Crude oil is transformed into the gasoline or diesel used in cars through a complex process called refining, which involves separating, breaking down, and restructuring its components.
Raw crude oil cannot be used directly in a car engine. It is a mixture of many different hydrocarbon molecules of varying sizes and complexities. To become usable fuel, it must undergo processing at an oil refinery.
The primary steps involved in turning crude oil into car fuel include:
- Fractional Distillation: The first step is heating the crude oil and separating it into different components, or "fractions," based on their boiling points. Lighter fractions (like gasoline components) rise higher in a distillation column, while heavier fractions (like diesel or fuel oil) condense lower down. However, this initial separation doesn't yield enough of the valuable lighter products like gasoline.
- Cracking: To increase the yield of gasoline and diesel from heavier fractions, refineries use processes that break apart larger, less valuable hydrocarbon molecules into smaller, more valuable ones. As mentioned in the reference, Cracking units use heat, pressure, and catalysts to break apart large, heavy molecules into smaller, higher-value ones like gasoline and diesel. This is a crucial step for maximizing gasoline production.
- Reforming: Gasoline fractions obtained from distillation and cracking often need further treatment to improve their quality, specifically their octane number, which prevents engine knocking. The reference notes that Reformers convert light streams like lower octane naphtha into higher octane gasoline with a small chemical change. This process rearranges the molecular structure of hydrocarbons.
- Treating: Various processes are used to remove impurities like sulfur, nitrogen, and water from the fuel components. These impurities can be corrosive and harmful to engines and the environment.
- Blending: Finally, the refined components (gasoline, diesel, etc.) are blended together in specific ratios to meet required specifications for octane rating, volatility, and other performance and environmental standards. Additives may also be mixed in at this stage.
Here's a simplified look at some common products from crude oil refining:
| Product | Typical Carbon Atoms per Molecule | Uses |
|---|---|---|
| Gases (Propane) | 1–4 | Heating, Cooking |
| Naphtha (Gasoline) | 5–12 | Car Fuel, Petrochemicals |
| Kerosene/Jet Fuel | 12–16 | Aircraft Fuel, Heating Oil |
| Diesel Fuel | 15–18 | Diesel Vehicles, Heating Oil, Power |
| Fuel Oil | >18 | Ships, Power Plants, Industrial Furnaces |
Through these steps, particularly cracking and reforming as highlighted, the complex mixture of crude oil is transformed into the refined fuels that power our cars.
