Page 358 - Design of Simple and Robust Process Plants

344 Chapter 8 Instrumentation, Automation of Operation and Control
The capacity set points for ramp-up conditions were calculated for reactants and
solvent flows as well as the inlet reactor temperature values over the projected capa-
city range at design conditions. These results were converted in polynomial correla-
tions for the feed-forward flow controller.
Heat balance control was realized by fine adjustment of the total solvent flow
with a feed-back signal from the level of the catalyst recycle drum.
Reactor temperature profile control. The catalyst flow was kept constant. The pro-
file was primarily reflected in the peak temperature, which was influenced by the
overall solvent flow and the split of the solvent flows. A feed-back loop was installed
between the peak temperature and the solvent flow and its split.
All the above controllers were installed in the basic control layer, and resulted in
robust control of the system, while the operators were no longer part of the control
loop (Figure 8.19). The whole control design was based on fundamental models,
which were also used to test and tune the controllers.
The above illustrates how, based on fundamental dynamic models, robust control
design can be developed for interactive process systems. The selection of the feed-
back loops was based on: de-coupling of capacity control, heat balance control and
conversion control through the temperature profile. The control design was imple-
mented at basic control level based on simple algebraic equations.
The example is a nice illustration how interaction between capacity ramping,
energy balance and conversion control can be de-coupled.
The same models are to be used to simulate the start-up of the reactor system
resulting in an operational procedure to achieve first pass prime production.
The control design effort resulted in hands-off operation of the reactor system.
The improved control supported the elimination of several decanting and off-spec
tanks which made the process much more complying with a simple and robust
design at lower cost.
P
Vapor
T. profile
control Product
T
Capacity Heat balance L
setpoints corrections

F
T
Solvent
T
F F T
T
F
F
L
T F
Reactants

Fig. 8.19. Reactor control design with hands-off control (Verwijs, 2001).