9.6 Separating Crude Oil

Introduction

Crude oil (petroleum) is a mixture of many different hydrocarbons. It cannot be used directly as a fuel or raw material because it contains too many different substances mixed together.

Refining is the process of separating crude oil into usable products. These useful products contain a group of hydrocarbon molecules that have similar chain lengths, similar boiling points, and condense over a range of temperatures.

This section explains how crude oil is separated into useful fractions by fractional distillation.

Definition

Refining: Refining is the process of separating crude oil into usable products.

Fractions: These useful products contain a group of hydrocarbon molecules that have similar chain lengths, similar boiling points, and condense over a range of temperatures. These groups of hydrocarbons are called fractions.

In simple words: Refining separates crude oil into different groups of hydrocarbons called fractions, which have similar properties and can be used for different purposes.

Main Idea

• Refining is the process of separating crude oil into usable products.
• This means that each useful fraction consists of a combination of different hydrocarbons that boil at a specific temperature.
• Oil refining takes place in a high-temperature fractionation tower.
• The temperature at the top of the tower is colder than at the bottom of the tower.
• Crude oil is heated under high pressure in a furnace to a temperature of 400^° C.
• The oil is then passed through a distillation column where vapour rises.
• As the hot vapour moves up the tower, it condenses into different fractions according to the boiling point.
• Compounds with the highest boiling point condense near the bottom.
• Compounds with the lowest boiling point condense at the top.
• Non-boiling compounds collect at the bottom.

Core Concept

Key point: Crude oil is separated into fractions by fractional distillation based on differences in boiling points.

Detailed Explanation

What is Refining?

Refining is the process of separating crude oil into usable products.

These useful products contain a group of hydrocarbon molecules that have:

• Similar chain lengths
• Similar boiling points
• Condense over a range of temperatures

These groups of hydrocarbons are called fractions.

Key point: Refining separates crude oil into fractions based on their boiling points.

How Does Fractional Distillation Work?

Oil refining takes place in a high-temperature fractionation tower, where the temperature at the top of the tower is colder than at the bottom of the tower.

Step-by-step process:

• Crude oil is heated under high pressure in a furnace to a temperature of 400^° C.
• The oil is then passed through a distillation column where vapour rises.
• As the hot vapour moves up the tower, it condenses into different fractions according to the boiling point.
• Compounds with the highest boiling point condense near the bottom.
• Compounds with the lowest boiling point condense at the top.
• Non-boiling compounds collect at the bottom.

Key point: The separation of crude oil into fractions is based on differences in boiling points.

Table 9.1: Fractions of Petroleum and Their Uses

The source provides the following table of fractions, their carbon atom ranges, boiling points, and uses:

Key point: Each fraction has a specific range of carbon atoms, boiling point, and uses.

Trends in the Fractionating Column

As the length of the hydrocarbon carbon chain increases, the strength of the intermolecular force also increases.

Bitumen has:

• The longest carbon chain
• The highest boiling point
• The highest viscosity
• The lowest volatility
• The darkest colour

As you move from bottom to top:

• The chain length of carbon atoms in different fractions gradually decreases.
• Thus, their boiling points and viscosity also decrease.
• However, the weakening of the strength of the intermolecular forces increases the volatility of the fractions.

For example:

- Refinery gas (LPG) has the shortest carbon chain, lowest boiling point, lowest viscosity, and highest volatility.

Key point: As you move from bottom to top of the fractionating column:

• Chain length decreases
• Boiling point decreases
• Viscosity decreases
• Volatility increases

Summary Table

Intermediate Level

Refining:

• Separating crude oil into usable products.
• Fractions = groups of hydrocarbons with similar chain lengths and boiling points.
• Process: Fractional distillation.

Fractional distillation:

• Crude oil is heated to 400^° C.
• Vapours rise in the fractionation tower.
• Fractions condense at different heights based on their boiling points.
• High boiling point → bottom; low boiling point → top.

Trends:

• As you move from bottom to top:
• Chain length decreases
• Boiling point decreases
• Viscosity decreases
• Volatility increases

Key point: Fractional distillation separates crude oil into fractions based on boiling points. The chain length, boiling point, and viscosity decrease from bottom to top, while volatility increases.

Important Points

• - Refining = separating crude oil into usable products.
• - Fractions = groups of hydrocarbons with similar chain lengths and boiling points.
• - Process = fractional distillation.
• - Fractionation tower = hotter at the bottom, colder at the top.
• - Crude oil is heated to 400^° C.
• - Compounds with the highest boiling point condense near the bottom.
• - Compounds with the lowest boiling point condense at the top.
• - Non-boiling compounds collect at the bottom.
• - Fractions and their uses:
• - LPG — cooking and heating
• - Petrol/gasoline — fuel for cars
• - Naphtha — chemical feedstock
• - Kerosene/paraffin — jet fuel, heating, lighting
• - Diesel/gas oil — diesel engines
• - Lubricating oil — lubricants, waxes, polishes
• - Fuel oil — power stations, ships, home heating
• - Bitumen — roads, roofing
• - Trends: As you move from bottom to top:
• - Chain length decreases
• - Boiling point decreases
• - Viscosity decreases
• - Volatility increases
• - Bitumen has the longest carbon chain, highest boiling point, highest viscosity, lowest volatility, and darkest colour.
• - Refinery gas (LPG) has the shortest carbon chain, lowest boiling point, lowest viscosity, and highest volatility.

Remember

• - Refining = separating crude oil into fractions.
• - Fractions = groups of hydrocarbons with similar boiling points.
• - Process = fractional distillation.
• - Tower = hotter at bottom, colder at top.
• - Crude oil heated to 400^° C.
• - High boiling point → bottom; low boiling point → top.
• - Fractions: LPG, petrol, naphtha, kerosene, diesel, lubricating oil, fuel oil, bitumen.
• - Trends (bottom to top): chain length ↓, boiling point ↓, viscosity ↓, volatility ↑.
• - Bitumen: longest chain, highest boiling point, highest viscosity, lowest volatility.
• - LPG: shortest chain, lowest boiling point, lowest viscosity, highest volatility.

Common Mistakes

Quick Revision

Separating Crude Oil:

Refining is the process of separating crude oil into usable products. These useful products contain a group of hydrocarbon molecules that have similar chain lengths, similar boiling points, and condense over a range of temperatures. These groups of hydrocarbons are called fractions. This means that each useful fraction consists of a combination of different hydrocarbons that boil at a specific temperature. Oil refining takes place in a high-temperature fractionation tower, where the temperature at the top of the tower is colder than at the bottom of the tower (see figure). Crude oil is heated under high pressure in a furnace to a temperature of 400^° C. The oil is then passed through a distillation column where vapour rises. As the hot vapour move up the tower, it condenses into different fractions according to the boiling point. Compounds with the highest boiling point condense near the bottom, compounds with the lowest boiling point at the top, and non-boiling compounds collect at the bottom (see Figure 9.1). Therefore, crude oil is divided into several fractions. Each fraction has its specific boiling range and composition. How many fractions are obtained?

Table 9.1: Fractions of petroleum and their uses:

As the length of the hydrocarbon carbon chain increases, the strength of the intermolecular force also increases. Bitumen has the longest carbon chain, highest boiling point, highest viscosity, lowest volatility, and darkest colour. As you move from bottom to top. The chain length of carbon atoms in different fractions gradually decreases. Thus, their boiling points and viscosity also decrease. However, the weakening of the strength of the intermolecular forces increases the volatility of the fractions. For example, refinery gas (LPG) has the shortest carbon chain, lowest boiling point, lowest viscosity, and highest volatility.

60-Second Revision

• Refining = separating crude oil into fractions.
• Fractions = groups of hydrocarbons with similar boiling points.
• Process = fractional distillation.
• Tower = hotter at bottom, colder at top.
• Crude oil heated to 400^° C.
• High boiling point → bottom; low boiling point → top.
• Fractions: LPG, petrol, naphtha, kerosene, diesel, lubricating oil, fuel oil, bitumen.
• Trends (bottom to top): chain length ↓, boiling point ↓, viscosity ↓, volatility ↑.
• Bitumen: longest chain, highest boiling point, highest viscosity, lowest volatility.
• LPG: shortest chain, lowest boiling point, lowest viscosity, highest volatility.
• ⚠️ Warning: the fractionating tower is hotter at the bottom and colder at the top — this is what allows the fractions to separate based on their boiling points.