Study in Batch Distillation using a Bubble Plate Column
Particular safety issues:
(1) Read the procedure below and then come back and re-read this paragraph.
(2) Make sure that the chiller (for the condenser) is activated well before
at least 20 minutes) the heating element for the still is switched on.
(3) At the end of the experiment, the chiller must be allowed to run after the heating
element is deactivated – until the still cools down enough that vapor is not
leaving the top of the column.
(4) Follow the instructor or assistants instructions about the distillate you’ve
collected. It should either be poured into the appropriate waste vessel or
returned to the still.
(5) The heating element, column, and still will become very hot over the course
of the experiment. Be careful not to touch them.
1. Calibrate the refractometer so you can identify compositions of samples. This is
done by making samples of known composition by mixing known volumes of
ethanol and water. Measure refractive index of each sample. Use a small syringe
to extract a few drops needed for the refractometer. Immediately close the sample
container to prevent vapor loss. Enter these measured refractive indices into
Excel. Since the volume composition is known, you can calculate the mole
fraction of each sample from the known densities. Show how you did this
calculation in your report. Graph refractive index against mole fraction of ethanol
and fit the data so you can relate refractive index to composition. Compare your
values against those from the literature (see for example the Handbook of
Chemistry and Physics). Note any differences and discuss that in your report.
How can you improve the accuracy of your data?
2. Add sufficient amount of a 30 % (by volume) of ethanol-water mixture to the
reboiler pot to bring the content of the reboiler pot to the 1000 ml mark. If the
mixture in the pot does not look clear, use a fresh batch of solution. Take about 1
ml of the mixture in the pot for analysis. Start the heater at about 80% power and
coolant circulation. (Note the full power rating of the heating mantle.)
3. Run the column at total reflux. Let system come to steady state. This can be
verified by taking temperature data at the top and bottom of the column at 2 min
intervals (It is suggested that you note the temperature to two decimal places even
though the accuracy of the temperature sensor is only .1 deg C). When the
changes in temperature are less than 1%/ min then steady state may be assumed.
Please note the temperature data in your lab notebook. Take a sample from of the
pot (1ml or less) and a sample from the top of the column (Make sure to flush the
exit port of any liquid left over from the past). Set aside in a closed container to
prevent evaporation losses. Measure the temperature at top and bottom. Repeat
this again after 10 minutes, just to make sure that the composition has not
changed. Note the pressure as well.
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4. Determine the boil up rate in the column by shutting off the reflux and letting
condensate accumulate at the top. Note the volume accumulated at 1 min
intervals. Stop after 15 minutes or after the accumulator is half full whichever
comes first.
5. Slowly release all the liquid in the accumulator back into the column. If you do it
too fast, you can damage the equipment. Let system equilibrate again.
6. Now set the reflux ratio at about 50% by opening both valves approximately the
same. You should get a distillate rate that is approximately half of the boilup rate
measured above. Start collecting the distillate in a graduated cylinder. Note the
amount accumulated as a function of time by taking readings every 5 minutes.
Also note the bottom and top temperatures every minute. Stop after you have
observed a measurable change in the top temperature. You should have collected
about 100-150 ml of distillate. At this point you may proceed to turn off the
power and shut down the column.
7. Use the refractometer to identify the compositions of the samples taken in step 2
after they have cooled to room temperature (why?).
The inner diameter of the column is 3.0 cm.
Clean up
Clean the refractometer and switch it off.
Shut of the power to the mantle. After the still has cooled, shut off the cooling system.
Clean all glass ware with tap water first. Rinse once with distilled water and keep in the
dishwasher for drying.
1. g at total reflux? Why or why not? Explain.
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