REDUCTION OF THE NOX WASTE GAS CONCENTRATION IN THE PRODUCTION OF NITRIC ACID DURING A SHUTDOWN AND/OR START-UP PROCESS OF THE PRODUCTION DEVICE

Abstract

An apparatus and a process for reducing the concentration of NOx nitrogen oxides in residual gas may be employed during shutdown and/or startup of apparatuses for preparing nitric acid. An example apparatus for reducing NOx nitrogen oxides may include a reactor that produces NOx nitrogen oxides, an absorption apparatus that absorbs at least part of the NOx nitrogen oxides produced in an aqueous composition, a residual gas purification plant that decomposes and/or reduces unabsorbed NOx nitrogen oxides, feed means for feeding the NOx nitrogen oxides to the absorption apparatus, discharge means for discharging the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant, and a bypass that transfers a gas mixture from the reactor to the residual gas purification plant while bypassing the absorption apparatus during startup and/or shutdown of the apparatus for preparing nitric acid.

Claims

1.-15. (canceled)

16. An apparatus for preparing nitric acid from NOx nitrogen oxides, the apparatus comprising: a reactor configured to produce NOx nitrogen oxides; an absorption apparatus configured to absorb at least part of the NOx nitrogen oxides produced in an aqueous composition; a residual gas purification plant configured to at least one of decompose or reduce unabsorbed NOx nitrogen oxides; feed means configured to feed the NOx nitrogen oxides from the reactor to the absorption apparatus; discharge means configured to discharge the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant; and a bypass configured to transfer a gas mixture from the reactor to the residual gas purification plant, bypassing the absorption apparatus during at least one of startup or shutdown of the apparatus for preparing nitric acid.

17. The apparatus for preparing nitric acid of claim 16 further comprising a control device configured to at least one of open and close the bypass; open and close the feed means; or open and close the discharge means.

18. The apparatus for preparing nitric acid of claim 17 wherein the control device is configured to open and close the discharge means.

19. The apparatus for preparing nitric acid of claim 16 wherein the bypass is configured to transfer the gas mixture from the reactor to the residual gas purification plant, bypassing the absorption apparatus and a heat exchanger.

20. The apparatus for preparing nitric acid of claim 16 wherein the absorption apparatus comprises sieve trays.

21. The apparatus for preparing nitric acid of claim 16 wherein the absorption apparatus comprises bubble cap trays.

22. A process for reducing a concentration of NOx nitrogen oxides in residual gas obtained during a startup or a shutdown of an apparatus for preparing nitric acid from NOx nitrogen oxides, the apparatus for preparing nitric acid comprising a reactor configured to produce NOx nitrogen oxides, an absorption apparatus configured to absorb at least part of the NOx nitrogen oxides produced in an aqueous composition, a residual gas purification plant configured to at least one of decompose or reduce unabsorbed NOx nitrogen oxides, feed means configured to feed the NOx nitrogen oxides from the reactor to the absorption apparatus, discharge means configured to discharge the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant, and a bypass configured to transfer a gas mixture from the reactor to the residual gas purification plant while bypassing the absorption apparatus during the startup or the shutdown of the apparatus for preparing nitric acid, the process comprising: stopping feeding of the gas mixture from the reactor to the absorption apparatus; and transferring the gas mixture from the reactor to the residual gas purification plant, bypassing the absorption apparatus.

23. The process of claim 22 wherein the transferring of the gas mixture from the reactor to the residual gas purification plant further comprises bypassing a heat exchanger.

24. The process of claim 22 further comprising stopping the discharge of the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant.

25. The process of claim 24 wherein at least one of the stopping feeding of the gas mixture, transferring the gas mixture, or stopping the discharge of the unabsorbed NOx nitrogen oxides is performed after the residual gas purification plant is taken out of operation during the shutdown of the apparatus for preparing nitric acid, or during the startup of the apparatus for preparing nitric acid before the residual gas purification plant is in operation.

26. The process of claim 22 wherein the bypassing of the absorption apparatus while transferring the gas mixture from the reactor to the residual gas purification plant is stopped as long as the residual gas purification plant is in operation during the shutdown of the apparatus for preparing nitric acid or as soon as the residual gas purification plant is put into operation during the startup of the apparatus for preparing nitric acid.

27. The process of claim 22 wherein at least a portion of the NOx nitrogen oxides that are present within the apparatus for preparing nitric acid are blocked-in in the absorption apparatus by the stopping of the feeding of the gas mixture from the reactor to the absorption apparatus.

28. The process of claim 27 wherein the blocking-in of the NOx nitrogen oxides in the absorption apparatus is also caused by stopping the discharge of the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant.

29. The process of claim 22 wherein the absorption apparatus comprises sieve trays that are filled with nitric acid when the apparatus for preparing nitric acid is started, wherein the filling with nitric acid is performed as soon as the residual gas purification plant is put into operation.

30. The process of claim 22 wherein the absorption apparatus comprises bubble cap trays that are pre-filled with nitric acid when the apparatus for preparing nitric acid is started.

31. The process of claim 22 comprising conveying at least a portion of nitric acid that condenses out in a heat exchanger into the absorption apparatus.

Description

[0043] In a preferred embodiment, the apparatus of the invention comprises further actively interconnected components. The apparatus preferably comprises at least a first heat exchanger which is arranged preferably downstream of the reactor and preferably upstream of the absorption apparatus and which is preferably configured for cooling the NO.sub.x nitrogen oxides which are fed via the feed means from the reactor to the absorption apparatus.

[0044] The apparatus of the invention preferably comprises one or more further heat exchangers which are arranged preferably downstream of the absorption apparatus and preferably upstream of the residual gas purification plant and are preferably configured for heating the NO.sub.x nitrogen oxides which are transferred via the discharge means from the absorption apparatus to the residual gas purification plant and/or are transferred via the bypass from the reactor to the residual gas purification plant. The NO.sub.x nitrogen oxides are preferably heated by at least 20 C., more preferably by at least 40 C., at least 60 C., at least 80 C., at least 100 C., at least 120 C., at least 140 C., at least 160 C., at least 180 C., at least 200 C., at least 220 C., at least 240 C., at least 260 C., at least 280 C., at least 300 C., at least 320 C., at least 340 C., at least 360 C., at least 380 C., at least 400 C., at least 420 C., at least 440 C. or at least 450 C., in the residual gas heater.

[0045] The heat exchangers according to the invention are, according to the invention, not restricted in terms of their structure. Suitable heat exchangers encompass shell-and-tube heat exchangers, plate heat exchangers, helical heat exchangers, U-tube heat exchangers, sheet-and-tube heat exchangers, etc.

[0046] The apparatus of the invention preferably comprises a mixing device which is arranged preferably downstream of the residual gas heater and preferably upstream of the residual gas purification plant. The mixing device is preferably configured for mixing the unabsorbed or desorbed NO.sub.x nitrogen oxides which are fed to the residual gas purification plant via the discharge means and/or the bypass with, preferably, ammonia.

[0047] In a preferred embodiment, the apparatus comprises a control device which is configured [0048] for opening and closing the bypass; and/or [0049] for opening and closing the feed means; and/or [0050] for opening and closing the discharge means.

[0051] The control device is preferably arranged downstream of the reactor and upstream of the absorption apparatus.

[0052] The control device is preferably configured for opening and closing the bypass and for opening and closing the feed means. The control device is preferably configured in such a way that the feed means are closed when opening the bypass. The control device is preferably configured in such a way that the feed means are opened when closing the bypass. The control device is preferably configured in such a way that opening of the bypass causes closing of the feed means. The control device is preferably configured in such a way that closing of the bypass causes opening of the feed means. The opening or closing of the bypass and the opening or closing of the feed means preferably occur simultaneously.

[0053] The control device preferably comprises closure devices for opening or closing a pipe, which preferably comprise at least one valve. The closure devices of the control device for opening or closing the pipes can be arranged at one position or at various positions. If the closure devices are installed at one position, the control device is preferably arranged at the place where the bypass branches off from the feed means. On the other hand, if the closure devices are arranged at various positions, preference is given to at least one closure device being arranged on the bypass and at least one closure device being arranged on the feed means.

[0054] The control device is preferably configured in such a way that the opening or closing of the bypass and the opening or closing of the feed means occur as a function of the operation of the apparatus for preparing nitric acid. During shutdown and/or start-up of the apparatus for preparing nitric acid, preference is given to the feed means being closed and the bypass being opened. In steady-state operation of the apparatus for preparing nitric acid, preference is given to the bypass being closed and the feed means being opened.

[0055] A person skilled in the art can distinguish the state of an apparatus for preparing nitric acid during shutdown and/or start-up thereof from the state of the apparatus during steady-state operation thereof. The shutdown of the apparatus follows steady-state operation, while start-up of the apparatus precedes steady-state operation.

[0056] In another preferred embodiment, the control device is configured in such a way that the opening or closing of the bypass and/or the opening or closing of the feed means occurs as a function of the operation of the residual gas purification plant. The control device is preferably configured in such a way that it brings about closing of the bypass and opening of the feed means during operation of the residual gas purification plant.

[0057] The control device preferably brings about opening of the bypass and closing of the feed means as soon as or as long as the residual gas purification plant is not in operation.

[0058] In a preferred embodiment, the control device is additionally or alternatively configured for opening and closing the discharge means.

[0059] The control device is preferably configured for opening and closing the bypass and for opening and closing the feed means and for opening and closing the discharge means.

[0060] If the control device is additionally or alternatively configured for opening and closing the discharge means, the control device preferably comprises a closure device which is arranged downstream of the absorption apparatus, preferably at the outlet of the absorption apparatus. The closure device preferably comprises a valve which is configured for opening and closing the discharge means. The control device is preferably configured for the simultaneous opening and closing of the feed means and discharge means, so that the control device simultaneously opens or closes the feed means and the discharge means.

[0061] The control device is preferably configured in such a way that the feed means and discharge means are closed on opening of the bypass. The control device is preferably configured in such a way that the feed means and discharge means are opened when the bypass is closed. The control device is preferably configured in such a way that the opening of the bypass causes closing of the feed means and the discharge means. The control device is preferably configured in such a way that the closing of the bypass causes the opening of the feed means and the discharge means. The opening or closing of the bypass and the opening or closing of the feed means and of the discharge means preferably occur simultaneously.

[0062] The control device is preferably configured in such a way that the opening or closing of the discharge means occurs as a function of the operation of the apparatus for preparing nitric acid. The discharge means are preferably closed during shutdown and/or start-up of the apparatus for preparing nitric acid. The discharge means are preferably opened in steady-state operation of the apparatus for preparing nitric acid.

[0063] In another preferred embodiment, the control device is configured in such a way that the opening or closing of the discharge means occurs as a function of the operation of the residual gas purification plant. The control device is preferably configured in such a way that it brings about opening of the discharge means during operation of the residual gas purification plant. The control device preferably brings about closing of the discharge means as soon as or as long as the residual gas purification plant is not in operation.

[0064] In another preferred embodiment, the opening or closing of the bypass occurs without closing of the feed and discharge means. The pressure drop in the absorption apparatus is preferably sufficiently high that the greater part of the gas mixture which flows through the apparatus for preparing nitric acid flows through the bypass.

[0065] The control device for blocking-in of the NO.sub.x nitrogen oxides in the absorption apparatus is preferably configured in such a way that the NO.sub.x nitrogen oxides cannot get out of the absorption apparatus. Closing of the discharge means and of the feed means preferably occurs during shutdown of the apparatus for preparing nitric acid and as soon as the residual gas purification plant is not in operation. The opening of the feed means and discharge means during start-up of the apparatus for preparing nitric acid preferably occurs as soon as the residual gas purification plant is in operation.

[0066] The absorption apparatus is preferably configured in such a way that at least 10% by volume of the NO.sub.x nitrogen oxides, based on the total volume of all apparatus parts of the apparatus for preparing nitric acid, can be blocked-in in the absorption apparatus, more preferably at least 20% by volume, at least 30% by volume, at least 40% by volume, at least 50% by volume, at least 60% by volume, at least 70% by volume, at least 80% by volume or at least 90% by volume.

[0067] In a preferred embodiment, the bypass is configured for transferring the gas mixture from the reactor to the residual gas purification plant with bypassing of the absorption apparatus and bypassing at least one heat exchanger. The bypass is preferably for transferring the NO.sub.x nitrogen oxides produced from the reactor to the residual gas purification plant with bypassing of the absorption apparatus and with bypassing of the heat exchanger which is arranged downstream of the reactor and upstream of the absorption apparatus and is configured for cooling the NO.sub.x nitrogen oxides. The opening of the bypass and the closing of the feed means during shutdown of the apparatus for preparing nitric acid preferably occurs after stopping of the combustion of NH.sub.3 in the reactor, so that the NO.sub.x nitrogen oxides present downstream of the reactor are conveyed to the residual gas purification plant and can there be decomposed or reduced with utilization of their residual heat.

[0068] A further aspect of the invention comprises a process for reducing the concentration of NO.sub.x nitrogen oxides in residual gas which is obtained during shutdown and/or start-up of the apparatus of the invention for preparing nitric acid, wherein the process comprises the following steps: [0069] (a) stopping of the feeding of a gas mixture from the reactor to the absorption apparatus; and [0070] (b) transferring of the gas mixture from the reactor to the residual gas purification plant with bypassing of the absorption apparatus.

[0071] All preferred embodiments which are described above in connection with the apparatus of the invention apply correspondingly and analogously to the process of the invention as well.

[0072] The gas mixture preferably comprises air. In another preferred embodiment, the gas mixture comprises air and NO.sub.x nitrogen oxides. The gas mixture can optionally comprise further constituents which are inert in the process of the invention.

[0073] The stopping of the feeding of the gas mixture from the reactor to the absorption apparatus preferably occurs as a function of the operation of the apparatus for preparing nitric acid. The introduction of the gas mixture into the absorption apparatus in step (a) is preferably stopped exclusively on shutdown and/or start-up but not in steady-state operation of the apparatus for preparing nitric acid.

[0074] In step (b) of the process of the invention, the gas mixture is transferred from the reactor to the residual gas purification plant with bypassing of the absorption apparatus to the residual gas purification plant. The gas mixture is preferably transferred from the reactor to the residual gas purification plant with bypassing of the absorption apparatus as a function of the operation of the apparatus for preparing nitric acid. The gas mixture is preferably transferred to the residual gas purification plant with bypassing of the absorption apparatus in step (b) during shutdown and/or start-up of the apparatus for preparing nitric acid. The gas mixture is preferably transferred to the residual gas purification plant with bypassing of the absorption apparatus before ignition or after stopping of the combustion of ammonia during shutdown and/or start-up of the apparatus for preparing nitric acid.

[0075] The steps (a) and (b) according to the invention preferably occur simultaneously.

[0076] In a preferred embodiment, the transfer of the gas mixture from the reactor to the residual gas purification plant in step (b) occurs with bypassing of the absorption apparatus and with bypassing of a heat exchanger. The transfer of the gas mixture from the reactor to the residual gas purification plant with bypassing of the absorption apparatus and bypassing a heat exchanger preferably occurs before ignition or after stopping of the combustion of ammonia during shutdown and/or start-up of the apparatus for preparing nitric acid. The transfer of the gas mixture from the reactor to the residual gas purification plant in step (b) preferably occurs with bypassing of the absorption apparatus and with bypassing of the heat exchanger which is arranged downstream of the reactor and upstream of the absorption apparatus and is configured for cooling the gas mixture and heating the residual gas. On shutting down the apparatus for preparing nitric acid, preference is given to opening the bypass and closing the feed means after stopping of the combustion of NH.sub.3 in the reactor so that the NO.sub.x nitrogen oxides present downstream of the reactor are conveyed to the residual gas purification plant and can be decomposed or reduced there with utilization of their residual heat.

[0077] The transfer of the gas mixture from the reactor to the residual gas purification plant in step (b) preferably occurs with bypassing of the absorption apparatus, with bypassing of a heat exchanger and with bypassing of at least one further apparatus part of the apparatus for preparing nitric acid. The further apparatus part preferably comprises a part of the apparatus for preparing nitric acid in which nitric acid can accumulate during the process of the invention. The further apparatus part which is bypassed in step (b) preferably comprises at least one further heat exchanger.

[0078] In a preferred embodiment, the process comprises the additional step [0079] (c) stopping of the discharge of unabsorbed NO.sub.x nitrogen oxides or desorbed NO.sub.x nitrogen oxides from the absorption apparatus to the residual gas purification plant.

[0080] The discharge of unabsorbed NO.sub.x nitrogen oxides or desorbed NO.sub.x nitrogen oxides from the absorption apparatus to the residual gas purification plant is preferably stopped as a function of the operation of the apparatus for preparing nitric acid. The discharge in step (c) is preferably stopped during shutdown and/or start-up of the apparatus for preparing nitric acid. Steps (a) and (c) of the process of the invention preferably occur simultaneously. Preference is given to the steps (a), (b) and (c) of the process of the invention occurring simultaneously.

[0081] In a preferred embodiment, step (a) and/or step (b) and/or step (c) are carried out as soon as the residual gas purification plant is taken out of operation during shutdown of the apparatus for preparing nitric acid or as long as the residual gas purification plant is not yet in operation during start-up of the apparatus for preparing nitric acid.

[0082] In step (a), the feeding of the gas mixture from the reactor to the absorption apparatus is preferably stopped as soon as the residual gas purification plant is taken out of operation during shutdown of the apparatus for preparing nitric acid or as long as the residual gas purification plant is not yet in operation during start-up of the apparatus for preparing nitric acid.

[0083] In step (b), the gas mixture is preferably transferred from the reactor to the residual gas purification plant with bypassing of the absorption apparatus as soon as the residual gas purification plant is taken out of operation during shutdown of the apparatus for preparing nitric acid or as long as the residual gas purification plant is not yet in operation during start-up of the apparatus for preparing nitric acid.

[0084] In step (c), the discharge of unabsorbed NO.sub.x nitrogen oxides or desorbed NO.sub.x nitrogen oxides from the absorption apparatus to the residual gas purification plant is preferably stopped as soon as the residual gas purification plant is taken out of operation during shutdown of the apparatus for preparing nitric acid or as long as the residual gas purification plant is not yet in operation during start-up of the apparatus for preparing nitric acid.

[0085] In a preferred embodiment, the bypassing of the absorption apparatus in step (b) is stopped as long as the residual gas purification plant is in operation during shutdown of the apparatus for preparing nitric acid or as soon as the residual gas purification plant is taken into operation during start-up of the apparatus for preparing nitric acid.

[0086] In a preferred embodiment, at least part of the NO.sub.x nitrogen oxides which are present within the apparatus for preparing nitric acid are blocked-in in the absorption apparatus by the stopping of the feeding of the gas mixture from the reactor to the absorption apparatus in step (a).

[0087] In a preferred embodiment, the blocking-in of the NO.sub.x nitrogen oxides in the absorption apparatus occurs in addition by the stopping of the discharge of unabsorbed or desorbed NO.sub.x nitrogen oxides from the absorption apparatus to the residual gas purification plant in step (c).

[0088] During shutdown and/or start-up of the apparatus for preparing nitric acid, preference is given to at least 10% by volume of the NO.sub.x nitrogen oxides present in the apparatus for preparing nitric acid being blocked-in in the absorption apparatus, more preferably at least 20% by volume, at least 30% by volume, at least 40% by volume, at least 50% by volume, at least 60% by volume, at least 70% by volume, at least 80% by volume or at least 90% by volume, in each case based on the total volume of all apparatus parts of the apparatus for preparing nitric acid.

[0089] The absorption apparatus of the process of the invention preferably comprises sieve trays which are filled with nitric acid on start-up of the apparatus for preparing nitric acid, with the filling with nitric acid being carried out as soon as the residual gas purification plant has been taken into operation.

[0090] In another preferred embodiment, the absorption apparatus comprises bubble cap trays which have already been filled with nitric acid on start-up of the apparatus for preparing nitric acid.

[0091] In a preferred embodiment, at least part of the nitric acid which condenses out in a heat exchanger is conveyed into the absorption apparatus. Preference is given to at least 10% by volume of the nitric acid which condenses out in a heat exchanger being conveyed into the absorption apparatus, more preferably at least 20% by volume, at least 30% by volume, at least 40% by volume, at least 50% by volume, at least 60% by volume, at least 70% by volume, at least 80% by volume or at least 90% by volume, in each case based on the total amount of nitric acid condensed out in a heat exchanger. The nitric acid condensed out in a heat exchanger is preferably conveyed in its entirety into the absorption apparatus.

[0092] In a preferred embodiment, the apparatus of the invention is used in the process of the invention.

[0093] The apparatus of the invention for preparing nitric acid from NO.sub.x nitrogen oxides is illustrated schematically and by way of example in FIG. 1, but the FIGURE should not be construed as constituting a restriction. Here, as is customary in nitric acid production, NO.sub.x nitrogen oxides (1) are preferably produced in a reactor which is preferably supplied with ammonia and air.

[0094] In the preparation of nitric acid in steady-state operation, these NO.sub.x nitrogen oxides (1) are fed, preferably via one or more heat exchangers (2) and preferably via the feed means (5), to the absorption apparatus (7), with at least part of the NO.sub.x nitrogen oxides being absorbed by an aqueous composition to form nitric acid which preferably accumulates in the bottom region (6) of the absorption apparatus. Unabsorbed or desorbed NO.sub.x nitrogen oxides are preferably conveyed via discharge means (8) to a mixing device (11) in which the NO.sub.x nitrogen oxides are preferably mixed with ammonia (10). They can optionally firstly be fed to a further heat exchanger (9) for this purpose. The NO.sub.x nitrogen oxides which are preferably mixed with ammonia are then preferably fed to the residual gas purification plant (12) and the excess residual gas (13) is preferably released into the environment.

[0095] During shutdown and/or start-up of the apparatus for preparing nitric acid, a gas mixture can preferably be conveyed through a bypass (4) around the absorption apparatus (7). The opening and/or closing of the bypass and the feed facility can preferably be regulated by means of a control device (3).

[0096] When shutting down the apparatus for preparing nitric acid, the combustion of NH.sub.3 in the reactor is preferably firstly stopped, so that no further NO.sub.x nitrogen oxides (1) are formed. The NO.sub.x nitrogen oxides (1) still present in the apparatus downstream of the reactor are preferably cooled in the heat exchanger (2). A gas mixture is preferably also conveyed through the apparatus for preparing nitric acid by means of a compressor after the combustion of NH.sub.3 has been stopped. As a result, the concentration of NO.sub.x nitrogen oxides in the feed means (5) drops, whereupon the feed means (5) are preferably closed by means of the closure device of the feed means (3). This preferably interrupts the air stream through the absorption apparatus, as a result of which at least part of the NO.sub.x nitrogen oxides are blocked-in in the absorption apparatus. As an alternative, the discharge means (8) can also optionally be closed in order to prevent outflow of NO.sub.x nitrogen oxides from the absorption apparatus (7). The bypass (4) is preferably opened simultaneously by means of the closure device of the bypass (3), so that the gas mixture which flows through the apparatus for preparing nitric acid preferably fully bypasses the absorption apparatus. Desorption of NO.sub.x nitrogen oxides (1) from the nitric acid which is present in the absorption apparatus (7) into the gas mixture which flows through the apparatus for preparing nitric acid is preferably prevented thereby.

[0097] As soon as the combustion of NH.sub.3 in the reactor is stopped, further cooling in the heat exchanger (2) of the NO.sub.x nitrogen oxides (1) still present in the apparatus downstream of the reactor can be disadvantageous since cooling of the NO.sub.x nitrogen oxides (1) acts counter to very long operation of the residual gas purification plant (12); the heat present in the NO.sub.X nitrogen oxides can instead keep the residual gas purification plant (12) at the required temperature for a certain time. For this reason, the feed means (5) are preferably closed after stopping of the combustion of NH.sub.3 in the reactor and a gas mixture is preferably fed via the bypass (4) going around both the absorption apparatus (7) and the heat exchanger (2) to the residual gas purification plant (12) (not shown in FIG. 1). Here, the residual heat of the NO.sub.x nitrogen oxides is preferably exploited for catalytic decomposition and reduction within the residual gas purification plant (12).

[0098] When starting up the apparatus for preparing nitric acid, the residual gas purification plant (12) has usually not yet attained the temperature which is necessary for the catalytic reduction or decomposition of NO.sub.x nitrogen oxides (1). In this state, the feed means (5) are preferably closed and the bypass (4) is preferably open. Before ignition of the combustion of ammonia when starting up the apparatus for preparing nitric acid, the residual gas purification is preferably heated until it has attained a temperature which allows the introduction of ammonia into the residual gas purification. As soon as the residual gas purification plant has attained its operating temperature, the bypass (4) is preferably closed and the feed means (5) and discharge means (8) are preferably opened. The NO.sub.x nitrogen oxides leaving the absorption apparatus (7) can preferably be reduced in the heated residual gas purification. The combustion of ammonia is then preferably started. This can preferably prevent NO.sub.x nitrogen oxides (1) which have remained in the absorption apparatus (7) during shutdown of the apparatus for preparing nitric acid from being able to leave the absorption apparatus before the residual gas purification plant has attained the necessary operating temperature. In addition, desorption of NO.sub.x nitrogen oxides from nitric acid present in the absorption apparatus (7) is preferably prevented. Preference is given to closing the bypass (4) and opening the feed means (5) as soon as the residual gas purification plant (12) is taken into operation. NO.sub.x nitrogen oxides (1) which have remained in the absorption apparatus (7) can then preferably flow through the discharge means (8) to the residual gas purification plant (12) and be decomposed or reduced there.

LIST OF REFERENCE NUMERALS

[0099]

TABLE-US-00001 1 NO.sub.x nitrogen oxides 2 heat exchanger 3 control device .sup.3 closure device for the feed means 3 closure device for the bypass 4 bypass 5 feed means 6 bottom region 7 absorption apparatus 8 discharge means 9 further heat exchanger 10 ammonia 11 mixing device 12 residual gas purification plant 13 residual gas