Author Topic: Different vac pumps, diff range of temp usage?  (Read 3263 times)

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gabd

  • Guest
Different vac pumps, diff range of temp usage?
« on: December 25, 2002, 11:25:00 AM »
First and foremost,
Merry Christmas to all that celebrate it.

I have a question regarding vacuum pumps. I was just looking at KNF website. They sell a couple of different laboport pumps. Most pull over 29 inches Hg. Of course the capacity in L/H changes, but the vacuum is always close, say between 7.5 and 1.5 Torr.

The question is: KNF suggests different pumps depending on what temp you need to run the distillation.
For example, they advise using a given vacuum pump for what they call high boiling stuff like xylene and for a different pump for DCM.

Why is that? Would someone use a vacuum pump to stripp off DCM?
I fail to understand the difference between a pump for high boiling solvents and low boiling solvents. The only differnce I can see is the capacity. Any pumps that sucks 29 inches should work for ketone distillation right?

A pump for stripping DCM?
I think it doesnt make any difference, but I still dont understand, maybe someone that knows can help me.

Thanks

metwurst

  • Guest
pumps, practically speaking.....
« Reply #1 on: December 26, 2002, 04:51:00 AM »
metwurst - and presumably others - frequently uses an aspirator station to strip residual low boiling solvents like DCM from reaction products. Distilling as much solvent off as is practical is followed by some time under vacuum to strip the remainder.

Generally speaking you'd be best to go for the pump that pulls the best vacuum of those you've mentioned, particularly if you're going to be vacuum distilling MDP2P. (metwurst would prefer not to get into an argment about separation of fractions at this point)
metwurst was initially successful using a fabricated aspirator station that pulled about 13mmHg when well stocked with ice.
Since then a proper mechanical vacuum pump has been procured, and it's a lot more practical for stuff like ketone distillation, but for solvent stripping the aspirator is still used.
This slows the contamination of the pump oil in the mechanical pump, because volatiles and corrosives are all handled by the aspirator.
When a cold-trap is also procured, metwurst reckons the aspirator may be archived, since the cold-trap will prevent the nasties from getting into the mechanical pump, and the mechanical pump is a great deal quieter.

As you say, any pump that pulls 29" (mmHg are preferable units methinx) will be suitable for ketone distillation as you'll be able to distill below the ~180°C decomposition temp.






gabd

  • Guest
Thanks metwurst
« Reply #2 on: December 26, 2002, 09:16:00 AM »
Thanks for the tip,
Yet I dont understand how a given vacuum pump that can pull 6 torr is used for stripping DCM when they advise for a different one that alos pulls 6 torr for high boiling constituents like xylene(hey thats what they say). Any pump that pull 6 torr could be used for ketone distillations right?
The good in this pump is that its a diaphragm, oiless pump, teflon coated so resistant to corrosive gases.

If anybody can reason why use different high vacuum pumps for distillation temperature, would make my day!
I dont know so much about vacuum pumps, but still I dont see why. Of courses there must be a reason other than just trying to sell u more then one pump.

Rhodium

  • Guest
pump
« Reply #3 on: December 26, 2002, 11:15:00 AM »
As so much of the distilled DCM will vaporize at room temp, such a pump need to be able to displace a lot more air per minute than one you use for distilling xylene, because the latter condenses considerably faster than DCM.

If you want to distill high-boiling compounds like certain freebases (tryptamines for example), you absolutely need a pump capable of pulling 1 mmHg or better, for general vacuum distillations 1-10 mmHg is pretty good too. However, if you want to use a roto-vap to recycle ether or DCM, there is no way you could use a stronger vacuum than 25-50 mmHg or so, possibly even less depending on the temperature of your cooling water and distillation rate. Therefore it is good to have a strong pump for product distillations, as well as one weaker which can take some abuse (water, corrosive fumes etc) for solvent evaporation and similar chores.