Fractional Distillation and Simple Distillation- A Comparison
While carrying out the process of separation of a mixture of different liquids sometimes we come across mixtures of two or more liquids that possess different boiling points. Separating these mixtures with the process of simple distillation will not work, because constituents of mixtures differ in their boiling points and difference in their boiling point is less than 25℃.
In such cases, the process of fractional distillation is used in which specially designed fractionating columns are used.
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The purpose of such a column is to restrict the movement of the vapours as they rise up and movement of descending liquid. It also provides more surface area for heat to exchange.
Involvement of fractionating column is the only difference in the apparatus used in the process of distillation and fractional distillation.
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Fractional Distillation Procedure
When a mixture of two or more liquid constituents is heated, vapours of all the constituents are formed and ascend up in the column. During their upward movement the vapours of liquid with higher boiling point will be able to condense first as compared to the vapours of liquid with low boiling point.
Liquids with high boiling point on condensation will release heat which is taken up by the vapours of liquid with low boiling point. As a result, the vapours of liquid with low boiling point tends to remain in vapour phase only and rises up, because they have extra energy. This way, eventually they escape the flask in which they are contained and are collected at the receiver site, which is a condenser and condense over there, contrary to which the vapours of liquid with high boiling point fall back in the distillation flask and heated up again.
After a series of successive distillations, the remaining liquid in the distillation flask get richer in liquid component with high boiling point. Hence the receiver contains the fraction mainly of liquids present in the mixture which are more volatile in nature.
Each successive condensation and vapourisation is called a theoretical plate.
Industrial Distillation
The process of separating the complex liquid-liquid mixture at large scale using heavy machinery following the process of distillation is termed as industrial distillation.
This large-scale distillation is carried out in large cylindrical shaped towers known as fractionating columns. The usual range of width of these columns id around 0.6m-6m.
The high efficiency of these towers is because of the use of reflux. Reflux is basically the fraction of the condensed liquid in the tower. When this reflux falls down, it helps in lowering the temperature of the liquid moving in an upward direction, which further facilitate the condensation of the boiling liquid at the specific place.
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Fractional Distillation of crude oil
Step 1: Neutralization of crude oil
The crude oil is first of all neutralized by washing it with an acidic or basic solution depending upon the chemical nature of impurities, i.e. impurities are basic or acidic in nature.
Step 2: Heating of neutralized Crude Oil in Furnace
The crude oil is heated to 723K in coiled pipes in a furnace and the pressure is reduced considerably to facilitate the formation of vapours of the constituents.
Step 3: In the process of Fractional distillation
The vapours obtained in step 2 are allowed to enter into the bottom of the fractionating tower called the bubble tower. As the vapours of the oil ascend, the portions of liquid with high boiling point condense in the lower portions of the tower. The lower boiling portions on taking up the heat evolved by condensation of liquid of high boiling point ascends up in the tower and condenses at different levels/plates in the tower depending upon the ranges of their boiling point.
Portions of Petroleum obtained after refining using fractionating column
Applications of Fractional Distillation
This process is mainly employed to separate the mixture of methanol (b.p.=338K) and acetone (b.p.=329K), Benzene and Toluene etc.
It is extensively used to separate the constituents of Petroleum/crude oil.
This process can be used to separate around 100 constituents of any liquid-liquid mixture.
Mixture of Liquids Usually Separated by the Process of Fractional Distillation
