Patent ID: 8522574

Claim:
A method of reducing the concentration of components having low boiling points in liquefied natural gas comprising: providing an initial LNG stream at an initial liquefaction temperature and pressure; passing the initial LNG stream through a first heat exchanger and a first liquid expander to reduce the temperature and dynamically decompress the LNG stream to obtain a first expanded LNG stream; decompressing the first expanded LNG stream in a second liquid expander to obtain a second expanded LNG stream containing a vapor phase; passing the second expanded LNG stream to a first pre-fractionation vessel for flash equilibrium separation to obtain a first vapor stream and a first liquid stream; decompressing the first vapor stream in a first vapor expander to produce an expanded first vapor stream; introducing the expanded first vapor stream to a fractionation column, wherein the expanded first vapor stream is introduced to an upper section of the fractionation column; decompressing the first liquid stream in a third liquid expander to obtain a second liquid stream comprising a vapor phase; separating the second liquid stream in a second pre-fractionation vessel for flash equilibrium separation to provide a second vapor stream and a third liquid stream; introducing the second vapor stream and the third liquid stream to the fractionation column; withdrawing a third vapor stream as an overhead from the fractionation column; withdrawing from a lower portion of the fractionation column a fourth liquid stream; passing at least a portion of the third vapor stream through the first heat exchanger to cool the initial LNG stream; passing at least a portion of the third liquid stream through the first heat exchanger to cool the initial LNG stream and then to a third pre-fractionation vessel for flash equilibrium separation into a fourth vapor stream and a fifth liquid stream; and introducing the fourth vapor stream and the fifth liquid stream to the fractionation column.