Effluent water from Spent caustic treatment plant has >10 which needs to be controlled continuously between 7-8 before marine disposal. Please suggest automatic process control scheme. Please highlight the issues with conventional PID controller if your recommendations is improved version. Do you prefer batch vs continuous control?
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With a pH greater than 10, I would suggest a two stage neutralization process: probably with sulfuric acid being added to bring the pH down because sulfuric acid is the cheapest acid in most areas. I have installed this process using a pH controller and an chemical injection pump. First of all a small circulating pump needs to be put in place to bring the solution from the first tank to the pH probe with a screen (probably a y strainer with a valve to easily clean) placed in-between the flow and the pH probe. The return water flow will have the sulfuric acid added by the drop into the stream, after the pH probe. The return flow is placed on the opposite side of the tank from the intake to create a circulation. Overflow from the tank near the intake will put a partially neutralized flow into the second tank, where the process is repeated.
I have done this in a single tank where the pH was below 10 (I did this when pH did not exceed 9.5) but above a 10.5 you are going to see free OH: thus the pH adjustment may not be complete in just a single tank operation.
I always try to put a pH recorder on the final discharge to prove that the discharge is in compliance with local regulations.
I found the same problem and I made an intermediate vessel where I throw the already treated water and feed with not treated water (I saw someone that throw it again to the main containers...) Ph decrease as well as the lectures of the on line ph meter starts to be more accurate.
You have to use the acid dosing pump with an acid-proof Metalic acid tank and have to inject sulphuric acid concentrated and control the acid dosing injection control when you get the required pH, just leave the pump running on the same setting thats it.