Plantwide control synthesis

The chapter is organized as follows. Section 2 presents brief reviews of the hierarchical steady-state design structure and a particular plantwide control synthesis structure. Both have relevance to this work. The former because some of the plantwide control methods rely on this hierarchy, and the latter to complement the proposed approach. Section 3 begins with preliminaries that define the state of the process design flowsheet, the modified Analytical Hierarchical Process (mAHP) method used for assessment, and the process flowsheet... 

McAvoy, T. J. (1999). Synthesis of plantwide control systems using optimization. Ind. Eng. Chem. Res., 38, 2984-2994. 

Zheng, A., Mahajanam, R. V., and Douglas, J. M. (1999). Hierarchical procedure for plantwide control system synthesis. AIChE J., 45, 1255-1265. 

Answering these questions is not at all a trivial matter. But these issues lie at the foundation of control system synthesis for an entire plant. The plantwide control problem is extremely complex and very much open-ended. There are a combinatorial number of possible choices and alternative strategies. And there is no unique correct" solution. 

The case study of vinyl acetate synthesis emphasises the benefits of an integrated process design and plantwide control strategy based on the analysis of the Reactor / Separation / Recycles structure. The core is the chemical reactor, whose behaviour in recycle depends on the kinetics and selectivity of the catalyst, as well as on safety and technological constraints. Moreover, the recycle policy depends on the reaction mechanism of the catalytic reaction. 

Hierarchical Approach is a simple but powerful methodology for the synthesis of process flowsheets. It consists of a top-down analysis organised as a clearly defined sequence of tasks grouped in levels. Each level solves a fundamental problem as, number of plants, input/output structure, reactor design and recycle structure, separation system, energy integration, environmental analysis, safety and hazard analysis, and plantwide control. At each level, systematic methods can be applied for the synthesis of subsystems, as chemical reaction, separations, or heat exchangers network. 

The current practice has shown that there is a gap between process and control engineers. Filling this gap is a challenge for education. Only very recently plantwide controllability issues have been included in a book dealing with process design (Seider, Seader Lewin, 1999). This chapter aims to give another perspective on the same subject, with emphasis on integrating controllability aspects in flowsheet synthesis. 

In this section we present a heuristic plantwide control procedure proposed by Luyben and Tyreus (1999). The methodology is founded on considerable industrial experience, and it is illustrated in the original reference by relevant case studies. The procedure is applicable for existing flowsheets. The synthesis of flowsheets for desirable controllability patterns remains an open research topic. 

This chapter presents a simple synthesis-oriented approach methodology for integrating plantwide control in hierarchical conceptual design (Bildea, 2001). Two ideas are central ... 

Be able to perform a conceptual synthesis of plantwide control structures (pairings) based on degrees-of-freedom analysis and qualitative guidelines. 

This chapter has introduced the importance of considering plantwide control early in the design process. A qualitative control synthesis method, combining the approaches suggested by Newell and Lee (1988) and Luyben and co-workers (1999), was presented to show how to generate alternative control configurations. The limitations of this qualitative approach have been highlighted and the need for the quantitative approach presented in Chapter 21, which involves analysis and dynamic simulation, has been established. 

Hoo and co-workers in Chapter C2 propose a modular decomposition of the plant flowsheet using a decision-based methodology for the synthesis of plantwide control structures. Design, operational and economic objectives are associated with those individual... 

Synthesis of plantwide control structures using a decision-based methodology... 

The goal of plantwide control structure synthesis is to develop feasible control structures that address the objectives of the entire chemical plant and account for the interactions associated with complex recycle and heat integration schemes, and the expected multivariate nature of the plant. Many strategies have been proposed for accomplishing this task, and the majority of them have been demonstrated using dynamic process simulations. However, none have been accepted as the universal approach, in a manner similar to the steady-state process design synthesis hierarchy of Douglas [1]. 

The process flowsheet of the hydro-dealkylation (HDA) of toluene to benzene is the ubiquitous example used in the pedagogy of the process design. Moreover, it has been studied extensively in the open literature [1,7]. Luyben et al. [7] have developed a plantwide control structure for the HDA process using their nine step approach to control structure synthesis Fisher et al. [19] provide a control structure development for the HDA process using a hierarchical approach. Stephanopoulos [47] also presents a control structure for the HDA process. These flowsheets provide a basis by which the outcome of the proposed methodology can be compared. 

This book addresses each of these questions and explains the fundamental ideas of control system synthesis. As its core, the book presents a general heuristic design procedure that generates an effective plantwide base-level regulatory control structure for an entire, complex process flowsheet and not simply individual units. 

The next example will revisit the plantwide problem of HDA plant, this time viewed as a process synthesis problem where flowsheet design and controllability are examined at the same levels of the hierarchical methodology. 

Simultaneous design is defined as the development of a chemical process by considering both steady-state economics and dynamic controllability at all stages of flowsheet synthesis. The basic notion is that the dynamics of the process are vitally important in its ability to operate efficiently and safely and to make on-specification products with little product-quality variability. There is no guarantee that a process flowsheet that has been developed to optimize some steady-state economic objective will provide good plantwide dynamic performance. 

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