Sciencemadness Discussion Board

Distillation rate, what's reasonable?

jwarr - 8-8-2010 at 15:59

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?

BromicAcid - 8-8-2010 at 16:36

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]

watson.fawkes - 8-8-2010 at 18:00

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.

jwarr - 8-8-2010 at 18:15

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).

watson.fawkes - 8-8-2010 at 18:41

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.

jwarr - 8-8-2010 at 20:00

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?

densest - 8-8-2010 at 21:22

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?

watson.fawkes - 8-8-2010 at 21:35

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?

jwarr - 9-8-2010 at 06:16

Thanks for the informative posts guys!

peach - 9-8-2010 at 18:26

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. :P

[Edited on 10-8-2010 by peach]

Chainhit222 - 31-8-2010 at 23:18

sulfuric acid distillation is so fucking slow

peach - 1-9-2010 at 04:07

Quote: Originally posted by Chainhit222  
sulfuric acid distillation is so fucking slow


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. ;)