US20180320961A1

US20180320961A1 - Optimization of a process for denitrogenation of a natural gas stream

Info

Publication number: US20180320961A1
Application number: US15/773,470
Authority: US
Inventors: Paul TERRIEN; Marie-Pascal Victor; Ludovic Granados; Michele MURINO; Bertrand Demolliens
Original assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2015-11-03
Filing date: 2016-10-05
Publication date: 2018-11-08
Also published as: EA036459B1; FR3042984B1; WO2017077205A1; EA201891034A1; FR3042984A1

Abstract

A process for the separation of the 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, including introducing the gas mixture to be treated into a first distillation column thereby producing, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of the column, another gas stream; and introducing the other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of the second column, thereby producing, at the top of this column, a second gas stream rich in methane.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application PCT/FR2016/052557, filed Oct. 5, 2016, which claims priority to French Patent Application 1560529, filed Nov. 3, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the separation of the components of a gas mixture containing methane, nitrogen and heavier hydrocarbons than methane.
The present invention thus applies to processes for the denitrogenation of natural gas, with or without recovery of helium.
Natural gas is desirable for use as fuel intended to be used for heating buildings, in order to provide heat for industrial processes, for the production of electricity, for use as starting material for various synthetic processes for producing olefins, polymers and the like.
Natural gas occurs in numerous fields which are at a distance from the users of natural gas. Natural gas is typically composed of methane (C₁), ethane (C₂) and heavier compounds, such as hydrocarbons having at least three carbon atoms, such as propane, butane, and the like (C₃+).
It can often be advantageous to separate the C₂and C₃+ compounds from the natural gas in order to market them as separate coproducts. This is because their value is generally greater than natural gas itself as they can be used directly for chemical processes (manufacture of ethylene from ethane, for example), as fuels (C₃/C₄is a conventional fuel known as LPG) or for many other applications.
Another component often present in natural gas is nitrogen. The presence of nitrogen in natural gas can cause difficulties in observing the specifications for natural gas (typically minimum net calorific value to be observed). This is all the more true when the heavier hydrocarbons than methane (C₂and C₃+ compounds) are removed as these have a higher net calorific value than methane; by removing them, the net calorific value is thus reduced, and it may then be necessary to increase it by the separation of nitrogen.
Consequently, a considerable effort has been devoted to the development of means for removing the nitrogen present in natural gas.
The natural gas deposits exploited increasingly contain nitrogen. This is explained in particular by the exhaustion and increased scarcity of fields which are sufficiently rich for no enriching treatment to be necessary before the marketing of the gas.
Frequently, these natural gas sources also contain helium. The latter can be made economic use of by carrying out a preconcentrating, before final treatment and liquefaction.
Unconventional resources, such as shale gases, also have the same problem: in order to make them marketable, it may prove to be necessary to increase their calorific value by means of a treatment which consists in denitrogenating the gas.
The most widely used method for separating nitrogen and heavier hydrocarbons than methane is “cryogenic separation”. A cryogenic process for the separation of nitrogen, more specifically a process employing a double column, is described in the patent application U.S. Pat. No. 4,778,498. Natural gas denitrogenation units generally treat gases which originate directly from wells at a high pressure. After denitrogenation, the treated gas has to be sent back to the network, often at a pressure close to its entry pressure.
During the exploitation of natural gas deposits, numerous steps may be provided. A relatively conventional step after the drying and the withdrawal of the impurities is the separation of the liquids associated with the natural gas (NGLs). This step can have many advantages but often it is a matter of making economic use of various “heavy” hydrocarbon products containing at least two carbon atoms (C₂, C₃, and the like, compounds) which are generally sold much more expensive than the natural gas product. If the natural gas contains nitrogen, there is a risk of re-encountering a natural gas having an excessively low calorific value because of the low resulting content of C₂, C₃, and the like, compounds. It is thus typical to then have to separate the nitrogen from this gas in order to make it marketable.
A conventional solution is to treat the two problems independently.
A first unit carries out the separation of the NGLs (subsequently known as NGL unit), while a second unit separates the nitrogen from the natural gas (subsequently known as NRU). This solution exhibits the advantage of operational flexibility. For example, if the NRU unit comprises a refrigeration cycle, the associated devices have a limited reliability, and a failure of a cycle compressor will result in the shutdown of the NRU but without resulting in the shutdown of the NGL.
Unfortunately, this shutdown cannot be lengthy in duration since it will then be necessary to send the production to the flare stack (because of its excessively low calorific value). In addition, this scheme is limited in terms of effectiveness as all the gas is cooled and then reheated in the NGL unit and then cooled and reheated in the NRU.
During a treatment in an NGL separation unit, a significant fraction (typically more than 10%) of the feed gas is condensed. During this condensation, methane is condensed with the heavier hydrocarbons (C₂+ and/or C₃+ compounds). It is then typically necessary to use a column known as a demethanizer in order to reboil the methane and not to lose methane in the C₂+ and/or C₃+ products. If nitrogen is present, the latter will, on the other hand, be only very slightly condensed and will be re-encountered predominantly in the gas phase introduced into the demethanization column.

SUMMARY

The inventors of the present invention have thus developed a solution which makes it possible to solve the problem raised above while optimizing the energy costs, such as, for example, those related to the electrical consumption during the implementation of such processes.
A subject matter of the present invention is a process for the separation of the 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) introduction of the gas mixture to be treated into a first distillation column in order to create, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of said column, another gas stream;
b) introduction of said other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of said second column, in order to create, at the top of this column, a second gas stream rich in methane,
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 comprised in the second gas stream.
More particularly, a subject matter of the present invention relates to:
A process as defined above, characterized in that it comprises the additional stage:
c) introduction of said first gas stream enriched in methane resulting from step a) into a denitrogenation unit in order to separate the nitrogen from the other components of this gas stream.
A process as defined above, characterized in that the second gas stream resulting from step b) is not treated by the denitrogenation unit.
A process as defined above, characterized in that, prior to step a), it comprises the following steps:

- at least partial condensation of said gas mixture to be treated in order to obtain a two-phase mixture;
- injection of the liquid phase from said two-phase mixture into said first demethanization column at a first injection stage;
- injection of the vapor phase from said two-phase mixture into said first demethanization column at an injection stage different from said first stage.

A process as defined above, characterized in that the gas stream, extracted from the first distillation column in step a), comprises at most half of the amount of hydrocarbons having more than two carbon atoms present in the feed gas.
A process as defined above, characterized in that said gas 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 said gas stream resulting from step b) is extracted directly from said second distillation column at a pressure of greater than 20 bara and comprises 95 mol % of methane.
A process as defined above, characterized in that a portion of the second gas stream rich in methane and depleted in nitrogen resulting from step b) at the outlet of the top of the second distillation column is compressed and then condensed in order to be introduced, for one portion, into the upper part of the first distillation column and, for the other portion, into the upper part of the second distillation column in order to carry out the reflux of said distillation columns.
Thus, the process which is a subject matter of the present invention makes it possible to take advantage of the fact that only the gas feed of the methanization column substantially contains nitrogen.
This is because the solution provided, in comparison with the known processes of the state of the art, is that of splitting in two the demethanization column conventionally used, one (that is to say, the first distillation column) producing a natural gas product poor in C₂+ and rich in nitrogen and the other (that is to say, 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% of C₂+ and at least 2% of nitrogen). According to a specific embodiment, the process according to the invention typically comprises the following steps:

- Pretreatment of the crude gas to be treated (separation of the water, CO₂, methanol, heavy hydrocarbons, for example).
- Cooling of the crude gas to a first subambient temperature (typically between −30° C. and −70° C.), making it possible to obtain a cooled two-phase stream.
- Separation of the cooled two-phase stream into a first gas and a first liquid.
- Reduction in pressure of at least a portion of said first gas in a turbine and introducing it, after reduction in pressure, into the middle of an upper demethanization column, known above as first distillation column.
- Reduction in pressure of at least a portion of the first liquid in order to inject it into the middle of an upper demethanization column, known above as first distillation column.
- Extraction of at least a portion of a gas stream at an intermediate stage of an upper demethanization column, known above as first distillation column, in order to inject it into a lower demethanization column, known above as the second distillation column, at least one theoretical stage below the column top.
- Obtaining, at the top of the upper demethanization column, a gas poor in C₂+ and rich in nitrogen (richer than the crude gas).
- Obtaining, at the top of the lower demethanization column, a gas poor in C₂+ and poor in nitrogen (typically containing at least two times less nitrogen than the top of the upper column and preferably containing less than 5% of nitrogen).

This also makes it possible to considerably simplify the sheet-metal working of the demethanization columns conventionally employed in the known processes of the state of the art. This is because, typically, the column top is much wider than the column bottom, which presents mechanical constraints and thus additional costs. The separation of the columns makes it possible to escape this constraint.
In order to deplete the top of the lower demethanization column, an additional reflux is provided. This reflux should, if possible, be very poor in nitrogen. Several means are possible for providing this reflux:

- Use of a dedicated condenser, for example with liquid methane at a lower pressure than the column top. This liquid methane can be produced by the downstream part of the process (NRU).
- Use of the recompressed and recondensed top gas from the lower demethanization column.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to the FIGURE, which illustrates a specific example of an implementation of a process according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the FIGURE, a flow 1 of natural gas pretreated beforehand (typically having undergone a separation of a portion of at least one of the following constituents: water, CO₂, methanol, sulfur compounds, very heavy hydrocarbons, that is to say having more than six or seven carbon atoms (such as C₈+ compounds, for example)) comprising at least 30 mol % of methane, at least 0.1 mol % of heavier hydrocarbons than methane (that is to say comprising at least two carbon atoms) and between at least 4 mol % et 50 mol %, indeed even 80 mol %, of nitrogen is introduced into a system 2 which makes possible an at least partial condensation of said flow 1.
The pressure of this flow 1 is between 20 bara (bar absolute) and 100 bara (typically between 30 and 70 bara) and the temperature is close to ambient temperature, for example between 0° C. and 60° C.
The system 2 is, for example, a heat exchanger. The mixture 3 exiting from this system 2 is in a two-phase (gas and liquid) state. This mixture 3 is introduced into a phase separator 4.
The operating pressure is between 20 and 100 bara, typically between 30 and 70 bara. The temperature of this separator is between −100° C. and 0° C., typically between −80° C. and −20° C.
At least a portion 8′ of the gas phase 8 resulting from the separator 4 is reduced in pressure by means of a turbine 9. The flow resulting from the turbine 9 is introduced into a first distillation column 7 at least one theoretical stage 10 located below the top of said column 7.
The liquid phase 5 resulting from the separator 4 is reduced in pressure through a valve 6 and is then injected, at a pressure of between 10 bara and 40 bara and a temperature, for example, of between −110° C. and −30° C., into said demethanization column 7. This liquid phase 5 is introduced at a theoretical stage 10′ below the top of said column 7 and below the stage 10 for introduction of the gas flow 9.
At the top 14 of the first distillation column 7 (top=highest outlet of the column), a gas flow 15 enriched in methane, typically containing less than 0.5 mol % of hydrocarbons having more than two carbon atoms (containing at most 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 less than −80° C.
At an intermediate stage 38 (that is to say at least one stage below that of the top 14 of the column 7) of the first distillation column 7, a gas stream 39 is extracted in order to be introduced into a demethanization column 7′, also known below as second distillation column 7′, at a stage 10′ located below that of the top 14′ of the column 7′.
This gas stream 39 resulting from the first distillation column 7 is depleted in nitrogen (typically containing less than 10%, preferably less than 5%), just like the liquid phase 5 resulting from the separator 4. Gas depleted in nitrogen is understood to mean a gas stream having a nitrogen content which is less than half the nitrogen content of the initial gas stream 1 to be treated and preferably less than a quarter of this content. The result of this is that very little nitrogen is introduced into the second distillation column 7′. Consequently, the gas stream which will be extracted from this second distillation column will not have to be introduced into an NRU unit, which will greatly lighten the burden on this NRU unit which will have to treat the gas stream 15 resulting from the first distillation column 7. Typically, between 10% and 20% of the methane initially present in the gas stream 1 to be treated will be re-encountered in this liquid stream 39 introduced into the second distillation column 7′ and thus will not have to be introduced into an NRU unit.
The liquid stream 39′ produced in the bottom 14″ of said second distillation column 7′ is extracted in order to be introduced into the lower part 50 of the first distillation column 7.
A liquid flow 12 of heavier hydrocarbons than methane is recovered in the bottom part 16 of the column 7.
A reboiler 11 is placed at a level which makes it possible to reboil the bottom liquid from the column 7 in order to reheat a portion of the liquid from said column with the aim of adjusting the maximum threshold for methane present in the flow 12 of heavy hydrocarbons.
At least 50 mol % (typically at least 85 mol %) of the heavy hydrocarbons present in the gas mixture 1 to be treated are recovered in this flow 12. Preferably, at least 90% are recovered.
Preferably, the liquid flow 12 of hydrocarbons does not contain more than 1 mol % of methane.
A heat exchanger 13 can be installed in order to reheat the bottom part of the column 7 (bottom part=below the introduction of the liquid originating from the separator 4). This exchanger is fed with the gas feed stream 1. This reheating improves the balance between search for maximum yield and purity of the outlet flow from demethanization column 7.
Demethanization column is understood to mean a distillation column intended to produce at least two streams which are different in composition starting from a feed stream to be treated according to the process of the present invention. The at least two streams are as follows: one, at the column top, gaseous, depleted in hydrocarbons having at least two carbon atoms, that is to say comprising less than half of the “heavy” hydrocarbons present in the feed gas (ethane, propane, butane, and the like), and the other, in the column bottom, in the liquid form, depleted in methane present in the feed stream to be treated.
The molar concentrations of the different components of the streams of the different steps of the process as illustrated according to the FIGURE are shown in the table below. It may then be observed that the stream 39 is a liquid comprising predominately methane and, to a minor extent, ethane and propane and contains virtually no nitrogen.


°	1	39	15	12	15′

Methane	88.6%	93.5%	93.9%	1.4%	97.4%
Ethane	4.8%	5.7%	0.4%	69.9%	0.4%
Propane	1.3%	0.2%	0.0%	19.7%	0.0%
Isobutane	0.2%	0.0%	0.0%	2.8%	0.0%
n-Butane	0.3%	0.0%	0.0%	4.5%	0.0%
Isopentane	0.1%	0.0%	0.0%	1.0%	0.0%
n-Pentane	0.0%	0.0%	0.0%	0.6%	0.0%
Helium	0.1%	0.0%	0.2%	0.0%	0.1%
Nitrogen	4.5%	0.6%	5.5%	0.0%	2.1%

Demethanization unit is understood to mean any system comprising at least one distillation column for enriching the top gas in methane and depleting the bottom liquid in methane.
It is possible to condense a gas enriched in methane under pressure in order to improve the performance qualities. This condensation is carried out by virtue of a heat exchanger 17 fed both with a portion 8″ of the gas flow 8 resulting from the separator 4 and with the gas stream enriched in methane 15 resulting from the top 14 of the distillation column 7. Only an exemplary embodiment of the process which is a subject matter of the invention is concerned here. However, according to a specific embodiment of the invention, a third stream to be condensed might be introduced into this exchanger. According to yet another embodiment of the invention, just one of the two streams described would need to be condensed.
Gas enriched in methane 15 is understood to mean a gas mixture containing methane, nitrogen and typically less than 0.5% 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 stream or streams 18 (18 a and 18 b) which has (have) been cooled in the exchanger 17 is (are) reduced in pressure by means, for example, of at least one valve 19 (19 a, 19 b) and is (are) then introduced into a top part (top part=above the feed 10 exiting from the turbine 9) of the column 7.
The gas stream 15′ is extracted at the top 14′ of the second distillation column 7′ at a temperature of between −80° C. and −120° C. and at a pressure of greater than 10 bara (typically of between 15 bara and 30 bara). This gas stream 15′ is introduced into a heat exchanger 17, 27 or 2 in order to be produced at the end of the process as natural gas at a pressure (before possible subsequent compression) close to the operating pressure of the column 7′ (typically between 10 and 30 bara) and a temperature close to ambient temperature (typically between 0° C. and 60° C.).
The reflux of the second distillation column 7′ is provided, in the same way as for the reflux of the first distillation column 7, by the introduction, into its upper part 41, of at least one stream (two are represented in the FIGS. 18c and 18d which has (have) been cooled in the exchanger 17 and reduced in pressure by means, for example, of at least one valve (19 c, 19 d).
Alternatively, the reflux of the two distillation columns 7 and 7′ can be provided, at least in part, by a portion of the gas stream 15′ extracted at the top of the second column 7′ and then cooled in a heat exchanger before said refluxes.
These reflux steps are necessary in order to feed the two columns 7 and 7′ with cold liquid poor in C₂₊.
The flow 20 which was reheated in exchanger 17 contains at most half of the amount of heavy hydrocarbons—having more than two carbon atoms—present in the feed gas 1.
The gas stream 20 reheated in the exchanger 17, at a temperature of between −40° C. and −70° C., preferably of the order of −60° C., is subsequently partially condensed by means, for example, of a heat exchanger 21. There emerges, at the outlet of this exchanger 21, a two-phase (gas/liquid) stream 22 (comprising from 20 mol % to 80 mol % of gas).
Alternatively, it is possible to be exempted from the preceding step, that is to say from the passage of the stream 15, extracted from the top of the demethanization column 7, into the heat exchanger 17. It is thus possible to keep the temperature of the stream 15 below −80° C. (or even below −100° C.) and to introduce said stream 15 directly into the heat exchanger 21 in order to obtain the stream 22.
The stream 22 is subsequently sent to a denitrogenation system A. In the denitrogenation system A, the two-phase stream 22 is, after an optional reduction in pressure in a valve or a turbine 23, introduced into a phase separator 25. The liquid phase 29 resulting from the phase separator 25 is, after an optional reduction in pressure in at least one valve 42′ (in the FIGURE, two valves 42″ and 42′ are represented), reheated through the heat exchangers 27, then 21 and finally 2 in order to rejoin the outlet stream 30 (in the FIGURE, two outlet flows 30 and 30′ are represented as, by way of the two prior reductions in pressure 42″ and 42′, a medium-pressure stream 30′ and a low-pressure stream 30 are produced) of gas rich in methane produced at the process outlet.
Medium-pressure is understood to mean a pressure of between 13 bara and 18 bara, typically 15.5 bara. Low-pressure is understood to mean a pressure of between 2 bara and 7 bara, typically 5.7 bara.
The outlet flows 30, 30′ and 40 contain less than 5 mol % of nitrogen.
The gas phase 26 resulting from the separator 25 is partially condensed in a heat exchanger 27 and then reduced in pressure at the outlet of said exchanger 27 by means of a turbine or of a valve 28 before being introduced into a distillation column 31.
The distillation column 31 is a column for stripping nitrogen, the aim of which is to separate the nitrogen from the liquid enriched in methane at the outlet, also known as denitrogenation column. The liquid enriched in methane comprises less than 5 mol % of nitrogen. In this instance, a distillation column joined to a reboiler 32 but not having available an associated condenser system is concerned.
A stream 33 very rich in methane in the liquid form is extracted at the bottom of the column 31, at a temperature of less than −100° C., preferably of less than −110° C. This stream 33 contains less than 5 mol % of nitrogen, preferably less than 4%. The liquid stream 33 is subsequently mixed with the liquid phase 29 resulting from the phase separator 25 and follows the same path as far as the outlet flows 30, 30′.
A gas flow 36 rich in nitrogen, at a temperature of less than −110° C., is produced at the top 35 of the column 31. Said flow 36 rich in nitrogen comprises at least 20 mol % of nitrogen.
The flow rich in nitrogen 36 is reheated through the successive exchangers 27, 21, then 2. It can also be one and the same exchanger, according to a specific embodiment of the invention. Moreover, according to another specific embodiment of the invention, more than three exchangers can be employed.
This then results in a stream 37, at a temperature close to ambient temperature (greater than −10° C. typically and less than 50° C.), sent to an additional denitrogenation system B. The aim of the denitrogenation system B is to produce a gas stream even richer in nitrogen than the stream 37. This system B can, for example, include at least one separator and one denitrogenation column. If the specification for nitrogen at the outlet of the system B is strict (<100 ppm typically), it may prove to be necessary to add, to the system B, a cycle compressor, for example a nitrogen or methane compressor, in order to contribute the reflux necessary in order to obtain the nitrogen purity at the top of the denitrogenation column of the system B.
A specific NRU unit has been described in this FIGURE but the process which is a subject matter of the present invention applies to any type of NRU unit downstream of an “NGL” unit.
The process which is a subject matter of the present invention makes it possible to achieve savings in terms of electricity consumption, for example. This is because only a portion of the methane included in the gas to be treated is sent to an NRU unit, as the other portion which occurs in the bottom of the first distillation unit in the liquid form does not contain nitrogen, with the result that the NRU unit downstream of the NGL unit is much less burdened.
Furthermore, if the FIGURE illustrating an embodiment of the present invention is taken up, it is found that the outlet stream 40 is already at high pressure, with the result that the final user does not need to use a compressor to increase the pressure of the stream 40 (or possibly has a very limited compression need); only the streams 30 and 30′ will require significant compression: this represents a saving in electricity consumption of the order of 10% to 30%.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.

Claims

1.-8. (canceled)

9. A process for the separation of the 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 thereby producing, at the column top, a first gas stream enriched in methane and, at a level lower than that of the top of said column, another gas stream;

b) introducing said other gas stream resulting from step a) into a second distillation column, at a level lower than that of that of the top of said second column, thereby producing, at the top of this column, a second gas stream rich in methane,

wherein 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 comprised in the second gas stream.

10. The process as claimed in claim 9, further comprising:

c) introducing said first gas stream enriched in methane resulting from step a) into a denitrogenation unit in order to separate the nitrogen.

11. The process as claimed in claim 10, wherein the second gas stream resulting from step b) is not treated by the denitrogenation unit.

12. The process as claimed in claim 9, further comprising, prior to step a), the following steps:

at least partially condensing said gas mixture to be treated in order to obtain a two-phase mixture;

injecting the liquid phase from said two-phase mixture into said first demethanization column at a first injection stage;

injecting the vapor phase from said two-phase mixture into said first demethanization column at an injection stage different from said first stage.

13. The process as claimed in claim 9, wherein the gas stream, extracted from the first distillation column in step a), comprises at most half of the amount of hydrocarbons having more than two carbon atoms present in the feed gas.

14. The process as claimed in claim 9, wherein said gas mixture to be treated comprises 70 mol % of methane, at least 4 mol % of nitrogen and 2 mol % of hydrocarbons having at least two carbon atoms.

15. The process as claimed in claim 9, wherein said gas stream resulting from step b) is extracted directly from said second distillation column at a pressure of greater than 20 bara and comprises 95 mol % of methane.

16. The process as claimed in claim 9, wherein a portion of the second gas stream rich in methane and depleted in nitrogen resulting from step b) at the outlet of the top of the second distillation column is compressed and then condensed in order to be introduced, for one portion, into the upper part of the first distillation column and, for the other portion, into the upper part of the second distillation column in order to carry out the reflux of said distillation columns.