2. Introduction
Some of the mixtures exhibit the same concentration in the
vapor phase and the liquid phase, They are called
as azeotropes.
An azeotrope or a constant boiling mixture is a mixture of two
or more liquids
Azeotropes cannot separate by simple distillation method as
in contrast to ideal solutions with one component typically
more volatile than the other, if the mixture forms an azeotrope
than vapor and liquid concentrations will be same that
prevents the separation through simple approach.
3. Azeotropes are mixtures of two or more different liquids
which can either have a higher boiling point than either of the
components or they can have a lower boiling point.
Unlike azeotropes, ideal solutions have uniform mixtures of
components. Ideal solutions always follow Raoult’s law.
Mixture of benzene and toluene is good example of ideal
solution.
Azeotropes do not follow Raoult’s law because during boiling,
the ratio of component in solution and vapor is same
Two types of azeotropes
1. Minimum boiling azeotrope and
2. Maximum boiling azeotrope
4. Minimum boiling
azeotrope
A solution that shows greater positive deviation
from Raoult's law forms a minimum boiling
azeotrope at a specific composition.
Example, an ethanol-water mixture (obtained by
fermentation) on fractional distillation yields -
approximately 95% by volume of ethanol.
This composition 95% v/v, the liquid and vapour
have the same composition, and no further
separation occurs.
A well-known example of a positive azeotrope
(95.63%w/w ethanol and 4.37% w/w water).
5. Maximum boiling azeotrope
A solution that shows large negative deviation from
Raoult's law forms a maximum boiling azeotrope at a
specific composition.
Ex. Nitric acid and water. This azeotrope has an approximate composition
of 68% nitric acid and 32% water by mass.
HCL at a concentration of 20.2% and 79.8% water (by weight).
Hydrogen chloride boils at −84 °C and water at 100 °C,
but the azeotrope boils at 110 °C, which is higher than
either of its constituents.
The maximum temperature at which any hydrochloric acid
solution can boil is 110 °C.
In general, a negative azeotrope boils at a higher
temperature than any other ratio of its constituents.
6. Methods of separation of azeotrope
1. Methods with no entrainer
◦ Pressure swing distillation
2. Methods with entrainer
◦ Homogenous azeotropic distillation
◦ Hetrogenous azeotropic distillation
◦ Extractive distillation
Entrainer: substance/ chemical that enhance the separation of
azeotrope
7. Pressure-swing distillation
Some binary azeotrope mixtures loss azeotropic behavior When the system
pressure is change.
Pressure-swing distillation (PSD) is the process to be utilized to separate the
Pressure sensitive mixture with closed boiling point or forming azeotrope.
Pressure-swing distillation is a special distillation process in which no new
additive is added.
Principle: A simple change in pressure can alter relative volatility of the
mixture with close boiling point or forming azeotrope.
8. Azeotropic distillation
The technique of addition of another component to
form a new low boiling point azeotropic solution such
as benzene can be added to the solution of ethanol and
water in azeotropic distillation.
9. Azeotropic distillation
The azeotropic distillation unit
consists of a container to feed the
azeotrope, decanter and steamer.
For example; the mixture of acetic
acid and water can be separate out
with the addition of an ester like n-
butyl acetate.
Remember the boiling point of
acetic acid is 118.1oC and water is
100oC.
Addition of ester whose boiling
point is 125oC forms a minimum-
boiling azeotrope with water with
boiling point 90.2oC.
10. Azeotropic distillation
Hence azeotropic mixture will be
distilled over as vapor and leave
acetic acid at bottoms. The
overhead vapor is condensed and
collected in a decanter.
Here it forms two insoluble layers
in which the top layer contains
pure butyl acetate with water, and
a bottom layer contains pure water
saturated with butyl acetate.
The top layer is returned to the
distillation column and bottom
layer is sent to another column for
the recovery of the ester by steam
stripping.
11. How Azeotropic Distillation Works
The entrainer decreases the boiling point of azeotropic solution and
separates the components of mixture at different boiling points.
When azeotropic mixture is heated with entrainer, the condensed overhead
vapor forms two liquid phases and collects in the decanter.
In decanter, the top layer contains benzene whereas bottom layer contains
water.
The top layer of benzene again back to first column as reflux and source of
entrainer whereas bottom layer of water back to 2nd column.
The bottom of 2nd column contains mixture of ethanol and water which
transfers to 3rd column for distillation.
Third column contains pure water at bottom and distillate is returned to the
1st column for recycling.
12. Azeotropic Distillation of Ethanol
Another example of azeotropic distillation is separation of
ethanol with water from its aqueous solution.
The boiling point of water is 100 °C and boiling point of ethanol
is 78.3°C.
By addition of benzene to the azeotropic mixture as entrainer,
ethanol can be separated out from the solution.
Presence of benzene forms a new solution of minimum boiling
point with 22.8 mole% ethanol, 23.3 mole% water and 53.9
mole% benzene which boils at 64.86 °C.
Pure water leaves as the overhead product and pure ethanol
leaves the column as bottoms product.
13. Heterogeneous azeotropic distillation:
The heterogeneous azeotrope contains the vapor phase with two liquid
phases.
Some common examples of heterogeneous azeotropic mixtures are
benzene with water, butanol with water and dichloromethane with water.
In the heterogeneous azeotropic distillation, the liquid phase of the mixture
is immiscible. In binary heterogeneous azeotropic mixture, during
distillation the system contains two columns with a decanter.
The fresh feed is added into the 1st column.
After feeder, it passes to decanter and the one of the component is
withdrawn as reflux into the first column.
Another phase is withdrawn as reflux into the 2nd column. So we can say
that 1st column produces one component and 2nd column produces another
component at bottom.
14. The simplest case of continuous heteroazeotropic
distillation is the separation of a binary
heterogeneous azeotropic mixture. In this case the
system contains two columns and a decanter. The
fresh feed (A-B) is added into the first column
In the industry the butanol-
water mixture is separated with
this technique.
Heterogeneous azeotropic distillation
Examples:
Benzene - Water NBP 69.2 °C
Dichloromethane - Water NBP
38.5 °C
n-Butanol - Water NBP 93.5 °C
Toluene - Water NBP 82 °C
15. Homogeneous Azeotropic Distillation
In the homogenous azeotrope the
constituents of the mixture are
completely miscible with each
other.
In homogeneous azeotropic
distillation method, entrainer may
or not form additional azeotropes
after addition.
This distillation process is carried
out in a sequence of columns.
The azeotropic mixture of A and B
forms azeotropic mixture with
minimum boiling point.
16. Homogeneous Azeotropic Distillation
Here both the components must
belong to the same distillation
region. Now fresh feed is mixed
with entrainer and distilled over.
The A component is taken as
bottom product in 1st column
whereas B is taken as top
product in 2nd column.
Entrainer (E) is recovered as
bottom product in 2nd column 2
and recycled to 1st column.
17. Extractive distillation
This process is carried out in two feed column in which
entrainer is introduced above the original mixture feed point
and largely removed as bottom product.
Entrainer is used to enhance the relative volatility of low
volatility component to precede the separation of mixture.
Entrainer has high boiling point (heavy entrainer) as
compared to the original mixture component. It does not
form any type of azeotrope.
18.
19. Steam distillation
Is a special type of distillation (a separation process)
for temperature sensitive materials like
natural aromatic compounds.
It once was a popular laboratory method for
purification of organic compounds, but has become
obsolete by vacuum distillation. Steam distillation
remains important in certain industrial sectors.
20. Many organic compounds tend to decompose at high sustained temperatures.
Separation by distillation at the normal (1 atmosphere) boiling points is not an
option, so water or steam is introduced into the distillation apparatus.
The water vapor carries small amounts of the vaporized compounds to the
condensation flask, where the condensed liquid phase separates, allowing for
easy collection.
This process effectively allows for distillation at lower temperatures, reducing
the deterioration of the desired products.
If the substances to be distilled are very sensitive to heat, steam distillation may
be applied under reduced pressure, thereby reducing the operating temperature
further.
After distillation the vapors are condensed.
Usually the immediate product is a two-phase system of water
and the organic distillate, allowing for separation of the
components by decantation, partitioning or other suitable method.
21. Principles
When a mixture of two practically immiscible liquids is heated while being
agitated to expose the surface of each liquid to the vapor phase, each
constituent independently exerts its own vapor pressure as a function of
temperature as if the other constituent were not present.
Consequently, the vapour pressure of the whole system increases.
Boiling begins when the sum of the vapour pressures of the two immiscible
liquids just exceeds the atmospheric pressure (approximately 101 kPa at sea
level).
In this way, many organic compounds insoluble in water can be purified at a
temperature well below the point at which decomposition occurs.
For example, the boiling point of bromobenzene is 156 °C and the boiling
point of water is 100 °C, but a mixture of the two boils at 95 °C.
Thus, bromobenzene can be easily distilled at a temperature 61 °C below its
normal boiling point.
