Injecting 93% sulfuric acid into a water tank 1

[Sean Rong]

We have a water treatment plant using the lime and soda ash process. The clarified product has a pH of 10-11 and is used as make-up for our cooling tower. Our Clarified Product water is saturated with calcium carbonate and has caused a lot of scaling in the forwarding pumps and make-up piping. We are investigating the option of injecting 93% sulfuric acid directly into the Clarified Product Tank (CPT) in order to lower the pH to about 9.0-9.5 and minimize scaling downstream. The CPT is made of carbon steel and uncoated. We intend to inject the sulfuric acid through a 3/8" stainless steel tubing into the tank from the top center where there is a blank flange. The estimated acid flow rate is no more than 5 GPD.

Tank dimensions:

​

Height:

21'

​

ID:

14'

​

Operating height:

150"​

The clarified product goes into the CPT by gravity from a height of 16'-6".

Does anybody have any similar experience in doing this, i.e. injecting sulfuric acid directly into a carbon steel water tank?
Without a mixer in the tank, how well is the acid mixed with water? The clarified product does drop from a height of 16'-6" to about a height of about 150" and provides some mixing.
What precautions are needed for this kind of set up, especially with regard to corrosion to tank metal?

Thanks very much in advance!

Sean

 

[EdStainless]

When I have seen this done successfully they have used a side stream flow through an injection and mixing line.
This side loop is all built of acid resistant materials (either FRP or Alloy 20) with inline (static) mixing elements.
The biggest corrosion risk is between 93% and 5%, so this needs to happen fast and reliably.
The side stream flows continuously but acid is only injected when called for.
Don't just run it into a steel tank without forced mixing.

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P.E. Metallurgy, consulting work welcomed

 

[don1980]

I agree with Ed about not injecting this acid directly into the CS vessel. I wouldn't inject directly into the tank even if the tank had a mixer. As a young engineer I did something kind of similar to this. It turned out a costly mistake for my employer but a valuable personal learning lesson for me. The plant had an incinerator which generated a waste aqHCl stream from the flue gas scrubber. As i recall this was about 2-3 gpm of 15-20% HCl which we needed to dispose of. The plant had a high-volume alkaline wastewater stream (several thouand gpm, pH of ~11-12) which went to a wastewater treatment plant. So I called for the acid stream to be injected into the 30" carbon steel wastewater line, using an injection quill so that the acid entered near the center of the 30" pipe. I was confident that this posed no risk to the CS pipe, but I was wrong. We later discovered severe pitting on the downstream side of the 30" pipe, caused by acid attack.

 

[hacksaw]

No, no, no.

Consult your acid plant designer, or your plant chemist for advice.

 

[LittleInch]

This will work, but only for a certain period of time - maybe 5 to 10 years. Then holes will start appearing in the tank if you do it like you're talking about.

Mixing in material which can withstand 100% of your acid is required before mixing into the mass of material in the tank.

Remember to put an actuated isolating valve on your acid line so that it doesn't leak in when it's not supposed to....

A depot I worked at put a whole tank full of fuel off spec because the CI injection skid kept pumping away =when the flow stopped so a large slug ended up in the tank. And another time it sucked in product doe to product cooling when stationary and did the same thing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[horacio Torres]

we had installed a mixer in the recycle line (PP line) with the acid injector before went into the CS tank. It work for a long time without corrosion.

Horacio

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