Fractional Distillation

[10fingers]

In a lot of synthesis it calls for the use of a Vigreux column to do fractional distillation. My question is, will it work just as well to take a standard condenser and fill it will broken glass or some stainless scrub pad and put this in your flask vertically?

[Mr Cool]

I use an air condenser full of crushed glass. Although I actually can't say if it works or not because I have never compared it to an identical distillation carried out without it.

[ALENGOSVIG1]

Ive heard of people filling a liebig or west condensor (or similar) with glass beads, and then stuffing glass wool in the bottom to prevent the beads from falling into the flask. Ive never tried it though so i dont know how effective it is.

[NERV]

From my experience a real Vigreux column is best because it has more air space to allow the gas to go through more smoothly. Although my cheap glass chip filled condenser worked well enough for me before I got my Vigreux column.

[Mr Cool]

I wouldn't use glass wool to keep the filling in place, IMHO it'd constrict gas flow way too much when soaked in whatever you're distilling.
The ID of a typical quickfit condenser reduces towards the end, what I did was find a bit of glass that would fit all the way down but then get stuck near the end, then this stops all the rest from falling through. This way only about 10% of the cross-sectional area is lost (a rough guess, judged by eye), so we still have 90% of the gas flow and no absorbent stuff to interfere.

[rikkitikkitavi]

actually filling a column with glass beads or small pieces of glass creates a separating coulmn of much higher efficency than a vigreux column, since the contact surface liquid - gas is larger.

Higher efficency means higher separation off high-boiling component and low-boiling compound, same as extending the length of the column.

However, pressure drop can be a problem, but not very likely causing any problem on laboratory scale, only industrial scale. becasue for most laboratory distillations gasflow is small

If problems with pressure drop arise (you have a higher pressure in the boiling vessel than the condenser and reciever ) , slowing down evaporation rate takes care of this.

/rickard

[Einsteinium]

actually filling a column with glass beads or small pieces of glass creates a separating coulmn of much higher efficency than a vigreux column I don't think so, there are no condensation plates in a glass-packed column thus the fractionnement isn't as good as with a Vigreux. Think about it, if a glass-packed column is better, why ALL serious laboratories use a vigreux instead of a glass-packed? I know for a fact that a glass-packed works quite well though and may be used as a substitute if you can't find a decent vigreux. For small quantities I used a glass-packed Claisen adapter with sucess before I got my vigreux.

[rikkitikkitavi]

true that there are no discrete disillation plates (or theorietical plates, bottoms ) in a glass packed column, but doesnt have to be. A theoretical plate is a mathematical abstraction used in calculation of gas-liquid contactors, and represents an equlibrium step between the gas and liquid.

The reason for large industrial columns are designed with plates is cost (cheaper than pall rings or similar fill bodies ) or weight, imagine a 2m wide, 30 m tall tower filled with 5 mm glass beads...

important is to increase the contact surface, the more surface the more theoretical plates. And thus better separation per height of column.


/rickard

[Einsteinium]

You don't need to tell me what is a theoretical plate... The need for discrete distillation plates is important as there is not enough free space in a packed column to ensure a good separation of the fractions. It's understandable that it would cost a lot to pack an industrial column with glass beads but then why no laboratory scale 30cm Vigreux are packed with glass if it's more efficient? Your argument just dosen't apply here. Surface area does promote condensation/evaporation cycles but still all columns listed in my Aldrich catalog has discrete plates and not much surface area compared to a glass-packed column.

[John Ashcroft]

Einsteinium, I recommend reading a o-chem lab manual.

Rikkitikkitavi is correct about condensors stuffed with, say, glass beads being much more efficient than vigreux columns. The reason why vigreux columns are more often used is because of less hold up. Not nearly as much liquid gets held up in the vigreux column as in a packed column.

[Arthis]

What would you use to stick glass pieces on the condenser ? Need something special if you're planning to distill, say, nitric acid...

[Mr Cool]

Don't stick anything, just arrange it so that everything is supported by something beneath it.

[Spudkilla]

Sorry for bringing up an old post, but when you mean stuffing the condensor full of glass pieces, are the glass pieces from say, a broken drinking glass? Do you just buy a window and smash it, and jam the pieces in the condensor?

[Arthis]

You take pieces of glass, from where you want. Smash a window or a glass, that doesn't matter. You just use them to act like the glass pikes in a Vigreux column.
You jam the pieces in, trying to make them hold altogether when the column is vertical.

[nbk2000]

If what you're distilling won't effect it, you might want to try using silica gel grains. They're nice and round little spheres of (essentially) glass and come in all sizes. Best of all, you can get them for free from the pharmacy.

Just tell them you want them to save the dissicant capsules that come in the pill bottles for flower drying. :) I've got bags and bags of the stuff, as well as activated carbon, which is in the 2 in 1 canisters.

[Arthis]

What will be the effects of those small spheres ? You have a larger contact area, but it will absord a lot of product, won't it ?

Moreover, it will be damn more difficult to make them hold, and you risk overpressure in the boiling flask...

[ALENGOSVIG1]

As I said at the beginning of the thread, the glass beads can be held in place with glass wool.

And as for them holding alot of product, they dont really. And besides, its usually good practaice not to collect the first and last few ml of distillate. Ofcourse you could always wash the beads with a solvent, and let it evaporate to recover the goodies. :)

[Imperial]

I have an idea for those who cannot be bothered with fractional distillation.

Instead, just heat the mixture to the boiling point of one product and collect using a standard distilling apparatus. Then, collect the next highest boiling point component. Continue until you have separated your mixture.

This is probably the best method for things like methyl and ethyl alcohol which have far apart boiling points, but for things like azeotropes (which are a pain in the arse as far as separation goes), fractional distillation will be the only way. And for that, the glass beads (or silica beads) work best in my experience.

[Anthony]

Methanol and Ethanol *do* form an azetrope.

If you try and collect the seperate fractions using a series of normal distillations, you will end up with them all contaminated. It doesn't work by going "ohh, the mixture is up to 60*C now I'm at my boiling point, myself and all my fellow atom/molecules must jump out of solution and nothing else shall".

[Imperial]

Of course methanol and ethanol form an azeotrope, but that is only when they are mixed in a certain proportion. For this reason, unless great purity is required (which is the case a lot of the time, I admit), normal distillation should work, unless they are mixed in that proportion.
Of course, as one liquid evaporates, then the proportions of the liquids may change enough to form an azeotrope, but this isn't usually the case.

Another factor is how far apart the boiling points are, as I said before. If one liquid boils at 50ºC and another at 120ºC, then not much contamination should occur, for the molecules of the other liquid simply will not have enough kinetic energy (except for a select few) to vaporise. And those few that do will probably condense again before they can contaminate anything.

This is why with mixtures such as the example above, fractional distillation doesn't need to be performed. For mixtures which have boiling points which are close together, or azeotropes, fractional distillation is really one of the only sure ways to separate them.

[Anthony]

But you've just summed up a simple, long held rule of thumb in a whole paragraph - pot still for mixtures with large differences in BP, fractional distill for mixtues with close boiling points.

Your original line was: "I have an idea for those who cannot be bothered with fractional distillation"

So your advice was "don't do fractional distillation when you don't need to". Sure it'll save some members the time and expense of running a fractional still to concentrate H2SO4, but it's hardly a relevation to the other members who have two or more brain cells to rub together...