distillation

Chemistry

2022

We explain what distillation is, examples of this separation method and the types of distillation that can be used.

Distillation uses vaporization and condensation to separate mixtures.

What is distillation?

Distillation is called a method of phase separation, which is among the methods of separation of mixtures. Distillation consists of the consecutive and controlled use of two physical processes: the vaporization and the condensation, using them selectively to separate the components of a mixture usually of a homogeneous type, that is, in which its components cannot be distinguished with the naked eye.

Mixtures that can be separated into their individual components using distillation can contain two liquids, a solid in a liquid or even gases smoothies. This method of separation is based on the difference in boiling points (an inherent property of the matter, what is the temperature to which the Pressure vapor of a liquid equals the pressure surrounding the liquid) of different substances. The substance with the lowest Boiling point have, then this substance will condense in another container, and it will be relatively pure.

In this way, for the distillation to be carried out correctly, we must boil the mixture until it reaches the boiling point of one of the substances components, which will then become vapor and can be conducted to a cooled container, where it condenses and becomes liquid again.

The other constituent substance, on the other hand, will remain in the container without alterations; but in both cases we will have pure substances, free from the initial mix.

Raoult's Law

When we have an ideal mixture of liquids (a mixture in which the interactions between the particles different are considered equal to the interactions between equal particles) the Law of Raoult is fulfilled.

This law states that the partial vapor pressure of each component in the gas mixture is equal to the vapor pressure of the pure component multiplied by its mole fraction in the liquid mixture.

The total vapor pressure is, then, the sum of the partial pressures of the components of the mixture in the gas phase. On the other hand, the mole fraction of a component in a mixture is a dimensionless measure of its concentration. The magnitudes mentioned above can be calculated using the following equations:

Where:

  •   px Y py are the partial pressures of the components x and Y respectively in the mixture of vapors surrounding the liquid mixture.
  • px * and py * are the vapor pressures of the components x and Y.
  • xx Y xy are the mole fractions of the components x and Y in the liquid mixture.
  • nx Y ny are the amount of substance of the components x and Y in the liquid mixture.

Raoult's Law raised above is valid for ideal mixtures (which are a model established by the human being to simplify studies), but in reality this law suffers deviations when the mixture is real.

Thus, if the different particles in the mixture have stronger intermolecular forces than the particles in the pure liquid, then the vapor pressure of the mixture is less than the vapor pressure of the pure liquid, which produces a negative deviation from the Law by Raoult.

If, on the other hand, the intermolecular forces between the particles in the pure liquid are greater than those of the particles in the mixture, the particles in the mixture will be able to escape to the vapor phase more easily, so that the vapor pressure of the mixture will be greater, producing a positive deviation from Raoult's law

When you want to distill an azeotropic mixture (for example, ethanol and water), it is necessary to add some component (benzene, in this case) so that the azeotrope is modified and in this way to be able to separate the components of the mixture. An azeotrope is a liquid mixture with a defined composition that when it boils, the vapors generated have the same composition as the mixture (so the components of the azeotropic mixture cannot be separated by simple or fractional distillation).

Azeotropes have a defined boiling temperature, for example, at a pressure of 1 atm, ethanol boils at 78.37 ° C and water boils at 100 ° C, while ethanol-water azeotrope boils at 78.2 ° C. Azeotropic mixtures have negative or positive deviations from Raoult's Law as the case may be.

Types of distillation

Distillation can occur in different ways:

  • Simple distillation. The most elementary consists of boiling the mixture until the different components are separated. It is an effective separation method when the boiling points of the components of the mixture differ greatly, (ideally they should have a difference of at least 25 ° C, otherwise it does not guarantee the total purity of the distilled substance).

  • Fractional distillation. It is carried out by means of a fractionation column, which is made up of different plates in which vaporization and condensation take place successively, guaranteeing greater purity in the separated components.

  • Vacuum distillation. By reducing the pressure until a vacuum is generated, the process is catalyzed to reduce the boiling point of the components, since some have very high boiling points that can be reduced when the pressure is greatly reduced and thus accelerate the process of distillation.
  • Azeotropic distillation. It is the distillation necessary to break an azeotrope, that is, a mixture whose substances behave as one, even sharing the boiling point, so they cannot be separated by simple or fractional distillation. To separate an azeotropic mixture it is necessary to modify the mixing conditions, for example by adding some separating component.
  • Steam distillation. The volatile and non-volatile components of a mixture are separated by direct injection of water vapor.
  • Dry distillation. It consists of heating solid materials without the presence of solvents liquids, obtain gases and then condense them in another container.
  • Improved distillation. Also called alternate or reactive distillation, it is adapted to the specific cases of mixtures that are difficult to separate or that have the same boiling point.

Examples of distillation

Coal uses dry distillation to obtain liquid organic fuels.
  • Oil refining. The separation of the various hydrocarbons present in the Petroleum It is carried out by fractional distillation, storing in different layers or separate compartments of a distillation column each compound derived from the cooking of crude oil. These gases rise and condense in the upper layers of the column, while denser substances such as asphalt and paraffin remain in the lower layers.
  • Catalytic cracking. This is the name given to certain usual vacuum distillations in oil processing, using vacuum towers to separate the gases from the crude cooking. Thus the boiling of the hydrocarbons is accelerated and the process. Cracking is a type of destructive distillation, in which larger hydrocarbons are broken (at high temperatures and using catalysts) into smaller hydrocarbons, which have lower boiling points.
  • Ethanol purification. To separate alcohols Like ethanol from water during its production in the laboratory, azeotropic distillation is used, adding to the mixture benzene or other components that promote or accelerate the separation, and that can then be easily removed without altering the chemical composition of the product.
  • Coal processing. To obtain liquid organic fuels, coal or wood are used by means of dry distillation procedures, thus the gases emitted in the combustion.
  • Thermolysis of mineral salts. By means of dry distillation, various mineral substances of high industrial utility are obtained, from the emanation and condensation of gases obtained by burning mineral salts.
  • The alembic. This is the name given to the device invented in Arab antiquity, whose purpose is to produce perfumes, medicines and alcohol from fermented fruits. In its operation, the principles of distillation are used: substances are heated in a small boiler and the gases produced are cooled in a coil that leads to another container where the liquid produced by the condensation of said gases is collected.
  • Perfume production. Steam distillation is used to obtain perfumes, boiling water together with preserved flowers, to produce a gas with the desired odor and which, when condensed, can then be used as a base liquid in perfumes.
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