Furch
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Failed fractional vacuum distillation.
I have a mixture of liquids with high boiling points. Somewhere in the range of 200-250 °C. I intend to fractionally distill these, with a vacuum...
In doing this, I ran into some trouble. The liquids condensed nicely all along the packing of the column, BUT, it wouldn't go any further than the
column. I.e. to the three way adapter... So I insulated it with quite a lot of aluminum foil. Still no success. And I gave it a good 30 minutes after
that the liquid had reached the upper part of the column...
Can someone please give me a few hundred pointers here? I have very limited experience with fractional distillation.
Thanks in advance!
- Furch
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bio2
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If the column is long and the heat barely adequate then it helps to heat the column to 10-15 deg below the boiling point. Use real insulation with
loose foil underneath shiny side in. If foil only for lower boilers don't wrap it tight then it's a radiator as the air is the insulator here not the
metal foil.
Insulate the flask, have the oil at least to the liquid level and stir like crazy. The take-off rate can be adjusted to some extent by stir speed and
if you screw up and flood the column while the condensate pours rather than drips into the reciever just return it and proceed more carefully.
Sometimes it can be frustrating and most likely you need more heat and a not too big flask for the volume.
[Edited on 17-10-2006 by bio2]
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Furch
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The column in question is 300 mm long. Worth mentioning is that I'm using a heating mantle, as my hotplate due to an error won't go to the desired
temperature (120-140 °C)... This means that either I heat the mixture just right, or slightly to little, or way too much, as the heating mantle only
has three pre-set working temperatures. And, to top it off, the mixture of liquids has a tendency to bump a lot.
Argh! 
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not_important
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A lamp dimmer rated for at least the wattage of the heating mantle will allow finer control of the temperature. An "infinite switch" as used to
control electric kilns is an alternative to the dimmer, as is a Variac transformer (likely to be the most expensive).
The range you're running in is starting to get high enough to require heating of the column; more so if doing under vacuum. Adding alternating layers
of fluffy cotton fabric and foil over the foil you have already wrapped around the column might give enough insulation.
Vacuum fractionation is tougher than at ordinary pressures. In part this is because of the greatly reduced pressure - there are fewer molecules per
cc. This means that there is less energy being transported that can heat the column, and less vapour to condense to get reflux. Increased bumping is
another problem, the trick of using a capillary as a slow leak to provide a stream of bobbles through the liquid in the pot.
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Furch
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That's a great idea! Thanks a lot for that. The dimmer thingy, that is.
I'll give it another go with better insulation... Thanks for all the tips, you've been most helpful!
- Furch
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bio2
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This will never work for this application which requires
steady rampable known heat. Not that it's impossible without these but previous experience with the exact same reagents and setup would be necessary.
Then it can be done by "feel". You could use a rated dimmer on the mantle set to high heat. This will work if the heat is enough and the incoming
voltage is steady.
A bath is much better, with a thermometer, which is slowly raised, after the vacuum is stable, to about 10 deg above the first fractions expected
boiling point. When a very slow drip take-off rate is established by again slowly raising the bath temp then when the next fraction is seen trying to
climb the column raise the heat again, You may have to have the bath up to 50deg above the boiling point but 15-30deg is more likely with adequate
insulation.
Without steady vacuum and stable slow heat input this becomes a more or less hopeless endeavor.
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not_important
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A mantle works well, you can always put a thermometer in the flask to measure the actual still pot temperature. Plenty of labs manage with heating
mantles and no bath. If you're really obsessive you can also measure the temperature of the mantle, many have thermocouples built into them.
A bath will be slightly more steady than a mantle, but a dimmer or a zero-crossing controller give very find control. Steady vacuum is very important,
no denying that, as is some way of measuring it.
Note that much research and discovery was done using no more than a manometer, gas flame with maybe a bath (but not aways), thermometer at the
take-off point or top of the column, and patience enough to not ramp the bath up too quickly and to monitor the vacuum and drip rate.
If the desired compound is the high boiling one, excluding side products, use a chaser in the still pot along with the crude product. That way you
don't run the flask to dryness while attempting to drive the last of the product out of the fractionation column.
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evil_lurker
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I'm working on getting a vacuum distillation setup as well.
From what I understand boiling stones don't work worth a damn under vacuum. I do know from experience with doing atmospheric distillations that a stir
bar and stirring will just about eliminate bumping. Right now I use a hot plate/stirrer/oil bath for heating flasks. IMO the biggest downside is that
it takes a very lon time to heat a flask with an oil bath. Other than that it works pretty good, but sometimes what happens when your running is that
the hot plate will cycle on and off which causes the distillation to want to surge. This problem is exaggerated with higher boiling materials or
larger flasks. Personally I'd like to have a mantle/stirrer running off a variac as a heat source but that is going to be a while down the road.
For vacuum control I'm going to be usin a J-KEM digital vacuum regulator with built in needle valve and a Lab Glas graduated "perkin triangle".
http://www.jkem.com/dvr.html
http://www.wilmad-labglass.com/group/943
With those two I should be able to maintain vacuum within a +/- 2 torr range and cut the fractions fairly easily with minimal disturbances to the
vacuum.
I'm also seriously considering gettin a refrigerated vacuum trap as well to eliminate the need for a dry ice bath to protect the pump.
The only thing left is fractionation. I'd like to have 20 theoretical plates of seperation... what would ya'll suggest for a column?
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not_important
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Try adding some thermal mass to the bath. Either make it larger, or try a thick slab of copper or aluminium ( or diamond 8-) between the hot plate and
bath.
Did you get lucky on finding the vac reg, or do you have the money to afford a new one? I'd assume you don't have the money for new, because you don't
have a heating mantle, but then you talk about getting a refrigerated trap.
While I have several Vigreux columns, for the tough jobs I also have 15 and 20 plate versions similar to these http://www.wilmad-labglass.com/group/897
The problem with the bubble plates is that you need to be pretty accurate about aligning the column to true vertical; packed columns tend not to be
quite so picky.
Packed columns can be quite inexpensive, especially if you just use glass beads in a Hempel. Better packings cost more, and flooding can be more of a
problem. Hempel columns can also be packed with active material such as acidic alumina, for reactive column type preparations.
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evil_lurker
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I got lucky and snagged the vacuum regulator off of Ebay. I paid a little more than I wanted to for it, but it was still less than a third of the
price of new one. The last one sold went for around $200. I picked up the distillation reciever on Ebay too... its only 50mls but for around $85 I
figure I didn't get hurt bad at all on that one. Also picked up two corrosive gas regulators, one for hydrogen halides using a CGA 330 fitting and the
other having a CGA 660 for chlorine, sulfur dioxide, and similar gasses. (no more making chlorine, yay!)  Got less than $200 in the both of them.
There has been a whole slew of refrigerated vacuum traps that have gone thru there lately manufactured by savant for several hundred or less. I don't
know too much about them, but they supposedly will chill a trap down to minus 104C. Glassware is not included but I'm sure you could have a trap
custom made.
Those oldershaw columns look pretty darned neat, but are probably over kill. Wilmad's vacuum jacketed hempel columns look rather attractive on the
other hand. Could someone explain to me the diffrence a vacuum jacketed column would have over a non-jacketed version? Why would you want one
silvered? It seems that the vacuum would insulate the column too much resulting in a lower reflux ratio. Then again the uninsulated top would create
some reflux.
Distillation truely is an art form.
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not_important
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Vacuum is a good insulator, silvering reduces radition of heat - the main loss with the jacked column at higher temperatures. The same holds true,
except heat flow direction is reversed, when distilling very low boiling substances, where the still pot and column could be below 0 C - vacuum is
hard to beat as an insulator.
Whether or not you need the jacketing depends on what temperature range you'll be working in, adjusting for pressures used - reduced pressure means
fewer molecules to carry heat so column losses become more important. The trade-offs is why some prefer heated columns, turn the heat off for lower
boiling mixes, turn it up as needed for higher boiling ones.
Reflux is usually created by a condenser above the column. The evaporation/condensation in the column is (mostly) working on the reflux flowing down
onto the column. Temperature gradiant comes from the cooler reflux and the hotter pot vapours.
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stricnine
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Use Al foil (as suggested by Bio2) with the shiny side in , and try wrapping the column (already wrapped with the Al foil) with corrugated cardboard.
Cut the cardboard pieces so that there is no overlapping of the edges, so that they hardly meet edges, and tie them with cotton string (tape can melt
or the adhesive could make a mess at high temps).
A few years ago I tried with some lubricant oil under low pressure and I managed to get some stuff distilled.
I am getting old. I used to push it aside with one hand... now I need both!
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