Transcript PowerPoint Presentation - FRACTIONAL DISTILLATION

Vapours pressure and distillation of
partially miscible & miscible liquids
&
Diffusion and Osmosis
By
Engr. Asadullah Memon
B.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
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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 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
In a Distillation Process a liquid is
heated 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
NAliquid is the mole fraction of A and NBliquid is the mole fraction of B
where
NAliquid = moles A/moles A + B and NBliquid = 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
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If A is more volatile than B, BPA < BPB and P˚A > P˚B
Then
NAvapor > NAliquid
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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
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Liquid-Vapor Composition Diagram
When a mixture AB of a specific
composition is heated, the total
vapor pressure (composed of the
contributions of PA 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 volatile
component. 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.
Fractional Distillation
AB 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 vaporizationcondensation 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.
Fractional distillation is the
separation of a mixture into its
component parts, or fractions, such
as in separating chemical
compounds 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.
Fractional distillation
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
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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 solution
called 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 result
Swell.
2)
Egg dissolved in salt
solution result
Shrink.
An experiment in osmosis.
A semipermeable membrane separating
water and an aqueous solution of glucose.
Osmotic Pressure
The HSP, Built up on the solution which just stops
the 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 second
solution.
 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 Osmosis
Molecular 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 pass
through the membrane while solute molecules being insoluble in the membrane do
not.
Theories Osmosis
Vapor 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 in
the 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. On
one 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 Pressure
The 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.
ЛV=nRT
Note: Determination of molecular weight from above equation.