Liquid pressurizing and cold returning type natural gas liquefaction device

Abstract

The invention relates to a liquid pressurizing and cold returning type natural gas liquefying device which is characterized in that pure liquid nitrogen, coarse liquefied natural gas or dirty liquid nitrogen generated by a rectifying tower in the natural gas liquefying device enters a main heat exchanger or a cold returning device after being pressurized by a hydraulic pump, and gas generated after heat absorption and gasification in the main heat exchanger or the cold returning device enters a high-pressure expansion machine to do work through expansion; exhaust gas of the high-pressure expansion machine enters the rectifying tower to provide cooling capacity needed by natural gas liquefaction, liquefied natural gas generated by the rectifying tower serves as a product to be output, and nitrogen discharged from the rectifying tower achieves closed-loop circulation through the circulating compressor. The purposes of energy conservation and consumption reduction of the natural gas liquefaction device are achieved through the processes of primary pressurization of the gas compressor, secondary pressurization of the hydraulic pump, cold returning, gasification and expansion.

Description

Technical Field

[0001] This invention relates to a natural gas liquefaction device, and more particularly to a natural gas liquefaction device that uses a hydraulic pump for pressurization, recooling, vaporization, and expansion refrigeration, belonging to the field of natural gas liquefaction technology. Background Technology

[0002] Natural gas is a high-quality and clean fossil energy source, and its liquefaction and storage are key technologies for its development and utilization.

[0003] It is estimated that by the middle of this century, if China consumes 5000*10... 8 m³ / a, of which 1000*10 are imported LNG 8 Based on m³ / a (equivalent to Japan's current import volume), the usable cold energy converted to electrical energy is 257*10. 8 kWh / a, equivalent to a 600*10 4 The annual power generation of a kW power plant. Therefore, how to achieve breakthroughs in LNG technology, management mechanisms, and market operations to significantly reduce LNG energy consumption, achieving substantial energy savings and economic benefits, while simultaneously promoting the rapid development of a large-scale cold energy industry chain including air separation and coal-enriched oxygenation, is crucial to contributing to my country's comprehensive realization of a circular and resource-saving economy. Furthermore, China's rapid economic development and model transformation necessitate the large-scale utilization of LNG.

[0004] There are three main traditional natural gas liquefaction processes:

[0005] 1. Cascade liquefaction process (also known as stepped liquefaction process, superimposed liquefaction process, or series evaporation-condensation liquefaction process) is mainly used in base load type natural gas liquefaction units.

[0006] 2. Mixed Refrigerant Liquefaction Process: Also known as the MRC liquefaction process, MRC uses a multi-component mixed refrigerant, consisting of C1 to C5 hydrocarbons and N2, as the working fluid. It involves staged condensation, evaporation, and throttling expansion to achieve different temperature levels of cooling capacity, thus gradually cooling and liquefying natural gas. MRC achieves similar results to cascaded liquefaction processes while overcoming their system complexity. Since the 1980s, for baseload-type natural gas liquefaction plants, both new and expanded baseload-type natural gas liquefaction processes have almost invariably adopted the propane pre-cooling mixed refrigerant liquefaction process.

[0007] 3. Liquefaction process with expander: The natural gas liquefaction process with expander refers to the process in which the gas expands and does work in the expander while the temperature is reduced and the work is recovered. It can be divided into nitrogen expansion liquefaction process and natural gas expansion liquefaction process.

[0008] Compared to cascaded liquefaction processes and mixed refrigerant refrigeration cycles, the nitrogen expansion liquefaction process is simpler, more compact, and slightly less expensive. It starts up quickly, achieving full load production within 2-4 hours of hot start-up. It is flexible, adaptable, easy to operate and control, and safe; venting does not pose a fire or explosion hazard. The refrigerant is a single-component gas, eliminating the hassle of refrigerant separation and storage found in mixed refrigerant refrigeration cycles, and avoiding the associated safety issues, making the liquefaction cold box simpler and more compact. However, energy consumption is approximately 40% higher than that of mixed refrigerant liquefaction processes. To reduce the power consumption of the expansion refrigeration cycle, a nitrogen-methane expansion refrigeration cycle has been developed, replacing pure nitrogen with a nitrogen-methane two-component mixed gas, saving 10%-20% of power consumption compared to the pure nitrogen expansion refrigeration cycle.

[0009] Because liquefaction processes with expanders are relatively simple to operate and require moderate investment, expander refrigeration technology has irreplaceable advantages in small-scale peak-shaving units, offshore FLNG, and high-nitrogen gas treatment due to its simplicity, safety, and reliability. However, its application is limited in large-scale baseload units due to its high energy consumption.

[0010] Chinese Patents 201310029518.3 - A natural gas isobaric liquefaction device (Authorization Announcement No.: CN103148673B; Authorization Announcement Date: 2015.01.07), 201310030104.2 - A natural gas isobaric liquefaction device (Authorization Announcement No.: CN103148674B; ​​Authorization Announcement Date: 2015.03.18), and 201310030948.7 - A natural gas isobaric liquefaction device (Authorization Announcement No.: CN103148674B; ​​Authorization Announcement Date: 2015.03.18), are hereby granted. (Announcement No.: CN103162511B; Authorization Announcement Date: 2015.10.21) proposes an external circulation liquid pressurization and recirculation cooling system based on a new theoretical foundation. This system replenishes the cooling capacity required for natural gas liquefaction and compensates for heat loss. The system adopts an external circulation cooling process and requires a separate refrigerant storage tank to construct a cooling circulation loop for the refrigerant. The material balance and energy balance organization of the cooling system are relatively complex, and the rationality of the process flow has not yet been experimentally verified.

[0011] Therefore, how to fully leverage the advantages of natural gas liquefaction processes with expanders while effectively reducing energy consumption in the liquefaction process, making it economically viable and applicable to large-scale baseload installations, is a question worthy of in-depth research and exploration by practitioners in the liquefied natural gas industry. Summary of the Invention

[0012] The purpose of this invention is to address the high energy consumption problem in natural gas liquefaction processes with expanders by proposing a liquid-pressurized and recooled natural gas liquefaction device. Utilizing the near-incompressible fluid characteristics of liquid during pressurization, a hydraulic pump is used for secondary pressurization and recooling. This effectively improves the refrigeration capacity of the expander while saving most of the energy required for the secondary pressurization process of the gas entering the expander, thereby achieving energy conservation and consumption reduction in the natural gas liquefaction device and expanding its application areas.

[0013] The objective of this invention is achieved through the following measures:

[0014] A liquid-pressurized recirculating natural gas liquefaction unit, characterized in that: the natural gas liquefaction unit includes a compressor 3, a purifier 6, a main heat exchanger 8, a distillation column, a circulating compressor 22, a hydraulic pump 11, and a high-pressure expander 13.

[0015] Natural gas, after being compressed by compressor 3, enters purifier 6 to remove moisture, carbon dioxide, hydrogen sulfide, hydrocarbons, etc., and then enters distillation column via main heat exchanger 8. The low-temperature pure nitrogen produced in distillation column enters main heat exchanger 8 to cool the natural gas fed from purifier 6. Pure liquid nitrogen, waste liquid nitrogen, or crude liquefied natural gas produced in distillation column is pressurized by hydraulic pump 11 and enters main heat exchanger 8. After absorbing heat and vaporizing in main heat exchanger 8, it enters high-pressure expander 13 to expand and do work. The exhaust gas from high-pressure expander 13 enters distillation column to provide the cooling required for distillation. The low-temperature pure nitrogen produced in distillation column recovers its cooling capacity in main heat exchanger 8, and then is pressurized by circulating compressor 22 before merging with the natural gas from compressor 3 outlet and entering purifier 6.

[0016] Alternatively, natural gas is compressed by compressor 3 and then enters purifier 6 to remove moisture, carbon dioxide, hydrogen sulfide, hydrocarbons, etc. It then enters distillation column via cooler 7 and main heat exchanger 8. The low-temperature pure nitrogen produced in distillation column enters main heat exchanger 8 to cool the natural gas fed from purifier 6. The pure liquid nitrogen, waste liquid nitrogen, or crude liquefied natural gas produced in distillation column is pressurized by hydraulic pump 11 and enters cooler 7. After absorbing heat and vaporizing in cooler 7, it enters high-pressure expander 13 to expand and do work. The exhaust gas from high-pressure expander 13 enters distillation column to provide the cooling capacity required for distillation. The low-temperature pure nitrogen produced in distillation column recovers the cooling capacity through main heat exchanger 8, and then is pressurized by circulating compressor 22. After merging with the natural gas from compressor 3 outlet, it enters purifier 6.

[0017] The distillation column includes a lower column 10, a condenser-evaporator 14, an upper column 20, and a subcooler.

[0018] Natural gas enters the lower tower 10 via compressor 3, purifier 6, and main heat exchanger 8. The crude liquefied natural gas produced in the lower tower 10 passes through a cooler and LNG throttle valve 17 before entering the upper tower 20. Nitrogen gas drawn from the upper part of the lower tower 10 is cooled by condenser-evaporator 14 to form liquid nitrogen; part of this liquid nitrogen returns to the lower tower 10, while the other part passes through a cooler and pure liquid nitrogen throttle valve 21 before entering the upper tower 20. The liquefied natural gas produced in the lower part of the upper tower 20 is vaporized by condenser-evaporator 14, and the resulting natural gas returns to the upper tower 20. The lower part of the upper tower 20 is then drawn off... Another portion of liquefied natural gas 24 is output as a product. The low-temperature pure nitrogen gas drawn from the top of the upper tower 20 recovers its cooling capacity through a cooler and main heat exchanger 8. The resulting pure nitrogen gas is then pressurized by the circulating compressor 22 and merges with the natural gas from the outlet of the compressor 3 into the purifier 6. The crude liquefied natural gas, waste liquid nitrogen, or pure liquid nitrogen drawn from the condenser-evaporator 14 drawn from the lower tower 10 enters the main heat exchanger 8 via the hydraulic pump 11 to absorb heat and vaporize. The resulting gas is expanded by the high-pressure expander 13 and then enters the lower tower 10 or the upper tower 20.

[0019] Alternatively, natural gas can enter the lower tower 10 via compressor 3, purifier 6, cooler 7, and main heat exchanger 8. The crude liquefied natural gas produced in the lower tower 10 passes through a cooler and LNG throttle valve 17 before entering the upper tower 20. Nitrogen gas drawn from the upper part of the lower tower 10 is cooled by condenser-evaporator 14 to form liquid nitrogen; part of this liquid nitrogen returns to the lower tower 10, while the other part passes through a cooler and pure liquid nitrogen throttle valve 21 before entering the upper tower 20. The liquefied natural gas produced in the lower part of the upper tower 20 is vaporized by condenser-evaporator 14, and the resulting natural gas returns to the upper tower 20. Another portion of liquefied natural gas 24 drawn from the lower part is output as a product. The low-temperature pure nitrogen drawn from the top of the upper tower recovers its cooling capacity after passing through the cooler and the main heat exchanger 8. The resulting pure nitrogen is then pressurized by the circulating compressor 22 and merged with the natural gas from the outlet of the compressor 3 into the purifier 6. The crude liquefied natural gas, waste liquid nitrogen, or pure liquid nitrogen drawn from the condenser evaporator 14 drawn from the lower tower 10 enters the return cooler 7 through the hydraulic pump 11 to absorb heat and vaporize. The resulting gas is then expanded by the high-pressure expander 13 and enters the lower tower 10 or the upper tower 20.

[0020] The subcooler includes a crude liquefied natural gas subcooler 16 and a liquid nitrogen subcooler 18, as shown in the attached diagram. Figure 1 To be continued Figure 2 The dashed box containing the crude liquefied natural gas subcooler 16 and the liquid nitrogen subcooler 18 is shown. The crude liquefied natural gas subcooler 16 and the liquid nitrogen subcooler 18 can be independent cold exchangers or they can be combined together to form an integrated cold exchanger.

[0021] The waste liquid nitrogen drawn from the lower column 10 enters the upper column 20 through the liquid nitrogen subcooler 18 and the waste liquid nitrogen throttling valve 19.

[0022] A liquefier 9 is provided: the low-temperature pure nitrogen gas drawn from the upper tower 20 is cooled by the crude liquefied natural gas subcooler 16, the liquefier 9, and the main heat exchanger 8. The resulting pure nitrogen gas is then pressurized by the circulating compressor 22 and merged with the natural gas at the outlet of the compressor 3 into the purifier 6.

[0023] A temperature controller 4 is provided: natural gas enters the purifier 6 through the compressor 3 and the temperature controller 4. The low-temperature liquid output from the hydraulic pump 11 absorbs heat and vaporizes through the main heat exchanger 8 or the cooler 7, and then enters the high-pressure expander 13 through the temperature controller 4.

[0024] A precooler 5 is provided: natural gas enters the purifier 6 through the compressor 3 and the precooler 5.

[0025] The braking device 12 driven by the high-pressure expander 13 includes a fan, a liquid booster pump, an air compressor, and a generator.

[0026] When the braking device 12 driven by the high-pressure expander 13 is a gas compressor, the low-temperature liquid output from the hydraulic pump 11 absorbs heat and vaporizes through the main heat exchanger 8 or the cooler 7, and then enters the high-pressure expander 13 after being pressurized by the braking device 12.

[0027] A heat exchanger 25 is provided: the low-temperature liquid output from the hydraulic pump 11 absorbs heat and vaporizes through the main heat exchanger 8 or the cooler 7, is pressurized and heated by the braking device 12, and is cooled by the heat exchanger 25 before entering the high-pressure expander 13.

[0028] A pretreatment unit 2 is provided: natural gas passes through the pretreatment unit 2 to remove carbon dioxide, hydrogen sulfide, moisture, heavy hydrocarbons, mercury, etc., from the raw natural gas before entering the compressor 3. The natural gas pretreatment unit 2 is a key link to ensure the safe and efficient operation of the subsequent liquefaction process.

[0029] The "high pressure" in the high-pressure expander 13 is relative to existing natural gas liquefaction units with expander cycles.

[0030] The recooler 7 adopts a partitioned heat exchange method.

[0031] The precooler 5 is used to reduce the temperature of the natural gas entering the purifier 6.

[0032] The subcooler is used to subcool the liquid entering the upper column 20, thereby reducing vaporization losses.

[0033] In practical applications, the crude liquefied natural gas subcooler 16, the liquid nitrogen subcooler 18, and the liquefier 9 can be integrated into a single design. A multi-flow plate-fin heat exchanger can be used to integrate the liquid nitrogen and crude liquefied natural gas channels into one device, forming an integrated composite heat exchanger.

[0034] When using a pressurized turbine expander for refrigeration and molecular sieve adsorption for purification, the cooling capacity regulation function of the liquefier 9 can be shared by the main heat exchanger 8 and the subcooler.

[0035] The purifier 6 is used to further remove moisture, carbon dioxide, and other substances contained in the natural gas.

[0036] The purifier 6 includes a molecular sieve purifier, an alumina purifier, a silica gel purifier, an activated carbon purifier, a catalytic converter, a stone regenerator, etc., which are used individually or in combination to remove components in natural gas that are harmful to gas separation.

[0037] The main function of the precooler 5 is to reduce the temperature of compressed natural gas from 80℃~120℃ to 8℃~15℃ to meet the purification temperature requirements of the purifier 6, and further recover the cooling capacity of the return gas. The main types include: nitrogen-water precooler (which uses the return waste nitrogen to exchange heat with compressed natural gas to reduce the temperature of natural gas), water cooling tower (which uses the cooling capacity of the return waste nitrogen to cool the cooling water to form a closed-loop circulating water system), shell and tube precooler (cooling water flows inside the tube and natural gas flows outside the tube, and heat exchange occurs through the tube wall), Freon / ammonia refrigeration precooler, etc., which play a role in cooling, removing water and impurities, saving energy and reducing consumption, and protection (preventing high-temperature natural gas from directly entering the molecular sieve, avoiding damage to the molecular sieve performance, and stabilizing the operating conditions of the subsequent distillation column, etc.).

[0038] One of the functions of the temperature controller 4 is to regulate the inlet gas temperature of the high-pressure expander 13 and ensure that the outlet gas of the high-pressure expander 13 is within a safe humidity range.

[0039] The heat exchanger 25 installed between the booster-type high-pressure expander 13 and the braking device 12 is used to regulate the inlet gas temperature and outlet gas humidity of the high-pressure expander 13 and enhance the cooling capacity of the expanded gas.

[0040] To prevent cavitation when the hydraulic pump 11 draws in liquid at saturation temperature, the liquid drawn in by the hydraulic pump 11 can be subcooled using low-temperature pure nitrogen gas drawn from the upper tower.

[0041] The liquid-pressurized recirculating natural gas liquefaction unit of the present invention adopts the start-up method of existing natural gas liquefaction units, or injects liquefied natural gas into the lower column of the distillation column and liquid nitrogen into the condenser-evaporator of the distillation column. First, the compressor is started to deliver compressed natural gas into the purifier, main heat exchanger and distillation column. Then, the hydraulic pump is started to output liquefied natural gas or liquid nitrogen, which enters the main heat exchanger or recirculating cooler to absorb heat and vaporize. Then, it enters the high-pressure expander for expansion and refrigeration, and enters the cooling and liquid accumulation stage to achieve the purpose of rapid start-up.

[0042] For details not provided in this disclosure, please refer to the following books: *Liquefied Natural Gas Receiving Terminal Technology and Engineering* (China Petrochemical Press, 1st edition, 1st printing, January 2022, ISBN: 978-7-5114-6119-3), *Liquefied Natural Gas (LNG) Technology and Engineering* (China Petrochemical Press, 1st edition, 1st printing, May 2015, ISBN: 978-7-5114-2718-2), *Liquefied Natural Gas Technology* (Machinery Industry Press, 2nd edition, 1st printing, October 2015, ISBN: 978-7-111-50344-6), and *Liquefied Natural Gas Technology Handbook* (Machinery Industry Press, 1st edition, 1st printing, January 2010, ISBN: 978-7-111-28573-1).

[0043] Equipment not described in this invention, such as backup systems, pipelines, instruments, valves, insulation, bypasses with regulating functions, and automatic control equipment, shall be equipped with known and mature technologies.

[0044] The present invention has the following advantages over the prior art:

[0045] 1. This invention retains the advantages of existing natural gas liquefaction units with expanders. By using a hydraulic pump to perform secondary pressurization on the crude liquefied natural gas, pure liquid nitrogen, or waste liquid nitrogen drawn from the distillation column, it utilizes the characteristic of liquids approaching incompressibility when pressure is increased. Compared with gas compression pressurization, this method can save a significant amount of pressurization power consumption, that is, it saves the large amount of power consumption required for pressurizing the gas entering the high-pressure expander. The cold energy is then used to cool the compressed natural gas. The gas produced by the low-temperature liquid absorbing heat and vaporizing then enters the high-pressure expander for expansion and refrigeration. This is equivalent to retaining the advantages of high expander inlet pressure and large expansion and refrigeration capacity in traditional Kraut and Hyland cycles. This forms a brand-new liquid pressurization and recooling type natural gas liquefaction unit that incorporates the advantages of the Kapitza, Kraut, and Hyland cycles. Moreover, the amount of liquid fraction entering the upper column is reduced, and the temperature difference between gas and liquid during upper column distillation is reduced, making the process setup of the entire unit more rational.

[0046] 2. Compared with the existing Chinese patents 201310029518.3 - A Natural Gas Isobaric Liquefaction Device, 201310030104.2 - A Natural Gas Isobaric Liquefaction Device, and 201310030948.7 - A Natural Gas Isobaric Liquefaction Device, the present invention proposes an external circulation liquid pressurization and recooling type supplementary cooling system based on a new theoretical foundation. This supplementary cooling system adopts an external circulation supplementary cooling process, which requires the setting up of a separate refrigerant storage tank, etc., to construct the refrigerant's cold power circulation loop. The present invention adopts an internal circulation liquid pressurization and recooling type natural gas liquefaction process, which makes the material balance and energy balance organization easier. It is equivalent to adding a hydraulic pump, a recooler and a high-pressure section expander to the existing successfully operating natural gas liquefaction device with an expander, forming a new type of high and low pressure composite natural gas liquefaction device.

[0047] 3. Since the boiling points of natural gas and nitrogen under normal pressure are much different from those of oxygen and nitrogen in air separation units, the expander in the liquid booster and recooling type natural gas liquefaction unit can provide more cooling capacity for natural gas liquefaction. Therefore, the outlet pressure of the natural gas compressor can be appropriately reduced, further reducing the power consumption of the natural gas liquefaction unit.

[0048] 4. This invention can be used to build new natural gas liquefaction plants, or to modify existing natural gas liquefaction plants with expanders to make new process modifications. That is, by adding hydraulic pumps, coolers, and adding high-pressure expanders to the original expanders, the process can be updated and modified.

[0049] 5. The natural gas liquefaction device of the present invention can also be superimposed with a propane pre-cooling mixed refrigerant liquefaction process to further reduce energy consumption and provide a more solid technical guarantee for the application of liquid booster recooling type natural gas liquefaction device in large base loads. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the process flow of a natural gas liquefaction device (pure liquid nitrogen pressurization) according to the present invention.

[0051] Figure 1 In the middle, 1-Natural gas, 2-Pretreatment unit, 3-Compressor, 4-Temperature controller, 5-Precooler, 6-Purifier, 7-Recooler, 8-Main heat exchanger, 9-Liquefier, 10-Lower tower, 11-Hydraulic pump, 12-Braking device, 13-High pressure expander, 14-Condenser evaporator, 15-Non-condensable gas, 16-Crude liquefied natural gas subcooler, 17-LNG throttle valve, 18-Liquid nitrogen subcooler, 19-Sludge liquid nitrogen throttle valve, 20-Upper tower, 21-Pure liquid nitrogen throttle valve, 22-Circulating compressor, 23-Nitrogen cooler, 24-Liquefied natural gas, 25-Heat exchanger.

[0052] Figure 2This is a schematic diagram of another natural gas liquefaction device process of the present invention (liquefied natural gas pressurization).

[0053] Figure 2 In the middle, 1-Natural gas, 2-Filter, 3-Compressor, 4-Temperature controller, 5-Precooler, 6-Purifier, 7-Recooler, 8-Main heat exchanger, 9-Liquefier, 10-Lower tower, 11-Hydraulic pump, 12-Braking device, 13-High pressure expander, 14-Condenser evaporator, 15-Non-condensable gas, 16-Crude liquefied natural gas subcooler, 17-LNG throttle valve, 18-Liquid nitrogen subcooler, 19-Sludge liquid nitrogen throttle valve, 20-Upper tower, 21-Pure liquid nitrogen throttle valve, 22-Circulating compressor, 23-Nitrogen cooler, 24-Liquefied natural gas, 25-Heat exchanger. Detailed Implementation

[0054] The present disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0055] Example 1:

[0056] As attached Figure 1 As shown, a liquid-pressurized recirculating natural gas liquefaction unit includes a compressor 3, a purifier 6, a main heat exchanger 8, a distillation column, a circulating compressor 22, a hydraulic pump 11, and a high-pressure expander 13.

[0057] The distillation column includes a lower column 10, a condenser-evaporator 14, an upper column 20, and a subcooler.

[0058] Natural gas enters the lower tower 10 via pretreatment unit 2, compressor 3, temperature controller 4, precooler 5, purifier 6, and main heat exchanger 8. The crude liquefied natural gas produced in the lower tower 10 passes through a cooler and LNG throttle valve 17 before entering the upper tower 20. Nitrogen gas drawn from the upper part of the lower tower 10 is cooled by a condenser-evaporator 14 to form liquid nitrogen. Part of this liquid nitrogen returns to the lower tower 10, while the other part passes through a cooler and pure liquid nitrogen throttle valve 21 before entering the upper tower 20. The liquefied natural gas produced in the lower part of the upper tower 20 absorbs heat and vaporizes in the condenser-evaporator 14, producing... The natural gas returns to the upper tower 20. Another portion of liquefied natural gas 24 drawn from the lower part of the upper tower 20 is output as a product. The low-temperature pure nitrogen drawn from the top of the upper tower 20 recovers its cooling capacity through a cooler and main heat exchanger 8, and is then pressurized by the circulating compressor 22 before merging with the natural gas from the compressor 3 outlet and entering the purifier 6. The pure liquid nitrogen drawn from the condenser-evaporator 14 is pressurized by the hydraulic pump 11 and enters the main heat exchanger 8 to absorb heat and vaporize. The resulting gas then enters the high-pressure expander 13 for expansion before entering the lower tower 10 or the upper tower 20.

[0059] Alternatively, natural gas can enter the lower tower 10 via pretreatment unit 2, compressor 3, temperature controller 4, precooler 5, purifier 6, return cooler 7, and main heat exchanger 8. The liquid air generated in the lower tower 10 passes through a cooler and liquid air throttling valve 17 before entering the upper tower 20. Nitrogen drawn from the upper part of the lower tower 10 is cooled by a condenser-evaporator 14 to form liquid nitrogen; part of this liquid nitrogen returns to the lower tower 10, while the other part passes through a cooler and pure liquid nitrogen throttling valve 21 before entering the upper tower 20. The liquefied natural gas generated in the lower part of the upper tower 20 is vaporized by condenser-evaporator 14 after absorbing heat. The generated natural gas returns to the upper tower 20. Another portion of liquefied natural gas drawn from the lower part of the upper tower 20 is output as a product. The low-temperature pure nitrogen drawn from the top of the upper tower recovers its cold energy through the crude liquefied natural gas subcooler 16 and the main heat exchanger 8. After being pressurized by the circulating compressor 22, it merges with the natural gas at the outlet of the compressor 3 and enters the purifier 6. The pure liquid nitrogen drawn from the condenser evaporator 14 enters the return cooler 7 through the hydraulic pump 11 to absorb heat and vaporize. The generated gas enters the high-pressure expander 13 for expansion and then enters the lower tower 10 or the upper tower 20.

[0060] The subcooler includes a crude liquefied natural gas subcooler 16 and a liquid nitrogen subcooler 18. The crude liquefied natural gas subcooler 16 and the liquid nitrogen subcooler 18 can be independent cold exchangers or they can be combined together to form an integrated cold exchanger.

[0061] The non-condensable gas 15, such as argon, generated at point 14 of the condenser-evaporator can be discharged periodically.

[0062] The waste liquid nitrogen drawn from the lower column 10 enters the upper column 20 through the liquid nitrogen subcooler 18 and the waste liquid nitrogen throttling valve 19.

[0063] A liquefaction unit 9 is provided: the low-temperature pure nitrogen gas drawn from the upper tower 20 passes through the crude liquefied natural gas subcooler 16, liquefaction unit 9, and main heat exchanger 8 to recover its cooling capacity. After being pressurized by the circulating compressor 22, it merges with the natural gas from the compressor 3 outlet and enters the purifier 6. The natural gas then passes through the pretreatment unit 2, compressor 3, temperature controller 4, precooler 5, purifier 6, main heat exchanger 8, and liquefaction unit 9 before entering the lower tower 10.

[0064] Alternatively, the low-temperature pure nitrogen gas drawn from the upper tower 20 recovers its cold energy through the crude liquefied natural gas subcooler 16, liquefaction unit 9, and main heat exchanger 8, and then is pressurized by the circulating compressor 22 and merged with the natural gas from the compressor 3 outlet into the purifier 6. The natural gas then enters the lower tower 10 through the pretreatment unit 2, compressor 3, temperature controller 4, precooler 5, purifier 6, cooler 7, main heat exchanger 8, and liquefaction unit 9.

[0065] The braking device 12 driven by the high-pressure expander 13 includes a fan, a liquid booster pump, an air compressor, and a generator.

[0066] When the braking device 12 driven by the high-pressure expander 13 is a gas compressor, the low-temperature liquid output from the hydraulic pump 11 absorbs heat and vaporizes through the main heat exchanger 8 or the cooler 7, and is pressurized by the braking device 12 before entering the high-pressure expander 13.

[0067] A heat exchanger 25 is provided: the low-temperature liquid output from the hydraulic pump 11 absorbs heat and vaporizes through the main heat exchanger 8 or the cooler 7, is pressurized and heated by the braking device 12, and is cooled by the heat exchanger 2525 before entering the high-pressure expander 13.

[0068] The "high pressure" in the high-pressure expander 13 is relative to the low-pressure expander in a traditional natural gas liquefaction plant with an expander.

[0069] The recooler 7 adopts a partitioned heat exchange method.

[0070] The precooler 5 is used to reduce the temperature of the natural gas entering the purifier 6.

[0071] The subcooler is used to subcool the liquid entering the upper column 20, thereby reducing vaporization losses.

[0072] In practical applications, the crude liquefied natural gas subcooler 16, liquid nitrogen subcooler 18, and liquefaction unit 9 can be integrated into a single design. A multi-flow plate-fin heat exchanger can be used to integrate the liquid nitrogen and liquid air channels into one device, forming an integrated composite heat exchanger.

[0073] When using a pressurized turbine expander for refrigeration and molecular sieve adsorption for purification, the cooling capacity regulation function of the liquefier 9 can be shared by the main heat exchanger 8 and the subcooler.

[0074] The purifier 6 is used to further remove moisture, carbon dioxide, hydrocarbons, etc. from the natural gas.

[0075] The purifier 6 uses a molecular sieve purifier to remove components from natural gas that are harmful to gas separation.

[0076] The main function of the precooler 5 is to reduce the temperature of natural gas from 80℃~120℃ to 8℃~15℃ to meet the purification temperature requirements of the purifier 6, and further recover the cooling capacity of the return gas. The main types include: nitrogen-water precooler (which uses the return waste nitrogen to exchange heat with natural gas to reduce the air temperature), water cooling tower (which uses the cooling capacity of the return waste nitrogen to cool the cooling water, forming a closed-loop circulating water system), shell and tube precooler (cooling water flows inside the tube and natural gas flows outside the tube, exchanging heat through the tube wall), Freon / ammonia refrigeration precooler, etc., which play a role in cooling, removing water and impurities, saving energy and reducing consumption, and protection (preventing high-temperature natural gas from directly entering the molecular sieve, avoiding damage to the molecular sieve performance, and stabilizing the operating conditions of the subsequent distillation column, etc.).

[0077] The main function of the temperature controller 4 is to regulate the inlet gas temperature of the high-pressure expander 13 and ensure that the outlet gas of the high-pressure expander 13 is within a safe humidity range.

[0078] A pretreatment unit 2 is provided: natural gas passes through the pretreatment unit 2 to remove carbon dioxide, hydrogen sulfide, moisture, heavy hydrocarbons, mercury, etc., from the raw natural gas before entering the compressor 3. The natural gas pretreatment unit is a key link to ensure the safe and efficient operation of the subsequent liquefaction process.

[0079] The outlet gas pipeline of the circulating compressor 22 is equipped with a nitrogen cooler 23 to reduce the temperature of its outlet gas, enhance its cooling capacity, and facilitate the safe operation of the purifier 6.

[0080] The heat exchanger 25 installed between the booster-type high-pressure expander 13 and the braking device 12 is used to regulate the inlet gas temperature and outlet gas humidity of the high-pressure expander 13 and enhance the cooling capacity of the expanded gas.

[0081] The liquid-pressurized recirculating natural gas liquefaction unit of the present invention adopts the start-up method of existing natural gas liquefaction units, or injects liquefied natural gas into the lower column of the distillation column and liquid nitrogen into the condenser-evaporator of the distillation column. First, the compressor is started to deliver compressed natural gas into the purifier, the main heat exchanger, and the distillation column. Then, the hydraulic pump is started to output liquefied natural gas or liquid nitrogen, which enters the main heat exchanger or recirculating cooler to absorb heat and vaporize. Then, it enters the high-pressure expander for expansion and refrigeration, and enters the cooling and liquid accumulation stage to achieve the purpose of rapid start-up.

[0082] Equipment not described in this invention, such as backup systems, pipelines, instruments, valves, insulation, bypasses with regulating functions, and automatic control equipment, shall be equipped with known and mature technologies.

[0083] Example 2:

[0084] As attached Figure 2 As shown, the crude liquefied natural gas drawn from the lower tower 10 is pressurized by the hydraulic pump 11, and then enters the high-pressure expander 13 through the main heat exchanger 8 or the cooler 7 and the temperature controller 4. The rest is the same as in Example 1.

[0085] Although the present invention has been disclosed above with reference to preferred embodiments, these are not intended to limit the invention. Any person skilled in the art can make various changes or modifications without departing from the spirit and scope of this disclosure, and these changes also fall within the protection scope of the present invention. Therefore, the protection scope of the present invention should be defined by the claims of this application.

Claims

1. A liquid-pressurized recirculating natural gas liquefaction unit, characterized in that: The natural gas liquefaction unit includes a compressor, a purifier, a main heat exchanger, a distillation column, a circulating compressor, a hydraulic pump, and a high-pressure expander. Natural gas is compressed by a compressor and then enters the purifier, followed by the main heat exchanger and then the distillation column. The low-temperature pure nitrogen produced in the distillation column enters the main heat exchanger to cool the natural gas fed from the purifier. Pure liquid nitrogen, waste liquid nitrogen, or crude liquefied natural gas produced in the distillation column is pressurized by a hydraulic pump and then enters the main heat exchanger. After absorbing heat and vaporizing in the main heat exchanger, it enters the high-pressure expander to expand and perform work. The exhaust gas from the high-pressure expander enters the distillation column. The low-temperature pure nitrogen produced in the distillation column recovers its cooling capacity in the main heat exchanger, is then pressurized by a circulating compressor, and finally merges with the natural gas from the compressor outlet before entering the purifier. Alternatively, natural gas is compressed by a compressor and enters a purifier, then passes through a cooler and a main heat exchanger before entering a distillation column. The low-temperature pure nitrogen produced in the distillation column enters the main heat exchanger to cool the natural gas fed from the purifier. The pure liquid nitrogen, waste liquid nitrogen, or crude liquefied natural gas produced in the distillation column is pressurized by a hydraulic pump and enters the cooler. After absorbing heat and vaporizing in the cooler, it enters a high-pressure expander to expand and do work. The exhaust gas from the high-pressure expander enters the distillation column. The low-temperature pure nitrogen produced in the distillation column recovers its cooling capacity through the main heat exchanger, and then is pressurized by a circulating compressor before merging with the natural gas from the compressor outlet and entering the purifier.

2. The natural gas liquefaction device according to claim 1, characterized in that: The distillation column includes a lower column, a condenser-evaporator, an upper column, and a subcooler. Natural gas enters the lower tower via a compressor, purifier, and main heat exchanger. The crude liquefied natural gas (LNG) produced in the lower tower passes through a cooler and an LNG throttling valve before entering the upper tower. Nitrogen gas drawn from the upper part of the lower tower is cooled by a condenser-evaporator to form liquid nitrogen; a portion returns to the lower tower, while the other portion passes through a cooler and a pure liquid nitrogen throttling valve before entering the upper tower. LNG produced in the lower part of the upper tower is vaporized by a condenser-evaporator, and the resulting natural gas returns to the upper tower. Another portion of LNG drawn from the lower part of the upper tower is output as a product. Low-temperature pure nitrogen gas drawn from the top of the upper tower recovers its cooling capacity through a cooler and main heat exchanger. The resulting pure nitrogen gas is then pressurized by a circulating compressor and merges with the natural gas from the compressor outlet before entering the purifier. Crude LNG, waste liquid nitrogen, or pure liquid nitrogen from the lower tower is pumped into the main heat exchanger for vaporization. The resulting gas is expanded by a high-pressure expander and then enters either the lower or upper tower. Alternatively, natural gas enters the lower tower via a compressor, purifier, cooler, and main heat exchanger. The crude liquefied natural gas produced in the lower tower enters the upper tower via a cooler and LNG throttle valve. Nitrogen gas drawn from the upper part of the lower tower is cooled by a condenser-evaporator to form liquid nitrogen. Part of the liquid nitrogen returns to the lower tower, while the other part enters the upper tower via a cooler and pure liquid nitrogen throttle valve. The liquefied natural gas produced in the lower part of the upper tower is vaporized by a condenser-evaporator and the resulting natural gas returns to the upper tower. Another part of the liquefied natural gas drawn from the lower part of the upper tower is output as a product. The low-temperature pure nitrogen gas drawn from the top of the upper tower recovers its cooling capacity by passing through a cooler and main heat exchanger. The resulting pure nitrogen gas is then pressurized by a circulating compressor and merges with the natural gas from the compressor outlet into the purifier. The crude liquefied natural gas, waste liquid nitrogen, or pure liquid nitrogen drawn from the condenser-evaporator from the lower tower enters the cooler via a hydraulic pump and vaporizes by absorbing heat. The resulting gas is then expanded by a high-pressure expander and enters the lower or upper tower.

3. The natural gas liquefaction apparatus according to claim 2, characterized in that: The subcooler includes a crude liquefied natural gas subcooler and a liquid nitrogen subcooler. The crude liquefied natural gas subcooler and the liquid nitrogen subcooler are independent cold exchangers, or they can be combined together to form an integrated cold exchanger.

4. The natural gas liquefaction device according to claim 2, characterized in that: The waste nitrogen drawn from the lower tower enters the upper tower through the liquid nitrogen subcooler and the waste nitrogen throttling valve.

5. The natural gas liquefaction apparatus according to claim 2, characterized in that: A liquefaction unit is provided: the low-temperature pure nitrogen gas drawn from the upper tower is discharged after recovering its cooling capacity through the crude liquefied natural gas subcooler, liquefaction unit, and main heat exchanger. Natural gas enters the lower tower via a compressor, purifier, main heat exchanger, and liquefaction unit, or via a compressor, purifier, cooler, main heat exchanger, and liquefaction unit.

6. The natural gas liquefaction apparatus according to claim 1, characterized in that: A temperature controller is provided: natural gas enters the purifier through the compressor and temperature controller, and the low-temperature liquid output from the hydraulic pump absorbs heat and vaporizes through the main heat exchanger or cooler, and then enters the high-pressure expander through the temperature controller.

7. The natural gas liquefaction apparatus according to claim 1, characterized in that: A precooler is provided: natural gas enters the purifier after passing through the compressor and precooler.

8. The natural gas liquefaction apparatus according to claim 1, characterized in that: When the braking device driven by the high-pressure expander is a gas compressor, the low-temperature liquid output from the hydraulic pump absorbs heat and vaporizes through the main heat exchanger or cooler, and after being pressurized by the braking device, it enters the high-pressure expander.

9. The natural gas liquefaction apparatus according to claim 8, characterized in that: It is equipped with a heat exchanger: the low-temperature liquid output from the hydraulic pump absorbs heat and vaporizes through the main heat exchanger or cooler, and then enters the high-pressure expander after being pressurized by the braking equipment and cooled by the heat exchanger.

10. The natural gas liquefaction apparatus according to claim 2, characterized in that: The natural gas liquefaction unit adopts the start-up method of existing liquefied natural gas units with expanders, or injects liquefied natural gas or liquid nitrogen into the lower column of the distillation column, or injects liquid nitrogen into the condenser-evaporator of the distillation column. First, the compressor is started to transport natural gas into the purifier, main heat exchanger, and distillation column. Then, the hydraulic pump is started to output liquefied natural gas or liquid nitrogen, which enters the main heat exchanger or cooler to absorb heat and vaporize. Then, it enters the high-pressure expander for expansion and refrigeration, and enters the cooling and liquid accumulation stage.

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