Central screen

Abstract

Methods for continuously producing pulp from lignocellulose-containing material in a digester are disclosed in which the porous lignocellulose-containing material is fed from the top to the bottom of the digester, liquor is pumped through the porous column of lignocellulose-containing material in a predetermined direction, and liquor is simultaneously withdrawn through at least one central screen assembly and through a wall screen assembly. A digester for continuously producing pulp from lignocellulose-containing material is also disclosed.

1 . A method for continuously producing pulp from lignocellulose-containing material in a digester including a top, a bottom, and a central pipe extending from said top to said bottom within said digester, at least one central screen assembly disposed on said central pipe, and a wall screen assembly disposed along the inner wall of said digester, said method comprising feeding a porous column of said lignocellulose-containing material from said top of said digester to said bottom of said digester, pumping liquor through said porous column of said lignocellulose-containing material in a predetermined direction, and simultaneously withdrawing said liquor through said at least one central screen assembly and said wall screen assembly.  
   
   
       2 . The method of  claim 1  wherein said predetermined direction comprises a direction selected from the group consisting of concurrent, countercurrent, and radial directions.  
   
   
       3 . The method of  claim 1  wherein said wall screen assembly and said central screen assembly are disposed at substantially the same level within said digester.  
   
   
       4 . The method of  claim 1  including withdrawing a plurality of separate streams of said liquor through said central screen assembly.  
   
   
       5 . The method of  claim 4  wherein said central screen assembly comprises a plurality of distinct central screens.  
   
   
       6 . The method of  claim 1  including recirculating at least a portion of said withdrawn liquor to said digester through a circulation loop.  
   
   
       7 . The method of  claim 6  including heating or supplying chemicals to said recirculating portion of said withdrawn liquor in said circulation loop.  
   
   
       8 . The method of  claim 1  wherein said digester includes a withdrawal screen disposed along said inner wall of said digester, said method including adding washing liquor to said digester adjacent to said withdrawal screen through said central pipe, whereby said washing liquor displaces said liquor through said withdrawal screen, and displacing at least a portion of said washing liquor through said central screen assembly by said washing liquor flowing countercurrent through said lignocellulose-containing material.  
   
   
       9 . A digester for continuously producing pulp from lignocellulose-containing material, said digester having a top and a bottom and comprising a top inlet for feeding said lignocellulose-containing material into said top of said digester, a bottom outlet for removing said cooked lignocellulose-containing material from said bottom of said digester, at least one wall screen assembly disposed along the inner wall of said digester for withdrawing liquor from said lignocellulose-containing material, a central pipe extending from said top to said bottom of said digester, and at least one annular central screen disposed on said central pipe for withdrawing liquor therethrough complementary with that of said at least one wall screen assembly.  
   
   
       10 . The digester of  claim 9  wherein said at least one annular central screen comprises a cylindrical central screen.  
   
   
       11 . The digester of  claim 9  wherein said at least one annular central screen comprises a conical central screen.  
   
   
       12 . The digester of  claim 9  including back-flushing means associated with said at least one annular central screen.  
   
   
       13 . The digester of  claim 9  wherein said central pipe includes an inlet for introducing washing liquor into said digester, said inlet being disposed adjacent to said at least one wall screen assembly.