Presaturation in Glycol Systems Fundamentals

1. The Hydrocarbon Absorption Problem

Triethylene glycol (TEG) is the most widely used liquid desiccant for natural gas dehydration, effectively removing water vapor from the gas stream to meet pipeline and processing specifications. However, TEG does not selectively absorb only water. As lean glycol contacts the wet gas stream in the contactor, it simultaneously absorbs significant quantities of hydrocarbons—particularly aromatic compounds—that create downstream operational, environmental, and regulatory challenges.

The hydrocarbon absorption problem is inherent to the glycol dehydration process. TEG is a polar organic solvent with moderate affinity for both water and hydrocarbons. While its selectivity for water over methane and ethane is very high, the selectivity decreases substantially for heavier hydrocarbons, and TEG actually exhibits preferential absorption of aromatic compounds relative to their concentration in the gas phase.

BTEX Absorption in Glycol

The most significant hydrocarbon absorption concern involves BTEX compounds—benzene, toluene, ethylbenzene, and xylene. These aromatic hydrocarbons are present in most natural gas streams at concentrations ranging from 50 to 5,000 ppmv, depending on the gas source and upstream processing. TEG absorbs BTEX compounds at rates disproportionate to their gas-phase concentration because of the strong thermodynamic interaction between the polar glycol and the aromatic ring structure.

The aromatic selectivity of TEG arises from the interaction between the glycol's hydroxyl groups and the delocalized electron system of the aromatic ring. This results in activity coefficients for BTEX in TEG that are significantly lower than those for paraffinic hydrocarbons of similar molecular weight, meaning the glycol has a higher thermodynamic capacity for aromatics. Typical absorption factors for BTEX compounds in a glycol contactor operating at standard conditions are:

Consequences of Hydrocarbon Absorption

The hydrocarbons absorbed by TEG in the contactor are carried through the glycol loop and ultimately released at the regenerator, where high temperatures drive the absorbed components out of the glycol. This creates several significant problems:

• BTEX emissions at the regenerator: The absorbed aromatic compounds are vaporized in the regenerator still column and exit with the overhead vapor. In conventional glycol units without emission controls, this overhead stream is vented to atmosphere or routed to a combustion device, making the glycol regenerator one of the largest point sources of BTEX emissions in natural gas processing. Benzene is classified as a known human carcinogen (Group 1), and all BTEX compounds are listed as hazardous air pollutants (HAPs) under the Clean Air Act
• Flash gas losses: In addition to BTEX, the rich glycol absorbs significant quantities of methane, ethane, and other light hydrocarbons under the high-pressure contactor conditions. When the rich glycol is depressured at the flash tank, these dissolved gases are released as flash gas. Higher hydrocarbon loading in the glycol increases the volume of flash gas, representing both a product loss and a potential emissions source
• Glycol foaming: Absorbed hydrocarbons, particularly the heavier C₆+ fraction, can act as surface-active agents that promote foaming in the contactor and regenerator. Foaming reduces contactor efficiency, causes glycol carryover into the dry gas, and can lead to operational instability requiring unit shutdown
• Increased stripping gas requirements: When the glycol is heavily loaded with hydrocarbons, the regenerator must work harder to achieve the same lean glycol concentration. The presence of hydrocarbons in the reboiler reduces the effective partial pressure of water, requiring more stripping gas or higher reboiler temperatures to achieve the target lean TEG concentration (typically 99.0–99.5 wt%)
• Glycol degradation: Heavy hydrocarbons that are not completely removed in the regenerator accumulate in the glycol over time, increasing viscosity and reducing dehydration effectiveness. Aromatic compounds can also participate in oxidation reactions that produce organic acids, accelerating glycol degradation

Environmental Regulations

The environmental significance of BTEX emissions from glycol dehydration units has driven increasingly stringent regulations. The EPA National Emission Standards for Hazardous Air Pollutants (NESHAP) for oil and natural gas production facilities (40 CFR Part 63, Subpart HH) and natural gas transmission and storage (Subpart HHH) establish emission limits for HAPs from glycol dehydrators. Key regulatory thresholds include:

• Major source threshold: Facilities emitting 10 tons per year (tpy) of any single HAP or 25 tpy of combined HAPs are classified as major sources and subject to Maximum Achievable Control Technology (MACT) requirements
• Area source requirements: Even facilities below the major source threshold may be subject to area source MACT standards requiring emission controls on glycol dehydrators with actual annual emissions exceeding 1 tpy of total HAPs
• Control requirements: MACT standards typically require 95% or greater reduction of HAP emissions from the glycol regenerator through combustion, condensation, or process modifications

Presaturation of the lean glycol is one of the recognized process modifications that can substantially reduce BTEX absorption in the contactor, thereby reducing emissions at the source rather than relying solely on end-of-pipe controls.

Hydrocarbon Loading vs. Dehydration Capacity

It is important to understand the scale of hydrocarbon absorption relative to the primary dehydration function. In a typical glycol contactor operating at 1,000 psig and 100°F, the lean glycol absorbs approximately 0.5–3.0 lb of water per gallon of circulated glycol (the design basis for dehydration). Simultaneously, the glycol may absorb 0.02–0.15 lb of hydrocarbons per gallon, depending on gas composition and contactor conditions. While the hydrocarbon mass is small relative to the water absorption, the environmental impact of the absorbed BTEX is disproportionately large because of the regulatory focus on HAP emissions.

At higher contactor pressures, more hydrocarbons dissolve in the glycol due to increased fugacity of the hydrocarbon components. Similarly, lower contactor temperatures increase hydrocarbon solubility in TEG. These trends are opposite to the water absorption behavior, where lower temperatures and higher pressures also favor water removal—meaning that operating conditions optimized for maximum dehydration also tend to maximize unwanted hydrocarbon absorption.

2. Presaturator Concept and Design

The presaturator is a vessel positioned in the glycol loop between the regenerator (lean glycol outlet) and the contactor (lean glycol inlet). Its purpose is to pre-expose the lean glycol to hydrocarbon vapor under controlled conditions before the glycol enters the high-pressure contactor. By equilibrating the glycol with hydrocarbons at low pressure, the driving force for additional hydrocarbon absorption in the contactor is substantially reduced, while the glycol's capacity for water absorption remains essentially unaffected.

Operating Principle

The presaturation concept exploits the difference between hydrocarbon solubility at low pressure (presaturator conditions) and high pressure (contactor conditions). When lean glycol from the regenerator contacts hydrocarbon gas at low pressure (50–100 psig), it absorbs a modest quantity of light hydrocarbons. The glycol is then pumped to contactor pressure (500–1,500 psig), where the pre-dissolved hydrocarbons reduce the glycol's remaining absorption capacity for additional hydrocarbons from the wet gas.

The key insight is that the water absorption capacity of TEG is governed primarily by the glycol concentration (wt% TEG) and temperature, and is relatively insensitive to the presence of dissolved hydrocarbons. Therefore, presaturating the glycol with hydrocarbons has a negligible effect on its dehydration performance while significantly reducing BTEX uptake in the contactor.

Presaturator Types

Several equipment configurations can be used to achieve glycol presaturation, each with distinct advantages depending on site constraints, gas availability, and the degree of BTEX reduction required:

Design Parameters

The effectiveness of presaturation depends on several design and operating parameters that must be carefully selected to achieve the target BTEX reduction without adversely affecting glycol system performance:

• Contact time: The lean glycol must have sufficient contact time with the presaturation gas to approach equilibrium hydrocarbon loading. Typical residence times range from 5 to 20 minutes for flash tank or sparger designs, while packed columns achieve equilibrium in shorter physical contact times due to higher mass transfer rates. Insufficient contact time results in incomplete presaturation and reduced BTEX reduction effectiveness
• Temperature: The presaturator operates at essentially the same temperature as the lean glycol entering the contactor, typically 10–20°F above the contactor gas temperature (to prevent hydrocarbon condensation on the trays). Typical presaturator temperatures range from 90 to 130°F. Lower temperatures increase hydrocarbon solubility in the glycol, providing better presaturation
• Gas-to-glycol ratio: The volume of presaturation gas must be sufficient to provide the hydrocarbon loading required for effective BTEX reduction in the contactor. Typical gas-to-glycol ratios range from 0.5 to 3.0 scf per gallon of glycol circulated, depending on gas composition, presaturator pressure, and the target degree of presaturation
• Presaturator pressure: The presaturator operates at significantly lower pressure than the contactor, typically 50–150 psig. This lower pressure limits the total hydrocarbon loading to a level that is effective for reducing additional absorption at contactor pressure without overloading the glycol with dissolved gas. Operating pressure is often dictated by the available gas supply pressure

Presaturation Gas Source

The hydrocarbon gas used for presaturation can come from several sources, each with trade-offs in terms of availability, composition, and system complexity:

• Flash tank gas: The gas released from the rich glycol flash tank is a convenient source because it is already available in the glycol system. However, this gas contains the hydrocarbons that were absorbed in the contactor, including BTEX, and using it for presaturation creates a recycle loop that can complicate mass balance calculations. Flash gas is available at 50–100 psig, which is appropriate for presaturator operation
• Treated (dry) gas: A small slip stream of the treated gas from the contactor outlet provides a clean presaturation gas source. The treated gas has been stripped of most water but retains its full hydrocarbon content. This is often the preferred source for new installations because it avoids BTEX recycle and provides a consistent composition
• Fuel gas: Plant fuel gas, if available at adequate pressure, can serve as the presaturation gas source. This simplifies the design because no additional gas supply connections are required, but the fuel gas composition may differ from the contactor gas
• Inlet (wet) gas: A slip stream of the inlet wet gas can be used but requires pressure reduction from contactor inlet pressure to presaturator operating pressure. This source provides gas with the same hydrocarbon composition as the contactor feed but adds complexity for pressure letdown and moisture content considerations

Equipment Sizing

Presaturator sizing is based on the glycol circulation rate, the required contact time, and the gas throughput. For the most common flash tank or sparger-type designs:

For packed column presaturators, the column is sized using conventional packed-column design methods. Random packing (1-inch Pall rings or equivalent) is common, with packed heights of 3–8 feet providing 3–5 theoretical stages. The small column diameter (typically 6–18 inches) reflects the low glycol and gas flow rates relative to the main contactor.

3. Benefits of Presaturation

Presaturation of lean glycol prior to contactor entry provides multiple interconnected benefits that extend beyond the primary objective of BTEX reduction. These benefits affect environmental compliance, process efficiency, operational reliability, and overall system economics.

BTEX Emission Reduction

The most significant benefit of presaturation is the reduction of BTEX absorption in the glycol contactor, which directly translates to lower BTEX emissions from the regenerator. Field data and simulation studies consistently demonstrate that properly designed presaturation systems can reduce BTEX absorption by 60–80%, with some installations achieving reductions exceeding 85%.

The degree of BTEX reduction depends on the presaturator type, contact efficiency, and operating conditions. Higher-efficiency presaturators (packed column type) and higher gas-to-glycol ratios produce greater BTEX reductions:

Benzene reduction is typically slightly lower than total BTEX reduction because benzene, as the lightest aromatic, has the highest volatility and lowest absorption factor in TEG. Nevertheless, even moderate presaturation can reduce benzene emissions sufficiently to bring facilities below the major source HAP threshold of 10 tpy.

Reduced Flash Gas Volume

By pre-loading the lean glycol with hydrocarbons at low pressure before it enters the high-pressure contactor, presaturation reduces the net hydrocarbon absorption that occurs at contactor pressure. This means less dissolved hydrocarbon must be released at the flash tank, resulting in lower flash gas volumes. Typical flash gas reductions of 20–40% have been reported, which translates to both reduced product losses and lower emissions from flash gas handling systems.

Improved Glycol Circulation Stability

Glycol systems without presaturation experience variable hydrocarbon loading depending on inlet gas composition fluctuations, contactor operating conditions, and seasonal temperature changes. This variability causes inconsistent flash gas volumes, fluctuating regenerator overhead composition, and difficulty maintaining stable emission controls. Presaturation provides a more consistent and predictable hydrocarbon loading, improving the stability of the entire glycol loop.

Better Dehydration Performance

Absorbed hydrocarbons in the glycol interfere with the water absorption process by occupying molecular interaction sites and reducing the effective glycol concentration available for water removal. By limiting additional hydrocarbon uptake in the contactor, presaturation preserves more of the glycol's water absorption capacity. Field observations indicate that presaturation can improve contactor dehydration efficiency by 5–15%, which may allow reduced glycol circulation rates or improved dewpoint depression without increasing circulation.

Reduced Foaming Tendency

Heavy hydrocarbons (C₆+), particularly aromatics, are among the primary contributors to glycol foaming in the contactor. By reducing the net absorption of these surface-active compounds, presaturation decreases the foaming tendency of the glycol in the contactor. Operators who have implemented presaturation frequently report reduced antifoam chemical consumption and fewer foaming-related operational upsets.

Environmental Permit Compliance

For facilities near the major source HAP emission threshold, presaturation can reduce BTEX emissions sufficiently to maintain area source classification, avoiding the significantly more stringent MACT requirements that apply to major sources. This can defer or eliminate the need for capital-intensive emission control equipment (condensers, thermal oxidizers, vapor recovery units) while still achieving meaningful emission reductions. Presaturation is recognized by the EPA as a legitimate process modification for HAP emission reduction.

Summary of Quantified Benefits

4. Integration with Glycol System

Integrating a presaturator into an existing or new glycol dehydration system requires careful consideration of the process flow sequence, equipment interconnections, control philosophy, and the effects on downstream equipment performance. The presaturator must be positioned correctly in the glycol loop and its gas handling integrated with the overall system material balance.

Position in the Glycol Loop

The presaturator is located in the lean glycol circuit between the regenerator system outlet and the glycol circulation pump suction (or between the pump discharge and the contactor, depending on the available gas pressure). The standard process sequence is:

1. Regenerator: Rich glycol is heated to drive off water and hydrocarbons, producing lean glycol at 380–400°F
2. Lean/rich heat exchanger: Hot lean glycol is cooled by exchanging heat with incoming rich glycol, recovering energy and reducing glycol cooler duty
3. Glycol cooler (trim cooler): Lean glycol is further cooled to within 10–20°F of contactor gas temperature, typically 90–130°F
4. Presaturator: Cooled lean glycol contacts hydrocarbon gas at low pressure, becoming pre-loaded with dissolved hydrocarbons
5. Glycol circulation pump: Pre-saturated lean glycol is pumped to contactor operating pressure
6. Contactor: Pre-saturated lean glycol enters the top of the contactor and absorbs water from the wet gas stream, with reduced BTEX uptake

Presaturator Gas Handling

The gas exiting the presaturator (after contacting the lean glycol) must be routed appropriately. The presaturator operates as a gas-liquid contactor, so the exit gas stream has been partially stripped of heavier hydrocarbons by the glycol and is enriched in lighter components. Typical gas disposal routes include:

• Fuel gas system: The presaturator exit gas can be routed to the plant fuel gas header if the pressure and composition are acceptable. This is the simplest disposal route and recovers the energy content of the gas
• Flash gas compression: If the plant has flash gas compression, the presaturator exit gas can be combined with the flash gas and compressed back into the sales gas stream
• Vent or flare: In some installations, the presaturator gas is vented (if volumes are small and regulations permit) or routed to a flare. This is the least desirable option due to product loss and potential emissions
• Recycle to inlet: The exit gas can be recycled back to the contactor inlet, but this reduces the net presaturation benefit by reintroducing absorbed hydrocarbons

Heat Integration Considerations

The presaturator operates at the lean glycol temperature entering the contactor (typically 90–130°F), so no additional heating or cooling is normally required for the glycol stream. However, the presaturation gas may require temperature conditioning:

• If flash gas is used, it exits the flash tank at essentially the flash tank temperature and pressure, which is generally suitable for presaturator operation without additional heating or cooling
• If treated gas is used, a pressure reduction from contactor outlet pressure to presaturator pressure produces Joule-Thomson cooling. Depending on the pressure drop and gas composition, the gas may need to be heated to prevent excessively low presaturator temperatures that could cause glycol viscosity problems or hydrocarbon condensation
• In cold-climate installations, heat tracing or insulation of the presaturator vessel may be required to prevent glycol viscosity from increasing to the point where gas-liquid contact efficiency is impaired

Effect on Lean Glycol Concentration

Presaturation has a negligible effect on the lean glycol TEG concentration because the hydrocarbons absorbed in the presaturator are dissolved in the glycol rather than diluting it with water. The lean glycol concentration leaving the presaturator is essentially the same as the concentration leaving the regenerator (typically 99.0–99.5 wt% TEG). The small amount of dissolved hydrocarbon (0.01–0.05 wt%) does not materially affect the glycol's water absorption capacity or the achievable dewpoint depression.

Interaction with Flash Tank Design

The flash tank in a glycol system with presaturation may require design modifications compared to a conventional system:

• Flash gas volume: The rich glycol from a presaturated system contains less dissolved hydrocarbon (because less was absorbed in the contactor), resulting in lower flash gas volumes. The flash tank may be oversized if the presaturator is retrofitted to an existing system, which is generally acceptable
• Flash gas composition: The flash gas composition shifts toward lighter components (more methane, less C₃+ and BTEX) because the presaturator has already pre-loaded the glycol with hydrocarbons, reducing the incremental heavy hydrocarbon absorption in the contactor
• Flash gas as presaturator feed: If flash gas is used as the presaturation gas source, a material balance recycle loop is created. The flash gas volume and composition become interdependent with the presaturator performance, requiring iterative mass balance calculations or process simulation to determine the steady-state operating conditions

Retrofit Considerations

Adding a presaturator to an existing glycol dehydration unit is generally straightforward because the presaturator is a relatively small, low-pressure vessel that can be installed in the lean glycol line with minimal disruption to the existing process:

• Piping modifications: The lean glycol line must be rerouted through the presaturator vessel, and a gas supply line must be installed from the chosen gas source. The glycol piping modifications are typically small-diameter (1–3 inch) and do not require significant structural modifications
• Foundation and space: The presaturator vessel is relatively compact (12–36 inch diameter, 4–8 feet tall for vertical designs) and can usually be installed on an existing equipment skid or a small dedicated foundation near the glycol pump
• Controls: Minimum instrumentation includes a level controller (to maintain glycol level in the presaturator), a pressure gauge, a temperature indicator, and gas flow measurement. These can typically be tied into the existing glycol unit control system
• Shutdown coordination: Installation of the presaturator connections requires a brief shutdown of the glycol circulation system, typically 4–8 hours for tie-in work. The contactor can often continue operating on a temporary glycol inventory during the tie-in

5. Alternative Approaches to BTEX Control

While presaturation is an effective source-reduction approach to BTEX control, several alternative technologies are available for managing BTEX emissions from glycol dehydration systems. Each approach has distinct advantages, limitations, capital and operating cost profiles, and regulatory acceptance. The choice of BTEX control method depends on the emission reduction required, the facility's regulatory classification, gas composition, site constraints, and economic factors.

Activated Carbon Adsorption

Activated carbon beds can be installed on the regenerator vent stream to adsorb BTEX and other VOCs from the overhead vapor before it is vented or combusted. The carbon acts as a polishing step that captures aromatic compounds while allowing water vapor and light hydrocarbons to pass through. Key characteristics include:

• BTEX removal efficiency: 90–99% when properly sized and maintained, with fresh carbon beds achieving near-complete BTEX removal. Efficiency declines as the carbon approaches saturation and must be monitored
• Carbon replacement: The carbon beds require periodic replacement (typically every 3–12 months depending on BTEX loading), generating a recurring operating expense and solid waste disposal requirement
• Moisture sensitivity: Activated carbon performance is degraded by water vapor in the regenerator overhead stream. Water occupies adsorption sites and reduces carbon capacity for BTEX. Pre-cooling the vent gas to condense water before the carbon bed improves performance
• Capital cost: Relatively low capital cost for the carbon vessel and initial carbon charge, but ongoing carbon replacement costs can be significant for high-BTEX applications

Condensation of Regenerator Overhead

Cooling the regenerator overhead vapor condenses BTEX and heavier hydrocarbons, which can then be collected and disposed of as a liquid rather than being emitted as a vapor. This approach requires an overhead condenser (typically air-cooled) and a separator to collect the condensed hydrocarbons. Performance characteristics include:

• BTEX removal efficiency: 70–90% depending on the condenser outlet temperature and ambient conditions. Lower condenser temperatures produce higher condensation efficiency but require more heat exchange area
• Temperature limitation: Benzene has a relatively high vapor pressure, and achieving high benzene condensation requires outlet temperatures below 100°F, which may be difficult in hot climates with air-cooled condensers
• Condensate disposal: The condensed BTEX-containing hydrocarbon liquid must be collected, stored, and disposed of properly, which adds operational complexity and cost
• Seasonal variability: Condensation efficiency varies with ambient temperature, providing less BTEX removal during summer months when air-cooled condenser performance is lowest

Drizo Process (BTEX Recovery)

The Drizo process is a modified glycol regeneration system that uses a closed-loop stripping gas circuit with hydrocarbon-based stripping gas instead of conventional atmospheric venting. The process recovers BTEX and other hydrocarbons from the regenerator overhead as a condensed liquid product rather than emitting them. The Drizo system provides:

• Near-complete BTEX recovery: 95–99%+ BTEX removal from the regenerator emissions, effectively eliminating HAP emissions from the glycol unit
• Enhanced lean glycol concentration: The closed-loop stripping gas system can achieve lean glycol concentrations of 99.9+ wt% TEG, improving dehydration performance and allowing reduced glycol circulation rates
• Hydrocarbon recovery: The condensed BTEX and associated hydrocarbons can be sold or blended into produced liquids, partially offsetting the system operating cost
• Higher capital and operating cost: The Drizo system requires additional equipment (stripping gas loop, condenser, separator, recirculation compressor) and is significantly more expensive than simpler alternatives

Comparative Analysis

Regulatory Context and Method Selection

The choice of BTEX control method is often driven by the facility's regulatory requirements under 40 CFR Part 63:

• Area sources (below major source threshold): Presaturation alone or in combination with overhead condensation may provide sufficient BTEX reduction to maintain compliance. These lower-cost options are often preferred for smaller facilities where capital investment must be minimized
• Major sources (MACT-regulated): Facilities exceeding the 10/25 tpy HAP threshold typically require 95%+ emission control, necessitating Drizo-type systems, thermal oxidizers, or enclosed combustion devices. Presaturation can be used in combination with these controls to reduce the hydrocarbon loading on the primary control device
• New Source Performance Standards (NSPS): New glycol dehydrators meeting certain throughput and emission thresholds under 40 CFR Part 60, Subpart OOOO/OOOOa may require specific emission controls regardless of the facility's major/area source classification

When Presaturation Is Preferred

Presaturation is the preferred BTEX control approach when the following conditions apply:

• The facility's uncontrolled BTEX emissions are moderate (5–20 tpy total HAPs), and a 60–80% reduction is sufficient to achieve or maintain compliance
• Capital budget is limited, and the facility cannot justify the cost of Drizo, thermal oxidation, or vapor recovery equipment
• The facility prefers source reduction over end-of-pipe treatment, avoiding the ongoing maintenance, consumable costs (carbon, fuel), and reliability concerns of add-on control devices
• Operational simplicity is valued—the presaturator has no moving parts, requires minimal maintenance, and does not create secondary waste streams (spent carbon, condensate)
• The presaturator provides ancillary benefits (reduced foaming, improved dehydration, lower flash gas) that improve overall system performance beyond just emission reduction
• The facility is considering a combined approach where presaturation reduces the BTEX load reaching a downstream control device, improving the control device's efficiency and reducing its size and operating cost

In many practical applications, presaturation is used as part of a layered BTEX control strategy, combining source reduction (presaturation) with end-of-pipe treatment (condensation or carbon adsorption) to achieve the overall emission reduction required for compliance. This combined approach often provides the most cost-effective solution for facilities that need 90%+ BTEX reduction but want to minimize the size and operating burden of the end-of-pipe equipment.

References

1. GPSA, Chapter 20 — Dehydration
2. EPA 40 CFR Part 63, Subpart HH — NESHAP for Oil and Natural Gas Production Facilities
3. EPA 40 CFR Part 63, Subpart HHH — NESHAP for Natural Gas Transmission and Storage Facilities
4. API Recommended Practice 12J — Specification for Oil and Gas Separators
5. GPA Midstream Standard 2261 — Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography