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Author: Subject: Speeding up vacuum distillations - Role of Diameter of Vacuum Intake
beerwiz
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[*] posted on 9-4-2019 at 21:41
Speeding up vacuum distillations - Role of Diameter of Vacuum Intake


Any ideas on how to speed up vacuum distillations? I have large volumes of alcohol to distill and I want to distill it in the least amount of time under vacuum. Would getting a sub 10 Torr vacuum pump with a larger vacuum intake and a larger diameter vacuum out barb on the glassware do the job quicker? Any other ideas?
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Deathunter88
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[*] posted on 9-4-2019 at 21:58


Quote: Originally posted by beerwiz  
Any ideas on how to speed up vacuum distillations? I have large volumes of alcohol to distill and I want to distill it in the least amount of time under vacuum. Would getting a sub 10 Torr vacuum pump with a larger vacuum intake and a larger diameter vacuum out barb on the glassware do the job quicker? Any other ideas?


There is no need to use vacuum distillation to distill large amounts of alcohol since it has such a low boiling point. By doing a vacuum distillation you are just increasing the load on your distillation column, as it now has to be colder to condense the alcohol coming over at a lower temperature.

Instead, increasing your heating ability and length of condenser column (so it doesn't get flooded) is the way to go.
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XeonTheMGPony
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[*] posted on 10-4-2019 at 04:23


Speed and quality distillation are mutually exclusive, Same for Quality, you do NOT get both, but you may chose any where between the two.
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RedDwarf
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[*] posted on 10-4-2019 at 05:05


An ideal vacuum distillation would have zero flow through the barb and vacuum intake once you had reached distillation temperature. Assuming that you're not bleeding air into the boiling flask to reduce bumping, any flow through the barb is either air leakage (undesirable) or un-condensed product loss (very undesirable). So increasing the cross sectional area of the pump feed has zero value at this point.
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SWIM
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[*] posted on 10-4-2019 at 08:27


If you're distilling to get the alcohol, the vacuum will be counterproductive in some ways because it will make re-condensing the vapors efficiently much tougher.
If you have a pump that can withstand alcohol vapors you might be able to just condense it after the pump, but that would take something like a diaphragm pump with internals that can take the alcohol.

I think the best answer is just a bigger still.
I've got big flasks, but for getting alcohol in a straight run distillation I use a big pressure cooker as a boiler with copper tubing for a condenser.

Added thoughts about bigger vacuum intakes and barbs:
I don't think they would help much unless what you've got is already very restricted.

The way I see it, you don't really want to pump ANYTHING out of your still during a conventional vacuum distillation.
You just want to maintain a vacuum. If your rig was perfectly airtight, you could just evacuate it and then not even run the pump during distillation.
If the pump has to run you'd want it to only pump out the air that leaks in. Any vapors pumped out are lost.
Of course you can't really do that, but you'd want to limit the pumping to a level that keeps air out of the system.

This would seem to imply that a bigger pump would only help if you either need to draw the boiler down to high vacuum faster, or if the leaks overwhelm the smaller pump.

Edit: Somehow I managed to fail to read the post right above, which makes the same point I was just guessing at.

[Edited on 10-4-2019 by SWIM]




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