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Southern African Natural Gas Advisory Service Service
How is it processed and produced?
The purpose of a natural gas processing plant is to produce what is referred to as ' pipeline quality dry natural gas '. The processing of natural gas is a multi-stage series of activities that clean raw natural gas to remove other non-methane constituents, impurities and fluids.
Raw natural gas as it comes out of the ground at the ' well head ' must be treated and purified to comply with the necessary quality standards specified by pipeline companies.
Raw gas from underground is gathered on the surface from a group of production wells for initial processing before being sent by pipeline to a gas processing plant. This initial stage of processing removes free liquid water and natural gas condensates.
In oil associated production situations the condensate typically contains methane gas, benzene, butane and other matter which exist in a liquid condition under pressure when underground. These are generally referred to as ' natural gas liquids '.
Raw gas with a high level of acid is also referred to as ‘ sour gas ‘. The separation of water and condensate from gas is part of the gas processing system, which includes removal of toxic elements that can cause corrosion.
Once separated this condensate is usually sent to an oil refinery for separate processing into finished products such as butane, propane and isobutene. Following the removal of associate water and condensate the residue of mainly natural gas is then passed to the gas processing plant.
There are many different configurations of the processes involved in oil and natural gas processing so the description that follows is intended to provide a general overview of the major activities that are involved in the processing of natural gas.
In large complex natural gas processing plants contaminants are removed and the heavier hydrocarbons are captured for other commercial applications.
Natural gas is referred to as ' sweet gas ' when it contains little hydrogen sulphide and ' sour gas ' when it does contain hydrogen sulphide. Gas that comes from methane deposits in coal seams is referred to as ' coal bed gas ' or ' coal bed methane ' (in Australia they refer to it as ' coal seam gas ').
Once in the processing plant the gas purification processes separate from the methane various unwanted constituents such as carbon dioxide and hydrogen sulphide (known as acid gas).
Nitrogen and helium are also removed together with water vapour and dissolved salts. Sometimes tiny amounts of mercury are present and have to be removed. Also radon, radioactive gas when it is sometimes present must also be treated.
Processed natural gas ready for dispatch to customers must comply with a demanding range of quality standards that will include matters such as the following:
- Have heating properties within a specified range. In the U.S. for example this will typically be about 1035 BTU per cubic foot of natural gas at 1 atmosphere and 60 degrees Fahrenheit.
- Be delivered at a specified dew point temperature.
- Be free of particulate solids and liquid water.
- Contain no more than trace amounts of constituents such as hydrogen sulfide, nitrogen and carbon dioxide.
- Be dehydrated of water vapor to avoid formation of methane hydrates within the processing plant or afterwards in the gas transmission pipelines.
