By Moizul Hasan Assistant Professor Distillation By Moizul Hasan Assistant Professor

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

Bubble cap trays

DOWN COMERS

4.1.2 Boiling-Point Diagrams and xy Plots Boiling-point diagram for system benzene (A)-toluene (B) at a total pressure of 101.32 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.

4.1.2 Boiling-Point Diagrams and xy Plots Boiling-point diagram for system benzene (A)-toluene (B) at a total pressure of 101.32 kPa. If we start with a cold liquid composition is xA1 = 0.318 (xB1 = 0.682) and heat the mixture, it will start to boil at 98ºC. The first vapor composition in equilibrium is yA1 = 0.532 (yB1 = 0.468). Continue boiling, the composition xA will move to the left since yA is richer in A.

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 101.32 kPa (1atm).

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.

Composition Mole fraction

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….

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….

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

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

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

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

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

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

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

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

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

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.

Thank U