Method and apparatus for liquefying a hydrocarbon stream

• US 9,310,127 B2
• Filed: 04/07/2009
• Issued: 04/12/2016
• Est. Priority Date: 04/09/2008
• Status: Active Grant

• Current Assignee: Shell USA Incorporated (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Teo, Chee Seng
• Primary Examiner(s): Jules, Frantz
• Assistant Examiner(s): RAYMOND, KEITH MICHAEL
• Application Number:
  US12/936,601
• Publication Number:
  US 20110056237A1
• Time in Patent Office: 2,562 Days
• Field of Search: 62611-614, 626/21
• US Class Current: 1/1
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/0035 by gas expansion with extraction of work
• 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 1/0055 originating from an incorporated cascade
• CPC Class Codes: F25J 1/0057 after expansion of the liquid refrigerant stream with extraction of work
• CPC Class Codes: F25J 1/021 using a deep flash recycle loop
• CPC Class Codes: F25J 1/0214 as a dual level refrigeration cascade with at least one MCR cycle
• CPC Class Codes: F25J 1/0216 using a C3 pre-cooling cycle
• CPC Class Codes: F25J 1/0219 in combination with an internal quasi-closed refrigeration loop, e.g. 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 1/0292 Refrigerant compression by cold or cryogenic suction of the refrigerant gas
• CPC Class Codes: F25J 1/0295 Shifting of the compression load between different cooling stages within a refrigerant cycle or within a cascade refrigeration system
• CPC Class Codes: F25J 2210/04 Mixing or blending of fluids with the feed stream
• 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 2230/08 Cold compressor, i.e. suction of the gas at cryogenic temperature and generally without afterstage-cooler
• CPC Class Codes: F25J 2230/20 Integrated compressor and process expander; Gear box arrangement; Multiple compressors on a common shaft
• CPC Class Codes: F25J 2230/30 Compression of the feed stream
• CPC Class Codes: F25J 2230/60 the fluid being hydrocarbons or a mixture of hydrocarbons
• CPC Class Codes: F25J 2245/02 : Recycle of a stream in general, e.g. a by-pass stream

Abstract

Method and apparatus for liquefying a hydrocarbon stream. A liquefaction system comprises at least an NGL recovery system 12, a main refrigerant circuit 42 and a first refrigerant circuit 100, a pressure reduction device 52 followed by an end gas/liquid separator 62. The main refrigerant circuit 42 comprises at least one or more main refrigerant compressors 45, 45a, 45b, and the first refrigerant circuit comprises one or more first refrigerant compressors 101. A hydrocarbon feed stream 10 is passed through the NGL recovery system 12 to produce a methane-enriched overhead stream 20, which is subsequently cooled and liquefied by the first and second refrigerant circuits. The pressure of the liquefied stream is reduced and the resulting mixed-phase stream 60 is passed through the end gas/liquid separator 62 to provide an end gaseous stream 70 and a liquefied hydrocarbon product stream 80. The loading power of the one or more main refrigerant compressors and the one or more first refrigerant compressors is maximized to their maximum load by adjusting temperature of the liquefied stream to change the amount of end gaseous stream and by controlling an amount of the end gaseous stream being fed into the methane-enriched overhead stream 20 in a recycle stream 90b.

Claims

1. A method of liquefying a hydrocarbon stream, comprising at least the steps of:

(a) providing a liquefaction system comprising at least an NGL recovery system, a main refrigerant circuit and a first refrigerant circuit and a pressure reducing device followed by an end gas/liquid separator, the main refrigerant circuit comprising at least one or more main refrigerant compressors, and the first refrigerant circuit comprising one or more first refrigerant compressors;

(b) passing a hydrocarbon feed stream through the NGL recovery system to produce a methane-enriched overhead stream from the hydrocarbon feed stream;

(c) passing the methane-enriched overhead stream through at least a first compressor to provide a methane-compressed stream;

(d) cooling the methane-compressed stream against a first refrigerant in the first refrigerant circuit and subsequently liquefying the methane-compressed stream against a main refrigerant in the main refrigerant circuit, to provide a first liquefied stream;

(e) reducing the pressure of the first liquefied stream to provide a mixed-phase stream;

(f) passing the mixed-phase stream through the end gas/liquid separator to provide an end gaseous stream and a liquefied hydrocarbon product stream;

(g) feeding at least a recycle fraction of the end gaseous stream into the methane-enriched overhead stream or the methane-compressed stream upstream of at least a part of said cooling against the first refrigerant in the first refrigerant circuit;

(h) maximizing the loading power of the one or more main refrigerant compressors and the one or more first refrigerant compressors to a predetermined maximum load by adjusting the temperature (T_x) of the first liquefied stream to change the amount of the end gaseous stream from the end gas/liquid separator and controlling the amount in the recycle fraction of the end gaseous stream being fed in step (g).

2. The method as claimed in claim 1, wherein said producing of the methane-enriched overhead stream in step (b) comprises extracting a C2+stream from the hydrocarbon feed stream and providing a C2+enriched bottom stream.

3. The method as claimed in claim 1, wherein the NGL recovery system comprises an expander, an NGL recovery column, and one or more turbo-compressors mechanically interlinked with the expander to be driven by the expander, and wherein step (b) further comprises:

passing at least a fraction of the hydrocarbon feed stream through the expander to provide a mixed-phase feed stream;

passing the mixed-phase feed stream into the NGL recovery column, which produces an overhead stream; and

,

passing the overhead stream through the turbo-compressor to produce the methane-enriched overhead stream.

4. The method as claimed in claim 3, wherein the pressure in the NGL recovery column is less than 40 bar.

5. The method as claimed in claim 3, wherein the pressure in the NGL recovery column is in the range from 15 to 45 bar.

6. The method as claimed in claim 1, wherein the first refrigerant circuit comprises at least one heat exchanger for cooling the hydrocarbon feed stream and at least one heat exchanger for cooling the methane-compressed stream.

7. The method as claimed in claim 1, further comprising passing the end gaseous stream through one or more end-compressors to provide an end-compressed stream prior to step (g) and wherein the recycle fraction of the end gaseous stream is drawn from the end compressed stream.

8. The method as claimed in claim 1, wherein the pressure of the methane-enriched overhead stream and the pressure of the recycle fraction of the end-compressed stream are in the range of 15 to 45 bar.

9. The method as claimed in claim 1, wherein the at least first compressor is commonly driven with at least one of the first refrigerant compressors.

10. The method as claimed in claim 1, wherein the hydrocarbon feed stream is a natural gas stream and the liquefied hydrocarbon product stream is a liquefied natural gas stream.

11. The method as claimed in claim 1, wherein said adjusting in step (h) of the temperature (T_x) of the first liquefied stream and said controlling of the amount in the recycle fraction of the end gaseous stream being fed in step (g) increases the production of the liquefied hydrocarbon product stream.

12. The method as claimed in claim 1, wherein said maximizing the loading power in step (h) comprises shifting of power load between the one or more first refrigerant compressors and the one or more main refrigerant compressors.

17. The method as claimed in claim 3, wherein the pressure in the NGL recovery column is ≦

35 bar.

18. The method as claimed in claim 3, wherein the pressure in the NGL recovery column is in the range from 20 to 35 bar.

20. The method as claimed in claim 1, wherein said methane-enriched overhead stream as obtained from the NGL recovery system is passed through said at least first compressor prior to said liquefying to provide said methane-compressed stream.

13. Apparatus for liquefying a hydrocarbon stream, the apparatus at least comprising:

an NGL recovery system to extract a C2+stream from a hydrocarbon feed stream to provide at least a methane-enriched overhead stream and a C2+enriched bottom stream;

at least a first compressor to provide a methane-compressed stream from the methane-enriched overhead stream;

a first cooling stage to cool the methane-compressed stream to provide a cooled methane-compressed stream, followed by a main cooling stage to liquefy the cooled methane-compressed stream to provide a first liquefied stream, wherein the first cooling stage comprises a first refrigerant circuit comprising one or more first refrigerant compressors and wherein the main cooling stage comprises a main refrigerant circuit comprising one or more main refrigerant compressors;

a pressure reducing device to reduce the pressure of the first liquefied stream to provide a mixed-phase stream;

an end gas/liquid separator to separate the mixed-phase stream into an end gaseous stream and a liquefied hydrocarbon product stream;

a recycle fraction line to feed at least a recycle fraction of the end gaseous stream into the methane-enriched overhead stream or the methane-compressed stream upstream of at least part of the first cooling stage; and

a control system arranged to maximize the loading power of the one or more main refrigerant compressors and the one or more first refrigerant compressors at their maximum load, by adjusting the temperature (T_x) of the first liquefied stream to change the amount of the end gaseous stream from the end gas/liquid separator, and to control the amount in the recycle fraction of the end-compressed stream in the recycle fraction line.

14. The apparatus of claim 13, wherein the NGL recovery system comprises

an expander arranged to expand at least a fraction of the hydrocarbon feed stream to provide a mixed-phase feed stream;

an NGL recovery column to receive the mixed-phase feed stream and to produce an overhead stream; and

one or more turbo-compressors mechanically interlinked with the expander to be driven by the expander to receive the overhead stream and produce the methane-enriched overhead stream.

15. The apparatus of claim 13, further comprising one or more end-compressors to compress the end gaseous stream to provide an end-compressed stream, wherein the recycle fraction line connects the end-compressed stream with the methane-enriched overhead stream.

16. The apparatus of claim 13, wherein at least one of the one or more first refrigerant compressors and the at least first compressor are mechanically interlinked and commonly driven.

19. The apparatus as claimed in claim 13, wherein the at least first compressor is provided between the NGL recovery system and the NGL recovery system and the main cooling stage such that the methane-enriched overhead stream as obtained from the NGL recovery system is passed through said at least first compressor prior to passing into the main cooling stage.