Sciencemadness Discussion Board

Vacuum distillation - cold trap

Blackbeltjawa - 26-1-2019 at 13:14

My vac is PTFE bonded. Can I forgo the use of a cold trap or is it still recommended?

walruslover69 - 26-1-2019 at 13:26

Ya its recommended. It will fuck up the oil otherwise. So you don't HAVE to, but you will have to change the oil a lot. What are you trying to run through it?

Heptylene - 26-1-2019 at 15:55

There isn't any oil if its a PTFE pump right? I have one and don't use a cold trap usually. I do use an empty flask with a suckback adapter before the pump, just to prevent the pump from flooding if I accidentally pump liquids or dusts. If there are lots of solvents to remove I just use ice water to condense them before the pump.

beerwiz - 6-2-2019 at 00:41

I got a brand new -120 C cold trap but as soon as I start the vac distillation, the temperature shoots way up, it goes from -120 C to +25 C relatively quickly making it useless. Constantly getting dry ice is a hassle, so I stopped using cold traps for my rotary vane oil pump, I just change the oil every time which is quick and easy.

One tip: The wider the diameter of your vacuum intake valve on the pump, the faster the vac distillation will go. If you have the typical small diameter vac intake, it could take way too long to vac distill.

XeonTheMGPony - 6-2-2019 at 03:56

Quote: Originally posted by beerwiz  
I got a brand new -120 C cold trap but as soon as I start the vac distillation, the temperature shoots way up, it goes from -120 C to +25 C relatively quickly making it useless. Constantly getting dry ice is a hassle, so I stopped using cold traps for my rotary vane oil pump, I just change the oil every time which is quick and easy.

One tip: The wider the diameter of your vacuum intake valve on the pump, the faster the vac distillation will go. If you have the typical small diameter vac intake, it could take way too long to vac distill.


If you have these effects you have an air leak of considerable proportion!

Durring initial pull down you get the most mass flow thus temp spike, (To avoid this you should throttle the initial pull down to what the trap can handle) and of course this is where most volatiles come over and load up the cold trap. Once at homeostasis (Under max vacuum the pump can provide) Your cold trap will cool back down to its nominal temp

At this point you begin warming your mother liquor to degas and drive off any other material that is volatile once this is don you ramp heat up to the target distillation temperature

SWIM - 6-2-2019 at 12:06

I use a PTFE diaphragm pump and don't usually bother with a cold trap.

If there are vapors I want to condense, or solvents I want to save, I run the output of the pump through a large flask and condenser.
That way the vapors are at atmospheric and easier to condense.

This is especially important for me because I don't have a low temp trap and am too lazy to go down to the dry ice place.

Blackbeltjawa - 7-2-2019 at 12:59

My pump is a PTFE diaphragm pump. I will be using it to distill oils, solvents and other pretty caustic reagents. Keeping a cold trap at operating temp for the duration of long distillations seems like such a hassle but if if saves my pump from total destruction then I guess I'll just have to do it. Thank you all for your answers. My decision isn't made yet but I am a little more informed.