Sciencemadness Discussion Board

Vacuum Oven vs Kiln for Molecular Sieve Drying

nimgoldman - 10-7-2019 at 12:19

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.

karlos³ - 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]

wg48temp9 - 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]

Ubya - 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)

nimgoldman - 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.

rockyit98 - 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.

wg48temp9 - 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.

monolithic - 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.

Pumukli - 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?

WGTR - 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.

nimgoldman - 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.

monolithic - 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.

Panache - 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!

nimgoldman - 31-8-2019 at 01:13

The steel balls are a pretty good idea!

I've come out with my "standard procedure" like this so far:


  1. remove any adhering solvent from sieves by either washing them with distilled water or by distillation
  2. pre-dry the sieves using regular oven set at max. temp.; pre-dry the RBF
  3. place sieves in RBF, flush with nitrogen/argon
  4. heat up RBF in a heating mantle up to 300 °C (careful about overheating) or until you see enough condensate in the flask
  5. pull vacuum, leave under vacuum for a few minutes
  6. put under nitrogen/argon by e.g. attaching a baloon and releasing stopcock
  7. repeat 4-6 several times

wg48temp9 - 31-8-2019 at 02:31

Quote: Originally posted by Panache  
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!


Most pressure cookers have a gasket that has a V profile with the top of the V facing in to the pressure cooker. When the internal pressure pressure is greater than the external pressure the sides of the V are forced apart to form more of U profile and seal against the lid and base.

With an internal vacuum the higher external pressure will tend to collapse the V and cause it to leak or possibly force part of the gasket into the pressure cooker.

wg48temp9 - 31-8-2019 at 03:05

Quote: Originally posted by nimgoldman  
The steel balls are a pretty good idea!

I've come out with my "standard procedure" like this so far:


  1. remove any adhering solvent from sieves by either washing them with distilled water or by distillation
  2. pre-dry the sieves using regular oven set at max. temp.; pre-dry the RBF
  3. place sieves in RBF, flush with nitrogen/argon
  4. heat up RBF in a heating mantle up to 300 °C (careful about overheating) or until you see enough condensate in the flask
  5. pull vacuum, leave under vacuum for a few minutes
  6. put under nitrogen/argon by e.g. attaching a baloon and releasing stopcock
  7. repeat 4-6 several times


If your going to fill a regular laboratory flask containing possibly damp sieves with a gas at near room temperature, seal it and then heat it to 300C then when the gas expands and the damp sieves release water vapor the stopper will probably pop or worse.

Hopefully your stopper is sufficiently leaky or loose to prevent too large a pressure build up.

Magpie - 31-8-2019 at 08:13

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?



I was regenerating 3A mole sieves used to dry ethanol. I placed the apparently dry sieves in a large glass cake pan in my kitchen oven at 400F. I then went into the next room to watch TV. After a while I heard a loud bang! I concluded that the ethanol vapor had exploded and pushed open the spring loaded oven door, then it self-closed. I continued the drying.

wg48temp9 - 31-8-2019 at 18:23

Below is snip of a set of isotherms for the absorption of water by 3A sieves.
You have to multiply the vertical axis by 1.8 to convert it to percent by mass. Unfortunately the pressure scale only extends to 0.3 of atmospheric pressure. A room temperature isotherm is not shown. That would be be similar to the 100C isotherm but with a sharper knee approaching 11.7 mol/kg (21% H2O by mass).

Interestingly at a pressure of 0.3 bar drying wet sieves at a temperature 200C only reduces the water content to about 9% from 21%. Note that the attached paper suggested regeneration temperature above 270C irreversibly reduces the absorption ability of the sieves.

At 200C to reduce the water content to less than 1% it requires pressures less than 10mbar. Flushing with an inert dry gas that has a negligible partial pressure of water vapor would be equivalent to reducing the pressure to zero which would remove that last 1% of moisture eventually.

Just drying drying the sieves at 200C give you about half the maximum drying capacity which is about 10% by mass. So to compensate for not using totally dry seives the amount used just need to be doubled.
.

3aisotherm.PNG - 104kB

From http://fennetic.net/irc/zeolite_adsorption_curves.pdf

Magpie - 31-8-2019 at 20:59

Isn't that "Pw" the partial pressure of water in the drying air?

wg48temp9 - 1-9-2019 at 03:43

Quote: Originally posted by Magpie  
Isn't that "Pw" the partial pressure of water in the drying air?


Yes if the sieves are in air and in equilibrium with it. ie the water vapor pressure of the sieves.

How can I relate those isotherms to drying alcohol? I guess a mixture of alcohol and water has a water vapor pressure. Assuming the alcohol does not effect the capacity of sieves then at equilibrium the water vapor pressure of the sieves and the alcohol would be equal. I could approximate
the water vapor pressure of the mixture using Raoults Law.

Magpie - 1-9-2019 at 10:28

From my psychrometric chart: at 68°F (20°C) and 50% relative humidity the vapor pressure of water in air is 0.18 psia = 1.2 kPa.

If the air is heated to 200°C the water content will stay the same and therefore its partial pressure will remain at 1.2 kPa. But the relative humidity will then be very low.

Therefore, from the isotherm at 200°C the water content of the zeolite will be 0.5mole H2O/kg, or 0.9%.

Does this look right?




[Edited on 1-9-2019 by Magpie]

wg48temp9 - 1-9-2019 at 13:22

Quote: Originally posted by Magpie  
From my psychrometric chart: at 68°F (20°C) and 50% relative humidity the vapor pressure of water in air is 0.18 psia = 1.2 kPa.

If the air is heated to 200°C the water content will stay the same and therefore its partial pressure will remain at 1.2 kPa. But the relative humidity will then be very low.

Therefore, from the isotherm at 200°C the water content of the zeolite will be 0.5mole H2O/kg, or 0.9%.

Does this look right?


No, its incorrect.

Your first paragraph may be correct.

Your second paragraph is incorrect. The result depends on how the air is heated. If it is heated at constant volume then the percent of water in the air will remain the same as will it its amount per unit volume. The pressure will increase and the humidity may change. If you heat the air and allow it to expand the ratio of water to air will remain the same and the amount of water per unit of volume will decrease. The humidity may change.

The third paragraph's "therefore" is in correct, that statement does not follow from paragraph 2.

The isotherms assume the sieves are in equilibrium with the water vapor surrounding them. That means at a given temperature the vapor pressure of water from the sieves is the vapor pressure of the water in the air surrounding the sieves and that will determine the percentage of water in that air, again at equilibrium.

What was the purpose of your question? Testing your understanding or mine?


Magpie - 1-9-2019 at 14:08

The purpose of my question is to try to determine what really happens when heating the zeolite in ambient air.

The ambient air at 20C will expand during the heating to 200C and much will leak out to maintain ambient pressure. This will decrease the amount of water in the oven air. Therefore the partial pressure of the water in the oven air will be even lower than I stated above. This will allow the zeolite to dry even further than the 0.9% water.

wg48temp9 - 1-9-2019 at 15:19

If you heat saturated sieves in a oven say in a long necked flask to 200C.
As the the temperature of the sieves increases their water vapor pressure will at some point exceed that of the atmosphere and steam will escape from the flask. Eventually the water content in the sieves will reduce until their water vapor pressure is equal to atmospheric pressure and the flask will be filled with water vapor. Unfortunately the isotherm I showed does not go up to 100kPa but it can be estimated from metaly extrapolating the graph to 100kPa. I estimated the percent water in sieves at that point as 11%

If the sieves are heated on tray in open air so that the reach equilibrium with the water vapor pressure of the atmosphere. then taking your figure of 1.2kPa at that pressure I estimate the percent of water in the sieves to be about 1.3%.

In practice the content of water will be between 1.3% and 11% depending on how well the sieves reach equilibrium with the atmospheric water vapor. If heated in an oven the air needs to be changed with fresh air to prevent a build up of water vapor. Most domestic ovens do not do that and the result will be closer to 11%.

One thing that surprised me was how effective flushing with a dry inert gas would be.

Combination MO when microwaving flush some cooling air through the oven but when used as a heated oven only they usual activate a flap that stops most of the cooling air flow thru the oven. Such an oven can be modified to allow some flow. As has already been stated using microwaves to heat the sieves risks over heating them.

The simplest and effective method would be to flush the sieves with heated air at 200C.

[Edited on 9/1/2019 by wg48temp9]

artemov - 1-9-2019 at 16:11

So an air fryer would be more effective than a conventional oven?

Put the sieves in a shallow dish and let the hot air swirl around them.

https://www.thekitchn.com/how-do-air-fryers-work-265185

wg48temp9 - 1-9-2019 at 16:31

Quote: Originally posted by artemov  
So an air fryer would be more effective than a conventional oven?

Put the sieves in a shallow dish and let the hot air swirl around them.

https://www.thekitchn.com/how-do-air-fryers-work-265185


That would just swirl damp air (air + the water vapor from the sieves) round them.You need a flow of fresh relatively dry air to get to the lowest levels of water in the sieves. I believe those fryers do have vents but not flowing air as that would cool them.

artemov - 1-9-2019 at 16:44

Quote: Originally posted by wg48temp9  
Quote: Originally posted by artemov  
So an air fryer would be more effective than a conventional oven?

Put the sieves in a shallow dish and let the hot air swirl around them.

https://www.thekitchn.com/how-do-air-fryers-work-265185


That would just swirl damp air (air + the water vapor from the sieves) round them.You need a flow of fresh relatively dry air to get to the lowest levels of water in the sieves. I believe those fryers do have vents but not flowing air as that would cool them.


Would stopping and opening the door for an air exchange every half an hour or so helps, as in the case for conventional oven?

As for conventional oven, mine has a small fan built-in as well, not sure if it's drawing air in or out or just recycling the air inside.

S.C. Wack - 1-9-2019 at 19:32

Quote: Originally posted by wg48temp9  
The simplest and effective method would be to flush the sieves with heated air at 200C.


Dried air. For some hours to constant weight. This is how alumina is dried...or at 400C on the stovetop with stirring for the same time; 200C costs less. It is said 3A sieves should not be heated above 260.

[Edited on 2-9-2019 by S.C. Wack]

wg48temp9 - 2-9-2019 at 02:11

Quote: Originally posted by artemov  


Would stopping and opening the door for an air exchange every half an hour or so helps, as in the case for conventional oven?


Yes i think that would help significantly.

magnet - 2-10-2019 at 16:59

Quote: Originally posted by Magpie  
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?



I was regenerating 3A mole sieves used to dry ethanol. I placed the apparently dry sieves in a large glass cake pan in my kitchen oven at 400F. I then went into the next room to watch TV. After a while I heard a loud bang! I concluded that the ethanol vapor had exploded and pushed open the spring loaded oven door, then it self-closed. I continued the drying.

I did the same thing in my convection/toaster oven. Kaboom haha.

morganbw - 3-10-2019 at 13:34

Quote: Originally posted by S.C. Wack  
Quote: Originally posted by wg48temp9  
The simplest and effective method would be to flush the sieves with heated air at 200C.


Dried air. For some hours to constant weight. This is how alumina is dried...or at 400C on the stovetop with stirring for the same time; 200C costs less. It is said 3A sieves should not be heated above 260.

[Edited on 2-9-2019 by S.C. Wack]


I always respect your post. Thank you.