Sciencemadness Discussion Board

Condensing water vapor (cold trap) without dry ice

tandpasta - 21-6-2015 at 12:10

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?

aga - 21-6-2015 at 13:23

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.

jock88 - 21-6-2015 at 13:33

phlogiston - 21-6-2015 at 15:50

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.

Metacelsus - 21-6-2015 at 17:38

Could you use a dessication agent (for example, calcium oxide)?

Crowfjord - 21-6-2015 at 18:11

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).

tandpasta - 22-6-2015 at 12:49

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]

jock88 - 22-6-2015 at 14:02

Another possibility is to use liquid propane or butane. Flammability hazard would need to be looked after.

tandpasta - 30-6-2015 at 15:29

That doesn't sound easy (or cheap).

macckone - 30-6-2015 at 19:53

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.

My advice is get a carbon dioxide cylinder and make your own sock.

violet sin - 30-6-2015 at 21:22

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)

or this bare 60W junction for $2.28,
which I just bought one, only 40mm/1.57" square, but big fun for the money & already have a larger one. had 3$ 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.

joemacone - 1-7-2015 at 06:58

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.

vmelkon - 7-7-2015 at 20:35

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.

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.