Fractionation (Fractional Distillation) of Essential Oils (Part 2)

The Distillation

With the apparatus set up as detailed, turn on the vacuum pump. When the correct vacuum has been achieved (usually the hardest vacuum the pump will give), check that the bleed is providing a fine stream of bubbles in the essential oil situated in the flask. Provided this is operating as it should then turn on the heat on the mantle. The essential oil should soon start to boil: lower the heat until a steady ‘rolling boil’ is achieved.

The vapour will be seen to slowly rise up through the column and through the helices. Eventually the first vapour will reach the thermometer and continue climbing vertically up to the condenser. Here it will liquefy and drip back down to the column and flow back into the flask. Once a stable equilibrium has been reached, whereby the oil is gently boiling and the vapour is condensing nicely, it I time to start the fractionation process.

After recording the initial boiling point temperature and vacuum, open the variable take-off valve to suit the appropriate stage in the distillation – at this point the process is intended to simply strip off the lightest terpenes, so a ratio of 50:50 would be satisfactory. In effect, this means that for each drop that is removed by travelling into the Perkins triangle, a similar drop will be returned down the column. This ratio is adjusted by opening or closing the Rotoflow valve, thus increasing or decreasing the rate of take-off. This rate will need to be slowed down at critical points; for example when approaching a terpeneless section that requires to be separated. Screwing the valve in decreases the flow to the Perkins triangle and returns more of the condensate back down the column.

The slower the removal of oil, the more accurate the fractionation process. If there a large percentage of a particular component present in the oil, e.g α pinene, then the take-off rate can be raised to a maximum of 1:1. This ratio should never be exceeded in order prevent any other components distilling over at the same time.

As the fractionation progresses, the distilled matter will slowly accumulate in the receiving flask that is fitted at the base of the Perkins triangle. Provided that the temperature of the distillate remains roughly constant, it is safe to assume that the same component is being removed on its own.

When the first 50ml aliquot is reached, turn off the tap A above the flask: the distillate will now collect ion the receiving section of the Perkins triangle. Gently turn the tap at the lower end of the Perkins triangle (B) which firstly turns off the vacuum to the flask and as the tap continues to rotate, will then vent the flask. Make sure that the flask is supported at this point: it is now vented and can drop as no longer held in place by the vacuum.

The vacuum is still operational on the rest of the Still, so the distillation will be proceeding normally. Quickly place the flask (now full of Fraction 1) in a safe area and place a second empty flask (with pre-greased socket) on to the Perkins triangle lower cone. It is important to get the following sequence correct: turn off the vacuum to the Still, using upper tap C on the Perkins triangle, then turn lower tap B so that the vacuum can evacuate the new empty receiving flask – this should take no more than a few seconds. As soon as the vacuum gauge shows that the flask is at the same vacuum as the Still, reconnect the two sections by gently opening the top tap C. Tap A can now be turned and this will allow any of the second fraction that has collected in the Perkins triangle reservoir to drop into the receiving flask. Make a note of the boiling point on the thermometer and also check that the vacuum has remained the same. Continue with the fractionation by repeating this procedure.

Notes:

From a purely practical standpoint, it is best to transfer each fraction to a labelled screw top vial, otherwise you are going to require dozens of 50ml or 100ml round bottomed collecting flasks. It is possible to use just one flask but two is better as it facilitates the removal and transfer of each fraction. However lots of bottles will be required, although fractions can be amalgamated once their odour has been assessed and this is especially true if the fractions have an identical boiling point. If the required product is for example, a terpeneless oil or a specific component is required from the oil, then a cleaner more distinct separation is achieved by gradually slowing down the take-off to allow the column to operate as efficiently as possible. As an example, if the vacuum pump is known to be operating at say 0.15mm Hg (millimetres of mercury) and it is known from published data that the terpeneless oil we are seeking has a boiling point of 136.2°C -139.4°C at that vacuum, then at about 115°C, slow the variable take-off down from say 2:1 (2 drips being returned down the column for each drip that goes into the Perkins triangle) to say 5:1.

When the thermometer shows that the vapour has a boiling point of say 130°C, slow it down to 10:1 and as it reaches 134°C, slow it yet again to 25:1.

There are often tests from the B.P or E.P that give criteria for various terpeneless oils – for example, for a particular terpeneless essential oil, it must be totally clear and free from turbidity when added to a 70% pure alcohol solution at 20°C. This means that all the fractions with the correct boiling point must be checked very regularly and their odour tested at the same time. It is probably a good idea to have smaller fractions at this point of the distillation, say 20ml or so: normally all the fractions that comply with the terpeneless criteria e.g. solubility with alcohol and also within the correct temperature range, are then combined to produce the pure terpeneless oil.

To maximise the yield, it is possible to carefully add back the fractions with boiling points on either side of the combined ones, so that the yield of terpeneless oil is maximised and all of the tests (for example, the alcohol solubility) are still met. This means it can still be classified as 100% terpeneless oil in accordance the criteria laid down by B.P or E.P. etc.

Since the terpeneless oils are usually the higher boiling components in the oil, it is common for fractional distillations to stop at this point. Nevertheless, some oils have valuable (and interesting) end-note components in the very highest boiling components. In this case, the distillation can be continued past the terpeneless stage to remove these last fractions. In fact most fractional distillations can be continued to the point that the boiling flask is virtually empty.