written by the great Dr HK

1. Simultaneous Heat and Mass Transfer;
“Multi-components Distillation”
An advanced method of distillation in chemical
engineering for the distillation of multi-
components mixtures

2. In binary distillation of two Components, vapour
becomes steadily richer in the more volatile component
on successive plates and thus we obtain it in top product
with higher %age. But some how in two cases this
steady increase in the concentration of the more volatile
component dose not take place after a certain limit;
i- either does not take place,
ii-or it takes place so slowly that an uneconomic number
of plates is required.

3. A mixture of ethanol and water is
distilled, the concentration of the alcohol
steadily increases until it reaches 96 per
cent by mass, when the composition of
the vapour equals that of the liquid, and
no further enrichment occurs. This
mixture is called an azeotrope, and it
cannot be separated by straightforward
distillation.

4. • water-nitric acid,
• water-hydrochloric acid, and
• water-alcohols.

5. When the activity coefficient
is greater than unity, giving a
positive deviation from
Raoult’s law, the molecules of
the components in the system
repel each other and exert a
higher partial pressure than if
their behavior were ideal. This
leads to the formation of a
“minimum boiling” azeotrope
shown in Figure.

6. When the value of the
activity coefficient is less
than a unity, negative
deviation from Raoult’s law
results in a lower partial
pressure and the formation
of a “maximum boiling”
azeotrope takes place as
shown in Figure.

7. When the relative volatility of a binary
mixture is very low, then continuous
distillation of the mixture to give nearly pure
products is difficult;
• you need high reflux ratios,
• high heat requirements,
• tower of large cross-section containing
many trays.
Example: separation of n-heptane from
methyl cyclohexane in which the relative
volatility is only 1.08

8. ● Azeotropic distillation refers to processes
whereby a new component (called
the entrainer) is added to the feed mixture
to form (or nearly form) an azeotrope with
the feed components.
●The azeotrope is then removed as either the
distillate or the bottoms.

9. ● A specific technique of adding another
component to generate a new, lower-boiling
azeotrope that is heterogeneous (e.g.
producing two, immiscible liquid phases),
such as the example in this presentation
below with the addition of benzene to water
and ethanol mixture.

10.  Entrainer addition leads to the formation of the
ternary azeotrope in the solution which facilitates the
distillation of the compound. Entrainer works by
affecting the volatility of one of the components of the
feed solution.
 The best known example of entrainer is benzene and
cyclohexane added to water/ethanol Azeotropic
mixture in order to form a azeotrope that will facilities
the distillation process.

11. ● Added new component to an original feed
mixture is to break the azeotrope formation
that otherwise would be formed by the feed
components.
● The purpose of deliberately adding the
entrainer is either to separate one component
of a closely boiling pair or to separate one
component of an azeotrope.

12. I. Method with no Entrainer
a) Pressure swing distillation
b) Hybrid method comprises of membrane + distillation
II. Method with Entrainer
a) Homogeneous Azeotropic (homoazeotropic) distillation
b) Heterogeneous Azeotropic (heteroazeotropic) distillation
c) Extractive distillation

13.  Pressure Swing distillation method is specifically employed in the
distillation of Tetrahydrofuran/water purification or separation which
works on the principle of changing pressure.
 Hybrid method is employed in the processes like Dehydration of
alcohols, removal Of Volatile Organic components, (VOC) from
wastewater
 Homogenous Azeotropic distillation is involved in the distillation of
Hydrochloric acid/water + sulphuric acid , nitric acid/water +
sulphuric water.
 Heterogeneous Azeotropic distillation comprises of the Dehydration
of ethanol by using benzene , diethyl ether or pentane , separate
acetic acid/water by using ethyl acetate.
 Extractive distillation is usually involved carry pout the
Acetone/methanol by using water

14. The entrainer E is a
medium boiler (i.e.
its boiling point in
intermediate
between components
A and B), or is a low
boiler that can form
an intermediate
boiling maximum
azeotrope with A.

15. The feed (A and B)
is mixed with the
entrainer E before
entering column C1.

16. Component B (which
is essentially free of
the azeotrope A-E) is
removed from the
bottom of column C1,
while the overhead
vapor from C1 is fed
to column C2.

17. Component A is
removed as overhead
product and
entrainer E as the
bottoms product.
The entrainer is
recycled back to
column C1.

18. Ethanol water Azeotropic Distillation

19. YOUNG(44), found in 1902, that if benzene is added to the
ethanol–water azeotrope, then a ternary azeotrope is formed with
a boiling point of 338.0 K (65 C), that is less than that of the
binary azeotrope, 351.3 K (78C). Separation requires the use of
three atmospheric pressure fractionating columns, and a
continuous two-phase liquid separator or decanter.

20. • The first tower in figure gives the ternary azeotrope as
an overhead vapour when benzene is added, and nearly
pure ethanol as bottom product.
• The ternary azeotrope is condensed and splits into two
liquid phases in the decanter. Top is benzene layer and
bottom is ethanol water layer.
• The benzene-rich phase from the decanter serves as
reflux, while the water–ethanol-rich phase passes to two
towers, one for benzene recovery and the other for water
removal.
• Over heads back to appropriate points

21. Minimum Boiling Azeotropes
 Each azeotrope has a characteristic boiling point.
The boiling point of an azeotrope is less than the
boiling point of any of its constituents known as
minimum boiling azeotrope.

22. Maximum Boiling Azeotropes
 Each azeotrope has a characteristic boiling point.
The boiling point of an azeotrope is more than the
boiling point temperatures of any of its constituents
is a Maximum boiling azeotrope.

23. • Extractive distillation is similar in purpose to
that of azeotropic distillation.
• A third component, termed a solvent, is added
which alters the relative volatility of the
original constituents, thus permitting the
separation.
• The added solvent is, however, of low volatility
and is itself not appreciably vaporized in the
fractionator.

24. For a non-ideal binary mixture the partial pressure may be
expressed as:
PA = γAPA
◦xA , and PB = γBPB
◦xB
where γA and γB are the activity coefficients for the two
components. The relative volatility α may thus be written as:
α = PA xB /PB xA
= γA PA◦ / γB PB
◦
solvent added, affects the activities of the two components, and
hence the relative volatility, α.

25. Solvent should be;
• Able to increase the relative volatility of the binary mixture
• Should be less volatile
• Should not make azeotrope with either or both feed
components
• Should have low cost
• Make possible extractive distillation on low temperature or
under vaccuume

26. An example of Ext. Distt. is the
separation of toluene from iso-
octane. Separation of this
mixture is very difficult due its
lower relative volatility. Phenol
is used as an entrainer or
solvent, which increases the RV
of this non-ideal mixture.

27. • Mixture entered into the
column (1) from middle.
• Solvent from top for uniform
composition.
• Iso-octane from top
• Bottom is mixture of solvent
and toluene
• Solvent recovery system ,
separate solvent from
bottom and recycled back.