Managing VOC & HAP Emissions from Glycol Dehydrators in Natural Gas Operations
1. Introduction
Glycol dehydration units are essential for natural gas processing, ensuring the gas meets pipeline specifications by removing water vapor. Without proper dehydration, water in the gas stream can form hydrates that may block pipelines and contribute to corrosion, leading to serious safety and operational risks. This blog explores how to calculate air emissions from glycol dehydration units.
Glycol dehydration units can use triethylene glycol (TEG), ethylene glycol (EG), or diethylene glycol (DEG). In oil and gas operations, TEG is used in the vast majority of glycol dehydrators. EG is commonly injected into pipelines to prevent hydrate formation and corrosion, while DEG is rarely used. This blog focuses exclusively on emissions from dehydration units using TEG.
2. Process Description
Natural gas, typically saturated with water vapor, enters the bottom of an absorber (contact tower) and flows upward through a countercurrent stream of lean, or dry, TEG. As the gas contacts the glycol on the packing or trays, water vapor is absorbed into the TEG, and the dehydrated gas exits through the top of the absorber. The typical required water content for dry gas is 7 lb per million standard cubic feet (MMSCF).
The rich (water-laden) glycol exits the bottom of the absorber and flows to a flash separator (aka, gas/condensate/glycol separator), which removes any entrained gas and light hydrocarbons released due to the pressure drop. From the flash separator, the rich glycol enters the reboiler regeneration system.
In the reboiler, the rich glycol is heated to drive off the absorbed water vapor, regenerating the glycol for reuse. The reboiler typically operates at or near atmospheric pressure, with a controlled temperature around 350°F (177°C), which is sufficient to strip water while avoiding thermal degradation of the glycol. The regenerated lean glycol is then cooled and recycled back to the absorber to repeat the dehydration process.
The absorber is a pressurized vertical contact vessel designed to maximize gas-liquid contact for efficient water vapor absorption. It may be equipped with structured or random packing, or with trays—such as sieve, valve, or bubble-cap trays—to enhance the contact area between the natural gas and the circulating glycol.
In addition to water, some hydrocarbons, including volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) such as benzene, are also absorbed by the TEG in the absorber. These compounds are ultimately released through the still column vent during glycol regeneration in the reboiler.
3. Air Pollutants Emitted
Natural gas contains methane, VOCs and HAPs such as benzene, toluene, ethylbenzene, and xylenes (BTEX) and n-hexane and 2,2,4-trimethylpentane. These pollutants are slightly soluble in the TEG and are released from the still column vent during regeneration.
Venting emissions from TEG units are primarily from the still column vent and the flash separator (if not routed to the gas handling system or to an emissions control device).
Reboilers that use natural as fuel will emit criteria pollutants such as NOx, CO, VOCs and particulate matter.
4. Dehydration Emission Sources
Key emission sources include:
- Still Column Vent: Emits water vapor, methane, VOCs, and HAPs.
- Flash Separator: Releases methane and VOCs during TEG pressure reduction between absorber and flash separator. This is not an emission source if the flash gas is routed to gas handling system.
- Glycol Pumps: If powered by pressurized natural gas, these may emit small amounts methane and VOCs.
Operators should always indicate whether a flash separator is installed between the absorber and the reboiler, as its presence significantly affects emissions from the still column vent. Without a flash separator, a much larger volume of flash gas is liberated in the reboiler and subsequently released through the still column vent. This directly influences the design and sizing of any control device used to condense liquids and/or recover or flare non-condensable natural gas.
5. Glycol Dehydrator Emission Regulations
Glycol dehydrators are subject to U.S. EPA federal regulations under 40 CFR 63 – National Emission Standards for Hazardous Air Pollutants (NESHAP):
- Subpart HH – Oil and Natural Gas Production Facilities
- Subpart HHH – Natural Gas Transmission and Storage Facilities
These federal rules regulate emissions of HAPs from glycol dehydration units and other affected equipment. HAPs of concern include benzene, toluene, ethylbenzene, xylenes, (BTEX), n-hexane, and 2,2,4-trimethylpentane. These federal emission standards apply to major sources of HAPs and area sources of HAPs. A major source of HAPs is one that emits 10 ton per year (tpy) of one HAP or 25 tpy of any combination of HAPs.
Regulatory compliance includes emissions calculations, control requirements, and reporting.
State-specific regulations that focus on VOCs and HAPs may also apply. Understanding applicable regulations is crucial for compliance and system design.
6. Gas Samples Used for Emission Calculations
Models such as GLYCalc used to estimate emission require a site specific wet gas sample collected upstream of the absorber. The gas analysis should include CO2, N2, H2S, C1-C10+, benzene, toluene, ethylbenzene, xylenes, n-hexane, and 2,2,4-trimethylpentane.
Standard conditions for gas mole percent inputs are 60°F and 14.7 psia.
7. Calculating Emissions
Models are used to estimate emissions from glycol dehydration units based on process conditions and gas composition. These models simulate the behavior of absorbed gas components and predict emissions from the still column vent and flash separator. The typical models used include ProMax, HYSYS and GLYCalc Ver. 4.
The models use the Peng–Robinson equation of state (EOS), a cubic EOS widely applied in the oil, gas, and chemical industries to predict phase behavior, particularly vapor–liquid equilibrium, for hydrocarbons and other fluids.
To obtain accurate emission estimates for total VOCs and individual HAPs, model inputs must include the mole percents of inlet gas components such as benzene, toluene, ethylbenzene, xylenes, n-hexane, and 2,2,4-trimethylpentane.
Note: The mass balance method, which relies on sampling rich and lean glycol streams, is not a valid approach for estimating total emissions, as it fails to account for methane released from the still column vent and omits flash gas emissions from the glycol flash separator.
7.1 Thermodynamic Models
ProMax (by BR&E) and Aspen HYSYS are more robust simulators using equations of state like Peng-Robinson. These two models are specifically listed in 40 CFR 98 subpart W as software for calculating emissions from glycol dehydrators. They offer comprehensive modeling of the dehydration process and emissions.
ProMax provides detailed emissions outputs for the still column vent and flash separator, including mass flow rates of methane, VOCs, and individual HAPs. It also delivers stream composition data, absorber and reboiler conditions, and the performance of any specified emission control devices.
Data inputs to thermodynamic models include site specific dehydrator operating parameters such as:
- Absorber pressure and temperature
- Wet gas water content
- Glycol circulation rate
- Mole percent of inlet gas components
- Dry gas water content
- Stripping gas usage and flow rate
- Flash separator operating conditions
- Flash separator gas destination
- Glycol pump type and gas usage
- Still column vent emission control operating conditions
7.2 GRI-GLYCalc Ver. 4
GRI-GLYCalc is a modeling tool specifically identified in 40 CFR 98 Subpart W as an acceptable method for calculating emissions from triethylene glycol (TEG) dehydration units. It is also widely accepted by state air permitting agencies for estimating emissions from dehydrators.
Originally developed by GTI Energy (formerly the Gas Research Institute), GRI-GLYCalc has been extensively used across the industry. The current version, GRI-GLYCalc 4.0, released in 2000, utilizes simplified thermodynamic models to estimate emissions based on site-specific operating data.
As of this writing, GTI Energy no longer sells new copies of the software and does not offer updates or technical support for current or previous versions. Interested users should contact GTI Energy directly to verify availability and licensing terms.
GRI-GLYCalc estimates emissions from both the still column vent and the flash separator, with results reported by component, including methane, total VOCs, and individual hazardous air pollutants (HAPs).
8. Factors Affecting Emission Rates
Some factors that affect emission rates include the following.
Glycol Circulation Rate
The VOC and HAP emission rates from TEG dehydrators modeled with GLYCalc and other thermodynamic models are affected by the glycol circulation rate. Increasing the TEG circulation rate raises VOC and HAP emissions from the still column vent when all other operating conditions remain constant. The relationship is positively correlated but constrained by equilibrium effects, resulting in diminishing emission increases at higher circulation rates.
Sparger (Stripping) Gas
Sparger (stripping) gas may be used by some systems where very low water content in dry gas streams are needed. The sparger gas is sparged (bubbled) into the bottom of the reboiler vessel to reduce the partial pressure of water vapor in the stripping section. This allows the glycol to release more water during regeneration and reduce the water content of the lean glycol that circulates back to the contact tower.
The use of stripping gas significantly increases VOC and BTEX emissions from the still column vent. This occurs because the sparging process carries more VOCs and BTEX with the water vapor into the vent stream. As a result, the still column vent gas has a higher flow rate and a greater total hydrocarbon content compared to systems that do not use stripping gas.
Atmospheric condensers are often used on still column vents to reduce emissions by condensing and recovering some of the vapors. When stripping gas is used there is increased gas flow and VOC content may overwhelm the condenser’s capacity. This leads to reduced condenser efficiency and higher uncontrolled emissions from the condenser. In such cases, atmospheric condensers alone are often insufficient for compliance with emissions limits.
Pneumatic Devices
Pneumatic devices used for process control in glycol dehydration units can be significant sources of vented methane and VOCs when powered by natural gas. Emission rates vary depending on the device’s make, model, and operating conditions.
Pneumatic devices used include:
- Glycol pumps such as Kimray energy exchange pumps that use absorber gas pressure to drive a reciprocating piston or diaphragm, transferring energy to pump glycol. Pump exhaust gas can be vented, recycled, or routed to a combustion device.
- Glycol pumps (Texsteam, Arrow, Williams) that use pressurized gas-driven positive displacement pumps. Pump exhaust gas can be vented, recycled or routed to a combustion device.
- Pneumatic controllers used for level, pressure and temperature control.
- Pressure relief devices
For permitting and regulatory compliance under applicable federal and state rules, glycol pumps are classified as components of the dehydration unit. Pneumatic controllers and pressure relief devices, however, are categorized as distinct emission sources separate from the dehydration unit.
9. Emission Controls – Including Cimarron’s Product Line
Cimarron offers a comprehensive suite of emission control technologies specifically designed for TEG dehydrator systems that we call BTEX systems.
- Jatco Ambient Air BTEX Eliminators®: These natural air-cooled condensers use no power and are ideal for remote sites. They are modular, allowing additional cooling capacity as needed, and integrate with JATCO blowcases for automated liquid transfer.
- Cimarron Forced-Draft BTEX Destructors: These systems utilize a powered fan to enhance cooling and vapor condensation. They include a robust enclosed combustor with cast ceramic fiber insulation and are available in fully enclosed or open-air configurations to suit diverse climate conditions.** **It provides over 98% destruction efficiency for methane, VOCs, and HAPs. This system prevents still column vapors from being routed to the reboiler, mitigating fire risks and improving reliability.
- Smart BTEX Combustors: These units feature real-time performance monitoring via Cimarron’s Opti Link® edge device. This systems enables sending glycol separator flash gas to an enclosed combustor for combustion. They provide cloud connectivity, data logging, and SCADA integration. Designed for compliance with the latest EPA standards, these combustors offer turnkey performance optimization.
Cimarron’s technologies are designed to meet increasingly stringent federal and state regulatory requirements while enabling operational efficiency and data-driven decision-making.
9. Summary
Calculating emissions from TEG dehydration units is essential for maintaining regulatory compliance, minimizing environmental impact, and improving operational transparency. These emissions primarily originate from the still column vent, flash separator, and gas-driven pumps, and they include methane, VOCs, and HAPs. To accurately estimate these emissions, operators must gather site specific gas composition data and use appropriate simulation tools.
GRI-GLYCalc offers a simplified, user-friendly model for typical field applications, while ProMax and Aspen HYSYS provide more advanced thermodynamic modeling for complex scenarios. The correct choice depends on operational scale, regulatory requirements, and the need for precision.
Cimarron enhances emission management efforts with its robust suite of emission control solutions, including EPA-certified combustors, air-cooled condensers, and remote monitoring for equipment leak capabilities through Opti Link®. These technologies not only support compliance but also drive operational efficiency and safety.
10. Conclusions
Accurately estimating and controlling emissions from TEG dehydrators is essential for regulatory compliance, environmental stewardship, and operational safety. By understanding the dehydration process, recognizing the sources of VOCs and HAPs, and applying reliable emission models, operators can proactively manage their environmental footprint.
Cimarron’s suite of BTEX control technologies—including ambient air condensers, forced-draft destructors, and Smart BTEX Combustors with real-time performance monitoring—provide operators with proven, turnkey solutions for reducing emissions and meeting stringent federal and state requirements.
Learn more at cimarron.com.