Chainhit222
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What molecular sieve sizes do I need for methanol, ethanol and diethyl ether?
I tried to research the topic on the forum, but I am not entirely certain as to what sieve I need.
http://en.wikipedia.org/wiki/Molecular_sieve
they can easily be bought on ebay, or on various websites (such as for alcohol brewing).
Wikipedia reads:
* 3A (pore size 3 Å): Adsorbs NH3, H2O, (not C2H6), good for drying polar liquids.
* 4A (pore size 4 Å): Adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, EtOH. Will not adsorb C3H8 and higher hydrocarbons. Good for drying nonpolar
liquids and gases.
* 5A (pore size 5 Å): Adsorbs normal (linear) hydrocarbons to n-C4H10, alcohols to C4H9OH, mercaptans to C4H9SH. Will not adsorb isocompounds or
rings greater than C4.
I am specifically interested in
-Methyl Alcohol
-Ethanol
-Diethyl Ether
From reading that describe, I understand that
-3A molecular sieve would absorb water, and leave Methanol, Ethanol and ether alone. (CH3OH, C4H10O, C2H5OH all are much larger molecules then water
or ammonia.
-4A molecular sieve will absorb Ethanol, Methanol, Water but not ether, as ether C4H10O, is a much longer molecule then C2H5OH)
-5A molecular sieve. Will a sieve of this size absorb diethyl ether? it says alcohols up to C4H9OH, but will it absorb ether?
Since 5A molecular sieve absorbs hydrocarbons up to C4H10, does that mean it would not absorb heptane or hexane? C6H14 and C7H16 are longer then
C4H10, so am I correct in assuming it will not absorb them?
The reason why I am asking is because perhaps 5A molecular sieve would be a good method of getting diethyl ether out of starting fluid (let a can of
the stuff soak in a flask of zeolite for some time, then filter it through a funnel, and then just heat the zeolite in a distillation setup to save
the ether. While distillation of starting fluid is easy, is this idea feasible regardless?
A problem I see is that the lubricant in starting fluid (im guessing some sort of heavy motor oil type stuff) would be difficult to remove from the
zeolite, even if its only 2% or so by volume.
4A molecular sieve could be used to dry ether after it is produced from a sulfuric acid/dripping ethanol type still (as the only contaminates should
be H2O and ethanol, and perhaps some H2SO4), without the hassle of additional distillations, and 3A molecular sieve could be used to get absolutely
dry alcohol without performing distillations.
I would like to buy 3a and 4a (maybe even 5a if you guys say its true) asap, so I have something to play with during my winter break, so quick
responses would be appreciated 
*additionally, am i correct in believing you can reuse the molecular sieve just by baking it, just like any other desiccant?
[Edited on 10-12-2009 by Chainhit222]
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not_important
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| Quote: | | 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 the 175-260° 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. |
http://www.tower-packing.com/Dir_molecular_sieve.htm
also
http://www.axens.net/pdf/products/Axens_adsorbents.pdf
when regenerating molecular sieves that absorbed organics or were used ti dry them, you need to remove the organics first or use a stream of N2 to
keep oxygen away from the organic material (and then use a cold trap to capture organics in the exhaust stream) Alternatively for water miscible
solvents, soak and rinse the sieves with water, then regenerate.
You can use slightly lower temparatures if you pull a good vacuum on the sieves when regenerating them.
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Chainhit222
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This is excellent news, but I cannot find the critical diameter of diethyl ether listed on that website, or on google for that matter! 
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not_important
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I think that you will find getting an absorbed substance out of the sieves is more difficult than simple distillation. You will have to heat it
considerably higher than the BP of ether, and even then not all will be release. And as you mention, there is the need to remove high boilers from
the sieves surface, meaning a wash with a solvent that is not absorbed and is fairly volatile.
But if you really want to try, I suspect that diethyl ether will have size/shape similar to n-pentane.
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smuv
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There is a high probability that in the process of trying to get the ether out of the sieves it will decompose to yield a mixture of ethanol, ethylene
ether and water...though i suppose you could then use 4A molecular sieves to remove those impurities 
You need essentially the same glassware to distill ether as you would to remove absorbed ether from molecular sieves...so why don't you just distill
the ether to separate it from starting fluid? I recommend a quick simple distillation to get a distillate with a high proportion of ether, followed
by fractionation to obtain a rather pure product.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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densest
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Two thoughts:
Use a (partial) vacuum to remove volatile substances from molecular sieves, and/or blow dry gases through a bed of the sieve beads. This is prompted
by the way oxygen separators work their beds of sieves.
Winter is a good time to distill ether - if it's -20C outside, you can circulate a glycol/water mix in your condenser and around your receiver vessel
for low escaped vapors. Done right, there isn't enough vapor to smell (and therefore, probably not enough to explode even if there were a spark).
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orbit_research
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Molecular Sieves
MeOH, EtOH - 3A
Et2O - 4A
I sell molecular sieves and anhydrous methanol, ethanol, and diethyl ether to 48 United States and Canada
http://www.orbitreagents.com/
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peach
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There's an excellent entry on Curly arrow's blog about drying solvents for anhydrous work, here
You'll want to have a read of the comments as well.
Seems like all the guys who do this on a routine basis are switching to column systems over the fireballs and explosions of sodium redistillation.
I think it may be time for the great lady Science Madness to set sail on a voyage to produce something similar at home.
The commercial arrays with taps and septums for all the common solvents are $30k. But it's basically stainless tubes packed with alumina and maybe
some Q5 to deoxygenate. Flash the solvent through with 10 - 15psi of argon and you get ppm's very close to sodium wire distillations.
We don't need earthquake proof arrays, it's only use using them (occasionally) and students who don't know what it is aren't using it, meaning it
could be possible to make one a lot cheaper than they cost prebuilt. Spool of stainless tubing or a flash column packed with alumina?
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maxidastier
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Can molecular sieve 3A be used for drying all those solvents, where literature actually recommends using 4A?
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vulture
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| Quote: |
The commercial arrays with taps and septums for all the common solvents are $30k. But it's basically stainless tubes packed with alumina and maybe
some Q5 to deoxygenate. Flash the solvent through with 10 - 15psi of argon and you get ppm's very close to sodium wire distillations.
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Quite true. But translating this into a amateur environment poses some problems:
1) The columns; the alumina needs to be dry and the right kind, then you need to transfer it into a sealed tube which can hold
pressure and won't block. It's also hard to know when they are saturated, unless you perform routine KF, which isn't exactly amateur...
2) The source solvent - the system we have at the uni is connected to kegs of solvent (much like beer taps) which are already quite dry from the start
- 0.1% water for THF for example.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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peach
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True vulture, particularly point number 1 and knowing when they're spent. I wonder if they are, or if it'd be possible, to regenerate the sieve by
baking the tube and passing dry gas through it. That'd probably increase the chance of it blocking though.
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maxidastier
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So, 3A is suitable for any solvent, where 4A is suitable, too?
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peach
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Drying agent selection guide
Solvents Drying and Drying agents
Have a look through the above for a suitable choice.
These links should be stickied somewhere, because it's a frequent question; I must have linked to them ten or more times.
[Edited on 16-7-2011 by peach]
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maxidastier
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I know this list. But my question is: Can I use 3A, when the list says use 4A?
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peach
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Yup, provided the only thing you're trying to remove is water or some other small molecule.
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maxidastier
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Ok, thank you. Would you recommend using 3A instead of 4A?
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peach
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If you pick one with a pore size that's just under your material's size, it will tend to absorb any other traces of contamination from it as well as
the water. But depending on how much 'other stuff' you have in that solution or gas, it may lower it's capacity to absorb water as it gets overloaded
with the other contaminants.
Say you were drying cyclohexane and used 3A. It'd only absorb really small molecules in the hexane, like water. That'd be great for drying it.
But you could also use 4A instead. That will absorb ethanol. Say your hexane had become contaminated with ethanol at some point in the past, that'd be
good. But you'd also need to add more sieve to account for that it's now soaking up water and the ethanol, and lots more small molecules.
If you want one that'll be compatible with (not absorb) the widest range of solutions and gases, that'd be 3A, since the pores will only soak up the
smallest of molecules.
If you're dealing with large molecules and want to soak up as many different contaminants as possible, then 4A will achieve that without also soaking
up the thing you want.
There are some examples on wiki;
| Quote: | | 3A (pore size 3 Å): Adsorbs NH3, H2O, (not C2H6) |
| Quote: | | 4A (pore size 4 Å): Adsorbs H2O, CO2, SO2, H2S, C2H4, C2H6, C3H6, ethanol. Will not adsorb C3H8 and higher hydrocarbons. |
So the latter will get more of those contaminants out, because the pores are big enough to fit them in, and you may have to use slightly more to
account for that, but it would unsuitable for drying / cleaning things like ethanol.
If all you're interested in is dryness, 3A.
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maxidastier
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Thanks a lot!
I think I`ll get 3A then, because I'm mainly interested in drying solvents (like Dioxane, DCM etc.) and with 3A I could also get absolute
ethanol/methanol.
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vulture
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| Quote: |
True vulture, particularly point number 1 and knowing when they're spent. I wonder if they are, or if it'd be possible, to regenerate the sieve by
baking the tube and passing dry gas through it. That'd probably increase the chance of it blocking though. |
I wouldn't want to bake alumina which has been soaked in unstabilized THF, certainly if has been exposed to the atmosphere after being spent.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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