Method and apparatus for continuously pulping cellulosic fibrous material

Abstract

In the production of chemical pulp, such as kraft pulp, it is possible to reduce the amount of ancillary equipment, and/or the size of the continuous digester vessel, while achieving enhanced uniformity of treatment by avoiding “hang ups”. Instead of providing vertical counter-current flow of liquid and comminuted cellulosic fibrous material being treated in the lower half of the digester, the counter-current flow may be a cross flow, that is substantially horizontal (e.g. radial) while the flow of the chip column is vertical. Regardless of the configuration of the digester vessel, a single pump may be utilized for all of the recirculations, or only a few pumps for all of the recirculations.

What is claimed is:  
     
         1 . A method of treating comminuted cellulosic fibrous material in a substantially upright vessel having an upper half and a lower half, comprising substantially continuously: 
 (a) feeding a liquid slurry of comminuted cellulosic fibrous material into a top portion of the vessel;    (b) withdrawing treated comminuted cellulosic fibrous material from a bottom portion of the vessel, so that the comminuted cellulosic fibrous material moves substantially continuously downwardly in the vessel; and    (c) in substantially the lower half of the vessel, causing treating liquid to flow substantially horizontally into and out of contact with the comminuted cellulosic fibrous material as the comminuted cellulosic fibrous material flows downwardly in the vessel, to effect treatment of the comminuted cellulosic fibrous material substantially without interfering with the substantially continuously downward movement of the comminuted cellulosic fibrous material.    
     
     
         2 . A method as recited in    claim 1    wherein (c) is practiced using a plurality of vertically spaced screen assemblies at peripheral portions of the vessel, and by introducing treating liquid into a central portion of the vessel at the approximate level of each screen assembly, and withdrawing the majority of the liquid introduced at the approximate level of any screen assembly through that screen assembly.  
     
     
         3 . A method as recited in    claim 2    wherein (c) is further practiced using cooking liquor as the treating liquid at at least one of the screen assemblies.  
     
     
         4 . A method as recited in    claim 3    wherein (c) is further practiced by recirculating liquid withdrawn from one screen assembly as introduced treating liquid at a central portion of another screen assembly.  
     
     
         5 . A method as recited in    claim 3    wherein (c) is further practiced by recirculating liquid withdrawn from one screen assembly as introduced treating liquid at a central portion of the immediately above screen assembly.  
     
     
         6 . A method as recited in    claim 3    further comprising (d) augmenting the flow of liquid introduced at at least one central portion with a non-cooking liquor having a dissolved organic material concentration less than one-half that of the dissolved organic material concentration of the liquid withdrawn through the screen assembly associated with that central portion.  
     
     
         7 . A method as recited in    claim 6    wherein (d) is practiced using washer filtrate.  
     
     
         8 . A method as recited in    claim 1    wherein the vessel is substantially cylindrical and has an UD ratio of 10 to 1 or more, or 6 to 1 or less, and wherein (c) is practiced by causing the treating liquid to flow substantially radially in the vessel.  
     
     
         9 . A method as recited in    claim 8    wherein (c) is practiced at two or more vertically spaced locations along the substantially upright vessel.  
     
     
         10 . A method as recited in    claim 9    wherein (c) is practiced using a cooking liquor as the treatment liquid at at least one of the locations.  
     
     
         11 . A method as recited in    claim 10    further comprising (d) augmenting the flow of liquid introduced at at least one location with a non-cooking liquor having a dissolved organic material concentration less than one-half that of the dissolved organic material concentration of the liquid existing in the vessel at that location.  
     
     
         12 . A method as recited in    claim 10    wherein (c) is further practiced by recirculating radially flowing liquid at one location as introduction liquid at another location.  
     
     
         13 . A method as recited in    claim 12    wherein (c) is further practiced by recirculating radially flowing liquid at one location as introduction liquid at the immediately vertically above location.  
     
     
         14 . A method as recited in    claim 1    wherein (a)-(c) are practiced so that the vessel is substantially devoid of internal counter-current flow of materials and liquid in substantially the lower half of the vessel.  
     
     
         15 . Apparatus for treating comminuted cellulosic fibrous material comprising: 
 a substantially upright vessel having a top and a bottom, a length or height L, a maximum width or diameter D, an upper half, and a lower half;    an inlet for a liquid slurry of comminuted cellulosic fibrous material adjacent said top of said vessel;    an outlet for a treated liquid slurry of comminuted cellulosic fibrous material adjacent said bottom of said vessel;    at least first and second vertically spaced screen assemblies disposed within said vessel;    at least first and second liquid introducing structures disposed at the approximate vertical level of each of said screen assemblies which introduce treating liquid that is ultimately withdrawn through said first and second screen assemblies, respectively; and    wherein said vessel is substantially devoid of internal countercurrent flow of materials and fluids in substantially said lower half of said vessel.    
     
     
         16 . Apparatus as recited in    claim 15    wherein said vessel comprises a continuous digester, and wherein the UD ratio is less than about 6to 1.  
     
     
         17 . Apparatus as recited in    claim 15    wherein said vessel comprises a continuous digester, and wherein said vessel is substantially cylindrical, and wherein the UD ratio is greater than about 10 to 1.  
     
     
         18 . Apparatus as recited in    claim 16    wherein said first screen is above said second screen; and further comprising means for recirculating liquid withdrawn from said second screen to said first liquid introducing structure.  
     
     
         19 . Apparatus as recited in    claim 18    wherein said at least first and second screens comprise at least first, second, third and fourth screens; and wherein said at least first and second liquid introducing structures comprise at least first, second, third and fourth liquid introducing structures; and further comprising liquid recirculating means for recirculating liquid from at least another screen besides said second screen to another liquid introducing structure besides said first liquid introducing structure.  
     
     
         20 . A continuous digester comprising: 
 a substantially upright continuous digester vessel having a top and a bottom;    an inlet for a liquid slurry of comminuted cellulosic fibrous material adjacent said top of said vessel;    an outlet for a treated liquid slurry of comminuted cellulosic fibrous material adjacent said bottom of said vessel;    at least first and second vertically spaced screen assemblies disposed within said vessel;    at least first and second liquid introducing structures which introduce treating liquid that is ultimately withdrawn through said first and second screen assemblies, respectively;    recirculating means for recirculating liquid from said screen assemblies to said liquid introducing structures; and    wherein said recirculating means comprises a single pump common to both said first and second screen assemblies and liquid introducing structures.