Plantwide Control Fundamentals

The nine steps of the design procedure center around the fundamental principles of plantwide control energy management production... 

In this chapter we examine some of the fundamental features and properties of the plantwide control problem. Our goal is to explain why we must design a control system from the viewpoint of the entire plant and not just combine the control schemes of each individual unit. 

Three basic features of integrated chemical processes lie at the root of our need to consider the entire plant s control system (1) the effect of material recycle, (2) the effect of energy integration, and (3) the need to account for chemical component inventories. If we did not have to worry about these issues, then we would not have to deal with a complex plantwide control problem. However, there are fundamental reasons why each of these exists in virtually all real processes. 

In this chapter we outline the nine basic steps of a general heuristic plantwide control design procedure (Luyben et al., 1997). After some preliminary discussion of the fundamentals on which this procedure is based, we outline each step in general terms. We also summarize our justification for the sequence of steps. The method is illustrated in applications to four industrial process examples in Part 3. 

Our plantwide control design procedure (Fig. 3.1) satisfies the two fundamental chemical engineering principles, namely the overall conservation of energy and mass. Additionally, the procedure accounts for nonconserved entities within a plant such as chemical components (produced and consumed) and entropy (produced). In fact, five of the nine steps deal with plantwide control issues that would not be addressed by simply combining the control systems from all of the individual unit operations. 

The plantwide control design procedure presented here was developed after many years of work and research in the fields of process control and process design. Research efforts by a number of people in industry and at universities have contributed essential ideas and concepts. We have assembled, analyzed, and processed this prior work to reach a logical, coherent, step-by-step procedure. We want to acknowledge these previous contributions and state that we are indeed fortunate to stand upon the shoulders of many giants. Listed below are some of the fundamental concepts and techniques that form the basis of the procedure. 

It should be noted that establishing the product-quality loops first, before the material balance control structure, is a fundamental difference between our plantwide control design procedure and Buckley s procedure. Since product quality considerations have become more important in recent years, this shift in emphasis follows naturally. 

We cannot hope to cover the vast subject of distillation fundamentals and control in a single chapter in this book. Our objective here is to review some of the basic principles about distillation and then to summarize the essentials of distillation column control, particularly as it relates to the plantwide control problem. Many more details are available in the books cited above. In the first section we review some of the important fundamentals about distillation. Sections 6.3 through 6.8 discuss distillation column control mostly from the perspective of an isolated column or column system, This treatment presents what may appear to be a laundry list of control structures for different types of columns. Although we feel this presentation is valuable, particularly for the young inexperienced student or engineer, it is also vital to retain a broader plantwide perspective. Section 6.9 addresses some of these plantwide distillation control issues. In the next section we review the process fundamentals of distillation as they relate to process operation and control. 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

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. 

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). 

The first topic developed in this chapter is the interrelation between steady state design and controllability. Particular attention will receive the generic stmcture Reactor-Separator-Recycle. We consider that material balance has priority in establishing the plantwide control strategy. That is why we will examine in the first place the effects induced by the recycle of mass. Particular attention will receive the feed policy of the reactants, as well as the handling of impurities, both fundamental plantwide control issues. Then we will examine the effect of recycling energy, with emphasis on heat... 

It is assumed that the reader is familiar with the fundamentals of process control of the key unit operations, as reactors, distillation columns, etc. A concise description of essential topics in Process Control can be found in the recent book of Luyben Luyben (1998). We also strongly recommend the recent monograph of Luyben and Tyreus on plantwide control (1999). Basic issues in dynamics and process control can be found in the classical textbooks of Stephanopoulos (1984), Luyben (1990), Marlin (1995) and Ray Ogunnaike (1998). 

The strategy of controlling the plants with recycles is known today as "plantwide control". The first monograph dedicated to this subject hcis been published only recently by Luyben, Tyreus and Luyben [1]. As they stressed "How a process is designed fundamentally... 

Two fundamental principles can be employed to develop material balance plantwide control structures, namely self-regulation and controlled-regulation. Control structures based on self-regulation set the flow rates of fresh reactants at values determined by the production rate and stoichiometry. Controlled-regulation consists of fixing one flow rate in each recycle... 

Two fundamental principles can be employed to develop material balance plantwide control structures, namely self-regulation and controlled-regulation. 

SVA, etc.) for multivariable control problems. In the absence of process models, one must resort to heuristic (rule-of-thumb) approaches. Although these approaches generally are based on prior experience, they also incorporate an understanding of the fundamental physics and chemistry that apply to all plants. In this chapter, several case studies are used to introduce important plantwide concepts. In the final chapter (Appendix H), we present a general strategy for designing plantwide control systems. 

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. 

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