This approach has always been widely used in industry, and has its main advantage that many effective control schemes have been established over the years for individual units
(e.g. Shinskey (1988)). However, with an increasing use of material recycle, heat integration
and the desire to reduce buffer volumes between units, this approach may result in too many
conflicts and become impractical.
As a result, one has to shift to plantwide methods, where a hierarchical decomposition
is used. The first such approach was Buckley’s (1964) division of the control system into
material balance control and product quality control, and three plantwide approaches partly
based on his ideas are described in the following.