A METHOD AND A DEVICE FOR DISCHARGING PULP FROM A PULP TOWER

Technical Field

The present invention relates to a method for discharging cellulose- containing pulp from a pulp tower, in which pulp tower the pulp is treated and/or stored, the pulp being introduced into the pulp tower through at least one inlet and discharged through at least one outlet. The present invention also relates to a device for discharging cellulose-containing pulp from a pulp tower, in which pulp tower the pulp is treated and/or stored, where the pulp tower comprises at least one inlet for introducing pulp, and the device comprises at least one outlet for discharging pulp from the pulp tower. Further, the present invention relates to a pulp tower for treating and/or storing cellulose-containing pulp, which pulp tower extends substantially vertically and comprises at least one inlet for introducing pulp and at least one device for discharging pulp from the pulp tower.

Background of the Invention

When storing cellulose-containing pulp, tanks called pulp towers are used, which extend in a vertical direction. These are usually cylindrical with a circular basis. The pulp towers are used for storing and also treating pulp, such as chemical treatment, e.g. bleaching the pulp, and can be included in a line for treating pulp or be an independent unit which is not included in a line. A common pulp tower is a so called upflow tower where the introduction of pulp is effected in the lower part of the pulp tower and the discharge of pulp is effected in the upper part of the pulp tower, whereby the general pulp flow in the pulp tower goes from below upwards. Today, discharge of pulp from a pulp tower, e.g. an upflow tower, is effected by means of a so called tower scraper or similar devices. The tower scraper is positioned at the top of the pulp tower and comprises a rotor having two or more arms which are provided with scraping blades. The rotating scraping blades are arranged to work into the pulp and feed the pulp towards the sides of the pulp tower. At the sides of the pulp tower, an outlet is provided for discharging the pulp.

However, there are problems with said discharge by means of a tower scraper or similar devices. The tower scraper is a heavy and relatively large apparatus resulting in an extensive and expensive installation, especially as the tower

scraper must be lifted up and positioned at the top of the pulp tower. Extensive monitoring, control and maintenance work of the tower scraper are required, as it comprises several movable parts and as its rotor in some cases rotates continuously, and additionally, the pulp tower must be provided with platforms, and lifting equipment is required for a secure maintenance work performed by a user. The weight of the tower scraper also puts stresses on the pulp tower. Further, power is required to keep the tower scraper running, which increases the operative expenses.

WO 99/25918 A1 discloses a pulp tower having a bottom which extends obliquely upwards from the outlet of the pulp tower, which outlet is provided in bottom part of the pulp tower.

US 3 053 067 describes a pulp tower provided with a pipe positioned centrally within the pulp tower for discharging cellulose-containing pulp. The inlet of the pipe is provided in the upper part of the pulp tower, and its outlet is provided in the bottom part of the pulp tower. The pulp is fed to the inlet of the pipe by means of a tower scraper and a feed-screw.

The Object of the Invention

The object of the present invention is thus to provide a more efficient discharge of cellulose-containing pulp from a pulp tower which requires less mainte- nance work in relation to prior art. Further, the object of the present invention is to overcome the other problems mentioned above.

Summary of the Invention

The above mentioned objects are achieved by providing a method of the kind defined in the preamble of claim 1 , which comprises the features which are mentioned in the characterizing portion of claim 1.

By providing said feeding surface, no power-driven discharge of pulp from the pulp tower is required. By this sloping feeding surface a continuous and controlled pulp flow to and through the outlet is obtained, whereby the amount of stagnant pulp at the outlet is minimized, and "jamming" of the pulp and clogging at the outlet is prevented, which would not be the case if there was only an outlet without said feeding surface. By the method of the invention, a pulp discharge is attained which does not require an extensive operation or monitoring as no apparatus having rotating parts is needed. This results in a radical reduction of the

power required for discharging the pulp. The only control required is the control of the general pulp flow within in the pulp tower. The method of the invention does not require any expensive and complicated installations of heavy and complicated mechanical equipment. The method is applicable to any pulp consistency. Advantageously, the pulp tower is a so called upflow tower where the pulp, contained in the pulp tower, before the entry onto the feeding surface, is forced to flow from below upwards, and the discharge of pulp is effected in the upper part of the pulp tower.

According to an advantageous embodiment of the method according to the present invention, the pulp is diluted by adding dilution fluid during the feeding of the pulp along the feeding surface. Hereby, the pulp is yet more easily fed downwards along the feeding surface and the amount of stagnant pulp is further reduced. Advantageously, the pulp can also be diluted after the discharge from the pulp tower by adding dilution fluid immediately after the pulp has been discharged from said outlet, which facilitates the onward transport of pulp.

According to the method of the present invention, the flow rate of the pulp is increased before entering onto the feeding surface by conveying the pulp towards a constriction provided above and in connection with entry of the pulp onto the feeding surface. Hereby, the pulp flow is subjected to a sufficient increase in speed before the entering onto the feeding surface in order to feed out the pulp along the feeding surface without any additional auxiliary means in the form of a tower scraper, or similar mechanically driven discharging devices, or dilution.

According to another advantageous embodiment of the method according to the present invention, the pulp fed along the feeding surface is controlled for preventing the pulp from laterally sliding off the feeding surface.

Further, the above-mentioned objects are attained by providing a device of the kind defined in the preamble of claim 6, which comprises the features which are mentioned in the characterizing portion of claim 6.

The device of the invention has an uncomplicated structure, which makes device easy to manufacture. Its uncomplicated structure and light weight contributes to an easy installation both when manufacturing the pulp tower and when installing the device in a pulp tower already built and which is in operation. The device of the invention does not require any direct power supply for its operation, does not require any extensive monitoring, and does not subject the pulp tower to

extensive stress. By the device of the invention an efficient discharge of pulp without any extensive measures with regard to operation, monitoring and maintenance is attained, since a power-driven apparatus having rotating parts is not longer needed. This results in a radical reduction of the power required for discharging the pulp. Further, the device of the invention can manage pulp of both high and low pulp consistency, such as dry pulp and wet pulp, without any modification of the device. The invention minimizes the risk of breakdown as the device does not comprise any moving parts and as any external operation of the device per se is not required in order to discharge the pulp. According to advantageous embodiments of the device according to the present invention, the device comprises dilution means for adding dilution fluid and diluting the pulp during the feeding of the pulp along the feeding surface and/or diluting the pulp immediately after the pulp has been discharged from said outlet. Hereby, the feeding of the pulp downwards the feeding surface and the discharge of the pulp from said outlet is facilitated, whereby jamming of the pulp, because of the amount of stagnant pulp, is minimized.

According to a further advantageous embodiment of the device according to the present invention, the feeding surface is provided with control means for laterally controlling the pulp in relation to the flow direction of the pulp on the feeding surface for preventing the pulp fed on the feeding surface from laterally sliding off the feeding surface. These control means can, for example, comprise longitudinal side walls provided at each longitudinal side of the feeding surface.

The feeding surface can be designed in several different ways within the scope of the invention. It can be all flat with side walls, have a curved shape, as a pipe longitudinally cut which opens upwardly, or be part of a complete pipe. When a feeding surface which opens upwardly to a great extent of its length, is involved, advantageously, the feeding surface has an inclination of at least 10 degrees, more precisely between 10 and 25 degrees, in relation to the horizontal plane. The device can also comprise several feeding surfaces which can feed pulp to several different outlets included in the device, or several feeding surfaces can be provided after one another in the flow direction of the pulp fed on the feeding surfaces, and together form a linked feeding surface which feeds the pulp to the same outlet. The feeding surface is advantageously provided and designed so that each individual pulp fibre of the general pulp flow has as short distance as possible to

the feeding surface and advantageously, the feeding surface an extension as long as possible for providing the pulp, which is fed on the feeding surface, with a long feeding-down distance, which results in a continuous feeding-down of the pulp along the feeding surface. The inclination of the feeding surface shall be of such an order that the pulp fed on the feeding surface obtains a flow rate which is high enough to minimize the stagnation of the pulp and at the same time low enough so that jamming at the outlet is prevented, which is the result if pulp having too high flow rate reaches the outlet.

According to the device of the present invention, the device comprises a guiding device, positioned at least above the entry of the pulp onto the feeding surface, for increasing the flow rate of the pulp before entering onto the feeding surface, by conveying the pulp past a constriction, provided by the guiding device, positioned above and in connection with the entry of the pulp onto the feeding surface. By providing such a guiding device a prominent flow constriction is ob- tained, which gives the pulp flow a sufficient velocity increase before the entering onto the feeding surface in order to feed the pulp along the feeding surface without any additional auxiliary means in the form of a tower scraper, or similar mechanically driven discharge devices, or dilution. This flow constriction can be provided in different ways, for instance by providing the guiding device with guiding plates an- gled in relation to the longitudinal axis of the pulp tower and mounted above the feeding surface, which additionally guide the pulp towards the feeding surface. The feeding surface per se can, to a limited extent, contribute to this flow constriction as the pulp first is forced to pass the feeding surface before the entering onto the feeding surface. The flow constriction of the guiding device can also be pro- vided by upwardly reducing the inner cross-section area of the upper part of the pulp tower, for example, by giving the top of the pulp tower a conical shape or a dome shape. The power consumed when throttling the pulp by means of said flow constriction is less than the power required to operate a tower scraper which discharges the pulp in a conventional manner. Further advantageous embodiments of the device according to the present invention emerge form the dependent claims.

Further, the present invention provides a pulp tower of the kind mentioned in the introductory part, which comprises the features which are mentioned in the characterizing portion of claim 16. Advantageously, the pulp tower is an upflow

tower adapted for a general pulp flow from below upwards, and where the outlet is provided in the upper part of the pulp tower. The pulp tower can be included in a line for treating pulp or be an independent unit which is not part of a line.

Brief Description of the Drawings The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:

Fig. 1 is a flow diagram illustrating an embodiment of the method according to the present invention, Fig. 2 is a schematic cross-section side view of a first embodiment of the device according to the present invention, Fig. 3 is a top view of the embodiment of Fig 2, Fig. 4 is a schematic cross-section side view of a second embodiment of the device according to the present invention, Fig. 5 is a top view of the embodiment of Fig 4,

Fig. 6 is a schematic cross-section side view of a third embodiment of the device according to the present invention, Fig. 7 is a schematic cross-section side view of a fourth embodiment of the device according to the present invention, Fig. 8 is a schematic cross-section side view of a fifth embodiment of the device according to the present invention, and

Fig. 9 is a cross-section of the feeding surface of Fig. 2 transverse to the flow direction of the pulp which is fed downwards along the feeding surface.

Detailed Description of Preferred Embodiments

Fig. 1 shows a flow diagram illustrating an embodiment of the method according to the present invention, comprising the following steps: Within a pulp tower, comprising an inlet for introducing cellulose-containing pulp and an outlet for discharging pulp positioned above the inlet, an elongated feeding surface is provided, at 101 , having a first end which connects to the said outlet and a second end which is positioned at a higher level than the first end. Cellulose-containing pulp is introduced into the pulp tower, at 102, via said inlet, which in this case is positioned in the lower part of the pulp tower, whereupon the pulp is forced to flow

in a direction from below upwards, at 103. Before the entry onto the feeding surface, the flow rate of the pulp is increased, at 104, by conveying the pulp past the feeding surface and towards a constriction provided above and in connection with the entry of the pulp onto the feeding surface. The pulp flows down onto the feed- ing surface, at 105, whereupon the pulp which is present on the feeding surface is diluted by adding dilution fluid, at 106. By the effect of gravity and the effect of the dilution fluid, the pulp is transported towards the outlet by feeding down the pulp along said feeding surface, at 107. The pulp fed on the feeding surface is laterally controlled in relation to the flow direction of the pulp on the feeding surface for preventing the pulp, which is fed on the feeding surface, from laterally sliding off the feeding surface. The pulp is discharged from the pulp tower, at 108, via the outlet and is additionally diluted by adding dilution fluid, at 109, immediately after the pulp has been discharged from said outlet, whereupon the pulp is subsequently transported, at 110. By above-mentioned steps, a controlled pulp flow through the outlet is provided.

Fig. 2 shows a first embodiment of the device according to the present invention provided in a substantially vertically extending flow tower 201 for cellulose- containing pulp, where the inlet for the pulp is provided in the lower part of the pulp tower, and the general pulp flow, illustrated by arrows 202, goes from below up- wards, the pulp tower 201 thus being a so called upflow tower 201. Only the upper part of the flow tower is shown in Fig. 2. The device comprises an outlet 203 for discharging pulp from the pulp tower 201 and an elongated feeding surface 204 adapted for installation inside the pulp tower 201 , and having a first end 205 connected to the said outlet 203 and a second end 206 positioned at a higher level than the first end 205, the second end 206 being attached to the inner wall of the pulp tower 201. The feeding surface 204 is arranged to transport the pulp to the outlet 203 by feeding the pulp downwards along this feeding surface 204 under the influence of gravity. The flow direction of the pulp which is fed along the feeding surface 204 is illustrated by arrow 220. The device comprises dilution means 207, 208 for adding dilution fluid and diluting the pulp. A first dilution means 207, in the form of a nozzle 207, is provided at the first end 206 of the feeding surface 204 and thus arranged to dilute the pulp during the feeding of the pulp downwards along the feeding surface 204, which additionally facilitates the feeding of the pulp downwards this feeding surface 204. A second dilution means 208, in the form of a

nozzle 208, is provided immediately outside and above the outlet 203 and inside an outlet pipe 209 which is included in the device, and is in the form of a vertically extending stand pipe 209, which is adapted to be installed outside the pulp tower 201 and into which stand pipe 209 the outlet 203 opens. In the stand pipe 209, the pulp falls freely for onward transport, the flow direction of the pulp in the stand pipe 209 being illustrated by arrow 221. Herein, the feeding surface 204 is in the form of a longitudinally divided pipe which forms a channel shaped feeding surface 204. The vertical distance between the first and second ends 205, 206 of the feeding surface 204 is approximately one unit of length and the horizontal distance be- tween the first and second ends 205, 206 is approximately three units of length.

Fig. 3 shows a top view of the embodiment of Fig. 2, which above all shows the flow direction of the pulp inside and above the feeding surface 204. The arrow 220 indicates, as mentioned above, the flow direction of the pulp fed along the feeding surface 204 in the direction towards the outlet 203. The arrows 331 and 332 illustrate the flow direction of the pulp which is located a ^" bove the feeding surface 204 before the pulp has entered onto the feedings surface 204.

Fig. 4 shows a second embodiment of the device according to the present invention which, in contrast to the device of Fig. 2, is equipped with a guiding device 441 , 442 for increasing the flow rate of the pulp before entering onto the feeding surface. This guiding device 441 , 442 is positioned at least above the entry of the pulp onto the feeding surface 204. The guiding device 441 , 442 is shown more clearly in Fig. 5 which is a top view of the device of Fig. 4. The guiding device 441 , 442 comprises two guiding plates 441 , 442 angled in relation to the longitudinal axis of the pulp tower 201 , which are positioned above the entry of the pulp onto the feeding surface 204, and provides a flow constriction above and in connection with the entry of the pulp onto the feeding surface 204, which results in an increase in flow rate of the pulp in the flow directions 531 and 532.

Fig. 6 shows a third embodiment of the device according to the present invention, installed in a pulp tower 201 as disclosed in connection with Fig. 2. In this embodiment, the device comprises a pipe 651 and the feeding surface 604 is part of the inner surface of this pipe 651. The inlet 652 of the pipe 651 is arranged to be positioned adjacent to the longitudinal central axis of the pulp tower 201. The feeding surface 604 is provided with a collecting plate 653 which forms an angle with the longitudinal axis of the pulp tower 201. This collecting plate 653 is ar-

ranged to collect pulp to the feeding surface 604, whereby the entry of the pulp onto the feeding surface 604 becomes more efficient, but also contributes to a flow constriction when the pulp is forced to pass the collecting plate 653 on its way upwards, whereby the flow rate of the general pulp flow increases. Further, this de- vice comprises the features, the equivalences of which are found in the device of Fig. 2, that the device comprises an outlet 603 for discharging pulp, that the first end 605 of the feeding surface 604 is connected to said outlet 603 and its second end 606 is positioned at a higher level than the first end 605, that device comprises dilution means 607, 608, in the form of nozzles 607, 608, for adding dilution fluid and diluting the pulp during the feeding down of the pulp along the feeding surface 604 and immediately outside the outlet 603 inside an outlet pipe 609, included in the device, into which outlet pipe 609 the outlet 603 opens and via which the pulp is subsequently transported.

Fig. 7 shows a fourth embodiment of the device according to the present invention installed in a pulp tower 761. Herein, more of the pulp tower 761 is shown, and in the lower part of the pulp tower 761 the inlet 762 for introducing cellulose-containing pulp is shown. This device is, in contrast to the device of Fig. 6, additionally provided with a guiding device 763, positioned at least above the entry of the pulp onto the feeding surface 604, for increasing the flow rate of the pulp before entering onto the feeding surface 604 by conveying the pulp past a constriction, provided by the guiding device 763 in connection with the entry of the pulp onto the feeding surface 604. This guiding device 763 comprises a guiding plate 763 extending along the inner circumference of the pulp tower 761 and angled in relation to the longitudinal axis of the pulp tower 761 , which guiding plate 763 extends above the entry of the pulp onto the feeding surface 604 and provides said flow constriction, whereby the upper part of the pulp tower obtains a conical shape.

Fig. 8 shows a fifth embodiment of the device according to the present invention installed in a pulp tower 871 , which device comprises a different guiding device 872 in relation to the device of Fig. 7, but with the same effect, i.e. an increase of the flow rate of the pulp. This guiding device 872 comprises an internal concave surface 872 which is provided in the top of the pulp tower 871 and extends above the entry of the pulp onto the feeding surface 604, whereby the upper part of the pulp tower 871 obtains a dome shape.

Fig. 9 shows a cross-section of the feeding surface of Fig. 2, transverse to the flow direction of the pulp which is fed downwards along the feeding surface, where it is shown that the feeding surface 204 of the device is provided with control means 981 , 982, in the form of longitudinal side walls 981 , 982 at each longi- tudinal side of the feeding surface 204, for laterally controlling the pulp in relation to the flow direction of the pulp on the feeding surface 204, for preventing the pulp fed on the feeding surface 204 from laterally sliding off the feeding surface 204. Herein, it is shown that the feeding surface 204 is provided with two collecting plates 983, 984 which are mounted on the side walls 981 , 982 of the feeding sur- face 204, and which form an angle with the longitudinal axis of the pulp tower 201. These collecting plates 983, 984 are arranged to collect pulp to the feeding surface 204, whereby the entry of the pulp onto the feeding surface 204 becomes more efficient, but also contribute to a flow constriction as the pulp is forced to pass the collecting plates 983, 984 on its way upwards, whereby the flow rate of the general pulp flow is increased.