Continuous pulping processes and systems

Abstract

A set of spaced apart screen assemblies are provided within a continuous kraft digester. Liquor is extracted from a first screen assembly of the set without recirculation of at least a portion of the extracted liquor into the digester above the first screen. In such a manner, the need for circulation flow below the first screen assembly of the set to the second screen assembly of the set is minimized, thereby also minimizing (if not eliminating entirely) the packing problems such flow may cause. The liquor extracted from the first screen assembly in the set could be reintroduced, however, into the digester at some other location in the cooking system (e.g., at a location just above the bottom screen). By employing the present invention, the distance between the first and second screen assemblies is greater than one digester diameter (D). In addition, the liquor discharge below the first screen assembly, to which the circulated liquor from the second screen assembly (plus added dilution liquor and heat, if desired) may be at a distance of less than one-half (½) of a digester diameter (D) below the first screen. A small upflow can thus be used to displace the liquor at the first screen assembly, with a relatively short upflow zone. Multiple sets of such first and second screen assemblies may also be provided within the digester along with other conventional screen assemblies to effect desired internal liquor flows.

What is claimed is:  
     
         1 . A continuous kraft pulping process comprising: 
 (a) passing a slurry of wood chips and liquor in a digester sequentially through a screen set which includes first and second screen assemblies longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester;    (b) extracting liquor from the first screen assembly of said set without recirculation to the digester to a location above the first screen assembly, while simultaneously    (c) extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor from the second screen assembly to a discharge location within the digester which is above the second screen assembly and spaced below the first screen assembly by a distance which is less than one-half the digester diameter.    
     
     
         2 . The process of  claim 1 , wherein dilution liquor is added to the liquor extracted from the second screen assembly and recirculated to the digester.  
     
     
         3 . The process of  claim 1 , wherein at least a portion of the liquor extracted from the first screen assembly is reintroduced to the digester at a location below the second screen assembly.  
     
     
         4 . The process of  claim 1 ,  2  or  3 , wherein there is at least one additional screen assembly set below the first mentioned screen assembly set, wherein said at least one additional screen assembly set includes third and fourth screen assemblies positioned in longitudinally spaced relationship by a distance which is greater than a diameter of the digester, and wherein said process includes: 
 (d) extracting liquor from the digester at the third screen assembly set without recirculation to the digester to a location above the third screen assembly, while simultaneously  
 (e) extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor from the fourth screen assembly to a discharge location within the digester which is above the fourth screen assembly and spaced below the third screen assembly by a distance which is less than one-half the digester diameter.  
 
     
     
         5 . The process of  claim 4 , wherein at least a portion of the liquor extracted from the first screen assembly is combined with the liquor extracted from the third screen assembly.  
     
     
         6 . The process of  claim 5 , wherein step (c) is practiced by introducing a dilution liquor into the portion of liquor extracted from the second screen assembly and recirculated to the digester to a location between the first and second screen assemblies.  
     
     
         7 . The process of  claim 6 , wherein said dilution liquor is selected from water, white liquor, wash liquor, bleach plant filtrate and mixtures thereof.  
     
     
         8 . The process of  claim 1 , wherein step (c) is practiced by replacing some or all of the extracted liquor with a liquor containing a substantially lower effective dissolved organic material level than the extracted liquor.  
     
     
         9 . The process of  claim 8 , wherein the digester has multiple other screen assemblies, and wherein liquor is extracted respectively at least at one of the multiple other screen assemblies and recirculated to the digester at a location above the at least one of the multiple other screen assemblies.  
     
     
         10 . The process of  claim 9 , which comprises introducing a dilution liquor to the liquor extracted respectively at least at one of the multiple other screen assemblies and recirculated to the digester at a location above the at least one of the multiple other screen assemblies.  
     
     
         11 . The process of  claim 10 , wherein the dilution liquor is selected from water, white liquor, wash liquor, bleach plant filtrate and mixtures thereof.  
     
     
         12 . A continuous kraft pulping system comprising: 
 a digester having a screen assembly set which includes first and second screen assemblies which are longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester;    a feed line for feeding a slurry of wood chips and liquor to a top of the digester above the screen assembly set so that the slurry can pass sequentially therethrough said first and second screen assemblies thereof;    a first extraction line for extracting liquor from the first screen assembly of the set in the absence of recirculation to the digester to a location above the first screen assembly;    a second extraction line for simultaneously extracting liquor from the second screen assembly of the set and recirculating at least a portion of the extracted liquor to a discharge location between the first and second screen assemblies of the set which is spaced below the first screen assembly of the set by a distance with is less than one-half the digester diameter.    
     
     
         13 . The system of  claim 12 , comprising a dilution liquor line for adding dilution liquor to the liquor extracted from the second screen assembly and recirculated to the digester.  
     
     
         14 . The process of  claim 12 , wherein at least a portion of the liquor extracted from the first screen assembly is reintroduced to the digester at a location below the second screen assembly.  
     
     
         15 . The system of  claim 14 , further comprising: 
 at least one additional screen assembly set positioned within the digester below the second screen assembly of the first mentioned set, said at least one additional screen assembly set including third and fourth screen assemblies longitudinally spaced apart from one another by a distance which is greater than a diameter of the digester,    a third extraction line for extracting liquor from the third screen assembly in the absence of recirculation to the digester to a location above the third screen assembly; and    a fourth extraction line for simultaneously extracting liquor from the fourth screen assembly and recirculating at least a portion of the extracted liquor to a discharge location between the third and fourth screen assemblies which is spaced below the third screen assembly by a distance with is less than one-half the digester diameter.    
     
     
         16 . The system of  claim 15 , wherein at least a portion of the liquor extracted from the first screen assembly is combined with the liquor extracted from the third screen assembly.  
     
     
         17 . The system of any one of claims  12 - 16 , comprising multiple other screen assemblies, at least one other extraction line for extracting liquor from at least at one of the multiple other screen assemblies, and at least one other recirculation line for recirculating at least a portion of the liquor extracted to the digester at a location above the at least one of the multiple other screen assemblies.