Transcript Distillation Lecture

CHEM 213
Fall 2007
Experiment 2 - Distillations
Penn State Erie, The Behrend College
Distillation techniques utilize the vaporization and re-condensation 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.
Distillation - Theory
A. Vaporization and Condensation – One component system
1. For any liquid, the individual molecules within the liquid are
continuously in motion
2. A small percentage of these molecules attain enough kinetic
energy to leave the liquid phase
3. This exerts an opposing pressure on the atmosphere above the
solution known as the vapor pressure, P
Atmospheric pressure, Patm
Vapor Pressure, P
Distillation - Theory
A. Vaporization and Condensation - One component system
4. When enough energy, in the form of heat, is imparted to the
solution the vapor pressure becomes equal to the atmospheric
pressure and the liquid begins to boil
P < Patm
P ≥ Patm
Distillation - Theory
A. Vaporization and Condensation – One component system
5. The vapor obtained from a boiling liquid, once cooled, will recondense to a liquid known as the distillate
6. The complete process is called a distillation
Distillation - Applications
B. Why perform a distillation?
1. Organic chemistry is driven to the preparation of organic
compounds – whereas other disciplines are concerned with
description of systems and analysis
2. For your first few experiments you are learning the means of
“cleaning up” a reaction mixture to obtain a pure product
Look at a typical organic reaction:
OH
H2SO4
+

major
minor
3. Distillation is one of the routine techniques for the separation and
purification of product – in this case distillation would be a good
technique for the separation of the two liquid alkene products
Distillation - Applications
C. Some applications you could encounter:
1. Separation of a reaction solvent from a non-volatile solute
A
+
B
C
Solvent
(bp typically below 100 oC)
Vacuum is
applied to
reduce the
boiling point
of the solvent
bp of most modern synthetic
targets is usually >> 200 oC
Mixture of solvent and solute is
rotated to increase the surface
area for evaporation which
helps alleviate the effects of
boiling point elevation
This is actually the most common
distillation performed in organic
laboratories – the simple removal of
solvent from a reaction mixture
Distillation - Applications
C. Some applications you could encounter:
2. Separation of one liquid from another – This experiment!
A
+
B
C
bp 60 oC
+
D
bp 100 oC
Pure C?
Pure D?
C + D
Mixture of C + D?
This situation is a little more complex, and we will use this
experiment to illustrate the separation process and the apparatus
required
Distillation – Back to Theory
D. Separation of Two Liquids
1. For each component: 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!
Distillation – Theory
D. Separation of Two Liquids
2. This relationship of vapor-pressure vs. temperature is given by the
Clausius-Clapeyron equation:
p = po exp
[
- H
(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)
H: heat of vaporization of the liquid
R: gas constant (8.314 J . mol-1 . ° K)
Distillation - Theory
D. Separation of Two Liquids
1. Above a mixture of two or more volatile liquids each liquid makes
a partial contribution to the overall vapor pressure.
Pmixture = PA + PB + …
2. When the sum of these partial pressures equals the atmospheric
pressure (or pressure above the mixture), the mixture boils
3. 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
more volatile = higher partial pressure, lower boiling point
4. This is the basis for using distillation as a technique for the
separation and purification of liquids.
Distillation - Theory
D. Separation of Two Liquids – Raoult’s Law
1. Raoult extended Dalton’s Law to illustrate that the contribution of
each components vapor pressure is related to its mole fraction in
the mixture at the interface of the liquid and vapor phases
Pmixture = XAPA + XBPB + …
2. Once again at the boiling point:
Patm = XAPA + XBPB (2 component system)
3. 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.
Distillation - Theory
Where does all of this get us?
Dalton + CC?
??? What do I get?
Raoult + CC?
Why am I doing this?
Organic chemists are interested in separations and purification not
necessarily physical derivations!
Distillation - Theory
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?
• An estimation of the given enrichment of the condensate collected
from the 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, and is it worth my time?
Which apparatus would I use?
Distillation - Theory
E. Combining Raoult with Clausius-Clapeyron
1. The sum of mole fractions of all components must equal one
1 = XA + XB
2. Substitute the equation for a single component into Raoult’s Law:
XB = 1 – XA
so
Patm = XAPA + (1-XA)PB
3. Expansion and rearrangement of the expression gives us the
variation of mole fraction versus partial and atmospheric pressure:
Patm - PB
XA = _______________
(PA - PB)
Distillation - Theory
E. Combining Raoult and Clausius-Clapeyron:
4. If we substitute this expression:
Patm - PB
XA = __________
(PA - PB)
into the Clausius-Clapeyron Equation:
p = po exp
[
- H
(1/T – 1/To)
R
]
We obtain an expression for the mole fraction of each component in
liquid that boils at a given temperature:
[
]
]
- HB
(1/T – 1/ToB)
Patm - P°B exp
R
XA = _________________________________________________________
- HA
- HB
_
(1/T – 1/ToA)
P°A exp
P°
exp
(1/T – 1/ToB)
B
R
R
[
[
]
Distillation - Theory
E. Combining Raoult with Clausius-Clapeyron:
Combining the expressions for each component in a two component
mixture we obtain the following graphical relationship:
Temperature
bp of pure A
Vapor
bp of pure B
Liquid
0.0, 1.0
0.5, 0.5
Mole Fraction, XA, XB
1.0, 0.0
This relationship gives us the boiling point for any mixture of A and B
Distillation - Theory
F. Dalton and Clausius-Clapeyron:
1. We have just described how the liquid composition relates to the
boiling temperature – what is happening in the vapor phase?
2. The composition of the vapor is given by Dalton’s Law:
P = PA + PB
3. 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
4. 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:
Distillation - Theory
G. 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:
Temperature
bp of pure A
Vapor
bp of pure B
Liquid
0.0, 1.0
0.5, 0.5
Mole Fraction, XA, XB
1.0, 0.0
Distillation - Theory
G. Now for any mixture of liquids, we can determine:
1. The boiling point of the mixture (liquid line)
2. The composition of the vapor (vapor line) which shows how much
enrichment in the lower boiling component occurs
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
Composition (mole%)
80
20
100
0
Distillation – Simple
A. What is it?
1. 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 distilling flask
is directly attached
to the distillation
head
The cooling jacket and
vacuum adapter function only
to cool the vapors to liquid
efficiently and direct them
into the receiver flask
Distillation – Simple
B. How efficient is it?
Let’s use the graphical representation we derived earlier to illustrate
what occurs in a simple distillation:
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
Composition (mole%)
80
20
100
0
Distillation – Simple
B. How efficient is it?
Suppose we have a 80:20 mixture of benzene and toluene and we
subject it to a simple distillation technique:
Vapor
line
From our
graph, we see
that this
mixture will
boil at ~100 °C
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
Composition (mole%)
80
20
100
0
Distillation – Simple
B. How efficient is it?
The vapor that is collected from this 80:20 mixture is enriched in the
lower boiling component
Vapor
line
110
Temperature °C
We can
determine that
the ratio of
components in
the vapor is now
55:45 toluene to
benzene
100
Liquid
line
90
80
0
Mole % Toluene
Mole % Benzene 100
20
80
40
60
60
40
Composition (mole%)
80
20
100
0
Distillation – Simple
C. Application
1. From the graphical analysis we see that a simple distillation is not
100% efficient at separating two liquids
2. 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 don’t have enough material to bring the more efficient
fractional distillation set-up to equilibrium
Distillation – Fractional
A. What is it?
A fractional distillation utilizes two or more vaporizationcondensation 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
let’s see how this works….
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 let’s see how this works….
Distillation – Fractional
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
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 80:20 mixture of toluene
to benzene we did in the simple distillation example
Distillation – Fractional
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
This surface begins to heat from the condensed vapors
which are now 55:45 toluene-benzene
Vapors from the
Distilling flask
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
Distillation – Fractional
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
These vapors are even further enriched in benzene (now
30:70, toluene:benzene) and condense on the next cold
surface
Vapors from the
Distilling flask
Distillation – Fractional
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
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
Distillation – Fractional
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
This vapor now condenses on the next cold surface (now
20:80, toluene:benzene) and the cycle continues
Vapors from the
Distilling flask
Distillation – Fractional
1:99 toluene:benzene
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
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
80:20 toluene-benzene
Distillation – Fractional
1:99 toluene:benzene
Vapor
line
Temperature °C
110
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
100
0
Composition (mole%)
Each vapor-condensation (or mini-distillation) cycle is
known as one theoretical plate
Vapors from the
Distilling flask
80:20 toluene-benzene
The length of distillation column required to provide one
theoretical plate of separation is known as the height
equivalent theoretical plate (HETP)
Distillation – Fractional
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
Vapor
line
110
Temperature °C
Important –
What we have discussed is
only true for the first drop of
distillate!
Liquid
line
100
90
80
Mole % Toluene 0
Mole % Benzene 100
20
80
40
60
60
40
80
20
Composition (mole%)
More and
more toluene
as distillation
proceeds
100
0
Industrially,
fractional
distillation is
very
common and
is typically
run as a
continuous
process
Distillation – Fractional
C. Applications
1. Because of the efficiency of the fractional distillation set-up it
should be used anywhere that two volatile liquids need to be
separated.
2. The only drawback is that each vaporization-condensation 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
3. For small amounts of liquid (<1 mL) chromatography (gas
chromatography, high-performance liquid chromatography or
column chromatography) is the separation method of choice.
Distillation – Experimental Setup
YOUR EXPERIMENT – TWO GOALS
• GOAL 1: 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 (50 mL each)
• Your receiver flask will be a graduated cylinder
• GOAL 2: Determine the identity of the unknown
alcohol – by bp, refractive index and gas
chromatography
Sand bath or
mantle heat
source
Distillation – Experimental Setup
• Set-up and perform your distillation
• 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
Distillation – Experimental Setup
To draw conclusions from YOUR 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 Sand bath or
mantle
boiling points
heat source
• therefore – a volume of liquid collected while
the temperature is intermediary is impure;
volumes of liquid collected while the temperature
is holding steady are probably pure
After the alcohol is purified it will be analyzed the following week by:
Gas Chromatography (GC) – to determine purity
– we will discuss GC in the next lecture
Refractive Index (RI) - to determine purity
– read Mohrig for a descrption
Infrared Spectroscopy (IR) – to determine identity and purity
– to be discussed in the CHEM 210 Lecture