What is Fractional Distillation?

by Lucy Bell-Young

Fractional distillation is the separation of various components from a liquid. Each component typically has a different boiling point and vapour pressure than the rest, so fractional distillation can be achieved by applying a temperature gradient.

The various components are condensed at different stages or levels of the setup, which can either be a small setup consisting of laboratory glassware and other equipment, or a large-scale industrial operation, such as in the oil and gas industry when refining petroleum into different types of fuel. 

Fractional distillation is typically used when there are more than two components in a liquid mixture that need to be separated. The differences in the boiling points of the liquids make it possible to condense the liquids at various points of the fractionating column.

Both small-scale and large-scale laboratory fractionating columns have the same basic designs: The various types of vapours are condensed and collected at various levels corresponding to their volatilities.

The three stages of fractional distillation

As the name suggests, fractional distillation involves the separation of the ‘fractions’ or components of a mixture. Each fractional component has its own boiling point, which is inversely proportional to its volatility. As a result, liquids that evaporate easily have relatively lower boiling points compared to those that don’t. 

The vapour pressure, on the other hand, is directly proportional to its volatility. This means that some vapours will condense at certain temperatures. The lightest and most volatile ones will be collected at the top while the heavier ones will settle at the bottom.

Although fractional distillation can have multiple levels of condensation and very precise temperature gradients, all liquid mixtures go through the same basic phases. The three main stages of fractional distillation are evaporation, condensation and, finally, collection. Below we explore each of these stages in a little more detail.

Step 1: Heating and evaporation

In this initial stage, a homogeneous mixture such as crude oil is gradually heated until it boils. The heat is kept at a constant temperature that corresponds to the highest boiling point of the liquid components at the bottom. 

The temperature gradient is achieved by the gradual drop in temperature as the vapours ascend to the top of the fractional distillation column. Once the liquid mixture evaporates, the vapour is directed to the fractionating column.

Step 2: Separate and condense

The various components of the mixture then separate during the condensation process. As the vapours rise, they condense at different temperatures and different heights in the fractionating column. 

The most volatile component condenses at the top of the column because this is where the temperature is lowest. Conversely, the least volatile and heaviest component condenses at the bottom. All other components are separated proportionally according to the temperatures at which they condense.

Step 3: Collect and Cooldown

Once the distillates condense, they’re collected into separate containers and left to cool. At this point, they become distinct products of the fractional distillation process. 

For example, crude oil is refined into different types of fuel such as kerosene, diesel, and gasoline. Although these separate petroleum products are still mixtures of hydrocarbons, it would be impractical and very difficult to separate them further because their boiling points are so similar.

What is the Purpose of Fractional Distillation?

Fractional distillation is very useful when separating more than two types of liquids from a homogeneous mixture. This method is particularly useful in separating liquids that have less than 25°C difference in boiling points. It can also be used to increase the purity of a particular distillate. During fractional distillation, fractions of the original mixture are evaporated and condensed at different levels as the temperature gradient changes.

Although an ordinary distillation method can also be used to separate mixtures consisting of more than two types of liquids, it would be tedious, time consuming, and impractical to redistill the same liquid mixture repeatedly at different temperatures just to separate the different components.

The elegant solution to this problem is using temperature gradients. This can be done either by using a fractionating column or separate distillation chambers in sequence. Here’s how the sequential distillations process works:

Diagram showing sequential distillations process

For a more in-depth look at this process, check out this YouTube video!

As you can see in the example, the molar purity of benzene distillate can be increased through a series of simple distillation steps. From 50% purity in the first flask, the distillate purity reached 97% in the last flask.

An Example of Fractional Distillation

The most well-known example of fractional distillation is the process of refining crude oil or fossil fuel. Various types of fossil fuels, like kerosene and gasoline, can be extracted from crude oil via the fractional distillation process.

On an industrial scale, a massive metal fractionating column is used. This column has trays inside at various levels that collect the specific type of distilled fuel. See the schematic illustration below for details:

Schematic illustration of how a fractioning tower works

Crude oil flows through a winding pipe that passes through a heating chamber. It’s then allowed to evaporate through the fractionating column. As the vapour ascends, the temperature decreases. This causes the heavier, less volatile components to condense first as the more volatile and lighter components remain in gaseous form until they reach cooler temperatures at the upper levels of the fractionating column. The condensed liquids are collected separately.

What is Fractional Distillation Used to Separate?

Fractional distillation is used to separate out several different liquid components from a mixture. It can be done either through a sequence of simple distillations or by using a fractionating column. Its most common application is the distillation process for crude oil, which refines it into different types of fuel and other petroleum products as the above image shows.

Fractional distillation can also be used for purifying water. Its other industrial uses include deriving highly concentrated and purified silicon from chlorosilanes. The electronics industry, which includes computer and mobile phone manufacturers, are dependent on this process. Without it, high quality semiconductor components would be difficult and expensive to manufacture.

The industrial process of extracting nitrogen, argon, and oxygen from the air also involves the large-scale fractional distillation of air. During this process, air is cooled down to −200°C in order for it to turn into liquid form. It’s then fractionally distilled to extract the various useful components. For example, oxygen can be separated from the mixture at the temperature of −183°C.

Stages of Fractional Distillation

As the name suggests, fractional distillation involves the separation of the “fractions” or components of a mixture. Each fractional component has its own boiling point, which is inversely proportional to its volatility. 

The vapour pressure, on the other hand, is directly proportional to its volatility. This means that some vapours will condense at certain temperatures. The lightest and most volatile ones will be collected at the top while the heavier ones will settle at the bottom.

Fractional distillation has three basic stages, regardless of the type of liquid mixtures that will be distilled. These steps are evaporation, condensation, and finally, collection. These steps happen at various levels of temperature, meaning that the process produces multiple products or distillates at various levels.

  • Step 1: Heat and Evaporate

At this stage, the liquid mixture, such as crude oil, is heated in order to make it evaporate. The vapour is then directed to the fractionating column.

  • Step 2: Separate and Condense

As the vapour rises, the heaviest and least volatile components start to condense. Some return to the bottom while others are condensed at certain levels corresponding to lower temperatures compared to the temperature at the bottom. The more volatile components rise to the topmost portion of the column, where they condense.

  • Step 3: Collect and Cooldown

The distilled fractions are collected and cooled down into separate containers. These are the distinct products of the distillation process.

Fractional distillation vector illustration

What is the Difference Between Distillation and Fractional Distillation?

In terms of the basic principles and basic steps involved, both simple distillation and fractional distillation are very similar. However, the main difference is in the details. The latter is more complex, consisting of sub-steps. The latter also produces multiple distillates as final products.


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