Hydrogen liquefaction system with mixed refrigerant precooling

Abstract

The application relates to a hydrogen liquefaction system pre-cooled by a mixed refrigerant, and belongs to the technical field of low-temperature liquefaction. The system comprises a raw hydrogen cooling pipeline, a pre-cooling unit and a deep cooling unit, the pre-cooling unit performs heat exchange in a pre-cooling cold box, the pre-cooling cold box comprises a first pre-cooling section and a second pre-cooling section connected in sequence, the pre-cooling circulating pipeline adopts two-stage heat exchange, the deep cooling unit performs heat exchange in a deep cooling cold box, the deep cooling circulating pipeline is provided, and a liquid hydrogen storage tank is connected with the hydrogen cooling pipeline. The pre-cooling circulating pipeline adopts two-stage heat exchange, the mixed refrigerant is used in the pre-cooling stage of hydrogen liquefaction, the mixed refrigerant (a hydrogen and helium mixture) is used in the deep cooling stage, the temperature section division of the hydrogen liquefaction process is optimized, the hydrogen and the mixed refrigerant are gradually cooled and multi-stage expanded, the temperature difference between the hydrogen and the mixed refrigerant is small, heat exchange loss is reduced, and energy consumption is lowered.

Description

Technical Field

[0001] This invention belongs to the field of cryogenic liquefaction technology, and specifically relates to a hydrogen liquefaction system with pre-cooling by a mixed refrigerant. Background Technology

[0002] Currently operating hydrogen liquefaction units, Efficiency is generally low, only 20-30%, and initial investment is large. For decades, many researchers have studied how to improve the efficiency of hydrogen liquefaction. Because the hydrogen liquefaction process has a wide temperature range, using single-stage or multi-stage precooling can improve thermal efficiency and reduce energy consumption. According to different refrigeration methods, the main hydrogen liquefaction systems are: precooled Linde-Hampson systems, precooled Claude systems, and helium-cooled hydrogen liquefaction systems. Each of these three process forms has its own characteristics. The Linde-Hampson cycle has high energy consumption and low efficiency. The Claude cycle, considering both equipment and operating economics, is suitable for large-scale hydrogen liquefaction plants. Helium-cooled hydrogen liquefaction plants, due to the significant development of helium refrigerators internationally and domestically in recent years, adopt a partitioned heat exchange form, but due to the heat exchange temperature difference, the overall efficiency is slightly lower than the Claude cycle. Researching and optimizing the liquid hydrogen production process to improve cycle efficiency is an important way to solve the problems of high energy consumption and low plant efficiency in the hydrogen liquefaction process. Summary of the Invention

[0003] The purpose of this application is to provide a hydrogen liquefaction system with pre-cooling by a mixed refrigerant to solve the problem of high energy consumption in the current hydrogen liquefaction process.

[0004] This invention provides a hydrogen liquefaction system employing multi-stage expansion refrigeration with mixed refrigerants for pre-cooling and deep cooling, the system comprising:

[0005] The raw material hydrogen cooling pipeline is connected in sequence to the compressor, purification unit, precooling unit, and cryogenic unit.

[0006] A precooling system, comprising a precooling cold box and a precooling circulation pipeline, wherein the precooling cold box exchanges heat with the precooling unit, and the precooling cold box includes a first precooling section and a second precooling section connected in sequence; the precooling circulation pipeline provides a cold source to the precooling cold box, and includes a precooling refrigeration unit and a heat exchange unit connected in sequence, the heat exchange unit including a gas-liquid separator, a first precooling pipeline, and a second precooling pipeline, the gas-liquid separator being connected to the precooling refrigeration unit for receiving and separating the precooled mixed refrigerant, one end of the first precooling pipeline being connected to the gas-liquid separator, and the other end of the first precooling pipeline being connected to the precooling refrigeration unit, the first... The precooling pipeline includes a first pipeline body, a first pressure-reducing device, and a first recooling pipeline connected in sequence. The first pipeline body exchanges heat with the first precooling section and the second precooling section. The first recooling pipeline exchanges heat with the first precooling section and the second precooling section. One end of the second precooling pipeline is connected to the gas-liquid separator, and the other end of the first precooling pipeline is connected to the first recooling pipeline. The second precooling pipeline includes a second pipeline body, a second pressure-reducing device, and a second recooling pipeline connected in sequence. The second pipeline body exchanges heat with the first precooling section, and the second recooling pipeline is connected to the first recooling pipeline to provide a cold source to the first precooling section.

[0007] A cryogenic system, comprising a cryogenic chamber for cooling raw material hydrogen and cryogenic cycle refrigerant, and cryogenic cycle pipelines for providing a cold source to the cryogenic chamber;

[0008] A liquid hydrogen storage tank, connected to the hydrogen cooling pipeline, is used to store the cooled product liquid hydrogen.

[0009] Optionally, the pre-cooling refrigeration unit includes a two-stage pre-cooling mixed refrigerant compression unit, a hydrogen purifier, and a positive-negative converter.

[0010] Optionally, the cryogenic circulation pipeline is precooled in the precooling box.

[0011] Optionally, the cryogenic box includes multiple cryogenic sections.

[0012] Optionally, the cryogenic circulation pipeline includes a cryogenic circulation pipeline body and multiple intermediate circulation pipelines. One end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the rear end of any cryogenic section, and the other end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the front end of the cryogenic cold box. The intermediate circulation pipeline exchanges heat with a portion of the cryogenic section.

[0013] Optionally, the number of cryogenic sections is 6, and the number of intermediate circulation pipelines is 2.

[0014] Optionally, the intermediate circulation pipeline is equipped with at least one hydrogen turbine expander.

[0015] Optionally, the cryogenic circulation pipeline body is provided with two expansion refrigeration units, each expansion refrigeration unit including a circulating refrigerant compressor and a heat exchanger, and the connection point between the intermediate circulation pipeline and the cryogenic circulation pipeline body is located between the two expansion refrigeration units.

[0016] Optionally, the intermediate circulation pipeline exchanges heat with the precooling box.

[0017] Optionally, the refrigerant in the cryogenic section includes one (elemental hydrogen) or two refrigerants (a mixture of hydrogen and helium), and the cryogenic section adopts a multi-stage expansion refrigeration method.

[0018] Optionally, the first pressure reducing device and the second pressure reducing device are throttle valves.

[0019] Optionally, the precooling mixed refrigerant includes at least two of methane, ethane, propane, butane, nitrogen, and hydrogen.

[0020] Optionally, the cryogenic unit includes multiple cryogenic heat exchangers, each of which is filled with a hydrogen-to-hydrogen conversion catalyst at intervals.

[0021] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0022] The hydrogen liquefaction system with mixed refrigerant precooling provided in this embodiment of the invention uses a two-stage heat exchange in the precooling circulation pipeline. A mixed refrigerant is used in the precooling stage of hydrogen liquefaction, and a multi-stage expansion refrigeration of a mixture of hydrogen and helium is used in the cryogenic section. Since the temperature difference between hydrogen and mixed refrigerant is small, heat exchange loss is reduced and energy consumption is lowered.

[0023] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the system provided in an embodiment of the present invention;

[0026] Reference numerals: 1-First precooling section, 2-Second precooling section, 3-First cryogenic section, 4-Second cryogenic section, 5-Third cryogenic section, 6-Fourth cryogenic section, 7-Fifth cryogenic section, 8-Sixth cryogenic section, 9-First stage refrigerant compressor, 10-Second stage refrigerant compressor, 11-First low-temperature section refrigerant compressor, 12-Second low-temperature section refrigerant compressor, 13-First expander, 14-Second expander, 15-Pump, 16-Cooler, 17-JT valve, 18-Ejector, 19-Gas-liquid separator. Detailed Implementation

[0027] The present invention will be described in detail below with reference to specific embodiments and examples, thereby making the advantages and various effects of the present invention more clearly apparent. Those skilled in the art should understand that these specific embodiments and examples are for illustrative purposes only and are not intended to limit the present invention.

[0028] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the event of any conflict, this specification shall prevail.

[0029] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0030] The technical solution of this application embodiment is to solve the above-mentioned technical problems, and the general idea is as follows:

[0031] To address the issues of high energy consumption and high cost of liquid hydrogen production in hydrogen liquefaction, this application proposes a hydrogen liquefaction process based on mixed refrigerant precooling and a high-pressure mixed refrigerant refrigeration cycle in the cryogenic stage, compared to the conventional nitrogen precooling LH system. The hydrogen liquefaction process was optimized using a Matlab genetic algorithm, and hydrogen liquefaction can be achieved under existing technical conditions, effectively solving the problem of high energy consumption in hydrogen liquefaction.

[0032] This hydrogen liquefaction system mainly includes: a raw material hydrogen compressor unit, a circulating hydrogen compressor unit, a hydrogen purification and refining device, a hydrogen-to-hydrogen conversion device, a pre-cooling refrigeration cycle, a pre-cooling cold box, a cryogenic cold box, a turbine expander unit, and a liquid hydrogen storage tank. In this invention, the compressed raw material hydrogen is cooled to its liquefaction temperature through a series of heat exchangers in the pre-cooling and cryogenic cold boxes. The cooling capacity in the pre-cooling stage is mainly provided by a mixed refrigerant, while the cooling capacity in the cryogenic stage is mainly generated by the expansion and refrigeration of the circulating mixed refrigerant. The raw material hydrogen undergoes a multi-stage hydrogen-to-hydrogen conversion process simultaneously with heat exchange. This invention optimizes the temperature range division of the hydrogen liquefaction process, effectively reducing the heat exchange temperature difference and minimizing energy loss through staged cooling and multi-stage expansion refrigeration, thus achieving high efficiency and low energy consumption for the entire hydrogen liquefaction system.

[0033] According to a typical embodiment of the present invention, a hydrogen liquefaction system with mixed refrigerant precooling is provided, the system comprising: a hydrogen cooling pipeline, a precooling cold box, a precooling circulation pipeline, a cryogenic cold box, a cryogenic circulation pipeline, and a liquid hydrogen storage tank.

[0034] A hydrogen cooling pipeline, wherein the hydrogen cooling pipeline is provided with a precooling unit and a cryogenic unit in sequence;

[0035] A precooling box, which exchanges heat with the precooling unit, the precooling box comprising a first precooling section and a second precooling section connected in sequence;

[0036] A precooling circulation pipeline is used to provide a cold source for the precooling cold box. The precooling circulation pipeline includes a precooling refrigeration unit and a heat exchange unit connected in sequence. The heat exchange unit includes a gas-liquid separator, a first precooling pipeline, and a second precooling pipeline. The gas-liquid separator is connected to the precooling refrigeration unit to receive and separate the precooled mixed refrigerant. One end of the first precooling pipeline is connected to the gas-liquid separator, and the other end is connected to the precooling refrigeration unit. The first precooling pipeline includes a first pipeline body, a first pressure reducing device, and a first recooling pipeline connected in sequence. The first pipeline body exchanges heat with the first precooling section and the second precooling section. The first recooling pipeline exchanges heat with the first precooling section and the second precooling section. One end of the second precooling pipeline is connected to the gas-liquid separator, and the other end of the first precooling pipeline is connected to the first recooling pipeline. The second precooling pipeline includes a second pipeline body, a second pressure reducing device, and a second recooling pipeline connected in sequence. The second pipeline body exchanges heat with the first precooling section, and the second recooling pipeline is connected to the first recooling pipeline to provide a cold source to the first precooling section.

[0037] In some embodiments, the precooling refrigeration unit includes two precooling mixed refrigerant compression units.

[0038] In this embodiment, the precooling mixed refrigerant includes at least two of the following: methane, ethane, propane, butane, nitrogen, and hydrogen.

[0039] Specifically, the first and second pressure-reducing devices are throttle valves.

[0040] A cryogenic cold box, wherein the cryogenic cold box exchanges heat with the cryogenic unit;

[0041] In some embodiments, the cryogenic box includes multiple cryogenic sections.

[0042] In this embodiment, there are 6 cryogenic sections, namely the first cryogenic section, the second cryogenic section, the third cryogenic section, the fourth cryogenic section, the fifth cryogenic section, and the sixth cryogenic section, and there are 2 intermediate circulation pipelines.

[0043] Cryogenic circulation pipeline, used to provide a cold source for the aforementioned cryogenic cold box;

[0044] In some embodiments, the cryogenic circulation pipeline also exchanges heat with the precooling box to provide a cold source for the precooling box.

[0045] In some embodiments, the cryogenic circulation pipeline includes a cryogenic circulation pipeline body and a plurality of intermediate circulation pipelines. One end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the rear end of any cryogenic section, and the other end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the front end of the cryogenic cold box. The intermediate circulation pipeline exchanges heat with a portion of the cryogenic section.

[0046] In some embodiments, the cryogenic circulation pipeline body is provided with two cooling units, each including a hydrogen compressor and a heat exchanger, and the connection point between the intermediate circulation pipeline and the cryogenic circulation pipeline body is located between the two cooling units.

[0047] Specifically, the central circulation pipeline is equipped with multiple hydrogen turbine expanders.

[0048] In some embodiments, the central circulation pipeline also exchanges heat with the precooling box to provide cooling capacity to the precooling box.

[0049] A liquid hydrogen storage tank, which is connected to the hydrogen cooling pipeline, is used to store the cooled product liquid hydrogen.

[0050] Specifically, the system mainly includes: a raw material hydrogen compressor, a refrigerant compressor, a refrigerant circulation pump, a hydrogen purification and refining device, a hydrogen-to-neutral conversion device, a pre-cooling box, a cryogenic box, a turbine expander, a liquid hydrogen storage tank, and a refrigerant storage tank.

[0051] The entire process of hydrogen cooling and liquefaction in this system includes: purified hydrogen entering the first and second precooling sections of the precooling box, cooling the hydrogen to 60K~120K. In this embodiment, the precooling positive-negative converter is placed in the heat exchanger channel of the cold box, increasing the secondary hydrogen content from 25% to 45%.

[0052] The raw hydrogen stream, cooled to 60K–120K, sequentially enters multiple cryogenic sections of the cryogenic box. The stream is depressurized by a JT valve, and some of the high-pressure liquid hydrogen is throttled and vaporized. After mixing with the low-temperature hydrogen in the liquid hydrogen storage tank introduced by the ejector, it is cooled to 20K–30K by a reflux heat exchanger, where the raw hydrogen is completely liquefied before entering the liquid hydrogen storage tank.

[0053] The cooling capacity inside the hydrogen liquefaction cold box in the cryogenic section is mainly provided by the expansion cooling of the high-pressure circulating refrigerant (a mixture of hydrogen and helium). The circulating refrigerant expands and cools at two pressure levels before being returned to the heat exchanger to provide cooling capacity.

[0054] The cooling capacity of the precooling heat exchanger is provided by a mixed refrigerant. The mixed refrigeration cycle uses a mixture of 4 to 6 working fluids, with the refrigerant components potentially being a mixture of methane, hydrogen, ethane, propane, butane, and nitrogen. Using a mixed refrigerant in the precooling stage of hydrogen liquefaction reduces heat exchange losses and lowers energy consumption due to the small temperature difference between hydrogen and the mixed refrigerant. The precooling cycle is a two-stage heat exchange.

[0055] The pressurized refrigerant mixture is a two-phase gas-liquid mixture (pressure 20-50 barg). It is separated into gaseous and liquid refrigerant by a gas-liquid separator, and then enters the pre-cooling chamber separately. The liquid refrigerant is depressurized after passing through the first pre-cooling section, becoming a liquid stream. The gaseous refrigerant passes through the first and second pre-cooling sections sequentially, then is depressurized again by a JT valve, becoming a liquid stream before entering the second pre-cooling section. The two streams merge to provide cooling for the first pre-cooling section. After heat exchange, the liquid stream, as a low-pressure gaseous refrigerant, enters the first-stage refrigerant compressor. It is pressurized again and cooled, then the gaseous phase enters the second-stage refrigerant compressor for compression and cooling. The liquid phase is pressurized by a pump and then separated into two refrigerant phases by a gas-liquid separator.

[0056] During the cryogenic stage, the refrigerant is a mixture of two refrigerants (hydrogen and helium). A two-stage hydrogen-helium JB refrigeration cycle is used to cool the hydrogen from 60K-120K to 15K-30K. The high-pressure hydrogen is completely liquefied, and after being depressurized by the JT valve, a portion of the high-pressure liquid hydrogen is throttled and vaporized. This vaporized liquid hydrogen mixes with the low-temperature hydrogen in the liquid hydrogen storage tank introduced by the ejector and then flows back to the heat exchanger to provide cooling. The feedstock hydrogen entering the final stage of the cryogenic heat exchanger is finally cooled to 15K-30K by the throttled liquid hydrogen, and the feedstock hydrogen is completely liquefied.

[0057] The high-pressure circulating refrigerant liquefies hydrogen in the low-temperature section to increase cooling capacity. The first cycle of the medium-pressure circulating pipeline is as follows: the high-pressure circulating refrigerant is extracted from the latter part of the first cryogenic section and enters the first expander for refrigeration. The medium-pressure low-temperature hydrogen exiting the expander is mixed with the low-temperature hydrogen from the third cryogenic section and then flows back to the first and second cryogenic sections to provide cooling capacity. After that, the material enters the inlet of the refrigerant compressor in the second cryogenic section.

[0058] The second cycle of the medium circulation pipeline is as follows: the high-pressure circulating refrigerant is partially extracted after the third cryogenic section and enters the second expander for refrigeration. The medium-pressure low-temperature hydrogen exiting the expander enters the third and fourth cryogenic sections to provide cooling.

[0059] The circulation process of the cryogenic circulation pipeline body is as follows: After the secondary evacuation, the remaining small amount of high-pressure circulating refrigerant continues to be cooled to a certain temperature and then throttled to form a gas-liquid mixture, which flows back to each cryogenic section to provide cooling capacity. The temperature is 250K~330K and the pressure is 50kpag~200kpag. The material flows back to the inlet of the refrigerant compressor in the first low-temperature section.

[0060] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0061] (1) In the system provided in this embodiment of the invention, the precooling cycle is a mixed refrigerant refrigeration cycle. The two-stage hydrogen-helium mixture JB cycle is used for the cryogenic liquefaction cycle of hydrogen and is suitable for large-scale hydrogen liquefaction devices;

[0062] (2) Compared to the Claude cycle hydrogen liquefaction process with liquid nitrogen precooling, the system provided in this embodiment of the invention effectively reduces energy consumption. The energy consumption per unit of liquid hydrogen product is only 9.89 kWh / kg LH2, which is significantly lower than that of the Claude cycle hydrogen liquefaction process with liquid nitrogen precooling, which is approximately 12.5–15 kWh / kg LH2. The precooling cycle is a two-stage heat exchange. The precooling cycle reduces the hydrogen temperature to 60K–120K. The energy consumption is 1.996 kWh / kg LH2, saving equipment costs.

[0063] Finally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0064] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0065] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A hydrogen liquefaction system pre-cooled with a mixed refrigerant, characterized in that, The system includes: A raw material hydrogen cooling pipeline, wherein the hydrogen cooling pipeline is sequentially connected to a compressor, a purification device, a precooling unit, and a cryogenic unit; A precooling system, comprising a precooling cold box and a precooling circulation pipeline, wherein the precooling cold box exchanges heat with the precooling unit, and the precooling cold box includes a first precooling section and a second precooling section connected in sequence; the precooling circulation pipeline provides a cold source to the precooling cold box, and includes a precooling refrigeration unit and a heat exchange unit connected in sequence, the heat exchange unit including a gas-liquid separator, a first precooling pipeline, and a second precooling pipeline, the gas-liquid separator being connected to the precooling refrigeration unit for receiving and separating the precooled mixed refrigerant, one end of the first precooling pipeline being connected to the gas-liquid separator, and the other end of the first precooling pipeline being connected to the precooling refrigeration unit, the first... The precooling pipeline includes a first pipeline body, a first pressure-reducing device, and a first recooling pipeline connected in sequence. The first pipeline body exchanges heat with the first precooling section and the second precooling section. The first recooling pipeline exchanges heat with the first precooling section and the second precooling section. One end of the second precooling pipeline is connected to the gas-liquid separator, and the other end of the first precooling pipeline is connected to the first recooling pipeline. The second precooling pipeline includes a second pipeline body, a second pressure-reducing device, and a second recooling pipeline connected in sequence. The second pipeline body exchanges heat with the first precooling section, and the second recooling pipeline is connected to the first recooling pipeline to provide a cold source to the first precooling section. A cryogenic system, comprising a cryogenic chamber for cooling raw material hydrogen and cryogenic cycle refrigerant, and cryogenic cycle pipelines for providing a cold source to the cryogenic chamber; A liquid hydrogen storage tank, which is connected to the hydrogen cooling pipeline, is used to store the cooled product liquid hydrogen; The cryogenic circulation pipeline includes a cryogenic circulation pipeline body and multiple intermediate circulation pipelines. One end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the rear end of any cryogenic section, and the other end of the intermediate circulation pipeline is connected to the cryogenic circulation pipeline body at the front end of the cryogenic cold box. The intermediate circulation pipeline exchanges heat with a portion of the cryogenic section. The cryogenic section employs a multi-stage expansion refrigeration method. The cooling capacity inside the hydrogen liquefaction cold box in the cryogenic section is mainly provided by a high-pressure circulating refrigerant that expands and cools at two pressure levels before being returned to the heat exchanger. The high-pressure circulating refrigerant is a mixture of hydrogen and helium.

2. The hydrogen liquefaction system with mixed refrigerant precooling according to claim 1, characterized in that, The precooling refrigeration unit includes a two-stage precooling mixed refrigerant compression unit, a hydrogen purifier, and a positive-negative converter.

3. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 1, characterized in that, The cryogenic circulation pipeline is precooled in the precooling box.

4. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 1, characterized in that, The cryogenic box includes multiple cryogenic sections.

5. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 4, characterized in that, The number of cryogenic sections is 6, and the number of intermediate circulation pipelines is 2.

6. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 5, characterized in that, The central circulation pipeline is equipped with at least one hydrogen turbine expander.

7. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 5, characterized in that, The cryogenic circulation pipeline body is provided with two expansion refrigeration units, each of which includes a circulating refrigerant compressor and a heat exchanger. The connection point between the intermediate circulation pipeline and the cryogenic circulation pipeline body is located between the two expansion refrigeration units.

8. The hydrogen liquefaction system with mixed refrigerant precooling according to claim 5, characterized in that, The central circulation pipeline exchanges heat with the pre-cooling cold box.

9. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 1, characterized in that, The first and second pressure-reducing devices are throttle valves.

10. The hydrogen liquefaction system with mixed refrigerant precooling according to claim 1, characterized in that, The precooling mixed refrigerant includes at least two of the following: methane, ethane, propane, butane, nitrogen, and hydrogen.

11. The hydrogen liquefaction system with pre-cooled mixed refrigerant according to claim 1, characterized in that, The cryogenic unit includes multiple cryogenic heat exchangers, and each cryogenic heat exchanger is filled with a hydrogen-to-hydrogen conversion catalyst at intervals.

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