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I was looking for some distillation equipment on Sigma Aldrich because I wanted to perform some vacuum distillation, when I came across a vacuum-jacketed distilling column.

Now as far as I understood my high-school chemistry, I can perform vacuum distillation perfectly fine with a 'normal' column as long as there is an entry to which I can connect my vacuum source (in my case a cheap aspirator, but he it does the trick).

Before I'm buying all this pretty expensive stuff, I want to make sure that I buy the right equipment. As far as I can see it, this vacuum-jacketed distilling column has nothing to do with vacuum distillation, it is simply a vacuum jacket around a normal column. But then, where it is used for and why, or am I wrong?

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The purpose of the vacuum jacket is to provide insulation of the column from cooling by ambient lab air. You don't want to have to heat the "pot" (the flask containing the crude material you want to purify) any more than you have to, otherwise heat sensitive material in the pot may degrade and turn to tar. Imagine if you cooled the exterior of your distillation column with cold water while performing the distillation. You'd have to heat the pot a LOT more to get the material in the pot to finally reach the top of the column, then if you weren't cooling the column. Air will have the same effect on your distillation, albeit to a smaller extent, than cold water. So to avoid putting unnecessary extra heat into the pot, you vacuum jacket your column to preclude undesired cooling by ambient air.

Remember, a distillation column is different than a condensing column. You don't want to cool a distillation column, you do want to cool a condensing column (or condenser). Back in the day, very long columns were sometimes needed in the organic lab (5-10 feet) to separate closely boiling materials, they are still used in industrial applications. If you had a 10-foot column that wasn't vacuum jacketed, you would turn most of your material to tar before it distilled over, because of the extra heat you had to supply to make up for the undesired cooling of the column by ambient air. Eventually improvements like column packing materials (glass beads for example) and spinning bands (a metal helix rotating in the column) came along to increase column efficiency and allow chemists to be able to get good separation efficiencies (measured as "height equivalent theoretical plates", HETP) without having to resort to long columns. Still a vacuum jacketed column is preferred to lessen unnecessary heating.

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  • $\begingroup$ That is interesting, thanks for your thorough explanation, I never though that ambient air could be such a problem, but now I come to think of it, it makes a lot of sense. Starting at what column length would I really have to worry about ambient lab air? I was planning on buying a 300mm vigreux. $\endgroup$
    – Jori
    May 13, 2014 at 20:56
  • $\begingroup$ So that's about a foot long. If the price difference between a 1' vacuum jacketed column and a 1' non-jacketed column is significant (it probably is), then I'd by the non-jacketed model and just wrap it good with insulating tape. $\endgroup$
    – ron
    May 13, 2014 at 20:59
  • $\begingroup$ @Jori Wrapping it in Aluminium foil should also do the trick, unfortunately at the expense of seeing your product in the column. Also helpful is applying a larger temperature gradient, i.e. cooling the collecting vessels with a water/ice/salt mixture. $\endgroup$
    – Martin - マーチン ♦
    May 14, 2014 at 4:47
  • $\begingroup$ @Ron, yes the price difference is indeed significant (100-150$, sometimes even more). $\endgroup$
    – Jori
    May 14, 2014 at 10:26
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    $\begingroup$ @Martin, I will try aluminium foil (cheap and easy). Cooling the collecting vessel seems interesting too. Does that work because a higher temperature difference means a higher heat flow and thus cooling the other end of the distillation setup will cause heat to be conducted along the glass, lowering the heat given off to the ambient air, resulting in a more effective spread of heat along the glass (and as a consequence increase the temperature at the top of the distillation column)? Are there any physical laws I could look into for some further research? Thanks a bunch both of you! $\endgroup$
    – Jori
    May 14, 2014 at 10:26

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