A zero-emission liquid hydrogen production and storage system and method

Abstract

The application provides a zero-emission liquid hydrogen production and storage system and method, which comprises a liquid hydrogen production system and a liquid hydrogen storage system connected with each other; the liquid hydrogen production system comprises a raw hydrogen feeding pipeline, a pressure regulating unit, a precooling unit and a cryogenic unit connected in sequence, and is used for liquefying raw hydrogen to produce liquid hydrogen and unliquefied hydrogen; the liquid hydrogen storage system comprises a separation unit, a storage unit and a evaporation gas recovery unit; the separation unit and the storage unit are used for separating and storing the liquid hydrogen and the unliquefied hydrogen; the evaporation gas recovery unit is used for liquefying evaporation gas, and the liquefied evaporation gas is recovered to the storage unit, and the unliquefied evaporation gas is returned to the cryogenic unit for recycling. In the application, the unliquefied hydrogen and the re-vaporized hydrogen in the storage unit are liquefied as evaporation gas, the liquefied evaporation gas is recovered to the storage unit, and the unliquefied evaporation gas is returned to the cryogenic unit for recycling, so that the zero emission of evaporation gas is realized, and the problems of great energy waste and safety risk are solved.

Description

Technical Field

[0001] The present invention relates to the field of liquid hydrogen preparation and storage, and in particular to a zero-emission liquid hydrogen production and storage system and method. Background Art

[0002] As the global energy system gradually shifts from fossil fuels to clean energy, hydrogen energy has garnered widespread attention due to its high energy density and pollution-free nature. The establishment of a supply chain is a crucial component in the development of hydrogen energy technology, and hydrogen storage and transportation technologies are essential for its large-scale utilization. Compared to gaseous hydrogen, liquid hydrogen has a higher energy density and lower storage pressure, offering significant transportation advantages. Therefore, the development of liquid hydrogen production technology is of great significance to the development of hydrogen energy technology.

[0003] The key to promoting liquid hydrogen technology lies in reducing production costs and developing storage technology. Hydrogen has a boiling point of -252.77°C at atmospheric pressure (101.325kPa), and the liquefaction process consumes a large amount of energy. Currently, the energy consumption per unit mass of liquid hydrogen in the production process is generally high, especially for large-scale liquid hydrogen production. High energy consumption has become a key constraint on the development of liquid hydrogen technology. Furthermore, during the storage process, especially for large-scale liquid hydrogen storage, liquid hydrogen storage tanks inevitably absorb heat from the outside world, generating re-vaporized gas. Efficiently recovering this vaporized gas, re-liquefying it, and achieving zero vaporized gas emissions are of great significance to the promotion of liquid hydrogen technology.

[0004] Based on this background, the present invention studies a zero-emission liquid hydrogen production and storage system and method. Summary of the Invention

[0005] In response to the deficiencies in the prior art, the present invention provides a zero-emission liquid hydrogen production and storage system and method. The system and method liquefy the unliquefied hydrogen and revaporized hydrogen in the storage unit as boil-off gas, and recycle the liquefied boil-off gas to the storage unit, while returning the unliquefied boil-off gas to the cryogenic unit for recycling, thereby achieving zero boil-off gas emissions and solving the problems of huge energy waste and safety risks.

[0006] To achieve the above objectives, the present invention provides a zero-emission liquid hydrogen production and storage system in a first aspect, the system comprising a liquid hydrogen production system and a liquid hydrogen storage system connected to each other;

[0007] The liquid hydrogen production system includes a raw hydrogen feed pipeline, a pressure regulating unit, a pre-cooling unit and a cryogenic unit connected in sequence, and is used to liquefy the raw hydrogen to produce liquid hydrogen and unliquefied hydrogen;

[0008] The liquid hydrogen storage system includes a separation and storage unit and a boil-off gas recovery unit that are interconnected;

[0009] The separation and storage unit comprises a separation unit and a storage unit connected to each other, and is used for separating and storing the liquid hydrogen and the unliquefied hydrogen gas;

[0010] The evaporation gas recovery unit is used for liquefying the evaporation gas and recovering the liquefied evaporation gas to the storage unit, and the unliquefied evaporation gas is returned to the cryogenic unit for recycling.

[0011] The evaporation gas is the unliquefied hydrogen gas and the revaporized hydrogen gas in the storage unit.

[0012] The second aspect of the present application provides a zero-emission liquid hydrogen production and storage method, which is carried out in the system described above, and the method comprises the following steps:

[0013] 1) sending the raw hydrogen gas to the pressure regulating unit to increase the pressure to obtain high-pressure hydrogen gas;

[0014] 2) sending the high-pressure hydrogen gas to the pre-cooling unit to pre-cool and obtain pre-cooled hydrogen gas;

[0015] 3) sending the pre-cooled hydrogen gas to the cryogenic unit to cool and depressurize to obtain liquid hydrogen and unliquefied hydrogen gas;

[0016] 4) sending the liquid hydrogen and the unliquefied hydrogen gas to the separation unit to separate, and sending the separated liquid hydrogen and unliquefied hydrogen gas to the storage unit;

[0017] 5) liquefying the unliquefied hydrogen gas and the revaporized hydrogen gas in the storage unit as evaporation gas, recovering the liquefied evaporation gas to the storage unit, and returning the unliquefied evaporation gas to the cryogenic unit for recycling.

[0018] The present application has the following effects:

[0019] (1) The zero-emission liquid hydrogen production and storage system provided by the present application liquefies the unliquefied hydrogen gas and the revaporized hydrogen gas in the storage unit as evaporation gas, recovers the liquefied evaporation gas to the storage unit, and returns the unliquefied evaporation gas to the cryogenic unit for recycling, thereby realizing zero emission of evaporation gas and solving the problems of huge energy waste and safety risk.

[0020] (2) The zero-emission liquid hydrogen production and storage system provided by the present application uses the helium refrigeration cycle assembly to provide cold energy for the evaporation gas in the liquid hydrogen storage tank, realizes direct condensation of part of the evaporation gas in the liquid hydrogen storage tank, and greatly reduces the energy consumption and cost in the recycling of evaporation gas.

[0021] (3) The zero-emission liquid hydrogen production and storage system provided by the present application recycles part of the evaporation gas by using the evaporation gas pressure recovery assembly to enter the liquid hydrogen preparation unit, and at the same time, uses the para-normal hydrogen conversion of the part of evaporation gas to provide cold energy for the helium cycle. The use of this part of cold energy avoids the introduction of external refrigerant, reduces the investment cost and operation and management difficulty, and at the same time realizes the zero emission of evaporation gas.

[0022] (4) The zero-emission liquid hydrogen production and storage system proposed in the present invention is provided with an automatic control interlock to ensure the smooth operation of the boil-off gas recovery unit, maintain the pressure of the liquid hydrogen storage tank, and at the same time increase the amount of boil-off gas recondensed in the storage tank as much as possible to reduce the system operation power consumption. Correspondingly, the present invention provides a boosting unit and a multi-stage throttling device in the hydrogen raw material pipeline of the liquid hydrogen preparation system to achieve multi-stage pressure control of the raw hydrogen to adapt to the pressure requirements of the boil-off gas boosting recovery component.

[0023] (5) The zero-emission liquid hydrogen production and storage system and method proposed in the present invention solves the storage problem of large-scale liquid hydrogen production, achieves zero evaporation gas emission, greatly reduces the power consumption of liquid hydrogen production and the cost of equipment operation and liquid hydrogen storage, and will greatly improve the level of liquid hydrogen production and storage technology.

[0024] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0026] Figure 1 It is a process flow diagram of a specific implementation of the zero-emission liquid hydrogen production and storage system proposed by the present invention.

[0027] Description of reference numerals:

[0028] 1. Pressure regulating unit; 2. Precooling unit; 3. Cryogenic unit; 4. Separation and storage unit; 5. Boil-off gas recovery unit; 6. Hydrogen compressor; 7. Precooling cold box; 8. First cryogenic cold box; 9. Second cryogenic cold box; 10. Separator; 11. Liquid hydrogen pump; 12. Liquid hydrogen storage tank; 13. Helium buffer tank; 14. Helium compressor; 15. Helium cooling heat exchanger; 16. Helium expander; 17. Boil-off gas condenser; 18. Ejector booster; 20. First raw hydrogen throttle valve; 21. Second raw material hydrogen throttle valve; 22. Circulating refrigerant throttle valve; 23. Circulating refrigerant expander; 24. Circulating refrigerant compressor; 25. Helium reflux regulating valve; 26. Pressure detection element; 27. Helium regulating valve; 28. Flow measuring element; 29. ​​Liquid hydrogen storage tank pressure measuring element; 30. High-pressure hydrogen regulating valve; 31. Pressure regulating pipeline; 32. Hydrogen cooling pipeline; 33. High-pressure circulating refrigerant pipeline; 34. Low-pressure circulating refrigerant pipeline; 35. Pre-cooling refrigerant pipeline. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0030] In this disclosure, unless otherwise stated, directional words such as "upper" and "lower" generally refer to the upper and lower positions of the device in normal use. Figure 1 In the drawing orientation, "inside" and "outside" refer to the outline of the device. In addition, the terms "first, second, and third" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, features specified as "first, second, and third" may explicitly or implicitly include one or more of such features. In the description of this disclosure, "plurality" means two or more, unless otherwise explicitly and specifically defined.

[0031] The present invention provides a zero-emission liquid hydrogen production and storage system, such as Figure 1 As shown, the system includes a liquid hydrogen production system and a liquid hydrogen storage system that are interconnected;

[0032] The liquid hydrogen production system includes a raw hydrogen feed pipeline, a pressure regulating unit 1, a pre-cooling unit 2 and a cryogenic unit 3 connected in sequence, and is used to liquefy the raw hydrogen to produce liquid hydrogen and unliquefied hydrogen;

[0033] The liquid hydrogen storage system includes a separation and storage unit 4 and a boil-off gas recovery unit 5 that are interconnected;

[0034] The separation and storage unit 4 comprises a separation unit and a storage unit connected to each other, and is used to separate and store liquid hydrogen and non-liquefied hydrogen gas;

[0035] The evaporation gas recovery unit 5 is used to liquefy the evaporation gas and recover the liquefied evaporation gas to the storage unit, and the unliquefied evaporation gas returns to the cryogenic unit 3 for recycling;

[0036] Boil-off gas is the unliquefied hydrogen and re-vaporized hydrogen in the storage unit.

[0037] In the present invention, the unliquefied hydrogen and revaporized hydrogen in the storage unit are liquefied as boil-off gas, and the liquefied boil-off gas is recovered to the storage unit, while the unliquefied boil-off gas is returned to the cryogenic unit 3 for recycling, thereby achieving zero discharge of boil-off gas and solving the problems of huge energy waste and safety risks.

[0038] According to the present invention, the pressure regulating unit 1 includes a hydrogen compressor 6 and a pressure regulating pipeline 31; the pre-cooling unit 2 includes a pre-cooling cold box 7 and a circulating refrigerant compressor 24; the cryogenic unit 3 includes a plurality of cryogenic cold boxes connected in sequence;

[0039] The hydrogen compressor 6 is arranged on the pressure regulating pipeline 31, and the inlet of the pressure regulating pipeline 31 is connected with the raw hydrogen feed pipeline;

[0040] The pre-cooling cold box 7 and the plurality of cryogenic cold boxes are communicated through the hydrogen cooling pipeline 32, the high-pressure circulating refrigerant pipeline 33 and the low-pressure circulating refrigerant pipeline 34 respectively;

[0041] The inlet of the hydrogen cooling pipeline 32 is connected with the outlet of the pressure regulating pipeline 31, and the outlet of the hydrogen cooling pipeline 32 is connected with the separation unit;

[0042] The first raw hydrogen throttle valve 20 is arranged on the hydrogen cooling pipeline 32, and the part of the hydrogen cooling pipeline 32 located in the cryogenic cold box is provided with a primary- and secondary-hydrogen conversion catalyst injection port.

[0043] According to the present application, the inlet of the high-pressure circulating refrigerant pipeline 33 and the outlet of the low-pressure circulating refrigerant pipeline 34 are arranged on the raw hydrogen feed side of the pre-cooling cold box 7 respectively, and are connected with the outlet and the inlet of the circulating refrigerant compressor 24 respectively;

[0044] The outlet of the high-pressure circulating refrigerant pipeline 33 and the inlet of the low-pressure circulating refrigerant pipeline 34 are arranged on the liquid hydrogen discharge side of the last cryogenic cold box respectively;

[0045] The circulating refrigerant throttle valve 22 is arranged on the pipeline between the outlet of the high-pressure circulating refrigerant pipeline 33 and the inlet of the low-pressure circulating refrigerant pipeline 34;

[0046] The high-pressure circulating refrigerant pipeline 33 is branched into at least one pipeline connected with the low-pressure circulating refrigerant pipeline 34, and the circulating refrigerant expander 23 is arranged on each pipeline;

[0047] The pre-cooling cold box 7 is also communicated with the pre-cooling refrigerant pipeline 35.

[0048] Preferably, the separation unit comprises the separator 10 and the liquid hydrogen pump 11, and the storage unit is the liquid hydrogen storage tank 12;

[0049] The outlet of the hydrogen cooling pipeline 32 is connected with the separator 10 through the second raw hydrogen throttle valve 21;

[0050] The gas phase outlet of the separator 10 is connected with the gas phase inlet of the liquid hydrogen storage tank 12, and the liquid phase outlet is connected with the liquid phase inlet of the liquid hydrogen storage tank 12 through the liquid hydrogen pump 11.

[0051] According to the present application, the evaporation gas recovery unit 5 comprises a helium refrigeration cycle assembly and an evaporation gas pressurization recovery assembly;

[0052] The helium refrigeration cycle assembly comprises a helium compressor 14, a helium cooling heat exchanger 15, a helium expander 16, a boil-off gas condenser 17, and a helium buffer tank 13, and the helium compressor 14, the helium cooling heat exchanger 15, the helium expander 16, the boil-off gas condenser 17 and the helium buffer tank 13 are sequentially connected in series to form a closed refrigeration cycle loop.

[0053] The boil-off gas pressurization and recovery assembly is an ejector pressurizer 18.

[0054] The gas phase outlet of the liquid hydrogen storage tank 12 is connected to the refrigerant inlet of the helium cooling heat exchanger 15, and the refrigerant outlet of the helium cooling heat exchanger 15 is connected to the boil-off gas inlet of the ejector pressurizer 18.

[0055] The hydrogen gas cooling pipeline 32 of the cryogenic unit 3 is provided with a high-pressure hydrogen gas outlet and a low-pressure hydrogen gas inlet.

[0056] The high-pressure hydrogen gas inlet of the ejector pressurizer 18 is connected to the high-pressure hydrogen gas outlet, and the mixed hydrogen gas outlet of the ejector pressurizer 18 is connected to the low-pressure hydrogen gas inlet.

[0057] The boil-off gas condenser 17 is arranged on the top of the liquid hydrogen storage tank 12 and is inserted into the gas space above the liquid hydrogen storage tank 12 at one end, and is used for condensing hydrogen gas above the liquid hydrogen storage tank 12.

[0058] The high-pressure hydrogen gas outlet is located upstream of the low-pressure hydrogen gas inlet.

[0059] In the present application, the helium refrigeration cycle assembly is used to provide cold energy for the boil-off gas in the liquid hydrogen storage tank 12, and direct condensation of part of the boil-off gas in the liquid hydrogen storage tank 12 is realized, so that the energy consumption and cost in the boil-off gas recovery and utilization are greatly reduced.

[0060] In the present application, the boil-off gas pressurization and recovery assembly is used to recover part of the boil-off gas into the liquid hydrogen preparation unit, and the use of the part of the boil-off gas for the conversion of para-hydrogen to ortho-hydrogen provides cold energy for the helium cycle, which avoids the introduction of external refrigerant, reduces investment cost and operation and management difficulty, and realizes zero emission of the boil-off gas.

[0061] In the present application, the boil-off gas passage in the helium cooling heat exchanger 15 is filled with an ortho-para hydrogen conversion catalyst, which is used to promote the conversion of para-hydrogen to ortho-hydrogen in the boil-off gas, release cold energy, and provide cold energy for helium.

[0062] According to a specific embodiment of the present application, an ortho-para hydrogen conversion catalyst injection port is arranged on the pipeline between the gas phase outlet of the liquid hydrogen storage tank 12 and the refrigerant inlet of the helium cooling heat exchanger 15, which is used to promote the conversion of para-hydrogen to ortho-hydrogen in the boil-off gas, release cold energy, and provide cold energy for helium.

[0063] A backflow pipeline is arranged on the helium compressor 14, and the backflow pipeline is in communication with the buffer tank.

[0064] According to a specific embodiment of the present invention, the evaporative gas recovery unit 5 further includes a plurality of regulating valves, a plurality of detection elements and a control system;

[0065] Multiple regulating valves include a helium reflux regulating valve 25, a helium regulating valve 27, and a high-pressure hydrogen regulating valve 30;

[0066] The plurality of detection elements include a pressure detection element 26, a flow measurement element 28, and a liquid hydrogen storage tank pressure measurement element 29;

[0067] A liquid hydrogen tank pressure measuring element 29 is provided above the liquid hydrogen storage tank 12, and a high-pressure hydrogen regulating valve 30 is provided on the high-pressure hydrogen inlet pipeline of the ejector supercharger 18. The pressure measuring element is used to feed back the pressure signal of the liquid hydrogen storage tank 12 to the control system, adjust the opening of the high-pressure hydrogen regulating valve 30 to adapt to the appropriate high-pressure hydrogen flow rate, and maintain the liquid hydrogen storage tank 12 at a stable pressure;

[0068] A flow measuring element 28 is provided on the gas phase outlet pipeline of the liquid hydrogen storage tank 12, and a helium regulating valve 27 is provided on the outlet pipeline of the evaporative gas condenser 17. The flow measuring element 28 feeds back the flow signal to the control system to adjust the opening of the helium regulating valve 27 to adapt to the appropriate helium circulation volume;

[0069] A pressure detection element 26 is provided on the outlet pipeline of the helium compressor 14, and a helium reflux regulating valve 25 is provided on the helium reflux pipeline. The pressure measuring element is used to feed back the pressure signal to the control system and adjust the opening of the helium reflux regulating valve 25 to adapt to the appropriate helium reflux amount and maintain the compressor outlet pressure at a stable value.

[0070] In the present invention, in the helium refrigeration assembly, the helium stored in the helium buffer tank 13 is pressurized by the helium compressor 14. At the same time, in order to ensure the normal operation of the helium compressor 14, a return pipeline is provided in the helium compressor 14, the return pipeline is connected to the buffer tank, and a regulating valve is provided on the return pipeline; the compressed high-pressure helium is cooled by the helium cooling heat exchanger 15, and then expanded and reduced in pressure and temperature by the helium expander 16, and then enters the condenser, providing cooling capacity in the condenser to partially liquefy the evaporated gas in the liquid hydrogen storage tank 12. The helium absorbs heat and heats up in the condenser and then enters the helium buffer tank 13, completing the cycle.

[0071] In the present invention, an automatic control interlock is provided to ensure the smooth operation of the boil-off gas recovery unit 5, maintain the pressure of the liquid hydrogen storage tank 12, and at the same time increase the amount of boil-off gas recondensed in the storage tank as much as possible to reduce the power consumption of the system operation. Accordingly, the present invention provides a boosting unit and a multi-stage throttling device in the hydrogen raw material pipeline of the liquid hydrogen preparation system to achieve multi-stage pressure control of the raw hydrogen to adapt to the pressure requirements of the boil-off gas boosting recovery component.

[0072] In the present invention, a helium refrigeration cycle component is used to provide cold energy for the boil-off gas in the liquid hydrogen storage tank 12, so that part of the boil-off gas is condensed; the cold energy released by the conversion of para-orthohydrogen in the boil-off gas is used to cool the high-pressure helium, providing cold energy for the helium cycle; and a boil-off gas pressurization recovery component is used to recover part of the boil-off gas, thereby achieving zero boil-off gas emissions.

[0073] In the present invention, a pressure detection element 26 is installed at the outlet of the helium compressor 14, and a helium reflux regulating valve 25 is installed in the helium reflux pipeline. The pressure measuring element feeds back a pressure signal to the control system. Under a specific compressor inlet flow rate, due to the fluctuation of the opening of the helium regulating valve 27, the outlet pressure of the helium compressor 14 is blocked. Therefore, according to the compressor outlet pressure, the opening of the helium reflux regulating valve 25 is adjusted to adapt to the appropriate helium reflux flow rate and maintain the compressor outlet pressure at a stable value.

[0074] In the present invention, the zero-emission liquid hydrogen production and storage system and method solve the storage problem of large-scale liquid hydrogen production, achieve zero boil-off gas emissions, greatly reduce the power consumption of liquid hydrogen production and the cost of equipment operation and liquid hydrogen storage, and will greatly improve the level of liquid hydrogen production and storage technology.

[0075] The present invention also provides a method for producing and storing liquid hydrogen with zero emission in the above system, such as Figure 1 As shown, the method includes:

[0076] 1) Feeding the raw hydrogen into the pressure regulating unit 1 to increase the pressure to obtain high-pressure hydrogen;

[0077] 2) sending the high-pressure hydrogen into the precooling unit 2 for precooling to obtain precooled hydrogen;

[0078] 3) sending the pre-cooled hydrogen into the cryogenic unit 3 for refrigeration, cooling, and pressure reduction to obtain liquid hydrogen and unliquefied hydrogen;

[0079] 4) sending the liquid hydrogen and the non-liquefied hydrogen gas into a separation unit for separation, and sending the separated liquid hydrogen and the non-liquefied hydrogen gas into a storage unit;

[0080] 5) The unliquefied hydrogen and re-vaporized hydrogen in the storage unit are liquefied as boil-off gas, and the liquefied boil-off gas is recovered to the storage unit, while the unliquefied boil-off gas is returned to the cryogenic unit 3 for recycling.

[0081] In the present invention, the unliquefied hydrogen and revaporized hydrogen in the storage unit are liquefied as boil-off gas, and the liquefied boil-off gas is recovered to the storage unit, while the unliquefied boil-off gas is returned to the cryogenic unit 3 for recycling, thereby achieving zero discharge of boil-off gas and solving the problems of huge energy waste and safety risks.

[0082] According to the present invention, the cooling capacity of the precooling unit 2 and the cryogenic unit 3 in step 2) and step 3) is jointly provided by the high-pressure circulating refrigerant and the low-pressure circulating refrigerant, the material direction of the high-pressure circulating refrigerant is the same as the material direction of the raw hydrogen gas, and the material direction of the low-pressure circulating refrigerant is opposite to the material direction of the raw hydrogen gas; on the raw hydrogen gas feed side of the precooling unit 2, the low-pressure circulating refrigerant is compressed by the circulating refrigerant compressor 24 and circulated as the high-pressure circulating refrigerant, and on the high-pressure liquid hydrogen discharge side of the cryogenic unit 3, the high-pressure circulating refrigerant is throttled and circulated as the low-pressure circulating refrigerant; the high-pressure circulating refrigerant is divided into multiple strands, and each strand is expanded by its own circulating refrigerant expander 23 to form multiple strands of low-pressure refrigerant, which are respectively injected into the low-pressure circulating refrigerant for circulation; the precooling unit 2 also precools the high-pressure hydrogen by the precooling refrigerant, and the material direction of the precooling refrigerant is opposite to the material direction of the raw hydrogen gas;

[0083] In step 3), when the pre-cooled hydrogen is fed into the cryogenic unit 3 for cooling, lowering the temperature and pressure, a para-hydrogen conversion catalyst is also injected into the cryogenic unit 3 for converting the para-hydrogen;

[0084] In step 5), the circulating helium is pressurized and refluxed, cooled by heat exchange, and expanded in sequence to provide cold to liquefy the boil-off gas. The unliquefied boil-off gas injected into the ortho-parahydrogen conversion catalyst and the circulating helium are heated by heat exchange and then sent to the ejector booster 18 together with the high-pressure hydrogen from the cryogenic unit 3 for mixing and then returned to the cryogenic unit 3 for recycling as low-pressure hydrogen.

[0085] In the present invention, in the helium refrigeration assembly, the helium stored in the helium buffer tank 13 is pressurized by the helium compressor 14. At the same time, in order to ensure the normal operation of the helium compressor 14, a return pipeline is provided in the helium compressor 14, the return pipeline is connected to the buffer tank, and a regulating valve is provided on the return pipeline; the compressed high-pressure helium is cooled by the helium cooling heat exchanger 15, and then expanded and reduced in pressure and temperature by the helium expander 16, and then enters the condenser, providing cooling capacity in the condenser to partially liquefy the evaporated gas in the liquid hydrogen storage tank 12. The helium absorbs heat and heats up in the condenser and then enters the helium buffer tank 13, completing the cycle.

[0086] Preferably, the method further comprises:

[0087] Liquid hydrogen storage tank 12 pressure control: by detecting the pressure of the liquid hydrogen storage tank 12 to control the high-pressure hydrogen flow rate, to maintain the liquid hydrogen storage tank 12 at a stable pressure;

[0088] Helium circulation volume control: The appropriate helium circulation volume is adjusted by detecting the flow rate of unliquefied boil-off gas;

[0089] Helium compressor 14 outlet pipeline pressure control: by detecting the outlet pressure of the helium compressor 14 outlet pipeline to control and adapt the appropriate helium return flow, maintain the compressor outlet pressure at a stable value.

[0090] In the present invention, an automatic control interlock is provided to ensure the smooth operation of the boil-off gas recovery unit 5, maintain the pressure of the liquid hydrogen storage tank 12, and at the same time increase the amount of boil-off gas recondensed in the storage tank as much as possible to reduce the power consumption of the system operation. Accordingly, the present invention provides a boosting unit and a multi-stage throttling device in the hydrogen raw material pipeline of the liquid hydrogen preparation system to achieve multi-stage pressure control of the raw hydrogen to adapt to the pressure requirements of the boil-off gas boosting recovery component.

[0091] In the present invention, a helium refrigeration cycle component is used to provide cold energy for the boil-off gas in the liquid hydrogen storage tank 12, so that part of the boil-off gas is condensed; the cold energy released by the conversion of para-orthohydrogen in the boil-off gas is used to cool the high-pressure helium, providing cold energy for the helium cycle; and a boil-off gas pressurization recovery component is used to recover part of the boil-off gas, thereby achieving zero boil-off gas emissions.

[0092] The present invention will be described in more detail below through examples.

[0093] Example 1

[0094] like Figure 1 As shown, this embodiment provides a zero-emission liquid hydrogen production and storage system, such as Figure 1 As shown, the system includes a liquid hydrogen production system and a liquid hydrogen storage system that are interconnected;

[0095] The liquid hydrogen production system includes a raw hydrogen feed pipeline, a pressure regulating unit 1, a pre-cooling unit 2 and a cryogenic unit 3 connected in sequence, and is used to liquefy the raw hydrogen to produce liquid hydrogen and unliquefied hydrogen;

[0096] The liquid hydrogen storage system includes a separation and storage unit 4 and a boil-off gas recovery unit 5 that are interconnected;

[0097] The separation and storage unit 4 comprises a separation unit and a storage unit connected to each other, and is used to separate and store liquid hydrogen and non-liquefied hydrogen gas;

[0098] The evaporation gas recovery unit 5 is used to liquefy the evaporation gas and recover the liquefied evaporation gas to the storage unit, and the unliquefied evaporation gas returns to the cryogenic unit 3 for recycling;

[0099] Boil-off gas is the unliquefied hydrogen and re-vaporized hydrogen in the storage unit;

[0100] The pressure regulating unit 1 includes a hydrogen compressor 6 and a pressure regulating pipeline 31; the pre-cooling unit 2 includes a pre-cooling cold box 7 and a circulating refrigerant compressor 24; the cryogenic unit 3 includes a first cryogenic cold box 8 and a second cryogenic cold box 9 connected in sequence;

[0101] The hydrogen compressor 6 is provided on the pressure regulating pipeline 31, and the inlet of the pressure regulating pipeline 31 is connected to the raw hydrogen feed pipeline;

[0102] The pre-cooling box 7, the first cryogenic box 8, and the second cryogenic box 9 are connected in sequence through the hydrogen cooling pipeline 32, the high-pressure circulating refrigerant pipeline 33, and the low-pressure circulating refrigerant pipeline 34;

[0103] The inlet of the hydrogen cooling pipeline 32 is connected to the outlet of the pressure regulating pipeline 31; the outlet of the hydrogen cooling pipeline 32 is connected to the separation unit;

[0104] The hydrogen cooling line 32 is provided with a first raw hydrogen throttle valve 20, and the portion of the hydrogen cooling line 32 located in the cryogenic cold box is provided with an injection port for a normal-para hydrogen conversion catalyst;

[0105] The inlet of the high-pressure circulating refrigerant pipeline 33 and the outlet of the low-pressure circulating refrigerant pipeline 34 are respectively arranged on the raw material hydrogen feed side of the pre-cooling cold box 7, and are respectively connected to the outlet and inlet of the circulating refrigerant compressor 24;

[0106] The outlet of the high-pressure circulating refrigerant pipeline 33 and the inlet of the low-pressure circulating refrigerant pipeline 34 are respectively arranged on the liquid hydrogen discharge side of the second cryogenic cold box 9;

[0107] A circulating refrigerant throttle valve 22 is provided on the pipeline between the outlet of the high-pressure circulating refrigerant pipeline 33 and the inlet of the low-pressure circulating refrigerant pipeline 34;

[0108] A branch of the high-pressure circulating refrigerant pipeline 33 is connected to the low-pressure circulating refrigerant pipeline 34, and a circulating refrigerant expander 23 is provided on the branch pipeline;

[0109] The pre-cooling cold box 7 is also connected to the pre-cooling refrigerant pipeline 35;

[0110] The separation unit includes a separator 10 and a liquid hydrogen pump 11; the storage unit is a liquid hydrogen storage tank 12;

[0111] The outlet of the hydrogen cooling pipeline 32 is connected to the separator 10 through the second raw hydrogen throttle valve 21;

[0112] The gas phase outlet of the separator 10 is connected to the gas phase inlet of the liquid hydrogen storage tank 12, and the liquid phase outlet is connected to the liquid phase inlet of the liquid hydrogen storage tank 12 via the liquid hydrogen pump 11;

[0113] The boil-off gas recovery unit 5 includes a helium refrigeration cycle component and a boil-off gas pressurization recovery component;

[0114] The helium refrigeration cycle assembly includes a helium compressor 14, a helium cooling heat exchanger 15, a helium expander 16, a boil-off gas condenser 17, and a helium buffer tank 13, and the helium compressor 14, the helium cooling heat exchanger 15, the helium expander 16, the boil-off gas condenser 17, and the helium buffer tank 13 are sequentially connected end to end to form a closed refrigeration cycle loop;

[0115] The evaporated gas pressurization recovery component is an ejector supercharger 18;

[0116] The gas phase outlet of the liquid hydrogen storage tank 12 is connected to the refrigerant inlet of the helium cooling heat exchanger 15, and the refrigerant outlet of the helium cooling heat exchanger 15 is connected to the boil-off gas inlet of the ejector supercharger 18;

[0117] The hydrogen cooling pipeline 32 of the cryogenic unit 3 is provided with a high-pressure hydrogen outlet and a low-pressure hydrogen inlet;

[0118] The high-pressure hydrogen inlet of the ejector supercharger 18 is connected to the high-pressure hydrogen outlet, and the mixed hydrogen outlet of the ejector supercharger 18 is connected to the low-pressure hydrogen inlet;

[0119] The evaporative gas condenser 17 is arranged on the top of the liquid hydrogen storage tank 12, and one end of the evaporative gas condenser 17 is inserted into the gas space above the liquid hydrogen storage tank 12 to condense the hydrogen above the liquid hydrogen storage tank 12;

[0120] The high-pressure hydrogen outlet is located upstream of the low-pressure hydrogen inlet;

[0121] A para-hydrogen conversion catalyst injection port is provided on the pipeline between the gas phase outlet of the liquid hydrogen storage tank 12 and the refrigerant inlet of the helium cooling heat exchanger 15 to promote the conversion of para-hydrogen in the boil-off gas into ortho-hydrogen, release cold energy, and provide cooling capacity for the helium gas.

[0122] A return line is provided on the helium compressor 14, and the return line is connected to the buffer tank;

[0123] The evaporative gas recovery unit 5 also includes a plurality of regulating valves, a plurality of detection elements and a control system;

[0124] Multiple regulating valves include a helium reflux regulating valve 25, a helium regulating valve 27, and a high-pressure hydrogen regulating valve 30;

[0125] The plurality of detection elements include a pressure detection element 26, a flow measurement element 28, and a liquid hydrogen storage tank pressure measurement element 29;

[0126] A liquid hydrogen tank pressure measuring element 29 is provided above the liquid hydrogen storage tank 12, and a high-pressure hydrogen regulating valve 30 is provided on the high-pressure hydrogen inlet pipeline of the ejector supercharger 18. The pressure measuring element is used to feed back the pressure signal of the liquid hydrogen storage tank 12 to the control system, adjust the opening of the high-pressure hydrogen regulating valve 30 to adapt to the appropriate high-pressure hydrogen flow rate, and maintain the liquid hydrogen storage tank 12 at a stable pressure;

[0127] A flow measuring element 28 is provided on the gas phase outlet pipeline of the liquid hydrogen storage tank 12, and a helium regulating valve 27 is provided on the outlet pipeline of the evaporative gas condenser 17. The flow measuring element 28 feeds back the flow signal to the control system to adjust the opening of the helium regulating valve 27 to adapt to the appropriate helium circulation volume;

[0128] A pressure detection element 26 is provided on the outlet pipeline of the helium compressor 14, and a helium reflux regulating valve 25 is provided on the helium reflux pipeline. The pressure measuring element is used to feed back the pressure signal to the control system and adjust the opening of the helium reflux regulating valve 25 to adapt to the appropriate helium reflux amount and maintain the compressor outlet pressure at a stable value.

[0129] The zero-emission liquid hydrogen production and storage process in the above system is as follows: Figure 1 As shown:

[0130] 1) Feeding raw hydrogen gas at a temperature of 300K into the pressure regulating pipeline 31 of the pressure regulating unit 1, and pressurizing it with the pressure regulating compressor to obtain high-pressure hydrogen gas with a pressure of 3.0 MPa(A);

[0131] 2) The high-pressure hydrogen is sent to the hydrogen cooling pipeline 32 of the pre-cooling cold box 7 to absorb cold energy to obtain pre-cooled hydrogen with a temperature of 83K;

[0132] 3) A portion of the pre-cooled hydrogen is fed into an ejector supercharger 18 as high-pressure hydrogen, and the remaining portion is sequentially cooled in a first cryogenic cold box 8 and throttled by a first raw hydrogen throttle valve 20 to obtain reduced-pressure hydrogen at a pressure of 0.8 MPa(A). The reduced-pressure hydrogen is sequentially cooled in a second cryogenic cold box 9 and throttled by a second raw hydrogen throttle valve 21 to obtain a gas-liquid mixture at a pressure of 0.13 MPa(A) and a temperature of 21 K, the gas-liquid mixture comprising 90 wt% liquid hydrogen and 10 wt% unliquefied hydrogen;

[0133] 4) sending the liquid hydrogen and the non-liquefied hydrogen gas into a separation unit for separation, and sending the separated liquid hydrogen and the non-liquefied hydrogen gas into a storage unit;

[0134] 5) liquefying the unliquefied hydrogen and revaporized hydrogen in the storage unit as boil-off gas, recovering the liquefied boil-off gas to the storage unit, and returning the unliquefied boil-off gas to the cryogenic unit 3 for recycling;

[0135] In step 2) and step 3), the cooling capacity of the precooling unit 2 and the cryogenic unit 3 is provided by a high-pressure circulating refrigerant and a low-pressure circulating refrigerant. The material direction of the high-pressure circulating refrigerant is the same as that of the raw hydrogen material, and the material direction of the low-pressure circulating refrigerant is opposite to that of the raw hydrogen material. On the raw hydrogen feed side of the precooling unit 2, the low-pressure circulating refrigerant is compressed by the circulating refrigerant compressor 24 and circulated as the high-pressure circulating refrigerant. On the high-pressure liquid hydrogen discharge side of the cryogenic unit 3, the high-pressure circulating refrigerant is throttled and circulated as the low-pressure circulating refrigerant. A stream of the high-pressure circulating refrigerant is separated and expanded by the circulating refrigerant expander 23 to form a low-pressure refrigerant which is injected into the low-pressure circulating refrigerant for circulation. The precooling unit 2 also precools the high-pressure hydrogen by the precooling refrigerant, and the material direction of the precooling refrigerant is opposite to that of the raw hydrogen material.

[0136] In step 3), when the pre-cooled hydrogen is fed into the cryogenic unit 3 for cooling, lowering the temperature and pressure, a para-hydrogen conversion catalyst is also injected into the cryogenic unit 3 for converting the para-hydrogen;

[0137] In step 5), the circulating helium is pressurized and refluxed, heat exchanged and cooled, and expanded in sequence to provide cooling capacity to liquefy the boil-off gas. The unliquefied boil-off gas injected with the normal-parahydrogen conversion catalyst and the circulating helium are heated by heat exchange and then sent to the ejector booster 18 together with the high-pressure hydrogen from the cryogenic unit 3 for mixing and then returned to the cryogenic unit 3 for recycling as low-pressure hydrogen.

[0138] The process also includes:

[0139] Liquid hydrogen storage tank 12 pressure control: by detecting the pressure of the liquid hydrogen storage tank 12 to control the high-pressure hydrogen flow rate, to maintain the liquid hydrogen storage tank 12 at a stable pressure;

[0140] Helium circulation volume control: The appropriate helium circulation volume is adjusted by detecting the flow rate of unliquefied boil-off gas;

[0141] Helium compressor 14 outlet pipeline pressure control: by detecting the outlet pressure of the helium compressor 14 outlet pipeline to control and adapt the appropriate helium return flow, maintain the compressor outlet pressure at a stable value.

[0142] In this embodiment, hydrogen is selected as the circulating refrigerant to provide cooling capacity for the cold boxes in the precooling unit 2 and the cryogenic unit 3, wherein the high-pressure circulating hydrogen pressure is 2.6 MPa(A) and the low-pressure circulating hydrogen pressure is 0.2 MPa(A); liquid nitrogen is selected as the precooling refrigerant to provide cooling capacity for the precooling cold box 7 through the precooling refrigerant pipeline 35.

[0143] In this embodiment, the pressure of the gas space above the liquid hydrogen storage tank 12 is maintained at 150 kPa(A).

[0144] In this embodiment, under the control of the automatic control system, the liquid hydrogen production and storage system is in a stable operating state, and the ratio of the helium circulation volume, the boil-off gas pressurization recovery volume, and the boil-off gas condensation recovery volume is 20:7:3, achieving efficient boil-off gas recovery and zero emissions.

[0145] While the embodiments of the present invention have been described above, the above description is intended to be exemplary, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A zero-emission liquid hydrogen production and storage system, characterized in that: The system includes a liquid hydrogen production system and a liquid hydrogen storage system connected to each other; The liquid hydrogen production system includes a raw hydrogen feed pipeline, a pressure regulating unit, a pre-cooling unit and a cryogenic unit connected in sequence, and is used to liquefy the raw hydrogen to produce liquid hydrogen and unliquefied hydrogen; The liquid hydrogen storage system includes a separation and storage unit and a boil-off gas recovery unit that are interconnected; The separation and storage unit includes a separation unit and a storage unit connected to each other, and is used to separate and store liquid hydrogen and non-liquefied hydrogen gas; The evaporation gas recovery unit is used to liquefy the evaporation gas and recover the liquefied evaporation gas to the storage unit, and the unliquefied evaporation gas is returned to the cryogenic unit for recycling; The boil-off gas is the unliquefied hydrogen and re-vaporized hydrogen in the storage unit; The boil-off gas recovery unit includes a helium refrigeration cycle component and a boil-off gas pressurization recovery component; The helium refrigeration cycle assembly includes a helium compressor, a helium cooling heat exchanger, a helium expander, a boil-off gas condenser, and a helium buffer tank, and the helium compressor, the helium cooling heat exchanger, the helium expander, the boil-off gas condenser, and the helium buffer tank are sequentially connected end to end to form a closed refrigeration cycle loop; The evaporated gas pressurization recovery component is an ejector supercharger; The gas phase outlet of the liquid hydrogen storage tank is connected to the refrigerant inlet of the helium cooling heat exchanger, and the refrigerant outlet of the helium cooling heat exchanger is connected to the boil-off gas inlet of the ejector supercharger; The hydrogen cooling pipeline of the cryogenic unit is provided with a high-pressure hydrogen outlet and a low-pressure hydrogen inlet; The high-pressure hydrogen inlet of the ejector supercharger is connected to the high-pressure hydrogen outlet, and the mixed hydrogen outlet of the ejector supercharger is connected to the low-pressure hydrogen inlet; The evaporative gas condenser is installed on the top of the liquid hydrogen storage tank, and one end of the evaporative gas condenser is inserted into the gas space above the liquid hydrogen storage tank to condense the hydrogen above the liquid hydrogen storage tank; The high-pressure hydrogen outlet is located upstream of the low-pressure hydrogen inlet.

2. The system according to claim 1, wherein: The pressure regulating unit includes a hydrogen compressor and a pressure regulating pipeline; the pre-cooling unit includes a pre-cooling cold box and a circulating refrigerant compressor; the cryogenic unit includes a plurality of cryogenic cold boxes connected in sequence; The hydrogen compressor is arranged on a pressure regulating pipeline, and the inlet of the pressure regulating pipeline is connected to the raw hydrogen feed pipeline; The pre-cooling cold box and the multiple cryogenic cold boxes are connected through a hydrogen cooling pipeline, a high-pressure circulating refrigerant pipeline and a low-pressure circulating refrigerant pipeline respectively; The inlet of the hydrogen cooling pipeline is connected to the outlet of the pressure regulating pipeline; the outlet of the hydrogen cooling pipeline is connected to the separation unit; A first raw material hydrogen throttle valve is provided on the hydrogen cooling pipeline, and a normal-para hydrogen conversion catalyst injection port is provided on the portion of the hydrogen cooling pipeline located in the cryogenic cold box.

3. The system according to claim 2, characterized in that The inlet of the high-pressure circulating refrigerant pipeline and the outlet of the low-pressure circulating refrigerant pipeline are respectively arranged on the raw material hydrogen feed side of the pre-cooling cold box, and are respectively connected to the outlet and inlet of the circulating refrigerant compressor; The outlet of the high-pressure circulating refrigerant pipeline and the inlet of the low-pressure circulating refrigerant pipeline are respectively arranged on the liquid hydrogen discharge side of the last cryogenic cold box; A circulating refrigerant throttle valve is provided on the pipeline between the outlet of the high-pressure circulating refrigerant pipeline and the inlet of the low-pressure circulating refrigerant pipeline; The high-pressure circulating refrigerant pipeline branches out at least one branch pipeline connected to the low-pressure circulating refrigerant pipeline, and each branch pipeline is provided with a circulating refrigerant expander; The pre-cooling cold box is also connected to the pre-cooling refrigerant pipeline.

4. The system according to claim 3, characterized in that The separation unit includes a separator and a liquid hydrogen pump; the storage unit is a liquid hydrogen storage tank; The outlet of the hydrogen cooling pipeline is connected to the separator via the second raw hydrogen throttle valve; The gas phase outlet of the separator is connected to the gas phase inlet of the liquid hydrogen storage tank, and the liquid phase outlet is connected to the liquid phase inlet of the liquid hydrogen storage tank via a liquid hydrogen pump.

5. The system according to claim 1, wherein: A para-hydrogen conversion catalyst injection port is provided on the pipeline between the gas phase outlet of the liquid hydrogen storage tank and the refrigerant inlet of the helium cooling heat exchanger to promote the conversion of para-hydrogen in the boil-off gas into ortho-hydrogen, release cold energy, and provide cooling capacity for the helium gas. A reflux pipeline is provided on the helium compressor, and the reflux pipeline is communicated with the buffer tank.

6. The system according to claim 5, characterized in that The evaporative gas recovery unit also includes a plurality of regulating valves, a plurality of detection elements and a control system; Multiple regulating valves include a helium reflux regulating valve, a helium regulating valve, and a high-pressure hydrogen regulating valve; Multiple detection elements include pressure detection elements, flow measurement elements, and liquid hydrogen storage tank pressure measurement elements; A liquid hydrogen storage tank pressure measuring element is provided above the liquid hydrogen storage tank, and a high-pressure hydrogen regulating valve is provided on the high-pressure hydrogen inlet pipeline of the ejector supercharger. The pressure measuring element is used to feed back the pressure signal of the liquid hydrogen storage tank to the control system, adjust the opening of the high-pressure hydrogen regulating valve to adapt to the appropriate high-pressure hydrogen flow rate, and maintain the liquid hydrogen storage tank at a stable pressure; A flow measuring element is provided on the gas phase outlet pipeline of the liquid hydrogen storage tank, and a helium regulating valve is provided on the outlet pipeline of the evaporative gas condenser. The flow measuring element feeds back the flow signal to the control system to adjust the opening of the helium regulating valve to adapt to the appropriate helium circulation volume; A pressure detection element is provided on the helium compressor outlet pipeline, and a helium reflux regulating valve is provided on the helium reflux pipeline. The pressure measuring element is used to feed back the pressure signal to the control system, adjust the opening of the helium reflux regulating valve to adapt to the appropriate helium reflux amount, and maintain the compressor outlet pressure at a stable value.

7. A zero-emission liquid hydrogen production and storage method, carried out in the system according to any one of claims 1 to 6, characterized in that: The method includes: 1) Feeding the raw hydrogen into the pressure regulating unit to increase the pressure and obtain high-pressure hydrogen; 2) Sending the high-pressure hydrogen into the pre-cooling unit for pre-cooling to obtain pre-cooled hydrogen; 3) The pre-cooled hydrogen is sent to a cryogenic unit for refrigeration, cooling, and pressure reduction to obtain liquid hydrogen and non-liquefied hydrogen; 4) sending the liquid hydrogen and the non-liquefied hydrogen gas into a separation unit for separation, and sending the separated liquid hydrogen and the non-liquefied hydrogen gas into a storage unit; 5) The unliquefied hydrogen and re-vaporized hydrogen in the storage unit are liquefied as boil-off gas, and the liquefied boil-off gas is recovered to the storage unit, while the unliquefied boil-off gas is returned to the cryogenic unit for recycling.

8. The method according to claim 7, characterized in that In step 2) and step 3), the cooling capacity of the precooling unit and the cryogenic unit is provided by a high-pressure circulating refrigerant and a low-pressure circulating refrigerant. The material direction of the high-pressure circulating refrigerant is the same as that of the raw hydrogen material, and the material direction of the low-pressure circulating refrigerant is opposite to that of the raw hydrogen material. On the raw hydrogen feed side of the precooling unit, the low-pressure circulating refrigerant is compressed by the circulating refrigerant compressor and circulated as the high-pressure circulating refrigerant. On the high-pressure liquid hydrogen discharge side of the cryogenic unit, the high-pressure circulating refrigerant is throttled and circulated as the low-pressure circulating refrigerant. The high-pressure circulating refrigerant is divided into multiple streams, and each stream is expanded by its own circulating refrigerant expander to form multiple streams of low-pressure refrigerant, which are respectively injected into the low-pressure circulating refrigerant for circulation. The precooling unit also precools the high-pressure hydrogen by the precooling refrigerant, and the material direction of the precooling refrigerant is opposite to that of the raw hydrogen material. In step 3), when the pre-cooled hydrogen is fed into the cryogenic unit for refrigeration, cooling, and pressure reduction, a para-hydrogen conversion catalyst is also injected into the cryogenic unit for conversion of para-hydrogen; In step 5), the circulating helium is pressurized and refluxed, cooled by heat exchange, and expanded in sequence to provide cold to liquefy the boil-off gas. The unliquefied boil-off gas injected into the ortho-parahydrogen conversion catalyst is heated by heat exchange with the circulating helium and then sent to the ejector booster together with the high-pressure hydrogen from the cryogenic unit for mixing and then returned to the cryogenic unit for recycling as low-pressure hydrogen.

9. The method according to claim 8, characterized in that The method further comprises: Liquid hydrogen storage tank pressure control: Control the high-pressure hydrogen flow by detecting the liquid hydrogen storage tank pressure to maintain the liquid hydrogen storage tank at a stable pressure; Helium circulation volume control: The appropriate helium circulation volume is adjusted by detecting the flow rate of unliquefied boil-off gas; Helium compressor outlet pipeline pressure control: by detecting the outlet pressure of the helium compressor outlet pipeline to control and adapt the appropriate helium return flow, maintain the compressor outlet pressure at a stable value.

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