3.2 Basic Concepts of Plantwide Control
3.2.1 Buckley basics
Page Buckley (1964), a true pioneer with DuPont in the field of process
control, was the first to suggest the idea of separating the plantwide
control problem into two parts: material balance control and product
quality controL He suggested looking first at the flow of material
through the system. Alogical arrangement oflevel and pressure control
loops is established, using the flowrates of the liquid and gas process
streams. No controller tuning or inventory sizing is done at this step.
The idea is to establish the inventory control system by setting up this
"hydraulic" control structure as the first step.
He then proposed establishing the product-quality control loops by
choosing appropriate manipulated variables. The time constants of the
closed-loop product-quality loops are estimated. We try to make these
as small as possible so that good, tight control is achieved, but stability
constraints impose limitations on the achieveable performance.
Then the inventory loops are revisited. The liquid holdups in surge
volumes are calculated so that the time constants of the liquid level
loops (using proportional-only controllers) are a factor of 10 larger than
the product-quality time constants. This separation in time constants
permits independent tuning ofthe material-balance loops and the prod