Lesson Video: Fractional Distillation of Crude Oil Chemistry

In this video, we will learn about the separation of crude oil into fractions, by the process of fractional distillation. We’ll look at the composition and properties of each fraction and discuss their common uses.


Video Transcript

In this video, we will learn about the separation of crude oil into separate fractions by the process of fractional distillation. We’ll look at each fraction, their composition and properties, and discuss their common uses.

Crude oil is unrefined, naturally occurring liquid rich in valuable hydrocarbons. Because of its origins, it’s often referred to as a fossil fuel. Crude oil is pumped, or pushed out, from natural reservoirs in colossal quantities and processed into many useful products. Depending on where it is found, the composition of crude oil varies. It can be any color from yellow to black, and it could be very thick or quite runny. Crude oil is a limited resource on Earth and was formed over thousands to millions of years from the bodies of small aquatic organisms put under high pressure and temperature.

Crude oil contains a range of hydrocarbons, linear and branched alkanes, alkenes, cyclic alkanes, aromatics, and so on. But in this video, we’re going to focus mainly on the alkanes. Some of these chemicals are extremely small containing fewer than five carbon atoms per molecule, while others are quite large containing many tens of carbon atoms per molecule. Crude oil on its own is not especially useful. It tends to burn with a sooty flame. And it has lots of contaminants that cause problems in machinery. In order to make it more useful, it has to be purified and separated.

Crude oil can contain thousands of chemicals. It’s too expensive to separate each pure chemical out from the mixture. Instead, it’s separated into different mixtures called fractions. Fractions contain a mixture of chemicals with similar boiling points. The precise composition of a fraction can change from batch to batch, but the overall behavior will be similar. The cheapest way to separate crude oil is by heating it and separating the chemicals by their boiling points.

This is a schematic of how crude oil is separated. In the first step, crude oil is heated in a furnace to about 400 degrees Celsius. What comes out of the furnace is a mixture of liquid and gas. It passes from the furnace into the fractional distillation column, otherwise known as a fractionating column. In practice, this column can be tens of meters tall. Anything still a liquid stays at the bottom, while anything that’s a gas begins to rise.

This liquid mixture at the bottom is called residue. It can be passed to another fractionating column, which uses even higher temperatures to fractionate it further. These fractions go on to be used as fuel oil, tar, asphalt, and so forth. Back in the first fractionating column, the bottom is constantly heated by the injection of very very hot steam. This stops any of the gases condensing too soon. But the higher up the column you go, the cooler it gets. Right at the top, the temperature is about 40 degrees Celsius. As the vapors rise, the chemicals with higher boiling points condense in trays. Devices called bubble caps in the trays help the vapors to mix with the liquid in the trays, keeping the temperature even and helping the fractions to separate.

Each tray contains a unique fraction. Above the residue is lubricating oil. Lubricating oil consists of hydrocarbons with roughly 20 to 50 carbon atoms with boiling points between 300 and 370 degrees Celsius. Above lubricating oil is diesel. Diesel consists of hydrocarbons with roughly 9 to 25 carbon atoms with boiling points between 250 and 350 degrees Celsius. Above diesel is kerosene. Kerosene consists of hydrocarbons with roughly 10 to 16 carbon atoms with boiling points between 175 and 325 degrees Celsius.

Above kerosene is gasoline. Gasoline consists of hydrocarbons with roughly 4 to 12 carbon atoms with boiling points between 40 and 205 degrees Celsius. Above gasoline is naphtha. Naphtha consists of hydrocarbons with roughly 5 to 9 carbon atoms with boiling points between 40 and 100 degrees Celsius. Naphtha is the last of the liquid fractions. Anything that is still a gas at the top of the column, which is around 40 degrees Celsius, is called petroleum gas. Petroleum gas consists of hydrocarbons with roughly 1 to 4 carbon atoms with boiling points around or below 40 degrees Celsius.

The fractions and their names are not standard around the world. Some people use different names or use the same names but for different fractions. The key thing to understand is that fractions boil at lower temperatures as you go up the column. Since crude oil fractions mostly consist of hydrocarbons, we can take a good guess at some of their properties. The general trend is that the bigger the hydrocarbon, the greater the number of carbon atoms per molecule, the greater its boiling point and viscosity. However, we also see a lower volatility and a lower flammability. The same trend appears with crude oil fractions.

Let’s have a look at the data for all the fractions. As each fraction is a mixture, we’ll see that the number of carbon atoms for the chemicals inside can vary. The boiling points of those chemicals, when pure, also fit within a broad range, but some of them overlap. The same chemical can appear in different fractions in different amounts. The properties of a fraction are a very complicated combination of all the properties of the components. But we can make some broad simplifications. As the average number of carbon atoms per molecule in each fraction increases, the boiling point will increase, as will the viscosity. And moving in the other direction, as we move to fractions with smaller molecules, they’ll have higher volatilities and higher flamabilities.

The combination of properties for each fraction is what makes a fraction good or bad for a particular application. Each fraction of crude oil is tailored to a particular application. Some aren’t that useful, so they’re converted to other products. On the whole, though, each faction has at least a few valuable applications. Petroleum gas is used for heating and cooking. It’s also processed into other chemicals that are made into plastics. Naphtha can be used as a fuel, but it’s generally turned into gasoline in a process called reforming or into petroleum gas in a process called cracking.

Gasoline is predominantly used as a fuel in automobiles. And kerosene is used in jet engines and tractors and is processed into other useful chemicals. Diesel is used as a fuel for diesel engines and as heating oil. It’s also processed into other useful chemicals. Lubricating oil is used in motor oils and greases. Fuel oil is an industrial fuel and is processed into other useful chemicals, which is the same for tar, which is also used for road surfaces.

It’s not often that you’ll need to remember all the details of the fractions of crude oil, just the names, the overall order, and some of their applications. There’s a trick to remembering the order going from the fractions with the lowest boiling point to the highest boiling point. Playful naughty gray kittens dance like ferocious tigers, petroleum gas, naphtha, gasoline, kerosene, diesel, lubricating oil, fuel oil, and tar. Now that we’ve looked at the process of fractional distillation of crude oil, the individual fractions, and their properties, and applications, let’s have some practice.

In which application is kerosene used as a primary fuel? A) Aircraft. B) Power stations. C) Cars. D) Domestic heating. Or E) trains.

Kerosene is the name of a fraction of crude oil. It sits between gasoline and diesel. In the fractionating column, diesel comes out earlier and gasoline comes out a little later. Kerosene is a very popular fuel for jet engines and for tractors. Now, we can go through the applications and see which one applies. Jets are a form of aircraft with jet engines, so we would expect kerosene to be used as a primary fuel in this case. So, we’ve already found our answer. Kerosenes are used as a primary fuel in some aircraft, but let’s have a look at the other options just in case.

Power stations tend to use coal much more than thay’d use liquid fuels. Liquid fuels are easier to transport and much better for small mobile applications. Coal, on the other hand, is cheaper and much better for mass energy production. So, B is not a correct answer. Cars are a form of automobile and are more likely to be using gasoline than other types of fuel. There are, however, diesel-powered cars, but those would use diesel and not kerosene.

The fuels in domestic heating will tend to be petroleum gas, natural gas, or wood. While kerosene is used for cooking on mobile stoves, it doesn’t tend to be used for heating. And lastly, trains, modern trains are mostly electric, but historically they used coal rather than oil or oil based products. So, kerosene would not be used as a primary fuel in this application. So, the answer to which application is kerosene used as a primary fuel is aircraft.

So, that was a question about the application of a fraction of crude oil. Now, let’s have a look at one that focuses on properties.

In which of the following are crude oil fractions arranged in order of increasing viscosity? A) Kerosene, diesel oil, gasoline. B) Diesel oil, gasoline, kerosene. C) Gasoline, kerosene, diesel oil. D) Kerosene, gasoline, diesel oil. Or E) gasoline, diesel oil, kerosene.

Crude oil fractions come from fractional distillation. The chemicals in crude oil, which is a huge mixture of different chemicals, are separated broadly by boiling point. Crude oil fractions are actually mixtures of many different chemicals, but the fraction as a whole will have a pretty reliable boiling point. You can easily remember the fractions of crude oil in order through the mnemonic playful naughty gray kittens dance like ferocious tigers. This puts the fractions in order from the lowest boiling point to the highest boiling point. The full names of the fractions are petroleum gas, naphtha, gasoline, kerosene, diesel, lubricating oil, fuel oil, and tar.

The three we’ll need for this question are gasoline, kerosene and diesel. Our job is to put these three fractions in order of their viscosity. We already know that moving down the list, the boiling point of the fraction increases. This is because, as we go down the list, the average size of hydrocarbon in each fraction tends to increase. Another key property of a bigger hydrocarbon is that it will have a higher viscosity. This is because as the hydrocarbons get bigger, they tend to entangle around one another and stick to each other more. This makes them thicker, more like syrup, less like water.

So, gasoline is less viscous than kerosene. And kerosene is less viscous than diesel. This means our answer is C. Gasoline, kerosene, diesel oil is the correct order of increasing viscosity for these three crude oil fractions.

It may interest you to know that gasoline is actually less viscous than water, while kerosene and diesel sit between water and linseed oil. Viscosity is something you can actually measure, so these are the numbers. The units poise are something you don’t really need to worry about. Just think about the relative values. There’s a very clear jump in viscosity between gasoline and kerosene, and kerosene and diesel. So, our answer is definitely gasoline less than kerosene less than diesel in terms of their viscosity.

Now we’ve looked at crude oil and done a few questions, let’s look at the key points. Crude oil is a natural fossil fuel made mostly of valuable hydrocarbons. Crude oil is separated into smaller mixtures called fractions by fractional distillation. Fractional distillation uses heating and a temperature gradient to separate chemicals by boiling point. Fractions with higher boiling points are more viscous, less volatile, and less flammable.

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