DISTILLATION

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

A small percentage of these molecules attain enough kinetic energy to leave the liquid phase

This exerts an opposing pressure on the atmosphere above the solution known as the vapor pressure, P

Atmospheric pressure, P atm

Vapor Pressure, P

P < P atm P ≥ P atm

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

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 = (PAo) x (NA)

Partial Pressure of B in solution = PB = (PBo) x (NB)

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

Ptotal = PA + PB

= PAo NA + PB

o NB

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:

NNAAvapor vapor == P˚P˚AA/P/PTT

And

NNBBvapor vapor == P˚P˚BB/P/PTT

If AA is more volatile than BB, BPAA < BPBB and P˚P˚AA > P˚P˚BB

Then

NNAAvapor vapor >> NNAA

liquidliquid

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 THE VAPOR IS ENRICHED IN THE MORE VOLATILE (LOWER BOILING) COMPONENTMORE VOLATILE (LOWER BOILING) COMPONENT.

TYPES OF DISTILLATIONSimple DistillationFractional DistillationVacuum DistillationAzeotropic Distillation

Simple Distillation In Simple distillation the

only one chain of vaporization and condensation takes place.

Its only applicable if difference of boiling points is very large like 25-35`C.

It is mostly use to separate volatile liquid from non-volatile liquids.

Its difficult get pure substance only by simple distillation.

Distillation of seawater.

Fractional Distillation

It is another common type of distillation.

It is used to separate liquids having very close boiling points.

It requires more evaporation/condensation cycles

For fractional distillation one fractionating column is introduced between flask and condenser. Each gap works as one cycle

It is use in petro chemical industry.

concentration

Vacuum Distillation Vacuum 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).

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

It is use for temperature sensitive materials like oil industry.

AZEOTROPIC DISTILLATION It is special type of distillation

which is specifically use for azeotropic mixture.

They can’t be distilled directly as their liquid and vapour composition are same.

In this some other component is added to create new low boiling azeotrope.

Which creates two heterogeneous layers.

This can be easily distill. In production of ethanol, to break

azeotrope of 96%ehanol and 4% water benzene is added.

SOME OTHER DISTILLATION Flash Distillation Steam Distillation Molecular Distillation Air-sensitive vacuum distillation Destructive Distillation Pervaporation Distillation Dry Distillation Extractive Distillation Freeze Distillation Codistillation

BASIC DISTILLATION EQUIPMENTS

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

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

A reboiler to provide the necessary vaporization 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

DISTILLATION OPERATION In distillation column feed is introduced at the

middle of the column on feed tray which divides the column in top and bottom section.

The feed slowly comes down in column where it is collected in reboliler which converts feed in to vapour.

Vapour is reintroduced in column from bottom and liquid remain in reboiler is known as bottom.

Vapour moves upward in column and comes out from top

It is then condensed by condenser and stored in vessel. It is known as distillate.

Some of this condensate is again introduced from top.It is known as reflux.

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

CLASSIFICATION OF COLUMN

Classification Of Column Continuous Column Batch Column

Types of Continues ColumnNature of Feed

Binary Column Multi Component Column

Column Internals Tray Column Packed Column

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