METHOD FOR PROCESSING A STREAM OF LNG OBTAINED BY MEANS OF COOLING USING A FIRST REFRIGERATION CYCLE AND ASSOCIATED INSTALLATION

• US 20070095099A1
• Filed: 10/09/2006
• Published: 05/03/2007
• Est. Priority Date: 10/10/2005
• Status: Active Grant

• Current Assignee: Technip France (TechnipFMC, plc)
• Original Assignee: Technip France (TechnipFMC, plc)
• Inventors: PARADOWSKI, Henri
• Granted Patent: US 7,628,035 B2
• Time in Patent Office: Days
• US Class Current: 62/600
• 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 1/0042 by liquid expansion with extraction of work
• CPC Class Codes: F25J 1/0045 by vaporising a liquid return stream
• CPC Class Codes: F25J 1/0052 by vaporising a liquid refrigerant stream
• CPC Class Codes: F25J 1/0208 in combination with an internal quasi-closed refrigeration loop, e.g. with deep flash recycle loop F25J1/021 takes precedence
• 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 1/0268 using a dedicated refrigeration means F25J1/0296 takes precedence
• CPC Class Codes: F25J 1/0274 Retrofitting or revamping of an existing liquefaction unit
• CPC Class Codes: F25J 1/0283 Gas turbine as the prime mechanical driver
• 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 1/0288 using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
• CPC Class Codes: F25J 2200/02 in a single pressure main column system
• 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 2210/06 Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2215/04 Recovery of liquid products
• CPC Class Codes: F25J 2240/30 Dynamic liquid or hydraulic expansion with extraction of work, e.g. single phase or two-phase turbine
• CPC Class Codes: F25J 2270/04 Internal refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/60 : Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons
• CPC Class Codes: F25J 2270/66 : Closed external refrigeration cycle with multi component refrigerant [MCR], e.g. mixture of hydrocarbons
• CPC Class Codes: F25J 2290/80 : Retrofitting, revamping or debottlenecking of existing plant
• 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
• CPC Class Codes: Y10S 62/927 : Natural gas from nitrogen

Abstract

In this method, the LNG stream (11) is cooled using a refrigerating fluid (83) in a first heat-exchanger (19). The refrigerating fluid (83) is subjected to a second semi-open refrigeration cycle (21) which is independent of the first cycle (15). The method comprises a step for introducing the stream (59) of sub-cooled LNG into a distillation column (49) and a step for recovering a stream (69) of gas at the top of the column (49).

The second refrigeration cycle (21) comprises a step for forming a stream (73) of refrigerating fluid from a portion of the top stream (69) of gas, a step for compressing the stream of refrigerating fluid (73) to a high pressure, then a step for expanding a portion (81) of the stream (75) of compressed refrigerating fluid in order to form a substantially liquid sub-cooling stream (83). The substantially liquid stream (83) is evaporated in the first heat-exchanger (19).

Claims

1. Method for processing a stream of LNG obtained by means of cooling using a first refrigeration cycle, the method being of the type comprising the following steps:

(a) the stream of LNG which has been brought to a temperature of less than −

100°

C. is introduced into a first heat-exchanger;

(b) the stream of LNG is sub-cooled in the first heat-exchanger by means of heat-exchange with a refrigerating fluid in order to form a stream of sub-cooled LNG; and

(c) the refrigerating fluid is subjected to a second semi-open refrigeration cycle which is independent of the first cycle,

characterised in that the method comprises the following steps;

(d) the stream of sub-cooled LNG is expanded in a dynamic manner in an intermediate turbine, maintaining this stream substantially in the liquid state;

(e) the stream from the intermediate turbine is cooled and expanded and then introduced into a distillation column;

(f) a stream of denitrogenated LNG at the bottom of the column and a stream of gas at the top of the column are recovered; and

(g) the top stream of gas is compressed in a stage compressor, and, at an intermediate pressure stage of the compressor, a first portion of the top stream of gas which is brought to an intermediate pressure PI is extracted in order to form a stream of combustible gas;

and in that the second refrigeration cycle comprises the following steps;

(i) an initial stream of refrigerating fluid is formed from a second portion of the top gas which has been compressed at the intermediate pressure PI;

(ii) the initial stream of refrigerating fluid is compressed to a high pressure PH which is greater than the intermediate pressure PI in order to form a stream of compressed refrigerating fluid;

(iii) the stream of compressed refrigerating fluid is cooled in a second heat-exchanger;

(iv) the stream of compressed refrigerating fluid from the second heat-exchanger is separated into a primary cooling stream and a sub-cooling stream of the LNG;

(v) the sub-cooling stream is cooled in a third heat-exchanger, then in the first heat-exchanger;

(vi) the sub-cooling stream from the first heat-exchanger is expanded to a low pressure PB which is lower than the intermediate pressure PI in order to form a substantially liquid sub-cooling stream of the LNG;

(vii) the substantially liquid sub-cooling stream is evaporated in the first heat-exchanger in order to form a reheated sub-cooling stream;

(viii) the main cooling stream is expanded substantially to the low pressure PB in a main turbine and the cooling stream from the main turbine is mixed with the reheated sub-cooling stream in order to form a mixed stream;

(ix) the mixed stream is reheated successively in the third heat-exchanger, then in the second heat-exchanger in order to form a reheated mixed stream; and

(x) the reheated mixed stream is introduced into the compressor at a low pressure stage located upstream of the intermediate pressure stage.

2. Method according to claim 1, wherein the high pressure PH is between approximately 40 and 100 bar, preferably between approximately 50 and 80 bar, and in particular between approximately 60 and 75 bar.

3. Method according to claim 1, wherein the low pressure PB is lower than approximately 20 bar.

4. Method according to claim 1, wherein during step (vi), the sub-cooling stream from the first heat-exchanger is expanded in a dynamic manner in a liquid expansion turbine.

5. Method according to claim 1, wherein during step (ii), the initial stream of refrigerating fluid is at least partially compressed in an auxiliary compressor which is coupled to the main turbine.

6. Method according to claim 1, wherein during step (i), a stream of C2 hydrocarbons is introduced into the compressor in order to form a portion of the initial stream of refrigerating fluid.

7. Method according to claim 1, wherein during step (iii), the compressed stream of refrigerating fluid is brought into a heat-exchange relationship with a secondary refrigerating fluid which circulates in the second heat-exchanger, the secondary refrigerating fluid being subjected to a third refrigeration cycle in which it is compressed at the outlet of the second heat-exchanger, it is cooled and condensed at least partially, then expanded before it is evaporated in the second heat-exchanger.

8. Method according to claim 7, wherein the secondary refrigerating fluid comprises propane and optionally ethane.

9. Method according to claim 1, wherein, before the expansion of step (e), the stream from the intermediate turbine is mixed with a supplementary stream of natural gas cooled by means of heat-exchange with the top stream of gas in a fourth heat-exchanger.

10. Method according to claim 1, wherein the content in terms of of the top gas is such that the stream cooled by the second heat-exchanger is purely gaseous.

20. Method according to claim 2, wherein the low pressure PB is lower than approximately 20 bar.

11. Installation for processing a stream of LNG obtained by means of cooling using a first refrigeration cycle, the installation, being of the type comprising:

means for sub-cooling the stream of LNG comprising a first heat-exchanger in order to bring the LNG stream into a heat-exchange relationship with a refrigerating fluid; and

a second semi-open refrigeration cycle which is independent of the first cycle, characterised in that it comprises;

an intermediate turbine for dynamic expansion of the stream of sub-cooled LNG from the first heat-exchanger;

means for cooling and expanding the stream from the intermediate turbine;

a distillation column which is connected to the cooling and expansion means;

means for recovering a stream of denitrogenated LNG at the bottom of the column, and means for recovering a stream of gas at the top of the column,

a stage compressor which is connected to the means for recovering the stream of gas at the top of the column; and

means for extracting a first portion of the top stream of gas tapped at an intermediate pressure stage of the compressor in order to form a stream of combustible gas;

and in that the second refrigeration cycle comprises;

means for forming an initial stream of refrigerating fluid from a second portion of the top gas compressed to the intermediate pressure;

means for compressing the initial stream of refrigerating fluid to a high pressure PH which is greater than the intermediate pressure PI in order to form a compressed stream of refrigerating fluid;

a second heat-exchanger in order to cool the compressed stream of refrigerating fluid;

means for separating the compressed stream of refrigerating fluid from the second heat-exchanger into a main cooling stream and a sub-cooling stream of the LNG;

a third heat-exchanger for cooling the sub-cooling stream;

means for introducing the sub-cooling stream from the third heat-exchanger into the first heat-exchanger;

means for expanding the sub-cooling stream from the first heat-exchanger to a low pressure PB which is lower than the intermediate pressure PI in order to form a substantially liquid sub-cooling stream of the LNG;

means for circulating the substantially liquid sub-cooling stream in the first heat-exchanger in order to form a reheated sub-cooling stream;

a main turbine for expanding the main cooling stream substantially to the low pressure PB;

means for mixing the cooling stream from the main turbine with the sub-cooling stream which has been reheated in order to form a mixed stream;

means for circulating the mixed stream successively in the third heat-exchanger then in the second heat-exchanger in order to form a reheated mixed stream;

means for introducing the reheated mixed stream in the compressor at a low pressure stage which is located upstream of the intermediate pressure stage.

12. Installation according to claim 11, that wherein the high pressure PH is between approximately 40 and 100 bar, preferably between approximately 50 and 80 bar and in particular between approximately 60 and 75 bar.

13. Installation according to claim 11, wherein the low pressure PB is lower than approximately 20 bar.

14. Installation according to claim 11, wherein the means for expanding the sub-cooling stream from the first heat-exchanger comprise a liquid expansion turbine.

15. Installation according to claim 11 wherein the means for compressing the initial stream of refrigerating fluid comprise an auxiliary compressor which is coupled to the main turbine.

16. Installation according to claim 11, wherein the second refrigeration cycle comprises means for introducing a stream of C2 hydrocarbons into the compressor in order to form a portion of the initial stream of refrigerating fluid.

17. Installation according to claim 11, wherein the second heat-exchanger comprises means for circulating a secondary refrigerating fluid, the installation comprising a third refrigeration cycle comprising secondary means for compressing the secondary refrigerating fluid from the third heat-exchanger, secondary means for cooling and expanding the secondary refrigerating fluid from the secondary compression means, and means for introducing the secondary refrigerating fluid from the secondary expansion means into the second heat-exchanger.

18. Installation according to claim 17, wherein the secondary refrigerating fluid comprises propane and optionally ethane.

19. Installation according to claim 11 wherein it comprises means for mixing the stream of sub-cooled LNG with a supplementary stream of natural gas, and a fourth heat-exchanger in order to bring the supplementary stream into a heat-exchange relationship with the top stream of gas.

21. Installation according to claim 12, wherein the low pressure PB is lower than approximately 20 bar.