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Simultaneous process and system control design: An actual industrial case
Abslracts in this section are from papers presented at: I F A C W O R K S H O P ON I N T E R A C T I O N S B E T W E E N P R O C E S S D E S I G N AND P R O C E S S C O N T R O L London, UK, 7-8 September 1992
Full papers appear in the Pergamon Press publication of the above meeting, to which the page numbers relate. (ISBN: 0 08 042063 X)
139
Effects of Design on the Controllability of Chemical Plants M. Morari, pp 3-16
143
First, the relevance of research on the area of distillation column dynantics and omRrol as well as trends in the equipment and system design are explained. After this, the optimum steady.state distillation control problem is evaluated, based on the F.igenstruetureCmcept, and the impact of the extensive process diversity on the distillation colunm control is reviewed. Finally, the previously suggested combined approach to process design (Mizsey and Fouyo, 1990) is extended by the mentioned impacts.
Controllability is defined as the "best" dynamic performance (setpc~nt following and disturbance rejection) achievable for a system under closed loop control. By defmition it does net depend on the controller but only on the plant itself. During the last decade valuable insights have been obtained into the effeas of plant design on controllability. Significant progress has been made in the development of tools which will allow the engineer to make controllability an integralpa~ of the design objectives. The different approaches are reviewed and furore needs are identified.
140
144
145
ControllablUty Measures for Disturbance Rejection S. Skogestad, E.A. Wolff, pp 23-29
Control System Design in Distillation Processes and its Dependence of the Original Process Design A. KarlstriJm, C. Breithollz, I. Jovtk, A. Lagerbeeg, pp 55-64
Process design today is characterized by the use of stationary simulations only. h is claimed here that the additional information obtained from dynamic process simulations must also be considered in the process design situation. In an example (the atm.osl~eric Benzene/l'duene system) it is shown that the relauve gain array (RGA) of the plant can be used as a measure. The results am compared with those obtained from a commercial package for stationary simulations. It is indicated that the condition number of the plant does not contain the same information as the l-non~ of RGA. Therefore, the condition number turns out to be a questionable tool.
This paper considers ~isturbance rejection in plants,by studying controller-independent disturbance measures for six classes of problems: opan-loop disturbance sensitivity, disturbance sensitivity for decemrulizod control, disturbance sensitivity for systems under partial control, input magnitudes for rejecting d~.mrbances, output magnitudes for disturbances in the presence ot mput limitations, and maximum disturbance range. For all six problenl.s frequency-dependent measures are obtained, and appropnate scaling of the variables is crucial for interpreting these measures. The paper also discusses the relationship between these measures and the Relative Disturbance Gain (RDG) of McAvoy and co-workers and the disturbance condition number of Skogestad and Morari.
142
Controllability Analysis for Unstable Process M. Hovd, S. Skogestad, pp 49-.54
The paper studies the controllability of unstable processes, with empimsis on selection of measurements and manipulated variables, and .~. Pa/.~S. problems for decenualized control. Wen-kya~ Imnng cntena have been gnne~o~d.to open-loop unstawe ptams. K~,ra Half Plane zeros m mdiv~dmd mmsfer function elements may make it practically impossible to stabilize the individual loapa, and in such cases these pairing criteriaare net very helpful RGA = I in the bandwidth region appears to in.di.cate good pairings, also when paired elements have slgnificant RHP zeros, but the Direa Nyqnist Array performs poorly as an indicator of good pairings. In some cases it is preferable to avoid p~rings giving narrow Gershgorin bands.
Early Integration of Process and Control Des/gn J.E. RUnsdorp, P. Bekkers, pp 17-22
Advanced process design leads to more-complicated flowsheets; hence it .r~.uims an eady assessment of process operability end controllability. In order to put the process-control engineers on a par with the process designers, the software prototype 'EPIC has been developed. It has the following main features: a library of dynamic process models; tools for the analysis of process sttb~ilty ana controllability; tools for selecting control structures and for designing control systems, and a visualizationof these structures (the VIM' table). The paper introduces a methodology for integrated process and control design and discusses the potential contributions of ~PIC'.
141
Design Medifkations and Proper Distillation Control Z. Fonyo, pp 41-48
146
Design and Control of Recycle Processes in Ternary Systems with Consecutive Reactions W.L. Luyben, pp 65.74
This paper explores some of the problems associated with the dynamics and control of recycle systems. A large recycle means high energy and capital costs, but the improved yield justifies a large recycle stream. Changes in fresh feed flow rate are shown to have. the most dramauc effect on the system when a convenumal control system is used. Increasingfresh feed by 5% can lead to 100% increase in the flow rate of the recycle stream. A variable-volume control strategy is proposed to eliminate this "snowball" effect. A generic rule for liquid recycle systems is l~mposed: one flow rate somewhere in the recycle loop should be flow-controlled.
Design and Operability of an Energy Integrated Dlstillatlea Column T.M. Schmidt, A. KoggersboI , S.B. Jergensen, pp 31-39
Operability issues are investigated on an energy-inmgramd distillationcohmm. Energy integration is achieved using an in,d'~.ct heat pump between the column condenser and the reboiler. The design aim of the integrated system is to enable operation of the column over its entire operating window, and to provide the operator with a set of standard distillation column actuators. Simulation and experimental results illustrate areas within the possible operating window where potential operabi/ity problems remain dependent upon the selected heat-pump control configuration. A very large part of the total operating window may be covered by just one heat-pump control structure. However muliivariable control avoids singularity of the muliiloop
147
structure.
Simultaneous Process and System Control Design: An Actual Industrial Case S.M.A.M. Bouwen, P.H. K6stor& pp 75.86
To reduce costs, a chemical plant was designed with highly
211
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
Full papers appear in the Pergamon Press publication of the above meeting, to which the page numbers relate. (ISBN: 0 08 042063 X)
139
Effects of Design on the Controllability of Chemical Plants M. Morari, pp 3-16
143
First, the relevance of research on the area of distillation column dynantics and omRrol as well as trends in the equipment and system design are explained. After this, the optimum steady.state distillation control problem is evaluated, based on the F.igenstruetureCmcept, and the impact of the extensive process diversity on the distillation colunm control is reviewed. Finally, the previously suggested combined approach to process design (Mizsey and Fouyo, 1990) is extended by the mentioned impacts.
Controllability is defined as the "best" dynamic performance (setpc~nt following and disturbance rejection) achievable for a system under closed loop control. By defmition it does net depend on the controller but only on the plant itself. During the last decade valuable insights have been obtained into the effeas of plant design on controllability. Significant progress has been made in the development of tools which will allow the engineer to make controllability an integralpa~ of the design objectives. The different approaches are reviewed and furore needs are identified.
140
144
145
ControllablUty Measures for Disturbance Rejection S. Skogestad, E.A. Wolff, pp 23-29
Control System Design in Distillation Processes and its Dependence of the Original Process Design A. KarlstriJm, C. Breithollz, I. Jovtk, A. Lagerbeeg, pp 55-64
Process design today is characterized by the use of stationary simulations only. h is claimed here that the additional information obtained from dynamic process simulations must also be considered in the process design situation. In an example (the atm.osl~eric Benzene/l'duene system) it is shown that the relauve gain array (RGA) of the plant can be used as a measure. The results am compared with those obtained from a commercial package for stationary simulations. It is indicated that the condition number of the plant does not contain the same information as the l-non~ of RGA. Therefore, the condition number turns out to be a questionable tool.
This paper considers ~isturbance rejection in plants,by studying controller-independent disturbance measures for six classes of problems: opan-loop disturbance sensitivity, disturbance sensitivity for decemrulizod control, disturbance sensitivity for systems under partial control, input magnitudes for rejecting d~.mrbances, output magnitudes for disturbances in the presence ot mput limitations, and maximum disturbance range. For all six problenl.s frequency-dependent measures are obtained, and appropnate scaling of the variables is crucial for interpreting these measures. The paper also discusses the relationship between these measures and the Relative Disturbance Gain (RDG) of McAvoy and co-workers and the disturbance condition number of Skogestad and Morari.
142
Controllability Analysis for Unstable Process M. Hovd, S. Skogestad, pp 49-.54
The paper studies the controllability of unstable processes, with empimsis on selection of measurements and manipulated variables, and .~. Pa/.~S. problems for decenualized control. Wen-kya~ Imnng cntena have been gnne~o~d.to open-loop unstawe ptams. K~,ra Half Plane zeros m mdiv~dmd mmsfer function elements may make it practically impossible to stabilize the individual loapa, and in such cases these pairing criteriaare net very helpful RGA = I in the bandwidth region appears to in.di.cate good pairings, also when paired elements have slgnificant RHP zeros, but the Direa Nyqnist Array performs poorly as an indicator of good pairings. In some cases it is preferable to avoid p~rings giving narrow Gershgorin bands.
Early Integration of Process and Control Des/gn J.E. RUnsdorp, P. Bekkers, pp 17-22
Advanced process design leads to more-complicated flowsheets; hence it .r~.uims an eady assessment of process operability end controllability. In order to put the process-control engineers on a par with the process designers, the software prototype 'EPIC has been developed. It has the following main features: a library of dynamic process models; tools for the analysis of process sttb~ilty ana controllability; tools for selecting control structures and for designing control systems, and a visualizationof these structures (the VIM' table). The paper introduces a methodology for integrated process and control design and discusses the potential contributions of ~PIC'.
141
Design Medifkations and Proper Distillation Control Z. Fonyo, pp 41-48
146
Design and Control of Recycle Processes in Ternary Systems with Consecutive Reactions W.L. Luyben, pp 65.74
This paper explores some of the problems associated with the dynamics and control of recycle systems. A large recycle means high energy and capital costs, but the improved yield justifies a large recycle stream. Changes in fresh feed flow rate are shown to have. the most dramauc effect on the system when a convenumal control system is used. Increasingfresh feed by 5% can lead to 100% increase in the flow rate of the recycle stream. A variable-volume control strategy is proposed to eliminate this "snowball" effect. A generic rule for liquid recycle systems is l~mposed: one flow rate somewhere in the recycle loop should be flow-controlled.
Design and Operability of an Energy Integrated Dlstillatlea Column T.M. Schmidt, A. KoggersboI , S.B. Jergensen, pp 31-39
Operability issues are investigated on an energy-inmgramd distillationcohmm. Energy integration is achieved using an in,d'~.ct heat pump between the column condenser and the reboiler. The design aim of the integrated system is to enable operation of the column over its entire operating window, and to provide the operator with a set of standard distillation column actuators. Simulation and experimental results illustrate areas within the possible operating window where potential operabi/ity problems remain dependent upon the selected heat-pump control configuration. A very large part of the total operating window may be covered by just one heat-pump control structure. However muliivariable control avoids singularity of the muliiloop
147
structure.
Simultaneous Process and System Control Design: An Actual Industrial Case S.M.A.M. Bouwen, P.H. K6stor& pp 75.86
To reduce costs, a chemical plant was designed with highly
211
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