Some organic compounds like those found in eucalyptus oil decompose or break down if heated to very high temperatures. This can be a problem for manufacturers who want to isolate eucalyptus oil, because the most convenient method to do so would be by distillation. Steam distillation overcomes this problem by reducing the boiling point so the eucalyptus oil can be boiled off at a lower temperature.
You already know that oils mix poorly with water. If you take some vegetable oil and pour it in a glass of water, for example, or if you try to add water to gasoline, you'll find they separate into two layers. The same holds true for eucalyptus oil; eucalyptol, which is one of the primary components of eucalyptus oil, dissolves very poorly in water. During steam distillation, eucalyptus oil and water or steam are combined in the same vessel or flask, but they do not, of course, mix properly.
As the vessel is heated, more and more of both the water and the oil begin to vaporise. The water and oil will boil when the total pressure exerted by the combined vapour from each is greater than atmospheric pressure. Since vapour from each contributes to this total vapour pressure, the boiling point is lower for both liquids than it would be for either alone. Ultimately, co-distilling the oil with water in this way dramatically reduces the boiling point of the oil.
Typically the fresh eucalyptus leaf is transported to a distillery and placed in a vat; pipelines run through the base of the vat along the floor. The lid on the vat is then closed and steam is introduced through the pipes. The oil in the steam-soaked leaves is vaporised and can be collected through outlets in the lid, while the tannins and other undesired components of the leaf material sink through a drain in the floor of the vat.
Subsequent Steps and Considerations
Following the steam distillation, the oil must be separated from the condensed steam. This part is not difficult, because the two elements do not mix; merely adding the oil and water to an open drum will cause them to separate into two layers just like water and gasoline, and the eucalyptus oil can be scooped off the top. The oil may later undergo further purification through another process called rectification. If you were distilling eucalyptus oil on a small scale in a laboratory, however, one alternative to steam distillation would be vacuum distillation.
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