Abstract:
The arrangement concerns an improved design for at least one of the cooling, dilution and washing at the bottom of a continuous digester for the production of cellulose pulp. By arranging at least one extra strainer section above the lowermost strainer section, with the addition of washing fluid or dilution fluid between the extra strainer section and the lowermost strainer section, more washing fluid can be added at the bottom of the digester without counteracting the flow of the column of chips. This provides space for the increase of production, for improvement of the flow of the column of chips, or for combinations of these effects while retaining good cooling, washing and dilution at the bottom of the digester.

Description:
PRIOR APPLICATION  
       [0001]     This application is a U.S. national phase application claiming priority from Swedish Patent Application No. 0502042-5, filed 15 Sep. 2005.  
         [0002]     1. Technical Area  
         [0003]     The present invention concerns an improvement of the cooling, washing, and exchange of fluid in a continuous digester for the production of cellulose pulp.  
         [0004]     2. The Prior Art  
         [0005]      FIG. 1  shows a typical design of the lower part of a continuous digester. A lower strainer section  2  is present in this digester from which consumed cooking fluid is withdrawn from the column of pulp in the digester. Dilution fluid or washing fluid WL is introduced into the bottom of the digester through vertical  4 V or horizontal  4 H dilution fluid nozzles or washing fluid nozzles. A certain amount of dilution fluid or washing fluid may also be added through nozzles  4 Sc in arms of the rotating bottom scraper and through a conventionally central pipe  4 C that opens out in the centre of the column of pulp in the digester.  
         [0006]     In the prior art design shown in  FIG. 1 , one or more rows of strainers  3   a / 3   b  may form the actual strainer section, where each row of strainers comprises strainer surfaces  22   a / 22   b  together with a withdrawal volume  20  arranged at each strainer surface, and a collection chamber  21  under the withdrawal volume from which consumed cooking fluid is led away to a recovery system, the flow labelled REC. The collection chamber  21  may be located also outside of the digester shell in what is known as an “external header”. When it is desired to increase the production capacity of the digester, i.e. to increase the number of tonnes of digested pulp per day, the speed of the chips and the column of pulp down through the digester increases, while it is necessary at the same time to withdraw a greater amount of consumed cooking fluid and a greater volume of added dilution fluid or washing fluid from the strainer section.  
         [0007]     This results in the lifting force from the upwards flow of fluid established at the bottom counteracting the tendency of the chips and column of pulp to sink, and this leads to the column of pulp easily becoming stuck such that output from the bottom of the digester is made more difficult, and sometimes even ceases completely.  
         [0008]     Increasing the amount of dilution fluid or washing fluid added per unit of time at the nozzles  4 V/ 4 H/ 4 Sc/ 4 C arranged at the bottom proportionally to the increase of production, with the aim of maintaining a constant degree of dilution and washing per tonne of digested pulp, ensures that the upward lifting force on the chips and column of pulp increases proportionally with the increase in production. There is thus an upper limit to the production capacity for each digester with a bottom of conventional design with a withdrawal section  2  and with the addition of dilution fluid or washing fluid.  
         [0009]     Other types of strainer design for continuous digesters are known, but these have been implemented for particular reasons and they solve totally different problems.  
         [0010]     U.S. Pat. No. 5,236,554 reveals a strainer design with which it is desired alternately to add new cooking fluid enriched with chemicals in one of four sections arranged at the periphery of the digester wall around the column of chips, and to withdraw cooking fluid from an opposite sector. The particular addition sector and the particular withdrawal sector of these four sectors are varied over time, such that it possible to reduce radial temperature gradients and obtain an even digestion of the chips over the complete cross-section of the column of chips. The addition sectors can be designed as wall sections lying next to strainer surfaces, with nozzles arranged in these wall sections. The technology is most suitable at high locations in the digester where it is desired to have internal circulation and adjustment of the alkali profile, and it suffers from the disadvantage that only 25% of the strainer surface seen in the direction of the circumference of the digester is actively used as withdrawal strainer at any moment in time. The technology is not suitable for withdrawal sections in which there is instead a very high demand placed on the strainers (i.e. a large volume of withdrawn cooking fluid per unit of strainer area) around the complete digester, as is the case for the bottom strainer sections in, principally, overloaded digesters.  
         [0011]     Thus U.S. Pat. No. 5,236,554 reveals something completely different than adding new cooking fluid enriched with chemicals through central pipes and only withdrawing consumed cooking fluid from the strainers in the wall of the digester, which technology ensures that only chips in the centre of the column of pulp are exposed to fresh cooking fluid and the chips in the column of pulp along the walls of the digester are exposed only to exposed cooking fluid.  
         [0012]     The technology with crossed or alternating addition and withdrawal around the wall of the digester is a technology that is revealed also in SE 145,257 (dated 1952).  
         [0013]     U.S. Pat. No. 6,123,808 describes another variant of the addition of dilution fluid or washing fluid at the bottom of the digester. A dispersion and strainer area that runs around the circumference is used in this case as a distributor of the added dilution fluid or washing fluid, which dispersion and strainer area is arranged directly under the lowermost withdrawal strainer. The aim here is to obtain a more even distribution of dilution fluid or washing fluid around the complete circumference of the digester, in a manner that differs totally from the distribution that can be achieved with local dilution fluid or washing fluid nozzles. An important aspect of this solution is that the relevant dispersion and strainer area must cover a larger diameter than that of the strainer area of the withdrawal strainer positioned above it. The disadvantage of this design is that the injection pressure for fluid into the column of pulp from the dilution fluid or washing fluid that is added though the dispersion and strainer area will be very low. The added dilution fluid or washing fluid can risk also being drawn directly to the strainer that lies above the dispersion and strainer area without passing in practice through any significant volume of pulp or chips in the column of pulp.  
       THE AIM OF THE INVENTION  
       [0014]     The primary aim of the invention is to improve the cooling, dilution and washing principally at the bottom of the digester in continuous digesters.  
         [0015]     A second aim is that of being able to increase the production of existing digesters without experiencing problems with the flow of the column of chips in the digester when the volume of dilution fluid or washing fluid that is added at the bottom of the digester increases in proportion with the increase in production while essentially maintaining constant the dilution fluid or washing effect.  
         [0016]     A further aim is to reduce the lifting force on the column of chips in the bottom wash, where the upwards flow from the fluid added at the bottom can be reduced by the establishment of several layers of upward flow on top of each other instead of these being formed at the same cross-section of the digester.  
         [0017]     A further aim is to be able to establish a further washing zone at the lower part of the digester without needing to reconstruct the central pipe of the digester, which central pipe is always otherwise used in a conventional manner for the addition of digester circulations above the row of strainers located lowermost in the digester.  
       SUMMARY OF THE INVENTION  
       [0018]     The arrangement concerns an improved design for at least one of the cooling, dilution and washing at the bottom of a continuous digester for the production of cellulose pulp. By arranging at least one extra strainer section above the lowermost strainer section, with the addition of washing fluid or dilution fluid between the extra strainer section and the lowermost strainer section, more washing fluid can be added at the bottom of the digester without counteracting the flow of the column of chips. This provides space for the increase of production, for improvement of the flow of the column of chips, or for combinations of these effects while retaining good cooling, washing and dilution at the bottom of the digester. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0019]      FIG. 1  shows a conventional design of a bottom strainer with the addition of dilution fluid at the bottom of a continuous digester;  
         [0020]      FIG. 2  shows a first embodiment of the invention where an extra row of strainers has been arranged directly above the existing bottom strainer;  
         [0021]      FIG. 3  shows an enlarged view of the design according to  FIG. 2 ;  
         [0022]      FIG. 4  shows a view seen in the section IV-IV in  FIG. 3 ;  
         [0023]      FIG. 5  shows an alternative embodiment of the invention with two extra rows of strainers arranged directly above the existing bottom strainer, where these extra rows of strainers are constituted by round strainers of the type known as “manhole strainers”. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 2  shows a first embodiment of the invention, where the bottom design comprises an arrangement for the addition and withdrawal of fluids to a digester that is used for the continuous cooking of cellulose pulp. Wood chips are continuously fed through an inlet at the top of the digester (not shown in the drawing) and cooked cellulose pulp is continuously output through an outlet  10  at the bottom of the digester. At least one strainer section  2  is arranged in the digester, in association with the bottom of the digester with strainer surfaces  22   c  (or similarly  22   a ,  22   b  in  FIG. 1 ) arranged in the strainer section arranged in the direction of the circumference of the wall of the digester for the withdrawal of consumed cooking fluid. Nozzles  4 V,  4 H,  4 Sc for the addition of dilution fluid or washing fluid are arranged under the lowermost strainer section  2  and between the lowermost strainer section  2  and the outlet  10  arranged in the bottom of the digester. A number of vertically directed nozzles  4 V are normally located in the curved bottom end wall of the digester evenly distributed around the circumference. These may typically constitute 10-30 nozzles, or more, in a digester with a diameter of 8 meters.  
         [0025]     The vertical nozzles  4 V are supplemented with a number of dilution nozzles  4 H directed in a horizontal direction that open out into the wall of the digester just above the curved bottom wall but under the lowermost row of strainers. The number of these nozzles may constitute 10-30 in a digester with a diameter of 8 meters.  
         [0026]     Addition of dilution fluid or washing fluid takes place in certain digesters also through the rotating bottom scraper through nozzles  4 Sc arranged in the bottom scraper. One outlet on each arm is shown in the drawing, but several of these outlets may be present across the arm of the bottom scraper, from the centre of the bottom scraper and out to the outer end of the arm of the bottom scraper.  
         [0027]     In addition to these dilution nozzles in the bottom of the digester, there is also an outlet from a central pipe positioned at the level of the lowermost row of strainers  2 , often just above this row of strainers, but the flow from this central pipe contributes to the dilution or washing process at the bottom of the digester.  
         [0028]      FIG. 2  shows that the strainer section is constituted by strainer surfaces  22   c  that are located in the pattern of the squares of a chessboard, a pattern that is known as “staggered screens”, where these strainer surfaces in each row of strainers  3   a ,  3   b  has a blind plate  22   d  between each strainer surface, which blind plate  22   d  has a surface area that essentially corresponds to that of the surrounding strainer surfaces  2   c . These types of rows of strainer are normally located in strainer sections with several rows of strainers, in which rows of strainers that lie above or below a row of strainers have strainer surfaces that are displaced such that a chessboard pattern is formed. This design is often chosen if it is desired to keep the cost of the strainer section low, while at the same time having a high withdrawal capacity, since it is the case that each strainer surface  22   c  has the capacity to drain the column of chips also in those parts that are located as neighbours to the blind plates, i.e. the strainer surfaces drain the column of chips in the direction of the circumference a good deal into half of the extent of the neighbouring blind plate in the direction of the circumference. The invention can, of course, be used also for strainer sections of the type that is shown in  FIG. 1 , where each row of strainers is constituted by a continuous strainer surface that runs in the direction of the circumference. All strainer surfaces in this description may be constructed of what are known as “rod strainers” or they may be simpler plates with slits.  
         [0029]     At least one extra strainer section  30  is arranged for the withdrawal of consumed cooking fluid according to the invention above the lowermost strainer section  2  at a distance between the uppermost part of the lowermost strainer section  2  and the lowermost part of the extra strainer section  30 . Furthermore, a number of extra nozzles  34  are arranged for the addition of dilution fluid or washing fluid distributed around the circumference of the digester between the uppermost part of the lowermost strainer section  2  and the lowermost part of the extra strainer section  30 , which extra nozzles are provided with fluid  33  with the aid of pumps, which fluid is continuously added into the column of pulp through the outlets of these nozzles  34 .  
         [0030]     The distance between the uppermost part of the lowermost strainer section  2  and the lowermost part of the extra strainer section  30  is the distance  31  in  FIG. 2 , which corresponds to a small section of blind plates where the extra nozzles  34  are arranged: this distance is less than the bottom diameter of the digester. This distance typically lies within the interval 0-8 metres. The variant in which this distance is zero means that the nozzles are located at the interface between the uppermost part of the lowermost strainer section  2  and the lowermost part of the extra strainer section  30 .  
         [0031]     In one advantageous embodiment, the distance between the uppermost part of the lowermost strainer section  2  and the lowermost part of the extra strainer section  30  is considerably less than the height of the extra strainer section  30 , i.e. the distance is less than 2 meters, and preferably less than 1 meter. A normal row of strainers, which may establish the extra strainer section, conventionally has a height of between 1.5 and 2 meters in digesters with production capacities of 1,500-3,000 tonnes per day.  
         [0032]     A compact reconstruction of the washing and dilution zone of the digester is obtained in this way that infringes to a minimal degree on the cooking zone that lies above it. The distance can, however, in certain cases be increased if changes to the cooking process are made at the same time, while even so retaining a sufficiently long cooking zone. This applies primarily to those digesters in which what is known as a long “Hi-heat” wash is used at the bottom of the digester, in which the process is changed such that parts of the original Hi-Heat zone are used as cooking zone. This zone may correspond to 30% or more of the total retention time of the chips in the digester, in older digesters with Hi-Heat wash.  
         [0033]      FIG. 3  and  FIG. 4  show in more detail the design with the extra nozzles  34  and the withdrawal volume  30 . The extra nozzles  34  are located arranged such that their openings have their outlet in the wall  40  of the digester between the uppermost part of the lowermost strainer section and the lowermost part of the extra strainer section. Each extra nozzle  34  is provided by the connecting pipes  37  with dilution fluid or washing fluid from a common distribution channel  38  that runs around the digester, and which is in its turn provided with dilution fluid or washing fluid by a pump shown schematically in  FIG. 4 .  
         [0034]     It is preferable that the strainer surface of the lowermost strainer section  2 , the strainer surface of the extra strainer section  30  and the openings of the extra nozzles  34  are all arranged at essentially the same diameter in the wall of the digester, something that is normally the case if manhole strainers are used that have been post-installed.  
         [0035]     The extra strainers may otherwise be mounted in an inner digester wall that constitutes a wall section that is extended downwards from a superior strainer section, which means that the strainer surface of the lowermost strainer section  2  and the openings of the extra nozzles  34  are both arranged at essentially the same diameter in the wall of the digester, while the strainer surface of the extra strainer section  30  is located at a smaller diameter in this wall section that has been extended downwards.  
         [0036]     The additional extra nozzles  34  are evenly distributed around the circumference of the digester and they are present in such a number that the distance around the circumference between neighbouring extra nozzles is less than 3 meters, preferably less than 2 meters.  
         [0037]     It is appropriate that the nozzles have an opening that delivers a concentrated jet into the column of pulp, but they may have openings that are oval or slits in the direction around the circumference. Addition of fluid may, in one extreme variant in which it is desired to achieve greater volumes of added fluid between the extra strainer section and the lower strainer section, also take place through what is essentially one single continuous slit that runs around the circumference. It is advantageous for achieving the best penetration effect into the column of pulp that the slit of the openings of the nozzles are subject to a controlled drop in pressure for the establishment of a high injection velocity of fluid into the column of pulp.  
         [0038]     The lower strainer section  2  is constituted by at least one row of strainers, preferably by at least two rows of strainers, as is shown in  FIG. 2 , where each row of strainers  3   a ,  3   b  consists of strainer plates or rod strainers arranged in the direction of the circumference around the digester. A collecting channel  20  is arranged at each row of strainers  3   a ,  3   b  for the cooking fluid that has been withdrawn through the strainers in this row of strainers, where each collection channel has at least one emptying arrangement  21  for the removal of the withdrawn cooking fluid.  
         [0039]     The extra strainer section  30  is constituted by at least one row of strainers  23 , where each row of strainers consists of strainer plates or rod strainers arranged in the direction of the circumference around the digester. A collecting channel  39  is arranged at each row of strainers for the cooking fluid that has been withdrawn through the strainers in this row of strainers, where each collection channel has at least one emptying arrangement  35 ,  36  for the removal of the withdrawn cooking fluid.  
         [0040]     Also the extra strainer section  23  may consist of at least one row of strainers with several strainer sections  23   b  where the strainer sectors have wall sections between them in the form of blind plates  23   d  that do not have strainer surfaces. A variant is shown in  FIG. 5  in which the strainer sectors are round, of the type known as manhole strainers, and they are arranged in two rows  30   a ,  30   b.    
         [0041]     The extra strainer section  23  may also consist of square strainer sectors of the type shown in  FIG. 2  for the rows of strainers  3   a ,  3   b , and arranged in a pattern that forms a chessboard around the circumference of the digester (an arrangement known as staggered screens).  
         [0042]     The invention can be modified in a number of ways within the framework of the claims. Several copies of the extra strainer section  30  and the nozzle section  31  may, for example, be located one above the other, such that several positions for the addition of dilution fluid are obtained at several heights in the bottom of the digester.  
         [0043]     An extra nozzle section can also be located above the extra row of strainers  30  in the variant that is shown in  FIG. 2 .  
         [0044]     While the present invention has been described in accordance with preferred compositions and embodiments, it is to be understood that certain substitutions and alterations may be made thereto without departing from the spirit and scope of the following claims.