Fraction: A Mixture of Similar Boiling-Point Hydrocarbons
The Building Blocks: Hydrocarbons and Their Properties
Everything starts with hydrocarbons. These are molecules made only of hydrogen ($H$) and carbon ($C$) atoms. They are the main components of crude oil and natural gas. The simplest hydrocarbon is methane ($CH_4$), the primary gas in the natural gas we use for cooking and heating.
A key property of hydrocarbons is their boiling point. This is the specific temperature at which a liquid turns into a gas. For hydrocarbons, the boiling point depends heavily on the size of the molecule, which we measure by the number of carbon atoms. A general rule is: the larger the hydrocarbon molecule, the higher its boiling point. This happens because larger molecules have stronger intermolecular forces (specifically, London dispersion forces) that hold them together, requiring more heat energy to break them apart into a gas.
Let's look at this in a table for the first few members of the alkane series, a common type of hydrocarbon:
| Hydrocarbon Name | Chemical Formula | Number of Carbon Atoms | Approximate Boiling Point (°C) | State at Room Temperature |
|---|---|---|---|---|
| Methane | $CH_4$ | 1 | -162 | Gas |
| Ethane | $C_2H_6$ | 2 | -89 | Gas |
| Butane | $C_4H_{10}$ | 4 | -1 | Gas |
| Octane | $C_8H_{18}$ | 8 | 126 | Liquid |
| Hexadecane | $C_{16}H_{34}$ | 16 | 287 | Liquid/Solid |
This predictable trend is the golden key. Crude oil contains thousands of different hydrocarbons, from small gases to huge waxy solids. To get useful products, we need to separate them. This is done using a process called fractional distillation, which cleverly uses their different boiling points.
The Separation Process: Fractional Distillation
Fractional distillation is a physical separation process, not a chemical reaction. It doesn't break molecules apart; it just sorts them based on their physical property of volatility (how easily they evaporate).
Imagine a giant vertical tower, called a fractionating column, attached to a furnace where crude oil is heated. Here is how it works, step by step:
- Heating: Crude oil is heated to about 400°C in a furnace. At this high temperature, most of the oil turns into vapor, but some of the largest, heaviest molecules remain as a liquid or solid residue.
- Rising Vapor: The hot vapor mixture enters the bottom of the fractionating column. The column is very hot at the bottom and gets progressively cooler towards the top.
- Condensation on Trays: The column is packed with horizontal trays or plates with bubble caps. As the hot vapor rises, it cools. Hydrocarbons with the highest boiling points (like heavy fuel oil) will condense first, turning back into liquid on the trays near the bottom where it's still hot enough for them to be liquid. Smaller molecules with lower boiling points continue rising.
- Collecting Fractions: At different heights, where the temperature matches the boiling range of a group of similar-sized hydrocarbons, they condense and are collected. Each collection is a fraction. For example, gasoline (petrol) hydrocarbons have lower boiling points and are collected near the top, while diesel, with higher boiling points, is collected lower down.
- Uncondensed Gases: The very smallest hydrocarbon molecules (like methane and propane) don't condense in the column at all and are collected as gases at the very top.
The beauty of this process is that it separates the complex crude oil mixture into several simpler mixtures (fractions), each containing hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range. No fraction is a single pure substance; it's always a mixture, but a much more useful one.
From Crude Oil to Useful Products: Common Fractions
What comes out of a refinery's fractionating column? Each fraction has distinct properties and uses. The following table outlines the typical fractions obtained, ordered from the top (lowest boiling point) to the bottom (highest boiling point) of the column.
| Fraction Name | Carbon Atoms per Molecule | Boiling Point Range (°C) | Common Uses |
|---|---|---|---|
| Refinery Gas | $C_1$ to $C_4$ | < 40 | Bottled gas (LPG1), chemical feedstock |
| Gasoline (Petrol) | $C_5$ to $C_{10}$ | 40 - 180 | Fuel for cars and motorcycles |
| Naphtha | $C_7$ to $C_{14}$ | 100 - 200 | Industrial solvent, feedstock for making chemicals/plastics |
| Kerosene (Paraffin) | $C_{11}$ to $C_{16}$ | 180 - 260 | Jet fuel, heating oil, lamps |
| Diesel Oil (Gas Oil) | $C_{15}$ to $C_{20}$ | 260 - 350 | Fuel for trucks, trains, some cars, and heating |
| Lubricating Oil / Wax | $C_{20}$ to $C_{30}$ | 350 - 450 | Engine oil, grease, candles |
| Fuel Oil / Bitumen | $C_{30}+$ | > 450 | Ship fuel, asphalt for roads, roofing |
The Science in Your Kitchen: A Simple Experiment
You can model fractional distillation at home with a simple, safe experiment using salt water and food coloring. This won't involve heat or chemicals, but it perfectly demonstrates the principle of separating a mixture based on a physical property.
What you need: A tall glass, warm water, table salt, blue food coloring, and red food coloring.
What to do:
- Fill the glass about 3/4 full with warm water.
- Add a teaspoon of salt and stir until it completely dissolves. The salt represents one component of a mixture (like a small, "low boiling point" hydrocarbon).
- Carefully add a single drop of blue food coloring. It will slowly sink and diffuse, creating a blue layer at the bottom. This represents a second, denser component (like a "medium boiling point" hydrocarbon).
- Even more carefully, add a single drop of red food coloring on top. It should stay mostly as a red layer on the surface. This represents a third, less dense component (like a "high boiling point" hydrocarbon that is actually collected low in the column because it's liquid).
The science connection: You now have a "mixture" of salt, blue dye, and red dye in water. If you could separate them based on their density (just like we separate based on boiling point), you could collect the red layer from the top first, the blue layer from the middle next, and finally evaporate the water to get the salt. In a fractionating column, the "sorting" property is boiling point, not density, but the idea of collecting different parts of a mixture from different levels is the same.
Important Questions
A fraction is a mixture. Fractional distillation does not produce single, pure hydrocarbons like pure octane ($C_8H_{18}$). Instead, it produces groups of hydrocarbons with similar boiling points. For example, the gasoline fraction contains many different molecules from pentane ($C_5H_{12}$) to decane ($C_{10}H_{22}$) and their isomers. The properties of the fraction are an average of the properties of all the molecules within its boiling range.
Smaller hydrocarbon molecules (like those in gasoline) have lower boiling points and are more volatile. This means they vaporize more easily and mix with air to form a combustible mixture that ignites smoothly in an engine. Larger molecules (like those in heavy fuel oil) are less volatile, thicker, and burn less cleanly. They are better suited for applications where controlled, steady heat is needed, like in ship engines or industrial furnaces, or for non-fuel uses like making asphalt.
The residue that does not vaporize in the initial heating is not wasted. It is drawn off from the bottom of the fractionating column. This residue often undergoes vacuum distillation, where pressure is lowered to allow these very large molecules to boil at lower temperatures without breaking down (cracking). From this, we get lubricating oils, waxes, and ultimately bitumen (asphalt), which is used to pave roads and waterproof roofs.
Conclusion
Footnote
1 LPG: Liquefied Petroleum Gas. A mixture of propane ($C_3H_8$) and butane ($C_4H_{10}$) gases, stored under pressure as a liquid. It is a common fuel for heating, cooking, and in vehicles.
2 Isomers: Molecules that have the same chemical formula but different structural arrangements of atoms. For example, octane ($C_8H_{18}$) has many isomers, all with slightly different properties, but they are all collected in the gasoline fraction because their boiling points are similar.