High consistency pulp tower

Abstract: The present invention relates to high consistency pulp towers and especially to improvements in discharging pulp therefrom. It is essential to the high consistency pulp tower (10) in accordance with the invention that a bottom pillar (30) is arranged at the center of the tower bottom and that cross-sectional area between pillar (30) and wall (12) of tower (10) is smaller at the upper end of pillar (30) than at its lower end.

Description: The present invention relates to high consistency pulp towers, and especially to improvements in discharging pulp therefrom. These towers are used in the wood processing industry, e.g., for peroxide bleaching and storage of high consistency pulp. According to prior art, pulp has to be discharged in a diluted form from high consistency pulp towers. This is because high consistency pulp cannot be pumped with, for example, a centrifugal pump which, however, in recent arrangements is practically the only way of conveying pulp from one process stage to the other. Therefore, high consistency pulp (having most commonly a consistency of 20 to 35%) is diluted to at least a medium consistency (of about 10 to 15%) in the bottom portion of the pulp tower. This makes the pulp pumpable with a so-called fluidizing centrifugal pump. Preferably, pulp is diluted to a consistency of about 3 to 5%, whereby it will be pumpable with a conventional centrifugal pump. Dilution is effected by introducing either clean water or filtrate from some suitable process stage into the bottom portion of the tower and mixing it with the pulp by mixers arranged for that purpose in the bottom portion, i.e, a so-called dilution zone of the tower. Depending on whether high consistency pulp towers are used for either bleaching or storage, the constructions and appearances of their bottom portions are much different from each other for a number of reasons. Specific to all types of towers is, however, that even dilution is almost unattainable. The reason for this is that high consistency pulp as well as medium consistency pulp flows downwards in the tower unevenly. This again is caused by friction between the pulp and the tower wall, which retards the pulp flow so much that in between the zone of diluted pulp in the bottom portion and the undiluted pulp in the upper part of the tower, there will be formed an arch, which, after having expanded enough, will collapse down to the bottom portion of the tower. Since dilution liquid is introduced as an even flow into the tower, the pulp to be discharged from the tower is continuously diluted during arching and, immediately after the arch has collapsed, the consistency will increase to a maximum, whereby the required pulp consistency will remain somewhere between the maximum and minimum values. In one high consistency pulp tower the discharge consistency has been established to range from 3.2 to 6.1% (Fig. 1a). As the pulp is in most cases conveyed from the high consistency pulp tower to some other process stage, whereby chemicals are mixed with it in pumping or soon thereafter, it is easy to understand that the chemicals dosage per pulp unit cannot be even when the consistency ranges so drastically. Another problem resulting from the collapse of high consistency pulp down to the bottom portion of the tower may also be difficult, namely because it is quite possible that the mixer may be damaged by the great volume of pulp falling onto it. In the worst case, the entire process has to be stopped for the repairs of the mixer. For example, FI patents 58522 and 64821 disclose means which aim to adjust the feed of high consistency pulp to the dilution zone so as to make the pulp flow evenly, whereby the discharge of pulp from the dilution zone would also be effected in a steady consistency. Said means comprise a rotative intermediate floor arranged in the pulp tower and preferably provided with blades for feeding pulp through a slot between the tower wall and the intermediate floor down to the dilution zone. However, said means involve several drawbacks. Firstly, the intermediate floor has to be of a very sturdy construction because it carries practically all the weight of the pulp inside the tower. Secondly, the shaft of the intermediate floor and the bracing of the shaft have to be also very strong, not to mention the axial bearings of the shaft. Thirdly, the rotative intermediate floor is susceptible to damage, for example because, irrespective of all preventive measures, pulp may arch above the intermediate floor in the tower. Collapsing, in the worst case one-sided collapsing of such arched pulp, causes a heavy impact on the intermediate floor as well as on the bearings and the shaft, which easily breaks the bearing system or bends the shaft and/or the intermediate floor. Fourthly, the bearing system and the rotative mechanism calls for space below the tower. So, the tower has to be located relatively high up. This in turn results in that the bearings of the upper end of the shaft for the intermediate floor, which bearings for practical reasons are right below the intermediate floor, must be applied high up in the tower wall an be supported by the wall. Thus, the tower wall has to be particularly sturdy from the base to a very high level. Because of these numerous drawbacks of the rotative intermediate floor, this arrangement is not in use at pulp mills today. Another way of arranging even downwards flow of pulp is described below. In the smallest towers having a diameter of about 3.5 to 7.0 m, the bottom portion may be either straight cylindrical or first somewhat narrowing and below that cylindrical. In bigger towers having a diameter typically larger than 5.0 m, a so-called bottom pillar is disposed at the center of the tower bottom. The purpose of the bottom pillar is to uphold pulp above the bottom portion and to divide the bottom portion into an annular mixing zone. Thus, for example, the maximum diameter of collapsing pulp may only be as long as the tower radius, whereas in the towers with no bottom pillar, it may equal the tower diameter. The shape of the prior art bottom pillars may be either an evenly converging cone (Fig. 2a), cylindrical pillar (Fig. 2b) or, according to the state of the art, a cylindrical pillar the upper end whereof is arranged with an upwardly converging cone (Fig. 2c). In all those towers which are provided with a bottom pillar, the dilution mixer/dilution mixers are disposed on sides of the bottom pillar so that they direct the flow to circulate along the annular mixing zone. The bottom pillars are of solid construction and when disposed on the tower bottom they are merely supported by the tower bottom or the foundation therebelow, in any case by the very point which would also otherwise carry the weight of the pulp in the tower. However, it has been shown in practice that no more conical or cylindrical bottom pillars than combinations thereof can eliminate the unevenness of the pulp discharge consistency. As shown in Fig. 1a, the discharge consistency fluctuates from 3.2 to 6.1% when a bottom pillar according to Fig. 2c is used. Correspondingly, also the volume flow of the pulp being discharged fluctuates from 210 to 240 m 3 /h because the centrifugal pump is not at all insensitive to remarkable changes in the consistency. A pulp tower according to the preamble of claim 1 is known from US-A-34 04 304 and comprises a plurality of rotary scrapers located eccentrically of a central outlet at the bottom of the tower to sweep across areas of the container bottom and advance the pulp towards the outlet. Additionally, a shaft extends along the vertical axis of the tower which serves to apply bleaching agents, washing liquid or the like into the tower and comprises a conical portion widening in the downward direction and terminating above the outlet in a plane annular face. The maximum diameter of the annular face approximately equals the diameter of the cylindrical outlet in the container bottom so that a narrow passage is formed at the lower part of the ranges of the scraper means to facilitate discharge of the pulp. A high-consistency pulp tower is also known from EP-A-0 269 124 and comprises scrapers at the bottom of the tower to convey pulp to the discharge outlet. A fluidizing rotor is arranged within the outlet to facilitate the discharge of the pulp through the outlet. It is the object of the invention to provide a high-consistency pulp tower which ensures efficient and reliable dilution at and discharge of the pulp from the tower bottom. This object is solved according to the present invention by high consistency pulp tower comprising the features of claim 1. Detailed embodiments are defined in dependent claims. Fig. 1b shows the test results received with a bottom pillar according to a preferred embodiment of the invention, which bottom pillar is illustrated in Fig. 3. In that test, all other variables were the same as those with the test for which the results are shown in Fig. 1a, except for the construction of the bottom pillar. The results of Fig. 1b indicates that when a bottom pillar according to the invention is used, the volume flow and the consistency of the pulp to be discharged do not practically change at all. High consistency pulp towers according to the present invention are explained more in detail in the following, by way of example, with reference to the accompanying drawings, in which Fig. 1a illustrates the discharge flow and the discharge consistency of discharge pulp as functions of time when pulp is being discharged from a prior art high consistency pulp tower; Fig. 1b illustrates pulp volume flows and discharge consistencies, provided by an improvement according to a preferred embodiment of the invention, as functions of time; Figs. 2a, 2b, and 2c illustrate bottom portions of prior art high consistency pulp towers; Fig. 3 illustrates the bottom portion of a high consistency pulp tower according to a preferred embodiment of the invention; Fig. 4 illustrates the bottom portion of a high consistency pulp tower according to a second preferred embodiment of the invention; Fig. 5 illustrates an embodiment according to Fig. 4 seen from above; Fig. 6 shows a flow pattern developed by the mixers in the bottom portion of the high consistency pulp tower of Fig. 3; Fig. 7 illustrates the bottom portion of a high consistency pulp tower according to a third preferred embodiment of the invention, Fig. 8 illustrates the bottom portion of a high consistency pulp tower according to a fourth preferred embodiment of the invention; Fig. 9 illustrates the bottom portion of a high consistency pulp tower according to a fifth preferred embodiment of the invention; Fig. 10 illustrates the bottom portion of a high consistency pulp tower according to a sixth preferred embodiment of the invention; and Fig. 11 illustrates the bottom portion of a high consistency pulp tower according to a seventh preferred embodiment of the invention. The following reference numerals are used in Figs. 2a to 2c as well as through the whole description to denote the main components of a high consistency pulp tower; high consistency pulp tower 10, tower wall 12, bottom portion 20, tower bottom 22, bottom pillar 30, mixer/mixers 40, conduit/conduits 50 for feeding dilution liquid, and discharge means 60 for diluted pulp. The high consistency pulp tower 10 illustrated in Figs. 2a to 2c is used for, e.g., peroxide bleaching of pulp in a high (20 to 30%) consistency. In our example (shown in Fig. 2a), the diameter of tower 10 is about 5.5 m in the upper part of the tower and the total height is about 7.5 m. The height of bottom pillar 30 from bottom is about 1.8 m and the diameter of its lower part is 2 m. In the arrangement of Fig. 2a, bottom portion 20 and the upper part of tower 10, have a conical wall section 14 between them. However, it is not necessary for the operation of the tower. Mixers 40 are arranged at about 1.1 m height from the tower bottom and so that the dilution liquid is fed through conduits 50 to the dilution zone of the bottom portion, to a level which is a little higher than the mixer shafts. The tower of Fig. 2b is either cylindrical or it widens slightly downwards. The purpose of the widening is to ensure that pulp flows evenly downwards in tower 10. Bottom pillar 30 of Fig. 2b is cylindrical and its upper end is flat. In Fig. 2c, bottom pillar 30 is cylindrical, but it is provided with a conical upper end. Fig. 3 shows an improved high consistency pulp tower 10 in accordance with the invention, bottom portion 20 thereof being provided with a bottom pillar 30, which is preferably cylindrical, although other cross-sectional shapes are also applicable. The upper end of pillar 30 has, however, been reshaped in comparison with prior art constructions. It is essential to the upper end of pillar 30 of this embodiment that the diameter of a parting member 31 disposed therein is at least in one point larger than the diameter of the lower part of pillar 30. More extensively, it is a feature of the invention that in the area of parting member 31, the cross section between parting member 31 and wall 12 of tower 10 is smaller than in the bottom area of the pillar 30. In the embodiment of Fig. 3, parting member 31 is formed of a first section 32, the diameter of which widens conically upwards, and a second section 34, the diameter of which converges conically upwards. In other words, at the point of contact between said first and second sections the diameter of the parting member is at its largest, whereby a throttle is formed between parting member 31 and tower wall 12. The purpose of this throttle is to even the downwards flow of high consistency pulp. It has to be noted, however, that the term "conical" has been used above and will be also used further below to specify a piece widening, or correspondingly converging, in some direction. So, in practice, the conical parting member is replaceable with, for example, quadrangular, pentangular, or hexagonal jacket. Correspondingly, the term "diameter" may as well refer to a diameter of an imaginary circle calculated on the basis of the area defined by the above-mentioned polygonal jackets. Fig. 4 shows a bottom pillar in accordance with Fig. 3 except that the second conical surface 34 of parting member 31 is provided with preferably radial fillets 36, one end of each fillet being attached to wall 12 of tower 10. It can also been seen from Fig. 4 that, according to a preferred embodiment of the invention, said fillets 36 are cross-sectionally triangular. The number of fillets may be two to six and they are intended to prevent the pulp in tower 10 from starting to rotate on the level of the second conical section 34 of parting member 31. Fig. 4 also indicates how mixer 40 is preferably disposed relative to bottom pillar 30 in bottom portion 20 of the tower. Fig. 5 shows the bottom portion arrangement of the high consistency pulp tower of Fig. 3 seen from above. It can be seen that this embodiment contains four mixers 40 (the number of mixers may range from two to six, mainly depending on the tower size), each mixer being connected with a feed conduit 50 for dilution liquid. Mixers 40 are disposed in bottom portion 20 of the tower so that they cause the pulp to be diluted to circulate fast around bottom pillar 30. Fig. 6 illustrates how bottom portion 20, i.e, a so-called dilution zone, of a high consistency pulp tower in accordance with the invention operates in practice. For simplicity reasons, Fig. 6 illustrates only one mixer 40 and pulp being discharged from only one side of parting member 31 to the mixing zone of the lower part of the tower. The shape of parting member 31 according to the invention purposes to exactly mark off the mixing zone below the largest diameter of parting member 31 or, more extensively said, below the smallest cross-sectional area between parting member 31 and the wall of tower 10, so that the circulating flow provided by mixers 40 is prevented from rising to a level of parting member 31 which is higher than the upper end of pillar 30. In prior art constructions, rising of the flow to the upper end of the pillar and even slightly above it caused uncontrolled discharge of pulp from the upper part of the tower to the mixing zone. Another object of the invention is that mixers 40 bring about a ring-shaped circulation of pulp in the mixing zone of the tower, which ring-shaped circulation of pulp, by means of the great difference in both the flow rate and direction, then evenly "cuts" pulp from the slowly downwardly flowing high consistency pulp to the dilution zone. It can be concluded, when reviewing measuring results of Fig. 1b and comparing them with the results measured in the tower of Fig. 1a that the invention functions as it was intended to. In Fig. 1b, both the consistency and the volume flow of the discharge pulp are practically constant. Fig. 7 illustrates a bottom pillar 30 according to a third preferred embodiment and a parting member 31 disposed at the upper end of the pillar. The tip angle of the lower conical section 32 of parting member 31 has been decreased, whereby the length of the first conical section 32 has increased. Fig. 8 illustrates a bottom pillar 30 according to a fourth preferred embodiment and a parting member 31' disposed at the upper end of the pillar, where the first conical section of Fig. 3 has been replaced with a radial plate 32', which correspondingly limits the mixing zone to bottom portion 20 of tower 10. Fig. 9 illustrates still another way of improving the function of both the bottom portion and the mixing/dilution zone arranged therein. On wall 12 of tower 10, essentially on the level of the largest diameter of parting member 31 is disposed on inwardly extending throttling/guide ring 38. Its task is also to limit the mixing/dilution zone and to prevent the circulating flow, brought about the mixers, from rising unnecessarily high up. The described guide ring 38 may naturally be also used alone without any parting member 31 at the upper end of bottom pillar 30, i.e., an extension of the upper end of the bottom pillar. Fig. 10 shows an arrangement which slightly deviates from the earlier described embodiments. In this arrangement, a parting member 31'' is attached to the tower wall with arms 36', which may be used as fillets 36 of Fig. 4, to prevent the pulp from starting to circulate on the side of the parting member. The biggest difference between this and the above described embodiments is naturally that there is no bottom pillar in this embodiment, but the parting member is totally resting on arms 36'. Fig. 11 illustrates a still further embodiment where the arrangement of Fig. 10 has been further developed, for example, for towers having a still larger diameter. In the arrangement of Fig. 11, a parting member 31''' is also carried by the tower wall alone, through arms 36'', but parting member 31''' comprises a ring, the cross section of which substantially corresponds to that of the parting member 31 described in the above embodiments. Example 1 In a practical test where an improvement in a high consistency pulp tower in accordance with the invention (disclosed in Fig. 3) was tested, the dimensions of the tower and the bottom pillar were as follows: tower diameter in the bottom portion 6000 mm, diameter of the lower section of bottom pillar 30 1400 mm, maximum diameter of the upper end of the bottom pillar 2400 mm, height of the cylindrical part Of the bottom pillar 2100 mm, tip angle of the first conical surface at the upper end of the bottom pillar 90°, and tip angle of the second conical surface 60°. The elevation of the mixer shaft from the tower bottom was 950 mm, and the mixers used were similar to those disclosed in FI patent application 902486, in which the dilution liquid to be mixed is fed among the pulp from inside the jacket encasing the mixer shaft. Example 2 Tests with various extensions of the bottom pillar and guide/throttling rings attached to the tower wall revealed that the best results were gained when the area of the largest surface defined by the bottom pillar extension and being perpendicular to the axis of the tower was 2.5 to 3.2 times, preferably 2.8 times the cross-sectional area of the bottom pillar lower end. Defined in a different manner, a preferred construction is such that the ratio of the area between said extension and tower wall (or throttling/guide ring) to the area between the bottom pillar and tower wall is 0.5 to 0.95, preferably 0.90. It was also established that an advantageous construction was such that, in the area of the bottom pillar upper end the cross section between the tower wall (or throttling/guide ring) and the pillar was 50 to 95%, preferably 90% of the area between the bottom pillar lower end and the tower wall. Further, the most suitable tip angle of the cone or a piece of a like shape at the bottom pillar upper end proved to be 45 to 70 degrees, preferably 60 degrees. This ensures that high consistency pulp flows evenly downwardly towards the mixing/dilution zone. Further, it is worth mentioning that also many other constructions of the bottom pillar upper end remain within the scope of the invention. The sectional view of the upper end of the pillar in the axial direction may also be curved in surface, but preferably so that the main shape is relatively near to the combination of two opposite cones, as shown in Fig. 3. As indicated by the above described various constructional arrangements and examples, and especially by the practical test results shown in Fig. 1b, a novel, previously unknown constructional arrangement has been developed for the dilution zone of a pulp tower. This constructional arrangement ensures that the discharge of pulp from high consistency pulp towers takes place at an even volume flow and steady pulp consistency. One has to remember, however, that the above described constructional arrangements are only preferred examples of the numerous alternatives embodying the invention. Thus, the above examples are by no means intended to limit the invention from the scope defined in the accompanying claims.

Claims: A high consistency pulp tower (10), comprising an upstanding tower wall (12), A bottom (22), a bottom portion (20) defining a so-called dilution zone of said tower, means (40, 50) for diluting high consistency pulp, arranged in the bottom portion (20), discharge means (60) for diluted pulp arranged in the bottom portion (20), and a parting member (31, 31', 31'', 31''') arranged in the upper section of the bottom portion (20) whereby the cross-sectional flow area defined by the wall (12) of tower (10) and said parting member (31, 31', 31'', 31''') is smaller than the corresponding cross-sectional flow area below the parting member, characterized in that said dilution means comprises at least one mixer arranged entirely below the smallest cross-sectional flow area in such manner as to generate a circulating pulp flow in the bottom portion (20) and prevent the pulp flow from rising to a level of the parting member (31, 31', 31'', 31''') which is higher than the largest diameter of the parting member or the smallest cross-sectional flow area between the parting member and the wall of the tower (10). A high consistency pulp tower as claimed in claim 1, characterized in that said parting member (31, 31') is arranged at the upper end of a so-called bottom pillar (30) attached to the bottom (22) of the tower (10). A high consistency pulp tower as claimed in claim 2, characterized in that parting member (31, 31') is formed of an extension (32, 34) at the upper end of the bottom pillar (30). A high consistency pulp tower as claimed in claim 1, 2 or 3, characterized in that the wall (12) of the tower (10) is provided with a ring (38) extending towards the parting member (31, 31', 31'', 31'''). A high consistency pulp tower as claimed in claim 1, 2, 3 or 4, characterized in that between the parting member (31, 31', 31'', 31''') and the wall (12) of the tower (10) are disposed substantially radial fillets (36). A high consistency pulp tower as claimed in claim 1, characterized in that the parting member (31, 31', 31'') is formed of two conical or like surfaces (32, 34) with their larger ends being opposite each other. A high consistency pulp tower as claimed in claims 2 and 6, characterized in that the area of the largest surface defined by said conical or like surfaces (32, 34) and being perpendicular to the axis of tower (10) is 2.5 to 3.2 times, preferably 2.8 times the cross-sectional area of the lower end of the bottom pillar. A high consistency pulp tower as claimed in claims 2 and 6, characterized in that the ratio of the area between the parting member (31, 31') and the wall (12) of the tower (10) to the area between the lower end of the bottom pillar (30) and the wall (12) of the tower (10) is 0.5 to 0.95, preferably 0.90. A high consistency pulp tower as claimed in claim 2, characterized in that in the area of the parting member (31, 31'), the cross-sectional area between the wall (12) of the tower (10) and the parting member (31, 31') is 50 to 95%, preferably 90% of the area between the lower end of the bottom pillar (30) and the wall (12) of the tower (10). A high consistency pulp tower as claimed in claim 1, characterized in that two to six mixers (40) are arranged in the dilution zone of the tower (10). A high consistency pulp tower as claimed in claim 10, characterized in that said mixers (40) are provided with feed means for dilution liquid. A high consistency pulp tower as claimed in claim 4, when dependent on claim 2 or 3, characterized in that in the area of an upper end of the parting member (31, 31'), the cross-sectional area between the ring (38) attached to the wall (12) of the tower (10) and the parting member (31, 31') is 50 to 95%, preferably 90% of the area between a lower end of the bottom pillar (30) and the wall (12) of the tower (10). A high consistency pulp tower as claimed in claim 1, characterized in that the tip angle of a conical or like surface (34) forming the upper end of the parting member (31, 31', 31'', 31''') is 45 to 70 degrees, preferably about 60 degrees. A high consistency pulp tower as claimed in claim 1, characterized in that the parting member (31'', 31''') is attached through arms (36', 36'') to the wall of the tower (10). A high consistency pulp tower as claimed in claim 14, characterized in that the parting member (31''') is a ring and has a substantially triangular cross-section.

Inventors: TOUKONUMMI OLAVI, TOUKONUMMI, OLAVI
Applicants: AHLSTROEM PUMPUT OY
Application year: 1995
Main classification: B65D88
IPC codes: D21C7, D21D5