Experiment 2 Distillations

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CHEM 36
Fall 2006
Experiment 2 - Distillations Penn State Erie,
The Behrend College
Distillation techniques utilize the vaporization
and recondensation of vapors of a liquid mixture
to effect purification of a liquid component. In
this experiment we will compare the apparatus for
simple and fractional distillation of organic
liquids. This comparison will be made on the
basis of scale, speed and the efficiency of the
separation for a mixture of containing an unknown
alcohol.
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Distillation - Theory

• Vaporization and Condensation One component
  system
• For any liquid, the individual molecules within
  the liquid are continuously in motion
• A small percentage of these molecules attain
  enough kinetic energy to leave the liquid phase
• This exerts an opposing pressure on the
  atmosphere above the solution known as the vapor
  pressure, P

Atmospheric pressure, Patm
Vapor Pressure, P
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Distillation - Theory

• Vaporization and Condensation - One component
  system
• When enough energy (in the form of heat, usually)
  is imparted to the solution the vapor pressure
  becomes equal to the atmospheric pressure and the
  liquid begins to boil

P lt Patm
P Patm
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Distillation - Theory

• Vaporization and Condensation One component
  system
• If vapor pressure is plotted versus temperature
  an exponential increase of vapor pressure is
  observed as the boiling point is approached Note
  even if two compounds have the same ultimate
  boiling point, the curvature of this line may be
  different!

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Distillation - Theory

• Vaporization and Condensation One component
  system
• This relationship of vapor-pressure vs.
  temperature is given by the Clausius-Clapeyron
  equation
• p po exp

- DH
(1/T 1/To)
R
The x,y (independent and dependent variables) for
this equation are the known temperature (T) and
the vapor pressure (p) calculated for that
temperature The constants for this equation
po and To known vapor pressure for a known temp.
(K) DH heat of vaporization of the liquid
R gas constant (8.314 J . mol-1 . K)
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Distillation - Theory

• Vaporization and Condensation One component
  system
• The vapor obtained from a boiling liquid, once
  cooled, will recondense to a liquid known as the
  distillate
• The complete process is called a distillation

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Distillation - Theory

• Distillation of a one (liquid) component system
  is useful
• Liquids that contain a non-volatile solute
• Most organic reactions involve the mixture of
  reactants within a solvent
• Solvents are typically low-boiling liquids (bps
  lt 100 C)
• After the reaction is complete, the solvent is
  removed by distillation to leave the products of
  the reaction (typically non-volatile solids)
  behind
• When a solvent containing non-volatile solutes is
  distilled, not all molecules at the surface of
  the liquid are volatile
• Consequently the vapor pressure of the volatile
  liquid is reduced, requiring increased heating to
  overcome the atmospheric pressure and boil
• This effect is known as molal boiling point
  elevation

DTb Kb m
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Distillation - Theory

• Distillation of a one (liquid) component system
  is useful
• Liquids that contain a non-volatile solute
• The removal of reaction solvent is routine and
  performed on a special distillation apparatus
  known as a rotary evaporator or rotavap

Mixture of solvent and solute is rotated to
increase the surface area for evaporation which
helps alleviate the effects of boiling point
elevation
Vacuum is applied to reduce the boiling point of
the solvent
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Distillation - Theory

• Mixtures of Liquids Daltons Law of Partial
  Pressures
• Above a mixture of two or more volatile liquids
  each liquid makes a partial contribution to the
  overall vapor pressure.
• Pmixture PA PB
• When the sum of these partial pressures equals
  the atmospheric pressure (or pressure above the
  mixture), the mixture boils
• This Law implies that if a mixture of different
  volatile liquids is heated to boiling and the
  condensed vapors are collected they will be
  enriched in the component that is more volatile
  (lower boiling point)
• This is the basis for using distillation as a
  technique for the separation and purification of
  liquids.

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Distillation - Theory

• Mixtures of Liquids Raoults Law
• Raoult extended Daltons Law to illustrate that
  the contribution of each components vapor
  pressure is related to its mole fraction in the
  mixture
• Pmixture XAPA
  XBPB
• Once again at the boiling point
• Patm XAPA XBPB
  (2 component system)
• The enrichment of a particular component in the
  condensed vapors of a boiling mixture is related
  to both their volatility (P) and their
  concentration (X) in the original mixture.

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Distillation - Theory

• Where does all of this get us?
• Organic chemists are interested in separations
  and purifications not necessarily physical
  derivations!
• What we need is
• A relation of the component mole fractions within
  a given mixture to the observed boiling point
• In English if I have an 80-20 mixture of A B
  at what temperature will it boil?
• 2. An estimation of the given enrichment of the
  condensate collected from the boiling/distilling
  mixture
• In English if I distill this 80-20 mixture of
  A B will I get more of one or the other in the
  condensed vapor?

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Distillation - Theory

• Combining Raoult with Clausius-Clapeyron
• Mole fractions of all components must equal one
• 1 XA
  XB
• Substitute the equation for a single component
  into Raoults Law
• XB 1 XA so
  Patm XAPA (1-XA)PB
• Expansion and rearrangement of the expression
  gives us the variation of mole fraction versus
  partial and atmospheric pressure
• XA _______________

Patm - PB
(PA - PB)
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Distillation - Theory

• Combining Raoult and Clausius-Clapeyron
• If we substitute this expression
• 
  
• XA __________
• into the Clausius-Clapeyron Equation
• p po exp
• We obtain an expression for the
  mole fraction of each component in liquid
  that boils at a given temperature

Patm - PB
(PA - PB)


- DH
(1/T 1/To)
R


- DHB
(1/T 1/ToB)
Patm - PB exp
R
XA _____________________________________________
____________




- DHA
- DHB
_
PA exp
(1/T 1/ToA)
PB exp
(1/T 1/ToB)
R
R
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Distillation - Theory

• Combining Raoult with Clausius-Clapeyron
• Combining the expressions for each component in a
  two component mixture we obtain the following
  graphical relationship

bp of pure A
Vapor
Temperature
bp of pure B
Liquid
0.0, 1.0
1.0, 0.0
0.5, 0.5
Mole Fraction, XA, XB
This relationship gives us the boiling point for
any mixture of A and B
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Distillation - Theory

• Dalton and Clausius-Clapeyron
• We have just described how the liquid composition
  relates to the boiling temperature
• The composition of the vapor is given by Daltons
  Law
• P PA PB
• Substituting in the Ideal Gas Law for each
  component
• (PA nA(RT)/V)
• and canceling similar terms we find that the
  ratio of each component to the total vapor
  pressure is given by
• PA/PTOTAL nA/nTOTAL
• Substituting mole fractions for number of moles
  we find that at 760 torr (1 atm) the vapor
  component for this system is given by
• XA vapor XA liquid (PA/760)
• If we substitute this XA into
  Clausius-Clapeyron

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Distillation - Theory

• We get an expression for the composition of the
  vapor.
• Now add this relationship, graphically, of the
  composition of the vapor to the mole-fraction to
  temperature relationship we illustrated earlier,
  we arrive at the goal

bp of pure A
Vapor
Vapor composition
Temperature
bp of pure B
Liquid composition
Liquid
0.0, 1.0
1.0, 0.0
0.5, 0.5
Mole Fraction, XA, XB
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Distillation - Theory

• Now for any mixture of liquids, we can determine
• The boiling point of the mixture (liquid line)
• The composition of the vapor of this boiling
  mixture (vapor line) which shows how much
  enrichment in the lower boiling component occurs
  if the mixture is distilled.

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Distillation Simple
A. What is it? A simple distillation
uses one vaporization-condensation cycle to
effect a separation
What we will be discussing occurs only in this
part of the apparatus
The cooling jacket and vacuum adapter function
only to cool the vapors to liquid efficiently and
direct them into the receiver flask
The distilling flask is directly attached to the
distillation head
Heat
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Distillation Simple
B. How efficient is it? Lets
use the graphical representation we derived
earlier to illustrate what occurs in a simple
distillation
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Distillation Simple
Suppose we have a 8020 mixture of benzene and
toluene and we subject it to a simple
distillation technique From our graph, we see
that this mixture will boil at 100 C
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Distillation Simple
The vapor that is collected from this 8020
mixture, by Raoults Law, is enriched in the
lower boiling component From the graph, we can
determine that the ratio of components in the
vapor is now 5545 toluene to benzene
Vapor line
110
100
Liquid line
Temperature C
90
80
Mole Toluene Mole Benzene
0 100
20 80
40 60
60 40
80 20
100 0
Composition (mole)
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Distillation Simple

• C. Application
• From the graphical analysis we see that a simple
  distillation is not 100 efficient at separating
  two liquids
• A simple distillation should therefore be used
  where
• The two components have boiling points that are
  more than 30-40 C apart
• One of the liquids is already 90 pure
• You are simply removing a pure solvent from a
  non-volatile solute - (we mentioned this as one
  of the most common distillation techniques,
  removal of a solvent from an organic reaction to
  obtain the product)
• You dont have enough material to bring the more
  efficient fractional distillation set-up to
  equilibrium

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Distillation Fractional
A. What is it? A fractional
distillation utilizes two or more
vaporization-condensation cycles, in succession,
to effect a separation. This is accomplished
by what distinguishes a fractional
distillation apparatus the
fractionating column The
fractionating column causes the
vaporization-condensation cycle to repeat
by providing multiple surfaces for the
cycle to take place
Using our graphical representation of the
benzene-toluene mixture as an example
lets see how this works.

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Distillation Fractional
A. What is it? The fractionating column
is placed between the distilling flask and the
distillation head Using our graphical
representation of the benzene-toluene mixture as
an example lets see how this works.

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Distillation Fractional
As the hot vapors leave the distilling
flask, they condense on the first cold surface,
completing one vaporization-condensation cycle.
Vapors from the Distilling flask
Suppose we distill the same 8020 mixture of
toluene to benzene we did in the simple
distillation example
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Distillation Fractional
This surface begins to heat from the condensed
vapors which are now 5545 toluene-benzene This
benzene enriched liquid now has a boiling point
of 94 C (lower than the incoming vapors) and it
begins to boil off this higher surface
Vapors from the Distilling flask
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Distillation Fractional
These vapors are even further enriched in benzene
(now 3070, toluenebenzene) and condense on the
next cold surface
Vapors from the Distilling flask
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Distillation Fractional
This condensed liquid has an even lower boiling
point (86 C) and as this surface heats it begins
to boil off this next higher surface
Vapors from the Distilling flask
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Distillation Fractional
This vapor now condenses on the next cold surface
(now 2080, toluenebenzene) and the cycle
continues
Vapors from the Distilling flask
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Distillation Fractional
199 toluenebenzene
This cycle will continue until the top of the
column is reached The liquid collected after
seven cycles is now 99 benzene!
Vapors from the Distilling flask 8020
toluene-benzene
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Distillation Fractional
199 toluenebenzene
Each vapor-condensation (or mini-distillation)
cycle is known as one theoretical plate The
length of distillation column required to provide
one theoretical plate of separation is known as
the height equivalent theoretical plate (HETP)
Vapors from the Distilling flask 8020
toluene-benzene
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Distillation Fractional
Important What we have discussed is only true
for the first drop of distillate! As the
distillation flask loses what the vapor is
enriched in, The starting point for the next drop
of distillate will be slightly different! In our
example, there will be more and more toluene in
the distillation flask more heat will need to
be applied to get the liquid to boil, and heat
the distillation column
More and more toluene as distillation proceeds
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Distillation Fractional

• C. Applications
• Because of the efficiency of the fractional
  distillation set-up it should be used anywhere
  that two volatile liquids need to be separated.
• The only drawback is that each vaporization-conden
  sation cycle requires a volume of liquid to
  attain equilibrium this is called the hold-up
  volume or column hold-up and places a lower limit
  on the amount of liquid we can distill AND how
  much liquid will be lost in performing the
  distillation
• For small amounts of liquid (lt1 mL)
  chromatography (gas chromatography,
  high-performance liquid chromatography or column
  chromatography) is the separation method of
  choice.

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Distillation Experimental Setup

• In this Experiment you will compare the
  efficiency of simple vs. fractional distillation
  by you and a partner distilling a mixture of
  methylene chloride (CH2Cl2, bp760 40 oC) and an
  unknown alcohol
• One partner will perform a simple distillation
  on half of the mixture, the other partner will
  perform a fractional distillation (25 mL each)
• Your receiver flask will be a graduated cylinder

Sand bath heat source
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Distillation Experimental Setup

• Every time an additional 2 mL of distillate is
  collected in the graduated cylinder, not the
  temperature on the thermometer
• You will construct a plot of temperature
  (dependent variable) vs. volume (independent
  variable)
• This plot is NOT the vapor-liquid phase diagrams
  we have discussed!!!

Sand bath heat source
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Distillation Experimental Setup

• To draw conclusions from the plot remember the
  following
• You have a finite amount of each liquid to
  distill, as one runs out and gives you x volume,
  there is only y volume left to distill
• A pure liquid (condensed hot vapor) will give a
  temperature within a few degrees of its accepted
  boiling point
• A mixture of two liquids (behaving ideally) will
  give a temperature between their accepted boiling
  points
• therefore a volume of liquid collected while
  the temperature is changing is impure volumes of
  liquid collected while the temperature is holding
  steady are probably pure

Sand bath heat source
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After the alcohol is purified that separated
liquid will be analyzed by GC (for
purity) Refractive Index (RI) (for
purity) IR (identity and purity)
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Industrially, fractional distillation is very
common and is typically run as a continuous
process