Distillation Concepts

By
Pankaj Khandelwal
Chemical Engineer & Corporate Trainer
Koncept Learning Center
DISTILLATION
CONCEPTS
Published By
KLC-201

Distillation Concepts
Contents
a.) Introduction
b.) Vapor – Carrier of Energy & High Volatiles
c.) Liquid – Carrier of Less Volatiles
d.) Contact of Vapor & Liquid
e.) Distillation System
f.) Available Theory from Literature
g.) Experiential Learning – I
h.) Experiential Learning – II
i.) Experiential Learning – III
j.) Experiential Learning - IV
P-1Koncept Learning Center klcenter@gmail.com

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.
Pankaj Khandelwal

Introduction
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’.
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
Fig.1Liquid
50 % v/v ‘A’
50 % v/v ‘B’
Vapor
70 % v/v ‘A’
30 % v/v ‘B’
Heat

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

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. For example, 1 kg of liquid water at
100oC carries 100 kcal heat while 1 kg of water vapors carries 640
(=100 + 540) kcal heat. (considering reference temperature for both
the cases as 0oC of liquid water). Thus vapors carries 540 kcal
additional heat energy which is latent heat of vaporization, than its
liquid phase.
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 shown in Fig.3.
Stage-I
Stage-II
Stage-III
Vapors – Heat Source for Stage-I
Vapors – Heat Source for Stage-II
Vapors – Heat Source for Stage-III
Fig.3
As discussed earlier, on
conversion of liquid to vapor,
concentration of volatile material
increases in the vapor phase.
This happens at every stage.
Thus the vapors generated at the
stage-I contain higher volatile
materials more than the vapors
generated at the stage-II. Same is
applicable for other stages too.
Thus 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.

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, as shown in Fig.4. 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.
Stage-I
Stage-II
Stage-III
Fig.4

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.
a.) Packed Column
Different types of packing (see Fig.5) 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
Fig.5

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.) as shown in
Fig.6.
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.
Fig.6
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
Fig.7
Outlet Weir
Down comer
Inlet WeirTray
Spacing

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.
As liquid falls from the upper tray to the lower tray through down
comer area, high turbulence causes fluctuations in the liquid level
over the tray. To minimize it, inlet weir is provided in the trays.
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:
a.) Feed Preheating
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.
b.) 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.
c.) Reflux Flow
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
Fig.8

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).
d.) 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.
e.) Product Stream 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.

Available Theory from Literature
Vapor-Liquid Equilibrium (VLE)
It describes the distribution of a chemical species between the
vapour phase and a liquid phase. Typical VLE for the binary liquid
mixture is shown in Fig.9.
Reflux Ratio
The Reflux ratio is the ratio between the boil up rate and the take-off
rate. Or in other words, it is the ratio between the amount of reflux
that goes back down the distillation column and the amount of
reflux that is collected in the receiver (distillate).
At total reflux, the number of theoretical plates required is a
minimum. As the reflux ratio is reduced (by taking off product), the
number of plates required increases. The Minimum Reflux Ratio (R
min) is the lowest value of reflux at which separation can be
achieved even with an infinite number of plates.
The total cost, which is the sum of fixed cost and operating cost,
must therefore passes through a minimum. The reflux ratio at this
minimum total cost is the optimum (or economical) reflux ratio.
Fig.9

Experiential Learning-I
In a binary liquid mixture of n-Heptane & n-Octane at 1 atm
pressure, find the equilibrium vapor concentrations for the
liquid of following concentrations:
Liquid Composition Vapor Composition
(Mole Fraction of n-Heptane) (Mole Fraction of n-Heptane)
0.10 ---------
0.20 ---------
0.30 ---------
0.40 ---------
0.50 ---------
0.60 ---------
0.70 ---------
0.80 ---------
0.90 ---------
0.95 ---------

Experiential Learning-II
Estimate the pressure inside distillation column at following
points for the scheme shown in the figure for three operating
conditions:
Atmospheric
Condition
Column in
Pressure
Column under
Vacuum
Column Top
Pressure
Feed Point
Pressure
Column Bottom
Pressure

Experiential Learning-III
Estimate the temperature inside distillation column at
following points for the scheme shown in figure for three
operating conditions:
Atmospheric
Condition
Column in
Pressure
Column under
Vacuum
Column Top
Temperature
Feed Point
Temperature
Column Bottom
Temperature

Experiential Learning-IV
Carry out trend analysis for following cases in the distillation
Columns.
What will happen if:
a.) Number of sieves provided in sieve tray column are more or less
than required
b.) Outlet weir height is lower or higher than required
c.) Energy consumption in distillation column if reflux ratio is more
or less than required at full capacity plant operation
d.) Column diameter if reflux rate is increased or reduced
e.) Column diameter if plant capacity is doubled or halved
f.) Feed concentration is increased or reduced
g.) Tray spacing less or more than required
h.) Feed temperature is less or more than required
i.) Down comer area provided less or more than required

NOTES

Koncept Learning Center (KLC) was established in
2001 with the aim of imparting knowledge and providing training programs from
students level to senior management level people of the industry. Our prime
objective is to upgrade conceptual knowledge of everyone for attaining
excellence in the chemical process industries.
Pankaj Khandelwal, a Chemical Engineer (BE-IIT,Roorkee; M.Tech-IIT,Kanpur;
DBM-IGNOU) with 14 Years of Industrial Work Experience (Research & Development –
Grasim Industries; Basic Engineering – Uhde India; Project Engineering – Atul Products; Process &
Commissioning Engineering – Praj Industries) and 17+ Years of Teaching/Training
Experience in various Engineering Colleges & Chemical Industries. Author of 35
Technical & Management Articles in International and Indian Journals, book
“Present Your Thoughts Effectively” and over 100+ Letters in English
Newspapers.
About the Trainer
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