METHOD AND DEVICE FOR PROCESSING NITROGEN-BASED GASES TECHNICAL FIELD OF THE INVENTION The present invention relates to a method used for processing nitro- gen-based gases containing compounds such as ammonia and amines. The method is adapted to purifying exhaust, process gases, ventilation air and similar. The purpose of the purification may be to prevent emission of small amounts of malodorous substances, so called odourants, or to simply oblit- erate large amounts. Emissions of nitrogen-based odourants interferes with the environment because the compounds are perceived even in low concen- trations. During the catalytic purification, decomposition of amines into am- monia first takes place followed by selective oxidation of ammonia into gase- ous nitrogen and water.

The present invention also relates to a device for processing such ni- trogen-based gases by the means of a catalyser comprising a zeolite cata- lyser and an oxidation catalyser. The present invention also relates to a method for manufacturing such device.

STATE OF THE ART There are three methods when it comes to purification of gases con- taining nitrogen-based odourants. The methods are: absorption, adsorption, and oxidation.

During absorption the gas is treated with an acid solution in a scrub- ber. The odourant is absorbed in the liquid resulting in a clean gas. The ab- sorption liquid is recirculated until it is considered to be consumed and should then be destroyed in a suitable fashion.

During adsorption the gas passes through a solid adsorption material such as active coal. The adsorption material is impregnated with an acid, metal ions, etc. to enhance the adsorption capacity so as to better bind the basic gases. Once consumed the adsorption material must be destructed or restored in a suitable manner.

Oxidation can be carried out either in a noncatalytic way by conducting the gases through a flame, or catalytically. For a catalytic oxidation the same metal oxide based or precious metal based catalyser is used as for reduction of VOC, i. e. volatile hydrocarbons.

Oxidation, both catalytically and noncatalytically, of nitrogen-containing compounds are effective, i. e. the odourants stop smelling. The odourants are, however, to a large extent oxidised to nitrogen oxides (NOx) and nitrous oxide (N2O). The development of these gases depends on the choice of catalyser, concentration and temperature. It is possible to oxidise ammonia to NOx with high selectivity across a precious metal catalyser, which is the industrial way for the production of nitric acid. The desired product, however, as far as oxidation of nitrogenous odourants is concerned, is nitrogen gas (N2). The selectivity for nitrogen gas in the case of the above-mentioned catalysers is poor. When they are used nitrogen oxides and laughing gas are always developed. Neither NOx or laughing gas is a desired product in a pu- rification plant.

Nitrogen oxides contribute to the global acidification, smog and forma- tion of near ground ozone. In addition, nitrogen oxides are acutely toxic and may cause damage to the respiratory passages. The harmful effects of the nitrogen oxides have resulted in actions taken to limit emissions of them.

Such exemplary actions are 3-way catalysis for cars and SCR technology at power plants. Laughing gas, N20, currently is unregulated but is considered to be hazardous to the environment due, on the one hand, to its participation in disintegrating the ozone layer of the stratosphere and, on the other hand, to its effect as a greenhouse gas.

In the prior art there are also described methods and devices for proc- essing gases containing nitrogen by the means of a catalyser comprising zeolite. One example of such method and device is disclosed in WO 9816300. WO 9816300 discloses a catalyser with a catalytic bed consisting of a zeolitic catalyser in the form of an extrudate. The catalyser further com- prise several oxidation nets, placed above and beneath the extrudate in the reactor, forming oxidation top nets and bottom oxidation nets.

One drawback with the device described in WO 9816300 is that it is rather large compared to the efficiency. Another drawback with prior art is the ungainly structure due to the different catalyser sections, the catalyser com- prising both extrudates and catalytic nets. It is also problematic selecting the proper size of the oxidation top net used in prior art since the size depends on gasload and temperature. If the oxidation top net is designed for one flow and one temperature, the operation of the oxidation top net will turn poorer if one or both parameters is changed.

One problem with using extrudates is that they consist of homogenous material, which require a large amount of active material. Devices according to prior art are thus expensive due to the different catalyser sections and the large amount of active material required in the extrudate. Still another draw- back with such methods and devices is the high energy consumption, due to the high processing temperature. Several of the drawbacks and problems are particularly unfavourable for domestic applications.

SUMMARY OF THE INVENTION One object of the present invention is to eliminate the above men- tioned drawbacks and problems with methods and devices for processing nitrogen-based gases described in the prior art. The present invention pro- vides an efficient method and device for purifying such malodorous gases with high selectivity for nitrogen gas (N2) formation. One embodiment of the present invention is excellent for industrial applications. One alternative em- bodiment is well adapted to domestic applications as well as industrial appli- cations. Further, the present invention provides a method for the manufactur- ing of a catalyser well adapted to the applications.

The present invention comprises a process tank, a heater and a reac- tor comprising a catalyser. The process tank is adapted to contain a material emitting the nitrogen-based gas to be processed.

In one embodiment of the invention the gas to be processed is con- ducted to the heater, the heater being arranged between a first reactor com-

partment and a second reactor compartment in a regenerative reactor. Thus, the nitrogen-based gas is introduced in the reactor between the first reactor compartment and the second reactor compartment and is heated by the heater to suitable processing temperature. The reactor compartments also comprise a catalyser and a recuperator or devices with similar function.

The reactor is connected to an air inlet via a first tubing and a second tubing, wherein an air flow may be introduced to the reactor in alternating directions by the means of a valve. By introducing the air flow to the reactor in one direction the nitrogen-based gas is brought into contact with one cata- lyser. For example, by introducing the air flow via the first tubing the nitrogen- based gas is brought into contact with the catalyser of the second reactor compartment absorbing and purifying the nitrogen-based gas. The heat en- ergy is recovered in the recuperator. The purified gas follow the air flow to the valve via the second tubing, the valve comprising an outlet for the puri- fied gas and the air.

When the direction of the air flow is changed, i. e. introducing the air flow to the reactor via the second tubing, the nitrogen-based gas remaining in the second reactor compartment and the new nitrogen-based gas intro- duced in the reactor is transported towards the first reactor compartment.

The first reactor compartment may be designed as the second reactor com- partment, comprising the catalyser purifying the nitrogen-based gas. The catalyser is described in more detail below. The purified gas follow the air flow to the outlet via the recuperator and the first tubing.

By alternating the direction of the air flow the heated nitrogen-based gas is transported between the first reactor compartment and the second reactor compartment, the nitrogen-based gas and the heat energy being trapped in the catalyst area. This imply an effective process for purifying ni- trogen-based gas at low consumption of energy and consequently at low costs. The processing temperature may be selected almost without consider- ing the costs for heating since the heat energy is trapped in the catalyst area.

For example the processing temperature is about 400-500°C.

One further advantage with this embodiment of the present invention is that only purified gas and air exits via the outlet, even when the direction of the air flow is changed.

Another advantage with this embodiment is that the dwell time for the nitrogen-based gas in the catalyser is indefinite.

In one alternative embodiment, the process tank is connected to a heat exchanger, designed to heat the nitrogen-based gas by means of puri- fied gas from an outlet of the reactor. The heat exchanger is connected to an inlet of the reactor via the heater, the heater being designed to heat the ni- trogen-based gas to proper processing temperature. For example the heater is designed to heat the nitrogen-based gas to about 350°C.

The reactor is provided with the catalyser, the catalyser being de- scribed in more detail below. The nitrogen-based gas is brought into contact with the catalyser, purifying the nitrogen-based gas.

The catalyser comprise at least one catalytic net. The nets may be coated with a porous and preferably ceramic material through a thermal forming process by spraying. The nets are preferably coated with titanium dioxide (TiO2) or another suitable substance. Several catalytic nets may be arranged in the reactor to obtain proper conversion and selectivity.

The catalytic net comprise a zeolite promoted with a precious metal, such as rhodium (Rh), platinum (Pt) and palladium (Pd), combining ammonia reduction properties with oxidation properties. Preferably the precious metal is Rh. Pt and Pd promotion gives similar increase in activity, but Rh gives superior selectivity towards nitrogen formation.

It is now proven feasible to catalytically oxidise, with high selectivity, nitrogen-containing odourants, such as ammonia and trimethylamine, to ni- trogen gas and water. The gases containing the odorant pass through at least one catalytic net, which consists of a zeolitic catalyser of the H- mordenite type, and an oxidation catalyser. The oxidation catalyser is com- bined with the zeolitic catalyser in the same catalytic net. The gases are

preferably processed at a temperature of about 400-500°C in one embodi- ment and about 350°C in one alternative embodiment. These embodiments will result in a technically speaking reasonable size of reactor and a very good selectivity for nitrogen gas formation.

For example, the catalytic nets are prepared by milling zeolite with a binder and water in a ball mill, producing a slurry. Zeolites are crystaline alumina silicates, usually produced in a crystal size of 50-100 jj, m and avail- able either as powder or as extrudates. To make extrudates, the zeolite powder is mixed with a binder, usually a silica or alumina based binder. Both zeolite powder and grounded extrudates may be used as source of zeolite.

The zeolite is grounded into a crystal size of about 2 lim.

As binder colloidal alumina, silica or sodium silicate may be used.

Preferably an alumina based slurry is used. The catalytic nets are then pre- pared by spraying, washcoating or dipping the nets in the slurry, producing a thin film of active material on the surface of the nets. Preferably the nets are coated with slurry by spraying. If required the catalytic nets may be coated repeatedly to obtain a certain amount of slurry on the net. The zeolite is, as mentioned above, then promoted with a precious metal. For example, the net may be dipped in a solution containing the precious metal.

Compared to the extrudates, consisting of homogenous material, the amount of active material required is only a small fraction, which reduces the costs.

In one embodiment of the invention at least one bottom oxidation net is also arranged in the reactor. The bottom oxidation nets may be coated with a porous and preferably ceramic material through a thermal forming process by spraying. The bottom oxidation nets may be surface area enlarged through sol treatment and coated with an active material having oxidation properties, such as precious metals Pt, Pd or metal oxides. Be- sides varying the layer formed by spraying, the surface area enlarging and the active phase, the properties of the bottom oxidation nets can be varied by electing an appropriate wire thickens and mesh size.

The ammonia entering the catalyser is oxidised with surprisingly high selectivity to become nitrogen gas and water.

When odourants of the amine type enter the catalyser they decom- pose into ammonia and, presumably, into corresponding alcohol. This step, already, reduces the problem with odour to a large extent because the am- monia odour threshold is significantly higher than that of amines. The am- monia oxidation then occurs as indicated above. The oxidation of amines, however, takes place at a lower rate and with a considerably lower selectivity to become nitrogen gas, that is the formation of nitrogen oxides and laughing gas is greater, than during oxidation of ammonia alone. By the operation of the catalytic nets in the reactor the amine rate of oxidation is substantially increased. At the same time, a quite unexpected improvement of the selectivity to yield nitrogen gas is achieved due to decreased formation of laughing gas. The introduction of catalytic oxidation properties in the nets has the additional effect of oxidising any incoming hydrocarbons to carbon dioxide and water. If additional bottom oxidation nets are placed after the catalytic nets, final combustion occurs of hydrocarbons (VOC) that are not oxidised in the catalytic nets.

In addition, this invention has the advantageous feature in common with other oxidising systems, when it comes to adsorption and absorption systems, that no solid or liquid waste is formed.

The invention has, in comparison with other oxidising systems, very high selectivity for formation of nitrogen gas utilised in the ammonia oxida- tion.

Compared to other oxidising systems, the present invention has a high selectivity for formation of nitrogen gas utilised in the amine oxidation.

The invention also requires substantially less active material and al- lows a supple construction able to process nitrogen-based gases at a lower temperature and/or with a lower energy consumption than prior art. These features result in a smaller and cheaper construction, without obtaining infe- rior purifying properties.

A good effect is achieved with catalyst material being in the form of zeolites having a low silicon to aluminium ratio. An unusually good effect is achieved with a silicon to aluminium ratio being lower than 20, especially in the range of 5-20. In this interval there is mordenite, primarily an H mor- denite. For example, a good effect can be attained also with a p-zeolite when the silicon to aluminium ratio is higher than for example 12. For certain appli- cations, zeolites with a silicon to aluminium ratio higher than for example 20 may also be used. For example, a suitable zeolite would then be ZSM-5.

The properties of the zeolites may change through an ion exchange with metal ions. Depending on what metal ion or ions that are used, different characteristics will be achieved. Alkaline metal ions, for example Na and K, lead to loss of the activity. Exchange of copper ions into the zeolite results in a strongly increased activity at the same time as the selectivity for the forma- tion of nitrogen gas continues to be good. Other metal ions with good proper- ties include Ce and Fe.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with the aid of ex- emplary embodiments and with reference to the accompanying drawings, in which Fig. 1 is a principal block diagram illustrating one embodiment of the invention, Fig. 2 is a principal block diagram illustrating one alternative embodi- ment of the invention, Fig. 3 is a diagram showing the conversion of ammonia using zeolite nets,

Fig. 4 is a diagram showing the conversion of ammonia using zeolite nets promoted with rhodium, and Fig. 5 is a diagram showing the selectivity of zeolite nets promoted with rhodium.

DESCRIPTION The block diagram of Fig. 1 shows a principal layout of a regenerative process device according to one embodiment of the present invention. The present invention comprise a reactor 10, a process tank 11 and a heater 12.

The process tank 11 is designed to contain a material emitting odourants such as nitrogen-based gases. In the process tank 11, amines and ammonia or other compounds which are to be treated are generated.

The process tank 11 is connected to the heater 12 adapted to heat the nitro- gen-based gas or compound to suitable processing temperature. For exam- pie the nitrogen-based gas is heated to about 400-500°C. Preferably the heater 12 is arranged between a first reactor compartment 10a and a second reactor compartment 10b. The nitrogen-based gas is then conducted to the reactor compartments 10a, 10b, respectively.

For example the reactor 10 is designed as a longish cylinder with a principally circular cross section. The reactor 10 comprise an inlet 15 for ni- trogen-based gas to be processed, the first reactor compartment 10a and the second reactor compartment 1 Ob.

The reactor compartments 10a, 10b comprise a catalyser and a recu- perator 16. The catalyser comprise a zeolite catalyser and an oxidation cata- lyser arranged as a catalytic net 14. The reactor compartments 10a, 10b comprise at least one catalytic net 14. Preferably several catalytic nets 14 are arranged in the reactor compartments 10a, 10b. The catalytic nets 14 are arranged between the heater 12 and the recuperator 16. The nitrogen-based gas, heated by the heater 12, is brought into contact with the catalytic nets

14, purifying the nitrogen-based gas. The catalytic nets 14 are described in more detail below.

The recuperator 16 is designed to recover the heat energy from the heated gases in the reactor 10. For example the recuperator 16 comprise a material with good thermal transmittance properties.

The reactor compartments 10a, 10b are connected to an air inlet 17, designed for introducing an air flow, via a valve 18. The first reactor com- partment 10a is connected to the valve 18 by the means of a first tubing 19 and the second reactor compartment 10b is connected to the valve 18 by the means of a second tubing 20. The valve 18 is designed for alternating the direction of the air flow, alternating the air flow into the first tubing 19 and the second tubing 20. The valve 18 also comprise an outlet for purified gas and air. The air flow, introduced through the air inlet 17 and conducted via the valve 18 and the tubings 19,20 to the reactor 10, transports the nitrogen- based gas and the purified gas in the same direction as the air flow. For ex- ample the air flow is about ten times the nitrogen-based gas introduced in the reactor 10.

For example, by introducing the air flow via the first tubing 19 the ni- trogen-based gas is brought into contact with the catalytic nets 14 of the second reactor compartment 10b absorbing and purifying the nitrogen-based gas. The heat energy is recovered in the recuperator 16 of the second reac- tor compartment 10b. The purified gas follow the air flow to the valve 18 via the second tubing 20, the valve 18 comprising the outlet 21 for the purified gas and the air.

When the direction of the air flow is changed by the means of the valve 18, i. e. introducing the air flow to the reactor 10 via the second tubing 20, the nitrogen-based gas remaining in the second reactor compartment 10b and the new nitrogen-based gas introduced in the reactor 10 through the inlet 15 is transported towards the first reactor compartment 10a. The cata- lytic nets 14 of the first reactor compartment 10a purifying the nitrogen-based gas. The purified gas follow the air flow to the outlet 21 via the recuperator 16 of the first reactor compartment 10a and the first tubing 19.

By alternating the direction of the air flow by the means of the valve 18, the heated nitrogen-based gas is transported between the first reactor compartment 10a and the second reactor compartment 1 Ob, the nitrogen- based gas and the heat energy being trapped in the catalyst area.

In one embodiment of such regenerative process device, the reactor compartments 10a, 10b each comprise 36 catalytic nets 14. The recuperator 16 comprise 6,5 kg material with good thermal transmittance properties and the air flow is 40 m3/h. During experimental studies of such device, injecting 200 ml/h of a 25% NH3-solution resulted in no detectable odourants. Only about 5-6 ppm N20 was detected.

The block diagram of Fig. 2 shows a principal layout of a recuperative process device according to one alternative embodiment of the present in- vention. The present invention comprise a reactor 30, a process tank 31, a heat exchanger 32 and a heater 33. A catalyser is arranged in the reactor 30.

The process tank 31 is designed to contain a material emitting nitro- gen-based gases. In the process tank 31, amines or other gases which are to be treated are generated. The process tank 31 is connected to a heat ex- changer 32 adapted to heat the nitrogen-based gases by the means of puri- fied gas from the reactor 30. The gases are conducted via the heat ex- changer 32 to the heater 33, in which the gases are heated to a suitable processing temperature. For example the gases are heated to about 350°C.

On heating of the gas in the heater 33, it is conducted into the reactor 30 containing a catalyser. Purified gas is sucked out from reactor 30 via heat exchanger 32 by means of a suction fan 34.

The reactor 30 has an inlet 35 for those gases that are to be proc- essed, and an outlet 36 for the processed gases. The catalyser comprise at least one catalytic net 14, provided in the reactor 30. The number of catalytic nets 14 arranged in the reactor 30 depends on the application.

If the catalyst is operated at 350°C, a gasload of 10 Ndm3/min on 10 catalytic nets with 16 mesh and 30 mm in diameter are required to obtain a

sufficient purification of the nitrogen-based gases. This is equal to 85 Nm3l (h m2). To a 75 Nm3/h unit, the catalyst needed is less than one square meter.

Preferably 10 catalytic nets 14 are arranged in the reactor 30.

In one embodiment at least one bottom oxidation net 22 is arranged in the reactor 30 between the catalytic nets 14 and the outlet 36. Preferably two bottom oxidation nets 22 are arranged in the reactor 30. Preferably 8 cata- lytic nets 14 and 2 bottom oxidation nets 22 are used in a reactor 30 with a diameter of 400 mm when the conditions are as described above.

The catalytic net 14 may be coated with a substance such as TiO2 through a thermal forming process by spraying, providing the catalytic net 14 with a surface appropriate for catalytic active material. The catalytic net 14 further comprise a zeolite catalyst promoted with a precious metal, such as rhodium, platinum and palladium. Preferably the precious metal is rhodium.

The catalytic net 14, for example coated with TiO2, is coated with the zeolite by spraying, washcoating or dipping the catalytic net 14 in a slurry containing the zeolite. The slurry is prepared by mixing the zeolite with a binder, such as alumina and silica binders, and water. The zeolite, which preferably is a zeo- lite powder, is grounded to a crystal size of approximately 2 p. m. The mixing and grinding is, for example, performed in a mill, such as a ball mill.

Fig. 3 shows the results from one experimental example of processing ammonia by the means of zeolite nets. Pre-prepared extrudated catalyst was grinded in a ball mill with water for 24 h until a smooth slurry was obtained.

The slurry was mixed with 30 wt% binder (AI203+Si02). The used nets was 10 mesh, 0,8 mm thread and sprayed with TiO2. The nets were immersed in the slurry, blown with compressed air and dried in a hot air oven (150°C).

This procedure was repeated 5 times. The catalyst was then calcined at 500°C for 1 h. The deposed amount was about 250 g per m2 net. 10 pieces of 32 mm in diameter circular nets, for a laboratory quartsreactor, was pre- pared.

The experiments was carried out in the quartsreactor (32 mm in di- ameter) at 10 dm3/min (STP). This flow corresponds to a space velocity of 57.000h-1 (10 I/min) for the net pack (32 mm in diameter and 15 mm high).

360 ppm of ammonia was injected into the pressurised gas flow. The con- version at a space velocity of 6000 h-'was calculated.

Fig. 4 and Fig. 5 shows the results using catalytic nets 14, i. e. zeolite nets promoted with rhodium, processing ammonia. A small batch of slurry used in this experimental study was obtained according to the following pro- cedure: 50 g zeolite powder of H mordenite type was mixed with 20 g of ce- rium nitrate (Ce (NO3) 2 x 6 H2O), 15 g of 30 weight% colloidal silica (Bindizil 30/NH3 220) and 100 mi of distilled water in a ball mill. The slurry was then grounded for 12 hours.

The nets 14 were prepared by dipping in the slurry, left to drip off, blown with pressurised air and dried at 150°C in a hot air oven. This was re- peated until at least 300 g/m2 was deposited on the net. The net was then calcined at 500°C for 12 hours.

To promote the catalytic net 14, comprising zeolite, with a precious metal, the catalytic nets 14 were dipped in a 0.02 M solution of RhCI3, dried at 150°C and then calcined at 500°C for 1 hour. 10 catalytic nets 16 mesh and 30 mm in diameter were used.

The bottom oxidation nets 22 may be coated with a porous and pref- erably ceramic material through a thermal forming process by spraying. The bottom oxidation nets 22 may be surface area enlarged through sol treat- ment and coated with an active material having oxidation properties, such as precious metals Pt, Pd or metal oxides. Besides varying the layer formed by spraying, the surface area enlarging and the active phase, the properties of the bottom oxidation nets 22 can be varied by electing an appropriate wire thickens and mesh size.

Alternatively, the bottom oxidation nets 22 may be arranged similar to the catalytic nets 14, i. e. with zeolite and Rh.