Crude Oil, Gasoline, and Diesel: A Simplified Chemistry Primer

Video summary generated by AI.

Crude oil, gasoline, and diesel are all mixtures of hydrocarbons: molecules made of only hydrogen and carbon. Understanding a few basic properties of hydrocarbons goes a long way toward understanding how a refinery works.

Boiling Point Increases with Molecule Size

The larger the hydrocarbon molecule, meaning more carbon atoms, the higher its boiling point. A small molecule like isopentane (5 carbons) boils at a much lower temperature than decane (12 carbons). This relationship is what makes distillation possible: heat crude oil, and different molecules boil off at different temperatures and can be separated.

Crude Oil Is a Wide Mix

Crude oil contains hundreds of different hydrocarbon molecules, from small ones like propane to very large ones with 70+ carbons. No two crude oils are identical. Producers publish a crude assay to describe their oil in bulk terms, including cut volume: the percentage of crude that falls within a given boiling point range. This tells a refinery how much gasoline- or diesel-range material to expect from a given crude.

How Boiling Ranges Define Fuels

Each fuel corresponds to a rough boiling point range:

• Gasoline: up to ~330°F
• Diesel: ~330°F to 650°F
• Heavy fuel oil: above the diesel range

A refinery's job, at its most basic, is to separate those slices from crude oil and upgrade them into finished products that meet fuel specifications. Everything else builds from here.