Page 345 - Design of Simple and Robust Process Plants

8.4 Control Design 331
During the controllability step 2 of process synthesis this information is required to
evaluate different flowsheet options, see Chapter 4.
Selection of final control loop pairings and any de-coupling algorithm.

8.4.7.2 Selection of the dominant variables
The selection of the dominant variables can be done in two ways ± either by static
simulations and determination of the process gains of selected variables, or by a
thermodynamic method for identification. Both are analytical techniques which
determines the dominant variables for control.
Analysis of different control variables by simulation is a straightforward approach
for simple systems where the variables are known from experience. In case of com-
plex reactor systems, it is not always easy to analyze in those cases, and the thermo-
dynamic method for identification might be preferred (TyrØus, 1999). This latter
methodology identifies the dominant variables that affect the rate of energy exchange
within the process. The method is based on earlier work of Schmid (1984) and
Fuchs (1996).
The methodology is based on the introduction of the term ªsubstance-likeº. This
is a terminology which describes a physical quantity that behaves like an actual sub-
stance, such as the mass of material within a process, the amount (moles) of
chemical components, the electrical charge, energy, entropy, and momentum.
These substance-like quantities can be characterized by three terms: density; flow;
and a balance or continuity equation.
Continuity equations have a production term included in them; however, mass
energy and momentum lack a production term. Because these last substance-like
quantities are conserved, the production terms are true for entropy and also for com-
ponents formed during a reaction.
While the balance equations are applicable for substance-like quantities under
conceivable conditions, they can only be solved if the production term can be formu-
lated. The production terms are called constitutive equations.
ªThe constitutive equations describe how substance-like quantities affect the state of
a particular dynamic system and how the quantities flow in and out of the system
depending on the system stateº
When it is accepted that a physical system can be described with balance and con-
stitutive equations, the question is which substance-like quantities should be used
to describe the dynamics of a physical system. Schmid and Fuchs provided the
answer to this question.
For mechanical systems, momentum was identified as the fundamental sub-
stance-like quantity, for electrical systems it was electrical charge. For chemical sys-
tems, the amount of component and for thermal systems entropy are used as the
independent substance-like quantities:

The amount of energy transported with the flow of a particular substance-like quantity
is the product of the quantity's flow rate times its intensity (potential).