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Hive BB  Methods
Discourse  Distillation
boil over
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| Author | Topic: Distillation boil over |
| Dope
Amine Hive Bee |
 posted 01-17-2000 06:13 PM 
No one was vac. distilling using an aspirator at the end of #1 for MD-P2P. The problem is that even though no one is using a 3 L flask which is not even a third full, it keeps on doing nothing as it heats up and then all of the sudden boils over. Yes, no one is using a stir bar. No one tried halving the batch and this still didn't prevent a boil over. What's in the flask is oil and charred stuff. It's coming over at about 125 deg.. Please help! |
| Ultraman Hive Bee |
posted 01-18-2000 12:24 AM
Did you try boiling stones ? Also, if memory serves me correctly (which sometimes it doesn't) you may get better results if the flask is one-half to no more than 2/3 full. i.e. Use a smaller flask. Might also try turning your temp up a little slower. Good luck. |
| equarius Hive Bee |
posted 01-18-2000 01:09 AM
I assume there's solvent in with the oil and by "boil over" you mean it suddenly shoots through the condenser in its unseperated state. Correct me if that's wrong. The boil over happens becuase your vac is suddenly rising which causes much of the solvent and / or oil to instantly go into the gaseous state and poof into the receiver flask. |
| Dope
Amine Hive Bee |
posted 01-18-2000 10:59 AM
Wait, you are saying it would be better for my flask to be more full? I dunno. Yes it boils up into the condenser in an unseparated state. |
| equarius Hive Bee |
posted 01-18-2000 01:46 PM
Between 1/3 and 1/2 full should be fine. The problem is your vac is fluctuating. Vacuum up + constant temp = bp down. |
| Methyl
Man Hive Bee |
posted 01-18-2000 03:08 PM
Mr. Amine, I know this is a typically "Hive-ish" response, but boy oh boy git yerself a KNF Neuberger teflon head diaphragm vac pump and yewl never look back. Yeeah shee-it it is 'bout $500 used (in great shape) for a 28.5" Hg model from online places but hell boy it'll suck the chrome off'n a trailer hitch for 3 days straight and never veary the vac, ah'll tell ewe whut... |
| Dope
Amine Hive Bee |
posted 01-19-2000 12:50 AM
If it is because of pressure fluxuation then why does does it only happen now after most of the water and acetone are gone and never before? Temp is about 125 deg c. Once it starts boiling over it doesn't stop. I would think it was cuz I needed boiling stones but the stir bar should be doing the job shouldn't it? |
| Icepick Hive Bee |
posted 01-19-2000 01:11 AM
If you have a distillation column handy, put that on there, itll take care of your problem. Insulate that fucker or use heating tape on it since yer doin such a high temp distillation. Between water and ketone lies (iso?)saf....it does sound as if your vac is fluctuating a bit, either cuz of the aspirator or a small leak on the system...Have actually had to stop b4 and regrease joints cuz the heat sometimes messes with the grease on the joints... Hope this helps, |
| Ultraman Hive Bee |
posted 01-19-2000 01:28 AM
OK. I pulled out a Org. Chem book. Here's a synopisis of what it has to say: Flask should not be more than 1/2 full. Heat by means of oil batch. Flask should be immersed to a depth above the liquid in the flask as an aid to prevent bumping problems. The volume of vapor formed by the volatilization of a given amount of liquid is pressure dependent.. i.e. the volume of vapor formed from vaporization of a drop of liquid will be approx. 20 time a great at 38mm as 760 mm. Serious bumping problems may occur in the distilling flask during vac distillation as large bubbles explosively escape from the liquid. The insertion of a Claisen connecting tube between the distilling flask and still head is a partial solution to the problem. Boiling stones are generally ineffective in preventing bumping under reduced pressure. The usual method is to provide a thin, flexible capillary tube extending to the boiling flask. Stir bars are good too. The 2nd problem with vac distill is that the measured pressure is affected by the rate of distillation,,,.... bla, bla, bla, essentially, keep a steady, slow distillation rate and avoid superheating by maintaining the oil bath temp no more than 15-25 higher than the head temp. So, forget those damn boiling stones. Sounds like maybe your going too fast. Try slowing down on your distillation rate. The 1st two components should be coming over at less than 100 degrees under vac. Sorry to ramble, hope this helps. |
| Dope
Amine Hive Bee |
posted 01-19-2000 02:05 AM
It definately could be heated slower. That was the only change I could think of but i was hoping for a more substantial answer. All I can say is thank you so much for going to such lengths to help out a fellow bee. The acetone/sulfuric ammounts were fucked up so I'm just hoping it is all worth it in the end. Bisulfite sux doesn't it? |
| equarius Hive Bee |
posted 01-19-2000 02:16 AM
The acetone should be removed before the H2SO4 reflux, unless another method? Yes, bisulphite sucks. |
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