# Choosing a vacuum pump for fractional distillation

I have a fractional distillation setup. I would like to purify a small mixture ($\pu{\approx 100-150 mL}$) of high boiling point compounds (between $200$ and $\pu{300^\circ C}$). The one that interests me has a b.p. of $80\rm~^\circ C$ at $\pu{13 mmHg}$. I don't know what this mixture is composed of.

Now the tricky part and the one that give me the most headaches is the vacuum pump. I need one that has enough vacuum power to be able to distill at $\pu{13 mmHg}$ and one that is not expensive too.

I also need a system to be able to monitor and control the vacuum pressure too, so the vacuum pressure remains constant.

I figured that a diaphragm pump is not suitable since the ones that have that kind of vacuum power are way more expensive than rotatory vane pumps. Can anyone help?

• Could you correct your orthography? Also I suggest not to thank in questions, but in comments. – Mithoron Feb 10 '16 at 23:39
• Have you tried if a water aspirator works? It can go as low as 10mmHg (I think) and should be OK for the scale you want – K_P Feb 11 '16 at 8:42
• Thank K_P i have thought about a water aspirator but the water temperature would need to be a very low to reach 13 mmHg, i have thought about using a water pump connected to a water aspirator, the water pump would use a precooled water in a container and then put it back there, but i assume i need i strong water pump but how much strength do i need ? – klaus Feb 11 '16 at 11:56

Here is an aspirator assembly with a gauge and valves I put together for about $50. Connecting it to a standard hose bib it can pull up to 27mmHg. The water valve controls the vacuum rate, and if you have a closed system once you reach the desired vacuum you can shut one of the other valves to hold it. Note: This was inspired by Lysander's answer to my similar question last year, which has additional useful details. I dont know what materials you'll be working with, but Ace usually has a pretty cheap 1.5 cfm two phase pump. They work alright. It pulls too fast in my opinion and not a very deep vacuum. It still hits -29.5 inHg (at the altitude im at), but you can regulate the pressure easily with a second cheap valve already placed on the pump itself. You can also add a inlet and exhaust trap for pretty cheap. If your pretty good with tools, spend an extra 200USD and buy a deep vacuum off Ebay. Something that just requires an added belt drive motor and you can build yourself a pretty diesel, multi thousand dollar rotary vane pump for about$200 dollars.