lab.student
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variable reflux ratio tips and advices
Hi members!
I recently got myself a nice aceglass variable reflux vacuum head like this
 and was wondering if any of you guys can give advice on the best technique for using it?
Im mainly distilling organic stuff, using a 25cm effective length silvered jacketed vigreux.
some chemists recommend while distilling to total reflux until the desired temp is reached, then start to draw sample till delta-T is 1-3degC, then
switching back to total reflux till temp again is in the wanted range, and repeating.
while other chemists ive talked with are certain constant distilling with a fixed reflux ratio (1:3 or 1:5 or 1:10 what ever suits the specific
distillation run) is the best.
Can anyone here give some more advice? maybe real life pointers?
Cheers!
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FrankMartin
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We use the procedure:
Ensure the distillation flask is heated evenly and there are boiling chips present (no bumping.)
Be sure the vigreux column is straight up (use a spirit level), and insulate further with a close-fitting foam outer jacket. (For moderate
temperatures use the material used in refrigeration lines which will slip on over the column.)
For the distillation, run the system at maximum total reflux until the minimum temperature is reached and this might take about 1/2 hour. Thereafter
it's trial and error to determine how great a rate the product may be taken off. For close-boiling mixtures it may be necessary to adopt this
procedure all the time, that is take off a small amount until the temperature starts to rise and then stop for 1/2 hour until the temp drops to a
minimum and then repeat. That is the column will be running at total reflux most of the time except when taking off the few drops of the lighter
component.
This can be speeded up only with a taller column with more efficient packing. For vacuum distillation denser packing may not be possible.
See Vogel for illustrations and methods.
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chemrox
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That's great FrankMartin but I haven't had much help from boiling chips in a hard vacuum, have you? I bleed in little gas (or air) or simply stir if
the material doesn't bump too badly.
lab.student: nice! I just picked up one like that in 14/20 now I need a tall 14/20 snyder column to compliment my semi-micro setup which I'm using
more and more.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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FrankMartin
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The only way I know to fix bumping in the distillation flask is to either use porous chips or an air/nitrogen bleed thru a capilliary.
Another way, for small amounts, is to stuff the flask with glass wool, so that the product is effectively baffled by it. Bumping is much reduced.
I have tried this with only small (200cc) amounts.
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lab.student
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Thanks for the replies. Used your tips FrankMartin. They work great, though ive found that full reflux till desired temp is reached and then 1/10
reflux ratio is good enough for most simple distillations. Thanks again!
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Texium
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Thread Moved
19-11-2023 at 16:13 |
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