Manufacture of NH3 and its chemicals properties

1. Manufacture of ammonia by Haber’s process
Principle:
Under ordinary conditions, nitrogen and hydrogen does not react. When a mixture of nitrogen and hydrogen gas in
a ratio of 1:3 by volume is heated at the temperature of 3800C to 4500C with the atmospheric pressure of 200 to 900
atm in the presence of iron as catalyst and molybdenum as a promotor, ammonia gas is produced. The reaction is :
N2 (g) + 3 H2 (g) 2NH3 (g) + 22.08 Kcal
Condition for maximum yield of ammonia gas:
The formation of ammonia by Haber’s process is reversible. Therefore, the process of formation of ammonia is
monitored by Le-Chatelier’s principle. The optimum yield of ammonia is favored by following conditions:
1. Temperature:
Formation of ammonia is exothermic process and as per rule it is expected to be favored by low
temperature. Although low temperature give high percentage conversion of ammonia the reaction is very slow.
Hence, a relatively higher temperature of 3800C to 4500C is favored for the maximum yield of ammonia.
3800C - 4500C
200-900 atm
Fe/Al2O3

2. 2. Pressure:
In the preparation of ammonia, Four volume of reactants gives two volume of products, i.e the
volume is reduced to half. Equilibrium shifts towards right at relatively high pressure. Therefore,
the pressure of 200- 500 atm is favored.
3. Use of catalyst:
Use of catalyst makes the reaction faster in forward direction. Hence, the catalyst like iron (Fe)
& promoter like aluminum oxide or platinum oxide are used for maximum yield of the product.
4. Concentration:
More the concentration of reactants, more is the production. So, high concentration of nitrogen
and hydrogen should be used.
5. Purity of gases:
Hydrogen and nitrogen gases used in the reaction should be pure for higher yield of product..
Presence of impurities like carbon monoxide, sulphur results in catalytic poisoining.

4. Raw materials:
The raw materials( Hydrogen and Nitrogen) are produced from hydrolysis of
water and fractional distillation of liquid air.
Working process of the plant:
A mixture of pure Nitrogen and hydrogen (Obtained from any of the above
process) at the volume ratio of 1:3 is passed through compressor, where the gases
are compressed to about 200 atm. The compressed gas mixture is then passed to
the catalyst chamber ( ammonia converter). The chamber contains heat exchanges
at the upper portion and catalyst at the central portion. Gases circulate through
preheated catalyst producing ammonia.
Ammonia and unreacted gases are now passed through condenser where
ammonia gas condensed to give liquid ammonia. Unreacted mixture of Hydrogen
and Nitrogen are recycled back by recycling pump.

5. Manufacture of Sulphuric acid (contact process):
principle Sulphur dioxide is first produced either by burning Sulphur
or by roasting iron pyrites.
S + O2 SO2
4FeS2 + 11O2 2Fe2O3 + 8SO2
The Sulphur dioxide is then catalytically oxidized to Sulphur trioxide; in the
presence of divanadium pentaoxide as catalyst at 450℃ and 2atm pressure.
2SO2 + O2 V2O5 2SO3 + heat
Thus formed sulphur trioxide is absorbed in conc. Sulphuric acid to form
pyrosulphuric acid (oleum)
SO3+ H2SO4 H2S2O7
Thus formed oleum is diluted in water to obtained sulphuric acid
H2S2O7 + H2O 2H2SO4

6. Conditions for higher yield of H2SO4:
Formations of Sulphur trioxide is main step. It is exothermic and reversible
reaction and occurs with diminution of volume . Hence according to Le-
Chatelier’s principles, it is favored by
i. Pressure : a pressure of only about 2atm pressure is applied. Very high pressure
is not applied in view of the high acidic nature of the gases and their effects.
ii. Temperature: since the reaction is exothermic, the lower the temperature, the
greater will the yield. But greater yield is obtained at low temperature only
after a long time. So the optimum temperature of about 450℃.
iii.High concentration: oxygen is used in excess to obtained greater conversion of
SO2 to SO3.
iv.Use of catalyst : the rate of reaction is enhanced by using platinum asbestos or
divanadium penta-oxide as catalyst.

8. Working principle
i. Pyrites burners: this is the place where Sulphur dioxide is produced
either by burning Sulphur or iron pyrites.
ii. Electrostatic precipitators: the gases are first passed through the
chamber fitted with metallic conductors between which very high
potential difference is maintained to precipitate dust particles.
iii.Scrubber: here the gases are washed by down flowing shower of
steam where by some soluble impurities and dust particles.
iv.Drying tower: it is a tower packed with stone or coke, down which
conc.H2SO4 is sprayed. Here gas made free from moisture.
v. Arsenic purifier: this chamber contains gelatinous ferric hydroxide
kept is shelves. Arsenious oxide is absorbed by it.

9. vi. Testing box : here the presence of dust particles is detected by
tyndal effect
vii.Preheater: the purified gases are mixed with oxygen, and passed
through a preheater to raise temperature to about 450℃.
viii.Contact converter: this is the most important chamber. It
consists metallic chamber fitted with vertical iron pipes packed
with a acid resistance catalyst. Here SO2 catalytically oxidized to
SO3.
ix.Absorption tower : it is also cylindrical steel tower lined with lead
and packed with acid resistance stones. Conc. H2SO4 is sprayed
down from top of this tower. The SO3 gas is absorbed by down
coming conc H2SO4 to form oleum.

10. Manufacture of sodium hydroxide by diaphragm cell:
Sodium hydroxide is an important alkali used industries as well as in
laboratories. It is commercially produced in large scale by electrolysis
of aqueous sodium chloride. Two types of electrolytic cells are in use
for the manufacture of sodium hydroxide. These are i. mercury cathode
cell and ii. Diaphragm cell. Here we discuss only the manufacture of
sodium hydroxide by diaphragm cell.
Diaphragm is an electrolytic cell composed of titanium oxide as anode,
steel mesh enclosed in an Teflon diaphragm as cathode and brine
solution (NaCl solution) as electrolyte. Brine solution is introduced
into anode compartment which later on passes into the cathode
compartment through the diaphragm.

11. Anode
compartment
The solution ionizes as follows;
NaCl Na+ + Cl-
H2O H+ + OH-
When electricity is passed into
the electrolyte solution,
following reactions takes place
in the corresponding electrodes
Reaction at anode: graphite can be used as anode material. At anode, Cl- and OH-
anions go to the anode and compete to get discharged. Diaphragm prevents the
entering of hydroxide ions into anode compartments and Cl- ions preferentially
get discharged forming chlorine gas. The chlorine gas is liberated. The OH – ions
remain intact.
2Cl- Cl2

12. Reaction at cathode: steel mesh or nickel can be used as cathode. The ion-
exchange membrane is used to selectively permit the flow of ions to the cathode
compartment. Both H+ and Na+ ions go to cathode and compete to get
discharged. Out of these two ions, the H+ ions has lower discharge potential and
gets preference to discharged forming hydrogen gas. The Na+ ions remain intact
in solution.
2H+ +2e- H2
once, the electrolysis is over, solution in the cathode compartment contains
excess of Na+ ions and OH- along with some sodium chloride solution.
Recovery of sodium hydroxide: during electrolysis process, the solutions from
the cathode compartment is periodically removed and concentration where by
sodium chloride gets crystallized leaving behind the concentrated solution of
sodium hydroxide. Evaporation of concentrated sodium hydroxide solution up to
the dryness gives solid sodium hydroxide, as per requirement, the solid sodium
hydroxide is made granules or pellet for commercial supply.

13. Manufacture of sodium carbonate(Na2CO3) by Solvay process or
ammonia-soda process:
Sodium carbonate is manufactured by Solvay process or ammonia-soda process.
The first successful commercial plant was designed by Ernest Solvay, a Belgian
chemical engineer as shown in the following figure.
Principle: Brine solution saturated with ammonia gas reacts with carbon dioxide to
form sodium bicarbonate.
NH3 + H2O + CO2 NH4HCO3
NaCl + NH4HCO3 NaHCO3 + NH4Cl
Sodium bicarbonate is only sparingly soluble in water in presence of unreacted
NaCl. The filtered product on heating gives Na2CO3.
2NaHCO3
150℃ Na2CO3 + CO2 + H2O
Sodium carbonate solution is crystallized in water to form washing soda.
Na2CO3(aq.) crystallization Na2CO3.10H2O

15. Working principle
i. Saturation of brine: In ammonia absorber, current principle. Here,
ammoniated brine is allowed to trickle down a tower and carbon dioxide is
passed from the bottom at a pressure about 2 atm, brine solution is saturated
with ammonia gas. Impurities like CaCl2 MgCl2 etc. ammoniated brine is then
pumped to the carbonation tower.
ii. Carbonation: Carbonation tower works on counter current principle. Here,
ammoniated brine is allowed to trickle down a tower CO2 is passed from
bottom at a, pressure and the temperature is maintained to about 30℃. Here,
sodium bicarbonate and ammonium chloride are formed. Sodium bicarbonate is
filtered and filtrate is pumped to the ammonia generator.
iii.Recovery of ammonia: In ammonia generator, ammonium chloride reacts with
slaked lime to generate ammonia gas.
iv.Generation of CO2 : in lime kiln, lime stone is heated to produce CO2 required
for carbonation.

16. v. Calcination: Sodium bicarbonate obtained by filtration is heated to get
anhydrous sodium carbonate.
vi. Crystallization: Aqueous sodium carbonate solution is subjected to
crystallization to get washing soda.

17. Fertilizer:
▪ Different types of components like air, water, carbon dioxide, carbon, hydrogen
are essential for plants.
▪ Some macronutrients like N, K, Ca, Mg, P and S and micronutrients like Co,
Ni, Na, and Si are obtained from growth media or soil.
▪ The elements or compounds which are essential to growth and to boost up the
cultivation are called fertilizers.
Classification
of fertilizers
i. Organic fertilizer or natural
fertilizer: the fertilizers which
are directly obtained by decayed
materials of plants and animals
are called natural and organic
fertilizers. E.g. cow dung, animal
bone powder
ii. Chemical or artificial fertilizer:
the fertilizers which are synthesized
artificially ensuring some specific
elements there in and soluble in water
also assimilate into the soil are called
chemical fertilizers. E.g. Urea,
phosphate and potash etc

18. Nitrogen fertilizers
Those chemical fertilizer which provide nitrogen in the soil as supplement element
are called nitrogen fertilizers. Usually nitrogen is provided in the form of nitrogen
rich compounds such as urea, ammonium sulphate, calcium ammonium nitrate,
sodium nitrate.
Phosphorous fertilizers:
Phosphorous is supplied from this type of fertilizer value here is measured by
available P2O5 for e.g. super phosphate, triple super- phosphate, Thomas slag, nitro
phosphate.
Potash fertilizer:
Potash fertilizers are important for tobaccos, potatoes , cottons, coffees. The
fertilizer value is measured as available K2O. For e.g. potassium chloride,
potassium nitrate, potassium sulphate.

19. Mixed (NPK) fertilizers
They are compound or complex fertilizers the mixture of nitrogen, phosphorus
and potash fertilizers. Their compositions are variable.
Production of urea:
Liquid ammonia and liquid carbon dioxide produced either by
industrial process or side product of a urea plant are used as raw
materials. Generally NH3 is produced by Haber process and CO2 is
produced by decomposition of limestone. Liquid ammonia is allowed
to react with liquid carbon dioxide in reactor at high temperature (180-
210℃) and high pressure (150 atm) whereby ammonia carbamate is
formed which upon decomposition from urea solution is evaporated up
to dryness to make granules of urea. The chemical reaction takes place
as follows.

20. a. Formation of ammonium carbonate:
Ammonia and carbon dioxide react under high temperature (180-210℃) and at
high pressure 150 atm to form ammonium cyanate. This reaction is exothermic
and fast. The reaction almost goes to completion.
NH3 + CO2 NH2COONH4 + Heat
b. Decomposition of ammonium carbamate: Ammonium carbamate is
decomposed to urea.
NH2COONH4 + heat NH2CONH2 + H2O.
The reaction is endothermic and slow. The reaction doesn’t go to the completion.
In the manufacture of urea, there may be unreacted ammonia, carbon dioxide, and
ammonium carbamate. Unreacted ammonia and carbon dioxide are recirculated.
Ammonium carbamate is removed by reducing the pressure.

21. Fertilizers as pollutants:
i. The biggest issue faced due to the use of fertilizer is water pollution.
ii. It affects the aquatic flora and fauna and aquatic animals.
iii. It affects drinking water which cause to decrease oxygen carrying capacity of
hemoglobin.
Liquid NH3
Liquid CO2
Reaction
chamber(1
80-210℃,150
atm)
evaporator
Packing
and storage
compressor
condensate
recirculation
Unreacted CO2 + NH3 + H2O
Fig: flow-sheet diagram of manufacture of urea