Advances in pH Modeling and Control Paper

by Jim Cahill

ModelingAndControl.com's Greg McMillan and Solutia's Mark Sowell will be presenting at the upcoming ISA 54th International Instrumentation Symposium. Their paper, Advances in pH Modeling and Control, describes the use of embedded simulation, coined "Virtual Plant" and model predictive control to improve the control of pH levels in a plant waste water treatment application.

The authors begin by describing the challenge of pH control:

The pH electrode offers by far the greatest sensitivity and rangeability of any industrial process measurement in terms of the measurement of concentration (hydrogen ions). To realize the full potential of this opportunity requires extraordinary performance of mixing equipment, control valves, reagent delivery systems, flow meters, control system design, and controller tuning.

The virtual plant is described:

A virtual plant can be used to sort out fact from fiction important for insuring performance and reducing capital and operating costs. The virtual plant consists of a download of the actual control system configurations and displays, embedded advanced control tools, and a dynamic process model running on personal computer...

The articles details the control strategy used:

We developed and prototyped model predictive controllers (MPC) to replace the fuzzy logic control system. MPC-1 adjusted the 1st stage pH set point to keep the second stage reagent valve at a minimum position for good response and reliability. MPC-2 trimmed the 2nd stage set point to keep the pH in the tank at an optimum pH.

The authors describe the interaction of the virtual plant with the real plant. They write:

In order to study and improve performance of the control system and the fidelity of the process model for actual process conditions, we put the virtual plant in a read-only mode online running real time. A simple interface module was configured that used object link[ing and] embedding for process control (OPC) to read indicated waste flows, controller set points, and controller modes from the actual plant.

If you are battling pH control in a waste water treatment application, you'll want to give this paper a read. You might also want to get your hands on one of Greg's books, Advanced pH Measurement and Control, if pH control is currently vexing you.

Update: Greg wrote me that the presentation went well and the room could have been bigger to hold all the folks interested in hearing about this topic. He has done a slight revision on page 1 to better tie in the results to the general situation with pH systems. This version is now posted on the original hyperlink above.

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April 21, 2008 in Control Strategies, in Process Optimization, in Regulatory Compliance, in pH Control | Comments (0)

pH Control Rules of Thumb, Facts and Humor

by Jim Cahill

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. You can see what I mean on the Process Control Insights area where we've been collecting his works.

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.

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November 29, 2007 in Education, in Regulatory Compliance, in pH Control | Comments (0)

Thanks for the Rules of Thumb

by Jim Cahill

If you're an automation professional and not already subscribed to the ModelingAndControl.com blog, you're missing some great stuff.

Greg McMillan has recently posted three "sensible sensor installation" posts:

Greg offers his rules of thumb based on his vast plant experience for installing temperature and pH sensors. Here's an example from his initial post:

The best sensitivity from a temperature or pH sensor can generally be achieved by an installation where the tip of the thermowell or electrode is in the center of the pipeline. This is particularly important when there is a high viscosity fluid such as a polymer for temperature control or concentrated sulfuric acid reagent for pH control. For temperature, it is also desirable to maximize the insertion length in the center line to reduce the thermal conduction error from the tip to the flange. The insertion of the thermowell into an elbow affords this opportunity.

I know when I was a young systems engineer I would have really appreciated more rules of thumb to give me grounding on some of the things I needed to consider. Experience teaches these things, so any shortcuts to gain these experiences are greatly appreciated.

As I mentioned in a Web 2.0 presentation at the last Emerson Exchange, many ways are emerging to share your process automation expertise. A blog is one way, but other ways include adding/modifying entries in Wikipedia, social bookmarking with Del.icio.us, and sharing interesting posts you come across with web-based RSS readers like Google Reader.

If you've not yet taken the plunge to see what subscribing to RSS feeds is all about, see the screencast of how to subscribe to this blog, and how to import my blogroll. This is my way of helping get you jumpstarted to these rules of thumb with many automation and process industry-based blogs, including Terry and Greg's ModelingAndControl.com.

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October 19, 2007 in Education, in Measurement, in pH Control | Comments (0)

Upcoming pH Control Web Seminar Featuring Greg McMillan

by Jim Cahill

For those of you challenged with the vagaries of pH control, I wanted to make sure you had seen the news of an upcoming pH Control web seminar, arranged by ISA, featuring ModelingAndControl.com's Greg McMillan. The web seminar covers the root causes of poor performance in pH control systems.

In a recent post, Greg describes how he plans to share his experiences:

I spent a lot of time on pH startups. I found most of the key design concepts needed for success where not discussed anywhere, For example, the normal dip tube design for reagent injection is disastrous and the mixing and valve resolution requirements are exceptional. I discovered how I could reduce the number of stages of neutralization, offer inexpensive alternatives to the classical neutralization vessel, and decide when signal characterization could help or hurt your control objectives.

Unlike his recently released free eBook, this May 16 web seminar (2:00pm - 3:30pm Eastern U.S. Time) does have a cost. It's $195 (USD) for ISA members and $225 (USD) for non-members. If you're not the lone person in your organization who struggles with pH control, Greg suggests:

The seminar is much more cost effective if the registrant connects in a conference room with a computer projector.

If you can't make this event, Greg has also published a book on this topic, Advanced pH Measurement and Control, 3rd Edition.

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May 10, 2007 in Control Strategies, in Education, in pH Control | Comments (0)