Fractional distillation

We can sort a mix of liquids.

First, we heat the mix. This makes the liquid turn to gas.

Some parts of the mix turn to gas easily. Other parts do not. The gas rises up a tall tube.

The tube is hot at the bottom. It is cool at the top. This helps the gas turn back into liquid.

This process helps us clean crude oil. We use it to make fuel.

It is a very useful way to sort things.

We can sort a mix of liquids into parts. This way is called fractional distillation.

It works by using heat. We heat a liquid mix in a flask. This makes the liquid turn into a gas.

The gas rises up a tall tube. This tube is a fractionating column. The bottom of the tube is very hot. The top of the tube is much cooler. This change in heat is called a temperature gradient.

As the gas rises, it hits the cool parts. Some gas turns back into liquid. This liquid drips down onto small shelves called trays.

Each part of the mix has a boiling point. This is the heat needed to make it a gas. Parts that boil easily rise to the very top. Parts that need more heat stay near the bottom.

Factories use giant towers for this work. These towers help sort crude oil. They make useful things like fuel.

This helps us get the parts we need.

Fractional distillation is a way to separate a mixture into its different parts, which are called fractions.

This method works by heating a mixture until one or more parts turn into a gas, or vapor. It is most useful when the different parts have boiling points that are close together. If the boiling points differ by less than 25 °C, fractional distillation is the best choice. If the difference is larger than 25 °C, scientists usually use a simpler method. This process is very important for making many things we use every day.

To see how it works, imagine a tall tube called a fractionating column.

A heat source sits at the bottom of a flask containing the mixture. As the vapor rises up the column, it moves through a temperature gradient. This means the bottom is very hot, but it gets cooler toward the top. The vapor hits small shelves inside the column called trays. When the vapor hits a cooler tray, it condenses back into a liquid. This liquid then drips back down, a process called reflux. Each time the vapor condenses and turns back into gas, it becomes more pure.

People have used this way of separating things for a very long time. In the 9th century, an alchemist named Jabir ibn Hayyan used it with plants and animals.

His work was later translated into Latin by Gerard of Cremona around the year 1114. Another famous work, the "Book of Seventy," also described these early experiments. These old studies helped scientists understand how to work with different substances. This history shows how important separating mixtures has been for hundreds of years.

In a laboratory, scientists use specific tools to do this work safely. They use a round-bottomed flask and a condenser to turn the gas back into liquid.

They often add small granules to the flask to stop the liquid from bubbling too wildly. In big factories, the setup looks very different and much larger. Industrial distillation towers can be huge cylinders. These towers can be over 60 meters tall and several meters wide. They often work in a steady state, meaning new liquid is added as fast as products are removed.

We can see this science in action at oil refineries.

Refineries use giant towers to separate crude oil into useful parts like fuel. The "lightest" parts with the lowest boiling points come out of the top. The "heavier" parts with the highest boiling points come out of the bottom.

This is also how we get pure gases like oxygen and nitrogen from the air. It is a way to take a messy mixture and turn it into many clean, useful things.

Fractional distillation is a chemical process used to separate a mixture into its individual components, which are known as fractions.

This technique is essential when the different substances in a mixture have boiling points that are very close to one another. Specifically, fractional distillation is typically used when the boiling points of the components differ by less than 25 °C (45 °F) under one atmosphere of pressure. If the difference between boiling points is greater than 25 °C, scientists generally opt for a simpler method called simple distillation. This process allows for the precise isolation of substances, making it vital for industries ranging from fuel production to air separation.

The mechanism of fractional distillation relies on a temperature gradient within a specialized piece of equipment called a fractionating column.

In a laboratory setting, the mixture is placed in a round-bottomed flask and heated by a source like a heating mantle. As the liquid heats, it begins to vaporize. The vapor rises into the fractionating column, which is hottest at the bottom and becomes progressively cooler toward the top. As the mixed vapor ascends this temperature gradient, it undergoes a repeating cycle of condensation and vaporization. Each time the vapor condenses on a surface and then re-evaporates, the concentration of the more volatile component—the substance with the lower boiling point—increases in the gas. Eventually, the vapor reaching the top of the column consists almost entirely of the most volatile component.

Inside the column, the physical structure plays a key role in the separation efficiency. Many columns contain internal platforms called trays where the vapor can condense into liquid. This liquid then flows back down the column, a process known as reflux.

At steady-state conditions, the vapor and the liquid on each tray reach a state of equilibrium. To improve the efficiency of this process, the column can be insulated with materials like wool, aluminum foil, or a vacuum jacket. In some laboratory setups, the column is a simple glass tube filled with packing material, such as small glass helices. The more trays or packing surfaces available, the more pure the final separation will be.

The history of this science stretches back many centuries to the work of early alchemists. In the 9th century, the Islamic alchemist Jabir ibn Hayyan performed experiments using fractional distillation on animal, vegetable, and mineral substances.

His findings were recorded in works like "The Book of Seventy," which was later translated into Latin by Gerard of Cremona between 1114 and 1187. These early alchemical texts became foundational resources for later scholars, such as Roger Bacon. These historical experiments laid the groundwork for the complex chemical engineering used in modern laboratories and factories today.

In industrial settings, fractional distillation is scaled up into massive structures called distillation or fractionation towers.

These vertical cylindrical columns are significantly larger than laboratory equipment, often reaching heights of 60 meters or more and diameters of several meters. Unlike batch processes used in labs, industrial towers often operate in a continuous, steady-state mode. This means that new feed material is constantly being added to the column while the separated products are simultaneously removed. To maintain efficiency, industries use reflux, where a portion of the condensed overhead product is returned to the top of the tower. This downward flow of liquid provides the necessary cooling to condense the rising vapors.

One of the most notable applications of this technology is in petroleum refineries.

Crude oil is a complex mixture of many different hydrocarbons. By using fractional distillation, refineries can separate this crude oil into useful fractions based on their boiling points. The "lightest" products, which have the lowest boiling points, exit from the top of the tower. The "heaviest" products, which have the highest boiling points, exit from the bottom. These heavier fractions typically contain more carbon atoms, have higher molecular weights, and are more viscous. This process is also used in air separation plants to produce high-purity liquid oxygen, liquid nitrogen, and argon.

While highly effective, fractional distillation has certain physical limitations. Some mixtures cannot be completely purified using this method alone. A classic example is a mixture of ethanol and water. Ethanol boils at 78.37 °C, while water boils at 100 °C. However, these two substances can form an azeotrope, which is a specific mixture that boils at a constant temperature of 78.15 °C. When this azeotrope forms, the mixture behaves like a single substance, making it nearly impossible to separate them further through standard fractional distillation. In such cases, scientists must use specialized methods like azeotropic distillation to achieve higher purity.