CS196549B1 - Process for selective catalytic reduction of nitrogen oxides from waste gases - Google Patents

Abstract

In the removal of N oxides from residual gases, esp. from the mfr. of nitric acid by selective redn. of the N oxides with NH3 in the presence of catalysts arranged in two layers and comprising base metal oxides and oxides of the Pt gp metals, deposited on a support e.g. of Al2O3 is characterised in that as the first step, the residual gas, heated to 200-450 degrees, is mixed with opt. preheated NH3 gas and, e.g. with several recycling steps, the mixt. is passed over a catalyst of V oxides, then to remove the residual NH3, over a catalyst contg. Pt gp. metals or chosen from oxides of Cu, Cr, Mn,Fe and Co, and chromites of Cu, Mn, Fe and/or Co, and their mixtures. The consumption of NH3 in the reaction is considerably less than that of e.g. natural gas, Co, hydrocarbons, etc. used during the total redn. The catalysts are highly selective in operation.

Description

The invention also relates to a process for the selective catalytic reduction of nitrogen oxides fed off with waste gases, particularly in the tail gases of nitric acid production.

Nitrogen oxides are present as a toxic flue gas in virtually all the exhaust gases of the combustion processes, in the exhaust gases of internal combustion engines and in the exhaust gases of various processes. The largest source of waste nitrogen oxides in the chemical industry is the nitrogen gases from nitric acid production. The most effective way of destroying oxides in these gases is by catalytic reduction to elemental nitrogen. One of the alternatives to this method is the so-called selective reduction method, in which ammonia is used as the reducing agent, which reacts with nitrogen oxides, and to a small extent with oxygen, in gases, under appropriate reaction conditions and on a suitable catalyst. .

Since the content of nitrogen oxides in the waste gases is of a lower order, both the acid content and the consumption of ammonia are of an order of magnitude less than the consumption of other reducing agents, e.g. , o, hydrocarbons or mixtures thereof, used in total or total, in which said components react first with oxygen and then with oxygen (e.g. U.S. Pat. 2,970,034, British Pat. 871,755, French Pat. 1,205,311, etc.).

198 549

The selective reduction process can therefore be economically more advantageous despite the relatively low cost of ammonia. It will be appreciated, however, that the economic performance of the process depends substantially on the amount of ammonia required to reduce the nitrogen oxides.

Suitable catalysts for selective reductions are proposed on the one hand

Platinum Metal I (U.S. Pat. Nos. 2,975,025, 3,053,613) and catalysts containing base metal oxides such as Co, Ni or Fe oxides (U.S. Pat. Nos. 3,008,796, 3,053,613). V, Mo and W (U.S. Pat. No. 3,279,184); ‘Mo, V, Mn, and Fe, mixtures or chromites thereof (NSC Pet No. 1,253 * 685) or metal oxides VI. to VIII. subgroups of the periodic system, especially Fe and Cr (NSR Pat. No. 1,259> 298). CuO (U.S. 3,559,427) or Pd-doped CuO (U.S. Pat. No. 3,449,063) has also been proposed as a catalyst to selectively reduce nitrogen oxides in automobile exhaust.

_{However, in} order to achieve a certain degree of removal of nitrogen oxides from the waste gases, due to the incomplete selectivity of the catalysts used, ammonia must be metered in a certain excess over its atechiomatically required amount, typically an excess of 20 to 50% nitrogen oxides according to the following equations:

NO + 4 NH ₃ »5 N ₂ ♦ 6 H ₂ 0

NO ₂ + 8 NH-J 7 N ₂ + 12 H ₂ O

Excess ammonia is not used in the reduction process as it is burned by side reaction with oxygen to form nitrogen ^:

NH ₃ * 3 0 ₂ = 2 N ₂ + 6 H ₂ 0

The selectivity of the catalyst used thus determines the economy of the process.

This disadvantage can be avoided and the savings on ammonia are achieved by using highly selective catalysts in which the undesired reaction of ammonia with oxygen is suppressed to the maximum extent possible. Common fluctuations in the content of nitrogen oxides in the tail gases and a change in their degree of prooxidation (the degree of pro-acidification means the ratio * of nitrogen dioxide to total nitrogen oxides) · dependent on technological parameters of nitric acid The amount of reduced nitrogen oxides is reduced, and if a certain value of this ratio is exceeded, depending on the type of catalyst used, the unregulated ammonia is then passed through the reactor. For example, when the concentration of nitrogen oxides in waste gases decreases from 0.2 vol% to 0.18 vol% and at the same time the degree of prooxidation changes from 40% to 30%

With a content of 0.21% by volume of ammonia in the gannet gas entering the vanadium catalyst, the ammonia concentration in the gases after reduction was increased from otherwise 30 ppm to 283 ppm under otherwise identical reaction conditions. However, it is desirable from an operational point of view; after the reduction the ammonia-free exhaust gas or? at lower temperatures, its reaction with residual nitrogen oxides could form ammonium salts, which would settle in the cold places of the plant.

The measurement of the nitrogen oxide content of the waste gases does not cause considerable difficulties when the process is carried out using less selective catalysts, in which case, from a stoichiometric point of view, the excess ammonia is burned with oxygen over a wide range of excess ammonia and outlet gases ammonia-free. However, due to the lower selectivity, ammonia consumption is considerably higher. For example, when performing the process of selective reduction of nitrogen oxides under pouužtí platinum catalyst containing .1% Pt deposited on active alumina, to achieve better than 90 # -ního remove oxides ^{ears d} u ^(T o. .About ^40% as N0 ^g ) - during simple ^t orov ^s rate of 3 ° ^{0 0} .C and the preheating temperature of the feed mixture to 240 ° C - requires almost 34% excess of ammonia over the stoichiometrically required π ^ ^ Αΐνί, as for example in vanadium catalyst suffices only 12.5% excess ammonia.

·. \

Considering the re-advantages of both mentioned solutions. a way was sought to combine these disadvantages at least substantially.

The aforementioned disadvantages have no method of catalytic reduction of nitrogen oxides contained in waste gases containing at the same time tysik by ammonia in the presence of a combination of catalysts based on platinum metals or oxides of base metals,. The invention is characterized in that the mixture of waste gases and ammonia is first contacted with a highly selective catalyst based on vanadium oxides and then with a catalyst containing platinum group metals and / or oxides. chromium, manganese, iron and coma, chromates of mixtures, manganese, iron and cobalt, or mixtures of at least two of these substances.

In the process invention, Poole SooíIí considerable saving in ammonia. (In ^{the example,} above-mentioned ^No. 'ora ^p III US AMO. ^Ia při.toižrté at ^0, 3 ¹ kg per ¹⁰⁰⁰ NRN terminal ^P ^ ^p l ^y nu), and The adverse effect of variations in the relative proportions of ammonia and nitrogen oxides in the feed gas mixture is subtracted.

Example 1

Waste gases from nitric acid production, containing 0,18 ·% of nitrogen oxides,

186 М9 of which 30% veformě N0 _2> and 3.5 vol% · oxygen ytek from nitrogen, steam and inerts, were not preheated temperature of 24 O C and mixed в ammonia preheated straighter; to the temperature of 24 ° C · Ammonia was dosed in such a quantity that its concentration in the resulting gas mixture was 0.21 vol ·% ·

The preheated gas mixture was then placed on a catalytic bed formed of two catalyst layers. The first layer consisted of a catalyst based on vanadium oxides and active alumina in the form of extrudates having a diameter of 6 mm and a length of 15-25 mm with a vanadium oxide content of approximately 15 wt. % VgO4. In the second layer was a platinum catalyst containing 1 wt. % Pt, coated on active alumina in the form of 5x5 mm tablets ·

The catalyst volumes were in such a ratio that the space velocity in the first layer was 15,000 h @ -1 and in the second layer 45,000 h @ ^-1 . Under these reaction conditions, the nitrogen oxide concentration in the first layer decreased to 135 ppm, ammonia concentration to 283 ppm. After passing through the second layer the nitrogen oxide content dropped to 100 ppm and the ammonia content dropped to less than 10 ppm ·

Example 2

A gaseous mixture of the same composition and under the same preheating as in Example 1 was introduced on a catalytic bed consisting of two catalyst layers. In the first layer the catalyst was again vanadium, the second layer consisted of a catalyst containing iron oxides activated by chromium trioxide; the content was approximately 7 wt. % ·

The catalyst volumes were 1: 1 and the space velocity was 15.000 h ¹ . Under these process conditions, the exhaust gases contained 65 ppm of nitrogen oxides and 20 ppm of ammonia, and after passing through the first layer the content of both reactants dropped to virtually the same values as in Example 1. The total temperature increase of the gas mixture after passing through the reactor was approximately 25 ° C.

Claims (1)

SUBJECT OF THE INVENTION Process for the selective catalytic reduction of nitrogen oxides contained in waste gases containing oxygen at the same time by ammonia in the presence of a combination of catalysts based on platinum metals or base metal oxides supported on a support, for example alumina, characterized in first in contact with a highly selective vanadium oxide catalyst and then with a catalyst containing platinum groups and / or copper, chromium, manganese, iron and cobalt oxides, copper, manganese, iron and cobalt chromates or mixtures thereof with at least one of these substances ·