Control structure design

The analysis presented here clearly indicates that the reactive distillation systems, regardless of the types of flowsheet, exhibit severe nonlinearities that include significant sign reversal, extremely large values of Allgower s nonlinearity measure ( ) ), and input multiplicity. Under these circumstances, control structure design becomes important. 

In this section a systematic approach is proposed to design the control structures for these three types of reactive distillation flowsheets. Because all five reactive distillation systems (Table 7.5) have almost equal molar feedflows (neat flowsheet), the stoichiometric balance has to be maintained. Here we adjust the feed ratio to prevent accumulation of unreacted reactants attributable to stoichiometric imbalance. The next issue is, how many product compositions or inferred product purities should be controlled For the esterification reactions with A -f B C + D with a neat flowsheet, controlling one-end product purity implied a similar purity level on the other end, provided the product flowrates are equal. Thus, a single-end composition (or temperature control) is preferred. This leads to 2 x 2 multivariable control, as opposed to a 3 x 3 multiple-input-multiple-output system. The 

Maintain the stoichiometric balance using the feed ratio. 

This leads to the following design procedure for temperature control of reactive distillation 

With the feed ratio as one manipulated variable, select a second manipulated variable. Typically, the other manipulated input is the heat input or the reflux ratio. 

---

R.D. Bartusiak, C. Georgakis, and M.J. Reilly. Nonlinear feedforward/feedback control structures designed by reference synthesis. Chemical Engineering Science, 44 1837-1851, 1989. 

Skogestad, S. (2004). Control structure design for complete chemical plants. Comput. Chem. Eng., 28, 219-234. 

Hazards related to the interaction among components, for example the interaction between attempts by air traffic control and by TCAS to prevent coUisions, need to be handled in the safety control structure design, perhaps by mandating how the pilot is to select between conflicting advisories.There may be considerations in handling these hazards in the subsystem design that will impact the behavior of multiple subsystems and therefore must be resolved at a higher level and passed to them as constraints on their behavior. 

General Safety Control Structure Design Principles... 

Spatial and Control Structure Design in Reactive Distillation... 

The results ofthis study show an attractive way for selection of proper dampers location and configuration, allowing the lowest and most effective control. In the authors opinion, efficiency of lever arm configuration in active controlled structures, designed using non-linear control algorithms, should be further investigated. 

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... 

J. O. Trierweiler, A Systematic Approach to Control Structure Design. Dr.-Ing. Dissertation, Dept, of Chemical Engineering, Universitat Dortmund, 1997. 

As mentioned, the focus of this chapter is on selecting controlled variables. More generally, the issue of selecting controlled variables is the first subtask in the plantwide control or control structure design problem (Foss 1973) (Morari 1982) (Skogestad and Postlethwaite 1996) ... 

Despite strong nonlinearity (Figs. 13.3 and 13.4), workable temperature control of these reactive distillation systems can be obtained using a systematic design procedure with rather simple control structures. It should be emphasized that the selections of controlled and manipulated variables (control structure design) play a cmcial role for these highly nonlinear processes and that decentralized control provides a better stmcture to cope with steady-state gain variations than model-inversion-based control stmctures. 

---