CHEG 281
Experiment 9: Determination of Vapor-Liquid Equilibrium
One of the most common chemical engineering operations is the separation of two or
more compounds based on differences in boiling points. At a given pressure a pure
compound boils at a very well defined temperature. Mixtures of compounds boil at
temperatures intermediate between the two pure compound boiling points; the exact
temperature depends upon the composition of the mixture. Unlike the pure compounds
in which the vapor and liquid have the same composition, boiling mixtures will have a
different composition in the liquid phase than in the vapor phase. Note that for a boiling
mixture, the vapor and liquid phases are at the same temperature. A plot of this behavior
is a VLE (Vapor-Liquid Equilibrium) envelope, as shown in Figure 1. This difference in
composition between the vapor and liquid phases becomes the basis for separating the
compounds. The industrial separation device is the distillation column. You will study
this in more detail in CHEG 3200 [Thermodynamics], CHEG 3810 [Modeling &
Simulation], CHEG 4600 [Process Design], and CHEG 4200 [Separations].
Not all VLE curves are as simple as that shown in Figure 1 and under certain
circumstances, the boiling point of the mixture can be above or below those of the pure
components when the mixture forms an azeotrope. Figure 2 is such a VLE curve.
Complete separation by simple distillation is then not possible, which is why ethanol
cannot be easily separated from water by distillation.
VLE - Furan / Carbon Tetrachloride
Temp ( F )
(1 atm)
180
170
160
150
140
130
120
110
100
90
80
y, Vapor Phase
x, Liquid Phase
0
0.2
0.4
0.6
0.8
Mole Fraction Furan
VLE Diagram at 1 atm for Furan / Carbon Tetrachloride
Figure 1
Experiment 9 -- VLE Determination
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CHEG 281
VLE - Ethanol /Toluene
240
230
Temp (F)
220
210
y, vapor phase
200
190
180
170
x, liquid phase
160
150
0
0.2
0.4
0.6
0.8
1
Mole Fraction Ethanol
VLE diagram at 1 atm for Ethanol and Toluene
Figure 2
Experimentally, VLE data are determined in the laboratory using a device that ensures
equilibrium between the liquid and the vapor. One such device, to be used in this
laboratory, is the Othmer still, shown in Figure 3. A volume of the material to be studied
is charged to the still and the contents heated. The vapor is condensed and returned to the
still-pot. When the temperature stabilizes, indicating thermal equilibrium, a sample of
both the liquid and vapor are withdrawn and analyzed. Material is then added to the still
to change the composition. In this unit, the heating element is in the recirculation leg
from the bottom of the condenser to the still pot. It is insulated both to conserve heat and
for user safety. The heating rate is controlled by a Variac (not shown).
In this experiment, you will determine the vapor-liquid equilibrium envelope (at
atmospheric pressure) for binary mixture made from two of the following compounds:
Water
Methanol
Ethanol
Compound pairs will be assigned to various teams after the semester begins.
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Othmer Still
Figure 3
Analysis of the vapor and liquid samples will be done using the Abbe refractometer. The
first task will be to build a calibration curve for the test solution. Note: this can be done
while the initial charge is heating in the still. You should use at least 10 (and preferably
about 15) composition samples to develop the calibration curve. Instructions for the
Abbe refractometer are next to instrument; READ them before using the device. Before
you come to lab, you should make up a table showing the volumes (or weights) of each of
your two compounds you will use to make the individual calibration samples.
(The Abbe refractometer is a more sophisticated version of the hand-held refractometer
used in Experiment 6. If you have not yet done Experiment 6, please read the discussion
on refractive index that is part of Experiment 6.)
Construction of a proper VLE curve requires the use of mole fractions for composition,
not mass fractions. However, you may find that the calibration curves is easier to read if
it is constructed using mass fraction data. Thus, you may want to develop two
refractometer calibration curves – one using mole fraction data and one using mass
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fraction data. Use whichever is easier, but remember to plot your VLE curves with the
x-axis in mole fraction units.
VLE Determination
Ensure water is flowing through the condenser and that the condensate and still pot
sample valves are closed. Add about 250 ml of the lower boiling compound (find normal
boiling points from a reference such as Perry's Chemical Engineers Handbook or The
Handbook of Chemistry and Physics) to the still pot. Hook up the resistance heater to
the Variac in the exhaust hood. Before applying power to the system, have the lab
instructor or TA inspect and verify your set-up. Set the Variac at the 75% level and
apply power to the resistance heater. It will take some time for the system to heat up and
reach a constant boiling rate, as evidenced by a stable temperature and a continuous flow
of condensed vapor in the recirculation leg. Even though this first sample should be a
pure compound, measure the refractive index of both liquid and vapor samples and verify
that both are pure components. Record the temperature and compare to the published
value of the normal boiling point. If it is not the same, be prepared to explain why it
might disagree. (The normal boiling point is the temperature at which a substance boils
at atmospheric pressure.)
To determine the VLE curves:
1. Drain off the condensed vapor (about 18 ml) and enough liquid from the still pot
such that the total volume removed is about 30 ml. Condensate and still pot
samples are collected in separate containers for later analysis.
2. Add about 30 ml of the higher boiling compound to the still pot and let the system
come to a new equilibrium as evidenced by a stable temperature AND a
continuous recirculation of the condensed vapor to the still pot.
3. Wait about 5 minutes after the temperature has stabilized and the there is
continuous recirculation flow of condensed vapor before taking samples. It is
critical that the system be at equilibrium for your data to be valid. You can’t
hurry the process!
4. Cool the samples and analyze using the Abbe refractometer. It is critical that the
samples be at the same temperature as the calibration samples. You can place
them in the water bath that cools the Abbe, but ensure that the caps are tight.
5. Repeat steps 1 - 4 until you have sufficient data to determine the VLE.
Determine (before you come to lab) your dilutions such that you take at least 10 samples
and preferably 15 samples. Plot all the data in form similar to Figure 1 (x is the liquid
mole fraction, y is the vapor mole fraction of one of the component). It is normal
practice to plot the mole fraction of the more volatile (lower boiling temperature) on the
x-axis.
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Your report should contain all the data, sample calculations as appropriate, the calibration
curve, and the T-x-y diagram. You should also discuss possible sources of error and, if
possible, compare to literature or theoretical results.
Important Note: This experiment can not be hurried and must be done carefully to
obtain good data. It may take more than the normal three hours allocated for lab so
be prepared to spend extra time if necessary.
Safety
Consider all compounds used in this lab to be poisonous if ingested. Do not drink any of
the compounds in this (or any) lab.
Wear safety glasses at all times
The fumes can be harmful if you are exposed for long periods of time. Be sure the
system is under the hood and the hood is working properly.
The Othmer still is glass (and expensive!) so be careful handling it.
During the distillation process, the glassware will be hot. Use caution when extracting
samples and wear the appropriate gloves when turning stopcocks or removing stoppers.
Do not put large volumes of cold fluid into the hot still. The thermal shock could break
the apparatus. When adding material to the system, use a pipette or syringe and add the
material slowly, injecting it into the pool of boiling liquid. Do not spray it on the sides of
the still.
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