Process and device for preparing liquid nitrogen tetroxide by recycling nitric oxide

Abstract

The invention discloses a production process and device for preparing liquid nitrogen tetroxide by recycling nitric oxide. Nitrogen oxides in the tail gas are respectively absorbed by water and alkali liquor to prepare a nitric acid solution and a sodium nitrate-nitrous acid solution; a dilute nitric acid solution and a sodium nitrate and nitrite solution are respectively stored and then mixed to enter a nitric oxide generation tower for reaction to generate nitric oxide gas and a sodium nitrate aqueous solution. The nitric oxide gas is lightly oxidized by using oxygen in the nitric oxide generation tower, then enters the oxidation absorption tower and is heavily oxidized by using concentrated nitric acid, so that the nitric oxide gas is completely converted into nitrogen dioxide gas, and the concentrated nitric acid is used for absorption at the upper section of the oxidation absorption tower to form fuming nitric acid. Fuming nitric acid is heated by steam, and liquid nitrogen tetroxide is produced after condensation. The non-condensable gas contains a small amount of nitrogen oxides, and then passes through a denitration device. By adopting the device and the process, the nitrogen oxide in the recovered waste gas can be effectively ensured to be prepared into liquid nitrogen tetroxide, and the content of the nitrogen oxide in the waste gas outlet is reduced.

Description

Technical Field

[0001] This invention belongs to the field of hazardous chemical production, specifically relating to an apparatus and process for recycling nitrogen oxides to produce liquid dinitrogen tetroxide. Background Technology

[0002] In the production of dimethyl sulfoxide (DMSO), nitrogen dioxide is used to oxidize dimethyl sulfide. To ensure the safety and stability of the oxidation reaction, nitrogen dioxide is always in excess to completely oxidize dimethyl sulfide and obtain DMSO. Therefore, a large amount of nitrogen dioxide is required in the DMSO production process, and excess nitrogen oxides precipitate from the material. Although absorbed, some nitrogen oxides are still directly released into the air, causing environmental pollution and wasting nitrogen oxides. To further recover nitrogen oxides from the waste gas and improve the environmental pollution situation, a production process and apparatus for recycling nitrogen oxides to produce liquid nitrogen tetroxide has been invented. The nitrogen tetroxide produced in the process is heated and vaporized to form nitrogen dioxide, which continues to participate in the DMSO production process, ensuring the recovery and utilization of nitrogen oxides.

[0003] With stricter environmental standards, the requirements for nitrogen oxide concentration at exhaust outlets are lower. The entire production and operation company uses a combination of wet absorption and ammonia oxidation-reduction to treat nitrogen oxides in exhaust gas. Summary of the Invention

[0004] This invention discloses a production process and apparatus for recycling nitrogen oxides to produce liquid nitrogen tetroxide. The process recovers nitrogen oxides from waste gas to produce liquid nitrogen tetroxide, generating economic benefits and further improving the control of environmental pollution sources by reducing the nitrogen oxide content in the exhaust gas.

[0005] To address the problems of low product added value, poor process continuity, easy equipment clogging, and low product purity in existing technologies for the resource utilization of nitrogen oxide-containing waste gas, this invention provides a novel method and system for the recycling and utilization of nitrogen oxides to prepare liquid dinitrogen tetroxide. This method aims to achieve near-zero emissions of nitrogen oxides from waste gas and full component resource utilization, producing high-purity liquid N₂O₄ that can be directly used in applications such as DMSO production, while ensuring long-term, stable, and continuous operation of the entire system.

[0006] Technical solution of the present invention To solve the above-mentioned technical problems, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing liquid nitrogen tetroxide by recycling nitrogen oxides, comprising the following steps: S1. Absorption: The waste gas containing nitrogen oxides is subjected to water absorption and alkaline absorption sequentially. Water absorption generates a nitric acid solution, while alkaline absorption (usually using NaOH solution) generates a mixed solution of sodium nitrite and sodium nitrate (hereinafter referred to as sodium nitrite solution). Through staged absorption, the capture rate of nitrogen oxides is maximized.

[0007] S2. Reaction occurs: The nitric acid solution and sodium nitrite solution obtained in step S1 are fed into the nitrogen oxide generating tower in a certain proportion, where a double decomposition reaction occurs (e.g., 3NaNO2 + 2HNO3 → 3NaNO3 + 2NO↑ + H2O), producing sodium nitrate solution and nitrogen oxide gas, mainly containing nitric oxide (NO). Simultaneously, oxygen-containing gas (such as air) is introduced into the tower to perform preliminary oxidation ("light oxidation") on the generated NO, partially converting it into nitrogen dioxide (NO2).

[0008] S3. Oxidation and Absorption: The nitrogen oxide gas (a mixture of NO and NO2) generated in step S2 is passed into an oxidation absorption tower. In the lower part of the tower, concentrated nitric acid (e.g., 98%) is used as the oxidant to deeply oxidize NO to NO2 (“re-oxidation”). In the upper part of the tower, low-temperature concentrated nitric acid is used to absorb NO2, generating fuming nitric acid (i.e., concentrated nitric acid containing a large amount of dissolved NO2).

[0009] S4. Separation and Purification: The fuming nitric acid obtained in step S3 is fed into a concentrated nitric acid distillation column and heated (e.g., 80-85℃) to desorb the dissolved NO2. The desorbed NO2 gas is condensed at low temperature in a condenser and polymerized into liquid N2O4, which is collected as the product. The regenerated concentrated nitric acid obtained in the column bottom can be returned to step S3 for recycling.

[0010] S5. Exhaust Gas Recirculation: All exhaust gases containing low concentrations of nitrogen oxides from the water absorption tower in step S1, the top of the oxidation absorption tower in step S3, and the condenser in step S4 are collected and returned to the inlet of the water absorption tower in step S1, forming a closed-loop circulation of nitrogen oxides, so that only trace amounts of exhaust gas are discharged in compliance with standards after terminal denitrification. The apparatus used in this invention comprises at least the following equipment: a waste gas outlet, a tail gas water absorption tower, a tail gas alkaline absorption tower, a nitrogen oxide generating tower, an oxidation absorption tower, a concentrated nitric acid distillation tower, a nitrogen dioxide condenser, a tail gas denitrification device, a nitric acid solution tank, a sodium nitrite solution tank, a sodium nitrate solution tank, and a nitrogen tetroxide storage tank, and the equipment is connected in the following manner: The exhaust outlet is connected to a tail gas water absorption tower, which is in turn connected to a water absorption tower circulation pump and a nitric acid solution tank. Water is added to the tail gas water absorption tower to spray and absorb nitrogen oxides in the exhaust gas, forming nitric acid. The nitric acid is then placed in the nitric acid solution tank and supplied to the nitrogen oxide generator.

[0011] The tail gas water absorption tower is connected to the tail gas alkali absorption tower, which in turn is connected to the sodium nitrite solution tank and the tail gas alkali absorption tower circulation pump. Nitrogen oxides that are not completely absorbed after absorption in the tail gas water absorption tower are sprayed and absorbed in the tail gas alkali absorption tower using liquid alkali to form a mixed aqueous solution of sodium nitrite and sodium nitrate. The absorbent is then placed in the sodium nitrite solution tank for use in the nitrogen oxide generation tower.

[0012] Sodium nitrite solution tank and nitric acid solution tank are connected to a nitric oxide generating tower, which is in turn connected to a generating tower circulation pump and a sodium nitrate solution tank. The sodium nitrite solution and nitric acid solution enter the nitric oxide generating tower to react and generate sodium nitrate solution and nitric oxide and nitrogen dioxide gases. Compressed air is introduced into the nitric oxide generating tower to lightly oxidize the nitric oxide gas. All the nitric oxide gas is collected from the top of the nitric oxide generating tower and sent to an oxidation absorption tower for use. The sodium nitrate solution produced in the reaction is circulated using the generating tower circulation pump to further recover any remaining nitric oxide gas in the solution.

[0013] The nitrogen oxide generator is connected to the oxidation absorption tower. Nitrogen oxide gas enters the oxidation absorption tower from the bottom, and concentrated nitric acid is used to oxidize the nitric oxide gas in the lower part. In the upper part of the oxidation absorption tower, concentrated nitric acid is used to absorb nitrogen dioxide to produce fuming nitric acid, which is then used in the concentrated nitric acid distillation tower.

[0014] The upper part of the oxidation absorption tower is connected to the concentrated nitric acid distillation tower, and the lower part is connected to the nitric acid solution tank. The waste gas from the top of the oxidation absorption tower is connected to the tail gas water absorption tower. Fuming nitric acid is collected from the upper part of the nitric oxide generator tower and sent to the concentrated nitric acid distillation tower, where it is heated to 80-85℃ to separate nitric oxide from the fuming nitric acid. The concentrated nitric acid at the bottom of the oxidation absorption tower oxidizes nitric oxide gas to form dilute nitric acid, which is recovered to the nitric acid solution tank for use in the nitric oxide generator tower. The gas from the top of the oxidation absorption tower enters the tail gas water absorption tower, where it is absorbed by water to form nitric acid for use in the nitric oxide generator tower.

[0015] The concentrated nitric acid distillation column is connected to a nitrogen dioxide condenser, which in turn is connected to a nitrogen tetroxide storage tank and a tail gas water absorption tower. The nitrogen dioxide gas produced in the concentrated nitric acid distillation column is condensed by the nitrogen dioxide condenser, and the condensed nitrogen dioxide polymerizes into liquid nitrogen tetroxide. The nitrogen tetroxide enters the nitrogen tetroxide storage tank for production use. The non-condensable gas, after passing through the nitrogen dioxide condenser, enters the tail gas water absorption tower to continue absorbing nitrogen oxides, thus recycling nitrogen oxides.

[0016] Beneficial effects of the present invention Compared with the prior art, the present invention has the following significant advantages: High degree of resource utilization and good economic benefits: It can effectively utilize pollutants such as NO. x It can be directly converted into high-value-added liquid N2O4 products, which can be directly reused in the production process of DMSO, realizing the circular economy model of "treating waste with waste and turning waste into treasure".

[0017] Significant environmental benefits: Through multi-stage absorption and closed-loop circulation design, NO in exhaust gas is reduced. x The recycling rate is extremely high, which significantly reduces the end-of-pipe emissions and reduces environmental pollution at the source.

[0018] The system operates stably and continuously: Through a series of key process parameters and structural optimizations (as described in the examples below), the problems of easy equipment blockage, reaction imbalance, and product purity fluctuation in traditional processes have been effectively solved, ensuring the long-term stable operation of the equipment.

[0019] High degree of process integration: It organically integrates multiple processes such as absorption, reaction, oxidation, absorption, and distillation, making rational use of energy and materials and achieving a high degree of automation. Attached Figure Description

[0020] Figure 1 Production processes and equipment for the recycling and reuse of nitrogen oxides.

[0021] 1. Exhaust gas outlet 2. Tail gas water absorption tower 3. Tail gas alkaline absorption tower 4. Nitrogen oxide generator tower 5. Oxidation absorption tower 6. Concentrated nitric acid distillation tower 7. Nitric acid solution tank 8. Sodium nitrite solution tank 9. Sodium nitrate solution tank 10. Nitrogen tetroxide storage tank 11. Nitrogen dioxide condenser 12. Tail gas denitrification device a. Tail gas water absorption tower circulation b. Tail gas alkaline absorption tower circulation pump c. Nitrogen oxide generator tower circulation pump d. Nitric acid feed pump e. Sodium nitrite feed pump Detailed Implementation The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the scope described in the embodiments.

[0022] Example 1 A production process and apparatus for recovering nitrogen oxides to produce liquid dinitrogen tetroxide, with specific connections as follows: exhaust gas outlet 1 is connected to tail gas water absorption tower 2; tail gas water absorption tower 2 is connected to tail gas alkaline absorption tower 3; tail gas water absorption tower 2 is connected to tail gas water absorption tower circulation pump a; tail gas water absorption tower 2 is connected to nitric acid solution tank 7; tail gas alkaline absorption tower 3 is connected to tail gas denitrification device 12; tail gas alkaline absorption tower 3 is connected to tail gas alkaline absorption tower circulation pump b; tail gas alkaline absorption tower 3 is connected to sodium nitrite solution tank 8; acid solution tank 7 is connected to nitric acid feed pump d; sodium nitrite solution tank 8 is connected to sodium nitrite solution tank 8. Sodium feed pump e is connected; nitric acid feed pump d is connected to nitrogen oxide generator tower 4; sodium sulfite feed pump e is connected to nitrogen oxide generator tower 4; nitrogen oxide generator tower 4 is connected to oxidation absorption tower 5; nitrogen oxide generator tower 4 is connected to generator tower circulation pump c; nitrogen oxide generator tower 4 is connected to sodium nitrate tank 9; oxidation absorption tower 5 is connected to concentrated nitric acid distillation tower 6; oxidation absorption tower 5 is connected to tail gas water absorption tower 2; concentrated nitric acid distillation tower 6 is connected to nitrogen dioxide condenser 11; nitrogen dioxide condenser 11 is connected to nitrogen tetroxide storage tank 10; nitrogen dioxide condenser 11 is connected to tail gas water absorption tower 2.

[0023] Example 2 A production process for recovering and utilizing nitrogen oxides to produce liquid nitrogen tetroxide, using the apparatus of Example 1, includes the following steps: Exhaust gas still containing nitrogen oxides from the exhaust outlet enters a tail gas water absorption tower. Water is added and circulated within the tail gas water absorption tower to absorb the nitrogen oxides and produce a nitric acid solution with a required concentration of 50-55%. After the nitric acid solution reaches the required concentration, it is discharged into a nitric acid solution tank for later use. The exhaust gas, after absorption by the tail gas water absorption tower, enters a tail gas alkaline absorption tower. A 15-20% sodium hydroxide solution is added and circulated within the tail gas alkaline absorption tower to absorb any incompletely absorbed nitrogen oxides from the tail gas water absorption tower, producing a mixed solution of sodium nitrate and sodium nitrite. The pH value of the absorbent in the tail gas alkaline absorption tower is tested and found to be neutral, indicating complete reaction of the sodium hydroxide. The absorbent from the tail gas alkaline absorption tower is then discharged into a sodium nitrite solution tank for later use. After replacing the absorbent in the tail gas water absorption tower and the tail gas alkaline absorption tower, fresh absorbent is replenished promptly to ensure that the exhaust gas, after absorption by the absorption tower, is delivered to the tail gas denitrification unit at a low concentration.

[0024] The sodium nitrite solution in the sodium nitrite tank is fed into the nitric oxide generator using a sodium nitrite feed pump. Before entering the nitric oxide generator, the sodium nitrite solution is heated to 70-80°C. The nitric acid solution in the nitric acid feed tank is fed into the nitric acid generator using a nitric acid feed pump. Inside the nitric acid generator, the nitric acid inlet is higher than the sodium nitrite inlet, facilitating the spraying and washing of the sodium nitrate solid produced by the reaction of sodium nitrite and nitric acid, as well as the acid mist carried out by the reaction gas. Steam heating is used at the bottom of the nitric oxide generator to control the solution temperature at 95-100°C and the generator pressure at 0.30-0.35 MPa. A generator circulation pump circulates the reacted solution. Compressed air is introduced into the solution at the bottom of the nitric oxide generator to strip and stir the sodium nitrate solution. The oxygen in the compressed air also preliminarily oxidizes the nitric oxide produced by the reaction of nitric acid and sodium nitrite, ensuring that the nitric acid and sodium nitrite solutions in the nitric oxide generator react as completely as possible. The feed ratio of nitric acid and sodium nitrite solution in the nitrogen oxide generator is controlled. The nitric acid feed flow rate is slightly excess over the sodium nitrite feed flow rate to ensure complete reaction of the sodium nitrite within the generator. The nitric oxide and nitrogen dioxide gases generated in the generator are extracted from the top and sent to an oxidation absorption tower for further reaction to produce nitrogen dioxide.

[0025] The nitrogen oxide gas extracted from the top of the nitrogen oxide generator contains nitric oxide and nitrogen dioxide, and enters the bottom of the oxidation absorption tower. At the bottom of the oxidation absorption tower, nitric oxide is oxidized to nitrogen dioxide by spraying with 98% concentrated nitric acid. At the top of the oxidation absorption tower, nitrogen dioxide rising from the bottom is absorbed by 98% concentrated nitric acid to form fuming nitric acid. The oxidation absorption tower is equipped with coils and circulated with chilled water to control the absorption of nitrogen dioxide by concentrated nitric acid in the upper section under low-temperature conditions. The fuming nitric acid is then fed into a concentrated nitric acid distillation tower for further use. The tail gas after absorption in the oxidation absorption tower enters a tail gas water absorption tower for water absorption to produce nitric acid for use in the nitrogen oxide generator.

[0026] Fuming nitric acid enters the top of the concentrated nitric acid distillation column and flows by gravity to the bottom. Steam is passed through the bottom of the column to heat the fuming nitric acid to 80-85°C. This heating process separates nitrogen dioxide from the fuming nitric acid, which is then collected from the top of the column. The nitrogen dioxide gas is condensed into liquid nitrogen tetroxide in a nitrogen dioxide condenser, and then the liquid nitrogen tetroxide is placed in a nitrogen tetroxide storage tank to obtain the finished nitrogen tetroxide product. The non-condensable gas remaining after condensation continues to enter the tail gas water absorption tower for water absorption to produce nitric acid for use in the nitrogen oxide generator.

[0027] Example 3 The method and steps are the same as in Example 2. The tail gas from the exhaust outlet, oxidation absorption tower, and nitrogen dioxide condenser enters the tail gas water absorption tower for water absorption to produce nitric acid. When the temperature of the tail gas water absorption tower is controlled at 20-40℃, a large amount of nitrogen oxides are emitted from the top of the tower. This is absorbed by liquid alkali in the tail gas alkali absorption tower, resulting in a large amount of mixed sodium nitrate and sodium nitrite solution produced. However, the nitric acid solution produced by the tail gas water absorption tower is relatively small, causing an imbalance between the materials entering the nitrogen oxide generator tower. This leads to inconsistent and unstable production. After experimental adjustments, the temperature of the tail gas water absorption tower is controlled at 10-20℃, and the temperature of the tail gas alkali absorption tower is controlled at 20-30℃, which precisely meets the requirements of the nitrogen oxide generator tower for nitric acid and sodium nitrite solution.

[0028] Example 4 The method and steps are the same as in Example 2. Nitric acid solution and sodium nitrite solution are introduced into the nitrogen oxide generator. The sodium nitrite inlet and the nitric acid inlet are at the same horizontal position. The sodium nitrate solid salt produced by the reaction of sodium nitrite and nitric acid precipitates and adheres to the packing material of the nitrogen oxide generator, causing blockage of the tower and affecting the reaction between nitric acid and sodium nitrite. After experimental adjustments, the nitric acid inlet was moved above the sodium nitrite inlet, and the sodium nitrate crystals adhering to the packing material were washed away by nitric acid spraying inside the nitrogen oxide generator. This ensures the continuous operation of the nitrogen oxide generator.

[0029] Example 5 The method and steps are the same as in Example 2. Nitrogen oxide gas enters the oxidation absorption tower. In the lower section, concentrated nitric acid oxidizes nitric oxide gas into nitrogen dioxide gas. In the upper section, concentrated nitric acid absorbs nitrogen dioxide gas to form fuming nitric acid. Chilled water is circulated into the oxidation absorption tower to cool it down. The temperature at the bottom of the oxidation absorption tower is controlled at -10 to -10℃, which affects the absorption of nitrogen dioxide by concentrated nitric acid in the upper section. If the temperature in the lower section of the oxidation absorption tower is too low, concentrated nitric acid will absorb some nitrogen dioxide, resulting in nitrogen dioxide waste and affecting the main content of fuming nitric acid. After experimental adjustments, the temperature in the lower section of the oxidation absorption tower is controlled at 23-40℃, and the feed flow rate of concentrated nitric acid is controlled at 100-120 kg / h. Controlling the temperature above the boiling point of nitrogen dioxide can effectively ensure that nitrogen dioxide is not absorbed and carried away by concentrated nitric acid. Controlling the feed of concentrated nitric acid in the lower section of the oxidation absorption tower can ensure that the oxidation of nitric oxide by concentrated nitric acid is incomplete, that some trace amounts of nitric oxide enter the upper section of the oxidation absorption tower, and that the absorption of nitrogen dioxide by a small amount of concentrated nitric acid does not carry away a large amount of nitrogen dioxide. This ensures that a small amount of nitric oxide and nitrogen dioxide gas enters the upper part of the oxidation absorption tower, where concentrated nitric acid absorbs the nitrogen dioxide, producing fuming nitric acid for use in the concentrated nitric acid distillation tower. Chilled water is circulated in the upper part of the oxidation absorption tower to condense the fuming nitric acid and nitrogen dioxide, ensuring that a large amount of nitrogen dioxide dissolves in the concentrated nitric acid solution. When the temperature in the upper section of the oxidation absorption tower exceeds 25℃, a large amount of nitrogen dioxide is drawn from the top of the oxidation absorption tower into the tail gas water absorption tower, resulting in excessive nitric acid production and affecting the continuous and stable operation of the entire production process. After experimental adjustments, controlling the temperature in the upper section of the oxidation absorption tower below 25℃ effectively ensures that the concentrated nitric acid in the upper section absorbs a large amount of nitrogen dioxide. The exhaust gas discharged from the top of the oxidation absorption tower contains a small amount of nitrogen oxides, which are then recycled in the tail gas water absorption tower. The pressure at the top of the oxidation absorption tower is controlled at 0.28-0.30 MPa. Controlling the pressure of the oxidation absorption tower effectively controls the flow rate of nitrogen oxide gas within the tower, allowing the concentrated nitric acid to absorb nitrogen dioxide as much as possible. Preliminary tests revealed that when the oxidation absorption tower is controlled at atmospheric pressure or below 0.2 MPa, the amount of nitric acid produced by the tail gas water absorption tower increases. This proves that strictly controlling the pressure of the oxidation absorption tower between 0.28 and 0.30 MPa and slowing down the flow rate of nitrogen dioxide gas in the oxidation absorption tower can more effectively allow concentrated nitric acid to absorb nitrogen dioxide and form fuming nitric acid.

[0030] Example 6 The method and steps are the same as in Example 2. Fuming nitric acid enters the lower part of the concentrated nitric acid distillation column. Steam is used to heat the fuming nitric acid at the bottom of the column, and when the temperature is controlled at 80-85℃, the main content of liquid nitrogen tetroxide produced after nitrogen dioxide rises from the bottom of the column to the top and passes through the nitrogen dioxide condenser can only reach 80%. When the fuming nitric acid enters the bottom of the concentrated nitric acid distillation column, the nitrogen dioxide gas rises after heating, failing to achieve the desired distillation effect. Only nitrogen dioxide carries a portion of the concentrated nitric acid into the nitrogen dioxide condenser, and the main content of the finished nitrogen tetroxide cannot be guaranteed. After experimental adjustments, the fuming nitric acid inlet of the concentrated nitric acid distillation column was moved to the upper part of the column. When fuming nitric acid is continuously fed into the column, it flows downwards from the top, contacting the rising nitrogen dioxide at the bottom and carrying away acid mist. The remaining, poorly absorbed or excessive nitrogen dioxide is collected from the top of the column and deposited into the nitrogen dioxide condenser. This achieves the effect of distillation, resulting in liquid dinitrogen tetroxide with a higher content or concentration.

[0031] Example 7 The methods and steps are the same as in Example 2. When the bottom temperature of the concentrated nitric acid distillation column is controlled below 80℃, the concentration of nitric acid collected from the bottom of the column is around 90%, and the nitrogen dioxide in the fuming nitric acid is not completely recovered. After experimental adjustment, the bottom temperature of the concentrated nitric acid distillation column is controlled at around 84℃. With a nitric acid concentration of 80% at the bottom, most of the dissolved nitrogen dioxide in the nitric acid can be recovered and reused to produce liquid nitrogen tetroxide. Controlling the bottom temperature of the concentrated nitric acid distillation column at 86℃ results in strong corrosion, as concentrated nitric acid has a boiling point of 86℃. High-temperature concentrated nitric acid can damage the distillation column equipment and reduce its service life. After experimental adjustment, the temperature of the concentrated nitric acid distillation column is strictly controlled between 80-85℃. Within this temperature range, the recovery of nitrogen dioxide from the concentrated nitric acid can be more effectively guaranteed, and the corrosion of the distillation column equipment can be reduced, extending its service life.

[0032] Example 8 The method and steps are the same as in Example 2. Nitrogen dioxide gas collected from the top of the concentrated nitric acid distillation column passes through a nitrogen dioxide condenser, where it condenses and polymerizes into liquid nitrogen tetroxide under low-temperature conditions. The temperature of the nitrogen dioxide condenser is controlled at -20 to -15°C to effectively and maximize the polymerization of nitrogen dioxide gas into liquid nitrogen tetroxide. When the nitrogen dioxide condenser temperature is controlled at -10 to 0°C, some nitrogen dioxide is not completely condensed into nitrogen tetroxide and enters the tail gas water absorption tower after the nitrogen dioxide condenser, forming nitric acid in the tail gas water absorption tower, increasing the production of nitric acid and sodium nitrite. Furthermore, the incompletely absorbed nitrogen oxides in the tail gas water absorption tower and tail gas alkaline absorption tower enter the tail gas denitrification device, causing energy consumption and non-compliance with high-altitude emissions standards. After experimental adjustments, circulating chilled water through the nitrogen dioxide condenser and controlling the condenser temperature at -20 to -15°C can better polymerize nitrogen dioxide into liquid nitrogen tetroxide and reduce the nitrogen oxide content in the tail gas, thus reducing environmental pollution.

Claims

1. A method for preparing liquid nitrogen tetroxide by recycling nitrogen oxides, characterized in that, Includes the following steps: S1. Absorption: The waste gas containing nitrogen oxides is subjected to water absorption and alkali absorption in sequence to obtain nitric acid solution and sodium nitrite / sodium nitrate mixed solution, respectively; S2. Reaction occurs: The nitric acid solution obtained in step S1 and the sodium nitrite / sodium nitrate mixed solution are fed into a nitrogen oxide generating tower to react and generate sodium nitrate solution and nitrogen oxide gas containing nitric oxide and nitrogen dioxide; oxygen-containing gas is introduced into the nitrogen oxide generating tower to perform preliminary oxidation of the generated nitric oxide; S3. Oxidation and absorption: The nitrogen oxide gas generated in step S2 is passed into an oxidation absorption tower. Nitric oxide is oxidized to nitrogen dioxide with concentrated nitric acid in the lower part of the tower, and nitrogen dioxide is absorbed at low temperature with concentrated nitric acid in the upper part of the tower to obtain fuming nitric acid. S4. Separation and purification: The fuming nitric acid obtained in step S3 is heated to separate nitrogen dioxide gas, which is then condensed to obtain liquid nitrogen tetroxide product; the remaining nitric acid after separating nitrogen dioxide is returned to the system for reuse. S5. Exhaust Gas Recirculation: The exhaust gas containing low concentrations of nitrogen oxides generated in steps S1, S3, and S4 is returned to step S1 for water absorption, thereby achieving the recycling of nitrogen oxides.

2. The method according to claim 1, characterized in that, In step S1, the temperature of the water absorption tower is controlled at 10-20℃, and the temperature of the alkali absorption tower is controlled at 20-30℃.

3. The method according to claim 1, characterized in that, In step S2, the reaction temperature inside the nitrogen oxide generating tower is controlled at 95-100℃ and the pressure is controlled at 0.30-0.35 MPa; the inlet of the nitric acid solution is located above the inlet of the sodium nitrite / sodium nitrate mixed solution.

4. The method according to claim 1, characterized in that, In step S3, the temperature of the lower part of the oxidation absorption tower is controlled at 23-40℃, and the temperature of the upper part is below 25℃; the pressure at the top of the oxidation absorption tower is controlled at 0.28-0.30 MPa, and the flow rate of concentrated nitric acid entering the lower part of the oxidation absorption tower is controlled at 100-120 Kg / h.

5. The method according to claim 1, characterized in that, In step S4, the heating is carried out in a concentrated nitric acid distillation column, and the bottom temperature of the distillation column is controlled at 80-85°C; the fuming nitric acid is fed from the top of the concentrated nitric acid distillation column, and the condensation is carried out in a nitrogen dioxide condenser, with the condensation temperature controlled at -20 to -15°C.

6. A nitrogen oxide recycling system for implementing the method according to any one of claims 1-5, characterized in that, Including those connected sequentially: The exhaust gas pretreatment unit includes a tail gas water absorption tower (2) for water absorption and a tail gas alkali absorption tower (3) for alkali absorption. The solution storage tank unit includes a nitric acid solution tank (7) connected to the tail gas water absorption tower (2) and a sodium sulfite solution tank (8) connected to the tail gas alkali absorption tower (3). The nitrogen oxide generating unit includes a nitrogen oxide generating tower (4), whose inlet is connected to the nitric acid solution tank (7) and the sodium sulfite solution tank (8) respectively via pumps; The oxidation absorption unit includes an oxidation absorption tower (5), whose inlet is connected to the top outlet of the nitrogen oxide generating tower (4), and whose upper liquid outlet is used to output fuming nitric acid. The product separation unit includes a concentrated nitric acid distillation column (6) and a nitrogen dioxide condenser (11). The feed inlet of the concentrated nitric acid distillation column (6) is connected to the upper liquid outlet of the oxidation absorption column (5), and the gas outlet at the top of the column is connected to the nitrogen dioxide condenser (11). The liquid phase outlet of the nitrogen dioxide condenser (11) is connected to a nitrogen tetroxide storage tank (10). The inlet of the tail gas water absorption tower (2) is also connected to the top outlet of the oxidation absorption tower (5) and the non-condensable gas outlet of the nitrogen dioxide condenser (11).

7. The system according to claim 6, characterized in that, The position of the nitric acid solution inlet of the nitrogen oxide generator (4) is higher than the position of its sodium nitrite / sodium nitrate mixed solution inlet.

8. The system according to claim 6, characterized in that, The oxidation absorption tower (5) is equipped with a coil for introducing the freezing medium, the bottom of the concentrated nitric acid distillation tower (6) is equipped with a steam heating device, and the nitrogen dioxide condenser (11) is a chilled water condenser.

9. The system according to claim 6, characterized in that, It also includes a sodium nitrate solution tank (9), and the bottom outlet of the nitrogen oxide generating tower (4) is connected to the sodium nitrate solution tank (9) through a generating tower circulation pump (c), and the outlet of the generating tower circulation pump (c) forms a circulation loop with the middle of the nitrogen oxide generating tower (4).

10. The system according to claim 6, characterized in that, The exhaust port of the tail gas alkaline absorption tower (3) is connected to a tail gas denitrification device (12), and the lower liquid outlet of the oxidation absorption tower (5) is connected to the nitric acid solution tank (7).

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