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Koya University

Faculty of Engineering

Chemical Engineering Department

Equipment Design

“Ammonia”

Preparation By

Aree Salah Alan Mawlud

Awat Qadr

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List of content:

Introduction …………………………………….…………….3

History of ammonia:………….……………………..…..….4-6

Properties ………….………………………….…………….7-8

Safety concerns with ammonia …………….……………..9-10

Manufacture of ammonia ………………………………..11-19

Waste Management …….........................................................20

Uses of ammonia.................................................................21-22

Fertilisers ………………………………..…………………...23

References ………………………………………..………......24

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Introduction:

Ammonia (NH3) is a common toxicant derived from wastes (Figure 1),

fertilizers, and natural processes. Ammonia nitrogen includes both the

ionized form (ammonium, NH4+) and the unionized form (ammonia, NH3).

An increase in pH favors formation of the more toxic unionized form

(NH3), while a decrease favors the ionized (NH4+) form.Temperature also

affects the toxicity of ammonia to aquatic life. Ammonia is a common

cause of fish kills, but the most common problems associated with

ammonia relate to elevated concentrations affecting fish growth, gill

condition, organ weights, and hematocrit (Milne et al. 2000). Exposure

duration and frequency strongly influence the severity of effects (Milne et

al. 2000).

Ammonia in sediments typically results from bacterial decomposition of

natural and anthropogenic organic matter that accumulates in sediment.

Sediment microbiota mineralize organic nitrogen or (less commonly)

produce ammonia by dissimilatory nitrate reduction. Ammonia is

especially prevalent in anoxic sediments because nitrification (the

oxidation of ammonia to nitrite [NO2-] and nitrate [NO3

-]) is inhibited.

Ammonia generated in sediment may be toxic to benthic or surface water

biota (Lapota et al. 2000).

Ammonia also exerts a biochemical oxygen demand on receiving waters

(referred to as nitrogenous biological oxygen demand or NBOD) because

dissolved oxygen is consumed as bacteria and other microbes oxidize

ammonia into nitrite and nitrate. The resulting dissolved

oxygen reductions can decrease species diversity and even cause fish

kills. Additionally, ammonia can lead to heavy plant growth

(eutrophication) due to its nutrient properties (see the Nutrients module).

Conversely, algae and macrophytes take up ammonia, thereby reducing

aqueous concentrations.

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History of ammonia:

Ammonia was first obtained in pure form in 1774 by an English chemist

Joseph Priestley. He heated sal ammoniac (ammonium chloride) with

slack lime (calcium hydroxide). The reaction 2NH4Cl + Ca(OH)2 ® NH3

+ CaCl2 is still used in laboratories if required a small quantities of this

gas. Priestley collected the released ammonia over mercury. He called it

"alkaline air" because water solution of ammonia had all attributes of

alkalis. In 1784, a French chemist Claude Louis Berthollet decomposed

ammonia into elements with the help of electric discharge and thus

identified the composition of this gas. Ammonia received its official name

as "ammoniac" in 1787 from the Latin name of ammonium chloride - sal

ammoniac; because that salt was obtained near the temple of Egyptian

god Amon. This name is still retained in the majority of West-European

languages (German Ammoniak, English ammonia, French ammoniaque);

the abbreviated Russian name "ammiak" was introduced in 1801 by

Russian chemist Yakov Dmitrievich Zakharov who was the first to

develop the Russian chemical nomenclature system. There are hydrogen

bonds between ammonia molecules. Although they are not as strong as

those between water molecules, these bonds promote strong attraction

between molecules. That is why physical properties of ammonia are in

many respects abnormal as compared to properties of other hydrides of

elements in the same subgroup (PH3, SbH3, AsH3). In solid ammonia,

each nitrogen atom is linked to six hydrogen atoms by three covalent

bonds and three hydrogen bonds. In ammonia melting, only 26% of all

hydrogen bonds break, and another 7% explode when heated to the

boiling point of a liquid. Only when the temperature is above that value,

almost all bonds remaining between molecules disappear. Ammonia

stands out among other gases because of its great solubility in water:

under normal conditions, 1 ml of water is able to absorb more than a litre

of gaseous ammonia (more exactly, 1,170 ml) with formation of 42.8%

solution. If we calculate relationship between NH3 and H2O in saturated

solution under normal conditions, we will find that for each one ammonia

molecule, there is one water molecule. Ammonia is quite active

chemically and interreacts with many substances. It burns with pale-

yellow flame in pure oxygen, turning, mainly, into nitrogen and water.

The mixtures of ammonia and air with ammonia content from 15% to

28% are explosive. Ammonia, due to its non-divided electron pair, forms

a huge number of complex compounds with metal ions - so called amino

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compounds or ammoniates. In contrast to organic amines, the nitrogen

atom in these complexes is always linked to three hydrogen atoms. The

most interesting of various substance solutions in liquid ammonia are,

doubtlessly, solutions of alkaline metals. Such solutions have raised a

great interest with scientists for more than a hundred years. Solutions of

sodium and potassium in liquid ammonia were obtained for the first time

in 1864. Several years later it was found that if ammonia is allowed to

evaporate normally, the residue would consist of pure metal, just as we

can observe in respect of solution of common salt in water. When metals

are dissolved in liquid ammonia, the solution volume is always higher

than the summary of component volumes. In the result expansion, its

density continually decreases with concentration increases (which is not

the case with water solutions of salts and other solid compounds).

Lithium concentrated solution in liquid ammonia is the lightest liquid

under normal conditions. Its density at 20° C is only 0.48 g/cm3 (only

hydrogen, helium and methane, liquefied at low temperatures are lighter

than this solution). How much metal can liquid ammonia dissolve? It

depends mainly on temperature. At boiling temperature, the solution

contains about 15% (molar) of alkali metal. As temperature rises,

solubility increases quickly and becomes infinitely great at the metal

melting temperature. This means that molten alkali metal (e.g., caesium

at 28.3°C already) is mixed with liquid ammonia in any proportions.

Evaporation of ammonia from concentrated solutions is slow since its

saturated vapor pressure tends to zero as metal concentration increases.

One more very interesting fact: diluted and concentrated solutions of

alkali metals in liquid ammonia do not mix with each other. This is a rare

phenomenon for aqueous solutions. If, for instance, 4 g of sodium is

added to 100 g of liquid ammonia at a temperature of –43°C, the solution

will split into two phases by itself. One of them, more concentrated but

less dense, will be on top while diluted solution with higher density will

be at the bottom. The boundary between the solutions is easily noticeable:

the upper liquid has metal bronze shine, and the lower one is dark blue.

Ammonia is used also for production of synthetic fibres, e.g., nylon and

kapron. In textile industry, it is used for cleaning and dyeing of cotton,

wool and silk. In petroleum-chemical industry, ammonia is used for

neutralization of acidic wastes, and in natural rubber production

ammonia helps to preserve latex in the course of carriage from the

plantation to the factory. Ammonia is used also in soda production by

Solve method. In steel industry, ammonia is used for nitride hardening –

saturation of surface layers of steel with nitrogen, which increases its

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hardness significantly. Medics use ammonia water solutions (ammonia

spirit) in everyday practice: a piece of cotton wool moistened in salt

ammoniac brings the person out of a fainting fit. Ammonia in such dose is

not hazardous for people. Nevertheless, it should be remembered that this

gas is quite toxic. Ammonia is one of the first products in the world by its

production volumes. Annually, around 100 million tons of compounds are

obtained worldwide. Ammonia is produced in liquid form or in water

solution - ammonia water, which usually contains 25% of NH3. Huge

quantities of ammonia are used for production of nitric acid, which is

used for production of fertilizers and many other products. Ammonia

water is applied also directly as a fertilizer. Ammonia is used for

production of various ammonium salts, urea, urotropine. It is also used

as a cheap cooling agent in industrial refrigerating plants.

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Properties:

Anhydrous (water-free) ammonia gas is easily liquefied under pressure

(at 20°C liquid ammonia has a vapor pressure of about 120 lb per sq in.)

It is extremely soluble in water; one volume of water dissolves about

1,200 volumes of the gas at 0°C (90 grams of ammonia in 100 cc of

water), but only about 700 volumes at room temperature and still less at

higher temperatures. The solution is alkaline because much of the

dissolved ammonia reacts with water, H2O, to form ammonium

hydroxide, NH4OH, a weak base. Liquid ammonia is used in the chemical

laboratory as a solvent. It is a better solvent for ionic and polar

compounds than ethanol, but not as good as water; it is a better solvent

for nonpolar covalent compounds than water, but not as good as ethanol.

It dissolves alkali metals and barium, calcium, and strontium by forming

an unstable blue solution containing the metal ion and free electrons that

slowly decomposes, releasing hydrogen and forming the metal amide.

Compared to water, liquid ammonia is less likely to release protons

(H+ ions), is more likely to take up protons (to form NH4+ ions), and is a

stronger reducing agent. Because strong acids react with it, it does not

allow strongly acidic solutions, but it dissolves many alkalies to form

strongly basic solutions.

Ammonia takes part in many chemical reactions. Ammonia reacts with

strong acids to form stable ammonium salts: with hydrogen chloride it

forms ammonium chloride; with nitric acid, ammonium nitrate; and with

sulfuric acid, ammonium sulfate. Ammonium salts of weak acids are

readily decomposed into the acid and ammonia. Ammonium carbonate,

(NH3)2CO3·H2O, is a colorless-to-white crystalline solid commonly

known as smelling salts; in water solution it is sometimes called aromatic

spirits of ammonia. Ammonia reacts with certain metal ions to form

complex ions called ammines. Ammonia also reacts with Lewis acids

(electron acceptors), e.g., sulfur dioxide or trioxide or boron trifluoride

Another kind of reaction, commonly called ammonolysis, occurs when

one or more of the hydrogen atoms in the ammonia molecule is replaced

by some other atom or radical. Chlorine gas, Cl2, reacts directly with

ammonia to form monochloramine, NH2Cl, and hydrogen chloride, HCl.

Products of such ammonolyses include amides, amines, imides, imines,

and nitrides. Ammonia also takes part inoxidation and

reduction reactions. It burns in oxygen to form nitrogen gas, N2, and

water. In the presence of a catalyst (e.g., platinum) it is oxidized in air to

form water and nitric oxide, NO. It reduces hot-metal oxides to the metal

(e.g., cupric oxide to copper).

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Physical properties of ammonia:

Ammonia is a colourless gas with a sharp, penetrating odour. Its boiling

point is −33.35 °C (−28.03 °F), and its freezing point is −77.7 °C

(−107.8 °F). It has a high heat ofvaporization (23.3 kilojoules

per mole at its boiling point) and can be handled as aliquid in thermally

insulated containers in the laboratory. (The heat of vaporization of a

substance is the number of kilojoules needed to vaporize one mole of the

substance with no change in temperature.) The ammonia molecule has

a trigonal pyramidal shape with the three hydrogen atoms and an

unshared pair of electronsattached to the nitrogen atom. It is a polar

molecule and is highly associated because of strong intermolecular

hydrogen bonding. The dielectric constant of ammonia (22 at −34 °C

[−29 °F]) is lower than that of water (81 at 25 °C [77 °F]), so it is a

better solvent for organic materials. However, it is still high enough to

allow ammonia to act as a moderately good ionizing solvent. Ammonia

also self-ionizes, although less so than does water.2NH3 ⇌ NH4+ + NH2

−

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Safety concerns with ammonia: There are several safety-related concerns with anhydrous ammonia and

with aqueous solutions of ammonia. It is a respiratory irritant that is a

highly hazardous chemical.Release could take place through a simple

industrial or transportation accident, a deliberate release caused by

terrorists, or by improper handling by those using it in the illegal

synthesis of methamphetamines.

Storage tanks on farms used for dispensing ammonia as fertilizer are

referred to as ''nurse'' tanks and contain approximately 2,500 pounds

(1134 kg) of anhydrous ammonia, so any farm with four or more nurse

tanks needs to assess its safety. In fact, the U.S. Environmental Protection

Agency (U.S. EPA) mandates the performance of an "Offsite

Consequence Analysis" (OCA) as part of their "Risk Management Plan"

(RMP) requirements for any facility that stores more than 10,000 pounds

(4,536 kg) of anhydrous liquid ammonia or 20,000 pounds (9,072 kg) of

aqueous solutions of ammonia. The RMP requirements apply for

ammonia refrigeration systems or any other ammonia storage facilities

as well as farms. The U.S. Occupational Health & Safety Administration

(OSHA) has mandated very similar requirements as part of their

"Process Safety Management" (PSM) regulations for hazardous

chemicals.

In transport, ammonia containers must have proper hazardous material

placards and, if the pertinent threshold quantity is exceeded, may need

additional safeguards such as reporting the shipment to industry

monitoring services such as CHEMTREC or additional local agencies.

There may be restrictions on transporting hazardous materials through

tunnels, or possibly streets in high-density areas.

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The U.S. Department of Homeland Security (DHS), citing its major

concern as toxic release, lists anhydrous ammonia, or mixtures

containing at least 1 percent ammonia, when stored in quantities of

10,000 pounds or more, as a chemical of interest, which falls under the

Risk for Chemical Facility Anti-Terrorism Standards (CFATS)

regulations and guidance Organizations that store or transport more

than the threshold quantity of 10,000 pounds, or believe they are at a

higher than normal risk, should use the Chemical Security Assessment

Tool.[

The U.S. EPA has issued an additional safeguards document, with special

emphasis on the theft of ammonia.

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Manufacture of ammonia:

The manufacture of ammonia from nitrogen and hydrogen takes place in

two main stages:

a) The manufacture of hydrogen

b) The synthesis of ammonia (the Haber Process)

The manufacture of hydrogen involves several distinct processes. Figure

2 shows their sequence and the location within an ammonia plant

(steps1-5). The converter used to make ammonia from the hydrogen is

also shown (step 6). What occurs in each of these steps is described

below the figure.

Figure 2 An ammonia plant in Western Australia:

1 -Desulfurisation units

2 -Primary reformer

3 -High temperature and low temperature shift reactors

4 -Carbon dioxide absorber

5 -Carbon dioxide stripper (recovery of the pure solvent, ethanolamine)

6 -Ammonia converter

7 -Ammonia storage as liquid

8 -Pipeline to the ship for export

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a) The manufacture of hydrogen

Hydrogen is produced from a variety of feedstocks, mostly from natural

gas, coal or naphtha. The ways in which hydrogen is obtained from these

feedstocks are dealt with separately.

Hydrogen from natural gas (methane)

This involves two stages:

i) the manufacture of synthesis gas (a mixture of carbon monoxide and

hydrogen (steam reforming))

>ii) the removal of the carbon monoxide and production of a mixture of

hydrogen and nitrogen (the shift reaction)

(i) The manufacture of synthesis gas

Whichever way the methane is obtained, it will contain some organic

sulfur compounds and hydrogen sulfide, both of which must be

removed. Otherwise, they will poison the catalyst needed in the

manufacture of synthesis gas. In the desulfurisation unit, the organic

sulfur compounds are often first converted into hydrogen sulfide, prior to

reaction with zinc oxide. The feedstock is mixed with hydrogen and

passed over a catalyst of mixed oxides of cobalt and molybdenum on an

inert support (a specially treated alumina) at ca 700 K.

Then the gases are passed over zinc oxide at ca 700 K and hydrogen

sulfide is removed:

Primary steam reforming converts methane and steam to synthesis gas,

a mixture of carbon monoxide and hydrogen:

High temperatures and low pressures favour the formation of the

products (Le Chatelier's Principle). In practice, the reactants are

passed over a catalyst of nickel, finely divided on the surface of a

calcium oxide/aluminium oxide support contained in vertical nickel

alloy tubes. The tubes, up to 350 in parallel, are heated in a furnace

above 1000 K and under a pressure of ca 30 atm. This is an example

of a tubular reactor.

Secondary steam reforming reacts oxygen from the air with some of the

hydrogen present and the resulting mixture is passed over a nickel

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catalyst. The steam and heat produced from the combustion reforms

most of the residual methane. Among the key reactions are:

The emerging gas from this net exothermic stage is at ca 1200 K and is

cooled in heat exchangers. The steam formed from the water used in

cooling the gases is used to operate turbines and thus compressors and to

preheat reactants.

Some recent designs use waste heat from the secondary reformer directly

to provide heat for the primary reformer.

At this stage the gas contains hydrogen, nitrogen, carbon monoxide and

carbon dioxide and about 0.25% methane. As air contains 1% argon,

this also accumulates in the synthesis gas.

(ii) The shift reaction

This process converts carbon monoxide to carbon dioxide, while

generating more hydrogen.

It takes place in two stages. In the first, the high temperature shift

reaction, the gas is mixed with steam and passed over an

iron/chromium(III) oxide catalyst at ca700 K in a fixed bed reactor. This

decreases the carbon monoxide concentration from 11%:

In the second stage, the low temperature shift reaction, the mixture of

gases is passed over a copper-zinc catalyst at ca 500 K. The carbon

monoxide concentration is further reduced to 0.2%.

The reaction is done in two stages for several reasons. The reaction is

exothermic. However, at high temperature, the exit concentration of

carbon monoxide is still quite high, due to equilibrium control. The

copper catalyst used in the low temperature stage is very sensitive to high

temperatures, and could not operate effectively in the high temperature

stage. Thus, the bulk of the reaction is carried out at high temperature to

recover most of the heat. The gas is then removed at low temperature,

where the equilibrium is much more favourable, on the very active but

unstable copper catalyst.

The gas mixture now contains about 18% carbon dioxide which is

removed by scrubbing the gas with a solution of a base, using one of

several available methods. One that is favoured is an organic base (in

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the carbon dioxide absorber), a solution of an ethanolamine, often 2,2'-

(methylimino)bis-ethanol (N-methyl diethanolamine).

The carbon dioxide is released on heating the solution in the carbon

dioxide stripper). Much of it is liquefied and sold, for example, for

carbonated drinks, as a coolant for nuclear power stations and for

promoting the growth of plants in greenhouses.

The last traces of oxides of carbon are removed by passing the gases over

a nickel catalyst at 600 K:

This process is known as methanation. A gas is obtained of typical

composition: 74% hydrogen, 25% nitrogen, 1% methane, together with

some argon.

Hydrogen from naphtha

If naphtha is used as the feedstock, an extra reforming stage is

needed. The naphtha is heated to form a vapour, mixed with steam and

passed through tubes, heated at 750 K and packed with a catalyst,

nickel supported on a mixture of aluminium and magnesium

oxides. The main product is methane together with oxides of carbon,

and is then processed by steam reforming, as if it was natural gas,

followed by the shift reaction.

Hydrogen from coal

If coal is used, it is first finely ground and heated in an atmosphere of

oxygen and steam. Some of the coal burns very rapidly in oxygen (in

less than 0.1 s) causing the temperature in the furnace to rise and the

rest of the coal reacts with the steam:

The gas emitted contains ca 55% carbon monoxide, 30% hydrogen,

10% carbon dioxide and small amounts of methane and other

hydrocarbons. This mixture is treated by the shift reaction.

The main problems of using coal includes the large amounts of sulfur

dioxide and trioxide generated in burning coal and the significant

amounts of other impurities such as arsenic and bromine, all of which

are very harmful to the atmosphere and all of which are severe poisons

to the catalysts in the process. There is also a massive problem with

disposal of the ash.

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(b) The manufacture of ammonia (The Haber Process)

The heart of the process is the reaction between hydrogen and nitrogen

in a fixed bed reactor. The gases, in stoichiometric proportions, are

heated and passed under pressure over a catalyst (Figure 3).

Figure 3 A diagram illustrating a conventional synthesis reactor (a

converter).

The proportion of ammonia in the equilibrium mixture increases with

increasing pressure and with falling temperature (Le Chatelier's

Principle). Quantitative data are given in Table 1. To obtain a

reasonable yield and favourable rate, high pressures, moderate

temperatures and a catalyst are used.

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Pressure/atm Percentage ammonia present at equilibrium at a range of

temperatures

373 K 473 K 573 K 673 K 773 K 973 K

10 - 50.7 14.7 3.9 1.2 0.2

25 91.7 63.6 27.4 8.7 2.9 -

50 94.5 74.0 39.5 15.3 5.6 1.1

100 96.7 81.7 52.5 25.2 10.6 2.2

200 98.4 89.0 66.7 38.8 18.3 -

400 99.4 94.6 79.7 55.4 31.9 -

1000 - 98.3 92.6 79.8 57.5 12.9

Table 1 Percentage, by volume, of ammonia in the equilibrium

mixture for the reaction

between nitrogen and hydrogen at a range of temperatures and

pressures.

A wide range of conditions are used, depending on the construction of

the reactor. Temperatures used vary between 600 and 700 K, and

pressures between 100 and 200 atmospheres. Much work is being

done to improve the effectiveness of the catalyst so that pressures as

low as 50 atmospheres can be used.

As the reaction is exothermic, cool reactants (nitrogen and hydrogen)

are added to reduce the temperature of the reactors (Figure 3).

The ammonia is usually stored on site (step 7) and pumped to another

part of the plant where it is converted into a fertilizer (urea or an

ammonium salt). However it is sometimes transported by sea (Figure

4) or by road, to be used in another plant.

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Figure 4 In a plant in Western Australia, the ammonia is transferred

by pipeline to a nearby harbour (Figure 2, step 8) and transported by

ship. This one is carrying about 40 000 tonnes of liquefied ammonia.

The original catalyst that Haber used was Fe3O4, which was reduced

by the reactant, hydrogen, to iron. Much work was done to improve the

catalyst and it was found that a small amount of potassium hydroxide

was effective as a promoter. Recently research has been focussed on finding even more effective

catalysts to enable the process to take place at lower pressures and

temperatures. Ruthenium on a graphite surface is a promising one.

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Postscript:

The Haber Process is of such importance to our lives that it has figured

in three Nobel Prizes in chemistry, all to German scientists, over a period

of nearly 90 years, a remarkable record.

The first was given in 1918, to Fritz Haber, the chemist who developed

the process in the laboratory. The second was to Carl Bosch, whose

brilliant engineering skills made the process viable on a massive scale,

but who waited until 1931 for his award.

In 2007, Gerhard Ertl was awarded the Prize for his work on catalysis of

gaseous reactions on solids. Among the wide range of reactions he

studied, he gained evidence for the adsorption of nitrogen molecules and

hydrogen molecules on the surface iron and that these adsorbed

molecules dissociate into atoms. These atoms then join up in stages to

form the ammonia molecule. It must be remembered that the conditions

used in these studies (at less than 10-10 atm) are very different from the

conditions used in industry, ca 150 atm.

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Waste Management:

• The Haber process maximises the conversion of nitrogen and

hydrogen into ammonia by recycling unreacted gases back into the

converter for further passes over the catalyst.

• In this way, almost complete conversion is achieved.

• Because the hydrogen is recycled, the amount of hydrogen

feedstock required from the hydrogen generation process is

reduced.

• Consequently the amount of raw materials, energy and waste

materials involved in the production of the hydrogen used in the

Haber process is reduced.

• In some section of the production process, aqueous solutions of

ammonia are produced when gases being released to the

atmosphere are purified by passing them through water.

• These solutions can also be used in the manufacture of urea.

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Uses of ammonia:

The manufacture of fertilizers is by far the most important use of

ammonia. These include urea, ammonium salts (ammonium phosphates,

ammonium nitrate, calcium ammonium nitrate) and solutions of

ammonia.

Figure 1 The uses of ammonia.

An increasing amount of ammonia, although still small compared with

other uses, is used as a concentrated solution in combating the discharge

of nitrogen oxides from power stations.

Annual production of ammonia

Ammonia ranks second, to sulfuric acid, as the chemical with the largest

tonnage. It is being increasingly made in countries which have low cost

sources of natural gas and coal (China and Russia account

for ca 40%). The largest plants produce about 3000 tonnes a day and

there are plans to build plants that produce 4000-5000 tonnes a day,

which would mean that the total world output could be managed with 100

such units. Current production is:

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World 140 million tonnes

Europe 16 million tonnes

North America 15 million tonnes

US 8 million tonnes

Asia 74 million tonnes

Russia 12.5 million tonnes1

Middle East 13 million tonnes

Data from:

Federal State Statistics Service: Russian Federation 2011

The world population is increasing by about 1.4% a year and the

increase in tonnage of ammonia made just about keeps pace.

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Fertilisers and much more:

China 51 352 000 tonnes

(about ⅓ of the world’s production)

India 13 600 000 tonnes

Russia 12 678 000 tonnes

USA 9 355 000 tonnes

Global production of ammonia

The annual production of ammonia remained

fairly constant over the past six years or so.

Although manufactured in countries around the

world, in 2009 the four biggest producers and the

quantities they produced are:

China 51 352 000 tonnes

(about ⅓ of the world’s production)

India 13 600 000 tonnes

Russia 12 678 000 tonnes

USA 9 355 000 tonnes

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References:

1-

http://www.essentialchemicalindustry.org/chemicals/ammonia.html

2-

http://www.essentialchemicalindustry.org/chemicals/ammonia.html

3-

http://www.thechemicalblog.co.uk/the-history-of-ammonia/

4-

http://www.britannica.com/EBchecked/topic/20940/ammonia-NH3

5-

http://www.chm.bris.ac.uk/~paulmay/haber/haber.htm

6-

https://chemengineering.wikispaces.com/Ammonia+production

7-

http://en.wikipedia.org/wiki/Ammonia_production