WO2008000902A2

WO2008000902A2 - Method and apparatus for taking a sample from a pulp suspension with an integrated agitator and sampler

Info

Publication number: WO2008000902A2
Application number: PCT/FI2007/050385
Authority: WO
Inventors: Antti Isola
Original assignee: Metso Paper Inc.
Priority date: 2006-06-27
Filing date: 2007-06-25
Publication date: 2008-01-03
Also published as: FI20065443A0; FI20065443L; WO2008000902A9; WO2008000902A3

Abstract

A method and an apparatus for taking a sample from a pulp suspension in a tank (1), said pulp suspension being mixed substantially continuously by means of an agitator (3) arranged in the tank (1). The sample is taken by a sampler (6, 8, 16) integrated in the agitator (3).

Description

Method and apparatus for taking a sample from a pulp suspension

Field of the invention

The invention relates to a method according to the preamble of the appended claim 1 for taking a sample from a pulp suspension. The invention also relates to an apparatus for implementing the aforementioned method in accordance with the preamble of the appended claim 11. In this context the term pulp suspension refers to fibrous pulp suspension treated in paper and pulp industry.

Background of the invention

In paper and pulp industry the consistency of a pulp suspension is measured and controlled when producing and treating fiber pulp suspension. Suitable consistency of the suspension is important in view of the quality of the final product.

For the measurement of the consistency of the pulp suspension there are several different measurement techniques, of which a suitable technique is selected in accordance with the consistency of the pulp 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 pulp to a measuring sensor. 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 pulp. Typically measuring devices and the sensors belonging thereto are attached directly to the tube conveying the suspension. It is very common to install a separate tube in the process, to which a part of the pulp suspension in the pipings or tanks is conveyed and in which the consistency measurement takes place.

The consistency of the pulp suspension in the tank is also measured by installing measuring equipment in the piping introducing the pulp to the tank and/or removing pulp therefrom, and by determining the consistency of the pulp 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 pulp, not the actual consistency value of the pulp in the tank. The consistency of the pulp suspension can also be measured directly from the pulp tank. Such a method is disclosed e.g. in the US publication 4,415,408, which is based on the use of ultrasound in the measurement.

In different stages of the pulping process there are several tanks with different tasks. They can be storage tanks, blending chests for mixing different pulps and additives, or pumping tanks. Some tanks comprise an agitator. The consistency measurements of the pulp suspension in a tank are often very unreliable. The reason for this is that the consis- tency of the pulp is not constant within the entire volume of the tank. In tanks not comprising an agitator, the fibers in the pulp as well as particles of solid matter tend to sink towards the bottom of the tank. In tanks comprising an agitator the situation is somewhat better but a pulp with completely uniform consistency is not attained in this way either. Thus, the bases of the measurement of the consistency of the pulp by means of a consistency measuring device fastened directly to the tank are unreliable as such.

The consistency measuring method disclosed in US 4,415,408, which is based on the use of ultrasound and in which the consistency of the pulp suspension is measured while it is in a tank where it is mixed with an agitator, is not reliable either. The problem of ultrasound measurements is, in fact, that they are not reliable in industrial environment, because the noise of the surroundings causes errors in the measurement results.

The measurement of the pulp sample taken from the tank is not reliable either, because the consistency of the sample depends from which point of the tank the sample is taken. Especially problematic it is to obtain a reliable consistency measurement result from a pulp suspension in a tank when pulp of different consistency, additives or auxiliary media or fluid is added into the suspension. The fluid can be diluted fibrous suspension, white liquor, water or other kind of fluid used in the pulping process.

One stage of the pulping process, in which the consistency of the pulp is measured is the discharge phase of the digester. Especially in the batch cooking method, problems occur in the measurement because in the discharge phase dilution liquor is added into the digester to dilute the bottom of the digester and to assist in the emptying of the digester. As a result of this, the consistency of the pulp contained in the tank during the discharge phase, and the consistency of the pulp discharged therefrom varies strongly from one batch to another. Therefore, especially the consistency measurements of pulp samples taken during the discharge phase of the batch digester are very unreliable.

In connection with pulp suspension measurements it is known to use for example a so-called filtrate sampler. These samplers contain a screen whose purpose is to prevent the access of fibers into the sample. Thus, such a sampler is not suitable for measurements for determining the consistency of the suspension. US patent 6,432,265 presents a sampler of the above-mentioned kind for taking a sample from pulp suspension in a tank. The sampler is installed on the wall of the tank. The sampler contains a screen that prevents the access of fibers and other particles into the sampler, but passes the liquid phase of the suspension.

Summary of the invention

The purpose of the present invention is to disclose a new solution for taking a representative sample from a pulp suspension in a tank for consistency measurement.

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 11.

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

The invention is based on the idea that a pulp suspension sample is taken from a tank equipped with an agitator by means of a sampler integrated in the agitator, said agitator mixing the pulp suspension in the tank substantially continuously.

The pulp suspension sample is taken from the immediate vicinity of the agitator positioned in the tank, i.e. from a mixing zone. When a sample is taken from the mixing zone, the actual consistency of the pulp in the tank is revealed, and the delay of the measurement is minimized when compared to a situation where the measurement is performed in the piping, far away from the mixing event. The short measurement delay time enables rapid reactions to the variations in the consistency and thus uniform feeding of pulp to the next process stage.

The sampler may be composed of one or several parts arranged in or in connection with the same. It can be any comprise any means suitable for taking a pulp suspension sample. In its simplest form it is merely a duct or tube. The essential aspect is that the sampler can be integrated in the agitator, i.e. it can be arranged in connection with the agitator. The sampler can also be arranged in connection with the agitator in such a manner that it forms a part of the structure of the agitator.

The sampler can be integrated in the agitator by arranging it inside the frame of the agitator. Thus, a sample chamber is arranged inside the frame of the agitator, to which a tube is connected for conveying the sample out of the sample chamber. The taking of the sample from the suspension takes place through at least one opening arranged in the frame of the agitator, said opening having a connection to the sample chamber. Each opening is covered by a screen that prevents the access of large particles, such as stones into the sample chamber.

The sampler may also be composed of a tube arranged inside a shaft extending in the longitudinal direction of the frame. The shaft itself may also function as a sampler. Thus, the sampler extends through the frame of the entire agitator to the front side of an agitating member and ends within a distance therefrom. Thus the sample is taken from the front side of the agitating member. The end of the sampler extending inside the pulp suspension is provided with a screen to prevent the access of large particles in the sampler.

The tube-type sampler may also be fastened on the frame of the agitator. The tube may be fastened either directly to the frame of the sam- pier or by means of separate fastening elements that position the sampler in parallel with the frame of the agitator, but within a small distance therefrom. The end of the sampler extending inside the pulp suspension is also in this embodiment provided with a screen to prevent the access of large particles in the sampler.

As description hereinabove shows, all the embodiments of the invention comprise a screen, the purpose of which is to prevent the access of large particles and impurities, such as stones in the suspension into the sampler. However, the gaps between the mesh bars and mesh wires of the screen are so large that substantially all parts of the suspension that affect the consistency of the pulp suspension are capable of entering the sampler. In other words, the screen does not filter the sample, but the fibers and other small particles as well as fluid contained therein are capable of passing therethrough.

The sampler is connected to a suitable actuator, such as a pump that causes the sample to flow to the sampler and through the same out of the tank. Samples may be taken continuously, wherein the sample constantly flows through the sampler to be measured. The taking of the sample may also take place at fixed intervals. The time interval between taking samples or the duration of the sampling depends on the desired determination interval consistency and the method used for measuring the consistency. Typically the measurement is continuous in a continuous process.

By means of the invention it is possible to obtain a representative sample of the pulp suspension in a tank. Because the suspension is mixed substantially continuously and the sample is taken from the mixing zone, the composition of the sample taken from the pulp suspension is as representative as possible, although the consistency of the pulp fed to the tank varies a great deal. In connection with consistency measurement the term representative sample refers to a sample that contains solid matter contained in the suspension, such as fibers and fillers, and fluid phase substantially in the same proportion than the entire amount of suspension in the tank.

Furthermore, if the shaft of the agitator is used as a sampler, or the sampler is positioned inside the shaft of the agitator, a separate duct is not required in the wall of the tank for the sampler.

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 schematically and in a partial cross-section a tank equipped with an agitator,

Fig. 2 shows a side view of an agitator in which a sampler is integrated for taking a sample,

Fig. 3 shows a side view of a second agitator in which a sampler is integrated for taking a sample, and

Fig. 4 shows a side view of a third agitator in which a sampler is inte- grated for taking a sample. In Figs. 1 to 4, the same numerals refer to corresponding parts and they will not be explained separately later on, unless required by the illustration of the subject matter.

Detailed description of the invention

Figure 1 shows a tank 1 containing pulp suspension. Pulp suspension is introduced into the tank and discharged therefrom via one or several ducts, which ducts are not shown in the figures for the sake of clarity. An agitator 3 is fastened to the wall 2 of the tank, said agitator penetrating through the wall of the tank 1. The agitator contains a frame 4, and an agitating member 5 is fastened to its end extending inside the suspension for mixing the pulp suspension. The shaft 6 of the agitator extends through the agitator 3 in its longitudinal direction. It is con- nected from one end to the agitating member 5 and from the other end to an actuator 7 rotating the shaft 6. The arrow A illustrates the rotating movement of the agitator. In the figure the agitator 3 has been placed in the lower part of the tank, but it can be positioned in any location in relation to the height of the tank.

Fig. 2 shows that part of the agitator 3 that is fastened to the wall 2 of the tank and extends inside the tank 1. The parts of the agitator that remain outside the tank and the actuator that enables the rotation of the agitating member are not shown in the figure for the sake of clarity. In order to take a pulp suspension sample, a sampler 8 is integrated in the agitator, said sampler being composed of a sample chamber 9 and a tube 10. At least one opening 11 extending through the frame of the agitator is formed in the frame 4 of the agitator in the section between the agitating member 5 and the wall 2 of the tank. There are three openings 11 shown in the figure. If there are several openings in the frame, they are positioned symmetrically in relation to the periphery of the frame. The sample chamber 9 is arranged inside the frame of the agitator and the tube 10 is connected thereto. The sample chamber 9 collects the pulp suspension flowing to the sample chamber via the openings 11. From the sample chamber 9 the suspension flows via the tube 10 out of the agitator 3. Each opening 11 is covered with a screen. The screen may be for example a bar screen that is composed of bars parallel to each other, arranged at determined intervals. It is possible to use other kinds of screens as well. It is essential that the distance between the bars of the screen and the mesh size of the screen are so large that the screen does not prevent the flow of fibers and other small particles contained in the pulp suspension or the flow of fluid through the screen.

As the description hereinabove shows, in this embodiment the pulp suspension sample used in the consistency measurement is taken from the back side of the agitating member 5, between the agitating member 5 and the wall 2 of the tank. The arrows B illustrate the movement of the pulp in the tank.

In the embodiment shown in Fig. 3, the taking of the sample for the consistency measurement takes place through the shaft 6 of the agitator. The shaft is a hollow tube, having an opening 18 in one end for taking the sample. The shaft 6 extends through the frame 4 of the agi- tator to the front side of the agitating member 5 and ends within a distance therefrom. The front side of the agitating member 5 refers to that side of the mixing member that is closest to the central axis penetrating the tank in the height direction. Thus, the shaft 6 functions as a sampler and leads the sample out of the tank. The sample is taken from the front side of the agitating member 5. The opening 18 of the sampler, i.e. the shaft 6 that extends inside the tank is covered with a screen 12. The screen 12 can be a conical screen, as shown in the figure. It may also be a planar screen covering the opening of the shaft.

In the embodiment shown in Fig. 4, the taking of the sample for the consistency measurement takes place by means of a sampler 16 fastened on the outer surface of the frame 4 of the agitator. The sampler 16 is a tube made of rigid material. The sampler 16 is fastened on the outer surface of the frame 4 of the agitator by fastening means 17. The fastening means 17 can be any fastening means suitable for the pur- pose. If desired, the sampler can also be fastened directly on the outer surface of the frame of the agitator.

The sampler is fastened on the back side of the agitating member 5 so that the other end of the sampler 16 that contains an opening 18 for taking a sample, is positioned within a distance from the agitating member 5. The taking of the sample thus takes place from the back side of the agitating member 5, between the agitating member 5 and the wall 2 of the tank. However, the sample is taken so close to the agitating member 5 that it takes place in the agitating zone. The opening 18 of the sampler 16 is covered with a screen 12. One end of the sampler 16 extends outside the tank for leading the sample out of the tank. For leading the sampler out of the tank, the wall of the tank is provided with a duct.

In the above-described embodiments the sample taken from the pulp suspension is led to consistency measurement. The sample can be measured when it flows in a measurement pipe arranged as an extension for the tube coming from the tank and conveying the sample. The sample is returned later to the main flow of the pulp, for the next process stage.

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.

Claims

1. A method for taking a sample from a pulp suspension in a tank (1), which pulp suspension is mixed substantially continuously by means of an agitator(3) arranged in the tank and comprising an agitating member (5), characterized in that the agitator is arranged in the wall (2) of the tank and that the sample is taken by means of a sampler (6, 8, 16) from between the agitating member (5) and the wall (2) of the tank, said sampler (6, 8, 16) being integrated in the agitator (3).

2. The method according to claim 1 , characterized in that the sample is taken by means of a sampler (8) arranged inside the frame (4) of the agitator (3), said sampler being composed of a sample chamber (9) and a tube (10).

3. The method according to claim 2, characterized in that the sample is led from the tank (1) to the sample chamber (9) through at least one opening (11) formed in the frame (4) of the agitator (3).

4. The method according to claim 3, characterized in that the sample is led to the sample chamber (9) through at least one opening (11 ) covered with a screen (12).

5. The method according to claim 1 , characterized in that the sample is taken through the shaft (6) of the agitator (3), said shaft functioning as a sampler.

6. The method according to claim 1 , characterized in that the sample is taken by means of a sampler (16) fastened on the outer surface of the frame (4) of the agitator (3).

7. The method according to claim 5 or 6, characterized in that the sample is led to the sampler (6, 16) through an opening (18) contained therein, the opening being covered with a screen (12).

8. The method according to any of the preceding claims 1 to 7, characterized in that the sample is taken by means of a sampler (6, 8, 16) integrated in the agitator (3) arranged in the wall (2) of the tank (1).

9. An apparatus for taking a sample from a pulp suspension in a tank (1), in which an agitator (3) is arranged in the tank (1) for mixing the pulp suspension substantially continuously, which agitator comprises an agitator frame (4), an agitator shaft (6) and an agitating member (5) connected to the shaft (6), characterized in that the agitator is ar- ranged in the wall (2) of the tank (1 ) and that the apparatus comprises a sampler (6, 8, 16) integrated in the agitator (3) for taking a sample from between the agitating member (5) and the wall (2) of the tank.

10. The apparatus according to claim 9, characterized in that the sampler (8) comprises a sample chamber (9) and a tube (10) and that the sampler is arranged inside the frame (4) of the agitator (3).

11. The apparatus according to claim 10, characterized in that at least one opening (11) is formed in the frame (4) of the agitator (3) for leading the sample from the tank (1 ) to the sample chamber (9).

12. The apparatus according to claim 11 , characterized in that the opening (11) is covered with a screen (12).

13. The apparatus according to claim 9, characterized in that the shaft (6) of the agitator (3) is arranged to function as a sampler.

14. The apparatus according to claim 13, characterized in that the shaft (6) extends to the front side of the agitating member (5).

15. The apparatus according to claim 9, characterized in that the sampler (16) is composed of a tube fastened on the outer surface of the agitator frame (4).

16. The apparatus according to claim 13 or 15, characterized in that the sampler (6, 16) comprises an opening (18) for leading the sample to the sampler (6, 16) and that the opening (18) is covered with a screen (12).

17. The apparatus according to claim 11 or 16, characterized in that at least one opening for leading the sample to the sampler (8, 16) is arranged in the part of the agitator frame (4) that extends between the agitating means (5) and the wall (2) of the tank.