A system and method for subcooling and depressurizing liquefied natural gas product within a liquefied natural gas plant

Abstract

The application provides a liquefied natural gas product cooling and pressure reducing system and method in a liquefied natural gas factory, which comprises a refrigeration unit for providing cold energy for normal-temperature natural gas and refrigerant, a precooling heat exchanger, a cold box, a liquefied natural gas storage tank and a BOG compressor. A low-temperature BOG compressor is used to reduce the pressure of the BOG gas phase space in the LNG storage tank, and the process of cooling the high-temperature liquid in the LNG storage tank through evaporation is realized. Through the cold energy provided for the system in the shallow cooling temperature range (not lower than -25 DEG C), and the adjustment of the mixed refrigerant ratio, the process of cooling the LNG product in the deep cooling temperature range (lower than -120 DEG C) is realized. Through the reduction of the temperature of the LNG product, the BOG gas generated in the transportation and storage process of the LNG product can be reduced, the utilization rate of natural gas is improved, and the emission of greenhouse gas CH4 is reduced.

Description

Technical Field

[0001] This invention relates to a cooling and depressurization system and method for liquefied natural gas products in a liquefied natural gas plant. Background Technology

[0002] Liquefied natural gas (LNG) is formed by liquefying ambient-temperature natural gas at its critical pressure by gradually lowering its temperature below its boiling point through heat exchange with a cryogenic medium or by throttling in a cold box. LNG products produced at LNG plants are stored in cryogenic storage tanks. Due to system heat leakage and other reasons, bottom-of-gas (BOG) gas is inevitably generated in these tanks, and the higher the temperature of the LNG product, the greater the amount of BOG generated. For existing LNG plants, as the performance of the storage tank insulation system deteriorates, the BOG generated by the storage system gradually increases. The generation of BOG reduces the effective production capacity of the final LNG product and leads to increased pressure in the cryogenic storage tanks, causing the safety valves to activate and releasing large amounts of gas. This not only wastes energy and increases operating costs but also poses air pollution problems. Importantly, the increased temperature of the LNG in the storage tanks degrades the quality of the LNG product, resulting in even more BOG gas being generated during the storage process when it is sold as LNG to downstream storage and distribution stations and LNG refueling stations.

[0003] The BOG (Boiled Gas) processing technology within an LNG plant needs to be selected based on factors such as the surrounding gas pipeline network, the user market, and the overall thermal balance to reduce the comprehensive costs of plant construction and operation. Currently, the following three BOG processing technologies are mainly used in LNG plants:

[0004] 1) After heating and pressurization, it enters the natural gas pipeline network for external transmission;

[0005] 2) After heating, it can be used as fuel gas in the factory;

[0006] 3) As fuel gas in the factory + reliquefaction.

[0007] Because the generation of BOG is closely related to the storage temperature inside the LNG storage tank, changes in LNG loading load, production process operation status, and external climate conditions, its generation is highly volatile. When it is gasified for external transmission or used as fuel gas in the plant, it is limited by the gas consumption and unevenness of downstream users, so peak-shaving gas storage facilities need to be set up. When reliquefaction is used, the reliquefaction equipment is required to have high operational flexibility, which increases investment costs. Summary of the Invention

[0008] The purpose of this invention is to solve the above-mentioned technical problems and provide a system that utilizes a newly added non-explosion-proof refrigeration unit to provide cooling capacity to the natural gas and mixed refrigerant before they enter the cold box of the LNG plant through a pre-cooling heat exchanger. Combined with adjustments to the mixed refrigerant ratio, this achieves cooling of the LNG product within the cryogenic temperature range (below -120°C). Furthermore, by adding a BOG compressor to reduce the pressure in the BOG gas phase space within the LNG storage tank, a process is achieved where the high-temperature liquid (-140°C to -155°C) in the LNG storage tank is cooled through evaporation. Through these two measures, the LNG temperature in the liquefaction plant is reduced from -140°C to -155°C to below -160°C, reducing the amount of natural gas evaporated during LNG tanker transportation and storage at downstream LNG regasification and refueling stations.

[0009] The liquefied natural gas (LNG) product cooling and depressurization system in the LNG plant of the present invention includes a refrigeration unit (refrigeration unit), a precooling heat exchanger, a cold box, an LNG storage tank, and a BOG compressor. The chilled water output pipe of the refrigeration unit is connected to the inlet of the chilled water channel of the precooling heat exchanger, and the outlet of the chilled water channel of the precooling heat exchanger is returned and connected to the inlet of the refrigeration unit.

[0010] The natural gas input pipeline connects to the inlet of the natural gas passage in the precooling heat exchanger. The outlet pipeline of the natural gas passage in the precooling heat exchanger splits into two branches after passing through the cold box. One branch connects to the bottom inlet of the liquefied natural gas (LNG) storage tank, and the other branch connects to the top inlet of the LNG storage tank. The BOG (Bottle-Off Gas) output pipeline of the LNG storage tank splits into two branches. One branch returns to the natural gas input pipeline after passing through the BOG compressor, and the other branch connects directly to the downstream pipeline via the BOG heater.

[0011] The refrigerant inlet pipe is connected to the inlet of the refrigerant passage of the precooling heat exchanger, and the refrigerant outlet pipe of the preheating heat exchanger is discharged from the system after passing through the cold box.

[0012] Furthermore, the refrigeration unit is a non-explosion-proof refrigeration unit, located in the auxiliary area of ​​the plant. The refrigeration unit is equipped with a chilled water circulation pump that powers the chilled water. Low-temperature circulating water or other non-flammable liquids are used as the refrigerant to provide cooling capacity to the shallow cooling zone of the natural gas liquefaction system.

[0013] The precooling heat exchanger provides cooling capacity to the raw natural gas and mixed refrigerant in front of the cold box of the liquefied natural gas plant. By providing cooling capacity to the system in the shallow cold temperature range (not lower than -25℃, for example -15 to -25℃), and in conjunction with the adjustment of the mixed refrigerant ratio, the LNG product can be cooled in the deep cold temperature range (for example, below -120℃).

[0014] Furthermore, the precooling heat exchanger is a plate-fin multi-flow heat exchanger, which provides cooling to the raw material natural gas and refrigerant before they enter the cold box, and performs preliminary cooling on them. The refrigerant is a mixed refrigerant. The use of a plate-fin multi-flow heat exchanger improves heat exchange efficiency and reduces equipment energy consumption.

[0015] Furthermore, the BOG compressor is a BOG cryogenic labyrinth compressor. By reducing the pressure of the BOG gas phase space inside the liquefied natural gas storage tank through the BOG compressor, the high-temperature liquid (-140℃ to -155℃) inside the LNG storage tank is cooled by evaporation. The inlet temperature of the BOG compressor is the same as the temperature of the liquefied natural gas storage tank, eliminating the need for a separate heater and improving the overall energy utilization efficiency.

[0016] Furthermore, the system also includes a PLC control unit, which can interlock and control the BOG compressor displacement based on the liquefied natural gas storage tank pressure to ensure that the pressure inside the liquefied natural gas storage tank is below 50 kPa, preferably below 40 kPa. All pressures mentioned here refer to gauge pressure.

[0017] Furthermore, the materials of the refrigeration unit, precooling heat exchanger and chilled water pipes are stainless steel or low-temperature carbon steel, preferably low-temperature carbon steel. When stainless steel equipment insulation material is used for insulation, the thickness of the insulation layer is greater than 100mm, preferably greater than 200mm. Using low-temperature carbon steel can improve the applicability of the material, and the insulation of equipment and pipes can reduce energy consumption.

[0018] Furthermore, a filter, pressure regulator, flow meter, and shut-off valve are sequentially installed on the branch pipe that returns to the natural gas input pipeline after passing through the BOG compressor.

[0019] Furthermore, a bypass pipe with a valve is connected between the chilled water output pipe and the chilled water return pipe of the refrigeration unit. This bypass pipe can be used to regulate the amount of chilled water entering the precooling heat exchanger.

[0020] Another aspect of the present invention is to provide a method for cooling and depressurizing liquefied natural gas products in a liquefied natural gas plant, the method comprising:

[0021] The refrigeration unit uses a refrigerant such as Freon to provide chilled water. The outlet chilled water temperature of the refrigeration unit is 5-9°C, preferably about 6-7°C. This chilled water is provided to a channel of the precooling heat exchanger. At the same time, natural gas and mixed refrigerant enter the corresponding channel of the precooling heat exchanger through their respective input pipes. The chilled water provides cooling capacity for the raw material natural gas and mixed refrigerant, reducing the temperature of the natural gas and mixed refrigerant to 10-15°C, preferably about 12-13°C, before entering the cold box in the liquefied natural gas plant. The LNG product produced in the cold box is cooled to -155°C to -165°C, preferably about -158°C to -160°C. The LNG product is then divided into two parts and fed into the liquefied natural gas storage tank through the bottom inlet and the top inlet of the liquefied natural gas storage tank, respectively (wherein the volume ratio of the bottom inlet to the top inlet can be 0.5-10:1, preferably 1-10:1).

[0022] The top-mounted bulk gas (BOG) from the LNG storage tank is led out through a pipeline, and a BOG compressor is used to reduce the pressure in the BOG vapor space within the LNG storage tank (for example, if the original LNG storage tank has a storage pressure of 0.3-0.5 MPa, say approximately 0.4 MPa). The BOG compressor is used to achieve high-temperature liquid evaporation within the LNG storage tank. The BOG compressor is designed with a flow rate of 1500-2500 Nm³ / hour. 3 / h, for example, about 2000 Nm 3 / h, inlet temperature -155~-162℃, for example, about -159.3℃, inlet pressure 0.04~0.06MPa(A), for example, about 0.05MPa(A), outlet pressure 5.2-5.8MPa(A), for example, about 5.4MPa(A). The BOG compressor can ensure that the pressure inside the LNG storage tank drops below 50KPa and the LNG temperature inside the LNG storage tank drops below -160℃. After the flash gas is compressed by the BOG compressor, it becomes room temperature natural gas, and the pressure rises to above 5.5MPa, for example, 5.6-6.0MPa. After being pre-cooled by the natural gas input pipeline, it enters the cold box for reliquefaction.

[0023] Furthermore, besides a portion being recovered by the BOG compressor, another portion of the BOG gas is heated by the BOG heater to become ambient temperature natural gas, which is used as fuel gas within the station or pressurized before entering the natural gas input pipeline. The proportion of the remaining portion can be determined based on actual usage, for example, 10-30% by volume.

[0024] The beneficial effects of this invention are:

[0025] (1) Expand the downstream market scope of some liquefaction plants. The temperature of liquefied natural gas (LNG) products has been reduced from -140℃ to below -158℃, which allows liquefaction plants that originally mainly provided LNG products to LNG regasification stations to provide LNG products to LNG regasification stations and LNG refueling stations. This is beneficial to the sales of LNG products by LNG plants and creates considerable economic benefits for enterprises.

[0026] (2) This reduces the direct emissions of hazardous gases such as natural gas from LNG refueling stations in the downstream market and improves the resource utilization rate of LNG refueling stations. This proposed solution is of great significance for reducing the operating costs of refueling stations and for energy conservation and emission reduction. Attached Figure Description

[0027] Figure 1 This invention relates to a cooling and depressurization system for liquefied natural gas products in a liquefied natural gas plant. Detailed Implementation

[0028] The following description, in conjunction with the accompanying drawings and embodiments, provides further details.

[0029] like Figure 1 As shown, the cooling and depressurization system for liquefied natural gas (LNG) products in a LNG plant provided by this invention includes a refrigeration unit (cooling unit) E1, a precooling heat exchanger E2, a cold box E3, an LNG storage tank V1, and a BOG compressor C1. The chilled water output pipe L1 of the refrigeration unit E1 is connected to the inlet of the chilled water channel of the precooling heat exchanger E2, and the outlet of the chilled water channel of the precooling heat exchanger E2 is returned and connected to the inlet of the refrigeration unit E1.

[0030] Natural gas input pipeline L2 connects to the inlet of the natural gas channel of precooling heat exchanger E2. The outlet pipeline L3 of the natural gas channel of precooling heat exchanger E2 branches into two branches after passing through cold box E3. One branch connects to the bottom inlet of liquefied natural gas storage tank V1, and the other branch connects to the top inlet of liquefied natural gas storage tank V1. The BOG output pipeline L4 of liquefied natural gas storage tank V1 branches into two branches. One branch L8 returns to the natural gas input pipeline L2 after passing through BOG compressor C1. The other branch L9 (normal temperature natural gas pipeline) connects directly to the downstream pipeline via BOG heater E4 for internal station use. Liquefied natural gas storage tank V1 also has an LNG product outlet pipeline L10.

[0031] The refrigerant inlet pipe L5 is connected to the inlet of the refrigerant passage of the precooling heat exchanger E2, and the refrigerant outlet pipe L6 of the preheating heat exchanger E2 is discharged from the system via the mixed refrigerant pipe L7 after passing through the cold box E3.

[0032] The refrigeration unit E1 is a non-explosion-proof refrigeration unit, located in the auxiliary area of ​​the plant. The unit is equipped with a chilled water circulation pump (not shown) that provides power to the chilled water.

[0033] The precooling heat exchanger E2 is a plate-fin type multi-flow heat exchanger.

[0034] The BOG compressor C1 is a BOG cryogenic labyrinth compressor.

[0035] The materials of the refrigeration unit E1, the precooling heat exchanger E2, and the chilled water pipes are stainless steel or low-temperature carbon steel, preferably low-temperature carbon steel. When stainless steel equipment insulation material is used for insulation, the thickness of the insulation layer is greater than 100mm, preferably greater than 200mm. Using low-temperature carbon steel can improve the applicability of the material, and the insulation of equipment and pipes can reduce energy consumption.

[0036] After passing through the BOG compressor, the branch pipe that returns to merge with the natural gas input pipeline can be further equipped with a filter, pressure regulator, flow meter and shut-off valve in sequence.

[0037] The BOG compressor is pressure-interlocked with the liquefied natural gas (LNG) storage tanks within the LNG plant. The BOG compressor displacement can be adjusted according to the LNG storage tank pressure to ensure the tank pressure remains below 50 kPa, preferably below 40 kPa. All pressures mentioned here refer to gauge pressure. This interlock can be implemented via a PLC control unit.

[0038] Example

[0039] In the first stage, refrigeration unit E1 uses Freon refrigerant to provide chilled water for the process unit. The outlet chilled water temperature of refrigeration unit E1 is about 7°C. A water pump is used to power the circulating chilled water, and the operating pressure is about 0.4 MPa. At the same time, natural gas and mixed refrigerant enter the corresponding channels of precooling heat exchanger E2 through their respective input pipelines, providing a cold source for the precooling heat exchanger and providing cooling capacity for the raw material natural gas and mixed refrigerant. The temperature of natural gas and mixed refrigerant is reduced to about 13°C before entering the cold box E3 in the liquefaction plant. The LNG product produced in cold box E3 can be cooled to -158°C to 160°C.

[0040] In the second stage, the BOG compressor C1 is used to reduce the BOG vapor phase space pressure within the LNG storage tank V1. The original LNG storage tank had a storage pressure of 0.4 MPa. The BOG compressor C1 is used to achieve high-temperature liquid evaporation within the LNG storage tank V1. The BOG compressor is designed with a flow rate of 2000 Nm³ / h. 3 / h, inlet temperature -159.3℃, inlet pressure 0.05MPa(A), outlet pressure 5.4MPa(A), BOG compressor C1 ensures that the pressure inside LNG storage tank V1 drops below 50KPa, and the LNG temperature inside LNG storage tank V1 drops below -160℃. After being compressed by BOG compressor C1, the flash gas becomes ambient temperature natural gas, with the pressure rising to above 5.5MPa. Except for a small portion that is depressurized and sent to the fuel gas system, the majority is piped into the natural gas input pipeline for pre-cooling before entering the cold box E3 for re-liquefaction. Ambient temperature natural gas pipeline L9 runs parallel to the BOG compressor recovery system, meaning that in addition to being recovered by the BOG compressor, the BOG gas can also be heated by BOG heater E4 to become ambient temperature natural gas. This portion of natural gas can be used as fuel gas within the station or pressurized before entering the inlet natural gas input pipeline.

Claims

1. A cooling and depressurization system for liquefied natural gas (LNG) products in a LNG plant, characterized in that, It includes a refrigeration unit, a precooling heat exchanger, a cold box, a liquefied natural gas storage tank, and a BOG compressor. The chilled water output pipe of the refrigeration unit is connected to the inlet of the chilled water channel of the precooling heat exchanger, and the outlet of the chilled water channel of the precooling heat exchanger is connected back to the inlet of the refrigeration unit. The natural gas input pipeline connects to the inlet of the natural gas channel of the precooling heat exchanger. The outlet pipeline of the natural gas channel of the precooling heat exchanger splits into two branches after passing through the cold box. One branch connects to the bottom inlet of the liquefied natural gas (LNG) storage tank, and the other branch connects to the top inlet of the LNG storage tank. The BOG (Bottle-Off Gas) output pipeline of the LNG storage tank splits into two branches. One branch returns to the natural gas input pipeline after passing through the BOG compressor, and the other branch connects directly to the downstream pipeline via the BOG heater. The LNG storage tank is also equipped with an LNG product outlet pipeline. The refrigerant inlet pipe connects to the inlet of the refrigerant channel of the precooling heat exchanger. The refrigerant outlet pipe of the precooling heat exchanger exits the system after passing through the cold box. The chilled water temperature at the outlet of the refrigeration unit is 5-9℃, and this chilled water is supplied to one channel of the precooling heat exchanger. Simultaneously, natural gas and mixed refrigerant enter the corresponding channels of the precooling heat exchanger through their respective inlet pipes. The chilled water provides cooling capacity for the raw material natural gas and mixed refrigerant, reducing their temperature to 10-15℃ before entering the cold box in the liquefied natural gas plant. After passing through the cold box, the LNG product is cooled to -155℃ to -165℃. BOG compressors are designed for an hourly flow rate of 1500-2500 Nm. 3 With an inlet temperature of -155 to -162°C, an inlet pressure of 0.04 to 0.06 MPaA, and an outlet pressure of 5.2 to 5.8 MPaA, the BOG compressor can ensure that the pressure inside the LNG storage tank drops below 50 kPa and the LNG temperature inside the tank drops below -160°C. The system also includes a PLC control unit, which controls the BOG compressor discharge volume based on the liquefied natural gas storage tank pressure interlock.

2. The cooling and pressure reduction system according to claim 1, characterized in that, The BOG compressor has an inlet temperature of -159.3℃, an inlet pressure of 0.05MPaA, and an outlet pressure of 5.4MPaA.

3. The cooling and pressure reduction system according to claim 1, characterized in that, The refrigeration unit is a non-explosion-proof refrigeration unit, located in the auxiliary area of ​​the plant. The refrigeration unit is equipped with a chilled water circulation pump that provides power to the chilled water.

4. The cooling and pressure reduction system according to claim 1, characterized in that, The precooling heat exchanger is a plate-fin type multi-flow heat exchanger.

5. The cooling and pressure reduction system according to claim 1, characterized in that, The BOG compressor is a BOG cryogenic labyrinth compressor.

6. The cooling and pressure reduction system according to claim 1, characterized in that, The materials of the refrigeration unit, precooling heat exchanger and chilled water pipes are stainless steel or low-temperature carbon steel, and when stainless steel equipment insulation materials are used for insulation, the thickness of the insulation layer is greater than 100mm.

7. The cooling and pressure reduction system according to claim 1, characterized in that, After passing through the BOG compressor, the branch pipe that returns to merge with the natural gas input pipeline is equipped with a filter, pressure regulator, flow meter, and shut-off valve in sequence.

8. The cooling and pressure reduction system according to claim 1, characterized in that, A bypass pipe with a valve is connected between the chilled water output pipe and the chilled water return pipe of the chiller unit.

9. A method for cooling and depressurizing liquefied natural gas (LNG) products in a LNG plant, wherein a cooling and depressurizing system for LNG products in a LNG plant as described in any one of claims 1-8 is used, the method comprising: The refrigeration unit uses refrigerant, specifically Freon, to provide chilled water. The outlet chilled water temperature of the refrigeration unit is 5-9℃. This chilled water is supplied to one channel of the precooling heat exchanger. At the same time, natural gas and mixed refrigerant enter the corresponding channels of the precooling heat exchanger through their respective input pipes. The chilled water provides cooling capacity for the raw material natural gas and mixed refrigerant, reducing their temperature to 10-15℃. The LNG then enters the cold box in the liquefied natural gas plant. The LNG product produced in the cold box is cooled to -155℃ to -165℃. The LNG product is then divided into two parts and fed into the liquefied natural gas storage tank through the bottom inlet and the top inlet of the liquefied natural gas storage tank, respectively. The top-mounted blowout gas (BOG) from the liquefied natural gas (LNG) storage tank is led out through a pipeline. A BOG compressor is used to reduce the pressure in the BOG vapor phase space within the LNG storage tank. After being compressed by the BOG compressor, the flash gas is converted to ambient temperature natural gas, with the pressure rising to over 5.5 MPa. It is then piped into the natural gas input pipeline for pre-cooling before entering the cold box for further liquefaction. BOG compressors are designed for an hourly flow rate of 1500-2500 Nm. 3 With an inlet temperature of -155~-162℃, an inlet pressure of 0.04~0.06MPaA, and an outlet pressure of 5.2-5.8MPaA, the BOG compressor can ensure that the pressure inside the LNG storage tank drops below 50KPa and the LNG temperature inside the LNG storage tank drops below -160℃.

10. The method according to claim 9, wherein, In addition to a portion being recovered by the BOG compressor, the remaining portion of the BOG gas is heated by the BOG heater to become ambient temperature natural gas, which is used as fuel gas within the station or pressurized before entering the natural gas input pipeline.

11. The method according to claim 9, wherein, The outlet chilled water temperature of the refrigeration unit is 6-7℃. The chilled water provides cooling capacity for the raw material natural gas and mixed refrigerant, reducing the temperature of the natural gas and mixed refrigerant to 12-13℃ before entering the cold box in the liquefied natural gas plant. The LNG products produced in the cold box have their temperature reduced to -158℃ to -160℃.

12. The method according to claim 9, wherein, The BOG compressor has an inlet temperature of -159.3℃, an inlet pressure of 0.05MPaA, and an outlet pressure of 5.4MPaA.

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