0620 · 11.3Fuels

Petroleum and hydrocarbons

0:00Car fuel, jet fuel, lubricating oil and road surfaces can all begin with the same raw material. That material is petroleum, often called crude oil. Its usefulness comes from containing many different hydrocarbons rather than one pure substance. By the end of this video, you will understand how these substances are separated and why the resulting fractions have different properties. Coal, natural gas and petroleum are the fossil fuels. They formed from ancient organic material over very long periods, so they are not replaced quickly when used.

0:30The main constituent of natural gas is methane. Natural gas is therefore mainly one particular hydrocarbon, whereas petroleum contains a mixture of many hydrocarbons. Hydrocarbons are compounds that contain hydrogen and carbon only. The word only is essential because a compound containing oxygen as well would not fit this definition. Different hydrocarbons contain different numbers of atoms arranged in different ways. Here, the important difference is the length of their carbon chains.

0:58In a carbon chain, carbon atoms are joined together within a molecule, with hydrogen atoms attached. Methane contains just one carbon atom, while propane has a chain of three carbon atoms. Petroleum contains hydrocarbons with a wide range of molecular sizes. This variation is the reason its components do not all boil at the same temperature. Before explaining the separation, remember the difference between boiling and condensation. Boiling changes a liquid into a gas, while condensation changes a gas into a liquid.

1:28Neither process changes the identity of the substance. A hydrocarbon molecule remains the same molecule when it passes between liquid and vapour. A mixture can contain substances with different boiling points. Heating it turns the components with lower boiling points into vapour most easily. Cooling those vapours can then turn different components back into liquids at different temperatures. Fractional distillation uses these differences to separate petroleum into useful groups called fractions.

1:55A fraction is a mixture of hydrocarbons with similar boiling points. It is not normally a single pure compound. Because molecular size affects boiling behaviour, the hydrocarbons within a fraction also have broadly similar chain lengths. Separating petroleum into fractions therefore sorts its contents into groups with similar properties.

Fractional distillation process

2:15Petroleum is first heated strongly in a furnace so that much of it vaporises. The hot mixture enters the lower part of a fractionating column. This column is hotter at the bottom and cooler at the top. The temperature difference allows hydrocarbons with different boiling points to be collected at different heights. As vapours rise, they enter cooler regions of the column. Hydrocarbons with higher boiling points condense in the hotter lower regions.

2:38Those with lower boiling points remain as vapour for longer and travel further upwards. They condense higher in the column, where the temperature is lower. Imagine following a hydrocarbon vapour with a relatively high boiling point. It does not need to reach the cool upper section before becoming liquid. Instead, it condenses lower down and is collected there. A hydrocarbon with a much lower boiling point stays gaseous through that region and continues upwards.

3:02The smallest hydrocarbons leave the top as the refinery gas fraction. At the other end, the heaviest material remains as residue at the bottom. Between these extremes, liquid fractions are collected through outlets at different levels. The column separates the original mixture rather than making new hydrocarbons. The separation works because of boiling points, not because heavy molecules simply fall while light molecules float. Position in the column reflects where each group condenses.

3:30The temperature gradient is therefore essential to the explanation. Without different temperatures at different heights, the fractions would not condense at separate levels.

Trends in fraction properties

3:38Moving from the bottom to the top, the fractions have decreasing chain length. Their boiling points become lower, and their volatility becomes higher. At the same time, their viscosity becomes lower. These are connected trends, rather than four unrelated facts to memorise. Volatility describes how readily a substance vaporises. A highly volatile liquid enters the gas state easily, so it generally has a low boiling point. Petrol is more volatile than the heavier liquid fractions collected below it.

4:07Higher volatility does not mean that a substance needs a higher temperature to boil. Viscosity describes a fluid’s resistance to flowing. A liquid with high viscosity flows less easily, rather like thick syrup compared with water. Fractions nearer the bottom are generally more viscous than those nearer the top. This explains why lubricating oil feels thicker and flows less readily than petrol. The chain length connection comes from attractions between molecules.

4:33Larger hydrocarbon molecules generally experience stronger attractions, so more energy is needed to separate them into a gas. Their boiling points are therefore higher. Longer chains also tend to move past one another less easily, helping explain their greater viscosity. Boiling overcomes attractions between separate molecules; it does not break the carbon chains inside them. This distinction explains why fractional distillation is a physical separation. The hydrocarbons are sorted according to their properties but remain chemically unchanged.

5:02The explanation needs molecular attractions, not a claim that carbon bonds are broken.

Fractions and their uses

5:07The fraction names follow a useful order down the column. At the top is refinery gas, followed by gasoline, also called petrol. Below these come naphtha, then kerosene, also called paraffin. Each lower position represents hydrocarbons with generally longer chains and higher boiling points than the fraction above. Continuing downwards, diesel oil is also called gas oil. Beneath it come fuel oil and then lubricating oil. Bitumen is collected at the bottom as the heavy residue.

5:35Keeping this order connected to the property trends makes the diagram easier to interpret. Refinery gas is used as gas for heating and cooking. Its position at the top fits its very low boiling behaviour. These small hydrocarbons remain gaseous after passing through the column. Do not confuse the fraction’s name with natural gas, whose main constituent is methane. The gasoline or petrol fraction provides fuel used in cars. Its relatively high volatility helps it form vapour that can mix with air inside an engine.

6:05This is a useful connection between a fraction’s properties and its application. For the required use, state that petrol is fuel for cars. Naphtha is used as a chemical feedstock. A feedstock is a raw material supplied to a chemical manufacturing process. This means petroleum is useful for making chemicals, not just for providing fuels. Naming every fraction as something that is burned would therefore miss an important distinction. Kerosene, also called paraffin, is used as jet fuel.

6:33The diesel oil or gas oil fraction provides fuel for diesel engines. Although both are fuels, they belong to different parts of the column. Keep each fraction linked to its specific use rather than describing both simply as engine fuel. Fuel oil is used as fuel in ships and home heating systems. The lubricating oil fraction is used for lubricants, waxes and polishes. Lubricants help moving surfaces slide past one another with less friction.

6:59These uses show that a fraction’s value is not always based on burning it. Bitumen is used for making roads. As the heavy bottom fraction, it is associated with long hydrocarbon chains and very high viscosity. Its thick, sticky character suits its role in binding road materials together. The contrast with the gaseous top fraction shows how varied petroleum’s components can be.

Comparisons and exam answers

7:21Let us work through a comparison using the column rather than a memorised list. Suppose a question asks you to compare petrol with lubricating oil. Begin by locating both fractions in the correct order. Petrol is higher in the column, so we apply the bottom to top trends in that direction. Petrol contains hydrocarbons with generally shorter chains than those in lubricating oil. Its boiling points are lower because the attractions between its smaller molecules are generally weaker.

7:47It has higher volatility, meaning it vaporises more readily. Petrol also has lower viscosity, so it flows more easily. The method matters as much as the answer. First identify which fraction is higher, then connect that position to the four property changes. If the question compares lubricating oil with petrol, reverse the wording of the comparison. Longer chains, higher boiling points, lower volatility and higher viscosity then describe lubricating oil.

8:15Several common mistakes come from treating related ideas as interchangeable. A fraction is not pure simply because it has one name. Petrol, for example, contains a mixture of hydrocarbons with similar boiling points. Likewise, petroleum is not one giant molecule that gets divided into smaller pieces during distillation. Another mistake is reversing the column’s temperature gradient. The bottom is hotter, while the top is cooler. Also keep high volatility separate from high viscosity, because these describe different behaviours.

8:43A fraction can vaporise readily and flow easily, giving high volatility but low viscosity. An examination question may ask you to describe how petroleum is separated into useful fractions. The command word describe calls for the process in a clear sequence. Start with heating, then explain the column and condensation. Your answer should show how different boiling points produce collection at different heights. Petroleum is heated so that much of it vaporises.

9:09The vapours enter a fractionating column that is hotter at the bottom and cooler at the top. Different hydrocarbons condense at different heights because they have different boiling points. The resulting fractions are collected separately. Other questions may ask you to state a definition or name a use. Hydrocarbons are compounds that contain hydrogen and carbon only. Naphtha is a chemical feedstock, and kerosene is used as jet fuel. When asked to describe changes from bottom to top, give the direction of every trend explicitly.

9:39The central idea is that petroleum is a mixture whose hydrocarbons have different boiling points. Fractional distillation separates them in a column with a hot bottom and a cooler top. Moving upwards means shorter chains, higher volatility, lower boiling points and lower viscosity. These relationships explain both where fractions are collected and why they have different useful applications.