Note: Descriptions are shown in the official language in which they were submitted.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
1
CONTROL ARRANGEMENT FOR REFINERS WITH TWO REFINING ZONES
i u Technical field:
The invention relates to a procedure, where among other measurements,
temperature
sensors are used directly in the refining zone, controlled by the changes in
the dilution
water flow rate and plate gap in refiners with two refining zones.
The main purpose with the invention is to obtain a more even distribution and
consistency in each refining zone. The distribution problem is mainly a result
of the
difficulties to measure the chip/pulp feed rate and to control the amount of
chips/pulp
to each refining zone. The procedure also copes with the problem associated
with the
pulp quality variations in time which can be minimized at the same time as the
production of pulp can be increased.
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
and for pre-treatment of fibers in paper-making for the pulp and paper
industry and
related industries through grinding, for example, thermo-mechanical pulp (TMP)
or
.3v 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,
35 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.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
2
Most refiners consist of two circular plates, discs, in between which the
material to be
treated passes from the inner part to the periphery of the plates, see Figure
1. Usually,
there is one static refiner plate and one rotating refiner plate, rotating at
a very high
speed.
The static refiner plate is placed on a stator holder (3), and is pushed
towards a
rotating one place on a rotor holder (4), electro mechanical or hydraulically
(5).
The chips or fibers (6) are often fed into the refiners together with the
dilution water
via the center (7) of the refiner plates and are grinded on its way outward to
the
periphery (8). The refining zone (9), between the plates (also called
segments) has a
variable gap (10) along the radius (11) dependent on the design of the plates.
The diameter of the refiner plates differ dependent on size (production
capacity) of the
refiner and brand. Originally the plates (also called segments 12, 13, see
Figure 1 and
Figure 2) were cast in one piece, but nowadays they usually consist of a
number of
modules (forming a disc) that are mounted together on the stator and rotor.
The
segments can be produced to cover the entire surface from the inner to the
outer part
of the holders or be divided into one inner part (14) often called "the
breaker bar
zone" and an outer part (15) called periphery zone.
These segments have grinding patterns (16), with bars (17) and troughs (18)
that differ
dependent on supplier. The bars can be seen as knives that defibrillate chips
or further
refine the already produced pulp. The plates wear continuously during the
refining
process and have to be replaced at intervals of around every 2 months or so.
In an HC
refiner, fibers, water and steam are also transported in the troughs between
the bars.
The amount of steam is spatially dependent, which is 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 fibers. Dependant on the segment design different
flow
patterns will occur in the refiner. In an LC-refiner no steam is generated and
thereby
12 A only two phases exist (liquid and pulp).
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 is 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 (10) of approximately 0.2-0.7
mm
dependent on what type of refiner is used.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
3
The plate gap is an important control variable and an increased or reduced
plate gap is
performed by applying an electro mechanical or hydraulic pressure (5) on one
or
several segments dependent on the type of refiner. With that an axial force is
applied
on the segments. The force which acting in opposite direction to the axial
force
consists in HC-refining processes by the forces obtained from the steam
generation
and the fiber network. In those cases LC-refining is considered the axial
force is
neutralized by the forces extracted from the increased pressure in the water
(liquid)
phase and the fiber network. If the plate gap is changed for example 10%, the
pulp
quality is changed considerably. Therefore, it is important to know the actual
plate
gap. Today, measurement units for plate gap are provided commercially.
Normally,
only one plate gap sensor is used to prevent plate clash and not as expected
in any
control algorithms. Other systems exist as well and one robust system is based
on
temperature measurements along the radius in the refining zone to visualize
the
~., temperature profile (19) alternatively the pressure profile (20) for
control purposes,
see Figure 3. For LC-refining the pressure is preferred to be measured but for
HC-
refining the temperature profile will be enough to measure.
When changing the process conditions in plate gap, production and the amount
of
20 added dilution water, the temperature is changed which gives an opportunity
to
control it in different ways. Several temperature- and/or the pressure sensors
are often
used and can be placed directly in the segments alternatively mounted in a
sensor
array holder (21) which can be placed between the segments (12 and 13), see
Figure 1,
Figure 2 and figure 4 as described in EP 0788 407. Usually, the sensor array
holder is
25 implemented between two segments in the outer part, see Figure 2.
The design of the segments has proven to be of great importance for
characteristics of
the temperature profile along the radius. Therefore, it is difficult in
advance to decide
where the temperature sensors (22) and/or the pressure sensors (22) should be
placed
,in in the sensor array holder (21).
According to traditional safety systems for plate clash protection,
accelerometers
placed on the stator holders (3) and/or the rotor holders (4) are used besides
the plate
gap sensors.
Technical problem
In the literature, extensive materials exist regarding refiner control by
using
consistency measurements and temperature measurement including safety systems
to
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
4
prevent plate clash of segments. The safety systems are often built on both
hardware
(typically accelerometers and plate gap sensors) and software in terms of
frequency
analyzers and specific functions for limit control et cetera.
The research results indicate that the measurements of vibration on the
holders shows
deviations from vibrations caused by actual local fluctuation in the fiber pad
inside the
refining zone, which can be a result of in-homogeneity in the fiber pad or the
other
phases (water and steam). When considering LC-refining, the in-homogeneity can
occur even though there exist only two phases.
The in-homogeneity in the fiber pad is central for the description of the
technical
problem. If the packing degree of the fiber pad varies locally both spatially
and in time
this can cause local areas where the spatial temperature alternatively the
pressure
increase or decrease dependent on if the packing degree increase or decrease.
This
leads to fluctuations in the pressure distribution in the refining zone which
cause non-
linear process conditions and thereby a varying residence time for the fibers
in the
refining zone which can cause bad pulp quality due to fiber cutting. Fiber
cutting
means that the length of the fibers is shortened too much when they hit the
segment
bars. The most unwanted situation is obtained when the fiber network is
collapsed, i.e.
20 the force related to the fiber network, which can be seen as a repulsive
force to the
axial force, is reduced drastically which certainly can lead to a plate clash.
Therefore,
it is of importance to keep the right spatial consistency in the refining zone
to prevent
phenomena like the fiber pad in-homogeneities.
25 In the literature, temperature measurements have shown to be an unusual
robust
technology for HC-refining control. When changing the production, dilution
water and
the hydraulic pressure the temperature profile is changed dynamically. This
dynamic
change is visualized in Figure 5a, where a step change in dilution water
affect the
temperature profile in different ways dependent on where on the radius (11) we
consider the process. It is seen that when the dilution water increase, the
temperature
(23) will decrease before the maximum (24) but increase (25) after the
maximum. The
reason is that the water added cools down the back-flowing steam at the same
time as
the steam which is going forward is warmed very fast.
When the production is increased the entire temperature profile (19) is lifted
to
35 another level (26), see Figure 5b. The same situation is valid when the
plate gap (10)
is reduced by increasing for instance the hydraulic pressure.
The non-linearities are affected also by the design of the segments. This can
result in
different temperature profiles (19, 27) and pressure profiles, see Figure 5c.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
The complex pattern of possible conditions has not so far been related to the
local
change in the refining zone consistency. The reason could be that the focus
has been
put on the rudimentary enthalpy balance for the entire refining zone without
any use
of temperature measurements.
5
All these process conditions, related to an increase in production and
dilution water,
will affect the active volume inside the refining zone at constant hydraulic
pressure
and hence affect the plate gap as well as the temperature and/or the pressure
profile.
This will result in a change in the fibers residence time which affect the
fluctuations in
the refining zone and finally the pulp quality. The process conditions can
also drive
the refiner into situations where another operating point is obtained which by
safety
reasons are forbidden due to the risk for damage. These forbidden areas are
difficult to
predict on beforehand with present technology.
In a research project a new theoretical physical model has been documented
("Refining models for control purposes" (2008), Anders Karlstrom, Karin
Eriksson,
David Sikter and Mattias Gustavsson, Nordic Pulp and Paper journal). The
model,
describes HC-refining and it is supposed that the temperature and/or the
pressure are
measured along the radius of a segment to span the material and energy
balances in
20 the refiners and thereby make it possible to estimate the plate gap. The
main
difference compared with earlier rudimentary trials to describe the physics of
the
grinding processes is that the model estimates both the reversible
thermodynamic
work and the irreversible defibration work applied on the fiber network where
the
shear forces have a central position when iterating to find the right plate
gap. Thereby,
25 the model is described from an entropy perspective instead of an enthalpy
based
approach which does not take into account the shear between the fibers,
flocks, water
and the segments. In this model it is assumed that the production rate is
possible to
measure which indeed is hard to prove due to for example the fluctuations in
raw
material density et cetera. Instead, in real processes the production is an
estimate
based on assumed consistencespeed rate of the refiner load conveyers and this
cause problems in most control concepts where the production is assumed to be
measured. The model is of course a simplification but it has been useful for
research
purposes.
35 In those cases where the refiners are constructed with two refining zones,
i.e. refiners
normally called Twin refiners, another problem occurs as the amount of chips
or pulp
distributed to each refining zone is unknown. Consequently, in those
situations where
two or more serially linked Twin refiners constitute the production line, the
final pulp
quality is affected and can vary considerably if the deviations in consistency
between
40 the two refining zones are not evened out.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
6
Hence, the problem is that the distributed consistency along the radius in the
refining
zone is not possible to measure which means that other solutions of the
problem must
be used. This can be expressed as follows:
In those cases where refiners with two refining zones, divided by a rotor, are
considered asymmetric temperature profiles can be obtained, see Figure 6.
Usually
these refining zones are called Front Side (28) and Drive Side (29). These two
sides
are below referred to as FS and DS, respectively.
The asymmetric temperature profiles can be a consequence of:
1. two different plate gaps, caused by the difficulties to position the rotor,
but
the same chip or pulp feed rate. This result in a situation where the
temperature profiles are displaced between themselves, see Figure 6a;
2. a situation where the chips to the primary refiner or the pulp to the
secondary
refiner are fed (unintentionally) differently to the FS and DS. This result in
a
uneven distribution of the chips or the pulp which is reflected in the shape
of
the temperature profile, see Figure 6b;
3. two different plate gaps, caused by a problem with the positioning of the
rotor
but with different chip or pulp feed rates. This result in a displacement of
the
temperature profile according to Figure 6c;
20 4. different fiber packing degrees dependent where in the refining zone the
pulp
occurs. This can be a consequence of a random situation where the fibers in
the refining zone are distributed differently during the start up procedure of
the refiners.
Summary of the invention
In The 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 according to claim 1.
The invention is based on a procedure where robust temperature- and/or
pressure
measurements in combination with available signals from the process, to
control the
amount of added dilution water to each refining zone and finally the hydraulic
pressure applied on each stator on the Front Side (FS) and Drive Side (DS).
As the temperature profiles in each refining zone is desired to converge to
each other,
instead of the described situation in Figure 6b, the dilution water must be
controlled in
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
7
another way compared with the situation faced today. The following procedure
is
therefore a solution of the problem.
1. By measuring and comparing the load on the conveyers (feed screws) to each
refining zone, i.e. FS and DS respectively, an indication is obtained how the
distribution of chips or pulp to each refining zone looks like in a relative
perspective. If the load for the feed screw on the FS is higher compared with
the
one on the DS it is possible to assume that more pulp is present in the FS
feed
screw and vice versa, see Figure 7. This of course requires similar motors and
mechanical construction for the screws which sometimes is hard to assume.
2. As a complement to the studies of the load conveyers, the vibrations on the
stators FS and DS respectively can be measured using accelerometers. If the
vibration, for instance, is higher on the DS less pulp is fed on that side
which
has not been known earlier. Instead it has been believed that large vibrations
always is related to high fiber packing degree of the pulp in the refining
zone.
3. At the same dilution water feed rate to each side i.e. the FS and DS, and
high
load on the feed screw to the FS a too high consistency is obtained compared
with the DS. This can result in the situation described in Figure 6b at the
same
plate gap. If we know that more pulp is fed to the FS it is possible to
compensate for that by controlling the dilution water feed rate to each
refining
20 zone in order to obtain the situation described in Figure 8. In this
specific case,
the dilution water is increased to the FS. It is also possible to decrease the
dilution water feed rate to the DS but this can sometimes cause in-homogeneous
refining. This can however, be followed by analyzing the inner temperature
variations as this part of the refining zone correlates to pumping effects
caused
25 by the in-homogeneities which also can be captured by analyzing the
measurements from the accelerometers on each side. Sometimes it is preferable
to change the dilution water flow rate in opposite directions to the refining
zones.
4. The remaining difference between the temperature profiles on FS (28) and DS
In (29) respectively, as seen in Figure 8, can thereafter be adjusted by
reducing the
hydraulic pressure on FS alternatively increase the hydraulic pressure on the
DS
in order to obtain the situation described in Figure 9. By using the same
arguments as above it is favorably to make the necessary changes on FS.
5. It is concluded that this procedure will result in a more symmetric
distribution
35 of pulp in the refining zone and thereby a similar consistency on the FS
and DS
respectively. This will also result in a similar residence time for the fibers
in the
refining zone and thereby a more homogeneous pulp quality. The relative local
consistency in the refining zone can preferably be estimated by using the
algorithms described in the paper "Refining models for control purposes",
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
8
Nordic Pulp and Paper Journal, 2008 but this is not required for the solution
described here.
6. If the consistency is controlled in the blow line, out from the refiners,
normally
the same control action is performed on the dilution water controller. In
present
solution of the problem a quotient between the dilution water to FS and DS
respectively can be introduced. This quotient can be maintained over long
periods and also be slightly modified to get a good balance between the FS and
DS. Thereby, both temperature profiles are changed in a similar way. In those
cases where the maximum temperature is controlled by the hydraulic pressure,
this control algorithm will adjust the temperature profiles to the right
level. No
problems with "wind up" will occur since this control loop is not affecting
the
consistency to any appreciable account, see further discussion in the paper
"Multi-rate optimal control of the TMP-refining processes, Anders Karlstrom
and Alf Isaksson, IMPC conference 2009.
7. If the pressure- or the temperature profile is not measured spatially in
the
refining zone the distribution of pulp to each zone can only be controlled
approximately by using the information from the load conveyers and the
accelerometers described in item 1 and item 2 above. This can be seen as the
last alternative when all sensors mounted in the refining zone are damaged.
20 However, no such control concepts have been implemented in refining
processes.
8. The complete procedure is preferably linked to the information about the
valve
opening for the dilution water as this information is directly related to the
back-
pressure on FS and DS refining zones, respectively. If the piping is equal on
25 each side and the valves are identical, a large opening degree corresponds
to a
high back-pressure and consequently a larger volume of pulp in the refining
zone.
9. At the same dilution water flow rate to FS and DS, it is sometimes possible
that
a larger load on the feed screw results in a higher consistency compared with
the one on the DS and this can result in a situation comparable with the one
described in Figure 6c. This is a consequence of different plate gaps on FS
and
DS, respectively. In those cases the maximum temperature will be displaced
forward the segment periphery. This situation is much more difficult to
control
compared with the one described in item 2 above. This will result in higher
35 vibration on DS as less pulp will be fed to this side. In this specific
case the
hydraulic pressure can be increased on FS and thereafter we have to
compensate and control the dilution water flow rate to each refining zone to
get
the same consistency.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
9
In Figure 10, a schematic controller for the process is shown. The unit (41)
consists of
a computer or similar electronic devices which are fed with the difference
between the
requested temperature profile vector (set point) (42) and the measured
temperature
profile vector (process value) from each refining zone (10). In the control
unit (41) the
difference vector is handled together with the algorithms for estimation of
the
distribution of chips or pulp to the refining zones by comparing the motor
loads on the
feed screws and/or the opening degree for the dilution water valves in
combination
with the difference between the temperature profiles on FS (28) and DS (29)
respectively. The difference and distribution is thereafter used for
controlling the
dilution water flow rate (43) and the hydraulic pressure (5) to each zone. In
this unit,
an acceptable distribution function is implemented (like the maximum and
minimum
difference in the distribution of the temperature profiles and/or the motor
load on the
feed screws to FS and DS).
From the process (44) the process variables (45), like the temperature profile
and
pressure profile vectors, are measured intermittently with high sampling rate
in a
measurement unit (47) required for control. The pulp is obtained from the
position
called (48).
In those cases where reliable plate gap measurement units are available, the
above
mentioned control concept can also include such devices as they can replace
the
20 hydraulic pressure and thereby give an understanding of the actual volume
in the
refining zone. This information will also correlate to the variations in the
load of the
feed screws to FS and DS which even more strengthen the concept.
Hence, the main purpose of the invention is to describe a procedure with high
25 accuracy which can present an on-line based controller for the temperature
profiles in
refiners with two refining zones by using the knowledge of the chip and pulp
distribution in each refining zone and thereby control the dilution water feed
rate to
each refining zone. The invention is thereby based on the assumption that the
temperature- and/or the pressure profile can be measure in each zone.
Different types
of functions can reflect the distribution between the FS and DS. 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.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
Brief description of the drawings:
Figure 1: Section of a stationary disc (circular plates) which is pushed
towards a
rotating disc.
5
Figure 2: Two segments where the sensor array holder, used for temperature-
and/or
pressure measurements, is placed in between.
Figure 3: Temperature profile and pressure profile as a function of the radius
in the
refining zone.
Figure 4: The sensor array holder with the sensors placed along the surface.
Figure 5a: The shape of the temperature profile before and after an increase
in the
dilution water feed rate.
Figure 5b: The shape of the temperature profile before and after an increase
in
production.
Figure 5c: The shape of the temperature profile before and after a change in
segments.
Figure 6a: The temperature profiles for a case with different plate gaps but
with the
same amount of chips or pulp to the refining zones.
Figure 6b: The temperature profiles for a case with asymmetric distribution of
chips or
pulp but with the same dilution water flow rate to the refining zones.
Figure 6c: The temperature profiles for a case with asymmetric distribution of
chips or
pulp but with different plate gaps and the same dilution water feed rate to
the refining
zones.
Figure 7: Load for the feed screws for a refiner with two refining zones.
Figure 8: Temperature profiles for a case with asymmetric distribution of
chips or
pulp to the refining zones. In this case, the dilution water feed rate to one
refining
zone is modified to get similar consistency to the corresponding refining
zone.
Figure 9: Temperature profiles for a case with asymmetric distribution of
chips or
pulp to the refining zones according to Figure 8 where the hydraulic pressure
is used
to even out the difference between the two refining zones.
CA 02769225 2012-01-26
WO 2011/002377 PCT/SE2010/000174
11
Figure 10: Schematic description of the how the different temperature profiles
are
controlled by using the dilution water feed rate and hydraulic pressure,
alternatively
the plate gap, to get similar temperature profiles and thereby similar
consistency in the
different refining zones.
