Dual pressure plant for the production of nitric acid and method for operating same

1 . A dual pressure plant for the production of nitric acid, comprising:

an air compressor, operable to pressurize air to a pressure ranging from 2 bar to 6 bar (low pressure, LP);

a converter, operating at the low pressure (LP), operable to be fed with a stream of an ammonia/pressurized air mixture at the low pressure (LP), to oxidize the ammonia and to produce an LP gaseous NO x gas/steam mixture, comprising water and NO;

a NO x gas compressor, operable to elevate the pressure of a gaseous NO x stream from

the low pressure (LP) to a pressure ranging from 9 bar to 16 bar (high pressure, HP), resulting, at its downstream side, in an HP gaseous NO x stream;

an absorber unit, operating at the high pressure (HP), operable to react the nitrogen oxides contained in the HP gaseous NO x stream with water, wherein the absorber unit provides an output product stream containing nitric acid and dissolved nitrogen oxides, and a tail gas; and

a bleacher unit, operable to strip dissolved nitrogen oxides away from the output product stream with a stripping medium, providing a stripped nitric acid stream and a NO x -loaded stripping gas;

wherein

the bleacher unit operates at the high pressure (HP), which is about equal to the pressure at which the absorber unit is operating;

the bleacher unit is in fluid communication with a source of high pressure (HP) oxygen-rich gas, for providing the bleacher unit with the high pressure (HP) oxygen-rich gas, comprising more than 21 vol % oxygen, as a stripping medium;

the plant comprising means for directing the NO x -loaded stripping gas to the downstream side of the NO x gas compressor;

the plant being characterised in that it further comprises

a high pressure (HP) water electrolyzer, operating at a gas pressure of at least 9 bar, in fluid communication with the bleacher unit for providing the high pressure (HP) oxygen-rich gas having a pressure ranging from 9 bar to 16 bar to be used as the stripping medium, alone or mixed with pressured air or any other suitable gas.

2 . The plant according to claim 1 , further comprising a flash vessel in fluid communication with the bleacher unit, comprising:

an inlet for the stripped nitric acid stream in fluid communication with the bleacher unit;

wherein the stripped nitric acid stream is flash reduced in pressure from the high pressure (HP) to the low pressure (LP);

a first outlet for gases produced in the flash vessel, in fluid communication with an upstream section of the NO x gas compressor; and

a second outlet for collecting the flashed nitric acid stream from the flash vessel.

3 . The plant according to claim 1 , further comprising a second bleacher unit, comprising:

a first inlet for the stripped nitric acid stream in fluid communication with the bleacher unit;

a second inlet for a stripping gas stream;

a first outlet for a NO x -loaded stripping gas stream in fluid communication with an upstream section of the NO x gas compressor; and

a second outlet for collecting the bleached nitric acid stream from the second bleacher unit;

wherein

the second bleacher unit operates at the low pressure (LP), which is lower than the pressure at which the bleacher unit is operating;

the second bleacher unit is in fluid communication with a source of HP oxygen-rich gas, for providing the second bleacher unit with an HP oxygen-rich gas as a stripping medium; and

the plant comprises means for directing the NO x -loaded stripping gas to the upstream side of the NO x gas compressor.

4 . The plant of claim 1 , wherein the HP oxygen-rich gas comprises more than 95 vol % of oxygen.

5 . The plant of claim 1 , wherein the water electrolyzer operates at a gas pressure of 9 bar to 30 bar.

6 . A method for operating a high pressure bleacher unit in a plant comprising:

an air compressor, operable to pressurize air to a pressure ranging from 2 bar to 6 bar (low pressure, LP);

a converter, operating at the low pressure (LP), operable to be fed with a stream of an ammonia/pressurized air mixture at the low pressure (LP), to oxidize the ammonia and to produce an LP gaseous NO x gas/steam mixture, comprising water and NO;

a NO x gas compressor, operable to elevate the pressure of a gaseous NO x stream from the low pressure (LP) to a pressure ranging from 9 bar to 16 bar (high pressure, HP), resulting, at its downstream side, in an HP gaseous NO x stream;

an absorber unit, operating at the high pressure (HP), operable to react the nitrogen oxides contained in the high pressure (HP) gaseous NO x stream with water, wherein the absorber unit-provides an output product stream containing nitric acid and dissolved nitrogen oxides, and a tail gas; and

a bleacher unit, operable to strip dissolved nitrogen oxides away from the output product stream with a stripping medium, providing a stripped nitric acid stream and a NO x -loaded stripping gas;

wherein

the bleacher unit operates at the high pressure (HP), which is about equal to the pressure at which the absorber unit is operating;

the bleacher unit is in fluid communication with a source of high pressure (HP) oxygen-rich gas, for providing the bleacher unit with the high pressure (HP) oxygen-rich gas as a stripping medium;

the plant comprising means for directing the NO x -loaded stripping gas to the downstream side of the NO x gas compressor;

the plant being characterised in that it further comprises:

a high pressure (HP) water electrolyzer, operating at a gas pressure of at least 9 bar, in fluid communication with the bleacher unit for providing the high pressure (HP) oxygen-rich gas at a pressure ranging from 9 bar to 16 bar to be used as the stripping medium, alone or mixed with pressured air or any other suitable gas, the method comprising steps of:

a) operating the bleacher unit at the high pressure (HP), which is about equal to the pressure at which the absorber unit is operating;

b) providing the bleacher unit with the high pressure (HP) oxygen-rich gas as the stripping medium at the pressure ranging from 9 bar to 16 bar, wherein the high pressure (HP) oxygen-rich gas is provided by the high pressure (HP) water electrolyzer; and

c) directing the NO x -loaded stripping gas to downstream of the NO x gas compressor.

7 . The method according to claim 6 , wherein the HP oxygen-rich gas comprises more than 21 vol % of oxygen.

8 . The method of claim 7 , wherein the HP oxygen-rich gas comprises more than 95 vol % of oxygen.

9 . The method according to claim 6 , wherein the method further comprises, before step b), the step of:

d) pre-heating the HP oxygen-rich gas as a stripping medium to a temperature ranging from ambient temperature to 120° C.

10 . The method of claim 9 , wherein the temperature of the preheated HP oxygen-rich gas is from 50° C. to 120° C.

11 . The method of claim 9 , wherein the water electrolyzer operates at a gas pressure of 9 bar to 30 bar.

12 . The method according to claim 6 , wherein the plant further comprises a flash vessel in fluid communication with the bleacher unit, the bleacher unit further comprising:

an inlet for the stripped nitric acid stream in fluid communication with the bleacher unit;

wherein the stripped nitric acid stream is flashed reduced in pressure from the high pressure (HP) to the low pressure (LP);

a first outlet for gases produced in the flash vessel, in fluid communication with an upstream section of the NO x gas compressor; and

a second outlet for collecting the flashed nitric acid stream from the flash vessel, and the method further comprising the steps of:

e) flashing the stripped nitric acid stream from the high pressure (HP) to the low pressure (LP), and

f) directing gases produced in the flash vessel, to an upstream section of the NO x gas compressor.

13 . The method according to claim 6 , wherein the method further comprises, in the plant, the steps of:

g) operating a second bleacher unit at the low pressure (LP), which is lower than the pressure at which the bleacher unit is operating;

h) providing the second bleacher unit with a high pressure (HP) oxygen-rich gas as a stripping medium; and

i) directing the NO x -loaded stripping gas to upstream of the NO x gas compressor.