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pH Control - Index

System Description

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.

Overview of the Example

It is recommended that you follow the example in order, but you may skip ahead if you wish:

Part 1 The basics
Part 2 Flow rate variations
Part 3 Control valve errors
Part 4 A 3 tank solution

Photos of the System

Below is a photo of an industrial pH neutralization system.

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