Why is TEG preferred over the shorter glycols for absorption drying?

It is a compromise that happens to sit in the right place. Monoethylene glycol and diethylene glycol are more volatile, so more of the inventory leaves with the dry gas at contactor conditions, and they cannot be regenerated as far before decomposition sets in. Tetraethylene glycol is less volatile still but is more viscous and costs more, which matters when a plant holds a large inventory. Triethylene glycol can be reconcentrated to a lean purity high enough to give a useful dew point depression while keeping vapour losses and pumping duty acceptable, and decades of operating experience exist for it.

Where does glycol actually go when losses are high?

Rarely into a single leak. The usual contributors are vaporisation with the dry gas leaving the contactor, mechanical carry over as droplets when the mist eliminator is damaged or the gas velocity is above design, entrainment out of the still column when the reboiler is over fired or the reflux is inadequate, losses with liquid drawn from the flash vessel, and pump seal leakage. Foaming multiplies carry over dramatically. A loss investigation that starts with a mass balance across each vessel finds the cause faster than one that starts by replacing equipment.

Why does the reboiler have a maximum operating temperature?

Because thermal degradation of the glycol begins below the temperature at which complete water removal would be possible, so the regenerator is limited by chemistry rather than by heat input. Pushed past that point the glycol darkens, its acidity rises and its decomposition products attack the metallurgy and promote foaming. That is why plants needing a deeper dew point depression use stripping gas, a vacuum still or an enhanced regeneration scheme instead of more fire tube duty. The specific limit belongs to the licensor and the equipment data sheet.

What does persistent foaming in the contactor indicate?

Contamination, almost always. Hydrocarbon condensate, compressor lubricant, corrosion inhibitor or other production chemicals carried over from upstream, fine solids and degradation products all stabilise foam. Antifoam addition treats the symptom and is justified as a temporary measure, but the durable fix is inlet separation that works, effective filtration of the circulating glycol, and control over which chemicals are injected upstream of the contactor. Foaming is worth chasing hard because it drives carry over losses and off specification gas at the same time.

Which analyses on a circulating glycol sample are worth running routinely?

Water content of the lean glycol, which is the direct measure of regeneration performance; pH, because acidity signals degradation and predicts corrosion; chlorides and total dissolved solids, which reveal carry over of produced water or brine; iron, as a corrosion indicator; solids content and particle sizing to judge filter performance; and hydrocarbon content to detect inlet separation problems. Trending these over time is far more informative than any single result, because most system failures announce themselves as a drift long before an alarm.

Triethylene glycol in gas dehydration: a field guide

Technical article · Eapearl Chemical · 11 May 2026

Almost every natural gas stream that reaches a pipeline has passed through a glycol contactor, and in most cases the liquid in that contactor was triethylene glycol. This article explains what the material does in the field, how the loop that holds it behaves, and which operating decisions protect the inventory.

The problem TEG is there to solve

Gas leaving a reservoir is saturated with water. Left in place, that water condenses as the stream cools and expands, and it causes three separate failures. It forms solid gas hydrates that plug valves, chokes and pipelines. It combines with acid gases to corrode carbon steel from the inside. It occupies pipeline capacity and reaches a customer who specified dry gas. Pipeline operators therefore impose a water content or dew point specification at the custody transfer point, and meeting that specification is the dehydration unit’s only job.

Triethylene glycol, CAS 112-27-6, molar mass 150.17 g/mol, is the material chosen for it. It is a clear, hygroscopic, high boiling liquid that is completely miscible with water and has a strong affinity for it. Contrast that with the role of ethylene glycol, which is injected into cold lines to suppress hydrate formation rather than to dry a gas stream in a tower. The two duties are often confused in conversation, and they call for different glycols and different equipment.

How the loop works

Absorption

Wet gas enters the base of a contactor, usually a trayed or packed column, and rises against lean glycol fed at the top. Water transfers into the liquid phase, dry gas leaves the top through a mist eliminator, and rich glycol leaves the bottom. The dew point depression achieved depends on three levers: the purity of the lean glycol, the circulation rate, and the number of theoretical stages in the tower. Of these, lean purity is by far the most powerful, which is why attention properly concentrates on the regenerator rather than on the pump.

Regeneration

Rich glycol is let down in pressure through a flash vessel, where absorbed hydrocarbon gas is released and can be recovered as fuel instead of being vented. It then passes through filtration and heat exchange into a still column mounted on a fired or heated reboiler, where water is boiled off and leaves overhead. The reconcentrated lean glycol is cooled against the incoming rich stream and returned to the top of the contactor. Where a deeper dew point is required, dry stripping gas is introduced into the reboiler or a packed stripping section below it, shifting the equilibrium and raising lean purity without raising temperature.

Filtration, and why it is not optional

A glycol loop accumulates two kinds of dirt: particulate matter from corrosion and formation solids, and dissolved hydrocarbons and degradation products. Particulate filtration protects pumps, exchangers and tower internals; an activated carbon bed removes the dissolved organics that cause foaming and darkening. Carbon is frequently the first element neglected during a cost reduction exercise and the first cause found during a foaming investigation. Filter differential pressure and carbon change out should be scheduled items, not responses to a problem.

What degrades the inventory

Thermal degradation — overheating in the reboiler, particularly at a fire tube with a hot spot or fouled surface, breaks the glycol down into acidic fragments.
Oxidation — air entering through a leaking pump seal, an unblanketed surge tank or a poorly designed storage arrangement darkens the glycol and raises acidity.
Acid gas pickup — carbon dioxide and hydrogen sulphide dissolve in the circulating liquid and lower its pH, accelerating corrosion of the loop.
Salt contamination — produced water carried past the inlet separator leaves brine solids behind, which deposit on fire tubes, cause local overheating and foul exchangers.
Hydrocarbon and chemical carry over — condensate, lubricant and upstream treatment chemicals promote foam and blind carbon beds.

Neutralising an acidic inventory with amine additions is a recognised remedy but a limited one; it does not remove the salts or the solids, and repeated dosing without addressing the cause ends with a tank of material that has to be replaced or reclaimed. Where degradation is advanced, vacuum reclamation or replacement against a fresh specification is the honest answer.

Buying and receiving glycol for field service

Glycol delivered to a remote site is rarely re tested before it goes into a loop, which makes what arrives on the truck important. A serious enquiry should settle the following before shipment.

Product identity and grade, distinguishing it clearly from diethylene glycol and tetraethylene glycol, which look the same in a drum.
A lot specific certificate reporting purity, water, colour, acidity or pH, ash and chlorides, with methods stated.
Packaging and prior cargo history for bulk movements, since a tank last carrying a different product can contaminate an entire charge.
A safety data sheet issued for the destination market, and transport documentation appropriate to the mode.
Whether the material is virgin or reclaimed, declared explicitly, because reclaimed glycol can be entirely fit for service and should still be identified as such.

Emissions and the flash vessel

The still overhead from a glycol regenerator is not pure water vapour. Aromatic components absorbed in the contactor, principally benzene, toluene, ethylbenzene and xylenes, follow the water out of the top of the column, along with light hydrocarbons that were not released earlier. Two design decisions govern how much of that reaches the atmosphere. The first is whether a flash vessel exists and is operated properly, since gas flashed there can be routed to fuel instead of travelling to the still. The second is whether the still overhead is condensed and separated, with the recovered hydrocarbon returned to the process and the vapour routed to a flare, a thermal oxidiser or a vapour recovery unit.

Permitting requirements differ by country and by installation size, and they have tightened in many jurisdictions over recent years, so the applicable limits and reporting duties are confirmed with the environmental regulator for the site. What is constant is the engineering logic: hydrocarbon recovered at the flash vessel is both an emission avoided and a fuel gained, which makes that vessel one of the better returns on attention in the whole unit.

Other field uses

Beyond dehydration, triethylene glycol appears in the same industry as a heat transfer medium in indirect heaters and utility loops, and as a desiccant in smaller skid mounted packages serving wellhead and compressor stations. In general industry the same properties support its use as a humectant, a plasticiser for cellulosic materials, a solvent for resins and dyes, and a component of air disinfection and moisture control systems. Those applications value low volatility and water affinity for exactly the reasons the gas plant does; see triethylene glycol for available grades.

Operating habits that protect the loop

Trend the lean glycol water content rather than reacting to it. Keep circulation at the rate the tower was designed for, since over circulation costs reboiler duty and increases losses without improving the dew point much. Watch the flash vessel, because hydrocarbon that leaves there does not reach the still. Treat every antifoam addition as an incident report rather than a routine dose. And sample regularly, from the same points, so that a slow drift is visible while it is still cheap to correct. Technical questions on grade selection can be raised through contact.