Ask Greg McMillan

Greg's Response:

In my 50+ year career in process automation, dynamic first principle simulations have played a more extensive and critical role in pH system design than any other process application, I was fortunate enough to home in on the charge balance started by Greg Shinskey’s book pH and pION Control in Process and Waste Streams for my Master’s Thesis in Process Control.  I found the charge balance could be readily setup for complex solutions and simply expanded to handle acids and bases with two or more dissociations, different solvents, salts, and ionic strength by the inclusion of multiple dissociation constants and activity coefficients. The books I had then and even now on ionic equilibrium focus on directly solving equations requiring a focus on simple aqueous solutions (e.g., single acid and single base water streams). I found that my much more general and extensive charge balance could provide a result within pH electrode accuracy in 8 iterations of a robust and simple interval halving search. 

Another major discovery on my part not mentioned in the literature is the need to add a small amount of carbonic acid from simple exposure to the carbon dioxide in air to match the lab titration curve. The simple addition of carbonic acid can reduce the change in process gain by orders of magnitude in the neutral region for a strong acid and strong base system. Besides an accurate titration curve, the inclusion in the model of reagent transportation delays in piping and dip tubes, mixing nonuniformity in pipes and vessels, valve stiction and backlash, and the 5Rs of electrode response (resolution, repeatability, rangeability, reliability, and response time), is critical. 

The Digital Twin simulation using a charge balance with carbon dioxide added to match the lab titration curve can be used to improve every aspect of pH system design justifying expenditures on better mixing, piping, control valves, and electrodes, as detailed in my 2024 ISA book Advanced pH Measurement and Control Fourth Edition. The capital savings from reducing the number of vessels required by better system design can be hundreds of thousands of dollars. 

A Digital Twin model can also update the titration curves and adapt PID controller tuning settings and signal characterization used to convert the controlled variable from pH to linear reagent demand. Just taking the time to read the best practices at the end of each chapter, will enormously help you get on the right path to the right solution.