Prehydrolysis sulfate cooking process

Abstract

Processes for the preparation of prehydrolyzed pulp from lignin-containing cellulosic material are described including prehydrolyzing the cellulosic material with steam at a first pressure, terminating the prehydrolyzing by at least one addition of an alkaline liquid at the first pressure followed by at least one pressure reduction and delignifying the prehydrolyzed cellulosic material in a cooking stage. The pressure reduction can be a single pressure reduction or a gradual pressure reduction carried out in incremental stages.

1 - 8 . (canceled) 
     
     
         9 . A process for the preparation of prehydrolyzed pulp from lignin-containing cellulosic material comprising prehydrolyzing said lignin-containing cellulosic material with steam in a first steam phase at a first pressure to obtain prehydrolyzed cellulosic material, terminating said prehydrolyzing of said cellulosic material by at least one addition of an alkaline liquid at said first pressure followed by at least one pressure reduction, and delignifying said prehydrolyzed cellulosic material in a cooking stage. 
     
     
         10 . The process of  claim 9  comprising a batch process. 
     
     
         11 . The process according to  claim 9  wherein said alkaline liquid comprises a white liquor. 
     
     
         12 . The process according to  claim 9  wherein said at least one addition of said alkaline liquid comprises a single addition of said alkaline liquid at said first pressure, and wherein said at least one pressure reduction comprises a single pressure reduction to a second pressure lower than said first pressure. 
     
     
         13 . The process according to  claim 12  wherein said first pressure comprises a pressure of between about 6 and 9 bar and said second pressure comprises a pressure of between about 3 and 5 bar. 
     
     
         14 . The process according to  claim 13  wherein said single pressure reduction is carried out for a period of from about 1 second to about 10 minutes. 
     
     
         15 . The process according to  claim 14  wherein said single pressure reduction is carried out for a period of from about 30 seconds to about 5 minutes. 
     
     
         16 . The process according to  claim 9  wherein said at least one addition of said alkaline liquid comprises a single addition of said alkaline liquid, and wherein said at least one pressure reduction comprises a gradual pressure reduction. 
     
     
         17 . The process according to  claim 16  wherein said first pressure comprises a pressure of between about 6 and 9 bar, and wherein said gradual pressure reduction comprises a gradual pressure reduction from said first pressure to a second pressure, said second pressure comprising a pressure of between about 3 and 5 bar, and wherein said gradual pressure reduction is carried out by reducing said pressure in increments of from about 0.5 to 2 bar.