VeritasC&E
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Easiest / Single Multi-Purpose Dessicant?
I'm looking for a single dessicant that I can use across a range of applications (from driving off excess water to dehydration) without contaminating
my products, and easily recycle.
Do you have any suggestion?
One thought I have in mind for this would be H2SO4 but I'm wondering if the data supports this (at least the its hygroscopic character when pure and
low VP makes it a nice candidate).
Along this thought is the idea that as I'd use the H2SO4 for the most extreme cases (dehydration) I'd recursively recollect the increasingly hydrated
H2SO4 grades in different bottles destined to gradually milder dessication applications), then once it a while dump it alltogether, heat it up with
H2O2, let it cool to RT, and then put it under vacuum with a receiver under an air condenser and heat it up again to drive of the water /
reconcentrate it (to 98%)? That would be for the bulk of the regeneration, for dehydration applications where purity is a concern I could distill a
small portion of it (starting off with what I condense in the receiver for instance, if it's enough)
Is this a good candidate? What are the flaws in this? Does anyone have data on how efficient of a dessicant/dehydrating agent H2SO4 is across it's
hydration range? (as for instance, down to which concentration will it still absorb water at a useable rate under 90% vacuum?)
[Edited on 7-7-2021 by VeritasC&E]
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Belowzero
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How about silicagel?
I also used calciumchloride in the past, bit of a pain to dehydrate (clumps/melts) but its cheap and easily available.
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Jenks
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Molecular sieves are what I use. Highly compatible, highly effective and, once I found they could be regenerated in a toaster oven, highly recyclable.
What is the role of hydrogen peroxide in recycling diluted sulfuric acid?
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VeritasC&E
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Quote: Originally posted by Jenks  | Molecular sieves are what I use. Highly compatible, highly effective and, once I found they could be regenerated in a toaster oven, highly recyclable.
What is the role of hydrogen peroxide in recycling diluted sulfuric acid? |
How much water can you at best absorb per Kg of molecular sieves? Do you know how much for H2SO4? (I don't)
H2O2 oxidates stuff. From what I understand mixed into H2SO4 and then brought to high temperatures degrades anything organic in there (one of the
advantages of the high BP). Recycling is partly about dehydrating the dessicant, but also about removing anything it could emit that would make it
contaminate products.
[Edited on 7-7-2021 by VeritasC&E]
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Oxy
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From my observations (but I didn't measure it so there is no science) - silica gel is good when it comes to absorb moisture when the humidity is
relatively high. When it comes to a situation when the humidity is low they are weak.
Maybe combined with KOH or H2SO4 (depending on the type of sample to dry) they will be good to use.
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Jenks
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I think ideally, sieves absorb about 14% of their weight in water. Mine get at least 10% after oven drying, which is closer to 200°C rather than the
300°C recommended. This isn't much of a limitation in a desiccator, but for drying solvents it can be wasteful if the solvent was decanted.
I don't know the capacity of sulfuric acid but would expect it to be more linear, as you have described, absorbing less at low humidity and more at
high humidity. You could take advantage of this if you don't need extreme drying by letting wet sulfuric acid, or phosphoric acid, sit in a casserole
dish on a hot plate and let it evaporate to equilibrium. The temperature could be safely below boiling while still providing an effective room
temperature desiccant. I would suggest covering the dish with a plate as it cools to avoid the hazard of handling hot acid.
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Bedlasky
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Manganese sulfate is cheap and can be easily dehydrated.
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VeritasC&E
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I didn't know about Manganese Sulfate! It's different hydrates look like it could be interesting (depending on hydration temperatures).
Would you know about the decomposition temperatures of its different hydrates? How much does it cost per 25Kg?
Do you know anything about H2SO4 (e.g. data about how hygroscopic it is vs concentration)?
What I like about H2SO4 is that it being liquid and having a low VP, I would think of it as less likely to contaminate your product than a solid
dessicant. When repressurizing a dessicator it's not uncommon for my clumsy self to see small crystals of my product being blown down into the
dessicant, I wouldn't like the same to happen the other way around).
[Edited on 7-7-2021 by VeritasC&E]
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unionised
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Do you mean magnesium sulphate?
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Bedlasky
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Yes, I mean magnesium sulfate  .
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Tsjerk
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Magnesium sulfate works for about anything where the last 0.01% of water isn't a problem. And you can dehydrate oin minutes in a microwave.
Sodium sulfate is also good for general purpose drying, I ran out of magnesium sulfate and bought the sodium salt. It was cheaper to begin with and
turned out to be dehydrated while I was expecting it to be the decahydrate.
When something has to be really dry I use sieves.
[Edited on 7-7-2021 by Tsjerk]
[Edited on 7-7-2021 by Tsjerk]
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VeritasC&E
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Sodium sulfate is a bliss, but solid.
Does anyone have data for how hygroscopic H2SO4 is at different concentrations? What is the point at which it just isn't anymore?
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VeritasC&E
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Ok, I compared a bunch of dessicants and Bedlasky's suggestion indeed is the best solution as a general purpose solid dessicant, sodium sulfate comes
second but is slightly slower and works on par at a lower temperature / higher pressure.
As for H2SO4 I believe it's the best general purpose liquid dessicant and though I'm still unsure about regeneration (which involves a much more
thorough process than for the two aforementionned solid options) if I understand correctly it maybe can hold on to / absorb >6x its weight in water
down under 18.5% concentration). It also performs better than both sodium and magnesium sulfates as soon as one gets into sufficiently low pressures /
higher temperatures / deeper dehydration.
Ok, I compared a bunch of dessicants and Bedlasky's suggestion indeed is the best solution as a general purpose solid dessicant, sodium sulfate comes
second but is slightly slower and works on par at a lower temperature / higher pressure.
As for H2SO4 I believe it's the best general purpose liquid dessicant and though I'm still unsure about regeneration (which involves a much more
thorough process than for the two aforementionned solid options) if I understand correctly it maybe can hold on to / absorb >6x its weight in water
down under 18.5% concentration). It also performs better than both sodium and magnesium sulfates as soon as one gets into sufficiently low pressures /
higher temperatures / deeper dehydration.
Price per Max Liters of H2O Absorbed at 25C, 30 torr:
H2 > Na2 > Mg (SO4)
Ease of Regeneration:
Na2 > Mg > H2 (SO4)
Depth / Applicable Vacuum Level of Dessication (per L H2O Absorbed):
H2 > Mg > Na2 (SO4)
[Edited on 7-7-2021 by VeritasC&E]
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SWIM
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If this is for use in a desiccator use calcium sulfate in the form of drierite.
It comes in big lumps that don't blow around easily, and is available with an indicator so it'll change colors when it is saturated and needs drying.
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Bedlasky
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VeritasC&E: 0,003 g/m^3 of water vapour is above 100% H2SO4 at 20°C. 0,3 g/m^3 of water vapour is above 95,1% H2SO4 at 20°C. I just found these
two values.
From experience - I often work with 85% and 70% H2SO4. If you add drop of <90% H2SO4 on to piece of paper towel, it make firstly black spot and
after few seconds it make hole throught it. 85% and 70% H2SO4 just make brown spot. So I wouldn't use >85% H2SO4 for drying. But 70% H2SO4 still
have enough drying propeties to dehydrate HCN to CO.
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Tsjerk
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Nice representation of the different hydrates of sulfuric acid:
link
link
[Edited on 8-7-2021 by Tsjerk]
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VeritasC&E
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Thank you all for your various contributions!
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macckone
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Agree magnesium sulfate. Calcium chloride is good for drying tubes but is not as general purpose.
[Edited on 18-7-2021 by macckone]
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Bezaleel
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I'm surprised NaOH has not been mentioned. I usually use hardware store prills of NaOH. Advantage is that it does not spread any vapours, but absorbs
acid vapours.
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VeritasC&E
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| Quote: Originally posted by Bezaleel | | I'm surprised NaOH has not been mentioned. I usually use hardware store prills of NaOH. Advantage is that it does not spread any vapours, but absorbs
acid vapours. |
NaOH acts fast. Do you know if it holds much water per g dessicant? And how easy it is to recycle? I think it might have subpar characteristics on
those points compared to Mg Sulfate.
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Bedlasky
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NaOH have similar efficiency as CaCl2 (at least according to literature). KOH is even better, comparable to 100% H2SO4 or CaSO4.
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Bezaleel
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| Quote: Originally posted by VeritasC&E | | Quote: Originally posted by Bezaleel | | I'm surprised NaOH has not been mentioned. I usually use hardware store prills of NaOH. Advantage is that it does not spread any vapours, but absorbs
acid vapours. |
NaOH acts fast. Do you know if it holds much water per g dessicant? And how easy it is to recycle? I think it might have subpar characteristics on
those points compared to Mg Sulfate. |
I never recycle it. Usually it has absorbed some acid as well. The process would require taking a lot of care to keep CO2 from reacting with it. And a
500ml bottle costs 3euro50 at present - not worth the effort.
I estimate (never really measured) the amount of water absorbed in the limit (reached in about a day) at about 4 times its volume. Usually I replace
it 2 or 3 times a day, because after a few hours the efficacy of water absorption has dropped significantly.
I started using NaOH when my CaCl2 didn't start absorbing water from a saturated EuCl3 solution. (EuCl3 solution is highly hygroscopic - the most
difficult compound I have ever dried (well, let crystallise).)
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VeritasC&E
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| Quote: Originally posted by Bezaleel | | Quote: Originally posted by VeritasC&E | | Quote: Originally posted by Bezaleel | | I'm surprised NaOH has not been mentioned. I usually use hardware store prills of NaOH. Advantage is that it does not spread any vapours, but absorbs
acid vapours. |
NaOH acts fast. Do you know if it holds much water per g dessicant? And how easy it is to recycle? I think it might have subpar characteristics on
those points compared to Mg Sulfate. |
I never recycle it. Usually it has absorbed some acid as well. The process would require taking a lot of care to keep CO2 from reacting with it. And a
500ml bottle costs 3euro50 at present - not worth the effort.
I estimate (never really measured) the amount of water absorbed in the limit (reached in about a day) at about 4 times its volume. Usually I replace
it 2 or 3 times a day, because after a few hours the efficacy of water absorption has dropped significantly.
I started using NaOH when my CaCl2 didn't start absorbing water from a saturated EuCl3 solution. (EuCl3 solution is highly hygroscopic - the most
difficult compound I have ever dried (well, let crystallise).) |
4x its weight is fairly good for a strong dessicant. I'm not a big fan of not being able to recycle dessicants myself as it increases dependence. One
day you wake up in a soft dictatorship and NaOH is banned from sale for individual slaves, alongside sulfuric acid, table salt and sugar >5Kg (as
well as all those other very basic things that have been core to any economy as far back as the Romans at the very least).
[Edited on 22-7-2021 by VeritasC&E]
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