Cracking: Definition, Types, and Process

Cracking is a process used in oil refineries to break down large and complex hydrocarbon molecules into lighter and smaller hydrocarbons that are more suitable for use in industry or by consumers.

• For the commercial production of petroleum and other petroleum products, it is the most essential step.
• Generally, cracking occurs either in the presence of catalysts or at high pressures and temperatures.
• Using catalysts can minimize the need for high temperatures, which results in an energy-saving measure.
• Cracking in chemistry is of two types: Thermal cracking and Catalytic Cracking.

Key Terms: Cracking, Thermal cracking, Catalytic Cracking, Hydrocarbons, Crude oil, Homologous series, Catalyst, Alkanes, Temperature, Pressure

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What is Cracking?

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The process of breaking down complex organic molecules, such as long-chain hydrocarbons or kerogens, into smaller molecules, such as light hydrocarbons, is known as cracking.

• The breakdown of carbon-carbon bonds is the cause of cracking.
• Temperature and catalyst presence are the two main factors that affect the rate of catalysts.
• This process is used in the refining of petroleum to produce diesel and gasoline.
• In chemistry, it is the process of breaking down large alkanes into smaller alkenes and alkanes.
• Similarly, hydrocarbon cracking involves breaking a long, complex chain of hydrocarbons into smaller ones.

The figure below shows the process of cracking of hydrocarbons.

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Process of Cracking in Organic Chemistry

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The chemical process of cracking involves various free radical reactions. These cracking reactions are mentioned below:

Initiation

Free radicals are important for the process of cracking. One molecule is broken down into 2 free radicals, in which the smaller portion undergoes initiation.

CH₃CH₃ → 2CH₃

Hydrogen Abstraction

Hydrogen is removed from the second molecule, hence it becomes a free radical as shown.

CH₃• + CH₃CH₃ → CH₄ + CH₃CH₂•

Radical Decomposition

Free radicals break into other free radicals and alkanes to produce alkene products.

CH₃CH₂• → CH₂=CH₂ + H

Radical Addition

Free radicals react with alkene and produce free radicals which form aromatic products.

CH₃CH₂• + CH₂=CH₂ → CH₃CH₂CH₂CH₂•

Termination

The termination process happens when two radicals react with each other to form a non-free radical product resulting in recombination and disproportionation.

CH₃ + CH₃CH₂ → CH₃CH₂CH₃

CH₃CH₂ + CH₃CH₂ → CH₂=CH₂ + CH₃CH₃

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Types of Cracking

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Cracking in chemistry is of two types:

• Thermal Cracking
• Catalytic Cracking.

Thermal Cracking

Thermal cracking involves the usage of pressure and heat to break down long-chained hydrocarbon molecules into smaller hydrocarbons.

It is further divided into

• Steam cracking
• Modern thermal cracking.

Steam Cracking

Steam Cracking is also known as Pyrolysis. It is important for the petrochemical industry to produce olefins from raw materials like propane-butane mixture, naphtha, and gas oil.

Modern Thermal Cracking

Modern thermal cracking uses high pressure and high temperature.

• The carbon bonds in the hydrocarbons go through homolytic fission.
• It is important to produce burner fuels.
• At low temperatures, coking takes place which produces needle coke used for carbon electrodes. 

Catalytic Cracking

Catalytic cracking means cracking in the presence of a catalyst. Gasoline is maximized in this process. There are three sub-processes for this method- Reaction, Regeneration, and Fractionation.

It is further divided into two categories:

• Hydrocracking
• Fluid Catalytic Cracking.

Hydrocracking

Hydrocracking is a combination process of hydrogenation and catalytic cracking.

• Heavier feedstocks are cracked in the presence of hydrogen to produce the desired products.
• It is done under high pressure, high temperature, hydrogen, and catalyst. 

The process is also highly dependent on the relative rates of the two reactions as well as the feedstock's type.

Fluid Catalytic Cracking

This process is critical for the conversion of long-chained hydrocarbons of petroleum into valuable smaller molecules like gasoline, olefinic gases, etc. The feedstock is contacted with powdered and hot catalysts with moderate pressure and high temperature.

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Things to Remember

• Cracking is defined as the process of breaking down long-chain hydrocarbons into simpler forms or lightweight hydrocarbons.
• Cracking in chemistry is of two types: Thermal cracking and Catalytic Cracking. 
• The chemical process of cracking involves various free radical reactions.
• These cracking reactions are Initiation, Hydrogen Abstraction, Radical Decomposition, Radical Addition and Termination.
• Thermal cracking involves the usage of pressure and heat to break down long-chained hydrocarbon molecules into smaller hydrocarbons.
• It is further divided into steam cracking and modern thermal cracking.
• Catalytic cracking means cracking in the presence of a catalyst.

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