The present invention is a process for the catalytic oxida- tion of ammonia to nitric oxide. In particular, in the pro cess according to the invention, nitric oxide is prepared by a two-step ammonia oxidation in presence of a catalyst comprising transition metal oxides. Industrial production of nitric acid is carried out by the Ostwald process. In this process, ammonia (NH ₃₎ is oxidized to nitric oxide (NO). The nitric oxide is subsequently oxi dized to nitrogen dioxide (NO2) which is absorbed in water to form nitric acid.

Oxidation of NH ₃ to NO is carried out by catalytic oxida tion, typically in a bed containing several layers of gauze catalyst made from Platinum Group metals (PGM), most typi cally a combination of Platinum (Pt), Palladium (Pd) and Rhodium (Rh).

PGM belong to the group of Ru, Rh, Pd, Os, Ir and Pt and are also called noble metals, which are expensive By contact with PGM, the oxidation to NO is most selective at temperatures above 800°C and typically the oxidation is carried out at temperatures around 900°C.

Absorption of NO2 in water later in the nitric acid produc- tion process is favoured by higher pressure and oxidation of NH ₃ to NO is carried out in an oxidation reactor, which is typically pressurized to reduce the equipment size and because pressure is required later in the process.

A disadvantage of the known NH ₃ oxidation process is that the PGM gauze catalyst has a loss of platinum during opera tion because platinum evaporates at the reaction condi tions.

NH ₃ oxidation is an exothermic process, which results in very high reaction temperatures, which lead to a large loss of platinum.

Campaign length of the PGM catalyst varies from a few months up to about 1 year at which time the process must be shut down and the PGM catalyst exchanged because the selec tivity and conversion has become too low.

The PGM gauzes are expensive as the prices for these metals are very high. Typically, around 80% of the platinum lost from the catalyst gauzes can be captured by a so called getter gauzes, which are installed directly underneath the PGM oxidation gauzes. These contain Palladium (Pd) and are therefore also expensive. This invention relates to an alternative process, in which the expensive PGM catalyst gauze and the getter gauze are avoided. In addition, the problem of losing platinum in the downstream equipment is also avoided. Oxides of transition metals (as opposed to noble metals such as PGM) have been tested over time but sufficient se lectivity to NO has so far not been achieved. A few plants worldwide (<1% of total nitric acid plants) operate on a transition metal oxide oxidation catalyst, but these do not obtain the same selectivity as the plants operating on PGM gauze.

The transition metal oxidation catalysts have a significant loss of selectivity towards NO at higher temperatures. Typ ically, the selectivity drops sharply above a temperature of about 800°C, meaning that metal oxides catalyst cannot be used in the majority of plants designed for PGM gauze operation.

We have found that splitting the oxidation step of NH ₃ to NO into two separate reaction steps with intermediate cool- ing and addition of NH ₃ , the issue of low selectivity of transition metal catalysts at high temperatures is avoided and makes it possible to obtain full NH ₃ conversion with a high selectivity towards NO when employing cheaper transi tion metal oxide catalysts.

Oxidation of NH ₃ by contact with the PGM gauze catalyst forms nitrous oxide (NO) as an unwanted by-product. NO is a potent greenhouse gas and different catalytic methods must be used to reduce emission of NO to the atmosphere from nitric acid production.

Reduction of NO emissions is therefore associated with significant costs, due to an increased pressure drop, cata lyst and equipment costs and possibly reducing agent costs. Transition metal oxide oxidation catalysts are known to have lower formation of NO during the NH ₃ oxidation. Em ploying catalyst comprising transition metals results in a lower selectivity for formation of the unwanted by-product NO during the oxidation of NH ₃ , the need for removing NO to avoid emissions to the atmosphere is reduced and so are the costs related thereto.

Pursuant to these findings, the present invention provides a process for the catalytic oxidation of ammonia to nitric oxide comprising the steps of

(a) providing a first gas stream comprising ammonia and ox ygen;

(b) providing a second gas stream comprising ammonia;

(c) oxidizing the ammonia contained in the first gas stream with oxygen contained in the gas stream by contact with a catalyst comprising one or more transition metal oxides in a first oxidation step ;

(d) cooling the oxidized first gas stream from step (c)to obtain a first effluent stream comprising an amount of ni- trie oxide;

(e) admixing the first effluent stream withdrawn from step

(d)with the second gas stream; (f) contacting the mixed first effluent stream from step

(e) with a catalyst comprising one or more transition metal oxides and oxidizing the ammonia contained in the second gas stream in a second oxidation step;

(g) withdrawing a second effluent stream from step (f)and cooling the second effluent stream to obtain a product stream comprising nitric oxide in amount higher than the amount in the first effluent stream.

When carrying out the oxidation of NH ₃ to NO in two steps, the reaction temperature is kept down and thus allowing the use of catalysts made from oxides of metals with a much lower price than PGM gauzes. In some applications it might be useful to add one or more additional steps with cooling the effluent stream from an oxidation step, introducing ammonia into the cooled efflu ent stream and subjecting the cooled effluent stream ad mixed with ammonia in a subsequent oxidation step.

As in the standard Oswald process, atmospheric air is fil tered and compressed. However, in the process according tto the invention only a portion of the total amount of NH ₃ is used in the first oxidation step of the process to provide the first gas stream with ammonia and oxygen.

The amount of ammonia introduced with compressed air into the first gas stream, is usually sub-stoichiometric and the first effluent stream will in many applications contain sufficient amounts of oxygen to oxidize the content of am monia introduced with the second gas stream into the second oxidation step. At certain conditions, it might be advantageous to add oxy gen into the effluent stream from the first oxidation step. Thus, in an embodiment of the invention, additional amounts of oxygen are added to the first effluent stream.

In further an embodiment of the invention, the second gas stream further comprises oxygen.

Alternatively, in an embodiment of the invention, the sec ond gas stream can be obtained from a split stream of the first gas stream prior to step (c). Preferably, oxygen in the first and optionally in the sec ond gas stream is provided by admixing ammonia into com pressed atmospheric air or oxygen enriched air.

The oxidation of NH ₃ is exothermic and cause the reacting gas stream to heat up. Thus, the concentration of NH ₃ in the air determines the reaction temperature, as apparent from Fig.4 in the drawings.

When in accordance with the invention introducing only a portion of the NH3 upstream step (c), the reaction tempera ture can be maintained at a lower level. This means that the oxidation catalyst employed in the process of the in vention will still have a high selectivity towards NO. Preferably, the content of the ammonia in the first gas stream is adjusted to result in an oxidation temperature in step (c) of between 650 and 850°C, preferably between 700 and 800°C.

The gas from the oxidized gas stream first NH3 oxidation step is cooled in step (d), preferably to a temperature of between 200 and 500°C, by indirect heat exchange and steam production. The second gas stream containing ammonia and optionally ox ygen is introduced into the cooled effluent from the first oxidation step (c) and the ammonia introduced with the sec ond gas stream is oxidized substantially to nitric oxide by contact with the catalyst of the invention.

Each ammonia oxidation step (c) and (f) will provide sub stantially full conversion of ammonia, primarily to nitric oxide. The second effluent from the second oxidation step (f) is cooled, e.g. by means of a boiler. The cooled second efflu ent is finally withdrawn as nitric oxide product stream for further processing, typically to nitric acid as described hereinbefore.

When employing a steam super heater for cooling of the first effluent stream and a boiler in the cooling of the second effluent stream, it is preferred to use steam pro duced in the boiler in the steam superheater. In all embodiments of the invention, the transition metal oxides of the catalyst comprise at least one of Mn, Fe, Co, Ni, Cu, Zn, Al, optionally promoted with Ce and/or La.

The catalyst is preferably arranged in fixed manner in the oxidation reactors.

As used herein "fixed bed manner" can also include a bed, which has a rake that occasionally moves pellets in the bed.

The main advantage of the process according to the inven tion is that the total amount of NH ₃ added into the oxida tion steps can be higher than in the known PGM catalyst process as there are not the same limitations of conversion decreases at ammonia concentrations near the stoichiometric ratio and the ammonia-air mixtures are kept far from the explosive limits.

The stoichiometric ratio of NH ₃ in air is about 14.3% NH ₃ in air for the oxidation of NH ₃ to NO. However, when using PGM gauze the used ammonia ratio is lower, mainly because conversion decreases when the ratio becomes too high and to stay safely below the lower explosion limits of air and am monia mixtures.

The present invention makes it possible to add ammonia into the oxidation steps up to the full stoichiometric ratio of 14.38% NH ₃ in air. This itself will increase the plant pro duction capacity as more ammonia can added to the oxidation steps. The oxidation temperature in the first and second ammonia oxidation step (c) and (f) is controlled by the individual ammonia injection and cooling between the steps to keep re action temperatures where the catalyst comprising one or more transition metal oxides has a high selectivity.

Preferably, the amount of ammonia in the first gas stream is adjusted to result in an oxidation temperature in step (c) of between 650 and 850°C, more preferably between 700 and 800°C.

The gas temperature at the inlet to the oxidation steps af fects the reaction temperature. The lower the inlet gas temperature, the lower the reaction temperature and thereby the better the selectivity.

Preferably the inlet temperature to the first oxidation step is between ambient temperature and 250°C, most pre ferred between 100°C and 200°C.

As used herein, "the amount of ammonia in the first gas stream" shall be understood as how much ammonia that is present in that gas stream relative to the other compo nents, this could for instance be measured in volume per- cent.

Typically the amount of ammonia in the first gas stream is controlled by adjusting the flow of ammonia in an ammonia dosing system injecting ammonia into air or compressed air, this could for instance be measured as kg/h, kmol/h or Nm ³ /h or similar unit. The first gas stream can also be obtained by injecting am monia into oxygen enriched air.

As the catalytic oxidation of ammonia is exothermic the re- suiting oxidation temperature will depend on the ammonia concentration prior to the oxidation.

Preferably the amount of ammonia in the second gas stream admixed in the first effluent in step (e) is adjusted to result in an oxidation temperature in step (f) of between 650 and 850°C, more preferably between 700 and 800°C.

As used herein, "the amount of ammonia in the second gas stream" shall be understood as how much ammonia that is present in that gas stream. The second gas stream can for instance be pure ammonia or it can for instance be a mix ture of air and ammonia. Such mixture of air and ammonia can be obtained from a split stream of the first gas stream prior to step (c).

The second gas stream is mixed into the first effluent stream and the resulting gas stream is led to the catalytic oxidation of ammonia in step (f). The concentration of am- monia prior to this oxidation step will determine the oxi dation temperature due to the exothermic reaction. The am monia concentration can for instance be adjusted by con trolling the flow of the second gas stream and can for in stance be measured as volume percent.

If the temperature during oxidation gets too high, the se lectivity towards NO is reduced, which is unwanted. In the present invention the oxidation temperature is therefore controlled and maintained at acceptable levels by control ling the amount of ammonia present before each of the oxi dation steps.

By splitting the oxidation of ammonia into two steps, the oxidation temperature can be lower than in one step ammonia oxidation, enabling the use of base metal oxide catalysts while maintaining a high selectivity towards NO.

The unwanted side reaction formation of NO is signifi cantly reduced when using catalyst of the invention com pared to using PGM catalysts. Another advantage of the pro cess according to the invention is that there is a signifi- cantly reduced need to reduce NO emissions from nitric acid production, thereby reducing the costs related thereto .

The process according to the invention is additionally use- ful for revamping existing PGM based plants or makes it possible to provide a capacity increase.

Preferably in a revamp from a standard PGM gauze process to the process of this invention, the steam superheater pro- duce the same temperature and pressure steam as before the revamp.

As mentioned above, it is advantageous to dose the NH ₃ in two steps, optionally more steps, each with a lower NH3 concentration and thus a greater distance to ammonia-air mixture explosion limits. Thus, the oxygen content in the air stream can be increased to higher levels than what is found in atmospheric air. Thereby, the stoichiometric ratio of NH ₃ increases and more NH ₃ can be injected. This is in turn results in higher NO concentration produced in the NH3 oxidation steps compared to a process with atmospheric air and thereby also an in creased nitric acid production.

This is an attractive way of boosting capacity of the plant. Typically, increase of the plant capacity involves increasing the gas flow through the plant, thereby making it possible to add more NH ₃ . Disadvantageously, this in creases the pressure drop in the plant and requires a very costly revamp of the compressor-turbine setup of the plant.

By instead increasing oxygen content and total amount of NH ₃ injected into the oxidation steps according to the in vention, more NO can be produced without the costly change of the compressor-turbine setup of the plant.

The process of this invention can be utilized in a plant designed for PGM gauze operation by only revamping the NH ₃ oxidation step. The rest of plant can remain the same. Thus, in a further aspect, the present invention provides a method for revamping an existing process for the catalytic oxidation of ammonia to nitric oxide, the existing process comprises the steps of

(a) providing a gas stream comprising ammonia and oxygen; (b) oxidizing the ammonia contained in the gas stream with oxygen contained in the gas stream by contact with a PGM group catalyst; (c) cooling the oxidized gas stream to obtain an effluent stream comprising an amount of nitric oxide; the method comprising the steps of

(i) providing a second gas stream comprising ammonia; (ii) replacing the PGM catalyst in step (b)with a catalyst comprising one or more transition metal oxides;

(iii) admixing the effluent stream withdrawn from step (c) with of the second gas stream;

(iv) contacting the mixed effluent stream from step (iv) with a catalyst comprising one or more transition metal ox ides and oxidizing the ammonia contained in the second gas stream;

(v) cooling the oxidized gas stream from step (iv); and

(vi) withdrawing an effluent stream from step (v)to obtain a product stream comprising nitric oxide.

Example In an existing nitric acid plant shown in Fig.l, NH3 is dosed corresponding to 11.5 volume % into compressed air upstream the ammonia oxidation reactor. Inside the ammonia reactor a platinum containing gauze catalyst oxidizes the NH3 to NO.

The oxidation reactions raises the temperature by approxi mately 750°C. When the compressed air has a temperature of 200°C, the temperature will increase to approximately 950°C after the oxidation. The hot gas product from the oxidation step enters subse quently a steam super heater where it is cooled by super heating steam from a downstream boiler below to around 400°C. The product gas subsequently enters the boiler where it is further cooled by producing steam (Fig.l).

The cooled product gas is passed to the nitric acid pro cess, which includes further cooling, an absorption tower and further air addition. It may further include an addi- tional compression step to increase the pressure which is favorable for the absorption process.

The existing plant is revamped to the process of the pre sent invention, as shown in Fig.2. The remaining nitric acid process remains the same. After revamp, the oxidation of NH3 is split into two oxidation steps. The addition of ammonia is also split into two steps.

In the first ammonia oxidation step, ammonia is dosed cor- responding to around 2/3 of the NH3 amount in the original setup, resulting in a temperature increase of around 500°C and thus a temperature of about 700°C after the first oxi dation step. The oxidized gas is subsequently cooled in the steam super heater and a boiler to about 450°C. Additional NH3 is dosed to the oxidized gas from the first oxidation step in the second oxidation step, corresponding to about the last 1/3 of the NH3 amount in the existing plant in creasing the temperature with about 250°C, and resulting in an exit temperature of about 700°C.

The oxidized gas withdrawn from the second oxidation step is then cooled in the existing boiler. Together with the boiler and superheater after the first oxidation step it is possible to produce the same amount of 400°C superheated steam as before the revamp, but keeping the NH ₃ oxidation reaction temperature below 800°C where the transition metal catalyst still has very high selectivity.

The total amount of NH3 reacted in the first and second ox idation step is the same amount of ammonia that corre sponded to 11.5 vol % in air in the existing plant prior to the revamp.

Optionally, the gas stream for the second oxidation step can be obtained by splitting the gas stream containing air and ammonia before the first oxidation step and adding more ammonia. This has the advantage that the flow to the first oxidation step is reduced and therefore the pressure drop across the first oxidation step is also reduced. This is particularly advantageous if the existing reactor can be reused. Such a reactor will be designed for PGM gauze oper- ation and therefore changing to a catalyst comprising tran sition metal oxide pellets or monolith may lead to a higher pressure drop.

Reducing the flow through the reactor can lower the pres- sure drop. The amount of ammonia dosed will have to result in the concentration required to stay at an acceptable tem perature in the first ammonia oxidation step. The reduced flow means that cooling duty after the first step is re duced compared to a full flow through the first oxidation step. Some amounts of ammonia will be present in the bypass gas stream, but more ammonia is added before the second ammonia oxidation step to obtain the adequate amount of ammonia for the second oxidation step. This additional ammonia addition controls the oxidation temperature in the second ammonia oxidation step (see Figure 3).

If a production increase is desired, then the ammonia addi tion can be increased as the ammonia concentrations accord- ing to this invention are far away from the explosive lim its of mixtures of air and NH3. Adding more NH3 means that the total temperature increase across the two oxidation steps is increased, so overall more cooling is needed and thus the steam production will increase. Even though more steam is produced, it can be with the same specifications as before the revamp. This can be advantageous to the plant as all equipment is designed for this type of steam. The steam export from the plant can then be increased. Consid ering the same setup as before, the second NH3 dosing can now be increased as the gas has been further cooled. If the boiler after the first oxidation step cools the gas to ap prox. 300°C instead of the 450°C, then the ammonia addition in the second step can be increased by about 55% compared to before. This will mean that the process gas will go from approx. 300°C to approx. 700°C in the second oxidation step. Such a setup would bring the total NH3 injection from the previous 11.5% NH ₃ in air to about 13.7% NH ₃ in air, which is an increase in ammonia dosing of about 19%. This can potentially translate directly into a 19% increase of plant nitric acid production.