A Thermodynamic Network Approach to Plantwide Control

Chemical processes rarely operate independently, but are part of a production line or an integrated facility containing many production lines.  The control of large-scale chemical processes has been the subject of significant research efforts and led to debates on the merits of decentralized versus centralized control or alternatively a hybrid distributed control approach.   Regardless of the proposed control approach, an accurate, compact dynamic description of large chemical processes is necessary to facilitate control design, analysis, and simulation.  Frameworks describing the interactions of chemical process networks (CPN)  have been previously described.[1] [2] [3] However, there has not been a concerted effort to apply the generalities of a thermodynamic network to real world problems.  The goal of this work is to complete and simplify the representation of thermodynamic CPNs, formulate tools to design and analyze CPN control systems, and solve a CPN control problem.

 A dual-edge thermodynamic network approach is presented to facilitate control design.   Final control elements  constitute a unique class of node in the network  that act directly on material and energy edges, and apply additional restrictions to network flows.  With the network defined,  unique topological properties of CPNs  analogous to KCL, KVL, and Tellegen’s Theorem from electrical circuit analysis are presented.  These properties are a function of  the 1^st and 2^nd laws of thermodynamics and the conservation of mass.  The properties of the resulting network places restrictions on the number and choice of manipulated variables in control system design.  An algorithm to determine acceptable sets of manipulated variables (control degrees of freedom) in the network is presented. 

Finally, we propose a procedure for plantwide control based on the control degree of freedom concept from the network analysis and passivity based control of chemical processes.  The stability conditions of the entire network are presented.  An industrial example is used to show the application of the network approach from initial analysis to implementation (via simulation).

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[1] Gilles, E. D. (1998). Network Theory for Chemical Processes. Chem. Eng. Tech. , 121-132.

 

[2]Jillson, K., & Ydstie, B. (2007). Process networks with decentralized inventory and flow control. Journal of Process Control , 399-413.

[3] Baldea, M., El-Farra, N., & Ydstie, B. (2013). Dynamics and control of chemical process networks : Integraing physics, communication and computation. Computers and Chemical Engineering .

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