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173 Discrete-event controlled systems in the chemical processing industry
568
Abstracts
by IMC-hased PI controllers. The results reveal that for feed flow rate and composition disturbances, the column can be controlled with a single-end structure.
167 Non-Linear Analysis of Distillation Control Structures J. Alvarez, J. Aivarez, C. Martinez, pp 225-230 With geometric control tools, the two-point control configuration problem for distillation columns is analyzed. The notion of left-invertability for nonlinear systems is used to establish solvability for the closedloop stabilization with noninteractive output dynamics. Results are analytic and use of first-principle modelling enables physical interpretation of derivations and results. The approach provides information about the necessity or not of integral action and about coupling of the underlying control strategy. Findings agree with and support earlier results obtained from finear techniques, with simulations and experimental testing.
application of the predictive controller to a nonlinear distillation system. The enhanced performance using the artificial neural network based control methodology is demonstrated.
171 Application of a Bilinear Long-Range Predictive Control Method in a Distillation Process Kun Lo, En Sup Yoon, Yeong.Koo Yeo, Hyung-Keun Song, pp 249-254 Lung-range predictive controllers for discrete-time MIMO bilinear processes are derived based on a new bilinear model with integral action. Using this model a multi-step-ahead optimal predictor is derived. Two alternative solution methods - rigorous and shortcut - of the minimization problem of a long-range objective function are established and used to calculate control inputs. Several simulation results show that the proposed control methods have robusmess to the limited a priori knowledge of the process. Dynamic simulation on dual composition control of a binary distillation process showed satisfactory servo and regulatory performances of the proposed algorithms.
168 A Comparative Study of Linear and Nonlinear Multivarlable Binary Distillation Column
Control R. Pullinen, P. Pletili, T. Jussila, P. Lautala, pp 231-236 Distillation columns present several problems for a control engineer because of their nonlinear nature and strong interactions. In this paper model predictive control and multivariable PI control are compared. Also, a method for constructing a nonlinear process model and linearizing it using Simulab software is presented. The controller simulation results indicate that model predictive control behaves better than PI control. Anyhow, the ease of tuning of the PI controller makes it a very attractive choice.
172 Combining Adaptive and Neural Control for Distillation Control M. Roele, K. Warwick, pp 255-260 Although developments in the computer industry have moved towards highly integrated parallel processing, the control industry generally only makes use o f the computer as a digital numerical manipulation tool for controlling the plant with a monitoring ability for failure detections. However, different types of control algorithms with a certain degree of intelligence can work in parallel, giving the possibility of better control performance under increased uncertainty, learning abilities and the possibility of intelligent fault tolerant design. In this paper, a simple parallel control scheme is presented for the control of the reflux, reboiler and condenser in a batch distillation process.
169 Feedforward/Feedback Control of a Binary High Purity Distillation Column J. Broil, H. Gelbe, pp 237-242 The objective of feedforward control is to measure the disturbances in process loads and to correct the manipulated variables so that the controlled variables remain unaffected. In this paper a nonlinear timediscrete rigorous model of a distillation column was used to calculate the manipulated variables relating to the measured disturbances, the controlled variables remaining constant. It is shown in simulations that the control behaviour can be improved, if the feedforward model is used in combination with a conventional feedback controller. The feedforward control is also advantageous when compared with a robust controller designed and tuned with the same model.
170 Predictive Control of Distillation Columns using Dynamic Neural Networks G.A. Montague, M.T. Tham, M.J. Willis, A.J. Morris, pp 243-248 In this paper a nonlinear multivariable predictive controller is proposed where the model used for control law synthesis is an artificial neural network. The controller makes use of an on-line optimisation routine which determines the future inputs that will minimise the deviations between the desired and predicted process outputs. Control is implemented in a receding horizon fashion. The paper highlights the importance of selection of the network training philosophy by
173 Discrete-Event Controlled Systems in the Chemical Processing Industry H.A. Preisig, pp 261-266 What are discrete-event dynamic systems and where do they show up in the chemical industry? The paper tries to fabricate a pair of glasses for the reader which will enable him to recognize the discrete-event dynamic nature of chemical processes. What are the modelling methods used and what are their strength and weaknesses? The paper tries to point them out, unbiased but brief. References are made to major cona'ibutions and information sources. What chemical processes have been analyzed and what techniques have been used? The paper lists the processes and briefly describes their nature. Where are things going? The author's view is presented.
174 The Dynamic Modeling and Optimization of an Industrial Batch Reactor S.E. Keeler, J.W. Hull, Jr., G.L. Agln, pp 267-272 The development of a kinetic mechanism for an industrially important process is presented. This mechanism is then implemented into a process model describing the operation of a batch reactor. The resulting model is used to examine the effects of alternative operating strategies on the production process. The Maximum Principle is used to determine optimal temperature profiles for the operation of the reactor,
Abstracts based on fluctuating product demand. The SimuSolvTM computer program is used to perform the parameter estimation, modelling and optimization discussed throughout this paper.
175 The Design and Synthesis of Batch/Semicontinuous Processes In-Beum Lee, Ho-Hyung Lee, Kun Soo Chang, pp 273-278 This study shows that solutions to the sizing problem for the optimal design of batch/semicontinuous processes are applicable to retrofitting. Economic structural design and efficient equipment utilization can reduce the overall ideal time in the processing units. A mathematical procedure is proposed here to improve the initial design of batch/semicontinuons processes, by constructing a superstructure which fully contains all possible structures, and then using an MINLP (mixed integer nonlinear programming) model to obtain the optimal process, network by mer~ingJsplitting tasks and using parallel equipment, t nus, economic savings are possible. The effectiveness of the model in a singleproduct batch plant is demonstrated.
176 Monitoring Discontinuous Reactors using Factor-Analytical Techniques O. Prlnz, D. Bonvin, pp 279-284 This work presents a novel approach based on target factor analysis (TFA), capable of monitoring on-line time-varying processes such as discontinuous chemical reactors. The technique relies on combining prior knowledge with directly available, generally unspecific measurements (multivariate observations). The factoranalytical approach is used at two levels. First, TFA helps monitor the main (but not necessarily all) species contained in the reaction system. Secondly, a composition-change data matrix of detectable species is constructed from the first-level results, and is analyzed via TFA. The underlying factors correspond to the reactions taking place in the system. Since incremental knowledge is used in subsequent target-testing steps, the technique is labelled 'incremental TFA'.
177 Dissolved Oxygen Control using an Automatic Tuning PID Controller K.O. Jones, D. Williams, D. Phipps, P.A. Montgomery, pp 285-290 The paper outlines a method of automatically tuning three-term (Proportional-Integral-Derivative) controllers that are used for the regulation of dissolved oxygen concentration in fermentation processes. The automatic tuner has been successfully applied to simulations of batch and fed-batch fermentations of Baker's Yeast, and practically applied on-line to Baker's Yeast batch and fed-batch fermentations. The results show how quickly the automatic tuner can determine new values for the PID controller. The performance of the automatic tuning system is compared to a conventionally tuned PID controller.
178 Optimization and Control of an Industrial Scale Multivarlable Nonlinear Microalgae Fermentation T. Pr611, A. Hilaly, M.N. Karim, D. Guyre, pp 291-296 In this work, optimization arid control techniques of industrial microalgae fermentation were verified in realtime application on the fermenter, and in a theoretical analysis on a dynamical model , which was also the
569 subject for control algorithm verification. The goal was to maximize the cellular productivity. The Direct Search Method was shown to be the preferable technique by applying it to the dynamical model. Real-time Feedback Sub-Optimal Policy closely resembled the theoretical results. The reference conditions obtained from the optimization algorithms are the set points for the multivariable controller. Linear State Variable Feedback has shown totally unsatisfactory behaviour. Globally Linearizing and Multivariable Adaptive Control performed very satisfactorily.
179 Batch Time Optimization using Both Reactant Dosing and External Cooling for Tight Temperature Control P. DJavdan, pp 297-302 The ability to remove the reaction heat of exothermic reactions depends, amongst other things, on the heat exchange coefficient in the reactor. In the case of a (semi)-batch reactor this varies during the reaction. This paper discusses a new control structure which controls the temperature of a semi-batch reactor by using both external cooling and reactant inlet flow adjustment. The controller maximizes the reaction rate by always using the cooling capacity of the external heat exchanger close to its maximum, leaving only some control margin for disturbance rejection. Simulation results show that this control structure significantly increases the production rate. It can also be used to decrease catalyst consumption.
180 Application of Nonlinear Model Predictive Control to Optimal Batch Distillation J.R. Bosley, Jr., T.F. Edgar, pp 303-308 The problem of properly determining and implementing the optimal operational trajectory for batch distillation is discussed. Modeling, control, and optimization aspects axe considered. A common modeling approach is shown to lead to significant errors; rigorous models with efficient solution methods are needed. Several industrially important problems are discussed and a control pairing which eliminates some interactions is suggested. Parameter sensitivity studies indicate that a priori parameter estimates must be applicable over the range of operation, or that on-line parameter estimation with reoptimization is required. Nonlinear Model Predictive Control (NMPC) offers an efficient, straightforward method of addressing difficulties in modeling, optimization, and control of batch distillation.
181 Operation Strategies for Reverse-Osmosis Membrane Fouling in the Dairy Industry A.J.B. van Boxtel, Z.E.H. Otten, pp 309-314 The relevance of optimal operation strategies for food processors and particularly for membrane fouling control is demonstrated for reverse-osmosis of cheese whey. Dynamic optimization is applied using a process model for membrane fouling and an objective function based on cash-flows. An analysis showed that trajectory control of state variables is well suited for the realization of reliable operation results. Although the application of optimal trajectories of the operation variables yields significant improvement of cash-flows (up to 28%), the consequences of additional investments have to be evaluated in terms of profitability. As indicated, for relatively high additional investments the application of suboptimal operation strategies is meaningful.
Abstracts
by IMC-hased PI controllers. The results reveal that for feed flow rate and composition disturbances, the column can be controlled with a single-end structure.
167 Non-Linear Analysis of Distillation Control Structures J. Alvarez, J. Aivarez, C. Martinez, pp 225-230 With geometric control tools, the two-point control configuration problem for distillation columns is analyzed. The notion of left-invertability for nonlinear systems is used to establish solvability for the closedloop stabilization with noninteractive output dynamics. Results are analytic and use of first-principle modelling enables physical interpretation of derivations and results. The approach provides information about the necessity or not of integral action and about coupling of the underlying control strategy. Findings agree with and support earlier results obtained from finear techniques, with simulations and experimental testing.
application of the predictive controller to a nonlinear distillation system. The enhanced performance using the artificial neural network based control methodology is demonstrated.
171 Application of a Bilinear Long-Range Predictive Control Method in a Distillation Process Kun Lo, En Sup Yoon, Yeong.Koo Yeo, Hyung-Keun Song, pp 249-254 Lung-range predictive controllers for discrete-time MIMO bilinear processes are derived based on a new bilinear model with integral action. Using this model a multi-step-ahead optimal predictor is derived. Two alternative solution methods - rigorous and shortcut - of the minimization problem of a long-range objective function are established and used to calculate control inputs. Several simulation results show that the proposed control methods have robusmess to the limited a priori knowledge of the process. Dynamic simulation on dual composition control of a binary distillation process showed satisfactory servo and regulatory performances of the proposed algorithms.
168 A Comparative Study of Linear and Nonlinear Multivarlable Binary Distillation Column
Control R. Pullinen, P. Pletili, T. Jussila, P. Lautala, pp 231-236 Distillation columns present several problems for a control engineer because of their nonlinear nature and strong interactions. In this paper model predictive control and multivariable PI control are compared. Also, a method for constructing a nonlinear process model and linearizing it using Simulab software is presented. The controller simulation results indicate that model predictive control behaves better than PI control. Anyhow, the ease of tuning of the PI controller makes it a very attractive choice.
172 Combining Adaptive and Neural Control for Distillation Control M. Roele, K. Warwick, pp 255-260 Although developments in the computer industry have moved towards highly integrated parallel processing, the control industry generally only makes use o f the computer as a digital numerical manipulation tool for controlling the plant with a monitoring ability for failure detections. However, different types of control algorithms with a certain degree of intelligence can work in parallel, giving the possibility of better control performance under increased uncertainty, learning abilities and the possibility of intelligent fault tolerant design. In this paper, a simple parallel control scheme is presented for the control of the reflux, reboiler and condenser in a batch distillation process.
169 Feedforward/Feedback Control of a Binary High Purity Distillation Column J. Broil, H. Gelbe, pp 237-242 The objective of feedforward control is to measure the disturbances in process loads and to correct the manipulated variables so that the controlled variables remain unaffected. In this paper a nonlinear timediscrete rigorous model of a distillation column was used to calculate the manipulated variables relating to the measured disturbances, the controlled variables remaining constant. It is shown in simulations that the control behaviour can be improved, if the feedforward model is used in combination with a conventional feedback controller. The feedforward control is also advantageous when compared with a robust controller designed and tuned with the same model.
170 Predictive Control of Distillation Columns using Dynamic Neural Networks G.A. Montague, M.T. Tham, M.J. Willis, A.J. Morris, pp 243-248 In this paper a nonlinear multivariable predictive controller is proposed where the model used for control law synthesis is an artificial neural network. The controller makes use of an on-line optimisation routine which determines the future inputs that will minimise the deviations between the desired and predicted process outputs. Control is implemented in a receding horizon fashion. The paper highlights the importance of selection of the network training philosophy by
173 Discrete-Event Controlled Systems in the Chemical Processing Industry H.A. Preisig, pp 261-266 What are discrete-event dynamic systems and where do they show up in the chemical industry? The paper tries to fabricate a pair of glasses for the reader which will enable him to recognize the discrete-event dynamic nature of chemical processes. What are the modelling methods used and what are their strength and weaknesses? The paper tries to point them out, unbiased but brief. References are made to major cona'ibutions and information sources. What chemical processes have been analyzed and what techniques have been used? The paper lists the processes and briefly describes their nature. Where are things going? The author's view is presented.
174 The Dynamic Modeling and Optimization of an Industrial Batch Reactor S.E. Keeler, J.W. Hull, Jr., G.L. Agln, pp 267-272 The development of a kinetic mechanism for an industrially important process is presented. This mechanism is then implemented into a process model describing the operation of a batch reactor. The resulting model is used to examine the effects of alternative operating strategies on the production process. The Maximum Principle is used to determine optimal temperature profiles for the operation of the reactor,
Abstracts based on fluctuating product demand. The SimuSolvTM computer program is used to perform the parameter estimation, modelling and optimization discussed throughout this paper.
175 The Design and Synthesis of Batch/Semicontinuous Processes In-Beum Lee, Ho-Hyung Lee, Kun Soo Chang, pp 273-278 This study shows that solutions to the sizing problem for the optimal design of batch/semicontinuous processes are applicable to retrofitting. Economic structural design and efficient equipment utilization can reduce the overall ideal time in the processing units. A mathematical procedure is proposed here to improve the initial design of batch/semicontinuons processes, by constructing a superstructure which fully contains all possible structures, and then using an MINLP (mixed integer nonlinear programming) model to obtain the optimal process, network by mer~ingJsplitting tasks and using parallel equipment, t nus, economic savings are possible. The effectiveness of the model in a singleproduct batch plant is demonstrated.
176 Monitoring Discontinuous Reactors using Factor-Analytical Techniques O. Prlnz, D. Bonvin, pp 279-284 This work presents a novel approach based on target factor analysis (TFA), capable of monitoring on-line time-varying processes such as discontinuous chemical reactors. The technique relies on combining prior knowledge with directly available, generally unspecific measurements (multivariate observations). The factoranalytical approach is used at two levels. First, TFA helps monitor the main (but not necessarily all) species contained in the reaction system. Secondly, a composition-change data matrix of detectable species is constructed from the first-level results, and is analyzed via TFA. The underlying factors correspond to the reactions taking place in the system. Since incremental knowledge is used in subsequent target-testing steps, the technique is labelled 'incremental TFA'.
177 Dissolved Oxygen Control using an Automatic Tuning PID Controller K.O. Jones, D. Williams, D. Phipps, P.A. Montgomery, pp 285-290 The paper outlines a method of automatically tuning three-term (Proportional-Integral-Derivative) controllers that are used for the regulation of dissolved oxygen concentration in fermentation processes. The automatic tuner has been successfully applied to simulations of batch and fed-batch fermentations of Baker's Yeast, and practically applied on-line to Baker's Yeast batch and fed-batch fermentations. The results show how quickly the automatic tuner can determine new values for the PID controller. The performance of the automatic tuning system is compared to a conventionally tuned PID controller.
178 Optimization and Control of an Industrial Scale Multivarlable Nonlinear Microalgae Fermentation T. Pr611, A. Hilaly, M.N. Karim, D. Guyre, pp 291-296 In this work, optimization arid control techniques of industrial microalgae fermentation were verified in realtime application on the fermenter, and in a theoretical analysis on a dynamical model , which was also the
569 subject for control algorithm verification. The goal was to maximize the cellular productivity. The Direct Search Method was shown to be the preferable technique by applying it to the dynamical model. Real-time Feedback Sub-Optimal Policy closely resembled the theoretical results. The reference conditions obtained from the optimization algorithms are the set points for the multivariable controller. Linear State Variable Feedback has shown totally unsatisfactory behaviour. Globally Linearizing and Multivariable Adaptive Control performed very satisfactorily.
179 Batch Time Optimization using Both Reactant Dosing and External Cooling for Tight Temperature Control P. DJavdan, pp 297-302 The ability to remove the reaction heat of exothermic reactions depends, amongst other things, on the heat exchange coefficient in the reactor. In the case of a (semi)-batch reactor this varies during the reaction. This paper discusses a new control structure which controls the temperature of a semi-batch reactor by using both external cooling and reactant inlet flow adjustment. The controller maximizes the reaction rate by always using the cooling capacity of the external heat exchanger close to its maximum, leaving only some control margin for disturbance rejection. Simulation results show that this control structure significantly increases the production rate. It can also be used to decrease catalyst consumption.
180 Application of Nonlinear Model Predictive Control to Optimal Batch Distillation J.R. Bosley, Jr., T.F. Edgar, pp 303-308 The problem of properly determining and implementing the optimal operational trajectory for batch distillation is discussed. Modeling, control, and optimization aspects axe considered. A common modeling approach is shown to lead to significant errors; rigorous models with efficient solution methods are needed. Several industrially important problems are discussed and a control pairing which eliminates some interactions is suggested. Parameter sensitivity studies indicate that a priori parameter estimates must be applicable over the range of operation, or that on-line parameter estimation with reoptimization is required. Nonlinear Model Predictive Control (NMPC) offers an efficient, straightforward method of addressing difficulties in modeling, optimization, and control of batch distillation.
181 Operation Strategies for Reverse-Osmosis Membrane Fouling in the Dairy Industry A.J.B. van Boxtel, Z.E.H. Otten, pp 309-314 The relevance of optimal operation strategies for food processors and particularly for membrane fouling control is demonstrated for reverse-osmosis of cheese whey. Dynamic optimization is applied using a process model for membrane fouling and an objective function based on cash-flows. An analysis showed that trajectory control of state variables is well suited for the realization of reliable operation results. Although the application of optimal trajectories of the operation variables yields significant improvement of cash-flows (up to 28%), the consequences of additional investments have to be evaluated in terms of profitability. As indicated, for relatively high additional investments the application of suboptimal operation strategies is meaningful.