MONO-PRESSURE PLANT FOR THE PRODUCTION OF NITRIC ACID AND METHOD FOR OPERATING SAME

Abstract

A mono-pressure plant for the production of nitric acid, comprising an ammonia convertor configured for operating at a working pressure ranging between 2 bar and 16 bar, for oxidizing ammonia, thereby producing a gaseous NOx gas/steam mixture; an air compressor in fluid communication with the ammonia convertor, for pressurizing air to the working pressure; an absorber unit, wherein the nitrogen oxides contained in a gaseous NO.sub.x stream react with water, thereby producing nitric acid; a first bleacher unit for stripping the dissolved nitrogen oxides from the output product stream, thereby producing a first NO.sub.x-loaded stripping gas; and a means for directing the first NO.sub.x-loaded stripping gas to an inlet of the absorber unit.

Claims

1. A mono-pressure plant for the continuous production of nitric acid, comprising: an ammonia convertor, configured for operating at a working pressure ranging between 2 bar and 16 bar, for oxidizing ammonia, thereby producing a gaseous NOx gas/steam mixture, the ammonia convertor comprising at least one inlet for a stream of ammonia and pressurized air, and an outlet for the gaseous NOx gas/steam mixture; an air compressor, in fluid communication with the ammonia convertor, for pressurizing air to the working pressure; an absorber unit, configured for operating at the working pressure, wherein the nitrogen oxides contained in a gaseous NO.sub.x stream react with water, thereby producing nitric acid, the absorber unit comprising an inlet for a gaseous NOx stream, an inlet for an aqueous solution, an outlet for a product stream containing a nitric acid solution and dissolved nitrogen oxides, and an outlet for a tail gas; a first bleacher unit, configured for operating at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, for stripping the dissolved nitrogen oxides from the output product stream, thereby producing a first NO.sub.x-loaded stripping gas, the bleacher unit comprising an inlet for a stripping medium (60) and an inlet for the product stream, an outlet for the first NOx-loaded stripping gas and an outlet for the stripped nitric acid stream; and a means for directing the first NO.sub.x-loaded stripping gas to the inlet of the absorber unit; the plant being characterized in that: the plant further comprises a pressurized water electrolyser as a source of oxygen-rich gas at a pressure of 2 to 30 bar, wherein the oxygen-rich gas comprises more than 21 vol % oxygen, for providing the bleacher unit with an oxygen-rich gas as a stripping medium via the inlet for the stripping medium; and optionally, the plant further comprises means for lowering the pressure of the first NO.sub.x-loaded stripping gas to a pressure that is 0.01 bar to 1 bar higher than the working pressure, reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber.

2. The plant according to claim 1, wherein the pressurized water electrolyser is connected to the mono pressure plant such as to be at least partially powered by energy provided by the nitric acid plant.

3. The plant according to claim 1, wherein the first bleacher unit is a vertical bleaching tower, comprising: a structured packing; and a liquid distributor comprising a feed box having a serrated weir for distribution of the output product stream comprising dissolved nitrogen oxides through upward-pointing serrations of the serrated weir into perforated trays of the liquid distributor, and located above the structured packing for distributing the aqueous nitric acid solution comprising the dissolved nitrogen oxides to the structured packing.

4. 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 first bleacher unit; a second inlet for a stripping gas stream; a first outlet for a second NO.sub.x-loaded stripping gas stream, in fluid communication with the inlet of the absorber unit; and a second outlet for collecting the bleached nitric acid stream from the second bleacher unit; wherein the second bleacher unit is configured for operating at a pressure that is 0.01 bar to 1 bar higher than the working pressure, and lower than the pressure at which the first bleacher unit is operating; the second bleacher unit is in fluid communication with the source of oxygen-rich gas, for providing the second bleacher unit with an oxygen-rich gas as a stripping medium; and the plant further comprises means for directing the second NO.sub.x-loaded stripping gas to the inlet of the absorber unit.

5. The plant according to claim 4, wherein the working pressure is selected from the range of 2 bar to 6 bar, and wherein the pressure in the first bleacher unit is at least 1 bar higher than the working pressure.

6. The plant according to claim 4, wherein the working pressure is selected from the range of 9 bar to 16 bar, and wherein the pressure in the first bleacher unit is at least 1 bar higher than the working pressure.

7. A method for continuously operating a bleacher unit in a plant according to claim 1, the ammonia convertor of the plant operating at a working pressure ranging from 2 bar to 16 bar, comprising the consecutive steps of: a) providing a first bleacher unit with an oxygen-rich gas from continuously operating a pressurized water electrolyser operating at a pressure of 2 bar to 30 bar, comprising more than 21 vol % oxygen as a stripping medium; b) operating the first bleacher unit at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, thereby generating a first NOx-loaded stripping gas; c) optionally, lowering the pressure of the first NO.sub.x-loaded stripping gas to a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber; and d) directing the first NO.sub.x-loaded stripping gas to the inlet of the absorber unit.

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

9. The method according to claim 1, wherein the pressurized water electrolyser is at least partially powered by energy provided by the nitric acid plant.

10. The method according to claim 7, wherein the method further comprises before step b), the step of: e) pre-heating the oxygen-rich gas as a stripping medium to a temperature of ambient temperature to 120° C.

11. A method four continuously operating a bleacher unit in a mono-pressure plant for the continuous production of nitric acid, the plant comprising: an ammonia convertor, configured for operating at a working pressure ranging between 2 bar and 16 bar, for oxidizing ammonia, thereby producing a gaseous NOx gas/steam mixture, the ammonia convertor comprising at least one inlet for a stream of ammonia and pressurized air, and an outlet for the gaseous NOx gas/steam mixture; an air compressor, in fluid communication with the ammonia convertor, for pressurizing air to the working pressure; an absorber unit, configured for operating at the working pressure, wherein the nitrogen oxides contained in a gaseous NO.sub.x stream react with water, thereby producing nitric acid, the absorber unit comprising an inlet for a gaseous NOx stream, an inlet for an aqueous solution, an outlet for a product stream containing a nitric acid solution and dissolved nitrogen oxides, and an outlet for a tail gas; a first bleacher unit, configured for operating at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, for stripping the dissolved nitrogen oxides from the output product stream, thereby producing a first NO.sub.x-loaded stripping gas, the bleacher unit comprising an inlet for a stripping medium and an inlet for the product stream, an outlet for the first NOx-loaded stripping gas and an outlet for the stripped nitric acid stream; a second bleacher unit, comprising: a first inlet for the stripped nitric acid stream, in fluid communication with the first bleacher unit; a second inlet for a stripping gas stream; a first outlet for a second NO.sub.x-loaded stripping gas stream, in fluid communication with the inlet of the absorber unit; and a second outlet for collecting the bleached nitric acid stream from the second bleacher unit; wherein the second bleacher unit is configured for operating at a pressure that is 0.01 bar to 1 bar higher than the working pressure, and lower than the pressure at which the first bleacher unit is operating, the second bleacher unit is in fluid communication with the source of oxygen-rich gas, for providing the second bleacher unit with an oxygen-rich gas as a stripping medium, and the plant further comprises a means for directing the first NO.sub.x-loaded stripping gas to the inlet of the absorber unit; and the plant further comprises a means for directing the second NO.sub.x-loaded stripping gas to the inlet of the absorber unit; and wherein the plant further comprises a pressurized water electrolyser as a source of oxygen-rich gas at a pressure of 2 to 30 bar, wherein the oxygen-rich gas comprises more than 21 vol % oxygen, for providing the bleacher unit with an oxygen-rich gas as a stripping medium via the inlet for the stripping medium; and optionally, the plant further comprises means for lowering the pressure of the first NO.sub.x-loaded stripping gas to a pressure that is 0.01 bar to 1 bar higher than the working pressure, reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber, the ammonia convertor of the plant operating at a working pressure ranging from 2 bar to 16 bar, the method comprising the consecutive steps of: a) providing a first bleacher unit with an oxygen-rich gas from continuously operating a pressurized water electrolyser operating at a pressure of 2 bar to 30 bar, comprising more than 21 vol % oxygen as a stripping medium; b) operating the first bleacher unit at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, thereby generating a first NOx-loaded stripping gas; c) optionally, lowering the pressure of the first NO.sub.x-loaded stripping gas to a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber; d) directing the first NO.sub.x-loaded stripping gas to the inlet of the absorber unit; and e) before b), pre-heating the oxygen-rich gas as a stripping medium to a temperature of ambient temperature to 120° C.; f) operating a second bleacher unit at a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop loss in the ammonia convertor and during transport of the gaseous NOx stream to the absorber unit, and lower than the pressure at which the first bleacher unit is operating; g) providing the bleacher unit with an oxygen-rich gas as a stripping medium, thereby obtaining a second NOx-loaded stripping gas stream; and h) directing the second NO.sub.x-loaded stripping gas stream to the inlet of the absorber unit.

12. The method according to claim 11, wherein the working pressure is selected from the range of 2 bar to 6 bar, and wherein the pressure in the first bleacher unit is at least 1 bar higher than the working pressure.

13. The method according to claim 11, wherein the working pressure is selected from the range of 9 bar to 16 bar, and wherein the pressure in the first bleacher unit is at least 1 bar higher than the working pressure.

14. (canceled)

15. The mono-pressure plant of claim 1 wherein the stream of ammonia and pressurized air is an ammonia/pressurized air mixture.

16. The mono-pressure plant of claim 1 wherein the absorber unit is configured for operating at the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit.

17. The mono-pressure plant of claim 1 wherein the absorber unit is configured for operating at the working pressure reduced by 0.01 bar to 1 bar.

18. The plant according to claim 4 wherein the second bleacher unit is configured for operating at the pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop loss in the ammonia convertor and during transport from the gaseous NOx stream to the absorber unit and lower than the pressure at which the first bleacher unit is operating.

19. The method of claim 10, wherein the stripping medium is pre-heated to a temperature of 50 to 120° C.

Description

LIST OF FIGURES

[0034] FIG. 1: schematic representation of a mono-pressure nitric acid production plant according to the state of the art (dual pressure plant).

[0035] FIG. 2: schematic representation of a mono-pressure nitric acid production plant according to an embodiment of the disclosure involving the supply of an oxygen-rich gas to a first bleacher unit of the mono-pressure nitric acid production plant.

[0036] FIG. 3: schematic representation of a mono-pressure nitric acid production plant according to an embodiment of the disclosure as depicted in FIG. 2, further involving a second, bleacher unit, operating at a lower pressure than the first bleacher unit.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0037] Throughout the description and claims of this specification, the words “comprise” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0038] Features, integers, characteristics, compounds, chemical moieties, or groups described in conjunction with a particular aspect, embodiment or example of the disclosure are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0039] The enumeration of numeric values by means of ranges of figures comprises all values and fractions in these ranges, as well as the cited end points. The term “ . . . to . . . ” as used when referring to a range for a measurable value, such as a parameter, an amount, a time period, and the like, is intended to include the limits associated to the range that is disclosed.

[0040] As defined herein, nitrogen oxides gases or NO.sub.x gases are assumed to comprise, as major NO.sub.x gas components, nitric oxide (NO), nitrogen dioxide (NO.sub.2) and dinitrogen tetroxide gases (N.sub.2O.sub.4).

[0041] As defined herein, and unless explicitly defined otherwise for a specific case, a medium-low pressure is defined as a pressure of 2 bar to 6 bar, and a high pressure is defined as a pressure of 9 bar to 16 bar. A medium pressure is always lower than a high pressure.

[0042] As defined herein, and unless explicitly defined otherwise for a specific case, “means for directing” are defined as means selected from the group of tubes, pipes, channels, conduits, ducts, and the like, capable of directing a fluid from a point A to a point B, which points A and B may be defined explicitly or implicitly, depending on the specific case.

[0043] As defined herein, and unless explicitly defined otherwise for a specific case, “about” is defined as a variation of maximum 10% of the value considered. Hence, about 100 is defined as a value from 90 to 110. This accounts a.o. for measurement accuracy and clearance.

[0044] As defined herein, the working pressure is the pressure at which an ammonia convertor of a mono pressure nitric acid plant is being operated.

[0045] The present disclosure relates to a mono-pressure plant for the production of nitric acid and to a method for operating the mono-pressure plant, the mono-pressure plant, particularly the ammonia convertor, operating or operable at a working pressure ranging between 2 bar and 16 bar, wherein the mono-pressure plant comprises an ammonia convertor; an air compressor for providing pressurized air to the ammonia convertor, an absorber unit and a first bleacher unit, wherein the first bleacher unit is configured for operating at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, and wherein the plant further comprises a source of oxygen-rich gas at a pressure of 2 to 30 bar as the stripper medium in the first bleacher unit.

[0046] According to a first aspect of the disclosure, a mono-pressure plant for the continuous production of nitric acid is provided. The plant comprises at least: [0047] an ammonia convertor (4), configured for operating at a working pressure ranging between 2 bar and 16 bar, for oxidizing ammonia, thereby producing a gaseous NOx gas/steam mixture (15), the ammonia convertor (4) comprising at least one inlet for a stream of ammonia and pressurized air, preferably a stream of an ammonia/pressurized air mixture (14), and an outlet for the gaseous NOx gas/steam mixture (15); [0048] an air compressor (2), in fluid communication with the ammonia convertor (4), for pressurizing air (12) to the working pressure; [0049] an absorber unit (6), configured for operating at the working pressure, particularly at the working pressure reduced by the pressure drop in the ammonia convertor (4) and during transport of the NOx gas stream to the absorber unit (6), particularly at the working pressure reduced by 0.01 bar to 1 bar, wherein the nitrogen oxides contained in a gaseous NO.sub.x stream (26) react with water, thereby producing nitric acid, the absorber unit (6) comprising an inlet (5) for a gaseous NOx stream, an inlet for an aqueous solution, an outlet for a product stream (27) containing a nitric acid solution and dissolved nitrogen oxides, and an outlet for a tail gas (30); [0050] a first bleacher unit (7), configured for operating at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor (4) and during transport of the NOx gas stream to the absorber unit (6), and up to 30 bar, for stripping the dissolved nitrogen oxides from the output product stream (27), thereby producing a first NO.sub.x-loaded stripping gas (19), the bleacher unit comprising an inlet for a stripping medium (60) and an inlet for the product stream (27), an outlet for the first NOx-loaded stripping gas and an outlet for the stripped nitric acid stream (29); and [0051] a means for directing the first NO.sub.x-loaded stripping gas (19) to the inlet (5) of the absorber unit (6);
the plant being characterized in that: [0052] the plant further comprises a pressurised water electrolyser as a source of oxygen-rich gas (60) at a pressure of 2 to 30 bar, wherein the oxygen-rich gas comprises more than 21 vol % oxygen, for providing the bleacher unit (7) with an oxygen-rich gas as a stripping medium (16) via the inlet for the stripping medium; and [0053] optionally, the plant further comprises means for lowering the pressure of the first NO.sub.x-loaded stripping gas (19) to a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor (4) and during transport of the NOx gas stream to the absorber.

[0054] As defined herein, an oxygen-rich gas is a gas comprising more oxygen than is on the average present in air. In particular, an oxygen-rich gas comprises more than 21 vol % of oxygen, more in particular more than 30 vol %, more than 40 vol %, more than 50 vol %, more than 60 vol %, more than 70 vol %, more than 80 vol %, more than 90 vol %, more than 95 vol %, and more than 99 vol %, more in particular 100 vol % of oxygen. The oxygen-rich gas differs from the gas (air) used in the prior art in that it contains more than 21 vol % of oxygen. The oxygen-rich gas may be obtained from mixing oxygen with air or any other gas, suitable for its primary purpose, i.e. as a stripping medium in a bleacher unit.

[0055] As defined herein, the bleacher unit can be any bleacher unit known in the prior art, such as, but not restricted to, a sieve tray bleacher unit, a random packing bleacher unit, or a structured packing bleacher unit.

[0056] It is to be understood that the pressure of the first NO.sub.x-loaded stripping gas, exiting the first bleacher unit, may be lowered by the corresponding means to a pressure that is from 0.01 bar to 1 bar, more in particular from 0.05 bar to 0.5 bar, even more in particular from 0.1 bar to 0.2 bar over the pressure of the gaseous NO.sub.x stream entering the absorber unit. Stated otherwise, it is to be understood that the pressure drops in the first bleacher unit, in the ammonia convertor and in the transport of the NO.sub.x-loaded stripping gas and of the NO.sub.x stream from the ammonia convertor to the inlet of the absorber unit, are taken into account for determining the pressure of the NO.sub.x-loaded stripping gas to be combined with the gaseous NO.sub.x stream and to be fed the inlet of the absorber unit. As a result, the pressure in the first bleacher unit is at least 0.01 bar to 1 bar higher, more in particular from 0.05 bar to 0.5 bar, even more in particular from 0.1 bar to 0.2 bar, than the working pressure reduced by the pressure drop in the ammonia convertor (4) and during transport of the NO.sub.x gas stream to the absorber unit (6).

[0057] By using a separate source of an oxygen rich gas, the air compressor is now only pressurizing primary air for the ammonia convertor and is not connected to the bleacher unit via a stripping gas conduit, since the secondary air supply for the bleacher unit has now been replaced by a stream of an oxygen-rich gas from a separate unit, in particular a water electrolyser unit which is operated at a gas pressure of 2 bar to 30 bar. As it requires less power to pump water than to compress gas, there is a significant overall energy savings. Furthermore, the air compressor is less loaded as it is only compressing primary air, thus requiring the air compressor to produce less work (for the same ammonia convertor throughput). Hence, the air compressor is able to handle an increased throughput of primary air (at an increased ammonia convertor throughput). In addition, the use of an electrolyser presents the benefit that renewable energy, such as solar or wind energy can be used to electrolyse water into oxygen gas and hydrogen gas. The hydrogen gas thereby produced can also be used for producing green ammonia, that is for producing ammonia from a fossil-free source. Further, as the pressurised water electrolyser can be continuously operated, either through the presence of renewable energy or through the presence of intermittent power, the continuous production of nitric acid, through the use of the high pressure and oxygen-rich gas provided by the electrolyser, is possible.

[0058] Moreover, as the drive power for the air compressor originates from the tail-gas turbine and a steam turbine (not shown), less power required on the air compressor translates into less steam consumption from the steam turbine (not shown), thereby resulting in additional steam export.

[0059] Furthermore, the use of an oxygen-rich gas as stripping medium results in an oxygen-rich tail gas, which results in additional power being delivered by the tail gas turbine.

[0060] Finally, due to the oxygen-rich gas source being fed to the first bleacher unit and, subsequently, to the absorber unit, the absorption in the absorber unit is improved and, therefore, the size of the absorption tower can be decreased. It is to be understood that, even following any reduction of the size of the absorber unit, the ammonia convertor and the absorber unit are still both operated at a working pressure of 2 bar to 16 bar and both the ammonia convertor and the absorber unit are operated at a pressure that is the same. Alternatively, if the same size is retained for the absorber unit, the tail gas will be cleaner and less work will be required from a De-NO.sub.x unit (not shown) for cleaning the tail gas, such that its content in NO.sub.x gases are according to the environmental regulations.

[0061] The plant comprises, as the source of oxygen-rich gas, a pressurised water electrolyser, in fluid communication with the bleacher unit, for providing a pressurised oxygen-rich gas to be used as a stripping medium, alone or mixed with pressured air or any other suitable gas. In any event, the air compressor is less loaded with respect to the prior art as the amount of secondary air to be compressed is reduced. In addition, as it requires less power to pump water than to compress gas, there is a significant overall energy savings.

[0062] A water electrolyser is a device for the electrolysis of water, being the decomposition of water into oxygen and hydrogen gas, due to the passage of an electric current there through. This technique can be used to make hydrogen gas, a main component of hydrogen fuel, and breathable oxygen gas.

[0063] A suitable water electrolyser may be comprised of an anode producing oxygen gas according to the reaction

2OH.sup.−═H.sub.2O+½O.sub.2+2e.sup.−; [0064] a cathode, producing hydrogen gas according to the reaction

2H.sub.2O+2e.sup.−═H.sub.2+2OH.sup.−; [0065] an electrolyte consisting of an alkaline solution such as potassium hydroxide; [0066] and a porous diaphragm separating the anode and the cathode, in order to avoid the mixing of hydrogen gas and oxygen gas that together form an explosive mixture. Alternatively, the anode and the cathode may be separated by a solid polymer electrolyte such as the fluoropolymer Nafion, where the electrolyte provides the selective transport of protons from the anode to the cathode, as well as the electrical insulation between the anode and the cathode, and avoids the mixing of hydrogen gas and oxygen gas that together form an explosive mixture. The anode and cathode can be made of nickel or steel, or mixtures thereof. Alternatively, for the purpose of enhancing the electrode reactions, the anode and cathode may contain catalysts that can be made of Iridium and Platinum, respectively. The diaphragm of an electrically insulating material is based on, for example, zirconia. The diaphragm has a porosity such that it forms a barrier against transport of hydrogen and oxygen gas bubbles, while containing a continuum of penetrated liquid electrolyte.

[0067] An anode-diaphragm-cathode assembly constitutes an electrolysis cell. Electrolysis cells are piled in series in stacks that compose the core of an electrolyser. The hydrogen and oxygen production for a given stack volume is proportional to the current density and inversely proportional to the stacking distance. Regardless of stack volume, the hydrogen and oxygen production is proportional to the total current.

[0068] In addition to the stack, the electrolyser comprises auxiliaries such as a current rectifier, a water demineralization unit, a water pump and a cooling system, a hydrogen purification unit, and instrumentation.

[0069] The electrolyser is operated by applying a voltage corresponding to the standard potential plus the overpotential over each cell. The total voltage depends on the total number of cells of which the electrolyser is comprised. Hydroxide OH− ions generated at the cathode migrate through the electrolyte in the diaphragm to the anode, where they are consumed by the anode reaction. Electrons travel the opposite direction in an external circuit.

[0070] The electrolyser may be operated at a temperature of 50° C. to 80° C., or 60° C. to 80° C., and a gas pressure of 2 bar to 30 bar, such as 3 bar to 30 bar, preferably 15 bar to 30 bar. The person skilled in the art will understand that, if needed, the pressure of the oxygen gas provided by the electrolyser can be adjusted and reduced, for example through the use of pressure release valves, in order for the pressure of the stripping gas to not exceed the targeted pressure in the first bleacher unit.

[0071] Currently, electrolysis units and nitric acid production facilities are not existing in close environments. However, with the emergence of green technologies, it is anticipated that water electrolysis, ammonia and nitric production will all be integrated on a single production site. Only the hydrogen produced by water electrolysers is currently used in the production of ammonia, whereas the oxygen gas is vented off. In the future, the water electrolyser and the nitric acid production being located nearby, also the oxygen produced by the water electrolyser can be used and benefit to the nitric acid production.

[0072] According to a specific embodiment of the disclosure, the pressurised water electrolyser 60 is connected to the mono pressure plant such as to be at least partially powered by energy provided by the nitric acid plant. Indeed, energy can be recovered from a nitric acid plant. Sources of energy that can be converted into electrical power suitable for operating the water electrolyser 60 are, for example, the tail-gas turbine 8 and the steam turbine. At times when renewable energy is not available, if the electrolyser is connected to the mono pressure plant to be provided with electrical power, the production of oxygen gas and hydrogen remains possible.

[0073] According to a specific embodiment of the disclosure, the stripping medium may be conditioned to a temperature of ambient temperature to 120° C., or from 50° C. to 120° C., or from 80° C. to 120° C., or from 90 to 120° C., before it is directed into the first bleacher unit. Bleaching of nitric acid in a bleacher unit in a nitric acid plant generally may be conditioned such that the temperature of the nitric acid is in the range of 30 to 60° C. (as measured inside the bleacher). By pre-conditioning the oxygen gas, the bleaching efficiency achieved in the first bleacher unit will be increased. Therefore, according to a specific embodiment, the plant according to the disclosure further comprises means for heating the stripping medium, such as, but not limited to, a pre-heater or a heat exchange system.

[0074] According to a specific embodiment of the disclosure, the first bleacher unit is a vertical bleaching tower, comprising: [0075] a structured packing; and [0076] a liquid distributor comprising a feed box having a serrated weir for distribution of the output product stream comprising dissolved nitrogen oxides through upward-pointing serrations of the serrated weir into perforated trays of the liquid distributor and located above the structured packing for distributing the aqueous nitric acid solution comprising the dissolved nitrogen oxides to the structured packing.

[0077] The above design of the bleacher unit has distinct advantages. The bleaching tower is a known restriction unit (bottle neck) in the nitric acid production. The problem is to meet the specifications of the product acid at high load. In general, the design of the column should ensure uniform distribution and contact between the upward flowing gas (stripping medium) and the downward liquid stream (aqueous nitric acid solution) through the entire column. The advantage of the specific design is that an increased nitric acid production capacity and a reduction of the amount of stripping gas used, can be obtained, while at least maintaining the quality of the aqueous nitric acid solution, i.e. the low level of dissolved nitrogen oxide gases.

[0078] Preferably, the structured packing has a surface area of at least 250 m.sup.2/m.sup.3, preferably 450-750 m.sup.2/m.sup.3.

[0079] Preferably, the liquid distributor has a drip-point density of at least 30 dripping points per m.sup.2, preferably from 60 to 200 dripping points per m.sup.2.

[0080] Preferably, the ratio between the height of the structured packing and the vertical bleaching tower diameter is at least 1, preferably at least 1.5, more preferably at least 2.

[0081] Preferably, the stripping gas is an oxygen-rich gas, moving in a counter-current direction to the acid solution, i.e. the output product stream and is in a gas/acid or enriched air/acid solution ratio of lower than 75 m.sup.3 air/m.sup.3 acid solution, preferably lower than 45 m.sup.3 air/m.sup.3 acid solution, more preferably lower than 30 m.sup.3 air/m.sup.3 acid solution, even more preferably lower than 20 m.sup.3 air/m.sup.3 acid solution.

[0082] Preferably, the pressure drop over the vertical bleaching tower is between 25 mbar and 65 mbar.

[0083] According to a specific embodiment of the disclosure, the plant further comprises a second bleacher unit, comprising: [0084] a first inlet for the stripped nitric acid stream in fluid communication with the first bleacher unit; [0085] a second inlet for a stripping gas stream; [0086] a first outlet for a second NO.sub.x-loaded stripping gas stream in fluid communication with the inlet of the absorber unit; and [0087] a second outlet for collecting the bleached nitric acid stream from the second bleacher unit; [0088] wherein [0089] the second bleacher unit is configured for operating at or is operable at a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and lower than the pressure at which the first bleacher unit is operating; [0090] the second bleacher unit is in fluid communication with the source of oxygen-rich gas, for providing the second bleacher unit with an oxygen-rich gas as a stripping medium; and [0091] the plant further comprises means for directing the second NO.sub.x-loaded stripping gas to the inlet of the absorber unit.

[0092] As defined herein, the bleacher unit can be any bleacher unit known in the prior art, such as, but not restricted to, a sieve tray bleacher unit, a random packing bleacher unit or a structured packing bleacher unit.

[0093] Since the second bleacher unit is operable or operates at a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and the first bleacher unit is operable or operates at a higher pressure than the second bleacher unit, the output product stream is fed to the first bleacher unit, through a pump, in order to produce a pressurised product stream at the pressure of the first bleacher unit, hence suitable for feeding to the first bleacher unit.

[0094] The stripping gas to the second bleacher unit is generated by using the stripping gas to the first bleacher unit: the pressure of the stripping gas fed to the first bleacher unit is adjusted, for example through the use of one or more pressure release valves, in order for the stripping gas for the second bleacher unit to be at a lower pressure than the stripping gas for the first bleacher unit.

[0095] It is to be understood that pressure drops in the ammonia convertor, in the first bleacher unit, in the second bleacher unit and in the transport of the NO.sub.x-loaded stripping gases to the inlet of the absorber unit, which allow for the transport of the correspond gas flows, are taken into account. As a result, the pressure in the first bleacher unit and in the second bleacher unit are adjusted, in total, by the sum of those pressure drops, in order for the pressure inside the absorber unit to be the about the same as the pressure in the ammonia convertor reduced by the sum of the pressure drops. It is thus understood that the sum of the pressure in the absorber unit and the above mentioned pressure drops correspond to the working pressure, i.e. the pressure in the ammonia convertor.

[0096] The pressure of the first NO.sub.x-loaded stripping gas from the first bleacher unit is adjusted by a pressure control valve to the pressure in the second bleacher unit. The first NO.sub.x-loaded stripping gas after pressure adjustment and the second NO.sub.x-loaded stripping gas may then be combined to generate a third (combined) NO.sub.x-loaded stripping gas, which is then fed to the inlet of the absorber unit.

[0097] In the presence of the second bleacher unit, the stripped nitric acid stream is further stripped, and the dissolved gases, containing in particular NO.sub.x and oxygen gases, are evacuated from the stripped nitric acid stream.

[0098] With this embodiment, comprising a second bleacher unit, the quality of the stripped nitric acid stream can be further improved, in the sense that it contains less dissolved gases, in particular NO.sub.x and oxygen gases. In addition, by further stripping the stripped nitric acid stream coming out of the first bleacher unit, the amount of oxygen gases that will be released when the stripped nitric acid stream is further stripped down to atmospheric pressure in the storage tank, may be reduced. A high concentration of oxygen forces the equilibrium for nitric oxide.Math.nitrogen dioxide towards nitrogen dioxide, which results in brown gas emissions from the ventilation system of the storage tank. Hence, further stripping of the stripped nitric acid stream downstream the bleaching tower and prior to storing the nitric acid product, can result in less brown gas emissions coming out of the ventilation system of the product storage tank.

[0099] Furthermore, there is an additional amount of oxygen being provided to the absorber unit. As a result, the tail-gas from the absorber will be cleaner and less chemical conversion will be required from a DeNO.sub.x unit (not shown) for treating the tail-gas. Moreover, the absorption in the absorber unit will be improved and, therefore, the size of the absorption tower can be decreased. It is to be understood that, even following any reduction of the size of the absorber unit, the ammonia convertor and the absorber unit are still both operated at a working pressure of 2 bar to 16 bar and both the ammonia convertor and the absorber unit are operated at a pressure that is the same.

[0100] As an alternative to the second bleacher unit, the person skilled in the art will understand that it is possible to use a flash vessel for achieving the same benefits as achieved upon using the second bleacher unit.

[0101] According to a specific embodiment of the disclosure, the bleacher unit is a vertical bleaching tower, comprising: [0102] a structured packing; and [0103] a liquid distributor comprising a feed box having a serrated weir for distribution of the output product stream from the first bleacher unit comprising dissolved nitrogen oxides through upward-pointing serrations of the serrated weir into perforated trays of the liquid distributor and located above the structured packing for distributing the aqueous nitric acid solution comprising the dissolved nitrogen oxides to the structured packing.

[0104] For the second bleacher unit, the same parameters apply as for the first bleaching tower.

[0105] According to a specific embodiment of the disclosure, the working pressure is selected from the range of 2 bar to 6 bar and the pressure in the first bleacher unit is at least 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, ranging between 3 bar to 16 bar. In other words, the disclosure allows for mono-pressure nitric acid plant operating at a medium-low working pressure of 2 bar to 6 bar.

[0106] According to a specific embodiment of the disclosure, the working pressure is selected from the range of 9 bar to 16 bar and the pressure in the first bleacher unit is at least 1 bar higher than the working pressure, ranging between 17 bar to 30 bar. In other words, the disclosure allows for mono-pressure nitric acid plant operating at a high working pressure of 9 bar to 16 bar.

[0107] According to a second aspect of the disclosure, a method is provided for continuously operating a bleacher unit in a plant according to the present disclosure, the ammonia convertor of the plant operating at a working pressure ranging from 2 bar to 16 bar, comprising the steps of: [0108] a) providing a first bleacher unit with an oxygen-rich gas from continuously operating a pressurised water electrolyser operating at a pressure of 2 bar to 30 bar, comprising more than 21 vol % oxygen as a stripping medium; [0109] b) operating the first bleacher unit at a pressure that is at least 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and up to 30 bar, thereby generating a first NOx-loaded stripping gas; [0110] c) optionally, lowering the pressure of the first NO.sub.x-loaded stripping gas to the working pressure or to a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit; and [0111] d) directing the first NO.sub.x-loaded stripping gas to the inlet of the absorber unit.

[0112] As defined herein, an oxygen-rich gas is a gas comprising more oxygen than is on the average present in air. In particular, an oxygen-rich gas comprises more than 21 vol % of oxygen, more in particular more than 30 vol %, more than 40 vol %, more than 50 vol %, more than 60 vol %, more than 70 vol %, more than 80 vol %, more than 90 vol %, more than 95 vol %, and more than 99 vol %, more in particular 100 vol % of oxygen. The oxygen-rich gas differs from the gas used in the prior art in that it contains more than 21 vol % of oxygen. The oxygen-rich gas may be obtained from mixing oxygen with air or any other gas, suitable for its primary purpose, i.e. as a stripping medium in a bleacher unit.

[0113] As defined herein, the bleacher unit can be any bleacher unit known in the prior art, such as, but not restricted to, a sieve tray bleacher unit, a random packing bleacher unit, or a structured packing bleacher unit. Suitable and particular embodiments for the bleacher unit are described in the main embodiment.

[0114] It is to be understood that the pressure of the first NO.sub.x-loaded stripping gas, exiting the first bleacher unit, is lowered by the corresponding means to a pressure that is from 0.01 bar to 1 bar, more in particular from 0.05 bar to 0.5 bar, even more in particular from 0.1 bar to 0.2 bar, over the pressure of the gaseous NO.sub.x stream entering the absorber. Stated otherwise, it is to be understood that the pressure drops in the first bleacher unit, in the ammonia convertor and in the transport of the NO.sub.x-loaded stripping gas and of the gaseous NO.sub.x stream from the ammonia convertor to the inlet of the absorber unit 6, which actually results in the flow of the NO.sub.x-loaded stripping gas and of the second gaseous NO.sub.x stream to the inlet of the absorber unit, are taken into account for determining the pressure of the NO.sub.x-loaded stripping gas to be combined with the gaseous NO.sub.x stream and to be fed the inlet of the absorber unit. As a result, the pressure in the first bleacher unit is at least the working pressure adjusted, i.e. increased by the sum of those pressure drops, and up to 30 bar. Accordingly, in the present disclosure, the first bleacher unit is operating at a minimum pressure corresponding to an overpressure from 0.01 bar to 1 bar, more in particular from 0.05 bar to 0.5 bar, even more in particular from 0.1 bar to 0.2 bar, over the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, in order to account for the above mentioned pressure drops.

[0115] By using a separate source of an oxygen rich gas, the air compressor is now only pressurizing primary air for the ammonia convertor since the secondary air supply for the bleacher unit has now been replaced by a stream of an oxygen-rich gas from a separate unit, in particular a water electrolyser unit which is operated at a gas pressure of 2 bar to 30 bar. As it requires less power to pump water than to compress gas, there is a significant overall energy savings. Furthermore, the air compressor is less loaded as it is only compressing primary air, thus requiring the air compressor to produce less work (for the same ammonia convertor throughput). Hence, the air compressor is able to handle an increased throughput of primary air (at an increased ammonia convertor throughput).

[0116] Moreover, as the drive power for the air compressor originates from the tail-gas turbine and a steam turbine (not shown), less power required on the air compressor translates into less consumption of steam from the steam turbine (not shown) and, thereby, additional steam export.

[0117] Furthermore, the use of an oxygen-rich gas as stripping medium results in an oxygen-rich tail gas, which results in additional power being extracted from the tail gas turbine.

[0118] Finally, due to the oxygen-rich gas source being fed to the first bleacher unit and, subsequently, to the absorber unit, the absorption in the absorber unit is improved and, therefore, the size of the absorption tower can be decreased. It is to be understood that, even following any reduction of the size of the absorber unit, the ammonia convertor and the absorber unit are still both operated at a working pressure of 2 bar to 16 bar and both the ammonia convertor and the absorber unit are operated at a pressure that is the same. Alternatively, if the same size is retained for the absorber unit, the tail gas will be cleaner and less work will be required from a De-NO.sub.x unit for cleaning the tail gas, such that its content in NO.sub.x gases is according to the environmental regulations.

[0119] The oxygen-rich gas to be used as the stripping medium, is provided, alone or mixed with air or any other suitable gas, by a pressurised water electrolyser operating at a pressure of 2 bar to 30 bar. Suitable electrolysers are described above, in the first aspect of the present disclosure.

[0120] It has been found that an improvement can be achieved by utilizing oxygen that already is pressurized. Indeed, the oxygen produced by a water electrolyser operating at higher pressure will be pressurized. Operating the water electrolyser at high pressure requires to pump water to the water electrolyser at a high pressure. However, pumping a liquid, in particular water, to a water electrolyser requires significantly less power than the compression of an oxygen-containing gas, in particular air, for use in a mono-pressure nitric acid production plant. Hence, the integration of a high pressure water electrolyser with a mono-pressure nitric acid production plant, in which pressurized oxygen is supplied to the mono-pressure nitric acid production plant, provides a large benefit in the form of energy saving associated with the compression of the oxygen-rich gas before its introduction in the mono-pressure nitric acid production plant.

[0121] Currently, electrolysis units and nitric acid production facilities are not existing in close environments. However, with the emergence of green technologies, it is anticipated that water electrolysis, ammonia and nitric production will all be integrated on a single production site. Only the hydrogen produced by water electrolysers is currently used in the production of ammonia, whereas the oxygen gas is vented off. In the future, the water electrolyser and the nitric acid production being located nearby, also the oxygen produced by the water electrolyser can be used and benefit to the nitric acid production. In addition, the use of an electrolyser presents the benefit that renewable energy, such as solar or wind energy can be used to electrolyse water into oxygen gas and hydrogen gas. The hydrogen gas thereby produced can also be used for producing green ammonia, that is for producing ammonia from a fossil-free source.

[0122] According to a particular embodiment, the pressurised water electrolyser 60 is at least partially powered by energy provided by the nitric acid plant. Indeed, energy can be recovered from a nitric acid plant. Sources of energy that can be converted into electrical power suitable for operating the water electrolyser 60 are, for example, the tail-gas turbine 8 and the steam turbine.

[0123] At times when renewable energy is not available, if the electrolyser is connected to the mono pressure plant to be provided with electrical power, the production of oxygen gas and hydrogen remains possible. Further, as the pressurised oxygen electrolyser can be continuously operated, either through the presence of renewable energy or through the presence of intermittent power, the continuous feeding of an oxygen-rich gas at a high pressure to the bleacher is possible.

[0124] According to a particular embodiment, the method further comprises, before step b), the step of: [0125] e) pre-heating the oxygen-rich gas as a stripping medium to a temperature of ambient temperature to 120° C., or from 50° C. to 120° C., or from 80° C. to 120° C., or from 90 to 120° C.

[0126] Bleaching of nitric acid in a bleacher unit in a nitric acid plant generally may be conditioned such that the temperature of the nitric acid is in the range of 30 to 60° C. (as measured inside the bleacher). By pre-conditioning the oxygen gas, the bleaching efficiency achieved in the first bleacher unit will be increased. Therefore, according to a specific embodiment and as described under the main embodiment, the plant according to the disclosure further comprises means for heating the stripping medium, such as, but not limited to, a pre-heater or a heat exchange system.

[0127] According to a particular embodiment of the present disclosure, the method further comprises the step of: [0128] f) operating a second bleacher unit at a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NO.sub.x gas stream to the absorber unit, and lower than the pressure at which the first bleacher unit is operating; [0129] g) providing the bleacher unit with an oxygen-rich gas as a stripping medium, thereby obtaining a second NO.sub.x-loaded stripping gas stream; and [0130] h) directing the second NO.sub.x-loaded stripping gas stream to the inlet of the absorber unit.

[0131] For the second bleacher unit, the same parameters apply as for the first bleaching tower as described herein.

[0132] Since the second bleacher unit operates at a pressure that is 0.01 bar to 1 bar higher than the working pressure reduced by the pressure drop in the ammonia convertor and during transport of the NOx gas stream to the absorber unit, and the first bleacher unit operates at a higher pressure than the second bleacher unit, the output product stream is fed to the first bleacher unit, through a pump, in order to produce a pressurised product stream at the pressure of the first bleacher unit, hence suitable for feeding to the first bleacher unit.

[0133] It is to be understood that pressure drops in the ammonia convertor, in the first bleacher unit, in the second bleacher unit and in the transport of the NO.sub.x-loaded stripping gases to the inlet of the absorber unit, which allow for the transport of the correspond gas flows, are taken into account. As a result, the pressure in the first bleacher unit and in the second bleacher unit are adjusted, in total, by the sum of those pressure drops, in order for the pressure inside the absorber unit to be the same as the pressure in the ammonia convertor reduced by the sum of the pressure drops.

[0134] The pressure of the first NO.sub.x-loaded stripping gas is adjusted by the pressure control valve to the pressure in the second bleacher unit. The first NO.sub.x-loaded stripping gas after pressure adjustment and the second NO.sub.x-loaded stripping gas are then combined to generate a third NO.sub.x-loaded stripping gas, which is then fed to the inlet of the absorber unit.

[0135] The stripping gas to the second bleacher unit may be generated by using the stripping gas to the first bleacher unit: the pressure of the stripping gas fed to the first bleacher unit is adjusted, for example through the use of one or more pressure release valves, in order for the stripping gas for the second bleacher unit to be at a lower pressure than the stripping gas for the first bleacher unit. In the presence of the second bleacher unit, the stripped nitric acid stream is further stripped, and the dissolved gases, containing in particular NO.sub.x and oxygen gases, are evacuated from the stripped nitric acid stream.

[0136] With this embodiment, the quality of the stripped nitric acid stream can be improved, in the sense that it contains less dissolved gases, in particular NO.sub.x and oxygen gases. In addition, by further stripping the stripped nitric acid stream coming out of the first bleacher unit, the amount of oxygen gases that will be released when the stripped nitric acid stream is further stripped down to atmospheric pressure in the storage tank, may be reduced. A high concentration of oxygen forces the equilibrium for nitric oxide.Math.nitrogen dioxide towards nitrogen dioxide, which results in brown gas emissions from the ventilation system of the storage tank. Hence, further stripping of the stripped nitric acid stream downstream the bleaching tower and prior to storing the nitric acid product, can result in less brown gas emissions coming out of the ventilation system of the product storage tank.

[0137] Furthermore, there is an additional amount of oxygen being provided to the absorber unit. As a result, the tail-gas from the absorber will be cleaner and less chemical conversion will be required from a DeNO.sub.x unit (not shown) for treating the tail-gas. Moreover, the absorption in the absorber unit will be improved and, therefore, the size of the absorption tower can be decreased. It is to be understood that, even following any reduction of the size of the absorber unit, the ammonia convertor and the absorber unit are still both operated at a working pressure of 2 bar to 16 bar and both the ammonia convertor and the absorber unit are operated at a pressure that is the same.

[0138] As an alternative to the second bleacher unit, the person skilled in the art will understand that it is possible to flash through a flash vessel instead of to further strip through a second bleacher unit, for achieving the same benefits as achieved upon further stripping through the second bleacher unit.

[0139] According to a specific embodiment of the disclosure, the working pressure is selected from the range of 2 bar to 6 bar and the pressure in the first bleacher unit is at least 1 bar higher than the working pressure, ranging between 3 bar and 16 bar. In other words, the disclosure allows to operate a nitric acid plant operating at a low-medium working pressure of 2 bar to 6 bar.

[0140] According to a specific embodiment of the disclosure, the working pressure is selected from the range of 9 bar to 16 bar and the pressure in the first bleacher unit is at least 1 bar higher than the working pressure, ranging between 17 bar to 30 bar. In other words, the disclosure allows to operate a nitric acid plant operating at a high working pressure of 9 bar to 16 bar.

[0141] Use

[0142] According to a third aspect of the disclosure, the use of any embodiment of a mono-pressure nitric acid production plant according to the disclosure, for recovery of energy from a bleacher unit operating in the mono-pressure nitric acid production plant, is disclosed.

EXAMPLES

Example 1

[0143] Reference is made to FIG. 2. The mono-pressure plant for the production of nitric acid comprised: [0144] an ammonia convertor (4), operating at a working pressure ranging between 2 bar and 16 bar, for oxidizing ammonia, thereby producing a gaseous NO.sub.x gas/steam mixture (15), and comprising at least one inlet for a stream of an ammonia/pressurized air mixture (14), and an outlet for a gaseous NO.sub.x gas/steam mixture (15); [0145] an air compressor (2), for pressurizing air (12) to the working pressure and for providing the pressurized air to the ammonia convertor 4; [0146] an absorber unit (6), operating at the working pressure decreased by the pressure drop losses occurring in the different units and transport elements of the plant, wherein the nitrogen oxides contained in a gaseous NO.sub.x stream (26) react with water, thereby producing nitric acid, the absorber unit (6) comprising an inlet (5) for a gaseous NOx stream, and an outlet for a product stream (27) containing a nitric acid solution and dissolved nitrogen oxides, and an outlet for a tail gas (30); [0147] a first bleacher unit (7), for stripping the dissolved nitrogen oxides from the output product stream (27), thereby producing a first NO.sub.x-loaded stripping gas (19), the absorber unit comprising an inlet for a stripping medium (60) and an inlet for the product stream (27), and an outlet for the stripped nitric acid stream (29); and [0148] a means for directing the first NO.sub.x-loaded stripping gas (19) to the inlet (5) of the absorber unit (6); and [0149] a source of oxygen-rich gas, particularly a pressurized water electrolyser (60), for providing the bleacher unit (7) with an oxygen-rich gas as a stripping medium (16) via the inlet for the stripping medium.

[0150] Pressurized oxygen gas 16 from an external, pressurised water electrolyser system 60 was fed into the nitric acid first bleacher unit 7 as the stripping medium at a pressure of 2 bar to 6 bar. The pressure in the first bleacher unit corresponded to the pressure in the ammonia convertor 4, taking into account pressure drop losses in the different units of the plant. The pressure from the air compressor 2 is 5.4 bar. The NO.sub.x-loaded stripping gas 19 was redirected to the inlet of the absorber 6. The pressure in the first bleacher unit 7 was 4.93 bar. At the inlet 5 of the absorber 6, the pressure of the NO.sub.x-loaded stripping gas 19 was 4.93 bar. Upon feeding of the NO.sub.x-loaded stripping gas 19 and of the second gaseous NO.sub.x-containing stream 26, the pressure in the absorber unit was 4.92 bar and corresponded to the pressure in the ammonia convertor 4 at 5.2 bar, decreased by the pressure drop losses occurring in the different units and transport elements of the plant. Compression work in the air compressor was reduced as no compression of secondary air was required. In the conventional plant the air compressor used 292.2 kWh/t 100% HNO.sub.3 produced. In example 1 the air compressor used 251.7 kWh/t 100% HNO.sub.3 produced, i.e a reduction of around 14%.

Example 2

[0151] Reference is made to FIG. 3. The mono-pressure plant for the production of nitric acid according to FIG. 3 corresponded to the mono-pressure plant discussed in example 1 (shown in FIG. 2), further comprising a second bleacher unit (32). The second bleacher unit 32 comprised: [0152] a first inlet for the stripped nitric acid stream (29), in fluid communication with the first bleacher unit (7); [0153] a second inlet for a stripping gas stream (16); [0154] a first outlet for a second NOx-loaded stripping gas stream (33), in fluid communication with the inlet (5) of the absorber unit (6); and [0155] a second outlet for collecting the bleached nitric acid stream (35) from the second bleacher unit (32).

[0156] In addition to the process described under Example 1, the stripped nitric acid stream 29 from the first bleacher unit 7 was further stripped into a second bleacher unit 32. The first bleacher unit 7 was operated by oxygen produced from a pressurised water electrolyser 60 operated at a pressure of 16 bar. Stream 34 was obtained by splitting the stream 16, used as stripping gas in the first bleacher unit 7 with a pressure of 15.95 bar, and expanding the part of the splitting stream to be fed to the second bleacher unit 32 over a pressure release valve 31, such that the stream 34 had a pressure of 5.02 bar. The NO.sub.x-loaded stripping gas 19 leaving the first bleacher unit 7 was expanded over a pressure release valve 65 and combined with the NO.sub.x-loaded stripping gas 33 leaving the second bleacher unit 32, to provide the NO.sub.x-loaded stripping gas 62. The NO.sub.x-loaded stripping gas 62 was redirected to the inlet of the absorber 6. At the inlet 5 of the absorber 6, the pressure of the NO.sub.x-loaded stripping gas 62 was 4.92 bar. Upon feeding of the NO.sub.x-loaded stripping gas 19 and of the second gaseous NO.sub.x-containing stream 26, the pressure in the absorber 6 was 4.92 bar and corresponded to the pressure in the ammonia convertor 4 at 5.2 bar, reduced by the pressure drop in the different units and transport elements of the plant. Compression work in the air compressor was reduced as no compression of secondary air was required. In addition, stripping of the liquid nitric acid stream 29 leaving the first bleacher unit 7 was improved. In example 2 the air compressor used 251.7 kWh/t 100% HNO.sub.3 produced. The stripping of the liquid nitric acid was improved from example 1 to example 2: the simulated mass-fraction of N.sub.2O.sub.4 in the bleached acid, was 2.7.106 in example 1 comprising one bleacher unit, while it was 2.3.106 in example 2 comprising two bleachers.

Example 3

[0157] Reference is made to FIG. 2.

[0158] Example 1 was repeated with a pressure in the first bleacher unit 7 being selected from the range of 9 to 16 bar. The inlet pressure to the bleacher was 9.0 bar, and the outlet pressure was 8.9 bar. Compression work in the air compressor was reduced as no compression of secondary air was required. In example 3 the air compressor used 251.7 kWh/t 100% HNO.sub.3 produced.

Example 4

[0159] Reference is made to FIG. 3.

[0160] In addition to the process described under Example 2, the stripped nitric acid stream 29 from the first bleacher unit 7 was further stripped into a second bleacher unit 32. The first bleacher unit 7 was operated by oxygen produced from a pressurised water electrolyser 60 operated at a pressure of 20 bar. Stream 34 was obtained by splitting the stream 16, used as stripping gas in the first bleacher unit 7, with a pressure of 19.54 bar, and expanding the part of the splitting stream to be fed to the second bleacher unit 32 over a pressure release valve 31, such that the stream 34 had a pressure corresponding to the pressure in the ammonia convertor 4 and was selected from the range of 2 bar to 6 bar, in this example 5.0 bar. The NO.sub.x-loaded stripping gas 19 leaving the first bleacher unit 7 was expanded over a pressure release valve 65 and combined with the NO.sub.x-loaded stripping gas 33 leaving the second bleacher unit 32, to provide the NO.sub.x-loaded stripping gas 62. The NO.sub.x-loaded stripping gas 62 was redirected to the inlet of the absorber 6. At the inlet 5 of the absorber 6, the pressure of the NO.sub.x-loaded stripping gas 62 was 4.92 bar. Upon feeding of the NO.sub.x-loaded stripping gas 19 and of the second gaseous NO.sub.x-containing stream 26, the pressure in the absorber unit was 4.92 bar and corresponded to the pressure in the ammonia convertor 4 at 5.2 bar decreased by the pressure drop losses occurring in the different units and transport elements of the plant. Compression work in the air compressor was reduced as no compression of secondary air was required. In example 4 the air compressor used 251.7 kWh/t 100% HNO.sub.3 produced.

[0161] The table below summarizes the reference case and examples:

TABLE-US-00001 Number in Conventional Example Description Unit drawing case Example 1 Example 2 Example 3 Example 4 Pressure from [bar abs] 13 5.401 5.401 5.401 5.401 5.401 air compressor Pressure from [bar abs] 15 5.201 5.201 5.201 5.201 5.201 converter Pressure in LP [bar abs] 5.026 5.026 5.036 5.036 bleacher inlet Pressure in HP [bar abs] 28 15.954 9.000 19.954 bleacher inlet Pressure at [bar abs] 5 4.916 4.916 4.916 4.916 4.916 absorber inlet Oxygen pressure [bar abs] 16 — 6 16 16 20 Split of bleaching 1 0.66 0 0.66 gas to LP absorber Mole flow of [kmol/h] 16 1107.3 790.2 808.7 790.0 808.7 bleaching gas to bleacher(s) Production rate [MTPD] 29 or 35 1800 1802 1802 1802 1802 Power to air [kW] 2 21915 18901 18901 18901 18901 compressor Power from tail [kW] 8 −10401 −9770 −9787 −9789 −9768 gas expander Power from steam [kW] −17326 −17181 −17124 −17126 −17158 turbine Power production [kW] −5612 −7851 −7811 −7815 −7826 from compressor train Specific values Power to air [kWh/t 292.2 251.7 251.7 251.7 251.7 compressor HNO3] Power from tail [kWh/t −138.7 −130.1 −130.3 −130.4 −130.1 gas expander HNO3] Power from steam [kWh/t −231.0 −228.8 −228.0 −228.1 −228.5 turbine HNO3] Power production [kWh/t −74.8 −104.5 −104.0 −104.1 −104.2 from compressor HNO3] train Volume flow gas [m3/h] 34 6658.09 4744.29 3181.27 3174.91 to LP bleacher Volume flow gas [m3/h] 519.66 2638.81 415.37 to HP bleacher Volume flow acid [m3/h] 28 93.06 93.33 93.33 93.10 93.35 to bleacher(s) Mass fraction NO 8.74E−18 1.05E−17 5.62E−25 5.95E−17 1.06E−24 in bleached acid Mass fraction NO2 1.96E−14 6.18E−14 3.31E−19 3.33E−13 5.75E−19 in bleached acid Mass fraction N2O4 6.76E−07 2.69E−06 2.34E−06 1.28E−05 2.80E−06 in bleached acid

TABLE-US-00002 LIST OF REFERENCE NUMERALS 1 Pre-heating unit 18 Gaseous NO.sub.x-containing stream 2 Air compressor 19 (First) NO.sub.x-effluent stream from 3 Mixing apparatus first bleacher unit 4 Ammonia burner 20, 22 Oxidising section (ammonia 26 Second gaseous NO.sub.x-containing convertor) stream 5 Inlet of 27 Absorber liquid nitric acid absorber unit output stream 6 Absorber unit 28 Pressurized absorber liquid 7 First bleacher nitric acid output unit stream after pumping 8 Tail-gas turbine 29 First Bleacher unit liquid nitric 9a, 9b Water cooler/ acid output stream condenser 30 Absorber tail-gas 9d Tail-gas heater 31 Absorption tower level 10 Ammonia stream control valve 11 Pre-heated 32 Second bleacher unit ammonia stream 33 Second NO.sub.x-loaded stripping 12 Air stream gas/NO.sub.x-effluent stream from 13 Compressed air second bleacher unit 13a Compressed 34 LP Bleacher unit secondary air 35 Second bleacher unit liquid 13b Compressed nitric acid output stream primary air 36 pump 14 Ammonia 60 Source of HP oxygen-rich gas, convertor input in particular a pressurized stream water electrolyser 15 First gaseous 61 Pressure control valve NO.sub.x for the stripping stream/ammonia gas to the second bleacher. convertor output 62 Combined (third) first and stream second NO.sub.x-loaded stripping gas 16 Stripping gas after pressure adjustment of 17, 21, 23 Liquid nitric acid first NOx-loaded stripping gas/ stream NO.sub.x-effluent stream 63 Pressure control valve for the stripping gas 64 Control valve for the liquid level in the first bleacher 65 Pressure control valve for the gas coming out of the outlet bleacher