Efficiency improvement of open-cycle cascaded refrigeration process for LNG production

• US 6,289,692 B1
• Filed: 12/22/1999
• Issued: 09/18/2001
• Est. Priority Date: 12/22/1999
• Status: Expired due to Term

• Current Assignee: Conocophillips Company (ConocoPhillips)
• Original Assignee: Phillips Petroleum Company (ConocoPhillips)
• Inventors: Baudat, Ned, Houser, Clarence G., Lee, Rong-Jwyn
• Primary Examiner(s): DOERRLER, WILLIAM CHARLES
• Application Number:
  US09/469,832
• Time in Patent Office: 636 Days
• Field of Search: 626/11, 626/13, 626/14
• US Class Current: 62/613
• 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 1/0042 by liquid expansion with extraction of work
• 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 1/0255 controlling the composition of the feed or liquefied gas, e.g. to achieve a particular heating value of natural gas
• CPC Class Codes: F25J 1/0285 Combination of different types of drivers mechanically coupled to the same refrigerant compressor, possibly split on multiple compressor casings
• CPC Class Codes: F25J 1/0294 Multiple compressor casings/strings in parallel, e.g. split arrangement
• CPC Class Codes: F25J 2220/64 Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
• CPC Class Codes: F25J 2230/20 Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2240/40 Expansion without extracting work, i.e. isenthalpic throttling, e.g. JT valve, regulating valve or venturi, or isentropic nozzle, e.g. Laval
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream

Abstract

This invention concerns a method and an apparatus for improving the efficiency of an open-cycle refrigeration process for LNG production by the employment of a liquid expander to recover energy associated with the flashing of a pressurized liquid stream and employing said recovered energy to compress the flashed vapor streams in the open cycle.

Claims

1. In a process wherein a LNG-bearing stream is produced at an elevated pressure and said stream is flashed in an open methane refrigeration cycle employing a multi-stage compressor via multiple stages of pressure reduction to a near-atmospheric pressure and employing a multi-stage compression step, the improvement comprising in at least one of said pressure reduction stages:

(a) flashing a pressurized LNG-bearing stream in an expander thereby generating a two-phase stream and energy;

(b) separating said two-phase stream into a gas stream and lower pressure predominantly LNG-bearing stream;

(c) compressing said gas stream in a compressor thereby producing a pressurized gas stream and wherein said compressor is powered at least in part by the energy of step (a); and

(d) returning said pressurized gas stream to the multi-stage compression step employed in the open methane refrigeration cycle.

2. A process according to claim 1 wherein said energy is mechanical energy, hydraulic energy or electrical energy.

3. A process according to claim 2 wherein the energy of step (a) is the sole source of energy for the compression step of step (c).

4. A process according to claim 2 wherein steps (a)-(d) are employed in at least the first pressure reduction stage.

5. A process according to claim 4 wherein said energy is mechanical energy.

6. A process according to claim 4 wherein the energy of step (a) is the sole source of energy for the compression step of step (c).

7. A process according to claim 6 wherein said energy is mechanical energy.

8. A process according to claim 2 wherein said LNG-bearing stream produced at an elevated pressure is flashed to near-atmospheric pressure by three pressure reduction stages and wherein said steps of claim 1 are employed in the first and second pressure reduction stages.

9. A process according to claim 8 wherein the energy of step (a) is the sole source of energy for the compression step of step (c).

10. A process according to claim 2 further comprising the step of

(e) contacting via indirect heat exchange means the LNG-bearing stream of step (a) prior to step (a) with the gas stream of step (b) prior to step (c) thereby cooling the LNG-bearing stream and warming the gas stream.

11. A process according to claim 10 wherein said LNG-bearing stream produced at an elevated pressure is flashed to near-atmospheric pressure by three pressure reduction stages and wherein said steps of claim 1 are employed in the first and second pressure reduction stages.

12. A process according to claim 11 wherein the energy of step (a) is the sole source of energy for the compression step of step (c).

13. A process according to claim 12 wherein said energy is mechanical energy.

14. A process according to claim 2 wherein the pressure of said compressed gas of step (c) is selected to approximate the preferred flash pressure for the corresponding stage of pressure reduction when expansion valves are employed in all stages of pressure reduction and the downstream pressure of step (a) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (b) to said preferred flash pressure for the pressure reduction stage of interest.

15. A process according to claim 14 further comprising the step of

(e) contacting via indirect heat exchange means the LNG-bearing stream of step (a) prior to step (a) with the gas stream of step (b) prior to step (c) thereby cooling the LNG-bearing stream and warming the gas stream.

16. A process according to claim 15 wherein said LNG-bearing stream produced at an elevated pressure is flashed to near-atmospheric pressure by three pressure reduction stages and wherein said steps of claim 1 are employed in the first and second pressure reduction stages.

17. A process according to claim 16 wherein said energy is mechanical energy.

18. In a process wherein a LNG-bearing stream at elevated pressure is flashed in an open methane refrigeration cycle via multiple stages of pressure reduction to a near-atmospheric pressure, the improvement comprising:

(a) cooling via indirect heat exchange said LNG-bearing stream at elevated pressure thereby producing a cooled LNG-bearing stream;

(b) flashing said cooled LNG-bearing stream in an expander thereby generating a first pressure reduction two-phase stream and energy;

(c) separating said first pressure reduction two-phase stream into a first gas stream and a second LNG-bearing stream;

(d) warming said first gas stream via indirect heat exchange with the stream of step (a) thereby producing a warmed first gas stream;

(e) compressing said warmed gas stream via a compressor thereby producing a compressed first gas stream and wherein said compressor is powered at least in part by energy of step (a);

(f) returning said compressed first gas stream to the high stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle;

(g) cooling via indirect heat exchange the second LNG-bearing stream thereby producing a cooled second LNG-bearing stream;

(h) flashing said cooled LNG-bearing stream via an expansion valve thereby generating a second pressure reduction two-phase stream;

(i) separating said second pressure reduction two-phase stream into a second gas stream and a third LNG-bearing stream;

(j) warming said second gas stream via indirect heat exchange with the stream of step (g) thereby producing a warmed second gas stream;

(k) returning said warmed second gas stream to the intermediate stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle;

(l) cooling via indirect heat exchange the third LNG-bearing stream thereby producing a cooled third LNG-bearing stream;

(m) flashing said cooled LNG-bearing stream via an expansion valve thereby generating a third pressure reduction two-phase stream;

(n) separating said third pressure reduction two-phase stream into a third gas stream and a fourth LNG-bearing stream;

(o) flowing said fourth LNG-bearing stream to storage;

(p) warming said third gas stream via indirect heat exchange with the stream of step (l) thereby producing a warmed third gas stream;

(q) returning said warmed third gas stream to the low stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle.

19. A process according to claim 18 wherein said energy is mechanical energy, hydraulic energy or electrical energy.

20. A process according to claim 19 further comprising:

(r) further warming said warmed second gas stream of step (q) via indirect heat exchange with the stream of step (a).

21. A process according to claim 19 further comprising:

(r) further warming said warmed third gas stream of step (q) via indirect heat exchange with the stream of step (g).

22. A process according to claim 21 further comprising:

(s) further warming said warmed third gas stream of step (r) via indirect heat exchange with the stream of step (a).

23. A process according to claim 22 further comprising:

(t) further warming said warmed second gas stream of step (j) via indirect heat exchange with the stream of step (a).

24. A process according to claim 19 wherein the energy of step (a) is the sole source of energy for the compression step of step (e).

25. A process according to claim 24 wherein said energy is mechanical energy.

26. A process according to claim 20 wherein the energy of step (a) is the sole source of energy for the compression step of step (e).

27. A process according to claim 26 wherein said energy is mechanical energy.

28. A process according to claim 23 wherein the energy of step (a) is the sole source of energy for the compression step of step (e).

29. A process according to claim 28 wherein said energy is mechanical energy.

30. A process according to claim 19 wherein the pressure of said compressed first gas stream of step (e) is selected to approximate the preferred flash pressure for the first stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (d) to said preferred flash pressure for the pressure reduction stage of interest.

31. A process according to claim 30 wherein said energy is mechanical energy.

32. A process according to claim 20 wherein the pressure of said compressed first gas stream of step (e) is selected to approximate the preferred flash pressure for the first stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (d) to said preferred flash pressure for the pressure reduction stage of interest.

33. A process according to claim 23 wherein the pressure of said compressed first gas stream of step (e) is selected to approximate the preferred flash pressure for pressure reduction stages when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (d) to said preferred flash pressure for the pressure reduction stage of interest.

34. In a process wherein a LNG-bearing stream at elevated pressure is flashed in an open methane refrigeration cycle via multiple stages of pressure reduction to a near-atmospheric pressure, the improvement comprising:

(a) cooling via indirect heat exchange the LNG-bearing stream at elevated pressure thereby producing a cooled LNG-bearing stream;

(b) flashing said cooled LNG-bearing stream in an expander thereby generating a first pressure reduction two-phase stream and energy;

(c) separating said first pressure reduction two-phase stream into a first gas stream and a second LNG-bearing stream;

(d) warming said first gas stream via indirect heat exchange with the stream of step (a) thereby producing a warmed first gas stream;

(e) compressing said warmed gas stream via a compressor thereby producing a compressed first gas stream and wherein said compressor is powered at least in part by energy of step (a);

(f) returning said compressed first gas stream to the high stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle;

(g) cooling via indirect heat exchange the second LNG-bearing stream thereby producing a cooled second LNG-bearing stream;

(h) flashing said cooled LNG-bearing stream in an expander thereby generating a second pressure reduction two-phase stream and energy;

(i) separating said second pressure reduction two-phase stream into a second gas stream and a third LNG-bearing stream;

(j) warming said second gas stream via indirect heat exchange with the stream of step (g) thereby producing a warmed second gas stream;

(k) compressing said warmed second gas stream via a compressor thereby producing a compressed second gas stream and wherein said compressor is powered at least in part by energy of step (h);

(l) returning said warmed second gas stream to the intermediate stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle;

(m) cooling via indirect heat exchange the third LNG-bearing stream thereby producing a cooled third LNG-bearing stream;

(n) flashing said cooled LNG-bearing stream via an expansion valve thereby generating a third pressure reduction two-phase stream;

(o) separating said third pressure reduction two-phase stream into a third gas stream and a fourth LNG-bearing stream;

(p) flowing said fourth LNG-bearing stream to storage;

(q) warming said third gas stream via indirect heat exchange with the stream of step (m) thereby producing a warmed third gas stream; and

(r) returning said warmed third gas stream to the low stage inlet port of the multi-stage compressor employed in the open methane refrigeration cycle.

35. A process according to claim 34 wherein said energy of steps (b) and (h) is mechanical energy, hydraulic energy or electrical energy.

36. A process according to claim 35 further comprising:

(s) further warming said warmed second gas stream of step (j) via indirect heat exchange with the stream of step (a).

37. A process according to claim 36 further comprising:

(t) further warming said warmed third gas stream of step (q) via indirect heat exchange with the stream of step (a).

38. A process according to claim 37 further comprising:

(u) further warming said warmed second gas stream of step (j) via indirect heat exchange with the stream of step (a).

39. A process according to claim 35 wherein the energy of step (a) is the sole source of energy for the compression step of step (e) and the energy of step (h) is the sole source of energy for the compression step of step (k).

40. A process according to claim 39 wherein said energy is mechanical energy.

41. A process according to claim 36 wherein the energy of step (a) is the sole source of energy for the compression step of step (e) and the energy of step (h) is the sole source of energy for the compression step of step (k).

42. A process according to claim 41 wherein said energy is mechanical energy.

43. A process according to claim 38 wherein the energy of step (a) is the sole source of energy for the compression step of step (e) and the energy of step (h) is the sole source of energy for the compression step of step (k).

44. A process according to claim 43 wherein said energy is mechanical.

45. A process according to claim 35 wherein the pressure of said compressed warmed gas stream of step (e) is selected to approximate the preferred flash pressure for the first stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (b) to said flash pressure for the first pressure reduction stage of interest and wherein the pressure of said compressed warmed second gas stream of step (k) is selected to approximate the preferred flash pressure for the second stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (i) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (i) to said flash pressure for the second pressure reduction stage.

46. A process according to claim 36 wherein the pressure of said compressed warmed gas stream of step (e) is selected to approximate the preferred flash pressure for the first stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (b) to said flash pressure for the first pressure reduction stage of interest and wherein the pressure of said compressed warmed second gas stream of step (k) is selected to approximate the preferred flash pressure for the second stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (i) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (i) to said flash pressure for the second pressure reduction stage.

47. A process according to claim 38 wherein the pressure of said compressed warmed gas stream of step (e) is selected to approximate the preferred flash pressure for the first stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (b) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (b) to said flash pressure for the first pressure reduction stage of interest and wherein the pressure of said compressed warmed second gas stream of step (k) is selected to approximate the preferred flash pressure for the second stage of pressure reduction when employing expansion valves in all stages of pressure reduction and the pressure of step (h) is selected such that the energy generated in this step is sufficient to compress the gas stream of step (i) to said flash pressure for the second pressure reduction stage.

48. A process according to claim 47 wherein said energy is mechanical.

49. An apparatus comprising

(a) a first indirect heat exchange means;

(b) a first directional flow control means;

(c) a first junction means;

(d) a liquid expander;

(e) an expansion valve;

(f) a gas liquid separator;

(g) a second indirect heat exchange means situated in close proximity to the first indirect heat exchange means and situated such that fluids flowing though such means flow generally countercurrent to one another;

(h) a single-stage compressor;

(i) a mechanical or hydraulic coupling between the liquid expander of (d) and the compressor of (h);

(j) a second directional flow control means;

(k) a second junction means;

(l) a multi-stage compressor;

(m) a conduit connected to the inlet of the first indirect heat exchange means;

(n) a conduit connected to the outlet of the first indirect heat exchange means and the inlet to the first directional flow control means;

(o) a conduit connected to an outlet of the first directional flow control means and to the liquid expander;

(p) a conduit connected to the liquid expander and to the first junction means;

(q) a conduit connected to an outlet of the first direction flow control means and to the expansion valve;

(r) a conduit connected to the expansion valve and the first junction means;

(s) a conduit connected to the first junction means and the gas-liquid separator;

(t) a conduit connected to the upper section of the gas-liquid separator and the inlet to the second indirect heat exchange means;

(u) a conduit connected to the outlet to the second indirect heat exchange means and the inlet to the second directional flow control means;

(v) a conduit connected to the outlet of the second directional flow control means and the single-stage compressor;

(w) a conduit connected to the single-stage compressor and the second junction means;

(x) a conduit connected to an outlet of the second directional flow control means and the second junction means; and

(y) a conduit between the second junction means and the high-stage inlet port of the multi-stage compressor.

50. The apparatus according to claim 49 wherein the expander of (d) is mechanically coupled to the compressor of (h).

51. The apparatus according to claim 49 wherein said multi-stage compressor has three stages.

52. The apparatus according to claim 50 wherein said multi-stage compressor has three stages.

53. The apparatus according to claim 49 additionally comprising

(z) a third indirect heat exchange means;

(aa) a third directional flow control means;

(bb) a third junction means;

(cc) a second liquid expander;

(dd) a second expansion valve;

(ee) a second gas-liquid separator;

(ff) a fourth indirect heat exchange means situated in close proximity to the third indirect heat exchange means and situated such that fluids flowing though such means flow generally countercurrent to one another;

(gg) a second single-stage compressor;

(hh) a second mechanical or hydraulic coupling between the second liquid expander and the second compressor;

(ii) a fourth directional flow control means;

(jj) a fourth junction means;

(kk) a conduit connected to the lower section of the gas-liquid separator and the third indirect heat exchange means;

(ll) a conduit connected to the outlet of the third indirect heat exchange means and the inlet to the third directional flow control means;

(mm) a conduit connected to an outlet of the third directional flow control means and to the second liquid expander;

(nn) a conduit connected to the second liquid expander and to the third junction means;

(oo) a conduit connected to an outlet of the third direction flow control means and to the second expansion value;

(pp) a conduit connected to the second expansion valve and the third junction means;

(qq) a conduit connected to the third junction means and the second gas-liquid separator;

(rr) a conduit connected to the upper section of the second gas-liquid separator and the inlet to the fourth indirect heat exchange means;

(ss) a conduit connected to the outlet to the fourth indirect heat exchange means and the inlet to the fourth directional flow control means;

(tt) a conduit connected to the outlet of the fourth directional flow control means and the second compressor;

(uu) a conduit connected to the second compressor and the fourth junction means;

(vv) a conduit connected to an outlet of the fourth directional flow control means and the fourth junction means; and

(ww) a conduit between the fourth junction means and either the intermediate-stage or low-stage inlet port at the multistage compressor.

54. The apparatus according to claim 53 wherein the second expander is mechanically coupled to the second single-stage compressor of (h).

55. The apparatus according to claim 53 wherein said multi-stage compressor has three stages and said conduit of (ww) is connected to the intermediate-stage inlet port.

56. The apparatus according to claim 54 wherein said multi-stage compressor has three stages and said conduit of (ww) is connected to the intermediate-stage inlet port.