Triethylene Glycol (TEG) 99%: High Performance Liquid Desiccant

Triethylene glycol at 99.0 wt% minimum purity is a high-boiling liquid desiccant identified by CAS 112-27-6 and linear formula C₆H₁₄O₄. The molecular weight is 150.17 g/mol. Water absorption is driven by hydrogen bonding with the terminal hydroxyl groups and ether oxygens, while the vapour pressure remains below 0.01 kPa at 20 °C. The release specification for TEG 99% therefore controls water content to 0.10 wt% maximum, preventing pre-dilution of the desiccant inventory.

The following properties apply to a standard 99% triethylene glycol liquid desiccant.

Property	Typical value	Test method
Purity, wt% minimum	99.0	ASTM E2409
Water, wt% maximum	0.10	ASTM E203
Density at 20 °C, g/cm3	1.1230–1.1260	ASTM D4052
Kinematic viscosity at 20 °C, cSt	47.8	ASTM D445
Boiling point at 101.3 kPa, °C	288	ASTM D1078
Flash point, PMCC, °C	165	ASTM D93
Colour, APHA maximum	15	ASTM D1209

In closed-loop natural gas dehydration, the desiccant is circulated between a high-pressure contactor and an atmospheric regenerator. Lean TEG at 99 wt% purity contacts water-saturated feed gas countercurrently on 6–10 theoretical stages. Feed gas entering at 21–49 °C and 3.4–10.3 MPa is dried to a water content of 48–112 mg/Sm³, equivalent to a water dew point of −10 °C to 5 °C depending on line pressure. Rich TEG leaves the contactor with 3–7 wt% water and is routed to a flash separator operating at 0.35–0.70 MPa to remove dissolved methane and ethane. The flash tank is sized for a liquid residence time of 10–20 min to allow dissolved light hydrocarbons to break out before the still column. The dried gas water dew point is verified by chilled-mirror analysers per ISO 18453, and a margin of 5–10 °C below the pipeline minimum operating temperature is specified to prevent condensation in the line.

What Limits the Achievable Water Dew-Point Depression in a 99% TEG Contactor?

The thermodynamic limit is set by the equilibrium partial pressure of water above the lean desiccant. Water absorption is a physical mass transfer process described by p_H2O = x_H2O γ_H2O p_H2O,sat. For a lean TEG stream at 99 wt%, the water mole fraction is near 0.015, and the activity coefficient is strongly depressed by the concentrated glycol. At 30 °C, the equilibrium water partial pressure can fall below 0.05 kPa, which corresponds to a water dew point below −20 °C at atmospheric pressure. At a contactor pressure of 6.9 MPa, the water content at the same partial pressure is reduced further because water concentration is proportional to partial pressure divided by total pressure. Therefore, high feed purity directly expands the available dehydration driving force before regeneration conditions are considered.

The practical limit is not thermodynamic alone. If the circulation rate is below 16 L TEG per kg of water removed, lean glycol loading becomes excessive and water breakthrough occurs before the full tower height is used. If the circulation rate is above 25 L/kg, the additional reboiler duty and hydrocarbon absorption outweigh the marginal dew-point improvement.

At the regenerator, rich glycol is preheated and flashed; the remaining water is distilled in a still column. Bulk reboiler temperature is held between 177 °C and 204 °C. A lower temperature leaves the lean TEG water content above 1.5 wt% and reduces dehydration capacity. A higher temperature accelerates thermal degradation and increases acid formation. Reboiler heat flux is typically specified at 6.3–9.5 kW/m² to avoid film boiling on the fire tube. In the still column, the overhead temperature is held at 99–102 °C at atmospheric pressure. A reflux coil or condenser keeps the overhead below 105 °C, which reduces glycol vapour losses. The regenerated lean TEG is cooled through a lean-rich exchanger and returned to the contactor.

Thermal Degradation Thresholds in TEG Reboiler Circuits

Thermal degradation above 204 °C proceeds by dehydration and oxidation. Oxygen ingress through pump seals or open storage tanks forms organic peroxides and acetic acid, and the acid catalyses further condensation to higher glycols and dioxane-related compounds. The pH of a 50 vol% aqueous solution falls below 7.0; when the lean glycol pH drops below 6.5, carbon steel corrosion rates can exceed 0.25 mm/year in hot sections. Production units typically maintain pH between 7.0 and 7.5 using triethanolamine or borax-based neutralisers. Over-neutralisation above 8.0 can precipitate iron salts and increase foam stability.

Operating variable	Typical control range	Analytical method or equipment
Reboiler bulk temperature	177–204 °C	thermowell and controller
Lean glycol water	0.5–1.5 wt%	ASTM E203
pH of 50 vol% aqueous solution	7.0–7.5	ASTM E70
Soluble iron	<25 mg/L	ICP-OES
Foam height in contactor	<0.15 m	level transmitter

In rich natural gas service, entrained liquid hydrocarbons condense in the contactor and lower the surface tension of the circulating TEG. Production-scale horizontal three-phase separators in TEG service show foam heights above 0.3 m when slug carryover exceeds the separator residence time. Coalescing filters with 0.3–1.0 µm nominal retention and activated carbon beds are installed on the rich-glycol stream to remove hydrocarbon contamination. Silicone-based antifoam is added at 5–20 mg/L only after compatibility testing, because excess antifoam can blind the activated carbon bed and increase the pressure drop across the filter.

When Regeneration Gas Stripping Is Applied, the Cost-Per-Degree of Additional Dew-Point Depression Must Be Evaluated

Stripping gas reduces the water vapour partial pressure in the regenerator and permits lower lean TEG water content at the same bulk reboiler temperature. With stripping gas injection into a reboiler operating at 200 °C, lean TEG water content can be reduced from 1.0 wt% to 0.5 wt% or lower. The stripping factor is controlled by the gas-to-liquid molar ratio and the humidity of the stripping gas. Increasing stripping gas rate from 0.5 Sm³/h per 100 kg/h lean TEG flow to 1.5 Sm³/h per 100 kg/h can increase TEG carryover into the off-gas by a factor of 2–3. A vertical overhead condenser with a demister is therefore required, and the recovered condensate must be checked for pH and dissolved iron.

In building and industrial air-handling units, TEG 99% has been evaluated as a direct-contact liquid desiccant for latent load control. Published data for this specific configuration is limited; most commercial liquid desiccant cooling systems use lithium chloride or calcium chloride because of lower viscosity and reduced organic contamination. A TEG solution at 95 wt% can reduce air humidity from 14 g/kg dry air to 7 g/kg at 25 °C, but the kinematic viscosity of TEG 99% at 20 °C is 47.8 cSt, which increases wetting and pump work in low-temperature air contactors. Re-concentration of the diluted TEG must be performed at 150–180 °C under vacuum or with stripping air, below the thermal degradation threshold of 204 °C.

Packed Bed Mass Transfer and Liquid Distribution Constraints

In a packed absorber, the effective interfacial area for water removal depends on liquid distribution quality and liquid load. Random or structured packing with a specific surface area of 125–250 m²/m³ is common for TEG contactors. A maldistribution of 5% of the tower cross-section can reduce the realised height equivalent to a theoretical plate by 20–40%, requiring additional packing height or lower gas throughput. The liquid load must be maintained above 10 m³/m²·h to achieve full wetting of the packing surface; below this value, channelling and early water breakthrough occur. Drip-point spacing of 100–150 mm on the liquid distributor is specified for high-viscosity service.

Batch-to-batch variance in TEG 99% is controlled by gas chromatographic purity, water content, colour, and acidity. ASTM E2409 quantifies ethylene glycol, diethylene glycol, and triethylene glycol homologues. ASTM E203 measures water by coulometric or volumetric Karl Fischer titration, and ASTM D4052 measures density. For trace chlorides, combustion ion chromatography with a detection limit of 1 mg/kg is preferred because residual chloride accelerates pitting corrosion in stainless steel reboilers. A production batch with water content above 0.20 wt% should be dried or rejected before charging to a gas dehydration unit, because the additional reboiler duty required to reach 0.5–1.0 wt% lean water can exceed 10% of the base duty.

In closed-loop operation, TEG 99% inventory should be pre-dried or nitrogen-blanketed when ambient humidity exceeds 60% RH to prevent water absorption during storage. The material is incompatible with strong oxidisers such as concentrated nitric acid or permanganates, which can initiate exothermic decomposition. Spent TEG from natural gas service may contain BTEX and must be managed under the applicable hazardous waste regulations; it is not suitable for direct discharge to surface water.