(145d) Updating the Process Controls and Dynamics Course for the 21st Century

Seames, W. S., University of North Dakota
The goal of this presentation . . .

In this presentation I will provide a topical outline/course schedule for an updated course in Process Controls and Dynamics and explain the contents including examples. The material is available in a new textbook published by the Taylor and Francis Division of the CRC Press entitled, “Designing Controls for the Process Industries”. Supplemental author resources will also be introduced.

Motivation . . .

A few years ago I finally had the opportunity to take over a one semester senior (4th year) course entitled “Process Dynamics and Controls”. I was looking forward to this as I had developed substantial applied controls experience during my 16 year industrial career. During that time, process controls underwent a complete transformation from electronic instruments to digital distributed control– based systems. I was excited to see new textbooks that reflected this “revolution” in process controls.

Imagine then my disappointment when I found that all the major textbooks in this field were still following the same format and with essentially the same content as textbook published in the 1960s and 1970s! These books emphasize simplified mathematical descriptions of process dynamics using time-dependent linear ordinary differential equations and their analytical solutions using Laplace transform solution methodologies.

The primary goal of these textbooks appears to be to help the reader understand the dynamics of the proportional-integral-derivative controller mathematically, so that the stability of control loops could be properly evaluated. This was an appropriate approach to the subject matter when process control was performed using a suite of stand-alone electronic controllers but is much less important now that control is most commonly performed using sophisticated, integrated distributed control and automation systems. While stability analysis is still of some interest to process control engineers, modern control system algorithms have reduced the importance from primary to secondary for control engineers.

Another big gap in the current literature is a lack of coverage of batch processes and, more importantly, the integration of batch unit operations within continuous processes (such a step being known as a semi-batch unit operation). Of the 17 most common textbooks that I reviewed, only one gave this topic any coverage at all. Yet almost every continuous commercial pathway includes at least one semi-batch step and batch processing represents a sizeable minority of process pathways employed (particularly in certain industries such as pharmaceutical).

Presentation Contents – an Overview of the Textbook . . .

“Designing Controls for the Process Industries” was conceived to address these deficiencies in the currently available literature. The goal is to completely transform chemical engineering process control and process dynamics education to focus on those aspects that are most important for process engineering in the 21st century.

Instead of starting with the controller, the book starts with the process and then moves on to how basic regulatory control schemes can be designed to achieve the process’ objectives while maintaining stable operations. Without a deep understanding of the process itself, the power of the modern plant automation system cannot be fully enabled.

As much as possible, I have tried to follow the International Society for Automation’s (ISA) guidelines for process control and instrumentation documentation. Some adjustments to the ISA guidelines were made where these improved the clarity of the concepts presented in the text. Most importantly, all of the process control schemes assume that field signals will be converted into digital form at the field device and that control will be accomplished in a distributed control system or programmable logic controller module(s).

In addition to continuous control concepts, I have embedded process and control system dynamics into the text with each new concept presented. I have also included sections on batch and semi-batch processes within new concept areas where appropriate. Finally, sections on safety automation are also included within concept areas.

The four most common process control loops - feedback, feedforward, ratio, and cascade - are introduced in Chapter 2 and the application of these techniques for process control schemes for the most common types of unit operations are provided in Chapters 3-6. For the practicing engineer, these chapters may prove to be the most useful for designing new control schemes or to help troubleshoot existing process instabilities. By comparing the schemes in these chapters to an existing situation, the engineer may be able to identify poorly designed control schemes. Modification of poorly designed control schemes may be an easy and cost effective way to solve process instability problems. This is often a better approach than to try to “tune” your way out of a problem.

More advanced and less commonly used regulatory control options are presented in Chapter 7 such as override, allocation, and split range controllers. These techniques provide additional ways to increase the overall safety, stability, and efficiency for many process applications.

Chapter 8 introduces the theory behind the most common types of controllers used in the process industries. For those instructors that prefer to start with a “what’s inside the box” approach, you might want to go through Chapters 1 and 2, and then jump to Chapter 8 prior to Chapters 3-7. For those instructors who are uncomfortable making a complete transition from the older course formats to that presented in this text, Appendix A provides content on how to solve simple linear ODEs using Laplace transforms while Appendix B provides information on PID controller tuning.

Chapters 9-12 provide various additional plant automation-related subjects. An instructor in a one semester course is unlikely to be able to use all of this material, but has the opportunity to emphasize those aspects that they feel are most important. Personally, I use Chapters 9 and 10. I then use Chapter 11 in a capstone design course. Chapter 12 is probably more appropriate for a graduate level class in process dynamic modeling or as part of an advanced transport phenomena course. However instructors who want to emphasize process modeling in their course may wish to use this material.