Page 359 - Design of Simple and Robust Process Plants

References 345
8.5
Summary

Simple and robust process plants require robust control designs for hands-off opera-
tion. The design of robust control is an effort which requires a fundamental model-
based design approach, and this is receiving an increasing amount of recognition.
The robustness of the control design needs to be achieved at the basic control layer
which is the fall-back control position when higher control hierarchies fail or are not
available. The requirement that operators should not be part of the control loops
demands quantitative process understanding to unable robust control design. Heur-
istic control approaches are still of value, but in time they will be superseded by
fundamental control design.
A methodology for the design of basic control design based on fundamental mod-
els is discussed, and consists of the following elements:

. Dynamic model building based on fundamental models.
. Preliminary selections of CVs and MVs, measure variables and disturbance
variables.
. Design of overall plant wide control layout.
. Selection of dominant control variables.
. Selection of variables based on self-optimizing control structures.
. Evaluation of loop pairings to minimize interactivity based on dynamic con-
trol parameters..
. Design of controllers which might include de-coupling algorithms based on
algebraic equations.
. Testing of controllers.
The approach has been completed with a control design example of a complex inter-
active reactor system implemented at the basic control level. The result was a hands-
off operated reactor system by de-coupling capacity ramping, energy balance and
conversion control. The system simulated was to comply with first pass prime
production. This approach supported the elimination of several decanting and off-
spec tanks and as such contributed to a simple and robust design.

References

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Bogle I.D.L., Rashid M. An Assessment of (Crowl, D., Ed.), New York, 1996.
Dynamic Operability Measures. Computers ISBN 0-8169-0703-X.
chem. Engng, (1989). 13, No. 11±12,
1277±1282.