texaspete
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Reducing bumping in vacuum distillation
I finally got my (vacuum) distillation setup up and running, and did some trial runs with water. The porous carbon boiling chips I have only work for
five minutes or so, and then are useless and heavy bumping follows. I've read that boiling chips don't work in a vacuum, and that the most common
solution is a fast spinning stirbar. Unfortunately, I don't have a stirring/heating plate.
After a bit more googling, I found this site(pdf), which mentions using a capillary tube to prevent bumping.
While searching these forums for information, I read that someone used an ebulliator to prevent bumping. I'm not quite sure what an ebulliator is,
however.
Has anyone used either of these to prevent bumping?
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Magpie
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Yes, I have used an ebulliator tube to prevent bumping while doing vacuum distillations. It works quite well. It's really nothing more than a glass
tube drawn to a fine end (capillary tube) as described in your attachment. I believe I needed it while vacuum concentrating phosphoric acid.
The single most important condition for a successful synthesis is good mixing - Nicodem
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texaspete
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Magpie, your's is the ebulliator I found. What did you use to connect the tube to the distillation head? A thermometer adaptor? Also, what diameter
was the glass tube?
Thanks!
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Magpie
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Yes, I used a thermometer adaptor. Diameter of the glass tube would be the same as that of a thermometer, right close to 6mm IIRC.
The single most important condition for a successful synthesis is good mixing - Nicodem
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pantone159
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My books say to use a long, fine capillary to let in air (or N2 if you must) into the distilling flask, as Magpie says.
Otherwise, the bumping is horrible. Boiling stones are useless. A magnetic stirrer does work in place of the capillary.
I haven't done this myself, but that's what the books say.
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Magpie
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I took a look back at the photo I posted in the phosphoric acid thread. What I used was an ordinary glass tube drawn down at the end (last 2-3") to a
small opening, ie, 0.5mm or so. Then on the top of the tube I placed a piece of rubber tubing and a pinch clamp. That way you have good control over
the air you let in.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Ozone
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Hello,
Or..you can regulate the vacuum manually. The severe bumping usually occurs at first (when the surface tension is broken by vapor pressure). Once you
have a solid vapor at the head, you can usually roll with it. This works well with higher boiling solvents (like toluene).
Just really *pay attention* at first-literally hold the hose onto the adaptor and remove/pinch when the boiling goes beserk. Do this until either the
bulk of the solvent is removed and/or stable boiling is acheived.
Alternatively, standard kits now include a thermometer adaptor and compatable bleed tube. I've found that the diameter (ID) on these is a bit large.
Take a cheap Pasteur pipette, roll it in a gas flame, and pull it when it sags. This will give you a fine capillary (also suitable for TLC spotting)
connected to a larger tube (which, coincidentally, will fit with the usual O-ring, a standard thermometer adaptor). This enables a much more
controllable vacuum whilst minimizing the influx of troublesome air/water vapor.
Happy New Year,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Sauron
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Usually you will want to feed an inert gas (N2 or Ar) in through that capillary because air might oxidize whatever you are distilling.
Spinbars are not idiot proof. If you lose power, or just switch off the mag stirrer without first slowing it gradually to a halt, the spinbar can and
will rattle around in the flask, I have had flasks broken that way, making a mess, a potential fire, and at least a loss of product.
An overhead stirrer with a glass or PTFE rod, appropriate bearing, and a PTFE paddle are better, but you have the hazard of dropping the rod through
and breaking the flask bottom. A mechanical stop ring secured weith a hex head screw on the shaft above the bearing will prevent that.
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Refinery
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I finally tested the bleeder tube. I had stock capillary tube, 500mm with 0.2mm bore and it was obviously too big, but I heated the tip and pulled it
to form a very fine needle capillary and I turned a ptfe seal for the neck joint on the flask to fit it. Letting in a very small, millimeter-scale
line of bubbles made vacuum distillation go absolutely smooth with zero bumping during the whole event. I don't have inert gas yet, but suppose to get
some, and I thought I just fill a balloon with it and attach it to a tube leading to the capillary because adjusting the bottle regulator to that very
small bleeder would be impossible. Now that I have it on hand and tested, it is very easy and fast to set up, considering the whole set-up time of
(vac) distillation apparatus.
Fascinating how such a small adjustment turns off the whole bumping thing. What phenomenon this is based on?
Last time I tested magstir on vacuum it still kept bumping. Boiling chips have only limited success and they appear to work better with some other
compounds and just aren't working at all with others.
[Edited on 5-7-2020 by Refinery]
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Ubya
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What phenomenon this is based on?
you are introducing bubbles, other bubbles can grow fron those, like seed bubbles.
porous materials help eith boiling because they have air pockets inside, with a vacuum that air is removed, so they don't work anymore as good
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feel free to correct my grammar, or any mistakes i make
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Fery
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There is a big temperature difference on the surface of the vacuum distilled liquid - there is rapid evaporation from the surface layer (which cools
it down) and there is much higher temperature close to the flask wall. Stream of minibubbles from the capillary mixes the content in the distillation
flask and prevents this high temperature gradient. You can also use magnetic stirring instead of capillary to prevent bumping.
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Refinery
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Hate to admit, but I've been avoiding vac distillations at all costs due to bumping issue. No matter what I tried to do, it always just remained
still, and then, BUMP > the whole apparatus jumps on the set and stuff shoots up to the stillhead (yea, column, but still).
[Edited on 6-7-2020 by Refinery]
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