US20210080175A1

US20210080175A1 - Method and apparatus for the cryogenic separation of a synthesis gas containing a nitrogen separation step

Info

Publication number: US20210080175A1
Application number: US16/772,357
Authority: US
Inventors: Antoine Hernandez
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: 2021-03-18
Also published as: RU2020121851A3; FR3075067B1; WO2019115966A1; RU2020121851A; EP3724573A1; FR3075067A1; CN111602020A

Abstract

A method for separating a gas mixture comprising carbon monoxide, nitrogen and hydrogen involves sending a hydrogen-depleted fluid to a denitrification column (K2) having a top condenser (C1) and a bottom reboiler (R2) in order 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 denitrification column by means of a nitrogen cycle using a nitrogen compressor (V1, V2, V3), vaporising, in the heat exchanger of the condenser, the liquid nitrogen (53) from the nitrogen cycle, and returning the nitrogen (55) vaporised in the heat exchanger to the nitrogen compressor.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application No. PCT/FR2018/053280, filed Dec. 13, 2018, which claims priority to French Patent Application No, 1762148, filed Dec. 14, 2017, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process and to an apparatus for cryogenic separation of a synthesis gas containing nitrogen. This gas commonly contains carbon monoxide, hydrogen, methane, and nitrogen. It preferably contains less than 50 mol % of methane. It preferably contains more than 10 mol % of carbon monoxide.
Units for producing carbon monoxide and hydrogen can be separated into two parts:

- Generation of Synthesis Gas (mixture containing H₂, CO, CH₄, CO₂, Ar, and N₂, essentially). Among the various industrial routes for the production of synthesis gas; that based on coal gasification appears to be increasingly expanding, in particular in countries rich in deposits of coal, such as China. The process of partial oxidation of natural gas may also be advantageous for production of CO, alone or with low H₂/CO production ratios. Another route is steam reforming.
- Purification of Synthesis Gas. The following are found:
  - a unit for scrubbing 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 adsorbents;
  - a unit for cryogenic separation, referred to as a cold box, for producing CO.

In the case where the synthesis gas is produced from an entrained bed or fluidized bed coal gasification, the cold box process is partial condensation. In the case where the synthesis gas is contaminated with methane, for MEG, TDI/MDI or PC applications; for example, it is necessary for the cold box to include a CH₄separation column. In the case where the synthesis gas is contaminated with nitrogen, if the nitrogen is used for transporting coal, for example, it is necessary for the cold box to include a nitrogen separation column.
In DE19541339, a CO/N₂column is sited upstream of a CO/CH₄column. Reboiling of the CO/CH₄column is provided by condensation of cycle nitrogen. The condensation at the CO/N₂column top is provided by vaporization of N2 liquid from the cycle at low pressure.
The nitrogen vaporized in the condensers of the CO/N₂and CO/CH₄columns returns to the intake of the compressor of the nitrogen cycle.
The CO/N₂column operates at a relatively low pressure (2.6 bar).
The pressure of the CO/CH₄column is relatively low.
The top condensation of the CO/N₂column is provided by vaporization of the bottoms from the CO/CH₄column and, in addition, by reheating of the hydrogen-rich fraction from the vessel for partial condensation of the synthesis gas.
The CO product at the outlet of the CO/N₂column goes back to the intake of the CO compressor for compression to the required pressure.
The top condenser of the CO/N₂column has a substantial volume, since the complement is supplied by reheating of the hydrogen, and hence a high gas flow: since this exchanger must be positioned in height relative to the top of the column CO/N₂column, its substantial volume would make it difficult to transport the packet containing the CO/N₂column.
The configuration leads to high energy consumption at the level of the CO cycle compressor, because the CO produced has to be compressed.
This necessitates the procurement of a CO compressor, which is more expensive than an N₂compressor.
The coupling of the CO/N₂condenser and CO/CH₄reboiler causes the unit operating difficulties when the amounts of CH₄and N₂in the incoming synthesis gas are varied.
DE2814660 describes a separation process utilizing a column for removal of methane, followed by a double column in which the top of the CO/argon separation column heats the bottom of the denitrification column.
The top denitrification condenser vaporizes the liquid from the bottom of the denitrification column after expansion and vaporization of a nitrogen cycle liquid. Conversely, the nitrogen cycle is not used as a refrigerant fluid for condensing the separation top from a CH₄separation column, either in the column 26 or in the column 13. The top of the CH₄separation column 13 is cooled with hydrogen.
This results in a larger exchanger at the top of the column 13, which takes up more space in the cold box packet and is therefore more difficult to transport. Moreover, the supply of cold is insufficient, and CH4 remains in the fluid sent to the second column, whereas, according to the invention, a single column removes all of the CH4.
DE2814660 has an N2 circuit to the reboiler of the bottoms from the argon/CO column, which is also at a pressure greater than that of the present invention, where the reboiling of the CO/CH4 column is performed by the synthesis gas.
According to this prior art, the reboiling of the CO/N2 separation is provided by the N2 cycle via the bottom reboiler of the argon/CO column, thus requiring a higher pressure than that according to the invention, where there is a need for reboiling nitrogen solely at the pressure of the CO/N2 column.

SUMMARY

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

- i) the mixture is cooled in a heat exchanger,
- ii) the mixture cooled in the heat exchanger is separated by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen,
- iii) the hydrogen-depleted fluid is sent to a denitrification column having a top condenser and a bottom reboiler, to produce a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom,
- iv) the condenser of the denitrification column is cooled by means of a nitrogen cycle using a nitrogen compressor having at least a first stage and a second stage, the entry pressure of the first stage being lower than that of the second stage,
- v) bottom liquid from the denitrification column is expanded and sent to the top condenser of the denitrification column for at least partial vaporization by heat exchange in a heat exchanger of the condenser with the nitrogen-enriched gas, which is thereby condensed,
- vi) the liquid nitrogen from the nitrogen cycle is also vaporized in the heat exchanger of the condenser and the vaporized nitrogen is returned to the heat exchanger at the inlet of the second stage of the nitrogen compressor, and
  - a) bottom liquid from the denitrification column is sent to a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, or
  - b) the separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column for separating a methane-depleted flow from a methane-enriched flow, and at least a portion of the methane-depleted flow constitutes the hydrogen-depleted fluid supplying the denitrification column, the methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, the bath of liquid of a) or b) being supplied with liquid nitrogen from the nitrogen cycle.

According to other, optional aspects of the invention:

- the mixture contains methane.
- the separation in step ii) comprises a step of distillation in a methane and carbon monoxide separation column, to separate a methane-depleted flow from a methane-enriched flow, and at least a portion of the methane-depleted flow constitutes the hydrogen-depleted fluid supplying the denitrification column.
- bottom liquid from the denitrification column is sent to a methane and carbon monoxide separation column.
- the methane and carbon monoxide separation column comprises a top condenser which is a bath vaporizer placed in a bath of liquid.
- the top condenser of the methane and carbon monoxide separation column is supplied with liquid nitrogen from the nitrogen cycle.
- liquid nitrogen from the top condenser of the methane and carbon monoxide separation column is sent to vaporize in the top condenser of the denitrification column.
- the mixture cooled in the heat exchanger is separated by at least one partial condensation step, to form a hydrogen-depleted gas, the hydrogen-depleted gas is sent to an intermediate level of a stripping column having a bottom reboiler, and bottom liquid from the stripping column is sent to a denitrification column in case a) or to the methane and carbon monoxide separation column in case b).
- the reboiler of the stripping column and/or the reboiler of the methane and carbon monoxide separation column are reheated with at least a portion of the gas mixture.
- the operating pressure of the denitrification column is at least 7 bar abs or even 8 bar abs.
- the operating pressure of the methane and carbon monoxide separation column is at least 5 bar abs or even 6 bar abs.
- the top condenser of the CO/CH4 column is cooled solely by cycle nitrogen.
- the reboiler of the denitrification column is reheated by means of cycle nitrogen.
- the nitrogen used for reheating the reboiler of the denitrification column is at the maximum pressure of the nitrogen cycle.
- the nitrogen sent to the bath of the condenser of the CO/CH4 column is condensed at the maximum pressure of the nitrogen cycle.

According to another subject of the invention, an apparatus is provided for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen, and optionally methane, comprising a heat exchanger for cooling the mixture, means for separating the mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen, a denitrification column having a top 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 column top and a nitrogen-depleted liquid at the column bottom, a nitrogen cycle using a nitrogen compressor having at least a first stage and a second stage, the entry pressure of the first stage being lower than that of the second stage, means for sending liquid of the nitrogen cycle to the condenser of the denitrification column, means for expanding bottom liquid from the denitrification column, means for sending the expanded liquid to the top condenser of the denitrification column for at least partial vaporization by heat exchange in a heat exchanger of the condenser with the nitrogen-enriched gas, which is thereby condensed, means for sending nitrogen 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 which is a bath vaporizer placed in a bath of liquid,

- a) means for sending the bottom liquid from the denitrification column to the methane and carbon monoxide separation column, or
- b) the 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 comprising means for sending liquid nitrogen from the nitrogen cycle to the top condenser of the methane and carbon monoxide separation column.

The apparatus may comprise means for sending liquid nitrogen from the top condenser of the methane and carbon monoxide separation column to the top condenser of the denitrification column.
The apparatus may comprise 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 stripping column.
According to the invention, the reboiling of the CO/CH₄column is by cooling of the synthesis gas, whereas in the DEXX it is by condensation of the cycle: the advantage of this in our scheme is that it allows the pressure of the CO/CH₄column to be increased without increasing the exit pressure of the N₂compressor.
The condenser of the CO/N₂column top is cooled by vaporization of at least a portion of the bottom liquid from the CO/N₂column after expansion and also by vaporization of N₂liquid at medium pressure. In the prior art, the cooling is performed by vaporization of N₂liquid from the cycle at low pressure. The vaporization of the bottom liquid after expansion enables a considerable reduction in the N₂cycle flow to be vaporized in the condenser, thereby reducing the N₂cycle flow and hence the power of the N₂cycle compressor. In the prior art, the nitrogen flow is relatively high relative to the flow of CO produced.
According to the invention, the nitrogen vaporized in the condensers of the CO/N₂and CO/CH₄columns returns to an inter-stage of the N₂compressor, whereas in the prior art it returns to the intake of the compressor. The prior art therefore results in an increase in the compression energy of the cycle. In the prior art, the flow 54 returns at the same pressure (2.4 bar) as the N₂flow required for the cooling of the synthesis gas in E2, whereas in our scheme the N₂flow vaporized in the condensers returns to the N₂compressor at a higher pressure than the N₂flow needed for the cooling of the synthesis gas. This is made possible in this case because the CO/N₂column is operated at a higher pressure (at least 7 bar, for example at 8.5 bar) relative to the prior art (2.6 bar).

- FIG. 3 of DE102012020469 includes a pump for increasing the pressure of the CO/CH₄column, but this pressure remains low (3.6 bar) relative to the schemes according to the invention (at least 5 bar, or even at least 6 bar), and, including in FIG. 3, the N₂liquid from the condenser of the CO/CH₄column is at low pressure and returns to the intake of the compressor, whereas in our scheme the nitrogen from the CO/CH₄column top condenser returns to an inter-stage of the N₂compressor (at a higher pressure than the nitrogen used for cooling the synthesis gas in the main exchanger).

As according to the invention the CO/N₂column is at a higher pressure, it is possible to produce the CO directly without recompression.
The energy for condensing the top of the CO/N₂column is provided by vaporization of the bottom liquid, after expansion, and additionally by vaporization of low-pressure nitrogen from the N₂cycle. This reduces the size of the top condenser and allows the packet containing the CO/N₂column and its top condenser to be transported.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates a schematic representation a separation process in accordance with one embodiment of the present invention.

FIG. 2 illustrates another schematic representation a separation process in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a gas mixture 1, resulting for example from coal gasification, contains carbon monoxide, hydrogen, methane, water, and nitrogen. The gas 1 is purified in adsorbent beds 3A, 3B and cooled in a cooler 4, Then it is sent to a first heat exchanger E1 to be cooled. Partial flows of the synthesis gas are used to reheat the reheaters R1, R2, which are drawn twice at different places in the drawing for reasons of clarity. Following expansion in a valve, separation takes place in a phase separator S1, forming a gas 5 and a liquid 7. The gas 5 is cooled in a heat exchanger E2, expanded, and sent to a phase separator S4, The gas 9 from this phase separator S4, which is rich in hydrogen, is reheated in the heat exchangers E2, E1, and a portion of the gas is used to regenerate the adsorbent beds 3A, 3B, A portion 11 of the liquid from the phase separator S4 is expanded and sent to the top of a stripping column K1 operating at 17.6 bar. The column K1 has no top condenser, but has a bottom reboiler R1. The remainder 13 of the liquid from the phase separator S4 is expanded and sent to a phase separator S3. The top gas 17 from the column K1 is reheated in the exchangers E1, E2.
The liquid 7 from the phase separator S1 mixes with other fluids (top gas from separator S3, derived from the liquid 13 from the separator S4) to form the flow 8, which is sent to a phase separator S2 and then to an intermediate level of the stripping column K1.
The gas from the phase separator S3 and the liquid from the phase separator 53, after vaporization in the exchanger E2, are mixed with the fluid 7 to supply the column K1.
Bottom liquid 19 from the column K1 is taken at −154° C., expanded at 8.3 bar, and sent to the phase separator S5, and the gas and the liquid from the phase separator are sent to an intermediate level of the CO/N₂column K2 operating at 8.3 bar. The column K2 has a top condenser C1, consisting of a plate-type heat exchanger, and a bottom reboiler R2.
The top gas 27 from the column K2 is partially condensed in the condenser C1, and the resulting liquid L, 29, is returned to the top of the column K2 and in part, and the remaining gas V, enriched in nitrogen, is reheated in the exchangers E2, E1 as gas 31.
A liquid 53 from the top condenser C2 of the column K3 is vaporized by heat exchange with the gas 27 in the condenser C1, forming the gas 55, which is sent to the entry of the compressor V3.
The bottom liquid 33 enriched in carbon monoxide and depleted in nitrogen is divided into two, 21, 35, and expanded. An expanded portion 21 at 6.5 bar is sent to a phase separator, the liquid from which is used in part to cool the condenser C1, Accordingly, the top condenser C1 of the CO/N₂column K2 is cooled by vaporization of at least a portion of the bottom liquid 33 from the CO/N₂column K2 after expansion and vaporization of the liquid nitrogen 53 at medium pressure. Vaporizing the bottom liquid 33 after expansion enables a considerable reduction in the nitrogen cycle flow to be vaporized in the condenser C1 thereby reducing the nitrogen cycle flow and hence the power of the nitrogen cycle compressor V1, V2, V3.
The remainder of the liquid from the separator S8 and the fraction 35 supply the CO/CH₄column K3 after passage through a phase separator S6, from which the gas and the liquid are sent to different intermediate levels of the column K3.
The column K3 has a top condenser C2, consisting of a plate-type heat exchanger disposed in a bath of liquid for vaporization, and a bottom reboiler R3, The carbon monoxide-enriched top gas is condensed in the condenser C2, and the methane-rich bottom liquid 39 is expanded and reheated in the exchanger E1. Column K3 functions at 6.6 bar.
The plate-type exchanger is surrounded by an annular barrier forming an overflow wall P. Accordingly, the liquid surrounding the exchanger is able to pass over the barrier P to be withdrawn as liquids 43, 53.
The top condenser C2 of the column K3 is cooled by compressed and expanded nitrogen 59 from the nitrogen cycle compressor V1, V2, V3 after cooling in the exchangers E1, E2. The vaporized nitrogen is returned upstream of the last stage V3 of the nitrogen cycle compressor. The nitrogen at the exit pressure of the stage V3 is also used to reboil the reboiler R2 of the column K2.
The reboilers R1 and R3 of the columns K1 and K3 are reheated by partial flows of the feed 1 downstream of the exchanger E1 and upstream of the phase separator S1, This reboiling of the CO/CH₄column K3 by cooling of the synthesis gas has the advantage of enabling an increase in the pressure of the column CO/CH₄without an increase in the exit pressure of the nitrogen cycle compressor. The partial flows sent to the reboilers R1, R3 are at the same temperature and at the same pressure.
Liquid nitrogen 53 from the bottom of the condenser C2 of the column K3 is sent for vaporization in the condenser C1 of the column K3 and is subsequently returned downstream of the stage V2 and upstream of the stage V3. Accordingly, the nitrogen vaporized in the condensers C1, C3 of the CO/N₂column K2 and CO/CH₄column K3 returns at an inter-stage of the nitrogen compressor V1, V2; the N₂flow 57 vaporized in the condensers C1, C2 returns to the N₂compressor at a higher pressure than the N₂flow required for the cooling of the synthesis gas. In this case, this is made possible by operating the CO/N₂column K2 at a higher pressure (8.5 bar) relative to the prior art (2.6 bar).
A carbon monoxide-rich gas 41 leaves the column K3 at 6.6 bar at −170.4° C. and is reheated in the heat exchangers E1, E2. Preferably no carbon monoxide compressor is used. It constitutes a product of the process and has not been compressed.
A supply of liquid nitrogen 69 enables compensation of the leaks from the nitrogen cycle. Sent to a phase separator S7, the liquid formed is vaporized in the exchanger E2, and mixes with the gas from the separator S7, and is sent to the entry of the compressor V1.
A portion 47 of the liquid nitrogen in the condenser C2 is expanded and sent to the separator SI, and the gas 49 formed enters at the entry of the compressor V1.
Another portion 45 of the same liquid is expanded at a lower pressure and is sent to the exit of the compressor V1 and the entry of the compressor V2.
The operating pressure of the denitrification column K2 is at least 7 bar abs or even 8 bar abs; the operating pressure of the methane and carbon monoxide separation column K3 is at least 5 bar abs or even 6 bar abs.
In FIG. 2, the order of the nitrification and methane and carbon monoxide separation columns is reversed.
Accordingly, the liquid 19 from the bottom of the stripping column is sent not to the denitrification column but instead to an intermediate point of the CO/CH₄separation column K3, after separation by a phase separator S5.
The CO/CH₄column K3 has a bottom reboiler R3 which is heated by the feed, and a top condenser C2, which is used to condense the top gas 51, which is returned to the column K3 in condensed form. The condenser is cooled with condensed nitrogen 61, 63 produced by condensing the cycle nitrogen 59 from the compressor V3 in the exchangers E1, E2 and in the reboiler R2. The liquid is partially vaporized, producing a gas 55 which is returned to the entry of the compressor V3, and a liquid which passes over the barrier P. A portion 31 of the liquid is vaporized in the exchanger E2 and returns to the entry of the compressor V3. The other portion, 53, is used to cool the top condenser C1 of the column K2, as before.
The bottom methane 39 of the column K3 is reheated in the exchanger E1, to leave the apparatus as a product. The top gas 26, enriched in carbon monoxide and containing nitrogen, leaves toward the middle of the denitrification column K2.
The column K2 has a top condenser C1, consisting of a plate-type heat exchanger, and a bottom reboiler R2, which is heated by cycle nitrogen. The top gas 27 from the column K2 is partially condensed in the condenser C1, and the liquid L formed, 29, is returned to the top of the column K2 and in part, and the remaining gas V, enriched in nitrogen, is reheated in the exchangers E2, E1 as gas 31.
A liquid 53 from the top condenser C2 of the column K3 is vaporized by heat exchange with the gas 27 in the condenser C1, forming the gas 55 which is sent to the entry of the compressor V3.
The carbon monoxide-enriched and nitrogen-depleted bottom liquid 21 is expanded. This liquid, at 6.5 bar, is sent to a phase separator, the liquid from which is used in part to cool the condenser C1. Accordingly, the top condenser C1 of the CO/N₂column K2 is cooled by vaporization of at least part of the bottom liquid 33 from the CO/N₂column K2, after expansion and vaporization of the liquid nitrogen 53 at medium pressure. The vaporization of the bottom liquid 33 after expansion enables a considerable reduction in the nitrogen cycle flow to be vaporized in the condenser C1, thereby reducing the nitrogen cycle flow and hence the power of the nitrogen cycle compressor V1, V2, V3.
The gas 31 is a carbon monoxide-rich product of the process.
The operating pressure of the denitrification column K2 is at least 7 bar abs or even 8 bar abs; the operating pressure of the methane and carbon monoxide separation column K3 is at least 5 bar abs or even 6 bar abs.
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.-13. (canceled)

14. A process for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen, and methane, the method comprising:

i) cooling the gas mixture in a heat exchanger,

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

iii) introducing the hydrogen-depleted fluid into a denitrification column comprising a column top, a top condenser, a column bottom, and a bottom reboiler, thereby producing a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom,

iv) cooling the top condenser by means of a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the first stage comprising a first entry pressure and the second stage comprising a second entry pressure, the first entry pressure being lower than that of the second entry pressure,

v) expanding the nitrogen-depleted liquid and sending the expanded nitrogen-depleted liquid to the top condenser wherein it is at least partially vaporized by heat exchange in the condenser with the nitrogen-enriched gas, thereby condensing the expanded nitrogen-depleted liquid,

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

a) sending the bottom liquid from the denitrification column to a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid, or

b) the separation in step ii) comprises a distillation step in a methane and carbon monoxide separation column for separating a methane-depleted flow from a methane-enriched flow, and at least a portion of the methane-depleted flow constitutes the hydrogen-depleted fluid supplying the denitrification column, the methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid,

the bath of liquid of a) or b) being supplied with liquid nitrogen from the nitrogen cycle.

15. The process of claim 14, wherein liquid nitrogen from the top condenser of the methane and carbon monoxide separation column is sent to vaporize in the top condenser of the denitrification column.

16. The process of claim 14, wherein the gas mixture cooled in the heat exchanger is separated by at least one partial condensation step so as to form a hydrogen-depleted gas, the hydrogen-depleted gas is sent to an intermediate level of a stripping column comprising a bottom reboiler, and bottom liquid of the stripping column is sent to a denitrification column in case a) or to the methane and carbon monoxide separation column in case b).

17. The process of claim 14, wherein the reboiler of the stripping column and/or the reboiler of the methane and carbon monoxide separation column is reheated with at least a portion of the gas mixture.

18. The process of claim 14, wherein the operating pressure of the denitrification column is at least 7 bar abs and/or the operating pressure of the methane and carbon monoxide separation column is at least 5 bar abs.

19. The process of claim 14, wherein the top condenser of the CO/CH₄column is cooled solely by cycle nitrogen.

20. The process of claim 14, wherein the reboiler of the denitrification column is reheated by means of the cycle nitrogen.

21. The process of claim 20, wherein the nitrogen used for reheating the reboiler of the denitrification column is at the maximum pressure of the nitrogen cycle.

22. The process of claim 14, wherein the nitrogen sent to the bath of the condenser of the column CO/CH₄is condensed at the maximum pressure of the nitrogen cycle.

23. The process of claim 14, wherein the operating pressure of the methane and carbon monoxide separation column is at least 5 bar.

24. An apparatus for separating a gas mixture comprising carbon monoxide, nitrogen, hydrogen and methane, comprising a heat exchanger for cooling the gas mixture, means for separating the gas mixture cooled in the heat exchanger by at least one scrubbing and/or distillation and/or partial condensation step, to form a hydrogen-depleted fluid containing carbon monoxide and nitrogen, a denitrification column comprising a top condenser and a bottom reboiler, a pipe for sending the hydrogen-depleted fluid to the denitrification column, to produce a nitrogen-enriched gas at the column top and a nitrogen-depleted liquid at the column bottom, a nitrogen cycle using a nitrogen compressor comprising at least a first stage and a second stage, the entry pressure of the first stage being lower than that of the second stage, means for sending liquid of the nitrogen cycle to the condenser of the denitrification column, means for expanding bottom liquid from the denitrification column, means for sending the expanded liquid to the top condenser of the denitrification column for at least partial vaporization by heat exchanger in a heat exchanger of the condenser with the nitrogen-enriched gas, which is thereby condensed, means for sending nitrogen vaporized in the heat exchanger of the condenser to the inlet of the second stage of the nitrogen compressor, a methane and carbon monoxide separation column comprising a top condenser which is a bath vaporizer placed in a bath of liquid,

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

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

the apparatus further comprising means for sending liquid nitrogen from the nitrogen cycle to the top condenser of the methane and carbon monoxide separation column.

25. The apparatus of claim 24, comprising means for sending liquid nitrogen from the top condenser of the methane and carbon monoxide separation column to the top condenser of the denitrification column.

26. The apparatus of claim 24, further comprising at least one phase separator for separating the gas 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 stripping column.