A Real-Time Approach to Process Control, 3rd Edition

Workshop 8

Plant Operability and Controllability

There's a better way to do it. Find it!

—Thomas Edison

Introduction

Prior to attempting this workshop, you should review Chapters 8, 9 and 10 in the book.

Traditionally, process design has been performed using steady-state analysis only. Simple rules-of-thumb have been used to size vessel hold-ups and to set other variables that affect the dynamic performance of a plant. This can sometimes lead to operability and controllability problems as a design might look good in the steady state but be very difficult to operate or control due to the presence of dead times or insufficient capacitance.

A key consideration for plant operability and controllability is variable interaction. We have learned that dead time is one of our enemies as it always makes tight control more difficult to achieve. Variable interaction places similar restrictions on the way we can control a process and can significantly reduce the overall control system performance. Three common sources of variable interaction are the nature of the process (i.e. distillation), the combination of multiple unit operations and heat integration. Each of these points can not only be highly advantageous in the steady state but also create operability and controllability problems that may not be evident without considering the process dynamics at the design phase.

This workshop will investigate several examples where variable interaction is significant and will introduce an analytical ...