JefferyH
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Is the avoidance of rapid distillations needlessly over-emphesized?
I always hear that distillations should be taken slowly, which is understandable in certain circumstances of similar boiling liquids, but what about
when the products being distilled are over 40-100 degrees different in BP's?
My lab group was debating over this today, but the reactants we are distilling have boiling points of 250 and 325, respectively. Both are destroyed by
excessive heat, but we only care about the lower boiling point one. Is there any reason to why we can't just crank up the vacuum as high as it will go
and turn the heat to a reasonable safe, but high temperature, and distill at the most rapidly optimal pace possible that is allowed without destroying
the product? I can't imagine any scenario where any of the higher BP liquid will leave the flask given the huge difference in temperatures.
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deltaH
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When vapour flow rates become too high, entrainment increases which would decrease the purity of your product when conducting single stage distillation.
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JefferyH
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Is there any correlation between entrainment and the difference in the BP's of the substrates? Not sure if it is relevant but some of the substrates
we will be distilling are over 100 c in different temperatures.
We are to be tasked with distilling up to 5 liters of liquid at a time to prepare the samples for class-lab usage. Not that we want to rush it, but we
also do not want to waste excessively long periods of time doing something that could be done in 1/10th the time.
Do you have any suggestions that wont limit the distillation to a couple drops per minute, for substrates that have such large differing boiling
points?
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deltaH
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Entrainment is typically only a minor contaminant, probably not an issue for you then. I assumed this was organic research chemistry where high purity
is important.
Entrainment is a function of many factors, but not differences in b.p. AFAIK. General rule of thumb is that the more 'violent' your boiling and the
faster the vapour flow rate, the higher the chance of mists or microdroplets being formed and carried away in the stream, but again, it sounds like
some impurity is tolerable for your application in which case it wouldn't matter.
You might also consider steam distillation, stripping the desired compound without vacuum. If your product is not water soluble, you simple decant the oil from the
condensed water. You can always dry the moisture in the oil afterwards with a drying agent if you want it very anhydrous.
[Edited on 8-5-2014 by deltaH]
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Nitrator
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The higher the temperature, the more molecules will have enough kinetic energy to evaporate. Even if the number of molecules transitioning into the
gas phase is still relatively small at 300, it's still significantly larger than at 250, perhaps by several orders of magnitude. While negligible, I
think it's generally bad practice to get any closer than necessary. Even if the occurrence of the one compound only increases from 0.2 to 0.6%, you
just tripled the impurity of your distillate.
Additionally, it's likely that under high vacuum, the vapor pressures of the reactants are actually closer together than at 1 atm. I would imagine
that entrainment would probably have a larger effect than the higher rate of evaporation, but just another thing to think about.
So really, I think it's generally bad practice, but since you mention having to distill 5L, I'd probably turn up the heat too. However, your original
post sounds rather half hazard. I can attest from personal experience that turning up the heat to distill something faster often results in
decomposition or in your case excessive evaporation/entrainment of the non distillate. So I highly recommend using a thermometer in the still head to
ensure you don't increase the temperature too much (perhaps you knew this already).
As to what temperature is the optimal balance between not taking forever yet maintaining high purity, I would recommend a difference of 30 C, as per
this: http://en.wikipedia.org/wiki/Fractional_distillation
Which indicates that 25 C is when excessive non distillate contamination occurs.
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JefferyH
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Quote: Originally posted by deltaH  | Entrainment is typically only a minor contaminant, probably not an issue for you then. I assumed this was organic research chemistry where high purity
is important.
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When you say minor contaminant, what do you mean, approximately? 10%? 5%? 1%? 0.05%? We would ideally and reasonably control the boiling to were it is
not violent (splashing everywhere, etc), but is reasonably energetic and roaring.
| Quote: Originally posted by Nitrator | The higher the temperature, the more molecules will have enough kinetic energy to evaporate. Even if the number of molecules transitioning into the
gas phase is still relatively small at 300, it's still significantly larger than at 250, perhaps by several orders of magnitude. While negligible, I
think it's generally bad practice to get any closer than necessary. Even if the occurrence of the one compound only increases from 0.2 to 0.6%, you
just tripled the impurity of your distillate.
Additionally, it's likely that under high vacuum, the vapor pressures of the reactants are actually closer together than at 1 atm. I would imagine
that entrainment would probably have a larger effect than the higher rate of evaporation, but just another thing to think about.
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Less than 1-2% is acceptable for our purposes.
We've been consulting a few charts to approximate the various BP's under pressure. We have a fairly good vacuum and plan to take the pressure down to
anywhere from 1 Torr to 0.2 Torr. Our target compounds around this pressure range would boil at 25-30 C, according to these charts, while the higher
BP substances won't start boiling until 100-110. Even under pressure this is quite a significant difference.
| Quote: Originally posted by Nitrator | | I can attest from personal experience that turning up the heat to distill something faster often results in decomposition or in your case excessive
evaporation/entrainment of the non distillate. So I highly recommend using a thermometer in the still head to ensure you don't increase the
temperature too much (perhaps you knew this already). |
Of course we will use a thermometer :p. Though, I didn't think it was possible for such compounds to decompose if the temperatures never rise above
100 degrees. In fact, I know for certain many of these compounds can be safely heated to 150-175 without decomposition, but I do not plan to take the
temperature high. We have a good vacuum, and I've never heard of too much of a vacuum hurting a compound (correct me if I'm wrong...)
I'm not sure what you mean by the optimal balance with regard to difference? Difference in what? Do you mean if the target boils at 30 C at a certain
pressure, then to set the heat of the mixture to 60 C in order to get a decent distillation rate without contamination?
I know about fractional distillation but I did not think a fractionating column could be used to distill at a reasonable speed due to their
properties. Again I could be wrong about this too because I don't have much experience specifically with fractionating as much as I do with standard
distillations (of low BP compounds).
My understanding was that all a fractionating column would do is assist in separating the compounds that are less than 25 degrees different in BP.
Even under a vacuum our compounds are still roughly 60-70 degrees apart.
[Edited on 8-5-2014 by JefferyH]
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Nitrator
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By decomposition I was referring to my own experiences (I made the distinction in my op), but I digress...
What I was trying to say with the reference to fractional distillation was that one can deduce that regardless of the reactants and temperatures
involved, approaching to within 25 C of a substance's boiling point results in significant carry over, as this is the point which fractional
distillation is recommended instead of simple distillation. Obviously, since you aren't fractionally distilling anything, you should not approach
within 25 C of the non distillate's boiling point, which in your case is 75 C. I would recommend 70 C as a precaution.
I hope this explanation makes better sense 
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deltaH
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It is very hard to say how much would be entrained as it's very conditions specific.
| Quote: | | We have a fairly good vacuum and plan to take the pressure down to anywhere from 1 Torr to 0.2 Torr. |
I don't know how much of your low boiling compound is in this 5l, but let's assume it's 10%. At 1 Torr the volume of your vapour will be 760 times the
volume it is at atmospheric pressure. If we assume that your substance has a vapour to liquid volume change of factor 500 at atmospheric pressure
(conservative?), then 5l will produce 190 m^3 of vapour at 1 Torr 
If you use glassware with a diameter of tubes of say 20mm. If you distill this over say 5 hours, then 10.6l/s of vapour will be boiling at 1 Torr.
That's a vapour velocity of about 34 m/s, but probably not so bad considering that the vapour density is very low. You would need a damn good
condenser though.
With ~11l of bubble volume popping on the boiling liquid every second, that sounds like something fun to watch  Might be ok if you're doing this in GIANT RB-flasks. In 2l flasks, you will probably have boil over and a mess!
Just trying to put some numbers to this situation to get a rough 'feel' for possibilities.
[Edited on 8-5-2014 by deltaH]
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JefferyH
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I probably will be using 5L flasks and splitting up the distillation. I don't see any reason to overloading the flask, since the speed of the
distillation isn't so much dependent on the amount of liquid in the flask as much as it is the heat input.
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BromicAcid
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If there is a large difference in boiling point, the faster the better, go for it! One of my favorite reactions to run was out of a 72L flask, the
process used 32L of THF as the solvent, after the reaction you stripped to dryness. We made a contest of who could strip to dryness the fastest. 1
hour 15 minutes was about the record. It was a thing of beauty to see and hear the solvent going to the receiver (it made a whistling sound), with
three variac zones each at 120V but the pot still at -10C and 10 mm Hg you just kind of had to be in awe.
One time you usually need to distill fast though is in the case of materials that are solids at room temperature. You take the slow route with those
and you're going to keep plugging your setup unless you use lots of heat tape or have a heat gun handy but if you get it going at a good clip the heat
of the vapors will keep everything molten.
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JefferyH
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72L!!! That's crazy! I imagine the hardest part of that process was handling the glassware. Are that size I didn't even think glassware would be used,
but instead steel reactors.
It will be interesting to try to distill these as I have never worked with a vacuum this strong. According to this chart I'm consulting, this lower
boiling point(roughly 200-230) will be able to boil around room temperature... that will be interesting to see for an oil.
As far as time is concerned... 1-2 hours would be awesome! This will be exciting.
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BromicAcid
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The largest interchangeable flask that I have seen is 250 L, I have a picture around here somewhere of it. The attached picture is of me with a 100 L
flask. The big open part at the top is for an interchangeable head, so you can take it off to reach inside and clean it out, but the actual head on
the top usually only has a 45/50 or 55/60 neck on it. I guess with respect to the topic of discussion, on that sort of scale you cannot afford to do
a slow distillation, especially in a production atmosphere.
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