Distillation
... A Separation Method
Background Concepts - Definitions
• Vapor Pressure –
Gas pressure created by the molecules
of a liquid which have acquired sufficient
Kinetic Energy to escape to the vapor
phase.
• As Temperature increases, the average
Kinetic Energy and the Vapor Pressure
increase until the boiling point is
reached.
Background Concepts -Definitions
• Boiling Point – The temperature at which
the vapor pressure of a liquid equals the
pressure applied by the surroundings to
the liquid.
• Boiling points for ethanol ...
Press = 760 torr B Pt = 78 oC
Press = 600 torr B Pt = 73 oC
Press = 10 torr B Pt = -2 oC
Background Concepts - Relationships
• For two pure liquids, which have different
boiling points…
...the vapor pressure at a given temperature
will be higher for the liquid of lower boiling
point.
Background Concepts
• The temperature of the vapors above a boiling
liquid will remain constant at the boiling point.
• At the boiling point for a pure liquid, the liquid
and vapor (gas) phases are in equilibrium.
• The boiling point (at a specified pressure) is a
characteristic property of a pure liquid.
Background – Raoult’s Law
• For an ideal solution with a volatile
solvent and a non-volatile solute
(e.g. water and salt) …
VPsolution = Xsolvent × VPsolvent
As you increase the concentration of the non-volatile
solute, you decrease the vapor pressure of the solution
and increase the boiling point. (Colligative property)
So What ??
• For an ideal solution with a volatile
solvent and a non-volatile solute
(e.g. water and salt) …
…separation of the two components
is as simple as heating the mixture and
collecting and condensing the vapor
– simple distillation.
Background – Dalton’s Law
• For an ideal solution made of two volatile
solvents, the total vapor pressure is the sum
of the vapor pressures of the two or more
solvents.
• Ptotal = Pa + Pb + . . .
• For a solution of liquid ‘a’ and liquid ‘b’
VPtotal = Xa (VPa) + Xb (VPb)
Deviation’s from Dalton’s Law
• For non-ideal solutions the combined vapor
pressure may be higher (positive deviation)
or lower (negative deviation) than predicted
by Dalton’s Law.
Deviation’s from Dalton’s Law
• For non-ideal solutions, there is either
strong forces of attraction (negative
deviation) or repulsion (positive
deviation) between the molecules of the
different components of the solution.
• Non-ideal solutions may distill with a
constant boiling point as if it were one
pure substance = Azeotrope.
Positive Deviation
• Classic example – ethanol and water
Pure ethanol: B. Pt of 78 oC
Pure water: B. Pt of 100 oC
Mixture of ethanol (95%) and water (5%)
distills with a constant B. Pt. of about 75 oC.
Background Concepts
• For an ideal solution of two miscible liquids of
different boiling points, the composition of
the liquid and vapor phases is not the same.
• The vapor will contain more of the liquid with
the higher vapor pressure or lower boiling
point.
(X’s and O’s represent percentage of each component at each level.)
essentially pure "O"
O O O O temp 50
O
O
O O OO
Po = 0.9 X 833 mmHg = 750 mmHg
Px = 0.1 X 100 mmHg = 10mmHg
760 mmHg
Po = 0.8 X 855 mmHg = 684 mmHg
Px = 0.2 X 380 mmHg = 76 mmHg
760 mmHg
Po = 0.7 X 867 mmHg = 608 mmHg
Px = 0.3 X 507 mmHg = 152 mmHg
760 mmHg
Po = 0.6 X 887 mmHg = 532 mmHg
Px = 0.4 X 570 mmHg = 228 mmHg
760 mmHg
Po = 0.5 X 912 mmHg = 456 mmHg
Px = 0.5 X 608 mmHg = 304 mmHg
760 mmHg
O O O O temp 60
O
O
O O XO
750/760 X 100 = 99% O
10/760 X 100 = 1% X
O O O O temp 70
O
X
O O XO
684/760 X 100 = 90% O
76/760 X 100 = 10% X
O X
O temp 80
O
O
X
X
O O O
O XO O
X
X temp 90
O
O X O
608/760 X 100 = 80% O
152/760 X 100 = 20% X
532/760 X 100 = 70% O
228/760 X 100 = 30% X
456/760 X 100 = 60% O
304/760 X 100 = 40% X
O X X O
O
O
X
O
O
X X
X X temp 100
X OO
Phase Diagram (Liquids A and B)
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Terms and Definitions
• Theoretical Plate – one of the horizontal lines
in the previous graph or 1 simple equilibration
between liquid and vapor phase
Simple Distillation
(Use a 10-mL graduated cylinder to collect fractions, instead of a flask)
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Simple Distillation
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Fractional Distillation
• Increasing the surface
area that the vapors contact
between the liquid and the
condenser, increases the
number of theoretical
plates.
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Fractional Distillation
• Greater plates = greater purity of distillate
= sharper transition in distillation plot
• The number of Theoretical Plates or the
height equivalent to a theoretical plate
(HETP) is a measure of the efficiency of a
column to separate components.