Sciencemadness Discussion Board

Storage of cryogenic materials

neutrino - 13-3-2006 at 16:57

A local store sells dry ice and I usually buy a pound or two to experiment with when I go by there. When I run out of ideas, I usually store my extra ice in an ordinary vacuum thermos bottle. A pound in the thermos will last around 12 hours if the thermos is stored at room temperature.

Recently, I had a pound left over and decided to see how long the ice would last if I put the bottle in my freezer. To my surprise, it lasted four days—well over what it normally lasts. My question is why does it last so much longer?

I have come up with two possible reasons, but neither can even come close to accounting for this difference.

The first is simple heat conduction. First I picked a point on the bottom of the inside vessel which will always be at the sublimation point of dry ice at 1 atm. (-80*C). Next I picked a point on the outer vessel close to where it is joined to the inner vessel and assumed that this point would remain at ambient temperature: 20*C in the open, -10*C in my freezer. Assuming that the rate of heat transfer in this case is proportional to the temperature difference, the ice will only last 50% longer.

The second is radiative heating. For the sake of this estimate I assumed that the outer vessel is a perfect emitter of electromagnetic radiation and that the inside vessel is a perfect absorber. Flux varies with the fourth power of temperature, so the ice should have lasted another 50% longer.

Still far short. What am I missing here?

chemoleo - 13-3-2006 at 17:59

How about the vapour pressure is much lower, therefore the rate of evaporation, and therefore the loss of solid CO2?

neutrino - 13-3-2006 at 19:30

I don't follow. The vapor pressure will always be 1 atm, the temperature of the dry ice will always be -78.5*C. The only thing I can think of that would determine the shelf life of the ice is the rate at which heat energy flows into the inner container containing the solid CO<sub>2</sub>.

chemoleo - 14-3-2006 at 04:31

Ok.. physical chem is not my strongest side.
Let's put it this way - water at 60 deg C evaporates faster, a hell of a lot faster in fact, than at 4 deg C, even though both temperatures are below the bp of water.
Same for iodine, I imagine, which sublimes. At RT it will evaporate/sublimate with time, but at 60 deg C it will last much shorter. Again it's below the actual melting/gas pt of I2.

The pressure of the gas you measure, say in a closed, evacuated container, is the vapour pressure, right? And this pressure would vary with temperature. The greater the temperature difference, the greater the vapour pressure. THat's why we use pressure cookers....

Quote:
vapor pressure, pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid. At equilibrium the rates of the two processes are equal and the system appears to be stationary (see chemical equilibrium). The vapor, like any gas, exerts a pressure, and this pressure at equilibrium is called the vapor pressure.
Temperature also affects the vapor pressure. If the system in equilibrium is perturbed by raising the temperature, then according to Le Châtelier's principle the system should react to relieve this stress; as the temperature is increased, the evaporation process, which absorbs heat, is speeded up to a greater degree than the condensation process, which gives off heat, so that the vapor pressure is higher when equilibrium is restored at the new temperature. If the temperature is increased enough to raise the vapor pressure until it equals atmospheric pressure, the liquid will boil. If the external pressure is reduced, as in a vacuum system, then the liquid will boil much more readily than under atmospheric pressure.
(dictionary definition, http://www.answers.com/topic/vapor-pressure).

In other words, if it evaporates faster, due to trying to achieve equilibrium at higher temp, then your loss of dry ice will be faster. Wha'ts wrong with this?

Magpie - 14-3-2006 at 10:02

Neutrino I think this problem is more complex than you have stated. I would model the thermos this way:

1. An inner, hollow, thin-walled, silvered glass cylinder filled with solid/gas CO2 at -80C.
2. Inner cylinder is surrounded by an evacuated annular space.
3. Annular space has an outer, thin wall of glass in close contact with a final thin wall of steel or plastic.

Then I would also assume that the heat transfer is:

1. Radiant + convective (in parallel) to the bottle outside wall.
2. Conductive throught the ouside wall.
3. Radiative to the inside of the inner wall.
4. Conductive through the inner wall.

If we neglect the convective mode and conductive resistances as minor, then we have

Radiative transfer to the outer wall = radiative transfer to the inner wall.

Solving this requires the geometrical factors and emissivities which we don't have. If we had the outer wall temperature (in each situation) we could solve it also, but we don't. So I don't see any easy way to answer your question.

I think it would be interesting to see how long it takes to sublime if you wrapped the thermos in a blanket and then put it in the freezer. :D

vulture - 14-3-2006 at 12:30

Quote:

In other words, if it evaporates faster, due to trying to achieve equilibrium at higher temp, then your loss of dry ice will be faster. Wha'ts wrong with this?


Once the temperature exceeds the boiling point, the vapor pressure has to be equal to the outside pressure.

Mr. Wizard - 14-3-2006 at 14:07

Maybe there is some gas in your thermos jug, that is to say, maybe it has lost it's vacuum. I would think dry ice should last longer than 12 hours in a thermos. I've read of LN being stored in them for a few days. If there was gas contamination in the jug, maybe the convective conduction is slowed down in the cooler? Have you tried a different jug?

[Edited on 14-3-2006 by Mr. Wizard]

neutrino - 14-3-2006 at 16:50

The outer jacket stayed at ambient temperature for the most part. In the open room this was 20*C, it was -10*C in the freezer. What else is needed here? I'm a little confused by the last part of Magpie's post.

I only have one thermos, unfortunately. What do you mean by 'convective conduction'?

Mr. Wizard - 14-3-2006 at 17:31

The fact that the outer jacket stayed at ambient makes it look like a good thermos. Obviously heat has to flow into the thermos, but if the heat transfer through the vacuum is much slower than through the jacket, the jacket will stay at 'ambient'. What I meant by convective conduction was the transfer of heat by the few remaining molecules of gas in the vacuum chamber. This should be very slow in a good dewar or thermos, but if you had a gassy one it might make a difference. Let's face it, it could be faulty. Vacuum tubes used to get gassy all the time, but you could tell by their operation something was wrong. I doubt the quality controls on thermos jugs are that rigorous. The rate of heat loss by a gas in low pressure is used in vacuum technology to measure the vacuum (Pirani Gauge). It fails after a certain level of evacuation is obtained (10-3 mbar), as the heat carrying ability of the 'gas' in the vacuum drops off severely after this level.

I've heard of shipping fish and game packed in dry ice with just styrofoam as insulation, and it would last for days. I've had a relative who used to do his own cryosurgery on skin blemishes and he said LN would last days in a big thermos. How long will your dry ice last in a styrofoam box in the freezer. I'd get a second thermos and try the experiment again.

Magpie - 14-3-2006 at 18:16

The outer wall temperature has to be lower than ambient otherwise there will be no heat transfer.