Any recommendations on what robust vacuum pump to attach to a rotary evaporator. I have tried several but they either die in a few years or require constant maintenance (oil pumps). I have a budget of about $2000 and would like a dry pump that can go to 5 Torr or less. No vacuum controller is required as we have a manual pressure release valve on the rotavap which we use to control the vacuum. We do not evaporate more than a liter of solvents at a time. I will appreciate the help.
Hi Syed, usually for a rotary evaporator you use a diaphragm pump, this ensures that the gas you are pumping is kept separate from the workings of the pump, as there is frequently liquid condensing in the vacuum line, which would otherwise cause havoc! We bought one of these:https://www.aliexpress.com/item/LH-85L-Good-Quality-Mini-Portable-Min-iDiaphragm-air-vacuum-pump/32217997137.html?spm=a2g0s.9042311.0.0.G0ibVe
However your 5 Torr spec is rather hard to achieve with this sort of pump!
Dear Syed,
As mentioned, oil pumps die when solvent gets in them ... look to your condenser! If you can improve the condenser performance and trap more solvent, less goes to the pump, this also helps to pull the vacuum. 5 Torr on a rotary is a challenge due to the inherent leakage due to the many joints. I'd suggestion considering alternative methods of evaporation as these actually may be better suited to your application - although it's hard to say without knowing the solvents concerned.
Kind regards,
Rob
Thank you Phil and Rob for your comments. The rotary evaporator I have is a Heidolph https://glutco.com/products/heidolph-036000030-model-hei-vap-value-the-collegiate-rotary-evaporator-with-standard-hand-lift-and-g1-diagonal-condenser-glassware-115v?gclid=EAIaIQobChMI48GBxtro1gIVgbjACh3hewuiEAYYAiABEgIPuPD_BwE
I use a number of solvents, including water. I do not want to increase the temperature of the bath more than 40 C most of the time (some products get decomposed). I know Buchi pumps can do that, but they are very expensive. 5 torr is just a number I cam up with. I have used 25 torr pumps earlier and they cannot evaporate water at 40 C at a reasonable rate (may take days to evaporate a liter).
Dear Syed,
Water is a solvent with which a rotary evaporator will struggle. Again - look to your condenser. All the heat you put in at the bath (which is quite efficient) needs removing at the condenser (which often is not so efficient in a rotary evaporator). The pressure set's the boiling point of the solvent, for water you want to operate at 6Torr = 8mbar. At this pressure the boiling point of water is +4°C. A condenser that maintains below +4°C is critical here. If you can't hold the temperature in the condenser, the pressure will rise, and solvent will go to your pump. An inexpensive freeze dryer, a basic lab one, will dry a liter of water in a day.
Kind regards
Rob
Thanks Rob for the quick reply. The condenser came with the rotavap (the one shown in the link above). It is hooked up to a chiller that maintains the temperature at 5 C. I am not aware of any specialized condensers for rotary evaporators. I will try to search.
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