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Unit 3•Lesson 3 2 min read

Petroleum Refining and Hydrocarbons

15/18

Learning Objectives

Describe crude oil as a mixture of hydrocarbons.
Explain the principle of fractional distillation for separating crude oil.
Identify the major fractions obtained from crude oil and their uses.
Define cracking and its importance in petroleum refining.

From Crude Oil to Useful Products

Petroleum, or crude oil, is a complex mixture of thousands of different hydrocarbons—organic compounds consisting entirely of hydrogen and carbon. To be useful, this mixture must be separated into its components, a process done in an oil refinery.

Fractional Distillation

The primary method for separating crude oil is fractional distillation.

Principle: This process separates a mixture into its component parts, or fractions, based on their different boiling points.
Process:
1.Crude oil is heated to a high temperature (around 400°C) in a furnace, causing most of the hydrocarbons to vaporize.
2.The hot liquid/vapor mixture is pumped into the bottom of a tall fractionating column (or tower).
3.The tower has a temperature gradient: it is hottest at the bottom and coolest at the top.
4.The hot vapors rise up the column. As they rise, they cool and condense back into liquids at different heights, according to their boiling points.
5.Trays are located at different levels to collect the condensed liquids, or 'fractions'.

Major Fractions

Long-chain hydrocarbons have strong intermolecular forces and high boiling points. They condense at the bottom of the tower.
Examples: Bitumen (for asphalt), fuel oil (for ships, power plants), lubricating oil.
Short-chain hydrocarbons have weak intermolecular forces and low boiling points. They continue to rise to the top of the tower before condensing.
Examples: Gasoline (petrol), naphtha (for chemicals), kerosene (for jet fuel), refinery gases (LPG).

Cracking

There is a much higher demand for valuable short-chain hydrocarbons like gasoline than is naturally present in crude oil.

Cracking is a process that breaks down large, complex hydrocarbon molecules into smaller, more useful ones.
Thermal Cracking: Uses high temperatures and pressures.
Catalytic Cracking: Uses a catalyst to allow the process to occur at lower temperatures and pressures, giving more control over the products.

Cracking is essential for increasing the yield of gasoline from a barrel of crude oil.

Key Terms

Hydrocarbon
A compound of hydrogen and carbon, such as any of those that are the chief components of petroleum and natural gas.
Fractional Distillation
The separation of a liquid mixture into fractions differing in boiling point by means of distillation, typically using a fractionating column.
Cracking (chemistry)
The process whereby complex organic molecules such as long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds.
Boiling Point
The temperature at which a liquid turns into a vapor when heated.

Check Your Understanding

1

What physical property is used to separate the components of crude oil in a fractionating column?

2

In a fractionating column, where would you collect the hydrocarbons with the lowest boiling points (e.g., gasoline), and where would you collect those with the highest boiling points (e.g., bitumen)?

3

What is the purpose of catalytic cracking in an oil refinery?

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