What is pyrolysis or cracking? Explain with an example.
What is pyrolysis or cracking? Explain with an example.
Pyrolysis is a thermal decomposition of materials at elevated temperatures in an inert atmosphere such as a vacuum or nitrogen gas. It involves the change of chemical composition and is irreversible. The word is coined from the Greek-derived elements pyro "fire" and lysis "separating".
Cracking: Alkanes, when subjected to high temperature (670-970 K.) in the presence of a catalyst, are decomposed into smaller molecules. This process of breaking down the less volatile higher hydrocarbons into different types of more volatile lower hydrocarbons by the application of heat is called cracking or pyrolysis. The cracking of alkanes involves the cleavage of C-C and C-H bonds.
For example, C4H10,C6H14,C8H18
C10H22Δ−−−→497oCC8H18+C2H4
Pyrolysis of alkanes is supposed to occur by the free radical mechanism. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, heating to 973K in the presence of Pt, Pd or Ni gives a mixture of heptane and pentene.
Cracking is of two types:
(i) Thermal cracking: It may be carried out in the vapour phase or in the liquid phase. It is difficult to control and gives rise to complex product mixtures.
(ii) Catalytic cracking. It is carried out at a temperature of 670- 820 K using silica and alumina as a catalyst.
Catalytic cracking is useful as it gives gasoline having higher octane number. The products formed during cracking depends upon:
(i) the structure of starting alkane
(ii) the pressure applied
(iii) use of a catalyst
Pyrolysis is a thermal decomposition of materials at elevated temperatures in an inert atmosphere such as a vacuum or nitrogen gas. It involves the change of chemical composition and is irreversible. The word is coined from the Greek-derived elements pyro "fire" and lysis "separating".
Cracking: Alkanes, when subjected to high temperature (670-970 K.) in the presence of a catalyst, are decomposed into smaller molecules. This process of breaking down the less volatile higher hydrocarbons into different types of more volatile lower hydrocarbons by the application of heat is called cracking or pyrolysis. The cracking of alkanes involves the cleavage of C-C and C-H bonds.
For example, C4H10,C6H14,C8H18
C10H22Δ−−−→497oCC8H18+C2H4
Pyrolysis of alkanes is supposed to occur by the free radical mechanism. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, heating to 973K in the presence of Pt, Pd or Ni gives a mixture of heptane and pentene.
Cracking is of two types:
(i) Thermal cracking: It may be carried out in the vapour phase or in the liquid phase. It is difficult to control and gives rise to complex product mixtures.
(ii) Catalytic cracking. It is carried out at a temperature of 670- 820 K using silica and alumina as a catalyst.
Catalytic cracking is useful as it gives gasoline having higher octane number. The products formed during cracking depends upon:
(i) the structure of starting alkane
(ii) the pressure applied
(iii) use of a catalyst
