Properties of Unit Operations

• They are physical in nature.

• They are used invariantly in all chemical

industries (approximately)

• For a specific unit operation basic concept and

principle behind it as same in all industries.
 Properties of Unit Operations…

• But may differ in method of application

• Unit operation may also be called separation

process.

• The separation is generally based on any

specific property called driving force.
 Driving force and Equilibrium
• This driving force is the deviation of that
property from the equilibrium

• When the driving force is high i.e.

deviation from equilibrium is large and
separation is easy

• When equilibrium is achieved separation

stops as equilibrium is the condition in
which net flow of the property across the
system is zero.
• For example if the separation is based on
concentration and the concentration of
the species is same on both the sides then
there will be no movement meaning
thereby no separation.
 Explanation of driving force
• Lets we have two tanks A & B
• Both are filled with water but
up to different levels.
• If we connect both the tanks,
what will happen?
“They come to the same
A B
level”
• This difference in level is the
driving force and the condition
of same level is equilibrium.
• When the difference in level is
high the flow from one side to
other is high whereas if
difference is low flow is slow.
 The driving force in different classes of
Unit Operations
• Heat Transfer Operation: Unit operations
Temperature difference is the driving force.

• Mass Transfer Operations: Unit operations where

concentration gradient is driving force.

• Fluid Flow Operations: Unit operations where

momentum gradient is driving force.

• Mechanical Operations: Operations based on

some external mechanical force.
 Unit Operation

Some important unit operations carried in

chemical industries are:

a) Distillation

b) Drying

c) Evaporation
Some important unit operations carried in
chemical industries are:….

e) Gas Absorption & Desorption

f) Liquid-Liquid Extraction

g) Solid-Liquid Extraction (Leaching)

h) Crystallization

i) Adsorption
 a) Distillation

• It is used to separate component of a mixture

on the basis of difference in volatilities in a
boiling liquid mixture.

• Distillation is a physical separation process,

and not a chemical reaction
• All components should be volatile.

• Vapour phase contains more % of more volatile

component and liquid contains less volatile
component in high proportion.
 a) Distillation…

• These phases are created on the expense of

heat energy.
 Uses of Distillation:-
1. Crude oil

2. Water is distilled to remove impurities

3. Air is distilled to separate its components

4. Distillation of fermented solutions to produce

distilled beverages with a higher alcohol content.
The premises where distillation is carried out,
especially distillation of alcohol, are known as a
distillery.
Typical Distillation Set-up
Distillation
Raoult’s Law –

States that in a solution of two miscible liquids (A & B)

the partial pressure of component “A” (P A ) in the
solution equals the partial pressure of pure “A” (P Ao )
times its mole fraction (N A )

Partial Pressure of A in solution = P A = (P A o ) x (N A )

Partial Pressure of B in solution = P B = (P B o ) x (N B )

– 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
P total = P A + P B = P A o N A + P B o N B

– 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
Vapor Pressure / Boiling Point

– According to Kinetic Theory, the molecules in a

liquid are in a constant state of Thermal Motion
and some of these molecules are moving fast
enough to escape from the liquid forming a vapor
above the liquid.

This vapor exerts a pressure on the surface of the

liquid, i.e., Vapor Pressure – Vapor Pressure
Vapor Pressure / Boiling Point (Con’t)
– Different liquid compounds or mixtures of
liquids have different vapor pressures at a
given temperature.

– Liquids with high vapor pressures (Volatile

compounds) require relatively little energy
(heat) to increase the vapor pressure to
match the applied (atmospheric) pressure,
and thus, boil, i.e. they have low boiling
points.
Liquids with low vapor pressures require
considerably more energy to increase the
vapor pressure to the point where it matches
the applied pressure, thus, they have
relatively high boiling points.

– The individual compounds in a mixture

each exert its own pressure – partial
pressure. – The sum of the partial pressures
equals to the total vapor pressure of the
solution
– The pressure of the vapor coexisting with a confined
liquid or solid, i.e., the pressure in an evacuated
container containing a liquid at constant temperature
after the liquid and escaping molecules near the
surface of the liquid – the vapor - reach equilibrium

– The Vapor Pressure of a liquid increases, generally

exponentially, with temperature – Boiling Point – As a
liquid is heated, the vapor pressure of the liquid
increases to the point at which it just equals the
applied pressure - usually atmospheric pressure. The
liquid now begins to bubble (boil)
Different methods of carrying out
distillation are:

Simple Distillation

Molecular Distillation

Vacuum Distillation

Batch Distillation
Different methods of carrying out
distillation are:…

Continuous Distillation

Flash Distillation

Fractional Distillation

Azeotropic Distillation
Simple Distillation
Single Vaporization/Condensation cycle of a
mixture
that produces a distillate that is always impure

Therefore, it is impossible to completely separate

the
components in a mixture with Simple Distillation

be attained
Simple Distillation …..
Fractional Distillation
Accomplishes the same thing as Multiple Simple Sequential
Vaporization / Condensation Cycles, by inserting a Fractionating
Column (a Vigreux Column) between the Distillation Flask and the
Distillation Head.

The Fractionating Column, of which there are many types

containing a variety of packing materials, subjects the mixture to
many Vaporization/Condensation Cycles as the material moves up
the column toward the Distillation Head, which is attached to the
Condenser.

With each cycle within the column, the composition of the vapor is
progressively enriched in the lower boiling liquid.
This process continues until most of the lower boiling compound is
removed from the original mixture and condensed in the receiving
flask
When the lower boiling liquid is effectively
removed from the original mixture, the
temperature rises and a second fraction
containing some of both compounds is produced.

As the temperature approaches the boiling point

of the higher boiling point compound, the
distillate condensing into the third receiving flask
is increasingly pure in the higher boiling point
compound.
Column Efficiency:-
A common measure of the efficiency of a
Fractionation Column is given by its number of
Theoretical Plates; One Theoretical Plate is
equivalent to a Simple Distillation, i.e., one
Vaporization / Condensation Cycle.

The smaller the boiling point difference, the greater

the number of theoretical plates a fractionating
column must have to achieve separation of mixtures
Azeotropic Distillation
An azeotrope is a mixture of two or more liquids
(chemicals) in such a ratio that its composition
cannot be changed by simple distillation. This
occurs because, when an azeotrope is boiled, the
resulting vapor has the same ratio of constituents
as the original mixture.

Because their composition is unchanged by

distillation, azeotropes are also called constant
boiling mixtures.
 Positive azeotrope is 95.63% ethanol and
4.37% water (by weight). Ethanol boils at
78.4°C, water boils at 100°C, but the
azeotrope boils at 78.2°C.

Negative azeotrope is hydrochloric acid 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.
Azeotropic distillation
Azeotropic distillation usually refers to the 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 below with the
addition of benzene to water and ethanol.

In actual fact, this practice of adding an entrainer

which forms a separate phase is a specific sub-set
of (industrial) azeotropic distillation methods, or
combination thereof. In some senses, adding an
entrainer is similar to extractive distillation.
Example - distillation of ethanol/water, using
normal distillation techniques, ethanol can only be
purified to approximately 96% . Some uses require
a higher percentage of alcohol, eg. when used as a
gasoline additive.

• Material separation agent- The addition of a

Material Separation Agent, such as benzene.

• Molecular sieves- For the distillation of ethanol

for gasoline addition, the most common means of
breaking the azeotrope is the use of molecular
sieves.