glass wool to control bumping during vacuum distillation

Lief · Mon Mar 14, 2005 6:09 am

An interesting page on the use of glass wool for controlling bumping during vacuum distillation:

http://home.ripway.com/2003-11/39191/pdfs/GlassWool.pdf

methyl_ethyl · Riedel De Haen

Hmm, are boiling chips useful for vacuum distillation? I have never added glass wool directly to a distillation flask, however does this not take the place of boiling chips? If it does, what are it's uses in vacuum distillation? this topic is new to me, so please bear with me, it sounds odd to me, I guess I need to be educated in the theory behind adding glass wool to a distillation flask........

On a similar note, Newbees performing vacuum distillation, have complained about "condensation" before the their condensor, i.e. on the upper most area of the distillation flask, the area not in contact with the heating mantle or oil bath and even the joint that connects the condensor to the distillation flask.

Glass wool is an excellent insulator that if applied to these areas of unwanted condensation, can greatly improve the efficiency of your distillation. I have heard of those that have destroyed their wanted anayte by just cranking up the heating mantle because it did not look like their product was getting "hot" enough to reach the condensor. Always try to optimize your experiment with regards to your target analyte, and make sure that you have the know how, and understanding pertaining to the reaction you are performing to further trouble shoot any problems, without the needless destruction of your target compound.

Take your time, learn the mechanism, and enjoy the ride.....

much_love,

methyl_ethyl

IndoleAmine · Dreamreader Deluxe

Violent magnetic stirring at maximum speed is also said to be an excellent method of preventing bumping during vacuum distillation *hint*hint*hint*

Mr. Green

i_a

icecool · Insistent Chemist

How are you going to warm it then while you are stirring?
I mean you can't use a hot oil bath or a water bath or heating mantle.
Wouldn't it be easier just to use a cappilair?
It is very simple just a very thin glass tube underneath the liquid...

Spacemonkey · Wed Mar 16, 2005 6:34 am

What you are talking about icecool?
Have you not heard of a magnetic-stirrer hotplate?

monkichi · Wed Mar 16, 2005 6:52 am

In a smooth container with no nucleating points, a liquid can be heated above its boiling point without boiling. Eventually, a vapor bubble will form. The bubble will grow rapidly in the superheated liquid with a 'bump'. To prevent bumping, nucleating points are added in the form of boiling stones or glass wool. Sharp points and edges on the boiling stones trigger bubble formation before significant superheating occurs. Perhaps rapid stiring has a similar effect.

loki · guinea pig

i've heard of these cappilary thingies - i think the idea is that post-boiling components of the liquid phase quickly aggregate when they strike the surface of the gas. thus a capillary tube immersed in the liquid such that it draws gas from the non-liquid filled part of the apparatus (or maybe it's from outside it, i may have this wrong) provides a very slow but constant source of gas within the hot liquid which promotes bubbling.

rapid stirring, on the other hand, i think its anti-bumping effect simply comes from distributing the superheated liquid more widely in the liquid, allowing greater transfer through the surface of the liquid, and limiting the formation of areas of superheated liquid which can very rapidly and suddenly form a bubble, known as a 'bump'.

as far as I know, the way that boiling stones and glass wool would decrease bumping problems would be by harbouring miniscule amounts of gas which, upon heating, slowly leak out from it and provide bubble nucleation surfaces for the superheated liquid to escape into.

monkichi · Fri Mar 18, 2005 4:58 am

Loki: I think you're correct. I was thinking of nucleating crystals. Does this mean that degassed boiling stones would have no effect? In a superheated liquid, what else could trigger bubble formation?

methyl_ethyl · Riedel De Haen

Vitriodor · Sat Mar 19, 2005 7:18 am

Somehow I wasn´t able to incorporate the picture. Maybe it is size related. Anyway, you can find some additional notes on bumping and the use of a capillary at this link:

http://img103.exs.cx/my.php?loc=img103&image=bumping8yg.png

IndoleAmine · Dreamreader Deluxe

Capillary tubes have the drawback that a very thin inlet is needed, which makes these damn things very fragile. Besides, you need a very good vacuum source, an aspirator often won't suffice when using a capillary.
They are mostyl used for distillations of volumes of 5L and more...

Mag. stirrers on the other hand have the advantage that they are essential tools in any laboratory anyway, magnetic stirbars come in all shapes and also in ENORMOUS sizes if you look aroud a bit, and with magnetic stirring the apparatus remains sealed - would you want to set up a protective gas argon/xenon bottle, regulator valve, hose, capillary etc., each time you distill a oxygen-sensitive compound?? And most mag.stirrers have hotplates capable of delivering at least 400W heating power (mine has 600W), combine that with a big aluminum cooking pot filled with peanut oil, a second stirbar to mix the heating oil, and you have a PERFECT vacuum distillation setup, capable of distilling everything with volumes up to 2000ml !without bumping!, even at aspirator vacuum (12mmHg), and this should suffice for most of our purposes I would think. Very Happy

For industrial plant operators, the capillary is the method of choice though... Laughing

Moncichi wrote:

MargaretThatcher · Sun Mar 20, 2005 1:33 am

IndoleAmine · Dreamreader Deluxe

Whoops! No, of course I still consider you my personal friend, and haven't forgotten anything! Why do you think I wouldn't like you? *hug*

I was just wondering how someone can question the usability of magnetic stirring as a method to control bumping during vacuum distillation.

Hence my statement, which has been quoted by you. Why?

And to keep it to the topic: superheating is a phenomenon that can only occur in liquids wich are a) heated too fast and b) not mixed well enough. This results in partial overheating and sudden bubble formation, which is the reason for bumping.

When you stir well enough, the heat is always distributed evenly, and superheating is very unlikely to occur. Very simple.

Another reason for superheating and subsequent bumping can be widely differing boiling points of mixed liquids - another problem which is solved through good mixing and even heat distribution.

Sorry if I sounded arrogant or something like that, but I thought you all knew that.

i_a

MargaretThatcher · Thu Mar 24, 2005 5:01 am

Sorry, I just had visions of the old Hive egomania, which seems to have been a mistake in this case.

As I understand, below a critical radius, the thermodynamic tendency is for bubbles to shrink. This is why liquids superheat. It is probably something to do with surface energy compared with vapour pressure. So long as voids larger than this critical radius are present, bubbles can form and grow. These voids may take the form of trapped gas pockets or capillary bubbles.

I imagine that various things can trigger bubble formation in a superheated liquid. E.g. a bubble chamber is triggered just from ions. Maybe sharp points can too. Clean liquids can certainly be superheated to a greater extent than dirty ones.