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Consideration of Variable Characteristics of Material in the Control of Pulp and Paper Production
CONSIDERATION OF VARIABLE CHARACTERISTICS OF MATERIAL IN THE CONTROL OF PULP AND PAPER PRODUCTION Round Table Discussion Summary Prepared by P. Soininen and V. Tinnis Chairman K. J. !strom
Q
Professor Astrom of Lund University, Sweden, opened the discussion with a review of the Paper Machine Computer System as applied today and touched the problems which arise in this connection. He stated that the paper machine computer control system mainly deals with the basis weight and moisture measuring systems both at the dry end and at the wet end. The basis weight control system manipulates the thick stock valve . The sheet moisture control system adjusts the steam pressure in the dryers. The main goal of computer control system is to cut down the variations, which naturally leads to considerable economic benefits. The measuring equipment consists of either beta gauges or infrared measuring heads for the detection of basis weight. Different methods for the measurement of sheet moisture are available. There are capacitive, infrared, microwave and neutron gauges. The gauges are traversible for the detection of cross machine profile.
characteristics. After the introduction of Professor Astrom an open discussion followed, which in addition to the chairman was participated in by the following persons: Bakke Balls Jefferis Kommonen Kontinen Makipaa Ojalehto Sandblom Striker Tinnis Valkama Williams Witting
(SF) (SF) (SF) (SF) (S) (H) (SF) (SF) ~~
(SF)
According to Mr. Sandblom, a very high accuracy is usually not required for basis weight and moisture measurement. The beta substance gauge is sufficient. It has been found that the various properties of the material have an influence on the measurement by the infrared method, which was confirmed by Mr. Striker, who referred to an English investigation and by Mr. Ojalehto, who had found this in practice. Because of the time delay the measurement should be done also in the wet end where the accuracy is 2. 5 to 3 times lower. Mr . Witting and Mr. Valkama added that the mechanical reliability of the traversing gauges is very important.
The control of headbox consistency is essential during the grade change and the jet to wire speed ratio has an influence on the quality, especially the formation. The following model could be developed for the system x(t+l)
(USA) (GB) ~~
Ax(t) + Bu(t)
where x the vector of the state variable u the vector of the measured variable t the time A, B parameters
The biggest difference between m1n1- and maxicomputers lies in the software. In the big computers Fortran or other high level languages are used, which make the programming much easier. On the other hand, Mr. Valkama said that the present high level languages don't have special instructions for handling the process interface and tend to be too slow. He believed that systems consisting of a mixture of assembler and high level codes like the PPCP package, would be successful in the future, The programming speed including system work, programming, testing and documentation is 10 - 20 assembler instructions per day. Mr . Valkama also stated that the profile cannot be controlled today by automatic manipulation of the slice opening.
When adjusting the control variables of the system according to the model, the parameters of the model have to be changed at a grade change. Changes in material characteristics lead to new calibrations of the parameters, since they have an influence on the drainage and on the dynamics of the wet end. There are certain requirements on the pulp characteristics in paper machine control . A uniform quality of the stock makes the paper machine control problem much easier to handle. The drainage on the wire is affected by the pulp
610
According to Mr. Ojalehto, the versatile measuring system contributed essentially to the good experiences at KaukQpaa mills.
the control loop every time the head comes to the same point of location.
As compared to the beta-ray gauge, the IRmeasurement area is much smaller. The traversing speed can be increased to about 15 m/min. The calibration of IR-gauges can be done with a beta standardization meter at a fixed point on the sheet.
The discussion of the fixed head system ended up with the statement that the method is necessary for a continuous MD control and that the information of the profile is needed for slice adjustment and paper sheet average moisture detection. So far it has not been proved that the fixed head issuperior for control purposes.
Mr. Sandblom mentioned that the temperatures has to be taken into account at the standardization. He also said that the control period should be shorter than the time delay of the machine (1-2 min.), which leads to the development of new control laws and this requires more space in the computer. The measuring head is not to be allowed to touch the web at the wet end. Mr. Balls took up the question of the fixed position measuring head versus the traversing one for the control of moisture and mentioned that a considerable interest exists for the fixed position control system. The CD profile and MD profile are two different functions with two different types of information for the machine tender. The fixed position gauge should be dedicated for the moisture control, and be movable to another position when necessary. The traversing gauge is used for data collection . In this way the unreliability of the traversing types does not affect the control system. Mr. Sandblom asked why two transmitting instruments are used when the same job can be done by a single one . Is the control frequency higher than the measuring frequency? Mr. Balls replied that one gets a continuous signal using the mentioned system. It is therefore possible to attain a closer control. o
Mr. Makipaa picked up the problem of the overall paper machine time delay and of its influence on control. He said that the high frequency noise in the otherwise stable paper machine is not possible to compensate for unless the disturbances can be measured at the wet end. Prof. Jefferis pointed out that the control system has to be considered with regard to the profit involved. An economic profit is achieved through better paper properties which can be obtained by the wet end measurement. Prof. Astrom concluded that the dry end gauge is important and that the wet end gauge doesn't replace the measurement at the dry end. According to Mr. Balls the wet end measurement should primarily be used for the basis weight control depending on the control strategy. The control is much closer than it is possible to detect at the dry end. Prof. Striker agreed in that the wet end measurement improves the control of the drying process. Mr. Sandblom said that the temperature profile anyhow has to be taken into account in the measurement. Therefore, the benefit of the accuracy and fast detection at the dry end is essential.
..
Prof. Astrom had good experiences at a system developed in 1964 with a fixed position measuring system for moisture control. The time delay has to be taken into consideration if the traversing speed is slow when using the average value for control purposes. Dr. Kontinen added that if the control action is made on the basis of the profile single point measurements, new disturbances are introduced into the system. Only the MD distrubances can be controlled automatically. Mr. Valkama said that it doesn't pay to control headbox variations because they are very fast. Modern paper machines are far more stable than the old ones. Therefore, it is not profitable to build complicated control systems. Prof. Striker emphasized the importance of CD profile data for the purpose of adjusting the dryer temperature profile in those cases where this is possible. But he stated that the benefit of the fixed head is the possibility to obtain reference data from the same location. Mr. Jefferis said that it is possible to compute the profile, even if you have an instantaneous measurement with traversing equipment and close
Prof . Striker took up the question of the possibilities of reaching a uniform CD moisture profile by controlling in some way the temperature profile of the drying cylinders, and that of obtaining a uniform MD moisture profile by adjusting the steam supply to the dryers. Mr. Balls mentioned that when producing certain grades of speciality papers, the curl is controlled by manual adjustment of the vent profile of the cylinder condensate system . It is possible to divide the dryers in sections and to control the individual cylinder temperatures. However, Mr . Sandblom stated that this depends on the machine construction . Mr. Bakke gave a survey of the paper mill coordination control using models . Methods are available to coordinate the material handling, the conversion, the inventory and the transportation procedure, inCluding the steam and power supply and pollution supervision. For the relevant models he referred to a paper at the AICA Tokyo Symposium on Simulation in September, 1971 and to a description in the proceedings of the PRP2 Automation Conference .
611
Prof. Jefferis recommended production planning rather in the production phase than plant by plant, since the transportation situation is reasonably good in general. A model design is under consideration for the Scott Paper Company, DuPont, Shell and others. Mr. Balls added that this kind of models have been treated on the BPBMA symposium in Oxford 1969, and that a multilevel system is the final approach to the production models.
profit maximum as the criterion, the problem can be solved. Prof. Striker replied that the problem is treated on the market basis in his country using manual decision making. He wondered whether the optimum techniques have been realized in practice and doubted the validity of the model parameter determination. Dr. Kontinen answered that a parameter estimation has to be made since the running period is long. But if the measurements reveal a different state, the production plan cannot be charged for that.
Prof. Striker emphasized that one output of the computerized mill is the overall material balance. The computer also calculates several production characteristics like the amount of raw material, power consumption, production cost per product unit as well as the total amount of produced material.
The model used in the production control system has its criteria on the order basis. Mr. Bakke further described the problem as an automation problem when regulating the setpoints and optimizing so that the economical criterion is satisfied.
Prof. Jefferis sketched the task by distinguishing the engineering type models from the production models. All production, except raw material, has a conversion aspect and every unit of production can be treated in a corresponding manner including both conversion, inventory and transportation and considering also delay of orders. Mr. Kommonen reviewed the activities in this field and mentioned the work at Enso-Gutzeit Company and Schauman far very little has been published. Ekono research on production control models is under way.
According to Mr. Makipaa, the complex problem can mainly be divided into two levels: production planning and running planning. The latter has a planning period of some days and it has been realized at the Ahlstrom Varkaus Mills. There is a great interest in hierarchial systems in Finland .
in Finland being done Mills. So Also at and mill
The problem is more relevant around the paper machine than on the company level, in the opinion of Mr. Balls. Majority of the control actions are done on the paper machine. One has to look for optimum performance at the wet end particularly in grade change, and it is essential to have a material balance.
Prof. ~strom referred to the work which was done in Sweden some years ago on production planning mentioning the thesis by Bengt Pettersson. While planning for a new paper machine, the question arose, how to coordinate the other operations, when the paper machine is shut down, without introducing too much disturbance to the chemical recovery and steam generation. A model was developed which calculates the production levels and facilitates the decisions the mill management has to make every morning. The whole problem is treated as an optimization problem using advanced optimal control theory.
Dr. Kontinen remarked that the actions at different levels can be either automatic or manual, but anyway the job has to be done. The question of material balancing around the machine and that of optimization are a part of the economical optimization and it can be done only if we have some values for the model parameters. Mr. Striker pointed out that some kind of fixed prices should be available in order to be able to solve the problem for best economical result. When discussing the programming languages Mr. Balls emphasized that since people often leave their companies, the computer projects should not be based on one specialized person only. He definitely recommended a high level language for a process control system because a system must be considered in its totality. Mr. Valkama, on the contrary, stressed the need of specialization because mere programming is only a small part of the whole system. Mr. Sandblom reminded that high level languages are necessary to facilitate testing, debugging, modification, and programming. Prof. Astrom referred to the special papers presented at this Conference and the discussions on the programming languages.
Prof . Striker asked if there are any experiences of the computerization of this problem using models based strictly on material balances and mill capacity data since, according to his opInIon, the economical optimization follows directly from the technical aspects. Dr. Kontinen explained the necessity of the business model by dividing the problem in four levels, namely regulation, running planning, production planning and business planning. He considered the case of several mills, each of which has several paper machines with different type of conversion producing several grades. Taking the maximum capacities and storage volumes as well as the prices of different qualities into account, it is possible to develop models using conventional techniques. The restrictions are for instance rawmaterial supply, market, and material flow with a different grade ratio at different machines. With the net
Mr. Tinnis held it necessary to discuss also other paper properties than basis weight and moisture. It is extremely important to take such properties into account like opacity, formation, porosity and others when designing
612
the wet end control system. For instanc e the head box consist ency and the jet/wir e speed ratio have a great influen ce on the formati on of the paper. There are some methods availab le for the continu ous measurement of these qualiti es but their on-line control has not been carried out so far. Anothe r area which is importa nt in the comput er control of the paper machine is the stock prepar ation. Some paper makers say that a really good paper is actuall y made in the stock prepara tion area. The control of the beaters , of the furnish and of the headbox consist ency should be incorpo rated in the overal l paper machine control system . Prof. Striker added that the continu ous basis wieght and moistu re measur ing system has been availab le over 15 years. Several other problem s exist which have to be solved in the future, and therefo re the measurement and control of the relevan t proper ties of materia l must be stresse d. Prof . Jefferi s told about the researc h made on low consist ency measur ing methods in the USA. The investi gated methods were the ultraso nic, the capacit ance and the bolome tric system s. Most of them are rather complex and the researc h on on-line equipm ent for the measurement of low consist ency is continu ing. Mr. Balls mention ed the optical device develop ed by the Univer site de Grenob le. This is a univer sal solutio n coverin g the measur ing span of 0.25 ... 8% consist ency. In a closing remark the Chairm an, referri ng to the course of the discuss ion, conside red it very importa nt that manufa cturers and users exchanged opinion s in the subjec t area.
613
Q
Professor Astrom of Lund University, Sweden, opened the discussion with a review of the Paper Machine Computer System as applied today and touched the problems which arise in this connection. He stated that the paper machine computer control system mainly deals with the basis weight and moisture measuring systems both at the dry end and at the wet end. The basis weight control system manipulates the thick stock valve . The sheet moisture control system adjusts the steam pressure in the dryers. The main goal of computer control system is to cut down the variations, which naturally leads to considerable economic benefits. The measuring equipment consists of either beta gauges or infrared measuring heads for the detection of basis weight. Different methods for the measurement of sheet moisture are available. There are capacitive, infrared, microwave and neutron gauges. The gauges are traversible for the detection of cross machine profile.
characteristics. After the introduction of Professor Astrom an open discussion followed, which in addition to the chairman was participated in by the following persons: Bakke Balls Jefferis Kommonen Kontinen Makipaa Ojalehto Sandblom Striker Tinnis Valkama Williams Witting
(SF) (SF) (SF) (SF) (S) (H) (SF) (SF) ~~
(SF)
According to Mr. Sandblom, a very high accuracy is usually not required for basis weight and moisture measurement. The beta substance gauge is sufficient. It has been found that the various properties of the material have an influence on the measurement by the infrared method, which was confirmed by Mr. Striker, who referred to an English investigation and by Mr. Ojalehto, who had found this in practice. Because of the time delay the measurement should be done also in the wet end where the accuracy is 2. 5 to 3 times lower. Mr . Witting and Mr. Valkama added that the mechanical reliability of the traversing gauges is very important.
The control of headbox consistency is essential during the grade change and the jet to wire speed ratio has an influence on the quality, especially the formation. The following model could be developed for the system x(t+l)
(USA) (GB) ~~
Ax(t) + Bu(t)
where x the vector of the state variable u the vector of the measured variable t the time A, B parameters
The biggest difference between m1n1- and maxicomputers lies in the software. In the big computers Fortran or other high level languages are used, which make the programming much easier. On the other hand, Mr. Valkama said that the present high level languages don't have special instructions for handling the process interface and tend to be too slow. He believed that systems consisting of a mixture of assembler and high level codes like the PPCP package, would be successful in the future, The programming speed including system work, programming, testing and documentation is 10 - 20 assembler instructions per day. Mr . Valkama also stated that the profile cannot be controlled today by automatic manipulation of the slice opening.
When adjusting the control variables of the system according to the model, the parameters of the model have to be changed at a grade change. Changes in material characteristics lead to new calibrations of the parameters, since they have an influence on the drainage and on the dynamics of the wet end. There are certain requirements on the pulp characteristics in paper machine control . A uniform quality of the stock makes the paper machine control problem much easier to handle. The drainage on the wire is affected by the pulp
610
According to Mr. Ojalehto, the versatile measuring system contributed essentially to the good experiences at KaukQpaa mills.
the control loop every time the head comes to the same point of location.
As compared to the beta-ray gauge, the IRmeasurement area is much smaller. The traversing speed can be increased to about 15 m/min. The calibration of IR-gauges can be done with a beta standardization meter at a fixed point on the sheet.
The discussion of the fixed head system ended up with the statement that the method is necessary for a continuous MD control and that the information of the profile is needed for slice adjustment and paper sheet average moisture detection. So far it has not been proved that the fixed head issuperior for control purposes.
Mr. Sandblom mentioned that the temperatures has to be taken into account at the standardization. He also said that the control period should be shorter than the time delay of the machine (1-2 min.), which leads to the development of new control laws and this requires more space in the computer. The measuring head is not to be allowed to touch the web at the wet end. Mr. Balls took up the question of the fixed position measuring head versus the traversing one for the control of moisture and mentioned that a considerable interest exists for the fixed position control system. The CD profile and MD profile are two different functions with two different types of information for the machine tender. The fixed position gauge should be dedicated for the moisture control, and be movable to another position when necessary. The traversing gauge is used for data collection . In this way the unreliability of the traversing types does not affect the control system. Mr. Sandblom asked why two transmitting instruments are used when the same job can be done by a single one . Is the control frequency higher than the measuring frequency? Mr. Balls replied that one gets a continuous signal using the mentioned system. It is therefore possible to attain a closer control. o
Mr. Makipaa picked up the problem of the overall paper machine time delay and of its influence on control. He said that the high frequency noise in the otherwise stable paper machine is not possible to compensate for unless the disturbances can be measured at the wet end. Prof. Jefferis pointed out that the control system has to be considered with regard to the profit involved. An economic profit is achieved through better paper properties which can be obtained by the wet end measurement. Prof. Astrom concluded that the dry end gauge is important and that the wet end gauge doesn't replace the measurement at the dry end. According to Mr. Balls the wet end measurement should primarily be used for the basis weight control depending on the control strategy. The control is much closer than it is possible to detect at the dry end. Prof. Striker agreed in that the wet end measurement improves the control of the drying process. Mr. Sandblom said that the temperature profile anyhow has to be taken into account in the measurement. Therefore, the benefit of the accuracy and fast detection at the dry end is essential.
..
Prof. Astrom had good experiences at a system developed in 1964 with a fixed position measuring system for moisture control. The time delay has to be taken into consideration if the traversing speed is slow when using the average value for control purposes. Dr. Kontinen added that if the control action is made on the basis of the profile single point measurements, new disturbances are introduced into the system. Only the MD distrubances can be controlled automatically. Mr. Valkama said that it doesn't pay to control headbox variations because they are very fast. Modern paper machines are far more stable than the old ones. Therefore, it is not profitable to build complicated control systems. Prof. Striker emphasized the importance of CD profile data for the purpose of adjusting the dryer temperature profile in those cases where this is possible. But he stated that the benefit of the fixed head is the possibility to obtain reference data from the same location. Mr. Jefferis said that it is possible to compute the profile, even if you have an instantaneous measurement with traversing equipment and close
Prof . Striker took up the question of the possibilities of reaching a uniform CD moisture profile by controlling in some way the temperature profile of the drying cylinders, and that of obtaining a uniform MD moisture profile by adjusting the steam supply to the dryers. Mr. Balls mentioned that when producing certain grades of speciality papers, the curl is controlled by manual adjustment of the vent profile of the cylinder condensate system . It is possible to divide the dryers in sections and to control the individual cylinder temperatures. However, Mr . Sandblom stated that this depends on the machine construction . Mr. Bakke gave a survey of the paper mill coordination control using models . Methods are available to coordinate the material handling, the conversion, the inventory and the transportation procedure, inCluding the steam and power supply and pollution supervision. For the relevant models he referred to a paper at the AICA Tokyo Symposium on Simulation in September, 1971 and to a description in the proceedings of the PRP2 Automation Conference .
611
Prof. Jefferis recommended production planning rather in the production phase than plant by plant, since the transportation situation is reasonably good in general. A model design is under consideration for the Scott Paper Company, DuPont, Shell and others. Mr. Balls added that this kind of models have been treated on the BPBMA symposium in Oxford 1969, and that a multilevel system is the final approach to the production models.
profit maximum as the criterion, the problem can be solved. Prof. Striker replied that the problem is treated on the market basis in his country using manual decision making. He wondered whether the optimum techniques have been realized in practice and doubted the validity of the model parameter determination. Dr. Kontinen answered that a parameter estimation has to be made since the running period is long. But if the measurements reveal a different state, the production plan cannot be charged for that.
Prof. Striker emphasized that one output of the computerized mill is the overall material balance. The computer also calculates several production characteristics like the amount of raw material, power consumption, production cost per product unit as well as the total amount of produced material.
The model used in the production control system has its criteria on the order basis. Mr. Bakke further described the problem as an automation problem when regulating the setpoints and optimizing so that the economical criterion is satisfied.
Prof. Jefferis sketched the task by distinguishing the engineering type models from the production models. All production, except raw material, has a conversion aspect and every unit of production can be treated in a corresponding manner including both conversion, inventory and transportation and considering also delay of orders. Mr. Kommonen reviewed the activities in this field and mentioned the work at Enso-Gutzeit Company and Schauman far very little has been published. Ekono research on production control models is under way.
According to Mr. Makipaa, the complex problem can mainly be divided into two levels: production planning and running planning. The latter has a planning period of some days and it has been realized at the Ahlstrom Varkaus Mills. There is a great interest in hierarchial systems in Finland .
in Finland being done Mills. So Also at and mill
The problem is more relevant around the paper machine than on the company level, in the opinion of Mr. Balls. Majority of the control actions are done on the paper machine. One has to look for optimum performance at the wet end particularly in grade change, and it is essential to have a material balance.
Prof. ~strom referred to the work which was done in Sweden some years ago on production planning mentioning the thesis by Bengt Pettersson. While planning for a new paper machine, the question arose, how to coordinate the other operations, when the paper machine is shut down, without introducing too much disturbance to the chemical recovery and steam generation. A model was developed which calculates the production levels and facilitates the decisions the mill management has to make every morning. The whole problem is treated as an optimization problem using advanced optimal control theory.
Dr. Kontinen remarked that the actions at different levels can be either automatic or manual, but anyway the job has to be done. The question of material balancing around the machine and that of optimization are a part of the economical optimization and it can be done only if we have some values for the model parameters. Mr. Striker pointed out that some kind of fixed prices should be available in order to be able to solve the problem for best economical result. When discussing the programming languages Mr. Balls emphasized that since people often leave their companies, the computer projects should not be based on one specialized person only. He definitely recommended a high level language for a process control system because a system must be considered in its totality. Mr. Valkama, on the contrary, stressed the need of specialization because mere programming is only a small part of the whole system. Mr. Sandblom reminded that high level languages are necessary to facilitate testing, debugging, modification, and programming. Prof. Astrom referred to the special papers presented at this Conference and the discussions on the programming languages.
Prof . Striker asked if there are any experiences of the computerization of this problem using models based strictly on material balances and mill capacity data since, according to his opInIon, the economical optimization follows directly from the technical aspects. Dr. Kontinen explained the necessity of the business model by dividing the problem in four levels, namely regulation, running planning, production planning and business planning. He considered the case of several mills, each of which has several paper machines with different type of conversion producing several grades. Taking the maximum capacities and storage volumes as well as the prices of different qualities into account, it is possible to develop models using conventional techniques. The restrictions are for instance rawmaterial supply, market, and material flow with a different grade ratio at different machines. With the net
Mr. Tinnis held it necessary to discuss also other paper properties than basis weight and moisture. It is extremely important to take such properties into account like opacity, formation, porosity and others when designing
612
the wet end control system. For instanc e the head box consist ency and the jet/wir e speed ratio have a great influen ce on the formati on of the paper. There are some methods availab le for the continu ous measurement of these qualiti es but their on-line control has not been carried out so far. Anothe r area which is importa nt in the comput er control of the paper machine is the stock prepar ation. Some paper makers say that a really good paper is actuall y made in the stock prepara tion area. The control of the beaters , of the furnish and of the headbox consist ency should be incorpo rated in the overal l paper machine control system . Prof. Striker added that the continu ous basis wieght and moistu re measur ing system has been availab le over 15 years. Several other problem s exist which have to be solved in the future, and therefo re the measurement and control of the relevan t proper ties of materia l must be stresse d. Prof . Jefferi s told about the researc h made on low consist ency measur ing methods in the USA. The investi gated methods were the ultraso nic, the capacit ance and the bolome tric system s. Most of them are rather complex and the researc h on on-line equipm ent for the measurement of low consist ency is continu ing. Mr. Balls mention ed the optical device develop ed by the Univer site de Grenob le. This is a univer sal solutio n coverin g the measur ing span of 0.25 ... 8% consist ency. In a closing remark the Chairm an, referri ng to the course of the discuss ion, conside red it very importa nt that manufa cturers and users exchanged opinion s in the subjec t area.
613