jwarr
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Distillation rate, what's reasonable?
I've read Zubrick and I know the recommended rate is 6-10 drops per minute. But realistically, if you don't have all day is it suitable to increase
this substantially?
For example:
I'm distilling nitromethane from a methanol blend. The 8% w/w nitromethane azeotrope comes over at 64C, as does pure methanol incidentally. At 101C
nitromethane comes over. If I've got a fractioning column attached and my thermometer is reading a steady 64C while collecting the azeotrope, am I
doing any harm running this distillation at 1 every second or so?
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BromicAcid
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It's one of those things that I've gotten a feeling for though hundreds of distillations. Usually I run my distillations and count my collection rate
in drops per second and occasionally I collect at steady stream. It depends on your column plenty too, what is it packed with, what is the length?
Also how much are you collecting and what is the bore of your glassware, it's a lot different at 14/20 vs 45/50. What kind of fractionating head do
you got on it? Of course if you don't have a GC handy it's probably best to just have a nice slow takeoff and hope for the best however.
[Edited on 8/9/2010 by BromicAcid]
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watson.fawkes
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The degree of fractionating decreases with increased takeoff rate. How to manage this trade-off depends on any number of factors, such as final
application, sensitivity to impurities, boiling points of other fractions, and so on, none of which is generic. In other words, your question is
under-specified.
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jwarr
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The distilled nitromethane will eventually end up in a solution of methanol, so a couple mL of methanol in say 30 or 40mL of nitromethane isn't a
problem. I gave the boiling points of the other constituents above (ignoring the 10% castor oil, BP 313C).
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watson.fawkes
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Quote: Originally posted by jwarr  | | The distilled nitromethane will eventually end up in a solution of methanol, so a couple mL of methanol in say 30 or 40mL of nitromethane isn't a
problem. |
You've specified a target impurity level of 5 - 7%. Thank you for providing quantitative estimation
for your impurity. "Purify" is pretty much always an underspecified word around here. "Purify to what degree" is always what's needed to get a
reasonable answer.
As your takeoff rate increases, the temperature at the top of the column will also increase, because you'll have some percentage of the higher-boiling
fraction coming over. Since the higher-boiling fraction is your desired product, the question really becomes one of yield. How much nitromethane are
you willing to have come over more than just the azeotropic amount?
If you want a quantitative estimate, you will need a phase diagram for the nitromethane-methanol system. You can look up the boiling point of a
candidate acceptable mixture and design accordingly.
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jwarr
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It really seems like it shouldn't be that hard though. If methanol boils at 64 degrees and the azeotrope boils at 64 degrees and my stillhead
thermometer reads 64 degrees am I not bringing over the azeotrope of 92% meoh 8% meno2? That is, aren't I getting a clean separation from the
remaining nitromethane?
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densest
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No. Depending on the vapor pressure of the other constituents at the various temperatures along your column, and the number of equivalent plates that
your column provides, you have -mostly- your azeotrope. The more equivalent plates = the more times the vapor and liquid interact & the better
temperature control along your column, the better the separation. Nothing is perfect! That's what all the numbers are about : how much & where?
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watson.fawkes
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| Quote: Originally posted by jwarr | | It really seems like it shouldn't be that hard though. If methanol boils at 64 degrees and the azeotrope boils at 64 degrees and my stillhead
thermometer reads 64 degrees am I not bringing over the azeotrope of 92% meoh 8% meno2? That is, aren't I getting a clean separation from the
remaining nitromethane? |
It's only this hard if you're doing science and trying to understand this
distillation. It's much easier if all you want is better recipe instructions.
Since the azeotrope is at 64.6 °C and the boiling point of methanol is 64.7 °C, aren't you at least a little worried about the
accuracy of your thermometer? If you don't have a phase diagram, how do you know what other percentage mixtures are within its resolution?
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jwarr
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Thanks for the informative posts guys!
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peach
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Unless you're trying to run 10l through, I can't imagine you're that pressed for time you can find some or use it more creatively (e.g. do the tidying
and washing up as it runs beside you).
That's a bit of a negative nancy answer, but this isn't a particularly time consuming mixture to distill. If you find that distillation boring, there
are some evil ones lurking in the shadows for you. 
[Edited on 10-8-2010 by peach]
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Chainhit222
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sulfuric acid distillation is so fucking slow
The practice of storing bottles of milk or beer in laboratory refrigerators is to be strongly condemned encouraged
-Vogels Textbook of Practical Organic Chemistry
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peach
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That's the massive button combination to charge it's special attack.
It's like jam on the cooker, but hotter and corrosive. My hardware stuff is even dyed pink.
You could try sucking, on the glass. 
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