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Author: Subject: Vacuum Oven vs Kiln for Molecular Sieve Drying
nimgoldman
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[*] posted on 10-7-2019 at 12:19
Vacuum Oven vs Kiln for Molecular Sieve Drying


I use plenty of 3A and 4A molecular sieves, mostly to dry alcohols.

Unfortunately, the reactivation is an ordeal as I can use only a 2L heating mantle for that - I usually VERY slowly heat it up to prevent melting of the glass and even then it takes few hours until last droplets of water escape. Then I pump out the air which releases more water. The whole procedure is very time consuming. I though about running dry nitrogen or argon slowly through the flask to drive the vapour off to speed things up a bit.

I wonder if one can use kiln instead as this provides high enough temperatures for drying sieves at atmospheric pressure (300 °C and more) and many kilns can be programmed for convenience.

Vacuum ovens reach lower temperatures (only about 250 °C), but enough for drying sieves under vacuum (I assume temperatures over 200 °C and pressure of 100 mbar is sufficient). Vacuum ovens are maybe more useful as I can use them to dry sensitive solids, glassware etc.

I am also thinking about some makeshit solution like putting sealed evacuated flask with sieves in ordinary oven set at max temperature (220-250 °C), leave it there and periodically flush it with dry gas to remove the released water vapour.

What would be your choice for drying molecular sieves?

I don't need them absolutely dry since I don't do any moisture-sensitive work, usually just need to break ethanol-water azeotrope or pre-dry solvents for precipitation of hygroscopic salts.
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karlos³
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[*] posted on 10-7-2019 at 13:06


You can simply use a microwave ;)
Nuke them 2min and you're good.

[Edited on 10-7-2019 by karlos³]
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wg48temp9
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[*] posted on 10-7-2019 at 13:14


Quote: Originally posted by nimgoldman  
I use plenty of 3A and 4A molecular sieves, mostly to dry alcohols.

Unfortunately, the reactivation is an ordeal as I can use only a 2L heating mantle for that - I usually VERY slowly heat it up to prevent melting of the glass and even then it takes few hours until last droplets of water escape. Then I pump out the air which releases more water. The whole procedure is very time consuming. I though about running dry nitrogen or argon slowly through the flask to drive the vapour off to speed things up a bit.

I wonder if one can use kiln instead as this provides high enough temperatures for drying sieves at atmospheric pressure (300 °C and more) and many kilns can be programmed for convenience.

Vacuum ovens reach lower temperatures (only about 250 °C), but enough for drying sieves under vacuum (I assume temperatures over 200 °C and pressure of 100 mbar is sufficient). Vacuum ovens are maybe more useful as I can use them to dry sensitive solids, glassware etc.

I am also thinking about some makeshit solution like putting sealed evacuated flask with sieves in ordinary oven set at max temperature (220-250 °C), leave it there and periodically flush it with dry gas to remove the released water vapour.

What would be your choice for drying molecular sieves?

I don't need them absolutely dry since I don't do any moisture-sensitive work, usually just need to break ethanol-water azeotrope or pre-dry solvents for precipitation of hygroscopic salts.


Putting a sealed flask containing a significant amount anything damp/wet in a 300c oven is probably a bad idea. Thing about the vapour pressure of water at 300C.

Usually the limitation of drying a powder or granules is the low thermal conductivity of them in combination with the need to supply heat to vaporize the liquid. So it inevitably take time.

Spread the granules out on a large tray so the depth is as shallow as possible put them in a clean oven at the recommended temperature and forget about them for a hour or more or ideally until they are a constant weight.

I would expect if you can pull a constant vacuum while heating the granules it would reduce the time significantly.

PS Microwaving the granules can be problematic. While they are damp the vaporization of the liquid will keep them no hotter than about 100C but once they are dry the microwave energy is still going into the oven and could even melt them. So the time required would be dependent the amount liquid to be evaporated.

I have on occasion dried item of cotton clothing in a microwave but the item has to be monitored almost contumaciously to avoid over heating as they approach dryness or the item will be scorched or even ignited.

I have also accidentally melted manganese oxides/carbon from a used dry cell while drying it in a microwave oven. Doing that deliberately is interesting.


[Edited on 10-7-2019 by wg48temp9]

[Edited on 10-7-2019 by wg48temp9]




i am wg48 but not on my usual pc hence the temp handle.
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Ubya
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[*] posted on 10-7-2019 at 13:14


Quote: Originally posted by nimgoldman  


I am also thinking about some makeshit solution like putting sealed evacuated flask with sieves in ordinary oven set at max temperature (220-250 °C), leave it there and periodically flush it with dry gas to remove the released water vapour.


this seems even more work.
you said that you heat the seaves and THEN youu vacuum it, why not doing it at same time? (lack of convection so uneven heating?) heat the flask while connected to a vacuum pump with a hose adapter (running continuously or on a cycle with a timer). if you need more volume for larger batches you could convert a stainless steel pot to a vacuum chamber (similar to those sold as vacuum chambers for degassing silicone) by adding a steel disk on top with the right hose fitting and a gasket that survuves 250 or 300 degrees. or maybe a pressure cooker with a hole tapped on the lid for a hose adapter, it already has a heat resistant gasket (maybe not for 300°C though)





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nimgoldman
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[*] posted on 10-7-2019 at 14:55


I see.

So perhaps I will pre-dry the sieves on tray in a simple electric oven (not microwave) for some hours (maybe 2-3 hrs?) then place them in RBF in a mantle for final drying under vacuum.

Once problem I had with vacuum drying is that my diaphragm pump pumps lots of hot water vapour which of course condenses. The pump has a reservoir but maybe it's better to remove bulk of the water first.
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[*] posted on 10-7-2019 at 15:20


i use old pressure cooker with induction heating connected to vacuum pump.bunch of steel balls to even out heating .
also before any of that simple distill the residual alcohol.
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wg48temp9
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[*] posted on 11-7-2019 at 03:27


Quote: Originally posted by nimgoldman  
I see.

So perhaps I will pre-dry the sieves on tray in a simple electric oven (not microwave) for some hours (maybe 2-3 hrs?) then place them in RBF in a mantle for final drying under vacuum.

Once problem I had with vacuum drying is that my diaphragm pump pumps lots of hot water vapour which of course condenses. The pump has a reservoir but maybe it's better to remove bulk of the water first.


Vacuum drying at 250C after heating in an open tray at 250C will not dry the sieves significantly more so its not required particularly as your only using them to break an azeotrope.

At 250C the vapor pressure of water is about 40bar at 300C its about 86bar.
So adding even a perfect vacuum will only add an additional 1bar.
Putting it a different way you could get the same effect as the vacuum by increasing the temperature by a few degrees.

I checked about 250+3C or 300+1C for an extra one bar.




i am wg48 but not on my usual pc hence the temp handle.
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[*] posted on 11-7-2019 at 03:43


Honestly, just drying them in a normal oven at 250 C for 3 hours is good enough. Will they be bone dry and 100% activated? Probably not. Will they work well enough for the vast majority of cases? Yes.
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[*] posted on 11-7-2019 at 08:13


Isn't it dangerous to put a few litres of wet sieves into a regular oven at high heat? I mean explosion hazard.

The op used them to dry alcohol(s), so they not only contain water but some alcohol as well.

Experience?
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WGTR
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[*] posted on 11-7-2019 at 08:37


Since you mentioned using nitrogen gas, if you have a house supply of dry nitrogen (some labs have it piped in) you can try drying the sieves by flushing them for some time in a column at room temperature. I don't know how well this works for molecular sieves, but it does work for silica gel. The indicating gel turns blue again after being flushed with dry nitrogen, even at room temperature.

I did buy some 3A sieves, but I took them home earlier, so I don't have them here with me in the lab. Otherwise I'd try this out.




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nimgoldman
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[*] posted on 11-7-2019 at 09:56


Unfortunately the sieves won't release water so easily as the H2O molecules are "stuck" in the zeolite crystals. It takes temperatures of 250 °C or so until they release the water.

As for the alcohol, I usually pre-dry the sieves with low heat (e.g. food dehydrator) and then apply high heat. When I use oven, I have the oven fan turned on so any residual alcohol vapours are flushed.

I am drying about 750 g of sieves at once, but will probably do more in future.
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monolithic
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[*] posted on 11-7-2019 at 15:04


Quote: Originally posted by Pumukli  
Isn't it dangerous to put a few litres of wet sieves into a regular oven at high heat? I mean explosion hazard.

The op used them to dry alcohol(s), so they not only contain water but some alcohol as well.

Experience?


Wash them several times with water and let them air dry for 24 hours -- they should no longer smell of solvent. I've done this with sieves soaked in DMF and methanol for weeks at a time. They didn't blow up my oven.
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[*] posted on 20-7-2019 at 21:00


Quote: Originally posted by rockyit98  
i use old pressure cooker with induction heating connected to vacuum pump.bunch of steel balls to even out heating .
also before any of that simple distill the residual alcohol.


Omg why have I never thought to use my large heavy never otherwise used pressure cooker as a vacuum oven!!
Thanks!
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