pH control is a common issue in many industrial processes. The basic idea to control the pH variations in some liquid flow, usually making the pH as close to 7 as possible. This influent liquid flows into a tank, where it is mixed with an amount of a concentrated reagent to alter its pH. If the effluent is acidic with a pH of 4, the reagent would be basic with a pH larger than 10. The reagent is more concentrated than the effluent, since it is desirable to as little volume to the effluent as possible.
The pH is controlled in a tank - the effluent flows into the top of the tank, and the reagent is also added to the top of the tank. The tank is well-stirred so that the pH is uniform throughout the tank. The effluent is pumped out the bottom of the tank, as shown in the figure below.
Note that pH is a very difficult control problem due to the non-linearities. This example simplifies the problem somewhat to highlight some points about control system design in general.
Below is a photo of an industrial pH neutralization system.
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