Determination of Vapor-Liquid Equilibrium
CHEG 2810 Data Acquisition
11/3/09
Ken McFarland & Pacscal M
Abstract:
In this lab we created a vapor-liquid equilibrium curve for ethanol and water. Vaporliquid equilibrium curves can be used to find the molar composition of a mixture of two
compounds, given the mixture’s boiling point. In order to do this we had to know find the molar
composition of our mixture at many different boiling points. We found these by taking samples,
and finding their molar composition using an Abbe refractometer. This entailed creating a
calibration curve for the Abbe refractometer, before we could use it’s readings to find the molar
composition of our samples.
While our data is approximately the same shape as the example azeotropic vapor-liquid
equilibrium curve that is provided in our lab manuals on page two of lab nine, our data does not
demonstrate the smooth curves of the example. Rather it jumps about, and is quite erratic. This is
most likely a result of our calibration curve, which rose steeply, to a maximum at the .5 molar /.5
molar mixture then plateaued, instead of dropping back to a low value as it should of according
to the TA.
Introduction:
Distillation is one of the most common chemical operations that chemical engineers
perform. It involves the separation of mixtures of compounds based on difference in the
compounds boiling points. When a mixture of compounds is boiling, the vapor and the liquid
components of the mixture have different compositions. The graph of the molar fractions of the
vapor and liquid components of the mixture in relation to the temperature of the boiling mixture
is the vapor-liquid equilibrium curve for that mixture.
Most simple mixtures of two compounds have boiling points that are between that of the
two compounds in their pure state. When this is true, the more volatile of the two compounds can
always be found in higher concentrations in the vapor than in the liquid whenever the mixture is
brought to a boil, no matter what the molar fraction of the more volatile compound in the mixture
is.
In mixtures of some compounds, the boiling point of the mixture can be higher than the
boiling point of the less volatile compound, or lower than the boiling point of the more volatile
compound at some molar compositions. When this happens the resulting mixture is called an
azeotrope. When this happened, it means that the mixture of the two compounds cannot be
further separated into pure compounds through distillation. Our two compounds, water and
ethanol, form such an azeotrope, with the boiling point of the mixture dropping below the boiling
point of pure ethanol, the more volatile of our two compounds. This can be seen in our vaporliquid equilibrium curve (Figure 1), where the molar fraction of the ethanol in the liquid in our
curve is higher than the molar fraction of ethanol in the vapor.
Results and Discussion:
The refractive indexes for the liquid and vapor samples that we pulled from the Othmer
still at each point and the boiling temperature of the mixture at equilibrium when we removed the
samples are attached below in the appendix in Table 1. Also attached are the data that we used to
create our calibration curve for the Abbe refractometer in Table 2. The calibration curve for the
Abbe refractometer was used to convert the refractive indexes found in Table 1 into the molar
fractions of ethanol that are also found in Table 1, for both the liquid and vapor samples.
This data can be seen plotted in graphical form in Figure 1, our vapor-liquid equilibrium
curve. In this, we have plotted the molar fractions of the ethanol in the liquid and vapor samples
with respect to the boiling temperatures for each of the thirteen samples we took from the
Othmer still. This curve can be used to see what the molar fractions of ethanol and water are in
the vapor and in the liquid for any mixture of ethanol and water that has a known boiling
temperature.
Procedure:
We followed the instructions in our lab manual in setting up the Othmer still portion of
the lab. The Othmer still was first filled up with pure ethanol, that being the more volatile of our
two compounds. This was brought to a boil in order to get a baseline temperature for the boiling
point of pure ethanol. We then removed 18 mL of the vapor, and 12 mL of the liquid, and added
30 mL of water to the Othmer still, in order to change the composition of the mixture. These
steps were repeated, each time recording the boiling temperature of the composition, and
measuring and recording the refractive index of both the vapor and liquid samples using an Abbe
refractometer, and adding more water to replace the removed portion of the mixture to chance
the composition. This was done until the boiling temperature approximated that of pure water,
the less volatile of our two components.
After we were done measuring the refractive index of the samples from the Othmer still
using the Abbe refractometer, we created a calibration curve for the Abbe refractometer, so that
we could convert the refractive indexes for each of our samples into the molar composition for
each sample. In order to do this we measured the refractive index of 14 different samples that we
created, each of a known molar fraction of each of our two compounds. These measured
refractive indexes were graphed with respect to their known molar fraction of ethanol in order to
create a calibration curve for the Abbe refractometer for mixtures of water and ethanol.
Conclusions:
Water and ethanol form an azeotropic mixture of approximately .85 molar fraction
ethanol and .15 molar fraction water. This can be seen in our vapor-liquid equilibrium curve
(Figure 1) where the molar fraction of the ethanol in the liquid sample is greater than the molar
fraction of the ethanol in the vapor sample, even though ethanol is more volatile than water.
Recommendations:
If this lab were to be dome again, we attempt to create a more accurate calibration curve
for the Abbe refractometer, as it is our feeling that most of the errors in our data came from a
lack of precision in our calibration curve. This led to an inability to precisely and accurately
determine the mole fractions of any of our samples from the Othmer still, and the overall
inaccuracy and imprecision in our vapor-liquid equilibrium curve.
To facilitate this, we would also create our calibration curve while the Othmer still was
running if we were to do this lab again, instead of waiting until we were finished with the
Othmer still to create the calibration curve. This would give us much more time to create the
calibration curve, and could potentially reduce or even eliminate errors created though hurrying
in order to finish the lab, and be able to leave.
Appendix:
Figure 1: Our vapor-liquid equilibrium curve
100
Temperature ( ⁰C)
95
90
liquid
vapor
85
80
75
0
0.1
0.2
0.3
0.4
0.5
0.6
Molar Fraction Ethanol
0.7
0.8
0.9
1
Figure 2: Abbe refractometer calibration curve molar fraction ethanol
Mol fraction ethanol Calibration curve
1.365
1.3625
1.36
1.3575
1.355
1.3525
1.35
Molar fraction ethanol
1.3475
1.345
Poly. (Molar fraction ethanol)
1.3425
1.34
1.3375
1.335
1.3325
1.33
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Table 1: Othmer still sample data
Liquid
Refraction
Index
1.3615
1.3616
1.3615
1.3598
1.3572
1.3542
1.3513
1.3469
1.3454
1.3406
1.3368
1.3351
1.3305
Liquid molar
fraction
ethanol
1
0.8
0.79
0.38
0.27
0.2
0.16
0.11
0.09
0.05
0.02
0.01
0
Vapor
Refraction
Index
1.3615
1.3614
1.3622
1.3624
1.3624
1.3627
1.3628
1.3625
1.362
1.3611
1.359
1.3548
1.3502
Vapor molar
fraction
ethanol
1
0.88
0.65
0.6
0.6
0.6
0.6
0.6
0.59
0.46
0.39
0.22
0.14
Boiling
temperature
(degrees
Celsius)
76
77.9
79
79.8
80.5
80.5
83
84.5
87
90
93
96
99.5
Table 2: Abbe refractometer calibration data
Mol fraction
ethanol
0
0.1
0.15
0.25
0.3
0.4
0.5
0.6
0.7
0.75
0.8
0.9
1
Refractive
Index
1.333
1.3467
1.3504
1.3564
1.3581
1.3601
1.3615
1.3621
1.3619
1.3619
1.3615
1.3613
1.3608