-To print this articleTheme 2.1 Technology of production of ammonia
Properties of ammonia
At ordinary temperatures ammonia is a colorless gas with a suffocating pungent odor and acrid taste, irritant acting on the mucous membranes, toxic.
Ammonia is highly soluble in water. At room temperature and atmospheric pressure in 1 liter of water dissolves 750 gaseous ammonia. When dissolved in water ammonia forms ammonium water
NH3+H2O= NH4IT
Due to the fact, the concentration of HE-ions is small ammonia water is one of the most important chemical reagents, diluted solutions which (ammonia) used in medicine and household.
At a temperature of -33,190With boils, and when -77,750With hardens. Ammonia is a very reactive substance, entering in the reaction. At ordinary temperature resistant, and when 3000With the presence of a catalyst dissociates into hydrogen and nitrogen. Ammonia reactionsost, in particular on the catalyst is oxidized to nitrogen oxide. In a mixture with air (15-28% by volume) explosive.
Ammonia is used in the production of nitric acid, urea, ammonium nitrate, of NPK. Liquid ammonia serves as the working substances of refrigeration machines, unnecessarily. upon evaporation of it from the environment is taken a lot of heat. Liquid ammonia is a good solvent for a large number of organic compounds, as well as many inorganic substances.
The value of nitrogen compounds
Gaseous nitrogen is one of the most stable chemicals. In the atmosphere nitrogen is in a free state in large quantities. It is estimated, over 1 hectare of the Earth's surface has about 80 thousand. t nitrogen. Essential nitrogen for life on Earth, as one of the elements, included in the protein structures, without which it is impossible the existence of a living cell.
Some nitrogen becomes biologically digestible form as a result of lightning discharges by the reaction
N2 + O2 = 2NO (2.1.1)
In the life of plants and animals involved are not elementary nitrogen, and associated, ie. its chemical compounds. Ammonia is an important compound of nitrogen, involved in nitrogen metabolism in nature.
Raw material for production of products in the nitrogen industry are air and different fuels. Air is a mixture of gases, forming the atmosphere. The main gases, included in the air are: nitrogen -78.03%, oxygen – 20.99%, argon -0.94%. In small amounts in the composition of the air consists of hydrogen, neon, helium, krypton, xenon and carbon dioxide. Nitrogen is used to produce ammonia, of calcium cyanamide and other products nitrogen technology. Oxygen is used for many industrial oxidation processes. Argon is used in lighting technology, neon – for filling cathode lamps, krypton and xenon for filling light bulbs. Due to the great inertness of nitrogen for a long time failed to find ways of connecting it with other elements. The inertness of nitrogen is due to the presence of triple bond between nitrogen atoms in the molecule.
Methods of production.
There are three methods for obtaining bound nitrogen: arc, cyanamide and ammonium.
Arc method based on the compound of nitrogen and oxygen at high temperatures –2500-30000With (in the flame of an electric arc) for reversible reactions:
N2 +In2 ↔2NО +Q (2.1.2)
Further, NO is oxidized to NO2 and absorb water, getting nitric acid, or associated with Sa(IT)2, receiving calcium nitrate. The energy consumption for receiving 1 t bound nitrogen arc method is very large – 60. kWh. This method worked the mills in Norway, Germany, USA, but with the advent of the method of binding ammonia nitrogen, these plants were closed.
Cyanamide method based on the ability of finely divided calcium carbide at a temperature of about 10000To interact with nitrogen according to the equation
SAS2 +N2↔СаСN2 + With +302кДж (2.1.3)
The energy consumption for the production 1 t bound nitrogen cyanamide method 10-12 thousand kW*hour. At present the role of this method in the production of bound nitrogen very small. In all of these methods receive nitrogen in the form of NO and the СаСN2.
The solution of the associated nitrogen was the reaction of synthesis of ammonia, industrial implementation of which helped to create a strong raw material base to produce a variety of nitrogen-containing compounds. The energy consumption for 1 t bound nitrogen in ammonia method is several times less, than cyanamide. So ammonia method is in n in. the main in the receipt of ammonia and all other compounds of nitrogen.
Getting ammonia ammonia method happens in reversible reactions
N2 +N2↔ NH3 + Q (2.1.4)
ie. the synthesis of ammonia is made from nitrogen and hydrogen.
Nitrogen for preparation of nitric mixture is obtained by liquefying air followed by rectification, the implementation of which is possible due to the different boiling points of the individual components. Under atmospheric pressure, nitrogen boils at -195.80With, oxygen at -1830With. The difference 180With ample, the liquid air is split by distillation in a commercially pure nitrogen and oxygen. Almost complete separation of air reaches the double rectification in a two-column apparatus.
Conversion of methane in the presence of oxygen in the air allows you to simultaneously produce hydrogen and nitrogen, that mingling, form a mixture of nitric, directly involved in the synthesis of ammonia.
The production of hydrogen and nitric mixture.
Because the resources of atmospheric nitrogen huge, the resource base nitrogen industry is mainly determined by the second kind of raw material – fuel, used for the production of hydrogen or hydrogen-containing gas.
Rice. 2. 1 Raw materials of ammonia production
Hydrogen for ammonia synthesis can be obtained:
1) the conversion of methane with steam;
2) the conversion of carbon monoxide with water vapor;
3) the cracking of methane;
4) the separation of coke-oven gas.
Of primary importance are methods of methane conversion and carbon oxide.
Currently the main raw material in the production of ammonia is natural gas. The synthesis gas from hydrocarbon gases (natural, associated gases, processing gases other fuels) he is currently the main source for production of ammonia and methanol. Natural gas contains 98% methane, the rest is ethane and propane. Impurities of ethane and propane involved in reactions similar to methane. As oxidizers use oxygen and steam. The last is added to compensate for the heat, absorbed in the conversion of methane. The oxidation of methane (the main component of hydrocarbon gases) when receiving the synthesis gas proceeds in the following main cumulative reactions:
CH4+0,5O2 = CO + 2H2; ΔH = -35,6 kJ (2.1.5)
CH4+ N2ABOUT = WITH + MN2; Δ N = 206,4 kJ (2.1.6)
Formed by reacting the carbon monoxide is converted with water vapor:
The process of conversion of carbon monoxide with water vapor proceeds according to the equation:
CO +N2O ↔ CO2 +N2 +Q (2.1.7)
All three reactions are reversible. For each of them there is a certain equilibrium ratio between the concentrations of the substances, which at constant temperature remains constant and is determined by the equilibrium constant.
Conversion of methane.
The processes of methane conversion with water vapor and oxygen flow with different thermal effect: the reaction of steam reforming endothermic, require a supply of warmth; the oxygen conversion reaction exothermic, and released enough heat not only for autothermal perform the actual oxygen conversion, but also to cover the consumption of heat for the endothermic reaction of steam reforming.
To achieve a residual content of SN4about 0,5% the conversion of lead into two stages: steam reforming under pressure {the first stage) and steam-air conversion with oxygen of air (the second stage). This yields a synthesis gas of stoichiometric composition and the necessity for the separation of air to produce process oxygen and nitrogen. The first stage conversion is carried out with water vapor in a tubular reactor at 8000With the degree of conversion 90%. In the second step, the conversion of residual methane is performed with the air in the shaft reactor at 10000With. In the converted gas contains 0.3% methane. In addition, the use of air as the oxidizing agent allows to obtain converted gas containing nitrogen (coming from air) in such quantity, needed to produce nitric mixture for the synthesis of ammonia.
The equilibrium composition of the gas mixture is determined by such process parameters, as temperature and pressure in the system, as well as the ratio of the reacting components.
Based on the principle of Le Chatelier to achieve maximum yield of hydrogen theoretically required for the following conditions when methane conversion: decompression, the temperature rise and the excess water vapor in comparison with the stoichiometric quantity. In practice, the conversion is generally carried out at elevated pressure (2-3MPa), despite the fact, the hydrogen content decreases with increasing pressure (the equilibrium shifts to the left). The conversion process is advantageously carried out at elevated pressure, since it increases the rate of reaction (as a result of increasing the concentration of the reactants), and besides, used natural pressure natural gas, he served on the enterprise. This affects the size reduction of apparatuses and pipelines and the reduction in capex for the construction of the plant. In addition to this reduced power consumption for compression of the converted gas before the next stage of the synthesis of ammonia, carried out at elevated pressure.
In terms of temperature, there are two types of conversion of methane: at high temperature 1350-14000With no catalyst and catalytic conversion at 800-10000With the presence of a catalyst. To create high temperatures and carrying out an exothermic reaction (2) it is necessary to draw warmth from the outside. In order to save the process is carried out, combining methane conversion with steam and oxygen, ie. using exothermic and endothermic processes.
The use of catalytic conversion has advantages over high temperature. The catalyst accelerates the reaction, reduces the process temperature and suppresses a side reaction CH4 →C +2H2, ie. prevents the formation of carbon and the additional step of cleaning.
The catalysts differ from each other not only in the content of the active component, but also the type and content of other components – media and promoters.
The highest catalytic activity in this process have Nickel catalysts on the media – alumina (A12In3).
Nickel catalysts for conversion of methane produced in the form of tableted and extruded Raschig rings. So, the catalyst GIAP-16 has the following composition: 25% NiO, 57% A12In3, 10%CaO, 8% MgO.
The service life of the catalysts conversion for proper operation of up to three years or more. Their activity decreases under the action of various catalytic poisons. Nickel catalysts are the most sensitive to the action of sulfur compounds. Poisoning occurs due to the formation on the surface of the catalyst of sulfides of Nickel, totally inactive against the reaction of conversion of methane and its homologues. Sulfur poisoned catalyst can be regenerated almost completely at certain temperatures in the feed to the reactor is pure gas. The zauglerozhenny catalyst activity can be restored, treating it with water vapor.
CH4+WITH2 = 2USING + 2N2; ΔH = 248,SCG (2.1.8)
To increase the yield of hydrogen conversion is carried out with an excess of water vapor in the SN ratio4: N2O= 1:2 of the stoichiometric amount.
The conversion of methane is carried out in reactors of two types: tubular shaft and. Tubular reactor (tube furnace) – the device, in tubes which are placed the catalyst, and into the annulus down the heat of the flue gases (often the combustion of natural gas). The mode of movement of the reactants the reactor, temperature – polythermal. Mine is a reactor apparatus of the capacitive type, lined inside with refractory bricks. Equipped with a water jacket, eliminating overheating of the housing in the case of local defects in the lining of. In the lower part of the reactor inject the condensate for removal of heat of converted gas and its moisture. The mode of movement of the reactants – the reactor, in terms of temperature – adiabatic.
The conversion of carbon monoxide.
The process of conversion of carbon monoxide with water vapor proceeds according to the equation:
CO +N2O ↔ CO2 +N2 +Q (2.1.9)
This reaction is partially carried out at the stage steam reforming of methane, however, the degree of conversion of carbon monoxide is very small in exiting gas contains up to 11,0% WITH and more. For additional quantities of hydrogen and to minimize the concentration of carbon monoxide in the converted gas has its own stage catalytic conversion of CO with water vapor. The process of conversion of carbon monoxide – heterogeneous, catalytic, reversible and exothermic.
In accordance with the conditions of thermodynamic equilibrium to increase the degree of conversion is possible WITH the removal of carbon dioxide from gas mixture, the increase in water vapor content or the conduct of the process possible at low temperature. The conversion of carbon monoxide, as can be seen from the equations of the reaction, occurs without volume changes, therefore, the increase in pressure does not cause displacement of the equilibrium. However carrying out the process at elevated pressure is economically feasible, since the rate of reaction, reduced size of the apparatus, useful use the energy previously compressed natural gas.
The process of conversion of carbon dioxide with intermediate removal of carbon dioxide is used in technological schemes of production of hydrogen in those cases, when you want to produce hydrogen with a minimum amount of impurities of methane.
The water vapor concentration in a gas is usually determined by the amount of dosing on methane conversion and remaining after its occurrence. The ratio of steam: gas to conversion of CO in large units of production of ammonia is 0.4—0.5. Conducting the process at low temperatures is a rational way to increase the equilibrium degree of conversion of carbon monoxide, but possible only in the presence of highly active catalysts. It should be noted, what is the lower temperature limit of the process is limited by the conditions of condensation of water vapor. In the case of carrying out the process under pressure 2-3 MPa this limit is 180-200°C. Lowering the temperature below the dew point causes water to condense on the catalyst, which is undesirable.
The reaction of CO conversion is accompanied by significant release of heat, what caused the carrying out the process in two stages at different temperatures on each. In the first stage high temperature high speed conversion of a large number of carbon monoxide; in the second stage at a low temperature, a high degree of conversion WITH the remaining. The exothermic heat of reaction is used for steam production. Thus, the desired degree of conversion is achieved while reducing steam consumption.
The temperature regime at each stage of the conversion is determined by the properties of the catalysts. In the first stage the catalyst used gelatobaby, which is produced in tableted and molded types. In industry widely used medium gelatobaby catalyst. |
For zhelezorudnogo catalyst poisons are sulfur compounds. The hydrogen sulfide reacts with Fe3O4, forming iron sulfide FeS. Organic sulfur compounds in the presence of a catalyst zhelezorudnogo interact with water vapour with formation of hydrogen sulphide. In addition to sulfur compounds poison the catalyst gelatobaby have phosphorus compounds, Bora, silicon, chlorine.
Low-temperature catalysts contain copper compounds, zinc, aluminum, sometimes chromium. Known two-, three, four- and multicomponent catalysts. As additives to the above-mentioned components are used magnesium compounds, titanium, palladium, manganese, cobalt, etc. The content of copper in the catalyst ranges from 20 to 50% (in terms of the oxide). The presence of low-temperature catalysts aluminum compounds, magnesium, manganese greatly improves their stability, makes it more resistant to temperature rise.
Service life low-temperature catalyst usually does not exceed two years.
The process is carried out in the Converter with a radial gas speed or by lowering the temperature on the shelves (according to the principle of Le Chatelier and optimization of the temperature regime) due to evaporation can be injected into the Converter condensate. This is a vertical cylindrical apparatus, filled with catalyst. Gas-vapor mixture is fed down the Central tube and through openings over its entire height is supplied to the catalyst. The reaction gas leaves the apparatus through the annular gap along the outer walls of the. To apply this model of reactor, and in terms of temperature – adiabatic.
Because the process is exothermic, he is avtoterminal, ie. without heat supply from outside, and the excess of it is used in heat-recovery steam generators to generate steam.
Technological registration of natural gas conversion.
At present in the nitrogen industry uses technological scheme of natural gas conversion under elevated pressure, including the conversion of carbon monoxide. The advantage of these schemes is lower energy consumption for compression of the reformed gas, the volume of which is substantially greater than the volume of source gases; with reduced dimensions of the devices, communications and valves; fully recuperated heat moist gases (as temperature is increased, condensation of water vapor), simplified design of nitric compressor, which creates prerequisites for the construction of units of large capacity using the principles of energy technologies. The latter allows to reduce the cost of production and capital expenditures and dramatically increase productivity in Wide use, how in the world, and in the domestic nitrogen industry has been the process of two-step steam and air-steam catalytic conversion under pressure. On its basis created large-capacity units for energotechnological scheme with deep heat recovery of catalytic reactions of conversion of CH4 and WITH, the methanation and ammonia synthesis.
In figure. 2.2 (Fig.14.4) the scheme of two-step unit conversion CH4 and under pressure performance 1360 t/day of ammonia.
Natural gas is compressed in the compressor up to pressure 4,6 MPa, mixed with nitric mixture (ABC: gas – 1 : 10) and served in a fired heater 2, where the reaction mixture is heated 130-140 up to 370-400°C. For heating use natural or other combustible gas. Next, the heated gas is purified from sulfur compounds: in the reactor 3 on alumocalcium the catalyst is the hydrogenation of organic sulfur compounds to hydrogen sulfide, and then in the adsorber 4 the hydrogen sulfide is absorbed by a sorbent based on zinc oxide. Usually the two sets of adsorbers, connected in series or in parallel. One of them may turn off the loading of the sorbent. H Content2S in the purified gas must not exceed 0,5 mg/m3 gas.
The purified gas is mixed with steam in respect 1:3,7 and obtained gas-vapor mixture enters the convection zone tube furnaces 12. In the radiation chamber of the furnace pipe placed, filled with a catalyst for conversion of methane, and burner, in which burning natural gas or fuel. Obtained in the burner, the flue gases heat pipe with catalyst, then the heat of these gases additionally is recovered in the convection chamber, the heaters steam-gas and steam mixture, superheater high pressure steam, feedwater heaters high pressure and natural gas.
Gas-vapor mixture is heated in a heater 10 to 5250 With and then under pressure 3.7 MPa is distributed from top to bottom by a large number of parallel connected pipes, filled with catalyst. Emerging from tubular reactor vapor-gas mixture contains 10% CH4. At a temperature of 8500With the converted gas supplied to the methane Converter of the second stage 13–the shaft-type reactor. In the upper part of the Converter 13 compressor 19 process air is supplied, heated in the convection zone of the furnace to 480-5000With. Gas-steam and air-steam mixture fed into the reactor by separate streams in a ratio of, required to ensure almost full conversion of methane and for producing process gas with respect to (CO+N2):N2 – 3,05-3.10. The content of water vapour corresponds to the ratio of p:gas=0,7:I. At a temperature of about 10000With the gas sent to the recovery boiler 14, generating steam pressure 10,5 MPa. Here, the reaction mixture is cooled to 380-4200C and goes into the Converter WITH the first stage 15, where the catalyst flows zhelezorudnom conversion the main quantity of carbon monoxide with water vapor. Coming out of the reactor at a temperature of 4500With the gas mixture contains about 3,6% WITH. In a steam boiler 16, which also produces steam, gas-vapor mixture is cooled to 2250With and fed into the Converter FROM the second stage 17, filled with low-temperature catalyst, where the CO content is reduced to 0,5%. The converted gas at the outlet of the Converter 17 has the following composition (%): N2-61,7; WITH–0.5; CO2 -17,4; N2+AG -20,1;CH4 -0,3. After cooling dalneishee utilization of heat of converted gas under ambient temperature and pressure 2,6 MPa is supplied to the cleaning.
Two-stage steam and air-steam catalytic conversion of hydrocarbon gases and carbon dioxide under pressure is the first stage energy technological scheme of production of ammonia. Heat chemical processes stages of conversion CH4, WITH, the methanation and ammonia synthesis is used to heat the high pressure water and obtain superheated steam pressure 10,5 MPa. This par, entering the steam turbine, drives the compressors and pumps of ammonia production, and is also used for technological purposes.
The main equipment of the unit conversion is a tubular furnace. Tube furnaces vary in pressure, type tubular screens, the shape of the combustion chambers, heating, the location of the cameras convective heating source flows. In industrial practice, common types of tubular furnaces: multi-row, terraced bunk, stacked with internal baffles, panel burners. In the modern production of synthetic ammonia and methanol are most commonly used in-line in-line tube furnace with the top flame heating.
Purification of the reformed gas.
In the converted gas along with nitrogen and hydrogen contains carbon monoxide and carbon dioxide. Oxygen-containing compounds are strong catalyst poisons for ammonia synthesis, therefore, the syngas must be thoroughly cleaned.
The cleaning gas is carried out by different methods: 1) adsorption of impurities by solid adsorbents; 2) absorption with liquid absorbents; 3) condensation of impurities deep cooling; 5) catalytic hydrogenation. Of great importance in industrial practice is the method of purification of liquid sorbents, based on typical absorption-desorption processes. Catalytic hydrogenation is used as a final purification step to remove small amounts WITH, WITH2, In2.
The synthesis of ammonia
In the heart of the process of synthesis of ammonia is a reversible exothermic reaction, proceeding with the reduction of gas volume:
N2 +3N2 ↔2NН3 +89 kJ (5000With and 30МПа) (2.1105)
According to the principle of Le Chatelier, the equilibrium reaction of ammonia synthesis is shifted to the right with decreasing temperature and increasing pressure. The equilibrium constant is determined by the equation:
TO= PN2 *P3H2 /P2NH3
PN2 , PH2,PNH3 –partial pressure of N2, N2, NH3.
However, with decreasing temperature decreases the speed of the process of catalysis, and, therefore, and the overall performance of the ammonia-synthesis unit. Even at relatively high temperatures the activation energy barrier of nitrogen molecules is too large, and the formation of ammonia occurs very slowly. To reduce the activation energy of the process is conducted at 400-5000With.
The reaction of ammonia formation proceeded at high speed, in industry this process is carried out under conditions, far from equilibrium systems. The duration of the reaction in this case is counted in seconds, but the ammonia content in the gas decreases compared to equilibrium.
Catalytically on the reaction of ammonia synthesis are many metals: manganese, iron, Osmi, platinum, rhodium, tungsten, etc. In the industry use iron catalyst, promoted with aluminum oxide, potassium, calcium and silicon.
Temperature, necessary for carrying out the synthesis of ammonia, is achieved by preheating the mixture of nitric and by releasing reaction heat. Calculations show, when education 1% ammonia gas temperature increases by 160Due to heat of reaction. For example, in the formation 12% NH3 the increase in temperature of 16*12 =From 192k.
The synthesis of ammonia proceeds through a series of successive stages:
1) the diffusion of hydrogen and nitrogen from the stream to the surface of the grains of the catalyst and inside of it then;
2) activated (chemical) the adsorption of gases onto the catalyst surface;
3) chemical interaction of nitrogen and hydrogen through the intermediate compounds with the catalyst to form ammonia;
4) desorption of ammonia and its diffusion into the gas stream.
Installed, that the slowest stage is the activated diffusion of gases inside the pores of the catalyst, ie. the whole process of synthesis proceeds in nutrititional region.
In an industrial environment is optimal stoichiometric ratio of N2 : H2 =1:3.
The synthesis of ammonia is a typical cyclic process, allowing the unreacted nitrogen-hydrogen mixture again to return to production after separation of the formed ammonia. Large flow rate process in conjunction with the observance of the optimum temperature mode, the use of a nitrogen-hydrogen mixture of high purity and sufficiently active catalyst to ensure high performance of ammonia synthesis units with high economic indicators of the process.
The synthesis of ammonia in industrial conditions is carried out in the apparatus with filter a layer of catalyst. The device can be shelf or tubular type (in the form of a column). To ensure reliable and safe operation of the ammonia synthesis column applications place great demands on steel, from which it is made. While taking into account corrosive properties of hydrogen and ammonia at high temperatures. Especially dangerous is the decarburization of the steel under the action of hydrogen. Therefore, to reduce the influence of high temperatures on the casing of the cold nitrogen-hydrogen mixture was fed through the annular gap between the column housing and the housing catalyst boxes. The gas mixture passes through a column of synthesis of ammonia with a filter bed of catalyst in the displacement mode. Temperature – polythermal.
Industrial methods of synthesis of ammonia depending on the applied pressure can be divided into three groups:
1) under low pressure from 10 to 16 MPA;
2) medium from 20 to 50МПа;
3) high pressure from 70 up to 100mpa.
High-potential heat of flue gases is converted and used to produce high pressure steam, used in turbines, employees drive compressors. Low-grade heat is used to produce process steam low pressure, water heating, refrigeration, etc.
Consider the basic technological scheme of modern production of ammonia at medium pressure performance 1360 tons/day (Fig.2.3). Its mode of operation is characterized by the following parameters: the temperature of contacting 450-5500With, pressure 32 MPa, the volumetric flow rate of the gas mixture 4*104nm3/m3*h, composition nitric mixture of stoichiometric.
The mixture of fresh and ABC circulation of gas under pressure is supplied from the mixer 3 in the condensation column 4, where the circulating gas is condensed out of the ammonia, where it comes to the synthesis column 1. Emerging from the column gas, containing up to 0.2 about.Dol. ammonia, water is directed to the cooler-condenser 2 and then in the gas separator 5, where it is separated liquid ammonia. The remaining gas after the compressor is mixed with fresh FAA and sent first to the condensing column 4, and then in the evaporator the liquid ammonia 6, where upon cooling to -200With also condense most of the ammonia. Then the circulating gas, containing about 0.03 about.Dol. ammonia, is supplied to the synthesis column 1. In the evaporator 6, simultaneously with the cooling of the circulating gas and the condensation of the contained ammonia, the evaporation of liquid ammonia with the formation of gaseous commodity product.
The main unit of the technological scheme – a column of synthesis of ammonia, representing the reactor RIVE-N. The column consists of a housing and nozzles of different devices, catalyst comprising a box placed inside the contact mass and the system heat exchanger tubes. For the process of ammonia synthesis is essential to the optimum temperature. To maximize the speed of the synthesis process should start at high temperature and with increasing degree of conversion to lower it. Temperature control and ensuring Autoterminal process is provided by means of heat exchangers, located in the layer of the contact mass and additionally, a part flow of the cold FAA to contact estate, bypassing the heat exchanger.
THE CIRCULATING GAS
Rice. 2.3 The scheme of synthesis of ammonia
1-synthesis column, 2- water capacitor, 3 the mixing of fresh and circulating gas AVS, 4-the condensing column, 5- a gas separator, 6 – evaporator liquid ammonia, 7-the boiler, 8- turbonormalizing compressor
The emission of ammonia from a gas mixture.
The resulting ammonia is isolated from the nitrogen-hydrogen mixture by cooling. In this part of the ammonia becomes liquid and is removed from the system, the remaining gas circulating compressors is recycled and joins the fresh gas.
The lower the temperature of condensation of ammonia, the smaller ammonia gas remains in the nitrogen-hydrogen mixture and the greater the amount of ammonia is condensed. For condensation of gaseous ammonia, the gas mixture is cooled first by water, then evaporating the ammonia.
Storage and transportation of liquid ammonia.
Storage of ammonia made in the form of a horizontal or vertical cylindrical tanks with a volume of 10 up to 55m3 with convex bottoms. In such storages you can pour from 6 up to 30 tons of liquid ammonia under maximum working pressure 16at alloy, which corresponds to the saturated vapor pressure of ammonia at 400With.
Transportation of liquid ammonia under pressure is produced in thick-walled steel tanks with a diameter of 2.2-2.4 m and a length of up to 10m. The tank is filled to 75% its volume.
The use of ammonia.
Ammonia is a key product to produce numerous nitrogen-containing substances, used in industry, agriculture and life. On the basis of ammonia in n in. made virtually all nitrogen compounds, used as the target products and intermediates of inorganic and organic technology.
Theme 2.2 Technology of production of nitric acid.
Properties of nitric acid
Nitric acid is one of the most important mineral acids and in terms of production takes the second place after sulfuric acid. It forms water-soluble salts (nitrates), has nitrating and oxidizing action in relation to organic compounds in a concentrated form, passivates ferrous metals. All this led to the widespread use of nitric acid in the national economy and enginery.
Anhydrous nitric acid (monohydrate NGOS3) is a colorless liquid with freezing point – 41.60With, boiling point – 82.60With and the density of 1.513 g/m3. Miscible with water in all respects, forming a personal connection – hydrates composition of NGOS3*N2Oh and NGOS3*3N2In, give three eutectic.
Boiling point of aqueous solutions of nitric acid depends on its concentration. With increasing concentration, the boiling point increases. Anhydrous nitric acid are unstable thermally and decomposed even when stored
4NGOS3↔4NО2 +2N2O +O2 +ΔH (2.2.1)
The rate of decomposition increases with increasing concentrations of, for 99% acid the temperature gradient is only 50With. When heated, the process is accelerated and proceeds according to the equation
2NGOS3 ↔ N2In3 +N2O +O2 +Days (2.2.2)
Released nitric oxide (4) soluble in acid and stains in the yellow-orange color. For removal of nitric oxide from the acid in the process of production provides the operation "bleaching" acid.
Upon dissolution of the oxide (4) in acid produces compound composition of NGOS3*NO.2 (nitrosonium), which is an intermediate product in the direct synthesis of nitric acid.
Nitric acid corrodes and dissolves all metals except gold, platinum, titanium, tantalum, rhodium, and
ride, however, in concentrated form passivates iron and its alloys.
Applications
Applications of nitric acid is very diverse. Most of it is spent on the production of nitrogen and complex fertilizers and a variety of nitrates, for the production of explosives and rocket fuel, the manufacture of dyes, in organic synthesis and non-ferrous metallurgy.
Rice. 2.2.1 Applications of nitric acid
Raw materials for production
Raw material for the production of nitric acid are synthetic ammonia, air and water. The purity of raw materials impose high demands. Get synthetic ammonia based on natural gas conversion. Ammonia, received from the departments of the synthesis of ammonia, catalyst contains dust and vapour compressor oil, poisons which at the stage of oxidation of ammonia. Therefore, ammonia is subjected to careful purification by filtration through cloth and ceramic filters and flushing liquid ammonia.
Similarly cleaned from mechanical and chemical impurities in the air, which enters the plant through the intake pipe. For air purification are irrigated scrubbers and fabric filters two-stage.
Water, necessary for the absorption of nitrogen dioxide and obtaining nitric acid, subjected to desalting.
Methods of obtaining
There are two ways of production of nitric acid:
– obtaining a dilute acid followed by concentration of it if necessary;
– direct delivery of concentrated nitric acid.
Most common first method, due to the use in the national economy as concentrated, and diluted acids. Methods differ in physico-chemical regularities of processes and technological schemes. However, regardless of the schema of the synthesis of nitric acid from ammonia is described by General chemical scheme.
The conversion of ammonia Processing waste gases
The method includes three stages:
1) the contact oxidation of ammonia to nitrogen oxide
4NH3 +5In2 = 4NО +6N2ABOUT –DAYS DAYS = 907.3 kJ (2.2.3)
2) oxidation of nitric oxide (2) to nitric oxide (4)
2NO. + In2 ↔ 2NО2-DN DN =112.3 kJ (2.2.4)
3) absorption of nitric oxide (4) water
3ΝО2 + N2ON ↔ 2НΝО3 + ΝО –DN DN =136кДж (2.2.5)
Released during this oxide (2) is oxidized to nitrogen oxide (4) and again absorbed
The first stage of the process is the same as to obtain a diluted, and for concentrated acids. The second stage has a number of features. Crucial when choosing parameters technical scheme has the optimal pressure at every stage of production, allowing the use of more sophisticated devices and massoobmena, in the end, reduces capital costs.
At the same time the pressure increase has a negative impact on the economic performance of the unit: accelerate side reactions at the stage of oxidation of ammonia, decreases the degree of conversion.
Techno-economic analysis shows, the application of uniform pressure at all stages of production is advisable only in the case, when the power plant does not exceed 600-800t/day. Install more power economically viable to create only with the use of different pressure at the stage of conversion of ammonia and the stage of processing waste gases.
The contact oxidation of ammonia
The oxidation of ammonia by air oxygen on the catalyst may flow following reactions:
4NH3 +5In2 = 4NО +6N2ABOUT –DAYS DAYS = 907.3 kJ (2.2.6)
4NH3 + 4In2 = 2N2O +6N2ABOUT –DAYS DAYS = 1104.9 kJ (2.2.7)
4NH3 + 3In2 = 2N2 + 6N2ABOUT –DAYS DAYS = 1269.1 kJ (2.2.8),
as well as the reaction involving the formed oxide (3)
4NH3 + 6N = 5N2 + 6N2ABOUT –DAYS DAYS = 110 kJ (2.2.9)
All reactions are practically irreversible. Of the three main reactions of ammonia oxidation reaction (2.2.8) thermodynamically most probable, as occurs with maximum heat. So, in the absence of catalyst the oxidation of ammonia is primarily to elemental nitrogen. To speed up the task oxidation to nitric oxide (2) apply selectively acting catalysts. In modern plants using platinum catalysts in the form of packet of grids from an alloy of platinum with 7.5% rhodium. Introduction of rhodium increases the mechanical strength and reduces the loss of platinum due to its entrainment by a current of gas. The surface of such catalysts reaches 1.5 m2/m3 volume.
The mechanism of heterogeneous catalytic oxidation of ammonia consists of the following successive stages:
– diffusion of molecules of ammonia and oxygen from the gas phase to the catalyst surface;
– the activated adsorption of oxygen molecules on the catalyst surface with the formation of intermediate compounds;
– chemisorption of molecules of ammonia and the formation of the complex;
decomposition of the complex with regeneration of the catalyst and the formation of molecules of nitric oxide (2) and water;
– diffusion of reaction products from the catalyst surface into the gas phase.
The defining stage of the whole process of oxidation is diffusion of oxygen to the catalyst surface. Therefore, catalytic oxidation of ammonia on a platinum catalyst proceeds predominantly in a diffusion region.
Platinum catalysts are very sensitive to catalytic poisons, contained in the ammonia and the air, forming an ammonia-air mixture (Amvs). As consequence of poisons, the catalyst activity decreases, it periodically regenerated by washing with hydrochloric acid, burning in a flame of hydrogen. In the process, the surface of the catalyst is destroyed, and its particles are carried away by the gas flow. Erosion of the catalyst the more, the higher the temperature, the pressure and volumetric velocity of the gas, passing through the catalyst. For systems, working under high pressure, the entrainment of the catalyst is 0.3-0.4 g per 1 ton of nitric acid.
To increase the strength of the platinum catalyst is in the form of grids of thin wire diameter 0.06-0.09 mm, having 1024 the holes in 1 cm2. Fasten the grid in the form of a package to provide the necessary time contacting. When working under atmospheric pressure in the apparatus installs the package from 18 nets.
The oxidation of ammonia under pressure of 0.8 MPa productivity of the catalyst in 5 times, than at atmospheric pressure, but ash and the platinum is greatly increased. To reduce losses of platinum in the oxidation of ammonia is carried out in two stages. The first stage uses platinum mesh, the second granular non-platinum catalyst (for example, gelatobaby). Application of two-stage catalyst makes it possible to reduce the loading of platinum into the machine three times, to reduce its losses 10-20% when the life of the catalyst 3-5 years.
Increasing the temperature increases the reaction rate and the diffusion coefficient of ammonia in the mixture and, so, is the most effective means of increasing the speed of the process, occurs predominantly in the diffusion region. At atmospheric pressure to maintain the temperature 8000With, with increased up to 9000With.
The ratio of ammonia and oxygen in the gas mixture affects the temperature regime and the overall speed of the process in that case, if it is limiting in a chemical reaction, ie. the process proceeds in the kinetic area. At stoichiometric ratio of components in AMS the degree of conversion of ammonia to nitric oxide (2) does not exceed 0.65 Dol. ed. To increase the output of nitric oxide (2) the process is conducted at About2:NH3=1.9–2.0, that corresponds to the content in Amvs 0.095–0.105 about. Dol. ammonia and 0.18–0.19 on. Dol. oxygen. The excess oxygen is used at the stage of final oxidation of nitric oxide (2), and the specified composition of AMS provides avtoterminal the oxidation process and lies beyond the limit of explosiveness of Amvs.
The pressure increase accelerates the process of oxidation of ammonia by increasing the concentration of reagents and the performance of the catalyst, reducing the size of the equipment. This reduces, however, the output of nitric oxide (2) and increases erosion and the entrainment of the catalyst, which increases the cost of production.
The rate of catalytic oxidation of ammonia to nitric oxide (2) very high. For ten thousandths of a second the degree of transformation is 0.98 $ -0.97. ed. at atmospheric pressure and 0.97 At a pressure of -0.98 0.8 -1.0 MPa.
The oxidation of ammonia is carried out in the contact apparatus of continuous action. The mode of movement of reagents, the displacement of. Temperature – adiabatic. The gas in the contact apparatus are fed. Catalyst gauzes, in order to avoid sagging rely on a metal grate.
Oxidation of nitric oxide (2).
Nitrous gases, obtained at the stage of oxidation of ammonia, contain nitric oxide (2), nitrogen, oxygen and water vapor. The oxidation of nitric oxide (2) in the oxide (4) in this system three parallel flow reaction:
2NO. + In2 ↔ 2NО2-DN DN =112.3 kJ (2.2.10)
2NO.2 ↔ Ν2In4 –DN DN =57.9 kJ (2.2.11)
ΝО2 + ΝО ↔ Ν2In3 –DN DN = 40.0 kJ (2.2.12)
All these reactions are reversible, proceed in a homogeneous system with heat and reducing the volume. As a consequence, the decrease of temperature and increase in pressure shifts the equilibrium to the right. Since nitrous gases exit from the reactor at a temperature of 8000With, in the oxide (4) there is practically no. For the conversion of nitric oxide (2) in the oxide (4) gases must be cooled below 1000With.
Usually the processing of waste gases is carried out at 10-500With. Under these conditions, the portion of the oxide (4) in dimerized tetroxide Ν2In4. Degree of coupling significantly depends on the temperature.
The oxidation number of N increases considerably with increasing pressure (which is equivalent to increasing the concentration), but at the same time increases the reaction rate. In plants, operating at elevated pressure, NO is almost completely oxidized to NO2. The volume of the equipment is hundreds of times less compared to the oxidation process at atmospheric pressure.
Absorption of nitric oxide (4) water.
Nitrous gases, coming to absorption, represent a complex mixture of various oxides of nitrogen, elementary nitrogen, oxygen and water vapor. Their composition depends on the conditions of oxidation. All the oxides of nitrogen, included in the composition of the nitrous gases, insoluble in water, but, with the exception of nitric oxide (2), interact with it. The absorption of water is accompanied by chemical reaction chemisorption, flowing in the system "gas – liquid", described by the equations:
2ΝО2 + N2ON ↔ НΝО3 + НΝО2 –DN DN =116кДж (2.2.13)
Ν2In4 + N2ON ↔NGOS3 + NGOS2 –DN DN = 59 kJ (2.2.14)
and the collapse of unstable nitrous acid according to the equation
3НΝО2 ↔ НΝО3 + 2ΝО +H2In + NAM NAM =76 kJ (2.2.15)
From these equations it follows, when the absorption of three moles of nitric oxide (4) form two moles of nitric acid and one mole of nitric oxide (2), which is returned into the cycle and again oxidized to nitrogen oxide (4). The state of the system "ΝО2–НΝО3–N2About" and, therefore, the concentration of the resulting nitric acid depends on temperature, pressure, the partial pressure of nitric oxide (4) in the absorbed gas mixture and the concentration of the formed acid. When the temperature and concentration of acid and pressure increase the degree of absorption of nitric oxide (4) aqueous nitric acid increases, while the more intense, the higher concentration in the nitrous gases.
The degree of absorption of nitric oxide (4) directly related to the absorption volume of the equipment. Increasing the degree of absorption requires a significant increase in absorption volume.
The absorption process at atmospheric pressure carried out in 6-8-mi absorption towers Packed type, filled with acid-resistant Raschig rings, also on the counterflow principle. The mode of movement of the gas and liquid phases in the tower corresponds to the displacement of.
Absorption of nitrous gases under pressure is carried out in a bubble column with cap plates or sieve type on the counterflow principle – bottom-piped gas, top of water or condensation. The column to apply the blend mode in the liquid phase and the displacement gas.
Clear advantages are absorption systems, working under high pressure: the high degree of transformation of nitrous gases (98-99%), a small number of absorption apparatus (1-2PCs.), no cumbersome installation for cleaning waste gases.
Regardless of the specific technological circuit schematic diagram of production of diluted nitric acid includes six basic operations.
the air
Steam NGOS3 ΝаΝО3, ΝаΝО2
Fig.2.2.2 Schematic diagram of production of diluted nitric acid
1–clearing the ammonia and air mix; 2- the oxidation of ammonia on the catalyst; 3, 4 – cooling of nitrous gases using heat from the oxidation process; 5 –the oxidation of nitric oxide (2) and the formation of nitric acid; 6– cleaning (neutralization) exhaust gases.
AMS –ammonia / air mixture; NG – nitrous gases; EXHAUST – waste gases.
Ammonia and air, cleaned of impurities, are mixed and sent to the stage of oxidation of ammonia. Warmed by the heat of reaction, the gas mixture (nitrous gases) is cooled in a waste heat boiler with production of process steam and in the fridge, where there is a partial oxidation of nitric oxide (2) to nitric oxide (4). Further oxidation is carried out simultaneously with the formation of nitric acid in the absorption process of nitrogen oxide (4) water. The exhaust gases, containing the balance of nitric oxide (4) unreacted, purified by neutralization with a solution of sodium, then emit.
Since the crucial parameter is pressure, all the existing technological schemes of production of diluted nitric acid are divided into three types:
– at atmospheric pressure (type 1);
– at high pressure (type 2);
– with two pressure levels (combined) (type 3).
Due to low productivity, cumbersome equipment, significant losses of ammonia, a small degree of absorption and, as a result, the need for costly treatment facilities, installation, working at atmospheric pressure, lost their value and are not based.
The basis of all XTC is a scheme for open-circuit and serial technology communications devices.
Flow sheet of production of diluted nitric acid under high pressure has the following main indicators:
– the pressure at the stage of oxidation of ammonia is 0.73 MPa;
– the pressure at the stage of absorption of nitric oxide(4) 0.65MPa;
catalyst – a platinum mesh;
– the concentration of nitric acid- 0.55 -0.58 wt.Dol.;
– the number of units -3
In the schema provided:
– catalytic cleaning of exhaust gases from nitrogen oxide (4), allowing to reduce its concentration with 0.3 to 0.002% about.;
– bleaching of the resulting nitric acid, reducing the content of nitric oxide (4) with 1.0 to 0.2% about.;
– utilization of heat energy and potential energy of compressed gases and, as a result, energy autonomy is the installation.
The main equipment of high pressure apparatus are the contact and absorption column. Contact device with the diameter of 1.6-2.0 m is made in two parts: the upper conical and lower cylindrical, between which are 12 platinoid catalyst gauzes, located in special cassettes. In the lower part of the integrated superheater of the recovery boiler. Performance contact apparatus -360 tons/day. The absorption column of the bubbling type has a diameter of 3.2 m and a height of 45m. It is equipped with sieve plates, which are located between the heat sink coils, water-cooled, which provide the necessary thermal regime of the absorption process. Purification of tail gases from nitrogen oxides is carried out in the reactor for catalytic purification, which over the catalyst AVK-10, consisting of palladium on aluminium oxide, when 7600With the reactions of reduction of nitrogen oxides gas, obtained by the conversion of methane:
CH4 +0.5In2 =CO +2H2 (11.2.4.14)
2NO.2 +4N2 = N2 +4N2In (11.2.4.15)
2NO. + 2N2 = N2 + 2N2In (11.2.4.16)
Technological scheme of production of nitric acid AK-72
This national scheme is the most modern. It is based on a closed energy cycle with two-stage conversion of ammonia and cooling of nitrous gases(1 stage) under pressure 0.42 MPa and processing of waste gases (2 stage) under pressure 0.108 MPa. This scheme provides the most optimal conditions of each stage of production is the oxidation of ammonia and recycling of waste gases. The scheme provides:
– the output in the form 60% nitric acid;
– thorough cleaning of ammonia and air;
– cooling of nitrous gases by washing them from the nitrite and ammonium nitrate;
– catalytic cleaning of exhaust gases;
– the use of secondary energy resources.
The contact apparatus in the system of the AK-72 has a cylindrical shape, diameter 4 m and height 5.6 m. The compressed air passes through the annular gap between the inner housing of the reaction part of the apparatus and the outer body and comes in built in the upper part of the mixer unit, where it is mixed with ammonia. The newly formed AMS passes the filter and is directed to the catalyst. In the lower part of the apparatus coils are located the waste heat recovery boiler, receiving nitrous gases after the catalyst.
Technical and economic indicators of production and consumption indices.
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| index | System type | ||
| combined | High pressure | AK-72 | |
| The capacity of the unit, kt/year |
|||
| The number of units | |||
| Unit capital costs, % | |||
| The cost | |||
| Productivity, % | |||
| The expenditure coefficients (per 1 t of ammonia) |
|||
| Ammonia, t | 0.293 | 0.293 | 0.293 |
| The platinum catalyst, g | 0.049 | 0.160 | 0.100 |
| Electricity, kWh |