Alkaline Process and System for Producing Pulp

Abstract

A process and an apparatus are provided for producing pulp from lignocellulosic material by an alkaline cooking process, in which process preheated chips are treated in an impregnation stage with impregnation liquor, which is taken from the impregnation stage, and concentrated to obtain both a high inorganic and organic dry solid content. The temperature is higher than in the heating of the chips and in the removal of air, and is sufficient to enable the finishing of the reactions in the impregnation stage before the cooking stage. The majority of the alkali is added to the impregnation stage. In the cooking stage alkali is added to enable the delignification to be performed to a desired degree, and at the end of the cook black liquor is added to regulate the liquid-to-wood ratio. The process can be applied both to a continuous cook and to a batch cook.

1 . An alkaline cooking process for producing pulp from lignocellulosic material, said process comprising impregnation of the raw material and cooking of the impregnated raw material in a steam/liquid phase or hydraulic digester, wherein the impregnation is performed with an alkaline impregnation liquor circulating in the impregnation stage, to which liquor fresh alkali is introduced as an alkali addition and which impregnation liquor circulating in the impregnation stage is concentrated by withdrawing impregnation liquid from the impregnation stage, removing water thereof and feeding the concentrated impregnation liquor back to the impregnation stage and that impregnation is performed at 135±35° C., the level of the inorganic and organic dry solid matter dissolved in the impregnation liquor is between 15 to 50%, and cooking is performed at 140±10° C.  
   
   
       2 . The process according to  claim 1 , wherein the cooking process is a sulfate cook, and the fresh alkali consists essentially or entirely of white liquor.  
   
   
       3 . The process according to  claim 1 , wherein the alkali addition is merely fresh alkali or essentially merely of fresh alkali.  
   
   
       4 . The process according to  claim 1 , wherein the alkali addition consists of fresh alkali and of black liquor which is introduced at the most 1.0 m 3 /Odt.  
   
   
       5 . The process according to  claim 1 , wherein the energy required for the evaporation to be carried out to concentrate the impregnation liquor is taken partly or entirely from the cooking stage.  
   
   
       6 . The process according to  claim 1 , wherein water is evaporated from the impregnation liquor by heating the impregnation liquor first indirectly with steam and by evaporating impregnation liquor.  
   
   
       7 . The process according to  claim 1 , wherein after the evaporation the impregnation liquor is heated by means of the black liquor from the cooking stage.  
   
   
       8 . The process according to  claim 1 , wherein after the evaporation the impregnation liquor is introduced into a level tank for stabilizing flow variations.  
   
   
       9 . The process according to  claim 8 , wherein the separation of extractives and soap is carried out in said tank.  
   
   
       10 . A cooking process according to  claim 1 , wherein the cooking process is a soda cook.  
   
   
       11 . The process according to  claim 1 , wherein the impregnation comprises a first step and a second step.  
   
   
       12 . The process according to  claim 11 , wherein in the second step the OH concentration and the HS concentration are essentially higher than the corresponding concentrations at the end of the first step.  
   
   
       13 . The process according to  claim 11 , wherein in the second step the HS concentration is essentially higher than at the end of the first step.  
   
   
       14 . The process according to  claim 11 , wherein in the second step the OH concentration is essentially higher than at the end of the first step.  
   
   
       15 . The process according to  claim 11 , wherein the second step of the impregnation is carried out at the beginning of the cooking stage.  
   
   
       16 . The process according to  claim 11 , wherein the impregnation comprises several steps.  
   
   
       17 . The process according to  claim 11 , wherein the duration of the first impregnation step is 15-120 minutes.  
   
   
       18 . The process according to  claim 11 , wherein the duration of the second impregnation step is 5-60 minutes.  
   
   
       19 . The process according to  claim 11 , wherein the impregnation stage is merely concurrent.  
   
   
       20 . The process according to  claim 1 , wherein the first step of the impregnation stage is concurrent and the second step is countercurrent.  
   
   
       21 . The process according to  claim 1 , wherein the impregnation stage is merely countercurrent.  
   
   
       22 . The process according to  claim 1 , wherein in the impregnation stage the effective alkali of the impregnation liquor does not exceed 1.5 mol/l NaOH.  
   
   
       23 . The process according to  claim 1 , wherein after the impregnation stage no energy is introduced to the process from the outside to elevate the temperature.  
   
   
       24 . The process according to  claim 1 , wherein the majority of the alkali to be added to the raw material is added to the steps of the impregnation.  
   
   
       25 . The process according to any  claim 1 , wherein the cooking stage comprises both a concurrent and a countercurrent zone, whose alkali concentrations and temperatures can be regulated.  
   
   
       26 . The process according to  claim 1 , wherein the dynamic liquid-to-wood ratio of the cooking stage is about 2-6 m 3 /Odt wood.  
   
   
       27 . The process according to  claim 1 , wherein anthraquinone is added to the impregnation stage and/or to the cooking stage.  
   
   
       28 . The process according to  claim 1 , wherein the cooking reactions are terminated in the digester by displacing the cooking liquor or by diluting using filtrate from a wash plant, whereby the blow is carried out at a temperature below 100° C.  
   
   
       29 . The process according to  claim 1 , wherein the pulp is blown out at a temperature essentially higher than 100° C., optionally to a blow tank provided with heat recovery.  
   
   
       30 . The process according to  claim 28 , wherein the pulp is blown out through a mechanically defibering stage to the blow tank.  
   
   
       31 . The process according to  claim 1 , wherein the cooking process is continuous.  
   
   
       32 . The process according to  claim 31 , wherein the continuous cooking process is with one or two vessels.  
   
   
       33 . The process according to  claim 32 , wherein middle pressure or low pressure steam is introduced to the steam/liquid phase digester for heating thereof.  
   
   
       34 . The process according to  claim 32 , wherein the impregnation takes place in a separate impregnation vessel or in the upper part of the digester before the cooking stage.  
   
   
       35 . The process according to  claim 1 , wherein the impregnation and cooking stages are carried out in a batch cook.  
   
   
       36 . The process according to  claim 35 , wherein the process is a displacement batch cook.  
   
   
       37 . The process according to  claim 35 , wherein the impregnation stages are carried out in a separate impregnation vessel outside the digester.  
   
   
       38 . The process according to  claim 1 , wherein the chips are heated with steam to remove gases prior to slurrying the chips with the impregnation liquor.  
   
   
       39 . The process according to  claim 1 , wherein the fresh alkali addition introduced to the impregnation liquor consists of green liquor or green liquor and other fresh alkali, especially of white liquor.  
   
   
       40 . The process according to  claim 1 , wherein water is removed in the form of steam.  
   
   
       41 . The process according to  claim 1 , wherein the amount of the inorganic and organic dry solid matter dissolved in the impregnation liquor is between 20-35%.  
   
   
       42 . A system for carrying out the process according to  claim 1 , comprising means for impregnating and cooking the raw material, means for circulating the impregnation liquor, means for introducing alkali addition, wherein the apparatus comprises means outside the impregnation apparatus for concentrating the impregnation liquor by removing water in the form of steam.  
   
   
       43 . A system according to  claim 42 , wherein the apparatus includes means for removing from the cooking process the steam or part of the steam generated in the said removal of water.