On the operational side,the measurements are
available:never,sometimes,or only after considerable
delay (dead-time).In addttion the instruments
measlaing key parameters are frequently unreliable or
require "excessive" maintenance support. These
situations are characteristics of the control system
design and operation problems common in
manufacturing pharmaceuticals, oil & gas, chemicals,
paper, pintmg, pipelines, and metals. For example,
Figure I shows a typical interactive nonlinear recovery
curve for a side-draw product from a crude unit
(distillation)tower.The nonlinear shape of the recovery
curve depends upon the operating conditions in the
tower (temperatures,pressures,liquid and gas flow
rates,flow rate from each side-draw,etc.)and upon the
composition and conditions (temperature:,flow rate) of
the feed stock oil going into the tower and the actual
operational history of the unit. Each and every one of
these unxhtions changes with the passage of time. In
addition, the response time of the tower can change
over time and as the state of the tower changes. The
control system design is further complicated since
instruments measuring the key compositions(true
bow point) are htstorically both ddticdt to maintain
and lag the process by 10 to 65 minute.