Introduction
Natural gas leaving a separator is saturated with water. Left untreated, that water condenses in the pipeline as temperature drops, forms hydrates with light hydrocarbons under pressure, and corrodes carbon steel via dissolved CO₂ or H₂S. Pipeline tariffs typically demand a water content of less than 4–7 lb H₂O / MMSCF (roughly 65–110 mg/Sm³), which corresponds to a water dewpoint well below the lowest expected pipeline temperature.
The standard solution: triethylene glycol (TEG) absorption. It is mature, robust, low-energy, and has been the upstream gas dehydration default for sixty years. But the design has subtleties — get the regenerator wrong and you can never reach the dewpoint; get the contactor wrong and you carry glycol downstream; get the heat integration wrong and you burn unnecessary fuel gas in the reboiler.
This post walks through the four key design decisions: contactor sizing, lean TEG concentration, regenerator design, and lean-rich heat integration.
Why TEG?
Glycols absorb water through hydrogen bonding. Of the three common glycols, TEG sits in the middle:
| Glycol | Boiling point | Typical lean conc. | Notes |
|---|---|---|---|
| Monoethylene (MEG) | 197°C | 80–95 wt% | Used as hydrate inhibitor injected upstream, not regenerated as dehydrator |
| Diethylene (DEG) | 245°C | 95–97 wt% | Older systems, high vapour losses |
| Triethylene (TEG) | 285°C | 99.0–99.95 wt% | Industry standard for dehydration |
| Tetraethylene (TREG) | 327°C | up to 99.99 wt% | Niche — very low dewpoints, but high cost and viscosity |
TEG's high boiling point allows the regenerator to be operated at near-atmospheric pressure with a reboiler at 200–204°C — hot enough to drive water out, cool enough to keep TEG degradation manageable. Above ~206°C, TEG degrades to acidic byproducts that corrode carbon steel.
Step 1 — Contactor Sizing
The contactor (or absorber) is a vertical column with structured packing or trays where lean TEG flows down and wet gas flows up. Key parameters:
Operating Pressure and Temperature
Contactors run at pipeline pressure — typically 50–100 barg. Higher pressure improves absorption (water is more soluble in TEG at higher partial pressures). Inlet gas temperature is typically 30–50°C. Below 30°C, TEG viscosity rises and mass transfer suffers; above 50°C, the equilibrium shifts unfavourably and you need more lean TEG to hit the same dewpoint.
Number of Theoretical Stages
For pipeline-spec dehydration (water dewpoint ~10°C below minimum pipeline temperature), 3 to 4 theoretical stages are typically sufficient. With structured packing, this maps to about 6–8 metres of packed height. With bubble-cap trays, 4–6 actual trays.
TEG Circulation Rate
Calculated from the water mass balance:
TEG rate (kg/h) = Water removed (kg/h) / (X_rich – X_lean)
Where X_rich and X_lean are the water mass fractions in rich and lean TEG. A typical lean-to-rich water uptake is 2–4 wt% — meaning the TEG circulation is roughly 25–50× the water removal rate. A useful rule of thumb: 2–6 gallons of TEG per pound of water removed (US units), or about 17–50 litres per kg.
Higher circulation rates give a closer approach to equilibrium (deeper dewpoint) but burn more reboiler fuel and increase glycol losses.
Diameter — Gas Capacity
Sized to the Souders-Brown equation, the same as a separator:
v_max = K × √((ρL – ρG) / ρG)
For TEG contactors with structured packing, K = 0.10–0.13 ft/s (0.030–0.040 m/s). With bubble-cap trays, K = 0.16–0.20 ft/s. Choose the larger diameter to accommodate operational upset and avoid TEG carryover.
Step 2 — Lean TEG Concentration
This is the dominant lever on water dewpoint. Lean TEG concentration determines the equilibrium water content of the gas leaving the top of the contactor:
| Lean TEG conc. | Approx. dewpoint depression |
|---|---|
| 98.5 wt% | 25–30°C below contactor temperature |
| 99.0 wt% | 35–40°C |
| 99.5 wt% | 50–55°C |
| 99.9 wt% | 65–75°C |
| 99.95 wt% | 80–90°C (with stripping gas) |
A simple atmospheric regenerator (just a reboiler and a still column) maxes out around 98.7–98.9 wt% because the boiling-point ratio of water/TEG at atmospheric pressure limits how dry you can get the regenerated solvent.
To go higher, you need either:
- Stripping gas — fuel gas injected into the regenerator below the reboiler, which lowers the partial pressure of water and shifts the equilibrium
- Vacuum regeneration — operate the regenerator at 50–80 kPa absolute
- Coldfinger / Drizo / Progylsol processes — proprietary stripping schemes using dry gas or solvent
For most onshore pipeline-spec applications, stripping gas is the standard answer. It typically gets you to 99.9 wt% with stripping gas rates of 2–5 standard m³ of gas per m³ of TEG circulated.
Step 3 — Regenerator Design
The regenerator (or "still column") sits on top of the reboiler. Its function: take rich TEG (containing absorbed water), heat it to drive off the water as vapour, and return lean TEG to the contactor.
Key elements:
- Reboiler: fired or heat-medium heated, holding TEG at 200–204°C
- Still column: 3–5 theoretical stages of packing, refluxes water back to the column to minimise TEG losses overhead
- Reflux condenser: a coil at the top of the still where the rising vapour is partially condensed; the condensed water-rich liquid refluxes back, the dry water vapour exits to atmosphere
- Stripping gas inlet: in the bottom of the reboiler or just above the liquid line in the surge drum
Reboiler Temperature Control
This is non-negotiable: reboiler temperature must not exceed 204°C (400°F). At 206°C, TEG starts thermal decomposition, producing acidic byproducts (formaldehyde, acetic acid) that corrode the regenerator and contaminate the lean TEG. A high-temperature shutdown on the reboiler is standard.
TEG Losses
Three loss paths:
- Vapour entrainment from the contactor top — minimised by adequate sizing and a coalescing element above the top tray
- Vapour losses overhead at the regenerator — minimised by good reflux condenser design
- Mechanical losses — leaks, sample points, filter changes
Typical losses are 0.05–0.2 lb TEG per MMSCF gas treated. If your make-up rate is materially higher than this, look for entrainment or thermal degradation first.
Step 4 — Lean-Rich Heat Exchange
The lean TEG leaves the regenerator at ~200°C and must be cooled to contactor temperature (~40°C). The rich TEG leaves the contactor at ~40°C and must be heated to near-boiling for the regenerator. The lean-rich exchanger does both in a single shell-and-tube unit, saving a significant fraction of the reboiler duty.
Typical lean-rich exchanger design:
- Lean TEG cools from 200°C to ~80°C (against rich)
- Rich TEG heats from 40°C to ~150°C (against lean)
- LMTD ~40–50°C
- Shell-and-tube, lean side on the tubes (cleaner stream)
- An additional aerial cooler downstream brings lean TEG to final 35–45°C contactor inlet
A well-designed lean-rich exchanger reduces reboiler duty by 50–60%. Skipping or undersizing it is the single biggest preventable source of inefficient operation.
Process Simulation
In any process simulator, the default Peng-Robinson EOS will not work for TEG dehydration. Use a dedicated Glycol property package or, in some packages, TST (Twu-Sim-Tassone). These are calibrated for the highly non-ideal water-glycol-hydrocarbon system. PR EOS will tell you the unit produces 99% lean glycol; the Glycol package will tell you the truth.
Set up the simulation with:
- Contactor as a column with 3–4 stages, structured packing
- Regenerator as a column with reboiler at 200°C and reflux condenser
- Lean-rich exchanger as a heat exchanger or LNG block
- Make-up TEG and stripping gas as feeds
- Recycle convergence on the lean TEG loop
Tune the simulation against vendor data or — if available — actual plant readings of contactor temperatures, regenerator overhead temperature, and reboiler duty.
Common Pitfalls
- Using PR EOS instead of the Glycol package. The simulation will look plausible and will be quantitatively wrong on dewpoint and TEG losses.
- Pushing the reboiler above 204°C. Drives TEG thermal degradation, acidifies the loop, corrodes carbon steel.
- Undersizing the lean-rich exchanger to save capex. Reboiler duty doubles, fuel gas consumption doubles, and operating cost dwarfs the saving in less than a year.
- Skipping the inlet separator and filter coalescer upstream of the contactor. Hydrocarbon liquid carryover into the TEG causes foaming, capacity loss, and emulsions in the regenerator.
- Ignoring BTEX emissions. TEG absorbs benzene, toluene, ethylbenzene, and xylene from the gas. These come off the regenerator overhead and are increasingly regulated. Some plants need a BTEX recovery condenser.
- Designing for full lean concentration without stripping gas. If you specify 99.5 wt% with no stripping gas, the regenerator cannot deliver and the contactor never hits the spec dewpoint.
Conclusion
TEG dehydration is a mature, forgiving process — but only within its design envelope. The four levers are contactor capacity, lean TEG concentration, regenerator design, and lean-rich heat integration. Get all four right and the unit holds pipeline spec for years with minimal operator intervention. Get one wrong — typically the lean concentration target or the reboiler temperature ceiling — and you spend the life of the asset chasing dewpoint with chemicals or chasing TEG losses with make-up.
The discipline is in the simulation property package, the reboiler temperature limit, and the heat integration. The rest is good vessel design.
