Device and method for co-producing nitrogen, hydrogen, CO gas and LNG (Liquefied Natural Gas) from synthesis gas

Abstract

The invention discloses a device and a method for co-producing nitrogen, hydrogen, CO gas and LNG (Liquefied Natural Gas) from synthesis gas, and belongs to the technical field of synthesis gas co-production. The device comprises a main heat exchanger, a liquid methane washing tower, a liquid nitrogen washing tower, a stripping tower, a methane rectifying tower, a reboiler and a methane rectifying tower condenser which are arranged in a cold box, and a circulating compressor outside the cold box, after carbon monoxide in upstream synthesis gas is washed and removed through the liquid methane washing tower, nitrogen and hydrogen are separated through the liquid nitrogen washing tower; methane liquid at the bottom of the liquid methane washing tower sequentially passes through a stripping tower and a methane rectifying tower to obtain CO gas and LNG. The device solves the problems that in the prior art, a liquid nitrogen washing device is low in yield, too large in nitrogen consumption and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthesis gas co-production, and in particular relates to a device and method for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas. Background Art

[0002] Synthesis gas is mainly a mixture of hydrogen and carbon monoxide, in addition to a small amount of methane, nitrogen, argon and other gases. The synthesis gas separation device mainly uses the boiling points of different substances to separate different components through heat exchange and distillation to obtain product gases such as hydrogen, CO gas, nitrogen and hydrogen, methane gas and liquid LNG.

[0003] Normally, the liquid nitrogen washing device and the CO gas production device are operated independently. When the raw gas contains a large amount of CO gas, the synthesis gas separation device may encounter the following problems: (1) Low yield. Due to the high concentration of CO gas, the yield of nitrogen and hydrogen in the liquid nitrogen washing device may drop significantly when processing raw gas with a high CO content; (2) High nitrogen consumption. In order to effectively separate nitrogen and hydrogen, the liquid nitrogen washing device needs to consume a large amount of nitrogen, which will increase operating costs and resource consumption. These problems will affect the overall performance and economic benefits of the synthesis gas separation device, so special attention should be paid to these factors during the design and operation process. In view of the above problems, it is urgent to develop a new gas separation device and method. Summary of the invention

[0004] The object of the present invention is to solve the problems in the prior art and to provide a device and method for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas.

[0005] The specific technical solutions adopted by the present invention are as follows:

[0006] In a first aspect, the present invention provides a device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas, comprising a main heat exchanger, a liquid methane washing tower, a liquid nitrogen washing tower, a stripping tower, a methane distillation tower, a reboiler and a methane distillation tower condenser arranged in a cold box, and a circulating compressor outside the cold box; carbon monoxide in the synthesis gas from upstream is washed and removed by the liquid methane washing tower, and then nitrogen and hydrogen are separated by the liquid nitrogen washing tower; the methane liquid at the bottom of the liquid methane washing tower passes through the stripping tower and the methane distillation tower in sequence to obtain CO gas and LNG.

[0007] Preferably, the first hot side inlet of the main heat exchanger receives the synthesis gas from the upstream, and the first cold side outlet of the main heat exchanger is connected to the synthesis gas inlet at the bottom of the liquid methane washing tower through a pipeline; the hydrogen-rich gas outlet at the top of the liquid methane washing tower is connected to the inlet at the bottom of the liquid nitrogen washing tower through a pipeline; the liquid phase outlet at the bottom of the liquid methane washing tower is connected to the inlet at the top of the stripping tower through a pipeline;

[0008] The second hot side inlet of the main heat exchanger receives medium-pressure nitrogen from outside the cold box, and the second cold side outlet of the main heat exchanger is connected to the nitrogen inlet of the liquid nitrogen washing tower through a pipeline; the nitrogen and hydrogen outlet at the top of the liquid nitrogen washing tower is connected to the first cold side inlet of the main heat exchanger through a pipeline; the first hot side outlet of the main heat exchanger is connected to an external nitrogen and hydrogen recovery device through a pipeline; the liquid phase outlet at the bottom of the liquid nitrogen washing tower is connected to the second cold side inlet of the main heat exchanger through a pipeline, and the second hot side outlet of the main heat exchanger is connected to an external liquid nitrogen washing tail gas recovery device through a pipeline;

[0009] The flash steam outlet at the top of the stripping tower is connected to the third cold side inlet of the main heat exchanger through a pipeline, and the third hot side outlet of the main heat exchanger is connected to the external flash steam recovery device through a pipeline; the liquid phase outlet at the bottom of the stripping tower is connected to the inlet in the middle of the methane distillation tower through a pipeline;

[0010] The carbon monoxide outlet at the top of the methane distillation tower is connected to the fourth cold side inlet of the main heat exchanger through a pipeline, and the fourth hot side outlet of the main heat exchanger is connected to an external carbon monoxide gas recovery device through a pipeline; the liquid phase outlet at the bottom of the methane distillation tower is divided into a first branch and a second branch, the first branch is connected to the fifth cold side inlet of the main heat exchanger through a pipeline, and the fifth hot side outlet of the main heat exchanger is connected to an external LNG recovery device through a pipeline; the second branch is connected to the third hot side inlet of the main heat exchanger after being pressurized by a liquid methane pump, and the third cold side outlet of the main heat exchanger is connected to the washing liquid inlet of the liquid methane washing tower through a pipeline for reflux;

[0011] A reboiler is provided between the stripping tower and the methane distillation tower, and the methane distillation tower is connected to the methane distillation tower condenser; the circulating gas outlet of the circulating compressor is connected to the fourth hot side inlet of the main heat exchanger through a pipeline, and the fourth cold side outlet of the main heat exchanger is connected to the reboiler through a pipeline; the condensation outlet of the reboiler is divided into two branches, one branch passes through the sixth cold side inlet of the main heat exchanger and the sixth hot side outlet of the main heat exchanger in sequence and refluxes to the circulating compressor, and the other branch passes through the methane distillation tower condenser, the seventh cold side inlet of the main heat exchanger, and the seventh hot side outlet of the main heat exchanger in sequence and refluxes to the circulating compressor.

[0012] Furthermore, the methane content in the upstream synthesis gas is not less than 2%; after being washed and reduced in the liquid methane washing tower, the carbon monoxide content in the hydrogen-rich gas entering the liquid nitrogen washing tower is less than 1%.

[0013] Furthermore, a throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the liquid methane washing tower and the inlet at the top of the stripping tower.

[0014] Furthermore, a throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the liquid nitrogen washing tower and the second cold side inlet of the main heat exchanger.

[0015] Furthermore, a throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the stripping tower and the middle inlet of the methane distillation tower.

[0016] Furthermore, a throttle valve is provided on the pipeline between the first branch at the bottom of the methane distillation tower and the fifth cold side inlet of the main heat exchanger.

[0017] Furthermore, the circulating compressor includes a primary compressor and a secondary compressor.

[0018] Furthermore, the circulating medium is nitrogen or carbon monoxide.

[0019] In a second aspect, the present invention provides a method for the device for co-producing nitrogen, hydrogen, CO and LNG using the synthesis gas described in the first aspect, wherein the synthesis gas purified from the upstream enters the liquid methane washing tower after being cooled by the main heat exchanger; the medium-pressure nitrogen from outside the cold box enters the liquid nitrogen washing tower after being cooled by the main heat exchanger, and is distilled with the hydrogen-rich gas from the top of the liquid methane washing tower; the low-temperature nitrogen and hydrogen at the top of the liquid nitrogen washing tower is reheated by the main heat exchanger and leaves the cold box to remove nitrogen and hydrogen recovery device, and the liquid phase at the bottom of the top of the liquid nitrogen washing tower enters the main heat exchanger after throttling, and leaves the cold box to remove the liquid nitrogen washing tail gas recovery device after vaporization and reheating;

[0020] The methane liquid from the bottom of the liquid methane scrubber enters the stripping tower for stripping after throttling; the flash steam at the top of the stripping tower is reheated in the main heat exchanger and leaves the cold box to go to the flash steam recovery device; the liquid phase at the bottom of the stripping tower enters the methane distillation tower for distillation after throttling; the low-temperature carbon monoxide at the top of the methane distillation tower is reheated in the main heat exchanger and leaves the cold box to go to the carbon monoxide recovery device; the liquid methane at the bottom of the methane distillation tower is divided into two paths, one path enters the main heat exchanger for reheating after throttling and leaves the cold box to go to the LNG recovery device, and the other path enters the main heat exchanger for cooling after being pressurized by the liquid methane pump, and then enters the methane liquid scrubber and distills with the synthesis gas after cooling in the main heat exchanger;

[0021] The circulating medium pressurized by the circulating compressor enters the main heat exchanger for cooling and then enters the reboiler between the stripping tower and the methane distillation tower for condensation. After condensation, it is divided into two paths. One path enters the main heat exchanger for heat exchange and then flows back to the circulating compressor. The other path enters the methane distillation tower condenser connected to the methane distillation tower for vaporization, and then enters the main heat exchanger for heat exchange and then flows back to the circulating compressor.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The device provided by the present invention first adopts a liquid methane washing tower to wash and reduce the CO gas in the synthesis gas with high methane content and high CO gas content to less than 1%. The hydrogen-rich gas produced at the top of the liquid methane washing tower also contains a small amount of carbon monoxide and methane, and the yield of hydrogen is relatively high. In this process, the purity of hydrogen in the hydrogen-rich gas can reach more than 98%, which fully meets the demand of the liquid nitrogen washing tower for nitrogen and hydrogen. At the same time, the carbon monoxide-rich gas at the bottom of the liquid methane washing tower can be further produced into methane gas and CO gas through stripping and distillation processes. The device solves the problems of low yield and excessive nitrogen consumption in the liquid nitrogen washing device in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the device for co-producing nitrogen, hydrogen, CO gas and LNG from synthesis gas provided in this embodiment.

[0025] In the figure: circulating compressor 1, main heat exchanger 2, liquid methane washing tower 3, liquid nitrogen washing tower 4, stripping tower 5, methane distillation tower 6, reboiler 7, methane distillation tower condenser 8, liquid methane pump 9, first hot side inlet N1-1, second hot side inlet N1-2, third hot side inlet N1-3, fourth hot side inlet N1-4, first cold side outlet N2-1, second cold side outlet N2-2, third cold side outlet N2-3, fourth cold side outlet N2-4, The first hot side outlet N3-1, the second hot side outlet N3-2, the third hot side outlet N3-3, the fourth hot side outlet N3-4, the fifth hot side outlet N3-5, the sixth hot side outlet N3-6, the seventh hot side outlet N3-7, the first cold side inlet N4-1, the second cold side inlet N4-2, the third cold side inlet N4-3, the fourth cold side inlet N4-4, the fifth cold side inlet N4-5, the sixth cold side inlet N4-6, the seventh cold side inlet N4-7. DETAILED DESCRIPTION

[0026] The present invention is further described and illustrated below in conjunction with the accompanying drawings and specific embodiments. The technical features of each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0027] like Figure 1 As shown, as a preferred embodiment of the present invention, this embodiment provides a device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas, which is mainly used to separate synthesis gas with high content of methane and carbon monoxide.

[0028] The device provided by the present invention comprises a main heat exchanger 2, a liquid methane washing tower 3, a liquid nitrogen washing tower 4, a stripping tower 5, a methane distillation tower 6, a reboiler 7 and a methane distillation tower condenser 8 arranged in a cold box, and a circulating compressor 1 outside the cold box. After carbon monoxide is washed and removed from the synthesis gas from the upstream by the liquid methane washing tower 3, nitrogen and hydrogen are separated by the liquid nitrogen washing tower 4. The methane liquid at the bottom of the liquid methane washing tower 3 passes through the stripping tower 5 and the methane distillation tower 6 in sequence to obtain CO gas and LNG.

[0029] Next, the specific connection method of the device provided by the present invention is described:

[0030] In the device, the first hot side inlet N1-1 of the main heat exchanger 2 receives the synthesis gas from the upstream, and the synthesis gas has a considerable methane content. The first cold side outlet N2-1 of the main heat exchanger 2 is connected to the synthesis gas inlet at the bottom of the liquid methane washing tower 3 through a pipeline. The hydrogen-rich gas outlet at the top of the liquid methane washing tower 3 is connected to the inlet at the bottom of the liquid nitrogen washing tower 4 through a pipeline. The liquid phase outlet at the bottom of the liquid methane washing tower 3 is connected to the inlet at the top of the stripping tower 5 through a pipeline, and a throttle valve is provided on the pipeline.

[0031] In the device, the second hot side inlet N1-2 of the main heat exchanger 2 receives medium-pressure nitrogen from outside the cold box, and the second cold side outlet N2-2 of the main heat exchanger 2 is connected to the nitrogen inlet of the liquid nitrogen washing tower 4 through a pipeline. The nitrogen and hydrogen outlet at the top of the liquid nitrogen washing tower 4 is connected to the first cold side inlet N4-1 of the main heat exchanger 2 through a pipeline. The first hot side outlet N3-1 of the main heat exchanger 2 is connected to the external nitrogen and hydrogen recovery device through a pipeline. The recovered nitrogen and hydrogen can be used as raw gas for synthetic ammonia. The liquid phase outlet at the bottom of the liquid nitrogen washing tower 4 is connected to the second cold side inlet N4-2 of the main heat exchanger 2 through a pipeline, and a throttle valve is provided on the pipeline. The second hot side outlet N3-2 of the main heat exchanger 2 is connected to the external liquid nitrogen washing tail gas recovery device through a pipeline.

[0032] In the device, the flash steam outlet at the top of the stripping tower 5 is connected to the third cold side inlet N4-3 of the main heat exchanger 2 through a pipeline, and the third hot side outlet N3-3 of the main heat exchanger 2 is connected to the external flash steam recovery device through a pipeline. The liquid phase outlet at the bottom of the stripping tower 5 is connected to the inlet in the middle of the methane distillation tower 6 through a pipeline, and a throttle valve is provided on the pipeline.

[0033] The carbon monoxide outlet at the top of the methane distillation tower 6 in the device is connected to the fourth cold side inlet N4-4 of the main heat exchanger 2 through a pipeline, and the fourth hot side outlet N3-4 of the main heat exchanger 2 is connected to the external carbon monoxide gas recovery device through a pipeline. The recovered carbon monoxide product gas can be used as a synthetic raw gas for ethylene glycol, acetic acid, etc.

[0034] The liquid phase outlet at the bottom of the methane distillation tower 6 in the device is divided into a first branch and a second branch. The first branch is connected to the fifth cold side inlet N4-5 of the main heat exchanger 2 through a pipeline, and a throttle valve is provided on the first branch. The fifth hot side outlet N3-5 of the main heat exchanger 2 is connected to the external LNG recovery device through a pipeline. LNG is liquefied natural gas. The second branch is connected to the third hot side inlet N1-3 of the main heat exchanger 2 after being pressurized by the liquid methane pump 9. The third cold side outlet N2-3 of the main heat exchanger 2 is connected to the washing liquid inlet of the liquid methane washing tower 3 through a pipeline for reflux.

[0035] In this embodiment, a reboiler 7 is provided between the stripping tower 5 and the methane distillation tower 6. In the synthesis gas co-production technology, the reboiler provided between the stripping tower and the methane distillation tower can provide heat: the reboiler heats the bottom liquid to partially vaporize it, providing the necessary heat to maintain the distillation process in the tower. This helps to improve the separation efficiency. The reboiler can also help maintain the temperature gradient in the tower, thereby ensuring that different components are effectively separated at different temperatures. The heat provided by the reboiler can more effectively separate high-purity methane and other gases, thereby improving the purity and quality of the product.

[0036] In this embodiment, the methane distillation tower 6 is connected to the methane distillation tower condenser 8. During the distillation process, the steam at the top of the methane distillation tower 6 will be cooled by the methane distillation tower condenser 8 and converted into liquid, which helps to separate pure methane. The condensed liquid part will be sent back to the tower as reflux liquid. This helps to improve the separation efficiency of the distillation tower.

[0037] The circulating gas outlet of the circulating compressor 1 is connected to the fourth hot side inlet N1-4 of the main heat exchanger 2 through a pipeline, and the fourth cold side outlet N2-4 of the main heat exchanger 2 is connected to the reboiler 7 through a pipeline. The condensation outlet of the reboiler 7 is divided into two branches, one branch passes through the sixth cold side inlet N4-6 of the main heat exchanger 2 and the sixth hot side outlet N3-6 of the main heat exchanger 2 in sequence and flows back to the circulating compressor 1, and the other branch passes through the methane distillation tower condenser 8, the seventh cold side inlet N4-7 of the main heat exchanger 2, and the seventh hot side outlet N3-7 of the main heat exchanger 2 in sequence and flows back to the circulating compressor 1. The circulating compressor 1 provided in this embodiment includes a primary compressor and a secondary compressor. By compressing the gas in stages, the compression ratio of each stage of the compressor can be reduced, thereby improving the efficiency of the overall system.

[0038] The syngas from the upstream purification also contains high methane and carbon monoxide content, of which methane is generally not less than 2% and carbon monoxide is higher than 5%. If the syngas is directly introduced into the liquid nitrogen scrubber, subsequent treatment is impossible due to the high carbon monoxide content. The method of using the above device to co-produce nitrogen, hydrogen, CO gas and LNG from syngas is as follows:

[0039] After the synthesis gas from the upstream is purified and cooled by the main heat exchanger 2, it enters the liquid methane scrubber 3 to wash and reduce the carbon monoxide content in the synthesis gas (which can be reduced to less than 1%). After the medium-pressure nitrogen from the outside of the cold box is cooled by the main heat exchanger 2, it enters the liquid nitrogen scrubber 4 and is distilled with the hydrogen-rich gas from the top of the liquid methane scrubber 3. The low-temperature nitrogen and hydrogen at the top of the liquid nitrogen scrubber 4 is reheated by the main heat exchanger 2 and leaves the cold box to remove nitrogen and hydrogen gas recovery device. The liquid phase at the bottom of the top of the liquid nitrogen scrubber 4 enters the main heat exchanger 2 after throttling, and leaves the cold box to go to the liquid nitrogen washing tail gas recovery device after vaporization and reheating.

[0040] The methane liquid from the bottom of the liquid methane washing tower 3 enters the stripping tower 5 for stripping after throttling. The flash steam at the top of the stripping tower 5 is reheated by the main heat exchanger 2 and leaves the cold box to go to the flash steam recovery device. The liquid phase at the bottom of the stripping tower 5 enters the methane distillation tower 6 for distillation after throttling. The low-temperature carbon monoxide at the top of the methane distillation tower 6 is reheated by the main heat exchanger 2 and leaves the cold box to go to the carbon monoxide recovery device. The liquid methane at the bottom of the methane distillation tower 6 is divided into two paths, one of which enters the main heat exchanger 2 for reheating after throttling and leaves the cold box to go to the LNG recovery device, and the other enters the main heat exchanger 2 for cooling after being pressurized by the liquid methane pump 9, and then enters the methane liquid washing tower 3 and distills with the synthesis gas after cooling by the main heat exchanger 2.

[0041] The circulating medium pressurized by the circulating compressor 1 enters the main heat exchanger 2 for cooling and then enters the reboiler 7 between the stripping tower 5 and the methane distillation tower 6 for condensation. After condensation, it is divided into two paths, one path enters the main heat exchanger 2 for heat exchange and then flows back to the circulating compressor 1, and the other path enters the methane distillation tower condenser 8 connected to the methane distillation tower 6 for vaporization, then enters the main heat exchanger 2 for heat exchange and then flows back to the circulating compressor 1. The circulating medium can usually be nitrogen or carbon monoxide product gas.

[0042] The nitrogen and hydrogen produced from the top of the liquid nitrogen washing tower are used to synthesize ammonia, the carbon monoxide gas produced from the top of the methane distillation tower is used for the synthesis of ethylene glycol, ethanol, acetic acid, etc. at the back end, and the LNG gas produced from the bottom of the methane distillation tower is used as fuel gas combustion or to supply city gas to the gas pipeline network.

[0043] The above embodiment is only a preferred solution of the present invention, but it is not intended to limit the present invention. A person skilled in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas, characterized in that: The invention comprises a main heat exchanger (2) arranged in a cold box, a liquid methane washing tower (3), a liquid nitrogen washing tower (4), a stripping tower (5), a methane distillation tower (6), a reboiler (7) and a methane distillation tower condenser (8), and a circulating compressor (1) outside the cold box; carbon monoxide in the synthesis gas from the upstream is reduced by washing through the liquid methane washing tower (3), and then nitrogen and hydrogen are separated by the liquid nitrogen washing tower (4); the methane liquid at the bottom of the liquid methane washing tower (3) passes through the stripping tower (5) and the methane distillation tower (6) in sequence to obtain CO gas and LNG.

2. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 1, characterized in that: The first hot side inlet (N1-1) of the main heat exchanger (2) receives the synthesis gas from the upstream, and the first cold side outlet (N2-1) of the main heat exchanger (2) is connected to the synthesis gas inlet at the bottom of the liquid methane washing tower (3) through a pipeline; the hydrogen-rich gas outlet at the top of the liquid methane washing tower (3) is connected to the inlet at the bottom of the liquid nitrogen washing tower (4) through a pipeline; the liquid phase outlet at the bottom of the liquid methane washing tower (3) is connected to the inlet at the top of the stripping tower (5) through a pipeline; The second hot side inlet (N1-2) of the main heat exchanger (2) receives medium-pressure nitrogen from outside the cold box, and the second cold side outlet (N2-2) of the main heat exchanger (2) is connected to the nitrogen inlet of the liquid nitrogen washing tower (4) through a pipeline; the nitrogen and hydrogen outlet at the top of the liquid nitrogen washing tower (4) is connected to the first cold side inlet (N4-1) of the main heat exchanger (2) through a pipeline; the first hot side outlet (N3-1) of the main heat exchanger (2) is connected to an external nitrogen and hydrogen recovery device through a pipeline; the liquid phase outlet at the bottom of the liquid nitrogen washing tower (4) is connected to the second cold side inlet (N4-2) of the main heat exchanger (2) through a pipeline, and the second hot side outlet (N3-2) of the main heat exchanger (2) is connected to an external liquid nitrogen washing tail gas recovery device through a pipeline; The flash steam outlet at the top of the stripping tower (5) is connected to the third cold side inlet (N4-3) of the main heat exchanger (2) through a pipeline, and the third hot side outlet (N3-3) of the main heat exchanger (2) is connected to an external flash steam recovery device through a pipeline; the liquid phase outlet at the bottom of the stripping tower (5) is connected to the inlet in the middle of the methane distillation tower (6) through a pipeline; The carbon monoxide outlet at the top of the methane distillation tower (6) is connected to the fourth cold side inlet (N4-4) of the main heat exchanger (2) through a pipeline, and the fourth hot side outlet (N3-4) of the main heat exchanger (2) is connected to an external carbon monoxide gas recovery device through a pipeline; the liquid phase outlet at the bottom of the methane distillation tower (6) is divided into a first branch and a second branch, the first branch is connected to the fifth cold side inlet (N4-5) of the main heat exchanger (2) through a pipeline, and the fifth hot side outlet (N3-5) of the main heat exchanger (2) is connected to an external LNG recovery device through a pipeline; the second branch is connected to the third hot side inlet (N1-3) of the main heat exchanger (2) after being pressurized by a liquid methane pump (9), and the third cold side outlet (N2-3) of the main heat exchanger (2) is connected to the washing liquid inlet of the liquid methane washing tower (3) through a pipeline for reflux; A reboiler (7) is provided between the stripping tower (5) and the methane distillation tower (6), and the methane distillation tower (6) is connected to the methane distillation tower condenser (8); the circulating gas outlet of the circulating compressor (1) is connected to the fourth hot side inlet (N1-4) of the main heat exchanger (2) through a pipeline, and the fourth cold side outlet (N2-4) of the main heat exchanger (2) is connected to the reboiler (7) through a pipeline; the condensation outlet of the reboiler (7) is divided into two branches, one branch passes through the sixth cold side inlet (N4-6) of the main heat exchanger (2) and the sixth hot side outlet (N3-6) of the main heat exchanger (2) in sequence and flows back to the circulating compressor (1), and the other branch passes through the methane distillation tower condenser (8), the seventh cold side inlet (N4-7) of the main heat exchanger (2), and the seventh hot side outlet (N3-7) of the main heat exchanger (2) in sequence and flows back to the circulating compressor (1).

3. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: The content of methane in the upstream synthesis gas is not less than 2%; after being washed and reduced in the liquid methane washing tower (3), the content of carbon monoxide in the hydrogen-rich gas entering the liquid nitrogen washing tower (4) is less than 1%.

4. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: A throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the liquid methane washing tower (3) and the top inlet of the stripping tower (5).

5. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: A throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the liquid nitrogen washing tower (4) and the second cold side inlet of the main heat exchanger (2).

6. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: A throttle valve is provided on the pipeline between the liquid phase outlet at the bottom of the stripping tower (5) and the middle inlet of the methane distillation tower (6).

7. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: A throttle valve is provided on the pipeline between the first branch at the bottom of the methane distillation tower (6) and the fifth cold side inlet of the main heat exchanger (2).

8. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: The circulating compressor (1) comprises a primary compressor and a secondary compressor.

9. The device for co-producing nitrogen, hydrogen, CO and LNG from synthesis gas according to claim 2, characterized in that: The circulating medium is nitrogen or carbon monoxide.

10. A method for co-producing nitrogen, hydrogen, CO and LNG using the synthesis gas according to any one of claims 1 to 9, characterized in that: The synthesis gas from the upstream after purification is cooled by the main heat exchanger (2) and then enters the liquid methane washing tower (3); the medium-pressure nitrogen from outside the cold box is cooled by the main heat exchanger (2) and then enters the liquid nitrogen washing tower (4) to be rectified with the hydrogen-rich gas from the top of the liquid methane washing tower (3); the low-temperature nitrogen and hydrogen gas at the top of the liquid nitrogen washing tower (4) is reheated by the main heat exchanger (2) and then leaves the cold box to remove nitrogen and hydrogen gas recovery device; the liquid phase at the bottom of the top of the liquid nitrogen washing tower (4) enters the main heat exchanger (2) after throttling, and leaves the cold box to go to the liquid nitrogen washing tail gas recovery device after vaporization and reheating; The methane liquid from the bottom of the liquid methane washing tower (3) enters the stripping tower (5) for stripping after being throttled; the flash steam at the top of the stripping tower (5) passes through the main heat exchanger (2) for reheating and leaves the cold box to go to the flash steam recovery device; the liquid phase at the bottom of the stripping tower (5) enters the methane distillation tower (6) for distillation after being throttled; the low-temperature carbon monoxide at the top of the methane distillation tower (6) passes through the main heat exchanger (2) for reheating and leaves the cold box to go to the carbon monoxide recovery device; The liquid methane at the bottom of the methane distillation tower (6) is divided into two paths. One path enters the main heat exchanger (2) after throttling and is reheated before leaving the cold box to go to the LNG recovery device. The other path is pressurized by the liquid methane pump (9) and enters the main heat exchanger (2) for cooling. It then enters the methane liquid washing tower (3) and is distilled with the synthesis gas after cooling by the main heat exchanger (2). The circulating medium pressurized by the circulating compressor (1) enters the main heat exchanger (2) to cool down and then enters the reboiler (7) between the stripping tower (5) and the methane distillation tower (6) for condensation. After condensation, it is divided into two paths. One path enters the main heat exchanger (2) for heat exchange and then flows back to the circulating compressor (1). The other path enters the methane distillation tower condenser (8) connected to the methane distillation tower (6) for vaporization and then enters the main heat exchanger (2) for heat exchange and then flows back to the circulating compressor (1).

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