Sublimation process of dry ice in a closed space

[adgirard]

Hello everybody,

I would like to understand a bit better the sublimation process for the purpose of my study.

I then have a dry ice block (m = 3kg, Vice = 2.10^-3 m3) in a closed space (Vtot = 4.10^-3 m3), in an ambient temperature. The temperature of the inside of the box (the closed space) is at Ti = -80°C and at Pi = 1 atm.
This dry ice block will thus sublimate due to the heat coming from the outside, and after 15 hours all the dry ice is sublimated and the Pressure P is thus Pf = 87.4 atm (I calculated by myself so it may be wrong).

With the help of

http://www.thermofluids.net/

I found the enthalpy of formation at the first state is h1 = -95.2 kJ/kg and at the end h2 = -298.6 kJ/kg. The value given by wikipedia for the sublimation of the dry ice is 574 kJ/kg. I would like to know how to interpret the increase of pressure on the energy absorb by the dry ice. How do I have to link the h1, h2 and the value given by wikipedia?
Thank you very much.

 

[willard3]

Sounds like homework to me.

 

[adgirard]

What do you need to prove you that this is not homework?
You want to check my linkedin profile?

 

[chicopee]

What makes you think that it will sublimate within the time stated? Your should consider that the rate of heat transfer thru the tank wall will slow down as you will develop an ice coating from condensed moisture on the outside of the tank wall and that ice thickness will build up until it reaches a steady state with ambient conditions.

 

[adgirard]

Thank you for your answer chicopee.

I already did quite a lot of experiments that proves that there is almost no dry ice left after 15 hours in our condition (T = 22°C and P = 1 atm), but with a NOT airtight box.
We now would like to learn if it is interesting (and how much it is) to build an airtight box around the dry ice to increase the cold capacity of the dry ice. I would like not to add to many parameters at the same time to understand it properly. So lets forget the heat transfer through the tank for now, assuming that by changing the material of the tank we can make sure that the sublimation is total at the end of the 15 hours.

 

[EnergyMix]

You might find this site somewhat helpful:

http://www.dryiceinfo.com/.

While it doesn't have the answer for your question with an exact number, it might give you some resources to call for people who work with dry ice who already would have the answers you need. Plus it points out that it's not a good idea to put dry ice in an airtight box because it could explode.

It could be you've already talked to them; I don't know. You haven't shared that information.

Want to know the do's and don'ts of Eng-Tips? Read FAQ731-376.
English not your native language? Looking for some help in getting your question across to others or understanding their answers? Go to forum1529.

 

[chicopee]

Assuming adiabatic condition, you'll have a time step (delta t) problem that involves heat transfer between CO2(s) and air in the box and mass transfer from CO2(s)and the vapor space containing air and CO2(g). Initial condition at time(t)=0, you have a chunk of ice of know mass and presumably square shape at a known temp(-110 dF) and air in the box of known temp and pressure. So, you'll have changes in internal energy of the ice and air, heat and mass transfer between the two throughout the time steps of equal interval; you'll have reduction in mass of the ice, an increase in mass of the CO2 vapor, a constant mass of air, a decrease in temperature and pressure of the gaseous mixture. You'll have to work the equations yourself. Eventually, the vapor mixture temperature will approach that of the ice (if any is left and the box is not too big)at which point the steady state is for all intended purposes is reached.

 

[IRstuff]

Where is the heat coming from that allows the dry ice to vaporize? -80ºC is below its freezing point, and unless there is a heat flow into the space, the dry ice theoretically does nothing.

TTFN
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[adgirard]

Thank you for all this information.
I will take a look at the equation.

The box around the dry ice is very small because it is only here to store the dry ice. So, let say 2 times the volume of dry ice.
The box is not specifically insulated and heat comes from the environment around the box.

 

[chicopee]

IRstuff, I am assuming that the air around the dry ice is warmer as the initial condition, therefore there will be heat and mass transfer between the two medium. Heat transfer, but not necessarily mass transfer, will stop once the air temperature is at the dry ice temperature if any dry ice is left. Theoretically the pressure within the box will drop and an implosion, not explosion as mentioned by one of the responders, could follow.

 

[racookpe1978]

So you just substantially changed the problem: Now you have a transient heat transfer problem.

(Does that theoretical box touch the ground (If so, what kind of ground/pavement/plate/wood/table/plastic is conducting heat upwards into the lower floor of the box?) , or is it in mid-air?

Heat from the outside (air or combination of air and support plate) heats the floor and walls of the small box, then that heat heats up the very cold CO2 bolck AND begins to sublimate the CO2.

You have both latent heat gain and sensible heat gain to think about inside the box.

 

[IRstuff]

Again, you stated that the air temperature is -80ºC, so what is the correct air temperature?

Your final answer should be consistent with the phase diagram

TTFN
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[chicopee]

Initially the air is at some ambient temperature and that temperature should go down to -110 df, then we have thermal equilibrium. Depending how big the box and CO2 block are, the heat transfer will be either by conduction if the air gap is small or you'll have convection. I would not dismiss, initially for a while, radiation cooling to complement, either conduction or convection.

 

[macmet]

I'm curious as to what the actual application of this study is. What is the intent?

 

[adgirard]

The intent is too keep a box (another one, not the one where the dry ice lay) at a certain temperature.
The dry ice is used as a buffer to suck hot from the environment.

 

[adgirard]

It is actually a very simple application but I would like to make it very optimized and make the better use of dry ice possible.
That is why I am investigating, maybe too much, where the company never investigate before.

 

[MortenA]

You can get lots of CO2 data here:

http://www.union.dk/media/CO2-GB.pdf

From this you can see that at 20ºC your dry ice will increase the pressure to around 70 barg

Best regards,

Morten

 

[dvd]

http://www.cnn.com/2013/10/16/travel/los-angeles-airport-dry-ice-arrest/

 

[dvd]

I red flagged this post several weeks ago based on this.

 

[25362]

Some threads that may be of interest:

thread71-231218
thread135-76882
thread391-193500