Method for alkaline batch cooking of fiber material

Abstract

An alkaline batch process for the production of pulp from wood chips, wherein the preheated chips are subjected to an extended impregnation step outside the digester for at least 60 min, preferably longer, at a temperature not exceeding the impregnation liquor boiling point at atmos-pheric conditions, and a rapid heating and cooking period in the digester of less than about 90 min, preferably shorter, followed by cooling to below reaction tempera-ture. Fresh alkali is added both during impregnation and the heating/cooking period.

1 . An alkaline batch process for the production of pulp from wood chips, said process comprising the steps of: 
 preheating the chips;    impregnating the chips outside the batch digester with an alkaline liquor at a temperature not exceeding the boiling point of said liquor at atmospheric conditions during a time period t 1  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 0.5 to 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;    transferring the chips to a digester,    heating the chips in the digester in a heating step to a temperature of at least 150° C. during a time period t 2  beginning after the charging of the chips when heat supply to the digester is started, during which period fresh alkali as well as possible additional liquor is added;    cooking the chips in a cooking step during a time t 3  ending when cooling of the digester starts, the total of t 2  and t 3  not exceeding about 90 min, the maximum temperature not exceeding 185° C., and the liquid-to wood ratio wood ratio during a substantial part of the heating and cooking steps being at least 2.5 m 3 /t odw; and    cooling the cooked material to a temperature where cooking reactions essentially cease.    
     
     
         2 . A process according to  claim 1 , wherein the total of t 2  and t 3  does not exceed 80 min, preferably 70 min.  
     
     
         3 . A process according to  claim 1 , wherein the total of t 2  and t 3  is between 10 and 60 min.  
     
     
         4 . A process according to any claim  1 - 3 , wherein t 2  is between 1 and 40 min, preferably between 1 and 30 min.  
     
     
         5 . A process according to any claim  14 , wherein the liquid-to-wood ratio during a substantial part of the heating and cooking steps is at least 3 m 3 /t odw, preferably at least 3.5 m 3 /t odw.  
     
     
         6 . A digesting system for performing the method according to  claim 1 , comprising at least one impregnation vessel; batch digesters in fluid communication with the impregnation vessel; transfer lines between the impregnation vessel and the bottom of each digester for transporting the impregnated material to the digester, a separator comprising a withdrawal space disposed in connection with each digester for separating a transport liquid from the impregnated material; first return lines attached to each separator to conduct the transport liquid from the separator back to the transfer lines; second return lines connected to the first return lines and to the impregnation vessel for transferring a portion of the transport liquid to an inlet of the impregnation vessel; a supply line for fresh alkali connected to an inlet of the impregnation vessel; whereby the volume V of the impregnation vessel is larger than 1.5 times the volume of each digester, preferably larger than 3 times the volume of each digester; more preferably larger than 5 times the volume of each digester.