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Author: Subject: Vac Distillation of Water keeps stalling? Too high vacuum?
beerwiz
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[*] posted on 20-9-2018 at 16:11
Vac Distillation of Water keeps stalling? Too high vacuum?


I'm using a 10 micron rotary vane pump 6CFM to vac distill water on a 2L rotavap.
At the start after turning the vac pump on I can see it boiling, then a minute later, no more boiling is seen and it sort of stalls, 2 hours into it and only about 30ml distills off out of 600ml. There are no leaks in the system, the bath temp is 45C, and the rotavap is at 120rpm. The condenser is at -50C.

Could the vacuum pump be pulling too strong of a vacuum and stalling the distillation?

Is there something I'm missing when doing vac distillations?

HEEEEEEEEEEEEEEEEEEEELPPPPPPP!

[Edited on 21-9-2018 by beerwiz]
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JJay
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[*] posted on 20-9-2018 at 16:26


If you put your finger over the exhaust of your vacuum pump, does pressure build up?



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beerwiz
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[*] posted on 20-9-2018 at 16:31


I haven't put my finger over it but I can see a lot of oil vapor coming out so I assume it does build up pressure at the exhaust port.

I did however put my finger on the addition port of the rotavap with the vac pump running and I could definitely feel the vacuum.
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JJay
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[*] posted on 20-9-2018 at 16:35


If you see a lot of oil vapor coming out, I practically guarantee that there's a leak somewhere.



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beerwiz
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[*] posted on 20-9-2018 at 16:40


When there's a leak there's the tell tale noise coming from the pump, it becomes louder, but when it's completely leak free, it runs with a humming sound. When I do the distillation it has the noise in the beginning while evacuating the air then it goes to a humming sound meaning there's no leak.

I have to admit, the first time I used the pump I put sulfuric acid into it (since the oil bottle and the sulfuric acid bottle was very similar and I didn't read the label) by accident instead of the oil. I'm not kidding, I immediately drained it out and put in the regular oil, but maybe this messed it up a little bit? I doubt it since my other vac pump also has the same problem of stalling the distillation of water.

edit: I have a gut feeling the culprit may be the oil I'm using. I normally use 10W-30 motor oil, but this time I used the oil from the pump manufacturer. I'll try plain old mineral oil next, I hope this solves the problem.

[Edited on 21-9-2018 by beerwiz]

[Edited on 21-9-2018 by beerwiz]
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Sidmadra
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[*] posted on 20-9-2018 at 17:41


You put sulfuric acid in it..? Was it concentrated? That's sketchy. Even if you drained it and put in new oil, there's almost certainly still sulfuric acid in there. Depending on the pump material, I think it is very very likely sulfuric acid could have corroded something in there. You would have to wash out the the vacuum pump like 3x with new oil to get all the sulfuric acid out...

But if you put concentrated sulfuric acid in there, then I think there's a very strong possibility your vacuum is toast. Vane pumps require tight contact between the metal surfaces to pull vacuum, and sulfuric acid would corrode even steel pretty quickly.


You can easily test if your vacuum pump is pulling a vacuum by turning it on and putting your hand over the hose, feeling the suction. Also, if a vacuum is being pulled, you will find it nearly impossible to pull your rotovap flasks off, because the vacuum sucks them on.

[Edited on 21-9-2018 by Sidmadra]
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Magpie
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[*] posted on 20-9-2018 at 18:27


Quote: Originally posted by beerwiz  

Could the vacuum pump be pulling too strong of a vacuum and stalling the distillation?


No.

Place a vacuum gauge or manometer on your system. You have lost vacuum for some reason.




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happyfooddance
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[*] posted on 20-9-2018 at 19:56


If your condenser is -50°C your water will be collecting there, as ice.
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beerwiz
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[*] posted on 21-9-2018 at 06:43


It was 98% sulfuric acid, I know it's crazy but it's true, mistakes happen.

So my question is how do I quickly distill water under vacuum? 15ml/hour seems to be way too slow or is this the norm for water?
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Deathunter88
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[*] posted on 21-9-2018 at 07:00


Quote: Originally posted by beerwiz  
It was 98% sulfuric acid, I know it's crazy but it's true, mistakes happen.

So my question is how do I quickly distill water under vacuum? 15ml/hour seems to be way too slow or is this the norm for water?


Get a vacuum gauge and check the pressure of the system. If the pump isn't pulling a good vacuum then you broke it, so go get a new pump.
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Swinfi2
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[*] posted on 21-9-2018 at 07:33


This is a funny coincide (not the pump thing, ouch) I've been thinking about distilled water for a while and just come up with this.

20180921_153901.jpg - 999kB

Obvious it doesn't go as quick as a vacuum system but the idea is to run it (99%) unattended. Just doing the first run now to make sure its safe to leave.

As for your issue could it be that the poor vacuum and low bath temp causes the water to cool below it's boiling point? 45 seems low to me. But then my best vacuum is an aspirator so it's relative.
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