Process and system for recovering nitrogen oxides in spray roasting nitric acid tail gas

Abstract

The application discloses a recovery process and system for nitrogen oxides in spray roasting nitric acid tail gas, and relates to the technical field of tail gas recovery. The recovery process comprises the following steps: S1, introducing the flue gas containing NOx into a cooling-multistage absorption system to form regenerated acid; S2, introducing the remaining NOx-containing gas after primary absorption into a VPSA crude denitration system, and adsorbing NO2 and water vapor by an adsorbent; S3, introducing the gas after VPSA crude denitration treatment into a TSA fine denitration system, and adsorbing the remaining NOx by a catalytic adsorbent to obtain purified tail gas; and S4, taking part of the purified tail gas, heating the purified tail gas, and using the heated purified tail gas as a regenerated gas source to flush the catalytic adsorbent to make the NOx desorption and regeneration, and returning the recovered gas 2 formed after cooling to the cooling-multistage absorption system. The application adopts the "VPSA crude denitration+TSA fine denitration" to replace the traditional SCR denitration technology, so that the concentration of NOx in the exhaust gas is less than or equal to 50 mg / Nm 3 , and the problem of over-standard NOx emission in the traditional process is effectively solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of tail gas recovery, in particular to a recovery process and system for nitrogen oxides in spray roasting nitric acid tail gas. BACKGROUND

[0002] As an advanced material preparation technology, spray roasting has been widely used in ceramic materials, catalyst carriers, nano-powder and other material science fields due to its controllability of product particle size, high purity, good dispersibility and low temperature process. For example, in the preparation of lithium battery cathode materials, the morphology and particle size distribution of lithium iron phosphate particles can be precisely controlled by spray roasting, significantly improving the electrochemical performance of the electrode material; in the production of environmental protection catalysts, this process can effectively avoid the agglomeration of active components caused by traditional solid phase method, and the specific surface area of the catalyst is increased by more than 30%. However, a large amount of nitrogen oxide (NOx) containing roasting furnace tail gas will be generated during the high temperature decomposition of nitrate, which will not only cause serious environmental pollution, but also lead to a large loss of nitrogen resources if not effectively treated.

[0003] In the traditional process, "low-temperature water absorption + SCR denitration" is mainly used to treat nitric acid tail gas. First, a multi-stage spray absorption tower is used to absorb NO2 in the tail gas with water or dilute nitric acid, and regenerated nitric acid is returned to the production system for recycling. However, due to the chemical equilibrium in the NOx system (3NO2+H2O=2HNO3+NO), NO gas will be generated during the absorption process, and the solubility of NO in water is very low, which limits the theoretical absorption rate of the system to 85%-93%. The 7%-15% of NOx (mainly NO) that is not absorbed needs to enter the SCR denitration system, where it reacts with the reducing agent (urea or ammonia water) to generate N2 and water under the action of the catalyst. This treatment method has significant defects: on the one hand, SCR denitration requires a large amount of energy and chemical raw materials, resulting in high operating costs; on the other hand, this process directly converts recyclable NOx into nitrogen gas, causing waste of nitrogen resources.

[0004] Under the background of continuous increase in manufacturing cost pressure, developing a new technology that can simultaneously achieve deep purification and efficient resource recovery of NOx has become a key problem that needs to be solved in the nitric acid production industry using spray roasting. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provide a recovery process and system for nitrogen oxides in spray roasting nitric acid tail gas, which effectively achieves deep purification (emission concentration ≤50mg / Nm³) and efficient resource recovery of NOx.

[0006] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0007] A recovery process of nitrogen oxides in spray roasting nitric acid tail gas, comprising:

[0008] S1, primary absorption: introducing the flue gas containing NOx into a cooling-multistage absorption system to form regenerated acid;

[0009] S2, introducing the remaining flue gas containing NOx after primary absorption into a VPSA crude denitration system, adsorbing NO2 and water vapor by the adsorbent, desorbing the formed recovery gas one and returning it to the cooling-multistage absorption system;

[0010] S3, introducing the gas after VPSA crude denitration into a TSA fine denitration system, adsorbing the remaining NOx by the catalytic adsorbent to obtain purified tail gas;

[0011] S4, TSA regeneration: taking part of the purified tail gas as a regeneration gas source after heating, washing the catalytic adsorbent to desorb and regenerate NOx, obtaining regenerated gas, and forming recovery gas two after cooling and returning it to the cooling-multistage absorption system;

[0012] S5, recycling: the cooling-multistage absorption system absorbs NOx in the recovery gas one and the recovery gas two and converts it into regenerated acid.

[0013] Further, in step S2, the adsorption pressure is 20-100 KPa, and the adsorption temperature is 10-50℃; the adsorbent is a combined bed formed by two or three of activated alumina, silica gel and molecular sieve, and the proportion of each component is 10%-60%.

[0014] Further, in step S2, the operation process of desorption is: under the condition of 10-50℃, the NOx is extracted from the adsorbent by a vacuum pump with a vacuum regeneration pressure of-50 to-90 KPa.

[0015] Further, in step S3, the catalytic adsorbent is one of 13X, ZSM-5, 10X and molecular sieve loaded with metal oxides; and the metal oxides are one or more of Pt, Pd, Rh, Cu, Mn and Ti oxides.

[0016] Further, in step S4, 5%-20% of the purified tail gas is heated to 180-250℃ as a regeneration gas source.

[0017] The application also provides a system for implementing the above-mentioned recovery process, comprising: a spray roasting system capable of generating NOx-containing gas, a cooling-multistage absorption system connected with the spray roasting system for primary absorption of NOx and generating regenerated nitric acid, a VPSA crude denitration system connected with the cooling-multistage absorption system for crude denitration and outputting recovery gas one, and a TSA fine denitration system connected with the VPSA crude denitration system for fine denitration and outputting recovery gas two.

[0018] Further, 2-6 adsorption towers are arranged in the VPSA coarse denitration system and the TSA fine denitration system respectively.

[0019] Further, a heater for heating a regeneration gas source and a cooler for cooling the regeneration gas to form recovered gas II are arranged in the TSA fine denitration system.

[0020] Further, a booster fan is arranged between the cooling-multistage absorption system and the VPSA coarse denitration system; a vacuum pump is connected to the VPSA coarse denitration system and can desorb the adsorbed NOx and water vapor on the adsorbent and form recovered gas I; and an outlet pressure regulating program control valve is arranged at the purified tail gas discharge port of the TSA fine denitration system.

[0021] Further, the output end of the vacuum pump and the output end of the cooler are connected to the cooling-multistage absorption system.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] (1) The present application adopts "VPSA coarse denitration + TSA fine denitration" to replace the traditional SCR denitration technology, so that the concentration of NOx in the exhaust tail gas is ≤50 mg / Nm 3 , effectively solving the problem of NOx emission exceeding the standard in the traditional process.

[0024] (2) The high-concentration NOx gas (recovered gas I and recovered gas II) desorbed by the VPSA and TSA systems is returned to the cooling-multistage absorption system, and finally converted into nitric acid product, with a resource utilization rate of NOx of more than 99.9%, which is significantly improved compared with the traditional process (85%-93%), and the waste of nitrogen resources is avoided.

[0025] (3) The present application eliminates the need for reducing agents such as urea and natural gas and energy consumption required by traditional SCR denitration, greatly reducing operating costs; in addition, the present application generates nitric acid product by recycling NOx, which can simultaneously reduce energy and raw material consumption, achieving a win-win of environmental protection and economic benefits.

[0026] (4) In the VPSA coarse denitration process, the present application uses a combination of activated alumina, silica gel and molecular sieve adsorbent to efficiently remove NO2 and water vapor, reducing the risk of corrosion in the subsequent system; in the TSA fine denitration process, an adsorbent with adsorption and catalytic functions (such as molecular sieve or metal oxide supported molecular sieve) is used, which can catalyze the oxidation of NO to NO2, enhance the adsorption capacity and ensure the purification accuracy. The present application arranges multiple towers for alternating operation (2-6 adsorption towers) in the VPSA coarse denitration system and the TSA fine denitration system, realizing continuous and stable operation.

[0027] (5) All NOx-containing desorption gas (recovery gas one and recovery gas two) in the application returns to the absorption system for recycling, without secondary waste gas emission; regenerated nitric acid can be transported to an external nitric acid storage tank for recycling or other purposes, forming a closed loop of "tail gas treatment-resource recovery-cyclic utilization", which conforms to the development direction of green chemical industry. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The overall structure schematic diagram of the recovery device provided by the application is shown in the figure;

[0029] Figure 2 The structure schematic diagram of the VPSA coarse denitration system in the application is shown in the figure;

[0030] Figure 3 The structure schematic diagram of the TSA fine denitration system in the application is shown in the figure.

[0031] Among them, the name corresponding to the reference sign is: 1-spray roasting system, 2-cooling-multistage absorption system, 3-VPSA coarse denitration system, 4-TSA fine denitration system, 5-heater, 6-boosting fan, 7-vacuum pump, 8-cooler, 9-outlet pressure regulating program control valve. DETAILED DESCRIPTION

[0032] The application will be further described below in combination with the accompanying drawings and examples, and the modes of the application include but are not limited to the following examples.

[0033] In this embodiment, the VPSA coarse NOx removal section and the TSA fine NOx removal section are arranged after the spray roasting furnace (nitric acid production) flue gas cooling and the multistage absorption tower to replace the traditional SCR denitration technology, so that the NOx in the exhaust tail gas is ≤50 mg / Nm 3 And the high-concentration NOx desorption gas desorbed by the VPSA coarse NOx removal section and the TSA fine NOx removal section returns to the nitric acid multistage absorption system for absorption, becomes a nitric acid product, and the NOx resource utilization rate is greater than 99.9%.

[0034] The spray roasting nitric acid tail gas nitrogen oxide recovery process provided by the embodiment includes:

[0035] (1) Primary absorption stage: the NOx-containing gas (NOx content 5-12%) generated by the spray roasting furnace (or nitric acid production) is introduced into the cooling-multistage absorption system, and 85%-93% of the NOx therein is absorbed to form regenerated nitric acid;

[0036] (2) VPSA coarse denox stage: the NOx-containing gas (NOx content 0.5%-3%) remaining after primary absorption is introduced into the VPSA coarse denox system, and part of the NOx (mainly NO2) and water vapor are adsorbed by the adsorbent, the adsorption pressure is 20-100 KPa, and the temperature is 10-50°C; the adsorbed NOx is extracted from the adsorbent by a vacuum pump (vacuum regeneration pressure-50 to-90 KPa) to form recovery gas 1 (containing NOx and water vapor, etc.), and the recovery gas 1 is returned to the cooling-multistage absorption system; the adsorbent is a combined bed formed by two or three of activated alumina, silica gel and molecular sieve (each component accounts for 10%-60%);

[0037] (3) TSA fine denox stage: the NOx gas (containing NOx 0.4%-2.8%) from which NO2 and water vapor are removed by VPSA is introduced into the TSA fine denox system, and the remaining NOx is adsorbed by the catalytic adsorbent in the TSA system, wherein the adsorbent needs to have both adsorption and catalytic functions, and can catalyze the reaction 2NO+O2=2NO2 to enhance the NOx adsorption capacity and purification precision, preferably, the catalytic adsorbent is molecular sieve (such as 13X, ZSM-5, 10X) or molecular sieve loaded with specific metal oxides (Pt, Pd, Rh, Cu, Mn, Ti); the treated purified tail gas has NOx≤50mg / Nm 3 , which can be directly discharged;

[0038] (4) TSA regeneration stage: 5%-20% of the purified tail gas is heated to 180-250°C by a heating system and then introduced into the TSA system to flush the adsorbent and regenerate the NOx at high temperature, and the high-temperature regeneration gas is cooled by a cooler to form recovery gas 2 (NOx content 5%-12%), and the recovery gas 2 is returned to the cooling-multistage absorption system;

[0039] (5) Recycling: the absorbed NOx (including the NOx absorbed in the primary absorption stage and the NOx in the recovery gas 1 and the recovery gas 2) is converted into regenerated nitric acid by the cooling-multistage absorption system, and is sent to an external storage tank for recycling or other purposes.

[0040] In the above process, the number of adsorption towers in the VPSA coarse denox system and the TSA fine denox system can be set to 2-6, and at least 2, so that one tower performs adsorption and the other tower performs regeneration desorption. After the tail gas is treated by the above process, the NOx content is ≤50mg / Nm 3 , and the recovery rate of NOx to form nitric acid is greatly increased, which is increased from 85-93% in the traditional process to more than 99.9%.

[0041] For example, Figure 1As shown, the embodiment also provides a system for implementing the above-mentioned recovery process, which comprises a spray roasting system 1, a cooling-multistage absorption system 2, a VPSA crude denitration system 3, and a TSA fine denitration system 4.

[0042] The spray roasting system 1 is the source of the NOx-containing gas, and generates flue gas containing NOx at a concentration of about 5%-12%, which is the raw gas source of the entire system. The cooling-multistage absorption system 2 is composed of a cooling device (such as a heat exchanger) and a multistage absorption tower (usually a spray absorption tower), which is used for cooling the NOx-containing gas from the spray roasting system 1 and absorbing 85%-93% of the NOx in the gas through multistage absorption (using water or dilute nitric acid as the absorbent) to form regenerated nitric acid. The outlet of the cooling-multistage absorption system 2 is divided into two routes: one route transports the NOx-containing gas (at a concentration of 0.5%-3%) that is not completely absorbed to the VPSA crude denitration system 3; and the other route transports the generated regenerated nitric acid to an external nitric acid storage tank for recycling or other uses.

[0043] The VPSA crude denitration system 3 is composed of 2-6 adsorption towers (such as Figure 2 The minimum number of adsorption towers is 2, which realize alternate operation of adsorption and regeneration, and are internally filled with adsorbents. The VPSA crude denitration system 3 is mainly used for adsorbing NO2 and water vapor to reduce the influence of water on the subsequent TSA system and the corrosiveness. The VPSA crude denitration system 3 performs crude denitration treatment on the NOx-containing gas (at a concentration of 0.5%-3%) from the cooling-multistage absorption system 2 to remove part of the NOx (mainly NO2) and water vapor, and the concentration of NOx in the treated gas is reduced to 0.4%-2.8%. A vacuum pump 7 is arranged in connection with the adsorption tower of the VPSA crude denitration system 3, and the NOx and water vapor adsorbed on the adsorbent can be desorbed by vacuum pumping (pressure-50 to-90 KPa) to form recovered gas one. The output end of the vacuum pump 7 is connected to the cooling-multistage absorption system 2, which is used for transporting the recovered gas one to the cooling-multistage absorption system 2. In the present VPSA crude denitration system, the adsorption pressure of the VPSA crude denitration system 3 is 20-100 KPa, and the adsorption and desorption temperature is 10-50°C; the single-tower cycle is usually 2-30 minutes, and the switching frequency of the adsorption tower is controlled according to the NO2 concentration and H2O content at the outlet of the VPSA. The operation process of the VPSA crude denitration system 3 is as follows:

[0044] Table 1 Operation process of the VPSA crude denitration system

[0045]

[0046] The TSA fine denitration system 4 is composed of 2-6 adsorption towers (such as Figure 3The TSA fine denitration system 4 is composed of at least two adsorption towers (as shown in the figure) and a catalytic adsorbent filled therein, which are alternately operated in adsorption and regeneration. The TSA fine denitration system 4 performs fine denitration treatment on the gas (NOx concentration 0.4%-2.8%) from the VPSA coarse denitration system 3, adsorbs the remaining NOx, and makes the NOx concentration in the purified tail gas ≤50mg / Nm³. An outlet pressure regulating program-controlled valve 9 is arranged at the purified tail gas outlet of the TSA fine denitration system 4, which is used to control the pressure of the TSA fine denitration system 4. In addition, a heater 5 is arranged in the TSA fine denitration system 4, which is used to heat 5-20% of the flue gas in the purified tail gas to 180-250°C, as a regeneration gas source of the TSA fine denitration system, to desorb and regenerate NOx by flushing the adsorbent. A cooler 8 is installed at the regeneration gas outlet of the TSA fine denitration system 4, which is used to cool the high-temperature regeneration gas containing NOx generated by desorption to form normal-temperature recovered gas two (NOx concentration 5%-12%). The output end of the cooler 8 is connected with the cooling-multistage absorption system 2, which is used to deliver the recovered gas two to the cooling-multistage absorption system 2 for reabsorption. In the TSA fine denitration system, the complete cycle period of a single adsorption tower for “adsorption-desorption regeneration” is 2-8 hours; the single-tower period is usually 120-180 minutes, which depends on the NO x The operation process of the TSA fine denitration system is shown in Table 2.

[0047] Table 2 Operation process of the TSA fine denitration system

[0048]

[0049] Further, a booster fan 6 is installed between the cooling-multistage absorption system 2 and the VPSA coarse denitration system 3, which provides conveying pressure for the NOx-containing gas entering the VPSA coarse denitration system 3 and the TSA fine denitration system 4, and ensures smooth flow of the gas.

[0050] The following is described by specific examples:

[0051] (1) Primary absorption stage: The NOx-containing gas generated by the spray roasting system enters the cooling-multistage absorption system, in which a large amount of NOx is absorbed to form regenerated nitric acid; the material data in the NOx-containing gas generated by the spray roasting system are shown in Table 3, and the material data in the NOx-containing gas after the treatment of the cooling-multistage absorption system are shown in Table 4.

[0052] Table 3 Material data in the NOx-containing gas generated by the spray roasting system

[0053]

[0054] Table 4 Data of each material in the NOx-containing gas after treatment by the cooling-multistage absorption system

[0055]

[0056] (2) VPSA rough denitration stage: the residual NOx-containing gas (NOx content 0.5%-3%) after the primary absorption enters the VPSA rough denitration system, and NO2 and water vapor therein are removed under the condition of 50 KPa and 25°C. The adsorbed NOx is extracted from the adsorbent by a vacuum pump (vacuum regeneration pressure -60 KPa) to form recovery gas one (containing NOx and water vapor, etc.), and the recovery gas one is returned to the cooling-multistage absorption system. The number of adsorption towers in the VPSA rough denitration system is 2, and the adsorbent is a combined bed of activated alumina and silica gel with a mass ratio of 1:1. The data of each material in the NOx-containing gas discharged after the VPSA rough denitration are shown in Table 5, and the data of each material in the recovery gas one are shown in Table 6.

[0057] Table 5 Data of each material in the residual NOx-containing gas after treatment by the VPSA rough denitration

[0058]

[0059] Table 6 Data of each material in the recovery gas one

[0060]

[0061] (3) TSA fine denitration stage: the NOx-containing gas after the rough denitration enters the TSA fine denitration system, and the NOx therein is catalytically adsorbed. The tail gas after purification contains NOx≤50 mg / Nm 3 , which can be directly discharged. Two adsorption towers are provided in the system, and the catalytic adsorbent is 13X molecular sieve. The data of each material in the purified tail gas obtained after the TSA fine denitration treatment are shown in Table 7.

[0062] Table 7 Data of each material in the purified tail gas obtained after the TSA fine denitration treatment

[0063]

[0064] (4) The purified tail gas after the TSA fine denitration system treatment contains NOx≤50 mg / Nm 3 . 15% of the flue gas in the purified tail gas is heated to 200°C and returned to the TSA fine denitration system as the regeneration gas source of the TSA. The high-concentration NOx desorption gas regenerated is cooled to form recovery gas two, and the recovery gas two is returned to the cooling-multistage absorption system. The data of each material in the recovery gas two are shown in Table 8.

[0065] Table 8 Data of each material in the recovered gas II

[0066]

[0067] (5) The regenerated nitric acid is discharged out of the system as a product for recycling or other use. The content of NOx in the treated calcination furnace tail gas is low, and the tail gas can be discharged into the atmosphere. The data of each material in the regenerated nitric acid are shown in Table 9.

[0068] Table 9 Data of each material in the regenerated nitric acid

[0069]

[0070] Note: Cooling - The excess H2O and O2 required for absorbing NOx to form nitric acid in the multi-stage absorption system are supplemented by an external unit. The concentration of the regenerated nitric acid can be adjusted by the amount of H2O supplement as needed.

[0071] The process provided in the embodiment is used to treat the NOx-containing tail gas generated by the spray calcination system. After the recycling of NOx, the discharge concentration of NOx in the purified tail gas is ≤50 mg / Nm 3 The above process is replaced by the traditional SCR technology, and the consumption of electricity, urea and natural gas is shown in Table 10:

[0072] Table 10

[0073]

[0074] By replacing the present process with the traditional SCR technology, the recovery rate of NOx (nitrogen oxide) is reduced by 15%. This is because part of the NOx reacts with urea to generate N2 in the SCR.

[0075] The above embodiment is only one of the preferred embodiments of the present application and should not be used to limit the protection scope of the present application. Any modification or polishing without substantial meaning made within the main design idea and spirit of the present application, which still solves the technical problems consistent with the present application, should be included in the protection scope of the present application.

Claims

1. A process for the recovery of nitrogen oxides from a spray roasting nitric acid off-gas, characterized in that, Comprising: S1, primary absorption: introducing NOx-containing flue gas into a cooling-multistage absorption system to form regenerated acid; S2, introducing the remaining NOx-containing gas after primary absorption into a VPSA crude denitration system, adsorbing NO2 and water vapor by the adsorbent, desorbing the recovered gas formed to return to the cooling-multistage absorption system; S3, introducing the gas treated by the VPSA crude denitration system into a TSA fine denitration system, adsorbing the remaining NOx by the catalytic adsorbent to obtain purified tail gas; S4, TSA regeneration: taking part of the purified tail gas as a regenerated gas source after heating, washing the catalytic adsorbent to desorb and regenerate NOx to obtain regenerated gas, and cooling to form recovered gas two to return to the cooling-multistage absorption system; S5, recycling: the cooling-multistage absorption system absorbs NOx in the recovered gas one and the recovered gas two and converts it into regenerated acid.

2. A process for the recovery of nitrogen oxides from the spray roasting of nitric acid off-gas according to claim 1, characterized in that, In step S2, the adsorption pressure is 20-100 KPa, and the adsorption temperature is 10-50℃; the adsorbent is a combined bed formed by two or three of activated alumina, silica gel, and molecular sieve, and each component accounts for 10%-60%.

3. A process for the recovery of nitrogen oxides from a spray roasting nitric acid tail gas according to claim 2, characterized in that, In step S2, the operation process of desorption is: under the condition of 10-50℃, NOx is extracted from the adsorbent by a vacuum pump with a vacuum regeneration pressure of-50 to-90 KPa.

4. A process for the recovery of nitrogen oxides from a spray roasting nitric acid tail gas according to claim 3, characterized in that, In step S3, the catalytic adsorbent is one of 13X, ZSM-5, 10X, and molecular sieve loaded with metal oxides; the metal oxides are one or more of Pt, Pd, Rh, Cu, Mn, and Ti oxides.

5. A process for the recovery of nitrogen oxides from a spray roasting nitric acid tail gas according to claim 4, characterized in that, In step S4, 5%-20% of the purified tail gas is heated to 180-250℃ as a regenerated gas source.

6. A system for carrying out the recycling process according to any one of claims 1 to 5, characterized in that, Comprising: a spray roasting system (1) capable of producing NOx-containing gas, a cooling-multistage absorption system (2) connected with the spray roasting system (1) for primary absorption of NOx and generation of regenerated nitric acid, a VPSA crude denitration system (3) connected with the cooling-multistage absorption system (2) for crude denitration and output of recovered gas one, and a TSA fine denitration system (4) connected with the VPSA crude denitration system (3) for fine denitration and output of recovered gas two.

7. The system of claim 6, wherein, 2-6 adsorption towers are arranged in the VPSA crude denitration system (3) and the TSA fine denitration system (4), respectively.

8. The system of claim 7, wherein, A heater (5) for heating the regenerated gas source and a cooler (8) for cooling the regenerated gas to form recovered gas two are arranged in the TSA fine denitration system (4).

9. The system of claim 8, wherein, A booster fan (6) is arranged between the cooling-multistage absorption system (2) and the VPSA crude denitration system (3); a vacuum pump (7) capable of desorbing NOx and water vapor adsorbed on the adsorbent and forming recovered gas one is connected to the VPSA crude denitration system (3); and an outlet pressure regulating program control valve (9) is arranged at the purified tail gas discharge port of the TSA fine denitration system (4).

10. The system of claim 9, wherein, The output end of the vacuum pump (7) and the output end of the cooler (8) are both connected with the cooling-multistage absorption system (2).

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