Page 375 - Design of Simple and Robust Process Plants

362 Chapter 9 Operation Optimization
1. Fractional recovery of the intermediate component B leaving the pre-fractio-
nator top.
2. Fractional net recovery of the bottom component C in the overhead of the
pre-fractionator versus the total net flow of C to the main column.
3. DTS ± a measure of temperature profile symmetry,

DTS = P (T 1, i ±T 4, i )+ P (T 2, i ±T 5, i )(3)
where T Ni is the temperature of tray i of section N, for the temperature location (see
Figure 9.6).
The last mentioned feed back variable can easily be measured by temperature,
while the others require a composition measurement.
The above self-optimizing technology can be seen as a suboptimization of a global
optimization problem (Figure 9.7), or as part of the control layer. The global optimi-
zation sets the feed composition (direct or indirect), next to the product specifica-
tions. The self-optimization reacts to any disturbance to keep the unit at its opti-
mum energy consumption. In the case of final products, the purities are often firm
constraints, while recycle streams often have economic specifications that are sub-
ject to change and need to be updated by global optimization.
The search for self-optimizing control structures is discussed by Skogestad (1999)
in a more generic paper that is illustrated with some other examples as a reactor
system and a simple distillation column. For a further description, see Chapter 8. In

Raw
materials Product B
Reactor Product A

Product C

Global optimizer Self optimizing control /
Unit optimizer

Fig. 9.7. Global optimization versus unit self optimizing control.