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Author: Subject: How do you distil a foaming liquid?
stoichiometric_steve
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[*] posted on 21-3-2008 at 12:36
How do you distil a foaming liquid?


Today i tried to distil an amine which heavily foamed, there was no way to prevent this - neither magnetic stirring nor silicone oil helped a bit. How can i cirvumvent this?
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chromium
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[*] posted on 21-3-2008 at 14:22


I distilled couple of years ago a lot of certain cleaning fluid wich consisted of 30% metanol, remarkable quantity of soaps and sufractants and of water.

At first it seemed impossible to distill due to intense foaming but problem was successfully solved through pouring some cooking oil to boiling flask.

I am not sure if this (or something similar) can be used in your case.

[Edited on 22-3-2008 by chromium]




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[*] posted on 21-3-2008 at 14:33


Lecithin or however its spelled has been used as an anti-foam agent.

Might try going to a farm supply place and get some anti-foam used in herbicide rigs.

Also, a vigreux column would probably help as well.




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leu
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[*] posted on 21-3-2008 at 17:48


This advice was posted long ago on the subject of bumping and foaming:

For reasons which are probably not fully understood it occasionally happens that the boiling of a liquid momentarily ceases. In consequence a portion of the liquid becomes somewhat superheated and this unstable condition is then relieved by the sudden formation of a correspondingly large volume of vapor, which causes the liquid to boil violently.
Ordinarily a liquid will boil without bumping, for the rising bubbles of vapor keep it moving and thus prevent superheating. Evidently we shall have an infallible method for the avoidance of bumping if we can keep the liquid moving. For example, when the liquid is being boiled in an open beaker, it is sufficient to make use of a mechanical stirrer. But when we are distilling, mechanical stirring is not conveniently provided. Here we can use the method of distillation of by drawing through it a continuous current of dry air. This technique has become common practice for distillations under reduced pressure, but it cannot be used when the objective is accurate measurement of the boiling point of the liquid, or when some of the compounds are decomposed by air. Another good method employs uses the capillary tube. They are best made from glass tubing having a thin wall and a bore of 2 - 3 mm. A suitable length of such tubing, supported at both ends and rotated, is heated at the in the edge of a flame until the walls melt and and then quickly drawn apart. One end of the tube is then cut off. The over-all length of the tube is then tube should be such that, when the lower end is resting on the bottom of the flask in which it is being used, the upper end will be supported by the neck of that vessel. The upper end of the tube may be closed by a rubber tube and a pinchcock clamp. The liquid in the flask must be deep enough to keep the lower end of the tube continously submerged. It is advisable to shield the equipment against the cooling effect of a possible current of air. And if several of these tubes are used simultaneously, trouble that might arise from the failure of a single tube may he avoided. This technique also is limited by exposure of the distilland to air. Other methods for the prevention of bumping succeed by providing conditions favoring the formation of bubbles of vapor at one or more points where the liquid is in contact with the heated surface of the vessel containing it. Fragments of unglazed porcelain or a small piece of pumice recently tone that has been weighted with platinum wire. Bumping can also be prevented by the introduction of 7-mm. discs of Vapoglas (an air-conditioning fiberglass filter material) cut from 3/16-inch sheets with a cork borer. The discs were cleaned with hot dichromate cleaning solution and rinsed with distilled water before being added to the hot digestion mixture. The porosity of the discs promotes a vigorous boiling and, because of their lightness, they move about rapidly in the solution and prevent development of localized hot spots. The discs are easily pulverized by pressure from a stirring rod thus assuring extraction of absorbed material. A small sliver or shred of polytetrafluoroethylene, makes an excellent boiling chip for the prevention of bumping during evaporation operations in the laboratory. This material is chemically inert to nearly known reagents (including fluorine and hydrogen fluoride) and is nonporous in nalure. Gas bubbles form very readily on its surface, and this property does not disappear on continued use and reuse in aqueous solutions or organic solvents. The numerous devices proposed for the alleviation of bumping during vacuum distillation may be classified generally as ebullators, packings, external capillary leaks, or mechanical agitators. Most ebullators become ineffective with time or on momentary interruption of boiling. Their use also involves the risk of contamination. Packings interfere with quantitative work. The use of an external capillary leak is generally quite satisfactory in prevent bumping. However, this results in a false (low) boiling point and in seriously in the loss of material by entraininent. The loss has been found be appreciable over the period of time required for a precision vacuum fractionation. Agitation is the most satisfactory method, except for the mechanical difficulties involved. The external electrically or air driven magnet will spin the magnet in the flask over a distance of about an inch through an electric heating mantle, an oil bath, or any other heating arrangement free of magnetic interference. The device has been used successfully in this laboratory for fractionations and distillations of a number of high boiling, moderately viscous compounds in the range of I - 5 mm. The dimensions of the stirring magnet(length and bulge) should be such as to provide fairly vigorous agitation.


If magnetic stirring didn't reduce the bumping adequately then the agitation wasn't enough to prevent superheating, you may need a more powerful magnetic stirrer and/or magnetic stir bar ;) In the case of air sensitive materials one can connect a tank of argon or perhaps nitrogen to the capillary tube :cool:




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MagicJigPipe
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[*] posted on 21-3-2008 at 18:07


I heard that simethicone (dimethicone and silica; GasX anti-gas liquid and gelcaps) is used to reduce foaming in many industrial applications. Just as it is used to reduce foaming your stomach (it actually doesn't decrease the amount of gas but decreases the amount of foaming, slowing the rate at which gas is realeased in the stomach).

It's definitely worth a try. Like I said, I've heard it's used extensively in industry and it is readily available OTC.




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Klute
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[*] posted on 21-3-2008 at 20:53


Octan-1-ol is a very efficient antifoaming agent. A drop or two is enough.
Try using a excessively large flask for the first half of the distn, and then use a more adapted one when what is left is clearly too small for the flask and gives irregular take-off. I have noticed that foaming often occurs only during the first 1/4-1/2 of the distn, when using solvants that foam under vacuum, such as toluene.
Was the distn at atm pressure?
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stoichiometric_steve
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[*] posted on 22-3-2008 at 01:06


wow, thanks for all your replies. as stated, i used silicone oil as an antifoaming agent which has helped me a lot in former distillations, but not this time. i suspect there's a lot of toluene left in the compound to be distilled, but it seems just impossible to get it out (except for distilling it out of there).

leu: the stir bar was 1/3 the size of the diameter of the distillation flask, so i guess it was adequately sized and the stirring speed was on max (~1200rpm).

klute: the distn pressure was varied from ~20mmHg to 120mmHg, with no change in the tendency to foam.
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[*] posted on 22-3-2008 at 07:49


Hum, if you really can't get by with distilling, and you think it's the toluene responsible, you could always perform a A/B extraction and recover your amine with DCM or ether...

Wait, you have a rotavap right? Then you could use it by putting the vacuum on/off as soon as the foaming comes up to high, with your finger on vacuum release. The disadvantage is that if your amine in a liitle sensitive or basic enough to form carbonates, it could suffer a little from the air current. IIRC, distilling will remove any co2 trapped. Surely the foaming will stop when enough of the toluene has been removed.
Have you already distilled much? Or does the foamming really prevent any take off?
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[*] posted on 22-3-2008 at 09:40


Sometimes stirring is not enough to control foaming.

A few drops of Castor oil breaks the foam and works best
when there is some aqueous content in the flask.

Castor oil available at the pharmacy works better than cooking oil as it's much higher boiling.

The vacuum release method usually works in the
beginning to break the foam until boiling is steady.
The vacuum needs to be reduced momentarily to a low value. Try a finger on an open bleed valve and
pulsate the vacuum using the finger every time the foam rises.

Otherwise resorting to the Castor oil or high boiling
alcohols is necessary.

Maximum headroom helps a lot, as mentioned, so try filling
the vessel maybe a third full.
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[*] posted on 22-3-2008 at 19:55
My trick


If i have problems with foams i don't ever bother with anything else but this, it takes some setting up but never ever fails.
Double back a glass rod on itself that can be inserted into your vessel opening, just above the top of your liquid level attach a stainless cable tie and trim it to the radius of your flask. This stirrer, for me, has never ever failed in destroying foams as they are created just above the level of the liquid. for an effective simple seal for the stirrer, simply run the stirrer on lowish speed, and wrap teflon tape quite firmly around until it is about the size of your opening. Stop the stirring and drop the stirrer into fitting, to remove just cut the teflon tape plug off.
As i said it may take some mucking around getting it running but you never have to sweat that halfway through your anti-foam will stop being as effective. The teflon tape against the ground glass joint provides a very effective seal and if your stirrer is good can be used under quite a decent vacuum.




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