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Method of extracting ethane from liquefied natural gas

  • US 8,156,758 B2
  • Filed: 08/17/2005
  • Issued: 04/17/2012
  • Est. Priority Date: 09/14/2004
  • Status: Active Grant
First Claim
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1. A method of processing liquefied natural gas (LNG), comprising two alternative modes of operation:

  • (a) operating in a first mode for recovering a portion of natural gas liquids (NGL) by;

    passing LNG through a heat exchanger to provide heated LNG;

    fractionating the heated LNG into a methane-rich vapor stream and a natural gas liquids (NGL) stream;

    passing the methane-rich vapor stream through the heat exchanger, without increasing the pressure of the methane-rich vapor stream, to transfer heat from the methane-rich vapor stream to the LNG passing through the heat exchanger and to provide a two-phase stream that includes a methane-rich liquid phase and a methane-rich vapor phase;

    separating the two-phase stream in a vapor liquid separator into at least a methane-rich liquid portion and a methane-rich gas portion;

    increasing the pressure of the methane-rich liquid portion to provide a sendout liquid stream;

    recovering the sendout liquid stream to provide a sales gas for delivery to a pipeline; and

    (b) operating in a second mode for rejecting a portion of NGL by diverting the LNG to a diverted flow path that bypasses the fractionating to provide sales gas that includes methane and ethane plus for delivery to the pipeline;

    wherein in mode (a), mode (b), or both mode (a) and (b) the following steps are performed;

    providing at least part of a refrigeration duty for the fractionation system by withdrawing a fraction of the LNG before being heated and passing the withdrawn fraction to the fractionation system and passing at least a portion of the methane-rich vapor stream produced by the fractionation system in heat exchange with the LNG to effect cooling of the methane-rich vapor stream and passing at least a portion of the cooled stream to the fractionation system, andheat exchanging the NGL stream with the heated LNG to provide a chilled NGL stream; and

    flashing the chilled NGL stream to substantially atmospheric pressure to provide a flashed NGL stream,wherein the fractionation system comprises a reflux input in fluid communication with a portion of the liquid recovered in the vapor-liquid separator.

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