TEG Regenerator Design — Reboiler Duty, Stripping Gas, and Lean Glycol Purity

Regeneration Sets the Dehydration Limit

The gas contactor gets most of the attention in a triethylene glycol dehydration unit, but the regenerator sets the performance ceiling. The contactor can only approach the equilibrium established by the glycol entering its top. If the regeneration package consistently produces 98.7 wt% lean TEG, increasing the contactor height or circulating more glycol will not make it behave like a 99.9 wt% system.

Regenerator design therefore begins with the required lean TEG water content, not with a standard reboiler size. That purity must be linked to the dry-gas water specification through the contactor model, using the actual pressure, temperature, stage efficiency, circulation rate, gas composition, and turndown cases.

This article develops the regeneration side in detail: how the still and reboiler remove water, what determines duty, why stripping gas increases purity, and how heat recovery and emissions control affect the final design. For the complete absorption and circulation loop, start with TEG Gas Dehydration — Designing the Contactor and Regenerator.

From Dry-Gas Specification to Lean TEG Purity

The required product-gas water content is the commercial requirement. Lean TEG concentration is an intermediate design variable selected to meet it with adequate operating margin.

A sound design sequence is:

1. Define the dry-gas water specification or water-dewpoint requirement.
2. Establish the governing contactor pressure and temperature.
3. Model the contactor with realistic stages or packing performance.
4. Test circulation rate and lean TEG concentration together.
5. Select the lean purity that meets specification across the operating envelope.
6. Design the regeneration system to deliver that purity continuously—not only at ideal conditions.

Lean purity and circulation rate are not interchangeable without limit. Increasing circulation exposes the gas to more glycol and can improve approach to equilibrium, but it also increases pump load, rich-glycol flashing, reboiler duty, still-vapour traffic, hydrocarbon absorption, and glycol losses. Once the gas approaches equilibrium with the lean solvent, additional circulation produces little benefit.

Typical performance regions are useful for concept screening:

These are not guarantees. Actual performance depends on pressure, temperature, still design, stripping rate, water loading, circulation, and the thermodynamic method used. The selected concentration must come from a validated glycol model or vendor performance basis.

What Happens Inside the Regenerator

Rich glycol leaves the contactor carrying absorbed water, dissolved gas, and soluble hydrocarbons. A typical regeneration path contains:

• A pressure letdown and flash separator to recover most dissolved light gas
• Particle and activated-carbon filtration where required
• A lean-rich exchanger that preheats rich glycol using hot regenerated glycol
• A packed still column that provides water–TEG separation and limits glycol overhead loss
• A reboiler that supplies the energy for desorption and vaporisation
• A surge or stripping section that provides residence time and, where fitted, counter-current stripping-gas contact
• Lean-glycol cooling before the solvent returns to the contactor

In the reboiler, the glycol–water mixture approaches its boiling condition. Water-rich vapour rises through the still packing while liquid returns as reflux. The column is not intended to boil TEG overhead; it is intended to remove water selectively while retaining glycol in the circulating inventory.

The still overhead contains more than water vapour. TEG also absorbs methane and aromatic hydrocarbons from the wet gas. Remaining methane, benzene, toluene, ethylbenzene, xylenes, and other volatile compounds can leave through the still vent unless they are separated or controlled. The flash tank, condenser, separator, vapour recovery, fuel routing, or flare arrangement is therefore part of regenerator design—not an environmental add-on at the end.

Why Atmospheric Regeneration Reaches a Limit

Raising temperature increases water removal, but the reboiler cannot be heated indefinitely. TEG is normally regenerated near atmospheric pressure with the bulk liquid controlled around 198–204°C. The exact operating target depends on the package and glycol condition, but sustained overheating accelerates thermal degradation and produces acidic degradation products, colour, fouling, and corrosion.

Once the allowable bulk temperature is reached, a conventional atmospheric reboiler cannot reduce the water partial pressure enough to keep increasing lean purity. More firing may increase vapour traffic and degradation without delivering the intended concentration.

There are three principal ways to move beyond this limit:

• Add stripping gas, reducing the water partial pressure in the vapour phase
• Reduce regenerator pressure using vacuum, allowing water removal at a lower temperature
• Use enhanced regeneration, such as proprietary vapour stripping or solvent-based arrangements

The correct choice depends on the required purity, available utilities, emissions constraints, offshore weight and complexity, turndown, maintenance philosophy, and value of dry gas.

Reboiler Duty: Build the Heat Balance

Reboiler duty is the sum of several loads. Treating it as only “the heat required to boil the absorbed water” will undersize the package.

The principal terms are:

1. Sensible heating of the rich TEG

The rich stream must be heated from its temperature after heat recovery to the reboiler operating temperature:

QTEG = mTEG × Cp,TEG × (Treboiler − Trich,in)

Because the glycol circulation rate is much larger than the absorbed-water rate, this term can dominate when lean-rich heat recovery is poor.

2. Sensible heating of absorbed water and other components

Water, dissolved hydrocarbons, and any stripping gas entering below reboiler temperature must also be heated.

3. Water vaporisation and desorption

Absorbed water must be separated from the glycol and carried overhead:

Qwater ≈ mwater × ΔHvap,water

The final calculation should use mixture enthalpies from the selected thermodynamic model rather than a single pure-water latent heat.

4. Reflux and still-column load

The vapour and liquid traffic needed for separation adds to the internal duty. Excessive reflux protects against glycol loss but increases energy consumption.

5. Heat loss

The reboiler shell, still, piping, and surge section lose heat to ambient conditions. This can be significant for small outdoor packages and should be included explicitly.

The process duty is converted to required heat input using the heater or heat-medium efficiency:

Required heat input = process duty / thermal efficiency

A direct-fired reboiler also requires checks on fire-tube heat flux, flame stability, burner turndown, stack temperature, and local hot spots. Average bulk temperature below the limit does not protect glycol that is being overheated at a poorly wetted fire-tube surface.

Worked Reboiler-Duty Example

Consider a gas-dehydration unit treating 20 MMSCFD. Wet gas contains 70 lb water/MMSCF, and the dry-gas target is 7 lb/MMSCF.

Water removal is:

(70 − 7) × 20 = 1,260 lb/day
                    = 23.8 kg/h

At a selected circulation of 3 US gal TEG/lb water, the glycol rate is:

1,260 × 3 = 3,780 US gal/day
          = 157.5 US gal/h
          ≈ 0.60 m³/h

Using an approximate hot-glycol density of 1,120 kg/m³ gives a circulating glycol mass rate of roughly 670 kg/h. Assume the lean-rich exchanger preheats rich TEG to 150°C and the reboiler operates at 202°C. With an average glycol-rich-stream heat capacity of 2.4 kJ/kg·K:

QTEG = 670 × 2.4 × (202 − 150)
     ≈ 83,600 kJ/h
     ≈ 23.2 kW

Using 2,100 kJ/kg as a screening enthalpy for water heating, desorption, and vaporisation:

Qwater = 23.8 × 2,100
       ≈ 50,000 kJ/h
       ≈ 13.9 kW

Adding 20% for still-column effects, other sensible loads, and heat loss gives a screening process duty of:

Qprocess ≈ (23.2 + 13.9) × 1.20
         ≈ 44.5 kW

At 75% fired thermal efficiency, required burner input is approximately:

Qfuel = 44.5 / 0.75 ≈ 59 kW

This is a screening calculation, not a vendor reboiler specification. The final duty must come from a converged TEG circulation model with the actual fluid, reflux, stripping gas, heat losses, exchanger performance, and design margin. Its value is in showing where the energy goes: even with strong heat recovery, heating the circulating glycol is comparable to vaporising the absorbed water.

Lean-Rich Heat Recovery Controls Fuel Use

The lean-rich exchanger is the main energy-recovery device in the loop. Hot lean TEG leaving regeneration preheats the rich stream while being cooled for its return to the absorber.

If the exchanger underperforms, two problems appear simultaneously:

• Rich TEG reaches the reboiler colder, increasing fired or heat-medium duty
• Lean TEG leaves the exchanger hotter, increasing the duty of the final glycol cooler

The exchanger should be evaluated across clean, fouled, design-rate, and turndown conditions. Important design checks include:

• Minimum temperature approach at both ends
• Viscosity and pressure drop, particularly during cold start-up
• Fouling from corrosion products, degradation solids, and heavy hydrocarbons
• Allowable pressure differential between lean and rich circuits
• Control arrangement during start-up and low circulation
• Thermal expansion and relief of blocked-in glycol

A declining rich-glycol outlet temperature at constant duty is an early sign of lost heat recovery. Before increasing reboiler firing, confirm exchanger bypass position, temperature instrumentation, fouling, flow distribution, and glycol circulation rate. The calculation method is developed further in Sizing a Shell-and-Tube Heat Exchanger.

How Stripping Gas Raises Lean Purity

Stripping gas does not dry TEG by reacting with water. It provides additional water-free vapour flow and lowers the water partial pressure above the hot glycol. This shifts the equilibrium so more water transfers from the liquid into the vapour at the same bulk temperature.

For effective stripping, the gas must contact the hot glycol in a suitable packed or contacting section. A gas line bubbling into a poorly mixed vessel is not equivalent to a designed stripping column. The arrangement should establish counter-current contact, adequate residence time, stable distribution, and low glycol entrainment.

Stripping-gas demand must be determined from the required purity and the specific regenerator configuration. A concept study may screen rates in the order of several standard cubic feet per US gallon of circulating TEG, but a universal ratio is not a design method. The final rate depends on:

• Target lean TEG concentration
• Reboiler temperature and pressure
• Rich-glycol water loading
• TEG circulation rate
• Number and efficiency of stripping stages
• Stripping-gas water content
• Hydrocarbon composition and available supply pressure
• Turndown and seasonal operating cases

The stripping gas itself must be dry enough to provide useful driving force. Wet fuel gas can add water to the system and undermine the intended benefit. Its composition also affects heating duty, emissions, and flammability of the still overhead.

The Cost of Stripping Gas

Stripping gas increases achievable purity, but it is not free.

First, gas used once-through becomes a continuous hydrocarbon loss unless the still overhead is recovered or used as fuel. Second, it increases non-condensable flow through the still and overhead condenser. This can reduce condensation effectiveness and increase the size of downstream vapour-control equipment. Third, the gas carries methane and can increase VOC and hazardous-air-pollutant emissions from an uncontrolled atmospheric vent.

The design should close a gas and emissions balance around:

• Stripping-gas source and metering
• Flash-tank gas recovery
• Still-condenser duty and condensate separation
• Skimmer-gas routing
• Vapour recovery, burner/firebox routing, or flare disposal
• Backpressure imposed on the atmospheric regenerator

Backpressure deserves special attention. A conventional regenerator relies on low operating pressure to achieve purity. Routing the still vent into a system with excessive pressure can raise the boiling temperature required, reduce regeneration performance, and encourage operators to compensate by overheating the glycol.

Stripping Gas, Vacuum, or Enhanced Regeneration?

The regeneration method should be selected against the whole facility rather than purity alone.

For a remote onshore unit with available fuel gas and moderate emissions requirements, stripping gas may be the most practical solution. For an offshore facility where venting is restricted and gas has high value, vacuum or overhead recovery may compare better. Where very deep dewpoints are required, molecular sieves may be more appropriate than forcing a TEG system beyond its economical range.

Controls and Safeguards

The regeneration controls must maintain purity without allowing temperature, pressure, or liquid inventory to drift into an unsafe region.

Key functions typically include:

• Reboiler temperature control through burner or heat-medium duty
• Independent high-temperature trip
• Reboiler or surge-vessel level control
• Low-level protection for fire tubes or heating surfaces
• Still-column pressure indication and high-pressure protection
• Stripping-gas flow control and low-flow alarm where it is required for specification
• Rich- and lean-glycol temperature monitoring around the exchanger
• Flash-tank pressure and level control
• Burner management and flame-failure protection for fired units
• Differential pressure monitoring across filters and contact sections

Lean concentration should be measured routinely using a method corrected for temperature and contamination. Density or refractive-index readings can be misleading when the glycol contains hydrocarbons, salts, degradation products, or suspended solids. Laboratory water analysis provides the reference needed to validate online or field measurements.

The control philosophy should not rely on reboiler temperature alone as proof of purity. The same temperature can produce different lean concentrations when pressure, circulation, stripping flow, rich loading, or still performance changes.

When the Regenerator Cannot Reach Purity

Off-spec lean glycol should be diagnosed as a system problem. A practical sequence is:

Increasing the temperature set point should not be the default response. Confirm the measurement, water loading, circulation, pressure, exchanger performance, and stripping-gas condition first. If the existing package cannot meet the target within its allowable operating envelope, the solution is a design change—not a hotter reboiler.

Simulation and Performance Testing

TEG regeneration involves a strongly non-ideal water–glycol–hydrocarbon system. A plain cubic equation of state can produce plausible but incorrect water and glycol distributions. Use a dedicated glycol method or an activity-coefficient/equation-of-state combination validated for the application; Choosing the Right Property Package explains why.

The simulation should close the complete loop:

• Gas contactor and dry-gas specification
• Rich-glycol flashing and hydrocarbon release
• Lean-rich heat exchange
• Still column, reboiler, and reflux
• Stripping-gas feed or vacuum system
• Lean-glycol concentration and circulation recycle
• Still-overhead water, hydrocarbon, and glycol losses

Performance testing should then reconcile the model against plant data: wet- and dry-gas water content, lean and rich glycol water content, circulation rate, key temperatures, reboiler fuel or heat-medium duty, flash-gas flow, stripping-gas flow, and still-overhead conditions. Sampling points and methods must be agreed in advance; a poorly taken lean-glycol or gas-dewpoint sample can send the troubleshooting effort in the wrong direction.

Design Deliverables

A complete regenerator design basis should provide:

• Required lean TEG purity and its link to dry-gas specification
• Rich-glycol water loading and circulation cases
• Reboiler process duty, design duty, efficiency, and turndown
• Lean-rich exchanger thermal rating
• Still-column diameter, packing, vapour traffic, and reflux basis
• Stripping-gas rate, quality, distribution, and control requirements
• Flash-tank and overhead material balances
• Condenser and vapour-disposal or recovery duties
• TEG and hydrocarbon loss estimates
• Operating envelope, start-up basis, alarms, trips, and safeguards
• Simulation files, assumptions, sensitivities, and vendor interfaces

These outputs connect process performance to the mechanical package, fuel system, emissions design, control narrative, and operating procedures. A single reboiler-duty number is not a regenerator design.

Conclusion

The TEG regenerator determines how dry the solvent can become and therefore how dry the gas can become. Its design is a balance: sufficient heat to remove water, sufficient vapour–liquid contact to retain glycol, sufficient stripping or pressure reduction to reach the required purity, and sufficient heat recovery to avoid wasting fuel.

The central discipline is to resist solving every problem with more circulation or more temperature. Higher circulation increases duty and overhead loading; higher temperature accelerates degradation. Stripping gas can raise purity substantially, but it consumes gas and changes the emissions and overhead system.

Start with the gas specification, calculate the required lean concentration, build the full heat and material balance, and select the regeneration method that delivers that concentration across real operating conditions. When the regenerator, heat recovery, stripping system, and overhead controls are designed as one package, lean purity becomes a controlled performance parameter rather than a number operators spend the life of the unit chasing.