Simple Distillation & Fractional Distillation

(Dr.) Mirza Salman BaigAssistant Professor (Pharmaceutics)

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

"Distillation is defined as the separation of the components of liquid mixture, by a processes involving vaporization and subsequent condensation"

Two steps1.Converting liquid into vapor phase

(evaporation)2.Recovering liquid from vapor by

condensation at another place.

Ideal Solution• When two liquids are mixed togther it

is known as binary mixture.• Ideal Solution is binary mixture of

liquid components and there is no change in properties of other substance.

• Heat is nither evolved nor absorbed during mixture formation.

Raoults Law• Raoults law states that, "the partial

vapour pressure of each volatile constituent is equal to vapour pressure of pure constituents multiplied by its mole fraction"

PA = P0A XA

Contd...Partial Vapour pressure of liquid= Vapour pressure of pure liquid × Mole

fraction of the liquidpA = p0A XA

pB = p0B XB

Example Mixture of Ethylene Chloride and benzene.

Daltons LawThe total pressure exerted by mixture

of ideal gas may be conciderd as sum of partial vapour prerssure of each gas (if alone were present and occupied same volume)

P= pA + pB

P= p0A XA + p0B XB

Application of Law• Component having higher vapour

pressure (lighter component) will be distilled first....

This principle is used in simple distillation

Ideal Mixture (obey Raoults Law)

Non Ideal Liquid mixture(real liquid)

Deviation from Raoults Law •Negative – (Adhesive Interaction)–Acetone /chloroform

•Positive – (Repulsion in two components)–Benzene /Ethyl Alcohol

VolatilityvA= Partial vap pressure of a (pA)/

Mole fraction of A (XA)in solution

Equilibrium curve

Plot of mole fraction of a more volatile componennt in vapours against its mole fraction in liquid.

VARIOUS TYPES OF DISTILLATION

Ø Simple Distillation Ø Fractional DistillationØ Vacuum Distillation

Molecular DistillationØ Azeotropic and Extractive distillationØ Steam distillation

Simple Distillation

Simple DistillationØ Single Vaporization/Condensation cycle of a

mixture that produces a distillate which is always impure

Ø Therefore, it is impossible to completely separate the components in a mixture with Simple Distillation

Ø Relatively pure substances can be obtained from a mixture with Simple Distillation if the boiling points of the components differ by a large amount (>25oC)

Simple Distillation

Simple Distillation• Distillation still is initially filled with a feed

mixture, which evaporates and leaves the still in the vapor form.

• This vapor, which is richer in the more volatile component, is collected in the condenser at the top and accumulated in a receiver.

• In this operation, no liquid is refluxed back to the still, and no plates or packing materials are present (fractionating column absent) inside the still.

• This simple distillation still is an example of a batch operation, often referred to as Rayleigh distillation.

Simple Distillation– It is mostly use to separate volatile liquid

from non/ volatile liquids.– Its difficult get pure substance only by

simple distillation. – If a small increment of the initial distillate

is separated and redistilled and this process is repeated many times, effectively producing multiple sequential Vaporization/ Condensation Cycles, an increasingly pure solution can be attained.

– Solution (feed) is Distilland– Vid

Evaporation vs Distillation• Distillation is used when condencate

is required.• Evaporation is used when

concentrated liquid resedue is needed as product.

Applications• Sepration of volatile oil• Purification of organic solvent• Refining of petroleum products• Seperation of drug obtained from

plant and animal.• Purification of drug from animal

source.

Fractional Distillation

Fractional Distillation

Ø Fractionating Column inserted between the Distillation Flask and the Distillation Head.

Ø The Fractionating Column, containing a variety of packing materials,

ØWith each cycle within the column, the composition of the vapor is progressively enriched in the lower boiling liquid.

Ø This process continues until most of the lower boiling compound is removed from the original mixture and condensed in the receiving flask

Ø Vid

Fractional Distillation

Fractionating column is important part of this assembly which is absent in simple distillation

Fractional Distillation (Cont.)

ØWhen the lower boiling liquid (more volatile liquid) effectively removed from the original mixture, the temperature of mixture rises and a second fraction containing some of both compounds is produced.

Ø As the temperature approaches the near the higher boiling point compound, the distillate condensing into next part of fractionating column will contain heavy component (higher boiling point compound).

Fractionating column types• Plate column

– Sieve plate– Bubble cap– Valve Plate

• Packed column

Column EfficiencyØ A common measure of the efficiency of

a Fractionation Column is given by its number of Theoretical Plates

ØOne Theoretical Plate is equivalent to one Simple Distillation cycle, i.e., one Vaporization / Condensation Cycle.

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

Plate Efficiency

Overall Plate Effic. = Theoretical plate required/ Actual no. of plates

Murphee Plate Effic. = Actual Change in vapour composition/ Change in

composition if perfect composition is achived

HETP• Hieght of packed section required to

give the change in composition that would provided by theoretical plate.

• HETP= Hieght of column/ No. of theoretical plate

Main Components of Distillation Columns

• A vertical shell where the separation of liquid components is carried out

• Column internals trays/plates and/or packings which are used to enhance component separations

• A reboiler to provide the necessary vaporisation for the distillation process

• A condenser to cool and condense the vapour leaving the top of the column

• A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

Contd...

The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column.

Basic Terminology

• The liquid mixture that is to be processed is known as the feed and this is introduced somewhere near the middle of the column to a tray known as the feed tray.

• The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section.

• The feed flows down the column where it is collected at the bottom in the reboiler.

Basic Operation

• Heat is supplied to the reboiler to generate vapour.

• The source of heat input can be steam.

• The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column.

• The liquid removed from the reboiler is known as the bottoms product.

Basic Operation (contd...)The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.

COLUMN INTERNALS• Bubble cap• Sieve plate• Column packing

Bubble cap tray

Bubble cap trays • A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. •The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. •Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.

Liquid and Vapour Flows in a Tray ColumnEach tray has 2 conduits, one on each side, called ‘downcomers’. Liquid falls through the downcomers by gravity from one tray to the one below it. The flow across each plate is shown in the diagram on the right.

A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.Being lighter, vapour flows up the column and is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is called the active tray area.

Sieve Plate (tray) • Sieve trays are simply metal

plates with holes in them. • Vapour passes straight

upward through the liquid on the plate.

• The arrangement, number and size of the holes are design parameters.

• Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced bubble cap trays in many applications.

Sieve Plate (tray)

• As the hotter vapour passes through the liquid on the tray above, it transfers heat to the liquid.

• In doing so, some of the vapour condenses adding to the liquid on the tray.

• The condensate, however, is richer in the less volatile components than is in the vapour.

• Additionally, because of the heat input from the vapour, the liquid on the tray boils, generating more vapour.

• This vapour, which moves up to the next tray in the column, is richer in the more volatile components.

• This continuous contacting between vapour and liquid occurs on each tray in the column and brings about the separation between low boiling point components and those with higher boiling

Plate (Tray) Designs

• A tray essentially acts as a mini- column, each accomplishing a fraction of the separation task.

• From this we can deduce that the more trays there are, the better the degree of separation and that overall separation efficiency will depend significantly on the design of the tray.

• Trays are designed to maximise vapour- liquid contact by considering the liquid distribution and vapour distribution on the tray.

• This is because better vapour/ liquid contact means better separation at each tray, translating to better column performance.

Packed column (Packings)Packings are passive devices that are designed to increase the interfacial area for vapour/ liquid contact. The following pictures show 3 different types of packings material pieces \..

These strangely shaped pieces are supposed to impart good vapour/ liquid contact when a particular type is placed together in numbers, without causing excessive pressure/ drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapour up the distillation column.

Packings versus Trays

• Packings provide extra inter/ facial area for liquid/ vapour contact

• efficiency of separation is increased for the same column height

• packed columns are shorter than trayed columns

• Packed columns are called continuous/contact columns while trayed columns are called staged/ contact columns because of the manner in which vapour and liquid are contacted.