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Ministry of Education and Science of the Russian Federation

State educational institution

Higher professional education

"Tver State Technical University"

Department of TPM

Course work

in the discipline: “General chemical technology”

Production of ammonium nitrate

Content

Introduction

2. Production methods

3. The main stages of the production of ammonium nitrate from ammonia and nitric acid

3.1 Preparation of ammonium nitrate solutions

3.1.1 Basics of the neutralization process

3. 1 5 Main equipment

4. Material and energy calculations

5. Thermodynamic calculation

6. Recycling and neutralization of waste in the production of ammonium nitrate

Conclusion

List of sources used

Appendix A

Introduction

In nature and in human life, nitrogen is extremely important. It is part of protein compounds (16-18%), which are the basis of the plant and animal world. A person consumes 80-100 g of protein daily, which corresponds to 12-17 g of nitrogen.

For normal plant development, many chemical elements are required. The main ones are carbon, oxygen, hydrogen, nitrogen, phosphorus, magnesium, sulfur, calcium, potassium and iron. The first three elements of a plant are obtained from air and water, the rest are extracted from the soil.

Nitrogen plays a particularly important role in the mineral nutrition of plants, although its average content in plant mass does not exceed 1.5%. Without nitrogen, no plant can live or develop normally.

Nitrogen is a component not only of plant proteins, but also of chlorophyll, with the help of which plants, under the influence of solar energy, absorb carbon from carbon dioxide CO2 in the atmosphere.

Natural nitrogen compounds are formed as a result of chemical processes of decomposition of organic residues, during lightning discharges, as well as biochemically as a result of the activity of special bacteria - Azotobacter, which directly absorb nitrogen from the air. The same ability is possessed by nodule bacteria that live in the roots of leguminous plants (peas, alfalfa, beans, clover, etc.).

A significant amount of nitrogen and other nutrients necessary for the development of agricultural crops is annually removed from the soil with the resulting harvest. In addition, some nutrients are lost as a result of their leaching by groundwater and rainwater. Therefore, to prevent a decrease in yield and depletion of the soil, it is necessary to replenish it with nutrients by applying various types of fertilizers.

It is known that almost every fertilizer has physiological acidity or alkalinity. Depending on this, it can have an acidifying or alkalizing effect on the soil, which is taken into account when using it for certain agricultural crops.

Fertilizers, the alkaline cations of which are more quickly extracted by plants from the soil, cause acidification; Plants that consume acidic anions from fertilizers more quickly contribute to soil alkalization.

Nitrogen fertilizers containing the ammonium cation NH4 (ammonium nitrate, ammonium sulfate) and the amide group NH2 (urea) acidify the soil. The acidifying effect of ammonium nitrate is weaker than ammonium sulfate.

Depending on the nature of the soil, climatic and other conditions, different amounts of nitrogen are required for different crops.

Ammonium nitrate (ammonium nitrate, or ammonium nitrate) occupies a significant place in the range of nitrogen fertilizers, the global production of which amounts to millions of tons per year.

Currently, approximately 50% of nitrogen fertilizers used in agriculture in our country are ammonium nitrate.

Ammonium nitrate has a number of advantages over other nitrogen fertilizers. It contains 34-34.5% nitrogen and in this respect is second only to urea CO(NH2) 2, containing 46% nitrogen. Other nitrogen and nitrogen-containing fertilizers have significantly less nitrogen (nitrogen content is given in terms of dry matter):

Table 1 - Nitrogen content in compounds

Ammonium nitrate is a universal nitrogen fertilizer, as it simultaneously contains ammonium and nitrate forms of nitrogen. It is effective in all zones, for almost all crops.

It is very important that the nitrogen forms of ammonium nitrate are used by plants at different times. Ammonium nitrogen, directly involved in protein synthesis, is quickly absorbed by plants during the growth period; Nitrate nitrogen is absorbed relatively slowly, so it lasts longer. It has also been established that the ammonia form of nitrogen can be used by plants without prior oxidation.

These properties of ammonium nitrate have a very positive effect on increasing the yield of almost all agricultural crops.

The high nitrogen content in ammonium nitrate, the relatively simple method of its production and the relatively low cost per unit of nitrogen create good preconditions for the further development of this production.

Ammonium nitrate is part of a large group of stable explosives. Explosives based on ammonium nitrate and ammonium nitrate, pure or treated with certain additives, are used for blasting operations.

A small amount of saltpeter is used to produce nitrous oxide, used in medicine.

Along with increasing the volume of ammonium nitrate production by modernizing existing production facilities and constructing new ones, measures are being taken to further improve the quality of the finished product (obtaining a 100% friable product and preserving granules after long-term storage of the product).

1. Physico-chemical properties of ammonium nitrate

In its pure form, ammonium nitrate is a white crystalline substance containing 35% nitrogen, 60% oxygen and 5% hydrogen. The technical product is white with a yellowish tint and contains at least 34.2% nitrogen.

Ammonium nitrate is a strong oxidizing agent for a number of inorganic and organic compounds. It reacts violently with the melts of some substances, even to the point of explosion (for example, with sodium nitrite NaNO2).

If gaseous ammonia is passed over solid ammonium nitrate, a very mobile liquid is quickly formed - ammonia 2NH4NO32NH3 or NH4NO33NH3.

Ammonium nitrate is highly soluble in water, ethyl and methyl alcohols, pyridine, acetone and liquid ammonia. With increasing temperature, the solubility of ammonium nitrate increases significantly.

When ammonium nitrate is dissolved in water, a large amount of heat is absorbed. For example, when 1 mole of crystalline NH4NO3 is dissolved in 220-400 moles of water and a temperature of 10-15 °C, 6.4 kcal of heat is absorbed.

Ammonium nitrate has the ability to sublimate. When ammonium nitrate is stored under conditions of elevated temperature and air humidity, its volume approximately doubles, which usually leads to rupture of the container.

Under a microscope, pores and cracks are clearly visible on the surface of ammonium nitrate granules. The increased porosity of nitrate granules has a very negative effect on the physical properties of the finished product.

Ammonium nitrate is highly hygroscopic. In the open air, in a thin layer of saltpeter, it quickly becomes moistened, loses its crystalline shape and begins to blur. The degree to which salt absorbs moisture from the air depends on its humidity and the vapor pressure above a saturated solution of a given salt at a given temperature.

Moisture exchange occurs between air and hygroscopic salt. Relative air humidity has a decisive influence on this process.

Calcium and lime-ammonium nitrate have a relatively low water vapor pressure over saturated solutions; at a certain temperature they correspond to the lowest relative humidity. These are the most hygroscopic salts among the above nitrogen fertilizers. Ammonium sulfate is the least hygroscopic and potassium nitrate is almost completely non-hygroscopic.

Moisture is absorbed only by a relatively small layer of salt immediately adjacent to the surrounding air. However, even such moistening of saltpeter greatly deteriorates the physical properties of the finished product. The rate at which ammonium nitrate absorbs moisture from the air increases sharply with increasing temperature. Thus, at 40 °C the rate of moisture absorption is 2.6 times higher than at 23 °C.

Many methods have been proposed to reduce the hygroscopicity of ammonium nitrate. One such method is based on mixing or fusing ammonium nitrate with another salt. When choosing a second salt, proceed from the following rule: to reduce hygroscopicity, the pressure of water vapor above a saturated solution of a mixture of salts must be greater than their pressure above a saturated solution of pure ammonium nitrate.

It has been established that the hygroscopicity of a mixture of two salts having a common ion is greater than the most hygroscopic of them (the exception is mixtures or alloys of ammonium nitrate with ammonium sulfate and some others). Mixing ammonium nitrate with non-hygroscopic but water-insoluble substances (for example, limestone dust, phosphate rock, dicalcium phosphate, etc.) does not reduce its hygroscopicity. Numerous experiments have shown that all salts that have the same or greater solubility in water than ammonium nitrate have the property of increasing its hygroscopicity.

Salts that can reduce the hygroscopicity of ammonium nitrate must be added in large quantities (for example, potassium sulfate, potassium chloride, diammonium phosphate), which sharply reduces the nitrogen content in the product.

The most effective way to reduce the absorption of moisture from the air is to coat the nitrate particles with protective films of organic substances that are not wetted by water. The protective film reduces the rate of moisture absorption by 3-5 times and helps improve the physical properties of ammonium nitrate.

A negative property of ammonium nitrate is its ability to cake - to lose its flowability (crumbly) during storage. In this case, ammonium nitrate turns into a solid monolithic mass, difficult to grind. Caking of ammonium nitrate is caused by many reasons.

Increased moisture content in the finished product. Particles of ammonium nitrate of any shape always contain moisture in the form of a saturated (mother) solution. The NH4NO3 content in such a solution corresponds to the solubility of the salt at the temperatures at which it is loaded into the container. As the finished product cools, the mother liquor often becomes supersaturated. With a further decrease in temperature, a large number of crystals with sizes of 0.2-0.3 mm fall out of the supersaturated solution. These new crystals cement the previously unbound particles of nitrate, causing it to turn into a dense mass.

Low mechanical strength of saltpeter particles. Ammonium nitrate is produced in the form of round-shaped particles (granules), plates or small crystals. Particles of granular ammonium nitrate have a smaller specific surface area and a more regular shape than flake and fine-crystalline ones, so the granules cake less. However, during the granulation process, a certain amount of hollow particles are formed, which have low mechanical strength.

When storing, bags with granulated saltpeter are placed in stacks 2.5 m high. Under the pressure of the upper bags, the least durable granules are destroyed with the formation of dust-like particles, which compact the mass of saltpeter, increasing its caking. Practice shows that the destruction of hollow particles in a layer of granular product sharply accelerates the process of caking. This is observed even if, when loaded into the container, the product was cooled to 45 °C and the bulk of the granules had good mechanical strength. It has been established that hollow granules are also destroyed due to recrystallization.

As the ambient temperature increases, saltpeter granules almost completely lose their strength, and such a product cakes heavily.

Thermal decomposition of ammonium nitrate. Explosion hazard. Fire resistance. From the point of view of explosion safety, ammonium nitrate is relatively little sensitive to shocks, friction, impacts, and remains stable when hit by sparks of varying intensity. Admixtures of sand, glass and metal impurities do not increase the sensitivity of ammonium nitrate to mechanical stress. It is capable of exploding only under the influence of a strong detonator or during thermal decomposition under certain conditions.

With prolonged heating, ammonium nitrate gradually decomposes into ammonia and nitric acid:

NH4NO3=NH3+HNO3 - 174598.32 J (1)

This process, which occurs with heat absorption, begins at temperatures above 110°C.

With further heating, ammonium nitrate decomposes to form nitrous oxide and water:

NH4NO3= N2O + 2H2O + 36902.88 J (2)

The thermal decomposition of ammonium nitrate occurs in the following successive stages:

· hydrolysis (or dissociation) of NH4NO3 molecules;

· thermal decomposition of nitric acid formed during hydrolysis;

· interaction of nitrogen dioxide and ammonia formed in the first two stages.

When ammonium nitrate is intensively heated to 220--240 °C, its decomposition may be accompanied by outbreaks of a molten mass.

Heating ammonium nitrate in a closed volume or in a volume with a limited release of gases formed during the thermal decomposition of nitrate is very dangerous.

In these cases, the decomposition of ammonium nitrate can proceed through many reactions, in particular, through the following:

NH4NO3 = N2+2H2O + S 02 + 1401.64 J/kg (3)

2NH4NO3 = N2 + 2NO+ 4H20 + 359.82 J/kg (4)

3NH4NO3= 2N2 + N0 + N02 + 6H20 + 966.50 J/kg (5)

From the above reactions it is clear that ammonia, formed during the initial period of thermal decomposition of nitrate, is often absent in gas mixtures; Secondary reactions take place in them, during which ammonia is completely oxidized to elemental nitrogen. As a result of secondary reactions, the pressure of the gas mixture in a closed volume sharply increases and the decomposition process can end in an explosion.

Copper, sulfides, magnesium, pyrites and some other impurities activate the decomposition process of ammonium nitrate when it is heated. As a result of the interaction of these substances with heated nitrate, unstable ammonium nitrite is formed, which at 70-80 ° C rapidly decomposes with an explosion:

NH4NO3=N2+ 2H20 (6)

Ammonium nitrate does not react with iron, tin and aluminum even in a molten state.

With increasing humidity and increasing particle size of ammonium nitrate, its sensitivity to explosion greatly decreases. In the presence of approximately 3% moisture, saltpeter becomes insensitive to explosion even when exposed to a strong detonator.

The thermal decomposition of ammonium nitrate increases with increasing pressure to a certain limit. It has been established that at a pressure of about 6 kgf/cm2 and the corresponding temperature, the entire molten nitrate decomposes.

Crucial to reducing or preventing the thermal decomposition of ammonium nitrate is maintaining an alkaline environment when evaporating solutions. Therefore, in the new technological scheme for the production of non-caking ammonium nitrate, it is advisable to add a small amount of ammonia to the hot air.

Considering that, under certain conditions, ammonium nitrate can be an explosive product, during its production, storage and transportation, the established technological regime and safety regulations must be strictly observed.

Ammonium nitrate is a non-flammable product. Only nitrous oxide, formed during the thermal decomposition of salt, supports combustion.

A mixture of ammonium nitrate with crushed charcoal can spontaneously ignite when heated strongly. Some easily oxidized metals (such as powdered zinc) in contact with wet ammonium nitrate with slight heat can also cause it to ignite. In practice, cases of spontaneous ignition of mixtures of ammonium nitrate with superphosphate have been observed.

Paper bags or wooden barrels that contained ammonium nitrate can catch fire even when exposed to sunlight. When a container containing ammonium nitrate ignites, nitrogen oxides and nitric acid vapors may be released. In case of fires arising from an open flame or due to detonation, ammonium nitrate melts and partially decomposes. The flame does not spread into the depth of the saltpeter mass.

2 . Production methods

ammonium nitrate neutralization acid

In industry, only the method of producing ammonium nitrate from synthetic ammonia (or ammonia-containing gases) and dilute nitric acid is widely used.

The production of ammonium nitrate from synthetic ammonia (or ammonia-containing gases) and nitric acid is multi-stage. In this regard, they tried to obtain ammonium nitrate directly from ammonia, nitrogen oxides, oxygen and water vapor by the reaction

4NH3 + 4NO2 + 02 + 2H20 = 4NH4NO3 (7)

However, this method had to be abandoned, since along with ammonium nitrate, ammonium nitrite was formed - an unstable and explosive product.

A number of improvements have been introduced into the production of ammonium nitrate from ammonia and nitric acid, which have made it possible to reduce capital costs for the construction of new plants and reduce the cost of the finished product.

To radically improve the production of ammonium nitrate, it was necessary to abandon the ideas that had prevailed for many years about the impossibility of working without appropriate reserves of basic equipment (for example, evaporators, granulation towers, etc.), about the danger of obtaining almost anhydrous ammonium nitrate melt for granulation.

It is firmly established in Russia and abroad that only the construction of high-power units, using modern achievements of science and technology, can provide significant economic advantages compared to existing ammonium nitrate production.

A significant amount of ammonium nitrate is currently produced from the ammonia-containing off-gases of some urea synthesis systems. According to one of the methods of its production, 1 ton of urea produces from 1 to 1.4 tons of ammonia. From this amount of ammonia, 4.6-6.5 tons of ammonium nitrate can be produced. Although more advanced schemes for the synthesis of urea are also working, ammonia-containing gases - waste from this production - will serve as raw materials for the production of ammonium nitrate for some time.

The method for producing ammonium nitrate from ammonia-containing gases differs from the method for producing it from gaseous ammonia only at the neutralization stage.

Ammonium nitrate is obtained in small quantities by exchange decomposition of salts (conversion methods).

These methods for producing ammonium nitrate are based on the precipitation of one of the resulting salts or on the production of two salts with different solubilities in water. In the first case, ammonium nitrate solutions are separated from sediments on rotating filters and processed into a solid product according to conventional procedures. In the second case, the solutions are evaporated to a certain concentration and separated by fractional crystallization, which boils down to the following: when cooling hot solutions, most of the ammonium nitrate is isolated in its pure form, then crystallization is carried out in separate equipment from the mother solutions to obtain a product contaminated with impurities.

All methods for producing ammonium nitrate by exchange decomposition of salts are complex and involve high steam consumption and loss of bound nitrogen. They are usually used in industry only when it is necessary to utilize nitrogen compounds obtained as by-products.

The modern method of producing ammonium nitrate from gaseous ammonia (or ammonia-containing gases) and nitric acid is constantly being improved.

3 . The main stages of the production of ammonium nitrate from ammonia and nitric acid

The ammonium nitrate production process consists of the following main stages:

1. Preparation of ammonium nitrate solutions by neutralizing nitric acid with gaseous ammonia or ammonia-containing gases.

2. Evaporation of ammonium nitrate solutions to a melt state.

3. Crystallization from melted salt in the form of round-shaped particles (granules), flakes (plates) and small crystals.

4. Cooling or drying salt.

5. Packaging of the finished product.

To obtain low-caking and water-resistant ammonium nitrate, in addition to the indicated stages, a stage of preparation of appropriate additives is also necessary.

3.1 P Preparation of ammonium nitrate solutions

3.1.1 Neutralization Process Basics

Ammonium nitrate solutions ry are obtained by reacting ammonia with nitric acid according to the reaction:

4NH3 + HNO3 = NH4NO3 + Q J (8)

The formation of ammonium nitrate is irreversible and is accompanied by the release of heat. The amount of heat released during the neutralization reaction depends on the concentration of nitric acid used and its temperature, as well as on the temperature of the ammonia gas (or ammonia-containing gases). The higher the concentration of nitric acid, the more heat is generated. In this case, water evaporates, which makes it possible to obtain more concentrated solutions of ammonium nitrate. To obtain solutions of ammonium nitrate, 42-58% nitric acid is used.

The use of nitric acid with a concentration higher than 58% to obtain ammonium nitrate solutions with the existing design of the process is not possible, since in this case a temperature develops in the neutralizer apparatus that significantly exceeds the boiling point of nitric acid, which can lead to its decomposition with the release of nitrogen oxides. When ammonium nitrate solutions are evaporated, juice steam is formed due to the heat of reaction in neutralizer apparatuses, having a temperature of 110-120 °C.

When obtaining ammonium nitrate solutions of the highest possible concentration, relatively small heat exchange surfaces of evaporators are required, and a small amount of fresh steam is consumed for further evaporation of the solutions. In this regard, together with the feedstock, they strive to supply additional heat to the neutralizer, for which they heat ammonia to 70 ° C and nitric acid to 60 ° C with juice steam (at a higher temperature of nitric acid, its significant decomposition occurs, and the heater pipes are subjected to strong corrosion if they are not made of titanium).

Nitric acid used in the production of ammonium nitrate must contain no more than 0.20% dissolved nitrogen oxides. If the acid is not sufficiently purged with air to remove dissolved nitrogen oxides, they form ammonium nitrite with ammonia, which quickly decomposes into nitrogen and water. In this case, nitrogen losses can amount to about 0.3 kg per 1 ton of finished product.

Juice vapor, as a rule, contains impurities NH3, NHO3 and NH4NO3. The amount of these impurities strongly depends on the stability of the pressures at which ammonia and nitric acid must be supplied to the neutralizer. To maintain a given pressure, nitric acid is supplied from a pressure tank equipped with an overflow pipe, and ammonia gas is supplied using a pressure regulator.

The neutralizer load also largely determines the loss of bound nitrogen with juice steam. Under normal load, losses with juice steam condensate should not exceed 2 g/l (in terms of nitrogen). When the neutralizer load is exceeded, side reactions occur between ammonia and nitric acid vapor, as a result of which, in particular, misty ammonium nitrate is formed in the gas phase, contaminating the juice steam, and the loss of bound nitrogen increases. The ammonium nitrate solutions obtained in the neutralizers are accumulated in intermediate containers with stirrers, neutralized with ammonia or nitric acid, and then sent for evaporation.

3.1.2 Characteristics of neutralization installations

Depending on the application required pressure, modern installations for producing ammonium nitrate solutions using neutralization heat are divided into installations operating at atmospheric pressure; in rarefaction (vacuum); at elevated pressure (several atmospheres) and combined installations operating under pressure in the neutralization zone and under vacuum in the zone of separation of juice vapors from the ammonium nitrate solution (melt).

Installations operating at atmospheric or slight excess pressure are characterized by simplicity of technology and design. They are also easy to maintain, start and stop; accidental violations of the specified operating mode are usually quickly eliminated. Installations of this type are most widely used. The main apparatus of these installations is the neutralizer apparatus ITN (use of neutralization heat). The ITN apparatus operates under an absolute pressure of 1.15--1.25 atm. Structurally, it is designed in such a way that almost no boiling of solutions occurs - with the formation of foggy ammonium nitrate.

The presence of circulation in the heat pump apparatus eliminates overheating in the reaction zone, which allows the neutralization process to be carried out with minimal losses of bound nitrogen.

Depending on the operating conditions of the production of ammonium nitrate, the juice steam of the ITN apparatuses is used for preliminary evaporation of nitrate solutions, for the evaporation of liquid ammonia, heating of nitric acid and gaseous ammonia sent to the ITN apparatuses, and for the evaporation of liquid ammonia when obtaining gaseous ammonia used in the production of diluted nitric acid.

Solutions of ammonium nitrate are produced from ammonia-containing gases in installations whose main apparatus operates under vacuum (evaporator) and at atmospheric pressure (scrubber-neutralizer). Such installations are bulky and it is difficult to maintain a stable operating mode in them due to the variability of the composition of ammonia-containing gases. The latter circumstance negatively affects the accuracy of regulation of excess nitric acid, as a result of which the resulting solutions of ammonium nitrate often contain an increased amount of acid or ammonia.

Neutralization installations operating under an absolute pressure of 5-6 atm are not very common. They require significant energy consumption to compress ammonia gas and supply pressurized nitric acid to the neutralizers. In addition, at these installations, increased losses of ammonium nitrate are possible due to the entrainment of splashes of solutions (even in separators of complex design, splashes cannot be completely captured).

In installations based on the combined method, the processes of neutralizing nitric acid with ammonia are combined and producing ammonium nitrate melt, which can be directly sent for crystallization (i.e., evaporators for concentrating nitrate solutions are excluded from such installations). Installations of this type require 58-60% nitric acid, which industry still produces in relatively small quantities. In addition, some of the equipment must be made of expensive titanium. The neutralization process to obtain nitrate melt must be carried out at very high temperatures (200-220 ° C). Considering the properties of ammonium nitrate, to carry out the process at high temperatures it is necessary to create special conditions that prevent thermal decomposition of the nitrate melt.

3.1.3 Neutralization plants operating at atmospheric pressure

These installations include They include ITN neutralizer devices (using the heat of neutralization) and auxiliary equipment.

Figure 1 shows one of the designs of the ITN apparatus used in many existing ammonium nitrate production plants.

Z1 - swirler; BC1 - external vessel (reservoir); VTs1 - inner cylinder (neutralization part); U1 - device for distributing nitric acid; Ш1 - fitting for draining solutions; O1 - windows; U2 - device for ammonia distribution; G1 - water seal; C1 - separator-trap

Figure 1 - ITN neutralizer apparatus with natural circulation of solutions

The ITN apparatus is a vertical cylindrical vessel (reservoir) 2, in which a cylinder (glass) 3 with shelves 1 (swirler) is placed to improve the mixing of solutions. Pipelines for introducing nitric acid and ammonia gas are connected to cylinder 3 (the reagents are supplied in countercurrent); the pipes end with devices 4 and 7 for better distribution of acid and gas. In the inner cylinder, nitric acid reacts with ammonia. This cylinder is called the neutralization chamber.

The annular space between vessel 2 and cylinder 3 serves for circulation of boiling solutions of ammonium nitrate. In the lower part of the cylinder there are 6 holes (windows) connecting the neutralization chamber with the evaporation part of the heating element. Due to the presence of these holes, the productivity of the ITN apparatus is somewhat reduced, but intensive natural circulation of solutions is achieved, which leads to a reduction in the loss of bound nitrogen.

The juice steam released from the solution is discharged through the fitting in the cover of the ITN apparatus and through the trap-separator 9. The solutions of nitrate formed in cylinder 3 in the form of an emulsion - mixtures with juice steam enter the separator through the water seal 5. From the fitting of the lower part of the trap-separator, ammonium solutions The nitrate is sent to the final neutralizer-mixer for further processing. The water seal in the evaporation part of the apparatus allows you to maintain a constant level of solution in it and prevents juice steam from escaping without flushing from the splashes of solution entrained by it.

Steam condensate is formed on the separator plates due to partial condensation of juice steam. In this case, the heat of condensation is removed by circulating water passing through coils laid on plates. As a result of partial condensation of juice steam, a 15--20% solution of NH4NO3 is obtained, which is sent for evaporation along with the main flow of ammonium nitrate solution.

Figure 2 shows a diagram of one of the neutralization units operating at pressure close to atmospheric.

NB1 - pressure tank; C1 - separator; I1 - evaporator; P1 - heater; SK1 - collection for condensate; ITN1 - ITN apparatus; M1 - stirrer; TsN1 - centrifugal pump

Figure 2 - Diagram of a neutralization installation operating at atmospheric pressure

Pure or with additives nitric acid is supplied to a pressure tank equipped with a constant overflow of excess acid into storage.

From pressure tank 1, nitric acid is directed directly into the glass of apparatus ITN 6 or through a heater (not shown in the figure), where it is heated by the heat of juice steam removed through separator 2.

Gaseous ammonia enters the liquid ammonia evaporator 3, then into the heater 4, where it is heated by the heat of secondary steam from the expander or by the hot condensate of the heating steam of the evaporators, and is then sent through two parallel pipes into the glass of the apparatus ITN 6.

In the evaporator 3, the liquid ammonia spray evaporates and the contaminants usually associated with gaseous ammonia are separated. In this case, weak ammonia water is formed with an admixture of lubricating oil and catalyst dust from the ammonia synthesis workshop.

The ammonium nitrate solution obtained in the neutralizer continuously flows through a hydraulic seal and a splash trap into the final neutralizer mixer 7, from where, after neutralizing the excess acid, it is sent for evaporation.

The juice steam released in the heating apparatus, after passing through separator 2, is sent for use as heating steam to the first stage evaporators.

Juice steam condensate from heater 4 is collected in collector 5, from where it is spent on various production needs.

Before starting the neutralizer, the preparatory work provided for in the operating instructions is carried out. Let us note only some of the preparatory work related to the normal conduct of the neutralization process and ensuring safety precautions.

First of all, you need to pour ammonium nitrate solution or steam condensate into the neutralizer up to the sampling valve.

Then it is necessary to establish a continuous supply of nitric acid to the pressure tank and its overflow into the warehouse storage area. After this, it is necessary to receive gaseous ammonia from the ammonia synthesis workshop, for which it is necessary to briefly open the valves on the line for releasing juice steam into the atmosphere and the valve for the solution outlet into the mixer-neutralizer. This prevents the creation of high pressure in the pumping apparatus and the formation of an unsafe ammonia-air mixture when starting the device.

For the same purpose, before start-up, the neutralizer and the communications connected to it are purged with steam.

After achieving normal operating conditions, juice steam from the heating apparatus is sent for use as heating steam].

3.1.4 Neutralization plants operating under vacuum

Co-processing of amm ammonia-containing gases and gaseous ammonia is impractical, as it is associated with large losses of ammonium nitrate, acid and ammonia due to the presence of a significant amount of impurities in ammonia-containing gases (nitrogen, methane, hydrogen, etc.) - These impurities bubbling through the resulting boiling solutions of ammonium nitrate , would carry away bound nitrogen with the juice steam. In addition, juice steam contaminated with impurities could not be used as heating steam. Therefore, ammonia-containing gases are usually processed separately from ammonia gas.

In installations operating under vacuum, the heat of reaction is used outside the neutralizer - in a vacuum evaporator. Here, hot solutions of ammonium nitrate coming from the neutralizer are boiled at a temperature corresponding to the vacuum in the apparatus. Such installations include: a scrubber-type neutralizer, a vacuum evaporator and auxiliary equipment.

Figure 3 shows a diagram of a neutralization installation operating using a vacuum evaporator.

HP1 - scrubber-type neutralizer; H1 - pump; B1 - vacuum evaporator; B2 - vacuum separator; NB1 - nitric acid pressure tank; B1 - tank (gate mixer); P1 - washer; DN1 - pre-neutralizer

Figure 3 - Scheme of a neutralization installation with a vacuum evaporator

Ammonia-containing gases at a temperature of 30--90 °C under a pressure of 1.2--1.3 atm are supplied to the lower part of the scrubber-neutralizer 1. A circulating solution of nitrate enters the upper part of the scrubber from the seal tank 6, which is usually continuously supplied from tank 5 nitric acid, sometimes preheated to a temperature not exceeding 60 °C. The neutralization process is carried out with an excess of acid in the range of 20-50 g/l. Scrubber 1 usually maintains a temperature 15-20 °C below the boiling point of solutions, which helps prevent acid decomposition and the formation of ammonium nitrate mist. The set temperature is maintained by irrigating the scrubber with a solution from a vacuum evaporator, which operates at a vacuum of 600 mm Hg. Art., so the solution in it has a lower temperature than in the scrubber.

The nitrate solution obtained in the scrubber is sucked into vacuum evaporator 5, where at a vacuum of 560-600 mm Hg. Art. partial evaporation of water occurs (evaporation) and an increase in the concentration of the solution.

From the vacuum evaporator, the solution flows into the water seal tank 6, from where most of it again goes to irrigate the scrubber 1, and the rest is sent to the after-neutralizer 8. The juice steam generated in the vacuum evaporator 3 is sent through the vacuum separator 4 to the surface condenser (not shown in the figure) or into a mixing-type capacitor. In the first case, juice steam condensate is used in the production of nitric acid, in the second - for various other purposes. The vacuum in the vacuum evaporator is created due to the condensation of juice steam. Non-condensed vapors and gases are sucked out of the condensers by a vacuum pump and discharged into the atmosphere.

Exhaust gases from scrubber 1 enter apparatus 7, where they are washed with condensate to remove drops of nitrate solution, after which they are also removed into the atmosphere. In the final neutralizer mixer, the solutions are neutralized to a content of 0.1-0.2 g/l of free ammonia and, together with the flow of nitrate solution obtained in the ITN apparatus, are sent for evaporation.

Figure 4 shows a more advanced vacuum neutralization scheme.

XK1 - refrigerator-condenser; CH1 - scrubber-neutralizer; C1, C2 - collections; TsN1, TsN2, TsN3 - centrifugal pumps; P1 - gas washer; G1 - water seal; L1 - trap; B1 - vacuum evaporator; BD1 - neutralizer tank; B2 - vacuum pump; P2 - juice machine washer; K1 - surface capacitor

Figure 4 - Vacuum neutralization diagram:

Distillation gases are directed to the lower part of the neutralizer scrubber 2, irrigated with a solution from the collector 3 using a circulation pump 4.

The collection 3 through the water seal 6 receives solutions from the scrubber-neutralizer 2, as well as solutions after the trap of the vacuum evaporator 10 and the juice steam washer 14.

Through a pressure tank (not shown in the figure), nitric acid solution from gas washer 5, irrigated with juice steam condensate, is continuously fed into collection 7. From here, the solutions are supplied by circulation pump 8 to washer 5, after which they are returned to collection 7.

Hot gases after the washer 5 are cooled in the refrigerator-condenser 1 and released into the atmosphere.

Hot solutions of ammonium nitrate from water seal 6 are sucked by vacuum pump 13 into vacuum evaporator 10, where the concentration of NH4NO3 increases by several percent.

The juice vapors released in the vacuum evaporator 10, after passing through the trap 9, the washer 14 and the surface condenser 15, are released into the atmosphere by the vacuum pump 13.

An ammonium nitrate solution with a given acidity is discharged from the discharge line of pump 4 into the neutralizer tank. Here the solution is neutralized with ammonia gas and pump 12 is sent to the evaporation station.

3.1. 5 Main equipment

ITN neutralizers. Several types of neutralizers are used, differing mainly in the size and design of devices for distributing ammonia and nitric acid inside the apparatus. Devices of the following sizes are often used: diameter 2400 mm, height 7155 mm, glass - diameter 1000 mm, height 5000 mm. Devices with a diameter of 2440 mm and a height of 6294 mm and devices from which the previously provided mixer has been removed are also used (Figure 5).

LK1 - hatch; P1 - shelves; L1 - sampling line; L2 - solution output line; BC1 - inner glass; C1 - external vessel; Ш1 - fitting for draining solutions; P1 - ammonia distributor; P2 - nitric acid distributor

Figure 5 - ITN neutralizer device

In some cases, for the processing of small quantities of ammonia-containing gases, ITP devices with a diameter of 1700 mm and a height of 5000 mm are used.

The ammonia gas heater is a shell-and-tube apparatus made of carbon steel. Case diameter 400--476 mm, height 3500--3280 mm. The tube often consists of 121 tubes (tube diameter 25x3 mm) with a total heat transfer surface of 28 m2. Gaseous ammonia enters the tubes, and heating steam or hot condensate enters the inter-tube space.

If juice steam from heating equipment is used for heating, then the heater is made of stainless steel 1Х18Н9Т.

The liquid ammonia evaporator is a carbon steel apparatus, in the lower part of which there is a steam coil, and in the middle there is a tangential input of gaseous ammonia.

In most cases, the evaporator operates with fresh steam at a pressure (excess) of 9 atm. At the bottom of the ammonia evaporator there is a fitting for periodic purging from accumulated contaminants.

The nitric acid heater is a shell-and-tube apparatus with a diameter of 400 mm and a length of 3890 mm. Tube diameter 25x2 mm, length 3500 mm; total heat exchange surface 32 m2. Heating is carried out by juice steam with an absolute pressure of 1.2 atm.

The scrubber-type neutralizer is a vertical cylindrical apparatus with a diameter of 1800-2400 mm and a height of 4700-5150 mm. Devices with a diameter of 2012 mm and a height of 9000 mm are also used. Inside the apparatus, for uniform distribution of circulating solutions across the cross-section, there are several perforated plates or a nozzle made of ceramic rings. In the upper part of the devices equipped with plates, a layer of rings with dimensions of 50x50x3 mm is laid, which acts as a barrier against splashes of solutions.

The gas velocity in the free section of the scrubber with a diameter of 1700 mm and a height of 5150 mm is about 0.4 m/sec. Irrigation of the scrubber-type apparatus with solutions is carried out using centrifugal pumps with a capacity of 175-250 m3/h.

Vacuum evaporator is a vertical cylindrical device with a diameter of 1000-1200 mm and a height of 5000-3200 mm. The nozzle is ceramic rings measuring 50x50x5 mm, laid in regular rows.

The gas washer is a vertical cylindrical apparatus made of stainless steel with a diameter of 1000 mm and a height of 5000 mm. The nozzle is ceramic rings measuring 50x50x5 mm.

Stirrer-neutralizer - a cylindrical apparatus with a stirrer rotating at a speed of 30 rpm. The drive is carried out from an electric motor through a gearbox (Figure 6).

Ш1 - fitting for installing a level meter; B1 - air vent; E1 - electric motor; P1 - gearbox; VM1 - mixer shaft; L1 - manhole

Figure 6 - Stirrer-neutralizer

The diameter of frequently used devices is 2800 mm, height 3200 mm. They operate under atmospheric pressure, serve for the final neutralization of ammonium nitrate solutions and as intermediate containers for solutions sent for evaporation.

Surface condenser is a vertical shell-and-tube two-pass (through water) heat exchanger designed to condense juice steam coming from a vacuum evaporator. Device diameter 1200 mm, height 4285 mm; heat transfer surface 309 m2. It operates at a vacuum of approximately 550-- 600 mm Hg. Art.; has tubes: diameter 25x2 mm, length 3500 m, total number 1150 pcs.; the weight of such a capacitor is about 7200 kg

In some cases, to eliminate emissions into the atmosphere of juice steam discharged during purging from evaporators, traps of heating equipment and water seals, a surface condenser is installed with the following characteristics: body diameter 800 mm, height 4430 mm, total number of tubes 483 pcs., diameter 25x2, total surface 125 m2.

Vacuum pumps. Different types of pumps are used. The VVN-12 type pump has a capacity of 66 m3/h, shaft rotation speed is 980 rpm. The pump is designed to create a vacuum in a vacuum neutralization unit.

Centrifugal pumps. To circulate ammonium nitrate solution in a vacuum neutralization installation, 7ХН-12 pumps with a capacity of 175-250 m3/h are often used. Installed power of the electric motor is 55 kW.

4 . Material and energy calculations

Let us calculate the material and thermal balance of the process. I calculate the neutralization of nitric acid with ammonia gas per 1 ton of product. I take the initial data from Table 2, using the methodology of manuals , , .

We accept that the neutralization process will proceed under the following conditions:

Initial temperature, °C

ammonia gas................................................... ...........................50

nitric acid........................................................ ......................................20

Table 2 - Initial data

Material calculation

1 To obtain 1 ton of nitrate by the reaction:

NH3+HNO3=NH4NO3 +Q J (9)

theoretically the following amount of raw materials is required (in kg):

ammonia

17 - 80 x = 1000*17/80 = 212.5

x - 1000

nitric acid

63 - 80 x = 1000*63/80 = 787.5

x - 1000

Where 17, 63 and 80 are the molecular weights of ammonia, nitric acid and ammonium nitrate, respectively.

The practical consumption of NH3 and HNO3 is slightly higher than the theoretical one, since during the neutralization process, losses of reagents with juice steam are inevitable through leaks in communications due to slight decomposition of the reacting components and nitrate, etc.

2. Determine the amount of ammonium nitrate in the commercial product: 0.98*1000=980 kg/h

or

980/80=12.25 kmol/h,

and also the amount of water:

1000-980=20 kg/h

3. I will calculate the consumption of nitric acid (100%) to obtain 12.25 kmol/h of nitrate. According to stoichiometry, the same amount of it is consumed (kmol/h) as nitrate is formed: 12.25 kmol/h, or 12.25*63=771.75 kg/h

Since the conditions set the complete (100%) conversion of the acid, this will be the amount supplied.

The process involves diluted acid - 60%:

771.75/0.6=1286.25 kg/h,

including water:

1286.25-771.25=514.5 kg/h

4. Similarly, ammonia consumption (100%) to produce 12.25 kmol/h, or 12.25*17=208.25 kg/h

In terms of 25% ammonia water, this will be 208.25/0.25 = 833 kg/h, including water 833-208.25 = 624.75 kg/h.

5. I will find the total amount of water in the neutralizer supplied with the reagents:

514.5+624.75=1139.25 kg/h

6. Let us determine the amount of water vapor formed by evaporation of the nitrate solution (20 kg/h remains in the commercial product): 1139.25 - 20 = 1119.25 kg/h.

7. Let’s draw up a table of the material balance of the ammonium nitrate production process.

Table 3 - Material balance of the neutralization process

8. Let's calculate technological indicators.

· theoretical expense coefficients:

for acid - 63/80=0.78 kg/kg

for ammonia - 17/80=0.21 kg/kg

· actual expense ratios:

for acid - 1286.25/1000=1.28 kg/kg

for ammonia - 833/1000=0.83 kg/kg

During the neutralization process, only one reaction took place, the conversion of the raw material was equal to 1 (i.e., complete conversion occurred), there were no losses, which means that the actual yield is equal to the theoretical one:

Qf/Qt100=980/980100=100%

Energy calculation

The arrival of warmth. During the neutralization process, the heat input consists of the heat introduced by ammonia and nitric acid, and the heat released during neutralization.

1. The heat contributed by ammonia gas is:

Q1=208.252.1850=22699.25 kJ,

where 208.25 is ammonia consumption, kg/h

2.18 - heat capacity of ammonia, kJ/(kg*°C)

50 - ammonia temperature, °C

2. Heat introduced by nitric acid:

Q2=771.752.7620=42600.8 kJ,

where 771.25 is the consumption of nitric acid, kg/h

2.76 - heat capacity of nitric acid, kJ/(kg*°C)

20 - acid temperature, °C

3. The heat of neutralization is preliminarily calculated per 1 mole of ammonium nitrate formed according to the equation:

HNO33.95H2O(liquid) +NH3(gas) =NH4NO33.95H2O(liquid)

where HNO3*3.95H2O corresponds to nitric acid.

The thermal effect Q3 of this reaction is found from the following quantities:

a) heat of dissolution of nitric acid in water:

HNO3+3.95 H2O=HNO3*3.95H2O (10)

b) heat of formation of solid NH4NO3 from 100% nitric acid and 100% ammonia:

HNO3 (liquid) + NH3 (gas) = NH4NO3 (solid) (11)

c) the heat of dissolution of ammonium nitrate in water, taking into account the reaction heat consumption for evaporation of the resulting solution from 52.5% (NH4NO3 H2O) to 64% (NH4NO3 2.5H2O)

NH4NO3 +2.5H2O= NH4NO3*2.5H2O, (12)

where NH4NO3*4H2O corresponds to a concentration of 52.5% NH4NO3

The value of NH4NO3*4H2O is calculated from the ratio

8047.5/52.518=4H2O,

where 80 is the molar weight of NH4NO3

47.5 - HNO3 concentration, %

52.5 - NH4NO3 concentration, %

18 - molar weight of H2O

The value of NH4NO3*2.5H2O corresponding to a 64% solution of NH4NO3 is calculated similarly

8036/6418=2.5H2O

According to reaction (10), the heat of solution q of nitric acid in water is 2594.08 J/mol. To determine the thermal effect of reaction (11), it is necessary to subtract the sum of the heats of formation of NH3 (gas) and HNO3 (liquid) from the heat of formation of ammonium nitrate.

The heat of formation of these compounds from simple substances at 18°C and 1 atm has the following values (in J/mol):

NH3(gas):46191.36

HNO3 (liquid): 174472.8

NH4NO3(s):364844.8

The overall thermal effect of a chemical process depends only on the heats of formation of the initial interacting substances and final products. It follows from this that the thermal effect of reaction (11) will be:

q2=364844.8-(46191.36+174472.8)=144180.64 J/mol

The heat q3 of dissolution of NH4NO3 according to reaction (12) is equal to 15606.32 J/mol.

The dissolution of NH4NO3 in water occurs with the absorption of heat. In this regard, the heat of solution is taken in the energy balance with a minus sign. The concentration of the NH4NO3 solution proceeds accordingly with the release of heat.

Thus, the thermal effect of Q3 reaction

HNO3 +3.95H2O(liquid)+ NH3(gas) =NH4NO32.5H2O(liquid)+1.45 H2O(steam)

will be:

Q3=q1+q2+q3= -25940.08+144180.64-15606.32=102633.52 J/mol

When producing 1 ton of ammonium nitrate, the heat of the neutralization reaction will be:

102633.52*1000/80=1282919 kJ,

where 80 is the molecular weight of NH4NO3

From the above calculations it is clear that the total heat gain will be: with ammonia 22699.25, with nitric acid 42600.8, due to the heat of neutralization 1282919 and a total of 1348219.05 kJ.

Heat consumption. When neutralizing nitric acid with ammonia, heat is removed from the apparatus by the resulting ammonium nitrate solution, spent on evaporating water from this solution and lost into the environment.

The amount of heat carried away by the ammonium nitrate solution is:

Q=(980+10)*2.55 tkip,

where 980 is the amount of ammonium nitrate solution, kg

10 - losses of NH3 and HNO3, kg

tboil - boiling temperature of ammonium nitrate solution, °C

The boiling point of the ammonium nitrate solution is determined at an absolute pressure in the neutralizer of 1.15 - 1.2 atm; This pressure corresponds to a temperature of saturated water vapor of 103 °C. at atmospheric pressure, the boiling point of the NH4NO3 solution is 115.2 °C. temperature depression is equal to:

?t=115.2 - 100=15.2 °C

Calculate the boiling point of a 64% NH4NO3 solution

tboil = tsat. steam+?tз =103+15.21.03 = 118.7 °С,

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Introduction
1. Production of ammonium nitrate
2. Raw materials
3. Ammonia synthesis
4. Characteristics of the target product
5. Physical and chemical substantiation of the main processes of production of the target product and environmental safety of production

Introduction

The most important type of mineral fertilizers are nitrogen fertilizers: ammonium nitrate, urea, ammonium sulfate, aqueous solutions of ammonia, etc. Nitrogen plays an extremely important role in the life of plants: it is part of chlorophyll, which is an acceptor of solar energy, and protein, necessary for the construction of a living cell. Plants can only consume fixed nitrogen - in the form of nitrates, ammonium salts or amides. Relatively small amounts of fixed nitrogen are formed from atmospheric nitrogen due to the activity of soil microorganisms. However, modern intensive agriculture can no longer exist without additional application of nitrogen fertilizers to the soil, obtained as a result of the industrial fixation of atmospheric nitrogen.

Nitrogen fertilizers differ from each other in their nitrogen content, in the form of nitrogen compounds (nitrate, ammonium, amide), phase state (solid and liquid), and there are also physiologically acidic and physiologically alkaline fertilizers.

1. Production of ammonium nitrate

Ammonium nitrate, or ammonium nitrate, NH 4 NO 3 - a white crystalline substance containing 35% nitrogen in ammonium and nitrate forms , both forms of nitrogen are easily absorbed by plants. Granulated ammonium nitrate is used on a large scale before sowing and for all types of fertilizing. On a smaller scale, it is used to produce explosives.

Ammonium nitrate is highly soluble in water and has high hygroscopicity (the ability to absorb moisture from the air). This is the reason that fertilizer granules spread out, lose their crystalline shape, caking of fertilizers occurs - bulk material turns into a solid monolithic mass.

Ammonium nitrate is produced in three types:

A and B - used in industry; used in explosive mixtures (ammonites, ammonials)

B is the effective and most common nitrogen fertilizer, containing about 33-34% nitrogen; has physiological acidity.

2. Raw materials

The starting materials in the production of ammonium nitrate are ammonia and nitric acid.

Nitric acid . Pure nitric acid HNO is a colorless liquid with a density of 1.51 g/cm3 at - 42 °C, solidifying into a transparent crystalline mass. In the air, it, like concentrated hydrochloric acid, “smoke”, since its vapors form small droplets of fog with the moisture in the air. Nitric acid is not durable, and already under the influence of light it gradually decomposes:

The higher the temperature and the more concentrated the acid, the faster the decomposition occurs. The released nitrogen dioxide dissolves in the acid and gives it a brown color.

Nitric acid is one of the most powerful acids; in dilute solutions, it completely decomposes into H and -NO ions. Nitric acid is one of the most important nitrogen compounds: it is used in large quantities in the production of nitrogen fertilizers, explosives and organic dyes, serves as an oxidizing agent in many chemical processes, and is used in the production of sulfuric acid. acids using the nitrous method, used for the manufacture of cellulose varnishes and film .

Industrial production of nitric acid . Modern industrial methods for producing nitric acid are based on the catalytic oxidation of ammonia with atmospheric oxygen. When describing the properties of ammonia, it was indicated that it burns in oxygen, and the reaction products are water and free nitrogen. But in the presence of catalysts, the oxidation of ammonia with oxygen can proceed differently. If a mixture of ammonia and air is passed over the catalyst, then at 750 ° C and At a certain composition of the mixture, almost complete conversion occurs

The resulting mixture easily passes into, which, with water in the presence of atmospheric oxygen, gives nitric acid.

Platinum-based alloys are used as catalysts for the oxidation of ammonia.

The nitric acid obtained by the oxidation of ammonia has a concentration not exceeding 60%. If necessary, it is concentrated,

The industry produces diluted nitric acid with a concentration of 55, 47 and 45%, and concentrated nitric acid - 98 and 97%. Concentrated acid is transported in aluminum tanks, diluted acid - in tanks made of acid-resistant steel.

3. Ammonia synthesis

ammonia nitrogen nitrate raw materials

Ammonia is a key product of various nitrogen-containing substances used in industry and agriculture. D.N. Pryanishnikov called ammonia “alpha and omega” in the metabolism of nitrogenous substances in plants.

The diagram shows the main applications of ammonia. The composition of ammonia was established by C. Berthollet in 1784. Ammonia NH 3 is a base, a moderately strong reducing agent and an effective complexing agent with respect to cations with vacant bonding orbitals.

Physico-chemical basis of the process . The synthesis of ammonia from elements is carried out according to the reaction equation

N 2 +3H 2 =2NH 3; ?H<0

The reaction is reversible, exothermic, characterized by a large negative enthalpy effect (?H = -91.96 kJ/mol) and at high temperatures becomes even more exothermic (?H = -112.86 kJ/mol). According to Le Chatelier's principle, when heated, the equilibrium shifts to the left, towards a decrease in the yield of ammonia. The change in entropy in this case is also negative and does not favor the reaction. With a negative value of ?S, an increase in temperature reduces the probability of a reaction occurring,

The reaction of ammonia synthesis proceeds with a decrease in volume. According to the reaction equation, 4 moles of initial gaseous reactants form 2 moles of gaseous product. Based on Le Chatelier's principle, we can conclude that, under equilibrium conditions, the ammonia content in the mixture will be greater at high pressure than at low pressure.

4. Characteristics of the target product

Physicochemical characteristics . Ammonium nitrate (ammonium nitrate) NH4NO3 has a molecular weight of 80.043; the pure product is a colorless crystalline substance containing 60% oxygen, 5% hydrogen and 35% nitrogen (17.5% each in ammonia and nitrate forms). The technical product contains at least 34.0% nitrogen.

Basic physical and chemical properties of ammonium nitrates:

Ammonium nitrate, depending on temperature, exists in five crystalline modifications that are thermodynamically stable at atmospheric pressure (table). Each modification exists only in a certain temperature range, and the transition (polymorphic) from one modification to another is accompanied by changes in the crystal structure, release (or absorption) of heat, as well as an abrupt change in specific volume, heat capacity, entropy, etc. Polymorphic transitions are reversible - enantiotropic.

Table. Crystal modifications of ammonium nitrate

The NH 4 NO 3 -H 2 O system (Fig. 11-2) refers to systems with simple eutectics. The eutectic point corresponds to a concentration of 42.4% MH 4 MO 3 and a temperature of -16.9 °C. The left branch of the diagram—the liquidus line of water—corresponds to the conditions for the release of ice in the system NN 4 MO 3 -H 2 O. The right branch of the liquidus curve is the solubility curve of MH 4 MO 3 in water. This curve has three break points corresponding to the temperatures of modification transitions NH 4 NO 3 1 = 11 (125.8 ° C), II = III (84.2 ° C) and 111 = IV (32.2 ° C). Melting point (crystallization) of anhydrous ammonium nitrate is 169.6 ° C. It decreases with increasing moisture content of salt.

Dependence of crystallization temperature of NH 4 NO 3 (Tcrystal, "C) on moisture content (X,%) up to 1.5% is described by the equation:

t crist = 169.6 - 13, 2x (11.6)

Dependence of the crystallization temperature of ammonium nitrate with the addition of ammonium sulfate on moisture content (X,%) up to 1.5% and ammonium sulfate (U, %) up to 3.0% is expressed by the equation:

t crystal = 169.6 - 13.2X+2, OU. (11.7).

Ammonium nitrate dissolves in water and absorbs heat. Below are the values of the heats of dissolution (Q dist) of ammonium nitrate of various concentrations in water at 25 ° C:

C(NH4NO3) % masses 59,69 47.05 38,84 30,76 22,85 15,09 2,17
Q solution kJ/kg. -202.8 -225.82 -240.45 -256.13 -271.29 -287.49 -320.95

Ammonium nitrate is highly soluble in water, ethyl and methyl alcohols, pyridine, acetone, and liquid ammonia.

Rice. 11-2. System State DiagramN.H.4 N03 - H20

Thermal decomposition . Ammonium nitrate is an oxidizing agent that can support combustion. When heated in a confined space, when thermal decomposition products cannot be freely removed, saltpeter can, under certain conditions, explode (detonate). It can also explode under the influence of strong shocks, for example when initiated by explosives.

During the initial period of heating at 110°C, endothermic dissociation of nitrate into ammonia and nitric acid gradually occurs:

NH 4 NO 3 > NH 3 + HNO 3 - 174.4 kJ/mol. (11.9)

At 165 °C, weight loss does not exceed 6%/day. The rate of dissociation depends not only on temperature, but also on the ratio between the surface of nitrate and its volume, the content of impurities, etc.

Ammonia is less soluble in the melt than nitric acid, so it is removed faster; the concentration of nitric acid increases to an equilibrium value determined by temperature. The presence of nitric acid in the melt determines the autocatalytic nature of thermal decomposition.

In the temperature range 200-270 °C, a mainly weakly exothermic reaction of the decomposition of nitrate into nitrous oxide and water occurs:

NH 4 NO 3 > N 2 O+ 2H 2 O + 36.8 kJ/mol. (11.10)

A noticeable effect on the rate of thermal decomposition is exerted by nitrogen dioxide, which is formed during the thermal decomposition of nitric acid, which is a product of the dissociation of ammonium nitrate.

When nitrogen dioxide reacts with nitrate, nitric acid, water and nitrogen are formed:

NH 4 NO 3 + 2NO 2 > N 2 + 2HNO 3 + H 2 O + 232 kJ/mol. (11.11 )

The thermal effect of this reaction is more than 6 times higher than the thermal effect of the reaction of the decomposition of nitrate into N 2 O and H 2 O. Thus, in acidified nitrate, even at ordinary temperatures, due to a significant exothermic reaction of interaction with nitrogen dioxide, spontaneous thermal decomposition occurs, which, with a large mass ammonium nitrate can lead to its rapid decomposition.

When nitrate is heated in a closed system at 210-220 °C, ammonia accumulates, the concentration of nitric acid decreases, and therefore the decomposition reaction is strongly inhibited. The thermal decomposition process practically stops, despite the fact that most of the salt has not yet decomposed. At higher temperatures, ammonia oxidizes faster, nitric acid accumulates in the system and the reaction proceeds with significant self-acceleration, which can lead to an explosion.

Additive to ammonium nitrate of substances that can decompose with the release of ammonia (for example, urea and acetamide) inhibits thermal decomposition. Salts with silver or thallium cations significantly increase the reaction rate due to the formation of complexes with nitrate ions in the melt. Chlorine ions have a strong catalytic effect on the thermal decomposition process. When a mixture containing chloride and ammonium nitrate is heated to 220-230 °C, very rapid decomposition begins with the release of large quantities of gas. Due to the heat of reaction, the temperature of the mixture increases greatly, and decomposition is completed within a short time.

If the chloride-containing mixture is maintained at a temperature of 150-200 ° C, then in the first period of time, called induction, decomposition will proceed at a rate corresponding to the decomposition of nitrate at a given temperature. During this period, in addition to decomposition, other processes will also occur, the result of which is, in particular, an increase in the acid content in the mixture and the release of a small amount of chlorine. After the induction period, decomposition proceeds at high speed and is accompanied by strong heat release and the formation of large amounts of toxic gases. With a high chloride content, the decomposition of the entire mass of ammonium nitrate quickly ends. In view of this, the chloride content in the product is strictly limited.

When operating mechanisms used in the production of ammonium nitrate, lubricants should be used that do not interact with the product and do not reduce the initial temperature of thermal decomposition. For this purpose, for example, VNIINP-282 lubricant (GOST 24926-81) can be used.

The temperature of the product sent for storage in bulk or for packaging in bags should not exceed 55 °C. Polyethylene or kraft paper bags are used as containers. The temperatures at which active processes of oxidation of polyethylene and kraft paper with ammonium nitrate begin are 270-280 and 220-230 °C, respectively. Empty plastic and kraft paper bags must be cleaned of product residues and, if they cannot be used, must be incinerated.

In terms of explosion energy, ammonium nitrate is three times weaker than most explosives. A granular product can, in principle, detonate, but initiation with a detonator capsule is impossible; this requires large charges of powerful explosives.

The explosive decomposition of nitrate proceeds according to the equation:

NH 4 NO 3 > N 2 + 0.5O 2 + 2H 2 O + 118 kJ/mol. (11.12)

According to equation (11.12), the heat of the explosion should be 1.48 MJ/kg. However, due to the occurrence of side reactions, one of which is endothermic (11.9), the actual heat of explosion is 0.96 MJ/kg and is small compared to the heat of explosion of hexogen (5.45 MJ). But for such a large-capacity product as ammonium nitrate, taking into account its explosive properties (albeit weak) is important for ensuring safety.

Consumer requirements for the quality of ammonium nitrate produced by industry are reflected in GOST 2-85, according to which two grades of commercial product are produced.

The strength of granules is determined in accordance with GOST-21560.2-82 using IPG-1, MIP-10-1 or OSPG-1M devices.

The friability of granulated ammonium nitrate packed in bags is determined in accordance with GOST-21560.5-82.

GOST 14702-79-" waterproof"

5. Physical and chemical substantiation of the main processes of production of the target product and environmental safety of production

To obtain practically non-caking ammonium nitrate, a number of technological methods are used. An effective means of reducing the rate of moisture absorption by hygroscopic salts is their granulation. The total surface of homogeneous granules is less than the surface of the same amount of fine-crystalline salt, so granular fertilizers absorb moisture from the air more slowly. Sometimes ammonium nitrate is fused with less hygroscopic salts, for example ammonium sulfate.

Ammonium phosphates, potassium chloride, and magnesium nitrate are also used as similarly acting additives. The process of producing ammonium nitrate is based on a heterogeneous reaction between gaseous ammonia and a solution of nitric acid:

NH 3 + HNO 3 = NH 4 NO 3

?H = -144.9 kJ (VIII)

The chemical reaction occurs at high speed; in an industrial reactor it is limited by the dissolution of gas in liquid. To reduce diffusion inhibition, mixing of the reagents is of great importance.

Intensive conditions for carrying out the process can be ensured to a large extent when developing the design of the apparatus. Reaction (VIII) is carried out in a continuously operating ITN apparatus (using the heat of neutralization). The reactor is a vertical cylindrical apparatus consisting of reaction and separation zones. In the reaction zone there is a glass /, in the lower part of which there are holes for circulation of the solution. A bubbler is located slightly above the holes inside the glass 2 for supplying ammonia gas, above it there is a bubbler 3 for supplying nitric acid. The reaction vapor-liquid mixture exits from the top of the reaction glass; part of the solution is removed from the ITN apparatus and enters the final neutralizer, and the rest (circulation) goes down again. The juice vapor released from the vapor-liquid mixture is washed on cap plates 6 from splashes of ammonium nitrate solution and nitric acid vapor with a 20% nitrate solution, and then juice steam condensate.

The heat of reaction (VIII) is used to partially evaporate water from the reaction mixture (hence the name of the apparatus - ITN). The difference in temperatures in different parts of the apparatus leads to more intense circulation of the reaction mixture.

The technological process for the production of ammonium nitrate includes, in addition to the stage of neutralization of nitric acid with ammonia, also the stages of evaporation of the nitrate solution, granulation of the melt, cooling of the granules, treatment of the granules with surfactants, packaging, storage and loading of nitrate, purification of gas emissions and wastewater.

In Fig. a diagram of a modern large-capacity unit for the production of ammonium nitrate AS-72 with a capacity of 1360 tons/day is shown. The initial 58-60% nitric acid is heated in a heater / up to 70-80 With juice steam from the ITN apparatus 3 and is sent for neutralization. In front of the devices 3 Phosphoric and sulfuric acids are added to nitric acid in such quantities that the finished product contains 0.3-0.5% P 2 O 5 and 0.05-0.2% ammonium sulfate.

The unit contains two ITN devices operating in parallel. In addition to nitric acid, they are supplied with ammonia gas, preheated in a heater. 2 steam condensate up to 120-130 °C. The amounts of supplied nitric acid and ammonia are regulated so that at the exit from the ITN apparatus the solution has a slight excess of acid (2-5 g/l), ensuring complete absorption of ammonia.

Nitric acid (58-60%) is heated in the apparatus 2 up to 80-90 °C with juice steam from an ITN apparatus 8. Ammonia gas in the heater 1 heated by steam condensate to 120-160°C. Nitric acid and gaseous ammonia in an automatically controlled ratio enter the reaction parts of two ITN 5 apparatuses operating in parallel. The 89-92% NH 4 NO 3 solution leaving the ITN apparatus at 155-170 °C has an excess of nitric acid in the range of 2-5 g/l, ensuring complete absorption of ammonia.

In the upper part of the apparatus, juice steam from the reaction part is washed away from splashes of ammonium nitrate; vapors of HNO 3 and NH 3 with a 20% solution of ammonium nitrate from a washing scrubber 18 and juice steam condensate from the nitric acid heater 2, which are served on the cap plates of the upper part of the apparatus. Part of the juice steam is used to heat nitric acid in heater 2, and the bulk of it is sent to the washing scrubber 18, where it is mixed with air from the granulation tower, with a steam-air mixture from the evaporator 6 and washed on scrubber wash plates. The washed steam-air mixture is released into the atmosphere by a fan 19.

Solution from ITN devices 8 sequentially passes through the neutralizer 4 and control neutralizer 5. To the neutralizer 4 sulfuric and phosphoric acids are dosed in an amount ensuring that the finished product contains 0.05-0.2% ammonium sulfate and 0.3-0.5% P20s. The dosage of acids by plunger pumps is adjusted depending on the load of the unit.

After neutralization of excess NMO3 in a solution of ammonium nitrate from ITN devices and introduced sulfuric and phosphoric acids in after-neutralizer 4, the solution passes the control after-neutralizer 5 (where ammonia is automatically supplied only in case of acid leakage from the neutralizer 4) and enters the evaporator 6. Unlike the AS-67 unit, the upper part of the evaporator 6 equipped with two sieve washing plates, onto which steam condensate is supplied, washing the steam-air mixture from the evaporator from ammonium nitrate

Nitrate melt from an evaporator 6, passing through the water seal and neutralizer 9 and filter 10, enters the tank 11, where it comes from a submersible pump 12 supplied through a pipeline with an anti-knock nozzle to a pressure tank 15, and then to the granulators 16 or 17. The safety of the melt pumping unit is ensured by a system of automatic maintenance of the melt temperature during its evaporation in the evaporator (not higher than 190 °C), control and regulation of the melt environment after the neutralizer 9 (within 0.1-0.5 g/l NH 3), by controlling the temperature of the melt in the tank 11, pump housing 12 and pressure pipeline. If the regulatory parameters of the process deviate, the pumping of melt automatically stops, and the melt in the tanks 11 and evaporator 6 when the temperature rises, dilute with condensate.

Granulation is provided by two types of granulators: vibroacoustic 16 and monodisperse 17. Vibroacoustic granulators, which are used on large-scale units, have proven to be more reliable and convenient to use.

The melt is granulated in a rectangular metal tower 20 with plan dimensions of 8x11 m. The flight height of the granules is 55 m, which ensures the crystallization and cooling of granules with a diameter of 2-3 mm to 90-120 ° C with a counter air flow in summer up to 500 thousand m/h and in winter (at low temperatures) up to 300- 400 thousand m/h. At the bottom of the tower there are receiving cones from which the granules are conveyed by a belt conveyor 21 sent to the CC cooling apparatus 22.

Cooling apparatus 22 divided into three sections with autonomous air supply under each section of the fluidized bed grate. In its head part there is a built-in screen, which sifts out lumps of nitrate formed as a result of disruption of the granulators’ operating mode. The lumps are sent for dissolution. Air supplied to the cooling apparatus sections by fans 23, heated in the apparatus 24 due to the heat of juice steam from ITN devices. Heating is carried out when the atmospheric humidity is above 60%, and in winter to avoid sudden cooling of the granules. Ammonium nitrate granules sequentially pass through one, two or three sections of the cooling apparatus, depending on the load of the unit and the ambient air temperature. The recommended cooling temperature for the granular product in winter is below 27 °C, in summer up to 40-50 °C. When operating units in southern regions, where a significant number of days the air temperature exceeds 30 °C, the third section of the cooling apparatus operates on pre-cooled air (in an evaporative ammonia heat exchanger). The amount of air supplied to each section is 75-80 thousand m³/h. Fan pressure 3.6 kPa. Exhaust air from sections of the apparatus at a temperature of 45-60°C, containing up to 0.52 g/m 3 of ammonium nitrate dust, is sent to the granulation tower, where it is mixed with atmospheric air and supplied for washing in a washing scrubber 18.

The cooled product is sent to a warehouse or for treatment with a surfactant (NP dispersant), and then for shipment in bulk or for packaging in bags. Treatment with NF dispersant is carried out in a hollow apparatus 27 with a centrally located nozzle spraying an annular vertical stream of granules, or in a rotating drum. The quality of processing of the granular product in all used devices satisfies the requirements of GOST 2-85.

Granulated ammonium nitrate is stored in a warehouse in piles up to 11 m high. Before shipping to the consumer, the nitrate is fed from the warehouse for sieving. The non-standard product is dissolved, the solution is returned to the park. The standard product is treated with an NF dispersant and shipped to consumers.

Tanks for sulfuric and phosphoric acids and pumping equipment for their dosing are arranged in a separate unit. The central control point, electrical substation, laboratory, service and household premises are located in a separate building.

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Technology and chemical reactions of the ammonia production stage. Feedstock, synthesis product. Analysis of the technology for purifying converted gas from carbon dioxide, existing problems and development of ways to solve identified production problems.

The method of producing ammonium nitrate from coke oven gas ammonia and dilute nitric acid was no longer used as economically unprofitable.

The ammonium nitrate production technology includes the neutralization of nitric acid with ammonia gas using the heat of reaction (145 kJ/mol) to evaporate the nitrate solution. After forming a solution, usually with a concentration of 83%, excess water is evaporated to a melt, in which the ammonium nitrate content is 95 - 99.5%, depending on the grade of the finished product. For use as a fertilizer, the melt is granulated in sprayers, dried, cooled and coated with compounds to prevent caking. The color of the granules varies from white to colorless. Ammonium nitrate for use in chemistry is usually dehydrated, since it is very hygroscopic and the percentage of water in it (ω(H 2 O)) is almost impossible to obtain.

In modern plants producing practically non-caking ammonium nitrate, hot granules containing 0.4% moisture or less are cooled in fluidized bed apparatus. The cooled granules are packaged in polyethylene or five-layer paper bitumen bags. To give the granules greater strength, ensuring the possibility of bulk transportation, and to maintain the stability of the crystalline modification during a longer shelf life, additives such as magnesite, hemihydrate calcium sulfate, products of decomposition of sulfate raw materials with nitric acid and others are added to ammonium nitrate (usually no more than 0.5 % by mass).

In the production of ammonium nitrate, nitric acid is used with a concentration of more than 45% (45-58%), the content of nitrogen oxides should not exceed 0.1%. In the production of ammonium nitrate, waste from ammonia production can also be used, for example, ammonia water and tank and purge gases removed from liquid ammonia storage facilities and obtained during purging of ammonia synthesis systems. In addition, distillation gases from urea production are also used in the production of ammonium nitrate.

With rational use of the released heat of neutralization, concentrated solutions and even melted ammonium nitrate can be obtained by evaporating water. In accordance with this, there are schemes for obtaining a solution of ammonium nitrate with its subsequent evaporation (multistage process) and for obtaining melt (single-stage or non-evaporation process).

The following fundamentally different schemes for producing ammonium nitrate using neutralization heat are possible:

Installations operating at atmospheric pressure (excess pressure of juice steam 0.15-0.2 at);

Installations with a vacuum evaporator;

Installations operating under pressure, with a single use of juice steam heat;

Installations operating under pressure, using double heat from juice steam (producing concentrated melt).

In industrial practice, they are widely used as the most efficient installations operating at atmospheric pressure, using neutralization heat and partially installations with a vacuum evaporator.

The production of ammonium nitrate using this method consists of the following main stages:

1. obtaining a solution of ammonium nitrate by neutralizing nitric acid with ammonia;

2. evaporation of the ammonium nitrate solution to a melt state;

3. crystallization of salt from melt;

4. drying and cooling salt;

5. packaging.

The neutralization process is carried out in a neutralizer, which allows the heat of reaction to be used for partial evaporation of the solution - ITN. It is designed to produce ammonium nitrate solution by neutralizing 58 - 60% nitric acid with ammonia gas using the heat of reaction to partially evaporate water from the solution under atmospheric pressure according to the reaction:

NH 3 + HNO 3 = NH 4 NO 3 + Qkcal

Ammonium nitrate is obtained by neutralizing nitric acid with ammonia gas according to the reaction:

NH 3 (g) + HNO 3 (l) NH 4 NO 3 +144.9 kJ

This practically irreversible reaction occurs at high speed and releases a significant amount of heat. It is usually carried out at a pressure close to atmospheric; In some countries, neutralization plants operate under a pressure of 0.34 MPa. In the production of ammonium nitrate, diluted 47-60% nitric acid is used.

The heat of the neutralization reaction is used to evaporate water and concentrate the solution.

Industrial production includes the following stages: neutralization of nitric acid with gaseous ammonia in a heat pump apparatus (use of neutralization heat); evaporation of saltpeter solution, granulation of saltpeter melt, cooling of granules, processing of granules with surfactants, packaging, storage and loading of saltpeter, purification of gas emissions and wastewater. Additives are introduced when neutralizing nitric acid.

Figure 1 shows a diagram of a modern large-capacity AC-72 unit with a capacity of 1360 t/day.

Rice. 1.

1 - acid heater; 2 - ammonia heater; 3 - ITN devices; 4 - pre-neutralizer; 5 - evaporator; 6 - pressure tank; 7, 8 - granulators; 9, 23 fans; 10 - washing scrubber; 11 - drum; 12.14 - conveyors; 13 - elevator; 15-fluidized bed apparatus; 16 - granulation tower; 17 - collection; 18, 20 - pumps; 19 - swimming tank; 21-filter for swimming; 22 - air heater

The incoming 58-60% nitric acid is heated in heater 1 to 70-80 o C with juice steam from apparatus ITN 3 and supplied for neutralization. Before apparatus 3, thermal phosphoric and sulfuric acids are added to nitric acid in an amount of 0.3-0.5% P 2 O 5 and 0.05-0.2% ammonium sulfate, based on the finished product.

Sulfuric and phosphoric acids are supplied by plunger pumps, the performance of which is easily and accurately controlled. The unit contains two neutralization devices operating in parallel. Gaseous ammonia is also supplied here, heated in heater 2 by steam condensate to 120-130 o C. The amount of supplied nitric acid and ammonia is regulated so that at the exit from the heat pump apparatus the solution has a slight excess of nitric acid, ensuring complete absorption of ammonia.

In the lower part of the apparatus, acids are neutralized at a temperature of 155-170°C to obtain a solution containing 91-92% NH 4 NO 3. In the upper part of the apparatus, water vapor (so-called juice vapor) is washed away from splashes of ammonium nitrate and HN0 3 vapor. Part of the heat from the juice steam is used to heat the nitric acid. Next, the juice steam is sent for cleaning to washing scrubbers and then released into the atmosphere.

The acidic solution of ammonium nitrate is sent to the final neutralizer 4, where ammonia is supplied in the amount necessary to complete the neutralization of the solution. Then the solution is fed into the evaporator 5 for additional steaming, which is carried out with water vapor under a pressure of 1.4 MPa and air heated to approximately 180°C. The resulting melt, containing 99.8-99.7% nitrate, passes filter 21 at 175 °C and is fed by a centrifugal submersible pump 20 into a pressure tank 5, and then into a rectangular metal granulation tower 16 with a length of 11 m, a width of 8 m and a height of top to cone 52.8 m.

At the top of the tower there are granulators 7 and 8; Air is supplied to the lower part of the tower, cooling the drops of nitrate, which turn into granules. The height of fall of nitrate particles is 50-55m. The design of granulators ensures the production of granules of a uniform granulometric composition with a minimum content of small granules, which reduces the entrainment of dust from the tower by air. The temperature of the granules at the exit from the tower is 90-110°C, so they are sent for cooling to the fluidized bed apparatus 15. The fluidized bed apparatus is a rectangular apparatus having three sections and equipped with a grate with holes. Air is supplied under the grate by fans, which creates a boiling layer of nitrate granules 100-150 mm high, which are supplied through a conveyor from the granulation tower. The granules are intensively cooled to a temperature of 40°C (but not higher than 50°C), corresponding to the conditions of existence of modification IV. If the temperature of the cooling air is below 15°C, then before entering the fluidized bed apparatus, the air is heated in a heat exchanger to 20°C. During the cold period, 1-2 sections may be in operation.

Air from apparatus 15 enters the granulation tower to form granules and cool them.

Ammonium nitrate granules from the fluidized bed apparatus are fed by conveyor 14 for treatment with a surfactant into a rotating drum 11. Here the granules are sprayed with a sprayed 40% aqueous solution of the NF dispersant. After this, the saltpeter passes through an electromagnetic separator to separate any accidentally caught metal objects and is sent to a bunker, and then for weighing and packaging in paper or plastic bags. The bags are transported by conveyor for loading into wagons or into a warehouse.

The air leaving the top of the granulation tower is contaminated with particles of ammonium nitrate, and the juice steam from the neutralizer and the steam-air mixture from the evaporator contains unreacted ammonia and nitric acid and particles of entrained ammonium nitrate. For cleaning, six parallel-operating disc-type scrubbers 10 are installed in the upper part of the granulation tower, irrigated with a 20-30% solution of ammonium nitrate, which is supplied by pump 18 from the tank. Part of this solution is taken to the ITN neutralizer for washing juice steam, and then mixed with the ammonium nitrate solution and, therefore, goes into production.

Part of the solution (20-30%) is continuously withdrawn from the cycle, so the cycle becomes depleted and is replenished by adding water. At the outlet of each scrubber, a fan 9 with a capacity of 100,000 m 3 /h is installed, which sucks air from the granulation tower and throws it into the atmosphere.

INTRODUCTION

The nitrogen industry is one of the fastest growing industries.

Nitric acid is one of the starting products for the production of most nitrogen-containing substances and is one of the most important acids.

In terms of production scale, nitric acid ranks second among various acids after sulfuric acid. The large scale of production is explained by the fact that nitric acid and its salts have acquired very significant importance in the national economy.

Nitric acid consumption is not limited to fertilizer production. It is widely used in the production of all types of explosives, a number of technical salts, in the organic synthesis industry, in the production of sulfuric acid, in rocket technology and in many other sectors of the national economy.

The industrial production of nitric acid is based on the catalytic oxidation of ammonia with atmospheric oxygen, followed by the absorption of the resulting nitrogen oxides by water.

The purpose of this course project is to consider the first stage of nitric acid production - contact oxidation of ammonia, as well as the calculation of the material and heat balances of the reactor.

In technological schemes for the production of nitric acid, the process of catalytic oxidation of ammonia is important, since it determines three main indicators - ammonia consumption, investments and losses of platinum metals, as well as the energy capabilities of the scheme. In this regard, improving the process of catalytic oxidation of ammonia is of great importance for the production of nitric acid and mineral fertilizers in general.

1. CHARACTERISTICS OF NITRIC ACID

1.1 Varieties of nitric acid

In industry, 2 types of nitric acid are used: diluted (weak) containing 30-60% HNO3 and concentrated, containing 97-99% HNO3, as well as a relatively small amount of reactive and highly pure nitric acid. The quality of produced nitric acid must meet established standards.

In terms of physicochemical indicators, concentrated nitric acid must meet the standards specified in Table 1.

Table 1 - Quality requirements for concentrated nitric acid (GOST 701-89)

The quality of the produced nitric acid must meet the established standards indicated in Tables 2 and 3.

Table 2 - Requirements for the quality of non-concentrated nitric acid (OST 6-03-270-76)

Table 3 - Requirements for the quality of nitric acid (GOST 4461-67)

Content in%, no more than chemically pure analytical grade Pure Nitric acid 61-68 54-6061-68 54-6061-68 54-60 Nitrogen oxides (NO2) 0.10.10.1 Residue after calcination 0.0010.0030, 005 sulfates (SO42) -0,00020,000,000.002 phosphates (PO43-) 0.000020,00020,002hlorides (CL-) 0.0000550,000,0005 Zhelezo (FE) 0.0000,000,000,000,0003-kARCHIC (CA) 0.000505050505050505050505050 .0010.002 Arsenic (As) 0.0000020.0000030.00001 Heavy metals (Pb) 0.000020.00050.0005

1.2 Application of nitric acid

Nitric acid is used in various fields of activity:

1)during galvanization and chrome plating of parts;

)for the production of mineral fertilizers;

)for the production of explosives (military industry);

)in the production of drugs (pharmaceuticals);

)obtaining silver nitrate for photography;

)for etching and engraving of metal forms;

)as a raw material for the production of concentrated nitric acid;

)in hydrometallurgy;

)in jewelry - the main way to determine gold in a gold alloy;

)for the production of aromatic nitro compounds - precursors of dyes, pharmacological drugs and other compounds used in fine organic synthesis;

)to obtain nitrocellulose.

1.3 Properties of nitric acid

3.1 Physical properties of nitric acid

Nitric acid is one of the strong monobasic acids with a sharp suffocating odor, is sensitive to light and, in bright light, decomposes into one of the nitrogen oxides (also called brown gas - NO2) and water. Therefore, it is advisable to store it in dark containers. In a concentrated state, it does not dissolve aluminum and iron, so it can be stored in appropriate metal containers. Nitric acid is a strong electrolyte (like many acids) and a very strong oxidizing agent. It is often used in reactions with organic substances.

Nitrogen in nitric acid is tetravalent, oxidation state +5. Nitric acid is a colorless liquid that fumes in air, melting point -41.59 , boiling +82.6 with partial decomposition. The solubility of nitric acid in water is not limited. Aqueous solutions of HNO3 with a mass fraction of 0.95-0.98 are called “fuming nitric acid”, with a mass fraction of 0.6-0.7 - concentrated nitric acid. Forms an azeotropic mixture with water (mass fraction 68.4%, d20 = 1.41 g/cm, Tbp = 120.7 )

When crystallized from aqueous solutions, nitric acid forms crystalline hydrates:

) monohydrate HNO3·H2O, Tm = -37.62 ;

2) trihydrate HNO3 3H2O, Tm = -18.47 .

Nitric acid, like ozone, can be formed in the atmosphere during lightning flashes. Nitrogen, which makes up 78% of the composition of atmospheric air, reacts with atmospheric oxygen to form nitric oxide NO. With further oxidation in air, this oxide turns into nitrogen dioxide (brown gas NO2), which reacts with atmospheric moisture (clouds and fog), forming nitric acid.

But such a small amount is completely harmless to the ecology of the earth and living organisms. One volume of nitric acid and three volumes of hydrochloric acid form a compound called “aqua regia.” It is capable of dissolving metals (platinum and gold) that are insoluble in ordinary acids. When paper, straw, or cotton are added to this mixture, vigorous oxidation and even combustion will occur.

1.3.2 Chemical properties of nitric acid

Nitric acid exhibits different chemical properties depending on the concentration and the substance with which it reacts.

If nitric acid is concentrated:

1) does not interact with metals - iron (Fe), chromium (Cr), aluminum (Al), gold (Au), platinum (Pt), iridium (Ir), sodium (Na) due to the formation of a protective film on their surface , which does not allow the metal to oxidize further. With all other metals<#"justify">HNO3 conc + Cu = Cu(NO3)2 + 2NO2 + H2O (1)

2) with non-metals<#"justify">HNO3 conc. + P = H3PO4 + 5NO2 + H2O (2)

If nitric acid is dilute:

1) when interacting with alkaline earth metals, as well as with zinc (Zn), iron (Fe), it is oxidized to ammonia (NH3) or to ammonium nitrate (NH4NO3). For example, when reacting with magnesium (Mg):

HNO3 diluted + 4Zn = 4Zn(NO3)2 + NH4NO3 + 3H2O (3)

But nitrous oxide (N2O) can also be formed, for example, when reacting with magnesium (Mg):

HNO3 diluted + 4Mg = 4Mg(NO3)2 + N2O + 3H2O (4)

Reacts with other metals to form nitrogen oxide (NO), for example, dissolves silver (Ag):

HNO3 diluted + Ag = AgNO3 + NO + H2O (5)

2) reacts similarly with non-metals, for example with sulfur<#"justify">HNO3 diluted + S = H2SO4 + 2NO (6)

Oxidation of sulfur to the formation of sulfuric acid and the release of gas - nitrogen oxide;

3) chemical reaction with metal oxides, for example, calcium oxide:

HNO3 + CaO = Ca(NO3)2 + H2O (7)

Salt (calcium nitrate) and water are formed;

) chemical reaction with hydroxides (or bases), for example, with slaked lime:

HNO3 + Ca(OH)2 = Ca(NO3)2 + H2O (8)

Salt (calcium nitrate) and water are formed - a neutralization reaction;

) chemical reaction with salts, for example with chalk:

HNO3 + CaCO3 = Ca(NO3)2 + H2O + CO2 (9)

A salt is formed (calcium nitrate) and another acid (in this case, carbonic acid, which breaks down into water and carbon dioxide).

6) depending on the dissolved metal, the decomposition of salt at temperature occurs as follows:

a) any metal (designated as Me) to magnesium (Mg):

MeNO2 + O2 (10)

b) any metal from magnesium (Mg) to copper (Cu):

3 = MeO + NO2 + O2 (11)

c) any metal after copper (Cu):

3 = Me + NO2 + O2(12)

2. METHODS FOR OBTAINING NITRIC ACID

nitric acid catalyst ammonia

Industrial methods for producing dilute nitric acid include the following steps:

) obtaining nitric oxide (II);

2) its oxidation to nitrogen oxide (IV);

3) absorption of NO2 by water;

4) purification of exhaust gases (containing mainly molecular nitrogen) from nitrogen oxides.

Concentrated nitric acid is obtained in two ways:

1) the first method is the rectification of ternary mixtures containing nitric acid, water and water-removing substances (usually sulfuric acid or magnesium nitrate). As a result, vapors of 100% nitric acid (which condense) and aqueous solutions of the dewatering agent are obtained, the latter is evaporated and returned to production;

2) the second method is based on the reaction:

N2O4(s) + 2H2O(l) + O2(g) = 4HNO3(l) + 78.8 kJ (13)

At a pressure of 5 MPa and using pure O2, 97-98% acid is formed, containing up to 30% by weight of nitrogen oxides. The target product is obtained by distilling this solution. Nitric acid of special purity is obtained by rectification of 97-98.5% nitric acid in silicate or quartz glass equipment. The impurity content in such an acid is less than 110-6% by weight.

3. RAW MATERIAL BASE IN THE PRODUCTION OF NON-CONCENTRATED NITRIC ACID

The main raw materials for the production of non-concentrated nitric acid are currently ammonia, air and water. Auxiliary material and energy resources are catalysts for ammonia oxidation and exhaust gas purification, natural gas, steam and electricity.

1. Ammonia. Under normal conditions, it is a colorless gas with a pungent odor, highly soluble in water and other solvents, and forms hemi- and monohydrates. A turning point in the development of synthetic ammonia production was the use of the now dominant method in industry for producing hydrogen by converting methane contained in natural gas, associated petroleum gases and refined petroleum products. The content of impurities in liquid ammonia is regulated by GOST 6221-82. The most typical impurities are: water, lubricating oils, catalyst dust, scale, ammonium carbonate, dissolved gases (hydrogen, nitrogen, methane). If GOST is violated, the impurities contained in ammonia can enter the ammonia-air mixture and reduce the yield of nitrogen oxide (II), and hydrogen and methane can change the explosive limits of the ammonia-air mixture.

Air. For technical calculations, it is assumed that dry air contains [%, (vol.)]: N2 = 78.1, O2 = 21.0, Ar2 = 0.9, H2O = 0.1-2.8. There may also be traces of SO2, NH3, CO2 in the air. In the area of industrial sites, the air is polluted with dust of various origins, as well as various components of fugitive gas emissions (SO2, SO3, H2S, C2H2, Cl2, etc.). The amount of dust in the air is 0.5-1.0 mg/m3.

3. Water. It is used in the production of nitric acid for refluxing an absorption column, for generating steam during heat recovery in waste heat boilers, for cooling reaction apparatuses. To absorb nitrogen oxides, steam condensate and chemically purified water are most often used. In some schemes it is allowed to use ammonium nitrate juice steam condensate. In any case, the water used to irrigate the columns should not contain free ammonia and suspended solids, the content of chloride ion should be no more than 2 mg/l, oil no more than 1 mg/l, NH4NO3 - no more than 0.5 g/l . Chemically purified water for waste heat boilers must comply with the requirements of GOST 20995-75. Process water intended for heat removal in heat exchangers and equipment cooling (recycled water) must meet the following requirements: carbonate hardness no more than 3.6 meq/kg, suspended solids content no more than 50 mg/kg, pH value 6.5-8 ,5.

4. Oxygen. It is used primarily in the production of concentrated nitric acid using the direct synthesis method. In some cases, it is used to enrich the ammonia-air mixture when producing non-concentrated nitric acid.

4. CONTACT OXIDATION OF AMMONIA

4.1 Physico-chemical basis of the process

Modern methods of producing nitric acid are based on contact oxidation of ammonia. During the oxidation of ammonia on various catalysts and depending on the conditions, the following reactions occur:

NH3 + 5O2 = 4NO + 6H2O + 907.3 kJ (14)

4NH3 + 4O2 = 2N2O + 6H2O + 1104.9 kJ (15)

4NH3 + 3O2 = 2N2 + 6H2O + 1269.1 kJ (16)

In addition to reactions (14-16), others are also possible, occurring in the near-surface layers of the catalyst. For example, NO decomposition, interaction of N2O, NO2 and NH3:

NO N2+O2 (17)

2NH3 + 3N2O = 4N2+3H2O (18)

NH3 + 6NO2 = 7N2 + 12H2O (19)

Naturally, reaction (14) will be “useful”. Thermodynamic calculations show that reactions (14-16) practically proceed to completion.

The equilibrium constants for reverse reactions (14-16) at 900°C have the following values

(20)

(21)

(22)

K1 = ,(23)

where k1 - NO + H2O; k2 - NH3 + O2.

At 900 the catalytic conversion of ammonia into final products reaches 100%, i.e. the process is practically irreversible.

However, equations (14-16) do not reflect the actual mechanism of the process, since in this case nine molecules would have to collide simultaneously in reaction (14); in reaction (16) - seven molecules. It's almost unbelievable.

Several mechanisms have been proposed for the oxidation of ammonia on catalysts. The differences in ideas about mechanisms are as follows:

1) formation of NO and N2 through an intermediate compound on the catalyst;

2) the formation of NO occurs on the catalyst, and the formation of N2 on the catalyst and in the gas volume.

Based on what has been said (about the equilibrium constant and oxidation mechanisms), it can be stated that the selected catalyst must have high activity (high reaction rate and short contact time: as it increases, the probability of N2 formation increases) and selectivity with respect to reaction (14).

Among several mechanisms proposed by our and foreign scientists, the most widespread is the mechanism proposed by L.K. Androsov, G.K. Boreskov, D.A. Epstein.

The mechanism can be presented step by step as follows:

Stage 1 - oxidation of the platinum surface. A catalyst-oxygen peroxide complex is formed (Figure 1).

Figure 1 - Structure of the catalyst-oxygen peroxide complex

stage - diffusion and adsorption of ammonia on the oxygen-coated surface of platinum. A catalyst-oxygen-ammonia complex is formed (Figure 2).

Figure 2 - Structure of the catalyst-oxygen-ammonia complex

stage - redistribution of electronic connections, breaking of old connections and strengthening of new connections.

stage - desorption of products and diffusion into the gas flow (stable compounds of NO and H2O are removed from the surface).

The released centers again adsorb oxygen, since the diffusion rate of oxygen is higher than that of ammonia, etc. According to scientists, oxygen included in the catalyst lattice (non-platinum contact) does not participate in the oxidation reaction of ammonia (proved using the labeled atom method).

The conversion of ammonia into nitrogen, according to I.I. Berger and G.K. Boreskov, can occur in volume as a result of reactions of ammonia with both oxygen and nitric oxide.

There are kinetic, transition and diffusion regions of the process. The kinetic region is characteristic of low temperatures: it is limited by the ignition temperature of the catalyst, at which rapid spontaneous heating of its surface is observed, i.e., up to the ignition temperature, the speed is limited by the speed of the chemical reaction at the contact. At T > Tzazh, diffusion already controls the process - the chemical reaction is fast. The process moves into the diffusion region. It is this area (600-1000 ) is typical for a stationary autothermal process in industrial conditions. This inevitably entails an increase in gas volumetric velocity and a decrease in contact time.

The oxidation of ammonia on active catalysts begins earlier: on palladium (Pd) at 100 , on platinum (Pt) at 145 , on iron (Fe) at 230 , on metal oxides the temperature at which the reaction begins fluctuates within wide limits. At the same time, it reaches a sufficient speed and degree of conversion at T > 600 .

4.2 Ammonia oxidation catalysts

Almost all nitric acid plants use platinum or its alloys as a catalyst for the oxidation of ammonia.

Platinum is an expensive catalyst, but it retains high activity for a long time, has sufficient stability and mechanical strength, and is easily regenerated. Finally, with the modern mesh form of the catalyst, the use of platinum makes it possible to use the simplest type of contact devices. It ignites easily, and its consumption per unit of production is insignificant.

In the production of nitric acid, carriers are not used for platinum and its alloys, since in the presence of carriers the activity of the catalyst drops relatively quickly and makes its regeneration difficult. In modern factories, platinum is used for catalysts in the form of grids. The mesh shape creates a large catalyst surface in the contact apparatus with a relatively low platinum consumption. Typically, meshes are used in which the wire diameter is 0.045-0.09 mm with dimensions on the cell side of 0.22 mm. The area of the mesh not occupied by wire is approximately 50-60% of its total area. When using threads of a different diameter, the number of weaves is changed so that the free area not occupied by the wire remains within the specified limits.

In contact devices operating under atmospheric pressure. install from 2 to 4 grids, mostly 3, and in devices operating under pressure up to 8 atm - from 13 to 16 grids. When installing one mesh, some of the ammonia molecules do not come into contact with the catalyst, which reduces the yield of nitrogen oxide. Under the best conditions, on one mesh the degree of contact can reach 86-90%, on two meshes 95-97%, and on three meshes 98%. When working under atmospheric pressure, do not use more than 4 meshes, since with a large number of meshes, although the productivity of the contact apparatus increases, the resistance to gas flow greatly increases. The grids must fit tightly to each other, since, otherwise, a number of homogeneous reactions take place in the free space between the grids, reducing the yield of nitric oxide.

During operation, platinum meshes are greatly loosened. Their smooth and shiny threads become spongy and matte, their elastic meshes become fragile. The formation of a spongy, loose surface increases the thickness of the threads. All this creates a highly developed network surface, which increases the catalytic activity of platinum. Only poisoning of the catalyst by impurities supplied with gases can subsequently cause a decrease in its activity.

Loosening of the surface of platinum mesh over time leads to severe destruction of the mesh, which causes large losses of platinum.

Platinum intended for the manufacture of the catalyst should not contain iron, which already at 0.2% significantly reduces the yield of nitrogen oxide.

Pure platinum is quickly destroyed at high temperatures, and its smallest particles are carried away with the gas flow. Other platinum group metals in their pure form are not used as catalysts. Palladium degrades quickly. Iridium and rhodium are little active. Osmium is easily oxidized.

Platinum alloys have been studied and used that are stronger and no less active than pure platinum. In practice, alloys of platinum with iridium or rhodium and sometimes palladium are used. At high temperatures, grids made of a platinum alloy with 1% iridium are more active than platinum ones. Significantly greater activity and, in particular, mechanical strength are characteristic of alloys of platinum and rhodium.

The best yield of nitrogen oxide is obtained when working on platinum alloys that contain 10% rhodium. However, given the higher cost of rhodium compared to platinum, its content in alloys is usually reduced to 7-5%.

When ammonia is oxidized under pressure on platinum-rhodium grids, a significantly higher yield of nitrogen oxide is obtained than on pure platinum grids.

Platinum catalysts are sensitive to certain impurities contained in the feed gas. Thus, the presence of 0.00002% phosphine (PH3) in the gas reduces the degree of conversion to 80%. Less strong poisons are hydrogen sulfide, acetylene vapors, lubricating oils, iron oxides and other substances. The grids are regenerated by treating them with a 10-15% solution of hydrochloric acid at 60-70°C for 2 hours. Then the grids are thoroughly washed with distilled water, dried and calcined in a hydrogen flame. During operation, the physical structure of the mesh changes and the mechanical strength of the alloy decreases, which increases metal losses and shortens the service life of the catalyst.

4.3 Composition of the gas mixture. Optimal ammonia content in the ammonia-air mixture

Air is predominantly used to oxidize ammonia. The oxygen consumption for the oxidation of ammonia according to reaction (24) with the formation of NO can be calculated as follows:

NH3 + 5O2 = 4NO + 6H2O (24)

According to reaction (24), per 1 mole of NH3 there are 1.25 moles of O2 = , then the NH3 content can be expressed as follows:

Where - amount of NH3 mixed with air; 100 - total amount of mixture (%).

However, this is theoretical. For practical purposes, a certain excess of oxygen is used, then the ammonia concentration will be less than 14.4% (vol.).

The optimal concentration of ammonia in the ammonia-air mixture is its highest content, at which a high yield of NO is still possible at the O2:NH3 ratio< 2.

A sharp decrease in the degree of conversion is observed with a decrease in the O2:NH3 ratio< 1,7 и содержании NH3 в смеси равном 11,5 % (об.). Если увеличивать соотношение O2:NH3, например, >2, then the conversion rate increases significantly.

So the important point is:

1) on the one hand, an increase in the NH3 content in the ammonia-air mixture, i.e., a decrease in the O2:NH3 ratio, leads to a decrease in the degree of ammonia conversion;

2) on the other hand, with an increase in the NH3 content in the ammonia-air mixture, the temperature of the system increases, since more heat is released through reactions (14-16), and the degree of conversion increases, as can be seen from Table 4.

Table 4 - Dependence of the degree of ammonia conversion on its content in the ammonia-air mixture (P = 0.65 MPa)

NH3 content in the mixture, % (vol.) O2:NH3 ratio Conversion temperatures, NH3 conversion degree, %9,531,9874391,8810,421,7878693,1610,501,7678993,3011,101,6782894,2111,531,5983495,30

From Table 4 it follows that increasing the temperature from 740 to 830°C with an O2:NH3 ratio of 1.6-2 has a beneficial effect on the process. At the ratio O2:NН3< 1,35 лимитирующая стадия процесса - диффузия кислорода.

An excess of O2 is necessary to ensure that the surface of platinum is always covered with oxygen to carry out the oxidation process according to the mechanism discussed earlier and to prevent the formation of N2 and N2O (in the absence of oxygen). It must be more than 30%, i.e. O2:NH3 ratio > 1.62.

The gas composition will also depend on the occurrence of the second stage of nitric acid production (NO oxidation)

2NO + 1.5O2 + H2O = 2HNO3 (25)

It also requires excess oxygen:

1) for systems operating under pressure - 2.5%;

2) for systems operating at atmospheric pressure - 5%.

The total reaction that determines the need for oxygen to produce nitric acid is written as follows:

NH3 + 2O2 = HNO3 + H2O (26)

There is one more circumstance due to which it is undesirable to increase the ammonia concentration above 9.5% (vol.). In this case, there is a decrease in the concentration of nitrogen oxides in the absorption towers due to the introduction of additional oxygen (i.e., NO is diluted). Thus, 9.5% (vol.) is the optimal ammonia content for all stages of producing dilute nitric acid.

You can use oxygen instead of air for oxidation. Then, in accordance with the total reaction (26), it is necessary to increase the ammonia concentration to 33.3% (vol.). However, safety precautions come into force here, since a mixture with such a concentration of ammonia becomes explosive (Table 5).

Table 5 - Lower (LEL) and upper (UEL) explosive limits of ammonia-oxygen-nitrogen mixture

As the humidity of the gas increases, the explosive limits narrow, i.e., it is possible to use steam-oxygen conversion of ammonia.

Mixtures of ammonia with oxygen ignite explosively (Tf = 700-800 ). Within these temperature limits, self-ignition occurs at any ammonia content in the ammonia-oxygen mixture.

Practically used ammonia-air mixtures (ammonia concentration 9.5-11.5% (vol.)) are not explosive (Table 5). There are dependences of the explosive limits of an ammonia-air mixture on the ammonia and oxygen content at different pressures.

However, it should be noted that the explosion propagation speed is low and for an ammonia-air mixture is 0.3-0.5 m/s. That is, to eliminate the possibility of an explosion spreading, it is necessary to create a gas velocity greater than this value (0.5 m/s). This is precisely achieved by using active platinum catalysts in the process, where the contact time is 10-4 seconds and, therefore, the linear speed is more than 1.5 m/s.

4.4 Ammonia oxidation under pressure

The purpose of increasing pressure is:

1) the need to increase the speed of the process;

2) compactness of installations.

It has been thermodynamically proven that even at high pressures the yield of NO is close to 100%. Converter productivity increases with increasing pressure and increasing the number of platinum catalyst grids. As the pressure increases, the process temperature also increases above 900 . However, with increasing pressure, in order to achieve a high degree of NH3 conversion, it is necessary to increase the residence time of the gas in the converter

which in turn leads to an increase in the number of grids.

The main disadvantage is increased loss of platinum (Pt) catalyst at high temperatures. These disadvantages (loss of platinum, reduction in the degree of conversion) can be eliminated by resorting to a combined production scheme, i.e., carrying out the oxidation of NH3 at atmospheric or close to atmospheric pressure, and NO oxidation and absorption at elevated pressure. This approach is often implemented in technological schemes in many countries. At the same time, energy consumption for gas compression increases the cost of nitric acid.

4.5 Optimal conditions for ammonia oxidation

Temperature. The reaction of ammonia on platinum begins at 145 , but proceeds with a low yield of NO and the formation of predominantly elemental nitrogen. An increase in temperature leads to an increase in the yield of nitric oxide and an increase in the reaction rate. In the range of 700-1000 NO yield can be increased to 95-98%. Contact time when temperature rises from 650 to 900 is reduced by approximately five times (from 5 10-4 to 1.1 10-4 sec). The required temperature regime of the process can be maintained due to the heat of oxidation reactions. For a dry ammonia-air mixture containing 10% NH3, at a conversion rate of 96%, the theoretical increase in gas temperature is approximately 705 or about 70 for each percentage of ammonia in the original mixture. Using an ammonia-air mixture containing 9.5% ammonia, due to the thermal effect of the reaction, it is possible to reach a temperature of about 600 , to further increase the conversion temperature, preheating of the air or ammonia-air mixture is necessary. It should be borne in mind that the ammonia-air mixture can only be heated to a temperature no higher than 150-200 at a heating gas temperature of no more than 400 . Otherwise, dissociation of ammonia or its homogeneous oxidation with the formation of elemental nitrogen is possible.

The upper limit for increasing the temperature of contact oxidation of ammonia is determined by the losses of the platinum catalyst. If up to 920 the loss of platinum is to some extent compensated by an increase in the activity of the catalyst, then above this temperature the increase in catalyst losses significantly outstrips the increase in the reaction rate.

According to factory data, the optimal temperature for ammonia conversion under atmospheric pressure is about 800 ; on installations operating under a pressure of 9 atm, it is equal to 870-900 .

Pressure. The use of increased pressure when producing dilute nitric acid is mainly associated with the desire to increase the rate of oxidation of nitrogen oxide and the conversion of the resulting nitrogen dioxide into nitric acid.

Thermodynamic calculations show that even at elevated pressure the equilibrium yield of NO is close to 100%. However, a high degree of contact in this case is achieved only with a large number of catalyst networks and a higher temperature.

Recently, in industrial conditions on multilayer catalysts with thorough gas purification and a temperature of 900 managed to increase the degree of ammonia conversion to 96%. When choosing the optimal pressure, it should be borne in mind that an increase in pressure leads to an increase in platinum losses. This is explained by an increase in the temperature of catalysis, the use of multilayer meshes and increased mechanical destruction under the influence of high gas speed.

3. Ammonia content in the mixture. Air is usually used to oxidize ammonia, so the ammonia content in the mixture is determined by the oxygen content in the air. At a stoichiometric ratio of O2:NH3 = 1.25 (the ammonia content in the mixture with air is 14.4%), the yield of nitric oxide is not significant. To increase the yield of NO, some excess oxygen is required, therefore, the ammonia content in the mixture should be less than 14.4%. In factory practice, the ammonia content in the mixture is maintained within the range of 9.5-11.5%, which corresponds to the ratio O2:NH3 = 21.7.

The total reaction (26), which determines the need for oxygen during the processing of ammonia into nitric acid, gives the ratio O2:NH3 = 2, which corresponds to an ammonia content in the initial mixture of 9.5%. This suggests that increasing the ammonia concentration in the mixture above 9.5% will not ultimately lead to an increase in NO concentration, since in this case additional air will have to be introduced into the adsorption system. If an ammonia-oxygen mixture is used as the initial reagents, then, in accordance with the equation of the total reaction, it would be possible to increase the concentration of ammonia in it to 33.3%. However, the use of high concentrations of ammonia is complicated by the fact that such mixtures are explosive.

Impact of impurities. Platinum alloys are sensitive to impurities contained in the ammonia-air mixture. In the presence of 0.0002% hydrogen phosphide in the gas mixture, the degree of ammonia conversion is reduced to 80%. Less strong contact poisons are hydrogen sulfide, acetylene, chlorine, lubricating oil vapors, dust containing iron oxides, calcium oxide, sand, etc.

Preliminary purification of gases increases the operating time of the catalyst. However, over time, the catalyst is gradually poisoned and the NO output decreases. To remove poisons and contaminants, the grids are periodically regenerated by treating them with a 10-15% solution of hydrochloric acid.

5. Contact time. The optimal contact time is determined by the rate of ammonia oxidation. Most often, the oxidation rate is defined as the amount of ammonia oxidized (kg) per unit surface area (m2) per day (catalyst intensity). The duration of contact of the gas with the catalyst, or contact time, is determined by the equation:

Vsv / W

where t is the residence time of the gas in the catalyst zone, sec; Vsv is the free volume of the catalyst, m3; W - volumetric velocity under contact conditions m3 sec-1.

The maximum degree of conversion of ammonia into nitrogen oxide is achieved at a very specific time of contact of the gas with the catalyst. The optimal contact time should not be considered the one at which the maximum NO yield is achieved, but somewhat less, since it is economically advantageous to work at higher productivity even at the expense of reducing the product yield. In practical conditions, the contact time of ammonia with the catalyst ranges from 1 10-4 to 2 10-4 sec.

Mixing ammonia with air. Complete homogeneity of the ammonia-air mixture entering the contact zone is one of the main conditions for obtaining a high yield of nitrogen oxide. Good mixing of gases is of great importance not only to ensure a high degree of contact, but also protects against the risk of explosion. The design and volume of the mixer must fully ensure good mixing of the gas and prevent ammonia from leaking in separate jets onto the catalyst.

5. CONTACT DEVICES

The most complex and has undergone significant improvements is the design of the contact apparatus itself.

Figure 3 - Ostwald contact apparatus: 1 - ammonia-air mixture collector; 2 - platinum spiral; 3 - viewing window; 4 - nitrous gas collector

The first industrial contact apparatus was the Ostwald apparatus (Figure 3), consisting of two concentric pipes: an outer cast iron with a diameter of 100 mm, enameled on the inside, and an inner one made of nickel with a diameter of 65 mm. The ammonia-air mixture entered the apparatus from below through the outer pipe and fell on the catalyst located in the upper part of the inner pipe. Nitrous gases were directed downward into the manifold through an inner pipe, giving off heat to the incoming mixture.

The catalyst consisted of platinum foil strips 0.01 mm thick and 20 mm wide, rolled together into a spiral. One of the tapes is smooth, the second is corrugated with bends of 1 mm. The degree of ammonia conversion reached 90-95%, the mixture with air contained 8% NH3 (vol.), the productivity of the apparatus was 100 kg of nitric acid per day.

This form of the catalyst did not allow increasing the productivity of the apparatus by increasing its size. In the Ostwald apparatus, the uniform supply of the gas mixture was not ensured, since before entering the catalyst, the gas flow changed its direction by 180° and only then entered it. In addition, the design of the apparatus did not allow nitrogen (II) oxides to be quickly removed from the high temperature zone.

In subsequent designs of the contact apparatus, a catalyst in the form of a mesh of filaments with a diameter of 0.06 mm was used.

Figure 4 - Andreev contact apparatus: 1 - platinum grids; 2 - viewing window

The first production of nitric acid in Russia was equipped with Andreev contact devices, which produced 386 kg of nitric acid per day and were considered the most advanced in the world. The cylindrical apparatus with a diameter of 300 mm and a height of 450 mm was made of cast iron. The mixture of gases came from below (Figure 4). The platinum catalyst grid was located across the apparatus, in the middle of it.

The use of cast iron for the manufacture of this apparatus had a number of disadvantages: the occurrence of adverse reactions, contamination of platinum with scale. The degree of conversion in it did not exceed 87%.

Figure 5 - Fischer contact apparatus: 1 - nozzle; 2 - platinum mesh; 3 - insulation

The Fischer apparatus was made of aluminum, its diameter was 1000 mm, height 2000 mm (Figure 5). The bottom of the apparatus was filled with porcelain Raschig rings, and the top part was lined with refractory bricks. The design of the apparatus did not ensure a uniform supply of the ammonia-air mixture to the catalyst; the yield of oxides was 89-92% at a contact temperature of 700-720°C. The ammonia productivity of the apparatus is 600-700 kg/day. Particles of refractory brick falling on the catalyst reduced its activity.

Figure 6 - Bamag device: 1 - nozzle; 2 - platinum mesh; 3 - viewing window

The apparatus proposed by the Bamag company (Figure 6) consisted of two truncated cones connected by wide bases, between which catalyst meshes were placed. The diameter of the apparatus in the wide part was 1.1 m or 2.0 m.

The ammonia-air mixture was fed into the apparatus from below. At first the device was made of aluminum, then its upper, hot part was made of stainless steel. To better mix the mixture, Raschig rings were poured into the lower part of the apparatus.

The main disadvantage of these devices was the direction of the gas mixture onto the catalyst from below, which led to vibration of the grids and an increase in the loss of platinum.

Studies of the design of the contact apparatus have shown that the direction of the gas mixture from top to bottom stabilizes the operation of catalyst grids, reduces losses of expensive, scarce platinum catalyst, helps to increase the degree of conversion by 1.0-1.5% and allows the use of a two-stage catalyst, in which the second stage is non-platinum oxide catalyst.

When feeding the gas mixture into the apparatus from above, in its lower part, it is possible to place a layer of insulating material, as well as the coils of the steam boiler and superheater, without the risk of contamination of the catalyst with refractory dust and iron scale. This reduces reaction heat loss to the environment.

A study of the temperature distribution over the surface of the catalyst showed that the edges of the catalyst adjacent to the walls have a lower temperature, and the degree of contact decreases accordingly, reducing the overall yield of nitrogen (II) oxide. In this regard, the geometry of the supply part of the contact apparatus is of great importance; it should be a smoothly diverging cone with an apex angle of no more than 30°.

Figure 7 - Parsons apparatus: 1 - cylindrical platinum mesh; 2 quartz bottom; 3 - viewing window; 4 - insulation

In the USA, a Parsons apparatus was created with a vertical arrangement of a catalyst mesh, rolled up in the form of a four-layer cylinder with a height of 33 cm and a diameter of 29 cm (Figure 7). The platinum cylinder was placed in a metal casing lined with refractory bricks, which ensured good heat exchange with the hot catalyst. The productivity of such a device was up to 1 ton of ammonia per day, the conversion rate was 95-96%.

The advantage of this device is the large catalyst surface compared to the volume of the device. Its disadvantage is the uneven supply of the ammonia-air mixture to the catalyst. More mixture passes through the bottom of the mesh catalyst than through the top.

A number of devices of various shapes were tested: in the form of two hemispheres, a cone and a hemisphere with the gas flow directed from bottom to top. These devices did not have any special advantages even when carrying out the process up to 0.51 MPa, the degree of conversion did not exceed 90%.

Figure 8 - DuPont device: 1 - platinum mesh; 2 - grate; 3 - water jacket

When carrying out the process at elevated pressure, a DuPont apparatus has become widespread (Figure 8), consisting of cones: the upper one is made of nickel and the lower one is made of heat-resistant steel. The lower housing was equipped with a water jacket for cooling. The catalyst placed on the grate is made in the form of a package of rectangular meshes.

Now all over the world they are designing and building units for the production of diluted nitric acid with large unit capacity - up to 400-600 tons/year. Contact devices with flat layers of mesh or a layer of granular material located across the gas flow for such units should have a large diameter of up to 5-7 m. However, with an increase in the diameter of the device, the uniformity of distribution of the ammonia-air mixture over the cross section of the device worsens, and the metal consumption per unit of productivity increases , difficulties in sealing flange connections are increasing. Devices with large diameters (over 4 m) cannot be transported by rail; their manufacture at the factory site is associated with serious difficulties.

In this regard, the most promising is a converter with a radial flow of the gas mixture through a catalyst made in the form of a cylinder or cone. With this arrangement of the catalyst, it is possible, without changing the diameter of the apparatus, to increase its height and, accordingly, productivity.

The designs of devices with a cylindrical catalyst arrangement have been known for a long time (Parsons devices), but with an increase in their productivity from 4.5 kg/h to 14.3 t/h of ammonia, problems arose in the distribution of gas mixture flows, heat exchange, catalyst fastening, etc.

Figure 9 - Improved Parsons apparatus: 1 - body; 2 - covers; 3 - refrigerant collector; 4 - support device; 5 - fitting for removing nitrous gases; 6 - catalyst grids; 7 - channels for coolant; 8 - channels for gases

One of the new devices is the improved Parsons apparatus (Figure 9). It consists of a housing with covers, fittings for the input of the ammonia-air mixture and the output of nitrous gases. The catalyst is platinum mesh, placed vertically on a cylindrical surface and secured under the covers. The grids are stretched over a ceramic support device, which has horizontal channels for supplying an ammonia-air mixture to the contact grids and vertical channels for supplying coolant. The disadvantage of such a support device is that the gas supplied to the catalyst is distributed in the form of separate jets, as a result of which the catalyst area does not fully operate.

Figure 10 - Contact device with radial gas stroke: 1 - housing; 2 - cover; 3 - system of supporting elements; 4 - catalyst; 5 - grate; 6 - solid bottom

A device with a radial gas flow is proposed (Figure 10), which consists of a housing 1 and a cover with a fitting for introducing an ammonia-air mixture. At the bottom of the housing there is a fitting for introducing nitrous gases. Catalyst grids in the form of a cylinder and a cone are located vertically. However, this device also does not ensure a uniform supply of gases to the catalyst.

Figure 11 - Contact device with granular catalyst: 1 cylindrical body; 2 - cover with a central hole; 3, 4 - coaxial cylindrical perforated distribution grids; 5 - annular bottom; 6 - outlet fitting

An apparatus with a radial gas flow and a granular catalyst is proposed. Platinum metals deposited on a carrier or non-platinum catalyst tablets are used as catalysts (Figure 11).

The apparatus in Figure 11 consists of a cylindrical body 1, into the upper part of which an ammonia-air mixture is introduced, and nitrous gases are discharged into the lower part. Inside there are two coaxial cylindrical perforated distribution grids 3 and 4, between which a layer of granular catalyst 7 is placed. The outer cylinder is closed at the top with a lid 2 with a central hole, and at the bottom with a blind annular bottom 5. The inner cylinder is closed at the top with a lid, and at the bottom is connected to the outlet fitting 6.

The ammonia-air mixture at the entrance to the apparatus is divided into two streams. The main part passes into the annular gap between the walls of the housing and the external distribution cylinder and flows radially onto the catalyst. The second, smaller part passes through the hole in the cover and enters the catalyst along the axis. Uniform distribution of the gas mixture in the catalyst is not ensured.

The disadvantage of these designs is the overheating of the ammonia-air mixture by more than 200 near the solid bottom due to a decrease in gas velocity to zero. Overheating of the gas causes overheating of the catalyst grids and increased wear.

Figure 12 - Apparatus with a cone-shaped catalyst: 1 - jacket for heating gas; 2 - catalyst; 3 - support pipe device; 4 - water jacket

The apparatus (Figure 12) contains a catalyst in the form of several layers of platinum mesh, welded from triangular pieces into a cone with an apex angle of about 60°. The mesh package rests on a structure consisting of 6-12 pipes along the generatrix of the cone, through which the coolant passes. This form of catalyst has a larger specific surface area (relative to the volume of the apparatus) compared to a flat catalyst located across the gas flow. However, compared to a cylindrical catalyst, its specific surface area is smaller.

Figure 13 - Contact apparatus for the oxidation of ammonia under high pressure: 1 - housing; 2 - internal cone; 3 - switchgear; 4 - igniter; 5 - catalyst grids; 6 - steam superheater; 7 - steam boiler packages; 8 - economizer

Figure 13 shows a contact apparatus for the oxidation of ammonia under a pressure of 0.71 MPa. The device consists of two cones inserted into each other. The ammonia-air mixture enters from below into the space between the inner and outer cones, rises up and from there falls down along the inner cone. On the way to the platinum catalyst, made in the form of grids, the mixture is well mixed in a distribution device made of Raschig rings.

To measure the temperatures of the incoming gas mixture and the conversion process, the apparatus is equipped with thermocouples: four before the catalyst and four after it. For gas sampling there are steam sampling tubes: four before the catalyst and four after it. The catalyst is ignited with a nitric-hydrogen mixture supplied using a rotary burner (igniter).

Figure 14 - Contact device from Grand Parouass: 1 - body; 2 grate; 3 - platinum catalyst; 4 - armored mesh; 5 - layer of rings; 6 perforated plate; 7 - steam superheater; 8 - waste heat boiler

Among the devices operating at an average pressure of 0.40-0.50 MPa, the device from Grand Parouasse, made of stainless steel, is of interest (Figure 14). It consists of a housing, closed on top with an elliptical lid, with an inlet fitting for introducing the gas mixture. Under the cover there is a perforated cone, then a reflective partition. A distribution grid is placed above the platinum grids, on which lies a layer of six grids that act as a damper for flow velocity pulsations. The disadvantage of the apparatus is the presence of stagnant zones in the region of high catalyst temperatures, where incoming ammonia can decompose.

6. SELECTION AND DESCRIPTION OF THE TECHNOLOGICAL SCHEME FOR PRODUCTION OF NON-CONCENTRATED NITRIC ACID

Depending on the conditions of the production process, the following types of nitric acid systems are distinguished:

1) systems operating at atmospheric pressure;

2) systems operating at elevated pressure (4-8 atm);

3) combined systems in which ammonia oxidation is carried out at a lower pressure, and oxide absorption is carried out at a higher pressure.

Let's look at these technological schemes.

1) systems operating at atmospheric pressure;

Figure 15 - Installation diagram for producing dilute nitric acid at atmospheric pressure: 1 - water scrubber; 2 - cloth filter; 3 - ammonia-air fan; 4 - cardboard filter; 5 - converter; 6 - steam recovery boiler; 7 - high-speed refrigerator; 8 - refrigerator-condenser; 9 - fan for nitrous gases; 10 - absorption towers; 11 - oxidation tower; 12 - tower for absorption of nitrogen oxides by alkalis; 13 - acid cooler; 14, 15 - pumps

These systems (Figure 15) are currently no longer in use due to the bulkiness of the equipment (a large number of acidic and alkaline absorption towers), low productivity, as well as the accumulation of a certain amount of chlorine, which in acidic and alkaline absorption systems has a strong corrosive effect on the equipment, which constantly has to be replaced, and this entails large economic costs.

2) combined systems;

Figure 16 - Production of nitric acid using a combined method: 1 - high-speed refrigerator; 2 - refrigerator; 3 - turbocharger engine; 4 - gearbox; 5 - turbocharger of nitrous gases; 6 - turbine for irrigation of exhaust gases; 7 - oxidizing agent; 8 - heat exchanger; 9 - refrigerator-condenser; 10 - absorption column; 11 - acid valve; 12 - condensate collector; 13, 14 - nitric acid collections

The main advantages of this scheme are:

1. These systems (Figure 16) operate without external energy consumption, since the heat of ammonia oxidation and nitrogen oxide oxidation is sufficient to generate energy for compressing air and nitrous gases to the required pressures;

2. Compactness of the equipment.

3. The productivity of such units is 1360 tons/day.

Disadvantages of the scheme:

The main disadvantage of this scheme is that during the oxidation of ammonia under a pressure of 9 atm, the degree of conversion is 2-3% less than at atmospheric pressure, and the loss of platinum catalyst is 2-3 times greater. Thus, it is more advantageous to carry out this process under atmospheric pressure. But for modern powerful workshops producing nitric acid, in this case a large number of large-sized devices will be required and, consequently, an increase in the cost of construction and installation work. These considerations force one to resort to increasing the pressure during the ammonia conversion process. In this regard, a pressure of the order of 2.5 atm is acceptable, since the volume of the equipment is reduced by 2.5 times compared to the volume in systems operating at atmospheric pressure, with moderate losses of ammonia and catalyst.

3) systems operating under high pressure.

Advantages of the scheme (Figure 17):

1. The unit is compact, all devices are transportable. The energy cycle of the unit is autonomous and, when chemical production is turned off, remains in operation until it is disconnected from the control panel. This allows you to quickly put the unit into operation in case of accidental shutdowns of the chemical process. The control of the unit in operating mode is automated.

2. The actual cost and energy intensity of nitric acid produced on units of a single pressure of 0.716 MPa remain the lowest compared to the AK-72 unit and the unit operating according to a combined scheme.

3. Instead of a waste heat boiler, a high-temperature heat exchanger is installed behind the contact apparatus to heat the exhaust gases in front of the turbine to 1120 K. Moreover, due to an increase in the power of the gas turbine, the electricity output increased by 274 compared to the AK-72 unit.

4. In the circuit, a constantly switched-on combustion chamber is installed parallel to the technological devices, which makes it possible to make the operation of the machine unit independent of the technological line, as well as to ensure a smooth transition from the operation of the machine in idle mode to the operation of the machine when the technology process is turned on.

Disadvantages of the scheme:

1. The process takes place in the unit at elevated temperatures, which puts very heavy loads on the palladium catalyst and it fails. According to literature data, the specific irreversible losses per 1 ton of nitric acid are 40-45 mg for the process at atmospheric pressure, 100 mg at 0.3-1.6 MPa, and 130-180 mg at 0.7-0.9 MPa. That is, platinum losses increase in installations operating under pressure due to higher catalysis temperatures compared to the temperature in installations operating at atmospheric pressure.

2. A very high degree of air purification is required before entering the gas turbine unit, since the air performance of the compressor can decrease by up to 10% and efficiency by up to 6%.

This course project examines in detail the scheme for the production of nitric acid under pressure with a compressor driven by a gas turbine (Figure 17).

The production capacity of nitric acid according to the scheme operating under a pressure of 0.716 MPa is determined by the number of units. The capacity of one unit is 120 thousand tons/year (100% HNO3). The number of units in the circuit is determined by the need for nitric acid in the processing shops.

Each unit carries out: preparation of the ammonia-air mixture (air purification and compression, evaporation of liquid ammonia, purification of gaseous ammonia and ammonia-air mixture); ammonia conversion; recovery of heat from the formation of nitrogen oxides; cooling of nitrous gases; obtaining nitric acid; heating of exhaust gases; cleaning them from nitrogen oxides and recovering gas energy in a gas turbine and waste heat boiler.

In addition, the scheme includes units for preparing feed water to power waste heat boilers, cooling condensate or demineralized water for irrigating absorption columns, reducing steam to the required parameters, storing produced nitric acid and distributing it to consumers.

Figure 17 - Diagram of the production of nitric acid under pressure with a compressor driven by a gas turbine: 1 - air filter; 2 - first stage turbocharger; 3 - intermediate refrigerator; 4 - second stage turbocharger; 5 - gas turbine; 6 - gearbox; 7 - motor-generator; 8 - air heater; 9 - ammonia-air mixer; 10 - air heater; 11 - porolit filter; 12 - converter; 13 - waste heat boiler; 14 - vessel for oxidation of nitrous gases; 15 - refrigerator - condenser; 16 - absorption column; 17 - converter; 18 - waste heat boiler

Atmospheric air is sucked through filter 1 by the first stage turbocompressor 2 and compressed to 0.2-0.35 MPa. Due to compression, the air heats up to 175 . After cooling to 30-45 in refrigerator 3, air enters the second stage turbocompressor 4, where it is compressed to a final pressure of 0.73 MPa and heated to 125-135 . Further heating of the air to 270 occurs in heater 8 due to the heat of hot nitrous gases leaving the converter. Hot air then enters mixer 9.

Ammonia under a pressure of 1.0-1.2 MPa is heated to 150 in the heater 10 with water vapor and enters the mixer 9, where it is mixed with air. The resulting ammonia-air mixture containing 10-12% NH3 is filtered in a porolyte filter 11 and enters converter 12, where it runs on a platinum-rhodium catalyst at a temperature of 890-900 ammonia is oxidized to nitric oxide. The heat of the gases leaving the converter is used in waste heat boiler 13 to produce steam, while the gases are cooled to 260 .

Next, the gases pass through a filter to capture platinum, located in the upper part of the empty vessel 14. In vessel 14, NO is oxidized to NO2 (oxidation degree 80%), as a result of which the gas mixture is heated to 300-310 and enters air heater 8, where it is cooled to 175 . Further use of the heat of nitrous gases becomes unprofitable, so they are cooled with water in the refrigerator 16 to 50-55 . Simultaneously with the cooling of the gas in the refrigerator 16, condensation of water vapor occurs and the formation of nitric acid as a result of the interaction of water with nitrogen dioxide. The concentration of the resulting acid does not exceed 52% НNO3, the yield is about 50% of the total productivity of the installation.

From refrigerator 15, nitrous gases enter absorption column 16 with sieve plates, where NO2 is absorbed by water to form nitric acid (concentration up to 55%). Coils (refrigeration elements) are placed on the plates of the absorption column 16, through which water circulates to remove the heat generated during the formation of nitric acid.

To purify exhaust gases from nitrogen oxides, they are heated to 370-420°C, a small amount of natural gas is added to them and sent to converter (reactor) 17. Here, in the presence of a palladium catalyst, the following reactions occur:

CH4 + O2 2СО + 4Н2 + Q (27)

2NO2 + 4H2 = N2 + 4H2O + Q (28)

2NO + 2H2 = N2 + 2H2O + Q (29)

Since these reactions occur with the release of heat, the temperature of the gases rises to 700-730 . These gases enter turbine 5 under a pressure of 0.5-0.6 MPa, which drives turbochargers 2 and 4, which compress the air. After this, the gases at a temperature of about 400 enter the waste heat boiler 19, in which low-pressure steam is produced.

Turbochargers of the first and second stages 2 and 4, as well as gas turbine 5 are a single unit. The first stage turbine 2 and the gas turbine 5 are located on a common shaft and are connected by a gearbox 6 to the second stage turbine 4 and an electric motor 7. Such a unit makes it possible to use the bulk of the energy spent on air compression, and thus significantly reduce electricity consumption.

7. CALCULATION OF MATERIAL AND HEAT BALANCES OF THE REACTOR

7.1 Calculation of the reactor material balance

1) Calculate the required volume of air:

2) Volumes supplied with air, nm3:

a) water vapor

b) dry air

3) Let us calculate the volumes of oxygen, nitrogen and argon supplied with the air based on their percentage in the air

) Find the volumes formed by reaction (14), nm ³ /h:

a) nitrogen oxides

b) water vapor

5) Determine the volumes formed by reaction (15), nm ³ /h:

a) nitrogen

b) water vapor

c) oxygen consumed during this reaction

6) Calculate the volumes in the gas after ammonia oxidation, nm ³ /h:

a) oxygen

b) nitrogen

c) argon

d) water vapor

7) The actual material balance can be calculated if the volumes of flows at the entrance to the contact apparatus and at the exit from it are converted into masses, while the material balance must be maintained.

Coming:

Consumption:

Let's fill out the table for material balance (Table 6).

Table 6

IncomeConsumptionComponentQuantityComponentQuantitykg/hm ³ /hkg/hm ³ /hNH34477,6795900NO7348,6615487O215608,57110926O25367,8573757,5N250729,69140583,755N250987,81640790,255Ar929,116520,305Ar928520 H2O1827.022273.625 H2O8938.62711123.625Total73572.07760203.68Total73570.96161678.38

Balance discrepancy

7.2 Calculation of the reactor heat balance

Let us find the temperature tx to which it is necessary to heat the ammonia-air mixture to ensure the autothermal process of ammonia oxidation.

1) Calculate the total volume of the ammonia-air mixture

) Determine the concentration of the components of the ammonia-air mixture, % (vol.):

a) ammonia

b) dry air

c) water vapor

3) Calculate the average heat capacity of the ammonia-air mixture

Сср = 0.01 · (35.8 · Pam + 28.7 · Psv + 32.6 · ПН2О)(59)

Сср = 0.01 · (35.8 · 9.8 + 28.7 · 86.4 + 32.6 · 3.8) = 29.544 kJ/(kmol K),

where 35.8; 28.7 and 32.6 are the heat capacities of ammonia, dry air and water vapor, kJ/(kmol K).

) Determine the heat introduced by the ammonia-air mixture

) We calculate the heats released during the reaction (14) and (16)

or 17030 kW, where 905800 and 126660 are the heats released during the formation of nitric oxide and nitrogen by reactions (14) and (16).

) Find the total volume of nitrous gas entering the waste heat boiler

7) Determine the concentration of nitrous gas components, % (vol.):

a) nitrogen oxides

b) oxygen

c) argon

d) nitrogen

d) water vapor

8) Calculate the average heat capacity of nitrous gas:

Ssr = 0.01(31.68 PNO + 32.3 PO2 + 20.78 Steam 30.8 PN2 + 37.4 Pvod 3(68)

Сср=0.01(31.68 · 8.9+32.3 · 6.1+20.78 · 0.84+30.8 · 66.1+37.4 · 18.0) = 32.17 kJ/(kmol K)

where 31.68; 32.3; 20.78; 30.8 and 37.4 - heat capacities of nitrous gas components at a temperature of 900 , kJ/(kmol·K).

9) For heating water steam from 198 up to 250 In the superheater it is necessary to remove heat:

1880 kW, where 800 10 ³ and 1082·10 ³ J/kg - specific enthalpy of superheated steam at temperatures of 198 and 250 and pressures of 1.5 MPa and 3.98 MPa.

10) The temperature of nitrous gases at the outlet of the contact apparatus is determined from the heat balance equation for this section:

6768 · 106 = 64631 · 1.66 · 10³(900 - t2)

11) We calculate the heat carried away by nitrous gases. Let's consider the case when the contact device and waste heat boiler are mounted as a single device:

12) Determine heat loss to the environment

Equating the heat inflow to the flow rate, we draw up a heat balance equation and solve it with respect to tx:

Let's fill out the table for heat balance (Table 7).

Table 7

Incoming, kWConsumption, kW Heat contributed by the ammonia-air mixture 6369.2 Heat for heating water vapor in the superheater 1880 Heat carried away by nitrous gases 20584.3 Heat released during the reaction (14) and (16) 17030.6 Losses to the environment 935.9 Total 23399.8 Total23400.2

Balance discrepancy:

8. OCCUPATIONAL SAFETY AND INDUSTRIAL ECOLOGY

To ensure a safe operating mode in the production of non-concentrated nitric acid under high pressure, it is necessary to strictly follow the technological regulations, instructions on labor protection for workplaces, instructions on labor protection and industrial safety of the department, and instructions on certain types of work.

Maintenance personnel are allowed to work in the work clothing and safety shoes required by the standards, and are required to have working personal protective equipment with them. Protective equipment (personal gas mask) must be checked every shift before starting work.

Persons servicing mechanisms must know the rules of Gosgortekhnadzor related to the equipment being serviced. Persons servicing boiler inspection equipment - boiler inspection rules.

Avoid disruption of the normal technological regime at all stages of the process.

Work should be carried out only on serviceable equipment, equipped with all necessary and properly functioning safety devices, instrumentation and control devices, alarms and interlocks.

When handing over for repair equipment and communications in which ammonia accumulation is possible, purge the equipment and communications with nitrogen until there are no flammable substances in the purging nitrogen.

Before filling devices and communications with ammonia after their repair, purge with nitrogen until the oxygen content in the purge nitrogen is no more than 3.0% (vol.).

Do not allow repairs to communications, fittings, or equipment under pressure. Repairs must be carried out after releasing the pressure and disconnecting the repaired area with plugs. Equipment and communications to be repaired must be purged or washed.

To avoid water hammer, supply steam to cold steam pipelines slowly, ensuring their sufficient heating with condensate discharge along the entire length of the pipeline. The release of dry steam from the drain indicates sufficient heating of the pipeline.

Do not allow electrical equipment to be turned on if the grounding is faulty.

Do not allow repairs to electrically driven equipment without removing the voltage from the electric motors.

Repair and adjustment of control and measuring instruments and electrical equipment should only be carried out by the services of the chief instrument operator department and electricians.

The use of open fire in production and storage areas is prohibited: smoking is permitted in areas designated for these purposes.

All rotating parts of equipment (coupling halves), impellers of rotating fans, on electric motor shafts must be securely fastened and protected, and painted red.

Flange connections of acid lines must be protected by protective covers.

Tightening the bolts of flange connections of pipelines, as well as working on equipment under pressure, is not allowed.

Apparatuses operating under pressure must meet the requirements set out in the technical specifications and rules for the design and safe operation of vessels and communications operating under pressure.

Work in closed vessels must be carried out with a work permit for gas hazardous work.

Ventilation must be in good condition and constantly in operation.

Servicing of lifting mechanisms and pressure vessels is carried out only by persons specially trained and holding a special certificate.

Approaches to emergency cabinets, fire detectors, telephones, and fire equipment must not be cluttered with foreign objects; they must be kept clean and in good condition.

Open openings in ceilings, platforms, and transition bridges must have fences 1 m high. At the bottom of the fence there should be a side or protective strip 15 cm high.

All instrumentation and automation and interlock systems must be in good condition.

To prevent the deposition of nitrite-nitrate salts on the internal surfaces of devices and pipelines, rotor blades, walls of nitrous gas compressors and other parts and devices, do not allow prolonged ignition of contact devices (more than 20 minutes), a decrease in the temperature of the catalyst grids, their rupture, leading to ammonia leaks , stopping irrigation of surfaces, which leads to the deposition of nitrite-nitrate salts.

Promptly wipe down, clean equipment from spills of technological products, and add oil to pump crankcases.

Workplaces for repair and other work and passages to them at a height of 1.3 m or more must be fenced.

If it is impossible or impractical to install fences for work at a height of 1.3 m and above, as well as when working from an extension ladder at a height of more than 1.3 m, it is necessary to use safety belts, and there must be auxiliary workers at the work site who are ready to assist the worker on high. The place where the carbine is attached is determined by the work supervisor.

Safety belts are tested before being put into operation, as well as during operation every 6 months. The safety belt must be labeled with the registration number and the next test date.

When working with nitric acid (sampling, inspection of communications, starting production acid pumps, etc.), it is necessary to use individual respiratory and eye protection (filtering gas mask with an “M” brand box, safety glasses with a rubber half mask or a protective shield made of plexiglass, or a gas mask helmet), rubber acid-proof gloves, special acid-proof clothing.

If any malfunctions in the operation of the equipment, defects in supports, walls, etc. are detected. promptly notify the department head and workshop mechanic. If necessary, stop the equipment and prepare it for repair.

Whenever the unit is stopped for repairs, open the lower hatch of the oxidizer and, if there are ammonium salts on the distribution grid, along the walls and bottom, steam it with live steam and drain the condensate.

Work with steam and steam condensate in special clothing, safety shoes, and gloves.

To prevent occupational poisoning and diseases in the department the following sanitary and hygienic requirements must be observed:

a) the air temperature should be:

23- transition and winter period;

18-27- summer period.

b) relative air humidity:

in summer - no more than 75%;

in winter - no more than 65%.

c) noise - no more than 65 dBA in soundproof cabins, in other places no more than 80 dBA;

d) vibration - no more than 75 dB in soundproof cabins, in the engine and contact rooms no more than 92 dB;

e) illumination of workplaces:

soundproof cabins - at least 200 lux;

at the sites of absorption columns - at least 50 lux;

in the engine and contact rooms - at least 75 lux.

f) maximum permissible concentration of harmful substances in the air of the working area of the premises:

ammonia - no more than 20 mg/m3;

nitrogen oxides - no more than 5 mg/m3.

In addition to individual gas masks, the department contains an emergency supply of filtering and insulating gas masks.

Emergency gas masks are stored in emergency cabinets.

CONCLUSION

During the course work, a reactor for the catalytic oxidation of ammonia was designed to produce nitrogen oxides in the production of non-concentrated nitric acid.

The physical and chemical foundations of the process were considered. The characteristics of the raw materials and the finished product are given.

The required volume of air for oxidation was calculated to be 5900 m ³ /h of ammonia, it amounted to 54304 m ³ /h. The volumes of oxygen, nitrogen and argon supplied with the air were calculated based on their percentage in the air. The volumes of oxygen, nitrogen, argon, and water vapor present in the gas after the oxidation of ammonia were also calculated.

The heat balance was calculated, as a result of which all heat flows were calculated. The temperature to which it is necessary to heat the ammonia-air mixture to ensure the autothermal process of ammonia oxidation was calculated; it was 288 . The temperature of nitrous gas after the superheater was calculated; it was 836.7 . Heat loss to the environment has been determined.

A literature review was carried out on the most effective scheme for the production of non-concentrated nitric acid. A system operating under high pressure was chosen, since this unit is compact, all devices are transportable, and the energy cycle of the unit is autonomous. In the considered scheme, electricity is not spent on technological needs. Electricity is consumed in small quantities only to drive the pumps necessary to pump acid and supply feedwater to the boilers. Work according to this scheme occurs without emissions of harmful gases into the atmosphere.

BIBLIOGRAPHICAL LIST

1. Atroshchenko V.I., Kargin S.I. Nitric acid technology: Textbook. A manual for universities. - 3rd ed., revised. and additional - M.: Chemistry, 1970. - 496 p.

Egorov A.P. Shereshevsky A.I., Shmanenko I.V. General chemical technology of inorganic substances: Textbook for technical schools. - Ed. 4th revision - Moscow, Leningrad: Chemistry, 1965 - 688 p.

Karavaev M.M., Zasorin A.P., Kleshchev N.F. Catalytic oxidation of ammonia/Ed. Karavaeva M.M. - M.: Chemistry, 1983. - 232 p.

Catalysts in the nitrogen industry./Ed. Atroshchenko V.I. - Kharkov: Vishcha school, 1977. - 144 p.

General chemical technology. Edited by prof. Amelina A.G. M.: Chemistry, 1977. - 400 s.

Pavlov K.F., Romankov P.G., Noskov A.A. Examples and tasks for the course on processes and apparatus of chemical technology. L.: Chemistry, 1976 - 552 p.

Perlov E.I., Bagdasaryan V.S. Optimization of nitric acid production. M.: Chemistry, 1983. - 208 p.

Calculations on the technology of inorganic substances: Textbook. A manual for universities/Pozin M.E., Kopylev B.A., Belchenko G.V. and etc.; Ed. Pozina M.E. 2nd ed. reworked and additional - L.: Chemistry. Leningr. department, 1977 - 496 p.

Rumyantsev O.V. Equipment for high-pressure synthesis workshops in the nitrogen industry; Textbook for universities. - M.: Chemistry, 1970 - 376 p.

10. Sokolov R.S. Chemical technology: textbook. aid for students higher textbook institutions: V 2 T. - M.: Humanit ed. VLADOS center, 2000. - T.1: Chemical production in anthropogenic activities. Basic issues of chemical technology. Production of inorganic substances. - 368 p.

Nitrogenist's Handbook./Ed. Melnikova E.Ya. - T.2: Production of nitric acid. Production of nitrogen fertilizers. Materials and basic special equipment. Energy supply. Safety precautions. - M.: Chemistry - 1969. - 448 p.

Chemical technology of inorganic substances: 2 books. Book 1. Textbook / T.G. Akhmetov, R.G. Porfiryeva, L.G. Gysin. - M.: Higher. school, 2002. 688 p.: ill.

Korobochkin V.V. Nitric acid technology. - Tomsk Polytechnic University Publishing House. 2012.

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Introduction

2. Production methods

3. The main stages of the production of ammonium nitrate from ammonia and nitric acid

3.1 Preparation of ammonium nitrate solutions

3.1.1 Basics of the neutralization process

3. 1 5 Main equipment

4. Material and energy calculations

5. Thermodynamic calculation

6. Recycling and neutralization of waste in the production of ammonium nitrate

Conclusion

List of sources used

Appendix A

Introduction

In nature and in human life, nitrogen is extremely important. It is part of protein compounds (16-18%), which are the basis of the plant and animal world. A person consumes 80-100 g of protein daily, which corresponds to 12-17 g of nitrogen.

For normal plant development, many chemical elements are required. The main ones are carbon, oxygen, hydrogen, nitrogen, phosphorus, magnesium, sulfur, calcium, potassium and iron. The first three elements of a plant are obtained from air and water, the rest are extracted from the soil.

Nitrogen plays a particularly important role in the mineral nutrition of plants, although its average content in plant mass does not exceed 1.5%. Without nitrogen, no plant can live or develop normally.

Nitrogen is a component not only of plant proteins, but also of chlorophyll, with the help of which plants, under the influence of solar energy, absorb carbon from carbon dioxide CO2 in the atmosphere.

It is known that almost every fertilizer has physiological acidity or alkalinity. Depending on this, it can have an acidifying or alkalizing effect on the soil, which is taken into account when using it for certain agricultural crops.

Fertilizers, the alkaline cations of which are more quickly extracted by plants from the soil, cause acidification; Plants that consume acidic anions from fertilizers more quickly contribute to soil alkalization.

Nitrogen fertilizers containing the ammonium cation NH4 (ammonium nitrate, ammonium sulfate) and the amide group NH2 (urea) acidify the soil. The acidifying effect of ammonium nitrate is weaker than ammonium sulfate.

Depending on the nature of the soil, climatic and other conditions, different amounts of nitrogen are required for different crops.

Ammonium nitrate (ammonium nitrate, or ammonium nitrate) occupies a significant place in the range of nitrogen fertilizers, the global production of which amounts to millions of tons per year.

Currently, approximately 50% of nitrogen fertilizers used in agriculture in our country are ammonium nitrate.

Table 1 - Nitrogen content in compounds

Ammonium nitrate is a universal nitrogen fertilizer, as it simultaneously contains ammonium and nitrate forms of nitrogen. It is effective in all zones, for almost all crops.

These properties of ammonium nitrate have a very positive effect on increasing the yield of almost all agricultural crops.

The high nitrogen content in ammonium nitrate, the relatively simple method of its production and the relatively low cost per unit of nitrogen create good preconditions for the further development of this production.

Ammonium nitrate is part of a large group of stable explosives. Explosives based on ammonium nitrate and ammonium nitrate, pure or treated with certain additives, are used for blasting operations.

A small amount of saltpeter is used to produce nitrous oxide, used in medicine.

1. Physico-chemical properties of ammonium nitrate

In its pure form, ammonium nitrate is a white crystalline substance containing 35% nitrogen, 60% oxygen and 5% hydrogen. The technical product is white with a yellowish tint and contains at least 34.2% nitrogen.

Ammonium nitrate is a strong oxidizing agent for a number of inorganic and organic compounds. It reacts violently with the melts of some substances, even to the point of explosion (for example, with sodium nitrite NaNO2).

If gaseous ammonia is passed over solid ammonium nitrate, a very mobile liquid is quickly formed - ammonia 2NH4NO3*2NH3 or NH4NO3*3NH3.

Ammonium nitrate is highly soluble in water, ethyl and methyl alcohols, pyridine, acetone and liquid ammonia. With increasing temperature, the solubility of ammonium nitrate increases significantly.

When ammonium nitrate is dissolved in water, a large amount of heat is absorbed. For example, when 1 mole of crystalline NH4NO3 is dissolved in 220-400 moles of water and a temperature of 10-15 °C, 6.4 kcal of heat is absorbed.

Ammonium nitrate has the ability to sublimate. When ammonium nitrate is stored under conditions of elevated temperature and air humidity, its volume approximately doubles, which usually leads to rupture of the container.

Under a microscope, pores and cracks are clearly visible on the surface of ammonium nitrate granules. The increased porosity of nitrate granules has a very negative effect on the physical properties of the finished product.

Moisture exchange occurs between air and hygroscopic salt. Relative air humidity has a decisive influence on this process.

Salts that can reduce the hygroscopicity of ammonium nitrate must be added in large quantities (for example, potassium sulfate, potassium chloride, diammonium phosphate), which sharply reduces the nitrogen content in the product.

A negative property of ammonium nitrate is its ability to cake - to lose its flowability (crumbly) during storage. In this case, ammonium nitrate turns into a solid monolithic mass, difficult to grind. Caking of ammonium nitrate is caused by many reasons.

As the ambient temperature increases, saltpeter granules almost completely lose their strength, and such a product cakes heavily.

With prolonged heating, ammonium nitrate gradually decomposes into ammonia and nitric acid:

NH4NO3=NH3+HNO3 - 174598.32 J (1)

This process, which occurs with heat absorption, begins at temperatures above 110°C.

With further heating, ammonium nitrate decomposes to form nitrous oxide and water:

NH4NO3= N2O + 2H2O + 36902.88 J (2)

The thermal decomposition of ammonium nitrate occurs in the following successive stages:

· hydrolysis (or dissociation) of NH4NO3 molecules;

· thermal decomposition of nitric acid formed during hydrolysis;

· interaction of nitrogen dioxide and ammonia formed in the first two stages.

When ammonium nitrate is intensively heated to 220--240 °C, its decomposition may be accompanied by outbreaks of a molten mass.

Heating ammonium nitrate in a closed volume or in a volume with a limited release of gases formed during the thermal decomposition of nitrate is very dangerous.

In these cases, the decomposition of ammonium nitrate can proceed through many reactions, in particular, through the following:

NH4NO3 = N2+2H2O + S 02 + 1401.64 J/kg (3)

2NH4NO3 = N2 + 2NO+ 4H20 + 359.82 J/kg (4)

3NH4NO3= 2N2 + N0 + N02 + 6H20 + 966.50 J/kg (5)

NH4NO3=N2+ 2H20 (6)

Ammonium nitrate does not react with iron, tin and aluminum even in a molten state.

With increasing humidity and increasing particle size of ammonium nitrate, its sensitivity to explosion greatly decreases. In the presence of approximately 3% moisture, saltpeter becomes insensitive to explosion even when exposed to a strong detonator.

The thermal decomposition of ammonium nitrate increases with increasing pressure to a certain limit. It has been established that at a pressure of about 6 kgf/cm2 and the corresponding temperature, the entire molten nitrate decomposes.

Considering that, under certain conditions, ammonium nitrate can be an explosive product, during its production, storage and transportation, the established technological regime and safety regulations must be strictly observed.

Ammonium nitrate is a non-flammable product. Only nitrous oxide, formed during the thermal decomposition of salt, supports combustion.

2 . Production methods

ammonium nitrate neutralization acid

In industry, only the method of producing ammonium nitrate from synthetic ammonia (or ammonia-containing gases) and dilute nitric acid is widely used.

The production of ammonium nitrate from synthetic ammonia (or ammonia-containing gases) and nitric acid is multi-stage. In this regard, they tried to obtain ammonium nitrate directly from ammonia, nitrogen oxides, oxygen and water vapor by the reaction

4NH3 + 4NO2 + 02 + 2H20 = 4NH4NO3 (7)

However, this method had to be abandoned, since along with ammonium nitrate, ammonium nitrite was formed - an unstable and explosive product.

A number of improvements have been introduced into the production of ammonium nitrate from ammonia and nitric acid, which have made it possible to reduce capital costs for the construction of new plants and reduce the cost of the finished product.

It is firmly established in Russia and abroad that only the construction of high-power units, using modern achievements of science and technology, can provide significant economic advantages compared to existing ammonium nitrate production.

The method for producing ammonium nitrate from ammonia-containing gases differs from the method for producing it from gaseous ammonia only at the neutralization stage.

Ammonium nitrate is obtained in small quantities by exchange decomposition of salts (conversion methods).

All methods for producing ammonium nitrate by exchange decomposition of salts are complex and involve high steam consumption and loss of bound nitrogen. They are usually used in industry only when it is necessary to utilize nitrogen compounds obtained as by-products.

The modern method of producing ammonium nitrate from gaseous ammonia (or ammonia-containing gases) and nitric acid is constantly being improved.

3 . The main stages of the production of ammonium nitrate from ammonia and nitric acid

The ammonium nitrate production process consists of the following main stages:

1. Preparation of ammonium nitrate solutions by neutralizing nitric acid with gaseous ammonia or ammonia-containing gases.

2. Evaporation of ammonium nitrate solutions to a melt state.

3. Crystallization from melted salt in the form of round-shaped particles (granules), flakes (plates) and small crystals.

If gaseous ammonia is passed over solid ammonium nitrate, a very mobile liquid is quickly formed - ammonia 2NH4NO32NH3 or NH4NO33NH3.