Furboffle - 15-10-2012 at 15:21
I recently purchased a vacuum aspirator on amazon. Not the most professional device, but I'm looking to use it for vacuum distillation along with
vacuum filtration. For filtration I couldn't be happier. It works awesome as should be expected. With distillation however the pump is rated to
achieve a vacuum as low as 1.5" Hg, or 38.1mm Hg. For some reason I've only been able to achieve a vacuum as low as 152mm Hg. I'm trying vacuum
distill methanol, water would be nice but I'm not expecting it to work unless I heat the water. One dilemma when I vacuum a flask the methanol just
gets super cold after 10-20 seconds. Anyone have any recommendations?
[Edited on 15-10-2012 by Furboffle]
DJF90 - 15-10-2012 at 15:58
You need a heat source on your flask of methanol. It gets cold due to the latent heat of vaporization. I suspect you are unable to reach your desired
vacuum because of the vapour pressure of methanol. Try with something that has a lower vapour pressure and you'll see what I mean. Supposedly MeOH has
a vapor pressure of ~100mmHg at 20*C...
zed - 16-10-2012 at 16:42
Colder water equals higher vacuum. One possible improvement is pre-cooling water before running it through your aspirator. An ice chest will do.
The pump can achieve no higher vacuum, than that of waters boiling point, at feed water temperature.
Colder water has a lower vapor pressure, and it can generate a higher vacuum.
So, how cold is your feed water?
Also, consider the temperature of your condenser water. It has to be cold enough to completely condense all methanol vapor. Otherwise, any
un-condensed vapor, increases the atmospheric pressure. Likewise, your receiving flask should be cooled.
[Edited on 17-10-2012 by zed]