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1. WO2000034575 - PROCEDE ET MATERIEL DE REGULATION DES PROPRIETES DU PAPIER

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

METHOD AND EQUIPMENT FOR CONTROLLING PROPERTIES OF PAPER
The invention relates to a method of controlling properties of paper, in which method the effect of at least one variable of the paper dryer section on the paper moisture is modelled, and the moisture is controlled by means of said modelling.
The invention further relates to equipment for controlling properties of paper, the equipment comprising a model of the effect of at least one variable of the paper dryer section on the paper moisture, and control means for adjusting the moisture on the basis of said model.
At present, the moisture of paper in a dryer section of a paper machine is controlled for example by adjusting steam pressure of steam-heated drying cylinders. The prior art also teaches how to model the effect of the steam pressure of the steam-heated drying cylinders on the paper moisture, and how the moisture of the paper is adjusted on the basis of the modelling. The basis weight of the paper, in turn, is controlled by means of stock flow control, which also takes into account changes in the stock consistency on the basis of total headbox consistency and/or a measurement result obtained from a measuring beam on the basis weight of the paper. As regards controls in the short circulation of the paper machine, for example white water total consistency is controlled by adjustment of the flow rate of retention agent. Controls in the drying section of the paper machine operate independently, without taking into account other controls in the short circulation, such as control of the flow rate of the retention agent. Controlling one property also affects the other properties; for example variation in the amount of retention agent and/or filler affects the moisture, and therefore one or more controls are adjusted to operate so slowly that they do not interfere with the faster controls. Such a slow control cannot naturally compensate for rapid changes occurring in the property it controls. Therefore, the result of the control does not have the desired effect and the control takes too much time. Further, during grade changes, for example, the controls are carried out at consecutive stages, wherefore the total time required for the changes is rather long.
The purpose of the present invention is to provide a method and equipment which provide rapid and effective control of paper properties.
The method according to the invention is characterized by modelling the effect of the stock flow and/or stock composition and the flow of retention agent on the paper moisture, and adjusting the moisture by simultaneously controlling at least one variable of the paper dryer section, the stock flow and the flow of retention agent.
Further, the equipment according to the invention is characterized in that the model also comprises the effect of the stock flow and/or stock composition and the flow of retention agent on the paper moisture, and that the control means comprise means for simultaneously controlling at least one variable of the paper dryer section, the stock flow and the flow of retention agent in order to adjust the paper moisture on the basis of said model.
The basic idea of the invention is to model both the effect of at least one variable of the paper dryer section on the paper moisture and the effect of the stock flow and/or stock composition and the flow of retention agent on the paper moisture, and to adjust the moisture by simultaneously controlling at least one variable of the paper dryer section, the stock flow and the flow of retention agent. The idea of a preferred embodiment is to also model the effect of the flow of filler on the paper moisture, and to adjust the paper moisture by simultaneously controlling at least one variable of the paper dryer section, the stock flow, the flow of retention agent and the flow of filler. In this application 'stock composition' refers to, for example, the ash content and the amount of fibres in the stock. The idea of another preferred embodiment is that the variable of the paper dryer section is a controlled variable, such as blow rate or blow temperature, of an impinged blowing unit operating as one of the drying units in the dryer section.
An advantage of the invention is that the paper properties can be controlled more rapidly and efficiently than previously as regards the dryer section in the paper machine. Further, the control can be carried out especially efficiently when the operation of the impinged blowing unit is adjusted.
In connection with the present application, 'paper' refers to board and soft tissue in addition to paper.
The invention will be described in greater detail in the accompanying drawings, in which
Figure 1 shows schematically a papermaking process,
Figure 2 is a diagram showing the structure of optimization according to the invention, and
Figure 3 shows schematically control alternatives for variables of the paper dryer section.

Figure 1 shows schematically a papermaking process. Stock is supplied to a paper machine via a wire pit silo 1. Water is mixed into the stock arriving from the wire pit silo in order to adjust the consistency to a suitable level. The stock is thereafter supplied to a headbox 2, from which it is further fed into a former 3, where it is formed into a fibre web 4. The former 3 is followed by a press section 5. The fibre web 4 is dried in a dryer section 7. The press section 5 is followed by a first measuring beam 6a, and after the dryer section 7 there is a second measuring beam 6b. The paper machine also comprises for example a reel and it may comprise size presses or a calender, which are not shown in the accompanying figure for the sake of clarity.

Further, the operation of a paper machine is known per se to those skilled in the art and therefore it will not be described in greater detail in this connection.
For the control of paper properties according to the invention, the paper moisture Moi and the basis weight of the paper BW are measured from the second measuring beam 6b. The paper ash content ASH can also be measured from the measuring beam 6b. Therefore, in the present application 'paper properties' refer to, for example, the moisture, the basis weight and/or the ash content of the paper. The variable 'paper ash content ASH' can be, for example, the proportion of ash from the basis weight or dry weight of the paper or the amount of ash, i.e. the mass flow, in a time unit. Further, the white water total consistency CS^ is measured, and the headbox ash content ASHhb can also be measured. Instead of the white water total consistency CS^ it is possible to measure some other variable that describes the filler content of white water, such as the consistency of the bottom, top, inner or outer white water, or for example the white water ash content. However, measurement of the white water total consistency CS^ is easy and simple.
The moisture Moiap after the press section 5 situated before the dryer section can be measured from the first measuring beam 6a, and this moisture can be used either as a disturbance variable or as a target value in the optimization. Measurement of the moisture Moiap after the press section provides measurement data on the moisture at as early a stage as possible. The measurement information about the moisture is also obtained before and after the dryer section 7 so that the process control will be very rapid and accurate. Another disturbance variable that is measured for the purpose of optimization is the machine speed S. The machine speed S can be measured from one or several points, for example from the former 3 or the reel, or from both. Further, the stock composition can also be used as a disturbance variable in the optimization. The stock composition refers to, for example, the stock ash content ASHts. Instead of the stock ash content ASHts it is possible to use a value calculated for the mass flow of the stock ash content QAts, such that
QAts = Fts * CSts * ASHts,

wherein QAts is the mass flow of the stock ash content,
Fts is the stock flow,
CSts is the total consistency of the stock, and
ASHts is the stock ash content.

The total consistency of the stock CSts is usually standardized with a separate control provided before a machine chest 8, but in the optimum control according to the invention the total consistency of the stock CSls is also introduced into the process as a disturbance variable.
The variable fD of the paper dryer section 7 is controlled according to the invention. The dryer section 7 can be, for example, a conventional cylinder dryer, a Yankee cylinder or a combination of conventional cylinder drying and impinged blowing, or some other suitable dryer section 7. The dryer section 7 can comprise a front dryer section 7a, a middle dryer section 7b and a rear dryer section 7c, each of which can be used for the control, i.e. control operations can be carried out in each dryer section. The controlled variable fD of the paper dryer section 7 can be, for example, the steam pressure or the steam flow in the steam-heated drying cylinders, the blow rate or blow temperature of the impinged blowing unit, or some other suitable controlled variable affecting the rate of drying of the paper in the dryer section 7, or a combination of some or all of the aforementioned variables. Further, the set value in the control can be the amount of energy supplied to or discharged from the dryer section, the amount of energy being dependent on the steam flow, steam pressure and steam temperature in the steam-heated drying cylinders, for example. In such a case the proportion of each element affecting the amount of energy from the effect of all the elements on the total amount of energy is determined by calculation. Furthermore, the stock flow Fts is controlled by a separate flow regulator, and the flow of retention agent Fra by another flow regulator. Retention agents increase the retention of fines and fillers and simultaneously speed up drainage in a manner known per se. Retention agents can be inorganic retention agents, natural organic retention agents or synthetic water-soluble organic polymers in a manner known per se.
If desired, the control system according to the invention can also be used to simultaneously adjust the flow of filler Ffl by a flow regulator. The purpose of a filler is, among other things, to improve paper formation, surface properties, opacity, brightness and printability and to decrease manufacturing costs. A filler can be for example kaolin, calcium carbonate, titanium dioxide or talc in a manner known per se.
A flow regulator that is used to control the stock flow or the flow of retention agent or filler can be for example a valve or a use-controlled pump or both. In the present application flow adjustment and control refer specifically to the adjustment of flow rate, which can be denoted for example in the following manners that are known per se: l/min or g/ton of production.
If there is no control for the flow of filler FfiI the flow of constant filler

Ffi- to be supplied to a mixing tank 9 can be controlled instead. However, in such a case it is necessary to take into account an additional time constant and therefore the accuracy of the final control may not necessarily be very good.
Figure 2 shows a structure of optimizing the control arrangement according to the invention. Parameters of process models include the necessary coefficients and time constants, which have been determined by utilizing both knowledge obtained from designing a paper machine and process tests carried out at different operating points. Models used by a predictor may differ from those used for the optimization. The predictor can calculate a new model for the optimization during each round of execution, and the model takes into account changes in the speed S and the rate of production PSts and changes which will take place in the future and which may be known in advance for example during a grade change. Determining a model is known per se to those skilled in the art, wherefore it will not be described in greater detail herein. The predictor obtains as input a disturbance variable that is the machine speed S, and the predictor takes it into account in case of change and provides for the optimization a model which is in a required form and which includes the change in the speed. The disturbance variable can also be the production rate PS,S, in which case PSts ~ Fts * CSts,
wherein
PSts is the production rate,
Fts is the stock flow, and
CSts is the total consistency of the stock.

The optimization is carried out on the basis of the control models, separate stored controls, measurements, disturbance variables and restrictions.
The optimization is a block with rather simple operation utilizing the models which are generated by the predictor and which describe, for example, the effect of a change in the variables fD of the paper dryer section and the flow of retention agent on the paper moisture. The predictor provides a prediction and generates a new process model for the optimization. The predictor comprises diverse functions and takes into account different situations and changes therein from various aspects. For example, the predictor takes into account the effect of variation in the machine speed and/or the draw on the basis weight of the paper.
The target values include the paper moisture SPMoi and the basis weight of the paper SPBW. Other possible target values include the moisture SPMoiap after the press section situated before the dryer section, the paper ash content SPASH, the headbox ash content SPASHhb and the white water total consistency SPCS^. Further, the target values can be denoted in the form of for example mass flow. For instance the paper moisture can be denoted in a manner known per se by kg/s, which describes the amount of water in kilograms per one second.
A process model is a dynamic model which comprises as input variables at least one variable of the paper dryer section fD, the stock flow Fts, the flow of retention agent Fra, the speed of the paper machine S and possibly also the flow of filler Ffi. Output variables of the dynamic process model can also include the paper moisture Moi and the basis weight of the paper BW, and possibly the white water consistency CS^ and, if desired, the headbox ash content ASHhb and the paper ash content ASH. Control variables used in an optimum control include at least one variable fD of the paper dryer section, the stock flow Fts, and the flow of retention agent Fra. In such a case it is possible to simultaneously control the variable of the paper dryer section fD, the stock flow Fts and the flow of retention agent Fra, which provides rapid and efficient control. If desired, the aforementioned control alternatives and flows can be compensated for by using either the machine speed S or the production rate PSts. Further, it is also possible to use the flow of filler Ffi simultaneously as a control variable so that the adjustment of the different paper properties, including the ash content, can be controlled very well. It is also possible to compensate for the variables by means of the concentration of the retention agent and/or filler. A dynamic model can be used to predict the future values of the output variables on the basis of the existing operating point and the previous values of the input variables if there are no new changes in the control.
A model-based optimum control algorithm calculates a guide value trajectory for a control variable on the basis of the target value trajectory of the controls and the predicted output variables. The guide value trajectory in turn guides the process optimally to the target values in the desired manner at each moment. This data is forwarded to an automation system. An essential feature of the method used is that the optimum control algorithm is independent of the dynamic model used, and during each control round it is possible to use a dynamic model that is determined separately and the optimum control algorithm can use different weighting coefficients in different situations during a run in principle on each control round. Such an arrangement is important particularly during grade changes where it is possible to predict the situation at each moment with this type of operation. Normally each of the 2 to 6 target values can be assigned a set value. If desired, the paper ash content ASH can be taken into account, but the headbox ash content ASHhb can be disregarded entirely. During a break, it is possible to predict the basis weight BW, to assign a set value to the headbox ash content ASHhb and to entirely disregard the paper ash content ASH. After the break, normal operation is resumed. On the other hand, during a break the paper ash content ASH can be replaced with a value provided by the model during the break, and normal operation based on measurements can be resumed after the break.
Figure 3 shows schematically a part of a preferred dryer section 7. The dryer section 7 comprises several conventional steam-heated drying cylinders 10. The dryer section 7 is also provided with one or more impinged blowing cylinders 11 , and impinged blowing hoods 12 are positioned in connection with the cylinders to blow hot air or gas or superheated steam to the fibre web to be dried. The impinged blowing can be directed either straight at the paper web or it is implemented through the wire. For the sake of clarity, the accompanying figure does not show a fire web, wires, auxiliary rolls, support structures and other corresponding parts of the dryer section 7, which are evident for those skilled in the art. The impinged blowing units considerably improve the drying efficiency and speed of the dryer section 7 compared to, for example, a conventional dryer section where the drying is based only on steam-heated cylinders 10.
The paper drying rate can be controlled by a control means 13. The control means 13 adjusts the steam flow and/or steam pressure of the steam-heated cylinders 10, for example. In connection with the impinged blowing hood 12 there is an air blower 14, which produces an air flow. The equipment further comprises a gas burner 15, which raises the temperature of the air to be blown to a sufficiently high level. The temperature of the air can be for example between about 320 and 380°C. Air is supplied to the equipment via an inlet duct 16 and excess moist air is discharged via a discharge conduit 17. The blow rate of the impinged blowing hood 12 is adjusted with the control means 13 through control of the speed of rotation of the blower 14, i.e. the pressure of the air to be blown. The temperature of the blown air can be controlled by adjusting a gas valve 18, which determines the amount of the gas flow to be supplied to the gas burner 15. The blow rate can be determined for example on the basis of a measurement result provided by an air pressure sensor 19 or a temperature sensor 20. To adjust the temperature of the air to be blown, data about the temperature of the air is supplied to the control means 13 on the basis of a measurement result given by the temperature sensor 20. The temperature sensor 20 can be placed for example in an air duct as shown in Figure 3 or, instead or in addition to the air duct, in the impinged blowing hood 12. The control can also utilize a measurement result given by a moisture meter 21 concerning the moisture of the exhaust air. It is also possible to utilize a measurement result given by a cylinder temperature sensor 22 concerning the temperature of the cylinder 11. Unlike in the arrangement shown in Figure 3, the variables fD of the dryer section 7 can also be controlled by means of a decentralized control apparatus.
In practice the steam-heated drying cylinders 10 can only be adjusted in groups. Further, the control of the cylinders is slow, wherefore the cylinders can be used for slow controls, such as determination of the level of drying. On the other hand, the control of the drying efficiency of the impinged blowing hoods 12 is very rapid, wherefore the hoods can be used to implement fast changes in the drying. The aforementioned control can be used, for example, to compensate for interference detected in the moisture following the press section. It is also possible to use simultaneously the control of the drying cylinders 10 with a slow response, and the control of the drying efficiency of the impinged blowing hoods 12 with a rapid response, to control the moisture. The control algorithm used determines how the controls are used. It is also possible to balance the use of these two different controls such that the effect of the costs and the desire to keep the variables at a desired level are included in the control, in other words minimum and maximum values are determined for the variables in question. Therefore the cost function can include both a change in the variables and the price.
There may be one or more impinged blowing units in the dryer section 7. When several impinged blowing units are used, they can all be controlled, if desired. On the other hand, it is possible to assign a particular set value to some of the units and to control only one or possibly a few impinged blowing units.
The drawings and the related description are only intended to illustrate the inventive idea. The details of the invention can vary within the scope of the claims. For example, when the moisture is measured immediately after the press section, it is possible to use the press section actively in the control of the paper web moisture.