Method for oxygen delignification of cellulose pulp at high pressure in several steps

Abstract

The method is for the improved oxygen delignification of cellulose pulp with a medium consistency of 8-16%. The fraction of dissolved oxygen can be maintained at a high level throughout the process by the use of high pressure, greater than 15.0 bar, and by repeated agitative mixing while maintaining the high pressure, such that as large a fraction as just over 20% of the total oxygen added is dissolved in the fluid phase, and such that the amount of oxygen in the fluid phase is maintained at a high level throughout the complete high pressure section. By the establishment of retention times between the remixing operations with successively increasing retention times, while retaining a high pressure, an optimal adaptation of the remixing is obtained at the time at which a certain fraction of the oxygen dissolved in the fluid phase has been consumed.

1 . A method for continuous alkali oxygen delignification of digested cellulose pulp and of cellulose pulp that has been washed after digestion, comprising: storing pulp in a storage tower or pulp chute at essentially atmospheric pressure, maintaining the pulp at a medium consistency in a range of 8-16%, maintaining the cellulose pulp to be delignified at a kappa value of at least 15 units, the oxygen delignification taking place in a reactor system with several oxygen reactors with a predetermined retention time of the cellulose pulp in the reactor system, adding alkali to the cellulose pulp in order to obtain an initial pH exceeding 9.0, adding oxygen to the cellulose pulp at an amount of 5-50 kg per ton of pulp at a position before a first oxygen reactor in the reactor system, and providing the pulp with a predetermined total retention time greater than 45 minutes in the reactor system, in association with an addition of chemicals and an initial mixing-in operation for oxygen delignification, placing the cellulose pulp under pressure in a high pressure section of the reactor system at an initial pressure of greater than 15.0 bar, the pulp passing at least two reactor volumes with intermediate remixing positions, setting a final pressure after a final reactor volume at least 13 bar at an end of the high pressure section, where a retention time t 1  is a retention time in a reactor volume before a first remixing position M 1  such that, when a number of high-pressure reactors is X, the retention time is t 1 -t x  for each reactor R 1 -R x  such that t 1 <t 2 <. . . t x . 
   
   
       2 . The method according to  claim 1 , wherein the retention times t 1 -t x  in the reactors R 1 -R x  in the high pressure section are expressed as:
   t   min =1 minute for  t   1 , after which (t x 2 *t   x−1 ) and T max =X*10 minutes;
 (t 1 =1-10 min., t 2 =2-20 min.; t 3 =4-30 min.; t 4 =8-40 min.),     where t x <t x+1 ,   
   
   
       3 . The method according to  claim 2 , wherein oxygen is added to the cellulose pulp immediately after the initial pressure of more than 15.0 bar has been established. 
   
   
       4 . The method according to  claim 3 , wherein the pressure of the pulp is reduced after the high-pressure section to a pressure that lies under 10 -12 bar, and the pulp is heated by steam such that the temperature of the pulp is raised by at least 5° C. by the addition of steam, followed by the heated pulp being led to a reactor system in a low pressure section with a retention time that exceeds the retention time in the high pressure section. 
   
   
       5 . The method according to  claim 4 , wherein the remixing positions are constituted by fluidizing mixers, either in a form of a fluidizing pump, a fluidizing restriction, a fluidizing mixer or a restriction in a flow that results in a fall in pressure of less than 1 bar. 
   
   
       6 . The method according to  claim 1  wherein a stirrer is present in at least one high pressure reactor, which stirrer acts in the principal part of the reactor volume, either in a form of a mechanical stirrer (S) or hydrodynamic stirrers that at least circulate free fluid in the reactor. 
   
   
       7 . The method according to  claim 1  wherein at least one of the oxygen and alkali additions are added to the cellulose pulp in association with the remixing positions in the high pressure section at an amount that is lower than the amount that is added at the initial mixing-in operation, and at least one of the oxygen and alkali additions are added batch-wise at a beginning of the low pressure section. 
   
   
       8 . The method according to  claim 1  wherein the cellulose pulp is dewatered before the oxygen delignification to a higher consistency and the cellulose pulp is rediluted before the oxygen delignification to a medium consistency with pure filtrate that has been previously oxidized, and alkali in a form of oxidized white liquor is used in the oxygen delignification.