Title of Invention

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

Abstract

The invention concerns a method which consists in cooling the LNG stream (11) with .a coolant (83) in a first heat exchanger (19). The coolant (83) is subjected to a second semi-open refrigerating cycle (21), independent of the first cycle (15). The method includes a step of introducing the under-cooled LNG stream (59) in a distillation column (49) and a step of recovering a gas stream (69) at the head of the column (49), The second refrigerating cycle (21) includes a step of forming a coolant stream (73) from part of the head gas stream (69), a step of compressing the coolant stream (73) up to a high pressure, then a step of expanding part (81) of the compressed coolant stream (75) to form an essentially liquid under-cooling stream (83). The essentially liquid stream (83) is evaporated in the first heat exchanger (19).

Full Text

TECH NT P FRASCE Method for processing a stream of LNG obtained by means of cooling using a first refrigeration cycle and associated installation. 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 sec ond semi -open refrigeration c ycle ( 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 (6 9) of gas at the top of the colum ( 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 f luid (7 3) 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-c ooling stream (83). The substantially liquid stream (83) is evaporated in the first heat - exchang er ( 19). Figure 1. The present invention relates to a method for processing a stream of LNG obtained by means of cooling using a first refrigeration cyc1ic, t he method beirnr of the t ype cornprsing the following steps: (a) the stream of LNG which has been brought to a temperature of 1ess than -100° C is introduced into a first heat -exchanger; ( b) the stream of LKG 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 cyc1e. US-B-6 30 8 5 31 discloses a method of the above-mentioned type, in which a stream of natural gas is liquefied using a first refrigeration cycle which uses the condensation and evaporation of a mixture of hydrocarbons. The temperature of the gas obtained is approximately -100° C. Then, the LNG produced is sub-cooled to approximately -170° C using a second refrigeration cycle of the type referred to as a semi-open "invert ed Brayt on c yc 1 e" comprising a stage compressor and a gas expansion t urbine. A method of this type is not entirely satisfactory. The maximum yield of the inverted Brayton cycle is limited to approxi mately 40%. Further more, the operation thereof in a semi -open cycle is difficult to implement. An object of the invention is therefore to provide an independent method for processing a stream of LNG which has an improved yield and which can be readily implemented in units of d i f f er ent structures. To this end, the invention relates to a processing method of the above-mentioned type, characterised in that the method comprises the fo11owing steps: (d) the stream of sub-cooled LING 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 int o a distillisation colum; (f) a stream of d enitr ogenat ed LNG at the bottom of the column and a stream of gas at the top of the colum 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 compressed at 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 stream of 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 compressed stream of refrigerating fluid; (iii) the compressed stream of refregerating fluid is cooled in a second heat-exchanger; ( iv) the compr essed stream of 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 -exchang er is expand ed to a low pressure which is lower than the intermediate pressure PI in order to form a substantial1 y 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 t ur bin e and the main c ooling 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 t o 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. The method according to invention may comprise one or more of the f ollowing f eatures , taken in isolation or according t o any technically possible combination: - the high pressure PH is between approximately 40 and 100 bar, preferably between approximately 50 and 80 bar, and in particular betw een approximately 60 and 75 bar; - the low pressure PB is lower than approximately 20 bar; - during step (vi) , the sub-cooling stream from the first heat-exchanger is expanded in a dynamic manner in a liquid expansion turbin e; - during step (ii), the initial stream of refrigerating fluid is at least partially compressed in an auxiliary compressor which is coupled to the main turbin e; - 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 f1uid ; - 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; - the secondary refrigerating fluid comprises propane and optionally ethane; and - 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; and - the content in terms of C2 of the top gas is such that the stream cooled by the second heat-exchanger is purely gaseous. The invention also relates to an installation f or 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 colum whic h is connected to the cooling and expansion means; - means for recovering a stream of denitrogenated L NG at the , bottom of the colurm, and means for recovering a stream of gas at the top of the colurm, - a stage compressor which is connected to the means for recovering the stream of gas at the top of the colurm; 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 cyc'le 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 which is greater than the intermediate pressure 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 h eat -exchanger to a low pressure which is low er than the intermediate pressure 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 preheated sub-cooling straem - a main turbine for expanding the main cooling stream to the low pressure; - 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. The installation according to the invention nay comprise one of more of the following features, taken in isolation or according to any technical combination possible: - 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; . - the low pressure PB is lower than approximately 20 bar; - the means for expanding the sub-cooling stream from the first heat-exchanger c omprise a liquid expansion turbine; - the means for compressing the initial stream of refrigerating fluid comprise an auxiliary compressor which is coupled to the main turbine; - 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; - the second heat-exchanger comprises means for circulating a secondary refrigerating fluid, the installation comprising a third refrigeration cycle comprising secondary means for coompressing 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; and - the secondary refrigerating fluid comprises propane and . optionall y ethane; and - 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. Embodiments of the invention will now be described with reference to the appended drawings, in which: - Figure 1 is an operational block diagram of a first installation according to the invention; - Figure 2 is a graph which illustrates the efficiency lines of the second refrigeration cycle of the installation of Figure 1, in accordance with the temperature of the LNG at the inlet of the first exchanger; - Figure 3 is a diagram similar to that of Figure 1 of a second installation according to the invention; - Figure 4 is a diagram similar to that of Figure 1 of a third installation according to the invention; and - Figure 5 is a diagram similar to that of Figure 1 of a fourth installation according to the invention. The first sub-cooling installation 9 according to the invention , illustrated in Figure 1, is intended to produce, from an initial stream 11 of liquefied natural gas (LNG) brought to a temperature of less than -90°C, a denitrogenated LNG stream 13. The installation 9 also produces a stream 16 of combustible gas which is rich in nitrogen. As illustrated in Figure 1, the initial stream 11 of L NG is produced by a unit 15 f or liquefaction of natural gas comprising a first refrigeration cycle 17. The first cycle 17 comprises, for example, a cycle comprising means for condensation and evaporation of a mixtur e of hydrocarbons. The installation 9 comprises a first sub-cooling heat-exchanger 19, a second semi -open refrigeration cycle 21 which is independent of the first cycle 17, and a d enitr ogenati on unit 23. The second refrigeration cycle 21 corrprises a stage compression device 25 comprising a plurality of compression stages 27. Each stage 27 c ompr ises a compressor 29 and a refrigeration unit 31. The second cycle 21 further comprises a second heat-exchanger 33, a third heat -exchanger 35, a expansi on valve 37 and an auxiliary compressor 39 which is coupled to a main expansion turbine 41. The second cycle 21 also corrprises an auxiliary refrigeration unit 4 3. in the example illustrated in Figure 1 , the stage compression device 25 corrprises four compressors 29. The four compressors 29 are driven by the same external energy source 45. The source 4 5 is, for example, a motor of the gas turbine type. The refrigeration units 31 and 43 are cooled by means of water and /or air. The denitrogenation unit 23 comprises an intermediate hydraulic turbine 47 which is c oupled to a stream generator' 48 / a distillation colum 4 9 , a heat- exchanger 51 f or t h e t op of the column and a heat-exchanger 53 for the bottom of the colum. It further corrprises a purrp 55 for discharging denitrogenated LNG 13, Below, a stream of liquid and the conduit which conveys it will be designated with the same reference numeral, the pressures in question are absolute pressures, and the percentages in question are molar percentages. The initial LNG stream 11 from the liquefaction unit 15 is at a temperature lower than -90° C, for example, at -130° C, This stream 11 comprises, for example, approximately 5% of nitrogen, 90% of methane and 5% of ethane, and the flow rate thereof is 50,000 kmol/h. The stream 11 of LNG is introduced into the first heat-exchanger 19, where it is sub-cooled to a temperature of -150°C in order to produce a stream 57 of sub-cooled LNG. The stream 57 is then introduced into the hydraulic turbine 4 7 and expand ed in a dynamic manner to a low pressure in order to form a expanded stream 59. This stream 59 is substantially Liquid, t hat is to say, it contains less than 2% mol of gas. The stream 59 is cooled in the bottom heat-exchanger 53, then introduced into a expansion valve 61 where it forms a stream 64 for supplying the column 49. The stream 64 is introduced at the top of the distillation column 49, at a low distillation pressure. The low distillation pressure is slightly higher than atmospheric pressure... In this example, this pressure is 1. 25 bar and the temperature of the stream 64 is appr oxi mat ely -165°C. A supplementary stream 63 of natural gas, substantially of the same composition as the initial stream 11 of LNG, is cooled in the top exchanger 51 then expanded in a valve 65 and mixed with the stream 59 of expanded sub-cooled LNG upstream of the valve 61. A reboiling stream 68 is extracted from the colum. 4 9 at an intermediate stage Ni, located in the region of the bottom of this colum The stream 68 is introduced into the exchanger 53, where it is reheated by means of heat-exchang e with the stream 5 9 of expanded sub-cooled LNG, before being reintroduced into the colurm 49 below the intermediate level Ni. A bottom liquid stream 67 containing less than 1% of nitrogen is extracted from the colurm 49. This bottom stream 67 is pumped by the purrp 5 5 in order to form the stream 13 of denitrogenated LNG which is intended to be sent to a storage d evic e. A top gaseous stream 6 9 which contains almost 50% of nitrogen is extracted from the distillation colurm 49, This stream 69 is rehetaed by means of heat-exchange with the s upplementary stream 63 in the top exchanger 51 in order to form a reheated. top stream 71. This stream 71 is introduced into the first stage 27 A of the compression device 25. The reheated top stream 71 is successively compressed in the first stage 27A and in the second stage 27B of the compressor 25 substantially to a low cycle pressure PB, then compressed in the third compression stage 27C before being introduced into the fourth conpression stage 27D. In each stage 27 of the compressor, the top stream 71 is subjected t o a conpression operation in the compressor 29 followed by cooling to a temperature of approximately 35°C in the associated refrigeration unit 31. A first portion 16 of the top stream compressed in the fourth compression stage 27D is extracted from the compressor 29D, at an intermediate pressure PI, in order to form the stream of combustible gas. The intermediate pressure PI is, for example, greater than 20 bar and preferably substantially equal t o 30 bar. The low cycle pressure PB is, for exarrpl e, lower than 20 bar. A second portion 73 of the top str earn c ontinues to be compressed in the compressor 29D to a mean pressure which is substantially equal to 50 bar in order to form an initial stream of refrigerating fluid. The stream 73 is cooled in the exchanger 31D then introduced into the auxiliary compressor 33, The flow rate of the initial stream 73 of refrigerating fluid is much higher tnan the flow rate of the stream 16 of combustible gas. The relationship between the two flow rates is, in this example, substantially equal to 6. 5 The stream 73 is then compressed in the compressor 39 to a high cycle pressure PH. This high pressure is between 40 and 100 bar, preferably between 50 and 80 bar and advantageously between 60 and 75 bar. The stream 73 from the compressor 39 forms, after passing through the refrigeration unit 43, a st ream 75 of compressed refrigerating fluid. The top stream 6 9- contains less than 5% by mass of . C+2 hydrocarbons, so that the stream 7 5 is purely gaseous. When the high pressure is greater than approximately 60 bar, the stream 75 is a supercritical fluid. The stream 75 is then cooled in the second heat-exchang er 33 and separated at the outlet of this exchanger 3 3 into a secondary sub-cooling stream 77 of the LING and a primary main cooling stream 79. The relationship of these two flow rates is in the order of 0.5. The sub-cooling stream 77 is cooled in the third exchanger 35, then in the first exchanger 19 in order to form a cooled sub-cooling stream 81. The stream 81 is expanded to the low cycle pressure PB in the valve 37 from where it is discharged in the form of a substantially liquid sub-cooling stream 83, that is to say, which contains less than 10% mol of gas. The stream 83 is then introduced into the first exchanger 19, where it evaporates and cools, by means of heat -exchange, the stream 81 and the initial LNG stream 11, in order to form, at the outlet of the first exchanger 19, a reheated sub-cooling strearn 8 5. The gaseous main stream 79 is expanded in the turbine 41 substantially to the low cycle pressure PB and mixed with the reheated stream 85 from the first exchanger 19 in order to form a mixed stream 87. The mixed stream 87 is then introduced successively into the third exchanger 35, then into the second exchanger 33 where it cools, by means of a heat-exchange relationship, the sub-cooling stream 77 and the stream 75 of compressed refrigerating fluid. Th e reheated mixed stream 89 from the exc han g er 3 3 is t h en introduced into the compression device 25 at the inlet of the third compression stage 27C, substantially at the low pressur e PB. By way of illustration, the pressure, temperature, and flow rate values when the high cycle pressure PH is substantially equal to 75 bar are set out in the table below. In Figure 2, the efficiency line 91 of the cycle 21 in the method according to the invention is illustrated in accordance with the temperature value of the stream 11 of LNG. As illustrated in this Figure, the yields are greater than 4 4%, which c onstitutes a significant increase compared with the methods of the prior art which involve a semi-open invert ed Brayton c ycle. This result is obtained in a simple manner since it is not necessary to provide means f or storing and preparing a refrigerating fluid, the refrigerating fluid 73 being continuously supplied by the installation 9. The method and the installation 9 of the present invention are used either in new liquefaction units or to improve the efficiency levels of existing LNG production units. In the latter case, with equal power consumption, the production of denitr ogenat ed LNG can be increased from 5% to 20%. The method and the installation 9 according to the invention can also be used to sub-cool and d enitrogenat e LNG produced in methods for extracting natural gas liquids ( NGT.). The installation 99 illustrated in Figure 3 differs from the first installation 9 in that the expansion valve 37 located downstream of the first exchanger is replaced with a turbine 101 f or d ynamic expansion coupled to a stream generator 103. The method for processing the stream of LNG in this installation is further identical to the method us ed in the installation 9 , t o w it h i n numer ica1 va1ues - In a variant which is illust rat ed with a dot-dash lin e in Figure 3, a stream 92 of ethane is mixed with the reheated mixed stream 8 9 before it is introduced into the third compression stage 27 C. The efficiency of the cycle 21 is then further increased as illustrated by the line 93 of Figure 2. The third installation 104 according to the invention is illustrated in Figure 4. This installation 10 4 differs from the second installation 9 9 in that it further comprises a third refrigeration cycle 105 which is closed and which is independent of the first and second cycles 17 and 21. The third cycle 105 comprises a secondary compressor 107, first and second secondary refrigeration units 109A and 109B, a expansion valve 111 and a separating flask 113. This cycle is implemented using a stream of secondary refrigerating fluid 115 which comprises propane. The gaseous stream 115 at the low pressure is introduced into the compressor 107, then cooled and condensed at the high pressure in the refrigeration units 109A and 109B in order to form a partially liquid stream 117 of propane. This stream 117 is cooled in the exchanger 33, then introduced into the expansion valve 111, where it is expanded and forms a biphase stream 119 of expanded propane. The stream 119 is introduced into the separating flask 113 in order to form a liquid fraction 121 which is extracted from the bottom of th e f1ask 113. Th e f raction 121 i s introduced into the exchanger 33 where it is evaporated by means of heat-exchang e with the stream 117 and with the stream 75 of compressed refrigerating fluid, before being introduced into the flask 113. The gaseous fraction from the top of the flask 113 forms the stream 115 of gaseous propane. As illustrated by the line 123 of Figure 2, the efficiency of the cycle 21 is then increased by 4% on average compared with the ef f ici enc y of the method implemented in the first installation 9. The fourth installation 25 according to the invention 125 illustrated in Figure 5 differs from that illustrated in Figur e 4 in that the third refrigeration cyc1e 105 has no separating flask 113, The stream 119 from the valve 111 is therefore introduced directly into the second exchanger 33 and completely evaporated in this exchanger. Furthermore, the refrigerating fluid 115 comprises a mixture of ethane and propane. The content in terms of ethane in the fluid 115 is substantially equal to the content. In terms of propane. As illustrated by the line 126 of Figure 2, the mean ef f i ciency of the second refrigeration cycle is then increased by approximately 0.5% compared with the efficiency of the method implemented in the third installation 104 when the temperature is lower than -130° C. Taking into account the energy produced by the turbine 47, the overall yield of the installation of Figure 5 is slightly greater than 50%, compared with approximately 47. 5% for that of Figure 1 , 47. 6% for that of Figure 3 and 49.6% for that of Figure 4. CLAIMS 1 . Method for processing a st rea m ( 11) of LNG obt ain ed by means of c ooling using a first r ef ri g erati on c yc 1 e ( 17 ) , the method being of the type comprising the following steps: (a) the stream (11) of LKG which has been brought to a temperature of less than -100°C is introduced into a first heat -exchanger ( 19) ; ( b) the stream (11) of L NG is sub-cooled in the first heat-exchanger by means of heat -exchange with a r ef rigerating fluid (83) in order to form a stream (57) of sub-cooled LNG; and (c) the ref rigerating f luid (83) is subjected to a sec ond semi-open refrigeration cycle (21) which is independent of the first cycle ( 15) , characterised in that the method c orrprises the f ollowing steps: (d) the stream (57) of sub-cooled LNG is expanded in a dynamic manner in an intermediate turbine (47), maintaining this stream substantially in the liquid state; (e) the stream (59) from the intermediate turbine (47) is cool ed and expanded and then introduced into a distillation column ( 49) ; (f) a stream (67) of denitrogenated LNG at the bottom of the colurm (49) and a stream ( 6 9) of gas at the top of the colurm are recoveresd; a nd ( g) the top stream (69) of gas is compressed in a stage compressor (25), and, at an intermediate pressure stage (29D) of the compressor (25), a first portion (16) of the top stream (69) of gas which is br ought t o an int er medi at e pressure PI is extracted in order to form a stream of combustible gas; and in that the second r ef rigeration cycle ( 21) c omprises the following steps: ( i ) a n i n i t ia1 strea m (7 3 ) of ref rigerating fluid i s formed from a second portion of the top gas (69) which has been compress ed at the intermediate pressure PI; (ii) the initial stream (73) of refrigerating fluid is compressed to a high pressure PH which is greater than the intermediate pressure PI in order to form a stream (75) of compressed refrigerating f luid; (iii) the stream (75) of compressed refrigerating fluid is cool ed in a second heat -exchanger ( 33) ; ( iv) the stream (75) of compressed refrigerating fluid from the second heat-exchanger (33) is separated into a primary cooling stream (79) and a sub-cooling stream (77) of the LNG; ( v) the Bub-cooling stream (77) is cooled in a third heat- exchanger (35), then in the first heat -exchanger (19); (vi) the sub-cooling stream (81) from the first heat- exchanger ( 19) 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 (83) of the LNG; (vii) the substantially liquid sub-cooling stream (83) is evaporated in the first heat-exchanger (19) in order to form a reheated sub-cooling stream (85); ( viii) the main cooling stream ( 79) is expanded substantially to the low pressure PB in a main turbine (41) and the cooling stream from the main turbin e (41) is mixed wit h the reheated sub-cooling stream (85) in order to form a mixed stream (87); ( i x) the mixed stream (87) is reheated successively in the third heat -exchanger ( 35) , then in the second heat-exchanger (33) in order to form a reheated mixed stream (89); and ( x) the reheated mixed stream (89) is introduced into the compressor ( 25 ) at a low pressure stage ( 29C) locat ed upstream of the intermediate pressure stage (29D). 2. Method according to claim 1, characterised in that the high pressure PH is betw een approxi matel y 40 and 100 bar , preferably between approximately 50 and 80 bar, and in particular between approximately 60 and 75 bar. 3. Method according to either claim 1 or claim 2, characterised in that the low pressur e PB is lower than approximately 20 bar. 4. Method according to any one of the preceding clai ms , characterised in that, during step ( vi ) , the sub-cooling stream (81) from the first heat -exchanger (19) is expanded in a dynamic manner in a liquid expansion turbine ( 101) . 5. Method according to any one of the preceding claims, characterised in that, during step (ii), the initial stream (73) of refrigerating fluid is at least partially compressed in an auxiliary compressor (39) which is coupled to the main turbine ( 41) . 6. Method according to any one of the preceding claims, characterised in that, during st ep ( i) , a str earn ( 92 ) of C2 hydrocarbons is introduced into the compressor (25) in order to form a portion of the initial stream (73) of refrigerating fluid. 7. Method according to any one of the preceding claims, characterised in that, during step ( iii) , the compressed stream ( 7 5) of refrigerating fluid is brought into a heat - exchange relationship with a secondary refrigerating fluid ( 117) which circulates in the sec ond heat -exchanger ( 33) , the secondary refrigerating fluid (117) being subjected to a third refrigeration cycle (105) in which it is conpressed at the outlet of the second heat-exchanger (33), it is cooled and condensed at 1east partially, then expandal before it is evaporat ed in the second heat -exchanger ( 33) , 8. Method according to claim 7, characterised in that the secondary refrigerating fluid (117) comprises propane and optiona11y ethane. 9. Method according to any one of the preceding claims, characterised in that, before the expansion of step ( e) , the stream from the intermediate turbine (47) is mixed with a supplementary stream (63) of natural gas cooled by means of heat-exchange with the top stream (69) of gas in a fourth heat -exchanger ( 51) . 10 . Method according to any on e of the prec eding claims , characterised in that the content in terms of C+2 of the top gas (69) is such that the stream cooled by the second heat- exchanger (33) is purely gaseous. 11, Installation (9; 9 9; 10 4; 125) f or processing a stream ( 11) of LNG obtain ed by means of cooling using a first refrigeration cycle (17), the installation (9; 99; 104; 125) being of the type comprising: - means for sub-cooling the stream (11) of LNG comprising a first heat-exchanger (19) in order to bring the LNG stream int o a heat -exchange relationship with a r ef rigerating fluid (83); and - a s ec ond semi -open refrigeration cyc1e ( 21) which is . independent of the first c yc 1 e ( 15 ) , characteris ed in that it c ompris es: - an intermediate turbine (47) for dynamic expansion of the stream (57) of sub-cooled LNG from the first heat-exchanger (19); - means (53, 61) 'for cooling and expanding the stream (59) from the intermediate turbine ( 47); - a distillation colurm (49) which is connected to the cooling and expansion means ( 53, 61); - means for recovering a stream (67) of denitrogenated LNG at the bottom of the colurm (49) , and means for recovering a strearn ( 6 9) of gas at the top of the colurm ( 49) , - a stage compressor (25) which is connected to the means for recovering the stream (69) of gas at the top of the colurm ( 49); and - means for extracting a first portion (16) of the top stream (69) of gas tapped at an intermediate pressure stage (29D) of the compressor (25) in order to form a stream of combustible gas; and in that the second refrigeration cycl e ( 21) c omprises: - means for forming an initial stream (73) of refrigerating fluid from a second portion of the top gas (69) compressed to the intermediate pressure; - means (39) for compressing the initial stream (73) of refrigerating fluid to a high pressure PH which is greater than r.he intermediate pressure PI in order to form a compressed stream ( 75) of refrigerating fluid; - a second heat -exchang er ( 33) in ord er to cool t h e compressed stream ( 75 ) of ref rigerating f luid; - means for separating the compressed stream (75) of refrigerating fluid from the second heat-exchanger (33) into a main cooling stream (79) and a sub-cooling stream (77) of the LNG; - a third heat-exchanqer (35) for cooling the sub-cooling steam (77); - means for introducing the sub-cooling stream (77) from the third heat-exchanger (35) into the first heat-exchanger (19); means (37; 101) for expanding the sub-cooling stream (31) from the first heat-exchanger (19) to a low pressure PB which is 1ower than the i ntermediat e pressure PI in order to form a substantially liquid sub-cooling stream (83) of the LNG; - means for circulating the substantially liquid sub-cooling stream (83) in the first heat-exchanger in order to form a reheated sub-cooling stream (85); - a main turbine (41) for expanding the main cooling stream (79) substantially to the low pressure PB; - means for mixing the cooling str earn from the main turbine (41) with the sub-cooling stream (85) which has been reheated in order to form a mixed stream ( 87 ) ; - means for circulating the mixed stream (87) successively in the third heat-exchanger (35) then in the second heat-exchanger (33) in order to form a reheated mixed stream (89); - means for introducing the reheated mixed stream (89) in the compressor (25) at a low pressure stage (29C) which is located upstream of the intermediate pressure stage (29D). 12. Installation (9; 99; 104; 125) according to claim 11, characterised in that the high pressure PH is between approximately 40 and 100 bar , preferably between approximately 50 and 8 0 bar and in particular between approximately 60 and 75 bar. 13. Installation (9; 99; 104; 125) according to either claim 11 or claim 12, characterised in that the low pressure PB is lower than approximately 20 bar. 14. Installation (99; 104; 125) according to any one of claims 11 to 13, characterised in that the means (37; 101) for expanding the sub-cooling stream (81) from the first heat-exchanger (19) comprise a liquid expansion turbine (101). 15. Installation (9; 99; 104; 125) according to any one of clai ms 11 to 14, char act eris ed in that the means ( 39) f or compressing the initial stream (73) of refrigerating fluid comprise an auxiliary compressor (39) which is coupled to the main turbine ( 41). 16 . Installation ( 9 9 ) according to any on e of claims 11 to 15 , characterised in that the second refrigeration cycle (21) comprises means for introducing a stream ( 92) of C2 hydrocarbons into the compressor (25) in order to form a portion of the initial stream (73) of refrigerating fluid. 17. Installation (104; 125) according to any one of claims 11 to 16 , char act erised in that the se: ond heat -exchanger ( 33) comprises means for circulating a secondary refrigerating fluid (117), the installation (104; 125) comprising a third refrigeration cycle (105) c omprising secondary means ( 107 ) for compressing the secondary refrigerating fluid (115) from the third heat -exchanger ( 33) , secondary means (109, 111) for cooling and expanding the secondary refrigerating fluid (117) from the secondary compression means (107), and means for introducing the secondary refrigerating fluid (119) from the seconder y expansion means (111) into t he second heat -exchanger ( 33 ) . 18. Installation (104; 125) according to claim 17, characterised in that the secondary refrigerating fluid (117) comprises propane and optionally ethane. 19. Installation (9; 99; 104; 125) according to any one of claims 11 to 18, characterised in that it comprises means for mixing the stream (59) of sub-cooled LNG with a supplementary stream (63) of natura1 gas , and a f ourth heat -exchang er ( 51) in order to bring the supplementary stream (63) into a heat -exchange relationship with the top stream (69) of gas.

Documents:

1763-CHENP-2008 AMENDED CLAIMS 24-04-2012.pdf

1763-CHENP-2008 CORRESPONDENCE OTHERS 14-03-2012.pdf

1763-CHENP-2008 CORRESPONDENCE OTHERS.pdf

1763-CHENP-2008 CORRESPONDENCE PO.pdf

1763-CHENP-2008 EXAMINATION REPORT REPLY RECEIVED 24-04-2012.pdf

1763-CHENP-2008 FORM-18.pdf

1763-CHENP-2008 FORM-3 24-04-2012.pdf

1763-CHENP-2008 OTHER PATENT DOCUMENT 24-04-2012.pdf

1763-CHENP-2008 POWER OF ATTORNEY 24-04-2012.pdf

1763-chenp-2008-abstract.pdf

1763-chenp-2008-claims.pdf

1763-chenp-2008-correspondnece-others.pdf

1763-chenp-2008-description(complete).pdf

1763-chenp-2008-drawings.pdf

1763-chenp-2008-form 1.pdf

1763-chenp-2008-form 18.pdf

1763-chenp-2008-form 3.pdf

1763-chenp-2008-form 5.pdf

1763-chenp-2008-pct.pdf