tandpasta
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Condensing water vapor (cold trap) without dry ice
I'm trying to learn how to freeze dry/lyophilize things. This tends to produce a lot of wator vapor which needs to be condensed in a cold trap so that
it doesn't wreck the vacuum pump. Everything I come across mentions dry ice with either ethanol or acetone (or just LN) to provide the cooling power.
But I can not get dry ice cheaply, I have to spend a minimum of about €30 for the smallest amount of dry ice, which is quite a lot for some beginner
experiments. Not to mention that it will all sublime in a couple of days. I've asked around at fish/meat places and with some friends who work in
labs, but there's none to be had cheaply.
Is there any alternative to using dry ice? Could I use a regular ice bath with a really long cold trap?
Or maybe build some kind of cooling contraption with a scrap fridge compressor?
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aga
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How cold do you need to go ?
Normal ice with table salt gets pretty cold, and if you can have multiple stages, then that might work.
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jock88
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http://www.engineeringtoolbox.com/freezing-mixtures-d_1614.h...
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phlogiston
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It only needs to be below the triple-point, ie. 0 deg C. Then, the lower pressure will cause a phase transition directly from solid to vapour
(sublimation).
The enthalpy of sublimation will keep your sample frozen until it has fully dried.
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Metacelsus
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Could you use a dessication agent (for example, calcium oxide)?
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Crowfjord
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For sub-zero cold baths, I like to use ice-alcohol. With either ethanol or isopropanol, the bath gets down to about -15 to -11 C (estimated, as my
thermometer's gradations only go as low as -10 C).
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tandpasta
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I'm not sure how cold I need to go, but my vacuum pump has a water vapor tolerance of 15 mbar.
jock88, thanks for that list! It seems that calcium chloride (or calcium chloride hexahydrate) + ice is the way to go. That should achieve
temperatures of around -40C.
Cheddite, I suppose that's possible, but I'm not sure how to build something like that. Maybe fit filter paper on the bottom of a column and fill it
with silica?
[Edited on 22-6-2015 by tandpasta]
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jock88
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Another possibility is to use liquid propane or butane. Flammability hazard would need to be looked after.
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tandpasta
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That doesn't sound easy (or cheap).
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macckone
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At that price for dry ice, you might want to look for liquid nitrogen.
My guess is that it will be cheaper.
You may also try finding a carbon dioxide supplier and make your own.
That is likely to be a fair price on the cylinder rental but the liquid
in a large cylinder will convert to about 50% solid when used with
a collection sock. Which is basically a loose knit material that is flexible
at those temperatures like a knit wool sock. They also
sell them for labs as that is how labs make dry ice.
http://www.scientificsonline.com/product/dry-ice-maker-attac...
My advice is get a carbon dioxide cylinder and make your own sock.
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violet sin
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what materials are you using for the components you already have assembled? is there any way you could integrate a peltier thermoelectric cooler for
the cold trap.
I would think that a months worth of experimenting electricity for cooling wouldn't be as costly as the minimum dry ice purchase? a guess, depends
on the wattage and time obviously. that is all only relevant if you can marry the device in easily and have a power supply for it.
bare 400W for just under $20-USD(I'm state side, so all $ in USD)
http://www.ebay.com/itm/NEW-400W-12V-Thermoelectric-Cooler-P...
or this bare 60W junction for $2.28,
http://www.ebay.com/itm/221489461411?_trksid=p2060353.m2749....
which I just bought one, only 40mm/1.57" square, but big fun for the money & already have a larger one. had 3$ left on the paypal so figured why
not.
just a suggestion for a solid state answer, instead of buying consumables every time. though not every one has a power supply that can crank out 26A
@ 12-15V for the 400W guy, the 60W bugger still takes 6A @ 12V, thinking ATX PSU from a computer. of which I have several laying around. there were
more than a few wattages available on ebay, so might find one you can use.
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joemacone
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What about windshield washer fluid that had been stored in the freezer? You could also store a container of some sort--a paint can wrapped in isolant
if you want.
It would most likely not be cold for that long, though.
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vmelkon
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A thermoelectric cooler is one way to go. As mentioned above. Mine was able to drop 22 °C, so from room temp of 20 °C, I obtained about -2 °C. You
just need to ice the hot side and the cold side should reach -25 °C.
One guy was able to run in series two TEC and he reached -58 °C.
https://www.youtube.com/watch?v=mqM4CsM4ewI
Another method is to buy compressed air. I can get a ~400 g can for 3$. It contains 1,1-difluoroethane. Bp is -25 °C.
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