Sciencemadness Discussion Board

vacuum fraction receivers?

Funkerman23 - 20-5-2013 at 23:58

While I am happy with my last order from Synthware/Laboy it has fallen just short of what I needed. I had wanted to collect larger fractions under a vacuum but so far no luck. I saw that Dr bob had a distilling receiver some time ago that might've worked but I'm almost certain that has already been sold. But all that aside hopefully you good folks can help me with ideas. If You/I wanted to take off larger( read 250mL+) fractions under a vacuum what would be the way to go about it assuming I wanted to change out flasks as needed but still maintain a vacuum for the rest of the apparatus? Would a pressure equalizing funnel with a stopcock on the equalizing arm ( assuming the glass side tube holds up and the stopcock doesn't leak) or is there really no way around something like this: https://www.aceglass.com/page.php?page=6638 ? Granted I may be over extending myself but so far nothing usable has come up using the search engine. I really thank you for your time & for your patience.

[Edited on 21-5-2013 by Funkerman23]

[Edited on 21-5-2013 by Funkerman23]

chirality - 21-5-2013 at 01:07

Don't know which author or artist deserves credit for this:

Homemade fraction cutter.jpg - 37kB

Dr.Bob - 21-5-2013 at 05:18

Exactly what size are you thinking? Are you talking 14/20 or 24/40?

Almost anything would work. What a lot of people don't seem to understand is that a high vacuum puts no more stain on glassware than a low vacuum, neither generates more than 1 Atm of pressure on the outside of the glassware. You cannot pull a negative pressure, so no high vacuum system can ever generate more than 1 atm stress on glassware. The main stress on glassware that I ever see if people clamping things poorly, holding weight with Keck clips, and not allowing glass to expand with heating. I have put far more than 1 Atm of pressure on many flasks, that is a much more dangerous risk, as pressure can easily go MUCH higher than 1 Atm with almost no effort, if you simply heat a closed system, which I have seen people do by mistake a few times. I have done it, even just by taking a cold flask from a freezer and then capping it and have had the cap/stopper fly off later.

So any decent addition funnel would work fine. The only problem with home made apparatus is the vacuum leaks from multiple joints. If the vacuum needed is easy to obtain, then no problem, if you need a very high vacuum, then that might not be easy to do, plus it uses multiple smaller pieces, so still a lot of glass to buy if you don't already have it. But it should work fine if you have the glassware and tubing to do it well. Just clamp it securely as well.

There are at least 2 simple solutions, one would be a cow or bull type fraction collector with large vessels, sold a beautiful 24/40 one a while back, but still have several in 14/20, might have something left in 24/40. The other would be a setup like the one on the left, I just sold a very nice one as part of an entire distillation head in 29/42, but might still have some in both 14/20 and 24/40 variations, which might not be that bad, depending on what all you want in it. I even still have some larger 24/40 fancy one-piece vacuum jacketed distillation setups that can do this type of fractionating, but they might cost a bit more.

Funkerman23 - 21-5-2013 at 09:02

Truth be told I have a decent amount of glass. from what I've bought from you to the other places. But I don't have anything that would allow me to replace a collection flask while maintaining a vacuum in the rest of the system, hence the post.I've looked here using the sites search engine but I haven't found much on vacuum fractional distillations, at least on larger scale runs.I must admit I have never seen such a one piece apparatus that could do that but I'd imagine in 24/40 that would be fairly large.

Hexavalent - 21-5-2013 at 09:16

What about collecting your distillate in a suitably-sized 2-neck flask; have the vacuum adapter connected to one socket, and a SubaSeal on the other. You can then withdraw the distillates from the RBF using a large syringe and needle/cannula.

The main issues with this method are contamination of samples from trace amounts of the previous fraction left behind in the collection flask, and potentially losing the pressure if the SubaSeal is punctured excessively and allows air to seep in.

Hexavalent - 22-5-2013 at 12:48

Quote: Originally posted by Hexavalent  
What about collecting your distillate in a suitably-sized 2-neck flask; have the vacuum adapter connected to one socket, and a SubaSeal on the other. You can then withdraw the distillates from the RBF using a large syringe and needle/cannula.

The main issues with this method are contamination of samples from trace amounts of the previous fraction left behind in the collection flask, and potentially losing the pressure if the SubaSeal is punctured excessively and allows air to seep in.


Edit: another idea I've had is to connect an addition funnel to the vacuum-take off port, letting the distillate collect into the funnel, and then connecting new flasks to the bottom of the funnel to drain off the fraction when necessary.

anotheronebitesthedust - 22-5-2013 at 14:01

http://www.laboyglass.com/distillation/distillation-receiver...

Hexavalent - 22-5-2013 at 14:04

Cows and pigs are fine for smaller flasks, but bigger flasks may not be able to physically fit onto it once you have more than one connected....

chemrox - 23-5-2013 at 08:25

You probably want a "cow" receiver. Depends on how low a pressure you're distilling with. I have one made from an ehrlemeyer. I will have one made from a Buchi evaporator flask. If its a 500 ml ehrlenmeyer 4x 250 ml flasks would work nicely.