OilAuthority.com

Liquid Desiccant

What Is a Liquid Desiccant?

Liquid desiccant (also called glycol dehydration fluid or hygroscopic absorption liquid) is a water-absorbing (hygroscopic) liquid used in gas dehydration systems to remove water vapor from natural gas streams before pipeline transmission, LNG liquefaction, or downstream processing. Triethylene glycol (TEG) is by far the most widely deployed liquid desiccant in the oil and gas industry; diethylene glycol (DEG) and ethylene glycol (MEG) are used in specific low-temperature or inhibition applications. Effective gas dehydration is essential because water vapor in a gas stream can form hydrate ice plugs at pipeline pressures and temperatures, cause severe corrosion when combined with carbon dioxide or hydrogen sulfide, and freeze solid inside LNG heat exchangers, damaging cryogenic equipment.

Key Takeaways

  • TEG is the dominant liquid desiccant for pipeline gas dehydration, typically achieving dew-point suppressions of 40 to 70 degrees Celsius in a standard absorber-reboiler unit.
  • The glycol dehydration cycle consists of four main steps: absorption in a contactor column, flash separation, filtration, and regeneration in a reboiler heated to approximately 204 degrees Celsius (400 degrees Fahrenheit).
  • Glycol circulation rates are typically 2 to 3 US gallons of lean TEG per pound of water removed, with lower rates risking inadequate dew-point suppression.
  • TEG dehydration units emit BTEX compounds (benzene, toluene, ethylbenzene, and xylene) from the reboiler still column, creating air quality and regulatory compliance obligations.
  • Molecular sieves (solid desiccants) are used instead of liquid desiccants where very deep dew-point suppression is required, such as upstream of LNG liquefaction trains.

How Liquid Desiccant Dehydration Works

A conventional TEG dehydration unit operates as a closed-loop absorption-regeneration cycle. Wet gas entering the unit first passes through an inlet separator to remove any free liquid water or hydrocarbon condensate droplets, which would otherwise contaminate and dilute the glycol. The cleaned gas stream then flows upward through a vertical absorber contactor column, which contains structured packing or valve trays designed to maximize intimate contact between the rising gas and the descending lean (dry) TEG solution. Lean TEG, typically at a concentration of 99 to 99.5 weight percent TEG with the balance being water, absorbs water vapor from the gas as the two phases contact each other on each tray or packing element. The dried (dehydrated) gas exits the top of the absorber and enters the sales pipeline or downstream processing unit, while the now water-laden rich TEG collects at the bottom of the contactor.

The rich TEG, containing approximately 4 to 6 weight percent water, flows from the absorber bottom through a glycol-glycol heat exchanger where it is pre-heated by the hot lean TEG returning from regeneration, then enters a flash tank operating at low pressure (typically 50 to 100 psig). In the flash tank, dissolved light hydrocarbons and some BTEX compounds that were co-absorbed in the contactor vaporize and are separated. The flashed gas can be used as fuel gas or directed to a vapor recovery unit. The pre-heated rich glycol then flows to the regeneration (reboiler) system, which consists of a still column sitting atop a fire-tube or direct-fired reboiler vessel. At the reboiler operating temperature of approximately 177 to 204 degrees Celsius (350 to 400 degrees Fahrenheit), water and remaining light hydrocarbons are boiled off the rich TEG through the still column, where rising vapors contact a small stream of stripping gas or a reflux condenser to improve separation. Lean TEG leaving the reboiler at high concentration is cooled through the glycol-glycol heat exchanger, pumped back to contactor pressure, and re-injected into the top of the absorber to complete the cycle.

Typical TEG losses from a properly maintained unit are 0.05 to 0.10 US gallons per million standard cubic feet (MMscf) of gas processed. Losses above this threshold indicate mechanical issues such as foam carryover from the absorber, deteriorated still column packing, or pump seal leaks. Glycol degradation products (organic acids and polymers) accumulate over time and must be monitored through periodic glycol sampling and analysis; heavily degraded glycol causes foaming, reduced absorption efficiency, and accelerated corrosion of carbon steel vessel walls.

Fast Facts: Liquid Desiccant (TEG Dehydration)
  • Most common desiccant: triethylene glycol (TEG), molecular formula C6H14O4
  • Reboiler operating temperature: 177 to 204 degrees Celsius (350 to 400 degrees Fahrenheit)
  • Typical lean TEG concentration: 99.0 to 99.9 weight percent
  • Circulation rate: 2 to 3 US gallons lean TEG per pound of water removed
  • Typical dew-point suppression: 40 to 70 degrees Celsius with standard TEG; up to 90 degrees Celsius with Drizo or Coldfinger enhancement
  • Normal TEG losses: 0.05 to 0.10 gal/MMscf gas treated
  • BTEX concern: benzene, toluene, ethylbenzene, xylene emitted at still column vent
  • Contactor pressure range: typically 300 to 1,200 psig for pipeline applications
Field Tip:

Foaming in a TEG absorber is the single most common operational problem and can reduce dehydration efficiency dramatically by carrying rich TEG droplets out of the contactor with the gas stream (glycol carryover). Foaming causes are typically hydrocarbon liquid carryover from a failing inlet separator, methanol or corrosion inhibitor injection upstream, or buildup of degradation products in the glycol. Before adding antifoam chemical (typically silicone-based), diagnose the root cause. Persistent antifoam addition without fixing the underlying problem masks the issue while gradually reducing glycol quality, eventually requiring a costly unit shutdown for system cleaning and glycol replacement.

BTEX Emissions and Environmental Considerations

The principal environmental concern associated with liquid desiccant dehydration is the emission of BTEX (benzene, toluene, ethylbenzene, and xylene) compounds from the reboiler still column vent. These aromatic hydrocarbons co-absorb in the glycol contactor along with water vapor and are volatilized during the high-temperature regeneration step. Benzene is a known human carcinogen; even small uncontrolled still vent emissions can trigger regulatory thresholds under the U.S. EPA's National Emission Standards for Hazardous Air Pollutants (NESHAP) for oil and gas production (40 CFR Part 63, Subpart HH). Common mitigation approaches include routing still column vapors to an enclosed combustor or flare (thermal oxidizer), using the Drizo regeneration process where hydrocarbon stripping solvent recovers BTEX from the still overhead before incineration, or switching to molecular sieve dehydration which does not produce BTEX still vent emissions. Operators in EPA-regulated areas are required to calculate annual site-level benzene emissions from all glycol dehydrators and report quantities above the 1-tonne-per-year (TPY) major source threshold.

  • glycol dehydration unit (GDU) -- the complete packaged skid containing the absorber, reboiler, flash tank, and associated equipment
  • TEG unit -- shorthand for a triethylene glycol dehydration system, the most common configuration encountered in the field
  • lean glycol -- regenerated, concentrated TEG returning to the absorber; contrasted with rich glycol leaving the absorber bottom
  • rich glycol -- water-laden TEG solution leaving the contactor bottom and flowing to the reboiler for regeneration

Related terms: gas dehydration, molecular sieve, hydrate, gas processing plant, dew point

Frequently Asked Questions About Liquid Desiccant

When should molecular sieves be used instead of TEG dehydration?

TEG dehydration is adequate for most pipeline gas specifications, achieving water content below 7 pounds per million standard cubic feet (lb/MMscf), corresponding to a dew point of approximately -10 degrees Celsius at pipeline pressure. However, LNG liquefaction trains require water content below 0.1 parts per million by volume (ppmv) to prevent ice formation in cryogenic exchangers, and some deep dew-point suppression applications (NGL recovery, LPG extraction) require extremely dry gas well below what TEG can achieve economically. In these cases, molecular sieves, solid alumino-silicate desiccants with controlled pore sizes of 3 to 4 angstroms, are used in temperature-swing adsorption (TSA) cycles. Molecular sieves achieve dew points below -100 degrees Celsius but require capital-intensive multiple-bed switching systems and periodic thermal regeneration at high temperatures using hot dry gas.

What is the Drizo process and how does it reduce emissions?

The Drizo process (licensed by Sivalls Inc.) is an enhanced TEG regeneration method that uses a hydrocarbon solvent, typically a natural gasoline fraction condensed from the still overhead, as a stripping medium in the reboiler still column instead of or in addition to natural gas stripping. The solvent strips BTEX and light hydrocarbons from the hot rich glycol more effectively than gas stripping alone, achieving lean TEG concentrations of 99.99 percent or higher (versus the approximately 99.0 to 99.5 percent achievable with conventional gas-fired reboilers). The BTEX-laden solvent vapors leaving the still column are then routed to a condenser and a small reflux separator where the BTEX-rich condensate is recovered as a liquid hydrocarbon product (usable as natural gasoline) rather than being vented or burned. This dramatically reduces atmospheric BTEX emissions while simultaneously improving glycol purity and gas dehydration performance.

How do operators know when TEG needs to be replaced or reclaimed?

Glycol quality is monitored through periodic sampling and laboratory analysis, typically quarterly for active units. Key indicators of glycol degradation include: pH below 6.5 (indicating organic acid buildup from thermal degradation or bacterial contamination), color darkening from straw-yellow to dark brown or black (oxidation products or iron sulfide contamination), high total dissolved solids or chloride content (indicating water quality issues or produced brine carryover), and viscosity increase (indicating polymer degradation products). When degradation products reach levels that cause persistent foaming or efficiency loss despite antifoam addition, glycol reclaiming is required: a slipstream of the glycol inventory is processed through a reclaimer vessel operating above the normal reboiler temperature (around 204 degrees Celsius) that boils clean TEG overhead while leaving degradation products, salts, and heavy contaminants in a concentrated bottoms residue for disposal. Severely degraded glycol inventories that cannot be adequately reclaimed on-site are replaced entirely.

Why Liquid Desiccant Matters in Oil and Gas

Gas dehydration using liquid desiccants is a non-optional step in nearly every natural gas gathering, processing, and transmission system worldwide. Water vapor in gas causes hydrate formation at the pressure and temperature conditions routinely encountered in subsea flowlines, transmission pipelines, and processing equipment, and even small quantities of liquid water with dissolved CO2 or H2S create strongly corrosive conditions that rapidly degrade carbon steel pipe. TEG dehydration units are simple, reliable, and low-maintenance relative to alternatives, which is why an estimated 40,000 or more glycol dehydration units are in operation in North America alone. For operators, optimizing glycol circulation rates, monitoring regeneration temperature, controlling BTEX emissions, and maintaining glycol quality directly affects both operating costs and regulatory compliance status. Understanding TEG dehydration fundamentals is a core competency for production engineers, facility engineers, and field operators managing gas handling systems at any scale.

Browse the Oil and Gas Glossary