Note: Descriptions are shown in the official language in which they were submitted.
CA 02744638 2011-05-25
WO 2010/063310 PCT/EP2008/066556
Procedure and system for control of a refiner to improve energy efficiency and
pulp quality
TECHNICAL FIELD.
The invention relates to an improved method for controlling one or more
refiners in a process
section for thermo-mechanical pulp (TMP) refining. The present invention is
applicable in all
technical areas where refiners are used, such as pulp and paper industry as
well as related
industries.
TECHNICAL BACKGROUND
Refiners of one sort or another play a central role in the production of high
yield pulp for the
pulp and paper industry and related industries through grinding, for example,
thermo-
mechanical pulp (TMP) or chemical thermo-mechanical pulp (CTMP) starting from
lignin-
cellulose material such as wood chips. Two types of refiners are important to
mention here;
low consistency (LC) refining where the pulp is refined at about 4 per cent
consistency (dry
content), and high consistency (HC) refining where the consistency is commonly
about 40 per
cent. LC refining is done in a two-phase system chips/pulp and water, while HC
refining has
three phases; chips/pulp, water and steam. Refiners are also used in other
industrial
applications, such as for example manufacturing of wood fiber board.
Most refiners consist of two circular plates, in between which the material to
be treated passes
from the inner part to the periphery of the plates. Usually there is one
static refiner plate and
one rotating refiner plate, rotating at a very high speed.
A more complete schematic illustration of a known refiner 1 is given in Figure
4. The raw
material which may consist of wood chips 5 or already treated pulp for a
previous stage enters
at the center C of the refiner. In a first stage refiner as the one
illustrated in the figure, the
material is transported via one or more screw feeders 7. Before entering the
actual refiner the
raw material is normally mixed with dilution water 2 whose flow is usually
measured and
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CA 02744638 2011-05-25
WO 2010/063310 PCT/EP2008/066556
controlled. Alternatively water may be added directly in the refiner. The
material is then treated
on its way to the periphery of the refiner plates. The static refiner plate 3,
or stator, is usually
pushed towards the rotating one 4, or rotor, either electro-mechanically or
hydraulically. The
rotating disc or discs are driven by one or two motors 10. The grinding zone
or as it is often
called the refining zone may also have a variable gap along the radius
dependent on the design
of the plates. The figure also shows the outlet position 6 where the pressure
Poutlet is
measured, and the point where production inlet pressure Pinlet 8 may be
measured.
The diameter of the refiner plates differ dependent on size (production
capacity) of the refiner
and brand. Originally the plates (also called segments) were cast in one
piece, but nowadays
they usually consist of a number of modules that are mounted together on the
stator or rotor.
These segments have grinding patterns, see Figure 5, with bars 15, 15' and
troughs 16 that
differ dependent on supplier. The bars act as knives that defibrillate chips
or further refine the
already produced pulp. In an HC refiner, fibers, water and steam is also
transported in the
troughs between the bars. The amount of steam is spatially dependent, why both
water and
steam may exist together with chips/pulp in the refining zone. In an HC
refiner water will
normally be bound to the fibres. Dependent on the segment design different
flow patterns will
occur in the refiner. In an LC refiner no steam is generated.
There are also other types of refiners such as double disc, where both plates
rotate counter to
each other, or conic refiners. Yet another type are called twin refiners,
where there are four
refiner plates. A centrally placed rotor has two refiner plates mounted one on
either side, and
then there are two static refiner plates that are pushed against each other
using, for example,
hydraulic cylinders thus creating two refining zones.
When refining wood chips or previously refined pulp the refiner plates are
typically pushed
against each other to obtain a plate gap of approximately 0.2-0.7 ITIM
dependent on what type
of refiner is used.
In traditional control concepts for refiner control the controlled variables
consist of the specific
energy (i.e. the ratio between the refiner motor load and the pulp
production), alternatively just
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CA 02744638 2014-06-03
the motor load, the pulp consistency out of the refiner, and best case also at
least one variable
describing the quality of the pulp (e.g. Canadian Standard Freeness, CSF). To
control these
process variables there are typically manipulated variables such as hydraulic
pressure, dilution
water flow, and wood chips or pulp production. Moreover, a typical refiner
line consists of two
refiners in series; a primary refiner (PR) and a secondary refiner (SR). Often
there is also a
processing step called reject refining.
Therefore control of a complete refiner line often becomes quite complex.
Examples of
coimnercially available control concepts may be found in the Licentiate thesis
by Lid& [1] and
in US 7,381,303 [2]. These control concepts based on Model Predictive Control
(MPC) using the
controlled variables described above, are used for large complex systems
consisting of multiple
refiner lines but also single lines or single refiners.
An alternative control variable central in, for example, [2] is the plate gap,
which is then
controlled by manipulating the hydraulic pressure Phydr. Today there are plate
gap sensors on
the market which are applied directly in the refiner plates. Usually only one
gap sensor is used
per refining zone, primarily to avoid the plates clashing together, and thus
not to control the
gap since they are not reliable enough.
There are also other systems on the market where the temperature and/or
pressure are measured
along the refining zone for the purpose of visualizing a temperature profile,
and/or a pressure
profile. When circumstances in the refiner are varied, for example gap,
production or dilution
water the temperature changes and can thus be controlled. Usually several
temperature and/or
pressure sensors are used placed directly in the plate or may be encapsulated
in a measurement
strip, also called sensor array, along the active radius of the refiner, see
EP0788407[3].
In US6024309[4] a control concept is described for refining zone control where
the
temperature profile is used for controlling the process. Similar subsequent
patents, for example
US634381 [5], which treats the same concept is also using the temperature
profile to control
refiners.
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CA 02744638 2014-06-03
=
The design of the refiners segments have proven very important for the shape
of the
temperature profile along the radius, and it is crucial to take this into
account when placing the
temperature and/or pressure sensors in the sensor array.
There are no other measurement devices that have been applied in the refining
zone, other than
the ones mentioned above. There are, however, instruments which may be placed
in the
blowline of the refiner, where the consistency of thc flowing pulp can be
calculated using
algorithms coupled to NIR (near infra red) measurements, which are assumed
available in, for
example, US 7,381,303 [2].
In US 7,381,303 entitled "System and Method for Controlling a Thermo-
Mechanical Wood
Pulp Refiner", assigned to Honeywell Inc, a system is described where separate
controllers are
used for the fast dynamics (motor load and blow line consistency) and the slow
dynamics (pulp
quality) respectively, with an optimizing coordinating control on top. It is
described that a
stabilizing controller preferably regulates the refiner motor loads and the
blow-line
consistencies, and that a Quality controller preferably controls the slow
dynamics associated
with pulp quality variable. It is also described that by operating the refiner
lines at the
maximum allowable motor loads the production is automatically maximized for a
given pulp
window.
However, it has become clear to the inventors that controlling the refining
process based on
measurements of specific energy as represented by motor load does not provide
reliable control
over pulp quality parameters because different pulp qualities can be produced
at the same
specific energy or motor load.
The aim with all refiner control systems is to secure the pulp quality at a
specified energy input
to the refiners. The problem, however, is that existing control systems on the
market are slow
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CA 02744638 2014-06-03
which results in difficulties to guarantee a specific pulp quality to a
minimum energy input
demand.
Traditionally, process control systems for refiners have no information from
the true refining
process, i.e. the process which occurs in the refining zone and the process
descriptions are
normally based on outputs to be controlled such as, specific energy E or the
motor load M and
if available also the measured consistency C in the blowline. These outputs
are normally
manipulated by changing e.g. the plate gap (controlled by the applied
hydraulic pressure Pk)*
which results in a force on the plates or e.g. a electro-mechanically based
force on the plates)
and the dilution water flow FD to the refiners as described by
Y
¨ ¨{1 GU = gll g12 Phydr
{g 21 g22 FD
where Y represents the vector to be controlled and G a transfer function
matrix which describes
the process dynamics by the elements gu. The vector U describes an input
vector with variables
possible to manipulate.
The drawback with traditional control systems, such as US 7,381,303 [2] which
is
based on the above type of system description, is that the specific energy or
the motor load are
affected by a number of process variables besides the two mentioned above
which makes it
difficult to pre-specify an optimal operating window where to run the
refiners.
Another problem with this structure is that the specific energy or the motor
load will always be
related to the integral of the force distribution along the radius in the
refining zone and it will
not give any information about the spatial energy consumption. It is dear,
from this aspect
alone, that the specific energy or motor load provides limited information
about how the local
process conditions affect the final pulp quality which is essential to get a
good control
performance.
Additional problems exist but one to be mentioned here is that traditional
control systems do
not handle natural non-linearities caused by e.g. plate wear, fluctuations in
fiber pad
distribution, different operating points et cetera which also reduce the
control performance.
CA 02744638 2014-06-03
As a consequence, the controllability in traditional control concepts as
described above is possible
to improve from a pulp quality perspective as different qualities can be
produced at the same
specific energy or motor load.
SUMMARY OF THE INVENTION
An aim of the present invention is to remedy one or more of the above
mentioned problems. In a
first aspect of the invention, this and other aims are obtained by a method
described herein.
In some embodiments of the invention, there is provided a method for
controlling a process section
for thermo-mechanical pulp (TMP) refining, said process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of said
at least one first refiner, said method comprising measuring, alternatively
estimating, one or more
process variables representing external states outside of the refiner for said
process section and
measuring, alternatively estimating, one or more values representing one or
more internal states
inside said at least one first refiner, and the possibility to manipulate one
or more variables,
characterised by calculating a change for said at least one manipulated
variable for said at least one
refiner using said measurement of an internal state of said at least one
refiner and a measurement of
said one or more process external states for said process section by means of
a mathematical
process model.
In some embodiments of the invention, there is provided a method for
controlling a process section
for thermo-mechanical, pulp (TMP) refining, said process section comprising at
least one first
refiner having a plurality of temperature or pressure sensors arranged in a
predetermined position
on a refiner plate of said at least one first refiner, said method comprising:
measuring or estimating at least one process variable representing external
states outside
of the at least one first refiner for said process section and measuring or
estimating, one or more
values representing at least one internal state inside said at least one first
refiner;
manipulating at least one variable; and
calculating a change for said at least one manipulated variable for said at
least one first
refiner using said measurement of said at least one internal state of said at
least one first refiner and
a measurement of said at least one process variable representing external
states outside of the at
least one first refiner for said process section by means of a mathematical
process model.
In some embodiments of the invention, there is provided a computer program
product for
controlling a process section for thenno-mechanical pulp (TMP) refining
comprising a computer
readable memory storing computer readable instructions thereon that when
executed by a computer
perform the method as described herein.
5a
= CA 02744638 2015-04-27
In some embodiments of the invention, there is provided a system including a
process section for
thermo-mechanical pulp (TMP) refining, said process section comprising at
least one first refiner
having a plurality of temperature or pressure sensors arranged in a
predetermined position on a
refiner plate of said at least one first refiner, said system comprising:
means for measuring or estimating at least one external process variable for
said process
section and measuring or estimating at least one internal state inside said at
least one first refiner;
an apparatus arranged for applying a process change calculated on at least one
manipulated variable for said at least one first refiner using said
measurement of said at least one
internal state of said at least one first refiner and a measurement of said at
least one external process
variable for said process section by means of a mathematical model to monitor
and control said at
least one first refiner.
In some embodiments of the invention, there is provided a method for
controlling a process section
for thermo-mechanical, pulp (TMP) refining, said process section comprising at
least one first
refiner having a plurality of temperature or pressure sensors arranged in a
predetermined position
on a refiner plate of said at least one first refiner, said method comprising:
a) providing at least one input process variable of said at least one refiner;
b) measuring or estimating state variables of said process section comprising
at least one
external state process variable representing external states outside of the at
least one first refiner for
said process section, and at least one internal state process variable
representing at least one internal
state inside said at least one first refiner;
c) producing a sequence of changes to said at least one input process variable
using said
measured or estimated state variables by means of a mathematical process
model;
d) controlling actuators to manipulate said at least one input process
variable based on said
sequence of changes to said at least one input process variable; and
e) repeating steps a) to d).
In some embodiments of the invention, there is provided a system including a
process section for
thermo-mechanical pulp (TMP) refining, said process section comprising at
least one first refiner
having a plurality of temperature or pressure sensors arranged in a
predetermined position on a
refiner plate of said at least one first refiner, said system comprising:
means for measuring or estimating state variables comprising at least one
external state
process variable for said process section, and at least one internal state
process variable inside said
at least one first refiner;
an apparatus arranged for applying a process change calculated on at least one
input
variable for said at least one first refiner using said measured or estimated
state variables by means
of a mathematical model to monitor and control said at least one first
refiner.
5b
. CA 02744638 2015-04-27
In some embodiments of the invention, there is provided use of a system as
described herein to
monitor and control a process handling of: pulp, paper, wood pulp, cellulose
pulp or any
combination thereof.
5c
CA 02744638 2014-06-03
In the first aspect of the invention a method is described for controlling a
process section for
thermo-mechanical pulp (TMP) refining, the process section comprising at least
one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring,
alternatively estimating, one or
more process variables representing external states outside of the refiner for
the process section
and measuring, alternatively estimating, one or more values representing one
or more internal
states inside said at least one first refiner, and the possibility to
manipulate one ore more
variables, wherein a change is calculated for said at least one manipulated
variable for said at
least one refiner using said measurement of an internal state of said at least
one refiner and a
measurement of said one or more process external states for said process
section by means of a
mathematical process model.
According to an embodiment of the invention a method is described for
controlling a process
section for therrno-mechanical pulp refining, the process section comprising
at least one first
refiner having a plurality of scnsors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring,
alternatively estimating, one or
more process variables representing external states outside of the refiner for
the process section
and measuring, alternatively estimating, one or more values representing one
or more internal
states inside said at least one first refiner, and the possibility to
manipulate one ore more
variables by means of a mathematical process model, and wherein a change is
calculated for
said at least one manipulated variable for said at least one refiner using a
mathematical process
model which is described by a set of nonlinear differential equations or
difference equations
with a vector valued non linear function.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
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one ore more variables using a mathematical process model, wherein the
mathematical process
model is described by a set of linear differential equations or difference
equations.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the proccss section
comprising at least
one first refiner having a plurality of sensors aiTanged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, wherein the
mathematical process
model is described by Laplace transforms and transfer functions.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, onc or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
using a measurement in which said internal state is temperature which is
calculated using an
array of temperature measurements along the radius of a disc inside said at
least one refiner.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process scction
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
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CA 02744638 2014-06-03
and changing at least one manipulated variable to affect a change in a measure
or estimate of
pulp quality.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by using the
mathematical process
model to mimimize the deviation between reference values and measured values,
or functions,
of internal and/or external states).
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refuter for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by using the
mathematical process
model to mimimize the deviation between reference values and estimated values
of internal
and/or external states.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
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CA 02744638 2014-06-03
one ore more variables using a mathematical process model, and by feeding the
output from
said at least one first refiner into a second refiner such that said process
section comprises a
two stage refiner.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
for at least one manipulated variable for the second refiner using said
measurement of an
internal state of the second refiner and a measurement of said one or more
external states for
said process section by means of a mathematical model.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables rcpresenting external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
for at least one manipulated variable of a process section comprising two or
more refiners
using said at least one measurement of an internal state for each refiner and
a measurement of
an external state for each refiner and at least a quality measurement after
the second refiner.
According to another embodiment of the invention a method is described for
controlling a
process section for therrno-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
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CA 02744638 2014-06-03
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
for at least one manipulated variable of a process section comprising two or
more refiners
using said measurement of an internal state for each refiner and a measurement
of an external
state for each refiner and a quality measurement of the second refiner in
order to optimize
energy input to said process section.
According to another embodiment of the invention a method is described for
controlling a
process section for thermo-mechanical pulp refining, the process section
comprising at least
one first refiner having a plurality of sensors arranged in a predetermined
position on a refiner
plate of said at least one first refiner, the method comprising measuring,
alternatively
estimating, one or more process variables representing external states outside
of the refiner for
the process section and measuring, alternatively estimating, one or more
values representing
one or more internal states inside said at least one first refiner, and the
possibility to manipulate
one ore more variables using a mathematical process model, by calculating a
process change
using a measurement in which said internal state is pressure (PI) which is
calculated using an
array of measurements along the radius of a disc inside said at least one
refiner.
In another aspect of the present invention, a system is described including a
process section for
thermo-mechanical pulp (TMP) refining, the process section comprising at least
one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least one manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
said one or more process variables for said process section by means of a
mathematical model
to monitor and control said at least one first refiner.
CA 02744638 2014-06-03
According to another embodiment of the invention, a system is described
including a process
section for thermo-mechanical pulp refining, the process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least onc manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
said one or more process variables for said process section by means of a
mathematical model
to control said at least one first refiner wherein one or more of said
plurality of sensors are
arranged on an active radius of a beating disc of a refiner in said process
section.
According to another embodiment of the invention, a system is described
including a process
section for thenno-mechanical pulp refining, the process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least one manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
said one or more process variables for said process section by means of a
mathematical model
to control said at least one first refiner wherein said process section
comprises two or more
refiners which are arranged connected in series, alternatively in parallel.
According to another embodiment of the invention, a system is described
including a process
section for thermo-mechanical pulp refining, the process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least one manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
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CA 02744638 2014-06-03
said one or more process variables for said process section by means of a
mathematical model
to control said at least one first refiner wherein the system further
comprises one or more
control units arranged as a control optimiser.
According to another embodiment of the invention, a system is described
including a process
section for thermo-mechanical pulp refining, the process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least one manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
said one or more process variables for said process section by means of a
mathematical model
to control said at least one first refiner wherein the system further
comprises one or more
control units arranged as a state estimator.
According to another embodiment of the invention, a system is described
including a process
section for thermo-mechanical pulp refining, the process section comprising at
least one first
refiner having a plurality of sensors arranged in a predetermined position on
a refiner plate of
said at least one first refiner, the method comprising measuring one or more
process variables
for said process section and measuring one or more internal states in said at
least one first
refiner wherein the system further comprises apparatus for applying a process
change
calculated on at least one manipulated variable for said at least one refiner
using said
measurement or estimate of an internal state of said at least one refiner and
a measurement of
said one or more process variables for said process section by means of a
mathematical model
to control said at least one first refiner wherein the system further
comprises a memory storage
device in which are stored one or more computer programs for carrying out a
method
as described herein.
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CA 02744638 2014-06-03
THE SOLUTION
In this invention the internal states, , Le. information (estimated or
measured) directly from
the refining zone plays a vital role to describe how to find improved
strategies for process
control. To distinguish the internal states from other states we introduce the
external states, 17,
as states obtained from measurement devices outside the refiners or estimated
from
mathematical models. These states are normally possible to sample relatively
fast but still at a
slower sampling rate compared with the internal states. Other external states,
Q, typically
representing measured pulp quality that may only be sampled at a much slower
sampling rate,
will be important to follow as well. All states which can be controlled are
elements in the state
vector x, i.e.
x=11
and with this terminology a clear distinction between traditional control
concepts and this
invention, with a new process optimization approach, can be described. Notice,
that each
component of the state vector x may in turn be a vector.
The relationship between the input vector tei and the state vector xi may be
described by a set of
nonlinear differential equations characterized by a vector valued nonlinear
function f;
= f (xi (t),u, (t))
where i represents the refiner to be described.
Normally, the input vector can vary in size dependent on the type of refiner
studied but could
typically be represented by chip or pulp production flow, dilution water flow,
and plate gap (or
means to influence the plate gap such as hydraulic pressure or alternatively
an electro-
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mechanic force). For some refiners also the inlet pressure in the pulp feeding
system is possible
to manipulate.
To describe this more concretely, typical internal states to be used in this
invention can be
described as e.g. temperature in the refining zone, pressure or force
measurements in the
refining zone or states estimated from physical or empirical models by using
different software
solvers. Examples of that is the predicted forces along the radius in the
refining zone, estimated
consistency in the zone, mean fiber residence time in different regions of the
zone, plate gap,
pulp quality from the refining zone et cetera. Pulp quality estimations can be
valuable when
comparing them with the measured pulp quality from the downstream analyzers.
Hence, the
variables can refer to a set of soft sensors (or estimates from the models)
describing the hidden
process variables in the refining zone. In this model e.g. temperature
measurements can be
used to get proper estimates of the energy balance which indirectly together
with zone specific
data give information normally difficult to measure directly, like the
residence time,
defibration/fibrillation work et cetera. The variables can also be referred to
as estimates
obtained from an algorithm which uses spatial refining zone measurements to
find some
optimum, like the position for the maximum temperature in the refining zone
which can be
controlled by e.g. Piwer as described by Sikter [6].
Typical examples of external states, which may be measured relatively fast
outside the refining
zone but slower than the internal states, will be the blow-line consistency,
pressure, motor load
et cetera. Such external states can also be estimated by a mathematical model.
Other typical
external states, which are only possible to sample much more slowly are placed
in the vector
Q. Such states are naturally the pulp quality variables CSF, the mean fiber
length and shives et
cetera which are measured downstream the refiners. If pulp quality variables
are estimated
from ordinary system identification procedures, e.g. using ARMAX models, to be
used as input
to the feedback control, such estimated variables will be placed in the vector
77 as the sampling
rate will be fast enough.
Note, external states like the motor load or combinations to get the specific
energy
(load/production) are not considered to be a part of the concept described by
this invention
which differs from traditional control concepts. The measured motor load in
the optimization
routines may be used as a constraint to the optimization.
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Moreover, it is worth to mention that the vector u can be truncated to a
smaller vector if
necessary and/or extended if e.g. the plate gap can be measured accurately (as
the hydraulic
pressure Phydr could be replaced) which of course affects the size of the
matrix. In the matrix i,
i.e. the number of refiners i.{1,2] included, is not specified and provides
many possible
combinations and a selection of each structure will be refiner specific.
By distinguishing the internal and external states from each other the fast
dynamics in the
refining zone can be handled and controlled faster compared with traditional
control concepts
and the reason why internal states such as the refining zone temperature
measurements give a
good contribution to the optimization procedure as well as the pulp quality
estimations is that
local non-linearities, i.e. spatially dependent information not captured by
the specific energy
(or the motor load), can be handled. An example of such non-linearities is the
different process
conditions encountered before and after the temperature maximum in the
refining zone. Earlier
work has shown that the residence time of pulp material in these two parts of
the refining zone
will be dependent on a complex set of phenomena. In other words the internal
states, such as
the temperature measurements provide information indirectly of how the
defibration/fibrillation is carried out in the refiner since it relates to the
difficulties for the
steam to evacuate from the refining zone.
Hence, from a mathematical point of view, the main variables to be controlled
are obtained
from the refining zone, i.e. not from the traditional concepts based on motor
load or specific
energy control. The essence will be that compared with the traditional concept
soft sensors and
measurements from the refining zone provide information which can be used
almost
momentarily. Of course, the consistency measured in the blow line can be used,
if available,
but the spatial consistency, which is available from the models, is to be
preferred.
If a refiner line comprising two refiners is controlled a model for the whole
line can be
constructed by combining the individual refiner models. For example,
neglecting the mixing
effect of the blow-line and only taking the time-delay Di into account (which
is typically in the
order of 5-10 s) we have
5c2 (t) = f2 (x, (t ¨ DI ), x, (t), u2 (t))
What is actually measured of course varies from installation to installation.
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Normally we would in our application expect these measurements to just be a
subset of the
process states, but more generally a model equation describing the
relationship between
measurements and states may include a nonlinear function, i.e.
y(t) = h(x(t),u(t))
In each iteration of the control, i.e. any time a new measurement is
collected, two optimization
problems have to be solved; one to estimate the state vector x and one to
optimize the future
control variables u. Then applying a receding control approach the control
variables for the
first time instant are sent to the process, and when at the next measurement
instant
optimizations are repeated.
The present invention describes a way, using among other things temperature
and/or pressure
measurements directly in the refining zone, to control and optimize process
conditions in
refiners to improve energy efficiency and pulp quality. The procedure means
that the internal
states, represented by temperature and/or pressure measurements, are primarily
used to
minimize the variations in pulp quality or energy consumption. Thanks to the
availability of the
internal states for use in the feedback system, the number of interaction
elements can be
minimized in the model based control.
The present invention presents the solution to the problems described, and
concerns use of
robust temperature and/or pressure measurements in combination with available
measurement
signals from the process together with a mathematical model to control both
pulp quality and
energy input to refiners much faster than what is the case today.
The principle advantage of the present invention is that thanks to measuring
internal states such
as temperature and/or pressure directly in the refining zone a faster control
response may be
produced of variables that better correlate to the final pulp quality than in
traditional refiner
control concepts. The present invention also provides a significant
improvement over the prior
art by introducing model based optimization involving internal states of the
refiner. In an
aspect of the invention the method:
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a) involves the temperature (and possibly force) measurements in the refiner,
but not
necessarily the motor load. More importantly
b) it treats the whole two-stage refiner line as one (multi-rate) optimal
control problem, which
is described in more detail below.
A computer program, and a computer program recorded on a computer-readable
medium is
disclosed in another aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the method and system of the present
invention may be had
by reference to the following detailed description when taken in conjunction
with the
accompanying drawings wherein:
Figure 1 shows a schematic block diagram of a method and system for
controlling a TMP pulp
refining process comprising a first refiner, according to an embodiment of the
invention;
Figure 2 shows the invention according to Figure 1 in which the diagram also
shows a process
comprising two refiners, according to an embodiment of the invention;
Figure 3 shows a schematic flowchart of the invention according to Figure 1 or
Figure 2 and in
particular steps for carrying out a method according to an embodiment of the
invention.
Prior Art
Figure 4 shows a schematic diagram for a known primary refiner, Figure 5 shows
a known
refiner grinding plate arranged with temperature or pressure sensors, and
Figure 6 shows a
known array of temperature sensors (or pressure sensors) arranged on a refiner
plate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 shows a schematic diagram for a method of controlling a TMP pulp
refiner. The
diagram shows a process with a single refiner 33, and a first control unit 32.
One or more
setpoints 30 are input to the first control unit or control optimiser 32, or
similar device with the
same function. The first control unit, the control optimiser, is arranged to
manipulate external
process variables such as the hydraulic pressure Phydr. pressing together the
refiner plates, flow
of wood chips 5flow, indicated as Fp and dilution water 2' indicated as FD ,as
inputs to the
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WO 2010/063310 PCT/EP2008/066556
process 33. Refined pulp is produced from the process 33. External variables
37 representing
pulp consistency 37 from the process and quality Q from sampling unit 35 are
input to a second
control unit, a state estimator 39. Internal values 36 representing a
temperature profile or a
pressure profile from inside the refiner using a sensors as shown 21 in Fig 6,
are fed to the state
estimator 39. The state estimator calculates and sends 40 a state estimation
.1c to the first
control unit, the control optimiser 32. In the control optimiser the estimated
state is used as a
starting point to calculate the future trajectories of all state variables
based on the process
model. The setpoints are then compared with the model outputs to obtain a
control error. This
control error is minimised with the future changes of the manipulated
variables as free
variables in the optimisation.
For the primary refiner the manipulated input variables are summarized below
in the vector u] ,
[_
Fp
u1 = FD
P
hydr _
where Fp denotes the chip production flow, FD the dilution water flow and
Phydi. the
hydraulic pressure applied on the refiner plates. Notice that there are other
potential means to
manipulate the force pressing the plates together, such as using
electromechanic devices. When
plate gap is measured a cascaded control structure is also an alternative,
where the setpoint for
the plate gap is considered as the manipulated varaiable in the control
described in this
invention. However, in the sequel Ph)dr will be used as a variable to describe
all such plate gap
changes. For some types of refiners the inlet pressure P11 may also be
available as
manipulated variable.
Similarly the corresponding process state variables are given in xi
-1 -
71
x1 =
CI
_Q1 _
where 6 is the force (or soft sensor of force) measured inside the refiner,
7'1 an array of
temperature measurements along the radius inside the refiner, C1 the blow-line
consistency and
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Qi a vector of pulp quality variables such as Canadian standard freeness CSF
and mean fiber
length MFL. An alternative to 6 or T/is to use the pressure P/ inside the
refiner as a state
variable.
The relationship between it] and .x/ may be described by a set of nonlinear
differential
equations characterized by a vector valued nonlinear function fi ;
= (xi (t), ui (t))
The main dynamics of the refiner are those for the actuation and sensing (more
about sensing
further below). Denoting the output from the controller by ujc the following
linear differential
equations approximately describe the inputs to the refining zone
= + /41c
The time constant t is roughly equal for all inputs and typically in the order
of 1-5 seconds.
Alternatively the relationship may be described by Laplace transforms and
transfer functions as
1
U1= ___
Figure 2 shows a schematic block diagram for a method of controlling a TMP
pulp refiner
line. The diagram shows a process with a first refiner 33, and a second
refiner 34, which are
preferably arranged as a primary and a secondary refiner. A first control unit
or control
optimiser 32 is supplied with one or more setpoints 30 to the first control
unit or, or similar
device with the same function. The first control unit, the control optimiser,
is arranged to
manipulate external process variables such as the hydraulic pressure Phydr.
pressing together the
refiner plates, flow of wood chips 5flow, and dilution water 2' as inputs to
the first refiner 33;
and hydraulic pressure Phydr and dilution water 2' to the secondary refiner.
Pulp from the first
refiner is led to the second refiner 34 through a blowline (not shown).
Refined pulp P is
produced from the secondary refiner 34. The refined pulp is sampled to measure
one or more
quality parameters Q. External variables 37 representing pulp consistency (C)
from the primary
refiner and quality 38 (Q) from sampling unit 35 are input to a second control
unit, a state
estimator 39. Internal values representing a temperature profile or a pressure
profile from
inside the refiner at T1 (primary) 36 and T2 (secondary) 36"using a sensors as
shown 21 in Fig
6, are fed to the state estimator 39. The state estimator calculates and sends
a state estimation
lc to the first control unit, the control optimiser 32. In the control
optimiser the estimated state
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is used as a starting point to calculate the future trajectories of all state
variables based on the
process model. The setpoints are then compared with the model outputs to
obtain a control
error. This control error is minimised with the future changes of the
manipulated variables as
free variables in the optimisation.
Commonly two refiners are arranged together in a process, as described for
example in relation
to Figure 2 above. For the secondary refiner the manipulated variables are
F D
142 =[n
rhydr
which again are dynamically related to the controller output as
= ¨U2 + U2c
Typical process state variables for the secondary refiner are
¨
T2
x=
2 C2
_.Q2 -
Again an alternative state variable is the pressure P2 inside the refiner.
If the blow-line between the primary and secondary refiners is considered
static
f2 (xi (t ¨ ), x2 (t), /42 (t))
However, as indicated in the equation the blow-line itself introduces a time
delay D1, typically
in the order of 5-10 s.
A typical set of measurements can be
T2
y=
C/
_Q2 _
where T1 and T2 are the temperature measurements (possibly vector valued) in
primary and
secondary refiner, respectively, C1 the measured blow-line consistency out of
the primary
refiner and Q2 the pulp quality after the secondary refiner.
CA 02744638 2011-05-25
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Here all signals may be measured every second except the final pulp quality Q2
which is
measured using a sampling analyzer equipment, typically needing 5 minutes per
sample.
Furthermore the same equipment may serve several measurement points why a
sampling
interval in practice often is 20-30 minutes. The measurement is often preceded
by a latency
chest, which then acts like an anti-aliasing filter.
The total model becomes a set of nonlinear Differential and Algebraic
Equations (DAE), which
are observed in a multi-rate sampled fashion
X(t) = f (x(t),u(t))
At) = h(x(t),u(0)
where
u(t) = [Ulc 1
U2c
and the state variable x(t) is built up by xl (t) and x2 (t)and possibly
additional states to
account for actuator and sensor dynamics.
In each iteration of the control, i.e. any time a new measurement is
collected, two optimization
problems have to be solved; one to estimate the state vector x and one to
optimize the future
control variables. Then applying a receding control approach the control
variables for the first
time instant are sent to the process, and when at the next measurement instant
optimizations
are repeated.
For the state estimation the optimization target is to the best estimate of
all states of the refiner
using the available measurements. This can be done using a Kalman filter [7]
(or if the model
is nonlinear extended Kalman filter) where a stochastic modelling of the
process and
measurement noises is deployed. Alternatively we may apply so-called moving
horizon
estimation [8]. Then the process and measurement noise are introduced using
slack variables
w and v in a discretized version of the model
Xk+1 =. g(Xk ,"k)+ 14?
k
yk = h(Xk ,Uk ) + Vk
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where the integer k denotes the k:th signal value which is available at time
t=kT.,, where Ts
is the sampling interval, i.e. we have for example xk = x(kTs).
Then moving horizon estimation corresponds to minimizing
min(xk_xf ¨ _)T p-1 (xk_m 1w IT, R1-1 wn v Rlv n
subject to, for example,
x 5 x(05. xõ.
Here P, R1 and R2 are weight matrices used for tuning of the estimator, which
have a similar
interpretation and importance as the estimate and noise covariance matrices in
Kalman
filtering.
As indicated this optimization is typically done over a horizon [ t¨MT,,t], if
t is the current
measurement time. Since this time interval is in the past we assume access to
historic values of
the applied manipulated variables uk . The first penalty term in the criterion
is to create a link
from one optimization window to the next, where IC k_iv denotes the estimate
for this particular
time instant from the optimization run at the previous cycle. What makes this
problem
nonstandard, though, is that not all elements of yk will be available at all
sampling instants,
creating a multi-rate state estimation problem. The state estimation produces
a starting point
for the optimization of future manipulated variables, where future setpoints
rk are compared
with some subset or combination of the state variables m(xk )calculated by use
of the
mathematical process model. A formulation of the optimization objective may
be, for example,
Ny N.
min E (rk m(xk))T W y(rk ¨ m(xk))¨ + EAuWAuk
k=0
subject to, for example,
.x xt.
umin umaõ
Au.õ 5 Auk 5 Au.
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Here the optimization is done using Auk = uk uk_1 as free variables, which
introduces
integral action in the controller.
In a traditional refiner control concept typical candidate variables to have
setpoints for are
primary refiner pulp consistency C1 andthe pulp quality after the secondary
refiner Q2 , but the
formulation above is in no way restricted to this choice.
Notice that since the number of manipulated variables are typically more than
2 and sometimes
as many as 6 , it should be possible to have setpoints on more then two
variables. One
possibility is, for example, to have setpoints for force and/or peak
temperature inside the
refiners. The radial location of the peak temperature is yet another candidate
for setpoints.
In the optimization problems described above the nonlinear model is used as an
equality
constraint, leading to a nonlinear model predictive control problem.
Alternatively, the model is linearized, resulting in a model of the form
xk+1 Axk Buk
yk =Cuk
Notice that due to the multi-rate nature of the measurements, the dimension of
the matrix C
will be time-varying. If such a model is used as equality constraint in the
optimization a
(multi-rate) linear model predictive control problem is thus solved instead.
Figure 3 shows a simplified flowchart for one or more methods according to
another aspect of
the invention. The figure shows that the method begins 50 by initializing the
time t. Each cycle
starts by retrieving the measured values from the sensors 52. These measured
values (and
possibly also historic values in a window of length M) are then used together
with the process
model to calculate 53 a state estimate :v. This state estimate is then used as
a starting point in
the forward optimisation which produces a sequence of changes 55 to the
manipulated
variables over a future horizon of length Nu . Applying the receding horizon
principle only the
first change of manipulated variables is sent to the actuators 57; after which
the time is
incremented 58 and the procedure (52-57) is repeated.
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CA 02744638 2015-04-27
The methods of condition monitoring as described above and elsewhere in this
specification
may be carried out by a computer application comprising computer program
elements or
software code which, when loaded in a processor or computer, causes the
computer or
processor to carry out the method stcps.
The methods of controlling and optimizing as described above and elsewhere in
this
specification may be carried out by a computer application comprising computer
program
elements or software code which, when loaded in a processor or computer,
causes the
computer or processor to carry out the method steps. The functions of the
methods, such as the
method shown in Figure 3, may be carried out by processing digital functions,
algorithms
and/or computer programs and/or by analogue components or analogue circuits or
by a
combination of both digital and analogue functions.
The methods of the invention may, as previously described, be carried out by
means of one or
more computer programs comprising computer program code or software portions
running on
a computer or a processor. The microprocessor (or processors) comprises a
central processing
unit CPU performing the steps of the method according to one or more facets of
the invention.
This is performed with the aid of one or more said computer programs, such as,
which are
stored at least in part in memory and/or and as such accessible by the one or
more proccssois.
The or each processor may be in a control unit, or as a separate control
optimizer unit or in a
state estimator unit or part thereof, or may as well run in a local or central
control system in a
local or distributed computerised control system. It is to be understood that
said computer
programs may also be run on one or more general purpose industrial
microprocessors or
computers instead of one or more specially adapted computers or processors.
The computer program comprises computer program code elements or software code
portions
that make the computer perform the method using equations, algorithms, data,
stored values
and calculations previously described. A part of the program may be stored in
a processor as
above, but also in a ROM, RAM, PROM, EPROM or EEPROM chip or similar memory
means. The program in part or in whole may also be stored on, or in, other
suitable computer
readable medium such as a magnetic disk, CD-ROM or DVD disk, hard disk,
magneto-optical
memory storage means, in volatile memory, in flash memory, as firmware, stored
on a data
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WO 2010/063310 PCT/EP2008/066556
server or on one or more arrays of data servers. Other known and suitable
media, including
removable memory media and other removable flash memories, hard drives etc.
may also be
used. The computer programs described may also be arranged in part as a
distributed
application capable of running on several different computers or computer
systems at more or
less the same time. Programs as well as data such as start positions, or flag-
related information
may be made available for retrieval, delivery or, in the case of programs,
execution over the
Internet. Data may be accessed by means of any of: OPC, OPC servers, an Object
Request
Broker such as COM, DCOM or CORBA, a web service.
It should be noted that while the above describes exemplifying embodiments of
the invention,
there are several variations and modifications which may be made to the
disclosed solution
without departing from the scope of the present invention as defined in the
appended claims.
REFERENCES
[1] J. Liden, Quality Control of Single Stage Double Disc Chip Refining,
Licentiate Thesis,
Mid Sweden University, 2003.
[2] M.S. Sidhu, R.J. van Fleet and M.R. Dion, System and Method for
Controlling a
Thermo-Mechanical Wood Pulp Refiner, US patent US 7,381,303, 2008.
[3] A. Karlstrom and P. Engstrand. System for continuously measuring
pressure and
temperature in the beating zone of refiners. European patent application EP 0
788 407, granted
February 1999.
[4] A. Karlstrom, A method for guiding the beating in a refiner and
arrangement for
performing the method, US Patent US6,024,309, 2000.
[5] O.M. Johansson. Refiner measurement system and method. US patent
US6,314,381
Granted Nov 6 2001.
[6] D. Sikter. Quality Control of Newsprint TMP Refining Process based on
Refining Zone
Temperature Measurements, Licentiate Thesis, Chalmers University of
Technology, 2007.
[7] B.D.0 Anderson and J.B. Moore. Optimal Filtering. Prentice-Hall, 1979.
[8]. C. V. Rao. Moving Horizon Strategies for the Constrained Monitoring and
Control of
Nonlinear Discrete-Time Systems, Ph.D. Thesis, University of Wisconsin, 2000.
