Distillation

1

Introduction

• Distillation is the process of converting liquid into its vapours by heating and reconverting it again into liquid by condensing the vapours. It is a method of separating substances which differ in their vapour pressures.

​

• It can separate the components from a mixture of liquids through selective boiling and condensation.

2

History of Distillation Chemistry: An age-old process

• The earliest evidence of distillation comes from a terracotta distillation apparatus dating to 3000 BC in the Indus valley of Pakistan.
• Distillation was known to be used by the Babylonians of Mesopotamia around 1200BC.
• Initially, distillation is believed to have been used to make perfumes.
• Distillation of alcoholic beverages in Italy and China started in the 12^th century.

3

• This apparatus originated from Alexandrian philosophy Schools in the 1^st century AD.
• The apparatus consists of four elements: (1) head (2) receiver (3) still (4) sand bath or water bath

4

1. Still of Democritos, also called alembic of Synesios, which originally meant kettle, in which water was boiled (4^th century)

​

​

• A distillation plant in Damascus consisiting of multiple units for producing rose-water (13^th century)

Ref. Norbert Kockmann et al., ‘History of distillation chemistry’ (2014), page no. 1-10

Terminology related to distillation

• Binary mixture: When two liquids are mixed together, they may be miscible with each other in all proportion. Such miscible liquids form binary mixture.

Example: (Ethanol + Water), (Acetone +Water)

• ldeal Solution: a solution in which the interaction between molecules of the components does not differ from the interactions between the molecules of each component.

Usually, a solution that conforms exactly to Raoult's law

• Real Solution: Most system shows varying degree of deviation from Raoult's law, depending on nature of liquids and temperature. These solutions are known as real solutions.
• Vapour pressure: the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate
• Azeotropic mixture: An azeotrope is a liquid mixture that has a constant boiling point and whose vapor has the same composition as the liquid.

​

​

5

6

Vapour-liquid diagram for a binary mixture of components P and Q , illustrating the principles of distillation

The principles : VAPOUR-LIQUID EQUILIBRIUM DIAGRAM.

The diagram relates to a binary mixture containing components P and Q. The lower curve gives the composition of the liquid boiling at various temperatures whilst the upper curve gives the composition of the vapor in equilibrium with the boiling liquid. Points x and y, therefore, give the boiling points of the individual components P and Q, respectively. For example, point A shows that at X degrees the vapor has a composition of approximately 90% P, whilst point B shows that the boiling liquid with which it is in equilibrium has a composition of approximately 80% P.

The principles : VAPOUR-LIQUID EQUILIBRIUM DIAGRAM.

• In a continuous distillation process, such as occurs in a distillation column, liquid of composition C (90% Q, 10% P) vaporizes to vapor of composition D, which condenses to liquid of composition E. Subsequently, liquid E becomes vapor F and liquid G(composition: 50% a, 50% P).This continuous process of vaporization and condensation occurs in the distillation column until a volatile fraction leaves the top of the column and is removed from the process by being collected in the collection flask. At the same time the liquid in the distillation flask becomes progressively more concentrated in the involatile component.

7

The main components : An overview

The distillation process is carried out in an apparatus which consists of

​

(a) Still, in which volatile material is boiled

(b) Condenser, in which vapours are condensed

(c) Receiver, in which distillate is collected.

8

(a) STILL

• It is a vaporizing chamber and used to place the material to be distilled.
• The still is heated by a suitable means of vaporization of the volatile constituents. On laboratory scale RBF ( round bottom flask) made of glass are used so that the progress of the distillation can be noticed.
• A condenser is attached to the still using appropriate joints. A trap is inserted between distillation flask and condenser.

9

(b) CONDENSER:

• Used to condense the vapor.
• It is kept cold by circulating water/air through jacket.

​

• TYPES
• Single surface condensers
• Straight tube
• Bulb type
• Spiral
• Coiled type

​

• Double surface condensers
• Multi-tubular condensers

10

11

(c)RECEIVER:

• It is used to collect distillate.
• It may be simple flask.
• It immersed in ice-bath to minimize loss of volatile matter.
• Florentine receivers are used for the separation of oil and water.

TYPES:

• Type-1 For separation of oil heavier than water.
• Type-2: For separation of oil lighter than water

12

TYPES OF DISTILLATION PROCESSES

The following are the various types of distillations:

1. Simple distillation
2. Distillation under reduced pressure
3. Fractional distillation
4. Steam distillation
5. Destructive distillation

13

1. Simple Distillation.

14

It is a process of converting a liquid into its vapour in a distillation still, transferring the vapour to another place and condensing it again into liquid

It consists of a distillation flask with a side arm sloping downward which is connected to a condenser. The condensed vapours are collected in a flask called 'receiver’. The whole apparatus is made of glass.

The distillation flask should be of such a size that it can contain half to two-thirds of the liquid to be distilled. The thermometer is fitted in distillation flask to note down the temperature, at which the vapours are distilled. Bumping is avoided by adding small pieces of porcelain or boiling chips before distillation

Applications of simple distillation

1. It is used for the preparation of distilled water.
2. Many volatile oils and aromatic waters are prepared by simple distillation.
3. Organic solvents are purified by distillation.
4. Many industrial compounds are prepared by distillation e.g.Spirit

15

2. Distillation Under Reduced Pressure

• Liquid boils when its vapour pressure is equal to the atmospheric pressure. The boiling point of the liquid may be lowered to the desired temperature by reducing the pressure on its surface

16

• Apparatus used for laboratory scale consists of a double-neck distillation flask known as Claisen flask. In one of its necks a thermometer is fitted and in the second neck a capillary tube is fitted which prevents bumping of the heated liquid. The capillary tube should be so line as to permit only a slow stream of bubble which can be controlled with a pinch cock.
• Thick walled glass apparatus with inter-changeable standard glass joints are used for vacuum distillation. The claisen flask is connected to a receiver through condenser. Vacuum pump is attached to the receiver to attain the desired degree of vacuum.
• Heating of claisen flask is not started-until the desired vacuum has been attained

​

17

Applications:

1. It is used for the concentration of compounds containing thermolabile (constituents which are sensitive to heat) in order to prevent their destruction.

2. It is used for separating substances which undergo decomposition when heated under normal atmospheric pressure.

3. This is mainly used in rotary evaporators.

18

VACUUM STILL

• The vacuum stills are employed for distilling substances under reduced pressure on a large scale. A vacuum still is generally made of stainless steel or any other metal which can withstand a high vacuum. The still is connected to condenser. The vacuum is created by means of a vacuum pump.

19

• The vacuum still is filled by attaching a pipe to a tap in the lower part of the hood and the pump is started. The other end of the pipe dips into the liquid to be distilled so that it can be drawn into the still. An observation window in the hood is very helpful to the operator to see the progress of distillation and also the level of the content of the liquid to be distilled. Two receivers are generally attached to the condenser in order to collect the distillate without stopping distillation.

​

• Applications:
• Distillation of substances that have a high boiling point at atmospheric pressure.
• Distillation of thermolabile substances that get damaged by a high temperature.
• Removal of the last traces of a volatile solvent.

20

3. Fractional Distillation

• When a substance is dissolved in a liquid, the vapour pressure of the liquid is lowered. When two miscible liquids are mixed together, each will act as solute or solvent for the other. So, when a mixture of two such liquids is heated, the vapour pressure of each is lowered. The pressure exerted by each liquid in the mixture is known as "partial pressure".
• The liquid boils when the sum of the partial pressures is equal to the atmospheric pressure. The vapours arising from two miscible liquids at boiling point is richer in the component exerting the greater partial pressure.

21

Apparatus used for laboratory scale

Fractionating column is fitted between the distillation flask and the condenser. Fractionating column is used for continuous separation of two miscible liquids. Long fractionating column is used in the mixture where the boiling point is quite close to each other and short fractionating column is used in those cases where there is a considerable difference in the boiling point of the mixture of miscible fluids

22

• In fractional distillation, the mixture of miscible liquids is heated in the still. The vapours formed are allowed to pass through the fractionating column, where a part of the vapour is condensed and while returning to the still comes into an intimate contact with the rising vapour resulting in further fractionation of the liquid being distilled. The liquid with higher boiling point is condensed first and vapour becomes richer with the liquid having the lower boiling point which gets condensed in a condenser.

​

• Applications
• Alcohol is purified from the mixture of alcohol and water obtained from fermentation tank.
• It is used for the separation of miscible liquids, such as, alcohol and water, acetone and water, chloroform and benzene.

23

4. Steam Distillation

• When two immiscible liquids are heated together, then mixture boils when the sum of the vapour pressure equals to the atmospheric pressure.
• The temperature at which mixture boils is lower than that of either of the liquids i.e. the boiling point of the mixture is low than that of the liquid with the lower boiling point.
• The temperature at which the mixture boils remains stationary until one of the liquids has been completely removed from the still.

24

Apparatus used for laboratory scale

• lt consists of a "Steam Can" fitted with a cork having two holes. Through one of the holes passes bent tube leading the steam to the flask containing the non-aqueous liquid. This tube should reach almost to the bottom of the flask. Another long tube which passes through the other hole reaches almost the bottom of the steam can. This tube acts as a safety tube.

25

• When steam starts coming out from the safety tube, it indicates that the steam can is almost empty. The delivery tube carrying vapours from the flask is connected to the condenser to convert it into liquid which gets collected in the receiver. The non-aqueous liquid is placed in the flask. A small quantity of water is added to it. The steam can and the flask are heated simultaneously, so that a uniform flow of steam passes through the boiling mixture. Distillation is continued until all the non-aqueous liquid has distilled over. The distillate is then collected in receiver.
• Applications : In purification of volatile oils.

26

27

Steam distillation of volatile oils

5. Destructive Distillation

• This is also known as Dry Distillation.
• The dried organic matter is heated in the absence of air, in a suitable apparatus, until all the volatile substances are driven off and the residue is left behind.
• Destructive distillation is mainly used in industry for obtaining many valuable products from wood and coal.
• Destructive distillation of wood gives acetone, menthol, Cresol, wood tar etc. while charcoal remains in the still.
• Destructive distillation of coal gives burning gases the ammonia, and the coke remains in the still.
• Destructive distillation of animal bones gives ammonia, amines and hydrocarbons.

28

29

Azeotropic Distillation

Azeotropic distillation is a process to break azeotrope where another volatile component, called the entrainer, the solvent, or the mass separating agent (MSA), is added to form a new lower-boiling azeotrope that is heterogeneous.

Azeotropic Distillation: Dehydration of 96% alcohol to absolute alcohol

• A continuous azeotropic distillation unit for dehydrating alcohol-water azeotrope is shown in figure. The azeotrope is fed to column A. The required quantity of benzene is added to the feed. The vapour from column A, which approaches the ternary azeotrope in composition, when cooled to atmospheric temperature, separates into a benzene phase and a water phase. These are separated in a decanter. The benzene phase is returned to the main column as a reflux and to serve as the entrainer and the water phase is fed to column Bin which the benzene is recovered as the ternary azeotrope which is returned to column A.
• Aqueous alcohol from the base of column B is fed to a column C. The overhead from this column, which is 96 % alcohol, is recycled to the main column for dehydration, and practically pure water is discarded from the base of the column. Anhydrous alcohol is taken out as the residue from the main column A. since benzene is cycled continuously back to column A.

30