EP2488695B1

EP2488695B1 - Procedure and system for refining a fibrous material with improved energy efficency and pulp quality

Info

Publication number: EP2488695B1
Application number: EP10720734.2A
Authority: EP
Inventors: Anders Karlström; Alf Isaksson; Lars Ledung; Simo Säynevirta
Original assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Current assignee: ABB Research Ltd Sweden
Priority date: 2009-10-14
Filing date: 2010-05-10
Publication date: 2016-09-21
Anticipated expiration: 2030-05-10
Also published as: CN102666977A; EP2488695A1; CN102666977B; WO2011045090A1

Description

TECHNICAL FIELD.

The invention relates to an improved method for controlling one or more refiners in a process section for refining a fibrous material. 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 pretreatment of fibres 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. 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 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 from 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 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 P_outlet is measured, and the point where production inlet pressure P _inlet 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 18, 19 Fig 6) were cast in one piece, but nowadays they usually consist of a number of modules that are mounted together on the stator and rotor. These segments have grinding patterns, see Figure 6, with bars 15, 15' and troughs 16 that differ dependent on supplier. The bars act with shear forces that defibrate and defibrillate chips or further refine the already produced pulp. The plates wear continuously in use and have to be replaced at intervals of around every 2 months or so. In an HC refiner, fibres, 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 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 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 of approximately 0.2-0.7 mm 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 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 for controlling plate gap, dilution water flow, and wood chips flow or pulp production. In a traditional production process, the refiners run at a constant speed for a specific product. In North America where the grid frequency is 60Hz, refiner motors are traditionally run at 1800 revolutions per minute (rpm) and in Europe and many countries of the world with a grid frequency of 50Hz, they typically run at 1500 rpm. Due to mechanical limitations (for example with a constant speed electrical machine) the refiner motor speed control is normally not even possible, and only on-off settings are possible. 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.
Control of a complete refiner line often becomes quite complex. Examples of commercially available control concepts may be found in the Licentiate thesis by Lidén [1] and in the patent application US2005/263259 (A1 ) [2]. These control concepts based on Model Predictive Control (MPC) using the traditional 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 P_hydr . Today there are plate gap sensors to measure the distance between the refiner plates 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.
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]. Figure 7 shows such a known array 21 of a number of sensors 22.
A Swedish patent SE530528 , entitled "Pulp refiner grinding gap calculating system", comprises separate or combined pressure and temperature sensors for providing data combinable with material and process variables, describes use of temperature or pressure sensors in pulp refiners. It describes a method to calculate grinding gap in a refiner as well as the central role of shear forces across the diameter of the refiner plates in determining how much work is applied to the fibrous mass to defibrillate and refine it.
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 also uses the temperature profile to control refiners.
The design of the refiner 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 blow line of the refiner, where the consistency of the 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].
WO2004076739 , entitled "Control method of a process for producing refiner mechanical pulp" describes a method controlled with a multivariable control algorithm. In the algorithm two or more of the following are used as control variables; a) mass flow of wood (via) speed of revolutions of the feed screw, b) width of the refining split, c) speed of revolutions of the rotor or its peripheral speed, d) rotation force of the motor. At least two measured variables are used of which one is tear, coupled to at least one of control variables a, b, c or d. However, although rotation force of the motor is easy to measure it is similar to motor load and has the disadvantage that the same value of motor load (or specific energy) can produce pulp of different qualities.
US2008/288090 entitled "Power Savings Method For Rotating Pulp And Paper Machinery" assigned to Johansson OM, describes a method in which the electric motor of a rotating machine is variable. The control system includes a control algorithm to optimize the efficiency of the rotating machine. The control system receives inputs of pressures, flows, consistency and position, and also receives input from a pulp quality measurement device. The motor speed is optimized relative to a pulp quality measurement such as freeness, shives, fibre length etc. However, experimental data for the control algorithm has to be established for each machine. The length of time required for measuring pulp quality such as freeness, shives, fibre length, tensile index etc means that the method is slow to react to variations in the characteristics of the incoming pulp material, chips etc., and more suitable for establishing values for a steady state process. In US6938843 , entitled "Method of high pressure high-speed primary and secondary refining using a preheating above the glass transition temperature", assigned to Andritz Inc., a method is disclosed in which refiners are run at constant speeds selected in one embodiment from a range of speeds between 1800-2000 rpm.
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 control coordinating and integrating 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.
US 6,336,602 presents, in the mechanical refining of wood chips to produce wood pulp, improving wood pulp quality by employing low refining intensity at least in a final refining stage; the refining is carried out in a double disc refiner or a single disc refiner at rotational speeds that are lower than those conventionally employed, specifically at less than 1200 RPM in a double disc refiner and at less than 1500 RPM in a single disc refiner.
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.

SUMMARY OF THE INVENTION

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. In the first aspect of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, the method comprising measuring, alternatively estimating, one or more process variables representing external states outside of the refiner for the process section and determining a value for said one or more process variables and calculating a setpoint for the variable motor speed of at least one refiner motor using a multivariable control method.
According to an embodiment of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed in a continuous manner, wherein the refiner also has 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 wherein a change is calculated for said at least one manipulated variable for said at least one refiner using a mathematical process model.
According to another embodiment of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises determining a value for one or more process variables wherein energy use by at least one refiner motor is minimum for a specific parameter of pulp quality for said at least one refiner of said process section.
According to another embodiment of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises determining a value for one or more process variables wherein energy use by at least one refiner motor is minimum for a specific parameter of pulp quality within an upper limit and a lower limit for pulp quality or consistency for said at least one refiner of said process section.
According to an embodiment of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises determining a value for at least one of the process variables hydraulic pressure (or plate gap), chip feed screw rpm, and calculating a setpoint for Inlet Pressure or variable motor speed of the at least one refiner motor using the multivariable control method.
According to another embodiment of the invention a method is described for controlling a process section for refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method further comprises calculating a process change and changing Inlet Pressure or steam valve opening 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 refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises 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 or 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 refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises 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 or 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 refining fibrous material, the process section comprising at least one first refiner with at least one motor having a variable speed controller arranged to vary the motor speed, wherein the method also comprises 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 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 or more variables using a mathematical process model, by calculating a process change 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 or 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 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 or more variables using a mathematical process model, by using the mathematical process model to minimize 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 one or 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 or 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 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 or 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 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 or 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 or more variables using a mathematical process model, by calculating a process change using a measurement in which said internal state is pressure (P₁) 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 refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the at least one first refiner is arranged with an apparatus for variably controlling the motor speed of the at least one refiner and where a control apparatus is arranged for calculating a setpoint for the variable motor speed using a multivariable control method for said at least one refiner of said process section. Said system is defined in claim 16.
According to an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the at least one first refiner is arranged with an apparatus for variably controlling the motor speed of the at least one refiner and where the at least one first refiner has 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.
According to an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the at least one drive motor of a refiner in said process section is arranged with the apparatus for variably controlling the motor speed with a control device for optimising energy use for a specific pulp quality in said at least one refiner.
According to another embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein at least one refiner in said process section is arranged with a steam valve control element for controlling a steam inlet into the said process section.
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 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 an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the process section comprises 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 an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the system further comprises one or more control units arranged as a control optimiser.
According to an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the system further comprises one or more control units arranged as a state estimator.
According to an embodiment of the invention a system is described including a process section for refining fibrous material, said process section comprising at least one first refiner having at least one motor to drive it, and at least one control unit arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner wherein the at least one first refiner is arranged with an apparatus for variably controlling the motor speed of the at least one refiner and wherein the system further comprises a control unit and/or memory storage device in which are stored one or more computer programs for carrying out a method according to claim 1.
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 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 P_hydr which results in a force on the plates or e.g. an electro-mechanically based force on the plates) and the dilution water flow F_D to the refiners as described by $Y = [\begin{matrix} E \\ C \end{matrix}] = GU = [\begin{matrix} g_{11} & g_{12} \\ g_{21} & g_{22} \end{matrix}] [\begin{matrix} P_{hydr} \\ F_{D} \end{matrix}]$
where Y represents the vector to be controlled and G a transfer function matrix which describes the process dynamics by the elements g_ij. The vector U describes an input vector with variables possible to manipulate.
The drawback with traditional control systems, such as US 7,381,303 / 263259 [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 approach 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 clear, 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.
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.

THE SOLUTION

This solution relates to achieving energy savings as well as stabilizing the product quality in a refining process by using at least one of two additional manipulated variables: Inlet Pressure (may also be expressed as steam valve opening), Motor Drive speed (which may be called refiner disc rpm). These two manipulated variables are not used in traditional refiner control.
The intention of the control actions is to stabilize the operation of the refiner, giving less variance in the quality variables. Achieving this, the setpoint can be changed without violating any quality constraints and by that the energy input into the refiner can significantly be reduced.
To implement this control innovation, the refiner is equipped with a variable speed motor apparatus, preferably an electric drive, or an electronic frequency converter, allowing flexible and continuously variable speed control. With the help of the accurate control, the right product quality can now be achieved with wide array of possible production speeds. Due to energy efficiency of an electronic variable speed drive, the speed of the refiner shall also reflect directly in the consumption of the electrical energy by the drive. Thus arranged with a variable speed controller or frequency converter, the motor speed is not limited to a multiple or other function of the power supply frequency, such as 50 Hz or 60 Hz. Neither is motor speed affected by variations in the frequency of the power supply.
For a full scale industrial refiner, the inventors suggest that refiner efficiency will initially improve with increased motor speed or disc rpm because higher rotational speed, especially at the periphery of the refiner disc, increases the shear force acting on the fibrous pulp. However, there is no reason to think that this improvement continues ad infinitum. Hence the inventors conclude that there will be an optimal speed or motor rpm per pulp property such as pulp quality or consistency measure Q, C for minimum energy use. In addition, the inventors propose that this optimal motor speed will change during the lifetime of the grinding plates of the refiner, which is typically 8-10 weeks or so. This means that the motor rpm must be varied over that period due to the wear of the grinding plates in order to maintain a given pulp quality as well as drive the refiner at optimum rpm for energy use. In addition, when running at higher motor speed, an increased amount of fibrous material can be produced, i.e. that a higher value of production, for example production amount as represented by the chip production flow, can be realized when the refiner is driven at a higher speed for the same value of specific energy (tons per energy unit). It is also advantageous to have measurements that directly or indirectly indicate the work input in the refining zone, measurements such as e.g. temperature, pressure, conductivity and/or shear force (internal states).
This innovation implements a control strategy that provides for the optimum speed set-point determination of the refiners, combined with the optimized and accurate control of the refiners to run within the set control boundaries for speed and product quality. The speed control is arranged such that the speed setpoint may be varied in a smooth or continuous fashion.
The control strategy is implemented through manipulated variables, for example motor speed and/or inlet pressure that may be changed instantaneously. The control strategy provides for the optimum speed set-point determination of the refiners within the set control boundaries for speed and product quality and optimizes energy continually and without delay despite variations in the pulp or fibre characteristics (e.g. chip moisture content) as well as changes in any other process inputs. Thus this control strategy can be implemented as a real-time control system to optimize the process on-line and avoiding delays for quality measurements or off-line quality sample testing and the like.
In an advantageous embodiment of this invention the internal states, ζ, i.e. information (estimated or measured) directly from the refining zone may also be used to play 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, η, 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. $x = [\begin{matrix} ζ \\ η \\ Q \end{matrix}]$
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 u_i and the state vector ẋ_i may be described by a set of nonlinear differential equations characterised by a vector valued nonlinear function ${\dot{x}}_{i} (t) = f_{i} (x_{i} (t), u_{i} (t))$
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-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 embodiment of the 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, 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. P_inlet as described by Sikter [6].
Typical examples of external states, η, 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 and infrequently 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 from 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 η as the sampling rate will be fast enough. The setpoints for the pulp quality variables may be configured as an upper limit and a lower limit for pulp quality or consistency.
Note, external states like the motor load or combinations of motor load and other external states 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 however be used as a constraint to the optimization.
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 P_hydr 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 line 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 D ₁ into account (which is typically in the order of 5-10 s) we have ${\dot{x}}_{2} (t) = f_{2} (x_{1} (t - D_{1}), x_{2} (t), u_{2} (t))$
What is actually measured of course varies from installation to installation.
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 as used in a preferred embodiment or development 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 for example by temperature and/or pressure measurements, are primarily used to minimize the variations in pulp quality and/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 a variable speed motor controller, which in a preferred embodiment used together with 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 with the implementation of the accurate process control, the right product quality can now be achieved with a wide array of possible production speeds. Due to this fact and to the energy efficiency of the variable speed drive, the speed of the refiner shall also reflect directly in the consumption of the electrical energy by the drive. In this way the specific energy (energy per ton of fibrous material produced) is minimized for a specific pulp quality parameter. In practice the motor speed will vary only a small amount around a nominal or previously fixed speed of say 1500 rpm or 1800 rpm for refiners designed to be driven at those speeds. However for refiners that are designed according to a design allowing for variable speed drive motors the amount of speed variation may be within a greater range than, say, within +/- 10-20% of a nominal or preferred motor speed.
In addition, in a preferred embodiment and 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 embodiment 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 embodiment the method:

a) involves the temperature (and possibly force) measurements in the refiner, but the motor load is not necessary. 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 refining process for fibrous material comprising a first refiner having a motor speed controller arranged in the process therewith, according to an embodiment of the invention;
Figure 2 shows the invention according to Figure 1 in which the diagram also shows a process section comprising a refiner but one that is further arranged with sensors in the refiner for providing additional control options according to an embodiment of the invention;
Figure 3 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 5 shows a schematic flowchart of the invention according to Figure 1 or Figure 2 or Figure 3 and in particular steps for carrying out a method according to an embodiment of the invention; and
Figure 8 shows a schematic diagram of the invention according to Figure 1 in which the process section is shown having a refiner arranged with a variable speed drive or speed controller, according to an embodiment of the invention;
Figure 9 shows the invention according to Figure 1 in which the diagram also shows a process section comprising a refiner arranged with a continuously variable speed motor and in particular where the location of the speed controller has been left open, according to an embodiment of the invention.

Prior Art

Figure 4 shows a schematic diagram for a known primary refiner, Figure 6 shows a known refiner grinding plate arranged with temperature or pressure sensors or conductivity sensors, and Figure 7 shows a known array of temperature sensors (or pressure sensors or conductivity sensors) arranged on a refiner plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a schematic diagram for a method of controlling a refiner for refining a fibrous material, such as for example, a TMP pulp refiner. The diagram shows a process with a single refiner 33, and a first control unit 32. A variable speed drive controller 11 for controlling the speed of the motor 10 (shown in Figs. 4, 8) driving the primary refiner 33 is shown. 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 P_hydr . pressing together the refiner plates, flow of wood chips indicated as F_P (wood chips indicated as 5, Fig 4) and dilution water 2 indicated as F_D , (water indicated as 2, Figs 8, 9) as inputs to the process 33. Refined pulp is produced from the process 33. External variables 37 and 38 representing pulp consistency C from the process and quality Q from sampling unit 35 are input to a second control unit, a state estimator 39. The state estimator calculates and sends 40 a state estimation x̂ 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.
Figure 8 shows a refiner similar to that shown in the Prior Art Figure 4 but with the crucial difference that the refiner motor 10 is arranged with a variable speed controller 11, or drive controller, to allow variable rotation speeds to be used as a manipulated variable. The measured or estimated motor speed may be compared to the calculated setpoint for rotation speed ω _m . Thus the refiner 1 of Figure 8 is otherwise similar and raw material, e.g. wood chips 5 or already treated pulp transported via one or more screw feeders 7, enters at the center C of the refiner. Before entering the actual refiner the raw material is normally mixed with dilution water 2 whose flow is usually measured and controlled. The material is then treated on its way to the periphery of the refiner plates. In single-rotating disc refiners, 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 speed of which is/are variable and controlled by speed controller 11. 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 P_outlet is measured, and the point where production inlet pressure, P _inlet 8, may be measured.
For the primary refiner the manipulated input variables are summarized below in the vector u₁ , $u_{1} = [\begin{matrix} F_{P} \\ F_{D} \\ F_{hydr} \\ ω_{m} \end{matrix}]$
where F_P denotes the chip production flow, F_D the dilution water flow, P_hydr the hydraulic pressure applied on the refiner plates and ω_m is the rotation speed setpoint for the refiner. Notice that there are other potential means to manipulate the force pressing the plates together, such as using electromechanical 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 variable in the control described in this invention. However, in the sequel P_hydr will be used as a variable to describe all such plate gap changes. For some types of refiners the inlet pressure P_inlet is preferably also available as manipulated variable.
Similarly the corresponding process state variables are given in x₁ $x_{1} = [\begin{matrix} ξ_{1} \\ T_{1} \\ C_{1} \\ Q_{1} \end{matrix}]$
where ξ ₁ is the force (or soft sensor of force) measured inside the refiner, T₁ an array of temperature measurements along the radius inside the refiner, C₁ the blow-line consistency and Q₁ a vector of pulp quality variables such as Canadian standard freeness CSF and mean fiber length MFL. An alternative to ξ₁ or T₁ is to use e.g. the pressure P₁ or shear force f_s inside the refiner as a state variable.
The relationship between u₁ and ẋ₁ may be described by a set of nonlinear differential equations characterised by a vector valued nonlinear function f₁ ; ${\dot{x}}_{1} (t) = f_{1} (x_{1} (t), u_{1} (t))$
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 u_1c the following linear differential equations approximately describe the inputs to the refining zone $τ {\dot{u}}_{1} = - u_{1} + u_{1 c}$
The time constant τ 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 $U_{1} = \frac{1}{τs + 1} U_{1 c}$
Figure 3 shows a schematic block diagram for a method of controlling a refiner or 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 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 P_hydr . pressing together the refiner plates, flow of wood chips F _P 5 , and dilution water F _D 2 as inputs to the first refiner 33; and hydraulic pressure P_hydr and dilution water flow F_D , 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_R 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. Referring also to Fig 8, a variable speed drive controller 11 for controlling the speed of the motor 10 is shown. The diagram of Fig 8 also shows an input 44 to the state estimator 39, where input 44 is the speed of the refiner as driven by the motor and controlled to a refiner speed setpoint ω _m . The input 44 may be provided to other control units in the processes 33, 34. The state estimator calculates and sends a state estimation x̂ 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. In addition and according to another embodiment, values for a temperature profile or a pressure profile (or the shear forces) from inside the refiner at T1 (primary) 36 and T2 (secondary) 36" representing Internal states and made using sensors as shown 21 in Fig 7, fed to the state estimator 39 may be used to control the process.
It can be seen that Figure 2 shows a similar schematic diagram to Figure 1 with the exception that measurements 36 of temperature (or pressure or shear force or conductivity, but not limited to these measurements) are provided from inside the refiner, the measurements denoted here as T₁. These Internal values 36 representing the temperature profile or a pressure profile or other measurement from inside the refiner using sensors as shown 21 in Fig 6, are fed to the state estimator 39 as well as the other values of consistency 37, quality 38 and refiner motor speed, shown as 44 in Figs 8, 2.
Commonly two refiners or more are arranged together in a process, as described for example in relation to Figure 3 above. For the secondary refiner the manipulated variables are $u_{2} = [\begin{matrix} F_{D} \\ P_{hydr} \\ ω_{m} \end{matrix}]$
which again are dynamically related to the controller output as $τ {\dot{u}}_{2} = - u_{2} + u_{2 c}$
Typical process state variables for the secondary refiner are $x_{2} = [\begin{matrix} ξ_{2} \\ T_{2} \\ C_{2} \\ Q_{2} \end{matrix}]$
Again an alternative state variable is the pressure P₂ or shear force inside the refiner.
If the blow-line between the primary and secondary refiners is considered static ${\dot{x}}_{2} (t) = f_{2} (x_{1} (t - D_{1}), x_{2} (t), u_{2} (t))$
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 $y = [\begin{matrix} T_{1} \\ T_{2} \\ C_{1} \\ Q_{2} \end{matrix}]$
where T ₁ and T ₂ are the temperature measurements (possibly vector valued) in primary and secondary refiner, respectively, C ₁ the measured blow-line consistency out of the primary refiner and Q ₂ the pulp quality after the secondary refiner. The setpoints for any of the pulp quality variables may be configured as an upper limit and a lower limit.
Here all signals may be measured every second except the final pulp quality Q ₂ which is measured using a sampling analyzer equipment, typically needing 5 minutes per sample. Furthermore the same equipment may serve several measurement points which is 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 $\dot{x} (t) = f (x (t), u (t))$
$y (t) = h (x (t), u (t))$
where $u (t) = [\begin{matrix} u_{1 c} \\ u_{2 c} \end{matrix}]$
and the state variable x(t) is built up by x ₁(t) and x ₂(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 e.g. 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 $x_{k + 1} = g (x_{k}, u_{k}) + w_{k}$
$y_{k} = h (x_{k}, u_{k}) + v_{k}$
where the integer k denotes the k:th signal value which is available at time t = kT_s , where T_s is the sampling interval, i.e. we have for example x_k = x(kT_s ).
Then moving horizon estimation corresponds to minimizing $\min {(x_{k - M} - {\hat{x}}_{k - M})}^{T} P^{- 1} (x_{k - M} - {\hat{x}}_{k - M}) + \sum_{n = k - M}^{k} w_{n}^{T} R_{1}^{- 1} w_{n} + v_{n}^{T} R_{2}^{- 1} v_{n}$
subject to, for example, $x_{\min} \leq x (k) \leq x_{\max}$
Here P, R ₁ and R ₂ 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_s ,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 u_k . The first penalty term in the criterion is to create a link from one optimization window to the next, where x̂_k-M 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 y_k 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 r_k are compared with some subset or combination of the state variables m(x_k ) calculated by use of the mathematical process model. A formulation of the optimization objective may be, for example, $\min \sum_{k = 0}^{N_{y}} {(r_{k} - m (x_{k}))}^{T} W_{y} (r_{k} - m (x_{k})) + \sum_{k = 0}^{N_{u}} Δ u_{k}^{T} W_{u} {Δ u}_{k}$
subject to, for example, $x_{\min} \leq x_{k} \leq x_{\max}$
$u_{\min} \leq u_{k} \leq u_{\max}$
${Δ u}_{\min} \leq {Δ u}_{k} \leq {Δ u}_{\max}$
Here the optimization is done using Δu_k = u_k - u _k-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 C ₁ and the pulp quality after the secondary refiner Q ₂. Here the objective is also to minimize energy usage which may, for example, be represented by specific energy E, either measured via motor loads or estimated using the internal states. Thus a possible objective function is $\min \sum_{k = 0}^{N_{y}} w_{1} {(C_{ref} - C_{1} (k))}^{2} + w_{2} {(Q_{ref} - Q_{2} (k))}^{2} + w_{3} E (k) + \sum_{k = 0}^{N_{u}} Δ u^{T} (k) W_{u} Δ u (k)$
subject to constraints as above. The objective function is in no way restricted to this choice. Instead of minimising energy use, it is also of course possible to minimise energy cost, for example, by letting $w_{3} = w_{3}^{'} c (k)$
where c(k) is the forecasted future electricity price (for example within a 24 hour period). An interesting alternative is to only have a constraint on the quality (min or max depending on what quality variable is chosen), and instead use the objective function $\min \sum_{k = 0}^{N_{y}} w_{1} {(C_{ref} - C_{1} (k))}^{2} + w_{2} E (k) + \sum_{k = 0}^{N_{u}} Δ u^{T} (k) W_{u} Δ u (k)$
subject to (assuming lower quality limit) $Q_{\min} \leq Q_{2} (k)$
$x_{\min} \leq x_{k} \leq x_{\max}$
$u_{\min} \leq u_{k} \leq u_{\max}$
${Δ u}_{\min} \leq {Δ u}_{k} \leq {Δ u}_{\max}$
Notice that since the number of manipulated variables is typically more than 2 and sometimes as many as 6, it should be possible to have setpoints on more than 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. Yet another alternative would be to organize the optimization in multiple levels; for example as one top level finding optimal setpoints for temperature that minimizes energy and one lower level using setpoints corresponding to the objectives described above.
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 linear, resulting in a model of the form $x_{k + 1} = {Ax}_{k} + {Bu}_{k}$
$y_{k} = {Cu}_{k}$
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 5 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, from consistency 37, from quality 38 and refiner (motor) speed 44. 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 x̂. 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 N_u. 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. In another embodiment measured values 36 of Internal states from the temperature or press sensors 22 inside the refiner may be included in step 52 to form part of the estimated states in step 53.
According to another embodiment, production amount as represented by the chip production flow, denoted above as F_P may be added as a manipulated variable for calculating a change using a multivariable control method to control or optimize the refiner process.
The methods 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 as described above and/or as described in relation to Figure 5.
Methods of the invention may also be practised, for example during an installation, or configuration phase, manually during operations, or remotely during any stage, by means of a Graphical User Interface, a graphical or textual or mixed display on an operator workstation, running on a user's logged-in computer. The user's computer may be set up connected directly to a process control system, or configured in some way as a remote workstation. Such a logged in workstation may be used to monitor energy consumption dependent on a motor speed setpoint ω_m, or motor speed 44, and/or monitor a temperature profile or pressure profile inside the refiner provided by sensors 22 and displayed by a GUI or other HMI arranged to display the parameters and values named in this description.
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 5, 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 processors. 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 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 starting values, historical measurement data or process 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, particulary as regards different selections of control variables in combination with different selections of internal variables without departing from the present invention as defined in the appended claims.

REFERENCES

[1] J. Lidén, 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. Karlström and P. Engstrand. System for continuously measuring pressure and temperature in the beating zone of refiners. European patent application EP 0 788 407 , granted 10 February 1999.
[4] A. Karlström, 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.O 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.

Claims

A method for controlling a process section for refining of fibrous material, said process section comprising at least one first refiner (33) driven by at least one motor (10), and having a control unit (32), said method comprising measuring, alternatively estimating, one or more process variables representing external states outside of the refiner (η₁ Q₁ ) for said process section, and the possibility to manipulate at least two variables (u ₁) using a multivariable control method, and calculating changes for said at least two manipulated variable (u ₁) for said at least one refiner using said measurement, alternatively estimate, of an external state (η₁, Q₁ ) and wherein a setpoint (ω_m) for any motor speed within a predetermined range of motor speeds for a variable motor speed of at least one refiner motor is one of said manipulated variables (u ₁), and wherein a setpoint (P_inlet) for inlet pressure is one of said manipulated variables (u ₁).
A method according to claim 1, characterised by calculating a value for said one or more manipulated variables wherein energy use by said at least one refiner motor is minimum for a specific parameter of a pulp property or quality (Q₁ C₁ ) for said at least one refiner of said process section.D2, D4
A method according to claim 1, characterised by calculating a value for said one or more manipulated variables wherein energy use dependent on a variable energy price by said at least one refiner motor is minimum for a specific parameter of a pulp property or quality (Q₁ C₁ ) for said at least one refiner of said process section.
A method according to claim 1, characterised by calculating a setpoint (ω_m) for a motor speed within a predetermined range of motor speeds for the variable motor speed which is calculated independent of a frequency of the electric power supply to the motor (10) or variation of that frequency.
A method according to any one of claims 1-4, characterised by determining a value or a setpoint for at least one of the manipulated variables hydraulic pressure (P_hydr ) or plate gap, dilution water flow F_D , chip feed screw rpm F_p , and calculating a setpoint for inlet pressure P_intlet or variable motor speed (ω _m ) of the at least one refiner motor using said multivariable control method.
A method according to claim 1, characterised by calculating a process change and changing inlet pressure (P_inlet ) or steam valve opening to affect a change in a measure or estimate of a pulp property or pulp quality (Q₁ , C₁ ),
A method according to claim 1, characterised by calculating said manipulated variables (u ₁) using a mathematical model.
A method according to any one of claims 1-4 , characterised by calculating said manipulated variables (u ₁) based in addition on one or more values representing one or more internal states (ζ₁) inside said at least one first refiner.
A method according to claim 1 or 7, characterised wherein the mathematical model is described by a set of non-linear differential equations or difference equations with a vector valued non-linear function.
A method according to claim 1 or 7, characterised wherein the mathematical model is described by a set of linear differential equations or difference equations.
A method according to claim 1 or 7, characterised wherein the mathematical model is described by Laplace transforms and transfer functions.
A method according to claim 1, characterised 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.
A method according to claim 1, characterised by calculating a process change by means of the multivariable control method for a second refiner (34) of said process section using at least one manipulated variable of the group of: motor speed ω_m, hydraulic pressure P_hydr , dilution water flow F_D .
A method according to claim 1, characterised by calculating a setpoint change for at least one manipulated variable of a process section comprising two or more refiners (33, 34) using at least one measurement of an internal state (ζ₁, ζ₂) for at least one refiner and a measurement of an external state (η₁, Q₁ ) for at least one refiner and at least one pulp property or pulp quality measurement (Q₂ , C₂ ) after the second refiner.
A computer program for controlling a process section for refining of fibrous material, comprising at least one code section that involves software code portions or computer code directly loadable into the internal memory of a digital computer to cause a computer or processor to carry out the steps of a method according claim 1.
A system including a process section for refining fibrous material, said process section comprising at least one first refiner (33) driven by at least one motor (10), and having a control unit (32) arranged to receive measurements or estimates of one or more process variables representing external states outside of the refiner (η₁, Q₁ ) and to manipulate at least two variables (u ₁) for use in a multivariable control method, and an apparatus (11) for variably controlling the motor speed (44) of the at least one refiner motor and the control unit arranged for calculating a setpoint (ω_m) for any motor speed within a predetermined range of motor speeds for the variable motor speed for said at least one refiner of said process section and a setpoint (P_inlet) for inlet pressure, wherein said setpoint (ω _m ) for any motor speed and said setpoint (P_inlet) for inlet pressure are said manipulated variables.
A system according to claim 16, characterised in that the at least one drive motor of a refiner in said process section arranged with the apparatus for variably controlling the motor speed (44) with a control device (32) for optimising energy use for a specific pulp property or pulp quality in said at least one refiner.
A system according to claim 16, characterised in that the at least one drive motor of a refiner in said process section arranged with the apparatus for variably controlling the motor speed (44) with a control device (32) for optimising energy use dependent on a variable energy price for a specific pulp property or pulp quality in said at least one refiner.
A system according to claim 16, characterised in that the at least one drive motor of a refiner in said process section arranged with the apparatus (11) for variably controlling the motor speed (44) with a control device (32) for optimising energy use dependent on an optimal energy cost for a specific pulp property or pulp quality in said at least one refiner.
A system according to claim 16, characterised in that the apparatus (11) for variably controlling the motor speed of the at least one drive motor of a refiner is a variable speed controller that controls speed of the motor (10) independently of a frequency of the electric power supply or variation of that frequency.
A system according to any one of claims 16-19 , characterised in that the at least one drive motor of a refiner in said process section arranged with the apparatus for variably controlling the motor speed (ω _m ) is also arranged with a state estimator (39).
A system according to claim 16, characterised in that the system comprises a variable motor speed controller (11) arranged as an electronic frequency converter.
A system according to claim 16, characterised in that at least one refiner in said process section is arranged with a steam valve control element for controlling a steam inlet Inlet Pressure P_inlet into the said process section.
A system according to claim 16, characterised in that at least one refiner in said process section is arranged with a plurality of sensors (22) arranged in predetermined positions in a sensor array (21) for measurement of values representing internal states inside said refiner.
A system according to claim 24, characterised in that one or more of said plurality of sensors (22) are arranged on an active radius of a beating disc of a refiner in said process section.
A system according to claim 16, characterised in that the system further comprises a memory storage device in which are stored one or more computer programs for carrying out a method according to claim 1.
Use of a system according to any of claims 16 - 26 to control a process section handling a fibrous material comprising any from the group of: pulp, paper, wood pulp, cellulose pulp.
Use of a system according to claim 27 to optimize an economic cost of energy use dependent on a variable energy price for one or more process sections.