ewel
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Is stirring necessary when performing vacuum distillation?
I have a stainless steel distiller I want to modify to perform vacuum distillation. Magnetic stirring can not be applied. Is there a necessity for it?
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greenlight
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I have always used a stir bar during vacuum distillations because I have forgotten a stir bar before and noticed that the liquid can start violently
bumping in the flask when it wasn't in.
I think it depends on what liquid you are distilling but I would recommend applying some kind of internal stirring.
I was going to say boiling chips but the vacuum sucks the trapped air from them and doesn't prevent bumping.
[Edited on 22-11-2014 by greenlight]
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BromicAcid
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Stirring helps to prevent hot spots in your distillation setup. It helps increase the rate of distillation. It works to help prevent bumping and
foaming. It also helps if there are reactants entering the system or if the system is biphasic. If these are not important to you (do not assume
they are not) then stirring would not be necessary.
You could always use overhead stirring if you have a good stir bearing. These are my recommended choice.
http://www.sigmaaldrich.com/catalog/product/aldrich/z154407?...
Of course you can probably use magnetic stirring anywhere with enough ingenuity. What is your setup like? Another option is, as greenlight said,
boiling chips. Teflon work pretty well for any material with a low viscosity at boiling temp. They help with the bumping. Growing up as a chemist I
always heard not to use boiling chips in a vacuum distillation but I have found out over the years that is not necessarily a genuine piece of advise.
There will be a little foaming in the middle and spattering at the end with boiling chips but they will do the job. Another option is a capillary
tube. Older tech that gets thrown around a lot but the few times I have used them they have been successful and you would be amazed what kind of
vacuum you can get.
There are plenty of threads on this subject, read around.
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macckone
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Many stainless steels are non-magnetic.
Meaning a magnetic stirrer will work fine.
A bleed tube may also help with bumping.
[Edited on 22-11-2014 by macckone]
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Magpie
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I'm a fan of using an ebuliator tube, aka a capillary tube, aka a bleed tube. As Bromic says they really don't affect the vacuum you can pull all
that much. After all you are likely pulling a certain amount of air into your system through leaks at the joints anyway. The usual vacuum pump is
rated for 2-3 CFM so some small leaks are immaterial for vacuum distillations, at least the ones I have done.
I have always read that boiling stones don't work in vacuum distillation - but as Bromic says they may well work to some degree. When I tried using
them I had mixed success. So I now just go right to the ebuliator tube. The tubes are easy to make by drawing the end of a 5 mm glass tube to a fine
end then break off the tip to get a 1mm (or less) hole size. Use a Claisen adapter and put the tube in the vertical arm. Use a 1 inch piece of
rubber tubing with a Hoffman clamp to regulate the air inflow.
One thing is clear: if you don't control the bumping somehow in many cases you are going to have a major problem.
[Edited on 22-11-2014 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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Quote: Originally posted by Magpie  | | I have always read that boiling stones don't work in vacuum distillation - but as Bromic says they may well work to some degree.
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I really don't see what difference vacuum would make. Boiling stones basically provide nucleation for bubbles (simply put).
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Magpie
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I'm guessing that the vacuum condition pulls all the air out of the pores so that this air is not available during the distillation.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Etaoin Shrdlu
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The air in the pores isn't important for nucleation.
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Magpie
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Some references say that the micropores or sharp edges provide sites for solvent vapor to form. That does support my observation that the porous
pottery chips I use never seem to run out of tiny bubbles. So why then do some say they cannot be used for vacuum distillation?
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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People say a lot of things, there was another Nessie sighting just the other day  .
Science people aren't necessarily immune to blind 'cut 'n paste' or parroting 'authorative' sources either.
I'm not saying vacuum doesn't make a difference, only that I can't see the rationale that would explain that. And in the absence of convincing
empirical data and/or a theoretical explanation I remain sceptical re. that claim.
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BromicAcid
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General issues with boiling chips are that they cause the nucleation from the bottom. The bubbles then rise to the top and become larger. This can
cause more issues with bumping vs magnetic or overhead stirring where you can vortex to the bottom. The stirring also works to break the bubbles at
the surface as they form due to the additional movement. Further, magnetic stirring, overhead stirring, and even capillary bubbling are continuous
processes. I do see an oscillation in starting a distillation under vacuum with boiling stones though it does level out. Since the heat is being
applied from the bottom, the boiling chips themselves do not supply agitation, only nucleation. So in the beginning until you get a steady stream of
bubbles from the stones to agitate you're competing with hot and cold areas of the flask.
The question then becomes, which of these are exasperated by vacuum? What I don't see with PTFE boiling chips but I did see with ceramic boiling
chips is that PTFE do not float. The ceramic chips that I have used before have occasionally floated to the surface during vacuum distillations. I
think that is because the whole surface of the ceramic chips nucleate and get covered in bubbles. That could be where some of these tales come from.
Another issue is that boiling stones are are not as efficient as intense stirring and it could just be that pulling down a distillation setup and
getting a distillation going the boiling chips are just not able to keep up so well as mechanical agitation.
At least these are speculative answers.
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aga
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Short answer: Yes.
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S.C. Wack
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IME with a capillary going to the bottom of the (not a stainless steel distiller) flask, there is plenty of agitation from the boiling, and also for
normal distillation with boiling stones, so it's generally unnecessary IMHO despite everything Bromic saying above sounding wise.
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