CS273192B2 - Method of catalyst production for nitrogen oxide removal from waste gases,resistant to arsenic deactivation - Google Patents

Description

(57) The additional impregnation of titanium oxide catalysts known in the art with dissolved molybdenum and / or tungsten compounds and optionally vanadium compounds yields catalysts substantially more resistant to arsenic deactivation. These arsenic-deactivation-resistant catalysts are suitable for removing nitrogen oxides from waste gases.

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The invention relates to the production of a catalyst which is resistant to arsenic deactivation, for the removal of nitrogen oxides from waste gases, based on titanium oxide, which optionally contains still further catalytically active components.

To date, the most important, large-scale method for reducing the nitrogen oxides content of waste gases from combustion plants is the selective catalytic reduction of nitrogen oxides with ammonia, using mainly titanium oxide catalysts. This method was originally developed for gas or oil-fired installations and for coal-fired dry ash installations. When transferred to a furnace with a melting chamber, especially where ash is recycled, the problem arises that the traces of arsenic compounds in coal in the flue gas are enriched to such a stationary concentration that these catalysts are subject to rapid deactivation (Energie Spektrum November 1986, p. 16). So far, the catalysts thus damaged must be replaced with fresh catalysts.

It would be considerably better to have a catalyst which would not deactivate on contact with arsenic compounds or deactivate only to a minor extent. However, such catalysts are not yet known.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a titanium oxide catalyst for the removal of nitrogen oxides from waste gases, in particular from flue gas, which is resistant to arsenic deactivation, i.e. which is not deactivated or only deactivated to a minor extent.

The above drawbacks have been overcome by the process of the present invention, which consists in impregnating the catalyst after the formation and calcination with a dissolved molybdenum compound and / or tungsten and optionally a vanadium compound.

Surprisingly, investigating the problem of catalytic toxin deactivation has revealed that titanium dioxide baei catalysts become substantially more resistant to inactivation by arsenic, which is predominantly arsenic trioxide under combustion conditions when subjected to impregnation with dissolved molybdenum and / or tungsten compounds after formation and calcination. and optionally vanadium compounds. The impregnation is preferably carried out by dipping, but also by other processes such as spraying and the like. The result of this measure is therefore particularly surprising because in the long-term known tungsten and / or molybdenum and / or vanadium-containing catalysts based on titanium dioxide, such as those described in DE-PS 24 58 888 and in U.S. Pat. DE-OS No. 34 38 367, this property has not yet been established.

The process of the present invention is applicable to any external catalyst forms, such as tablets, pellets, rings, moldings, plates or honeycombs, manufactured in a manner known per se by calendering, rolling, overlaying, pelletizing, tabletting, compression or extrusion, and after forming subjected to calcination and which contain titanium oxide utilized for the catalytic reaction as a major component of cells in its entirety, or in part, used for the catalytic reaction. The term main component refers to a higher proportion by weight than any other catalytically active individual component.

Preferably, the major component, such as titanium dioxide, is present wholly or predominantly in the non-crystallized form of anatase. The catalysts may consist solely of titanium oxide as the catalytically active components, or may contain other catalytically active components, such as preferably molybdenum and / or tungsten compounds and / or vanadium compounds. For this purpose, they may contain other catalytically active components, inter alia in the form of their oxides and / or sulphates. Oxides are of particular technical importance. It is preferable to finely disperse these additional components in the catalyst mass, which can be achieved by adsorption to the titanium dioxide or by co-precipitation of the mixture, or by stirring, kneading or grinding, preferably in the wet state, with titanium dioxide or suitable precursors. This is exemplified in DE-PS 24 58 888.

In addition to titanium oxide, the catalysts of the invention may contain other inorganic components such as silica, zirconia, alumina, aluminosilicates, clay and the like. These other components, which are not themselves catalytically active or only to a minor extent, may be admixed by co-precipitation in the titanium oxide production process, or may be admixed at the most appropriate stage of the process, for example before molding, in powder, sol or gel form. These materials improve the processability of the molding and, in particular, the use of binder salts or gels increases the strength values of the molded bodies. Optionally, the catalytic properties are also stabilized or enhanced.

In addition, additives of fibrous materials, for example glass, ceramic, carbon and / or sufficiently thermally stable organic fibers, serve to improve the mechanical properties, especially the strength.

Still from the titanium oxide production process or later added, the catalysts may comprise sulphates, optionally in the form of barium sulphate.

The process of the present invention is not limited to a particular type of production of catalytically active titanium oxide or mixtures thereof with other catalytically active components, nor to any kind of further processing into formed catalysts.

The process is preferably based on a metatitanic acid slurry which precipitates as an intermediate in the sulfate process of titanium dioxide production. This starting material still contains about 6 to 10 wt. sulfuric acid, based on tritium dioxide, which either remains in the material, or is partially or also virtually completely removed by basic washing or is converted into sparingly soluble barium sulphate by addition of barium salt. If desired, the above additional catalytically active components or special inorganic components may be incorporated into or added to the metatitanic acid suspension.

The metatitanic acid may be dried after optional filtration, preferably at a temperature in the range of 100 to 200 ° C. Drying may be carried out without limitation to a particular method, for example in molds, in a paddle dryer, in a rotary tube furnace or in a spray dryer. The subsequent calcination is carried out at a temperature in the range of 300 to 800 ° C, preferably in the range of 400 to 600 ° C, for example, on strips, in a fluid bed or in a rotary tube furnace.

The calcining time is usually in the range of 0.5 to 12 hours, preferably 2 to 8 hours.

The acid or acid treatment, such as, for example, nitric acid, sulfuric acid, acetic acid or oxalic acid, may be added before or after calcination to improve the hardness and durability of the material. Generally, the calcined material needs to be ground prior to further processing.

Thereafter, the powder is mixed or kneaded with processing aids such as water, higher alcohols, methylcellulose, polyalkylene oxides, oils, waxes and the like. Other components such as additional catalio active components, fibers and similar inorganic components. The addition of sols or gels such as silicic acid or titanium dioxide alone as a binder improves the strength of the end product. Additional additional catalytically active components are preferably added in dissolved form, for example as concentrated aqueous solutions of vanadyloxalate, ammonium metavanadate, ammonium meta-tungstate, ammonium dimolybdate or ammonium heptamolybdate.

The material is then shaped. The shaped bodies are then dried at room temperature to about 200 ° C and calcined at a temperature up to 650 ° C in the usual manner.

In particular, catalysts in the form of honeycombs or plates are of technical importance in reducing the nitrogen oxide content of combustion plants.

Thus, further description is provided by means of a honeycomb catalyst; without being <?

the invention has been limited to this. The new process is readily transferable to all conventional catalyst forms.

It can be seen from FIG. 1 that prior art catalysts K1 to K3, which are also used on a large scale, are strongly deactivated upon contact with arsenic-containing gas without exception. Giant. 1 also shows that the deactivation rate of the catalyst produced by the process of the present invention is drastically reduced.

Molded and calcined catalysts are impregnated. These may consist of titanium oxide itself as catalytic components, or may contain other catalytically active components, such as vanadium, molybdenum and tungsten compounds, preferably oxides of these compounds, and the like. The impregnation can be carried out either in one step with a solution of molybdenum compound and / or tungsten and optionally vanadium, or successively with a solution of molybdenum compound and / or a solution of vanadium compound and optionally a solution of vanadium compound.

Preferably, the impregnation is carried out by dipping in a corresponding solution, optionally in appropriate solutions or with suitable precursors with a corresponding concentration, which is determined by the desired content of molybdenum and / or tungsten compound and optionally vanadium in the catalyst. An effective soaking time is 0.5 to 30 minutes, while the soaking process is kept as short as possible, that is, it can only be a few seconds to achieve uniform application. The concentration in the catalyst, calculated from the liquid uptake of the unprocessed catalyst and the concentration of the impregnating solution, is generally determined at lower soak times of about one minute or less. Longer soak times generally lead to higher catalyst concentrations than calculated, for example, ammonium heptamolybdate solution instead of the calculated 7.5 wt%. The molybdenum trioxide reached 10.4 wt. Even at shorter soaking times, it may be expedient to eliminate the deviation of the concentration in the finished catalyst from the theoretically expected value by detecting a correction factor for the special catalyst and the desired impregnation time in a preliminary experiment.

It is preferred that the additionally impregnated molybdenum and / or tungsten and optionally vanadium components be uniformly distributed throughout the catalyst body.

This is achieved at soak times of about one minute or less. Longer soak times, on the other hand, lead to uneven distribution in the catalyst.

The impregnation is carried out on pre-dried and heated honeycombs whose temperature is in the range of 50 to 200 ° C, preferably 80 to 120 ° C. Heating of the impregnating solution is not necessary. In contrast, it is normally expedient to cool the solution to avoid evaporative concentration changes or temperature-dependent density changes.

As an alternative to or in addition to preheating the honeycombs, the honeycombs may be evacuated before soaking. For this purpose, an evacuation using a water pump is sufficient.

After impregnation, the catalyst is allowed to drip or the channels are purged with compressed air. Finally, the impregnated catalyst is dried at a temperature in the range of 20 to 200 ° C. The drying may be carried out by treating the catalyst bodies in an air stream either at a lower temperature, preferably in the range of 20 to 50 ° C for 4 to 16 hours, and then at a higher temperature, preferably in the range of 80 to 200 ° C for 1 hour. up to 8 hours, or by treating them immediately at higher temperatures, preferably in the range of 50 to 200 ° C for 2 to 12 hours.

Finally, the impregnated and then dried catalyst is calcined at a temperature in the range of 300 to 800 ° C, preferably in the range of 400 to 600 ° C.

The calcination time is 15 minutes to 12 hours, preferably 2 to 8 hours. The drying and calcining can be carried out in one operation. However, the times indicated for drying and calcining are not exclusive values. The minimum times at the temperatures indicated depend on the residual moisture required or the time required to convert the impregnation component into the catalyst required, while the upper limit, regardless of the economic usefulness of the unnecessarily long processing time, is determined by the onset of the thermal catalyst due to the growth of cristallite. BET surface loss and anatase conversion to rutile. In view of the large number of different catalysts for which the present invention is applicable, it is necessary to determine in each case the optimum drying and calcining temperatures and the time at which these processes are carried out, but this can be accomplished with little experimental effort.

In the process according to the invention, the catalyst bodies are impregnated with a molybdenum and / or tungsten compound and optionally a vanadium compound, preferably a molybdenum compound and particularly preferably a molybdenum and vanadium compound. Direct addition of a small amount of vanadium, for example in the form of vanadium pentoxide, results in a particularly good resistance to deactivation of the catalyst by arsenic.

The solution may at the same time contain other components which are also to be applied to the catalyst or to prevent the component already present in the catalyst from being impregnated,

In the impregnation solution, molybdenum or tungsten or vanadium compounds can be used in the form in which they are required in the finished catalyst or in another form which is converted to the desired form by further processing, such as drying or calcination.

In the catalyst of particular technical importance, molybdenum, tungsten and vanadium compounds are present, such as molybdenum oxide or tungsten oxide or vanadium pentoxide. In order to deposit these oxides by impregnation, it is particularly advantageous to use solutions of ammonium dimolybdenum, or ammonium heptamolybdenum, or ammonium meta-tungsten or ammonium metavanadate or mixtures thereof. These compounds are obtainable on a technical scale and are very well soluble in water and in technically usable and economically acceptable solvents so that they can achieve high catalyst contents in one process. In the case of poorly soluble starting materials, the catalyst must be subjected to several times or a number of operating steps.

In order to determine the concentration of molybdenum, tungsten or vanadium compounds in the impregnation solution for the predicted amount of these components in the finished catalyst, the following procedure may be followed:

First, the ability of the catalyst to hold the liquid (liters of liquid / kg of catalyst) was determined by drying for several hours at elevated temperature, e.g. From the ability to absorb the liquid, the formula I alpha (1 - C _K ) (I), which denotes kg MoOO, possibly WO ^, or Vg ° 5.

C - liter of impregnating solution (concentration of molybdenum or tungsten or vanadium compound calculated as KoO2 or WO2, optionally ^at 2 ° 5 ^{in the} impregnating solution).

kg MoO 2 or WO 3 or Vg 5 kg of total catalyst mass (proportion of molybdenum or tungsten or vanadium compound calculated as MoO 3 or WO 3, optionally in the finished catalyst) liters of water kg of catalyst mass prior to impregnation (water uptake by catalyst) prior to impregnation) calculates the concentration of molybdenum or tungsten or vanadium compound, which must be in the impregnating solution, in order to introduce into the catalyst the necessary amount of the corresponding component, calculated as molybdenum oxide or tungsten oxide or vanadium pentoxide.

Particularly good resistance to deactivation of the catalyst by arsenic is obtained when impregnating 1 to 20 wt.%, Preferably 5 to 15 wt. that is, the catalyst impregnated with the dissolved molybdenum compound contains 1 to 20% by weight, preferably 5 to 15% by weight, of the molybdenum compound, calculated on the basis of the total weight of the finished catalyst. molybdenum, calculated as molybdenum trioxide and based on the total mass of the catalyst. For example, a catalyst consisting essentially only of titanium dioxide may be impregnated.

however, catalysts in the production of which molybdenum and / or tungsten compounds are added, for example in a kneader, have a better mechanical strength, so that they already contain at least 2 to 6% by weight before impregnation. of these compounds, calculated as molybdenum oxide or tungsten oxide, respectively. In such cases, the impregnation, preferably molybdenum, already leads to very good results when the total content of this compound, calculated as molybdenum oxide or tungsten oxide, is adjusted to 6 to 12% by weight in the finished catalyst. A part of the optionally present vanadium compound can also be added during the production of the catalyst.

An even better resistance to deactivation of the catalyst by arsenic can be achieved by impregnating the catalyst impregnated with the dissolved molybdenum compound and vanadium with a 5 to 15 wt. molybdenum and 0.1 to 3% by weight, preferably 0.3 to 1.5% by weight, of vanadium, calculated as molybdenum trioxide and vanadium trioxide, based on the total weight of the catalyst.

Although a higher additional vanadium content is possible, it cannot be recommended in the case of sulfur dioxide-containing off-gases, since it can at the same time largely cause an undesirable conversion of sulfur dioxide to sulfur trioxide.

The process for producing the arsenic inactivation catalyst of the present invention avoids the drawbacks that can be caused by compressing the air enclosed in the pore volume and provides catalysts with a defined content of molybdenum and / or tungsten and / or vanadium impregnation components in short impregnation times, e.g. and / or sulphates (deviation from the nominal value is generally not less than 10%), as well as their uniform distribution over the length and cross section of the catalyst shapes, for example honeycombs of 15 x 15 x 100 cm and baffle thickness of 1.4 mm, such as has been proven by EDX (energy dispersive x-ray).

Example 1

Of a honeycomb catalyst K1 which contains 8 wt. % tungsten oxide and 0.6 wt. a vanadium oxide, a few percent of alumina and silica as an admixture and as a major component of titanium dioxide and manufactured according to the prior art as described above, is cut off a 30 c length with 3 x 3 channels of 6 mm 3-light width and thickness The wall was dried at 150 ° C and weighed. Then the still warm water is soaked in fully desalinated water and weighed again after the liquid drips. The dried honeycomb weighs 0.0912 kg and 0.1217 kg wet. From the difference in weight, the alpha value - the water-holding capacity of the catalyst - is 0.334 l / kg.

For a content of molybdenum oxide in the finished catalyst of about 7.0% by weight, the required concentration of molybdenum, calculated as molybdenum oxide, in an aqueous ammonium heptamolybdate impregnation solution to 0.2253 kg MoO4 / l is determined according to formula (I). The ° C is now soaked in the above pre-cooled impregnating solution in one second and left there for one minute. After dripping, the honeycomb is first dried for 6 hours at 150 ° C in a stream of air and then caloinated at 450 ° C for 5 hours in a stream of air.

The calcined honeycomb contains 7.5% by weight of molybdenum oxide according to X-ray fluorescence analysis.

Example 2

A catalyst K4 of 96 wt. titanium dioxide and the remainder are clay and glass fibers made according to the state of the art, as shown above, in the form of a honeycomb.

For the same catalyst section, the water-binding capacity of the alpha catalyst is measured according to Example 1. It amounts to 0.345 l / kg.

For a molybdenum oxide content of the finished catalyst of about 8.5 wt. ee according to formula (I), the corresponding molybdenum concentration, calculated as molybdenum oxide 0.2693 kg MoO 2 / l, is determined and the concentration of the ammonium heptamolybdate impregnation solution is set at this value.

AND

The impregnation and thermal post-treatment were carried out as described in Example 1.

X-ray fluorescence analysis revealed a molybdenum trioxide content of 9.1% by weight in the calibrated honeycomb.

Example 3>

Catalyst K5, containing 86% by weight, titanium dioxide and 7.7% by weight, tungsten oxide and the remainder of clay and glass fibers, is prepared according to the state of the art in the form of honeycombs, as clearly described above.

As shown in Example 1, the same honeycomb cut was prepared and the water-holding capacity of the alpha 'catalyst was measured. Sine here 0.266 1 / kg,

For a content of about 7.0 wt. % molybdenum oxide and about 0.7 wt. The vanadium trioxide in the finished catalyst is determined according to formula (I) corresponding to a concentration of molybdenum, calculated as molybdenum trioxide, 0.283 kg MoO 2 / l and a vanadium concentration, calculated as vanadium oxide, 0.027 kg VgO 2 / l. For molybdenum impregnation, a solution of ammonium heptamolybdate is prepared at this concentration, and for vanadium impregnation an aqueous solution of vanadyloxalate prepared from vanadium pentoxide and oxalic acid at the indicated concentration is prepared.

The impregnation is carried out in two stages, first with the above-mentioned ammonium heptamolybdate solution according to Example 1. After drying and calinating according to Example 1, the honeycomb is impregnated in the same way with said solution of vanadyloxalate. Calibration of the honeycomb after impregnation with vanadium is not necessary, it is sufficient to dry the catalyst according to Example 1. According to X-ray fluorescence analysis, a finished catalyst with a molybdenum oxide content of 7.5% by weight is obtained. % and a vanadium oxide content of 0.65 wt.

Activity testing

Activity testing was performed on the honeycomb slots described in the exemplary 30 µm length with 3 x 3 channels of 6 mm wide width and 1.4 mm wall thickness under the following test conditions:

experimental gas loading temperature area velocity AV gas composition

400 ° C 1 Nm ³ / h

Nm ³ / m ² .h 100 mg As / Nm ³

000 ppm vol ΝΟ _χ / 95% NO / 1000 ppm vol, NH4

000 ppm by volume SOg% by volume 0 ₂ residue Ng.

Arsenic is supplied to the gas stream by means of a temperature controlled vaporizer in the form of arsenic trioxide. The high concentration of arsenic, which is one hundred to thirty times the typical values found in power plant flue gas, permits the evaluation of the catalyst behavior after a short test period of 10 to 50 hours.

The conversion of nitrogen oxides on the fresh catalyst and after certain contact times of the catalyst with the arsenic-containing gas stream was measured. From the decimal conversion ΙΊΟ _χ eta were by formula II

K and - AV. ln (1 - eta) (II) calculated the activity values of the fresh catalyst K _Q and the activity values for the arsenic-damaged catalyst at a certain point in time, as well as the relative residual activity α = K i / K q. The results are shown in Figures 1, 2, 3 and 5, which indicate the dependence of a 3 f (t). Giant. 4 represents the residual activity and as a function of the molybdenum trioxide content by weight impregnated, based on the total weight of the catalyst.

Giant. 1 shows a rapid de-inactivation of K1 to K3 down to low levels of residual activity of catalysts of 0.25 to 0.4 and, on the other hand, compared to a high residual activity of 0.7 of catalyst K1 impregnated according to the present invention 7.5 wt. The catalysts K1 to K3 have the following contents of catalytically active components:

Kl 85% wt. Wt. WOj

0.5 wt. VgOjj rest glass fibers

K2 85.5 wt. TiOg

7.9 wt. WO ^ rest glass fibers

K3 74.5 wt. TiOg

12.3 wt.%, SiO2

7.4 wt.%, WOj 0.7 wt. VgOj rest glass fibers.

Catalysts K1, K2 and K3 were made according to the prior art as described above.

Giant. 2 shows the deactivation of the catalyst K4, which contains only titanium dioxide as active ingredient (catalyst substance) up to a value of 0.3 and, on the other hand, a slight decrease in the activity of the catalyst K4 after impregnation of 9.1% by weight. molybdenum trioxide, designated B1, where α is 0.82.

Giant. 3 shows catalyst K4 the effect of impregnated molybdenum oxide content of varyingly high, namely 2.85? 4.30; 5.90; 8.84; 9.09 and 13.56 wt.%, Respectively, to resist this catalyst to arsenic deactivation. These catalysts are designated B1, B2, B3, B4, B5 and B6 in the figure,

Giant. 4 includes the results of FIG. 3 by depositing residual activity and test samples against the concentration of molybdenum oxide deposited by impregnation. It is clearly seen that the maximum residual activity is achieved at a molybdenum oxide content of about 9% by weight.

This also means that ae for this catalyst K4 achieves optimum resistance to arsenic deactivation at this molybdenum oxide content impregnated.

Giant. 5 shows the behavior of the following catalysts towards arsenic:

(1) catalyst K5 unimpregnated chemical composition see Example 35 content 7.7 wt. (2) Catalyst K4 impregnated with 7.7 wt. tungsten oxide chemical composition (see example 2j) was additionally impregnated with 7.7 wt.%, tungsten oxide (3) catalyst K5 impregnated with 9.3 wt. Molybdenum oxide chemical composition (see Example 3) Additionally, impregnation of 9.3 wt. molybdenum oxide (4) catalyst K5 impregnated with 7.5 wt. % molybdenum oxide and 0.6 wt. vanadium oxide chemical composition see Example 3, additionally impregnation of 7.5 wt. % molybdenum oxide and 0.6 wt. vanadium pentoxide.

It is clear

- that impregnation of the catalyst K4 with a tungsten component (see curve / 2 /) compared to a catalyst with the same amount of tungsten oxide previously ignited as catalyst K5 (see curve / 1 /) also improves the resistance of titanium dioxide to arsenic deactivation, albeit somewhat less efficiency than using a molybdenum component (cf. Figure 3) 5

- that additional impregnation with the molybdenum component (see curve (3)) substantially improves the impregnated catalyst K5 already containing tungsten oxide (see curve (1));

that additional impregnation of the catalyst K5, which is already impregnated with 7.5 wt. molybdenum trioxide, with a small amount of vanadium component, leads to further improvement (see curve (4)). This behavior is particularly surprising because in further experiments it was found that the catalyst K4, impregnated with only 0.5% by weight. vanadium pentoxide is even more inactivated than K4 alone (a β 0, 27 vs. a «0.3). Thus, impregnation by the method according to the invention carried out with only one vanadium component remains unsuccessful.

Claims (9)

SUBJECT OF THE INVENTION 1. A process for the preparation of a catalyst for the removal of nitrogen oxides from arsenic deactivation-resistant waste gases based on titanium oxide, further optionally containing further catalytically active components and / or binders, characterized in that the Be catalyst additionally impregnates the dissolved compound after forming and calcining. molybdenum and / or tungsten and optionally a vanadium compound and then calcined at a temperature in the range of 300 to 800 ° C. 2. The method according to claim 1, characterized in that the impregnation is carried out in one step with a solution of the molybdenum compound and / or tungsten and optionally a solution of the vanadium compound, 3. 5. The process according to claim 1, wherein the impregnation is carried out after each other with a solution of a molybdenum compound and / or a tungsten compound and optionally a solution of a vanadium compound. 4. The process according to claims 1 to 3, characterized in that the catalyst impregnated with the soluble molybdenum compound contains, after calcination, additionally 1 to 20% by weight of molybdenum, calculated as molybdenum oxide and based on the total weight of the catalyst. 5. The process according to claims 1 to 3, characterized in that the catalyst impregnated with soluble molybdenum and vanadium compounds additionally contains 5 to 15% by weight of molybdenum and 0.1 to 3% by weight of vanadium, calculated as molybdenum oxide and vanadium oxide and based on the total weight of the catalyst. 6. 5. The process of claims 1 to 5 wherein the impregnated catalyst is dried at a temperature in the range of 20 to 200 [deg.] C. 7. 5. A process according to any one of claims 1 to 6, wherein the impregnated and then dried catalyst is calcined at a temperature in the range of 400 to 600 ° C. 8. 5. The process of claim 1 wherein the impregnated catalyst is honeycomb-shaped. 9. 5. The process of claim 1, wherein the impregnated catalyst is plate-shaped.