Friday, October 22, 2010

Steam distillation


Steam distillation is a special type of distillation (a separation process) for temperature sensitivematerials like natural aromatic compounds.

Many organic compounds tend to decompose at high sustained temperatures. Separation by normal distillation would then not be an option, so water or steam is introduced into the distillation apparatus. By adding water or steam, the boiling points of the compounds are depressed, allowing them to evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the material becomes appreciable. If the substances to be distilled are very sensitive to heat, steam distillation can also be combined with vacuum distillation. After distillation the vapors are condensed as usual, usually yielding a two-phase system of water and the organic compounds, allowing for simple separation.


Steam distillation
Notice that in the presence of water, phenylamine (or any other liquid which is immiscible with water) boils well below its normal boiling point. This has an important advantage in separating molecules like this from mixtures.
Normal distillation of these liquids would need quite high temperatures. On the whole these tend to be big molecules we are talking about. Quite a lot of molecules of this sort will be broken up by heating at high temperatures. Distilling them in the presence of water avoids this by keeping the temperature low.That's what steam distillation achieves.

Carrying out steam distillation
We will carry on with the phenylamine example for now. During the preparation of phenylamine it is produced as a part of a mixture containing a solution of all sorts of inorganic compounds. It is removed from this by steam distillation.
Steam is blown through the mixture and the water and phenylamine turn to vapour. This vapour can be condensed and collected.
The steam can be generated by heating water in another flask (or something similar).
As the hot steam passes through the mixture it condenses, releasing heat. This will be enough to boil the mixture of water and phenylamine at 98°C provided the volume of the mixture isn't too great. For large volumes, it is better to heat the flask as well to avoid having to condense too much steam and increase the volume of liquid in the flask too much.
The condensed vapour will consist of both water and phenylamine. If these were truly immiscible, they would form two layers which could be separated using a separating funnel. In fact, the phenylamine has a slight solubility in water and various other techniques have to be used in this particular case to get the maximum yield of phenylamine. These aren't relevant to this topic.

Some other applications of steam distillation
Steam distillation can be used to extract some natural products - for example, to extract eucalyptus oil from eucalyptus, citrus oils from lemon or orange peel, and to extract oils used in perfumes from various plant materials

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