WO/2000/078458 | A METHOD AND MEANS FOR MEASURING STRESS FORCES IN REFINERS |
JPH0783606 | NON-CONTACT TYPE GAP MEASURING APPARATUS |
WO/2023/107579 | HYDRAULIC AXIAL ADJUSTMENT APPARATUS FOR CHIPPER DISC |
WO2004004909A1 | 2004-01-15 | |||
WO1997038792A1 | 1997-10-23 | |||
WO1992005874A1 | 1992-04-16 |
US5398876A | 1995-03-21 |
See also references of EP 2424671A4
Claims 1 A method for intermittently calculating the difference between the distributed axial force acting on a segment and the distributed force that arises in the refining zone of the refiner, where the estimated difference is fed to a computer unit provided with a chosen set point, from which the deviation from the chosen set point is fed to a control unit that controls the pressure applied to the refiner plates in the refiner, characterised in that a number of temperature sensors and/or pressure sensors positioned at known positions, arranged along the active radius of the segment of the refiner, in conjunction with spatial information on the grinding patterns of the segments, are used with information on at least one of the processing variables chip or pulp supply, measured motor load of the refiner, dilution water supply, temperature of input flows, temperature of output flows, pressure of input flows, pressure of output flows or the pressure applied to the refiner plates of the refiner in order to minimize to difference using a model. 2 A method according to claim 1, characterised in that the estimated difference between the distributed axial force and the distributed steam force, alternatively when liquid/chips/fibers are pressurized at low consistency refining (actual value function) is fed into a computer unit provided with a desired limiting function (set point values), and the deviations from the set point values are fed to a control unit that controls the influx of chips or pulp and dilution water and inflow and outflow pressure to the refining zone, alternatively combinations of these in order to compensate displacements of the distributed steam force. 3 A method according to any one of claims 1-2, characterised in that the estimated difference between the distributed axial force and the distributed steam force, alternatively when liquid/chips/fibers are pressurized at low consistency refining (actual value function) is fed into a computer unit provided with a desired limiting function (set point values), in order to control the average fibre length alternatively the arising fibre fractions of varying fibre length and/or the dehydration of the pulp and/or other quality variables specific to pulp by controlling the pressure applied to the refiner plates or the influx of chips/pulp or dilution water or inlet pressure to the refining zone or outlet pressure from the refining zone, or combinations of these. |
Technical field:
The invention relates to a procedure, where among other measurements, temperature sensors are used directly in the refining zone when refining fibrous material such as wood pulp, cellulose pulp and the alike. The main purpose with the invention is to reduce the risk for fiber cutting and refiner damages. The procedure also cope with the problem associated with the pulp quality variations in time which can be minimized as the distributed force related to the fibrous fiber network is estimated and thereby possible to be controlled. 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 and one rotating refiner plate, rotating at a very high speed.
The static refiner plate is placed on a stator holder (3), and is pushed towards a rotating one place on a rotor holder (4), electro mechanical or hydraulically (5).
The chips or fibers (6) are often fed into the refiners together with the dilution water via the center (7) of the refiner plates and are grinded on its way outward to the periphery (8). The refining zone (9), between the plates (also called segments) has a variable gap (10) along the radius (11) dependent on the design of the plates.
The diameter of the refiner plates differ dependent on size (production capacity) of the refiner and brand. Originally the plates (also called segments 12, 13, see Figure 1 and Figure 2) were cast in one piece, but nowadays they usually consist of a number of modules (forming a disc) that are mounted together on the stator and rotor. The segments can be produced to cover the entire surface from the inner to the outer part of the holders or be divided into one inner part (14) often called "the breaker bar zone" and an outer part (15) called periphery zone.
These segments have grinding patterns (16), with bars 17 and troughs 18 that differ dependent on supplier. The bars act as knives that 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, 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 fibres. Dependent on the segment design different flow patterns will occur in the refiner. In an LC-refmer no steam is generated and thereby only two phases exist(liquid and plup).
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. Therefore, it is important to know the actual plate gap. Today, measurement units for plate gap are provided commercially. Normally, only one plate gap sensor is used to prevent plate clash and not as expected in any control algorithms. Other systems exist as well and one robust system is based on temperature measurements along the radius in the refining zone to visualize the temperature profile (19) alternatively the pressure profile (20) for control purposes, see Figure 3. For LC-refming the pressure is preferred to be measured but for HC- refming 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).
According to traditional safety systems for plate clash protection, accelerometers placed on the stator holders (3) and/or the rotor holders (4) are used besides the plate gap sensors.
Technical problem
In the literature, extensive materials exist regarding refiner control by using consistency measurements and temperature measurement including safety systems to prevent plate clash of segments. The safety systems are often built on both hardware (typically accelerometers and plate gap sensors) and software in terms of frequency- analyzers and specific functions for limit control et cetera.
The research results indicate that the measurements of vibration on the holders shows deviations from vibrations caused by actual local fluctuation in the fiber pad inside the refining zone, which can be a result of in-homogeneity in the fiber pad or the other phases (water and steam). When considering LC-refming, the in-homogeneity can occur even though there exist only two phases.
The in-homogeneity in the fiber pad is central for the description of the technical problem. If the packing degree of the fiber pad varies locally both spatially and in time this can cause local areas where the spatial temperature alternatively the pressure increase or decrease dependent on if the packing degree increase or decrease. This leads to fluctuations in the pressure distribution in the refining zone which cause non- linear process conditions and thereby a varying residence time for the fibers in the refining zone which can cause bad pulp quality due to fiber cutting. Fiber cutting means that the length of the fibers is shortened too much when they hit the segment bars. The most unwanted situation is obtained when the fiber network is collapsed, i.e. the force related to the fiber network, which can be seen as a repulsive force to the axial force, is reduced drastically which certainly can lead to a plate clash.
Hence, neither accelerometers nor plate gap sensors can measure and prevent a fiber pad collapse as important information about the local fluctuations inside the refining zone is filtered and not handled properly.
In the literature, temperature measurements have shown to be an unusual robust technology for HC-refining control. When changing the production, dilution water and the hydraulic pressure the temperature profile is changed dynamically. This dynamic change is visualized in Figure 5a, where a step change in dilution water affect the temperature profile in different ways dependent on where on the radius (11) we consider the process. It is seen that when the dilution water increase, the temperature (23) will decrease before the maximum (24) but increase (25) after the maximum. The reason is that the water added cools down the back-flowing steam at the same time as the steam which is going forward is warmed very fast. When the production is increased the entire temperature profile (19) is lifted to 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.
All these process conditions, related to an increase in production and dilution water, will affect the active volume inside the refining zone at constant hydraulic pressure, and hence affect the plate gap as well as the temperature and/or the pressure profile. This will result in a change in the fibers residence time which affect the fluctuations in the refining zone and finally the pulp quality. The process conditions can also drive the refiner into situations where another operating point is obtained which by safety reasons are forbidden due to the risk for damage. These forbidden areas are difficult to predict on beforehand with present technology.
However, neither the temperature and/or the pressure profiles alone cannot give information about how to prevent fiber cutting and plate clashes.
Another problem with the HC-refming of today is that the local fluctuation cannot be captured by a simplified force balance where the axial force F d (27) is the sum of the steam force F s (28) and the force associated with the fiber network F p (29), see Figure 6a. To simplify, these forces can be seen as the integral of the distributed forces for all segments and this gives no added value to the solution compared with the measurements of the vibration on the holders if it is not developed further to describe also the distributed forces/. / (30),/, (31) and/, (32) along the radius, see Figure 6b.
To simplify the description below for the special case LC-refming we assume that the force for the water phase/ includes/ as it is hard to divide the information about the forces obtained from the water and fibers network. When referring to HC-refming we will use the notation distributed forces to describe the axial distribution, /, / , Steam force distribution,/, and the fiber pad's force distribution,/, which are formed by the fiber network inside the refining zone.
In a research project a new theoretical physical model has been documented ("Refining models for control purposes" (2008), Anders Karlstrδm, Karin Eriksson, David Sikter and Mattias Gustavsson, Nordic Pulp and Paper journal). The model, describes HC-refming and presuppose that the temperature and/or the pressure is measured along the radius of a segment to span the material and energy balances in the refiners and thereby make it possible to estimate the plate gap. The main difference compared with earlier rudimentary trials to describe the physics of the grinding processes is that the model estimates both the reversible thermodynamic work and the irreversible defibration work applied on the fiber network where the shear forces have a central position when iterating to find the right plate gap. Thereby, the model is described from an entropy perspective instead of an enthalpy based approach which does not take into account the shear between the fibers, flocks, water and the segments.
In the research project, a new sensor array holder was developed to meet the demands when following faster fluctuations in the steam phase. Thereby, a possibility was obtained to estimate the absolute pressure along the radius in the refining zone which can be used for predicting the force related to the steam phase. Using this information it was clear that earlier safety systems on the market fail to prevent from running into situations where a plate clash can occur. On reason, which the model above can indicate, is the dynamic changes for different steps in production, dilution water and hydraulic pressure are strongly non-linear which means that at certain circumstances for example at low consistency in the refining zone, the temperature profile is not affected so much while at other process conditions it is affected considerably, see Figure 5a-Figure 5b.
The non-linearities are affected also by the design of the segments. This can result in different temperature profiles (19, 33) and pressure profiles, see Figure 5c. This means that it is difficult to describe how the pulp is affected by the distributed fluctuations, which can cause local collapse of the fiber network along the radius. Moreover, the distributed axial force/. / , see Figure 6b, will also be strongly dependent on the design parameters related to the segments and its taper. The taper can mathematically be described as a vector which is important when it comes to the estimation of the shear forces formed in the refining zone.
For LC-refming similar phenomena exist but in this process the physical conditions are described based on two phases only.
However, knowing all this, the problem to measure the distributed fluctuations in the force balance is impossible and therefore other solutions to the problem must be formulated.
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, design parameters for the segments and a model to estimate the distributed axial force/./ and the obtained steam force/ alternatively the liquid related force for LC-refming/.
In those cases where HC-refming is considered it is assumed the temperature measurements will do fine according to EP 0 907416, as the conditions are assumed to be saturated, i.e. the pressure in the refining zone can be estimated from the temperature profile. In those cases where superheating occurs both temperature and pressure must be measures to estimate the steam force /. As the sensors are placed along the radius in the refining zone a temperature vector can be created. A radius vector, describing the sensor positions, must be formed as well to describe where the sensors are located on the sensor array holder.
To reproduce the non-linear phenomena in the process it is assumed that the model can describe the process good enough to secure a useful measure for/, / . The main variables for the model are the hydraulic pressure, inlet and outlet pressure to the zone, segment specific design parameters in terms of taper radius and in certain situations also production, added dilution water ant motor load.
Interpolation is a common way to describe the radius dependent variables as accurate as possible. This is important when discontinuities are approximated as continuous (34) in the processes. Examples of such discontinuities are the changes (35) in the taper from on part to the other on the segment, see Figure 7a.
If the steam pressure is saturated and measured alternatively estimated from the temperature profile the distributed steam force can be estimated as
f s {r) = P s (r)A{r)= P s {r)2πdr
where P s (r) is the distributed steam force for HC -refining and A(r) is the area for the infinitesimal element dr. The distribution of the radius into a number of elements dr is performed based on the length of the interpolated temperature- and pressure vector. For LC-refming the following will be applicable
f ι {r) = P ι {r)A{r)= P, {r)2πdr
where P t (r) is the liquid related pressure.
How the distributed axial force, f d can looks like can be obtained from the physical model mentioned earlier which describes the shear force profile (36), ξ{r) , se Figur 7b. This model is, however, quite complicated as a number of other process variables must be measured simultaneously or estimated along the radius ξ(r) ~ ξ ] (r) = a ] (r)μ ι (r)^- )
where, Cu represents the angle velocity, a } (r) the fiber concentration, Δ(r) the plate gap and //, (r) the fiber viscosity. Of course, this description is a simplification as it does not include the shear forces for the steam and water. However, it is verified as a good approximation in "Study of tangential forces and temperature profiles in commercial refiners" (2003), Hans-Olof Backlund, Hans Hoglund, Per Gradin, International Mechanical Pulping Conference, p.379-388, Quebec City.
A simplification of the concept described above is to create a similar distribution vector which for example can be based on knowledge about the segment taper, ψ{r) in combination with the shear force distribution as the segments taper and shear force is correlated to each other. By studying Figure 7a and Figure 7b it is easy to understand that the shear should be higher at the periphery (8) of the segments compared with the inner part (7) of the segments. An example of a function (37) to be used for a description of the distribution is
This function, see Figure 7b, looks similar to the shear force (36). By knowing the axial force where r ιn and r out corresponds to the segments inner and outer radius, f d can be estimated.
When the electro-mechanical pressure alternatively the hydraulic pressure are increased the distributed axial force f d (30) increase to f d (37) in Figure 8. As the temperature in HC-refmmg also increase the distributed steam forced (31) increase to s (38) in Figure 8, especially in areas around the maximum temperature, see Figure 5b. It is significant that/ increase faster and approach/, / when the refiner limitations are near. This is a consequence of the non-linear behavior of the process. Thereby, a local collapse occur as the fiber network cannot withstand a high enough force f p at the same time as it increase dramatically primarily in the periphery but also close to the inner part of the segments. This is shown in Figure 9 where f p (32) locally can be reduced to f p (39) which lies under the lowest acceptable level for the force f p (also defined as the threshold value (40). Similarly, this means that when the production is increased the temperature profile will increase as seen in Figure 5b. This means that/ will move closer tof d and thereby result in the same type of network collapse as seen in Figure 8 and Figure 9. The collapse of the network is hence a result of large local fluctuations at the maximum temperature but also close to the periphery. This also result in a refiner positioning in a non-linear operating point which is difficult to handle. Of special interest is that large fluctuations in/, close to the periphery can be observed early which can be used to indicate the risk for fiber cutting. Besides the setting of the simplified threshold value (40), more sophisticated threshold functions can be introduced. An example is the derivative of f p as a function of time, especially in regions close to the inner part and the periphery of the segments.
It is of course difficult to exactly know when the fiber pad is going to collapse if the distributed steam force / is not estimated. However, fiber cutting can occur already when/ is about 80% of/, / dependent on the age of the segment or if the refiner is run at operating points where we have a local high consistency. A plate clash can occur at any time when fiber cutting is reached and it can be difficult to back out from that state to a more stable position without closing the production and start up the refiners again. Hence, it is important to pinpoint the need for measuring the temperature-, or the pressure profile or a combination of these two in the refining zone to find the distributed steam force in HC-refming. When LC-refming is considered the procedure is simplified as / / always must be less than / to prevent the machine from a plate clash. Whether HC- or LC-refming will be used or not the method is possible to be used for control purposes.
The acceptable difference between / / and / in HC-refming must be well-specified, especially in regions close to the maximum temperature and should be controlled preferably by the hydrulic pressure. The difference between/ / and/, i.e. f p , can also be affected by other variables as the dilution water fed to the inner part and the inlet pressure to the refining zone as these two affect the volume in the refining zone and hence the temperature profile. However, these two variables will not affect the profile as much as a change in hydraulic pressure or production. In Figure 10 the control of the process is shown schematically. The unit (41) which is a computer or similar electronic equipment receives the difference between the set points (42) and the process values of the estimated difference between the / / and/ (10). The unit (41) controls the applied electromechanical pressure alternatively the hydraulic pressure (5) but also the chip- or pulp feed rate (6) in combination with added water (43) can occur.
The measured process signals (45), such as
• production, dilution water added, hydraulic pressure, temperature- and/or pressure profiles, motor load, the temperatures and pressures of the in- and outlet flows, consistencies et cetera, together with
• the geometric and material specific parameters (46), such as the segments taper, positions of the sensors, density, viscosity, et cetera, and • the difference between f d and/ 5 , are fed into a computer (47). The pulp from the process is symbolized in the drawing as well (48). When LC-refining is considered the difference between f d and β are fed into the computer (47) instead. In those cases where a reliable plate gap sensor is available it can be included as well.
Hence, the main purpose with the inventions is to describe a procedure, which can present a reliable on-line based estimation of the distributed forces f d and f s alternatively the difference between them, i.e. f p (alternatively the difference between f d and /l when applied to LC-refming concepts.) in the refiners grinding zone and thereby also implement a threshold or limit which the mentioned difference is not allowed to be below, see Figure 9. As the difference can be estimated, a more homogeneous pulp quality, in terms of for example mean fiber length, fractions of the fibers with specific length or dewatering of the pulp, can be produced if the difference can be controlled so it does not exceed a specified threshold.
A necessity for the invention is to measure the temperature- and/or the pressure profile in the refining zone and moreover that the segment taper is available and/or the shear force distribution is known obtained from the entropy model for example. Other functions describing the distribution of the axial force can be used as well and in the text above two examples are given, se Figure 7b.
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.
Brief description of the drawings:
Figure 1 : Section of a stationary disc (circular plates) which is pushed towards a rotating disc.
Figure 2e: Two segments where the sensor array holder, used for temperature- and/or pressure measurements, is placed in between.
Figure 3: Temperature profile and pressure profile as a function of the radius in the refining zone.
Figure 4: The sensor array holder with the sensors placed along the surface. Figure 5a: The shape of the temperature profile before and after an increase in the dilution water feed rate.
Figure 5b: The shape of the temperature profile before and after an increase in production.
Figure 5c: The shape of the temperature profile before and after a change in segments.
Figure 6a: The integral of the axial force which is balanced by the sum of the steam force and the force obtained from the fiber network.
Figure 6b: The distributed axial force in combination with the distributed steam force and the distributed force related to the fiber network,
Figure 7a: The true plate gap as a function of the radius.
Figure 7b: Shear force and the distribution function versus the radius used to describe the variable distribution of the axial force.
Figure 8: Example of the distributed axial force and the steam force as a function of radius for two different hydraulic pressures alternatively two different plate gaps.
Figure 9: Example of the distributed force related to the fiber network as a function of the radius for two different hydraulic pressures alternatively two different plate gaps.
Figure 10: Schematic description of the how the process will be controlled by using the hydraulic pressure or the plate gap to prevent fiber cutting and a plate clash of the segments.
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