US2764011A - Bleaching tower with mixing device - Google Patents

Description

MAQ

Sept 25, 1956 J. c. F. C. RICHTER 2,764,011

BLEACHING TOWER WITH MIXING DEVICE Filed Sept. 8, 1954 2 Sheets-Sheet 1 Sept. 25, 1956 J. c. F. c. RICHTER BLEACHING TOWER WITH MzxlNG DEVICE 2 SheetS-Sheet 2 Filed Sept, 8, 1954 l/VVE/V TOR.'

Johan C'. EC. Rvhi'er,

United States Patent() BLEACHING rowER WITH MIXING DEVICE Johan Christoifer Fredrik Carl Richter, Karlstad, Sweden, assignor to Alrtiebolaget Kamyr, Karlstad, Sweden, a Swedish company Application September 8, 1954, Serial No. 454,686

Claims priority, application Sweden September 11, 1953 Claims. (Cl. 68--181) When continuously bleaching cellulose pulp in towers the general rule has been that at low consistencies, i. e. 3 to 8 per cent dry content, the pulp is fed into the bottom of the tower and discharged at the top, whereas at high consistencies the pulp is supplied at the top and discharged at the bottom of the tower. The main reason therefor has been the diiculty of feeding thick pulp upwards through towers, while downward feed is facilitated by the gravity of the pulp. One would believe that it always were preferable to supply the pulp at the tower top and discharge it at the bottom, but that does not hold true, because diiculties are encountered due to the tendency of the liquid to drain, whereby the pulp thickens. Said tendency often causes that the movement of the pulp is slowed down at certain parts of the cross section of the tower, so that the pulp runs through at an increased speed along a path of restricted cross section, or it may even cause the formation of bridges or plugs whereby the movement of the pulp is completely blocked.

In this respect a tower with upward feed of the pulp works more favourably, and the feeding devices have also proved to be cheaper and to require less power in this case. However, it has not been possible to use upward feed for a higher consistency than 8 per cent, except in one case, viz. in applicants concentrator tower shown in U. S. Patent No. 2,383,684, where a screw press feeds thick pulp from below into a tower which in order to counteract the tendency to unequal ilow distribution is provided with a pointed conical bottom portion. In said design a considerable tower volume is lost and besides, in towers of large diameter there is nevertheless still a risk of unequal flow distribution.

The present invention is based thereupon that the applicant has recently found it possible to dispense with said screw press and to feed high density pulp against a considerable static pressure by means of pressure gas or air. When using said new method of feeding pulp to the bottom of a bleach tower two problems are encountered, Viz. the problem of eliminating the tendency of unequal flow distribution, whereby the pulp moves in the centre of the tower but slows down along the walls of the tower, and the problem of simultaneously mixing the pulp with the bleaching agent in an effective manner.

According to the invention these problems are solved thereby that the tower is provided with a disk rotary about a vertical axis, which disk wholly covers the central pulp inlet opening in the bottom of the tower and which together with the part of the tower bottom surrounding the inlet opening forms an annular passage for directing the pulp radially outwards, the under side of said disk having pins, obliqnely set blades or similar stirring members adapted to mix into the pulp a bleaching agent supplied at or before the inlet opening. By said means the pulp is forced towards the periphery of the tower and the upward directed feeding force will be better distributed over the whole cross section of 'the tower, and therefore the bottom portion of the tower ICC need not be shaped as pointed as has been necessary earlier. By the radial spreading of the pulp the mixing effect will be particularly effective, as the rotary stirring members, which are located on different radial distances, move with different absolute speeds.

An embodiment of the tower according to the invention is shown in the accompanying drawings, Fig. l being an elevation and vertical cross section of the tower and the mixing device, and Fig. 2 being :a cross section of the mixing device shown on an enlarged scale.

In the drawing, l1 designates the bleaching tower, preferably consisting of concrete. The tower is in the shape of a standing cylinder having a downwardly tapering conical or partly spherical portion 13 merging with a smooth bend into the substantially plane and horizontal bottom 15. Inserted in the centre of the bottom is an inlet piece 17 in the general shape of a tube bend. Said inlet is connected to the end of the tube 19. Pulp is forced through said tube by pressure air feed or by some other method affording a pressure suciently high to push the pulp up through the tower and over its overflow edge 21. A rotary scraper 23 is arranged at the top of the tower in order to facilitate an even discharge of the pulp.

In and above the inlet piece 17 is a rotor consisting of a horizontal disk 25 and a hub 26 therefor keyed to the upper end of of a vertical shaft 29. Said shaft is journalled in bearings 26, 28, is driven by a driving means 27, and passes through a sealing sleeve 31 positioned in the centre of the inlet piece 17. Keyed to the shaft 29 between said sleeve and the hub 26 is a second hub 30 which carries a number of stirring arms 35, 36. Said arms move in the part of the inlet piece 17 located immediately below the level of the upper face of the tower bottom. The disk 25 and its hub 26 form the upper wall of an annular passage 33 being the extension of the upwardly directed tubular passage located around the sleeve 31. In said passage 33 the moving pulp is diverted to the sides and the pulp leaves the same between the periphery of the disk 25 and the tower bottom in a radially outward and mainly horizontal direction and with an even distribution all around the circumference.

Attached to the hub 26 and to the under side of the disk 25 are a number of stirring members 37, 38; 39 and 40. These members are `located at different radial dis'A tances, so that their circular paths are offset, as clearly shown by the dot-dash-lines. The stirring members may be shaped as pins, vanes, obliqnely set plates or blades, or similar, and may be present in a number depending upon the size of the disk, the pulp consistency, the capacity and the power consumption. Similar stirring members 41, 42 are attached to the tower bottom and project in between the above-mentioned stirring members, so that the pulp alternately meets rotary and stationary stirring members. Preferably, the members 41, 42 are shaped as twisted blades, whereby the pulp is displaced also in the vertical direction in order to improve the stirring elfect. The rotor has a step 43 opposite the end of the stirring member 41 towards which said member projects, thereby catching the pulp following along the under surface of the disk 25. The bleaching agent which may be chlorine gas, is supplied through a conduit 45 debouching into the inlet piece 17 immediately below the path of the rotary stirrer 36.

The diameter of the disk is of the order of 0.2 to 0.5 of the maximum diameter of the tower. Thus, in towers having a diameter of 3 to 5 meters, the diameter of the disk preferably is between 1 and 2 meters. The upper side of the disk is provided with an upwardly directed point 47, preferably formed approximately as a cone.

The rotary disk not only gives an even pulp flow through the tower but has also proved to serve eiciently as -a mixer.

3 Thespeed of the rotor is held between 100 and V3.01)11. -P` M The .stirring 4pins or V,blades which preferably are set somewhat obliquely in order to whirl the pulp around, extend across the major part of the :annular .passage to Athe neighborhood :of the opposite r wall v'thereof 4and haven height orlength of @il to 0.3 meter. Usually the volume ofi-the space between the Vdiskland the Vtower .bottom sis Vof the order of 0.25 to il cubic :meten The pulp quantity continuously vthrough the inlet is ofthe order of 0,3 'to 2 cubic meters `per minute, yand fro'm these vfigures Aitlfollowsthat it will Stake each fpulp Hparticl'esa time of A0.5'to lrminute to passfrom the inlet to ,the'periphery ofthedisk. Thisltime :is --suflicient for a thoroughmixing Vof the bleaching chemicals -with^the pulp AAn essential advantage ofthe above-described device lies therein that-.it effectively iprevents `a gaseous bleaching agent from rising in the .tower kbefore it lh-as been thoroughly mixed into andpartlylabsorbed by the pulp.

`When chlorine `gasris used, no gas can 'escape-for during the passage of the pulp in the channel covered by the disk, thechlorine is mixedwith `and absorbed in the pulp to such-adegree that there is no risk of free chlorine seeking its way out .ofthe tower.

1I. claim:

1. Iny a plant for bleaching cellulosic pulp, 'a cylindri- Y.cal tower having A.a tapering lower end, fa pulp supply tube having an inlet at the centre of the lowerend of said tower, a horizontal -disklocated labove said inlety said disk forming with lsaid .inlet and the bottom of `said -tower a passage for diverting an upward axial-How of pulpV into an annular radial ow, a rotary vertical vshaft coaxial with the tower fand journalled below its bottom, said shaft carrying said disk, a first `group Vofi-varies fat tach'ed tosaid disc and spaced about the outer periphery thereof, ysaid vanes extendingdownwardly from-said disc :towards the lower yend-of saidtower, and a second group of vanes rattached to the lower end of said tower adja- Icent lsaid inlet and projecting inwardly towards said vertical 4shatt `to a position intermediate said shaft and said first group of vanes.

l2. In va plant for bleaching -cellulosic pulp, a con- `crete `tower having a main cylindrical `part andan `essentially'semi-spherical bottompart, a pulp inlet at the centre offsaid bottom part, means for` continuously fee'ding pulp upwardsthrough the tower, a rotary displace- -ment body located in and yabove said inlet forspreading the pulp supplied through the inlet radially outwards along the surface 0f said bottom part of -the"tower, vsaid rotary displacement body including a disc member positioned vWithin said ltower over `said inlet and spaced a distance therefrom, and vanes attachedto said disc and extending downwardly therefrom atleast half the distance between said disc and said bottom `part of said tower for 4agitating the -pulp simultaneously with the spreading thereof.

3. In a plant for bleaching cellulos'icfpulp, Ia'cylindri- L4.in .the center thereof, `passage means AVfor .supplying `lpulp and bleaching chemicals continuously into said tower through said bottom wall opening, means for discharging bleached pulp from the top of said tower and a mixing device for mixing the pulp and bleaching chemicals fed through said bottom wall opening, said mixing device comprising ahead member positioned over Isaid vbottom wall opening and forming an annular, substantially horizontally directed peripheral inlet for'spreading pulp yand bleaching chemicals vfrom said passage means radially outwards along said bottom wall, a shaft rotatably'positioned withiny said -f-bottom wall opening and pulp agitating means associated with said shaft and projecting into said passage Ameans in vthe path of the pulpsupplied to said tower to agitate said pulp and bleaching chemicals.

4. In a plant for continuously bleaching cellulosic pulp, a cylindrical bleaching tower having a bottom wall with ian opening in Kthe center thereof, means for feeding pulp to said ytower through said central opening, and

:mixing means ,positioned withinsaid vopening .for mixing saidfipulp with bleaching chemicals, 'said mixing means .comprising a rotatable horizontally disposeddisc posi- 4tioned within said ytower and spaced from the bottom 'wall thereof, ;a lvertical shaft `extending upwardlyv through said opening .and .carrying fsaidv disc, means-for .driving said .shaft to rotate said disc, La 'plurality of agit-ating membersiixed `to said disc and `extending Adownwardly therefrom .and terminating ,short 'of `said bottom wall, secondgroup of agitating members, means :to move said second group tof agitating .members 'by said shaft, ,said

ysecond group ,of agitatingvrnembers extending into the path fof pulp "suppliedxtoaid tower, and a plurality of agitatingV members xed tosaid tower .andrpositioned between theimembers xed to said disc and saidsecond group ofagitating members.

5. In :a plantforbleaching'rcellulosic pulp, a reaction tower having a bottom Wall, an annular, substantially :horizontally directed ,peripheral ,pulp inlet in the center of vsaidxbottom wall, tpassage means Vfor `supplying .pulp and bleachingichemicalsto :said tower through saidrpulp inletfmeans fordischargingbleached pulp 'fromthe'top Vof said tower, andiagitating means-positionedwithin. said passage means for mixing said pulp and-bleaching :chemicals priors-to theirppassagethrough said inlet `into said tower.

References .Cited "in the le of this .patent "UNITED 'STATES PATENTS Fraser.. 'Man 16, 1954

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