Distillation

1. Distillation

2. The Basics of Distillation

3. Definition
 Distillation An important organic process used to separate
two or more than two liquids having different boiling points from
a liquid mixture.
A process in which a liquid or vapour mixture of two or more
substances is separated into its component fractions of desired
purity, by the application and removal of heat.
Distillation is a physical separation process, not a chemical
reaction to purify an impure liquid.

4. Principle of distillation
1. Distillation method basis on the difference in boiling point of the
components of the mixture at standard pressure conditions.
2. Also, depending on the concentrations of the components present, the
liquid mixture will have different boiling point characteristics. Therefore,
distillation processes depends on the vapour pressure characteristics of
liquid mixtures.
3. At any boiling temperature a liquid is in equilibrium with its vapor.
4. Distillation is based on the fact that the vapour of a boiling mixture will
be richer in the components that have lower boiling points.
5. Therefore, when this vapour is cooled and condensed, the condensate
will contain more volatile components. At the same time, the original
mixture will contain more of the less volatile material.

5. DISTILLATION PRINCIPLES
Vapour Pressure and Boiling
The vapour pressure of a liquid at a particular temperature is the equilibrium
pressure exerted by molecules leaving and entering the liquid surface. Here are
some important points regarding vapour pressure
• vapour pressure is related to boiling
• a liquid is said to ‘boil’ when its vapour pressure equals the surrounding
pressure
• a liquid boils depends on its volatility
• liquids with high vapour pressures (volatile liquids) will boil at lower
temperatures
• the vapour pressure and hence the boiling point of a liquid mixture depends
on the relative amounts of the components in the mixture
• distillation occurs because of the differences in the volatility of the
components in the liquid mixture

6. History
• The first clear evidence of distillation comes from Greek alchemists
working n Alexandria in the firs t century AD.
• Distilled water has been known since at least ca. 200 AD.
• Arabs learned the process from the Egyptians and used it extensively
in their alchemical experiments.
• The first clear evidence from the distillation of alcohol comes from the
School of Salerno in the 12th century.
• Fractional distillation was developed by Tadeo Alderotti in the 13th
century Alderotti in the 13th century.
• In 1500, German alchemist Hieronymus Braunschweig published the
Book of the Art of Distillation the first book solely dedicated to the
Distillation, the first book solely dedicated to the subject of distillation.
• In 1651, John French published The Art of Distillation the first major
English compendium of practice This includes diagrams with
compendium of practice. This includes diagrams with people in them
showing the industrial rather than bench scale of the operation.

7. • Early forms of distillation were batch processes using one
vaporization and one condensation Purity was one vaporization
and one condensation.
• Purity was improved by further distillation of the condensate.
Greater volumes were processed by simply repeating the
distillation.
• In the early 19th century the basics of modern techniques
including pre techniques including pre -heating and reflux were
developed, particularly by the French, then in 1830 a British
Patent was issued for a whiskey distillation column, which
worked continuously and may be regarded as the archetype of
modern petrochemical units.
• In 1877, Ernest Solvay was granted a U.S. Patent for a tray
column for ammonia distillation and the subsequent years saw
developments o f this theme for oil and spirits.

8. Boiling point:
• The boiling point is defined as the temperature at which the
saturated vapor pressure of a liquid is equal to the surrounding
atmospheric pressure.
• If the liquids are heated at constant pressure, for example at
atmospheric pressure, the vapor pressure increases in proportion
to the heat supplied. Once the vapor pressure of the liquid is equal
to the pressure of the outside atmosphere, the liquid begins to
boil. The temperature at which the vapor pressure equals the
atmospheric pressure of the outside is called the boiling point of
the liquid.
• If more heat is given to a liquid at the boiling point, the
temperature of the liquid does not increase, but the heat supplied
ensures that the liquid becomes vapor and the temperature
remains constant until the liquid evaporates completely.

9. The Boiling Point Diagram (BPD)
• The BPD shows how the equilibrium compositions of the components in a
liquid mixture vary with temperature at a fixed pressure. Consider an
example of a liquid mixture containing 2 components (A and B) - a binary
mixture. This has the following boiling point diagram.
• The boiling point of A is that at which the
mole fraction of A is 1. The boiling point
of B is that at which the mole fraction of A
is 0. In this example, A is the more volatile
component and therefore has a lower
boiling point than B. The upper curve in
the diagram is called the dew-point curve
while the lower one is called the bubble-
point curve.
• The dew-point is the temperature at which
the saturated vapour starts to condense.
• The bubble-point is the temperature at
which the liquid starts to boil.

10. How does it work
Distillation is a three step process, heating of liquid to change
the phase in vapor form, condense the liquid and last one is
collecting the condensed liquid.
1. As the mixture is heated, the temperature rises until it reaches the
temperature of the lowest boiling substance in the mixture,
2. The other components of the mixture remain in their original phase in
the mixture.
3. The resultant hot vapor passes into a condenser and is converted to
the liquid, and then collected in a receiver flask.
4. The other components of the mixture remain in their original phase
until the most volatile substance has all boiled off.
5. Then the temperature of the gas phase rises again until it reaches the
boiling point of a second component in the mixture, and so on.

11. Distillation techniques depends on
several factors
• The difference in vapor pressure (related to the difference in the
boiling points) of the components present.
• the size of the sample.
• The distillation apparatus.
It consumes enormous amounts of energy, both in terms of cooling
and heating requirements
It can contribute to more than 50% of plant operating costs
The best way to reduce operating costs of existing units, is to improve
their efficiency and operation via process optimization and control. To
achieve this improvement, a thorough understanding of distillation
principles and how distillation systems are designed is essential.
Idealized models of distillation are essentially governed by Raoult's law
and Dalton's law, and assume that vapor-liquid equilibria are attained.

12. Relative volatility
Volatility
• The volatility of A, It is defined as the ratio of the partial pressure of A to
the mole fraction of A in a liquid phase.
• Volatility of A = pA/xA Volatility of B = pB/xB
• Relative volatility is a measure of the differences in volatility between 2
components, and hence their boiling points. It indicates how easy or difficult a
particular separation will be. The relative volatility of component A with
respect to component B is the ratio of the volatility of A (the more volatile
component) to the volatility of B. It is also known as the volatility of A
with respect to B and is denoted by the symbol αAB.
•
αab=
𝑌𝑎
𝑋𝑎
𝑌𝑏
𝑋𝑏
Ya = mole fraction of component ‘a’ in the vapour
Xa = mole fraction of component ‘a’ in the liquid
• Thus if the relative volatility between 2 components is very close to one, it
is an indication that they have very similar vapour pressure characteristics.
This means that they have very similar boiling points and therefore, it will
be difficult to separate the two components via distillation.

13. Some optimal relative volatility that are used for distillation process design
Relative volatility is the measure of separability by distillation. When @=1 a separation
by distillation is not possible. The separation by distillation is possible for relative
volatility values greater than one. The larger the value of the relative volatility, the
easier the separation by distillation.

14. Role of Raoult’s Law and Dalton’s Law
The boiling point of a liquid: temperature, at which the liquid is
converted into its vapor.
• It changes with the surrounding pressure. E.g. the boiling point of water
at sea level is 100oC but its boiling point at an altitude of 1905 meters is
93.4oC (since the atmospheric pressure is lower at high altitudes).
• It is a misconception that in a liquid mixture at a given
pressure, each component boils at the boiling point
corresponding to the given pressure, allowing the vapors of
each component to collect separately and purely.
• However, this does not occur, even in an idealized system.
Idealized models of distillation are essentially governed
by Raoult's law and Dalton's law and assume that vapor–
liquid equilibria are attained.

15. • As per Raoult’s law, the partial pressure of a single liquid component in an ideal liquid mixture
equals the product of the vapor pressure of the pure component and its mole fraction.
• Raoult’s law assumes that a component contributes law assumes that a component contributes to
the total vapor pressure of the mixture in proportion to its percentage of the mixture and its vapor
pressure when pure partial pressure equals mole fraction multiplied by vapor pressure when pure
• According to Dalton’s law of partial pressures, the total pressure exerted by a mixture of gases is
equal to the sum of the partial pressures of all the constituent gases.
• When a mixture of liquids (a multi-component liquid) is heated, the vapor pressure of the
individual components increases, which in turn increases the total vapor pressure.
• When the total vapor pressure reaches the pressure surrounding the liquid,
reaches the pressure surrounding the liquid, boiling occurs and liquid turns to
gas throughout the bulk of the liquid
• Therefore, the mixture cannot have multiple boiling points at a given composition and pressure.

16. Vapour-Liquid Equilibrium (VLE)
A two phase multicomponent mixture is said to be in equilibrium if;
 The temperature Tv of the vapor phase is equal to Tl of the liquid phase.
 The total pressure Pv throughout the vapor phase is equal to the total
pressure Pl throughout the liquid phase.
 The tendency of each component to escape from the liquid phase to the
vapor phase is exactly equal to its tendency to escape from the vapor phase
to the liquid phase.
In the following analysis it is assumed that Tv = Tl = T, Pv = Pl = P
and the escaping tendencies are equal. A special case of the third
condition for equilibrium is represented by Raoult’s law.

17. Raoult's law
• It states that the vapor pressure of a solvent above a solution is equal to the
vapor pressure of the pure solvent at the same temperature scaled by the mole
fraction of the solvent present:
• This law applies to ideal solutions, or solutions that have
different components but whose molecular interactions are the
same as or very similar to pure solutions.
• P (solution) =χ(solvent) P(solvent)
– P (solvent) = vapour pressureof pure 100% A
– X= the mole fraction of liquid A (solvent) in a solution (mixture)
• A volatile liquid is one with a relatively high vapor pressure at room
temperature.
• Ideal solutions are uniform mixtures of components that have physical
properties connected to their pure components. These solutions are supported
by Raoult’s law stating that interactions between molecules of solute and
molecules of solvent are the same as those molecules each are by
themselves. An example of ideal solutions would be benzene and toluene.
• Azeotropes fail to conform to this idea because, when boiling, the component
ratio of unvaporized solution is equal to that of the vaporized solution. So an
azeotrope can be defined as a solution whose vapor has the same composition
its liquid. Virtually all liquids, when mixed and heated, will display azeotropic
behavior.

18. • That plot would be all there is if the second component was just a
dissolved solid with no vapor pressure of its own.
• However, if you have the second component to be another volatile
liquid (say liquid B) then you have to also apply Raoult's Law to that
liquid as well and get
• P yB = PB xB,
• In a mixture of two liquids A and B, then we can actually show both
the vapor pressure of A and the vapor pressure of B on the same
plot.
• Daltons law, It states that the total pressure exerted by a gas/vapour
mixture is equal to the sum of the partial pressures of components
present in it. Thus, it expresses the addictive nature of the partial
pressures.
• We can even show that the mixture's vapor pressure is just a
combination of the two separate liquids following the law...
• P Total= P yA +P yB = PA xA + PB xB,

19.  This relationship, derived from Raoult’s law, is capable of describing
the boiling point behavior of compound A in a mixture of compounds
under a variety of different circumstances.
The boiling point of the solution is reached when PT is equal to the
pressure applied to the surface of the solution.
One might expect the vapor pressure of a solution of ethanol and
water to be directly proportional to the sums of the values predicted
by Raoult's law for the two liquids individually, but in general, this so-
called "ideal" behavor is not observed. The reason for this can be
understood if you recall that Raoult's law reflects a single effect: the
smaller proportion of vaporizable molecules (and thus their reduced
escaping tendency) when the liquid is diluted by an otherwise "inert"
substance.

21. • Application of distillations
Distillation is used to:
The process of distillation can be used to purify liquids or to separate
them into their individual components.
• Separation of crude oil components.
• Water purification techniques: Remove impurities from Water. In
desalination plants to obtain drinking water from seawater.
• Distilled water has numerous applications, such as in lead-acid
batteries and low-volume humidifiers.
• Distillation of fermented solutions to produce distilled beverages
with a higher alcohol content.
• in the production of alcohol, essential oils,

22. • Many perfumes and food flavorings are obtained from herbs and
plants via distillation.
• Oil stabilization is an important type of distillation that reduces the
vapor pressure of the crude oil, enabling safe storage and
transportation.
• Air can be separated into nitrogen, oxygen, and argon by
employing the process of cryogenic distillation.
• Distillation is also employed on an industrial scale to purify the
liquid products obtained from chemical synthesis.

23. According to the differences in boiling points
between the liquids, distillation process
classified into six types:-
(Several laboratory scale techniques)
1. Simple distillation.
2. Steam distillation.
3. Vacuum distillation.
4. Fractional distillation.
5. Azeotropic Distillation
6. Extractive Distillation

24. 1. Simple Distillation

25. Simple distillation
 Simple distillation is the basic type of distillation process in which a liquid mixture
of two components is heated until the substance with a lower boiling point starts
vaporizing.
 The vapor is then condensed with the help of a condenser with suitable utility and
then collected in another vessel.
 Simple distillation is used to separate the mixture of the liquids dissolved in each
other and the difference in boiling point between various liquid components
should be at least 25˚C.
 Separating liquids boiling below 150 ˚C at 1 atm.
 The purity of the distillate (the purified liquid) is governed by Raoult’s law.
 Simple distillation involves a single equilibration between the liquid and vapor. This
distillation is referred to as involving one theoretical plate.

26. Separation of liquid mixture by simple distillation:-
1. For example; hydrocarbons, alcohols, esters, small molecule fatty
acids, amines are purified by this method.
2. A mixture composed of (acetone) and (water) with boiling point
(56 & 100) °C respectively is heated.
3. The lowest boiling point (acetone) will vaporized and ascended
(elevated) from the solution till it reach the top of the system, with
recording its real b.p. with the help of thermometer.
4. The ascended (rises) vapor (acetone) will converts to the liquid
form by the action of the condenser, then collect at the receiver.
5. Finally the highest boiling point (water) will remain in the
distillation flask.

28. 2. Steam Distillation

29. Different Parts of Distillation
Rectifying
Section
Stripping Section
Overhead Vapor
Condenser
Reflux
Overhead Product
Reboiler
Bottoms Product

30. 2. Steam distillation
• The basic type of distillation process in which steam is introduced to the
mixture which is to heat the liquid mixture which will increase the vapor
pressure of the components.
• When the vapor pressure of the immiscible components outplace the
atmospheric pressure, the high boiling component will start evaporating
at low temperatures and form a mixture with water.
• Steam distillation is used to separate out the heat-sensitive components
which can decompose at high temperatures. These compounds should be
immiscible with water. The goal is to heat and separate the components
at temperatures below their decomposition point.
• A common example of Steam distillation is it is used to extract oil from
plant matter.
• Steam distillation is useful for the purification of organic compounds,
although vacuum distillation is more common. When organics are
distilled, the vapor is condensed. Because water and organics tend to be
immiscible, the resulting liquid generally consists of two phases: water
and the organic distillate.
• Steam distillation is the preferred method used to isolate essential oils. It
is also used for "steam stripping" in petroleum refineries and to separate
commercially important organic compounds, such as fatty acids.

31. 3. Vacuum Distillation
• The distillation process can be classified as atmospheric distillation
and vacuum distillation.
• The distillation process which is conducted under vacuum or negative
pressure is known as vacuum distillation.
• It is similar to atmospheric distillation but the only difference is
vacuum distillation is conducted under vacuum.
• The boiling point is directly proportional to pressure.
• If the boiling point of any substance present in the mixture is high, in
that case we need to use utility which can help to achieve the boiling
point of that high boiling component which will be costly.
• So to avoid using that utility, vacuum distillation is used in which
vacuum is created in the column which will decrease the boiling point
of component and hence separation can be achieved at low
temperature.

32. Vacuum distillation
• Distillation under reduced Pressure (or) Vaccum
distillation.
• It is used for organic compounds which decompose at or
below their boiling points. Example: Glycerol.
• If we have an organic substance which decompose at its
boiling point, we can make it to boil at a temperature
lower than its boiling point. All that we have to do is to
create a partial vaccum. Under reduced pressure, the
substance boils at a much lower temperature and distils
over undecomposed.

33. 4. Fractional Distillation
• Fractional Distillation the principle behind fractional distillation is
different components in the mixtures boils at different temperatures.
• In fractional distillation, so the mixture is heated and the low boiling
substance starts evaporating first, condenses the liquid first, and
separate out.
• Now increase the temperature and similarly separate out the
components from lower boiling point to higher boiling point.
• Fractional distillation is carried out commonly in refineries where the
distillation of crude oil is carried out.
• Different components like petrol, diesel, kerosene, naptha are
separated from the crude oil using fractional distillation as the boiling
points of these substances present in the crude oil are different.

34. 5. Azeotropic distillation
• Azeotropic distillation is carried out when we have a mixture of
immiscible liquid which cannot be separated using simple distillation as
the difference in boiling point is very low and such mixtures are called
an azeotropic mixture.
• Azeotropic mixtures are the constant boiling mixture in which they
boils at a constant boiling point and the composition of components in
vapor are the same as the in the liquid.
• If we carried out the simple distillation of the azeotropic mixture,
vapor generated by heating the mixture will contain both the
component, hence separation though the simple distillation
method can’t be done.
• In this case, the Azeotropic distillation process is carried out.

35. There are two types of azeotropes i.e.
1. Minimum boiling azeotrope
• In 95.63% ethanol and 4.37% water (by mass) mixture, the
boiling point of ethanol is 78.4 °C and the boiling point of
water is 100 °C. They form an azeotrope and the boiling point
of the azeotrope is 78.2 °C which is lower than both the
component, hence it is a minimum boiling azeotrope.
2. Maximum boiling Azeotrope.
• In a mixture of hydrogen chloride, the boiling point of
hydrogen chloride is -84 °C and boiling point of water is 100
°C and the boiling point of azeotrope is 110 °C. As the boiling
point of the azeotrope is more than all both the component, it
is called maximum boiling point.

36. • In the Azeotropic distillation method, in the mixture of
supposing A and B, an additional substance i.e. C is added
which has the ability to alter the activity coefficient of the
components and herby change the relative volatility of the
mixture.
• The third component added to the mixture is known as
entrainer which will create either minimum-boiling with
one of the components.

37. Entrainer
• In Azeotropic distillation, entrainer plays an important role as this makes the
azeotropic distillation process happen.
• a specific chemical that is being added to the mixuture which alters the
relative volatility of the mixture and form a new azeotrope with either of the
components.
• The entrainer selected must have the same volatility as the original
azeotropic mixture to be separated.
The selection criteria for entrainer
• It should enhance the relative volatility of the key component significantly.
• Require solvent quantity to be added to the azeotropic mixture should not
be excessive.
• It should be soluble in the feed components.
• It should be relatively inexpensive and easily available.
• It should be stable at the operating condition of both the column.
• it should be non-reactive with the feed components.
• It should have low latent heat of vaporization.
• it should be non-corrosive and non-toxic.
• It should not form an immiscible liquid mixture at any point in the column.

38. Azeotropic distillation ethanol-water
• also be referred to as dehydration of 92.4 wt% ethanol
using cyclo-hexane as a entrainer
• Component Boiling Point
• Ethanol 78.25 °C
• Water 100 °C
• Azeotrope 78.09 °C
• Cyclo-Hexane 80.74 °C

39. Azeotropic distillation ethanol-water

40. Two distillation columns C1 and C2 and one decanter.
• In Column C1, 89% mole fraction ethanol is feed where the
cyclohexane is added as an entertainer (recycled from the
decanter).
• This entrainer forms a Minimum boiling azeotrope with water.
• From the column C1, where we get pure ethanol from the bottom
and from the top we get tertiary azeotrope. The vapor from the
top of column C1 is condensed which is a heterogeneous mixture
into the decanter and here get two different phases i.e. organic
layer and the aqueous layer.
• The organic layer is rich in entrainer and is recycled back to
column C1 where the aqueous layer is feed to column C2 for
further separation.
• In column C2, again separation is performed and we get pure
water from the water and the top product containing ethanol-
water and entrainer.
• The vapor from the top of column C2 is condensed and is feed
back to column C1.

41. 6. Extractive Distillation
• Extractive Distillation can be defined as distillation
performed in the presence of a miscible, high-boiling,
relatively non-volatile component, the solvent, that forms
no azeotrope with the other components in the mixture.
• This method is used for mixtures having a low value of
relative volatility, nearing unity.
• In this method, an additional component called entrainer
is added to the mixture but it will not form any azeotrope
but act differently with the component of the mixture and
cause a change in relative volatility to allow the new
three-part mixture to be separated by normal distillation.

42. Laboratory scale distillations
• almost exclusively run as batch distillations exclusively
run as batch distillations.
• The device used in distillation, sometimes referred to as a
referred to as a still, consists at a minimum of
• a reboiler or pot in which the source material is heated,
• a condenser in which the heated vapour is cooled back to the liquid
state, and
• a receiver in which the concentrated or purified liquid, called the
purified liquid, called the distillate is collected.

43. Industrial distillation of alcohol
• To further amplify the alcoholic content created during
fermentation, spirits take the next step called distillation.
To distill spirits means to purify or concentrate it. In
layman's terms, the fermented spirit is heated at a low and
controlled temperature using a still.
• The two basic types of still are:
• Pot still and Column Still
• Column stills are more industrial in nature and used for
large-scale manufacturing of alcohol. They are said to
deliver purer and lighter spirits.

44. Pot Still
Pot stills are more traditional and are known for producing fuller-bodied
and fuller-flavoured spirits.
The distillation of craft spirits is conducted in classic copper pot stills.
Process of distillation of spirits inside a pot still.
1. The fermented spirit, which at this point is called the "wash" or the
"mash," is transferred into the pot still.
2. It will be heated to a low temperature, which could be anywhere from
175 degrees to 212 degrees Fahrenheit.
3. The goal of the heating is to vaporize only alcohol. When condensation
happens, the alcohol-rich steam will rise above the surface of the liquid
and will be caught into a tube known as the lyne arm or swan's neck,
which is connected to the condenser.
4. The process condenses back the steam into a liquid, which now has
significantly higher amounts of alcohol concentration.
5. The distilled spirit will flow out of the still and collected.

45. Industrial Distillation processes
• Large scale industrial distillation applications include both
batch and continuous fractional, vacuum, azeotropic,
extractive, and steam distillation.
• The most widely used industrial applications of continuous,
steady-state fractional distillation are in petroleum
refineries, petrochemical and chemical plants and natural
gas processing plants, alcohol industry.
• To control and optimize such industrial distillation, a
standardized laboratory method, ASTM D86, is
established.

46. Industrial distillation tower
• Industrial distillation is typically performed in
large, vertical cylindrical columns known as
distillation towers or distillation columns with
diameters ranging from about 0.65 to 16
metres (2 ft 2 in to 52 ft 6 in) and heights
ranging from about 6 to 90 meters (20 to
295 ft) or more.
• When the feed has a diverse composition,
liquid outlets at intervals up the column allow
for the withdrawal of different products having
different products having different boiling
points boiling points.
• The "lightest" products (those with the lowest
boiling point) exit from the top of the columns
and the "heaviest" products (the highest
boiling point) exit from "heaviest" products
(the highest boiling point) exit from the
bottom and are often called the bottoms.

47. Main Components of Distillation Columns
Distillation columns are made up of several components, each of which is used
either to transfer heat energy or enhance material transfer. A typical distillation
contains several major components:
– A vertical shell where the separation
of liquid components is carried out
– column internals such as trays/plates
and/or packing's which are used to
enhance component separations
– A Reboilers to provide the necessary
vaporization for the distillation
process
– a condenser to cool and condense the
vapour leaving the top of the column
– a reflux drum to hold the condensed
vapour from the top of the column so
that liquid (reflux) can be recycled
back to the column

48. Cont..
• The liquid mixture that is to be processed is known as the feed and this is
introduced usually somewhere near the middle of the column to a tray
known as the feed tray. The feed tray divides the column into a top
(enriching or rectification) section and a bottom (stripping) section. The
feed flows down the column where it is collected at the bottom in the
reboiler.
Heat is supplied to the reboiler to generate
vapour. The source of heat input can be
any suitable fluid, although in most
chemical plants this is normally steam. In
refineries, the heating source may be the
output streams of other columns. The
vapour raised in the reboiler is re-
introduced into the unit at the bottom of
the column. The liquid removed from the
reboiler is known as the Bottoms product
or simply,

49. Cont...
• The vapour moves up the column,
and as it exits the top of the unit, it
is cooled by a condenser. The
condensed liquid is stored in a
holding vessel known as the reflux
drum. Some of this liquid is
recycled back to the top of the
column and this is called the reflux.
The condensed liquid that is
removed from the system is known
as the distillate or top product.
• Thus, there are internal flows of
vapour and liquid within the
column as well as external flows of
feeds and product streams, into and
out of the column.

50. Design & operation of a distillation tower
• depends on the feed and desired products
• Given a simple, binary component feed, analytical
methods such as the McCabeThiele method or the
Fenske equation can be used.
• For a multi-component feed, simulation models are used
both for design and operation
• Moreover, the efficiencies of the vapor-liquid contact
devices (referred to as "plates" or "trays") used in
distillation towers are typically lower than that of a
theoretical 100% efficient equilibrium stage.
• Hence a distillation tower needs more trays Hence, a
distillation tower needs more trays than the number of
theoretical vapor-liquid equilibrium sta ges.

51. TYPES OF DISTILLATION COLUMNS
There are many types of distillation columns, each designed to perform
specific types of separations, and each design differs in terms of complexity.
Batch and Continuous Columns
One way of classifying distillation column type is to look at how they are
operated. Thus we have:
batch and
continuous columns.
Batch Columns
 In batch operation, the feed to the column is introduced batch-wise. That is,
the column is charged with a 'batch' and then the distillation process is
carried out. When the desired task is achieved, a next batch of feed is
introduced.

52. Continuous feed stream.
• In Continuous distillation a liquid mixture is continuously
(without interruption unless there is a problem with the
column or surrounding process units) fed into the process
and separated fractions are removed continuously as
output streams as time passes during the operation.
• Continuous distillation produces at least two out put
fractions, including
– at least one volatile distillate fraction, which has boiled and
been separately captured as a vapor condensed to a liquid.
– There is always a bottoms (or residue) fraction, which is the
least volatile residue that has not been separately captured as a
condensed vapor.
They are capable of handling high throughputs and are the
most common of the two types.

53. • Continuous distillation differs from batch distillation in the
respect that concentrations do not change over time.
• Continuous distillation can be run at a steady state for an
arbitrary amount of time.
• For any source material of specific composition, the main
variables that affect the purity of products in continuous
distillation are the reflux ratio and the number of
theoretical equilibrium ratio and the number of theoretical
equilibrium stages (practically, the number of trays or the
height of packing).
• Reflux is a flow from the condenser back to the column
which generates a recycle that allows a column, a better
separation with a given number of trays.

54. • Equilibrium stages are ideal steps where compositions
achieve vapor-liquid equilibrium repeating the
separation process equilibrium, repeating the
separation process and allowing better separation
given a reflux ratio.
• A column with a high reflux ratio may have fewer
stages but it refluxes a large amount fewer stages, but
it refluxes a large amount of liquid, giving a wide
column with a large holdup.
• Conversely, a column with a low reflux ratio must have
a large number of stages, thus requiring a taller column

55. MPR distillation
• The MultiPressure system saves energy by recovery and reuse of
secondary energy without impacting the overall plant performance.
• The advanced MultiPressure system operates a number of columns
at different pressure levels. The finely-balanced application of
vacuum, atmospheric and overpressure allows to reuse the heat
input multiple times.
• This brings about a significant reduction in live steam consumption,
thus generating equivalent (cost) savings in the energy required to
produce it.

56. Cont..
Types of Continuous Columns
Continuous columns can be further classified according to:
The nature of the feed that they are processing,
 binary column - feed contains only two components
 multi-component column - feed contains more than two components
The number of product streams they have
– multi-product column - column has more than two product streams
where the extra feed exits when it is used to help with the separation,
– extractive distillation - where the extra feed appears in the bottom product stream
– Azeotropic distillation - where the extra feed appears at the top product stream
The type of column internals
– Tray column - where trays of various designs are used to hold up the liquid to provide
better contact between vapour and liquid, hence better separation
– Packed column - where instead of trays, 'packings' are used to enhance contact between
vapour and liquid

57. Types of Distillation Columns
• Packed Bed Columns
 Used more often for absorption and
distillation of vapor-liquid mixtures.
 The liquid flows downward through the packing and
the vapor flows upward through the column.
• Advantages
 Cost efficient
 Lower pressure drop
 Good for thermally sensitive liquids
• Disadvantages
 Packing can break during installation

58. Types of Distillation Columns
•Tray Column
 The number of trays or stages is dependent
• Advantages
 Cost efficient
 Can handle high liquid flow rates
• Disadvantages
 Higher pressure drops than packed columns
 Foaming can occur due to agitation

59. Multi-stage Continuous distillation column design
• The vapour-liquid equilibrium characteristics (indicated by the shape of the
equilibrium curve) of the mixture will determine the number of stages, and
hence the number of trays, required for the separation.
• This is illustrated clearly by applying the McCabe-Thiele method to design a
binary column.
McCABE-THIELE DESIGN METHOD
• The McCabe-Thiele approach is a graphical one, and uses the VLE plot to
determine the theoretical number of stages required to effect the separation
of a binary mixture.
• It assumes constant molar overflow and this implies that:
Molar heats of vaporization of the components are roughly the same
Heat effects (heats of solution, heat losses to and from column, etc.) are negligible
For every mole of vapour condensed, 1 mole of liquid is vaporized

60. Cont...
The design procedure is simple. Given the VLE diagram of the binary
mixture, operating lines are drawn first.
• Operating lines define the mass balance relationships between the
liquid and vapour phases in the column.
• There is one operating line for the bottom (stripping) section of the
column, and one for the top (rectification or enriching) section of
the column.
• Use of the constant molar overflow assumption also ensures the
operating lines are straight lines.

61. Cont..
The steps to be followed to determine the
number of theoretical stages by McCabe-
Thiele Method:
1. Determination of the Rectifying
section operating line (ROL).
2. Determination the feed condition (q).
3. Determination of the feed section
operating line (q-line).
4. Determination of required reflux ratio
(R).
5. Determination of the stripping
section operating line (SOL).
6. Determination of number of
theoretical stage.

62. Advanced Distillation
Reactive Distillation
Vacuum Distillation
Extractive Distillation