Distillation

Distillation is the phenomenon of separation of mixtures into the individual components at their boiling points. Distillation is almost widely used method in chemical, petrochemical and other process industries, and is generally used to carry out the separation of those mixtures, which are impossible from other separation methods like the separation of fractions from crude oil, distillation of salt water, separating components of air, distillation of alcohols etc.

The distillation method is based on the difference in composition between the liquid mixture and the vapors formed from this mixture. However there is also a specific type of the distillation, known as “Azeotropic distillation”. In chemical engineering, the azeotropic distillation is the one, in which the vapors and liquids have exactly the same composition at equilibrium. The most common example of the Azeotropic distillation is the ethanol-water system. Experimentally the composition of vapor and liquids at the equilibrium is determined by the vapor-liquid equilibrium, and the results are expressed as the temperature-composition diagram.

There are many types of distillation based on the application criteria and the number of components in the mixture. Some important types for a chemical engineer to know are described as under:

1)     Differential Distillation

It is the most elementary form of the batch distillation, in which the liquids mixture is brought to the boiling temperature of the low volatile liquid. The lower volatile liquid is vaporized and condensed. So the mixture is separated as the two components. This method is also carried out for a single liquid having impurities. I.e. it utilizes single equilibrium stage to obtain separation. But this is the single stage process, so complete separation is impossible in this system, so this system is used for the processes where high purification is not needed.

2)     Flash Distillation

It is also named as the equilibrium distillation. This is a single stage continuous process, in which the liquid mixture is partially vaporized, so the produced vapors are in equilibrium with the residual liquid. The process is carried out in such a way, that the feed stream is pumped through heater to the separator through a valve, where the pressure is reduced. So the liquid and vapors produced due to the reduction reaches to the equilibrium. Then the vapors are removed from the top of the separator, while the liquid is collected from the bottom.

3)     Rectification

The rectification is also named as the fractional distillation. It is undoubtedly the most important distillation method, as in rectification, art of the separated vapors are condensed and returned back to the separator, to achieve highest purification. Now, how does fractional distillation work? The fractionating column is the special column used for this type of distillation having number of plates. The solution at the bottom is heated, and the vapors rises in the column, to the first plate, cools and condensed on the first plate, then the vapors from lower rises to the first plate, heats the liquid of this plate, whose vapors condensed at the second plate, and this vaporization-condensation cycle goes on, giving the high purity components. But the above describes fractionating column is an ideal column, in real does not occur at the same exact position, that’s why the concept of theoretical plate occurs.

4)     Azeotropic Distillation

As described above, this is the distillation in which the vapors and liquids have the same composition at equilibrium. The water-Ethanol mixture is the perfect example of the azeotropic distillation. If this water-ethanol mixture is distilled, then the alchohol concentration increases in vapors, until it reaches 96% by mass, i.e. the concentration of vapors and liquid is same. In process engineering, the azeotropic distillation is handled by adding a new element to the mixture (termed as entrainer in the case of azeotropic distillation), which can increase the relative volatility of the two main components. In actual, the third added component makes an azeotrope with one or more components of the mixture. Benzene is added to the water-Ethanol mixture as the third component, and makes a ternary azeotrope with a boiling point lesser then that of the binary mixture.

5)     Extractive Distillation

This method is almost similar to that of the azeotropic distillation, i.e. for those mixtures, which are unable to distilled by any other method, a third component (termed as solvent for extractive distillation), is added which changes the relative volatility of the components , thus leading to separation. The extractive distillation is used, when the relative volatility of the binary component is very low, and if the continuous distillation is carried out, then it will give almost same purity products with high reflux rations, and thus the greater heat consumption and the requirement for the tower having large cross section areas, and number of plates.

The basic difference between above two methods of distillation is very simple, the substance added in azeotropic distillation forms an azeotrope with the components of the mixture, while the substance added in extractive distillation in non-volatile in comparison with other components and thus added continuously from top of the column, so it run down as the reflux.

There are several other types of distillation like steam distillation, vacuum distillation, air sensitive vacuum distillation,

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