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A multiobjective optimization approach for analyzing the interaction of design and control
212
Abstracts
integrated process-unit operations, with special emphasis on system controllability. This paper discusses the optimal design and the best control structure for the process heater. Design and cot).trollability were studied as stand-alone, as well as integrated, umts. Dynamic modelling and controllability behaviour was studied using SpeedUp and Speecon software packages. The work showed that heavily integrated processes require designstage controllability analysis, which should cover complete process networks, as illustrated by the fact that the best control structure of the integrated heater came out as the least good one for the stand-alone unit. 148
Concurrent Integrated Design of Process and Operating System D.M. Laing, J.W. Ponton, pp 87-93
Research into processes with improvedThe°perabilityse-has focused on providing measures or indices, are useful in selecting between alternative designs, but are principally tools for analysis rather than synthesis. This paper describes the generalisation of the established hierarchical model of process design to indude the control system and operating policy, in order to achieve design for operability. This procedure allows and encourages exploration of both process and operating-system alternatives. .The latterinclude .controlin itsbroadest sense, development of an mtegrated operating strategy, and planning for expected constraintsand varied demands. 149
A MultiobJective Optimization Approach for Analyzing the Interaction of Design and Control M.L. Luyben, C.A. Flondas, pp 101.106
This work presents a systematic procedure for analyzing the interaction of design and control. The mathematical programming framework of process synthesis is used to formulate and solve an optimization problem with multiple objectives involving open-loop controllability measures and economics. A muhiobjecuv" e optimization algorithm based on cutting planes is "~olied to determine the optimal trade-off using partial derivative trnation from the nouinferior solution set. A binary distillation column example illustrates the proposed approach. 150
Integrated Design of Effluent Treatment Systems S. Walsh, J. Perkins, pp 107-112
This paper presents some tools for integrated design and their application to an effluent treatment problem. Process configurations for which time delays prevent the performance requirements being met can be eliminated by a controllability test, allowing many possible process and control systems to be eliminated. The use of pre-screening tests simplifies the final design problem of finding a minimum-cost system which would actually satisfy the performance reqmrements for all combinations of disturbances and uncertain process or measurement parameters. In the example, the final design was tackled successfully. The paper shows that integrated design methods can be applied successfully to constrained, nonlinear, dynamic design problems with uncertainty.
1. Generate the model 2. Scale the plant 3. Compute controllability measures 4. Analyze controllability In the paper issues are raised in all of these categories. An FCC reactor is used as an example. 153
Increasingly, companies will come to expect guarantees on both the strady-stateand the dynamic performance of process designs. Most previous approaches treat the integration of flowsheet design and control considerations very much from the steadystate design poin_t of view. For example, a measure of (closedloop) dynamic flowsbeet bchavionr may be provided by openloop indicators based on a linearisedmodel. Such indicatorsdo not specifically address the problem of the trade-off between m o d e l uncenainty, controller complexity and performance specifications, although they may provide the designer with some information. This paper presents a methodology for analysing these trade-offs within a robust controller design frmnework which is described in detail elsewhere. 154
155
152
A Procedure for Controllability Analysis E.A. Wolff, S. Skogestad, M. Hovd, K.W. Mathisen, pp 127-132
This paper gives an overview of some of the tools available for linear controllability analysis. A procedure is presented, which may be described by the following main steps:
Inferential Control using Nonlinear Model.Based Observer Control R.W. Jones, P.J. Gawthrop, pp 151-156
Model-based Observer Control is introduced and its use in the inferential control of the vapour composition of a simulated flash separation process is demonstrated. As the underlying control problem is mulfivariable in nature the fundamental consideration in the control design problem is the choice of a control structure that allows all the practicalperformancc criteria to be satisfied. By providing estimates of the unmeasurable process variables, model-based observer control allows a greater range of possible control structures to be considered. The most desirable control structure does not control the inferred primary control variable direcdy and an inferential setpoint scheme is implemented.
Interactions Between Design and Operation of Batch Plants S. Macchietto, pp 113-126
The interaction between design and operations for batch plants are considered, in particular with reference to three problems: the design and operation of a typical dynamic single processing unit (batch distillation); the design and operations scheduling of a multipurpose plant, and the design of operating sequences, procedures and controllers when discrete events and control actions are imDortanL The issues involved, some practical results and the benefits obtained are discussed with reference to current research within the Centre. The needs for a more integrated approach and the problems and challenges to achieve it are highlighted.
Early Stage Process Controllability Assessment M j . Oglesby, T.I. Malik, S. Fararooy, pp 145.150
Traditionally, process design has concentrated on the design of a plant intended to operate at a set of well-defined flowsheet conditions. The problem of how to achieve, and maintain, these conditions is postponed. This sequential approach can resuh in plants which are difficult to control. Often, remedial work (possibly even plant redesign) is needed, post start-up, for the plant to achieve its control objectives. The pressures to reduce capital cost and plant inventories, together with the need for flexible operation, make this approach now unacceptable. To enable early consideration of control, the process engineer requires a set of simple control indicators, easily understood by non -control-engineenng specialists.
156 151
Robust Control Indicators in Process Design and Control J. Figueron, J. Romagnoli, G. Barton, pp 139-144
Control Configuration Selection for the Inferential Control of a High-purity Distillation Column D. AIkaya, D. Ariburnu, C. Ozgen, T Gurkan, pp 157-166
The regulatory control structure to achieve the best product quality is established by utilizing the recent control configuration selection methods for the inferential control of an industrial, multicomponent, high-purity ethylbenzette distillation column. The possible use of single-end control is investigated within the context of Eigenstructum concept. It is found that for feed flow rate and composition disturbances, the column can be successfully controlled with a single-end structure. The results obtained for the existing industrial column have indicated that the control performance can be significantly improved by the proper choice of the size of the drums at an earlier stage of the design.
157
The Model ProJection Problems in the Gobal Instrumentation of a Process N. Karcanlas, pp 167-170
A number of important control theory problems are involved in the selection of input-output schemes of a given process. One
Abstracts
integrated process-unit operations, with special emphasis on system controllability. This paper discusses the optimal design and the best control structure for the process heater. Design and cot).trollability were studied as stand-alone, as well as integrated, umts. Dynamic modelling and controllability behaviour was studied using SpeedUp and Speecon software packages. The work showed that heavily integrated processes require designstage controllability analysis, which should cover complete process networks, as illustrated by the fact that the best control structure of the integrated heater came out as the least good one for the stand-alone unit. 148
Concurrent Integrated Design of Process and Operating System D.M. Laing, J.W. Ponton, pp 87-93
Research into processes with improvedThe°perabilityse-has focused on providing measures or indices, are useful in selecting between alternative designs, but are principally tools for analysis rather than synthesis. This paper describes the generalisation of the established hierarchical model of process design to indude the control system and operating policy, in order to achieve design for operability. This procedure allows and encourages exploration of both process and operating-system alternatives. .The latterinclude .controlin itsbroadest sense, development of an mtegrated operating strategy, and planning for expected constraintsand varied demands. 149
A MultiobJective Optimization Approach for Analyzing the Interaction of Design and Control M.L. Luyben, C.A. Flondas, pp 101.106
This work presents a systematic procedure for analyzing the interaction of design and control. The mathematical programming framework of process synthesis is used to formulate and solve an optimization problem with multiple objectives involving open-loop controllability measures and economics. A muhiobjecuv" e optimization algorithm based on cutting planes is "~olied to determine the optimal trade-off using partial derivative trnation from the nouinferior solution set. A binary distillation column example illustrates the proposed approach. 150
Integrated Design of Effluent Treatment Systems S. Walsh, J. Perkins, pp 107-112
This paper presents some tools for integrated design and their application to an effluent treatment problem. Process configurations for which time delays prevent the performance requirements being met can be eliminated by a controllability test, allowing many possible process and control systems to be eliminated. The use of pre-screening tests simplifies the final design problem of finding a minimum-cost system which would actually satisfy the performance reqmrements for all combinations of disturbances and uncertain process or measurement parameters. In the example, the final design was tackled successfully. The paper shows that integrated design methods can be applied successfully to constrained, nonlinear, dynamic design problems with uncertainty.
1. Generate the model 2. Scale the plant 3. Compute controllability measures 4. Analyze controllability In the paper issues are raised in all of these categories. An FCC reactor is used as an example. 153
Increasingly, companies will come to expect guarantees on both the strady-stateand the dynamic performance of process designs. Most previous approaches treat the integration of flowsheet design and control considerations very much from the steadystate design poin_t of view. For example, a measure of (closedloop) dynamic flowsbeet bchavionr may be provided by openloop indicators based on a linearisedmodel. Such indicatorsdo not specifically address the problem of the trade-off between m o d e l uncenainty, controller complexity and performance specifications, although they may provide the designer with some information. This paper presents a methodology for analysing these trade-offs within a robust controller design frmnework which is described in detail elsewhere. 154
155
152
A Procedure for Controllability Analysis E.A. Wolff, S. Skogestad, M. Hovd, K.W. Mathisen, pp 127-132
This paper gives an overview of some of the tools available for linear controllability analysis. A procedure is presented, which may be described by the following main steps:
Inferential Control using Nonlinear Model.Based Observer Control R.W. Jones, P.J. Gawthrop, pp 151-156
Model-based Observer Control is introduced and its use in the inferential control of the vapour composition of a simulated flash separation process is demonstrated. As the underlying control problem is mulfivariable in nature the fundamental consideration in the control design problem is the choice of a control structure that allows all the practicalperformancc criteria to be satisfied. By providing estimates of the unmeasurable process variables, model-based observer control allows a greater range of possible control structures to be considered. The most desirable control structure does not control the inferred primary control variable direcdy and an inferential setpoint scheme is implemented.
Interactions Between Design and Operation of Batch Plants S. Macchietto, pp 113-126
The interaction between design and operations for batch plants are considered, in particular with reference to three problems: the design and operation of a typical dynamic single processing unit (batch distillation); the design and operations scheduling of a multipurpose plant, and the design of operating sequences, procedures and controllers when discrete events and control actions are imDortanL The issues involved, some practical results and the benefits obtained are discussed with reference to current research within the Centre. The needs for a more integrated approach and the problems and challenges to achieve it are highlighted.
Early Stage Process Controllability Assessment M j . Oglesby, T.I. Malik, S. Fararooy, pp 145.150
Traditionally, process design has concentrated on the design of a plant intended to operate at a set of well-defined flowsheet conditions. The problem of how to achieve, and maintain, these conditions is postponed. This sequential approach can resuh in plants which are difficult to control. Often, remedial work (possibly even plant redesign) is needed, post start-up, for the plant to achieve its control objectives. The pressures to reduce capital cost and plant inventories, together with the need for flexible operation, make this approach now unacceptable. To enable early consideration of control, the process engineer requires a set of simple control indicators, easily understood by non -control-engineenng specialists.
156 151
Robust Control Indicators in Process Design and Control J. Figueron, J. Romagnoli, G. Barton, pp 139-144
Control Configuration Selection for the Inferential Control of a High-purity Distillation Column D. AIkaya, D. Ariburnu, C. Ozgen, T Gurkan, pp 157-166
The regulatory control structure to achieve the best product quality is established by utilizing the recent control configuration selection methods for the inferential control of an industrial, multicomponent, high-purity ethylbenzette distillation column. The possible use of single-end control is investigated within the context of Eigenstructum concept. It is found that for feed flow rate and composition disturbances, the column can be successfully controlled with a single-end structure. The results obtained for the existing industrial column have indicated that the control performance can be significantly improved by the proper choice of the size of the drums at an earlier stage of the design.
157
The Model ProJection Problems in the Gobal Instrumentation of a Process N. Karcanlas, pp 167-170
A number of important control theory problems are involved in the selection of input-output schemes of a given process. One