# Water Sublimnation at low presures

1. Feb 27, 2013

### NSS

Say we have 2 liter of water ICE in a 2 liter soda bottle.

Assuming sea level air pressure is, 14.7 PSI

We lower the temperature of the atmosphere around the ice to -40C

And we lower the pressure to 0.5 psi (roughly 75,000 feet)

The air around the ice is pure dry no water vapor in it at all. It's container, (probably a 2 liter soda bottle) will be vented via a tube to a balloon, that is filled with Hydrogen.

assuming this environment, at what rate would the ice Sublime to water vapor?

And how many cuft or cubic meters of water vapor would it produce for "X" amount of time?

Thanks

2. Feb 28, 2013

### SteamKing

Staff Emeritus
Unless you add heat to the ice, I think the rate of sublimation will be very low, if not zero. At the conditions you specify, the ice will be happy to stay solid.

3. Feb 28, 2013

### NSS

Low is a relative number.

How much do you think as a low number?

4. Feb 28, 2013

### SteamKing

Staff Emeritus

5. Feb 28, 2013

### Staff: Mentor

Even at -40 degrees, ice has a non-zero vapor pressure. If the air is bone dry, the ice will certainly evaporate into it. The rate at which this happens depends on a number of factors, but obviously will be very low. By the way, it's very unlikely that atmospheric air at 75000 ft will be bone dry.

6. Feb 28, 2013

### NSS

But what the water is exposed to is bone dry, 100% Hydrogen only.

Well at least 100% Hydrogen only until the water starts to enter it, by the sublimation.

7. Feb 28, 2013

### Staff: Mentor

Well, yes. Hydrogen. But that wasn't really relevant to what I was saying. At the interface between the ice and the hydrogen, the partial pressure of the water is the equilibrium vapor pressure of ice at -40. The difference between this partial pressure and the partial pressure of water vapor in the bulk of the hydrogen phase (zero) provides the driving force for evaporation. The mass transfer occurs by water vapor diffusion through the hydrogen in the immediate vicinity of the interface. The more that the hydrogen is mixed/stirred/agitated, the thinner the diffusion boundary layer at the ice interface, and the higher the rate of mass transfer (evaporation).

8. Feb 28, 2013

True,