Sulphuric acid corrosion (pH control) on effluent pipework 2

[Quentin]

I am working on a project to neutralise alkaline spikes in our plant effluent to between pH 6 to 8, and am looking to use 50% or 98% sulphuric acid for this.
The place in the system where I am considering injeting the acid is in a 200mm galvanised pipe which feeds the DAF (dissolved air flotation) unit. The idea I have is to insert an injection point into this pipe, with a static mixer immediately downstream, and pH measurement directly after the mixer.
I am concerned at the potential effects of any localised corrosion from the acid on the galvanised sched 40 pipe (8.2mm wall thickness) around the injection point and on the static mixer.
Will I need to consider replacing the section of galv pipe which houses the injector and static mixer with some other material as well as needing to use a particular material for the static mixer, or will I get sufficient life (eg 5 years) out of the galv pipe and be able to use stainless steel (eg 316) for the static mixer?

 

[metengr]

For your application, I would consider a PVC or CPVC static mixer housing to install in the galvanized carbon steel line at the acid injection point.

http://westfallmfg.com/sim.htm

The sulfuric acid injection line can be piped out of 316 ss to handle 93%-98% concentrated sulfuric acid from the acid storage tank up to the point of injection at the static mixer. I would stick with concentrated (93%-98%) sulfuric acid because handling dilute sulfuric acid (50 to 80%) makes no sense and will be expensive. You can control pH of the effluent by metering the amount of concentrated sulfuric acid during mixing.

 

[Quentin]

Thanks metengr,

the static mixers on offer there should do the trick from both a materials and efficiency point of view.
My only concern now is to prevent any backflow of waste water into the acid line tank...I'm not sure if a non-return valve would be sufficient. Perhaps I need to install an isolation valve on the acid line either out of the tank or after the metering pump, and link it via the PLC to the pump operation, as well as installing a non-return valve close to the injection point. Any ideas or experience with this? I'm very aware of the dangers of mixing acid and water in an exclosed pipe!

 

[quark]

Ball type non return valves are a standard supply with dosing pumps and they suffice your requirement. You can also use LDPE tubing for transfer of sulfuric acid.

 

[bimr]

Question,

Your question should be over on the Waste disposal & treatment Forum.

An inline pH correction system will probably never work. You need mixing and reaction times. See this thread:
pH system - Design criteria
thread161-110441

You will also need to make the equipment corrosion proof. Galvanized pipe will not work.

 

[NickelMet]

Quentin:

I had the same comments as BIMR, with the addition that you could probably consult with a water treatment specialist (DREW Chemical, Betz-Dearborne, etc.) for better information.

~NiM