Methods for obtaining ammonia

The raw material for ammonia production is a nitric-hydrogen mixture (ABC) of stoichiometric composition N2:H2 = 1:3. fuel, conversion of natural gas (Fig. 14.5).

Rice. 14.5. Raw materials for ammonia production

The structure of the raw material base for the production of ammonia has changed and over 90% of ammonia is produced on the basis of nature - 14.3 shows the dynamics of changes in the structure of the main types of raw materials for ammonia production.

Table 14.3. Changes in the raw material base of ammonia production

The nitric-hydrogen mixture, regardless of the method of its preparation, contains impurities of substances, some of which are catalytic poisons, causing both reversible (oxygen, carbon oxides, water vapor) and irreversible (various compounds of sulfur and phosphorus) poisoning of the catalyst.

In order to remove these substances, ABC undergoes pre-treatment, the methods and depth of which depend on their nature and content, that is, on the method of production of ABC. Usually, ABC obtained by natural gas conversion contains carbon monoxide (IV), methane, argon, traces of oxygen and up to 0.4% vol. carbon monoxide (II).

Absorption with liquid scavengers (wet method) and adsorption with solid scavengers (dry method) are used in industry to purify ABC. At the same time, the cleaning process can be carried out at various stages of production:

Source gas before submitting it for conversion;

converted gas to remove carbon monoxide (IV) from it;

Nitrogen mixture immediately prior to ammonia synthesis (ABC fine purification).

The first two processes are considered in the description of the respective industries.

Fine purification of ABC is achieved by chemisorption of impurities with liquid reagents and, finally, by their catalytic hydrogenation or washing of ABC with liquid nitrogen.

To remove carbon monoxide (IV) and hydrogen sulfide, ABCs are washed in packed towers with alkaline reagents that form thermally unstable salts with them: aqueous solution ethanolamine or hot, activated by the addition of diethanolamine, a solution of potassium carbonate. In this case, the following reactions take place:

H 2S+CH 2OH-CH 2NH 2+HS- - ?Н,

SO 2+ K 2CO3 + H 2Oh? 2KNSO3 - ?N.

Carbon monoxide (II) is removed from ABC by washing it with a copper-ammonia solution of copper acetate:

CO + NH3 + +Ac? +Ac -?H,

where: AC \u003d CH3 SOO.

The absorbents used for chemisorption form unstable compounds with those absorbed from ABC. Therefore, when their solutions are heated and the pressure is reduced, the dissolved impurities are desorbed, which makes it easy to regenerate the absorbent, return it to the process and ensure the absorption operation cycles according to the scheme:

where: P is the admixture absorbed from ABC, A is the absorbent, PA is the combination of the admixture and the absorbent.

More effective method purification of ABC from carbon monoxide (II) is the washing of ABC with liquid nitrogen at -190 ° C, which is used in modern installations, during which, in addition to carbon monoxide (II), methane and argon are removed from it.

The final purification of ABC is achieved by catalytic hydrogenation of impurities, called methanation or pre-catalysis. This process is carried out in special methanation units (Fig. 14.6) at a temperature of 250-300 ° C and a pressure of about 30 MPa on a nickel-aluminum catalyst (Ni + Al 2O 3). In this case, exothermic reactions of reduction of oxygen-containing impurities to methane, which is not a poison for an iron catalyst, proceed, and water condenses when the purified gas is cooled and is removed from it:

CO + ZN 2? CH 4 + H 2HE,

SO 2+ 4H 2?CH 4 + 2H 2HE,

O 2+ 2H 2?2H 2HE

Rice. 14.6. Scheme of the ABC methanation plant: 1 - compressor, 2 - heater, 3 - methanation reactor, 4 - water heater, 5 - condenser, 6 - dehumidifier

If an iron catalyst is used in the precatalysis, some ammonia is also formed in the hydrogenation process, in which case the precatalysis is called blowing.

The methanation process is simple, easy to control, and the heat released due to the ongoing exothermic hydrogenation reactions is used in the general energy-technological scheme for the production of ammonia. Purified ABC supplied for synthesis contains up to 0025 vol. share of argon, 0.0075 vol. share of methane and no more, 00004 vol. share of carbon monoxide (II), which is the most powerful catalytic poison.

The process of ammonia synthesis is based on a reversible exothermic reaction that proceeds with a decrease in gas volume:

2+3H 2+2NH 3+Q.

In accordance with the Le Chatelier principle, as the pressure increases and the temperature decreases, the equilibrium of this reaction shifts towards the formation of ammonia. To ensure the optimal speed of the process, a catalyst, increased pressure, temperature of 400 ... 500 ° C and a certain volumetric velocity of the reacting components are required. In industry, an iron catalyst with additives of Al oxides is used. 2O 3, TO 2O, CaO and SiO2 .

The following industrial systems of ammonia synthesis units are distinguished: low pressure(10…20 MPa), medium (20…45 MPa) and high pressure (60…100 MPa). In world practice, medium-pressure systems are widely used, since in this case the issues of ammonia separation from a nitrogen-hydrogen mixture are most successfully solved at a sufficiently high process speed.

CH 4+ H2 O? CO + 3H 2

Partial combustion of hydrogen in atmospheric oxygen occurs:

H 2+ O 2 = H 2O(steam)

As a result, at this stage, a mixture of water vapor, carbon monoxide (II) and nitrogen is obtained.

The main unit of the installation for the production of ammonia is the synthesis column (Fig. 1.1). The tubular column in the medium pressure system is a cylinder 4 made of chrome-vanadium steel with a wall thickness of up to 200 mm, a diameter of 1 ... 1.4 m and a height of about 20 m. From above and below it is closed with steel covers 2.

Structurally, the columns differ mainly in the size of the body and the device of the internal packing. In the upper part of the column under consideration, there is a catalyst box 3, and in the lower part there is a heat exchanger 8, which ensures the autothermal process. The catalyst box is connected to the heat exchanger by a central tube 7. The column body has thermal insulation 5. The catalyst is loaded onto the grate 6. To ensure uniform temperature distribution, double pipes 1 are introduced into the catalyst bed.

Rice. 1.1. Ammonia synthesis column with double countercurrent heat exchange tubes

At present, ammonia synthesis columns are combined with steam boilers for waste heat recovery (1 ton of ammonia accounts for 0.6...1 ton of steam at a pressure of 1.5...2 MPa). Medium pressure ammonia synthesis columns have a capacity of about 150 tons of ammonia per day and operate without replacing the catalyst for four years.

In the synthesis of ammonia under medium pressure (Fig. 1.1), a nitrogen-hydrogen mixture (N 2:N 2=1:3) is fed into column 1, where ammonia is synthesized on the catalyst; a nitrogen-hydrogen-ammonia gas mixture leaves the column (ammonia content - 14 ... 20%), having a temperature of about 200 ° C. This mixture is sent to the water cooler 2, cooled to 35 °C and enters the separator 3. Here, up to 60% of the ammonia formed in the column is released from the gas (at a pressure of 30 MPa, ammonia cannot condense completely in the cooler). Ammonia is released more fully when the nitrogen-hydrogen mixture is cooled to more low temperatures. This mixture with ammonia residues from the separator 3 is sent to the circulating compressor 4 and then to the filter 6 to separate the compressor oil. At the inlet to the filter, a fresh nitrogen-hydrogen mixture is added to the circulating gases, compressed to operating pressure using a multi-stage compressor 5. From the filter, the gas mixture is fed into the ammonia secondary condensation system, consisting of a condensation column 7 and an evaporator of liquid ammonia 8. In the condensation column, gas it is pre-cooled in a heat exchanger located in the upper part of the column and then sent to the evaporator 8, where, due to the evaporation of the incoming liquid ammonia, the gas is cooled to -5 ° C and the ammonia is condensed from the gas to a residual content of about 2.5% NH3 in it. The condensed ammonia is released in the lower part of the condenser column 7, which is the separator. After the separation of ammonia, the nitrogen-hydrogen mixture cools the gas entering it in the upper part of the column 7, and then is again sent to the synthesis column 1.

In the case of ammonia synthesis at a higher pressure (45 MPa and higher), there is no need for its secondary condensation, since the residual ammonia content in the nitrogen-hydrogen mixture at the outlet of the water cooler is insignificant.

Rice. 17.16. Scheme of the installation for the synthesis of ammonia under medium pressure

Description of the technological process of ammonia production and its characteristics.

. Arc method.The arc method consists in blowing air through the flame of an electric arc. At a temperature of about 3000 ° C, a reversible reaction occurs

2 + O 2?2NO - Q.

The resulting nitric oxide (II) can be oxidized to nitric oxide (IV) and processed into nitric acid and other connections. To obtain 1 ton of bound nitrogen by this method, 60,000 ... 70,000 kWh of electricity are consumed.

2. Cyanamide method.The first industrial process that was used to produce ammonia was the cyanamide process. When lime CaO and carbon were heated, calcium carbide CaC2 was obtained. The carbide was then heated under nitrogen to give calcium cyanamide CaCN2; further ammonia was obtained by hydrolysis of cyanamide:

CaCN 2(tv) + 3N 2O = 2NH 3? + CaCO3 (TV)

This process required a lot of energy and was economically unprofitable.

Modern Process production of ammonia is based on the ability of finely ground calcium carbide at a temperature of about 1000 ° C to interact with nitrogen according to the equation

CaS 2+ N 2= CaCN2 + C + 302 kJ

The share of production of bound nitrogen by the cyanamide method is very small.

The ammonia method of nitrogen fixation consists in its synthesis from nitrogen and hydrogen using a special catalyst:

2+ 3H 2? 2NH3 ? + 45.9 kJ

This method has an economic and technological advantage over other methods of elemental nitrogen fixation.

3. Ammonia method.The ammonia method of binding atmospheric nitrogen consists in combining nitrogen with hydrogen and obtaining ammonia:

N 2+3H 2?2NH 3+Q.

It is the most economical (electricity consumption is 4000…5000 kWh per 1 ton of ammonia), technologically easier to implement compared to other methods of atmospheric nitrogen fixation. In the total production of nitrogen compounds, over 90% is accounted for by ammonia. Hydrogen for this reaction is obtained by thermal cracking of hydrocarbons, the action of water vapor on coal or iron, the decomposition of alcohols with water vapor, or the electrolysis of water.

4. A variant of the ammonia method.In 1909, an original method was developed for the simultaneous production of ammonia and aluminum oxide from bauxite through aluminum nitride according to the scheme shown in Fig. 14.4.

Rice. 14.4. Production of ammonia from bauxite

Industrial installations according to this method were built in the period 1909-1918. in a number of countries, but the method has not found application due to low production efficiency.

Chemical and principal schemes of production.

The main stage of the ammonia synthesis process from a nitric-hydrogen mixture is described by the equation:

N 2+ 3H2 = 2NH 3

However, since the predominant method for producing ABC is air and steam reforming of methane, the chemical scheme for the production of ammonia includes, in addition to this reaction, several reactions of air and steam reforming:

CH 4+ H 2O = ZH2 + CO,

CH 4+ 0.5O 2(N 2) = 2H 2(N 2) + CO

and subsequent conversion of carbon monoxide (II) to carbon monoxide (IV):

CO + H 2O = H2 + CO 2

ammonia production absorption column

After removing carbon monoxide (IV) from the gas mixture and correcting its composition, ABC is obtained with a nitrogen and hydrogen content in a ratio of 1: 3.

Thus, modern ammonia production consists of two stages: preparation of ABC and its conversion into ammonia, representing a single energy-technological scheme that combines the operations of obtaining ABC, its purification and ammonia synthesis and effectively uses the thermal effects of all stages of the process, which allows several times reduce electricity costs.

Rice. 14.7. circuit diagram ammonia production

1 - purification of natural gas from sulfur compounds, 2 - steam reforming of methane, 3 - air reforming of methane, 4 - conversion of carbon monoxide (II), 5 - chemisorption purification of ABC, 6 - methanation, 7 - synthesis of ammonia, 8 - absorption ammonia, 9-ammonia compression, I-natural gas, II-converted gas, III-ABC, IV-methane

The basic scheme of ammonia production consists of three stages:

The first stage is the production of ABC (nitrogen mixture):

I operation: purification of natural gas from sulfur compounds;

I operation: steam conversion of methane;

I operation: air conversion of methane;

I operation: conversion of carbon monoxide (II).

The second stage is gas purification from ballast impurities and impurities that poison the catalyst:

I operation: purification of ABC by absorption methods from carbon monoxide (II) and carbon monoxide (IV);

I operation: fine purification of ABC from carbon monoxide (II) and carbon monoxide (IV) by methanation or pre-catalysis.

The third stage is the synthesis of ammonia from ABC in the presence of a catalyst.

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The nitrogen industry today is one of the leading industries. The use of ammonia has spread to refrigeration applications (R717, medicine or Agriculture(fertilizers).

Primary attention is paid to the production of nitrogen fertilizers (and, therefore, their bases, including ammonia, the demand for which has grown by 20% over the past two decades).

But the production of ammonia is distinguished, first of all, by high energy intensity. The whole history of this production is a struggle to reduce the energy used (mechanical, thermal, electrical).

The synthesis of ammonia reveals the formula:

N2 + 3H2 = 2NH3 + Q

The reaction is exothermic, reversible, with a decrease in volume. Since the reaction is exothermic, lowering the temperature will shift the equilibrium to form ammonia, but will decrease significantly. Ammonia production must take place at high temperatures (synthesis takes place at 500 degrees Celsius). An increase in t° will lead to pressure from 15 to 100 MPa, which allows you to counteract the influence of temperature (low pressure - from 10 to 15 MPa, medium pressure - from 25 to 30 MPa, high pressure- over 50 MPa). Of these, the average is preferable.

Serves as a catalyst with additions of calcium, silicon, potassium, aluminum oxides.

Harmful impurities (water, hydrogen sulfide) adversely affect the rate of the reaction, poisoning the catalyst, thereby reducing its activity and reducing the service life. This means that the hydrogen sulfide mixture must be thoroughly cleaned. But even after purification, only part of this mixture turns into ammonia. Therefore, the remaining unreacted fraction is again sent to the reactor.

How is ammonia produced?

An already prepared mixture of three parts of hydrogen and one nitrogen is fed into the pipeline. It passes through a turbocharger, where it is compressed to the pressure indicated above, and is sent to the synthesis column with a catalyst on built-in shelves. The process, as we found out, is strongly exothermic. The released heat heats up the nitric-hydrogen mixture. About 25 percent of ammonia and unreacted nitrogen with hydrogen come out of the column. The entire composition enters the refrigerator, where the mixture is cooled. Ammonia becomes liquid under pressure. Now the separator comes into operation, the task of which is to separate the ammonia into the collector at the bottom and the unreacted mixture, which is returned back to the column. Thanks to this circulation, the nitric-hydrogen mixture is used by 95 percent. Liquid ammonia is delivered to a special warehouse through the ammonia pipeline.

All devices used in production are as tight as possible, which eliminates leakage. Only the energy of the exothermic reactions occurring inside is used. The circuit is closed, low-waste. Costs are reduced thanks to a continuous and automated process.

Ammonia production cannot but affect environment. Gas emissions are inevitable, including ammonia, carbon and nitrogen oxides and other impurities. Low potential heat is released. Water is discharged after washing the cooling systems and the reactor itself.

Therefore, in the production of ammonia, it is necessary to include catalytic purification with the presence of a reducing gas. Quantity reduction Wastewater can be achieved by replacing the turbochargers. Low potential heat can be utilized by introducing high potential heat. However, this will increase the pollution by flue gases.

An energy-technological scheme that includes a steam-gas cycle, where both steam heat and fuel combustion products are used, will simultaneously increase production efficiency and reduce emissions.

1) 4FeS 2 + 11O 2 → 2Fe 2 O 3 + 8SO 2

2) 2SO 2 + O 2 V 2 O 5 → 2SO 3

3) nSO 3 + H 2 SO 4 → H 2 SO 4 nSO 3 (oleum)

Crushed purified wet pyrite (sulfur pyrite) is poured from above into the kiln for firing in " fluidized bed". From below (counterflow principle) air enriched with oxygen is passed through.
Furnace gas comes out of the furnace, the composition of which is: SO 2, O 2, water vapor (pyrite was wet) and the smallest particles of cinder (iron oxide). The gas is purified from impurities of solid particles (in a cyclone and electrostatic precipitator) and water vapor (in a drying tower).
In the contact apparatus, sulfur dioxide is oxidized using a V 2 O 5 catalyst (vanadium pentoxide) to increase the reaction rate. The process of oxidation of one oxide to another is reversible. Therefore, the optimal conditions for the flow of the direct reaction are selected - increased pressure (because the direct reaction proceeds with a decrease in the total volume) and a temperature not higher than 500 C (because the reaction is exothermic).

In the absorption tower, sulfur oxide (VI) is absorbed by concentrated sulfuric acid.
Water absorption is not used, because sulfur oxide dissolves in water with the release of a large amount of heat, so the resulting sulfuric acid boils and turns into steam. In order to avoid the formation of sulfuric acid mist, use 98% concentrated sulfuric acid. Sulfur oxide dissolves very well in such an acid, forming oleum: H 2 SO 4 nSO 3

Industrial production of ammonia

Preliminarily, a nitric-hydrogen mixture is obtained. Hydrogen is obtained by conversion of methane (from natural gas):

CH 4 + H 2 O (g) → CO + ZH 2 - Q

2CH 4 + O 2 → 2CO + 4H 2 + Q

CO + H 2 O (g) → CO 2 + H 2 + Q

Nitrogen is obtained from liquid air.

In the turbocharger, the mixture is compressed to the required pressure of 25·10 6 Pa. In the synthesis column, the gases react at 450-500 °C in the presence of a catalyst (porous iron with impurities of Al 2 O 3 and K 2 O):
N 2 + 3H 2 ↔ 2NH 3 + 92 kJ (output 10-20% ammonia)

The resulting ammonia is separated from unreacted nitrogen and hydrogen by liquefaction in a refrigerator, returning the unreacted nitric-hydrogen mixture to the synthesis column.
The process is continuous, circulating.

Application: production of nitrogen fertilizers, explosives, plastics, etc.

Production of methyl alcohol

Before the industrial development of the catalytic method of obtaining methanol was obtained by dry distillation of wood (hence its name "wood alcohol"). At this time, this method is of secondary importance.

Modern way:

Raw material: synthesis gas - a mixture of carbon monoxide (II) with hydrogen (1:2).

Auxiliary materials: catalysts (ZnO and CuO).

Basic chemical process: synthesis gas at a temperature of 250 ° C and a pressure of 7 MPa is converted catalytically into methanol:

CO + 2H 2 ↔ CH3OH + Q

Features of the technological process: when the gas mixture passes through the catalyst bed, 10-15% methanol is formed, which is condensed, and the unreacted mixture is mixed with a fresh portion of synthesis gas and, after heating, is again sent to the catalyst bed (circulation). The overall yield is 85%.

The conditions for the synthesis of methanol and ammonia at medium pressure are similar, and the raw material (natural gas) is common for both processes. Therefore, most often the production of methanol and ammonia is combined (nitrogen-fertilizer plants).

Ammonia (NH3) is a chemical compound of hydrogen and nitrogen. It got its name from the Greek word "hals ammniakos" or the Latin "sal ammoniacus" which are translated the same way - "ammonia". It was such a substance called that was obtained in the Libyan desert in the Ammonium oasis.

Ammonia is considered a highly toxic substance that can irritate the mucous membranes of the eyes and respiratory tract. The primary symptoms are profuse lacrimation, shortness of breath, and pneumonia. But at the same time, ammonia is a valuable chemical that is widely used to obtain inorganic acids, for example, nitric, hydrocyanic, as well as urea and nitrogen-containing salts. Liquid ammonia is an excellent working medium for refrigerated containers and machines, as it has a high specific heat of vaporization. Water ones are used as liquid fertilizers, as well as for ammonization of superphosphates and fertilizer mixtures.

The production of ammonia from waste gases in the process of coal coking is the oldest and most accessible method, but today it is already outdated and practically not used.

The modern and main method is the production of ammonia in industry based on the Haber process. Its essence lies in the direct interaction of nitrogen and hydrogen, which occurs as a result of the conversion of hydrocarbon gases. Oil refineries, associated petroleum gases, residual gases from the production of acetylene usually act as feedstock. The essence of the ammonia conversion method is the decomposition of methane and its homologues at high temperature into components: hydrogen and with the participation of oxidizing agents - oxygen and water vapor. At the same time, air enriched with oxygen or atmospheric air is mixed with the converted gas. Initially, the ammonia production reaction based on the convertible gas proceeds with heat release, but with a decrease in the volume of the initial reaction products:

N2 + 3H2 ↔ 2NH3 + 45.9 kJ

However, the production of ammonia on an industrial scale is carried out using a catalyst and under artificially created conditions that allow increasing the yield of the finished product. In the atmosphere where ammonia is produced, the pressure increases to 350 atmospheres, and the temperature rises to 500 degrees Celsius. Under such conditions, the ammonia yield is about 30%. The gas is removed from the reaction zone using the cooling method, and nitrogen and hydrogen that have not reacted are returned back to the synthesis column and can again participate in the reactions. In the course of synthesis, it is very important to purify the mixture of gases from catalytic poisons, substances that can negate the effect of catalysts. Such substances are water vapor, CO, As, P, Se, O2, S.

Porous iron with impurities of aluminum and potassium oxides acts as a catalyst in the reactions of nitrogen and hydrogen synthesis. Only this substance, out of all 20 thousand previously tested, allows reaching the equilibrium state of the reaction. This principle of obtaining ammonia is considered the most economical.

Obtaining ammonia in the laboratory is based on the technology of its displacement from ammonium salts with strong alkalis. Schematically, this reaction is represented as follows:

2NH4CI + Ca(OH)2 = 2NH3 + CaCl2 + 2H2O

NH4Cl + NaOH = NH3 + NaCl + H2O

To remove excess moisture and dry ammonia, it is passed through a mixture of caustic soda and lime. Very dry ammonia is obtained by dissolving sodium metal in it and then distilling the mixture. Most often, such reactions are carried out in a closed metal system under vacuum. Moreover, such a system must withstand high pressure, which is achieved by the released ammonia vapor, up to 10 atmospheres at room temperature.

Ammonia It is a light colorless gas with an unpleasant pungent odour. It is very important for chemical industry, since it contains a nitrogen atom and three hydrogen atoms. Ammonia is mainly used to produce nitrogen-containing fertilizers, ammonium sulfate and urea, to produce explosives, polymers and other products, and ammonia is also used in medicine.

Production of ammonia in industry not a simple, time-consuming and expensive process based on its synthesis from hydrogen and nitrogen using a catalyst, high temperature and pressure. Activated by oxides potassium and aluminum sponge iron is used as a catalyst. Industrial plants for the synthesis of ammonia are based on the circulation of gases. It looks like this: the reacted mixture of gases, which contains ammonia, is cooled and condensation and separation of ammonia occurs, and nitrogen and hydrogen that did not react are mixed with a new portion of gases and re-fed to the catalyst.

Let us consider this process of industrial synthesis of ammonia, which occurs in several stages, in more detail. At the first stage, sulfur is removed from natural gas using a desulfurizer technical device. At the second stage, the methane conversion process is carried out at a temperature of 800 degrees Celsius on a nickel catalyst: hydrogen reaction is suitable for the synthesis of ammonia and air containing nitrogen is supplied to the reactor. At this stage partial combustion of carbon also occurs after its interaction with oxygen, which is also contained in the air: 2 H2O + O2-> H2O (steam).

The result of this stage production is to obtain a mixture of water vapor and oxides of carbon (secondary) and nitrogen. The third stage goes in two processes. The so-called "shift" process takes place in two "shift" reactors. In the first one, the Fe3O4 catalyst is used and the reaction proceeds at high temperatures, on the order of 400 degrees Celsius. The second reactor uses a more efficient copper catalyst and runs at a lower temperature. The fourth stage includes purification of the gas mixture from carbon monoxide (IV).

This cleaning is carried out by washing the gas mixture with an alkaline solution that absorbs the oxide. The reaction 2 H2O + O2H2O (steam) is reversible, and after the third stage, approximately 0.5% carbon monoxide remains in the gas mixture. This amount is enough to spoil the iron catalyst. At the fourth stage, carbon monoxide (II) is eliminated by the conversion of hydrogen to methane on a nickel catalyst at temperatures of 400 degrees Celsius: CO + 3H2 -> CH4 + H2O

gas mixture, which roughly contains? 74.5% hydrogen and 25.5% nitrogen, subjected to compression. Compression leads to a rapid increase in the temperature of the mixture. After compression, the mixture is cooled to 350 degrees Celsius. This process is described with the reaction: N2 + 3H2 - 2NH3 ^ + 45.9 kJ. (Gerber process)

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