Safe production of nitric acid according to the dual-pressure process

Abstract

The present invention relates to a device for producing nitric acid from ammonia, wherein: the device has a first oxidation reactor (10), at least one first heat exchanger (12), a first condenser (20), an NO compressor (24) and an absorption tower (30); the oxidation reactor (10) is connected to the at least one first heat exchanger (12) via a first connection (41) for transfer of the NOx gas mixture; the at least one first heat exchanger (12) is connected to the first condenser (20) via a second connection (42) for transfer of the NOx gas mixture; the first condenser (20) is connected to the NO compressor (24) via a third connection (43) for transfer of the NOx gas mixture; the NO compressor (24) is connected to the absorption tower (30) via a fourth connection (44) for transfer of the NOx gas mixture; the condenser (20) has a first acid outlet; the first acid outlet is connected to a first acid inlet of the absorption tower (30) via a first acid circulating line (51); the absorption tower (30) has a second acid outlet; and the second acid outlet is connected to a product outlet (36) via a second acid circulating line (52); characterized in that the first acid outlet or the second acid outlet is connected to the first condenser (20) and / or the third connection (43) via a third acid circulating line (53).

Description

[0001] Safe production of nitric acid using the dual-pressure process

[0002] The invention relates to a device and a method for preventing potentially dangerous deposits within a plant for the production of nitric acid.

[0003] Nitric acid is produced industrially using the Ostwald process. The first step is the oxidation of ammonia to NO, followed by further oxidation and absorption to produce, for example, 60 wt.% nitric acid.

[0004] DE 10 2017 201 180 A1 shows a process for the production of nitric acid and a suitable plant for this purpose.

[0005] A process for producing nitric acid is known from US 7 118 723 B2.

[0006] A plant for the production of nitric acid is known from US 11 292 719 B2

[0007] EP 4 238 932 A1 discloses a two-pressure process for the production of nitric acid.

[0008] If the ammonia is not fully converted, ammonium nitrate and / or ammonium nitrite may form within the device. Both are particularly critical in dry form, as they can be mechanically converted. Ammonium nitrite, which is more likely to form under alkaline ambient conditions, is generally more critical than ammonium nitrate, which is more likely to form under acidic conditions.

[0009] Such solids can be particularly critical if they enter the compressor between the condenser and the absorption tower, or if they separate from the gas phase in the compressor as solids, as this is where mechanical stress is most likely to occur. Such compressors are used in a variant of the Ostwald process (“dual-pressure process”). Ammonia slip, i.e. incomplete conversion of ammonia to NO, can occur, for example, if the catalyst is damaged (in some places) or if there is an inhomogeneous flow over the catalyst. Ammonia slip occurs particularly when starting up nitric acid plants, as the primary catalyst has not yet reached its operating temperature and therefore has a lower reactivity.

[0010] If such ammonia slip occurs, both NO and the subsequent products up to nitric acid are found in the downstream system alongside the ammonia, so that ammonium nitrate or ammonium nitrite can be formed.

[0011] The object of the invention is to ensure safety even in the event of ammonia slip.

[0012] This object is achieved by the device having the features specified in claim 1 and by the method having the features specified in claim 9. Advantageous further developments emerge from the subclaims, the following description, and the drawings.

[0013] The device according to the invention is used to produce nitric acid from ammonia. This is the so-called Ostwald process. The device comprises a first oxidation reactor, at least one first heat exchanger, a first condenser, a NO compressor, and an absorption tower. The oxidation reactor is connected via a first connection for transferring the NO x-gas mixture (the oxidation product) is connected to the at least one first heat exchanger. Typically, several first heat exchangers are operated in series to achieve sufficient cooling of the oxidation product. The oxidation takes place at temperatures of around 1000°C, usually between 800°C and 950°C, while in the first condenser the temperature is closer to 100°C, usually between 50°C and 60°C. The first condenser is usually designed as a heat exchanger itself, in which the final cooling step takes place, which leads to condensation. The different heat exchange stages of the several first heat exchangers enable optimal use of the thermal energy. The at least one first heat exchanger is connected via a second connection for transferring the NO x-gas mixture is connected to the first condenser. The first condenser is usually also designed as a heat exchanger, in which the temperature level is lowered to such an extent that the water produced during the combustion of the ammonia can largely condense out. In this process, a portion of the NO X so that the condensate is diluted nitric acid. The first condenser is connected via a third connection to transfer the NO x -gas mixture is connected to the NO compressor. The pressure is increased using a dual-pressure process to improve the efficiency of absorption and thus achieve the highest possible NO X into nitric acid. The NOx compressor is connected to the absorption tower via a fourth connection for transferring the NOx gas mixture. During compression, the NO X- gas mixture is reheated, therefore the fourth connection usually has a second heat exchanger and / or a second condenser. The first condenser has a first acid outlet. This is usually arranged on the underside of the first condenser in the region of a collection tank for the condensed nitric acid. The second condenser has a third acid outlet. This is usually arranged on the underside of the second condenser in the region of a collection tank for the condensed nitric acid. The first acid outlet is usually connected via a first acid line to a first acid inlet of the absorption tower. The third acid outlet is usually connected via a fourth acid line to the collection tank for condensed nitric acid of the first condenser or to the line for nitric acid that connects the absorption tower and bleaching column or to the second acid line.The absorption tower has a second acid outlet. The second acid outlet is connected to a product outlet via a second acid line. A bleaching column and / or a storage tank, for example, can be arranged between the second acid outlet and the product outlet. The absorption tower is typically constructed such that the first acid inlet is positioned so that the diluted nitric acid interacts with the NO. x -richer NO x -Gas is brought into contact with the corresponding acid concentration in a section of the column and is thereby enriched. A water inlet is usually located at the top of the absorption tower. Thus, the water (with the lowest nitric acid concentration) comes into contact with the NO x -poorest gas, so that absorption is improved. The NO X - Gas flow is supplied at the bottom, the gas outlet is located at the top. The NO x-containing gas flows upwards, while the liquid flows downwards. As a result, the NO x Concentration of the gas decreasing in contact with increasingly diluted nitric acid to achieve particularly good absorption. In addition, NO x Oxygen-rich gas, such as air, is added to the gas stream upstream of the NO compressor to obtain as much nitric acid as possible through oxidizing conditions and minimize the loss of NO. Furthermore, the absorption tower typically has an integrated cooling device to dissipate the reaction heat.

[0014] According to the invention, the first acid outlet, the second acid outlet or the third acid outlet is connected to the first condenser and / or the third connection via a third acid line. Thus, NO xNitric acid is sprayed into the gas stream in a condensing temperature window where no immediate evaporation occurs. The advantage of using nitric acid over water is that it creates an acidic level, which in turn leads to the preferential formation of ammonium nitrate over ammonium nitrite. If water is used, a basic environment can develop in the presence of ammonia, which disfavors the formation of ammonium nitrite over ammonium nitrate. Furthermore, the use of additional water leads to dilution of the nitric acid product, which is undesirable. Furthermore, the separation takes place just upstream of the NO compressor. Such deposits would be most dangerous in the NO compressor due to the high mechanical stress, so separation just upstream of the NO compressor is advantageous, especially compared to directly supplying water upstream of and thus to the NO compressor.

[0015] It is advantageous to conduct the injected acid in a circulating stream. The acid can be conducted via the sump of the first condenser or the sumps of the condensers, the absorption tower, or other tanks and lines. It is important that the nitric acid ultimately originates from the first acid outlet, the second acid outlet, or the third acid outlet, even if it is taken from a downstream storage tank, for example. Examples of these sources include a trickling acid tank, a buffer tank for off-spec nitric acid, a product acid tank, or a tank installed specifically for this purpose. Extraction from a nitric acid-carrying line is also possible, in particular the first acid line, the second acid line, or the fourth acid line. The corresponding nitric acid sources are connected to the first condenser for injection into the third acid line.To provide the required pressure for injection, the third acid line can be equipped with a pump. Alternatively, a process-related pressure drop can be utilized. It is advantageous to use existing pumps such as acid condensate pumps, product acid pumps, filling pumps, or drop acid pumps, which already generate such a pressure drop.

[0016] In a further embodiment of the invention, the third connection has a liquid separator. For example, and preferably, the liquid separator is designed as a droplet separator. The third acid line preferably opens into the third connection upstream of the liquid separator in the NO flow direction. This enables reliable separation of the nitric acid before it can reach the NO compressor. More preferably, the liquid separator is located at the lowest point of the third connection. This assists in the separation of the nitric acid.

[0017] In a further embodiment of the invention, the first condenser has at least one nozzle. The nozzle is connected to the third acid line. The nozzle thus represents the connection between the third acid line and the interior of the first condenser. Introduction into the first condenser is particularly preferred because liquid is separated there anyway, i.e., ideal formations are present in terms of temperature, and the liquid, the diluted nitric acid, is collected anyway and transferred to the absorption tower. This does not require any additional equipment. In addition, the material of the first condenser is already designed for direct contact with the corrosive nitric acid, so that here too, no additional material expenditure is required when designing equipment that has not previously come into contact with the liquid acid.The use of nozzles ensures good contact between liquid nitric acid and gaseous ammonia. The nozzles can be solid cone, hollow cone, or flat jet nozzles, for example.

[0018] In a further embodiment of the invention, the third connection has at least one nozzle. The nozzle is connected to the third acid line. Thus, nitric acid can be sprayed into the third connection from the third acid line.

[0019] In a further embodiment of the invention, the first condenser has a NO side. Since the first condenser is usually designed as a heat exchanger, it usually has, in addition to the NO side, a heat exchange medium side, on which, for example, cooling water flows and the NO x-gas mixture cools down to such an extent that condensation occurs. At the same time, the condensation heat can be easily and efficiently removed. The NO side has a condenser gas inlet and a condenser gas outlet. The condenser gas inlet is connected to the second connection, through which the NO x-gas mixture is supplied. The condenser gas outlet is connected to the third connection, from there the NOx gas mixture is transferred to the NO compressor. Condensation thus takes place between the condenser gas inlet and the condenser gas outlet. In one embodiment, the NO side has at least a first region with an upward gas flow and a second region with a downward gas flow. The first region is arranged upstream of the second region in the flow direction. Preferably, the first condenser can have several successive regions with alternating upward and downward gas flow. Upward and downward, respectively, in the sense of the invention, are arranged against and with gravity. The nozzle is arranged above the first region with upward flow.Thus, the nitric acid is sprayed into an upward gas stream and can then more easily settle again in the subsequent downward gas stream, together with the condensed nitric acid from the gas stream.

[0020] For example, the first condenser is designed as a cross-flow heat exchanger, in which the heat exchange medium, specifically cooling water, flows horizontally, while the gas flow alternates upwards and downwards, perpendicular to the heat exchange medium. The multiple changes of direction enable effective heat exchange.

[0021] In a further embodiment of the invention, the first condenser has a first part and a second part. The first condenser is constructed with mirror symmetry with respect to the NO side. The condenser gas inlet is arranged centrally. Accordingly, the first part preferably has the condenser gas outlet, and the second part has a further condenser gas outlet. The condenser gas outlet and the further condenser gas outlet are connected to the NO compressor via a Y-shaped third connection. This allows the contact area and thus the heat transfer in the first condenser to be increased in a simple design. The mirror symmetrical design means that in the first part and in the second part, the nitrate side is sprayed into an upwardly directed gas flow.

[0022] In a further embodiment of the invention, the condenser housing and the nozzle are made of corrosion-resistant material. Particularly preferably, the condenser housing and the nozzle are made of the same corrosion-resistant material.

[0023] In a further aspect, the invention relates to a process for producing nitric acid. The process comprises the following steps: a) oxidizing ammonia with oxygen, b) cooling the oxidation product, c) condensing the oxidation product in a first condenser, d) compressing the oxidation product, e) cooling the compressed oxidation product, f) absorbing the nitric acid in an absorption tower.

[0024] To this extent, the process corresponds to the known Ostwald process for producing nitric acid from ammonia and is extensively known to those skilled in the art. According to the invention, a portion of the nitric acid produced in step c) or in step f) is recycled, in particular sprayed, into the gas stream of the oxidation product in step c) and / or between step c) and step d). The use of nitric acid ensures an acidic environment, which favors the formation of ammonium nitrate over ammonium nitrite and thus reduces the safety risk. Likewise, the removal of any ammonia present before compression in step d) prevents ammonium nitrate and / or ammonium nitrite from settling, in particular within the NOx compressor, and is thus exposed to particular mechanical stress. The spraying achieves optimal contact between the recycled liquid phase and the gas phase.

[0025] In a further embodiment of the invention, the sprayed nitric acid is conducted in a circulating stream.

[0026] In a further embodiment of the invention, the spraying takes place countercurrent to the gas flow of the oxidation product.

[0027] In a further embodiment of the invention, a liquid phase is separated after spraying. Since the liquid phase may also potentially contain ammonium nitrate and / or ammonium nitrite, it is advantageous to reliably separate the liquid phase upstream of the NO compressor.

[0028] In a further embodiment of the invention, the spraying takes place at a gas stream temperature of below 150 °C.

[0029] In a further embodiment of the invention, the method is applied only temporarily, for example, only at certain time intervals, for example, every four hours for five minutes. This minimizes the effort involved.

[0030] In a further embodiment, the method according to the invention is carried out during the start-up of a device according to the invention until the catalyst has reached its operating temperature of, for example, over 700°C. In a further embodiment of the invention, the ammonia concentration in the oxidation product is additionally detected via a sensor, and the method is carried out when a predetermined ammonia concentration, which is preferably just above the detection limit, is exceeded. Thus, the system can continue to operate safely, for example, in the event of a localized defect in the oxidation catalyst, until the oxidation catalyst can be repaired or replaced.

[0031] The device according to the invention is explained in more detail below using an embodiment shown in the drawings.

[0032] Fig. 1 first exemplary device

[0033] Fig. 2 second exemplary device

[0034] Fig. 3 Capacitor

[0035] Fig. 4 third exemplary device

[0036] Fig. 1 shows a first exemplary device according to the invention in a highly schematic and simplified manner. In particular, the heat exchange streams were not shown in the illustration, as this creates highly intersecting material streams and is irrelevant to the actual invention, as well as being known to those skilled in the art. Compression stages and the expansion of the product gas are also not shown for these reasons. The illustration serves only to place the invention in the context of a device known to those skilled in the art for producing nitric acid from ammonia.

[0037] The device comprises an oxygen source 1 and an ammonia source 2. The air source 1 is typically ambient air, but can also be, for example, an air separation plant or another suitable oxygen source. The ammonia source 2 is typically an ammonia evaporator. The device also comprises a water source 3. The product is ultimately removed via the product outlet 36. The ammonia and oxygen are fed to an oxidation reactor 10 and converted to NO. The oxidation product is passed through a first connection 41 into a series arrangement of four first heat exchangers 12, where it is gradually cooled. From the last first heat exchanger 12, the pre-cooled oxidation product is passed via the second connection 42 into the first condenser 20, which is also designed as a heat exchanger and cools the oxidation product below the condensation temperature, so that dilute nitric acid condenses out.The oxidation product is fed from the first condenser 20 via the third connection 43, which has a liquid separator 22, to the NO compressor 24. Since the oxidation product is heated during compression, the compressed oxidation product is fed into the absorption tower 30 via a fourth connection 44, which has a second heat exchanger 26.

[0038] The absorption tower 30 serves to ensure that NO produced from the ammonia during oxidation Xto dissolve in water as nitric acid and thus provide the product. For this purpose, water from the water source 3 is fed at the upper end and the oxidation product is fed in countercurrent to it at the bottom. The diluted nitric acid from the first condenser 20 is accordingly fed via the first acid line 51 and the first acid inlet 61 more centrally into the absorption tower 30. At the same time, the absorption tower 30 is usually cooled in order to remove the heat of reaction and thus increase the yield. The gas leaving the absorption tower 30 is fed to an exhaust gas purification system 32. The product, the nitric acid, is removed via the second acid outlet 67 and the subsequent second acid line 52 and fed via a bleaching column 34 to the product outlet 36. In the bleaching column 34, NO dissolved in the nitric acid from the oxygen source 1, for example, is Xoutgassed and fed as a gas mixture to the third compound 43 to enable oxidation of the NO or NO2 to HNO3.

[0039] To this extent, the device corresponds to a device according to the state of the art.

[0040] Essential to the invention is a third acid line 53, which recirculates nitric acid from the second acid outlet 67 of the absorption tower 30 to the first condenser 20. This allows ammonia that has passed through the oxidation reactor unoxidized to be bound as ammonium nitrate and thus reliably removed from the gas stream.

[0041] The second exemplary device shown in Fig. 2 differs from the first exemplary device shown in Fig. 1 in that a second condenser 28 is arranged instead of the second heat exchanger 26.

[0042] Fig. 3 shows an exemplary and preferred first condenser 20 in cross-section of the NO side. The first condenser 20 is constructed symmetrically with respect to the NO side, which means that a condenser gas inlet 71 is arranged centrally, through which the NO coming from the second connection 42 x -gas mixture is supplied. A condenser gas outlet 72 is arranged on the right and left, through which the NOx gas mixture is supplied to the third connection 43. In addition, the first condenser 20 has a first acid outlet 61 on its underside, through which the diluted nitric acid is supplied to the absorption tower 30 through the first acid line 51.

[0043] The gas path in the first condenser 20 has several regions on each side with alternating downward flow and upward flow. The condenser gas inlet 71 is followed by a zeroth region with a downward flow.

[0044] current, followed by a first area with an upward flow

[0045] current, followed by a second area with a downward flow

[0046] current, followed by a third area with an upward flow

[0047] Flow. A nozzle 21 is arranged above each of the first two regions, which are connected to the third acid line 53. Nitric acid from the absorption tower 30 is sprayed into the gas stream through the nozzles 21, where it settles well again in the second region and can thus be returned via the first acid outlet 51.

[0048] The third exemplary device shown in Fig. 4 differs from the first exemplary device shown in Fig. 1 in that the third acid line 53 returns nitric acid from the first acid outlet 66 to the first condenser 20. Reference numerals

[0049] 1 oxygen source

[0050] 2 Ammonia source

[0051] 3 water source

[0052] 10 Oxidation reactor

[0053] 12 first heat exchanger

[0054] 20 first capacitor

[0055] 21 nozzle

[0056] 22 liquid separators

[0057] 24 NO compressors

[0058] 26 second heat exchanger

[0059] 28 second capacitor

[0060] 30 absorption tower

[0061] 32 Exhaust gas purification

[0062] 34 Bleaching column

[0063] 36 Product outlet

[0064] 41 first connection

[0065] 42 second connection

[0066] 43 third connection

[0067] 44 fourth connection

[0068] 51 first acid line

[0069] 52 second acid line

[0070] 53 third acid line

[0071] 61 first acid inlet

[0072] 66 first acid outlet

[0073] 67 second acid outlet

[0074] 71 Condenser gas inlet

[0075] 72 Condenser gas outlet

Claims

Patent claims 1. Device for producing nitric acid from ammonia, the device comprising a first oxidation reactor (10), at least one first heat exchanger (12), a first condenser (20), a NO compressor (24) and an absorption tower (30), the oxidation reactor (10) being connected via a first connection (41) for transferring the NO x -gas mixture is connected to the at least one first heat exchanger (12), wherein the at least one first heat exchanger (12) has a second connection (42) for transferring the NO x -gas mixture is connected to the first condenser (20), wherein the first condenser (20) has a third connection (43) for transferring the NO x -gas mixture is connected to the NO compressor (24), wherein the NO compressor (24) is connected via a fourth connection (44) for transferring the NO x-gas mixture is connected to the absorption tower (30), wherein the first condenser (20) has a first acid outlet (66), wherein the first acid outlet (66) is connected via a first acid line (51) to a first acid inlet (61) of the absorption tower (30), wherein the absorption tower (30) has a second acid outlet (67), wherein the second acid outlet (67) is connected via a second acid line (52) to a product outlet (36), characterized in that the first acid outlet (66) or the second acid outlet (67) is connected via a third acid line (53) to the first condenser (20) and / or the third connection (43).

2. Device according to claim 1, characterized in that the third Connection (43) has a liquid separator (22).

3. Device according to claim 2, characterized in that the third Acid line (53) opens into the third connection (43) in the NO flow direction upstream of the liquid separator (22).

4. Device according to one of claims 2 to 3, characterized in that the liquid separator (22) is arranged at the lowest point of the third connection (43).

5. Device according to one of the preceding claims, characterized in that the first condenser (20) has at least one nozzle (21), wherein the nozzle (21) is connected to the third acid line (53).

6. Device according to claim 5, characterized in that the first condenser (20) has a NO side, the NO side having a condenser gas inlet (71) and a condenser gas outlet (72), the condenser gas inlet (71) being connected to the second connection (42), the condenser gas outlet (72) being connected to the third connection (43), the NO side having at least a first region with an upward gas flow and a second region with a downward gas flow, the nozzle (21) being arranged above the first region.

7. Device according to claim 6, characterized in that the first condenser (20) has a first part and a second part, wherein the first condenser (20) is constructed mirror-symmetrically with respect to the NO side, wherein the condenser gas inlet (71) is arranged centrally.

8. Device according to one of claims 5 to 7, characterized in that the housing of the first capacitor (20) and the nozzle (21) are made of corrosion-resistant material.

9. A process for producing nitric acid, the process comprising the following steps: a) oxidizing ammonia with oxygen, b) cooling the oxidation product, c) condensing the oxidation product in a first condenser (20), d) compressing the oxidation product, e) cooling the compressed oxidation product, f) absorption to form nitric acid in an absorption tower (30), wherein a portion of the nitric acid produced in step c) or in step f) is recycled, in particular sprayed, into the gas stream of the oxidation product in step c) and / or between step c) and step d).

10. The process according to claim 9, characterized in that the sprayed nitric acid is conducted in a circulating stream.

11. The process according to one of claims 9 to 10, characterized in that the spraying takes place countercurrent to the gas stream of the oxidation product.

12. Method according to one of claims 9 to 11, characterized in that a liquid phase is separated after spraying.

13. Method according to one of claims 9 to 12, characterized in that the spraying takes place at a gas stream temperature of below 150 °C.

Images