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Author: Subject: External heating for bath to allow use of magstir?
Fyndium
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[*] posted on 20-11-2020 at 11:41
External heating for bath to allow use of magstir?


How could I employ a larger heating bath with more heating capacity than what is achievable with ordinary hotplate? Are there immersion heaters that could be used directly to the bath so a stirrer without heating can be used?

I tested that if metal is thin enough, it has little to no effect on magstir, but any thicker bottomed vessels like ordinary kettles, and especially magnetic metals prohibit the stir ability.

Is there actual reasonable power limit to magstir? If I would use a fan motor from tabletop fan and attach two very strong neodymium magnets, they would enable to use larger stirbars from longer distances. What I always wonder how weak motors are used for magstir, they almost always need to be turned up in rpm to even start, and struggle to keep up with little resistance, and the magnets are very weak, they sometimes struggle to make contact even in normal circumstances.

All the general equipment are primarily purposed to be used for small volumes like 500mL, but even stirring 2-4 liter vessels render them not powerful enough.

And finally, what is the best method to stir the actual heating/cooling bath?

[Edited on 20-11-2020 by Fyndium]
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monolithic
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[*] posted on 20-11-2020 at 12:51


Probably something like this (or cheaper, from China): https://in.omega.com/prodinfo/immersionheaters.html or https://www.thomassci.com/Equipment/Immersion-Heaters/_/HEET... or https://www.carlroth.com/com/en/microwave-devices-immersion-...

But you would need a PWM or variable AC controller to run it, if it's not integrated into the immersion heater itself, otherwise you would risk burning it out or maybe even localized superheating (autoignition of flammable solvents.) I know they make magnetic stir bars capable of stirring tens of gallons of solution like https://www.belart.com/giant-polygon-without-pivot-ring.html but who knows if it's practical in real life. Imagine what would happen if a huge stir bar decoupled at high speed in a 10,000 ml+ glass reactor and smashed the bottom out, a huge fucking mess. If overhead stirring is feasible it seems the preferred route, especially when you consider the torque capabilities and the fancy overhead stirrer paddle geometries that aren't possible with magnetic stirring https://www.ika.com/ika/pdf/flyer-catalog/201903_Flyer_Mixer...

[Edited on 11-20-2020 by monolithic]

[Edited on 11-20-2020 by monolithic]
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Fyndium
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[*] posted on 20-11-2020 at 13:21


I forgot to mention overhead stirring indeed. I have two PTFE paddle foldable stirrers, but not actual overhead stirrer motor, instead I use my own, which does work well but is somewhat improvised - the biggest issue with it is that it's too powerful. With more rotation speed, it makes the liquid centrifuge around the walls of the RBF instead of just churning it, so I need to order a smaller one.

So far my go-to for distillations has been no-stir with boiling media or ebulliator, and only exceptions have been with air sensitive compounds which I used stirplate because they were capable, but for bulk distillations they hardly reach. For them, I use CaCl2 bath and induction plate, and it can easily reach 160C.

I looked up some immersion heaters and they start from 500W. As it's passive element, it should work with simple dimmer setup. I was hoping there were circular ready elements that could be immersed in the bath around the flask to heat evenly, but I suppose these should be DIY, hence dangerous because insulating can turn to be an issue.

30x150mm stirbar that contains significant core of metal gets so heavy quickly that it indeed can be harmful to glass. 10L reaction volume containing NFPA 4/4/4 stuff at high temp vacuum having a bottom fall out could cause some issues.
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Dr.Bob
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[*] posted on 21-11-2020 at 05:32


You can buy oil baths with a heater built in, Ace Glass sells them, I would guess others do also. They are designed to work with a variac or temp controller, but some can get quite hot. Most immersion heaters are not strong enough and many are designed for water use, so they might overheat in oil. Water has a much higher heat capacity and transfer rate than oil, I believe.

You don't typically have to stir the oil in a bath, as convention does much of that for you. Many people do put paperclips in glass crystallizing dishes used as oil bathes on hot plates. As stated, these are fine for 500-1000 ml rbfs, not much more. For bigger reactions, a lot depends on the temps needed. If the temp is below 100C, a water bath can be easier to manage, or usuing a steam bath (for older chemists that shouold bring back bad memories...) If you want to go much above 100C on a large scale, the best solution on a budget is a heating mantle and some form of controller. Jacketted reactors are spliffy, but costly, and most heat transfer fluids are quite limted in temp range.

For most any reaction above 2-3 L, an overhead stirrer will be needed, hard to stir things bigger with a simple magnet stirbar, unless not viscous and no solids present. If you keep the paddle off od the bottom of the flask, you can minimize glass sracthing, but if solids are there, it will eventually happen.
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Fyndium
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[*] posted on 13-1-2021 at 12:16


I have had high success using overhead stirrer paddle and ptfe bearing joints. I use hand stirrer with turned coupling to rotate the paddle, and it has an excellent speed control and having 500W of power the only issue is that it's a bit noisy. It seems to have hold on for tens of hours of stirring, up to over 8 hours non-stop at a time. I tried to use a floor standing fan motor, and it is powerful enough, but speed control is an issue and with speed controller it hums and heats up and doesn't keep constant stir speed as the power seems to fluctuate.

Now, the question. Are those single stage ptfe bearing seals good for vacuum distillations? Does the o ring keep enough air out so it can be used as such, as this would prove beneficial in vacuum distilling air sensitive stuff that cannot be agitated with ebulliator. I have, although, a bottle of argon gas which I could use and I tested filling air balloons with CO2 for the purpose, but I'm not quite sure about it.
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macckone
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[*] posted on 13-1-2021 at 13:11


Only mercury stirrer seals are really good for vacuum.
There are probably PTFE seals that work but they aren't the cheap ones.
I would expect a single stage o ring is not going to suffice for high vacuum work.
It is probably sufficient for air sensitive stuff that doesn't require a vacuum which is what they are made for.
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[*] posted on 13-1-2021 at 13:23


Would 15mbar be too much to ask? Of course, find it out yourself is the correct answer. Distilling easily oxidized stuff like benzaldehyde required minimum oxygen introduction. Perhaps I should look at the argon ebulliator instead, it is proven and simple technique.
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[*] posted on 13-1-2021 at 18:23


I built a water bath vessel to circulate around the outside of a 5L RBF. I used a pancake griddle for the heat element and hammered out aluminum walls for water boundary. Insulation and wood on the outside for rigidity. The griddle element is driven by a $50 PID controller for beer brewing. Water circulates with a household hot water pump.



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[*] posted on 14-1-2021 at 10:01


https://www.homedepot.com/p/Presto-10-qt-Black-Kitchen-Kettl...

Something like this can be used as a thermostatic oil bath/water bath.

They are generally aluminum casings so you can power a stirbar right through the bottom.
Sometimes you can just stick a regular stir plate right under the thing.

Their thermostats are pretty good in most cases, and they generally go up to over 200 C.

The shallower electric frying pans are deep enough for most uses, but the deep fryer models like this with the higher sides also provide a bit of extra protection for working under vacuum.

These can often be found used and in great shape at used appliance shops, garage sales, and whatever they call fundraising sales for charities in your neck of the woods.

Here's one of those shallower electric frying pans:https://www.homedepot.com/p/HomeCraft-36-sq-in-Black-Non-Stick-Electric-Skillet-HCSK6BK/314835466




[Edited on 14-1-2021 by SWIM]




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[*] posted on 14-1-2021 at 11:14


boiling benzaldehyde does not require a vacuum.
The amount of benzoic acid created from the oxygen in a 50ml flask is going to be negligible.
The stirrer with one o ring is appropriate if you just don't want to get more oxygen in.

A slight vacuum will hold a 15mm of mercury, probably not.
Get a double o-ring or better stirrer or a mercury stirrer.
Otherwise do what everyone else does.
Use a beaker and put a stir bar in the beaker.
If you are doing really large runs of benzaldehyde then you can probably spend the money on a mercury seal stirrer or a better ptfe one.
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[*] posted on 14-1-2021 at 11:16


ps. by putting a stir bar in the beaker used as a heating bath, the bath gets stirred and it helps couple to the boiling flask.
If you need something bigger the same trick applies with cheap thin aluminum pots.
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[*] posted on 14-1-2021 at 11:27


Smallest flask I own is 500mL. I usually carry vac distillations in 1L 3n flask though to allow for splatter headspace. Anyway, I find vacuum distilling a much more convenient than putting almost 200C of heat, especially when my heating appliances pretty much limit to 150C.

I currently use 10L ikea kettle as my heating bath for the larger flasks 2L and above, with an induction hotplate. The volume allows for adjustments, including on-the-go turning into a cooling bath. A smaller kettle of 4L is used for smaller flasks, and for certain low intensity reactions I use just pyrex baking dish as heating bath with stirplate, with the benefit I can see through it.
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[*] posted on 14-1-2021 at 13:00


Wow how big flasks do you use for vacuum distillation? I know there is some secure limit in flask size as the force is proportional to the surface of the flask. Of course different manufacturers = different glass and different wall thickness.



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[*] posted on 14-1-2021 at 16:28


For that size distillation, a soft sided heating mantle (like Glascol brand) works best, they can be controlled well via a variac or temp controller, they don't bother your magnetic stirring, and they keep the flask insulated, so the flask is easier to heat. And if by some chance your flask breaks under vacuum, they help keep the glass contained. Oil and water baths work OK for smaller sizes, under 1 L, but above that, I prefer mantles, plus less mess with a mantle. A friend is doing 5L reactions with distillation as I type with a 5L heating mantle and a variac, they are working well at 120 C or so (takes about 58 volts to get there). You always want to heat those size flasks slowly and gently, so start at 20 V and slowly ramp it up by 10 volts at a time, as you watch the temp. I have overheated a few reactions, and it can make a real mess.
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