DE1267236B - Process for liquefying natural gas - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

Number:
File number:
Registration date:
Display day:

F25j

German KL: 17 2 -1

1,267,236
P 12 67 236.5-13
April 4th 1963
May 2, 1968

The invention relates to a method for liquefying natural gas, the larger or smaller Contains quantities of volatile neutral gases. Under the term volatile neutral gases are in the To understand gases contained in natural gas, such as nitrogen, helium, argon, etc. Carbon dioxide gas does not fall under this term, as it is less volatile than the natural gas obtained at the end of the process.

When liquefying such natural gases, several successive cooling circuits are used, the liquefied natural gas in a flash chamber or in a denitration column relaxed and the natural gas, which remained liquid, through heat exchange with that of the Relaxation evaporated, liquefied again and further relaxed gas partial flow, subcooled and in a storage tank is passed.

In such a known method, the invention is characterized in that the reliquefaction of the vaporized partial gas flow takes place through heat exchange with the last cooling circuit and the natural gas that has remained liquid after exiting the flash evaporation chamber or of the denitration column before its last subcooling also by heat exchange with the last cooling circuit is cooled further.

The inventive management of the process between the flash evaporation chamber and the subcooler achieves a significant improvement in terms of liquefaction, purification and storage of natural gases with a considerable reduction in losses during storage. In the process according to the invention it is important that the last subcooling of the natural gas takes place to a temperature which is about 5 ^° C. lower than the boiling point of the gas at atmospheric pressure.

Another feature is that of the denitration column or the flash chamber incoming gas substream partially condensed and in a fractionation column in a most of the gas portion containing more volatile neutral gases and a liquid portion are separated is, which after subcooling partly returned to the top of the fractionation column and is partly used as a cooling liquid for subcooling the natural gas, and that the nitrogen and the Helium are deposited, which are contained in the gas part from the fractionation column. Here it is essential that the nitrogen and helium are separated off using a molecular sieve.

The invention is further characterized by a method for liquefying natural gas

Applicant:

Societe d'fitude du Transport et de la Valorisation des Gas Naturalis du Sahara S. E. G. A. N. S.,

Paris

Representative:

Dipl.-Ing. H. Seiler and Dipl.-Ing. J. Pfenning,

Patent Attorneys, 1000 Berlin 19, Oldenburgallee 10

Named as inventor:

Yves Roche, Ville d'Avray, Seine-et-Oise;

Jean Perret, Paris (France)

Claimed priority:

France of April 5, 1962 (893 475)

that the liquid natural gas expanded in a flash evaporation chamber to precipitate CO ₂ and the CO _{2 is} separated by filtration in the flash evaporation chamber. It is important here that the gas stream from the flash evaporation chamber is cooled and that the condensed vapors of the stream are injected again into the liquid natural gas to be expanded at the moment of its expansion.

Another characteristic of the invention is that the supercooled liquid portion from the fractionation column is expanded in a flash evaporation chamber and part of the liquid from the flash evaporation chamber is introduced into the supercooled liquefied natural gas.
Another feature of the invention is that a layer of more volatile neutral gases is maintained in the storage container above the liquid level. It is important here that the vapors escaping from the liquid natural gas during storage are reliquefied in direct heat exchange with the supercooled liquefied natural gas introduced into the container.

The drawings show some embodiments of the invention and it means

F i g. 1 a schematic representation of a plant for carrying out the method according to the invention, provided that the exit in the liquefied

809 574/120

3 4

Natural gas remaining amount of neutral gas unbe- output of the exchanger E ₃ , the natural gas has a

is limited, pressure of the same order of magnitude as

F i g. 2 and 3 are schematic representations of the pressure prevailing in the chamber B ₁ , as well as one

which can be used in the case, however, since the amount of the liquefied natural gas remaining at the temperature of the boiling point of methane is close to _{new-5 at the pressure of the exchanger E 3}

central gas is limited to a certain value above her. The natural gas then flows through you

is, exchanger E ₄ in such a way that it is

F i g. 4 shows a schematic representation of a with the course of this exchanger at 5 in a printing system according to FIG. 3 matching system, which lasts, but which provides for the separation of the neutral gas close to that prevailing in chamber B ₁ , is io pressure of 15 to 20 kg / cm ² , and at a temperature

F i g. 5 and 6 are schematic representations of temperature which is equal to or lower than the boiling temperature

are for carrying out the valve according to the invention of the natural gas at atmospheric pressure.

driving, in which a stage of the separation of the gas coming from chamber B _{1 at 6}

CO ₂ during the liquefaction process, the previous part of the rapid evaporation, which is later seen as 15 combustible gas, ie as propellant or heating gas.

Fig. 7 is a schematic representation of a system turns, flows through exchangers JS ₅ and E _e , to carry out the method according to the invention ie through the medium pressure stage or the low pressure stage, in which a stage for controlling the in the pressure stage of the selected cooling circuit, and Liquefied amount of nitrogen containing natural gas is partially or fully condensed at 7. That is provided at 7, and 20 obtained in greater or lesser proportions

8, 9, 10 and 11 are schematic representations of the gas-liquid mixture flowing through a system for storing or storing the exchanger E ₇ , at the outlet of which at 8 the gas in the case of liquefied natural gas. approximately the pressure in the flash evaporation

In the various figures of the drawing , the pressure corresponding to chamber B _{1 has been} completely concomitant with the corresponding parts being condensed and supercooled in the same way. The resulting liquid draft, whereby for the sake of clarity only the speed is then shown in the exchangers E ₄ and E ₇ to essential parts of each system, since at atmospheric or a slightly higher pressure this type of system is sufficiently relaxed for the person skilled in the art, which of the in is unfamiliar to the combustible gas. held quantity of neutral gas as well as of the for

The pressurized natural gas flow is before the liquid natural gas emerging from the system at 5 cooled, whereupon it depends on the temperature conditions to be achieved at a temperature gas -100 ° C with the help of the usual cascade gig.

or other process condenses, with the fact that the combustible gas which is liquid at 8 is essentially

Pressure of the natural gas flow is determined so that one borrowed from a mixture of neutral gas (Stickdas Gas near this temperature in liquid 35 substance) and methane, the boiling point changes.

Condition. At 1, the natural gas flow is corresponding to the temperature of this relaxed liquid.

Attachments according to Fig. 1 to 4 introduced. accordingly those achieved in exchangers E ₁ and E ₇

Depending on the amount of the liquid-vapor equilibrium state in the natural gas flow,

hold more volatile neutral gas and amount, consequently these exchangers are countercurrent cutoffs one in the 40 exchanger with multiple recirculation of the vapors obtained by the process. she

wants to retain liquefied natural gas, used are divided into successive departments,

one or the other of the in F i g. 1, 2 and 3, the number of departments corresponding to the

provided systems. between the liquid natural gas flow and the itself

It is clear that the method according to the invention is to maintain the liquid in the boiling state.

is only applicable to a natural gas flow which is chosen a 45 tendency minimal approximation. At the

non-zero amount more volatile neutral output of each compartment will be the fumes of the

Gas (nitrogen, helium, etc.). mixed with leaking liquid from the department,

pii -χ so that the gas-liquid mixture feeding the next department is in liquid-vapor equilibrium-

The amount contained in the liquid natural gas is _{50 by} weight.

neutral gas is unlimited (Fig. 1) At the exit of exchangers E ₇ and E ₄ ,

The combustible one arriving at 1 in a liquid state and not in a gaseous state

The pressurized natural gas stream is used as a propellant in a gas via a line 9

Exchanger E ₁ , ie supplied to the high pressure stage for example or heating medium,

way of the methane cycle of the cascade cycle conventionally discussed 55 Case 2 above, cooled and flowing

over 2 before it came in a flash evaporation- The amount contained in the liquefied natural gas

mer B _{1 is} relaxed. Relaxation takes place when the gas is neutral to a certain value

a pressure that is limited between that prevailing at 2

Pressure and a between 15 and 20 kg / cm ² lie- 60 The systems according to F i g. 2 and 3 are practically correct

the pressure lies. This pressure is caused by the high pressure stage of the cooling circuit, except for some

Amount of neutral gas contained in natural gas flow, for example of the methane cycle, the usual one

as well as through the details pertaining to the liquid natural gas retaining cascade,

The amount of neutral gas is determined. According to FIG. 2 becomes the pressurized, in

The liquid part of the content of the 65 liquid state exiting at 3, natural gas flow arriving at 1

Chamber B ₁ is in exchangers E ₂ and E ₃ , ie in exchanger E ₁ , ά. H. the high pressure stage

the medium pressure stage or the low pressure stage of the methane cycle, for example, and cooled

same cooling circuit as before, cooled. At 4 am then relaxed in a denitration column V _1.

5 6

The pressure is released at a pressure that has the necessary conditions to maintain the pressure exiting at 5 see the pressure prevailing at 2 and a liquid natural gas stream not only under one pressure

is between 15 and 20 kg / cm ² pressure. close to 10 to 20 kg / cm ² , but also a This pressure is determined by the in the liquid natural temperature, which clearly, namely around

gas contained amount of neutral gas determined. 5 5 ° C lower than the boiling point of the gas at

The liquid part at the bottom that emerges at 3 is atmospheric pressure.

Column V ₁ is in exchangers E ₂ and E _s , the pressure of the liquefied natural gas, i.e. the medium-pressure stage or the low-pressure stage, allows the liquid stream to be transferred from the same cooling circuit, for example the liquid plant to the storage or storage or storage tank circuit, chilled. At 4 the natural gas has an ioing system. The subcooling of this liquid is of the same order of magnitude as possible due to the change in its heat that prevails in the denitration column F ₁ , equal to the external heat input, on the one hand in pressure and one of the boiling temperature of methane in the transport line and, on the other hand, in the position of the pressure of the Exchanger adjacent, but storage or storage systems, as below, temperature lying above her. 15 explained in more detail.

The natural gas then flows through the exchanger F i g. 4 shows a system for carrying out the

E ₄ in such a way that it is at the output of this process according to the invention that the deposition of the

exchanger at 5 at a pressure close to that of the neutral gases contained in the natural gas stream

in the denitration column V ₁ prevailing pressure and optionally the increase in the content of in

and at a temperature which is equal to or lower than the helium contained in the neutral gas for the purpose of

riger than the boiling temperature of the natural gas with ease of its later separation allows,

atmospheric pressure is. In this system, the in

The return F i g leaving the column V _{1 at 6.} 3 illustrated system, but the new flow, which is later used as a combustible gas, ie as a propellant element also all with the systems according to FIG. 1 or heating gas, flows through the outlet 25 or 2 could be combined,
exchangers E ₅ and E ₆ , ie through the medium pressure stage The combustible gas taken from the denitration column at 6 or the low pressure stage of the selected cooling circuit becomes in the over the low pressure run. The combustible gas is partially or fully condensed at 7 stage of the cooling circuit, for example the methane, so that it is partially liquid and subcooled in the outlet circuit, fed exchanger E _5, partially re- _{exchanger £ 7.} Condenses at output 30. The liquid-vapor stream is treated in the exchanger E ₇ at 8, the completely flüs- the column V ₂ , in which the combustible gas sige combustible gas in the exchangers E ₁ and E ₇ from most of the contained in it again expanded which are of the same design as the upstream gases are separated.

Exchangers E _i and E ₇ described above . The at 10 from the bottom of the column F _{2 from the} plant according to FIG. 3 differs from the condensation products occurring after 35 flow through the FIG. 2 in the mode of operation of the denitration column exchanger E ₆ , ie the low pressure stage of the MeV ₁ , which is calculated so that the amount of new exchanger E ₇ remaining at the bottom of the condensate cooling circuit and via 7 through the outlet and exiting at 3 according to FIG. 1 to 3, the neutral gas in this case is essentially zero, which consists of two elements E _7a and E _7b . The liquid phase obtained at 8 is contained in all of the neutral gas in the gas exiting at 6 exit of element E _7b. In this arrangement, the is divided into two streams, one of which is the high pressure stage of the methane cycle for the natural gas via the top condenser of the column V ₂ and gas through the evaporator of the denitration column, the other the exchangers E _7a and E _7b and JE ₄ un - V _{1 is} replaced, with the meter being fed indirectly in order to achieve cooling. The return from the condenser than the cooling circuit for the purpose of its use 45 of the column F ₂ via 12 is used with the liquid-vapor chamber B ₂ emerging from the medium-pressure stage from a high-speed evaporation exchanger E _7b . mixed back into exchanger E ₇₀ . At the

With the aid of the three inputs of the exchanger E _7a described above, the combustible gas is available

layers (Fig. 1, 2 and 3) can be completely present in vapor form according to the invention and is available via the

the process according to the invention, the 50 line 9 liquefied under pressure is supplied to its use. At the

Get saturated natural gas at a temperature that consists of the _{top of the column F 2 at 13}

equal to or lower than the boiling point of Na- vapors from a mixture of nitrogen and volatile

turgases is at atmospheric pressure. The advantage of higher components (helium), their concentration

the process of the invention is that of the mole percent of these ingredients in the

the working pressures of the low-pressure and medium-pressure 55 natural gas flow is dependent. The temperature of this

pressure stage of the process cycle. Vapors and their concentrations are sufficient for

going cooling circuit can be increased. Achieving the separation of the constituents, for example

In the procedure described above, using a molecular sieve 14, which is thus the fracture form, a liquid natural gas can be produced when the nitrogen and the more volatile loading pressure are close to 10 to 20 kg / cm ² and at the boiling point 60 constituents. The vapors of the neutral gases available at the outlet of the plant temperature of this natural gas at atmospheric pressure have received the corresponding temperature conditions. For the fractionation of the combustible gas from the natural gas stream, the process according to the invention also enables the selected pressure (pressure of the cores corresponding to the lonne F _{1 contained} in the natural gas stream).

tenen amount of neutral gas as well as that at 6 am 65 The process according to the invention enables, in addition to the exit of the flash evaporation chamber or the amount of neutral gas contained by filtering in the course of the liquefaction pre-denitration column, the elimination of the gas in the system in the exchangers JS ₄ , E ₁ such boiling natural gas present CO ₂ .

7 8

The pressurized natural gas flow contains a flow to the second expansion stage or in the direction of

in addition to the more volatile neutral gases, more or more of the storage systems via an open valve

less significant amounts of CO ₂ . Generally collected in a collecting container 19,

the CO ₂ is prior to liquefaction by switching off the wash at 21 from the flash vaporization chamber with soda or ethanolamine from the ground 5 B _3a exiting gas phase is an open

gas stream removed. The use of such a process valve 22 is fed to a condenser 23,

Ren increased to eliminate the CO ₂ , regardless of the emerging at 24 from the condenser 23

Their investment costs, the operating costs of the re-condensed vapors are again included in the

liquid of natural gas. On the other hand, the natural rapid evaporation chamber B _{30 is} injected through the valve 17 of the gas flow at the outlet of these systems at an expansion element 16 that is fed via io,

In this mode of operation, the valves 17, 20 and 22 are

Saturated water, so that for this reason the De- opened while the valves 25, 26, 27 and 28 closed

hydration systems for the gas are considerably closed.

be increased. Since the chamber B _3b was assumed to be in the process of being refreshed. The equilibrium state curves of CO ₂ in FIG. 15, this refreshing takes place

Hydrocarbons are perfectly known and an influx of about - 50 ⁰ C heated

enable the precipitation point of the combustible gas that arrives via line 8 to be determined

CO ₂ in the solid state for the intended concentration and the filter via an open valve 29

tration as well as temperature. In this way, one can determine on the basis of these curves that the rapid evaporation chamber B _{$ b countercurrently shows that the CO 2 is} flowing through in zo.

solid form at temperatures close to −140 ° C at The warm gas sublimes that at the filter

Concentrations on the order of 0.20 mol- stored CO ₂ , the mixture being combustible gas

percent is failed. and CO ₂ via line 31 for its use.

The method according to the invention does not relate to that which is carried out by opening a valve 35 while forming this precipitate, but rather its elimination, the end of the refreshment of the filter has been reached

generation from the liquid natural gas flow. It consists in is when the temperature is at a value of 34

the elimination of the by relaxation, preferably approaches -50 ° C.

of the liquid in a flash evaporation chamber, FIG. 6 shows a complete plant in which the precipitates obtained are filtered. The formation of the filtering out of carbon dioxide in two successive precipitates actually took place from the point of the 30 following relaxation processes. Each expansion chamber B ₃ and B ₄ provided with the formation of the first crystal continuously, if filters, the temperature of the liquid continues to drop. Since the precipitate shown schematically only by one chamber, however, tends to clump together these two solid masses to a rend the device according to FIG. the presence of heavier hydrocarbons 35 The exiting at 3 from the column V ₁ Current material in the precipitate brittle is, comes dieKüh- liquid natural gas is cooled in exchanger E _2, the liquid development in a normal exchanger at the output of the exchanger E ₂ is the liquid is not cooled due to the clogging and sticking in the bottom of the column F ₂ and is over in question. the expansion nozzle 16 is fed to the chamber B _3. The 40 The filtered liquid part obtained during the expansion in the rapid evaporation chamber is emptied via the line 36 due to the cooling into the rapid evaporation chamber B ₄ , the liquid contains a large amount of CO ₂ in solid is released via the expansion opening. The state. Since this precipitate only occurs when the filtered liquid exits at about 38, the flow velocities, which are zero, are drained natural gas.

it is filtered out by flowing it through a gaseous part exiting at 39, a plate of sintered metal fungskammer B ₃ or any other filtering material through the exchanger E ₃ , then through the This carries out the intended temperature exchanger E _ia , emerges at 40, with a chamber B _{withstanding 5} and stress conditions. optionally the deposition of non-condensate from Fig. 5 of the drawing, which allows a detail of the 50 ized gas, while the liquid from the system illustrates, it can be seen that two of the chamber B ₅ via a line 41 directly in flash evaporation chambers B _3a and B _3b via which the expansion element 16 is fed to the flash evaporation line 15 in parallel with the pressurized chamber B _3. The steam, which is liquid in the chamber B ₁ and which is released in a nozzle 16 and which is suddenly created by nature, will be charged with gas via a line. The actuation of the valves 17, 55 42 in the exchanger E _ib recondenses and via ΪΊα, 20, 28, 22, 25, 27, 29, 26, 35 enables one line 43 to be passed directly into the alternating expansion operation of the rapid evaporation element 37. The entire scheme of the system chambers B ₃₀ or B _3b . except for this additional element, agrees with the

It is assumed that the chamber S _{30 is located} in the system according to FIG. 4 match. Is in operation while chamber B _{3b is being} refilled with the system discussed above. The valve 17 is open while the,. ,, ,,. Valve 17a is closed, ^the cooling in B _{3a being} cooled to remove the CO ₂ in tensioned liquid and splitting into a solid state by the expansion of this liquid part and a gaseous part. In the ^{liquid natural gas containing CO} 2, the liquid part _{precipitates all of the CO 2} , 65 2. the recondensation of the vapors created from the despandas in a filter 18 arranged at the lower section of the rapid venting via the cooling circuit vaporization chamber or the additional cooling circuit is believed to be. The liquid natural gas is converted into highly flammable gas,

3. the re-injection of the condensed vapors into the liquid to be relaxed at the moment of its relaxation.

The expansion can take place in one or more stages according to the amount of CO ₂ contained in the gas introduced, whereby the heat available in the cooling circuits and the temperatures at which these heat calories are available, thus the steam condensation pressures and the value of the expansion in the flash evaporation chamber determine.

The condensation of the vapors that suddenly arise as a result of the relaxation is in turn immediate Function of the nitrogen percentage contained in the natural gas stream introduced. The more These vapors are enriched with neutral gases, the more the recondensation requires Vape a coolant with a low boiling point. Now the low boiling points are an immediate one Function of the amount of neutral gas remaining in the combustible gas. So it exists as a function of the amount of neutral gas in the natural gas introduced between that in the combustible Gas contained amount of neutral gas and the amount remaining in the liquid natural gas neutral gas an optimal equilibrium point. At percentages of neutral gas in the introduced Natural gas on the order of 7 mole percent can reduce the amount of neutral gas in the liquid Gas do not exceed 0.5%. However, according to the method according to the invention, it is possible to increase this remaining amount of neutral gas.

The re-injection of the condensed vapors via one at the expansion point of the one to be cooled and the liquid to be filtered has been installed to prevent the formation of Solid deposits at the relief opening and consequently from blockages and immediate Restoration of the liquid-vapor equilibrium in the flash chamber and thus the maximum reduction in the neutral gas content of the steam formed for the purpose by so its Recondensation is facilitated.

In the above-described method according to the invention it has been found that with normal Neutral gas concentrations in the natural gas flow are those left in the liquefied natural gas The amount of neutral gas could not exceed 0.5%. Now it is time to limit the evaporation the storage and during the transport of interest to increase this amount of neutral gas, since they constitutes the most essential element of this evaporation. Namely, if the content of neutral gas (for example nitrogen) is higher, this forms a significant part of the evaporation, which the losses of liquefied natural gas during storage are significantly reduced. To increase the amount of nitrogen contained in the liquefied natural gas, proceed as follows (Fig. 7):

The flammable gas, which is completely liquid at 8, is no longer expanded in exchanger U ₇₀ , but in a rapid evaporation chamber _{B 6} , the vaporous part of the sudden evaporation being conducted via a line 44 into exchanger E _7b. The liquid part is divided into three streams, one of which feeds the top condenser of column V ₂ via line 11, the second feeds exchanger E _7b , while the third feeds the liquid coming from flash evaporation chamber B ₁ via a valve 46 in a line 45 is added and the current is emptied from the system via line 38.

In this way some of the liquid from the flash chamber becomes that from the last one The liquid is mixed into the filter stage, so that by mixing these two liquids the Content of neutral gas contained in the liquefied natural gas is increased. Thus, an accurate Control of the amount of neutral gas added to the liquefied natural gas can be achieved, whereby the loss of natural gas during storage can be reduced by more than 50%.

In Figs. 8-11 are storage or storage facilities for the liquefied natural gas shown schematically.

A. Storage without re-liquefaction of the

Evaporation (Fig. 8)

The liquefied natural gas is fed at 50 to a storage container 51 as a source. Above the liquid level, a cushion of neutral gas is formed, as is produced by the fractionation column V _{2 of} the systems described above. This neutral gas arriving at 52 is stored in a buffer container, which consists of a normal gas container 53, which is connected to the liquid container 51 at its top, where a pressure relief valve 54 also allows the combustion point to be over 55 the excess pressure of neutral gas due to the Filling or supplying the overpressure of natural gas when the storage tank 51 is full. Then the heat losses are no longer compensated for by the considerable heat supplied by the supercooled liquefied natural gas.

B. Storage with re-liquefaction of the

Evaporation (Fig. 9 to 11)

At the top of each container 51 there is attached a column 56 which is provided with baffle plates that allow intensive mixing of the vapors from the container with the liquid introduced. The supercooled liquefied natural gas is introduced at the top of the column at 57. It flows from plate to plate and is collected in a funnel 58 located at the bottom of the column is attached and makes it possible to feed the container 51 via a dip tube 59 as a source. the Vapors generated by the boiling of the stored liquid escape after passing through the Mass of the incoming liquid has flown through 57, between the baffles of the Column 56 through via line 60. As it travels between these plates, it becomes the largest Part of these vapors are recondensed by the supercooled liquid.

The manifold 60 is connected to a gas container 61 (Fig. 10) which receives neutral gases, as they are preferably achieved with the aid of the method according to the invention, the excess pressure is flared in this gas container 61 via a line 62. Such a device is on Systems can be used with a single storage tank or with several storage tanks fed in parallel are provided. When the plant

809 574/120

includes multiple separately-powered storage containers, there is preferably used an apparatus, as shown in Fig. 11, which includes a common manifold 63, the pipe stub 60 of the container 51 a, 51b connects with the gas container 61. When the first container is completely full, the evaporation due to the considerable heat input of the supercooled liquid natural gas can no longer be compensated for. In the course of the reheating of the storage material vapors generated are supplied so the common manifold 63, from where they b for the most part by the container 51 sucked during its filling and reliquefied by the subcooled liquid natural gas in this container.

This method of liquefying the vapors generated can be used while a methane cargo ship is being loaded use. In this case, the evaporations are fed to the common collecting line of the storage container, in which case the liquefied natural gas produced by the factory is cooled to such a temperature that this Much of the vapors re-liquefied in the storage tanks during the loading of the ship will. In addition to the gas surge at the beginning of loading or storage, almost all of the vapors are liquefied again.

Due to the use of the supercooled obtained with the aid of the method according to the invention liquefied natural gas and a perfectly dehydrated neutral gas, as can be done with this process the losses during storage are significantly reduced.

The liquefaction process according to the invention thus enables a significant improvement in liquefaction, Purification and storage of natural gases.

Claims (9)

Patent claims: 1. Process for the liquefaction of natural gas containing larger or smaller quantities of volatile neutral gases by means of several successive cooling circuits, in which the liquefied natural gas is expanded in a flash evaporation chamber or in a denitration column and the natural gas that has remained liquid is liquefied again through heat exchange with the one that evaporated during the expansion and further expanded gas partial flow is subcooled and passed into a storage container, characterized in that the reliquefaction of the vaporized gas partial flow takes place by heat exchange with the last cooling circuit and the natural gas remaining liquid after exiting the flash evaporation chamber (B ₁ ) or the denitration column (F ₁ ) before its last subcooling is also further cooled by heat exchange with the last cooling circuit. 2. The method according to claim 1, characterized in that the last subcooling of the Natural gas takes place at a temperature that is around 5 ° C lower than the boiling temperature of the gas at atmospheric pressure. 3. The method according to claim 1 or 2, characterized in that the gas substream coming from the denitration column (F ₁ ) or the flash evaporation chamber (B ₁ ) partially condenses and in a fractionation column (F ₂ ) into a most of the more volatile neutral gases containing Gas part and a liquid part is separated, which after subcooling is partly returned to the top of the fractionation column (F ₂ ) and partly used as a cooling liquid for subcooling the natural gas, and that the nitrogen and helium are separated out in the gas part from the fractionation column (F ₂ ) are included. 4. The method according to claim 3, characterized in that the deposition of nitrogen and helium is carried out through a molecular sieve. 5. The method according to any one of claims 1 to 4, characterized in that the liquid natural gas is expanded in a _{flash evaporation chamber (B 30} , B sb) to precipitate CO ₂ and the CO ₂ is deposited by filtration in the flash evaporation chamber. 6. The method according to claim 5, characterized in that the gas stream from the rapid evaporation chamber (B ₃ , B ₄ ) is cooled and that the condensed vapors of the stream are injected again into the liquid natural gas to be expanded at the moment of its expansion. 7. The method according to any one of claims 1 to 6, characterized in that the supercooled liquid part from the fractionation column (F ₂ ) is expanded in a flash evaporation chamber (B ₆ ) and part of the liquid from the flash evaporation chamber (B ₆ ) into the supercooled liquefied natural gas is introduced. 8. The method according to any one of claims 1 to 7, characterized in that in the storage container (51) a layer of more volatile neutral gases is maintained above the liquid level. 9. The method according to claim 8, characterized in that the out during storage The vapors escaping from the liquid natural gas in direct heat exchange with that in the container (51) imported supercooled liquefied natural gas can be reliquefied. Documents considered: French Patent No. 1227903; U.S. Patent No. 2,960,837. For this purpose 2 sheets of drawings 809 574/120 4.68 © Bundesdruckerei Berlin