Page 141 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: ZCK/FLS P2: FYK/FLS QC: FYK Revised Pages
Encyclopedia of Physical Science and Technology EN002G-100 May 19, 2001 18:49

Chemical Process Design, Simulation, Optimization, and Operation 763

in feasible operation. This computation is reasonably sim-
ple for a convex problem, as illustrated, but becomes much
more complicated for nonconvex problems and those with
a large number of degrees of freedom.

C. Design for Operability

Process design modifications usually have a bigger impact
on operability (dynamic resilience). Dynamic resilience
depends on controller structure, choice of measurements,
and manipulated variables. Multivariable frequency-
FIGURE 8 Schematic of the flexibility index. response techniques have been used to determine resi-
lience properties. A primary result is that closed-loop
Additionally,theincreaseduseofmodelpredictivecontrol control quality is limited by system invertability (nonmin-
techniques allows for more degrees of freedom (associated imum phase elements). Additionally, it has been shown
with having more manipulated variables) which in turn that steady-state optimal designs are not necessarily
tends to improve controllability and therefore decrease optimal in dynamic operation.
process variability. Principal component analysis (PCA) Another consideration is the dynamic controllability of
and projection to latent structures (PLS) are tools that can a process design. If there are no uncertainties or distur-
be used to monitor process performance and behavior. bances, then the optimum economic design normally oc-
curs at a constraint. An actual process cannot be operated
B. Design for Flexibility at a constraint, because any disturbance may force the
system to violate the constraint (a product purity limit, for
A process design is flexible if it can tolerate uncertainties example). In this case, the desired steady-state operating
in parameters and can handle disturbances. A flexibility point must be “backed-off” from the economic optimum
index is a measure of the amount of uncertainty that can so that the control strategy can tolerate disturbances. The
be tolerated with the desired process operation remaining basic idea is shown in Fig. 9.
feasible. A schematic of the flexibility index, δ, is shown
in Fig. 8. Here, the two degrees of freedom represent un-
certain parameters or disturbance variables, which have VII. PROCESS OPERATION
assumed upper and lower bounds. The feasible operating
region lies within the cross-hatched area. The flexibility The actual operation of a chemical process involves de-
index is the fraction of the parameter range that still results cisions and actions that occur at a number of levels and

FIGURE 9 Illustration of best feasible steady-state operating point. (Adapted from Morari and Perkins, 1995).