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1. PREETHI.M

2.  An azeotrope or a constant boiling point
mixture is a mixture of two or more liquids
whose proportions cannot be altered or
changed by simple or distillation.
 This happens because when an azeotrope is
boiled, the vapour has the same proportions
of constituents as the unboiled mixure

4.  The principle of azeotropic distillation and
extractive distillation is the addition of a new
third substance to the mixture so as to
increase the relative volatility of one of the
two key components and thus making
separation relatively easy.
 Example: Ethanol, water, Benzene

5.  Azeotropic distillation is a distillation method
in which azeotropic mixture is broken by the
addition of a third substance, which forms a
new azeotropic with one of the components
 The relative volatility of the liquid mixture
can be changed by addition of the
components.
 For example: benzene is added to the
azeotropic mixture of water and ethyl
alcohol.
 Benzene breaks the mixture water-ethyl
alcohol. And forms a new azeotrope between
benzene and ethyl alcohol. The volatility of
the water(more polar liquid) is enhanced

6. contd…
 On distillation, water distills at 65.85 c leaving
alcohol and benzene behind.
 The boiling point of this binary mixture is 68.2c
and benzene and benzene gets distills leaving
pure alcohol behind. It can be distilled off at
78.3c.
 The benzene can be recycled. Thus using
fractional distillation method, absolute alcohol
can be prepared.
 When glycerin is added to the above mixture the
vapour pressure of water is lowered. Practically
pure ethanol can be obtained from the
fractionating tower.

7.  Extractive distillation is defined
as distillation in the presence of a miscible,
high-boiling, relatively non-volatile
component, the solvent, that forms no
azeotrope with the other components in the
mixture.
 The method is used for mixtures having a low
value of relative volatility, nearing unity.

8.  In extractive distillation, the third substance
added to the azeotropic mixture is relatively
nonvolatile liquid compared to the
components to be separated.
 The third component is withdrawn at the base
of the fractionating column
 For example: Is separation of toluene from
paraffin hydrocarbons of approximately same
molecular weights. The separation of toluene
and iso-octane(example of hydrocarbons) is
difficult.

9.  In the presence of phenol,the relative
volatility of iso-octane increases, therefore,
separation of toluene is relatively easy.
 In another example, furfural is added for the
separation of butadiene from its mixture
containing butane and butene.

10.  The main application of azeotropic distillation is
to separate ethanol and water mixtures, also
called dehydration of ethanol. Absolue alcohol
can be prepared by azeotropic distillation.
 Petroleum refineries and distilleries use these
types of distillation.
 The liquor from fermentation process is a
common source of ethanol and contains
approximately 8 to 10%.