Distillation is the process of separating components of a mixture based on different boiling points. Examples of uses of distillation include purification of alcohol, desalination, crude oil refining, and making liquefied gases from air.

1. DISTILLATION
Ms. Nisha Vijayan
SJIPR

2. Contents
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1. Introduction
2. Applications
3. Vapour –Liquid Equilibrium Relation
4. Distillation Methods
5. Flash Distillation
6. Fractional Distillation
7. Molecular Distillation
8. Azeotropic and Extractive Distillation
9. Simple Distillation
10. Steam Distillation

3. Introduction
 The separation of the components of a liquid
mixture by a process involving vaporization
and subsequent condensation at another
place.
 Involves 2 steps:
1. Converting a liquid into vapour
2. Transferring the vapour to another place and
recovering the liquid by condensation.
 Feed liquid: Distilland
 Condensed liquid : Distillate / condensate
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4. Applications
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5. Vapour – Liquid Equilibrium relation
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 Vapour Liquid Equilibrium relation of binary
systems can be studied for 3 different classes
of system:
1. Miscible system
2. Immiscible system
3. Partially miscible system
 Raoult’s Law: “A components vaporising
tendency depends on its concentration in
the mixture and its vapour pressure at that
temperature”
In such system the components have no
effect on each other except dilution.

6. Miscible System
Ideal System Non – Ideal System
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 Obeys Raoult’s Law
 E.g. A mixture of
ethylene chloride
and benzene
 Raoult’s Law is
obeyed by few
liquids. These
solutions are known
as “perfect
solutions”
 Shows varying
degree of deviation
from Raoult’s Law
depending on the
nature of liquids and
the temperature.
1. Positive deviation
(benzene and
ethanol)
2. Negative deviation
(water and nitric

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IDEAL CURVE RAOULTS
LAW

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9. Immiscible Systems
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 In a mixture of 2 immiscible liquids each
components will vaporise independently
hence their vapour pressure are additive.
 Thus the system starts boiling when the
summation of 2 vapour pressures is equal to
the atmospheric pressure (boiling point of
mixture)
PA + PB = 760 mmHg
 Naturally this temperature is below boiling
points of either of the individual components.

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 During distillation of such a system the
composition of vapours produced will be in
proportion of the relative vapour pressures of
the 2 components. Therefore in vapour phase:
Moles of A = PA
Moles of B PB
Weight of A in vapour phase = PA MA
Weight of B in vapour phase PB MB
 This forms the basis of distillation of an
immiscible system.

11. Partial Miscible System
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 Exhibited by some mixture e.g.. Water and n-
butanol.
 Such system are special case of minimum
boiling azeotrope.
 In this system over a range of liquid
composition, the vapour composition and
boiling point are constant.

12. Distillation Methods
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1. Equilibrium or Flash Distillation
2. Simple or Differential Distillation
3. Fractional Distillation
4. Distillation under reduced pressure: Molecular
Distillation
5. Special Distillation Methods (Non Ideal
Mixtures)
 Azeotropic Distillation
 Extractive Distillation

13. Equilibrium or Flash Distillation
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 A process in which the entire liquid mixture is
suddenly vaporised (flash) by passing the feed
from a high pressure zone to a low pressure
zone.
 Called Equilibrium Distillation as separation is
attempted when the liquid and vapour phases
are in equilibrium.
 Equilibrium or Flash Distillation is carried out
as a continuous process and does not involve
rectification.
 Advantages: used for obtaining multi
component systems of narrow boiling range.
(petroleum ether)

14. Equilibrium or Flash Distillation
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15. Fractional Distillation
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 A process in which vaporisation of liquid mixture give
rise to a mixture of constituents from which the
desired one is separated in pure form.
 Also known as Rectification Distillation as a part of the
vapour is condensed and returned as a liquid.
 Used to separate miscible volatile liquids whose boiling
points are close by means of fractional column.
 In fractional distillation the vapour must pass through
fractionating column in which partial condensation of
vapour is allowed to occur.
 Condensation takes place in the fractionating column
so that a part of the condensing vapour returns to the
still.

16. Fractional Distillation
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17. Fractionating Columns
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 In fractional distillation special type of still-
heads are required to provide continuous
affect on condensation and revapourisation.
 Classified as :
A. Plate Columns: Sieve Column, Bubble Cap
Column, Valve Plate Columns
B. Packed Columns

18. Sieve Plate Columns
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19. Bubble Cap Column
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20. Valve Plate Column
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21. Packed Columns
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 Some form of packing is used in the column to
affect the necessary liquid / vapour contact.
 The packing may consist of wire or glass,
glass rings, cylindrical glass beads etc.
 Applications : Uniform Packing, used in
laboratories

22. Examples of Packed Columns
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23. Fractional distillation Lab Scale Assembly
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24. Plate efficiency (Overall Efficiency)
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 Theoretical plate: The liquid on a plate is completely
mixed having uniform composition and the vapours
reach perfect equilibrium with the liquid on each plate.
 But on actual plate such conditions does not exist.
 Plate Efficiency is a factor which is introduced to
express the performance of an actual plate in relation
to the theoretical plate.
 Plate Efficiency is the ratio of calculated number of
theoretical plate required to the actual number of
plates in the column.
E0 = N* / NA
 N* - Number of Theoretical Plate
NA - Number of Actual Plate

25. Height Equivalent To Theoretical Plate
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 In a packed column there is no one definite
plate upon which the vapours and liquid
streams approach equilibrium.
 But the interchange between vapour and
liquid takes place gradually as they flow in
counter-current manner.
 A definite length of packing is required to
produce the same fractionating effect as one
theoretical plate.
H.E.T.P. = Height of columns
No. of theoretical plate
 Limitation : the calculation of number of
theoretical plates.

26. Distillation Under Pressure
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 This is a distillation process in which the liquid
is distilled at a temperature lower than its
boiling point by application of vaccum.
 Applications : 1. Minimize chemical change
due to heat.
2. To change physical form of material.
 Molecular Distillation is a type of Distillation
Under Pressure.

27. Molecular Distillation
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 This is a distillation process in which each
molecule in the vapour phase travels mean free
path and gets condensed individually without
intermolecular collisions on application of vaccum.
 Also called as Evaporative distillation or Short
Path distillation.
 It is necessary to design the equipment based on
the requirement of the Molecular Distillation:
1. The evaporating surface must be close to the
condensing surface.
2. The molecular collisions should be minimized
3. The liquid surface must be as large as possible.

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 Based on the method of formation of the
liquid film, apparatus maybe divided into 2
types:
1. Falling Film Molecular Still
2. Centrifugal Molecular Still

29. Falling Film Molecular/
Wiped Film Molecular Still
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 Vaporization occurs from a film of liquid
flowing down a heated surface under high
vaccum.
 The vapour travels a short distance and
strikes the condenser nearby.
 Each molecule is condensed individually and
distillate is subsequently collected.

30. Falling Film Molecular
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31. Centrifugal Molecular Still
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 Liquid feed is introduced into a vessel which is
rotated at very high speed (centrifugal force).
 On account of heating vaporization occurs
from a film of liquid on the sides of the vessel.
 The vapours travels short distance and gets
condensed on the adjacent condenser.
 Each molecule is condensed individually and
distillate is subsequently collected.

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33. Azeotropic Distillation
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 The principle of azeotropic distillation lies in
the addition of new substance to the mixture
so as to increase the relative volatility of one
of the 2 key components and thus making
separation relatively easy.
 Azeotropic ternary mixtures with minimum
boiling point are pharmaceutically important.
 In this distillation the azeotropic mixture
present is broken down by addition of 3rd
substance which forms a new azeotrope with
one of the components.
 The relative volatility of the liquid mixture can
be changed by adding 3rd substance.

34. Azeotropic Distillation
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E.g. Benzene is added to the azeotropic mixture of
Water and Ethanol

35. Extractive Distillation
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 The 3rd
substance added to the azeotropic
mixture is relatively non-volatile liquid
compared to the components to be separated.
 E.g. Separation of Toluene from Paraffin
Hydrocarbons of approximately same
molecular weights.

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 Applications of Azeotropic and Extractive
Distillation:
1. Absolute alcohol can be prepared by
Azeotropic Distillation.
2. Petroleum refineries and distilleries use
these type of distillation.

37. Steam Distillation
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 Carried on with the aid of steam and is used
for the separation of high boiling substances
from non- volatile impurities.
 Used for separation of immiscible liquids, high
boiling liquids.
 Applications: For extracting volatile oils,
purification of liquids with high boiling points.
 Advantages: Volatile oils can be separated at
lower temperatures without decomposition.
 Disadvantages: Not suitable when immiscible
mixture reacts with water.

38. Steam Distillation
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39. Simple Distillation
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 Is the process of converting a single constituent
from a liquid (or mixture) onto its vapour, vapour is
transferred to another place and liquid is recovered
by condensation of the vapour (usually by allowing
it to come in contact with cold surface).
 Also known as Differential Distillation as it is based
on difference in volatilities of the of the
components of the mixture.
 Applications : - For Preparation of distilled water
and water for injection.
-Volatile and aromatic waters are prepared
- Organic solvents are purified

40. Simple Distillation
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