Note: Descriptions are shown in the official language in which they were submitted.
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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 cn 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 andlor 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 andlor 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 nave 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
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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 andlor 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 andlor 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.
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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 Moiao 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
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both. Further, the stock composition can also be used as a disturbance
variable in the optimization. The stock composition refers to, for example,
the
stack ash content ASH,S. Instead of the stock ash content ASHts it is possible
to use a value calculated for the mass flow of the stock ash content QA,S,
such
that
QA;S = F.5 * CS,; * ASH25,
wherein QA,S is the mass flow of the stock ash content,
F,5 is the stock flow,
CSis is the total consistency of the stock, and
ASHts is the stock ash content.
The total consistency of the stock CS,S 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 CS,S is
also
introduced into the process as a disturbance variable.
The variable f~ 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
f~
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 c ach element affecting
the amount of energy from the effect of all the elemen::_ cn the total amount
of
energy is determined by calculation. Furthermore, the stock flow F~5 is
controlled by a separate flow regulator, and the flow of retention agent F~a
by
another flow regulator. Retention agents increase the retention of fines and
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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
5 used to simultaneously adjust the flow of filler F,; 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: I/min or g/ton of production.
If there is no control for the flow of Pilfer Ff" the flow of constant filler
Ff~ 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 info 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 PSts, in which case
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PSts = F;5 * 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 f~ 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 fo, the stock flow
F~,
the flow of retention agent F~a, the speed of the paper machine S and possibly
also the flow of filler Ff,. 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 fo of the paper dryer
section,
the stock flaw FMS, and the flow of retention agent F~a. In such a case it is
possible to simultaneously control the variable of the paper dryer section f~,
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the stock flow F,5 and the flow of retention agent F~a, 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 PS,S. Further, it is also possible to use the flow of filler
Ff
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 filter. 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 mode!-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 B 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
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connection with the cylinders to blow hot air or gas or superheated steam to
the fbre 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 r ,
which
are evident for those skilled in the art. The impinged blowing units
considerably improve the drying efficiency and speed of the dryer section r
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 an 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 M°asurement result given by a cylinder
temperature
sensor 22 concerning the temperature cf the cylinder 11. Unlike in the
arrangement shown in Figure 3, the variables fp 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 stow, wherefore
the
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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 al! 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.
