patrick (Stranger)
08-30-01 00:12
No 208545
      Strong vacuum  Bookmark   

I had the chance to get an old 5.5m3/h pump with an ultimate vacuum of 0.01mBar. Is this pressure too low to make a proper fractional distillation of safrol and ketone. Will my pump really deliver such a vacuum in real conditions (small leakage and maybe some vapor coming in). Will the separation between the different fractions be OK. And finally, what kind of distillation temp can I expect for the safrol and the ketone (Rhodium's page nomograph give a really low temp, out of the nomograph in fact).

Thanks a lot

(Stoni's sexual toy)
08-30-01 02:03
No 208574
      Re: Strong vacuum  Bookmark   

The pump will not even come near that vacuum during a distillation. The vacuum you will be able to achieve depends on many factors, like oil quality, amount of volatiles present, what kind of vacuum line you use, flask size, condition of the pump and so on.

The pump is probably a bit too strong for sassy distillation, but should work perfectly for ketone and final honey. Usually you can expect a vacuum of 1-2mbar from such pumps under real world conditions.
08-30-01 04:57
No 208611
      Re: Strong vacuum  Bookmark   

Thank you Osmium,

Do you think that the final vacuum obtained with such a pump (pfeiffer duo5) will be constant or do I need to monitor it with a vacuum gauge.

08-30-01 18:06
No 208820
      Re: Strong vacuum  Bookmark   

I have a one-stage rotary vane pump, I have used it for safrole and ketone, I believe I can give you some tips :
If I were you I would just do simple distillation.
I have done one dry run with my pump, with my vacuum meter connected. It started at 0.4 mm Hg, then improved to 0.2 about 20 min later. Then it slowly started to degrade, but I think it was still at 0.8 mm Hg or so two hours later.
This was probably caused by the temperature increase.
I have distilled safrole, very easy, final distillation temperature was 70 C. When I did ketone, first I got some isosafrole at about 70 C, then ketone at 90 C, final distillation temperature. There was a very good gap from the iso to the ketone.
After the ketone distillation, if I remember things well the vacuum was at 2.0 mm Hg, this was probably due to gas being absorbed into the pump oil. I suggest if you don't use it too often change the oil every time, otherwise do a dry run with the ballast open to clean the oil.
If yours is not a rotary vane then I suggest you to learn more about it.
08-30-01 18:59
No 208833
      Re: Strong vacuum  Bookmark   

There seems to be a big misunderstanding of the vacuum traject.

When you start to distill f.e. MDP2P ketone with f.e. a diaphragm pump with an ultimate vacuum of 2 mbar, the meter will drop fast at start from 1012 mbar to ~20 mbar, no more then a few minutes.
Then, it will very slowly increase to the highest vacuum reading, ~2 mbar, as the last drops of ketone come over.

I'll give you a clear explanation why :

You are evacuating a closed system, with on one side an evaporation flask in hot oil, then a 15 C water cooled condenser, and then a preferably also cooled collector flask.
At start, billions of ketone vapour molecules will be vapourize, forcing the ultimate vacuum up to circa 20 mbar, but during the process of condensation in the condenser and collecting in a cool collector flask, the speed (Browns movement) of the molecules degrades fast, until they reach the coolest spot, the collector flask.
Fast moving(oscillating) 165 C ketone molecules have a higher vapour pressure, thus you read at start not 2 mbar, but 20 mbar, because the pump has to work against a resistance of billions of molecules.
Slow moving (cold) 15 C ketone molecules all collected at the end of the process, have a low vapour pressure, so you read when no ketone condenses anymore, a value of 2 mbar, there is nearly no resistance for the pump left in the system.

The pumps which go to 10-3 mbar will in practice, the real world, never reach that value if you work with a wetted system, like we do.
Only researchers who work f.e. in the field of thin molecule layer depositing techniques will reach and need these values, because they work with minimal amounts of molecules, heated on a white hot needle point, to bring anorganic molecules fast into a vapour state, to let them condense on f.e. a chip. There these pumps do their real job.

For us, in your case, the only important step to take is protecting your pump(oil) from contaminating with solvent or ketone molecules, which will immediately turn your precious pump capacity back to that of a simple diaphragm pump, if you'r lucky that the contamination is just minimal, if bigger, your capacity will be worse than a simple waterdriven aspirator, about 12 mbar.

To protect the pump, ALWAYS place a cold trap filled with icecubes and alcohol, or beter, a mix of acetone and dry ice (-75 C) inbetween the hose from your system to the pump. Any vapours escaping your system will directly be condensed in the cold trap.

Owners of a diaphragm chemical resistance vacuumpump don't have to worry that much, because the pump, if it's a new one, will have a selfcleansing mechanism, which will flush eventual fluids buildups away from the pumping head(s), which are btw chemical resistant, PTFE or so, and can be picked up in a cooled condenser at the exhaust port of the pump, but most of the time our beloved members do not even consider this and blow the exhaust gasses freely into their living area. wink