I'm looking at a 2 stage rotary vane vacuum pump that uses oil. Let's say all I have in a distillation flask are high boiling organics above 150 C at
760 torr. Is a cold trap (e.g., dry ice/acetone or lN2) necessary?
What about vacuum filtrations--is a cold trap necessary?macckone - 27-4-2013 at 19:53
A cold trap will help keep your vacuum oil clean.
With a good vacuum pump you will be able to make lots of things boil at room temperature.
The oil will absorb things that gets into the pump as a vapor.
As the oil absorbs chemicals it will start to outgas at low pressure.
This can cause problems achieving good vacuum.
The cold trap will prevent a lot of vapor from reaching the oil.
A bubbler trap with vacuum oil can also be used but it increase your pressure on the other side of the trap.BromicAcid - 28-4-2013 at 05:42
Let's say all I have in a distillation flask are high boiling organics above 150 C at 760 torr. Is a cold trap (e.g., dry ice/acetone or lN2)
necessary?
What matters is the pressure you are planning to actually distill the material at and what kind of cooling you are using in the setup. Here are two
separate examples:
1) You are using a condenser running ice water for the initial cooling and you have wet ice (i.e., not dry ice, just ice from water) around your
receiver to keep it cool. You are distilling at just about 5 torr which should give a head temp of about 25C. In this case you would not
need a trap as the vast majority of your material would be condensed in the condenser and then stay cool in the receiver to prevent
it from volatilizing during the course of the vacuum distillation. Further if you wanted to add a trap, you wouldn't need to go with dry ice, just
wet ice would work. Dry ice would be nice but wet ice will still give you a decent buffer.
2) You are again using a condenser running ice water and same as before your receiver is buried in wet ice. However in this scenario you're trying
to strip off the lower boiling organics (i.e., if you had anything right at the bottom end of your range). You are distilling at the best available
vacuum which puts the boiling points of your most volatile components at <0C. In this case those are going to sweep right through your setup and
end up in your pump. You would need a dry ice trap to capture them.
Dry ice traps are great for high vacuum work where you are trying to distill a product of low volatility because under those conditions, residual
solvents become much harder to capture. If indeed all of your components boil above 150C at atmospheric and you are not going to full vacuum you
could get away with not using a dry ice trap. However in order to keep your pump oil it's best for the very longest time it is good practice to use
one. In which case it becomes a matter of do you have access to dry ice or do you have access to pump oil. You'd be amazed at the torture one can do
to a vacuum pump then change the oil and it will run like a champ again. It's all a matter of what risks you want to take.
