Methane Emissions Reduction Methods and Technologies in Dehydration Facilities

Stringent methane emissions limits for Alberta Energy Regulator (AER) Directive 60 have come into effect since January 2022, which target methane associated with fugitive emissions (leaks) and vented emissions (due to equipment design or operational procedures). The AER has implemented specific limits on a variety of equipment outlined in Directive 60 (D60), including compressors, pumps, controllers, tanks, and dehydrators/refrigerators.

According to the April 2022 issue of AER D60, glycol dehydrators installed after January 1, 2022, have a methane emissions limit of 68 kg/day and for those installed before January 1, 2022, the limit is 109 kg/day. This has created a requirement to reduce emissions for a significant number of dehydrators which could end up being very costly without first considering optimization opportunities. Fortunately, dehydrators and refrigerators offer very strong opportunities for reduction with simple, cost-efficient operation changes and retrofit plans.

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There are a variety of options for companies to consider when deciding how to reduce emissions from dehydration and refrigeration units, while dehydration facilities tend to have better optimization opportunities than refrigeration units. The main sources of methane emissions in dehydration facilities include:

1. Absorption of hydrocarbons by the triethylene glycol solvent (TEG) commonly used to absorb water from the gas feed. When the TEG is regenerated, water is boiled off which can contain significant quantities of absorbed hydrocarbons. Glycol regeneration loops often also incorporate a flash tank to remove light hydrocarbons (e.g., methane and carbon dioxide) in the rich glycol from the contactor, before the glycol is routed to the regenerator. 
2. Kimray pumps require additional energy supplied by the gas at the absorber pressure. Most of the absorbed gas and what is used for the Kimray pump will be emitted in the flash tank and regenerator overheads.
3. Stripping gas which is used to improve dehydration efficiency is also vented from the still overhead.

This article summarizes the main optimization approaches for reducing methane emissions in glycol regeneration facilities. The risks that come with adopting these methods are also highlighted.

Process Optimization

• Circulation Rate Reduction: Typically, at low to moderate glycol circulation rates, water removal is proportional to glycol circulation rate. By reducing the circulation rate, operators can save money and reduce methane emissions, as less fuel gas will be required for the still reboiler and the Kimray pump (if present). 

Risk: There is a minimum circulation rate that is sufficient to meet sales gas specifications. If the rate requires further reduction below the pump’s minimum circulation rate, it may be necessary to downsize the glycol pump which will come with a capital cost. 

• Stripping gas Reduction: Stripping gas is used to improve the purity of glycol by removing more water during the regeneration process; however, it is usually emitted from the still vent. By reducing the amount of stripping gas, installing an emission control, or removing stripping gas completely, methane emissions drop significantly. There are no extra costs to this modification.

Risk: Not meeting water dewpoint spec on warm days.

• Reboiler Temperature Optimization: Optimization of the reboiler temperature in the still column reduces fuel gas use, especially when glycol purity is higher than required. Optimally, the reboiler temperature should be close to 200 C while also ensuring that there isn’t over circulation of glycol. There would be no extra costs to this modification.

Risk: Temperature should not exceed the degradation temperature of TEG (207 C).

Process Optimization: Equipment Replacement

• Glycol Pump Electrification: The use of Kimray pumps results in higher methane emissions since the gas that provides the motive power for the pump is partially entrained in the rich glycol and will eventually be emitted. Electric pumps don’t require the use of gas, significantly reducing methane emissions.

Risk: Minimal risk, usually a good opportunity if electric power is available.

• Flash Tank Installation: When a flash tank is installed, substantial vapours (which include methane) can be separated from the rich glycol stream. These vapours can be captured and used as fuel gas or sent to a flare/incinerator for destruction, resulting in a large decrease in methane emissions.

Risk: Minimal Risk.

Thermal Combustion

• Flare: A flare can be installed to capture gases from the still column or the flash tank. The gas leaving the still column/flash tank would then be combusted before entering the atmosphere. Combustion of methane produces CO₂ and water vapour, which are less potent greenhouse gases than methane. 

Risk: There are potential operational issues with the flare.

• Incinerator: Similar to a flare stack, an incinerator combusts the gas. However, for a number of reasons including improved mixing of the waste gas and air, the combustion efficiency is improved, compared to flaring. 

Risk: There are potential operational issues with the incinerator.

Vapour Recovery and Combustion in reboiler

• Kenilworth: The Kenilworth is one type of vapour recovery technology that captures the effluent from the still and returns it as a low-pressure fuel gas to the system. The vapours are sent to a temporary condenser tank where the water and hydrocarbon liquids condense out. This is then sent to the still column reboiler. 

Risk: Potential condensate issue by not meeting fuel gas spec due to low higher heating value.

• Jatco BTEX Eliminator: The Jatco captures the gas emissions from the still vent and feeds it through an air-cooled exchanger where condensable liquids are dropped out. The remaining vapors are fed through a closed-loop system and eventually used for burner fuel. 

Risk: Potential condensate issue by not meeting fuel gas spec due to low higher heating value

Other Vapour Recovery

• Slipstream: A Slipstream module captures the emissions from the still vent or downstream condensing tank and uses it as feed into a reciprocating engine. Normally, the gas is not enough to meet all the gas demand of the engine, and the engine is in continuous operation. This requires a condensing tank, must be close to natural gas engines (100-4000 hp) and the still vent needs H2S concentration to be less than 1%.

Risk: Solid/liquid carrying over into engine; Engine internal corrosion and plugging flame arrestor, low higher heating value.

• Vapor Recovery Unit: A vapor recovery unit typically captures the gas emissions from the still vent, which then compresses the gas and routes it to the facility inlet. 100% GHG and BTEX reduction can generally be assumed for this technology, although downtime should be considered. 

Risk: Potential operational issues with compressor. 

Summary:

There are many alternatives for reducing methane emissions in dehydrators and refrigerators. Some options simply require the optimization of existing facilities through circulation rate reduction, stripping gas reduction, and reboiler optimization which also offers opportunities for cost reduction. These options should be considered before evaluating technology installation, as those will be more costly. Each method has its limits and risks and depending on how each unit operates, it is important to assess all options to meet methane reduction goals.

References: 

Alberta Energy Regulator: “Directive 060: Upstream Petroleum Industry Flaring, Incinerating, and Venting”, 2022.

Process Ecology Inc., “PTAC Methane Emissions from Dehys”, 2019.