Distillation - Concept E-Learning Program - Study Material

Concept E-Learning Program
Distillation
Study Material
Koncept Learning Center

Distillation Concepts
Preface
In the distillation, difference in volatilities is used in the
separation of two or more components from their miscible
mixtures. The term relative volatility which is worked out from
the experiments and available as vapor-liquid equilibrium data.
Higher the relative volatility, easier will be the separation of two
components from their mixture using distillation process and
vice versa.
Fluid nature, operational requirements, economics, etc. affects
the selection of type of distillation column internals viz. packing
or trays. These internals are provided to ensure proper contact
of uprising vapors & down flowing liquids. Overall distillation
system contains various components like feed pre-heater,
distillation column, condenser, reboiler, etc.
Knowledge of complete distillation system along with column
internals is required to design & operate it more efficiently.
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INDEX
i. Why Distillation is required?
ii. How Distillation Works?
iii. Types of Distillation
iv. Distillation Requirements
v. Distillation Column Hardware
vi. Distillation System
vii. Column Operating Parameters
viii. Column Design
ix. Energy Conservation
x. Troubleshooting

Why Distillation is required?
In the chemical plants, raw materials are converted to the
product chemicals. Often solvents are added in the reaction
processes. So after the reaction completion, a mixture of
solvent with product chemicals are obtained.
This mixture, may also contain un-reacted raw materials
and any other side chemicals produced during the reaction.
In short, the product stream coming out from the reactor,
will be a mixture of many components including product
chemical.
What is required, is to separate out desired product
chemicals, from the mixture product stream containing
desired product as well as undesired other chemicals like,
solvent, un-reacted raw materials, side products, etc.
To match the gap between what is produced during the
chemical reaction, and, what is required by the next
processing step, or to match the final product specification,
separation of components from their mixture is required.
Multiple separation techniques may be technically possible,
but the best cost effective one is selected for a specific fluid
mixture application.

To decide a separation technique, material properties
govern a major role. Distillation is the separation technique
used for the separation of two miscible liquids. And, this
separation is done based on their relative volatilities of two
different components.
Though we have limited our discussion for binary mixtures
only, but the same concepts applies for the multi-
component miscible liquid mixtures also.

How Distillation Works?
Distillation is a process for the separation of two or more
components of desired composition from their miscible
liquid mixtures. Like every separation technique which
uses the difference in properties of two components e.g.
density difference used in the liquid solid separation from
their slurry by gravity settling method, difference in
volatility of two components is used for their separation
using distillation method.
Being one component is more volatile than the other in
their binary mixture, on heating liquid boils and generates
vapors which are also a mixture of both the components
but with relatively higher concentration of more volatile
components (as observed experimentally). As shown in the
Fig.1, ‘A’ is relatively more volatile than ‘B’.
Fig.1Liquid
50 % v/v ‘A’
50 % v/v ‘B’
Vapor
70 % v/v ‘A’
30 % v/v ‘B’
Heat

Though vapors generated are enriched with high volatile
component in single stage of boiling, to achieve desired
concentration it may undergo many more similar kind of
stages. Like 70% v/v ‘A’ liquid mixture may generate 85%
v/v ‘A’ in vapors. And so on till the desired concentration is
not achieved.
Separation by distillation becomes easier if the relative
volatility between two components is higher and the
desired separation is achieved with less number of
separation stages. Which reduces down the hardware cost.
Increased reflux rate reduces the number of stages
required which means lesser hardware cost. While energy
consumption which means the operating cost increases
with the increase of reflux rate. Thus an optimum design of
the distillation system is done by minimizing the total of
fixed cost (due to number of stages) and operating cost (due
to energy input). Due to high energy and capital cost
involved, distillation system is usually the last choice of
selected separation technique.
In case relative volatility is closer to ‘1’, distillation alone
becomes uneconomic. Other alternatives like liquid-liquid
extraction followed by the distillation, absorption, etc. are
recommended for the desired separation.

When relative volatility is exactly ‘1’, separation by
distillation technique becomes impossible. As the
composition of generated vapor and the liquid remain
same, as shown in Fig.2. This
is called as Azeotropic mixture and distillation fails to work
here as both the components are having same volatility.
Fig.2Liquid
95 % v/v ‘A’
5 % v/v ‘B’
Vapor
95 % v/v ‘A’
5 % v/v ‘B’
Heat

Types of Distillation
According to the separation requirements, various types of
distillations are evolved in the chemical plants. Different
types of distillation commonly used in the chemical plants.
a.) Simple Batch Distillation
b.) Simple Continuous Distillation
c.) Fractional Batch Distillation
d.) Fractional Continuous Distillation
a.) Simple Batch Distillation
In this type of distillation, a Distilling Vessel is filled with
the liquid mixture and is heated to generate vapors.
Generated vapors are condensed in Condenser, and
collected in a separate Collection Vessel.

b.) Simple Continuous Distillation
This is also called as Flash Distillation. Hot Liquid feed is
flashed inside the flash vessel to generate vapor and liquid
in equilibrium to each other.
c.) Fractional Batch Distillation
If we look that in both, batch and continuous simple
distillation, concentration of volatile material increases up
to a limit only which is governed by its equilibrium data.
Flash
Vessel
Feed
Vapor
Liquid
A : 0.5
B : 0.5
A : 0.83
B : 0.17
A : 0.46
B : 0.54

Now, if we are looking for higher concentration of, ‘A’ in the
vapor stream, as well as lower concentration of ‘A’ in liquid
stream, we need to go for multiple stages, which is also
called as “Fractional Distillation”.
For increasing the product purity, we need to convert liquid
to vapor in which concentration of volatile material will be
increased. These vapors need to be condensed and again
heated to convert to vapor of further higher concentration
of vapors and so on. This is called as Fractional
Distillation.
A batch still, is filled with the initial liquid mixture, and
heat is supplied to boil it. Generated vapors of
concentration, in equilibrium with the boiling liquid, rises
up through a fractionating column. These fractionating
column are either packed, or tray column.

As the vapors moves up, due to heat loss, part of the
vapors get condensed inside the column itself, and rest top
vapors are condensed and sent back to the column. This is
also called as, reflux.
In short, fractionating column is having liquid falling down,
while uprising vapors which are having higher heat, comes
in contact with the liquid.
Simultaneous Heat and mass transfer takes place between
vapor and liquid stream, which are in direct contact to each
other.
Every time liquid converts to vapor, concentration of
volatile material ‘A’, increases. Thus, as vapors moves up,
concentration of ‘A’, increases.
Since purity of ‘A’ is increasing through fractionating
column, purity of less volatile material ‘B’, also increases in
the distilling vessel.
But Still, in most of the cases, purity of liquid in distilling
vessel is not achieved very high. Thus, we can achieve
better separation in the fractional distillation, as compared
to simple distillation, especially when two liquid mixtures
are not having high relative volatility.

d.) Fractional Continuous Distillation
For large capacity chemical plants, continuous fractional
distillation operations are used.
In the distillation terminology, section above feed point,
where we are enriching volatile material is called as,
Rectification Section. While section below feed point, where
we are increasing concentrating less volatile component, or
stripping out remaining volatile material, from the bottom
product stream, is called as Stripping Section of distillation
column.

Distillation Requirements
Following 3- major things are required in distillation:
1.) Vapor : Carrier of Energy & High Volatiles
2.) Liquid : Carrier of Less Volatiles
3.) Contact of Vapor & Liquid
1.) Vapor : Carrier of Energy & High Volatiles
On supply of heat to the liquid, it converts to vapor. For the
distillation process to occur, liquid need to be converted to
vapor at different stages which are arranged in the vertical
column.
Vapors are having high thermal energy (heat) as compared
to liquid of nearly same composition. As heat is required at
every stage in the distillation column and can’t be supplied
from the external heat energy sources, generated vapors
from one stage becomes the heat source for the next higher
stage internally. Being vapors are having natural property
to move up, so the vapors generated in the lower stages
naturally moves up for higher stages.

As the vapors move up, concentration of high volatile
materials increase. Vapors naturally moving up carries
heat as well as high volatile materials upside in a
distillation column.
2.) Liquid : Carrier of Less Volatiles
To carryout the distillation process, liquid inside the
distillation column is essentially required as the conversion
of liquid mixture to vapor only increases the concentration
of more volatile material in the vapor phase. As the vapors
moves up along with high concentration of more volatile
materials, they leave behind less volatile materials in the
liquid phase only or in other words concentration of high
volatile materials in the liquid phase reduces.
Liquid is having natural property to flow down i.e. under
gravity. Liquid flows down from the higher stages to lower
stages by the natural gravity. Thus liquid flows from the
higher stage of separation to the lower stage within
distillation column. As at every stage, high volatile material
is stripped off and leaving behind low volatile material in
the liquid phase.

Liquid along with downward movement carries less volatile
material and finally drawn from the bottom of the
distillation column as bottom product stream.
Sufficient amount of liquid should be present in the
distillation column to ensure its better contact with the
vapors required for simultaneous heat and mass transfer
within distillation column.
3.) Contact of Vapor & Liquid
Uprising vapors which carries heat as well as high volatile
materials need to be forcibly put in contact with down
flowing liquids which carries low volatile materials.
Thus inside a distillation column either packing or trays
(depending upon the fluid nature, operational
requirements, economics, etc.) are provided which ensures
sufficient space for the good contact between vapor and
liquid. Simultaneous heat transfer (by the condensation of
vapors in the liquid phase) and mass transfer (by mixing
high volatile material in the vapor phase with low volatile
material in the liquid phase followed by flashing of

resultant liquid to vapor phase containing higher
concentration of more volatile materials as per its
equilibrium) takes place in every stage. These equilibrium
stages are provided in the form of either packing or trays
within the distillation column.

Distillation Column Hardware
a.) Packed Column
Different types of packing are used in the packed type
distillation column are used to provide required surface for
the contact of vapors and liquids. Selection of packing type
depends upon its efficiency (fixed cost) & pressure drop
(operating cost).
Being liquid is supplied from the top, it is distributed
across the column cross section by the ‘Liquid Distributor’
provided at the top section of the distillation column.
Some times, depending upon the higher height of packing,
‘Liquid Redistributors’ are provided in the intermediate

sections of the distillation column. Vapors provided at the
bottom, naturally spread throughout the column cross
section. Flow of liquid is maintained so that the packing is
just wet and not flooded. Otherwise upward movement of
the vapors through the column will be obstructed. Even
less liquid flow rate will dry up part of the packing material
resulting less space or wet area available for vapor to
carryout simultaneous heat & mass transfer required for
the material separation.
b.) Tray Column
As the name suggests, distillation column is provided with
different types of trays (Sieve, Bubble Cap, Valve, Grid, etc.)

Selection of type of tray depends upon the their efficiency &
economics. Each tray is provided with the space for contact
of vapor and liquid to carryout simultaneous heat and
mass transfer.
Liquid coming from the top gets time over the tray for
better contact with the vapor. Each tray is provided with
the opening (in the form of sieve, riser, variable valve
opening, etc.) for vapors to bubble out through liquid level
maintained by the outlet weir provided at the end of each
tray. Liquid overflows from higher tray to lower through
down comer area provided with the help of down comer
plate which is sealed with the liquid at the next below tray.
This blocks the movement of vapors through the down
comer area. Thus vapors are forced to move through the
tray opening and the liquid flowing over the tray.
All the trays are maintained at certain distance which is
called as tray spacing. It is fixed based on the liquid type
and tray hydraulic calculations. It shall always be higher
than liquid level in the down comer side. Liquid
entrainment is also considered while fixing tray spacing.

Distillation System
Overall distillation system comprises of various hardware
component like equipment (distillation column, pump, heat
exchangers, etc.), pipelines (for feed, vapor, reflux, utilities,
etc.), instrumentation (for parameters like pressure,
temperature, etc. and controls). Overall system is divided
into following sub-systems:
1. Mixture Feeding,
2. Vapor Condensation,
3. Reflux,
4. Column Heating, and
5. Column Draws.

1.) Mixture Feeding
Feed condition (cold liquid, saturated liquid, saturated
vapor, etc.) required at the feed point of distillation column
and the available feed from the upstream process decides
the need of its conditioning. Generally feed pre-heater
provided with fresh heating utility or any other available
process stream required to be cooled, is used for
maintaining desired feed condition.
2.) Vapor Condensation
Top vapors generated in the distillation column are
condensed in the condenser with the help of fresh cooling
utility or any other process stream require heating.
3.) Reflux
To ensure sufficient liquid in the distillation column during
the start as well as normal plant operation, part of
condensed liquid from the condenser is refluxed back at
the top most tray of the column. This may be achieved
directly by the gravity flow from the condenser (if it is
located above the distillation column) or set of reflux tank
and pumps are used (if condenser is located below reflux
point).

4.) Column Heating
Throughout the distillation column, heating of liquid is
required to generate vapors. As discussed earlier, these
vapors generated at one stage becomes the heat source for
another. Bottom most tray or the bottom section of the
packing in the distillation column requires fresh heat
source. This is achieved either by putting direct steam (if
allowed) at the bottom of distillation column or steam or
any other heating utility indirectly through reboiler.
5.) Column Draws
As per the composition required, product draws are taken
from the top (for high volatile components), from the
bottom (for low volatile components) or from the
intermediate stage (for desired mixture of low and high
volatile components). Being these draws are at high
temperatures, they are cooled by top, bottom and
intermediate product coolers respectively.

Column Operating Parameters
Distillation column operation, is very critical for getting the
desired separation. Even small disturbance in the column
parameters, may greatly affect quality of separated product
streams.
These disturbances, are sensed by the column parameters
like pressure, composition, & temperatures, at various
locations.
Column Pressure
Distillation column pressure increases from the top of the
distillation column to its bottom.
PTop
PBottom
Increases

Composition
Vapor and liquid composition, varies across the complete
distillation column. More volatile materials, goes to top of
the distillation column, while less volatile materials, comes
out from the bottom. Concentration of more volatiles,
increases from bottom to top, while, concentration of less
volatiles, increases from top to bottom of distillation
column.
In the tray type column, every tray concentration profile
remains constant. Composition of liquid, as well as, vapor
phase remain constant at a specific tray location, though it
varies from the top to bottom.
Trend in Change in concentration of vapor and liquid
composition, in packed column remains same. But, the
concentration varies at every point throughout the
distillation column packed height.

Temperature
Similar to pressure and composition, temperature in
distillation column varies from top to bottom. As in the
distillation column, every time, liquid remains in the boiling
condition at every stage.
Boiling temperature, is dependent upon the concentration
of mixture, as well as pressure condition. For example,
pure water boils at 100 degree centigrade, and pure ethanol
boils at 78.3 degree centigrade. While their mixture of 50
% weight by weight, ethanol concentration, boils at 81.9
degree centigrade in atmospheric condition.
Top of the column will have nearly pure volatile material.
Also, its pressure will be close to atmospheric pressure, for
atmospheric distillation column. Thus, column
temperature will be nearly equal to, normal boiling point of
volatile material. While column bottom, will be nearly, pure,
less volatile material. But due to higher bottom pressure,
even in the atmospheric distillation column, its
temperature is higher than normal boiling temperature of
corresponding liquid. Thus, in the distillation column, top
temperature is low, while bottom temperature is high.

Column Design
Following steps are used for the tray type distillation
column design:
Step-1: Fix up Stream Flow Rates & Composition
Step-2: Fix up Thermal Condition of Feed
Step-3: Fix up Reflux Ratio
Step-4: Calculate Liquid & Vapor Load
Step-5: Find number of theoretical stages required
Step-6: Decide type of distillation column
Step-7: Design of Tray Type Distillation Column
Step-8: Height of Tray Type Distillation Column
Step-9: Diameter of Tray Type Distillation Column
For packed type distillation column design, Step-1 to 6,
remains same. And other steps for packing height &
column diameter calculations are added:
Step-7A: Calculate Packing Height
Step-8A: Calculate Column Diameter

Energy Conservation
Distillation is one of the most widely used unit operation,
in the chemical process industries. It is the major
consumer of energy, either in the form of heat, or
electricity. Thus, reduced consumption of energy in the
distillation operations, is the focus area of most of the
chemical plant stake holders.
Various energy conservation ways, which can be adopted to
conserve energy in the distillation system are:
1.) Good House Keeping.
2.) Optimization of Plant Operation.
3.) Hardware modification or replacement, and,
4.) Process Modification.

Troubleshooting
Major troubles of distillation are
• Low Plant Capacity
• Poor Quality of Separated Products
• High Consumption of Utilities
• Poor Efficiency/ Recovery
• Frequent Shut Downs
For solving any trouble, identification of its cause, is very
important. Once cause is identified, the given trouble can
be easily solved.
Here, we are classifying various causes into three major
categories, which are,
1. Fluid Specific
2. Hardware Specific
3. Manpower Specific

Fluid Specific Causes
Some of the Fluid Specific Causes, drawn from various
chemical industries, are listed here. They are,
•Liquid with Suspended Solids
• Liquid with Dissolved Solids
• Liquid with Trapped Gases
• Foaming Liquids, and,
• Corrosive Liquids
Hardware Specific Causes
Some of Tray Column related causes are,
•Outlet Weir Height
•Number of seives
• Down comer Area
• Wrong Location of Feed Nozzle
• Gap between Tray & Shell
Packed Column related causes are,
•Packing Size
•Choked Liquid Distributor
•Damaged Packing
•Choked Vapor Riser
•Excessive Liquid Entrainment

Manpower Specific Causes
People are involved in different phases, starting from the
plant design, to its erection at plant location. Any fault
done by them, becomes the cause of trouble in the plant
operation.
Various activities covering for the distillation systems are,
•Process Design
•Mechanical Design
•Procurement
•Fabrication
•Site Work

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