Plantwide Controllability Assessment

Detailed Design, Equipment Sizing, and Optimization—Configured Product Design 

Safety, Health, and Loss Prevention in Chemical Processes Problems for Undergraduate Engineering Curricula—Student Problems (1990). 

Guidelines for Hazard Evaluation Procedures, Second Edition with Worked Examples (1992). 

The latter reference is particularly noteworthy for instructors because it provides a PowerPoint file that can be integrated into a safety lecture. 

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NF Thornhill and A Horch. Advances and new directions in plantwide controller performance assessment. In Prep. IFAC ADCHEM 2006, pages 29-36, Gramado, Brazil, 2006. 

This chapter presents fundamental elements from the process control theory necessary in assessing the controllability of a design. It prepares also the computational tools used in Chapter 13 devoted to plantwide control. Prior knowledge is welcome. If the material is too difficult, the reader should go back to specialized undergraduate books. A concise presentation of the process control essentials viewed from process engineer s perspective has been recently published by Luyben Luyben (1998). In the field of feedback control of multivariable systems with emphasis on controllability analysis we recommend the book of Skogetsad and Postletwhite (1998). 

In a multi-input multi-output (MIMO) control system (Fig. 12.14), there are several controlled variables (vector y) that should be kept on set-points (vector r) faced to disturbances (vector d) by means of appropriate manipulated variables (vector u). The feedback controller K provides the algorithm that will ensure the link between the manipulated (inputs) and controlled (outputs) variables. In this chapter we will consider a decentralised control system that makes use of multi-SISO control loops, which means that a single controlled variables is controlled by a single manipulated variable. This arrangement is typical for plantwide control purposes. However, there will be interactions between different loops. These Interactions can be detrimental, or can bring advantages. Therefore, the assessment of interactions is a central issue in the analysis of MIMO systems. 

Chemical conversion is an effective way to counteract the accumulation of impurities due to positive feedback. Also, changing the connectivity of units may be used to modify the effect of interactions, for example by preventing an excessive increase in recycles due to snowball effects. Effective plantwide control structures may imply controlled and manipulated variables belonging to different but dynamically neighbouring units. The methodology to evaluate the dynamic inventory of impurities consists of a combination of steady state and dynamic flowsheeting with controllability analysis. This is used to assess the best flowsheet alternative and propose subsequent design modifications of units. Case Study 3 in Chapter 17 will present this problem in more detail. 

Assess steady state design and dynamic control objectives. The manipulations must have sufficient power to reject disturbances and drive the set points. The choice of the control objectives is essential, because different objectives may lead to different structures. Typically the plantwide control objectives could include reactor and separation yields, quality specifications, product grades, as well as constraints set by... 

For the vinyl chloride process synthesized in Section 3.4, a preliminary design of its plantwide control system helps to assess the ease of maintaining the desired production level. As shown in Figures 20.17 and 20.18, this is achieved following the design procedure of Luyben and co-workers (1999) ... 

The quantitative assessment of the controllability and resiliency of chemical processes has generated considerable interest. The term resiliency was introduced by Morari (1983), who also pioneered qualitative measures for its assessment. Furthermore, Perkins (1989) presented an approach for the simultaneous design of processes and their control systems that addresses plantwide controllability directly. 

To our knowledge, this book is the first design text to emphasize the importance of assessing plantwide controllability. Modem computing tools are enabling practitioners and students to be more aware that processes selected on the basis of steady-state economics alone often perform poorly and are less profitable. 

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... 

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