1. By Moizul Hasan Assistant Professor
Distillation
By Moizul Hasan
Assistant Professor

2. Valve trays
(b) valve cap
(a) perforation
(c) Tray with valve caps

3. Bubble cap trays

6. DOWN COMERS

7. 4.1.2 Boiling-Point Diagrams and xy Plots
Boiling-point diagram for system benzene (A)-toluene (B) at a total pressure of kPa.
Dew point is the temperature at which the saturated vapour starts to condense.
Bubble-point is the temperature at which the liquid starts to boil.
The difference between liquid and vapour compositions is the basis for distillation operations.

8. 4.1.2 Boiling-Point Diagrams and xy Plots
Boiling-point diagram for system benzene (A)-toluene (B) at a total pressure of kPa.
If we start with a cold liquid composition is xA1 = (xB1 = 0.682) and heat the mixture, it will start to boil at 98ºC.
The first vapor composition in equilibrium is yA1 = (yB1 = 0.468).
Continue boiling, the composition xA will move to the left since yA is richer in A.

9. A common method of plotting the equilibrium data is shown in Fig
A common method of plotting the equilibrium data is shown in Fig. 2 where yA is plotted versus xA for the benzene-toluene system. The 45º line is given to show that yA is richer in component A than is xA.
Fig. 2 Equilibrium diagram for system benzene(A) – toluene(B) at kPa (1atm).

10. 4.1 Vapor-Liquid Equilibrium Relations
4.1.2 Boiling-Point Diagrams and xy Plots
Ideal boiling point diagram
Minimum-boiling azeotrope
Maximum-boiling azeotrope
An azeotrope is a mixture of two or more liquids 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.

11. Composition Mole fraction

12. This is accomplished by what distinguishes a
Distillation – Fractional
A. What is it?
A fractional distillation utilizes two or more vaporization- condensation cycles, in succession, to effect a separation.
This is accomplished by what distinguishes a
fractional distillation apparatus:
the fractionating column
The fractionating column causes the
vaporization-condensation cycle to repeat
by providing multiple surfaces for the cycle
to take place
Using our graphical representation of the
benzene-toluene mixture as an example
let’s see how this works….

14. Distillation – Fractional
A. What is it?
The fractionating column is placed between the distilling flask and the distillation head
Using our graphical representation of the benzene-toluene mixture as an example let’s see how this works….

15. As the hot vapors leave the distilling flask,
Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
As the hot vapors leave the distilling flask,
they condense on the first cold surface, completing
one vaporization-condensation cycle.
Vapors from the
Distilling flask
Suppose we distill the same 80:20 mixture of toluene to benzene we did in the simple distillation example

16. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
This surface begins to heat from the condensed vapors which are now 55:45 toluene-benzene
This benzene enriched liquid now has a boiling point of ~94 °C (lower than the incoming vapors) and it begins to boil off this higher surface
Vapors from the
Distilling flask

17. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
These vapors are even further enriched in benzene (now 30:70, toluene:benzene) and condense on the next cold surface
Vapors from the
Distilling flask

18. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
This condensed liquid has an even lower boiling point (86 °C) and as this surface heats it begins to boil off this next higher surface
Vapors from the
Distilling flask

19. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
This vapor now condenses on the next cold surface (now 20:80, toluene:benzene) and the cycle continues
Vapors from the
Distilling flask

20. This cycle will continue until the top of the column is reached
Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
1:99 toluene:benzene
This cycle will continue until the top of the column is reached
The liquid collected after seven cycles is now 99% benzene!
Vapors from the
Distilling flask
80:20 toluene-benzene

21. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
1:99 toluene:benzene
Each vapor-condensation (or mini-distillation) cycle is known as one theoretical plate
The length of distillation column required to provide one theoretical plate of separation is known as the height equivalent theoretical plate (HETP)
Vapors from the
Distilling flask
80:20 toluene-benzene

22. Distillation – Fractional
Vapor
line
Liquid
Temperature °C
110 90 80
100
Mole % Toluene
Mole % Benzene 20 40 60
Composition (mole%)
Important –
What we have discussed is only true for the first drop of distillate!
As the distillation flask loses what the vapor is enriched in,
The starting point for the next drop of distillate will be slightly different!
In our example, there will be more and more toluene in the distillation flask – more heat will need to be applied to get the liquid to boil, and heat the distillation column
More and more toluene as distillation proceeds

23. Industrially,
fractional distillation is very common and is typically run as a continuous process

24. Distillation – Fractional
C. Applications
Because of the efficiency of the fractional distillation set-up it should be used anywhere that two volatile liquids need to be separated.
The only drawback is that each vaporization-condensation cycle requires a volume of liquid to attain equilibrium; this is called the hold-up volume or column hold-up and places a lower limit on the amount of liquid we can distill AND how much liquid will be lost in performing the distillation
For small amounts of liquid (<1 mL) chromatography (gas chromatography, high-performance liquid chromatography or column chromatography) is the separation method of choice.

25. Thank U