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Renin Angiotensin Mechanism.

Fractional Distillation / Rectification.

 

  • Fractional distillation is a process in which vapourization of a mixture of volatile miscible liquids gives rise to a mixture of constituents (vapours of constituents) from which the desired one is separated in pure form. 

  • This method is also known as rectification because a part of the vapour is condensed in a fractional column and returned as a liquid. 

  • This method is used to separate miscible volatile liquids, whose boiling points are close. 

Principle:

  • In fractional distillation, partial condensation of the vapours occurs due to the obstructions in the fractionating column. 

  • The vapour of a more volatile component must pass through this fractionating column before reaching the condenser. 

  • In the fractionating column, the vapours of the less volatile components get condensed and increase in concentration in the liquid phase. 

  • During this condensation, heat of condensation is released which further evaporates the more volatile components of the liquid phase. 

  • These processes in the column increase the vapour phase concentration of more volatile components moving from the bottom to the top of the column and then to the condenser to get condensed. 

  • The liquid phase concentration of less volatile components increases in the opposite direction, from the top to the bottom of the column. 

  • This condensate which returns to the liquid mixture is known as reflux.

Construction:

  • A lab scale fractional distillation unit is consists of,

    • A source of heat.

    • A distillation flask.

    • A fractionating column.

    • A condenser.

    • A receiver.

    • (All the parts are made up of glass).

Working:

  • The unit is assembled as shown in the above fig.

  • The liquid mixture is put into a distilla on flask along with a few pieces of porcelain to avoid bumping. 

  • Then the fractionating columns fitted over the mouth of the flask. 

  • Heat the mixture. 

  • As the mixture boils, vapour rises up the column. 

  • The vapour condenses on the glass platforms, known as trays, inside the fractionating column, and runs back down into the liquid below. 

  • This is known as "refluxing" the distillate.

  • Only the most volatile fraction of the vapours remains in gaseous form all the way to the top of the column. 

  • This fraction passes into the condenser, which cools it down to a liquid and this liquid is collected in the receiver.

Uses:

  1. To separate a volatile mixture of two miscible liquids.

    1. e.g. Acetone water mixture.

  2. Separation of air components.

Advantages:

  1. Easy operation.

  2. Efficient and highly effective.

Disadvantages:

  1. Expensive.

  2. Not useful for azeotropic mixtures.

Commonly Asked Questions.

  1. Define distillation and write in detail about Fractional distillation.

  2. With help of a well labelled diagram describe the principle and working of the apparatus used for Fractional distillation.
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