captainC11
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smaller rbf flask in larger heating mantle
Hi guys would say a 500ml rbf flask be ok to use in a 3L heating mantle ? any problems etc? this for refluxing and distillation purposes
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XeonTheMGPony
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pushing it a bit on efficiency terms of heat transfer, but it will work if you use a lid, you'll need be very careful with heating power as not to
overdrive the heating wire in it, so a very very very slow ramp up of the heat and crap load of patience.
Basically you're using the mantle as a hot air bath.
I use the general rule of 2 size difference, so for my 500ml mantle I will go as low as a 150ml max, 2L mantle will go as low as a 500Ml one. This
keeps the dead air volume moderately in check, so changes in out put of the mantle translates a bit better (faster) changes to the flask.
I all so embedded a thermal couple into the mantle it self to measure its temp directly to ensure I do not over heat it too much.
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Dan Vizine
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The best solution is to fill the empty space with sand.
With that much "dead" space, thermal lag will be the worst side-effect.
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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Ubya
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Quote: Originally posted by Dan Vizine  | The best solution is to fill the empty space with sand.
With that much "dead" space, thermal lag will be the worst side-effect. |
sand produces a big thermal lag, plus if you ever are in neet to quickly lower remive the heating mantle (with a lab jack for example) the sand would
collapse making it hard to reposition the flask
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greenlight
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Although not heat efficient, I use my 1000ml heating mantle for every size down to a 50ml round bottom once for a small distillation.
As long as the bottom of the flask is touching the mantle it works just fine apart from losing a lot of heat to the air.
You can always shroud the flask and mantle with al foil to increase efficiency.
[Edited on 27-1-2019 by greenlight]
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macckone
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BBs are nice small round and if lead, not likely to scratch glass.
But as others have noted are not ideal if you need to remove the flask and replace it.
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Sulaiman
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When using a 500ml rbf in a 500ml heating mantle I clamp the flask to not touch the mantle
on the assumption that this gives the most even heating of the rbf
- mainly by infra red.
CAUTION : Hobby Chemist, not Professional or even Amateur
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captainC11
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would a 500ml fit nicely in a 2L mantle then no heating problems? assuming it touches the bottom .. anyone have problems refluxing on a hot plate with
a flat bottom flask for say 48 hours or so?
cheers
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XeonTheMGPony
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500mL will fit nicely in a 500ml mantle!
Any thing ells is sub optimal with thermal lag and require care full temp control to not over shoot or cook the mantle.
It can be don, but you'll need to take good diligent care to get it to a stable temp and reflux as if it where a simple hot air bath
As for the hot plate some aluminium foil and some due care will be fine, pending the quality of the hot plate naturally. I use a thin sheet of mineral
wool my self jacketed with aluminium foil.
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Dan Vizine
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| Quote: Originally posted by Sulaiman | When using a 500ml rbf in a 500ml heating mantle I clamp the flask to not touch the mantle
on the assumption that this gives the most even heating of the rbf
- mainly by infra red. |
Hi Sulaiman,
You should let the flask touch the mantle. Mantles really don't even like to be operated without touching a medium which conducts heat away.
Even-heating is easily assured by adequate stirring.
Dan
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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BromicAcid
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Air is not a very good conductor of heat. If you're patient this isn't a problem but if you're not you will ruin your mantle. Turning up a variac
all the way will make the entire mantle glow a dull orange, even before the glow though you can start fusing the glass wool in the mantle. I run
across mantles all the time that are 'glassed' where someone had likely used a flask that was too small and tried to overcome the poor heat transfer
with more heat. Sand isn't a good option, copper shot could be.
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SWIM
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Some mantles are MUCH more flexible than others.
Most of my Glass-Col mantles are stiff little bird's nests with no give at all, but a few are much floppier and can 'mold' to different size flasks to
some extent. Those ones are much flatter and shallower than the 'bird's nest' models.
I've got a garbage can full of Glass-cols I need to test some time soon and I believe there are some soft-sided ones in there.
I'll check the serial numbers if I find any.
Sometimes you see adaptor blocks on Ebay which let you use a smaller flask in the mantle.
I think Opti-therm makes them. Little metal hemispheres.
They probably don't transmit the heat as well as a flexible surface though.
I believe they are limited to use at moderate temperatures (<200C).
There's somebody on here who uses copper plated steel air gun ammo (.177) as a shot bath for small flasks in big mantles.
That's a mighty cheap way to make a shot bath, and it was reported to work well.
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teodor
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I try to used 1L mantle for 250ml flask so I've tested 3 materials to fill the gap: small glass balls, pieces of steel wool wrapped in aluminium foil
and a iron chain from "Praxis" shop (this is a network of hardware stores in the Netherlands). I found that the chain is the best as with thermal
conductivity and with how easily/accurate and fast it could be got in and out of the mantle (actually to fill the volume I use 3 pieces of chains but
it is possible to connect them).
Alas, I didn't find true copper chain in those stores. All chains in the store I tested with a piece of magnet and I didn't find any not magnetic
chain. So, I have concern that it will make unable to use a magnetic stirrer with the mantle, at least with flasks < 1L.
[Edited on 8-12-2020 by teodor]
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itsallgoodjames
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I have a 1000ml heating mantle, and use it for flasks all the way down to 100ml. Just insulate it with aluminum foil (assuming you're not distilling
bromine or something) and there shouldn't be any issues
[Edited on 8-12-2020 by itsallgoodjames]
Nuclear physics is neat. It's a shame it's so regulated...
Now that I think about it, that's probably a good thing. Still annoying though.
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Sulaiman
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Using an undersize flask I would expect the operating temperature of the heating wire in the mantle to be above the autoignition temperature of most
flammable solvents,
so in this circumstance you need to treat the mantle as if it is an open flame.
(similar to the high temperature required of a hotplate to get a useful flow of heat through a sand bath)
A 500ml heating mantle costs less than a solvent fire 
you could diy a 500ml heating mantle using your 3l mantle as thermal insulation,
just a cheap mantle heating basket/element/liner, a lid for the 3l mantle with a hole for the 500ml mantle, plug, wire, 'dimmer' and diy.
CAUTION : Hobby Chemist, not Professional or even Amateur
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B(a)P
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| Quote: Originally posted by teodor |
Alas, I didn't find true copper chain in those stores. All chains in the store I tested with a piece of magnet and I didn't find any not magnetic
chain. So, I have concern that it will make unable to use a magnetic stirrer with the mantle, at least with flasks < 1L.
[Edited on 8-12-2020 by teodor] |
Why not use something like this?
Copper mesh wool
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teodor
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Good point, Sulaiman. Especially because I thought about making diethyl ether. So, I definitely will use 1L flasks in the mantle for this purpose.
This mantle has 2 zones, so it could be only half-heated for smaller flasks.
There are mantles which are qualified for several sizes. Actually I bought one which should be 250-500-1000ml and 3 zones. I don't know whether that
model had some adjustable heating surface or not, the seller sent me this one marked as 1L saying it should be OK to use with 250ml. Because I don't
see an alternative for the same price (it was a second-hand) and found the solution with metal chain working very good (excepting probably the point
you mentioned) I think to keep this one.
Good idea, I should try that. Also I will try copper chain and this: https://www.kentronmicrobiology.nl/nl/webshop/lab-armor/lab-... .
But comparing with steel wool the steel chain transfers heat MUCH MUCH better, so I think the diameter of wire (and the mass = pressure) also matters.
So, it will be interesting to compare copper chain and copper wool.
[Edited on 9-12-2020 by teodor]
[Edited on 9-12-2020 by teodor]
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Sulaiman
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I have been using a 5l rbf and mantle with 24/29 columns and condensers lately,
(previously 500ml with similar columns and condensers),
the 10x higher pot volume makes fractional distillation of larger quantities of solvent more convenient.
A heating mantle with 2 or 3 'bands' (eg lower, middle, upper) would be nice because I found that once the level of liquid in the pot drops below the
level of the heating element, the upper part of the rbf can get VERY hot.
Hot enough to heat up the thermometer adapter and thermometer to give a falsely high indication of vapour temperature, and in one case hot enough to
explode a thermometer.
(I bought a ptfe thermometer adapter due to this problem, and I just sealed a thermocouple in a 7mm glass tube to replace the thermometer... not
tested yet)
The facility to heat only the lower part of an rbf is really useful, even if only for the single rbf size.
CAUTION : Hobby Chemist, not Professional or even Amateur
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