I recently used some 3A molecular sieves to dry some ethanol. I'd recently run a low-yielding experiment using ethanol dried with the same sieves, so
I checked the specific gravity, and it came up as corresponding to 88% ethanol. I'd thought the ethanol had started at 90%, and the quantity of sieves
added were sufficient to fully dehydrate it twice over according to their spec sheet, so I dehydrated them again and added them again to the ethanol.
They only removed about 2% of the water. I had to run through several cycles to obtain dry ethanol.
I've used these sieves numerous times, and this was the first time I had noticed diminished efficiency... they are the expensive $0.20/gram variety. I
was pretty surprised at how ineffective they were, although I'd guess some of them have been through 20 or so cycles drying various solvents.
I'm thinking that I will be throwing these away and getting some new ones, but is there typically a limit to the number of times molecular sieves can
be used, or is it likely that something in my solvents have degraded the sieves? Also, is there a big difference between the $0.05/gram 3A sieves and
the $0.20/gram 3A sieves?Boffis - 22-11-2017 at 12:27
It requires a lot of zeolite to absorb the water from 88-90% alcohol. Its far better to fractionate the alcohol first a couple of times with
decreasing distillation rates. This will give you something clos to 95% and then you need to use one of the methods to break the azeotrope. This will
give you 99%+ which you can then dry with the zeolite. I don't know for certain whether zeolites loose their activity with repeated drying but I can
quite see that eventually they could undergo degradation both through structural collapse during drying (many natural zeolites do this spontaneously,
famously laumontite) or through partial leaching of the alkali component which are often readily exchangeable.aga - 22-11-2017 at 12:46
Definitely only ever use sieves for getting things dryer, not as the go-to solution to get large chunks of water out.
Ethanol is easy to distill to azeo, then dry with sieves.
Alternatively use tripot phos without any distillation at all :-
These no longer useful as a dessicant sieves, are rhey in the form of small diameter spheres?
If so, they make good cores to start rolling round stars on. Hydrate them, drain the excess water/alcohol off, and toss a bunch in a star roller.
(If you think about it the right way, EVERYTHING is related to pyrotechnics...) aga - 22-11-2017 at 13:12
Careful Bert - you could get a hay-bale of suggestions !
unionised - 22-11-2017 at 13:17
These sieves like water more than they like (almost) anything else, so, if you leave them in very humid air you will displace almost anything volatile
from them.
Ideally, you blow a stream of air though water and then through a trap to remove liquid water then through the sieve. The air carries in water and
flushes out other stuff.
(It also gets round the potential problem of blowing up your oven when you drive alcohol vapour off as part of the "drying" process.)
Once you have some that's saturated with water, weigh some of it, dry it and weigh it again. Then dry it again and reweigh it.
If it loses more weight then your "drying" process isn't up to scratch.
It takes quite a long time to fully dry these things, even at elevated temperatures (Ideally you need a thermocouple in the middle of the batch of
sieve you are drying)
Keep on drying it until the weight is more or less constant, and find out what % water the stuff holds.
That should give you an idea of whether or not it is working to the specification. aga - 22-11-2017 at 13:26
... also gets round the potential problem of blowing up your oven when you drive alcohol vapour off as part of the "drying" process. ...
Ah.
So you're NOT supposed to set fire to them after drying the ethanol.
OK. Noted.Bert - 22-11-2017 at 17:12
Aga, as your attraction to alcohol may exceed that of the sieves? Perhaps lick them off?
[Edited on 23-11-2017 by Bert]JJay - 23-11-2017 at 00:15
I always soak them in water before drying them in the oven to reduce the risk of fire. I've tried setting fire to them while they were still wet with
ethanol, and I think that causes them to crack and break, so I usually avoid doing that....
Oh and yes, they are small diameter spheres.
[Edited on 23-11-2017 by JJay]aga - 23-11-2017 at 10:36
