EP0644996A1

EP0644996A1 - Fluid cooling process and plant, especially for natural gas liquefaction

Info

Publication number: EP0644996A1
Application number: EP94913137A
Authority: EP
Inventors: Maurice Grenier
Original assignee: Gaz de France SA
Current assignee: Engie SA
Priority date: 1993-04-09
Filing date: 1994-04-05
Publication date: 1995-03-29
Anticipated expiration: 2014-04-05
Also published as: NO944701D0; DE69415454T2; HK1012700A1; JP3559283B2; AU669628B2; FR2703762B1; WO1994024500A1; CA2136755C; ES2125448T3; RU94046343A; AU6540494A; NO308969B1; NO944701L; US5535594A; CA2136755A1; ATE175019T1; RU2121637C1; US5613373A; DE69415454D1; JPH07507864A

Abstract

In this process, which incorporates an integral cascade, the coolant mixture issuing from the penultimate stage (1B) of the compressor cycle (1) is delivered to a distillation apparatus (5) the head vapor of which is cooled (in 24) to a temperature significantly lower than the ambient temperature, then separated into two phases (in 6C); the vapor stage is supplied to the last stage (1C) of the compressor, and the liquid phase constitutes a coolant fluid for the hot part (8) of the heat exchange line (7).

Description

METHOD AND PLANT FOR COOLING A FLUID, PARTICULARLY FOR LIQUEFACTION OF NATURAL GAS.

The present invention relates to the cooling of fluids, and applies in particular to the liquefaction of natural gas. It relates firstly to a process for cooling a fluid, in particular for liquefying natural gas, of the integral incorporated cascade type, in which a refrigerant mixture composed of constituents of different volatilities is compressed in at least two stages and, after at least each of the intermediate stages of compression, the mixture is partially condensed, at least some of the condensed fractions as well as the high pressure gas fraction being cooled, expanded, brought into heat exchange relationship with the fluid to be cooled and then compressed again . The pressures discussed below are absolute pressures.

It has long been proposed to liquefy natural gas using a so-called "incorporated cascade" refrigeration cycle using a mixture of fluids.

The refrigerant mixture consists of a certain number of fluids including, among others, nitrogen, hydrocarbons such as methane, ethylene, ethane, propane, butane, pentane, etc. The mixture is compressed, liquefied and then sub-cooled at the high pressure of the cycle which is generally between 20 and 50 bars. This liquefaction can be carried out in one or more stages with separation of the condensed liquid at each stage. The liquid or liquids obtained are, after their sub-cooling, expanded at the low pressure of the cycle, generally between 1.5 and 6 bars, and vaporized in counter-current of the natural gas to be liquefied and of the cycle gas to be cooled.

After reheating near room temperature, the refrigerant mixture is again compressed until the high pressure of the cycle. For operation to be possible, it is necessary to have a fluid capable of condensing at room temperature at the high pressure of the cycle. This poses a particular difficulty, arising from the fact that the mixture and the pressures are generally optimized for the cold part of the liquefaction installation and are ill suited to equally efficient refrigeration in the hot part, that is to say between ambient temperature (generally of the order of + 30 to + 40 ° C in regions producing natural gas) and an intermediate temperature of the order of - 20 to -40 ° C.

Many existing installations thus use, for the hot part, a separate refrigeration cycle, with propane or propane-ethane mixture. A relatively low specific energy expenditure is thus obtained, but at the cost of a significant increase in the complexity and cost of the installation.

The object of the invention is to eliminate the separate refrigeration cycle, and therefore to use a single compressor unit, that is to say a refrigeration cycle known as "with integral incorporated cascade", so as to make it possible to obtain both relatively specific process energy and relatively low investment.

To this end, the invention relates to a cooling process of the aforementioned type, characterized in that the gas from the penultimate is distilled compression stage in a distillation apparatus, the head of which is cooled with a liquid having a temperature significantly below ambient temperature, in order to form on the one hand the condensate of this penultimate stage, and on the other hand a vapor phase which is sent at the last stage of compression.

For the sake of clarity, the "ambient temperature" will be defined as the thermodynamic reference temperature corresponding to the temperature of the cooling fluid (notably water) available on the site and used in the cycle, increased by the temperature difference that the it is fixed, by construction, at the outlet of the refrigerating machines (compressors, exchangers ...). In practice, this difference is around 3 to 10 ° C, and preferably of the order of 5 to 8 ^β C.

It will also be noted, as of now, that the cooling temperature of the head of the distillation apparatus (corresponding substantially to the temperature of the "liquid" acting for this purpose) will be between approximately 0 and 20 "C, and generally between 5 and 15 "C, for an" ambient temperature "(or inlet temperature in the exchange line) of the order of 15 to 45 ° C., and generally between 30 and 40 ° C.

The method can also include one or more of the following characteristics:

- The head vapor of the distillation apparatus is cooled and partially condensed by heat exchange with at least the said expanded fractions, and the head of the distillation apparatus is cooled with the liquid phase thus obtained;

- The gas from the last stage of compression is partially cooled and partially condensed near ambient temperature, the liquid phase obtained is expanded, and the head of the device is cooled distillation by means of this expanded liquid phase;

- operating a dephlegmation of the gas from the last stage of compression during its cooling;

- There is an indirect heat exchange between the liquid resulting from the cooling of the gas from the last stage of compression and the overhead vapor of the distillation apparatus before sending this vapor to the last compression stage and expanding said liquid;

- At least part of the condensate from the first compression stage is pumped up to the outlet pressure from the second compression stage, and it is mixed with the gas from this second compression stage;

- When the process is intended for the liquefaction of natural gas containing nitrogen, the liquefied natural gas resulting from refrigeration and then denitrogenated is sub-cooled, by heat exchange with expanded non-denitrogenated liquefied natural gas;

- when the process is intended for the liquefaction of natural gas containing nitrogen, a primary denitrogenation of the natural gas is carried out under its treatment pressure in an auxiliary column, part of the liquefied natural gas which has undergone is expanded to an intermediate pressure this primary denitrogenization, the liquid thus expanded is vaporized by cooling the head of the auxiliary column, which produces a combustible gas under intermediate pressure, this combustible gas is sent to a gas turbine driving the compressor, and the rest of the liquefied natural gas having undergone the primary denitrogenation as well as the overhead vapor of the auxiliary column in a denitrogenation column final under low pressure producing in tanks the nitrogenous liquefied natural gas intended to be stored.

The subject of the invention is also an installation for cooling a fluid, in particular for liquefying natural gas, intended for the implementation of such a process.

This installation, of the type comprising a refrigeration circuit with integral incorporated cascade in which a refrigerant mixture circulates and which comprises a compressor with at least two stages, at least the intermediate stages of which are each provided with a refrigerant and a heat exchange line, is characterized in that it comprises a distillation apparatus supplied by the penultimate stage of the compressor and whose head is connected to the suction of the last stage of the compressor, and means for cooling the head of the apparatus distillation by means of a liquid having a temperature markedly below ambient temperature. In a particular embodiment, the heat exchange line consists of two plate exchangers in series, in particular of the same length, connected to each other by end domes and optionally welded together end to end.

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

- Figure 1 schematically shows a natural gas liquefaction installation according to the invention;

- Figure 2 shows schematically another embodiment of the following installation

The invention; - Figure 3 shows in more detail an element of the installation of Figure 2; - Figure 4 schematically shows part of a variant of the installation of Figure 1;

- Figure 5 shows schematically a variant of the cold part of the installation of Figure 1 or Figure 2; and

- Figure 6 is a partial schematic view of another alternative installation according to the invention. The natural gas liquefaction installation represented in FIG. 1 essentially comprises: a single cycle compressor 1 with three stages 1A, 1B and 1C, each stage discharging, via a respective pipe 2A, 2B and 2C, into a respective refrigerant 3A , 3B and 3C cooled with sea water, this water typically having a temperature of the order of + 25 to + 35 "C; a pump 4; a distillation column 5 having a few theoretical plates; separator pots 6B, 6C, the top of which communicates respectively with the suction of stages 1B and 1C; a heat exchange line 7 comprising two exchangers in series, namely a "hot" exchanger 8 and a "cold" exchanger 9; an intermediate separator pot 10 ; an auxiliary coolant circuit 11; an auxiliary heat exchanger 12; a denitrogenation column 13; and a liquefied natural gas (LNG) storage 14.

The outlet of the refrigerant 3A opens into the separator 6, the bottom of which is connected to the suction of the pump 4, while the latter flows back into the pipe 2B. The outlet of the refrigerant 3B communicates with the tank of the column 5, and the bottom of the separator 6C is connected by gravity, via a siphon 15 and an adjustment valve 16, to the head of the column 5. The exchangers 8, 9 are parallelepipedic exchangers with aluminum plates possibly brazed, with counter-current circulation of the fluids in heat exchange relation, and have the same length. They each include the passages necessary to ensure the operation which will be described below.

The refrigerant mixture, consisting of C1 to C5 hydrocarbons and nitrogen, leaves the top (hot end) of the exchanger 8 in the gaseous state and arrives via a line 17 at the suction of the first stage of compressor 1A .

It is thus compressed to a first intermediate pressure PI, typically of the order of 8 to 12 bars, then is cooled to + 30 to + 40 ^β C in 3A and separated into two phases in the pot 6B. The vapor phase is compressed to a second intermediate pressure P2, typically of the order of 14 to 20 bars, at IB, while the liquid phase is brought by the pump 4 to the same pressure P2 and injected into line 2B. The mixture of the two phases is cooled and partially condensed in 3B, then distilled in 5.

The liquid in the bottom of the column 5 constitutes a first cooling liquid, suitable for ensuring most of the cooling of the hot exchanger 8. For this, this liquid is introduced laterally, via an inlet box 18, into the part upper side of this exchanger, sub-cooled in passages 19 to the cold end of the exchanger, at around -20 to -40 ° C., released laterally via an outlet box 20, expanded at the low pressure of the cycle, which is typically of the order of 2.5 to 3.5 bars, in an expansion valve 21, and reintroduced in two-phase form at the cold end of the same exchanger via a side box 22 and an appropriate distribution device, to be vaporized in the low pressure passages 23 of the exchanger. The overhead vapor of column 5 is cooled and partially condensed in passages 24 of exchanger 8 to an intermediate temperature significantly lower than ambient temperature, for example up to + 5 to + 10 "C, then introduced in the pot 6C. The liquid phase returns to reflux by gravity, via the siphon 15 and the valve 16, at the head of the column 5, while the vapor phase is compressed at the high pressure of the cycle, typically of the order of 40 bars, in 1C, then brought back to + 30 to + 40 ^, 'C in 3 C. This vapor phase is then cooled from the hot end to the cold end of the exchanger 8 in high pressure passages 25, and separated into two phases in 10. To complete the refrigeration of

1'exchangeur 8, we can, as shown in broken lines, sub-cool to an intermediate temperature part of the liquid collected in 6B, then take it out laterally from the exchanger, relax it at low pressure in a valve trigger 26, and reintroduce it laterally into the exchanger to vaporize it in the intermediate part of the low pressure passages 23.

The refrigeration of the exchanger 9 is obtained by means of the high pressure fluid, as follows.

The liquid collected at 10 is sub-cooled in the hot part of the exchanger 9, in passages 27, then taken out of the exchanger, expanded at low pressure in an expansion valve 28, reintroduced into the exchanger and vaporized in the hot part of the low pressure passages 29 thereof. The vapor phase from separator 10 is cooled, condensed and sub-cooled from the hot end to the cold end of the exchanger 9, and the liquid thus obtained is expanded at low pressure in an expansion valve 30, and reintroduced at the cold end of the exchanger to be vaporized in the cold part of the low pressure passages 29 and then combined with the expanded fluid at 28.

The treated natural gas, arriving at around + 20 ^β C, after drying, via a pipe 31, is introduced laterally into the exchanger 8 and cooled to the cold end of the latter in passages 32.

At this temperature, natural gas is sent to an apparatus 33 for removing C2 to C5 hydrocarbons, and the remaining mixture, consisting essentially of methane and nitrogen, with a small amount of ethane and propane, is divided into two streams: a first stream, cooled, liquefied and sub-cooled from the hot end to the cold end of the auxiliary exchanger 12 and then expanded to 1.2 bar in an expansion valve 34, and a second stream, cooled, liquefied and sub-cooled from the hot end to the cold end of the exchanger 9 in passages 35, sub-cooled again by about 8 to 10 "C in a coil 36 forming the column reboiler

13, and expanded to 1.2 bar in an expansion valve 37. The two relaxed streams are combined and then introduced under reflux at the top of the column 13, which thus ensures the denitrogenation of natural gas. The tank liquid in this column constitutes the nitrogenous LNG produced by the installation and is sent to storage.

14, while the overhead steam is reheated to - 20 to - 40 ^β C from the cold end to the hot end of

The exchanger 12 and is sent via a line 38 to the "fuel gas" network to be burned or used in a gas turbine of the installation serving to drive the compressor 1.

It should be noted that an additional shutdown on natural gas can be made in the exchanger 9 at a temperature allowing recover additional quantities of C2 and C3 hydrocarbons from the apparatus 33.

As shown, taking into account the very high flow rates generally used in such an installation, it may be desirable to expand a portion of the cold liquids in liquid turbines or "expanders" 39 to produce cold as well as 'part of the electrical current required. In addition, the hottest part of 1'échangeur 8 can be used for cooling from + 40 to + 20 ^* 'C about a suitable liquid, in particular pentane, circulated through the passages 40 by a pump 1'échangeur 41 and used to refrigerate another part of the installation, for example raw natural gas intended to be dried before its treatment in the liquefaction installation. This circulation of liquid constitutes the above-mentioned refrigerant circuit 11.

The arrangement described above makes it possible both to accelerate the condensation of the mixture coming from the second compression stage IB, by means of the injection of liquid into the line 2B by means of the pump 4, to simplify the exchanger 8 if all of the liquid from the pot 6B is pumped, and to obtain a high pressure mixture sufficiently free of heavy products, more precisely, in the example considered, almost all of the hydrocarbons in C5 and the majority of the hydrocarbons in C4 , to be completely vaporized at the hot end of the passages 29 of the cold exchanger 9. This has the important advantage that these passages can lead into an upper dome 42 of the exchanger 9 communicating directly with a lower dome 43 of the exchanger 8, without any two-phase redistribution being necessary for the break between the two exchangers. We can then simplify still 1 • installation by butt welding the two exchangers 8 and 9.

It can also be noted that the suction of the compressor stage 1C at a relatively cold temperature is favorable to the performance thereof.

The cutoff at around - 20 to - 40 ° C between the two exchangers also corresponds to heat exchange surfaces of the same order above and below this cutoff, so that two exchangers can be used 8 and 9 of maximum length under conditions of optimal thermal performance, and a single separator pot 10, with the abovementioned cut-off, for the high pressure fluid. It is understood that the control of the temperature and the pressure (+ 5 to + 10 ° C, 14 to 20 bars) of the coolant at the head of the column 5 makes it possible to obtain a single-phase gas both at the outlet of the 3C refrigerant and at the outlet (at 42) from the cold exchanger 9 (- 20 ° C to - 40 "C, 2.5 to 3.5 bars).

It should be noted that in practice, n exchangers 8 are mounted in parallel, and n exchangers 9 in parallel. The installation shown in FIG. 2 differs from that of FIG. 1 only by the addition, between the compression stages IB and 1C, of another intermediate compression stage 1D, as well as by the method of cooling the liquid. reflux from column 5. Thus, the outlet of the refrigerant 3B opens into a separator pot 6D, the vapor phase of which feeds the stage 1D. The discharge of this is cooled by a 3D coolant and then introduced at the base of the column 5. The liquid in the pot 6D constitutes an additional coolant, sub-cooled in additional passages 45 provided in the hot part of the exchanger 8, taken out of it, expanded at low pressure in an expansion valve 46 and reintroduced into the exchanger to be vaporized in the intermediate part of the low pressure passages 23. Furthermore, the overhead vapor from column 5 is sent directly to the suction of the last compression stage 1C, and the high pressure fluid is sent to the base of a dephlegmator 47 cooled by trickling seawater around vertical tubes 48. The majority of products heavy are collected at the base of the dephlegmator, expanded in an expansion valve 49 and introduced into reflux at the top of column 5, and the top vapor of the dephlegmator forms, as before, the high pressure refrigerant, which is cooled to the end exchanger 8 cold then, after phase separation at 10, to the cold end of exchanger 9.

Figure 3 shows an embodiment of a heat exchanger which can be used as an intermediate refrigerant 3B. This exchanger comprises a calender 50 in which a certain number of vertical tubes 51 open at their two ends extend between an upper plate 52 and a lower plate 53. Between these plates, and outside the tubes, are mounted a certain number of horizontal baffles 54. The cooling water arrives by a lower pipe 55 on the plate 52, circulates upwards in the tubes 51 and is evacuated by an upper pipe 56. The two-phase mixture conveyed by line 2B enters laterally into the grille under the plate 52 and descends along the baffles, then exits via the outlet pipe 57 of the exchanger, located a little above the plate 53. Such an arrangement makes it possible to homogenize the two-phase mixture well during its cooling, and to obtain to a high degree the advantage of acceleration of the condensation in the second stage of the compressor 1 brought by the loop comprising the pump 4. FIG. 4 represents another alternative arrangement of the column of distillation 5. In this variant, the column overhead vapor is heated by a few degrees Celsius in an auxiliary heat exchanger 58, then sent to the suction of the last compression stage 1C. The high pressure fluid, after cooling and partial condensation in 3C around + 30 to + 40 "C, is separated into two phases in a separator pot 59. The vapor from this pot constitutes the high pressure refrigerant, while the liquid phase , after sub-cooling a few degrees Celsius in the exchanger 58, is expanded in an expansion valve 49 as in FIG. 2 and then introduced under reflux at the top of the column 5. It is understood that this variant can be applied to a installation with either three or four stages of compression, and subcooling 58 is optional.

Whichever embodiment is considered, the nitrogen removal column 13 must operate at around 1.15 bar or 1.2 bar, and consequently the nitrogen-free LNG leaving the tank of this column must be expanded to atmospheric pressure at storage inlet 14, which produces flash gas. This gas, as well as the gas resulting from the heat inputs into the storage 14, must therefore be taken up and compressed by an auxiliary compressor to be distributed to the "fuel gas" network. Figure 5 shows an arrangement which makes it possible to omit this auxiliary compressor, in the case where the LNG leaving the exchanger 9 contains a few% of nitrogen. For this, the LNG leaving the exchanger 9 is sub-cooled in the coil 36 of the column 13 and again sub-cooled in an auxiliary heat exchanger 60. The liquid is then expanded to 1.2 bar in the valve. expansion 37 and the turbine 39, then divided into two streams: a stream which is vaporized in an exchanger 60 and then introduced at an intermediate level into the column 13, and a stream which is sent at reflux at the head of the latter. The bottom liquid of column 13, which is LNG without nitrogen, is then, for each storage, divided into two streams, one of which is sub-cooled in the exchanger 60 while the other passes through a bypass 61 to adjust the degree of overall sub-cooling, the circulation of the liquid being ensured by a pump 62.

In this way, it is supercooled liquid of about 2 ° C. which is sent to the storages 14, which eliminates practically any flash at the entry of these storages and any evaporation due to the heat entries during the time. As can be understood, it is the difference in the compositions of the LNG before and after denitrogenation which makes it possible to obtain such sub-cooling in the exchanger 60. Likewise, the overhead vapor of column 5 is generally sufficiently rich in methane to be recovered as "fuel gas", in the sense indicated above. It is therefore necessary to provide another auxiliary compressor for this purpose. If, in addition, the cycle 1 compressor is driven by a gas turbine, it is necessary to supply the latter with combustible gas at a pressure of the order of 20 to 25 bars, which leads to installing an auxiliary compressor of significant power. The arrangement of Figure 6 shows how the need for such an auxiliary compressor can be eliminated. In this FIG. 6, an additional column 63 for primary deaeration of natural gas under pressure is used, provided with an overhead condenser 64. The part of the natural gas coming from the apparatus 33 which is treated in the exchanger 12 n ' is cooled there only to an intermediate temperature TI, then is introduced into the tank of the column 63, via a pipe 65, while the rest of this natural gas is cooled in the exchanger 9 only to a temperature intermediate T2 lower than TI then introduced at an intermediate level of the same column, via a pipe 66.

The condenser 64 is cooled by expanding part of the column tank liquid to around 25 bars in an expansion valve 67. The gas resulting from this vaporization has the same composition as the column tank liquid, that is that is to say has a low nitrogen content, and therefore constitutes a combustible gas at 25 bars which can be directly used, via a pipe 68, in the gas turbine 69.

The remainder of the bottom liquid of the column 63 is, after sub-cooling partly in the cold part of the exchanger 9 and in the coil 36 of the column 13, and partly in the cold part of the exchanger 12, expanded at 37, respectively at 70, and introduced at an intermediate level of column 13. The overhead vapor of column 63, containing 30 to 35% of nitrogen, is cooled and condensed in the cold part of the exchanger 9 , sub-cooled in that of the exchanger 12, and, after expansion in an expansion valve 71, introduced under reflux at the top of the column 13.

The nitrogen enrichment of the column 13 washing liquid thus obtained has the consequence that the nitrogen vapor of this column is sufficient poor in methane, for example contains 10 to 15% of methane, to be put into the atmosphere via line 38 after heating in 12.

In total, two waste gases are thus obtained, one of which is rich in methane and at 25 bars, and feeds the gas turbine, and the other of which, under low pressure, is poor in methane and is not recovered. .

As shown in FIG. 6, a fraction of the natural gas to be treated conveyed by the pipe 31 can be cooled in the hot part of the exchanger 12 before being sent to the device 33.

Claims

1. A method of cooling a fluid, in particular for the liquefaction of natural gas, of the integral incorporated cascade type, in which a refrigerant mixture composed of constituents of different volatilities is compressed in at least two stages and, after at least each of the intermediate stages of compression, the mixture is partially condensed, at least some of the condensed fractions as well as the high pressure gas fraction being cooled, expanded, brought into heat exchange relation with the fluid to be cooled and then compressed again, characterized in that that the gas from the penultimate stage of compression (IB; 1D) is distilled in a distillation apparatus (5) of which the head is cooled with a liquid having a temperature markedly below ambient temperature, to form d on the one hand the condensate of this penultimate stage, and on the other hand a vapor phase which is sent to the last stage of compression (1C).

2. Method according to claim 1, characterized in that the head vapor of the distillation apparatus (5) is partially cooled and condensed by heat exchange (at 24) with at least said expanded fractions, in order to obtain a vapor phase and a liquid phase, and the head of the distillation apparatus (5) is cooled with the liquid phase thus obtained (in 6C), the vapor phase constituting said vapor phase which is sent to the last stage of compression.

3. Method according to claim 1, characterized in that it is cooled and partially condensed in the vicinity of room temperature (at 47, Figure 2: 3C, Figure 4) the gas from the last stage of compression (1C), relaxes (at 49) the liquid phase obtained, and the head of the apparatus is cooled distillation (5) by means of this expanded liquid phase.

4. Method according to claim 3, characterized in that one operates a dephlegmation of the gas from the last stage of compression (1C) during its cooling.

5. Method according to claim 3 or 4, characterized in that one carries out (in 58) an indirect heat exchange between the liquid resulting from the cooling of the gas from the last compression stage (1C) and the overhead vapor of the distillation apparatus (5) before sending this vapor to the last compression stage (1C) and expanding said liquid (at 49).

6. Method according to any one of claims 1 to 5, characterized in that at least part of the condensate of the first compression stage (1A) is pumped up to the outlet pressure of the second stage of compression (IB), and it is mixed (in 2B) with the gas resulting from this second stage of compression.

7. Method according to any one of claims 1 to 6, for the liquefaction of natural gas containing nitrogen, characterized in that sub-cools (in 60) the liquefied natural gas resulting from refrigeration (in 7 , 8) then denitrogenated (in 13), by heat exchange with expanded non-denitrogenated liquefied natural gas (in 37).

8. Method according to any one of claims 1 to 7, for the liquefaction of natural gas containing nitrogen, characterized in that one carries out (at 63) a primary denitrogenation of natural gas under its treatment pressure in a auxiliary column (63), part of the liquefied natural gas having undergone this primary desazotation is expanded to an intermediate pressure (at 67), the liquid is vaporized thus relaxed by cooling the head (64) of the auxiliary column, which produces a combustible gas under the intermediate pressure, this combustible gas is sent to a gas turbine (70) driving the compressor (1), and treating the remainder of the liquefied natural gas having undergone the primary denitrogenation as well as the head vapor of the auxiliary column (63) in a column (13) of final denitrogenation under low pressure producing in tank the denitrogenated liquefied natural gas intended to be stored (in 14).

9. Installation for cooling a fluid, in particular for liquefying natural gas, of the type comprising a refrigeration circuit with integral incorporated cascade in which a refrigerant mixture circulates and which comprises a compressor (1) with at least two stages (1A to 1C ) at least the intermediate stages (1A, IB; 1A, IB, 1D) are each provided with a coolant (3A, 3B; 3A, 3B, 3D), and a heat exchange line (7, 8), characterized in that it comprises a distillation apparatus (5) supplied by the penultimate stage (IB; 1D) of the compressor and the head of which is connected to the suction of the last stage (1C) of the compressor, and means (24, 6C; 47, 49; 58 to 60) for cooling the head of the distillation apparatus (5) by means of a liquid having a temperature significantly below ambient temperature.

10. Installation according to claim 9, characterized in that said means (24, 6C) for cooling the head of the distillation apparatus comprise cooling passages (24) of the hot part (8) of the line heat exchange (7), and a separator pot (6C), the bottom of which is connected to the top of the distillation apparatus (5) and the top, to the suction of the last compression stage (1C).

11. Installation according to claim 9, characterized in that said means (47, 49) comprise a device (3C; 47) for cooling around the ambient temperature of the gas coming from the last stage (1C) of the compressor (1), and a valve (49) for expansion of the liquid from this cooling device, the outlet of this valve being connected to the top of the distillation apparatus (5).

12. Installation according to claim 11, characterized in that the cooling device (47) is a dephlegmator.

13. Installation according to claim 11 or 12, characterized in that an auxiliary heat exchanger (58) is provided for bringing the liquid from the cooling device (47) and the overhead vapor into an indirect heat exchange relationship. of the distillation apparatus (5).

14. Installation according to any one of claims 9 to 13, characterized in that a separator pot (6B) is interposed between the refrigerant (3A) of the first stage (1A) of the compressor (1) and the second stage (IB ) of this compressor, and in that there is provided a pump (4) whose suction is connected to the bottom of this separator pot and whose discharge is connected to the discharge of the second stage of the compressor.

15. Installation according to any one of claims 9 to 14, for the liquefaction of natural gas containing nitrogen, characterized in that it comprises a column of denitrogenation (13) and a subcooling exchanger (60) suitable for sub-cooling the nitrogenous liquefied natural gas coming from the tank of this column by heat exchange with the expanded non-nitrogenous natural gas (in 37).

16. Installation according to any one of claims 9 to 15, for the liquefaction of natural gas containing nitrogen, characterized in that that it comprises a deaeration column (63) supplied with natural gas under its treatment pressure and comprising a head condenser (64) supplied with tank liquid from this expanded column (at 67) at an intermediate pressure, a gas turbine (69) supplied with the gas resulting from the vaporization of this expanded tank liquid, and a column (13) for final nitrogen removal under low pressure producing in the tank the denitrogenated liquefied natural gas intended to be stored (at 14).

17. Installation according to any one of claims 9 to 16, characterized in that the heat exchange line (7) consists of two plate exchangers (8, 9) in series, in particular of the same length, connected to the one to the other by end domes (42, 43) and possibly welded together end to end.