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pH Control

Introduction

Most process plants generate a wastewater effluent that must be neutralized prior to discharge or reuse. Consequently, pH control is needed in just about every process plant, and yet a large percentage of pH loops perform poorly. Results are inferior product quality, environmental pollution, and material waste. With ever increasing pressure to improve plant efficiency and tighter regulations in environmental protection, effective and continuous pH control is highly desirable.

However, implementing a pH system is like putting a puzzle together. It will only work when all the components are in place. The pH puzzle includes effective pH probes, actuators, and controllers. While various pH probes and actuators for pH control are available, commercial adaptive pH controllers are still in demand. The challenge is to provide a controller that is able to deal with large nonlinear gain changes in the pH loop. It will be useful for not only wastewater neutralization, but also chemical concentration control, since concentration is a key quality variable.

Solution - Why is pH Control Difficult?

A strong-acid-strong-base pH process is highly nonlinear. The pH value versus the reagent flow has a logarithmic relationship. Away from neutrality, the process gain is relatively small. Near neutrality where pH=7, the process gain can be a few thousand times higher. It is impossible for a fixed controller like PID to effectively control this process.

In practice, most pH loops are in a “bang-bang” type of control with pumps cycling on and off, which causes large oscillations. Since acid and caustic neutralize each other, over-dosing acid and caustic is prohibitively expensive. Statistics show that a poorly controlled pH process can cost tens of thousands of dollars in chemical usage each month, not counting the penalties imposed by violating EPA or local government discharge codes.

MFA pH Controller vs. PID Controller

The following trends show the MFA pH controller (top) is able to control the pH in its full range when the pH setpoint is changed from 7 to 12 and back to 7. PID (bottom) is either sluggish or oscillating.



MFA pH Controller Configuration

As shown in the following bitmap, one can easily enter Break Point A and B to define the estimated shape of the titration curve of the pH process. Then the MFA controller gain Kc for the flat portion and steep slope can be entered. For example, the controller gain for the flat portion is set to1 for a strong-acid-strong-base pH process. The estimated gain for the steep slope can then be set to 0.001, which is 1000 times smaller. Due to the adaptive capability of the MFA pH controller, the shape of the titration curve does not have to be accurately estimated, and, in actual applications, the shape can vary in real-time. Lastly, the flow rate and the pH value of the inflows may vary significantly, and even with these large disturbances, the MFA pH controller will be effective.



pH Control

The MFA pH Controller is specially designed to control pH value for continuous water neutralization. A special Anti-delay MFA pH controller can effectively control pH processes with large time delays.

MFA pH control users include the following companies:

Rohm Haas, Shell Oil, PetroBras, Atofino, McDermott International, Chiron, Unilever, PetroChina, Akzo Nobel, Morningstar, Sinopec and Baosteel.

To read more about implementations of CyboSoft’s MFA pH solutions, click on the following case studies:

Model-Free Adaptive Control on pH Loops - Rohm and Haas

Model-Free Adaptive Control on Wastewater Neutralization - Chiron

MFA Control and Optimization of Oil Recovery Boilers - PetroChina

View controller video  .9MB ( Macromedia Shockwave plug-in required )

 

 

 

 
     
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