[berlinjamal]

Hello, this is my first post to this forum.  Looks like a great site. 

I am doing some testing on dry ice shipments on airplanes, a special project my company assigned to me. Dry ice sublimates and therefore releases CO2 in the airplane. The FAA restricts CO2 concentration thru 14 CFR §25.831(b)(2), which limits the maximum concentration of CO2 during flight at 0.5% by volume in compartments normally occupied by passengers or crewmembers. 

My question is regarding the "sublimation" during change of pressure and change of ambient temperature. The sublimation point of the dry ice is -109.3F. Atmospheric pressure decreases with increasing elevation. Atmospheric pressure is 14.7 psi and aircraft pressure is around 10.3 psi, maintains to an equivalent altitude of 8000 feet.

1. At what ambient temperature does dry ice sublimate faster?
2. Does the lower cabin pressure increase sublimation of dry ice?

The FAA's average sublimation rate of dry ice is 2% per hour. That is what airlines use to calculate dry ice shipments (dry Ice limits are derived from the accepted method to calculate the mass of Dry Ice, provided in the FAA Advisory Circular 91.76A).

So if the sublimation rate of 2% has been given, does temperature and pressure have a very minimum effect on sublimation, like +-0.3%. Very insignificant?

In other words, should I be concerned about temperature and pressure?

[Zyklonb]

Yes the lower pressure will increase the rate of sublimation. The temperature will not change at all until all of the carbon dioxide has evaporated, just like water boils at 100 deg. C until its all vapor. Its because the energy is used not to increase the temperature, but rather to change the state of matter, this is called heat of vaporization.
How much faster it sublimates isn't really possible to calculate with the information given. It will depend more on the quantity and the surface area of the dry ice, the bigger the chunks the slower it will sublimate.
However you can look up the boiling curve under various pressures.
The best way to figure this out will be experimentation. 

[billnotgatez]

I did a internet search on
sublimation rate of dry ice at altitude
and got the below link
dtic.mil/cgi-bin/GetTRDoc?AD=ADA461451
Plus many others

What is interesting is that there are other factors like
pellet or solid block that effect rate

Also looking at some of the other links it appears that the 2.0 +/- 0.3%/h was determined empirically.
http://trid.trb.org/view.aspx?id=790971

It seems the aviation industry several official documents on this issue. So there is no shortage of experimentation already done on this topic.

[Borek]

What is interesting is that there are other factors like pellet or solid block that effect rate

They have different surface per mass unit, and that's what counts (sublimation takes place at the surface only).

[berlinjamal]

Reply back to Zyklonb:
Assuming i have 10lbs of dry ice in pellet form, the ambient outside temperature will have no effect on the sublimation? Whether its 100F or 50F ambient temp, the sublimation on a 10lbs pellet of dry ice will not differ? Ambient temperature has almost no effect on the sublimation.

So in essence we are saying, its the size the dry ice comes in that is the most important factor for sublimation rate. The bigger the block, the slower the sublimation. 

Airplane cabin pressure of 10.3psi has a very very insignificant impact on the sublimation. As Borek mentions, the sublimation happens at the surface area of dry ice. So a 100 block of dry ice will sublimate much faster than a 10lb of dry ice, I guess it was calculated 1% sublimation vs 2% sublimation. Its really surface area i have to be concerned, and not cabin pressure. I know now that temperature has no effect on dry ice.  And pressure has a very insignificant impact on sublimation.  Please correct if I am wrong...and also thank you very much.

[billnotgatez]

From what I see of the studies on this by the aviation community, they kept the temperature constant and varied other attributes like altitude and surface area.

@berlinjamal
What is the typical cabin temperature range kept at when flying at the pressure to maintain 8000 feet?

Your statement about the rate of sublimation

Whether its 100F or 50F ambient temp

based on your interpretation of what @Zyklonb posted has me puzzled.

[Zyklonb]

No that's not what I said. The temperature will affect the rate of sublimation, but it will not change the temperature of the dry ice itself. The energy from the increased temperature will be used to evaporate the carbon dioxide, just like a stove uses heat to boil water, but once the water is boiling it's temperature stays the same.

[berlinjamal]

cabin temperature can be set from 50F-85F.

I am still confused how ambient temperature would make a difference to 10lb pellet of dry ice.

I will think thru this...

[billnotgatez]

An observation
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50F = 10C = 283.15K
86F = 30C = 303.15K

(303.15-283.15)/283.15 = 20/283.15 = 0.070633939607981635175701924774854
which is close to 7%
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