What is #"steam-distillation"#?

1 Answer
Oct 9, 2016


Steam distillation is a softer method of distillation, in which steam is used to elevate the combined vapour pressures of the steam and an organic component ABOVE atmospheric pressure.


The result? When the pressure of the steam and the vapour pressure of the component, which vapours are entrained, exceeds the atmospheric pressure, the distillate is enriched with the component, which normally separates on standing.

Thus when #P_"stillpot"=P_"steam"+P_"volatile component">1*atm#, the volatile component will begin to distil over. Of course, you collect a lot of water with the distillate, but water is cheap and plentiful, and very easily separated, and the volatile component may be a high value essential oil. Often when you do this the distillate comes as a milky, turbid, liquid, and on standing you see a fine, clean oil accumulate at the bottom of the flask; the collected water is discarded (or could be back washed with an organic solvent if the organic product is a high value essential oil), and the oil retained.

Of course, you could attempt to distill the desired liquid directly. But you certainly would need to use higher temperatures, the which may decompose or chemically alter the component you are after. If you are in the perfume caper, (and such distillations are widely used in this trade), you could claim that the product is #"ethically steam distilled"#, without the use of dem there nasty kimicles (of course water is not a chemical, yeah right!).

So I can give you a specific example from a first year undergrad organic lab. Bromobenzene is often steam distilled. Its normal boiling point is #156# #""^@C#. In a steam distillation, water and bromobenzene co-distil at #95# #""^@C#; a substantially lower temperature than the normal boiling point. Mind you, I would not put bromobenzene in any perfume (it would bring literal tears to your eyes!).

Anyway, there is nothing that I say here that you won't learn better by doing the experiment in a laboratory. When you do it, ask for the insight of your teaching assistant; they are usually very experienced, and more than willing to share their knowledge and experience. Good luck.

PS It seems that I have addressed this question before. C'est la vie.