The process of thermal cracking has been around for quite some time. Being discovered back in the late 1800’s, it is a very effective way of breaking down longer straight chain paraffin into shorter chian ones through the use of high temperatures and free radical reactions. The most commonly used cracking methods utilizing free radical reactions are hydrogen abstraction and beta bond scission. Hydrogen abstraction is when a free radical “plucks” away hydrogen from the carbon chain causing it to become shorter. Beta bond scission is when the beta bond, in respect to the position of the free radical, becomes “cut” due to the movement of the free radical along the carbon-carbon chain. After doing some quick research it is apparent that the process of more conventional thermal cracking as we recognize it was invented back in 1912 and operated under temperatures between 700-750F and a constant pressure of 90psi. However more modern techniques of thermal cracking involve what we know as vis breaking and coking. Coking has three main types which are flexi, fluid, and delayed coking. Each has a distinctive process that separates itself from the other two. For example delayed coking always utilizes two drums. Vis breaking occurs when the viscosity of the crude oil is reduced while using high temperatures to accomplish this. Overall the thermal cracking process has become an invaluable part of the petroleum, refining industry and is a growing field that is constantly making minor advancements to get the best result out of its feedstock.

Source Wikipedia: http://en.wikipedia.org/wiki/Cracking_(chemistry)