WO2002090654A1

WO2002090654A1 - Method for controlling the amount of treating agent in the manufacture of a paper or board

Info

Publication number: WO2002090654A1
Application number: PCT/FI2002/000374
Authority: WO
Inventors: Janne Poranen; Markku Kataja
Original assignee: Metso Paper, Inc.
Priority date: 2001-05-03
Filing date: 2002-05-02
Publication date: 2002-11-14
Also published as: FI109370B; EP1397556A1; FI20010925A0

Abstract

The invention serves to provide to a method by means of which the amount of a web treatment agent remaining on a web, a film-transfer roll or the moving surface can be computed in order to control the final amount of the web treatment agent to a desired value under changes in process conditions. The invention is based on a computational formula suited for pre-estimating the applied amount of the web treatment agent, whereby a change in the value of a process variable can be compensated for even earlier than the effect of the change is detectable by measurements.

Description

Method for controlling the amount of treating agent in the manufacture of a paper or board

The present invention relates to a method according to the preamble of claim 1 for controlling the amount of coating mix or other web treatment agent during the manufacture of a paper or paperboard .and for pre- estimation of the final coat weight.

Paper and paperboard can be treated in various ways to improve their qualities or to convert them for different applications. Generally, the treatment agent contains solids such as printability improving calcium carbonate and clay. The treatment agent may be applied directly to a moving web or, alternatively, via a particular transfer surface. Conventionally, a rotating roll is used as the transfer surface, but a belt or band, for instance, may as well be used in lieu of the roll. Most typically, a doctor blade or leveling rod is employed for smoothing the surface of the applied furnish. However, a problem arises in the control of the applied amount of web treatment agent therefrom that a great number of variables affect the final weight of applied treatment agent requiring each applicator system to be provided with a table listing the relationship between the applied amount of coating furnish and the proper lineal doctoring pressure, on the basis of which table it is possible under given circumstances or variations in the desired coat weight to preselect the lineal loading pressure of the leveling rod or doctor blade. The final coat weight is then controlled to a desired value by way of measuring the actual coat weight of the finished product and, based thereon, adjusting the lineal doctoring pressure via a feedback control so as to bring the treatment process into a desired state, whereupon the actual applied amount of the treatment agent becomes equal to the desired coat weight of the finished product. This kind of a control strategy slows down the startup of the apparatus and its response to varying conditions. Moreover, changes in the qualities of the applied treatment agent make it react in different ways under a given lineal doctoring pressure, thus requiring a great number of pretabulated guideline values of the doctoring pressure for different treatment agent compositions or, if the treatment agent composition is changed, the system startup phase is further retarded because of the slower tuning of the doctoring pressure to a correct level. The properties of the web treatment agent, such as its solids content for instance, may vary when the treatment agent is passed through the machine circulation system, thus affecting the applied amount of the treatment agent or the thickness of the applied coat layer. If the properties of the web treatment furnish are not measured on-line, a deviation may be detected as late as at the gauge measuring the coat weight of the finished product. While the measurement of the different qualities of a web treatment furnish is a relatively easy task as an off-line operation, instrumentation developed for online measurements in a factory environment is scarce.

It is an object of the present invention to provide a method capable of computing the amount a web treatment agent adhering to a web, film-transfer roll or other moving surface so as to make it possible to control the applied amount of the treatment agent to a desired target value under varying process conditions.

The goal of the invention is achieved by means of computation formula suited for pre-estimating the amount of applied web treatment agent under varying process conditions .

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention offers significant benefits

To achieve rapid control of the amount of applied web treatment agent, it is necessary to provide a facility of pre-estimating the deviation caused by a change in the applied amount of treatment agent, the composition of the treatment agent itself or the web speed. Inasmuch at the high web speeds currently used it takes only a short time to run a substantial amount of broke if the tuning of the control system to correct values is slow, it is obvious that a control strategy utilizing only the gauging of the applied coat layer with subsequent feedback control based on the measurement data is excessively sluggish. Herein, the present invention offers a method of computing the change in the applied amount of the web treatment agent due to a deviation in the process conditions and to alter, e.g., the lineal pressure of the leveling rod imposed on the application surface so as to control the amount of applied treatment agent immediately to its desired target value after the detection of the deviation. Thus, the invention makes it possible to react to the most typical ones of the changes affecting the amount of applied web treatment agent, such as a deviation in the solids content, in the viscosity of the furnish and, naturally, in the speed of the web being treated. While the most important use of the invention is related to application by different kinds of coating pigment furnishes, it may obviously as well be applied to surface sizing or other treatment of a paper or paperboard web.

Next, the invention will be examined in greater detail by making reference to the appended drawing in which

FIG. 1 shows diagrammatically the function of a leveling- rod applicator.

A leveling rod is used for metering the amount of a web treatment agent applied to the surface of a moving paper or paperboard web or of a film-transfer roll and for smoothing the surface of the applied layer. Additionally, the invention is suited for use in conjunction with film- transfer belts and other similar moving surfaces. Typically, a leveling rod applicator comprises a coating pan 2 with a smooth-surface or grooved-surface rod 1 adapted to rotate therein. The rod 1 is rotated in the coating pan 2, whereby the qualities of the layer applied to the web surface can be controlled by proper choice of the rod surface properties and rotating direction/speed of the rod. The leveling rod unit may be arranged to operate as a separate doctoring apparatus or, alternatively, it may be adapted to function as metering element that de- lineates the applicator chamber of a short-dwell coater, for instance. In any case, application to a surface 4 passing from under the leveling rod occurs always in one and the same fashion. The furnish 3 that is applied to the surface 4 at an earlier stage or from the coating pan located in the applicator chamber becomes squeezed in the nip formed between the surface 4 and the rod 1 that thus meters the amount of web treatment furnish passing through the nip so as to leave on the surface only a coating layer 5 of a desired thickness.

The thickness of the coating layer passing through the nip between the leveling rod 1 and the web surface 4 is chiefly controlled by altering the force, more precisely, the lineal doctoring pressure, imposed by the leveling rod 1 onto the surface . Inasmuch the thickness and properties of the hydrodynamic film formed between the moving surface and the rotating leveling rod are dependent on very many variables, the thickness of the film is extremely difficult to determine by computational means. The present invention is based on the concept that the leveling process can be modeled using a finite number of properly selected variables. The most important variables affecting the leveling process are the thickness h of the smoothed layer 5, the specific size D of solids particles, the viscosity μ of the web treatment furnish, the velocity V of the moving surface 4 and the maximum lineal pressure P in the nip. Accordingly, these variables are selected to define the process, whereby the relationship between the variables can be written as a dimensionless equation:

As can be seen from the equation, it includes an unknown function g that must be determined.

Experimental techniques may be applied to determine the unknown function. In a test performed on a film-transfer coater using a given coating furnish, the speed of the film-transfer roll was varied over the range of 800 to 2000 m/min, while the lineal pressure of the leveling rod was varied over the range of 0.5 to 3 kN/m. The size of solids particles was assumed to be constant (1 μm) , since the same pigment type was used in all the test runs. The viscosity of the coating furnish was measured using a capillary viscosity meter that was operated at a constant shear rate of 10⁶ s^"1. Table 1 below shows the measured values of maximum lineal pressure plotted against the respective values of film thickness.

Table 1

Mixiroum pressure P [kPi]

As can be seen from the table above, a given maximum lineal pressure can produce a wide variety of different coat layer thicknesses meaning that it is impossible to estimate from the lineal leveling pressure the final thickness of the coat layer applied to the surface of a moving web. Therefore, the above-mentioned unknown function must be defined in order to make it possible to estimate the final thickness of the smoothed coat layer by mathematical means. Herein, if the ratio h/D is plotted as a function of the dimensionless variable μV/PD of function g as shown in Table 2 below, it can be seen that the value of said ratio is a linear function of said dimensionless variable, whereby function g may be assumed to be a linear function whose parameters can be determined by a linear approximation.

Table 2

WD

Now, Eq. 1 can be solved for the layer thickness, whereupon Eq. 2 is obtained: μV h = -bD ( 2 )

As can be seen from Eq. 2, the values of constants a and b remain to be determined by experimental methods. After the constants are known, the coating layer thickness can be computed. As the constants are specific for each leveling system, the following example is carried out using values obtained from one test run: a = 0.0416 and b = 0.0484.

In order to make the above equation practicable, the value of nip pressure used in the equation must be linked to a force loading the leveling rod. In the art are known a number of different ways for loading the leveling rod, whereby the loading force may be, e.g., the pressure of a blade loading hose or the mechanical force that provides the loading torque of the leveling rod pan. The lineal pressure P in the nip can be obtained by measurement means, whereupon a linear approximation gives the lineal pressure from the following equation:

P = cL + d (3)

In the above equation, is the loading pressure of the rod (in bars) and the values of the constants in the exemplary case are c = 407 and d = 38. When these are substituted in Eq. 2, the equation can be written as: μV h = ^■ bD (4)

4071 +38 a

Now, Eq. 4 gives directly the final value of the applied or leveled coat weight C_w multiplied by the density of the web treatment agent : μV

Ow — — bD £ ( 5 )

407Z + 38 a

Using Eq. 5, it is possible to pre-estimate computationally the final amount of applied web treatment agent as a function of the loading force of the leveling rod. As is now obvious, the above equations include four constants a, b e and d, whose values must be found experimentally. As the qualities of the web treatment agent, such as its viscosity, density and particle size are substantially constant unless the composition of the agent is not changed, the only real variables in the use of one and the same web treatment agent are the surface speed of the moving web and loading force of the leveling rod. As the speed of the web surface can be obtained directly from the machine control system and the qualities of the coating furnish are relatively easy to measure, the desired amount of applied coating can be preset with the help of Eq. 5 through altering the loading force of the leveling rod. Since the equation makes it possible to compute the required loading force of the leveling rod even before a change occurs in the applied coat weight, the novel technique of coat weight control becomes substantially faster than the prior-art control scheme based on coat weight measurement combined with subsequent ad- justment of level rod loading. Obviously, coat weight control may nonetheless be performed by combining the invention with the prior-art technique of coat weight gauging and leveling rod loading in such a fashion, for example, that the estimating control is based on compu- tational means, while the final fine-tuning is carried out based on coat weight measurements. To the function of the invention it is essential that the ratio h/D is expressed by means of formula in which the coat weight measurement results are mapped in a linear fashion, whereby the parameters of the unknown function g are relatively easy to determine. Table 3 below shows the values of pre-estimated amount of applied plotted against the actual coat weight values measured from the finished paper web.

Table 3

Measured coat weight Ig/m']

As can be seen from the table above, the correlation between the measured and computed values of coat weight is good. Inasmuch also the deviation of the dimensionless variables in Table 2 is small, the variables of Eq. 2 can be determined using only a small number of test results. Obviously, required number of test results is dictated by the desired accuracy of the method.

Claims

What is claimed is:

1. A method for setting the amount of applied web treatment agent at a leveling rod unit, the method comprising the steps of

- applying a furnish of a web treatment agent to a moving surface, and - metering the amount of coating furnish applied to the surface by way of pressing the leveling rod (1) of a leveling rod unit against the surface,

characterized in that

- controlling the loading of the leveling rod at least partially based the loading value obtained from equation: μV

C = -bD P c +d a

where C_w is the amount of applied treatment agent per unit area μ is the web treatment agent viscosity V is the speed of the moving surface L is the lineal loading of the leveling rod

D is the specific size of furnish particles p is the furnish density a, b, c and d are constants.

2. The method of claim 1, characterized in that constants a and b are determined by experimental techniques through measuring the lineal pressure in the nip between the leveling rod (1) and the surface (4) , and then expressing ratio h/D as a function of a variable written as μV/PD, whereby it is possible to determine in equation:

where h is the thickness of the smoothed layer of applied furnish μ is the web treatment agent viscosity V is the speed of the moving surface P is the lineal pressure in nip between the leveling rod and the moving surface D is the specific size of furnish particles,

the function g thereof that is characterizable by constants a and b.

3. The method of claim 1 or 2 , characterized in that constants c and d are determined from the lineal pressure P at the nip and the respective loading of the leveling rod so that the lineal pressure is expressed as a function of the loading force, whereby said constants are solved from equation P = cL + d.

4. The method of any one of foregoing claims, character- ized in that the moving surface is the surface of a film- transfer roll.

5. The method of any one of foregoing claims 1 - 4, characterized in that the moving surface is the surface of a web of paper or paperboard.