Visbreaking is the process that converts heavy oil residues into lighter hydrocarbons using heat alone.
The name comes from “viscosity breaking.” In a refinery, heavy residual oil is heated under controlled conditions so some large hydrocarbon molecules thermally crack into smaller molecules. The result is a less viscous product that can be blended into fuel oil or sent to further processing, along with gases and lighter distillates.
Visbreaking reduces the viscosity and pour point of residue, improving its handling and sometimes increasing the yield of marketable products. It is a relatively mild thermal-cracking process, so the operating conditions are controlled to limit excessive coke formation.
Visbreaking differs from catalytic cracking because it does not rely on a catalyst. It also differs from hydrocracking, which uses hydrogen and a catalyst to produce cleaner, more extensively converted products. A delayed coker applies more severe thermal conditions and deliberately produces petroleum coke.