How can the pH value be better controlled?
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Operating conditions: Acidification of wastewater – The pH value of the wastewater is around 13; the wastewater enters the static mixer via a pump, a control valve, and a flow meter, while hydrochloric acid is introduced into the static mixer through a metering pump； After mixing, keep the pH value between 5 and 6 ； My idea is that the flow rate of hydrochloric acid entering the mixer is fixed ； Series control is employed for wastewater treatment: pH is controlled by the main loop, while wastewater flow rate serves as the secondary loop to regulate the pH value ； However, the delay in pH value is relatively large; are there better control methods, such as using ratio control?
pH control exhibits strong non-linearity. The sensitivity varies with different operating points; it is highest at pH around 7, while it is very low near pH 14 and pH 1, approaching saturation. In other words, the further the deviation from pH 7, the more neutralizing agent is required; the closer it is to pH 7, the less neutralizing agent is needed. That is to say, near pH 7, a small amount of neutralizing agent can cause a significant change in pH. In summary, it is better to adopt nonlinear control (a variable gain controller).
For this type of control regulation based on chemical reactions, it is advisable to examine the overall conditions of the medium as well as the actual progress of the chemical reactions; based on the data obtained from such examinations, a control method can be selected or a control scheme tailored to the actual reaction conditions can be developed. Chemical reactions take time, and it is also necessary to have a thorough understanding and consideration of the uniformity of the entire system undergoing neutralization. This is because different alkaline substances may exhibit varying reaction characteristics when neutralized with hydrochloric acid; especially those reactions that result in the formation of pH-buffering systems require careful adjustment. Find suppliers who understand neutralization reactions and can provide good technical solutions to discuss with, and then proceed with actual implementation; it would be best if a pilot or scale-up test can be carried out. Although the pH value is a simple parameter, its actual control can be much more complex than you might think.
Thank you very much. We need to determine the actual parameters through pilot testing or production trials; currently, we are considering using ratio control for the acid and base flow rates, but this may not be the best approach.
It was tried in the early years; it can be considered a failure, but some materials can serve as reference. The key is to fully understand the technical characteristics, as well as the requirements regarding control precision and the operating range. . . It’s not easy to switch to automatic mode using a standard PID controller (non-linear controllers or multi-variable controllers are required). You can check out the work of Greg McMillan – he is an expert in PH control, has written books on the subject, and is an expert in the ISA Hall of Fame. https://modelingandcontrol.com/author/greg-mcmillan/ https://www.controlglobal.com/articles/2013/mcmillan-ph-response-accuracy-reliability/
Bro on the 7th floor, is there a Chinese version?