EP1946026B1 - Method for treating a liquefied natural gas stream obtained by cooling using a first refrigerating cycle and related installation - Google Patents

Description

1. (a) on introduit le courant de GNL porté à une température inférieure à - 100°C dans un premier échangeur thermique ;
2. (b) on sous-refroidit le courant de GNL dans le premier échangeur thermique par échange thermique avec un fluide réfrigérant pour former un courant de GNL sous-refroidi ; et
3. (c) on fait subir au fluide réfrigérant un deuxième cycle de réfrigération semi-ouvert, indépendant du premier cycle.

The present invention relates to a method of treating an LNG stream obtained by cooling by means of a first refrigeration cycle, the process being of the type comprising the following steps:

1. (a) introducing the LNG stream brought to a temperature below -100 ° C into a first heat exchanger;
2. (b) the LNG stream is subcooled in the first heat exchanger by heat exchange with a coolant to form a subcooled LNG stream; and
3. (c) the cooling fluid is subjected to a second semi-open refrigeration cycle, independent of the first cycle.

We know US-B-6,308,531 a process of the aforementioned type, in which a stream of natural gas is liquefied using a first refrigeration cycle which involves the condensation and the vaporization of a hydrocarbon mixture. The temperature of the gas obtained is about -100 ° C. The LNG product is then subcooled to about -170 ° C using a second semi-open "reverse Brayton cycle" refrigeration cycle consisting of a stage compressor and an expansion turbine. gas.

Such a method is not entirely satisfactory. Indeed, the maximum yield of the inverted Brayton cycle is limited to about 40%. Moreover, its operation in semi-open cycle is difficult to implement.

The article "High Efficiency 6MTPA LNG Design Train Via Two Different Mixed Refrigerant Processes" XP009052299 discloses a method of treatment including in particular steps (a) to (g) defined in claim 1.

• le courant de fluide réfrigérant comprimé issu du deuxième échangeur thermique est séparé en un courant de refroidissement principal et en le courant de sous-refroidissement du GNL ;
• le courant de refroidissement principal est détendu sensiblement jusqu'à la pression basse dans une turbine principale,
• le courant de sous-refroidissement du GNL issu du premier échangeur thermique après détente forme un courant essentiellement liquide de sous-refroidissement du GNL ;
• le courant essentiellement liquide de sous-refroidissement est vaporisé dans le premier échangeur thermique pour former le courant de sous-refroidissement réchauffé ;
• le courant de sous-refroidissement issu de la turbine principale est mélangé avec le courant de sous-refroidissement réchauffé pour former un courant de mélangé;
• le courant de mélange est réchauffé successivement dans le troisième échangeur thermique, puis dans le deuxième échangeur thermique pour former le courant de fluide réfrigérant réchauffé qui est par la suite comprimé dans le compresseur à étages.

However, this method does not include the following steps:

• the compressed coolant stream from the second heat exchanger is separated into a main cooling stream and the LNG subcooling stream;
• the main cooling stream is expanded substantially to the low pressure in a main turbine,
• the LNG subcooling stream from the first heat exchanger after expansion forms a substantially liquid subcooling stream of the LNG;
• the substantially liquid subcooling stream is vaporized in the first heat exchanger to form the heated subcooling stream;
• the subcooling stream from the main turbine is mixed with the heated subcooling stream to form a mixed stream;
• the mixing stream is successively heated in the third heat exchanger and then in the second heat exchanger to form the heated coolant stream which is subsequently compressed in the stage compressor.

An object of the invention is therefore to provide an autonomous process for treating a stream of LNG, which has an improved yield and which can easily be implemented in units of various structures.

For this purpose, the subject of the invention is a method according to claim 1.

The method according to the invention may comprise one or more of the features of claims 2 to 10.

The invention also relates to an installation according to claim 11.

The installation according to the invention may comprise one or more of the features of claims 12 to 19.

• la Figure 1 est un schéma synoptique fonctionnel d'une première installation selon l'invention ;
• la Figure 2 est un graphe représentant les courbes d'efficacité du deuxième cycle de réfrigération de l'installation de la Figure 1, en fonction de la température du GNL à l'entrée du premier échangeur ;
• la Figure 3 est un schéma analogue à celui de la Figure 1 d'une deuxième installation selon l'invention ;
• la Figure 4 est un schéma analogue à celui de la Figure 1 d'une troisième installation selon l'invention ; et
• la Figure 5 est un schéma analogue à celui de la Figure 1 d'une quatrième installation selon l'invention.

Examples of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• the Figure 1 is a functional block diagram of a first installation according to the invention;
• the Figure 2 is a graph representing the efficiency curves of the second refrigeration cycle of the installation of the Figure 1 , depending on the temperature of the LNG at the inlet of the first exchanger;
• the Figure 3 is a diagram similar to that of the Figure 1 a second installation according to the invention;
• the Figure 4 is a diagram similar to that of the Figure 1 a third installation according to the invention; and
• the Figure 5 is a diagram similar to that of the Figure 1 of a fourth installation according to the invention.

The first subcooling installation 9 according to the invention, represented on the Figure 1 , is intended for the production, from a stream 11 of liquefied natural gas (LNG) starting at a temperature below -90 ° C, a denitrogenated LNG stream 13. The installation 9 also produces a fuel gas stream 16 rich in nitrogen.

As illustrated by Figure 1 the starting LNG stream 11 is produced by a natural gas liquefaction unit including a first refrigeration cycle 17. The first cycle 17 comprises, for example, a cycle comprising means for condensing and vaporizing a mixture of hydrocarbons.

The installation 9 comprises a first subcooling heat exchanger 19, a second half-open refrigeration cycle 21, independent of the first cycle 17, and a denitrogenation unit 23.

The second refrigeration cycle 21 comprises a stage 25 compression apparatus having a plurality of compression stages 27. Each stage 27 comprises a compressor 29 and a refrigerant 31.

The second cycle 21 further comprises a second heat exchanger 33, a third heat exchanger 35, an expansion valve 37 and an auxiliary compressor 39 coupled to a main expansion turbine 41. The second cycle 21 also comprises an auxiliary refrigerant 43.

In the example shown on the Figure 1 the stage compressor comprises four compressors 29. The four compressors 29 are driven by the same source 45 of external energy. The source 45 is for example a gas turbine engine type.

The refrigerants 31 and 43 are cooled by water and / or air.

The denitrogenation unit 23 comprises an intermediate hydraulic turbine 47 coupled to a current generator 48, a distillation column 49, a heat exchanger 51 at the top of the column and a heat exchanger 53 at the bottom of the column. It further comprises a pump 55 for evacuating the de-nitrogenated LNG 13.

In what follows, will be designated by the same reference a liquid stream and the pipe that carries it, the pressures considered are absolute pressures, and the percentages considered are molar percentages.

The starting LNG stream 11 from the liquefaction unit 15 is at a temperature below -90 ° C, for example at-130 ° C. This stream 11 comprises for example substantially 5% nitrogen, 90% methane and 5% ethane, and its flow rate is 50,000 kmol / h.

The LNG stream 11 is introduced into the first heat exchanger 19, where it is subcooled to a temperature of-150 ° C to produce a subcooled LNG stream 57.

The stream 57 is then introduced into the hydraulic turbine 47 and dynamically expanded to a low pressure, to form a stream 59 expanded. This stream 59 is essentially liquid, that is to say that it contains less than 2 mol% of gas. The stream 59 is cooled in the foot heat exchanger 53, then introduced into an expansion valve 61 where it forms a feed stream 64 of 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 above atmospheric pressure. In this example, this pressure is 1.25 bar, and the temperature of stream 64 is about -165 ° C.

A make-up stream 63 of natural gas, substantially of the same composition as the starting LNG stream 11, is cooled in the head exchanger 51 and then expanded in a valve 65 and mixed with the depressurized subcooled LNG stream 59. upstream of the valve 61.

A reboiling stream 68 is extracted from the column 49 at an intermediate stage Ni, located in the vicinity of the bottom of this column. The stream 68 is introduced into the exchanger 53, where it is heated by heat exchange with the expanded sub-cooled LNG 59 stream, before being reintroduced into the column 49 under the intermediate level Ni.

A liquid foot stream 67 containing less than 1% nitrogen is withdrawn from column 49. This foot stream 67 is pumped by pump 55 to form the denitrogenated LNG stream 13 to be sent to a storage.

A gaseous overhead stream 69, containing about 50% nitrogen, is withdrawn from the distillation column 49. This stream 69 is heated by heat exchange with the makeup stream 63 in the head exchanger 51 to form a heated stream 71. This stream 71 is introduced into the first stage 27A of the compression apparatus 25.

The heated overhead stream 71 is successively compressed in the first stage 27A and in the second stage 27B of the compressor 25 to substantially a low cycle pressure PB, then compressed in the third compression stage 27C before being introduced into the fourth compression stage 27D. In each stage 27 of the compressor, the overhead stream 71 is compressed in the compressor 29 followed by cooling to a temperature of about 35 ° C in the associated refrigerant 31.

A first portion 16 of the compressed head stream in the fourth compression stage 27D is extracted from the compressor 29D at an intermediate pressure P1 to form the fuel gas stream.

The intermediate pressure PI is for example greater than 20 bar, and preferably substantially equal to 30 bar. The low cycle pressure PB is, for example, less than 20 bar.

A second portion 73 of the overhead current continues its compression in the compressor 29D to a mean pressure substantially equal to 50 bar to form a flow of refrigerant starting fluid.

The current 73 is cooled in the exchanger 31D and then introduced into the auxiliary compressor 39.

The flow rate of the starting coolant stream 73 is much greater than the flow rate of the fuel gas stream 16. The ratio between the two flow rates is, in this example, substantially equal to 6.5.

Current 73 is then compressed in compressor 39 to a high pressure cycle PH. This high pressure is between 40 and 100 bar, preferably between 50 and 80 bar and advantageously between 60 and 75 bar.

de sorte que le courant 75 est purement gazeux. Lorsque la pression haute est supérieure à 60 bars environ, le courant 75 est un fluide supercritique.The stream 73 coming from the compressor 39 forms, after passing through the refrigerant 43, a stream of compressed refrigerant 75. The overhead stream 69 contains less than 5% by mass of hydrocarbons ${\mathrm{VS}}_2^{+},$

so that the stream 75 is purely gaseous. When the high pressure is greater than about 60 bar, the stream 75 is a supercritical fluid.

The stream 75 is then cooled in the second heat exchanger 33 and separated at the outlet of this exchanger 33 into a minority sub-cooling flow stream 77 and a main cooling stream 79. The ratio of these two flows is of the order of 0.5.

The subcooling stream 77 is cooled in the third exchanger 35 and then in the first exchanger 19 to form a cooled subcooling stream 81. The stream 81 is expanded to the low cycle pressure PB in the valve 37, from which it exits as a substantially liquid subcooling stream 83, i.e. containing less than 10 % mol of gas.

The stream 83 is then introduced into the first exchanger 19, where it vaporizes and cools the stream 81 and the starting LNG stream 11 by heat exchange, to form, at the outlet of the first exchanger 19, a stream 85 of heated cooling.

The main gas stream 79 is expanded in the turbine 41 to substantially the low cycle pressure PB and mixed with the heated stream 85 from the first heat exchanger 19 to form a mixing stream 87. The mixing stream 87 is then introduced successively into the third heat exchanger 35, then into the second heat exchanger 33, where it cools by heat exchange relation, respectively the heat flow. -cooling 77 and the compressed coolant stream 75.

The heated mixing stream 89 from the exchanger 33 is then introduced into the compression apparatus 25 at the inlet of the third compression stage 27C, substantially at the low pressure PB.

By way of illustration, the pressure values, the temperatures and the flow rates in the case where the high cycle pressure PH is substantially equal to 75 bars are given in the table below. <u> TABLE 1 </ u> Current Temperature ° C Pressure (bars) Flow (kmol / h) 11 -130.0 49.1 50000 13 -161.1 5.3 46724 16 67.0 30.0 4876 57 -150.0 49.0 50000 59 -150.7 5.0 50000 63 -34.0 50.0 1600 64 -164.9 1.3 51600 67 -161.1 1.2 46724 69 -165.2 1.2 4876 71 -48.6 1.2 4876 73 124.0 50.9 31768 75 35.0 74.7 31768 77 -38.2 74.2 11496 79 -38.2 74.2 20272 81 -150.0 73.6 11496 83 -155.2 11.0 11496 85 -132.0 10.9 11496 87 -130.3 10.9 31768 89 34.38 10.7 31768

On the Figure 2 , curve 91 of efficiency of cycle 21 in the process according to the invention is represented as a function of the temperature value of the LNG stream 11. As illustrated in this Figure, the yields are greater than 44%, which constitutes a significant gain over the methods of the state of the art involving a so-called inverted semi-open Brayton cycle.

This result is obtained in a simple manner, since it is not necessary to provide means for storing and preparing a refrigerant, the refrigerant 73 being delivered continuously by the installation 9.

The method and plant 9 of the present invention are used either in new liquefaction units or to improve the performance of existing LNG production units. In the latter case, at equal power consumption, the production of nitrogenized LNG can be increased from 5% to 20%. The method and plant 9 according to the invention can also be used to subcool and de-nitrogen LNG produced in natural gas liquids extraction (NGL) processes.

The installation 99 shown on the Figure 3 differs from the first installation 9 in that the expansion valve 37 located downstream of the first exchanger is replaced by a dynamic expansion turbine 101 coupled to a current generator 103.

The method of treating the LNG stream in this installation is also identical to the method implemented in the installation 9, to the numerical values.

In a variant shown in dotted lines on the Figure 3 a stream of ethane 92 is mixed with the heated mixture stream 89 before it is introduced into the third compression stage 27C.

The efficiency of cycle 21 is then further increased, as illustrated by curve 93 of FIG. Figure 2 .

The third installation according to the invention 104 is represented on the Figure 4 . This installation 104 differs from the second installation 99 in that it also comprises a third refrigeration cycle 105 closed, independent of the first and second cycles 17 and 21.

The third cycle 105 comprises a secondary compressor 107, first and second secondary refrigerants 109A and 109B, an expansion valve 111 and a separator tank 113.

This cycle is carried out using a secondary refrigerant fluid stream 115 made of propane. The gaseous stream 115 at low pressure is introduced into the compressor 107, then cooled and condensed at the high pressure in the coolers 109A and 109B to form a stream 117 of partially liquid propane. This stream 117 is cooled in the exchanger 33, then introduced into the expansion valve 111, where it is expanded and forms a two-phase stream of expanded propane 119.

The stream 119 is introduced into the separator tank 113 to form a liquid fraction 121 extracted from the base of the balloon 113. The fraction 121 is introduced into the exchanger 33, where it is vaporized by heat exchange with the stream 117 and with the stream 75 compressed refrigerant, before being introduced into the balloon 113.

The gaseous fraction from the head of the flask 113 forms the gaseous propane stream 115.

As shown in curve 123 of the Figure 2 , the efficiency of the cycle 21 is then increased by 4% on average with respect to the efficiency of the process implemented in the first installation 9.

The fourth installation 25 according to the invention 125, represented on the Figure 5 , differs from that shown on the Figure 4 in that the third refrigerant cycle 105 is devoid of a separator tank 113. The stream 119 coming from the valve 111 is thus directly introduced into the second exchanger 33 and completely vaporized in this exchanger.

Moreover, the refrigerant 115 is composed of a mixture of ethane and propane. The ethane content in the fluid 115 is substantially equal to the propane content.

As shown in curve 126 of the Figure 2 , the average efficiency of the second refrigeration cycle is then increased by about 0.5% with respect to the efficiency of the process implemented in the third installation 104 when the temperature is below - 130 ° C. Taking into account the energy produced by the turbine 47, the overall efficiency of the installation of the Figure 5 slightly above 50%, compared with around 47.5% for Figure 1 , 47.6% for that of Figure 3 and 49.6% for that of Figure 4 .

Claims (19)

1. Method for processing a stream (11) of LNG obtained by cooling using a first refrigeration cycle (17), the method being of the type comprising the following steps:

   (a) the stream (11) of LNG 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 LNG is sub-cooled in the first heat-exchanger by heat-exchange with a refrigerating fluid (83) in order to form a stream (57) of sub-cooled LNG; and

   (c) the refrigerating fluid (83) is subjected to a second semi-open refrigeration cycle (21) which is independent of the first cycle (15),

   the method comprising the following 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 cooled and expanded and then introduced into a distillation column (49) ;

   (f) a stream (67) of denitrogenated LNG at the bottom of the column (49) and a stream (69) of gas at the top of the column are recovered; and

   (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 brought to an intermediate pressure PI is extracted in order to form a stream of combustible gas;

   the second refrigeration cycle (21) comprising the following steps:

   (i) an initial stream (73) of refrigerating fluid is formed from a second portion of the top gas (69) which has been compressed 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 fluid;

   (iii) the stream (75) of compressed refrigerating fluid is cooled in a second heat-exchanger (33);

   (iv) the stream (75) of compressed refrigerating fluid from the second heat-exchanger (33) is separated into a main cooling stream (79) and a sub-cooling stream (77) of the LNG;

   (v) the sub-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 turbine (41) is mixed with the reheated sub-cooling stream (85) in order to form a mixed stream (87);

   (ix) 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) located upstream of the intermediate pressure stage (29D).
2. Method according to claim 1, characterised in that 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 either claim 1 or claim 2, characterised in that the low pressure PB is lower than approximately 20 bar.
4. Method according to any one of the preceding claims, 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 step (i), a stream (92) of 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 (75) of refrigerating fluid is brought into a heat-exchange relationship with a secondary refrigerating fluid (117) which circulates in the second heat-exchanger (33), the secondary refrigerating fluid (117) being subjected to a third refrigeration cycle (105) in which it is compressed at the outlet of the second heat-exchanger (33), it is cooled and condensed at least partially, then expanded before it is evaporated in the second heat-exchanger (33).
8. Method according to claim 7, characterised in that the secondary refrigerating fluid (117) comprises propane and optionally 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 heat-exchange with the top stream (69) of gas in a fourth heat-exchanger (51).
10. Method according to any one of the preceding claims, characterised in that the content in terms of ${\mathrm{C}}_2^{+}$

    

    of the top gas (69) is such that the stream cooled by the second heat-exchanger (33) is purely gaseous.
11. Installation (9; 99; 104; 125) for processing a stream (11) of LNG obtained by 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 into a heat-exchange relationship with a refrigerating fluid (83) ; and

    - a second semi-open refrigeration cycle (21) which is independent of the first cycle (15),

    - 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 column (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 column (49), and means for recovering a stream (69) of gas at the top of the column (49),

    - a stage compressor (25) which is connected to the means for recovering the stream (69) of gas at the top of the column (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;

    second refrigeration cycle (21) comprises:

    - 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 the intermediate pressure PI in order to form a compressed stream (75) of refrigerating fluid;

    - a second heat-exchanger (33) in order to cool the compressed stream (75) of refrigerating fluid;

    - 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-exchanger (35) for cooling the sub-cooling stream (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 (81) from the first heat-exchanger (19) 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;

    - 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 stream 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 80 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 claims 11 to 14, characterised in that the means (39) for compressing the initial stream (73) of refrigerating fluid comprise an auxiliary compressor (39) which is coupled to the main turbine (41).
16. Installation (99) according to any one of claims 11 to 15, characterised in that the second refrigeration cycle (21) comprises means for introducing a stream (92) of 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, characterised in that the second heat-exchanger (33) comprises means for circulating a secondary refrigerating fluid (117), the installation (104; 125) comprising a third refrigeration cycle (105) comprising secondary means (107) for compressing the secondary refrigerating fluid (115) from the third heat-exchanger (35), 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 secondary expansion means (111) into the 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 natural gas, and a fourth heat-exchanger (51) in order to bring the supplementary stream (63) into a heat-exchange relationship with the top stream (69) of gas.

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