A PLANT FOR THE PRODUCTION OF NITRIC ACID, A RELATED PROCESS AND METHOD OF REVAMPING

Abstract

A dual-pressure plant for the synthesis of nitric acid comprising: a reactor (4) providing a gaseous effluent (15) containing nitrogen oxides; an absorption tower (6) nitrogen oxides react with water providing raw nitric acid and, said absorption tower operating at a pressure greater than the pressure of the reactor; a compressor (5) elevating the pressure of the reactor effluent (15) to the absorption pressure; said plant also comprising a first bleacher (37) and a second bleacher (7), said first bleacher (37) stripping with air (39) nitrogen oxides from the output stream (27) of the absorption tower (6) providing a partially stripped nitric acid stream (40) and a nitrogen oxides-loaded air stream (41), the former being fed to the second bleacher (7) and the latter being recycled to the delivery-side of said compressor (5).

Claims

1-15. (canceled)

16. A dual-pressure process for the synthesis of nitric acid comprising the following steps: a) oxidation of a stream of ammonia, providing a gaseous effluent containing nitrogen oxides; b) subjecting said gaseous effluent to a process of absorption of nitrogen oxides, obtaining an output product stream containing nitric acid and nitrogen oxides and a tail gas mainly composed of nitrogen; said step a) being operated at a reaction pressure and said step b) being operated at an absorption pressure greater than the reaction pressure, c) compression of the gaseous effluent obtained from said step a) from the reaction pressure to the absorption pressure in a suitable compressor; d) subjecting said output product stream from the absorption step to a first bleaching process, wherein nitrogen oxides are stripped with a first stripping medium from said output stream, providing a partially stripped nitric acid stream and a nitrogen oxides-loaded stripping medium; e) subjecting said partially stripped nitric acid stream to a second bleaching process, wherein nitrogen oxides are stripped with a second stripping medium from said output stream, providing a stream of nitric acid; f) said nitrogen oxides-loaded stripping medium is recycled to the discharge-side of the compressor.

17. The process according to claim 16, said step c) of bleaching being performed substantially at said absorption pressure and said nitrogen oxides-loaded stripping medium being directly recycled to the discharge-side of said compressor.

18. The process according to claim 16, wherein said step d) of bleaching is performed substantially at said reaction pressure and provides a nitrogen oxides-loaded stripping medium which is recycled at the suction-side of said compressor.

Description

DESCRIPTION OF FIGURE

[0054] FIG. 1 shows a plant for the nitric acid production according to the prior art.

[0055] FIG. 2 shows the plant of FIG. 1 modified according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0056] The plant shown in FIG. 1 essentially comprises an ammonia pre-heater 1, an air compressor 2, an air-ammonia mixer 3, a reactor 4 for the oxidation of ammonia, a nitrous gas compressor 5, an absorption tower 6, a bleacher 7, a tail-gas turbine 8, water cooler-condensers 9a-9c and a tail-gas pre-heater 9d.

[0057] An ammonia stream 10 is heated to a temperature of about 150° C. in an ammonia pre-heater 1, resulting in a hot ammonia stream 11.

[0058] An air flow 12 is compressed from atmospheric pressure to the reaction pressure, for example of around 2-6 bar, in the air compressor 2, resulting in a compressed air stream 13. Said stream 13 splits into a first portion 13a and a second portion 13b.

[0059] Said first portion 13a is sent to the ammonia pre-heater 1, where it is cooled down providing an air stream 16 used as stripping medium in the bleacher 7.

[0060] Said second portion 13b is mixed with ammonia 11 inside the mixer 3 to provide the input stream 14 of the reactor 4, wherein ammonia is catalytically oxidized at around 900° C. to provide a gaseous effluent 15 essentially containing nitrogen oxides and water.

[0061] The term of “nitrogen oxides” denotes the following: nitrogen monoxide (NO), nitrogen dioxide (NO.sub.2), dinitrogen tetroxide (N.sub.2O.sub.4) and dinitrous monooxide (N.sub.2O). For the sake of simplicity, nitrogen oxides are also referred to as NO.sub.x.

[0062] The heat content of the gaseous effluent 15 is partially recovered in a series of heat exchangers HX upstream of the water cooler-condenser 9a.

[0063] The gaseous effluent 15 is then further cooled in said cooler-condenser 9a, wherein it reaches a temperature lower than 50° C. and the water contained in said effluent partially condenses, thus providing a nitric acid solution 17 and a NO.sub.x-containing stream 18.

[0064] Said stream 18 is mixed with NO.sub.x-loaded air 19 which is recycled from the bleacher 7 and the resulting mixture 20 is sent to a subsequent cooler-condenser 9b, which provides a nitric acid solution 21 and a NO.sub.x-containing stream 22 with a water content lower than the stream 18.

[0065] Said nitric acid solution 21 mixes with the solution 17 obtained from the previous cooler-condenser 9a and the resulting mixture 23 is supplied to the absorption tower 6.

[0066] Said NO.sub.x-containing stream 22 is compressed to an absorption pressure, for example of around 9-16 bar, in the nitrous gas compressor 5, obtaining a compressed NO.sub.x-containing stream 24 at a temperature of around 130-160° C.

[0067] Said stream 24 passes through a tail-gas pre-heater 9d and then through a cooler-condenser 9c. Inside the cooler-condenser 9c, the stream 24 is cooled to a temperature of about 50° C., obtaining a nitric acid solution 25 and a NO.sub.x-containing stream 26.

[0068] Said nitric acid solution 25 mixes with the stream of raw nitric acid 27 from the absorption tower 6, thus providing a stream 28.

[0069] Said NO.sub.x-containing stream 26 enters the absorption tower 6, where it is contacted with water to provide the stream of raw nitric acid 27 also containing NO.sub.x. Generally, said absorption tower 2 is a tray or packed column where NO.sub.x are absorbed in water to form nitric acid.

[0070] Said stream 27 leaves the absorption tower 6 and, upon mixing with the above referred nitric acid solution 25, is fed to the bleacher 7 as stream 28 after partial flashing in a valve 31. Said bleacher 7 substantially operates at the same pressure as the reactor 1, preferably at a pressure of 2-6 bar.

[0071] Inside said bleacher 7, NO.sub.x are stripped with the air stream 16 to provide a stream 29 of purified nitric acid and the above mentioned NO.sub.x-loaded air stream 19. Said air stream 19 is recycled at the suction-side of the nitrous gas compressor 5, preferably it is mixed with the NO.sub.x-containing stream 18 leaving the cooler-condenser 9a.

[0072] The absorption tower 6 also provides a tail gas 30 as overhead product, which is mostly composed of nitrogen and also contains oxygen and NO.sub.x. Said tail gas 30 exits the absorption tower 6 at around 20° C. and is preheated in the tail-gas pre-heater 6d, before being expanded in the tail-gas turbine 8.

[0073] The turbine 8 supplies around 60-70% of the power required by the air compressor 2 and the nitrous gas compressor 5. The remaining power can be obtained from a steam turbine (not shown).

[0074] FIG. 2 shows the plant of FIG. 1 modified according to the invention.

[0075] A bleacher 37 operating substantially at the same pressure as the absorption tower 6, preferably at a pressure of 9-16 bar, is installed upstream of the bleacher 7.

[0076] For simplicity, the bleacher 37 and the bleacher 7 will be referred to as high-pressure (HP) bleacher and low-pressure (LP) bleacher, respectively.

[0077] The output stream of the absorption tower 6 is supplied via the line 28 to the HP bleacher 37, wherein NO.sub.x are stripped by means of air 39 to provide a partially stripped nitric acid stream 40 and a nitrogen oxides-loaded air stream 41.

[0078] Said partially stripped nitric acid stream 40 is sent to the LP bleacher 7, wherein nitrogen oxides are further stripped to provide said stream of purified nitric acid 29. Since the bleacher 37 operates at a greater pressure (e.g. 9-16 bar) than the bleacher 7 (e.g. 2-6 bar), the stripped nitric acid stream 40 is properly flashed in the valve 31 before entering the bleacher 7.

[0079] The nitrogen oxides-loaded air stream 41 is advantageously sent to the discharge-side of said nitrogen oxides compressor 5.

[0080] A further air compressor 42 is also provided, which supplies the HP bleacher 37 with stripping air 39 at a suitable pressure of e.g. 9-16 bar. A heat exchanger 43 is further provided at the discharge-side of said air compressor 42 to cool down the stripping air 39 before its admission into the HP bleacher 37. Said further air compressor 42 is advantageously smaller than the existing air compressor 2.

[0081] In the specific case where the plant of FIG. 2 is obtained by revamping the plant of FIG. 1, the line 28 feeding the raw nitric acid from the absorption tower 6 to the existing LP bleacher 7 is modified to accommodate the newly installed HP bleacher 37.