LuckyWinner - 9-3-2019 at 10:37
trying to distill an oil with a boiling point of 292 Celsius.
prepared a vacuum distillation with magnetic stirring and a rotary vane vacuum pump.
heating plate is set to
temperature 280C (MAX)
Stirring (900 of 1500)
vacuum gauge shows a vacuum of over 30mmHg.
New BP under vacuum should be at 178 C.
1st run:
Oil did condense on the outside of the 500ml round bottom flask but did not
reach the condenser.
little oil droplets are on the complete inside of the RBF 500ml.
the oil started turning dark, polymerizing...
insulated flask with aluminium foil 4x wraps around it. , a bit of oil came over but it
looked polymerized / dark.
question:
will insulating my RBF with aluminium foil 4x wraps around it be sufficient to
keep the temp uniform till the condenser or is glass wool essential?
many people say alu foil by itself is not enough.
any recommendations about the temp setting, I read you always should crank it up to the max and stirring to 2/3 - 3/3 full power.
there are no leaks in my setup , it holds the vacuum very well.
UPDATE
Im using a metal heating block that sits on my heating plate.
in this metal block is my 500ml RBF.
with vacuum higher then 30mmHG i mean a higher vacuum = lower mmHG then 30
(water boils at room temperature)
[Edited on 10-3-2019 by LuckyWinner]
Sulaiman - 9-3-2019 at 13:37
for most things aluminium foil works ok.
it is not the foil that insulates, it is the trapped air,
so I crumple up then stretch out my foil to look something like this
loosely wrapped layers hardly touch each other = trap air nicely,
pre-crumpled foil also seems easier to remove, store and re-use than plain foil.
Dan Vizine - 9-3-2019 at 16:18
Your vacuum is pretty bad for a rotary vane pump. You could get a much better vacuum with an aspirator.
BromicAcid - 9-3-2019 at 21:25
What was the pot temp? Were you heating it directly with a hot plate? What was it resting on? My first worry is that you're just not heating hot
enough, if you've just got a flask hovering or sitting on a flat-top hot plate you're just not doing a good job of it. If you have a proper mantle
you'll do much better. Also, as Sulaiman said, foil is actually not a good insulator, the air between it can do an acceptable job. But overall
you're better off just using fiberglass insulation. Also when you say your vacuum is over 30 mm Hg that's confusing since over could mean it is
literally higher than 30 mm or since you're usually trying to get a better vacuum you could be saying your vaccuum is lower (better) than 30 mm.
SWIM - 10-3-2019 at 07:35
Sounds like you've got an RBF sitting on a hot plate stirrer. hard to transfer heat evenly that way.
I use oil baths for vacuum distillations. The oil level should be a little higher than the level of fluid in the pot as this helps control bumping.
(but still stir it. That's important)
I set the oil bath temp from 10 to 25 degrees above the expected boiling point of the material to be distilled.
And I run a thermometer in the boiling flask too. The more info, the easier it is to diagnose any problem.
Insulation for the top of the flask can be just foil, but I put fiberglass insulation on the flask with foil over that.
With an oil bath I try to insulate the whole top of the bath, and not just the flask if it's high temperature distillation.
If you're really dedicated to doing good vacuum fractionations then a spherical heating mantle might be worth a try.
I've never found any useful information on how to wire them up though.
I assume the top and bottom get different power levels, or that maybe the top is run thermostatically; but I just don't know. Sauron uses them. Maybe
he has some useful posts about this somewhere.
I agree with above posts that you'd be better off with lower vacuum.
BTW: Not sure what you're doing here, but maybe steam distillation could do the job if you're just after a water-white product freed of non-volatiles.
(assuming it doesn't hydrolize or anything like that)
Sulaiman - 10-3-2019 at 08:34
For higher b.p. liquids I too easily get a 50oC temperature diference between inside and outside an rbf doing a gentle distillation,
just distilling an ethanol:water mixture I get 70+oC temperature difference across the wall of a 2l rbf in an oil bath.
rbf size makes a significant difference because larger RBFs have a lower (area)/(volume) ratio,
and more significantly, require thicker glass for physical strength.
If you make the crumpled foil into a cone/pyramid shape then you can cover the entire hotplate and you effectively have an air bath,
but I would use an oil bath or preferably a heating mantle - either requires insulation of the RBF
(and in my opinion, all parts where vapours flow upwards)
EDIT: except for air-cooled condensers or columns)
[Edited on 10-3-2019 by Sulaiman]
LuckyWinner - 10-3-2019 at 09:44
UPDATE
Im using a metal heating block that sits on my heating plate.
in this metal block is my 500ml RBF.
with vacuum higher then 30mmHG i mean a higher vacuum = lower mmHG then 30
(water boils at room temperature)
I try to use multiple layers of loosely alu foil wrapped around it.
thanks for the advice
I've never used a heating block ... but ...
Sulaiman - 10-3-2019 at 11:14
At these temperatures I would consider a little oil in the heating block for better and more even heat transfer.
(messier but so what)
If the rbf:block is a good fit then only a little oil would be required,
if it is not a good fit then oil would help a lot.
178 + 50 = 228 which is high for cooking oils,
but may be ok if a higher smoke point oil (or silicone grease) is used.
If does not matter if water boils at lower pressures, the water is not at low pressure, so try lower pressure hence temperature.
In fact, cooling 178oC vapours with a water-cooled condenser is rather stressful on the glassware,
consider an air condenser before the water-cooled condenser ?
EDIT: corrected spelling and math errors 
[Edited on 10-3-2019 by Sulaiman]
Dan Vizine - 10-3-2019 at 16:54
For every time you half the distillation pressure you decrease the distillation temperature by 10 C. If you were getting a more normal vacuum for a
rotary-vane pump (I can get to almost 10 or 12 mm Hg with just an aspirator) which is <1 mm, you'd lower the distillation temperature by at least
50 C. This is way more important than more insulation or better heat transfer. What kind of rotary-vane pump do you have?
LuckyWinner - 10-3-2019 at 18:56
UPDATE 2
vacuum distillation worked with 4 lose layers of aluminum foil wrapped around RBF and heat cranked up to full blast 280C.
(thanks sulaiman)
vacuum pump got resealed with teflon tape and achieved a deeper vacuum then before.
success!