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Topic: Can gas pass a wall?  (Read 5029 times)

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Offline Palladipeloarancione

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Can gas pass a wall?
« on: September 05, 2012, 04:21:46 AM »

Hi everybody,

I'm investigating a fire that burn insulating foam ((inside a wall) and I wanna know where the gases gone.

That my ideas:


The volume of gas increased because of stoichiometric transformation – the foam was of Polystyrene
 -  ( with n=1:  6 O2 + (C8H8)n → 8CO + 4 H2O   from 6 mole to 12 mole) .

I find that Polystyrene
 has got:
Flash point 345-360 °C (618,15 - 633,15 K)
Melting point    
~240 °C[3] (decomposes at lower T)


CO Gas  was  drive outside the wall from the pressure?

But how much smoke does pass a wall?

This is the questions

I'm wondering if it's possible to estimate the flux rate of gas (essentially CO) out coming from the wall.

I know which are the wall layers:
mortar 1 cm
brick 20 cm
polyester 2 cm

Unfortunately I know only “water vapor permeability” of these materials.

Do you think I can use the water vapor permeability to assess whether and how a gas (carbon monoxide) has gone through a wall?

Recalling the definition of 'resistance to water vapor μ
“The value μ of a material construction is a parameter, without dimension of the matter itself, which indicates how many times the building material is more insulating vapor, compared with a layer of still air of the same thickness.
The larger the parameter μ, the greater will be the impermeability to vapor of the material construction!”

I can use it to get an indication (or exact values, note the differential pressure) of a gas other than water vapor?

Here the wall values, with μ equal to:
mortar 38
brick 8
polyester 45


Using Fick's law - for one dimension - that ∂ c / ∂ t = - D ∂ ² c / ∂ x ² ; dimensionally
∂ c / ∂ t [kg / m³ / s] = - D [m · s] · ∂ ² c / ∂ x ² [kg / m³ / m²]

in steady state I can transform this one as:
0 = - D · ∂ ² c / ∂ x ² and integrating - K/D = ∂ c / ∂ x
but I can go further...
Any idea?

It seems Fick's law is not the right way to think the problem...


Thanks.


Andrea

Offline Enthalpy

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Re: Can gas pass a wall?
« Reply #1 on: September 05, 2012, 12:28:44 PM »
Grüss Dich Andrea!

- Yes, some gases (the smallest molecules) pass through some solids...
- But normal gases don't pass through normal solids under normal conditions in normal amounts!
- In your case, only holes, cracks, crevices are relevant, diffusion is not.

-----

Examples where diffusion can be noticed :
- A helium balloon, because He diffuses so quickly, and macromolecules, especially elastomers, are so permeable.
- A silicon chip in an epoxy package, because vapour diffuses quickly (less so than He and H2), epoxy is a polymer, and minute amounts of vapour over years corrode <1µm of metal thickness on a chip.
- Hydrogen (of course!) in some metals like palladium in thin film or powder form, and then it may be adsorption (of individual protons in the metal).
- Hydrogen embrittlement of steel. Put 1/4h at 180°C (kitchen oven), the hydrogen is gone.
- Water purification by reverse osmosis. Again, chemical or Van der Waals forces act. And just check the surface and the thickness of the polymer required to get a little throughput!
- Washing water in polyamide clothes
- Semiconductors, because the transistors are in the first 100nm depth, manufacturing involves heat, and the slightest impurity has tremendous consequences.

-----

Normal gases like N2, CO, CO2... do not diffuse through metals, compact ceramic... under normal conditions. You would need your bricks to be porous, and then a diffusion equation won't tell it, because it's not a property of the compact material.

So much that people willing to test the hermeticity of a ceramic package for electronic chips heat then, use helium, wait for long, and have an ultra-sensitive helium detector.

-----

Fick's law would have been the proper one and you used it properly (and its answer, zero, would be right as well...). You need to complete it with one experimental data that tell the equilibrium ratio of gas concentration in the air versus in the solid - hard to find.

My suggestion: CO permeability is much lower, so use vapour permeability to prove that diffusion is negligible. Or neglect the whole hypothesis anyway.

Offline Palladipeloarancione

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Re: Can gas pass a wall?
« Reply #2 on: September 06, 2012, 05:19:20 AM »
Thank you,  Enthalpy, for you detailed answer.
 ;)
Andrea

Offline Palladipeloarancione

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Re: Can gas pass a wall?
« Reply #3 on: September 06, 2012, 02:08:16 PM »
Wonderful.
I found a specific EN norm that's about this thing:  DIN 4108, or Italian  UNI EN 12086:1999.

In these the flux is
ΔQ/Δt [mg/h]= λD [mg/(m·h·Pa)] · A [m²] · Δp/Δx [Pa/m]

with permeability to vapor µ = λDL/λD   
(see also http://www.wufi-wiki.com/mediawiki/index.php5/Details:WaterVaporDiffusion)


I don't know how much pressure can achieve the combustion, and a lot of other thing...
but - by the way - I get a tip :D

Quote:
Normal gases like N2, CO, CO2... do not diffuse through metals, compact ceramic... under normal conditions. You would need your bricks to be porous, and then a diffusion equation won't tell it, because it's not a property of the compact material.


It seems that construction material are very porous...
May be because of the wall transpiration...

 

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