ENHANCED NITROGEN REMOVAL IN AN LNG FACILITY

• US 20090277217A1
• Filed: 05/08/2008
• Published: 11/12/2009
• Est. Priority Date: 05/08/2008
• Status: Active Grant

• Current Assignee: Conocophillips Company (ConocoPhillips)
• Original Assignee: Conocophillips Company (ConocoPhillips)
• Inventors: Ortego, J. Dale JR., Ransbarger, Weldon L.
• Granted Patent: US 9,528,759 B2
• Time in Patent Office: Days
• US Class Current: 62/612
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/004 by flash gas recovery F25J1/0267 takes precedence
• CPC Class Codes: F25J 1/0052 by vaporising a liquid refrigerant stream
• CPC Class Codes: F25J 1/021 using a deep flash recycle loop
• CPC Class Codes: F25J 1/023 for the combustion as fuels, i.e. integration with the fuel gas system
• CPC Class Codes: F25J 1/0238 Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
• CPC Class Codes: F25J 1/0241 wherein the overhead cooling comprises providing reflux for a fractionation step
• CPC Class Codes: F25J 1/0265 comprising cores associated exclusively with the cooling of a refrigerant stream, e.g. for auto-refrigeration or economizer
• CPC Class Codes: F25J 1/0283 Gas turbine as the prime mechanical driver
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• CPC Class Codes: F25J 2200/08 in a triple pressure main column system
• CPC Class Codes: F25J 2200/10 in a quadruple, or more, column or pressure 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 2200/76 Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
• CPC Class Codes: F25J 2200/78 Refluxing the column with a liquid stream originating from an upstream or downstream fractionator column
• CPC Class Codes: F25J 2215/04 Recovery of liquid products
• CPC Class Codes: F25J 2220/62 Separating low boiling components, e.g. He, H2, N2, Air
• CPC Class Codes: F25J 2220/64 Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/88 : Quasi-closed internal refrigeration or heat pump cycle, if not otherwise provided
• 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 3/0257 : separation of nitrogen from air F25J3/04, production of nitrogen in general C01B21/00

Abstract

An LNG facility employing an enhanced nitrogen removal system that concentrates the amount of nitrogen in the feed stream to a nitrogen removal unit (NRU) to thereby increase the separation efficiency of the NRU. In one embodiment, the nitrogen removal system comprises a multistage separation vessel operable to separate nitrogen from a cooled natural gas stream. At least a portion of the resulting nitrogen-containing stream exiting the multistage separation vessel can be used as a refrigerant, processed to a nitrogen removal unit, and/or utilized as fuel gas for the LNG facility.

Claims

1. A process for liquefying a natural gas stream in an LNG facility, said process comprising;

(a) cooling at least a portion of said natural gas stream in a first heat exchanger of a first upstream refrigeration cycle via indirect heat exchange with a first pure-component refrigerant to thereby provide a cooled natural gas stream;

(b) cooling at least a portion of said cooled natural gas stream in a cooling pass of a second heat exchanger in an open-loop methane refrigeration cycle to thereby provide a cooled predominantly methane stream;

(c) separating at least a portion of said cooled predominantly methane stream in a multistage separation vessel to thereby provide a predominantly vapor stream and a predominantly liquid stream; and

(d) passing at least a portion of said predominantly vapor stream through a warming pass of said second heat exchanger to thereby accomplish at least a portion of said cooling of step (b),

wherein said multistage separation vessel is positioned downstream of said cooling pass and upstream of said warming pass of said second heat exchanger,

wherein the nitrogen mole fraction of said predominantly vapor stream is at least about 1.25 times greater than the nitrogen mole fraction of said cooled predominantly methane stream introduced into said multistage separation vessel.

2. The process of claim 1, wherein step (d) causes warming of said predominately vapor stream to thereby provide a warmed predominately vapor stream, further comprising separating said warmed predominately vapor stream into a refrigerant fraction and a removed fraction and introducing said refrigerant fraction into a methane compressor of said open-loop methane refrigeration cycle.

3. The process of claim 2, further comprising introducing at least a portion of said removed fraction into a nitrogen removal unit.

4. The process of claim 2, further comprising utilizing at least a portion of said removed fraction as fuel gas in one or more locations within said LNG facility.

5. The process of claim 1, further comprising using a heavies removal column located upstream of said open-loop methane refrigeration cycle to separate said cooled natural gas stream into a heavies-depleted stream and a heavies-rich stream, wherein said at least a portion of said cooled natural gas stream introduced into said second heat exchanger comprises at least a portion of said heavies-depleted stream.

6. The process of claim 5, further comprising combining a predominately methane refrigerant stream from said open-loop methane refrigeration cycle with at least a portion of said heavies-depleted stream to thereby form a combined predominantly methane stream, wherein said cooled natural gas steam introduced into said second heat exchanger comprises at least a portion of said combined predominantly methane stream.

7. The process of claim 1, wherein the nitrogen mole fraction of said predominately vapor stream is at least 2 times greater than the nitrogen mole fraction of said cooled predominately methane stream introduced into said multistage separation vessel.

8. The process of claim 1, wherein said cooled predominately methane stream introduced into said multistage separation vessel has a nitrogen concentration of less than about 15 mole percent, wherein said predominately vapor stream has a nitrogen concentration of at least 20 mole percent.

9. The process of claim 8, wherein said predominately vapor stream has a nitrogen concentration of at least 30 mole percent.

10. The process of claim 1, further comprising flashing at least a portion of said predominately liquid stream to thereby provide a two-phase stream and using at least a portion of the flash vapor from said two-phase stream to provide at least a portion of said cooling of step (b).

11. The process of claim 1, further comprising flashing said cooled predominately methane stream prior to introduction into said multistage separation vessel.

12. The process of claim 1, wherein said multistage separation vessel comprises at least three theoretical stages.

13. The process of claim 1, further comprising introducing a stripping gas stream and/or a reflux stream into said multistage separation vessel.

14. The process of claim 13, wherein said stripping gas stream and/or said reflux stream comprise at least a portion of said predominantly vapor stream.

15. The process of claim 13, further comprising withdrawing a liquid stream from the lower portion of said multistage separation vessel and warming at least a portion of the withdrawn liquid stream via indirect heat exchange with said reflux stream prior to introducing said reflux stream into said multistage separation vessel.

16. The process of claim 13, further comprising flashing at least a portion of said stripping gas stream prior to introducing said stripping gas stream into said multistage separation vessel.

17. The process of claim 1, wherein said first pure-component refrigerant comprises predominantly propane, propylene, ethane, or ethylene.

18. The process of claim 17, further comprising, prior to step (b), further cooling said cooled natural gas stream via indirect heat exchange with a second refrigerant in a second upstream refrigeration cycle to thereby produce a further cooled natural gas stream, wherein said at least a portion of said cooled natural gas stream introduced into said second heat exchanger comprises at least a portion of said further cooled natural gas stream.

19. The process of claim 18, wherein said first refrigerant comprises predominantly propane or propylene and said second refrigerant comprises predominantly ethane or ethylene.

20. A process for liquefying a natural gas stream in an LNC facility, said process comprising:

(a) cooling said natural gas stream in an upstream refrigeration cycle to thereby provide a cooled natural gas stream;

(b) separating at least a portion of said cooled natural gas stream in a heavies removal column to thereby provide a predominantly methane overhead stream and a bottoms stream;

(c) cooling at least a portion of said predominantly methane overhead stream in a heat exchanger of an open-loop methane refrigeration cycle to thereby provide a cooled predominantly methane stream;

(d) flashing at least a portion of said cooled predominantly methane stream to thereby provide a two-phase predominantly methane stream;

(e) separating at least a portion of said two-phase predominantly methane stream in a multistage separation vessel to thereby produce a predominantly vapor stream and a predominantly liquid stream;

(f) passing at least a portion of said predominantly vapor stream through said heat exchanger to thereby accomplish at least a portion of said cooling of step (c), wherein said at least a portion of said predominantly vapor stream passed through said heat exchanger is withdrawn from said heat exchanger as a warmed vapor stream;

(g) dividing at least a portion of said warmed vapor stream into a refrigerant fraction and a removed fraction;

(h) compressing at least a portion of said refrigerant fraction in a methane compressor of said open-loop methane refrigeration cycle to thereby produce a compressed refrigerant stream;

(i) cooling at least a portion of said compressed refrigerant stream in said upstream refrigeration cycle to thereby produce a cooled refrigerant stream; and

(j) introducing at least a portion of said cooled refrigerant stream into said multistage separation vessel as a separation-enhancing stream.

21. The process of claim 20, wherein said separation-enhancing stream comprises a reflux stream.

22. The process of claim 21, further comprising, prior to step (j), withdrawing a predominantly liquid side stream from the lower portion of said multistage separation vessel and using at least a portion of the withdrawn side stream to further cool said cooled refrigerant traction to thereby provide a cooled predominantly liquid stream, wherein said reflux stream comprises at least a portion of said cooled predominantly liquid stream.

23. The process of claim 20, wherein said separation-enhancing stream comprises a stripping gas stream.

24. The process of claim 23, further comprising, prior to step (j), flashing at least a portion of said cooled refrigerant stream to thereby provide a two-phase refrigerant stream, wherein said stripping gas stream comprises at least a portion of said two-phase refrigerant stream.

25. The process of claim 20, wherein the nitrogen mole fraction of said predominantly vapor stream is at least 2 times greater than the nitrogen mole fraction of said two-phase predominately methane stream introduced into said multistage separation vessel.

26. The process of claim 20, wherein said cooled predominantly methane stream comprises less than 15 mole percent nitrogen, wherein at least a portion of said removed fraction is used as fuel gas at one or more locations within said LNG facility.

27. The process of claim 20, wherein at least a portion of said removed fraction is routed to a nitrogen removal unit.

28. A facility for liquefying a stream of natural gas, said facility comprising:

a first refrigeration cycle comprising a first heat exchanger, wherein said first heat exchanger defines a first cooling pass, wherein said first cooling pass comprises a first warm fluid inlet and a first cool fluid outlet;

a second refrigeration cycle comprising a second heat exchanger, wherein said second heat exchanger defines a second cooling pass and a second warming pass, wherein said second cooling pass comprises a second warm fluid inlet and a second cool fluid outlet, wherein said second warming pass comprises a second cool fluid inlet and a second warm fluid outlet; and

a multistage separation vessel defining a first fluid inlet, an upper vapor outlet, and a lower liquid outlet,

wherein said multistage separation vessel is positioned downstream of said first cooling pass of said first heat exchanger and upstream of said second warming pass of said second heat exchanger,

wherein said first cool fluid outlet of said first cooling pass is in fluid flow communication with said second warm fluid inlet of said second cooling pass,

wherein said second cool fluid outlet of said second cooling pass is in fluid flow communication with said first fluid inlet of said multistage separation vessel, and

wherein said upper vapor outlet of said multistage separation vessel is in fluid flow communication with said second cool fluid inlet of said second warming pass.

29. A facility according to claim 28, wherein said second warm fluid outlet of said second heat exchanger is in fluid flow communication with at least one fuel gas user located within said LNG facility.

30. A facility according to claim 28, further comprising a refrigerant compressor defining a suction port and a discharge port, further comprising a nitrogen removal unit (NRU) defining a feed gas inlet, a nitrogen-rich outlet, and a nitrogen-depleted outlet, wherein said second warm fluid outlet of said second heat exchanger is in fluid flow communication with said suction port of said refrigerant compressor and said feed gas inlet of said NRU.

31. A facility according to claim 30, wherein said nitrogen-depleted outlet of said NRU is in fluid flow communication with said suction port of said refrigerant compressor.

32. A facility according to claim 30, wherein said first heat exchanger or said second heat exchanger further comprises a third cooling pass defining a third warm fluid inlet and a third cool fluid outlet, wherein said multistage separation vessel further comprises a second fluid inlet, wherein said discharge port of said refrigerant compressor is in fluid flow communication with said third warm fluid inlet of said third cooling pass, wherein said third cool fluid outlet of said third cooling pass is in fluid flow communication with said second fluid inlet of said multistage separation vessel.

33. A facility according to claim 32, further comprising an expander fluidly disposed between said third cool fluid outlet of said third cooling pass and said second fluid inlet of said multistage separation vessel.

34. A facility according to claim 32, wherein said second fluid inlet is located near the upper portion of said multistage separation vessel and is configured to receive a reflux stream.

35. A facility according to claim 32, wherein said second fluid inlet is located near the lower portion of said multistage separation vessel and is configured to receive a stripping gas stream.

36. A facility according to claim 32, wherein said first heat exchanger further comprises said third cooling pass, wherein said second heat exchanger further comprises a fourth cooling pass defining a fourth warm fluid inlet and a fourth cool fluid outlet, wherein said third cool fluid outlet of said first heat exchanger is in fluid flow communication with said fourth warm fluid inlet of said second heat exchanger, wherein said fourth cool fluid outlet of said second heat exchanger is in fluid flow communication with said second fluid inlet of said multistage separation vessel.

37. A facility according to claim 28, wherein said first refrigeration cycle comprises a propane, propylene, ethane, or ethylene refrigeration cycle.

38. A facility according to claim 28, further comprising an expansion cooling section defining a liquid feed inlet and an LNG outlet, wherein said lower liquid outlet of said multistage separation vessel is in fluid flow communication with said liquid feed inlet of said expansion cooling section.