METHOD AND PRODUCTION PLANT FOR PRODUCING NITRIC ACID

Abstract

In a process for preparing nitric acid, nitrogen oxides are first produced in an ammonia combustion plant and cooled in a condenser to form a nitric acid-containing solution. The nitric acid-containing solution is then supplied to at least one absorption tower in which the nitrogen oxides are brought into contact with water and oxygen, wherein the nitrogen-containing gas mixture reacts with the water and the oxygen at least in part to form an aqueous nitric acid-containing solution which accumulates at the base of the absorption tower and is then compressed and recycled via a conduit back into the absorption tower. In order to minimize the concentration of nitrogen oxides in the offgas from such a plant and to increase the efficiency of the process, the invention proposes injecting ozone into a connection conduit which leads from the condenser to a first absorption tower and conducts the nitric acid-containing solution.

Claims

1. A process for preparing nitric acid, in which: a. ammonia is reacted with oxygen to give nitrogen oxides and steam in an ammonia combustion plant; b. the nitrogen oxides and the steam from step (a.) are cooled in a condenser to a temperature at which at least some of the steam condenses, with the nitrogen oxides in part reacting with the condensed steam and oxygen to give an aqueous, nitric acid-containing solution and in part remaining in a nitrogen oxide-containing gas mixture; c. the nitric acid-containing solution from step (b.) is supplied from the condenser via a connection conduit to a first absorption tower; d. the nitrogen oxide-containing gas mixture from step (b.) is supplied to the first absorption tower, in which it is brought into contact with water or with an aqueous solution, with the nitrogen oxide-containing gas mixture reacting with water at least in part to form an aqueous, nitric acid-containing solution which, together with the nitric acid-containing solution from step (c.), accumulates at the base of the first absorption tower; and e. ozone is introduced into the nitric acid-containing solution from step (b.) conducted through the connection conduit prior to the supply thereof to the first absorption tower.

2. The process as claimed in claim 1, wherein the nitrogen oxide-containing gas mixture, after having passed through the first absorption tower, is supplied to a second absorption tower in which the nitrogen oxide-containing gas mixture is brought into contact with water or with an aqueous nitric acid-containing solution nitric acid solution, with the nitrogen oxide-containing gas mixture reacting at least in part to form a nitric acid-containing solution which accumulates at the base of the second absorption tower and from there is supplied to an upper region of the first absorption tower via a riser conduit, wherein ozone and/or oxygen is introduced into the nitric acid-containing solution conducted through the riser conduit.

3. The process as claimed in claim 1, wherein nitric acid-containing solution is discharged from the base of the first absorption tower and supplied via a conveying conduit to an upper region of the first absorption tower and/or to a bleaching column, wherein ozone and/or oxygen is introduced into the nitric acid-containing solution conducted through the connection conduit.

4. The process as claimed in claim 1, wherein the connection conduit is in the form of a riser conduit and the ozone and/or the oxygen is introduced downstream of a conveying device arranged in the connection conduit, in a geodetically lower region of the respective connection conduit.

5. The process as claimed in claim 3, wherein a substream is branched off from the nitric acid-containing solution conducted through the riser conduit, is compressed and is enriched with ozone and/or oxygen before being supplied to the absorption tower or the bleaching column.

6. The process as claimed in claim 1, wherein the ozone injected into the nitric acid-containing solution in the connection conduit has a temperature of below 10° C., preferably below 0° C.

7. A production plant for preparing nitric acid, the production plant comprising: an ammonia combustion plant for reacting ammonia with oxygen to give nitrogen oxides and steam; a condenser connected to the ammonia combustion plant for cooling the reaction products from the ammonia combustion plant to a temperature at which at least some of the reaction products condense; a first absorption tower arranged downstream of the condenser for scrubbing the gas mixture formed in the condenser with water or with an aqueous nitric acid solution; at least one riser conduit, leading from the condenser to the first absorption tower and equipped with a conveying device, for introducing a nitric acid-containing solution into the first absorption tower; and a conveying conduit connecting the first absorption tower to a bleaching column; wherein the riser conduit running between the condenser and the first absorption tower is connected in terms of flow via an ozone supply conduit to a source for ozone.

8. The production plant as claimed in claim 7, wherein the riser conduit is equipped with a bypass conduit into which the ozone supply conduit opens.

9. The production plant as claimed in claim 8, wherein means for compressing the nitric acid-containing solution are provided in the riser conduit and/or the bypass conduit at least in the region of the mouth of the ozone supply conduit.

10. The production plant as claimed in claim 7, wherein a conduit is provided, which leads from the bottom of the first absorption tower into a higher region of the first absorption tower and/or into a bleaching column and into which opens a supply conduit that is connected in terms of flow to an additional source for ozone and/or a source for oxygen.

11. The production plant as claimed in claim 7, wherein at least one second absorption tower is connected downstream of the first absorption tower and is connected to same via a riser conduit leading from the base of the second absorption tower to a headspace of the first absorption tower, wherein a supply conduit that is connected in terms of flow to an additional source for ozone and/or a source for oxygen opens into the riser conduit.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0027] An exemplary embodiment of the invention shall be described in more detail on the basis of the drawing. The sole drawing (FIG. 1) schematically shows a connection diagram of a production plant according to the invention for preparing nitric acid.

DETAILED DESCRIPTION

[0028] The production plant 1 shown in FIG. 1 for preparing nitric acid comprises, in a manner known per se, an ammonia combustion plant 2, a condenser 3, a plurality of, in the exemplary embodiment two, absorption towers 4, 5, and a bleaching column 6 The absorption towers 4, 5 in the exemplary embodiment are low- and medium-pressure columns that operate at an operating pressure of from 2 to 5 bar(g); however, medium-pressure or high-pressure columns with an operating pressure of up to 15 bar(g) may also be used.

[0029] The ammonia combustion plant 2 serves to react gaseous ammonia and oxygen at a temperature of between 600° C. and 900° C. over a gauze catalyst made from a noble metal, such as for example platinum or a platinum/rhodium alloy, to give nitrogen monoxide and steam. As oxygen, atmospheric oxygen is typically used. The reaction products of the reaction taking place in the ammonia combustion plant 2, essentially nitrogen monoxide and steam, and also excess oxygen, are supplied to the condenser 3 in which the reaction products are cooled by indirect thermal contact with a cooling medium, for example water or liquefied or cold gaseous nitrogen, conveyed via a cooling medium supply conduit 8, to a temperature at which at least some of the steam condenses, for example to 60° C. to 80° C. The cooling medium heated in the heat exchange is discharged via a cooling medium discharge conduit 9 and released to atmosphere or sent for another use. Some of the nitrogen oxides react with the water to give nitric acid, which settles at the base of the condenser 3 in an aqueous solution. The gas mixture present in the condenser 3 is introduced as process gas into a lower region of the absorption tower 4 via a gas supply conduit 11. Some of the nitrogen monoxide is oxidized with excess oxygen to give nitrogen dioxide and the dimer thereof dinitrogen tetraoxide. The aqueous, nitric acid-containing solution from the base of the condenser 3 is supplied via a connection conduit 12 to a region of the absorption tower 4 that is higher in relation to the mouth of the gas supply conduit 11. If the connection conduit 12 is a riser conduit, a conveying device 13 provides the pressure necessary to overcome the hydrostatic pressure.

[0030] The aqueous, nitric acid-containing solution from the condenser 3 is sprayed into the absorption tower 4 by means of a nozzle arrangement (not explained in more detail here), falls downwards and in the process comes into contact with the nitrogen oxide-containing process gases rising from the below. Further proportions of the nitrogen oxides present in the gas mixture react here to give nitric acid, which accumulates in an aqueous solution at the bottom of the absorption tower 4. This aqueous, nitric acid-containing solution is discharged via a conduit 14, transported by means of a conveying device 15 to the bleaching column 6, and sprayed therein. In turn, a nitrogen oxide-containing gas is formed in the bleaching column 6 and is introduced via a conduit 18 into the gas supply conduit 11 and via the latter into the absorption tower 4. The product, the bleached acid, is conducted away via conduit 17.

[0031] The nitrogen oxide-containing gas mixture remaining in the absorption tower 4 is discharged via a process gas conduit 19 and introduced into a lower region of the absorption tower 5. At the same time, water is sprayed from a water supply conduit 20 into the headspace of the absorption tower 5. The nitrogen oxide-containing gas mixture rising from below comes into contact in the absorption tower 5 with the water sprayed in and reacts at least in part with the latter to give nitric acid, which accumulates at the base of the absorption tower 5 in an aqueous solution. This aqueous, nitric acid-containing solution is discharged via a riser conduit 21 and conducted by means of a conveying device 22 to the headspace of the absorption tower 4, sprayed in there, and passes through the absorption tower 4 in countercurrent to the process gas stream to form increasingly highly concentrated nitric acid.

[0032] Gas mixture still present in the absorption tower 5 is discharged via an offgas conduit 23 and supplied to a unit (not shown here) for denoxing, in which the remaining nitrogen oxides are to a very great extent removed from the gas mixture.

[0033] For reasons of clarity, the exemplary embodiment shown in FIG. 1 only comprises two absorption towers 4, 5; of course, also conceivable in the context of the invention are exemplary embodiments comprising three or more absorption towers through which the nitrogen oxide-containing gas streams and the aqueous, nitric acid-containing solutions pass in countercurrent in a known way.

[0034] In order to intensify the oxidation of the nitrous gases and the nitrous acid, ozone is supplied to the process. The ozone is taken from a source for ozone which in the exemplary embodiment is an ozonizer 29 in which the ozone is produced on site from oxygen. The ozone from the ozonizer 29 is injected, together with oxygen still present, via an ozone supply conduit 30 into a bypass conduit 31 which branches off from the connection conduit 12 and opens back into it downstream of the mouth of the ozone supply conduit 30. In order to be able to achieve as high as possible a pressure in the region of the mouth of the ozone supply conduit 30, in the bypass conduit 31 a compressor 32 is arranged upstream thereof and a pressure reducer 33 is arranged downstream thereof, the latter reducing the pressure back down to the pressure prevailing in the connection conduit 12. In this way, a pressure of 10 bar(g) to 15 bar(g) or even higher can be achieved in the bypass conduit 31 in the region of the mouth of the ozone supply conduit 30, which promotes the dissolution of the ozone in the nitric acid-containing solution. Alternatively, the bypass conduit 31 can also open directly into the absorption tower 4.

[0035] In addition, in the exemplary embodiment shown here, oxygen is taken from an oxygen source, for example a tank 24, and introduced into the process. To this end, in a manner known per se, the oxygen passes through an air evaporator 25 and is supplied cold, but in gas form, via oxygen supply conduits 26, 27 to the conduit 14 and/or to the riser conduit 21 and/or to an air supply conduit 28 leading to the bleaching column 6. Instead of conversion to gas in an air evaporator 25, the cold content of the liquefied oxygen may furthermore also be used for cooling the reaction products from the ammonia combustion plant 2 in the condenser 3, for example by subjecting the cooling medium used there to a heat exchange with the liquid oxygen from the tank 24, or supplying the liquid oxygen from tank 24 directly to the condenser 3 as cold medium.

[0036] The oxygen conducted by the oxygen supply conduit 26 into the conduit 14 and/or into the air supply conduit 28 facilitates the oxidation of nitrogen oxides still present in the nitric acid and of the nitrous acid. An oxygen-rich gas phase accumulates in the headspace of the bleaching column 6 and is taken off via conduit 18 and combined with the gas mixture conducted through the gas supply conduit 11 from the condenser 3.

[0037] Instead of oxygen, ozone or an ozone-containing gas mixture may furthermore also be used, this being generated in an ozonizer 35 and injected via an ozone supply conduit 36 into at least one of the conduits 14, 21, 28.

[0038] In the riser conduit 21 and in the conduits 12, 14, if these are also riser conduits, the oxygen or the ozone of the ozone-containing gas mixture is preferably introduced in a geodetically lower region downstream of the respective conveying device 13, 15, 22, 32, in order to make use of the hydrostatic pressure of the liquid column in the conduit 12, 14, 21 and possibly of an additional pressure generated by the respective conveying device 13, 15, 22, 32. Within the section of the conduits 12, 14, 21 that adjoins downstream of the introduction point for the oxygen or the ozone, the oxygen or the ozone partially dissolves and reacts with nitrogen oxides that are dissolved in the nitric acid-containing solution, and possibly with water. Some of the oxygen introduced in excess and ozone decomposing into oxygen, if there is any, does not react with the nitrogen oxides and passes in gas form into the respective absorption tower 4, 5, where it results in a higher oxygen partial pressure which in turn promotes the formation of nitric acid in the respective absorption tower 4, 5. The formation of nitric acid is additionally facilitated by the low temperature of the oxygen or ozone supplied.

[0039] The invention is in particular also suitable for retrofitting existing plants which typically operate with absorption towers having an operating pressure that lies in the low- and medium-pressure region, i.e. at about 1 to 5 bar(g). The objective invention is, however, also usable for retrofitting high-pressure and dual-pressure plants. In this case, NO.sub.x concentrations in the offgas that are much lower still could even be achieved, as a result of which the operating costs of denoxing plants could be markedly reduced or the use of same might even be avoided entirely. This can result in substantial cost savings by saving on ammonia and/or natural gas, these typically being used as reducing agent for the denoxing.

LIST OF REFERENCE SIGNS

[0040] 1. Production plant [0041] 2. Ammonia combustion plant [0042] 3. Condenser [0043] 4. Absorption tower [0044] 5. Absorption tower [0045] 6. Bleaching column [0046] 7. — [0047] 8. Cooling medium supply conduit [0048] 9. Cooling medium discharge conduit [0049] 10. — [0050] 11. Gas supply conduit [0051] 12. Connection conduit [0052] 13. Conveying device [0053] 14. Conduit [0054] 15. Conveying device [0055] 16. — [0056] 17. Product discharge conduit [0057] 18. Conduit [0058] 19. Process gas conduit [0059] 20. Water supply conduit [0060] 21. Riser conduit [0061] 22. Conveying device [0062] 23. Offgas conduit [0063] 24. Tank [0064] 25. Air evaporator [0065] 26. Oxygen conduit [0066] 27. Oxygen conduit [0067] 28. Air supply conduit [0068] 29. Ozonizer [0069] 30. Ozone supply conduit [0070] 31. Bypass conduit [0071] 32. Compressor [0072] 33. Pressure reducer [0073] 34. — [0074] 35. Ozonizer [0075] 36. Ozone supply conduit