"Home made" desiccant

[FlaMtnBkr]

So I was looking at a container of 'Damp Rid' brand desiccant that contains calcium chloride. It got me wondering how do you know how well a desiccant will work? If you have 2 hygroscopic chemicals how do you know which one will absorb the water? What I imagine happens is they both absorb water to a certain extent, one more than the other. And if that is the case then it's only somewhat helpful. Anyone know what dictates how well a desiccant works?

 

The way I use desiccant I guess it doesn't matter but it also doesn't allow me to take a chemical that has some water in it and have that water pulled out over time which would be nice. I dry my chemical (which can be a pain if large quantities and an old toaster oven are on hand) and place in a sealed container which I then put in another sealed container that contains the desiccant. Depending on how hygroscopic it is I might even put it in another container. I'm in the process of drying 50# of material that is about 8% water and it's not fun, quick, or energy efficient. Once dry I don't want to have to worry about it again.

 

Back to the Damp Rid. It is calcium chloride which is not only hygroscopic but also deliquescent, meaning it will absorb so much water that it will dissolve itself and turn into a puddle of water. So one of their products is a plastic container that has a upper part with holes in the bottom that holds the CC, so that the CC absorbs water and then drips through the holes into the bottom catch container. So I was wondering with all the plastic containers we put in the recycle bin, surely I can rig something similar. I don't live in the snowy north so big bags of de-icer (calcium chloride) are unavailable, just little expensive bags of Damp Rid refill. So I was thinking that ammonium nitrate is also deliquescent and should work pretty good pulling water out of the air (and maybe less hygroscopic chemicals?) Any downside anyone can think of? I can get 50# bags for quite a bit less than some of the other common desiccants. It wont be as easy as throwing a sock full of silica gel into a double bag of chemicals, but it might work good sitting in the top of a 5 gallon bucket.

 

Any thoughts, criticisms, or off topic ramblings are welcome. Thanks!

 

Edit: Can molecular sieves be used to pull water out of the air? Or do they only work in the bottom of a container of liquid?

Edited April 1, 2012 by FlaMtnBkr

[Mumbles]

If they're activated, molecular sieves most certainly can pull water out of the air. It's a very common way to dry gases. For this the gas is passed through a column of mol sieves. If just placed into a container with still air, it'd still probably absorb water over a period of time. I don't know if just heating them in the oven will activate them enough though. There is a more thorough method where they are heated under vacuum. I don't understand adsorbents (ie mol sieves) all that well, so I really don't know the dynamics of how they work with gases all that well. With liquids they will have an equilibrium that they can attain, so I assume there is a maximum level that gases can be dried too.

 

Desiccant strength is determined via the vapor pressure of water over them. The lower the vapor pressure, the stronger the desiccant. Nothing has a vapor pressure of 0 above it. Ultimately, if you have two different desiccants in a closed system one will dry the other to the extent an equilibrium is established. At approximately room temperature 100% RH is 17mmHg of water. Above CaCl2*6H2O (saturated CaCl2, probably a liquid) you have a vapor pressure of 10 mmHg. Above an excess of totally anhydrous CaCl2, you can achieve something on the order of 1mmHg. Thus an excess of totally dry CaCl2 will dry down to 1mmHg of water or about 6% Relative Humidity. Any hygroscopic material will still establish this type of equilibrium. Sodium Nitrate, ammonium nitrate, etc all have values like these (which I don't have by the way). As long as you can keep the relative humidity of the atmosphere lower than it's equilibrium vapor pressure of that material it will not absorb water, and will actually eventually give up any water it has absorbed. I'd be willing to bet that calcium chloride is a stronger desiccant than anything we use in pyro.

[Peret]

Move to the sunny south west! We get below 10% in the open air on an average day and it's an unusual star that's still wet after 24 hours outside, especially when the wind is blowing. As a plus, unlike your state, we have actual mountains to bike on.

[warthog]

We have neither mountains nor dry air here.

 

I find that a barrier helps a lot to determine which will do the absorbing. I put my hygroscopic chemicals into a zip-lock baggie I squeeze as much air out of as I can when sealing and then this goes into a tub than has a snap on lid into which I put a sachet of desiccant. The tubs all then go into either a plastic storage tub with a good, tight lid that also has several or one large sachet of desiccant OR a gamma sealed bucket that has sachet of desiccant. Fo far at least, this seems to keep most of the moisture troubles to a minimum but I still wind up drying the chemicals before using. The drying time is drastically reduced since adopting this storage method though.