Cooking process using hot black liquor

Abstract

This invention refers to a method for producing pulp in an alkaline digesting process comprising the following steps: charging lignocellulosic material to a digester system; pre-treating said lignocellulosic material with an impregnation liquor and subsequently with a hotter liquor containing black liquor, simultaneously displacing spent liquor; heating and cooking said lignocellulosic material to produce cooked lignocellulosic material and cooking liquor; displacing a portion of said cooking liquor at cooking temperature from said digester to a first section by supplying a washing liquid; further displacing a second portion of displaced liquor from said digester system to at least a second section by supplying further washing liquid until the temperature of said second portion of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure, said second portion of displaced liquor having a dry solids content substantially lower than the initial dry solids content of the cooking liquor and substantially lower than that of said first portion of displaced liquor. The method is characterized in that substantial parts of said second portion of displaced liquor is heated by the heat of the first portion of displaced liquor, whereby said first portion having a high solids content is prevented from being reused in a digester and said heated second portion is further used as hot liquor in the pre-treatment and heating of said lignocellulosic material.

What we claim:  
     
         1 . A method for producing pulp in an alkaline digesting process comprising the following steps: 
 charging lignocellulosic material to a digester system;    pre-treating said lignocellulosic material initially with an impregnation liquor and subsequently with at least a hotter liquor containing substantially black liquor, spent liquor being displaced from the digester during said treatment;    heating and cooking said lignocellulosic material at a cooking temperature producing cooked lignocellulosic material and cooking liquor, said cooking liquor being at said cooking temperature and having an initial dry solids content;    displacing a portion of said cooking liquor at said cooking temperature from said digester to a first section by supplying a washing liquid to said digester in order to obtain a first portion of displaced cooking liquor having a first temperature and a first dry solids content;    further displacing a second portion of displaced liquor from said digester system to at least a second section by supplying further washing liquid until the temperature of said second portion of displaced liquor drops to a temperature of at least about the boiling point of said cooking liquor at atmospheric pressure, resulting in a second portion of displaced liquor having a dry solids content substantially lower than said initial dry solids content of said cooking liquor and substantially lower than said first portion of displaced liquor,    characterized in that substantial parts of said second portion of displaced liquor is heated by the heat of the first portion of displaced liquor, whereby said first portion having a high solids content is prevented from being reused in the digester, and said heated second portion is further used as hot liquor in the pre-treatment and heating of said lignocellulosic material.    
     
     
         2 . Method according to  claim 1 , characterized in that the dry solids content and temperature of displaced liquor are monitored.  
     
     
         3 . Method according to  claim 2 , characterized in that displacement of cooking liquor to the first section is continued until the dry solids content of the displaced cooking liquor drops below said initial dry solids content of cooking liquor resulting in a first portion of displaced liquor having a temperature of at least over about 85% of the cooking temperature value in Celsius degrees and a dry solids content substantially corresponding to at least over about 85% of the dry solids content of said cooking liquor.  
     
     
         4 . Method according to  claim 3 , characterized in that liquors displaced during the cooking stage is conducted to first or second section dependent on monitored dry solids content of said liquors.  
     
     
         5 . Method according to  claim 1 , characterized in that the first portion of displaced liquor having a high dry solids content is conducted through heat recovery equipments to a chemical recovery plant, i.e. an evaporation plant.  
     
     
         6 . Method according to  claim 1 , characterized in that the second portion of displaced liquor is heated by the first portion of displaced liquor in a liquid-liquid heat exchanger before introduction into the digester.  
     
     
         7 . Method according to  claim 5 , characterized in that heat of first portion of displaced liquor is used in heating white liquor.  
     
     
         8 . Method according to  claim 6 , characterized in that the temperature of the second portion of displaced liquor is adjusted by steam before it is introduced into the digester.  
     
     
         9 . Method according to  claim 8 , characterized in that alkalinity of the second portion of displaced liquor is adjusted by fresh alkali before said liquor is introduced into the digester.  
     
     
         10 . Method according to  claim 9 , characterized in that the alkalinity of the second portion of displaced liquor is adjusted in a line from said second section to the digester.  
     
     
         11 . Method according to  claim 9 , characterized in that fresh liquor, i.e. white liquor is added to the pre-treatment stage and/or to the heating/-cooking stage in order to adjust the alkalinity of the liquor in the digester.  
     
     
         12 . Method according to  claim 1 , characterized in that gas space of the first section communicates with the second section, whereby released steam from said first section heats the liquor in said second section simultaneously diluting said liquor and increasing the dry solids content of the liquor in said first section.  
     
     
         13 . Method according to  claim 12 , characterized in that sulphur compounds released from the liquor in said first section absorb into the liquor in said second section.  
     
     
         14 . Method according to  claim 1 , characterized in that liquor displaced from the digester during the pre-treatment is led to the second section, as soon as the temperature of said liquor is above about 100° C. and the dry solids content of said liquor is at about the same level as that of the second portion of displaced liquor and/or the dry solids content of said liquor is below that of the first portion of displaced liquor.  
     
     
         15 . Method according to  claim 1 , characterized in that said second section is divided into a plurality of subsections containing liquors of various dry solids content.  
     
     
         16 . Method according to  claim 15 , characterized in that substantial parts of the liquors of the subsections of second section are used as hot liquor in the pre-treatment such that the order of addition to the digester system is from highest to lowest dry solids content.  
     
     
         17 . Method according to any of the preceding claims, characterized in that said second section and its subsections is/are provided with mixing.  
     
     
         18 . Method according to any of the preceding claims, characterized in that the dry solids content of the wash liquor is lower than that of said initial cooking liquor, said first portion of displaced liquor and said second portion of displaced liquor.  
     
     
         19 . Method according to  claim 1 , characterized in that heat from first portion of displaced liquor is used to heat the lignocellulosic material during charging to the digester.  
     
     
         20 . Method according to  claim 19 , characterized in that said heating is carried out by flashing, using flash steam.  
     
     
         21 . Method according to  claim 1 , characterized in that the temperature of the impregnation liquor is below about 120° C., preferably below the boiling point of said impregnation liquor  
     
     
         22 . Method according to  claim 1 , characterized in that the temperature of the hotter liquor is about 120 to 180° C.  
     
     
         23 . Method according to  claim 1 , charact rized in that pre-treatment of the lignocellulosic material is carried out for a time and temperature corresponding to an H-factor of below 300.  
     
     
         24 . Method according to  claim 1 , characterized in that the temperature of the washing liquid is below the boiling point of said washing liquid, preferably about 85 to 50° C.  
     
     
         25 . Method according to  claim 1 , characterized in that said cooking temperature is 140 to 185° C.  
     
     
         26 . Method according to  claim 1 , characterized in that the invention is applicable both on batch and continuous digesting processes.