I am working on a large scale natural product Rotovapping project. There are two botanical extracts to evolve, combined in processing. The solvents are 50% EtOH and 50% glycerin. There is a large volume of material to dry that will require several weeks of run time. I am using a diaphragm pump that has specs for chemical resistance; I am not too concerned for any issue with EtOH. My concern is that the efficiency of the pump may be impacted by water vapors if I push the parameters to expedite the process. I know that cold traps are encouraged for high volatiles...but would it be helpful to add one to condense any water vapor also, for a run this large? The pump does not have any trap system in place currently. Not sure if it is worth the trouble to create an inline trap as or if it will enhance the speed.
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$\begingroup$ How large is "large" exactly? Depending on what exactly you are trying to achieve, if you give us additional information we may be able to recommend a better way to do it. $\endgroup$– IT TsoiJul 11, 2016 at 5:58
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$\begingroup$ Thanks for the reply. The system is a 20L Buchi; I think the total volume to dry is ~300gallons. Because the solvents are not extremely volatile, no one seems to be concerned about secondary trapping...based on the parameters I am concerned about the water impacting the pump efficiency. I don't think water vapor will damage a diaphragm pump, but could conceive it may affect the vacuum eventually. Do you think a solution could be to just incorporate a few minutes every hour to vent the pump? I am more experienced with aspirator pumps then diaphragms...I have skittle learning curve! $\endgroup$– RyanJul 11, 2016 at 14:01
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$\begingroup$ It does help that the solvents are not particularly volatile and I imagine you're not going to heat it to boiling point so it should be all right if you're just applying the vacuum. 20L is huge, especially to rotavap. No way to extract it into something more volatile first, I presume?? $\endgroup$– IT TsoiJul 11, 2016 at 14:17
2 Answers
You will always want a cold trap not only to protect the pump but also to make sure it's running at peak efficiency at all times. Especially when you push the vacuum and heating to try to expedite the process, you never know when the whole thing might suddenly bubble up. A clean cold trap will also allow you to recover the material that has escaped the main chamber, whereas without one you'd probably be faced with loss of compound and having to repair the pump.
Ideally you should have pump attached to dry ice/acetone trap followed by a cold water condenser (from the rotovap). There are several reasons to have a cold trap. Protection of the pump is one of them. The other reason is that a pump can maintain deep vacuum OR pump large volume of material, but not both. By capturing solvents in the trap you decrease the amount of solvents a pump should push through, so you can evaporate solvents faster.
Do not increase the water bath temperature to expedite the process. This might lead to decomposition of biomolecules.
