DE2618510A1 - PROCEDURE FOR THE REMOVAL OF NITROGEN OXIDES FROM OXYGEN GASES - Google Patents

Description

The invention relates to a method for removing nitrogen oxides from the oxygen-containing Exhaust gases, more precisely a method of selective catalytic reduction of nitrogen oxides contained in exhaust gases, primarily from the exhaust gases from the production of nitric acid, in which a reduction in the demand for Re

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reducing agent - the ammoi can essentially be prevented in good time.

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formation of ammonia salts

As a toxic product, nitrogen oxides are found in practically all incineration processes Contained in exhaust gases, in exhaust gases and in the exhaust gases of various chemical companies. In the chemical Industry are those involved in the production of nitric acid

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The largest source of nitrogen oxides emitted. The most effective method of combating the nitrogen oxides in these gases are their reduction to elemental nitrogen. One of the possible An alternative to this type of process is the so-called selective reduction method, which is used as a reducing agent gaseous ammonia is used. The ammonia gas reacts under suitable conditions and when used a suitable catalyst mainly with the nitrogen oxides and only to a small extent with that in the gases oxygen present.

Because the level of nitrogen oxides in the exhaust gases is lower than that of oxygen, the consumption of ammonia is correspondingly much lower than that of other reducing agents, such as. B. from natural gas, coal gas, carbon monoxide, hydrocarbons or mixtures thereof, which are used in the total reduction (eg., US-PS 2 970 03 ¹ I-, GB-PS 871 755, PR-PS 1,205,311). In the case of total reduction, the components listed react first with the oxygen and only afterwards with the nitrogen oxides. The process of selective reduction can thus be economically more favorable despite the relatively higher price of ammonia. The economic advantage of the process depends primarily on the amount of ammonia required to reduce the nitrogen oxides.

On the one hand, the catalysts, the platinum metals, became suitable catalysts for selective reduction (ζ. B. US-PS 2,975,025, 3,053,619 and others), on the other hand catalysts, the oxides of cobalt, nickel or

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Iron (US-PS 3,088,796 and 3,053,613), vanadium, Molybdenum and tungsten (US Pat. No. 3,279,884), molybdenum, vanadium, manganese and iron, their mixtures or chromites (DAS 1 253 685) or oxides of metals of the VI. until . VIII. Subgroup of the periodic table, particularly iron and chromium (DAS 1 259 298), proposed. Copper (II) oxide was also used for the selective reduction of nitrogen oxides in motor vehicle exhaust gases (U.S. Patent 3,559,427) or palladium activated cupric oxide (U.S. Patent 3,449,063).

In order to achieve a sufficient degree of elimination of the nitrogen oxides in the exhaust gases, the ammonia gas must be dosed in a certain excess over the stoichiometric amount, in view of the imperfect selectivity of the catalysts used. This excess normally represents 20 to 50 % of the amount of ammonia required to reduce the nitrogen oxides according to the following equations:

6 NO + 4 NH-, = 5 N ₂ + 6 H ₂ O 6 NO ₂ + 8 NH ₃ = 7 N ₂ + 12 H ₂ O.

The excess ammonia gas then reacts with the oxygen to form nitrogen according to the equation

4 NH ₃ + 3 O ₂ = 2 N ₂ + 6 H ₂ O.

The selectivity of the catalyst used is decisive for the economy of the whole Procedure.

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Ammonia savings can therefore be achieved if highly selective catalysts, in which the course the undesired reaction of ammonia with oxygen is suppressed to the greatest possible extent, can be applied. The usual fluctuations in the content of nitrogen oxides and their degrees of oxidation in the Exhaust gases (the term "degree of oxidation" is used here as the ratio of the content of nitrogen dioxide to the total content of nitrogen oxides), which depends on the technological parameters of the relevant Are dependent on the process, but inevitably also manifests itself in the fluctuations in the ratio of the amount of dosed ammonia to the amount of reduced nitrogen oxides, with it when a certain value is exceeded to occur depending on the type of catalyst used of the ammonia in the exhaust gas comes after the reactor. For operational reasons, however, it is desirable that the The gases escaping after the reduction do not contain any ammonia gas, as this could occur at lower temperatures Form ammonium salts with the remaining nitrogen oxides, which settle in cooler parts of the system could.

The fluctuations in the nitrogen oxide content in the exhaust gases cause less selective catalytic converters to be used no particular difficulties, because the excess ammonia reacts with the oxygen in one very broad range of excess ammonia, and the gases emitted are then almost always free of ammonia.

If a less selective catalyst, e.g. B. a platinum catalyst containing 1 % Pt

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active alumina, is used for more than 90% removal of nitrogen oxides from exhaust gases containing 0.2 yol-fo nitrogen oxides (of which 40 fo as NO ₀ ) at a space velocity of

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50,000 h and preheating of the incoming gas mixture to 240 ⁰ C requires an almost 34 ^ iger excess of ammonia over the stoichiometric requirement. When using a vanadium catalyst, on the other hand, an excess of only 12.5 % ammonia is sufficient.

As a result of a possible increase in the ratio of the ammonia content to the content of nitrogen oxides in the incoming gases, ammonia can occur downstream of the reactor. If z. B. the concentration of nitrogen oxides in the exhaust gases from 0.2 Vol- # to 0.18 Vol- $ and at the same time the degree of oxidation from 40 % to 30 % , then at a content of 0.21 Vol- ^ ammonia in the gas mixture , which is fed to the vanadium catalyst, the concentration of the ammonia after the reduction under otherwise identical conditions increase from originally 30 ppm to 283 ppm.

The object of the invention is to develop such a process with both low ammonia consumption as well as with changing gas compositions always lower final ammonia concentration of the treated exhaust gas. It is said to be particularly useful for removing nitrogen oxides from such oxygen-containing exhaust gases that arise in the production of nitric acid.

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The solution according to the invention is based on a selective reduction of nitrogen oxides by ammonia in two stages in the presence of catalysts which consist either of oxides of base metals or of platinum metals and are supported on a carrier, e.g. B. on aluminum oxide, are applied, wherein in the first stage for the purpose of nitrogen oxide reduction, the exhaust gases, the temperature of which is in the range of 200 to 450 ⁰ C, mixed with gaseous, possibly preheated ammonia and z. B. be repeatedly passed over a catalyst consisting of vanadium oxides and in the second stage the gases are brought into contact with a catalyst which either contains metals of the platinum group or from which the oxides of copper, chromium, manganese, iron and cobalt, chromites des Copper, manganese, iron and cobalt and mixtures of at least two of these substances-containing groups is selected, whereby ammonia residues are removed.

The ammonia gas must be preheated if the temperature of the gases should drop ^{below 200 ° C. after mixing with the exhaust gases.}

The process is thus carried out in two stages using two types of catalysts, in which first stage predominantly the reduction of nitrogen oxides by ammonia takes place, while in the second Stage mainly the reaction of the unreacted ammonia with the oxygen is realized. the technological parameters of the process divided in this way remain the same as those at the previously known processes.

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The advantage of selective catalytic. reduction the nitrogen oxides according to the invention are based on that as a result of the use of a highly selective catalyst, the economics of the process through Reduction of the ammonia requirement improves and at the same time the possibility of the formation of ammonia salts be reduced.

The process according to the invention enables considerable savings in ammonia. In the one mentioned above Example will be approximately 0.31 kg of ammonia per 1 000 NnP saved and the unfavorable influence of the fluctuations the ratio of the ammonia and nitrogen oxide content in the incoming gas mixture is eliminated.

Some exemplary embodiments are given below.

example 1

Exhaust gases from a nitric acid production plant, which contain 0.18 Vol Ji nitrogen oxides, of which 30 % NO ₂ , and 3.5 Vol Ji Og, the remainder being nitrogen, water vapor and inert gases, are ^{preheated to 2 ^ 0 0} C and with the to 240 ⁰ C preheated ammonia gas mixed. The ammonia gas is dosed so that its concentration in the resulting gas mixture is 0.21 vol- #.

The preheated gas mixture is then passed into the catalyst bed consisting of two layers.

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In the first layer 15 is a catalyst based on vanadium oxides of active alumina in the form of tablets, having a diameter of 6 mm - 25 mm long and are about 15 wt -.% Of vanadium oxides, on VpOcumgerechnet contain. The second layer contains a platinum catalyst which has 1 wt .- ^ Pt applied to active aluminum oxide and is in the form of 5 × 5 mm tablets.

The amounts of catalyst are in such a ratio that the space velocity in the first layer is 15,000 h " ¹ and in the second layer 45,000 h" ¹ . Under these reaction conditions, the concentration of nitrogen oxides in the first layer drops to 135 ppm and that of ammonia to 283 ppm. After passing through the second layer, the nitrogen oxide content drops to 100 ppm and the ammonia content drops to less than 10 ppm.

Example 2

A gas mixture of the same composition, which has been preheated to the same temperature as in Example 1, is passed into a catalytic bed consisting of two catalyst layers. In the first layer is again the vanadium oxide catalyst, the second layer forms a catalyst comprising activated by chromium (III) oxide, iron oxides and the Cr ₂ 0, content of about 7 wt -. <Fo is.

The volumes of the catalysts are in a ratio of 1: 1, and the space velocity is 15,000 h ~ " Under these technological conditions, the

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escaping gases 65 ppm nitrogen oxides and 20 ppm ammonia, with the content of both in the first layer reacting constituents practically drops to the same values as in Example 1.

Example 3

A gas mixture of the same composition and at the same temperature as in Example 1 is passed into a catalyst bed consisting of two layers. The first layer of the catalytic bed forms a vanadium oxide catalyst, while the second layer contains a mixture of copper oxides, chromium oxides and copper (II) -hromite (CuO-CrgO ·,) as a catalyst. The active substances, the content of which is approximately 15 % , are applied to active Al ₂ O in the form of 5x5 mm tablets. The volumes of the two catalysts are in a ratio of 1: 1, and the space velocity of the gas mixture in the individual layers is 15,000 hours

The analytically determined content of nitrogen oxides and ammonia in the exiting gases is 50 ppm Nitrogen oxides or 22 ppm ammonia. The content of nitrogen oxides and the ammonia in the gas mixture before the second layer is approximately the same as in Example 1.

Example 4

Under the same technological conditions of the selective reduction, as they are given in example 3, is the gas mixture over a catalytic bed, which

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consists of a combination of the vanadium oxide catalyst in the first layer and a manganese (IV) oxide catalyst in the second layer. The catalyst of the second layer is in the form of tablets, the dimensions of which are 5 x 5 mm, and contains 10 % MnOp on active AlpO. The volumes of the two catalysts in the individual layers and the space velocity of the gas mixture are the same as in Example 3. The gas mixture emerging behind the second layer contained approximately 40% ppm nitrogen oxides and less than 10 ppm ammonia.

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Claims (1)

- li - Claim Process for the removal of nitrogen oxides from oxygen-containing exhaust gases, in particular from the production of nitric acid, by selective reduction of nitrogen oxides by means of ammonia in the presence of catalysts arranged in two layers, which consist of oxides of base metals or metals of the platinum group, which are supported on a carrier, e.g. B. aluminum oxide, are applied, characterized in that in the first stage the 200 to 450 ⁰ C warm exhaust gas for the purpose of nitrogen oxide reduction with gaseous, possibly preheated ammonia is mixed and, for example repeatedly, passed over a catalyst consisting of vanadium oxides, and that in the second stage the gas to remove the remaining ammonia is brought into contact with a catalyst which either contains metals of the platinum group or from which the oxides of copper, chromium, manganese, iron and cobalt, chromites of copper, manganese, iron and Cobalt, as well as mixtures of at least two of these substances contained group is selected. 609848/0596