Plantwide Control System Design - NTNU

1 A-63 AppendixGPlantwide Control System DesignIn this chapter, we describe a hierarchical Design pro-cedure that can be used to develop multiloop andmultivariable measurement and Control strategies forplantwide Control systems. The procedure assists theengineer in determining how to choose the best con-trolled, manipulated, and measured variables in theplant, when to use advanced Control techniques suchas MPC, and how to select appropriate multiloopcontrol structures with minimum interactions amongthe coupled processes in the plant. The proposed de-sign procedure is based on the hierarchy of processcontrol activities described in Chapter 1, the controlsystem Design guidelines discussed in Chapter 12,RGA and SVA multivariable methods presented inChapter 16, the model predictive Control approach ofChapter 20, Plantwide Control concepts of AppendixF, and designers experience.

2 It is important to real-ize that the Design of Plantwide Control systems is anart as well as a science. Typically, more than one de-sign will be satisfactory; thus, there is no single solu-tion to the Design problem. Furthermore, a designprocedure generally involves iteration of individualsteps until a satisfactory Design results. Thus, the ap-plication of a systematic Design procedure, such asthe hierarchical approach of this chapter, producespreliminary designs that are subject to further explo-ration and refinement. Simulation methods should beemployed to examine alternative Control configura-tions while exploring the effect of controller tuningon the response of key process variables. The hierar-chical procedure recommended in this chapter is il-lustrated by a case goal is a Plantwide Control System Design thatis no more complicated or expensive than necessaryand that, when built, can be operated easily by typicalplant operators.

3 Ultimately, the only definitive way of validating a selected Plantwide Control System Design is by plant tests and by the operating plant PROCEDURES FOR THE Design OFPLANTWIDE Control SYSTEMSThe Design of a Plantwide Control System consists offour major overall specifications for the plant and itscontrol System are Control System structure is developed. Thisstep includes selecting controlled, measured, andmanipulated variables; choosing multiloop or mul-tivariable Control ; deciding how to Control produc-tion rate, product quality, and inventories; andhandling operating constraints. Decomposition ofthe Plantwide Control problem into smaller prob-lems for the purpose of analysis may also be em-ployed is followed by a detailed specification of allinstrumentation/hardware and software, cost esti-mation, evaluation of alternatives, and the order-ing and installation of Design and construction of the plant,plant tests, including startups, operation at designconditions, and shutdowns, are carried out priorto commissioning of the chapter is concerned with the first two steps, beginning with the plant Control System Design principle, a comprehensive top-downformulationcould be used to develop the required Plantwide con-trol Design .

4 We assume that general requirements forthe plant, such as product specifications and productionrates, have been established at the plant, division, orcorporate level. The specifications for plant operatingconditions have been developed by the plant designgroup working in collaboration with product develop-ment and process Control specialists. Starting with theabove specifications plus knowledge of the potentialmeasured, manipulated, and controlled variables, opti-mization methods could be employed to develop thecontrol System Design based on a comprehensive dynamic model of the plant. Unfortunately, such an A-64 Appendix GPlantwide Control System Designapproach is impractical because of the large number ofprocess variables involved in modern processing the other hand, some aspects of a top-down designapproach may be quite useful as part of a realistic de-sign traditional Design procedure used for industrialcontrol systems has been a bottom-up, unit-by-unit ap-proach.

5 Even though it incorporates systematic meth-ods to develop the Control structure, this approachalso relies on heuristic Design methods and rules ofthumb developed from previous designs and the expe-rience of both the process and Control System System Design has been organized into alogical, sequential, process-oriented methodology bynumerous Control researchers for example, Buckley(1964), Downs (1992), Luyben et al. (1998), Larssonand Skogestad (2000), and Stephanopoulos and Ng(2000). However, it would be naive to assume that anyproposed Design procedure can generate a suitableplantwide Control System Design after one passthrough a set of sequential steps. At virtually everystep of a given procedure, alternative choices will pre-sent themselves, each leading to a different final de-sign alternative.

6 The knowledge, skill, intuition, andpersistence of the Plantwide Control Design team willalways be key elements in the Design process. In addi-tion, the Control Design specifications and modelsused for Design will exhibit uncertainty, which meansthat multiple designs may need to be developed (Sko-gestad, 2002).An effective way to make the large number of deci-sions is to organize the procedures in a generally hier-archical manner. Thus, detailed studies should not beundertaken until important general questions havebeen answered. Skogestad (2002) has developed a de-sign procedure based on the intrinsically hierarchicalnature of Plantwide Control systems while incorporat-ing the best aspects of top-down and bottom-up designapproaches. As shown in Fig. , the most criticalcontrol tasks deal with the safety System (Chapter 9)and with regulating the integrating response modesusually associated with liquid levels (holdups in thevessels).

7 Thus, the basic objective at Levels 1, 2, and 3is to provide safe, stable Control of the plant. Level 4 isconcerned with economic optimization of plant oper-ating conditions, and this step is usually decoupledfrom the Control System from many Control System Design methodolo-gies, even hierarchical ones, is the important role thatdecompositionand decentralizationplay in a plantwidedesign approach. Procedures that lead to decompositionof the overall Design into smaller subproblems can beadvantageous. Even highly integrated plants do not re-quire a multivariable approach linking all of the con-trolled variables with all of the manipulated extent to which a Plantwide Control System can bedecentralized into smaller Control systems designed towork at the process unit level invariably determines howeasily the Control System can be designed, tuned, and un-derstood by plant operators.

8 Decentralized Control sys-tem designs generally are more robust when operatingconditions change and are more tolerant to individualcomponent A SYSTEMATIC PROCEDURE FORPLANTWIDE Control SYSTEMDESIGNT able provides the key steps in a systematic procedurerecommended here for Design of Plantwide Control struc-tures. It is based on the combined top-down/bottom-up ap-proach of Larsson and Skogestad (2000) and Skogestad(2002) and the hierarchical organization that generallymatches Fig. The proposed systematic plantwidecontrol Design approach consists of the four major stepsshown in Table Control System Design ObjectivesPlant operating/ Control objectives must be establishedat the outset of the Design process. Two categories of5. Planning andscheduling4. Real-timeoptimization3a.

9 Regulatorycontrol1. Measurementand actuationProcess3b. Multivariableand constraintcontrol2. Safety andenvironmentalequipmentprotection(days -months)(hours-days)(minutes-hours)(seco nds-minutes)(< 1 second)(< 1 second)Figure of process Control Systematic Procedure for Plantwide Control System DesignA-65information must be provided: (1) plant productionand Control objectives and (2) process constraints(Step I).In this chapter we use box outlines to summarize thetasks in each step. A full case study and references to related work are provided to clarify the detailed Top-Down AnalysisThe top-down analysis identifies both the scope andcomplexity of a Plantwide Control Design project and itsStep I. Specify the Control System Design the plant production, economic, and controlobjectives, including composition and productionrates of all process constraints that must be satis-fied, including safety, environmental, and qual-ity structure.

10 (See Step II for an outline of individ-ual tasks.) Among the conceptual issues considered atthis point in the Design are where to Control the keyproduction and quality measurements, how the overallplant might be divided into smaller subsystems (decom-position) to simplify Control System Design , and wherevariable coupling or constraint handling may justify, oreven require, the use of multivariable Control . For ex-ample, it is important to identify certain subsystemswhose Control System designs cannot be developed sep-arately because the processes are so closely coupled,such as in heat Procedure to Design a PlantwideControl the Control System Design State the plant production, economic, and controlobjectives, including composition and productionrates of all Identify process constraints that must be satisfied,including safety, environmental, and a top-down Identify the process variables, Control degrees offreedom, Control structure, and options Establish the overall Control structure (in conceptualform).