A device and method for preparing low-temperature distillation liquid nitrogen for offshore platforms or ships

Abstract

The invention discloses a low-temperature distillation liquid nitrogen preparation device for an offshore platform or ship, comprising an air compressor unit, a precooling and purification system, a liquid nitrogen preparation low-temperature cold box and a liquid nitrogen storage gasifier, wherein the air inlet of the air compressor unit is connected to an air source, the compressed air outlet of the air compressor unit is connected to the compressed air inlet of the precooling and purification system, the raw air outlet of the precooling and purification system is connected to the raw air inlet of the liquid nitrogen preparation low-temperature cold box, the dirty nitrogen regeneration gas outlet of the liquid nitrogen preparation low-temperature cold box is connected to the dirty nitrogen regeneration gas inlet of the precooling and purification system, and the first liquid nitrogen product outlet and the second liquid nitrogen product outlet of the liquid nitrogen preparation low-temperature cold box are respectively connected to the liquid nitrogen product inlet of the liquid nitrogen storage gasifier. The beneficial effects of the invention are as follows: the device of the invention introduces secondary distillation, which makes up for the loss of the distillation capacity of the original system, is energy-saving and efficient, and improves the system's ability to resist tilting and swaying conditions under offshore platform and ship environments while improving product extraction rate and product purity.

Description

Technical Field

[0001] The present invention relates to the technical field of cryogenic distillation, and in particular to a device and method for preparing cryogenic distillation liquid nitrogen for an offshore platform or a ship. Background Art

[0002] New large-tonnage ships require high-purity, low-dew-point, high-pressure nitrogen, with the highest required purity reaching 99.999%, so a nitrogen system needs to be designed, and the functions of production, high-pressure storage, transfer and decompression on board are realized. The space on sea platforms and ships is small, and the vibration and impact caused by sea voyages, high air humidity, and a series of specific environmental conditions such as oil mist, salt mist and mold make the overall design of the gas system quite difficult.

[0003] Realizing the preparation of liquid nitrogen products for offshore platforms and ships is the only way to solve the problems of difficult storage and use of nitrogen products on ships and large floor space. Cryogenic distillation technology can not only produce nitrogen with a purity of more than 99.999%, but also realize the preparation of liquid nitrogen products. The principle of cryogenic distillation technology is that the boiling point of nitrogen components is low and it is easier to volatilize into the gas phase, while the boiling point of oxygen components is high and most of them remain in the liquid phase. Therefore, after multiple distillations, repeated separation of gas and liquid phases and partial condensation of gas phase are carried out to achieve oxygen and nitrogen separation in the air. The strict requirements for equipment height under offshore platform and ship conditions limit the application of cryogenic distillation devices. At the same time, the tilting and swaying conditions brought about by offshore platform and ship conditions also have a great adverse effect on traditional distillation devices. Summary of the invention

[0004] The object of the present invention is to provide a device and method for preparing liquid nitrogen by cryogenic distillation for offshore platforms or ships with compact structure and high efficiency and energy saving in view of the deficiencies in the prior art.

[0005] The technical solution adopted by the present invention is: a low-temperature distillation liquid nitrogen preparation device for an offshore platform or ship, comprising an air compressor unit, a precooling and purification system, a liquid nitrogen preparation low-temperature cold box and a liquid nitrogen storage gasifier, wherein the air compressor unit is provided with an air inlet and a compressed air outlet, the precooling and purification system is provided with a compressed air inlet, a raw air outlet and a contaminated nitrogen regeneration gas inlet, the liquid nitrogen preparation low-temperature cold box is provided with a raw air inlet, a first liquid nitrogen product outlet, a second liquid nitrogen product outlet and a contaminated nitrogen regeneration gas outlet, the liquid nitrogen storage gasifier is provided with a liquid nitrogen product inlet and a nitrogen gas outlet, product outlet; the air inlet of the air compressor unit is connected to the air source, the compressed air outlet of the air compressor unit is connected to the compressed air inlet of the precooling and purification system, the raw air outlet of the precooling and purification system is connected to the raw air inlet of the liquid nitrogen preparation low-temperature cold box, the dirty nitrogen regeneration gas outlet of the liquid nitrogen preparation low-temperature cold box is connected to the dirty nitrogen regeneration gas inlet of the precooling and purification system, the first liquid nitrogen product outlet and the second liquid nitrogen product outlet of the liquid nitrogen preparation low-temperature cold box are respectively connected to the liquid nitrogen product inlet of the liquid nitrogen storage gasifier, and the nitrogen product outlet of the liquid nitrogen storage gasifier is connected to the subsequent nitrogen supply pipeline.

[0006] According to the above scheme, the liquid nitrogen preparation low temperature cold box includes a first nitrogen making distillation tower and a second nitrogen making distillation tower; a first raw material inlet and a first liquid air outlet are arranged at the bottom of the first nitrogen making distillation tower, and a first condenser, a first liquid air inlet, a first dirty nitrogen outlet, a first nitrogen outlet and a first reflux liquid nitrogen inlet are arranged at the top of the tower; a second raw material inlet and a second liquid air outlet are arranged at the bottom of the second nitrogen making distillation tower, and a second condenser, a second liquid air inlet, a second dirty nitrogen outlet, a second nitrogen outlet and a second reflux liquid nitrogen inlet are arranged at the top of the tower; the first raw material inlet of the first nitrogen making distillation tower is connected to the raw material air outlet; the first nitrogen outlet is connected to the inside of the first condenser through a side pipe, and the first nitrogen outlet is connected to the second The first liquid air outlet is connected with the first liquid air inlet and the second liquid air inlet respectively; the first dirty nitrogen outlet is connected with the dirty nitrogen regeneration gas inlet, the interior of the first condenser is connected with the first liquid nitrogen product outlet through the first stop valve, and the interior of the first condenser is connected with the first reflux liquid nitrogen inlet; the second raw material inlet of the second nitrogen making distillation tower is connected with the first nitrogen outlet through the second stop valve, the second liquid air outlet is connected with the second liquid air inlet outside the second condenser, and the second dirty nitrogen outlet is connected with the dirty nitrogen regeneration gas outlet; the second nitrogen outlet is connected with the interior of the second condenser through the side pipeline, and the interior of the second condenser is connected with the second liquid nitrogen product outlet; the interior of the second condenser is connected with the second reflux liquid nitrogen inlet.

[0007] According to the above scheme, the device is also equipped with a main heat exchanger, a subcooler and a turbine expander, the raw air inlet is connected to the heat source inlet of the main heat exchanger, and the heat source outlet of the main heat exchanger is connected to the first raw material inlet; the cold source outlet of the main heat exchanger is connected to the contaminated nitrogen regeneration gas outlet, the cold source inlet of the main heat exchanger is connected to the outlet of the turbine expander, the inlet of the turbine expander is respectively connected to the first raw material inlet and the cold source outlet of the subcooler, and the cold source inlet of the subcooler is respectively connected to the first contaminated nitrogen outlet and the second contaminated nitrogen outlet; the heat source inlet of the subcooler is connected to the first liquid air outlet; the heat source outlet of the subcooler is connected to the first liquid air inlet, the second liquid air inlet and the second liquid air outlet through pipelines.

[0008] The present invention also provides a method for preparing liquid nitrogen by the liquid nitrogen preparation device as described above, the method comprising: under normal working conditions, the first stop valve is partially opened and the second stop valve is fully closed, and the raw air enters the first nitrogen production distillation tower for distillation; under the working conditions of the offshore platform or the ship swaying and tilting, the first stop valve is fully closed and the second stop valve is partially opened, the second nitrogen production distillation tower participates in the distillation, the liquid nitrogen at the first liquid nitrogen product outlet is returned to the tower body of the first nitrogen production distillation tower as reflux liquid, part of the nitrogen drawn out from the first nitrogen outlet enters the first condenser, and the rest enters the second nitrogen production distillation tower, and the liquid nitrogen at the first liquid nitrogen product outlet is returned to the tower body of the first nitrogen production distillation tower as reflux liquid, part of the nitrogen drawn out from the first nitrogen outlet enters the first condenser, and the rest enters the second nitrogen production distillation tower. As the nitrogen fractionation tower rises in the tower body, the purity of the nitrogen component increases, and it rises to the top of the tower to become high-purity nitrogen, and then enters the inside of the second condenser; at the same time, the liquid air drawn out from the first liquid air outlet is reheated and cooled, and a part of it enters the first liquid air inlet after throttling, and the rest of it enters the outside of the second condenser after throttling, and exchanges heat with the high-purity nitrogen in the second condenser. The liquid air is vaporized into liquid air and then merged into the first dirty nitrogen outlet. The high-purity nitrogen is condensed into high-purity liquid nitrogen in the second condenser, and a part of it is drawn out from the second liquid nitrogen product outlet as a high-purity product, and the rest of it is returned to the second nitrogen production distillation tower as a reflux liquid.

[0009] Under normal working conditions, the first stop valve is partially opened and the second stop valve is fully closed, and the raw air enters the liquid nitrogen preparation distillation cold box; under the swaying and tilting conditions of the offshore platform or ship, the first stop valve is fully closed and the second stop valve is partially opened, the second nitrogen production distillation tower participates in the distillation, the liquid nitrogen at the first liquid nitrogen product outlet is returned to the first nitrogen production distillation tower body as reflux liquid, part of the nitrogen drawn out from the first nitrogen outlet enters the first condenser, and the rest enters the second nitrogen production distillation tower. While rising in the second nitrogen production distillation tower body, the purity of the nitrogen component increases. It rises to the top of the tower and becomes high-purity nitrogen, and then enters the inside of the second condenser; at the same time, the liquid air drawn out from the first liquid air outlet is reheated and cooled, and part of it enters the first liquid air inlet after throttling, and the rest enters the outside of the second condenser after throttling, and exchanges heat with the high-purity nitrogen in the second condenser. The liquid air is vaporized into liquid air and then merged into the first dirty nitrogen outlet. The high-purity nitrogen is condensed into high-purity liquid nitrogen in the second condenser, and part of it is drawn out from the second liquid nitrogen product outlet as a high-purity product, and the rest is returned to the second nitrogen production distillation tower as reflux liquid.

[0010] The beneficial effects of the present invention are as follows: the device of the present invention introduces secondary distillation under the rocking and tilting conditions of offshore platforms and ships, thereby compensating for the loss of distillation capacity of the original system; the first nitrogen-making distillation tower adopts a sieve plate tower structure to achieve a higher effective number of plates under strict height restrictions; the second nitrogen-making distillation tower adopts a packed tower structure to achieve better adaptability under variable working conditions, energy saving and high efficiency; while improving the product extraction rate and product purity, the system's ability to resist the tilting and rocking conditions under offshore platform and ship environments is improved, thereby ensuring the normal operation of the system; compared with traditional offshore platform and ship nitrogen supply devices, the present invention realizes the preparation and storage of liquid nitrogen products, has a compact structure, eliminates the cumbersome inspection procedures and high operation and maintenance costs required for the nitrogen system, and has high social and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The figure is a schematic diagram of the overall process of a specific embodiment of the present invention.

[0012] Figure 2 Schematic diagram of the structure of the low-temperature cold box prepared by liquid nitrogen in this embodiment.

[0013] Among them: 1-air compressor unit; 1-1-air inlet; 1-2-compressed air outlet; 2-precooling purification system; 2-1-compressed air outlet; 2-2-raw air outlet; 2-3-contaminated nitrogen regeneration gas inlet; 3-liquid nitrogen preparation low temperature cold box; 3-1-raw air inlet; 3-2-first liquid nitrogen product outlet; 3-3-second liquid nitrogen product outlet; 3-4-contaminated nitrogen regeneration gas outlet; 3-5-main heat exchanger; 3-6-subcooler; 3-7-first nitrogen distillation tower; 3-7-1-first raw material inlet; 3-7-2-first liquid air outlet; 3-7-3-first condenser; 3-7-4-first liquid air inlet; 3-7-5-first contaminated nitrogen outlet; 3-7-6-first nitrogen inlet; 3-7-7-first reflux liquid nitrogen inlet; 3 -8-second nitrogen production distillation tower; 3-8-1-second raw material inlet; 3-8-2-second liquid air outlet; 3-8-3-second condenser; 3-8-4-second liquid air inlet; 3-8-5-second dirty nitrogen outlet; 3-8-6-second nitrogen outlet; 3-8-7-second reflux liquid nitrogen inlet; 3-9-turbine expander; 3-10-first throttle valve; 3-11-second throttle valve; 3-12-third throttle valve; 3-13-fourth throttle valve; 3-14-first stop valve; 3-15-second stop valve; 3-16-third stop valve; 3-17-fourth stop valve; 3-18-fifth stop valve; 3-19-sixth stop valve; 4-liquid nitrogen storage vaporizer; 4-1-liquid nitrogen product inlet; 4-2-nitrogen product outlet. DETAILED DESCRIPTION

[0014] In order to better understand the present invention, the present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0015] like Figure 1 The offshore platform or ship cryogenic distillation liquid nitrogen preparation device comprises an air compressor unit 1, a precooling and purification system 2, a liquid nitrogen preparation low-temperature cold box 3 and a liquid nitrogen storage gasifier 4, wherein the air compressor unit 1 is provided with an air inlet 1-1 and a compressed air outlet 1-2, the precooling and purification system 2 is provided with a compressed air inlet 2-1, a raw air outlet 2-2 and a dirty nitrogen regeneration gas inlet 2-3, the liquid nitrogen preparation low-temperature cold box 3 is provided with a raw air inlet 3-1, a first liquid nitrogen product outlet 3-2, a second liquid nitrogen product outlet 3-3 and a dirty nitrogen regeneration gas outlet 3-4, and the liquid nitrogen storage gasifier 4 is provided with a liquid nitrogen product inlet 4-1 and a nitrogen product outlet 4-2; The air inlet 1-1 of the air compressor unit 1 is connected to the air source, the compressed air outlet 1-2 of the air compressor unit 1 is connected to the compressed air inlet 2-1 of the precooling and purification system 2, the raw air outlet 2-2 of the precooling and purification system 2 is connected to the raw air inlet 3-1 of the liquid nitrogen preparation low temperature cold box 3, the dirty nitrogen regeneration gas outlet 3-4 of the liquid nitrogen preparation low temperature cold box 3 is connected to the dirty nitrogen regeneration gas inlet 2-3 of the precooling and purification system 2, the first liquid nitrogen product outlet 3-2 and the second liquid nitrogen product outlet 3-3 of the liquid nitrogen preparation low temperature cold box 3 are respectively connected to the liquid nitrogen product inlet 4-1 of the liquid nitrogen storage gasifier 4, and the nitrogen product outlet 4-2 of the liquid nitrogen storage gasifier 4 is connected to the subsequent nitrogen supply pipeline. When working, the air compressor unit 1 and the precooling and purification system 2 provide clean high-pressure raw gas, and then the raw gas enters the low temperature cold box 3 for distillation oxygen and nitrogen separation, and the produced liquid nitrogen product is stored in the liquid nitrogen storage gasifier 4 and supplied for use after gasification. In this embodiment, the air compressor unit 1, the precooling and purification system 2 and the liquid nitrogen storage gasifier 4 are all prior art and will not be described in detail here. After the raw air enters the air compressor unit 1 and the compression pressure is increased, impurities such as CO2, H2O and C2H2 are removed in the precooling and purification system 2, and then enters the liquid nitrogen preparation low temperature cold box 3. When the turbine expander 3-9 is supported by a static pressure gas bearing, an additional bearing gas branch is separated from the outlet of the precooling and purification system 2 and connected to the turbine expander 3-9.

[0016] like Figure 2As shown, the liquid nitrogen preparation cryogenic cold box 3 includes a first nitrogen production distillation tower 3-7 and a second nitrogen production distillation tower 3-8; a first raw material inlet 3-7-1 and a first liquid air outlet 3-7-2 are arranged at the bottom of the first nitrogen production distillation tower 3-7, and a first condenser 3-7-3, a first liquid air inlet 3-7-4, a first dirty nitrogen outlet 3-7-5, a first nitrogen outlet 3-7-6 and a first reflux liquid nitrogen inlet 3-7-7 are arranged at the top of the tower; a second raw material inlet 3-8-1 and a second liquid air outlet 3-8-2 are arranged at the bottom of the second nitrogen production distillation tower 3-8, and a second condenser 3-8-3, a second liquid air inlet 3-8-4, a second dirty nitrogen outlet 3-8-5, a second nitrogen outlet 3-8-6 and a second reflux liquid nitrogen inlet 3-8-7 are arranged at the top of the tower. In this embodiment, the internal structures of the first nitrogen production distillation tower 3-7 and the second nitrogen production distillation tower 3-8 are both prior art, which will not be repeated here. The interior of the first condenser 3-7-3 and the second condenser 3-8-3 are both heat source channels, and the exteriors are both cold source channels.

[0017] The first raw material inlet 3-7-1 of the first nitrogen-making distillation tower 3-7 is connected to the raw air outlet 2-2 of the precooling and purification system 2; the first nitrogen outlet 3-7-6 is connected to the interior of the first condenser 3-7-3 through a side pipeline, and the first nitrogen outlet 3-7-6 is connected to the second raw material gas inlet 3-8-1; the first liquid air outlet 3-7-2 is respectively connected to the first liquid air inlet 3-7-4 (which is the cold source inlet of the first condenser 3-7-3) and the second liquid air inlet 3-8-4; the first dirty nitrogen outlet 3-7-5 is connected to the dirty nitrogen regeneration gas inlet 2-3 of the precooling and purification system 2, and the interior of the first condenser 3-7-3 is connected to the first liquid nitrogen product outlet 3-2 through a pipeline (the first stop valve 3-14 is arranged on the connecting pipeline); the interior of the first condenser 3-7-3 is connected to the first reflux liquid nitrogen inlet 3-7-7 through a pipeline (the third stop valve 3-16 is arranged on the connecting pipeline).

[0018] Preferably, the second raw material inlet 3-8-1 of the second nitrogen making distillation tower 3-8 is connected to the first nitrogen outlet 3-7-6 at the top of the first nitrogen making distillation tower 3-7 through the second stop valve 3-15, the second liquid air outlet 3-8-2 at the bottom of the second nitrogen making distillation tower 3-8 is connected to the second liquid air inlet 3-8-4 (which is the cold source inlet of the second condenser 3-8-3) outside the second condenser 3-8-3 through the fourth throttle valve 3-13, and the second dirty nitrogen outlet 3-8-5 is connected to the dirty nitrogen regeneration gas outlet 3-4 through a pipeline; the second nitrogen outlet 3-8-6 is connected to the inside of the second condenser 3-8-3 through a side pipeline, and the inside of the second condenser 3-8-3 is connected to the second liquid nitrogen product outlet 3-3 through a pipeline (the fifth stop valve 3-18 is arranged on the connecting pipeline); the inside of the second condenser 3-8-3 is connected to the second reflux liquid nitrogen inlet 3-8-7 through a pipeline (the fourth stop valve 3-17 is arranged on the connecting pipeline).

[0019] Preferably, the device is also equipped with a main heat exchanger 3-5, a subcooler 3-6 and a turbine expander 3-9, the raw air inlet 3-1 is connected to the heat source inlet of the main heat exchanger 3-5, and the heat source outlet of the main heat exchanger 3-5 is connected to the first raw material inlet 3-7-1 of the first nitrogen production distillation tower 3-7 through a pipeline (the pipeline is equipped with a first throttle valve 3-10); the cold source outlet of the main heat exchanger 3-5 is connected to the dirty nitrogen regeneration gas outlet 3-4, the cold source inlet of the main heat exchanger 3-5 is connected to the outlet of the turbine expander 3-9, the inlet of the turbine expander 3-9 is respectively connected to the first raw material inlet 3-7-1 and the cold source outlet of the subcooler 3-6, and the cold source inlet of the subcooler 3-6 is connected to the first raw material inlet 3-7-1. They are respectively connected with the first contaminated nitrogen outlet 3-7-5 and the second contaminated nitrogen outlet 3-8-5; the heat source inlet of the subcooler 3-6 is connected with the first liquid air outlet 3-7-2; the heat source outlet of the subcooler 3-6 is respectively connected with the first liquid air inlet 3-7-4, the second liquid air inlet 3-8-4 and the second liquid air outlet 3-8-2 through pipelines (the heat source outlet of the subcooler 3-6 is connected with the first liquid air inlet 3-7-4 via the second throttle valve 3-11; the heat source outlet of the subcooler 3-6 is connected with the second liquid air inlet 3-8-4 via the third throttle valve 3-12, and the heat source outlet of the subcooler 3-6 is connected with the second liquid air outlet 3-8-2 via the third throttle valve 3-12 and the fourth throttle valve 3-13). In this embodiment, the main heat exchanger 3-5 adopts a three-stream low-temperature plate-fin structure; the hot end of the turbine expander 3-9 adopts an open cycle fan brake; when the turbine expander 3-9 is supported by a static pressure gas bearing, an additional bearing gas branch is separated from the outlet of the pre-cooling purification system 2 and connected to the turbine expander.

[0020] A method for preparing liquid nitrogen by the liquid nitrogen preparation device as described above, the method comprising:

[0021] Under normal operating conditions (normal operating conditions are when the offshore platform or ship does not sway or tilt), the liquid nitrogen product of the first distillation tower 3-7 can reach the required purity; the raw air enters the liquid nitrogen preparation distillation cold box 3 from the raw air inlet 3-1, firstly recovers heat and cools down in the main heat exchanger 3-5, and then enters the first nitrogen production distillation tower 3-7 from the first raw material inlet 3-7-1 for distillation after throttling to a two-phase state through the first throttle valve 3-10, wherein the gas phase part rises in the tower body of the first distillation tower 3-7 while the purity of the nitrogen component increases, rising to the top of the tower to become high-purity nitrogen, and the liquid phase part refluxes in the tower body to the bottom of the tower to become liquid air; the high-purity nitrogen at the top of the tower is drawn out from the first nitrogen outlet 3-7-6 and enters the inner side of the first condenser 3-7-3, and the liquid air at the bottom of the tower is drawn out from the first liquid air outlet 3-7-2, and successively passes through the cooler 3-6 for cooling After being throttled by the second throttle valve 3-11, it enters the outside of the first condenser 3-7-3 from the first liquid air inlet 3-7-4; the liquid air and the high-purity nitrogen perform heat exchange in the first condenser 3-7-3, the liquid air is vaporized into dirty nitrogen and discharged from the first dirty nitrogen outlet 3-7-5, and successively passes through the cooler 3-6 and the main heat exchanger 3-5 for heat recovery and then enters the turbine expander 3-9 for expansion and cooling, and then passes through the main heat exchanger 3-5 again for heat recovery and is discharged from the dirty nitrogen regeneration gas outlet 3-4 to enter the pre-cooling purification system 2 as regeneration gas; the high-purity nitrogen is condensed into liquid nitrogen in the first condenser 3-7-3, part of which is drawn out from the first liquid nitrogen product outlet 3-2 as a high-purity product, and the rest is returned to the tower body from the first reflux liquid nitrogen inlet 3-7-7 as reflux liquid, and the reflux liquid becomes liquid air when the purity of the descending nitrogen component decreases to the bottom of the tower;

[0022] Under the swing and tilt conditions, the number of effective plates of the first distillation tower 3-7 decreases. At this time, the liquid nitrogen drawn out of the first liquid nitrogen product outlet 3-7-8 will be lower than the purity standard. Correspondingly, the nitrogen drawn out from the first nitrogen outlet 3-7-6 becomes medium purity nitrogen. Figure 2, under the tilting and swinging working conditions, the second nitrogen production distillation tower 3-8 begins to participate in distillation to make up for the loss of distillation capacity of the first nitrogen production distillation tower 3-7; at this time, no liquid nitrogen is drawn out from the first liquid nitrogen product outlet 3-7-8 as a product, and all liquid nitrogen is returned to the first nitrogen production distillation tower 3-7 tower body from the first reflux liquid nitrogen inlet 3-7-7, and the medium-purity nitrogen drawn out from the first nitrogen outlet 3-7-6 enters the first condenser 3-7-3, and the rest enters the second nitrogen production distillation tower 3-8 from the second raw material inlet 3-8-1 for further distillation and purification; while the medium-purity nitrogen rises in the tower body of the second nitrogen production distillation tower 3-8, the purity of the nitrogen component further increases, rises to the top of the tower to become high-purity nitrogen, and then is drawn out from the second nitrogen outlet 3-8-6 and enters the inner side of the second condenser 3-8-3; at the same time, the liquid air drawn out from the first liquid air outlet 3-7-2 is reheated and cooled by the cooler 3-6, except for a part of the liquid air that passes through the second After throttling by the throttle valve 3-11, the liquid enters the first liquid air inlet 3-7-4. The remaining part enters the second condenser 3-8-3 from the second liquid air inlet 3-8-4 after throttling by the third throttle valve 3-12. The liquid air and high-purity nitrogen are heat-exchanged in the second condenser 3-8-3. The liquid air is vaporized into liquid air and is led out from the second dirty nitrogen outlet 3-8-5 and then merged into the dirty nitrogen flow path led out from the first dirty nitrogen outlet 3-7-5. The high-purity nitrogen is discharged from the second condenser 3-8-3. The internal condensation is high-purity liquid nitrogen, part of which is drawn out from the second liquid nitrogen product outlet 3-3 as a high-purity product, and the rest is returned to the second nitrogen distillation tower 3-8 from the second reflux liquid nitrogen inlet 3-7-7 as reflux liquid; when the cooling capacity of the second condenser 3-8-3 is insufficient, part of the liquid air at the bottom of the tower is drawn out from the second liquid air outlet 3-8-2, and after throttling by the fourth throttle valve 3-13, it enters the second condenser 3-8-3 from the second liquid air inlet 3-8-4 to supplement the required cooling capacity.

[0023] Under normal working conditions, only the first nitrogen distillation tower 3-7 participates in distillation, and under tilting and swinging working conditions, the second nitrogen distillation tower 3-8 also participates in distillation, and the switching between the two working conditions is achieved by changing the opening of the first stop valve 3-14 and the second stop valve 3-15. Under normal working conditions, the first stop valve 3-14 is partially opened and the second stop valve 3-15 is fully closed, and under tilting and swinging working conditions, the first stop valve 3-14 is fully closed and the second stop valve 3-15 is partially opened; at the same time, the reflux ratio working condition of the first nitrogen distillation tower 3-7 can be adjusted by changing the opening of the first stop valve 3-14 and the third stop valve 3-16, and correspondingly, the reflux ratio working condition of the second nitrogen distillation tower 3-8 can be adjusted by changing the opening of the fourth stop valve 3-17 and the fifth stop valve 3-18. Flexible operation mode adjustment and working condition adjustment enable the system to obtain higher extraction rate and lower power consumption. In addition, when the temperature in the liquid nitrogen preparation low-temperature cold box 3 drops slowly at the initial start-up of the device, the opening of the sixth stop valve 3-19 increases, and the raw air inlet 3-1 flows through the main heat exchanger 3-5 for cooling and then partially leads to the inlet of the turbine expander 3-9. The cooling output is increased by increasing the gas volume of the turbine expander 3-9. When the device is cooled to the operating condition, the sixth stop valve 3-19 is closed. The setting of this branch improves the cooling output of the liquid nitrogen preparation low-temperature cold box 3 at the initial stage of cooling, accelerates the cooling of the device and the initial accumulation of the liquid level in the distillation tower, and greatly shortens the startup time of the system.

[0024] The above description is only a preferred embodiment of the present invention. It should be pointed out that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications are also considered to fall within the scope of protection of the present invention.

Claims

1. A cryogenic distillation liquid nitrogen preparation device for an offshore platform or ship, characterized in that: It comprises an air compressor unit, a precooling and purification system, a liquid nitrogen preparation low temperature cold box and a liquid nitrogen storage gasifier, wherein the air compressor unit is provided with an air inlet and a compressed air outlet, the precooling and purification system is provided with a compressed air inlet, a raw air outlet and a contaminated nitrogen regeneration gas inlet, the liquid nitrogen preparation low temperature cold box is provided with a raw air inlet, a first liquid nitrogen product outlet, a second liquid nitrogen product outlet and a contaminated nitrogen regeneration gas outlet, and the liquid nitrogen storage gasifier is provided with a liquid nitrogen product inlet and a nitrogen product outlet; the air inlet of the air compressor unit is connected with an air source, the compressed air outlet of the air compressor unit is connected with the compressed air inlet of the precooling and purification system, the raw air outlet of the precooling and purification system is connected with the raw air inlet of the liquid nitrogen preparation low temperature cold box, the contaminated nitrogen regeneration gas outlet of the liquid nitrogen preparation low temperature cold box is connected with the contaminated nitrogen regeneration gas inlet of the precooling and purification system, the first liquid nitrogen product outlet and the second liquid nitrogen product outlet of the liquid nitrogen preparation low temperature cold box are respectively connected with the liquid nitrogen product inlet of the liquid nitrogen storage gasifier, and the nitrogen product outlet of the liquid nitrogen storage gasifier is connected with the subsequent nitrogen supply pipeline; The liquid nitrogen preparation cryogenic cold box comprises a first nitrogen making distillation tower and a second nitrogen making distillation tower; a first raw material inlet and a first liquid air outlet are arranged at the bottom of the first nitrogen making distillation tower, and a first condenser, a first liquid air inlet, a first contaminated nitrogen outlet, a first nitrogen outlet and a first reflux liquid nitrogen inlet are arranged at the top of the tower; a second raw material inlet and a second liquid air outlet are arranged at the bottom of the second nitrogen making distillation tower, and a second condenser, a second liquid air inlet, a second contaminated nitrogen outlet, a second nitrogen outlet and a second reflux liquid nitrogen inlet are arranged at the top of the tower; the first raw material inlet of the first nitrogen making distillation tower is connected to the raw material air outlet; the first nitrogen outlet is connected to the inside of the first condenser through a side pipe, and the first nitrogen outlet is connected to the second raw material gas outlet The first liquid air outlet is connected with the first liquid air inlet and the second liquid air inlet respectively; the first dirty nitrogen outlet is connected with the dirty nitrogen regeneration gas inlet, the interior of the first condenser is connected with the first liquid nitrogen product outlet through the first stop valve, and the interior of the first condenser is connected with the first reflux liquid nitrogen inlet; the second raw material inlet of the second nitrogen making distillation tower is connected with the first nitrogen outlet through the second stop valve, the second liquid air outlet is connected with the second liquid air inlet outside the second condenser, and the second dirty nitrogen outlet is connected with the dirty nitrogen regeneration gas outlet; the second nitrogen outlet is connected with the interior of the second condenser through the side pipeline, and the interior of the second condenser is connected with the second liquid nitrogen product outlet; the interior of the second condenser is connected with the second reflux liquid nitrogen inlet.

2. The cryogenic distillation liquid nitrogen production device for offshore platforms or ships according to claim 1, characterized in that: The device is also equipped with a main heat exchanger, a subcooler and a turbo expander. The raw air inlet is connected to the heat source inlet of the main heat exchanger, and the heat source outlet of the main heat exchanger is connected to the first raw material inlet; the cold source outlet of the main heat exchanger is connected to the dirty nitrogen regeneration gas outlet. The cold source inlet of the main heat exchanger is connected with the outlet of the turbine expander, the inlet of the turbine expander is respectively connected with the first raw material inlet and the cold source outlet of the subcooler, the cold source inlet of the subcooler is respectively connected with the first dirty nitrogen outlet and the second dirty nitrogen outlet; the heat source inlet of the subcooler is connected with the first liquid air outlet; the heat source outlet of the subcooler is respectively connected with the first liquid air inlet, the second liquid air inlet and the second liquid air outlet through pipelines.

3. A method for preparing liquid nitrogen using the liquid nitrogen preparation device according to claim 2, characterized in that: The method comprises the following steps: under normal working conditions, the first stop valve is partially opened and the second stop valve is fully closed, and the raw air enters the first nitrogen making distillation tower for distillation; under the working conditions of the offshore platform or the ship swaying and tilting, the first stop valve is fully closed and the second stop valve is partially opened, and the second nitrogen making distillation tower participates in distillation, the liquid nitrogen at the first liquid nitrogen product outlet is returned to the tower body of the first nitrogen making distillation tower as reflux liquid, part of the nitrogen drawn out from the first nitrogen outlet enters the first condenser, and the rest enters the second nitrogen making distillation tower, and the nitrogen rises in the tower body of the second nitrogen making distillation tower. The purity of the component increases and rises to the top of the tower to become high-purity nitrogen, and then enters the inside of the second condenser; at the same time, the liquid air drawn out from the first liquid air outlet is reheated and cooled, and part of it enters the first liquid air inlet after throttling, and the rest enters the outside of the second condenser after throttling, and exchanges heat with the high-purity nitrogen in the second condenser. The liquid air is vaporized into liquid air and then merged into the first dirty nitrogen outlet. The high-purity nitrogen is condensed into high-purity liquid nitrogen in the second condenser, and part of it is drawn out from the second liquid nitrogen product outlet as a high-purity product, and the rest is returned to the second nitrogen production distillation tower as reflux liquid.

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