WO2012022528A2

WO2012022528A2 - Method for filtering aqueoous suspensions of cellulose fibres

Info

Publication number: WO2012022528A2
Application number: PCT/EP2011/060889
Authority: WO
Inventors: Eirik Kultom Karlsen
Original assignee: Voith Patent Gmbh
Priority date: 2010-08-19
Filing date: 2011-06-29
Publication date: 2012-02-23
Also published as: CN103069073A; DE102010039506A1; WO2012022528A3; EP2606177A2

Abstract

The method serves to filter suspensions (S1, S2), in particular cellulose fibres in aqueous suspensions. Here, at least two different suspensions (S1, S2) are filtered in a filter device, in particular a disc filter, with the result that filtrates (F1, F2) are formed as through-feed and thick stock is formed as residue. The method is preferably regulated by virtue of the fact that both the vacuums for filtrate extraction and the rotational speed of the hollow shaft (3) are incorporated into the regulating concept.

Description

The invention relates to a process for filtering suspensions, in particular aqueous-suspended pulp fibers, according to the preamble of claim 1.

Filter processes are known to serve for the separation of solids from a suspension. The filter elements used in this case hold back most of the solids at the inlet side of their filter surfaces and allow the liquid, usually water, to pass through as filtrate. Often, as little residue of solids as possible is desired in the filtrate, resulting in e.g. by the separate discharge of a primary turbid filtrate (small amount) and a secondary clear filtrate (large amount) can be achieved. The retained solids, often referred to as filter cake, should have the lowest possible water content. Frequent use is in pulp and paper making, where fibrous suspensions are to be filtered. The filtrates formed are very well suited for reuse in the production process.

The typical filter device for the process is the disc filter whose filter trough has a plurality of chambers for different in their composition suspension streams. This offers the possibility to filter several suspension streams in a single device, which reduces costs and space requirements. On the other hand, the operation of such a machine becomes complex and the control more difficult. There is therefore a risk of a non-optimal effectiveness of the process, which can make up for the aforementioned savings again.

The invention is based on the object to provide a method of filtering, with which it is possible to carry out the filtering economically and at the same time particularly effective. This object is achieved by the features mentioned in claim 1.

With the aid of the invention, the parameters for a plurality of filtration processes in the same device can be influenced independently of one another. An additional degree of freedom is provided, in particular for the control strategy of the method, whereby it is possible to apply different pressure differences in the same machine.

If z. B. the rotational speed of the hollow shaft on which all rotating filter elements are mounted, so regulated that the one (first) suspension is optimally filtered, can be made to optimize the filtration of the other (second) suspension, a change in the pressure difference, with the Filtrate of the second suspension is withdrawn. In this way, the different dewatering behavior and / or the different solids content of the various suspensions can be taken into account. It may also be that different dry contents of the filter cake and / or different filtrate purities to be set, which is also achievable with this method.

The invention will be explained with reference to schematic drawings. 1 shows the principle of the method according to the invention with only one sketched

Filter device;

Figure 2: schematically: a disc filter with a view of the front side;

Figure 3: a simplified illustrated control scheme for carrying out the

process;

Figure 4: the scheme of Figure 3, varied and extended with dilution water addition.

1 schematically shows a disk filter serving as a filter device with a plurality of disk-shaped segmented filter elements 1 and 1 ^' (see FIG. partly immersed in a filled with suspension S1 and S2 filter trough 4. Of the Filter trough 4 is divided by a partition wall 14 into the chambers 12 and 13, which are fed by the pumps 8 and 9 with the suspension S1 and S2, respectively. Instead of a divided filter trough 4, two adjacent troughs could be used. In this example, five filter elements 1 dip in the chamber 12 and three filter elements 1 ^' in the chamber 13 a. Depending on the requirements of the process, the filter elements can also be assigned differently to the corresponding chambers. All filter elements 1 and are mounted on a horizontally arranged rotating hollow shaft 3, from which the filtrates F1 and F2 are withdrawn at both ends, for which purpose the pumps 10 and 11 are provided in this example. In this way, the filtrate F1 is formed from the suspension S1 of the chamber 12 and the filtrate F2 from the suspension S2 of the chamber 13. The drive 7 sets the hollow shaft in rotation, wherein a rotational speed between 0.5 and 1, 5 revolutions per minute is common. Advantageously, the rotational speed of the hollow shaft 3 and the flow rates of the pumps 8, 9, 10 and / or 1 1 are integrated into the control of the filtering process. The majority of the solids is retained on the filter elements 1 and 1 ^' , and forms a filter cake, which is discharged as thick matter 5 and 6 from the filter device.

The filter trough 4 is open at the top, so it works with ambient pressure. There are also other filter devices suitable for the process in which the filter trough is closed and under positive pressure. This is more complex, but allows greater pressure differences in the filtering process.

In Fig. 2 the disc filter is shown with a view of the front side and thereby the segmentation of the filter elements 1 can be seen. The individual segments divided against each other take up the filtrate F1 and guide it at its radially inner side into the interior of the hollow shaft 3. By a control head, not shown here, a hydraulic connection to the segments, depending on their respective rotational position produced. This example also shows the already mentioned possibility of forming a primary turbid filtrate TF and a clear filtrate KF and stripping them separately. The clear filtrate KF is an important product of the process, the usually intended for dilution elsewhere. The quantitatively far lower turbid filtrate TF z. B. recycled as dilution water and again subjected to the filtering process. The operation of disc filters is known, which is why it will not be discussed further here.

The diagram in FIG. 3 serves to explain and explain various advantageous control strategies in the implementation of the method. The filter device used is a disk filter as in FIG. 1. Of the filter elements are exemplified a filter element 1, which dips into the chamber 12 and a filter element 1 ^' , which dips into the chamber 13. Normally, there are a total of about 4 to 35 filter elements. The chamber 12 is equipped with a level transmitter 15 and the chamber 13 with a level transmitter 16, each of which their signals (measured values) 19 and 20 forward the process control system PLS. The pumps 8 and 9 for the suspensions S1 and S2 and the pumps 10 and 1 1 for the filtrates F1 and F2 are controlled in this example by signals 17, 18, 21 and 22. The pumps can be speed-controlled or equipped with adjustable throttles. In addition, the drive 7 of the hollow shaft 3 is controlled by signal 23 to influence the rotational speed. Frequently, instead of the pumps F1 and F2 conveying the filtrates 10 and 1, geodesic drop sections over several meters in height are used (see Fig. 4), in which the downpipes dip into the filtrate bundles.

Some possible variants of the control strategy are as follows: Variant 1:

The flow rates of the suspensions S1 and S2 are adjusted to temporally constant values by the signals 17 and 18. The pump 10 generates the negative pressure in the conduit system for the filtrate F1, which may be set relatively high in order to obtain the highest possible filtrate. Signal 23 controls the drive 7 of the hollow shaft so that in the chamber 12, the signal level 19 reported by the signal level is within the prescribed range. It lowers a higher speed the liquid level of the chamber 12 and vice versa. Of course, this speed change also affects the fluid level of the chamber 13. However, since other conditions may be present here than in the chamber 12, it is advantageous to be able to set a further parameter acting only in chamber 13. Therefore, a signal 21 is formed from the liquid level of the chamber 3 reported by the signal 20, which controls the pump 10 for the filtrate F2 so that the liquid level remains in the prescribed range even in the chamber 13. In this way, the operation of the disc filter can be automated perfectly. The flow rate of the filtrate F1 from the first chamber 12 can be at a high

Value regulated to maximize utilization of the disc filter. However, other requirements for the process can also be taken into account, such as: B. the dry content of the thick stock 5 or the quality of the turbidity filtrate TK (see Fig. 2).

What has been said here about the filtrates F1 and F2 generally refers to the clear filtrate KF (see Fig. 2). However, it is also conceivable to provide the control processes for the negative pressure of the cloudy filtrate TK, since their deduction quantity can also affect the liquid levels in the chambers.

Variant 2:

Variation 2 is modified such that the liquid level of the second chamber 13 is controlled by changing the rotational speed of the hollow shaft 3, while liquid level of the first chamber 12 by controlling the filtrate F1 deducting pump 10 in the given

Area is held.

When choosing which of these two variants is preferable, take into account that the changes in the parameters of negative pressure rotational speed have different effects on filtration to have. Thus, the adjustment of the rotational speed is a common means to make an effective adjustment to changing drainage capacity of the suspension. However, an increase in the rotational speed may lead to increased and / or higher solids turbid filtrate, which may be undesirable. The applied for filtrate vacuum is purchased with energy expenditure; an increase can therefore lead to higher operating costs. It is positive that the thick material can reach a higher dry content. However, in the case of poorly dewaterable or higher-consistency suspension, the influence of the reduced pressure on the throughput is less than that of the rotational speed of the hollow shaft.

Variant 3:

The aim of this variant is to set the process in each case optimally to the requirements. It therefore provides that by an intelligent

Regulation during operation of the plant a selection is made, whether variant 1 or variant 2 provides the highest benefit, and is switched accordingly. It can be considered that changes in the properties of the suspensions to be filtered S1 and / or S2 or by changing the operational requirements, such as required quality or

Quantity of filtrates and / or slurries switching from one variant to the other leads to better results.

Another possibility for influencing the operation of a filter device carrying out the method is the addition of dilution water into the filter trough 4 or in its feed lines. This is shown in FIG. 4, in which the scheme of FIG. 3 has been supplemented accordingly. Thus, the dilution water 24 via control valves 25 and 26 is optionally added to the streams of the suspensions S1 and S2, in this example, in the suction lines of the pumps 8 and 9. The control valves 25 and 26 receive signals 27 and 28 from the process control system PLS. As a further embodiment of the method, the negative pressure sucking the filtrates is hereby generated in each case with a geodetic drop height H1 or H2. These are the filtrate 33 and 34, dip in the downpipes z. B. placed in a lower level / floor of the building. The liquid levels in the filtrate beds 33 and 34 have in practice an upper and a lower limit, which leads to a minimum or maximum value of the negative pressure for filtrate suction. These fluid levels can each be adjusted according to the required negative pressure. For this purpose, the signals 29 and 31 give the level values to the process control system PLS. The output from the process control system PLS signals 30 and 32 then adjust the control valves 35 and 36, the z. B. in the drain lines of Filtratbütten 33 and 34 are located. In order to regulate the negative pressure, controlled control valves 35 ^' and 36 ^{' can} also be located in the downpipes between the hollow shaft 3 and the filtrate chest 33 or 34. The use of downpipes for vacuum generation is particularly favorable and may save two pumps. It is not limited to the example of FIG. 4. The negative pressures in the Filtratbütten 33 and 34 can also by other means, for. B. pumps are generated.

The regulation of the process can actually take place in the case of a disc filter having two chambers 12 and 13 as follows:

1 . The liquid level of a chamber (eg, chamber 12 in Fig. 4) is regulated as a "Level Master." The maximum pressure for the filtrate suction is set and no dilution water is added, and the liquid level is continuously measured as a reaction

a) if the value is too high, the rotational speed of the hollow shaft 3 is increased, b) if the value is tolerable, the rotational speed is equal, and c) if the rotational speed is too low, the rotational speed is lowered. 2. The liquid level of the second chamber (eg, chamber 13 in Fig. 4) becomes regulated as "Level Slave." The liquid level is constantly measured.

a) becomes too high

if dilution water 24 flows, its addition to the control valve 26 is reduced,

- otherwise the negative pressure for the filtrate suction increases (higher

Power of the pump 1 1 or level reduction in the filtrate chest 33 or the control valve 35 ^' open),

b) at tolerable value, everything is left equal, and

c) if the value is too low

- if the negative pressure for the filtrate suction is greater than minimum

(Power of the pump 1 1 reducible or level in the Filtratbütte 33 still below the upper limit), this lowered.

otherwise the addition of dilution water 24 on the control valve 26 increases or the control valve 36 ^' throttled / closed.

3. If the liquid level of the second chamber (chamber 13 in Figure 4) exceeds the set point by a predetermined maximum value (eg twice) and is

at the same time the liquid level of the first chamber (chamber 12 in Figure 4) is lower than the setpoint, so there is a master-to-slave and slave-on-master

Switching. In this case, therefore, the first chamber (chamber 12 in Figure 4) is regulated as before the second chamber (chamber 13 in Fig. 4).

Criteria for deciding in which chamber (12 or 13) the operation is best controlled only by changing the negative pressure, z. B. In which chamber is the suspension S1 or S2 with the lower

Dewatering resistance fed, or

in which chamber is the suspension S1 or S2 with the lower

Fed solids content, or

in which chamber is the one with the lower on the filter surface related volume flow rate driven, or

which chamber has the most free capacity. The previously described methods, each with two different suspensions S1 and S2 and two separately withdrawn filtrates F1 and F2 are certainly the most important applications of the invention. In principle, however, a disk filter can also be subdivided into three or chambers and provided with a corresponding number of filtrate systems in the hollow shaft and filtrate outlets. With this constructive

Thus, additional expenditure could be carried out simultaneously in one machine using three or more filtrations. The regulation would then have to make a negative pressure influence on at least two filtrates.

Claims

claims

1. A method for filtering suspensions in a filter device with a plurality of disk-shaped segmented filter elements (1, 1 ^' ) which rotate in at least one of the suspension (S1, S2) receiving the filter trough (4) and which are fixed to a rotating hollow shaft (3) through which the filtrate (F1, F2) formed during filtering is discharged, while at least some of the solids are retained on the filter elements (1, 1 ^' ) and removed as thick matter (5, 6) from the filter device,

characterized in that at least two filter elements (1, 1 ^' ) each with different suspensions (S1, S2) are acted upon, and that between the suspension (S1, S2) in the filter trough (4) and the filtrate (F1, F2) applied Pressure difference of different suspensions (S1, S2) acted upon filter elements (1, 1 ^' ) is set differently.

2. The method according to claim 1,

characterized in that the filtering process is controlled by both at least two independent pressure differences and the rotational speed of the hollow shaft (3) are used as a manipulated variable.

3. The method according to claim 1 or 2,

characterized in that a filter trough (4) having at least two chambers (12, 13) is used, and that from the hollow shaft (3) carrying all filter elements (1, 1 ^' ) the filtrates (S1, S2) originating from different suspensions (S1, S2) F1, F2) are derived separately.

4. The method according to claim 3,

characterized in that the liquid level of the one chamber (12, 13) by changing the rotational speed of the hollow shaft (3) and the Level of the other chamber (13, 12) is adjusted by changing the applied to the associated Filtratleitung the hollow shaft (4) negative pressure.

5. The method according to claim 3 or 4,

characterized in that the process control system (PLS) makes a selection in which chamber (12, 13), the regulation of the liquid level by changing the rotational speed of the hollow shaft (3) is made.

6. The method according to claim 5,

characterized in that the selection to maximize the throughput of the filter device is made upon satisfaction of the required quality requirements.

7. The method according to any one of claims 3 to 6,

characterized in that the regulation of the liquid level in all chambers (12, 13) is adjusted by changing the vacuum applied to the associated filtrate line of the hollow shaft (4), and that the change in the rotational speed of the hollow shaft (3) in a chamber (12, 13) has priority over the change in negative pressure.

8. The method according to any one of the preceding claims 3 to 7,

characterized in that in at least one chamber (12, 13)

Dilution water (24) is added, and that the amount of this

Dilution water (24) is adjustable and is included in the control concept.

9. Method according to one of the preceding claims,

characterized in that the filter trough (4) is operated at ambient pressure.

10. The method according to any one of claims 1 to 8,

characterized in that the filter trough (4) is operated with positive pressure.

1 1. Method according to one of the preceding claims,

characterized in that the majority of the solids, preferably at least 80% retained on the filter elements (1, Γ) and as thick matter (5, 6) is discharged from the filter device.