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.

CyboSoft, a NI Alliance Partner in Rancho Cordova, California offers an innovative Model-Free Adaptive (MFA) control solution for controlling pH processes without having to build process models and retune controller parameters. The MFA pH controller can effectively control a wide range of pH processes within the full pH range. It is useful for wastewater neutralization and also enables automatic control of acid or alkaline concentration, both of which are critical quality variables for the chemical process industry.

Leveraging National Instruments LabVIEW Real-Time development environment, the embedded MFA pH controller can run in a PC or be downloaded into NI's FieldPoint distributed I/O or PXI instruments.

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 in LabVIEW 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 in LabVIEW

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.



FieldPoint Based MFA pH Control System

National Instruments FieldPoint distributed I/O products are ideal hardware platforms to implement a pH control system. The following table lists all the components and part numbers required. With both options, at least 2 MFA pH controllers can be implemented. You may select NI's Compact FieldPoint products to implement this system as well. See the "Nonlinear Control" solution for detailed product information and part numbers.

Option 1. Remote I/O Option (MFA running in PC)

Option 2. Embedded Real-Time Option. (MFA running inside FieldPoint)

CyboSoft's Embedded MFA Control Software in LabVIEW

The following table lists the required embedded MFA control software to be ordered from CyboSoft. A SISO (single-input-single-output) MFA controller can be configured as one of the following MFA controllers.

• SISO MFA to replace PID to eliminate manual tuning,
• Nonlinear MFA to control nonlinear processes,
• MFA pH controller to control pH processes,
• Feedforward MFA to deal with measurable disturbances,
• Anti-delay MFA to control processes with large time delays, and
• Robust MFA to protect the process variable from running outside certain boundaries.

Summary

This user-friendly and robust solution can be easily implemented. No special knowledge or experience with advanced control or process modeling is required. Truly automated production operations can be achieved by using NI's FieldPoint and CyboSoft's embedded MFA pH controllers. Difficult pH loops can be effectively controlled and immediate savings in chemical reagents and elimination of violation penalties will be realized.