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4 – 1
CHE 4115 Chemical Processes Laboratory 2 Experiment 1
Batch Distillation
BACKGROUND
Distillation is one of the most commonly used unit operations in chemical
engineering. In general, a distillation operation may be designed as either a batch or a
continuous process and either with or without reflux. In this experiment, batch
distillation with total reflux will be studied.
OBJECTIVE
To evaluate the efficiency of a plate column for batch distillation.
PROCEDURE
1. Measure volume of each component (500 mL of water, 1000 mL of acetic acid).
Calculate mole fractions for the initial mixture. [Note: the specific gravity of
acetic acid = 1.049]
2. Mix together and charge into pot. Caution: get an instructor to help you with the
next step. Slip glass joints together until they align and seal. Close distillate
stopcock.
3. Turn on condenser cooling water to a moderate rate.
4. Turn on low heat. Gradually increase until slow boil occurs.
5. Adjust heat upward gradually until water vapor can be seen rising up into the
condenser. If too much heat is applied, the column will flood with liquid and
make a rattling noise. Turn off heat immediately until noise stops, then gradually
increase heat until there is vapor in the condenser but not flooding occurs.
CAUTION: An excessive amount of heat could cause the condenser to receive
more vapor than it can condense. If this happens, vapor may be released into the
room. Shut off the heat immediately and increase rate of condenser cooling
water.
6. When steady state occurs, take samples (approx. 50 mL each) of liquid and vapor
in stillpot, and of distillate product (close stopcock again). Measure both the
volume and the weight of each sample. Record temperatures in the stillpot and
the top of the distillation column. Measure the input and output temperatures and
4 – 2
the flow rate of the cooling water. When taking a vapor sample, submerge the
receiving flask in an ice bath to ensure that the entire sample is condensed.
[Note: if the technique used to analyze sample composition is temperature
dependent (see Appendix), allow samples to cool to approximately room
temperature before analysis.]
7. Repeat step 6 a few minutes later: Collect all samples and apply all
measurements, as above.
ANALYSIS
1. Using the equilibrium data provided below, construct the y-x equilibrium curve
and the T-x-y plot of the system.
2. The given y-x equilibrium values were calculated using a constant value of
relative volatility in
A
A
Ax
xy
)1(1 −+=
α
α (11.8)
Plot 1/y versus 1/x to calculate that value from the slope or intercept of the plot.
3. Using the McCabe-Thiele graphical method, determine the theoretical number of
stages and the overall column efficiency, given that the column is equipped with
27 plates, and operated under nearly total reflux.
4. Using Equation (11.51) in the text, calculate another value of the number of
theoretical stages at total reflux. Compare the values and discuss any differences.
av
sA
B
dB
A
x
x
x
x
nαlog
log
1
=+ (11.51)
5. Calculate the water mole fraction (xs1) and total moles (S1) in the initial stillpot
mixture (from the given volumes of water and acetic acid used to prepare it).
6. Calculate the ratio of final to initial moles in the stillpot, S2/S1
a. from the measured values of xd av
, xs1, xs2
b. from the measured xs1, xs2 and assuming xd=k xs (since xs varies very little)
21
2211
SS
SxSxx SSav
d−
−= ∫
−=
1
22
1lnS
S
x
x sd
s
xx
dx
S
S
4 – 3
Both methods should give the same result. Use it to derive S2, compare the result
to S2=S1-Db (Db: total moles of collected distillate), and discuss.
7. From the measured flow rate and temperature change of the cooling water,
calculate the heat removal in the condenser. Write a heat balance on the overall
system (column, stillpot, heater, and condenser) and use it to calculate the net heat
supplied to the system by the heater (i.e., heat supplied by the heater minus any
heat losses to the environment).
EQUILIBRIUM DATA
The following data has been calculated for the equilibrium mole fractions of water
in the liquid (xA) and vapor (yA) phases for the water (A)/acetic acid (B) binary system:
T (ºC) xA yA
118.3 0.000 0.000
110.6 0.188 0.306
107.8 0.308 0.447
105.2 0.450 0.597
104.3 0.520 0.658
103.5 0.582 0.711
102.8 0.675 0.780
102.1 0.726 0.824
101.5 0.795 0.867
100.8 0.879 0.919
100.5 0.913 0.941
100.2 0.958 0.971
100.0 1.000 1.000
REFERENCE
Coulson, J.M., and Richardson, J.F., Chemical Engineering, Vol. 2, 4th
Ed., Pergamon
(1991).
4 – 4
APPARATUS
The glass distillation column is a versatile apparatus, which can be used in several
modes to determine useful data. The vacuum jacketed column has multiple plates with
disc bubble caps and liquid downcomers. Two still pots are available each with a
corresponding size electric mantle heater. On top, a condenser and flow splitter complete
the apparatus. All connections (joints) are ground glass and usually need no grease for
sealing, but must be handled in a careful way so that only vertical and no bending forces
are imposed on the joints. Since most liquids of interest are flammable and/or toxic,
safety precautions must be observed to prevent skin contact or emission to the room.
Figure 4.1. Distillation Unit and Gas Chromatograph
4 – 5
Figure 4.2. Distillation Unit
1 - Condenser 5 - Heating mantel
2 - Distillate 6 - Still pot – vapor sample
3 - Distillation column 7 - Still pot –liquid sample
4 - Still pot 8 - Temperature readout
1
6
2
3
4
5
7
8
4 – 6
Figure 4.3. Distillation Unit – Closer View
4 – 7
APPENDICES
A. SAFETY PRECAUTIONS
Heater: Avoid spilling liquid on the heater. Always heat liquids slowly to prevent rapid
boiling and overflow.
Stillpot: The pot is under pressure. A sudden opening may permit hot liquid to flow out
rapidly, potentially causing a burn or a fire. Handle the connections with care.
Do not run the flask dry, keep at least 200 mL of liquid at all times. Do not force
glass materials. (See the instructor.)
Column: Any blow or bending motion will break the column and your experiment will be
over. Please use extreme care in handling the column. Rotate ground glass joints
slowly to free them. Lift slightly while rotating. Do not force them.
Condenser: This is delicate glass. When adjusting the condenser or any glassware, be
careful and concentrate fully on what you are doing.
Cooling Water: Be absolutely sure this is on at all times during the experiment.
Thermocouples: Be sure that they are set firmly in their connectors.
Sampling: (a) liquid from stillpot- open the stopcock slowly and have a cold flask in
position ready to catch liquid. Cover sample immediately after drawing it. (b)
Vapor from stillpot: this is under pressure, so open valve slowly and catch sample
with flask in ice bath. Either of these samples may be hot enough to burn your
skin. Cool all samples to room temperature before attempting analysis.
Liquid or Vapor Spills: Be sure the lab door is open. Wipe up the spill with sponge and
flush down a sink. Wash hands with lots of water. Avoid excessive inhalation of
vapors.
B. pH METER
A pH meter may be used to measure the molarity of an acetic acid solution
assuming that an equilibrium constant may be defined as Keq = [H+][Ac
-]/[HAc]. You
may assume Keq = 1.75 x 10-3
mol/L.
Note: the accuracy of this measurement is limited, especially at high
concentrations (low pH). That should not affect the ability to perform the calculations
required for the experiment. It may, however, affect the values of calculated parameters
and thus be worth “discussing” in the final report!
4 – 8
Gas Chromatograph Operating Instructions
These are the steps that you should follow to start the GC properly. If this is your first
time, you may want to read the next section, “Choosing the GC Settings.”
1. Turn the helium cylinder valve so that the gas is fed to the GC.
2. Adjust the helium flow rate to approximately the desired value using the bubble
flow meter.
3. Turn on the power supply to the GC.
4. Set the oven, detector and injector temperatures.
5. Set the detector current and attenuation.
6. When all the settings have reached their set point values, adjust the helium flow
rate again (when the GC oven temperature changes, so does the flow rate). Turn
on the integrator. Wait for about 5 minutes before injecting a sample to let the
system stabilize.
7. Check that the integrator baseline does not drift, and then inject the sample.
8. When you are ready to turn the GC off, set the oven, detector and injector
temperatures and the detector current at their minimum. Let the GC cool down
for at least 20 minutes; if you turn it off when the oven temperature is still high,
the packing will melt and the column will have to be replaced.
9. Turn off the power supply and stop the helium flow rate.
4 – 9
Choosing the GC Settings
Helium flowrate
The standard flow rates are 30 cc/min for 1/8” columns and 60 cc/min for 1/4”
columns. You can change these flow rates according to your needs. For example, if two
key components are overlapped you may want to use a lower flow rate, but keep in mind
that the peaks are not going to be as sharp as before. Conversely, if you want to reduce
the run time, you can increase the flow rate, although the peaks may overlap after doing
so.
Oven Temperature
You should try to use the oven temperature that the manufacturer of the column
recommends. If this does not yield satisfactory results, you may change it taking in mind
the following:
• The maximum oven temperature that the GC can handle is 400 ºC.
• All column packings have a maximum permissible temperature; never use an
oven temperature higher than that.
• Increasing the temperature reduces the elution time and produces sharper peaks
(and vice versa).
• It is not recommended to operate the oven at a temperature more than 10 ºC below
the boiling point of any of the components of the sample because the elution time
will become too long.
Injector and Detector Temperature
The injector and detector temperatures are usually chosen to be 5 to 15 ºC and 20
to 40 ºC above the oven temperature respectively.
Detector Current
Changing the detector current changes the sensibility of the apparatus. The
maximum current that can be used depends on the temperature and the carrier gas used.
You should check the GC manual to find the maximum allowed current for your
temperature and carrier gas.
When more sensitivity is needed, it is better to reduce the attenuation than to
increase the detector current; this will prolong the detector’s life.
4 – 10
Attenuation
The attenuation has the same effect as the detector current. Using the lowest
attenuation without getting “noise” in the chromatogram will enable you to use the
smallest detector current and make it last longer.
Gas Chromatograph Parameters
Column Temperature → Tc = 170 °C
Injector Temperature → Ti = 180 °C
Detector Temperature → Td = 185 °C
Detector Current Setting → 85 mA
Attenuation → 64
Sample size → 2 µm
Volume and Mol Percentage versus GC Area Percentage for Acetic Acid/Water
Volume Percentage = 2
AA
3
AA X107666.3X3664.10831.1 ×−+
Mol Percentage = 2
AA
3
AA X103534.7X24127.05197.1 −×++