Nitrogen filling inerting and exhaust liquid nitrogen subzero treatment system and method for external floating roof tank

Abstract

The invention relates to a nitrogen filling inerting and exhaust liquid nitrogen subzero treatment system of an external floating roof tank, which comprises the external floating roof tank (1), an external floating roof (2), a primary seal (3), a sealed space (4), a secondary seal (5), a pressure transmitter (6), a frequency conversion fan (7), a sampling pump (8), a gas sensor, a subzero condenser (11), a liquid nitrogen tank (12), a nitrogen injection valve (15) and a nitrogen buffer tank (16), the sealed space (4) is connected with the cryogenic condenser (11) through the pressure transmitter (6) and the frequency conversion fan (7), the sampling pump (8) is connected with the sealed space (4) and the cryogenic condenser (11), the gas sensor is arranged between the sampling pump (8) and the cryogenic condenser (11), the cryogenic condenser (11) is connected with the liquid nitrogen tank (12) and the nitrogen buffer tank (16), and the nitrogen buffer tank (16) is connected with the sealed space (4). The problems of nitrogen source of active safe nitrogen charging and oil gas recycling and up-to-standard emission of exhausted gas during nitrogen charging are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil storage and transportation, and relates to a system and method for actively and safely filling nitrogen and recovering and treating exhaust gas generated during oil storage and transportation, and in particular to a system and method for actively and safely filling nitrogen and recovering and treating exhaust gas containing VOCs in an external floating roof tank. Background Art

[0002] Crude oil is a flammable and explosive dangerous good. There are many ways to store crude oil, and external floating roof oil storage tanks have become the most commonly used type of storage tank. Because there are primary and secondary sealing devices between the floating roof edge and the inner wall of the tank, a mixture of explosive gas and oxygen appears between the primary and secondary seals for a long time, and this space belongs to the explosion hazard area. Apart from human factors, factors such as lightning, static electricity generated by the tank itself, and sparks can easily cause oil and gas explosions in this sealed space.

[0003] At present, the fire-fighting methods for large oil storage tanks mainly use foam and water to rescue fire accidents, and active safety systems have also been developed.

[0004] For example, Chinese invention patent CN113003028B discloses a safety protection system and method for the whole process operation of an external floating roof storage tank, which is provided with a floating roof gas collection system on the floating plate, and the floating roof gas collection system connects several automatic vent valves and edge breathing valves on the floating plate to form a closed gas collection environment, and is provided with a nitrogen injection port and an air extraction port on the floating roof gas collection system, and the safety protection system also includes a pressure detection unit; the nitrogen injection port is connected to the nitrogen recovery supply system through a nitrogen pipeline, and the air extraction port is connected to the nitrogen recovery supply system through an adaptive telescopic ventilation pipe network, and the floating roof gas collection system is connected to a protective device through a nitrogen injection and air extraction pipe network and a sampling pipe network, respectively, and the protective device is connected to the nitrogen recovery supply system. This invention targets the safety hazards existing in the whole process operation of the floating plate of the external floating roof storage tank, controls the VOCs leakage points of the floating plate from the source, reduces the unorganized emission of VOCs, and realizes the intelligent nitrogen sealing and orderly collection of VOCs in the whole operation process of the external floating roof storage tank. However, the invention requires a separate nitrogen generator to meet the demand for nitrogen, and also requires a separate oil and gas processing device to pressurize, condense, adsorb and purify the oil and gas. The system has many moving equipment, a long process, high investment and a high operating failure rate.

[0005] Chinese utility model patent CN206032280U discloses an active safety protection and oil and gas recovery device for an external floating roof storage tank, which includes a sampling and analysis system, an inerting system and an oil and gas recovery system. The sampling and analysis system includes a sampling probe, a gas analyzer and a negative pressure buffer tank, and multiple sampling probes are arranged in the secondary sealed space of the external floating roof storage tank; the inerting system includes an inerting gas filling head and an inerting gas storage tank, and multiple inerting gas filling heads are arranged in the secondary sealed space of the external floating roof storage tank. The utility model analyzes the oil and gas components in the secondary sealed space of the external floating roof storage tank through the sampling and analysis system, comprehensively calculates the oil and gas explosion limit, and refers to the oxygen content to determine whether the oil and gas reach the explosion range, so as to perform gas inerting and achieve the purpose of active inerting. At the same time, the oil and gas recovery system is used to recover the oil and gas by molecular sieve adsorption first and then refrigeration deliquidation, so as to achieve zero emission of the device and avoid the pollution of the atmospheric environment caused by oil and gas emissions. However, the utility model needs to set up an inerting gas storage tank, but does not mention the source of the inerting gas, and the oil and gas recovery system is complex and costly.

[0006] Therefore, it is necessary to develop a solution with convenient nitrogen source, low oil and gas condensation temperature, and sufficient liquefaction and recovery of VOCs. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system and method for an external floating roof tank. The system takes liquid nitrogen cryogenic oil and gas recovery technology as the core, and solves the nitrogen source of the active safety nitrogen filling system and the oil and gas recovery and standard emission problems of the exhaust gas during nitrogen filling, which has significant safety benefits, environmental and social benefits and economic benefits.

[0008] In order to achieve the above object, the present invention provides the following technical solutions: A nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank, comprising an external floating roof tank, an external floating roof, a primary seal, a sealed space and a secondary seal, wherein the external floating roof is installed in the external floating roof tank and the primary seal and the secondary seal are installed between the external floating roof and the external floating roof tank, and the sealed space is formed between the primary seal and the secondary seal. The invention is characterized in that it also comprises a pressure transmitter, a variable frequency fan, a sampling pump, a gas sensor, a cryogenic condenser, a liquid nitrogen tank, a nitrogen injection valve and a nitrogen buffer tank, wherein the sealed space is connected to the pressure transmitter and the pressure transmitter is connected to the variable frequency fan, the variable frequency fan is connected to the cryogenic condenser, one end of the sampling pump is connected to the sealed space and the other end is connected to the cryogenic condenser, and the gas sensor is installed between the sampling pump and the cryogenic condenser, the cryogenic condenser is connected to the liquid nitrogen tank and the nitrogen buffer tank, and the nitrogen buffer tank is connected to the sealed space through the nitrogen injection valve.

[0009] Preferably, the gas sensor includes an oxygen sensor and a VOCs concentration sensor.

[0010] Preferably, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank further comprises a condensate tank, and the condensate tank is connected to the cryogenic condenser.

[0011] Preferably, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank further comprises a condensate pump, and the condensate tank is connected to the external floating roof tank via the condensate pump.

[0012] Preferably, the nitrogen filling inerting and exhausting liquid nitrogen cryogenic treatment system of the external floating roof tank further comprises an exhaust pipe, and the exhaust pipe is connected to the cryogenic condenser.

[0013] In addition, the present invention also provides a method for nitrogen filling inerting and exhausting liquid nitrogen cryogenic treatment of an external floating roof tank, which is characterized in that it adopts the above-mentioned nitrogen filling inerting and exhausting liquid nitrogen cryogenic treatment system of an external floating roof tank and comprises the following steps: 1) The oil and gas in the sealed space are extracted by the sampling pump and detected by the oxygen sensor and the VOCs concentration sensor to determine whether it is in a safe state. If it is in a safe state, proceed to step 2. If it is in an unsafe state, proceed to step 3); 2) No nitrogen injection measures are required, the nitrogen injection valve is closed, and the oil and gas extracted by the sampling pump is sent to the cryogenic condenser, and the liquid nitrogen stored in the liquid nitrogen tank is also sent to the cryogenic condenser. The oil and gas are cooled by liquid nitrogen to condense the VOCs in the oil and gas into condensate, and the condensate flows into the condensate tank. The liquid nitrogen absorbs heat in the cryogenic condenser and gasifies into nitrogen and enters the nitrogen buffer tank. The non-condensable tail gas in the oil and gas after deep cooling is discharged through the exhaust pipe; 3) Nitrogen injection measures need to be taken, the nitrogen injection valve is opened, and the nitrogen in the nitrogen buffer tank is injected into the sealed space. The injected nitrogen reduces the oxygen concentration and VOCs concentration in the sealed space, and causes the pressure in the sealed space to increase. After the reading of the pressure transmitter increases, the frequency conversion fan is controlled to increase the frequency, and the oil and gas in the sealed space after nitrogen injection are extracted to prevent the oil and gas from escaping unorganizedly through the secondary seal. The oil and gas extracted by the frequency conversion fan are sent to the cryogenic condenser, and the liquid nitrogen stored in the liquid nitrogen tank is also sent to the cryogenic condenser. The oil and gas are cooled by liquid nitrogen to condense the VOCs in the oil and gas into condensate, and the condensate The liquid flows into the condensate tank, the liquid nitrogen absorbs heat in the cryogenic condenser and vaporizes into nitrogen which enters the nitrogen buffer tank, and the non-condensable tail gas in the oil and gas after deep cooling is discharged through the exhaust pipe. During this period, the oil and gas extracted by the sampling pump are continuously detected by the oxygen sensor and the VOCs concentration sensor, and the detected oil and gas are sent to the cryogenic condenser to condense and recover the VOCs in the oil and gas, and the non-condensable tail gas is discharged through the exhaust pipe, and the condensate enters the condensate tank, and when the oxygen sensor and the VOCs concentration sensor detect that they are in a safe state, the nitrogen injection valve is closed, and the variable frequency fan is operated at a reduced frequency or turned off.

[0014] Preferably, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment method of the external floating roof tank further comprises the following steps: 4) When the liquid level in the condensate tank reaches a high liquid level, the condensate pump is started to transport the condensate in the condensate tank back to the external floating roof tank to recycle resources.

[0015] Compared with the prior art, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system and method of the external floating roof tank of the present invention has one or more of the following beneficial technical effects: 1. The present invention uses liquid nitrogen as a cold source and nitrogen source, which comprehensively solves the nitrogen source problem during nitrogen injection for active safety protection in a sealed space and the VOCs oil and gas condensation recovery problem during nitrogen injection exhaust, without the need to set up a separate nitrogen generator and oil and gas recovery and processing equipment.

[0016] 2. Since the liquid nitrogen produced by large-scale air separation units is easy to obtain and cheap, and the oil and gas condensation temperature is as low as -120~-160℃, it has a higher recovery rate for crude oil and gas with a high light component content than mechanical refrigeration (generally the oil and gas condensation temperature is -50~-70℃), meeting the relevant national environmental protection emission standards. In addition, the nitrogen purity after liquid nitrogen absorbs heat and vaporizes is high, generally reaching 99.9%, and has better performance when used for nitrogen injection inerting.

[0017] 3. Compared with the existing active safety protection nitrogen injection technology and oil and gas processing technology, the solution provided by the present invention has low cost and simple operation, which significantly reduces the owner's comprehensive operating cost and has significant safety benefits, environmental and social benefits and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the structure of the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention.

[0019] In the figure: 1. External floating roof tank; 2. External floating roof; 3. Primary seal; 4. Sealed space; 5. Secondary seal; 6. Pressure transmitter; 7. Variable frequency fan; 8. Sampling pump; 9. Oxygen sensor; 10. VOCs concentration sensor; 11. Cryogenic condenser; 12. Liquid nitrogen tank; 13. Condensate tank; 14. Condensate pump; 15. Nitrogen injection valve; 16. Nitrogen buffer tank; 17. Exhaust pipe. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The present invention relates to a nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system and method for an external floating roof tank. Aiming at the explosive oil and gas in the sealed space of the oil product external floating roof tank, the oil and gas concentration and oxygen content are detected. When the oil and gas concentration and oxygen content are in an explosion risk state, nitrogen is injected for inerting to reduce the oil and gas concentration and oxygen content to a safe range. After nitrogen injection, the pressure of the sealed space increases, and the VOCs oil and gas in the sealed space are extracted through a pressure transmitter interlocked with a variable frequency fan. The extracted oil and gas are sent to a liquid nitrogen cryogenic condenser for condensation treatment. The VOCs are cooled and condensed into liquid. The tail gas after cryogenic treatment meets the emission standards. The condensate returns to the condensate tank. The liquid nitrogen absorbs the latent heat of liquefaction of the VOCs. The gasified nitrogen is used as an inerting gas source. With the liquid nitrogen cryogenic oil and gas recovery technology as the core, the nitrogen source of the active safety nitrogen filling system and the oil and gas recovery and utilization of the exhaust gas during nitrogen filling and the emission standards are solved in combination, which has significant safety benefits, environmental and social benefits and economic benefits.

[0023] Figure 1 The schematic diagram of the structure of the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention is shown. Figure 1 As shown, similar to the prior art, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention comprises an external floating roof tank 1, an external floating roof 2, a primary seal 3, a sealed space 4 and a secondary seal 5. The external floating roof 2 is installed in the external floating roof tank 1 and the primary seal 3 and the secondary seal 5 are installed between the external floating roof 2 and the external floating roof tank 1. The sealed space 4 is formed between the primary seal 3 and the secondary seal 5.

[0024] Different from the prior art, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention also includes a pressure transmitter 6, a variable frequency fan 7, a sampling pump 8, a gas sensor, a cryogenic condenser 11, a liquid nitrogen tank 12, a nitrogen injection valve 15 and a nitrogen buffer tank 16.

[0025] The sealed space 4 is connected to the pressure transmitter 6 and the pressure transmitter 6 is connected to the variable frequency fan 7. The variable frequency fan 7 is connected to the cryogenic condenser 11. Thus, the pressure transmitter 6 can sense the pressure change in the sealed space 4 and drive the variable frequency fan 7 to increase the frequency when the pressure increases, so that the variable frequency fan 7 can extract oil and gas from the sealed space 4 and extract the oil and gas into the cryogenic condenser 11 for condensation recovery.

[0026] One end of the sampling pump 8 is connected to the sealed space 4, and the other end is connected to the cryogenic condenser 11, and the gas sensor is installed between the sampling pump 8 and the cryogenic condenser 11. Thus, the sampling pump 8 can pump the oil and gas in the sealed space 4, and detect it through the gas sensor to determine whether it is in a safe state. In addition, the pumped oil and gas can be sent to the cryogenic condenser 11 for condensation and recovery.

[0027] Preferably, the gas sensor includes an oxygen sensor 9 and a VOCs concentration sensor 10. The oxygen sensor 9 can detect the oxygen concentration in the sealed space 4, and the VOCs concentration sensor 10 can detect the VOCs concentration in the sealed space 4. Thus, whether the oxygen concentration and the VOCs concentration exceed the standard can be used to determine whether the space is in a safe state.

[0028] The cryogenic condenser 11 is connected to the liquid nitrogen tank 12 and the nitrogen buffer tank 16. The nitrogen buffer tank 16 is connected to the sealed space 4 through the nitrogen injection valve 15. Thus, the liquid nitrogen in the liquid nitrogen tank 12 can enter the cryogenic condenser 11, and the oil and gas entering the cryogenic condenser 11 are cooled by the liquid nitrogen so that the VOCs in the oil and gas are condensed into condensate. The liquid nitrogen absorbs heat in the cryogenic condenser 11 and is gasified into nitrogen and enters the nitrogen buffer tank 16. In addition, by opening the nitrogen injection valve 15, the nitrogen in the nitrogen buffer tank 16 can be injected into the sealed space 4, and the injected nitrogen can reduce the oxygen concentration and VOCs concentration in the sealed space 4 to ensure safety.

[0029] Preferably, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention further comprises a condensate tank 13 and the condensate tank 13 is connected to the cryogenic condenser 11. Thus, the VOCs in the oil and gas can be stored in the condensate tank 13 after condensation into condensate.

[0030] More preferably, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention further comprises a condensate pump 14, and the condensate tank 13 is connected to the external floating roof tank 1 through the condensate pump 14. Thus, when the liquid level in the condensate tank 13 reaches the high liquid level, the condensate pump 14 is started, and the condensate in the condensate tank 13 can be transported back to the external floating roof tank 1 for recycling.

[0031] Moreover, the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention can further include an exhaust pipe 17 and the exhaust pipe 17 is connected to the cryogenic condenser 11. Thus, the non-condensable tail gas in the oil gas can be discharged through the exhaust pipe 17 to achieve standard emission.

[0032] When the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank of the present invention is in use, the oil and gas in the sealed space 4 can be extracted by the sampling pump 8, and the oxygen concentration and VOCs concentration in the sealed space 4 are detected by the oxygen sensor 9 and the VOCs concentration sensor 10 to determine whether it is in a safe state.

[0033] When in a safe state, that is, when the oxygen concentration and VOCs concentration do not exceed the standard, there is no need to take nitrogen injection measures, the nitrogen injection valve 15 is closed, and the oil and gas extracted by the sampling pump 8 is sent to the cryogenic condenser 11. The liquid nitrogen stored in the liquid nitrogen tank 12 is also sent to the cryogenic condenser 11, and the oil and gas are cooled by liquid nitrogen in the cryogenic condenser 11 so that the VOCs in the oil and gas are condensed into condensate. The condensate flows into the condensate tank 13, and the liquid nitrogen absorbs heat in the cryogenic condenser 11 and vaporizes into nitrogen and enters the nitrogen buffer tank 16. The non-condensable tail gas in the oil and gas after deep cooling is discharged through the exhaust pipe 17.

[0034] Due to the continuous volatilization of VOCs components contained in crude oil and the infiltration of air, the oxygen concentration and VOCs concentration in the sealed space 4 will continue to increase. When it is in an unsafe state, that is, when the oxygen concentration or VOCs concentration exceeds the standard, nitrogen injection measures need to be taken. The nitrogen injection valve 15 is opened, and the nitrogen in the nitrogen buffer tank 16 is injected into the sealed space 4 to achieve active protection. The injected nitrogen reduces the oxygen concentration and VOCs concentration in the sealed space 4, and at the same time causes the pressure in the sealed space 4 to increase. After the reading of the pressure transmitter 6 increases, the variable frequency fan 7 is controlled to increase the frequency to extract the oil and gas in the sealed space 4 after nitrogen injection to prevent the oil and gas from escaping unorganized through the secondary seal 5. The oil and gas extracted by the variable frequency fan 7 contains VOCs, which are precious resources. On the one hand, venting will cause waste of resources, and on the other hand, it will also cause environmental pollution. Therefore, the oil and gas extracted by the variable frequency fan 7 are sent to the cryogenic condenser 11, and the liquid nitrogen stored in the liquid nitrogen tank 12 is also sent to the cryogenic condenser 11. In the cryogenic condenser 11, the oil and gas are cooled by liquid nitrogen to condense the VOCs in the oil and gas into condensate, and the condensate flows into the condensate tank 13. The liquid nitrogen absorbs heat in the cryogenic condenser 11 and gasifies into nitrogen and enters the nitrogen buffer tank 16. The non-condensable tail gas in the oil and gas after deep cooling is discharged through the exhaust pipe 17. During this period, the oil and gas extracted by the sampling pump 8 is continuously detected by the oxygen sensor 9 and the VOCs concentration sensor 10. The detected oil and gas are sent to the cryogenic condenser 11, and the VOCs in the oil and gas are condensed and recovered. The non-condensable tail gas is discharged through the exhaust pipe 17, and the condensate enters the condensate tank 13. Furthermore, when the oxygen sensor 9 and the VOCs concentration sensor 10 detect that they are in a safe state, the nitrogen injection valve 15 is closed, and the variable frequency blower 7 is operated at a reduced frequency or is turned off.

[0035] When the liquid level in the condensate tank 13 reaches a high liquid level, the condensate pump 14 is started to transport the condensate in the condensate tank 13 back to the external floating roof tank 1 to recycle resources.

[0036] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank, comprising an external floating roof tank (1), an external floating roof (2), a primary seal (3), a sealed space (4) and a secondary seal (5), wherein the external floating roof (2) is installed in the external floating roof tank (1) and the primary seal (3) and the secondary seal (5) are installed between the external floating roof (2) and the external floating roof tank (1), and the sealed space (4) is formed between the primary seal (3) and the secondary seal (5), characterized in that: The invention also comprises a pressure transmitter (6), a variable frequency fan (7), a sampling pump (8), a gas sensor, a cryogenic condenser (11), a liquid nitrogen tank (12), a nitrogen injection valve (15) and a nitrogen buffer tank (16), wherein the sealed space (4) is connected to the pressure transmitter (6) and the pressure transmitter (6) is connected to the variable frequency fan (7), the variable frequency fan (7) is connected to the cryogenic condenser (11), one end of the sampling pump (8) is connected to the sealed space (4) and the other end is connected to the cryogenic condenser (11), and the gas sensor is installed between the sampling pump (8) and the cryogenic condenser (11), the cryogenic condenser (11) is connected to the liquid nitrogen tank (12) and the nitrogen buffer tank (16), and the nitrogen buffer tank (16) is connected to the sealed space (4) through the nitrogen injection valve (15).

2. The nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank according to claim 1 is characterized in that: The gas sensor comprises an oxygen sensor (9) and a VOCs concentration sensor (10).

3. The nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank according to claim 2, characterized in that: It further comprises a condensate tank (13), and the condensate tank (13) is connected to the cryogenic condenser (11).

4. The nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank according to claim 3 is characterized in that: It further comprises a condensate pump (14), and the condensate tank (13) is connected to the external floating roof tank (1) via the condensate pump (14).

5. The nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system for an external floating roof tank according to claim 4, characterized in that: It further comprises an exhaust pipe (17), and the exhaust pipe (17) is connected to the cryogenic condenser (11).

6. A method for nitrogen inerting and exhausting liquid nitrogen cryogenic treatment of an external floating roof tank, characterized in that: The method adopts the nitrogen filling inerting and exhaust liquid nitrogen cryogenic treatment system of the external floating roof tank as claimed in claim 5, and comprises the following steps: 1) The oil and gas in the sealed space (4) are extracted by the sampling pump (8) and detected by the oxygen sensor (9) and the VOCs concentration sensor (10) to determine whether it is in a safe state. If it is in a safe state, the process proceeds to step 2; if it is in an unsafe state, the process proceeds to step 3); 2) No nitrogen injection measures are required, the nitrogen injection valve (15) is closed, and the oil and gas extracted by the sampling pump (8) is sent to the cryogenic condenser (11), and the liquid nitrogen stored in the liquid nitrogen tank (12) is also sent to the cryogenic condenser (11). The oil and gas are cooled by the liquid nitrogen so that the VOCs in the oil and gas are condensed into condensate, and the condensate flows into the condensate tank (13). The liquid nitrogen absorbs heat in the cryogenic condenser (11) and is gasified into nitrogen and enters the nitrogen buffer tank (16). The non-condensable tail gas in the oil and gas after being cryogenically cooled is discharged through the exhaust pipe (17); 3) Nitrogen injection measures need to be taken, the nitrogen injection valve (15) is opened, and the nitrogen in the nitrogen buffer tank (16) is injected into the sealed space (4). The injected nitrogen reduces the oxygen concentration and VOCs concentration in the sealed space (4), and at the same time causes the pressure in the sealed space (4) to increase. After the reading of the pressure transmitter (6) increases, the variable frequency fan (7) is controlled to increase the frequency of operation, and the oil and gas in the sealed space (4) after nitrogen injection are extracted to prevent the oil and gas from escaping unorganizedly through the secondary seal (5). The oil and gas extracted by the variable frequency fan (7) are sent to the cryogenic condenser (11), and the liquid nitrogen stored in the liquid nitrogen tank (12) is also sent to the cryogenic condenser (11). The oil and gas are cooled by the liquid nitrogen so that the VOCs in the oil and gas are condensed into condensate, and the condensate flows into the In the condensate tank (13), liquid nitrogen absorbs heat in the cryogenic condenser (11) and is gasified into nitrogen and enters the nitrogen buffer tank (16). The non-condensable tail gas in the oil gas after being cryogenically cooled is discharged through the exhaust pipe (17). During this period, the oil gas extracted by the sampling pump (8) is continuously detected by the oxygen sensor (9) and the VOCs concentration sensor (10). The detected oil gas is sent to the cryogenic condenser (11) to condense and recover the VOCs in the oil gas. The non-condensable tail gas is discharged through the exhaust pipe (17), and the condensate enters the condensate tank (13). In addition, when the oxygen sensor (9) and the VOCs concentration sensor (10) detect that they are in a safe state, the nitrogen injection valve (15) is closed, and the variable frequency fan (7) is operated at a reduced frequency or is turned off.

7. The method for nitrogen inerting and exhausting liquid nitrogen cryogenic treatment of an external floating roof tank according to claim 6, characterized in that: Further comprising the steps of: 4) When the liquid level in the condensate tank (13) reaches a high liquid level, the condensate pump (14) is started to transport the condensate in the condensate tank (13) back to the external floating roof tank (1) to recover resources.

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