method for producing cellulose pulp in a continuous digester in an energy-efficient manner

Abstract

The method is for producing cellulose pulp in a continuous digestion system in an energy-efficient manner. Cellulose chips are fed continuously with a transfer liquid in a transfer line to a top separator arranged at the top of a digester, for the separation of free liquid from the cellulose chips. The top separator has a withdrawal space in which free liquid collects. At least a fraction of the free liquid that has separated, with a temperature T out , is led in a return line from the withdrawal space of the top separator to a position to be used as transfer liquid. At least one pressurised withdrawal of black liquor is carried out in the digester essentially at the cooking temperature of at least 135° C. An additive liquid with a temperature T in is added at the withdrawal space of the top separator, where the temperature T in is greater than the temperature T out .

1 . A method for producing cellulose pulp in a continuous digestion system in an energy-efficient manner, comprising:
 continuously feeding cellulose chips with a transfer liquid in a transfer circulation to a top separator arranged at a top of a digester, the top separator having a withdrawal space defined therein,   collecting free liquid in the withdrawal space;   separating at least a fraction of the free liquid from the cellulose chips, the separated free liquid having a temperature T out ,   leading the separated free liquid in a return line extending from the withdrawal space of the top separator to a position,   at the position, using the free liquid as the transfer liquid to transfer cellulose chips to the top separator;   carrying out at least one pressurised withdrawal of black liquor in the digester essentially at a cooking temperature of at least 135° C.; and   adding an additive liquid, having a temperature T in , at the withdrawal space of the top separator, so that the temperature T in  is greater than the temperature T out .   
     
     
         2 . The method according to  claim 1 , wherein the cellulose chips have been impregnated with impregnation liquid in an impregnation vessel before the cellulose chips are fed in the transfer liquid to the top separator. 
     
     
         3 . The method according to  claim 2 , wherein the free liquid that has been separated is led in the return line from the withdrawal space of the top separator to an outlet end of the impregnation vessel in order there to be used as the transfer liquid. 
     
     
         4 . The method according to  claim 1  wherein the additive liquid is added at the withdrawal space downstream of the return line, as seen in a direction of motion of the cellulose chips. 
     
     
         5 . The method according to  claim 1  wherein the additive liquid is mixed with the cellulose chips under an influence of a screw that feeds material upwardly in the top separator, and wherein the digester is a steam phase digester. 
     
     
         6 . The method according to  claim 1  wherein the additive liquid is constituted by a liquid that has been heated. 
     
     
         7 . The method according to  claim 6 , wherein the liquid that has been heated is constituted at least partially by a liquid withdrawn from the top separator. 
     
     
         8 . The method according to  claim 1  wherein the additive liquid is constituted at least a part of the pressurised black liquor. 
     
     
         9 . The method according to  claim 1  wherein the additive liquid is constituted by at least a part of the pressurised black liquor and at least a part of a withdrawal from the top separator that has been heated. 
     
     
         10 . The method according to  claim 1  wherein a liquid/wood ratio in the transfer line is greater than or equal to 7, and a residual liquid in the cellulose chips after the top separator lies at a liquid/wood ratio that is greater than or equal to 3, where an amount of the additive liquid that is added lies at a liquid/wood ratio greater than or equal to 1. 
     
     
         11 . The method according to  claim 1  wherein the withdrawal space extends over at least 50% of a threaded height of a screw in the top separator. 
     
     
         12 . The method according to  claim 1  wherein the additive liquid that is added to the top separator has a lower COD level than a liquid that is withdrawn from the top separator. 
     
     
         13 . The method according to  claim 1  wherein the additive liquid that is added to the top separator has a higher COD level than a liquid that is withdrawn from the top separator. 
     
     
         14 . The method according to  claim 1  wherein the additive liquid is added to the upper part of the withdrawal space through an addition line, and the return line is connected to a lower part of the withdrawal space.