Method and apparatus to decrease steam consumption in conventional batch digesters

Abstract

Method and apparatus for conventional batch digesters to retain thermal energy typically lost in the flashing blow operation by transferring the thermal energy to fresh cooking liquors from hot spent cooking liquor.

1 . In a process for producing pulp using at least two batch digesters the improvement comprising: 
 removing hot spent cooking liquor from a first digester;    passing said hot spent cooking liquor in heat exchanger with a supply of fresh cooking liquor to be supplied to a second digester thereby heating said fresh cooking liquor.    
   
   
       2 . A process according to  claim 1  including the step of withdrawing said spent cooking liquor from one of prior to or after said spent cooking liquor is passed through a heater used to heat recirculating cooking liquor in said first digester.  
   
   
       3 . A process according to  claim 1  including the step of an accumulation to store pre-heated fresh cooking liquor from said first digester.  
   
   
       4 . A process according to  claim 1  the step of, at the conclusion of a cooking cycle in said second digester is completed, hot spent cooking liquor from said first digester is used to pre-heat fresh cooking liquor supplied to said first digester.  
   
   
       5 . A process according to  claim 1  including the step of using spent cooking liquor to heat water for use in one of said pulping process or selected processes or equipment related to said pulping process.  
   
   
       6 . A process according to  claim 1  including the step of withdrawing said cooked pulp from said first digester by means of a pump upstream of a blow tank for receiving said cooked pulp.  
   
   
       7 . A system for producing pulp from lignocellulosic material comprising in combination: 
 at least two batch digesters each digester having means to introduce lignocellulosic material and cooking liquor into each of digesters, means to re-circulate cooking liquor with intermediate heating as required, and means to withdraw cooked pulp from said digesters; and    heat exchange means to use spent cooking liquor from one digester at the end of a cooking cycle to pre-heat fresh cooking liquor prior to introduction into said other digester at the beginning of a cooking cycle.