Chemistry of Distillation

7/27/2019 Chemistry of Distillation

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Vapours pressure and distillation of

partially miscible & miscible liquids&

Diffusion and Osmosis

By

Engr. Asadullah MemonB.E (Petroleum and Natural Gas)



Immiscible and miscible liquids.

Partially miscible liquids are liquids that do not mix at all proportions at all temperatures.



THE 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).

• 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 pureliquid. At the boiling point, the liquid

and vapor are in equilibrium...if the

composition of each phase remains

constant, the temperature will remain

constant

In a Distillation Process a liquid isheated to its boiling point, the vapors

expand out of the container and are

then cooled below the boiling point

temperature, where they recondense

as a liquid



Raoult’s Law

• For a mixture of two miscible liquids (A and B), the total vapor pressure is the

sum of the individual vapor pressures:

Ptotal = PA + PB

where

PA = NAliquid

x P˚A And PB = NBliquid x P˚B

where

P ̊A is the vapor pressure of pure A and P˚B is the vapor pressure of pure B

and

N Aliquid is the mole fraction of A and NB

liquid is the mole fraction of B

where

N Aliquid = moles A/moles A + B and NB

liquid = moles B/moles A +B



Vapor Enrichment

• 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, BP A < 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 .



Distillation Process

• When a mixture AB of a specific

composition is heated, the total

vapor pressure (composed of thecontributions of P A and PB) will rise

until it is equal to the external vapor

pressure. The mixture will begin to

boil.

• The vapor which first forms is

enriched in the more volatilecomponent. This behavior is shown

at right,

• Assume a two component mixture with a composition of 30%A:70%B (point W).

The boiling point of this mixture is found by drawing a vertical line from W to where

it intersects the lower curve (point X). A horizontal line drawn from X to where it

intersects the vertical axis (the temperature) gives the bp of composition W. From

the point ( Y) where this horizontal line intersects the upper curve (vapor) drop a

vertical line to intersect the lower axis (the composition). Point Z gives the

composition of the vapor which is in equilibrium with a liquid of composition W at its

boiling point.

Liquid-Vapor Composition Diagram



Fractional DistillationAB at composition of 5% A boils at temperature L1 and the vapors with composition V1 enter the column at

that temperature. The vapor will condense to a liquid 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, after which the temperature at the

thermometer rises to the boiling point of B.


http://slidepdf.com/reader/full/chemistry-of-distillation 8/18Fractional distillation

Fractional distillation is the

separation of a mixture into its

component parts, or fractions, such

as in separating chemicalcompounds by their boiling point

by heating them to a temperature at

which several fractions of the

compound will evaporate.

It is a special type of distillation.

Generally the component parts boil

at less than 25 °C from each

otherunder a pressure of one

atmosphere (atm).

If the difference in boiling points is

greater than 25 °C, a simple

distillation is used.



Diffusion

• Consists of solute molecules moving upward into

the solvent and solvent molecules moving down

into concentrated solution.• Bilateral process.

• Its occur in unequal concentrations.



Osmosis It is Greek word means to push.

The flow of solvent through a semi-permeable membrane from pure

solvent to solution or from dilute solution to concentrated solutioncalled osmosis.

A membrane which is permeable to solvent and not to solute is

called semi-permeable membrane.

Reverse Osmosis

Examples:

1) Egg dissolved in

water solution resultSwell.

2) Egg dissolved in salt

solution result

Shrink.



A semipermeable membrane separating

water and an aqueous solution of glucose.

An experiment in osmosis.



Osmotic Pressure

The HSP, Built up on the solution which just stopsthe osmosis of pure solvent into the solution through

semi-permeable membrane is called osmotic

pressure.

OR

The external pressure applied to the solution in

order to stop the osmosis of solvent into solution

separated by a semipermeable membrane



Determination of Osmotic Pressure

1. Pfeffer’s Method (Descriptions and its Disadvantages)

2. Berkeley & Hartley’s Method (Descriptions and its Advantages)

3. Modern Osmometer (Descriptions and its features )

The pfeffer’s method suffers from two disadvantages:

a) It is slow and it takes a few days before the highest pressure is reached.

b) It can not be used for measuring high osmotic pressure.

Berkely & Hartley’s method: This method is superior than pfeffer’s

method in such a way that,

a) It is quick and accurate.

b) It can be used for determining high osmotic pressures.



Osmotic Solutions (Hypotonic, Isotonic &

Hypertonic)

When two solutions are separated by a semipermeable membrane and there is

no flow of water across membrane, the solutions are said to be Isotonic.

When two solutions are separated by a semipermeable membrane, if one of

lower osmotic pressure, it is said to be Hypotonic relative to the secondsolution.

When two solutions are separated by a semipermeable membrane, if one of

higher osmotic pressure, it is said to be Hypertonic relative to the second

solution.



Theories Osmosis

1. Molecular Sieve Theory.

2. Membrane Solution Theory.

3. Vapor Pressure Theory

4. Membrane Bombardment Theory



Theories OsmosisMolecular Sieve Theory:

The membrane contains a lot of fine pores and act as sort of molecular sieve.

Smaller solvent molecules can pass through the pores but the large molecules can

not. Some membrane can act as sieve even through the solute molecules are

smaller than the solvent molecules. Therefore, this theory remain in doubt.

Membrane Solution Theory:Membrane proteins bearing functional groups such as COOH, OH, NH2 etc

dissolve water molecules by hydrogen bonding or chemical interaction. This

membrane dissolves water from the pure water (Solvent) forming what may be

called Membrane solution. The dissolved water flows into the solution across the

membrane in a bid to equalize concentration. In this way water molecules passthrough the membrane while solute molecules being insoluble in the membrane do

not.



Theories OsmosisVapor Pressure Theory:

It suggests that a semipermeable membrane has many fine hole or capillaries. The

walls of these, capillaries are not wetted by water (Solvent ) or Solution. Thus

neither solution nor water can enter the capillaries. Therefore, each capillary will

have in it solution at one end and water at the other, separated by a small gap.

Since the VP of a solution is lower than that of the pure solvent, the diffusion of

vapor will occur across the gap from water side to solution side. This will result inthe transfer of water into solution.

Membrane Solution Theory:

This theory suggests that osmosis result from unequal bombardment pressure

caused by solvent molecules on the two sides of the semipermeable membrane. Onone side solvent while other side solute molecules occupying some of the surface

area. Thus, there are fewer bombardment per unit area of surface on the solution

side than on the solvent side. Result flow of solvent from the pure solvent to the

solution across membrane.



Laws of Osmotic PressureThe osmotic pressure of a solution at a given temperature is directly proportional to

the concentration.

The osmotic pressure of a solution at a given Concentration is directly proportional to

the temperature.

1. Boyle-van’t Hoff law of solutions.

2. Charle’s-van’t Hoff law of solutions.

3. Van’t Hoff law of solution.

Л V = n R T

Note: Determination of molecular weight from above equation.

Chemistry of Distillation

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