Emerson technologist and ModelingAndControl.com blogger, Greg McMillan, coauthors with Solutia’s Mark Sowell an article, Virtual Control of Real PH in the November issue of Control magazine. The wonderful thing about Greg’s writing is that it seems to always include experienced-based rules of thumb, a lack of sugarcoating the facts, and large amounts of humor.
In this article, the authors waste no time in mentioning why we should be interested in reading the article. Most plants have pH control applications, even if in their waste-treatment areas. These areas usually have environmental compliance issues and for applications like crystallizers, fermenters, reactors, and strippers, pH control is critical.
My example of Greg not sugarcoating the facts is:
While we tend to focus on the configuration of the DCS, achieving the full potential of the pH measurement requires exceptional attention to every aspect of the system design. Deficiencies in the equipment, piping, valves or sensor selection or installation can cause the system to fail miserably.
This advice alone may save someone loads of troubleshooting time by first looking at the field equipment and installation before fiddling with the automation system’s configuration and tuning.
He’s also very good at simplifying the approach to pH control problem solving by helping the reader form a quick mental picture:
The name of the game with pH is to minimize the loop dead time to minimize the excursion along the highly nonlinear titration curve.
The solution described in the article is to use a virtual plant–a dynamic simulation of the waste-treatment system–based on a first-principle dynamic model of the pH system and control system configuration. These all run in the same PC. Dynamic simulations can be quite complex but here’s where Greg’s rules of thumb based on his experience come in. The key is to focus on simplification and attention to the details that really matter. An example of a rule of thumb:
For pH modeling for process control of environmental systems, about 20 acids and bases cover about 90% of the applications. The physical properties requirements are much less (just molecular weight, density and dissociation constants of each acid and base). The waste treatment systems are normally dilute enough so that activity coefficients are not needed.
They used the virtual plant to see if the existing fuzzy logic control could be replaced with a straightforward model predictive control (MPC) strategy. You’ll have to read the article to see the full approach but the bottom line was that:
The MPC did a much better than expected job of chasing the acid concentration… We confirmed later that the production unit that was the source of most of the strong acid was having issues. A comparison of the virtual plant and actual plant control valve positions and pH response revealed there was no flow going through one of the second-stage reagent valves. The problem cleared a day after a phone call.
I had to wrap up this post with an example of Greg’s ever-present humor that engineers can appreciate:
It takes more and more interesting opportunities to get weathered engineers excited. However, the almost limitless opportunities to explore advanced control ideas make us downright tingly.
If you’ve been fighting pH control, the article is well worth it as is the “Extra-Credit Reading” they cite.
