Abstract:
The method and arrangement is for the feed of a chips suspension from one vessel to a subsequent digester in a continuous cooking process for the production of chemical cellulose pulp. The vessel has an inlet defined therein for the input of chips and an outlet defined therein for the output of a chips suspension. The chips suspension in the vessel has a first fluid/wood ratio established above a second fluid/wood ratio that is established at the bottom of the vessel. The second fluid/wood ratio is at least as great as, preferably greater than, the first fluid/wood ratio. After the output of the chips suspension from the vessel and before the chips suspension is placed under pressure for onwards transport to a subsequent digester, a fraction of fluid is withdrawn from the chips suspension, whereby a third fluid/wood ratio is established in the chips suspension.

Description:
PRIOR APPLICATION 
     This application is a U.S. national phase application based on International Application No. PCT/SE2006/050037, filed 21 Mar. 2006, claiming priority from Swedish Patent Application No. 0500672-1, filed 23 Mar. 2005. 
     TECHNICAL AREA 
     The present invention concerns a method and an arrangement for the feed of a chips suspension from one vessel to a subsequent digester in a continuous cooking process for the production of chemical cellulose pulp. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The use of scraper devices at the bottom of digesters and impregnation vessels in the continuous cooking of chemical cellulose pulp has been long known. The aim of these scraper devices is to ensure a continuous output of the cellulose pulp or chips from the vessel. The scraper device consists of a number of scraper arms that are arranged on the shaft that is arranged to be vertical during production. The motion of the arms in the suspension of pulp or chips counteracts the formation of blockages, the formation of channels, and other undesired effects. 
     The above-mentioned shaft for the operation of the said scraper arms has been used since early times for the addition of fluid at the lower part of the digester or impregnation vessel. The addition of fluid occurs in this case by making the shaft hollow and leading fluid in through this way. The primary purpose of adding fluid has been to wash the pulp. This addition of fluid through the shaft has more recently been used for the dilution of the pulp with the aim of ensuring output from the vessel. U.S. Pat. No. 5,736,005 reveals a variant of such a hollow shaft in which fluid is added to a continuous digester with the aim of ensuring output from the digester. 
     An alternative to the above-described addition of fluid with the aim of diluting and ensuring output of the pulp or chips from the digester or the impregnation vessel is to add the fluid at the lower part of the vessel through a fluid supply device through the vessel. It is preferable that this addition takes place in the vicinity of the scraper device. SE 180 289 reveals an embodiment in which the fluid supply device adds fluid close to the bottom of a container with the aim of preventing the formation of blockages of cellulose fibres. 
     Addition of fluid by the methods that have been described above, however, involves a number of disadvantages, particularly when the addition is made to an impregnation vessel. 
     In the cases in which the fluid is added to an impregnation vessel, the extra addition of fluid must be dealt with by the top separator in subsequent digesters, which involves a considerable extra expense at the top separator. 
     Furthermore, the added fluid involves large volumes of fluid that the system must deal with, and this in turn involves expensive investment and high operating costs of pumps and high-pressure taps, or both. 
     The same problem arises, naturally, also in those cases in which no fluid has been added at the bottom of the impregnation vessel due to the fluid/wood ratio of the chips suspension being so high that it is not necessary to add fluid in order to ensure output from the impregnation vessel. 
     THE AIMS OF THE INVENTION 
     The principal aim of the present invention is to either eliminate or reduce the above-described problems and disadvantages in association with the output of cellulose pulp from an impregnation vessel to a transfer line, where the invention allows:
         a reduction in the amount of fluid in the chips suspension that is fed out from the impregnation vessel to the digester, i.e. a reduction in the fluid/wood ratio;   the ability initially to establish a stable flow out from the bottom of the impregnation vessel with only instantaneously increased fluid volumes, or the opportunity for increased dilution in the bottom of the impregnation vessel without the increased amounts of fluid needing for this reason to be pumped onwards into the transfer line;   the ability to use a smaller and cheaper top separator in subsequent digesters as a consequence of the lower volumes of fluid, and preferably the ability to eliminate completely a top separator;   the ability to use smaller and cheaper pumps or high-pressure taps, or both, that consume lower power, due to the lower volumes of fluid.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be described in more detail below with the aid of the attached drawings, of which: 
         FIG. 1  shows one preferred embodiment of an impregnation vessel in which the arrangement according to the invention is included. 
         FIG. 2   a  shows a side view with a section A-A and  FIG. 2   b  shows a top view of a first preferred embodiment of the bucket-shaped outlet  201 , 
         FIG. 3   a  shows a side view with a section B-B and  FIG. 3   b  shows a top view of a second preferred embodiment of the bucket-shaped outlet  201 , 
         FIG. 4   a  shows a side view and  FIG. 4   b  shows a top view of a third preferred embodiment of the bucket-shaped outlet  201 , 
         FIG. 5   a  shows a side view with a section C-C and  FIG. 5   b  shows a top view of a fourth preferred embodiment of the bucket-shaped outlet  201  and outlet line  301 , 
         FIGS. 6   a ,  6   b  and  6   c  show different embodiments of the appearance of different strainer surfaces of the bucket-shaped outlet. 
         FIG. 7  shows an embodiment of how scraper arms  207  are arranged around shaft  106  in order to maintain the holes or slits in the strainer clean. 
         FIG. 8  shows an embodiment of a variant of the embodiment in  FIG. 4 , in which a debris trap is arranged under the bottom surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The concept “chips suspension” will be used in the following detailed description of the invention. This term is here used to denote chips together with fluid, which suspension is treated in an impregnation vessel and fed out from the said impregnation vessel to a subsequent digester in a continuous cooking process for the production of cellulose pulp. 
     A further expression that will be used is “fluid/wood ratio”. This expression is here used to denote the relationship between fluid and wood that is prevalent in the chips suspension. 
     Furthermore, the expression “perforated strainer hole or slit” will be used in the description of strainer surfaces. This expression is here used to denote penetrating openings in the surface with no requirements placed on their shape. Thus, these openings may be round, square, triangular, etc. Furthermore, it is also possible to conceive that the perforations consist of penetrating slits that may be straight, bent, curved, etc. 
     Finally, the concept “feed device” will be used. This term is here used to denote a device that is intended to feed the chips suspension from an impregnation vessel to a digester by the application of pressure. Examples of such feed devices are pumps and high-pressure taps. 
       FIG. 1  shows the lower part of a principally cylindrical vertically arranged impregnation vessel  101  for the impregnation of chips, which impregnation vessel precedes a digester  401  in a continuous cooking process for the production of chemical cellulose pulp. The impregnation vessel has a diameter D 1 , an inlet  107  at the top of the vessel into which untreated chips are fed, and a bucket-shaped outlet  201  at the bottom of the vessel from which a chips suspension, i.e. impregnated chips with fluid, is fed out. The chips suspension in the impregnation vessel has a first fluid/wood ratio, which first fluid/wood ratio preferably lies within the interval  2 - 7 . 
     In order to facilitate the output of the chips suspension from the impregnation vessel  101 , a mechanical stirrer  102  is arranged at the bottom of the impregnation vessel  101 , in order to obtain stirring of the chips suspension. The stirrer  102  comprises a number of scraper arms  105 , preferably two, that are arranged at the upper end of a shaft  106  that is vertically arranged. The shaft  106  is driven at its lower end by means of a directly acting driver device  107 . The stirring of the chips suspension breaks the orientation of the chips in association with the output process, such that the output from the impregnation vessel is facilitated. 
     In order to ensure further the output of the chips suspension from the impregnation vessel  101 , dilution fluid is added in a known manner in an amount of Q 1  in the vicinity of the bottom by means of at least one dilution fluid supply nozzle  103 . The dilution fluid supply nozzles  103  are most often arranged through the wall of the impregnation vessel  101  or in the scraper arms  105 . In the embodiment in which the dilution fluid supply nozzles  103  are arranged in the scraper arms  105 , the fluid is led to the scraper arms  105  through a hole in the shaft  106  (not shown in the drawing) through which fluid flows. The total amount of dilution fluid that is added to the impregnation vessel  101  from the dilution fluid supply nozzles  103  will hereafter be referred to as Q 1 . The chips suspension after the addition of the dilution fluid has a second fluid/wood ratio, which is higher than the first fluid/wood ratio further up in the impregnation vessel, which second fluid/wood ratio is established in order to ensure an even output that is free of disturbances. This second fluid/wood ratio preferably lies in the interval  6 - 10 . Operating conditions can, however, occur in which Q 1 =0, i.e. no dilution fluid is added through the dilution fluid supply nozzles  103 , and in the cases in which the first and the second fluid/wood ratios are equal, this ratio lies in the interval  6 - 10 . 
     In order to summarise briefly the relationship between the first and the second fluid/wood ratios, it can be stated that the chips suspension in the vessel  101  has the first fluid/wood ratio established above the second fluid/wood ratio, where the second fluid/wood ratio is established at the bottom of the vessel. The second fluid/wood ratio is at least as large as the first fluid/wood ratio, preferably larger. 
     The chips suspension, i.e. the impregnated chips together with the fluid, is continuously fed out from the impregnation vessel  101  through a bucket-shaped outlet  201  arranged in and under the bottom of the impregnation vessel  101  below the scraper device  102 . The bucket-shaped outlet  201  has a diameter D 2  that is less than the diameter of the impregnation vessel D 1 , i.e. D 2 &lt;D 1 . The diameter D 2  of the bucket-shaped outlet is approximately 1-1.5 m for an impregnation vessel  101  with a diameter D 1  of 3-5 m. For an impregnation vessel with a diameter D 1  of 10 m, D 2  can have a dimension of approximately 2 m. The diameter D 2  is thus less than 50% of D 1  and preferably in the interval  15 —40% of D 1 . Parts of the wall of the bucket-shaped outlet, or the complete wall, consist of perforated strainer holes or slits. The strainer holes or slits are surrounded by a withdrawal space  206  at the outer wall of the outlet from which withdrawal space  206  the partial fluid volume Q 2  is withdrawn from the chips suspension by means of a pump  303 , before the remainder of the chips suspension is sent in the outlet line  301  to subsequent digesters  401  through being placed under pressure by a pressure device  302 . The outlet line  301  is connected to the wall section of the bucket-shaped outlet, which outlet line  301  has a diameter D 3 , where D 1 , D 2  and D 3  have the following relationship: D 1 &gt;D 2 &gt;D 3 . The chips suspension after the withdrawal of fluid has a third fluid/wood ratio, which is lower than the second fluid/wood ratio. This third fluid/wood ratio lies in the interval  5 - 9 , and is at least 1 unit, preferably at least 2 units, lower than the second fluid/wood ratio, which lies in the interval  6 - 10 . 
     The withdrawn fluid Q 2  can then be sent to any one or to a combination of the following:
         Q 2  is sent in a circulation line that is connected at its first inlet end to at least one withdrawal space ( 206 ) arranged at the bucket-shaped outlet ( 201 ) and where a second end of the circulation line is connected to a recovery process (REC). A natural position if it is desired to withdraw consumed impregnation fluid, which in turn has been partly constituted by a withdrawal from the digester.   Q 2  is sent in a circulation line that is connected at its first inlet end to at least one withdrawal space ( 206 ) arranged at the bucket-shaped outlet ( 201 ) and where a second end of the circulation line is connected to a dilution fluid supply nozzle ( 103 ). In this case it is solely a question of a local dilution.   Q 2  is sent in a circulation line that is connected at its first inlet end to at least one withdrawal space ( 206 ) and where the second end of the circulation line is connected to a position (A) close to the top of the impregnation vessel ( 101 ).   Q 2  is sent in a circulation line that is connected at its first inlet end to at least one withdrawal space ( 206 ) and where the second end of the circulation line is connected to a position (B) in a subsequent digester ( 401 ). This is done with the aim of, if it is desired at any cooking phase, to modify the digestion conditions, possibly to raise the sulphidity, or to initiate precipitation of early dissolved XYLAN onto the fibres in the digester.       

       FIGS. 2   a  and  2   b  show a first preferred embodiment of the bucket-shaped outlet  201  where parts of, and preferably the complete, surface  204  of the outlet is perforated with strainer holes or slits  205 , and from which perforated surface  204  a fraction Q 2  of the fluid in the chips suspension is withdrawn with a pump  303  through a withdrawal space  206  arranged around the strainer holes or slits of the outer surface  204 . The shaft  106  (not shown in this drawing) passes through a penetrating opening  202  in the bucket-shaped outlet  201 . 
       FIGS. 3   a  and  3   b  show a second preferred embodiment of the bucket-shaped outlet  201  where the surface  204  of the outlet  201  is perforated with strainer holes or slits  205  over a surrounding angle α between 90° and 270°, preferably 180°, and from which perforated surface  204  a fraction Q 2  of the fluid in the chips suspension is withdrawn by a pump  303  through a withdrawal space  206  arranged around the strainer holes or slits of the outer surface  204 . The shaft  106  (not shown in this drawing) passes through a penetrating opening  202  in the bucket-shaped outlet  201 . 
       FIGS. 4   a  and  4   b  show a third preferred embodiment of the bucket-shaped outlet  201  where the outlet has a bottom surface  203 . Parts of or, preferably, the complete bottom surface  203  are perforated with strainer holes or slits  205 . From the perforated bottom surface  203  a fraction Q 2  of the fluid in the chips suspension is withdrawn by a pump  303  through a withdrawal space  206 . The shaft  106  (not shown in this drawing) passes through a penetrating opening  202  in the bucket-shaped outlet  201 . 
       FIGS. 5   a  and  5   b  show a fourth preferred embodiment where the surface of the outlet line  302  is partially or fully perforated strainer holes or slits  205 . From the perforated surface a fraction Q 2  of the fluid in the chips suspension is withdrawn by a pump  303  through a withdrawal space  206  arranged around the perforated strainer holes or slits  205  in the outer surface of the outlet line. 
       FIG. 6   a  shows a fifth preferred embodiment of how the strainer surface of the bucket-shaped outlet, which consists of strainer holes or slits  205 , may appear. The complete surface is perforated in this case. 
       FIG. 6   b  shows a sixth preferred embodiment in which parts of the strainer surface are perforated by strainer holes or slits  205 . 
       FIG. 6   c  shows a seventh preferred embodiment in which parts of the strainer surface are perforated with strainer holes or slits  205 . 
       FIGS. 7   a  and  7   b  shows a side view and a top view of the bucket-shaped outlet  201  where scraper arms  207  have been arranged on a shaft  106  with the aim of maintaining the strainer holes or slits in the strainer surfaces of the bucket-shaped outlet clean, such that they do not become clogged. 
       FIGS. 8   a  and  8   b  show an eighth preferred embodiment of the bucket-shaped outlet  201  where the outlet has a bottom surface  203 , similar to that shown in  FIGS. 4   a  and  4   b . Parts of, preferably the complete, bottom surface  203  are perforated with strainer holes or slits  205 . From the perforated bottom surface a fraction Q 2  of fluid is withdrawn from the chips suspension with the pump  303  through the withdrawal space  206 . An outlet  801  is present in the bottom surface  203  with a space arranged under the bottom surface. Sluice valves  802  are arranged in the space of the outlet, which valves can be emptied of coarse material  804  that collects in this space during operation. It is an advantage if the outlet is arranged in the vicinity of the outlet line  301 , since the chips suspension passes the outlet, such that the heavy or coarse material falls down into the outlet  801 . It is an advantage if a fluid line  803  is arranged after the pump  303  at the space in the outlet  803 . In this way, output from the outlet  803  is facilitated, in that a dilution is achieved. The scraper arms  207 , which are shown in  FIG. 7 , aid in transporting the material  804  to the outlet  801 . 
     The following advantages, among others, are achieved with the invention, compared with conventional technology described above as the prior art:
         A reduced flow of fluid to the top separator of the digester from the pre-ceding impregnation vessel, which results in the ability to use a smaller and cheaper top separator. It is possible with a optimal embodiment to dispense completely with the top separator on the digester.   a reduced fluid content of the chips suspension that leaves the impregnation vessel, which results in the ability to use smaller, cheaper and less energy-consuming pumps or high-pressure taps, or both.       

     The invention is not limited to the embodiments described: several variants are possible within the scope of the attached patent claims. All of the following combinations, for example, are possible, individually or in combination:
     1) strainer holes or slits  205  at a location on the outer surface  204  of the bucket-shaped outlet   2) strainer holes or slits  205  at a location on the bottom surface  203  of the bucket-shaped outlet   3) strainer holes or slits  205  in the outer surface  301  of the line.   

     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.