Desiccant Dehydrators
Summary
Glycol dehydrators are used in the oil and gas industry to remove water from the natural gas stream. Replacing glycol dehydrators with desiccant dehydrators that use moisture absorbing (deliquescent) salts to remove water reduces emissions of methane, volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) and reduces operating and maintenance costs.
Description
Desiccant dehydrators use moisture absorbing (deliquescent) salts to remove moisture from natural gas. Deliquescent salts used by the oil and gas industry include calcium chloride, potassium chloride, and lithium chloride. The amount of moisture that can be removed from natural gas depends on the type of desiccant used, as well as the temperature and pressure of the gas. Calcium chloride, the most common and least expensive desiccant, can achieve pipeline-quality moisture content at temperatures below 59 degrees Fahrenheit (°F) and pressures above 250 pounds per square gauge (psig). Lithium chloride has a wider operating range of up to 70 °F and above 100 psig.
A desiccant dehydrator is a very simple device that operates without the need of an external power supply, making it ideal for remote sites. Figure 1 shows a simple schematic of a desiccant dehydrator. A desiccant dehydrator consists of the following equipment:
- Vessel with a filler hatch at the top (for pouring in replenishment salt);
- Support grid to hold the solid salt while allowing the wet gas to pass up through the salt bed;
- Wet gas inlet nozzle and distributor pipe below the salt bed support;
- Dry gas outlet collection pipe and nozzle;
- Bottom nozzle to drain the waste brine; and
- Sight glass to observe the salt level when the bed needs replenishing.
As the wet gas passes up through the salt bed, the salt absorbs moisture from the gas and forms a brine solution that drips down through the bed to the “claim” area on the bottom of the vessel. Salt is gradually dissolved in the process and must be replenished periodically at a frequency depending on the size of the dehydrator, wet gas flow rate, moisture content of the gas, and the type of salt. With two desiccant dehydrator vessels located at a site, one unit can be dehydrating gas while the other is shut down for salt replenishment. Replenishing the salt requires the vessel to be depressurized, which does vent a small amount of methane gas to the atmosphere. Brine may be combined with produced water from the well for offsite disposal.
Desiccant dehydrators can also be used temporarily. Glycol dehydrator maintenance often requires a complete shutdown of the dehydrator unit during the service period which in turn requires production wells to either be shut in or vented to the atmosphere. A desiccant dehydrator can be used in place of a glycol dehydrator during the service periods to avoid interruptions and unnecessary emissions.
Applicability
Desiccant dehydrators can be used permanently in place of a glycol dehydrator where the wellhead gas is cold--below 59 to 70 °F, depending on the desiccant salt used--and pressure is above 100 to 250 psig, again, depending on the desiccant salt used.
Desiccant dehydrators can be used temporarily in situations where a large amount of gas would otherwise be vented during glycol dehydrator maintenance. Desiccant dehydrators can also be used for dehydrating gas captured by reduced emissions completion units following gas well hydraulic fracturing.
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. 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). A default emission factor is also not available to estimate emissions from desiccant dehydrators, and a formula-based methodology reasonably estimates emissions. Further information on calculating desiccant dehydrator emissions is also 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, replacing a glycol dehydrator with a desiccant dehydrator may:
- Reduce air pollution: Reduces emissions of volatile organic compounds and hazardous air pollutants.
- Reduce material and disposal costs: Eliminates glycol chemical costs and degraded glycol disposal costs.
- Reduce maintenance: Eliminates the maintenance time and costs associated with glycol circulation pump maintenance.
Use of a desiccant dehydrator temporarily, during glycol unit maintenance shutdown, may:
- Improve gas supply continuity: Allows a continuity of supply to downstream processing, transmission, and sales outlets, in spite of necessary maintenance shutdown.
- Provide supplemental control: Eliminates the need for venting or flaring during maintenance shutdown.
Lessons Learned
References
Croft, C. P. (2020, May 21). Glycol dehydration versus solid desiccant dehydration.https://www.croftsystems.net/oil-gas-blog/teg-versus-passive-dehydration-systems/
Currie, T. (n.d.). Natural gas deliquescent dehydrator application [Interview]. Compressed Air Best Practices. https://www.airbestpractices.com/industries/oil-gas/natural-gas-deliquescent-dehydrator-applications
Oesterling, D. (2012, November 29). Deliquescent dryer fundamentals: An engineer’s perspective. https://www.vanairsystems.com/deliquescent-dryer-fundamentals-an-engineers-perspective/
Vavro, M. E. (1996, October 23). Minimizing natural gas dehydration costs with proper selection of dry bed desiccants and new dryer technology. SPE Eastern Regional Meeting, Columbus, OH, United States. https://onepetro.org/SPEERM/proceedings-abstract/96ERM/All-96ERM/SPE-37348-MS/59328
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