Method and apparatus for determining the consistency of a suspension

Field of the invention

The invention relates to a method according to the preamble of the appended claim 1 for determining the consistency of a suspension. The invention also relates to an apparatus for implementing the afore- mentioned method in accordance with the preamble of the appended claim 9.

The term suspension refers to fluid containing solid matter. The amount of solid matter in the suspension is not restricted in any way. Thus, the suspension may be nearly pure fluid or it may be dense suspension containing solid matter. The solid matter may consist of colloidal substances, particles or clusters of particles. The size of the particles may vary from very fine particles to particles of larger size. The suspension may also be a mixture of colloidal substances and particles of different sizes.

Background of the invention

For the measurement of the consistency of a suspension there are several different measurement techniques, of which a suitable technique is selected in accordance with the consistency level of the suspension that is being measured. The most common methods are the mechanical, optical of microwave method. Mechanical methods are based on the measurement of shear force produced by the suspension to a measuring sensor. One such measurement method and apparatus is disclosed in Fl patent 75424 (corresponding US patent 4,757,708). In the solution according to the patent, a sensing element is installed in a process tube, said sensing element being in contact with the paper pulp flowing in the tube. The flowing paper pulp produces a shear force into th _, e sensing element, said force being dependent on the consistency of the pulp.

Optical methods include methods based on fiber absorption, reflection and polarization of visible light and IR light. Microwave methods are based on monitoring the changes of the speed of microwave when microwaves travel through the suspension. Typically the measuring devices and the sensors belonging thereto that are used in the above- described measurement methods are attached directly to the tube conveying the suspension. It is also very common to install a separate tube in the process, to which a part of the suspension in the pipings or tanks is conveyed and in which the consistency measurement takes place.

The consistency of the suspension in the tank is also measured by installing measuring equipment in the piping introducing the suspension to the tank and/or removing suspension therefrom, and by determining the consistency of the suspension in the tank by means of calculations based on these measurements. Thus, as a result of this method it is possible to obtain a calculated estimate on the consistency of the suspension, not the actual consistency of the suspension in the tank.

The consistency of the suspension can also be measured directly from the suspension tank. These measurements are often very unreliable. The reason for this is that the consistency of the suspension is not constant within the entire volume of the tank. In tanks not comprising an agitator, the particles of solid matter contained in the suspension tend to sink towards the bottom of the tank. In tanks comprising an agitator the situation is somewhat better but a suspension with completely uniform consistency is not attained in this way either. Thus, the bases of the measurement of the consistency of the suspension by means of a consistency measuring device fastened directly to the tank are unreliable as such.

US publication 4,415,408 discloses a consistency measurement method, which is based on the use of ultrasound in the measurement. In the solution according to the publication, the consistency of a fiber pulp suspension contained in a tank and mixed with an agitator is

measured directly from the tank. The problem of ultrasound measurements is that they are not reliable in industrial environment, because the noise of the surroundings causes errors in the measurement results.

In the manufacture and processing of a fiber pulp suspension in paper and pulp industry, there are several tanks having different tasks in different stages of the process. They can be storage tanks, mixing tanks for mixing different pulps and additives, or pumping tanks. Some tanks comprise an agitator. The consistency of pulp suspensions in the tanks is measured and controlled on-line. The purpose of controlling the consistency is to produce pulp having correct consistency to the pulp processing stage following the tank.

The consistency of the pulp suspension contained in the discharge and/ or intermediate tanks is regulated by adding suitable dilution fluid thereto. The dilution fluid is mixed to the pulp suspension by means of an agitator. The controlling of the consistency of the pulp suspension contained in the tank is typically conducted by measuring the electric power taken by the agitator and determining the consistency of the suspension therefrom. The amount of dilution fluid added to the tank is adjusted on the basis of the measured power. The problem of this method is that the correlation between the power taken by the agitator and the consistency is not unambiguous and the obtained consistency value is not accurate.

Brief description of the invention

Therefore, the purpose of the present invention is to provide a method for determining the consistency of a suspension in a tank, which avoids the above-mentioned problems and with which it is possible to determine the consistency of a suspension in a tank in a simple and reliable manner. Furthermore, it is an aim of the invention to provide an apparatus implementing the aforementioned method.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The apparatus according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 9.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that the consistency of the suspension mixed in the tank by means of an agitator is determined by means of measurement results obtained by measuring the frequency and force or pressure of the pressure pulses produced by the agitator in the suspension.

It has, in fact, been observed that a suspension diluted and/or mixed in a tank transmits the pressure waves i.e. pressure pulses produced during the dilution and/or mixing in different ways. In other words, as the mixing proceeds, and mixing time becomes longer, the frequency and force or pressure of the pressure pulses change although the amount of components in the suspension remain the same. This results from the fact that a homogenous suspension transmits pressure pulses in a different manner than a non-homogenous suspension. The so-called free liquid between the components in the suspension transmits pressure pulses. If the amount of free liquid is small, the pressure pulses produced by the agitator are transmitted/transferred poorly to the measuring device, i.e. the pressure pulses are weaker. On the other hand, if the amount of free liquid is large, the pressure pulses are strong and clear. The measured signal thus correlates directly with the consistency of the mixed suspension.

The agitator is arranged on the wall of the tank so that it is located underneath the surface of the suspension. The agitator mixes the suspension substantially constantly. The measurements are conducted

by means of at least one measuring device arranged on the wall or at the bottom of the tank. If the measurement device is arranged on the wall of the tank, it may be positioned either on the same or on a different level with the agitator. Advantageously the measuring device is positioned on the same level with the agitator, on the opposite side of the tank with respect to the agitator.

The agitator produces pressure waves, i.e. pressure pulses in the suspension, the frequency of the pressure pulses being measured constantly by means of the measuring device. The measurement results are conveyed to a computing and control unit in which the consistency of the suspension is determined from the measurement results. The measuring device may be any measuring device measuring pressure or force, which can be arranged in connection with the tank.

The advantage of the invention is that the consistency of the suspension contained in the tank may be determined directly from the tank, and separate sampling devices or tubes are not required for the consistency measurement. In the act of implementing the method it is possible to utilize an agitator already installed in the tank, said agitator producing the pressure pulses required in the measurement. The only new component required is the measuring device, various of such devices being commercially available. Thus, the solution is economically advantageous. Furthermore, it is possible to determine the consistency of the suspension according to the invention in a rapid and reliable manner.

Furthermore, as it is possible to monitor the mixing of the suspension continuously, homogenous suspension is attained, the consistency of the suspension being substantially constant within the entire volume of the suspension. This is a significant improvement when compared to methods of prior art. Especially in the manufacture and processing of fiber pulp suspension, it is possible to utilize the invention when diluting the suspension and/or mixing different pulps, and by means of the

same it is possible to detect when the pulp suspension is sufficiently homogenous to be passed to the following process stage.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a schematic side view of a cross-section of a tank, and

Fig. 2 shows the tank of Fig. 1 from the top in a cross-section.

Detailed description of the invention

Fig. 1 shows a tank containing a suspension S. The suspension is brought into the tank and discharged therefrom via one or several ducts, which ducts are not shown in the figures for the sake of clarity. The longitudinal cross-section of the tank 1 is annular, and a agitator 2 is arranged therein. The agitator 2 is installed underneath the liquid level of the suspension on the wall 3 of the tank 1 so that it extends through the wall 3 of the tank. The agitating member 4 of the agitator 2 extends inside the tank 1 , within a distance from the wall 3 of the tank. The parts of the agitator that remain outside the tank and the actuator that enables the rotation of the agitating member 4 are not shown in the figure for the sake of clarity.

To measure frequency and pressure or force of the pressure pulses produced by the agitator in the agitating member 4, the tank 1 is equipped with at least one measuring device 5a to 5c that performs measurements continuously. The measuring device 5a is installed on the wall 3 of the tank, substantially on the same level with the agitator 2. The measuring device can also be installed on a different level with the agitator 2 with respect to the height of the tank. This embodiment is shown in the figure by a measuring device 5b drawn by means of broken lines in the tank. The essential aspect is that the measuring

device is located underneath the surface of the suspension S contained in the tank. The measuring device can be arranged in the tank in any position with respect to its perimeter, advantageously, however, the measuring device is installed on the opposite side with respect to the agitator 2. The measuring device can also be installed at the bottom 9 of the tank 1 , said embodiment being illustrated by means of a measuring device 5c shown in broken lines in the figure. There may, of course, be more than one measuring device arranged in the contained on different levels with respect to the height of the tank. Thus, the measuring devices perform measurements simultaneously.

Figure 2 shows a cross-section of the tank of Fig. 1 in a top view. The agitating member 4 of the agitator 2 produces pressure waves i.e. pressure pulses when it rotates around the shaft 6 of the agitator, said pulses being illustrated by means of curved lines in the figure.

The measuring device 5 positioned on the same level with the agitator, on the opposite side of the tank, measures the frequency and force or pressure of the pressure pulses continuously. When placing the measuring device 5a and the agitator 2 in the tank, it must be taken into account that the distance between the agitating member 4 and the measuring device 5a is suitable so that the pressure pulses are not attenuated too much as they propagate in the suspension.

The measuring device 5a is positioned in such a manner with respect to the tank 1 that the measurement sensor 7 belonging to the measuring device penetrates through the wall 3 of the tank at least so far that the measuring head of the sensor is on the same level with the inner surface of the wall 3 of the tank. The measuring device 5a can also be positioned in such a way that the measuring head of the sensor is so far inside the tank that the fiber layer that has possibly adhered to the walls of the tank does not cover the measuring head of the sensor.

To determine the consistency of the suspension, the measurement signals obtained from the measuring device 5a are conveyed to a computing and control unit 8 that is either integrated in the measuring

device or is a separate unit. Measurement signals transmitted to a separate control unit are illustrated by means of broken lines 8 ¹ in the figure. The software components contained in the control unit comprise calculation algorithms by means of which the consistency of the suspension in the tank is determined. The determining of the consistency of the suspension may be based on empirical measurement results obtained from the measurements conducted earlier or on mathematical models derived therefrom. The simplest way to determine the consistency is to use a reference curve obtained from earlier measurements. By means of the control unit it is also possible to monitor the state of the suspension being mixed in the tank.

As a measuring device 5a it is possible to use any measuring device measuring the force or pressure of a pulse produced in the suspension, such as a measuring device using a diaphragm as a means for transmitting a pressure shock. The measuring device can also be positioned entirely inside the tank. Thus, for transmitting the measurement signal out of the measuring device, a cable must be arranged through the wall of the tank to transmit the measurement signals to control unit arranged outside the tank. The measuring signals can also be transmitted outside the tank wirelessly.

The control unit 8 may also contain software components for controlling the consistency of the suspension in the tank. Thus, the ducts arranged in the tank (not shown in the figure) for bringing a dilution fluid or suspension having different consistency to the tank, are provided with actuators for regulating the dilution fluid or suspension having different consistency. On the basis of the determined consistency of the suspension in the tank the control unit transmits control commands to the actuator regulating the amount of dilution liquid or suspension of different consistency, thus controlling the dilution of the suspension.

The ducts conveying the suspension before and after the tank can also be provided with measuring devices of prior art to determine the consistency of the suspension. Thus, the computing and control unit 8 can utilize all the measurements, i.e. the measurements obtained from

inside the tank and before and after the tank to determine the consistency of the suspension and to control the amount of dilution fluid supplied to the tank. The programs used for the control can determine a calculatory value for the feeding consistency. The absolute consistency values are measured conventionally in the laboratory of the mill as well. The samples for the laboratory measurements are taken from different parts of the process stage, and ' the function of the measuring devices is monitored by means of the results.

In the measurements conducted at later stages of the process, i.e. stages following the mixing tank, there is a time delay between the mixing and the measurement. This must be taken into account in the control unit so that the amount of dilution liquid conveyed to the mixing zone would be accurate and the desired consistency level would be uniform.

The method according to the invention eliminates the time delay, because the measurement and the mixing take place in the same surroundings, i.e. where the dilution liquid necessary for controlling the consistency is conveyed.

When the control unit receives real-time information on the mixing zone, the controlling of the consistency is better and the obtained feeding consistency for the next process stage is more uniform. This has the advantage that the apparatus of the next process stage, for example a pulp washing apparatus may be run close to its maximum capacity without the risk that a large variation in the consistency would block the washing apparatus and cause a stoppage in the production. This results in that the process devices can be acquired without a capacity reserve for variations in the consistency that affects the size of the apparatus. In other words, it is possible to select an apparatus of correct size in accordance with the production. This also brings about a considerable economical advantage in view of the investment costs of the apparatus.

The invention is especially advantageous for determining the consistency of a fibrous suspension processed in paper and pulp industry. The tank may for example be a discharge tank or intermediate tank following the cooking or bleaching stages. As it is possible to monitor the mixing of the suspension by means of the invention, a suspension can be produced whose consistency is substantially the same within the entire volume of the suspension. The dilution of the pulp suspension in the tank and for example the mixing of different pulp grades can be performed in a reliable manner. This is essential in view of the quality of the final product.

In such a tank which is symmetrically circular and whose bottom is even, the measuring device is advantageously positioned on the same level with the agitator, on the opposite side of the tank with respect to the agitator. If there are surfaces guiding the flow at the bottom of the tank, the measuring device must be positioned in an advantageous position with respect to the propagation of the pressure wave.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. Thus, the tank can be pressurized or have an atmospheric pressure therein.