Nitric acid is one of the most popular types of industrial raw materials. Its production can be carried out by different methods - depending on in which variety the acid should be supplied to the customer. What is the essence of relevant technologies? How do they relate to the type of nitric acid produced at the factory?
Industrial production of nitric acid: a history of technological development
First of all, it will be useful to study historical facts about how the production of nitric acid developed in Russia. In domestic industry, the release of this substance, if we follow the available information, was initiated under Peter I. Subsequently, M.V. Lomonosov proposed a method for the production of nitric acid from nitrate - this substance was actually considered the main raw material for the release of the substance in question until the beginning of the 20th century.
Along with nitrate, the production of nitric acid in industry was carried out using sulfuric acid. The two substances under consideration, interacting with each other, formed nitric acid and sulfuric oxide of sodium. The advantage of this method was the ability to obtain nitric acid in a concentration of the order of 96-98% (subject to the use of raw materials of the required quality).
The corresponding technology has been quite actively improved - in favor of increasing the processing speed of raw materials and ensuring the yield of a larger volume of product. But gradually, it gave way to the concept that nitric acid was produced through contact oxidation of ammonia.
A method was also invented in which nitric oxide, obtained by the arc oxidation of the corresponding gas from the atmosphere, was used as the main raw material for acid production. There is a widespread view that the first method is more economically viable.
With the improvement of technological approaches to the production of nitric acid, an approach has been formed according to which the most optimal option for the production of the corresponding substance is the use of industrial infrastructure operating under high pressure. An alternative is the production of acid at atmospheric pressure, which is considered less cost-effective from an economic point of view.
The release of a substance under normal or elevated pressure involves the production of nitric acid from ammonia. There is a combined method that combines the advantages of the other two. Features of the production of nitric acid by the combined method are, firstly, in the oxidation of ammonia at atmospheric pressure, and the implementation of its absorption at high.
Ammonia is now considered the main raw material for the release of the substance in question, along with water and atmospheric air. We study the specifics of their application in the production of acid in more detail.
Raw materials for the release of nitric acid
So, the main raw materials used in the production of the substance in question are ammonia, air, and also water.
This requires the use of ammonia in purified form. To do this, it is cleaned using special evaporation and distillation equipment within various production cycles. Similarly, clean air should be used to release nitric acid. It is also filtered using special equipment. In turn, the water that is used in the release of nitric acid is purified from impurities and salts. In many cases, pure condensate must be used to obtain the substance in question.
We will study in which varieties the substance in question can be represented, as well as how each type of nitric acid is produced.
Varieties of nitric acid and the main stages of its release
There are 2 varieties of nitric acid produced in modern industrial enterprises - diluted and concentrated. The production of diluted nitric acid is carried out in 3 main stages:
- ammonia conversion (its final product is nitric oxide);
- obtaining nitrogen dioxide ;
- the implementation of the absorption of nitrogen oxides when using water.
At modern industrial enterprises, the production of diluted nitric acid in the framework of the so-called AK-72 scheme is widespread. But there are, of course, other technologies for the release of this substance.
In turn, the production of concentrated nitric acid can be carried out by increasing the level of the corresponding substance in dilute form or by direct synthesis. The first method, as a rule, allows to obtain an acid solution in a concentration of about 68%, which may not be enough for the use of the substance in question in a number of areas. Therefore, the direct synthesis method is also widespread, allowing one to obtain a substance in a concentration of the order of 97-98%.
Let us consider in more detail how nitric acid is produced in a particular variety. Above, we noted that the release of the diluted substance can be carried out according to the scheme AK-72. Let us first examine its specifics.
Production of dilute acid using AK-72 technology
The scheme under consideration, through which the production of nitric acid is carried out, involves the use of a closed cycle, accompanied by:
- ammonia conversion;
- cooling of associated gases at a pressure of about 0.42-0.47 MPa:
- the implementation of the absorption of oxides under pressure of the order of 1.1-1.26 MPa.
The final product of the AK-72 scheme is nitric acid at a concentration of about 60%. The production of nitric acid in the framework of the technology under consideration is carried out in the framework of such stages as:
- ensuring the flow of air from the atmosphere into the industrial unit and the implementation of its purification;
- air compression, its separation into technological flows;
- the implementation of the evaporation of ammonia, purification of the corresponding gas from oil and other impurities, as well as its subsequent heating;
- mixing purified ammonia and air, subsequent purification of the mixture and its transfer to the catalyst;
- production of nitrous gases and their cooling;
- condensate collection with nitric acid;
- concentration and absorption of nitric acid;
- cooling and purification of the resulting product.
Finished acid is sent to the store or to the customer.
Along with the technology under consideration, which produces nitric acid - AK-72, another popular concept for the release of the corresponding substance is used, which involves ensuring the functioning of the industrial infrastructure under pressure of about 0.7 MPa. Consider its features.
Technology of product release under pressure of 0.7 MPa: nuances
Using the technology in question, non-concentrated nitric acid is produced - as an alternative to the AK-72 concept. It involves the implementation of the following stages of release of the substance in question.
First of all, as in the previous technology, atmospheric air is purified. For this, as a rule, a two-stage filter is used. Further, the air that has been cleaned is compressed by means of an air compressor - up to approximately 0.35 MPa. At the same time, the air heats up to a temperature of about 175 degrees, and it must be cooled. After this problem is solved, it is sent to the area of additional compression, in which its pressure increases to approximately 0.716 MPa. The forming air stream, in turn, is heated to a high temperature - about 270 degrees through exposure to nitrotase gases. After it is mixed with ammonia in a special area of the industrial unit. The corresponding substance is involved in the release of acid initially in a gaseous state, which is formed due to evaporation of the liquid. In addition, ammonia must be purified. After preparation, the gas is heated and fed into the mixer simultaneously with air. This mixture is also filtered and after purification is fed to the conversion of ammonia. The corresponding procedure is carried out using platinum and rhodium alloy meshes at a very high temperature of about 900 degrees. The conversion level is about 96%.
The production of weak nitric acid by the technology under consideration involves the formation of nitrous gases. They are displayed in a special area of the industrial unit in which they are cooled. Due to this, the evaporation of purified water and the appearance of steam with high pressure. Conducted through the corresponding region of the industrial unit, nitrous gases are transferred to the oxidizing agent. It should be noted that their oxidation partially occurs already at the previous stage of acid production. But in the oxidizing agent, it becomes even more intense. In this case, nitrous gases are heated to a temperature of about 335 degrees. Subsequently, they are cooled in a special heat exchanger, and then sent to a condenser.
After this, nitric acid is formed in a weak concentration. It is necessary to separate the remaining nitrous gases from it - a separator is used for this. From it, nitric acid is fed into the absorption region of an industrial unit. Acid subsequently flows into areas of the apparatus located below. In this case, it interacts with nitrogen oxides, as a result of which its concentration increases. At the output, it is about 55-58%. As a rule, it contains dissolved oxides, which must be removed: for this, the substance is sent to the purge region of the unit. Using heated air, oxides are extracted from the acid. The finished product is placed in stock or shipped to the customer.
Concentrated Acid Production: Direct Synthesis
Having considered how the production of diluted nitric acid is carried out, we study the specifics of the release of concentrated matter. Among the most economically viable technologies involved in enterprises of the corresponding profile is the production of acid through direct synthesis from raw materials in the form of nitrogen oxides.
The essence of this method is to stimulate a chemical reaction between the specified substance, water, and oxygen under a pressure of about 5 MPa. The technology by which the production of concentrated nitric acid based on dilute is carried out has a nuance: ensuring the transition of nitrogen dioxide into liquid form is possible at a pressure and temperature close to atmospheric. However, in some reactions, the concentration of the corresponding substance is insufficient for its translation into a liquid state at ordinary pressure, and it has to be increased.
Diluted Concentrated Acid Production
In this case, it is intended to concentrate the acid by means of absorbent substances such as sulfuric, phosphoric acid, various solutions of nitrates. The main stages of the production of concentrated nitric acid based on dilute sulfuric acid are as follows.
First, the raw material is divided into 2 streams: the first is fed to the evaporator, the second goes to the cold region of the industrial unit. Sulfuric acid is fed into the area of the apparatus located above the second diluted nitric acid stream. In the lower part of the unit, in turn, steam is supplied, which heats the mixture used, as a result of which nitric acid evaporates from it. Her pairs rise up the apparatus, and then displayed in the refrigerator. There, acid fumes condense - until its concentration reaches 98-99%.
At the same time, some of the nitrogen oxides present in this production stage are absorbed by acid. They must be removed from the product: most often nitric acid vapors are used for this purpose, which are sent to the condenser. The extracted nitrogen oxides, as well as acid fumes that did not form condensate, are sent to another area of the apparatus - for absorption, where they are treated with water. As a result, a dilute acid is formed, which is again fed to condensation and cooling. The finished product is sent to the warehouse or customer.
Features of concentration using sulfuric acid
The main task that characterizes the production of nitric acid in industry is the implementation of its cost-effective and efficient concentration. There are several approaches to developing an optimal solution scheme. It will be useful to consider those that are among the most common.
We noted above that sulfuric acid can be used to concentrate the substance in question. There is a fairly common way to increase the effectiveness of its use - a preliminary increase in the concentration of nitric acid through evaporation. It is optimal that before treatment with sulfuric acid, the corresponding substance will have a concentration of about 59-60%. It should be noted that in practice this technology for the production of nitric acid is characterized by a low level of environmental friendliness. Therefore, as an alternative to the use of sulfuric acid, the use of nitrates is widespread. We study their specifics in more detail.
Nitrate Concentration
Most often, to solve the problem under consideration, which characterizes the production of nitric acid from ammonia, magnesium nitrates or zinc are used. First of all, it is significantly more environmentally friendly than the method involving the use of sulfuric acid. In addition, this technology allows to ensure the highest quality of nitric acid as a final product.
However, it has a number of disadvantages, which determine the appearance of difficulties in its widespread use. First of all, this is a rather high cost of the production process. In addition, this technology in many cases involves the appearance of solid waste, the processing of which can be complicated.
The use of catalysts in the production of nitric acid
It will be useful to consider what other substances are used along with the main raw materials for the release of the product in question in the industrial (most often considered as such catalysts) production of nitric acid. The use of the substances in question is due to the need to increase the profitability of acid production, to increase the dynamics of its output at the industrial unit.
The main requirement for a catalyst in the production of the product in question is selectivity of action. That is, it should act on the main chemical reaction without affecting side processes. Most often, catalysts are used in the production of acid, in which platinum is present.
We noted above that when releasing a weakly concentrated substance at elevated pressure, catalysts based on platinum and rhodium are used. In some cases, alloys with the addition of palladium are also used. But the main metal in them is platinum, its content is usually not less than 81%. The essence of the catalyst in this case is to stimulate the rapid passage of the main chemical reaction. As a rule, it takes place within the external diffusion region.
The process depends on the diffusion limits of oxygen relative to the surface of the catalyst. This feature causes a higher concentration of ammonia - the main raw material for the release of nitric acid, on the surface of one or sometimes a catalyst, if we compare the corresponding indicator with the concentration in air. It is possible to increase the specific gravity of various adverse reactions in which incomplete oxidation and the formation of nitrogen or its oxide are observed. In this regard, the oxygen at the surface must be in an amount sufficient to displace ammonia. In this case, it will be possible to achieve a sufficiently deep oxidation.
It can be noted that, along with platinum catalysts, combined substances are also involved in the production of nitric acid. In particular, iron chromium. They can significantly reduce the economic costs that characterize the release of the substance in question.
So, we examined the methods for the production of nitric acid, identified its main types. How many stages of production of nitric acid need to be implemented depends on its variety, as well as on the specific technology for the release of the corresponding substance. It will now be useful to consider what difficulties characterize the industrial production of this product, which is in demand in many areas of the economy.
The main problems in the production of nitric acid
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Another problem characterizing the production of nitric acid is environmental. We noted above that sulfuric acid can be used at a concentration of raw materials, and harmful substances are formed upon passing through the corresponding production cycle. An alternative in this case may be the use of nitrates - but this, again, implies an increase in economic costs. However, the environmental factor for modern manufacturers today is as significant as the level of profitability of production.