Liquefaction method and system

1. A method of liquefaction using a closed loop refrigeration system, the method comprising the steps of:

(a) compressing a gaseous refrigerant stream in at least one compressor;

(b) cooling at least a portion of the compressed gaseous refrigerant stream in a first heat exchanger;

(c) expanding a first portion of the cooled, compressed gaseous refrigerant stream from the first heat exchanger in a first expander to provide a first expanded gaseous refrigerant stream;

(d) cooling and substantially liquefying a feed gas stream to form a substantially liquefied feed gas stream in a second heat exchanger through indirect heat exchange against the first portion of the first expanded gaseous refrigerant stream from the first expander; and

(e) further cooling a second portion of the cooled, compressed gaseous refrigerant stream from the first heat exchanger in a third heat exchanger by indirect heat exchange with a second portion of the first expanded gaseous refrigerant stream from the first expander,

wherein the first expanded gaseous refrigerant stream exiting the first expander is substantially vapor.

2. The method of claim 1 , further comprising subcooling the cooled and substantially liquefied feed gas stream through indirect heat exchange in a subcooler exchanger against a second expanded gaseous refrigerant stream exiting a second expander.

3. The method of claim 2 , wherein the second portion of the cooled, compressed gaseous refrigerant stream, after being further cooled in the third heat exchanger, is fed to the second expander to provide the second expanded gaseous refrigerant stream.

4. The method of claim 3 , wherein the second expanded gaseous refrigerant stream exiting the second expander is substantially vapor.

5. The method of claim 4 , wherein the second expanded gaseous refrigerant stream exiting the subcooler exchanger is compressed in a low pressure compressor; combined with the first portion of the first expanded gaseous refrigerant stream exiting the second heat exchanger and the second portion of the first expanded gaseous refrigerant stream exiting the third heat exchanger; and the mixed stream further compressed in a high pressure compressor.

6. The method of claim 1 , further comprising warming at least a portion of the first portion of the first expanded gaseous refrigerant stream exiting the second heat exchanger in the first heat exchanger.

7. The method of claim 1 , further comprising warming the second portion of the first expanded gaseous refrigerant stream exiting the third heat exchanger in the first heat exchanger.

8. The method of claim 1 , further comprising splitting the compressed gaseous refrigerant stream exiting the at least one compressor into a first portion and a second portion, cooling said first portion in a supplemental refrigeration system that comprises at least one stage of a vaporizing liquid refrigerant, cooling said second portion in the first heat exchanger in step (b) of claim 1 , and combining the cooled first portion with a portion of the cooled second portion for expansion in the first expander in step (c) of claim 1 .

9. The method of claim 1 , further comprising splitting the compressed gaseous refrigerant stream exiting the at least one compressor into a first portion and a second portion, expanding said first portion in a third expander, warming the resultant expanded first portion in the first heat exchanger, and cooling said second portion in the first heat exchanger in step (b) of claim 1 .

10. The method of claim 1 , wherein the feed gas stream for liquefaction is a natural gas stream.

11. The method of claim 1 , wherein the gaseous refrigerant stream is a nitrogen stream.

12. A closed loop system for liquefaction comprising:

a refrigeration circuit, the refrigeration circuit comprising:

a first compressor;

A first heat exchanger fluidly coupled to the first compressor and adapted to accept a gaseous refrigerant stream from the first compressor;

a first expander fluidly coupled to the first heat exchanger and adapted to accept a first stream of refrigerant from the first heat exchanger;

a second heat exchanger fluidly coupled to the first expander and adapted to accept a first expanded gaseous refrigerant stream from the first expander and a feed gas stream;

a third heat exchanger fluidly coupled to the first heat exchanger and to the first expander and adapted to accept a second stream of refrigerant from the first heat exchanger and a second expanded gaseous refrigerant stream from the first expander;

a second expander fluidly coupled to the third heat exchanger and adapted to accept a stream of refrigerant from the third heat exchanger; and

a subcooler exchanger fluidly coupled to the second heat exchanger and the second expander and adapted for acceptance of the feed gas stream from the second heat exchanger and an expanded gaseous refrigerant stream from the second expander,

wherein the first heat exchanger and the third heat exchanger may be combined into one heat exchanger, and

wherein the second heat exchanger and the subcooler exchanger may be combined into one heat exchanger.

13. A method of liquefaction using a closed loop refrigeration system, the method comprising the steps of:

(a) compressing a gaseous refrigerant stream in at least one compressor;

(b) cooling at least a portion of the compressed gaseous refrigerant stream in a first heat exchanger;

(c) removing from an intermediate section of the first heat exchanger a cooled, compressed gaseous refrigerant stream and expanding said portion of the cooled, compressed gaseous refrigerant stream from the first heat exchanger in a first expander to provide a first expanded gaseous refrigerant stream;

(d) cooling and substantially liquefying a feed gas stream to form a substantially liquefied feed gas stream in a second heat exchanger through indirect heat exchange against the first portion of the first expanded gaseous refrigerant stream from the first expander; and

(e) further cooling a second portion of the cooled, compressed gaseous refrigerant stream in the first heat exchanger by indirect heat exchange with a second portion of the first expanded gaseous refrigerant stream from the first expander,

wherein the first expanded gaseous refrigerant stream exiting the first expander is substantially vapor.

14. The method of claim 13 , further comprising subcooling the cooled and substantially liquefied feed gas stream through indirect heat exchange in the second exchanger against a second expanded gaseous refrigerant stream exiting a second expander.

15. The method of claim 14 , wherein the second portion of the cooled, compressed gaseous refrigerant stream, after being further cooled in the first heat exchanger, is fed to the second expander to provide the second expanded gaseous refrigerant stream.

16. The method of claim 15 , wherein the second expanded gaseous refrigerant stream exiting the second expander is substantially vapor.

17. The method of claim 13 , further comprising subcooling the cooled and substantially liquefied feed gas stream through indirect heat exchange in a subcooler exchanger against a second expanded gaseous refrigerant stream exiting a second expander.

18. The method of claim 13 , wherein the feed gas stream for liquefaction is a natural gas stream.

19. The method of claim 13 , wherein the gaseous refrigerant stream is a nitrogen stream.

20. A closed loop system for liquefaction, comprising:

a refrigeration circuit, the refrigeration circuit comprising:

a first compressor;

a first heat exchanger fluidly coupled to the first compressor and adapted to accept a compressed gaseous refrigerant stream from the first compressor;

a first expander fluidly coupled to the first heat exchanger and adapted to accept a first portion of the cooled, compressed gaseous refrigerant stream from an intermediate location of the first heat exchanger;

a second heat exchanger fluidly coupled to the first expander and adapted to accept a first expanded gaseous refrigerant stream from the first expander and a feed gas stream;

a second expander fluidly coupled to the first heat exchanger and adapted to accept a second portion of the cooled, compressed gaseous refrigerant stream from the first heat exchanger;

wherein the first heat exchanger is adapted to accept a second expanded gaseous refrigerant stream from the first expander, and wherein the second heat exchanger is adapted to receive an expanded gaseous refrigerant stream from the second expander.