Fractional Distillation of Crude Oil

Introduction

Crude oil, pumped from deep underground, is a thick, dark liquid that is of little use as it is. Yet it becomes fuel for cars and aircraft, gas for cooking, lubricating oil and the tar on roads. The first step in turning crude oil into these products is fractional distillation in a refinery. The same principle used to separate ethanol from water, carried out in a column tens of metres tall, sorts crude oil into useful groups of substances called fractions .

Core explanation

Crude oil is a mixture. Crude oil contains hundreds of different hydrocarbons — compounds of hydrogen and carbon only. Their molecules range from one carbon atom to more than fifty. Each has its own boiling point, and many boiling points are close together, so separating every single compound would be impractical. Instead, the oil is separated into fractions, each a mixture of hydrocarbons with a similar boiling range .

Molecule size and boiling point. Small hydrocarbon molecules have weak forces of attraction between them, so they have low boiling points. Larger molecules have stronger attractions, so they have higher boiling points. Size also affects other properties: larger molecules make liquids that are more viscous (thicker), less volatile and harder to ignite.

The industrial column. Crude oil is heated in a furnace to about 350–400 °C, so that most of it vaporises. The hot mixture enters near the bottom of a tall steel column. The column has a temperature gradient: hot at the bottom and cool at the top . It contains many horizontal trays with openings that let vapour rise.

- Vapour rises up the column and cools. - When a hydrocarbon reaches a level where the temperature is below its boiling point, it condenses on a tray and is piped off. - Hydrocarbons with lower boiling points keep rising to higher, cooler levels. - The smallest molecules stay as gases and leave at the top. - The largest molecules never vaporise; they collect at the bottom as a thick residue.

Unlike laboratory fractional distillation, this is a continuous process: crude oil is fed in constantly and fractions are drawn off at every level at the same time.

Main fractions (approximate; exact ranges vary between refineries):

Fraction Carbon atoms Boiling range (°C) Main use --- --- --- --- Refinery gases 1–4 below 25 Bottled gas for heating and cooking Petrol 5–10 40–110 Fuel for cars Naphtha 8–12 110–180 Feedstock for making chemicals and plastics Kerosene 10–16 180–260 Jet fuel, paraffin Diesel oil 14–20 260–340 Fuel for lorries, buses and trains Fuel oil 20–40 340–400 Fuel for ships and power stations Bitumen over 40 above 400 Road surfaces and roofing

Step-by-step reasoning

To predict where a hydrocarbon leaves the column:

1. Find its number of carbon atoms, or its boiling point. 2. Larger molecules mean higher boiling points. 3. Higher boiling points condense lower in the column, where it is hotter. 4. So large molecules leave near the bottom and small molecules near the top.

Visual explanation

Picture a tall column with a thermometer scale on its side, from about 25 °C at the top to 350 °C at the bottom. Pipes stick out at different heights, each labelled: gases at the top, then petrol, naphtha, kerosene, diesel, fuel oil, and a thick black stream of bitumen at the base.

Real-world analogy

The column is like a building where people get off a lift at the floor that suits them. Everyone enters at the ground floor; those who cannot climb far leave on the lower floors, and only the lightest travellers reach the roof. Each hydrocarbon "gets off" at the level whose temperature matches its boiling point.

Real-world example

Aircraft run on kerosene rather than petrol. Kerosene is less volatile, so it is less likely to form an explosive vapour in fuel tanks, and it releases a large amount of energy per kilogram. Its boiling range places it in the middle of the column.

Why?

Why do larger hydrocarbons have higher boiling points? Larger molecules have more atoms and more surface in contact with neighbouring molecules, so the attractive forces between molecules are stronger. More energy is needed to separate them into a gas, which means a higher temperature.

Common misconception

"Each fraction is a single pure compound." A fraction is still a mixture of many hydrocarbons, but with a narrow range of molecule sizes and boiling points. Petrol, for example, contains dozens of different compounds.

Worked example

Question: A hydrocarbon has 12 carbon atoms. Using the table, name a fraction in which it could be found and state whether it leaves the column above or below the diesel fraction.

Reasoning: Twelve carbon atoms fall within the kerosene range (10–16). Kerosene has a lower boiling range than diesel, so it condenses higher up.

Answer: Kerosene (or naphtha); it leaves above the diesel fraction.

Quick check

1. Where in the column do the fractions with the highest boiling points leave? Answer: At the bottom, where the column is hottest.

Exam focus

Learn the order of the main fractions from top to bottom and one use of each. Explain the trends: as molecules get larger, boiling point and viscosity increase, while volatility and ease of ignition decrease. Remember that crude oil is vaporised before it enters the column.

Advanced insight

Demand for small molecules like petrol is greater than the supply from distillation, while there is a surplus of heavier fractions. Refineries therefore break large molecules into smaller, more useful ones by cracking , using heat and catalysts. Crude oil is also a finite resource, and burning its fractions releases carbon dioxide, which is why alternatives are being developed.

Summary

Crude oil is a mixture of hydrocarbons that is separated into fractions by continuous fractional distillation. It is vaporised and fed into a tall column that is hot at the bottom and cool at the top. Each fraction condenses at the level matching its boiling range: small molecules such as refinery gases leave at the top, and large ones such as bitumen at the bottom. Larger molecules have higher boiling points and viscosity.

Practice questions

1. Why must crude oil be separated before it is useful? Answer: It is a mixture of many hydrocarbons with different properties; each fraction has different uses. 2. State how boiling point and viscosity change as hydrocarbon molecules get larger. Answer: Both increase. 3. Explain why petrol leaves the column higher up than diesel. Answer: Petrol molecules are smaller with a lower boiling range, so they stay as vapour until they reach cooler levels higher in the column. 4. Give one use each of refinery gases and bitumen. Answer: Refinery gases: bottled gas for heating or cooking. Bitumen: surfacing roads or roofs. 5. State one way in which the industrial process differs from laboratory fractional distillation. Answer: It is continuous, with crude oil fed in constantly and all fractions drawn off at the same time, and the column uses trays rather than glass beads.