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How is thermal cracking different from visbreaking in petroleum refining?

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Aerial view of an oil refinery complex near New Orleans, Louisiana, with storage tanks, industrial structures, and smoke rising from the facility, surrounded by open land. — petroleum refining Aerial view of an oil refinery near New Orleans.

In petroleum refining, both visbreaking and thermal cracking are conversion processes that use heat to break down heavy hydrocarbon molecules into lighter, more valuable products. The primary difference lies in the severity of the process and the intended output.12

Visbreaking: This is a form of mild thermal cracking designed primarily to lower the viscosity of heavy residual oils. It typically converts only about 15 percent of the feedstock into lighter components like naphtha and diesel oils, resulting in a lower-viscosity residual fuel that is easier to handle and transport.32
Thermal Cracking: This process is more severe than visbreaking. It is used to convert a significantly larger portion of the feedstock—often 50 to 60 percent—into lighter products such as naphtha and light diesel oils.2

While both processes utilize similar furnace and reaction chamber setups to heat feedstock to temperatures between 450 and 500 °C, thermal cracking units operate under conditions that promote more extensive molecular breakdown compared to the milder conditions maintained in a visbreaker.2