Diluted Sulfuric Acid- Corrosion problem 2

[Mike4chemic]

Dear All,

In our geothermal power plant we used a diluted sulfuric acid in the geothermal water pH modification system.

We "manufacture" the diluted sulfuric acid (about 0.8 -0.9 % w) by means of the dilution of the 98% sulfuric acid with the slip stream of the untreated geothermal water.

The dilution ratio is about 1: 100. The concentrated acid flow is 92 kg/hr and water flow is 10,000 kg/hr. The dilution water temperature is about 170 deg. C.
We are experiencing quite a few problems with the dilution system when the biggest problem is that we are getting extremely high corrosion rates (about 0.1" in two weeks) in the Hastelloy C276 piping, especially around the 98% acid injection point in the water slip stream and through the 2" Static Mixer (Hastelloy C276) which located 2 meter downstream of the concentrated acid injection "T".
Do you have an idea how I could find the cause of this problem?

Thanks in advance,

Mike

 

[Leclerc]

what is in this geothermal slip water?

I have known massive corrosion rate increase when ppm concentration of sulphides are present.

 

[eahrens]

We predilute our concentrated sulfuric to 5% in a pH control application, and it works fine. We are using a PTFE lined steel pipe spool static mixer with glass filled PTFE static mixing elements. The concentrated acid is injected using a PTFE lined instrument tee. No quill is needed, as this piping can handle the heat from the exothermic reaction. Komax Systems, supplied the mixer. PTFE lined mixers are a fraction of the cost of a hastelloy mixer.

 

[pumpking]

Non metallic pumps and lined pipework all the way...

We have had so many instances of concentrations droppingonly very slightly and eating metal pumps all too quickly. ETFE or PFA lined systems have many years experience of safely handling varying degrees of Sulphuric.

Have you looked at PTFE/Teflon surface treatment such as Belzona coating, Edlon etc etc. I know they do them successfully to tanks, pump components ertc, and may well be worth investigating this further to cover your quill ?

Ash Fenn

http://www.cdrpumps.co.uk

 

[agf94510]

From my experience, it is the method of adding strong acid to an aqueous solution that is actually the cause of high corrosion. In the chemistry lab we added strong acid to water, never water to strong acid. Partial reason was that the large volume of water absorbed the heat of dilution.

In-line mixing of acid into water has zones of strong acid gradually becoming weak acid. That transition develops high localized heat and the injection quill corrodes rapidly at the tip. As the quill tip is destroyed, additional corrosion moves gradually to the source of the strong acid. The quill is destroyed rapidly.

As an alternate mixing method, consider adding acid into vapor space above the aqueous solution. A spray nozzle is used to control droplet size and improves distribution. Small droplets are good. Large droplets might not mix and reach bottom of the tank causing problems there. Given some agitation in the tank, the small droplets will be dispersed and absorbed by the moving liquid.

Material selection for strong acid is simplified - it is strong acid. Material selection for dilute acid is simplified - it is weak acid. Reagents can be fed to the mix tank using flow controllers to obtain desired ratio.

 

[unclesyd]

Here is information on Danfoss Tantalum and Tantalum coatings for metals. This might be a way to approach your problem. They can supply either full Ta or coated Ta components. I would definately look at the molten salt applied Ta.

We used a molten salt applied Ta coating for a heated SS boss used in our process to resist 120C 65% HNO3 + organics that worked extremely well. A 304L SS boss would last at best 6 months as some of Ta ones were over 7 yrs old when the MOC was changed to all Ti. In fact none of the Ta coated components failed.

http://www.tantalum-coating.com/index.htm

http://www.tantalum-coating.com/Prodcoat.htm

http://www.tantalum-coating.com/Literat/UserGuide.pdf

Once you resolve the quill problem, just remember that you may not have to have all the mixing elements coated. You can make your mixer in segments.

 

[moltenmetal]

unclesyd: thanks for this one- definitely worth trying someday!