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Natural gas is treated to remove contaminants including CO2, water and mercury, then chilled to about –260°F in successively colder heat exchangers that use propane, ethylene and methane as refrigerants. The resulting LNG is pumped into insulated storage tanks before being loaded on ships. Gases which boil out as LNG warms in the storage tank are returned to the process to be reliquefied. Receiving terminals transfer LNG to storage tanks where it is re-gasified and piped to users.
This approach is used at the Darwin LNG plant in Australia, for example. At that site, the GE LM2500+G4 is used along with GE compressors (Figures 2 & 3) as follows.
Propane: 2 X PGT25+G4 and 3MCL1405
Ethylene: 2 X PGT25+G4 and 2MCL1006
Methane: 2 X PGT25+G4 and the following in series: MCL806 + MCL 806 + BCL608
The scale of the Darwin and Sabine Pass facilities is far from unusual. A joint venture comprised of Zachry Industrial and CB & I are engineering the Freeport Liquefaction Project near Freeport, Texas. This will comprise three LNG liquefaction trains (each rated at 4.4 million tons per year).