Process Control is the study of automatic control principles applied to chemical processes. It applies principles of mathematics and engineering science to the regulation of the dynamic operation of process systems. To be successful, you need strong applied mathematics skills and process understanding (most of which is just common sense).
The skills and tasks you've been exploring in the first three years of ChE classes are predominantly analytical. They are used for diagnosis and understanding of processes and problems. This year, your design classes will work on synthesis skills for devising new processes. In Process Control, we will use some analytical tools (old and new) and synthetic skills to understand the dynamic (time dependent) behavior of processes and ways to regulate plant operation.
Since the primary function of control systems is to compensate for dynamic changes in process systems, we need to understand the dynamics of processes -- how their behavior changes with time -- if we are to develop workable solutions. We address this need through dynamic modeling of the chemical processes. Mathematically, this means we will be dealing with differential equations.
Process plants do not operate at steady state, no matter what you have been assuming in other classes.
Consider what might happen to a distillation column operating in a plant:
Putting all this together, the two main functions of control systems are:
Disturbances can never be completely eliminated; however, a good control system can greatly attenuate their consequences and reduce the variability of process parameters. If we can reduce variability, we need smaller margins of error and contingency allowances, and so can operate much closer to optimum conditions, reducing waste and saving money.
Control courses can be difficult for an instructor to organize. There are often multiple ways of approaching concepts, each with its own "dialect" of terminology and equation. Topics often wrap back around, so books and instructors sometimes have a tendency to use terms and ideas before they are fully defined. I'm guilty of this myself -- so PLEASE do not hesitate to ask questions when you spot things that feel out of place.
The same basic control methods, principles, and tools apply whether the "process" is chemical, electrical, or mechanical. Control theory has been developed by ChEs, EEs, and MEs, so the terminology reflects concepts from all three disciplines (as well as mathematical systems and optimization theory).
Differences in the application are what separate ChE control from other practitioners. Chemical process systems are distinguished by:
No feedback control loop, no matter how well-designed and tuned, can guarantee safe operation. Consequently, a regulatory process control system cannot be trusted as the primary safety system. Almost all chemical plants have a second, parallel control system to handle safety alarms and shutdown. While we will always consider the safety aspects of control systems, we will not study the design of these alarm/shutdown systems.
The objectives of a control system fit into a hierarchy -- that is, some objectives are given priority over others. One way of ordering the hierarchy is by the purpose of the control system components:
Be aware that a loop can serve more than one purpose and that its place in the hierarchy is not always cut-and-dried.
One of the themes of our study this semester will be the tradeoffs between plant design and plant operation. Control systems are part of the day to day operation of a plant. This suggests another way of ordering the hierarchy of control objectives: "achievability". After all, until your plant is operating, controls aren't needed at all. This sort of hierarchy tends to group loops by function as much as it does by objective:
Original: 9/29/93; 12/7/96
Modified: 1/10/94, 1/6/95, 12/15/95, 12/7/96, 1/6/98, 4/24/2003, 8/10/2004
Copyright 1993, 1996, 2003, 2004 by R.M. Price -- All Rights Reserved