Imagine you’ve got a big, dark, smelly liquid that looks like treacle… and somehow it ends up as petrol in cars, fuel in planes, and even the plastic in your phone case. That liquid is crude oil, and the key trick that sorts it into useful parts is crude oil distillation.
So how do scientists and engineers turn a messy mixture into neat “fractions” you can actually use? Let’s break it down.
What crude oil really is (and why it’s so useful)
Crude oil is a mixture of hydrocarbons. A hydrocarbon is a molecule made only of hydrogen (H) and carbon (C)atoms.
Here’s the important idea: crude oil isn’t one chemical. It’s thousands of different hydrocarbons mixed together, with different chain lengths. Some have short chains like 3–8 carbons. Others have long chains with 30, 40, or even more carbon atoms.
That difference in chain length changes the properties a lot:
- Short-chain hydrocarbons are runny, evaporate easily, and burn quickly.
- Long-chain hydrocarbons are thicker, less flammable, and have higher boiling points.
A memorable fact: a typical barrel of crude oil is about 159 litres—and it can be split into many different useful products.
Light vs heavy crude: the “runny vs treacle” problem
Not all crude oil is the same. Scientists often talk about light and heavy crude.
- Light crude oil has a higher proportion of smaller hydrocarbons. It flows more easily (lower viscosity) and tends to produce more petrol and diesel after refining.
- Heavy crude oil has more long-chain hydrocarbons. It’s thicker, harder to pump, and produces more fuel oil and bitumen unless you process it further.
Think of it like this: light crude is more like cooking oil, heavy crude is more like cold syrup.
This matters because the next step—fractional distillation—works by separating hydrocarbons based on their boiling points, and chain length strongly affects boiling point.
Why boiling point changes with chain length
If you line up hydrocarbons from short to long, their boiling points rise as chains get longer. That’s because longer molecules have stronger intermolecular forces (the weak attractions between molecules). More attractions mean you need more energy (higher temperature) to pull the molecules apart into a gas.
So:
- Short chains → weaker forces → lower boiling point → evaporate easily
- Long chains → stronger forces → higher boiling point → stay liquid/solid until very hot
This is the science behind why crude oil distillation works so well.
Inside a fractional distillation column (the giant temperature sorter)
A fractional distillation column is a tall tower that’s hot at the bottom and cooler at the top.
Here’s what happens:
- Crude oil gets heated strongly so most of it turns into vapour.
- The vapour enters the column at the bottom.
- As vapour rises, it cools.
- Different hydrocarbons condense (turn back to liquid) at different heights because each fraction has a different boiling point range.
- Engineers collect each fraction from trays or pipes at those heights.
So you don’t “filter” crude oil like you would with sand and water. You separate it by boiling point.
Common fractions you’ll hear about (from top to bottom) include:
- Refinery gases (very small molecules)
- Petrol (gasoline)
- Kerosene (jet fuel)
- Diesel
- Fuel oil
- Bitumen (for roads and roofing)
The higher up you go, the smaller the molecules tend to be.
What if we need more petrol? Cracking to the rescue
Here’s a twist: the world often wants lots of smaller molecules (like petrol) but crude oil can contain loads of long chains, especially in heavy crude.
That’s why refineries use cracking.
Cracking breaks long-chain hydrocarbons into smaller molecules. It usually needs:
- High temperature (thermal cracking), and/or
- A catalyst like zeolite (catalytic cracking)
Cracking can make:
- More petrol-range hydrocarbons
- Alkenes (hydrocarbons with a double bond), which are brilliant for making polymers like poly(ethene) and poly(propene)
So, cracking doesn’t just make fuels. It also helps make the starting chemicals for plastics.
So, as you think about crude oil distillation, ask yourself: next time you see plastic packaging or step onto a bus, how many chemical steps do you think happened between sticky crude oil and that final product?