System and method for producing nitric acid

Abstract

The application discloses a nitric acid preparation system and method, and relates to the technical field of chemical industry, and can reduce tail gas emission and nitrogen oxide content in the tail gas during nitric acid preparation.

Description

Technical Field

[0001] This invention relates to the field of nitric acid production technology, and more specifically, to a nitric acid preparation system and method. Background Technology

[0002] With the rapid development and application of renewable energy, the ways to obtain electricity resources are becoming increasingly diversified, providing more options for industrial production. Especially with the promotion and application of clean energy sources such as wind power, hydropower, and photovoltaic power, inexpensive electricity is increasingly being used in industrial production. In this context, industrialized devices for water electrolysis using electricity are gradually emerging, ensuring a stable and inexpensive oxygen source, making it possible to use pure oxygen to replace air in nitric acid production.

[0003] Currently, nitric acid is an important basic chemical raw material, widely used in the production of dyes, explosives, pharmaceuticals, plastics, nitrogen fertilizers, chemical reagents, as well as in metallurgy and organic synthesis. However, traditional nitric acid production processes have many problems. For example, while the atmospheric pressure process has simple equipment, it has low production intensity and high NOx content in the tail gas, easily generating serious pollution. The medium-pressure process, although with more compact equipment, suffers from slightly lower oxidation rates and insufficient absorption efficiency due to the oxidation of ammonia under pressure. The high-pressure process, while improving production intensity to some extent, still suffers from high platinum consumption, high energy consumption, and large tail gas emissions. In particular, the dual-pressure process, although improving production efficiency to some extent, still suffers from large tail gas emissions, large equipment size, and high compressor power.

[0004] Therefore, there is an urgent need to develop a new nitric acid production process to solve the problems of large tail gas emissions and high nitrogen oxide content in the tail gas in the existing technology. Summary of the Invention

[0005] The purpose of this invention is to provide a nitric acid preparation system and method, which helps to overcome the problems of large tail gas emissions and high nitrogen oxide content in the tail gas in existing nitric acid preparation processes.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a nitric acid preparation system, comprising: An oxidation unit includes a raw gas mixer and an oxidizer. The raw gas mixer is provided with an ammonia delivery pipe, an oxygen delivery pipe, and a mixed gas output pipe for connection with the oxidizer. The oxidizer is also provided with a high-temperature conversion gas output pipe. The heat exchange and impurity removal unit includes a heat exchange assembly and a conversion gas separator that are sequentially connected to the high-temperature conversion gas output pipe of the oxidizer. The circulation control unit includes a circulating gas compressor and a circulating gas superheater that are sequentially connected to the circulating gas output pipe of the gas separator, and the circulating gas output pipe of the circulating gas superheater is connected to the raw material gas mixer. The absorption unit includes a conversion gas cooler, an inlet gas cooler, an absorber, and a bleacher, which are sequentially connected to the conversion gas output pipe of the conversion gas separator. The bleacher is provided with a dilute nitric acid product outlet.

[0007] In an optional embodiment, an ammonia evaporation unit is also included, comprising a liquid ammonia filter, a liquid ammonia evaporator, a gaseous ammonia separation filter, and a gaseous ammonia heater connected in sequence, wherein the gaseous ammonia outlet of the gaseous ammonia heater is connected to the gaseous ammonia delivery pipe. And / or, it also includes an oxygen purification unit, comprising an oxygen preparation device, an oxygen separator, an oxygen filter, an oxygen compressor and a bleaching gas heat exchanger connected in sequence, wherein the outlet of the bleaching gas heat exchanger is connected to the oxygen delivery pipe; And / or, it also includes the heat exchange assembly comprising a primary heat exchanger group, a tail gas heater, a circulating gas superheater, and a boiler water heater that are sequentially connected to the high-temperature converted gas output pipe; And / or, it also includes an exhaust gas treatment unit, comprising an exhaust gas preheater, an exhaust gas heater, and a denitrification reactor that are sequentially connected to the exhaust gas output pipe of the absorber.

[0008] In a second aspect, the present invention provides a method for preparing nitric acid using the nitric acid preparation system described in any one of the foregoing embodiments, comprising: In the oxidation step, gaseous ammonia and oxygen are mixed in a feed gas mixer and then enter the oxidizer for catalytic oxidation to produce high-temperature converted gas. Heat exchange and impurity removal are performed to cool the high-temperature converted gas through a heat exchange component and separate the dilute nitric acid therein through a converted gas separator. The circulation control allows a portion of the converted gas leaving the converted gas separator to pass sequentially through the circulating gas compressor and the circulating gas superheater before returning to the feed gas mixer; The remaining converted gas from the gas separator is absorbed and then passed sequentially through a gas cooler, a tower gas cooler, an absorber, and a bleacher to obtain dilute nitric acid product.

[0009] In an optional embodiment, the process further includes ammonia evaporation: liquid ammonia is sequentially passed through a liquid ammonia filter, a liquid ammonia evaporator, a gaseous ammonia separator, and a gaseous ammonia heater for filtration, evaporation, separation of liquid ammonia, and heating to obtain gaseous ammonia at 100-150°C, and then the heated gaseous ammonia is transferred to the raw material gas mixer. And / or, in the liquid ammonia evaporator, liquid ammonia undergoes phase change evaporation by exchanging heat with the cooling water of the absorber at an operating pressure of 0.4~0.8MPa.

[0010] In an optional embodiment, the oxygen discharged from the oxygen compressor has a pressure of 0.3~0.6 MPa and a temperature of 220~240℃. And / or, the bleaching gas heat exchanger is a bleaching heat exchanger, and the heating medium of the bleaching heat exchanger is oxygen discharged from the top of the bleaching unit; And / or, the oxygen temperature exiting the bleaching gas heat exchanger is 110–130°C.

[0011] In an optional embodiment, the temperature of the catalytic oxidation is 750~900°C.

[0012] In an optional embodiment, the temperature of the converted gas at the outlet of the primary heat exchanger group is 360~380°C, and superheated steam is generated, wherein the pressure of the superheated steam is 3.8~4.2 MPa and the temperature is 380~420°C. And / or, the converted gas leaving the primary heat exchanger group exchanges heat with the exhaust gas from the exhaust gas preheater in the exhaust gas heater, the exhaust gas leaving the exhaust gas heater having a temperature of 250~260°C.

[0013] In an optional embodiment, the converted gas leaving the exhaust gas heater is heat-exchanged in the recirculating gas superheater until the temperature drops to 290~330°C, and then transferred to the catalyst recovery unit; the heat exchange medium of the recirculating gas superheater is the converted gas from the recirculating gas compressor, and the outlet temperature of the heat exchange medium of the exhaust gas heater is 300~360°C. And / or, the converted gas leaving the catalyst recovery unit enters the boiler water heater to exchange heat with the demineralized water, cooling it to 120~160°C, and then enters the converted gas separator.

[0014] In an optional embodiment, in the converter gas cooler, the converter gas from the converter gas separator exchanges heat with the cooling water from the inlet gas cooler, so that the converter gas at the outlet of the converter gas cooler is cooled to 45~60°C. And / or, a conversion gas compressor is also provided between the conversion gas outlet of the conversion gas cooler and the conversion gas inlet of the tower cooler. The water vapor in the conversion gas cooler condenses and reacts with the nitrogen oxide gas in the conversion gas to generate dilute nitric acid intermediate. After separation, the dilute nitric acid intermediate is sent to the middle of the absorber. The conversion gas at the outlet of the conversion gas cooler is mixed with the nitrogen oxide gas from the bleacher and then sent to the conversion gas compressor. The outlet gas pressure of the conversion gas compressor is 1.0~1.2 MPa and the temperature is 150~180℃. And / or, the outlet gas of the converter compressor is cooled to 45~48°C by heat exchange with cooling water from the absorber.

[0015] In an optional embodiment, the oxygen exiting the bleaching heat exchanger is subjected to gas stripping in the bleacher by countercurrent contact with a nitric acid solution from the absorber. And / or, in the exhaust gas preheater, the exhaust gas from the absorber exchanges heat with the exhaust gas from the denitrification reactor until the temperature rises to 170~190°C, and the exhaust gas from the denitrification reactor cools down to below 100°C. And / or, in the exhaust gas heater, the exhaust gas from the exhaust gas preheater exchanges heat with the conversion gas from the primary heat exchanger group and is heated to 250~260°C.

[0016] The present invention has the following beneficial effects: The nitric acid preparation system and method provided by this invention significantly improves the safety, energy efficiency, and environmental protection of nitric acid production by employing pure oxygen oxidation and gas-phase circulation processes. The system uses pure oxygen instead of air, greatly reducing nitrogen dilution, lowering raw material gas flow and equipment load, and reducing compressor power while simultaneously generating byproducts, achieving efficient energy recovery. A circulation control unit recirculates a portion of the converted gas containing water vapor, precisely controlling the reaction temperature and gas mixture composition to avoid the risk of overheating and explosion, ensuring stable operation of the catalyst and equipment. Furthermore, in a nitrogen-free environment, the partial pressures of NOx and oxygen increase, enhancing absorption efficiency and significantly improving product quality. Tail gas emissions and nitrogen oxide emissions are reduced, demonstrating significant environmental advantages. The system has a compact structure and flexible process, adaptable to medium- or high-pressure operating modes, achieving efficient, low-consumption, safe, and clean nitric acid production, demonstrating significant technological advancement and industrialization value. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart of the preparation of nitric acid in this application; Figure 2 This is a process diagram of the preparation of nitric acid by double-pressure oxidation with pure oxygen in Example 1; Figure 3 This is a process diagram of the preparation of nitric acid by medium-pressure oxidation with pure oxygen in Example 2.

[0019] Diagram: 1.1-Liquid ammonia filter; 1.2-Liquid ammonia evaporator; 1.3-Gasmic ammonia separation filter; 1.4-Gasmic ammonia heater; 2.1-Oxygen separator; 2.2-Oxygen filter; 2.3-Oxygen compressor; 2.4-Bleach gas heat exchanger; 3.1-Raw gas mixer; 3.2-Oxidizer; 3.3-Primary heat exchanger assembly; 3.4-Steam drum; 3.5-Tail gas heater; 3.6-Recirculating gas superheater; 3.7-Catalyst recovery unit; 3.8-Boiler water heater; 3.9-Converter gas separator; 3.10-Converter gas cooler; 3.11-Recirculating gas compressor; 4.1-Converter gas compressor; 4.2-Inlet gas cooler; 4.3-Absorber; 4.4-Bleacher; 5.1-Denitrification reactor; 5.2-Tail gas preheater; 5.3-Pressure regulating valve; 5.4-Chimney. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] This invention provides a nitric acid preparation system, comprising: An oxidation unit includes a raw gas mixer 3.1 and an oxidizer 3.2. The raw gas mixer 3.1 is provided with an ammonia delivery pipe, an oxygen delivery pipe and a mixed gas output pipe for connection with the oxidizer 3.2. The oxidizer 3.2 is also provided with a high-temperature conversion gas output pipe. The heat exchange and impurity removal unit includes a heat exchange assembly and a conversion gas separator 3.9 that are sequentially connected to the high-temperature conversion gas output pipe of the oxidizer 3.2; The circulation control unit includes a circulating gas compressor 3.11 and a circulating gas superheater 3.6 connected in sequence to the circulating gas output pipe of the circulating gas separator 3.9, and the circulating gas superheater 3.6 has a circulating gas output pipe connected to the raw material gas mixer 3.1. The absorption unit includes a conversion gas cooler 3.10, an inlet gas cooler 4.2, an absorber 4.3, and a bleacher 4.4, which are sequentially connected to the conversion gas output pipe of the conversion gas separator 3.9. The bleacher 4.4 is provided with a dilute nitric acid product outlet.

[0022] This invention provides a nitric acid preparation system that achieves efficient, energy-saving, and safe nitric acid production through the synergistic design of an oxidation unit, a heat exchange and impurity removal unit, a circulation control unit, and an absorption unit. Using pure oxygen instead of air as the oxidant significantly reduces nitrogen introduction, substantially lowers the raw material gas flow rate and equipment load, and achieves compressor energy savings of over 80%. High-temperature conversion gas recovers heat stage by stage via heat exchange components, producing superheated steam as a byproduct, thus improving system thermal efficiency. The circulation control unit compresses and heats a portion of the conversion gas before returning it to the raw material gas mixer 3.1, effectively controlling reactant concentration, regulating the temperature and composition of the mixed gas, preventing ammonia-oxygen mixture from reaching explosive limits, and ensuring operational safety. In a nitrogen-free dilution environment, the partial pressures of NOx and oxygen increase, enhancing absorption driving force. Combined with multi-stage cooling and a tray-type absorber 4.3, the nitric acid concentration is higher, significantly improving product quality, reducing tail gas emissions and nitrogen oxides, and greatly reducing environmental treatment pressure. The bleacher 4.4 uses dry pure oxygen for gas stripping, avoiding moisture introduction and improving bleaching efficiency and product stability. The system has a compact structure and can be adapted to medium or high voltage operating modes, possessing good engineering flexibility and industrial application prospects.

[0023] The present invention also provides a method for preparing nitric acid using the nitric acid preparation system described in any one of the foregoing embodiments, the process of which is as follows: Figure 1 As shown, it includes: In the oxidation step, gaseous ammonia and oxygen are mixed in the raw material gas mixer 3.1 and then enter the oxidizer 3.2 for catalytic oxidation to prepare high-temperature converted gas. Heat exchange and impurity removal are performed so that the high-temperature conversion gas is cooled by the heat exchange component and separated from the dilute nitric acid by the conversion gas separator 3.9; The circulation control ensures that a portion of the converted gas leaving the converted gas separator 3.9 passes sequentially through the circulating gas compressor and the circulating gas superheater 3.6 before returning to the raw material gas mixer 3.1; The remaining converted gas from the gas separator 3.9 is absorbed and then passed sequentially through the gas cooler 3.10, the inlet gas cooler 4.2, the absorber 4.3, and the bleacher 4.4 to obtain dilute nitric acid product.

[0024] This application achieves efficient, energy-saving, and safe continuous nitric acid synthesis by integrating oxidation, heat exchange and impurity removal, circulation control, and absorption processes. The method uses pure oxygen instead of air as the oxidant. After mixing with gaseous ammonia in the raw material gas mixer 3.1, it enters the oxidizer 3.2, where a high-temperature catalytic oxidation reaction occurs under the action of a catalyst, generating high-temperature converted gas mainly composed of NO, O2, and H2O. Subsequently, through heat exchange and impurity removal steps, the high-temperature converted gas undergoes multi-stage waste heat recovery via a primary heat exchanger group 3.3, a tail gas heater 3.5, a circulating gas superheater 3.6, and a boiler water heater 3.8, generating high-quality superheated steam and significantly improving energy utilization. A portion of condensed dilute nitric acid is separated by the converted gas separator 3.9 to adjust the system's water content. In the circulation control step, most of the converted gas is pressurized by the circulating gas compressor and heated by the circulating gas superheater 3.6 before returning to the raw material gas mixer 3.1. This serves as a circulating medium to dilute the reactant concentration, effectively reducing the intensity of the ammonia-oxygen mixture reaction, preventing excessive local temperature rise, and stabilizing the oxidation reaction temperature, ensuring catalyst activity and equipment safety. Simultaneously, by adjusting the circulating gas flow rate, the mixture composition is controlled to stay away from the explosion limits, improving process safety. The remaining converted gas enters the absorption step, where it is further cooled by the converted gas cooler 3.10 and the feed gas cooler 4.2 before being sent to the absorber 4.3. There, it comes into countercurrent contact with demineralized water, utilizing the high partial pressure to enhance NO2 absorption and NO re-oxidation, significantly improving absorption efficiency and obtaining dilute nitric acid with a concentration of over 70%. The final product is then removed from dissolved nitrogen oxides using pure oxygen in the bleacher 4.4, ensuring product stability and purity. The entire process significantly reduces tail gas emissions and nitrogen oxide emissions, greatly alleviating environmental pressure. This method achieves the unity of energy cascade utilization, equipment miniaturization, low operating consumption, and high product quality, and has significant technological advancement and industrial application value.

[0025] In some alternative implementations, the nitric acid preparation process diagram is as follows: Figure 1 As shown, the preparation method specifically includes the following steps: Step 1, the ammonia evaporation process, specifically includes: Step 101: Liquid ammonia produced by the ammonia synthesis unit enters the liquid ammonia filter 1.1 to filter out impurities in the liquid ammonia; Step 102: The filtered liquid ammonia enters the liquid ammonia evaporator 1.2 and evaporates into gaseous ammonia at 0.4~0.8 MPa. It exchanges heat with the cooling water in the absorber 4.3, using the heat absorbed from oxidation to evaporate the liquid ammonia and cool the cooling water to below 15℃~25℃. Step 103: After evaporation, the gaseous ammonia first passes through a gaseous ammonia separation filter 1.3 to separate the liquid ammonia entrained in the evaporation. After separation, it passes through a gaseous ammonia heater 1.4 to be heated to 120°C. The heat source is steam. The condensed water is returned to the system for recycling.

[0026] Step 2, oxygen refining process, specifically includes: Step 201: Oxygen from an electrolysis water unit or an air separation unit, including but not limited to an oxygen separator 2.1 and an oxygen filter 2.2, enters an oxygen compressor 2.3 and is pressurized to 0.3~0.6 MPa and heated to 220~240℃. Step 202: The compressed oxygen first enters the bleaching gas heat exchanger 2.4 to exchange heat with the gas coming out from the top of the bleacher 4.4 and is cooled to 120°C. 50-90% of the total oxygen coming out from the top of the bleacher 4.4 goes to the raw material gas mixer 3.1; 10-50% of the total oxygen goes to the absorption process as supplementary oxygen.

[0027] Step 3, oxidation process, specifically includes: Step 301: Oxygen from the bleaching gas heat exchanger 2.4 and the converted circulating gas from the circulating gas compressor enter the raw material gas mixer 3.1 for mixing; then it is mixed with gaseous ammonia from the gaseous ammonia heater 1.4. Step 302: The mixed feed gas enters the oxidizer 3.2. The shell of the oxidizer 3.2 is equipped with water-cooled coils and filled with a highly selective catalyst. At a temperature of 750~900℃, the gaseous ammonia is oxidized into nitrogen oxide gas. The main components of the converted gas are nitric oxide, oxygen, and water vapor. The high-temperature converted gas first enters the primary heat exchanger group 3.3, where it is cooled to 360~380℃. The resulting superheated steam (3.8~4.2 MPa, 380~420℃) is sent to the steam user through the steam drum 3.4. Step 303: After cooling, the converted gas continues to enter the tail gas heater 3.5 to exchange heat with the tail gas from the tail gas preheater 5.2, heating the tail gas to 250~260℃ to ensure the activity of the denitrification catalyst. It then enters the circulating gas superheater 3.6 to cool to 300~360℃, heating the converted circulating gas to 200~350℃. The converted gas then enters the catalyst recovery unit 3.7, where catalyst recovery reduces precious metal loss. Subsequently, the converted gas enters the boiler water heater 3.8 to exchange heat with demineralized water from the demineralized water system, cooling to 120~160℃. The cooled converted gas then enters the converted gas separator 3.9 to separate the generated condensed nitric acid, used to control the water vapor content of the circulating converted gas. The main components of the converted gas are nitric oxide, nitrogen dioxide, oxygen, water, and a small amount of nitrogen as a reaction byproduct.

[0028] Step 304: Most of the converted gas from the outlet of the converter gas separator 3.9 enters the circulating gas compressor for pressurization and then enters the circulating gas superheater 3.6 for heating to 200~350℃. Then it goes to the raw material gas mixer 3.1 as a circulating cooling medium to control the temperature of the oxidizer 3.2. Step 305: The remaining converted gas enters the converted gas cooler 3.10 and exchanges heat with the cooling water from the inlet gas cooler 4.2. The converted gas is cooled to 45~60℃. The water vapor in the converted gas condenses and reacts with nitrogen oxide gas to generate dilute nitric acid with a mass concentration of 30~35%. After the dilute nitric acid is separated, it is sent to the middle of the absorber 4.3. The cooling water is returned to the external circulating cooling water station, and the converted gas enters the converted gas compressor 4.1.

[0029] Step 4, absorption process, specifically includes: Step 401: The converted gas cooled by the converted gas cooler 3.10 is mixed with oxygen containing nitrogen oxides from the bleacher 4.4 and enters the converted gas compressor 4.1 to be pressurized to 1.0~1.2 MPa and the temperature to 150~180℃. Then it enters the inlet gas cooler 4.2 to exchange heat with the cooling water from the absorber 4.3 and is cooled to 45~48℃. The cooled converted gas enters an absorber 4.3 with a tray and cooling water coil. The NO2 in the converted gas reacts with the demineralized water added from the top of the absorber 4.3 to produce nitric acid and release NO. The NO then reacts with the excess oxygen in the converted gas to convert it into NO2. The nitrogen dioxide continues to react with the demineralized water to produce nitric acid and nitric oxide. The absorption reaction and oxidation reaction are carried out in a cycle within the absorber 4.3. Step 402: The exhaust gas at the top outlet of the absorber 4.3 consists of nitrogen, excess oxygen, and trace amounts of nitrogen oxides produced by the reaction process of the oxidizer 3.2; the exhaust gas then goes to the exhaust gas preheater 5.2. Step 403: The heat released by the absorption and oxidation reaction at the top of absorber 4.3 is carried away by the cooling water circulating inside the system and sent to liquid ammonia evaporator 1.2 to evaporate liquid ammonia. After the cooling water is cooled down, it returns to absorber 4.3 for recycling. The heat at the bottom is carried away by cooling water provided from the outside and goes from absorber 4.3 to converter gas cooler 3.10. Step 404: The generated nitric acid solution is discharged from the bottom of the absorber 4.3 and sent to a bleacher 4.4 with a tray, where it comes into countercurrent contact with the oxygen from the oxygen compressor 2.3. This process extracts the nitrogen oxide gas dissolved in the nitric acid solution. After exiting the bleacher 4.4, 10-50% of the total gas mixes with the converted gas from the outlet of the converted gas cooler 3.10 and re-enters the converted gas compressor 4.1; 50-90% of the total gas goes to the bleaching gas heat exchanger 2.4. Step 405: The nitric acid solution at the bottom of the bleacher 4.4 is sent to an external storage tank as a product.

[0030] Step 5, exhaust gas treatment process, specifically includes: The exhaust gas from the top of absorber 4.3 is first sent to exhaust gas preheater 5.2 to exchange heat with the exhaust gas after denitrification in denitrification reactor 5.1, recovering heat and raising the temperature to 180°C. It is then sent to exhaust gas heater 3.5 to exchange heat with converted gas, raising the temperature from 180°C to 250-260°C. Next, it enters denitrification reactor 5.1, where nitrogen oxides in the exhaust gas react with supplemented ammonia gas to form N2. The temperature rise varies depending on the nitrogen oxide absorption efficiency. Finally, the exhaust gas exiting denitrification reactor 5.1 is cooled to below 100°C by exhaust gas preheater 5.2. The exhaust gas pressure is controlled by pressure regulating valve 5.3 and discharged into the atmosphere via chimney 5.4.

[0031] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0032] Example 1 This embodiment provides a production method for preparing nitric acid using pure oxygen via a gas-phase circulation method, such as... Figure 2 As shown, taking a unit with an annual production capacity of 150,000 tons of nitric acid as an example, the specific steps include: Step 1, Ammonia Evaporation Process: Step 101: Liquid ammonia (5875 kg / h) produced by the ammonia synthesis unit enters the liquid ammonia filter 1.1 to filter out impurities in the liquid ammonia; Step 102: The filtered liquid ammonia enters the liquid ammonia evaporator 1.2 and evaporates into gaseous ammonia at 0.5 MPa. It exchanges heat with the cooling water in the absorber 4.3, using the heat absorbed from oxidation to evaporate the liquid ammonia and cool the cooling water to 12°C. Step 103: After evaporation, the gaseous ammonia first passes through a gaseous ammonia separator to separate the liquid ammonia entrained in the evaporation. After separation, it passes through a gaseous ammonia heater 1.4 to be heated to 120°C. The heat source is steam. The condensed water is returned to the system for recycling.

[0033] Step 2, Oxygen Refining Process: Step 201: Oxygen (22560 kg / h) from the water electrolysis unit passes through oxygen separator 2.1 and oxygen filter 2.2 and then enters oxygen compressor 2.3, where it is pressurized to 0.4 MPa and heated to 230°C. Step 202: The compressed oxygen first enters the bleaching gas heat exchanger 2.4 to exchange heat with the gas coming out from the top of the bleacher 4.4 and is cooled to 120°C. 70% of the total oxygen coming out from the top of the bleacher 4.4 goes to the raw material gas mixer 3.1; 30% of the total oxygen goes to the absorption process as supplementary oxygen.

[0034] Step 3, Oxidation Process: Step 301: Oxygen from the bleaching gas heat exchanger 2.4 and the converted circulating gas from the circulating gas compressor enter the raw material gas mixer 3.1 for mixing; then it is mixed with gaseous ammonia from the gaseous ammonia heater 1.4. Step 302: The mixed feed gas enters oxidizer 3.2. The shell of oxidizer 3.2 is equipped with water-cooled coils and filled with a platinum-rhodium-palladium three-way catalyst. At a temperature of 860℃, the gaseous ammonia is oxidized into nitrogen oxide gas. The main components of the converted gas at the outlet of oxidizer 3.2 are nitrogen monoxide, oxygen, and water vapor. The high-temperature converted gas first enters the primary heat exchanger group 3.3, where the converted gas is cooled to 370℃. The resulting superheated steam (34000 kg / h, 4.0 MPa, 400℃) is sent to the steam user. Step 303: After cooling, the converted gas continues to enter the tail gas heater 3.5 to exchange heat with the tail gas from the tail gas preheater 5.2, heating the tail gas to 255°C to ensure the activity of the denitrification catalyst. Then it enters the circulating gas superheater 3.6 to cool down to 320°C, heating the converted circulating gas to 350°C. Then the converted gas enters the catalyst recovery unit 3.7, where catalyst recovery reduces precious metal loss. Subsequently, the converted gas enters the boiler water heater 3.8 to exchange heat with the demineralized water from the demineralized water system, cooling down to 130°C. The cooled converted gas enters the converted gas separator 3.9 to separate the generated condensed nitric acid, which is used to control the water vapor content of the circulating converted gas.

[0035] Step 304: Most of the converted gas from the outlet of the converter gas separator 3.9 enters the circulating gas compressor for pressurization and then enters the circulating gas superheater 3.6 for heating to 350°C. Then it goes to the raw material gas mixer 3.1 as a circulating cooling medium to control the temperature of the oxidizer 3.2. Step 305: The remaining converted gas enters the converted gas cooler 3.10 and exchanges heat with the cooling water from the inlet gas cooler 4.2. The converted gas is cooled to 50°C. The water vapor in the converted gas condenses and reacts with nitrogen oxide gas to generate dilute nitric acid with a mass concentration of 32%. After the dilute nitric acid is separated, it is sent to the middle of the absorber 4.3. The cooling water is returned to the external circulating cooling water station, and the converted gas enters the converted gas compressor 4.1.

[0036] Step 4, Absorption Process: Step 401: The converted gas cooled by the converted gas cooler 3.10 is mixed with oxygen containing nitrogen oxides from the bleacher 4.4 and enters the converted gas compressor 4.1 to be pressurized to 1.2 MPa and 160°C. It first enters the inlet gas cooler 4.2 to exchange heat with cooling water from the absorber 4.3 and is cooled to 47°C. The cooled converted gas enters an absorber 4.3 with a tray and cooling water coil. The NO2 in the converted gas reacts with the demineralized water added from the top of the absorber 4.3 to produce nitric acid and release NO. The NO then reacts with the excess oxygen in the converted gas to convert it into NO2. The nitrogen dioxide continues to react with the demineralized water to produce nitric acid and nitric oxide. The absorption reaction and oxidation reaction are carried out in a cycle within the absorber 4.3. Step 402: The exhaust gas at the top outlet of the absorber 4.3 consists of nitrogen, excess oxygen, and trace amounts of nitrogen oxides produced by the reaction process of the oxidizer 3.2; the exhaust gas then goes to the exhaust gas preheater 5.2. Step 403: The heat released by the absorption and oxidation reaction at the top of absorber 4.3 is carried away by the cooling water circulating inside the system and sent to liquid ammonia evaporator 1.2 to evaporate liquid ammonia. After the cooling water is cooled down, it returns to absorber 4.3 for recycling. The heat at the bottom is carried away by cooling water provided from the outside and goes from absorber 4.3 to converter gas cooler 3.10. Step 404: The generated nitric acid solution is discharged from the bottom of the absorber 4.3 and sent to a bleacher 4.4 with a tray, where it comes into countercurrent contact with the oxygen from the oxygen compressor 2.3. This process extracts the nitrogen oxide gas dissolved in the nitric acid solution. After exiting the bleacher 4.4, 30% of the total gas mixes with the converted gas from the outlet of the converted gas cooler 3.10 and re-enters the converted gas compressor 4.1; the remaining 70% of the total gas goes to the bleaching gas heat exchanger 2.4. Step 405: The nitric acid solution (70% wt, 29760 kg / h) at the bottom of the bleacher 4.4 is sent to an external storage tank as a product. Step 5, Exhaust gas treatment process: Step 501: The exhaust gas (500 kg / h) at the top of absorber 4.3 is first sent to exhaust gas preheater 5.2 to exchange heat with the exhaust gas after denitrification in denitrification reactor 5.1 to recover the heat of the exhaust gas and raise its temperature to 180°C. Then it is sent to exhaust gas heater 3.5 to exchange heat with the converted gas, raising its temperature from 180°C to 255°C. Then it enters denitrification reactor 5.1, where nitrogen oxides in the exhaust gas react with the supplemented ammonia gas to convert into N2. The temperature rise varies depending on the nitrogen oxide absorption efficiency. Step 502: Finally, the exhaust gas from the denitrification reactor 5.1 is cooled to 100°C by the exhaust gas preheater 5.2. The exhaust gas is discharged as part of the system pressure regulation, controlled by the pressure regulating valve 5.3, and finally discharged into the atmosphere through the chimney 5.4.

[0037] Comparative Example 1 – Double Pressure Method This comparative example provides a conventional dual-pressure method for preparing nitric acid, taking a 150,000-ton-per-year nitric acid plant as an example, specifically including the following steps: Step 1, ammonia-air mixing gas preparation. Liquid ammonia is mixed with air that has been pressurized by an air compressor after evaporation, separation, and heating. Part of the compressed air is sent to the bleaching unit.

[0038] Step 2, ammonia oxidation and reaction heat recovery. The mixed ammonia and air enter the oxidizer to undergo an ammonia oxidation reaction on the catalyst surface. The high-temperature nitrogen oxide gas after the reaction is cooled by heat exchange with water and the tail gas from the absorber outlet, and the heat is recovered.

[0039] Step 3, dilute acid condensation and separation. The cooled nitrogen oxide gas continues to exchange heat with circulating water to cool down to below 60°C. The water vapor condenses and reacts with the nitrogen oxide gas to produce dilute acid. The dilute acid is sent to the absorption tower for recovery, while the nitrogen oxide gas is sent to the compressor.

[0040] Step 4, nitrogen oxide gas compression and cooling. After the nitrogen oxide gas enters the compressor for pressurization, it exchanges heat with the exhaust gas at the absorber outlet to recover heat, and then exchanges heat with circulating water to cool it down to below 50°C before entering the absorber.

[0041] Step 5: Nitric oxide gas is absorbed to produce nitric acid. The cooled nitrogen oxide gas reacts with water in the absorber to produce nitric acid and release nitric oxide. The nitric oxide then reacts with excess oxygen to produce nitrogen dioxide. The tail gas after the reaction, mainly composed of nitrogen, is discharged from the top of the absorber, while the nitric acid is discharged from the bottom of the absorber to the bleaching unit. There, it comes into counter-current contact with a portion of the air diverted from the air compressor, extracting the dissolved nitrogen oxides from the acid, which are then sent to the tank area for storage.

[0042] Step 6: Exhaust gas recirculation for heat recovery and denitrification. The exhaust gas discharged from the top of the absorber is returned to the pre-system for gas-to-gas heat exchange with nitrogen oxide gas to recover heat. Then, it enters the denitrification reactor to decompose the nitrogen oxide gas to below the emission standard. The denitrified exhaust gas enters the exhaust gas turbine to recover energy and is then discharged through the chimney.

[0043] Table 1

[0044] Note: The compressor power in the table refers to the sum of the power of all compressors in the system.

[0045] Example 2: This embodiment provides a production method for preparing nitric acid using pure oxygen via a gas-phase circulation method, such as... Figure 3 As shown, taking a unit with an annual production capacity of 150,000 tons of nitric acid as an example, using the medium-pressure process, the conversion gas compressor 4.1 in Example 1 is eliminated, and the specific steps include: Step 1, Ammonia Evaporation Process: Step 101: Liquid ammonia (5875 kg / h) produced by the ammonia synthesis unit enters the liquid ammonia filter 1.1 to filter out impurities in the liquid ammonia; Step 102: The filtered liquid ammonia enters the liquid ammonia evaporator 1.2 and evaporates into gaseous ammonia at 0.70 MPa. It exchanges heat with the cooling water in the absorber 4.3, using the heat absorbed from oxidation to evaporate the liquid ammonia and cool the cooling water to 20°C. Step 103: After evaporation, the gaseous ammonia first passes through a gaseous ammonia separator to separate the liquid ammonia entrained in the evaporation. After separation, it passes through a gaseous ammonia heater 1.4 to be heated to 120°C. The heat source is steam. The condensed water is returned to the system for recycling.

[0046] Step 2, Oxygen Refining Process: Step 201: Oxygen (22560 kg / h) from the water electrolysis unit passes through oxygen separator 2.1 and oxygen filter 2.2 and then enters oxygen compressor 2.3, where it is pressurized to 0.5 MPa and heated to 230°C. Step 202: The compressed oxygen first enters the bleaching gas heat exchanger 2.4 to exchange heat with the gas coming out from the top of the bleacher 4.4 and is cooled to 120°C. 80% of the total oxygen coming out from the top of the bleacher 4.4 goes to the raw material gas mixer 3.1; 20% of the total oxygen goes to the absorption process as supplementary oxygen.

[0047] Step 3, Oxidation Process: Step 301: Oxygen from the bleaching gas heat exchanger 2.4 and the converted circulating gas from the circulating gas compressor enter the raw material gas mixer 3.1 for mixing; then it is mixed with gaseous ammonia from the gaseous ammonia heater 1.4. Step 302: The mixed feed gas enters oxidizer 3.2. The shell of oxidizer 3.2 is equipped with water-cooled coils and filled with a platinum-rhodium-palladium three-way catalyst. At a temperature of 860℃, the gaseous ammonia is oxidized into nitrogen oxide gas. The main components of the converted gas are nitric oxide, oxygen, and water vapor. The high-temperature converted gas first enters the primary heat exchanger group 3.3, where the converted gas is cooled to 370℃. The resulting superheated steam (34000 kg / h, 4.0 MPa, 400℃) is sent to the steam user. Step 303: After cooling, the converted gas continues to enter the tail gas heater 3.5 to exchange heat with the tail gas from the tail gas preheater 5.2, heating the tail gas to 255°C to ensure the activity of the denitrification catalyst. Then it enters the circulating gas superheater 3.6 to cool down to 315°C, heating the converted circulating gas to 320°C. Then the converted gas enters the catalyst recovery unit 3.7, where catalyst recovery reduces precious metal loss. Subsequently, the converted gas enters the boiler water heater 3.8 to exchange heat with the demineralized water from the demineralized water system, cooling down to 130°C. The cooled converted gas enters the converted gas separator 3.9 to separate the generated condensed nitric acid, which is used to control the water vapor content of the circulating converted gas.

[0048] Step 304: Most of the converted gas from the outlet of the converter gas separator 3.9 enters the circulating gas compressor for pressurization and then enters the circulating gas superheater 3.6 for heating to 320°C. Then it goes to the raw material gas mixer 3.1 as a circulating cooling medium to control the temperature of the oxidizer 3.2. Step 305: The remaining converted gas enters the converted gas cooler 3.10 and exchanges heat with the cooling water from the inlet gas cooler 4.2. The converted gas is cooled to 50°C. The water vapor in the converted gas condenses and reacts with nitrogen oxide gas to generate dilute nitric acid with a mass concentration of 32%. After the dilute nitric acid is separated, it is sent to the middle of the absorber 4.3. The cooling water is returned to the external circulating cooling water station, and the converted gas enters the absorber 4.3.

[0049] Step 4, Absorption Process: Step 401: The converted gas, cooled by the converted gas cooler 3.10, first enters the inlet gas cooler 4.2 and exchanges heat with the cooling water from the absorber 4.3 to cool down to 47°C. The cooled converted gas then enters an absorber 4.3 with a tray and cooling water coil. The NO2 in the converted gas reacts with the demineralized water added from the top of the absorber 4.3 to produce nitric acid and release NO. The NO then reacts with the excess oxygen in the converted gas to convert back into NO2. The nitrogen dioxide continues to react with the demineralized water to produce nitric acid and nitric oxide. The absorption and oxidation reactions circulate within the absorber 4.3. Step 402: The exhaust gas at the top outlet of the absorber 4.3 consists of nitrogen, excess oxygen, and trace amounts of nitrogen oxides produced by the reaction process of the oxidizer 3.2; the exhaust gas then goes to the exhaust gas preheater 5.2. Step 403: The heat released by the absorption and oxidation reaction at the top of absorber 4.3 is carried away by the cooling water circulating inside the system and sent to liquid ammonia evaporator 1.2 to evaporate liquid ammonia. After the cooling water is cooled down, it returns to absorber 4.3 for recycling. The heat at the bottom is carried away by cooling water provided from the outside and goes from absorber 4.3 to converter gas cooler 3.10. Step 404: The generated nitric acid solution is discharged from the bottom of the absorber 4.3 and sent to a bleacher 4.4 with a tray, where it comes into countercurrent contact with the oxygen from the oxygen compressor 2.3. This process extracts the nitrogen oxide gas dissolved in the nitric acid solution. After exiting the bleacher 4.4, 20% of the total gas mixes with the converted gas from the outlet of the converted gas cooler 3.10 and re-enters the converted gas compressor 4.1; the remaining 80% of the total gas goes to the bleaching gas heat exchanger 2.4. Step 405: The nitric acid solution (60%wt, 34720kg / h) at the bottom of the bleacher 4.4 is sent to an external storage tank as a product. Step 5, Exhaust gas treatment process: Step 501: The exhaust gas (500 kg / h) at the top of absorber 4.3 is first sent to exhaust gas preheater 5.2 to exchange heat with the exhaust gas after denitrification in denitrification reactor 5.1 to recover the heat of the exhaust gas and raise its temperature to 180°C. Then it is sent to exhaust gas heater 3.5 to exchange heat with the converted gas, raising its temperature from 180°C to 255°C. Then it enters denitrification reactor 5.1, where nitrogen oxides in the exhaust gas react with the supplemented ammonia gas to convert into N2. The temperature rise varies depending on the nitrogen oxide absorption efficiency. Step 502: Finally, the exhaust gas from the denitrification reactor 5.1 is cooled to 100°C by the exhaust gas preheater 5.2. The exhaust gas is discharged as part of the system pressure regulation, controlled by the pressure regulating valve 5.3, and finally discharged into the atmosphere through the chimney 5.4.

[0050] Table 2

[0051] As can be seen from the above embodiments and comparative examples, the nitric acid preparation method of this application has the following advantages: 1. High efficiency and energy saving. It is more energy-efficient than the current traditional nitric acid process. The raw material gas is entirely replaced by oxygen, eliminating the need to compress and transport nitrogen, which accounts for 77% of the raw material gas volume and is unrelated to the reaction, resulting in energy savings of over 80%. 2. Equipment investment is significantly reduced. Because the volume of gas participating in the absorption reaction is reduced by more than 75%, the size of the equipment and pipelines required for production will be reduced by more than 30% to 50%. Since the amount of exhaust gas is very small and the energy carried away is small, the exhaust gas turbine unit used to recover the energy of the exhaust gas and the large gas-to-gas heat exchanger are eliminated.

[0052] 3. Controllable reaction temperature. The oxidation reaction temperature for preparing nitric acid from theoretically pure oxygen is high, reaching over 2200℃, which poses a problem for catalyst and reactor materials. Therefore, a conversion gas circulation process was designed. After cooling and recovering heat, the reaction products are pressurized by a compressor and returned to the reactor to remove the heat of reaction, thus controlling the reactor temperature.

[0053] 4. Improved process safety. In pure oxygen processes, the mixture of ammonia and oxygen can reach explosive limits. This invention ensures process safety by controlling the flow rate of the circulating conversion gas to keep the ammonia concentration below the explosive limits, and by controlling the water vapor content to alter the explosive limits of the ammonia-oxygen mixture.

[0054] 5. High product concentration. Without the dilution effect of nitrogen, the partial pressures of oxygen and nitrogen oxides participating in the reaction are very high, allowing nitric oxide to be fully oxidized to nitrogen dioxide. Therefore, the partial pressure of nitrogen dioxide is 4 to 5 times that of conventional processes. At the same time, due to the reduction of moisture introduced by the air, nitric acid with a mass concentration of over 70% can be produced. In contrast, the highest concentration of dilute nitric acid produced by the traditional double-pressurization process can reach 65%, and the nitrogen oxides in the exhaust gas will exceed the exhaust gas emission standards, increasing the cost of denitrification.

[0055] 6. Environmental protection and emission reduction, ultra-low emissions. Conventional nitric acid processes produce a large amount of tail gas emissions, while this process only emits nitrogen, a byproduct of the reaction process, with emissions less than 2% of those of traditional processes, equivalent to zero emissions.

[0056] 7. High heat recovery rate. Traditional processes take away a large amount of heat from the exhaust gas. This process, due to its minimal emissions, takes away only a small amount of heat from the system. The heat is fully recovered and utilized within the system, with more heat used to generate steam.

[0057] 8. Large steam surplus. Because the required gas flow rate for compression and transportation is small, the compressor is small in size and power, and can be driven by an electric motor, resulting in high energy efficiency.

[0058] 9. Long catalyst life. The oxygen provided by the oxygen generator is cleaner than air, with fewer impurities such as dust and metal powder, which extends the lifespan of the filter and catalyst. The replacement frequency can be increased from once every 6 months to once a year.

[0059] 10. The production capacity combines large-scale operation with flexibility, applicable to multiple processes, increasing the number of application scenarios. This process requires a small flow rate of the compressed and transported process medium, allowing for more than three times the production capacity of equipment with the same capacity.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A nitric acid preparation system, characterized in that, include: An oxidation unit includes a raw gas mixer and an oxidizer. The raw gas mixer is provided with an ammonia delivery pipe, an oxygen delivery pipe, and a mixed gas output pipe for connection with the oxidizer. The oxidizer is also provided with a high-temperature conversion gas output pipe. The heat exchange and impurity removal unit includes a heat exchange assembly and a conversion gas separator that are sequentially connected to the high-temperature conversion gas output pipe of the oxidizer. The circulation control unit includes a circulating gas compressor and a circulating gas superheater that are sequentially connected to the circulating gas output pipe of the gas separator, and the circulating gas output pipe of the circulating gas superheater is connected to the raw material gas mixer. The absorption unit includes a conversion gas cooler, an inlet gas cooler, an absorber, and a bleacher, which are sequentially connected to the conversion gas output pipe of the conversion gas separator. The bleacher is provided with a dilute nitric acid product outlet.

2. The nitric acid preparation system according to claim 1, characterized in that, It also includes an ammonia evaporation unit, comprising a liquid ammonia filter, a liquid ammonia evaporator, a gaseous ammonia separation filter and a gaseous ammonia heater connected in sequence, wherein the gaseous ammonia outlet of the gaseous ammonia heater is connected to the gaseous ammonia delivery pipe; And / or, it also includes an oxygen purification unit, comprising an oxygen preparation device, an oxygen separator, an oxygen filter, an oxygen compressor and a bleaching gas heat exchanger connected in sequence, wherein the outlet of the bleaching gas heat exchanger is connected to the oxygen delivery pipe; And / or, it also includes the heat exchange assembly comprising a primary heat exchanger group, a tail gas heater, a circulating gas superheater, and a boiler water heater that are sequentially connected to the high-temperature converted gas output pipe; And / or, it also includes an exhaust gas treatment unit, comprising an exhaust gas preheater, an exhaust gas heater, and a denitrification reactor that are sequentially connected to the exhaust gas output pipe of the absorber.

3. A method for preparing nitric acid using the nitric acid preparation system according to any one of claims 1-2, characterized in that, include: In the oxidation step, gaseous ammonia and oxygen are mixed in a feed gas mixer and then enter the oxidizer for catalytic oxidation to produce high-temperature converted gas. Heat exchange and impurity removal are performed to cool the high-temperature converted gas through a heat exchange component and separate the dilute nitric acid therein through a converted gas separator. The circulation control allows a portion of the converted gas leaving the converted gas separator to pass sequentially through the circulating gas compressor and the circulating gas superheater before returning to the feed gas mixer; The remaining converted gas from the gas separator is absorbed and then passed sequentially through a gas cooler, a tower gas cooler, an absorber, and a bleacher to obtain dilute nitric acid product.

4. The method for preparing nitric acid according to claim 3, characterized in that, It also includes ammonia evaporation: liquid ammonia is sequentially passed through a liquid ammonia filter, a liquid ammonia evaporator, a gas ammonia separator, and a gas ammonia heater for filtration, evaporation, separation of liquid ammonia, and heating to obtain gas ammonia at 100-150°C, and then the heated gas ammonia is transferred to the raw material gas mixer; And / or, in the liquid ammonia evaporator, liquid ammonia undergoes phase change evaporation by exchanging heat with the cooling water of the absorber at an operating pressure of 0.4~0.8MPa.

5. The method for preparing nitric acid according to claim 4, characterized in that, The oxygen discharged from the oxygen compressor has a pressure of 0.3~0.6 MPa and a temperature of 220~240℃. And / or, the bleaching gas heat exchanger is a bleaching heat exchanger, and the heating medium of the bleaching heat exchanger is oxygen discharged from the top of the bleaching unit; And / or, the oxygen temperature exiting the bleaching gas heat exchanger is 110–130°C.

6. The method for preparing nitric acid according to claim 3, characterized in that, The temperature for the catalytic oxidation is 750~900℃.

7. The method for preparing nitric acid according to claim 3, characterized in that, The temperature of the converted gas at the outlet of the primary heat exchanger group is 360~380℃, and superheated steam is generated. The pressure of the superheated steam is 3.8~4.2 MPa and the temperature is 380~420℃. And / or, the converted gas leaving the primary heat exchanger group exchanges heat with the exhaust gas from the exhaust gas preheater in the exhaust gas heater, the exhaust gas leaving the exhaust gas heater having a temperature of 250~260°C.

8. The method for preparing nitric acid according to claim 7, characterized in that, The converted gas leaving the exhaust gas heater is heat-exchanged in the circulating gas superheater until the temperature drops to 290~330°C, and then transferred to the catalyst recovery unit; the heat exchange medium of the circulating gas superheater is the converted gas from the circulating gas compressor, and the outlet temperature of the heat exchange medium of the exhaust gas heater is 300~360°C. And / or, the converted gas leaving the catalyst recovery unit enters the boiler water heater to exchange heat with the demineralized water, cooling it to 120~160°C, and then enters the converted gas separator.

9. The method for preparing nitric acid according to claim 3, characterized in that, In the converter gas cooler, the converter gas from the converter gas separator exchanges heat with the cooling water from the inlet gas cooler, so that the converter gas at the outlet of the converter gas cooler is cooled to 45~60℃. And / or, a conversion gas compressor is also provided between the conversion gas outlet of the conversion gas cooler and the conversion gas inlet of the tower cooler. The water vapor in the conversion gas cooler condenses and reacts with the nitrogen oxide gas in the conversion gas to generate dilute nitric acid intermediate. After separation, the dilute nitric acid intermediate is sent to the middle of the absorber. The conversion gas at the outlet of the conversion gas cooler is mixed with the nitrogen oxide gas from the bleacher and then sent to the conversion gas compressor. The outlet gas pressure of the conversion gas compressor is 1.0~1.2 MPa and the temperature is 150~180℃. And / or, the outlet gas of the converter compressor is cooled to 45~48°C by heat exchange with cooling water from the absorber.

10. The method for preparing nitric acid according to claim 3, characterized in that, The oxygen leaving the bleaching heat exchanger is stripped in the bleaching unit by countercurrent contact with a nitric acid solution from the absorber. And / or, in the exhaust gas preheater, the exhaust gas from the absorber exchanges heat with the exhaust gas from the denitrification reactor until the temperature rises to 170~190°C, and the exhaust gas from the denitrification reactor cools down to below 100°C. And / or, in the exhaust gas heater, the exhaust gas from the exhaust gas preheater exchanges heat with the conversion gas from the primary heat exchanger group and is heated to 250~260°C.

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