Thermal cracking was initially developed to attempt to solve an energy source issue for the automobile and for aircrafts. Gasoline is produced by cracking gas oils at high temperature and initially at high pressures for hydrogen abstraction, then at low pressures for the cracking or breaking of the bonds. The heavy and light gas oils are separated to avoid heating the reactive longer alkane chains to maintain coke formations. This was a great means for our country to produce light middle distillates and heavier ends by excessive heating. The US has since discovered catalytic cracking which allows for a conversion process that produces higher yields of gasoline and higher octane number. Although the process of thermal cracking is in the past for the US, other countries still use this conversion process as a principal application of petroleum refining for diesel fuel production.

Visbreaking is known as a mild thermal cracking process which reduces the viscosity of the vacuum distillation residue to produce fuel oil, which is converted to lighter distillates. Thermal severity is a measurement of temperature and time and is an index number that helps describe how viscosity will change in visbreaking. Thermal severity can inform a refinery on how asphaltene and carbon content of feedstocks are characteristics to be aware of when considering possibilities of coking in the visbreaker reactor.