Method for alkaline cooking of fiber material

Abstract

A continuous, alkaline process for the production of pulp from wood chips, wherein the preheated chips are subjected to an extended impregnation step for at least 60 min, preferably longer, at a temperature not exceeding the impregnation liquor boiling point at atmospheric conditions, and a rapid heating and cooking period of less than 65 min, preferably shorter, followed by cooling to below reaction temperature. Fresh alkali is added both during impregnation and the heating/cooking period.

1 . A continuous, alkaline process for the production of pulp from wood chips, said process comprising the steps of: 
 preheating the chips;    impregnating the chips in an impregnation step with an alkaline liquor at a temperature not exceeding the boiling point of said liquor at atmospheric conditions, during a time period of at least 60 min, during which impregnation step fresh alkali is added, the average effective alkali concentration of the added liquor being in the range from about 0.5 to about 2.2 mol/l as OH ions and the liquor to wood ratio during a substantial part of the impregnation step being between 3 and 10 m 3 /t odw;    heating the impregnated chips to a temperature of at least about 150° C. and cooking the chips, the maximum temperature of the cooking step being less than 185° C. and the total time of the heating and cooking steps being less than 65 min, during which period fresh alkali is added as well as possible additional liquor, so as to result in a liquor to wood ratio of at least 2.5 m 3 /t odw during a substantial part of the heating and cooking steps, and    cooling the cooked material to a temperature where cooking reactions essentially cease.    
     
     
         2 . A process according to  claim 1 , wherein at the most 90% of the total fresh alkali calculated as total titrable alkali per unit charged wood is added during the impregnation step.  
     
     
         3 . A process according to  claim 1  or  2 , wherein the time t 1  of the impregnation step is above 120 min.  
     
     
         4 . A process according to ay claim  1 - 3 , wherein the total of time t 2  of the heating step and the time t 3  of the cooking step is in the range from 10 to 50 min, preferably in the range from 10 to 40 min.  
     
     
         5 . A process according to any claim  14 , wherein the time t 2  of the heating step is below 30.  
     
     
         6 . A process according to  claim 1 , wherein the liquid-to-wood ratio is in the range from 3 to 6 m 3 /t odw during a substantial part of the impregnation step.  
     
     
         7 . A process according to  claim 1 , wherein the liquid-to-wood ratio is at least 3 m 3 /t odw during a substantial part of the heating and cooking steps.  
     
     
         8 . A two-vessel digesting system for performing the method according to  claim 1 , comprising an impregnation vessel and a digester in fluid communication with the impregnation vessel, whereby the volume V of the impregnation vessel is larger than 1/1, preferably larger than 2/1, more preferably larger than 3/1 of the volume of the digester.