SE533701C2 - Procedure for minimizing the difference between temperature profiles in refiners with two grinding zones - Google Patents

Description

The stationary grinding wheel, which is clamped with bolts in a stator holder (3), is usually pressed electromechanically or by means of a hydraulic pressure (5) against the rotating grinding wheel, which is clamped on a rotor holder (4). The aggregate (6) is in most cases dried into the rafts together with dilute water via the center (7) of the milling discs and if the aggregate consists, for example, of wood ice or processed pulp from a previous rafter, this aggregate is distributed on its way to the periphery of the milling discs (8) . The grinding zone (9), or as it is also called the refining zone, between the grinding wheels may have a variable grinding gap (10) along the radius (11) of the grinding wheels depending on the grinding applied to the surfaces of the grinding wheels.

The diameter of the grinding wheels varies depending on the raw material and the production capacity of the grinders. Previously, the grinding wheels were cast in one piece, but today it is also common to modularly manufacture the grinding wheels in a number of grinding segments (12 and 13), see Figure 1 and Figure 2. The segments can, for example, extend from the center of the grinding wheels to its periphery or be divided into two rings. , an inner (14) and an outer ring (15). The zones between the inner and outer rings are often called "breaker bar zone" and peripheral zone, respectively.

The surfaces (16) of the grinding segments are designed in different ways with characteristic patterns in the form of booms (17) and ponds (18). The booms act as knives and they 'break the ice alternatively refine the mass formed. In addition to the direct grinding zone, during HC refining, both glaciers, water and steam are also transported in the ponds between the barriers. Through different pattern designs, it is possible to make the grinding segments become feeding or stopping the mass of pulp in order to influence the flow conditions and thereby create special pulp qualities. The frictional work to which the ice and pulp are subjected in the malting zone causes the incoming water to evaporate during HC refining. The amount of steam produced is spatially dependent, which is why both water and steam can occur together with fl ice or mass in the malt zone. It is usually assumed in this case that the water in the malting zone is bound to the alternative or the meter. No steam is generated during LC cleaning.

There are also other types of grinders such as concave mills or grinders where both grinding wheels rotate in the opposite direction or retainers consisting of four grinding wheels, where a rotating center in the middle has grinding wheels mounted on both sides and two stationary grinding wheels which are pressed together by hydraulic pistons to toe two malzoner. The latter type of pipe is often called Twin raff mortars.

When setting pulp from the outside of wood ice or alternatively previously refined pulp, the grinding discs are compressed so that the gap (10) of the grinding zone becomes approximately 0.2-0.7 mm, depending on the type of raw material used. 10 15 20 25 30 35 533 701 3 The grinding gap is a central control variable and an increase or decrease of the gap often takes place electromechanically or by means of hydraulic pistons which apply a hydraulic pressure (5) to one or more grinding wheels depending on the type of grinder. This creates an axial force that is applied to the grinding wheels. The force that withstands the axial force in HC refining consists partly of the force obtained by evaporation of water and partly of the force generated by the grinding of the grinding material. In cases where LC refining is used, it is the cracks caused by the pressure increase in the aqueous phase and the fiber network of the grinding material that withstand the axial force. If the grinding gap is changed by, for example, 10%, the pulp quality is significantly affected. There are grinding column sensors on the market today that are applied directly to the grinding wheels. Usually only one grinding gap sensor is used per grinding zone, mainly to prevent the grinding discs from collapsing and thus not primarily to control the grinding gap.

There are also other systems on the market where the temperature is measured along the template zone in order to visualize a temperature profile (19) or alternative pressure profile (20) for control purposes, see Figure 3. In LC refining, it is preferably the pressure profile that is interesting to follow. When HC refining, it is usually sufficient to follow the temperature profile.

In the event of a changed condition in the grinding gap, production (ie fl ice- or pulp supply) and diluent addition, the temperature changes, which can thus be controlled.

Several temperature and / or pressure sensors are commonly used and can be placed directly in the grinding segments or alternatively enclosed in a parallelepiped longitudinal rail (21) extending along the active radius (l1) of the grinding segments (l2 and 13), see Figure 1, Figure 2 and Figure 4, according to the procedure in EP 0788 407 - Measurement in Malzon. Usually, the parallelepiped rail is implemented between two grinding segments in the outer ring on larger refiners, see Figure 2.

The design of the grinding segments has proven to be of great importance for the temperature profile along the active radius, which is why it is difficult to determine in advance where the temperature sensors (22) and / or the pressure sensors (22) are to be placed in the rail (21).

Otherwise, there are really no other measurement systems that have been applied in the operators' malzone. On the other hand, there is the instrument that can be placed in the blow line outside the interior where the dry content of the flowing mass stream can be calculated using algorithms connected to NIR (Near Infra Redy measurements.

In the case of safety systems to prevent the grinding discs from collapsing, in addition to grinding gap sensors, vibration sensors located on 10 15 20 25 30 35 40 533? 0 '| 4 the stator holder (3) and / or the rotor holder (s) (4), hereinafter referred to as holders, on which the grinding unit is mounted.

Description of the Invention Technical problem: Extensive material regarding electron control using dry matter measurement, grinding gap measurement and temperature measurement including safety systems to prevent aggregation of grinding segments has been reported in the literature. The safety systems are also made up of both hardware in the form of, for example, accelerometers and grinding column sensors, and software in the form of, for example, frequency analysis algorithms and limitation functions.

Measurement systems based on vibration measurement can be used but usually provide integrated information about what is happening in the grinding zone as they cannot resolve the collected information based on local conditions.

Recent research tests show that measurement of vibrations on the holder o fi a shows a clear deviation from vibrations caused by actual local ktctuations in the mold inside the malt zone, which can occur due to inhomogeneity either in the mold or in the two additional phases of HC rat fi n liquid and vapor alternatively combinations of the three which o fi a is the case. This means that accelerometers in the tiden am time should only be used as a complement to more robust solutions. In LC refining, inhomogeneities can also occur, even if in this case there are only two phases.

The inhomogeneities in the earthenware are central to the description of the technical problem.

If the degree of packing of the mold varies locally in time and space, this can create local areas where the spatial temperature or the pressure increases or decreases due to the degree of packing of the mold increasing or decreasing. This is already known and leads to ktctuations in the pressure distribution in the grinding zone which causes non-linear process conditions and as a result a varying residence time of fibrema in the grinding zone which can cause deteriorated pulp quality due to fi cutting. Fiber cutting means that the length of the ñbrers is shortened unnecessarily when they hit the bars of the grinding segments. Therefore, it is central to be able to maintain the right spatial dry matter content to counteract local inhomogeneity phenomena.

For a long time, it has been believed that by designing absolute measuring grinding gap sensors, it should be possible to improve and supplement traditional vibration measuring systems and thereby obtain an estimate of how inhomogeneity can be counteracted by, for example, controlling supplied diluted water. The problem with this technology, however, is that important information about the local uctuations is filtered out.

According to the literature, temperature measurement has proven to be an unusually robust technology for control in HC refining.

When measuring the temperature profile in the grinding zone, it has emerged that when production, diluent supply and grinding gap change, the temperature profile is dynamically affected.

The dynamic change is suitably illustrated by studying Figure 5a, where a step change of the dilution water affects the temperature profile in different ways depending on where along the radius (11) the course of events is viewed. As the diluent supply increases, the temperature (23) decreases before the temperature maximum (24). After the temperature maximum, the temperature increases (25). The reason for this is that the incoming water cools the return steam at the same time as the forward steam is heated by the fi icing between segments and fi glaciers.

When production increases, this usually means that the entire temperature profile (19) rises to another level (26), see Figure 5b. This usually also applies when the grinding gap (10) decreases, which is equivalent to the electromechanical pressure or the hydraulic pressure (5) being applied to the grinding wheels via the hydraulic pistons.

The nonlinearities are also affected by how the grinding segments are designed, which has the consequence that the appearance of temperature (19, 27) and pressure problems can vary both spatially, see Figure 5c, and dynamically. A relatively large (open) grinding gap closer to the center of the peripheral segments usually means that the temperature maximum is shifted towards the periphery, see Figure 5c. This means that it is not always possible to describe in advance how the consistency of the grinding material in the grinding zone is affected by the distributed ktuctuations, which can cause local collapse of the fi bear network along the radius of the grinding segments.

However, the complex pattern of possible events has not previously been linked to the local change in the dry matter content of the malt zone. The reason for this may be that they have been so focused on calculating the outgoing dry content from the inside through a rudimentary enthalpy balance across the generator, preferably without the use of temperature measurement.

All process conditions, for example increased production or dilution water supply that changes the active volume in the grinding zone at constant hydraulic pressure, consequently affect both the grinding gap and the appearance of the temperature and / or pressure profile. This has the consequence that the residence time of the pulp in the grinding zone can vary, which affects the fluctuations in the grinding zone and finally the pulp quality during normal operation. It can also happen that the process conditions are negatively affected so that the operator drives away to work points that are prohibited for safety reasons due to the risk of an accident.

These forbidden areas are difficult to predict in advance with current technology until it is too late and a merging of the segments is a fact. 10 15 20 25 30 35 40 533 70'l In a recently filed Swedish patent application "Procedure for controlling process conditions in miners to prevent" rock cutting and breakdown of grinding segments "this is described as a balancing problem between different forces acting in the grinding zone. The method provides a procedure for how the temperature profile and the applied azial force are distributed over the grinding zone and how to prevent the grinding segments from collapsing and / or getting bare cutting. Thus, this patent application is not about how the local dry matter content can be controlled in the malt zone.

In connection with a number of research projects carried out at Chalmers University of Technology, a completely new theoretically based physical model has been documented (“Reining models for control purposes” (2008), Anders Karlström, Karin Eriksson, David Sikter and Mattias Gustavsson, Nordic Pulp and Paper journal) . The model, which describes HC refining, thus assumes that the temperature and / or the absolute pressure are measured along a segment, preferably along the outer ring of the refiner where the actual refining takes place, in order to mathematically tighten both the material balance and the energy balance in the refiner and thus calculate the grinding gap. What distinguishes the model from previous rudimentary attempts to describe the physics of the grinding process itself is that it calculates both the reversible thermodynamic work and the irreversible refining work carried out on the brema, where the shearers have a central role in iterating the correct grinding gap. It can be said that the model is described from an entropy perspective instead of an enthalpy-based approach that does not take into account the shear between fibers, fiber yokes, liquid and grinding segments. This model assumes that the measured production is correct. However, this is problematic because it is an estimate based on, among other things, assumed dry matter content and speed.

This is of course a simplification, but the model has proven to be useful for armat spatial dry matter estimation.

In cases where the refiner is constructed with two grinding zones, so-called Twin refiners, a problem also arises in that it is not known how much ice or mass is distributed to each zone. In cases where there are two series-connected refiners, where the first is usually called the primary thinner and the second sectarian thinner, a direct effect on the final quality arises if it is not possible to even out the differences between the mal zones in each thinner.

The problem is thus that the distributed dry matter along the radius in the malt zone cannot yet be measured with any equipment, so other solutions to the problem must be resorted to. This can take the following expression: In cases where we have Twin radiators with two grinding zones, separated by a rotor, you can get asymmetric temperature profiles according to Figure 6, Usually the different sides are called Front Side (28) and Drive Side (29). 10 15 20 25 30 35 40 533 701 The problem with the asymmetry can be caused by having: 1) two different grinding gaps, caused by problems with the rotor positioning, but with the same feed, which means that the temperature problems are shifted among themselves according to Figure 6a; 2) a situation where fl the ice to the primary refiner or the pulp to the secondary refiner is involuntarily fed differently on the Front Side (F S) and Drive Side (DS) respectively, which results in a skewed distribution of the pulp which is reflected in the shape of the temperature problems, see Figure 6b; 3) two different grinding gaps, caused by problems with the rotor positioning, but with different feedings, which means that the temperature coils are displaced from each other according to Figure 6c; 4) different degree of packing depending on where in the grinding zone the pulp is located, which may be caused by the random situation that arises where the grains in the grinding zone are distributed differently at the start of the raff butter.

The solution: The present invention is the solution to this problem and relates to a method that uses robust temperature and / or pressure measurement in combination with available measuring gears, especially the load of ice or mass feed screws and estimated production from the process and vibration measurement on FS and DS respectively to control the amount added. dilution water to the malzone and finally hydraulic pressure for each malzone.

Since the measuring sensors are located along the radius in the grinding zone, a temperature vector is formed which forms the so-called temperature profile, see Figure 5. In order to be able to describe this, a radius vector is also created which describes the position of the sensors according to Figure 4.

Since the desire is for the temperature profiles to converge towards each other instead, as in the case shown in Figure 6b, we must know how to control the dilution of the water supply. We therefore have the following solution to the problem: 1) By measuring and comparing the load on the feed screws on FS and DS, we give an indication of how the distribution of fl ice or mass to each malzone looks relative. If the load on the feed screw to FS is greater than for DS, we can assume that there is more mass in the FS feed screw and vice versa, see Figure 7. 10 15 20 25 30 35 40 533? 0'l 8 2) A complement to studies of the load on the feed screws is to measure the vibrations on the FS and DS respectively with accelerometers. A large vibration on, for example, DS means that less mass goes on this side, which has not been known before as it is believed that large vibration is related to a large degree of packing of the mass. 3) At the same dilution fl fate to FS and DS, respectively, at high load on the feed screw on FS, a too high dry content is obtained compared to DS, which can result in the situation reflected by Figure 6b at the same grinding gap. Because we know that more pulp goes to FS, we can therefore compensate for this by controlling the supply of dilute water to each malzone to strive for the condition prevailing in Figure 8. In this particular case, the dilution water to FS is increased. It is also possible to reduce the dilution water supply to the DS, but this can create inhomogeneous grinding with large machine vibrations as a result, which can cause downtime. See also the patent application "Procedure ßr to control process conditions in miners to prevent clogging and breakdown of grinding segments". However, this can be indicated by following the variation of the internal temperature sensor units over time as they directly correlate with the resulting pumping phenomena and thus measurable vibrations in the machine. 4) The discrepancy remaining between the temperature curves in Figure 8 can then be adjusted by reducing the hydraulic pressure on the FS or increasing the hydraulic pressure on the DS to achieve the situation shown in Figure 9. With the same argument as above, it is preferable to make the necessary changes on FS. 5) This dry process means that a more symmetrical distribution of the pulp is obtained in the malt zone and thereby a similar dry content of DS and FS, respectively, which in turn creates a more consistent residence time for the mills in the mill zones and thus a more homogeneous pulp quality. The local dry matter content in the malt zone can preferably be calculated using the algorithms reproduced in the document “Cleaning models for control purposes”, Nordic Pulp and Paper Journal, 2008 but is not a requirement for this solution. 6) If the dry matter content is measured in the effluent outlet and used to control the dilution water fate, the ratio between the dilution water on FS and DS can be maintained in order to continue to have a balance between the temperature problems on FS and DS. Thus, both temperature profiles change in a similar way to another level. In cases where the temperature maximum is controlled by the hydraulic pressure, this control algorithm will adjust the temperature profile to the correct level. No problems with Wind-up will arise because this control loop does not affect the dry content, see the discussion on natural decoupling in the document "Multi-rate optimal control of the 'FMP-re fi riing processes", Anders Karlström and Alf J. Isaksson, IMPC conference, 2009. 5 10 15 20 25 30 35 533 701 9 7) In the event that the pressure or temperature profile is not measured in the grinding zone, the feed distribution can only be used to control the dry content spatially in the grinding zone approximately. This can be seen as a last resort in cases where all sensors have failed. 8) The whole process is preferably also connected to the degree of opening of the dilution water valves, which indicates whether there is a large or small back pressure where the water is fed into the rafter on the FS and DS, respectively. Provided that the supply systems for dilution water supply are comparable, they can be used together with the information from the load on the feed screws to control the distribution. A large degree of opening means that you have a large back pressure and consequently a larger mass volume. 9) At the same dilution water flow to FS and DS, respectively, a high dry content is sometimes obtained at high load on the feed screw on FS compared with DS, which can result in the situation reflected by Figure 6e. This is caused by the fact that you have different template gaps on FS and DS, respectively. In these cases, consequently, the temperature maximum will be shifted towards the periphery of the grinding segments. This situation is much more difficult to control than that reproduced in point 2 above and will result in greater vibrations on the DS as a relatively smaller amount of mass goes to this malzone. In this specific case, you want to increase the hydraulic pressure on the FS and then compensate and control the dilution water supply to the respective mold zone to get the same dry content.

Figure 10 schematically shows the control of the process. The unit (41) consisting of a computer or similar electronic equipment is fed with the difference between the desired temperature sample vector (setpoints) (42) and (actual values) measured temperature sample vectors respektive from the respective malzone (10). In the Control Unit (41) the difference vector is handled together with algorithms for calculating the distribution of fl ice or mass to the malzones by comparing the motor loads on the feed screws supplying the malzones with fl ice or mass and / or the degree of opening of the dilution valves in combination with the difference between the temperature 28 ) and DS (29). The difference and distribution are then used for the regulation of the dilution water (43) and the electromechanical pressure or the hydraulic pressure (5) to the respective grinding zone. This unit also adds an acceptable distribution function (46) (such as max and min deviation in distribution between estimated ice or mass feed to FS and DS).

From the process (44), the measured process signals (45) (such as temperature sample vectors and / or pressure profile vectors 10 15 20; 533 70'l 10 fi from the different template zones) are measured intermittently at high sampling rate in a measuring unit (47) required for control. The ground pulp is removed from the process at (48).

In cases where sufficiently accurate grinding column sensors are available, the above control procedure can also include these in the introduction as this information can replace the hydraulic pressure and provides information on how much volume and mass each grinding zone can de facto handle after calculation. This information correlates well with the signals obtained from the load on the feed screws to FS and DS.

The main object of the invention is thus to describe a method which can with great reliability present an on-line based control of the temperature zones of twin drivers in the malt zones by knowledge of the distribution of ice or mass in the feed screws to the malt zones. The invention is thus based on the fact that the temperature profile and / or the absolute pressure profile can be measured in the grinding zone. Different types of functions can represent the distribution between the feed screws on the FS and DS and the invention is not linked to any specific algorithm in this control concept.

The method according to the present invention is also not limited to any particular device for reading temperature or pressure in the grinding zone. However, such devices are known from, for example, Swedish patent 94037433-9 and EP 0907416.

The invention is not limited to the embodiment shown, but it can be varied in various ways within the scope of the claims. 10 15 20 25 30 35 533 701 l l Description of the drawing base: Figure 1: Section of a stationary grinding wheel that presses against a rotating grinding wheel.

Figure 2: Two grinding segments with intermediate parallelepiped elongated rail before measuring temperature and / or pressure.

Figure 3: Temperature profile and pressure profile as a function of the malzone radius.

Figure 4: Parallel pipedisk elongate rail with discreetly placed temperature and / or pressure sensors.

Figure Sa: Appearance of the temperature profile before and after an increase in the dilution of the water supply.

Figure Sh: The appearance of the temperature profile before and after an increase in production.

Figure See: The appearance of the temperature profile before and after a template segment change.

Figure 6a: Temperature profile for a case with different grinding gaps but with the same amount of ice or mass for the grinder's grinding gaps.

Figure 6b: Temperature samples show a case with asymmetric distribution of ice or mass but with the same diluent addition to the generator's malzones.

Figure 6c: Temperature profile for a case with an asymmetrical distribution of ice or mass but with different grinding gaps and the same dilution water additive to the grinder's grinding zones.

Figure 7: Load for the motor-driven feed screws on an operator with two grinding zones.

Figure 8: Temperature profile for a case with an assyrinetic distribution of chips or pulp to the generator's grinding slits where the diluent flow to one of the grinding zones has been modified to obtain a similar dry content as in the corresponding grinding zone.

Figure 9: Temperature profiles for a case with asymmetric distribution of ice or mass to the grinder's grinding slits according to Figure 8 where the hydraulic pressure has been used to even out the difference between the two grinding zones. 533 701 12 Figure 10: Schematic description of how the different temperature problems are controlled by means of dilution water and hydraulic pressure or grinding gap to achieve similar temperature problems and thereby similar dry content in the different grinding zones.

Claims (2)

A device for controlling the grinding process in refractors with a first and a second grinding zone, the device comprising at least a first series of sensors (22) arranged radially at different distances from the center of a refiner in the first grinding zone. and a second series of sensors (22), arranged radially at different distances from the center of a radiator in the second grinding zone, the series of sensors being arranged to measure at least either of pressure or temperature, the device therefor comprising a first and a second load meter for the load on the feed devices to the first and second grinding zones, and furthermore a first and a second dilution control means for regulating the dilution supply to the first and the second grinding zone, characterized in that the device comprises a control device (41) receiving measurement data at least from the two series of sensors (22) and the two load meters, where the control device is arranged to control at least the first o and other dilute water control means in such a way that the difference between the measured values from the two series of the sensors (22) is minimized. A device according to claim 1, characterized in! in that it further comprises a first and a second pressure control means which control the applied axial pressure on grinding segments in the first and second grinding zones, the control device (41) therefor regulating at least one pressure control means so that the difference between the measured values from the two series of sensors (22) minimized. A device according to claim 1 or 2, characterized in that the device further comprises a dry content sensor that measures the dry content in the outlet of the batteries and / or a calculated dry content from each grinding zone where the control device (41) receives measured values from the dry content sensor and uses these in the control process. A device according to any one of claims 1-3, characterized in that the device further comprises a grinding gap sensor in each grinding zone which measures the width of the grinding gap and where the control device (41) receives measured values å from the grinding gap sensor and uses these in the control process. A device according to any one of claims 1-4, characterized in that the device further comprises measuring the degree of opening of the diluent valves to the first and second grinding zones and where the control device (41) receives measured values fi from the diluent valves and uses them in the control process. A device according to any one of claims 1-5, characterized in that the device further comprises measuring the vibrations related to the first and second grinding zone and where the control device (41) receives measured values from accelerometers and uses these in the control process. 15