WO1998016300A1

WO1998016300A1 - Method and device for processing gases containing nitrogen

Info

Publication number: WO1998016300A1
Application number: PCT/SE1997/000986
Authority: WO
Inventors: Jan Brandin
Original assignee: Katator Ab
Priority date: 1996-10-16
Filing date: 1997-06-05
Publication date: 1998-04-23
Also published as: AU3199197A; SE9603796D0; SE9603796L

Abstract

A method and a device for processing nitrogen-based gases, such as ammonia and amines, by means of a catalyser. The gas is brought into contact with a zeolite catalyser and an oxidation catalyser. The catalysers are provided in a reactor (10) having an inlet (11) for the nitrogen-based gas, and an outlet (12) for the purified gas.

Description

METHOD AND DEVICE FOR PROCESSING GASES CONTAINING NITROGEN

Technical field of the invention

The present invention relates to an efficient method of purifying exhaust, process gases, ventilation air and similar, of nitrogen-based compounds such as ammonia and amines. The purpose of the purification may be to prevent emission of small amounts of malodorous substances, so called odourants, or to simply obliterate large amounts. Emissions of nitrogen-based odourants interferes with the environment because the compounds are perceived even in low concentrations. During the catalytic purification, decomposition of amines into ammonia first takes place followed by selective oxidation of ammonia into gaseous nitrogen and water.

State of the art

There are three methods when it comes to purification of gases containing nitrogen-based odourants. The methods are: absorption, adsorption, and oxidation.

During absorption the gas is treated with an acid solution in a scrubber. The odourant is absorbed in the liquid resulting in a clean gas. The absorption 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 purification plant. Nitrogen oxides contribute to the global acidification, smog and formation 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, N₂O, 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.

Summary of the invention

It is now proven feasible to catalytically oxidise, with high selectivity, nitrogen-containing odourants, such as ammonia and trimethylamine, to nitrogen gas and water. The gases containing the odourant pass through a catalytic bed, which consists of a zeolitic catalyser of the H-mordenite type, and an oxidation catalyser. Preferably the ocidation catalyser is formed as one or several catalytic nets, but also other geometrical embodiments are possible, such as rings, pellets, screws and the like. The gases are preferably treated at a temperature between 400 and 600°C. These embodiments, especially when catalytic nets are included, and the temperature intervals will result in a technically speaking reasonable size of reactor and a very good selectivity for nitrogen gas formation.

The reaction, however, will continue down to a temperature as low as 250°C. In the upper temperature range, the process is limited by an increasingly larger formation of NOx, and above 700°C the process can no longer be regarded as selective. The catalytic nets are coated with a porous and preferably ceramic material through a thermal forming process by spraying. The nets are surface area enlarged through sol treatment 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 proporties of the nets can be varied by electing an appropriate wire thickens and mesh size. As a result the performance of the nets, such as the activity and the presume drop, can be very carefully adopted to the present needs. It is possible to use sizes from coarse grates to very fine nets having hundreds of meshes per inch (mesh).The ammonia entering the bed is oxidised with surprisingly high selectivity to become nitrogen gas and water.

When odourants of the amine type enter the catalytic bed they decompose quite unexpectedly into ammonia and, presumably, into corresponding alcohol. This step, already, reduces the problem with odour to a large extent because the ammonia odour threshold is significantly higher than that of amines. The ammonia 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 placing one or several catalytic nets in the bed 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 nets in the bed has the additional effect of oxidising any incoming hydrocarbons to carbon dioxide and water. If the nets are placed after the bed, final combustion occurs of hydrocarbons (VOC) that are not oxidised in the catalytic bed. Also ammonia and amines that pass through the catalytic bed, for instance during start-up, will be oxidised here before the bed has reached its operational temperature. This will not occur selectively but the start-up occurs only during a limited time.

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 oxidation.

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

The invention presents the advantage of incoming amines being decomposed first into ammonia, which has a higher odour threshold than amines, before the ammonia is oxidised to nitrogen gas and water.

A good effect is achieved with catalyst material being in the form of zeolites having a low silicon to aluminum ratio. An unusually good effect is achieved with a silicon to aluminum ratio being lower than 12, especially in the range of 5-12. In this interval there is mordenite, primarily an H mordenite. A good effect can be attained even with a silicon to aluminum ration higher than 12. In such a case, an example of zeolite would be β-zeolite. For certain applications, zeolites with a silicon to aluminum ratio higher than 20 may also be used. 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 formation of nitrogen gas continues to be good. Other metal ions with good properties include Ce and Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with the aid of exemplary embodiments and with reference to the accompanying drawings, in which

Figure 1 is a principal block diagram illustrating an embodiment of the invention,

Figure 2 is a principal section view showing an embodiment of the invention,

Figure 3 is a principal section view representing an alternate embodiment of the invention,

Figure 4 presents the conversion of 0.5% by volume of NH₃ as a function of the temperature for SV 3000 h^"1 and SV 6000 h^"1 across an H mordenite catalyser,

Figure 5 illustrates the formation of NOx during oxidation of NH₃ across an H mordenite catalyser,

Figure 6 shows the conversion of trimethylamine based on NH₃ output,

Figure 7 illustrates the formation of N₂O laughing gas during oxidation of trimethylamine,

Figure 8 illustrates the formation of NOx during oxidation of trimethylamine and, Figure 9 presents the conversion of NH3 as a function of the temperature, and a comparison between the Cu and H form of the zeolite.

The block diagram of Fig. 1 shows a principal layout of a process device according to one embodiment of the present invention. In a process tank 15, amines or other gases which are to be treated are generated.

The gases are conducted via a heat exchanger 16 to a heater 17, in which the gases are heated to a suitable temperature in the range of 250°C- 700°C. A particularly good effect is achieved in the interval between 400°C and 600°C. The reaction, on the other hand, will continue down to a temperature as low as 250°C. In the upper temperature range, the process is limited by an increasingly larger formation of NOx, and above 700°C the process can no longer be regarded as selective. On heating of the gas in a heater 17, it is conducted into a reactor 10 (See the description below with regard to Fig. 2). Purified gas is sucked out from reactor 10 via heat exchanger 16 by means of a suction fan 18.

Figure 2 shows the principal design of a reactor 10. The reactor 10 has an inlet 11 for those gases that are to be processed, and an outlet 12 for the processed gases. There is provided a bed 14 of catalyst material in reactor 10. The bed may consist of a particulate material, such as pellets, an extrudate or granules, or a monolithic block. It is possible also to provide the bed material as zeolite coated nets of which the zeolite has been applied on the net by for instance slip casting or thermal spraying, or by a combination thereof. According to a preferred embodiment the reactor also includes catalyser nets 13A-13C. The nets may be arranged in diverse fashions in relation to bed 14. There are different effects achieved depending on the placement of the nets in the reactor. If the nets are positioned above the catalytic bed at 13A, the rate of reaction as well as the selectivity are increased during the amine oxidation. If, on the other hand, the nets are positioned in the bed at 13B, the rate of reaction is increased for the selective ammonia oxidation. It is also possible to place the nets downstream of the bed, at 13C. If so, a final combustion takes place of the hydrocarbons that are not oxidised in the catalytic bed. The placements can be selected on an individual basis, or be combined in various ways, for instance in the way shown in Fig. 2, wherein all three placements are used simultaneously. Figure 3 illustrates an alternate process layout with a so called regenerative design. Incoming gas passes through an inert thermal buffer 19, such as sand. The heat stored in the buffer heats the gas before the gas enters the catalytic bed 14. The temperature of the gas is shown by the curve at 20. To compensate for heat losses and to enable a start-up, there is provided a heater 21 in position between the two bed halves. The heat of the exiting gas is stored in a second thermal buffer, which is identical to the first one. Before the heat front reaches the exit, the flow direction changes, from A to B. This kind of heat exchange in a particulate bed renders a very high efficiency with respect to heat recycling. Oxidation nets are placed between the thermal buffer and the catalytic bed, and in the catalytic bed itself and, thus, in the way corresponding to the process layout as per Fig. 2.

DESCRIPTION OF THE EXAMPLES

The testing was carried out in a laboratory reactor of a tube type. The gases were premixed from pressure vessels containing liquid ammonia and trimethylamine in pressure air which had been moistened with water to reach 100% air moisture at an ambient temperature. Input and output gases were analysed with an IR instrument (Bruel Kjar Multigas Analyser) in addition to NOx (NO and NO₂) which were analysed with a chemical luminescence instrument (API NOx Analyser 200).

Example 1 In this Example, selective oxidation of ammonia is carried out in a tube type laboratory reactor. The catalyser of H mordenite is in an extruded form. 0.5% by volume of ammonia is premixed in air. Figure 4 presents the conversion of ammonia as a function of the reactor temperature for two different gas loads: SV 3000 h^"1 and SV 6000 IT¹, respectively. Figure 5 illustrates the surprisingly low formation of NOx. Laughing gas was not detected.

Example 2

In this Example, selective oxidation is carried out in the same reactor system as above. Figure 6 shows the conversion for 1% by volume of trimethylamine in moist pressure air without a catalytic net, on the one hand, and 0.5% by volume of trimethylamine with and without a catalytic oxidation net in the bed.

The conversion is based upon the content of ammonia output since, surprisingly enough, no output of trimethylamine can be detected. All incoming trimethylamine is first decomposed to ammonia, which in turn is oxidised to nitrogen gas and water.

Figure 7 illustrates laughing gas formed during oxidation of trimethylamine. The introduction of a catalytic net in the bed reduces the formation of laughing gas by 30-50%, which is surprising considering that use of a precious metal net should normally increase such formation. Figure 8 illustrates that a certain increase of the formation of NOx occurs through the introduction of a precious metal net. At temperatures lower than 550°C the problem is not very troublesome. It is also possible to eliminate this NOx through suitable placement of nets before and/or after the zeolite catalyser.

Example 3

In this Example, the activity is compared between the Cu ion exchange and the H form of zeolite.

Figure 9 shows the conversion of ammonia as a function of the temperature for the same gas load and content of ammonia input. The fresh Cu zeolite exhibits a superior activity as compared to the H form. The selectivity for N₂ formation is also relatively good.

Cu and H mordenite, however, exhibit different ageing and deactivation characteristics, and the catalyser choice is governed not only by catalytic activity, but also, to a large extent, by process conditions.

Example 4

In this example odourants were removed from a biological process. The gas flow was 75 Nm³/k and 25 kg of Ce-mordenite were used. Three nets were applied in the reactor: one over, one within and one under the bed.

limit of detection

Besides ammonium and amines for which the process specifically is designed also aldehydes which count as VOC:s, are combusted completely. It was also shown flat even small contents of sulphur compounds were removed effectively.

Claims

Patent Claims:

1. A method used for processing nitrogen-based gases, such as ammonia and amines, with a catalyser, characterised in that the gas is brought into contact with a zeolite catalyser.

2. A method according to Claim 1 , characterised in that the gas is also brought into contact with an oxidation catalyser formed as at least one catalytic net.

3. A method according to Claim 1 , characterised in that the gas is conducted through a catalytic bed with zeolite.

4. A method as claimed in any of Claims 1-3, characterised in that the gas is supplied to the catalyser in a temperature range of 250-700°C.

5. A method as claimed in any of Claims 1-3, characterised in that the gas is supplied to the catalyser in a temperature range of 400-600°C.

6. A method as claimed in any of Claims 1-4, characterised in that the gas is supplied to the catalyser in a gas load range of 0-100,000h^~1.

7. A device for processing nitrogen-based gases, such as ammonia and amines, with a catalyser, characterised in that the catalyser is provided in a reactor (10) having an inlet (11) for the nitrogen-based gas, and an outlet (12) for the purified gas, and that the catalyser comprises a zeolite catalyser and an oxidation catalyser.

8. A device according to Claim 7, characterised in that the silicon to aluminum ratio of the zeolite in the catalyser is lower than approximately 20.

9. A device according to Claim 7, characterised in that the silicon to aluminum ratio of the zeolite in the catalyser is lower than approximately 12.

10. A device according to Claim 7, characterised in that the silicon to aluminum ratio of the zeolite in the catalyser is in the range of 5-12.

11. A device as claimed in any of Claims 7-10, characterised in that there is used, as zeolite, mordenite of the H form or ion-exchanged with one or several of the following ions: Cu, Ce or Fe.

12. A device as claimed in any of Claims 7-11 , characterised in that the zeolite is provided as a bed in the reactor.

13. A device as claimed in any of Claims 7-12, characterised in that the oxidation catalyser is formed as at least one catalytic net.

14. A device according to Claim 12, characterised in that at least one catalytic net is provided in the reactor between the inlet and the bed.

15. A device as claimed in Claims 13 or 14, characterised in that catalytic nets existing in the reactor are coated with a porous material.

16. A device according to Claim 15, characterised in that the porous material is surface area enlarged through sol treatment.

17. A device as claimed in Claims 15 or 16, characterised in that the porous material is coated with an active material having oxidation properties.

18. A device as claimed in any of Claims 7-17, characterised in that the flow load in the reactor is 0-100,000fr¹.

19. A device according to any of claims 7-17, characterised in that the zeolite catalyser is formed as zeolite coated nets.

20. A device according to claim 19, characterised in that the zeolite is applied to a net by slip casting or thermal spraying or a combination thereof.