Tail gas circulation nitric acid production process

Abstract

The invention provides a nitric acid production method with tail gas circulation. The method comprises the following steps: enriching feed air to form oxygen-enriched gas, and mixing the oxygen-enriched gas with high-pressure tail gas absorbed by a nitric acid absorption tower to obtain mixed gas; mixing the preheated mixed gas in the reactor with the gasified ammonia raw material, and oxidizing or reducing nitrogen oxides in the tail gas into NO2, N2O5 and N2; recovering high-grade heat in the process gas at the outlet of the reactor; process gas is subjected to gas-liquid separation, a gas phase is subjected to heat exchange, and a liquid phase enters a nitric acid absorption tower and faces a concentration tower plate. According to the method, unit consumption and energy consumption in the nitric acid production process can be reduced, NOx in tail gas emission is reduced, and the investment and operation cost in the nitric acid production process is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical engineering processes, and particularly relates to a nitric acid production method with tail gas recycling. Background Art

[0002] Nitric acid is one of the important products in the basic chemical industry and has a wide range of uses. After hundreds of years of development, it has become an industrial system with mature technology, reasonable design, complete product specifications, comprehensive energy utilization, and environmental protection compliance. In the rapid development of the nitric acid industry, the double-pressure process has become the preferred process for many enterprises to newly build nitric acid plants due to its advanced and reliable technology, large production scale, low energy consumption, and excellent comprehensive economic indicators. In the double-pressure process for ammonia oxidation, a pressure of 0.35 - 0.4 MPa(G) is used, and for nitrogen oxide absorption, a pressure of 1.0 - 1.2 MPa(G) is used. This method is more suitable for the production process of higher-concentration nitric acid, with a higher absorption efficiency, and the nitric acid concentration can reach 68%.

[0003] In the simplified process of double-pressure nitric acid production, during the production process, the air compressor in the four-in-one unit is used to compress air, which is mixed with the raw materials after ammonia vaporization, so that the raw material mixture is compressed to a pressure of 0.35 - 0.4 MPa(G) and preheated to above 150 - 250 °C. The tail gas coming out of the absorption tower at 30 - 50 °C and 0.8 - 1 MPa(G) needs to pass through an ammonia reduction system to remove the unreacted NOx gas in the reaction system. It is necessary to gradually preheat the tail gas coming out of the absorption tower to above 300 °C, and then the NOx in the tail gas reacts with NH3 through a catalyst to be removed. On the one hand, this causes an increase in ammonia consumption and energy consumption. On the other hand, more heat exchange and reaction equipment are required, resulting in an increase in investment costs and on-site space.

[0004] Currently, in the double-pressure nitric acid preparation process, for the tail gas at 0.8 - 1 MPa(G) coming out of the absorption tower, which contains a small amount of unabsorbed NOx, it needs to go through a series of heat exchanges and ammonia reduction before it can be removed, resulting in an increase in ammonia unit consumption and energy waste in production, causing relatively large economic losses.

[0005] Therefore, how to reasonably recover the NOx gas in the discharged tail gas and utilize it resourcefully can not only reduce the material consumption and energy consumption in the nitric acid production process, reduce costs and increase efficiency, but also reduce the NOx in the tail gas emissions, reduce air pollution, and can also simplify the nitric acid process flow, reduce the equipment, land use, and device operation costs of the nitric acid process, which has important environmental protection significance and economic value. Summary of the Invention

[0006] One of the objectives of the present invention is to provide a nitric acid production method with tail gas recycling, which can reduce the unit consumption and energy consumption during nitric acid production, reduce NOx in tail gas emissions, and decrease the investment and operation costs during nitric acid production.

[0007] To achieve the above-mentioned invention objective, the present invention adopts the following technical solutions:

[0008] A nitric acid production method with tail gas recycling, the method comprising the following steps:

[0009] S1: Enrich the feed air to form an oxygen-rich gas, and mix it with the high-pressure tail gas after being absorbed by the nitric acid absorption tower to obtain a mixed gas;

[0010] S2: Mix the preheated mixed gas in the reactor with the vaporized ammonia raw material, and oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, N2;

[0011] S3: Recover the high-grade heat in the process gas at the reactor outlet;

[0012] S4: Perform gas-liquid separation on the process gas, heat exchange the gas phase, and feed the liquid phase into the nitric acid absorption tower for the concentration tray.

[0013] In the present invention, it contains incompletely absorbed NO, N2O, NO2, N2O3 and N2O5, and oxidize NO and N2O3 to NO2 and N2O5, the specific reaction formulas are:

[0014] NO + O2 → NO2, N2O3 + O2 → N2O5

[0015] The products generated by the reaction are absorbed again to convert them into nitric acid; reduce N2O to N2, the specific reaction formula is:

[0016] N2O + NH3 → N2 + H2O

[0017] And NH3 and O2 do not react under this condition.

[0018] In an embodiment of the present invention, the pressure of the enriched air in S1 is 0.01 - 0.6 MPaG; the volume concentration of the air enrichment is 30 - 70%.

[0019] In an embodiment of the present invention, the oxygen volume content in the mixed gas in S1 is 19 - 23%.

[0020] In an embodiment of the present invention, the high-pressure tail gas in S1 contains incompletely absorbed NO, N2O, NO2, N2O3 and N2O5; preferably, the volume ratio of the total amount of incompletely absorbed nitrogen oxides in the high-pressure tail gas is 3000 - 5000 ppm, based on the composition when the high-pressure tail gas is discharged.

[0021] In one embodiment of the present invention, the preheating temperature in S2 is 180°C - 250°C.

[0022] In one embodiment of the present invention, the volume ratio of the mixed gas to the gasified ammonia raw material in S2 is 0.09 - 0.11.

[0023] In one embodiment of the present invention, the reaction temperature in S2 is 180 - 350°C, and the pressure is 0.01 - 0.6 MPaG. Under these conditions, NO and N2O3 are respectively oxidized to NO2 and N2O5, and N2O is reduced to N2, while NH3 and O2 do not react under these conditions.

[0024] In one embodiment of the present invention, the high-grade heat of 380 - 450°C in the recycled process gas is recovered in S3.

[0025] In one embodiment of the present invention, after heat exchange, the gas phase in S4 is compressed by a NO2 compressor and then sent into the absorption tower.

[0026] In one embodiment of the present invention, after condensation and separation of the gas phase in S4, the liquid phase enters the nitric acid absorption tower for the concentration tray.

[0027] Another object of the present invention is to provide a use of a nitric acid production method with tail gas recycling.

[0028] A use of a nitric acid production method with tail gas recycling, the method being the above-mentioned method, and the method being used for a nitric acid production method of an ammonia reduction system that does not require tail gas recycling and does not require tail gas treatment.

[0029] Compared with the prior art, the positive effects of the present invention are as follows:

[0030] (1) Reduce material consumption and energy consumption during nitric acid production, and reduce costs and improve efficiency;

[0031] (2) Reduce NOx in tail gas emissions and reduce air pollution;

[0032] (3) Simplify the nitric acid process flow, and reduce the equipment, land, and device operation costs of the nitric acid process. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the production process of the nitric acid production method with tail gas recycling of the present invention. DETAILED DESCRIPTION

[0034] The following examples are used to further illustrate the specific implementation of the present method. However, the present invention is not limited to the listed examples, and should also include any other well-known changes within the scope of the rights required by the present invention.

[0035] Nitrogen Oxide Detection Instruments and Methods: Agilent (8890) Gas Chromatograph.

[0036] Example 1

[0037] As Figure 1 shown in the process flow chart, the volume concentration of oxygen in the air is 70%, the pressure is 0.01 MPa (G), the volume ratio of the recycle tail gas to the discharged tail gas is 9.1, the volume ratio of the feed air to the recycle tail gas is 0.33, the oxygen volume content in the mixed air after mixing is 23%, the volume ratio of the mixed gas to the gasified ammonia raw material is 0.11, the process gas temperature before entering the oxidation furnace is controlled at 350 °C, and NO and N2O3 with a volume ratio of 3000 ppm in the tail gas are oxidized to NO2 and N2O5 to eliminate NO and N2O3 in the recycle tail gas, and N2O decomposes to produce N2. In this example, the energy and the recovery utilization rate of NO and N2O3 in the tail gas are 99.1%.

[0038] The high-pressure tail gas discharged from the top of the nitric acid absorption tower and the enriched feed air form an oxygen-rich gas to obtain a mixed gas; the preheated mixed gas in the reactor is mixed with the gasified ammonia raw material to oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, and N2; the process gas passing through the reactor is subjected to a series of heat exchanges to recover heat, on the one hand, heating the mixed raw material gas, and on the other hand, generating steam. Then, the process gas is subjected to gas-liquid separation, the gas phase is cooled and compressed, and enters the bottom of the absorption tower, and the liquid phase enters the nitric acid absorption tower for the concentration tray.

[0039] For the tail gas after absorption, according to the above requirements, the circulation ratio is controlled. Part of it returns to the air feed system to become a pressure-increased supplement and recover the incompletely absorbed NOx; the other part enters the tail gas turbine to recover a part of the energy. The pressure of this part of the tail gas after the turbine is 30 KPa (G), and the energy recovery rate is 95%.

[0040] Example 2

[0041] As Figure 1 shown in the process flow chart, the volume concentration of oxygen in the air is 60%, the pressure is 0.05 MPa (G), the volume ratio of the recycle tail gas to the discharged tail gas is 5.5, the volume ratio of the feed air to the recycle tail gas is 0.4, the oxygen volume content in the mixed air after mixing is 22%, the volume ratio of the mixed gas to the gasified ammonia raw material is 0.105, the process gas temperature before entering the oxidation furnace is controlled at 320 °C, and NO and N2O3 with a volume ratio of 3500 ppm in the tail gas are oxidized to NO2 and N2O5 to eliminate NO and N2O3 in the recycle tail gas, and N2O decomposes to produce N2. In this example, the energy and the recovery utilization rate of NO and N2O3 in the tail gas are 99.4%.

[0042] The high-pressure tail gas discharged from the top of the nitric acid absorption tower forms an oxygen-rich gas with the enriched feed air to obtain a mixed gas; the preheated mixed gas in the reactor is mixed with the gasified ammonia raw material to oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, and N2; the process gas passing through the reactor is subjected to a series of heat exchanges to recover heat, on the one hand, heating the mixed raw material gas, and on the other hand, used to generate steam. Then, the process gas is subjected to gas-liquid separation, the gas phase is cooled and compressed, and enters the bottom of the absorption tower, and the liquid phase enters the nitric acid absorption tower for concentration trays.

[0043] For the tail gas after absorption, according to the requirements, the circulation ratio is controlled. A part returns to the air feed system to become a pressure-increased supplement and recover the incompletely absorbed NOx therein; another part enters the tail gas turbine to recover a part of the energy. The pressure of this part of the tail gas after the turbine is 30 KPa(G), and the energy recovery rate is 95.1%.

[0044] Example 3

[0045] As Figure 1 shown in the process flow chart, the volume concentration of oxygen-rich in the air is 50%, the pressure is 0.1 MPa(G), the volume ratio of the circulating tail gas to the discharged tail gas is 3.3, the volume ratio of the feed air to the circulating tail gas is 0.55, the oxygen volume content in the mixed air after mixing is 21%, the volume ratio of the mixed gas to the gasified ammonia raw material is 0.1, the temperature of the process gas before entering the oxidation furnace is controlled at 280 °C, oxidize NO and N2O3 with a volume ratio of 4000 ppm in the tail gas into NO2 and N2O5, eliminate NO and N2O3 in the circulating tail gas, and N2O decomposes to produce N2. In this example, the recovery rate of energy and NO and N2O3 in the tail gas is 99.2%.

[0046] The high-pressure tail gas discharged from the top of the nitric acid absorption tower forms an oxygen-rich gas with the enriched feed air to obtain a mixed gas; the preheated mixed gas in the reactor is mixed with the gasified ammonia raw material to oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, and N2; the process gas passing through the reactor is subjected to a series of heat exchanges to recover heat, on the one hand, heating the mixed raw material gas, and on the other hand, used to generate steam. Then, the process gas is subjected to gas-liquid separation, the gas phase is cooled and compressed, and enters the bottom of the absorption tower, and the liquid phase enters the nitric acid absorption tower for concentration trays.

[0047] For the tail gas after absorption, according to the requirements, the circulation ratio is controlled. A part returns to the air feed system to become a pressure-increased supplement and recover the incompletely absorbed NOx therein; another part enters the tail gas turbine to recover a part of the energy. The pressure of this part of the tail gas after the turbine is 30 KPa(G), and the energy recovery rate is 95.2%.

[0048] Example 4

[0049] AsFigure 1 As shown in the process flow chart, the enriched volume concentration of oxygen in the air is 40%, the pressure is 0.2 MPa (G), the volume ratio of the recycled tail gas to the discharged tail gas is 2, the volume ratio of the feed air to the recycled tail gas is 0.82, the oxygen volume content in the mixed air after mixing is 20%, the volume ratio of the mixed gas to the gasified ammonia raw material is 0.098, the process gas temperature before entering the oxidation furnace is controlled at 200 °C, NO and N2O3 with a volume ratio of 4500 ppm in the tail gas are oxidized into NO2 and N2O5, NO and N2O3 in the recycled tail gas are eliminated, and N2O decomposes to produce N2. In this example, the energy and the recovery utilization rate of NO and N2O3 in the tail gas are 99.1%.

[0050] The high-pressure tail gas discharged from the top of the nitric acid absorption tower and the enriched feed air form an oxygen-rich gas to obtain a mixed gas; the preheated mixed gas in the reactor is mixed with the gasified ammonia raw material, and the nitrogen oxides in the tail gas are oxidized or reduced to NO2, N2O5, and N2; the process gas passing through the reactor is subjected to a series of heat exchanges to recover heat. On the one hand, it heats the mixed raw material gas, and on the other hand, it is used to generate steam. Then, the process gas is separated into gas and liquid. The gas phase is cooled and compressed and enters the bottom of the absorption tower, and the liquid phase enters the nitric acid absorption tower for the concentration tray.

[0051] After the absorbed tail gas, according to the requirements, the circulation ratio is controlled. Part of it returns to the air feed system to become a supplementary with increased pressure and recover the incompletely absorbed NOx in it; the other part enters the tail gas turbine to recover part of the energy. The pressure of this part of the tail gas after the turbine is 30 KPa (G), and the energy recovery rate is 94.5%.

[0052] Example 5

[0053] As Figure 1 As shown in the process flow chart, the enriched volume concentration of oxygen in the air is 30%, the pressure is 0.3 MPa (G), the volume ratio of the recycled tail gas to the discharged tail gas is 0.85, the volume ratio of the feed air to the recycled tail gas is 1.66, the oxygen volume content in the mixed air after mixing is 19%, the volume ratio of the mixed gas to the gasified ammonia raw material is 0.09, the process gas temperature before entering the oxidation furnace is controlled at 180 °C, NO and N2O3 with a volume ratio of 5000 ppm in the tail gas are oxidized into NO2 and N2O5, NO and N2O3 in the recycled tail gas are eliminated, and N2O decomposes to produce N2. In this example, the energy and the recovery utilization rate of NO and N2O3 in the tail gas are 99.3%.

[0054] The high-pressure tail gas discharged from the top of the nitric acid absorption tower forms an oxygen-rich gas with the enriched feed air to obtain a mixed gas; the preheated mixed gas in the reactor is mixed with the gasified ammonia raw material to oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, and N2; the process gas passing through the reactor is subjected to a series of heat exchanges to recover heat. On the one hand, it heats the mixed raw material gas, and on the other hand, it is used to generate steam. Then, the process gas is separated into gas and liquid. The gas phase is cooled and compressed and enters the bottom of the absorption tower, and the liquid phase enters the nitric acid absorption tower for the concentration tray.

[0055] After the absorption of the tail gas, according to the requirements, the circulation ratio is controlled. A part of it returns to the air feed system to become a pressure-increased supplement and recover the incompletely absorbed NOx therein; another part enters the tail gas turbine to recover a part of the energy. The pressure of this part of the tail gas after the turbine is 30 KPa(G), and the energy recovery rate is 95%.

[0056] Comparative Example 1

[0057] In the existing conventional process, the raw material gas used is air. After reacting with ammonia, it generates NO, NO2, and other N x O y , and after a series of heat exchanges, it is absorbed by desalted water in the nitric acid absorption tower to produce nitric acid. The incompletely absorbed NO, NO2, and other N x O y After leaving the absorption tower, it needs to undergo a series of heat exchanges, be heated to 400 °C, and enter the ammonia reduction system, consuming a certain amount of ammonia to denitrify NO and NO2 to generate nitrogen, while N2O and other N x O y After passing through the turbine with the process tail gas, it is directly discharged into the atmosphere. The complex heat exchange system and ammonia reduction system therein cause construction land, equipment investment, energy waste, and environmental pollution.

Claims

1. A nitric acid production method with tail gas circulation, characterized in that The method comprises the following steps: S1: Enrich the feed air to form an oxygen-rich gas, mix it with the high-pressure tail gas after being absorbed by the nitric acid absorption tower, and obtain a mixed gas; S2: Mix the preheated mixed gas in the reactor with the gasified ammonia raw material, and oxidize or reduce the nitrogen oxides in the tail gas to NO2, N2O5, N2; S3: Recover the high-grade heat in the process gas at the reactor outlet; S4: Separate the process gas into gas and liquid phases, heat exchange the gas phase, and feed the liquid phase into the nitric acid absorption tower for the concentration tray; 2. The method according to claim 1, wherein In S1, the pressure of the enriched air is 0.01 - 0.6 MPaG; the volume concentration of the air enrichment is 30 - 70%; And / or, the oxygen volume content in the mixed gas in S1 is 19 - 23%; And / or, the high-pressure tail gas in S1 contains unabsorbed NO, N2O, NO2, N2O3, and N2O5; Preferably, the volume ratio of the total amount of unabsorbed nitrogen oxides in the high-pressure tail gas is 3000 - 5000 ppm, based on the composition of the high-pressure tail gas when discharged; 3. The method according to claim 1, characterized in that, In S2, the preheating temperature is 180°C - 250°C; And / or, the volume ratio of the mixed gas to the gasified ammonia raw material in S2 is 0.09 - 0.11; And / or, in S2, the reaction temperature is 180 - 350°C, the pressure is 0.01 - 0.6 MPaG. Under this condition, NO and N2O3 are respectively oxidized to NO2 and N2O5, N2O is reduced to N2, and NH3 and O2 do not react under this condition; 4. The method according to claim 1, wherein In S3, recover the high-grade heat of 380 - 450°C in the process gas; 5. The method according to claim 1, characterized in that, In S4, after the gas phase is heat exchanged, it is compressed by a NO2 compressor and then fed into the absorption tower; And / or, after the gas phase in S4 is condensed and separated, the liquid phase is fed into the nitric acid absorption tower for the concentration tray; 6. Use of a nitric acid production method with tail gas recycling, the method being the method according to any one of claims 1 - 5, and the method being used for a nitric acid production method of an ammonia reduction system without tail gas recycling and without tail gas treatment.

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