Method of purifying nitric acid production tail gases

Abstract

The invention relates to a method and unit for purifying nitric acid production tail gases and can be used in the chemical industry and in the production of fertilizers. What is disclosed is a method of purifying nitric acid production tail gases, comprising the steps of a) heating tail gases in a combustion chamber of a turbine, using heat from flue gases produced by the combustion of natural gas in the turbine combustion chamber; and b) catalytically purifying the tail gases produced in step a) in a selective purification reactor, using gaseous ammonia as a reducing agent, to produce a stream of purified tail gases. A unit for purifying nitric acid production tail gases is also proposed. The aim and technical result of the present invention is that of providing more efficient purification of nitric acid production tail gases by simultaneously reducing NOx and N2O emissions into the atmosphere.

Description

[0001] METHOD FOR PURIFYING TAIL GASES FROM NITRIC ACID PRODUCTION

[0002] Field of technology

[0003] The proposed invention relates to a method and unit for cleaning tail gases from nitric acid production and can be used in the chemical industry and in the production of fertilizers.

[0004] Prior art

[0005] The following steps are traditionally used to obtain nitric acid from ammonia:

[0006] • air preparation and compression;

[0007] • preparation of gaseous ammonia;

[0008] • preparation of ammonia-air mixture;

[0009] • conversion of ammonia into nitrous gases;

[0010] • cooling of nitrous gas with heat recovery;

[0011] • absorption of nitrogen oxides;

[0012] • catalytic purification of exhaust (tail) gases from residual nitrogen oxides;

[0013] • energy recovery of purified exhaust gas;

[0014] • storage and distribution of production acid.

[0015] The tail gas purification stage is a key step in the process, due to the high toxicity of nitrogen oxides and the negative impact their emissions have on the environment. Various methods exist for tail gas purification.

[0016] Thus, from D1 (A.P. Ilyin et al., Nitric Acid Production. Textbook. Ivan. State Chemical-Technological University, Ivanovo, 2011, Fig. 2.1) a method for cleaning tail gases from nitric acid production is known, including: a) the stage of heating the tail gases by cooling nitrous gases; b) the stage of heating the tail gases obtained in stage a) in the combustion chamber of the reactor; c) the stage of catalytic cleaning (using a palladium-containing catalyst) of the tail gases obtained in stage b) from nitrogen oxides with the release of a stream of purified tail gases; d) the stage of mixing the purified tail gases with the flue gases of the combustion chamber of the turbine.

[0017] In the proposed method, the catalytic tail gas cleaning stage precedes the heating of the tail gases in the turbine combustion chamber. Nitrogen oxides are formed in the combustion chamber and subsequently released into the atmosphere, reducing the environmental friendliness of the technology. Furthermore, the proposed method for catalytic tail gas cleaning uses a catalyst containing rare, hard-to-find palladium. This method also cools the gas turbine components using a compressed air flow, reducing the amount of compressed air that can be supplied to the process to increase nitric acid production.

[0018] The closest analogue of the present invention is the method known from D2 (A.P. Ilyin et al., Nitric Acid Production. Study Guide. Ivan. State Chemical-Technological University, Ivanovo, 2011, Fig. 2.15) for purifying tail gases from nitric acid production, which includes a) a stage of heating the tail gases by cooling the nitrous gases; b) a stage of catalytic purification (using a vanadium-containing catalyst) of the tail gases obtained in stage a) in a selective purification reactor using gaseous ammonia as a reducing agent to obtain a stream of purified tail gases; c) a stage of heating the purified tail gases obtained in stage b) in a turbine combustion chamber using the heat of the flue gases obtained from burning natural gas in the turbine combustion chamber.

[0019] In this method, the catalytic tail gas cleaning stage precedes the tail gas heating stage in the turbine combustion chamber. Nitrogen oxides are formed in the combustion chamber and subsequently released into the atmosphere, reducing the environmental friendliness of the technology. Furthermore, the vanadium catalyst used in the method generates additional nitrous oxide rather than promoting its removal, further reducing the environmental friendliness of the technology. This method also cools the gas turbine components using a compressed air flow, reducing the amount of compressed air that can be supplied to the technology to increase nitric acid production.

[0020] Disclosure of the essence of the invention

[0021] The objective and technical result of the present invention is to improve the efficiency of cleaning tail gases from nitric acid production by simultaneously reducing NO emissions. Xand N2O into the atmosphere. Additionally, increasing the energy efficiency of nitric acid production technology while improving environmental performance of production, increasing the efficiency of NO reduction X at the system outlet up to > 98.5%, increase in the efficiency of N2O reduction at the system outlet up to > 97%, increase in the service life of the catalyst, with calculated indicators from the start of operation: > 5 years after the first exposure to gas.

[0022] In order to solve the problem and achieve a technical result, a method for purifying tail gases from nitric acid production is proposed, which includes a) a stage of heating the tail gases in a combustion chamber using the heat of flue gases obtained from burning natural gas in the combustion chamber; b) a stage of catalytic purification of the tail gases obtained in stage a) in a selective purification reactor using gaseous ammonia as a reducing agent to obtain a stream of purified tail gases.

[0023] Nitrogen oxides (oxides) formed in the combustion chamber enter the selective purification reactor, where they are reduced to nitrogen. Thus, the proposed method reduces the nitrogen oxide content in the purified tail gas, which negatively impacts gas emissions. Conditions are provided for the removal (thermal decomposition) of nitrous oxide to nitrogen.

[0024] Under nitrogen oxides (NO X ) in this application refers to the oxides NO and NO2.

[0025] In this application, tail (exhaust) gases are understood to mean gases from the production of nitric acid, leaving the absorption column, with a content of more than 0.05% nitrogen oxides.

[0026] In this application, purified tail (exhaust) gases are understood to mean tail (exhaust) gases after their catalytic purification from nitrogen oxides. Preferably, a portion of the purified tail gas stream obtained in step (b) is diverted as a purified tail gas side stream for cooling in a purified tail gas cooling unit and then sent to cool gas turbine components.

[0027] In standard nitric acid production technologies, gas turbine components are cooled using a compressed air flow. The present invention proposes cooling turbine components using a purified tail gas flow. This allows the released air to be used in the process, further increasing the energy efficiency of the proposed method.

[0028] Preferably, in the cooling unit for purified tail gases, a two-stage cooling of the purified tail gases takes place: in the first stage - with water or by generating steam, in the second stage - with water.

[0029] In the case of using a single stage, an increased temperature difference between the refrigerant and the heat transfer fluids may be observed, which will lead to additional complexity of the design and to an additional decrease in the reliability of the heat exchanger.

[0030] Preferably, a zeolite catalyst, preferably a honeycomb zeolite catalyst, is used in the selective purification reactor.

[0031] The use of a honeycomb zeolite catalyst ensures a lower pressure drop and also leads to a reduction in the use of active components of the catalyst due to the ceramic carrier, ensuring ease of loading and operation of the catalyst - in particular, due to the absence of dust.

[0032] Preferably, Fe-ZSM-5, Fe-BEA, Fe-ZSM-12, Fe / HZSM-5, Fe-Ferrerite, Cu-ZSM-5, Cu-BEA, Cu-ZSM-12, Cu / HZSM-5, Cu-Ferrerite are used as the zeolite catalyst.

[0033] By modifying iron or copper to the beta position, the catalyst achieves increased efficiency in the nitrous oxide removal process without generating nitrous oxide, which further improves the environmental friendliness of the process.

[0034] Preferably, stage b) of catalytic purification of tail gases is carried out at a temperature of 450-700°C, preferably 550-600°C.

[0035] The specified temperature range ensures the most suitable conditions for the removal of nitrous oxide (i.e., for the thermal decomposition of nitrous oxide to nitrogen). Also, to solve the stated problem and achieve the technical result, a nitric acid production tail gas purification unit is proposed, comprising a tail gas heating unit, including a natural gas combustion chamber and connected to the tail gas supply line L1, and also configured to heat the tail gases with flue gases generated in the combustion chamber and connected to the tail gas discharge line L2; a tail gas purification unit, including a selective purification reactor, configured to purify the tail gases using gaseous ammonia as a reducing agent, connected to the tail gas supply line L2 and a purified tail gas discharge line.

[0036] Preferably, the line for removing the flow of purified tail gases is configured to remove a side flow of purified tail gases.

[0037] Preferably, the unit includes a purified tail gas cooling unit connected to a supply line for a side stream of purified tail gases.

[0038] Preferably, the tail gas cooling unit includes a tail gas cooling unit with water or by generating steam and a tail gas cooling unit with water.

[0039] Preferably, the selective purification reactor comprises a zeolite catalyst, preferably a honeycomb zeolite catalyst.

[0040] Preferably, the zeolite catalyst is Fe-zeolite or Si-zeolite, preferably Fe-ZSM-5, Fe-BEA, Fe-ZSM-12, Fe / HZSM-5, Fe-Ferrerite, Cu-ZSM-5, Cu-BEA, Cu-ZSM-12, Cu / HZSM-5, Cu-Ferrerite.

[0041] It is further noted that all advantages of the present invention stated in relation to the method for producing nitric acid are equally applicable to the claimed installation and are not repeated here in order to avoid unnecessary duplication.

[0042] Brief description of the drawings

[0043] The drawings are provided to better understand the invention, but it will be apparent to those skilled in the art that the disclosed invention is not limited to the embodiment shown therein. Fig. 1 shows a schematic diagram of a nitric acid production system with a tail gas purification unit according to the prior art.

[0044] Fig. 2 shows a diagram of the production of nitric acid with a tail gas purification unit according to the present invention.

[0045] Fig. 3 shows a tail gas purification unit for nitric acid production according to the present invention.

[0046] The best embodiment of the invention

[0047] The described embodiments are provided for illustrative purposes only. Those skilled in the art will readily recognize that other embodiments are possible without changing the essence of the invention.

[0048] In this application, a line is understood to mean a means for delivering a flow from one place to another, which, in particular, includes the pipes and connecting elements necessary for this, as well as, if necessary, control means and devices.

[0049] In this application, a block is understood to be a device or a set of devices that ensure the implementation of the function specified for a given block.

[0050] It is further noted that, unless otherwise indicated, all positions (blocks, devices, lines) of the process flow diagrams designated identically in Figs. 1-3 imply complete functional identity.

[0051] Example 1 (prototype).

[0052] Fig. 1 shows a schematic diagram of the nitric acid production process according to the prototype. A stream of liquid ammonia is fed to unit 1 for preparing gaseous ammonia via line 100 from the plant network (not shown). Unit 1 prepares the gaseous ammonia stream, which is then fed via line 103 to unit 3 for preparing the ammonia-air mixture.

[0053] The atmospheric air flow is drawn into the axial compressor OK via line 200 through the air intake pipe (not shown) into the air preparation unit 2 (air filter), where it is purified. The air entering the axial compressor is preheated by feeding it hot air from the supercharger CN.

[0054] From air preparation unit 2, the purified air flow is fed via line 201 to the axial compressor OK, where it is compressed and heated. The air flow is then fed via line 202 to the intermediate air cooler VP, where it is cooled by recirculating water (not shown), and then via line 203 to the centrifugal supercharger CN, where it is compressed and heated. The axial compressor OK and the centrifugal supercharger CN of the GTT-3M gas turbine unit are driven by the gas turbine GT, which is structurally integrated into the same casing as the compressor.

[0055] From the blower, the air flow is fed through line 303 to block 3 for preparing the ammonia-air mixture.

[0056] Also, air is sent through line 208 to the purge column 8 as additional air for the oxidation of nitrogen oxide into dioxide and the purge of nitrogen oxides from the product acid.

[0057] In addition, the air flow through line 204 is directed into the combustion chamber of the UKST turbine.

[0058] In addition, part of the air flow is directed to the gas turbine of the GTT-3M unit to cool the elements of the flow part of the housing (not shown).

[0059] Next, in block 3 for preparing the ammonia-air mixture, where a flow of air is supplied through line 303 and ammonia through line 103, an ammonia-air mixture is formed, which is then purified from impurities and supplied to contact apparatus 4 through line 304.

[0060] In the contact apparatus, ammonia is oxidized to nitric oxide (II) and, partially, to nitric oxide (IV). The hot nitrous gases formed during ammonia oxidation enter the waste heat boiler (not shown). In the waste heat boiler, the cooling of the nitrous gases causes the feedwater to evaporate, producing superheated steam (not shown).

[0061] The nitrous gas stream obtained in contact apparatus 4 is sent via line 400 to oxidizer O with a built-in tail gas heater (UGS2), where nitrogen oxides (II) are oxidized to nitrogen oxides (IV). Next, a portion of the nitrous gas stream is sent to UGS2 and cooled by heating the tail gases in UGS2. The second portion of the nitrous gas stream from O is bypassed by UGS2, then both streams are combined and fed via line 500 to the first-stage tail gas heater of the UGS. In the tail gas heater, the nitrous gases are cooled, heating the tail gases.

[0062] From the UGS, the flow of nitrous gases is fed through line 600 into the inter-tube space of the refrigerator-condensers 5, 6, where the flow is cooled by circulating water (not shown) coming from the absorption unit 7.

[0063] From the cooler-condensers, nitrous gases are fed via line 607 to absorption unit 7, where nitric acid is formed. The nitric acid stream after absorption unit 7 is fed to purge column 8, where nitrogen oxides are purged from it with hot make-up air. The purged (bleached) nitric acid from the purge column is discharged to one of the storage facilities via line 800. The purged gases from the purge column, to ensure complete oxidation of nitrogen oxide (II) (NO) to nitrogen oxide (IV) (NO2), are fed into the nitrous gas pipeline upstream of the absorption column via line 807.

[0064] The tail gas stream exiting the absorption column enters the UGS facility via line 706, where it is heated by nitrous gases. The tail gas stream is then sent via line 605 to the UGS2 tail gas heater, integrated into the oxidizer, where it is heated by the heat of the nitrous gases.

[0065] The heated tail gas stream is sent via line 606 to the selective purification reactor (RSO), where ammonia is also supplied via line 101. In the selective purification reactor, the process of reducing nitrogen oxides primarily to molecular nitrogen occurs at temperatures of 200–350°C.

[0066] A purified tail gas stream is generated in the RSO, which is fed via line 900 to the universal combustion chamber of the UKST turbine. The purified tail gas then enters the GT gas turbine via line 901. In the gas turbine, the internal energy of the gases is converted into mechanical energy (turbine rotational energy).

[0067] Next, the purified tail gases enter the gas turbine waste heat boiler (GTU) via line 902, then via line 904 to the economizer E, where they are cooled and then released into the atmosphere via line 905. In the waste heat boiler, superheated steam with a pressure of no more than 1.5 MPa is generated by cooling the purified gases, which is discharged via line 903 to the plant network (not shown). Heated deaerated chemically treated water (not shown) is used as feedwater for the waste heat boilers. The feedwater passes through the economizer E, where it is heated by the purified tail gases to a temperature and enters the GTU waste heat boiler. In the waste heat boiler, the heat from the purified tail gases causes the feedwater to evaporate, producing steam.

[0068] Example 2,

[0069] Fig. 2 shows a modernized flow chart for producing nitric acid according to the present invention. The steps of implementing the example, which coincide with the prototype, are not repeated in order to avoid duplication. The heated tail gas stream through line 608 (L1) is directed into the universal combustion chamber of the UKST turbine, where it is heated to 61СН-620°С by mixing it with flue gases obtained from the combustion of natural gas supplied through line 909, in a stream of air entering through line 204. The tail gas stream leaving the combustion chamber is diverted through line 609 (L2), mixed with a stream of ammonia entering through line 101, and fed to the RSO.

[0070] The purification process is carried out with ammonia at a temperature of 45CH-700°C on a zeolite catalyst with a honeycomb structure in a vertical reactor according to the following reactions:

[0071] 4NO + O2 + 4NH3 4N2 + 6H2O

[0072] NO + NO2 + 2NH3 2N2 + 3H2O

[0073] 6NO2+ 8NH37N2+ 12H2O

[0074] The presence of NOx will promote the decomposition of N2O:

[0075] 2N2O 2N2+ O2

[0076] The purified tail gas stream leaving the RSO is diverted via line 901 to the gas turbine (GT). In the gas turbine, the internal energy of the gases is converted into mechanical energy (turbine rotational energy).

[0077] A portion of the purified tail gas stream is diverted via line 906 to the purified tail gas cooling unit 9. In the purified tail gas cooling unit 9, the purified tail gas is cooled in two stages: in the first stage, by generating steam, and in the second stage, by using water.

[0078] At the first stage, cooling of purified tail gases can also be carried out by additional heating of the feedwater of the GKU boiler or by using circulating water.

[0079] In the second stage of cooling the purified tail gases, recycled water from the plant's water circulation cycle can be used, as well as chemically purified or partially desalinated water to feed the waste heat boiler from the chemical water treatment shop.

[0080] The cooled, purified tail gas stream is diverted via line 907 and sent further to cool turbine components (not shown). Compressed air is not supplied to cool the turbine components and is entirely consumed in the process to increase nitric acid production. Fig. 3 shows a tail gas purification unit according to the present invention.

[0081] All of the above lines are equipped with flow rate, temperature, and pressure meters. Lines 606, 608, and 905 are additionally equipped with automated gas analyzers capable of measuring nitrogen oxide content.

[0082] The table below shows the results of the experiment:

[0083] The table below shows the results of the experiment (volume flow rate is adjusted to standard conditions: 20°C, 1 atm):

[0084] Thus, the proposed group of inventions allows for:

[0085] - improving the efficiency of cleaning tail gases from nitric acid production by simultaneously reducing NO emissions X and N2O into the atmosphere

[0086] - increasing the energy efficiency of nitric acid production technology while improving the environmental performance of production

[0087] - NO reduction efficiency X at the system output > 98.5%

[0088] - N2O reduction efficiency at the system outlet > 97%

[0089] - increase in the service life of the catalyst, with calculated indicators from the start of operation: > 5 years after the first exposure to gas.

Claims

Invention formula 1. A method for purifying tail gases from nitric acid production, comprising a) a step of heating the tail gases in a combustion chamber using the heat of flue gases obtained from burning natural gas in the combustion chamber; b) a step of catalytically purifying the tail gases obtained in step a) in a selective purification reactor using gaseous ammonia as a reducing agent to obtain a stream of purified tail gases.

2. The method according to item 1, characterized in that part of the flow of purified tail gases obtained in step b) is diverted as a side flow of purified tail gases for cooling in a purified tail gas cooling unit and then sent to cool elements of the gas turbine.

3. The method according to item 2, characterized in that in the cooling unit for purified tail gases, two-stage cooling of the purified tail gases is carried out: in the first stage with water or by generating steam, and in the second stage with water.

4. The method according to claim 1, characterized in that a zeolite catalyst, preferably a zeolite catalyst with a honeycomb structure, is used in the selective purification reactor.

5. The method according to item 4, characterized in that Fe-zeolite or Cu-zeolite, preferably Fe-ZSM-5, Fe-BEA, Fe-ZSM-12, Fe / HZSM-5, Fe-Ferrerite, Cu-ZSM-5, Cu-BEA, Cu-ZSM-12, Cu / HZSM-5, Cu-Ferrerite, is used as the zeolite catalyst.

6. The method according to claim 1, characterized in that stage b) of catalytic purification of tail gases is carried out at a temperature of 450-700°C, preferably 550-600°C.

7. A tail gas purification unit for a nitric acid plant, comprising a tail gas heating unit, including a natural gas combustion chamber, and connected to a tail gas supply line L1, and also configured to heat the tail gases with flue gases generated in the combustion chamber, and connected to a tail gas discharge line L2; a tail gas purification unit, including a selective purification reactor, configured to purify the tail gases using gaseous ammonia as a reducing agent, connected to a tail gas supply line L2 and a purified tail gas discharge line.

8. The unit according to claim 7, characterized in that the line for removing the flow of purified tail gases is designed with the possibility of removing a side flow of purified tail gases.

9. The unit according to claim 8, characterized in that it includes a cooling unit for purified tail gases, connected to a supply line for a side stream of purified tail gases.

10. The unit according to paragraph 9, characterized in that the unit for cooling purified tail gases includes a unit for cooling tail gases with water or by generating steam and a unit for cooling tail gases with water.

11. The unit according to claim 7, characterized in that the selective purification reactor contains a zeolite catalyst, preferably a zeolite catalyst with a honeycomb structure.

12. The unit according to claim 11, characterized in that the zeolite catalyst is a Fe-zeolite or a Cu-zeolite, preferably Fe-ZSM-5, Fe-BEA, Fe-ZSM-12, Fe / HZSM-5, Fe-Ferrerite, Cu-ZSM-5, Cu-BEA, Cu-ZSM-12, Cu / HZSM-5, Cu-Ferrerite.

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