Page 346 - Design of Simple and Robust Process Plants

332 Chapter 8 Instrumentation, Automation of Operation and Control
The intensity with momentum is velocity, while the intensity for electrical charge is
voltage. The intensity for a chemical component is chemical potential, while for
entropy the intensity is the absolute temperature. The intensities are a subset of the
intensive variables used for equilibrium thermodynamics.
Now reactive and physical separation systems can be described with substance-
like carriers. Since energy as such can neither be produced nor consumed, every
process might be seen as an exchanger of energy. TyrØus (1999) referred as example
to a fuel cell where (entropy flow rate ” absolute temperature (intensity)) = power
generated by the chemical reaction. Other examples cited were flash and distillative
separations. This is summarized as follows: a process system can be described by a
generalized balance equations applicable for substance-like quantities to identify
variables that determine the rate of energy exchange. For a chemical system, the
process can be modeled by the number of component balances and one entropy
balance. The balance equations need to complemented with the constitutive equa-
tions which depend primarily on the intensive variable (such as temperature, pres-
sure) of the process.
8.4.7.3 Thermodynamics and partial control
Partial control has been defined by TyrØus et al. (1999) as a form of decentralized
(basic) control in which the controlled variables are explicitly paired with the
manipulated variables such that the main feedback loop are easily identified.
A physical justification needs to be derived to identify the dominant variables from
the thermodynamically description of the process. The economic performance of a
process is determined by the production rate and the conversion and selectivity per-
formance. During operation optimization, the operational conditions are pushed
against their constraints, operational, safety environmental and product specifica-
tions to achieve the maximum economic performance. The economic objectives are
mostly tied to the flow and production rates of substance-like quantities, and to the
intensive variables that help to establish these rates. To use thermodynamic models
for guidance in the identification of substance-like quantities, the focus must be on
the constitutive equations instead of the balance equations. As a help in the selec-
tion of the constitutive equation the energy principle was introduced by Fuchs (1996),
which is applicable to all process systems. It states that at steady state, the net energy
flow in and out of a process should be zero. The energy might, however, flow in and
out with different carriers.

The internal rate of exchange of energy between the various carriers will thus be taken
as the most relevant rate indicator of a process and the process variables affecting the
energy exchange will be considered dominant.

The rate of energy exchange is expressed as the power release expressed in an
entropy substance-like quantity for a flash unit (see Figure 4.16) appears as:
P flash =I s (T out ±T in )

where P is power in Watts, I s is entropy current W/ K, and T is absolute temperature K