Technical field:

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 condition which can cause unacceptable pulp quality.

The procedure means that the pulp quality variations can be minimized by using pre- specified operating conditions visualized by one or several operating windows.

The present invention is applicable in all technical areas where refiners are used, such as pulp and paper industry as well as related industries.

Technical background: Refiners of one sort or another play a central role in the production of high yield pulp and for pre-treatment of fibers in paper-making for the pulp and paper industry and related industries through grinding, for example, thermo-mechanical pulp (TMP) or 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, discs, in between which the material to be treated passes from the inner part to the periphery of the plates, see Figure 1. Usually, there is one static refiner plate (1) and one rotating refiner plate (2), rotating at a very high speed. The static refiner plate is placed on a stator holder (3), and is pushed towards a rotating one place on a rotor holder (4), electro mechanical or hydraulically (5).

The chips or fibers (6) are often fed into the refiners together with the dilution water via the center (7) of the refiner plates and are grinded on its way outward to the periphery (8). The refining zone (9), between the plates (also called segments) has a variable gap (10) along the radius (11) dependent on the design of the plates.

The diameter of the refiner plates differ dependent on size (production capacity) of the refiner and brand. Originally the plates (also called segments 12, 13, see Figure 1 and Figure 2) were cast in one piece, but nowadays they usually consist of a number of modules (forming a disc) that are mounted together on the stator and rotor. The segments can be produced to cover the entire surface from the inner to the outer part of the holders or be divided into one inner part (14) often called "the breaker bar zone" and an outer part (15) called periphery zone.

These segments have grinding patterns (16), with bars (17) and troughs (18) that differ dependent on supplier. The bars can be seen as knives that defibrillate chips or further refine the already produced pulp. The plates wear continuously during the refining process and have to be replaced at intervals of around every 2 months or so. In an HC refiner, fibers, water and steam are also transported in the troughs between the bars. The amount of steam is spatially dependent, which is why both water and steam may exist together with chips/pulp in the refining zone. In an HC refiner water will normally be bound to the fibers. Dependent on the segment design different flow patterns will occur in the refiner. In an LC-refiner no steam is generated and thereby only two phases exist (liquid and pulp).

There are also other types of refiners such as double disc, where both plates rotate counter to each other, or conic refiners. Yet another type is called twin refiners, where there are four refiner plates. A centrally placed rotor has two refiner plates mounted one on either side, and then there are two static refiner plates that are pushed against each other using, for example, hydraulic cylinders thus creating two refining zones. When refining wood chips or previously refined pulp the refiner plates are typically pushed against each other to obtain a plate gap (10) of approximately 0.2-0.7 mm dependent on what type of refiner is used. The plate gap is an important control variable and an increased or reduced plate gap is performed by applying an electro mechanical or hydraulic pressure (5) on one or several segments dependent on the type of refiner. With that an axial force is applied on the segments. The force which acting in opposite direction to the axial force consists in HC-refining processes by the forces obtained from the steam generation and the fiber network. In those cases LC-refining is considered the axial force is neutralized by the forces extracted from the increased pressure in the water (liquid) phase and the fiber network. If the plate gap is changed for example 10%, the pulp quality is changed considerably.

One system available on the market today is based on temperature measurements along the radius in the refining zone to visualize the temperature profile (19) alternatively the pressure profile (20) for control purposes, see Figure 3. For LC- refining the pressure is preferred to be measured but for HC-refining the temperature profile will be enough to measure.

When changing the process conditions in plate gap, production and the amount of added dilution water, the temperature is changed which gives an opportunity to control it in different ways. Several temperature- and/or the pressure sensors are often used and can be placed directly in the segments alternatively mounted in a sensor array holder (21) which can be placed between the segments (12 and 13), see Figure 1, Figure 2 and figure 4 as described in EP 0788 407. Usually, the sensor array holder is implemented between two segments in the outer part, see Figure 2. The design of the segments has proven to be of great importance for characteristics of the temperature profile along the radius. Therefore, it is difficult in advance to decide where the temperature sensors (22) and/or the pressure sensors (22) should be placed in the sensor array holder (21). In the outlet from the refiners, preferable the primary refiner, 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 in the refiners.

In the literature, temperature measurements have shown to be an unusual robust technology for HC-refining control US2000/6024309. When changing the production, dilution water and the hydraulic pressure the temperature profile is changed dynamically. This dynamic change is visualized in Figure 5a, where a step change in dilution water affect the temperature profile in different ways dependent on where on the radius (11) we consider the process. It is seen that when the dilution water increase, the temperature (23) will decrease before the maximum (24) but increase (25) after the maximum. The reason is that the water added cools down the back- flowing steam at the same time as the steam which is going forward is warmed very fast.

When the production is increased the entire temperature profile (19) is lifted to another level (26), see Figure 5b. The same situation is valid when the plate gap (10) is reduced by increasing for instance the hydraulic pressure.

The non-linearities are affected also by the design of the segments. This can result in different temperature profiles (19, 32) and pressure profiles, see Figure 5c.

In traditional control concepts for refiner control, 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. ^Y P =

where G represents the transfer funcition matrices with its elements g _y - 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 6. Sometimes another structure is used with parallel designs which make the control concepts more complex.

Examples of 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 ", J oar 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.

These concepts are not based on measurements obtained directly from the refining zones as described in US2000/6024309. Instead these concepts are focused on available process variables which are measured outside each refiner.

In some research projects prediction of pulp quality out from the refiner lines has been performed off-line. The method used has been based on "Auto Regressive Moving Average eXogenous"(ARMAX) modeling procedures, for details see "System identification, Theory for the user", Lennart Ljung, 2nd edition, Prentice Hall, New Jersey (1999), and can be seen as a subset of a number of system identification tools available on the market today. The off-line trials resulted in an article "Refining zone temperature control: A good choice for pulp quality control?", Karin Eriksson och Anders Karlstrom, IMPC09, 2009 where the dynamic effects on the pulp quality were studied by using a new type of ARMAX-modeling procedure which still can be characterized as a state space model which is easy to translate to transfer functions G if required. The aim was to investigate if an empirical correlation exists between the pulp quality and step changes in dilution water flow rate, production and hydraulic pressure from a refiner. The primary result obtained was that the prediction of the pulp quality was slightly better when information from the temperature profile in the primary refiner was included in the vector U.

All refiners are different in terms of construction, types of refiner segments and lon- linearities in the process which has been documented in "Refining models for control purposes" (2008), Anders Karlstrom, Karin Eriksson, David Sikter and Manias Gustavsson, Nordic Pulp and Paper journal.

The model, which describes the HC-refining requires the knowledge about the temperature profile and/or the absolute pressure along the radius of a refining segment in order to span the material and energy balances and thereby estimate the plate gap, see the Swedish patent 0502784-2. The model, however, does not show how the temperature profile is affected by the wood chip mixture or any other disturbances. As the energy distribution in each refiner, normally two serially linked refiners called primary and secondary refiners, is not estimated in the model a clear suggestion how the required amount of power should be split in the refiners is impossible to get.

Technical problem

In the literature, extensive materials exist regarding refiner control by using consistency measurements and temperature measurement including safety systems to prevent plate clash of segments. The safety systems are often built on both hardware (typically accelerometers and plate gap sensors) and software in terms of frequency analyzers and specific functions for limit control et cetera.

Reports on refiner control using measurements of pulp quality are, however, not so common. This has resulted in situations where not enough data is available to perform overall analyses of how the variations in e.g. wood chip content in details affect the refining conditions.

Measurement systems for pulp quality are most often empirically based for the estimated Mean fiber length (MFL), Canadian Standard Freeness (CSF) and shives and the measurement systems are often required to be calibrated to get information valuable in the analysis of the refining conditions in terms of defibration and fibrillations of the fibers.

As described above, refiner lines are often built on two serially linked refiners (primary and secondary refiners) or refiners in parallel and most often also a reject refiner which can be common for several production lines.

The refiners can be run in many different ways and dynamically different operating points can be used. This means that the pulp quality is set in both the primary and secondary refiners which must be taken into account in control applications.

As the pulp quality is measured after the latency chest (38), see Figure 6, it indeed difficult to decide how the energy input to the production line should be distributed in the primary and secondary stage to reach a specified pulp quality.

During a long time it has been believed that the pulp characteristics and thereby indirectly the final pulp quality after the secondary refiner can be controlled by the specific energy, i.e. the ratio between the sum of the motor loads and the production rate, and this is of course a simplification which has been commented on in "Towards Improved Control of TMP Refining Processes", Karin Eriksson, 2009, Dept. of Signals and Systems, Chalmers Univ. Of Technology.

Normally, in situ measurements are not installed in the refining zones which have been a problem during a long time. However, lately technics for temperature and/or pressure measurements in the refining zone have been introduced on the market and now the research results can be used for optimizing the processes. This statement is important for the patent application.

The problem to face, when neither temperature nor pressure measurements are available is that the energy split in the refiners are difficult to set. Usually the refiners are loaded to the production limit without taking notice of the process from a system perspective.

On-going research shows that the temperature profile and/or the pressure profile are affected differently dependent on the type of refiner, pattern of the refiner segments, were in the refining zone the fibers are defibrated/fibrillated et cetera. This has not, so far, been used for classification of required operating conditions in each refiner to reach a specific pulp quality.

Another problem which has not been considered is that the variations in the raw material will affect the consistency in the refining zones. Earlier, the operators have been satisfied with a good enough pulp quality within a certain specification, often described by CSF and MFL. The spread can be large as seen in Figure 7. It is, of course, a desire to minimize the operating window (39) to a smaller window (40) which has been difficult to perform earlier.

Besides the variations in the raw material fed into the refiners, all controllable process variables like dilution water feed rate, hydraulic pressure or mechanical pressure on the segments which affect the plate gap, will affect the active volume in the refining zone. This will result in a complex pattern of process conditions which must be controlled in one way or another. This has been a problem as the variables are presented in time domain only which makes it difficult to see how the fluctuations affect and differ from normal process disturbances, see Figure 8.

So far, no results have been published where both primary- and secondary refiners are running with sensor arrays for temperature and/or pressure measurements which will be vital for the technical solution described below.

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.

The invention is based on a procedure where robust temperature- and/or pressure measurements in combination with available signals from the process, to control the complete production line for different raw material fed to the refiners. The refiner line comprises at least two refiners called primary- and secondary refiners. The different raw materials which affect the process are sometimes related to the amount of saw mill chips in contrast to other situations where virgin chips are used.

To characterize the process conditions it is not enough to measure only the refining zone temperature in the primary refiner. Instead it is important to measure the temperature- and/or the pressure profiles from both the primary and secondary refiners as both refiners are affected by the disturbances in the feed stock and thereby also produce different pulp qualities. In those cases where the maximum temperature is used for control it is important to measure the temperature along the radius in the refining zone. Otherwise it will be difficult to find the maximum. In some cases also a reject refiner can be included in the concept if it significantly affects the final product, i.e. the pulp quality. It is known that the refiners can be controlled by a cascade control concept see "Quality Control of a Newsprint TMP Refining Process based on Refining Zone Temperature Measurements", David Sikter, 2007, Dept. of Signals and Systems, Chalmers Univ. of Technology, which describes a system where only the primary refiner is controlled by using the maximum temperature.

When both refiners are controlled a complex MIMO(Multi-Input-Multi-Output) system will be obtained with a consistency control and a temperature control in each refiner. In summary the complete production line, if it consists of two serially linked refiners, will be controlled by at leas four input signals and four outputs. If the maximum temperature (T^) along the profile is controlled the process description becomes

For each refiner where the hydraulic pressure P _hydr and the dilution water flow rate F _D are considered as element in a vector comprising the inputs U and the variables and the consistency C belongs to the vector Y. The advantage with this description is that the anti-diagonal elements in the transfer function G will be negligible as the impact from the dilution water on is small at the same time as the plate gap changes do not affect the consistency to any appreciable account. This description can thereafter easily be implemented in a control context for each refiner. Note, in this summary the is used as it can be found by using the sensor array (21) and its temperature sensors (22), see Figure 4. This means that in those cases where a sensor fails to measure a proper value, another sensor near the maximum temperature can be used to represent T^. Hence, from a control perspective, the does not necessarily need to give the exact physical maximum temperature along the radius.

It is obvious that the distribution of the energy in the primary and secondary refiners must refer to the requested pulp quality to produce instead of refining the pulp on the edge of the capacity which is the common situation of today. Besides the need to produce a good pulp quality it is also important to minimize the risk for machine damage et cetera. The best way to prevent problems like that is to define an operating window (40), see Figure 7, which defines the boundary valus for the process. In Figure 7 two pulp quality variables are given an span the operating window. This discussion can also be expanded and comprise specific operating windows for other variables like motor load or T _max according to the discussion above. Hence, instead of controlling each refiner as an individual process the solution is to comprise the complete production line using an operating window. Therefore, the operating window should be described according to the variables obtained in the primary and secondary refiners, respectively. Using this, a study of the process conditions impact on the pulp quality at different raw material mixture can be performed. Below an example is given:

By visualizing the T _max in the secondary refiner versus T _max in the primary refiner, see Figure 9, a more complete picture of the situation at different raw material mixtures can be obtained. The names R21 (primary refiner) and R22(secondary refiner) have been included in Figure 8 to clarify which refiner to follow.

In Figure 9, data obtained for two different raw material mixtures (41a and 41b) are given and it is obvious that the data sets are separated from each other. This is interesting as T _max is used instead of the motor load which data sets are difficult to separate from each other, see Figure 10.

The fact that the temperatures in the refiners show a distinct classification for different raw materials and process conditions have not been known until now and will be further studied in future research projects.

The fact that the motor loads cannot be clearly associated with the raw material mixture is one of the reasons why it has been difficult to classify the process conditions earlier.

Hence, by using the operating window for _, the operators can pre-design the distribution between the primary and secondary refiners. This makes it easy to get a fast start up procedure to get requested process conditions after a production stop.

By implementing a simple PID-control concept for controlling T _max , using the hydraulic pressure or mechanical pressure applied on the holders, the control error ( i.e. the difference between the set point (SP) and the process values (PV)) can be obtained. This is shown in Figure 11 where the control error in T _max can be assumed to land within an interval of +/- 1 degree Celcius.

When the maximum temperatures in the primary and secondary refiner are controlled it is obvious that the process can be controlled to a pre-specified operating condition (43), see Figure 12, which approximately corresponds to the operating window (40) in Figure 7. Hence, as a classification can be performed also the residence time of the fibers inside the refining zone can be reduced and the means a reduction in the pulp quality variations compared with the normal situation (39) described in Figure 7.

Another consequence of controlling in both refiners will also be that the motor load variations will be reduced, see Figure 13, which shows two data sets ((44) and (45)) for two different raw material mixtures.

Hence, the main purpose of the invention is to describe an arrangement which can present an on-line based tool defining an optimal operating window connected to different raw material mixtures and at the same time pre-specify the distribution between the refiners.

It should be noted that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention as defined in the appended claims.

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Brief description of the drawings:

Figure 1 : Section of a stationary disc (circular plates) 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.

Figure 3: Temperature profile and pressure profile as a function of the radius in the refining zone.

Figure 4: The sensor array holder with the sensors placed along the surface.

Figure 5a: The shape of the temperature profile before and after an increase in the dilution water feed rate.

Figure 5b: The shape of the temperature profile before and after an increase in production. Figure 5c: The shape of the temperature profile before and after a change in segments.

Figure 6: Schematic drawing how two refiners are linked to a a latency chest and pulp quality analyzer. In the figure also a reject refiner is included. Figure 7: Description of an operating window for pulp quality described by

Freeness(CSF) and mean fiber length(MFL).

Figure 8: Freeness(CSF) versus time. Figure 9: Operationg window for in the secondary refiner versus T _max in the primary refiner for two different raw material mixtures fed to the refiner line.

Figure 10: Operating window for the motor load in the secondary refiner versus the motor load in the primary refiner for two different raw material mixtures fed to the refiner line. Figure 11 : Obtained control error, i.e. the differense between the set point (SP) and the process value (PV) for when controlling the process.

Figure 12: Exemple of the reduction of the operating window for T _max (secondary refiner) versus (primary refiner) when in both refiners are controlled.

Figure 13: Reduction of the operating window for motor load (secondary refiner) versus (primary refiner) when in bothe refiners are controlled.