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Question
What is cracking? Explain in detail – fixed bed catalytic cracking.
Solution
1. Cracking is a process of converting heavy oil with higher molecular weight hydrocarbons to the oil with lower molecular weight hydrocarbons which is known as gasoline.
2. Generally on cracking a mixture of hydrocarbons is obtained which is allowed to undergo fractional condensation to separate gasoline.
Thermal Cracking:
a. Liquid phase thermal cracking: By this any type of oil can cracked. In this method the oil is pumped into the coil kept at 4200C to 5500C under pressure of 15 to 100 kg/cm2 .
b. Vapour phase thermal cracking: in this method the heavy is treated at 4000c to convert it into the vapour and then these vapours are passed to the reaction chamber.
Catalytic cracking:
a. It is a process in which heavy is oil is heated in presence of catalyst. Generally used catalyst are crystalline substances.
Eg: bauxite, zeolite, crystalline alumina silicate, etc.
Fixed bed catalytic cracking:
1. In this method vapours of heavy oil is treated in the presence of catalyst due which a better yield of petrol is obtained.
2. In this method heavy oil is vapourised by heating in an electrical heater. Then the vapours are passed over a series of trays containing catalyst. Generally the catalyst used are crystalline alumino-silicate, bauxite and zeolite.
3. The reaction chamber is maintained at 4250c to 4500c and under pressure of 1.5 kg/cm2.
4. The cracked gases are taken out from the top of the reaction chamber and are allowed to pass in the fractionating tower, where gasoline fraction is collected. The octane value of this gasoline is about 80-85.
5. During the cracking, free carbon is also formed which deposits in catalyst, then the flow of vapours of heavy oil is passed over the second set of reaction chamber and catalyst in earlier chamber in regenerated by burning the carbon deposits with help of air.
