After distillation, deasphalting, and dewaxing, refineries recovered most available fractions. However, gasoline and kerosene production still lagged rising demand driven by early automobile growth.

To boost light-product yields, refineries moved beyond separation and adopted chemical conversion, breaking and reforming hydrocarbon bonds to make more valuable products.

The first widely adopted conversion process was thermal cracking, which uses high temperature to break long-chain paraffins in heavy fractions into smaller molecules in the gasoline boiling range.

As octane requirements increased, thermally cracked gasoline became less suitable. Around World War II, refineries broadly adopted catalytic cracking to produce higher-octane fuels.

Thermal cracking proceeds mainly via free-radical chain reactions, while catalytic cracking proceeds mainly via carbocation (ionic) reactions; both pathways are covered later in the course.

Thermal cracking still upgrades very heavy streams such as Vacuum Distilled Residue (VDR). At moderate severity it lowers viscosity and makes residual fuel oil; at high severity it produces more light products and petroleum coke (“thermal upgrading”).

Key point: Thermal cracking represents transition from separation to conversion.