Flash Tank Separators

Glycol dehydrators are used in the oil and gas industry to remove water from the natural gas stream. Most glycol dehydrators use triethylene glycol (TEG) as the dewatering agent.  During the dehydration process, as TEG is regenerated through heating in a reboiler, absorbed methane, volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) are vented to the atmosphere with the water vapor. A flash tank separator can be used in the dehydration process to remove the gas absorbed by the glycol in the gas contactor. Circulating the rich glycol/gas mixture through a flash tank separator prior to the regenerator reduces emissions, if the gas is captured, through beneficial use or combustion.

Using a flash tank separator in the dehydration process enables the capture of gas for beneficial use or combustion, instead of venting emissions to the atmosphere. Most glycol dehydrators in the natural gas industry use TEG to remove water from the natural gas stream and to meet pipeline quality standards. During the dehydration process, as lean TEG absorbs water, it also absorbs methane, VOCs, and HAPs when contacted with the wet gas. Rich TEG (glycol/gas mixture) is then regenerated (i.e., water and impurities are removed) through heating in a reboiler, where absorbed methane, VOCs, and HAPs are vented to the atmosphere along with the water vapor. Furthermore, if the glycol dehydration unit uses a gas-assist pump to circulate regenerated (lean) glycol back to the contactor, the assist gas is also exhausted from the pump into the rich glycol stream prior to the regenerator resulting in additional emissions. Figure 1 shows a typical TEG glycol dehydrator system using a gas-assist glycol circulation pump. 

Retrofitting a glycol dehydrator system by installing a flash tank separator to process the rich glycol/gas mixture prior to entering the regenerator eliminates most of the methane emissions from the process. Figure 2 shows a glycol dehydrator with a flash tank separator. In the flash tank separator, the majority of the absorbed gas, and the assist gas if a gas-assisted glycol circulation pump is used, is separated from the rich glycol at a lower pressure than the contactor pressure – generally in the 40 to 100 pounds per square inch gauge (psig) range. At this lower pressure, and without added heat, the flash tank overhead gas is rich in methane and lighter VOCs, but water remains in solution with the glycol. In this way, the flash tank captures a portion of the methane and VOCs entrained by the glycol, thereby reducing emissions if the captured gas is then routed to beneficial use or combustion. The rich glycol, largely depleted of methane and light hydrocarbons, then flows to the glycol regenerator where it is heated to boil off the absorbed water, remaining methane, and VOCs. These gases are normally vented to the atmosphere and the dry (i.e., lean) glycol is circulated back to the gas contactor.

Figure 1. Glycol Dehydrator with Gas-Assist Glycol Circulation Pump

Figure 2. Glycol Dehydrator with Flash Tank Separator

Retrofitting a glycol dehydrator system with a flash tank separator is most applicable to remote production and gas gathering dehydrators that use gas-assist glycol circulation pump because they do not have reliable electrical power. Where there is reliable electrical power, most of the methane emissions can be avoided using an electric glycol circulation pump, and thereby avoiding the extra pneumatic gas required for the gas-assist pump.

Methane emission reductions can be determined by taking the difference in emissions from the source before and after the specific mitigation action was applied. Glycol dehydrators are an integrated system with multiple components and methods to operate and reduce emissions. As a result, circulating the rich glycol/gas mixture through a flash tank separator prior to the regenerator will impact emissions throughout the entire system. Because there are multiple glycol dehydrator configurations and unique parameters to consider, such as the volume of natural gas and water content, a default emission factor is not available to adequately estimate emissions. Alternate methodologies for estimating emissions from glycol dehydrators include the use of simulation software, which can model emissions from the glycol dehydrator for the existing configuration and after implementation of the mitigation option. Further information on calculating glycol dehydrator emissions using simulation software is available in subpart W of EPA’s Greenhouse Gas Reporting Program at 40 CFR 98.233(e).

The calculation methodology in this emissions reduction section is based upon current information and regulations (as of August 1, 2023). EPA will periodically review and update the methodology as needed.

In addition to reducing emissions of methane, installing flash tank separators may: 

• Reduce air pollution: Reduces emissions of volatile organic compounds and hazardous air pollutants that are recovered in the flash tank separator.  
• Increase gas production and revenue: Allows for beneficial use of recovered gas, such as on-site fuel use or additional sale opportunities.

Cimarron. (2021, January 5). Glycol dehydration process and emission controls. https://cimarron.com/glycol-dehydration-process-and-emission-controls/

Kimray. What is a flash separator?https://kimray.com/training/what-flash-separator

Stewart, M. & Arnold, K. (2008). Gas-liquid and liquid-liquid separators. Gulf Professional Publishing. https://doi.org/10.1016/B978-0-7506-8979-3.X0001-3