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pH Modeling and Control at AIChE Spring Meeting

by , | Mar 31, 2022 | Event, Production

Jim Cahill

Jim Cahill

Chief Blogger, Social Marketing Leader

I was always better at physics than chemistry, which is probably why I chose electrical engineering over chemical engineering. But I do appreciate my very smart Emerson chemical engineering colleagues and the contributions they make to safe, reliable and efficient manufacturing plants.

I mention this as background for the chemical engineers’ professional organization, AIChE. The 2022 AIChE Spring Meeting and 18th Global Congress on Process Safety will be held April 10-14 in San Antonio, Texas. Emerson’s Greg McMillan, one of the Process Automation Hall of Fame members, will be presenting pH Modeling and Control. Here is his presentation abstract:

Titration curves from labs can be misleading due to sample temperature and composition changes and contamination, poor mixing, split range uncertainties, reagent dilution or substitution, and insufficient data points. Guidance is offered to obtain titration curves that most closely match the process and provide the information needed for control valve size and precision, signal characterization, control system design, and controller tuning.

First principle models can be greatly improved by a universal charge balance solved rapidly by a simple interval halving search for the pH that provides a zero-excess charge. The ability of this universal charge balance to handle a large spectrum of salts, acids, and bases is essential to be able to exactly model the process pH response and corresponding actual titration curve. Theoretical titration curves can have slopes many orders of magnitude larger than the actual titration curves when strong acids and strong bases are used due to omission of conjugate salts and small amounts of carbon dioxide absorption just from exposure to air. High salt concentrations can cause errors due to changes in ionic strength. Means of including these effects in the charge balance are provided. Finally, critical dynamics are included by addition of reagent injection delays, mixing delays, and measurement lags and transportation delays.

pH control can be extremely challenging due to exceptional nonlinearity and sensitivity particularly when strong acids and strong bases are used. Also, electrode life is relatively short even in the best conditions and can decrease to a matter of days due to high temperature and high acid concentration, abrasion, dehydration, and poisoning. Advances in electrode design and signal selection can greatly help. The need to reduce oscillations from valve resolution limit and backlash can be addressed by valve design and use of a PID feature. Finally, the system dynamics and performance can be significantly improved and the cost of the system reduced by better control strategy design and installation.

In his presentation, Greg will discuss:

  • Extraordinary Nonlinearity, Sensitivity and Rangeability
  • Charge Balance Universality
  • Effect of Process Temperature
  • Uses of and Problems with Titration Curves
  • Digital Twin Synergy
  • Common Mistakes in Control System Design
  • Equipment Performance
  • Control Strategy Alternatives
  • Advances in Electrode Technology
  • Middle Signal Selection and Wireless Advantages

I’ll share one quick example from the presentation where he shows a basic neutralization system before and with feedforward control added after.


Typical pH neutralization system control strategy


pH neutralization system control strategy with feedforward control

If you’ll be in San Antonio the week after next for the AIChE Spring Meeting, check out Greg and the other Emerson experts presenting there. If you can’t make it but want to learn more about effective and efficient pH control, check out Greg’s book, Advanced pH Measurement and Control, 3rd Edition. While the 3rd edition is now available, Greg is working on an update and the 4th edition should be available later this year.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.