2.11: Fractional Distillation

1. Set-up of Fractional Distillation Apparatus

Table 1. Fractional distillation apparatus components.
Note: all glassware, with the exception of the graduated cylinders, should have ground-glass joints

1. Gather all of the equipment needed to assemble the fractional distillation apparatus (Table 1) and place them in a fume hood.
2. Place the heating mantle and stir plate at the foot of the retort stand, sitting on top of a screw jack.
3. Using the screw jack, elevate the heating mantle to a height of 20 cm.
4. Attach a clamp on the retort stand. Secure the round-bottom flask to the retort stand using the clamp so that the flask sits snuggly in the heating mantle.
5. Remove the screw jack and the heating mantle, putting them aside for later.
6. Add a magnetic stir bar to the round-bottom flask.
7. Add a fractionating column to the top of the round-bottom flask, securing it to the retort stand with a clamp.
8. Add a "Y" adaptor to the top of the fractionating column, securing it to the retort stand with a clamp and to the fractionating column with a Keck clip.
9. Add a condensing column to the downward sloping arm of the "Y" adaptor, securing it with a Keck clip. For additional stability, the condenser can be secured with a clamp to a second retort stand.
10. Connect tubing from the water source to the lower connection on the condenser and attach the water return tube to the higher connection on the condenser. Ensure the tubing is securely fitted to the condenser (consider using copper wire to hold the tubing in place).
11. Connect a collection adaptor to the open end of the condenser and secure the connection with a Keck clip.
12. Have three 25 mL graduated cylinders available for collection of the distillate.

Figure 5. Apparatus for fractional distillation.

2. Preparation for the Distillation

1. Remove the round bottom flask from the assembly by lowering it to the base of the retort stand.
2. Place a stemmed funnel into the top of the round-bottom flask, and add the liquid to be distilled. Do not fill the flask more than half full.
3. After filling the flask, remove the funnel.
4. Raise the flask once again and place the flask back in its original position on the retort stand.
5. Using the thermometer adaptor, place the thermometer into the remaining open port on the "Y" adaptor. The thermometer should be added last, as it is most susceptible to breakage.
6. Position the bulb of the thermometer so that it is just below the side arm of the "Y" adaptor. This ensures an accurate reading of the vapor temperature.
7. Replace the heating mantle and screw jack to their original position, snuggly enveloping the round-bottom flask.
8. Place a graduated cylinder underneath the condenser adaptor, ready to collect the distillate.

3. Performing the Distillation

1. Increase the temperature of the heating mantle gradually until the liquid in the round-bottom flask begins boiling.
2. As soon as boiling commences, hold the temperature for 2 min.
3. Note the ring of condensate rising slowly up the fractionating column. The rise should be gradual, which ensures proper separation of the mixture components across the theoretical plates. If the ring of condensate stops rising, increase the temperature slightly. It should take at least 5 min for the ring to reach the top of the column.
4. Once the first drops of liquid start to fall down the condenser, the temperature should remain nearly constant. Record the temperature as each 2 mL of distillate collects in the graduated cylinder.
5. Adjust the heat accordingly to allow for 1—2 drops/s from the condenser into the graduated cylinder.
6. Each time the graduated cylinder reaches 4 mL, quickly replace it with an empty one.
7. Pour the distillate into a clean, labeled vial for safekeeping.
8. If the distillation is occurring very slowly, wrap the column with aluminum foil or glass wool to protect it from cold drafts.
9. When the temperature begins to drop significantly, nearly all the cyclohexane has been distilled. Increase the temperature until the toluene starts to boil and distill. Again, record the temperature as each 2 mL of distillate collects.
10. Before the entire content of the flask evaporates, stop heating the flask. It is important to ensure that the flask does not become dry, which might cause it to crack.
11. Remove the heating mantle by lowering the screw jack to the base of the retort stand
12. Analyze the contents of the vials of distillate to confirm the purity using a chosen technique, such as ¹H NMR spectroscopy.

Distillation is one of the most commonly used techniques for purifying liquids in laboratory settings and accounts for up to 95% of all industrial separation processes.

This method separates and purifies liquids based on their volatility, or the tendency for its molecules to escape from the liquid phase as gaseous vapor. To distill a solution, it is heated until the most volatile compounds begin to vaporize. The resultant vapors are then condensed into a purified liquid, known as the distillate, and collected.

The most common distillation methods are: simple distillation, which employs just one vaporization-condensation cycle, and fractional distillation, which employs multiple vaporization-condensation cycles.

This video will review the principles behind simple and fractional distillation, typical laboratory distillation apparatuses, and demonstrate an example fractional distillation procedure. Finally, applications of distillation will be covered.

When a liquid is heated, the kinetic energy of its molecules increases, causing some of the molecules to transition from the liquid into the gaseous state. This process, called vaporization, increases the vapor pressure above the liquid relative to the atmospheric pressure. The temperature at which the liquid's vapor pressure equals the surrounding environment's air pressure, at which point 'bubbles' of vapor form within the liquid, is known as its boiling point. Above this temperature, the liquid will completely vaporize into a gas.

In a simple distillation, a mixture is heated in a flask, and the resultant vapors pass into a condensing column where they are cooled and condensed into a liquid called the distillate. This method uses just one vaporization-condensation cycle, and generates pure distillates from both mixtures of liquids and solids, and mixtures of liquids with vastly different boiling points.

Mixtures of liquids with boiling points differing by less than about 30 ?C cannot be completely separated using simple distillation. The distillate composition, however, can be predicted using a 'boiling point-diagram', which plots temperature as a function of liquid and vapor composition. For example, an equal mixture of cyclohexane and toluene has a boiling point of 90 ?C, resulting in a vapor composition of 80% cyclohexane and 20% toluene, and yielding an 80% pure distillate.

The boiling point diagram predicts that a second vaporization-condensation cycle, achieved by boiling the 80:20 distillate at roughly 84 ?C, results in a 95% purity. Each consecutive cycle, referred to as a 'theoretical plate,' increases the purity of the, where 3 theoretical plates?result in a 99% purity. Although it is possible to generate this by chaining multiple 'simple distillation' apparatuses together, 'fractional distillation' achieves this more efficiently.

The setup adds a "fractionating column," between the starting flask and the condenser. This column is typically filled with glass beads or metal wool, providing a large surface area for the liquid to condense and re-evaporate onto numerous times. This generates a large number of theoretical plates.

As vapors rise through the column, a ring of condensate forms and slowly moves up as the vapors separate across the theoretical plates. When the vapor reaches the top of the column, the molecules with the highest volatility pass into the condenser where they are collected as a high purity distillate.

Now that we've reviewed the basic principles behind distillation, let's walk through an example for a cyclohexane: toluene mixture.

Begin by assembling the components of the fractional distillation apparatus in the fume hood. Place the heating mantle and stir plate on top of the laboratory jack at the foot of the retort stand and raise everything 8 in.?with the jack.

Attach three clamps to the retort stand. Position the round-bottom flask so that it fits snuggly into the heating mantle and secure it to the retort stand.

Add a magnetic stir bar to the flask, place the fractionating column on top, and secure it.

Fit the "Y" adaptor into the top of the fractionating column. Secure it to the retort stand with a clamp and then secure the ground-glass joint between it and the fractionating column with a keck clamp.

Fit the condenser into the downward sloping arm of the "Y" adaptor and secure the joint. For added stability, clamp the condensing column to a second retort stand.

Connect a tube from the water source to the bottom connection on the condenser. Connect a second tube from the upper condenser connection to the water return port.

Secure the tubing, turn on the water, and verify that water flows through the condenser.

Add a collection adaptor to the condensing column. Secure this joint with a keck clip and place a graduated cylinder beneath it.

Before installing the thermometer, lower the flask. Use a funnel to fill less than half of the flask with the starting mixture.

Raise the flask, fitting it back into the fractionating column, and securing it. Secure this final ground-glass joint with a keck clamp and then replace and reposition the jack and heating mantle.

Finally, fit the thermometer into an adaptor and place it into the remaining port of the "Y" adaptor. Adjust the height of the thermometer bulb, situating it just below the side arm of the "Y" adaptor to ensure accurate vapor temperature readings.

Turn on the hot plate, gradually increasing the temperature and heating the mantle until the starting mixture begins to boil. Adjust the hotplate temperature as needed to ensure that the mixture continues to boil, maintaining a constant vapor temperature for the first 2 min?of the distillation.

Watch as the condensation ring forms, and rises, to the top of the column. When the first drops of liquid condense and drip into the graduated cylinder, record the vapor temperature. Keep the vapor temperature constant by adjusting the hotplate setting until drops fall from the condenser at a rate of 1?2 per second.

For each 2 mL?increment of distillate volume, record the vapor temperature. After 4 mL?has been collected, record the vapor temperature and then quickly replace the partially filled graduated cylinder with an empty one. Save the 4 mL?sample in an individually labeled vial for future analysis.

Continue recording vapor temperatures at 2 mL intervals, and saving distillate samples at 4 mL intervals, until the vapor temperature drops significantly and the mixture stops boiling.

Turn off the hotplate and the water running to the condenser. The distillation of cyclohexane is now complete and the 4 mL distillate samples can be prepared for NMR analysis.

Evaluate the purity of both the starting mixture and the distillate using NMR spectroscopy, a common technique for assessing the composition and purity of mixtures. In our example, the NMR spectrum of the initial mixture shows the characteristic peaks associated with both toluene and cyclohexane. The NMR spectra of the first distillate we collected after fractional distillation, however, was pure cyclohexane.

Distillation has applications in a broad range of fields, from large-scale petroleum refineries to small-scale whiskey stills.

To generate distilled spirits such as vodka or whiskey, a mixture of grain fermentation products known as 'wash,' which is 10?12 % alcohol by volume, is boiled in a "still" and the resultant vapors separated by either simple or fractional distillation, depending on the still's design and the type of spirit. This allows 'the heart', ethanol, to be separated from 'the tails', like propanol and water, which have higher boiling points. Additionally, distillation allows for the elimination of 'the heads' like methanol, which famously caused blindness in poorly made moonshine. The distillate may be around 50 % alcohol if produced from simple distillation, or as much as 95 % if fractionally distilled.

Gas chromatography uses thousands, if not millions, of theoretical plates to separate volatile mixtures using fractional distillation on a micro-scale. Here, a mixture of volatiles used to stimulate the olfactory nerves of bees were injected into the gas chromatograph, which was used to separate and identify the compounds based on the amount of time they took to pass through the chromatography column.

Trace explosive vapors of TNT and RDX were selectively separated from a sample headspace using the principles of distillation. These samples were collected in temperature desorption tubes and introduced into a temperature desorption stage, where they were heated them to between 350 and 900 degrees to increase their volatility. Finally, they were selectively condensed using a cryotrap and introduced into a gas chromatograph for analysis.

You've just watched JoVE's introduction to fractional distillation. You should now understand the basic principles behind distillation, the apparatuses for simple and fractional distillation, and a basic fractional distillation procedure.

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