CN111602020A

CN111602020A - Process and plant for the cryogenic separation of synthesis gas comprising a nitrogen separation step

Info

Publication number: CN111602020A
Application number: CN201880086543.6A
Authority: CN
Inventors: 安托万·埃尔南德斯
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: 2017-12-14
Filing date: 2018-12-13
Publication date: 2020-08-28
Also published as: FR3075067A1; EP3724573A1; US20210080175A1; WO2019115966A1; RU2020121851A; RU2020121851A3; FR3075067B1

Abstract

A method for separating a gas mixture comprising carbon monoxide, nitrogen and hydrogen involves feeding a hydrogen-depleted fluid to a nitrogen rejection column (K2) having an overhead condenser (C1) and a bottom reboiler (R2) to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column, cooling the condenser of the nitrogen rejection column with a nitrogen compressor (V1, V2, V3) through a nitrogen cycle, vaporizing liquid nitrogen (53) from the nitrogen cycle in a heat exchanger of the condenser and returning the nitrogen (55) vaporized in the heat exchanger to the nitrogen compressor.

Description

Process and plant for the cryogenic separation of synthesis gas comprising a nitrogen separation step

The invention relates to a method and a device for the cryogenic separation of synthesis gas containing nitrogen. This gas typically contains carbon monoxide, hydrogen, methane and nitrogen. It preferably contains less than 50 mol% methane. It preferably contains more than 10 mol% of carbon monoxide.

The unit for the production of carbon monoxide and hydrogen can be divided into two parts:

·-synthesis gas(mainly containing H)₂、CO、CH₄、CO₂Ar and N₂Mixture of (d) was generated. Among the various industrial routes for the production of synthesis gas, the route based on coal gasification seems to be increasingly enlarged, especially in countries rich in coal deposits, such as china. For H alone or at low₂The process for producing CO, partially oxidising natural gas, at a/CO production ratio may also be advantageous. Another route is steam reforming.

·Purification of synthesis gas. The following was found:

a unit for washing with a liquid solvent in order to remove most of the acid gases present in the synthesis gas;

a unit for purification on a bed of adsorbent;

cold box for the production of CO.

In the case where synthesis gas is produced by entrained flow or fluidized bed gasification, the cold box process is partial condensation. In the case where the syngas is contaminated with methane, for example, for MEG, TDI/MDI or PC applications, the cold box must include CH₄And (5) separating the column. In the case where the syngas is contaminated with nitrogen, for example, if nitrogen is used to transport coal, the cold box must include a nitrogen separation column.

In DE19541339, CO/N₂The column is located in CO/CH₄Upstream of the column. CO/CH₄Reboiling of the column is provided by condensation of recycle nitrogen. CO/N₂Condensation at the top of the column is provided by vaporization of the N2 liquid from the low pressure cycle.

In the presence of CO/N₂And CO/CH₄The nitrogen vaporized in the column condenser was returned to the inlet of the nitrogen recycle compressor.

CO/N₂The column was run at a relatively low pressure (2.6 bar).

CO/CH₄The column pressure is relatively low.

CO/N₂Overhead condensation of the column from CO/CH by gasification₄The bottom of the column and, in addition, is provided by reheating a hydrogen-rich fraction from the vessel for partially condensing the synthesis gas.

CO/N₂The CO product at the column outlet is returned to the inlet of the CO compressor for compression to the desired pressure.

CO/N₂The top condenser of the column has a large volume, since the make-up is supplied by the reheating of the hydrogen, and therefore has a high gas flow: the exchanger must therefore be aligned to the CO/N₂The top of the column is placed at a height, the large volume of which would make it difficult to transport a column containing CO/N₂A pack of columns.

This configuration results in high energy consumption at the level of the CO recycle compressor, since the CO produced must be compressed.

This requires the purchase of a CO compressor, which is compared to N₂The compressor is more expensive.

When entering CH in syngas₄And N₂When the amount of (A) is varied, CO/N₂Condenser and CO/CH₄The coupling of the reboiler causes difficulties in the operation of the unit.

DE2814600 describes a separation process which utilizes a column for methane removal followed by a dual column, wherein the top of the CO/argon separation column heats the bottom of the denitrification column.

After the nitrogen recycle liquid expands and vaporizes, the overhead denitrogenation condenser vaporizes the liquid from the bottom of the denitrogenation column. In contrast, in column 26 or in column 13, the nitrogen recycle is not used as a refrigerant liquid for condensing the separated overhead from the CH4 separation column. The top of CH4 separation column 13 was cooled with hydrogen.

This results in a larger exchanger at the top of column 13 which takes up more space in the cold box and is therefore more difficult to transport. Furthermore, the cold supply was insufficient and CH4 remained in the fluid sent to the second column, whereas according to the invention, a single column removed all CH 4.

DE2814660 has a N2 loop to the reboiler from the bottom of an argon/CO column, the pressure of which is also higher than that of the present invention, wherein the reboiling of the CO/CH4 column is carried out by synthesis gas.

According to this prior art, reboiling of the CO/N2 separation is provided by N2 recycle through the bottom reboiler of the argon/CO column, thus requiring a higher pressure than according to the present invention where reboiling of nitrogen only at the pressure of the CO/N2 column is required.

According to one subject of the invention, a process is provided for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and optionally methane, wherein:

i) the mixture is cooled in a heat exchanger,

ii) separating the mixture cooled in the heat exchanger by at least one washing and/or distillation and/or partial condensation step to form a hydrogen-depleted gas stream containing carbon monoxide and nitrogen,

iii) passing the hydrogen-depleted stream to a denitrification column having an overhead condenser and a bottoms reboiler to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column,

iv) cooling the condenser of the denitrification column by nitrogen recycle using a nitrogen compressor having at least a first stage and a second stage, the inlet pressure of the first stage being lower than the inlet pressure of the second stage,

v) expanding the bottom liquid from the denitrogenation column and feeding it to the overhead condenser of the denitrogenation column to at least partially vaporize it by heat exchange with the nitrogen-rich gas in the heat exchanger of the condenser, thereby condensing the nitrogen-rich gas,

vi) vaporizing liquid nitrogen from the nitrogen cycle also in the heat exchanger of the condenser and returning the vaporized nitrogen to the heat exchanger at the inlet of the second stage of the nitrogen compressor, and

a) the bottom liquid from the denitrogenation column is sent to a methane and carbon monoxide separation column comprising a top condenser which is a bath gasifier placed in a liquid bath, or

b) The separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column to separate a methane depleted fluid from a methane rich fluid, and at least a portion of the methane depleted fluid constitutes the hydrogen depleted fluid that supplies the denitrogenation column, the methane and carbon monoxide separation column comprising an overhead condenser, the overhead condenser being a bath gasifier placed in a liquid bath,

a) the liquid bath of b) is supplied with liquid nitrogen from the nitrogen gas cycle.

According to other optional aspects of the invention:

-the mixture contains methane.

-the separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column to separate a methane depleted fluid from a methane rich fluid, and at least a portion of the methane depleted fluid constitutes the hydrogen depleted fluid supplying the denitrogenation column.

-sending the bottom liquid from the denitrification column to a methane and carbon monoxide separation column.

-the methane and carbon monoxide separation column comprises an overhead condenser, which is a bath vaporizer placed in a liquid bath.

-the top condenser of the methane and carbon monoxide separation column is supplied with liquid nitrogen from the nitrogen cycle.

-feeding liquid nitrogen from the top condenser of the methane and carbon monoxide separation column to be gasified in the top condenser of the denitrogenation column.

-separating the mixture cooled in the heat exchanger by at least one partial condensation step, so as to form a hydrogen-depleted gas, sending the hydrogen-depleted gas to an intermediate level of a stripping column having a bottom reboiler, and sending the bottom liquid from the stripping column to a denitrogenation column in case a) or to the methane and carbon monoxide separation column in case b).

-reheating the reboiler of the stripping column and/or the reboiler of the methane and carbon monoxide separation column with at least a portion of the gas mixture.

-the operating pressure of the denitrification column is at least 7 bar absolute or even 8 bar absolute.

-the operating pressure of the methane and carbon monoxide separation column is at least 5 bar absolute or even 6 bar absolute.

The overhead condenser of the CO/CH4 column is cooled only by circulating nitrogen.

-the reboiler of the denitrification column is reheated by the circulating nitrogen.

-the nitrogen used to reheat the reboiler of the denitrification column is at the maximum pressure of the nitrogen cycle.

-condensing the nitrogen in the bath of the condenser sent to the CO/CH4 column at the maximum pressure of the nitrogen cycle.

According to another subject of the invention, there is provided an apparatus for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and optionally methane, the apparatus comprising: a heat exchanger for cooling the mixture; means for separating the mixture cooled in the heat exchanger by at least one washing and/or distillation and/or partial condensation step to form a hydrogen-depleted stream comprising carbon monoxide and nitrogen; a denitrification column with an overhead condenser and optionally a bottom reboiler; a pipe for sending the hydrogen-depleted fluid to the denitrification column to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column; a nitrogen recycle using a nitrogen compressor having at least a first stage and a second stage, the first stage having an inlet pressure lower than the inlet pressure of the second agent; means for feeding the liquid of the nitrogen gas recycle to the condenser of the denitrification column; means for expanding the bottoms liquid from the denitrification column; means for passing the expanded liquid to the overhead condenser of the denitrification column to at least partially vaporize the nitrogen-rich gas by heat exchange with the nitrogen-rich gas in a heat exchanger of the condenser, thereby condensing the nitrogen-rich gas; means for feeding the nitrogen gas vaporized in the heat exchanger to the inlet of the second stage of the nitrogen compressor; a methane and carbon monoxide separation column comprising a top condenser, the top condenser being a bath vaporizer placed in a liquid bath;

a) means for feeding the bottom liquid from the denitrification column to the methane and carbon monoxide separation column, or

b) Methane and carbon monoxide separation column means forming part of the means for separating the mixture cooled in the heat exchanger by at least one distillation step,

the apparatus further includes means for sending liquid nitrogen from the nitrogen recycle to the overhead condenser of the methane and carbon monoxide separation column.

The apparatus may include means for sending liquid nitrogen from the overhead condenser of the methane and carbon monoxide separation column to the overhead condenser of the denitrification column.

The apparatus may include: at least one phase separator for separating the mixture cooled in the heat exchanger by a partial condensation step to form a hydrogen-depleted gas; a stripping column; and means for sending the hydrogen-depleted gas to an intermediate level of the stripper column.

According to the invention, CO/CH₄Reboiling of the column is by cooling the syngas, while in dehx it is by condensing the following cycles: the advantage of this in our scheme is that it allows to increase N without increasing it₂Increasing CO/CH at compressor discharge pressure₄The pressure of the column.

CO/N₂The condenser at the top of the column is operated by gasifying CO/N after expansion₂At least part of the bottom liquid of the column and also by vaporizing N under moderate pressure₂Liquid to cool. In the prior art, N from the cycle is vaporized at low pressure₂Liquid to cool. The vaporization of the expanded bottom liquid makes it possible to significantly reduce the N to be vaporized in the condenser₂Recycle stream, thereby reducing N₂Recycle stream and thus N₂The power of the compressor is cycled. In the prior art, the nitrogen flow is relatively high with respect to the CO flow produced.

According to the invention, in CO/N₂And CO/CH₄The nitrogen gas vaporized in the column condenser is returned to N₂Interstage of the compressor, whereas in the prior art it returns to the inlet of the compressor. Thus, the prior art results in an increase in the compression energy of the cycle. In the prior art, stream 54 is combined with the cooling of syngas in E2Required N₂The same pressure (2.4 bar) of the flow is returned, while in our case the N vaporized in the condenser₂Flow ratio of N required for cooling the synthesis gas₂High pressure return N₂A compressor. This is possible in this case, since the CO/N ratio is such that it is comparable to the prior art (2.6 bar)₂The column is operated at a higher pressure (at least 7 bar, e.g. 8.5 bar).

FIG. 3 of-DE 102012020469 includes means for increasing CO/CH₄The pump of the pressure of the column, but still low (3.6 bar) with respect to the solution according to the invention (at least 5 bar, or even at least 6 bar), and included in figure 3, from CO/CH₄The N2 liquid from the column condenser is at low pressure and is returned to the inlet of the compressor, whereas in our case, it comes from the CO/CH₄Nitrogen return to column overhead condenser to N₂Interstage (at a higher pressure than the nitrogen used to cool the syngas in the main exchanger) compressor.

As according to the invention, CO/N₂The column is at a higher pressure and can produce CO directly without recompression.

For condensing CO/N₂The energy at the top of the column is vaporized by the expanded bottom liquid and additionally by the energy from N₂Recycled low pressure nitrogen is provided by gasification. This reduces the size of the overhead condenser and allows transport of the product including CO/N₂Column and its overhead condenser package.

The invention will be described in more detail with reference to two figures, each representing a separation method according to the invention.

In fig. 1, a gas mixture 1, for example resulting from coal gasification, comprises carbon monoxide, hydrogen, methane, water and nitrogen. Gas 1 is purified in adsorbent beds 3A, 3B and cooled in cooler 4. It is then sent to a first heat exchanger E1 for cooling. This partial flow of syngas is used to reheat the reheaters R1, R2 (drawn twice in different positions in the drawing for clarity). After expansion in the valve, separation takes place in phase separator S1, forming gas 5 and liquid 7. Gas 5 was cooled in heat exchanger E2, expanded and sent to phase separator S4. The hydrogen-rich gas 9 from this phase separator S4 is reheated in heat exchangers E2, E1 and a portion of this gas is used to regenerate the adsorbent beds 3A, 3B. A portion 11 of the liquid from the phase separator S4 was expanded and sent to the top of a stripper column K1 operating at 17.6 bar. Column K1 has no overhead condenser but has a bottom reboiler R1. The remaining portion 13 of the liquid from phase separator S4 was expanded and sent to phase separator S3. The top gas 17 from column K1 is reheated in exchangers E1, E2.

Liquid 7 from phase separator S1 was mixed with other fluid (overhead gas from separator S3 derived from liquid 13 from separator S4) to form stream 8, which was sent to phase separator S2 and then to an intermediate level of stripping column K1.

After vaporization in exchanger E2, the gas from phase separator S3 and the liquid from phase separator S3 were mixed with stream 7 to supply column K1.

The bottom liquid 19 from column K1 was taken at-154 ℃, expanded at 8.3 bar and sent to phase separator S5, and the gas and liquid from the phase separator were sent to CO/N operating at 8.3 bar₂Middle level of column K2. Column K2 has a top condenser C1 consisting of a plate heat exchanger and a bottom reboiler R2.

The top gas 27 from column K2 is partially condensed in condenser C1 and the resulting liquid L, 29, is returned to the top of column K2 and partially, and the remaining nitrogen-rich gas V is reheated in exchangers E2, E1 as gas 31.

The liquid 53 from the overhead condenser C2 of column K3 is vaporized in condenser C1 by heat exchange with gas 27 to form gas 55, which is sent to the inlet of compressor V3.

The carbon monoxide rich and nitrogen lean bottom liquid 33 is split into two 21, 35 and expanded. The expanded portion 21 at 6.5 bar is sent to a phase separator, the liquid portion coming from which is used to cool the condenser C1. Thus, after the liquid nitrogen 53 has been expanded and gasified at moderate pressure, it is passed through the process from the CO/N₂Vaporization of at least a portion of the bottoms liquid 33 of column K2 to cool the CO/N₂Overhead condenser C1 of column K2. Vaporizing the bottom liquid 33 after expansion can be significantly reducedThere is less nitrogen recycle stream to be vaporized in condenser C1, thereby reducing the nitrogen recycle stream and hence the power of the nitrogen recycle compressors V1, V2, V3.

The remaining part of the liquid from separator S8 and fraction 35 were supplied with CO/CH after passing through phase separator S6₄Column K3, the gas and liquid from the phase separator are sent to different intermediate levels of column K3.

Column K3 has a top condenser C2 consisting of a plate heat exchanger for gasification arranged in a liquid bath and a bottom reboiler R3. The top gas rich in carbon monoxide is condensed in condenser C2 and the bottom liquid rich in methane 39 is expanded and reheated in exchanger E1. Column K3 worked at 6.6 bar.

The plate exchangers are surrounded by an annular barrier forming an overflow wall P. Thus, the liquid surrounding the exchanger can pass over the barrier P and be withdrawn as

liquids

43, 53.

The overhead condenser C2 of column K3 was cooled by compressed and expanded nitrogen 59 from nitrogen recycle compressors V1, V2, V3 after cooling in exchangers E1, E2. The vaporized nitrogen is returned upstream of the last stage V3 of the nitrogen recycle compressor. Nitrogen at the outlet pressure of stage V3 was also used to reboil reboiler R2 of column K2.

Reboilers R1 and R3 of columns K1 and K3 were reheated by a partial stream of feed 1 downstream of exchanger E1 and upstream of phase separator S1. This CO/CH is produced by cooling the synthesis gas₄Column K3 reboiling has the following advantages: the column CO/CH can be increased without increasing the outlet pressure of the nitrogen recycle compressor₄The pressure of (a). The partial stream sent to the reboiler R1, R3 is at the same temperature and the same pressure.

Liquid nitrogen 53 from the bottom of condenser C2 of column K3 is sent to be vaporized in condenser C1 of column K3 and then returned downstream of stage V2 and upstream of stage V3. Thus, in CO/N₂Column K2 and CO/CH₄The nitrogen vaporized in the condensers C1, C3 of column K3 is returned interstage the nitrogen compressors V1, V2; the vaporized N2 stream 57 in the condensers C1, C2 is at a ratio of N required to cool the syngas₂The higher pressure of the flow returns to N₂A compressor. In this case, this is achieved by operating the CO/N at a higher pressure (8.5 bar) than in the prior art (2.6 bar)₂Column K2 makes this possible.

The carbon monoxide rich gas 41 leaves column K3 at-170.4 ℃ at 6.6 bar and is reheated in heat exchangers E1, E2. Preferably, no carbon monoxide compressor is used. It constitutes the product of the process and is not yet compressed.

The supply of liquid nitrogen 69 makes it possible to compensate for leaks from the nitrogen circulation. The formed liquid sent to phase separator S7 was vaporized in exchanger E2 and mixed with the gas from separator S7 and sent to the inlet of compressor V1.

A portion 47 of the liquid nitrogen in condenser C2 is expanded and sent to separator S7, and the gas 49 formed enters at the inlet of compressor V1.

Another portion 45 of the same liquid is expanded at a lower pressure and sent to the outlet of compressor V1 and the inlet of compressor V2.

The operating pressure of the denitrification column K2 is at least 7 bar absolute or even 8 bar absolute; the operating pressure of the methane and carbon monoxide separation column K3 is at least 5 bar absolute or even 6 bar absolute.

In fig. 2, the order of nitration and methane and carbon monoxide separation columns is reversed.

Thus, after separation by the phase separator S5, the liquid 19 from the bottom of the stripping column is not sent to the denitrogenation column, but is sent to the CO/CH₄Middle point of separation column K3.

CO/CH₄Column K3 has a bottom reboiler R3 heated by the feed and a top condenser C2 for condensing the top gas 51 which is returned in condensed form to column K3. The condenser is cooled with condensed nitrogen 61, 63 produced by condensing recycled nitrogen 59 from compressor V3 in exchangers E1, E2 and reboiler R2. The liquid is partially vaporized, producing gas 55 (which returns to the inlet of compressor V3) and liquid (flowing through barrier P). A portion 31 of this liquid is vaporized in exchanger E2 and returned to the inlet of compressor V3. As previously mentioned, the other section 53 is used for coolingOverhead condenser C1 of column K2.

The bottom methane 39 of column K3 is reheated in exchanger E1 to leave the plant as product. The top gas 26, which is rich in carbon monoxide and contains nitrogen, leaves to the middle of the denitrification column K2.

Column K2 had a top condenser C1 consisting of a plate heat exchanger and a bottom reboiler R2 heated by circulating nitrogen. The top gas 27 from column K2 is partially condensed in condenser C1 and the formed liquid L, 29, is returned to the top of column K2 and partially and the remaining nitrogen-rich gas V is reheated in exchangers E2, E1 as gas 31.

The carbon monoxide rich and nitrogen lean bottoms liquid 21 is expanded. This liquid at 6.5 bar was sent to a phase separator, the liquid fraction from which was used to cool the condenser C1. Thus, after the liquid nitrogen 53 has been expanded and gasified at moderate pressure, it is passed through the process from the CO/N₂Vaporization of at least a portion of the bottoms liquid 33 of column K2 to cool the CO/N₂Overhead condenser C1 of column K2. Vaporizing the bottom liquid 33 after expansion can significantly reduce the nitrogen recycle stream to be vaporized in condenser C1, thereby reducing the nitrogen recycle stream and hence the power of the nitrogen recycle compressors V1, V2, V3.

Gas 31 is the carbon monoxide rich product of the process.

Claims

1. A process for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and methane, wherein:

i) cooling the mixture in a heat exchanger (E1, E2),

iii) feeding the hydrogen-depleted gas stream to a nitrogen removal column (K2) having an overhead condenser (C1) and a bottoms reboiler (R2) to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column,

iv) cooling the condenser of the denitrogenation column by nitrogen recycle using a nitrogen compressor (V1, V2, V3) having at least a first stage and a second stage, the inlet pressure of the first stage being lower than the inlet pressure of the second stage,

v) expanding the bottom liquid (21) from the denitrification column and feeding it to the overhead condenser of the denitrification column to at least partially vaporize it by heat exchange with the nitrogen-rich gas in the heat exchanger of the condenser, thereby condensing the nitrogen-rich gas,

vi) vaporizing liquid nitrogen (53) from the nitrogen cycle also in the heat exchanger of the condenser and returning the vaporized nitrogen (55) to the heat exchanger at the inlet of the second stage (V3) of the nitrogen compressor, and

a) the bottom liquid (33) from the denitrogenation column (K2) is sent to a methane and carbon monoxide separation column (K3) comprising a top condenser (C2) which is a bath gasifier placed in a liquid bath, or

b) The separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column (K3) to separate a methane depleted fluid from a methane rich fluid, and at least a portion of the methane depleted fluid (41) constitutes the hydrogen depleted fluid that supplies the denitrogenation column, the methane and carbon monoxide separation column (K3) comprising an overhead condenser (C2), which is a bath gasifier placed in a liquid bath,

a) or b) is supplied with liquid nitrogen (61, 63, 65) from the nitrogen gas circuit (59).

2. The process of claim 1, wherein liquid nitrogen (53) from the top condenser (C2) of the methane and carbon monoxide separation column is fed to be gasified in the top condenser (C1) of the denitrogenation column.

3. The process of any one of the preceding claims, wherein the mixture cooled in the heat exchanger (E1, E2) is separated by at least one partial condensation step to form a hydrogen-depleted gas (5), which is fed to an intermediate level of a stripping column (K1) having a bottom reboiler (R1), and the bottom liquid (19) of the stripping column is fed in case a) to a denitrification column (K2) or in case b) to the methane and carbon monoxide separation column (K3).

4. The method as claimed in one of the preceding claims, wherein the reboiler (R1) of the stripping column (K1) and/or the reboiler (R3) of the methane and carbon monoxide separation column (K2) is reheated with at least a part of the gas mixture (1).

5. The method as claimed in one of the preceding claims, wherein the operating pressure of the denitrification column (K2) is at least 7 bar absolute or even 8 bar absolute and/or the operating pressure of the methane and carbon monoxide separation column (K3) is at least 5 bar absolute or even 6 bar absolute.

6. The process of one of the preceding claims, wherein the overhead condenser (C2) of the CO/CH4 column (K3) is cooled only by circulating nitrogen.

7. The method of one of the preceding claims, wherein the reboiler (R2) of the denitrogenation column (K2) is reheated by the circulating nitrogen.

8. The method of claim 7, wherein the nitrogen for reheating the reboiler (R2) of the denitrogenation column (K2) is at the maximum pressure of the nitrogen cycle.

9. The method of one of the preceding claims, wherein nitrogen in the bath of the condenser (C2) sent to the column CO/CH4(K3) is condensed at the maximum pressure of the nitrogen cycle.

10. The process of any one of the preceding claims, wherein the operating pressure of the methane and carbon monoxide separation column is at least 5 bar absolute or even 6 bar absolute.

11. An apparatus for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and methane, the apparatus comprising: a heat exchanger (E1, E2) for cooling the mixture; means for separating the mixture cooled in the heat exchanger by at least one washing and/or distillation and/or partial condensation step to form a hydrogen-depleted stream comprising carbon monoxide and nitrogen; a denitrogenation column (K2) with an overhead condenser (C1) and optionally a bottom reboiler (R2); a pipe for sending the hydrogen-depleted fluid to the denitrification column to produce a nitrogen-enriched gas at the top of the column and a nitrogen-depleted liquid at the bottom of the column; a nitrogen cycle using a nitrogen compressor (V1, V2, V3) having at least a first stage and a second stage, the inlet pressure of the first stage being lower than the inlet pressure of the second agent; means for sending the liquid of the nitrogen recycle to the condenser (C1) of the denitrification column; means for expanding the bottom liquid (21) from the denitrification column; means for passing the expanded liquid to the overhead condenser of the denitrification column to at least partially vaporize the nitrogen-rich gas by heat exchange with the nitrogen-rich gas in a heat exchanger of the condenser, thereby condensing the nitrogen-rich gas; means for feeding the nitrogen (55) vaporized in the heat exchanger of the condenser to the inlet of the second stage (V3) of the nitrogen compressor; a methane and carbon monoxide separation column (K3) comprising an overhead condenser (C2), which is a bath vaporizer placed in a liquid bath;

a) means for feeding the bottom liquid (33) from the denitrification column to the methane and carbon monoxide separation column, or

the apparatus further comprises means for sending liquid nitrogen (61, 63, 65) from the nitrogen recycle to the overhead condenser of the methane and carbon monoxide separation column.

12. The plant of claim 11, comprising means for sending liquid nitrogen (53) from the top condenser (C2) of the methane and carbon monoxide separation column (K3) to the top condenser (C1) of the denitrogenation column (K2).

13. The apparatus of any of the preceding claims 11 and 12, comprising at least one phase separator (S1, S3) for separating the mixture cooled in the heat exchanger by a partial condensation step to form a hydrogen-depleted gas; a stripper column (K1); and means for sending the hydrogen-depleted gas to an intermediate level of the stripper column.