Engineered large scale subcooled liquid methane production system with back pressure and method of production

Abstract

The present application relates to a kind of liquefied natural gas preparation system and its preparation method, specifically to a kind of engineering large-scale back pressure supercooled liquid methane preparation system and preparation method, solve the existing LNG when storage, transfer, the method for selecting LNG supercooling is prone to excessive heat exchange, cause LNG to be frozen after plugging heat exchanger, lead to using liquid nitrogen to join heat exchanger heat exchange efficiency is low, the technical problem of large-scale preparation supercooled liquid methane cost increase.This engineering large-scale back pressure supercooled liquid methane preparation method, including liquid methane supercooling device, vacuum extraction device, liquid nitrogen gasification device, liquid accumulator and liquid nitrogen pump;Vacuum extraction device's injection medium mouth is connected with external nitrogen source, and its outlet is connected with liquid methane supercooling device;The inlet of liquid nitrogen gasification device is connected with the outlet of liquid nitrogen pump, and the outlet of liquid nitrogen gasification device is connected with the inlet of liquid accumulator;The outlet of liquid accumulator is connected with liquid methane supercooling device;Realize the supercooling of large-scale LNG.

Description

TECHNICAL FIELD

[0001] The present application relates to a liquefied natural gas preparation system and a preparation method thereof, in particular to an engineering large-scale subcooled liquid methane preparation system with back pressure and a preparation method thereof. BACKGROUND

[0002] The existing large-scale LNG (Liquefied Natural Gas, referred to as LNG, the main component of which is methane, also known as liquid methane) is usually stored and transported in a saturated state at normal pressure or with pressure, and the temperature Tb of the liquid methane depends on the back pressure Pb corresponding to the saturated vapor pressure of the temperature Tb of the liquid methane. When the back pressure Pb of the liquid methane changes, the temperature Tb also changes, accompanied by the phenomenon of gasification or liquefaction of the gas phase methane; the saturated state transportation mode of LNG leads to the gasification phenomenon caused by heat transfer during transportation, or the temperature Tb of the liquid methane increases and the density decreases due to the increase of the back pressure Pb caused by heat absorption gasification, resulting in temperature differences of LNG from different vehicles and different manufacturers, and rolling phenomenon during transportation; the storage and transportation of LNG in a saturated state, whether it is normal pressure storage and transportation or pressure storage and transportation, inevitably causes evaporation loss, which increases the cost of LNG storage and transportation.

[0003] With the profound changes in the world energy pattern and the vigorous development of the LNG industry, large-scale LNG storage and transportation technology has become an important technology in the LNG application industry. Therefore, how to achieve efficient and low-cost storage and transportation of large-scale LNG is a major issue in the LNG industry. With the development of civil aerospace, LNG as an important liquid propellant, how to achieve efficient transportation and storage has also become an important issue to reduce the cost of civil aerospace launch.

[0004] In the existing engineering, when storing and transporting LNG, the method of subcooling LNG is usually selected, and liquid nitrogen is generally used as the cooling medium to cool the liquid methane to a certain temperature through a corresponding heat exchanger. This method is prone to excessive heat exchange because the normal pressure liquid nitrogen temperature is already lower than the freezing temperature of the liquid methane at normal pressure, which can cause the liquid methane to freeze and block the heat exchanger, resulting in low heat exchange efficiency of the heat exchanger with liquid nitrogen and increased cost of large-scale preparation of subcooled liquid methane. SUMMARY

[0005] The present application aims to solve the technical problems of the existing LNG storage and transportation, which selects the method of subcooling LNG, which is prone to excessive heat exchange, causing the LNG to freeze and block the heat exchanger, resulting in low heat exchange efficiency of the heat exchanger with liquid nitrogen and increased cost of large-scale preparation of subcooled liquid methane, and provides an engineering large-scale subcooled liquid methane preparation system with back pressure and a preparation method thereof, to realize large-scale subcooling of LNG.

[0006] The present application is:

[0007] The present application adopts a large-flow vacuum extraction device to first extract the liquid methane in the liquid methane supercooling device to below 15 kPa, then start the liquid nitrogen gasification device to generate saturated nitrogen, ensure the liquid methane supercooler to generate corresponding back pressure, set a saturated liquid methane filling valve V5 at the lower part of the liquid methane supercooling device to fill saturated liquid methane, and after generating supercooled liquid methane in the liquid methane supercooling device, discharge the supercooled liquid methane through a supercooled liquid methane discharge valve V6.

[0008] To solve the above technical problems and realize the above application concept, the technical scheme adopted by the present application is:

[0009] An engineering large-scale supercooled liquid methane preparation system with back pressure, which is characterized in that: comprising a liquid methane supercooling device, a vacuum extraction device, a liquid nitrogen gasification device, a liquid accumulator and a liquid nitrogen pump;

[0010] The extraction medium port of the vacuum extraction device is used to connect with external extraction medium, the inlet of the vacuum extraction device is connected with the gas outlet of the liquid methane supercooling device, and a vacuum isolation valve V4 is arranged on the connecting pipeline between the inlet of the vacuum extraction device and the gas outlet of the liquid methane supercooling device;

[0011] The inlet of the liquid nitrogen gasification device is connected with the outlet of the liquid nitrogen pump, and the outlet of the liquid nitrogen gasification device is connected with the inlet of the liquid accumulator;

[0012] The inlet of the liquid nitrogen pump is used to connect with external liquid nitrogen source;

[0013] The outlet of the liquid accumulator is connected with the nitrogen gas inlet of the liquid methane supercooling device, and a pressurization isolation valve V3 is arranged on the connecting pipeline between the outlet of the liquid accumulator and the nitrogen gas inlet of the liquid methane supercooling device;

[0014] The lower part of the liquid methane supercooling device is provided with a liquid methane inlet and outlet, a saturated liquid methane filling valve V5 and a supercooled liquid methane discharge valve V6 are arranged on the liquid methane inlet and outlet, saturated liquid methane is filled into the liquid methane supercooling device through the saturated liquid methane filling valve V5, and the supercooled liquid methane in the liquid methane supercooling device is discharged through the supercooled liquid methane discharge valve V6.

[0015] Further, a buffer tank is arranged between the vacuum extraction device and the liquid methane supercooling device;

[0016] A nitrogen isolation valve V9 and a nitrogen supply valve V 10 are arranged on the connecting pipeline between the extraction medium port of the vacuum extraction device and external extraction medium, the inlet of the vacuum extraction device is connected with the outlet of the buffer tank, and a vacuum isolation valve V8 is arranged on the connecting pipeline between the inlet of the vacuum extraction device and the outlet of the buffer tank;

[0017] The inlet of the buffer tank is connected with the outlet of the liquid methane supercooling device through the vacuum isolation valve V4, and a vacuum isolation hand valve V7 is arranged on the connecting pipeline between the inlet of the buffer tank and the vacuum isolation valve V4.

[0018] Further, a low-temperature check valve is arranged between the liquid nitrogen gasification device and the liquid nitrogen pump.

[0019] The inlet of the liquid nitrogen gasification device is connected with the overflow port of the liquid accumulator, and a liquid nitrogen backflow valve V2 is arranged on the pipeline between the inlet of the liquid nitrogen gasification device and the overflow port of the liquid accumulator.

[0020] Further, a liquid nitrogen supply valve V1 is arranged on the connecting pipeline between the inlet of the liquid nitrogen pump and the external liquid nitrogen source.

[0021] Further, a gas release valve V is arranged on the outlet of the liquid methane supercooling device. 11 and a safety valve, the gas release valve V 11 is used for controlling the internal pressure of the liquid methane supercooling device.

[0022] Further, a pressure monitor is arranged outside the buffer tank.

[0023] An adiabatic layer is arranged on the liquid methane supercooling device.

[0024] Further, the liquid methane supercooling device is a vacuum adiabatic container or an insulation container.

[0025] The vacuum injection device is a nitrogen ejector, and the flow rate of the nitrogen ejector is 0-1 m 3 / s.

[0026] The liquid nitrogen gasification device is an air-cooled liquid nitrogen vaporizer.

[0027] The liquid nitrogen pump is a small-flow liquid nitrogen pump.

[0028] Further, the volume of the insulation container is greater than 10 m 3 .

[0029] The volume of the buffer tank (6) is 0.5 m 3 .

[0030] The flow rate of the small-flow liquid nitrogen pump is 0.15-0.3 kg / s.

[0031] Further, a heating device is arranged.

[0032] The heating device is arranged on the pipeline between the outlet of the buffer tank and the vacuum isolation valve V8, and the heating device is used for heating the injection medium of the vacuum injection device, and the temperature of the injection medium is greater than 5℃.

[0033] The vacuum ejector is a water ring vacuum pump or a water ejector.

[0034] In addition, the application also provides an engineering large-scale supercooled liquid methane preparation method with back pressure, which adopts the engineering large-scale supercooled liquid methane preparation system.

[0035] Step 1: open the saturated liquid methane filling valve V5, and fill the supercooled liquid methane device with saturated liquid methane;

[0036] Step 2: close the saturated liquid methane filling valve V5, open the vacuum isolation valve V4, start the vacuum ejector, and vacuumize the supercooled liquid methane device;

[0037] Step 3: close the vacuum isolation valve V4, start the liquid nitrogen pump and the pressurization isolation valve V3, start the liquid nitrogen pump, and fill the supercooled liquid methane device with low-temperature saturated nitrogen;

[0038] Step 4: close the liquid nitrogen pump and the pressurization isolation valve V3, open the supercooled liquid methane discharge valve V6 to discharge the supercooled liquid methane in the supercooled liquid methane device, and complete the preparation of the supercooled liquid methane.

[0039] Compared with the prior art, the application has the following beneficial effects:

[0040] (1) The engineering large-scale supercooled liquid methane preparation system with back pressure provided by the application can obtain supercooled liquid methane with back pressure by using the vacuum ejector to vacuumize the supercooled liquid methane device filled with saturated liquid methane, and then using the liquid nitrogen gasification device to fill the supercooled liquid methane device with nitrogen to supplement the pressure in the supercooled liquid methane device, so that the temperature of the liquid methane is reduced and the density is increased; the storage efficiency is improved by supplementing the pressure in the supercooled liquid methane device with nitrogen, and since the LNG is in a pressurized supercooled state, there is no waste of BOG (Boil Off Gas, simply referred to as BOG, flash steam) gas, so that the pressurized and supercooled liquid methane has great advantages in storage and transportation.

[0041] (2) In the engineering large-scale supercooled liquid methane preparation system with back pressure provided by the application, the liquid nitrogen gasification device is used to generate saturated nitrogen to provide back pressure for the obtained supercooled liquid methane at 93-110K. In order to obtain saturated nitrogen, a liquid accumulator is arranged at the outlet of the liquid nitrogen gasification device, and the liquid accumulator can realize gas-liquid separation to ensure that the pressurized gas entering the supercooled liquid methane device is saturated nitrogen at the corresponding pressure.

[0042] (3) In the engineering large-scale supercooled liquid methane preparation system with back pressure of the present application, the vacuum extraction device adopts the way of vacuum extraction to extract the liquid methane in the liquid methane supercooling device to below 15 kPa, so that the temperature of the finally obtained liquid methane is 93-110 K. Since the vacuum extraction device will extract low-temperature liquid methane gas in the later working period, a buffer tank is arranged between the vacuum extraction device and the liquid methane supercooling device, which can not only ensure the temperature of the liquid methane gas, but also realize the stability of the external extraction medium flow during the vacuum extraction process of the vacuum extraction device.

[0043] (4) The engineering large-scale supercooled liquid methane preparation system of the present application realizes the propellant densification of the liquid rocket with liquid methane as the propellant through the supercooled liquid methane, which can provide important technical support for the related fields such as civil aerospace.

[0044] (5) The engineering large-scale supercooled liquid methane preparation method of the present application realizes the supercooling of large-scale LNG, and cools the LNG to about 93 K, and ensures that it has a back pressure of 0.1-0.46 MPa; at the same time, it can produce supercooled liquid methane with back pressure, which provides important technical support for large-scale LNG storage and transportation in the LNG industry. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 It is a structural schematic view of the first embodiment of the engineering large-scale supercooled liquid methane preparation system of the present application.

[0046] Figure 2 It is a flow chart of the engineering large-scale supercooled liquid methane preparation method of the present application using the system in the first embodiment.

[0047] Figure 3 It is a structural schematic view of the second embodiment of the engineering large-scale supercooled liquid methane preparation system of the present application.

[0048] In the drawings, the reference signs are:

[0049] 1-liquid methane supercooling device, 2-vacuum extraction device, 3-liquid nitrogen gasification device, 4-liquid accumulator, 5-liquid nitrogen pump, 6-buffer tank, 7-low-temperature one-way valve, 8-safety valve, 9-pressure monitor, 10-thermal insulation layer, 11-heating device. DETAILED DESCRIPTION

[0050] The technical solutions of the present application will be described below in conjunction with the drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the technical solutions in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0051] As shown in Figure 1 A kind of engineering large-scale back pressure supercooled liquid methane preparation system, including liquid methane supercooling device 1, vacuum ejector device 2, liquid nitrogen gasification device 3, liquid accumulator 4, liquid nitrogen pump 5, buffer tank 6, cryogenic one-way valve 7, gas release valve V 11 With safety valve 8;

[0052] The ejector medium port of vacuum ejector device 2 is used to be connected with external ejector medium;Buffer tank 6 is arranged between vacuum ejector device 2 and liquid methane supercooling device 1;The ejector medium port of vacuum ejector device 2 is connected with external ejector medium through nitrogen isolation valve V9, nitrogen supply valve V 10 With external ejector medium, the inlet of vacuum ejector device 2 is connected with the outlet of buffer tank 6 through vacuum isolation valve V8;The inlet of buffer tank 6 is connected with the gas outlet of liquid methane supercooling device 1 through vacuum isolation valve V4, and vacuum isolation hand valve V7 is arranged on the connecting pipeline between the inlet of buffer tank 6 and vacuum isolation valve V4;Pressure monitor 9 is arranged outside buffer tank 6.

[0053] The function of vacuum ejector device 2 is to draw vacuum for liquid methane supercooling device 1 by external ejector medium, in the embodiment, high-pressure nitrogen is used as external ejector medium, and the pressure of nitrogen is 1-5 MPa;The function of vacuum isolation valve V4 is to control the vacuum pipeline of liquid methane supercooling device 1;The function of vacuum isolation hand valve V7 is to control the inlet of buffer tank 6;The function of vacuum isolation valve V8 is to control the outlet of buffer tank 6;The function of nitrogen isolation valve V9 is to manually isolate and control the external ejector medium of vacuum ejector device 2;The function of nitrogen supply valve V 10 Is the control solenoid valve of ejector medium of vacuum ejector device 2.

[0054] The inlet of liquid nitrogen gasification device 3 is connected with the outlet of liquid nitrogen pump 5, and the outlet of liquid nitrogen gasification device 3 is connected with the inlet of liquid accumulator 4;The inlet of liquid nitrogen pump 5 is connected with external liquid nitrogen source through liquid nitrogen supply valve V1;The function of liquid nitrogen supply valve V1 is to control external liquid nitrogen source and supply normal-pressure liquid nitrogen for liquid nitrogen pump 5;The inlet of liquid nitrogen gasification device 3 is connected with the overflow port of liquid accumulator 4, and liquid nitrogen backflow valve V2 is arranged on the connecting pipeline between the inlet of liquid nitrogen gasification device 3 and the overflow port of liquid accumulator 4;The outlet of liquid accumulator 4 is connected with the nitrogen gas inlet of liquid methane supercooling device 1, and pressurization isolation valve V3 is arranged on the connecting pipeline between the outlet of liquid accumulator 4 and the nitrogen gas inlet of liquid methane supercooling device 1;The function of liquid nitrogen backflow valve V2 is to control the liquid level of liquid accumulator 4, to ensure that there is enough gas phase space in liquid accumulator 4, and then to ensure that liquid methane supercooling device 1 is saturated with nitrogen gas;The function of pressurization isolation valve V3 is to control the internal pressurization of liquid methane supercooling device 1;

[0055] The cryogenic check valve 7 is located between the liquid nitrogen vaporization device 3 and the liquid nitrogen pump 5; the function of the cryogenic check valve 7 is to ensure that the pressure inside the liquid nitrogen vaporization device 3 does not affect the operation of the liquid nitrogen pump 5.

[0056] The lower part of the liquid methane subcooling device 1 is provided with a liquid methane inlet and outlet. A saturated liquid methane filling valve V5 and a subcooled liquid methane discharge valve V6 are provided on the liquid methane inlet and outlet. The saturated liquid methane filling valve V5 is used to fill the liquid methane subcooling device 1 with room temperature saturated liquid methane, and the subcooled liquid methane discharge valve V6 is used to discharge the pressurized subcooled liquid methane from the liquid methane subcooling device 1.

[0057] The vent valve V is installed on the liquid methane subcooling device 1 11 With safety valve 8, vent valve V 11 Used to control the pressure inside the liquid methane subcooling device 1; the liquid methane subcooling device 1 is also equipped with an insulation layer 10.

[0058] The liquid methane subcooling device 1 used in this embodiment is a vacuum insulated container. Alternatively, it can be a container with good insulation performance and a volume greater than 10m³. 3 The insulated container. The vacuum ejector device 2 uses a nitrogen ejector to evacuate the liquid methane in the liquid methane subcooling device 1 to below 15 kPa via ejection, ultimately achieving a liquid methane temperature of 93–110 K. The nitrogen ejector's gas source pressure is 1–5 MPa, and the ejector flow rate is 0–1 m³ / s. 3 / s. Since the nitrogen ejector will extract cryogenic liquid methane gas in the later stages of operation, a 0.5m... 3 The buffer tank 6 can both ensure the temperature of the liquid methane gas and achieve stability of the external ejector gas flow rate during the vacuuming process.

[0059] The liquid nitrogen vaporization unit 3 employs an ambient air-temperature (AAT) liquid nitrogen vaporizer. This AAT vaporizer generates saturated nitrogen gas, providing back pressure for the obtained subcooled liquid methane at 93–110 K. The heat exchange area of ​​the AAT liquid nitrogen vaporizer is 50–80% of the calculated area required to completely vaporize the liquid nitrogen. To obtain saturated nitrogen gas, a liquid collector 4 is installed at the outlet of the AAT liquid nitrogen vaporizer, enabling gas-liquid separation and ensuring that the pressurized gas entering the liquid methane subcooling unit 1 is saturated nitrogen gas at the corresponding pressure.

[0060] Liquid nitrogen pump 5 uses a small flow rate liquid nitrogen pump to pressurize the ambient temperature liquid nitrogen vaporizer. The flow rate of the small flow rate liquid nitrogen pump is 0.15 kg / s, so that the highest back pressure in the liquid methane subcooling device 1 depends on the pressure generated by the corresponding head of the ambient temperature liquid nitrogen vaporizer and liquid nitrogen pump 5. Atmospheric pressure liquid nitrogen enters the small flow rate liquid nitrogen pump, and after pressure is generated, it enters the ambient temperature liquid nitrogen vaporizer through the cryogenic check valve 7.

[0061] As shown in Figure 2 The engineered large-scale subcooled liquid methane preparation method with back pressure is specifically as follows:

[0062] First, open the saturated liquid methane filling valve V5, the exhaust valve V 11 , and the rest of the valves are in the closed state; the saturated liquid methane is 0.1 MPa, 111.5 K, and the liquid methane subcooling device 1 is filled with normal-temperature saturated liquid methane.

[0063] Then, close the saturated liquid methane filling valve V5, the exhaust valve V 11 , open the vacuum isolation valve V4, the vacuum isolation hand valve V7, the vacuum isolation valve V8, the nitrogen supply valve V9, and the nitrogen supply valve V 10 , start the vacuum ejector device 2, and perform vacuumization of the liquid methane subcooling device 1.

[0064] Then, close the vacuum isolation valve V4, the vacuum isolation hand valve V7, the vacuum isolation valve V8, the nitrogen supply valve V9, and the nitrogen supply valve V 10 , open the liquid nitrogen supply valve V1, the liquid nitrogen return valve V2, and the pressurization isolation valve V3, and start the liquid nitrogen pump 5 to fill the liquid methane subcooling device 1 with low-temperature saturated nitrogen.

[0065] Finally, close the liquid nitrogen supply valve V1, the liquid nitrogen return valve V2, the pressurization isolation valve V3, and the liquid nitrogen pump 5, open the subcooled liquid methane discharge valve V6, and discharge the pressurized subcooled liquid methane in the liquid methane subcooling device 1, with the pressurized subcooled liquid methane being 0.1-0.46 MPa and 93 K.

[0066] Example Two

[0067] As shown in Figure 3 , the difference between Example Two and Example One is that it further includes a heating device 11 arranged on the pipeline between the outlet of the buffer tank 6 and the vacuum isolation valve V8; the heating device 11 is used to heat the ejector medium of the vacuum ejector device 1, and the temperature of the ejector medium is greater than 5°C; in Example Two, the vacuum ejector device 2 is a water ring vacuum pump or a water ejector, and the ejector medium is water. The inlet of the vacuum ejector device 2 is connected with a water source through the nitrogen isolation valve V9.

[0068] In this embodiment, the vacuum ejector device 2 is a water ejector (in other embodiments, a water ring vacuum pump can also be used), but a heating device 11 needs to be arranged at the inlet of the water ejector; the heating device 11 is determined according to the actual ejector flow of the water ejector, and needs to ensure that the temperature of the water source entering the water ejector is greater than 5°C; if a water ring vacuum pump is used, it should have sufficient explosion-proof performance.

[0069] Combining low-temperature saturated nitrogen filling with vacuumizing can obtain supercooled liquid methane with back pressure, so that the density of liquid methane increases, which not only improves the storage efficiency, but also does not produce BOG gas waste because LNG is in a supercooled state. Such liquid methane with pressure and supercooling has great advantages in storage and transportation.

[0070] The realization of propellant densification of liquid rocket with liquid methane as propellant through supercooled liquid methane will provide important technical support for future civil aerospace related fields.

Claims

1. An engineered large scale subcooled liquid methane production system with back pressure, characterized by: The system comprises a liquid methane subcooling device (1), a vacuum ejector device (2), a liquid nitrogen gasification device (3), a liquid accumulator (4), a liquid nitrogen pump (5), a buffer tank (6) arranged between the vacuum ejector device (2) and the liquid methane subcooling device (1), and a low-temperature check valve (7) arranged between the liquid nitrogen gasification device (3) and the liquid nitrogen pump (5); The ejector medium port of the vacuum ejector device (2) is connected with an external ejector medium, the inlet of the vacuum ejector device (2) is connected with the gas outlet of the liquid methane subcooling device (1), and a vacuum isolation valve V4 is arranged on the connecting pipeline between the inlet of the vacuum ejector device (2) and the gas outlet of the liquid methane subcooling device (1); The inlet of the liquid nitrogen gasification device (3) is connected with the outlet of the liquid nitrogen pump (5), and the outlet of the liquid nitrogen gasification device (3) is connected with the inlet of the liquid accumulator (4); The inlet of the liquid nitrogen pump (5) is connected with an external liquid nitrogen source; The outlet of the liquid accumulator (4) is connected with the pressurized nitrogen inlet of the liquid methane subcooling device (1), and a pressurization isolation valve V3 is arranged on the connecting pipeline between the outlet of the liquid accumulator (4) and the pressurized nitrogen inlet of the liquid methane subcooling device (1); The lower part of the liquid methane subcooling device (1) is provided with a liquid methane inlet and outlet, a saturated liquid methane filling valve V5 and a subcooled liquid methane discharge valve V6 are arranged on the liquid methane inlet and outlet, saturated liquid methane is filled into the liquid methane subcooling device (1) through the saturated liquid methane filling valve V5, and the subcooled liquid methane in the liquid methane subcooling device (1) is discharged through the subcooled liquid methane discharge valve V6; Nitrogen isolation valve V9 and nitrogen supply valve V 10 The vacuum suction device (2) is connected with the outlet of the buffer tank (6), and a vacuum isolation valve V8 is arranged on the connecting pipeline between the inlet of the vacuum suction device (2) and the outlet of the buffer tank (6). The inlet of the buffer tank (6) is connected with the gas outlet of the liquid methane subcooling device (1) through the vacuum isolation valve V4, and a vacuum isolation hand valve V7 is arranged on the connecting pipeline between the inlet of the buffer tank (6) and the vacuum isolation valve V4; The inlet of the liquid nitrogen gasification device (3) is connected with the overflow port of the liquid accumulator (4), and a liquid nitrogen backflow valve V2 is arranged on the pipeline between the inlet of the liquid nitrogen gasification device (3) and the overflow port of the liquid accumulator (4).

2. The system according to claim 1, wherein: A liquid nitrogen supply valve V1 is arranged on the connecting pipeline between the inlet of the liquid nitrogen pump (5) and an external liquid nitrogen source.

3. The system according to claim 2, wherein: Also included is a vent valve V provided on the gas outlet of the liquid methane subcooling device (1) 11 with the safety valve (8), the vent valve V 11 for controlling the internal pressure of the liquid methane subcooling device (1).

4. The system according to claim 3, wherein: A pressure monitor (9) is arranged outside the buffer tank (6); An adiabatic layer (10) is arranged on the liquid methane subcooling device (1).

5. The system according to claim 4, wherein: The liquid methane subcooling device (1) is a vacuum insulated container or a heat preservation container; The vacuum ejector device (2) is a nitrogen ejector, and the flow rate of the nitrogen ejector is 0-1 m³ / s; The liquid nitrogen gasification device (3) is an air-cooled liquid nitrogen vaporizer; The liquid nitrogen pump (5) is a small-flow liquid nitrogen pump.

6. The engineered large-scale subcooled liquid methane production system with back pressure according to claim 5, wherein: the volume of the heat preservation container is greater than 10 m³; the volume of the buffer tank (6) is 0.5 m³; the flow rate of the small flow liquid nitrogen pump is 0.15-0.3 kg / s.

7. The engineered large scale subcooled liquid methane production system with back pressure of claim 5, wherein: a heating device (11) is further included; the heating device (11) is arranged on the pipeline between the outlet of the buffer tank (6) and the vacuum isolation valve V8; the heating device (11) is used for heating the motive medium of the vacuum motive device (2), and the temperature of the motive medium is greater than 5℃; the vacuum motive device (2) is a water ring vacuum pump or a water ejector.

8. An engineered large scale subcooled liquid methane production process with back pressure, using the engineered large scale subcooled liquid methane production system with back pressure as claimed in any one of claims 1 to 6, characterized in that, the following steps are included: Step 1: open the saturated liquid methane filling valve V5 to fill the liquid methane subcooling device (1) with saturated liquid methane; Step 2: close the saturated liquid methane filling valve V5, open the vacuum isolation valve V4, start the vacuum motive device (2), and perform vacuumization on the liquid methane subcooling device (1); Step 3: close the vacuum isolation valve V4, start the liquid nitrogen pump (5) and the pressurization isolation valve V3, and fill the liquid methane subcooling device (1) with low-temperature saturated nitrogen; Step 4: close the liquid nitrogen pump (5) and the pressurization isolation valve V3, open the subcooled liquid methane discharge valve V6 to discharge the subcooled liquid methane in the liquid methane subcooling device (1), and complete the preparation of the subcooled liquid methane.

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