Reroute Glycol Skimmer Gas
Summary
The oil and gas industry relies heavily upon glycol dehydrators using triethylene glycol (TEG) as the desiccant agent to remove water from the natural gas stream. Some glycol dehydrators have a glycol reboiler vent condenser and condensate separator. The non-condensable gas, or skimmer gas, from the condensate separator contains mostly methane and is often vented to the atmosphere. Rerouting the skimmer gas to the reboiler firetubes or a storage tank with a vapor recovery unit reduces methane emissions.
Description
In the glycol dehydration process, rich glycol is circulated through a reboiler where the dissolved water, methane, VOCs, and HAPs are vaporized and vented to the atmosphere. Figure 1 shows a typical glycol dehydrator system. As an alternative to venting this stream, some glycol dehydrators have a vent condenser and condensate separator on the reboiler vent. In the condenser, water and heavy hydrocarbons are removed from the vent stream, reducing VOC and HAP emissions. The non-condensable gas from the condensate separator, commonly known as skimmer gas, is mostly methane and also includes light hydrocarbon VOCs. Instead of venting the skimmer gas to the atmosphere, it can be rerouted to a nearby atmospheric pressure source, such as the dehydrator reboiler firetubes or to a storage tank with a vapor recovery unit. Figure 2 shows a glycol dehydrator where the skimmer gas is routed to the reboiler firetubes. Skimmer gas should not be routed to a higher-pressure outlet because that raises the temperature required in the reboiler and degrades the glycol more quickly. Using a flash tank separator also increases the efficiency of the vent condenser and reduces the skimmer gas volume.
Applicability
This practice can be employed on all dehydrators with vent condensers and condensate separators with the assumption that reboiler fuel gas consumption can accommodate the extra volume of the rerouted skimmer gas. Skimmer gas is essentially at atmospheric pressure, so it cannot be directly injected into the reboiler fuel gas ahead of the burner; however, it can be combined with the air drawn into the burner flame tubes. In addition, the skimmer gas can be routed to a storage tank with vapor recovery.
Methane Emissions Reductions
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, rerouting the skimmer gas to the reboiler firetubes or a storage tank with a vapor recovery unit 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.
Other Benefits
In addition to reducing emissions of methane, rerouting glycol skimmer gas may:
- Increase revenue. If sufficient light hydrocarbons can be recovered in a vapor recovery unit, these products can provide additional sales revenue
- Reduce costs. The significant gas savings from rerouting glycol skimmer gas to a fuel gas system will offset the low capital, operating, and maintenance costs.
- Reduce air pollution. Emissions of VOCs, HAPs, and other toxic contaminants will be reduced.
Lessons Learned
References
Acevedo, W. R., Stevens, D., Rhode, M., & Chu, B. (2019). Troubleshooting mono ethylene glycol carryover in a Canadian gas plant. Laurance Reid Gas Condition Conference, Norman, OK, United States. https://tartanenergygroup.com/wp-content/uploads/2020/07/TROUBLESHOOTING-MONO-ETHYLENE-GLYCOL-CARRYOVER.pdf (9 MB)
Stewart, M., & Arnold, K. (2011). Gas dehydration field manual. Gulf Professional Publishing. https://doi.org/10.1016/C2009-0-62053-5
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