Glycol Dehydrator Quantification
Glycol dehydrators in the natural gas industry are used to remove water from a wet gas stream using mono-ethylene glycol, diethylene glycol, or, most commonly, triethylene glycol. Operators employ a glycol dehydrator quantification methodology to ensure the consistent assessment of glycol dehydrator methane emissions and perform an informed evaluation of mitigation options.
In natural gas systems, removing water vapor reduces potential pipeline corrosion and can eliminate line blockage caused by a hydrate formation that obstructs the gas flow. Glycol is passed through the natural gas and absorbs water from the gas stream. Lesser amounts of methane, volatile organic compounds, and hazardous air pollutants, are removed from the natural gas producing a “dry” gas and a “wet” glycol.
In the oil and gas sector, a dry substance refers to a substance’s state without water contamination, and when it becomes wet, it means that water has been mixed in.
Wet glycol is sent to a reboiler and heated to boil off the water. The methane, HAPs, and VOCs are boiled out and routed with the steam to be vented or rerouted. Capturing, redirecting, or eliminating the dehydrator’s rerouted vent emissions to an alternative outlet allows for significant fuel gas savings or waste monetization.
The recommended methane emission factor varies by industry segment and does not include the emissions from gas-assisted glycol pumps or stripping gas, which can be a significant source of methane emissions. Surface Solutions’ experts can work with operators to design and schedule a regular testing program for their glycol dehydrator quantification systems.
Glycol Dehydration Systems
Glycol dehydration is a liquid desiccant system for removing water from natural gas and natural gas liquids(NGL). It is the most common and most economical means of removing water.
In the most common glycol dehydration systems, wet gas will enter a tower at the bottom and flow upward. Dry, or what is sometimes referred to as lean glycol, is pumped via a pneumatic or electric pump to a contactor to mix with the natural gas stream. Dry glycol flows down the tower, across trays, or through various packing materials absorbing the water from the natural gas. The dehydrated gas then leaves the tower, returns to the pipeline, or is further processed.
The water-logged glycol leaves the tower and goes to a reconcentration or reboiler system, which is filtered to remove larger impurities. The wet glycol is then heated to allow the trapped water to escape as steam along with other impurities. From there, the purified glycol returns to the tower, and the process repeats.
Glycols used in the oil and gas industry include triethylene glycol(TEG), ethylene glycol(MEG), diethylene glycol(DEG), and tetraethylene glycol(TREG). Triethylene glycol dehydration systems are the most commonly used dehydrators in the oil and gas industry. Operators favor them because of their efficiency at removing water from a natural gas stream and other volatile organic compounds(VOCs) like toluene, ethylbenzene, benzene, and xylene.
Alternative Glycol Dehydrator
Each glycol dehydrator can have various configurations, affecting methane emission levels from their operation. Operators can evaluate the gasses emitted from glycol dehydrators routed to a flare to estimate methane emissions from the flare combustion efficiency.
Alternatively, some dehydrators inject gas into the glycol feed to the reboiler to act as a “stripping gas” to aid in removing water from the glycol at a lower temperature. This gas vents with steam and other gasses. In energy exchange pumps, low-pressure glycol is pumped into the absorber by pistons driven by the high-pressure glycol leaving the absorber.
These alternative dehydrators are less common because wet glycol can leak through the internal pump seals and contaminate the dry glycol, causing the dehydrator to operate less efficiently and requiring a higher circulation rate to achieve dry gas moisture specifications. Surface Solutions can help determine the best solution and testing program for glycol dehydrator quantification systems.
Issues With Inaccurate Measurements
Accurate emission level measurements ensure operators minimize their economic costs and max out the value of gas saved. While measurements are highly encouraged whenever possible, it is important first to determine where the gas stream from the regenerator is routed to help ensure that the measurement points are accurately identified.
For triethylene glycol (TEG) systems, the best method for calculating glycol dehydrator emissions of methane as well as volatile organic compounds, benzene, toluene, ethylbenzene, and xylenes. The water content of natural gas at saturation depends on temperature and pressure. With the gas’s increasing pressure, the water content decreases, and with increasing temperature, the water content in the gas increases.
Natural gas, in combination with liquid water, can form methane hydrate. Methane hydrate is a solid in which a large amount of methane is trapped within the crystal structure of water, forming a solid similar to ice. The methane hydrate production from a unit amount of water is higher than the ice formation. The methane hydrates formed by cooling may plug the valves, the fittings, or pipelines.
