FRACTIONAL DISTILLATION

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FRACTIONAL DISTILLATION

• ORG I LAB
• Dr. W. J. KELLY

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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.
• P µ e -C/T
• 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
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THE TEMP/TIME RELATIONSHIP

• 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
  BPAltBPB 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.

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Raoults 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 PA And PB NBliquid x PB
• where
• PA is the vapor pressure of pure A and PB 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

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Vapor Enrichment

• From Raoults Law we can obtain the following
  relationships
• NAvapor PA/PT
• And
• NBvapor PB/PT
• If A is more volatile than B, BPA lt BPB and PA gt
  PB
• Then
• NAvapor gt NAliquid
• 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.

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Distillation Process
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 30A70B (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.

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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
  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.

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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.

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Distillation Setups
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Fractional Distillation Set-up
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Proper Thermometer Depth