Sciencemadness Discussion Board - the proper use of heating tape on a vigreux type column...

the proper use of heating tape on a vigreux type column...

stoichiometric_steve - 3-1-2008 at 06:44

please help me on this one:

i want to fracionally distill an organic compound, yet i have no idea what temperature to choose for the heating tape wound around the column. right at the compounds estimated bp.?

kmno4 - 3-1-2008 at 08:19

Using heating tape has the same purpose as isolation of column.
Do you really have/want to use this tape ? What is bp ?

chemrox - 3-1-2008 at 23:11

I'd like some discussion on this too. I was discouraged, here, from using one. I undertsnad the idea is to maintain the column at the temperature of the vapour rising from the still pot. I was told a shitty separation was from runing the column too hot. However a column with an evacuated, silvered jacket would accomplish the same thing as having a heating tape or wire at the temperature of the vapour wouldn't it?

evil_lurker - 3-1-2008 at 23:34

The #1 killer of efficiancy on fractional distillations is the use of a hot plate instead of a mantle.. there is absolutely no way for the column to equalize worth a shit with the heat constantly cycling on and off.

Also, vigreaux columns have a much much lower height equivalent to theoretical plate ratio vs a properly packed hempel column.

I've pretty much come to the conclusion that in most instances heating tapes are unecessary and all that is needed is some decent pipe foam insulation... in fact, some cooling is actually beneficial as that helps to generate reflux.

And you cannot run a column too hot, only too fast. Faster you go, the lower the efficiancy. Only way to counter that is to lower the heat or increase the width and length of the column.

stoichiometric_steve - 4-1-2008 at 01:09

i'm limited to using my 60cm vigreux column (lacking the funds to buy a packed column, which also has much more holdup - i'm distilling 50-100ml quantities).

the compound has an approximate bp of 260°C@760mmHg. i'm distilling it in vacuo so it comes over at around 115°C. there is a lot of heat loss in the column, since it only has a simple glass jacket fitted with two rubber o-rings - hence the use of heating tape.

i have a lot of glass wool which would make some nice insulation, yet i'm trying not to work with glass wool due to the fiber dust being quite some health hazard.

evil_lurker, you can in fact run a distillation "too hot" (not a column in particular) - i have had one incident where i cranked the heat up too much, to the point where the covalent bonding energy in the compound was lower than the heat of vaporisation. this ended up with distilling over a copious amount of very, very expensive tar

solo - 4-1-2008 at 04:43

There is an interesting discussion on the use and modification in using this type of distilling column ............solo

http://www.sciencemadness.org/library/books/Research_Techniq... -

stoichiometric_steve - 4-1-2008 at 06:25

thanks, solo! this really is a useful document. the grignard soxhlet method on page 55 rocks, especially if you want to prepare e.g. BenzylMgBr.

MagicJigPipe - 4-1-2008 at 11:20

So, what if you have a very expensive, well made hotplate that creates very stable temps?

Nicodem - 4-1-2008 at 11:39

There is no such hotplate as far as I'm aware. Hotplates have an on/off regulation, hence there is no way for them to maintain a stabile temperature of the plate. The best you can have is a hotplate magnetic stirrer with an oil bath equipped with an external thermostat sensor. These are however much more expensive than the simple ones without an external sensor, and they still run on an on/off regulation resulting in an deltaT of about 10K (though depending on the model). This is still a much higher deltaT that can be achieved by simply using a triac rheostat or other continuous current regulation like the ones on the heating mantles.

evil_lurker - 4-1-2008 at 12:52

Quote:

Originally posted by Nicodem
There is no such hotplate as far as I'm aware. Hotplates have an on/off regulation, hence there is no way for them to maintain a stabile temperature of the plate. The best you can have is a hotplate magnetic stirrer with an oil bath equipped with an external thermostat sensor. These are however much more expensive than the simple ones without an external sensor, and they still run on an on/off regulation resulting in an deltaT of about 10K (though depending on the model). This is still a much higher deltaT that can be achieved by simply using a triac rheostat or other continuous current regulation like the ones on the heating mantles.

Think of it like this... distillation removes heat. By the time hot plate's thermostat realizes that the temperature has cooled down due to all the oil, glass, and metal that interferes with efficiant heat transfer, the distillation rate has slowed.

As the hot plate thermostat cycles back on, by the time the flask gets the heat the hot plate has thrown more heat than necessary to bring the flask back to temperature, resulting in a "surge" effect.

stoichiometric_steve - 5-1-2008 at 10:18

Quote:

Originally posted by evil_lurker
As the hot plate thermostat cycles back on, by the time the flask gets the heat the hot plate has thrown more heat than necessary to bring the flask back to temperature, resulting in a "surge" effect.

i have been using a hot plate with oil bath for this distillation, which has a socket for a thermometer that you can dip in the bath and have your temp regulation this way. yet i havent checked how much the "overshoot" was when the hot plate cycled on heat.

i have also used a heating mantle (PILZ brand), hooked up to a regulating device which modulates power by switching the current on and off at defined intervals, corresponding to the 1-10 dial setting on the device. i had to use this since the 3 level switch on the mantle delivered either too much or too little heat.

i guess either setup is faulted with temp fluctuation.

evil_lurker - 5-1-2008 at 11:24

Get on Ebay and buy a Glas-Col Series O heating mantle and controller... you can usually find them in good shape quite cheap ($30-75).

Hell, this last week I got a 12L TM series mantle brand new for $150... they normall list for around $550-600.

chemrox - 8-1-2008 at 21:38

I don't know how hot plates crept in to the discussion, I must have missed the beginning sentence. I do believe that an oil bath heated by a hot plate will work out to being a better heat source, thpugh messy and inconvenient, than a mantle. Reasons: the oil can hold a nearly constant temp while the plate cycles .. buffering the temperature, so to speak, by way of its heat capacity. The oil bath provides nearly uniform heat to the flask whereas the mantle will have hotter and cooler zones, nearer or away from the heating element. I had a chem prof who believed so sincerely in them we were required to use oil baths in ug organic classes and he tried to convince us to use them later on. He had a killer Podbielnak column he made from tubing and cork.

I wanted an opinion on Snyder columns while we're talking efficiency. Seems like a bubble plate column in essence. How much separation per cell? cell = plate?? This is P Chem or ChemE or something equally obscure to me.

Solo's reference, cited above, offers these observations on distillation columns.

"Since any effective distillation requires that a close to true equilibrium
exists between the boiling liquid and condensing vapors, it is important to
minimize external heat losses in the system. This is most easily accomplished
by enclosing the fractionating tube in a vacuum jacket and placing the
jacketed tube in a second jacket that can be heated electrically. After the
liquid is refluxing steadily in the column, the external jacket temperature
should be adjusted to about 1-2° below the reflux temperature. If the column
floods at the bottom, either the boil-up rate is too great or the column jacket
is too cold. If the column floods at the top, the column jacket is probably
too hot. Another common source of flooding with packed columns is the
presence of a constriction due either to improper packing or to a rearrangement
of packing caused by bumping or a prior flooding condition."

[Edited on 8-1-2008 by chemrox]

Sauron - 9-1-2008 at 00:09

It is my view that the first best choice for any fractionation column is a vacuum jacketed and silvered one, regardless of the type of column (Hempel, Oldershaw, Snyder, Vigreaux).

This removes all variations due to good/bad technique of lagging. Therefore, the operator's attention can be focused on the other vital parameters of the still design and use. These include

Proper matching of heat input to condenser and cooling capacity
Proper pot size for column length and diameter (and that varies with type)
Reflux ratio
Boil-up rate
And time to allow system equilibration at 100% reflux before gingerly reducing the RR and starting to take off lowest boiling fraction.

I would think that even a cursory review of the theory and practice of fractionation would clearly indicate that if an external heat input is to be made to the column, in lieu of normal cacuum jacketing or insulation lagging, the temperature of the heating tape or cord needs to be less than or equal to the LOWEST BOILING compound in the mixture, as determined by analysis or preliminary testing. Preferably, lower than that. Otherwise, there cannot possibly be any equilibration. The still head will be overloaded with vapor. The desired liquid-vapor continuum cannot be achieved in the column and thus, seperation will suffer.

So I would run a tape or cord at 10-20 C less than the bp of the lowest boiling component. But I would still rather do without the tape and either use a vac.jacketed column properly packed, or else a well lagged unsilverered column.

The use of a heating tape makes sense if your mixture is high boiling even under reduced pressure and your are operating in a draft, particularly if a silvered vac.jacketed column of suitable dimensions is unavailable or prohibitively costly - as they often are. I have a 1 meter, jacketed silvered Hempel of 29mm ID that I bought on LabX for $70, but a new column like this would cost me several thousands of dollars. Oldershaw columns are even worse cost wise.

I'd be inclined to spand a little money on a upper spherical mantle or a safety shield/poncho, any of which will prevent excess reflux from the upper part of the pot. Inexperienced operators often just compensate by upping the mantle temp, to their regret - you ALWAYS want the MINIMUM heat input that gets the mix to the top of the column eventually under 100% reflux, and good lagging.

I hope these remarks are helpful. Entire books have been written on this subject and at least one excellent one has been posted on this forum previously, UTFSE. Doing fractionation without studying it is like trying to do brain surgery by reading a Classic comic book.

kmno4 - 9-1-2008 at 09:21

BTW:

Heating tape (cord or so...) does not set any temperature. "Temperature of column" changes continuously, in batch processes, during distillation - such warming supplies only heat, losing via natural convection (mainly). It is obvious. The longer column, heat loses are the bigger but these loses cannot be compensated by increased warming of RBF ( because lower part of column changes into steam cone - length of working part of columd decreases). Of course, losig heat is sine qua non of practical fractional distilltion but in the case of too long column, vapors "get stuck" in the middle of it and heating/insulating is needed.

Magpie - 10-7-2013 at 18:19

One of my goals is to gain the ability to perform difficult separations via fractional distillation. Several times I have needed that ability for alcohols and esters with bps, say, 5-10°C apart.

One of the most important contributors to success is having a sufficiently high reflux ratio. At present I do not have a distillation head that would provide control of this ratio. HS Martin makes such equipment but it is horribly expensive. The image below shows one such example:

Perhaps with a clever arrangement one could jury rig a still head along the same lines using condensers, stopcocks, and such.

Has anyone else been thinking along these lines? What are your thoughts? What have been your successes?

[Edited on 11-7-2013 by Magpie]

[Edited on 11-7-2013 by Magpie]

bfesser - 10-7-2013 at 18:29

Have you considered a <a href="http://en.wikipedia.org/wiki/Spinning_band_distillation" target="_blank">spinning band distillation</a> <img src="../scipics/_wiki.png" />? (Sorry, I don't have anything at the moment that can open a .doc file.)

Magpie - 10-7-2013 at 18:38

I'm actually open to any kind of equipment that will allow difficult fractional distillations. I know that spinning band is probably the ultimate for column design. The design with the little ball floats (Snyder) is also quite good, I understand. Right now I'm just concentrating on trying to get good control over the reflux ratio as I know that it can be of prime importance.

Sauron has written profusely on this subject and seems to speak from experience. Some of this is upthread. Some is found in other threads.

Don't you have Word?

[Edited on 11-7-2013 by Magpie]

[Edited on 11-7-2013 by Magpie]

bfesser - 10-7-2013 at 19:12

Well, it's customary to provide an image as an image file, not a Word document.

Also, no, I don't have Word. This laptop came with Windows 8 (GAH!), and I haven't downloaded OpenOffice (yuck) yet.

adamsium - 10-7-2013 at 20:05

Here is the image, bfesser (and others):

Magpie - 10-7-2013 at 20:08

Yes, thank you adamsium. I will go back and edit my original post. It seems I'm still learning about these things.

watson.fawkes - 11-7-2013 at 05:37

Quote: Originally posted by Magpie

Right now I'm just concentrating on trying to get good control over the reflux ratio as I know that it can be of prime importance.

Yes. The separation ratio in the column is different issue than the reflux ratio accomplished in the still head.

There's a good treatment of this in the book The Compleat Distiller, which I mention specifically because it's targeted at people building their own gear (because they focus on alcohol production). The mention three ways of doing this: management by vapor, liquid, or cooling. Vapor and liquid management mechanically divide a fluid flow into two flows. Cooling management splits the incoming vapor flow into a fluid flow (refluxed) and a vapor flow (passed on an condensed). All this is tantamount to saying that you can build such a device before, within, or after the reflux condenser.