Fractional Distillation of Crude Oil

Introduction

Crude oil contains hydrocarbons with anything from one to more than seventy carbon atoms. To turn it into petrol, jet fuel, diesel and road surfacing, a refinery must sort these molecules by size. It does this without any chemical reaction, using only the fact that different-sized molecules boil at different temperatures. This physical separation is fractional distillation , and it takes place in towering columns that are a familiar sight at oil refineries.

Core explanation

Boiling point and size. In alkanes, larger molecules have stronger intermolecular forces because they have more surface contact with their neighbours. More energy is needed to separate them, so the bigger the molecule, the higher its boiling point . Crude oil therefore contains substances boiling over a very wide range.

The fractionating column.

1. Crude oil is heated in a furnace until most of it has vaporised . 2. The hot vapour enters near the bottom of a tall fractionating column . 3. The column has a temperature gradient : it is very hot at the bottom and much cooler at the top. 4. Vapour rises through trays in the column. As it rises it cools. 5. When a substance cools to its boiling point, it condenses into a liquid and collects on a tray at that level, where it is piped off. 6. Small molecules with low boiling points stay as gases and leave from the top . The largest molecules never vaporise and are drained from the bottom as a thick residue.

The main fractions (approximate values):

Fraction Carbon atoms Boiling range Main use --- --- --- --- Refinery gases C₁–C₄ below 25 °C bottled gas for heating and cooking Petrol (gasoline) C₅–C₁₀ 40–100 °C fuel for cars Naphtha C₈–C₁₂ 90–160 °C feedstock for chemicals and plastics Kerosene C₁₀–C₁₆ 150–250 °C jet fuel, paraffin Diesel (gas oil) C₁₄–C₂₀ 220–350 °C diesel engines, trains Fuel oil C₂₀–C₄₀ above 350 °C ships, power stations Bitumen over C₄₀ residue road surfaces, roofing

The ranges overlap because each fraction is itself a mixture.

Trends down the column. Moving from the top to the bottom, molecules get larger and fractions become:

- higher boiling — they condense at higher temperatures; - more viscous — thicker and slower to pour; - less volatile — they evaporate less easily; - harder to ignite — less flammable, burning with smokier flames.

Step-by-step reasoning

To predict where a hydrocarbon will be collected:

1. Find its number of carbon atoms. 2. Use size to judge its boiling point: more carbons, higher boiling point. 3. Higher boiling point means it condenses lower in the column, where it is hotter. 4. Match it to a fraction: for example, C₈H₁₈ condenses with petrol.

Visual explanation

Picture a tall tower with a thermometer scale painted up its side: about 350 °C at the base, falling to roughly 25 °C at the top. Side pipes branch off at several heights, each labelled with a fraction. Gas leaves at the top, a black, tar-like residue at the bottom.

Real-world analogy

A fractionating column works like a stack of sieves with the coarsest at the top, except it sorts by boiling point rather than size of hole. Each tray "catches" the molecules that can no longer stay as vapour at that temperature, while the rest pass on upwards.

Real-world example

Aircraft use kerosene rather than petrol. Kerosene is less volatile, so it is less likely to form explosive vapour in fuel tanks, yet it still flows well at the very low temperatures experienced at cruising altitude.

Why?

Why do large molecules condense low in the column? They have the strongest intermolecular forces and the highest boiling points. As soon as the vapour rises into a region cooler than their boiling point, they condense — and that happens near the hot bottom. Small molecules must travel much higher before it is cool enough.

Common misconception

"Fractional distillation breaks crude oil molecules into smaller ones." It does not. It is a physical process that only separates molecules already present. Breaking large molecules into smaller ones is a chemical reaction called cracking.

Worked example

Question: Hydrocarbon A has 7 carbon atoms and hydrocarbon B has 18. Which is collected higher in the column, and which is more viscous?

Reasoning: A is smaller, so it has weaker intermolecular forces and a lower boiling point; it condenses higher up, in the petrol fraction. B is larger, with stronger forces, so it condenses lower (diesel) and is thicker.

Answer: A is collected higher; B is more viscous.

Quick check

1. Where in the column is the temperature highest? Answer: At the bottom.

Exam focus

Describe the process in order: heat, vaporise, rise, cool, condense at different heights. Link molecule size to boiling point, viscosity, volatility and ease of ignition, and remember that the process is physical, not chemical.

Advanced insight

Very heavy residues would need such high temperatures to boil that they would start to break down. Refineries therefore distil the residue again at reduced pressure ("vacuum distillation"), which lowers boiling points and allows heavy lubricating oils to be separated without decomposition.

Summary

Fractional distillation separates crude oil into fractions by boiling range. Vaporised crude enters a column that is hot at the bottom and cool at the top; each fraction condenses where the temperature falls to its boiling range. Larger molecules condense lower down and are more viscous, less volatile and harder to ignite. The process is physical.

Practice questions

1. Explain why the fractions condense at different heights in the column. Answer: They have different boiling points; each condenses where the column temperature falls below its boiling point, and the column gets cooler towards the top. 2. Which fraction is used for jet fuel, and roughly how many carbon atoms do its molecules contain? Answer: Kerosene, with about 10–16 carbon atoms. 3. Compare the viscosity and flammability of petrol and fuel oil. Answer: Petrol is runny and ignites easily; fuel oil is much more viscous and harder to ignite. 4. Is fractional distillation a physical or chemical process? Explain. Answer: Physical, because no new substances are made — molecules are only separated by boiling point.