Thermal cracking was the first commercially used conversion process to process crude oils. It was initially implemented to produce more gasoline as well as higher octane gasoline. The higher octane gasoline was mainly used for fuels for aircrafts. The main reason thermal cracking came along was to produce light to middle distillates from heavier fractions of the crude oil. Thermal cracking became obsolete for gasoline production in modern refineries when catalytic cracking was initiated. Catalytic cracking came around in the 1930’s and could produce more gasoline at a higher octane number, then thermal cracking. Thermal cracking is still used in some modern refineries with visbreaking and coking. Visbreaking is a thermal cracking process that uses VDR as a feedstock to produce fuel oil and light products to increase distillation output of a refinery. The main goal of visbreaking is to reduce viscosity in a feedstock, but could also be used to produce lighter distillates from fuel oil. The second thermal cracking process that is used in refineries is called coking. Coking is the most severe thermal cracking process used in a refinery. It cracks the heaviest of the crude oil fractions, such as vacuum residue. Three coking processes are used to maximize the yield of distillation products. Thermal cracking is no longer the main process of a refinery but it is still plays a small role in increasing the yields of distillates.