3 replies to this topic

[chemks2012]

Hi all,

I need your help once again please.

We have a Nitric Acid [40% nitric acid] pickle tank at about 85 deg C and another pickle tank containing a mixture of hydrofluoric acid and nitric acid [2% hydrofluoric and 30% nitric acid] at about 30 to 35 deg C. Please note that vents from both tanks are connected to a common ducting header.

I wish to select the material of construction for the ducting with approx diameter of 700mm and not sure which one is most suitable material of construction for above chemicals.

I am aware that hydrofluoric acid attacks glass and therefore, GRP is a non-starter. PVC and PVDF not suitable for higher temperature?

I believe PTFE would be suitable [PS:  I am aware of cost issue ] but wondering if someone has past experience and PTFE gives of chlorine at such high temperature.

 

Thanks in advance for your help.

KS

[fseipel]

Polypropylene should hold up; it is cheaper than teflon.  If it is extremely low pressure as for a scrubber, stormboss or similar polypropylene piping might be very economical compared to teflon lined carbon steel.  What is meant by teflon giving off chlorine?  It withstands 85 deg C with no problem, it's just expensive. 

[chemks2012]

fseipel,

 

Thanks very much for your input. The ducting vendor mentioned chlorine evolution from PTFE during hot work e.g. welding.

However, it seems that it's a different issue i.e. PTFE is suitable for our application and I believe welding is not our problem.

 

[fseipel]

If you heat teflon to 350-400 deg C, decomposition products are formed.  You should not see any chlorine containing byproducts as teflon contains only Carbon and Fluorine -- so you would see organofluorides, primarily (CF2)=(CF2).  These compounds effect birds more than humans -- if you heat an empty teflon coated pan to 400 C on your stove, and have a parrot in the room, it will be severely effected, whereas humans typically exhibit flu-like symptoms.  If you're welding above teflon or plastic ductwork or pipe, simply cover it with a fireproof tarp so that the slag does not melt through it or damage it while welding.  Teflon chemical resistance is great but it is just so expensive -- I just bought some 6" PTFE lined carbon steel teflon ells, they were around $1400 ea (Resistoflex brand) -- duct would probably be appreciably cheaper, though. In contrast a 6" polypropylene ell from Simtech is $60.  You can buy PTFE ducting with spiral reinforcement but you have to support it.  If you have a short segment of polypropylene tubing you could immerse it in a hot acid bath or hang the tubing segment above it to see if it yellows or exhibits signs of breakdown.  e.g. cut a 1/2" tubing segment.

Edited by fseipel, 20 November 2013 - 07:25 PM.