Page 417 - Design of Simple and Robust Process Plants

404 Chapter 9 Operation Optimization
Quality ofcontrol
± H Process that requires many operator interventions to compensate for dis-
turbances and requires ongoing adjustments to maintain quality operation.
The cause might be frequent changes in feedstock rates other external distur-
bances, and difficult quality control caused by interaction.
± M Processes that are less subject to disturbances and interaction, like con-
stant feed composition and major quality loops closed.
± L Process only minimal subject to external disturbances and interaction such
as constant feed streams and most quality loops closed.
± NA Process not impacted by external disturbances and interaction such as
total automated unit operations as refrigeration systems.

Constraint operations
± H Process is running against different constraints at different units, like
capacity constraints and environmental constraints (by processing different
feeds, different constraints might become active also constraints at low capa-
city or changing meteorological conditions).
± M Process have a moderate number of constraints, like processes with feed
streams with constant composition and those running only incidentally
against constraints.
± L Process running below capacity and only incidentally forced to an extreme
operational regime which also may include low-capacity operation or
equipped with constraint controllers on most activated constraints.
± NA Almost never running against a constraint or fully provided with con-
straint controllers. Floating pressure controllers or maximum duty control on
a refrigeration system can also be considered as constraint controller next to
MIMO controllers.
Note: after implementation of an off-line optimization, constraints become more
visible and often there are more constraints than would be noticed without optimi-
zation results at hand.

Optimization closed loop
± H Processes with many economic disturbances at a frequency of a few times
per day, such as processes with wide feed slate and product distributions, a
high number of DOFs likely with a large number of units, highly constrained
operations which is implicitly coupled to the processing of wide feed and
product slates. Parameters that need a regular update.
± M Processes with several economic disturbances and a moderate number of
DOFs (5±10)on a daily basis.
± L Processes with a limited number of disturbances and DOFs (<5)on a less
than daily basis, with off-line optimization results available for manual imple-
mentation.
± NA process with low number of disturbances and DOFs that are already
manually implemented based on off-line optimization results.