Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/20—Methods of refining
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/04—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres
  • D21B1/12—Fibrous raw materials or their mechanical treatment by dividing raw materials into small particles, e.g. fibres by wet methods, by the use of steam
  • D21B1/14—Disintegrating in mills
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/002—Control devices
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/20—Methods of refining
  • D21D1/30—Disc mills
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/20—Methods of refining
  • D21D1/30—Disc mills
  • D21D1/303—Double disc mills
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21D—TREATMENT OF THE MATERIALS BEFORE PASSING TO THE PAPER-MAKING MACHINE
  • D21D1/00—Methods of beating or refining; Beaters of the Hollander type
  • D21D1/20—Methods of refining
  • D21D1/30—Disc mills
  • D21D1/306—Discs
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/34—Paper
  • G01N33/343—Paper pulp

Definitions

• the invention relates to a procedure, where among other measurements temperature sensors are used directly in the refining zone for linked refiners to minimize the risk to get operating conditions which can cause unacceptable pulp quality.
• the procedure means that the pulp quality variations can be minimized as the pulp quality variables can be controlled by using an inner loop comprising two or more control objects in terms of a primary refiner and a secondary refiner or more refiners.
• the outer loop is designed to facilitate control of the pulp quality variables simultaneously.
• a selective estimation of the pulp quality is obtained dynamically by using a model which uses the temperature measurements in the refining zones.
• the estimation provides an inter-sampling which makes it possible to design faster control concepts compared with traditional concepts.
• the present invention is applicable in all technical areas where refiners are used, such as pulp and paper industry as well as related industries.
• 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 chemical thermo-mechanical pulp (CTMP) starting from lignin-cellulose material such as wood chips.
• TMP thermo-mechanical pulp
• CMP chemical thermo-mechanical pulp
• 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, discs, in between which the material to be treated passes from the inner part to the periphery of the plates, see Figure 1.
• 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 ( 1 1) dependent on the design of the plates.
• the diameter of the refiner plates differ dependent on size (production capacity) of the refiner and brand.
• 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.
• refiners there are also other types of refiners such as double disc, where both plates rotate counter to each other, or conic refiners.
• 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.
• 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.
• 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.
• 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 the sensor array holder (21).
• a near infra red measurement unit is sometimes installed. This unit measures the pulp consistency and is used for controlling the flow of dilution water to the refiner.
• the pulp quality is not measured in the blow-line from each refiner. Instead the pulp quality is normally measured after a large chest called the latency chest. This makes it possible to measure the pulp quality about 20-30 minutes after the treatment refiners.
• the non-linearities are affected also by the design of the segments. This can result in different temperature profiles (19, 32) and pressure profiles, see Figure 5c.
• the specific energy, E i.e. the ratio between the refiners motor load and the chip feed rate (30) F P , alternatively only the motor load, the consistency, C for each refiner below indicated by using the sub- indices p for the primary refiner and s for the secondary refiner, see below.
• Pulp quality related variables (37) e.g. the Canadian standard freeness, CSF, which normally are analyzed after the latency chest (38), see Figure 6a which shows schematically a flow sheet, is normally controlled manually without automatic control concepts.
• the elements in the output vector Y are thereby affected by the elements in the input vector U which often comprises hydraulic pressure (5) Phydr, dilution water feed rate (29) F D , and the chip feed rate (30) F P dependent on the refiner to study, i.e. the primary or the secondary refiner.
• G represents the transfer function matrices with its elements g, describing the dynamics in the system.
• the pulp properties (31) out from each refiner are not controlled and varies dependent on the conditions inside the refining zones.
• the linear function G represents a simplification of the process dynamics as it is strongly non-linear which will be commented below.
• the refining processes are often designed by using two serially linked refiners; one called primary stage (34) and one called secondary stage (35), and often also a process stage called a reject refiner (36), see Figure 6a.
• primary stage (34) and one called secondary stage (35)
• reject refiner (36) a process stage
• Figure 6a Sometimes another structure is used with parallel designs which make the control concepts more complex.
• control concepts available on the market today is presented in the thesis from the Mid university in Sweden, "Quality Control of Single Stage Double Disc Chip Refining ", Joar Liden, 2003 and US2005/0263259 where the control concept is based on a Model Productive Controller, MPC, which is used for large complex systems comprising several refiner lines but also single refiner lines and refiners.
• MPC Model Productive Controller
• Direction dependent dynamics means that the pulp quality from different step changes in the input signals (5, 29, 30, 31), see Figure 6b, will be different dependent on the direction of the step, negative or positive step, see “Realisation and estimation of piecewise-linear output-error models ", Rosenqvist and Karlstrom, Automatica (2005). In some process operating point the direction dependency not importnat while in other operating points it will be central.
• the direction dependent dynamics reflect a non-linear dynamics which can cause problems when designing control systems.
• a non-linear function (with the time constants T mt and T m2 and the gains k ml , k m2 respectively) which approximately can be describes as two linear transfer function where t and i describes the direction of the steps in the elements u m in the input vector U.
• 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.
• 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.
• 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.
• the model shows that variations in the temperature profile also affect the local consistency estimation which results in a variable irreversible defibration work and hence correlates to the final pulp quality.
• Measurement systems for pulp quality are often constituted by both hardware, such as sampling devices from the process, and image analysis systems for fiber characterization. The latter is connected to software that calculates pulp fractions which constitute for example mean fiber length (MFL) calculations.
• MFL mean fiber length
• CSF Canadian Standard Freeness
• Some results indicate that measurement of pulp quality variables using such instruments show a clear deviation from actual conditions achieved during the direct refinement of the fibers. This may, for example, be caused by two serially connected refiners, often denoted primary and secondary refiner, may be dynamically operating in a number of different ways, while at the same time various operating points may be used and this has not been taken into account in previously published patents. Deviations in the pulp quality measurement may also be caused by local fluctuations in the latency tank (38) in Figure 6a which causes inhomogeneous pulp makes the control of the quality harder.
• Figure 8 shows a signal (42) averaged over five samples (corresponding to 125 minutes) with a variation of +/- 7 ml (43) which is more acceptable.
• having such time horizons to handle is unacceptable when the pulp quality in the refining zones is set within a second.
• pulp quality characterization being uncertain, there are also other problems when control related technical solutions are to be formulated.
• a central issue that has to be addressed is the long lead times in the process.
• the lag time for a typical system with two serially connected refiners and an adjoining latency chest may be anything from 20 to 30 minutes depending on the process design.
• the present invention constitute a solution to these problems and relates to a method that uses robust temperature and/or pressure measurement directly in the refining zone combined with accessible signals from the process and a model for estimating the spatial dry content in each refining zone and/or the estimated pulp quality in order to control a complete refiner line.
• a temperature vector is formed that constitutes the so called temperature profile, see Figure 5.
• pressure profile In the case pressure sensors are used it is then called pressure profile.
• a dry content measurement is made in the blow-line from the refiners, it is preferably controlled by the dilution water flow rate.
• the physical model presented in “Refining models for control purposes” (2008), Anders Karlstrom, Karin Eriksson, David Sikter and Mattias Gustavsson, Nordic Pulp and Paper journal has turned out to be useful for calculating the dry content output from the refiners.
• a comparison between measured and predicted dry content shows that the predicted dry content correspond better with laboratory samples taken from the blowout pipe.
• the measuring device in the blow-line is unable to handle variations in dry content locally in the refining zone. Instead, the physical model must be used to access these internal states along the radius of the refining zone. This therefore means that the physical model gives access to the dry content profile in the refining zone.
• the model does though assume that the energy balance in the process uses the temperature profile or pressure profile as input variable in the calculation. A large change in , for example, the inner part of the temperature profile gives a large change in the estimated dry content in the same region, see Figure 5.
• A(q)y(t) B(q)u(t-nk)+C(q)e(t), is an "Auto Regressive Moving Average eXogenous"(ARMAX) -based equation where the output signal vector y(t) contains the signals we want to model.
• the input signal vector u(t) can, as mentioned above, for example contain some variables from the temperature profile of the refining zone, production, dilution water flow rate, and hydraulic pressure, that also may be replaced by the plate gap if it is measured, nk represents lag time for respective input signal to the output signal, e(t) is the error and A(q), B(q) and C(q) are polynomials in the operator q l .
• the output signal vector y(t) must therefore be coupled to the slowly varying part (40) of the signal that is being modeled in order to achieve the absolute value of the signal that is being modeled, such as the pulp quality variables CSF (39) or other dynamic characteristic measurement signals, see Figure 7.
• the slowly varying part of the signal is constituted by an average or a heavily filtered linear trend, see "System identification, Theory for the user ", Lennart Ljung, 2nd edition, Prentice Hall, New Jersey (1999).
• the input signal vector u(t) has a tendency to become complex and vast for cases where complete refiner lines are viewed, so it is preferably divided into vectors for the process variables V (prim) (44) and V (sec) (46), while measurement signals from the refining zones are represented by the vectors T (prim ) (45) and T (sec) (47), see Figure 9a. Another reason for this subdivision is also that sometimes not all information is needed as input signals in making the model. For certain types of refiners it is enough to only have measurement signal from the refining zone T (prim ) (45) och T (sec) (47). In other cases it may be important to also include the process variables V (prim) (44) and V(sec) (46), especially if the operating points of the refiner change over time intensely.
• ARMAX- models or other models may for example be used.
• the output signals (48, 49) from each model in combination with the slow part of the signal (51) may be added and constitute the final pulp quality vector (50).
• the lag time nk must be included in order to get a good dynamic adherence to the measured pulp quality.
• the modified output signal y mo d(t) should then be coupled with a modified slowly varying signal in order to get a new estimated pulp quality without lag time, see Figure 10.
• a slowly varying signal (56) is used, created for example from the averaged signal (42) in Figure 8 or a signal averaged over 2-10 samples, depending on which filtration is preferred.
• the quality of the pulp can be prevented from drifting outside the specifications due to changes in the process conditions.
• This is preferably done by inter-sampling the pulp quality (57) for example every minute, see Figure 10, so that it is possible to use the predicted pulp quality in various control concepts.
• the predicted pulp quality (57) is continually compared with the measured pulp quality (39) even if the estimated quality usually is more precise and better suited for control purposes.
• FIG. 1 a schematically shows the control of general processes with a traditional control concept, where a controller (C) is provided with the difference between a set point value (SP) and a measured signal (M) and then via a device which controls a process (//).
• Figure l ib schematically shows the control of the process using the new control concept.
• the controller ( ) constituted by a computer or similar electronic equipment, is fed with the difference between (the set point values (SP)) and (the actual values (PV)) of the fast estimated pulp quality ⁇ M moc j).
• the controller ( ) gives information to a distribution block (D) which divides the control signal to the interior control loops for each refiner that constitute part of the control concept.
• the interior control loops may be constituted by a number of interior control loops for controlling the spatially estimated dry content using the dilution water flow rate and/or the measured temperature profiles and/or the pressure profiles in the primary and secondary refiners, respectively.
• the temperature profiles and/or pressure profiles preferably control the hydraulic pressures (5) in respective refiner, but also in combination with supplied water (43) or chip supply (6).
• Relevant variables from the measurement sensors is the plate gap in combination with a modeled dry content at various positions in the plate gap are then fed to a model block that sums up the final estimated variation of the pulp quality (58). This signal is added to the measured signal M via a filter Mf llt if the signal qualitative of M varies a lot over time.
• a number if distribution routines may be formulated and one is given in Figure 11c, where two serially connected blocks K and R describes a distribution set by an operator and R is a routine that relates to the actual gain distribution for the refiner zones measurements/estimations at normal production.
• K the operator may thus increase or diminish the effect of R on respective internal control loop.
• the internal loop assumes measurement of some physical quantity in the refining zone, see Figure l id.
• Each control loop has its own controller C to handle the dynamics H that de facto may vary from refiner to refiner.
• the model, see Figure l ie may be described in various ways, but a common feature of all models is that is deals with fast variations on the same time scale as the internal control loops.
• each partial model F (59) is summed up and forms (55) that is then summed up with a trend signal (60), see Figure l ib and Figure l lf.
• the final modifying signal ⁇ M mod ) is then fed back to create the necessary control difference that enters the controller ( ). Note that in Figure l lf an arbitrary number of serially connected refiners and/or refiners connected in parallel that are controlled by the valid control concept.
• the calculation method above may include that too as a follow up.
• the main purpose of the invention is thus to disclose a method that with a large degree of accuracy is able to present an on-line based estimation of pulp quality while it concurrently is used for controlling the pulp quality output from the last refiner and then the latency chest.
• the invention is based on that the temperature profile and/or the absolute pressure profile can be measured in the primary and/or the secondary refiner's refining zones and/or that the spatial dry content in the refining zone can be estimated using a model.
• the method according to the present invention is not restricted to any specific device for sensing temperature or pressure in the refining zone. Such devices are however known through for example the Swedish patent 9601420-4.
• the invention of neither limited to the illustrated embodiment, but may be varied in various ways within the scope of the claims.
• Figure 1 Cross section of a stationary disc which is pushed towards a rotating disc.
• Figure 2 Two segments where the sensor array holder, used for temperature- and/or pressure measurements, is placed in between.
• FIG. 3 Temperature profile and pressure profile as a function of the radius in the refining zone.
• Figure 4 Array shaped as a parallelepiped with discrete temperature and/or pressure sensors.
• 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 Simplified view of two refiners connected to a latency chest with
• Figure 7 Example of a measured pulp quality variable in the form of CSF.
• Figure 8 Example of a signal averaged over 5 samples based on the signal in Figure 7.
• Figure 9a Simplified view of how the modeling blocks are connected and summed to represent the variation in the pulp quality.
• Figure 9b Simplified figure for describing how a modified pulp quality variables behavior changes as lag time and time constants are modified to better represent what happens in each refiner.
• Figure 10 Example of an averaged signal (DC-level) and a signal with a fast variation (AC-level) which are to be added and represent the final pulp quality.
• Figure 11a Traditional control loop concept.
• Figure 11c Distribution block distributing the output signal from the regulator to respective internal control loop.
• Figure l id Example of an internal control loop with an arbitrary number of refiners that are connected in series or in parallel.
• Figure l ie Example of a model block with subsequent summation for final pulp quality variation.
• Figure 1 If: Compilation of the entire control concept in its detail.

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Citations (4)

• 2009
  • 2009-12-21 SE SE0901588A patent/SE0901588A1/sv unknown
• 2010
  • 2010-12-20 EP EP10839881A patent/EP2517006A1/en not_active Withdrawn
  • 2010-12-20 WO PCT/SE2010/000309 patent/WO2011078760A1/en active Application Filing

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