WO2017077202A1

WO2017077202A1 - Process for separating components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms

Info

Publication number: WO2017077202A1
Application number: PCT/FR2016/052486
Authority: WO
Inventors: Paul Terrien; Marie-Pascal Victor; Ludovic Granados; Michele MURINO
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2015-11-03
Filing date: 2016-09-29
Publication date: 2017-05-11
Also published as: SA518391515B1; FR3042982B1; FR3042982A1

Abstract

Process for separating components of a gas mixture to be treated, comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following steps: a) introducing the gas mixture to be treated into a first distillation column in order to create, at the bottom of the column, a first liquid stream enriched in hydrocarbon having at least two carbon atoms and, at the top of the column, a first methane-enriched gas stream; b) introducing said first liquid stream enriched in hydrocarbon having at least two carbon atoms derived from step a) into a second distillation column in order to create, at the top of this column, a second methane-enriched gas stream and, at the bottom of this column, a second liquid stream comprising at least 85 mol% of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated; characterized in that the nitrogen content of said second gas stream is at least 1.5 times lower than the nitrogen content of the first gas stream and in that from 5 mol% to 30 mol% of the methane initially present in the gas mixture to be treated is within the second gas stream.

Description

Process for separating the components of a gaseous mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms The present invention relates to a process for separating the components of a gaseous mixture containing methane , nitrogen and hydrocarbons heavier than methane.

The present invention therefore applies to denitrogenation processes of natural gas with or without helium recovery.

Natural gas is desirable for use as a fuel for use in heating buildings, to provide heat for industrial processes for the generation of electricity, for use as a feedstock for various synthesis processes for produce olefins, polymers and the like.

Natural gas is found in many areas that are remote from natural gas users. Natural gas typically consists of methane (C1), ethane (C2) and heavier compounds such as hydrocarbons having at least three carbon atoms, such as propane, butane ... (C3 +). Often it may be advantageous to separate C2 and C3 + from natural gas for commercialization as separate co-products. Indeed, their recovery is generally greater than the natural gas itself because they can be used directly for chemical processes (manufacture of ethylene from ethane for example), as fuels (C3 / C4 is a fuel classic called GPL) or for many other applications.

Another component often found in natural gas is nitrogen. The presence of nitrogen in natural gas can lead to difficulties in meeting the specifications for natural gas (typically the minimum heating value to be met). This is all the more true when hydrocarbons heavier than methane (C2 and C3 +) are removed because they have a lower heating value than methane, so removing them reduces the lower calorific value than the methane. it may then be necessary to increase by means of nitrogen separation. As a result, considerable effort has been devoted to developing ways to remove nitrogen from natural gas.

The exploited natural gas fields contain more and more nitrogen. This is particularly due to the depletion and scarcity of fields rich enough that no enrichment treatment is necessary before the marketing of gas.

It is common for these natural gas sources to also contain helium. This can be valorized by performing a pre-concentration, before final treatment and liquefaction.

Unconventional resources such as shale gas also have the same problem: to make them marketable, it may be necessary to increase their calorific value by means of a treatment that consists in de-gasing the gas.

The most widely used method for separating nitrogen and hydrocarbons heavier than methane is "cryogenic separation". A cryogenic nitrogen separation process, more specifically a method employing a double column, is described in patent application US-A-4778498. Natural gas denitrogen units generally treat gases that come directly from wells at high pressure. After denaturing, the treated gas must be returned to the network, often at a pressure close to its inlet pressure.

When operating natural gas deposits, many steps can be planned. A relatively conventional step after drying and removal of impurities is the separation of natural gas-associated liquids (NGL). The interest of this stage can be multiple but often it is a question of valorizing various so-called heavy hydrocarbon products containing at least two carbon atoms (C2, C3 ...) which are generally sold much more expensive than the product natural gas. If the natural gas contains nitrogen, it is possible to find a natural gas with a calorific value too low because of the low content of C2, C3 ... resultant. It is therefore typical to have to separate the nitrogen from this gas to make it marketable.

A classic solution is to treat both problems independently. A first unit operates the separation of NGL (later called NGL unit) while a second unit separates nitrogen from natural gas (later called NRU unit).

This solution has the advantage of flexibility in operation. For example, if the NRU has a refrigeration cycle, the associated machines have limited reliability, and failure of a cycle compressor will cause the NRU to shut down, but without stopping the NGL.

Unfortunately, this stop can not be long since it will then send to the flare production (because of its calorific value too low). In addition, this scheme is limited in efficiency because all the gas is cooled and then warmed in the NGL unit and then cooled and heated in the NRU.

During a treatment in an NGL separation unit, a large fraction (typically more than 10%) of the feed gas is condensed. During this condensation, methane is condensed with heavier hydrocarbons (C2 + and / or C3 +).

It is then typically necessary to use a column called demethanizer to reboil methane and not lose methane in C2 + and / or C3 + products. If nitrogen is present, it will however very little condensed and will be found mainly in the gas phase introduced into the demethanizer column.

The inventors of the present invention have then bridged a solution to solve the problem raised above while optimizing energy costs such as those related to the power consumption during the implementation of such methods.

The present invention relates to a process for separating the components of a gaseous mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the steps following:

a) Introducing the gaseous mixture to be treated in a first distillation column to create, in a column vessel, a first hydrocarbon-enriched liquid stream having at least two carbon atoms and, at the top of the column, a first gaseous stream enriched in methane ; b) introducing said first hydrocarbon-enriched liquid stream having at least two carbon atoms from step a) into a second distillation column to create, at the head of this column, a second gaseous stream rich in methane and in the tank; of this column, a second liquid stream comprising at least 85 mol% of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated; characterized in that the nitrogen content of said second gas stream is at least 1.5 times lower than the nitrogen content of the first gas stream and in that from 5 mol% to 30 mol% of the methane initially present in the gas mixture to be treated is included in the second gas stream.

More particularly, an object of the present invention relates to:

A method as defined above characterized in that it comprises the additional step:

c) introducing said first gas stream enriched in methane from step a) into a denitrogenation unit to separate the nitrogen from the other components of this gas stream.

A process as defined above characterized in that the second gaseous stream from step b) is not treated by the denitrogenation unit.

- A method as defined above characterized in that

10 mol% to 20 mol% of the methane initially present in the gas mixture to be treated is included in the first hydrocarbon-enriched liquid stream having at least two carbon atoms from step a).

A method as defined above characterized in that prior to step a) it comprises the following steps:

At least partial condensation of said gaseous mixture to be treated in order to obtain a diphasic mixture;

Injecting the vapor phase of said diphasic mixture into said first distillation column;

- Injecting the liquid phase of said two-phase mixture into said second distillation column.

A process as defined above characterized in that the gaseous stream, extracted from the first distillation column in step a), comprises at most half the amount of hydrocarbons having more than two carbon atoms present in the feed gas.

A process as defined above characterized in that the liquid extracted from said first distillation column in step a) comprises at least 90 mol% of hydrocarbons having at least two carbon atoms and preferably at least 95%.

A process as defined above characterized in that said gaseous mixture to be treated comprises at least 70 mol% of methane, at least 4 mol% of nitrogen and 2 mol% of hydrocarbons having at least two carbon atoms.

A process as defined above characterized in that, during step b), the hydrocarbon-enriched liquid stream having at least two carbon atoms derived from step a) is introduced into said second distillation column at a temperature of theoretical floor below the head of this said second column.

A process as defined above characterized in that said gaseous stream from step b) is extracted directly from said second distillation column at a pressure greater than 20 bara and comprises 95 mol% of methane.

Thus, the method which is the subject of the present invention makes it possible to take advantage of the fact that only the gaseous feed of the methanation column contains nitrogen substantially.

Indeed, the solution proposed by comparison with the known processes of the state of the art is to split the demethanization column conventionally used, one (that is to say the first distillation column) producing a natural gas product C2 + poor and rich in nitrogen, the other (i.e. the second distillation column) containing a natural gas product poor in C2 + and denitrogenated.

The process of the invention makes it possible to separate a crude gas rich in C2 + and in nitrogen (typically at least 1% C2 + and at least 2% nitrogen). In a particular embodiment, the method according to the invention typically comprises the following steps:

• Pre-treatment of the raw gas to be treated (separation of water, CO2, methanol, heavy hydrocarbons, for example). • Cooling of the raw gas at a first subambient temperature (typically between -30 ° C and -70 ° C) to obtain a cooled two-phase current.

• Separation of the cooled two-phase current into a first gas and a first liquid.

• Relaxing at least a portion of the first liquid for injection into a lower demethanizer column, referred to above as the second distillation column.

• Relaxation of at least a portion of the gas in a turbine and introduce it after expansion down an upper demethanizer column, above called the first distillation column.

Condensation of at least a portion of the gas for use as reflux in the upper demethanizer column.

• Introduction of the liquid of the upper demethanization column in the demethanization column less than at least one theoretical stage below the column head.

• At the top of the upper demethanizer column, obtain a gas that is low in C2 + and rich in nitrogen (richer than the raw gas).

• Obtain at the top of the lower demethanizer column a low C2 + gas and low in nitrogen (typically containing at least two times less nitrogen than the head of the upper column and preferably containing less than 5% nitrogen).

This also makes it possible to considerably simplify the boilermaking of the demethanization columns conventionally used in the methods known from the state of the art.

Indeed, typically the column head is much wider than the column vessel which poses mechanical constraints and therefore additional costs. The separation of the columns makes it possible to be free of this constraint.

In order to deplete the head of the lower demethanizer column additional reflux is provided.

This reflux should if possible be very low in nitrogen. Several ways are possible to provide this reflux: • Use of a dedicated condenser, for example with liquid methane at a lower pressure than the column head. This liquid methane can be produced by the downstream part of the process (NRU).

• Use of the overhead gas from the lower demethanization column recompressed and recondensed.

The invention will be described in more detail with reference to the figure which illustrates a particular example of an implementation of a method according to the invention.

In the figure, a stream 1 of pretreated natural gas (having typically undergone a separation of a part of at least one of the following constituents: water, CO ₂ , methanol, sulfur compounds, very hydrocarbons heavy, that is to say having more than six or seven carbon atoms (such as C8 + for example)) comprising at least 30 mol% of methane, at least 0.1 mol% of hydrocarbons heavier than methane (that is to say comprising at least two carbon atoms) and between at least 4 mol% and 50 mol%, or even 80 mol% of nitrogen is introduced into a system 2 allowing at least partial condensation of said stream 1 .

The pressure of this stream 1 is between 20 bara (absolute bar) and 100 bara (typically between 30 and 70 bara) and the temperature is close to room temperature, for example between 0 ° C and 60 ° C.

The system 2 is for example a heat exchanger. The mixture 3 leaving this system 2 is in a two-phase state (gas and liquid). This mixture 3 is introduced into a phase separator pot 4.

The operating pressure is between 20 and 100 bara, typically between 30 and 70 bara. The temperature of this pot is between -100 ° C and 0 ° C, typically between -80 ° C and -20 ° C.

At least a portion 8 'of the gas phase 8 from the separator pot 4 is expanded by means of a turbine 9. The flow from the turbine 9 is introduced into a first distillation column 7 at a stage 10 located in the part lower of said column 7.

The liquid phase 5 coming from the separator pot 4 is expanded through a valve 6 and then injected at a pressure of between 10 bara and 40 bara and a temperature for example between -1 10 ° C. and -30 ° C. a demethanizer column 7 ', hereinafter also called second distillation column.

This liquid phase 5 is introduced at a theoretical stage 10 'below the head of said column 7'.

A liquid stream 12 of hydrocarbons heavier than methane is recovered in the lowest part 16 (in the tank) of column T.

A reboiler 1 1 is placed at a level to reboil the bottom liquid of the column 7 'in order to heat a portion of the liquid of said column in order to adjust the maximum threshold of methane contained in the stream 12 of hydrocarbons heavy.

At least 50% (typically at least 85%) molar heavy hydrocarbons present in the gaseous mixture 1 to be treated are recovered in this stream 12. Preferably at least 90% is recovered.

Preferably, the hydrocarbon liquid stream 12 does not contain more than 1 mol% of methane.

A heat exchanger 13 can be put in place to warm the lower part of the column 7 '(lower part = below the introduction of the liquid from the pot 4). This exchanger is fed by the feed gas stream 1. This warming improves the balance between the search for maximum yield and purity of the flow at the outlet of this second column 7 'of so-called demethanization distillation.

At the top 14 of the first distillation column 7 (head = highest outlet of the column), a gas stream 15 enriched in methane, typically containing less than 0.5 mol% of hydrocarbons having more than two carbon atoms ( containing not more than half the amount of heavy hydrocarbons - having more than 2 carbon atoms - present in the feed gas) is extracted. The temperature of the gas stream 15 is below -80 ° C.

In the tank 38 of the first distillation column 7, a liquid stream 39 is extracted to be introduced into said second distillation column 7 'at a stage 10 "substantially at the same level as that 10' where the liquid phase 5 coming from the phase separator pot 4. This liquid stream 39 coming from the first distillation column 7 is depleted in nitrogen (typically containing less than 10%, preferably less than 5%), as is the liquid phase 5 coming from the separator pot 4.

By nitrogen-depleted gas is meant a gas stream having a nitrogen content less than half the nitrogen content of the initial gas stream 1 to be treated and preferably less than a quarter of this content. As a result, very little nitrogen is introduced into the second distillation column 7 '.

Consequently, the gaseous stream which will be extracted from this second distillation column will not have to be introduced into a NRU unit, which will considerably lighten the load of this NRU unit which will have to treat the gas stream coming from the first column of distillation 7.

Typically, between 10% and 20% of the methane initially present in the gaseous stream 1 to be treated are found in this liquid stream 39 introduced into the second distillation column 7 'and therefore will not have to be introduced into a NRU unit.

By demethanizer column is meant a distillation column for producing at least two streams of different composition from a feed stream to be treated according to the method of the present invention. The at least two streams are the following: one at the top of the gaseous column, depleted in hydrocarbons having at least two carbon atoms, that is to say comprising less than half of the so-called heavy hydrocarbons contained in the feed gas (ethane, propane, butane, etc.) and the other, in the bottom of the column, in liquid form, depleted of methane present in the feed stream to be treated.

In the table below, the molar concentrations of the various components of the streams of the various steps of the process as illustrated in the figure are indicated.

It can then be seen that the stream 39 is a liquid comprising mainly methane and a minority of ethane and propane and contains virtually no nitrogen. o 1 39 15 12 15 '

Methane 88.6% 48.2% 93.9% 1, 4% 97.4%

Ethane 4.8% 41, 1% 0.4% 69.9% 0.4%

Propane 1, 3% 7.3% 0.0% 19.7% 0.0% i-Butane 0.2% 1.0% 0.0% 2.8% 0.0% n-Butane 0.3 % 1, 5% 0.0% 4.5% 0.0% i-Pentane 0.1% 0.3% 0.0% 1.0% 0.0% n-Pentane 0.0% 0.2 % 0.0% 0.6% 0.0%

Helium 0.1% 0.0% 0.2% 0.0% 0.1%

Nitrogen 4.5% 0.1% 5.5% 0.0% 2.1%

By demethanization unit is meant any system comprising at least one distillation column to enrich the methane in the overhead gas and to lower the methane tank liquid.

Cold can be recovered by condensing a gas enriched in methane under pressure. This condensation is carried out by means of a heat exchanger 17 supplied at the same time by a portion 8 "of the gas stream 8 coming from the separator pot 4 and by the gas stream enriched in methane 15 coming from the head 14 of the distillation column 7. This is only an example of implementation of the method object of the invention.

But according to a particular embodiment of the invention, a third current to be condensed could be introduced into this exchanger.

According to yet another embodiment of the invention, only one of the two described currents would be to condense.

By methane-enriched gas is meant gas mixture containing methane, nitrogen and typically less than 0.5% of hydrocarbons having more than two carbon atoms (containing at most half the amount of heavy hydrocarbons). - having more than two carbon atoms - present in the feed gas). The flow (s) 18 (18a and 18b) which has (have) been cooled in the exchanger 17 is (are) expanded (s) by means of, for example, at least one valve 19 (19a, 19b). ) and is (are) introduced into an upper part (upper part = above the supply 10 leaving the turbine 9) of the column 7.

The gaseous stream 15 'is extracted at the top 14' of the second distillation column 7 'at a temperature between -80 ° C and -120 ° C and at a pressure greater than 10 bara (typically between 15 bara and 30 bara ). This gaseous stream 15 'is introduced into a heat exchanger 17, 27 or 2 in order to be produced at the end of the process as a natural gas at a pressure (before any subsequent compression) close to the operating pressure of the column 7' (typically between 10 and 30 bara) and a temperature close to room temperature (typically between 0 ° C and 60 ° C)

The reflux of the second distillation column 7 'is ensured, in the same manner as for the reflux of the first distillation column 7, by introducing at its upper part 41 at least one stream (two are represented on the 18c and 18d which has (have) been cooled in the exchanger 17 and expanded (s) by means of, for example, at least one valve (19c, 19d).

These reflux steps are necessary to feed the two columns 7 and 7 'cold liquid and poor C2 +.

The stream which has been reheated in the exchanger 17 contains at most half the amount of heavy hydrocarbons - having more than two carbon atoms - present in the feed gas 1.

The stream of gas heated in the exchanger 17 at a temperature between -40 ° C and -70 ° C, preferably of the order of -60 ° C, is then partially condensed by means of, for example, a heat exchanger 21. At the outlet of this exchanger 21, a two-phase (gas-liquid) stream 22 (comprising from 20% to 80% molar of gas) is obtained.

Alternatively, it is possible to exonerate from the previous step, that is to say the passage of the stream 15, extracted from the head of the demethanizer column 7, in the heat exchanger 17. It is therefore possible to keep the temperature of the stream below -80 ° C (or even below -100 ° C) and to introduce said stream 15 directly into the heat exchanger 21 to obtain the stream 22. Current 22 is then sent to a denitrogenation system. In the denitrogenation system A, the diphasic current 22 is, after a possible expansion in a valve or a turbine 23, introduced into a phase separator pot 25.

The liquid phase 29 coming from the phase separator pot 25 is, after a possible expansion in at least one valve 42 '(in the figure, two valves 42 "and 42' are shown), heated through the heat exchangers 27 and 21 and finally 2 in order to join the output stream 30 (in the figure two output streams 30 and 30 'are shown because, thanks to the two detents 42 "and 42' beforehand, a current 30 'at medium pressure and a current 30 at low pressure) are produced) gas rich in methane produced at the output of the process. By medium pressure, it is understood a pressure of between 13 bara and 18 bara, typically 15.5 bara. By low pressure, it is understood a pressure of between 2 bara and 7 bara, typically 5.7 bara.

The outlet streams 30, 30 'and 40 contain less than 5 mol% of nitrogen.

The gaseous phase 26 coming from the separator pot 25 is partially condensed in a heat exchanger 27 and then expanded at the outlet of said exchanger 27 by means of a turbine or a valve 28 before being introduced into a distillation column 31.

The distillation column 31 is a so-called "stripping" column of nitrogen for the purpose of separating the nitrogen from the liquid enriched in output methane, also called denitrogenation column. The methane-enriched liquid comprises less than 5 mol% of nitrogen. This is a distillation column joined to a reboiler 32 but does not have an associated condenser system.

At the bottom of column 31, at a temperature below -100 ° C., preferably at -1 10 ° C., a stream 33 very rich in methane in liquid form is extracted. This stream 33 contains less than 5 mol% of nitrogen, preferably less than 4%. The liquid stream 33 is then mixed with the liquid phase 29 from the phase separator pot 25 and follows the same path to the output streams 30, 30 '.

At the top of column 31, a nitrogen-rich gas stream 36 at a temperature below -1 10 ° C is produced. Said stream 36 rich in nitrogen comprises at least 20 mol% of nitrogen. The nitrogen-rich stream 36 is heated through the successive exchangers 27, 21 and then 2. It can also be a single exchanger according to a particular embodiment of the invention. And according to another particular embodiment of the invention, more than three exchangers can be implemented.

There then emerges a flow 37, at a temperature close to ambient temperature (typically above -10 ° C. and below 50 ° C.), sent to an additional denitrogenation system B. The denitrogenation system B aims to produce a gaseous flow even richer in nitrogen than the stream 37. This system B may for example include at least one separator pot and a denitrogenation column. If the nitrogen specification at the outlet of system B is strict (<100ppm typically), it may be necessary to add a cycle compressor, for example a nitrogen or methane compressor, to the system B. provide the necessary reflux to obtain nitrogen purity at the top of the B system denazotation column.

A particular NRU unit has been described in this figure, but the method of the present invention applies to any type of NRU unit downstream of a unit called NGL.

The method which is the subject of the present invention makes it possible to achieve savings in terms of electrical consumption, for example. Indeed, only a part of the methane included in the gas to be treated is sent to a NRU unit, because the other part which is in the tank of the first distillation unit in liquid form does not contain nitrogen so that the NRU unit downstream of the NGL unit is much less loaded.

Furthermore, if we take again the figure illustrating an embodiment of the present invention, it is found that the output current 40 is already at high pressure, so that the end user does not need to use a compressor to increase the pressure of this current 40 (or possibly has a very limited need for compression), only the currents 30 and 30 'will require significant compression: this represents a saving in power consumption of the order of 10 to 30%.

Claims

1. A process for separating the components of a gaseous mixture (1) to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following steps:

a) introduction of the gaseous mixture (1) to be treated in a first distillation column (7) to create, in a tank (38) of column (7), a first liquid stream (39) enriched in hydrocarbon having at least two atoms of carbon and at the top (14) of column (7), a first gaseous stream (15) enriched in methane;

b) introducing said first hydrocarbon-enriched liquid stream (39) having at least two carbon atoms from step a) into a second distillation column (7 ') to create at the top (14') of said column ( 7 '), a second gaseous stream (15') rich in methane and, in the tank (16) of this column (7 '), a second liquid stream (12) comprising at least 85 mol% of hydrocarbons having at least two atoms carbon initially present in the mixture (1) to be treated;

characterized in that the nitrogen content of said second gas stream (15 ') is at least 1.5 times lower than the nitrogen content of the first gas stream (15) and in that from 5 mol% to 30 mol% methane initially present in the gaseous mixture to be treated (1) is included in the second gaseous stream (15 ').

2. Method according to the preceding claim characterized in that it comprises the additional step:

c) introducing said first gaseous stream (15) enriched in methane from step a) into a denitrogenation unit (A) to separate the nitrogen from the other components of this gaseous stream (15).

3. Method according to the preceding claim characterized in that the second gas stream (15 ') from step b) is not processed by the denitrogenation unit (A).

4. Method according to one of the preceding claims characterized in that from 10 mol% to 20 mol% of the methane initially present in the gaseous mixture to be treated (1) is included in the first hydrocarbon enriched liquid stream (39) having at least minus two carbon atoms from step a).

5. Method according to any one of the preceding claims, characterized in that prior to step a) it comprises the following steps:

At least partial condensation of said gaseous mixture (1) to be treated in order to obtain a diphasic mixture (3);

Injecting the vapor phase (8) of said diphasic mixture (3) into said first distillation column (7);

Injecting the liquid phase (5) of said diphasic mixture (3) into said second distillation column (7 ').

6. Method according to any one of the preceding claims, characterized in that the gaseous stream (15), extracted from the first distillation column (7) in step a), comprises at most half the amount of hydrocarbons. having more than two carbon atoms present in the feed gas (1).

7. Process according to any one of the preceding claims, characterized in that the liquid (39) extracted from said first distillation column (7) during step a) comprises at least 90 mol% of hydrocarbons having at least two atoms. carbon and preferably at least 95%.

8. Process according to any one of the preceding claims, characterized in that the said gaseous mixture (1) to be treated comprises at least 70 mol% of methane, at least 4 mol% of nitrogen and 2 mol% of hydrocarbons having at least two carbon atoms.

9. Method according to any one of the preceding claims characterized in that, during step b), the liquid stream (39) enriched in hydrocarbon having at least two carbon atoms from step a) is introduced in said second distillation column (7 ') at a theoretical stage (10 ") below the head (14') of said second column (7 ').

10. Method according to any one of the preceding claims characterized in that said gaseous stream (15 ') from step b) is extracted directly from said second distillation column (7') at a pressure greater than 20 Bara and comprises 95 mol% of methane.