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Mateo_swe
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[*] posted on 14-6-2020 at 05:23
Distillation of solvents


I just spent many hours distilling some isopropanol using a 3L flask and my hotplate stirrer.
Due to the large volume of solvent and a big flask it took quite a while.
This made me think about how i can speed up this process.
I read a PDF about a DIY rotary evaporator and i sure would want a rotovap but building a DIY one is quite a project and one need a good lathe and other tools.
A new or even a used one is out of my budget right now.
But what caught my interest was in the paper there was a comparison of evaporation rates of acetone.
Evaporation without vacuum and rotation compared to Evaporation with vacuum but without rotation and finally with vacuum and rotation.

Evaporation of 30ml in a 100ml flask without vacuum and rotation was about 2ml/min.
Evaporation of 30ml in a 100ml flask with vacuum but without rotation was about 8ml/min.
Evaporation of 30ml in a 100ml flask with vacuum and rotation was about 8.5ml/min.

Its quite clear that the vacuum is what makes the biggest difference.
The rotation seemed to add very little evaporation rate.
Now this was with a small volume of acetone and a small flask but it seems bigger volumes and bigger flasks make the evaporation go much faster.

Now i wonder, can i get same big reduction in distillation times just by applying vacuum to my distillation?
If the PDF he got from 2ml/min to 8ml/min just by applying the vacuum.
That is much more than doubble the evaporation rate.
Is it really so simple as applying vacuum and the distillation goes 3-4 times quicker?

I dont remember if i found the PDF with the DIY rotary evaporator here on this forum or if it was somewhere else on the net.
I add the PDF if you want to see it.

Attachment: An Affordable Modular Rotary Evaporator.pdf (979kB)
This file has been downloaded 164 times
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Tsjerk
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[*] posted on 14-6-2020 at 05:35


Rate of evaporation is dependent on how much energy you can get into the system and of course the amount of energy that is needed to evaporate the solvent.

You can indeed bring the energy needed way down by applying a vacuum, as you bring down the boiling temperature.

Rotating giving such a small difference in evaporation rate is not surprising as the energy applied and energy needed stay approximately the same, only the efficiency goes a bit up. The reason rotovaps are used in the lab is mostly convenience and to avoid bumping.
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Sulaiman
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[*] posted on 14-6-2020 at 05:40


If you distill acetone at low pressures you will need a very cold condenser,
typically dry ice temperatures.




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brubei
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[*] posted on 14-6-2020 at 05:45


Rotation prevents the formation of bubbles and liquid splashing. The process in then cleaner, with less product loss in the vaccum system.



I'm French so excuse my language
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Refinery
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[*] posted on 14-6-2020 at 06:10


I was just thinking the same about ethanol.

I can get 95% ethanol otc for cheap, but it contains few % of all kind of stuff for car windshield wipe, including denaturant MEK.

Since it contains practically no water, it could be distilled from few % of NaOH at any speed the device can handle and there is no issue with separation since basically all there is is ethanol.

I've performed my distillations in similar instances with propane burner so it goes as fast as my condenser allows. I just recently distilled 3.5 liters off from 4L flask, which I had to run slower at first because it was practically filled to the brim. I had total of 4 liters, and I added the rest with dropping funnel when there was enough room. Last time when I was separating acetone from reactants that contained acetone, water and 150-200c BP compounds, I maxed out my condenser and changed the coolant frequently. Even more power could be obtained by using cryogenic coolant.

On the other hand, when fractionating, strong burner is also needed to get the reflux ratio over 10:1 to enrich the condensate at the top.

[Edited on 14-6-2020 by Refinery]
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Mateo_swe
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[*] posted on 14-6-2020 at 11:36


So if i apply vacuum to my distillation of isopropanol by using an aspirator it will indeed speed up the distillation.
But can i still use water with a few pieces of ice to cool the condenser or is colder temps needed if vacuum is applied?

Also, can i increase the temperature setting on the hotplate to speed the distillation up?
I mean, how high can i go?
My hotplate stirrer temp setting goes to 450°C and i usually have it at somewhere around 170-200°C during this distillation.
Im quite new to chemistry and it seem little hmm.. dangerous to set the hotplate to very high temps when the isopropanol boils at about 80-81°C.
I guess it should be ok but still feels like somewhat dangerous to go very high.
Im reducing the isopropanol volume after an extraction where i used quite a bit isopropanol, way more than needed.
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[*] posted on 14-6-2020 at 11:50


Quote: Originally posted by Tsjerk  


You can indeed bring the energy needed way down by applying a vacuum, as you bring down the boiling temperature.


You would think so, but dropping the pressure drops the boiling temperature, and that raises the heat of vap.

https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#/media/File:Heat_of_Vaporization_(Benzene+Acetone+Methanol+Water).png
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[*] posted on 14-6-2020 at 15:24


Quote: Originally posted by Mateo_swe  
...Also, can i increase the temperature setting on the hotplate to speed the distillation up?
I mean, how high can i go?...

It would be unwise to operate the hotplate near to or above the autoignition temperature,
Isopropanol = 399oC, Acetone = 465oC
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If you use a heating mantle with RBF,
the speed of distillation will be limited mainly by your condenser.
You could use an efficient condenser, or a couple of cheap ones in series.




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[*] posted on 14-6-2020 at 15:50


Are you distilling to purify? If you boil the snot out of it at full blast you are carrying along most of what you do not want. A somewhat mild distillation might be what you need.
Of course you may boil the hell out of it, but what are the results?
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Mateo_swe
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[*] posted on 14-6-2020 at 21:44


Im not distilling to purify the isopropanol.
I just made a practice extraction following a YT video and now im just removing the most of the isopropanol and then i will precipitate the extracted chemical.
I probably recrystallize also to get some experience on this.
I understand that pushing the destillation will result in a not so pure distillate.
If i was distilling for high purity a smooth distillation not overheating the boiling flask would produce a better purity distillate.

[Edited on 2020-6-15 by Mateo_swe]
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Tsjerk
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[*] posted on 15-6-2020 at 00:20


Quote: Originally posted by unionised  

You would think so, but dropping the pressure drops the boiling temperature, and that raises the heat of vap.

https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#/media/File:Heat_of_Vaporization_(Benzene+Acetone+Methanol+Water).png


Edit: if I understand this graph correctly, the heat of evaporation goes up by around 10% when bringing the boiling point (water for example) from 373 K degrees to 273 K. Correct?

[Edited on 15-6-2020 by Tsjerk]
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[*] posted on 15-6-2020 at 03:35


in vacuum you drop the boiling temp AND condensing temp so you need a colder condenser.

Sorry the only way you will get a good quality distillate, and economy is a cheap dose of patience and a good coffe' / tea

Even if you are just concentrating a solution why double up on steps?

Set it to get a nice steady drip rate in the receiving flask and leave it there, once the system is stable just come back and check on it every once in a while and let it do its thing.

for every variable you change several others go with it, or invest in a Soxhlet extractor

https://en.wikipedia.org/wiki/Soxhlet_extractor
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[*] posted on 15-6-2020 at 09:23


You are right, I should just let it do its thing with a steady drip rate and spend the time finishing my fumehood.
That way i can keep an eye on the progress while working.

I actually already have a soxhlet extractor, a clever little apparatus i must say.
I havent used it yet but im sure it will get some good use eventually.
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[*] posted on 16-6-2020 at 03:15


just to be annoying to everyone, distillation rate is directly and unequivocally tied to the surface area of the liquid gas interface. Its why round bottoms should be used half full, to maximise surface area. Aside from being correct it makes logical sense.
Adding more heat when you are already at your maximium distillation rate for the system only creates problems.
So rotary evaporators increase dramatically the surface area of the liquid gas interface. its why the distillation rate increases if you spin it faster.

There is an enormous amount of physical chemistry going on even in the simplest of distillations, they are fascinating.
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[*] posted on 16-6-2020 at 04:59


Quote: Originally posted by Panache  
just to be annoying to everyone, distillation rate is directly and unequivocally tied to the surface area of the liquid gas interface. Its why round bottoms should be used half full, to maximise surface area. Aside from being correct it makes logical sense.
Adding more heat when you are already at your maximium distillation rate for the system only creates problems.
So rotary evaporators increase dramatically the surface area of the liquid gas interface. its why the distillation rate increases if you spin it faster.

There is an enormous amount of physical chemistry going on even in the simplest of distillations, they are fascinating.


Quite correct, which is why I tell people the only thing they can do is learn patience.

With distillation the faster you go the more time you waste as you'll just have to do it again for a pure product. It is one of those things where the more painfully slow and tedious it is the better the result.

My method is I get it till it is just evaporating calmly with minimal active boiling, with a stable drip rate at the receiver then jacket it in cloth and start doing other work or watching a movie with a check up every 30 minutes or so till it is don.

get a very high purity distillate with out wasting energy for nothing.

there really is just nothing you can practically do other then violate the laws of reality.
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[*] posted on 18-6-2020 at 12:49


Quote: Originally posted by Panache  
So rotary evaporators increase dramatically the surface area of the liquid gas interface. its why the distillation rate increases if you spin it faster.


It seems the high rate of solvent evaporation in a rotary evaporator is mainly because of the vacuum applied.
If you read the PDF about the DIY rotary evaporator in first post in this thread you find comparison measurements of evaporation rates.
The measurement
without vacuum and without rotation got evap rate 1.97ml/min.
With Vacuum but without rotation got evap rate 8.00ml/min.
With Vacuum and rotation got evap rate 8.49ml/min.
All measurements used same heating.

So the biggest factor for high evaporation rate is the vacuum.
The rotation didnt add so much on the evaporation rate.
A quick calculation gives,
6% of evaporation speed was due to rotation.
71% was due to the vacuum and
23% was due to heating.

But every increase is of cause valuable for a chemical company as it saves time and running a lab isnt cheap.
For an chemistry hobbyist its not the same as its the hobbyists own free time spent and also a rotovap can be a too big investment for many.
Especially if you can get 94% of the results by just using vacuum and heating.
The figures probably vary some if using bigger flasks and volume solvent.

So i figure one can come to pretty good results just using vacuum, if one just want to get rid of the solvent in the flask, like a rotovap do.
If distilling for purity its different.
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[*] posted on 19-6-2020 at 03:06


Quote: Originally posted by Panache  
just to be annoying to everyone, distillation rate is directly and unequivocally tied to the surface area of the liquid gas interface. Its why round bottoms should be used half full, to maximise surface area.

I disagree ;
if you use boiling stones the shape of the vessel or how far you fill it is immaterial.




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Panache
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[*] posted on 19-6-2020 at 10:44


Quote: Originally posted by Mateo_swe  
Quote: Originally posted by Panache  
So rotary evaporators increase dramatically the surface area of the liquid gas interface. its why the distillation rate increases if you spin it faster.


It seems the high rate of solvent evaporation in a rotary evaporator is mainly because of the vacuum applied.
If you read the PDF about the DIY rotary evaporator in first post in this thread you find comparison measurements of evaporation rates.
The measurement
without vacuum and without rotation got evap rate 1.97ml/min.
With Vacuum but without rotation got evap rate 8.00ml/min.
With Vacuum and rotation got evap rate 8.49ml/min.
All measurements used same heating.

So the biggest factor for high evaporation rate is the vacuum.
The rotation didnt add so much on the evaporation rate.
A quick calculation gives,
6% of evaporation speed was due to rotation.
71% was due to the vacuum and
23% was due to heating.

But every increase is of cause valuable for a chemical company as it saves time and running a lab isnt cheap.
For an chemistry hobbyist its not the same as its the hobbyists own free time spent and also a rotovap can be a too big investment for many.
Especially if you can get 94% of the results by just using vacuum and heating.
The figures probably vary some if using bigger flasks and volume solvent.

So i figure one can come to pretty good results just using vacuum, if one just want to get rid of the solvent in the flask, like a rotovap do.
If distilling for purity its different.


You misunderstand what they are comparing, the systems are not identical, if they were then they would all boil at the same point, clearly if the same heating regime is applied the system under vacuum performs better.
Im not pretending to be a phys chemist, they would have a very definite way of approaching this discussion.
A very real issue with fast vapping off of solvents is the entrained product in the mist. its essential to use a splash head of some sort. The worst possible scenario is if theres line of sight between your boiling surface and the bend of no return. For a simple splash head a loose ball of fibreglass or teflon wool in the stillhead suffices.
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Panache
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[*] posted on 19-6-2020 at 10:49


Quote: Originally posted by Sulaiman  
Quote: Originally posted by Panache  
just to be annoying to everyone, distillation rate is directly and unequivocally tied to the surface area of the liquid gas interface. Its why round bottoms should be used half full, to maximise surface area.

I disagree ;
if you use boiling stones the shape of the vessel or how far you fill it is immaterial.


Is it? I think you are quite incorrect there. Boiling stones ensure theres adequate no-wetted surface for the vapour to form on at the place where the temperature gradient is most severe. They in essence simple help to ensure that no superheating takes place.
I much prefer to use a stirrer bar to ensure adequate mixing and no superheating, it often also serves as a boiling surface.
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