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I know that fractional distillation is used to separate two liquids that are miscible in one another and have close boiling points.

However, I am really confused about vacuum and steam distillation - in which cases do we use these separation techniques? Can someone please give examples.

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Vacuum distillation is employed when the boiling point of the substance is too high (400 Celsius for example). Under reduced pressure, the boiling point will be reduced to the range that is easier to handle. Steam distillation take advantage of azeotropic effect, water steam is bubbled into your liquid (aromatic compound for example) and carry out the mixture of water vapor and your liquid vapor. This technique can also avoid handling the high boiling point distillation because the azeotropic boiling point is usually lower than pure liquid.

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  • $\begingroup$ Now I understand why vacuum distillation is used but I am still confused about steam distillation...Could you please explain the azeotrophic effect (I know that azeotropes are mixtures that have no change in composition when distilled) which has a role to play in steam distillation. $\endgroup$ – Eliza May 5 '14 at 17:57
  • $\begingroup$ I am actually little wrong on the steam distillation in the answer. It is not about azeotrope. Sorry for my confusion. I have a link to a good web page that explains it very well: chemguide.co.uk/physical/phaseeqia/immiscible.html $\endgroup$ – Ian Fang May 10 '14 at 13:31
  • $\begingroup$ I'm confused again. I understand that steam distillation is used when the substance is immiscible in water. However, I am confused about the difference in the purpose of the two distillations. They both allow the substance to boil at a temperature below its normal boiling point. How will I know which procedure to use? $\endgroup$ – Eliza May 10 '14 at 17:42
  • $\begingroup$ Steam distillation is mostly used in industry. Vacuum distillation is more preferred in lab. $\endgroup$ – Ian Fang May 11 '14 at 1:58
  • $\begingroup$ But they both serve the same purpose of boiling a substance below its normal boiling point where it would otherwise decompose? $\endgroup$ – Eliza May 11 '14 at 5:05
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http://www.umsl.edu/~orglab/documents/distillation/Image169.gif
boiling point vs. pressure nomograph

The separation unit of a distillation (or countercurrent exchanges overall) is the theoretical plate (originally from petroleum distillation towers),

http://fch.upol.cz/skripta/fcc_and_zvem_english/rektif/rektif_teorie_clip_image002.jpg
http://www.vaxasoftware.com/alen/MCTH_EN.jpg

Each separation step must be at equilibrium. Your basic lab rig is might be three theoretical plates at best, vapor vs. liquid in a hollow vertical column. Efficient physical separation demands thin film countercurrent flow to maximize surface area/volume, hence packed columns then

http://www.solvent--recycling.com/spinning_band_packed_column.html
spinning band distillation

wherein a tightly fitting teflon screw thread spinning like all get out jams fluid condensate downward against vapor flowing upward. Net output is a drop every now and again.

The natural evolution of phase exchange is gas chromatography then liquid chromatography (much denser medium - look up the exchange maths). HPLC using monodisperse spheres with surface exchange only can have theoretical plate heights of a few microns, giving tens of thousands of theoretical plates in a rather short column.

http://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Supelco/Posters/1/T413105H.pdf.

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  • $\begingroup$ This was a bit too complicated for me to understand... could you please explain to me when we should use steam distillation and when to use distillation under reduced pressure. $\endgroup$ – Eliza May 7 '14 at 8:40

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