METHOD AND PLANT FOR PRODUCING NITRIC ACID

Abstract

A process and a plant for producing nitric acid involves oxidizing ammonia in the presence of catalysts to provide nitrogen monoxide-containing process gas in an oxidation reactor. The formed nitrogen monoxide may be supplied with oxygen-containing gas, and nitrogen monoxide is oxidized to provide nitrogen dioxide that is reacted with water in an absorption apparatus to give nitric acid, nitrous acid, and/or solutions of nitrates and/or nitrites. Oxidation of the nitrogen monoxide may be effected in an additional reactor positioned between the oxidation reactor and the absorption apparatus and traversed by the process gas. The oxidation of the nitrogen monoxide may be effected in an additional reactor parallel and connected to the absorption apparatus and traversed by the process gas. The disclosed processes and plants feature a high energy efficiency combined with a simple construction, and existing plants are easily upgradeable.

Claims

1.-21. (canceled)

22. A process for producing nitric acid comprising: oxidizing ammonia with oxygen in a presence of catalysts to provide a process gas containing nitrogen monoxide in an oxidation reactor; supplying oxygen-containing gas to the nitrogen monoxide; and oxidizing the nitrogen monoxide in a second reactor to provide nitrogen dioxide that is reacted with water in an absorption apparatus to provide nitric acid, nitrous acid, and/or solutions of nitrates and/or nitrites, wherein the second reactor is a container charged with a catalyst for oxidizing the nitrogen monoxide to provide the nitrogen dioxide, wherein the second reactor is at least one of: positioned between the oxidation reactor and the absorption apparatus with respect to a flow direction of the process gas such that the process gas traverses the second reactor, or positioned in parallel with the absorption apparatus and connected to the absorption apparatus such that the process gas traverses the second reactor.

23. The process of claim 22 comprising causing the process gas to enter the second reactor at a temperature of 160 C.-350 C.

24. The process of claim 22 wherein the second reactor is positioned downstream of the oxidation reactor and upstream of a residual gas heater with respect to the flow direction of the process gas.

25. The process of claim 22 wherein the second reactor is positioned downstream of a compression stage for compressing the process gas to an absorption pressure and upstream of a heat exchanger with respect to the flow direction of the process gas.

26. The process of claim 22 wherein the second reactor is positioned downstream of the oxidation reactor and upstream of an economizer with respect to the flow direction of the process gas.

27. The process of claim 22 wherein the second reactor is positioned between an economizer and a heat exchanger with respect to the flow direction of the process gas.

28. The process of claim 22 wherein the second reactor is in parallel with the absorption apparatus, wherein the process comprises absorbing the nitrogen dioxide in water in the absorption apparatus, wherein the second reactor comprises connection and feed points to the absorption apparatus.

29. The process of claim 22 wherein, with respect to the flow direction of the process gas, the second reactor is positioned downstream of the oxidation reactor and upstream of a heat exchanger that transfers heat into a system that provides a thermodynamic process for converting heat into mechanical energy.

30. The process of claim 22 wherein the absorption apparatus is a first absorption apparatus, wherein, with respect to the flow direction of the process gas, the second reactor is positioned downstream of the oxidation reactor between the first and second absorption apparatuses that absorb nitrogen dioxide in water.

31. The process of claim 22 wherein the second reactor is a container whose total gas content is large enough for the oxidation of the nitrogen monoxide to provide the nitrogen dioxide as a gas-phase reaction to proceed substantially completely.

32. The process of claim 22 wherein the second reactor is a radial bed reactor comprising a concentric catalyst bed that is traversed by the process gas.

33. The process of claim 22 wherein the second reactor is a pipeline for the process gas.

34. The process of claim 33 wherein the catalyst for the oxidation of the nitrogen monoxide is a honeycomb body coated with catalytically active materials that is integrated into the pipeline.

35. A plant for producing nitric acid comprising: an oxidation reactor for ammonia oxidation, the oxidation reactor including a feed conduit for a reactant gas mixture containing ammonia and oxygen, and a discharge conduit for a process gas containing nitrogen monoxide; a catalyst for oxidizing ammonia with oxygen in an interior of the oxidation reactor; an absorption apparatus for absorbing nitrogen dioxide and forming nitric acid, nitrous acid, or solutions of nitrates and/or nitrites; and a second reactor for oxidizing nitrogen monoxide over a catalyst to provide nitrogen dioxide, the second reactor being disposed at least one of between the oxidation reactor and the absorption apparatus or downstream of the absorption apparatus such that the process gas traverses the second reactor, or parallel with the absorption apparatus and connected to the absorption apparatus such that the process gas traverses the second reactor.

36. The plant of claim 35 comprising a residual gas heater having a heat exchanger function that is operatively connected to the absorption apparatus and is traversed by a residual gas stream exiting the absorption apparatus, wherein the second reactor is positioned between the oxidation reactor and the residual gas heater.

37. The plant of claim 35 comprising: a residual gas heater having a heat exchanger function that is operatively connected to the absorption apparatus and is traversed by a residual gas stream exiting the absorption apparatus; and a compression stage for compressing the process gas to an absorption pressure, wherein the second reactor is positioned between the compression stage and the residual gas heater or between the compression stage and the absorption apparatus.

38. The plant of claim 35 comprising: a residual gas heater having a heat exchanger function that is operatively connected to the absorption apparatus and is traversed by a residual gas stream exiting the absorption apparatus; and an economizer that is traversed by the process gas, wherein the second reactor is positioned between the residual gas heater and the economizer.

39. The plant of claim 35 comprising a third reactor for oxidizing nitrogen monoxide to provide nitrogen dioxide, wherein at least one of the second or third reactors is positioned between the oxidation reactor and a residual gas heater, wherein at least one of the second or third reactors is positioned between a compression stage for compressing the process gas to an absorption pressure and the absorption apparatus.

40. The plant of claim 35 comprising a third reactor for oxidizing nitrogen monoxide to provide nitrogen dioxide, wherein at least one of the second or third reactors is positioned between the oxidation reactor and a residual gas heater, wherein at least one of the second or third reactors is positioned between a residual gas heater and an economizer that is traversed by the process gas.

41. The plant of claim 35 comprising a third reactor for oxidizing nitrogen monoxide to provide nitrogen dioxide, wherein the second reactor is positioned between the oxidation reactor and the absorption apparatus with respect to a flow direction of the process gas, wherein the third reactor is parallel with the absorption apparatus with respect to the flow direction of the process gas, downstream of the absorption apparatus with respect to the flow direction of the process gas, or between the absorption apparatus and a second absorption apparatus in which the absorption of nitrogen dioxide in water occurs.

42. The plant of claim 35 wherein the second reactor is parallel with the absorption apparatus and connected to an upper third and middle of the absorption apparatus, or to a lower third of the absorption apparatus.

Description

[0077] The present invention is hereinbelow more particularly elucidated by means of exemplary embodiments with reference to the accompanying drawings. In the figures:

[0078] FIG. 1 shows a nitric acid plant according to the prior art in the two-pressure process;

[0079] FIGS. 2 to 5 each show modified nitric acid plants according to possible exemplary embodiments of the present invention, by way of example in the two-pressure process;

[0080] FIG. 6 shows a modified nitric acid plant according to a possible exemplary embodiment (E) of the present invention, by way of example in the monopressure process.

[0081] FIG. 1 shows a simplified process flow diagram of a typical conventional two-pressure plant for producing nitric acid. The plant comprises an NH.sub.3 oxidation reactor 1 in which the oxidation of the ammonia to afford nitric oxide (NO) proceeds according to the reaction scheme (I) hereinabove. This NH.sub.3 oxidation reactor 1 is supplied via a compressor 15 with combustion air. Gaseous ammonia is mixed with the combustion air and this mixed gas is then supplied to the NH.sub.3 oxidation reactor 1. Said reactor usually has a steam generator 4 connected directly downstream of it for recovery of the high-caloric-level combustion heat. The NO gas produced by the reaction in the NH.sub.3 oxidation reactor 1 then flows to a residual gas heater 2 and then through an economizer 3 (which has a heat exchanger function). In the cooler/condenser 6a the process gas is then supercooled, i.e. cooled to below its dew point. This results in partial condensation of the water proportion present in the process gas and in a proportion of acid formation by absorption (reaction III, cooler/condenser already operates as an absorption apparatus). Since the example concerns a two-pressure plant it includes a subsequent additional compression stage 7 performing a compression to the desired absorption pressure. The NO gas then optionally traverses further heat exchangers 10d and a further cooler and condenser 6b and then arrives in a (main) absorption apparatus 8 in which the nitric acid product is formed by absorption of NO.sub.2 in water according to the above reaction scheme (III and II).

[0082] There is generally no limitation on the number and sequence of the heat exchangers used for cooling the process gas above the dew point (residual gas heater, economizer, heat exchanger with other cooling media). The number and sequence is determined by manufacturing, construction or infrastructural technical factors and design requirements. Further residual gas heaters, economizers or heat exchangers with other cooling media may occupy each of positions 10a, 10b, 10c, 10d.

[0083] The residual gas exits the absorption apparatus 8 in the top region thereof and is heated in the residual gas heater 2 to then arrive into a residual gas reactor 11 in which residual NO.sub.x and optionally N.sub.2O are removed generally by catalytic means. The residual gas finally traverses a residual gas turbine 18 for energy recovery during decompression of the residual gas into the atmosphere.

[0084] There is generally no limitation on the number and sequence of the heat exchangers used for heating the residual gas (residual gas heater, heat exchanger with other heating media). The number and sequence are determined by manufacturing, construction or infrastructural technical factors and design requirements. Further residual gas heaters or heat exchangers with other heating media may occupy each of positions 12a, 12b.

[0085] The plant further comprises a functional unit 9 for workup of the product acid produced in the absorption apparatus 8 using the secondary air stream. This secondary air stream is a substream diverted upstream of the reactor 1 from the combustion air produced by the air compressor 15. After exiting the functional unit 10 this secondary air stream may be supplied to the process gas stream via conduit 9b, for example downstream of the cooler and condenser 4. This increases the oxygen content of the process gas.

[0086] FIG. 2 shows a simplified process flow diagram of a two-pressure plant for producing nitric acid which has been modified according to the invention. According to the invention the plant comprises not only the above-described elements 1 to 18 but also the additional reactor A in which the NO present in the gas stream is oxidized to afford NO.sub.2 as completely as possible. In FIG. 2 this is the exemplary additional reactor A, a catalyst being present as an irregular dumped bed in a radial reactor. The concentric catalyst bed (irregular dumped bed) is traversed from the inside outward in this example. It is generally also possible according to the invention for a plurality of inventive reactors A to be provided. The additional reactor A is traversed by the NO-containing process gas. The oxidation reaction proceeding in these additional reactors evolves additional heat which effects further heating of the process gas. The residual gas can thus be subjected to stronger heating in the downstream residual gas heater 2. This necessarily results in a particularly efficient recuperation of the additionally generated usable heat by decompression in the residual gas turbine 18. This energy may then be used directly for propulsion of the compressors 15, 7 for example, as indicated in FIG. 2 by the dashed shaft 20.

[0087] FIG. 3 shows a simplified process flow diagram of a two-pressure plant for producing nitric acid which has been modified according to the invention. According to the invention the plant comprises not only the above-described elements 1 to 18 but also the additional reactor B in which the NO present in the gas stream is oxidized to afford NO.sub.2 as completely as possible. In FIG. 3 this is the exemplary additional reactor B, a catalyst being present as a coating in the pipeline system or in a suitable container. It is generally also possible according to the invention for a plurality of inventive reactors B of this type to be provided. The additional reactor B is traversed by the NO-containing process gas. The oxidation reaction proceeding in this additional reactor evolves additional heat which effects further heating of the process gas. This allows more heat to be recovered in a downstream heat exchanger 10d of any desired type. In modern two-pressure plants a temperature preferred for the oxidation of NO to NO.sub.2 may be obtained directly by compression in the second compression stage and the compression heat associated therewith.

[0088] FIG. 4 shows a simplified process flow diagram of a further two-pressure plant for producing nitric acid which has been modified according to the invention. According to the invention the plant comprises not only the above-described elements 1 to 18 but also the additional reactor C in which the NO present in the gas stream is oxidized to afford NO.sub.2 as completely as possible. In FIG. 4 this is the exemplary additional reactor C, a catalyst as a structured packing in the form of honeycomb bodies being integrated into the pipeline system or being present as such in a suitable reactor container. It is generally also possible according to the invention for a plurality of inventive reactors C of this type to be provided. The additional reactor C is traversed by the NO-containing process gas. The oxidation reaction proceeding in these additional reactors evolves additional heat which effects further heating of the process gas. Water can thus be subjected to stronger heating in the downstream economizer 3. This additionally generated usable heat can be utilized internally and without substantial additional infrastructure for steam generation in the steam generator 4 as indicated in the drawing. Generated steam may be used for example for machine propulsion via a steam turbine, for electricity generation or, similarly to water in the case of using the economizer 3 for hot water generation, for heating purposes for example.

[0089] FIG. 5 shows a simplified process flow diagram of a further two-pressure plant for producing nitric acid which has been modified according to the invention. According to the invention the plant comprises not only the above-described elements 1 to 18 but also the additional reactor D in which the NO present in the gas stream is oxidized to afford NO.sub.2 as completely as possible. It is generally also possible according to the invention for a plurality of inventive reactors D of this type to be provided. The additional reactor D is traversed by the NO-containing process gas. The oxidation reaction proceeding in these additional reactors evolves additional heat which effects further heating of the process gas. This allows any desired suitable heat transfer medium to be subjected to stronger heating in the downstream heat exchanger 22. This additionally generated usable heat may, as indicated in the drawing, be recovered in the indicated system for conversion of heat into mechanical energy. Such external systems, here represented generally through compression 21 and decompression 22 of the heat transfer medium, allow particularly flexible utilization of the additional usable heat generated according to the invention for generation of mechanical energy.

[0090] FIG. 6 shows a simplified process flow diagram of a further monopressure plant for producing nitric acid which has been modified according to the invention. The (main) absorption apparatus 8 in which NO.sub.2 is absorbed in water to afford nitric acid according to reaction 3 (in countercurrent here) is divided into two apparatuses 8a, 8b in this exemplary embodiment. According to the invention the plant comprises not only the above-described elements 1 to 18 but also the additional reactor E in which the NO present in the gas stream is oxidized to afford NO.sub.2 as completely as possible. It is generally also possible according to the invention for a plurality of inventive reactors E of this type to be provided. The additional reactor E is traversed by NO-containing process gas. The oxidation reaction proceeding in these additional reactors evolves additional heat which may be recovered in the downstream heat exchanger 25 of any desired type. The arrangement between the two (main) absorption apparatuses 8a and 8b reduces the required total volume for absorption in the apparatuses 8a, 8b.

[0091] Similar alternative variants of this invention both in the monopressure and in the two-pressure process are the arrangement of the additional reactor (E) in parallel with only one absorption apparatus 8 by means of an intermediate takeoff or else the arrangement of the additional reactor E between cooler/condenser 6 and absorption apparatus 8.

LIST OF REFERENCE NUMERALS

[0092] 1 NH.sub.3 oxidation reactor

[0093] 2 residual gas heater

[0094] 3 economizer

[0095] 4 steam generator

[0096] 6a cooler/condenser

[0097] 6b cooler/condenser

[0098] 7 compression stage/compressor

[0099] 8 absorption apparatus

[0100] 8a absorption apparatus

[0101] 8b absorption apparatus

[0102] 9 functional unit

[0103] 9b conduit

[0104] 10 residual gas reactor

[0105] 10a further residual gas heater, economizer or heat exchanger

[0106] 10b further residual gas heater, economizer or heat exchanger

[0107] 10c further residual gas heater, economizer or heat exchanger

[0108] 10d further residual gas heater, economizer or heat exchanger

[0109] 11 residual gas reactor

[0110] 12a further residual gas heater or heat exchanger

[0111] 12b further residual gas heater or heat exchanger

[0112] 15 air compressor/compressor

[0113] 18 residual gas turbine

[0114] 20 shaft

[0115] 21 compression

[0116] 22 decompression/downstream heat exchanger

[0117] 25 downstream heat exchanger

[0118] A additional reactor

[0119] B additional reactor

[0120] C additional reactor

[0121] D additional reactor

[0122] E additional reactor