FI84299B - Method and device for determination of content of different fractions in a pulp suspension - Google Patents

Description

1 84299

Method and apparatus for determining the concentrations of different fractions of a pulp suspension

The present invention relates to a method and apparatus 5 for determining the concentrations of said fractions of a pulp suspension containing mainly water, fibers and air, which method is particularly well applicable for determining the consistency and air content of fiber suspensions in the pulp and paper industry.

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A number of more or less accurate and useful methods and apparatus for determining the consistency of a pulp are known in advance. Most known devices are based on measuring the viscosity of a liquid. As the measuring device 15, a rotor rotating in, for example, a liquid or a liquid suspension, which is rotated either at a constant torque or at a constant speed, can be used. In this case, either the rotational speed or the torque required for rotation are proportional to the viscosity, which in turn is assumed to be directly proportional to the consistency. However, this is not known to be the case, for most liquids or suspensions the temperature has a very large effect on the viscosity without changing the consistency. However, temperature is easy to measure and the changes it causes are compensated for, but many other factors also affect viscosity. Fiber suspensions in the cellulose industry often contain a variety of chemicals that affect viscosity without changing the consistency, the effect of which is difficult to compensate for. The air entrained in the fiber suspensions also changes the values of the viscosity measurement.

Many different ways of measuring consistency are also known, using the friction of a liquid or suspension against the walls of a flow path or a special measuring device, but the disadvantage of these methods is that the result is affected by chemicals and air in the measured mass P646HENR / 8902 2 84299. .

In the current paper and pulp industry, very accurate information on the consistency and air content of the pulp to be treated is required at 5 different stages of the process. For example, to dispense chemicals or adjust headbox densities to their optimum. The measurements should, of course, be so-called. on-line measurements, i.e. the measurements 10 should be continuous, in which case the properties of the mass would be continuously known. If a method and equipment meeting the requirements could be developed, it would be simple, for example, to arrange automatic consistency control or chemical dosing.

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With the method and apparatus according to the present invention, the above requirements can be realized so that the consistency value of the pulp, e.g. the fiber suspension, i.e. the total amount of the fiber suspension and the amount of fiber contained therein, can be read continuously, as well as the air contained in the pulp. The method according to the invention is closely related to the measuring method and apparatus disclosed in FI patent 69372 for determining the mass flow and moisture or some other property of a solid, granular substance. The said patent mainly focuses on the equipment used and its application, for example, for measuring the flow of chips. The said patent uses a radio frequency measurement method which studies the effect of a fluid on the resonant frequency and the goodness of the resonator. From said quantities, it is possible to determine, for example, the average density of the substance to be measured and another quantity using a calibration graph. The measurement method according to FI patent 69372 is thus based on the determination of both the resonant frequency of the resonator 35 and the goodness of the resonator, which method cannot be applied to P646HENR / 8902 3 84299 fiber suspensions in the cellulose industry. The reasons have been found to be the salts, lyes and air in the cellulose, the variations in the concentrations of which have an immediate effect on the quality factor. In order to take account of the changes caused by those substances, their concentrations should be determined. However, fiber suspensions in the cellulose industry contain a large number of different chemicals with different effects, the effects of which on the quality factor are almost impossible to take into account as a whole. Thus, the solution according to the proposed patent cannot be applied to the needs of the mass industry.

15 In an effort to develop a device for continuous consistency measurement, it has surprisingly been found that in a pulp suspension consisting of water, fibers and air, the dielectric constant of the fibers is substantially the same as air and the dielectric constant of water or aqueous solutions is significantly higher (typically about 80 times). than air and fibers, with electromagnetic waves detecting only water or aqueous solution in suspension. In addition, by utilizing the compressibility of air under pressure, the air content, fiber content, and consistency of any of the suspensions can be readily determined.

The method according to the invention described above can be used to determine both consistency and air content of fiber suspensions without having to resort to complex compensations for changes caused by different chemicals, which would greatly increase the potential for error, which must of course be minimized. , but still accurate. Another advantage of the invention is that P646HENR / 8902 4 84299, if the flow contains steam, as is often the case in mechanical processes, the steam does not interfere with the measurement by the measurement method in question. Thus, the process according to the invention can also be used in mechanical pulping processes such as TMP, RMP and PGW. Especially in pulping processes, the lack of an on-line consistency meter significantly impairs process control.

A method according to the invention which meets said requirements is characterized, inter alia, in that the air contained in the suspension is taken into account by measuring the water content of the suspension at two pressures, using the water contents obtained and calculating the air, water and / or fiber content of the suspension without known co-compressibility.

The device according to the invention, in turn, is characterized in that it consists of a meter for determining the water content of the suspension arranged in connection with the space 20 containing the pulp suspension and at least one pressure sensor for determining the pressure of the material flowing in the pipe.

The method according to the invention and the devices used in connection therewith will be explained in more detail with reference to the accompanying figures, in which Fig. 1 shows an apparatus solution for applying the method according to the invention, Fig. 2 shows an apparatus arrangement preferably applying the method according to the invention, Fig. 3 shows another method according to the invention Fig. 4 shows a preferred embodiment of the device according to the invention, Fig. 6 shows a preferred embodiment of the device according to the invention applied to laboratory use, Fig. 6 shows a device arrangement used in tests performed to test the accuracy of the measuring method according to the invention, P646HENR / 8902 Fig. 7 shows the change of resonant frequencies in different measurements, Fig. 8 shows with 1 according to Fig. 5 values of the consistencies measured by the apparatus in the laboratory, and Fig. 9 shows the correspondence between the resonant frequency and the consistency.

According to Figure 1, the device according to a preferred embodiment of the invention consists of a tube 2 connected to or inherently belonging to the mass flow channel 1 and a surrounding chamber 6 delimited by the walls 3, 4 and 5. The chamber 6 can be either rectangular or cylindrical in shape. In the following, the focus will be on dealing with the mainly cylindrical resonator chamber, the so-called a cavity-20 resonator because its use has certain advantages over a rectangle. The tube sizes used in industry usually range from 0.1 to 0.5 ra, so that since the diameter of the resonator must be about 5 times the diameter of the tube, the diameter of the resonator 25 is between 0.5 and 2.5 m. an electromagnetically insulating material, for example plastic or fiberglass. Another essential aspect of the accuracy of the measurement results is that the resonator placed around the flow tube has wall sections 3 extending far enough to prevent electromagnetic wave movement of the surrounding space from entering the resonator chamber and to prevent the internal electric field 35 from discharging into the chamber. From a flow point of view, it is also essential that the pipe 2 has at least the dimensions P646HENR / 8902 6 84299 corresponding to the flow channel 1, unless it is directly part of it, in which case it does not cause changes in the flow itself. The walls 3, 4 and 5 are made of an electromagnetically conductive material, for example copper or the like, the chamber 6 being substantially isolated from the rest of the surrounding space so that any electromagnetic radiation acting there does not interfere with the measurement. In connection with the walls 4 or 5, switch loops 7 or similar means are arranged 10 for supplying a high-frequency electromagnetic wave movement to the resonance chamber 6.

In the commissioning of the described device, the first task is to calibrate the resonator 15, which can be done, for example, by placing the device around an empty flow tube and adjusting the frequency of electromagnetic wave motion from the switching loops until a resonant frequency is found. The flow tube is then filled with water and the resonant frequency corresponding to a water content of 100% is re-determined. Through these two points obtained, a line can be drawn and water concentrations corresponding to other resonant frequencies can be read from that line. Principle 25 is exactly the same line as in Figure 9. Thereafter, as the mass containing mainly water, air and fibers flows in the tube 1, a resonant frequency is determined, the magnitude of which is proportional to the amount of water in the flow as described above. This is based on the fact that the dielectric constant of the pulp suspension flowing in the tube differs from the dielectric constant of air. However, the dielectric constant of the fibers is practically the same as that of air, so it need not be taken into account. Water has a decisive effect on the wavelength of the electromagnetic wave motion and thus also on the resonant frequency, P646HENR / 8902 7 84299 so that the amount of water in the tube can be known. In Tols, the resonant frequency is proportional to the ratio N * ητ / (η ¥ + η *), where nv * is the amount of water and «is the number of fibers in the suspension, 5 when there is no air in the mass or when its amount is irrelevant. The consistency of the fiber suspension is easily achieved, consistency (%) = 100 (1-N).

When it is desired to determine the consistency, air content or some other quantity of the air-containing pulp suspension 10, the resonance frequency is proportional to the ratio N- ητ / (nv + Π ,, + nl), where, in addition to the above, n £ * is the amount of air in the suspension. Now, however, the expression for N has three unknowns nv, 15 n „and nA, of which only the amount of water n ¥ can be determined in one measurement, so it is not possible to directly determine the consistency or air content from the expression. It is helpful to pressurize the suspension to be measured, i.e. either to raise the pressure of the suspension so high that the air is compressed to such a small space that it is practically irrelevant, in which case the measurement and result processing are exactly as described above, or to take measurements at two different pressures. the pressure values are used to assist in determining the desired suspension characteristics.

By simultaneously measuring the amount of water contained in two different high-pressure mass streams with a resonator chamber of a similar type, the amount of water and fibers in the liquid as well as the amount of air in the mass can be determined, since only air is compressible with available pressure differences. Thus, measurements in said two resonator-35 chambers measuring different masses give different high ratios N to the amount of water n ¥, i.e. a measurement at higher pressure P646HENR / 8902 β 84299 gives a higher amount of water due to the compressed air content of the mass, the proportion by mass is higher. That is, Nj = nv / (nT + ¾ + ηχ x) and 5 N2 = nv / (nv + n * + n12), where N:> N2 because air is compressed at a higher pressure. Utilizing the formula pV = nRT known from physics for gases, and further p ^ j-PjVj (Fig. 2), i.e. when the effect of pressure rise on air volume is known, the amount of air and water contained in the mass can be calculated as well as the consistency of the mass.

The above-mentioned pressure difference between the different measuring points can be achieved either by arranging the measuring points at different heights as in Fig. 2, points 10 and 11. According to the figure, the pulp is pumped by a pump 12 to a pipe 13 with a resonator chamber 10 immediately after the pump. next to the supply connection 15 of the tower 14. In this case, the mass flow 20 is the same at both measuring points, but the pressure p2 measured by the sensor 16 next to the point 10 is considerably higher than the pressure p2 measured by the sensor 17 next to the point 11.

Another possibility is to arrange the measuring points according to Fig. 3, which illustrates, for example, the pumping of pulp from the pulp tower 20. The pulp is pumped by a pump 21 to a pipe 22 branching back to the pulp tower 20 via valves 24 and 25. After the valve 24 there is a first resonator measuring point. 26, followed by a pressure sensor 27 measuring the pressure of the mass flow. A second resonator measuring point 28 and a second pressure sensor 29 are arranged after the second valve 25. The valves 24 and 25 can be used to adjust the desired pressure levels for the mass flow 35 from which the resonator measurements are performed. From the results measured with said measuring devices P646HENR / 8902 9 84299, the amount of water, the amount of air and the consistency of the mass can be determined as described above. In some cases, it is also possible to do without a valve 24. In this case, throttling the valve 25 causes the pressure difference to act over the valve 5, whereby the resonator measuring points are at different pressures and the measurements and the determinations performed by them can thus be performed.

Figure 4 shows a preferred and useful embodiment of the device 10 according to the invention, in which the resonator chamber consists of two parts, preferably halves 31 and 32, which can be easily fastened to each other by quick connectors around the flow tube. In this case, the measuring device is easy to install or move from one location to another during the operation of the process, as long as the flow tube size used is suitable and the tube material is electromagnetically insulating, i.e. non-conductive. The only condition, of course, is that the pipes in the process are electromagnetically insulating material, so that no changes need to be made to the pipelines of the 20 factories for measurement. If this is not the case, the necessary modifications must be made to the appropriate points in the piping.

Figure 5 shows another preferred embodiment of the method and apparatus according to the invention as a laboratory measuring device. In this case, a resonator chamber 41 is arranged around the measuring vessel 40 used in the laboratory, which of course can be either fixed or detachable. The device can be used, for example, by placing a mass in a measuring vessel 30 and measuring the resonator reading, whereby the amount of water in the vessel is known. In the second stage, the air contained in the pulp is displaced and / or the device is pressurized, whereby the air bubbles are removed and / or compressed. Re-measure the amount of water 35 with a resonator and compare with the total volume of the chamber by subtracting the amount of mass if air has escaped or P646HENR / 8902 10 84299 if air is compressed, the amount of air in the mass and other desired quantities such as consistency can be determined. In some cases, the mass sample can be compressed at such a high pressure that the volume required for air decreases below the required dimensional accuracy. Alternatively, the method already used to calibrate the process measuring device can be used, in which the resonator readings are determined from both a water-filled measuring vessel and an empty, i.e. air-filled measuring vessel, whereby the consistency of the mass can be directly deduced from the mass-filled vessel.

As one possibility to determine the water content of the pulp suspension under laboratory conditions, the following procedure can be used. The content of a readily identifiable and also easily measurable chemical, such as table salt, is determined from the mass sample, after which a certain amount of either the chemical itself or pure water is added to the suspension, so that by re-determining the concentration of the same chemical, ie ending with a certain difference. water content easily calculated. By then pressurizing the pulp sample, the air content of the suspension is determined as described above, whereby the fiber content is further calculated.

If necessary, two resonator probes can be arranged in succession in the measuring equipment, in which case, using the correlation technique, it is possible to determine the flow rate in the channel and thereby also the amount of mass flow in the desired time interval. Of course, the flow can also be determined in many other known ways, such as, for example, the radar principle or magnetic methods.

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In order to achieve the highest possible accuracy, it must be taken into account that the resonant frequency of the resonator is affected by the dielectric constants of water, fibers and also dry matter. In this case, we end up with three unknown groups of 5 equations, the solution of which can be performed, for example, on the basis of the values obtained from experimental determinations. In addition, the corrections to be considered are due to temperature, which in some cases can be large.

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Preliminary tests were performed with the arrangement according to Figure 6 to determine the accuracy of the measurement method, which already showed the functionality and accuracy of the method according to the invention, despite the simplicity of the experimental arrangement. Figure 6 shows a pulp collection tank 50, from which the pulp is pumped by a pump 51 through a flow meter 52 and a temperature sensor 53 to a resonant device 54. After the meters, the pulp was returned to the collection tank 50. At the start of measurements 20, the pulp consistency was 12.3%. was calculated by adding water to the pulp in the collection tank 50 so that the final consistency was 6.9%. The effect of the air was tested by supplying compressed air to the mass, with the first test passes from a total of 55 to the collecting tank 50 and finally from a total of 56 to the flow channel before the resonance measurement. As additional tests, the effect of changes in the mass flow rate on the measurement results and also the effects of switching off the pump deaeration pump were tested. A total of 24 measurements were made. A microcomputer was used to locate the re-30 resonance peak. For additional information, temperature data were needed for both the pulp and the resonator chamber, as the pulp temperature varied significantly during the experiments due to the recirculation of the pulp in the pump and measuring equipment and the addition of water to the 35 pulp.

P646HENR / 8902 12 84299

Figure 7 shows the change in resonant frequencies with different measurement times taking into account the temperature correction. Frequency (MHz) on the vertical axis and sequence number (1 - 24) on the horizontal axis. A stepwise decrease in frequency is observed just as in Figure 8, which compares the consistency values measured in the laboratory (rectangles, accuracy 0.3%) and with the resonator (x-mark) at different measurement factors (vertical axis consistency%, horizontal axis sequence number). The inaccuracy of the resonator measurements (x-10 mark) in Figure 8 at the low consistency end is due to the fact that several calibration curves had to be used in the assays and that the temperature rise in the mass causes complex corrections. However, since the ultimate purpose of the measurements was to determine whether the measurements can be performed on moving mass and whether air moving with the mass is detected, it can be stated that these preliminary studies provide a positive answer to both basic questions.

20 Looking at the results, it can be seen that the accuracy of these preliminary mass measurements is already about 1%, although the accuracy of the resonance measurements was not optimal due to an inaccurate measuring device and the residual air mass was not taken into account, ie the measurement pressure already described above was not used. . It was also found that the mass flow rate has no effect on the resonant frequency at all. Thus, it can be stated that by using more accurate measuring devices, it is possible to achieve laboratory accuracy 30 in continuous and instantaneous measurement.

Figure 9 (horizontal axis consistency%; vertical axis resonant frequency MHz) shows the correspondence between resonant frequency and consistency measured under laboratory conditions, where it is found that the resonant frequency is directly proportional to the consistency of the pulp, and that at 8902HEN get to determine the consistency accurately.

Thus, it can be seen from the above that a new and unprecedented method for determining the consistency and air volume of fiber suspensions in the paper and pulp industry, for example, has been developed which allows accurate process control based on on-line measurement. thus, has posed a major challenge for product developers in the industry. It should be noted, however, that only some preferred embodiments of the invention have been described above, which in no way seek to limit the invention to what is set forth in the appended claims, which alone determine the scope of the invention and its scope. For the reason just mentioned, the description of the invention has not focused in more detail on describing the different types of devices available, which are described e.g. in the FI patent mentioned as prior art.

20 P646HENR / 8902

Claims (8)

1. Method for controlling the content of the various fractions; The fiber, liquid and gas fractions in a pulp suspension, characterized in that the air contained in the suspension is observed to measure the water content of the suspension at two pressures, the air, water and / or fiber content of the suspension being determined by the calculation of utilize the obtained water contents and the known compressibility of the air 10. 2. A method according to claim 1, characterized in that the air content of the suspension is determined by measuring the water content of the suspension at at least two spaced apart measurement points which differ under different pressures, wherein the air, water and / or fiber content of the suspension is determined by the calculation. by utilizing the obtained water contents and the known compressibility of the air. 20 Method according to claim 1, characterized in that the air content of the suspension is determined by measuring the water content of the suspension at the same measuring point at least two times, during which the measurement pressure of the suspension is changed relative to the previous measurements, wherein the air, water and / or fiber content of the suspension. is determined by the calculation using the obtained water contents and the known compressibility of the air. 30 Method according to claim 1, characterized in that a saturation vessel is filled with the air-containing suspension which is subject to the determination, the water content of the suspension is determined, the saturation vessel and the suspension in it is pressurized and the water content of the suspension is again determined, whereby the air content of the original suspension is obtained. - P646HENR / 8902 i7 84299 by comparing the water content determined at the later measurement stage with the water content determined at the previous measurement. 5 Method according to claim 2, characterized in that two water content meters and two pressure sensors are used in the measurements. 10 Method according to claim 4, characterized in that two water content meters and at least one pressure sensor are used in the measurements. Process according to claim 1, characterized in that the water content of a pulp suspension is determined by calculations after the content of a readily determinable chemical in the pulp suspension is first determined and by effecting a change in the level of the chemical in question in the suspension either by to add the chemical or pure water, the water content being determined by the change in the chemical content and the addition of said substance. 8. Device for determining the content of the various fractions; the fiber, liquid and gas fractions in a pulp suspension, characterized in that it consists in connection with a space (1,2,40) containing pulp suspensions, which determines the water content of the suspension and at least one pressure sensor (16,17; 27 Which determines the pressure of the material flowing in a tube (2.40). P646HENR / 8902