ABSORPTION TOWER FOR A NITRIC ACID PLANT METHOD FOR PRODUCING NITRIC ACID

Abstract

An absorption tower for production of nitric acid by the Ostwald process may include sieve trays that are arranged on top of one another and each spaced apart from one another, a water inlet in an upper region of the absorption tower, an inlet for gaseous nitrogen oxides in a lower region of the absorption tower, and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray and is divided by a dividing wall into a first, radially inner region and at least a second, radially outer region. Nitric acid that trickles down from the lowermost sieve tray with a higher concentration can be collected in a middle region. The less-concentrated nitric acid that then effluxes from sieve trays higher up can then be collected separately in a region farther outward.

Claims

1.-20. (canceled)

21. An absorption tower for a plant for production of nitric acid by the Ostwald method, the absorption tower comprising: sieve trays disposed one on top of another and each spaced apart from one another; an inlet for water in an upper region of the absorption tower; an inlet for gaseous nitrogen oxides in a lower region of the absorption tower; and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray of the sieve trays, the column bottom being divided by a dividing wall into mutually separated regions, wherein the dividing wall is disposed with such a progression that the dividing wall divides the column bottom into a first, radially inner region and a second, radially outer region.

22. The absorption tower of claim 21 wherein the dividing wall is cylindrical at least in sections.

23. The absorption tower of claim 22 wherein the dividing wall is disposed in the column bottom in a concentric arrangement with a radial distance from an outer wall and divides the column bottom into a first cylindrical central region within the dividing wall and a second annular radially outer region outside the dividing wall.

24. The absorption tower of claim 22 wherein the dividing wall comprises: a first, radially inner dividing wall that is disposed in the column bottom in a concentric arrangement with a radial distance from an outer wall of the column bottom; and a second, radially outer dividing wall that is disposed in a concentric arrangement with a radial distance from the first, radially inner dividing wall and between the first, radially inner dividing wall and the outer wall.

25. The absorption tower of claim 24 wherein the second, radially outer dividing wall extends less far in an upward direction than the first, radially inner dividing wall.

26. The absorption tower of claim 24 wherein the dividing wall comprises a third dividing wall that is concentric with the second, radially outer dividing wall, between the second, radially outer dividing wall and the outer wall of the column bottom.

27. The absorption tower of claim 26 wherein the third dividing wall is shorter than the second, radially outer dividing wall.

28. The absorption tower of claim 21 wherein the dividing wall comprises mutually parallel dividing walls that divide the column bottom into one rectangular, radially inner region and two segment-shaped radially outer regions.

29. The absorption tower of claim 28 wherein the dividing wall comprises at least four mutually parallel dividing walls that divide the column bottom into one rectangular, radially inner region, two middle regions, and two segment-shaped radially outer regions.

30. The absorption tower of claim 21 comprising a cover above the dividing wall, wherein the cover is inclined at an angle relative to horizontal toward a middle of the column bottom, wherein the cover fully covers a shell space between the dividing wall and an outer wall of the column bottom.

31. The absorption tower of claim 21 comprising a cover disposed in the upper region at the dividing wall, the cover being inclined at an angle to horizontal toward a middle of the column bottom at least in sections.

32. The absorption tower of claim 31 wherein the dividing wall comprises a first dividing wall and a second dividing wall, wherein the cover is disposed on each of the first dividing wall and the second dividing wall in the upper region.

33. The absorption tower of claim 31 wherein the dividing wall comprises a first dividing wall and a second dividing wall, wherein the cover is sized and shaped such that the cover only partly covers a radially inner region of an annular gap between the first dividing wall that is radially further inward and the second dividing wall that is adjacent thereto and is radially further outward.

34. The absorption tower of claim 21 comprising: a first conduit for nitric acid that extends from a first, radially inner region to draw off nitric acid therefrom; and a second conduit for nitric acid that is separate from the first conduit and extends from a second, radially outer region to draw off nitric acid from the second, radially outer region.

35. A plant for producing nitric acid, the plant comprising an absorption tower that includes: sieve trays disposed one on top of another and each spaced apart from one another; an inlet for water in an upper region of the absorption tower; an inlet for gaseous nitrogen oxides in a lower region of the absorption tower; and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray of the sieve trays, the column bottom being divided by a dividing wall into mutually separated regions, wherein the dividing wall is disposed with such a progression that the dividing wall divides the column bottom into a first, radially inner region and a second, radially outer region.

36. A process for producing nitric acid by the Ostwald process, the process comprising absorbing gaseous nitrogen oxides in water to produce nitric acid in an absorption tower, wherein the nitrogen oxides are introduced into the absorption tower in a lower region and water is introduced into the absorption tower in an upper region, wherein the nitric acid formed flows downward through sieve trays arranged one on top of another and each spaced apart from one another, wherein the nitrogen oxides flow upward from below through the absorption tower in countercurrent with an aqueous liquid, wherein in a column bottom of the absorption tower concentrated nitric acid accumulates, wherein a dividing wall in the column bottom creates mutually divided regions in which nitric acid of different concentration is collected, wherein by a cover above one of the mutually divided regions nitric acid that effluxes first from the sieve trays flows only into one of the mutually divided regions.

37. The process of claim 36 wherein concentrated nitric acid effluxing from the sieve trays is collected in a first, central region of the mutually divided regions of the column bottom, wherein only after this first, central region has filled completely does less concentrated nitric acid pass via overflow over the dividing wall into a second region of the mutually divided regions that is radially farther outward and is divided from the first, central region.

38. The process of claim 37 wherein after completely filling the second region less concentrated nitric acid passes via overflow over a second dividing wall into a third region of the mutually divided regions that is radially farther outward and is divided from the second region.

39. The process of claim 36 wherein positions of the dividing wall in the column bottom and of the cover above the mutually divided regions determines a ratio of volumes of more highly concentrated to less highly concentrated nitric acid that is produced.

40. The process of claim 36 wherein the process is performed in a plant that includes an absorption tower that comprises: sieve trays disposed one on top of another and each spaced apart from one another; an inlet for water in an upper region of the absorption tower; an inlet for gaseous nitrogen oxides in a lower region of the absorption tower; and a column bottom that is disposed in the lower region of the absorption tower beneath a lowermost sieve tray of the sieve trays, the column bottom being divided by a dividing wall into mutually separated regions, wherein the dividing wall is disposed with such a progression that the dividing wall divides the column bottom into a first, radially inner region and a second, radially outer region.

Description

[0051] The present invention is described in detail by two working examples with reference to the appended drawings. The figures show:

[0052] FIG. 1: a schematically simplified diagram of an absorption tower of an illustrative plant of the invention for production of nitric acid;

[0053] FIG. 2: a schematically simplified diagram of the lower region of an absorption tower in an alternative execution variant of an illustrative plant for production of nitric acid according to the present invention.

[0054] Reference is made hereinafter firstly to FIG. 1, and this diagram is used to elucidate a first illustrative execution variant of the invention in detail. The diagram of the absorption tower 10 in FIG. 1 is schematically highly simplified, and only those plant components that are of significance in the context of the present invention are shown. FIG. 1 shows only the absorption tower 10. The other plant components of a plant for production of nitric acid that are not shown here are known per se; the incorporation of the absorption tower into the overall plant for production of nitric acid is likewise known per se and is therefore not described in detail.

[0055] The inventive absorption tower 10 is a column having a fundamentally cylindrical geometry, wherein the column in the top region and in the region of the bottom 15 may be executed as a vessel rounded at each end. In an upper region, process water is introduced into the absorption tower 10 via a feed conduit 12 and, in a lower region, NOx gases or acid condensate are supplied to the absorption tower 10 via a feed conduit 13, such that these NOx gases flow in countercurrent to the water within the absorption tower 10, which flows from the top downward. The residual gas is removed via a conduit 9 at the top of the absorption tower 10 and is then subjected to further processing, which is not shown here in detail.

[0056] In the interior, the absorption tower has a relatively large number of, for example, twenty or more (typically 25 to 40) sieve trays 14, which are preferably each installed horizontally and arranged in parallel one on top of another and at a distance from one another. By way of simplification, FIG. 1 shows just some of these sieve trays. The incoming process water at first arrives at the uppermost sieve tray and flows proceeding therefrom successively further downward to the further sieve trays. The NOx gases ascend in the absorption tower, flow upward through the perforations in the sieve trays 14 and hence come into contact with the aqueous liquid on the sieve trays 14, in which they are absorbed, which results in formation of nitric acid. In steady-state operation, a concentration profile develops within the absorption tower 10, with increasing concentration of the nitric acid on the sieve trays from the top downward. The pressure of the NOx gases flowing from the bottom upward results in formation of a liquid column on each of the sieve trays. But if this gas pressure decreases when the plant is shut down, the effect of this is that the nitric acid trickles downward through the perforations of the sieve trays.

[0057] FIG. 1 shows a first simplified solution variant of the invention, in which a cylindrical dividing wall 16 is installed in the bottom 15 of the absorption column 10 below the lowermost sieve trays 14 in a roughly central region, and preferably runs vertically and divides a first middle, radially inner region 20 of the bottom 15 from an annular second, radially outer region 21 of the bottom. Above the dividing wall 16, spaced apart somewhat therefrom, is disposed a cover 17 in a radially outer region, which runs radially, is connected to the outer cylindrical wall 11 of the absorption tower 10, and extends radially inward therefrom to such an extent that it fully covers the second radially outer region 21 when projected onto the horizontal plane. This cover, which may take the form, for example, of a cover plate or the like made from an acid-resistant material, runs inclined at an angle α to the horizontal from radially outward to radially inward, such that nitric acid that trickles down from the sieve trays 14 above the cover 17 at first flows inward over the cover 17 and then flows exclusively into the first radially inner region 20, since the second, radially outer region 21 is covered by the cover 17. This first middle region 20 is provided with an output conduit 18, such that concentrated nitric acid that collects in the first middle region 20 can be removed separately via this first output conduit 18.

[0058] When the plant is shut down or run down, the first nitric acid to trickle down is that from the lower sieve trays 14, which is the most concentrated, such that predominantly concentrated nitric acid arrives in the middle region 20, which can be applied again to the lower sieve trays on restarting of the plant, in order thus to shorten the initial phase in which the plant produces solely dilute nitric acid. When the plant is being run down, only when such an amount of nitric acid has run down from the lower sieve trays that the first volume of the first middle region 20 is filled does the nitric acid that then continues to efflux from the sieve trays 14, the concentration of which decreases gradually, flow over the upper end of the dividing wall 16 and hence arrive in the second, radially outer region 21. This second region is likewise provided with an output conduit 19, such that the less concentrated nitric acid that collects in this second region 21 can be removed separately from the bottom 15 of the absorption tower 10 via the output conduit 19.

[0059] The nitric acid produced in the absorption tower 10, which is removed via the two output conduits 18, 19, is subsequently generally supplied in a manner known per se to a bleaching tower in which the nitric acid is purified, which is not shown here in detail.

[0060] A second alternative variant of the present invention is elucidated in detail hereinafter with reference to FIG. 2. In this further working example, only the lower region with the bottom 15 of an absorption tower 10 is shown on an enlarged scale compared to FIG. 1. By contrast with the variant of FIG. 1, in the embodiment according to FIG. 2, a total of three dividing walls 16, 22, 23 are provided, which, for example, are in the form of concentric cylindrical rings. Similarly to the variant of FIG. 1, the radially inner dividing wall 16 divides a first middle region 20 from a second region 21 radially further outward, which surrounds the first middle region 20 in the form of a ring. A further dividing wall 22 divides this second region 21 on the outside from a third region 24 even further radially outward, which in turn surrounds the second region 21 on the outside in the form of a ring. Finally, a third dividing wall 23 is provided, which is radially even further outward and divides the third region 24 from an outer fourth region 25, which surrounds the third region 24 on the outside in the form of a ring and which is bounded to the outside by the outer wall 11 of the absorption tower 10.

[0061] As soon as the first inner region 20 is filled with concentrated nitric acid, this flows over the first dividing wall 16 into the second region 21. When the latter is also filled with nitric acid, nitric acid flows over the dividing wall 22 into the third region 24, and, finally, nitric acid, when the third region 24 is also filled, flows over the dividing wall 23 into the fourth outer region 25. As apparent in the working example, the individual regions divided from one another by the dividing walls may have different volumes, which is dependent on the radial width of the respective region and on the respective distance from the center of the absorption tower. It is thus possible via the choice of the respective position of the individual dividing walls 16, 22, 23 to determine the intended size of the volumes of the differently concentrated acids in the regions. The first middle region accommodates a liquid volume V.sub.1, the second region a liquid volume V.sub.2, the third region a liquid volume V.sub.3 and the fourth, radially outer region a liquid volume V.sub.4, and the liquid volumes V.sub.1 to V.sub.4 may all be of different sizes.

[0062] Each of the four divided regions mentioned is preferably provided with a separate conduit 18, 19, 26, 27, via which the nitric acid can be removed. As also apparent in FIG. 2, the height of the dividing walls decreases in each case from radially inward to outward in the absorption tower, such that, with the second region 21 filled, the nitric acid overflows into the third region 24 further radially outward, and cannot flow back into the first inner region 20.

LIST OF REFERENCE NUMERALS

[0063] 9 conduit for tail gas [0064] 10 absorption tower, absorption column [0065] 11 outer cylindrical wall [0066] 12 feed conduit for process water [0067] 13 feed conduit for NOx gases [0068] 14 sieve trays [0069] 15 column bottom [0070] 16 dividing wall [0071] 17 cover [0072] 18 output conduit for concentrated nitric acid [0073] 19 output conduit for less concentrated nitric acid [0074] 20 first radially inner region [0075] 21 second radially outer region [0076] 22 middle dividing wall [0077] 23 outer dividing wall [0078] 24 third annular region [0079] 25 fourth outer annular region [0080] 26 conduit for nitric acid to be removed [0081] 27 conduit for nitric acid to be removed