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How does visbreaking reduce the viscosity of petroleum residue?

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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.

Visbreaking, or viscosity breaking, is a mild thermal cracking process used in petroleum refining to reduce the viscosity of heavy residual oils.12

The process achieves this reduction by heating the feedstock to temperatures between 450 and 500 °C (840 to 930 °F) at moderate pressures. This thermal treatment breaks down large, heavy hydrocarbon molecules into smaller, lighter molecules. By converting a portion of the heavy residue—typically about 15 percent—into lighter products like naphtha and diesel oils, the overall viscosity of the remaining residual fuel is significantly lowered.23

To ensure the process remains efficient and prevents the formation of unwanted solid coke, the residence time in the furnace is strictly limited. The fluid is then moved to a reaction chamber maintained at a pressure high enough to facilitate the cracking of large molecules while restricting coke formation, before being cooled to inhibit further reactions.2