LNG – How Natural Gas is Liquified?

There are a series of processing stages to produce LNG from the natural gas already extracted.

Stage 1 – Remove Impurities

  • Natural gas is separated from other substances in order to produce ‘pipeline quality’ dry Natural gas
  • The level and types of impurities within the gas will depend upon the composition of the gas provided to the processing plants
  • Processing involves four main steps
Step NumberProcess
1Oil and Condensate Removal
2Water Removal
3Separation of Natural Gas Liquids
4Hydrogen Sulphide  and Carbon Dioxide Removal
  • Carbon Dioxide, Hydrogen Sulphide and Water need to be removed as otherwise they would freeze and cause blockages in the pipes when the gas is cooled
  • The gas goes through a process called fractionation which separates components of gas by their boiling points.

Stage 2 – Liquefication

  • What is left after these processes is a mixture of methane, ethane, and small amounts of propane and butane. Together these make purified natural gas.
  • The gas is now ready to be liquefied.
  • The liquefaction plant acts like a giant refrigerator, chilling the gas until it reaches -162°C.
  • LNG is stored in large, insulated storage tanks and transported. When it reaches it’s destination the LNG is kept in storage tanks until required

Regasification

  • This involves re-heating the gas in order to make it vaporise.
  • This is typically done with seawater which is much warmer than LNG, or by passing the LNG through a heated water bath.
  • Sometimes, the purification done during the earlier processes has an impact on this. The gas can become ‘too clean’
  • To counter this LNG may need to be blended with an inert gas such as nitrogen, or have propane added to make it compatible with other natural gas in the local network
  • The composition of natural gas in the local network can differ from country to country, so the dilution formula is specific to each individual country or network
  • Once successfully regasified the LNG is transported in pipelines and used as any other natural gas.