Simple rules for economic plantwide control

CATEGORY: PROCESS CONTROL
Simple rules for economic plantwide control
PSE2015 - ESCAPE25
12th International Symposium on Process Systems Engineering and 25th European Symposium on Computer Aided Process Engineering, Copenhagen, Denmark, 31 May - 4 June 2015
Vladimiros Minasidis a, Sigurd Skogestad a,*, Nitin Kaistha b
a Department of Chemical Engineering, Norwegian University of Science and Technology, 7491, Trondheim, Norway
b Department of Chemical Engineering, Indian Institute of Technology, 208016 Kanpur, India
When designing new-technology based plants process engineers often operate under a strict time schedule and inevitably, the additional time required to devise a control strategy that achieves not only safe and stable operation but also economic- operation using optimization-based/mathematical methodologies, is often not available. One alternative is to employ practical guidelines that lead to a close-to-optimal control structure design, since those are usually easier to understand, thus easier and faster to implement without requiring high expertise. In this work we exemplify the rules that can be used solely based on engineering intuition a from a systematic procedure plantwide control design procedure proposed by Skogestad (2004). We attempt to present the them in an easy-tounderstand fashion and to demonstrate that even though the procedure is based on a combination of heuristics and plant model optimization, it can be used as a supplement to the traditional practical guidelines by suggesting what to control -or what not to- in order to improve the economics of the plant operation.
Additionally, we propose a new simple rule that supplements the existing economic controlled variable selection rules, stated as follows: ”Do not control an unconstrained controlled variable that reaches a minimum or a maximum at the optimum”. We apply the newly suggested and the existing rules to a simple but quite famous reactor-separator-recycle (RSR) plant operation case study and investigate their effectiveness. We successfully demonstrate that using the practical rules from Skogestad’s procedure can potentially improve a plant operation even if the process model is not available. 
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