CFR Part 60, Subpart OOOO/OOOOa). Operators must quantify BTEX emissions from glycol dehydrators in air quality permits and may be required to route still vent vapors to a combustor (enclosed flare) or condenser to reduce emissions below threshold levels.

How do operators know when to replace TEG?

Glycol quality is monitored by measuring the lean glycol concentration (target: 99.0%–99.9% by weight), pH (target: 6.5–8.0), and appearance. Dark coloration indicates thermal degradation or hydrocarbon contamination. Acidic pH accelerates corrosion of the still column and reboiler. Glycol that has accumulated salts, iron, or degradation products (glycolate salts) must be reclaimed by a glycol reclaimer — a small distillation unit that purges contaminants — or replaced entirely. Most operators send glycol samples to a laboratory quarterly for full compositional analysis and track glycol losses (target: less than 1 gallon per MMscf processed) as an operating KPI.

Why Dehydrators Matter in Oil and Gas

Dehydration is a non-negotiable step in the natural gas value chain. Every molecule of natural gas sold through a pipeline in North America has passed through some form of dehydration system before reaching the custody transfer meter. Without reliable dehydration, pipelines hydrate and plug, compressors corrode and fail, and LNG and NGL plants cannot operate safely. As gas production from tight formations and deepwater fields continues to grow, and as LNG export capacity expands globally, the demand for reliable, efficient dehydration — from field-edge TEG skids to large molecular sieve beds at liquefaction terminals — will grow with it.