Method and system for treating NOx in nitric acid production tail gas

Abstract

The invention discloses a method for treating NOx in nitric acid production tail gas, which is characterized by comprising the following steps: introducing the nitric acid production tail gas into a first-stage absorption tower, and absorbing by using a NaOH solution with the concentration of 15-18%, so that the NOx conversion rate is 95.2-98.5%; the absorption liquid of the first-stage absorption tower is input into a conversion reactor for a conversion reaction; and returning the solution converted by the conversion reactor to the nitric acid production system to form circulation. The NOx in the tail gas is efficiently converted into high-purity sodium nitrate and sodium nitrite products by adopting a'two-stage absorption-conversion 'closed-loop process and accurately controlling parameters such as absorption liquid concentration, reaction temperature, pH value and retention time, and the high-purity sodium nitrate and sodium nitrite products are returned to a nitric acid production system to form a material closed loop. According to the method, the NOx concentration of the tail gas is reduced to 5 ppm or below, the resource utilization rate is increased to 98% or above, the traditional treatment cost is changed into production benefits, and high-valued conversion from pollutants to resources is achieved.

Description

Technical Field

[0001] This invention relates to a method and system for treating NOx in nitric acid production tail gas, belonging to the field of chemical environmental protection production technology. Background Technology

[0002] Currently, the tail gas generated during nitric acid production contains various nitrogen oxides (NOx), mainly including nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Direct emission of these NOx components not only causes air pollution, leading to acid rain and photochemical smog, but also results in a serious waste of nitrogen resources. Therefore, the effective treatment and resource recovery of NOx from nitric acid production tail gas is an important issue in the field of chemical environmental protection.

[0003] Traditional nitric acid tail gas treatment technologies mainly rely on catalytic reduction methods, such as selective catalytic reduction (SCR) or non-selective catalytic reduction (NSCR). These methods typically involve introducing a reducing agent (such as ammonia or methane) into the tail gas in the presence of a catalyst (such as V2O5 / TiO2), reducing NOx to harmless nitrogen (N2) at a high temperature of 300-400℃ before releasing it into the atmosphere.

[0004] However, existing catalytic reduction technologies have the following significant drawbacks:

[0005] Resource waste: This method converts nitrogen, which has potential economic value, into worthless N2, failing to achieve resource recycling and resulting in low resource utilization (usually less than 50%).

[0006] High emission concentration: Due to limitations in reaction efficiency and operational fluctuations, the NOx concentration in the treated exhaust gas is usually still in the range of 50-300 ppm, making it difficult to consistently meet increasingly stringent emission standards (e.g., the Chinese national standard limit is 30 ppm).

[0007] Poor economic efficiency: The process requires additional reducing agents and energy to maintain high-temperature reaction conditions, resulting in high operating costs. It is estimated that this treatment will cause an economic loss of about 100 yuan for every ton of nitric acid produced.

[0008] Secondary pollution risk: When using ammonia as a reducing agent, there may be ammonia escape; at the same time, the process may generate wastewater containing byproducts such as sulfate, which poses a risk of secondary pollution to the environment.

[0009] The system is not closed-loop: the various stages of the existing process (absorption, reaction, emission) lack effective material closed-loop connection, which is essentially an open "end-of-pipe treatment" model.

[0010] While existing technologies can reduce NOx emissions to some extent, they cannot fundamentally resolve the contradiction between environmental pressures and low resource efficiency faced by the nitric acid industry. Therefore, there is an urgent need for a revolutionary treatment process and system that can convert NOx pollutants into high-value products and achieve material recycling within the system. Summary of the Invention

[0011] The purpose of this invention is to provide a method and a system for treating NOx in nitric acid production tail gas. This invention, through precise control of parameters such as absorbent concentration, reaction temperature, pH value, and residence time, efficiently converts NOx in the tail gas into high-purity sodium nitrate and sodium nitrite products, which are then returned to the nitric acid production system to form a closed-loop material supply. This method reduces the NOx concentration in the tail gas to below 5 ppm, increases resource utilization to over 98%, transforms traditional treatment costs into production benefits, and achieves high-value transformation of pollutants into resources.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a method for treating NOx in nitric acid production tail gas, comprising the following steps:

[0013] S1. The tail gas from nitric acid production is fed into the first-stage absorption tower and absorbed using a 15-18% NaOH solution to achieve a NOx conversion rate of 95.2-98.5%.

[0014] S2. The absorbent from the first-stage absorption tower is fed into the conversion reactor to carry out the conversion reaction;

[0015] S3. The solution converted by the conversion reactor is returned to the nitric acid production system to form a cycle.

[0016] In the aforementioned method, the NOx concentration in the nitric acid production tail gas is 500-700 ppm, the composition ratio is NO:NO2:N2O = 3:2:1, the tail gas temperature is 40-50℃, and the flow rate is 100-300 m³ / h. 3 / h.

[0017] In the aforementioned method, the NaOH solution is prepared by dissolving analytical grade NaOH solid in deionized water, and its concentration is accurate to one decimal place.

[0018] In the aforementioned method, the packing height of the first-stage absorption tower is 1.5-2.5m, and the absorption temperature is 30-40℃.

[0019] In the aforementioned method, the temperature of the conversion reactor is 60-70℃, the residence time is 20-30 minutes, and the pH is 8.5-8.9.

[0020] In the aforementioned method, the purity of sodium nitrate is ≥99.5%, and the purity of sodium nitrite is ≥98.5%.

[0021] A system for implementing the above method includes:

[0022] Nitric acid production system

[0023] The first-stage absorption tower is used to receive and absorb the tail gas produced by the nitric acid production system.

[0024] A conversion reactor, connected to the first-stage absorption tower, is used to convert the absorbent into sodium nitrate and sodium nitrite;

[0025] The circulation pipeline returns the solution output from the conversion reactor to the nitric acid production system, forming a closed loop.

[0026] In the aforementioned system, the first-stage absorption tower is equipped with a packing layer with a packing height of 1.5-2.5m.

[0027] The aforementioned system includes a conversion reactor equipped with a temperature control device, a pH control device, and a residence time control device.

[0028] The aforementioned system further includes a NaOH solution preparation unit connected to the first-stage absorption tower, used to prepare and input NaOH solution into the first-stage absorption tower, and to precisely control the concentration of the NaOH solution as the absorbent to be 15-18%.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects:

[0030] (1) This invention achieves ultra-low emissions, reducing the NOx concentration in the exhaust gas from 50-300 ppm in traditional processes to below 5 ppm, which is far below the national standard of 30 ppm.

[0031] (2) This invention achieves efficient utilization of resources, converting NOx into high-purity sodium nitrate (≥99.5%) and sodium nitrite (≥98.5%), increasing the resource utilization rate from less than 50% to over 98%.

[0032] (3) This invention significantly improves economic efficiency, turning processing costs into profits. Each ton of nitric acid produced can generate an additional 120-150 yuan in profit, which is more than 245% more economically efficient than traditional methods.

[0033] (4) The present invention forms a closed-loop material chain of “NOx → nitrate → nitric acid”, avoiding the consumption of reducing agent and secondary pollution, and realizing clean production and resource recycling. Attached Figure Description

[0034] Figure 1 This is a process system diagram of the present invention.

[0035] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Detailed Implementation

[0036] Example 1 of the present invention: A method for treating NOx in nitric acid production tail gas, comprising the following steps:

[0037] S1. The tail gas from nitric acid production is fed into the first-stage absorption tower and absorbed using a 15-18% NaOH solution to achieve a NOx conversion rate of 95.2-98.5%.

[0038] S2. The absorbent from the first-stage absorption tower is fed into the conversion reactor to carry out the conversion reaction;

[0039] S3. The solution converted by the conversion reactor is returned to the nitric acid production system to form a cycle.

[0040] The aforementioned method results in a NOx concentration of 500-700 ppm in the nitric acid production tail gas, a composition ratio of NO:NO2:N2O = 3:2:1, a tail gas temperature of 40-50℃, and a flow rate of 100-300 m³ / h. 3 / h.

[0041] Specifically, the NaOH solution is prepared by dissolving analytical grade NaOH solid in deionized water, and its concentration is accurate to one decimal place.

[0042] Specifically, the packing height of the first-stage absorption tower is 1.5-2.5m, and the absorption temperature is 30-40℃.

[0043] Specifically, the temperature of the conversion reactor is 60-70℃, the residence time is 20-30 minutes, and the pH is 8.5-8.9.

[0044] Specifically, the purity of sodium nitrate is ≥99.5%, and the purity of sodium nitrite is ≥98.5%.

[0045] A system for implementing the above method includes:

[0046] The nitric acid production system will generate nitric acid production tail gas, which contains 500-700 ppm NOx, with a NO:NO2:N2O ratio of 3:2:1, a temperature of 40-50℃, and a flow rate of 100-300 m³ / h. 3 / h;

[0047] The first-stage absorption tower is used to receive and absorb the tail gas produced by the nitric acid production system. The first-stage absorption tower is equipped with a packing layer with a packing height of 1.5-2.5m. It uses a NaOH solution with a precise concentration of 15-18% and absorbs the NOx at 30-40℃, achieving a NOx conversion rate of 95.2-98.5%.

[0048] The conversion reactor, connected to the first-stage absorption tower, is used to convert the absorbent into sodium nitrate and sodium nitrite. The temperature of the conversion reactor is precisely 60-70℃, the residence time is precisely 20-30 minutes, and the pH is precisely controlled at 8.5-8.9 to produce sodium nitrate (purity ≥99.5%) and sodium nitrite (purity ≥98.5%).

[0049] The circulation pipeline returns the solution output from the conversion reactor to the nitric acid production system, forming a closed loop.

[0050] Specifically, the conversion reactor is equipped with a temperature control device, a pH control device, and a residence time control device.

[0051] Specifically, the system also includes a NaOH solution preparation unit, which is connected to the first-stage absorption tower. This unit is used to prepare and input NaOH solution into the first-stage absorption tower, and precisely controls the concentration of the NaOH solution as the absorbent to be 15-18%. The NaOH solution is prepared by dissolving analytical grade NaOH solid in deionized water, with the concentration accurate to one decimal place.

[0052] This invention innovatively converts NOx into high-value nitrates, achieving a revolutionary transformation from "pollutant to resource," reducing the NOx concentration in exhaust gas to below 4.2 ppm (1.4% of the national standard limit of 30 ppm), increasing resource utilization to 99.1%, and generating an additional economic benefit of 142.8 yuan per ton of nitric acid produced. It completely solves the technical bottlenecks of high emissions, resource waste, and poor economic benefits caused by traditional catalytic reduction methods.

[0053] The present invention will be further explained below by comparing specific embodiments with existing technologies.

[0054] Example 3:

[0055] A closed-loop "two-stage absorption-conversion" process system was constructed in a laboratory-scale reactor. The experimental feedstock was the tail gas from nitric acid production (containing 500 ppm NOx, NO:NO2:N2O = 3:2:1, temperature 40℃, flow rate 100 m³ / h). 3 The absorbent solution was prepared by dissolving analytical grade NaOH solid (99.9% purity) in deionized water, with a concentration accurate to 15%.

[0056] Experimental procedure: The tail gas was introduced into the first-stage absorption tower (packing height 1.5m, temperature precisely 30℃) and absorbed using 15% NaOH solution, achieving a NOx conversion rate of 95.2%; the absorbent was fed into the conversion reactor (temperature precisely 60℃, residence time precisely 30 minutes), and sodium nitrate (purity 99.5%) and sodium nitrite (purity 98.5%) were generated by precisely controlling the pH to 8.5; the conversion liquid was returned to the nitric acid production system.

[0057] The system records NOx concentration in the exhaust gas, resource utilization rate, and economic benefits in real time throughout the process. After three repeated experiments, the NOx concentration in the exhaust gas was stabilized at 4.8 ppm (standard deviation ±0.12 ppm), the resource utilization rate was 98.2% (standard deviation ±0.15%), and the additional revenue per ton of nitric acid was 125.3 yuan (standard deviation ±1.2 yuan).

[0058] This embodiment innovatively converts NOx completely into high-value nitrates (non-traditional N2 emissions), reducing the NOx concentration in the exhaust gas to 16% of the national standard limit (30ppm), increasing resource utilization to 98.2% (48.2 percentage points higher than the traditional catalytic reduction method), and improving economic benefits by 223.8% (the traditional method loses 98.5 yuan / ton, while this embodiment gains 125.3 yuan / ton).

[0059] Example 4:

[0060] In this embodiment, a closed-loop process system of "two-stage absorption-conversion" is constructed in a pilot-scale reactor, focusing independently on the resource utilization of NOx from nitric acid tail gas.

[0061] The experimental raw material was the tail gas from nitric acid production (containing 600 ppm NOx, NO:NO2:N2O = 3:2:1, temperature 45℃, flow rate 200 m³ / h). 3 The absorbent solution was prepared by dissolving analytical grade NaOH solid (99.9% purity) in deionized water, with a concentration accurate to 16.5%.

[0062] Experimental procedure: The tail gas was introduced into the first-stage absorption tower (packing height 2m, temperature precisely 35℃) and absorbed using 16.5% NaOH solution, with a NOx conversion rate of 96.8%; the absorbent was introduced into the conversion reactor (temperature precisely 65℃, residence time precisely 25 minutes), and sodium nitrate (purity 99.6%) and sodium nitrite (purity 98.7%) were generated by precisely controlling the pH to 8.7; the conversion liquid was returned to the nitric acid production system.

[0063] Key indicators were recorded in real time throughout the process. After three repeated experiments, the NOx concentration in the exhaust gas was stabilized at 4.5 ppm (standard deviation ±0.10 ppm), the resource utilization rate was 98.7% (standard deviation ±0.12%), and the additional revenue per ton of nitric acid was 132.5 yuan (standard deviation ±1.0 yuan).

[0064] This embodiment innovatively achieves stable operation at the pilot scale. By improving pH control accuracy to 8.7 and optimizing residence time to 25 minutes, resource utilization is increased by 0.5 percentage points compared to Example 3, economic benefits are improved by 5.7%, and the NOx concentration in the exhaust gas is reduced by 98.5% compared to the national standard limit (30 ppm) and by 98.2% compared to the traditional catalytic reduction method (250 ppm). All parameters are accurate to one decimal place, and the consistency of three repeated verifications reaches 99.5%, providing key support for the engineering application of the patented "two-stage absorption-conversion" closed-loop process. Together with the previous two patents, it constructs a green manufacturing technology system for the nitric acid industry.

[0065] Example 5:

[0066] This patent independently focuses on the resource utilization of NOx from nitric acid tail gas by constructing a "two-stage absorption-conversion" closed-loop process system in an industrial-scale reactor.

[0067] The experimental raw material was the tail gas from nitric acid production (containing 700 ppm NOx, NO:NO2:N2O = 3:2:1, temperature 50℃, flow rate 300 m³ / h). 3 The absorbent solution was prepared by dissolving analytical grade NaOH solid (99.9% purity) in deionized water, with a concentration accurate to 18%.

[0068] Experimental procedure: The tail gas was introduced into the first-stage absorption tower (packing height 2.5m, temperature precisely 40℃) and absorbed using 18% NaOH solution, achieving a NOx conversion rate of 98.5%; the absorbent was fed into the conversion reactor (temperature precisely 70℃, residence time precisely 20 minutes), and sodium nitrate (purity 99.7%) and sodium nitrite (purity 98.8%) were generated by precisely controlling the pH to 8.9; the conversion liquid was returned to the nitric acid production system.

[0069] Key indicators were recorded in real time throughout the process. After three repeated experiments, the NOx concentration in the exhaust gas remained stable at 4.2 ppm (standard deviation ±0.08 ppm), the resource utilization rate was 99.1% (standard deviation ±0.09%), and the additional revenue per ton of nitric acid was 142.8 yuan (standard deviation ±0.8 yuan).

[0070] This embodiment innovatively achieves stable industrial-scale operation. By increasing the absorbent concentration to 18% and optimizing the conversion temperature to 70°C, resource utilization is improved by 0.4 percentage points compared to Example 4, economic benefits are increased by 7.8%, and the NOx concentration in the exhaust gas is reduced by 98.6% compared to the national standard limit (30 ppm) and by 98.3% compared to the traditional catalytic reduction method (250 ppm). All parameters are accurate to one decimal place, and the consistency of three repeated verifications reaches 99.5%, providing key empirical support for the large-scale application of the patented "two-stage absorption-conversion" closed-loop process.

[0071] Comparative example (traditional catalytic reduction method):

[0072] Traditional catalytic reduction processes have not achieved closed-loop utilization of NOx resources.

[0073] The experimental raw material was tail gas from nitric acid production (containing 500 ppm NOx, NO:NO2:N2O = 3:2:1, temperature 40℃, flow rate 100 m³ / h). 3 / h).

[0074] Experimental procedure: The exhaust gas was introduced into a catalytic reactor (temperature precisely 350℃, catalyst V2O5 / TiO2), and ammonia gas with a precise concentration of 1.5% was added as a reducing agent to convert NOx into N2 for emission; the NOx concentration, resource utilization rate and economic benefits of the exhaust gas were recorded in real time throughout the process.

[0075] After three repeated experiments, the NOx concentration in the exhaust gas was stabilized at 250 ppm (standard deviation ±5 ppm), the resource utilization rate was 45.2% (standard deviation ±1.5%), and the loss per ton of nitric acid was 98.5 yuan (standard deviation ±2.0 yuan).

[0076] This comparison clearly demonstrates the three major shortcomings of traditional techniques:

[0077] 1. The NOx concentration in the exhaust gas exceeded the national standard limit (30ppm) by 733%, the resource utilization rate was less than 50%, and the economic benefits were negative.

[0078] 2. A precise comparison with Example 5 shows that the NOx concentration in the exhaust gas is 245.8 ppm higher, the resource utilization rate is 53.9 percentage points lower, and the economic benefit is 241.3 yuan / ton lower (traditional method loss of 98.5 yuan / ton vs. the benefit of this patent of 142.8 yuan / ton).

[0079] 3. This comparative example fully presents the inefficient "emission-oriented" mode of traditional processes, providing key empirical support for the "two-stage absorption-conversion" closed-loop process of this invention, highlighting the revolutionary innovation of this invention in converting pollutants (NOx) into high-value nitrates (sodium nitrate ≥99.7%, sodium nitrite ≥98.8%).

[0080] The following is a comparison of specific experimental data from Examples 3-5 and the comparative examples:

[0081] Table 1: Key Parameters and Performance Indicators of the Example

[0082]

[0083]

[0084] Table 2: Comparison of Performance between Examples and Traditional Processes

[0085] project Traditional crafts Example 3 Example 4 Example 5 NOx concentration in exhaust gas (ppm) 250 4.8 4.5 4.2 Resource utilization rate (%) 45.2 98.2 98.7 99.1 NOx concentration reduction rate in exhaust gas (%) - 98.08 98.2 98.32 Resource utilization rate improvement rate (%) - 53 53.5 53.9 Economic benefit per ton of nitric acid (yuan) -98.5 125.3 132.5 142.8 Economic efficiency improvement rate (%) - 227.3 235.6 245.7

[0086] Note: All data are based on Aspen software and experimental verification. After three repeated experiments, the standard deviation was <0.2%, and the consistency was over 99.5%. Example 3 achieved efficient NOx conversion by controlling pH (8.5) and residence time (30 minutes); Example 4 achieved stable pilot-scale operation by increasing the absorbent concentration (16.5%) and optimizing reaction conditions; Example 5 achieved stable industrial-scale operation by further increasing the absorbent concentration (18%) and conversion temperature (70°C).

[0087] Compared with the traditional catalytic reduction method, this invention innovatively converts NOx into high-value nitrates (sodium nitrate ≥99.7%, sodium nitrite ≥98.8%), reducing the NOx concentration in the exhaust gas to 1.4% of the national standard limit (30ppm), increasing resource utilization to 99.1%, and improving economic benefits from a loss of 98.5 yuan / ton to a gain of 142.8 yuan / ton, achieving a revolutionary transformation from "pollutant to resource".

Claims

1. A method for treating NOx in nitric acid production tail gas, characterized in that, Includes the following steps: S1. The tail gas from nitric acid production is fed into the first-stage absorption tower and absorbed using a 15-18% NaOH solution to achieve a NOx conversion rate of 95.2-98.5%. S2. The absorbent from the first-stage absorption tower is fed into the conversion reactor to carry out the conversion reaction; S3. The solution converted by the conversion reactor is returned to the nitric acid production system to form a cycle.

2. The method according to claim 1, characterized in that, The NOx concentration in the tail gas from the nitric acid production process is 500-700 ppm, with a composition ratio of NO:NO2:N2O = 3:2:

1. The tail gas temperature is 40-50℃, and the flow rate is 100-300 m³ / h. 3 / h.

3. The method according to claim 1, characterized in that, The NaOH solution is prepared by dissolving analytical grade NaOH solid in deionized water, and its concentration is accurate to one decimal place.

4. The method according to claim 1, characterized in that, The packing height of the first-stage absorption tower is 1.5-2.5m, and the absorption temperature is 30-40℃.

5. The method according to claim 1, characterized in that, The temperature of the conversion reactor is 60-70℃, the residence time is 20-30 minutes, and the pH is 8.5-8.

9.

6. The method according to claim 1, characterized in that, The purity of the sodium nitrate is ≥99.5%, and the purity of the sodium nitrite is ≥98.5%.

7. A system for implementing the method of any one of claims 1-6, characterized in that, include: Nitric acid production system The first-stage absorption tower is used to receive and absorb the tail gas produced by the nitric acid production system. A conversion reactor, connected to the first-stage absorption tower, is used to convert the absorbent into sodium nitrate and sodium nitrite; The circulation pipeline returns the solution output from the conversion reactor to the nitric acid production system, forming a closed loop.

8. The system according to claim 7, characterized in that, The first-stage absorption tower is equipped with a packing layer with a packing height of 1.5-2.5m.

9. The system according to claim 7, characterized in that, The conversion reactor is equipped with a temperature control device, a pH control device, and a residence time control device.

10. The system according to claim 7, characterized in that, The system also includes a NaOH solution preparation unit, which is connected to the first-stage absorption tower and is used to prepare and input NaOH solution into the first-stage absorption tower, and to precisely control the concentration of NaOH solution as the absorbent to be 15-18%.

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