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

• Current Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Original Assignee: Exxonmobil Upstream Research Company (Exxon Mobil Corporation)
• Inventors: Denton, Robert D., Oelfke, Russell H., Brimm, Allen E.
• Primary Examiner(s): Jules, Frantz
• Assistant Examiner(s): BALDRIDGE, LUKAS M
• Application Number:
  US11/662,027
• Publication Number:
  US 20080087041A1
• Time in Patent Office: 2,435 Days
• Field of Search: 62/501, 626/17, 626/20, 626/18, 626/19, 626/21, 626/22, 626/28
• US Class Current: 62/620
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/74 Refluxing the column with at least a part of the partially condensed overhead gas
• CPC Class Codes: F25J 2205/02 using simple phase separation in a vessel or drum
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2215/02 Mixing or blending of fluids to yield a certain product
• CPC Class Codes: F25J 2230/08 Cold compressor, i.e. suction of the gas at cryogenic temperature and generally without afterstage-cooler
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2245/90 the recycled stream being boil-off gas from storage
• CPC Class Codes: F25J 2280/02 Control in general, load changes, different modes ("runs"), measurements
• CPC Class Codes: F25J 2290/34 Details about subcooling of liquids
• CPC Class Codes: F25J 3/0214 Liquefied natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0238 separation of CnHm with 2 carbon atoms or more

Abstract

Methods and systems for recovery of natural gas liquids (NGL) and a pressurized methane-rich sales gas from liquefied natural gas (LNG) are disclosed. In certain embodiments, LNG passes through a heat exchanger, thereby heating and vaporizing at least a portion of the LNG. The partially vaporized LNG passes to a fractionation column where a liquid stream enriched with ethane plus and a methane-rich vapor stream are withdrawn. The withdrawn methane-rich vapor stream passes through the heat exchanger to condense the vapor and produce a two phase stream, which is separated in a separator into at least a methane-rich liquid portion and a methane-rich gas portion. A pump pressurizes the methane-rich liquid portion prior to vaporization and delivery to a pipeline. The methane-rich gas portion may be compressed and combined with the vaporized methane-rich liquid portion or used as plant site fuel.

Claims

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, and

heat 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.

2. The method of claim 1, wherein the methane concentration of the sales gas is substantially the same as the methane concentration of the methane-rich liquid portion.

3. The method of claim 1, wherein fractionating the heated LNG occurs in a fractionating tower, which produces the methane-rich vapor stream at a tower output pressure, and wherein the pressure of the methane-rich vapor stream entering the heat exchanger is substantially the same pressure as the tower output pressure.

4. The method of claim 1, further comprising increasing the pressure of the LNG before passing the LNG through the heat exchanger.

5. The method of claim 1, further comprising:

mixing a compressed boil-off vapor stream from an LNG tank with an LNG liquid stream from the LNG tank increased to a first pressure, wherein the mixing provides an LNG feed stream; and

increasing the pressure of the LNG feed stream to a second pressure to provide the LNG for passing through the heat exchanger.

6. The process of claim 5, wherein the first pressure ranges from 400 psia to 600 psia.

7. The process of claim 5, wherein the second pressure ranges from 1000 psia to 1300 psia.

8. The method of claim 1, wherein the methane-rich liquid phase constitutes at least 85 weight percent of the two-phase stream.

9. The method of claim 1, wherein the methane-rich liquid phase constitutes at least 95 weight percent of the two-phase stream.

10. The method of claim 1, wherein passing the methane-rich vapor stream through the heat exchanger occurs without increasing the pressure of the methane-rich vapor stream, and wherein the methane-rich liquid phase occupies at least 85 weight percent of the two-phase stream.

11. The method of claim 1, wherein the sendout liquid stream is at a pressure of at least 1000 psia.

12. The method of claim 1, wherein delivery of sales gas to a pipeline includes transporting methane-rich gas at a pressure of at least 800 psia via the pipeline.

13. The method of claim 1, wherein the methane-rich vapor stream and the sendout liquid stream each has a methane concentration of at least 98 mole percent.

14. The method of claim 1, wherein the NGL stream has an ethane plus concentration of at least 98 mole percent.

15. The method of claim 1, further comprising utilizing at least part of the methane-rich gas portion as a plant site fuel.

16. The method of claim 1, further comprising boosting the pressure of at least part of the methane-rich gas portion for delivery to the pipeline.

17. The method of claim 1, further comprising heat exchanging the NGL stream with the heated LNG to chill the NGL stream.

18. The method of claim 1, further comprising passing the flashed NGL stream to storage.

19. The method of claim 1, further comprising:

splitting a part of the methane-rich liquid portion into a reflux stream; and

chilling the reflux stream against the heated LNG to provide a reflux for fractionating the heated LNG.

20. The process of claim 1, wherein the NGL stream has ethane as a predominant component.

21. The process of claim 1, wherein the pressure of LNG of step (a) is at or near atmospheric pressure.