What is Thermal Cracking?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 26 September 2015
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Thermal cracking is a process in which hydrocarbons such as crude oil are subjected to high heat and temperature to break the molecular bonds and reduce the molecular weight of the substance being cracked. This process is used to extract usable components, known as fractions, which are released during the cracking process. It is one among several cracking methods used in the petroleum industry to process crude oil and other petroleum products for commercial use.

In the thermal cracking process, the compound to be cracked is subjected to high heat and pressure. Sometimes a catalyst is added to control the chemical reactions which occur during the process, with the goal of promoting the development of specific molecules. Fractions which have a low boiling point, such as gasoline, will be released first. As the cracking proceeds, fractions of various molecular weights can be extracted and processed further for additional uses, or packaged for transport and sale.

This process creates free radicals at the sites where molecular bonds are broken, which can be harnessed in chemical reactions such as polymerization to create new chemical compounds. A wide variety of compounds are extracted or derived through these types of processes, making it a valuable part of petroleum refining. The process can be supervised by petroleum engineers or chemists who are familiar with the needs of the market, the product being worked with, and the cracking process.


Sometimes known as pyrolysis because it involves the controlled decomposition of a chemical compound under heat and pressure, thermal cracking is designed to create more useful fractions. The cracking can be adjusted to meet needs like a rise in demand for a particular product, or a shortage of a product caused by changes in refinery capacity. Thermal cracking of petroleum is also often discussed in chemistry classes while introducing students to basic chemical concepts which come up in the refinery industry.

The term thermal cracking is also used in reference to concrete, asphalt, and similar materials. In this case, low temperatures make the material prone to cracking. Things like asphalt are elastic at temperate and high temperatures, deforming as they are stressed, but when it is cold, the material may crack or fracture. Thermal cracking in concrete and similar materials is a cause for concern in very cold environments, especially when the material is being used for structures such as bridges and buildings, where failure could cause a catastrophe.


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Post 3

I'm doing a project on cracking and I was wondering if someone could tell me the pros and cons of both catalytic and thermal cracking. I tried searching online but it didn't help much. Any help is greatly appreciated.

Post 2

Cracking oil using catalysts not only promotes development of specific molecules, it also reduces the amount of pressure required. The catalysts are usually solids like aluminum hydrosilicate, certain clays, or bauxite, in the form of beads or powders. It does sound a bit odd to discuss "cracking" a liquid but it's really cracking the molecular bonds.

Post 1

So thermal cracking is useful in a variety of ways and dangerous in others. We've probably all seen the bad effects of concrete cracking and worry about it on bridges.

But thermal cracking as part of the oil refining process is how we get our gasoline! More than 50% of the crude oil is converted to gas and one way of doing that is by cracking. It breaks down the molecules of the heavy oil.

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