FRACTIONAL DISTILLATION
ORG I LAB
Dr. W. J. KELLY
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.
• P  e -C/T
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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
THE TEMP/TIME RELATIONSHIP
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A thermometer placed in the
vapor of a boiling pure liquid
registers the liquids boiling
point.
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
The temperature of a liquid
mixture AB, where BPA<BPB
will rise steadily over time. The
composition of the liquid and
vapor in equilibrium changes
constantly over time. At the
beginning the vapor contains
more A, at the end more B.
Boiling Point of Pure B
Boiling point of pure Liquid
T
E
M
P
Boiling point of pure A
TIME or VOLUME
Boiling Temperature Behavior of Pure Liquid
Boiling Temperature Behavior of Mixture A
and B where BP of A < BP of B
Raoult’s Law
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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
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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.
Distillation Efficiency
•The efficiency of a fractional distillation is determined by the amount of “pure” liquid
components obtained. Keep in mind that if a liquid is “pure” it will have a constant boiling point.
The temperature of vapors in equilibrium with liquid at the boiling point will be constant. A plot of
temperature vs. time for a pure liquid will look like A below.
•The efficiency of a fractional distillation can be demonstrated graphically by plotting the change
in temperature of the distillate over time (or over volume of distillate, as in this experiment). In a
fractional distillation with low efficiency, separation will be poor. There will be little or no “pure”
component as distillate. The composition of the distillate will be constantly changing and the bp
of the vapor in equilibrium with liquid will be constantly changing. It will give a plot such as B.
•An efficient distillation will give pure components which will have constant boiling points. Such
a process is shown below in plot C. The relatively “flat: horizontal regions at the beginning and
end of the plot indicate “pure” components A and B are obtained.
The closer to this ideal sigmoid shape the better the fractional distillation.
Distillation Setups
Fractional Distillation Set-up
Proper Thermometer Depth