MOC for nitric acid 2

[andmil]

I received some very good info re. MOC from this thread: thread124-190648. However, further investigation has revealed how difficult they are to get hold of!

Can anyone advise on the suitability of glass lined reactors for the following process conditions:

70% Nitric acid @ 121ºC (BP).

The following issues are associated with using stainless / duplex / noble metals:
304L - unsuitable
Titanium - suitability questionable. Corrosion rates are quoted from 0.05 - 0.9 mm/year under these conditions!
Hastelloy G30 - Very scarce
Proprietary duplex grades - Only used for certain applications e.g tubing or require very large orders
Zirconium - Cost?

Considering the above, glass lining seems to be the best option.

In addition, the vessels will be trace-heated with possible cable sheath temperatures of 450ºC. Can this accelerate corrosion? And is this an issue if you have glass-lining?

Thanks in advance

 

[unclesyd]

Can you comeback with a better description of your proposed process using HNO3?

We run 65% HNO3 + organics in Titanium Equipment 24/7/365 with no discernable corrosion. We have several in house fabricated Ti tube bundles that have been in service since the late 60's. Our process is exothermic and we maintain our process at 121C with the heating surface not any greater than 125C by resorting to vacuum processing. If we get into the high 120's the process goes boom.

If your process has organics I wouldn't allow anything near your 450C near the equipment.

If you don't have organics only inorganics other metal ions will cause accelerated corrosion at higher temperatures.

 

[andmil]

No significant amount of organics present.

HNO3 is pre-heated through an external heat exchanger from ambient to 115ºC.

Then transferred from an intermediate vessel into the reactors where it is then trace-heated from 115ºC - 121ºC.

My main concern is whether a material of construction such as Ti can handle this duty, or alternatively whether glass lined reactors are suitable for trace-heating?

 

[kenvlach]

What levels of Cl^-, F^- and metallic impurities?

From C. P. Dillon's book, Materials Selection for the Chemical Process Industries, 2nd Edn.,:
Titanium Grades 2 & 7 are suitable for 65-90% nitric acid.
Zirconium is more resistant than titanium for 65% nitric and is even used up to 230 °C.
Tantalum is even better, resistant to all compositions to at least boiling, but more expensive.

“Glass and ceramic materials resist nitric acid up to 65% to at least 125 °C (260°F) and, with reduced strength and life, to almost 205 °C (400°F). Because of the leaching of iron and siliceous components, the maximum temperature should be limited to 100 °C (212°F) in the 65 to 100% acid range.”

I'm not familiar with glass linings, but a purer glass (borosilicate such as Pyrex or 96% silica such as Vycor) should give better chemical resistance. However, the lower COE of these glasses might result in cracking due to substrate's greater expansion.

Corrosion Resistance of Titanium to nitric acid:

http://titanex.com/tiabout/specific...on-applications.php?sel=7#Resistance to Acids

http://www.timet.com/pdfs/corrosion.pdf

(pages 12-14 for nitric acid)