Ammonium nitrate is one of the main types of nitrogen fertilizers; contains at least 34.2% nitrogen. Raw material for granulated ammonium nitrate are non-concentrated 30-40% nitric acid and gaseous ammonia.

As a conditioning agent, 92.5% sulfuric acid is sometimes used, which is neutralized with ammonia together with nitric acid to form ammonium sulfate. For spraying finished granules, a surfactant is used - a 40% aqueous solution of the "NF" dispersant.

The main stages of ammonium nitrate production are: neutralization of nitric acid with gaseous ammonia; obtaining a highly concentrated melt of ammonium nitrate; melt granulation; cooling of ammonium nitrate granules; treatment of granules with a surfactant - dispersant "NF"; purification of air and juice vapor before being released into the atmosphere; packaging and storage of the finished product.

Technological scheme of production

Ammonium nitrate is one of the most common nitrogen fertilizers. It is obtained by neutralization of dilute nitric acid (40--50%) with gaseous ammonia.

Nitric acid from the receiving tank 1 (Fig. 9.8) passes through the heat exchanger 2 and enters the neutralizer 3. Gaseous ammonia preheated in the heat exchanger 5 is also supplied there. The main amount of ammonia comes in a gaseous state from the ammonia synthesis shop. Additionally, liquid ammonia is supplied from the warehouse, which evaporates in apparatus 4.

In the neutralizer 3, at atmospheric pressure and a certain temperature, the neutralization process takes place

in parallel with it, a partial evaporation of the solution occurs due to the heat of neutralization. Partially one stripped off weakly acid solution of ammonium nitrate with a concentration of 60--80% (the so-called weak lye) enters the tank with a stirrer - doneutralizer 6, where it is finally neutralized with ammonia. The vapor generated during the evaporation of the solution (juice vapor) is removed from the upper part of the neutralizer. If the process is not carried out correctly, part of the ammonia and nitric acid can be carried away from the neutralizer with juice steam.

Evaporation of weak liquor to 98.5% NH4NO3 is carried out under vacuum in two stages. Initially, in the evaporator 8, the concentration of liquor is brought to 82% NH4NO3, and then in the evaporator 12, to the predetermined one.

Weak liquor is fed into the lower part of the evaporator 8. Juice steam is mainly used as a heating agent in the evaporator of the first stage. Additionally, water vapor is supplied to it. As the concentration of juice vapor increases, inert gases accumulate in the heating chamber of the evaporator, which impair heat transfer. To ensure the normal operation of apparatus 8, the annular space is purged with the release of inert gases into the atmosphere.

One stripped off liquor from the apparatus 8 is moved to the collector 10. Here, to improve the quality of the resulting saltpeter, a solution of dolomite is added to the liquor, which reduces the caking of saltpeter.

From the collector 10, the liquor is pumped into the evaporator 12. In the separator 13, the evaporated solution is separated into juice vapor and a concentrated solution - melt. Juice vapor passes into the barometric condenser 14, and the melt is fed into the granulation tower 15. Granulated ammonium nitrate (final product) is removed from the tower through the outlet pipe 16 by the conveyor 17.

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

Ammonium nitrate dissolves well in water and has a high hygroscopicity (the ability to absorb moisture from the air), which causes the fertilizer granules to spread, lose their crystalline shape, fertilizer caking occurs - the bulk material turns into a solid monolithic mass.

circuit diagram ammonium nitrate production

To obtain a practically non-caking ammonium nitrate, a number of technological methods are used. An effective remedy reducing the rate of absorption of moisture 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, therefore, granular fertilizers absorb moisture more slowly from

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

NH 3 + HNO 3 \u003d NH 4 NO 3; ΔН = -144.9kJ

The chemical reaction proceeds at a high rate; in an industrial reactor, it is limited by the dissolution of the gas in the liquid. Mixing of the reactants is of great importance to reduce diffusion retardation.

Technological process 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 granules with surfactants, packaging, storage and loading of nitrate, purification of gas emissions and wastewater. On fig. 8.8 shows a diagram of a modern large-capacity unit for the production of ammonium nitrate AS-72 with a capacity of 1360 tons / day. The original 58-60% nitric acid is heated in the heater to 70 - 80°C with juice vapor from the apparatus ITN 3 and is fed to neutralization. Before apparatuses 3, phosphoric and sulfuric 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 is equipped with two ITN devices operating in parallel. In addition to nitric acid, gaseous ammonia is supplied to them, preheated in the heater 2 with steam condensate to 120-130°C. The amounts of supplied nitric acid and ammonia are regulated in such a way that at the outlet of the ITN apparatus the solution has a slight excess of acid (2-5 g/l), which ensures the complete absorption of ammonia.

In the lower part of the apparatus, a neutralization reaction takes place at a temperature of 155-170°C; this produces a concentrated solution containing 91-92% NH 4 NO 3 . In the upper part of the apparatus, water vapor (the so-called juice vapor) is washed from splashes of ammonium nitrate and nitric acid vapor. Part of the heat of the juice vapor is used to heat the nitric acid. Then the juice steam is sent for purification and released into the atmosphere.

Fig. 8.8. Scheme of the AS-72 ammonium nitrate unit:

1 – acid heater; 2 – ammonia heater; 3 – ITN devices; 4 - after-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 - tank for swimming; 21 - filter for swimming; 22 - air heater.

An acid solution of ammonium nitrate is sent to the neutralizer 4; where ammonia enters, necessary for interaction with the remaining nitric acid. Then the solution is fed into the evaporator 5. The resulting melt, containing 99.7-99.8% nitrate, at 175 submersible pump 20 is fed into the pressure tank 6 and then into the rectangular metal granulation tower 16.

In the upper part of the tower there are granulators 7 and 8, the lower part of which is supplied with air, which cools the saltpeter drops falling from above. During the fall of saltpeter drops from a height of 50-55 m, fertilizer granules are formed when air flows around them. The temperature of the pellets at the outlet of the tower is 90-110°C; the hot granules are cooled in a fluidized bed apparatus 15. This is a rectangular apparatus having three sections and equipped with a grate with holes. Fans supply air under the grate; this creates a fluidized bed of nitrate granules coming through the conveyor from the granulation tower. The air after cooling enters the granulation tower. Granules of ammonium nitrate conveyor 14 is served for treatment with surfactants in a rotating drum. Then the finished fertilizer is sent to the packaging by the conveyor 12.

The air leaving the granulation tower is contaminated with ammonium nitrate particles, and the juice vapor from the neutralizer and the vapor-air mixture from the evaporator contain unreacted ammonia and nitric acid, as well as particles of carried-away ammonium nitrate.

To clean these streams in the upper part of the granulation tower, there are six parallel-operating washing tray-type scrubbers 10, irrigated with a 20-30% solution of ammonium nitrate, which is supplied by pump 18 from collection 17. Part of this solution is diverted to the ITN neutralizer for washing juice vapor, and then mixed with a solution of saltpeter, and, therefore, used to make products. The purified air is sucked out of the granulation tower by fan 9 and released into the atmosphere.

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• Introduction
• 1. Production of ammonium nitrate
• 2. Raw material
• 3. Synthesis of ammonia
• 4. Characteristics of the target product
• 5. Physical and chemical substantiation of the main processes for the production of the target product and environmental safety of production

Introduction

The most important view mineral fertilizers are nitrogenous: ammonium nitrate, carbamide, 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 building a living cell. Plants can only consume bound nitrogen - in the form of nitrates, ammonium salts or amides. Relatively small amounts of bound nitrogen are formed from atmospheric nitrogen due to the activity of soil microorganisms. However, modern intensive farming can no longer exist without the 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), physiologically acidic and physiologically alkaline fertilizers are also distinguished.

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 assimilated by plants. Granular ammonium nitrate is used on a large scale before sowing and for all types of top dressing. On a smaller scale, it is used for the production of 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, lose their crystalline form, caking of fertilizers occurs - loose 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 - effective and most common nitrogen fertilizer containing about 33-34% nitrogen; has physiological acidity.

2. Raw material

The feedstock in the production of ammonium nitrate is 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, “smokes”, since its vapors form small droplets of fog with “moisture in the air. Nitric acid does not differ in strength, Already under the influence of light, it gradually decomposes:

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

Nitric acid is one of the strongest acids; in dilute solutions, it completely decomposes into H and -NO ions. Nitric acid is one of the most important connections nitrogen: in large quantities ah, it is consumed in the production of nitrogen fertilizers, explosives and organic dyes, serves as an oxidizing agent in many chemical processes, is used in the production of sulfuric acid by the nitrous method, is used for the manufacture of cellulose varnishes, 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 you pass a mixture of ammonia with air over the catalyst, then at 750 ° C and a certain composition of the mixture, almost complete transformation occurs

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

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

Nitric acid obtained by oxidation of ammonia has a concentration not exceeding 60%. If necessary, concentrate

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

3. Synthesis of ammonia

ammonia nitric nitrate raw material

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.

Physical and chemical bases of the process . The synthesis of ammonia from the elements is carried out according to the reaction equation

N 2 + 3H 2 \u003d 2NH 3; ?H<0

The reaction is reversible, exothermic, characterized by a large negative enthalpy effect (?H = -91.96 kJ/mol) and becomes even more exothermic at high temperatures (?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? S, an increase in temperature reduces the likelihood of a reaction occurring,

The ammonia synthesis reaction proceeds with a decrease in volume. According to the reaction equation, 4 mol of the initial gaseous reactants form 2 mol of the gaseous product. Based on Le Chatelier's principle, it can be concluded 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; pure product - 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) belongs to systems with a simple eutectic. 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 HH 4 MO 3 -H 2 O system. The right branch of the liquidus curve is the solubility curve of MH 4 MO 3 in water. This curve has three breaking points corresponding to the temperatures of the 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 salt moisture content.

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

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

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

t crist \u003d 169.6 - 13.2X + 2, OU. (11.7).

Ammonium nitrate dissolves in water with heat absorption. Below are the values ​​​​of the heats of dissolution (Qsolv) of ammonium nitrate of various concentrations in water at 25 ° C:

C (NH 4 NO 3) % 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, liquid ammonia.

Rice. 11-2. System State DiagramNH4 N03 - H20

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

In 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, the weight loss does not exceed 6%/day. The dissociation rate depends not only on temperature, but also on the ratio between the surface of the 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 character of thermal decomposition.

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

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

Nitrogen dioxide, which is formed during the thermal decomposition of nitric acid, which is a product of the dissociation of ammonium nitrate, has a noticeable effect on the rate of thermal decomposition.

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 greater than the thermal effect of the decomposition reaction of saltpeter into N 2 O and H 2 O. ammonium nitrate can lead to its rapid decomposition.

When saltpeter is heated in a closed system at 210-220 ° C, ammonia accumulates, the concentration of nitric acid decreases, 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 amounts of gas. Due to the heat of reaction, the temperature of the mixture is greatly increased, and the 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 the induction period, decomposition will proceed at a rate corresponding to the decomposition of saltpeter at a given temperature. During this period, in addition to decomposition, other processes will also take place, the result of which, in particular, is 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 a high rate and is accompanied by a strong release of heat and the formation of a large amount of toxic gases. At great content chloride decomposition of the entire mass of ammonium nitrate quickly ends. In view of this, the content of chlorides 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 grease (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. As a container, bags made of polyethylene or kraft paper are used. 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 polyethylene and kraft paper bags must be cleaned of product residues and, if not usable, 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 by a detonator capsule is impossible, this requires large charges of powerful explosives.

The explosive decomposition of saltpeter 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 have been 1.48 MJ / kg. However, due to side reactions, one of which is endothermic (11.9), the actual heat of explosion is 0.96 MJ/kg, which is small compared to the heat of RDX explosion (5.45 MJ). But for such a large-tonnage product as ammonium nitrate, taking into account its explosive properties (albeit weak ones) is important for ensuring safety.

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

The strength of the 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 for the production of the target product and environmental safety of production

To obtain a practically non-caking ammonium nitrate, a number of technological methods are used. An effective means of reducing the rate of absorption of moisture 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 alloyed with less hygroscopic salts, such as ammonium sulfate.

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

NH 3 + HNO 3 \u003d NH 4 NO 3

?H = -144.9 kJ (VIII)

The chemical reaction proceeds at a high rate; in an industrial reactor, it is limited by the dissolution of the gas in the liquid. Mixing of the reactants is of great importance to reduce diffusion drag.

Intensive conditions for carrying out the process can be ensured to a large extent by 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 the circulation of the solution. A bubbler is placed slightly above the holes inside the glass. 2 for supplying gaseous ammonia, above it - a bubbler 3 to supply nitric acid. The reaction vapour-liquid mixture exits the top of the reaction beaker; part of the solution is removed from the ITN apparatus and enters the after-neutralizer, and the rest (circulating) goes down again. Juice vapor released from the vapor-liquid mixture is washed on capped plates 6 from splashes of ammonium nitrate solution and nitric acid vapors with a 20% solution of nitrate, and then juice steam condensate.

The heat of reaction (VIII) is used to partially evaporate water from reaction mixture(hence the name of the apparatus - ITN). The temperature difference in different parts apparatus leads to more intensive 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 granules with surfactants, packaging, storage and loading of nitrate, purification of gas emissions and wastewater.

On 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 given. Initial 58-60% nitric acid is heated in the heater / up to 70-80 With juice steam from the ITN apparatus 3 and sent for neutralization. In front of the machines 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 is equipped with two ITN devices operating in parallel. In addition to nitric acid, they are supplied with gaseous ammonia, preheated in a heater. 2 steam condensate up to 120-130 °С. The amount of supplied nitric acid and ammonia is regulated in such a way that at the outlet of the ITN apparatus the solution has a slight excess of acid (2-5 g/l), which ensures the complete absorption of ammonia.

Nitric acid (58-60%) is heated in the apparatus 2 up to 80-90 °С with juice steam from the ITN apparatus 8. Gaseous ammonia 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% solution of NH 4 NO 3 leaving the ITN devices at 155-170 ° C has an excess of nitric acid in the range of 2-5 g / l, which ensures the complete absorption of ammonia.

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

Solution from ITN devices 8 sequentially passes after-neutralizer 4 and control converter 5. To the neutralizer 4 dose sulfuric and phosphoric acids in an amount that ensures the content in the finished product of 0.05-0.2% ammonium sulfate and 0.3-0.5% P20s. The dosage of acids by plunger pumps is regulated depending on the load of the unit.

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

Saltpeter melt from the evaporator 6, after passing the water seal 9 and filter 10, enters the tank 11, from where its submersible pump 12 through a pipeline with an anti-knock nozzle is fed into a pressure tank 15, and then to the granulators 16 or 17. The safety of the melt pumping unit is ensured by the system of automatic maintenance of the temperature of the melt during its evaporation in the evaporator (not higher than 190 °C), control and regulation of the melt medium after the after-neutralizer 9 (within 0.1-0.5 g/l NH 3), temperature control of the melt in the tank 11, pump housing 12 and pressure pipeline. If the regulatory parameters of the process deviate, the pumping of the 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 operated on large-capacity units, turned out to be more reliable and convenient in operation.

The melt is granulated in a rectangular metal tower 20 with dimensions in terms of 8x11 m. The flight height of granules of 55 m provides 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. In the lower part of the tower there are receiving cones, from which the granules are conveyed by a belt conveyor 21 sent to the CS 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, on which lumps of saltpeter formed as a result of a violation of the granulator operation are screened out. Lumps are sent for dissolution. Air supplied to the cooler sections by fans 23, heated in the apparatus 24 due to the heat of juice vapor from the ITN apparatus. Heating is carried out at atmospheric air humidity above 60%, and in winter time to avoid sudden cooling of the pellets. Granules of ammonium nitrate sequentially pass one, two or three sections of the cooling apparatus, depending on the load of the unit and the temperature of the atmospheric air. The recommended temperature for cooling the granular product in winter is below 27 °C, in summer it is up to 40-50 °C. When operating units in the 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. The pressure of the fans is 3.6 kPa. The exhaust air from the 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 enters the washing scrubber 18.

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

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

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

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

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

This almost irreversible reaction proceeds at a high rate and with the release of a significant amount of heat. Usually it is carried out at a pressure close to atmospheric; in some countries, neutralization plants operate at a pressure of 0.34 MPa. In the production of ammonium nitrate, dilute 47-60% nitric acid is used.

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

Industrial production includes the following stages: neutralization of nitric acid with gaseous ammonia in the ITN apparatus (use of neutralization heat); saltpeter solution evaporation, saltpeter melt granulation, granule cooling, surfactant granule processing, saltpeter packaging, storage and loading, gas emissions and wastewater treatment. Additives are introduced during the neutralization of nitric acid.

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

Rice. 1.

1 - acid heater; 2 - ammonia heater; 3 - ITN devices; 4 - 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 - tank for swimming; 21-filter for swimming; 22 - air heater

The incoming 58-60% nitric acid is heated in the heater 1 to 70-80 o C with juice vapor from the apparatus ITN 3 and is fed to 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, counting on the finished product.

Sulfuric and phosphoric acids are supplied by plunger pumps, the performance of which is easily and precisely regulated. The unit is equipped with two neutralization devices operating in parallel. Gaseous ammonia is also supplied here, heated in the heater 2 by steam condensate to 120-130 ° C. The amount of nitric acid and ammonia supplied is regulated so that the solution has a slight excess of nitric acid at the outlet of the ITN apparatus, ensuring the complete absorption of ammonia.

In the lower part of the apparatus is the neutralization of acids 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 (the so-called juice vapor) is washed from splashes of ammonium nitrate and HN0 3 vapor. Part of the heat from the juice vapor is used to heat the nitric acid. Next, the juice steam is sent for cleaning in washing scrubbers and then released into the atmosphere.

An acid solution of ammonium nitrate is sent to the neutralizer 4, where ammonia is supplied in the amount necessary to neutralize the solution. Then the solution is fed into the evaporator 5 on the doupar, which is conducted by water vapor at a pressure of 1.4 MPa and air heated to about 180°C. The resulting melt, containing 99.8-99.7% saltpeter, passes through a 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 the cone 52.8 m.

In the upper part of the tower are granulators 7 and 8; air is supplied to the lower part of the tower, cooling drops of saltpeter, which turn into granules. The drop height of saltpeter particles is 50--55m. The design of the 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 outlet of 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, thus creating a fluidized layer of saltpeter granules 100--150 mm high, which come through the conveyor from the granulation tower. There is an intensive cooling of the granules to a temperature of 40°C (but not higher than 50°C), corresponding to the conditions for the 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 the heat exchanger to 20°C. In the cold period of time, 1-2 sections can be in operation.

The air from the apparatus 15 enters the granulation tower for the formation of granules and their cooling.

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 that, the saltpeter passes through an electromagnetic separator to separate accidentally trapped metal objects and is sent to the bunker, and then for weighing and packaging in paper or plastic bags. Bags are conveyed by a conveyor for loading into wagons or to a warehouse.

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

Part of the solution (20-30%) is continuously withdrawn from the cycle, so the cycle is depleted and replenished with the addition of 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 releases it into the atmosphere.

Production of ammonium nitrate

Ammonium nitrate is a ballast-free fertilizer containing 35% nitrogen in the ammonia and nitrate form, so it can be used on any soil and for any crop. However, this fertilizer has unfavorable physical properties for its storage and use. Crystals and granules of ammonium nitrate spread in the air or caked into large aggregates as a result of their hygroscopicity and good solubility in water. In addition, when the temperature and humidity of the air change during storage of ammonium nitrate, polymorphic transformations can occur. To suppress polymorphic transformations and increase the strength of ammonium nitrate granules, additives are used that are introduced during its manufacture - ammonium phosphates and sulfates, boric acid, magnesium nitrate, etc. The explosive nature of ammonium nitrate complicates its production, storage and transportation.

Ammonium nitrate is produced in factories producing synthetic ammonia and nitric acid. The production process consists of the stages of neutralization of weak nitric acid with gaseous ammonia, evaporation of the resulting solution and granulation of ammonium nitrate. The neutralization step is based on the reaction

NH 3 + HNO 3 \u003d NH 4 NO 3 +148, 6 kJ

This chemisorption process, in which the absorption of a gas by a liquid is accompanied by a rapid chemical reaction, occurs in the diffusion region and is highly exothermic. The heat of neutralization is rationally used to evaporate water from ammonium nitrate solutions. By using high concentration nitric acid and heating the initial reagents, it is possible to directly obtain ammonium nitrate melt (with a concentration above 95-96% NH 4 NO 3) without the use of evaporation.

The most common schemes with incomplete evaporation of the ammonium nitrate solution due to the heat of neutralization (Fig. 2).

The bulk of the water is evaporated in a chemical reactor-neutralizer ITN (using the heat of neutralization). This reactor is a stainless steel cylindrical vessel, inside which is another cylinder, where ammonia and nitric acid are directly introduced. The inner cylinder serves as the neutralization part of the reactor (zone chemical reaction), and the annular space between the inner cylinder and the reactor vessel is the evaporative part. The resulting solution of ammonium nitrate is supplied from the inner cylinder to the evaporative part of the reactor, where the evaporation of water occurs due to heat exchange between the neutralization and evaporative zones through the wall of the inner cylinder. The resulting juice steam is removed from the HP neutralizer and then used as a heating agent.

The sulphate-phosphate additive is dosed into nitric acid in the form of concentrated sulfuric and phosphoric acids, which are neutralized together with nitric ammonia in the ITN neutralizer. When neutralizing the original nitric acid, a 58% solution of ammonium nitrate at the outlet of the ITN contains 92-93% NH 4 NO 3; this solution is sent to an after-neutralizer, into which gaseous ammonia is supplied in such a way that the solution contains an excess of ammonia (about 1 g / dm 3 free NH 3), which ensures the safety of further work with the NH 4 NO 3 melt. The neutralized solution is concentrated in a combined plate tubular evaporator to obtain a melt containing 99.7-99.8% NH 4 NO 3 . For granulation of highly concentrated ammonium nitrate, the melt is pumped by submersible pumps to the top of a granulation tower 50-55m high. Granulation is carried out by spraying the melt with the help of acoustic cell-type vibrogranulators, which provide a uniform particle size distribution of the product. The granules are cooled with air in a fluidized bed cooler, which consists of several successive cooling stages. Cooled granules are sprayed with surfactants in a drum with nozzles and transferred to packaging.

In view of the shortcomings of ammonium nitrate, it is advisable to manufacture complex and mixed fertilizers on its basis. By mixing ammonium nitrate with limestone, ammonium sulfate, lime-ammonium nitrate, ammonium sulfate nitrate, etc. are obtained. Nitrophoska can be obtained by fusing NH 4 NO 3 with salts of phosphorus and potassium.

Urea production

Carbamide (urea) among nitrogen fertilizers ranks second in terms of production after ammonium nitrate. The growth of carbamide production is due to the wide scope of its application in agriculture. It has great resistance to leaching compared to other nitrogen fertilizers, i.e. less susceptible to leaching from the soil, less hygroscopic, can be used not only as a fertilizer, but also as an additive to cattle feed. Urea is also widely used in compound fertilizers, time-controlled fertilizers, and in plastics, adhesives, varnishes, and coatings.

Carbamide CO(NH 2) 2 is a white crystalline substance containing 46.6% nitrogen. Its production is based on the reaction of the interaction of ammonia with carbon dioxide

2NH 3 + CO 2 \u003d CO (NH 2) 2 + H 2 O H=-110.1 kJ (1)

Thus, the raw material for the production of urea is ammonia and carbon dioxide obtained as a by-product in the production of process gas for the synthesis of ammonia. Therefore, the production of urea in chemical plants is usually combined with the production of ammonia.

Reaction (1) - total; it proceeds in two stages. At the first stage, the synthesis of carbamate occurs:

2NH 3 + CO 2 \u003d NH 2 COONH 4 H \u003d -125.6 kJ (2)

gas gas liquid

At the second stage, an endothermic process of water splitting off from carbamate molecules occurs, as a result of which carbamide is formed:

NH 2 COONH 4 \u003d CO (NH 2) 2 + H 2 O H \u003d 15.5 (3)

liquid liquid liquid

The reaction of formation of ammonium carbamate is a reversible exothermic, proceeds with a decrease in volume. To shift the equilibrium towards the product, it must be carried out at elevated pressure. In order for the process to proceed at a sufficiently high speed, elevated temperatures are also necessary. An increase in pressure compensates for the negative effect of high temperatures on the shift of the reaction equilibrium in the opposite direction. In practice, the synthesis of urea is carried out at temperatures of 150-190 C and pressure 15-20 MPa. Under these conditions, the reaction proceeds at a high rate and to the end.

The decomposition of ammonium carbamate is a reversible endothermic reaction that proceeds intensively in the liquid phase. To prevent crystallization of solid products in the reactor, the process must be carried out at a temperature below 98C (eutectic point for the CO (NH 2) 2 - NH 2 COONH 4 system).

More high temperatures shift the equilibrium of the reaction to the right and increase its rate. The maximum degree of conversion of carbamate into carbamide is reached at 220C. To shift the equilibrium of this reaction, an excess of ammonia is also introduced, which, by binding the reaction water, removes it from the reaction sphere. However, it is still not possible to achieve complete conversion of carbamate into urea. The reaction mixture, in addition to the reaction products (carbamide and water), also contains ammonium carbamate and its decomposition products, ammonia and CO 2 .

For the full use of the feedstock, it is necessary either to provide for the return of unreacted ammonia and carbon dioxide, as well as carbon ammonium salts (intermediate reaction products) to the synthesis column, i.e. creation of a recycling, or separation of carbamide from the reaction mixture and sending the remaining reagents to other industries, for example, to the production of ammonium nitrate, i.e. conducting an open process.

In a large-capacity unit for the synthesis of urea with a liquid recycle and the use of a stripping process (Fig. 3), one can distinguish a high-pressure unit, a unit low pressure and a granulation system. An aqueous solution of ammonium carbamate and carbon ammonium salts, as well as ammonia and carbon dioxide, enter the lower part of the synthesis column 1 from the high-pressure carbamate condenser 4. In the synthesis column at a temperature of 170-190C and a pressure of 13-15 MPa, the formation of carbamate ends and the urea synthesis reaction proceeds . The consumption of reagents is selected so that the molar ratio of NH 3:CO 2 in the reactor is 2.8-2.9. The liquid reaction mixture (melt) from the urea synthesis column enters the stripping column 5, where it flows down the pipes. Carbon dioxide compressed in the compressor to a pressure of 13-15 MPa is fed countercurrently to the melt, to which air is added in an amount that provides an oxygen concentration of 0.5-0.8% in the mixture to form a passivating film and reduce equipment corrosion. The stripping column is heated with steam. The gas-vapor mixture from column 5, containing fresh carbon dioxide, enters the high-pressure condenser 4. Liquid ammonia is also introduced into it. It simultaneously serves as a working stream in the injector 3, which supplies the condenser with a solution of ammonium carbon salts from the high-pressure scrubber 2 and, if necessary, part of the melt from the synthesis column. Carbamate is formed in the condenser. The heat released during the reaction is used to produce steam.

From the upper part of the synthesis column, unreacted gases continuously exit, entering the high-pressure scrubber 2, in which most of them are condensed due to water cooling, forming a solution of carbamate and carbon ammonium salts.

The aqueous solution of carbamide leaving the stripping column 5 contains 4-5% carbamate. For its final decomposition, the solution is throttled to a pressure of 0.3-0.6 MPa and then sent to the upper part of the distillation column 8.

The liquid phase flows in the column down the packing in countercurrent to the vapor-gas mixture rising from the bottom up. NH 3 ,CO 2 and water vapor exit from the top of the column. Water vapor condenses in the low pressure condenser 7, while the main part of ammonia and carbon dioxide is dissolved. The resulting solution is sent to scrubber 2. The final purification of gases emitted into the atmosphere is carried out by absorption methods.

A 70% solution of urea leaving the bottom of the distillation column 8 is separated from the gas-vapor mixture and sent after reducing the pressure to atmospheric, first to evaporation, and then to granulation. Before spraying the melt in the granulation tower 12, conditioning additives, such as urea-formaldehyde resin, are added to it in order to obtain a non-caking fertilizer that does not deteriorate during storage.

Environmental protection in the production of fertilizers

In the production of phosphorus fertilizers, there is a high risk of atmospheric pollution with fluorine gases. The capture of fluorine compounds is important not only from the point of view of protection environment, but also because fluorine is a valuable raw material for the production of freons, fluoroplasts, fluororubbers, etc. Fluorine compounds can get into wastewater at the stages of fertilizer washing, gas cleaning. It is expedient to reduce the amount of such wastewater to create closed water circulation cycles in the processes. For wastewater treatment from fluorine compounds, ion exchange methods, precipitation with iron and aluminum hydroxides, sorption on aluminum oxide, etc. can be used.

Waste water from the production of nitrogen fertilizers, containing ammonium nitrate and carbamide, is sent for biological treatment, pre-mixing them with others. sewage in such proportions that the concentration of carbamide does not exceed 700 mg / l, and ammonia - 65-70 mg / l.

An important task in the production of mineral fertilizers is the purification of gases from dust. The possibility of polluting the atmosphere with fertilizer dust at the granulation stage is especially great. Therefore, the gas leaving the granulation towers is necessarily subjected to dust cleaning by dry and wet methods.

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General chemical technology: Proc. for chemical engineering. specialist. universities.

In 2 volumes. T.2. The most important chemical production / I.P. Mukhlenov, A.Ya. Kuznetsov and others; Ed. I.P. Mukhlenov. – 4th ed., revised. and additional - M .: "Higher. school”, 1984.-263 p., ill.

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