artemov
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Molecular Sieves - First Use and Regeneration
Bought 500g of 3A molecular sieves from Ebay.
Arrived weighing 480g (including the sealed bag  ), and much smaller than I
thought (~1mm in diameter).
Just wondering if I need to wash and soak them, then chuck them in the oven at 250C for 3 hrs before first use? Do I need to wait for them to cool
down in the oven before use or storage? Will they start absorbing water vapour while cooling?
For regeneration after use, do I need to wash and clean them first before oven?
Cheers.
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Sulaiman
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Put about 1cm depth of sieves in a test tube,
just cover in water and feel the temperature,
if warm then there is probably no need to dehydrate your sieves.
You can boil off the water then dehydrate the sieves in the test tube,
then when cool do the test again to compare activity.
My Chinese 3A sieves contained a fair quantity of dust, that normally settles out..
The sieves may seem small, but to water molecules they are about as big as the Earth is to us 
P.S. If you use a wide-mouthed glass jar to store your sieves
be prepared for the partial vacuum making the lid very difficult to unscrew.
[Edited on 1-11-2018 by Sulaiman]
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Tsjerk
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I wouldn't worry about absorption of moisture while cooling down after drying in the oven, it will be neglectable.
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artemov
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Thanks guys!
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DavidJR
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Quote: Originally posted by artemov  | Bought 500g of 3A molecular sieves from Ebay.
Arrived weighing 480g (including the sealed bag ), and much smaller than I
thought (~1mm in diameter).
Just wondering if I need to wash and soak them, then chuck them in the oven at 250C for 3 hrs before first use? Do I need to wait for them to cool
down in the oven before use or storage? Will they start absorbing water vapour while cooling?
For regeneration after use, do I need to wash and clean them first before oven?
Cheers.
|
I actually prefer the 1mm beads since you can more easily pour them down funnels etc, and the extra surface area probably makes them work faster than
if they were larger diameter.
I'm not convinced 250C for 3h will get them completely dry. It's probably workable though. The method I use for regeneration is:
If the sieves are still wet with organic solvent, then put them in a pie dish and warm gently on the hotplate until the bulk of the solvent is
gone. This is to avoid fireballs later.
Get a 600ml beaker and fill it 3/4 full with the sieves. Record the total mass of beaker+sieves.
Put the beaker in the microwave
Zap it for 3 minutes
Open the door to vent the pocket of water vapour, and give the sieves a quick stir with a glass rod
Repeat the previous two steps until two consecutive cycles where the sieves lose no mass
Warning: the sieves get extremely hot during this process. If you are too overzealous it is possible to quite literally melt the sieves into a glowing
ball. Use beaker tongs and put a cork ring or similar on the scales to protect them.
Once done, cover the beaker with foil to minimise reabsorption of atmospheric moisture and allow to cool for a couple of hours, before transferring to
a glass bottle for storage.
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Tsjerk
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Drying them for a couple of hours at around 130 degrees is a lot less laborious and with less risk of producing a mess.
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DavidJR
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| Quote: Originally posted by Tsjerk | | Drying them for a couple of hours at around 130 degrees is a lot less laborious and with less risk of producing a mess. |
That definitely won't get them completely dry, unless you mean in a vacuum oven.
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unionised
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If you use them with solvent, it's a good idea to leave them exposed to the air for a while before "drying" them. They will pick up water from the
air and, because they bind water better than practically anything else, you will ensure that there's nothing flammable present when you heat them.
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wg48
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From Sigma https://www.sigmaaldrich.com/chemistry/chemical-synthesis/le...
"A. Regeneration (activation)
Regeneration in typical cyclic systems constitutes removal of the adsorbate from the molecular sieve bed by heating and purging with a carrier gas.
Sufficient heat must be applied to raise the temperature of the adsorbate, the adsorbent and the vessel to vaporize the liquid and offset the heat of
wetting the molecular-sieve surface. The bed temperature is critical in regeneration. Bed temperatures in the175-260° range are
usually employed for type 3A. This lower range minimizes polymerization of olefins on the molecular sieve surfaces when such materials are present in
the gas. Slow heat up is recommended since most olefinic materials will be removed at minimum temperatures; 4A, 5A and 13X sieves require temperatures
in the 200-315 °C range."
Borosilicate glass:
Good temperature resistance and good thermal shock resistance but finite.
For normal, standard service typically 200-230°C, for short-term (minutes) service max 400°C
Maximum thermal shock resistance is 160°C
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unionised
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And it is important to ensure good ventilation too.
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S.C. Wack
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Depends on how much drying ability you want the sieves to have...presumably half-ass scores accordingly. Instead of seeking wisdom here it may not be
unwise to read in to something like PoLC:
"After use, molecular sieves can be regenerated by heating at between 300-350C for several hours, preferably in a stream of dry inert gas such as
nitrogen or preferably under vacumm, then cooling in a desiccator."
[Edited on 2-11-2018 by S.C. Wack]
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artemov
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Thanks guys. I have bare basic equipments here ... trying to make do with what I have ... a conventional oven or maybe a microwave it will have to be
(which will have to double for cooking duties as well ... so nothing too toxic 
)
So no washing with water necessary? Just let the organic solvent evaporate off? Cheers.
PS. For microwave ... is there any "power" setting to use?
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DavidJR
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| Quote: Originally posted by artemov | Thanks guys. I have bare basic equipments here ... trying to make do with what I have ... a conventional oven or maybe a microwave it will have to be
(which will have to double for cooking duties as well ... so nothing too toxic
)
So no washing with water necessary? Just let the organic solvent evaporate off? Cheers.
PS. For microwave ... is there any "power" setting to use? |
I just use my kitchen microwave oven for this. I leave it on the full power setting and it has a power rating of 1000W.
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beerwiz
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I use the microwave to dry them. Set the time to 10 minutes. Put the sieves in a round bottom flask or flat purex dish. Turn on microwave. Every 30
seconds, take out the vessel and shake and put back in the microwave. This has two purposes, to vent the moisture out of the microwave and to
shake/turn over the sieves. You are done in 10 minutes. Let them cool down in the microwave, when they are at room temperature put them in a sandwich
bag and seal it to keep out moisture.
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