Distillation(part 2)

(Dr.) Mirza Salman BaigAssistant Professor (Pharmaceutics)

AIKTC, School of Pharmacy,New PanvelAffiliated to University of Mumbai (INDIA)

Distillation Types• Azeotropic Distillation• Extractive distillation• Vacuum distillation• Steam distillation• Molecular Distillation

Azeotropic Distillation

Azeotropic Distillation• Principle:- Addition of new

substance to the liquid mixture in order to increase relative volitility of one of two components and make seperation easier.

• Ex- Benzene is added to mixture of water and EtOH

Azeotropic DistillationØ This occurs because, when an azeotrope is

boiled, the resulting vapor has the same ratio of constituents as the original mixture.

Ø Because their composition is unchanged by distillation, azeotropes are also called constant boiling mixtures.

Ø Positive azeotrope is 95.63% ethanol and 4.37% water (by weight).

Ø Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.2°C.

ØNegative azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water (by weight). Hydrogen chloride boils at −84°C and water at 100°C, but the azeotrope boils at 110°C.

Azeotropic distillation Ø Azeotropic distillation usually refers to the

specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two, immiscible liquid phases), such as the example below with the addition of benzene to water and ethanol.

Ø This practice of adding an entrainer which forms a separate phase is a specific sub-set of (industrial) azeotropic distillation methods, or combination thereof.

Contd...• Example - distillation of ethanol/water, using

normal distillation techniques, ethanol can only be purified to approximately 96% .

• Some uses require a higher percentage of alcohol, eg. when used as a gasoline additive.

• Material separation agent- The addition of a Material Separation Agent, such as benzene.

• Molecular sieves- For the distillation of ethanol for gasoline addition, the most common means of breaking the azeotrope is the use of molecular sieves.

Extractive distillation

Extractive distillation• Extractive distillation is defined as

distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.

• When third substance added to azeotropic mixture is non volatile.

• Ex- Glycerien addion reduce vapour pressure of water.

Extractive Distillation (Contd...)

Ø The method of extractive distillation uses a separation solvent, which is generally nonvolatile, has a high boiling point and is miscible with the mixture, but doesn't form an azeotropic mixture.

Ø The solvent interacts differently with the components of the mixture thereby causing their relative volatilities to change.

Extractive Distillation (Contd...)

• The solvent must alter the relative volatility by a wide enough margin for a successful result. The quantity, cost and availability of the solvent should be considered.

• The solvent should be easily separable from the bottom product, and should not react chemically with the components or the mixture, or cause corrosion in the equipment.

• A classic example is of aniline as suitable solvent.

Distillation under reduced pressure (Vacuum distillation)

Distillation under reduced pressure (vacuum distillation)

• Liquid distillation at lower pressure than normal boiling point by applying vacuum.

• Mass of vapour formed = Vapour pressure of evaporating liquid / External pressure

• Application- – Preventing degradation of active components– Changing physical form (to produce light porous

mass of Cascara sagrada extract.

Vacuum DistillationØVacuum distillation is a method of

distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually less than atmospheric pressure) causing evaporation of the most volatile liquid(s) (those with the lowest boiling points).

Ø This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient (sorrounding) pressure.

Vacuum distillation (Contd...)• Vacuum distillation is used with or without

heating the solution.• Temperature sensitive materials (such as

beta carotene) require vacuum distillation to remove solvents from the mixture without damaging the product.

• Vacuum distillation is sometimes referred to as low temperature distillation.

• This type of distillation is in use in the oil industry.

Advantages of Vacuum Distillation

Ø Industrial-scale vacuum distillation has several advantages.

• One tool to reduce the number of stages needed is to utilize vacuum distillation. Vacuum distillation columns typically used in oil refineries have diameters ranging up to about 14 meters (46 feet), heights ranging up to about 50 meters (164 feet), and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

• Vacuum distillation increases the relative volatility of the key components in many applications. Lower pressures increase relative volatilities in most systems.

Advantages of Vacuum Distillation• A second advantage of vacuum distillation is

the reduced temperature requirement at lower pressures. For many systems, the products degrade at high temperatures.

• Vacuum distillation can improve a separation by:

• Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures.

• Increasing capacity, yield, and purity. • Another advantage of vacuum distillation is

the reduced capital cost, at the expense of slightly more operating cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a distillation column.

Distillation under reduced pressure (Vacuum distillation)

Distillation under reduced pressure (industrial scale)

Steam distillation

Steam distillation• Principle - Mixture of immiscible

liquids bigin to boil when the sum of there vapour pressure is equal to atmospheric pressure.

• Application Separation of high boiling point liquid from non volatile impurity.

• Ex- to separate toulene / water.

Steam Distillation still (industrial scale)

Molecular Distillation

Molecular DistillationØ A special application of the simple

distillation is molecular distillation, known also as evaporative distillation or short path distillation.

Ø Theory of molecular distillation:- The mean free path of a molecule is

defined as the average distance through which a molecule can move without coming into collision with another.

Ø For material that are regarded as non volatile under ordinary conditions of temperature and pressure are generally removed by increasing the mean free path.

Mean free path

• Average distance through a molecule can move without collision with other molecules.

Molecular Distillation• It is the distillation processes where

each molecule in vapour phase travels mean free path and get condense on condenser surface without intercollision.

• Distance between evaporating surface and condensor surface is approx. equal to mean free path.

• Short path distillation

Molecular Distillation (cont.)Characteristics of the molecular

distillation process:-• Very high vacuum• Evaporating surface must be close to

the condensing surface• The liquid area is large to avoid

boiling and evolution of the vapors is from surface only.

Molecular Distillation (cont.)Applications of molecular distillation:-Ø Purification of oils Ø Separation of vitamins

Wiped film molecular still

• Evaporative distill insted of boiling

• High vacuum• short path

Centrifugal molecular still

• Feed introduced in center

• Rotation of bucket form the film which is heated by radiation.

• Vapouts condense at condensor surface.