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

 

[EmmanuelTop]

Corrosion rates increase almost exponentially with temperature. Is there any way to decrease your dillution water temperature?

Turbulence is another point of concern. Check if this mixing system is designed properly.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[SeanB]

How are you injecting the acid? Are you using a quill?

 

[Mike4chemic]

Thanks SeanB. We are using an injection quill (hastelloy C),which suffers a strong corrosion.

 

[SeanB]

Are you experiencing the corrosion in the quill or just at the tip?

 

[ash9144]

Acid dilution processes will have areas that have high corrosion due to localized concentration and temperature effects. I would recommend going to all teflon and teflon lined parts for the mixing section of pipe.

 

[SeanB]

98% sulfuric acid should not be corrosive to Hastelloy C. But as the concentration drops (particulary in the 30% - 80% range)and at your temperature it is corrosive. So I would have to agree with ash9144 that you should examine your choice of metallurgy in the mixing section.

 

[Mike4chemic]

We are you experiencing the corrosion in the dipped part of the quill.

ash 9144,
I can't sure if teflon will survive at 170 degC.

 

[ash9144]

Can you cool it down or find a different injection spot.
There is glass impregnated teflon which holds up at higher temps but I don't know if it can handle that environment.

 

[SeanB]

I checked our specs for metallugy for your process conditions. Based on that one would use tantalum. I have never specifed this material before and quite frankly I am not sure what it even is.

 

[hacksaw]

you should consider two stage dilution with a mix tank, injecting 98%acid is fool hardy

 

[jistre]

Tantalum is expensive, that's what it is. It's is a rare metallic element that can withstand certain acidic corrosions that even some of the superalloys can't (things like 98% sulfuric out to 175C).

http://www.cabot-corp.com/cws/produ...Cabot Corrosion Product Sheet.pdf?OpenElement

That link is to a company that produces superalloys and it outlines the corrosion resistance of tantalum.

 

[Mike4chemic]

Gentlemen,

Thank you very much for your responses.

I would like to ask you a generally question, if the Hastelloy C276 could be considered for use (piping,static mixer) in a diluted sulfuric acid (particulary in the 20% - 80% range)at 95-100 deg. C ?

Thanks in advance,

Mike

 

[SeanB]

Hastelloy C is resistant to all concentrations of sulfuric acid up to a maximum temperature of 93 °C.

 

[sulphuric]

Addition of concentrated sulphuric acid into water to form a weak acid solution should be done in PTFE lined pipe. The high temperature and weak acid concentration is extremely corrosive to most metals. The lined pipe should extend from the injection point through to the static mixer and another metre downstream of the static mixer. The static mixer elements should also be PTFE.

Acid and water do not mix readily so you are probably getting a wide variation of acid concentrations at high temperatures (170 deg C) in the line before it reaches the static mixer. Only PTFE is resistant to the wide range of acid concentrations that are possible after the inject point.

 

[jistre]

With a name like that, how could you argue with him?

 

[moltenmetal]

Not to flog a dead horse, but you should choose corrosion immunity in this case (ie. teflon) rather than mere corrosion resistance. Teflon-lined pipe can be operated to 170 C but solid PTFE components (ie. unreinforced injection quills or static mixer components) should be used with caution as the material will tend to creep. Don't forget about the heat of dilution either- locally it can be intense.

 

[Mike4chemic]

moltenmetal

Thanks, I think the tantalum in spite of an high cost will be optimal solution for the static mixer.

What do you think?

 

[moltenmetal]

The answer depends on whether or not you've got differential pressure available to induce this mixing. A static mixer in tantalum is going to be serious money- tantalum components are usually 0.020" thick at most to save money, and that's too thin for pressure drop elements in a static mixer. A complex part such as a static mixer would be difficult to tantalum-clad, and a tantalum coating on something else is going to be risky.

If you've got pressure loss (ie. pump head) available, there are plenty of ways to introduce turbulence to induce mixing without resorting to tantalum. If the pump itself is PFA lined, the pump can be used for mixing. Downstream of the pump, an option worth considering is a couple inexpensive PFA-lined ductile iron plug valves (Xomox, Durco etc.) in series. Fit them with gearwheel operators so they can be used to throttle flow, and have their ports arranged at alternating angles- that will generate plenty of turbulence to induce mixing.

You could also fabricate a simplified static mixer design from Hastelloy or even stainless steel and have it PFA spray/powder coated. All components would need to be generously radiused and free of porosity/pinholes/crevices etc. to make sure the coating was durable. Someone like ITT or Resistoflex probably sell such a beastie already.

 

[Rocco8]

We have designed numerous sulfuric battery acid dilution systems.

In each case we used a teflon lined tee and then a teflon heat exchanger to remove the heat of mixing.

ash9144 said it on Dec 10.