SYSTEM FOR EVACUATING NOx GASES FROM A NITRIC ACID STORAGE TANK

Abstract

A system for evacuating NO.sub.x gases from a nitric acid storage tank in a nitric acid production plant. The production plant includes a gas ejector with at least two gas inlets and at least one gas outlet, wherein a first gas inlet is branched from a gas conduit through which NO.sub.x-containing gas is flowing at a pressure P1 ranging from 2 to 16 bar; a second gas inlet is fluidically connected to the nitric acid storage tank essentially maintained at atmospheric pressure; and a gas outlet is fluidically connected to a gas conduit through which NO.sub.x-containing gas is flowing at a pressure P2 lower than P1. A method for evacuating NO.sub.x gases from a nitric acid storage tank, a method for revamping a system of a nitric acid plant, and the use of a gas ejector for evacuating NO.sub.x gases from a nitric acid storage tank in a production plant.

Claims

1. A system for evacuating NO.sub.x gases from a nitric acid storage tank in a nitric acid production plant, characterized in that the nitric acid production plant comprises a gas ejector comprising at least two gas inlets and at least one gas outlet, wherein: a first gas inlet of the gas ejector is branched from a gas conduit of the nitric acid production plant through which NO.sub.x-containing gas is flowing at a pressure P1 ranging from 2 bar to 16 bar; a second gas inlet of the gas ejector is fluidically connected to the nitric acid storage tank essentially maintained at atmospheric pressure; and a gas outlet of the gas ejector is fluidically connected to a gas conduit of the nitric production plant through which NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1.

2. The system according to claim 1 wherein the first gas inlet of the gas ejector is branched from a gas conduit downstream from an absorption tower having an outlet from which a NO.sub.x-containing tail gas is obtained, through which the tail gas is flowing, and wherein the gas outlet of the gas ejector is fluidically connected to a gas conduit downstream from a tail gas expander for expanding the tail gas and/or downstream from a tail gas heater of the nitric acid production plant for heating the tail gas to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas.

3. The system according to claim 1 wherein the first gas inlet of the gas ejector is branched from a gas conduit downstream from a tail gas heater for heating a NO.sub.x-containing tail gas obtained from an outlet of an absorption tower to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas, and wherein the gas outlet of the gas ejector is fluidically connected to a gas conduit downstream from a tail gas expander for expanding the heated tail gas.

4. The system according to claim 1 wherein the first gas inlet of the gas ejector is branched from a gas conduit downstream from a NO.sub.x gas compressor and upstream from an absorption tower, and wherein the gas outlet of the gas ejector is fluidically connected to a gas conduit upstream from the NO.sub.x gas compressor.

5. The system according to claim 4, wherein the pressure P1 ranges from 4 bar to 16 bar.

6. The system according to claim 5, further comprising a tail gas treatment unit located downstream from the absorption tower and/or downstream from a tail gas expander.

7. A method for evacuating NO.sub.x gases from a nitric acid storage tank in a nitric acid production plant, wherein a first NO.sub.x-containing gas from a gas conduit of the nitric acid production plant at a pressure P1 ranging from 2 bar to 16 bar and a second NO.sub.x-containing gas from the nitric acid storage tank at atmospheric pressure are respectively flown in through a first inlet and a second inlet of a gas ejector and flown out to a gas conduit of the nitric acid production plant through which a NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1.

8. The method according to claim 7 wherein the first NO.sub.x-containing gas is a tail gas obtained from an outlet of an absorption tower and wherein an outflowing gas stream of the gas ejector flows to a gas conduit downstream from a tail gas expander for expanding the tail gas and/or downstream from a tail gas heater for heating the tail gas to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas.

9. The method according to claim 7 wherein the first NO.sub.x-containing gas is obtained from a gas conduit downstream from a tail gas heater for heating a tail gas obtained from an outlet of an absorption tower to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas, and wherein an outflowing gas stream of the gas ejector flows to a gas conduit downstream from a tail gas expander for expanding the heated tail gas.

10. The method according to claim 7 wherein the first NO.sub.x-containing gas is obtained from a gas conduit downstream from a NO.sub.x gas compressor and upstream from an absorption tower, and wherein an outflowing gas stream of the gas ejector flows to a gas conduit upstream from the NO.sub.x gas compressor.

11. The method according to claim 10, wherein the pressure P1 ranges from 4 bar to 16 bar.

12. The method according to claim 11, further comprising a step of treating a tail gas downstream from the absorption tower and/or downstream from a tail gas expander.

13. The method according to claim 7, wherein the method is a periodical or continuous process.

14. A method for revamping a nitric acid production plant comprising a step of installing a gas ejector for executing the method according to claim 7.

15. A method for evacuating NO.sub.x gases from a nitric acid storage tank, in a production plant for producing nitric acid, utilizing a gas injector.

16. The system according to claim 1, wherein the pressure P1 ranges from 4 bar to 16 bar.

17. The system according to claim 1, further comprising a tail gas treatment unit located downstream from an absorption tower and/or downstream from a tail gas expander.

18. The method according to claim 7, wherein the pressure P1 ranges from 4 bar to 16 bar.

19. The method according to claim 7, further comprising a step of treating a tail gas downstream from an absorption tower and/or downstream from a tail gas expander.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 shows a conventional gas ejector device;

[0019] FIG. 2 shows a schematic representation of a dual nitric acid plant;

[0020] FIG. 3 shows a schematic representation of an embodiment of the system of the disclosure;

[0021] FIG. 4 shows a schematic representation of an embodiment of the system of the disclosure;

[0022] FIG. 5 shows a schematic representation of an embodiment of the system of the disclosure; and

[0023] FIG. 6 shows a schematic representation of an embodiment of the system of the disclosure.

DETAILED DESCRIPTION

[0024] Throughout the description and claims of this specification, the words comprise and variations thereof 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 disclosure, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the disclosure is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0025] 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 disclosure (including the description, 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 disclosure (including the description, claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0026] 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 ranges from . . . 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.

[0027] As defined herein a gas inlet that is branched from means that the inlet only receives the gas from the conduit to which it is branched from, divided from or split from.

TABLE-US-00001 Table of numerals 2 nitric acid storage tank 3 outlet of nitric acid storage tank 5 tail gas 6 outlet of nitric acid absorption tower 7 tail gas expander 8 inlet of tail gas expander 9 outlet of tail gas expander 10 ejector as used in the disclosure 11 first inlet of the gas ejector as used in the disclosure 12 second inlet of the gas ejector as used in the disclosure 13 outlet of the gas ejector as used in the disclosure 14 ammonia/air mixture 15 low-pressure NO.sub.x gas/steam mixture 16 gaseous stripping medium 17 aqueous diluted nitric acid mixture 18 gaseous NO.sub.x stream 19 NO.sub.x-loaded stripping gas 21 aqueous diluted nitric acid mixture 22 gaseous NO.sub.x stream 24 pressurized gaseous NO.sub.x stream 27 stream of raw nitric acid-containing residual NO.sub.x gas 29 stripped nitric acid stream 31 Valve 32 Ammonia 33 pre-heater 34 compressed air 35 mixing apparatus 36 air compressor 37 ammonia converter 38 water cooler/condenser 39 cooler/separator 40 NO.sub.x gas compressor 41 absorption tower 42 bleacher unit 43 tail gas heater 44 NO.sub.x gas compressor inlet 45 NO.sub.x gas compressor outlet 48 Air 49 heated tail gas 100 conventional gas ejector 200 first inlet for a high-pressure gas in a conventional gas ejector 300 second inlet for a low-pressure gas in a conventional gas ejector 400 outlet in conventional ejector
System for Evacuating NO.sub.x Gases from a Nitric Acid Storage Tank

[0028] In one aspect of the disclosure, a system for evacuating NO.sub.x gases from a nitric acid storage tank in a nitric acid production plant, is disclosed. The system is characterized in that the nitric acid production plant comprises a gas ejector comprising at least two gas inlets and at least one gas outlet, wherein: [0029] a first gas inlet of the gas ejector is branched from a gas conduit of the nitric acid production plant through which NO.sub.x-containing gas is flowing at a pressure P1 ranging from 2 bar to 16 bar; [0030] a second gas inlet of the gas ejector is fluidically connected to the nitric acid storage tank essentially maintained at atmospheric pressure; and [0031] a gas outlet of the gas ejector is fluidically connected to a gas conduit of the nitric production plant through which NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1.

[0032] The inventors have now established that this is possible to use a gas ejector to evacuate the NO.sub.x gases from a storage tank containing nitric acid. As there are gases at a pressure of at least 2 bar in the nitric acid process suitable for use as a motive gas and the gases from the storage tank are at atmospheric pressure, it is possible to use a gas ejector in combination with these two types of gases. The result is that the gases of the storage tank are evacuated and, instead of spreading in the surroundings of the storage tank, they are diluted, hence treated, through dilution using the tail gas, at the outlet of the gas ejector. In this way, not only are the gases of the storage tank evacuated, they are also handled in a manner that does not involve the washing of the NO.sub.x gases with water, as is commonly done, for example through the use of scrubbing systems. Therefore, the system of the disclosure, when compared to the systems of the prior art, is simplified, has a smaller area footprint and is cheaper.

[0033] The person skilled in the art will understand that what is required in the system of the disclosure is the presence of a gas that can act as a motive gas in a gas ejector and of an acid storage tank in which gases at a low pressure are released. Further, in the gas conduit in fluid communication with the outlet of the gas ejector, that is in the gas conduit where the gases leaving the gas ejector are released, the pressure should be lower than the pressure of the motive gas, such that a pressure drop is created, resulting in under-pressure in the storage tank, in turn resulting in the proper functioning of the gas ejector, meaning that the NO.sub.x gases released in the storage tank are evacuated out of the storage tank, and to a safe location. As long as those criteria associated with the motive gas and with the pressure drop are met, the production plant needs not be a nitric acid plant specifically. Therefore, the system of the disclosure can be to the system of any process comprising a gas that can be used as a motive gas and low pressure gases in a storage tank that are to be evacuated.

[0034] Reference is made to FIGS. 3 to 6. In one aspect of the disclosure, a system for evacuating NO.sub.x gases from the nitric acid storage tank 2 in a nitric acid production plant, is disclosed. The system is characterized in that the nitric acid production plant comprises the gas ejector 10 comprising at least two gas inlets and at least one gas outlet, wherein: [0035] the first gas inlet 11 of the gas ejector 10 is branched from a gas conduit of the nitric acid production plant through which NO.sub.x-containing gas is flowing at a pressure P1 ranging from 2 bar to 16 bar; [0036] the second gas inlet 12 of the gas ejector 10 is fluidically connected to the nitric acid storage tank 2 essentially maintained at atmospheric pressure; and [0037] the gas outlet 13 of the gas ejector 10 is fluidically connected to a gas conduit of the nitric production plant through which NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1.

[0038] The inventors have now established that this is possible to use a gas ejector 10 to evacuate the NO.sub.x gases from a storage tank 2 containing nitric acid. As there are gases at a pressure of at least 2 bar in the nitric acid process suitable for use as a motive gas and the gases from the storage tank 2 are at atmospheric pressure, it is possible to use the gas ejector 10 in combination with these two types of gases. The result is that the gases of the storage tank 2 are evacuated and, instead of spreading in the surroundings of the storage tank 2, they are diluted, hence handled, through dilution using the tail gas, at the outlet of the gas ejector. In this way, not only are the gases of the storage tank 2 evacuated, they are also handled in a manner that does not involve the washing of the NO.sub.x gases with water, as is commonly done, for example through the use of scrubbing systems. Therefore, the method of the disclosure, when compared to the methods of the prior art, is simplified and cheaper, due to the simple and cheap operation of the gas ejector 10.

[0039] In one embodiment according to the system of the disclosure, the first gas inlet of the gas ejector is branched from a gas conduit downstream from an absorption tower having an outlet from which a NO.sub.x-containing tail gas is obtained, through which the tail gas is flowing, and the gas outlet of the gas ejector is fluidically connected to a gas conduit downstream from a tail gas expander for expanding the tail gas and/or downstream from a tail gas heater for heating the tail gas to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas.

[0040] As defined herein, a tail gas heater is a system which comprises one or more, such as one to four, heaters installed in series, for gradually heating the tail gas from a temperature ranging from 2? C. to 50? C., to a temperature ranging from 200? C. to 650? C., thereby generated a heated tail gas.

[0041] Reference is made to FIGS. 3 and 4. In one embodiment according to the system of the disclosure, the first gas inlet 11 of the gas ejector 10 is branched from the gas conduit downstream from the absorption tower 41 having the outlet 6 from which the NO.sub.x-containing tail gas 5 is obtained, through which the tail gas 5 is flowing, and the gas outlet 13 of the gas ejector 10 is fluidically connected to the gas conduit downstream from the tail gas expander 7 for expanding the tail gas 5 and/or downstream from the tail gas heater 43 for heating the tail gas 5 to a temperature ranging from 200 to 650? C., thereby generating the heated tail gas 49.

[0042] As defined herein, the tail gas heater 43 is a system which comprises one or more, such as one to four, heaters installed in series, for gradually heating the tail gas from a temperature ranging from 2? C. to 50? C., to a temperature ranging from 200? C. to 650? C., thereby generating the heated tail gas 49.

[0043] In one embodiment according to the system of the disclosure, the first gas inlet of the gas ejector is branched from a gas conduit downstream from a tail gas heater for heating a NO.sub.x-containing tail gas obtained from an outlet of an absorption tower to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas, and the gas outlet of the gas ejector is fluidically connected to a gas conduit downstream from a tail gas expander for expanding the heated tail gas.

[0044] As defined herein, a tail gas heater is a system which comprises one or more, such as one to four, heaters installed in series, for gradually heating the tail gas from a temperature ranging from 2? C. to 50? C., to a temperature ranging from 200? C. to 650? C., thereby generating a heated tail gas.

[0045] The person skilled in the art will understand that, since the tail gas heater comprises one or more heaters in series, it is possible to have the first inlet of the tail gas ejector downstream a first heater of the tail gas heater, and the outlet of the gas ejector is in fluid communication with the outlet of a second heater of the tail gas heater, as long as the second heater is located downstream the first heaterthat is as long as there is the necessary pressure drop between the gas conduit in fluid communication with the first inlet of the gas ejector and the gas conduit in fluid communication with the outlet of the gas ejector.

[0046] Reference is made to FIG. 5. In one embodiment according to the system of the disclosure, the first gas inlet 11 of the gas ejector 10 is branched from the gas conduit downstream from the tail gas heater 43 for heating the NO.sub.x-containing tail gas 5 obtained from the outlet of the absorption tower 41 to a temperature ranging from 200 to 650? C., thereby generating the heated tail 49, and the gas outlet 13 of the gas ejector 10 is fluidically connected to the gas conduit downstream from the tail gas expander 7 for expanding the heated tail gas (49).

[0047] As defined herein, the tail gas heater 43 is a system which comprises one or more, such as one to four, heaters installed in series, for gradually heating the tail gas from a temperature ranging from 2? C. to 50? C., to a temperature ranging from 200? C. to 650? C., thereby generating the heated tail gas 49.

[0048] The person skilled in the art will understand that, since the tail gas heater 43 comprises one or more heaters in series, it is possible to have the first inlet 11 of the tail gas ejector 10 downstream a first heater of the tail gas heater 43, and the outlet 13 of the ejector 10 is in fluid communication with the outlet of a second heater of the tail gas heater 43, as long as the second heater is located downstream the first heaterthat is as long as there is the necessary pressure drop between the gas conduit in fluid communication with the first inlet 11 of the ejector 10 and the gas conduit in fluid communication with the outlet 13 of the ejector 10.

[0049] In one embodiment according to the system of the disclosure, the first gas inlet of the gas ejector is branched from a gas conduit downstream from a NO.sub.x gas compressor and upstream from an absorption tower, and the gas outlet of the gas ejector is fluidically connected to a gas conduit upstream from the NO.sub.x gas compressor.

[0050] Reference is made to FIG. 6. In one embodiment according to the system of the disclosure, the first gas inlet 11 of the gas ejector 10 is branched from the gas conduit downstream from the NO.sub.x gas compressor 40 and upstream from the absorption tower 41, and the gas outlet 13 of the gas ejector is fluidically connected to a gas conduit upstream from the NO.sub.x gas compressor 40.

[0051] In one embodiment according to the system of the disclosure, the pressure P1 ranges from 4 bar to 16 bar.

[0052] The higher the pressure of the motive gas directed to the first inlet of the gas ejector, the higher the pressure drop between the gas conduit in fluid communication with the first inlet of the gas ejector and the gas conduit in fluid communication with outlet of the gas ejector will be, resulting in higher under-pressure above the storage tank, hence in a more efficient evacuation of the NO.sub.x gases released in the storage tank.

[0053] Reference is made to FIGS. 3 to 6. In one embodiment according to the system of the disclosure, the pressure P1 ranges from 4 bar to 16 bar.

[0054] The higher the pressure of the motive gas directed to the first inlet 11 of the gas ejector 10, the higher the pressure drop between the gas conduit in fluid communication with the first inlet 11 of the ejector 10 and the gas conduit in fluid communication with outlet 13 of the ejector 10 will be, resulting in higher under-pressure above the storage tank 2, hence in a more efficient evacuation of the NO.sub.x gases released in the storage tank 2.

[0055] In one embodiment according to the system of the disclosure, the system further comprises a tail gas treatment unit located downstream from an absorption tower and/or downstream from a tail gas expander.

[0056] The addition of such tail gas treatment unit (not shown) allows for a reduction of the emissions downstream the tail gas expander. Indeed, the lower the concentration on NO.sub.x gases at the tail gas expander inlet or downstream the tail gas expander outlet, and upstream the location of the release of the gases to the atmosphere, the lower the concentration in NO.sub.x gases of the gases that are released to the atmosphere. Therefore, the use of a tail gas treatment unit results in enhanced control of the emissions from the nitric acid production plant.

[0057] Reference is made to FIGS. 3 and 6. In one embodiment according to the system of the disclosure, the system further comprises a tail gas treatment unit (not shown) located downstream from the absorption tower (41) and/or downstream from the tail gas expander (7).

[0058] The addition of such tail gas treatment unit (not shown) allows for a reduction of the emissions downstream the tail gas expander 7. Indeed, the lower the concentration on NO.sub.x gases at the tail gas expander inlet 8 or downstream the tail gas expander outlet 9, and upstream the location of the release of the gases to the atmosphere, the lower the concentration in NO.sub.x gases of the gases that are released to the atmosphere. Therefore, the use of a tail gas treatment unit results in enhanced control of the emissions from the nitric acid production plant.

Method for Evacuating NO.sub.x Gases from a Nitric Acid Storage Tank

[0059] In another aspect of the disclosure, a method for evacuating NO.sub.x gases from a nitric acid storage tank in a nitric acid production plant, wherein a first NO.sub.x-containing gas from a gas conduit of the nitric acid production plant a at a pressure P1 ranging from 2 bar to 16 bar and a second NO.sub.x-containing gas from the nitric acid storage tank at atmospheric pressure are respectively flown in through a first inlet and a second inlet of a gas ejector and flown out to a gas conduit of the nitric production plant through which a NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1, is disclosed.

[0060] The inventors have now established that this is possible to use a gas ejector to evacuate the NO.sub.x gases from a storage tank of a nitric acid. As there are gases at a pressure of at least 2 bar in the nitric acid process suitable for use as a motive gas and the gases from the storage tank are at atmospheric pressure, it is possible to use a gas ejector in combination with these two types of gases. The result is that the gases of the storage tank are evacuated and, instead of spreading in the surroundings of the storage tank, they are diluted, hence handled, through dilution using the tail gas, at the outlet of the gas ejector. In this way, not only are the gases of the storage tank evacuated, they are also handled in a manner that does not involve the washing of the NO.sub.x gases with water, as is commonly done, for example through the use of scrubbing systems. Therefore, the method of the disclosure, when compared to the methods of the prior art, is simplified and cheaper, due to the simple and cheap operation of the gas ejector.

[0061] The person skilled in the art will understand that what is required to apply the method of the disclosure is the presence of a tail gas that can act as a motive gas in a gas ejector and of an acid storage tank in which gases at a low pressure are released. Further, in the gas conduit in fluid communication with the outlet of the gas ejector, that is in the gas conduit where the gases leaving the gas ejector are released, the pressure should be lower than the pressure of the motive gas, such that a pressure drop is created, resulting in under-pressure in the storage tank, in turn resulting in the proper functioning of the gas ejector, meaning that the gases released in the storage tank are actually directed out of the storage tank, and at a safe location. As long as those criteria associated with the motive gas and with the pressure drop are met, the process needs not be nitric acid specifically. Therefore, the system of the disclosure can be applied to any process comprising a gas that can be used as a motive gas and low pressure gases in a storage tank that are to be evacuated.

[0062] Reference is made to FIGS. 3 and 6. In another aspect of the disclosure, a method for evacuating NO.sub.x gases from the nitric acid storage tank 2 in a nitric acid production plant, wherein a first NO.sub.x-containing gas from a gas conduit of the nitric acid production plant a at a pressure P1 ranging from 2 bar to 16 bar and a second NO.sub.x-containing gas from the nitric acid storage tank 2 at atmospheric pressure are respectively flown in through the inlets 11 and 12 of the gas ejector 10 and flown out to a gas conduit of the nitric production plant through which a NO.sub.x-containing gas is flowing at a pressure P2 which is lower than P1, is disclosed.

[0063] The inventors have now established that this is possible to use a gas ejector 10 to evacuate the NO.sub.x gases from a storage tank 2 containing nitric acid. As there are gases at a pressure of at least 2 bar in the nitric acid process suitable for use as a motive gas and the gases from the storage tank 2 are at atmospheric pressure, it is possible to use the gas ejector 10 in combination with these two types of gases. The result is that the gases of the storage tank 2 are evacuated and, instead of spreading in the surroundings of the storage tank 2, they are diluted, hence handled, through dilution using the tail gas, at the outlet of the gas ejector. In this way, not only are the gases of the storage tank 2 evacuated, they are also handled in a manner that does not involve the washing of the NO.sub.x gases with water, as is commonly done, for example through the use of scrubbing systems. Therefore, the method of the disclosure, when compared to the methods of the prior art, is simplified and cheaper, due to the simple and cheap operation of the gas ejector 10.

[0064] In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is a tail gas obtained from an outlet of an absorption tower and the outflowing gas stream of the gas ejector flows to a gas conduit downstream from a tail gas expander for expanding the tail gas and/or downstream from a tail gas heater for heating the tail gas to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas.

[0065] Reference is made to FIGS. 3 and 4. In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is the tail gas (5) obtained from the outlet (6) of the absorption tower (41) and the outflowing gas stream of the gas ejector (10) flows to the gas conduit downstream from the tail gas expander (7) for expanding the tail gas (5) and/or downstream from the tail gas heater (43) for heating the tail gas (5) to a temperature ranging from 200 to 650? C., thereby generating the heated tail gas (49).

[0066] In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is obtained from a gas conduit downstream from a tail gas heater for heating a tail gas obtained from an outlet of an absorption tower to a temperature ranging from 200 to 650? C., thereby generating a heated tail gas, and the outflowing gas stream of the gas ejector flows to a gas conduit downstream from a tail gas expander for expanding the heated tail gas.

[0067] The person skilled in the art will understand that, since the tail gas heater comprises one or more heaters in series, it is possible to have the first inlet of the tail gas ejector downstream a first heater of the tail gas heater, and the outlet of the gas ejector is in fluid communication with the outlet of a second heater of the tail gas heater, as long as the second heater is located downstream the first heaterthat is as long as there is the necessary pressure drop between the gas conduit in fluid communication with the first inlet of the gas ejector and the gas conduit in fluid communication with the outlet of the gas ejector.

[0068] Reference is made to FIG. 5. In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is obtained from the gas conduit downstream from a tail gas heater 43 for heating the tail gas 5 obtained from the outlet 6 of the absorption tower 41 to a temperature ranging from 200 to 650? C., thereby generating the heated tail gas 49, and the outflowing gas stream of the gas ejector (10) flows to a gas conduit downstream from a tail gas expander (7) for expanding the heated tail gas (49).

[0069] The person skilled in the art will understand that, since the tail gas heater 43 comprises one or more heaters in series, it is possible to have the first inlet 11 of the tail gas ejector 10 downstream a first heater of the tail gas heater 43, and the outlet 13 of the gas ejector 10 is in fluid communication with the outlet of a second heater of the tail gas heater 43, as long as the second heater is located downstream the first heaterthat is as long as there is the necessary pressure drop between the gas conduit in fluid communication with the first inlet 11 of the gas ejector 10 and the gas conduit in fluid communication with the outlet 13 of the gas ejector 10.

[0070] In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is obtained from a gas conduit downstream from a NO.sub.x gas compressor and upstream from an absorption tower, and the outflowing gas stream of the gas ejector flows to a gas conduit upstream from the NO.sub.x gas compressor.

[0071] Reference is made to FIG. 6. In one embodiment according to the method of the disclosure, the first NO.sub.x-containing gas is obtained from the gas conduit downstream from the tail gas heater 43 for heating the tail gas 5 obtained from the outlet 6 of the absorption tower 41 to a temperature ranging from 200 to 650? C., thereby generating the heated tail gas 49, and the outflowing gas stream of the gas ejector 10 flows to the gas conduit downstream from a tail gas expander 7 for expanding the heated tail gas 49.

[0072] In one embodiment according to the method of the disclosure, the pressure P1 ranges from 4 bar to 16 bar.

[0073] The higher the pressure of the motive gas directed to the first inlet of the gas ejector, the higher the pressure drop between the gas conduit in fluid communication with the first inlet of the gas ejector and the gas conduit in fluid communication with outlet of the gas ejector will be, resulting in higher under-pressure above the storage tank, hence in a more efficient evacuation of the NO.sub.x gases generated in the storage tank.

[0074] Reference is made to FIGS. 3 to 6. In one embodiment according to the method of the disclosure, the pressure P1 ranges from 4 bar to 16 bar.

[0075] The higher the pressure of the motive gas directed to the first inlet 11 of the gas ejector 10, the higher the pressure drop between the gas conduit in fluid communication with the first inlet 11 of the ejector 10 and the gas conduit in fluid communication with outlet 13 of the ejector 10 will be, resulting in higher under-pressure above the storage tank 2, hence in a more efficient evacuation of the NO.sub.x gases released in the storage tank 2.

[0076] In one embodiment according to the method of the disclosure, the method further comprises the step of treating a tail gas downstream from an absorption tower and/or downstream from a tail gas expander.

[0077] Such treatment of the tail gas allows for a reduction of the emissions downstream the tail gas expander. Indeed, the lower the concentration on NO.sub.x gases at the tail gas inlet or downstream the tail gas outlet and upstream the location of the release of the gases to the atmosphere, the lower the concentration in NO.sub.x gases of the gases leaving the gas ejector is. As a result, the lower the concentration on NO.sub.x gases in the tail gas is, the lower the concentration in NO.sub.x gases that are released to the atmosphere. Therefore, the treatment of the tail gas results in enhanced control of the emissions from the nitric acid production plant.

[0078] Reference is made to FIGS. 3 to 6. In one embodiment according to the method of the disclosure, the method further comprises the step of treating the tail gas 5 downstream from the absorption tower 41 and/or downstream from the tail gas expander 7.

[0079] Such treatment of the tail gas allows for a reduction of the emissions downstream the tail gas expander 7. Indeed, the lower the concentration on NO.sub.x gases at the tail gas expander inlet 8 or downstream the tail gas expander outlet 9, and upstream the location of the release of the gases to the atmosphere, the lower the concentration in NO.sub.x gases of the gases that are released to the atmosphere. Therefore, the use of a tail gas treatment unit results in enhanced control of the emissions from the nitric acid production plant.

[0080] In one embodiment according to the method of the disclosure, the method is a periodical or continuous process.

Method for Revamping a System for Evacuating NO.sub.x Gases from a Nitric Acid Storage Tank into a System According to the Disclosure

[0081] In another aspect of the disclosure, a method for revamping a nitric acid production plant comprising the step of installing a gas ejector for executing the method of the disclosure, is disclosed.

[0082] Reference is made to FIGS. 2 and 6. In another aspect of the disclosure, a method for revamping a nitric acid production plant comprising the step of installing the gas ejector 10 for executing the method the disclosure, is disclosed.

Use of a Gas Ejector in a Nitric Acid Plant for Evacuating NO.sub.x Gases from a Nitric Acid Storage Tank

[0083] In another aspect of the disclosure, the use of a gas ejector for evacuating NO.sub.x gases from a nitric acid storage tank, in a production plant for producing nitric acid, is disclosed.

[0084] Reference is made to FIGS. 3 to 6. In another aspect of the disclosure, the use of the gas ejector (10) for evacuating NO.sub.x gases from the nitric acid storage tank (2), in a production plant for producing nitric acid, is disclosed.

Example

[0085] Reference is made to FIG. 3. The NO.sub.x gas 22 was compressed in the NO.sub.x gas compressor 40, thereby generating the compressed NO.sub.x gas 24 at a pressure of 11 bar. The compressed NO.sub.x gas 24 was absorbed in the absorption tower 41, thereby generating the tail gas 5 having a pressure ranging of 10 bar and a stream of raw nitric acid 27 that was treated in the bleacher 42 out of which nitric acid was sent to the storage tank 2. The tail gas 5 was heated in the tail gas heater 43, thereby generating a heated tail gas having a temperature of 450? C. and a pressure of 9.5 bar at the tail gas heater outlet.

[0086] The pressure at the nitric acid storage tank outlet 3 equaled atmospheric pressure. The NO.sub.x gases from the nitric acid storage tank 2 were directed, through the storage tank outlet 3, to the second inlet 12 of the gas ejector 10. Part of the tail gas 5 at the tail gas expander outlet 6 of the absorption tower 41, was directed to the first inlet of the gas ejector 11. The gases directed to both inlets 11 and 12 of the gas ejector 10 were mixed and ejected at the outlet 13 of the gas ejector 10 which was in fluid communication with a gas conduit located the tail expander outlet 9. The part of the tail gas 5 not directed to the first inlet 11 of the gas ejector 10 was directed to the inlet of the tail gas expander 8 and was mixed with the tail gas ejected from the gas ejector 10 at the outlet 13, to a gas conduit downstream the tail gas expander outlet 9. The NO.sub.x emissions in this latter conduit were 600 ppm. With the presence of a treatment unit downstream the absorption tower 41 and upstream the tail gas expander 7, the NO.sub.x emissions were reduced to below 50 ppm.