FSc Notes Part 2 Chemistry Important Chemicals Sulphuric Acid (Contact Process), Nitric Acid (Ostwald’s Process) and Caustic Soda Or NaOH (Castner Kellner’s Process)
Sulphuric Acid (Contact Process)
Introduction
On industrial scale two methods are employed for the manufacture of sulphuric acid, the Lead Chamber Process and the Contact Process.
The Contact Process
When a mixture of pure and dry sulphur dioxide and air is passed over a heated catalyst such as Platinum Asbestos, then Sulphur dioxide is oxidized into sulphur trioxide. This gas gives sulphuric acid when treated with water.
The plant used in Contact in Contact Process is composed of following parts.
1. Sulphur and Pyrite Burner
In this part, SO2 is produced by either burning sulphur in air or roasting ores. Like ZnS, PbS, FeS2 in air.
S + O2 —-> SO2
4FeS2 + 11O2 —-> 2Fe2O3 + 8S2
2. Dust Chamber
The SO2 obtained from pyrite burner contains impurities such as unreacted coal, dust, moisture, arsenic compounds etc. These impurities are removed from the gas in dust chamber. In this chamber hot air is blown as a result the heavy particles are removed and unreacted sulphur convert into SO2. Now the gases are passed through a cooling pipe into water chamber.
3. Water Chamber
In this chamber a stream of water is showed on the gas to remove the dust particles. The SO2 gas is now proceed into another chamber, which is known as drying tower.
4. Drying Tower
In drying tower moist SO2 gas is mixed with concentrated H2SO4. As a result the gas become dry and dilute H2SO4 is sink at the bottom.
5. Oxidation Tower
Before oxidation the dry gas is allowed to pass through arsenic purifier containing ferric hydroxide here the arsenic compounds are absorbed. The purified gases are then passed through a strong beam of light to remove any remained dust particle.
The purified and dry SO2 gas is passed through the oxidation tower. This tower consist of iron pipes packed with plantanized asbestos coated with Vanadium Penta Oxide (V2O5), which act as catalyst. In this tower the temperature is kept constant at about 480ºC, under high pressure and high concentration of sulphur dioxide and oxygen, oxidation of SO2 takes place in a favourable condition to yield maximum amount of SO3.
2SO2 + O2 <—-> 2SO3 (45000 cal)
6. Absorption Tower
Sulphur trioxide formed in Oxidation Tower is passed up to Absorption tower, where it meets a stream of concentrated sulphuric acid and hence absorbed to form OLEUM. The maximum absorption of sulphur trioxide is 60% at which oleum has maximum density.
SO3 + H2SO4 —-> H2S2O7
Oleum can be diluted to obtain any desired concentration of sulphuric acid.
H2S2O7 + H2O —-> 2H2SO4
Sulphuric acid is obtained from this process is nearly 99.99% pure
Nitric Acid (Ostwald’s Process)
Introduction
On industrial process three methods are employed for the manufacture of nitric acid. Ostwald’s Process uses Ammonia as raw material.
Ostwald’s Process
When a mixture of ammonia and air is passed through a catalytic chamber containing electrically heated Platinum Gauze, (NO) nitric oxide is formed. This nitric oxide combines with more oxygen to form nitrogen dioxide (NO2). This gas is then absorbed by water to form nitric acid. The plant used in Ostwald’s Process consist of following parts.
1. Catalytic Chamber
A mixture of pure and dry ammonia is passed through a catalytic chamber containing heated (800ºC) platinum, which serves as catalyst. Here ammonia is completely oxidized to nitric oxide.
2. Oxidation Tower
The nitric oxide is passed into oxidation tower where the temperature of gases falls to 200ºC – 250ºC. At this temperature nitric is oxidized to nitrogen dioxide.
2NO + O2 —-> 2NO2
3. Absorption Tower
Now the gases containing NO2 and oxygen are than sent to a series of absorption tower, which is filled with broken quartz and water is showered from top of the tower. Then NO2 reacts with H2O in the presence of oxygen to form HNO3.
4NO2 + 2H2O + O2 —-> 4HNO3
Caustic Soda Or NaOH (Castner Kellner’s Process)
Introduction
Caustic Soda or Sodium Hydroxide is one of the most important chemicals of industrial use. Caustic Soda must be handled carefully because it is caustic to touch and causes painful burns.
Castner Kellner’s Process
Caustic Soda is manufacture by an electrolytic process. This process is carried out in an electrolytic cell known as Castner-Kellner’s cell and the process is called Castner-Kellner’s Process.
Construction Of Castner-Kellner’s Cell
The electrolytic solution is a 25% of NaCl solution. The anode consist of a number of titanium plates whereas cathode is a steam of flowing mercury.
Working of Castner-Kellner’s Cell
Sodium chloride dissociates in water to give Na+ and Cl- ions are
2NaCl —-> 2Na+ + 2Cl-
The Cl- ions migrate towards titanium plates, it gains electron and convert into chlorine gas.
2Cl- —-> Cl2 + 2e- (Oxidation)
In castner-kellner’s process, H+ ions are not easily discharged due to high voltage of H+ ions, on the contrary Na+ ions are easily discharge over mercury surface. The sodium, thus liberated dissolves in mercury forming an amalgam.
2Na+ + 2e- —-> 2Na (Reduction)
Na + Hg —-> Na/Hg (Amalgum)’
The mercury containing dissolved sodium is sent to another chamber called Denuder where sodium reacts with water forming sodium hydroxide and hydrogen. Denuder is packed with graphite blocks as hydrogen is easily liberated over graphite surface.
2Na/Hg + 2H2O —-> 2NaOH + H2 + 2Hg
The mercury is recycled to dissolve more of sodium.
The solution, which flows out from denuder, is a NaOH solution, which is evaporated to dryness.
Post a Comment