Safety control device and method for preventing dry baking in nitration pyrolysis process

Abstract

The invention discloses a safety control device and method for preventing dry baking in a nitration pyrolysis process, and belongs to the technical field of chemical process safety. The device comprises a spiral plate heat exchanger which is provided with a gas phase inlet, a gas phase outlet, a condensate outlet, a circulating water inlet and a circulating water outlet; the three-way valve is connected to the downstream of the condensate outlet; the condensate reflux pipeline is connected with the three-way valve and the nitrolysis machine; the condensate extraction pipeline is connected with the three-way valve and the light acetic acid receiving tank; in the nitration and pyrolysis process, gas-phase vinegar smoke generated by the nitrolysis machine is introduced into the spiral plate type heat exchanger to be cooled, and condensate is controlled by the three-way valve, is extracted and recovered in the nitration stage and flows back to the nitrolysis machine in the hydrolysis / pyrolysis stage to automatically supplement the liquid level. Through an automatic backflow compensation mechanism, the safety risk of'dry baking 'caused by liquid level drop is fundamentally eliminated, meanwhile, operation automation and resource recovery are achieved, and the device has the advantages of being simple in structure, reliable in operation and remarkable in economic benefit.

Description

Technical Field

[0001] This invention relates to the technical field of process route in nitration reaction, specifically to a safety control device and method for preventing dry roasting during nitration pyrolysis. Background Technology

[0002] Nitration is a crucial unit operation in chemical, pharmaceutical, pesticide, and explosives industries. The process is typically accompanied by a severe exothermic effect, and the materials involved are often flammable, explosive, highly corrosive, and highly toxic. In the production of specific nitro compounds such as "Product 101," a large number of unstable byproducts and intermediates often remain in the reaction mixture after the nitration reaction. To ensure the quality of the final product and the stability of the production process, these impurities must be subsequently treated.

[0003] The commonly used method in the industry is as follows: after the nitration reaction (which typically includes stages such as primary nitration, primary heat preservation, secondary nitration, and secondary heat preservation), a certain amount of water is added to the nitration machine, and jacketed steam is introduced for heating to complete the hydrolysis and pyrolysis operations. The purpose of this process is to use high temperatures to promote the decomposition, volatilization, or transformation of unstable byproducts, thereby removing them from the reaction system.

[0004] However, this hydrolysis and pyrolysis process itself introduces significant safety risks. As the internal temperature rises, a large amount of acetic acid, water, and other low-boiling-point components in the material vaporize, forming so-called "acetic acid fumes" vapor. The escape of these vapors directly leads to a continuous drop in the liquid level inside the nitration machine. When the liquid level drops to a height where it cannot completely submerge and cover the heating jacket (especially the half-jacket), the reactants come into direct contact with the high-temperature walls, resulting in a "dry roasting" phenomenon. Dry roasting can trigger a series of serious consequences: 1. It can cause localized overheating, potentially leading to the decomposition and carbonization of residual nitration products or byproducts, or even triggering violent decomposition reactions, posing an explosion risk. 2. It accelerates the coking and solidification of materials on the equipment walls, not only affecting heat transfer efficiency but also potentially clogging pipes and creating hidden dangers for subsequent production. 3. It can severely damage the equipment and shorten its service life.

[0005] To address the aforementioned risks, existing technologies primarily rely on manual intervention based on operator experience. Specifically, the operator continuously monitors the nitration machine's level gauge and manually adds dilution water when the level is too low to maintain a safe operating level. This method has the following inherent drawbacks: (1) Poor safety: Manual monitoring is subject to lag and negligence, and cannot achieve instantaneous and accurate liquid level compensation. The risk of dry baking is not fundamentally eliminated, posing a continuous threat to production safety. (2) High operational intensity and low efficiency: Operators need to be highly focused throughout the process and frequently perform water replenishment operations, which increases labor intensity and is not conducive to the automation and continuity of production. (3) Impact on process economy: In order to be on the safe side, excessive dilution water is often added during operation. This not only increases the load and cost of subsequent wastewater treatment, but may also affect the process efficiency of subsequent sections due to excessive dilution. (4) Resources are not effectively recovered: The acetic acid vapor generated by volatilization is usually directly fed into the tail gas system for treatment. Valuable components such as acetic acid contained therein are not condensed and recovered, resulting in waste of materials.

[0006] Therefore, there is an urgent need in this field for a safety technology and process control method that can automate and fundamentally solve the risk of dry roasting caused by the drop in liquid level during nitration and pyrolysis, so as to ensure the inherent safety of production while realizing the effective utilization of resources and the optimization and upgrading of processes. Summary of the Invention

[0007] The purpose of this invention is to provide a safety control device and method for preventing dry roasting during the nitration pyrolysis process, in order to solve the problem mentioned in the background art that the prior art lacks a technical solution that can automate and fundamentally solve the risk of dry roasting caused by the drop in liquid level during the nitration pyrolysis process.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A safety control device for preventing dry baking during nitration pyrolysis includes: A spiral plate heat exchanger is provided with a gas phase inlet, a gas phase outlet, a condensate outlet, a circulating water inlet, and a circulating water outlet; A three-way valve is connected downstream of the condensate outlet; The condensate return line connects to the nitration machine via a three-way valve. The condensate collection pipeline is connected to a three-way valve and a dilute acetic acid receiving tank. The gas phase inlet is used to connect to the exhaust pipe of the nitration machine, and the gas phase outlet is used to connect to the tail gas treatment system.

[0009] Furthermore, the spiral plate heat exchanger adopts a double spiral channel structure, including an alternating spirally wound gas phase channel and a circulating water channel. The gas phase inlet and condensate outlet are respectively located at both ends of the gas phase channel, and the circulating water inlet and circulating water outlet are respectively located at both ends of the circulating water channel.

[0010] Furthermore, the three-way valve is an electrically or pneumatically controlled valve and is equipped with a valve position feedback signal interface.

[0011] The further control system is connected to the three-way valve signal to automatically control the opening and closing status of the three-way valve according to the current stage of the nitration pyrolysis process, so as to switch the flow direction of the condensate; the specific control logic is as follows: a) Receive stage signals from the production control system or determine the current process stage of the nitration-pyrolysis process based on the preset process sequence. b) When the current process stage is the nitration reaction stage or the heat preservation stage, the first control signal is output to the three-way valve to drive it to switch to the extraction state, so that the condensate flows to the dilute acetic acid receiving tank through the condensate extraction pipeline. c) When the current process stage is converted to the hydrolysis stage or the pyrolysis stage, a second control signal is output to the three-way valve to drive it to switch to the reflux state, so that the condensate is returned to the nitrification machine through the condensate reflux pipeline.

[0012] Furthermore, the control system is also connected to a liquid level detection element installed inside the nitration machine to monitor the liquid level inside the nitration machine in real time; when the liquid level is lower than the preset threshold, the control system controls the three-way valve to switch to the reflux state.

[0013] A safety control method for preventing dry heating in a nitration pyrolysis process, employing the aforementioned safety control device for preventing dry heating in a nitration pyrolysis process, includes the following steps: Step S1: During the nitration pyrolysis process, the gaseous acetic acid fumes generated by the nitration machine are introduced into the spiral plate heat exchanger through the gas phase inlet; Step S2: Indirectly cool the gaseous acetic acid fumes with circulating water, causing some of them to condense into condensate; Step S3: Discharge the uncondensed gas phase through the gas phase outlet to the exhaust gas treatment system; Step S4: Control the flow direction of the condensate through a three-way valve, switching it between returning to the nitration machine and being collected into the dilute acetic acid receiving tank; Step S5: During the pyrolysis stage, the condensate is returned to the nitration machine to replenish the liquid level inside the machine and prevent dry burning.

[0014] Furthermore, during the nitration stage, the three-way valve is controlled to allow the condensate to be drawn out to the dilute acetic acid receiving tank; during the hydrolysis or pyrolysis stage, the three-way valve is controlled to allow the condensate to flow back to the nitrification machine.

[0015] Furthermore, by monitoring the liquid level inside the nitration machine in real time and dynamically adjusting the opening and closing status of the three-way valve according to the liquid level height, automatic liquid level compensation is achieved.

[0016] Furthermore, it also includes the control of the circulating water system: adjusting the flow rate of the circulating water according to the temperature or flow rate of the acetic acid flue gas at the gas phase inlet in order to control the condensation efficiency.

[0017] Furthermore, before or during system startup, maintenance steps include backflushing or chemical cleaning of the spiral plate heat exchanger to maintain heat exchange efficiency.

[0018] Compared with the prior art, the beneficial effects of the present invention are: (1) Enhanced safety. Through the reflux design of the vinegar fume condenser, automatic and real-time replenishment of the liquid level in the nitration machine is achieved during the pyrolysis process, eliminating the risk of "dry baking" caused by excessively low liquid level from the source. This changes the traditional manual intervention mode that relies on the experience of operators and is prone to delays and negligence, transforming passive protection into proactive prevention, which greatly improves the safety level of the production process; (2) Automation and intelligence of operation. Through the linkage between the control system and the three-way valve, the flow direction of the condensate (return or extraction) can be automatically switched according to the preset process stage or real-time liquid level signal. This realizes precise automatic control of the production process, greatly reduces the labor intensity of operators and the possibility of human error, and lays the foundation for building an "unmanned" or "less manned" smart factory; (3) Optimization of process economy and resource utilization. In the nitration stage, the condensed dilute acetic acid is collected and recovered, realizing the resource utilization of valuable materials (acetic acid), turning waste into treasure, and reducing raw material costs. In the hydrolysis / pyrolysis stage, the condensate is reused, reducing the amount of external dilution water replenishment, and reducing the load and operating costs of subsequent wastewater treatment; (4) Advantages of system structure and maintenance. The spiral plate heat exchanger has a compact structure, high heat transfer efficiency, and small footprint, which effectively reduces equipment investment and space costs. The entire system has a simple structure, no complex moving parts, low failure rate, reliable operation, convenient maintenance and low cost. Attached Figure Description

[0019] Figure 1 This is a first-view structural schematic diagram of a safety control device for preventing dry baking during the nitration pyrolysis process according to the present invention; Figure 2 This is a second-view structural schematic diagram of a safety control device for preventing dry baking during the nitration pyrolysis process according to the present invention; Figure 3 This is a third-view structural schematic diagram of a safety control device for preventing dry baking during the nitration pyrolysis process according to the present invention; Figure 4 This is a control flowchart of a safety control method for preventing dry baking in the nitration pyrolysis process according to the present invention.

[0020] In the diagram: 1. Spiral plate heat exchanger (vinegar fume condenser); 2. Circulating water inlet; 3. Circulating water outlet; 4. Vapor outlet; 5. Condensate outlet; 6. Vapor inlet; 7. Three-way valve; 8. Condensate return pipeline; 9. Condensate extraction pipeline. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Please see Figures 1 to 3 This invention provides a technical solution: a safety control device for preventing dry roasting during nitration pyrolysis. The complete dry roasting prevention safety control device consists of a spiral plate heat exchanger (vinegar fume condenser), a circulating water inlet 2, a circulating water outlet 3, a gas phase outlet 4, a condensate outlet 5, a gas phase inlet 6, a three-way valve 7, a condensate return pipeline 8, a condensate collection pipeline 9, and matching liquid level detection element LIT-101, circulating water flow regulating valve FCV-201, and a PLC / DCS control unit.

[0023] Specifically, the gas phase inlet 6 is connected to the flange of the exhaust pipe at the top of the nitration machine, and the gas phase outlet 4 is connected to the tail gas absorption tower; the condensate outlet 5 is divided into two paths after passing through the three-way valve 7: one path returns to the upper return port of the nitration machine via pipeline 8, and the other path is sent to the dilute acetic acid receiving tank V-102 via pipeline 9; the circulating water inlet 2 and outlet 3 are connected to the main circulating water pipe of the workshop, and an FCV-201 is installed on the inlet pipeline to automatically adjust the cooling water volume according to the gas phase temperature TIC-301; the three-way valve 7 is a pneumatic flange ball valve with 4-20mA valve position feedback, and the signal is connected to the PLC / DCS; a magnetostrictive level gauge LIT-101 is installed on the side wall of the nitration machine cylinder to output a 4-20 mA level signal to the same PLC / DCS in real time.

[0024] Working principle: The nitration-pyrolysis process is divided into two stages: "nitration-heat preservation" and "hydrolysis-pyrolysis".

[0025] a) Nitrification-Heating Stage: Acetic acid, water, and low-boiling-point substances inside the machine vaporize upon heating to form "acetic acid fumes," which enter the gas phase channel of the spiral plate heat exchanger 1 through gas phase inlet 6. Circulating water flows counter-currently in adjacent spiral channels, cooling the acetic acid fumes to 40-60°C, partially condensing them into dilute acetic acid with a mass fraction of approximately 35%. At this time, the PLC / DCS outputs an "extraction" signal, the three-way valve 7 is in position AB, and the condensate flows through pipeline 9 into the dilute acetic acid receiving tank V-102 for recovery; the uncondensed gas phase is discharged to the tail gas system through outlet 4.

[0026] b) Hydrolysis-pyrolysis stage: The decomposition temperature of byproducts rises, and the liquid level drops rapidly due to evaporation. The PLC / DCS outputs a "reflux" signal based on the process sequence (or when the liquid level is below 30% of the set value). The three-way valve 7 switches to the AC position, and the condensate flows back to the nitration machine via pipeline 8, instantly replenishing the liquid level and ensuring the heating jacket is always submerged, fundamentally eliminating "dry heating." The reflux rate can be indirectly controlled by adjusting the cooling water flow through FCV-201, achieving precise liquid replenishment. The entire switching process requires no manual intervention, and the system operates in a closed loop.

[0027] The specific control logic of the three-way valve is as follows: Step S1: The PLC / DCS receives the stage signal or internal timing from the upper batch control system and determines whether it is currently in "nitration / heat preservation" or "hydrolysis / pyrolysis"; Step S2: If it is nitration / heat preservation, output the first control signal and keep the three-way valve 7 in the production position; if it is hydrolysis / pyrolysis, output the second control signal and switch the three-way valve 7 to the reflux position. Step S3: When the liquid level LIT-101 is lower than the set lower limit (e.g., 30%), regardless of the stage, the PLC / DCS forces the three-way valve 7 to enter the reflux position until the liquid level is restored to the upper limit (e.g., 50%), thus achieving double protection.

[0028] Equipment maintenance methods: Set a temperature difference ΔT≥8 ℃ alarm on the circulating water side to indicate fouling on the heat exchanger surface; clean the heat exchanger with 2% Na2CO3 solution for 30 min in reverse circulation once a month to maintain the design heat transfer coefficient; add a sight glass to the condensate return line 8 for easy on-site observation of the return status; A flow meter is installed on the dilute acetic acid extraction pipeline 9. The cumulative recovery amount is included in the workshop's acetic acid balance, with an annual recovery rate of ≥65%, significantly reducing raw material consumption.

[0029] By employing a reflux design in the vinegar fume condenser, automatic and real-time replenishment of the liquid level within the nitration machine is achieved during pyrolysis, eliminating the risk of "dry roasting" due to excessively low liquid levels. This transforms the traditional manual intervention model, which relies on operator experience and is prone to delays and negligence, into proactive prevention, significantly improving the safety level of the production process. Through the linkage between the control system and the three-way valve, the flow direction of the condensate (reflux or extraction) can be automatically switched based on preset process stages or real-time liquid level signals. This achieves precise automatic control of the production process, significantly reducing the labor intensity of operators and the possibility of human error, laying the foundation for building "unmanned" or "minimal-manned" smart factories.

[0030] After the safety control device was put into actual use, it immediately generated huge social and economic value: taking the 3000t / year 101 product unit as an example: the number of dry baking accidents decreased from 2 times / year before the modification to 0 times; the number of times operators need to replenish water per batch decreased from 5-6 times to 0 times; the amount of dilution water used decreased by 8%; 210t of dilute acetic acid was recovered annually, equivalent to 73t of pure acetic acid, saving about 380,000 yuan in raw material costs; the spiral plate heat exchanger occupies only 0.8m2, saving 60% of the space compared to the original shell and tube cooler, and reducing maintenance costs by 50%.

[0031] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A safety control device for preventing dry-out in a nitric acid production process, characterized by comprising: The application relates to a safety control device for preventing dry burning in a nitro-thermal cracking process. The application comprises: a spiral-plate heat exchanger (1) provided with a gas phase inlet (6), a gas phase outlet (4), a condensate outlet (5), a circulating water inlet (2) and a circulating water outlet (3); a three-way valve (7) connected downstream of the condensate outlet (5); a condensate return pipeline (8) connecting the three-way valve (7) and a nitro-thermal cracking machine; a condensate extraction pipeline (9) connecting the three-way valve (7) and a dilute acetic acid receiving tank; 2. A safety control device for preventing dry-out in a nitric acid production process according to claim 1, wherein wherein the gas phase inlet (6) is used for connecting a smoke duct of the nitro-thermal cracking machine, and the gas phase outlet (4) is used for connecting an exhaust gas treatment system.

3. A safety control device for preventing dry-out in a nitric acid production process according to claim 1, wherein The spiral-plate heat exchanger (1) is internally provided with a double-spiral channel structure, including alternately spirally-wound gas phase channels and circulating water channels, the gas phase inlet (6) and the condensate outlet (5) are respectively arranged at two ends of the gas phase channels, and the circulating water inlet (2) and the circulating water outlet (3) are respectively arranged at two ends of the circulating water channels.

4. A safety control device and method for preventing dryout in a nitric acid process according to claim 1, wherein The three-way valve (7) is an electric or pneumatic control valve and is provided with a valve position feedback signal interface. The control system is signal-connected with the three-way valve (7) and is used for automatically controlling the opening and closing states of the three-way valve (7) according to a current stage of the nitro-thermal cracking process, so as to switch the flow direction of the condensate; the specific control logic is as follows: a) receiving a stage signal from a production control system or according to a preset process timing, judging a current process stage of the nitro-thermal cracking process; b) when the current process stage is a nitration reaction stage or a heat preservation stage, outputting a first control signal to the three-way valve (7) to drive the three-way valve (7) to switch to an extraction state, so that the condensate flows to the dilute acetic acid receiving tank through the condensate extraction pipeline (9); 5. A safety control device for preventing dry-out in a nitric acid production process according to claim 4, wherein c) when the current process stage is converted into a hydrolysis stage or a pyrolysis stage, outputting a second control signal to the three-way valve (7) to drive the three-way valve (7) to switch to a return state, so that the condensate returns to the nitro-thermal cracking machine through the condensate return pipeline (8).

6. A safety control method for nitric pyrolysis process anti-dryout, characterized by, The control system is further connected with a liquid level detection element arranged in the nitro-thermal cracking machine and is used for monitoring the liquid level in the nitro-thermal cracking machine in real time; when the liquid level is lower than a preset threshold, the control system controls the three-way valve (7) to switch to the return state. The application adopts the safety control device for preventing dry burning in a nitro-thermal cracking process and comprises the following steps: Step S1: in the nitro-thermal cracking process, introducing gas phase acetic smoke generated by a nitro-thermal cracking machine into a spiral-plate heat exchanger (1) through a gas phase inlet (6); Step S2: indirectly cooling the gas phase acetic smoke through circulating water to make the gas phase acetic smoke partially condensed into condensate; Step S3: discharging the uncondensed gas phase to an exhaust gas treatment system through a gas phase outlet (4); Step S4: controlling the flow direction of the condensate through a three-way valve (7) to switch the condensate between returning to the nitro-thermal cracking machine and being extracted to a dilute acetic acid receiving tank; 7. A safety control method for preventing dry-out in a nitric- pyrolysis process according to claim 6, characterized in that, Step S5: in a pyrolysis stage, returning the condensate to the nitro-thermal cracking machine to supplement the liquid level in the nitro-thermal cracking machine and prevent dry burning. In the nitration reaction stage, the three-way valve (7) is controlled to extract the condensate to the dilute acetic acid receiving tank; in the hydrolysis or pyrolysis stage, the three-way valve (7) is controlled to return the condensate to the nitro-thermal cracking machine.

8. A safety control method for preventing dry-out in a nitric- pyrolysis process according to claim 6, characterized in that, By real-time monitoring the liquid level in the nitration machine, and dynamically adjusting the open-close state of the three-way valve (7) according to the liquid level, the automatic compensation of the liquid level is realized.

9. A method for safe control of run-away in a nitric- pyrolysis process according to claim 6, characterized in that, The control of the circulating water system is also included: according to the temperature or flow of the gas phase inlet (6), the flow of the circulating water is adjusted to control the condensation efficiency.

10. A method for safe control of run-away in a nitric- pyrolysis process according to claim 6, characterized in that, Before the system starts or during operation, the maintenance step of backwashing or chemical cleaning of the spiral plate heat exchanger (1) is also included to maintain the heat exchange efficiency.

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