Distillation CourseMSO2015

DistillationDistillation is a process in which a liquid or vapour mixture of two ormore substances is separated into its component fractions of desiredpurity, by the application and removal of heat.

Distillation is done on the basis of differences in their volatilities in aboiling liquid mixture. Distillation is a physical separation process, andnot a chemical reaction.

Principle:Separation of components from a liquid mixture via distillation depends

on the differences in boiling points of the individual components. Andalso depends on the vapour pressure characteristics of liquid mixtures.

For any liquid, the individualmolecules within the liquidare continuously in motion

A small percentage of thesemolecules attain enoughkinetic energy to leave theliquid phase

This exerts an opposingpressure on the atmosphereabove the solution known asthe vapor pressure, P

Atmospheric pressure, P atm

Vapor Pressure, P

P < P atm P ≥ P atm

When enough energy, in the form of heat, is imparted tothe solution the vapor pressure becomes equal to theatmospheric pressure and the liquid begins to boil

Raoult’s LawIn a solution of two miscible liquids (A & B) the partial

pressure of component “A” (PA) in the solution equalsthe partial pressure of pure “A” (PA

o) times its mole fraction(XA) Partial Pressure of A in solution = PA = (PA

o) x (XA) Partial Pressure of B in solution = PB = (PB

o) x (XB)When the total pressure (sum of the partial pressures) is

equal to or greater than the applied pressure, normallyAtmospheric Pressure (760 mm Hg), the solution boils

Ptotal = PA + PB= PA

o XA + PBo XB

If the sum of the two partial pressures of the two compoundsin a mixture is less than the applied pressure, the mixturewill not boil. The solution must be heated until the combinedvapor pressure equals the applied pressure

Vapor EnrichmentFrom Raoult’s Law we can obtain the following relationships:

XAvapor = P˚A/PT

AndXB

vapor = P˚B/PT

If A is more volatile than B, BPA < BPB and P˚A > P˚BThen

XAvapor > XA

liquid

The result of this process is that when a mixture of two miscibleliquids with different boiling points is heated, the vapor will have adifferent composition than the liquid. THE VAPOR IS ENRICHED INTHE MORE VOLATILE (LOWER BOILING) COMPONENT.

Vapour Pressure and Boiling Point

Boiling Point , Pressure , Vapour and Liquid Phases

Boiling Point Curve

Vapour – Liquid Equilibrium Curve (VLE)

VLE – Ideal mixtures

VLE - Non-ideal mixtures

Minimum Boiling Point Azeotrope

Maximum Boiling Point Azeotrope

Examples of Azeotropic Mixtures

Minimum-boiling azeotropes

Carbon-disulfide - acetone (61.0 mole% CS2, 39.25 oC, 1 atm)

Benzene - water (29.6 mole% H2O, 69.25 oC, 1 atm)

Maximum-boiling azeotropes

Hydrochloric acid - water (11.1 mole% HCl, 110 oC, 1 atm)

Acetone - chloroform (65.5 mole% chloroform, 64.5 oC, 1 atm)

Separation of azeotropic mixtures

Relative Volatility

Binary Mixture

Separation area

Degree of Separability

Constant Relative Volatility

Examples

Toluene – Benzene Mixture at 1 atm

Effect of Pressure

- Although most distillations are carried out at atmospheric or near atmospheric pressure, it is not uncommon to distill at other pressures.

- High pressure distillation (typically 3- 20 atm) usually occurs in thermally integrated processes. 1 atm = 101.325 kPa; 1 bar = 105 Pa

- In those cases the equilibrium curve becomes narrower at higher pressures .

Separability becomes less at higher pressures

Effect of Pressure* Pressure is a critical factor in normal distillation.

* As the pressure is increased, the temperatures throughout the column will also increase and visa versa.

* Low pressures are often used to the separation of chemicals that are sensitive to high temperatures.

* High pressures are used to reduce the cooling requirements in the condenser.

* The pressure in a distillation column also has an effect on the vapor flow within the distillation column, as the pressure increases the vapor flow decreases.

Cont’d* When the normal boiling point of the vapour product is lower than thetemperature of the cooling water required to condense it.

* In such cases, it is normally cheaper to pressurize the column in order toraise the boiling point of the vapour product than to install a refrigerationsystem to condense it.

*As shown in the earlier figure, the phase diagram becomes narrower athigher pressures; and the corresponding temperatures also becomeshigher.