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Q & A: Storing dry ice

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Most recent answer: 06/18/2013
Q:
Hi, Is there a way to keep dry ice from sublimating? My company recently looked into buying an ultra low temperature freezer (-86 degrees F.)for this purpose. We were informed that the dry ice would still sublimate due to the air movement inside the freezer. Could dry ice be kept from sublimating if stored inside some sort of pressure tank?
- Daniel (age 30)
Las Cruces, NM, USA
A:

Judging from the CO2 phase diagram () it looks like your freezer isn't quite cold enough to keep the dry ice from sublimating even in pure CO2 vapor at atmospheric pressure. At that temperature, it looks like the CO2 vapor pressure reaches almost 3 atm. That's not too high. A typical bike tire fills to about 4 atm.

So yes, a pressure-sealed container with a standard little relief valve to keep the pressure from going over 3 atm (for safety)  would work. However, you'd have to be very careful opening the container. Say you had a 1000 cm2 door. The pressure differential on it would give over 500 lb of force. I think ideally you'd have some interlock, so you couldn't open it without first releasing the pressure inside. Pressure relief valves are also easy and cheap- you just press until the hissing stops. I bet the interlocks aren't all that hard to find either. You could even just use a door that swings inward. It would waste a little space but wouldn't blow up at you.

Have you thought about a much cheaper solution? A  very thick styrofoam box provides good thermal isolation. The inside will soon reach the evaporation point of dry ice at atmospheric pressure. −109.3 °F. You'd get some heat leaks each time you opened it up, but a good design should minimize that. There are plenty of commercial versions of this passive storage available. I hope you haven't bought the expensive freezer. Just googling "dry ice storage" turns up lots of useful safety tipes, etc.

Mike W.


(published on 06/18/2013)

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