From Wellhead to Sales Gas: The Gas Plant in Outline
The units of a typical gas train in the order the gas meets them, and the job each one does.
12 min
Activities: 0 of 2
Gas from a well is not a product. It arrives hot or cold, at high pressure, saturated with water, often carrying liquid hydrocarbons, sand, carbon dioxide (CO2) and sometimes hydrogen sulphide (H2S). A pipeline company or a power station will not accept it in that state. A gas plant turns this raw stream into sales gas that meets a written specification, and it recovers valuable liquids on the way.
Two words are used for this work. Gas conditioning usually means the treatment needed to make gas safe and acceptable for transport: removing water, acid gases and enough heavy hydrocarbons to stop liquids forming in the pipeline. Gas processing is the wider term. It also includes recovering natural gas liquids (NGL) — ethane, propane, butanes and heavier components — and separating them into products.
A typical gas train, in the order the gas meets it
- 1
Step 1
Inlet separation and slug catching
Free liquids (condensate and water) and solids are removed. A slug catcher absorbs surges of liquid arriving from the pipeline. - 2
Step 2
Acid gas removal
An amine unit or another process removes H2S and CO2. The removed acid gas goes to sulphur recovery or disposal. - 3
Step 3
Dehydration
Triethylene glycol (TEG) or molecular sieves remove water vapour so that hydrates and free water cannot form downstream. - 4
Step 4
Hydrocarbon dew point control or NGL recovery
The gas is chilled so that heavier hydrocarbons condense and are removed. - 5
Step 5
Fractionation
Recovered liquids are split into products such as ethane, propane, butane and natural gasoline. - 6
Step 6
Compression, metering and export
Sales gas is compressed to pipeline pressure, measured and analysed, then delivered to the customer.
Why the order matters
The sequence is not random. Acid gas removal with water-based amine solutions comes before dehydration, because the amine leaves the gas saturated with water again. Deep dehydration comes before any cold process, because water would freeze or form hydrates in a chiller or a cryogenic exchanger. Not every plant has every unit. A sweet, lean gas field may only need inlet separation, a TEG unit and compression. A sour, rich gas field may need every unit in the list, plus sulphur recovery.
The rest of the course takes each unit in turn and explains how it works, how to operate it, and what goes wrong. It assumes the separation, pressure and flow basics taught in Oil and Gas Processing Fundamentals.
Why is gas usually dehydrated after an amine unit rather than before it?
Put the gas train in order
Arrange the units in the order the gas meets them in a full gas plant.
- 1Acid gas removal (amine unit)
- 2Dehydration (TEG or molecular sieve)
- 3NGL recovery or hydrocarbon dew point control
- 4Sales gas compression and metering
- 5Inlet separation and slug catcher