Hardwood alkaline pulping processes and systems

Abstract

Systems are provided for continuously producing chemical cellulose pulp from a slurry of comminuted hardwood material by subjecting the slurry of comminuted hardwood material to a first cooking stage under conditions sufficient to reduce syringyl lignin (s-lignin) content in the hardwood material as compared to guaiacyl lignin (g-lignin) content therein, and thereafter subjecting the slurry of comminuted hardwood material to a second cooking stage under conditions sufficient to reduce the g-lignin content remaining therein after the first cooking stage.

1 . A system for continuously producing chemical cellulose pulp from a slurry of comminuted hardwood material, comprising:
 (a) a first digester for subjecting the slurry of comminuted hardwood material to a first cooking stage under conditions sufficient to reduce syringyl lignin (s-lignin) content in the hardwood material as compared to guaiacyl lignin (g-lignin) content therein; and   (b) a second digester downstream from the first digester for subjecting the slurry of comminuted hardwood material to a second cooking stage under conditions sufficient to reduce the g-lignin content remaining therein after the first cooking stage.   
     
     
         2 . The system of  claim 1 , wherein the first digester subjects the slurry of hardwood material to a temperature of between about 120° C. to about 170° C. at about 2 to about 10% effective alkaline charge as (EA) NaOH on wood. 
     
     
         3 . The system of  claim 2 , wherein the cooked hardwood material which exits the first cooking stage is between about 30 to about 100. 
     
     
         4 . The system of  claim 1 , wherein the first digester is operated under a low sulfidity condition of less than about 20% sulfidity. 
     
     
         5 . The system of  claim 4 , wherein the low sulfidity condition of the first cooking stage is achieved by adding a sufficient amount of anthroquinone and/or polysulfide. 
     
     
         6 . The system of  claim 4 , wherein the low sulfidity condition of the first cooking stage is achieved without adding anthroquinone and/or polysulfide. 
     
     
         7 . The system of any one of  claims 1 - 6 , wherein the second digester subjects the slurry of hardwood material to a temperature of between about 130° C. to about 180° C. at about 2 to about 10% effective alkaline charge as (EA) NaOH on wood. 
     
     
         8 . The system of  claim 7 , wherein the cooked hardwood material which exits the second cooking stage is between about 15 to about 30. 
     
     
         9 . The system of  claim 1 , wherein the second digester is conducted under a high condition of greater than about 20% sulfidity. 
     
     
         10 . The system of  claim 1 , wherein the high sulfidity condition of the second cooking stage is achieved by adding a sufficient amount of anthroquinone and/or polysulfide. 
     
     
         11 . The system of  claim 1 , wherein the high sulfidity condition of the second cooking stage is achieved without adding anthroquinone and/or polysulfide. 
     
     
         12 . The system of  claim 1 , further comprising prior to the first digester, a pretreatment vessel for pretreating the slurry of comminuted hardwood material at an alkaline charge of between about 20 to about 70% of total effective alkaline charge as (EA) NaOH on wood at a temperature between about 80 to about 120° C.