Material selection for Dry gas transportation 1

[Jbgois]

Hi all,

I'm seeking for some help as I am putting together a specification for pneumatic tubing used to transport "pure & dry"gas.

The main contrainst is the following:

-The material has to be plasticiser-free (to limit risk of contamination)
-As we are working with "Dry gas", the water vapour permeability must be low. Our "wettest" gas has a dew point of -20C
-The gas has a max CO2 content of 2%, so the material should have a low CO2 (and other gases if possible) permeabilty.
-Working temperature: Application 1: 0C to 50C Application 2: 0C to 180C

Unfortunately, I don't have accurate value to give as until this moment no one in my company seems to have been looking into this. (If ain't broke, don't fix it...)

Although, I struggled getting data for most of the material I was looking at (I ended up using this Table), I have put aside, Nylon (water vapour permeability too high) and PVC (phthalate).

For application 1, 0C up to 50C, I'm curious about Polyurethane, (ether rather than ester based), but I can't find any relevant information on this material. Could anyone communicate me some value, please? Otherwise, based on the value I have at the moment, my preference goes to PP over PET due to lower water vapour permeability.

For application 2, 0C up to 180C, PTFE is currently used but after looking at the values, I was wondering if PFA would be better for this application. Again, I can't find gas permeability value for PFA and I was wondering if someone could communicate me some values, please?

My background being mechanical engineering, I would more than welcome advices, critics or helpful link.

Thank you for your help

 

[patprimmer]

My first intuition tells me biaxially oriented PET will be good to most gasses.

Maybe a fluorine gas treatment will improve the performance of whatever you choose.

Multilayer may also be a possibility with nylon or PET for all but water and PP for water barrier.

Regards
Pat
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[Jbgois]

Thank Pat for you reply.

And whitout going that far: I was originally looking at something a little bit more "standard", Off-the shelf type of product, as the volume of our production can't really justify a tailor-made tubing.
What about basic grades? PP? PC? PET? PU? and any other grade you may know about? i understand the level of gas barrier will be inferior to your solution, but i think it will be easier for me to talk people into this solution.

Also I have never heard anything on this subject, but how high is the risk of having product leeching from fluoro tubing and under which condition this can happen?

Cheers

JB

 

[patprimmer]

It is quite possible that co=ex tubing is commercially available unless you are tied to an unusual bore and wall thickness combo.

Does this need flexibility.

Nano clay inclusion and/or thick walls all helps.

I know fluorine treated MDPE is suitable for storing tooth paste.

Regards
Pat
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[patprimmer]

The 180 deg C application will have a much shorter list.

Does the tubing need to be flexible.

Regards
Pat
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[cloa]

Funnily enough that document actually graphically lists its PFA permeability for CO2- its quite close to PTFE. Assuming its used in air and you don't mind you lose if you lose CO2- Air's CO2 is 300 ppm- you are more likely to lose CO2 than gain especially since the hot side in interior.

 

[Jbgois]

Sorry pat, I was trying to find you some value in case that would help:

For O.D 6mm, wall thickness 1mm --> Min bending radius 34 mm
For O.D 8mm, wall thickness 1mm --> Min bending radius 60 mm
For O.D 10mm, wall thickness 1mm --> Min bending radius 95 mm

So, it's relatively flexible and that suits our systems well as they can be quite "compactly arranged".

Cloa, thanks for your feedback. If I understand your comment correctly you are saying Greater permeability for letting more CO2 out? Can that be applied to the water vapour permeability/water absorption as well? I'm slighlty concern, that during down time, when the pressure drops, the opposite might happen and the system would fill with CO2. Would this process be driven by the pressure difference or the CO2 gradient?

We were previously using some nafion tubing on another range of systems and the problem we had was that it balanced the humidity between the two environments (inside/outside), which means that in dry environment, it was working fine, but in more humid environment it reduced our gas dryness.

Regards,
JB