Unit operation II lab

Distillatio
n

1. Introduction :
Distillation is method of separation of components from a liquid mixture which
depends on the differences in boiling points of the individual components and the
distributions of the components between a liquid and gas phase in the mixture.
The liquid mixture may have different boiling point characteristics depending on
the concentrations of the components present in it. Therefore, distillation
processes depends on the vapor pressure characteristics of liquid mixtures. The
vapor pressure is created by supplying heat as separating agent. In the distillation,
the new phases differ from the original by their heat content. During most of the
century, distillation was by far the most widely used method for separating liquid
mixtures of chemical components (Seader and Henley, 1998). This is a very
energy intensive technique, especially when the relative volatility of the
components is low. It is mostly carried out in multi tray columns. Packed column
with efficient structured packing has also led to increased use in distillation [1].

2. Types of distillation :
2.1 Batch distillation
In batch operation, the feed is introduced batch-wise to the column. 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. Consider a
binary mixture of components A (more volatile) and B (less volatile). The system
consists of a batch of liquid (fixed quantity) inside a kettle (or still) fitted with
heating element and a condenser to condense the vapor produced as shown in
Figure 1.1. The condensed vapor is known as the distillate. The distillate is
collected in a condensate receiver. The liquid remaining in the still is known as
the residual. The process is unsteady state. The concentration changes can be
analyzed using the phase diagram, and detailed mathematical calculations carried
out using the Rayleigh Equation [1].

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Figure 1.1: Simple batch or differential distillation process

2.2. Continuous distillation columns

In contrast, continuous columns process a continuous feed stream. No
interruptions occur unless there is a problem with the column or surrounding
process units. They are capable of handling high throughputs. Continuous column
is the more common of the two types.Types of Continuous Columns Continuous
columns can be further classified according to the nature of the feed that they are
processing:
Binary distillation column: feed contains only two components Multi-
component distillation column: feed contains more than two components the
number of product streams they have
Multi-product distillation 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 distillation column: where trays of various designs are used to hold up the
liquid to provide better contact between vapor and liquid, hence better separation.
The details of the tray column are given in Module 4.

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Packed distillation column: where instead of trays, 'packings' are used to enhance
contact between vapor and liquid. The details of the packed column are given in
Module 4 [1].

Figure 1.2: Flash distillation process

Figure 1.3: Multi-stage binary distillation column

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3. Objectives
some common objectives of the distillation process include:

1. Separation of components: Distillation is primarily used to separate different

components in a liquid mixture based on their differences in boiling points. By heating
the mixture and collecting the vapors, the more volatile components with lower boiling
points can be separated from the less volatile components with higher boiling points

2. Purification: Distillation is also used to purify compounds by separating them from

impurities or contaminants. The process takes advantage of the different boiling points of
the compound and the impurities, allowing for the separation of the pure compound from
the impurities

3. Concentration: Distillation can be used to concentrate a specific component in a

mixture by selectively removing the other components. By collecting and condensing the
vapors, the desired component can be obtained in a more concentrated form

4. Identification: Distillation is often used in the laboratory to determine the boiling point
of a compound, which is a characteristic physical property used for identification
purposes. By measuring the temperature at which the compound boils, its boiling point
can be compared to known values to aid in identification [2].

4. Distillation device components:

Distillation columns are made up of several components, each of which is used either to
tranfer heat energy or enhance materail 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 packings which are used to enhance
component separations

 a reboiler to provide the necessary vaporisation for the distillation process

 a condenser to cool and condense the vapour leaving the top of the column

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 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 [1]

Figure 1.4: composition of distillation

5. Relative volatility
Relative volatility is a measure of the differences in volatility between two 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’ in a
binary mixture is defined as :

where, yA = mole fraction of component ‘A’ in the vapor, x A = mole fraction of

component ‘A’ in the liquid. In general, relative volatility of a mixture changes with the
mixture composition. For binary mixture, x B = 1-xA. So Equation (5.3) can be rearranged,
simplifying and expressed by dropping subscript 'A' for more volatile component as:

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The Equation (2) is a non-linear relationships between x and y. This Equation can be used
to determine the equilibrium relationship (y vs. x) average relative volatility, ave is
known. If the system obeys Raoult’s law, i.e pA=pyA ,pB=pyB, the relative volatility can be
expressed as:

where pA is the partial pressure of component A in the vapor, p B is the partial pressure of
component B in the vapor and P is the total pressure of the system. Thus if the relative
volatility between two components is equal to one, separation is not possible by
distillation. The larger the value of , above 1.0, the greater the degree of separability,
i.e. the easier the separation.

6. description of the process

The concepts of mass transfer and vapor-liquid equilibrium form the foundation of the
theory of distillation columns. A distillation column is a type of separation apparatus used
to separate mixtures of two or more substances according to their respective boiling
points. The idea underlying distillation is that the more volatile components will be
concentrated in the vapour that is created when a mixture is boiled. The vapour
encounters a liquid flowing in the opposite direction as it rises through the column. The
more vaporous components condense in this liquid, which is called the reflux, and return
to the liquid phase.Creating as many stages as you can in a distillation column is the key
to effective separation. each having a liquid-vapor balance. This is accomplished by
increasing the surface area for vapor-liquid contact by using interior structures like trays
or packing. The reflux ratio, the number of theoretical stages, and the location of the feed
tray are some of the elements that affect the distillation process. The amount of
condensed vapour that is permitted to exit the column as a product divided by the amount
of vapour that is refluxed back into the column is known as the reflux ratio. The number
of vapor-liquid equilibrium stages required to attain the intended separation is indicated
by the number of theoretical stages.A crucial component of the design of a distillation
column is the feed tray location. because it chooses when to insert the feed into the
column. The required separation performance and the relative volatility of the mixture's

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constituents should be taken into consideration when choosing the feed tray placement.
The general idea behind the theory of the distillation column is to separate a mixture
effectively according to the boiling points of its constituent parts by establishing as many
vapor-liquid equilibrium phases as feasible. Key parameters like the reflux ratio and the
number of theoretical stages can be adjusted to maximise separation efficiency and
produce the required product characteristics.

figure1.5: description of the process

7. Calculation and results:

The folowing vapour pressure were obtain for phenol and ortho-cresol.assuming rault’s
low find the folowing data for a totoal pressure of 10 KN/m2 .

A. A Temperature-Composition Diagram.

B. A Vapour-Liquid Equilibrium Data.

C. Relative Volatility Against Mole Fration Of Phenol In Liquid

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P°A P°B XA YA αAB Temp(K)

10 7.7 1.0000 1.0000 1.2987 387
10.4 7.94 0.8374 0.8709 1.3098 387.9
10.8 8.21 0.6911 0.7464 1.3155 388.7
11.2 8.5 0.5556 0.6222 1.3176 389.6
11.6 8.76 0.4366 0.5065 1.3242 390
12 9.06 0.3197 0.3837 1.3245 391.1
12.4 9.4 0.2000 0.2480 1.3191 391
12.9 9.73 0.0852 0.1099 1.3258 392.7
13.3 10 0.0000 0.0000 1.3300 393.3

A)
394

392

390

388 temp,Xa
temp,Ya
386

384

382
0.0000 0.2000 0.4000 0.6000 0.8000 1.0000 1.2000

B)

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C)

relative volatility
1.3400

1.3300

1.3200
relative volatility
1.3100

1.3000

1.2900

1.2800
0.0000 0.2000 0.4000 0.6000 0.8000 1.0000 1.2000

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References:

1. McCabe, W. L., Smith, J. C., & Harriott, P. (1993). Unit operations of

chemical engineering. McGraw-hill.

2. Sawadogo, S., Samikannu, R., Karpagam, R., Priya, N., Bhavani, N. P.

G., & Yahya, A. (2023, December). Comprehensive Review on the Recent
Trends in Solar Photo Voltaic distillation. In 2023 International Conference
on Energy, Materials and Communication Engineering (ICEMCE) (pp. 1-5).
IEEE.

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