Method of digesting cellulose fibrous material in a continuous digester

Abstract

The invention discloses a method of continuous cooking cellulose fibrous material in a digester, said digester having a top and a bottom. The method of the invention comprises the steps of:—feeding a slurry comprising cellulosic fibrous material and cooking liquor into the top the digester,—feeding a first washing liquor into the bottom portion of the digester, which first washing liquor moves counter currently up through at least part of the digester,—removing pulp from the bottom of the digester,—feeding a second washing liquor into the center of the digester at least at the level of a first screen section,—feeding a third washing liquor into the center of the digester at the level of a second screen section, and—withdrawing the cooking liquor and the washing liquors from the first screen section and from the second screen section, which first screen section is arranged in a lower part of the digester and which second screen section is arranged below said first screen section. Thus, in accordance with the invention, a third washing liquor is added to the center of digester at the level of an additional, second extraction screen section arranged in the lower part of the digester. The method makes it possible to increase the production rate in the digester but still maintain or even increase the washing efficiency.

1 . A method of continuous cooking cellulose fibrous material in a digester, said digester having a top and a bottom, comprising the steps of
 feeding a slurry comprising cellulosic fibrous material and cooking liquor into the top of the digester,   feeding a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester,   removing pulp from the bottom of the digester,   feeding a second washing liquor into the center of the digester at least at the level of a first screen section,   feeding a third washing liquor into the center of the digester at the level of a second screen section, and   withdrawing the cooking liquor and the washing liquors at least from the first screen section and from the second extraction screen section, which first screen section is arranged in a lower part of the digester and the second screen section is arranged below said first screen section.   
     
     
         2 . A method according to patent  claim 1 , wherein the digester comprises at least one central pipe and wherein the second and/or the third washing liquor is fed into the center of the digester through said central pipe. 
     
     
         3 . A method according to  claim 1 , wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 0-30 minutes. 
     
     
         4 . A method according to  claim 1 , wherein said first screen section is arranged above said second screen section at a distance of between 0.1-15 m. 
     
     
         5 . A method according to  claim 1 , wherein cooking and wash liquor is withdrawn from the digester in an additional third screen section and wherein wash liquor is fed into the digester at the level of or close to said third screen section. 
     
     
         6 . A continuous digester, having a top and a bottom, for the production of chemical pulp by cooking wood chips in cooking liquor, which digester comprises;
 an inlet arranged at the top of the digester for feeding the wood chips and the cooking liquor into the digester,   an outlet arranged at the bottom of the digester for removal of the pulp,   at least a first and a second screen section, which first screen section is arranged in the lower part of the digester and said second screen section is arranged below said first screen section,   means for feeding a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester,   means for feeding a second washing liquor into the center of the digester at least at the level of the first screen section, and   means for feeding a third washing liquor into the center of the digester at the level of the second screen section,   means for withdrawal of the cooking liquor and the washing liquors from at least the first screen section and from the second screen section.   
     
     
         7 . A continuous digester, having a top and a bottom, for the production of chemical pulp by cooking wood chips in cooking liquor, which digester comprises;
 an inlet arranged at the top of the digester for feeding the wood chips and the cooking liquor into the digester,   an outlet arranged at the bottom of the digester for removal of the pulp,   at least a first and a second screen section, which first screen section is arranged in the lower part of the digester and said second screen section is arranged below said first screen section,   a first inlet pipe arranged to feed a first washing liquor into the bottom of the digester, which first washing liquor moves counter currently up through at least part of the digester,   a second inlet pipe arranged to feed a second washing liquor into the center of the digester at least at the level of the first screen section,   a third inlet pipe arranged to feed a third washing liquor into the center of the digester at the level of the second screen section, and   outlet pipes arranged for withdrawal of the cooking liquor and the washing liquors from at least the first screen section and from the second screen section.   
     
     
         8 . A method according to  claim 1 , wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 0.1-20 minutes. 
     
     
         9 . A method according to  claim 1 , wherein said first screen section is arranged above said second screen section at a distance corresponding to a retention time of between 5-15 minutes. 
     
     
         10 . A method according to  claim 1 , wherein said first screen section is arranged above said second screen section at a distance of between 0.5-5 m.