PROCESS AND PLANT FOR PREPARING NITRIC ACID

Abstract

A process for producing nitric acid by the Ostwald process involves reacting ammonia with atmospheric oxygen as primary air to afford a NOx-containing gas stream in an ammonia oxidation reactor at a first pressure and absorbing the NOx-containing gas stream in water in an absorption apparatus at a higher, second pressure. Nitric acid is bleached with bleach air as secondary air at approximately the first pressure. The secondary air is brought to an operating pressure of the bleaching operation via a separate secondary air compressor or compressor stage. The separate secondary air compressor is independent of the compressor that brings the primary air to the first pressure. Compression to the second higher pressure at which the absorption of the NOx gases is performed in the absorption apparatus is provided only downstream of the bleaching operation.

Claims

1.-21. (canceled)

22. A process for producing nitric acid by the Ostwald process, comprising: reacting ammonia with atmospheric oxygen as primary air to afford a NOx-containing gas stream in an ammonia oxidation reactor at a first pressure; absorbing the NOx-containing gas stream in water in an absorption apparatus at a second pressure that is higher than the first pressure; and bleaching nitric acid with bleach air as secondary air, wherein the bleaching is performed at an operating pressure that is closer to the first pressure than the second pressure; wherein the secondary air is brought to the operating pressure of the bleaching with a separate secondary air compressor or a secondary air compressor stage, wherein the separate secondary air compressor or the secondary air compressor stage is independent of a compressor that brings the primary air to the first pressure of the ammonia oxidation reactor, wherein downstream of the bleaching, a gas mixture formed in the bleaching is brought to the second pressure with a bleach air compressor or a bleach air compressor stage at which second pressure the absorption of the NOx-containing gas stream in water occurs, wherein the bleach air compressor or the bleach air compressor stage operates independently of an NOx compressor that compresses a product gas mixture of the ammonia oxidation reactor to the second pressure.

23. The process of claim 22 wherein an output stream of a low-pressure bleaching apparatus, which is brought to the second pressure in the bleach air compressor or the bleach air compressor stage, at an entrance to the bleach air compressor or the bleach air compressor stage comprises a residual content of NOx (NO+NO.sub.2) of 0-5000.

24. The process of claim 22 wherein an output stream of a low-pressure bleaching apparatus, which is brought to the second pressure in the bleach air compressor or the bleach air compressor stage, has monitoring means therein for a residual content of NOx (NO+NO.sub.2) and/or of water and/or for parameters that depend on the residual content of NOx and the water.

25. The process of claim 22 wherein an acid feed stream to a low-pressure bleaching apparatus includes means therein for adhering to a preferred residual content of NOx (NO+NO.sub.2).

26. The process of claim 25 wherein the means for adhering to the preferred residual content of NOx (NO+NO.sub.2) at an entrance to the bleach air compressor or the bleach air compressor stage includes an upstream bleaching tower that is operated at the second pressure level of the absorption apparatus and is supplied on a gas side with an output stream of the bleach air compressor or the bleach air compressor stage.

27. The process of claim 23 wherein the output stream of the low-pressure bleaching apparatus includes means for adhering to a preferred residual content of NOx (NO+NO.sub.2) at the entrance to the bleach air compressor or the bleach air compressor stage.

28. The process of claim 27 wherein the means for adhering to the preferred residual content of NOx is configured as a catalytic measure for a selective catalytic reduction of NOx with NH.sub.3.

29. The process of claim 27 wherein the means for adhering to the preferred residual content of NOx is configured as a dilution of the output stream of the low pressure bleaching apparatus with additional air that is provided from the bleach air compressor or from the bleach air compressor stage.

30. The process of claim 22 wherein the bleach air compressor or the bleach air compressor stage is operatively connected to the separate secondary air compressor or the secondary air compressor stage.

31. The process of claim 30 wherein the bleach air compressor or the bleach air compressor stage is coupled to the separate secondary air compressor or the secondary air compressor stage via transmission means.

32. The process of claim 22 wherein the secondary air brought to the first pressure by the separate secondary air compressor or the secondary air compressor stage is supplied to a bleaching apparatus operating at the first pressure via a separate conduit.

33. The process of claim 32 comprising supplying the gas mixture of NOx-laden bleach air exiting the bleaching apparatus to the bleach air compressor or the bleach air compressor stage via a conduit.

34. The process of claim 22 comprising passing the gas mixture that is brought to the second pressure and exits the bleach air compressor or the bleach air compressor stage into a region downstream of the NOx compressor via a conduit, with the gas mixture being subsequently supplied to the absorption apparatus.

35. A plant for producing nitric acid, comprising: a primary air compressor; a secondary air compressor disposed in a conduit tract separate from the primary air compressor; an NOx compressor disposed on a same shaft as the primary air compressor; an ammonia oxidation reactor; an absorption apparatus; a bleaching apparatus; a bleach air compressor or a bleach air compressor stage disposed downstream of the secondary air compressor, wherein the bleach air compressor or the bleach air compressor stage is configured to bring a laden gas stream from the bleaching apparatus to the second pressure at which absorption of an NOx-containing gas stream occurs in water.

36. The plant of claim 35 wherein an exit of the secondary air compressor is connected via a conduit to a bleaching apparatus, wherein a conduit connects an exit of the bleaching apparatus to an entrance of the bleach air compressor or the bleach air compressor stage, wherein the bleach air compressor or the bleach air compressor stage is distinct from the NOx compressor.

37. The process of claim 35 wherein a conduit that extends from a high-pressure side of the bleach air compressor or the bleach air compressor stage opens into a conduit in a high-pressure region downstream of the NOx compressor.

38. The process of claim 35 wherein the secondary air compressor and the bleach air compressor are configured as two separate compressors coupled to one another via transmission means.

39. The process of claim 35 wherein the secondary air compressor is configured as a first compressor stage of a compressor having at least two compressor stages and the bleach air compressor stage is configured as a second compressor stage of the same compressor, wherein the two compressor stages are coupled to one another via transmission means.

40. The process of claim 35 wherein the secondary air compressor and the bleach air compressor or a secondary air compressor stage and the bleach air compressor stage are configured as transmission turbo compressors, wherein respective compressor shafts of the transmission turbo compressors or of respective compressor stages are coupled to one another via transmission means.

41. The process of claim 35 wherein the primary air compressor and the NOx compressor are disposed on a same drive shaft while the secondary air compressor or a secondary air compressor stage and the bleach air compressor or the bleach air compressor stage are arranged in a drive train separate from the drive shaft.

42. The process of claim 35 comprising a high-pressure acid condenser that is disposed downstream of the bleach air compressor or the bleach air compressor stage.

Description

[0045] The present invention is more particularly described hereinbelow by means of an exemplary embodiment and with reference to the accompanying figure. In this connection:

[0046] FIG. 1 shows a schematically simplified plant scheme of an exemplary inventive plant for producing nitric acid by the dual-pressure process;

[0047] FIG. 2 shows a schematically simplified plant scheme of an exemplary plant for producing nitric acid by the dual-pressure process according to an alternative variant of the present invention.

[0048] The following refers initially to FIG. 1, and a first exemplary variant of the process according to the invention is more particularly elucidated with reference thereto. The representation of FIG. 1 is a much-simplified schematic diagram and only the plant components that are relevant in the context of the present invention are shown.

[0049] The inventive plant comprises an ammonia oxidation reactor 11 to which ammonia is supplied via a conduit 12 (see arrow 12). In this ammonia oxidation reactor 11, ammonia is catalytically oxidized to nitrogen oxides (NOx) using atmospheric oxygen.

[0050] The supply of air for this oxidation reaction (so-called primary air) is effected via conduit 13, wherein the primary air is brought using the air compressor 14 to a first pressure (for example 2 to 6 bar) at which the reaction in the ammonia oxidation reactor is carried out. The oxidation of the ammonia is carried out catalytically at high temperatures of for example about 900° C., wherein the ammonia is initially oxidized to nitrogen monoxide (NO) and then, after temperature reduction, to nitrogen dioxide (NO.sub.2). The high temperature in the ammonia oxidation reactor 11 is utilized for producing steam, wherein the steam 15 produced is supplied to a steam turbine 16, by means of which at least a portion of the propulsion energy for the compressors used in the plant is obtained. To this end, the steam turbine 16, a first air compressor 14, a NOx compressor 17 and a residual gas turbine 18 are arranged on the same shaft 19.

[0051] The NOx-containing product gas produced in the ammonia oxidation reactor 11 exits the reactor via the conduit 20 and is initially supplied to one or more heat exchangers 21 for heat recovery and subsequently supplied via conduit 22 to an intermediate-pressure acid condenser 23 in which a cooling is effected, thus condensing water present in the product gas stream to form, together with the NOx gas, a first proportion of acid condensate which may be supplied using pump 24 to the absorption tower 25 (the relevant connecting conduit is not shown here). The NOx-containing gas stream flows from the acid condenser 23 via conduit 26 to the NOx compressor 17 and is compressed therein to a higher pressure (for example to about 9 to 16 bar) since this is a dual-pressure process and is subsequently supplied via conduit 27 to a high-pressure acid condenser 28, in which a portion of acid condensate is likewise obtained, and the product mixture then passes through conduit 29 into the absorption tower 25 in which the NOx gas is absorbed in water and nitric acid (HNO.sub.3) is produced. The process water required for the reaction may be supplied to the absorption tower 25 via a conduit indicated by arrow 30.

[0052] The residual gas (so-called tail gas) exits at the top of the absorption tower 25 via conduit 31, is passed through a residual gas heater 32 and then supplied via conduit 33 to a residual gas reactor 34 which is also supplied with ammonia which reacts with the nitrogen oxides remaining in the residual gas to afford nitrogen and water, so that the residual gas is freed of NOx and purified. The residual gas is then supplied to the residual gas turbine 18 via conduit 35 and decompressed therein. This drives the residual gas turbine 18 and, since it is on the same shaft 19 as the two compressors 14 and 17, the residual gas turbine 18 likewise provides a proportion of the propulsion energy for the two compressors 14, 17. The decompressed residual gas may be discharged via a chimney for example.

[0053] The nitric acid produced in the absorption tower 25 is discharged via the arrow 36 (the representation of FIG. 1 is a simplified plant scheme which does not show all plant parts and conduits) and is then supplied, as indicated by the further arrow 37, to a bleaching tower 38 in which the nitric acid is purified by stripping the NOx gases present in the product mixture using air. This is done using the so-called secondary air which in conventional dual-pressure processes for producing nitric acid is provided by the same compressor as the primary air for oxidation of the ammonia to NOx. However, the process according to the invention employs an assembly of at least two compressors 40, 41 or a two-stage or multi-stage compressor, in particular a transmission turbo compressor as indicated in the exemplary embodiment. This transmission turbo compressor comprises a first compressor 40 (also referred to herein as secondary air compressor) or a first compressor stage to which air is supplied, wherein the air compressed to an elevated pressure is supplied via the conduit 42 to the bleaching tower 38, so that, similarly to a conventional dual-pressure plant, the bleaching operation is carried out at a first lower pressure which is approximately of the same order of magnitude as the pressure in the ammonia oxidation reactor 11. The bleaching tower 38 may also have a cooler 43 arranged upstream of it. The purified nitric acid is discharged in the lower region from the bleaching tower 38 via the conduit 44 (see arrow).

[0054] After the bleaching operation in the bleaching tower 38 the gas stream in the upper region is discharged from the bleaching tower via the conduit 45 and then supplied to the second compressor 41 (also referred to herein as further bleach air compressor) or the second compressor stage and via this second compressor 41 compressed to a higher pressure, so that the compressed gas stream can flow via the conduit 46 downstream of the second compressor 41 into the high-pressure region of the plant where the conduit 46 opens into the conduit 27 in which the NOx gases compressed by the NOx compressor 17 flow, so that the gas stream from the conduit 46 may be supplied to the high-pressure acid condenser 28 together with the NOx gas stream from conduit 27 and is recycled into the nitric acid production.

[0055] It is a particular feature of the plant according to the invention that the two compressors 40, 41, i.e. the first compressor 40 which compresses the secondary air to a first pressure and the second compressor which compresses the gas stream from the bleaching tower 38 to a second pressure higher than the first pressure, are effectively mechanically coupled to one another via a transmission 39. Transmission turbo compressors are known in principle from the prior art and are described for example in EP 3 106 670 A1. Transmission turbo compressors generally comprise a drive shaft and at least two compressor shafts of the respective compressor, wherein the compressor shafts are each coupled to the drive shaft via transmission means and therefore also coupled to one another. The drive shaft and the compressor shafts may each be arranged parallel to one another or else perpendicular to one another when using bevel gears as described for example in the abovementioned European patent specification. FIG. 1 shows merely a schematic of the transmission turbo compressor used according to the invention with the two compressors 40, 41, wherein the coupling is indicated via a shaft 47 and the transmission 39 is illustrated by a rectangular box.

[0056] An exemplary alternative variant of the present invention is more particularly elucidated hereinbelow with reference to FIG. 2. In the variant according to FIG. 2, the plant for producing nitric acid by the dual-pressure process is in large parts configured in a similar fashion as in the variant according to FIG. 1, and reference is therefore made to the preceding description in this respect. Therefore, only the regions of the plant which are divergent are more particularly elucidated hereinbelow. These divergences are essentially in the lower right-hand region of the drawing. In a departure from the variant of FIG. 1 the NOx-laden gas stream from the bleach air compressor 41 compressed in the second bleach air compressor 14, via conduit 46, is not passed directly into the high-pressure region downstream of the NOx compressor 17 and upstream of the high-pressure acid condenser 28 but rather is initially supplied to an upstream bleaching tower 48 where this gas stream bleaches the acid stream from the absorption tower 25 (by stripping), wherein the gas stream then exits from the top of the upstream bleaching tower 48 and is passed from there into the high-pressure region where it is admixed with the NOx gas stream in conduit 27. The acid from the absorption tower 25 and from the high-pressure acid condenser 28 enters the upstream bleaching tower 48 in countercurrent to the gas stream and the prepurified acid exits the lower region of the bleaching tower 48 and is then passed via the conduit 36 (see arrow) to the bleaching tower 38 in the low-pressure region which is also referred to herein as the low-pressure bleaching apparatus. This makes it possible to adhere to a preferred residual content of NOx (NO+NO.sub.2) at the entrance to the further bleach air compressor 41 or a further bleach air compressor stage since effectively two bleaching measures are arranged in series.

[0057] Another alternative variant of the process, which is not shown in the figures, provides that for adhering to the preferred residual content of NOx (NO+NO.sub.2) at the entrance to the further bleach air compressor 41 or the further bleach air compressor stage the output stream of the low-pressure bleaching apparatus is provided with a catalytic measure therein for reduction of the NOx content, which especially consists of a selective catalytic reduction of NOx with NH.sub.3. To this end, ammonia may for example be diverted from the conduit 12 shown in FIG. 1 and introduced into the conduit 45 downstream of the bleaching tower to reduce the NOx content in the laden bleach air by reaction with ammonia and thus protect the bleach air compressor 41.

[0058] In a further alternative variant of the process according to the invention, which is likewise not shown in the figures, a measure envisaged for adhering to the preferred residual content of NOx (NO+NO.sub.2) in the output stream from the low-pressure bleaching apparatus at the entrance to the further bleach air compressor or the further bleach air compressor stage is a dilution of the output stream from the low-pressure bleaching apparatus with additional air which may be provided for example by the first bleach air compressor 40 or the first bleach air compressor stage. To this end air may be diverted for example from the conduit 42 downstream of the bleach air compressor 40 via a branch conduit which is preferably closable via a valve and which then opens into the conduit 45 downstream of the bleaching tower 48. Alternatively, this branch conduit could also open out between the cooler 43 and the bleaching tower 38 and supply additional air there.

LIST OF REFERENCE NUMERALS

[0059] 11 Ammonia oxidation reactor [0060] 12 Conduit for ammonia [0061] 13 Conduit for primary air [0062] 14 Air compressor [0063] 15 Steam [0064] 16 Steam turbine [0065] 17 NOx compressor [0066] 18 Residual gas turbine [0067] 19 Shaft [0068] 20 Conduit for NOx-containing product gas [0069] 21 Heat exchanger [0070] 22 Conduit [0071] 23 Intermediate-pressure acid condenser [0072] 24 Pump [0073] 25 Absorption tower, absorption apparatus [0074] 26 Conduit [0075] 27 Conduit [0076] 28 High-pressure acid condenser [0077] 29 Conduit [0078] 30 Arrow, supply of process water [0079] 31 Conduit for residual gas [0080] 32 Residual gas heater [0081] 33 Conduit [0082] 34 Residual gas reactor [0083] 35 Conduit [0084] 36 Arrow [0085] 37 Arrow [0086] 38 Bleaching tower/low-pressure bleaching apparatus [0087] 39 Transmission [0088] 40 Secondary air compressor or compressor stage [0089] 41 Bleach air compressor or compressor stage [0090] 42 Conduit for secondary air (bleach air) [0091] 43 Cooler [0092] 44 Conduit for nitric acid [0093] 45 Conduit [0094] 46 Conduit [0095] 47 Shaft [0096] 48 Upstream bleaching tower