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

10

Plant-Wide Control

The fundamental questions in plant-wide control are whether the feed rates can simply be set for a process and left unattended and whether the process is meeting the desired purity and quality specifications [1,2]. What happens when common disturbances occur such as feed composition changes, production rate changes, product mix or purity specification changes and ambient temperature changes or measurement sensors either fail or are in error? This chapter covers some of the most common problem areas encountered when designing a plant-wide control scheme.

10.1 Short-Term versus Long-Term Control Focus

When applying a plant-wide control scheme, it is important to be aware of the propagation of variation and the transformation that each control system performs. Management of that variation is the key to good plant-wide operation and control. A healthy variation management strategy should have both a short-term and a long-term focus. The short-term focus is to use control strategies to transform the variation to less harmful locations in the plant. The long-term focus should concentrate on improvements which reduce or eliminate either the variations or the problems caused by variations.

To better illustrate the idea of short- and long-term control focus, consider an acid recovery plant [3,4]. An example of a short-term-focused control scheme for the plant is shown in Figure 10.1. In this system acid feeds of varying concentrations are pumped to four storage tanks. ...