Artificial graphite negative electrode material coating production control system and control method based on DCS control

Abstract

The invention discloses an artificial graphite negative electrode material coating production control system based on DCS control and a control method. The control system mainly comprises a temporary feeding storage bin, a mixing machine, a vacuum feeding machine, a reaction kettle, a cooling kettle and a tail gas tank, the temporary feeding storage bin is connected with the mixing machine through a negative pressure conveying pipeline, the mixing machine is connected with the vacuum feeding machine through a negative pressure conveying pipeline, a feeding stop valve is arranged between the vacuum feeding machine and the reaction kettle, a tail gas stop valve is arranged between the reaction kettle and the tail gas tank, and the tail gas tank is provided with an oxygen concentration sensor; a first discharge valve is arranged between the reaction kettle and the cooling kettle, the reaction kettle is connected with a first nitrogen inlet valve through a nitrogen inlet pipeline, and the reaction kettle is provided with an emergency emptying valve, a temperature sensor and a pressure sensor; the feeding stop valve, the tail gas stop valve, the oxygen concentration sensor, the first discharging valve, the first nitrogen inlet valve, the emergency emptying valve, the temperature sensor and the pressure sensor are all connected into a DCS, and data collection, instruction issuing and logic interlocking centralized management are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical production safety control, and in particular to a control system and a control method for artificial graphite negative electrode material coating production based on DCS control. Background Art

[0002] In the traditional artificial graphite negative electrode material coating process, the horizontal kettle, tail gas treatment and public auxiliary system control have the following technical defects: 1) Decentralized equipment control: The horizontal kettle, feeding system, tail gas system, etc. are controlled by a single PLC machine. The public auxiliary systems (such as machine seal cooling water and cooling kettle circulating water) lack effective interlocking with the main equipment and rely on manual operation to adjust parameters, which is inefficient and prone to errors. 2) Risks of key parameter monitoring: Key parameters such as machine seal cooling water and cooling water flow are only monitored on-site through field instruments. The data is not centrally managed, and abnormal situations cannot be warned in real time, posing a great safety hazard. 3) Insufficient control of the tail gas system: The tail gas valve is not interlocked with the working conditions in the kettle. When the pressure or temperature in the kettle exceeds the limit, manual intervention is required to open and close the valve. The delayed response can easily lead to safety accidents (such as overpressure explosion and tail gas combustion). 4) Low degree of automation: The start and stop of the process requires operation of each device, which is time-consuming and labor-intensive, and lacks a unified safety interlock logic, making it difficult to ensure production continuity.

[0003] In view of the problems existing in the existing technology, such as lack of safety interlock between equipment, frequent manual intervention, disconnection between key parameter monitoring and emergency response, lack of exhaust system control, and prominent safety hazards, it is urgent to develop a DCS-based coating process safety interlock control method. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a production control system and control method for artificial graphite negative electrode material coating based on DCS control, so as to ensure safe production operation and stable and controllable product quality.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a production control system for coating artificial graphite negative electrode materials based on DCS control, the production control system mainly includes a feeding temporary storage bin, a mixer, a vacuum feeder, a reactor and a cooling kettle connected in sequence, the reactor is also connected to the tail gas tank; the feeding temporary storage bin is connected to the mixer through a negative pressure conveying pipeline, the mixer is connected to the vacuum feeder through a negative pressure conveying pipeline, a feed shut-off valve is provided between the vacuum feeder and the reactor, and a tail gas shut-off valve is provided between the reactor and the tail gas tank. The tail gas tank is provided with an oxygen concentration sensor; a first discharge valve is provided between the reactor and the cooling kettle, the reactor is connected to the first nitrogen inlet valve through a nitrogen inlet pipe, an emergency vent valve is provided on the top of the reactor, and the emergency vent valve is connected to the tail gas tank; a temperature sensor and a pressure sensor are provided in the reactor; the feed shut-off valve, tail gas shut-off valve, oxygen concentration sensor, first discharge valve, first nitrogen inlet valve, emergency vent valve, temperature sensor and pressure sensor are all connected to the DCS to realize centralized management of data acquisition, command issuance and logical interlocking.

[0006] Furthermore, the cooling kettle is connected to a circulating water flow switch through a circulating water input pipe, the cooling kettle is connected to a second nitrogen inlet valve through a nitrogen inlet pipe, and a second discharge valve is provided on the cooling kettle discharge pipe; the circulating water flow switch, the second nitrogen inlet valve and the second discharge valve are all connected to the DCS.

[0007] Furthermore, the reaction kettle and the cooling kettle are respectively provided with a first agitator and a second agitator, and the cooling kettle is provided with a first weighing sensor; the first agitator, the second agitator and the first weighing sensor are all connected to the DCS.

[0008] Furthermore, the reactor is connected to a heating system, which is controlled by a heating system shut-off valve, which is interlocked with a temperature sensor signal; and the heating system shut-off valves are all connected to a DCS.

[0009] Furthermore, an intermediate weighing bin is provided between the temporary storage bin and the mixer; the intermediate weighing bin is used for batching and unloading via an automatic weighing system; the intermediate weighing bin is provided with a second weighing sensor and a unloading rotary valve; the second weighing sensor and the unloading rotary valve are both connected to the DCS.

[0010] A control method for an artificial graphite negative electrode material coating production control system based on DCS control, the control method comprising the following contents:

[0011] 1) Feeding: Add the reaction materials into the temporary storage bin and automatically add them to the intermediate weighing bin through the negative pressure conveying system;

[0012] 2) Batching: The second weighing sensor in the intermediate weighing bin is interlocked with the signal of the discharge rotary valve. The material is fed into the mixer through a negative pressure conveying system. When the weight in the intermediate weighing bin reaches the required value, the discharge rotary valve is automatically closed.

[0013] 3) Mixing: Turn on the mixer and stir to mix the materials;

[0014] 4) Feeding: The material in the mixer is conveyed to the vacuum feeder through the negative pressure conveying system; the first agitator in the reactor is automatically turned on, and the tail gas shut-off valve is automatically opened;

[0015] 5) Nitrogen replacement: When the vacuum loader finishes unloading, the first nitrogen inlet valve is automatically opened and the tail gas shut-off valve is automatically closed for nitrogen replacement; when the tail gas oxygen concentration sensor meets the requirements, the agitator in the reactor is automatically turned on first, and then the heating system of the reactor is automatically turned on;

[0016] 6) Heating: The temperature sensor in the reactor is interlocked with the cut-off valve signal of the heating system to control the reaction temperature and time until the reaction is completed;

[0017] 7) Cooling: After the reaction is completed, the cooling kettle is automatically replaced with nitrogen, and the circulating cooling water and stirring are turned on. Then the reactor automatically opens the first discharge valve and puts the material into the cooling kettle for cooling;

[0018] 8) Discharging: When the temperature drops to the target value, the second discharge valve of the cooling kettle is automatically opened, and the material is transported to the next process through the negative pressure conveying system; the first weighing system is interlocked with the second discharge valve, and when the weighing requirements are met, the second discharge valve is automatically closed.

[0019] Furthermore, when the value of the pressure sensor in the reactor exceeds the set upper limit, the DCS automatically triggers the interlock, and the exhaust gas shut-off valve is fully opened to release pressure; when the temperature and pressure in the reactor both exceed the design upper limit, the DCS automatically triggers the interlock, and the exhaust gas shut-off valve and the emergency vent valve are opened at the same time.

[0020] Furthermore, when the oxygen concentration sensor for exhaust gas detects an oxygen concentration ≥5%, the DCS immediately executes the following: cuts off the exhaust gas emission path; starts nitrogen replacement, introducing nitrogen to dilute the oxygen concentration to <3%; triggers an audible and visual alarm and records the event.

[0021] Furthermore, in the control system, a one-button start-stop interlock function is implemented through DCS. The one-button start is specifically:

[0022] When the operator clicks "One-Click Start" on the DCS interface, the system completes the following actions in a pre-set sequence: 1) Utility systems (cooling water and nitrogen supply) pre-start and self-test; 2) Material loading and verification of the material level in the feeding system; 3) Reactor temperature and pressure are raised to process setpoints; 4) The exhaust system enters automatic monitoring mode. Specifically, when "One-Click Shutdown" is triggered in an emergency, the DCS executes the following: 1) disconnects the reactor heating power supply; 2) closes the feed valve and initiates reactor pressure relief; 3) switches the exhaust system to nitrogen protection mode; and 4) records the shutdown reason and a snapshot of key parameters.

[0023] The control system also features intelligent alarms and fault self-diagnosis. The tiered alarm system includes: 1) Level 1 (early warning): If a parameter approaches a threshold (e.g., pressure reaches 90% of the upper limit), the DCS prompts the operator to check; 2) Level 2 (emergency): If a parameter exceeds a limit, an interlock is triggered and forced intervention is initiated. Historical data tracing: The DCS stores three years of production data and supports retrieval by time and event type for accident analysis and process optimization. The DCS will also issue an alarm if the cooling kettle's circulating water flow switch is not activated.

[0024] Further, in step 5), the emergency vent valve is first closed, and then the first nitrogen inlet valve is opened. The pressure sensor in the reactor is interlocked with the first nitrogen inlet valve signal. When the pressure in the reactor reaches the requirement, the first nitrogen inlet valve is automatically closed, and then the emergency vent valve is opened. The pressure sensor in the reactor is interlocked with the emergency vent valve signal. When the pressure in the reactor reaches the requirement, the emergency vent valve is automatically closed. At this point, a nitrogen replacement operation is completed. The nitrogen replacement operation is repeated 3 to 10 times until there is basically no air in the reactor. At this time, the nitrogen replacement step is completed.

[0025] The beneficial effects of the present invention are as follows: compared with the prior art, the present invention provides a DCS-based control system and control method for coating artificial graphite negative electrode materials, which realizes centralized management of data acquisition, instruction issuance and logic interlocking by uniformly connecting corresponding valves, sensors, etc. in the production system to the DCS; key parameters (such as pressure, temperature, cooling water flow, exhaust gas oxygen concentration, etc.) in the reactor are transmitted to the DCS in real time through sensors, and data storage and visualization interface are updated synchronously; it has multi-level safety interlock control logic such as reactor pressure interlock, exhaust gas oxygen concentration interlock, cooling water alarm, etc.; in addition, it also has one-button start-stop interlock, intelligent alarm and fault self-diagnosis functions; interlock control shortens the abnormal response time to within 5 seconds, reduces the accident rate by 70%, and greatly improves the safety performance of the production system; manual operation links are reduced by 85%, process startup time is shortened by 50%, and the automation level is significantly optimized. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic structural diagram of the control system provided by the present invention.

[0027] 1-Feed shut-off valve; 2-Emergency vent valve; 3-Tail gas shut-off valve; 4-First discharge valve; 5-Circulating water flow switch; 6-Second discharge valve; 7-Second nitrogen inlet valve; 8-First nitrogen inlet valve. DETAILED DESCRIPTION

[0028] The present invention will be further described below by way of specific examples, which are only intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0029] Example

[0030] like Figure 1 As shown, a production control system for coating artificial graphite negative electrode materials based on DCS control is shown. The production control system mainly includes a feeding temporary storage bin, a mixer, a vacuum loader, a reactor and a cooling kettle connected in sequence, and the reactor is also connected to a tail gas tank; the feeding temporary storage bin is connected to the mixer through a negative pressure conveying pipe, and the mixer is connected to the vacuum loader through a negative pressure conveying pipe. A feed shut-off valve 1 is provided between the vacuum loader and the reactor, a tail gas shut-off valve 3 is provided between the reactor and the tail gas tank, and the tail gas tank is provided with an oxygen concentration sensor; a first discharge valve 4 is provided between the reactor and the cooling kettle, the reactor is connected to a first nitrogen inlet valve 8 through a nitrogen inlet pipe, and an emergency vent valve 2 is provided on the top of the reactor; a temperature sensor and a pressure sensor are provided in the reactor; the feed shut-off valve 1, the tail gas shut-off valve 3, the oxygen concentration sensor, the first discharge valve 4, the first nitrogen inlet valve 8, the emergency vent valve 2, the temperature sensor and the pressure sensor are all connected to the DCS to realize centralized management of data acquisition, command issuance and logical interlocking.

[0031] In this embodiment, the cooling kettle is connected to a circulating water flow switch 5 through a circulating water input pipe, the cooling kettle is connected to a second nitrogen inlet valve 7 through a nitrogen inlet pipe, and a second discharge valve 6 is provided on the cooling kettle discharge pipe; the circulating water flow switch 5, the second nitrogen inlet valve 7 and the second discharge valve 6 are all connected to the DCS.

[0032] In this embodiment, the reaction kettle and the cooling kettle are respectively provided with a first stirrer and a second stirrer, and the cooling kettle is provided with a first weighing sensor; the first stirrer, the second stirrer and the first weighing sensor are all connected to the DCS.

[0033] In this embodiment, the reactor is connected to a heating system, which is controlled by a heating system shut-off valve. The heating system shut-off valve is interlocked with a temperature sensor signal; the heating system shut-off valve is connected to a DCS.

[0034] In this embodiment, an intermediate weighing bin is provided between the temporary storage bin and the mixer; the intermediate weighing bin is used for material dispensing and unloading through an automatic weighing system; the intermediate weighing bin is provided with a second weighing sensor and a material unloading rotary valve; the second weighing sensor and the material unloading rotary valve are both connected to the DCS.

[0035] A control method for an artificial graphite negative electrode material coating production control system based on DCS control, the control method comprising the following contents:

[0036] 1) Feeding: Add the reaction materials into the temporary storage bin and automatically add them to the intermediate weighing bin through the negative pressure conveying system.

[0037] 2) Batching: The second weighing sensor in the intermediate weighing bin is interlocked with the signal of the discharge rotary valve. The material is added to the mixer through the negative pressure conveying system. When the weight in the intermediate weighing bin reaches the requirement, the discharge rotary valve is automatically closed.

[0038] 3) Mixing: Turn on the mixer to mix the materials.

[0039] 4) Feeding: The materials in the mixer are conveyed to the vacuum feeder through the negative pressure conveying system; the first agitator in the reactor is automatically turned on, and the tail gas shut-off valve 3 is automatically opened.

[0040] 5) Nitrogen Replacement: After the vacuum loader finishes unloading, the first nitrogen inlet valve 8 is automatically opened and the tail gas shut-off valve 3 is automatically closed to perform nitrogen replacement. The specific steps of nitrogen replacement are as follows: first close the emergency vent valve, then open the first nitrogen inlet valve 8. The pressure sensor in the reactor is interlocked with the signal of the first nitrogen inlet valve 8. When the pressure in the reactor reaches the required level, the first nitrogen inlet valve 8 is automatically closed, and then the emergency vent valve 2 is opened. The pressure sensor in the reactor is interlocked with the signal of the emergency vent valve 2. When the pressure in the reactor reaches the required level, the emergency vent valve 2 is automatically closed. At this point, one nitrogen replacement operation is completed. Repeat the nitrogen replacement operation 3 to 10 times until there is essentially no air in the reactor. At this point, the nitrogen replacement step is completed. When the tail gas oxygen concentration sensor reaches the required level, the agitator in the reactor is automatically turned on, followed by the heating system of the reactor.

[0041] 6) Heating: The temperature sensor in the reactor is interlocked with the signal from the heating system's shutoff valve to control the reaction temperature and time until the reaction is complete. For example, if the temperature is raised in steps to 600°C, the reaction pressure in the reactor should be between 0.06 and 0.08 MPa, and the reaction should continue for 12 hours. The reaction temperature in the reactor is automatically controlled, with the reactor temperature being the controlled variable and the heating system being the manipulated variable.

[0042] 7) Cooling: After the reaction is completed, the cooling kettle automatically performs nitrogen replacement, and the circulating cooling water and stirring are turned on. Then the reactor automatically opens the first discharge valve 4 and puts the material into the cooling kettle for cooling.

[0043] 8) Discharging: When the temperature drops to the target value, the second discharge valve 6 of the cooling kettle is automatically opened, and the material is transported to the next process through the negative pressure conveying system; the first weighing system is interlocked with the second discharge valve 6, and when the weighing requirements are met, the second discharge valve 6 is automatically closed.

[0044] During the reaction process, safety interlock protection controls include: The pressure sensor for the reaction pressure in the reactor is interlocked with the tail gas shut-off valve 3. When the value of the pressure sensor in the reactor exceeds the set upper limit (e.g., 0.1 MPa), the DCS automatically triggers the interlock, fully opening the tail gas shut-off valve 3 to release pressure. The temperature sensor and pressure sensor in the reactor are interlocked with the emergency vent valve 2. When the temperature (greater than or equal to 700°C) and pressure (greater than 0.1 MPa) in the reactor both exceed the designed upper limits, the tail gas shut-off valve 3 and the emergency vent valve 2 are automatically opened. When the exhaust gas oxygen concentration sensor detects an oxygen concentration of 5% or higher, the DCS immediately shuts off the exhaust gas discharge path (closing the tail gas shut-off valve 3); initiates nitrogen replacement, diluting the oxygen concentration to <3%; and triggers an audible and visual alarm and records the event.

[0045] In the control system, a one-button start-stop interlock function is implemented through the DCS. Specifically, one-button start is as follows: the operator clicks "One-button Start" on the DCS interface, and the system completes the following actions in a preset sequence: 1) the public auxiliary systems (cooling water, nitrogen supply) are pre-started and self-checked; 2) the feeding system is fed and the material quantity is verified; 3) the reactor temperature and pressure are raised to the process setpoints; 4) the exhaust system enters automatic monitoring mode. Specifically, one-button shutdown is as follows: in an emergency, if "One-button Shutdown" is triggered, the DCS executes the following: 1) cuts off the reactor heating power supply; 2) closes the feed valve and starts depressurizing the reactor; 3) the exhaust system switches to nitrogen protection mode; and 4) records the shutdown reason and a snapshot of key parameters.

[0046] The control system also features intelligent alarms and fault self-diagnosis. The tiered alarm system includes: 1) Level 1 (early warning): If a parameter approaches a threshold (e.g., pressure reaches 90% of the upper limit), the DCS prompts the operator to check; 2) Level 2 (emergency): If a parameter exceeds a limit, an interlock is triggered and forced intervention is initiated. Historical data tracing: The DCS stores three years of production data and supports retrieval by time and event type for accident analysis and process optimization. The DCS will also issue an alarm if the cooling kettle's circulating water flow switch is not activated.

[0047] The above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. Ordinary technicians in the relevant technical field may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention. The scope of patent protection of the present invention should be defined by the claims.

Claims

1. A DCS-based artificial graphite negative electrode material coating production control system, characterized by: The production control system mainly includes a feeding temporary storage bin, a mixer, a vacuum loader, a reactor and a cooling kettle connected in sequence, and the reactor is also connected to a tail gas tank; the feeding temporary storage bin is connected to the mixer through a negative pressure conveying pipe, and the mixer is connected to the vacuum loader through a negative pressure conveying pipe. A feed shut-off valve is provided between the vacuum loader and the reactor, a tail gas shut-off valve is provided between the reactor and the tail gas tank, and the tail gas tank is provided with an oxygen concentration sensor; a first discharge valve is provided between the reactor and the cooling kettle, the reactor is connected to a first nitrogen inlet valve through a nitrogen inlet pipe, an emergency vent valve is provided on the top of the reactor, and the emergency vent valve is connected to the tail gas tank; a temperature sensor and a pressure sensor are provided in the reactor; the feed shut-off valve, tail gas shut-off valve, oxygen concentration sensor, first discharge valve, first nitrogen inlet valve, emergency vent valve, temperature sensor and pressure sensor are all connected to the DCS to realize centralized management of data acquisition, instruction issuance and logical interlocking.

2. The artificial graphite negative electrode material coating production control system based on DCS control according to claim 1, characterized in that: The cooling kettle is connected to a circulating water flow switch through a circulating water input pipe, the cooling kettle is connected to a second nitrogen inlet valve through a nitrogen inlet pipe, and a second discharge valve is provided on the cooling kettle discharge pipe; the circulating water flow switch, the second nitrogen inlet valve and the second discharge valve are all connected to the DCS.

3. The artificial graphite negative electrode material coating production control system based on DCS control according to claim 1, characterized in that: The reaction kettle and the cooling kettle are respectively provided with a first stirrer and a second stirrer, and the cooling kettle is provided with a first weighing sensor; the first stirrer, the second stirrer and the first weighing sensor are all connected to a DCS.

4. The DCS-based artificial graphite negative electrode material coating production control system according to claim 1, characterized in that: The reactor is connected to a heating system, which is controlled by a heating system shut-off valve. The heating system shut-off valve is interlocked with a temperature sensor signal; the heating system shut-off valve is connected to a DCS.

5. The DCS-based artificial graphite negative electrode material coating production control system according to claim 1, characterized in that: An intermediate weighing bin is provided between the temporary storage bin and the mixer; the intermediate weighing bin is used for batching and unloading via an automatic weighing system; the intermediate weighing bin is provided with a second weighing sensor and a rotary unloading valve; both the second weighing sensor and the rotary unloading valve are connected to the DCS.

6. The control method of a DCS-based artificial graphite negative electrode material coating production control system according to claim 1, characterized in that: The control method includes the following contents: 1) Feeding: Add the reaction materials into the temporary storage bin and automatically add them to the intermediate weighing bin through the negative pressure conveying system; 2) Batching: The second weighing sensor in the intermediate weighing bin is interlocked with the signal of the discharge rotary valve. The material is fed into the mixer through a negative pressure conveying system. When the weight in the intermediate weighing bin reaches the required value, the discharge rotary valve is automatically closed. 3) Mixing: Turn on the mixer and stir to mix the materials; 4) Feeding: The material in the mixer is conveyed to the vacuum feeder through the negative pressure conveying system; the first agitator in the reactor is automatically turned on, and the tail gas shut-off valve is automatically opened; 5) Nitrogen replacement: When the vacuum loader finishes unloading, the first nitrogen inlet valve is automatically opened and the tail gas shut-off valve is automatically closed for nitrogen replacement; when the tail gas oxygen concentration sensor meets the requirements, the agitator in the reactor is automatically turned on first, and then the heating system of the reactor is automatically turned on; 6) Heating: The temperature sensor in the reactor is interlocked with the cut-off valve signal of the heating system to control the reaction temperature and time until the reaction is completed; 7) Cooling: After the reaction is completed, the cooling kettle is automatically replaced with nitrogen, and the circulating cooling water and stirring are turned on. Then the reactor automatically opens the first discharge valve and puts the material into the cooling kettle for cooling; 8) Discharging: When the temperature drops to the target value, the second discharge valve of the cooling kettle is automatically opened, and the material is transported to the next process through the negative pressure conveying system; the first weighing system is interlocked with the second discharge valve, and when the weighing requirements are met, the second discharge valve is automatically closed.

7. The control method of the DCS-based artificial graphite negative electrode material coating production control system according to claim 6, characterized in that: When the value of the pressure sensor in the reactor exceeds the set upper limit, the DCS automatically triggers the interlock, and the tail gas shut-off valve is fully opened to release pressure; when the temperature and pressure in the reactor both exceed the design upper limit, the DCS automatically triggers the interlock, and the tail gas shut-off valve and the emergency vent valve are opened at the same time.

8. The control method of the DCS-based artificial graphite negative electrode material coating production control system according to claim 6, characterized in that: When the exhaust gas oxygen concentration sensor detects an oxygen concentration ≥5%, the DCS immediately executes the following: cuts off the exhaust gas emission path; starts nitrogen replacement, introducing nitrogen to dilute the oxygen concentration to <3%; triggers the sound and light alarm and records the event.

9. The control method of the artificial graphite negative electrode material coating production control system based on DCS control according to claim 6, characterized in that: In the control system, a one-button start-stop interlocking function is realized through DCS.

10. The control method of the DCS-based artificial graphite negative electrode material coating production control system according to claim 6, characterized in that: In step 5), the emergency vent valve is first closed, and then the first nitrogen inlet valve is opened. The pressure sensor in the reactor is interlocked with the first nitrogen inlet valve signal. When the pressure in the reactor reaches the requirement, the first nitrogen inlet valve is automatically closed, and then the emergency vent valve is opened. The pressure sensor in the reactor is interlocked with the emergency vent valve signal. When the pressure in the reactor reaches the requirement, the emergency vent valve is automatically closed. At this point, a nitrogen replacement operation is completed. The nitrogen replacement operation is repeated 3 to 10 times until there is basically no air in the reactor. At this time, the nitrogen replacement step is completed.

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