Distillation Course MSO2015
Distillation Distillation is 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 done on the basis of differences in their volatilities in a boiling liquid mixture. Distillation is a physical separation process, and not a chemical reaction. Principle: Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. And also depends on the vapour pressure characteristics of liquid mixtures.
For any liquid, the individual molecules within the liquid are continuously in motion Atmospheric pressure, P atm A small percentage of these molecules attain enough kinetic energy to leave the liquid phase Vapor Pressure, P This exerts an opposing pressure on the atmosphere above the solution known as the vapor pressure, P
When enough energy, in the form of heat, is imparted to the solution the vapor pressure becomes equal to the atmospheric pressure and the liquid begins to boil P < P atm P P atm
Raoult s Law In a solution of two miscible liquids (A & B) the partial pressure of component A (P A ) in the solution equals the partial pressure of pure A (P Ao ) times its mole fraction (X A ) Partial Pressure of A in solution = P A = (P Ao ) x (X A ) Partial Pressure of B in solution = P B = (P Bo ) x (X B ) When the total pressure (sum of the partial pressures) is equal to or greater than the applied pressure, normally Atmospheric Pressure (760 mm Hg), the solution boils P total = P A + P B = P Ao X A + P Bo X B If the sum of the two partial pressures of the two compounds in a mixture is less than the applied pressure, the mixture will not boil. The solution must be heated until the combined vapor pressure equals the applied pressure
Vapor Enrichment From Raoult s Law we can obtain the following relationships: X vapor A = P A/P T And X vapor B = P B/P T If A is more volatile than B, BP A < BP B and P A > P B Then X vapor A > X liquid A The result of this process is that when a mixture of two miscible liquids with different boiling points is heated, the vapor will have a different composition than the liquid. THE VAPOR IS ENRICHED IN THEMORE 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 o C, 1 atm) Benzene - water (29.6 mole% H2O, 69.25 o C, 1 atm) Maximum-boiling azeotropes Hydrochloric acid - water (11.1 mole% HCl, 110 o C, 1 atm) Acetone - chloroform (65.5 mole% chloroform, 64.5 o C, 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 = 10 5 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 the temperature of the cooling water required to condense it. * In such cases, it is normally cheaper to pressurize the column in order to raise the boiling point of the vapour product than to install a refrigeration system to condense it. *As shown in the earlier figure, the phase diagram becomes narrower at higher pressures; and the corresponding temperatures also becomes higher.