Nitrogen-containing methane gas denitrification equipment

Abstract

The present invention provides a nitrogen-containing methane gas denitrification device to solve the technical problem that the existing denitrification equipment has high cost and use cost. The nitrogen-containing methane gas denitrification device includes a raw gas supply device, a cold box and a distillation tower; the distillation tower has a reboiler at the bottom and a tower top cooler at the top; the raw gas supply device is connected to the inlet of the reboiler through a heating branch, and the heating branch passes through the cold box; the outlet of the reboiler is connected to the tower top cooler through a cooling branch, and the cooling branch passes through the cold box; the cooling branch passes through the tower top cooler and is connected to one end of the distillation feed branch, and the other end of the distillation feed branch is connected to the distillation tower; the upper end of the distillation tower is connected to a distillation branch, and the distillation branch passes through the tower top cooler and is connected to the top discharge branch; the bottom of the distillation tower is connected to a bottom discharge branch. The invention simplifies the structure of the denitrification equipment, saves equipment cost and operating cost, and improves energy utilization efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of distillation, in particular to a nitrogen-containing methane gas denitrification device. Background Art

[0002] The main components of nitrogen-containing methane gas are methane and nitrogen. Common nitrogen-containing methane gases include flash gas of liquefied natural gas, purified gas after treatment of nitrogen-rich wellhead gas, and natural gas produced by methanation of coke oven gas.

[0003] Since the atmospheric boiling point of methane is -161.5℃ and the atmospheric boiling point of nitrogen is -195.8℃, the difference between the two boiling points is usually used for separation. In the industry, nitrogen-containing methane gas is generally pressurized, first deep-cooled to below the boiling point of methane at the corresponding pressure through a heat exchanger, and then passed into a nitrogen flash tank or a cryogenic distillation tower to separate the nitrogen to obtain a methane gas product with higher purity. High-purity methane gas can be further deep-cooled to below -162℃ to obtain LNG products, or reheated to room temperature and exported as a natural gas product. The separated nitrogen can be used as chemical raw material gas.

[0004] In the existing denitrification equipment, a higher temperature heat medium needs to be provided for the reboiler at the bottom of the distillation tower, and a lower temperature cold medium needs to be provided for the top cooler at the top of the distillation tower. The cost and use cost of the denitrification equipment are both high. Summary of the invention

[0005] The object of the present invention is to provide a nitrogen-containing methane gas denitrification device to solve the technical problem that the existing denitrification equipment has high cost and use cost.

[0006] The present invention provides a nitrogen-containing methane gas denitrification device, which includes a raw gas supply device, a cold box and a distillation tower;

[0007] The distillation tower has a reboiler at the bottom and a tower top cooler at the top;

[0008] The raw gas supply device is connected to the inlet of the reboiler through a heating branch, and the heating branch passes through the cold box;

[0009] The outlet of the reboiler is connected to the tower top cooler through a cooling branch, and the cooling branch passes through the cold box;

[0010] The cooling branch passes through the tower top cooler and is connected to one end of the rectification feed branch, and the other end of the rectification feed branch is connected to the rectification tower;

[0011] The upper end of the distillation tower is connected with a distillation branch, and the distillation branch passes through the tower top cooler and is connected with the top discharge branch;

[0012] The bottom of the distillation tower is connected with a bottom discharge branch.

[0013] As a further embodiment of the present invention, the nitrogen-containing methane gas denitrification equipment further comprises a tower top separator, and the distillation branch is connected to the tower top separator after passing through the tower top cooler;

[0014] The tower top separator can perform gas-liquid separation, the tower top separator is connected to the top discharge branch through a separation branch, and the tower top separator is connected to the distillation tower through the distillation reflux branch.

[0015] As a further solution of the present invention, the top discharge branch and the bottom discharge branch both pass through the cold box.

[0016] As a further solution of the present invention, a first throttle valve is provided on the top discharge branch, and the medium in the top discharge branch flows through the cold box after being decompressed by the first throttle valve; and / or,

[0017] A second throttle valve is provided on the bottom discharge branch, and the medium in the bottom discharge branch flows through the cold box after being decompressed by the second throttle valve.

[0018] As a further solution of the present invention, the nitrogen-containing methane gas denitrification equipment also includes a refrigeration pipeline, the refrigeration pipeline passes through the cold box, and the cooling medium flows in the refrigeration pipeline.

[0019] As a further solution of the present invention, the nitrogen-containing methane gas denitrification equipment also includes a pressurizing device, which is arranged in the heating branch and located between the raw gas supply device and the cold box.

[0020] As a further solution of the present invention, the nitrogen-containing methane gas denitrification equipment also includes a refrigeration device, and the refrigeration device is connected to the cold box for cooling the cold box.

[0021] As a further embodiment of the present invention, the refrigeration device comprises a compressor, a first refrigerant separator and a second refrigerant separator;

[0022] The outlet of the compressor is in communication with the inlet of the first refrigerant separator;

[0023] One outlet of the first refrigerant separator is connected to the inlet of the second refrigerant separator through a first phase branch, and a third throttle valve is provided on the first phase branch, and gas phase refrigerant or liquid phase refrigerant enters the second refrigerant separator through the first phase branch;

[0024] Both outlets of the second refrigerant separator are communicated with a reflux cooling branch, and the reflux cooling branch passes through the cold box and is communicated with the inlet of the compressor.

[0025] As a further solution of the present invention, the refrigeration device further includes a third refrigerant separator;

[0026] The other outlet of the first refrigerant separator is connected to the inlet of the third refrigerant separator through a second phase branch, and a fourth throttle valve is provided on the second phase branch, and the refrigerant in another phase enters the third refrigerant separator through the second phase branch;

[0027] Both outlets of the third refrigerant separator are communicated with the reflux cooling branch.

[0028] As a further solution of the present invention, both the first phase branch and the second phase branch pass through the cold box;

[0029] The third throttle valve is located between the second refrigerant separator and the cold box, and the fourth throttle valve is located between the third refrigerant separator and the cold box.

[0030] Combined with the above technical solutions, the beneficial effects brought by the present invention are analyzed as follows:

[0031] The present invention provides a nitrogen-containing methane gas denitrification device, wherein a raw gas supply device supplies raw gas to the outside through a heating branch, the heating branch passes through a cold box, the raw gas in the heating branch is pre-cooled by the cold box, the heating branch is connected to a reboiler, and heat is provided to the reboiler. The raw gas flows out of the reboiler and enters a cooling branch, the cooling branch passes through a cold box, the raw gas in the cooling branch is cooled, the cooling branch is connected to a tower top cooler, and cold capacity is provided for the tower top cooler, the raw gas flowing out of the tower top cooler enters a rectification feed branch, the rectification feed branch transports the raw gas to a rectification tower, the rectification tower rectifying the raw gas, the light component flows out from the distillation branch at the upper end of the rectification tower and flows to the top discharge branch, and the heavy component flows out from the bottom discharge branch at the lower end of the rectification tower. The equipment uses the raw gas itself as the cooling medium of the top cooler of the low-temperature distillation tower and the heating medium of the bottom reboiler, realizing the heat transfer between the cold box-tower bottom reboiler and the cold box-tower top cooler, simplifying the structure of the denitrification equipment, saving equipment cost and operating cost, and improving energy utilization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0033] Figure 1A schematic structural diagram of a first embodiment of a nitrogen-containing methane gas denitrification device provided in an embodiment of the present invention;

[0034] Figure 2 A schematic structural diagram of a second embodiment of a nitrogen-containing methane gas denitrification device provided in an embodiment of the present invention;

[0035] Figure 3 A schematic structural diagram of a third embodiment of the nitrogen-containing methane gas denitrification equipment provided in an embodiment of the present invention.

[0036] Icons: 10-raw gas supply device; 12-boosting device; 14-cold box; 16-distillation tower; 18-reboiler; 20-tower top cooler; 22-heating branch; 24-cooling branch; 26-distillation feed branch; 28-distillation branch; 30-top discharge branch; 32-bottom discharge branch; 34-tower top separator; 36-separation branch; 38-distillation reflux branch; 40-first throttle valve; 42-second throttle valve; 44-refrigeration pipeline; 46-refrigeration device; 48-compressor; 50-first refrigerant separator; 52-second refrigerant separator; 54-first phase branch; 56-third throttle valve; 58-second phase branch; 60-fourth throttle valve; 62-third refrigerant separator; 64-refrigeration cooling branch. DETAILED DESCRIPTION

[0037] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0039] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0040] The present invention provides a nitrogen-containing methane gas denitrification device. Please refer to the attached drawings in the specification for details. Figures 1 to 3 .

[0041] like Figures 1 to 3 As shown, the nitrogen-containing methane gas denitrification equipment includes a raw gas supply device 10, a cold box 14 and a distillation tower 16. The distillation tower 16 has a reboiler 18 at the bottom and a tower top cooler 20 at the top. The raw gas supply device 10 is connected to the inlet of the reboiler 18 through a heating branch 22, and the heating branch 22 passes through the cold box 14. The raw gas in the heating branch 22 enters the reboiler 18 after precooling, and provides heat to the reboiler 18. The outlet of the reboiler 18 is connected to the tower top cooler 20 through a cooling branch 24. The raw gas in the heating branch 22 flows into the reboiler 18 and exchanges heat with the reboiler 18, the temperature of the raw gas is reduced, and the cooling branch 24 passes through the cold box 14, and the raw gas in the cooling branch 24 is further cooled by the cold box 14. The cooling branch 24 passes through the tower top cooler 20 and is connected to one end of the distillation feed branch 26. The raw gas in the cooling branch 24 provides cooling capacity for the tower top cooler 20. The other end of the distillation feed branch 26 is connected to the distillation tower 16. The raw gas enters the distillation tower 16 through the distillation feed branch 26 for distillation. The upper end of the distillation tower 16 is connected to a distillation branch 28. After the raw gas is distilled in the distillation tower 16, the light component rises and flows out from the distillation branch 28. The distillation branch 28 passes through the tower top cooler 20 and is connected to the top discharge branch 30. The heavy component is liquefied in the cooler and flows back to the distillation tower 16. The bottom of the distillation tower 16 is connected to a bottom discharge branch 32, and the heavy component flows out from the bottom of the distillation tower 16.

[0042] It should be noted that in the distillation process, the component with a lower boiling point is the light component, and the component with a higher boiling point is the heavy component; in nitrogen-containing methane gas, the light component is nitrogen and the heavy component is methane gas.

[0043] The equipment utilizes the raw gas itself as the cooling medium of the top cooler 20 of the low-temperature distillation tower 16 and the heating medium of the bottom reboiler 18, thereby realizing the heat transfer between the cold box 14—the bottom reboiler 18 and the cold box 14—the top cooler 20, simplifying the structure of the denitrification equipment, saving equipment cost and operation cost, and improving energy utilization efficiency.

[0044] like Figures 1 to 3As shown, the nitrogen-containing methane gas denitrification equipment also includes a tower top separator 34, and the distillation branch 28 is connected to the tower top separator 34 after passing through the tower top cooler 20. The tower top separator 34 can perform gas-liquid separation, and the tower top separator 34 is connected to the top discharge branch 30 through the separation branch 36. The light component flows into the top discharge branch 30 through the separation branch 36, and finally flows out from the top discharge branch 30. The tower top separator 34 is connected to the distillation tower 16 through the distillation reflux branch 38. The heavy component flowing out of the distillation branch 28 is liquefied in the tower top cooler 20 and separated from the light component in the tower top separator 34. The heavy component flows back to the distillation tower 16 through the distillation reflux branch 38. The above structure reduces the heavy component mixed in the light component, improves the purity of the nitrogen flowing out of the top discharge branch 30, reduces resource waste, and is more environmentally friendly.

[0045] Continue reading Figures 1 to 3 , the top discharge branch 30 and the bottom discharge branch 32 both pass through the cold box 14. The nitrogen in the top discharge branch 30 is reheated to room temperature when passing through the cold box 14, and the methane in the bottom discharge branch 32 is reheated to room temperature when passing through the cold box 14.

[0046] like Figure 3 As shown, a first throttle valve 40 is provided on the top discharge branch 30, and the medium in the top discharge branch 30 flows through the cold box 14 after being depressurized by the first throttle valve 40. After the light component in the top discharge branch 30 is depressurized by the first throttle valve 40, the temperature drops. When the top discharge branch 30 passes through the cold box 14, the light component provides cold energy for the cold box 14, and is reheated to room temperature to obtain room temperature nitrogen. In addition, a second throttle valve 42 is provided on the bottom discharge branch 32, and the medium in the bottom discharge branch 32 flows through the cold box 14 after being depressurized by the second throttle valve 42. After the heavy component in the bottom discharge branch 32 is depressurized by the second throttle valve 42, the temperature drops. When the bottom discharge branch 32 passes through the cold box 14, the heavy component provides cold energy for the cold box 14, and is reheated to room temperature to obtain room temperature methane gas.

[0047] Figure 3 The structure in the embodiment utilizes the product gas's own pressure to expand and refrigerate to provide cooling capacity for the cold box 14. A natural gas product with high purity can be obtained without a complicated refrigerant refrigeration system, which greatly saves equipment cost and operation cost.

[0048] Of course, it is also possible to set the first throttle valve 40 only on the top discharge branch 30 without setting the second throttle valve 42 on the bottom discharge branch 32; it is also possible to set the second throttle valve 42 only on the bottom discharge branch 32 without setting the first throttle valve 40 on the top discharge branch 30.

[0049] In addition to adopting Figure 3 In addition to providing cold capacity for the cold box 14, it is also possible to use Figure 2 The cold box 14 is provided with cold energy in the manner of Figure 2 The nitrogen-containing methane gas denitrification device further includes a refrigeration pipeline 44, which passes through the cold box 14, and a cooling medium flows in the refrigeration pipeline 44. The cooling medium can be liquid nitrogen, liquid hydrogen, liquid helium or other refrigerants. The cooling medium exchanges heat with the cold box 14 to provide cold capacity for the cold box 14.

[0050] like Figures 1 to 3 As shown, the nitrogen-containing methane gas denitrification equipment also includes a booster 12, which is arranged in the heating branch 22, and the booster 12 is located between the raw gas supply device 10 and the cold box 14. The booster 12 pressurizes the raw gas in the heating branch 22 to reduce the boiling points of the nitrogen and methane in the raw gas, so that the cold box 14 can more easily meet the use requirements.

[0051] In addition to adopting Figure 2 and Figure 3 In addition to providing cold capacity for the cold box 14, it is also possible to use Figure 1 The cold box 14 is provided with cold energy in a manner as follows. Figure 1 In the present invention, the nitrogen-containing methane gas denitrification equipment further includes a refrigeration device 46, which is connected to the cold box 14 and is used to cool the cold box 14 so that the cold box 14 has a lower temperature.

[0052] Continue reading Figure 1 , the refrigeration device 46 includes a compressor 48, a first refrigerant separator 50 and a second refrigerant separator 52. The outlet of the compressor 48 is connected to the inlet of the first refrigerant separator 50, and the refrigerant is compressed by the compressor 48 and sent to the first refrigerant separator 50, and is separated into gaseous refrigerant and liquid refrigerant in the first refrigerant separator. One of the outlets of the first refrigerant separator 50 is connected to the inlet of the second refrigerant separator 52 through the first phase branch 54, and the first phase branch 54 is provided with a third throttle valve 56, and the gaseous refrigerant or the liquid refrigerant enters the second refrigerant separator 52 through the first phase branch 54. When the refrigerant in the first phase branch 54 passes through the third throttle valve 56, the pressure drops and the temperature drops. After the refrigerant enters the second refrigerant separator 52, it is further separated into gaseous refrigerant and liquid refrigerant. Both outlets of the second refrigerant separator 52 are connected to the reflux cooling branch 64, and the reflux cooling branch 64 passes through the cold box 14 and is connected to the inlet of the compressor 48. After the gaseous refrigerant and the liquid refrigerant flow out from the two outlets of the second refrigerant separator 52, they both enter the reflux cooling branch 64. When the reflux cooling branch 64 passes through the cold box 14, it provides cooling for the cold box 14, so that the cold box 14 maintains a relatively low temperature.

[0053] Continue reading Figure 1, the refrigeration device 46 also includes a third refrigerant separator 62. The other outlet of the first refrigerant separator 50 is connected to the inlet of the third refrigerant separator 62 through the second phase branch 58, and the second phase branch 58 is provided with a fourth throttle valve 60, and the refrigerant of the other phase enters the third refrigerant separator 62 through the second phase branch 58. When the refrigerant of the other phase passes through the fourth throttle valve 60, the pressure drops and the temperature drops. After the refrigerant enters the third refrigerant separator 62, it is further separated into gaseous refrigerant and liquid refrigerant. Both outlets of the third refrigerant separator 62 are connected to the reflux cooling branch 64. After the gaseous refrigerant and the liquid refrigerant flow out from the two outlets of the third separator, they both enter the reflux cooling branch 64. When the reflux cooling branch 64 passes through the cold box 14, it provides cold capacity for the cold box 14, so that the cold box 14 maintains a lower temperature.

[0054] Figure 1 The first phase branch 54 shown contains gaseous refrigerant, and the second phase branch 58 contains liquid refrigerant. Of course, the first phase branch 54 may contain liquid refrigerant, and the second phase branch 58 may contain gaseous refrigerant.

[0055] Specifically, the first phase branch 54 and the second phase branch 58 both pass through the cold box 14. In addition, the third throttle valve 56 is located between the second refrigerant separator 52 and the cold box 14, and the fourth throttle valve 60 is located between the third refrigerant separator 62 and the cold box 14. The refrigerant in the first phase branch 54 is cooled by the cold box 14 and then depressurized and further cooled by the third throttle valve 56, and the refrigerant in the second phase branch 58 is cooled by the cold box 14 and then depressurized and further cooled by the fourth throttle valve 60, thereby reducing the working pressure of the third throttle valve 56 and the fourth throttle valve 60.

[0056] The nitrogen-containing methane gas denitrification equipment in this embodiment is used to distill the coke oven gas after methanation to separate and obtain methane-rich gas and nitrogen-rich gas. The product composition is shown in the following table.

[0057] Cold box inlet raw gas Nitrogen Methane-rich gas Flow rate (Nm3 / h) 6341 454.5 5887 Pressure(Mpa.G) 5.2 0.005 0.21 Temperature(℃) 40 30 -162 Composition (mol / %) CH4 89.67 5.68 96.14 C2H6 2.26 0 2.43 O2 0.71 1.6 0.65 N2 7.36 92.72 0.78

[0058] From the above table, it can be seen that the methane recovery rate is 99.53%, and the content of light hydrocarbons (methane and ethane) in the methane-rich gas is 98.57%.

[0059] The heat load of the tower bottom reboiler 18 is 286kW, and its heat comes from the heat of the raw gas itself and does not need to be introduced from the outside.

[0060] The heat load of the tower top cooler 20 is 51kW. Figure 3In the illustrated embodiment, the cooling capacity of the tower top cooler 20 is basically derived from the cooling capacity generated by the throttling expansion of the methane-rich gas and the nitrogen-rich gas. The present invention uses the raw gas itself as the heat exchange medium, and does not require an external refrigeration system to be separately set up for the tower top cooler 20, thereby saving equipment purchase costs and operating costs.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A nitrogen-containing methane gas denitrification device, characterized in that: It comprises a raw gas supply device (10), a cold box (14) and a distillation tower (16); The distillation tower (16) has a reboiler (18) at the bottom and a top cooler (20) at the top; The raw gas supply device (10) is connected to the inlet of the reboiler (18) through a heating branch (22), and the heating branch (22) passes through the cold box (14); The outlet of the reboiler (18) is connected to the tower top cooler (20) via a cooling branch (24), and the cooling branch (24) passes through the cold box (14); The cooling branch (24) passes through the tower top cooler (20) and is connected to one end of the distillation feed branch (26), and the other end of the distillation feed branch (26) is connected to the distillation tower (16); The upper end of the rectification tower (16) is connected to a distillation branch (28), and the distillation branch (28) passes through the tower top cooler (20) and is connected to a top discharge branch (30); The bottom of the distillation tower (16) is connected to a bottom discharge branch (32); It also includes a tower top separator (34), wherein the distillation branch (28) is connected to the tower top separator (34) after passing through the tower top cooler (20); The tower top separator (34) is capable of performing gas-liquid separation, the tower top separator (34) is connected to the top discharge branch (30) via a separation branch (36), and the tower top separator (34) is connected to the distillation tower (16) via a distillation reflux branch (38); It also includes a refrigeration device (46), wherein the refrigeration device (46) is connected to the cold box (14) and is used to cool the cold box (14).

2. The nitrogen-containing methane gas denitrification equipment according to claim 1, characterized in that: The top discharge branch (30) and the bottom discharge branch (32) both pass through the cold box (14).

3. The nitrogen-containing methane gas denitrification equipment according to claim 2, characterized in that: The top discharge branch (30) is provided with a first throttle valve (40), and the medium in the top discharge branch (30) is decompressed by the first throttle valve (40) and then flows through the cold box (14); and / or, A second throttle valve (42) is provided on the bottom discharge branch (32), and the medium in the bottom discharge branch (32) is decompressed by the second throttle valve (42) and then flows through the cold box (14).

4. The nitrogen-containing methane gas denitrification equipment according to claim 1, characterized in that: It also includes a refrigeration pipeline (44), wherein the refrigeration pipeline (44) passes through the cold box (14), and a cooling medium flows in the refrigeration pipeline (44).

5. The nitrogen-containing methane gas denitrification equipment according to claim 1, characterized in that: It also includes a pressure boosting device (12), which is arranged in the heating branch (22) and located between the raw gas supply device (10) and the cold box (14).

6. The nitrogen-containing methane gas denitrification equipment according to claim 1, characterized in that: The refrigeration device (46) comprises a compressor (48), a first refrigerant separator (50) and a second refrigerant separator (52); The outlet of the compressor (48) is in communication with the inlet of the first refrigerant separator (50); One outlet of the first refrigerant separator (50) is connected to the inlet of the second refrigerant separator (52) via a first phase branch (54), and a third throttle valve (56) is provided on the first phase branch (54), so that gas-phase refrigerant or liquid-phase refrigerant enters the second refrigerant separator (52) via the first phase branch (54); Both outlets of the second refrigerant separator (52) are connected to a return cooling branch (64), and the return cooling branch (64) passes through the cold box (14) and is connected to the inlet of the compressor (48).

7. The nitrogen-containing methane gas denitrification equipment according to claim 6, characterized in that: The refrigeration device (46) further includes a third refrigerant separator (62); The other outlet of the first refrigerant separator (50) is connected to the inlet of the third refrigerant separator (62) via a second phase branch (58), and a fourth throttle valve (60) is provided on the second phase branch (58), so that the refrigerant in the other phase enters the third refrigerant separator (62) via the second phase branch (58); Both outlets of the third refrigerant separator (62) are in communication with the return cooling branch (64).

8. The nitrogen-containing methane gas denitrification equipment according to claim 7, characterized in that: The first phase branch (54) and the second phase branch (58) both pass through the cold box (14); The third throttle valve (56) is located between the second refrigerant separator (52) and the cold box (14), and the fourth throttle valve (60) is located between the third refrigerant separator (62) and the cold box (14).

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