2. Distillation
Distillation is a process of
separating components where
more volatile substances in a
mixture are separated from
less volatile substances.
This separation process is
carried out by vaporizing a
liquid mixture or by
condensing a vapour mixture.
Distillation is a Unit Operation
or Physical separation
method.
3. Principle :
• Relative volatility
• Boiling point of the components to be
separated.
Types of distillation:
• Conventional distillation method
• Non-Conventional distillation
Non-Conventional distillation method can be
classified as
(a) Extractive Distillation
(b) Azeotropic Distillation
(c) Reactive Distillation etc.
4. To understand the Principle of Non-Conventional
distillation process, we have to get a primary idea on
some Parameters. i.e. as follows
(a)Raoult’s law (Ideal Solution)
(b)Deviation from Raoult’s law (Non-ideal )
(c)Azeotrope
(d)Types of Azeotrope
5. Raoult’s Law
The partial pressure of any volatile component of a solution at
any temperature is equal to the vapour pressure of pure
component multiply by the mole fraction of that component in
the solution.
In a solution of two miscible liquids (A & B) the partial pressure
of component “A” (PA) in the solution equals the partial pressure
of pure “A” (PA
o) times its mole fraction (xA)
Partial Pressure of A in solution = PA = (PA
o) x (xA)
Partial Pressure of B in solution = PB = (PB
o) x (xB)
When the total pressure (sum of the partial pressures) is
equal to or greater than the applied pressure, normally
Atmospheric Pressure (760 mm Hg), the solution boils
Ptotal = PA + PB = PA
o xA + PB
o xB
If the sum of the two partial pressures of the two compounds
in a mixture is less than the applied pressure, the mixture
will not boil. The solution must be heated until the combined
vapor pressure equals the applied pressure.
6. Raoult’s law is followed by only Ideal solution
Vapour pressure of ideal binary solution of two
components A and B having different mole fractions
are shown
in Fig-1
Fig-1 Vapour Press vs Composition
7. DEVIATIONS FROM RAOULT’S LAW
(NON-IDEAL SOLUTIONS)
There are two types of deviations may possible
from Raoult’s law
(i) Vap. Press curve of liq.pairs showing positive deviation
(ii) Vap. Press curve of liq.pairs showing negative deviation
8. Deviations from Raoult’s law (cont.)
Positive deviation :
Cohesive (Attraction) forces between
unlike components (say A-B) are
weaker than those of the pure liq (say
A-A or B-B)
i.e. PA>PA
oxA ,PB>PB
oxB and Ptotal > PA
o
xA + PB
o xB
Negative deviation :
Attraction forces between unlike
molecules in solution (A-B) are
stronger than those between like
molecules (A-A and B-B)
i.e. PA<PA
oxA ,PB<PB
oxB and Ptotal < PA
o xA
+ PB
o xB
9. Azeotrope
An azeotropic mixture is a mixture of
two liquids having same boiling point.
These mixture can’t be separated by
simple fractional distillation because
no change in composition of these
mixture on boiling i.e. same boiling
point.
These mixture is called Constant
Boiling Point mixture.
These mixtures are formed by non-
ideal solution
10. AZEOTROPE
• Constant boiling mixture
• Non-ideal solution
• Cannot be seperated by simple distillation
• Same composition in distillate and residue at
minimum temperature.
11. TYPES OF AZEOTROPES
Based on the boiling point on temperature-composition
or press-composition diagram, Azeotrope can be
classified as follows:
Positive azeotrope/ minimumboiling mixture
• When boiling temperature of the azeotrope is less
than that of the pure component.-minimum boiling
mixture.
• It is shown by those liq.pair which show positive
deviation from ideal behavior.
•Example – Ethanol (96%) and Water mixture (4%)
Negative azeotrope/maximumboiling mixture.
• Boiling temperature of the azeotrope is higher than
that of pure component
• Liq pair which shows negative deviation
• Example –Water (20.2%) and HCl(79.8%)
15. PRINCIPLE OF NON-CONVENTIONAL
DISTILLATION PROCESS
At low to moderate pressure, with the assumption
of ideal-gas model for the vapor phase
Vapor-liquid phase equilibrium (VLE) of many
mixture can be adequately describe by the
following Modified Raoult’s Law: i.e.
yi P = xi γi Psat for i = 1, …, c [1]
Where, yi = mole fraction of component i in vapor
phase , xi = mole fraction of component i in
liquid phase, P = system pressure , P sat = vapor
pressure of component i , γi = liquid-phase activity
coefficient of component i
16. CONT.
When γi = 1, the mixture is said to be ideal and equation 1
simplifies to Raoult’s Law.
Non-ideal mixtures (γi ≠ 1) can exhibit either positive (γi > 1) or
negative deviations (γi < 1) from Raoult’s law.
In many highly non-ideal mixtures these deviations become so
large that the pressure- composition (P-x, y) and temperature-
composition (T-x, y) phase diagrams exhibit a minimum or
maximum azeotrope point.
In the content of T - x, y phase diagram, these points are so
called minimum boiling azeotrope (where the boiling temperature
of the azeotrope is less than that of the pure component) or
maximum boiling azeotrope (the boiling temperature of the
azeotrope is higher than that of the pure components).
About 90% of the known azeotropes are of the minimum variety.
At these minimum and maximum boiling azeotrope, the liquid
phase and its equilibrium vapor phase have the same
composition, i.e.,
xi = yi for i = 1, …, c [2]
17. EXTRACTIVE DISTILLATION
Extractive distillation is a vapor-liquid process
operation that uses a third component or solvent or
Entrainer.
The extractive agent creates or enhances the
volatility difference between the components to be
separated.
The extractive agent and the less volatile
component flow to the bottom of the distillation
column, where the extracted component is
recovered by a subsequent distillation.
The non-extracted species are distilled to the top of
the extractive distillation tower.
18. A mixture of components A
and B which are close
boiling. In order to separate
the components by
extractive distillation, a
suitable non-volatile solvent,
E, is introduced near the top
of the column. The solvents
preferentially associates one
of the component, say B,
and there by increases the
relative volatility of the other
component, A. A stream of
relatively pure A is obtained
as the top product of the
column. The mixture of B
and E leaves at the bottom
and is further processed to
recover the solvent for
recycle and to separate the
product B. The solvent E is
called an entrainer since it
entrains or extracts one of
the component.
Process flow diagram showing an extractive
distillation apparatus
19. CHOICE OF SOLVENT / ENTRAINER
Entrainer / Solvent - is an agent, which affects
the volatility of one of the azeotrope constituents
more than another.
The most important factors in the selection of
solvent are Solvent Selectivity and Solvency.
Selectivity refers to the solvents ability to enhance
the separation of the key component(s) in the
mixture. It is expressed as the ratio of the
distribution coefficients of the two components in
the solvent.
Solvency is the concentration ratio of the entrained
component B in the solvent phase and raffinate
phase.
20. CONTD..
The solvent should be low-cost, non-corrosive,
non-toxic, non-reactive and should have low
latent heat.
The solvent should not form an azeotrope with
any of the component(s) but it should definitely
have more affinity to for the less volatile
component.
21. APPLICATION OF EXTRACTIVE DISTILLATION
Extractive distillation is widely used in
Petrochemicals industry. It is used for
Separation of butadiene from a mixture of
butane, butene and small quantities of other
unsaturated hydrocarbon can be separated by
using extractive distillation. Low polarity
solvents like furfural, N-methyl pyrrolidone
(NMP) etc are good entrainers for butadiene.
Separation of Benzene from the mixture of
benzene-cyclohexane during the production of
cyclohexane from benzene via hydrogenation.
Here propylene glycol is used as a
Solvent/Entrainer.
22. The basic
configuration for
LLE requires four
units, in contrast
to the two
required for ED
ED systems now
use better heat
integration.
Smaller and
fewer pieces of
equipment
Enhanced mass
transfer devices
Liquid-liquid Extraction vs
Extractive distillation
23. AZEOTROPIC DISTILLATION
Azeotropic distillation is any of a range of techniques
used to break an azeotrope in distillation.
Azeotropic distillation usually refers to the specific
technique of adding another component, called solvent
or entrainer.
Entrainer is added to the mixture for generating new or
lower boiling azeotrope (minimum boiling azeotrope) or
new upper boiling azeotrope (maximum boiling
azeotrope).
Example- (i) minimum boiling azeotrope or positive
azetrope is 95.6% ethanol and 4.4% water(by mass),
ethanol boils at 78.4ºC and water boils at 100ºC but the
azeotrope boils at 78.2ºC.
(ii) maximum boiling azeotrope or negetive
azeotrope is hydrochloric acid at a concentration of
20.2% and 79.8% water( by mass).HCl boils at -84ºC
and water at 100ºC but the azeotrope boils at 110ºC.
24. It is classified as
positive azeotrope or
minimum boiling
azeotrope.
Here as an entrainer
cyclohexane or
benzene is used.
A minimum boiling
azeotrope of water
and cyclohexane
leaves as the overhead
vapour.
The condensate is
phase separated as
usual and
cyclohexane phase is
sent back as reflux.
Dehydrated ethanol
leaves the bottom of
the column. The
water rich phase flows
to the cyclohexane
stripper. The
recovered
hydrocarbon is
recycled.
AZEOTROPIC DISTILLATION FOR
ETHANOL WATER SYSTEM
25. Uses of pure Ethanol(anhydrous)
Why produce 100% ethanol using
azeotropic distillation when
standard distillation gives 96%
anyway?
• Pure ethanol is used as a solvent
for laboratory and industrial
application- paint and varnishes.
• Pure ethanol is used as a motor
fuel alcohol ( gasohol)
Water could react with other
chemicals that the solvent is
mixed with and engines are only
suitable for anhydrous ethanol.
26. USES OF AZEOTROPIC DISTILLATION
Seperation of isobutanol and water
Seperation of benzene and cyclohexane
However, the main application of azeotropic
distillation is to separate ethanol and water
mixture, also called dehydration of ethanol.
27. EXTRACTIVE DISTILLATION VS AZEOTROPIC
DISTILLATION
ED method is used when mixture
boiling pt. is very close to each
other.
Entrainer, which is used for ED
process, is used to create a
difference of relative volatility b/w
the component(s) of mixture.
For ED, additional component
(Entrainer) appears mostly at the
bottom of the column.
Example-Separation of
Butadiene from the mixture of
butane, butene or other
unsaturated hydrocarbon.
AD is used for the mixture of
close boiling component.
Entrainer, which is used for AD
process, forms a new lower
boiling azeotrope with one or
more components of the feed.
For AD, additional component
(Entrainer) appears in
appreciable amount at the top of
the column.
Example- Dehydration of Ehanol
by using cyclohexane or
benzene as entrainer.
Extractive Distillation Azeotropic Distillation
28. REACTIVE DISTILLATION
Reactive distillation involves a chemical reaction and product
separation in the same column or vessel.
Add reactant &/or catalyst to cause a reversible/selective
reaction with one of the feed components.
Example- Production of methyl tertiary butyl ether (MTBE), a
synthetic octane-booster produced by the reaction between
iso-butene and methanol.
29. COMPARISON BETWEEN EXTRACTIVE,
AZEOTROPIC AND REACTIVE DISTILLATION
Extractive Distillation Azeotropic Distillation Reactive Distillation
Higher boiling solvent
eg- NMP, Propylene glycol
Add entrainer that forms lower
boiling point (min boiling AZ) or
upper boiling point (max boiling AZ)
eg.-Benzene, cyclohexane etc
Add reactant &/or catalyst to cause
a reversible/selective reaction with
one of the feed components
Enters near top of column Added near top or bottom
depending upon the AZ is min/max
BP
Reaction and distillation occur in
same vessel
Interacts with other components
to affect volatility or activity
coefficients
Entrainer forms a new and lower
boiling AZ with one or more of the
components in the feed.
Example – Butadiene extraction
from butane, butene and other
unsaturated hydrocarbon.
Example- Dehydration of ethanol Example- Production of methyl
tertiary butyl ether (MTBE) by the
reaction between iso-butane and
methanol.
30. ADVANTAGES AND DISADVANTAGES OF NON-
CONVENTIONAL DISTILLATION PROCESS
Advantages :
Allowing the separation of chemicals that cannot feasibly be
separated by conventional distillation, such as systems
containing azeotropes or pinch points.
Improving the economics of the separation by saving energy and
increasing recovery.
Disadvantages :
Larger diameter column required to allow for increased vapour
volume due to the azeotropic agent.
An increase in control complications compared with simple
distillation.
31. REFERENCES :
1. Principle of Mass Transfer and Separation Process by
B.K.Dutta 5th edition
2. Unit operations in Chemicals engineering by Mc-
Cabe-Smith 7th edition
3.http://www.cheresources.com/content/articles/separati
on-technology/extractive-5.distillation-an-in-depth-look
4.http://www.rccostello.com/distil/distileqp.htm
5.http://www.differencebetween.net/science/chemistry-
science/differences-between-azeotropic-and-extractive-
distillation/
6.https://en.wikipedia.org/wiki/Azeotropic_distillation
7.https://en.wikipedia.org/wiki/Extractive_distillation
