Field Operations

Glycol Dehydrators: Drying the Gas and Controlling the Vent

Natural gas comes out of the well saturated with water vapor, and that water cannot stay in the gas if it is to be sold. Moisture in a gas line forms hydrates, drives corrosion, and freezes in cold weather, all of which plug the line and stop the flow of sales gas. That is why gas well sites and processing facilities dry the gas before it enters the gathering lines, and the workhorse of gas dehydration is the glycol dehydrator. This article covers how the units work, what happens to the benzene, toluene, ethylbenzene and xylene they capture, and how the EPA regulates their emissions.

Why natural gas has to be dried

Raw natural gas leaving the wellhead is wet. It carries water vapor picked up from the reservoir, and the amount varies with temperature and pressure. If that wet gas moves into gathering lines and sales pipelines, several problems follow:

• Hydrates: solid, ice-like crystals of water and gas that form at pipeline pressure and temperature, blocking valves, fittings and the line itself
• Corrosion: water in the line reacts with the gas and with steel, eating away pipe walls from the inside
• Freezing: water condenses and freezes in cold weather, plugging chokes and regulators
• Product quality: pipeline companies set water content limits on the gas they accept, and wet gas fails the spec

Dehydration removes the water before these problems start. The dried gas moves on to the pipeline and the removed water is handled separately. In the same way that gas compression equipment keeps pressure up through the gathering system, dehydration keeps the gas dry enough to flow and sell.

How a glycol dehydrator works

The heart of the system is the contactor, or absorber, a vertical vessel where wet gas meets a stream of glycol. Wet gas enters the bottom and rises through trays or packing, while lean glycol is pumped in at the top and falls through the same internals. Glycol absorbs water out of the gas, so the gas leaves the top of the contactor dry and the glycol leaves the bottom water-rich. The glycol is almost always triethylene glycol, or TEG, chosen because it absorbs water strongly and can be reused.

Rich glycol is not thrown away; the unit regenerates it. The water-rich glycol flows to the reboiler and regenerator, where it is heated to boil the water off. Water vapor leaves through the still vent, and the lean, dry glycol is cooled and pumped back to the top of the contactor to start the cycle again. Pumps move the glycol through the loop, and filters remove solids and decomposition products that would otherwise foul the trays. A well-tuned unit runs that loop continuously at a steady rate and at temperatures that drive the water off.

What BTEX is and where it goes

Benzene, toluene, ethylbenzene and xylene, known together as BTEX, are volatile aromatic compounds that come out of the ground with the gas. When gas flows through the contactor, the glycol absorbs BTEX along with the water. The BTEX travels with the rich glycol into the regenerator, and when the glycol is heated it boils off and leaves through the still vent or reboiler vent with the water vapor.

The same vent stream carries volatile organic compounds, or VOC, and methane. A dehydrator doing its job on water also concentrates a small hydrocarbon emission source at one point, and that point is what the regulations target. The amount emitted depends on the size of the unit and the composition of the gas, but for any dehydrator treating rich gas the still vent deserves attention.

How the EPA regulates dehydrator emissions

The EPA regulates oil and gas dehydration under the New Source Performance Standards, or NSPS, in 40 CFR 60, specifically Subparts OOOO, OOOOa and OOOOb. These rules set emission control requirements for glycol dehydrators, and the newest subpart, OOOOb, applies to facilities that commenced construction, modification or reconstruction after its effective date. For larger dehydrators, the rules generally require emission controls, such as routing the vent to a combustion device, recovering the vapors, or meeting an emissions limit instead.

The key point for operators: applicability depends on how much the unit processes and what it emits, so a small dehydrator can fall outside the requirements while a large one cannot ignore them. Because the rules have changed over time, a unit built years ago may fall under a different subpart than one installed this year. The EPA methane rule for oil and gas builds on the same NSPS structure and tightens the picture further.

Beyond the rules themselves, the EPA's Natural Gas STAR program documents practices that cut dehydrator emissions, including combustion devices that destroy the BTEX, VOC and methane in the vent stream, vapor recovery units that capture the vapors instead of venting them, and process improvements such as controlling the circulation rate so the unit does not strip more than it needs to.

Field checks that keep a dehydrator compliant

Emissions control on a dehydrator is not a one-time installation; it only works when the unit runs as designed. The checks that matter in the field are straightforward:

• Inspect the still vent and any control device on a regular schedule, looking for visible vapors, leaks, or signs the device is not operating
• Check for liquid carryover, meaning glycol or water leaving the contactor with the gas instead of staying in the loop
• Monitor the glycol circulation rate and the reboiler and contactor temperatures, because off-spec conditions change what the unit vents
• Do leak checks on fittings, pumps and flanges, where small methane and VOC leaks hide
• Keep the records that show the unit meets its control requirements: inspections, maintenance, circulation rates, temperatures and any monitoring data

That last point is where operations and compliance meet. A regulator audit is built on records, and a unit with no paper trail looks like one that was never checked. Teams that log dehydrator inspections and maintenance against field tickets keep the evidence attached to the job, showing when the work happened, on which unit, and by whom. OpsFlo tracks dehydrator maintenance and emissions recordkeeping alongside field tickets, dispatch, approvals, timesheets and documents, which keeps the compliance trail current without a second filing system.

Sources and further reading

• EPA Natural Gas STAR Program
• 40 CFR 60 Subpart OOOOb, NSPS for oil and gas
• EPA oil and natural gas air pollution control