Cracking is defined as a process, wherein complex organic molecules namely long chain hydrocarbons or kerogens are broken down into smaller molecules namely light hydrocarbons. It is caused by the breaking of carbon-carbon bonds. The rate of catalyst greatly depends upon the factors such as the presence of catalyst and temperature. Cracking of hydrocarbons is illustrated in the below diagram.
In petroleum refining, this process is used in the production of petrol, diesel, and gasoline. In chemistry, it is a process of breaking down large alkane into simpler alkenes and alkanes. Similarly cracking of hydrocarbons includes breaking down of a complex long chain of hydrocarbons into smaller ones.
During this process, it involves numerous chemical reactions based on free radicals. Some of the vital reactions that take place are stated below.
Initiation: Here a single molecule breaks down into 2 free radicals. Only a smaller portion of freed radicals undergoes initiation, but it is sufficient to produce free radicals that are necessary to carry forward the entire reaction.
CH3CH3 → 2 CH3
Abstraction of Hydrogen: Here the second molecule becomes a free radical as it removes a hydrogen atom from another molecule.
CH3• + CH3CH3 → CH4 + CH3CH2•
Radical Decomposition: Here free radicals break into other free radical and an alkane. This reaction gives rise to alkene products.
CH3CH2• → CH2=CH2 + H
Radical Addition: This reaction results in the formation of aromatic products. Here radical reacts with an alkene to produce a free radical.
CH3CH2• + CH2=CH2 → CH3CH2CH2CH2•
Termination: Here 2 radicals react with each other to form products that are not free radicals. This reaction results in two forms namely recombination and disproportionation.
CH3• + CH3CH2• → CH3CH2CH3
CH3CH2• + CH3CH2• → CH2=CH2 + CH3CH3
Types of Cracking
- FCC – Fluid Catalytic Cracking: It is mainly used in petroleum refiners. This process involves the conversion of high molecular weight, high boiling hydrocarbons into olefinic, gases, gasoline and other products.
- Hydrocracking: It is a catalytic cracking process, where it uses hydrocracking to break C – C bonds. Products produced by this process include diesel, jet fuel, and LPG.
- Steam Cracking: It is a petrochemical process that involves the breakdown of saturated hydrocarbons into smaller unsaturated hydrocarbons.
Thermal Cracking: It is a process that involves breaking of large non-volatile hydrocarbons into gasoline.