Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
1998-11-17
2001-11-20
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S190000, C162S258000, C162S259000, C162S263000, C162SDIG006, C162SDIG004, C700S128000
Reexamination Certificate
active
06319362
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for controlling properties of paper, which method comprises the steps of measuring a variable describing the amount of filler in the white water and the paper ash content and/or the headbox ash content, and adjusting the amount of filler in the white water by controlling the flow of retention agent on the basis of the measurements of the variable describing the amount of filler in the white water, and adjusting the paper ash content by controlling the flow of filler on the basis of the measurement result of the paper ash content and/or the headbox ash content.
The invention also relates to equipment for controlling properties of paper, comprising means for measuring a variable describing the amount of filler in the white water and the paper ash content and/or the headbox ash content, and control means for controlling the flow of retention agent in order to adjust the amount of filler in the white water on the basis of the measurement result of the variable describing the amount of filler in the white water, and for controlling the flow of filler in order to adjust the paper ash content on the basis of the measurement result of the paper ash content and/or the headbox ash content.
BACKGROUND
As regards controls in so-called short circulation of a paper machine, paper properties are controlled at present for example by adjusting the white water total consistency through adjustment of the flow rate of the retention agent. In such a case, the result of the measurement of the white water consistency forms the basis for controlling a retention agent pump or valve in order to adjust the flow rate of the retention agent to be supplied. Further, in the prior art the ash content of paper is controlled by measuring the ash content for example from the finished paper with a measuring beam and/or by measuring the ash content in the headbox and/or the ash content of the stock and by controlling the flow rate of the filler to be added to the stock. The basis weight of the paper 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 the measuring beam on the basis weight of the paper. Each control operates independently regardless of the other controls. Such an arrangement is disclosed for example in the reference “Lyhyen kierron sakeussäädöt, AEL/INSKO P906202/96 V, Nokelainen J., 1996” (consistency controls in the short circulation). Controlling one property also affects other properties; for example variation in the amount of retention agent and/or filler affects the basis weight, 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. On the other hand, for example during grade changes the controls are carried out at consecutive stages and therefore the total time required for the changes is rather long. The situation is especially difficult with paper grades having a considerable ash content or with paper grades utilizing coated broke, which means that the papermaking process will be subjected to problems with the ash due to variation in the amount of broke or in the ash content of the broke, since in such a case changes in the amount of retention agent strongly affect the paper ash content and, correspondingly, changes in the amount of filler strongly affect the white water consistency.
SUMMARY OF THE INVENTION
The purpose of the present invention is to provide a method and equipment for rapid and effective control of paper properties.
The method according to the invention is characterized by modelling the effect of the flow of retention agent on the amount of filler in the white water and on the paper ash content, and the effect of the flow of filler on the paper ash content and on the amount of filler in the white water, and adjusting, on the basis of said modelling, the amount of filler in the white water and the paper ash content by simultaneously controlling the flow of retention agent and the flow of filler.
Further, the equipment according to the invention is characterized in that the equipment comprises a model of the effect of the flow of retention agent on the amount of filler in the white water and on the paper ash content, and of the effect of the flow of filler on the paper ash content and on the amount of filler in the white water, and that the control means comprise means for controlling simultaneously the flow of retention agent and the flow of filler in order to adjust the amount of filler in the white water and the paper ash content on the basis of said model.
The invention is based on the idea of modelling the effect of the amount of retention agent both on the variable describing the amount of filler in the white water and on the paper ash content, and the effect of the amount of filler both on the paper ash content and on the variable describing the amount of filler in the white water, and the executed modelling is used to simultaneously adjust, by means of the amount of retention agent, the amount of filler in the white water and the paper ash content, and by means of the amount of filler the paper ash content and the amount of filler in the white water. The idea of a preferred embodiment is to model the effect of the amounts of retention agent and filler on the basis weight of the paper and to control the basis weight of the paper by means of the aforementioned amounts by controlling simultaneously the basis weight by means of the stock flow.
In connection with the present application, the term “paper” also refers to board in addition to paper.
The invention has the advantage that it provides faster and more effective control of paper properties in paper machine short circulation than the previous methods.
REFERENCES:
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patent: 5381341 (1995-01-01), Herrala et al.
patent: 5944957 (1999-08-01), Fagerlund et al.
patent: 5954923 (1999-09-01), Chase et al.
Camacho, E.F., et al., Model Predictive Control in The Process Industry (1995, Springer Verlag London Limited.
David Clarke, “Advances in Model-Based Predictive Control” Oxford University Press 1994.
I.C. Roberts, “Paper Chemistry” Blackie Academic and Professional 1996.
Haggman, B, “Matematiska Modeller for Pappersmaskiner”Svensk Papperstidning, vol. 79 (Feb. 1976) pp. 44-49.
Nokelainen, J.,Consistency Controls in the Short Circulation (Lyhyen Kierron Sakeussaadot), 1996.
Pirgach, N.C., Microprocessor-Based System for Automatic Control of Paper-Web Ash, Jul. 1998.
Huhtelin Taisto
Kuusisto Risto
Nokelainen Jukka
Rantala Timo
Tian Lin
Fortuna Jose
Ladas & Parry
Metso Paper Automation Oy
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