Method and apparatus for producing liquified natural gas

• US 6,269,656 B1
• Filed: 09/18/1998
• Issued: 08/07/2001
• Est. Priority Date: 09/18/1998
• Status: Expired due to Term

• Current Assignee: Richard P. Johnston
• Original Assignee: Richard P. Johnston
• Inventors: Johnston, Richard P.
• Primary Examiner(s): DOERRLER, WILLIAM CHARLES
• Application Number:
  US09/156,241
• Time in Patent Office: 1,054 Days
• Field of Search: 626/11, 626/13, 626/05, 626/15
• US Class Current: 62/613
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/0037 of a return stream
• CPC Class Codes: F25J 1/004 by flash gas recovery F25J1/0267 takes precedence
• CPC Class Codes: F25J 1/0201 using only internal refrigeration means, i.e. without external refrigeration
• CPC Class Codes: F25J 1/0232 integration within a pressure letdown station of a high pressure pipeline system
• CPC Class Codes: F25J 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways

Abstract

Both a method and apparatus for producing liquified natural gas (LNG) at a well head or other source where cool high pressure natural gas is provided. The natural gas flow from the source is cleaned, if required, and is split into two portions. The first flow portion goes to a primary heat exchanger, then to a second heat exchanger or super cooler, and is thereafter throttled into a LNG tank wherein a part thereof flashes to liquid natural gas and a part thereof becomes a very cold saturated vapor to be vented from the LNG tank. This vent remainder of the first flow portion enters the super cooler as a coolant therefor. The second flow portion enters an expander where work is extracted and the temperature and pressure of the second flow portion are lowered. The cold lower pressure second portion combines with the partially warmed vent remainder of the first portion from the super cooler and passes into the primary heat exchanger as the initial coolant for the first portion. The combined warmed second portion and first portion vent remainder then pass out of the system into a low pressure receiver such as a pipeline. The combined portions are at a pressure equal to or greater than that in the receiver. In a second embodiment the supercooler is eliminated, somewhat reducing the liquid natural gas produced.

Claims

1. A method for converting a fraction of natural gas from a source to liquid natural gas comprising the steps of providing a source of pressurized, cool, clean natural gas, splitting said natural gas into first and second portions, causing said first portion to pass through at least a first heat exchanger to cool said first gas portion, causing said first gas portion to pass through a restriction into a liquid natural gas collector wherein a part of said first gas portion flashes to liquid natural gas, the remainder comprising a very cold saturated natural gas, venting said remainder through said at least first heat exchanger to serve as a cooling medium therefor, directing said second portion of said cleansed natural gas to an expander, expanding said second portion by extracting expander work, combining said cooled and expanded second portion with said vent remainder prior to passage thereof through said at least first heat exchanger, thereafter conducting said combined vent remainder of said first gas portion and said second gas portion to a receiver, and assuring that said combined vent remainder of said first gas portion and said second gas portion are at a pressure equal or greater than the pressure in said receiver.

2. The method claimed in claim 1 including the steps of providing a second heat exchanger to serve as a supercooler, locating said second heat exchanger between said first heat exchanger and said restriction, conducting said first natural gas portion from said first heat exchanger to said second heat exchanger and from said second heat exchanger to said restriction, conducting said vent remainder of said first gas portion as a cooling medium through said second heat exchanger, thereafter combining said vent remainder with said second gas portion from said expander, directing said combined vent remainder and second gas portion through said first heat exchanger as a cooling medium therefor, conducting said combined second gas portion and said vent remainder of said first gas portion to a receiver, and assuring that said combined second gas portion and vent remainder and said first gas portion are at a pressure equal to or greater than the pressure in said receiver.

3. The method claimed in claim 1 including the step of determining the split of said natural gas from said source by the pressure relationship between said source and said receiver, by the properties of the liquid natural gas, by optimization of the heat exchange process and by the thermodynamic efficiency of said at least first heat exchanger and said expander.

4. The method claimed in claim 1 including the steps of conveying said natural gas directly from said source to a purifier, removing unwanted constituents therefrom and thereafter splitting said purified natural gas into said first and second flow portions.

5. The method claimed in claim 1 wherein said restriction is a throttle valve.

6. The method claimed in claim 1 wherein said collector is a liquid natural gas tank.

7. The method claimed in claim 1 wherein said at least first heat exchanger is of the cross-counter flow type.

8. The method claimed in claim 1 wherein said expander comprises a positive displacement piston expander, a turbo expander, or a radial vane expander.

9. The method claimed in claim 1 including the step of reducing the pressure of said second flow portion in said expander to a level equal to or greater than the pressure in said receiver.

10. The method claimed in claim 1 including the step of reducing the pressure of said first portion by said restriction to a value equal to or greater than the pressure in said receiver.

11. The method claimed in claim 1 wherein said receiver is a pipeline.

12. The method claimed in claim 1 wherein said receiver comprises a gas pipeline, the inlet of a gas turbine, or the inlet of a chemical process, a pump inlet, or a burner head.

13. The method claimed in claim 1 wherein said source of said natural gas comprises a well head.

14. The method claimed in claim 2 including the step of determining the split of said natural gas from said source by the pressure relationship between said source and said receiver, by the properties of the liquid natural gas, by optimization of the heat exchange process and by the thermodynamic efficiency of said first and second heat exchangers and said expander.

15. The method claimed in claim 2 including the steps of conveying said natural gas directly from said source to a purifier, removing unwanted constituents therefrom and thereafter splitting said purified natural gas into said first and second flow portions.

16. The method claimed in claim 2 wherein said restriction is a throttle valve.

17. The method claimed in claim 2 wherein said collector is a liquid natural gas tank.

18. The method claimed in claim 2 wherein said first and second heat exchangers are of the cross-counter flow type.

19. The method claimed in claim 2 wherein said expander comprises a positive displacement piston expander, a turbo expander, or a radial vane expander.

20. The method claimed in claim 2 including the step of reducing the pressure of said second flow portion in said expander to a level equal to or greater than the pressure in said receiver.

21. The method claimed in claim 2 including the step of reducing the pressure of said first portion by said restriction to a value equal to or greater than the pressure in said receiver.

22. The method claimed in claim 2 wherein said receiver is a pipeline.

23. The method claimed in claim 2 wherein said receiver comprises a gas pipeline, the inlet of a gas turbine, or the inlet of a chemical process, a pump inlet or a burner head.

24. The method claimed in claim 2 wherein said source of said natural gas comprises a well head.

43. The method claimed in claim 1 including the steps of modifying flow and pressure at various points in said method to maintain design levels of pressure and flow.

44. The method claimed in claim 2 including the steps of modifying flow and pressure at various points in said method to maintain design levels of pressure and flow.

25. An apparatus for converting a fraction of the natural gas from a source thereof to liquid natural gas, said apparatus comprising a source of pressurized, cool, clean natural gas, said source being connected to a point where said natural gas from said source is split into first and second portions, at least one heat exchanger, said first gas portion being connected to said at least one heat exchanger, a restriction, said at least one heat exchanger being operatively connected to said restriction, a liquid natural gas collector, said collector being connected to said restriction whereby said first portion of said natural gas passes through said at least one heat exchanger and is throttled into said collector wherein it is partially converted to liquid natural gas, the remainder comprising a very cold saturated natural gas vapor, said collector being connected by a conduit to said at least one heat exchanger whereby to vent said vapor through said at least one heat exchanger as a cooling medium therefor, an expander, a conduit for directing said second gas portion to said expander for expansion and cooling, a conduit from said expander being connected to said conduit from said collector to said at least one heat exchanger whereby to combine said cooled and expanded second gas portion with said vented remainder of said first gas portion before entry into said at least one heat exchanger, a receiver, a conduit from said at least one heat exchanger to said receiver to convey said combination of said cooled and expanded second gas portion and said vented remainder of said first gas portion from said at least one heat exchanger to said receiver.

26. The apparatus claimed in claim 25 wherein said at least one heat exchanger comprises a first heat exchanger, a second heat exchanger comprising a super cooler, said first heat exchanger being connected to said second heat exchanger and said second heat exchanger being connected to said restriction and thereafter to said liquid natural gas collector, whereby said first gas portion can pass through both heat exchangers and said restriction to partially flash in said collector to liquid natural gas with a gaseous remainder, said collector being connected to said second heat exchanger whereby to vent said remainder of said first gas portion in said collector through said second heat exchanger as a cooling medium therefor, a conduit between said first and second heat exchangers for the passage of said remainder of said first gas portion from said second heat exchanger to said first heat exchanger, said conduit from expander being connected to said conduit from said second heat exchanger to said first heat exchanger whereby to combine said expanded and cooled second gas portion with said first gas portion remainder from said second heat exchanger to form a cooling medium for said first heat exchanger, a conduit from said first heat exchanger to said receiver whereby to convey said second gas portion and said remainder of said first gas portion to said receiver.

27. The apparatus claimed in claim 25 including the steps of conveying said natural gas directly from said source to a purifier, removing water, other liquids, heavier molecules and other unwanted constituents therefrom and thereafter splitting said cleansed natural gas into said first and second flow portions.

28. The apparatus claimed in claim 25 wherein said restriction is a throttle valve.

29. The apparatus claimed in claim 25 wherein said collector is a liquid natural gas tank.

30. The apparatus claimed in claim 25 wherein said at least one heat exchanger is of the cross-counter flow type.

31. The apparatus claimed in claim 25 wherein said expander comprises a positive displacement piston expander, a turbo expander, or a radial vane expander.

32. The apparatus claimed in claim 25 wherein said receiver is a pipeline.

33. The apparatus claimed in claim 25 wherein said receiver comprises a pipeline, the inlet of a gas turbine, or the inlet of a chemical process, a pump inlet or a burner head.

34. The apparatus claimed in claim 25 wherein said source of natural gas comprises a well head.

35. The apparatus claimed in claim 26 including the steps of conveying said natural gas directly from said source to a purifier, removing water, other liquids, heavier molecules and other unwanted constituents therefrom and thereafter splitting said cleansed natural gas into said first and second flow portions.

36. The apparatus claimed in claim 26 wherein said restriction is a throttle valve.

37. The apparatus claimed in claim 26 wherein said collector is a liquid natural gas tank.

38. The apparatus claimed in claim 26 wherein said first and second heat exchangers are of the cross-counter flow type.

39. The apparatus claimed in claim 26 wherein said expander comprises a positive displacement piston expander, a turbo expander, or a radial vane expander.

40. The apparatus claimed in claim 26 wherein said receiver is a pipeline.

41. The apparatus claimed in claim 26 wherein said receiver comprises a pipeline, the inlet of a gas turbine, or the inlet of a chemical process, a pump inlet or a burner head.

42. The apparatus claimed in claim 26 wherein said source of natural gas comprises a well head.

45. The apparatus claimed in claim 25 including a number of regulators added to said apparatus to regulate and modify flow and pressure at various points in said apparatus to maintain design levels of pressure and flow.

46. The apparatus claimed in claim 26 including a number of regulators added to said apparatus to regulate and modify flow and pressure at various points in said apparatus to maintain design levels of pressure and flow.

47. A method for converting a fraction of natural gas from a source to liquid natural gas, comprising the steps of:

a. providing a flow of pressurized natural gas having an initial pressure;

b. passing a first portion of said flow through at least a first heat exchanger to cool said first portion of said flow;

c. reducing the pressure of said first portion of said flow thereby flashing a first part of said first portion of said flow to liquid natural gas, leaving a second part of said first portion of said flow which comprises a saturated natural gas;

d. reducing the pressure of a second portion of said flow by passing said second portion through an expander, thereby cooling said second portion of said flow;

e. passing said second portion of said flow and said second part of said first portion of said flow through said at least a first heat exchanger to serve as a cooling medium therefor.

48. The method claimed in claim 47 comprising the step of combining said second portion of said flow with said second part of said first portion of said flow prior to the step of passing said second portion of said flow and second part of said first portion of said flow through said at least a first heat exchanger.

49. The method claimed in claim 47 including the steps of:

a. passing said first portion of said flow through at least a second heat exchanger after said first portion of said flow has passed through said at least a first heat exchanger, prior to the step of reducing the pressure of said first portion of said flow;

b. passing said second part of said first portion of said flow through said at least a second heat exchanger prior to the step of passing said second portion of said flow through said at least a first heat exchanger.

50. The method claimed in claim 49 comprising the step of combining said second portion of said flow with said second part of said first portion of said flow prior to the step of passing said second portion of said flow and second part of said first portion of said flow through said at least a first heat exchanger.

51. The method claimed in claim 47, 48, 49 or 50 including the step of determining respective flow rates of said first and second portions of said flow

a. by the relationship between said initial pressure of said flow and the respective pressures of said second part of said first portion of said flow and of said second portion of said flow after the step of passing said second portion through said at least a first heat exchanger,

b. by the properties of the liquid natural gas,

c. by optimization of the heat exchange process, and

d. by the thermodynamic efficiency of said at least a first heat exchanger and of said step of passing said second portion of said flow through said expander.

52. The method claimed in claim 47, 48, 49 or 50 including the step of removing unwanted constituents from said flow of pressurized natural gas.

53. The method claimed in claim 47, 48, 49 or 50 wherein the step of reducing the pressure of said first portion of said flow includes passing said first portion of said flow through a throttle valve.

54. The method claimed in claim 47, 48, 49 or 50 wherein said at least a first heat exchanger is of the cross-counter flow type.

55. The method claimed in claim 47, 48, 49 or 50 wherein said expander comprises a positive displacement piston expander, a turbo expander, or a radial vane expander.

56. The method claimed in claim 47, 48, 49 or 50 wherein said step of reducing the pressure of said second portion of said flow includes reducing the pressure of said second portion of said flow to a pressure equal to or greater than the pressure of said second part of said first portion of said flow.

57. The method claimed in claim 47, 48, 49 or 50 wherein said flow of pressurized natural gas comes from a well head.