RogueRose
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Moisture absorbing (from air) capacity of MgCl2 & CaCl2 (both anhydrous) - which is better?
In almost every article I read about drying air or gases people often mention CaCl2 in anhydrous form. Both are inexpensive side walk de-icers in
50lb bags. I have some that I ensured was anhydrous by heating in a cast iron skillet over a propane flame up to 700-800F. The MgCl2 was advertised
as anhydrous and the weight verified this but I also tried heating but found the melting be an issue and the decomp noted at 572F (in wiki) was a
concern (though anhydrous melts at 1,317F so IDK how hexahydrate can decompose at 572F..?
I had 2 gallon jugs, with top cut off and both filled 1/2 full with 3kg of salt (both anhydrous). They sat in the attic starting in Nov and a year
later the MgCl2 was a solid block, while the CaCl2 could still be poured but had changed to a glossy look instead of a chalky look. In the summer
after the 1 year mark (about 8 months later - total 20 months), the MgCl2 has leaked all the fluids out of the punctures in the bottom of the jug,
loosing about 1/2 the volume and there was slushy crystals in the bottom 1/4 of the jug. The CaCl2 was now a solid block with beads of moisture being
able to be seen on the side of the plastic so it had reached the point of starting to pool as a saturated solution
So, from what I can see, the MgCl2 absorbed water at what seems like 2-3x faster than the CaCl2, and both form the hexahydrate at highest hydrate
form, so they should both be equal in absorption properties.
The only thing I can see that may be an issue is drying the MgCl2 as it turns to a hot, thick liquid at about 270F and boils for what seems like a
long time. If re-use is needed, then CaCl2 works nicer (unless there is something I don't know about MgCl2), but I can say the MgCl2 sucks water out
of air MUCH faster.
Both gallon jugs were in the same area, seperated by about 5-6 ft, each with same ventilation, so there is no reason one should have acted so
differently.
This makes me wonder how excatly hygroscopic compounds work as I need a strong/active compound to dry high flow of air - but it needs to be able to be
"recharged" - preferably within the drying vessel (was thinking of a heating element) and the CaCl2 would work well.
I finally have some silica gel that I know can be dried in the vessel but I don't know how it will work in drying air.
I can use a vessel 8 or 12" wide and 3-4ft long if needed. The flow of air will be 5-12 CFM for a compressor that needs dry air (as dry as possible).
I was considering using something like a standard freezer or mini fridge turned into a freezer and blowing air over perforated 1/4 - 1/2" aluminium
allowing the moisture to freeze on the surfaces. A quick defrost overnight and it's ready to go again.
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Sulaiman
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I don't know your climate, but hot humid air carries a surprising ammount of water.
e.g. 20oC, 100% RH, c 1.07 lb water per 1000 cu.ft.
12 cfm x 60 min/hr = 720 cfm/hour = 0.77 lb H2O per hour.
Or, in sensible units, about 350 ml/hr.
So you want to keep the water fluid, not frozen solid.
Look up water content vs. temp here
https://www.engineeringtoolbox.com/maximum-moisture-content-...
and multiply by Relative Humidity.
[Edited on 11-12-2017 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Bert
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Mood: " I think we are all going to die. I think that love is an illusion. We are flawed, my darling".
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We use the straight Calcium chloride ice melter to dehumidify inside explosive magazines without needing electricity or mechanicals (sparks?!).
A 50 lb. bag, split evenly between two 5 gallon plastic buckets, one at each side of a 40'×8' space. We sit each of the buckets in a large, shallow
plastic pan that is intended to go under a laundry machine to catch leaks, the Calcium chloride brine will continue to draw moisture out of the air
even after completely dissolved, it will overflow the bucket if you don't keep it somewhat emptied of the brine that forms.
The brine, diluted and used in moderation will make your tomato and pepper plants happy after a heavy rain. They love Calcium...
At $20.00 US per 50 lb. bag, we don't bother to dry it out for reuse. If you wanted to do a small quantity, a microwave does the job. Just be sure
your container can take the conditions, including some pretty fierce bumping as the steam from water trapped inside chunks explodes out.
Rapopart’s Rules for critical commentary:
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that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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NEMO-Chemistry
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Quote: Originally posted by Bert  | We use the straight Calcium chloride ice melter to dehumidify inside explosive magazines without needing electricity or mechanicals (sparks?!).
A 50 lb. bag, split evenly between two 5 gallon plastic buckets, one at each side of a 40'×8' space. We sit each of the buckets in a large, shallow
plastic pan that is intended to go under a laundry machine to catch leaks, the Calcium chloride brine will continue to draw moisture out of the air
even after completely dissolved, it will overflow the bucket if you don't keep it somewhat emptied of the brine that forms.
The brine, diluted and used in moderation will make your tomato and pepper plants happy after a heavy rain. They love Calcium...
At $20.00 US per 50 lb. bag, we don't bother to dry it out for reuse. If you wanted to do a small quantity, a microwave does the job. Just be sure
your container can take the conditions, including some pretty fierce bumping as the steam from water trapped inside chunks explodes out.
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I will need to check if Calcium Chloride is used for ice salt here, otherwise its expensive!! Would be a nice cheap way to get it though, Here however
even deicing salt isnt as cheap as in the states.
But many roads have little plastic bunkers at the end of the street with it in.......
The above has reminded me, it cant be calcium Chloride here, the bunkers would be full of water and they are dry all year! So maybe we use normal rock
salt??
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j_sum1
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No need for de-icing where I live. (Sigh.)
CaCl2 is sold as a dehumidifier at reasonably high prices. But it is also sold as a calcium booster for plaster-sealed swimming pools.
A couple of years ago we had a gutter block and during a storm a large amount of water flowed down the inside of the wall and saturated the carpet and
underlay of our guest room. Ten buckets, ten kg of CaCl2 a couple of fans and a week with the carpet lifted made a big difference.
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clearly_not_atara
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According to some things I've read, MgCl2 hydrates cannot be dehydrated by simple heating, because they release HCl and turn into magnesium
hydroxychlorides. CaCl2 on the other hand can be dehydrated by heating. So while MgCl2 is probably stronger it is much less practical.
[Edited on 04-20-1969 by clearly_not_atara]
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CaCl2
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CaCl2 is probably more practical due to being easier to obtain in large quantities. (Though I may be a bit biased.)
[Edited on 16-12-2017 by CaCl2]
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