Distillation

Dr. Nasir Abbas

Definition

• Distillation is a method of separating mixtures based on differences in volatility (vapour pressure) of the components in a boiling liquid mixture

• It is a unit operation, or a physical separation process, and not a chemical reaction.

• It is used in pharmacy either to extract volatile active principles from vegetable drugs or to separate volatile substances from less volatile impurities.

Types of Distillation

• Distillation can be classified into four main types

1. Simple distillation

i. Under atmospheric pressure

ii. Under reduced pressure

2. Fractional distillation

3. Steam distillation

4. Destructive distillation

Theoretical consideration (Basic Concepts)

• Vapour Pressure – According to Kinetic Theory, the molecules in a liquid are in a constant

state of thermal motion and some of these molecules are moving fast enough to escape from the liquid forming a vapor above the liquid. This vapour exerts a pressure on the surface of the liquid, i.e., Vapor Pressure

– The Vapour Pressure of a liquid increases, generally exponentially, with temperature

– As a liquid is heated, the vapour pressure of the liquid increases to the point at which it just equals the applied pressure - usually atmospheric pressure. The liquid now begins to bubble (boil)

– Liquids with high vapour pressures (Volatile compounds) require relatively little energy (heat) to increase the vapor pressure to match the applied (atmospheric) pressure, and thus, boil, i.e. they have low boiling points

– Liquids with low vapour pressures require considerably more energy to increase the vapor pressure to the point where it matches the applied pressure, thus, they have relatively high boiling points

Theoretical consideration (Basic Concepts) Boiling point

• The Boiling Point is the temperature at which internal vapor pressure of the liquid is equal to the pressure exerted by its surroundings

• If the liquid is open to the atmosphere, the boiling point is the temperature at which the internal vapor pressure of the liquid becomes equal to atmospheric pressure ( ̴760 mm Hg = 1 atm).

• The internal vapor pressure of a pure liquid rises steadily as the temperature is increased until the boiling point is reached.

• The temperature remains constant throughout the boiling process of a pure liquid. At the boiling point, the liquid and vapor are in equilibrium, if the composition of each phase remains constant, the temperature will remain constant

Vapour pressure/ boiling point of Miscible Liquid mixtures

– Liquid/ liquid mixtures • Miscible and non miscible liquid mixtures • Miscible liquid mixture is also called as solution

– liquid compounds making a miscible mixtures normally have different vapor pressures at a given temperature

– The individual compounds in a mixture each exert its own pressure - i.e. called partial pressure

– The sum of the partial pressures equals to the total vapor pressure of the solution/mixture (Dalton’s Law for ideal gas)

Ptotal = PA + PB – When the total pressure of a mixture (sum of the partial pressures)

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

– Solutions/mixtures have higher vapour pressure and lower boiling point than their constituent pure liquids

Vapour pressure/ boiling point of Miscible Liquid mixtures

• Raoult’s Law – In a solution of two miscible liquids (A & B) the partial pressure of

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

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

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

o) x (NB) – When the total pressure (sum of the partial pressures) is equal to or

greater than the applied pressure the solution boils Ptotal = PA + PB = PA

o NA + PBo NB

• Conclusion When the two component of a binary mixture are completely

miscible, the vapour pressure of the mixture is a function of the composition as well as the vapour pressure of the two pure components. – Boiling point/ total vapour pressure of a mixture changes with its

composition

Vapour pressure of Miscible liquid

• Boiling point of a mixture changes with its composition (Raoult’s Law)

To get the total vapour pressure of the mixture, you need to add the values for A and B together at each composition. The net effect of that is to give you a straight line as shown in blue.

• Solutions/ mixture may or may not follow the Raoult’s Law. Ptotal = PA + PB

• Binary mixtures that follows the Raoult’s law are called ideal solution, where the attraction between A and B molecules is the same as those for the pure components, i.e A-B=A-A & A-B, Example benzene/toluene mixture

• When the interaction of A and B molecules (A-B) is less than between the molecules of pure constituents (A-A, A-B), then vapour pressure of solution is now greater than the expected from Raoult’s Law. (positive deviation) example benzene /ethyl alcohol

• When the interaction of A and B molecules (A-B) is more than between the molecules of pure constituents (A-A, A-B), then vapour pressure of solution is now lesser than the expected from Raoult’s Law. (negative deviation) example Chloroform /acetone

Vapour pressure of Miscible liquid

Boiling point diagram of an ideal solution

• Constructing a boiling point / composition diagram

• To remind you - we've just ended up with this vapour pressure / composition diagram:

• it is more convenient to work with boiling point diagram (phase diagram)

• We'll start with the boiling points of pure A and B.

• B has the higher vapour pressure. That means that it will have the lower boiling point.

To make this diagram really useful (and finally get to the phase), we are going to add another line. This second line will show the composition of the vapour over the top of any particular boiling liquid (dew point).

Boiling point diagram for an ideal solution

• Composition of the liquid phase differ than the vapour phase • If you boil a liquid mixture, you would expect to find that the more volatile

substance escapes to form a vapour more easily than the less volatile one. • you would expect to find a higher proportion of B (the more volatile component)

in the vapour than in the liquid. You can discover this composition by condensing the vapour and analysing it. That would give you a point on the diagram.

• The difference between liquid and vapour compositions is the basis for distillation operations.

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.

• From Raoult’s Law we can obtain the following relationships:

NAvapor = P˚A/PT

And

NBvapor = P˚B/PT

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

Then

NAvapor > NA

liquid

• 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 THE MORE VOLATILE (LOWER

BOILING) COMPONENT.

Conclusion/ vapour enrichment

• Assume a two component mixture with a composition of 50% heane:50% pentane . • The boiling point of this mixture is found by drawing a vertical line from to where it

intersects the lower curve (point L1) , Point L1 indicates a boiling point of 44 oC . • Upon removing a sample of the vapor, we find that it has a molar composition of 87%

pentane and 13% hexane as indicated by point V1. • The mole fraction of the component with the lower boiling point is greater in the vapor

than in the liquid. • If the vapor at V1 condenses, the liquid that collects (L2) will have the same

composition as the vapor (V1). Now, if the condensed liquid (L2) is revaporized, the new vapor will be even richer in pentane (V2).

• Repeating the boiling and condensing several more times allow us to obtain pure pentane, uncontaminated by hexane.

Types of Distillation

• Distillation can be classified into four main types

1. Simple distillation

i. Under atmospheric pressure

ii. Under reduced pressure

2. Fractional distillation

3. Steam distillation

4. Destructive distillation

Simple distillation under atmospheric pressure

• Simple distillation is the process of converting a liquid into its vapour, transferring the vapours to another place, and recovering the liquid by condensing the vapours

• In simple distillation, the vapor is immediately channeled into a condenser.

• Consequently, the distillate is not pure but rather its composition is identical to the composition of the vapors at the given temperature and pressure. That concentration follows Raoult's law.

• As a result, simple distillation is effective only when the liquid boiling points differ greatly (rule of thumb is 25 °C or when separating liquids from non-volatile solids or oils.

• For these cases, the vapor pressures of the components are usually sufficiently different that the distillate may be sufficiently pure for its intended purpose.

Simple distillation under atmospheric pressure

• Simple distillation is a Single Vaporization / Condensation cycle process that produces a distillate

• Consequently, the distillate is not pure but rather its composition is identical to the composition of the vapors at the given temperature and pressure. That concentration follows Raoult's law.

Distillation apparatus for simple distillation

Common distillation apparatus consists of

three parts

1. Still: consists at a minimum of

a reboiler or pot in which the source

material is heated,

2. Condenser: in which the heated vapour is

cooled back to the liquid state

3. Receiver: in which the concentrated or

purified liquid, called the distillate, is

collected.

The anti-bumping granules give a smoother boiling action. This can be used to purify water because the dissolved solids have a much higher boiling point and will not evaporate with the steam, BUT it is too simple a method to separate a mixture of liquids especially if the boiling points are relatively close.

Simple distillation under reduce pressure

• Effect of pressure on boiling point has already been discussed (reducing pressure will decrease the boiling point)

• The liquids which are unstable at their boiling point under atmospheric pressure , are distilled at a much lower temperature under reduce pressure with less likelihood of decomposition.

• Also used for concentrating thermo-labile substances.

• It is used for the evaporation of the manstrum in the preparation of extracts.

• This process is used in the purification of vitamins

• Same procedure as of simple distillation, but at lower or vacuum pressure. the vapor is immediately channeled into a condenser.

Distillation apparatus for simple distillation under reduce pressure

• Vacuum distillation is mostly carried out in Claisen flask, which has two

necks

• Second neck prevents splashing of the violently agitated liquid.

• Small pressure gauge should be inserted between pump and receiver

• At large scale vacuum still is used

Fractional distillation (rectification)

Basic principle is same as of simple distillation but Multiple Simple Sequential

Vaporization / Condensation Cycles,

multiple step process, partial condensation is allowed to occur by inserting a

Fractionating Column (a Vigreux Column) between the Distillation Flask and the

Distillation Head

This process continues until most of the lower boiling compound is removed from

the original mixture and condensed in the receiving flask

Lets start with the liquid at L1 (5% A and 95 %B). boils at temperature L1 and the

vapors with composition V1 enter the column at that temperature. The vapor will

condense to a liquid (L2) with composition V1.

The condensate L2 has a lower boiling point (because it has more of the lower boiling

liquid A) and will thus vaporize at a lower temperature (warmed up by coming in

contact with the additional vapors from below) to give vapors of composition V2.

These vapors will condense somewhat farther up the column to give a condensate L3.

If the column is long enough or contains sufficient surface area that many successive

vaporization-condensation steps (theoretical plates) can occur, the distillate that comes

over the top is nearly pure A.

Distillation yielding pure A continues until all of A is removed

Distillation apparatus for fractional distillation

• Fractionating Column (a Vigreux Column)

between the Distillation Flask and the

Distillation Head

• The Fractionating Column, of which there

are many types containing a variety of

packing materials, subjects the mixture to

many Vaporization/Condensation Cycles as

the material moves up the column toward

the Distillation Head, which is attached to

the Condenser

• With each cycle within the column, the

composition of the vapor is progressively

enriched in the lower boiling liquid

• This process continues until most of the lower boiling compound is

removed from the original mixture and condensed in the receiving

flask

Fractionating columns

• Fractionating column is inserted between the still and the condenser

• Bringing about repeated distillation throughout the length of the column

• The action of the column is partially to condense the vapours rising from the boiling liquid; this condensate will be richer in more volatile component

• It is vaporised again by the condensation of more ascending vapours; containing more volatile component

• More condensation and vaporizations take place further up in the column, further enrichment of volatile component occur

• At the end in ideal conditions lower boiling point liquid arrives at the top of column and the higher boiling point liquid left in the column

• Thus a temperature gradient will be established in the column when distillation is in the progress

Design of Fractionating columns

• The purpose of a fractionating column is to achieve an extensive liquid-vapour interface so that equilibrium between ascending vapour and reflux can be rapidly attained.

• Different types of fractionating column are used e.g. • Packed columns: cylindrical glass beads, stainless steel rings, single turn

helices of wire or glass etc. • Vigreux Column: best type has indentation in the walls, spirally arranged

occupying most of the interior

Column Efficiency - How pure can you get!!

• A common measure of the efficiency of a Fractionation Column is given by its number of Theoretical Plates

• One Theoretical Plate is equivalent to a Simple Distillation, i.e., one Vaporization / Condensation Cycle

• The smaller the boiling point difference, the greater the number of theoretical plates a fractionating column must have to achieve separation of mixtures

Design of Fractionating columns

Distillation of Azeotropic mixture

Azeotropic mixture:

• 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.

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

• Thus a mixture behaves like a pure liquid

• Such a mixture cannot be separated into pure liquid.

• Azeotropic mixtures of pairs of compounds have been documented (listed). Many azeotropes of three or more compounds are also known.

Distillation of Azeotropic mixture

Two types

• Minimum boiling point azeotropic mixture • Maximum boiling point azeotropic mixture

When you do the fractional distillation they will end up to azeotropic ratio, not to pure liquid

Steam distillation

• Steam distillation is a special type of distillation for temperature sensitive materials like natural aromatic compounds.

• Many organic compounds tend to decompose at high sustained temperatures. Separation by normal distillation would then not be an option, so water or steam is introduced into the distillation apparatus.

• By adding water or steam, the boiling points of the compounds are depressed, allowing them to evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable.

• If the substances to be distilled are very sensitive to heat, steam distillation can also be combined with vacuum distillation.

• After distillation the vapors are condensed as usual, usually yielding a two-phase system of water and the organic compounds, which can be separated because they are immiscible

• Conclusion: • Steam is used to reduce the boiling point of a liquid to be distilled

Steam distillation apparatus

Steam generator still

receiver

Basic Principle of steam distillation

• When a mixture of two practically immiscible liquids are heated to the vapor phase, each constituent independently exerts its own vapor pressure as a function of temperature as if the other constituent were not present.

• Consequently, the vapor pressure of the whole system increases. Boiling begins when the sum of the partial pressures of the two immiscible liquids just exceeds the atmospheric pressure.

• In this way, many organic compounds insoluble in water can be purified at a temperature well below the point at which decomposition occurs.

• For example, the boiling point of bromobenzene is 156 °C and the boiling point of water is 100 °C, but a mixture of the two boils at 95 °C. Thus, bromobenzene can be easily distilled at a temperature 61 °C below its normal boiling point.

Application of steam distillation

• Steam distillation is employed in the manufacture of essential oils, for use in perfumes, for example. In this method, steam is passed through the plant material containing the desired oils.

• Eucalyptus oil and orange oil are obtained by this method on the industrial scale.

• Steam distillation is also sometimes used to separate intermediate or final products during the synthesis of complex organic compounds.

• Steam distillation is also widely used in petroleum refineries and petrochemical plants.