Plantwide control

Process Control. The traditional process control will be expanded toward new applications such as nonlinear process control of biosystems. However, in the commodity chemicals industry there will be increased need for synthesizing plantwide control systems, as well as integrating dynamics, discrete events, and safety functions, which will be achieved through new mathematical and computer science developments in hybrid systems. 

The downside of this design is that the excess A must be recovered and recycled, which means high capital and energy costs. However, the resulting improvement in the yield of C is typically well worth the added cost. This is particularly true when the undesirable product D is toxic, corrosive, explosive or an environmental pollutant that is difficult to dispose of. Environmental and safety concerns have pushed the designs of many chemical processes to include several large recycle streams so that the yields of desirable products are increased and the yields of undesirable products are decreased. These recycle streams increase the difficulty of the plantwide control problem. 

The dynamics and control of the reactor-column system studied in Chapter 2 (Section 2.9.3) are investigated in this section. Mathematical models of both the reactor and the column are developed, and a plantwide control structure is evaluated. 

Figure 3.38 Reactor-column plantwide control structure.

Plantwide Control Structure There are several alternative control structures for this process, and there is no claim that the one developed is the best from whatever perspective you consider. What is claimed is that it provides effective base-level regulatory control of this process. 

Chen, R. and McAvoy, T. J. (2003). Plantwide control system design methodology and application to a vinyl acetate process. Ind. Eng. Chem. Res., 42, 4753-4771. 

Downs, J. J. and Skogestad, S. (2009). An industrial and academic perspective on plantwide control. In IFAC Symposium on Advanced Control of Chemical Processes, pp. 119-130. 

Larsson, T. and Skogestad, S. (2000). Plantwide control - a review and a new design procedure. Modeling, Identification Contr., 21, 209-240. 

Vasudevan, S. and Rangaiah, G.P. (2009). Development of guidelines for plantwide control of gas-phase industrial processes, from reactor-separator-recycle results. Ind. Eng. Chem. Res., 50, 297-337. 

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

Examine plantwide control aspects, including safety, environment protection, flexibility with respect to production rate, and quality control. 

However, only the static evaluation of process performance around an operation point is not sufficient. The flexibility in operation and the dynamic behavior should be explored by dynamic simulation, including the implementation of the main features of the control system. Therefore, process dynamics and plantwide control issues are largely addressed in this book. 

Selectivity is a key topic for the design of reactors in recycles. From the standalone viewpoint the means to influence selectivity are reactor type, conversion level and mixing method. In the standalone view low conversion and PFRs are recommended for achieving good selectivity. By contrast, following the results of RSR analysis, the recycle policy and the plantwide control of reactant feeds can play the determinant role the reactor type or the conversion level is less important... 

The advent of powerful and user-friendly dynamic simulation software makes it possible to handle the plantwide control strategy directly with nonlinear plant model. This advantage will be greatly exploited in this book. Chapter 4 will treat in more detail the problem of nonlinear analysis of plants with recycle with consequences on the plantwide control strategies. 

The task of plantwide control is to harmonize the BFSs in such a way that the whole system operates in the required manner. This is achieved by assigning control objectives to BFFs. The controllability of the BFSs is a necessary (but not sufficient) condition for the controllability of the entire plant. Consequently, integrating design and control consists of two steps ... 

By including the reactor/separation/recycle level in the hierarchical approach, plantwide control can be considered at an early stage of design. In most cases, the separation is considered as a black-box. By black-box we mean that some targets are set, for example as species recovery or product purities. The separation is then modeled based on simple input-output component balances. The decisions to be taken and the detail of the results obtained depend on the information about the chemical reactor that is available. 

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