Optimize Glycol Circulation
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. 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 along with the water vapor. The amount of methane absorbed and vented is directly proportional to the TEG circulation rate. Many wells produce gas far below the original design capacity of the dehydrator which may continue to circulate TEG at rates two or three times higher than necessary, resulting in little improvement in gas moisture quality but much higher methane emissions and fuel use. The TEG circulation rate may also be increased in dehydrators using gas-assist glycol circulation pumps to postpone maintenance of the pump seals, which leads to additional methane emissions. Optimization of TEG circulation rates can reduce methane emissions at negligible cost.
Description
Most glycol dehydration systems in production, gathering, processing, and transmission use TEG as the desiccant agent to remove moisture from the natural gas stream. Gas production wells experience declining production as pressure is drawn off the reservoir. Wellhead glycol dehydrators and their TEG circulation rates are designed for the initial (i.e., highest) production rate, and therefore, become oversized as the well matures. As a result, it is not uncommon for the TEG circulation rate to be much higher than necessary to meet the sales gas specification for moisture content. In addition, dehydrators using gas-assist glycol circulation pumps experience gradual degradation of the rubber seals separating wet glycol motive power from dry glycol pumped to the contactor. Increasing the glycol circulation rate to maintain the gas stream moisture specification allows postponement of maintenance of the pump seals.
As the methane emissions from a glycol dehydrator are directly proportional to the amount of TEG circulated through the system, the higher the circulation rate, the more methane that is vented from the regenerator. Over-circulation results in more methane emissions without significant reduction in gas moisture content. Dehydrator systems often recirculate TEG at rates two or more times higher than necessary as a well matures. Operators can reduce the TEG circulation rate, and the associated methane emissions rate, without affecting dehydration performance or adding any additional cost. Figure 1 shows a typical glycol dehydrator system using a gas-assist glycol circulation pump. Figure 2 shows a glycol dehydrator system with optimized glycol circulation (represented by a smaller number of “glycol arrows”).
Operators can calculate the optimal TEG circulation rate by following a few simple calculations, as follows:
- Obtain the current circulation rate by reading the TEG flow controller (where there is one) or count circulation pump strokes per minute. For each gallon of TEG circulated, one standard cubic foot of methane is absorbed, and if the unit has a gas-assist glycol circulation pump, two additional cubic feet of gas will be necessary to drive the pump. All this gas is vented to the atmosphere from the regenerator when there is no flash tank separator.
- Determine the minimum TEG circulation rate necessary to meet the gas stream moisture specification. The minimum TEG circulation rate at a particular site is a function of the gas flow rate, the water content of incoming gas, and the desired water content of outgoing gas. The water removal rate is a function of the gas flow rate and the amount of water to be removed from the gas stream. The TEG-to-water ratio (i.e., how many gallons of TEG are required to absorb 1 pound of water) varies between 2 and 5 gallons of TEG per pound of water; the industry accepted rule-of-thumb is 3 gallons of TEG per pound of water removed. The greater the water removal rate or the higher the TEG-to-water ratio, the higher the TEG circulation rate must be.
Problems can arise if the TEG circulation rate is too low, therefore, a certain amount of over-circulation is desired. For instance, an overly restricted circulation rate can cause problems with tray hydraulics, contactor performance, and fouling of glycol-to-glycol heat exchangers. As such, operators should include a margin of safety, or “comfort zone,” when calculating reductions in circulation rates.
It should be noted that this mitigation option is focused on reducing the TEG circulation rate to optimal level and not reducing the quantity of glycol in the system. Reducing the quantity of glycol in the system will not achieve the desired methane reductions.
Applicability
This practice applies to all types of glycol dehydrators: triethylene glycol (TEG), diethylene glycol (DEG), ethylene glycol (MEG), and tetraethylene glycol (TREG); however, TEG is by far the most common desiccant glycol used in the natural gas industry.
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, optimization of glycol circulation rates 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, optimizing glycol circulation may:
- Reduce air pollution: Reduces emissions of volatile organic compounds and hazardous air pollutants will be reduced.
- Reduce operational and maintenance costs: Reduces the frequency of glycol replacement and fuel consumption in the reboiler.
Lessons Learned
References
Bassey, P., Johnson, G., & Obonukut, M. (2022). Simulation and optimization of a natural gas dehydration plant with triethylene glycol. London Journal of Engineering Research. https://journalspress.com/LJER_Volume22/Simulation-and-Optimization-of-a-Natural-Gas-Dehydration-Plant-with-Triethylene-Glycol.pdf (476 KB)
Chebbi, R., Qasim, M., & Jabbar, N. A. (2019, July 2). Optimization of triethylene glycol dehydration of natural gas. Energy Reports. https://doi.org/10.1016/j.egyr.2019.06.014
Jacob, N. C. G. (2014, June). Optimization of triethylene glycol (TEG) dehydration in a natural gas processing plant. International Journal of Research in Engineering and Technology. https://doi.org/10.15623/ijret.2014.0306064
Kamin, Z., Bono, A., & Leong, L. Y. (2017, January). Simulation and optimization of the utilization of triethylene glycol in a natural gas dehydration process. Chemical Product and Process Modeling. https://doi.org/10.1515/cppm-2017-0017
Mokhatab, S., Poe, W. A., & Mak, J. Y. (2019). Handbook of natural gas transmission and processing, Fourth Edition. Gulf Professional Publishing. https://doi.org/10.1016/C2017-0-03889-2
Stewart, M., & Arnold, K. (2011). Gas dehydration field manual. Gulf Professional Publishing. https://doi.org/10.1016/C2009-0-62053-5
Stewart, M. I. (2014). Surface production operations – Volume 2: Design of gas-handling systems and facilities, Third Edition. Gulf Professional Publishing. https://doi.org/10.1016/C2009-0-64501-3
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