EA035004B1 - Reflux of demethanization columns - Google Patents

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 60 mol.% of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated; characterized in that a portion of the second methane-rich gas stream derived from step b), on leaving 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 (18c), into the upper part of the second distillation column in order to effect the reflux of said distillation columns.

Description

The present invention relates to a method for separating components of a mixture of gases containing methane, nitrogen and hydrocarbons heavier than methane.

Therefore, the present invention is applied to methods for de-nitriding natural gas with or without helium recovery.

Natural gas is in demand for use as a fuel intended for use in heating buildings, for providing heat for industrial methods of generating electricity, for use as a raw material for various synthetic methods for producing olefins, polymers, etc.

Natural gas is found in many fields remote from natural gas users. Natural gas usually consists of methane (C1), ethane (C2) and heavier compounds, such as hydrocarbons containing at least three carbon atoms, such as propane, butane, etc. (C3 +). It can often be beneficial to separate C2 and C3 + compounds from natural gas in order to sell them as separate by-products.

This is because their value is usually higher than that of natural gas itself, since they can be used directly for chemical methods (for example, the manufacture of ethylene from ethane), as fuels (C3 / C4 is a traditional fuel known as LPG) ), or for many other applications.

Another component often found in natural gas is nitrogen. The presence of nitrogen in natural gas can cause difficulties in meeting the requirements for natural gas (usually the required minimum net calorific value). Even more correct is the removal of hydrocarbons heavier than methane (C2 and C3 +), since they have a higher value of lower calorific value than methane, so when they are removed, the value of lower calorific value decreases, and then it can be must be increased by nitrogen separation.

Therefore, some efforts have been made to create means for removing nitrogen present in natural gas.

The exploited natural gas deposits contain increasing amounts of nitrogen. This, in particular, is explained by the development and increase in the deficit of deposits rich enough so that processing for enrichment is not required before the gas is brought to the market.

These natural gas sources often also contain helium. The latter can be introduced into commercial consumption by pre-concentration before final processing and liquefaction.

Non-traditional resources, such as shale gases, also have the same problem: in order to make them marketable, it may be necessary to increase the value of their calorific value through processing consisting in gas de-nitration.

The most widely used method for the separation of nitrogen and hydrocarbons heavier than methane is cryogenic separation. A cryogenic nitrogen separation method, more specifically a double column method, is described in US-A-4,778,498.

Natural gas de-nitriding plants typically process gases coming directly from wells at high pressure. After de-nitriding, the treated gas must be returned to the network, often under pressure close to the inlet pressure.

When exploiting natural gas deposits, many stages can be envisaged. A relatively common step after drying and removing impurities is the separation of liquids associated with natural gas (NGL).

This stage can have many advantages, but often they include the commercialization of various heavy hydrocarbon products containing at least two carbon atoms (compounds C2, C3, etc.), which are usually sold significantly more expensive than the natural product gas.

If natural gas contains nitrogen, there is a risk of producing natural gas with too low a calorific value due to the resulting low content of compounds C2, C3, etc. Therefore, there is usually a need then to separate nitrogen from this gas to make it suitable for sale.

A common solution is to deal with two problems independently.

In the first unit, NGL is separated (hereinafter referred to as the NGL unit), while in the second unit, nitrogen is separated from natural gas (hereinafter referred to as NRU). The advantage of this solution is operational flexibility.

For example, if the NRU installation includes a cooling cycle, the associated devices have limited reliability, and a failure in the compressor of the cycle will disable the NRU, but will not disable the NGL.

Unfortunately, this shutdown cannot be long, because then it will be necessary to burn products (due to its excessively low value of calorific value).

In addition, this scheme is limited in terms of efficiency.

When processed in an NGL separation unit, a significant portion (usually more than 10%) of the supplied 1,035,004 gas is condensed. During this condensation, methane is condensed with heavier hydrocarbons (compounds C2 + and / or C3 +).

Then, it is usually necessary to use a column called a demethanizer to re-evaporate the methane so that methane is not lost in the C2 + and / or C3 + products. If nitrogen is present, it will, on the other hand, be condensed in very small amounts and will predominantly revert to the gaseous phase, which is fed to the demethanation column.

Thus, the authors of the present invention have developed a solution that allows to solve the above problems and at the same time optimizes energy costs, such as, for example, associated with the consumption of electricity in the implementation of these methods.

An object of the present invention is a method for separating components of a gas mixture to be processed containing methane, nitrogen and at least one hydrocarbon containing at least two carbon atoms, or a mixture of such hydrocarbons, comprising the following steps:

a) introducing the gas mixture to be treated into the first distillation column to obtain in the cube of the column a first liquid stream enriched in at least two carbon atoms and in the upper part of the column of the first methane-enriched gas stream;

b) introducing said first liquid stream enriched in a hydrocarbon containing at least two carbon atoms obtained in step a) into a second distillation column to obtain a second gas stream with a high methane content in the upper part of the column and in the cube of the second liquid column a stream containing at least 85 mol% of hydrocarbons containing at least two carbon atoms initially present in the mixture to be treated;

characterized in that part of the second gas stream with a high methane content obtained in stage b) is compressed at the outlet of the upper part of the second distillation column to a pressure value exceeding 1 bar the pressure in the second distillation column, and then condensed to introduce one part in the upper part of the first distillation column and another part in the upper part of the second distillation column to ensure the return of reflux in these distillation columns.

More specifically, an object of the present invention relates to the following.

The method defined above, which is characterized in that the pressure of part of the second gas stream in the turbine is reduced to a pressure value that is at least 1 bar lower than the pressure value in the distillation column.

The method defined above, which is characterized in that the nitrogen content in the specified second gas stream is at least 1.5 times less than the nitrogen content in the first gas stream.

The method defined above, further comprising step c): introducing said first methane-enriched gas stream obtained in stage a) into a de-nitration unit to separate nitrogen from the remaining components of this gas stream.

The method defined above, which is characterized in that the second gas stream obtained in stage b) is not processed in a de-nitriding apparatus.

The method defined above, which is characterized in that from 5 to 30 mol% of methane, which is initially present in the gas mixture to be treated, is included in the first liquid stream enriched in hydrocarbon containing at least two carbon atoms obtained in stage a )

The method defined above, which is characterized in that from 10 to 20 mol% of methane, which is initially present in the gas mixture to be treated, is included in the first liquid stream enriched in hydrocarbon containing at least two carbon atoms obtained in stage a )

The method defined above, which is characterized in that before stage a) it includes the following stages:

at least partial condensation of the gas mixture to be treated to form a two-phase mixture;

injecting a vapor phase of said biphasic mixture into said first distillation column;

injecting a liquid phase of said biphasic mixture into said second distillation column.

The method defined above, which is characterized in that the gas stream that is extracted from the first distillation column in stage a) contains not more than half of the amount of hydrocarbons containing more than two carbon atoms present in the feed gas.

The method defined above, characterized in that the liquid that is extracted from the specified first distillation column in stage a) contains at least 90 mol.% Hydrocarbons containing at least two carbon atoms, and it preferably contains at least least 95%.

The method as defined above, characterized in that said gas mixture to be treated contains 70 mol% of methane, at least 4 mol% of nitrogen and 2 mol% of hydrocarbons containing at least two carbon atoms.

The method defined above, which is characterized in that during stage b) a liquid stream enriched in a hydrocarbon containing at least two carbon atoms obtained in stage

a), introduced into the specified second distillation column at a theoretical stage below the upper part of this specified second column.

The method defined above, which is characterized in that the gas stream obtained in stage b) is extracted directly from the specified second distillation column at a pressure above 20 bar abs., And while it contains 95 mol.% Methane.

Thus, the method that is the object of the present invention makes it possible that only the gas which is fed to the methanation column contains a significant amount of nitrogen.

This is because the proposed solution, compared with methods known from the prior art, consists in dividing the standardly used column for demethanation into two, while in one (in other words, in the first distillation column) a natural gas product with a low content of C2 + and high nitrogen content and in another (in other words, in the second distillation column) contains a natural gas product with a low content of C2 + and de-nitrated.

The method of the present invention allows the release of a crude gas with a high content of C2 + and nitrogen (usually at least 1% C2 + and at least 2% nitrogen). In accordance with a specific embodiment, the method in accordance with the present invention typically includes the following steps.

Pre-treatment of the untreated gas to be treated (e.g., separation of water, CO ₂ , methanol and heavy hydrocarbons).

Cooling the crude gas to a first temperature below ambient (usually from -30 to -70 ° C) with the possibility of obtaining a cooled two-phase stream.

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

Pressure reduction of at least a portion of the first liquid to be pumped into the lower demethanation column, referred to above as the second distillation column. Alternatively, a gas stream containing less nitrogen than the original gas stream to be treated is diverted below the top of the first demethanation column for injection into the lower demethanation column, referred to above as the second distillation column.

Decreasing the pressure of at least a portion of the gas in the turbine and introducing it after decreasing the pressure into the cube of the upper demethanation column, referred to above as the first distillation column.

Condensation of at least a portion of the gas with its use as a reverse flow in the upper demethanation column.

The introduction of fluid from the upper column for demethanation in the lower column for demethanation at least one theoretical stage below the upper part of the column.

Obtaining in the upper part of the upper column for demethanation of gas with a low content of C2 + and a high nitrogen content (higher than in untreated gas).

Obtaining in the upper part of the lower column for demethanation of gas with a low content of C2 + and a low nitrogen content (usually containing at least two times less nitrogen compared to the gas in the upper part of the upper column and below a quarter of the nitrogen content compared to the nitrogen content in the upper top column).

This also provides the possibility of significantly simplifying the processing of sheet metal columns for demethanation, standardly used in the methods known from the prior art.

This is because the top of the column is usually much wider than the cube of the column, which imposes mechanical limitations and, thus, leads to additional costs. Separation of columns provides an opportunity to eliminate this limitation.

For depletion of the upper part of the lower column for demethanation provide an additional return of reflux. This reflux should, if possible, be characterized by a very low nitrogen content. There are several ways to ensure this return phlegm.

The use of a special condenser, for example with liquid methane, at a pressure lower than in the upper part of the column. This liquid methane can be obtained through part of the process downstream (NRU).

The use of re-compressed and re-condensed gas from the bottom column for demethanation.

The invention will be described in more detail with reference to the drawing, illustrating the method of the present invention.

In the drawing, pre-treated natural gas stream 1 (typically subjected to separation of at least one of the following components: water, CO ₂ , methanol, sulfur compounds, very heavy hydrocarbons, in other words, having more than six or seven carbon atoms, for example, such as compounds C8 +) containing at least 30 mol.% methane, according

- 3 035004 at least 0.1 mol.% Of hydrocarbons heavier than methane (in other words, containing at least two carbon atoms) and from at least 4 to 50 mol.%, Or even 80 mol.% Of nitrogen are introduced into the system 2 s enabling at least partial condensation of said stream 1.

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

System 2 is, for example, a heat exchanger. Mixture 3 emerging from this system 2 is in a two-phase (gas and liquid) state. This mixture 3 is introduced into the separator 4 phases.

The operating pressure is from 20 to 100 bar abs., Usually from 30 to 70 bar abs. The temperature of this separator is from -100 to 0 ° C, usually from -80 to -20 ° C.

In at least a portion 8 ′ of the gaseous phase 8 obtained from the separator 4, the pressure is reduced by means of a turbine 9. The stream obtained from the turbine 9 is introduced into the first distillation column 7 to a stage 10 located in the lower part of the said column 7.

In the liquid phase 5 obtained from the separator 4, the pressure is reduced by means of valve 6 and then at a pressure of 10 to 40 bar abs. and a temperature, for example, from -110 to -30 ° C, is pumped into said demethanation column 7 ′, hereinafter also referred to as the second distillation column. The liquid phase 5 is introduced to the theoretical stage 10 ′ below the upper part of said column 7 ′.

A liquid stream of hydrocarbons 12 heavier than methane is recovered in the lowest part 16 (in the cube) of the column 7 '.

The reboiler 11 is placed at a level that allows the boiling of the bottled liquid from the column 7 'to reheat part of the liquid from the specified column in order to control the maximum limit of methane contained in the heavy hydrocarbon stream 12.

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 stream 12. Preferably, at least 90% is recovered.

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

To reheat the bottom of the column 7 '(bottom = below the liquid inlet from the separator 4), a heat exchanger 13 can be installed. A gas feed stream 1 is supplied to this heat exchanger. This reheating improves the balance between finding the maximum yield and the purity of the effluent from the second distillation column 7 ′ for demethanation.

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

In the cube 38 of the first distillation column 7, a liquid stream 39 is recovered for introduction into said second distillation column 7 'to the stage 10, essentially at the same level as the stage 10', to which the liquid phase 5 obtained from the phase separator 4 is introduced .

This liquid stream 39, obtained from the first distillation column 7, is depleted in nitrogen (usually contains less than 10%, preferably less than 5%), in the same way as the liquid phase 5 obtained from the separator 4. By a nitrogen depleted gas is meant a stream gases with a nitrogen content below half the nitrogen content in the initial gas stream 1 to be treated, and preferably below a quarter of the content. As a result, a very small amount of nitrogen is introduced into the second distillation column 7 '.

Therefore, the gas stream that will be extracted from this second distillation column will not need to be introduced into the NRU unit, which will significantly reduce the load on this NRU unit, which will process the gas stream 15 obtained from the first distillation column 7.

Typically, from 10 to 20% of methane, which is initially present in the gas stream 1 to be treated, will again transfer to this liquid stream 39, which is introduced into the second distillation column 7 ', and thus, it will not need to be introduced into the NRU unit.

Under the column for demethanation refers to a distillation column designed to receive at least two streams of different compositions, based on the feed stream to be processed in accordance with the method of the present invention.

At least two streams are the following: one of them, gaseous, at the top of the column, depleted in hydrocarbons containing at least two carbon atoms, in other words, containing less than half of the heavy hydrocarbons present in the feed gas (ethane, propane, butane, etc.), and another in the cube of the column, in liquid form, depleted in methane, which is present in the feed to be treated.

The table below shows the molar concentrations of the various components of the streams at various stages of the process depicted in the drawing.

Thus, it can be seen that stream 39 is a liquid containing mainly methane and, to a lesser extent, ethane and propane, and practically does not contain nitrogen.

- 4 035004

ABOUT 1 39 fifteen 12 fifteen' 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-bhutan 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%

Under the installation for demethanation refers to any system that includes at least one distillation column for the enrichment of methane gas from the upper part and the methane depletion of bottoms liquid.

The gas stream 15 'is recovered in the upper part 14' of the second distillation column 7 'at a temperature of from -80 to -120 ° C and at a pressure above 10 bar abs. (usually from 15 to 30 bar abs.). A part of this gas stream 15 'is introduced into the heat exchanger 17 for reheating to a temperature of from -50 to -110 ° C, then it is introduced into the turbine 43 before combining the methane-enriched stream 30 at the outlet of the de-nitriding unit A and is obtained as a natural gas. The pressure of this produced natural gas is, for example, from 15 to 30 bar abs. (before re-compression), and the temperature is above 0 ° C after re-heating in the heat exchanger 2.

Methane-enriched gas can be condensed under pressure to improve its performance.

This condensation is carried out using a heat exchanger 17, which serves part 8 of the gas stream 8 obtained from the phase separator 4, and part 44 of the gas stream 15 'extracted from the upper part 14' of the second distillation column 7 '.

Before being introduced into the heat exchanger 17, this gas stream 44 is compressed using a compressor 46, preferably a refrigerated compressor, in other words, a compressor whose temperature is less than 0 ° C. It is this compressed stream 45 that is supplied to the heat exchanger 17. The power of the compressor 46 can advantageously be generated by the turbine 43, which makes it possible to optimize the compression.

The pressure of flow 44 increases by several bar abs. only to allow re-condensation of stream 45 in countercurrent. In the stream or streams 18 (18a and 18b), which (which) were cooled in the heat exchanger 17, the pressure is reduced, for example by means of at least one valve 19 (19a, 19b), and then introduced into the upper part 42 (upper part = higher supply 10 exiting the turbine 9) of the column 7.

The reflux in the second distillation column 7 'is provided as well as the reflux in the first distillation column 7 by introducing at least one stream 18c into its upper part 41, which was cooled in the heat exchanger 17 and its pressure was reduced, for example, by at least one valve 19c.

These stages of reflux recovery are necessary for supplying cold liquid with a low C2 + content to two columns 7 and 7 '.

The use of part 44 of a methane-enriched and nitrogen-free gas stream 15 'avoids the need for recirculation of the effluent in order to increase the yield of C2 + products.

The methane-enriched gas stream 15 obtained from the upper part 14 of the distillation column 7 is partially condensed, for example, by means of a heat exchanger 21. At the outlet of this heat exchanger 21, a two-phase (gas / liquid) stream 22 (containing from 20 to 80 mol% of gas )

The temperature of this stream 15 is maintained below -80 ° C (or even below -100 ° C), and the specified stream 15 enters directly into the heat exchanger 21 to obtain a stream 22.

Then, stream 22 is sent to system A for de-nitration.

In system A for de-nitriding, a two-phase stream 22, after an optional decrease in pressure in the valve or turbine 23 (stream 24), is introduced into the phase separator 25.

The liquid phase 29 obtained from the separator 25 of the phases, after an optional decrease in pressure, is reheated using heat exchangers 27, then 21 and finally 2 to return to the effluent stream 30 a gas with a high methane content obtained at the outlet of the process. Exit stream 30 contains less than 5 mol.% Nitrogen.

The gaseous phase 26 obtained from the separator 25 is partially condensed in the heat exchanger 27 and then its pressure is reduced at the outlet of the heat exchanger 27 using a turbine or valve 28, after which it is introduced into the distillation column 31.

- 5,035,004

The distillation column 31 is a nitrogen stripping column, the purpose of which is to separate nitrogen from the methane-rich liquid at the outlet, also called a de-nitriding column. The methane-enriched liquid contains less than 5 mol.% Nitrogen. In this case, a distillation column is considered, combined with a reboiler 32, but without an associated condensation system.

Stream 33 with a high methane content in liquid form is recovered in the bottom of the column 31 at a temperature below −100 ° C., preferably below −110 ° C. This stream 33 contains less than 5 mol.% Nitrogen, preferably less than 4%. Then, the liquid stream 33 is mixed with the liquid phase 29 obtained from the phase separator 25 and transferred in the same way to the effluent 30.

A stream of gases with a high nitrogen content 36 is obtained at a temperature below -110 ° C. in the upper part 35 of the column 31. Said stream 36 with a high nitrogen content contains at least 20 mol% of nitrogen.

High nitrogen stream 36 is reheated using sequential heat exchangers 27, 21, then 2. It can also be the same heat exchanger in accordance with a particular embodiment of the present invention. Moreover, in accordance with another specific embodiment of the present invention, more than three heat exchangers can be used.

This then leads to a stream 37 at a temperature close to ambient temperature (typically above -10 and below 50 ° C), sent to additional system B to remove nitrogen.

The purpose of system B for de-nitriding is to produce a gas stream with an even higher nitrogen content than stream 37. This system B may, for example, include at least one separator and one column for de-nitriding.

If the nitrogen requirements at the outlet of system B are strict (typically <100 ppm), it may be necessary to add a cycle compressor, such as a nitrogen or methane compressor, to system B to ensure that the necessary portion of the reflux is returned to produce a nitrogen purity at the top of the decontamination column B. systems

This drawing depicts a specific NRU installation, but the method that is an object of the present invention applies to any type of NRU installation downstream of the NGL installation.

Even more preferably, the method, which is the object of the present invention, provides the opportunity to reduce costs in relation to, for example, electricity consumption. This is because only part of the methane contained in the gas to be treated is transferred to the NRU unit, while the other part, which is in the cube of the first distillation column in liquid form, does not contain nitrogen, so that the NRU unit is downstream regarding the installation of NGL is much less loaded.

This corresponds to a reduction in energy consumption costs of about 10 to 30%.

Advantageously, the method which is the object of the present invention allows, in relation to the device in which it will be used for processing, to avoid reduced productivity due to recirculated flow coming from outside, which is necessary in conventional methods known from the prior art: only the outgoing stream is used processing a mixture of 1 gases. This may correspond to a reduction in gas consumption, usually used to return reflux in distillation columns, of the order of 10%.

Claims (11)

CLAIM 1. The method of separation of the components to be processed mixture (1) of gases containing methane, nitrogen and at least one hydrocarbon that contains at least two carbon atoms, or a mixture of such hydrocarbons, comprising the following stages: a) introducing the gas mixture (1) to be treated into the first distillation column (7) to obtain in the cube (38) of the column (7) the first liquid stream (39) enriched in a hydrocarbon containing at least two carbon atoms, and in the upper part (14) columns (7) of a first methane-rich gas stream (15); b) introducing said first liquid stream (39) enriched in a hydrocarbon containing at least two carbon atoms obtained in step a) into a second distillation column (7 ') to obtain this column (7') in the upper part (14 ') ) a second stream (15 ') of gases with a high methane content and in the cube (16) of this column (7') of the second liquid stream (12) containing at least 85 mol.% hydrocarbons containing at least two carbon atoms, initially present in the mixture to be processed (1); c) introducing said first methane-enriched gas stream (15) obtained in step a) into a de-nitration unit (A) to separate nitrogen from the remaining components of this gas stream (15); characterized in that - 6 035004 part (44) of the second stream (15 ') of gases with a high methane content obtained in stage b), at the outlet of the upper part (14') of the second distillation column (7 ') is compressed to a pressure value exceeding at least at least 1 bar the pressure value in the specified second distillation column (7 '), and then condensed to introduce one part (18a) into the upper part (42) of the first distillation column (7) and the other part (18c) into the upper part (41) of the second distillation column (7 ') to ensure the return of reflux in the specified distillation columns (7 and 7'); the second gas stream (15 ′) obtained in step b) is not treated in the de-nitration unit (A). 2. The method according to the preceding paragraph, characterized in that the pressure of a part of the second gas stream (15 ′) in the turbine is reduced to a pressure value that is at least 1 bar lower than the pressure value in said second distillation column (7 ′). 3. A method according to any one of the preceding paragraphs, characterized in that the nitrogen content in said second gas stream (15 ') is at least 1.5 times less than the nitrogen content in the first methane-rich gas stream (15). 4. The method according to any one of the preceding paragraphs, characterized in that from 5 to 30 mol% of methane, which is initially present in the gas mixture (1) to be treated, is included in the first liquid stream (39) enriched in hydrocarbon containing at least at least two carbon atoms obtained in stage a). 5. The method according to any one of the preceding paragraphs, characterized in that from 10 to 20 mol.% Methane, which is initially present in the gas mixture (1) to be treated, is included in the first liquid stream (39) enriched in hydrocarbon containing at least at least two carbon atoms obtained in stage a). 6. The method according to any one of the preceding paragraphs, characterized in that before stage a) it includes the following additional stages: at least partial condensation of said gas mixture (1) to be treated to obtain a two-phase mixture (3); injecting a vapor phase (8) of said two-phase mixture (3) into said first distillation column (7); injection of a liquid phase (5) of said two-phase mixture (3) into said second distillation column (7 '). 7. The method according to any one of the preceding paragraphs, characterized in that the gas stream (15) that is extracted from the first distillation column (7) in step a) contains no more than half of the amount of hydrocarbons containing more than two carbon atoms present in the feed gas (1). 8. The method according to any one of the preceding paragraphs, characterized in that the liquid (39), which is extracted from the specified first distillation column (7) in stage a), contains at least 90 mol.% Hydrocarbons containing at least two carbon atoms , and preferably contains at least 95%. 9. The method according to any one of the preceding paragraphs, characterized in that the gas mixture to be treated (1) contains 70 mol.% Methane, at least 4 mol.% Nitrogen and 2 mol.% Hydrocarbons containing at least two carbon atoms . 10. The method according to any one of the preceding paragraphs, characterized in that during stage b) a liquid stream (39) enriched in a hydrocarbon containing at least two carbon atoms obtained in stage a) is introduced into said second distillation column (7 ' ) one theoretical stage (10) below the upper part (14 ') of this second column (7'). 11. The method according to any one of the preceding paragraphs, characterized in that the gas stream (15 ') obtained in stage b) is extracted directly from the specified second distillation column (7') at a pressure above 20 bar abs., And he contains 95 mol.% methane.