How crude oil becomes the fuels and chemicals we use. Explore the main stages and play with the distillation tower.
Tip: Try the distillation tower below to see which products come off at which temperatures.
Drag the slider or click a band to see each cut. Tooltips on the tower describe typical uses; choose a crude type below to see approximate yield of the selected fraction.
Distillation separates by boiling point; conversion and quality-upgrading units then improve the value of each stream. The main units and their typical outputs:
Heavy gas oil from the crude tower is cracked at high temperature with a zeolite catalyst. The workhorse for gasoline production in many refineries.
Main outputs: Gasoline, LPG, light cycle oil (diesel blend), heavy cycle oil (recycle or fuel). Increases gasoline yield from each barrel.
Heavy feed is cracked in the presence of hydrogen and a catalyst. Higher pressure than FCC; produces more middle distillates (diesel, jet) and less coke.
Main outputs: Naphtha (gasoline blend), jet fuel, diesel — all with good quality. Favoured when diesel demand is strong; more flexible than FCC for middle distillates.
The heaviest residue is heated until it “cracks” into lighter liquids and solid coke. Used when the refinery must dispose of bottom-of-the-barrel residue.
Main outputs: Petroleum coke (sold or fuel), coker naphtha (treated and blended), coker gas oil (fed to FCC or hydrocracker). Maximises liquid yield from residue; coke is a by-product.
Hydrotreating is a cleanup unit that removes sulfur, nitrogen, and metals before or after conversion. It is essential for low-sulfur gasoline, diesel, and jet fuel specifications.
Main outputs: Low-sulfur gasoline and diesel blend components, cleaner intermediate streams, and improved emissions compliance.
Reforming upgrades low-octane naphtha into high-octane reformate for gasoline and co-produces hydrogen used in hydrotreaters and hydrocrackers.
Main outputs: High-octane gasoline blendstock and hydrogen supply for sulfur-removal units, improving both product quality and refinery integration.
The 3:2:1 crack spread is a benchmark gross refining margin: 3 barrels of crude are assumed to produce 2 barrels of gasoline and 1 barrel of distillate (diesel/heating oil).
Real refinery margins differ from this benchmark because actual yields vary by crude quality and configuration, and because operating costs are excluded.
Each processing unit is assigned a complexity factor based on its cost and capability relative to atmospheric distillation (which has a factor of 1.0). The index is the sum of (unit capacity × factor) / (crude capacity), so it is dimensionless. Adding FCC, hydrocracking, coking, or hydrotreating increases the NCI.
| Unit type | Typical factor (relative to distillation) |
|---|---|
| Atmospheric distillation | 1.0 (base) |
| Vacuum distillation | ~2.0 |
| Catalytic cracking (FCC) | ~6.0 |
| Hydrocracking | ~6.0–7.0 |
| Coking | ~6.5 |
| Hydrotreating | ~2.0–4.0 |
| Reforming | ~5.0 |
Refineries with more and heavier upgrading units have higher NCI and can run heavier or sour crudes while turning more of the barrel into light products.
Typical NCI ranges:
Desulphurisation (e.g. hydrotreating) removes sulphur from distillates to meet fuel specifications. Road diesel and gasoline in most regions must be low-sulphur (e.g. <10 ppm in Europe). Refineries invest in hydrotreating capacity to comply; sour crude requires more treating.
Emissions regulations affect refinery operations (flue gas, wastewater, CO₂) and the specs of products they sell.
Euro VI (and equivalent standards elsewhere) set limits on vehicle NOx and particulates, which in turn require clean, well-refined fuels. Tighter specs increase refining cost and favour complex refineries with strong treating and conversion.
For how product quality and margins interact, see Crack spread explained for margin analysis and regional differences.
Refining turns crude oil into usable products. Crude is a mixture of thousands of hydrocarbons; refineries separate and transform them into fuels (gasoline, diesel, jet) and feedstocks for chemicals. Margin comes from selling products for more than the cost of crude and running the plant.
Distillation uses the fact that lighter molecules boil at lower temperatures. Heating crude in a tower lets us draw off “cuts” at different heights — no chemistry yet, just separation. This gives the refiner streams that can then be converted, treated, and blended into final products.
Demand is skewed toward light products (gasoline, diesel). Conversion units — FCC, hydrocracking, and coking — break heavy molecules into lighter ones and change product yields. See the “Conversion units” section above for how each unit works and its main outputs.
Streams are treated to remove sulfur and other impurities (e.g. hydrotreating). Blending combines different streams to meet specs — e.g. gasoline blend for octane and vapor pressure. What leaves the gate is saleable product.
Environmental specs (e.g. Euro VI, low-sulphur fuels) drive treating investment.
The Nelson complexity index (NCI) measures how sophisticated a refinery is. Each unit is assigned a factor relative to atmospheric distillation (1.0); the index is the weighted sum. See the “Nelson complexity index” section above for the calculation and typical values (simple ~2–5, conversion ~5–9, deep conversion ~9–15+).