Hardwood alkaline pulping processes and systems

Abstract

Hardwood pulp rejects at a given Kappa number can be reduced by subjecting the hardwood pulp to specified cooking conditions at the end of cooking. In this regard, it has been discovered that s-lignin reacts faster than g-lignin and that there exists a higher proportion of g-lignin at the end of a hardwood cook as compared to the g-lignin present at the beginning of the cook. Thus, when cooking hardwoods, relatively harsher cooking conditions (e.g., higher cooking temperatures) at the end of the cooking have been discovered to decrease the g-lignin content which in turn decreases the percentage of pulp rejects at a given Kappa number. In preferred embodiments, methods and 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 method of continuously producing chemical cellulose pulp from a slurry of comminuted hardwood material, comprising:
 (a) 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   (b) subjecting the slurry of comminuteed 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 method of  claim 1 , wherein step (a) comprises subjecting 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 method of  claim 2 , wherein step (a) is practiced so that the Kappa number for the cooked hardwood material which exits the first cooking stage is between about 30 to about 100. 
   
   
       4 . The method of  claim 1 , wherein step (a) is conducted under a low sulfidity condition of less than about 20% sulfidity. 
   
   
       5 . The method 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 method of  claim 4 , wherein the low sulfidity condition of the first cooking stage is achieved without adding anthroquinone and/or polysulfide. 
   
   
       7 . The method of any one of  claims 1 - 6 , wherein step (b) comprises subjecting 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 method of  claim 7 , wherein step (b) is practiced so that the Kappa number for the cooked hardwood material which exits the second cooking stage is between about 15 to about 30. 
   
   
       9 . The method of  claim 1 , wherein step (b) is conducted under a high condition of greater than about 20% sulfidity. 
   
   
       10 . The method of  claim 9 , wherein the high sulfidity condition of the second cooking stage is achieved by adding a sufficient amount of anthroquinone and/or polysulfide. 
   
   
       11 . The method of  claim 9 , wherein the high sulfidity condition of the second cooking stage is achieved without adding anthroquinone and/or polysulfide. 
   
   
       12 . The method of  claim 1 , further comprising prior to step (a), the step of pretreating the slurry of comminuted hardwood material at 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. 
   
   
       13 . 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.   
   
   
       14 . The system of  claim 13 , 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. 
   
   
       15 . The system of  claim 14 , wherein the cooked hardwood material which exits the first cooking stage is between about 30 to about 100. 
   
   
       16 . The system of  claim 13 , wherein the first digester is operated under a low sulfidity condition of less than about 20% sulfidity. 
   
   
       17 . The system of  claim 16 , wherein the low sulfidity condition of the first cooking stage is achieved by adding a sufficient amount of anthroquinone and/or polysulfide. 
   
   
       18 . The system of  claim 16 , wherein the low sulfidity condition of the first cooking stage is achieved without adding anthroquinone and/or polysulfide. 
   
   
       19 . The system of any one of  claims 13 - 18 , 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. 
   
   
       20 . The system of  claim 19 , wherein the cooked hardwood material which exits the second cooking stage is between about 15 to about 30. 
   
   
       21 . The system of  claim 13 , wherein the second digester is conducted under a high condition of greater than about 20% sulfidity. 
   
   
       22 . The system of  claim 13 , wherein the high sulfidity condition of the second cooking stage is achieved by adding a sufficient amount of anthroquinone and/or polysulfide. 
   
   
       23 . The system of  claim 13 , wherein the high sulfidity condition of the second cooking stage is achieved without adding anthroquinone and/or polysulfide. 
   
   
       24 . The system of  claim 13 , 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.