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Author: Subject: vacuum distillation of cyclohexane
thioacetone
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[*] posted on 15-1-2023 at 21:58
vacuum distillation of cyclohexane


How would I design a cold trap to stop cyclohexane vapors getting into my vacuum pump? The melting point of cyclohexane is 6.6 degrees C.
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[*] posted on 16-1-2023 at 01:34


I think that running chilled water in your distillation and putting your receiving flask in a cold water bath, you should be fine. Obviously you don't want it freezing and blocking your cond|nser. So, keep everything above 6.6.



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gemar14
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[*] posted on 16-1-2023 at 02:25


Quote: Originally posted by thioacetone  
How would I design a cold trap to stop cyclohexane vapors getting into my vacuum pump? The melting point of cyclohexane is 6.6 degrees C.


Alternatively, you could just do atmospheric pressure distillation. The boiling point of cyclohexane is only around 80 C
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thioacetone
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[*] posted on 16-1-2023 at 15:14


I can't use heat as I am evaporating solvent from organic plant material (to make an extract) and heat will degrade the product.

I have tried using chilled water in the condenser and submerging the receiving flask in ice water, but half of my solvent just disappears (into the pump).

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[*] posted on 16-1-2023 at 16:44


Cold temperatures:
Chilled water
Ice and salt bath
Dry ice
Dry ice and acetone
Liquid nitrogen

Pick what works for the solvent and level of vacuum.

Sounds like you might need something colder for the condenser. You could use car antifreeze chilled down to the temperature you need.

I have seen on this site a table of salt/ice baths and the temperatures that can be sustained with each combination. Worth a search.




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[*] posted on 16-1-2023 at 17:45


Or use less of a vacuum.
Without a proper manometer you can eye ball the pressure. But it is tricky

Keep your coolant in ice as well as the receiver.
Heat your solvent to something like 5 degrees above room temp,
then ever so slowly open a valve leading to your vacuum.

When you start hearing air move through the vacuum pump, close the valve just a little.
Now wait, first your solvent will degas. Take it slow.
If it doesnt then open the valve a little more
When it starts boiling close the valve.
If your appratus is properly sealed,
the condenser will maintain the vacuum level and your set
If your reciving flask or condenser get too hot then the pressure will increase,
as well as the boiling point

If you have a leak, same effect




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thioacetone
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[*] posted on 16-1-2023 at 18:28


I will try using a lower boiling point solvent (pentane) and just use conventional distillation

Thank you for the suggestions

I think I would have to buy a proper cold trap and use dry ice/acetone if I want to properly capture the cyclohexane vapours. I can't afford a cold trap and the cost of ongoing purchases of dry ice every time I want to vacuum distil is excessive.

The reason I want to capture all/most of the vapours is because cyclohexane is quite expensive here. If I am vacuum distilling a different cheaper solvent (e.g. ethanol) I would not really care as much about the vapours. I can just change vac pump oil as needed.

[Edited on 17-1-2023 by thioacetone]
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Sulaiman
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[*] posted on 16-1-2023 at 20:04


You could add a second chilled condenser above the receiving flask and attach that to the pump.



CAUTION : Hobby Chemist, not Professional or even Amateur
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thioacetone
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[*] posted on 17-1-2023 at 01:07


Quote: Originally posted by Sulaiman  
You could add a second chilled condenser above the receiving flask and attach that to the pump.


That's a good idea. I did consider using a condenser as a makeshift cold trap.
I am worried about the cyclohexane freezing in the condenser and blocking it though. It freezes at 6.6 deg C. I can imagine this problem would be worse with a spiral condenser compared to a Liebig

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