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Fractional distillation: process, equipment, applications, examples

In this sense, the fractional distillation method involves the evaporation of the liquid species, the condensation of the most volatile species in increasing order of their boiling point and the subsequent collection of the substance that was initially desired to be obtained.

Fractional distillation
Fractional Distillation Laboratory Equipment

It is a method used for centuries in human civilizations in a rudimentary way. The efficiency of the distillation allows it to continue to be used today in both industrial and laboratory settings.

Fractional distillation process

Heating

Fractional distillation consists of separating a solution into its constituents that are in the liquid state , based on the difference between their boiling points and being applied when this difference is less than approximately 25 ° C.

First liquid boiling

In this way, when a mixture whose boiling points differ considerably is placed under heating, when the boiling temperature of the most volatile component is reached, a vapor phase will form, which will mostly contain this substance at the beginning.

Second liquid boiling

Then, as the temperature continues to increase and over time, multiple cycles of evaporation and condensation continually occur (each cycle is known as a “theoretical plate”), until the constituent with the lowest boiling point emerges first.

In each cycle, the constitution of the vapor phase found in the column accumulates a larger quantity of the component with the highest volatility, which is why this substance is essentially in its pure state when it reaches the top of the fractionation column.

Flask

In laboratories, equipment is used that consists in the first place of a flask or distillation flask made of glass, in which the solution is placed, which is placed directly under heating. A few boiling stones are placed inside this balloon to control this process.

Thermometer

In addition, a thermometer is required to record the temperature over time, so that the distillation process can be under control.

Fractionation column

The flask is attached to a fractionating column via a three-neck connector, where the length of the column determines how complete the distillation will be. That is, the longer the column, the more effective the separation will be.

Likewise, the internal structure of the column is designed to simulate several successive simple distillations, which happens because the vapor gradually rises up the column, temporarily condenses at the top and rises repeatedly.

Condenser

Subsequently, the outlet of this column is connected to a condenser that causes the vapor of the separated and purified substance to cool down.

This is stored in a specific container to collect it, increasing the temperature again until reaching the boiling point of the next component, the second most volatile, repeating the process described, in which each component is stored in a specific container for it.

Applications

Being one of the most important and widely used methods in the separation of liquid mixtures, this physical separation technique has many benefits that are observed in the large number of applications that have been given both in industry and in the laboratory.

– Starting with the industrial-scale uses of fractional distillation, in oil refining facilities it is used to separate crude oil into its component fractions. 

In this sense, it is used to obtain and process the natural gas that is extracted in these industrial processes. In addition, it is used in chemical plants and petrochemical processes for the processing of substances such as phenol or formaldehyde.

– It is used in cryogenic air separation plants to decompose the air in the atmosphere into its main constituents.

– Fractional distillation is used for desalination of seawater.

– On a laboratory scale, it is used in the purification of reagents and products, such as in obtaining cyclopentadiene through the distillation of commercial dicyclopentadiene.

– It is used to recycle solvents that have already been used, through purification through this technique.

Examples

Fractional distillation of petroleum

In the case of oil, fractional distillation is carried out in equipment of enormous dimensions, called distillation towers, which emulate fractionation columns and are specifically designed for the separation of crude oil in different cuts or streams according to its range. boiling.

This boiling range refers to the range of the boiling points of each separated fraction, since they are mixtures of hydrocarbons with different components and, therefore, have different boiling points.

Before entering the distillation tower, the crude oil is heated to a temperature of approximately 400 ° C, to vaporize this substance and it is separated in the column in increasing order of its boiling range.

In this way, the most volatile cuts such as gases (propane, butane…), gasoline and naphtha are in the upper part of the column, and as it descends, the “heavier” streams are found, such as lubricants or residual components.

Some fractions extracted from the tower (such as gasoline) are added and improved for later commercialization; other cuts such as diesel are used as feed or fuel for other processes within the industry.

Other streams such as residual substances are introduced into other processes that separate them into their constituents and give them other uses, or their commercial value is increased.

Fractional distillation of LPG

When liquefied petroleum gas (LPG) is associated with natural gas, the most common process for its extraction is fractional distillation. 

This is because LPG, being in a gaseous state , behaves like a component with lower vapor pressure and, therefore, has higher boiling points.

This makes it easier for the process by fractional distillation to separate the dry natural gas from the LPG, as well as the rest of the hydrocarbons or liquids that are associated with it.

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