Method for Nitrogen Rejection and or Helium Recovery in an Liquefaction Plant

• US 20100162755A1
• Filed: 12/31/2008
• Published: 07/01/2010
• Est. Priority Date: 12/31/2008
• Status: Active Grant

• Current Assignee: Kellogg Brown & Root LLC (KBR Incorporated)
• Original Assignee: Kellogg Brown & Root LLC (KBR Incorporated)
• Inventors: Anantharaman, Bharthwaj, Crawford, Duffer, Coyle, David A.
• Granted Patent: US 8,522,574 B2
• Time in Patent Office: Days
• US Class Current: 62/621
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/40 Features relating to the provision of boil-up in the bottom of a column
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• 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 2215/04 Recovery of liquid products
• CPC Class Codes: F25J 2240/02 Expansion of a process fluid in a work-extracting turbine (i.e. isentropic expansion), e.g. of the feed stream
• CPC Class Codes: F25J 2240/30 Dynamic liquid or hydraulic expansion with extraction of work, e.g. single phase or two-phase turbine
• CPC Class Codes: F25J 2270/02 Internal refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/04 Internal refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 3/0209 Natural gas or substitute natural gas
• CPC Class Codes: F25J 3/0233 separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0242 separation of CnHm with 3 carbon atoms or more
• CPC Class Codes: F25J 3/0257 separation of nitrogen from air F25J3/04, production of nitrogen in general C01B21/00
• CPC Class Codes: F25J 3/029 of helium

Abstract

Methods of reducing the concentration of low boiling point components in liquefied natural gas are disclosed. The methods involve dynamic decompression of the liquefied natural gas and one or more pre-fractionation vessels. Particular embodiments are suited for recovering helium and/or nitrogen enriched streams from a liquefied natural gas stream.

Claims

1. ) A method of reducing the nitrogen concentration in liquefied natural gas comprising:

passing an 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 static liquid expander to obtain a second expanded LNG stream that contains a vapor phase;

passing the second expanded LNG stream to one or more pre-fractionation vessels for flash equilibrium separation to obtain one or more vapor streams that have increased concentration of nitrogen as compared to the initial LNG stream and a liquid stream that has a reduced concentration of nitrogen as compared to the initial LNG stream;

passing the liquid stream that has a reduced concentration of nitrogen as feed to a fractionation column and withdrawing from an upper portion of the fractionation column a nitrogen enriched stream as compared to the feed to the fractionation column; and

withdrawing from a lower portion of the fractionation column a LNG product stream that has a reduced concentration of nitrogen as compared to the initial LNG stream, wherein at least a portion of at least one of the vapor or liquid streams from the one or more pre-fractionation vessels passes through the first heat exchanger to provide cooling to the initial LNG stream.

2. ) The method of claim 1, further comprising dynamically decompressing at least one of the vapor streams from the one or more pre-fractionation vessels in one or more vapor expanders.

3. ) A method of recovering helium and reducing the nitrogen concentration in liquefied natural gas comprising:

passing an 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 static liquid expander to obtain a second expanded LNG stream that contains a vapor phase;

passing the second expanded LNG stream to one or more helium flash drums for flash equilibrium separation to obtain a helium enriched vapor stream and a LNG stream that has reduced helium concentration;

passing the LNG stream that has reduced helium concentration to one or more pre-fractionation vessels for flash equilibrium separation to obtain a nitrogen enriched vapor stream and a liquid stream that has a reduced concentration of nitrogen;

passing at least a portion of the vapor and liquid streams from the one or more pre-fractionation vessels as feed to a fractionation column and withdrawing from an upper portion of the fractionation column a nitrogen enriched vapor stream as compared to the feed to the fractionation column; and

withdrawing from a lower portion of the fractionation column a LNG product stream that has a reduced concentration of nitrogen as compared to the initial LNG stream, wherein at least one of the vapor or liquid streams from the one or more pre-fractionation vessels pass through the first heat exchanger to provide cooling to the initial LNG stream.

4. ) The method of claim 3, further comprising further processing of the helium enriched vapor stream in a helium recovery facility.

5. ) The method of claim 3, further comprising utilizing the nitrogen enriched vapor stream as fuel gas.

6. ) The method of claim 3, further comprising dynamically decompressing at least one of the vapor streams from the one or more helium flash drums or the one or more pre-fractionation vessels in one or more vapor expanders.

7. ) The method of claim 3, further comprising passing the LNG stream that has reduced helium concentration through a second heat exchanger for cooling prior to entering the one or more pre-fractionation vessels, wherein at least one of the vapor or liquid streams from the one or more pre-fractionation vessels, the helium enriched vapor stream, or the nitrogen enriched vapor stream from the fractionation column pass through the second heat exchanger to provide cooling to the LNG stream that has reduced helium concentration prior to entering the one or more pre-fractionation vessels.

8. ) A method of reducing the concentration of components that have low boiling points in liquefied natural gas comprising:

providing an initial LNG stream at an initial liquefaction temperature and pressure;

providing a first heat exchanger and a first liquid expander in fluid communication with the first heat exchanger;

passing the initial LNG stream through the first heat exchanger and the first liquid expander to reduce the temperature and dynamically decompress the LNG stream to obtain a first expanded LNG stream that has a temperature and pressure less than or equal to the initial liquefaction temperature and pressure;

providing a second liquid expander in fluid communication with and located after the first heat exchanger and the first liquid expander, and decompressing the first expanded LNG stream in the second liquid expander to obtain a second expanded LNG stream that contains a vapor phase;

providing a first pre-fractionation vessel in fluid communication with and located after the second liquid expander and passing the second expanded LNG stream to the first pre-fractionation vessel for flash equilibrium separation to obtain a first vapor stream that has increased concentration of low boiling point components and a third liquid stream that has a reduced concentration of low boiling point components;

providing a fractionation column in fluid communication with and located after the first pre-fractionation vessel and injecting the first vapor stream and third liquid stream to the fractionation column;

withdrawing from an upper portion of the fractionation column a second vapor stream that has an increased concentration of low boiling point components as compared to the initial LNG stream; and

withdrawing from a lower portion of the fractionation column a fourth liquid stream that has a reduced concentration of low boiling point components as compared to the initial LNG stream, wherein at least a portion of one of the first vapor stream or third liquid stream from the pre-fractionation vessel passes through the first heat exchanger to provide cooling to the initial LNG stream.

9. ) The method of claim 8, further comprising passing at least a portion of the second vapor stream through the first heat exchanger to provide cold energy to the initial LNG stream.

10. ) The method of claim 8, further comprising

passing at least a portion of the fourth liquid stream through the first heat exchanger to provide cold energy to the initial LNG stream and to form a partially vaporized fourth liquid stream;

and

reinjecting the partially vaporized fourth liquid stream to the fractionation column to provide heat duty to the fractionation column.

11. ) The method of claim 8, further comprising passing the portion of the third liquid stream from the first heat exchanger to a subsequent pre-fractionation vessel for flash equilibrium separation into subsequent vapor and liquid streams prior to entering into the fractionation column.

12. ) The method of claim 8, further comprising providing a first vapor expander in fluid communication with the first pre-fractionation vessel and the fractionation column, wherein the first vapor expander decompresses the first vapor stream prior to injection into the fractionation column.

13. ) The method of claim 8, further comprising:

providing a second pre-fractionation vessel in fluid communication with and located after the first pre-fractionation vessel;

providing a third liquid expander in fluid communication with and located between the first pre-fractionation vessel and the second pre-fractionation vessel, and decompressing the third liquid stream in the third liquid expander to obtain a fifth liquid stream that contains a vapor phase;

flowing the fifth liquid stream into the second pre-fractionation vessel for flash equilibrium separation to form a third vapor stream that has increased concentration of low boiling point components as compared to the fifth liquid stream and a sixth liquid stream that has a reduced concentration of low boiling point components as compared to the fifth liquid stream; and

flowing the third vapor stream and the sixth liquid stream into the fractionation column.

14. ) The method of claim 13, further comprising providing a second vapor expander in fluid communication with the second pre-fractionation vessel and the fractionation column, wherein the second vapor expander dynamically decompresses the third vapor stream prior to injection into the fractionation column.

15. ) The method of claim 14, further comprising:

flowing at least a portion of the first vapor stream after the first vapor expander through the first heat exchanger to provide cold energy to the initial LNG stream and obtain a warmed first vapor stream; and

combining said warmed first vapor stream exiting the first heat exchanger with the third vapor stream prior to the second vapor expander.

16. ) The method of claim 13, further comprising:

flowing at least a portion of the sixth liquid stream through the first heat exchanger to provide cold energy to the initial LNG stream and obtain a seventh liquid stream with a warmer temperature than the sixth liquid stream; and

flowing the seventh liquid stream to the fractionation column.

17. ) The method of claim 16, wherein the seventh liquid stream provides vapor to the fractionation column needed to strip low boiling point components.

18. ) The method of claim 16, further comprising:

providing a third pre-fractionation vessel in fluid communication with the second pre-fractionation vessel;

flowing the seventh liquid stream to the third pre-fractionation vessel for flash equilibrium separation to obtain a fourth vapor stream that has increased concentration of low boiling point components as compared to the sixth liquid stream and an eighth liquid stream that has a reduced concentration of low boiling point components as compared to the sixth liquid stream; and

flowing the fourth vapor stream and the eighth liquid stream into the fractionation column.

19. ) The method of claim 8, further comprising:

providing a first helium flash drum in fluid communication with and located before the first pre-fractionation vessel;

passing the second expanded LNG stream to the first helium flash drum for flash equilibrium separation to obtain a first helium enriched vapor stream and a first helium reduced liquid stream; and

providing a fourth liquid expander in fluid communication with and located between the first helium flash drum and the first pre-fractionation vessel, and decompressing the first helium reduced liquid stream in the fourth liquid expander prior to entering the first pre-fractionation vessel.

20. ) The method of claim 19, further comprising passing at least a portion of the first helium enriched vapor stream through the first heat exchanger to provide cold energy to the initial LNG stream.

21. ) The method of claim 19, further comprising:

providing a third heat exchanger in fluid communication with and located between the second pre-fractionation vessel and the fractionation column that can cool the third vapor stream by heat exchange with the first helium enriched vapor stream, and

passing at least a portion of the first helium enriched vapor stream through the third heat exchanger to provide cold energy to the third vapor stream prior to the third vapor stream entering the fractionation column.

22. ) The method of claim 19, further comprising providing a second heat exchanger in fluid communication with and located between the first helium flash drum and the fourth liquid expander that cools the first helium reduced liquid stream by heat exchange with the first helium enriched vapor stream.

23. ) The method of claim 22, further comprising flowing at least a portion of the second vapor stream through the second heat exchanger to provide cold energy to the first helium reduced liquid stream.

24. ) The method of claim 22, further comprising flowing at least a portion of the sixth liquid stream through the second heat exchanger prior to the first heat exchanger, to provide cold energy to the first helium reduced liquid stream.

25. ) The method of claim 19, further comprising:

providing a second helium flash drum in fluid communication with and located between the first helium flash drum and the fourth liquid expander;

providing a fifth liquid expander in fluid communication with and located between the first helium flash drum and the second helium flash drum, and decompressing the first helium reduced liquid stream in the fifth liquid expander to obtain a second helium reduced liquid stream that contains a vapor phase;

flowing the second helium reduced liquid stream into the second helium flash drum for flash equilibrium separation to form a second helium enriched vapor stream that has increased concentration of helium as compared to the first helium reduced liquid stream and a third helium reduced liquid stream that has a reduced concentration of helium as compared to the first helium reduced liquid stream;

combining the second helium enriched vapor stream with the first helium enriched vapor stream;

flowing the third helium reduced liquid stream through the second heat exchanger and the fourth liquid expander prior to flowing into the first pre-fractionation vessel.