RU2070865C1 - Method of producing nitrogen oxides - Google Patents

Abstract

FIELD: production of nitrogen-containing substances. SUBSTANCE: nitrogen oxides are produced by molecular oxygen oxidation of ammonia on two-step catalyst at elevated temperature and pressure. Nitrous gases produced in the 1st step are combined with stream of gases containing oxygenated nitrogen compounds. In the 2nd step, molecular nitrogen from Nitrous gases is oxidized to nitrogen oxides on a catalyst based on refractory metals with additives. EFFECT: enhanced efficiency of process due to additional oxidation of molecular nitrogen from Nitrous gases. 6 cl, 2 tbl, 1 dwg

Description

 The invention relates to chemical technology, mainly the production of oxygen compounds of nitrogen, can be used in the chemical industry for the production of nitric acid, mineral fertilizers and other nitrogen compounds.

A known method of producing nitrogen oxides by oxidizing ammonia with atmospheric oxygen to nitric oxide (II) on catalysts at 800-900 ^o With and at elevated or atmospheric pressure; while the heat of the reaction of oxidation of ammonia is used to produce steam, and nitric oxide (II) is converted into nitric acid.

 The disadvantage of this method is the use of expensive ammonia for the production of nitric oxide II, the latter being oxidized on scarce and expensive platinum catalysts.

 The closest technical solution is the production of nitrogen oxides at elevated temperatures and pressures by oxidizing ammonia on a combined (two-stage) catalytic system consisting of one or more platinum chains in the 1st stage and the next layer of oxide catalyst (2nd stage) . In this method, up to 85-92% of the incoming ammonia is oxidized with atmospheric oxygen on the first catalyst bed (one or more platinoid networks) along the gas path. Unreacted ammonia in the presence of nitrogen (II) oxides is oxidized by the oxygen remaining in the nitrous gases on the second catalyst bed (non-platinum oxide catalyst). The total oxidation state of ammonia is the same as in the case of using only the platinum catalyst. The heat of the ammonia oxidation reaction is used to produce steam (Karavaev M.M. et al. Catalytic oxidation of ammonia. M. Chemistry, 1983, Ch. 5).

The disadvantage of the method of producing nitrogen oxides is the use of expensive ammonia as a raw material and a rather high investment and loss of platinum. In the prototype, the consumption coefficient for ammonia per 1 t of НN0 ₃ remains as high as in the well-known method discussed above, although the investments and losses of platinoids are reduced by 30-35%
The aim of the invention is to increase the efficiency of the process for producing nitrogen (II) oxides by catalytic oxidation of ammonia due to the additional oxidation of molecular nitrogen of nitrous gases to nitrogen oxides.

 This is achieved by the fact that in the method for producing nitrogen (II) oxides by oxidizing ammonia with atmospheric oxygen on a two-stage catalyst at elevated temperature and pressure, ammonia is oxidized in the first stage of the catalyst to produce nitrous gases consisting of nitrogen oxides, molecular nitrogen, oxygen, then before the second the oxidation step introduces into the nitrous gases a gas stream containing oxygen compounds of nitrogen, and molecular nitrogen is oxidized on the oxide catalyst to nitrogen oxides.

Newly introduced nitrogen compounds in the volume of the mixed gas are in amounts from 0.1 to 2.2 vol. The process of nitrogen oxidation is carried out on platinum or oxide catalysts at a temperature of from 900 to 450 ^o C and linear gas velocities from 0.5 to 5 m / s working.

 As an additionally introduced oxidizing agent, nitric acid vapors or mixtures thereof with nitrogen dioxide (tetravalent nitrogen) are used. The process of nitrogen oxidation itself can be carried out in the presence of a reducing agent, for example, ammonia in an amount of from 0.03 to 1.2 vol.

 The novelty and essential distinguishing features of the proposed invention are elements that are interconnected and in total determine the high efficiency of the proposed method, namely: the oxidation reaction of molecular nitrogen with its oxygen compounds is endothermic.

For example, during a reaction
2НN0 ₃ + N ₂ 3N0 + N0 ₂ + Н ₂ 0 335.0 kJ (1)
167.5 kJ are required per 1 mol of newly formed nitric oxide (actually per 1 g atom of oxidized nitrogen).

 It is proposed that the nitrogen oxidation reaction (1) be carried out due to the heat of the exothermic oxidation reaction of ammonia with atmospheric oxygen. Heat is transferred with nitrous gases.

Oxidation of nitrogen is carried out by oxygen compounds of nitrogen, in which it has a valency of N ⁺⁴ and higher: specifically at this stage, nitric acid vapors and mixtures thereof with nitrogen dioxide are used.

 Ammonia conversion is carried out on a complete set of platinum networks (ammonia content in nitrous gases is less than 0.03 vol.) Or on a part of the set when ammonia is oxidized by 85-92% (combined catalyst, see prototype).

 In the oxidation of only part of the ammonia, the process of nitrogen oxidation on the second catalyst bed is carried out not only in the presence of nitric oxide, but also an ammonia reducing agent.

 Known, for example, platinum catalysts are used as ammonia oxidation catalysts.

 Mesh platinum catalysts, as well as oxide catalysts based on heat-resistant and rare metals, for example, cobalt, nickel, iron and others, or mixtures thereof, are used as nitrogen oxidation catalysts.

The method of producing nitrogen oxides is carried out in a laboratory installation of quartz glass. The installation has systems for preparing two gas streams: a stream of ammonia-air mixture (ABC) and a stream of air saturated with HN0 ₃ and H ₂ 0 vapors (PCVS vapor-air-air mixture). These gas streams are introduced into a quartz glass reactor, in which a catalytic system consisting of two stages of catalyst layers is placed.

The resulting nitrous gases (n / gases) are processed in a system of absorption bottles, and the remaining gases (N ₂ , O ₂ , H ₂ O) are released into the atmosphere.

 The amount of gas is controlled by rheometers, the composition of the mixtures by analyzing them at the places of entry into the reactor and at the outlet of the reactor after each layer.

 The drawing shows a diagram of the process and selection of control analyzes.

 The set temperature is provided by external electric furnaces and controlled by thermocouples.

Example 1. A stream of air in an amount of 330 l / h is mixed with 38.7 l / h of ammonia. The resulting ABC is heated to 250 ^o With and served on a platinum catalyst (mesh). Before the catalyst, a gas sample was taken for the ammonia content (TA ₁ ), which was 10.5 vol. (see drawing).

On a platinum catalyst at 800 ^o With the conversion of ammonia with atmospheric oxygen. The process may be represented by a reaction
4NH ₃ + 5O ₂ 4NO + 6H ₂ O + 904 kJ (2)
Possible adverse reaction
4NH ₃ + 3O ₂ 2N ₂ + 6H ₂ O + 1268.8 kJ (3)
The degree of conversion to NO (2), determined analytically, (TA ₂ ) is 96.4%
The ammonia content in nitrous gases, calculated on the basis of kinetic data, was 0.35 vol. (taking into account reaction 3).

The second air stream (75 l / h) is fed into the saturator vessel, into which the nitric acid solution is poured. Here, under given conditions, it is saturated with vapors of nitric acid and water. The obtained PCVS contains 6.2 vol. HNO ₃ (analysis at point A ₃ ), i.e. based on air flow, the amount of HNO ₃ vapor was 5.0 l / h.

The PCVS stream having a temperature of 100 ^{° C.} is fed into the reactor in the space between the 1st and 2nd layers of the catalyst system. Here it is mixed with the 1st stream (nitrous gases) having a temperature of 800 ^o C.

The resulting gas mixture, having a temperature of 650-660 ^o With enters the oxide catalyst (heat-resistant oxide with additives). At the second stage of the catalyst, reaction (1) proceeds, the oxidation of molecular nitrogen by nitric acid vapors.

In parallel with reaction (1), adverse reactions can occur. The main one is the high-rate thermal decomposition reaction of НNO ₃ vapors.

4НNO ₃ = 4NO ₂ + 2Н ₂ O + O ₂ 191.0 kJ (4)
Reactions 1 and 4 are endothermic, so the temperature along the height of the catalyst layer decreased from 640 to 550 ^o C.

In nitrous gases at the exit from the layer (TA ₄ ), 11.6 vol. NO _x .

 The process is evaluated on the basis of the material balance performed on the computer, experience on three indicators.

Степень увеличения соединений азота в газе

Cтепень превращения НNO₃ по реакции (1)

где

объем NO после платиновидной сетки;

сумма объемов NO и NO₂ в выходящем из реактора газе.The degree of conversion of ammonia to nitric oxide

The degree of increase in nitrogen compounds in the gas

The degree of conversion of HNO ₃ according to reaction (1)

Where

the amount of NO after the platinum mesh;

the sum of the volumes of NO and NO ₂ in the gas leaving the reactor.

 The following examples were carried out on the laboratory setup described above, under the conditions reflected in Example 1.

 The table shows the indicators for examples 1-21.

 In the first series of experiments (op. 2-4), the test results are presented in the existing catalytic system (prototype) (1 platinum mesh + NK layer 4 cm high).

As can be seen from the table, the degree of conversion of ammonia to nitric oxide (2) is 94.6-96.0%, which is usual for the accepted conversion conditions, which are close to industrial ones. Since in these experiments no additional gas flow with an oxidizing agent is introduced, the values of the degree of conversion α correspond to the values of b _o .

Under the conditions described in example 1 (PT Rh Pd net + catalyst layer of heat-resistant oxides with additives), the effect of the concentration of an oxidizing agent (НNO ₃ vapor with N ₂ ) is shown in the 2nd series. The load and the technological mode of operation of the 1st stage were kept constant (see table). The change in the vapor content of HNO ₃ in the mixed gas is achieved by changing the amount of acid vapor with an additional stream, and the air load in this stream remains constant.

). В опытах 5-6 значения

превышают 100% Это связано с дополнительными реакциями окисления азота (кроме реакции 1), которые не учитывались в расчетах (содержание оксидов азота устанавливалось экспериментально, что не допускает ошибки).From the data of experiments 5-9, an increase in the degree of conversion of НNO ₃ by reaction (1) (

) In experiments 5-6 values

exceed 100% This is due to additional reactions of nitrogen oxidation (except reaction 1), which were not taken into account in the calculations (the content of nitrogen oxides was established experimentally, which does not allow errors).

 The third series of experiments relates to identifying the effect of gas velocity on process performance. The increase in speed is achieved by increasing the gas load in both streams (ABC and PCVS) and by changing the diameter of the apparatus (experiments 15,16).

 In this case, the process conditions on the 1st and 2nd catalyst layers were changed.

 The temperature across the catalyst beds is maintained almost constant.

снижаются).In this case (experiments 10-16), all process indicators deteriorate with increasing gas velocity (

are decreasing).

 In experiments 17-21, the ammonia content in front of the oxide catalyst layer changes. This is achieved by increasing the number of grids in the 1st stage, as well as the gas load. The temperature across the catalyst beds and the concentration of the oxidizing agent (in front of the oxide catalyst bed) are kept constant. As follows from the results of experiments 17-21, a decrease in the total yield of nitric oxide was observed at ammonia concentrations in the mixture of 1.7 vol.

An analysis of the experimental data presented shows that the process of nitrogen oxidation proceeds at temperatures lying outside the thermodynamic stability of НNO ₃ molecules. Therefore, in the mixture of gases in front of the catalyst, the oxidizing agent will always be present in the form of НNO ₃ vapors, its intermediate decomposition products (ОН, NO ₃ ), and NO ₂ molecules (final decomposition product).

 The amount of nitric acid vapor introduced is limited by the energy capabilities of nitrous gases and the conditions for preparing an additionally introduced gas stream with an oxidizing agent.

In particular, nitrous gases can be cooled from an ammonia conversion temperature of up to 400-450 ^o С. The released energy is spent mainly on the endothermic process of nitrogen oxidation.

The proposed method for producing nitrogen oxides can be primarily used in the production of nitric acid, while the consumption coefficient for ammonia (due to the reaction of nitrogen oxidation) is reduced in schemes with conversion under atmospheric pressure by 40-50 kg / t HNO ₃ , and in the schemes pressure 100-110 kg / t НNO ₃ .

Claims (6)

 1. A method of producing nitrogen oxides by catalytic oxidation of ammonia by atmospheric oxygen on a two-stage catalyst at elevated temperature and pressure, characterized in that a gas stream containing oxygen compounds of nitrogen is introduced into the nitrous gases obtained during the oxidation of ammonia at the first stage of the catalyst and they oxidize molecular nitrogen nitrous gases in the second stage of the catalyst.  2. The method according to p. 1, characterized in that the concentration of the introduced oxygen compounds of nitrogen in the mixture obtained by mixing with nitrous gases is maintained at a level of 0.1 to 2.2 vol. 3. The method according to PP. 1 and 2, characterized in that the process of nitrogen oxidation is carried out at 900 450 ^o C and linear gas velocities of 0.8 to 5.0 m / s rus.  4. The method according to PP.1 to 3, characterized in that as the oxygen compounds of nitrogen, nitric acid vapors or a mixture thereof with nitrogen oxides are used.  5. The method according to claims 1 to 4, characterized in that platinum, its alloys or oxides of heat-resistant metals with additives are used as a catalyst for the oxidation of nitrogen.  6. The method according to PP.1 to 5, characterized in that the process of nitrogen oxidation is carried out in the presence of a reducing agent, for example ammonia, in an amount of up to 1.2 vol.

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