Plantwide control design procedure

Luyben, M.L, B.D. Tyreus, W.L. Luyben, Plantwide control design procedure. 

While we use conventional control systems here, our plantwide control design procedure does not preclude the use of MPC at a certain level. Our focus is on the issues arising from the operation of an integrated process. We find that a good control structure provides effective control, independent of any particular controller algorithm, while a poor one cannot be greatly improved with any algorithm (MPC or PID controllers). 

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. 

Optimize Economics and Improve Dynamic Controllability Figure 3.1 Nine steps of plantwide control design procedure. 

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 have discussed in detail each of the nine steps in our plantwide control design procedure. The first two steps establish the control objectives and control degrees of freedom for the plant. In the third step we discuss how the plantwide energy management problem can be converted to a local unit operation energy management problem by using the plant utility system. 

Even though unit operation control is not addressed in our plantwide control design procedure until Step 8, it is important to understand up front what all the dominant variables are and their relationship with potential manipulators. This is particularly true if appropriate manipulators are unavailable, in which case design changes must be made. The dominant variables influence several steps in the design procedure, in particular our choice of controlling reactor temperature, production rate, and recycle stream compositions. 

Step 3 of our plantwide control design procedure involves two activities. The first is to design the control loops for the removal of heat from exothermic chemical reactors. We dealt with this problem in Chap. 4, where we showed various methods to remove heat from exothermic reactors and how to control the temperature in such reactors. At that point we assumed that the heat was removed directly and permanently from the process (e.g., by cooling water). How-ever. it is wasteful to discard the reactor heat to plant utilities when we need to add heat in other unit operations within the process. Instead, a more efficient alternative is to heat-integrate various parts of the plant by the use of process-to-process heat exchangers. 

A plantwide control design procedure was used to develop a simple but effective regulatory control system for the Eastman process with an on-demand product control objective. With this strategy, control of production rate is essentially instantaneous. Drastic upsets and disturbances are handled by simple proportional-only overrides. 

We first presented the process for the hydrodealkylation (HDA) of toluene in Chap. 1 as part of our explanation of what the term plantwide process control means. We now return to it as the third illustration of our plantwide control design procedure. 

In this chapter we have applied the plantwide control design procedure to the HDA process. The HDA process is typical of many chemical process with many chemical components, many unit operations, several recycle streams, and energy integration. The steady-state design of the HDA process has been extensively studied in the literature, but no quantitative study of its dynamics and control has been reported. 

The final example to illustrate our plantwide control design procedure comes from Luyben and Tyreus (1998), who present design details of an industrial process for the vapor-phase manufacture of vinyl acetate monomer. This process is uniquely suited for researchers pursuing process simulation, design, and control studies. It has common real chemical components in a realistically large process flowsheet with standard chemical unit operations, gas and liquid recycle streams, and energy integration. 

Figures 12.1 through 12.11 give several flowsheets of typical chemical processes. You might find it instructive to apply the plantwide control design procedure to these processes. Very little process information is provided in these figures. You will have to make assumptions about a number of process conditions, flowrates, and control objectives. Keep a list of these as you work through the nine stages.

The inventory of impurities is a plantwide control problem, because it involves both the reaction and separation subsystems through recycles. Ideally, the inventory of each component should be traced from the source to its final destination. Recent systematic studies on the dynamics and control of the recycle systems have been started, as described in the Chapter 13. Luyben and Tyreus (1998) proposed a ten steps plantwide control design procedure (section 13.7). The step 7 consists of Checking component balances, identify how chemical components enter, leave, and are generated or consumed in the process. At this stage it is necessary to find the specific mechanism or control loop to guarantee that there will be no uncontrollable build-up of any chemical component within the process . 

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