Operators are often required to dehydrate natural gas streams that are saturated with water vapor to meet pipeline specifications. Water in a natural gas pipelines can result in hydrates that obstruct or plug the pipe. Also, water vapor in a pipeline can cause corrosion due to the presence of carbon dioxide (CO2) or hydrogen sulfide (H2S) in the natural gas.
Most natural gas producers use triethylene glycol (TEG) dehydrators to remove water from natural gas to meet pipeline water content requirements.
In the process, wet gas enters near the bottom of the glycol contactor and comes into contact with lean glycol (water poor) in the absorber contact tower. In the contact tower, water in the natural gas is absorbed by circulating glycol and the natural gas is dehydrated and the gas dew point is reduced. The dehydrated gas is referred to as dry gas and exits through the top of the glycol contactor. The glycol that absorbed the water is called rich glycol. The rich glycol then exits from the bottom of the glycol contactor and flows to the regeneration system. The regeneration system typically includes a glycol flash tank (gas-condensate-glycol separator) and a reboiler.
The glycol flash tank (gas-condensate-glycol separator) serves as a separator to recover entrained flash gas and condensate. It also reduces the pressure of the rich glycol prior to entering the reboiler. In the reboiler, the glycol is heated to boil off water from the glycol to produce lean glycol. The lean glycol is cooled using a heat exchanger and pumped back to the glycol contactor to continue the cycle.
Typical dry gas pipeline requirements can range from 4 to 7 lbs water per MMSCF of natural gas.
Figure 1 below is a diagram of a typical basic glycol dehydrator process flow diagram from Figure 20-58 of Volume II of the GPSA Engineering Data Book, 13th Edition. (Used with permission of Gas Processors Association).
A glycol circulation pump is used to circulate glycol through the system. There are many varieties of pumps used including Kimray positive displacement (gas-injection) pumps, other pneumatic pumps and electric reciprocating and centrifugal pumps. Larger glycol dehydrators often use electric motor-driven pumps.
The reboiler uses a still column (reflux condenser coil) to separate water from the glycol. The still column’s vent gas will contain water vapor and hydrocarbons such as methane, benzene, toluene, ethylbenzene, xylenes, n-hexane and other VOCs.
Glycol Dehydrator Air Pollutants
Natural gas streams contain varying amounts of methane, VOCs and hazardous air pollutants (HAP). HAPs in natural gas include benzene, toluene, ethylbenzene, xylenes, (BTEX), n-hexane and 2,2,4-trimethylpentane. These HAPs are slightly soluble in the TEG used and as a result, HAPs are absorbed in the glycol contactor. Also methane and VOCs (other than BTEX) will be entrained in the rich glycol due to the high operating pressure of the glycol contactor (600 to >1000 psig).
Flash gas liberated from the flash tank (located between glycol contactor and reboiler) will be natural gas that is mostly methane and some VOCs and small amounts of BTEX.
Regeneration of the rich glycol in the glycol reboiler causes methane, VOCs and HAPs to be released with the water vapor exiting the still column vent.
Glycol Dehydrator Emission Sources
The sources of and types of air pollution from a TEG glycol dehydrator include the following:
- Still Column Vent – water, methane, VOCs, BTEX, n-hexane, 2,2,4-trimethylpentane
- Flash Tank – primarily natural gas similar to fuel gas (primarily methane and some VOC and BTEX)
- Glycol pump using high pressure natural gas – primarily natural gas similar to fuel gas
Still Column Vent Emission Control
- Air cooled condensers with noncondensable gases vented to the atmosphere
- Water or glycol cooled condensers with noncondensable gases vented to the atmosphere
- Air cooled, water cooled and glycol cooled condensers with noncondensable gas routed to reboiler burner as fuel or routed to an enclosed combustor or flare
- Air cooled or water cooled condensers with noncondensable gas routed to a vapor recovery unit (VRU)
- Route still column vent gas to the vapor space of storage tank and recover storage tank VOC emissions using a VRU
Glycol Flash Tank Emission Control
- Since the glycol flash tank is a pressure vessel (operating pressure range of 60 to 120 psig) and has a similar makeup as fuel gas it can be routed back to the fuel gas system or sent back to the process.
- Flare or enclosed combustion device.
Optimization Techniques to Reduce Emissions
The EPA Natural Gas STAR Program gives recommended technologies and practices reported by operators as proven ways to reduce emissions from glycol dehydrators. Some of these include:
- Limit glycol circulation rate to only what is needed to dehydrate the gas to the required lbs/MMSCF. NOTE: EPA approved models indicate that the VOC and BTEX emissions are directly proportional to the circulation rate of glycol. Based on the GRI-GLYCalc model, if the glycol circulation rate is reduced by 50%, then BTEX and VOC emission can be reduced by 50%.
- Use electric glycol circulating pumps instead of gas operated pumps.
Air emissions are tightly regulated by air permits and federal and state air quality regulations. These regulations typicall require the units to control still column vent and flash tank emissions. EPA regulations affecting glycol dehydration units include the hazardous air pollutant rules (HAPs) in 40 CFR 63 Subpart HH—National Emission Standards for Hazardous Air Pollutants From Oil and Natural Gas Production Facilities. This regulation impacts onshore oil and gas production facilities glycol dehydrators and some oil storage tanks.