Dual pressurized nitric acid process off-gas treatment system
By employing multi-stage heat recovery and deep denitrification technologies, the problems of excessive NOx and wasted heat energy in traditional exhaust gas treatment systems have been solved. Stable control of NOx in exhaust gas and cascade utilization of heat energy have been achieved, reducing operation and maintenance costs and equipment corrosion risks, and improving the energy efficiency of the system.
Patent Information
- Application Number
- CN202522115720.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-09-30
- Publication Date
- 2026-08-25
- Estimated Expiration
- 2035-09-30
AI Technical Summary
Traditional dual-pressure nitric acid process tail gas treatment systems cannot effectively treat residual NOx in the tail gas, especially when production load fluctuates or equipment malfunctions, they are prone to exceeding emission standards. Furthermore, they fail to effectively recover low-grade heat energy, resulting in energy waste, frequent equipment corrosion, and high operation and maintenance costs.
Employing multi-stage heat recovery units and deep denitrification technology, including a pre-separator, a low-temperature heat exchanger, an SCR denitrification reactor, a multi-stage heat exchanger, and an emergency buffer unit, the NOx content in the exhaust gas is kept stable below 200×10-6 through pretreatment, deep denitrification, and cascaded heat energy utilization, while recovering the heat energy from the exhaust gas.
It achieves stable control of NOx content in exhaust gas and cascade utilization of thermal energy, reduces operation and maintenance costs, improves the self-sufficiency rate of low-pressure steam, reduces equipment corrosion and downtime risks, and meets environmental protection standards and energy conservation and carbon reduction requirements.
Smart Images
Figure CN224672467U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of nitric acid production technology, specifically to a tail gas treatment system for a dual-pressure nitric acid process. Background Technology
[0002] The dual-pressure nitric acid production process, as the current mainstream nitric acid preparation technology, is based on a combination of "medium-pressure oxidation (0.45MPa) + high-pressure absorption (1.1MPa)" to achieve efficient conversion of ammonia and preparation of nitric acid. At the same time, it relies on the tail gas expander to recover the heat energy of the tail gas and reduce energy consumption.
[0003] However, after years of application, the exhaust gas treatment system of this process has gradually revealed many problems. The traditional system relies solely on high-pressure absorption (1.1 MPa) and a single exhaust gas preheating-heat exchange process (exhaust gas is heated to approximately 360°C via an exhaust gas separator, secondary air cooler, exhaust gas preheater, and high-temperature gas-air heat exchanger before entering the expander), without addressing the residual NOx in the exhaust gas after absorption. When production load fluctuates (ammonia-air ratio deviates from the design value of 9.5%), platinum mesh activity decreases (ammonia oxidation rate is lower than the design value), or the absorption tower cooling effect is poor (high-pressure water cooler outlet temperature is higher than 45°C), the NOx content in the exhaust gas easily exceeds <200 × 10⁻⁶. -6 The emission standards, especially during the start-up, shutdown, or restart phase after maintenance, significantly increase the risk of exceeding the standards. In traditional systems, the exhaust gas temperature is approximately 45°C when it exits from the top of the absorption tower. Although it is heated to 360°C by a secondary air cooler, exhaust gas preheater, and high-temperature gas-to-gas heat exchanger, this process only achieves unidirectional heat exchange between "exhaust gas and secondary air" and "exhaust gas and NOx gas," without effectively recovering the low-grade heat energy (45-100°C range) in the exhaust gas. At the same time, the exhaust gas (temperature approximately 150-200°C) after the exhaust gas expander has performed its work is directly discharged through the exhaust stack, and this part of the heat energy is not utilized, resulting in energy waste, which does not meet the current development needs of "energy conservation and carbon reduction" in the industrial sector. Traditional exhaust gas treatment systems lack buffer and emergency response units. When upstream processes (temperature fluctuations in the ammonia oxidizer, insufficient pressure in the NOx compressor) malfunction, the composition and flow rate of the exhaust gas change instantaneously, easily leading to unstable operating conditions (speed fluctuations) of the exhaust gas expander, and even triggering shutdown protection. In addition, equipment within the system (exhaust gas preheater, high-temperature gas-to-gas heat exchanger) is prone to corrosion by dilute nitric acid formed by the combination of NOx and water vapor, requiring frequent shutdowns for maintenance, resulting in high operation and maintenance costs. Utility Model Content
[0004] In view of the shortcomings of the prior art, the purpose of this utility model is to provide a dual-pressurized nitric acid process tail gas treatment system, which realizes the tiered utilization of tail gas heat energy in the full temperature range from 35℃ to 100℃ through a multi-stage heat energy recovery unit, which can greatly improve the system's low-pressure steam self-sufficiency rate.
[0005] This utility model is achieved using the following technical solution: The aforementioned dual-pressurized nitric acid process tail gas treatment system includes an absorption tower. The top of the absorption tower is connected to a low-temperature heat exchanger via a pre-separator. The low-temperature heat exchanger is connected to an SCR denitrification reactor via a demister. The SCR denitrification reactor is connected to a primary heat exchanger via a catalyst layer. The primary heat exchanger is connected to a tertiary heat exchanger via a secondary heat exchanger. The tertiary heat exchanger is connected to a tail gas expander via a pipeline.
[0006] The exhaust gas pretreatment unit includes a pre-separator, a cryogenic heat exchanger, and a demister. It removes dust, demisters mist, and initially cools the initial exhaust gas (temperature 45℃, pressure ≈1.1MPa, containing NOx and trace amounts of dilute nitric acid droplets) discharged from the top of the absorption tower to prevent corrosion of subsequent equipment. The pre-separator operates at a pressure of 0.9-1.0MPa and a temperature of 45℃, employing a cyclone separation structure with a separation efficiency ≥95%. The cryogenic heat exchanger uses demineralized water as the cooling medium to reduce the exhaust gas temperature to 35-40℃, while the outlet temperature of the cooling medium rises to 60-65℃ (sent to the boiler feedwater system for heat recovery). The demister uses corrugated packing made of polypropylene, with a demisting efficiency ≥99%, ensuring that the droplet content in the exhaust gas is <5mg / m³. 3 .
[0007] The exhaust gas expander is connected to the buffer tank via a waste heat boiler, and the buffer tank is equipped with an online NOx monitor.
[0008] The buffer tank is connected to an exhaust pipe via a pipeline, and the buffer tank is equipped with an emergency denitrification bypass.
[0009] An absorption tower outlet pipe connects the absorption tower to the pre-separator, and an ammonia injection system is connected to the SCR denitrification reactor.
[0010] The advanced denitrification unit includes an SCR denitrification reactor, an ammonia injection system, and a catalyst bed (using a honeycomb vanadium-titanium catalyst); it performs advanced denitrification on the pretreated exhaust gas, reducing the NOx content to 150 × 10⁻⁶. -6 The following serves as a "double guarantee" for emission standards: The inlet of the SCR denitrification reactor is connected to the demister outlet of the tail gas pretreatment unit via a pipeline. One end of the ammonia injection system is connected to the gaseous ammonia pipeline (0.52MPa, 100℃) of the ammonia synthesis system, and the other end extends into the inlet pipeline of the SCR denitrification reactor to achieve precise mixing of gaseous ammonia and tail gas. The operating pressure of the SCR denitrification reactor is 0.8-0.9MPa, the reaction temperature is 280-320℃ (achieved through preheating by subsequent multi-stage heat recovery units), the ammonia-to-nitrogen ratio (NH3 / NOx) is controlled at 1.0-1.2, and the catalyst space velocity is 1500-2000h⁻¹. -1 The denitrification efficiency is ≥90%.
[0011] The low-temperature heat exchanger has a low-temperature heat exchanger coil inside, which is connected to the primary heat exchanger through the boiler feedwater pipeline.
[0012] The secondary heat exchanger is connected to a NOx gas pipeline, and the tertiary heat exchanger is connected to a secondary air pipeline.
[0013] The multi-stage heat recovery unit includes a primary heat exchanger (exhaust gas-boiler feedwater heat exchanger), a secondary heat exchanger (exhaust gas-NOx gas heat exchanger), a tertiary heat exchanger (exhaust gas-secondary air heat exchanger), an exhaust gas expander, and a waste heat boiler. It recovers heat energy from the exhaust gas in stages, achieving cascade utilization through "low-grade heat energy preheating boiler feedwater, medium-grade heat energy preheating NOx gas, and high-grade heat energy driving the expander to perform work and generate steam." The inlet of the primary heat exchanger is connected to the outlet of the SCR denitrification reactor, and boiler feedwater (104℃ after deoxygenation) is introduced into the shell side. After heat exchange between the exhaust gas and boiler feedwater, the exhaust gas temperature drops from 320℃ to 220℃, while the boiler feedwater temperature rises to 150-160℃ (sent to the economizer). The inlet of the secondary heat exchanger is connected to the outlet of the primary heat exchanger, and NOx gas (45℃) from the nitrogen oxide separator is introduced into the shell side. After heat exchange between the exhaust gas and NOx gas, the exhaust gas temperature drops from 220℃ to 220℃. The temperature drops to 180℃, and the NOx gas temperature rises to 120-130℃ (sent to the NOx compressor inlet to reduce compressor energy consumption); the inlet of the third-stage heat exchanger is connected to the outlet of the second-stage heat exchanger, and secondary air (from the air filter, at room temperature) is introduced into the shell side. After heat exchange between the exhaust gas and the secondary air, the exhaust gas temperature rises from 180℃ to 360℃ (meeting the exhaust gas expander inlet temperature requirements), and the secondary air temperature rises to 150-160℃ (sent to the bleaching tower to improve bleaching efficiency); the inlet of the exhaust gas expander is connected to the outlet of the third-stage heat exchanger, and the exhaust gas after work (temperature 150-200℃) is sent to the waste heat boiler; the inlet of the waste heat boiler is connected to the outlet of the exhaust gas expander, adopting a natural circulation structure, using demineralized water as the medium, and utilizing the waste heat of the exhaust gas to generate 0.5MPa low-pressure steam (sent to the low-pressure steam network). After being cooled by the waste heat boiler, the temperature of the exhaust gas drops to 80-100℃ and enters the emergency buffer unit.
[0014] The emergency buffer unit includes a buffer tank, an emergency denitrification bypass, and an exhaust valve group. When the system malfunctions (such as a failure of the SCR denitrification reactor or a sudden increase in NOx in the exhaust gas), it provides temporary buffering and emergency treatment of the exhaust gas to avoid shutdown and excessive emissions.
[0015] The working principle of this utility model is as follows: Exhaust gas pretreatment process: Initial exhaust gas discharged from the top of the absorption tower (temperature 45℃, pressure 1.0MPa, NOx content 180-220×10⁻⁶) -6The exhaust gas enters the pre-separator 1 via pipeline 16, where cyclone separation removes over 95% of the dilute nitric acid mist droplets. The exhaust gas then enters the low-temperature heat exchanger 2, where it exchanges heat with the cooling medium (25°C) from the demineralized water system, reducing the exhaust gas temperature to 38°C. The cooling medium is then heated to 62°C before being sent to the boiler feedwater pipeline 17. Finally, the exhaust gas undergoes further demisting in the demister 3 (polypropylene corrugated packing) to ensure a droplet content of <5mg / m³. 3 It then enters the deep denitrification unit.
[0016] Deep denitrification process: Pretreated tail gas enters SCR denitrification reactor 4. Simultaneously, ammonia injection system 5 introduces gaseous ammonia at 0.52 MPa and 100℃ from the ammonia synthesis system, precisely injecting it into the reactor inlet pipe at an ammonia-to-nitrogen ratio of 1.1. After mixing with the tail gas, it flows through catalyst layer 6 (vanadium-titanium honeycomb catalyst). Under conditions of 290-310℃ and 0.85 MPa, NOx and NH3 undergo a reduction reaction, and the NOx content in the tail gas after denitrification is reduced to 120-150 × 10⁻⁶. -6 It then enters the multi-stage heat recovery unit.
[0017] Multi-stage heat recovery process: First-stage heat exchange: The denitrified tail gas (310℃) enters the first-stage heat exchanger 7 and exchanges heat with the deoxygenated water (104℃) sent from the boiler feedwater pipeline 17. The tail gas is cooled down to 220℃, and the boiler feedwater is heated up to 155℃ before being sent to the economizer. Secondary heat exchange: The exhaust gas enters the secondary heat exchanger 8 and exchanges heat with the NOx gas (45°C) sent from the NOx gas pipeline 18. The exhaust gas is cooled to 180°C, and the NOx gas is heated to 125°C and then sent to the NOx compressor inlet. Three-stage heat exchange: The exhaust gas enters the three-stage heat exchanger 9 and exchanges heat with the ambient temperature secondary air sent from the secondary air pipeline 19. The exhaust gas is heated to 360°C and the secondary air is heated to 155°C before being sent to the bleaching tower. Expander work and waste heat recovery: The exhaust gas at 360°C enters the exhaust gas expander 10 to perform work (driving an air compressor or generator). After performing work, the exhaust gas (180°C) enters the waste heat boiler 11, where it exchanges heat with the demineralized water to generate 0.5MPa low-pressure steam (sent to the low-pressure steam pipeline). After the exhaust gas cools down to 90°C, it enters the emergency buffer unit.
[0018] Emergency buffer procedure: 90℃ exhaust gas enters buffer tank 12 (50m³). 3 Operating pressure 0.65 MPa), NOx online monitor 15 monitors the NOx content in the exhaust gas in real time; when the monitored value is ≤200×10 -6 When the main exhaust valve opens, the exhaust gas is discharged through exhaust pipe 20; when the monitored value > 200×10 -6(If the SCR catalyst activity decreases), the main exhaust valve closes, the emergency denitrification bypass 13 opens, and a 30% concentration urea solution is injected as needed. After emergency denitrification, the NOx in the exhaust gas drops to 200 × 10⁻⁶. -6 Next, the system will be vented through an emergency exhaust valve to ensure there is no risk of exceeding the standard.
[0019] Compared with the prior art, the beneficial effects of this utility model are: (1) Through a three-tiered protection system of "pretreatment + deep SCR denitrification + emergency buffer", the NOx content in the exhaust gas can still be stably controlled at 200×10⁻⁶ even when production load fluctuates or equipment malfunctions. -6 The following meet stringent environmental standards. (2) The multi-stage heat recovery unit realizes the cascade heat energy utilization of the tail gas in the full temperature range from 35℃ to 100℃, which can increase the self-sufficiency rate of low-pressure steam in the system by 15-20% and save about 500-800 tons of standard coal per year (calculated based on a 1000-ton-per-day nitric acid plant).
[0020] (3) This system can cope with upstream process fluctuations and avoid frequent shutdowns of the exhaust gas expander; the pre-fogging and low-temperature heat exchange design reduces equipment corrosion, extends the maintenance cycle from the original 6 months to 12 months, and reduces operation and maintenance costs by more than 30%. Attached Figure Description
[0021] Figure 1 This is a schematic diagram of the tail gas treatment system for the dual-pressure nitric acid process of this utility model; In the diagram: 1. Pre-separator; 2. Low-temperature heat exchanger; 3. Demister; 4. SCR denitrification reactor; 5. Ammonia injection system; 6. Catalyst layer; 7. Primary heat exchanger; 8. Secondary heat exchanger; 9. Tertiary heat exchanger; 10. Exhaust gas expander; 11. Waste heat boiler; 12. Buffer tank; 13. Emergency denitrification bypass; 14. Absorption tower; 15. NOx online monitor; 16. Absorption tower outlet pipe; 17. Boiler feed water pipe; 18. NOx gas pipe; 19. Secondary air pipe; 20. Exhaust gas stack; 21. Low-temperature heat exchanger coil. Detailed Implementation
[0022] To make the objectives and technical solutions of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings.
[0023] Example 1 like Figure 1As shown, the dual-pressure nitric acid process tail gas treatment system includes an absorption tower 14. The top of the absorption tower 14 is connected to a low-temperature heat exchanger 2 via a pre-separator 1. The low-temperature heat exchanger 2 is connected to an SCR denitrification reactor 4 via a demister 3. The SCR denitrification reactor 4 is connected to a primary heat exchanger 7 via a catalyst layer 6. The primary heat exchanger 7 is connected to a tertiary heat exchanger 9 via a secondary heat exchanger 8. The tertiary heat exchanger 9 is connected to a tail gas expander 10 via a pipeline. The inlet of the primary heat exchanger 7 is connected to the outlet of the SCR denitrification reactor 4. The shell side of the primary heat exchanger is fed into the boiler feedwater, which is deoxygenated to a temperature of 104°C. After heat exchange between the tail gas and the boiler feedwater, the tail gas temperature drops from 320°C to 220°C, while the boiler feedwater temperature rises to 150-160°C before being sent to the economizer. The tail gas expander 10 is connected to a buffer tank 12 via a waste heat boiler 11. The buffer tank 12 is equipped with an online NOx monitor 15. A tail gas exhaust stack 20 is connected to the buffer tank 12 via a pipeline, and an emergency denitrification bypass 13 is provided on the buffer tank 12. An absorption tower outlet pipe 16 connects the absorption tower 14 to the pre-separator 1, and an ammonia injection system 5 is connected to the SCR denitrification reactor 4. A low-temperature heat exchanger coil 21 is installed inside the low-temperature heat exchanger 2, and the low-temperature heat exchanger coil 21 is connected to the primary heat exchanger 7 via a boiler feedwater pipe 17. A NOx gas pipe 18 is connected to the secondary heat exchanger 8, and a secondary air pipe 19 is connected to the tertiary heat exchanger 9.
[0024] The above-mentioned dual-pressure nitric acid process tail gas treatment system includes the following steps during operation: (1) The initial tail gas discharged from the top of the absorption tower enters the pre-separator 1 through the absorption tower outlet pipe 16. More than 95% of the dilute nitric acid mist droplets are removed by cyclone separation. Then the tail gas enters the low temperature heat exchanger 2, and the temperature decreases after exchanging heat with the demineralized water. The heated demineralized water is sent to the boiler feed water pipe 17 to recover heat energy. Finally, the tail gas is further demisted by the demister 3 to ensure that the droplet content meets the requirements of subsequent treatment and enters the deep denitrification unit. (2) The pretreated tail gas enters the SCR denitrification reactor 4. At the same time, the ammonia injection system 5 introduces gaseous ammonia from the synthetic ammonia system and injects it into the reactor inlet pipe in a set ratio to mix with the tail gas. The mixed gas flow undergoes a reduction reaction through the catalyst layer 6. After denitrification, the NOx content of the tail gas is significantly reduced and enters the multi-stage heat recovery unit. (3) The denitrified tail gas first enters the primary heat exchanger 7 to exchange heat with the boiler feedwater. The heated boiler feedwater is then sent to the economizer. Next, the tail gas enters the secondary heat exchanger 8 to exchange heat with the NOx gas from the nitrogen oxide separator. The heated NOx gas is then sent to the NOx compressor inlet. Subsequently, the tail gas enters the tertiary heat exchanger 9 to exchange heat with the secondary air. The heated secondary air is then sent to the bleaching tower. The tail gas that reaches the required temperature enters the tail gas expander 10 to perform work. The tail gas that has performed work enters the waste heat boiler 11 to generate low-pressure steam using waste heat and then sends it to the pipeline network. (4) The cooled tail gas enters the buffer tank 12. The NOx online monitor 15 monitors the NOx content of the tail gas in real time. When the monitoring value meets the standard, the main exhaust valve is opened, and the tail gas is discharged through the tail gas exhaust stack 20. When the monitoring value exceeds the standard, the main exhaust valve is closed, the emergency denitrification bypass 13 is opened, and urea solution is injected for emergency denitrification. After meeting the standard, the tail gas is discharged through the emergency exhaust valve.
Claims
1. A tail gas treatment system for a dual-pressure nitric acid process, characterized in that, The absorption tower (14) is connected to the top of the absorption tower (1) via a pre-separator (1) and a low-temperature heat exchanger (2). The low-temperature heat exchanger (2) is connected to the SCR denitrification reactor (4) via a demister (3). The SCR denitrification reactor (4) is connected to the primary heat exchanger (7) via a catalyst layer (6). The primary heat exchanger (7) is connected to the tertiary heat exchanger (9) via a secondary heat exchanger (8). The tertiary heat exchanger (9) is connected to the exhaust gas expander (10) via a pipeline.
2. The tail gas treatment system for the dual-pressurized nitric acid process according to claim 1, characterized in that, The exhaust gas expander (10) is connected to the buffer tank (12) via the waste heat boiler (11), and the buffer tank (12) is equipped with an online NOx monitor (15).
3. The tail gas treatment system for the dual-pressurized nitric acid process according to claim 2, characterized in that, The buffer tank (12) is connected to the exhaust pipe (20) via a pipe, and the buffer tank (12) is equipped with an emergency denitrification bypass (13).
4. The tail gas treatment system for the dual-pressurized nitric acid process according to claim 1, characterized in that, The absorption tower (14) is connected to the pre-separator (1) by an absorption tower outlet pipe (16), and the SCR denitrification reactor (4) is connected to an ammonia injection system (5).
5. The tail gas treatment system for the dual-pressurized nitric acid process according to claim 1, characterized in that, The low-temperature heat exchanger (2) is equipped with a low-temperature heat exchanger coil (21) inside, which is connected to the primary heat exchanger (7) through the boiler feed water pipe (17).
6. The tail gas treatment system for the dual-pressurized nitric acid process according to claim 1, characterized in that, The secondary heat exchanger (8) is connected to a NOx gas pipeline (18), and the tertiary heat exchanger (9) is connected to a secondary air pipeline (19).