Method and production plant for producing nitric acid

Abstract

In a method for producing nitric acid, nitrogen oxides are first created in an ammonia combustion plant (2) by burning ammonia with primary air and are cooled in a condenser (3), thereby producing a solution containing nitric acid. The solution containing nitric acid is then supplied to at least one absorption tower, (4, 5), in which the nitrogen oxides are brought into contact with water and oxygen, the nitrogen-containing gas mixture reacting at least partially with the water and the oxygen, forming an aqueous solution containing nitric acid, which solution collects at the bottom of the absorption tower, (4, 5) and is then compressed and supplied back to the absorption tower (4, 5) via a line (13, 15, 21). To minimise the formation of dinitrogen monoxide during ammonia oxidation in such a plant and to increase the efficiency of the process, it is proposed in accordance with the invention to use a gas, the oxygen content of which is between 21.3 vol.% and 35 vol.% as an oxidant in the ammonia combustion plant (2). According to the invention, this oxidant is produced by introducing pure oxygen or an oxygen-rich gas into the primary air or into the ammonia combustion plant (2).

Description

[0001] Process and production plant for the production of nitric acid

[0002] The present invention relates to a method for producing nitric acid according to the preamble of claim 1. The invention further relates to a production plant for producing nitric acid according to the preamble of claim 8.

[0003] The invention thus relates to a process or plant for the industrial production of nitric acid, in which a multi-stage catalytic ammonia oxidation process (Ostwald process) is used. In the first step, ammonia and oxygen are reacted in a reactor (hereinafter also referred to as an "ammonia combustion plant") over a network catalyst, usually consisting of precious metals, for example platinum-rhodium, to form nitrogen monoxide and water vapor:

[0004] This reaction is carried out at high temperatures, for example, around 890 °C. Air (hereinafter also referred to as "primary air") is typically used as the oxidant. Generally, a superstoichiometric amount of air is used to prevent the formation of an explosive mixture and to provide additional oxygen for subsequent oxidation reactions. The atmospheric nitrogen carried in the primary air does not participate in the reactions described here, or only to a negligible extent. The discharge from the ammonia combustion plant is cooled in a condenser by heat exchange with a cooling medium to a temperature at which some of the components contained in the process gas stream condense. In this process, some of the nitrogen monoxide reacts with water and oxygen to form an aqueous solution containing nitric acid, which also contains nitrogen oxides, particularly nitrogen monoxide.The remaining, non-dissolving gas mixture is fed, optionally after passing through one or more optionally available oxidation column(s), to an absorption tower (column) in which a portion of the gaseous nitrogen monoxide is oxidized with oxygen, which is provided in the form of atmospheric oxygen or in the form of pure oxygen, to nitrogen dioxide or its dimer dinitrogen tetroxide, which are then reacted with water to form nitric acid.

[0005] The water, or the resulting nitric acid solution (weak acid), typically flows through the absorption tower countercurrently to the rising gas stream. The liquid phase, which previously formed in the condenser, is usually fed onto one of the lower trays of the column. The nitric acid collects at the bottom of the absorption tower in an aqueous solution. This nitric acid is then fed to the top of the bleaching column. Air is introduced countercurrently into the bleaching column to drive off any remaining nitrous gases in the solution. In many cases, several absorption towers are connected in series, with the gas stream or nitric acid flowing through the series countercurrently. To increase the solubility of the nitrous gases, the absorption tower(s) are operated at a higher pressure of 1 to 15 bar(g).In plants whose absorption towers operate at a relatively low pressure of 1 to 5 bar(g) (low- and medium-pressure plants), the proportion of nitrous exhaust gases is comparatively high. While applying higher pressures reduces the residual nitrogen oxide content in the exhaust gas, this entails considerable additional costs for compression and the corresponding design of plant components suitable for higher pressures. EP 0 799 794 A1, EP 1 013 604 B1, and EP 2 953 894 A1 propose introducing pure oxygen at various points in the nitric acid production process outlined above to increase process efficiency, improve the quality of the nitric acid produced, or reduce the formation of undesirable NOx gases. Another possibility for increasing efficiency is the use of ozone, as described, for example, in EP 4 126 761 A1.

[0006] EP 0 808 797 A1 and EP 2 953 894 A1 also mention the possibility of using pure oxygen instead of air as an oxidizing agent in the ammonia combustion plant, or of introducing pure oxygen into the primary air supply. However, this would result in significantly higher temperatures in the ammonia combustion chamber, which could damage or even destroy the catalyst and could only be avoided with extensive temperature control measures. Therefore, this approach has been avoided in practice to date.

[0007] Another problem in the production of nitric acid is the formation of climate-damaging nitrous oxide (N2O), which is produced as a byproduct during the oxidation of ammonia and which has to be separated using secondary or tertiary catalysis.

[0008] The object of the present invention is therefore to reduce the N2O content in the process gas during the production of nitric acid compared to conventional production methods, and to increase the efficiency of the process.

[0009] This problem is solved by a method with the features of claim 1 and by a production plant for the manufacture of nitric acid with the features of claim 8. Advantageous embodiments of the invention are specified in the dependent claims.

[0010] The inventive process for producing nitric acid is therefore characterized in that an oxidizing agent is supplied to the ammonia combustion plant as a whole, the oxygen content of which is between 21.5 vol.% and 35 vol.%, preferably between 22 vol.% and 32 vol.%, particularly preferably between 23% and 28 vol.%, and the remainder of which consists of gases that are largely inert with respect to the nitric acid production process.

[0011] The term "gases largely inert with respect to the nitric acid production process," hereinafter referred to simply as "inert gases," refers to gaseous components that are carried along with oxygen in the oxidizing agent stream but contribute little or nothing to the reactions of the Ostwald process described above. Examples include nitrogen, noble gases, and other components of air.

[0012] The term "oxidizing agent supplied to the ammonia combustion plant" here refers to the totality of all oxygen-containing gases or gas mixtures that are supplied directly or indirectly, via one or more supply lines, to the ammonia combustion plant for the purpose of burning ammonia.

[0013] According to the invention, an oxidizing agent is supplied to the ammonia combustion plant whose oxygen content is only slightly higher than that of air. Surprisingly, it has been found that such a moderately increased oxygen concentration of the oxidizing agent, compared to the use of air, leads to a significant reduction in N₂O formation by up to 50%. At the same time, the increased oxygen concentration of the oxidizing agent leads to increased reaction efficiency in the direction of the desired products in the process, in particular nitric oxide (NO).

[0014] Due to the higher oxygen content compared to air, the proportions of inert gases in the oxidizer are correspondingly reduced. For example, if primary air (with a composition of 78 vol.% N2, 21 vol.% O2 and 1 vol.% other gases) and pure oxygen (with an oxygen content of at least 95 vol.%) are supplied to the ammonia combustion plant in a volume flow ratio of between 100:1 and 100:20, the composition of the total oxidizer supplied to the ammonia combustion plant changes to values ​​between approximately 77.2 vol.% N2, 21.8 vol.% O2 and 1 vol.% other gases, and approximately 65.1 vol.% N2, 34.1 vol.% O2 and 0.8 vol.% other gases.

[0015] Preferably, the oxidizing agent supplied to the ammonia combustion plant is air ("primary air") to which pure oxygen or an oxygen-rich gas has been added, so that the oxygen content in the total oxidizing agent supplied to the ammonia combustion plant corresponds to a value from the ranges mentioned above.

[0016] By appropriately selecting the temperatures of the preheated primary air and the added pure oxygen or oxygen-rich gas, the reaction temperature in the ammonia combustion plant can be regulated to the desired temperature, for example, approximately 890 °C. Any temperature increase that might be expected and could cause damage to the grid catalyst and other plant components was not observed in tests.

[0017] Here, "pure oxygen" refers to oxygen in gaseous form with a purity of at least 95% by volume. "Oxygen-rich gas" refers to a gas mixture whose oxygen content exceeds that of air (21% by volume), for example, oxygen-enriched air with an oxygen content of 22% to 50% by volume.

[0018] Preferably, for the production of the oxidizing agent, pure oxygen or oxygen-rich gas is injected, for example, via a lance into a primary air supply line. In this case, the primary air is first enriched with oxygen before the enriched primary air is fed to the ammonia combustion plant. Alternatively or additionally, however, it is also conceivable to feed the pure oxygen or oxygen-enriched gas in the appropriate quantity directly into the ammonia combustion plant and / or to add it to the ammonia stream.

[0019] Furthermore, it is advantageous that the volume flow of pure oxygen or oxygen-rich gas supplied to the ammonia combustion plant in addition to the primary air has the lowest possible temperature, preferably below 10°C, and particularly preferably below 0°C. This is easily achievable if the oxygen is provided in a cryogenically liquefied state and evaporated to a gas of the appropriate temperature before being supplied to the primary air supply or the ammonia combustion plant. An air evaporator is generally used to evaporate the liquid oxygen; however, process streams from the plant itself that require cooling can also be used for this purpose. For example, the liquid oxygen can be used as a cooling medium in the condenser and evaporated in the process.

[0020] In a preferred embodiment of the invention, the volume flow of the oxidizing agent supplied to the ammonia combustion plant contains a superstoichiometric amount of oxygen relative to the ammonia flow supplied, in relation to reaction (1). This further enhances the N₂O reduction effect. While a superstoichiometric supply of primary air can also reduce the IShO values ​​to some extent, this effect is considerably weaker than that achieved by the slightly increased oxygen concentration in the oxidizing agent according to the invention (or by the combination of both measures). Furthermore, the process efficiency suffers due to the relatively higher proportion of inert gases that must be supplied.

[0021] In a further advantageous embodiment of the invention, the procedure described, for example, in EP 2 953 894 A1 or EP 4 126 761 A1 is used, in which ozone and / or oxygen are introduced into the connecting line and / or a conveying line, through which a solution containing nitric acid is drawn from the bottom of the first absorption tower and fed to an upper section of the first absorption tower and / or a bleaching column. This further increases the process efficiency.

[0022] The object of the invention is also solved by a production plant with the features of claim 8.

[0023] A production plant according to the invention for the manufacture of nitric acid comprises an ammonia combustion plant equipped with a supply line for ammonia and a supply line for primary air for the conversion of ammonia with oxygen to nitrogen oxides and water vapor, a condenser connected to the ammonia combustion plant for cooling the reaction products from the ammonia combustion plant, wherein at least a portion of the reaction products condenses, at least one absorption tower arranged downstream of the condenser for washing the gas mixture formed in the condenser with water or an aqueous solution, at least one connecting line for supplying a solution containing nitric acid from the condenser to the at least one absorption tower, and a conveying line connecting the absorption tower or one of the absorption towers to a bleaching column for conveying crude acid, and is characterized according to the invention in thatthat the ammonia combustion plant is connected to at least one supply line for pure oxygen and / or for an oxygen-rich gas, which leads into the supply line for primary air and / or into the supply line for ammonia and / or directly into the ammonia combustion plant. Optionally, one or more oxidation columns are arranged between the condenser and the (first) absorption tower in the flow path of the gaseous reactants to support oxidation.

[0024] The device according to the invention is particularly intended and suitable for carrying out the method according to the invention and has at least one supply line connected to a source of pure oxygen or an oxygen-rich gas, by means of which pure oxygen or an oxygen-rich gas (according to the definitions above) can be fed directly or indirectly into the ammonia combustion plant in addition to the primary air, in order to increase the oxygen content of the total oxidizer supplied to the ammonia combustion plant. If several supply lines are present, one or more of them can open into the supply line for primary air, while another supply line or several other supply lines open into the supply line for ammonia and / or directly into the ammonia combustion device.

[0025] If the supply line connects to the primary air supply line, it is advantageous to position the connection downstream of a compressor located in the primary air supply line. Also advantageous are means that enable intensive mixing of primary air and supplied pure oxygen or oxygen-rich gas before the primary air reaches the ammonia combustion plant. For example, the pure oxygen or oxygen-rich gas can be introduced via a lance that exits into the primary air supply line against the primary air flow.

[0026] The device according to the invention preferably comprises a control unit, preferably electronic, by means of which the oxygen content and / or the temperature of the oxidizing agent can be adjusted as a function of a measured control parameter. The measurement of the control parameter, for example the temperature in the ammonia combustion plant or a concentration of a specific component, for example N₂O, in the exhaust gas of the device, is carried out continuously or at predetermined time intervals by means of a sensor that transmits the measurement data to the control unit. The control unit processes the measurement data according to a predetermined program and issues control commands to other components of the device.For example, the control unit actuates a valve that is located in at least one supply line for pure oxygen or for oxygen-rich gas, and by means of which the volume flow of the oxidizing agent can be continuously adjusted to the respective requirements.

[0027] An embodiment of the invention will be explained in more detail with reference to the drawing. The single drawing (Fig. 1) schematically shows the circuit diagram of a production plant according to the invention for manufacturing nitric acid.

[0028] The production plant 1 for the manufacture of nitric acid shown in Fig. 1 comprises, in a manner known per se, an ammonia combustion plant 2, a condenser 3, several (in the exemplary embodiment two) absorption towers 4, 5, and a bleaching column 6. In the exemplary embodiment, the absorption towers 4, 5 are low- and medium-pressure columns operating at a pressure of 2 to 5 bar(g); however, medium- or high-pressure columns with an operating pressure of up to 15 bar(g) can also be used. The ammonia combustion plant 2 serves to convert gaseous ammonia and oxygen to nitrogen monoxide and water vapor at a temperature between 600°C and 950°C over a network catalyst made of a noble metal, such as platinum or a platinum / rhodium alloy. The oxygen is usually supplied in the form of air (primary air), which is fed into the ammonia combustion plant 2 via an air supply line 7.The ammonia is supplied to the ammonia combustion plant 2 via an ammonia supply line 9. The temperature in the ammonia combustion plant 2 is usually regulated by preheating the primary air flow in the air supply line 7 by means of a heating device not shown here.

[0029] In the embodiment of the invention shown here, the primary air guided through the air supply line 7 is also enriched with pure oxygen, which is fed into the air supply line 7 via an oxygen supply line 8.

[0030] The reaction products of the reaction taking place in the ammonia combustion plant 2, essentially nitrogen monoxide and water vapor, as well as excess oxygen, are fed to the condenser 3. There, the reaction products are cooled by indirect thermal contact with a cooling medium, such as water or a cryogenic medium, supplied via a cooling medium supply line 10, to a temperature at which at least some of the water vapor condenses, for example, to 60°C to 80°C. The cooling medium, heated during heat exchange, is discharged via a cooling medium outlet 11 and released into the atmosphere or used for another purpose. Some of the nitrogen oxides react with the water to form nitric acid, which precipitates at the bottom of the condenser 3 in an aqueous solution. The gas mixture present in the condenser 3 is introduced as process gas via a gas supply line 12 into a lower section of the absorption tower 4.A portion of the nitric oxide is oxidized with excess oxygen to nitrogen dioxide or its dimer, dinitrogen tetroxide. The aqueous solution containing nitric acid from the bottom of the condenser 3 is fed via a connecting line 13 to a higher section of the absorption tower 4 compared to the inlet of the gas supply line 12. If the connecting line 13 is a riser, a pumping device 14 provides the necessary pressure to overcome the hydrostatic pressure.

[0031] The aqueous solution containing nitric acid from condenser 3 is sprayed into absorption tower 4 via a nozzle arrangement (not described in detail here). It sinks to the bottom and comes into contact with the nitrogen oxide-containing process gases rising from below. Further nitrogen oxides in the gas mixture react to form nitric acid, which accumulates in an aqueous solution at the bottom of absorption tower 4. This aqueous solution containing nitric acid is discharged via line 15, transported by a conveying device 16 to the bleaching column 6, and sprayed into it. Nitrogen oxide-containing gas is again produced in the bleaching column 6 and is introduced via line 18 into the gas supply line 12 and from there into the absorption tower 4. The product, the bleached acid, is discharged via a product outlet 17.

[0032] The nitrogen oxide-containing gas mixture remaining in absorption tower 4 is discharged via a process gas line 19 and introduced into a lower section of absorption tower 5. Simultaneously, water from a water supply line 20 is sprayed into the headspace of absorption tower 5. The nitrogen oxide-containing gas mixture rising from below comes into contact with the sprayed water in absorption tower 5 and reacts, at least partially, with it to form nitric acid, which accumulates at the bottom of absorption tower 5 in an aqueous solution. This aqueous solution containing nitric acid is discharged via a riser line 21 and conveyed to the headspace of absorption tower 4 by means of a conveying device 22, where it is sprayed in and passes through absorption tower 4 countercurrently to the process gas flow, thereby forming increasingly concentrated nitric acid.Any remaining gas mixture in absorption tower 5 is discharged via an exhaust gas line 23 and fed to a denitrification device (not shown here) in which the remaining nitrogen oxides are largely removed from the gas mixture.

[0033] For the sake of clarity, the embodiment shown in Fig. 1 comprises only two absorption towers 4, 5; embodiments with three or more absorption towers are of course also conceivable within the scope of the invention, through which the nitrogen oxide-containing gas streams and the aqueous, nitric acid-containing solutions pass in countercurrent flow in a known manner. Likewise, the nitrogen oxide-containing gas streams can pass through one or more oxidation columns arranged in the gas supply line 12 before reaching the absorption tower 4; these are not shown here.

[0034] As mentioned above, in the embodiment shown here, pure oxygen is additionally introduced into the process. This is taken from an oxygen source, for example a tank 24, in a cryogenically liquefied state, passes through an air evaporator 25 in a manner known per se, and is supplied cold, but in gaseous form, via the oxygen supply line 8. Instead of using an air evaporator 25, the liquid oxygen can also be used, for example, as a cooling medium in the condenser 3 and thereby evaporated (not shown here). The temperature of the pure oxygen supplied via the oxygen supply line 8 is, for example, between -100°C and 10°C. In the embodiment shown here, the pure oxygen is fed from the oxygen supply line 8 into the air supply line 7.For this purpose, a lance (not shown here) is used, for example, which exits into the air supply line 7 in the opposite direction to the primary airflow, thus ensuring thorough mixing of the primary air and the supplied pure oxygen. Alternatively, the pure oxygen can also be fed directly into the ammonia combustion unit 2 or into the ammonia supply line 9 (not shown here) via a lance, or an additional oxygen line can be provided for this purpose.

[0035] Furthermore, cold, gaseous oxygen can be supplied via oxygen supply lines 26, 27 to line 15 and / or riser line 21 and / or an air supply line 28 leading to bleaching column 6 to support the oxidation processes in absorption tower 4 and / or bleaching column 6. Instead of or in addition to oxygen, ozone or an ozone-containing gas can be introduced into one or more of lines 13, 15, 21, 28, which also increases the efficiency of the process. The advantage of such a procedure is described in EP 2 953 894 A1 or EP 4 126 761 A1, to which explicit reference is made here. The volumetric flow rate of the oxygen supplied via oxygen supply line 8 can be regulated by means of an electronic control unit 30.For this purpose, the control unit 30 is in data communication with a sensor 31 for recording a measurement parameter; for example, the sensor 31 serves to record a temperature in the ammonia combustion plant 2. According to a predefined program, the control unit 30 determines a suitable value for the volume flow in the oxygen supply line 8 and regulates this by means of corresponding control commands to a valve 32 arranged in the oxygen supply line 8.

[0036] The invention is particularly suitable for retrofitting existing systems and leads to a significant reduction in the N2O concentration.

[0037] List of reference signs

[0038] 1 production plant

[0039] 2 Ammonia combustion plants

[0040] 3 Capacitor

[0041] 4 Absorption tower

[0042] 5 Absorption tower

[0043] 6 bleaching columns

[0044] 7 Air supply

[0045] 8 Oxygen supply

[0046] 9 Ammonia supply line

[0047] 10 Cooling medium supply line

[0048] 11 Cooling medium discharge

[0049] 12 Gas supply line

[0050] 13 Connecting line

[0051] 14 Funding facility

[0052] 15 Management

[0053] 16 Funding institution

[0054] 17 Product Discharge

[0055] 18 Management

[0056] 19 Process gas pipeline

[0057] 20 Water supply

[0058] 21 Riser

[0059] 22 Funding institution

[0060] 23 Exhaust pipe

[0061] 24 Tank

[0062] 25 air evaporators

[0063] 26 Oxygen line

[0064] 27 Oxygen line

[0065] 28 Air supply line

[0066] 29 -

[0067] 30 control unit

[0068] 31 Sensor

[0069] 32 Valve

Claims

Patent claims 1. A process for producing nitric acid, in which a. ammonia and an oxygen-containing oxidizing agent are supplied to an ammonia combustion plant (2), wherein oxygen from the oxidizing agent reacts with ammonia to form nitrogen oxides and water vapor, b. the nitrogen oxides and the water vapor from step (a.) are cooled in a condenser (3) to a temperature at which at least a portion of the water vapor condenses, wherein the nitrogen oxides partially react with the condensed water vapor and oxygen to form an aqueous solution containing nitric acid and partially remain in a nitrogen oxide-containing gas mixture, c. the nitric acid-containing solution from step (b.) is supplied from the condenser (3) via a connecting line (13) to a first absorption tower (4), d. the nitrogen oxide-containing gas mixture from step (b.), optionally after passing through one or more oxidation columns, is fed to the first absorption tower (4), in which it is brought into contact with water or an aqueous solution, wherein the nitrogen oxide-containing gas mixture reacts with water at least partially to form an aqueous solution containing nitric acid, which, together with the nitric acid-containing solution from step (c.), accumulates at the bottom of the first absorption tower (4), characterized in that e. an oxidizing agent is supplied to the ammonia combustion plant (2) in step (a.) whose oxygen content is between 21.5 vol.% and 35 vol.% and which otherwise consists of gases that are largely inert with respect to the nitric acid production process.

2. Method according to claim 1, characterized in that the oxygen content of the oxidizing agent supplied to the ammonia combustion plant is between 22 vol.% and 32 vol.%, preferably between 23 vol.% and 28 vol.%.

3. Method according to claim 1 or 2, wherein the oxidizing agent supplied to the ammonia combustion plant (2) consists of primary air and an additional volume flow of pure oxygen or an oxygen-rich gas, the oxygen content of which is greater than that of the primary air, supplied to the primary air.

4. Method according to claim 4, characterized in that the volume flow of pure oxygen or an oxygen-rich gas is fed into a supply line (7) for primary air and is supplied together with the primary air contained therein to the ammonia combustion plant (2).

5. Method according to claim 3 or 4, characterized in that the volume flow of pure oxygen or an oxygen-rich gas supplied to the ammonia combustion plant (2) has a temperature of less than 10°C, preferably less than 0°C.

6. Method according to one of the preceding claims, characterized in that the volume flow of the total oxidizing agent supplied to the ammonia combustion plant (2) has a superstoichiometric amount of oxygen compared to the volume flow of ammonia supplied to the ammonia combustion plant (2).

7. Method according to one of the preceding claims, characterized in that ozone and / or oxygen are introduced into the connecting line (13) and / or a conveying line (15), via the solution containing nitric acid, which is discharged from the bottom of the first absorption tower (4) and supplied to an upper region of the first absorption tower (4) and / or a bleaching column (6).

8. Production plant for the manufacture of nitric acid, comprising an ammonia combustion plant (2) equipped with a supply line (9) for ammonia and a supply line (7) for primary air for the conversion of ammonia with oxygen to nitrogen oxides and water vapor, with a condenser (3) connected to the ammonia combustion plant (2) for cooling the reaction products from the ammonia combustion plant (2) to a temperature at which at least a portion of the reaction products condenses, with at least one absorption tower (4, 5) arranged downstream of the condenser (3) for washing the gas mixture formed in the condenser (3) with water or an aqueous nitric acid solution, with at least one riser line (13) leading from the condenser (3) to the absorption tower (4) and equipped with a conveying device (14) for introducing a solution containing nitric acid into the absorption tower (4), and with a conveying line (15) connecting the absorption tower (4) to a bleaching column (6), characterized in that the ammonia combustion plant (2) is connected by flow to at least one supply line for pure oxygen and / or for an oxygen-rich gas,which leads into the supply line (7) for primary air and / or into the supply line (9) for ammonia and / or directly into the ammonia combustion plant (2).

9. Production plant according to claim 8, characterized by a control device (30) by means of which the oxygen content and / or the temperature of the total volume flow of oxidizing agent supplied to the ammonia combustion plant (2) can be controlled as a function of a parameter measured at a sensor (31).

Images